AMCF Materials Characterization School 2012

X-Ray Photoelectron Spectroscopy

Tim Morgan

Overview

• What is XPS?• The Physics Behind XPS• Instrumentation• Data Analysis– Elemental Analysis– Chemical State Identification– Quantification

• Capabilities

What is it?

XPS is a technique designed to give chemical information.

light electrons

e-

What is on the surface?

XPS is a technique designed to analyze the surface of a material

Light penetrates

microns

Inelastic electrons only escape a few nanometers

e-e-

e-

e-

e-

e-

e-

e- e-

EF

EVac

1s

2s2p

φs

BE2

Fingerprinting Atoms

X-ray (hν)

KE = hν-BE-φs

EF

EVac

1s

2s2p

φs

BE1

ph

oto

ele

ctro

n

KE

Inte

nsity

ph

oto

ele

ctro

n

X-ray (hν)

BE1

BE2

Photoelectron Detection

EF

EVac

1s

2s2p

X-ray (hν)

φs

BE

Excited Ion

ph

oto

ele

ctro

n

EVac

φan

KE = hν-BE-φs-(φan-φs)

KE = hν-BE-φan

Analyzer

EF

KE = hν-BE-φs

XPS is independent of the sample’s work function.

Basic XPS Instrumentation

MCD

Quartz crystal monochromator

Al Anode

Electron Gun Rowland

Circle

Al Kα x-rays

(1486eV)

Lens

Energy Analyzer

(SCA)

Photoelectrons

Sample

15-20kV electrons

V

E0

UHV Chamber

ΔE

Charge Neutralization

Conducting Sample

+++

X-ray beam

Electron neutralizer

Surface Charge Neutralization

Insulating Sample

+++

X-ray beam

Electron neutralize

r

- - - - - - - - - - - - - - - - - - -

Ion Gun

The Internal Workings

MCD

Quartz crystal monochromator

Al Anode

Electron Gun Rowland

Circle

Al Kα x-rays

(1486eV)

Lens

Energy Analyzer

(SCA)

Photoelectrons

Sample

15-20kV electrons

Electron Neutralizer

1eV electronsIon Gun

V

E0

UHV Chamber

ΔE

PHI VersaProbe 5000

Spherical Capacitve Analyzer

C-60 Ion Gun

Ar Ion Gun

Entry/Exit Chamber

PHI VersaProbe 5000

Ar Ion Gun

Lens

Electron Gun

Sample Stage

Important Operational Concerns

• Ultra High Vacuum: 10-10 Torr Base Pressure• Monochromated X-ray beam– Spot size ranges from 8 – 200 microns

• Electron Gun for Positive Ion Charge Neutralization

• Argon Ion Gun for Insulators• 5 axis stage

THE EXPERIMENT

Vacuum Watcher

Control:- Venting E/E- Pumping Down E/E- Gate Valve

Operations

Monitor - Pressure in E/E &

Main Chamber- Gate Valve Status

Summit: Image

Control:- Sample Stage- X-ray Setup- SXI Image Capture- Neutralizer Settings- Sputter Settings

Monitor - Analysis Position- Ion Gun Pressure- SXI Image

Summit: Acquisition

Control:- Experimental Setup- Scan Ranges- Data Save Location- Advanced Setup

Setting Up an Experiment

• Average over an area to avoid anomalies• Balance is key to getting good statistics in a

reasonable amount of time• Always perform a survey scan before detailed

scans• Understand your resolution needs• Is your sample an insulator?

DATA ANALYSIS

Typical Spectra Features

•Reverse Energy Scale•Sharp Photoelectron peaks•Broader Auger peaks with fine structure•Background

BE = hν-KE-φan

Complete Chemical Analysis

1. Identify All Elements2. Determine Chemical Environment3. Calculate the Stoichiometry

Survey Scan

Qualitative Data Analyis:

1. Identify All Major Peaks

2. Identify All Other Peaks

3. Look up Reference Peaks

What material do you think this is?

Quantitative Analysis

Carbon

Iron

Oxygen

How do we prove if carbon is a surface contaminent?

Comparing Pre & Post Sputter

Cleaning the surface removes atmospheric contamination and real analysis of the sample

Postsputter

Presputter

Peak Fitting Carbon for PET

ADVANCED FEATURES

Additional Questions

• How do I examine layers below the surface?• How surface sensitive can I get?• How can I differentiate regions with different

chemical species?• How can I examine polymers?

How to examine the orange material?

X-ray beam

Ion Gun

Detector

High Energy Ions (2keV)

X-ray beam

Ion Gune-

e-

e-

Detector

X-ray beam

Ion Gun

Detector

X-ray beam

Ion Gun

e-

e-e-

Detector

Depth Profiling Si/SiO2

Si

ODouble Si Peak

Depth Profiling

• 1-2 nm depth resolution• Variable Energy Argon Ion Gun– 5 kV to 100 V

• C-60 Gun for Polymers

Angle Resolved XPS Detector

Decreasing the Take Off Angle decreases the analysis depth.

Elemental Mapping

Chemical Mapping

Questions?

X-ray Sources

Line Energy (eV) Width (eV)

Y Mζ 132.3 0.47

Mg Kα 1253.6 0.7

Al Kα 1486.6 0.85

Cu Kα 8048.0 2.6

Ultraviolet (UV) 10-100

What is on the surface?

XPS is a technique designed to analyze the surface of a material

light electrons

e-

The Internal Workings

MCD

Quartz crystal monochromator

Al Anode

Electron Gun Rowland

Circle

Al Kα x-rays

(1486eV)

Lens

Energy Analyzer

(SCA)

Photoelectrons

Sample

15-20kV electrons

Electron Neutralizer

1eV electronsIon Gun

V

E0

UHV Chamber

ΔE

Quantitative Analysis

• Higher Resolution– Peak energy Accuracy– Chemical State Identification

• Peak Fitting– Deconvolution– Detailed Peak Information

• Stoichiometry Calculations