The effect of target temperature on the erosion ratio of ionically bombarded fused silica

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1 The effect of target temperature on the erosion ratio of ionically bombarded fused silica Item type text; Thesis-Reproduction (electronic) Authors Sanger, Gregory M. Publisher Rights The University of Arizona. Copyright is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Downloaded 9-May :54:04 Link to item

2 THE EFFECT OF TARGET TEMPERATURE ON THE EROSION RATIO OF IONICALLY BOMBARDED FUSED SILICA by G regory M arshall Sanger A T h esis S ubm itted to th e F a c u lty o f th e COMMITTEE ON OPTICAL SCIENCES In P a r t i a l F u lfillm e n t o f th e Requirem ent For th e Degree o f MASTER OF SCIENCE In th e G raduate C ollege THE UNIVERSITY OF ARIZONA

3 STATEMENT BY AUTHOR T his th e s is has been su bm itted in p a r t i a l f u lf illm e n t o f r e quirem ents fo r an advanced degree a t The U n iv e rsity o f A rizona and is d e p o site d in th e U n iv e rsity L ib rary to be made a v a ila b le to borrow ers under ru le s o f th e L ib rary. B rie f q u o ta tio n s from t h i s th e s is are allo w ab le w ithout s p e c ia l p e rm issio n, provided th a t a c c u ra te acknowledgement o f source is made. R equests fo r p erm issio n fo r extended q u o ta tio n from or rep ro d u c tio n o f t h i s m anuscript in whole o r in p a r t may be g ranted by th e head of th e m ajor departm ent or th e Dean o f th e G raduate C ollege when in h is ju d g ment th e proposed use o f th e m a te ria l is in th e i n t e r e s t s o f s c h o la r s h ip. In a l l o th e r in s ta n c e s, however, p erm issio n must be obtained. from th e a u th o r. SIGNED: APPROVAL BY THESIS DIRECTOR T his th e s is has been approved on th e d a te shown below: R. H. NOBLE P ro fe sso r o f O p tic a l S ciences

4 ACKNOWLEDGEMENTS The a u th o r s in c e r e ly w ishes to thank Dr. R. H. Noble fo r h is ad vice and co n tin u ed a s s is ta n c e in com pleting t h i s experim ent and t h e s i s, and Dr. D. B. McKenney fo r h is many su g g estio n s and h e lp fu l d isc u ssio n s on ex p erim en tal tech n iq u e. Many people c o n trib u te d to t h i s work and I am in d eb ted to them a l l. S p e c ia l thanks a re due to. C. B urkhart and th e s t a f f o f th e O p tic a l S ciences C enter In stru m en t Shop fo r c o n stru c tio n o f th e vacuum com ponents, to J. Poulos f o r a s s is ta n c e in s p e c ia l vacuum problem s and J. Harvey fo r h is a s s is ta n c e in g a th e rin g in fo rm atio n on th e e le c tro n guns and h e lp in a c q u irin g d a ta. T his work was a s s is te d by C o n tract N A-G from th e O ffic e o f Naval R esearch.

5 TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS v i LIST OF TABLES v i i ' ABSTRACT v i i i CHAPTER I..INTRODUCTION, I I. A QUALITATIVE MODEL OF SPUTTERING / T arg et Tem perature C o n sid e ra tio n s.. 11 Therm al Spikes T arg et A nnealing T herm ally Induced Momentum S c a tte rin g I I I. EXPERIMENTAL CONFIGURATION The Duoplasm atron and E in zel Lens The Vacuum S y s te m T arg et P latfo rm and E le c tro n ic s The T arg et C a r rie rs The E le c tro n Gun The Thermocouples The Ion Beam Faraday Cup.., IV. EXPERIMENTAL MEASUREMENTS Method P re p a ra tio n W eighing.. 39 Bombardment. 39 C a lc u la tio n s : V. RESULTS E rro r D iscu ssio n Degree o f P a r tic le Io n iz a tio n Contam inant Film s, S u rface Charge N e u tra liz a tio n Ion Beam Inhom ogeneities i v - %' ' :

6 V. TABLE OF CONTENTS C ontinued V I. SUMMARY AND RECOMMENDATIONS FOR FUTURE RESEARCH Page S uggestio n s f o r F uture R esearch APPENDIX A. THE DERIVATION OF THE EQUATION FOR EROSION RATIO REFERENCES.... '.

7 LIST OF ILLUSTRATIONS F igure Page 1 The Median Range o f Argon and Krypton in Fused S i l i c a C o llis io n o f Ion w ith T arg et Atom a t R est Q u a lita tiv e Model o f S p u tte rin g.., Eroded Depth due to-bombardment o f Fused S ilic a by Argon a t 12 KeV, as a F unction o f Tem perature , S p u tte rin g Y ield Rate as a F unction o f Tem perature Thermal S p ik e. Tem perature - Time - D istance P r o f ile s E xperim ental A pparatus , Texas N uclear C o rp o ratio n D uoplasm atron, Model Texas N uclear C o rp o ratio n E in zel Lens, Model Duoplasm atron and Lens. G eneral Schem atic ' Focusing P ro p e rtie s o f th e E in zel Lens Vacuum S y s te m T a rg e t P latfo rm and W orkstand * Block Diagram. T a rg e t Tem perature Experim ent E le c tro n Gun Schem atic : 16. D e ta il e f th e E le c tro n Gun and Faraday Cup Thermocouple Schem atic Tem perature Dependence of th e E ro sio n R atio Tem perature Dependence o f th e E ro sio n R atio w ith Neurt r a l i z a t i o n Anomaly '

8 LIST OF TABLES Table.. Page 1. S tandard D ev iatio n s o f th e D ata from 1 st to 5 th O rder Curves o f B est F it C a lc u la te d C o e ffic ie n ts o f th e T h ird O rder Polynom ial

9 ABSTRACT A stu d y has been made o f th e e f f e c t o f e le v a te d ta r g e t tem pera tu re s on th e e ro sio n r a t i o o f fu sed s i l i c a. The tem p eratu re range cons id e r e d, extended from 50 C to 500 C and th e bombarding io n s were 10 KeV + '. : Ar. The e ro sio n r a t i o (Atoms p e r io n ) was determ ined by w eight lo s s, io n c u rre n t and tim e m easurem ents. The fused s i l i c a showed an e ro sio n r a t i o in c re a s e o f 118% over th e tem p eratu re range c o n sid e re d, as d e te r mined by th e s t a t i s t i c a l b e s t f i t o f a 3rd o rd e r polynom ial curv e. The slo p e o f th e tem p eratu re dependent e ro sio n r a t i o curve g e n e ra lly d ecreased as tem p eratu re in c re a se d. The sp read o f th e observed d a ta, however, d id n o t allow an a c c u ra te d e term in a tio n o f th e a c tu a l dependence. D uring th e course o f th e experim ent, an unexpected in v e rse r e la tio n between th e e ro sio n r a t i o and s u rfa c e charge n e u tr a liz a tio n c u r r e n t, was observed. I t appears th a t th e h ig h e r th e su rfa c e charge n e u tr a liz a tio n c u r r e n t, th e low er th e e ro sio n r a t i o. Many u n c e r ta in tie s are a sso c ia te d w ith t h i s, making f u r th e r re s e a rc h and ex p erim en tatio n n e cessa ry b e fo re any e x p lan a tio n o f t h i s i n i t i a l o b serv a tio n can be g iv en. v i i i

10 CHAPTER I INTRODUCTION I t was d isco v ered in 1965 by M einel, Bashkin and Loomis th a t m a te ria l could be removed, in a p re d ic ta b le fa s h io n, from d ie le c tr ic s exposed to a beam o f p o s itiv e ly charged io n s. This d isco v ery le d them to propose th a t io n bombardment tech n iq u es be a p p lie d to th e p o lis h in g and fig u rin g o f o p tic a l elem ents. Since th e n, re s e a rc h in io n ic p o lis h ing (S chroeder, Bashkin and N e ste r 1966; and Narodny and T erasev ich 1967) has been d ire c te d tow ard determ ining th e u sefu ln e ss o f t h i s tech n iq u e in producing th e v ery smooth s u rfa c e s and p re c is e c o n tro l o f th e o p tic a l fig u rin g p ro c e ss, e s s e n tia l to th e f a b r ic a tio n o f p r e c is io n o p tic a l s y s tem s. T arg et tem p eratu re ("T arg et" w ill be s u b s titu te d fo r " o p tic a l. elem ent" in t h i s pap er) is one o f s e v e ra l v a ria b le s known to have an i n flu e n c e on a q u a n tity c a lle d th e e ro sio n r a t i o (to be d iscu ssed l a t e r ), fundam ental to th e developm ent o f th e io n ic p o lis h in g te ch n iq u e. I t i s th e purpose o f th is p ap er to examine j u s t how v a r ia tio n s in ta r g e t temp e ra tu re e f f e c t th e e ro sio n r a t i o and th e r e f o r e, i t s s ig n ific a n c e to io n ic p o lis h in g. The e v en tu a l hope o f io n ic p o lis h in g ex p erim en tatio n i s to fin d methods o f re p la c in g th e co n v en tio n al p itc h lap and rouge p o lis h in g te c h nique w ith an a c c u ra te ly c o n tro lle d beam o f p o s itiv e io n s. Such an ion beam i s commonly produced by an a c c e le r a to r such as a duoplasm atron and ; v 1. 1 / v ' - ;

