AP 5301/8301 Instrumental Methods of Analysis and Laboratory Lecture 11 Ion Beam Analysis

Transcription

1 1 AP 5301/8301 Instrumental Methods of Analysis and Laboratory Lecture 11 Ion Beam Analysis Prof YU Kin Man Tel: Office: P64

2 Lecture 8: outline Introduction o Ion Beam Analysis: an overview Rutherford backscattering spectrometry (RBS) o Introduction-history o Basic concepts of RBS Kinematic factor (K) Scattering cross-section Depth scale o Quantitative thin film analysis o RBS data analysis Other IBA techniques o Hydrogen Forward Scattering o Particle induced x-ray emission (PIXE) Ion channeling o Minimum yield and critical angle o Dechanneling by defects o Impurity location

3 Detection Range TXRF XRF Analytical Techniques 3 100at% 10at% AES XRD 1at% 0.1at% IBA RBS 100ppm 10ppm TOF- SIMS 1ppm 100ppb 10ppb 1ppb 100ppt 10ppt Chemical bonding Elemental information Imaging information Thickness and density information DynamicS IMS Analytical Spot size But IBA is typically quantitative and depth sensitive

4 Ion Beam Analysis: an overview 4 Inelastic Nuclear Reaction Analysis (NRA) p, a, n, g Incident Ions Elastic Rutherford backscattering (RBS), resonant scattering, channeling Particle induced x-ray emission (PIXE) Defects generation Elastic recoil detection Ion Beam Modification Absorber foil

5 Ion Beam Analysis

6 Why IBA? 6 Simple in principle Fast and direct Quantitative (without standard for RBS) Depth profiling without chemical or physical sectioning Non-destructive Wide range of elemental coverage No special specimen preparation required Can be applied to crystalline or amorphous materials Simultaneous analysis with various ion beam techniques (RBS, PIXE, NRA, channeling, etc.) Can obtain a lot of information in one measurement

7 A typical Ion Beam Analysis Facility 7 Experimental chamber bending focusing Accelerator Ion source Beam collimators Fixed particle detector PIXE detector f Moveable particle detector

8 IBA Equipment: accelerator 8 Van de Graaff (single ended) Tandem Pelletron

9 IBA Equipment: tandem pelletron 9

10 Equipment: particle detector 10

11 11 Rutherford Backscattering Spectrometry (RBS) Relatively simple physics Requires some experimental skills Requires some data simulation to obtain the most out of the spectrum The only quantitative and absolute method to determine atomic concentration with no matrix effect composition variation with depth layer thickness damage distribution and impurity lattice location of multi-elemental, multi-layered films

12 Rutherford Backscattering Spectrometry (RBS) a brief history The original experiment Alpha particles at a thin gold foil experiment by Hans Geiger and Ernest Marsden in ) Almost all of the alpha particles went through the gold foil ) Some of the alpha particles were deflected only slightly, usually or less. 3) A very, very few (1 in 8000 for platinum foil) alpha particles were turned through an angle of 90 or more. "We shall suppose that for distances less that 10-1 cm the central charge and also the charge on the alpha particle may be supposed to be concentrated at a point." (1911)

13 RBS: The Surveyor V experiment 13 Surveyor V, first of its spacecraft family to obtain information about the chemical nature of the Moon's surface, landed in Mare Tranquillitatis on September 11, "Surveyor V carried an instrument to determine the principal chemical elements of the lunar-surface material," explained ANTHONY TURKEVICH, Enrico Fermi institute and Chemistry Department, University of Chicago. "After landing, upon command from Earth, the instrument was lowered by a nylon cord to the surface of the Moon

14 General applications of RBS 14 Quantitative analysis of thin films thickness, composition, uniformity in depth solid state reactions interdiffusion Crystalline perfection of homo- and heteroepitaxial thin films Quantitative measurements of impurities in substrates Defect distribution in single-crystal samples Surface atom relaxation in single crystals Lattice location of impurities in single crystals

15 RBS: basic concepts 15 Kinematic factor: elastic energy transfer from a projectile to a target atom can be calculated from collision kinematics mass determination Scattering cross-section: the probability of the elastic collision between the projectile and target atoms can be calculated quantitative analysis of atomic composition Energy Loss: inelastic energy loss of the projectile ions through the target perception of depth These allow RBS analysis to give quantitative depth distribution of targets with different masses

16 Kinematic factor: K 16 Conservation of energy : 1 m v 1 m v 1 m v Conservation of momentum: m v m v cos m v cosf m v sin m v 1 1 sinf K ( 1 sin ) 1 1 cos E m m m m Eo ( m m1 ) K, m, m 1

