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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?. light. electrons. e -.
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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? light electrons e- XPS is a technique designed to give chemical information.
What is on the surface? Inelastic electrons only escape a few nanometers e- e- e- e- e- Light penetrates microns e- e- e- e- XPS is a technique designed to analyze the surface of a material
Fingerprinting Atoms KE = hν-BE-φs EVac EVac φs φs EF photoelectron EF photoelectron 2p BE2 2s 2p BE1 2s X-ray (hν) X-ray (hν) 1s 1s BE2 BE1 Intensity KE
Photoelectron Detection KE = hν-BE-φs EVac EVac φan φs EF EF photoelectron Analyzer 2p BE 2s X-ray (hν) 1s KE = hν-BE-φs-(φan-φs) Excited Ion KE = hν-BE-φan XPS is independent of the sample’s work function.
V MCD Basic XPS Instrumentation UHV Chamber Quartz crystal monochromator Energy Analyzer (SCA) Electron Gun Rowland Circle Al Kα x-rays (1486eV) 15-20kV electrons Photoelectrons ΔE Lens E0 Al Anode Sample
Conducting Sample Charge Neutralization X-ray beam Electron neutralizer +++
Insulating Sample Ion Gun Surface Charge Neutralization X-ray beam Electron neutralizer - - - - - - - - - - - - - - - - - - - +++
V MCD The Internal Workings UHV Chamber Quartz crystal monochromator Energy Analyzer (SCA) Electron Gun Rowland Circle Al Kα x-rays (1486eV) 15-20kV electrons Photoelectrons ΔE Lens E0 Electron Neutralizer Al Anode Ion Gun 1eV electrons Sample
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: 10-10Torr 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
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?
Typical Spectra Features • Reverse Energy Scale • Sharp Photoelectron peaks • Broader Auger peaks with fine structure • Background BE = hν-KE-φan
Complete Chemical Analysis • Identify All Elements • Determine Chemical Environment • Calculate the Stoichiometry
Survey Scan Qualitative Data Analyis: Identify All Major Peaks Identify All Other Peaks Look up Reference Peaks What material do you think this is?
Quantitative Analysis Carbon Oxygen Iron How do we prove if carbon is a surface contaminent?
Comparing Pre & Post Sputter Postsputter Presputter Cleaning the surface removes atmospheric contamination and real analysis of the sample
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?
Ion Gun Detector X-ray beam High Energy Ions (2keV)
Ion Gun Detector X-ray beam e- e- e-
Ion Gun Detector X-ray beam
Ion Gun Detector X-ray beam e- e- e-
Depth Profiling Si/SiO2 Double Si Peak O Si
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.
What is on the surface? light electrons e- XPS is a technique designed to analyze the surface of a material
V MCD The Internal Workings UHV Chamber Quartz crystal monochromator Energy Analyzer (SCA) Electron Gun Rowland Circle Al Kα x-rays (1486eV) 15-20kV electrons Photoelectrons ΔE Lens E0 Electron Neutralizer Al Anode Ion Gun 1eV electrons Sample
Quantitative Analysis • Higher Resolution • Peak energy Accuracy • Chemical State Identification • Peak Fitting • Deconvolution • Detailed Peak Information • Stoichiometry Calculations