11 focused onto a ta r g e t by a beam c o n tro l device such as an E in zel len s in a vacuum chamber. The n e t e f f e c t i s to produce an a re a on th e ta r g e t where m a te ria l has been removed by a p ro cess o f atom ic a b la tio n. C o n tro l o f th is p ro cess is achieved by changing v a rio u s p ro p e rtie s o f th e io n : beam and ta r g e t. Ion beam p o lis h in g has i t s th e o r e tic a l b a s is in th e w idely known f i e l d o f s p u tte rin g (W ilson 1969). S p u tte rin g th e o ry c o n sid ers th e m utual in te r a c tio n o f high v e lo c ity ions w ith su rfa c e and n e a r s u r fa c e atoms o f a ta r g e t m a te ria l. These in c id e n t io n s, on e n te rin g a ta r g e t s u rfa c e, may s tr ik e one o r more ta r g e t atom s, tr a n s f e r r in g energy to them in s u f f ic ie n t q u a n titie s, riot only to b reak t h e i r in te ra to m ic bonds b u t to d riv e them through th e t a r g e t m a te ria l a t h ig h v e l i c i t i e s. G enerally, th e se in c id e n t ions and now e n e rg e tic atoms go on to c o llid e w ith o th e r ta r g e t atom s, th u s i n i t i a t i n g a number o f ta r g e t atom - atom and ion - atom c o llis io n sequences. The r e s u l t o f a number o f such c o l lis io n sequences i s th a t some ions and atoms a cq u ire s u f f i c i e n t momentum in th e r ig h t d ir e c tio n to escape th e ta r g e t su rfa c e. P a r tic le s th a t escape in t h i s manner a re c a lle d eroded p a r t i c l e s and i f enough o f them escap e, a lte r a tio n s (e ro sio n and p o lis h in g ) to th e ta r g e t su rfa c e become o b serv ab le. I t i s th e se lib e r a te d ions and atoms th a t a re o f s p e c ia l i n t e r e s t to io n ic p o lis h in g fo r an o th er re a so n. T h e ir number is e s s e n t i a l : in determ ining how much and how f a s t m a te ria l i s removed from a ta r g e t. The m ajor aim o f p re s e n t re se a rc h in io n ic p o lis h in g i s to determ ine th e r a t i o o f th e amount o f ta r g e t m a te ria l removed to th e number o f in c id e n t

12 3 p a r t i c l e s and how t h i s is m odified by v a rio u s c o n tro lla b le param eters in an io n ic p o lis h in g system. This r a t i o is c a lle d th e e ro sio n r a t i o. For purposes o f th is p a p er, th e e ro sio n r a t i o, i s d efin ed to be a r a t i o o f th e number o f eroded p a r t i c l e s to th e number o f in c id e n t io n s. This is p ro p o rtio n a l to th e ta r g e t w eight d if f e r e n c e, b e fo re and a f t e r bombardment d iv id e d by th e in c id e n t charge and i s m easured in micrograms p e r m icroam pere-hour (y g /y a -h r). The m agnitude o f t h i s e ro sio n r a t i o i s found to be a ffe c te d by: 1) The energy and sp e c ie s (mass) o f th e in c id e n t io n s. 2) The an g le th e ion beam makes w ith th e ta r g e t s u rfa c e. 3) The vacuum p re ssu re in which th e bombardment tak es p la c e. 4) The amount o f accum ulated e l e c t r o s t a t i c s u rfa c e charge. 5) The am bient tem perature o f th e ta r g e t. 6) The ta r g e t m a te ria l.. As m entioned b e fo re, i t i s th e am bient tem p eratu re o f th e t a r g e t th a t i s o f i n t e r e s t h e re. P revious work (Dugdale and Ford 1966) has a lre ad y shown th a t th e e ro sio n r a te appears to in c re a s e as tem p eratu re i s in c re a s e d, b u t th e ex act dependence i s s t i l l undeterm ined. The work done h e re is an attem p t to fin d what th a t tem p eratu re dependence i s. S p e c if ic a lly, th e v a r ia tio n s o f th e e ro sio n r a tio are m easured as a fu n c tio n o f th e bombarded ta r g e t tem p eratu re from 50 to 500 C. The rem ainder o f t h i s p aper w ill p re s e n t a q u a lita tiv e d is c u s sio n o f th e g e n eral s p u tte rin g model follow ed by c o n sid e ra tio n of th e tem p eratu re dependent phenomena b e lie v e d to in flu e n c e i t. Then, a s h o rt

13 p re s e n ta tio n o f th e experim ent, used in a tte m p tin g to v e r if y th is m odel, and a d iscu ssio n o f th e observed r e s u lts w ill be g iv en.

14 CHAPTER I I ' A QUALITATIVE MODEL OF SPUTTERING C o n tro lle d e ro sio n o f d i e l e c t r i c s by a beam o f low energy p o s i tiv e io n s i s, as p re v io u sly m entioned, based on th e well-known th e o ry and ex p erim en tal tech n iq u es o f s p u tte r in g. S p u tte rin g th eory c o n sid ers th e e ro sio n o f a m a te ria l ( ta r g e t) by a momentum o r energy tr a n s f e r p ro cess which r e s u l t s, s t a t i s t i c a l l y, in th e e je c tio n o f one o r more ta r g e t atoms fo r each in c id e n t ion s tr ik in g th e s u r f a c e. This momentum tr a n s f e r i s accom plished by sequences o f su b su rface atom - atom and io n - atom c o llis io n s. Each sequence o r ig in a te s from "prim ary" c o llis io n s o f an in c id e n t h ig h v e lo c ity ion and ta r g e t atoms on o r n e a r th e t a r g e t s s u rfa c e. There are s e v e ra l approaches to a th e o r e tic a l tre a tm e n t o f t h is problem (Brandt and L aubert 1967; Keywell 1955), most o f which attem p t to advance a model o f s p u tte r in g, e x p la in in g observed v a r ia tio n s in th e e ro sio n r a t i o. C o n sid eratio n w ill be given, h ere o nly to a q u a lita tiv e form o f t h i s m odel, and to a review o f phenomena th a t a l t e r th e e ro sio n r a t i o as ta r g e t tem p eratu re is in c re a se d. An ex act d e term in a tio n o f th e e ro sio n.ra tio from a th e o r e tic a l tre a tm e n t o f s p u tte rin g i s extrem ely complex. T h e re fo re, a d iscu ssio n o f th e s p u tte rin g mechanism and e ro sio n r a t i o is g e n e ra lly made, s u b je c t to some sim p lify in g assum ptions. A ccording to S ilsb e e (S ilsb e e 1957), most modern s p u tte rin g th e o ry is.p r e s e n te d s u b je c t to two such " ' 5 ;;.. ', :.. /

15 assum ptions. F i r s t, th e e ro sio n p ro c e ss can be c o n sid ered as a s e r ie s 6 o f two body c o llis io n s and second, such c o llis io n s are tr e a te d as "h ard sphere" c o llis io n s, s u b je c t to th e computed c o llis io n c ro ss s e c tio n s o f th e c o llid in g p a r t i c l e s. A com plete tre a tm e n t o f. th e se two body hard sp here c o llis io n s is given by C a rte r and C o llig o n (1968). C onsider now, what occurs in th e case o f a p o s itiv e ly charged ion w ith betw een 10 and 30 KeV o f en erg y, in c id e n t,at an angle n e ar th e normal to a fu se d s i l i c a ta r g e t s u rfa c e. When such an io n is d ire c te d tow ard a ta r g e t s u rfa c e, i t i s expected th a t i t w ill c o llid e w ith a s u rfa c e o r n e a r su rfa c e atom. I f a c o llis io n does n o t occur w ith a s u r face atom th e ion w ill p e n e tr a te some d is ta n c e in to th e t a r g e t. This p e n e tra tio n d is ta n c e i s in th e o rd e r o f a range d efin ed as th e v e r tic a l d is ta n c e from th e p o in t o f e n tra n ce to a p o in t where h a l f th e p a r tic le s have no rem aining energy. mass (M'i) and energy (E0). T his range is r e la te d to th e in c id e n t io n 's A pl.ot o f t h i s f o r Argon and Krypton is shown in F ig. 1. R egardless o f w hether th e i n i t i a l ion-atom c o llis io n occurs a t o r somewhere b eneath th e ta r g e t s u rfa c e, some f r a c tio n o f th e in c id e n t ion energy w ill be tr a n s f e r r e d to th e atom h i t. The ev en ts t h a t follow th e i n i t i a l c o llis io n, w hether i t be s u rfa c e o r su b su rfa c e, a re id e n tic a l and th e range only g iv e s approxim ate in fo rm atio n as to th e depth over which th e subsequent ev en ts can ta k e p la c e. A ccording to C a rte r and C o llig o n (1968), i f E0 and are th e energy and mass o f th e in c id e n t ion and is th e mass o f a ta r g e t atom.

16 600 r ARGON 500 RANGE KRYPTON E(KeV) Fig. I. The Median Range of Argon and Krypton in Fused Silica. ( Ref. L. Narodny 1968 )

17 th e maximum energy (Ejmax) th a t can be tr a n s f e r r e d in a s in g le c o llis io n is 4 MjMa :;E ax= (m^+m2)2 < 2 "! i This would be th e energy o b ta in e d fo r a head on c o llis io n. As one would expect how ever, th e energy tr a n s f e r r e d to a ta r g e t atom d ecreases as th e c o llis io n angle ( 8, F ig. 2) in c re a s e s u n t i l i t reach es zero fo r g razin g in cid en ce o f th e io n. This d ecrease in energy tr a n s f e r r e d i s found in term s o f th e c o llis io n angle to be, ET ETmax s in A lso, according to Symon (1960), th e momenta o f th e two p a r t i c l e s a f t e r c o llis io n can be found from 2 v r Mi Mx _ M2 -Mi P i = cos9 ± cos26 + Pi (2-3) F M1+M2 M1+M2 M1+M2 I and P2 = P i 2 + P i 2-2Pi Pi cos 8 (2-4) F F I I F Where Mi, P i and Pi re p re s e n t th e in c id e n t ion m ass, and momentum b e : F fo re and a f t e r c o llis io n r e s p e c tiv e ly. P2 and M2 re p re s e n t th e ta r g e t F atom momentum a f t e r c o llis io n and i t s m ass. P2 is assumed to be zero I in e q u atio n s 2-3 and 2-4 as th e t a r g e t atom i s c o n sid ered to be a t r e s t b efo re, c o llis io n. I t is found th a t t h i s assum ption becomes in c re a s in g ly poor a t e le v a te d tem p eratu res because th e am plitude o f th e ta r g e t atom