17 Kinematic Factor ) ( cos ) sin ( m m m m m E E K o m 1 1 ) ( ) ( ) 180 ( m m m m K o m ) ( ) ( ) 90 ( 1 1 m m m m K o m ~ 1 ) 180 ( o m K When m >>m 1 :

18 Element identification 18.5 MeV He ion with =170 o K( 197 Au) = 0.93 K( 109 Ag) = K( 107 Ag) = 0.86 K( 65 Cu) = K( 63 Cu) = 0.777

19 Mass Resolution, dm For 180 o scattering: E 1 ( m m1 ) K m E 0 ( 1) m m d E m m 1 d E m m For a fixed de 1 /E 0 (~0.01), heavier projectiles result in better mass resolution However, de 1 for heavier projectiles is higher

20 Mass Resolution: examples 0 With system energy resolution de = 0keV and E 0 =MeV For m =40 For m = 70 dm dm 3 (40 4) 4 4(40 4) a. m u Isotopes of Ga (68.9 and 70.9 a.m.u.) cannot be resolved 3 (70 4) 4 4(70 4) a. m. 000 u. 4 MeV 3 MeV MeV 1 MeV We can also improve mass resolution by increasing E o d m m m d E 4m m m E

21 RBS Quantification 1 Backscattering Yield Q For a given incident number of particles Q, a greater amount of an element present (N s ) should result in a greater number of particles scattered. Thus we need to know how often scattering events should be detected (A) at a characteristic energy (E = KE 0 ) and angle, within our detector s window of solid angle W.

22 Scattering cross-section If particles are coming in with impact parameters between b and b +db, they will be scattered through angles between and +d. For central forces, we have circular symmetry. Rutherford cross-section: bdb ( ) sin d d dw Z 1/ 1 Ze cos (1 A sin ) Z1Z ~ E o 4 1/ Eo sin (1 A sin ) A m m 1

23 Scattering Yield 3 Yield,Y 1/ Z cos 1 A sin 1Ze E Z 1Z1 ( E 0 ) 0 4 ~ 10 4 sin 1 A sin cm [ barn] 1/ Example: Mixed Si and W target analzed by a MeV He ion beam at 165 o scattering angle. (Si)=.5x10-5 cm /str (W)=7.4x10-4 cm /str Total incident ions Q=1.5x10 14 ions W=1.8mstr Area under Si, A(Si)=00 counts Area under W, A(W)=6000 counts (Nt) Si =3x10 15 atoms/cm (Nt) W =3x10 15 atoms/cm

24 Energy Loss Electronic stopping de dx ele Nuclear Stopping de dx nucl When an He or H ion moves through matter, it loses energy through interactions with e - by raising them to excited states or even ionizing them. Direct ion-nuclei scattering Since the radii of atomic nuclei are so small, interactions with nuclei may be neglected de dx total de dx ele de dx nucl de dx ele

25 How to read a typical RBS spectrum 5 A thin film on a light substrate Most projectile ions experience electronic stopping that results in a gradual reduction of the particle s kinetic energy (de/dx). At the same time a small fraction of projectile ions come close enough to the nucleus for largeangle scattering (KE). A detected backscattered particle has lost some energy during initial penetration, then lost a large fraction of its remaining energy during the large-angle scattering event, then lost more energy in leaving the solid.

26 Depth Scale E E 1 1 K( E 0 E in ) E out K( E 0 de dx E 0 de t) ( dx KE 0 t ) cos KE o Total energy loss E=KE 0 -E 1 E 1 KE E 0 K de 1 de E ( ) t [ So] t cos 1 dx cos dx E 0 [S o ] is the effective stopping power Stopping cross-section KE 0 1 N de dx : ev cm 3 cm atom evcm atom E K 1 E Nin Nout t N[ o] t cos1 cos

27 Depth scale: thin film 180 o - t E o KE o E 1 Thickness of film: de de dx KE0 E ( K ) t [ So ] t dx cos E 0 E 1 KE o t E E S ] o N[ o E obtained from TRIM code or empirical energy loss data fitting E

28 Example: layer thickness 8 consider the Au markers E Au =E AuF - E AuB = [K Au de/dx Eo + 1 / (cos10º) de/dx EAuB ] t de/dx 3MeV = N Al Al 3MeV = 6.0x x10-15 =.x10 9 evcm -1 de/dx EAuB = N Al Al.57MeV = 6.0x10 x 39.34x10-15 =.37x10 9 evcm -1 t = 3945Å E Al = 165keV E Au = 175keV K Au = 0.95 K Al = consider the Al signals E Al = [K Al de/dx Eo + 1 / (cos10º) de/dx KEo ] t t = 3937Å

