XPS /AES

Surface Analysis >1000 nm 100 nm <10 nm Bulk Analysis Thin- - film Analysis Surface Analysis Primary beam (source) Secondary beam (spectrometers, detectors) Ions Electrons Photons Ions Electrons Photons Vacuum XPS AES XPS, AES 2

History 1905 Photoelectric effect by Einstein while studying electron emission induced by X-ray excitation in a Wilson cloud chamber, Pierre Victor Auger observed electron emitted with constant energy independent of the X-ray energy Robinson and Young, who observed the line shift caused by chemical bonding The development of electron spectrometers in Uppsala by Kai Siegbahn, the first XPS spectra with high resolution, investigation of core level binding energies and their shifts due to chemical bonding Siegbahn received the Nobel Prize in physics for his achievements in ESCA ( he also coined the acronym ESCA) The first commercial AES instruments for surface analysis the use of XPS in materials increased dramatically with the development of digital instruments with multichannel detection and with higher analyzer transmission. 3

XPS and AES can answer questions: 1.Which elements are present at the surface? 2.What chemical states of these elements are present? 3.How much of each chemical state of each element is present? 4.What is the spatial distribution of the materials in three dimensions? 5.If material is present as a thin film at the surface, how thick is the film? how uniform is the thickness? how uniform is the chemical composition of the film? K-Alpha VG Scientific 220i-XL imaging multitechnique surface analysis system 4

Basic Principles and General Aspects Primary radiation: -X-ray (XPS) AlKα (1486,6 eV) MgKα (1253,6 eV) -Electrons (AES) LaB 6 el. gun Components of a typical AES or XPS instrument The same energy analyzer may be used for XPS and AES. 1Pa =10 -2 mbar = 0,75 * Torr; 1Torr = 133,3Pa = 1,333mbar: defines mean free path, avoid surface reactions/ contaminations ~10 -8 to torr 5

Primary Electron Sources Tungsten cathode source, wire filament Lateran resolution limited (current densities only about 1,75 A/cm 2 ) Lanthanum hexaboride crystal (LaB 6 ) cathodes Provide higher current density (lower work function and greater emission), 100 A/cm 2 Field Emission electron sources Brightest beam ( A/cm 2 ) with diameter 10 nm. Tungsten tip 6

XPS instrumentation UHV System Ultra-high vacuum keeps surfaces clean Allows longer photoelectron path length Electron analyser Lens system to collect photoelectrons Analyser to filter electron energies Detector to count electrons X-ray source Typically Al Ka radiation Monochromated using quartz crystal Low-energy electron flood gun Analysis of insulating samples Ion gun Sample cleaning Depth profiling X-ray source Mono crystal Flood gun Electron transfer lens Hemispherical analyzer Detector 7

Dual anode X-ray source wedge-shaped anode → two target materials the end of the Cu supported anode has two faces, one coated by an Al film, the other by a Mg film (typically, ~10 μm or more in thickness) either filament, F1/F2, can be switch-selected for electron bombardment of the desired anode face Al window prevents detection of secondary electrons from the X-ray source and isolates the pumping of the outgassing anode target from the UHV measurement chamber 8

Analyzer Most popular: concentric hemispherical analyzer (CHA) Potential difference ΔV placed across the hemispheres Centre line: radius R 0 = /200 mm (typically) Crucial parameter: pass energy E 0 Electrons entering at the entrance slit S with energy E 0 travel along the trajectory of mean radius R 0 and are focused at the exit slit F -V 1 and -V 2 negative, V 2 > V 1 Electrons entering at S with energy different from E 0 - follow different trajectories, not able to emerge at F Opposite to the entrance slit is a 2D detector consisting of a multi-channel plate, a phosphor screen. 9

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Basic Principles and General Aspects Schematic diagram of electron emission processes in solids. Left side: Auger process, right side: photoelectron process XPS: E binding =E photon -(E kinetic +Ф S ) E KL 1 L 2,3 (Z)=E K (Z)-1/2[E L 1 (Z)+ E L 1 (Z+1)]-1/2[E L 2,3 (Z)+ E L 2,3 (Z+1)] Empirical approach Kinetic energy of Auger electron AES: E KL 1 L 2,3 =E K -E L 1 -E L 2,3 -E inter (L 1 L 23 )+E R -F S depends on Z, characteristic for the element 11 BE: C 1s=285eV Mg 1s=1304 eV Au 1s=81000 eV

