X-Ray Photoelectron Spectroscopy (XPS)Inelastic mean free path ( ) is the mean distance that an...

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?