SPU TTERIN G F R O M A LIQ U ID -PH A SE G A -IN EUTECTIC ALLOY KEVIN M A R K H U B B A R D YALE UNIVER SITY M A Y

SPU TTERIN G F R O M A LIQ U ID -PH A SE G A -IN EUTECTIC ALLOY KEVIN M A R K H U B B A R D YALE UNIVER SITY M A Y 1 9 8 9

ABSTRACT S p u t t e r i n g f r o m a L i q u i d - P h a s e G a - I n E u t e c t i c A l l o y T a r g e t K e v i n M a r k H u b b a r d Y a l e U n i v e r s i t y 1 9 8 9 S p u t t e r i n g e x p e r i m e n t s w e r e p e r f o r m e d w i t h a l i q u i d - p h a s e G a - I n e u t e c t i c a l l o y t a r g e t. T h e b u l k c o m p o s i t i o n o f t h e e u t e c t i c a l l o y i s 1 6. 5 a t% I n, b u t t h e r m o d y n a m ic s u r f a c e - s e g r e g a t i o n r e s u l t s i n a s u r f a c e m o n o l a y e r o f > 9 4 a t% I n. A n g u l a r d i s t r i b u t i o n s a n d p a r t i a l s p u t t e r i n g y i e l d s w e r e m e a s u r e d f o r 3, 2 5, a n d 5 0 k e V A r + b o m b a r d m e n t. C o m p a r is o n o f t h e s p u t t e r e d - f l u x c o m p o s i t i o n w i t h t h e a l l o y s u r f a c e c o m p o s i t i o n p r o f i l e a l l o w e d c a l c u l a t i o n o f t h e f r a c t i o n o f s p u t t e r e d a to m s o r i g i n a t i n g f r o m t h e f i r s t a t o m i c l a y e r. T h e r e s u l t w as f o u n d t o b e = 0. 8 7 ± o '03 a t 2 5 a n d 5 0 k e V, a n d i n c r e a s e d t o 0. 9 4 ± o io? a t 3 k e V. T h e i n c r e a s e m ay r e f l e c t a d e c r e a s e i n t h e a v e r a g e e n e r g y o f r e c o i l a to m s w i t h i n c o l l i s i o n c a s c a d e s, b e c a u s e f e w e r s e c o n d l a y e r r e c o i l a to m s w o u ld b e a b l e t o p e n e t r a t e t h e s u r f a c e l a y e r. L o w - e n e r g y ( 4-6 e V ) s e c o n d a r y - i o n m ass s p e c t r a w e r e m e a s u r e d f o r N e +, A r +, K r +, a n d X e + b o m b a rd m e n t i n t h e e n e r g y r a n g e o f 2 5-2 5 0 k e V. T h e i n t e n s i t y r a t i o I ( I n + ) / I ( G a + ) w a s f o u n d t o b e i n d e p e n d e n t o f p r o j e c t i l e e n e r g y, a s e x p e c t e d f r o m t h e c o l l i s i o n c a s c a d e t h e o r y o f s p u t t e r i n g. H o w e v e r, a s y s t e m a t i c i n c r e a s e, o f = 2 5 %, w a s o b s e r v e d as t h e p r o j e c t i l e m as s w a s i n c r e a s e d f r o m t h a t o f N e t o t h a t o f X e. T h i s w a s e s t i m a t e d t o c o r r e s p o n d t o a n i n c r e a s e o f 5% i n t h e f r a c t i o n o f

s p u t t e r e d a to m s o r i g i n a t i n g fro m t h e f i r s t a to m ic l a y e r. M o l e c u l a r / a t o m i c i o n y i e l d r a t i o s w e r e m e a s u r e d f o r 2 5-2 5 0 k e V N e +, A r +, K r +, a n d X e + b o m b a rd m e n t o f Ga a n d I n e l e m e n t a l s t a n d a r d s, a n d w e r e f o u n d t o b e n e a r l y i n d e p e n d e n t o f b o t h p r o j e c t i l e e n e r g y a n d m a s s. A p u b l i s h e d t h e o r y b a s e d o n t h e r e c o m b i n a t i o n o f i n d e p e n d e n t l y s p u t t e r e d n e u t r a l c o n s t i t u e n t s p r e d i c t s a r e l a t i v e y i e l d p r o p o r t i o n a l t o t h e s p u t t e r i n g y i e l d. T h e d i s c r e p a n c y m ay b e e x p l a i n e d i f s p u t t e r e d m o l e c u l e s a r e p r i m a r i l y f o r m e d w h e n a s i n g l e r e c o i l a to m c a u s e s t h e c o r r e l a t e d e j e c t i o n o f s e v e r a l a to m s f r o m a l o c a l i z e d r e g i o n o f t h e s u r f a c e.

Sputtering from a Liquid-Phase Ga-In Eutectic Alloy A D i s s e r t a t i o n P r e s e n t e d t o t h e F a c u l t y o f t h e G r a d u a t e S c h o o l o f Y a l e U n i v e r s i t y i n C a n d i d a c y f o r t h e D e g r e e o f D o c t o r o f P h i l o s o p h y b y K e v i n M a r k H u b b a r d M a y 1 9 8 9

/ ACKNOWLEDGMENTS I am h a p p y t o a c k n o w le d g e t h e a s s i s t a n c e o f t h e m a n y p e o p l e w i t h o u t whom t h i s w o r k c o u l d n o t h a v e b e e n c o m p l e t e d. F i r s t, I w o u l d l i k e t o t h a n k my a d v i s o r, B ob W e l l e r, who p r e s i d e d o v e r my t r a i n i n g a s a s c i e n t i s t a n d ( a l l e g e d l y ) u s e f u l m em b er o f s o c i e t y. H i s e n t h u s i a s m a n d s e n s e o f h u m o r c o n t r i b u t e d s i g n i f i c a n t l y t o t h e e n j o y m e n t o f my w o r k. I a l s o w i s h t o t h a n k my " a d o p t e d " a d v i s o r, P e t e r P a r k e r, f o r h i s a s s i s t a n c e b o t h b e f o r e a n d a f t e r B o b 's d e p a r t u r e, a n d Tom T o m b r e l l o f o r h i s h e l p f u l a d v i c e. O u r e n t i r e g r o u p i s g r a t e f u l t o D. A l l a n B r o m le y f o r h i s c o n t i n u e d i n t e r e s t i n a n d g e n e r o u s s u p p o r t o f o u r r e s e a r c h e f f o r t. I w o u l d a l s o l i k e t o t h a n k t h e m e m b e rs o f my d i s s e r t a t i o n c o m m i t t e e, D. A l l a n B r o m l e y, B ob W e l l e r, P e t e r P a r k e r, V i c t o r H e n r i c h, a n d S u b i r S a c h d e v, f o r t h e i r t i m e aijid t h o u g h t f u l c o m m e n ts o n t h e w o r k. S p e c i a l t h a n k s go t o t h e o t h e r m e m b e rs o f o u r e x p e r i m e n t a l g r o u p : M a r i W e l l e r, J o h n O 'C o n n o r, P a t t y B l a u n e r, L a u r i e B a u m e l, a n d M i c h a e l K a u r i n. O u r t i r e l e s s e f f o r t s i n t h e p u r s u i t o f d a t a w i l l n o t s o o n b e f o r g o t t e n. T h e s t a f f a t WNSL h a s p r o v i d e d i n v a l u a b l e a s s i s t a n c e i n a l l p h a s e s o f t h e w o r k. T h e y i n c l u d e R i c h a r d H y d e r a n d J o h n M c K a y ( a c c e l e r a t o r e n g i n e e r s ) ; K e n z o S a t o, P h i l C l a r k i n, T e d D u d a, a n d E d S t e p e n s k y ( a c c e l e r a t o r o p e r a t o r s ) ; A l J e d d r y, J o e C i m i n o, R a y C o m e a u, Tom L e o n a r d, a n d A l O u e l l e t t e ( m a c h in e s h o p ) ; C h a r l e s G i n g e l l, Tom B a r k e r, a n d D i c k W a g n e r ( e l e c t r o n i c s t e c h n i c i a n s ) ; J o h n B a r i s ( c o m p u t e r t e c h n i c i a n ) ; D ee B e r e n d a ( t e c h n i c a l d r a w e r ) ; M a r y A n n e S c h u l z, R i t a B o n i t o, L i s a C l o s e, ii

a n d K a r e n D e F e l i c e ( c l e r i c a l s t a f f ) ; a n d P r o f. M o s h e G a i. S p e c i a l t h a n k s a l s o t o S a r a B a t t e r, w h o k e e p s t h e P h y s i c s D e p a r t m e n t r u n n i n g. I a l s o w i s h t o t h a n k A l a n R i c e a n d D u n c a n W e a t h e r s f o r t h e i r a s s i s t a n c e w i t h t h e e x p e r i m e n t a l m e a s u r e m e n ts p e r f o r m e d a t C a l T e c h. T h e f r i e n d s h i p o f f e l l o w s t u d e n t s h a s m ad e my e x p e r i e n c e a t Y a l e m uch m o re e n j o y a b l e. I n m o re o r l e s s c h r o n o l o g i c a l o r d e r, t h e y i n c l u d e : S t e v e S t e r b e n z, A n n a H a y e s, B r o n e k D i c h t e r, T z u F a n g W a n g, T h e E n n is B r o t h e r s, P a u l a n d T o k o M a g n u s, P a u l C o t t l e, S o o n k e o n N am, D e b b i B u r n s, E r h a n G u l m e z, B. S h i v a k u m a r, M i c h a e l S m i t h, S t e v e R u g a r i, F a b i o D e l l a g i a c o m a, Z h i p i n g Z h a o, H e p i n g L i, J o e G e r m a n i, A n n C a r e l y, a n d D an B l u m e n t h a l. F i n a l l y, a n d m o s t i m p o r t a n t l y, I w i s h t o t h a n k my f a m i l y f o r t h e i r c o n s t a n t l o v e, s u p p o r t, a n d e n c o u r a g e m e n t d u r i n g t h i s a d v e n t u r e. i i i

T a b le o f C o n t e n ts A ck n o w led g m e n ts... i i T a b l e o f C o n t e n t s... i v CHAPTER O NE: IN T R O D U C T IO N... 1 1. 1 I n t r o d u c t i o n t o S p u t t e r i n g... 1 1. 2 T h e o r y o f S p u t t e r i n g... 3 1. 3 B a c k g r o u n d... 8 1. 4 E s c a p e D e p t h M e a s u r e m e n t s... 1 0 1. 5 M o l e c u l a r S p u t t e r i n g... 11 CHAPTER TWO: A L L O Y C O M P O S IT IO N A N A L Y S IS... 18 2. 1 I n t r o d u c t i o n... 18 2. 1. 1 I o n S c a t t e r i n g S p e c t r o s c o p y... 18 2. 1. 2 A u g e r E l e c t r o n S p e c t r o s c o p y... 19 2. 2 T a r g e t P r e p a r a t i o n... 2 0 2. 3 IS S A n a l y s i s... 22 2. 3. 1 A p p a r a t u s a n d E x p e r i m e n t a l P r o c e d u r e... 22 2. 3. 2 R e s u l t s.... 2 5 2. 4 AES A n a l y s i s... 27 2. 4. 1 A p p a r a t u s a n d E x p e r i m e n t a l P r o c e d u r e... 27 2. 4. 2 R e s u l t s... 28 2. 5 D i s c u s s i o n... 31 CHAPTER T H R E E : ANGULAR D IS T R IB U T IO N MEASUREMENTS... 4 9 3. 1 I n t r o d u c t i o n... 4 9 3. 2 C o l l e c t i o n o f S p u t t e r e d A to m s... 51 i v

3. 3 RBS A n a l y s i s... 52 3. 4 R e s u l t s... 5 4 3. 5 D i s c u s s i o n... 56 3. 6 A n a l y s i s o f U n c e r t a i n t i e s... 64 CHAPTER FOUR: SECONDARY-ION MEASUREMENTS... 81 4. 1 I n t r o d u c t i o n... 81 4. 2 A p p a r a t u s a n d E x p e r i m e n t a l P r o c e d u r e... 81 4. 3 R e s u l t s... 8 6 4. 4 D i s c u s s i o n... 91 4. 4. 1 M o l e c u l a r - I o n M e a s u r e m e n t s... 91 4. 4. 2 D e p t h o f O r i g i n M e a s u r e m e n t s... 96 CHAPTER FIVE: CONCLUSIONS... 117 5. 1 S um m ary a n d C o n c l u s i o n s... 1 1 7 5. 2 F u t u r e E x p e r i m e n t s... 1 2 0 L i s t o f R e f e r e n c e s... 1 2 2 v

CHAPTER ONE INTRODUCTION 1. 1 I n t r o d u c t i o n t o S p u t t e r i n g A n u m b e r o f i n t e r e s t i n g a n d i n t e r - r e l a t e d p h e n o m e n a o c c u r w h e n a m e t a l l i c t a r g e t i s b o m b a r d e d b y i o n s i n t h e e n e r g y r a n g e w h e r e t h e n u c l e a r s t o p p i n g p o w e r i s s i g n i f i c a n t. T h e p r o j e c t i l e l o s e s e n e r g y i n p a r t t h r o u g h e l a s t i c c o l l i s i o n s w i t h t a r g e t n u c l e i, d i s p l a c i n g som e o f th e m f r o m t h e i r o r i g i n a l l a t t i c e l o c a t i o n s. I f t h e e n e r g y t r a n s f e r i s s u f f i c i e n t, t h e p r i m a r y r e c o i l a to m s w i l l t h e m s e l v e s d i s p l a c e a d d i t i o n a l t a r g e t a t o m s, r e s u l t i n g i n t h e f o r m a t i o n o f a c a s c a d e o f m o v in g r e c o i l a t o m s. T h e d e v e l o p m e n t o f s u c h a c o l l i s i o n c a s c a d e i s i l l u s t r a t e d i n F i g u r e 1. 1. T h e c o l l i s i o n c a s c a d e m ay h a v e a n u m b e r o f e f f e c t s w i t h i n t h e t a r g e t. R e c o i l a to m s t h a t r e a c h t h e s u r f a c e w i t h e n o u g h e n e r g y t o o v e r c o m e t h e s u r f a c e p o t e n t i a l b a r r i e r a r e e j e c t e d ( s p u t t e r e d ), a n d i t i s s p e c i f i c a l l y t h i s p r o c e s s w h ic h w a s s t u d i e d i n t h e p r e s e n t e x p e r i m e n t s. I n a d d i t i o n, t h e c a s c a d e s m ay c a u s e c o n s i d e r a b l e d a m a g e t o t h e t a r g e t s t r u c t u r e, p o s s i b l y i n d u c i n g p h a s e t r a n s i t i o n s w i t h i n t h e a f f e c t e d v o lu m e. I f t h e t a r g e t i s m u l t i - c o m p o n e n t, t h e c o m p o s i t i o n o f t h e n e a r - s u r f a c e r e g i o n m ay b e a l t e r e d t h r o u g h b a l l i s t i c m i x i n g p r o c e s s e s, o r b y d i f f u s i o n e n h a n c e d b y t h e p r e s e n c e o f b o m b a rd m e n t i n d u c e d d e f e c t s. F u r t h e r m o r e, i f m o b i l e i n t e r s t i t i a l s ( v a c a n c i e s ) c o u p l e p r e f e r e n t i a l l y t o o n e c o m p o n e n t, t h e r e w i l l b e a n i n d u c e d s e g r e g a t i o n o f t h i s c o m p o n e n t t o w a r d s (a w a y f r o m ) d e f e c t s i n k s, i n c l u d i n g t h e t a r g e t s u r f a c e. 1

T h e e f f e c t s o f s p u t t e r i n g a r e o f t e n u n d e s i r a b l e. H i s t o r i c a l l y, 2 s p u t t e r i n g w a s f i r s t d i s c o v e r e d t o b e t h e c a u s e t h e e r o s i o n o f c a t h o d e e l e m e n t s i n g a s d i s c h a r g e d e v i c e s [ G o 0 2 ], a n d i t s e v e r e l y l i m i t s t h e l i f e t i m e o f f i l a m e n t s i n h o t - c a t h o d e p l a s m a - d i s c h a r g e i o n s o u r c e s. S p u t t e r i n g m ay a l s o b e a n im p e d im e n t t o t h e d e v e l o p m e n t a n d o p e r a t i o n o f m a g n e t i c c o n f i n e m e n t f u s i o n r e a c t o r s [ B h 8 7 ]. T h e f l u x o f e n e r g e t i c i o n s d i f f u s i n g f r o m t h e c o n f i n e d p la s m a m ay c a u s e s i g n i f i c a n t e r o s i o n o f t h e f i r s t - w a l l a n d l i m i t e r o r d i v e r t e r e l e m e n t s. P e r h a p s m o re i m p o r t a n t l y, t h e s p u t t e r e d a t o m s, w h i c h a r e t y p i c a l l y h i g h - Z, m ay e n t e r t h e c e n t r a l p l a s m a r e g i o n, w h e r e t h e y w o u ld c o n s t i t u t e a m a j o r s o u r c e o f p la s m a c o o l i n g t h r o u g h r a d i a t i o n l o s s e s. S p u t t e r i n g d o e s h a v e a n u m b e r o f u s e f u l a p p l i c a t i o n s, t h e e f f e c t i v e n e s s o f w h i c h m ay b e i m p r o v e d t h r o u g h a b e t t e r u n d e r s t a n d i n g o f t h e p r o c e s s. S p u t t e r - i o n s o u r c e s, f o r i n s t a n c e, a r e common i n a c c e l e r a t o r t e c h n o l o g y. T h r o u g h d e p o s i t i o n o f s p u t t e r e d m a t e r i a l o n t o a s u b s t r a t e, i t i s p o s s i b l e t o g r o w u n i f o r m, h i g h q u a l i t y t h i n f i l m s. By t h e i n v e r s e p r o c e s s, i t i s p o s s i b l e t o p e r f o r m m i c r o - m a c h i n i n g o f s u r f a c e s, r e m o v i n g s m a l l a m o u n ts o f m a t e r i a l i n a h i g h l y c o n t r o l l e d m a n n e r. S u b - m i c r o n r e s o l u t i o n i o n - b e a m e t c h i n g i s e m p lo y e d i n t h e m i c r o - c i r c u i t f a b r i c a t i o n i n d u s t r y, a n d t h e a c h i e v a b l e r e s o l u t i o n w i l l s o o n b e l i m i t e d b y t h e p h y s i c a l d i m e n s i o n s o f c o l l i s i o n c a s c a d e s. F i n a l l y, a n a l y s i s o f s p u t t e r e d a to m s o r i o n s b y m ass s p e c t r o s c o p i c t e c h n i q u e s p r o v i d e s a p o w e r f u l m e th o d f o r s u r f a c e c o m p o s i t i o n a n a l y s i s, a n d a l l o w s t h e d e t e r m i n a t i o n o f c o m p o s i t i o n d e p t h p r o f i l e s w h e n p e r f o r m e d i n c o n j u n c t i o n w i t h a h i g h s p u t t e r i n g r a t e i o n b e a m. S p u t t e r i n g h a s a l s o h a d a s i g n i f i c a n t i n f l u e n c e o n t h e e v o l u t i o n o f

t h e s o l a r s y s t e m. S p u t t e r i n g o f t h e M a r t i a n a t m o s p h e r e b y s o l a r w i n d 3 i o n s i s t h o u g h t t o h a v e r e m o v e d a s m uch a s 99% o f t h e N2 o r i g i n a l l y p r e s e n t, a n d t o h a v e c a u s e d s i g n i f i c a n t i s o t o p i c f r a c t i o n a t i o n o f t h e r e m a i n i n g N2 [ T o 8 2 ]. I t h a s b e e n e s t i m a t e d t h a t t h e 1 5 N / 1 4 N r a t i o h a s i n c r e a s e d b y a f a c t o r o f tw o a s a r e s u l t, i n r e a s o n a b l e a g r e e m e n t w i t h a m e a s u r e m e n t o f 1. 6 ± 0. 2 f r o m t h e V i k i n g s p a c e c r a f t. S p u t t e r i n g b y h i g h e n e r g y i o n s t r a p p e d i n t h e J o v i a n m a g n e t o s p h e r e h a s h a d a s i g n i f i c a n t e f f e c t o n t h e i n n e r G a l i l e a n s a t e l l i t e s [ J o 8 1 ]. I o n b o m b a r d m e n t o f I o m ay b e r e s p o n s i b l e f o r t h e l a r g e c l o u d s o f n e u t r a l N a a n d K a s s o c i a t e d w i t h t h a t s a t e l l i t e ' s o r b i t, a n d s p u t t e r i n g i s t h e d o m i n a n t e r o s i o n m e c h a n is m f o r H 2 0 i c e o n t h e s u r f a c e o f E u r o p a, h a v i n g r e m o v e d a t h i c k n e s s o f 5 0-5 0 0 m o v e r t h e a g e o f t h e s o l a r s y s t e m. 1. 2 T h e o r y o f S p u t t e r i n g T h e t h e o r e t i c a l d e s c r i p t i o n o f s p u t t e r i n g i s g e n e r a l l y d i v i d e d i n t o t h r e e r e g i m e s, w h i c h a r e d i s t i n g u i s h e d b y t h e m a g n i t u d e o f t h e n u c l e a r s t o p p i n g p o w e r o f t h e p r o j e c t i l e / t a r g e t c o m b i n a t i o n [ S i 81 ]. T h e t h r e e r e g i m e s a r e i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 1. 2. I n t h e p r i m a r y k n o c k o n r e g i m e, p r i m a r y - r e c o i l a to m s a r e s p u t t e r e d i f t h e y r e a c h t h e s u r f a c e w i t h s u f f i c i e n t e n e r g y t o o v e r c o m e t h e s u r f a c e p o t e n t i a l b a r r i e r, b u t i n g e n e r a l t h e y h a v e i n s u f f i c i e n t e n e r g y t o d i s p l a c e a d d i t i o n a l t a r g e t a to m s f r o m t h e i r l o c a t i o n s. S p u t t e r i n g y i e l d s a r e t h e r e f o r e q u i t e s m a l l. T h i s m e c h a n is m i s d o m i n a n t f o r p r o j e c t i l e e n e r g i e s u p t o s e v e r a l h u n d r e d e V, o r a f e w k e V f o r l i g h t i o n s s u c h as H e + i n c i d e n t o n h e a v y t a r g e t s. I n t h e l i n e a r c o l l i s i o n - c a s c a d e r e g i m e, t h e p r i m a r y - r e c o i l a to m s

4 o b t a i n s u f f i c i e n t e n e r g y t o d i s p l a c e a d d i t i o n a l t a r g e t a t o m s, a n d t o g e n e r a t e a c o l l i s i o n c a s c a d e. T h e d e n s i t y o f m o v in g r e c o i l a to m s w i t h i n t h e c a s c a d e v o lu m e, h o w e v e r, r e m a i n s s m a l l e n o u g h s o t h a t c o l l i s i o n s b e t w e e n tw o m o v in g a to m s do n o t o c c u r. N o n - l i n e a r, o r s p i k e, c a s c a d e s o c c u r w h e n t h e e l a s t i c e n e r g y l o s s o f t h e p r o j e c t i l e i s s o g r e a t t h a t t h e m a j o r i t y o f a to m s w i t h i n t h e c a s c a d e v o lu m e a r e s e t i n t o m o t i o n. I n t h i s c a s e, a t h e r m o d y n a m ic e q u i l i b r i u m m ay b e e s t a b l i s h e d am o ng t h e m o v in g a t o m s, a n d l o c a l m e l t i n g m ay o c c u r w i t h i n t h e c a s c a d e v o lu m e. S p u t t e r i n g y i e l d s f r o m a n o n l i n e a r c a s c a d e a r e s i g n i f i c a n t l y e n h a n c e d. T h i s m e c h a n is m b e c o m e s i m p o r t a n t f o r b o m b a r d m e n t b y h e a v y i o n s, s u c h a s K r + a n d X e +, n e a r t h e m axim um o f t h e i r n u c l e a r s t o p p i n g p o w e r s. F o r t h e p r o j e c t i l e / t a r g e t c o m b i n a t i o n s s t u d i e d i n t h e p r e s e n t e x p e r i m e n t s, t h e l i n e a r c o l l i s i o n - c a s c a d e i s t h e d o m i n a n t s p u t t e r i n g m e c h a n is m. T h e s t a n d a r d t h e o r y o f l i n e a r c p l l i s i o n - c a s c a d e s p u t t e r i n g i s d u e t o S ig m u n d [ S i 6 9, S i 8 1 ], T h e t h e o r y c o n s i d e r s a p r o j e c t i l e o f e n e r g y E w h i c h t r a n s f e r s e n e r g y t o t a r g e t a to m s t h r o u g h e l a s t i c c o l l i s i o n s, g e n e r a t i n g a c o l l i s i o n c a s c a d e. T h e d i s t r i b u t i o n o f r e c o i l - a to m m o t i o n w i t h i n t h e c a s c a d e i s d e t e r m i n e d a s a s t a t i s t i c a l a v e r a g e o v e r a l a r g e n u m b e r o f c a s c a d e s b y s o l u t i o n o f a B o l t z m a n n - t y p e t r a n s p o r t e q u a t i o n. T h e y i e l d o f s p u t t e r e d a t o m s, a n d t h e s p u t t e r e d - a to m e n e r g y a n d a n g u l a r d i s t r i b u t i o n s, a r e c a l c u l a t e d b y e v a l u a t i o n o f t h e r e c o i l - a t o m f l u x a c r o s s t h e t a r g e t s u r f a c e, a c c o u n t i n g f o r t h e e f f e c t s o f t h e s u r f a c e p o t e n t i a l b a r r i e r. A n u m b e r o f a s s u m p t i o n s a r e g e n e r a l l y m ade i n t h e c a l c u l a t i o n. Th e t a r g e t s t r u c t u r e i s a s s u m e d t o b e ra n d o m, s o t h a t c r y s t a l l o g r a p h i c

e f f e c t s a r e n o t c o n s i d e r e d. A l l c o l l i s i o n s a r e a s s u m e d t o b e b i n a r y a n d 5 e l a s t i c, w i t h i n e l a s t i c e n e r g y l o s s a c t i n g a s a c o n t i n u o u s e n e r g y d r a i n. T h e c o l l i s i o n c r o s s s e c t i o n i s d e t e r m i n e d b y a s c r e e n e d C o u lo m b p o t e n t i a l, a n d i s a p p r o x i m a t e d b y t h e p o w e r l a w e x p r e s s i o n 1. 1 ) f o r 0 < m < 1, w h e r e Cm i s a p a r a m e t e r d e p e n d in g o n t h e a t o m i c n u m b e rs a n d m a s s e s o f t h e p a r t i c i p a t i n g a t o m s, a n d T i s t h e k i n e t i c e n e r g y t r a n s f e r d u r i n g t h e c o l l i s i o n. T h e p a r a m e t e r m v a r i e s f r o m m - 0 f o r v e r y lo w e n e r g y c o l l i s i o n s, t o m - 1 f o r v e r y h i g h e n e r g y c o l l i s i o n s, f o r w h ic h E q u a t i o n 1. 1 r e d u c e s t o t h e R u t h e r f o r d s c a t t e r i n g c r o s s s e c t i o n. T h e t a r g e t i s a s s u m e d t o b e i n f i n i t e i n v o lu m e, w i t h t h e s u r f a c e r e p r e s e n t e d b y a n i m a g i n a r y p l a n e. B e c a u s e t h e i r c o l l i s i o n s p e c t r u m i s d o m i n a t e d b y s o f t c o l l i s i o n s, o n l y a s m a l l f r a c t i o n o f r e c o i l a to m s m ak e m u l t i p l e c r o s s i n g s o f t h e i m a g i n a r y s u r f a c e. T h e c o l l i s i o n c a s c a d e s a r e a s s u m e d t o b e l i n e a r, w h ic h h a s t h e e f f e c t o f l i n e a r i z i n g t h e t r a n s p o r t e q u a t i o n. F i n a l l y, i t i s a s s u m e d t h a t t h e i n c i d e n t p r o j e c t i l e i s o f s u f f i c i e n t l y h i g h e n e r g y s o t h a t t h e m o s t p r o b a b l e r e c o i l - a t o m e n e r g y i s m uch l e s s t h a n t h e m axim u m p o s s i b l e e n e r g y. I n t h i s l i m i t, t h e c a s c a d e i s d o m i n a t e d b y l o w - e n e r g y ( t y p i c a l l y < 1 0 e V ), l a t e - g e n e r a t i o n r e c o i l a to m s w h o s e m o t i o n i s i s o t r o p i c. B e c a u s e t h e l a t e - g e n e r a t i o n r e c o i l a to m s h a v e n o m e m o ry o f t h e i n i t i a l p r o j e c t i l e / t a r g e t a to m c o l l i s i o n s, t h e s h a p e s o f t h e d i f f e r e n t i a l s p u t t e r i n g y i e l d s a r e e x p e c t e d t o b e i n d e p e n d e n t o f p r o j e c t i l e p a r a m e t e r s. T h e s p u t t e r i n g y i e l d ( d i f f e r e n t i a l i n e n e r g y, E x, a n d a n g l e o f e m i s s i o n w i t h r e s p e c t t o t h e s u r f a c e n o r m a l, 8X) i s g i v e n b y