18 Fig. 2. Collision of Ion Mj with Target Atom Mr, at rest. (Ref. Symon, I960)

19 10 therm al o s c i l l a t i o n s, becomes more s i g n i f i c a n t, th a t i s, P2 can no I lo n g er be consid ered zero. In t h i s s i t u a t i o n, th e problem i s somewhat d if f e r e n t and w ill n o t be co n sid ered h e re. I t can be seen from Eq. 2-1 th a t i f th e m asses o f th e two c o l lid in g p a r t i c l e s a re n o t equal (th e s i tu a tio n o f i n t e r e s t h e r e ), th e maximum energy tr a n s f e r r e d to a ta r g e t atom in a c o llis io n i s le s s th an th e energy o r ig i n a lly p o ssessed by th e in c id e n t, io n. T h e re fo re, a f t e r c o l l i s i o n, th e in c id e n t ion w ill s t i l l have c o n sid e ra b le energy and con- tin u e. on in to th e s o lid, making f u r th e r c o llis io n s w ith o th e r ta r g e t atom s, lo sin g energy w ith each c o llis io n. The maximum energy tr a n s f e r r e d to each ta r g e t atom encountered i s le s s th an every p re c ed in g one. T his d ecrease in io n energy w ill reach a p o in t where th e maximum energy th a t can be tr a n s f e r r e d to a ta r g e t atom d u rin g c o llis io n is le s s th an th e ta r g e t atom b in d in g energ y. From t h i s p o in t, f u r th e r ion-atom c o llis io n s r e s u l t only in in c re a se d v ib r a tio n a l m otion o f th e atoms h i t, r e s u ltin g in lo c a l h e a tin g o f th e t a r g e t. The io n w ill e v e n tu a lly lo se a l l i t s energ y.an d come to r e s t, embedded w ith in th e su rfa c e la y e rs o f th e t a r g e t. ' When ta r g e t atoms are h i t by an io n w ith s u f f i c i e n t energy, th e y a re broken fre e o f t h e i r bound p o s itio n s and d riv e n through th e mat e r i a l. These atoms th en c o llid e w ith o th e r ta r g e t atoms in a manner id e n tic a l to th a t d e sc rib e d above fo r ion-atom c o l l i s i o n s, th a t i s, th e same energy and momentum eq u atio n s h o ld. - T arget atoms h i t in th e se secondary c o llis io n s can a c q u ire enough energy to cause f u r th e r damage to th e s o lid and so on. The n e t p ro c e ss, fo r i n i t i a l io n s o f s u f f i c i e n t

20 11 energy, is a cascade o f e l a s t i c c o l l i s i o n s, f i r s t betw een ions and atoms and th en betw een th e atoms, them selves as i l l u s t r a t e d in F ig. 3. Excluding th e case o f a head on c o llis io n s, b o th c o llid in g p a r t i c l e s, a f t e r c o llis io n, w ill tr a v e l in new d ir e c tio n s. In a sequence o f th e se c o llis io n s some o f th e o r ig in a l momentum p o ssessed by th e in c id e n t ion may now be co nverted to momentum tow ard th e s u rfa c e. I f t h i s occurs b e fo re most o f th a t o r ig in a l ion energy i s co n v erted to h eat and momentum p o ssessed by o th e r ta r g e t atom s, one o r more p a r t i c l e s may acq u ire s u f f ic ie n t momentum to escape th e ta r g e t su rfa c e as eroded p a r t i c l e s. G e n era lly, th e 10 to 60 KeV ions used fo r s p u tte r in g have s u f f ic ie n t energy to accom plish t h i s momentum re v e rs a l and e je c t m a te ria l. This i s evidenced by an observed e ro sio n r a t i o range betw een.1 and 4 atoms p e r io n. T arget Tem perature C o n sid eratio n s The concept o f an in c id e n t io n iz e d p a r t i c l e s tr ik in g a d ie le c -. t r i e ta r g e t and s e ttin g o f f sequences o f atom - atom c o llis io n s is n o t adequate to give a com plete model o f th e s p u tte rin g p ro c e ss. I t s e f f i c ien c y, m easured in term s o f th e e ro sio n r a t i o a t e le v a te d ta r g e t. tem p eratu res must be consid ered. While th e p ro cess o f momentum t r a n s f e r is s t i l l predom inant, s e v e ra l a d d itio n a l phenomena need to be consid ered in d e sc rib in g th e observed v a r ia tio n s a t e le v a te d te m p e ratu res. E xperim ental r e s u lts o f ta r g e t tem perature s tu d ie s a re few and sk etch y b u t they g e n e ra lly re v e a l an e ro sio n r a t i o in c re a s e w ith e le v a te d ta r g e t tem p eratu re. Dugdale and Ford (1966) m easured th e e ro sio n r a t i o

21 Sputtered Atoms Incident Ions s Sputtered Atom 0 Collision Chain Atom O Target Atom S Ion Fig. 3. Qualitative Model of Sputtering.

22 o f fu sed s i l i c a in d ir e c tly by co n sid e rin g v a r ia tio n s in th e depth o f th e eroded a re a on th e ta r g e t s u rfa c e. T h e ir r e s u lts a re p lo tte d in F ig. 4 which shows th a t th e eroded depth and th e re fo re, th e e ro sio n r a t i o in» creased w ith ta r g e t tem p eratu re. C. E. C a rlsto n e t a l (1965) measured e ro sio n r a tio s o f s in g le and p o ly s r y s ta llin e Cu, A l, Mo, W and Ta in a tem p eratu re range o f 20Q to 1000 K elvin. Using 2, 5 and 10 KeV Ar io n s, they found th a t th e e ro sio n r a t i o, in some c a se s, gave a very s l i g h t, n e a rly lin e a r in c re a s e w ith te m p eratu re. O ther experim ents howe v e r, in d ic a te d no tem p eratu re dependence o r even a s l i g h t in v e rse tem p eratu re dependence. This is shown in F ig. 5 fo r C u ( l l l ). While t h i s and o th e r work is beg in n in g to show some tem p eratu re dependent tre n d s in th e e ro sio n r a t i o, th e o v e r a ll e f f e c t i s n o t y e t f u lly u n d ersto o d. There a re th re e tem p eratu re dependent phenomena, "therm al s p ik e s ", ta r g e t an n ealin g and th e rm a lly induced momentum s c a t te r in g, th a t may d e sc rib e what occurs a t e le v a te d te m p e ra tu re s. Thermal S pikes "Thermal s p ik e s ", d e sc rib e s a phenomenon re s u ltin g.fro m th e e l e c t r o s t a t i c in te r a c tio n o f a moving charged ion and th e e le c tro n cloud su rrounding each ta r g e t atom (Narodny 1968). There a re two le v e ls on which t h i s in te r a c tio n can tak e p la c e. The f i r s t is sim ply to e x c ite th e ta r g e t atoms im m ediately su rro u n d in g th e ion p a th in to r a th e r v io le n t v ib ra tio n s by e l e c t r o s t a t i c fo rc e s. Second, i f th e io n c o llid e s w ith an atom, th e b re a k in g o f i t s netw ork bonds tra n sm its energy to th e su rro u n d ing atom s, fo rc in g them to v ib r a te even more v io le n tly. T h e re fo re, a t

23 14 8 Eroded Depth ( /im ) Temperature ( C ) Fig.4. Eroded depth due to bombardment of fused silica by argon at 12 kev, as a function of temperature. ( Dugdale and Ford, 1966 )

24 15 Sputtering Yield ( Atoms/Ion ) Sputtering Yield (Atoms/Ion ) Mo Al Target Temperature ( K) Cu(lll) Mo(IIO) Cu(IIO) J ( Single Crystals ) Target Temperature ( K ) (Polycrystalline Materials ) Mo Fig.5. Sputtering Yield Rate as a function of Temperature. ( Ref. Carlston etal.1965).

25 th e in s ta n t when th e bombarding ion is tra v e r s in g th e t a r g e t, a c y lin d e r 16 o f ta r g e t atoms is h e ate d to a high te m p eratu re. When am plitudes o f th is v ib r a tio n a l m otion a re compared w ith a s im ila r system o f atoms in therm al e q u ilib riu m where th e v ib r a tio n a l m otion can be ex p ressed in - : ' ' - - ' ' o u n its o f tem p e ratu re, one o b ta in s tem p eratu res in excess o f 10,000 K elv in (Narodny 1968). q u ic k ly d is s ip a te s. The volume h e ate d is sm all and t h i s therm al energy A fte r a s h o rt tim e, i t is found th a t th e h eated re g io n i s s e v e ra l tim es la r g e r th an th e o r ig in a l b u t a t a low er tem peratu r e. As tim e c o n tin u e s, th e h e ate d re g io n co n tin u es to expand and cool u n t i l th e tem p eratu re d iffe re n c e a t th e s i t e o f th e therm al sp ik e and th e r d s t o f th e ta r g e t i s in d is tin g u is h a b le. Narodny (1968) s ta t e s th a t when th e tem p eratu re is p lo tte d as a fu n c tio n o f th e r a d ia l d ista n c e from th e ion p a th in a therm al s p ik e, a t a given tim e a f t e r th e passage o f th a t io n, curves o f th e form shown in F ig. 6 are o b ta in e d. These tem pera tu re - tim e - d is ta n c e p r o f ile s a re shown to expand r a d i a l l y, and cool w ith tim e as d e sc rib e d e a r l i e r. I f th e tem p eratu re produced in a therm al sp ik e i s s u f f ic ie n tly h ig h, th e v ib ra tin g atom s, p a r tic u la r ly n e a r th e s u rfa c e may contain enough energy to overcome t h e i r in te ra to m ic b in d in g energy and e v ap o rate. The h ig h e r th e am bient ta r g e t tem p eratu re th e g r e a te r th e p ro b a b ility t h a t an atom, in a therm al s p ik e, w ill acq u ire enough energy to ev ap o rate. This i s sim ply because a t h igh ta r g e t te m p e ra tu re s, le s s a d d itio n a l e n e r gy is re q u ire d to evap o rate such an atom. The e ro sio n r a te is th e re fo re expected to in c re a s e w hen:the ta r g e t, tem p eratu re is in c re a se d.

26 17 II t =0.5x10 SEC. TEMPERATURE IN DEGREES KELVIN t = 2 x l6 MSECV t = 4xl0 SEC t = 0 0 _ -II RADIUS, r, IN ANGSTROMS Fig.6. Thermal Spike. Temperature-Time-Distance Profiles. ( Narodny 1968)

27 T arg et A nnealing 18 A second ta r g e t tem p eratu re dependent phenomena in s p u tte rin g th e o ry is ta r g e t annealin g (C a rlsto n e t a l 1965). When an ion c o llid e s w ith a ta r g e t atom and tr a n s f e r s energy to i t in s u f f ic ie n t q u a n titie s to remove i t from i t s p o s itio n in th e atom ic netw ork o f th e ta r g e t, a h o le o r "netw ork d e fe c t" is. l e f t in. t h a t p o s itio n. These d e fe c ts te n d to reduce th e s p u tte rin g r a te w ith bombardment tim e as th e number o f su rfa c e atoms p re se n te d to th e beam is red u ced. T hat i s, an ion in c id e n t on a ta r g e t su rfa c e a t th e b eg in n in g o f bombardment w ill see a la r g e r f r a c tio n o f th e su rfa c e a re a occupied by ta rg e t, atoms than an ion in c id e n t on th a t same su rfa c e l a t e r in th e exposure. C onsequently, as bombardment tim e in c re a s e s, in c id e n t io n s m ust, on th e av erag e, p e n e tra te deeper and deeper in to th e ta r g e t b e fo re c o llis io n o ccu rs. The r e s u ltin g deep c o llis io n sequence has a low er p r o b a b ility o f producing s p u tte re d m a te ria l and th e e ro sio n r a t i o should be reduced.. A la rg e f r a c tio n how ever, o f th e in c id e n t ions and ta r g e t atoms t h a t a re removed from t h e i r o r ig in a l p o s itio n s become lodged elsew here in th e t a r g e t, in o th e r bombardment d e fe c ts o r sim ply betw een th e bound atoms o f th e netw ork. These p a r tic le s a re c a lle d i n t e r s t i t i a l p a r t i c l e s. As ta r g e t tem p eratu re is in c re a s e d, th e m o b ility o f th e s e i n t e r s t i t i a l s in c re a se s and th e p r o b a b ility th a t th e y w ill f i l l o r "an n eal" out netw ork d e fe c ts in c re a s e s. T h e re fo re, w ith in c re a se d te m p e ra tu re, i t would seem t h a t ta r g e t an n ealin g would p a r t i a l l y can cel out th e p ro d u c tio n o f def e c ts in th e ta r g e t and te n d to in c re a s e th e observed e ro sio n r a t i o.

