X-Ray Photoelectron
Spectroscopy (XPS)
Prof. Paul K. Chu
X-ray Photoelectron Spectroscopy
Introduction
Qualitative analysis
Quantitative analysis
Charging compensation
Small area analysis and XPS imaging
Instrumentation
Depth profiling
Application examples
Photoelectric Effect
Einstein, Nobel Prize 1921
Photoemission as an analytical
tool
Kai Siegbahn, Nobel Prize 1981
XPS is a widely used surface analysis technique because of its
relative simplicity in use and data interpretation.
KE = hn - BE - FSPECT
BE = hn - KE - FSPECT
hu: Al Ka(1486.6eV)
P 2s
P 2p1/2-3/2
Kinetic Energy
Peak Notations
L-S Coupling ( j
e-
=
l
s= 12
j=
l + 12
s)
s=
1
2
j=
l
1
2
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.
Spin orbital splitting and peak area
ratios assist in elemental identification
Au
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
(1) Check spin orbital splitting and area ratios for p, d, f peaks
A marine sediment sample from Victoria Harbor
Si 2s
Si 2p
Al 2s
Al 2p
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
Analysis Depth
Inelastic mean free path () is the mean distance that
an electron travels without energy loss

3
0


0
For XPS,  is in the range of 0.5 to 3.5 nm
-
x
-
x
e  dx
e  dx
1 - e -3

 0.95
1
Only the photoelectrons in the near surface region can
escape the sample surface with identifiable energy
Measures top 3 or 5-10 nm
B.E. = Energy of Final state - Energy of initial state
B
A
+
B
(one additional
+ve charge)
Redistribution of
electron density
B
A
B
B.E. provides information on chemical environment
Example of Chemical Shift
Example of Chemical Shift
Chemical Shifts
Chemical Shifts
Factors Affecting Photoelectron Intensities
For a homogenous sample, the measured photoelectron intensity is given by
Ii,c  f  Ni  i,c    cos  F  T  D  A
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
d
i,c: Photoelectric cross-section for core level c of element i
: Inelastic mean free path of the photoelectron in the sample matrix
: 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
Detector
Quantitative Analysis
Peak Area of element A
IA
SA
Atom ic % 
 100%
Ii
i S
i
Sensitivity factor of
element A
Peak Areas / Sensitivity
factors of all other elements
Au 4f
Peak Area measurement
Need background subtraction
Empirical Approach
I A  k S AM A
k = constant
SA = sensitivity factor of a
core level of element A
MA = No. of A in the empirical
formula
I A S AM A
IA MF

