MS482 Materials Characterization ( 재료분석 ) Lecture Note 4: XRF

2016 Fall Semester MS482 Materials Characterization ( 재료분석 ) Lecture Note 4: XRF 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)

XRF: X-ray Fluorescence XRF measures composition and impurities of bulk materials and films. Copyright Evans Analytical Group

Primary & Secondary Processes X-ray (XPS, XRF), UV (UPS), Electrons (AES, EDS) Excited Ion Relaxation Process 1 Fluorescent X-ray (XRF, EDS) Auger electron emission (AES) Emitted photoelectron (XPS, UPS) Relaxation Process 2

XRF Principle Moseley s law 1 λ = k(z σ) k : constant for a particular spectral series Z : atomic number σ : screening constant for the repulsion λ : wavelength of an X-ray characteristic line correction due to other electrons

XRF Overview XRF spectra look similar to EDS spectra Sampling depth is ~10X greater than EDS (~10 µm) Detection limits are ~100X lower than EDS Smallest analysis area is ~50 µm Vacuum not required, but often used to improve sensitivity Light element detection very difficult (typically Na-U detected) Nondestructive Copyright Evans Analytical Group

Types of XRF Wavelength-dispersive XRF (WDXRF): wavelength of the emitted X-rays determined using a diffracting crystal Energy-dispersive XRF (EDXRF): energy of the emitted X-rays directly measured by collecting the ionization produced in a suitable detecting medium Better energy resolution with WDXRF than EDXRF Higher detection efficiency with EDXRF than EDXRF Simultaneous detection of X-rays of different energies (wavelengths) possible with EDXRF, not with WDXRF

WDXRF large l (small E) à large 2q For a given diffracting crystal (fixed d), Bragg condition at different q for different l of emitted X-rays small l (large E) à small 2q Angular dispersion (resolution) dθ dλ = n 2d cos θ (better resolution with smaller d)

WDXRF Limitation in spectral range that can be measured 2q from a few degrees to ~150 o possible, but optimal range is 15 o 70 o - at high 2q: angular dispersion widens peak profile - at low 2q: only a small fraction of X-ray from the sample intercepted by the diffracting crystal sample q crystal sample Low Z-limit (low E limit): l < 2d High Z-limit (high E limit) - by primary X-ray source (e.g. W anode ~50 kv à Z < 63) - by diffracting crystal, 2q 15 o q Crystal 2d (nm) Element range LiF (420) 0.18 LiF (220) 0.29 LiF (200) 0.40 Si (111) 0.65 Ni (28) U V (23) U K (19) U PX-1* 5.1 O (8) Mg (12) PX-2* 12 B (5) C (6) PX-3* 20 B (5) OV-H** 24.2 Be (4) B (5) repeating multilayer * From Philips ** Ovonic Synthetic Materials Company

WDXRF: Detector (Photon) detector: photon à electrical pulse Gas-filled detector Current pulse I proportional to n ~ KE/F ~ hn/f, called proportional counters F = 27.8 ev (He), 27.4 ev (Ne), 26.4 ev (Ar), 22.8 ev (Kr), 20.8 ev (Xe) Further amplification of n by the applied voltage V e.g. B Ka (185 ev) creates ~6 pairs; Mo Ka (17.4 kev) ~580 pairs, V

WDXRF: Detector Scintillation detector High energy photons à ionize NaI (Tl), F ~ 50 ev with e - (KE = hn F hn) à excitation of iodide atoms to 3 ev above the ground state, # of excited atoms hn / 3 ev à deexcitation with light, hn ~ 3 ev, emitted à photoelectrons from the photocathode (such as InSb) à multiplication of e - as large as 10 6 Poorer resolution compared to gas-filled detector

WDXRF at KARA https://kara.kaist.ac.kr http://www.rigaku.com/en/products/xrf/primus2

EDXRF Less restrictive geometry in placing a detector à large solid angle and increased detection efficiency Simultaneous detection of X-rays over a wide range Poor resolution compared to WDXRF Semiconductor detector p-n junction at reverse bias cross-section of Si(Li) detector n + hn # of e - -h + pairs generated in the intrinsic region proportional to hn n p qv bias (intrinsic) (serves as p + contact) # of e - -h + pairs collected by drift at V bias

XRF Intensity of a Pure Element µ K / (µ K + µ L +.) K a / (K a + K b +.) W K / (W K + W A ) # of incident photons between l and l + dl linear absorption coefficient attenuation of K a fluorescence created at dx dn RS x, λ = N V λ dλ W exp μ [ (λ) W # of Ka fluorescence photons from dx intensity at a depth x x sin φ W P RS W exp μ [ (α) W x sin ψ d a b N RS = ` ` dn RS x, λ V a c dλ dx h (for a thin film with h < 100 nm)

