x-ray photoelectron spectroscopy: theory and practicephysicsweb.phy.uic.edu/581/xps lecture fall...

University of Illinois at Chicago RRC - Electron Microscopy Service

X-Ray Photoelectron Spectroscopy: Theory and Practice

PHYS-481 (Fall 2014)


Contact Information for EMS in RRC-East

Alan Nicholls, PhD Director of Research Service Facility - Electron Microscopy Research Resources Center-East 845 West Taylor Street SES Building, Room 110 Email: [email protected] Office: (312) 996-1227

Tad Daniel, PhD Senior Research Specialist - Electron Microscopy Research Resources Center-East 845 West Taylor Street SES Building, Room 112 Email: [email protected] Office: (312) 355-2087


Outline of Lecture

(1) Background

(2) Vacuum 101

(3) Analytical Capabilities

(4)  Instrumentation

(5) Spectrum Simulation

(6) Summary


Background

Photoelectric effect discovered by Albert Einstein

Nobel Prize

1921

Photoemission as an analytical tool demonstrated by Kai Siegbahn (Electron Spectroscopy for Chemical Analysis – ESCA)

Nobel Prize

1981


specimen

Background


Background


Quantum Numbers Spectroscopist Notation n l s j nlj 1 0 ± 1/2 1/2 1s1/2 2 0 ± 1/2 1/2 2s1/2 2 1 + 1/2 3/2 2p3/2 2 1 - 1/2 1/2 2p1/2 3 0 ± 1/2 1/2 3s1/2 3 1 + 1/2 3/2 3p3/2 3 1 - 1/2 1/2 3p1/2 3 2 + 1/2 5/2 3d5/2 3 2 - 1/2 3/2 3d3/2

j = |l ± s| j: Total angular momentum l: Orbital angular momentum s: Spin angular momentum

Background


XPS probes core-levels → Binding energies in the range of 10 – 103 eV → Kinetic energies of similar magnitudes when Al-Kα or Mg- Kα radiation is used → Electrons with such low KE easily scattered (REMEMBER THIS)

Background


1000

10

1

IMFP

, λ (n

m)

100

Kinetic Energy (eV)

‘Universal’ IMFP vs. KE Curve

Background

‘Universal Curve’ shows that photoelectrons with KE in the 10 – 103 eV range have inelastic mean-free-paths (IMFPs) from 1 – 3.5 nm IMFP depends on: (1) Material (atomic #, density) (2) Kinetic energy


95% of all photoelectrons detected are generated within 3λ of the surface = Sampling Depth (65% within 1λ). ‘3λ’ is used as the ‘benchmark’ definition for Sampling Depth in XPS. So the sampling depth for XPS is typically 3 - 10 nm → Surface-Sensitive! Instrument must be run under ultra-high vacuum!

Background


VISCOUS FLOW - When diameter of tube > 100λ, gas molecules more likely to bump into each other. Molecules in general move towards lower pressure end of tube. Unlikely to get backstreaming.

MOLECULAR FLOW - When diameter of tube is < λ, gas molecules are more likely to collide with the tube wall than each other. There is free movement of molecules in either direction, the numbers directly related to ratio of pressures at each end of tube. At high vacuum this ratio is likely to be close to 1. Backstreaming a concern.

TRANSITIONAL FLOW - Intermediate between Viscous and Molecular.

Pressure (Torr)

Average distance gas molecules between

collisions λ

Time for formation of a monolayer of gas

Number of gas molecules per litre

Atmospheric Pressure 760 0.000066 3.3ns 2.5x1022

1 0.066 3.3 µsec 2.5x1019

10-1 660 µm 33 µsec 2.5x1018 Medium Vacuum 10-3 66 mm 3.3 msec 2.5x1016

High Vacuum 10-6 66 m 3.3 sec 2.5x1013

10-9 66 km 55 mins 2.5x1010 Ultra-high Vacuum 10-10 660 km 550 mins 2.5x109

Vacuum 101


So for an 60mm diameter tube

VISCOUS > 0.1 Torr > TRANSITIONAL > 1 mTorr > MOLECULAR

PUMPS •  Used from atmosphere down to 0.1Pa

•  Problems with corrosive of condensable gases (H2O)

•  Potential source of oil contamination of vacuum system if pressure in line to system is not kept in viscous flow regime.

•  At or near atmospheric pressure an oil mist is ejected through outlet valve. Must vent outside or through a filter.

•  Used from 1Pa to 10-7 Pa

•  Historically most widely used high vacuum pump, really a vapor jet pump.

•  Pumping speed virtually constant below 10-1 Pa

•  Major problem - Backstreaming; minimised by using a low vapour pressure oil.

•  BAD NEWS - do not let air into a diffusion pump!