1-2 ) d E x dzq 1 " F* ( E * 0 ) 4 * NCm ( E ^ l ^ ) 3 ' 2 c o s ^ i ) r m i s a p a r a m e t e r d e p e n d e n t o n t h e v a l u e o f m, N i s t h e a t o m i c d e n s i t y o f t h e t a r g e t, U 0 i s t h e s u r f a c e b i n d i n g e n e r g y ( g e n e r a l l y a p p r o x i m a t e d b y t h e s u b l i m a t i o n e n e r g y ), a n d FD ( 0 ) i s t h e a m o u n t o f e n e r g y d e p o s i t e d a t t h e t a r g e t s u r f a c e, w h ic h i s p r o p o r t i o n a l t o t h e n u c l e a r s t o p p i n g p o w e r. T h e a n g u l a r d i s t r i b u t i o n o f s p u t t e r e d a to m s i s p r e d i c t e d t o h a v e a c o s i n e d e p e n d e n c e w i t h r e s p e c t t o t h e s u r f a c e n o r m a l. T h i s i s a c o n s e q u e n c e o f t h e a s s u m e d i s o t r o p y o f t h e l a t e - g e n e r a t i o n r e c o i l - a t o m m o t i o n, a n d i s a n a l o g o u s t o g a s d i f f u s i n g t h r o u g h a p i n h o l e. T h e m axim u m i n t h e s p u t t e r e d - a t o m e n e r g y d i s t r i b u t i o n i s p r e d i c t e d t o b e a t a p p r o x i m a t e l y h a l f t h e s u r f a c e b i n d i n g e n e r g y, s o t h a t m o s t s p u t t e r e d a to m s a r e o f v e r y lo w e n e r g y, t y p i c a l l y s e v e r a l e V. I n t e g r a t i o n o f E q u a t i o n 1. 2 g i v e s t h e t o t a l s p u t t e r i n g y i e l d, 1 3N V 3_ FnlE ^ O). 1 > 3 ) Y 4 N C 0 U 0 * 2 E x p e r i m e n t a l a n d t h e o r e t i c a l s p u t t e r i n g y i e l d s a r e c o m p a r e d i n F i g u r e 1. 3 f o r N e +, A r +, K r +, a n d X e + b o m b a r d m e n t o f C u. E q u a t i o n 1. 3 a d e q u a t e l y p r e d i c t s t h e s p u t t e r i n g y i e l d o v e r a w i d e r a n g e o f p r o j e c t i l e e n e r g y a n d m a s s. A q u a l i t a t i v e e s t i m a t e o f t h e c h a r a c t e r i s t i c e s c a p e d e p t h o f s p u t t e r e d a to m s w a s m ade b y S ig m u n d [ S i 6 9 ]. T h e c a l c u l a t e d r e c o i l - a t o m d i s t r i b u t i o n w a s u s e d t o d e t e r m i n e t h e n u m b e r o f r e c o i l a t o m s, w i t h i n a s u r f a c e l a y e r o f t h i c k n e s s d x, m o v in g t o w a r d s t h e s u r f a c e w i t h s u f f i c i e n t e n e r g y t o o v e r c o m e t h e s u r f a c e p o t e n t i a l b a r r i e r, b u t n e g l e c t i n g t h e e f f e c t o f e n e r g y l o s s. T h e r e s u l t i s p r o p o r t i o n a l t o t h e t h i c k n e s s d x, a n d c o m p a r is o n w i t h t h e s p u t t e r i n g y i e l d o f E q u a t i o n 1. 3

7 g i v e s 1. 4 ) T h e c a l c u l a t i o n w as r e f i n e d b y F a l c o n e a n d S ig m u n d [ F a 8 1 ], b y a s s u m in g a c o n t i n u o u s s t r a i g h t - l i n e s l o w i n g do w n o f t h e r e c o i l a t o m s. A n e x p r e s s i o n f o r t h e s p u t t e r i n g y i e l d d i f f e r e n t i a l i n d e p t h o f o r i g i n was o b t a i n e d, w i t h a c h a r a c t e r i s t i c e s c a p e d e p t h o f 1. 5 ) e s s e n t i a l l y e q u i v a l e n t t o t h e r e s u l t o f E q u a t i o n 1. 4. T h e p h y s i c a l i n t e r p r e t a t i o n o f t h e e j e c t i o n p r o c e s s w h i c h e m e r g e d f r o m t h e s e r e s u l t s i s t h a t r e c o i l a to m s p a r t i c i p a t e i n a c o l l i s i o n c a s c a d e, a n d a r e t h e n s p u t t e r e d i f t h e y r e a c h t h e s u r f a c e w i t h s u f f i c i e n t e n e r g y. T h e q u a n t i t y x 0 i s i n t e r p r e t e d a s t h e r a n g e o f l o w - e n e r g y r e c o i l a to m s i n t h e s o l i d. F o r a p u r e Ga t a r g e t, NGa - 5. 1 x 1 0 22 a t o m s /c m 3. T h e s t a n d a r d v a l u e o f C0-1. 8 0 8 A 2 t h e n g i v e s d x - 8. 1 A - 3. 3 a t o m i c l a t t i c e s p a c i n g s, s o t h a t a s i g n i f i c a n t f r a c t i o n o f s p u t t e r e d a to m s a r e e x p e c t e d t o o r i g i n a t e f r o m r a t h e r d e e p w i t h i n t h e t a r g e t. T h e e j e c t i o n p r o c e s s h a s b e e n r e i n t e r p r e t e d b y F a l c o n e ( F a 8 6 ). I f i t i s a s s u m e d t h a t c o l l i s i o n s b e t w e e n tw o m o v in g a to m s do n o t o c c u r, a n d t h a t p a r t i c l e p e n e t r a t i o n i s d e s c r i b e d b y P o i s s o n s t a t i s t i c s ( c o l l i s i o n p r o b a b i l i t i e s a r e d e t e r m i n e d o n l y b y t h e c u r r e n t r e c o i l - a t o m d i s t r i b u t i o n f u n c t i o n, a n d n o t b y p r i o r c o l l i s i o n e v e n t s ), t h e n i t c a n b e s h o w n t h a t t h e q u a n t i t y 1 /N C 0 i s t h e m ea n f r e e p a t h f o r r e c o i l - a t o m e l a s t i c c o l l i s i o n s, a n d n o t t h e r e c o i l - a t o m r a n g e. T h i s s u g g e s t s t h a t t h e m o s t p r o b a b l e t r a j e c t o r y f o r a s p u t t e r e d a to m a f t e r i t s i n i t i a l d i s p l a c e m e n t i s t o l e a v e t h e t a r g e t w i t h o u t e x p e r i e n c i n g a n a d d i t i o n a l

c o l l i s i o n. T h e v e r y l o w e n e r g y o f t h e m a j o r i t y o f r e c o i l a to m s 8 t h e r e f o r e i m p l i e s t h a t m o s t o f th e m o r i g i n a t e f r o m t h e f i r s t a t o m i c l a y e r. 1. 3 B a c k g r o u n d E x p e r i m e n t a l d e t e r m i n a t i o n o f t h e s p u t t e r e d - a t o m e s c a p e d e p t h i s q u i t e i m p o r t a n t f o r t h e u n d e r s t a n d i n g o f t h e e j e c t i o n p r o c e s s. M e a s u r e m e n t s o f t h e a n g u l a r d i s t r i b u t i o n o f s p u t t e r e d a to m s p r o v i d e a n a d d i t i o n a l p r o b e o f t h e n e a r - s u r f a c e r e c o i l - a t o m m o t i o n w i t h i n c a s c a d e s, a n d w h e n c o m b in e d w i t h m e a s u r e m e n ts o f t h e e s c a p e d e p t h, c o n s i d e r a b l e i n f o r m a t i o n a b o u t t h e s p u t t e r i n g p r o c e s s m ay b e o b t a i n e d. K n o w le d g e o f t h e e s c a p e d e p t h i s a l s o c r u c i a l i n s t u d i e s o f s p u t t e r i n g f r o m m u l t i - c o m p o n e n t t a r g e t s. T h i s i s e s p e c i a l l y t r u e w h e n s h a r p c o n c e n t r a t i o n g r a d i e n t s e x i s t a t t h e s u r f a c e, s u c h a s w h e n a t o m i c m o b i l i t y i s s u f f i c i e n t f o r s u r f a c e s e g r e g a t i o n t o o c c u r. T h e r e s u l t s o f p r e v i o u s e x p e r i m e n t s a n d c o m p u t e r s i m u l a t i o n s g e n e r a l l y i n d i c a t e t h a t 7 5-9 5 % o f s p u t t e r e d a to m s o r i g i n a t e f r o m t h e f i r s t a t o m i c l a y e r [ K e 8 6 ], T h e f i r s t e x p e r i m e n t a l m e a s u r e m e n t o f t h e s p u t t e r e d - a t o m e s c a p e d e p t h w a s p e r f o r m e d b y Dum ke e t a l. [ D u 8 3 ], u s i n g a l i q u i d G a - I n e u t e c t i c a l l o y t a r g e t. T h e a l l o y h a s a b u l k c o m p o s i t i o n o f 1 6. 5 a t% I n, b u t th e r m o d y n a m ic s u r f a c e s e g r e g a t i o n w as f o u n d t o r e s u l t i n a h i g h d e g r e e o f I n e n r i c h m e n t a t t h e s u r f a c e. A c o m b i n a t i o n o f i o n s c a t t e r i n g s p e c t r o s c o p y ( I S S ) a n d A u g e r e l e c t r o n s p e c t r o s c o p y (A E S ) a n a l y s e s i n d i c a t e d t h a t t h e a l l o y s u r f a c e c o m p o s i t i o n w a s > 9 4 a t% I n, a n d t h a t t h e I n e n r i c h m e n t w a s l i m i t e d a l m o s t c o m p l e t e l y t o t h e f i r s t a t o m i c l a y e r.

D um ke e t a l. [ D u 8 3 ] m e a s u r e d a n g u l a r d i s t r i b u t i o n s o f Ga a n d I n 9 a to m s s p u t t e r e d f r o m t h e a l l o y t a r g e t u n d e r 1 5 a n d 2 5 k e V A r + b o m b a r d m e n t, a n d f i t t h e r e s u l t s. b y a f u n c t i o n o f t h e f o r m Y ( 0 ) a c o s n ( 0 ). T h e a n g u l a r d i s t r i b u t i o n s a n d c o m p u t e r f i t s a r e show n i n F i g u r e 1. 4. F o r t h e 1 5 k e V b o m b a r d m e n t, t h e I n d i s t r i b u t i o n w a s f i t b y n I n - 2. 0 0 ± 0. 0 5, b u t f o r G a, r ig, - 3. 6 ± 0. 2. W hen t h e A r + e n e r g y w a s i n c r e a s e d t o 2 5 k e V, t h e I n d i s t r i b u t i o n w a s u n c h a n g e d, b u t t h e Ga d i s t r i b u t i o n b r o a d e n e d s i g n i f i c a n t l y, t o nga - 2. 0 ± 0. 2. T h e r a t i o o f t h e p a r t i a l s p u t t e r i n g y i e l d s, Y l n / Y G a, d e c r e a s e d f r o m 5. 6 ± 0. 4 t o 2. 6 ± 0. 2, i n d i c a t i n g a s i g n i f i c a n t i n c r e a s e i n t h e r e l a t i v e a m o u n t o f s p u t t e r e d G a. A c o m p a r is o n o f t h e s p u t t e r e d - f l u x c o m p o s i t i o n w i t h t h e a l l o y s u r f a c e c o m p o s i t i o n p r o f i l e i n d i c a t e d t h a t t h e i n c r e a s e i n t h e Ga p a r t i a l s p u t t e r i n g y i e l d r e f l e c t e d a n i n c r e a s e i n t h e r e l a t i v e c o n t r i b u t i o n o f s p u t t e r e d a to m s f r o m b e n e a t h t h e f i r s t a t o m i c l a y e r. T h e f r a c t i o n o f s p u t t e r e d a to m s o r i g i n a t i n g f r o m t h e f i r s t a t o m i c l a y e r, F x, w a s f o u n d t o d e c r e a s e f r o m 0. 8 5 ± 0. 0 3 f o r 1 5 k e V b o m b a r d m e n t, t o 0. 7 0 ± 0. 0 3 a t 2 5 k e V. T h e s t u d y o f P e l l i n e t a l. [ P e 8 5 ] p r o v i d e s e v i d e n c e f o r a s i g n i f i c a n t v a r i a t i o n i n F x w i t h p r o j e c t i l e m ass a s w e l l. L a s e r f l u o r e s c e n c e s p e c t r o s c o p y w a s u s e d t o o b s e r v e T i a n d 0 a to m y i e l d s fr o m a T i t a r g e t w i t h a s i n g l e a d s o r b e d o x y g e n u n d e r - l a y e r b e n e a t h a s i n g l e s u r f a c e l a y e r o f T i. T h e l a s e r f l u o r e s c e n c e t e c h n i q u e p e r m i t t e d t h e u s e o f a v e r y l o w i n c i d e n t i o n d o s e, w h i c h w a s n e c e s s a r y t o a v o i d t h e p e r t u r b a t i o n o f t h e n e a r - s u r f a c e c o m p o s i t i o n p r o f i l e. U n d e r 3 k e V A r + b o m b a r d m e n t, 80% o f t h e s p u t t e r e d a to m s w e r e T i, n e a r l y a l l o f w h ic h a r e e x p e c t e d o n t h e b a s i s o f c o m p u t e r s i m u l a t i o n s t o h a v e o r i g i n a t e d i n t h e

f i r s t a to m ic l a y e r. W ith a n i n c i d e n t beam o f 3 kev He+, t h e p e r c e n t a g e w a s f o u n d t o i n c r e a s e t o 87%. 10 T h e s t u d i e s o f Dum ke e t a l. [ D u 8 3 ] a n d o f P e l l i n e t a l. [ P e 8 5 ] r e p r e s e n t t h e o n l y e x p e r i m e n t a l m e a s u r e m e n ts o f t h e i n c i d e n t i o n e n e r g y o r m as s d e p e n d e n c i e s o f t h e c h a r a c t e r i s t i c e s c a p e d e p t h o f s p u t t e r e d a t o m s. I n c o n t r a s t t o t h e p r e d i c t i o n o f t h e l i n e a r c o l l i s i o n - c a s c a d e t h e o r y, t h e r e s u l t s s u g g e s t a c o n s i d e r a b l e c h a n g e i n t h e c a s c a d e d y n a m ic s a s t h e p r o j e c t i l e p a r a m e t e r s a r e v a r i e d. T h e y a r e c o n s i s t e n t w i t h a n i n c r e a s e i n t h e a v e r a g e e n e r g y o f r e c o i l a to m s a s t h e e l a s t i c e n e r g y l o s s o f t h e p r o j e c t i l e n e a r t h e t a r g e t s u r f a c e i n c r e a s e s. T h i s w o u l d a l l o w a g r e a t e r n u m b e r o f s u b - s u r f a c e r e c o i l a to m s t o p e n e t r a t e t h e s u r f a c e l a y e r a n d b e s p u t t e r e d. 1. 4 E s c a p e D e p t h M e a s u r e m e n t s T h e p r e s e n t e x p e r i m e n t s w e r e p e r f o r m e d t o i n v e s t i g a t e v a r i a t i o n s i n t h e s p u t t e r e d - a t o m e s c a p e d e p t h o v e r a w i d e r a n g e o f p r o j e c t i l e e n e r g y a n d m a s s. T h e l i q u i d G a - I n e u t e c t i c a l l o y w a s c h o s e n f o r u s e a s t h e t a r g e t m a t e r i a l t o f a c i l i t a t e c o m p a r i s o n w i t h t h e r e s u l t s o f [ D u 8 3 ]. C o n c e r n h a d b e e n e x p r e s s e d f o l l o w i n g p u b l i c a t i o n o f [D u 8 3 ] t h a t t h e s u r f a c e c o m p o s i t i o n o f t h e a l l o y m ay h a v e b e e n i n d u c e d b y t h e d i f f u s i o n o f i m p u r i t i e s [ K e 8 6 ]. T h e r e f o r e, a d d i t i o n a l IS S a n d AES a n a l y s e s o f t h e a l l o y a n d p u r e e l e m e n t a l s t a n d a r d s w e r e p e r f o r m e d, i n c o n j u n c t i o n w i t h s e c o n d a r y - i o n r e s u l t s, t o f i r m l y e s t a b l i s h t h e I n e n r i c h m e n t o f t h e f i r s t a t o m i c l a y e r o f a c l e a n a l l o y t a r g e t. A n g u l a r d i s t r i b u t i o n s a n d p a r t i a l s p u t t e r i n g y i e l d s o f Ga a n d I n a to m s s p u t t e r e d f r o m t h e a l l o y w e r e o b t a i n e d f o r 3, 2 5, a n d 5 0 k e V A r +

b o m b a r d m e n t, b y c o l l e c t i o n o f s p u t t e r e d m a t e r i a l o n t o c o l l e c t o r f o i l s, 11 a n d s u b s e q u e n t a n a l y s i s o f t h e f o i l s b y R u t h e r f o r d b a c k s c a t t e r i n g s p e c t r o s c o p y ( R B S ). T h i s a l l o w e d a d e t e r m i n a t i o n o f t h e q u a n t i t y F1, t h e f r a c t i o n o f s p u t t e r e d a to m s o r i g i n a t i n g f r o m t h e f i r s t a t o m i c l a y e r. I t a l s o a l l o w e d a c o r r e l a t i o n t o b e d e v e l o p e d b e t w e e n t h e a n g u l a r d i s t r i b u t i o n a n d d e p t h o f o r i g i n o f s p u t t e r e d a t o m s. S e c o n d a r y - i o n m ass s p e c t r a w e r e o b t a i n e d f o r 2 5-2 5 0 k e V N e +, A r +, K r +, a n d X e + b o m b a r d m e n t o f t h e a l l o y, a n d a l s o o f p u r e Ga a n d I n s t a n d a r d s. A l t h o u g h t h e c o m p o s i t i o n o f t h e s p u t t e r e d - i o n f l u x c o u l d n o t b e d e t e r m i n e d a b s o l u t e l y, r e l a t i v e v a r i a t i o n s, w h i c h w o u ld i n f e r c h a n g e s i n t h e f r a c t i o n F x, c o u l d b e a c c u r a t e l y m e a s u r e d. 1. 5 M o l e c u l a r S p u t t e r i n g T h e u s e o f s e c o n d a r y - i o n m ass s p e c t r o s c o p y a l l o w e d t h e s e p a r a t e m o n i t o r i n g o f b o t h a t o m i c a n d m o l e c u l a r - i o n s p e c i e s, s o t h a t t h e s p u t t e r i n g o f m o l e c u l a r i o n s c o u l d b e s t u d i e d a s w e l l. S p u t t e r e d m o l e c u l e s p r o v i d e i n f o r m a t i o n f r o m a v e r y l o c a l i z e d r e g i o n o f t h e s u r f a c e, a n d s o c a n b e u s e d t o s t u d y p r o b le m s o f s h o r t - r a n g e o r d e r a t s u r f a c e s. F o r e x a m p l e, W in o g r a d e t a l. [ W g 7 9 ] s t u d i e d s e c o n d a r y - i o n e m i s s i o n f r o m a N i t a r g e t w i t h a d s o r b e d l a y e r s o f 0 2 o r CO. By o b s e r v i n g t h e r e l a t i v e y i e l d s o f t h e a t o m i c a n d m o l e c u l a r i o n s, i t was p o s s i b l e t o d i s t i n g u i s h b e t w e e n t h e d i f f e r e n t p o s s i b l e b o n d i n g s i t e s a n d s y m m e t r i e s o f t h e a d s o r b e d l a y e r s. B a r b e r e t a l. [ B a 7 7 ] p e r f o r m e d a s i m i l a r e x p e r i m e n t f o r CO a d s o r p t i o n o n a v a r i e t y o f m e t a l s u r f a c e s, a n d w e r e a b l e t o d i s t i n g u i s h b e t w e e n m o l e c u l a r a n d d i s s o c i a t i v e a d s o r p t i o n. E n e r g y t r a n s f e r c a l c u l a t i o n s sh o w t h a t s p u t t e r e d m o l e c u l e s c a n n o t

b e e m i t t e d i n t a c t, u n l e s s t h e m o l e c u l e s h a v e r a t h e r l a r g e d i s s o c i a t i o n 12 e n e r g i e s ( a b o u t t w i c e t h e s u r f a c e b i n d i n g e n e r g y ) [ G e 7 5 a ]. T h i s i s i n a g r e e m e n t w i t h t h e r e s u l t s o f m o l e c u l a r d y n a m ic s s i m u l a t i o n s t h a t, f o r c l e a n m e t a l t a r g e t s, t h e s p u t t e r i n g o f m o l e c u l e s p r o c e e d s b y t h e r e c o m b i n a t i o n a b o v e t h e s u r f a c e o f i n d e p e n d e n t l y s p u t t e r e d a to m s [W g 7 8, H a 7 6 ], T h i s p r e d i c t i o n i s s u p p o r t e d e x p e r i m e n t a l l y b y m e a s u r e m e n ts o f t h e c o m p o s i t i o n o f m o l e c u l a r i o n s s p u t t e r e d f r o m w e l l c h a r a c t e r i z e d m u l t i - c o m p o n e n t t a r g e t s. S l u s s e r a n d W in o g r a d [ S 1 8 0 ] s t u d i e d s e c o n d a r y - i o n e m i s s i o n f r o m a P da g a l l o y c o v e r e d b y a n a d s o r b e d CO o v e r - l a y e r, u n d e r 3 k e V A r + b o m b a r d m e n t. A l a r g e r e l a t i v e AgCO+ y i e l d w as o b s e r v e d. B e c a u s e t h e a d s o r b e d CO h a d b e e n f o u n d t o b i n d a l m o s t e x c l u s i v e l y t o Pd a t o m s, t h e AgCO+ m u s t h a v e b e e n f o r m e d a b o v e t h e s u r f a c e fr o m i n d e p e n d e n t l y s p u t t e r e d c o n s t i t u e n t s. T h e d o m in a n c e o f t h i s m e c h a n is m f o r m e t a l t a r g e t s i s a l s o s u p p o r t e d b y t h e o b s e r v a t i o n o f n a r r o w e n e r g y d i s t r i b u t i o n s o f s p u t t e r e d m o l e c u l a r i o n s [ S 1 7 2, W i 7 9 ], S u c h a d i s t r i b u t i o n i s p r e d i c t e d b e c a u s e o n l y a to m s w i t h lo w r e l a t i v e k i n e t i c e n e r g i e s a r e a b l e f o r m s t a b l e m o l e c u l e s [ K o 7 4 ]. T o d e s c r i b e t h e p r o j e c t i l e e n e r g y a n d m as s d e p e n d e n c e o f t h e n e u t r a l d i m e r y i e l d, G e r h a r d d e v e l o p e d a t h e o r y t o c a l c u l a t e t h e s t a t i s t i c a l f r e q u e n c y o f s p u t t e r e d - a t o m r e c o m b i n a t i o n a b o v e t h e t a r g e t s u r f a c e [ G e 7 5 a ]. T h e t h e o r y i n c o r p o r a t e s t h e p r e d i c t e d e n e r g y a n d a n g u l a r d i s t r i b u t i o n s o f s p u t t e r e d a t o m s, a n d a s s u m e s a b i n o m i a l d i s t r i b u t i o n f o r t h e n u m b e r o f a to m s s p u t t e r e d f r o m a n i n d i v i d u a l c a s c a d e. A t y p i c a l r e l a t i v e d i m e r y i e l d o f s e v e r a l p e r c e n t i s p r e d i c t e d, w i t h a d e p e n d e n c e f o r t h e a b s o l u t e d i m e r y i e l d, Y 2, o n t h e t o t a l s p u t t e r i n g y i e l d, Y, o f

1.6 ) Y2 «Y2/W. 13 H e r e W i s t h e a v e r a g e a r e a o f t h e s u r f a c e f r o m w h i c h a to m s s p u t t e r e d f r o m a s i n g l e i o n i m p a c t a r e e j e c t e d. E x p e r i m e n t a l r e s u l t s f o r p r o j e c t i l e s i n t h e k e V e n e r g y r a n g e h a v e s h o w n l i t t l e v a r i a t i o n o f t h e m o l e c u l a r / a t o m i c i o n y i e l d r a t i o w i t h e i t h e r p r o j e c t i l e e n e r g y o r m a s s. F o r e x a m p l e, S t a u d e n m a i e r [ S t 7 2 ] b o m b a r d e d p o l y c r y s t a l l i n e t u n g s t e n w i t h 1 5 0 k e V N e +, A r +, K r +, X e +, a n d Cu+, a n d m e a s u r e d t h e s e c o n d a r y - i o n m as s s p e c t r a. T h e r e l a t i v e m o l e c u l a r - i o n y i e l d s a t a s e c o n d a r y - i o n e n e r g y o f 1 5 e V w e r e f o u n d t o b e i n d e p e n d e n t o f p r o j e c t i l e m a s s. I n t h e p r e s e n t e x p e r i m e n t, t h e I ( I n 2 ) / I ( I n + ), I ( G a 2 ) / I ( G a + ), a n d I ( G a ^ ) / l ( G a 2 ) y i e l d r a t i o s w e r e m o n i t o r e d o v e r t h e e n t i r e p r o j e c t i l e e n e r g y a n d m ass r a n g e s t u d i e d, t o o b t a i n a d d i t i o n a l i n f o r m a t i o n a b o u t t h e m o l e c u l a r s p u t t e r i n g p r o c e s s. U

F ig u r e 1.1 I l l u s t r a t i o n o f t h e d e v e l o p m e n t o f a c o l l i s i o n c a s c a d e, s h o w in g t h e t r a j e c t o r i e s o f t h e g e n e r a t e d r e c o i l a t o m s. Top: M o n t e C a r l o c o m p u t e r s i m u l a t i o n o f 4 k e V A r + b o m b a r d m e n t o f C u ( s u p e r p o s i t i o n o f t e n c a s c a d e s ). F ro m ( I s 7 4 ]. Bottom: F r a c t a l r e p r e s e n t a t i o n o f a s i n g l e c a s c a d e. F ro m [ R i 8 8 ].

ION BEAM 4 0 2 0 A i 0 2 0 4 0 ( A ) 2 0-4(H 60 60 1^ -» 0 1 2 0

15 F i g u r e 1. 2 I l l u s t r a t i o n o f t h e t h r e e s p u t t e r i n g r e g i m e s : a ) p r i m a r y k n o c k - o n, b ) l i n e a r c o l l i s i o n - c a s c a d e, a n d c ) n o n - l i n e a r, o r s p i k e, c a s c a d e. F ro m [ S i 8 1 ].

1 N l N o 0 o o o 0 / \ o o o m m \ o o o o o o o o I o o 0 o o / o \ o \ o o I a) b) \ \ r t \ o o o o o o o o o o o o o o o o o o o o c) )

16 F i g u r e 1. 3 M e a s u r e d t o t a l s p u t t e r i n g y i e l d s f r o m a Cu t a r g e t, a s a f u n c t i o n o f p r o j e c t i l e e n e r g y, f o r v a r i o u s i n c i d e n t i o n s. T h e d a s h e d l i n e s a r e t h e p r e d i c t i o n s o f t h e l i n e a r c o l l i s i o n - c a s c a d e t h e o r y. F ro m [ S i 8 1 ].

- ------------1 I 1 1 I I I I" n-------1---- 1 i i i i i l------ 1----1 i r r i T N*>. N* #, No**,»*, P'O, S*. Ar? Cl*0, K* Of*. J', ROC.»t d (I 317) N*P, N/*, Nt**, Af*d,Kr*A, Ar*d, I<i*A. Cu*. *»*, U'A Cu**, YONISd Ptf * ot ALMEN, tt 3ISI BRUCEII 33) N>** N#**, Ar*dKr*AX*> Ar*«, Kr*A, Xf*» WEIJSENFEIO»t ANOERSEN,BAY(t al 12) Nr'-f, A/d, K/A, X/» OUPP, SCHARMANN(t It 760, 3t3) NV*,Ar*d N/*,Ai*0,X«*d LAEGREIO. OECMSNERIt WEHNERU 170,373) Kr*+, */ ROSENBERG,WEHNERIt 798) N*» HOFER(t 336) 799) Ar*0i FERTCl 3U) >,Kr*A,Xt*d,Cu**: GUSEVACl 311] KREBSCl 311) I AKAISHI ot all t 63) I, SCHIRRWITZIt 601 I Xf**. CHENEY,PITKINIt 315) BRAUNd SOUTHERNflattl al ft 1591 3191 ), HAYWARO, WOtTER11 691 r OALEY, WEHNER,ROSENBERGft PEREt Cl 3171 3711 COPPER J I 111 l.l. t ISMAIL,5EP1IERIt I 1 I I I I II 3091 i i i 11 _j i i i i i 0.1 1 5 1.0 1 5 10 7 * 100 2 5 1000 ENERGY (kev)

17 F i g u r e 1. 4 A n g u l a r d i s t r i b u t i o n d a t a, m e a s u r e d b y Dum ke e t a l. [ D u 8 3 ], f o r 15 a n d 2 5 k e V A r + b o m b a r d m e n t o f a l i q u i d G a - I n e u t e c t i c a l l o y t a r g e t. A l s o s h o w n a r e c o m p u t e r f i t s b y a f u n c t i o n o f t h e f o r m N ( 0 ) a c o s n ( 0 ). Top: R e s u l t s f o r s p u t t e r e d G a ( i \ ; a * 3. 6 a n d 2. 0 a t 1 5 a n d 2 5 k e V, r e s p e c t i v e l y ). Bottom: R e s u l t s f o r s p u t t e r e d I n ( n I n - 2. 0 a t e a c h e n e r g y ). F ro m [ T o 8 7 ].