28 19 A ccording to C a rlsto n e t a l.(1965) the in c re a se in m o b ility o f i n t e r s t i t i a l p a r t i c l e s w ith in c re a se d tem p eratu re i s sm all sin ce i t is known, from r a d ia tio n damage th e o ry, th a t i n t e r s t i t i a l s are very m obile, even a t low te m p e ratu res. I t must be concluded th en t h a t although d e fe c t p ro d u c tio n and ta r g e t annealin g a re tem p eratu re dependent s p u tte rin g phenomena th ey ten d to cancel one an o th er and w ill have l i t t l e s ig n ific a n c e to th e tem p eratu re dependence of e ro sio n r a t i o s. Therm ally Induced Momentum S c a tte rin g A th ir d tem p eratu re dependent phenomenon th a t m ight be used to e x p lain th e e ro sio n r a t i o tem p eratu re dependence w ill be c a lle d th e rm a lly induced momentum s c a tte r in g. This has been s tu d ie d in d e ta i l under th e name o f c o llis io n chain defocusing (Saunders and F lu it 1964; N elson, Thompson and Montgomery 1962) and is based on th e average in c re a se in amp litu d e o f therm al v ib ra tio n s th a t ta r g e t atoms make about t h e i r netw ork p o s itio n s as ta r g e t tem p eratu re in c re a s e s. At room te m p e ratu re, an ion ; w ill i n i t i a t e s e v e ra l c o llis io n sequences t h a t p ro p ag ate in to th e ta r g e t losing: energy as they go. The energy is r e ta in e d by a sm all number o f atom s, most o f which have enough energy fo r p o te n tia l s p u tte r in g. Nelson e t a l (1962) s t a t e th a t a t e le v a te d te m p e ra tu re s, th e in c re a se d therm al v ib r a tio n o f th e ta r g e t atoms is expected to s c a t t e r energy out o f th e c o llis io n sequences. The i n i t i a l io n energy i s r a p id ly sp read over a la rg e volume o f th e ta r g e t w ith v ery few p a r t i c l e s having enough energy to s p u tte r. This therm al s c a tte r in g e f f e c t then should d ecrease th e e ro sio n r a t i o w ith in c re a se d tem p eratu re and according to N elson e t a l (1962)

29 ' 20 i t could be ex p ected to f a l l as much as 50%. betw een.room tem p eratu re and th e m eltin g p o in ts o f some m a te ria ls co n sid ered. In th e above d is c u s s io n, th re e b a s ic s p u tte rin g phenomena have been co n sid ered in an e f f o r t to q u a lita tiv e ly d e sc rib e v a r ia tio n s in th e e ro sio n r a t i o as ta r g e t tem p eratu re is in c re a se d. The phenomena cons id e re d have been therm al s p ik e s, ta r g e t annealin g and th e rm a lly induced momentum s c a tte r in g. What each p r e d ic ts as to how.the e ro sio n r a t i o w ill change when ta r g e t tem p eratu re i s in c re a se d has tu rn e d out to be v a rie d. E xperim ental r e s u l t s, c ite d e a r l i e r in th is s e c tio n how ever, show th a t a g e n eral in c re a s e in th e e ro sio n r a te occurs as tem p eratu re in c re a s e s. This in c re a s e p e rh a p s, says th a t th o se phenomena p re d ic tin g an ero sio n r a te in c re a se s w ith tem p eratu re are p ro bably more s ig n if ic a n t than th o se which do n o t.

30 CHAPTER I I I EXPERIMENTAL CONFIGURATION The dependence o f th e e ro sio n r a t i o on th e am bient ta r g e t temp e ra tu re was determ ined by bombarding a s e r ie s o f fu sed s i l i c a ta r g e ts, h e ate d to tem p eratu res between 50 and 500 C in approxim ately 50 in crem ents. The equipm ent used c o n s is te d p r in c ip a lly o f an e x is tin g io n ic p o lis h e r, and a ta r g e t p la tfo rm w ith su p p o rtin g e le c tr o n ic s which were s p e c if ic a lly designed and b u i l t fo r th is experim ent as shown in F ig. 7. The fu n c tio n o f th e io n ic p o lis h e r is tw o fo ld. F i r s t, i t p ro duces, a c c e le r a te s, and focuses p o s itiv e ions in to a beam, used fo r bombarding t a r g e ts. Second, i t serv es as a framework f o r p la c in g th e ta r g e ts d ir e c tly in th e beam p a th. This p o rtio n o f th e equipment p ro v id es th e n e cessa ry c o n tro l o f io n en erg y, ion beam c u rre n t and vacuum p re s s u re. The ta r g e t p la tfo rm, which is in s e r te d in to th e io n ic p o lis h e r, p ro v id es m ounting and h e a t fo r ta r g e t su rfa c e charge n e u tr a liz a tio n, and th e n e c e ssa ry in stru m e n ta tio n, fo r m onito rin g te m p e ratu re, e le c tro n beam c u rre n t and ion beam c u rre n t. This ap p aratu s can be broken down in to components according to t h e i r fu n c tio n in th e t o t a l system. These components a re : (1) th e duoplasm atron and E in zel le n s ; (2) th e vacuum system ; and (3) th e ta r g e t p la tfo rm and su p p o rtin g e le c tr o n ic s. A b r i e f d e s c rip tio n o f each is now given w ith emphasis on th e ta r g e t p l a t form and i t s a sso c ia te d e le c tr o n ic s. 21

31 Fig. 7. Experimental Apparatus 22

32 The Duoplasm atron and E in zel Lens 23 The combined fu n c tio n o f th e duoplasm atron and E in zel lens is to produce a beam o f p o s itiv e io n s. In t h i s p a r t i c u l a r system th e ions a re produced by a Texas N u clear C o rp o ra tio n, Model 9406 duoplasm atron shown in F ig. 8. I t i s capable o f d e liv e rin g about one m ih iam pere o f p o s itiv e ion c u rre n t when 10KV (th e h ig h v o lta g e a c c e le r a tio n p o te n tia l used in th is work) is a p p lie d to th e e x tr a c to r e le c tro d e (th e f i r s t elem ent o f X th e E in zel lens shown in F ig. 9 ). These e x tra c te d ions a re focused onto th e ta r g e t by an e l e c t r o s t a t i c E in zel len s (Texas N u clear C o rp o ratio n, Model 9426), lo c a te d im m ediately fo llo w in g th e duoplasm atron and is shown in F ig. 9. P ro d u ctio n o f a focused p o s itiv e ion beam by th e se two u n its can be d e sc rib e d as fo llo w s. The duoplasm atron (D.P.) f i r s t g e n e ra te s e le c tro n s by b o ilin g them o f f a h e ate d fila m e n t. These e le c tro n s are a c c e l e ra te d from th e fila m e n t, through an a p e rtu re in th e in te rm e d ia te e le c tro d e, to th e anode b ase o f th e D.P. (see F ig. 8 ). T his a c c e le r a tio n forms an e le c tro n beam which extends from th e fila m e n t to th e anode b a se. The c o n fig u ra tio n o f th e in te rm e d ia te e le c tro d e d e fin e s th e shape o f t h i s e le c tro n beam and e v e n tu a lly d e fin e s th e shape o f th e io n -e le c tro n plasm a to be g e n erate d. When a g a s. Argon in th is c a se, is adm itted in to th e fila m e n t chamber, i t d iffu s e s in to th e e le c tro n beam where io n iz a tio n ta k es p la c e. A plasm a, from which th e D.P. d e riv e s i t s name, is th en form ed, c o n ta in in g b o th e le c tro n s and atom ic io n s. As w ith th e e le c tr o n s, t h i s plasm a extends from th e filam e n t to th e anode b ase o f th e D.P.. A vacuum p re s su re d i f f e r e n t i a l i s m ain tain ed between th e D.P. and ta r g e t

33 ITEM DWG. NO. DESCRIPTION ITEM DWG. NO- DESCRIPTION DB BODY D A D D D A D D DB D A CENTER RING TOP FLANGE INTERMED. ELECTRODE FILAMENT BASE ANODE BASE BASE ANODE CLAMP 10 D A II D D D ANODE INSERT VALVE MODIFICATION FILAMENT SUPPORT FILAMENT CLAMP D A MAGNET ASSY. D MAGNET SHIM D D D MAGNET SUPPORT FILAMENT INSULATOR 19 MAGNET SCREW F ig. 8. Texas Nuclear Corporation Duoplasmatron, Model 9406

34 25 1 Einzel Base 2 Front Lens 3 Center Lens 4 Rear Lens 5 Guard Ring 6 Guard Ring 7 Extractor 8 Zap Ring 9 Assy. Ring 10 Insulator Fig. 9 Texas Nuclear Corp. Einzel Lens. Model 9426

