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X-Ray Photoelectron Spectroscopy (XPS)
Prof. Paul K. Chu
X-ray Photoelectron Spectroscopy
IntroductionQualitative analysisQuantitative analysisCharging compensationSmall area analysis and XPS imagingInstrumentationDepth profilingApplication 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
h: Al K(1486.6eV)
P 2s P 2p1/2-3/2
KE = hBE SPECT BE = hKE SPECT
Peak Notations
L -S C o u p lin g ( 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 2pAl 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
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
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
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
D e te c to r
%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
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
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
X-ray damage
Some samples can be damaged by x-rays
For sensitive samples, repeat the measurement to check for x-ray damage.
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
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.
Shift in B.E. of a polymer surface
B ro ad en in g o f p ea k
S am p le
Differential (non-uniform) surface charging
Effects of Surface Charging
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
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
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
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
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
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
Sample Introduction Chamber
Ion pump
Turbopump
500 x 500m
+ 1
+ 2
X-ray induced secondary electron imaging for precise location of the analysis area
x-ray secondary electrons
Sputteredmaterials
Pea
k A
rea
Sputtering Time
Depth Profiling
Ar+
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
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)
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
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
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
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
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
+ ++
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
Polyethylene
Substrate
Adhesion Layer
Base Coat
Clear Coat
Mapping Area
695 x 320µm
1072 x 812mm
XPS study of paintPaint 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
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
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
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
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?