XRF Intensity of an Element in Multicomponent Mixture # of Ka fluorescence intensity of element A from dx average mass absorption coefficient µ(l) = C A µ A (l) + C B µ B (l) + C C µ C (l) +. fraction absorbed by the element A dn RS,g x, λ = N V λ dλ W exp μ(λ) W xρ sin φ W C gμ g λ μ λ W P RS,g W exp μ(α) W xρ sin ψ d a b N RS,g = ` ` dn RS,g x, λ V a c dλ dx h g Matrix Effect Calibration curve for Pb La, Sn Ka, Sn La in Pb-Sn binaries Best practice is using standards (pure A, pure B, pure C,., of known thickness & mixture of ABC with known composition and thickness).

Example: RoHS and WEEE analysis Can detect Hg, Pb, Cd, Br, and Cr down to <100ppm Is a widely accepted measurement technique Can analyze small areas and individual components Spectral interferences and quantification issues may affect some analyses XRF Spectrum from reference sample, 1% Br in polystyrene Cr Fe Hg,Pb Br Element Rh (source) Cr Br Copyright 2007 Evans Cd Analytical Group Cd Hg Pb Concentration <300 ppm* 1% (reference) <30 ppm <100 ppm <100 ppm *Interference from high Fe conc. affects Cr detection limit RoHS: Restriction of Hazardous Substances WEEE: Waste Electrical and Electronic Equipment Copyright Evans Analytical Group

TXRF: Total Reflection XRF TXRF is a non-destructive, elemental survey technique which can measure wafers up to 300 mm. Copyright Evans Analytical Group

TXRF Instrumentation Overall Beamline Layout Monochromator Si(Li) Detector EDS Scintillation Detector X-Ray Source Slit Slit Wafer Chuck/ Goniometer Slit Incident Angle << Critical Angle for reflection TXRF Spectra Fluorescent X-Rays To EDS detector Monitor Beam Reflected X-Rays To SC Detector ~ 0.05 o

X-ray Source Selection Elements quantified by W Lb excitation. 9 kw rotating anode (~ 9.67 kev) Elements quantified by Mo Ka excitation. 2 kw tube, Mo target (~ 17.4 kev)

Example of TXRF: Si Wafers Location specific Spectrum With tabulated concentrations Copyright Evans Analytical Group

Typical Detection Limits Interference free, practical detection limits (10 10 atoms/cm 2 ) on Silicon* Element DL Element DL S 50 Fe 0.3 Cl 20 Ni 0.3 K 10 Cu 0.3 Ca 10 Zn 0.8 Ti 2 As 3 V 2 Rh 20 Cr 0.7 Sb 20 Mn 0.6 Ta 3 W 3 * In some cases, spectral interferences prevent detection at low levels. Copyright Evans Analytical Group

Example of TXRF: Si Wafers Surface Metal Contamination: Control vs. Implanted wafers Si W Si W Cl S Cl Ar Fe S Ar Fe Control Wafer Implant Wafer Table 1. TREX 630-T TXRF Results (Units of 1e10 atoms/cm2) S Cl K Ca Ti Cr Mn Fe Ni Cu Zn Control Wafer Center 125±11 118±9 <10 <10 <1.1 <0.6 <0.5 0.4±0.2 <0.3 <0.3 <0.4 Implant Wafer Center 270±19 390±20 <10 <10 <0.9 1.5±0.3 <0.4 6±0.5 <0.3 <0.5 0.7±0.2 Copyright Evans Analytical Group

TXRF Mapping 2D Elemental Mapping capabilities: Determination metal contamination distribution across the wafer Best suited for gross contamination, such as particles Common usage for wafer backside / handling contamination Cu Mapping Cu < 3e9 at/cm 2 Cu= 2e13 at/cm 2 VPD-ICPMS would yield a single value of 1e11 at/cm 2 Cu for the whole wafer with no distribution information. Copyright Evans Analytical Group

TXRF at KARA https://kara.kaist.ac.kr

TXRF Strengths and Limitations Strengths Elemental survey analysis Quantitative, High Sensitivity Non-destructive Fast/Automated analysis Full wafers (up to 300 mm) 2D Mapping capable (sensitive to single particles) Limitations Cannot detect low-z elements (Li, Na, K, Al) No depth distribution 10 mm lateral spot size Polished surface required for best sensitivity