35 26 chamber, th e l a t t e r having th e low er p re s s u re. This p re s su re d i f f e r n t i a l fo rc e s some o f th e charged p a r t i c l e s o f th e plasm a through a sm all (.013" d ia.) h o le in th e c e n te r o f th e anode b ase in to th e ta r g e t chamber. The h ig h v o lta g e a c c e le r a tio n p o te n tia l, su p p lie d by Sorensen 30KV power supply (Model ), is p la ced betw een th e anode b ase and th e e x tr a c to r e le c tro d e. The anode c a r r ie s th e p o s itiv e p o te n tia l and th e e x tr a c to r c a r r ie s th e n e g a tiv e (ground) as shown in th e g en eral schem atic diagram (F ig. 10). The r e s u l t is th a t p o s itiv e p a r t i c l e s (th e argon ions) are fo rced through t h i s gap, g ain in g th e a c c e le r a tio n energy and n e g a tiv e p a r t i c l e s a re fo rced back in to th e plasm a above the anode b a se. A fte r a c c e le r a tio n, th e beam o f p o s itiv e ly io n iz e d atoms tr a v e ls through an E in z e l len s which c o n s is ts o f th re e e le c tro d e s. The e l e c t r i c ' f ie ld s formed when p o te n tia ls a re a p p lie d to th e len s e le c tro d e s p ro v id e s th e e l e c t r o s t a t i c fo rc e s n e cessa ry to focus th e ions in to a beam. As i n d ic a te d by th e sm all fo rc e arrow s in F ig. 11, th e se fo rc e s a re a p p lie d in both th e r a d ia l and a x ia l d ire c tio n s o f th e le n s. I f th e high v o lta g e. (+) i s a p p lie d o nly,to the. c e n te r e le c tro d e as shown (F ig. 1 1 ), p o s itiv e ions tr a v e r s in g th e elem ent w ill ex p erien ce a x ia l d e c e le ra tio n a t th e f i r s t f i e l d en countered and f e a c c e le r a tio n a t th e second. Because th e se f ie ld s a re a x ia lly id e n t i c a l, how ever, th e d e c e le ra tio n ex p erienced a t th e f i r s t f i e l d i s e x a c tly equal to th e a c c e le r a tio n ex p erien ced a t th e second. The ions th e re fo re leave th e len s w ith th e same a x ia l v e lo c ity p o ssessed b e fo re fo c u sin g. The r a d ia l fo rc e s in t h i s s itu a tio n how ever, have th e combined e f f e c t o f focu sin g th e p a r tic le s in to a narrow, n e a rly

36 27 IN 1202 A C L IN E R E G U LA TO R TO 115 VAC L IN E I0 0 W 0 - SOpOO VDC 2 0 MA INSULATOR ANODE BASE INS FRONT CENTER LENS - INS REAR LENS 2 M SOW 5K 2 -IO O K 1/2 W SOW ^.O l^ t 77*7 250V BALLAST W (+) H.V. F IL T E R E IN Z E L LENS POWER SUPPLY 0 - IOKV, 10 MA F IL A M E N T SUPPLY 3 0 A, 5 VAC ARC SUPPLY VDC 3 A MAGNET SUPPLY 100 VDC 3 A (-) E «EA R TH GROUND FLO ATING GROUND, SAME AS E X T R A C T IO N VOLTAGE 5 0 K V DC IS O LATIO N TRANSFORMER - > TO 115 VAC t L IN E FIG 10 DUOPLASMATRON AND LENS GENERAL SCHEMATIC cy lin d rical beam leaving the lens. This beam is allowed to continue unaltered to the ta rg e t. The Vacuum System The vacuum system used is diagrammed in Fig. 12 and employs conventional diffusion pump techniques. The system is centered around a Corning 6" I.D. Conical Pyrex cross which served as a chamber for the ta rg e ts and instrum entation. Having four openings, its use in th is app lic a tio n has been id eal. As diagrammed (Fig. 12), one opening was used to mount the duoplasmatron and Einzel lens while another served as a pumping port connecting the pumps and vacuum instrum entation. This leaves two openings for arranging the experimental apparatus.

37 i ^ i S equipotentials 1 Focused Ion Beam 2 Rear Electrode 3 Center Electrode 4 Extractor Electrode 5 Anode Plate 6 Anode Aperture 7 HV. Acceleration Gap Fax = Axial Force component Fr = Radial Force component Fig. II Focusing Properties of the Einzel Lens. K> 00

38 29 REGULATOR CONTROLLED GAS LEAK GAS CYLINDER DUOPLASMATRON AND El NZ EL LENS LIQUID NITROGEN TRAP TARGET CHAMBER SIX INCH MERCURY DIFFUSION PUMP GATE VALVE THERMOCOUPLE GAUGE TUBE VACUUM CONTROL SWITCH GATE VALVE ROUGHING LINE TRAP GATE VALVE AIR INLET VALVE MECHANICAL VACUUM PUMP EXHAUST TO ROOF OUTLET VACUUM FORELINE TRAP W 2 - I F ig. 12. Vacuum System The chamber is evacuated by a six inch, three stage, mercury d i f fusion pump (Consolidated Vacuum Corporation Type MGH-300A), equipped with a liq u id nitrogen cold trap. Rough pumping the chamber and diffusion pump backing is done with a Welch S c ie n tific Company Model 1397 mechanical pump. By using the above described pumps which are much larg er than would be r e quired for the size chamber used here, rapid cycling from vacuum to a ir

39 p re s su re to vacuum was o b ta in e d. This ra p id cy clin g c a p a b ility i s n e cessary as ta r g e ts are removed and re p la c e d q u ite fre q u e n tly d u ring the course o f th e experim ent. With th e se pumps, th e system is capable o f reach in g 1 X 10~6 T orr in about 30 m inutes. The u ltim a te p re s su re a t t a i n ab le i s 4 to 5 X 10-7 T o rr w ith th e ion beam o f f and 1 to 5 X 10~6 T o rr w ith th e beam on. T arg et P latform and E le c tro n ic s The ta r g e t p la tfo rm and su p p o rtin g e le c tr o n ic s a re th e h e a r t o f th e ta r g e t tem perature experim ent. The p latfo rm i t s e l f c o n s is ts o f a vacuum fla n g e, which s e a ls to th e Pyrex c ro s s, and two h o riz o n ta l s ta g e s, one d ir e c tly above th e o th e r, which c a rry th e ta r g e ts and r e la te d i n s t r u m en tatio n. A photograph o f th e p la tfo rm in i t s w orkstand is shown in F ig. 13. The elem ents mounted to th e p la tfo rm s ta g e s, n e cessa ry to th e e x p erim ent, in clu d e two ta r g e t c a r r ie r s co n tain in g th e h e a te r s, an e le c tro n gun, therm ocouples and two Faraday cups w ith a c tu a to rs,-o n e each, f o r th e ion and e le c tro n beams. A schem atic diagram o f th e whole p la tfo rm and e le c tr o n ic s i s shown in F ig. 14. Each o f th e se w ill now be d e sc rib e d, n o tin g t h e i r fu n c tio n in th e experim ent. The T arget C a rrie rs The ta r g e t c a r r ie r s a re 1 3 /4" d ia. by 7/8" long s o lid copper c y lin d e rs, machined to accomodate th e t a r g e t s, therm ocouples, and h e a te r s. The ta r g e ts are n e a rly en clo sed in th e se c a r r i e r s, having only th e bomb a rd in g su rfa c e exposed. This i s done to m ain tain th e tem perature throughout th e ta r g e t as uniform as p o s s ib le. The te m p eratu re i t s e l f was

40 Fig. 13 Target Platform and Workstand

41 32 1 Target Carriers 2 Ion Beam Faraday Cup 3 Electron Beam Faraday Cup 4 Ion Beam Cup Solenoid 5 Electron Gun 6 Electron Beam Cup Solenoid 7 Target Advance Drive Mechanism 8 Vacuum Flange Ion beam current Q,. Q Faraday cup bias supply..q -.Q Electron beam current Filament supply Bias grid supply ± 1- Acceleration supp. Ref. junc't ice bath Differential o voltmeter Faraday solenoid cup supply 0 0 Target heater supp..q Q. Fig. 14. Block diagram. Target temperature experiment.

42 measured by p la c in g one therm ocouple d ir e c tly b eneath th e ta r g e t c e n te r and an o th er p re sse d d ir e c tly a g a in s t th e edge o f th e bombarded s u rfa c e. T arg et h e a tin g was accom plished by h e a tin g th e e n tir e ta r g e t c a r r ie r w ith two 75 w a tt immersion h e a te rs (Vulcan E le c tr ic Company, T hunderbolt s e r ie s ) p la c e d d ir e c tly below th e ta r g e ts in th e c y lin d r ic a l copper b lo ck. Experim ent showed th e se h e a te rs capable o f h e a tin g the ta r g e t to o about 800 C in th e above d e sc rib e d c a r r i e r s. Two ta r g e t c a r r ie r s are fa ste n e d by ceram ic sta n d o ffs (fo r th e r mal is o la tio n ) to a r o ta r y ta b le lo c a te d between th e main sta g e s of th e p la tfo rm. This ta b le r o ta te s about a v e r t i c a l s h a f t d riv en from th e o u t sid e by a r o ta r y vacuum feedthrough (Sloan In stru m en ts C o rp o ratio n, P a rt No ) and a worm-wormgear d riv e. T his m otion allow s more than one ta r g e t to come under th e beam each tim e th e p latfo rm i s p la c e d in th e vacuum. The ta b le is a lso designed to h o ld a l l ta r g e ts a t a 45 angle to th e v e r t i c a l ion beam. The E le c tro n Gun P o s itiv e charge b u ild u p on a d i e l e c t r i c ta r g e t su rfa c e occurs whenever p o s itiv e ions are allow ed to bombard i t fo r a le n g th o f tim e. Continued b u ild u p reduces th e in c id e n t ion energy which in tu rn reduces e ro sio n r a t i o. T his i s u n d e sira b le and can be e lim in a te d by in tro d u c in g e le c tro n s to n e u tr a liz e th e p o s itiv e charge b u ild u p. S urface charge n e u tr a liz a tio n was accom plished h ere by in t r o ducing a low energy beam o f e le c tro n s g enerated in an e le c tro n gun manufa c tu re d by th e S u p e rio r E le c tro n ic s Company (Model SE-83E). Complete

43 34 charge n eu tralizatio n should occur when the ion and electron beams are of equal magnitude. I t was found th at by using the c irc u it shown in Fig. 15, up to 400ua of 400ev electrons could be obtained on a f l a t plate anode placed where the ta rg e t is normally positioned. Secondary emission was not suppressed so somewhat lower currents were actu ally obtained. A Faraday cup had been in sta lle d during the orig in al construction for measuring these electron currents, but was rendered useless la te r by a modific a tio n made to actu ally get an electron current to the ta rg e t. D etails of the electron gun, the orig in al Faraday cup and its actuator linkage are shown in Fig. 16. Cathode Filament \.G L Electron beam 5-10 VDC Fil.su pp. JoW o o 0-I00VDC Bias grid SUPP V D C < > Accelleration supply Target Fig.15. Electron Gun Schematic

