Introduction to X-ray Photoelectron Spectroscopy (XPS) Sources of Information Principles of XPS and Auger How to prepare samples for XPS Instrumentation, X rays, Photoelectron detection Data acquisition Quantitative and Qualitative analyses Spin-orbit splitting, Plasmons, Shake-up, etc. Sample charge control Overlayer effects Ion sputtering 8/18/2010 1 Sources of Information Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics ~$600 (2 CCMR copies, 1 copy on reserve in Engineering Library) Surface Analysis, Briggs & Grant, ~$300 (1 CCMR copy) 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 E-mail list-serv ccmr-surfaceanalysis@ccmr.cornell.edu Subscribe at http://lists.ccmr.cornell.edu CCMR system updates, announcements, questions, etc. Sources for IMFP: Quases-IMFP-TPP2M software (10.6MB) free download at www.quases.com NIST program IMFPWIN (1 CCMR copy) 8/18/2010 2 Introduction to X-ray Photoelectron Spectroscopy (XPS) Sources of Information Principles of XPS and Auger How to prepare samples for XPS Instrumentation, X rays, Photoelectron detection Data acquisition Quantitative and Qualitative analyses Spin-orbit splitting, Plasmons, Shake-up, etc. Sample charge control Overlayer effects Ion sputtering 8/18/2010 3 Surface Analysis The Study of the Outer-Most Layers of Materials (<100 Å). Electron Spectroscopies XPS: X-ray Photoelectron Spectroscopy AES: Auger Electron Spectroscopy EELS: Electron Energy Loss Spectroscopy Ion Spectroscopies SIMS: Secondary Ion Mass Spectrometry SNMS: Sputtered Neutral Mass Spectrometry ISS: Ion Scattering Spectroscopy 8/18/2010 4 Comparison of Sensitivities 1% 1ppm H Ne Co Zn Zr Sn Nd Yb Hg Th SIMS PIXE AES and XPS 1ppb 20 40 60 80 100 5E13 0 8/18/2010 ATOMIC NUMBER 5 RBS 5E19 5E16 X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate the chemical composition of surfaces. X-ray Photoelectron spectroscopy, based on the photoelectric effect, 1,2 was developed in the mid-1960 s by Kai Siegbahn and his research group at the University of Uppsala, Sweden. 3 1. H. Hertz, Ann. Physik 31,983 (1887). 2. A. Einstein, Ann. Physik 17,132 (1905). 1921 Nobel Prize in Physics. 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 Incident X-ray Ejected Photoelectron Free Electron XPS spectral lines are identified Level by the shell from which the Conduction Band electron was ejected (1s, 2s, 2p, Fermi etc.). Level The ejected photoelectron has Valence Band kinetic energy: KE=hv-BE- 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 Scofield cross-sections are proportional rate of emitted photoelectrons Typically the C 1s transition is given a value of 1, sometimes F 1s Peaks are created with areas proportional to Scofield cross-sections 8/18/2010 8 Elemental XPS Spectrum Auger Relation of Core Hole Incident X-ray or electron Emitted Auger Electron 2p 2s 1s Conduction Band Valence Band Free Electron Level Fermi Level L2,L3 L1 K L electron falls to fill core level vacancy (step 1). KLL Auger electron emitted to conserve energy released in step 1. The kinetic energy of the emitted Auger electron is: KE=E(K)-E(L2)-E(L3). A 3-step process which often makes Auger peaks more difficult to characterize than 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 scan the sample surface Electron beams can be focused to much smaller spot sizes (~5 nm) than x-rays Electrons from the beam are collected along with photoemitted electrons Typically the derivative spectrum is used to quantify peak intensities Derivative peaks can vary greatly depending on the broadness of the signal peak. X-ray penetration depth ~1mm. Electrons can be excited in this entire volume. Monochromatic X-ray Beam Electrons are extracted only from a narrow solid angle. ~1 mm 2 ~10 nm 8/18/2010 11 8/18/2010 SSI system: X-ray spot 150 to 1000 microns 12
