MS482 Materials Characterization ( 재료분석 ) Lecture Note 5: RBS. Byungha Shin Dept. of MSE, KAIST

2015 Fall Semester MS482 Materials Characterization ( 재료분석 ) Lecture Note 5: RBS Byungha Shin Dept. of MSE, KAIST 1

Course Information Syllabus 1. Overview of various characterization techniques (1 lecture) 2. Chemical analysis techniques (7 8 lectures) 2.1. X-ray Photoelectron Spectroscopy (XPS) 2.2. Ultraviolet Photoelectron Spectroscopy (UPS) 2.3. Auger Electron Spectroscopy (AES) 2.4. X-ray Fluorescence (XRF) 3. Ion beam based techniques (4 lecture) 3.1. Rutherford Backscattering Spectrometry (RBS) 3.2. Secondary Ion Mass Spectrometry (SIMS) Mid-term Exam 1 (30%) 4. Diffraction and imaging techniques (6 7 lectures) 4.1. Basic diffraction theory 4.2. X-ray Diffraction (XRD) & X-ray Reflectometry (XRR) 4.3. Reflection High Energy Electron Diffraction (RHEED) & Low Energy Electron Diffraction (LEED) 4.3. Scanning Electron Microscopy (SEM) & EDS 4.4. Transmission Electron Microscopy (TEM) Mid-term Exam 2 (30%) 5. Scanning probe techniques (2 lectures) 5.1. Atomic Force Microscopy (AFM) & associated techniques 5.2. Scanning Tunneling Microscopy (STM)? 6. Summary: Examples of real materials characterization (1 lecture) Final Exam (40%)

Course Information Exams & Make-up Class Lecture 9 Lecture 13 Lecture 14 Lecture 10 Lecture 11 Lecture 15 Lecture 16 No Lecture No Lecture Mid exam 1? Mid exam 1? Mid-term exam period No Lecture No Lecture Mid exam 2? Mid exam 2? Lecture 17 Lecture 18 No Lecture Make-up Lecture 12 Lecture 1 (7 pm)? Make-up Lecture (7 pm)? Lecture 19 Lecture 20 Lecture 21 Lecture 22 Final exam period Final exam Mid-term exam 1: 10/13 (Tue) or 10/15 (Thurs) Mid-term exam 2: 11/17 (Tue) or 11/19 (Thurs) Final exam: 12/17 (Thurs), 09:00 11:45 Make-up class 1: 7 pm on 10/28 (Wed) or 10/30 (Fri) Possibly second make-up class in Nov or Dec

RBS: Rutherford Backscattering Spectrometry Copyright 2007 Evans Analytical Group RBS accurately measures the composition and depth profile of thin films, including hydrogen. Quantitative? Yes Destructive? No Detection limits? 0.001 10 at% Lateral resolution (Probe size)? 1 mm Chemical bonding? No Depth resolution? 5 20 nm ~

Key Applications & Instrument Configuration Determine thickness and composition of thin films Measure hydrogen Determine film density from a film of known thickness Assess damage to crystal structure as a result of processing Quantification of films on whole wafers (up to 300 mm) Qualify & monitor deposition systems (also fab-to-fab comparisons) Copyright 2007 Evans Analytical Group MeV ions from an electrostatic accelerator are focused on a sample in a vacuum chamber for analysis. Typically, 2 MeV He ++ ions are used.

