1. X-Ray Photoelectron Spectroscopy (XPS)
Prof. Paul K. Chu

2. X-ray Photoelectron Spectroscopy
Introduction
Qualitative analysis
Quantitative analysis
Charging compensation
Small area analysis and XPS imaging
Instrumentation
Depth profiling
Application examples

3. Photoelectric Effect Einstein, Nobel Prize 1921
Photoemission as an analytical tool
Kai Siegbahn, Nobel Prize 1981

5. XPS is a widely used surface analysis technique because of its relative simplicity in use and data interpretation.

6. KE = hn - BE - FSPECT BE = hn - KE - FSPECT
hu: Al Ka(1486.6eV)
P 2s
P 2p1/2-3/2
Kinetic Energy

7. Peak Notations

8. 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

9. Relative binding energies and ionization cross-section for U

10. 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

11. 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

12. B.E. provides information on chemical environment
Redistribution of electron density
B.E. provides information on chemical environment

13. Example of Chemical Shift

14. Example of Chemical Shift

15. Chemical Shifts

16. Chemical Shifts

17. 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

18. 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

19. Empirical Approach
Usually assume SF=1
For example, Teflon (-CF2-)

20. Examples of Sensitivity Factors
N = number of compounds tested

21. 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.

22. X-ray damage Some samples can be damaged by x-rays
For sensitive samples, repeat the measurement to check for x-ray damage.

23. Charging Compensation
Electron loss and compensation

24. Note: for conducting samples, charging may also occur if there is a high resistance at the back contact.

25. Shift in B.E. of a polymer surface

26. Differential (non-uniform) surface charging

27. Effects of Surface Charging

28. Charge Compensation Techniques Low Energy Electron Flood Gun

29. Electron source with magnetic field Low energy electrons and Ar+
A single setting for all types of samples

30. Small area analysis and XPS Imaging

31. 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

32. Dual Anode X-ray Source

33. X-ray monochromator Advantages of using x-ray monochromator
Narrow peak width
Reduced background
No satellite & ghost peaks

34. Commonly used

35. Cylindrical Mirror Analyzer
CMA: Relatively high signal and good resolution ~ 1 eV

36. Concentric Hemispherical Analyzer (CHA)
Resolution < 0.4 eV

38. XPS system suitable for industrial samples

39. Sample Introduction Chamber
Vacuum Chamber
Control Electronics
Ion pump
Turbopump
Sample Introduction Chamber

40. X-ray induced secondary electron imaging for precise location of the analysis area
secondary electrons
+ 1
+ 2
500 x 500mm

41. Focused X-ray Source Ellipsoidal Monochromator Scanning Focused
Electron Beam
Scanning Focused
X-ray Beam
Analyzer Input Lens
Aluminum Anode
Sample

42. Depth Profiling
Sputtered
materials
Peak Area
Sputtering Time

43. 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

44. Angle Resolved XPS

45. Plasma Treated Polystyrene
Angle-Resolved XPS Analysis
High-resolution C 1s spectra

46. 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

47. Angle-resolved XPS analysis Oxide on silicon nitride surface

48. Typical Applications

49. Silicon Wafer Discoloration

50. Depth Profiling Architectural Glass Coating
~100nm thick coating
Sputtered crater
Sample platen 75 X 75mm

51. 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)

52. 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

53. 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

54. 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

55. 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)

56. 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

57. 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

58. Elemental ESCA Maps using C 1s, O 1s, Cl 2p, and Si 2p signals
695 x 320mm

59. C 1s Chemical State Maps
C 1s
695 x 320mm

60. 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

61. Quantitative Analysis

62. 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

63. 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?