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Introduction to X-Ray Photoelectron Spectroscopy (XPS)

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Introduction to X-Ray Photoelectron Spectroscopy (XPS) Introduction to Sources of Information • Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics X-ray Photoelectron ~$600 (2 CCMR copies, 1 copy on reserve in Engineering Library) • Surface Analysis, Briggs & Grant, ~$300 (1 CCMR copy) Spectroscopy (XPS) • XPS of Polymers Database, ~$600 (1 CCMR copy on CD) • UK Surface Analysis forum, www.uksaf.org • XPS Short Courses (John Grant), www.surfaceanalysis.org • Sources of Information • Principles of XPS and Auger • E-mail list-serv [email protected] • How to prepare samples for XPS – Subscribe at http://lists.ccmr.cornell.edu • Instrumentation, X rays, Photoelectron detection – CCMR system updates, announcements, questions, etc. • Data acquisition • Sources for IMFP: – Quantitative and Qualitative analyses – Quases-IMFP-TPP2M software (10.6MB) free download at – Spin-orbit splitting, Plasmons, Shake-up, etc. www.quases.com – Sample charge control – NIST program IMFPWIN (1 CCMR copy) – Overlayer effects – Ion sputtering 8/18/2010 1 8/18/2010 2 Introduction to Surface Analysis X-ray Photoelectron The Study of the Outer-Most Layers of Materials (<100 Å). Spectroscopy (XPS) • Electron • Ion Spectroscopies Spectroscopies • Sources of Information SIMS: Secondary Ion • Principles of XPS and Auger XPS: X-ray Mass Spectrometry Photoelectron • How to prepare samples for XPS Spectroscopy • Instrumentation, X rays, Photoelectron detection SNMS: Sputtered Neutral Mass • Data acquisition AES: Auger Electron Spectrometry – Quantitative and Qualitative analyses Spectroscopy – Spin-orbit splitting, Plasmons, Shake-up, etc. ISS: Ion Scattering Spectroscopy – Sample charge control EELS: Electron Energy Loss – Overlayer effects Spectroscopy – Ion sputtering 8/18/2010 3 8/18/2010 4 Comparison of Sensitivities X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy H Ne Co Zn Zr Sn Nd Yb Hg Th for Chemical Analysis (ESCA) is a widely AES and XPS used technique to investigate the chemical 1% composition of surfaces. 5E19 RBS X-ray Photoelectron spectroscopy, PIXE based on the photoelectric effect,1,2 was 1ppm 5E16 developed in the mid-1960’s by Kai Siegbahn and his research group at the SIMS University of Uppsala, Sweden.3 1. H. Hertz, Ann. Physik 31,983 (1887). 1ppb 5E13 0 20 40 60 80 100 2. A. Einstein, Ann. Physik 17,132 (1905). 1921 Nobel Prize in Physics. 8/18/2010 ATOMIC NUMBER 5 8/18/2010 3. K. Siegbahn, Et. Al.,Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 20 (1967). 6 1981 Nobel Prize in Physics. The Photoelectric Process Photoionization Cross Section Ejected Photoelectron Incident X-ray • Scofield cross-sections are proportional rate of Free Electron XPS spectral lines are identified emitted photoelectrons Level by the shell from which the • Typically the C 1s Conduction Band electron was ejected (1s, 2s, 2p, transition is given a value etc.). Fermi of 1, sometimes F 1s Level The ejected photoelectron has • Peaks are created with Valence Band kinetic energy: areas proportional to KE=hv-BE- Scofield cross-sections 2p L2,L3 XPS typically uses relation 2s L1 BE=hv-KE- Kinetic energy of the exciting x- ray must be known 1s K Work function,, of the detector is known and constant Each pathway has a 8/18/2010 photoionization cross-section 7 8/18/2010 8 Elemental XPS Spectrum Auger Relation of Core Hole Incident X-ray or Emitted Auger Electron electron Free Electron L electron falls to fill core level Level vacancy (step 1). Conduction Band KLL Auger electron emitted to Fermi conserve energy released in Level step 1. Valence Band The kinetic energy of the emitted Auger electron is: 2p L2,L3 KE=E(K)-E(L2)-E(L3). 2s L1 A 3-step process which often makes Auger peaks more difficult to characterize than 1s K XPS peaks 8/18/2010 9 8/18/2010 10 Auger Spectrum X-ray Photoelectron Spectroscopy Small Area Detection •Auger utilizes an electron beam to Monochromatic X-ray Beam Electrons are extracted scan the sample surface only from a narrow solid •Electron beams can be focused to angle. much smaller spot sizes (~5 nm) than x-rays •Electrons from the beam are collected along with photoemitted X-ray penetration electrons depth ~1mm. Electrons can be ~10 nm •Typically the derivative spectrum is excited in this entire volume. ~1 mm2 used to quantify peak intensities •Derivative peaks can vary greatly depending on the broadness of the signal peak. 8/18/2010 11 8/18/2010 SSI system: X-ray spot 150 to 1000 microns 12 Inelastic Mean Free Path (IMFP or ) Inelastic Mean Free •IMFP is the average distance an electron travels before it undergoes Path (IMFP or ) for: an inelastic collision (and therefore loses energy and can become part of the XPS background) •Electron elastic scattering is neglected •IMFP depends on: Nickel •The kinetic energy of the electron •The material in