Fundamental Principles Kinematics of elastic scattering Conservation of energy and momentum 1 2 MM 1vv 2 = 1 2 MM 1vv 2 1 + 1 2 MM 2 2vv 2 MM 1 vv = MM 1 vv 1 cos θθ + MM 2 vv 2 cos φφ 0 = MM 1 vv 1 sin θθ + MM 2 vv 2 sin φφ v 2 v v 1 For M 1 < M 2, kinematic factor K EE 1 = MM 2 2 MM 2 1 sin 2 θθ 1/2 + MM 1 cos θθ EE 0 MM 2 + MM 1 (listed in Handout #4, Append. 1) 2 For a given M 1 and M 2, smallest K is at θ = 180 o Different types of atoms, M 2 largest change in E 1 when θ = 180 o Hence, backscattering spectrometry, though θ ~170 o in practice

Fundamental Principles Scattering Cross Section THIN TARGET: N S ATOMS/cm 2 (= N t), Q E 0 of incident particle of M 1 K M2 E 0 that this particle possesses at any angle θ after an elastic collision with an initially stationary M 2 How frequently such a collision occur at a certain angle θ? differential scattering cross section Number of particles scattered into ddω = QQ NN SS Number of particles scattered into the detector with Ω = QQ NN SS Ω dddd θθ ddω (in RBS, solid angle Ω is small, 10-2 steradian or less, so average can be used instead of differential) ddω ddσσ θθ ddω ddω = QQ NN SS 1 Ω Ω = QQ NN SS σσ θθ Ω dddd θθ ddω ddω Ω (average) scattering cross section [cm 2 /steradian]

Fundamental Principles Scattering Cross Section Without considering recoil of the target atom (M 1 << M 2 ), i.e., the target atom is stationary all the time σσ θθ = qq2 ZZ 1 ZZ 2 4EE 2 1 sin 4 θθ/2 Including the recoil effect, Z 1 : atomic number of incident particle Z 2 : atomic number of target atom q: elemental charge E: energy of incident particle (For derivation see pp. 21-24 of Handout #5) σσ θθ = qq2 ZZ 1 ZZ 2 4EE 2 sin 4 θθ 2 2 MM 1 MM 2 2 + ~4% correction in the case of He (M 1 =4) incident on Si (M 1 =28) Rutherford Scattering Cross Section, σ(θ) for 1 MeV 4 He is listed in Handout #4, Appendix 2.

Fundamental Principles Deviation from Rutherford Scattering Assumption to derive Rutherford scattering cross section: scattering due to the repulsion of two positively charged nuclei of atomic number Z 1 and Z 2 Meaning that incident atom penetrates well inside the orbital of the atomic electrons closest distance < K shell electron radius EE > qq2 ZZ 1 ZZ 2, where a 0 is Bohr radius. aa 0 ~10 kev for He scattering from Si ~340 kev for He scattering from Au At low energy, correction from the screening should be considered. σσ ssss = σσ θθ F At higher energy, departure from the Rutherford scattering cross section due to the interaction of the incident particle with the nucleus of the target atom ~9.6 MeV for He ions incident on Si

Fundamental Principles Stopping power (stopping cross section) Energy loss of MeV light ions (such as He) in solids: electronic energy loss (interaction with electrons, excited or ejected) Negligible nuclear energy loss dddd dddd = 2ππqq4 2 ZZ 1 NNZZ EE 2 MM 1 mm 2mmvv2 ln II de/dx : ev / Å (1/ρ) de/dx : ev / (µg/cm 2 ), where ρ is mass density (1/N) de/dx : ev / (atoms/cm 2 ), where N is atomic density listed in Appendix 3 (Handout #4)

Fundamental Principles Energy transfer from a projectile to a target atom in an elastic two-body collision concept of kinematic factor and capability of mass perception Likelihood of occurrence of such a two-body collision concept of scattering cross section and capability of quantitative analysis of atomic composition Average energy loss of an atom moving through a dense medium concept of stopping cross section and capability of depth perception