44 Fig. 16. Detail of Electron Gun and Faraday Cup.

45 36 The Thermocouples Target temperature was measured by two junction Chrome1-Alumel thermocouples. The junctions were constructed of.010" diameter wire and fused by spot welding. The measured junctions were placed on the ta rg e t c a rrie rs and the reference junctions were placed in an ice bath providing a 0 C reference. The p o te n tia ls generated by the thermocouples were measured with a d iffe re n tia l voltm eter manufactured by John Fluke Manufacturing Company (Model 803R) and the c irc u it used is shown in Fig. 17. The d iffe re n tia l voltm eter functions sim ilarly to a p o te n tia l bridge and was used in conjunction with its null meter for measurement of the th e r mocouple p o te n tia ls. At n u ll, the thermocouples see no load so th e ir fu ll p o ten tial can be accurately measured. Differentia Voltmeter X. Measuring ^ Junctions Digital 6 Voltmeter o- Reference Junctions Fig. 17. Thermocouple Schematic

46 37 The Ion Beam F araday Cup The ion beam Faraday cup c o n s is ts o f a sm all s ta i n le s s s te e l c y lin d r ic a l cup w ith grooves cu t in th e bottom to d e f le c t secondary em issio n in to th e cup w a lls. I t is used to m easure th e ion beam c u rre n t and is b u i l t onto th e upper p la tfo rm stag e j u s t below th e ion beam lim itin g a p e rtu re, d ir e c tly above th e ta r g e t. W hatever beam p a sse s through th e a p e rtu re is c o lle c te d by the cup. As th e io n s have 10 KeV o f energy, secondary em ission in th e cup becomes s ig n if ic a n t. I t was accounted fo r by i n s t a l l i n g two secondary em ission su p p ressin g b ia s p la te s. One is fa ste n e d on th e t i p o f th e cup and in s u la te d by a rin g o f Boran N itrid e (a vacuum in s u la to r ) and th e o th e r was mounted to th e frame h o ld in g th e beam lim itin g a p e rtu re above the cup. To bombard th e ta r g e t, th e Faraday cup is moved a sid e out of th e. beam. This i s done by mounting th e cup on a tra c k and moving i t back and fo r th w ith a vacuum so le n o id and connecting lin k a g e.

47 CHAPTER IV EXPERIMENTAL MEASUREMENTS Method ' o Fused s i l i c a ta r g e ts, h eated to tem p eratu res ran g in g from 50 o 500 C, were bombarded w ith 10 KeV p o s itiv e Argon ions a t an in cid en ce to angle o f From d a ta acq u ired f o r each t a r g e t, an e ro sio n r a t i o was c a lc u la te d and p lo tte d a g a in s t th e mean ta r g e t tem p eratu re o v er th e exp osure p e rio d.. I t was b e lie v e d th a t two ta r g e ts bombarded a t each o f th e te n 50 increm ents between 50 and 500 C would be s u f f ic ie n t to o b ta in th e d e s ire d curve r e la tin g th e e ro sio n r a t i o and tem p eratu re. In a l l, 25 ta r g e ts were a c tu a lly bombarded in o b ta in in g th is d a ta. Each ta r g e t r e q u ire d p re p a ra tio n, w eighing, bombardment, and rew eighing to acq u ire th e n e cessa ry d a ta f o r making c a lc u la tio n s o f i t s ero sio n r a t i o. P re p a ra tio n. '.. Sample p re p a ra tio n c o n siste d p r in c ip a lly o f a thorough c leanin g p r io r to w eighing and bombardment. T his was done to reduce su rfa c e contam in an ts th a t can s ig n if ic a n tly a l t e r th e f in a l e ro sio n r a t i o. S tandard chem ical clean in g te c h n iq u e s, u sin g chrom ic a c id, su g g ested by L. H olland (1964, pp ) were used w ith th e a d d itio n o f a s e r ie s o f m onitored d i s t i l l e d w ater r in s e s in an u ltr a s o n ic c le a n e r. Such m o n ito rin g, allow ed placem ent o f a num erical v alu e on ta r g e t ' c le a n lin e s s ". M onitoring was done by m easuring th e r e s i s t i v i t y o f th e r in s e b a th in which

48 39 ta r g e ts were immersed./. I t was found th a t r e s i s t i v i t y in c re a se d monoton- i c a l l y w ith continued rin s in g and replacem ent of th e w ater b a th a t regul a r in te r v a ls. A ll ta r g e ts were rin s e d u n t i l a g iv en, a r b itr a r y v alue (9.5 X 105fi-cm) o f r e s i s t i v i t y was a tta in e d. This m ain tain ed some degree o f u n ifo rm ity in th e f in a l s t a t e o f th e p repared ta r g e ts. Weighing - T arg ets p re p a red in th e above manner were weighed on an a n a ly t- / ic a l b alan ce (M e ttle r Instrum ent C o rp o ra tio n, Type M5) having a r a te d a c curacy o f ±0.001 m illig ra m s. made to o b ta in a mean v a lu e. Five read in g s o f each t a r g e t s mass were A sta n d a rd mass was a ls o weighed w ith each s e t o f ta r g e ts thro u g h o u t th e course o f th e experim ent. Data from th e sta n d a rd showed th a t th e b alan ce was a c c u ra te to ±0.008 m illig ram s over long p e rio d s o f tim e, re q u irin g a p p ro p ria te d a ta ad ju stm en t. Bombardment The ta r g e ts were loaded in to th e copper ta r g e t c a r r ie r s on th e p la tfo rm and th e e n tir e assem bly p la ced in th e ta r g e t chamber o f th e io n ic p o lis h e r. The chamber was then ev acuated and th e ta r g e t su rfa c e h e ate d to and h e ld a t a d e sire d te m p e ratu re. By moving th e ion beam Faraday cup a s id e, th e ta r g e t was exposed to th e ion beam. As th is occured, th e a c c e le ra tio n was a p p lie d to th e e le c tro n gun fo rc in g e le c tro n s o u t o f th e gun and onto th e ta r g e t s u rfa c e fo r n e u tr a liz a tio n. At r e g u la r in te r v a ls during th e exposure, th e Faraday cup was re tu rn e d to i t s o r ig in a l p o s itio n above th e ta r g e t to re c o rd io n beam c u r r e n t, exposure tim e, and ta r g e t tem p eratu re. The t o t a l exposure fo r a l l

49 40 ta r g e ts run (measured in io n beam y a -h rs) was lim ite d to approxim ately 200 y a -h rs. When an exposure o f a ta r g e t reached t h i s l i m i t, th e run was co n sid ered com plete. The ta r g e ts were th en co o led, and removed from th e io n ic p o lis h e r. The d a ta were c o n sid ered com plete f o r th a t s p e c if ic t a r g e t a f t e r th e f in a l w eighing. C a lc u la tio n s The d a ta o b tain ed fo r each ta r g e t c o n s is ts o f two s e ts o f mass readin g s and bombardment exposure d a ta in c lu d in g te m p e ratu re, tim e and. ' - * ion beam c u rre n t re a d in g s. Id e a lly th e e ro sio n r a t i o (e) could be c a l c u la te d from th is by r t t- i. = k [ / i B(t) a tj (4-i) ' TO where and a re th e ta r g e t masses b e fo re and a f t e r bombardment r e s p e c tiv e ly and I g ( t) is th e ion beam c u rre n t over th e exposure in te r v a l. t Q and tj, are th e i n i t i a l and f in a l tim es bounding th e exposure in te r v a l and K i s a c o n sta n t depending on ta r g e t m a te ria l (K =4/3 fo r fu sed s i l i c a ). In th e equipment used h e re, how ever, beam c u rre n t was measured p e r io d i c a l l y, re q u irin g use o f an approxim ate form ula, d e riv e d in Appendix A and given by e " ( W [ J. V b ] (4-2). 1=1 i th where I D is th e average ion beam c u rre n t in ya over th e i tim e in c re - i. ment At^ (h rs) o f th e t o t a l exposure in te r v a l. I f t^ ^ and t ^ are th e

50 i n i t i a l and f i n a l tim e s.o f th e i increm ent then 41 Ati - t f i - ^ (4~3) The approxim ation o f equatio n 4-1 by 4-2 is q u ite good as th e e rro r i n tro d u ced i s sm all. E xperim ental measurement in d ic a te s t h i s e r r o r to be about 1.34%.

51 CHAPTER V RESULTS ' The observed dependence o f th e e ro sio n r a t i o on ta r g e t tem peratu r e i s shown in F ig. 18. Each p o in t corresponds to th e e ro sio n r a t i o fo r one ta r g e t w ith th e e r r o r b a rs re p re s e n tin g s t a t i s t i c a l v a r ia tio n s in th e observed v a lu e s. While th e expected in c re a se in e ro sio n r a t i o i s observ ed as th e tem p eratu re o f th e t a r g e t in c re a s e s, th e sp read o f th e d a ta p o in ts i s so la rg e th a t a curve could n o t be drawn through them d ir e c tly. The curve shown in F ig. 18 re p re s e n ts a s t a t i s t i c a l l y determ ined curve capable o f g iv in g o n ly g en eral tre n d s in th e d a ta. This s t a t i s t i c a l curv e, drawn through th e d a ta, was c a lc u la te d by a computer program u sin g th e concept o f b e s t f i t by lin e a r le a s t s q u a re s. The program f i t s a polynom ial curve o f th e form e = A0 + A]T + A2T2 + A3T3 + «+ A^T (5-1) to a s e t o f d a ta p o in ts, where e i s th e dependent v a r ia b le (e ro sio n r a t i o ), T i s th e independent v a ria b le (tem p eratu re) and m i s th e o rd e r o f th e polynom ial o r f i t. The f i r s t o rd e r f i t (m=l) is a s t r a i g h t lin e cu rv e, th e second o rd e r f i t (m=2) i s a p a ra b o lic curve and so on. A stan d a rd d e v ia tio n (a) o f th e observed d a ta p o in ts from th e c a lc u la te d curve. 42

52 3.01 Erosion Ratio ( Atoms / Ion ) * 1.0-I.70 e b i J t r Target Temperature C r Fig. 18. Temperature Dependence of Erosion Ratio.