1.0 0. 8 0. 6 0.4 0. 2 S (fl) S (0 ) 0 1.0 0. 8 0. 6 0.4 0. 2 0-9 0-4 5 0 4 5 9 0

CHAPTER TWO ALLOY COMPOSITION ANALYSIS 2. 1 I n t r o d u c t i o n 2. 1. 1 I o n S c a t t e r i n g S p e c t r o s c o p y A c o m b i n a t i o n o f i o n s c a t t e r i n g s p e c t r o s c o p y ( I S S ) a n d A u g e r e l e c t r o n s p e c t r o s c o p y (A E S ) a n a l y s e s w a s u s e d t o d e t e r m i n e t h e c o m p o s i t i o n o f t h e f i r s t tw o a t o m i c l a y e r s o f t h e G a - I n e u t e c t i c a l l o y t a r g e t. T h e t e c h n i q u e o f IS S i s s i m i l a r i n p r i n c i p l e t o R u t h e r f o r d b a c k s c a t t e r i n g a n a l y s i s ( R B S ), b u t i s g e n e r a l l y p e r f o r m e d u s i n g h e a v y i o n s ( N e + o r A r + ) w i t h e n e r g i e s o f s e v e r a l k e V. T h e e n e r g y s p e c t r u m o f i o n s e l a s t i c a l l y b a c k s c a t t e r e d a t a k n o w n a n g l e i s m e a s u r e d, a n d t h e m a s s e s o f t h e t a r g e t a to m s a r e d e t e r m i n e d b y c l a s s i c a l k i n e m a t i c s. F o r b a c k s c a t t e r i n g a t a f i x e d a n g l e 4>, t h e r a t i o o f t h e f i n a l / i n i t i a l i o n e n e r g i e s, Ej/Eq, i s d e t e r m i n e d o n l y b y t h e p r o j e c t i l e / t a r g e t m ass r a t i o, M1 / M 2, a c c o r d i n g t o [ V a 8 1 ] 2.1} h. B K _ { -I1 -.lm)zm?.)± s in 2^ ]* + (M ^ M^ co^. Eq ( 1 + (Mj /M2) T h e i n f o r m a t i o n d e p t h f o r IS S i s c o n s i d e r a b l y s m a l l e r t h a n t h a t f o r R B S. E l a s t i c s c a t t e r i n g c r o s s s e c t i o n s f o r l o w - e n e r g y h e a v y i o n s a r e q u i t e l a r g e, t y p i c a l l y 1 0 " 18 cm2 [ R o 7 0 ] ( t y p i c a l R u t h e r f o r d c r o s s s e c t i o n s a r e = 1 0 ' 24 cm2 [ C h 7 8 ] ). T h e r e f o r e r e l a t i v e l y f e w i o n s p e n e t r a t e b e n e a t h t h e f i r s t o r s e c o n d a t o m i c l a y e r w i t h o u t e x p e r i e n c i n g a s i g n i f i c a n t d e f l e c t i o n [ B c 7 5 ]. F u r t h e r m o r e, i f i n e r t g a s i o n s a r e u s e d, t h e n e u t r a l i z a t i o n p r o b a b i l i t y d u r i n g i n t e r a c t i o n w i t h t h e t a r g e t i s q u i t e h i g h. T h e i o n f r a c t i o n o f p r o j e c t i l e s s c a t t e r e d f r o m t h e s u r f a c e l a y e r i s g e n e r a l l y < 1 0 %, a n d t h e i o n f l u x p e n e t r a t i n g t h e f i r s t 18

a t o m i c l a y e r i s a l m o s t c o m p l e t e l y n e u t r a l i z e d [ L u 8 0 a, L u 8 0 b ]. B e c a u s e 19 o f t h e l a r g e i o n i z a t i o n p o t e n t i a l s o f i n e r t g a s i o n s, t h e m o s t i m p o r t a n t n e u t r a l i z a t i o n m e c h a n is m i s a n A u g e r - t y p e m e c h a n is m [ W o 8 2 ], i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 2. 1. C o m p o s i t i o n i n f o r m a t i o n t h e r e f o r e d e r i v e s a l m o s t e x c l u s i v e l y f r o m t h e f i r s t a t o m i c l a y e r i f b a c k s c a t t e r e d io n s o n l y a r e a n a l y z e d. I t i s d i f f i c u l t t o e x t r a c t q u a n t i t a t i v e i n f o r m a t i o n f r o m IS S s p e c t r a, b e c a u s e t h e s c a t t e r i n g c r o s s s e c t i o n s a r e n o t w e l l e s t a b l i s h e d. F o r h e a v y - i o n s c a t t e r i n g a t l o w e n e r g i e s, e l e c t r o n i c s c r e e n i n g o f t h e n u c l e i i s s i g n i f i c a n t, s o t h a t t h e i n t e r a c t i o n p o t e n t i a l s a n d c r o s s s e c t i o n s c a n o n l y b e a p p r o x i m a t e d. A m o re s e r i o u s p r o b l e m i s t h e q u a n t i f i c a t i o n o f t h e n e u t r a l i z a t i o n p r o b a b i l i t i e s, w h ic h m u s t b e d e t e r m i n e d e x p e r i m e n t a l l y. Q u a n t i t a t i v e r e s u l t s c a n b e o b t a i n e d b y t h e u s e o f p u r e e l e m e n t a l s t a n d a r d s. R e l a t i v e e f f e c t i v e c r o s s s e c t i o n s, w h i c h i n c o r p o r a t e t h e d i f f e r e n c e s i n t h e n e u t r a l i z a t i o n p r o b a b i l i t i e s, c a n b e d e t e r m i n e d e x p e r i m e n t a l l y f r o m p u r e e l e m e n t a l t a r g e t s, a n d t h e r e s u l t s a p p l i e d t o s p e c t r a o b t a i n e d f r o m a m u l t i - c o m p o n e n t t a r g e t. T h e v a l i d i t y o f t h i s m e t h o d d e p e n d s o n w h e t h e r t h e n e u t r a l i z a t i o n p r o b a b i l i t y f o r a n i o n s c a t t e r i n g f r o m a n a to m i s d e p e n d e n t o n t h e e n v i r o n m e n t o f t h a t a to m ( a m a t r i x e f f e c t ). E x p e r i m e n t s s u g g e s t t h a t f o r c l e a n m e t a l t a r g e t s, i o n n e u t r a l i z a t i o n i s d e p e n d e n t o n l y o n t h e p r o j e c t i l e v e l o c i t y, w i t h l i t t l e m a t r i x e f f e c t [ V a 8 1 ]. 2. 1. 2 A u g e r E l e c t r o n S p e c t r o s c o p y M a t e r i a l s c h a r a c t e r i z a t i o n b y AES r e l i e s o n t h e p r o d u c t i o n o f c o r e e l e c t r o n - h o l e s t a t e s b y k e V e l e c t r o n b e am b o m b a r d m e n t. T h e p r o c e s s i s

i l l u s t r a t e d i n F i g u r e 2. 2. T h e c o r e - h o l e i s f i l l e d b y a n o u t e r s h e l l 20 e l e c t r o n, a n d t h e t r a n s i t i o n e n e r g y i s a b s o r b e d b y a s e c o n d o u t e r s h e l l e l e c t r o n ( t h e A u g e r e l e c t r o n ), w h i c h e s c a p e s f r o m t h e i o n. F o r i n c i d e n t e l e c t r o n s w i t h e n e r g i e s o f s e v e r a l k e V, o n l y r e l a t i v e l y s h a l l o w c o r e l e v e l s a r e e f f i c i e n t l y i o n i z e d, s o t h a t t h e b r a n c h i n g r a t i o o f t h e c o m p e t i n g p r o c e s s o f p h o t o n e m i s s i o n i s < 0. 0 4 [ B s 6 9 ]. B e c a u s e d i f f e r e n t e l e m e n t s h a v e d i f f e r e n t c h a r a c t e r i s t i c A u g e r t r a n s i t i o n s, c o m p o s i t i o n a n a l y s i s i s p o s s i b l e. T h e i n f o r m a t i o n d e p t h i s d e t e r m i n e d b y t h e r a n g e o f t h e A u g e r e l e c t r o n s d e t e c t e d, a n d m ay e x t e n d t o s e v e r a l a t o m i c l a y e r s. T h e r e f o r e, i f t h e c o m p o s i t i o n o f t h e f i r s t a t o m i c l a y e r i s d e t e r m i n e d b y I S S, A u g e r s p e c t r a c a n b e u s e d t o d e t e r m i n e t h e a v e r a g e c o m p o s i t i o n d e e p e r w i t h i n t h e t a r g e t. I n p a r t i c u l a r, t h e c o m p o s i t i o n o f t h e s e c o n d a t o m i c l a y e r c a n b e c a l c u l a t e d i f t h e c o m p o s i t i o n b e n e a t h i t i s a s s u m e d k n o w n. A u g e r e x c i t a t i o n p r o b a b i l i t i e s a r e n o t k n o w n w i t h s u f f i c i e n t a c c u r a c y, s o t h a t a s f o r I S S, r e l a t i v e s e n s i t i v i t i e s m u s t b e d e t e r m i n e d b y a n a l y s i s o f p u r e e l e m e n t a l s t a n d a r d s. I t h a s b e e n e s t a b l i s h e d t h a t f o r c l e a n m e t a l t a r g e t s, A u g e r e l e c t r o n i n t e n s i t i e s a r e p r o p o r t i o n a l t o c o n c e n t r a t i o n, a n d A u g e r e x c i t a t i o n p r o b a b i l i t i e s a r e m a t r i x i n d e p e n d e n t [ T u 8 4 ]. 2. 2 T a r g e t P r e p a r a t i o n A l l t a r g e t m a t e r i a l s w e r e o b t a i n e d c o m m e r c i a l l y f r o m A l p h a P r o d u c t s. T h e I n t a r g e t s w e r e 9 9.9 9 9 % p u r e p o l y c r y s t a l l i n e f o i l s o f t h i c k n e s s 0. 1 2 7 mm. B e f o r e b e i n g m o u n t e d i n t h e t a r g e t c h a m b e r, t h e f o i l s w e r e d e g r e a s e d i n F r e o n T F, s u c c e s s i v e l y r i n s e d i n w a rm d i s t i l l e d w a t e r a n d i s o p r o p y l a l c o h o l, a n d t h e n d r i e d. T h e Ga t a r g e t s w e r e m e l t e d

f r o m 9 9. 9 9 9 9 9 % p u r e i n g o t s. A l t h o u g h t h e m e l t i n g p o i n t ( T m - 2 9. 8 C ) i s 21 a b o v e ro o m t e m p e r a t u r e, m e l t e d Ga r e a d i l y r e m a i n s l i q u i d, i n a s u p e r c o o l e d s t a t e, b e l o w t h i s t e m p e r a t u r e. T h e e u t e c t i c a l l o y, o f c o m p o s i t i o n 8 3. 5 a t% G a, 1 6. 5 a t% I n, i s a l i q u i d a t ro o m t e m p e r a t u r e ( T m - 1 5. 7 C ). W h e n e x p o s e d t o a i r, l i q u i d G a, b o t h p u r e a n d a l l o y e d, o x i d i z e s r a p i d l y, f o r m i n g a g r e y o x i d e l a y e r w h i c h f l o a t s o n t h e s u r f a c e o f t h e l i q u i d. T h e r e f o r e, t h e Ga a n d t h e G a - I n a l l o y w e r e a lw a y s h a n d l e d i n a g l o v e b o x f i l l e d w i t h a r g o n, a n d e x p o s u r e t o a i r d u r i n g t r a n s f e r t o t h e v a c u u m c h a m b e r w a s l i m i t e d t o l e s s t h a n t e n m i n u t e s. B e f o r e r e m o v i n g t h e t a r g e t s f r o m t h e g l o v e b o x, t h e s u r f a c e s w e r e s k im m e d i n o r d e r t o re m o v e a n y v i s i b l e g a l l i u m - o x i d e. No o t h e r s p e c i a l c l e a n i n g w a s d o n e o n t h e s e t a r g e t s b e f o r e i n t r o d u c t i o n i n t o t h e v a c u u m c h a m b e r. S p e c i a l t a r g e t h o l d e r s w e r e r e q u i r e d t o h o l d t h e l i q u i d Ga a n d G a i n a l l o y t a r g e t s i n a v e r t i c a l p o s i t i o n. T h e d e s i g n i s s h o w n i n F i g u r e 2. 3. T h e s h a l l o w w e l l w as f i l l e d w i t h l i q u i d, a n d w i t h t h e m ask p l a c e d a b o v e i t, t h e s u r f a c e t e n s i o n w a s a d e q u a t e t o h o l d t h e t a r g e t m a t e r i a l i n p l a c e. T h e t a r g e t h o l d e r w a s m ade f r o m t a n t a l u m, b e c a u s e o f i t s r e s i s t a n c e t o a t t a c k b y G a. P r i o r t o m a k i n g m e a s u r e m e n t s, t h e t a r g e t s w e r e s p u t t e r - c l e a n e d b y a r a s t e r e d 3 k e V A r + b e a m p r o d u c e d b y a n i o n g u n m o u n t e d i n t h e c h a m b e r. T a r g e t c l e a n l i n e s s w a s p e r i o d i c a l l y m o n i t o r e d b y s e c o n d a r y - i o n m ass s p e c t r o m e t r y, a n d c l e a n i n g w a s c o n t i n u e d u n t i l t h e s e c o n d a r y - i o n i n t e n s i t i e s b e c a m e c o n s t a n t a s a f u n c t i o n o f c l e a n i n g d o s e. B e c a u s e t h e i o n i z a t i o n p r o b a b i l i t y o f s p u t t e r e d a to m s i s s e n s i t i v e t o t h e p r e s e n c e o f i m p u r i t i e s s u c h a s o x y g e n a t t h e t a r g e t s u r f a c e, t h i s i s a n e f f e c t i v e

m e a s u r e o f t a r g e t c l e a n l i n e s s. T h e r e q u i r e d c l e a n i n g d o s e f o r t h e I n 22 f o i l w a s t y p i c a l l y s s l. 5 x l 0 18 i o n s / c m 2. T h e d o s e r e q u i r e d f o r t h e l i q u i d Ga a n d G a - I n t a r g e t s w a s s i g n i f i c a n t l y h i g h e r, = * 6 x l0 18 i o n s / c m 2. A f t e r c l e a n i n g i n t h i s f a s h i o n, A u g e r e l e c t r o n s p e c t r o s c o p i c a n a l y s i s d e t e c t e d n o s u r f a c e c o n t a m i n a n t s. 2. 3 IS S A n a l y s i s 2. 3. 1 A p p a r a t u s a n d E x p e r i m e n t a l P r o c e d u r e T h e i o n b e a m s u s e d f o r t h e IS S a n a l y s i s w e r e p r o d u c e d b y a 3 0 0 k V C o c k c r o f t - W a l t o n a c c e l e r a t o r, s h o w n s c h e m a t i c a l l y i n F i g u r e 2. 4. T h e i o n s o u r c e w a s a h o t - c a t h o d e p la s m a d i s c h a r g e d e v i c e, w i t h t h e p la s m a a r c s u s t a i n e d b y t h e r m i o n i c e m i s s i o n o f e l e c t r o n s f r o m a t u n g s t e n f i l a m e n t. I o n s w e r e e x t r a c t e d fr o m t h e i o n s o u r c e i n t o a n e i n z e l l e n s, a n d t h e n a c c e l e r a t e d. T h e b e am w a s f o c u s e d b y a n e l e c t r o s t a t i c q u a d r u p o l e l e n s, a n d m om entum a n a l y z e d b y a 3 0 b e n d i n g m a g n e t. T h e u l t r a - h i g h v a c u u m b e a m l i n e i s s h o w n i n F i g u r e 2. 5. D i f f e r e n t i a l p u m p in g d o w n s t r e a m f r o m t h e c o l d - t r a p w a s p r o v i d e d b y i o n p u m p s. T h e b e a m l i n e i n c l u d e d a s e c o n d e l e c t r o s t a t i c q u a d r u p o l e l e n s, a n d tw o m u t u a l l y p e r p e n d i c u l a r s e t s o f d e f l e c t o r p l a t e s f o r s c a n n i n g t h e b eam a c r o s s t h e t a r g e t. A d o u b l e - a p e r t u r e c o l l i m a t o r d e f i n e d t h e b e a m s p o t s i z e a n d l o c a t i o n o n t h e t a r g e t s u r f a c e. T h e d o w n s t r e a m a p e r t u r e w as g e n e r a l l y n e g a t i v e l y b i a s e d t o s u p p r e s s e l e c t r o n s. T h e t a r g e t c h a m b e r h a d a b a s e p r e s s u r e o f 1. 0 x 1 0 ' 10 T o r r, a c h i e v a b l e a f t e r a 4 8 h o u r b a k e - o u t a t 1 0 0-1 2 0 C. D i f f e r e n t i a l p u m p in g w a s a d e q u a t e t o m a i n t a i n a p r e s s u r e o f 2-3 x 1 0 ' 10 T o r r w i t h t h e v a l v e s t o t h e b e a m l i n e a n d i o n s o u r c e o p e n, t h o u g h d u r i n g a c c e l e r a t o r o p e r a t i o n t h e p r e s s u r e i n c r e a s e d

23 t o 2-4 x 1 0 " 9 T o r r b e c a u s e o f i n e r t g a s f l o w f r o m t h e i o n s o u r c e. E n e r g y a n a l y s i s o f b a c k s c a t t e r e d i o n s w a s p e r f o r m e d w i t h a d o u b l e - f o c u s i n g, s p h e r i c a l - s e c t o r e l e c t r o s t a t i c a n a l y z e r, w h i c h h a s b e e n d e s c r i b e d i n d e t a i l e l s e w h e r e [ O C 8 6 ]. I t i s i l l u s t r a t e d i n F i g u r e 2. 6. T h e a n a l y z e r e l e c t r o d e s w e r e 1 6 0 s p h e r i c a l s e c t o r s, o f r a d i i 3. 2 5 a n d 4. 0 5 cm. T h e e n t r a n c e a s s e m b l y h a d a d e f i n i n g a p e r t u r e o f 1 mm d i a m e t e r, s u b t e n d i n g a s o l i d a n g l e o f AO - 0. 7 m s r, a n d c e n t e r e d o n t h e z e r o p o t e n t i a l r a d i u s o f t h e tw o e l e c t r o d e s. F o r t h e 1SS a n a l y s i s, b o t h g r i d s i n t h e e n t r a n c e a s s e m b l y w e r e g r o u n d e d. I o n s p a s s i n g t h r o u g h t h e s e c t o r f i e l d s e x i t e d t h e a n a l y z e r t h r o u g h a 1 mm a p e r t u r e a n d w e r e d e t e c t e d b y a G a l i l e o E l e c t r o - O p t i c s # 4 0 3 9 c h a n n e l e l e c t r o n m u l t i p l i e r, w h i c h w a s o p e r a t e d i n t h e p u l s e s a t u r a t i o n m o d e. T h e p u l s e - h e i g h t s p e c t r u m a t t h e c h a n n e l t r o n e x i t w a s t h e r e f o r e a G a u s s i a n d i s t r i b u t i o n, w i t h t h e e q u i l i b r i u m n u m b e r o f e l e c t r o n s i n t h e d e t e c t o r p u l s e d e t e r m i n e d b y s p a c e - c h a r g e e f f e c t s [ S c 6 6 ], T h i s a l l o w e d a m o re e f f e c t i v e d i s c r i m i n a t i o n a g a i n s t n o i s e p u l s e s, t h e r e b y i n c r e a s i n g s e n s i t i v i t y. T h e a n a l y z e r w a s o p e r a t e d i n t h e s e c t o r s w e e p i n g m o d e, w i t h t h e p a s s e n e r g y v a r i e d b y r a m p i n g t h e v o l t a g e s, ± V, o n t h e tw o s e c t o r e l e c t r o d e s. A p r e v i o u s c a l i b r a t i o n o f t h e a n a l y z e r e s t a b l i s h e d t h e r e l a t i o n s h i p b e t w e e n t h e p o t e n t i a l d i f f e r e n c e, AV - 2 V, a n d t h e p a s s e n e r g y, E, t o b e [O C 8 6 ] 2. 2 ) E - ( 2. 3 3 0 ± 0. 0 0 5 ) AV. T h e e l e c t r o d e p o w e r s u p p l y h a d a m axim um v o l t a g e o u t p u t o f ± 1 k V, s o t h a t i o n s w i t h e n e r g i e s u p t o 4. 6 6 k e V c o u l d b e a n a l y z e d. T h e f r a c t i o n a l e n e r g y r e s o l u t i o n o f t h e a n a l y z e r w a s a c o n s t a n t,

A E /E - 0. 0 1 1 5, s o t h a t t h e b a n d p a s s, A E, v a r i e d l i n e a r l y w i t h t h e p a s s 24 e n e r g y. T h e e l e c t r o n i c s u s e d f o r d a t a a c q u i s i t i o n a r e s h o w n i n F i g u r e 2. 7. T h e 5 0 Mn r e s i s t o r c o n n e c t e d t o t h e c h a n n e l t r o n a n o d e a c t e d a s a v o l t a g e d i v i d e r, c r e a t i n g a p o t e n t i a l d i f f e r e n c e t h a t a c c e l e r a t e d t h e e l e c t r o n s t o w a r d s t h e c h a n n e l t r o n c o l l e c t o r. T h e e l e c t r o n p u l s e s w e r e t h e n a m p l i f i e d, a n d c o u n t e d b y a m u l t i - c h a n n e l a n a l y z e r (M C A ) o p e r a t i n g i n t h e m u l t i - s c a l i n g m o d e. T h e MCA s w e e p w a s e x t e r n a l l y c o n t r o l l e d b y a P D P - 1 1 / 2 3 + c o m p u t e r, w h i c h s i m u l t a n e o u s l y c o n t r o l l e d t h e v o l t a g e s w e e p o f t h e s e c t o r e l e c t r o d e s. T h e p i c o - a m m e t e r h a d a DC v o l t a g e o u t p u t w h ic h w a s p r o p o r t i o n a l t o t h e m e a s u r e d b e a m c u r r e n t. T h i s o u t p u t w as p r o c e s s e d b y a v o l t a g e - t o - f r e q u e n c y c o n v e r t e r w h i c h p r o d u c e d a s e r i e s o f o u t p u t p u l s e s w i t h a f r e q u e n c y p r o p o r t i o n a l t o t h e b e am c u r r e n t ( 1 k H z f o r a f u l l s c a l e r e a d i n g o n t h e p i c o - a m m e t e r ). W hen a p r e s e t n u m b e r o f p u l s e s h a d b e e n c o u n t e d b y t h e c o m p u t e r, t h e v o l t a g e o u t p u t o f t h e d i g i t a l / a n a l o g c o n v e r t e r, a n d t h e p o t e n t i a l d i f f e r e n c e a c r o s s t h e s e c t o r e l e c t r o d e s, w a s i n c r e m e n t e d. T h e a c t i v e c h a n n e l o f t h e MCA was s i m u l t a n e o u s l y i n c r e m e n t e d, s o t h a t t h e c h a n n e l n u m b e r w a s p r o p o r t i o n a l t o t h e p a s s e n e r g y o f t h e a n a l y z e r. T h e v o l t a g e r a n g e o f a f u l l s w e ep w a s d e t e r m i n e d b y t h e g a i n a n d b i a s l e v e l s o f t h e b i a s a m p l i f i e r. F o r t h e IS S a n a l y s i s, t h e b i a s a m p l i f i e r w a s s e t t o s w e e p a r a n g e o f ± 0-1 0 0 0 V. I t w a s a l s o p o s s i b l e t o p r e s e t n a r r o w e r s u b - r a n g e s w i t h t h e c o m p u t e r, i n o r d e r t o r e d u c e t h e a n a l y s i s t i m e. A 6. 7 k e V N e + b e a m f r o m t h e C o c k c r o f t - W a l t o n a c c e l e r a t o r w a s u s e d f o r t h e IS S a n a l y s i s. T y p i c a l b e a m c u r r e n t s w e r e 9 na f o r t h e a n a l y s i s o f t h e p u r e s t a n d a r d s, a n d 2 n A f o r t h e a l l o y. T h e b e a m - s p o t d i a m e t e r

i n e a c h c a s e w a s < 1 mm. T h e a n a l y z e r g e o m e t r y w a s f i x e d t o o b s e r v e io n s 25 b a c k s c a t t e r e d a t a n a n g l e o f ^ - 1 3 5, a n d t h e t a r g e t w a s p o s i t i o n e d so t h a t t h e s u r f a c e n o r m a l b i s e c t e d t h e 4 5 a n g l e b e t w e e n t h e b e am a n d t h e a n a l y z e r e n t r a n c e. T h e s p e c t r a w e r e o b t a i n e d w i t h n o b i a s o n t a r g e t, so b e f o r e a n d a f t e r e a c h s p e c t r u m w a s t a k e n, t h e b e a m c u r r e n t w a s m e a s u r e d w i t h a + 1 8 0 V b i a s t o c o r r e c t t h e m e a s u r e d t o t a l i n c i d e n t c h a r g e f o r t h e e f f e c t o f s e c o n d a r y - e l e c t r o n e m i s s i o n. I n o r d e r t o a v o i d c r o s s - c o n t a m i n a t i o n o f t h e t a r g e t s d u r i n g t h e s p u t t e r - c l e a n i n g p r o c e s s, o n l y o n e t a r g e t w a s m o u n t e d o n e a c h s i d e o f t h e t a r g e t l a d d e r. I t w a s n o t, t h e r e f o r e, p o s s i b l e t o o b t a i n m e a s u r e m e n t s f r o m b o t h t h e p u r e s t a n d a r d s, a n d f r o m t h e a l l o y, i n t h e sam e v a c u u m c y c l e. T h e s p e c t r a f o r t h e p u r e G a a n d p u r e I n t a r g e t s w e r e t a k e n t o g e t h e r i n o n e c y c l e. T h e r e l a t i v e s e n s i t i v i t y w a s d e t e r m i n e d, a n d a p p l i e d t o t h e s e p a r a t e l y o b t a i n e d a l l o y d a t a. 2. 3. 2 R e s u l t s F i g u r e 2. 8 s h o w s a s u p e r p o s i t i o n o f t y p i c a l IS S s p e c t r a f o r t h e p u r e G a a n d p u r e I n s t a n d a r d s. T h e s p e c t r a h a v e b e e n c o r r e c t e d f o r t h e e n e r g y d e p e n d e n t b a n d p a s s, A E, a n d n o r m a l i z e d t o e q u a l i n c i d e n t c h a r g e, a c c o u n t i n g f o r t h e d i f f e r e n c e s i n s e c o n d a r y - e l e c t r o n e m i s s i o n. A s m o o th b a c k g r o u n d w a s t h e n s u b t r a c t e d b y a c o m p u t e r f i t t i n g p r o c e d u r e. A c c o u n t i n g f o r t h e d i f f e r e n c e s i n t a r g e t d e n s i t y, a n d a v e r a g i n g o v e r s e v e r a l p a i r s o f s p e c t r a, t h e r e l a t i v e s e n s i t i v i t y f o r t h e p u r e e l e m e n t s w a s f o u n d t o b e " 1-75 ± -06 H e r e a r e p r e s e n t s t h e e f f e c t i v e c r o s s s e c t i o n, w h i c h i n c l u d e s t h e a c t u a l

s c a t t e r i n g c r o s s s e c t i o n, t h e i o n n e u t r a l i z a t i o n p r o b a b i l i t y, a n d 26 i n s t r u m e n t a l f a c t o r s. F i g u r e 2. 9 s h o w s a t y p i c a l s p e c t r u m f r o m t h e a l l o y t a r g e t, w h i c h i s s h o w n i n t h e i n s e t p r i o r t o s u b t r a c t i o n o f t h e b a c k g r o u n d. T h e b a c k g r o u n d a t l o w e n e r g i e s i s p r i m a r i l y f r o m s p u t t e r e d G a+ a n d I n + i o n s. A t h i g h e r e n e r g i e s i t r e s u l t s f r o m m u l t i p l e s c a t t e r i n g e v e n t s, b o t h fr o m t h e s u r f a c e l a y e r a n d f r o m d e e p e r w i t h i n t h e t a r g e t. A l s o e v i d e n t i s a s m a l l p e a k a t 1. 8 k e V, w h i c h r e p r e s e n t s N e 2+ i o n s b a c k s c a t t e r e d f r o m I n a t o m s. F i g u r e 2. 1 0 i l l u s t r a t e s t h e b a c k g r o u n d s u b t r a c t i o n p r o c e d u r e f o r t h e a l l o y s p e c t r u m. T h e f i t t i n g p r o c e d u r e w a s p e r f o r m e d u s i n g a p r o g r a m b a s e d o n t h e r o u t i n e C U R F IT [ B e 6 9 ]. B o t h p e a k s w e r e f i t t o a s k e w e d G a u s s i a n s u p e r im p o s e d o n a q u a d r a t i c b a c k g r o u n d. C o n s i d e r a b l e I n e n r i c h m e n t o f t h e a l l o y s u r f a c e i s e v i d e n t i n t h e s p e c t r u m o f F i g u r e 2. 9. T h e r a t i o o f t h e i n t e g r a t e d p e a k a r e a s, a v e r a g e d o v e r s e v e r a l s p e c t r a, a n d c o r r e c t e d f o r t h e r e l a t i v e s e n s i t i v i t y, w a s f o u n d t o b e C ( I n ) / C ( G a ) - 4. 3 2 ± 0. 1 6. T h i s c o r r e s p o n d s t o a s u r f a c e c o m p o s i t i o n o f 8 1. 2 ± 0. 6 a t% I n. T h e m a j o r p o t e n t i a l s o u r c e o f e r r o r i n t h e c a l c u l a t i o n i s t h e u n c e r t a i n t y i n t h e r a t i o o f t h e s e c o n d a r y - e l e c t r o n c o r r e c t i o n s t o t h e i n c i d e n t c h a r g e f o r t h e p u r e G a a n d I n s t a n d a r d s. T h i s i s e s t i m a t e d t o b e ± 3%. T h e o t h e r s o u r c e o f p o t e n t i a l e r r o r i s t h e c o m p u t e r f i t t i n g p r o c e d u r e. T h e u n c e r t a i n t y i n p e a k a r e a s f o r t h e p u r e G a a n d I n s t a n d a r d s w a s a lw a y s < 1 %. B e c a u s e o f t h e d e p l e t i o n o f Ga a t t h e a l l o y s u r f a c e, t h e s t a t i s t i c a l u n c e r t a i n t y f o r t h e G a p e a k s i n t h e a l l o y s p e c t r a w as g r e a t e r, < 3 %. A n y s y s t e m a t i c e r r o r i n t h e b e a m c u r r e n t m e a s u r e m e n t w i l l