 SA  
I F SF M F
IF M A
For example, Teflon (-CF2-)
IC 2
SC  
IF 1
Usually assume SF=1
Examples of Sensitivity Factors
1s
1
SA 
N
Li2CO3
Li2SO4
KBF4
NH4BF4
Na2SO3
CuSO4
K2SO4
Ag(COCF3)3
Na5P3O10
C6H2NS2K3O9
C 1s
S 2p
K 2p
N 1s
S 2p
S 2p
S 2p
F 1s
Na 2s
K 2p
0.067
0.069
0.50
0.55
2.95
3.25
2.90
2.62
3.40
2.89
0.069
0.067
0.50
0.57
2.85
2.81
3.05
N
S
i 1
Ai
N = number of compounds tested
X-ray damage
Some samples can be
damaged by x-rays
For sensitive samples,
repeat the
measurement to check
for x-ray damage.
Charging Compensation
Electron loss and compensation
For metal or other conducting
samples that grounded to the
spectrometer
-
e eX-ray
-
e
e esample
-
Electrons move to the surface
continuously to compensate
the electron loss at the surface
region.
For resistive samples
+
+
+
+
+
+
+
"current" net loss of electrons
from the surface
+
V  RI
-
e
Potential developed Resistance between the
at the surface
surface and the ground
I
R
V
10nA 10nA 10nA
1k 1M 1000M
10-5V 0.01V 10V
Not important
Important
for accurate B.E.
measurements
Note: for conducting
samples,
charging
may also occur if
there is a high
resistance at the back
contact.
Shift in B.E.
of a polymer
surface
Differential (non-uniform) surface charging
B
r
o
a
d
e
n
i
n
g
o
f
p
e
a
k
Sample
Effects of Surface Charging
Charge Compensation Techniques
Low Energy Electron Flood Gun
filament
~2eV-20eV
Electrons
optics
-
e
Electron source
with magnetic field
Low energy
electrons and Ar+
analyser
+
electrons
Low energy Ar beam
-ve
filament
X-ray
e
Low energy
electron beam
Sample
Sample
Magnet
A single setting for all types
of samples
Small area analysis and XPS Imaging
Photoelectrons
Aperture of
Analyzer lens
X-ray
Photoelectrons
Aperture of
Analyzer lens
X-ray
Sample
Spot size determined by the analyser
Both monochromated and dual anode
x-ray sources can be used
Sample
Spot size determined by the x-ray beam
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
Dual Anode X-ray Source
X-ray monochromator
n=2dsin
For Al Ka
  8.3Å
use (1010) planes
of quartz crystal
d = 4.25Å
o
 = 78.5
Advantages of using x-ray monochromator
• Narrow peak width
• Reduced background
• No satellite & ghost peaks
Commonly used
Cylindrical Mirror Analyzer
CMA: Relatively high signal and good resolution ~ 1 eV
Concentric Hemispherical Analyzer (CHA)
Resolution < 0.4 eV
XPS system suitable for industrial samples
Vacuum Chamber
Control Electronics
Ion pump
Turbopump
Sample Introduction Chamber
X-ray induced secondary electron
imaging for precise location of the
analysis area
x-ray
secondary
electrons
+1
+2
500 x 500mm
Depth Profiling
Sputtered
materials
Peak Area
Ar+
Sputtering Time
Depth Scale Calibration
1. Sputtering rate determined from the time required to sputter
through a layer of the same material of known thickness
Peak Area
Concentration
2. After the sputtering analysis, the crater depth is measured using
depth profilometry and a constant sputtering rate is assumed
Sputtering Time
Depth
Angle Resolved XPS
Plasma Treated Polystyrene
Angle-Resolved
XPS Analysis
High-resolution
C 1s spectra
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
Angle-resolved
XPS analysis
Oxide on silicon
nitride surface
Typical Applications
Silicon Wafer Discoloration
Depth Profiling Architectural Glass Coating
• Architectural glass coating
• ~100nm thick coating
Sputtered crater
Sample platen 75 X 75mm
Depth profile of Architectural Glass Coating
100
80
O 1s
O 1s
O 1s
60
Ti 2p
40
Si 2p
Ti 2p
Nb 3d
N 1s
Si 2p
N 1s
20
Al 2p
0
0 Surface
Sputter Depth (nm)
200
Nickel (30.3 nm)
Chromium (31.7 nm)
Chromium Oxide (31.6 nm)
Nickel (29.9 nm)
Chromium (30.1 nm)
Depth profiling
of a multilayer
structure
Silicon (substrate)
100
80
60
40
Ni 2p
Cr 2p metal
Cr 2p oxide
Ni 2p
Cr 2p metal
Si 2p
O 1s
20
0
0
Sputter Depth (nm)
185
Depth Profiling with Sample Rotation
High energy ions
100
Ions: 4 keV
Sample still
80
Cr 2p
60 Ni 2p
Ni 2p
Atomic concentration (%)
40
Cr 2p
Si 2p
Sample
Cr/Si interface width (80/20%) = 23.5nm
O 1s
20
00
100
185
Si 2p
80
60 Ni 2p
Ions: 4 keV
With Zalar rotation
Ni 2p
Cr 2p
40
Cr 2p
O 1s
High energy ions
Sample rotates
20
00
185
Cr/Si interface width (80/20%) = 11.5nm
100
60 Ni 2p
40
Ions: 500 eV
With Zalar rotation
Si 2p
80
Cr 2p
Ni 2p
Cr 2p
Low energy ions
O 1s
20
00
Sample rotates
Sputtering depth (nm)
185
Cr/Si interface width (80/20%) = 8.5nm
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
Photograph of cross-section
Polymer coating ‘A’
10µm x-ray beam
30 minutes
Al
foil
-Si 2s
-Si 2p
++ +
1000
Binding Energy (eV)
0
1072 X 812µm
Polymer ‘A’ / Al foil Interface
10µm x-ray beam
30 minutes
1000
Binding Energy (eV)
Polymer coating ‘B’
10µm x-ray beam
30 minutes
0 1000
Binding Energy (eV)
0
Polymer coating ‘A’
10µm x-ray beam
11.7eV pass energy
30 minutes
Photograph (1072 X 812um)
C 1s
Al foil
Interface
C
H
CCl
O=C-O
298
288
Binding Energy (eV)
Atomic Concentration (%)
Area
A
Interface
B
C
82.6
83.2
85.9
O
12.2
12.2
9.8
N Si
---- 0.7
---- 1.3
4.3 ----
++ +
278
Polymer coating ‘B’
10µm x-ray beam
11.7eV pass energy
30 minutes
C 1s
CH
CNO
O=C-O
A silicon (Si) rich layer is present at
the interface
298
288
Binding Energy (eV)
278
XPS study of paint
Paint Cross Section
Polyethylene
Substrate
Mapping Area
Adhesion Layer
Base Coat
Clear Coat
695 x 320µm
1072 x 812mm
Elemental ESCA Maps using C 1s,
O 1s, Cl 2p, and Si 2p signals
C
695 x 320mm
O
Cl
Si
C 1s Chemical State Maps
C 1s
695 x 320mm
CH
CHCl
O=C-O
Small Area Spectroscopy
High resolution C 1s spectra from each layer
Polyethylene Substrate
Base Coat
Polyethylene Substrate
CHn
CHn
CN
C-O
O-C=O
Adhesion Layer
300
280
300
Clear Coat
Adhesion Layer
Base Coat
CHn
280
CHn
C-O
Clear Coat
O-C=O
CHCl
300
Binding Energy (eV)
300
280
800 x 500µm
CN
280
Binding Energy (eV)
Quantitative Analysis
Atomic Concentration* (%)
Analysis Area
Substrate
Adhesion Layer
Base Coat
Clear Coat
*excluding H
C
O
N
Cl
Si
Al
100.0
90.0
72.0
70.6
----16.4
22.2
----3.5
7.2
--10.0
3.3
---
----2.6
---
----2.2
---
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
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?