•  Used from 10-1 Pa to 10-9 Pa

•  Gas molecules that are pumped are trapped inside pump by the gettering action of the sputtered Ti - limited lifetime.

•  Absolute freedom from oil contamination with no moving parts.

•  Ideal for high vacuum systems but are not well suited on systems that are cycled frequently to atmosphere.

•  Used from 10 Pa to 10-8 Pa

•  Extremely high speed (10,000rpm) mechanical pumps typically with magnetic levitation bearings for EM use.

•  Works efficiently in Molecular Flow region - needs to be backed.

•  No backstreaming of oil when operating at full speed.

•  Major concern - preventing physical damage to pump!

ROTARY DIFFUSION ION PUMP Turbo Molecular Pump OTHER PUMPS

•  Cryosorption - oil free, capture, Atmosphere to 10-1 Pa

•  Diaphragm - oil free, transfer, Atmosphere to 1Pa

•  Claw pump - dry, transfer, Atmosphere to 10Pa

•  Molecular Drag - dry, transfer, 10Pa to 10-6 Pa

•  Sublimation - oil free, capture, 10-1 Pa to 10-9 Pa

REMEMBER -- No Pump exerts a force that drags or pulls gas molecules to it. Pumping is purely diffusion of gas molecules from high pressure to low pressure regions

Vacuum 101


Analytical Capabilities of XPS

(1)  Identify elements/compounds (except H and He)

(2) Determine oxidation states (e.g. Ti3+ or Ti4+)

(3)  Identify types of chemical bonds (e.g. Si-O or Si-C)

(4) Semi-quantitative analysis (10-15% error)

(5) Determine adsorbate/film thickness

(6) Highly surface-sensitive (3 – 10 nm from the surface) → Detection limit 0.1 to 1 at% → Ultra-high vacuum required!!! → Minimize/delay surface reactions and contaminations


KINETIC ENERGY, eV


Survey spectrum for element identification


Nickel



XPS spectra show characteristic "stepped" background. Due to inelastic processes (extrinsic losses) from deep in bulk. Electrons deeper in surface loose energy and emerge with reduced KE, apparent background increase at higher BE



Typical Features and ‘Artifacts’ of Core-Level Peaks Can get multiple peaks from core levels – must be aware of where they come from in order to carry out chemical analysis (not all may be present) 1.  Spin orbit splitting leads to additional peaks (no splitting for s, splitting for

p,d,f etc.) 2.  Additional peaks due to, for example, chemical shifts and oxidation

states 3.  Ghost peaks at lower binding energies (achromatic X-ray only) – no

useful info! 4.  Shake up/ off peaks at higher binding energies (result of energy being

transferred from the ejected photoelectron electron to a valence electron).

5.  Plasmon loss peaks (due to electron excitations) 6.  Photon-induced Auger peaks 7.  Effects of charging of non conductive specimens



537.0 534.0 531.0 528.0 525.0 Binding Energy (eV)

O 1s

No spin-orbital splitting for s Spin-orbital splitting for p, d, f



Ti 2p1/2 and 2p3/2 chemical shift for Ti and Ti4+. Charge withdrawn Ti → Ti4+ so 2p orbital relaxes to higher BE



Schematic of Ghost and Shake-up peaks

0

100

200

300

400

500

600

293.8

293.4

293

292.6

292.2

291.8

291.4

291

290.6

290.2

289.8

289.4

289

288.6

288.2

287.8

287.4

287

286.6

286.2

285.8

285.4

285

284.6

284.2

283.8

283.4

283

282.6

282.2

281.8

281.4

281

280.6

280.2

279.8

279.4

279

278.6

278.2

277.8

277.4

277

276.6

276.2

MonochromaticAchromatic

Binding Energy Kinetic Energy

Main peak

Shake-up Peak

Ghost Peak



Electrical insulators cannot dissipate charge generated by photoemission Process. Surface picks up excess positive charge - all peaks shift to higher BE Can be reduced by exposing surface to neutralizing flux of low energy electrons - "flood gun" or "neutralizer“. BUT must have good reference peak.