Inelastic Mean Free Path (IMFP or ) IMFP is the average distance an electron travels before it undergoes an inelastic collision (and therefore loses energy and can become part of the XPS background) Electron elastic scattering is neglected IMFP depends on: 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: 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 Inelastic Mean Free Path (IMFP or ) for: Nickel Polymer 8/18/2010 13 8/18/2010 14 Mean Escape Depth (MED) Mean escape depth is defined as the average depth with respect to the surface normal, from which electrons escape MED = cos( ) where is the angle with respect to the surface normal At high angles, elastic scattering of electrons may be significant More Surface Sensitive =75 = 0 Less Surface Sensitive, greater electron escape depth Information Depth (ID) or Analysis Depth Information depth can be identified as the sample thickness from which a specified percentage (95% or 99%, e.g.) of the detected signal originates The 95% ID corresponds to 3 if elastic scattering effects are neglected Practical information depth is 3 cos( ) in the SSI system is 55, unless using an angled stage. 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 Introduction to X-ray Photoelectron Spectroscopy (XPS) Sources of Information Principles of XPS and Auger How to prepare samples for XPS Instrumentation, X rays, Photoelectron detection Data acquisition Quantitative and Qualitative analyses Spin-orbit splitting, Plasmons, Shake-up, etc. Sample charge control Overlayer effects Ion sputtering 8/18/2010 17 Samples Ideal sample: UHV compatible, nothing with high vapor pressure Very clean, will discuss sample handling Conductive, metals or metal thin films on conducting substrate Flat, polished surface (deposited on silicon substrate, e.g.) About 1cm x 1cm square or larger Things to consider: Do you need the sample back? Can it be broken or modified for mounting? Maximum sample size ~100 mm wide and ~50 mm tall 8/18/2010 18
Si 2p Al 2p Ca 2p CPS O 1s C 1s Insulating Samples: Sample Charging Insulating Samples: Charge Neutralization Incident X-ray + - Ejected Photoelectron 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 kineticenergy photoelectrons and shifts peaks to higher binding energies. Non-uniform charging of the surface can lead to peak broadening Grid aids in keeping electric field uniform 8/18/2010 19 8/18/2010 20 Types of Surfaces Surface Contact Ideal Surface Contamination layer Surface Microstructure Laterally Inhomogeneous- Emitted intensity May vary with Orientation Deposited Thin Film Rough Surface- May get shadowing effects 8/18/2010 21 Use non-magnetic, ultraclean tweezers to handle the sample Try not to touch the surface to be analyzed Any dust generated can end up on the sample surface after going into vacuum 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 sticking If you must handle the sample directly, use of silicone-based, powderfree gloves is recommended 8/18/2010 23 In Reynolds wrap: Aluminum signal is much lower due to a thicker hydrocarbon layer Silicon peaks could be due to siliconebased mineral oil Background at high BE indicates presence of overlayer 24oct06b_2.dat Data Set 2 d Total Acquisition Time 17.067 (mins) (1000.0 (ms) x 1 x 1024) Source: Al 8/18/2010 24 40 35 30 25 20 15 10 5 x 10 3 Name Ca 2p C 1s O 1s Si 2p Al 2p Pos. 345.36 282.64 530.58 99.38 70.96 FWHM 5.754 3.377 3.746 3.170 3.682 Reynolds Aluminum foil Area 3965.5 119061.1 77774.8 3300.3 3699.3 At% 0.497 75.683 16.873 2.568 4.379 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 Drop off samples My office, Clark D21 In the dessicator outside of D21B. Fill out a drop-off 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, should talk with me if you don t. View system schedule online CCMR Coral Surface Analysis, XPS Do NOT schedule time for yourself This is only a