x-ray photoelectron spectroscopy (xps) prof. paul k. chu
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![Page 1: X-Ray Photoelectron Spectroscopy (XPS) Prof. Paul K. Chu](https://reader034.vdocuments.net/reader034/viewer/2022050703/56649d485503460f94a246ea/html5/thumbnails/1.jpg)
X-Ray Photoelectron Spectroscopy (XPS)
Prof. Paul K. Chu
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X-ray Photoelectron Spectroscopy
IntroductionQualitative analysisQuantitative analysisCharging compensationSmall area analysis and XPS imagingInstrumentationDepth profilingApplication 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
h: Al K(1486.6eV)
P 2s P 2p1/2-3/2
KE = hBE SPECT BE = hKE SPECT
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Peak Notations
L -S C o u p lin g ( 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 2pAl 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
dxex
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 . = E n e rg y o f F in a l s ta te - E n e rg y o f in itia l s ta te
(o n e a d d i tio n a l+ v e c h a rg e )
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
D e te c to r
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%100% i i
i
A
A
SISI
Atomic
Quantitative AnalysisPeak 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 = c o n s ta n t S = s e n s itiv ity fa c to r o f a c o re le v e l o f e le m e n t AM = N o . o f A in th e e m p iric a l fo rm u la
A
AAAA 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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1 s L i2C O 3 C 1 s 0 .0 6 7 0 .0 6 9L i2S O 4 S 2 p 0 .0 6 9 0 .0 6 7K B F 4 K 2 p 0 .5 0 0 .5 0
N H 4B F 4 N 1 s 0 .5 5 0 .5 7N a 2S O 3 S 2 p 2 .9 5C u S O 4 S 2 p 3 .2 5K 2S O 4 S 2 p 2 .9 0 2 .8 5
A g (C O C F 3)3 F 1 s 2 .6 2 2 .8 1N a 5P 3O 1 0 N a 2 s 3 .4 0
C 6H 2N S 2K 3O 9 K 2 p 2 .8 9 3 .0 5
Examples of Sensitivity Factors
N = number of compounds tested
N
iAiA S
NS
1
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 CompensationF or m eta l o r o th er co n d u ctin g sa m p les th at gro u n d ed to th esp ectro m eter
E lec tro n s m o v e to th e su rfa ceco n tin u o u sly to c o m p en sa te th e e lec tro n lo ss a t th e su rfac ereg io n .
e -
e -e -
X -r a y
sa m p le
e -e -
Electron loss and compensation
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F o r re sis tiv e sa m p les
e -
+ ++ ++ ++ +
V R I
" c u rre n t" n e t lo s s o f e le c tro n s f ro m th e su rfa c e
R e s is ta n c e b e tw e e n th e su r fa c e a n d th e g ro u n d
P o te n tia l d e v e lo p e d a t th e su r fa c e
IR
1 0 n A1 k
1 0 n A1 M
1 0 n A1 0 0 0 M
V 1 0 -5 V 0 . 0 1 V 1 0 V
N o t im p o r ta n t Im p o r ta n t fo r a cc u ra te B .E .m e a s u re m e n ts
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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B ro ad en in g o f p ea k
S am p le
Differential (non-uniform) surface charging
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Effects of Surface Charging
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e -
~ 2 e V -2 0 eV
filam e n t
E lec tro n so p tic s
Charge Compensation Techniques
Low Energy Electron Flood Gun
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S a m p le
-v e
f ila m e n t e
a n a ly se r
M a g n e t
X -ra y
e le c tro n s
L o w e n e rg ye le c tro n b e a m
L o w e n e rg y A r b e a m+
S a m p le
Electron source with magnetic field
Low energy electrons and Ar+
A single setting for all types of samples
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S a m p le S a m p le
A p e r tu re o f A n a ly z e r le n s
A p e r tu re o f A n a ly z e r le n s
X -r a y X -r a y
P h o to e le c tr o n s P h o to e le c tr o n s
S p o t s iz e d e te r m in e d b y th e x -r a y b e a mS p o t s iz e d e te r m in e d b y th e a n a ly s e r
B o th m o n o c h ro m a te d a n d d u a l a n o d e x -r a y s o u rc e s c a n b e u s e d
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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n = 2 d sin
F o r A l K 8 .3
Å
u se (1 0 1 0 ) p la n e so f q u a rtz c ry s ta l d = 4 .2 5 = 7 8 .5 o
Å
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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Vacuum Chamber Control Electronics
Sample Introduction Chamber
Ion pump
Turbopump
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500 x 500m
+ 1
+ 2
X-ray induced secondary electron imaging for precise location of the analysis area
x-ray secondary electrons
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Sputteredmaterials
Pea
k A
rea
Sputtering Time
Depth Profiling
Ar+
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Pea
k A
rea
Sputtering TimeC
once
ntra
tion
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)
Ato
mic
Con
cen
trat
ion
(%
)
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)
Ato
mic
Co
nce
ntr
atio
n (
%)
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
nce
ntr
atio
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 )
H igh energy ions
S am p le
H igh ene rgy ions
S am p le ro ta tes
Low energy ions
S am p le ro ta tes
Ions: 4 keVSample still
Ions: 4 keVWith Zalar rotation
Ions: 500 eVWith 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 layerat interface ?
Multi-layered Drug Package
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01000 Binding Energy (eV)
1000 Binding Energy (eV) 0
-O
KL
L -O
1s
-C 1s
-C
l 2p
-Si 2
p
1000 0Binding Energy (eV)
-O
KL
L
-O
1s
-N
1s
-C 1s
+ ++
Photograph of cross-section
1072 X 812µm
-O
KL
L -O
1s
-C 1s
-C
l 2p
Polymer coating ‘A’
Al foil
Polymer coating ‘B’Polymer ‘A’ / Al foil Interface
10µm x-ray beam30 minutes
10µm x-ray beam30 minutes
10µm x-ray beam30 minutes
-Si 2
p-S
i 2s
-Si 2
s
-Al 2
p -A
l 2s
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+ ++
278288298Binding Energy (eV)
Polymer coating ‘B’
C 1s
CHCNO
O=C-O
Atomic Concentration (%)
Area C O N SiA 82.6 12.2 ---- 0.7Interface 83.2 12.2 ---- 1.3B 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’
CH
CClO=C-O
10µm x-ray beam11.7eV pass energy30 minutes
10µm x-ray beam11.7eV pass energy30 minutes
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Polyethylene
Substrate
Adhesion Layer
Base Coat
Clear Coat
Mapping Area
695 x 320µm
1072 x 812mm
XPS study of paintPaint 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
CNC-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 2.2Clear 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 m 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?