amcf materials characterization school 2012
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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
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