Is = Io exp (-d / λcosθ) Is: Intensity at surface Io: Intensity from infinitely-thick sample d: depth λ: Inelastic mean-free-path (IMFP) θ: Spectrometer take-off angle

Beer-Lambert relationship (Numerical expression Describing the photoelectron Intensity generated from a material)



⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−−= ∞

FilmFilmFilm

dIIλ

exp1⎟⎟⎠

⎞⎜⎜⎝

⎛−= ∞

FilmSubstrateSubstrateSubstrate

dII,

expλ

SiO2 Surface Layer

Si Substrate


Using the Beer-Lambert expression to estimate film thickness…


ISi: 1411.6

nm 0.58

1lncos 2

22

=

⎪⎭

⎪⎬

⎫

⎜⎜⎝

⎛+⎟⎟⎠

⎞

⎪⎩

⎪⎨

⎧

⎜⎜⎜

⎝

⎛

⎟⎟

⎠

⎞=

∞

∞

oxide

Si

SiO

SiO

SiSiOoxide

d

II

IId θλ

ISiO2: 241.4

When λ, θ and all the respective intensities are known, film thickness can be determined by taking the ratio of Ifilm to Isubstrate and solve for d



Semi-Quantitative Analysis Photoelectron intensity from a homogeneous material is also dependent on instrumental factors, and can be alternatively-described by

I = JCσζTλ J: X-ray flux C: Concentration of the element-of-interest σ: Ionization cross-section ζ: Spectrometer angular acceptance T: Spectrometer transmission function λ: IMFP of the element



C = I/(JσζTλ) = I/JF F is termed the Relative Sensitivity Factor: - incorporates all the terms associated with the spectrometer and material -  empirically-determined by XPS manufacturer

-  values are normalized against Fluorine



Atomic fraction of an element (A) in a multi-component material (ABCD…) can be estimated using the following,

Atomic % A = (IA/FA) / Σ (In/Fn) Quantification using this expression is valid only if: (1)  Material is homogeneous, (2)  Material surface is smooth and flat.



Ion Gun

Charge Neutralizer (built into lenses)

Instrumentation


Instrumentation

Kratos Axis-165 XPS system in RRC-East


Instrumentation

Total resolution (i.e. peak’s full-width-half-maximum) of instrument is convolution of: (1) X-ray energy spread, (2) Spectrometer broadening, and (3) Intrinsic line-width of the element-of-interest.

Total FWHM = {FWHMx-ray2 + FWHMspectrometer

2 + FWHMintrinsic2}1/2

Inte

nsity

Energy

50 %

100 %

Total FWHM


Instrumentation

Binding energy shifts due to different chemical states or bonding configurations can be subtle (1eV or less) for certain elements. Say, if achievable total FWHM of a peak is 2 eV, the XPS instrument will not be able to resolve 2 peaks that are separated only by 0.5eV!!! Total FWHM can only be decreased by minimizing the FWHM of: (1) X-ray; (2) Spectrometer * Intrinsic line-widths is a non-controllable term!


Ideal Candidates for X-ray source: Al- and Mg-Kα (1) High energy

(2) Narrow energy spread

Instrumentation


Instrumentation

Twin-Anode Achromatic X-ray source: Bombard metallic anode with 10-25kV electrons with ~10mA of current to generate X-rays. Can generate high X-ray flux producing high signal BUT specimen may be damaged by heat generated by the X-rays and continuum radiation and source emits X-ray satellites (additional weak lines at lower binding energies) Simple, relatively inexpensive High flux (1010-1012 photons·s-1) Beam size ~ 1cm


Instrumentation

Rowland Circle

Quartz Crystal

X-ray Source

Sample

Electron Spectrometer

Monochromatic X-Ray source: Diffraction from bent SiO2 crystal focusing primary λ at specimen. Other λ 's focused at different points in space (filtered). Always use Al Kα which is diffracted from quartz (no equivalent crystal for Mg Kα)

Beam size ~ 1 cm to 50 mm

Eliminates satellites peaks – simpler spectra

Decreases FWHM of X-ray energy

Flux decreases at least an order of magnitude leading to less damage, improves S/N (no X-ray continuum) but lower signal More complicated and expensive


Instrumentation

Most common type of electrostatic deflection-type analyzer: Concentric Hemispherical Analyzer (CHA) or spherical sector analyzer Energy resolution dependant on radius. Capable of collecting photoelectrons of larger angular distribution. Photoelectrons of a specific energy are focused by the lens at the slit of the spectrometer. Lens also controls sampling area. Photoelectrons travel through a circular path and exit into a series of channeltrons (electron multipliers).


Instrumentation

- Hollow glass tube with semiconducting layer on inner surface

- Electrons/ions entering Channeltron produce secondary electrons (SEs)

- ‘Avalanche’ effect as SEs accelerate down the tube under HV

- Signal amplified by ~ 106 to 108

Single-Channel Electron Multipliers (Channeltron)


Instrumentation

Negative potential on two hemispheres V2 > V1

Potential of mean path, Ro through analyzer is

Vo = (V1R1+V2R2)/2Ro

An electron of kinetic energy eV = Vo will travel a circular orbit through hemispheres at radius Ro Since Ro, R1 and R2 are fixed, in principle changing V1 and V2 will allow electrons of different KE to be detected.