which it is traveling •Similar to, but not to be confused with Effective Attenuation Length (EAL) which tries to account for elastic scattering effects •IMFP is usually denoted by •IMFP generally larger for softer materials like polymers (up to 10 nm) •IMFP for metals typically 1-3 nm Sources for IMFP: Polymer .Quases-IMFP-TPP2M software (10.6MB) free download at www.quases.com .NIST program IMFPWIN (can obtain copy from me) .Online IMFP Grapher at www.lasurface.com 8/18/2010 13 8/18/2010 14 Mean Escape Depth (MED) Information Depth (ID) or Analysis Depth •Mean escape depth is defined as the average depth with respect to the surface normal, from which electrons escape •Information depth can be identified as the sample thickness •MED = cos() where is the angle with respect to the surface normal from which a specified percentage (95% or 99%, e.g.) of the •At high angles, elastic scattering of electrons may be significant detected signal originates •The 95% ID corresponds to 3 if elastic scattering effects are neglected =75° = •Practical information depth is 3cos() 0° Less Surface More Surface Sensitive, Sensitive greater electron in the SSI system is 55°, unless using an angled stage. escape depth cos(55)= 0.573 CCMR system has detector 55° from surface-normal of a horizontal sample Tilt stages are used for Angle-resolved analyses cos(55) = 0.573 8/18/2010 15 8/18/2010 16 Samples Introduction to X-ray Photoelectron Spectroscopy •Ideal sample: (XPS) •UHV compatible, nothing with high vapor pressure •Very clean, will discuss sample handling •Conductive, metals or metal thin films on conducting • Sources of Information substrate • Principles of XPS and Auger •Flat, polished surface (deposited on silicon substrate, • How to prepare samples for XPS e.g.) • Instrumentation, X rays, Photoelectron detection •About 1cm x 1cm square or larger • Data acquisition •Things to consider: – Quantitative and Qualitative analyses •Do you need the sample back? – Spin-orbit splitting, Plasmons, Shake-up, etc. •Can it be broken or modified for mounting? – Sample charge control •Maximum sample size ~100 mm wide and ~50 mm tall – Overlayer effects – Ion sputtering 8/18/2010 17 8/18/2010 18 Insulating Samples: Sample Charging Insulating Samples: Charge Neutralization - Ejected Photoelectron Incident X-ray Photoemission of electrons leaves the sample with a net positive charge The positive charge makes it more difficult for electrons to escape the surface This results in lower kinetic- energy photoelectrons and shifts peaks to higher binding energies. Non-uniform charging of the surface can lead to peak Grid aids in keeping electric broadening field uniform + 8/18/2010 19 8/18/2010 20 Types of Surfaces Surface Contact Contamination layer Ideal Surface Surface Deposited • Use non-magnetic, ultra- Microstructure Thin Film clean tweezers to handle the sample • Try not to touch the surface to be analyzed Laterally •Any dust generated can end Inhomogeneous- Rough Surface- up on the sample surface Emitted intensity May get shadowing after going into vacuum May vary with effects Orientation 8/18/2010 21 8/18/2010 22 Use of Gloves Aluminum Foil UHV oil-free Aluminum foil • Plastic ziploc bags and Aluminum foil often has an oil film on it to prevent Reynolds Aluminum foil 24oct06b_2.dat Data Set 2 d sticking Total Acquisition Time 17.067 (mins) (1000.0 (ms) x 1 x 1024) Source: Al 3 •If you must handle the x 10 40 sample directly, use of Name Pos. FWHM Area At% Ca 2p 345.36 5.754 3965.5 0.497 C 1s C 1s 282.64 3.377 119061.1 75.683 silicone-based, powder- 35 O 1s 530.58 3.746 77774.8 16.873 Si 2p 99.38 3.170 3300.3 2.568 Al 2p 70.96 3.682 3699.3 4.379 free gloves is 30 recommended In Reynolds wrap: 25 • Aluminum signal is much lower due to a O 1s thicker hydrocarbon layer CPS 20 •Silicon peaks could be due to silicone- based mineral oil 15 • Background at high BE indicates 10 Ca 2p presence of overlayer 5 Si 2p 8/18/2010 23 8/18/2010 24 Al 2p 1000 800 600 400 200 0 Binding Energy (eV) CasaXPS (T his string can be edited in CasaXPS.DEF/PrintFootNote.txt) Sample Handling Sample Handling 8/18/2010 25 8/18/2010 26 Sample Drop-off Introduction to .Drop off samples X-ray Photoelectron .My office, Clark D21 .In the dessicator outside of D21B. Fill out a drop-off Spectroscopy (XPS) sheet AND email me to let me know it is there. You can your ID entered into the D21 door lock. .Better if you know what scan regions you need, • Sources of Information should talk with me if you don’t. • Principles of XPS and Auger .View system schedule online • How to prepare samples for XPS .CCMR Coral • Instrumentation, X rays, Photoelectron .Surface Analysis, XPS detection .Do NOT schedule time for yourself • Data acquisition .This is only a tentative schedule and may be offset – Quantitative and Qualitative analyses due to longer runs, system breakdowns, maintenance, – Spin-orbit splitting, Plasmons, Shake-up, etc.
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