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X-Ray Photoelectron Spectroscopy (XPS)
Prof. Paul K. Chu
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X-ray Photoelectron Spectroscopy
Introduction Qualitative analysis Quantitative analysis Charging compensation Small area analysis and XPS imaging Instrumentation Depth profiling Application examples
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Photoelectric Effect Einstein, Nobel Prize 1921
Photoemission as an analytical tool Kai Siegbahn, Nobel Prize 1981
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XPS is a widely used surface analysis technique because of its relative simplicity in use and data interpretation.
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KE = hn - BE - FSPECT BE = hn - KE - FSPECT
hu: Al Ka(1486.6eV) P 2s P 2p1/2-3/2 Kinetic Energy
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Peak Notations
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For p, d and f peaks, two peaks are observed.
The separation between the two peaks are named spin orbital splitting. The values of spin orbital splitting of a core level of an element in different compounds are nearly the same. The peak area ratios of a core level of an element in different compounds are also nearly the same. Au Spin orbital splitting and peak area ratios assist in elemental identification
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Relative binding energies and ionization cross-section for U
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General methods in assisting peak identification
(1) Check peak positions and relative peak intensities of 2 or more peaks (photoemission lines and Auger lines) of an element Check spin orbital splitting and area ratios for p, d, f peaks A marine sediment sample from Victoria Harbor Si 2p Si 2s Al 2s The following elements are found: O, C, Cl, Si, F, N, S, Al, Na, Fe, K, Cu, Mn, Ca, Cr, Ni, Sn, Zn, Ti, Pb, V Al 2p
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Analysis Depth Measures top 3 or 5-10 nm
Inelastic mean free path (l) is the mean distance that an electron travels without energy loss For XPS, is in the range of 0.5 to 3.5 nm Only the photoelectrons in the near surface region can escape the sample surface with identifiable energy Measures top 3 or 5-10 nm
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B.E. provides information on chemical environment
Redistribution of electron density B.E. provides information on chemical environment
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Example of Chemical Shift
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Example of Chemical Shift
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Chemical Shifts
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Chemical Shifts
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Factors Affecting Photoelectron Intensities
For a homogenous sample, the measured photoelectron intensity is given by Ii,c: Photoelectron intensity for core level c of element i f: X-ray flux in photons per unit area per unit time Ni: Number of atoms of element i per unit volume si,c: Photoelectric cross-section for core level c of element i l: Inelastic mean free path of the photoelectron in the sample matrix q: Angle between the direction of photoelectron electron and the sample normal F: Analyzer solid angle of acceptance T: Analyzer transmission function D: Detector efficiency A: Area of sample from which photoelectrons are detected
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Need background subtraction
Quantitative Analysis Peak Area of element A Sensitivity factor of element A Peak Areas / Sensitivity factors of all other elements Au 4f Peak Area measurement Need background subtraction
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Empirical Approach Usually assume SF=1 For example, Teflon (-CF2-)
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Examples of Sensitivity Factors
N = number of compounds tested
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Quantification Uncertainties
An error of 15% is generally quoted If 20% of Cu is calculated, the Cu concentration should be in the range of 17-23%. An error of 1-2% can be achieved if samples with known concentrations are used as standards e. g., determination of the concentrations of Si and N in SiNx films with a Si3N4 standard.
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X-ray damage Some samples can be damaged by x-rays
For sensitive samples, repeat the measurement to check for x-ray damage.
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Charging Compensation
Electron loss and compensation
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Note: for conducting samples, charging may also occur if there is a high resistance at the back contact.