Instrumentation

But ΔE/Eo = S/2Ro

ΔE = (S/2Ro)Eo

Peak FWHM Spectrometer

Term (constant)

Photoelectron KE

Spectrometer broadening (i.e. FWHMspectrometer) is a function of photoelectron KE entering spectrometer…

i.e. Resolution is non-uniform across XPS energy spectrum!


To circumvent the problem… (1) Hemisphere potentials are fixed to allow electrons with a fixed KE (the pass energy, Ep) to reach the electron detectors (Channeltrons); (2) Electrostatic lens before the slit decelerate photoelectrons of a particular KE to Ep; (3) Magnetic lens focus these electrons with Ep at the slit, so that only electrons with that pass energy are allowed to enter the slit. Scanning of the energy-scale is achieved by varying the decelerating potential on the electrostatic lens instead of the hemisphere potentials → Fixed energy-resolution across the energy-scale!!!

Instrumentation


Instrumentation

Most XPS systems also come equipped with an ion gun. Purpose: (1)  Removes surface contaminants (usually oxides and hydrocarbons);

(2)  Allows depth-profiling study. How does it ‘clean’ a surface? (1)  Ionizes an inert gas (usually Ar)

(2)  Focuses and accelerates the ions towards the specimen surface

(3)  Rasters the ion beam across the surface

(4)  Ions impart energy to surface contaminants → Sputtering them away!


Instrumentation

Atomic Fraction

of Cd

Atomic Fraction

of Te

Cd: Te Ratio

As-received 0.65 0.35 1.86

10 mins Sputter

0.51 0.49 1.04

20 mins Sputter

0.55 0.45 1.22

Example: Successful removal of Cr-contamination from CdTe


Instrumentation

Potential Problems Associated with Sputter-Cleaning: •  Surface roughening;

•  Alter oxidation states of some elements;

•  Selective ‘etching’ of surface due to different sputtering rates in a multi-component material.


Instrumentation

Example: Oxidation-state changed in Pd after sputtering…

100

150

200

250

300

350

400

450

500

326328330332334336338340342344346348350352

Binding Energy (eV)

Inte

nsity

(CP

S)

Pd 3d (As-received)

Pd 3d (10 min Sputter)

Elemental Pd 3d5/2

PdO 3d5/2

Pd 3d3/2 Orbitals


Spectrum Simulation (Peak-Fitting)

Simulation of XPS spectra (i.e. peak fitting) to match experimentally-observed spectra. Purpose: (1)  Background noise subtraction to reveal true peak intensities;

(2)  Precise determination of peak position;

(3)  Deconvolute spectra into individual components when 2 or more peaks are in close proximity.


Select Background

Model

Adding Synthetic Peak

Providing Initial-Guess For Peak Parameters: (1)  Line position (2)  Area (3)  FWHM (4)  Gaussian-Lorentzian Ratio (5)  Asymetry (6)  S.O.S.

Software Fitting

End of

Fitting

Modify Peak Parameters

Background Quality?

Good

Poor

Fit Quality?

Good

Poor

Option 1

Option 2



Types of Background: (1)  Linear (2)  Shirley (typically-used) (3)  Tougaard

Shirley Tougaard

Linear



Types of Line-Shapes: (1)  Gaussian Function

→ Describes the measurement process

(e.g. instrumental response, X-ray line-shape, Doppler and thermal broadening)

(2)  Lorentzian Function

→ Describes lifetime broadening (intrinsic line-width)

XPS peaks can usually be described by varying G-L ratio



Peaks associated with pure metals may tend to be asymmetic… → Need to introduce a ‘Tail Modifier’ term to the G-L functions



XPSPEAK version 4.1 is a free windows-based XPS peak fitting program available online. Installation file and user manual downloadable at: http://www.phy.cuhk.edu.hk/~surface/XPSPEAK

You will need this software to complete the XPS assignments!



Summary

X-ray Photon Spectroscopy (XPS) (Originally called ESCA) is a surface-sensitive technique that probes the chemical properties of the top ~10 nm of a solid surface that must be UHV-compatible – no wet specimens! It is the most versatile and quantifiable of all the surface chemical analysis methods: elemental ID, oxidation-states, chemical bonds, film thickness, depth-profiling, semi-quantitative analysis… Limitations: (1) does not detect H or He; (2) Radiation damage possible (worse for achromatic sources); (3) Charge neutralization needed for insulating material; (4) Chemical analysis can be limited to functional groups and in some cases chemical shifts are not resolvable.


Electron Microscopy Service @ UIC

JEM-1220 JEM-3010 JEM-2010F HB601UX JSM-6320F

S-3000N AXIS-165 XPS Ramascope 2000 VT-SPM

x-ray photoelectron spectroscopy: theory and practicephysicsweb.phy.uic.edu/581/xps lecture fall...

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