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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.
Kinetic Energy
hu: Al Ka(1486.6eV)
P 2s P 2p1/2-3/2
KE = hn - BE - FSPECT BE = hn - KE - FSPECT
Peak Notations
L-S Coupling ( j = l s )
e-s=
12
s=12
12j = l + 1
2j = l
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
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
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
Al 2s
Si 2pSi 2s
Only the photoelectrons in the near surface region can
escape the sample surface with identifiable energy
Measures top 3 or 5-10 nm
95.01
1 30
3
0
-
-
-
-
e
dxe
dxe
x
x
Inelastic mean free path () is the mean distance that
an electron travels without energy loss
Analysis Depth
For XPS, is in the range of 0.5 to 3.5 nm
B.E. = Energy of Final state - Energy of initial state
(one additional+ve charge)
A
A
B
B
B
B+
Redistribution of
electron density
B.E. provides information on chemical environment
Example of Chemical Shift
Example of Chemical Shift
Chemical Shifts
Chemical Shifts
Factors Affecting Photoelectron Intensities
ADTFNfI ciici cos,,
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
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
d
Detector
%100%
i i
i
A
A
S
I
S
I
Atomic
Quantitative Analysis
Peak Area of element A
Sensitivity factor of
element A
Peak Areas / Sensitivity
factors of all other elements
Peak Area measurement
Need background subtraction
Au 4f
Empirical Approach
k = constant S = sensitivity factor of a
core level of element AM = No. of A in the empirical
formula
A
A
AAA MSkI
A
F
F
AA
FF
AA
F
A
M
M
I
IS
MS
MS
I
I
For example, Teflon (-CF2-)
1
2
F
CC
I
IS
Usually assume SF=1
1s Li2CO3 C 1s 0.067 0.069
Li2SO4 S 2p 0.069 0.067
KBF4 K 2p 0.50 0.50
NH4BF4 N 1s 0.55 0.57
Na2SO3 S 2p 2.95
CuSO4 S 2p 3.25
K2SO4 S 2p 2.90 2.85
Ag(COCF3)3 F 1s 2.62 2.81
Na5P3O10 Na 2s 3.40
C6H2NS2K3O9 K 2p 2.89 3.05
Examples of Sensitivity Factors
N = number of compounds tested
N
i
AiA SN
S1
1
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
For metal or other conducting samples that grounded to thespectrometer
Electrons move to the surfacecontinuously to compensate the electron loss at the surfaceregion.
e-
e-e
-
X-ray
sample
e-e
-
Electron loss and compensation
For resistive samples
e-
+ ++ ++ ++ +
V R I
"current" net loss of electrons from the surface
Resistance between the surface and the ground
Potential developed at the surface
I
R
10nA
1k
10nA
1M
10nA
1000M
V 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.
Broadening of peak
Sample
Differential (non-uniform) surface charging
e-
~2eV-20eV
filament
Electronsoptics
Charge Compensation Techniques
Low Energy Electron Flood Gun
Sample
-ve
filament e
analyser
Magnet
X-ray
electrons
Low energyelectron beam
Low energy Ar beam+
Sample
Electron source
with magnetic field
Low energy
electrons and Ar+
A single setting for all types
of samples
Shift in B.E.
of a polymer
surface
Effects of Surface Charging
Sample Sample
Aperture of
Analyzer lensAperture of Analyzer lens
X-ray X-ray
Photoelectrons Photoelectrons
Spot size determined by the x-ray beamSpot size determined by the analyser
Both monochromated and dual anode
x-ray sources can be used
Small area analysis and XPS Imaging
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
XPS system suitable for industrial samples
Vacuum Chamber Control Electronics
Sample Introduction Chamber
Ion pump
Turbopump
Dual Anode X-ray Source
Commonly used
n =2dsin
For Al K
8.3a
Å
use (1010) planesof quartz crystal d = 4.25
= 78.5o
Å
X-ray monochromator
Advantages of using x-ray monochromator
• Narrow peak width
• Reduced background
• No satellite & ghost peaks
Cylindrical Mirror Analyzer
CMA: Relatively high signal and good resolution ~ 1 eV
Concentric Hemispherical Analyzer (CHA)