h a v e n o e f f e c t, b e c a u s e o n l y r a t i o s o f t h e t o t a l i n c i d e n t c h a r g e o c c u r 27 i n t h e a n a l y s i s. 2. 4 AES A n a l y s i s 2. 4. 1 A p p a r a t u s a n d E x p e r i m e n t a l P r o c e d u r e A u g e r e l e c t r o n s p e c t r a w e r e m e a s u r e d w i t h a P e r k i n - E l m e r P H I 1 0-1 5 5 s i n g l e - p a s s, e l e c t r o s t a t i c c y l i n d r i c a l - m i r r o r e n e r g y a n a l y z e r, i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 2. 1 1. T h e u n i t w a s e q u i p p e d w i t h a c o a x i a l e l e c t r o n g u n a n d e l e c t r o n - m u l t i p l i e r d e t e c t o r. T h e e l e c t r o n b e a m e n e r g y w a s 3 k e V, w i t h a t y p i c a l b e a m c u r r e n t o f 6 0 pk. E l e c t r o n s e m i t t e d f r o m t h e t a r g e t w i t h i n a n a n g u l a r r a n g e o f 4 8 ± 6 C w i t h r e s p e c t t o t h e s u r f a c e n o r m a l w e r e e n e r g y a n a l y z e d. T h e e n e r g y s p e c t r a o f e m i t t e d e l e c t r o n s a r e g e n e r a l l y d o m i n a t e d b y i n e l a s t i c a l l y s c a t t e r e d a n d s e c o n d a r y e l e c t r o n s, s o t h a t p e a k s f r o m A u g e r t r a n s i t i o n s a r e o f t e n d i f f i c u l t t o d e t e c t. T h e r e f o r e t h e d e r i v a t i v e s p e c t r u m, d N / d E, was m e a s u r e d b y m o d u l a t i n g t h e p a s s e n e r g y o f t h e a n a l y z e r, a n d o b s e r v i n g t h e AC s i g n a l f r o m t h e d e t e c t o r w i t h a l o c k - i n a m p l i f i e r r e f e r e n c e d t o t h e m o d u l a t i o n v o l t a g e. A m o d u l a t i o n a m p l i t u d e o f 3 e V w a s u s e d f o r r e c o r d i n g A u g e r f e a t u r e s a t e n e r g i e s b e l o w 6 0 0 e V, b u t w a s i n c r e a s e d t o 6 e V f o r h i g h e r e l e c t r o n e n e r g i e s t o i n c r e a s e s e n s i t i v i t y. T h e s p e c t r a w e r e p l o t t e d o n a n x - y r e c o r d e r, a n d p e a k - t o - p e a k a m p l i t u d e s w e r e m e a s u r e d f o r t h e 4 0 4 e V (M N N ) A u g e r e l e c t r o n t r a n s i t i o n i n I n, a n d f o r t h e 1 0 7 0 e V (L M M ) A u g e r e l e c t r o n t r a n s i t i o n i n G a. A s w i t h t h e IS S a n a l y s i s, t h e r e l a t i v e s e n s i t i v i t y f o r t h e p u r e e l e m e n t s w a s f i r s t d e t e r m i n e d, a n d t h e n a p p l i e d t o t h e s e p a r a t e l y o b t a i n e d a l l o y d a t a.

28 2.4.2 Results T h e h i g h d e g r e e o f I n e n r i c h m e n t a t t h e a l l o y s u r f a c e w a s c o n f i r m e d b y t h e AES a n a l y s i s. S how n i n F i g u r e 2. 1 2 a r e t y p i c a l A u g e r s p e c t r a f r o m t h e p u r e G a a n d I n s t a n d a r d s, a f t e r s p u t t e r - c l e a n i n g. A u g e r s p e c t r a f r o m t h e G a - I n a l l o y t a r g e t a r e s h o w n i n F i g u r e 2. 1 3, b o t h b e f o r e a n d a f t e r s p u t t e r - c l e a n i n g. A u g e r s p e c t r a f r o m t h e a l l o y p r i o r t o s p u t t e r - c l e a n i n g a r e d o m i n a t e d b y t h e p r e s e n c e o f GaO a n d C a t t h e s u r f a c e. V i r t u a l l y n o I n i s o b s e r v e d. A f t e r s p u t t e r - c l e a n i n g, C a n d 0 a r e n o l o n g e r d e t e c t a b l e, a n d t h e I n e n r i c h m e n t a t t h e s u r f a c e i s e v i d e n t. H o w e v e r t h e i n t e n s i t y o f t h e G a ( 1 0 7 0 e V ) e l e c t r o n s, m o s t o f w h i c h o r i g i n a t e f r o m b e n e a t h t h e f i r s t a t o m i c l a y e r, r e m a i n s s t r o n g. T h i s s u g g e s t s t h a t t h e I n e n r i c h m e n t w a s l i m i t e d a l m o s t c o m p l e t e l y t o t h e f i r s t a t o m i c l a y e r. T h e r e l a t i v e s e n s i t i v i t y, S, f o r t h e p u r e G a a n d I n s t a n d a r d s w as c a l c u l a t e d f r o m t h e p e a k - t o - p e a k a m p l i t u d e s o f t h e d N /d E s p e c t r a. T h e r e s u l t, a v e r a g e d o v e r s e v e r a l p a i r s o f s p e c t r a, w as f o u n d t o b e S - I n ( 4 0 4 e V ) / G a ( 1 0 7 0 e V ) - 6. 4 0 ± 0. 2 6. T h i s a g r e e s w e l l w i t h t h e r e f e r e n c e v a l u e o f 6. 6 5 [ D v 7 8 ], s i n c e t h e d e n s i t y o f l i q u i d Ga ( u s e d i n t h e p r e s e n t e x p e r i m e n t ) i s 3% g r e a t e r t h a n t h e d e n s i t y o f s o l i d G a. T h e c o r r e s p o n d i n g r a t i o, A, f o r t h e a l l o y t a r g e t w a s f o u n d t o b e A - I n ( 4 0 4 e V ) / G a ( 1 0 7 0 e V ) - 3. 8 9 ± 0. 1 2, a l s o a v e r a g e d o v e r s e v e r a l s p e c t r a. i I n o r d e r t o c a l c u l a t e t h e a l l o y c o m p o s i t i o n, t h e r a t i o s m u s t b e c o r r e c t e d f o r t h e d i f f e r e n c e s i n t a r g e t d e n s i t y, t h e c o n t r i b u t i o n t o A u g e r e x c i t a t i o n f r o m b a c k s c a t t e r e d p r i m a r y e l e c t r o n s, a n d t h e e s c a p e

29 d e p t h s o f t h e o b s e r v e d A u g e r e l e c t r o n s. T h i s w a s d o n e f o l l o w i n g the f o r m a l i s m o f O v e r b u r y a n d S o m o r j a i [ O v 7 6 ]. T h e r a t i o o f t h e b a c k s c a t t e r i n g f a c t o r s, r x, f o r t h e p u r e s t a n d a r d s w a s o b t a i n e d f r o m the M o n t e C a r l o c a l c u l a t i o n s o f I c h i m u r a [ I c 8 1 ], w h i c h g i v e r G a / r In = p - 0. 8. T h e s e c a l c u l a t i o n s d e s c r i b e t h e p e n e t r a t i o n o f e l e c t r o n s i n m a t t e r o v e r a w i d e r a n g e o f a t o m i c n u m b e r, i n c o r p o r a t i n g b o t h e l a s t i c a n d i n e l a s t i c e n e r g y l o s s. T h e v a l u e o f p - 0. 8 is c o n s i s t e n t w i t h t h e e x p e r i m e n t a l r e s u l t s o f T a g a e t a l. [ T a 7 9 ], i n t e r p o l a t e d f o r t h e r e l e v a n t a t o m i c n u m b e r s, a n d e x t r a p o l a t e d to 3 k e V i n c i d e n t e l e c t r o n e n e r g y. T h e a t t e n u a t i o n w i t h d e p t h o f t h e A u g e r e l e c t r o n i n t e n s i t y w a s a s s u m e d t o b e e x p o n e n t i a l w i t h t h e r a n g e s,, c a l c u l a t e d f r o m t h e w o r k o f P e n n [ P n 7 6 ], H e r e, t h e s u p e r s c r i p t e is t h e A u g e r e l e c t r o n e n e r g y in ev, a n d t h e s u b s c r i p t i r e f e r s t o t h e t a r g e t m a t e r i a l. T h e r e s u l t s a r e Aj 4-6. 8 7 A a n d Aj 70-1 4. 8 A f o r e l e c t r o n s i n a n I n t a r g e t, a n d 5. 8 4 A a n d AG a 70-1 2. 4 A f o r e l e c t r o n s i n Ga. I f t h e d e v i a t i o n f r o m t h e b u l k c o m p o s i t i o n is l i m i t e d e n t i r e l y to t h e f i r s t a t o m i c l a y e r, t h e n t h e m e a s u r e d r a t i o s S a n d A a r e r e l a t e d a c c o r d i n g t o 2 A _ r B. x, ( l - A* *)NX + x h A * * N L n.. j j L 2 * r lb (1 - X j ) ( l - Aj 0 7 0 ) N ^ n + (1 - x b ) A X 7 NGa N f n w h e r e x x (xb ) is t h e I n c o n c e n t r a t i o n i n t h e f i r s t a t o m i c l a y e r ( b u l k ), a n d N x is t h e a t o m i c d e n s i t y o f e l e m e n t i i n t h e l i q u i d (L) o r s o l i d (S) p h a s e. T h e p a r a m e t e r A x is t h e f r a c t i o n o f d e t e c t e d A u g e r e l e c t r o n s o f e n e r g y e o r i g i n a t i n g f r o m b e n e a t h t h e f i r s t a t o m i c l a y e r. T h i s is g i v e n i n g e n e r a l f o r t h e j 't h a t o m i c l a y e r b y

30 2.4) AJ - exp[v ;fd (#)] w h e r e d is t h e a t o m i c l a t t i c e s p a c i n g a n d 6 is t h e e j e c t i o n a n g l e, w i t h r e s p e c t t o t h e s u r f a c e n o r m a l, o f t h e d e t e c t e d A u g e r e l e c t r o n s. C a l c u l a t e d v a l u e s o f Aj a r e s h o w n i n T a b l e 2. 1 f o r 4 0 4 e V a n d 1 0 7 0 e V e l e c t r o n s i n b o t h p u r e G a a n d p u r e In. B e c a u s e o f t h e h i g h d e g r e e o f In e n r i c h m e n t i n t h e f i r s t a t o m i c l a y e r, v a l u e s a p p r o p r i a t e t o p u r e I n w e r e u s e d f o r t h e c a l c u l a t i o n s. S o l v i n g E q u a t i o n 2. 3 f o r Xj, a n d s u b s t i t u t i n g f o r t h e o t h e r p a r a m e t e r s, t h e f i r s t l a y e r I n c o n c e n t r a t i o n w a s f o u n d t o b e x x - 0. 9 1 ± 0. 1 1. U s i n g t h e v a l u e o f x x - 0. 8 1 2 o b t a i n e d f r o m t h e I S S a n a l y s i s, a n d a s s u m i n g a b u l k c o m p o s i t i o n b e n e a t h t h e s e c o n d a t o m i c l a y e r, t h e A u g e r d a t a c a n b e u s e d to c a l c u l a t e x 2, t h e I n c o n c e n t r a t i o n i n t h e s e c o n d a t o m i c l a y e r. A n e q u a t i o n s i m i l a r to E q u a t i o n 2. 3 is e m p l o y e d : 2. 5 ) A _ Ifi*. Xl (1-A* 0 4 ) N j t, + x, ( A * * - A * * ) N L + x_h A j ^ N L s r In ( l - x 1 ) ( l - A l 0 7 0 ) N ^ + ( l - X 2 ) ( A j 0 7 -A 0 7 0 ) N k a + ( l - x b )Ai 0 7 0 N a N f n S o l v i n g f o r x 2 g i v e s a s e c o n d l a y e r I n c o n c e n t r a t i o n o f x 2 = 0. 2 9 ± 0. 1 3, a s l i g h t e n r i c h m e n t r e l a t i v e t o t h e b u l k c o m p o s i t i o n. T h e m a i n s o u r c e o f p o t e n t i a l e r r o r i n t h e c a l c u l a t i o n o f Xi a n d x 2 is t h e u n c e r t a i n t y i n t h e v a l u e s o f p a n d A. T h e q u o t e d e r r o r s f o r the I n c o n c e n t r a t i o n s h a v e i n c o r p o r a t e d a 1 0 % u n c e r t a i n t y f o r e a c h. T h e m a j o r s o u r c e o f e x p e r i m e n t a l u n c e r t a i n t y is t h e r e c o r d i n g o f t h e A u g e r s p e c t r a, a n d t h e m e a s u r e m e n t o f t h e p e a k - t o - p e a k a m p l i t u d e s, b e c a u s e o f t h e r e l a t i v e l a c k o f p r e c i s i o n o f t h e x - y r e c o r d e r. T h e a m p l i t u d e s o f t h e A u g e r f e a t u r e s w e r e f o u n d to b e r e p r o d u c i b l e to w i t h i n ±3% f r o m s p e c t r u m t o s p e c t r u m. A n o t h e r p o t e n t i a l s o u r c e o f e r r o r is t h e p o s i t i o n

31 o f t h e t a r g e t w i t h r e s p e c t t o t h e a n a l y z e r e n t r a n c e. T h e c o r r e c t t a r g e t p o s i t i o n w a s d e t e r m i n e d b y m o v i n g t h e t a r g e t a l o n g t h e a n a l y z e r a x i s u n t i l t h e m a g n i t u d e o f t h e e l a s t i c b a c k s c a t t e r i n g p e a k w a s a m a x i m u m. T h e p o s i t i o n d e t e r m i n e d i n t h i s m a n n e r w a s r e p r o d u c i b l e t o w i t h i n ± 0.03 m m. B e c a u s e d e v i a t i o n s f r o m t h e o p t i m u m p o s i t i o n o n t h e o r d e r o f ±0.5 m m w e r e r e q u i r e d t o c a u s e s i g n i f i c a n t c h a n g e s i n t h e p e a k - t o - p e a k a m p l i t u d e s o f t h e A u g e r f e a t u r e s, t h e t a r g e t p o s i t i o n is n o t b e l i e v e d to b e a s i g n i f i c a n t s o u r c e o f e r r o r. I f o n l y t h e e x p e r i m e n t a l u n c e r t a i n t i e s a r e i n c l u d e d, t h e r e s u l t s a r e x x - 0. 9 1 ± 0. 0 5 a n d x 2-0. 2 9 ± 0. 0 6. 2. 5 D i s c u s s i o n T h e I S S r e s u l t o f x x - 0. 8 1 2 ± 0. 0 0 6 f o r t h e s u r f a c e I n c o n c e n t r a t i o n o f t h e G a - I n e u t e c t i c a l l o y is c o n s i d e r a b l y l o w e r t h a n t h a t o f Xj > 0. 9 4 m e a s u r e d b y D u m k e e t a l. [ D u 8 3 ]. T h e m o s t p r o b a b l e r e a s o n f o r t h e d i s c r e p a n c y is a n i n c r e a s e d c o n t r i b u t i o n i n t h e p r e s e n t e x p e r i m e n t f r o m s i n g l e a n d q u a s i - s i n g l e s c a t t e r i n g e v e n t s f r o m the s e c o n d a t o m i c l a y e r. A n a n a l y s i s b e a m o f 2 k e V A r + w a s u s e d i n [ D u 8 3 ], w h e r e a s 6. 7 k e V N e + w a s u s e d i n t h e p r e s e n t e x p e r i m e n t. T h e s c a t t e r i n g c r o s s s e c t i o n f o r N e + is c o n s i d e r a b l y l e s s t h a n t h a t f o r A r + (a f a c t o r o f f o u r a t a s c a t t e r i n g a n g l e o f 30 [ R o 7 0 ] ), s o t h a t N e + i o n s a r e m o r e l i k e l y t o p e n e t r a t e t h e f i r s t a t o m i c l a y e r w i t h o u t e x p e r i e n c i n g a s i g n i f i c a n t d e f l e c t i o n. F u r t h e r m o r e, t h e n e u t r a l i z a t i o n p r o b a b i l i t y o f t h e h i g h e r v e l o c i t y N e + i o n s is e x p e c t e d t o b e s i g n i f i c a n t l y l o w e r t h a n f o r A r +. M e a s u r e m e n t s o f i o n f r a c t i o n s f o r 4> 30 s c a t t e r i n g o f b o t h N e + a n d A r + f r o m a C u t a r g e t h a v e b e e n p e r f o r m e d b y L u i t j e n s e t

32 a l. [ L u 8 0 a, L u 8 0 b ]. F o r s i n g l e s c a t t e r i n g e v e n t s, t h e i o n f r a c t i o n f o r 7 k e V N e + w a s f o u n d t o b e 6%. F o r 5 k e V A r +, i t w a s f o u n d t o b e 1. 2 %, a n d e x t r a p o l a t i n g t o a n e n e r g y o f 2 k e V g i v e s a v a l u e o f o n l y = 0. 5 %. S e c o n d l a y e r s i n g l e s c a t t e r i n g e v e n t s a r e t h e r e f o r e n o t e x c l u d e d f r o m t h e b a c k s c a t t e r e d N e + s p e c t r u m t o t h e s a m e e x t e n t a s t h e y a r e f o r A r +. A n I S S a n a l y s i s u s i n g a n A r + b e a m w a s a l s o a t t e m p t e d i n t h e p r e s e n t e x p e r i m e n t, w i t h a b a c k s c a t t e r i n g a n g l e o f <f> - 135. T h e r e s u l t s o b t a i n e d f o r t h e p u r e G a a n d p u r e I n s t a n d a r d s a r e s h o w n i n F i g u r e 2. 1 4, f o r a n i n c i d e n t A r + e n e r g y o f 7. 9 k e V. A l t h o u g h u s e f u l s p e c t r a w e r e o b t a i n e d f o r t h e I n t a r g e t, n o d i s t i n c t s i n g l e s c a t t e r i n g p e a k w a s o b s e r v e d i n t h e s p e c t r a f o r t h e G a t a r g e t. A k i n e m a t i c a n a l y s i s s u g g e s t s t h a t t h e b a c k s c a t t e r i n g s p e c t r a f r o m a G a t a r g e t a r e d o m i n a t e d b y d o u b l e s c a t t e r i n g e v e n t s, r e s u l t i n g i n a b r o a d e n e r g y d i s t r i b u t i o n. S i m i l a r r e s u l t s w e r e o b t a i n e d w i t h a n i n c i d e n t A r + e n e r g y o f 1 2. 8 k e V, p r e v e n t i n g q u a n t i t a t i v e a n a l y s i s w i t h t h e A r + b e a m. T h e p r e s e n t r e s u l t f o r N e + b o m b a r d m e n t is u s e f u l a s a l o w e r l i m i t, a n d d o e s v e r i f y t h e s u b s t a n t i a l I n e n r i c h m e n t o f t h e f i r s t a t o m i c l a y e r. T h e A E S r e s u l t o f x 2 «0. 9 1 ± 0. 1 1, w h i c h a s s u m e s s i n g l e m o n o l a y e r s e g r e g a t i o n, a g r e e s w e l l w i t h t h e A E S r e s u l t o f D u m k e e t a l. [ D u 8 3 ] o f x : = 0. 9 8 ± o 'oi- H o w e v e r, t h e v a l u e o f t h e r a t i o A / S w a s f o u n d i n the p r e s e n t e x p e r i m e n t t o b e A / S - 0. 6 0 8 ± 0. 0 3 1, w h e r e a s i n [ D u 8 3 ], A / S - 0. 8 5 3. T h e d i s c r e p a n c y i n t h e d a t a is m a s k e d b y a d i f f e r e n c e in t h e a n a l y s i s p r o c e d u r e, s p e c i f i c a l l y t h e i n c l u s i o n o f t h e e l e c t r o n b a c k s c a t t e r i n g f a c t o r s. I f t h e b a c k s c a t t e r i n g f a c t o r s a r e n o t i n c o r p o r a t e d i n t o t h e p r e s e n t a n a l y s i s, t h e r e s u l t f o r t h e I n s u r f a c e c o n c e n t r a t i o n is x 2-0. 7 2 ± 0. 0 5. T h i s d i f f e r e n c e b e t w e e n t h e t w o

33 e x p e r i m e n t s i s n o t u n d e r s t o o d. O n e p o t e n t i a l s o u r c e o f i n a c c u r a c y f o r b o t h t h e I S S a n d A E S a n a l y s e s a r i s e s f r o m t h e p h a s e s o f t h e p u r e e l e m e n t a l s t a n d a r d s. I n t h e p r e s e n t e x p e r i m e n t, t h e G a s t a n d a r d w a s u s e d i n t h e l i q u i d p h a s e, b u t t h e I n s t a n d a r d w a s a s o l i d. C o n s i d e r a b l e s u r f a c e r o u g h n e s s o f t h e In t a r g e t m u s t h a v e d e v e l o p e d d u r i n g t h e s p u t t e r - c l e a n i n g p r o c e s s, b u t it is n o t c l e a r h o w i t m i g h t h a v e e f f e c t e d t h e m e a s u r e m e n t s. I n the e x p e r i m e n t o f [ D u 8 3 ], b o t h e l e m e n t a l s t a n d a r d s w e r e u s e d i n t h e s o l i d p h a s e, s o t h a t s i m i l a r c o n c e r n s e x i s t f o r t h o s e r e s u l t s. \ I n o r d e r t o c a l c u l a t e F x, t h e f r a c t i o n o f s p u t t e r e d a t o m s o r i g i n a t i n g f r o m t h e f i r s t a t o m i c l a y e r, k n o w l e d g e o f t h e f i r s t a n d s e c o n d l a y e r I n c o n c e n t r a t i o n s o f t h e a l l o y t a r g e t is r e q u i r e d. A n u p p e r l i m i t o n t h e f i r s t l a y e r I n c o n c e n t r a t i o n is g i v e n b y x 7 = 1.0. F o r t h i s v a l u e o f x 7, t h e A E S d a t a f r o m t h e p r e s e n t e x p e r i m e n t i m p l y a s e c o n d l a y e r I n c o n c e n t r a t i o n o f x 2-0. 0 4 5. T h e A E S r e s u l t o f x x - 0. 9 1, o b t a i n e d a s s u m i n g a b u l k c o m p o s i t i o n f o r t h e s e c o n d a t o m i c l a y e r, w i l l b e t a k e n a s a l o w e r l i m i t. T h i s is a l s o t h e s m a l l e s t v a l u e o f x 7 c o n s i s t e n t w i t h t h e p h y s i c a l r e s u l t o f Fx < 1.0, f o r t h e 3 k e V a n g u l a r d i s t r i b u t i o n m e a s u r e m e n t. F o r t h e c a l c u l a t i o n o f F 7, t h e i n t e r m e d i a t e v a l u e x 7 0. 9 6, b a s e d o n t h e r e s u l t s i n [ D u 8 3 ], w i l l b e u s e d. T h e A E S d a t a t h e n i m p l y a c o r r e s p o n d i n g v a l u e o f x 2-0. 0 9 7, a s l i g h t d e p l e t i o n o f I n r e l a t i v e to t h e b u l k c o n c e n t r a t i o n.

34 F i g u r e 2. 1 S c h e m a t i c i l l u s t r a t i o n o f t h e A u g e r n e u t r a l i z a t i o n p r o c e s s f o r low- e n e r g y i n e r t - g a s i o n s a b o v e a s o l i d s u r f a c e. r

Vacuum level Valance band Ion

35 F i g u r e 2. 2 S c h e m a t i c i l l u s t r a t i o n o f t h e s e q u e n c e o f e v e n t s l e a d i n g t o t h e e j e c t i o n o f a n A u g e r e l e c t r o n. T h e a t o m is l e f t i n a d o u b l y - i o n i z e d s t a t e. F r o m [ R v 7 3 ].

Vac. Ei Vac. Ei E 2 E 2 ( E j - E 2) t) c) Auger e (Ei - E 2 - E 3) Vac. Ei E 2 Vac. E, E 2 Vac. Ei Ez 0 f)

36 F i g u r e 2. 3 I l l u s t r a t i o n o f t h e t a r g e t h o l d e r u s e d f o r t h e l i q u i d G a a n d G a - I n a l l o y t a r g e t s. T h e s u r f a c e t e n s i o n s o f t h e l i q u i d m e t a l s w e r e a d e q u a t e to h o l d t h e t a r g e t s i n p l a c e w i t h t h e a s s e m b l y i n a v e r t i c a l p o s i t i o n.

L iq u id M e ta l

Figure 2.4 S c h e m a t i c i l l u s t r a t i o n o f t h e 3 0 0 k V C o c k c r o f t - W a l t o n a c c e l e r a t o r Y a l e U n i v e r s i t y. F r o m [ B n 8 7 ].

3 0 0 k V C o c k c r o f t - W a l t o n A c c e l e r a t o r H o t C a t h o d e I o n S o u r c e E i n z e l L e n s A c c e l e r a t i n g C o l u m n E l e c t r o s t a t i c Q u a d r u p o l e L e n s A n a l y z i n g M a g n e t Zj S \ 3 0 e Differentially P u m p e d B e a m L i n e

38 F i g u r e 2. 5 I l l u s t r a t i o n o f t h e u l t r a - h i g h v a c u u m b e a m l i n e c o n n e c t e d t o t h e Y a l e C o c k c r o f t - W a l t o n a c c e l e r a t o r. F r o m [ B n 8 7 ].

DIFFERENTIALLY PUMPED B E A M LINE BEAM VIEWER ELECTROSTATIC QUAORUPOLE LENS DEFLECTION PLATES VALVE COLLIMATOR^, TARGET CHAMBER TAROET MANIPULATOR (X.Y.Z.0) if tb lljo r J LEAK VALVE^ 30cm 230 IIMr/ M ION PUMP 120 III* A IM - x. 3 C

39 F i g u r e 2. 6 I l l u s t r a t i o n o f t h e 160 s p h e r i c a l - s e c t o r, e l e c t r o s t a t i c e n e r g y a n a l y z e r u s e d f o r t h e I S S a n a l y s i s. T h e a n g l e o f i n c i d e n c e w a s 22.5, a n d t h e b a c k s c a t t e r i n g a n g l e w a s 135. F r o m [ O C 8 6 ].

1 E S A E n t r o n c e Structure ( e x p a n d e d view)

40 F i g u r e 2. 7 B l o c k d i a g r a m o f t h e d a t a a c q u i s i t i o n e l e c t r o n i c s u s e d f o r t h e I S S a n a l y s i s.

41 F i g u r e 2. 8 A s u p e r p o s i t i o n o f t y p i c a l N e + I S S s p e c t r a f r o m p u r e G a a n d p u r e In e l e m e n t a l s t a n d a r d s. T h e b a c k g r o u n d h a s b e e n s u b t r a c t e d b y a c o m p u t e r f i t t i n g p r o c e d u r e.

Relative Counts E n e r g y ( k e V )

42 F i g u r e 2. 9 A t y p i c a l N e + I S S s p e c t r u m f r o m t h e G a - I n e u t e c t i c a l l o y. T h e i n s e r t s h o w s t h e s p e c t r u m p r i o r t o s u b t r a c t i o n o f t h e b a c k g r o u n d.