Energy Width in Backscattering t= Q: which one is larger, E Au or E Au? E Au Surface Au peak at higher energy than surface Al peak tt dddd EE iiii = dddd dddd dddd dddd tt iiii At t, EE tt = EE 0 EE iiii = EE 0 EE 1 tt = KK AAAA EE tt tt cos θθ dddd = tt KK AAAA + dddd iiii dddd dddd oooooo tt cos θθ dddd tt dddd iiii dddd + KK dddd AAAA EE 0 oooooo EE 1 KK AAAA EE 0 KK AAAA EE? 0 EE AAAA = KK AAAA EE 0 EE 1 = tt[s] dddd Surface energy approximation (for < 100 nm): ~ dddd iiii dddd dddd Mean energy approximation: ~, dddd ~ dddd iiii dddd EEoo 1 4 EE dddd iiii dddd, dddd EEoo dddd dddd EE1 + 1 4 EE dddd ~ dddd oooooo dddd dddd KKEEoo

How to Interpret RBS Data Pt on Si 200nm Pt Channel number = Backscattering energy Surface is on the right (high energy) greater depths to the left (lower energy) Heavier elements produce higher energy backscattering. Why? Rutherford scattering cross section σσ θθ = qq2 ZZ 1 ZZ 2 4EE 2 sin 4 θθ 2 2 MM 1 MM 2 2 + Heavier elements produce larger peaks per unit concentration. Why? Shape of spectrum. Why?

Depth Profiles E 1 HH NNNN = NN NNNN σσ NNii (EE 0 ) HH SSSS NN SSSS σσ SSii (EE 1 ) NN NNNN (ZZ NNii /EE 0 ) 2 NN SSSS (ZZ SSSS /EE 1 ) 2 σσ θθ = qq2 ZZ 1 ZZ 2 4EE 2 sin 4 θθ 2 2 MM 1 MM 2 2 + HH NNNN HH SSSS = NN NNNN NN SSSS σσ NNii (EE 0 ) σσ SSii (EE 0 ) NN NNNN NN SSSS or better approximation HH NNNN EE NNii HH SSSS EE SSSS = NN NNNN NN SSSS σσ NNii σσ SSii NN NNNN NN SSSS 2

Effect of Film Density on Thicknesses 26 24 22 20 18 16 2.27 mev He, 160 RBS The total atoms in each film are equal (1.13 x 10 18 atoms/cm 2 ) Both samples produce this spectrum Si Ti density = 5.66 x 10 22 Ti atoms/cm 3 14 12 200 nm 10 8 6 4 Si Ti Si Ti density = 2.83 x 10 22 Ti atoms/cm 3 2 0 0 Channel Number 200 400 400 nm Fundamental unit of measurement for RBS is atoms/cm 2 Density thickness = atoms/cm 2 To calculate a film thickness using RBS alone one must assume a film density If the film thickness is known (by TEM, SEM, profilometry, etc.), then the film density can be calculated

Hydrogen Forward Scattering Spectrometry (HFS) He is heavier than H, so no He backscatters from H (or D) He does forward scatter H at significant energy

Channeling Conceptual image of channeling process Ref. Scientific American

Applications of Channeling Quantitative crystal damage profiling Ion Implants Regrowth of damaged crystals Polishing damage Ion etching Epitaxial layers Thickness of amorphous layers Damage detection limit: 1 x 10 15 to 1 x 10 17 displaced at/cm 2 Substitutionality of dopants / impurities

Channeling: Crystal Damage Measurement of damage in crystal structure Disorder in crystal structure results in higher backscattering yield Random MeV He ions Yield Crystal with disorder Aligned Region without disorder Region with disorder 0 0 Perfect crystal Energy

Channeling: Epitaxial Growth Deposited atoms are in perfect registry with the substrate (i.e., epitaxy) shadow cones by the absorbed atoms

Channeling: Substituionality of impurities Yb-implanted Si Yb in not substitutional but is located near the <110> channels

Strengths and Weaknesses Strengths Non destructive depth profiling Quantitative without standards Analysis of whole wafers (150, 200, 300 mm), irregular and large samples Can analyze conductors and insulators Can measure hydrogen Weaknesses Large analysis area (1 mm) Poor sensitivity for low Z elements In many cases, useful information limited to thin films (<0.5 µm) Generally not good for bulk samples