tentative schedule and may be offset due to longer runs, system breakdowns, maintenance, etc. I will try to update often. Introduction to X-ray Photoelectron Spectroscopy (XPS) Sources of Information Principles of XPS and Auger How to prepare samples for XPS Instrumentation, X rays, Photoelectron detection Data acquisition Quantitative and Qualitative analyses Spin-orbit splitting, Plasmons, Shake-up, etc. Sample charge control Overlayer effects Ion sputtering 8/18/2010 27 8/18/2010 28 Instrumentation for XPS Instrumentation for XPS Surface analysis by XPS requires irradiating a solid in an Ultra-high Vacuum (UHV) chamber with monoenergetic soft X- rays and analyzing the energies of the emitted electrons. 8/18/2010 29 8/18/2010 30
Why UHV for Surface Analysis? Degree of Vacuum Low Vacuum Medium Vacuum High Vacuum Ultra-High Vacuum Pressure Torr 102 10-1 10-4 10-8 10-11 Remove adsorbed gases from the sample. Eliminate adsorption of contaminants on the sample. Prevent arcing and high voltage breakdown. Increase the mean free path for electrons, ions and photons. 8/18/2010 31 Schematic of SSI system Analyzer Electron gun (10 kev Anode (aluminum) produces characteristic x-rays Crystal Monochromator focuses x-rays and reduces x-ray energy width Sample must be at focus of both the monochromator and collection lens Collection lens collects photoelectrons Detector measures incidence of photoelectrons Some systems scan the anode to scan the sample. This system moves the sample 8/18/2010 32 SSI system Anode: X-ray Source X-rays are produced by hitting a metal anode with high-energy electrons (5-15keV) >99.9% of this energy is dissipated as heat, therefore anode cooling is critical AlKa x rays have an overall line width of ~0.85eV 8/18/2010 33 8/18/2010 34 Anode: X-ray Source Mg, Al, and Cu are common XPS anodes Mg has a lower x- ray output than Al Al Ka x-rays can probe to larger BE s than Mg E(Al-Mg) = 233 ev 8/18/2010 35 Non-Monochromated vs. Monochromated X-rays Non-monochromated x- rays contain Bremsstrahlung radiation Peak width ~0.85 ev X-rays scatter throughout chamber, creating photoelectrons on all surfaces. These photoelectrons help to neutralize insulating samples Greater sample heating may occur Monochromators typical cut the characteristic x-ray line to ~0.3 ev Focus beam onto sample Insulating samples require an electron flood gun to neutralize charge build-up 8/18/2010 36
Monochromated vs. non-monochromated X-rays XPS Analyzers Non-monochromated Bremsstrahlung radiation creates a higher background 8/18/2010 37 8/18/2010 38 Photoelectron Detection Hemispherical Analyzer Analyzer resolution is typically 1% of the pass energy In order to get 0.3eV resolution, need a pass energy of 30 ev SSI uses fixed pass energy. A retarding lens at the input of the detector enables this. Some other systems vary the pass energy. 8/18/2010 39 8/18/2010 40 SSI Analyzer Resolution Analyzer Transmission Calibration SSI Resolution# 1 2 High Res 3 4 Low Res SSI instrument is only calibrated for pass energy 150eV (Resolution 4) 8/18/2010 41 8/18/2010 42
Pass Energy Calibration Peak Widths Photon width typically ~0.3 ev for monochromated x-rays Peak x-ray type Binding Energy (ev) Al Ka Mg Ka Monochrom -ated Al Ka Au 4f 7/2 83.95 83.95 83.96(84) Cu 2p 3/2 932.63 932.62 932.62 Ag3d 5/2 --- --- 368.21 SSI system uses Au 4f7/2 peak and the Cu 2p3/2 peak to calibrate the energy scale Calibration will be performed bi-weekly Typical drift between calibrations is <~0.1eV Natural peak width Analyzer/detector width (0.25 to 1.5 ev) At high resolution (i.e. low pass energy), E a is typically the smallest value E p typically ~0.3 ev for monochromated x-rays Possible to calculate/estimate x-ray decay lifetime from E n if this is the largest contribution to peakwidth 8/18/2010 43 8/18/2010 44 SSI Instrument Parameters