29 Energy Loss: Bragg s rule 9 For a target A m B n, the stopping crosssection is the sum of those of the constituent elements weighted by the abundance of the elements. AmBn = m A + n B Example: the stopping cross-section AlO3 of Al O 3. Given: Al = 44 x evcm O = 35 x evcm AlO3 = /5 x Al +3/5 x O = (/5 x /5 x 35) x = 38.6 x ev-cm /atom de/dx(al O 3 )=N AlO3 =(1.15x10 )(38.6x10-15 )ev/cm =44.4 ev/å

30 Quantitative analysis: composition and thickness A A = A WQ(Nt) A A B = B WQ(Nt) B n m Z Z Nt Nt A A B A B A B A B A ) ( ) ) ( ) ( ( ) ( ) ( ) ( A B B A Z Z A A n m n m n m B A B o B B A A o A S E S E t ] [ ) ( ] [ ) ( n m n m B A B o B B A A o A N E N E t ] [ ) ( ] [ ) ( 30

31 Example: As implanted Si 31 K As =0.809; K Si =0.566 [ ] o [ ] o E Si Si Si As ( FWHM ) R p R Si As 9.6x x10 68keV E As N[ ] p As Si o keV Si As ev cm ev cm 140Å ( FWHM ) As.355 N[ ] As o Si As Total As dose: A ( Nt) As As ( W Q) / atom 540Å / atom

32 RBS Application: impurity profile 3 Ge inplanted region Si SiO 1.95 MeV 4 He + I. Sharp et al., LBNL (004)

33 Thin film analysis 33 Y x Ba y Cu z O y x A Z Y ( Ba ) Y Ba ABa Z ( ) Y x H Ba ZY y H Z

34 Example: silicide formation ( s) 34 Before reaction After reaction Wei-Kan Chu, James W. Mayer and Marc-A. Nicolet, Backscattering Spectrometry, (Academic Press, New York 1978).

35 RBS application: silicide formation Reaction kinetics: K. N. Tu et al. Thin Solid film 5, 403 (1975). K. N. Tu et al. Japn, J. Appl. Phys. Suppl. Pt (1974). J. O. Olowolafe et al. Thin Solid film 38, 143 (1976).

36 IBA Data Analysis 36 Starts with a simple guess sample structure (film thickness and composition) Simulates spectrum and directly compare to experimental data Iterates simulated structure to fit experimental data (either automatically or manually) An ideal software: able to handle different data files have large data base for various ion-solid interaction: resonant scattering, nuclear reaction, etc. User friendly interface Fast simulation Can simulate sample non-uniformity (roughness, compositional gradient, etc.) Able to correct for electronic errors (pile up, dead time, charge up, etc)

37 Thin Film Analysis: single layer GaN 1-x Bi x on sapphire 550 nm GaN 0.9 Bi MeV a 000 Ga 1000 Bi A.X. Levendar, LBNL Energy (kev)

38 Thin film analysis: multilayers Mo CIGS solar cell structure 3.05 MeV a 150 ITO nm ITO 35 i- ZnO nm ZnO 45 nm CdS CdS mm CuGa CIGS 0.16 In 0.84 Se In 000 O Se In Cu Zn S Cd Ga Sn Energy (MeV) 1 Momm Mo glass

39 Impurity profile 39 Ge inplanted region Si SiO 1.95 MeV 4 He + I. Sharp et al., LBNL (004)

40 Strengths of RBS 40 Simple in principle Fast and direct Quantitative without standard Depth profiling without chemical or physical sectioning Non-destructive Wide range of elemental coverage No special specimen preparation required Can be applied to crystalline or amorphous materials Simultaneous analysis with various ion beam techniques (PIXE, PIGE, channeling, etc.)

