MS482 Materials Characterization ( 재료분석 ) Lecture Note 5: RBS

2016 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 (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) 4. Diffraction and imaging techniques (7 lectures) 4.1. Basic diffraction theory 4.2. X-ray Diffraction (XRD) & X-ray Reflectometry (XRR) 4.3. Scanning Electron Microscopy (SEM) & Energy Dispersive X-ray Spectroscopy (EDS) 4.4. Transmission Electron Microscopy (TEM) 5. Scanning probe techniques (1 lecture) 5.1. Scanning Tunneling Microscopy (STM) 5.2. Atomic Force Microscopy (AFM) 6. Summary: Examples of real materials characterization (1 lecture) * Characterization techniques in blue are available at KARA (KAIST analysis center located in W8-1)

RBS: Rutherford Backscattering Spectrometry Copyright 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)? ³ 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.

Principles Kinematics of elastic scattering Conservation of energy and momentum 1 2 M Dv F = 1 2 M Dv D F + 1 2 M Fv F F M D v = M D v D cos θ + M F v F cos φ 0 = M D v D sin θ + M F v F sin φ v 2 v v 1 E D E M = For M 1 < M 2, kinematic factor K M F F M F D sin F θ D/F + M D cos θ M F + M D F (listed in Handout #5, Append. 1) For a given M 1 and M 2, smallest K is at q = 180 o Different types of atoms, DM 2 à largest change in DE 1 when q = 180 o Hence, backscattering spectrometry, though q ~170 o in practice

Principles Scattering Cross Section THIN TARGET: N S ATOMS/cm 2 (= N t), Q E 0 of incident particle of M 1 à K M2 E 0 that this particle possesses at any angle q after an elastic collision with an initially stationary M 2 s(q), how frequently such a collision occur at a certain angle q? differential scattering cross section Number of particles scattered into dω = Q W N Y W Number of particles scattered into the detector with W = Q W N Y W [ (in RBS, solid angle W is small, 10-2 steradian or less, so average can be used instead of differential) \ dσ θ dω dω dσ θ dω W dω = Q W N Y W 1 dσ θ [ Ω dω \ = Q W N Y W σ θ W Ω dω W Ω (average) scattering cross section [cm 2 /steradian]

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 σ θ = qf Z D Z F 4E F 1 sin`θ/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 the derivation, read pp. 21-24 of Handout #8) σ θ = qf Z D Z F 4E F θ a` sin 2 2 M D M F F + ~4% correction in the case of He (M 1 =4) incident on Si (M 1 =28) Rutherford Scattering Cross Section, s(q) for 1 MeV 4 He is listed in Handout #5, Appendix 2.

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 à closest distance < K shell electron radius à E > qf Z D Z F, where a 0 is Bohr radius. a M ~10 kev for He scattering from Si ~340 kev for He scattering from Au At low energy, correction from the screening should be considered. σ fg = σ θ 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

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 de dx = 2πq`Z F D W NZ E F W M D m 2mvF ln I de/dx : ev / Å (1/r) de/dx : ev / (µg/cm 2 ), where r is mass density (1/N) de/dx : ev / (atoms/cm 2 ), where N is atomic density listed in Appendix 3 (Handout #5) N: target atom concentration (#/cm 3 ) m: electron mass I: average excitation energy of an electron

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

How to Interpret RBS Data σ θ = Pt on Si 200nm Pt (t) qf Z D Z F 4E F Si from Si/Pt interface energy corresponding to Si at surface Rutherford scattering cross section θ sina` 2 2 M D M F F Pt at surface + DE Channel number = Backscattering energy Surface is on the right (high energy) greater depths to the left (lower energy) à DE ~ t Heavier elements produce higher energy backscattering. Why? Heavier elements produce larger peaks per unit concentration. Why? Shape of spectrum. Why?

Energy Width t= Q: which one is larger, DE Au or DE Au? DE Au Surface Au peak at higher energy than surface Al peak t E rs = [ de dx dx de dx v rs W t At t, E t = E M E rs = E M xy xz { rs W t E D = K no E t t }~ = t K no xy xz { rs + xy { xz pt t xy }~ xz { pt + K no E M E D K no E M? K np E M E no = K no E M E D = t[s] Surface energy approximation (for < 100 nm): xy xz { rs ~ xy xz { y, xy xz { pt ~ xy Mean energy approximation: xy { ~ xy { xy, xz rs xz y a ˆ y { ~ xy { xz pt xz y ˆ y xz { y

Depth Profiles E 1 H r = N r σ r (E M ) H Yr N Yr σ Yr (E D ) N r (Z r /E M ) F N Yr (Z Yr /E D ) F H r H Yr = N r N Yr σ r (E M ) σ Yr (E M ) N r N Yr Z r Z Yr or better approximation F σ θ = qf Z D Z F 4E F θ sina` 2 2 M D M F F + H r E r H Yr E Yr = N r N Yr σ r σ Yr

Measuring Concentration & Thickness Comparison of Three WSi x Films with varying W concentrations Comparison of Three Ti Films with varying thickness

Example of RBS Spectrum Depth O at Surface Si at Surface 2.27 mev He, 160 RBS Si SiO 2 Depth O Si Si in SiO 2

Scattering Geometry Affects Depth Resolution Grazing Exit Detector ~100 Incident He ++ Ions Backscattered He Ion Normal Angle Detector Sample Copyright 2007 Evans Analytical Group ~160

Scattering Geometry Affects Depth Resolution Grazing angle detector improves depth resolution for thin layers Copyright 2007 Evans Analytical Group

Effect of Film Density on Thicknesses 26 24 22 20 18 16 14 12 10 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 200 nm density = 5.66 x 10 22 Ti atoms/cm 3 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) Also called Forward Recoil Spectrometry He is heavier than H, so no He backscatters from H (or D) He does forward scatter H at significant energy Energy of recoiling H is measured

RBS/HFS Analysis of Silicon Nitride Film RBS spectrum HFS spectrum Film composition: Si - 38.4% N - 49.1% H - 12.5% Copyright Evans Analytical Group

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) à 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