x-ray photoelectron spectroscopy (xps)inelastic mean free path ( ) is the mean distance that an...
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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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Kinetic Energy
hu: Al Ka(1486.6eV)
P 2s P 2p1/2-3/2
KE = hn - BE - FSPECT BE = hn - KE - FSPECT
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Peak Notations
L-S Coupling ( j = l s )
e-s=
12
s=12
12j = l + 1
2j = l
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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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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
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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
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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
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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
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
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%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
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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
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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
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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
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
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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.
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Broadening of peak
Sample
Differential (non-uniform) surface charging
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e-
~2eV-20eV
filament
Electronsoptics
Charge Compensation Techniques
Low Energy Electron Flood Gun
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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
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Shift in B.E.
of a polymer
surface
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Effects of Surface Charging
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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
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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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XPS system suitable for industrial samples
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Vacuum Chamber Control Electronics
Sample Introduction Chamber
Ion pump
Turbopump
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Dual Anode X-ray Source
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Commonly used
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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
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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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500 x 500mm
+ 1
+ 2
X-ray induced secondary electron
imaging for precise location of the
analysis area
x-ray secondary
electrons
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Sputtered
materials
Pea
k A
rea
Sputtering Time
Depth Profiling
Ar+
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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
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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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• 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
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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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Sample platen 75 X 75mm
Sputtered crater
•Architectural glass coating
• ~100nm thick coating
Depth Profiling Architectural Glass Coating
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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
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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
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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
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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
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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
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+ ++
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
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Polyethylene
Substrate
Adhesion Layer
Base Coat
Clear Coat
Mapping Area
695 x 320µm
1072 x 812mm
XPS study of paint
Paint Cross Section
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C O Cl Si
695 x 320mm
Elemental ESCA Maps using C 1s,
O 1s, Cl 2p, and Si 2p signals
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C 1s CH CHCl O=C-O
695 x 320mm
C 1s Chemical State Maps
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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
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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
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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?