Relative Counts E n e r g y ( k e V )

43 F i g u r e 2. 1 0 A n i l l u s t r a t i o n o f t h e b a c k g r o u n d s u b t r a c t i o n a n d p e a k i n t e g r a t i o n p r o c e d u r e f o r a n I S S s p e c t r u m f r o m t h e G a - I n e u t e c t i c a l l o y. T h e p e a k s r e p r e s e n t N e * b a c k s c a t t e r e d f r o m : a) G a a t o m s, a n d b) I n a t o m s. T h e d a s h e d l i n e s a t t h e b o t t o m o f t h e p l o t s s h o w t h e d i f f e r e n c e b e t w e e n t h e d a t a a n d t h e c o m p u t e r f i t s. >

Figure 2.11 A n i l l u s t r a t i o n o f a n e l e c t r o s t a t i c, c y l i n d r i c a l - m i r r o r e n e r g y a n a l y z e r o f t h e t y p e u s e d i n t h e p r e s e n t e x p e r i m e n t f o r A E S a n a l y s i s. F r o m [ R v 7 3 ].

-V(t) Multiplier

45 F i g u r e 2. 1 2 T y p i c a l A E S s p e c t r a f r o m : a) p u r e G a, a n d b ) p u r e I n e l e m e n t a l s t a n d a r d s, a f t e r s p u t t e r - c l e a n i n g o f t h e t a r g e t s.

PlieG a Epp3keV 1x iffv- Energy

b) Pure In.27x 5>=3keV dn de I ln(404ev) MNN E n e r g y ----------

46 F i g u r e 2. 1 3 T y p i c a l A E S s p e c t r a f r o m a G a - I n e u t e c t i c a l l o y, b o t h : a) b e f o r e, a n d b) a f t e r s p u t t e r - c l e a n i n g o f t h e t a r g e t. (

E n e r g y Ga-ln alloy

Ga-ln alloy Sputter-deaned ^>=3keV Ga(1070eV) LMM In (404 ev) MNN

47 F i g u r e 2. 1 4 I S S s p e c t r a f o r A r + b a c k s c a t t e r e d f r o m : a) p u r e Ga, a n d b) p u r e I n e l e m e n t a l s t a n d a r d s.

Counts ~ C o u n t s 160 140 120 100 C h a n n e l N u m b e r 500 1000 1500 2000 2500 3000 3500 4cac C h a n n e l N u m b e r

48 T a b l e 2. 1 C a l c u l a t e d v a l u e s f o r t h e f r a c t i o n o f A u g e r e l e c t r o n s o r i g i n a t i n g f r o m b e n e a t h t h e j 't h a t o m i c l a y e r. V a l u e s a r e p r e s e n t e d f o r 4 0 4 e V a n d 1 0 7 0 e V e l e c t r o n s i n b o t h G a a n d In. T h e d e t a i l s o f t h e c a l c u l a t i o n a r e d e s c r i b e d in t h e text.

Electron Energy Layer Number Ga In j = 1 0.525 0.534 = 404 ev I to 0.275 0.286 j = 3 0.144 0.153 j = 1 0.738 0.748 e = 1070 ev j = 2 0.545 0.559 j = 3 0.402 0.418

CHAPTER THREE ANGULAR DISTRIBUTION MEASUREMENTS 3.1 Introduction A n g u l a r d i s t r i b u t i o n s o f a t o m s s p u t t e r e d f r o m a G a - I n e u t e c t i c a l l o y t a r g e t w e r e d e t e r m i n e d b y a t w o - s t e p p r o c e d u r e. F i r s t, m a t e r i a l s p u t t e r e d f r o m t h e a l l o y w a s c o l l e c t e d o n t o a s u b s t r a t e foil, c y l i n d r i c a l i n s h a p e w i t h t h e t a r g e t a t t h e c e n t e r o f t h e r a d i u s o f c u r v a t u r e. T h e t h i c k n e s s o f m a t e r i a l s p u t t e r e d o n t o t h e f o i l ( g e n e r a l l y 1-2 m o n o l a y e r s a t 8-0 ) w a s t h e n m e a s u r e d a s a f u n c t i o n o f the s p u t t e r i n g a n g l e b y R u t h e r f o r d b a c k s c a t t e r i n g s p e c t r o s c o p y ( R B S ). S p u t t e r i n g y i e l d s w e r e c a l c u l a t e d b y i n t e g r a t i o n o f t h e m e a s u r e d a n g u l a r d i s t r i b u t i o n o v e r a h e m i s p h e r e. T h e s p u t t e r e d - f l u x c o m p o s i t i o n w a s t h e n c o m p a r e d w i t h t h e c o m p o s i t i o n o f t h e f i r s t t w o a t o m i c l a y e r s o f the a l l o y t a r g e t t o d e t e r m i n e t h e f r a c t i o n o f s p u t t e r e d a t o m s o r i g i n a t i n g in t h e f i r s t a t o m i c l a y e r. W i t h R B S a n a l y s i s, t a r g e t c o m p o s i t i o n is d e t e r m i n e d b y c l a s s i c a l k i n e m a t i c s f r o m t h e e n e r g y s p e c t r u m o f i o n s e l a s t i c a l l y b a c k s c a t t e r e d f r o m t h e t a r g e t a t a k n o w n a n g l e (as i n E q u a t i o n 2. 1 ). H e l i u m i o n s o f e n e r g y 1-4 M e V a r e t y p i c a l l y u s e d a s t h e a n a l y s i s b e a m. T h e e l e c t r o n i c s c r e e n i n g o f t h e n u c l e i d u r i n g c o l l i s i o n s i s n e g l i g i b l e, s o t h a t s c a t t e r i n g c r o s s s e c t i o n s c a n b e c a l c u l a t e d e x a c t l y. Q u a n t i t a t i v e r e s u l t s c a n t h e r e f o r e b e o b t a i n e d w i t h o u t t h e u s e o f p u r e e l e m e n t a l s t a n d a r d s. T h e i n f o r m a t i o n d e p t h o f R B S is d e t e r m i n e d b y t h e r a n g e o f t h e a n a l y s i s b e a m w i t h i n t h e t a r g e t, a n d is t y p i c a l l y = 1 p m [C h 7 8]. R B S is t h e r e f o r e a n i d e a l t e c h n i q u e f o r t h i n - f i l m a n a l y s i s. 49

T h e p e r f o r m a n c e o f R B S f o r t h e a n a l y s i s o f m o n o l a y e r f i l m s, s u c h as a r e g e n e r a l l y o b t a i n e d i n c o l l e c t i o n e x p e r i m e n t s, c a n b e e n h a n c e d c o n s i d e r a b l y b y t h e u s e o f h e a v y - i o n b e a m s [ W e 8 2 ]. W h e n h e l i u m is u s e d a s t h e a n a l y s i s b e a m, p u l s e p i l e - u p f r o m s u b s t r a t e b a c k s c a t t e r i n g e v e n t s l i m i t s t h e u s e f u l b e a m c u r r e n t t o s e v e r a l n a n o a m p s. W i t h h e a v y i o ns, s i n g l e - c o l l i s i o n b a c k s c a t t e r i n g f r o m t h e s u b s t r a t e c a n b e c o m p l e t e l y e l i m i n a t e d i f a l o w - m a s s m a t e r i a l s u c h a s g r a p h i t e is e m p l o y e d. T h i s e l i m i n a t e s t h e m a j o r s o u r c e o f b a c k g r o u n d i n t h e e n e r g y s p e c t r u m, a n d p e r m i t s t h e u s e o f b e a m c u r r e n t s a s h i g h a s 5 0-1 0 0 na. B e c a u s e t h e s c a t t e r i n g c r o s s s e c t i o n s f o r h e a v y i o n s a r e c o m p a r a b l e t o t h o s e f o r h e l i u m, m u c h g r e a t e r s e n s i t i v i t y is a c h i e v e d. M a s s s e p a r a t i o n is a l s o i m p r o v e d, b e c a u s e o f t h e i n c r e a s e in t h e e n e r g y s e p a r a t i o n o f h e a v y i o n s b a c k s c a t t e r e d f r o m d i f f e r e n t m a s s t a r g e t a t o m s. T h i s f a c i l i t a t e s the a n a l y s i s o f m u l t i - c o m p o n e n t f i l m s, a n d r e d u c e s t h e e f f e c t o f p u l s e p i l e - u p i f it h a s n o t b e e n k i n e m a t i c a l l y e l i m i n a t e d. B e c a u s e o f t h e s e a d v a n t a g e s, o x y g e n w a s u s e d a s t h e a n a l y s i s b e a m i n t h e p r e s e n t e x p e r i m e n t. T h e r e a r e d i s a d v a n t a g e s to t h e u s e o f h e a v y - i o n b e a m s f o r R B S a n a l y s i s. S u r f a c e - b a r r i e r d e t e c t o r s a r e g e n e r a l l y e m p l o y e d, w h i c h p r o d u c e a n o u t p u t p u l s e w i t h a n a m p l i t u d e p r o p o r t i o n a l o n l y t o the e l e c t r o n i c e n e r g y l o s s o f t h e b a c k s c a t t e r e d i o n w i t h i n t h e d e t e c t o r. B e c a u s e a s i g n i f i c a n t f r a c t i o n o f t h e e n e r g y l o s s o f h e a v y i o n s o c c u r s t h r o u g h e l a s t i c n u c l e a r c o l l i s i o n s ( p u l s e - h e i g h t d e f e c t ), a n a c c u r a t e e n e r g y c a l i b r a t i o n o f t h e b a c k s c a t t e r i n g s p e c t r u m is n o t p o s s i b l e. F o r c o l l e c t i o n - t y p e s p u t t e r i n g e x p e r i m e n t s, w h e r e t h e m a s s e s o f the d e p o s i t e d a t o m s a r e k n o w n a p r i o r i, t h i s is n o t a c o n c e r n. F l u c t u a t i o n s

i n t h e e l a s t i c e n e r g y l o s s w i t h i n t h e d e t e c t o r d o c a u s e a d e c r e a s e in 51 e n e r g y r e s o l u t i o n, b u t t h e d e g r a d a t i o n is g e n e r a l l y m u c h l e s s t h a n the i m p r o v e m e n t r e s u l t i n g f r o m t h e s c a t t e r i n g k i n e m a t i c s [ C h 7 8 ]. F i n a l l y, t h e i n d u c e d c o l l i s i o n c a s c a d e s c a u s e c o n s i d e r a b l e d a m a g e t o t h e d e t e c t o r c r y s t a l, r e s u l t i n g i n a n i n c r e a s e i n t h e l e a k a g e c u r r e n t, a n d l i m i t i n g i t s u s e f u l l i f e t i m e. 3. 2 C o l l e c t i o n o f S p u t t e r e d A t o m s S p u t t e r e d - a t o m a n g u l a r d i s t r i b u t i o n s w e r e o b t a i n e d f o r t h e a l l o y t a r g e t o n l y. T h e c o l l e c t i o n s w e r e p e r f o r m e d w i t h 2 5 a n d 50 k e V A r + b e a m s f r o m t h e C o c k c r o f t - W a l t o n a c c e l e r a t o r, a n d w i t h a 3 k e V A r + b e a m f r o m t h e i o n g u n m o u n t e d i n t h e t a r g e t c h a m b e r. A t b o t h 2 5 a n d 50 k e V, t w o c o l l e c t i o n s w e r e p e r f o r m e d i n o r d e r t o v e r i f y t h e r e p r o d u c i b i l i t y o f t h e r e s u l t s. o f < 0. 9 m m. B e a m c u r r e n t s w e r e 1 0 0-2 0 0 na, w i t h a b e a m - s p o t d i a m e t e r T h e b e a m s w e r e i n c i d e n t a l o n g, t h e t a r g e t n o r m a l i n e a c h c a s e. T h e c o l l e c t i o n a p p a r a t u s is s h o w n s c h e m a t i c a l l y i n F i g u r e 3.1. G r a p h i t e c o l l e c t o r f o i l s o f t h i c k n e s s 0. 1 3 m m w e r e m o u n t e d o n t h e i n s i d e o f t h e c o l l e c t o r - f o i l c y l i n d e r, w h i c h h a d a r a d i u s o f c u r v a t u r e o f 3. 6 4 cm, a n d w a s a t t a c h e d to a v e r t i c a l m o t i o n m a n i p u l a t o r. T h e m a s k i n g c y l i n d e r h a d a h o r i z o n t a l s l o t a t b e a m l e v e l. D u r i n g t a r g e t c l e a n i n g, t h e c o l l e c t o r - f o i l c y l i n d e r w a s m o v e d a b o v e t h e h o r i z o n t a l s l o t, s o t h a t s p u t t e r e d a t o m s h a d n o d i r e c t p a t h t o t h e c o l l e c t o r f o i l s. F o r c o l l e c t i o n, t h e c y l i n d e r w a s l o w e r e d, e x p o s i n g a n a r r o w s t r i p o f t h e c o l l e c t o r f o i l s t o s p u t t e r e d a t o m s. T h e b e a m - s p o t d i a m e t e r a n d p o s i t i o n o n t h e t a r g e t w e r e d e f i n e d b y t h e u p s t r e a m a p e r t u r e o f t h e c o l l i m a t o r

a s s e m b l y, w h i c h w a s r i g i d l y a t t a c h e d t o t h e c o l l e c t o r - f o i l c y l i n d e r. 52 T h e s e c o n d a p e r t u r e p l a t e w a s n e g a t i v e l y b i a s e d t o s u p p r e s s e l e c t r o n s, w i t h a v o l t a g e o f - 1 8 0 V f o r t h e 3 k e V b o m b a r d m e n t, a n d - 3 5 0 V f o r t h e 2 5 a n d 5 0 k e V b o m b a r d m e n t s. B e a m c u r r e n t w a s m e a s u r e d f r o m t h e c o m b i n a t i o n o f t h e t a r g e t, m a s k i n g c y l i n d e r, a n d c o l l e c t o r - f o i l c y l i n d e r, w h i c h f o r m e d a n e f f e c t i v e F a r a d a y c a g e. T h e c o l l e c t o r - f o i l c y l i n d e r h a d s u f f i c i e n t v e r t i c a l m o t i o n t o e x p o s e i n t u r n f o u r w e l l - s e p a r a t e d s t r i p s o n a s i n g l e s e t o f c o l l e c t o r f o i l s. W i t h t h e c o l l e c t i o n a p p a r a t u s i n p l a c e, t h e r e w a s i n s u f f i c i e n t r o o m i n t h e t a r g e t c h a m b e r f o r t h e q u a d r u p o l e m a s s s p e c t r o m e t e r. T h e r e f o r e, t h e i n i t i a l c l e a n i n g d o s e f o r t h e c o l l e c t i o n e x p e r i m e n t s w a s c o n s e r v a t i v e l y d e t e r m i n e d o n t h e b a s i s o f p r e v i o u s e x p e r i m e n t s w i t h t h e q u a d r u p o l e i n p l a c e. D u r i n g t h e c o l l e c t i o n, t h e t a r g e t w a s c l e a n e d b r i e f l y a t i n t e r v a l s o f t y p i c a l l y 3. 5 h o u r s, i n o r d e r t o p r e v e n t t h e a c c u m u l a t i o n o f a d s o r b e d a t o m s. 3. 3 R B S A n a l y s i s R u t h e r f o r d b a c k s c a t t e r i n g a n a l y s i s o f t h e c o l l e c t o r f o i l s w a s p e r f o r m e d a t t h e E N t a n d e m V a n d e G r a a f f f a c i l i t y a t t h e C a l i f o r n i a I n s t i t u t e o f T e c h n o l o g y. T h e a n a l y s i s b e a m w a s 1 0 M e V 1 6 0 2 +, w i t h a t y p i c a l b e a m c u r r e n t o f 8 0-1 0 0 na. T h e b e a m - s p o t s i z e a n d l o c a t i o n o n t h e c o l l e c t o r f o i l w e r e d e f i n e d b y a d o u b l e a p e r t u r e c o l l i m a t o r u p s t r e a m f r o m t h e s c a t t e r i n g c h a m b e r, w i t h t h e downstream a p e r t u r e b i a s e d at- 6 0 0 V t o s u p p r e s s e l e c t r o n s. T h e b e a m - s p o t d i a m e t e r a t t h e t a r g e t w a s = l m m. T h e b a c k s c a t t e r i n g a p p a r a t u s is s h o w n i n F i g u r e 3.2. O x y g e n i o n s

b a c k s c a t t e r e d f r o m t h e f o i l s a t a n a v e r a g e a n g l e o f 6-171.9 w e r e 53 d e t e c t e d b y a n a n n u l a r Si s u r f a c e - b a r r i e r d e t e c t o r, c o - a x i a l w i t h t h e i o n b e a m. T h e e f f e c t i v e s o l i d a n g l e o f A n - 4 9. 7 m s r w a s d e f i n e d b y a n e l e c t r i c a l l y i s o l a t e d a p e r t u r e w h i c h s h i e l d e d t h e a r e a o f t h e d e t e c t o r n e a r t h e e d g e o f i t s a c t i v e r e g i o n. T h e c o l l e c t o r f o i l s w e r e m o u n t e d h o r i z o n t a l l y o n a h o l l o w c y l i n d r i c a l t a r g e t l a d d e r, s o t h a t t h e a n g u l a r d i s t r i b u t i o n w a s o b t a i n e d b y a v e r t i c a l s w e e p. T h e v e r t i c a l p o s i t i o n w a s r e p r o d u c i b l e t o w i t h i n ± 0.02 cm, w h i c h c o r r e s p o n d e d t o a c h a n g e in t h e s p u t t e r i n g a n g l e o f ± 0.33". T h e t a r g e t l a d d e r w a s s u r r o u n d e d b y a F a r a d a y c a g e, s o l i d e x c e p t f o r o n e o p e n i n g f o r t h e b e a m a n d b a c k s c a t t e r e d i o n s, a n d a n o t h e r o n t h e o p p o s i t e s i d e f o r o b s e r v i n g a q u a r t z b e a m - v i e w e r m o u n t e d o n t h e t a r g e t l a d d e r. T h e F a r a d a y c a g e a n d d e t e c t o r a p e r t u r e w e r e b i a s e d a t - 6 0 0 V w i t h r e s p e c t t o t h e t a r g e t l a d d e r, a n d b e a m c u r r e n t w a s m e a s u r e d f r o m t h e c o m b i n a t i o n o f t h e t h r e e. T h e c h a m b e r p r e s s u r e d u r i n g t h e a n a l y s i s w a s = 5 x 1 0 " 7 T o r r. T h e e l e c t r o n i c s f o r d a t a a c q u i s i t i o n a r e s h o w n s c h e m a t i c a l l y in F i g u r e 3.3. D e t e c t o r p u l s e s w e r e p r o c e s s e d b y a p r e - a m p a n d a m p l i f i e r, a n d c o u n t e d b y a m u l t i - c h a n n e l a n a l y z e r o p e r a t i n g i n t h e p u l s e - h e i g h t a n a l y s i s m o d e. D e a d - t i m e w a s m o n i t o r e d t h r o u g h o u t t h e e x p e r i m e n t. A 4 8 H z o u t p u t s i g n a l f r o m a f u n c t i o n g e n e r a t o r w a s c o u n t e d b y a s c a l e r, a n d a c t e d a s t h e e x t e r n a l t r i g g e r f o r a t a i l - p u l s e g e n e r a t o r, t h e o u t p u t o f w h i c h w a s c o n n e c t e d t o t h e p r e - a m p t e s t i n p u t. T h e r e s u l t i n g p e a k in t h e M C A s p e c t r u m w a s i n t e g r a t e d, a n d t h e n u m b e r o f c o u n t s c o m p a r e d w i t h t h e s c a l e r r e a d i n g. T h e m e a s u r e d d e a d - t i m e n e v e r e x c e e d e d 1%. T h e b e a m - c u r r e n t i n t e g r a t o r a l l o w e d d a t a t o b e c o l l e c t e d f o r a p r e s e t i n c i d e n t c h a r g e. W h e n t h e l i m i t w a s r e a c h e d, a b e a m b l o c k w a s

a c t i v a t e d, a n d c o u n t i n g b y t b e s c a l e r a n d M C A w a s i n h i b i t e d. A t y p i c a l R B S s p e c t r u m is p r e s e n t e d i n F i g u r e 3.4. I t s h o w s t h e s p u t t e r e d - f l u x c o m p o s i t i o n a t a n a n g l e o f ( 40 w i t h r e s p e c t t o the s u r f a c e n o r m a l, f o r 2 5 k e V A r + b o m b a r d m e n t. T h e r a t i o o f t b e R u t h e r f o r d s c a t t e r i n g c r o s s s e c t i o n s is a I n /(7G a - 2. 6 8, b u t c o n s i d e r a b l e I n e n r i c h m e n t o f t b e s p u t t e r e d f l u x is s t i l l e v i d e n t. B e c a u s e o x y g e n i o n s w e r e u s e d f o r t h e a n a l y s i s, t h e b a c k g r o u n d r e s u l t i n g f r o m s c a t t e r i n g f r o m t h e g r a p h i t e s u b s t r a t e h a s b e e n e l i m i n a t e d. F u r t h e r m o r e, t h e m a s s r e s o l u t i o n w a s a d e q u a t e t o s e p a r a t e t h e t w o G a i s o t o p e s a t m a s s 6 9 u a n d 71 u. 3. 4 R e s u l t s F i g u r e 3. 5 s h o w s t h e a n g u l a r d i s t r i b u t i o n s o f G a a n d I n a t o m s s p u t t e r e d o n t o t h e c o l l e c t o r f o i l s f o r 3, 25, a n d 5 0 k e V A r + b o m b a r d m e n t o f t h e a l l o y t a r g e t. T h e a n g u l a r d i s t r i b u t i o n s w e r e f i t b y a f u n c t i o n o f t h e f o r m 3. 1 ) - A c o s n ( 0 + 0 o ), u s i n g t h e n o n - l i n e a r l e a s t - s q u a r e s a n a l y s i s p r o g r a m D A T A P L O T [F i 82]. T h e p a r a m e t e r 60 r e p r e s e n t s t h e a n g u l a r o f f s e t r e s u l t i n g f r o m s m a l l m i s a l i g n m e n t s d u r i n g t h e c o l l e c t i o n o r R B S a n a l y s i s. B e c a u s e t h e s h a p e o f t h e a n g u l a r d i s t r i b u t i o n is d e t e r m i n e d p r i m a r i l y b y e j e c t i o n a t l a r g e a n g l e s, f i t s w e r e a l s o o b t a i n e d b y w e i g h t i n g t h e d a t a ( c o r r e c t e d f o r the o f f s e t, 80) b y t h e s o l i d a n g l e f a c t o r s i n ( 0 ). T h i s is e q u i v a l e n t to f i t t i n g t h e d a t a b y a f u n c t i o n o f t h e f o r m

55 w h e r e A N - ( d N / d 0 ) A 0 is t h e t o t a l n u m b e r o f a t o m s s p u t t e r e d i n t o t h e a n g u l a r r a n g e A 8, c e n t e r e d a t 6. T h e f i t s s u p e r i m p o s e d o n t h e d a t a in F i g u r e 3. 5 w e r e o b t a i n e d b y u s e o f t h e w e i g h t i n g p r o c e d u r e. F i g u r e 3. 6 i l l u s t r a t e s t h e d i f f e r e n c e b e t w e e n t h e s t a n d a r d a n d w e i g h t e d c o m p u t e r f i t s f o r t h e 5 0 k e V A r + d a t a. T h e f i t b y t h e f o r m dn/dff e n h a n c e s t h e q u a l i t y o f t h e f i t a t l a r g e a n g l e s, b u t t h e n o r m a l i z a t i o n, A x, is n o t a s a c c u r a t e l y d e t e r m i n e d. F o r t h e I n d i s t r i b u t i o n s, t h e v a l u e s o f t h e p a r a m e t e r n A ( A a ) a r e s y s t e m a t i c a l l y = 1 2 % ( = 8 % ) g r e a t e r t h a n t h o s e o b t a i n e d f r o m f i t s b y t h e f o r m dn/dfi. F o r t h e G a d i s t r i b u t i o n s, t h e y a r e = 6 % (=5%) l e s s. P a r t i a l s p u t t e r i n g y i e l d s ( t h e s e p a r a t e y i e l d s o f t h e i n d i v i d u a l c o n s t i t u e n t s ) w e r e c a l c u l a t e d u s i n g t h e p a r a m e t e r r e s u l t s f r o m t h e w e i g h t e d f i t s, b y i n t e g r a t i o n o f t h e a n g u l a r d i s t r i b u t i o n s ; 3. 3 ) H e r e N 0 is t h e n u m b e r o f i o n s i n c i d e n t o n t h e t a r g e t d u r i n g the c o l l e c t i o n. T h e r e s u l t s f o r e a c h c o l l e c t i o n a r e s h o w n i n T a b l e 3.1, w h i c h a l s o p r e s e n t s t h e r a t i o Y l n / Y G a, a n d t h e f i t t i n g p a r a m e t e r s f o r t h e t o t a l d i s t r i b u t i o n o f m a t e r i a l, N I n (0) + N G a (0), o n t h e c o l l e c t o r f o i l s. A l t h o u g h t h e r e s u l t s f o r t h e f i t t i n g p a r a m e t e r s a r e d e p e n d e n t o n t h e w e i g h t i n g p r o c e d u r e, t h e v a r i a t i o n s i n A x a n d n x l a r g e l y c o m p e n s a t e e a c h o t h e r i n t h e c a l c u l a t i o n o f t h e p a r t i a l s p u t t e r i n g y i e l d s. T h e p a r t i a l s p u t t e r i n g y i e l d s c a l c u l a t e d f r o m t h e t w o f i t t i n g p r o c e d u r e s d i f f e r b y a t m o s t 1. 7 %, c o n s i d e r a b l y l e s s t h a n t h e o t h e r e x p e r i m e n t a l u n c e r t a i n t i e s.

