X-Ray Photoelectron Spectroscopy (XPS) Prof. Paul K. Chu.

transcript

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

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.

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

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 )

Redistribution of electron density

B.E. provides information on chemical environment

Example of Chemical Shift

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 e te c to r

%100% i i

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

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

AAAA MSkI

For example, Teflon (-CF2-)

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

iAiA S

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 -

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

+ ++ ++ ++ +

" 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

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

~ 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

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

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

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

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

X-ray induced secondary electron imaging for precise location of the analysis area

x-ray secondary electrons

Sputteredmaterials

Sputtering Time

Depth Profiling

Sputtering TimeC

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

Sputter Depth (nm)

Si 2pNb 3d N 1s

O 1s O 1s

Si 2pTi 2p

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

Sputter Depth (nm)

Cr 2p oxideCr 2p metal Ni 2p

Si 2pNi 2p Cr 2p metal

Depth profiling of a multilayer structure

Cr/Si interface width (80/20%) = 23.5nm

0 1850

Cr 2pSi 2pCr 2pNi 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

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 beam30 minutes

278288298Binding Energy (eV)

Polymer coating ‘B’

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

Polymer coating ‘A’

CClO=C-O

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

Binding Energy (eV)280300

280300

280300Binding Energy (eV)

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?

X-Ray Photoelectron Spectroscopy (XPS) Prof. Paul K. Chu.

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