Resolution < 0.4 eV
500 x 500mm
+ 1
+ 2
X-ray induced secondary electron
imaging for precise location of the
analysis area
x-ray secondary
electrons
Sputtered
materials
Pea
k A
rea
Sputtering Time
Depth Profiling
Ar+
Pea
k A
rea
Sputtering TimeC
on
cen
trat
ion
Depth
Depth Scale Calibration
1. Sputtering rate determined from the time required to sputter
through a layer of the same material of known thickness
2. After the sputtering analysis, the crater depth is measured using
depth profilometry and a constant sputtering rate is assumed
Angle Resolved XPS
Plasma Treated Polystyrene
Angle-Resolved
XPS Analysis
High-resolution
C 1s spectra
• 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
Plasma Treated Polystyrene
Angle-resolved
XPS analysis
Oxide on silicon
nitride surface
Typical Applications
Silicon Wafer Discoloration
Sample platen 75 X 75mm
Sputtered crater
•Architectural glass coating
• ~100nm thick coating
Depth Profiling Architectural Glass Coating
0 2000
20
40
60
80
100
Sputter Depth (nm)
Al 2p
Si 2pNb 3d
N 1s
Ti 2p
O 1s O 1s
O 1s
Si 2pTi 2p
N 1s
Surface
Depth profile of Architectural Glass Coating
Chromium (31.7 nm)
Silicon (substrate)
Nickel (29.9 nm)
Nickel (30.3 nm)
Chromium (30.1 nm)
Chromium Oxide (31.6 nm)
0 1850
20
40
60
80
100
Sputter Depth (nm)
Cr 2p oxideCr 2p metal Ni 2p
O 1s
Si 2pNi 2p Cr 2p metal
Depth profiling
of a multilayer
structure
Cr/Si interface width (80/20%) = 23.5nm
Cr/Si interface width (80/20%) = 11.5nm
Cr/Si interface width (80/20%) = 8.5nm
Ato
mic
co
nc
en
tra
tio
n (
%)
0 1850
20
40
60
80
100
Si 2p
O 1s
0 1850
20
40
60
80
100
Si 2p
O 1s
0 1850
20
40
60
80
100
Cr 2pSi 2pCr 2pNi 2p
O 1s
Ni 2p
Ni 2p Cr 2p Ni 2p Cr 2p
Ni 2pCr 2p Ni 2p Cr 2p
Sputtering depth (nm)
High energy ions
Sample
High energy ions
Sample rotates
Low energy ions
Sample rotates
Ions: 4 keV
Sample still
Ions: 4 keV
With Zalar rotation
Ions: 500 eV
With Zalar rotation
Depth Profiling with Sample Rotation
Optical photograph of
encapsulated drug tablets
100 X 100mm
SPS Photograph
Cross-section of Drug Package
1072 X 812µm
Polymer
Coating ‘A’
Polymer Coating ‘B’
Al foil
Adhesion layer
at interface ?
Multi-layered Drug Package
01000 Binding Energy (eV)
1000 Binding Energy (eV) 0 1000 0Binding Energy (eV)
+ ++
Photograph of cross-section
1072 X 812µm
Polymer coating ‘A’
Al
foil
Polymer coating ‘B’Polymer ‘A’ / Al foil Interface
10µm x-ray beam
30 minutes
10µm x-ray beam
30 minutes
10µm x-ray beam
30 minutes
-Si
2p
-Si
2s
+ ++
278288298Binding Energy (eV)
Polymer coating ‘B’
C 1s
CHCNO
O=C-O
Atomic Concentration (%)
Area C O N Si
A 82.6 12.2 ---- 0.7
Interface 83.2 12.2 ---- 1.3
B 85.9 9.8 4.3 ----
A silicon (Si) rich layer is present at
the interface
Photograph (1072 X 812um)
Al foil
Interface
Binding Energy (eV)
278288298
C 1s
Polymer coating ‘A’
C
HCClO=C-O
10µm x-ray beam
11.7eV pass energy
30 minutes
10µm x-ray beam
11.7eV pass energy
30 minutes
Polyethylene
Substrate
Adhesion Layer
Base Coat
Clear Coat
Mapping Area
695 x 320µm
1072 x 812mm
XPS study of paint
Paint Cross Section
C O Cl Si
695 x 320mm
Elemental ESCA Maps using C 1s,
O 1s, Cl 2p, and Si 2p signals
C 1s CH CHCl O=C-O
695 x 320mm
C 1s Chemical State Maps
Polyethylene Substrate
Adhesion Layer
Base Coat
Clear Coat
800 x 500µm
280300
CHn
Binding Energy (eV)280300
CHn
CHCl
280300
CHn
CN
C-O
O-C=O
280300Binding Energy (eV)
CHn
CN
C-O
O-C=O
Polyethylene Substrate
Adhesion Layer
Base Coat
Clear Coat
Small Area SpectroscopyHigh resolution C 1s spectra from each layer
Atomic Concentration* (%)
Analysis Area C O N Cl Si Al
Substrate 100.0 --- --- --- --- ---
Adhesion Layer 90.0 --- --- 10.0 --- ---
Base Coat 72.0 16.4 3.5 3.3 2.6 2.2
Clear Coat 70.6 22.2 7.2 --- --- ---
*excluding H
Quantitative Analysis
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?