56 3.5 D i s c u s s i o n A l t h o u g h a n g u l a r d i s t r i b u t i o n s w e r e n o t o b t a i n e d f r o m p u r e e l e m e n t a l t a r g e t s, s o m e c o m m e n t s c a n b e m a d e a b o u t t h e t o t a l d i s t r i b u t i o n o f m a t e r i a l, N I n (fl) + N G a (0), s p u t t e r e d o n t o t h e c o l l e c t o r f o i l s f r o m t h e a l l o y t a r g e t. T h e d i s t r i b u t i o n s a r e a l l o v e r - c o s i n e, w i t h n «2, i n c o n t r a s t t o t h e c o l l i s i o n - c a s c a d e t h e o r y p r e d i c t i o n o f n 1. A l t h o u g h s u c h r e s u l t s a r e t y p i c a l o f c o l l e c t i o n e x p e r i m e n t s, m o s t o f t h e e x p e r i m e n t s t o d a t e h a v e b e e n p e r f o r m e d o n p o l y c r y s t a l l i n e t a r g e t s w i t h l a r g e i n c i d e n t i o n b e a m d o s e s [ A n 8 5 ], U n d e r s u c h c o n d i t i o n s, it is e x p e c t e d t h a t t h e t a r g e t w i l l d e v e l o p a c o m p l i c a t e d s u r f a c e t o p o g r a p h y, w h i c h m a y d i s t o r t t h e a n g u l a r d i s t r i b u t i o n o f s p u t t e r e d a t o m s [Li78, W h 8 0 ]. F u r t h e r m o r e, m a n y o f t h e e x p e r i m e n t s w e r e p e r f o r m e d u n d e r p o o r v a c u u m c o n d i t i o n s. T h e p r e s e n c e o f a n a d s o r b e d g a s l a y e r o n a t a r g e t s u r f a c e h a s b e e n s h o w n t o h a v e a s i g n i f i c a n t e f f e c t o n t h e a n g u l a r d i s t r i b u t i o n, b y b l o c k i n g t h e e j e c t i o n o f s p u t t e r e d a t o m s at o b l i q u e a n g l e s [ S z 8 6 ]. S e v e r a l e x p e r i m e n t s h a v e b e e n p e r f o r m e d u n d e r U H V c o n d i t i o n s, u s i n g e i t h e r v e r y l o w i n c i d e n t i o n d o s e s, o r t a r g e t s w h i c h d i d n o t d e v e l o p s u r f a c e t o p o g r a p h y. T h e r e s u l t s o f t h e s e e x p e r i m e n t s g e n e r a l l y i n d i c a t e s h a r p l y o v e r - c o s i n e a n g u l a r d i s t r i b u t i o n s [ D u 8 3, A n 8 5, W e 8 3, B x 8 6 ). A n a l y t i c a l c a l c u l a t i o n s [ G a86, L i 7 5 ] a n d M o n t e C a r l o c o m p u t e r s i m u l a t i o n s [ H a 8 5 ] i n d i c a t e t h a t o n e c a u s e o f t h e o v e r - c o s i n e d i s t r i b u t i o n s is a n i s o t r o p y o f t h e r e c o i l - a t o m m o t i o n w i t h i n t h e t a r g e t. T h e n e t r e c o i l f l u x n e a r t h e t a r g e t s u r f a c e is f o u n d t o b e s t r o n g l y o u t w a r d d i r e c t e d, t h e r e s u l t o f t h e f l o w o f r e c o i l a t o m s t o w a r d s t h e

s u r f a c e f r o m d e e p e r w i t h i n t h e t a r g e t. A n a d d i t i o n a l c a u s e o f t h e d i s c r e p a n c y b e t w e e n e x p e r i m e n t a n d t h e o r y m a y b e t h e d e f l e c t i o n o f o b l i q u e l y e j e c t e d a t o m s t o w a r d s t h e s u r f a c e n o r m a l b y n e i g h b o r i n g s u r f a c e a t o m s [ S i 8 7 ]. T h i s p r o c e s s, i l l u s t r a t e d i n F i g u r e 3.7, is t h e r e s u l t o f t h e a b s e n c e o f s c a t t e r i n g c e n t e r s a b o v e t h e t a r g e t s u r f a c e, a n d w o u l d p r e s u m a b l y n o t b e a c c o u n t e d f o r i n a n y m o d e l t h a t a s s u m e s a n i n f i n i t e t a r g e t. C a l c u l a t i o n s h a v e s u g g e s t e d t h a t s u c h a n e f f e c t w o u l d a l t e r a c o s i n e d i s t r i b u t i o n i n t o o n e w i t h n» 1. 5 5. T h e b e h a v i o r o f t h e t o t a l s p u t t e r i n g y i e l d, Y In + Y G a, w a s f o u n d to b e c o n s i s t e n t w i t h t h e c o l l i s i o n - c a s c a d e t h e o r y p r e d i c t i o n t h a t it b e p r o p o r t i o n a l t o t h e e l a s t i c e n e r g y l o s s o f t h e p r o j e c t i l e a t t h e t a r g e t s u r f a c e [ S i 6 9, S i 8 1 ]. F i g u r e 3. 8 i l l u s t r a t e s t h e p r o j e c t i l e e n e r g y d e p e n d e n c e o f t h e n u c l e a r s t o p p i n g p o w e r f o r A r i n p u r e G a [ Z i 8 0 ], T h e r e is l i t t l e v a r i a t i o n i n t h e e n e r g y r a n g e b e t w e e n 25 a n d 5 0 k e V, in a g r e e m e n t w i t h t h e e x p e r i m e n t a l y i e l d. F u r t h e r m o r e, t h e d e c r e a s e i n t h e y i e l d m e a s u r e d a t 3 k e V is c o n s i s t e n t w i t h t h e s h a r p d e c r e a s e i n t h e n u c l e a r s t o p p i n g p o w e r. T h e p a r t i a l s p u t t e r i n g y i e l d s f r o m t h e a l l o y t a r g e t w e r e u s e d in c o n j u n c t i o n w i t h t h e a l l o y s u r f a c e c o m p o s i t i o n p r o f i l e t o d e t e r m i n e Fj, t h e f r a c t i o n o f s p u t t e r e d a t o m s o r i g i n a t i n g f r o m t h e f i r s t a t o m i c l a y e r. I t w a s a s s u m e d i n t h e c a l c u l a t i o n t h a t t h e s p u t t e r i n g c r o s s s e c t i o n s f o r G a a n d I n a t o m s f r o m t h e a l l o y t a r g e t w e r e e q u a l. F o r P - Y I n / Y G a, a n d n e g l e c t i n g t h e c o n t r i b u t i o n o f a t o m s s p u t t e r e d f r o m b e n e a t h t h e s e c o n d a t o m i c l a y e r, F x is g i v e n b y

58 T a b l e 3. 2 p r e s e n t s t h e r e s u l t s f o r e a c h c o l l e c t i o n. T h e f i r s t s e t o f t a b u l a t e d u n c e r t a i n t i e s w a s o b t a i n e d f r o m t h e d i f f e r e n c e s o f t h e v a l u e s o f F x c a l c u l a t e d f o r t h e l i m i t i n g v a l u e s o f t h e f i r s t l a y e r In c o n c e n t r a t i o n d i s c u s s e d i n C h a p t e r 2. N o t e t h a t t h e u p p e r l i m i t o f x x - 1. 0 g i v e s a lower l i m i t f o r F x. T h e e r r o r s i n t h e s e c o n d s e t i n c l u d e e x p e r i m e n t a l u n c e r t a i n t i e s f r o m t h e c o l l e c t i o n, R B S a n a l y s i s, a n d f i t t i n g p r o c e d u r e. A t e a c h e n e r g y, n e a r l y a l l s p u t t e r e d a t o m s w e r e f o u n d t o o r i g i n a t e f r o m t h e f i r s t a t o m i c l a y e r. T h e v a l u e s o b t a i n e d f o r t h e a n g u l a r d i s t r i b u t i o n p a r a m e t e r s n A, a n d f o r Fj, a r e n e a r l y e q u a l f o r t h e 2 5 a n d 5 0 k e V A r + b o m b a r d m e n t s. A t e a c h e n e r g y, t h e a n g u l a r d i s t r i b u t i o n o f s p u t t e r e d G a a t o m s w a s f o u n d to b e s i g n i f i c a n t l y s h a r p e r t h a n t h a t o f s p u t t e r e d In. T h i s is e m p h a s i z e d i n F i g u r e 3. 9, w h i c h p r e s e n t s t h e N I n / N Ga r a t i o a s a f u n c t i o n o f s p u t t e r i n g a n g l e f o r t h e 5 0 k e V A r + b o m b a r d m e n t. T h e h i g h I n c o n c e n t r a t i o n o f t h e f i r s t a t o m i c l a y e r, a n d t h e h i g h ( a p p r o x i m a t e l y b u l k ) G a c o n c e n t r a t i o n b e n e a t h it, i m p l y t h a t m o s t o f t h e s p u t t e r e d G a a t o m s o r i g i n a t e d f r o m t h e s e c o n d a t o m i c l a y e r. T h i s f r a c t i o n, r, is g i v e n b y 3 5) r - -----------------------, ' ' F 1 ( l - x 1 ) + ( l - F 1 ) ( l - x 2 ) n e g l e c t i n g s p u t t e r i n g f r o m b e n e a t h t h e s e c o n d a t o m i c l a y e r. F o r f i r s t a n d s e c o n d l a y e r I n c o n c e n t r a t i o n s o f x x - 0. 9 6 a n d x 2 * 0. 0 9 7, r - 0. 7 7, a l t h o u g h t h e v a l u e d e c r e a s e s r a t h e r q u i c k l y a s t h e a s s u m e d v a l u e o f x x is l o w e r e d. T h i s is c o n s i s t e n t w i t h t h e r e l a t i v e d e p l e t i o n o f s p u t t e r e d G a o b s e r v e d a t l a r g e e j e c t i o n a n g l e s. G a a t o m s f r o m t h e s e c o n d a t o m i c l a y e r a r e p h y s i c a l l y b l o c k e d b y t h e f i r s t l a y e r I n a t o m s f r o m b e i n g e j e c t e d a t o b l i q u e a n g l e s. T h i s e f f e c t w a s p r e d i c t e d b y

S i g m u n d [ S i 8 2 ], a n d h a s b e e n o b s e r v e d e x p e r i m e n t a l l y f o r s o l i d a l l o y 59 t a r g e t s i n w h i c h t h e r a d i a t i o n - e n h a n c e d d i f f u s i o n r a t e w a s s u f f i c i e n t f o r s u r f a c e s e g r e g a t i o n t o o c c u r [ W e 8 3, A n 8 2 ). I t h a s a l s o b e e n o b s e r v e d i n m o l e c u l a r - d y n a m i c s s i m u l a t i o n s o f t h e s p u t t e r i n g o f p u r e C u [ S h 8 6 ]. T h e l a r g e r e l a t i v e d e c r e a s e i n t h e G a p a r t i a l s p u t t e r i n g y i e l d o b s e r v e d f o r t h e 3 k e V A r + b o m b a r d m e n t is a r e f l e c t i o n o f a s i g n i f i c a n t d e c r e a s e i n t h e r e l a t i v e c o n t r i b u t i o n o f s p u t t e r e d a t o m s o r i g i n a t i n g f r o m t h e s e c o n d a t o m i c l a y e r. T h e d e c r e a s e i n Y Ga w a s a c c o m p a n i e d b y a s i g n i f i c a n t n a r r o w i n g o f t h e a n g u l a r d i s t r i b u t i o n o f t h e ( m o s t l y s e c o n d l a y e r ) s p u t t e r e d G a a t o m s. T h i s m a y b e t h e r e s u l t o f a d e c r e a s e i n the a v e r a g e e n e r g y o f l a t e - g e n e r a t i o n r e c o i l a t o m s w i t h i n t h e c o l l i s i o n c a s c a d e s. F e w e r s e c o n d l a y e r a t o m s w o u l d b e a b l e t o p e n e t r a t e t h e s u r f a c e l a y e r, a n d t h o s e t h a t d i d w o u l d b e p r i m a r i l y d i r e c t e d a l o n g the s u r f a c e n o r m a l. T h e i n c r e a s e o b s e r v e d i n b o t h n ^ a n d Y I n / Y G a, a s t h e v a l u e o f F x i n c r e a s e d f r o m 0. 8 8 t o 0. 9 4, a l s o s u g g e s t s t h a t t h e f r a c t i o n o f s p u t t e r e d G a o r i g i n a t i n g f r o m t h e s e c o n d a t o m i c l a y e r m u s t b e c l o s e t o u n i t y, a n d h e n c e t h a t x x > 0. 9 6. T h e r e s u l t s o f t h e p r e s e n t e x p e r i m e n t a r e i n q u a l i t a t i v e a g r e e m e n t w i t h t h e r e s u l t s o f D u m k e e t a l. [ D u 8 3 ] f o r 15 k e V A r + b o m b a r d m e n t o f the G a - I n e u t e c t i c a l l o y, f o r w h i c h a n a r r o w G a a n g u l a r d i s t r i b u t i o n w a s a l s o o b s e r v e d. I n t h i s c o n t e x t, it is d i f f i c u l t to u n d e r s t a n d the r e s u l t s o f [ D u 8 3 ] f o r 2 5 k e V A r + b o m b a r d m e n t, f o r w h i c h t h e G a d i s t r i b u t i o n b r o a d e n e d t o n<;a - 2. 0, a n d t h e r a t i o o f t h e p a r t i a l s p u t t e r i n g y i e l d s, Y I n / Y G a, d e c r e a s e d b y a f a c t o r o f 2. 1 4 f r o m i t s v a l u e a t 1 5 k e V. O n e p o s s i b l e e x p l a n a t i o n is t h e p r e s e n c e o f g a l l i u m - o x i d e

i n c l u s i o n s a t t h e a l l o y s u r f a c e, t h o u g h i t is c o n s i d e r e d u n l i k e l y 60 b e c a u s e o f t h e h i g h c o v e r a g e t h a t w o u l d r e q u i r e d t o p r o d u c e t h e e f f e c t. I t is a l s o p o s s i b l e t h a t t h e b u l k c o m p o s i t i o n o f t h e a l l o y t a r g e t u s e d f o r t h e 2 5 k e V b o m b a r d m e n t w a s i n e r r o r. T h e v a l u e s o f n x m e a s u r e d i n t h e p r e s e n t e x p e r i m e n t a r e s y s t e m a t i c a l l y l e s s, b y = 1 0 %, t h a n t h o s e r e p o r t e d i n [ D u 8 3 ] f o r the 15 k e V A r * b o m b a r d m e n t. T h i s m a y b e t h e r e s u l t o f i n a c c u r a c y i n t h e p o s i t i o n i n g o f t h e t a r g e t r e l a t i v e t o t h e c o l l e c t o r f o i l s i n o n e o r b o t h o f t h e e x p e r i m e n t s. T h e e r r o r s f r o m t h e p o s i t i o n i n g u n c e r t a i n t y, t o b e d i s c u s s e d i n s e c t i o n 3.6, h a v e b e e n i n c o r p o r a t e d i n t o t h e u n c e r t a i n t y e s t i m a t e s f o r n A i n T a b l e 3.1. T h e i n c l u s i o n o f s u c h u n c e r t a i n t i e s in t h e a n a l y s i s o f [ D u 8 3 ] w o u l d p r o b a b l y b r i n g t h e t w o e x p e r i m e n t s i n t o a g r e e m e n t. S e v e r a l p r e v i o u s e x p e r i m e n t a l m e a s u r e m e n t s o f t h e s p u t t e r e d - a t o m e s c a p e d e p t h h a v e b e e n p e r f o r m e d, a l t h o u g h n o n e h a v e i n v e s t i g a t e d t h e p r o j e c t i l e e n e r g y d e p e n d e n c e. T h e s t u d y o f P e l l i n e t a l. [ P e 8 5 ], w h i c h u s e d l a s e r f l u o r e s c e n c e s p e c t r o s c o p y t o o b s e r v e T i a n d 0 a t o m y i e l d s f r o m a T i O t a r g e t, w a s d i s c u s s e d i n C h a p t e r 1. U n d e r 3 k e V Ar* b o m b a r d m e n t, 8 0 % o f t h e s p u t t e r e d a t o m s w e r e T i, n e a r l y a l l o f w h i c h a r e e x p e c t e d t o h a v e o r i g i n a t e d f r o m t h e f i r s t a t o m i c l a y e r. W i t h a n i n c i d e n t b e a m o f 3 k e V H e *, t h e p e r c e n t a g e o f T i i n c r e a s e d t o 87%. B u r n e t t e t a l. [ B u 8 8 ] u s e d l a s e r i o n i z a t i o n o f s p u t t e r e d a t o m s to o b s e r v e C u a n d R u y i e l d s as a f u n c t i o n o f C u c o v e r a g e o n a c r y s t a l l i n e R u t a r g e t. F o r 3. 6 k e V A r * b o m b a r d m e n t, a n d a C u c o v e r a g e o f 1 m o n o l a y e r, t h e r a t i o o f t h e p a r t i a l s p u t t e r i n g y i e l d s w a s f o u n d to b e Y c u / Y R u 3. 2 ± 0. 6. I t w a s a s s u m e d i n t h e a n a l y s i s t h a t t h e r e l a t i v e

s p u t t e r i n g c r o s s s e c t i o n s w e r e e q u a l t o t h o s e o f t h e p u r e e l e m e n t a l 61 t a r g e t s, w h i c h i m p l i e s a v a l u e o f F x «* 0. 6 6 ± 0. 1 2. B e c a u s e t h e c a s c a d e d e v e l o p s i n p u r e Ru, a m o r e p l a u s i b l e a s s u m p t i o n m a y b e t h a t o f e q u a l s p u t t e r i n g c r o s s s e c t i o n s. T h e r e s u l t w o u l d t h e n b e F x - 0. 7 6. A s i m i l a r e x p e r i m e n t w a s p e r f o r m e d b y N i e h u s a n d B a u e r [ N i 7 4 ], u s i n g s e c o n d a r y - i o n m a s s s p e c t r o s c o p y. S e c o n d a r y i o n s w e r e o b s e r v e d f o r 1 k e V A r + b o m b a r d m e n t o f a W t a r g e t, a s a f u n c t i o n o f A g c o v e r a g e. A t a c o v e r a g e o f 1 m o n o l a y e r, t h e V* i n t e n s i t y w a s r e d u c e d to z e r o, i n d i c a t i n g t h a t v i r t u a l l y a l l o f t h e s p u t t e r e d i o n s o r i g i n a t e d f r o m the f i r s t a t o m i c l a y e r. K r a u s s e t a l. [ K r 8 4 ] s t u d i e d s e c o n d a r y - i o n e m i s s i o n f r o m a d i l u t e L i - C u a l l o y u n d e r 1 k e V A r + b o m b a r d m e n t. A s t h e t a r g e t w a s h e a t e d, Li s e g r e g a t e d t o t h e s u r f a c e, s u p p r e s s i n g t h e e j e c t i o n o f C u + ions. F o r a s u r f a c e c o m p o s i t i o n o f 8 0 a t % Li, t h e C u + i n t e n s i t y w a s r e d u c e d b y a f a c t o r o f 3. A s s u m i n g t h a t t h e i o n i z a t i o n p r o b a b i l i t y o f t h e s p u t t e r e d C u a t o m s w a s c o n s t a n t, t h i s i m p l i e s a v a l u e o f Fx = 0. 8 4. F i n a l l y, L a m e t a l. [ L a 8 5 ) i n v e s t i g a t e d t h e s p u t t e r i n g o f h i g h - t e m p e r a t u r e N i C u a l l o y s u n d e r 3 k e V N e + b o m b a r d m e n t. T h e s t e a d y s t a t e C u c o n c e n t r a t i o n a t t h e a l l o y s u r f a c e w a s f o u n d t o i n c r e a s e w i t h t a r g e t t e m p e r a t u r e. B e c a u s e t h e s t e a d y s t a t e s p u t t e r e d - f l u x c o m p o s i t i o n is a s s u m e d t o b e e q u a l t o t h e b u l k a l l o y c o m p o s i t i o n, t h i s i m p l i e s a s i g n i f i c a n t c o n t r i b u t i o n to t h e s p u t t e r e d f l u x f r o m t h e s e c o n d a t o m i c l a y e r, a n d a c o r r e s p o n d i n g d e c r e a s e i n t h e C u c o n c e n t r a t i o n i n t h a t l a y e r. T h e v a r i a t i o n s w e r e m o d e l e d, i n c l u d i n g t h e e f f e c t s o f t h e r m o d y n a m i c a n d b e a m - a s s i s t e d d i f f u s i o n a n d s e g r e g a t i o n. T h e r e s u l t s i n d i c a t e d t h a t a t l e a s t 3 5 % o f s p u t t e r e d a t o m s o r i g i n a t e d f r o m b e n e a t h

62 t h e f i r s t a t o m i c l a y e r. T h e r e a s o n f o r t h e r e l a t i v e l y s m a l l v a l u e o f Fj is u n c e r t a i n. O n e p o s s i b i l i t y is t h a t t h e d i f f u s i o n o f C u t o t h e t a r g e t s u r f a c e w a s s u f f i c i e n t l y r a p i d, r e l a t i v e t o t h e s p u t t e r r e m o v a l, t h a t t h e s t e a d y s t a t e s p u t t e r e d - f l u x c o m p o s i t i o n w a s n o t e q u a l t o t h e a l l o y b u l k c o m p o s i t i o n. T h e l i q u i d G a - I n a l l o y u s e d i n t h e p r e s e n t e x p e r i m e n t p r o v i d e s a n e x t r e m e e x a m p l e o f s u c h a n o c c u r r e n c e. C o m p u t e r s i m u l a t i o n s h a v e g e n e r a l l y p r e d i c t e d v a l u e s o f Fj in t h e r a n g e o f 0. 7 5-0. 9 5 [ K e 8 6 ]. F o r e x a m p l e, S h a p i r o e t a l. [ S h 8 6 ] p e r f o r m e d a m o l e c u l a r - d y n a m i c s s i m u l a t i o n o f t h e s p u t t e r i n g o f l i q u i d C u b y 5 k e V A r +. A v a l u e o f F x - 0. 8 1 w a s o b t a i n e d, a v e r a g e d o v e r s e v e r a l c o m p u t e r r u n s. L i k o n e n a n d H a u t a l a [ L k 8 8 ] a l s o s i m u l a t e d t h e s p u t t e r i n g o f C u b y 5 k e V A r +, b u t u s e d a b i n a r y - c o l l i s i o n c o d e. F o r a p o l y c r y s t a l l i n e t a r g e t, F x - 0. 8 9, a n d f o r a r a n d o m t a r g e t, Fj = 0. 8 1. T h e r e s u l t s o f t h e p r e s e n t e x p e r i m e n t a r e g e n e r a l l y i n a g r e e m e n t w i t h t h e p r e v i o u s e x p e r i m e n t a l m e a s u r e m e n t s o f t h e s p u t t e r e d - a t o m e s c a p e d e p t h. T h e s i m u l a t i o n p e r f o r m e d b y R o s e n e t a l. [ R s 8 3 ], F i g u r e 3.10, i l l u s t r a t e s t h e p h y s i c a l i n t e r p r e t a t i o n o f t h e e j e c t i o n p r o c e s s t h a t e m e r g e s f r o m t h e s e e x p e r i m e n t s. P r o j e c t i l e / t a r g e t - a t o m c o l l i s i o n s w h i c h u l t i m a t e l y r e s u l t i n t h e s p u t t e r i n g o f a n a t o m o c c u r f a i r l y u n i f o r m l y w i t h i n t h e t a r g e t, t o a d e p t h o f a b o u t 1 0 a t o m i c l a y e r s. N e v e r t h e l e s s, t h e s p u t t e r e d a t o m s o r i g i n a t e a l m o s t e x c l u s i v e l y f r o m t h e f i r s t a t o m i c l a y e r. T h i s is c o n s i s t e n t w i t h t h e c o n t e n t i o n o f F a l c o n e [ F a 8 6 ], b a s e d o n a r e - i n t e r p r e t a t i o n o f t h e c o l l i s i o n - c a s c a d e t h e o r y, t h a t t h e c h a r a c t e r i s t i c e s c a p e d e p t h o f s p u t t e r e d a t o m s is d e t e r m i n e d b y t h e m e a n f r e e p a t h b e t w e e n e l a s t i c c o l l i s i o n s o f r e c o i l a t o m s. B e c a u s e o f the l o w a v e r a g e e n e r g y o f r e c o i l a t o m s, t h e p r e d i c t i o n i m p l i e s t h a t m o s t

63 s p u t t e r e d a to m s o r i g i n a t e fro m t h e f i r s t a to m ic l a y e r. S e v e r a l c o m p u t e r s i m u l a t i o n s h a v e i n v e s t i g a t e d t h e d e p e n d e n c e o f Fj o n p r o j e c t i l e e n e r g y. B i e r s a c k [ B i87] s i m u l a t e d t h e s p u t t e r i n g o f N i b y 1, 10, a n d 1 0 0 k e V A r *. T h e f r a c t i o n o f s p u t t e r e d a t o m s o r i g i n a t i n g f r o m b e n e a t h t h e f i r s t a t o m i c l a y e r w a s f o u n d t o d e c r e a s e w i t h d e c r e a s i n g A r * e n e r g y, f r o m ~0.20 a t 1 0 0 k e V, t o *=0.11 a t 1 k e V. A s i m i l a r c a l c u l a t i o n f o r N e * b o m b a r d m e n t o f N i w a s p e r f o r m e d b y B i e r s a c k a n d E c k s t e i n [ B i 8 4 ]. T h e r e s u l t s i n d i c a t e d t h a t t h e a v e r a g e e n e r g y o f s p u t t e r e d a t o m s a l s o d e c r e a s e d as t h e p r o j e c t i l e e n e r g y w a s l o w e r e d, f r o m 8 0 e V f o r 1 0 0 k e V b o m b a r d m e n t, t o 15 e V f o r 1 k e V b o m b a r d m e n t. T h e s e l f - s p u t t e r i n g o f U i n t h e e n e r g y r a n g e o f 1-9 k e V w a s s i m u l a t e d b y R o b i n s o n [ R o 8 3 ]. T h e c o n t r i b u t i o n o f s p u t t e r e d a t o m s f r o m b e n e a t h t h e f i r s t a t o m i c l a y e r w a s f o u n d t o i n c r e a s e f r o m 1% a t 1 kev, t o 7% a t 9 k e V. T h e a v e r a g e e n e r g y o f t h e s p u t t e r e d a t o m s a l s o i n c r e a s e d, f r o m 17 e V t o 37 ev, t h e r e s u l t o f a n i n c r e a s i n g n u m b e r o f h i g h e n e r g y a t o m s i n t h e e n e r g y d i s t r i b u t i o n. H a r r i s o n [ H a 8 3 ] u s e d a m o l e c u l a r - d y n a m i c s c o d e t o s i m u l a t e t h e s p u t t e r i n g o f C u b y A r *. U n d e r a l l b o m b a r d m e n t c o n d i t i o n s, a t l e a s t 8 5 % o f t h e s p u t t e r e d a t o m s w e r e f o u n d t o o r i g i n a t e f r o m t h e f i r s t a t o m i c l a y e r. T h e p e r c e n t a g e w a s c o n s t a n t i n t h e e n e r g y r a n g e o f 8-2 0 k e V, b u t w a s f o u n d t o i n c r e a s e s t e a d i l y a s t h e p r o j e c t i l e e n e r g y w a s d e c r e a s e d to 0. 6 k e V. T h e p r e s e n t r e s u l t s, i n c o n j u n c t i o n w i t h t h e 1 5 k e V A r * d a t a r e p o r t e d i n [ D u 8 3 ], i n d i c a t e t h a t f o r A r * b o m b a r d m e n t t h e v a l u e s o f Fj a n d n A a r e i n d e p e n d e n t o f p r o j e c t i l e e n e r g y i n t h e r a n g e o f 1 5-5 0 kev. T h i s s u g g e s t s t h a t t h e r e is n o s i g n i f i c a n t v a r i a t i o n i n t h e c a s c a d e

d y n a m i c s i n t h i s e n e r g y r a n g e, i n c o n t r a s t t o t h e c o n c l u s i o n s d r a w n f r o m 64 [ D u 8 3 ]. I t is, h o w e v e r, c o n s i s t e n t w i t h t h e a s s u m p t i o n o f t h e l i n e a r c o l l i s i o n - c a s c a d e t h e o r y t h a t t h e l a t e - g e n e r a t i o n r e c o i l a t o m s t h a t d o m i n a t e t h e r e c o i l f l u x h a v e n o m e m o r y o f t h e i n i t i a l p r o j e c t i l e / t a r g e t - a t o m c o l l i s i o n s [Si69, S i 8 1 ]. S i g n i f i c a n t v a r i a t i o n s w e r e o b s e r v e d w h e n t h e i n c i d e n t A r + e n e r g y w a s l o w e r e d t o 3 k e V. T h e i n c r e a s e i n F x is w e l l r e p r o d u c e d b y c o m p u t e r s i m u l a t i o n s t u d i e s, w h i c h s u g g e s t t h a t a p r o b a b l e c a u s e is a d e c r e a s e i n t h e a v e r a g e e n e r g y o f r e c o i l a t o m s w i t h i n t h e t a r g e t [Bi84, B i 8 7, R o 8 3 ], T h i s is a l s o s u g g e s t e d i n t h e p r e s e n t e x p e r i m e n t b y t h e n a r r o w i n g o f t h e a n g u l a r d i s t r i b u t i o n o f s p u t t e r e d G a a t o m s, m o s t o f w h i c h o r i g i n a t e i n t h e s e c o n d a t o m i c l a y e r. 3. 6 A n a l y s i s o f U n c e r t a i n t i e s T h e r e a r e s e v e r a l s o u r c e s o f e x p e r i m e n t a l u n c e r t a i n t y r e s u l t i n g f r o m p o t e n t i a l m i s a l i g n m e n t s o f t h e a p p a r a t u s. D u r i n g t h e c o l l e c t i o n, t h e t a r g e t n o r m a l m a y n o t h a v e b e e n a c c u r a t e l y a l i g n e d p a r a l l e l t o the b e a m a x i s. A c c o r d i n g t o t h e c o l l i s i o n - c a s c a d e t h e o r y, s m a l l d e v i a t i o n s f r o m n o r m a l i n c i d e n c e s h o u l d h a v e n o e f f e c t o n t h e a n g u l a r d i s t r i b u t i o n o f a t o m s s p u t t e r e d f r o m a s m o o t h t a r g e t [ S i 8 1 ], a n d t h i s h a s b e e n c o n f i r m e d e x p e r i m e n t a l l y [ M n 8 1 ]. T h e r e f o r e t h e e f f e c t o n t h e m e a s u r e d a n g u l a r d i s t r i b u t i o n w o u l d b e a s i m p l e r o t a t i o n, w h i c h w o u l d b e c a l c u l a t e d b y t h e f i t t i n g p r o c e d u r e. T h e s a m e p r o b l e m c o u l d r e s u l t b e c a u s e o f c u r v a t u r e o f t h e l i q u i d t a r g e t s u r f a c e. A c o m p o n e n t o f c u r v a t u r e i n t h e c o l l e c t i o n p l a n e w o u l d a l s o c a u s e a s i m p l e r o t a t i o n o f t h e a n g u l a r d i s t r i b u t i o n. A c o m p o n e n t p e r p e n d i c u l a r t o t h e c o l l e c t i o n

p l a n e w o u l d n o t c a u s e a v a r i a t i o n i n t h e p a r a m e t e r n A, b u t w o u l d r e d u c e 65 t h e o v e r a l l a p p a r e n t n o r m a l i z a t i o n. F o r a v a l u e o f n ( - 2, a t i l t o f 10 w i t h r e s p e c t t o t h e n o m i n a l t a r g e t p l a n e w o u l d d e c r e a s e t h e m e a s u r e d v a l u e o f Al b y 3%. B e c a u s e t h e c o l l i m a t o r a s s e m b l y w a s r i g i d l y a t t a c h e d t o t h e c o l l e c t o r - f o i l c y l i n d e r, a n d c o u l d b e a c c u r a t e l y a l i g n e d (± 0. 2 m m ), it is e x p e c t e d t h a t t h e b e a m - s p o t w a s w e l l p o s i t i o n e d a t t h e c e n t e r o f t h e c o l l e c t o r - f o i l s w i t h r e s p e c t t o t r a n s v e r s e d e v i a t i o n s. S m a l l v a r i a t i o n s i n t h e t r a n s v e r s e p o s i t i o n o f t h e t a r g e t w o u l d h a v e v e r y l i t t l e e f f e c t o n t h e m e a s u r e d a n g u l a r d i s t r i b u t i o n. F o r a n o m i n a l v a l u e o f n - 2, a t r a n s v e r s e s h i f t o f 0. 4 m m w o u l d r e s u l t i n n o s i g n i f i c a n t v a r i a t i o n in t h e v a l u e s o f e i t h e r o r n A ( w i t h i n t h e u n c e r t a i n t y o f t h e f i t t i n g p r o c e d u r e ), a n d a n a n g u l a r o f f s e t o f 0.4. T h e t a r g e t l a d d e r d i d h a v e s o m e f r e e d o m o f m o v e m e n t i n t h e d i r e c t i o n p a r a l l e l t o t h e b e a m, e v e n w h e n l o c k e d i n t o p o s i t i o n. T h e e s t i m a t e d u n c e r t a i n t y i n t h e p o s i t i o n w a s ±0.8 m m. S u c h a v a r i a t i o n w o u l d h a v e l i t t l e e f f e c t o n t h e n o r m a l i z a t i o n o f t h e a n g u l a r d i s t r i b u t i o n, b u t w o u l d d i s t o r t i t s s h a p e. T h e n o m i n a l a n d a c t u a l m e a s u r e d a n g l e s o f e m i s s i o n, 8 a n d if> r e s p e c t i v e l y, a r e r e l a t e d a c c o r d i n g to,, c o s (8) - h / R 3.6) cos(v-) - [(h/r)2. 2hcos(0)/R + 1]^ w h e r e R is t h e r a d i u s o f c u r v a t u r e o f t h e c o l l e c t o r f o i l, a n d h is t h e o f f s e t i n t h e t a r g e t p o s i t i o n i n t h e d i r e c t i o n p a r a l l e l t o t h e b e a m. T o e s t i m a t e t h e p o t e n t i a l e f f e c t o n t h e f i t t i n g p a r a m e t e r s, t h e e x p e r i m e n t a l d a t a w e r e t r a n s f o r m e d a c c o r d i n g t o E q u a t i o n 3.6, a n d r e fit. A d e v i a t i o n o f 0. 8 m m i n t h e t a r g e t p o s i t i o n w o u l d t y p i c a l l y