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Shift in B.E. of a polymer surface
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Differential (non-uniform) surface charging
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Effects of Surface Charging
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Charge Compensation Techniques Low Energy Electron Flood Gun
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Electron source with magnetic field Low energy electrons and Ar+
A single setting for all types of samples
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Small area analysis and XPS Imaging
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Instrumentation Electron energy analyzer X-ray source Ar ion gun
Neutralizer Vacuum system Electronic controls Computer system Ultrahigh vacuum < 10-9 Torr (< 10-7 Pa) Detection of electrons Avoid surface reactions/ contamination
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Dual Anode X-ray Source
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X-ray monochromator Advantages of using x-ray monochromator
Narrow peak width Reduced background No satellite & ghost peaks
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Commonly used
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Cylindrical Mirror Analyzer
CMA: Relatively high signal and good resolution ~ 1 eV
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Concentric Hemispherical Analyzer (CHA)
Resolution < 0.4 eV
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XPS system suitable for industrial samples
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Sample Introduction Chamber
Vacuum Chamber Control Electronics Ion pump Turbopump Sample Introduction Chamber
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X-ray induced secondary electron imaging for precise location of the analysis area
secondary electrons + 1 + 2 500 x 500mm
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Focused X-ray Source Ellipsoidal Monochromator Scanning Focused
Electron Beam Scanning Focused X-ray Beam Analyzer Input Lens Aluminum Anode Sample
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Depth Profiling Sputtered materials Peak Area Sputtering Time
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Depth Scale Calibration
Sputtering rate determined from the time required to sputter through a layer of the same material of known thickness After the sputtering analysis, the crater depth is measured using depth profilometry and a constant sputtering rate is assumed Peak Area Concentration Sputtering Time Depth
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Angle Resolved XPS
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Plasma Treated Polystyrene
Angle-Resolved XPS Analysis High-resolution C 1s spectra
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Plasma Treated Polystyrene
O concentration is higher near the surface (10 degrees take off angle) C is bonded to oxygen in many forms near the surface (10 degrees take off angle) Plasma reactions are confined to the surface
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Angle-resolved XPS analysis Oxide on silicon nitride surface
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Typical Applications
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Silicon Wafer Discoloration
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Depth Profiling Architectural Glass Coating
~100nm thick coating Sputtered crater Sample platen 75 X 75mm
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Depth profile of Architectural Glass Coating
100 80 O 1s O 1s O 1s 60 Ti 2p Atomic Concentration (%) 40 Nb 3d N 1s Si 2p Ti 2p Si 2p 20 N 1s Al 2p Surface 200 Sputter Depth (nm)
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Depth profiling of a multilayer structure
Nickel (30.3 nm) Depth profiling of a multilayer structure Chromium (31.7 nm) Chromium Oxide (31.6 nm) Nickel (29.9 nm) Chromium (30.1 nm) Silicon (substrate) 185 20 40 60 80 100 Sputter Depth (nm) Atomic Concentration (%) Cr 2p oxide Cr 2p metal Ni 2p O 1s Si 2p
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Depth Profiling with Sample Rotation
Ions: 4 keV Sample still Cr/Si interface width (80/20%) = 23.5nm Ions: 4 keV With Zalar rotation Cr/Si interface width (80/20%) = 11.5nm Ions: 500 eV With Zalar rotation Cr/Si interface width (80/20%) = 8.5nm
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encapsulated drug tablets SPS Photograph Cross-section of Drug Package
Multi-layered Drug Package Optical photograph of encapsulated drug tablets SPS Photograph Cross-section of Drug Package Al foil Polymer Coating ‘A’ Polymer Coating ‘B’ Adhesion layer at interface ? 100 X 100mm 1072 X 812µm
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Photograph of cross-section Polymer coating ‘A’
10µm x-ray beam 30 minutes -C 1s Al foil -O 1s -O KLL -Cl 2p + + + -Si 2s -Si 2p 1000 Binding Energy (eV) 1072 X 812µm Polymer ‘A’ / Al foil Interface Polymer coating ‘B’ 10µm x-ray beam 30 minutes 10µm x-ray beam 30 minutes -C 1s -C 1s -O 1s -O 1s -O KLL -Al 2s -Al 2p -O KLL -N 1s -Cl 2p -Si 2s -Si 2p 1000 Binding Energy (eV) 1000 Binding Energy (eV)
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A silicon (Si) rich layer is present at the interface
Binding Energy (eV) 278 288 298 C 1s Polymer coating ‘A’ CH CCl O=C-O 10µm x-ray beam 11.7eV pass energy 30 minutes Photograph (1072 X 812um) Al foil Interface + + + 278 288 298 Binding Energy (eV) Polymer coating ‘B’ Atomic Concentration (%) Area C O N Si A Interface B A silicon (Si) rich layer is present at the interface 10µm x-ray beam 11.7eV pass energy 30 minutes C 1s CH CNO O=C-O
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XPS study of paint Paint Cross Section 1072 x 812mm Polyethylene
Mapping Area Substrate Adhesion Layer Base Coat Clear Coat 695 x 320µm 1072 x 812mm
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Elemental ESCA Maps using C 1s, O 1s, Cl 2p, and Si 2p signals
695 x 320mm
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C 1s Chemical State Maps C 1s 695 x 320mm
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Small Area Spectroscopy High resolution C 1s spectra from each layer
Polyethylene Substrate Base Coat Polyethylene Substrate CHn CHn CN Adhesion Layer C-O O-C=O 300 280 300 280 Clear Coat Adhesion Layer Base Coat CHn CHn C-O Clear Coat CN O-C=O CHCl 300 280 300 280 Binding Energy (eV) Binding Energy (eV) 800 x 500µm
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Quantitative Analysis
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Summary of XPS Capabilities
Elemental analysis Chemical state information Quantification (sensitivity about 0.1 atomic %) Small area analysis (5 mm spatial resolution) Chemical mapping Depth profiling Ultrathin layer thickness Suitable for insulating samples
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Sample Tutorial Questions
What is the mechanism of XPS? What are chemical shifts? How is depth profiling performed? What is angle-resolved XPS? Is XPS a small-area or large-area analytical technique compared to AES? Is XPS suitable for insulators? What kind of applications are most suitable for XPS?
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