53 44 determ ined b y.th e program from cr = f\ ( e e,)? (5-2 ) V " nc. v (e c and a re th e c a lc u la te d and observed e ro sio n r a t i o s re s p e c tiv e ly, n i s th e number o f observed d a ta p o in ts.) was used as a fig u re o f m e rit. fo r d eterm in in g th e low est o rd e r curve th a t adeq u ately re p re s e n ts th e d a ta. The v a lu e s o f th e sta n d a rd d e v ia tio n s fo r each o rd e r o f f i t consid e re d a re given in T able 1. T able 1 S tandard D eviatio n s o f th e D ata from 1 st to 5th O rder Curves o f B est F it O rder o f F it S tandard D ev iatio n ' As would be expected, in c re a s in g th e o rd e r o f f i t d e crease s th e c a lc u la te d sta n d a rd d e v ia tio n. A fte r th e 3rd o rd e r however, in c re a se d o rd er does n o t S ig n if ic a n tly a l t e r th e sta n d a rd d e v ia tio n. The t h i r d o rd e r curve th e n, was used in F ig. 18 and th e c a lc u la te d c o e f f ic ie n ts o f th e corresponding polynom ial a re g iv en in T able 2. t

54 45 Table 2 C alcu la te d C o e ffic ie n ts o f th e T hird O rder Polynom ial C o e ffic ie n ts Value. A X 10"1 0 Al X 10-3 ( C -1) a X 10"5 ( V 2) As X 10"8 ( C -3) - E rro r D iscussion The accuracy in o b ta in in g th e e ro sio n r a t i o s were e stim ate d by s t a t i s t i c a l l y e v a lu a tin g th e random e r r o r o f th e tem p eratu re and mass d iffe r e n c e. The r e s u lts have been in d ic a te d w ith h o riz o n ta l (tem p eratu re) and v e r t i c a l (e ro sio n r a tio ) e r r o r b a rs drawn through each p o in t in F ig. 18. The magnitude's o f th e se e r r o r b a rs are p lu s and minus one stan d a rd d e v ia tio n. O ther in c lu s io n s in th e se e r r o r b ars were e stim a te s of e r r o r in readin g io n beam c u rre n t p e r io d ic a lly in s te a d o f c o n tin u a lly and in c a lib r a tio n e rro rs o f th e therm ocouples. In comparing th e m agnitude o f th e f in a l computed e rro rs w ith th e observed spread o f th e d a ta, i t b e comes ev id en t th a t o th e r ex p erim ental in c o n s is te n c ie s e x i s t. A d d itio n a l, sy ste m a tic so u rces o f e r r o r which m ight account fo r th e la rg e d a ta sp read o r ig in a te from d i f f i c u l t y in c o n tro llin g, th e degree o f p a r t i c l e

55 46 io n iz a tio n, th e d e p o sitio n o f co n taim inant film s on th e ta r g e t s u rfa c e, th e b u ild u p o f su rfa c e charge on th e t a r g e t, and inhom ogenieties in th e beam. Each o f th e se w ill now be d e sc rib e d w ith p o s s ib le s o lu tio n s fo r fu tu re re s e a rc h. Degree o f P a r tic le Io n iz a tio n I t was assumed a t th e o u ts e t o f th is study th a t a beam, leav in g th e duoplasm atron and E in zel len s assem bly, c o n siste d t o t a l l y o f s in g ly io n iz e d p a r t i c l e s. While i t is tru e th a t most o f th e p a r t i c l e s in th e beam a re s in g ly io n iz e d, a number o f them were doubly o r even t r i p l y io n iz e d. The number o f p a r tic le s in c id e n t on a ta r g e t was determ ined by d iv id in g th e t o t a l in c id e n t charge o f th e exposure (determ ined from r e cording c u rre n t and tim e measurements) by th e charge on a s in g ly io n iz e d p a r t i c l e. A doubly io n iz e d p a r t i c l e s tr ik in g th e ta r g e t would th e re fo re be counted as two p a r t i c l e s. I f a la rg e enough q u a n tity o f th e se m u lti- io n iz e d p a r t i c l e s a re p re s e n t in th e beam, th e c a lc u la te d number o f in c id e n t p a r t i c l e s w ill be much h ig h e r th an th e a c tu a l number s tr ik in g th e ta r g e t s u rfa c e. This r e s u lts in th e c a lc u la te d e ro sio n r a t i o being somewhat d if f e r e n t th an i t r e a l l y i s. A sim ple c o rre c tio n f o r th is (n o t a v a ila b le f o r t h i s w ork), is - m agnetic beam a n a ly s is. By in tro d u c in g a m agnetic f i e l d around th e beam a f t e r fo c u sin g, p a r tic le s can be s o rte d in to s e v e ra l beams according to t h e i r degree o f io n iz a tio n. When a ta r g e t is now p la ced under th e a n a l yzed beam co n tain in g only s in g ly io n iz e d p a r tic le s c a lc u la tio n o f th e number o f p a r t i c l e s s tr ik in g th a t t a r g e t w ill be a c c u ra te by beam c u rre n t m easurem ents.

56 C ontam inant Film s - 47 Whenever an ion beam s tr ik e s a p o rtio n o f an a p e rtu re or an obj e c t in i t s p a th, m a te ria l from th a t o b je c t or ap ertu re, is s p u tte re d o f f. This s p u tte re d m a te ria l can re d e p o s it i t s e l f on o th e r s u rfa c e s in th e vacuum chamber, th e ta r g e t su rfa c e in clu d ed. While t h i s pro cess is used to g re a t advantage in th in film s work, a c o atin g o f extran eo u s m a te ria l on th e ta r g e t su rfa c e h e re can a l t e r th e observed e ro sio n r a t i o. Two th in g s r e s u l t when such a contam inant film is p re s e n t on a ta r g e t s u rfa c e. F i r s t, i f th e n a tu re o f th e ta r g e t s u rfa c e is a lte r e d by th e a d d itio n o f fo re ig n m a te r ia l, some change in th e e ro sio n r a t i o w ill occur as e ro sio n r a t i o i s a fu n c tio n o f ta r g e t m a te ria l. Secondly, a f t e r com pletion o f a bombarding ru n, a s ig n if ic a n t q u a n tity o f t h i s s p u tte re d m a te ria l can rem ain embedded in th e ta r g e t s u rfa c e. This a d d itio n a l mat e r i a l reduces th e m easured mass d iffe re n c e on th a t t a r g e t, low ering i t s apparen t e ro sio n r a t i o. In t h i s stu d y, p a r ts o f th e beam lim itin g a p e rtu re and ta r g e t c a r r i e r were exposed to th e s p u tte rin g p ro c e s s. This r e s u lte d in many ta r g e ts showing dark p a tch e s o f embedded m a te ria l. An in v e s tig a tio n o f th e e n tir e s e t o f bombarded ta r g e ts and t h e i r re s p e c tiv e e ro sio n r a tio s re v e a le d th a t whenever a ta r g e t was observed to have s p u tte re d m a te ria l on i t, i t s e ro sio n r a t i o g e n e ra lly f e l l on th e low s id e o f th e main s e quence o f d a ta p o in ts in F ig. 18. Poor c o n tro l o f th e focus on th e beam is co n sid e re d to be th e m ajor cause o f th is problem. With p ro p e r care in ta r g e t p la tfo rm desig n and b e t t e r beam fo c u sin g tech n iq u es in th e f u tu r e, t h i s problem should

57 48 be e lim in a te d. I t s im portance to io n ic p o lis h in g is co n sid e ra b le sin c e ex tran eo u s m a te ria l in io n ic a lly bombarded o p tic a l elem ents w ill n o t only a l t e r t h e i r index o f r e f r a c tio n n ear th e s u rfa c e, b u t in tro d u c e a reas o f a b so rp tio n u n d e sira b le in most a p p lic a tio n s. S u rface Charge N e u tra liz a tio n ' For reaso n s given in C hapter I I I, an attem p t was made to n e u tr a l iz e p o s itiv e s u rfa c e charge b u ild u p in t h i s stu d y. I t was b e lie v e d th a t th e e ro sio n r a t i o would in c re a s e w ith in c re a se d charge n e u tr a liz a tio n. This is because th e e l e c t r i c f i e l d b u ild u p by th e accum ulation o f p o s itiv e charge on th e t a r g e t, which ten d s to re p e l f u r th e r io n s, would be reduced. D uring th e course o f th e experim ent, how ever, v a r ia tio n s in th e e le c tro n guns and ion beam c u rre n t from ta r g e t to ta r g e t in d ic a te d th e e ro sio n r a t i o d ecreased w ith in c re a se d e le c tro n beam c u rre n t o r charge n e u tr a liz a tio n. Experim ental v e r if ic a tio n was o b ta in e d by bombarding a s e r ie s o f f iv e ta r g e ts. A ll c o n d itio n s h e ld on th e se ta r g e ts were id e n tic a l to th e r e s t. w ith th e excep tio n o f e x cessiv e n e u tr a liz a tio n c u rre n t ( tim es th e ion beam c u r r e n t). As shown in F ig. 19, th e observed r a t i o s fo r th e s e ta r g e ts (en clo sed in a dashed box) f e l l c o n sid e ra b ly below th e main s e quence o f ta r g e ts run w ith low er n e u tr a liz a tio n charge p re s e n t. This i n v e rse dependence was com pletely unexpected and as y e t no e x p la in a tio n can be o ffe re d w ith o u t f u r th e r.r e s e a r c h. I t s e f f e c t, how ever, appears to be q u ite s ig n if ic a n t, changing th e e ro sio n r a tio s over th e tem p eratu re range co n sid ered in th e fiv e t e s t ta r g e ts by about 20%.

58 3.0 1 Erosion Ratio ( Atoms/ Ion ) 2.5- T T T T T T Target Temperature C Fig. 19. Temperature Dependence of the Erosion Ratio with Neutralization Anomaly. KO

59 50 Ion Beam Inhom ogeneities Atoms, o th e r th an argon, can be p re s e n t in th e beam b e fo re reach in g th e p la tfo rm. Iro n, f o r in s ta n c e, can be s p u tte re d from th e duoplasm atron e le c tro d e s by th e plasm a. These iro n atoms are io n iz e d, a c c e le r a te d, and focused in to th e beam w ith th e argon, and a re allow ed to s tr ik e th e ta r g e t. Because th e mass o f iro n atoms is g r e a te r than argon, th e number o f ta r g e t atoms e je c te d by i t a re somewhat la r g e r. This r e s u l t s in th e observed e ro sio n r a t i o b ein g s l i g h t l y h ig h e r th an i t should b e. The s o lu tio n to t h i s problem i s a lso m agnetic beam a n a ly s is. S in g ly io n iz e d iro n and argon atoms have d if f e r e n t charge to mass r a tio s and w ill th e re fo re emerge from th e m agnetic f i e l d in d i f f e r n t d ir e c tio n s. By p la c in g th e ta r g e t under th e d e sire d beam and b lo ckin g o f th e iro n and o th e r beams r e s u ltin g from a n a ly s is, t h i s problem can be e lim in a te d.