66 r e s u l t i n a v a r i a t i o n o f 5 % i n t h e v a l u e s o f n ^ a n d n I n, a n d 0. 1 % ( 0. 3 % ) i n t h e v a l u e o f A g g ( A I n ). T h e v a r i a t i o n s i n t h e f i t t i n g p a r a m e t e r s w e r e a d d e d i n q u a d r a t u r e t o t h e u n c e r t a i n t i e s o b t a i n e d f r o m t h e s t a n d a r d f i t t i n g p r o c e d u r e. U n c e r t a i n t y i n t h e v a l u e o f t h e s p u t t e r i n g a n g l e 6 m a y a l s o r e s u l t f r o m s m a l l m i s a l i g n m e n t s o f t h e c o l l e c t o r f o i l s o n t h e c o l l e c t o r - f o i l c y l i n d e r, o r f r o m t h e d e t e r m i n a t i o n o f t h e c o l l e c t o r f o i l a n g l e d u r i n g t h e R B S a n a l y s i s. T h e e s t i m a t e d u n c e r t a i n t y i n t h e v a l u e o f 8 is ±0.4. T h e p o t e n t i a l e f f e c t o n t h e f i t t i n g p a r a m e t e r s w a s e s t i m a t e d b y a d d i n g 0.4 t o t h e a b s o l u t e m a g n i t u d e s o f t h e n o m i n a l a n g l e s ( s o t h a t it d i d n o t c a u s e a s i m p l e r o t a t i o n ) a n d r e - f i t t i n g t h e d a t a. T h e v a r i a t i o n in t h e v a l u e o f n x w a s t y p i c a l l y = 1. 5 %, a n d f o r A x, = 0. 2 %. T h e v a r i a t i o n s w e r e a d d e d i n q u a d r a t u r e w i t h t h e o t h e r u n c e r t a i n t i e s. A d d i t i o n a l e x p e r i m e n t a l u n c e r t a i n t i e s r e s u l t f r o m b e a m c u r r e n t i n t e g r a t i o n a n d c o u n t i n g s t a t i s t i c s. D u r i n g t h e c o l l e c t i o n, t h e o p e n i n g i n t h e a p p a r a t u s t h r o u g h w h i c h t h e b e a m p a s s e d s u b t e n d e d 0. 5 % o f t h e s o l i d a n g l e i n t o w h i c h s e c o n d a r y e l e c t r o n s c o u l d b e e m i t t e d, s o t h a t c u r r e n t i n t e g r a t i o n is e x p e c t e d t o b e q u i t e a c c u r a t e. T h e c o r r e s p o n d i n g v a l u e f o r t h e R B S a n a l y s i s w a s 1%. S t a t i s t i c a l u n c e r t a i n t i e s f o r t h e i n t e g r a t i o n o f t h e I n p e a k s i n t h e R B S s p e c t r a w e r e t y p i c a l l y 0. 6 %, a n d f o r G a, 2%. T w o p o t e n t i a l s o u r c e s o f i n a c c u r a c y i n t h e c o l l e c t i o n d a t a w e r e i n v e s t i g a t e d. I t h a s b e e n o b s e r v e d t h a t d u r i n g h e a v y - i o n R B S a n a l y s i s o f c o l l e c t o r f o i l s, t h e a m o u n t o f m a t e r i a l a t a g i v e n p o i n t b e i n g a n a l y z e d m a y d e c r e a s e w i t h a n a l y s i s b e a m f l u e n c e [ W e 8 8 ]. T h e c a u s e o f t h e d e c r e a s e is u n c e r t a i n, a s t h e s p u t t e r i n g y i e l d is e x p e c t e d to b e

67 q u i t e s m a l l. I n t h e p r e s e n t e x p e r i m e n t, t h e p e a k h e i g h t s w e r e c a r e f u l l y m o n i t o r e d a s a f u n c t i o n o f t h e a n a l y s i s b e a m f l u e n c e, a n d n o s u c h e f f e c t w a s o b s e r v e d. I f t h e s t i c k i n g p r o b a b i l i t y o f s p u t t e r e d m a t e r i a l o n the s u r r o u n d i n g m e t a l s u r f a c e s is s i g n i f i c a n t l y l e s s t h a n o n e, t h e n G a a n d I n a t o m s s p u t t e r e d d u r i n g t a r g e t c l e a n i n g m a y b e d e p o s i t e d o n t h e c o l l e c t o r f o i l s, e v e n t h o u g h t h e r e is n o d i r e c t p a t h t o them. S i m i l a r l y, m a t e r i a l s p u t t e r e d d u r i n g t h e c o l l e c t i o n s m a y b e d e p o s i t e d o n a d j a c e n t c o l l e c t o r s t r i p s n o t e x p o s e d to, t h e t a r g e t. S e v e r a l p o i n t s b e t w e e n s t r i p s w e r e, t h e r e f o r e, a n a l y z e d f o r t h e p r e s e n c e o f a G a o r In b a c k g r o u n d. T h e a m o u n t o f m a t e r i a l w a s a l w a y s f o u n d t o b e < 1 % o f t h a t o n t h e a d j o i n i n g c o l l e c t i o n s t r i p s. A r e l a t e d c o n c e r n is t h a t t h e s t i c k i n g p r o b a b i l i t i e s o f G a a n d I n a t o m s m a y b e u n e q u a l, o r m a y d e p e n d o n t h e a m o u n t o f m a t e r i a l d e p o s i t e d o n t h e c o l l e c t o r f o i l s. A l t h o u g h n o s t i c k i n g p r o b a b i l i t y m e a s u r e m e n t s w e r e p e r f o r m e d i n t h e p r e s e n t e x p e r i m e n t, D u m k e e t a l. [ D u 8 3 ] i n v e s t i g a t e d t h e p r o b l e m i n d e t a i l. T h e r e s u l t s o f [ D u 8 3 ] i n d i c a t e t h a t f o r b o t h e l e m e n t s t h e s t i c k i n g p r o b a b i l i t i e s a r e i n d e p e n d e n t o f c o l l e c t o r f o i l c o v e r a g e o v e r t h e r e l e v a n t r a n g e, a n d a r e p r o b a b l y n e a r u n i t y. B e c a u s e t h e a m o u n t o f s p u t t e r e d m a t e r i a l c o l l e c t e d w a s s i m i l a r f o r t h e t w o e x p e r i m e n t s, a n d b e c a u s e e a c h u s e d g r a p h i t e c o l l e c t o r f o i l s, t h e c o n c l u s i o n s s h o u l d b e a p p l i c a b l e t o t h e p r e s e n t e x p e r i m e n t a s w e l l. A f i n a l c o n c e r n is t h a t t h e i o n b e a m b o m b a r d m e n t m i g h t h a v e a f f e c t e d t h e n e a r - s u r f a c e c o m p o s i t i o n p r o f i l e o f t h e a l l o y t a r g e t. D u r i n g t h e c o l l e c t i o n, t h e m a x i m u m s p u t t e r i n g r a t e w a s l. l x l O 15 a t o m s / c m 2 / s e c, c o n s i s t i n g m o s t l y o f I n s p u t t e r e d f r o m t h e f i r s t

68 a t o m i c l a y e r, w h i c h a r e r e p l e n i s h e d b y d i f f u s i o n f r o m t h e b u l k. T h e d i f f u s i o n - d r i v e n f l u x, J, is r e l a t e d t o t h e c o n c e n t r a t i o n g r a d i e n t, d C / d x, b y F i c k ' s law, w h e r e D is t h e d i f f u s i o n c o e f f i c i e n t. E q u a t i o n 3. 7 c a n b e u s e d to e s t i m a t e t h e c h a n g e i n s u r f a c e I n c o n c e n t r a t i o n w h i c h w i l l d r i v e a f l u x o f I n t o t h e s u r f a c e e q u a l i n m a g n i t u d e t o t h e s p u t t e r - r e m o v a l r a t e. F o r l i q u i d Ga, D - 1. 9 7 x 1 0 " 5 c m 2 / s e c [ W a 8 0 ], s o t h a t e q u i l i b r i u m is a t t a i n e d f o r a v a l u e o f d C / d x - 5. 6 x 1 0 19 a t o m s / c m 4. A s s u m i n g d x - o n e a t o m i c l a t t i c e s p a c i n g, d C - 1. 5 x 1 0 12 a t o m s / c m 3, w h i c h c o r r e s p o n d s t o a f r a c t i o n a l c h a n g e i n t h e I n c o n c e n t r a t i o n o f 4 x 10"11. T h e e f f e c t o f i o n b o m b a r d m e n t o n t h e s u r f a c e c o m p o s i t i o n is t h e r e f o r e n e g l i g i b l e.

69 F i g u r e 3. 1 S c h e m a t i c i l l u s t r a t i o n o f t h e a p p a r a t u s u s e d f o r t h e c o l l e c t i o n o f s p u t t e r e d a t o m s. T h e g r a p h i t e c o l l e c t o r f o i l s w e r e m o u n t e d o n the i n s i d e o f t h e c o l l e c t o r f o i l c y l i n d e r, w h i c h h a s a r a d i u s o f c u r v a t u r e o f 3. 6 4 cm.

collector foil cylinder isking cylinder collim ator assembly current integration

70 F i g u r e 3. 2 S c h e m a t i c i l l u s t r a t i o n o f t h e a p p a r a t u s u s e d f o r t h e R B S a n a l y s i s o f c o l l e c t o r f o i l s. T h e a n a l y s i s w a s p e r f o r m e d a t t h e E N t a n d e m V a n de G r a a f f f a c i l i t y a t t h e C a l i f o r n i a I n s t i t u t e o f T e c h n o l o g y. /

C ollim ators Detector and Aperture Faraday Cage /\,6q 2* Beam D3! Q -600 V v

71 F i g u r e 3. 3 B l o c k d i a g r a m o f t h e d a t a a c q u i s i t i o n e l e c t r o n i c s u s e d f o r t h e R B S a n a l y s i s.

72 Figure 3.4 T y p ic a l RBS spectrum obtained fo r oxygen backscattered from a c o lle c to r f o i l. The c o lle c tio n was performed fo r 25 kev Ar+ bombardment o f a Gain e u te c tic a llo y ta rg e t. I

c o u n t s 0 1000 2000 3000 c h a n n e l

73 Figure 3.5 Angular d is trib u tio n s o f Ga and In atoms sputtered from the G a-in e u te c tic a llo y under Ar* bombardment a t: a) 3 kev, b) 25 kev, and c) 50 kev. Also shown are computer f i t s o f the angular d is trib u tio n s by a fu n c tio n o f the form N (0) oc cosn ( 0 ). The angular d is trib u tio n s have been norm alized so th a t N (0) - 1.0. Results fo r the f i t t i n g parameters are presented in Table 3.1.

Relative Counts A n g l e ( d e g r e e s )

Relative Counts A n g l e ( d e g r e e s )

Relative Counts A n g l e ( d e g r e e s )

74 Figure 3.6 An ill u s t r a t i o n o f the d iffe re n c e between non-weighted and weighted computer f i t s o f the angular d is trib u tio n s obtained fo r 50 kev Ar+ bombardment o f the G a-in e u te c tic a llo y : a) sputtered Ga, non-weighted f i t ; b) sp uttered Ga, weighted f i t ; c) sputtered In, non-weighted f i t ; and d) sputtered In, weighted f i t.

An^e (degrees) Angle (degrees)

An^e (degrees) Angle (degees)

75 Figure 3.7 Schematic ill u s t r a t i o n o f the e ffe c t on the e je c tio n angle o f a sp u ttered atom caused by the absence o f s c a tte rin g centers above the ta rg e t surface. The s tra ig h t lin e in d ic a te s the tr a je c to r y the atom would fo llo w i f the ta rg e t were i n f i n i t e, w ith an im aginary surface plane (as g e n e ra lly assumed in the c o llis io n -c a s c a d e th e o r y ). The open c ir c le s would then represent f i l l e d l a t t i c e s ite s. The curved lin e shows the tru e t r a je c to r y, d e fle c te d towards the surface normal because o f the unbalanced in te ra c tio n w ith the surface atoms ( f i l l e d c ir c le s ). From [ S i8 7 ].

vacuum target 4

76 Figure 3.8 The n uclear stopping power o f Ar in Ga, as a fu n c tio n o f p r o je c t ile energy. C alculated using the formalism o f [ Z i8 0 ].

Sn(eV-cnf/IO^atoms) E n e r g y (kev)

77 Figure 3.9 In/G a y ie ld r a t io as a fu n ctio n o f s p u tte rin g an gle, fo r 50 kev Ar+ bombardment o f a G a-in e u te c tic a llo y ta rg e t.

Anc/e (degrees)

78 Figure 3.10 R esults o f a Monte Carlo computer sim u la tio n o f 90 kev s e lf-s p u tte r in g o f Cu. The s o lid lin e represents the depth d is trib u tio n o f sp uttered atoms. The dashed lin e represents the depth d is trib u tio n o f atomic displacements th a t u ltim a te ly re s u lt in the s p u tte rin g o f an atom. From [R s83].

RELATIVE NUMBER OF SPUTTERED ATOMS DEPTH (UNITS OF 3.6I5A)

79 Table 3.1 Results o f computer f i t s o f the angular d is trib u tio n s o f Ga and In atoms, sputtered from the G a-in e u te c tic a llo y, by a fu n c tio n o f the form N (0) - Acos" (6). The f i t t i n g parameter na is also given fo r the t o t a l d is tr ib u tio n o f m a te ria l, NGa(0 ) + NI n ( 0 ), sp u ttered onto the c o lle c to r f o i l s. The p a r t ia l s p u tte rin g y ie ld s were c a lc u la te d using Equation 3.3.

Ar+ energy (kev) Ar+ dose (mc) Atonic species Fitting parameter Partial sputtering yield Ys Y.nAca 3 4.20 Ga 4.9 ± 0.3 0.18 ± 0.02 10 ± 1 In 1.8 ± 0.1 1.7 ± 0.1 Ga + In 2.2 ± 0.1 25 5.72 Ga 3.3 ± 0.2 0.68 ± 0.05 5.7 ± 0.6 In 1.8 ± 0.1 3.9 ± 0.3 Ga + In 2.0 ± 0.1 25 6.47 Ga 3.2 ± 0.2 0.71 ± 0.05 5.8 ± 0.6 In 1.8 ± 0.1 4.1 ± 0.3 Ga + In 2.0 ± 0.1 50 4.50 Ga 3.2 ± 0.2 0.67 ± 0.05 5.4 ± 0.5 In 1.8 ± 0.1 3.6 ± 0.2 Ga + In 2.0 ± 0.1 50 6.57 Ga 3.2 ± 0.2 0.69 ± 0.05 5.5 ± 0.5 In 1.8 ± 0.1 3.8 ± 0.3 Ga + In 2.0 ± 0.1

80 Table 3.2 The fra c tio n o f sp u ttered atoms o rig in a tin g from the f i r s t atomic la y e r, c a lc u la te d using Equation 3.4, and assuming f i r s t and second la y e r In co ncentrations o f 0.96 and 0.0 9 7, re s p e c tiv e ly. The two sets of u n c e rta in tie s are discussed in the te x t.

Ar+ energy (kev) First layer fraction Alloy composition uncertainty Sputtering yield uncertainty 3 0.94-0.04 ±0.01 +0.06 25 0.88-0.03 ±0.01 (both runs) +0.05 50 0.87-0.03 ±0.01 (both runs) +0.05

CHAPTER FOUR SECONDARY-ION MEASUREMENTS 4.1 In tro d u c tio n The re s u lts presented in Chapter 3 were extended to a wide range o f p r o je c t ile energy and mass using the technique o f secondary-ion mass spectroscopy. I t was not p ossible to o b tain q u a n tita tiv e re s u lts fo r the absolute In +/Ga+ ion y ie ld r a tio w ith our apparatus, because o f u n c e rta in tie s in the io n iz a tio n p r o b a b ility o f sp u ttered atoms and in the ion mass dependence o f the spectrom eter tran sm issio n. However, v a ria tio n s in the in te n s ity r a t io I ( I n + ) /I( G a + ) could be ac cu ra te ly measured. Such v a ria tio n s would be in d ic a tiv e o f changes in the to ta l s p u tte rin g y ie ld r a tio and, th e re fo re, in the escape depth o f sp uttered atoms. The technique o f secondary-ion mass spectroscopy is a co nsiderably more e f f i c ie n t means than c o lle c tio n experiments fo r o b ta in in g such in fo rm a tio n. Furtherm ore, i t p e rm itted the simultaneous measurement o f v a ria tio n s in the m olecular/ato m ic ion y ie ld r a t io, which provided im portant in fo rm ation on the s p u tte rin g o f molecules. 4.2 Apparatus and Experim ental Procedure Secondary-ion mass spectra were measured fo r pure Ga and pure In ta rg e ts, and fo r the G a-in e u te c tic a llo y. The ta rg e ts were prepared and s p u tter-c lea n ed in the manner described in Chapter 2. Measurements were performed w ith in c id e n t ion beams o f 20Ne+, * A r+, 84Kr+, and 132Xe+ over an energy range o f 25-250 kev. The beams were obtained from the Cockcroft-W alton a c c e le ra to r. Typ ical beam 81

cu rren ts were <1 na, w ith a beam-spot diam eter o f =1 mm. S table beams o f such low cu rrent were obtained by o p erating the ion source w ith out a plasma a rc. The ion beams were in c id e n t on the ta rg e t a t an angle o f 30 w ith respect to the ta rg e t normal, which was alig n e d p a r a lle l to the mass spectrom eter a x is. Secondary-ion mass spectra were measured w ith a quadrupole mass spectrom eter. The p h ysical p rin c ip le s and o p eratio n o f quadrupole spectrom eters have been described in d e t a il by Dawson [D a76], The id e a l design co nsists o f two p a irs o f electro d es w ith h yperb o lic cross s e ctio n s, as illu s t r a t e d in Figure 4.1. In p ra c tic e, these are g e n e ra lly approximated by rods w ith ap p ro p riate c ir c u la r cross sectio n s. The opposing electrod es are separated by a distance o f 2 r0, and by convention the quadrupole axis lie s along the z-co o rd in ate a x is. The two sets o f electrod es are biased a t equal and opposite p o te n tia ls, which are superpositions o f d ir e c t and a lte r n a tin g components o f the form 4.1 ) 4> - ±(U-Vcoswt). In th is c o n fig u ra tio n, the tra je c to r ie s in the xz and yz planes are decoupled, and the equations o f motion are 4.2 a ) + - r[u -V c o s (w t)]x - 0 dt^ mr^ and 4.2 b ) ^ - ^ 2-[U-Vcos(Wt ) ] y - 0. These are examples o f Mathieu equations, fo r which so lu tio n s and s t a b il i t y co nditions have been analyzed a t length [Da76]. For ty p ic a l o p e ra tio n, V - 7U, and the frequency o f the a lte r n a tin g

component is several MHz. The fu n ctio n o f the quadrupole as a mass f i l t e r is possible because only lig h t ions are able to respond 83 a p p reciably to the high frequency component o f the p o te n tia l. In the xz p lan e, the ion motion induced by the p o s itiv e d ir e c t component is s ta b le. However, the a lte r n a tin g component causes a net in s t a b ilit y in the t r a je c to r ie s o f lig h t ions, so th a t only heavy-io n tra je c to r ie s remain s ta b le in th is p lan e. The n egative d ir e c t component in the yz plane g e n e ra lly induces unstable motion. However, fo r s e le c t lig h t ions, the frequency o f the a lte r n a tin g component is such th a t the t r a je c t o r ie s are p ro p e rly corrected as the am plitude o f the motion increases, and the tr a je c to r y remains s ta b le. For a proper choice o f the parameters U and V, the quadrupole w i l l pass only ions w ith in a narrow range o f mass/charge r a t io. Figure 4.2 i llu s t r a t e s the p r in c ip le set o f values fo r the parameters a = 4eU/mw2r 2 and q * 2eV/mw2ro fo r which ion tra je c to r ie s are s ta b le in both planes. A mass scan is obtained by ramping the vo ltag es U and V, w hile m ain tain in g a constant value fo r t h e ir r a t io, U /V. fo r U/V determines the mass re s o lu tio n o f the scan, The value chosen as illu s t r a t e d by the lin e s o f constant U/V in Figure 4.2. For a p p lic a tio n s such as the measurement o f secondary-ion mass sp ectra, where good re s o lu tio n is re q u ire d, the quadrupole is operated near the t ip o f the s t a b ilit y reg io n. The r e la t iv e s im p lic ity o f quadrupole mass spectrom eters provides an advantage over magnetic spectrom eters. The mass re s o lu tio n is e a s ily v a rie d by an e x te rn a l e le c tro n ic adjustment o f the r a t io U /V, and the mass/charge r a t io o f tran sm itted ions is a simple lin e a r fu n c tio n o f the

84 v o ltag es U and V. However, ion transm ission through the quadrupole is a strong fu n c tio n o f both mass and energy, h in d e rin g q u a n tita tiv e a n a ly s is. Above -4 0 u /e, the ion transm ission decreases e x p o n e n tia lly w ith mass [U t8 1 ]. For ions o f fix e d mass/charge r a t io, the transm ission decreases w ith decreasing energy, as the tim e (measured in r f cycles) spent in the quadrupole fie ld s increases. P a rtly fo r th is reason, i t is d e s ira b le to equip the quadrupole w ith an energy p r e f i l t e r to l im i t the energy range o f ions to be analyzed. Secondary-ion mass spectra were measured w ith a Uthe Technology, In c. model 100C quadrupole mass spectrom eter, shown in Figure 4.3. The u n it is a m odified re s id u a l gas a n a ly z e r, w ith the io n iz e r assembly replaced by a K ratos, In c. e le c tro s ta tic energy p r e f i l t e r. The p r e f i l t e r is shown in d e ta il in Figure 4.4. The c e n tra l stop blocks the passage o f sputtered n e u tra ls and high-energy ions, which would otherw ise c o n s titu te a s ig n ific a n t source o f background in the mass sp e c tra. The elements o f the p r e f i l t e r were biased as shown in Figure 4.4 to pass p o s itiv e ions o f energy 5 ev, w ith a bandpass o f 2 ev FWHM [Bn87]. In th is c o n fig u ra tio n, the entrance to the p r e f i l t e r was biased a t -5 V, c re a tin g a small c o lle c tio n f i e l d between the ta rg e t and p r e f i l t e r entrance. The f i l t e r capacito rs connected to the p r e f i l t e r power supplies were necessary to e lim in a te r f p ick-u p from the a lte r n a tin g p o te n tia l on the quadrupole rods [Bn87]. The angular acceptance o f the p r e f i l t e r was estim ated to be 13e±50 w ith respect to the ta rg e t surface norm al. This estim ate was made sim ply by c a lc u la tin g the t r a je c to r ie s o f 5 ev ions between the ta rg e t and the p r e f i l t e r en trance, assuming a uniform e le c t r ic f i e l d in th a t regio n.

85 Secondary ions were detected by an o f f-a x is G a lile o E le c tro - O ptics #4876 channeltron o p eratin g in the pulse s a tu ra tio n mode, and sp ectra were accumulated by a m u lti-ch a n n e l an alyzer o p e ra tin g in the m u lti-s c a lin g mode. The data a c q u is itio n e le c tro n ic s are illu s t r a t e d schem atically in Figure 4.5. The e le c tro n ic s were com pletely analogous to those used fo r the ISS a n a ly s is, described in d e t a il in Chapter 2. The ramp vo ltag e output o f the d ig ita l/a n a lo g co n verter in th is case drove the quadrupole c o n tro l u n it, which in tu rn ramped the rod voltag es U and V. The gain and bias o f the b ias a m p lifie r were adju sted to give a f u l l range sweep o f 0-250 u. The mass sub-regions to be scanned by the quadrupole, and the in c id e n t charge per mass step, were p reset on the computer. During measurements the ta rg e t was unbiased, but before and a fte r each spectrum was taken the beam c u rre n t was measured tw ice both w ith and w ith out a +180 V b ias on ta rg e t, in order to c o rre c t the in c id e n t charge fo r seco ndary-electron emission. In order to assure th a t the measurements were not in flu enced by the gradual accum ulation o f an adsorbed la y e r between clean in g s, or by a change in the surface composition o f the a llo y due to h eatin g o f the ta rg e t during clean in g, several mass spectra were taken fo r each in c id e n t-io n energy. The quadrupole re s o lu tio n was norm ally set to give a peak FWHM of 1 u. The s lig h t ly less than optimum re s o lu tio n was a t r a d e -o ff fo r the increase in s e n s it iv it y d esired a t high ion mass fo r m onitoring Ga and In m olecular ions. Because the s ig n ific a n t Ga and In isotopes are separated by 2 u, th is introduced no in te rfe re n c e s in the regions of in te r e s t.

86 4.3 Results Figure 4.6 shows ty p ic a l p o s itiv e secondary-ion mass spectra fo r clean Ga and In ta rg e ts, and fo r the clean G a-in e u te c tic a llo y. These sp ectra, and (unless noted) a l l oth er data presented, have not been co rrecte d fo r the u n c e rta in mass dependent quadrupole tran sm ission, or fo r the mass dependent channeltron d e te c tio n e ffic ie n c y. Lack o f a s u ita b le theory has also prevented accurate c o rre c tio n fo r d iffe re n c e s in ta rg e t work fu n c tio n and atomic io n iz a tio n p o te n tia l, which can cause la rg e changes in the sputtered-atom io n iz a tio n p r o b a b ility [Y u 86]. For the clean Ga and In ta rg e ts, a la rg e fra c tio n o f the detected low-energy ions were dimers and, fo r Ga, even trim e rs (h e a v ie r molecules are beyond the scan range o f the quadrupole). However, before s p u tte r c le a n in g, m o lecular-io n y ie ld s were v i r t u a l ly undetectable even though ato m ic-io n y ie ld s were g re a tly enhanced.. Only as the ta rg e t was s p u tte r-c le a n e d d id the m olecular/atom ic ion y ie ld r a t io become s ig n if ic a n t, dem onstrating th a t th is r a t io is a s e n s itiv e in d ic a to r o f ta rg e t c le a n lin e s s. S im ila r behavior has been described by Wittmaack [W i77]. No m u ltip ly charged ions were detected fo r e ith e r ta rg e t. The secondary-ion mass spectrum fo r the clean G a-in a llo y ta rg e t in d ic a te s considerable In enrichment o f the s p u tte re d -io n f lu x. The In 2 c o n trib u tio n was also stro ng, w ith the I ( I n 2 ) / I ( I n * ) r a t io n e a rly equal to th a t o f the pure In ta rg e t. In c o n tra s t, v i r t u a l l y no Ga2 was detected, though a sm all Gain* peak was observed. For comparison, Figure 4.7 shows a spectrum obtained fo r the a llo y ta rg e t p r io r to s p u tte r-c le a n in g. C onsistent w ith the re s u lts o f Auger e le c tro n

87 a n a ly s is, the spectrum is dominated by Ga+, because o f the presence o f g a lliu m oxide a t the ta rg e t su rface. As the ta rg e t was s p u tte r-c le a n e d, the secondary-ion flu x became enriched w ith In, and the m o lecular-io n species became s ig n ific a n t. Figure 4.8 illu s t r a t e s the dependence o f the Ga+ y ie ld from a pure Ga ta rg e t on the energy and mass o f the prim ary ion beam. Also shown is the Ga s p u tte rin g y ie ld as c a lc u la te d from the sem i-em p irical form ula o f Matsunami e t a l.[m a 8 4 ], The th e o re tic a l y ie ld has been norm alized to f i t the d ata, using the same n o rm a liza tio n fa c to r fo r each p r o je c t ile. The o v e r a ll agreement o f the p r o je c tile energy and mass dependence of the c a lc u la te d s p u tte rin g y ie ld s and the measured Ga* y ie ld s is q u ite s a tis fa c to ry, e s p e c ia lly fo r Ne+ and Ar+. For Kr* and Xe*, the c a lc u la te d y ie ld s d eviate from the ion y ie ld s a t h ig h er in c id e n t ion energies (and hig her nuclear stopping pow er). This may in d ic a te the onset o f n o n -lin e a r e ffe c ts in the c o llis io n cascades, although c a lo rim e tric measurements by Andersen and Bay [An75] suggest th a t the excess atoms e je c te d from n o n -lin e a r cascades are o f very low energy, and should th e re fo re not c o n trib u te to th is measurement. S im ila r re s u lts were obtained from a pure In ta rg e t, as illu s t r a t e d in Figure 4.9. However, fo r the In d ata, th ere are v a ria tio n s (10% fo r Ne+ and 20% fo r Kr+ ) in the value o f the n o rm a liza tio n fa c to r req u ired to best f i t the ion y ie ld s. The agreement between the data o f Figures 4.8 and 4.9, and the c a lc u la te d s p u tte rin g y ie ld s (s o lid lin e s ), im ply th a t the io n iz a tio n p r o b a b ility o f 4-6 ev sp uttered atoms is independent o f p r o je c tile mass and energy fo r the p r o je c tile /ta r g e t combinations stu d ied. This

suggests th a t v a ria tio n s o f the measured I ( I n + ) /I( G a + ) r a t io fo r the a llo y ta rg e t would r e f le c t v a ria tio n s in the y ie ld r a t io fo r sp uttered n e u tra ls as w e ll, and would th e re fo re s ig n a l changes in the 88 c h a ra c te ris tic escape depth o f sputtered atoms. ' The dependence o f the d im er-io n y ie ld s on p r o je c t ile energy d e viates from th a t o f the atomic ions, although the p r o je c t ile mass dependence is s t i l l adequately described by the s e m i-e m p irical formula o f [Ma84]. For the pure In ta r g e t, I ( I n 2 ) / I ( I n + ) is shown in Figure 4.1 0 as a fu n c tio n o f p r o je c tile energy fo r Ne+, Ar+, Kr+, and Xe+ bombardment. For the Ar+, Kr+, and Xe+ beams, I ( I n 2 ) / I ( I n + ) is constant above =100 kev in c id e n t ion energy, but increases a t lower energy. The magnitude o f the increase v a rie s fo r the th ree beams, but the general p r o je c tile energy dependence is very s im ila r in each case. There is no s ig n ific a n t v a r ia tio n w ith p r o je c tile energy fo r Ne+ bombardment. At a fix e d p r o je c tile energy o f ( fo r example) 200 kev, th ere is l i t t l e v a r ia tio n o f I ( I n 2 ) / I ( I n + ) w ith p r o je c tile mass. S im ila r b e h a v io r is observed fo r the I(G a 2 ) /I( G a + ) and I(G a 2 ) /I( G a 2 ) data from a pure Ga ta rg e t, as illu s t r a t e d in Figures 4.11 and 4.1 2, re s p e c tiv e ly. The s ig n ific a n t amount o f s c a tte r in these data is probably the r e s u lt o f sm all (perhaps m icroscopic) g alliu m -o xid e in c lu s io n s a t the surface o f the liq u id ta rg e t. A tom ic-ion y ie ld s from such regions are considerably enhanced. For Ar+, Kr+, and Xe+ bombardment o f the Ga ta rg e t, the r a tio s increase fo r in c id e n t-io n energies below =200 kev, as compared w ith =100 kev fo r the In ta rg e t, but the magnitude o f the increase is s im ila r fo r each ta rg e t. For Ne+ bombardment, th ere is again l i t t l e v a r ia tio n w ith p r o je c tile energy.