41 Weaknesses of RBS 41 Poor lateral resolution (~0.5-1mm) Moderate depth resolution (>50Å) No microstructural information No phase identification Poor mass resolution for target mass heavier than 70amu (PIXE) Detection of light impurities more difficult (e.g. Li, B, C, O, etc) (non-rutherford scattering, Nuclear Reaction Analysis) Data may not be obvious: require knowledge of the technique (simulation software)

42 4 Hydrogen Forward Scattering (HFS) (Elastic Recoil Detection Analsyis ERDA)

43 Hydrogen Forward Scattering (HFS) 43 Generally known as elastic recoil detection analysis (ERDA)

44 Hydrogen Forward Scattering (HFS) 44 Generally known as Elastic Recoil Detection (ERD) Charles Evans and Assoc., RBS APPLICATION SERIES NO. 3 Quantitative hydrogen and deuterium profiling Good sensitivity (~0.01at% of H) Can be perform simultaneously with RBS and PIXE Profiling with any light element in solid (using heavy ion beam, ERD)

45 HFS: a-sin:h film Energy (MeV) RBS Normalized Yield nm SiN 1.15 H Channel Energy (MeV) Si HFS Normalized Yield Channel Courtesy: F. Hellman group, 008

46 Particle Induced X-ray Emission (PIXE)

47 PIXE: Light impurity in heavy matrix 47 GaAs MeV 4 He + RBS Ga 1-x Mn x As PIXE K. M. Yu et al. 00.

48 PIXE Application: Geology, Art, Archeology, Biology 48 External Beam

49 Ion channeling 49

50 Ion Channeling 50 Kobelco Steel Group

51 Ion Channeling random Planar channel Axial channel

52 Ion Channeling: minimum yield and critical angle 5 Two important parameters to characterize channeling results: 1. Minimum yield: min Y channeled Y random ~ Critical half-angle, y 1/ indicates presence of defects responsible for beam dechanneling

53 Ion Channeling: minimum yield and critical angle Minimum Yield, min Critical half-angle, y 1/ min min Y r channeled Y random min ro ~ c 1 Z ( Ed 1 Z 1 e Ca ) ln[( ) 1 ] where d is the distance of atoms in a row, a is the Thomas-Fermi screeing distance, r is rms thermal vibration Z 1 1/ ~ c ~ ( ) ~ 0.5 E Z 1 o 1

54 Dechanneling by defects 54 Small angle dechannelingline defects Perfect crystal channeling Direct scatteringpoint defects

55 Flux Channeling: Impurity Lattice Location 3 r r o Atomic row y/y 1 = Channel center Distance from atom row, r/r o

56 Experimental techniques : combined channeling RBS/PIXE 56 Rutherford backscattering (RBS) Mn Particle-induced x-ray emission (PIXE) c-rbs: crystalline quality of film (from GaAs backscattering yields) c-pixe: substitutionality of Mn atoms w.r.t. host GaAs

57 Channeling: Ga 1-x-y Be y Mn x As 57 <100> Channeling RBS/PIXE: -presence of interstitial Mn in GaAs -[Mn I ] increases with Be doping -Mn I reduces T C <110> <111> K. M. Yu et al. 003.

58 Homo- and Heteroepitaxy 58

59 Channeling: Heteroepitaxy nm ~530nm InN on 10 nm GaN Z. Liliental-Weber K. M. Yu et al., LBNL 004.

60 Amorphous layer analysis 60

61 Strengths of Ion Beam Analysis Techniques 61 Simple in principle Fast and direct Quantitative (without standard for RBS) Depth profiling without chemical or physical sectioning Non-destructive Wide range of elemental coverage No special specimen preparation required Can be applied to crystalline or amorphous materials Simultaneous analysis with various ion beam techniques (RBS, PIXE, channeling, etc.)

62 Pitfalls in IBA 6 Yield: dσ E, Y i = N i ( dω ) QΩ Q-total charge accurate charge integration W-solid angle d/dw- detection angle, double/plural scattering Other issues: Radiation damage Sputtering Detector response Surface roughness Non-uniformity Charging on insulators Count rate effects

63 Charge integration 63 Charge Integration Accurate charge integration is important for absolute quantitative measurements Good faraday cup design

64 Deviation from Rutherford scattering 64 Correction factor F, which describes the deviation from pure Rutherford scattering due to electron screening for He + scattering from atoms, Z, at variety of incident kinetic energies. Electronic screening Rutherford Nuclear Interaction Energy At very high energy and very low energy, scattering will deviate from the Rutherford type. At low energy : screening of e - must be considered At high energy : nuclear short range force will enhance the cross-section, the so-called resonance scattering.

65 Charging effect for insulating samples 65 Severely distort the RBS spectrum Provide a supply of low-e electrons from a small, hot filament located nearby Coating the surface with a very thin layer of conducting material

66 Target non-uniformity 66 Surface roughness and interface roughness cannot be distinguished Target non-uniformity will resemble diffusion Campisano et al., 1978