Manufacturer: Surface Science Instruments (SSI) Model: X-Probe (SSX-100) X-ray kv and ma emission: 10kV, 1.5 to 22.5 ma (spot-size dependent) X-ray Energy: 1486.6 ev (8.3393 Å) Analyzer Type: 180-degree hemispherical Binding energy range: -50 to 1100 ev System Base Pressure: < 10-9 Torr Normal Operating Pressure: 1.6 x 10-9 Torr Angle of X-ray incidence: a = 71 (relative to sample normal) Standard Electron emission angle: = 55 (relative to sample normal) Angle between X-ray and Analyzer axes = 71 (fixed, non-variable) 8/18/2010 45 SSI Instrument Parameters Spot Sizes: 150, 300, 600, 1000 microns Energy Resolution (Au4f) 0.8 to 1.8 ev Pass Energies: 150 V for Resolution 4 setting (must use for calibrated analysis) 100 V for Resolution 3 setting 50 V for Resolution 2 setting (most common for high-resolution analysis) 25 V for Resolution 1 setting Detector type: SSI Position Sensitive Detector, resistive anode, 40mm x 40mm, electronically defined as 128 active channels with maximum count rate of 1,000,000 Typical Information given: Sample was analyzed using a Surface Science Instruments SSX-100 with operating pressure < 2x10-9 Torr monochromatic AlKa x rays at 1486.6 ev. Photoelectrons were collected at an angle of 55-degrees from the surface normal hemispherical analyzer with pass energy of 8/18/2010 46 External Labs A new system can easily cost $500k ($25k/year depreciated over 20 years) Can charge >$300/hr machine time Can charge for all time that your sample is in the system, including pumpdown time You work on their schedule. Can take weeks to get your data or sample back. Basis for User Fees Academic machine time currently $30/hour Technician time $85/hour Cornell-subsidized academic rates Typical sample costs: ~0.5 to 2 hours per analysis spot, depends on: surface cleanliness/roughness count rates 1/2 hour tech time ($85/hr) + 1 hour machine time, includes: Sample prep and setup Pump-down time Assistance with data analysis, etc. Requires over $30k/year to run this system in Maintenance Upgrades Personnel costs User fees and usage by appropriate groups justifies having/keeping the system. Rates can be adjusted at any time 8/18/2010 47 Introduction to X-ray Photoelectron Spectroscopy (XPS) Sources of Information Principles of XPS and Auger How to prepare samples for XPS Instrumentation, X rays, Photoelectron detection Data acquisition Quantitative and Qualitative analyses Spin-orbit splitting, Plasmons, Shake-up, etc. Sample charge control Overlayer effects Ion sputtering 8/18/2010 48
Relative Sensitivity Factor (RSF) I peak a n electrons (E) scofield T detector (E) more simply I peak a n atoms RSF Scofield I peak is also referred to as a relative sensitivity factor (RSF) or atomic sensitivity factor(asf) n electrons is the electron population scofield is the Scofield cross-section T detector (E) is the transmission function of the detector at peak energy E RSF values are somewhat welldefined and vary between instrument brands 8/18/2010 49 Quantitative Analysis Typically use the peak with largest RSF value in calculations Can use survey scan data or peak scan data to calculate atomic%, if taken at Resolution4 If doublet peaks are close together, use combined RSF values. RSF Au 4f7/2 = 9.58 Au 4f5/2 = 7.54 Au 4f = 17.12 = sum of both Au 4f peaks 8/18/2010 50 Detection Limits Typical detection limit is 0.1 to 1 atomic % Factors that affect the detection limit: RSF Signal-to-noise ratio in the spectrum Time to acquire data Energy resolution of the analyzer Peak overlap issues Light elements may have fewer peaks Interference from other XPS and Auger peaks Overlayer effecting the information depth. Background choice Accurate Precise Accurate & precise Quantification accuracy is about 10-20%, so a 50-50 alloy may be seen as a 50% (+/- 5 to 10%) Detection precision is excellent and