60 CHAPTER VI SUMMARY AND RECOMMENDATIONS FOR FUTURE RESEARCH T h is re s e a rc h has been concerned w ith th e dependence of e ro sio n r a t i o on in c re a s e d ta r g e t te m p e ratu re. S p e c if ic a lly, th e change in e ro sio n r a t i o of fu sed s i l i c a ta r g e ts which were h e ate d to tem p eratu res ran g in g from 50 to 500 C and bombarded w ith 10 KeV argon io n s, was examined. The d a ta a cq u ired has been in te r p r e te d as showing a d e f in ite i n cre ase w ith in c re a se d te m p e ratu re. T his g e n e ra lly ag rees w ith work done p re v io u s ly by Dugdale and Ford (F ig. 4) and w ith a sim ple th e o r e tic a l model (Chap. I I ) concerned w ith therm al s p ik e s, t a r g e t a n n ealin g, and th e rm a lly induced momentum s c a tte r in g. The e ro sio n r a t i o, m easured in e je c te d ta r g e t atoms p e r in c id e n t io n, ranged from to 2.48 w ith a s t a t i s t i c a l l y determ ined p e rc e n t in c re a s e o f 118%. The la rg e spread o f d a ta re q u ire d s t a t i s t i c a l methods be used in d eterm in in g th e b e s t curve to be drawn through th e p o in ts. The la b o ra to ry procedure employed was one o f d ir e c t m easurem ent. T w enty-five ta r g e ts were bombarded a t v ary in g tem p eratu res w ith in th e above range, each re q u irin g p re p a ra tio n, w eighing, bombardment and r e - w eighing to a c q u ire th e n e c e ssa ry raw d a ta. This raw d a ta o b ta in e d, was used to c a lc u la te th e e ro sio n r a t i o s o f each t a r g e t. Each t a r g e t 's e ro sio n r a t i o was then p lo tte d a g a in s t tem p eratu re to.o b ta in th e d e sire d curve shown in F ig

61 52 In th e course o f t h i s in v e s tig a tio n, s e v e ra l unaccounted fo r phenomena were observed to have an in flu e n c e.o n th e e ro sio n r a t i o. These were ta r g e t su rfa c e contam inant film s, a lack o f m agnetic beam a n a ly s is and an unexpected in v e rse r e la tio n s h ip between s u rfa c e charge n e u tr a liz a tio n and th e e ro sio n r a t i o. S uggestions f o r F u ture Research. The m agnitude of th e observed in c re a s e in e ro sio n r a t i o o v er th e tem p eratu re range consid ered shows th a t ta r g e t tem p eratu re w ill be a s ig n if ic a n t f a c to r in a ccura te c o n tro l o f ion bombardment tech n iq u es app lie d to th e p o lis h in g and fig u rin g o f o p tic a l elem ents. B efore a c c u ra te fig u rin g can be r e a liz e d, however, a d d itio n a l work needs to. be done on th e tem p eratu re dependence o f th e e ro sio n r a t i o, e s p e c ia lly fo r d ie le c t r i c s over a w ider tem p eratu re ra n g e, p re fe ra b ly above 500 C. One im p o rtan t f a c to r a sso c ia te d w ith any o p tic a l elem ent is th e q u a lity o f i t s o p tic a l s u rfa c e s. A stu d y o f th e ta r g e t s u rfa c e s, p a r t i c u l a r l y th e roughness a f t e r e le v a te d tem p eratu re bombardment m ight be p e r tin e n t fu tu re re se ra c h. Such a stu d y would a lso be v ery u se fu l in a t tem pting to a s c e r ta in th e q u a lity and u se fu ln e ss o f p r e c is io n o p tic a l elem ents produced by io n ic p o lis h in g te ch n iq u es. Io n ic p o lis h in g w ill g e n e ra lly be done on d i e l e c t r i c ta r g e ts ( o p tic a l e le m e n ts). Such ta r g e ts have been shown to re q u ire su rfa c e charge n e u tr a liz a tio n f o r a c o n sta n t bombardment p ro c e ss to tak e p la c e. over a len g th o f tim e. Since th e attem p t h ere to n e u tr a liz e su rfa c e charge met w ith unexpected r e s u l t s, work needs to be done in determ in in g

62 ;; i v v- ' ; 53 j u s t what t h i s means and why i t occurs in th e in v e rse fa sh io n observed. A lso, i f e le c tro n beam n e u tr a liz a tio n tech n iq u es should prove f r u i t l e s s, thought must be given to o th e r tech n iq u es which w ill e lim in a te t h i s very s ig n if ic a n t problem.

63 54 APPENDIX A THE DERIVATION OF THE EQUATION FOR EROSION RATIO The e q u atio n for. e ro sio n r a t i o u sin g mass and ion beam d a ta can be d eriv ed from th e b a s ic d e f in itio n o f e ro sio n r a t i o. The e ro sio n r a t i o (e) i s d e fin e d as th e r a t i o o f th e number o f s p u tte re d p a r t i c l e s (N ) to th e number o f in c id e n t io n s (N^.) o r: N ; ; - H P u > E x pressions f o r N and N^ a re now o b ta in e d in term s o f th e mass and ion beam d a ta. The number o f s p u tte re d atoms removed i s sim ply th e t o t a l t a r g et mass removed d iv id ed by th e mass o f an average t a r g e t atoms (Mave) o r: ' where and M a re th e masses o f th e ta r g e t b e fo re and a f t e r bombardment r e s p e c tiv e ly... Assuming a l l p a r t i c l e s in th e beam are s in g ly io n iz e d, Nj is sim ply th e t o t a l in c id e n t charge (Q) d iv id e d by th e charge on a s in g le io n ( q ). Q, th e p ro d u ct o f th e ion beam c u rre n t ( I g ( t ) ) and th e t o t a l

64 55 exposure tim e, i s given by * r Q = / i BCt) d t (3) t o where t Q and. a re th e tim es a t th e beg in n in g and end o f th e exposure in te r v a l r e s p e c tiv e ly. For in crem en tal ion beam c u rre n t m easurem ents, Q i s approxim ated by Q = I TB A ti (4) i= l i 1 where I R i s th e average beam c u rre n t ober th e i ^ tim e increm ent (A t.) i and n i s th e number o f increm ents in th e t o t a l exposure. T his gives A - i [ i v i] :. (5) so th a t th e e ro sio n r a t i o can be w ritte n as where e = K C V ^ ) [: j r B i t. ] 1 : ; (6) L1=1 1 J 1 ;* r 2- (7) ave S u b s titu tin g v alu es o f q (1.602 X lo "1^ c o u l.) and fo r fused s i l i c a (S i0 2), given by, % ' %, 28amu + 32am,, ^ (8) ave 3, 3

65 56 where Mg^ and are th e atom ic w eights o f S ilic o n. (28amu) and Oxygen (16amu) re s p e c tiv e ly, in to equatio n 7 g iv e s. '1.6 0 X 1 0 '19 ctiul. hr.' ' K = 20amu X (1.67 X 10"21tgm/amu) X 3.6 X 103se c. K = = 4/3 coul (9) gm. sec, T h is g iv e s th e d e s ire d r e l a t i o n, w ritte n as e = t [ j T.:. j ^ L 1=1 1 J

66 REFERENCES B ran d t, W., and L aubert-, R., "U n ified S p u tte rin g T heory," N uclear Ir is tf u - ' merits-' and M ethods. 47(6) : , C a rlsto n, C. E., G. D. Magnuson, A. Comeaux, and P. Mahadevan, " E ffe c t o f E lev ated Tem peratures on S p u tte rin g Y ie ld s," P h y sic a l Review.. 138(3A):A759-A763, May C arter-, G., and J. S. C o llig o n, Ion Bombardment o f S o lid s. New York: American E ls e v ie r P u b lish in g Company, I n c., pp. 6-30, Dugdale, R. A., and S. D. Ford, "The E tching o f Amina and Fused S ilic a by S p u tte r in g," T ra n s la tio n s o f th e B r itis h Ceramic S o c ie ty. 65: , H o llan d, L., The '.P ro p e rtie s o f G lass S u rfa c e s. New York: John Wiley and Sons, I n c., pp , Keywell, F., "Measurements and C o llis io n -R a d ia tio n Damage Theory o f High Vacuum S p u tte r in g," P h y sical Review. 97(6): , M einel, A. B., S.. B ashkin, and D. A. Loomis, "C o n tro lle d F ig u rin g o f Opt i c a l S u rfaces by E n e rg e tic Io n ic Beams," A pplied O p tic s. 4(12) : 1674, Narodny, L., Ion Beam O p tic a l F ig u rin g. T echnical R e p o r t,. C o n tract No. F C-1146, P repared f o r U. S. A ir Force Systems Command (SEKNB), S y o sset, New York: Kollsman In stru m e n t C o rp o ratio n, pp. 6-37, Narodny, L., and M. T a ra sev ic h, "P arabolo id Figured by Ion Bombardment," A pplied O p tic s. 6(1 1 ): , N elson, R. S., M. W. Thompson, and H. Montgomery, "The In flu en c e o f T hermal V ib ra tio n s on Focused C o llis io n Sequences," P h ilo so p h ic a l M agazine. 7: , S aunders, J. B., and J. M. F l u i t, "Tem perature Dependence o f th e Range o f Focused C o llis io n Sequences o f Copper S in g le C r y s ta ls," P h y sic a. 30: , S ch ro ed er, J. B., S. B ashkin, and J. F. N e ste r, " Io n ic P o lish in g of Opt i c a l S u rfa c e s," A pplied O p tic s. 5 ( 6 ) : ,

67 58 S ils b e e, R. H., "Focusing in C o llis io n Problems in S o lid s," Jo u rn a l o f A pplied P h y s ic s. 28(11): , Symon, K. R., M echanics. Reading, M assa c h u se tts: Addison-W esley Publis h in g Company, I n c., pp , W ilson, R. G., " Io n ic P o lish in g o f Fused S ilic a and G la ss," T echnical Rep o rt No. 36, Tucson: O p tic al S ciences C en ter, p. 1, 1969.

68