89 For a l l beams, the ra tio s are equal to w ith in 15% a t 200 kev in c id e n t energy. The I ( I n + ) /I( G a + ) r a t io is shown as a fu n c tio n o f p r o je c tile energy in Figure 4.1 3 fo r Ne+, Ar+, Kr+, and Xe+ bombardment o f the G a-in e u te c tic a llo y ta rg e t. For Ne+, Ar+, and Xe+ bombardment, the r a t io is independent o f the in c id e n t ion energy w ith in the range o f s c a tte r o f the d a ta. There is a sm all decrease in the r a t io above 150 kev fo r Kr+ bombardment, and a more s ig n ific a n t system atic in crease, o f about 25%, as the p r o je c tile mass increases from th a t o f Ne to th a t o f Xe. The same general behavior is observed in Figure 4.1 4, where the Ga+ y ie ld from the G a-in a llo y ta rg e t is compared w ith th a t from the pure Ga t a r g e t, as a fu n c tio n o f p r o je c tile mass and energy. The data presented are the averages o f the ion y ie ld s fo r a l l spectra taken a t each in c id e n t ion energy. I t is b e lie v e d th a t the s c a tte r in the data observed in Figures 4.1 3 and 4.1 4 r e fle c ts the in te r m itte n t in flu e n c e of GaO in clusions a t the surfaces o f the Ga and G a-in a llo y ta rg e ts. S everal sp ectra were measured a t low re s o lu tio n, where the quadrupole transm ission is approxim ately independent o f ion mass, to estim ate the tru e values o f the ion y ie ld r a tio s. For 100 kev Ar+ bombardment o f a G a-in a llo y ta rg e t, i t was found th a t I ( I n 2 ) / I ( I n + ) = 0.6 5 ± 0.02 and I ( l n + ) /I( G a + ) - 5.9 ± 0.2. The quoted u n c e rta in tie s are the standard d e viatio n s o f the averages o f severa l measurements. An approximate c o rre c tio n can be made fo r the mass dependent channeltron d e te c tio n e ffic ie n c y, based on a (m ass)*' dependence [Ku79]. The re s u lts are I ( l n 2 ) / I ( I n + ) = 0.92 and I ( I n + ) /I( G a + ) = 7.5. The In/G a data were fu rth e r co rrected fo r the lim ite d angular acceptance o f the

quadrupole, using the values o f na obtained a t 50 kev fo r sp uttered n e u tra ls. Then ln +/Ga+ ~ 9. The surface co n centration o f an a llo y su bject to thermodynamic segregation is p red ic te d to be dependent on the ta rg e t tem perature, according to 90 4,3) l-xi J-'Xb explaa T ]. where x2 (xb ) is the surface (b u lk ) co n centratio n o f the segregating component, and a is a species dependent constant [Du83]. In order to v e r if y th a t h e atin g o f the G a-in a llo y ta rg e t during clean in g was not a ffe c tin g the secondary-ion measurements or the c o lle c tio n experim ents, an a llo y ta rg e t was exposed fo r 30 minutes to a 2.2 pk 100 kev Ar+ beam. This represents a deposited power o f about 5 times th a t a c tu a lly used fo r c le anin g. The in c id e n t ion cu rren t was then lowered to the le v e l norm ally used fo r c o lle c tin g secondary-ion mass sp ectra, and the Ga+ and In + y ie ld s were monitored fo r more than one hour. This was achieved by d iv id in g the MCA memory in to fo u r secto rs. Each sector was assigned an ion mass from the computer, and the masses were s e q u e n tia lly monitored by the quadrupole fo r a p re -s e t t o t a l in c id e n t charge. The sequence was repeated fo r a larg e number o f ite r a tio n s, developing a tim e v a r ia tio n spectrum fo r each designated ion mass. The r e s u ltin g d a ta, shown in Figure 4.1 5, in d ic a te no s ig n ific a n t change in the I ( I n + ) /I( G a + ) r a t io. This demonstrates th a t even i f the a llo y ta rg e t does become hot enough to a lt e r the surface com position, i t cools down r a p id ly enough not to a ffe c t the measurements.

91 4.4 Discussion 4.4.1 Molecular-Ion Measurements As illu s t r a t e d by Figure 4.6, the r e la t iv e In 2 y ie ld from the Gain a llo y ta rg e t was found to be n e a rly equal to th a t from the pure In ta rg e t, although v i r t u a l l y no Ga2 ions were d etected. Because the Ga2 y ie ld from a pure Ga ta rg e t is q u ite h ig h, th is suggests th a t Ga is g re a tly depleted in the low-energy sputtered-atom flu x r e la t iv e to the bulk a llo y com position, and is co n sistent w ith a high degree o f In enrichment a t the f i r s t atomic la y e r. The re s u lts o f the low re s o lu tio n mass spectra in d ic a te th a t in the 4-6 ev energy range, sp uttered m olecular ions are n e a rly as numerous as atomic ions fo r the p r o je c t ile /t a r g e t combinations observed. Such large r e la t iv e dimer y ie ld s have p revio u sly been observed when only secondary ions were detected. For example, Wittmaack and Staudenmaier [VJi75] measured secondary-ion emission fo r 10 kev Ar+ bombardment o f T i, V, Nb, Mo, Ta, and W ta rg e ts. The m olecular/ato m ic ion y ie ld r a tio s, measured a t the peaks o f the resp ective energy d is tr ib u tio n s, were found to be 0.7 0-2.3. Corresponding ra tio s fo r the noble m etals Cu and Au were low er, 0.33 and -0.0 1 re s p e c tiv e ly. Reported m olecular/ato m ic y ie ld ra tio s fo r sp u ttered n e u tra ls are much lower than those fo r ions. Oechsner and Gerhard [Oe74] observed the y ie ld s o f n e u tra ls sputtered from several m etals under 1 kev Ar* bombardment. For N i, Z r, Mo, Pd, H f, Ta, W, and Pt ta rg e ts, I(M 2 ) /I( M ) was measured to be 0.0 1 5-0.0 3 8. The r a tio s fo r the noble m etals Cu, Ag, and Au were h ig h e r, 0.0 9 5-0.1 1 7. For 200 ev Ar+ bombardment o f Cu,

Cooper and Hamed [Co84] measured I(C u 2 ) /I( C u ) fo r sp u ttered n e u tra ls to be 0.0 4 6. These re s u lts are w e ll reproduced by m olecular-dynamics 92 computer sim ulations o f 600 ev Ar+ bombardment o f c r y s ta llin e Cu. The Cu2/Cu y ie ld r a t io was c a lc u la te d to vary from 0.035 to 0.1 2, depending on the c r y s ta l o r ie n ta tio n (la rg e r r a tio s fo r the more c lo s e ly packed c r y s ta l faces) [Wg78]. This suggests th a t molecules are s u b s ta n tia lly o ver-represen ted in the s p u tte re d -io n flu x r e la t iv e to t h e ir abundance in the n e u tra l f lu x. Wittmaack and Staudenmaier [Wi75] have suggested th a t the io n iz a tio n p r o b a b ility o f sp u ttered molecules is s ig n ific a n tly g re a te r than th a t o f atoms, because the e le c tro n ic e x c ita tio n energies o f the m olecular co n stitu e n ts may be a d d itiv e, and the m olecular and atomic io n iz a tio n p o te n tia ls are n e a rly equal. The behavior o f the In 2 y ie ld may th e re fo re be neglected when using the measured s p u tte re d -io n flu x composition to in fe r changes in the n e u tra l flu x composition. The m olecular/ato m ic ion y ie ld ra tio s measured in the present experim ent d id not scale w ith the s p u tte rin g y ie ld s, as is p re d ic te d by the s t a t i s t ic a l recom bination theory (Equation 1.6 ). For a p r o je c tile energy o f 200 kev, the measured ln + y ie ld fo r Xe* bombardment o f pure In is a fa c to r o f 16 g re a te r than th a t fo r Ne+ bombardment. However, the I ( I n 2 ) / I ( I n * ) ra tio s are n e a rly equal fo r each beam, as w e ll as fo r the Ar* and Kr* beams. As a fu n ctio n o f p r o je c tile energy, th ere is no c o rr e la tio n between the observed y ie ld r a t io v a ria tio n s and the atom icion y ie ld s. This is eviden t from a comparison o f Figures 4.8-4.1 2. S im ila r experim ental re s u lts have been rep o rted fo r studies w ith in c id e n t ion energies above several kev. Staudenmaier [S t72] bombarded p o ly c r y s ta llin e W w ith 150 kev N e *, A r *, K r *, X e *, and C u*, and observed

W* y ie ld s fo r n -1-5. The r e la t iv e m o lecula r-io n y ie ld s a t a secondary- ion energy o f 15 ev were observed to be independent o f p r o je c tile mass. 93 Over the mass range stu d ied, the c a lc u la te d s p u tte rin g y ie ld increases from 0.45 fo r Ne+ bombardment, to 11.2 fo r Xe+ [M a84]. Wittmaack [Wi79] studied secondary-ion emission from Si bombarded by Ne+, Ar+, Kr+, and Xe+, in the energy range o f 3-30 kev. The in c id e n t ion energy dependence o f the I ( S i * ) / I ( S i 2 ) y ie ld ra tio s fo r n -3-6 were q u ite s im ila r, w ith l i t t l e v a r ia tio n (<30%) w ith energy, desp ite a fa c to r o f two increase in the s p u tte rin g y ie ld. As a fu n ctio n o f p r o je c tile mass, the S i2 y ie ld was found to be p ro p o rtio n a l to the s p u tte rin g y ie ld Y, ra th e r than to Y2. Experiments a t lower in c id e n t ion energy were performed by Gerhard and Oechsner [G e75b]. Eleven d iffe r e n t ta rg e ts were bombarded by Ar+ in the energy range o f 0.1-1 kev, and sp uttered n e u tra ls were observed by RF p o s t-io n iz a tio n. The I(M 2 ) /I( M ) y ie ld r a tio s fo r Ag, Cu, N i, Pd, and Mo ta rg e ts were found to be approxim ately p ro p o rtio n a l to the s p u tte rin g y ie ld, as p re d ic te d by Equation 1.6. For Au, P t, W, Ta, Z r, and Hf ta rg e ts, the r a t io was found to increase sh arply up to 0.3 kev in c id e n t ion energy, and to be n e a rly constant a t h ig h er energy. For each ta rg e t, the s p u tte rin g y ie ld increases m onotonically over the energy range s tu d ied. W ith the exception o f Z r, the two groups o f elements are w e ll separated by atomic mass. W ith even lower beam energies o f 60-300 ev, Cooper and Hamed [Co84] bombarded Cu w ith Ar+ and observed sp u ttered n e u tra ls, using a low- pressure argon plasma as a p o s t-io n iz a tio n medium. The Cu2 y ie ld was measured to be p ro p o rtio n a l to the square o f the s p u tte rin g y ie ld, as

94 predicted by Equation 1.6. I f sputtered atoms were d is trib u te d randomly about the in c id e n t ion impact p o in t, as assumed in the d e riv a tio n o f Equation 1.6, then the data from the present experiment would suggest th a t the area about the impact p o in t from which sp uttered atoms are e je c te d is n e a rly p ro p o rtio n a l to the s p u tte rin g y ie ld. However, fo r a fix e d p r o je c t ile /t a r g e t mass r a t io, m1/m2, i t is more probable th a t the l a t e r a l ex te n t o f a c o llis io n -c a s c a d e near the surface is re la te d to the magnitude o f the energy tra n s fe r in the f i r s t few p r o je c tile /ta r g e t-a to m c o llis io n s, and so to the i n i t i a l p r o je c tile energy. S im ila r ly, fo r a r a t io m1/m2 < 1, such as fo r Ne+ bombardment o f In, the p r o je c tile is more l ik e l y to experience a larg e s c a tte rin g angle c o llis io n near the s u rface, so th a t the tran sverse momentum o f the scatte re d io n, and the la t e r a l ex te n t o f the c o llis io n -c a s c a d e, may be r e la t iv e ly la rg e. For m: /m2 > 1, such as fo r Xe+ bombardment o f In, the p r o je c tile w i l l p rim a rily experience only sm all angle c o llis io n s, so th a t the momentum o f the sc a tte re d ion is more forward d ire c te d. These e ffe c ts are w e ll illu s t r a t e d by computer sim u latio n s. B iersack [B i87] used a b in a r y -c o llis io n code to sim ulate Ga+ bombardment o f A l. In creasin g the p r o je c tile energy from 15 kev to 100 kev increased by a fa c to r o f 2.0 the radius o f the c ir c le from in sid e which 50% o f the sputtered atoms were e je c te d, d esp ite a fa c to r o f three decrease in the s p u tte rin g y ie ld. Kang e t a l.[k a 8 5 ] also used a b in a ry - c o llis io n code to sim ulate the 3 kev Ar+ bombardment o f Cu and Au. The l a t e r a l e x te n t o f the c o llis io n -c a s c a d e volume, and the area about the ion impact p o in t from which sputtered atoms were e je c te d, were

95 s ig n if ic a n tly la rg e r fo r the h e avie r Au ta rg e t than fo r Cu, although the s p u tte rin g y ie ld was le s s. The data may instead suggest th a t the form ation o f m ultiraers occurs p r im a rily when an in d iv id u a l r e c o il atom causes the c o rre la te d e je c tio n o f severa l atoms from a r e la t iv e ly lo c a liz e d reg io n o f the surface. A m olecular-dynam ics sim u la tio n by Jakas and H arrison [Ja86] o f the 3 kev Ar+ bombardment o f Cu shows an e x c e lle n t example o f a s in g le 40 ev r e c o il atom causing the low-energy e je c tio n o f fo u r neighboring surface atoms. As mentioned by Andersen [An87], such an event may be r e la t iv e ly l ik e l y to r e s u lt in the form ation o f a m olecule. This mechanism is in te rm e d iate between the s t a t i s t ic a l recom bination model o f Gerhard [G e75a], in which the sp u ttered atoms are assumed com pletely independent, and the d ire c t emission mechanism resp onsib le fo r the s p u tte rin g o f adsorbate m olecules. The m olecular form ation p r o b a b ility would be determined p rim a rily by the nature o f the atom-atom in te r a c tio n, because i t is th is in te ra c tio n which governs the e je c tio n process and the s t a b ilit y above the surface o f m olecules. Therefo re the m o lecular/ato m ic y ie ld r a t io would remain approxim ately co nstan t, even as the s p u tte rin g y ie ld v a rie d. The sm all v a ria tio n s o f I(M 2 ) / I ( M + ) observed in the present experim ent as a fu n c tio n o f p r o je c t ile energy are n o t, however, understood. For those p r o je c tile - ta r g e t combinations fo r which Equation 1.6 has been found to be v a lid [Co84, Ge75b], the generated c o llis io n cascades may be o f s u f f ic ie n t ly low energy th a t the e je c tio n o f severa l atoms by a s in g le r e c o il atom is an extrem ely ra re occurrence. The s p a tia l d is tr ib u tio n o f sputtered atoms would then be t r u ly random, and

96 Equation 1.6 would be expected to be v a lid. 4.4.2 Depth o f O rig in Measurements The re s u lts o f the I ( I n * ) / I ( G a * ) data fo r the a llo y ta rg e t in d ic a te th a t the escape depth o f low-energy sp u ttered p a r tic le s is n e arly independent o f p r o je c tile energy and mass over the ranges stu d ied. The s lig h t decrease o f I ( I n * ) / I ( G a + ) observed fo r Kr* bombardment above 150 kev is not reproduced by the data o f Figure 4.1 4, and may be spurious. The system atic increase o f l ( I n * ) / l ( G a * ) as the p r o je c tile mass was increased is more s ig n ific a n t. Assuming a value o f Fx - 0.87 fo r 50 kev Ar* bombardment, i t is estim ated th a t the fra c tio n o f lowenergy sputtered ions o r ig in a tin g from the f i r s t atomic la y e r increased by =5% as the p r o je c tile mass was increased from th a t o f Ne to th a t of Xe. The data presented in Chapter 3 show th a t the fra c tio n o f sp uttered atoms o r ig in a tin g from the f i r s t atomic la y e r is constant (a t Fj = 0.8 7 ) fo r Ar* bombardment between 25 and 50 kev. The secondary ion re s u lts presented in the cu rrent chapter fu rth e r suggest th a t th is fr a c tio n is constant fo r energies up to 250 kev. In a d d itio n, only a small v a r ia tio n is observed w ith p r o je c tile mass. For the secondary-ion measurements, only sp u ttered ions in a narrow energy and angular window were d e tected, whereas by use o f c o lle c tio n techniques a l l sputtered p a r tic le s were observed. The p o s s ib ility must th e re fo re be considered th a t a s ig n ific a n t v a r ia tio n o f YIn /Y Ga may have been masked by changes in the io n iz a tio n p r o b a b ility, or the energy and angular d is trib u tio n s, o f sputtered atoms. However, as discussed p re v io u s ly, the agreement of

97 the p r o je c tile energy and mass dependence o f the measured ato m ic-io n y ie ld s from elem ental ta rg e ts w ith the c a lc u la te d s p u tte rin g y ie ld s in d ic a te s th a t the io n iz a tio n p r o b a b ility o f low-energy sp u ttered atoms is independent o f p r o je c tile energy and mass. I t also suggests, because o f the narrow d e te c to r energy window, th a t th ere are no dram atic changes in the shape o f the secondary-ion energy d is trib u tio n over the p r o je c tile energy and mass range observed. A c a lc u la tio n shows th a t the mass spectrom eter would also be s e n s itiv e to changes in the angular d is tr ib u tio n o f sp u ttered ions. For in stan ce, the angular d is tr ib u tio n measured in [Du83] fo r Ga sputtered from the G a-in a llo y ta rg e t broadened when the in c id e n t ion energy was increased, from rigg - 3.7 a t 15 kev, to n^a - 2.0 a t 25 kev. This was accompanied by a change o f a fa c to r o f 2.14 in the r a t io o f the p a r t ia l s p u tte rin g y ie ld s, which would be observed as a fa c to r o f *=1.7 change w ith the quadrupole spectrom eter. I t is th e re fo re u n lik e ly th a t v a ria tio n s in the sp uttered atom io n iz a tio n p r o b a b ility, or energy and angular d is trib u tio n s, have masked a s ig n ific a n t change in the In/G a y ie ld r a tio over the p r o je c tile energy and mass range studied. The re s u lts o f the secondary-ion measurements are in reasonable agreement w ith the c o llis io n -c a s c a d e th eo ry, which assumes th a t a s u f f ic ie n t number o f c o llis io n s occurs w ith in the cascade to randomize the motion o f the la te -g e n e ra tio n r e c o il atoms [S i6 9, Si81]. In th is l i m i t, the r e c o il atoms lose a l l memory o f the i n i t i a l p r o je c t ile /t a r g e t atom c o llis io n s. Small v a ria tio n s were observed only as a fu n c tio n of p r o je c t ile mass, fo r which v a ria tio n s in the nuclear stopping power are q u ite la rg e. The re s u lts cast a d d itio n a l doubt on the measurements by

Dumke e t a l.[d u 8 3 ], which suggested a s ig n ific a n t change in the cascade dynamics r e s u ltin g from a modest v a r ia tio n in the p r o je c tile energy. The only a d d itio n a l experim ental data on the p r o je c tile mass dependence o f the sputtered-atom escape depth are those o f P e ll in e t a l.[p e 8 5 ], discussed in Chapter 1. For 3 kev A r+ bombardment o f a TiO ta rg e t, Fx = 0.8 0, w h ile fo r 3 kev H e*, Fx * 0.8 7. Several re le v a n t computer sim ulations have been perform ed. The work o f B iersack [B i8 7 ], and B iersack and E ckstein [B i8 4 ], was discussed in Chapter 3. B in a ry -c o llis io n sim ulations were performed fo r He*, A r *, and Xe* bombardment o f Ni a t 1, 10, and 100 kev in c id e n t ion energy. For each p r o je c t ile, the r e la t iv e fra c tio n o f atoms sp u ttered from the f i r s t atomic la y e r was found to decrease w ith in creasin g p r o je c tile energy. For Xe* bombardment, Fj = 0.91 a t 1 kev, and =0.82 a t 100 kev. The magnitude o f the v a r ia tio n was s im ila r fo r a l l three beams. The decrease o f F: is p a r t ia l ly the r e s u lt o f an in creasin g c o n trib u tio n of high-energy sp u ttered atoms, which would not be detected w ith the present apparatus, from deeper w ith in the ta rg e t. However, the r e la t iv e c o n trib u tio n o f such atoms was always <3%. For a fix e d in c id e n t ion energy, the value o f Fx was less fo r the Ar* and Xe* p r o je c tile s, fo r which i t was n e a rly eq ual, than fo r He*. The v a r ia tio n is co n sistent w ith the experim ental re s u lts o f P e llin e t a l.[p e 8 5 ]. H arrison [Ha81b] used a m olecular-dynamics code to sim ulate the s p u tte rin g o f Cu. At 0.6 kev in c id e n t ion energy, the value o f Fx was independent o f p r o je c tile mass. At 5 kev, Fx was found to increase w ith in c re a s in g p r o je c t ile mass, from =0.91 fo r Ne* bombardment, to =0.95 fo r X e *. The magnitude and d ire c tio n o f the e f fe c t are co n sis te n t w ith the

99 re s u lts o f the present experim ent. A d d itio n a l sim u la tio n s, in which the in c id e n t ion s iz e and hardness were separa te ly v a rie d w h ile the mass remained constan t, showed no v a r ia tio n in the second la y e r c o n trib u tio n fo r 5 kev bombardment [H a81a]. W ith the exception o f He+ bombardment, the re s u lts o f [ B i87, Bi84, Ha81b] in d ic a te l i t t l e v a r ia tio n w ith p r o je c tile mass o f the sp u ttered - atom escape depth. The sm all c o n trib u tio n o f atoms sp uttered from beneath the f i r s t atomic la y e r fo r He+ bombardment is co n sistent w ith i t s very low nuclear stopping power ( a t 50 kev, in a Ga ta r g e t, a fa c to r o f 50 less than th a t o f Ne+). The sim u la tio n re s u lts obtained by H arrison [Ha81b] are co n sistent w ith the present experim ent, but the cause o f the v a r ia tio n w ith p r o je c tile mass is not understood. The re s u lts from the lo w -re s o lu tio n secondary-ion mass spectra in d ic a te th a t the flu x o f low-energy sp uttered ions is h ig h ly enriched w ith In, as is the flu x o f sp uttered n e u tra ls. An approximate value of In +/G a+ s= 9 was obtained fo r 4-6 ev secondary ions. Because the io n iz a tio n p o te n tia l o f In is 0.213 ev less than th a t o f Ga, the io n iz a tio n p r o b a b ility, P+, o f sp u ttered In is expected to be s ig n if ic a n tly h ig h e r. A crude c o rre c tio n can be made by examining the data o f Bernhein and LeBourse [Br87] on the ion y ie ld v a ria tio n s caused by changes o f the ta rg e t work fu n c tio n. Measurements were performed fo r a v a r ie ty o f m etals, and the average re s u lts suggest a value of pi n / pga ~ 2 - T*1611 YinA G a *4-5, in reasonable agreement w ith the r e s u lt determined fo r sp uttered n e u tra ls, given the q u a lita tiv e nature o f the a n a ly s is. Such agreement was expected on the basis o f the s c a lin g o f the low

energy secondary-ion y ie ld s w ith the c a lc u la te d s p u tte rin g y ie ld s, 100 and is co n sis te n t w ith an e le c tro n tu n n e lin g model o f io n iz a tio n [Yu86], The tu n n e lin g mechanism is illu s t r a t e d schem atically in Figure 4.1 6. The sp u ttered p a r t ic le has an energy le v e l Ea (th e io n iz a tio n p o te n tia l) which is increased in energy and broadened by the image p o te n tia l in te r a c tio n w ith the ta rg e t su rface. As the p a r t ic le leaves the s u rface, the energy le v e l w i l l cross the Fermi le v e l o f the s o lid a t a c r i t i c a l distan ce zc, and resonant e le c tro n tra n s fe r between the p a r t ic le and the valence band o f the s o lid can occur. The io n iz a tio n p r o b a b ility is determined by the c r i t i c a l d istan ce, the degree o f energy le v e l broadening a t th a t d istan ce, and the p a r t ic le v e lo c ity. The r e s u lt is given by 4.4 ) P+ = e x p [- ( I - ^ ) / f 0 ], where <f> is the ta rg e t work fu n c tio n, I is the atomic io n iz a tio n p o te n tia l, and t 0 is a constant. The d is tin g u is h in g fe a tu re o f th is model is th a t the f in a l charge s ta te is determined a t some distance from the ta rg e t surface. The io n ize d component o f the sp uttered flu x p a rtic ip a te s in the c o llis io n cascade in the same manner as the n e u tra l component, and the f in a l charge s ta te is in s e n s itiv e to the p r o je c tile param eters. This model has been q u ite successful in describ in g secondary-ion emission from clean m etal ta rg e ts [Yu86].

101 Figure 4.1 Illu s t r a t i o n o f the id e a l electro d e c o n fig u ra tio n o f a quadrupole mass f i l t e r. The ap p lied p o te n tia l is given by Equation 4.1. From [Da76].

102 Figure 4.2 The p r in c ip le set o f values o f the parameters a = 4eU/mw2r 2 and q be 2eV/mw2r 2 fo r which ion tra je c to r ie s are s ta b le in both the xz and yz planes. From [Da76].

Operating line

103 Figure 4.3 Illu s t r a t i o n o f the quadrupole mass spectrom eter and e le c tro s ta tic energy p r e f i l t e r used fo r measuring secondary-ion mass sp ectra. The ta rg e t normal was p a r a lle l to the quadrupole a x is, a t an angle o f 30 w ith respect to the in c id e n t beam. From [Bn87].

Channeltron

104 Figure 4.4 D e ta ile d ill u s t r a t i o n o f the e le c tr o s ta tic energy p r e f i l t e r attached to the quadrupole mass spectrom eter. The p r e f i l t e r was b iased, as shown, to pass p o s itiv e ions o f energy 5 ev, w ith a bandpass o f 2 ev FWHM.