generally very repeatable 8/18/2010 51 Background Subtraction Background is produced by inelastic scattering of photoelectrons Shirley background assumes the background is proportional to the # of electrons with kinetic energies higher than the peak energy. Generally better than a straight line for metal peaks. Linear background is determined by the endpoints. Generally works well for polymers and insulators 8/18/2010 52 Tougaard Background Subtraction Tougaard utilizes whole survey spectrum to create background Artifacts X-ray damage. A sample that undergoes a material change due to x- ray exposure or heat. Ghost peaks. Impurity elements in the x-ray source A dual anode source can have up to 2% cross-talk (2% of x-rays coming from the other anode) Charging of poorly conducting samples Charging may vary over time or a sample may take a long time to equilibrate Causes a shift in measured peak energies Peak broadening If only parts of a sample charge, peaks from those areas will shift 8/18/2010 53 8/18/2010 54
Sample Damage Due to Irradiation Spin-Orbit Splitting Can perform multiple scans of the sample over time to check for degradation or damage Peak doublets can make analysis trickier due to: Making background choice more difficult Greater likelihood of interference with other peaks or artifacts Gold 4f 5/2 and 4f 7/2 peaks 8/18/2010 55 8/18/2010 56 Overlayer Effects Overlayer Thickness Hill, JM et. al., Chem. Phys. Lett. 1976; 44: 225 t is overlayer thickness is the electron attenuation length in the film I o and I s are peak intensities from film and substrate, respectively s o and s s are film/substrate sensitivity factors a) Copper thin film on gold b) Heterogeneous structure c) Buried thick copper layer between gold d) Copper substrate beneath gold???? Can measure layers covering film or substrate, including organic layers Instrument independent Works for large and small film thicknesses Assumes I o and I s originate from similar photoelectron energies Thickogram studies account for differing photoelectron energies 8/18/2010 57 8/18/2010 58 Thickogram Surface Plasmon Effects A C Calculate intensity ratios (A) Calculate KE ratios (B) Plot and draw a line to calculate thickness (C) dotted line denotes thicknesses that are difficult to measure in practice (low-signal to noise) Photoemitted electrons can interact with surface plasmons and generate resonance at integer multiples of the plasmon frequency This interaction reduces the primary peak intensity and is distributed to the plasmon peaks Seen typically in metals or materials with free electrons B E o and E s are KE (not BE) of film and substrate peaks Applicable to a wide range of KE above ~500eV Applicable for emission angles up to ~60 from surface normal 45 emission angles minimize errors due to elastic scattering and surface roughness 8/18/2010 59 8/18/2010 60
Chemical Shifts Chemical Shifts- Electronegativity Effects There is a redistribution of charge of the outer electrons when a chemical bond is formed This results in a shift in binding energies of core electrons Consult Briggs/Grant textbook for more info calculating/estimating shifts Many chemical shifts listed in the XPS handbook 8/18/2010 61 Chemical shift (ev) is proportional to the summation of nearest neighbor interactions There are several electronegativity scales (Pauling, e.g.) Double-bonds have twice contribution of single bonds 8/18/2010 62 Shake-up Shake up generally due to electron interaction with ringed-carbon structures Sputter/ion Cleaning Utilize as a last resort or necessity Sputtering a sample surface can remove impurities Depth profiling can be very informative and can produce A LOT of data Depth profiling can cause Sample damage Surface roughening due to varying sputtering rates of elements Implantation of sputtering gas 8/18/2010 63 8/18/2010 64