Basic Principles and General Aspects 12

Qualitative analysis 13 BE: Au 1s=81000 eV

Spin orbital splitting For p, d and f peaks, two peaks are observed. The separation between the two peaks are named spin orbital splitting. The values of spin orbital splitting of a core level of an element in different compounds are nearly the same. The peak area ratios of a core level of an element in different compounds are also nearly the same. Spin orbital splitting and peak area ratios assist in element identifications 14

Inelastic mean free path λ i = inelastic mean free path of an electron (electron of intensity I O emitted at a depth d below) According to the Beer-Lambert law the intensity I S of the same electron as it reaches the surface is Sampling Depth - depth from which 95% of all photoelectrons are scattered ( 3 λ ) XPS nm at 3λ, I/I 0 = 0.05 at 1000 eV, λ ≈ 1.6 nm 15

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XPS Photo electron spectrum of lead showing the manner in which electrons escaping from the solid can contribute to discrete peaks or suffer energy loss and contribute to the background; 17 Z

Elemental shift 18 Nitrogen

Chemical shift XPS of polymethylmethacrylate Sensitivity to chemical structures with XPS is short-ranged. 19 Lets think about chemical shift due to electronegativity

Chemical Shifts 20 N 1s spectra of First-Row Transition Metal Nitrides: ScN, TiN, VN, and CrN More covalent More ionic

Quantitative XPS Some XPS quantitative measurements are as accurate as ± 10% I i = intensity of photoelectron peak for element “i” N i = average atomic concentration of element “i” in the surface under analysis σ i = photoelectron cross-section (Scofield factor) for element “i” λ i = inelastic mean free path of a photoelectron from element “i”, varies with the kinetic energy of the photoelectron K = all other factors related to quantitative detection of a signal (assumed to remain constant during exp’t) I i = N i σ i λ i K σ i have been calculated for each element from scattering theory, specifically for AlKα and MgKα radiation For a multi-element surface with elements i, j, k 21

Angle-resolved XPS 22

Angle-resolved XPS (Si with native oxide) θ = 0º θ = 75º Si 2p Si oxide Si O 1s C 1s Si 2s Si 2p O 1s C 1s Si 2s Si 2p Si oxide Si 23

Depth analysis Calibration of depth scale 1. Sputtering rate determined from the time required to sputter through a layer of the same material of known thickness. 2. After the sputtering analysis, the crater depth is measured using depth profilometer. A constant sputtering rate is assume Ion Sputtering (0.5 – 5 keV Ar +) Causes physical and chemical damage 24

Depth analysis 25

Charging Compensation Electron loss and compensation Electrons move to the surface continuously to compensate the electron loss at the surface region. For metal or other conducting samples that grounded to the spectrometer 26

Spectroscopy Mode: Hemispherical energy analyzer With the entrance aperture closed, an energy dispersed image (reciprocal image I.e. spectrum) is projected onto the detection plane. Imaging Mode: Spherical mirror analyzer With the entrance aperture open, a spatially dispersed image (real image) is Kratos AXIS Ultra image (real image) is projected onto the detection plane. XPS imaging 27

Spectroscopy Mode: Hemispherical energy analyzer With the entrance aperture closed, an energy dispersed image (reciprocal image I.e. spectrum) is projected onto the detection plane. Imaging Mode: Spherical mirror analyzer With the entrance aperture open, a spatially dispersed image (real image) is Kratos AXIS Ultra image (real image) is projected onto the detection plane. XPS imaging 28

XPS non-destructive surface-sensitive Quantitative measurements Provides information about chemical bounding expensive High vacuum is required Slow processing H and He can not be identified 29

Electron beam – Sample interaction 30

Basic Principles and General Aspects Schematic diagram of electron emission processes in solids. Left side: Auger process, right side: photoelectron process XPS: E binding =E photon -(E kinetic +Ф S ) E KL 1 L 2,3 (Z)=E K (Z)-1/2[E L 1 (Z)+ E L 1 (Z+1)]-1/2[E L 2,3 (Z)+ E L 2,3 (Z+1)] Empirical approach Kinetic energy of Auger electron AES: E KL 1 L 2,3 =E K -E L 1 -E L 2,3 -E inter (L 1 L 23 )+E R -F S depends on Z, characteristic for the element 31

Auger electron vs x-ray emission yield 32

Auger spectrum Can use peak areas or peak-to-background ratios Auger spectrum - quantification Concentration of element N A is: N A = I A /(I A + F AB I B +F AC I C +....) I is the element intensity F is a sensitivity factor determined from binary standards such that: F AB = (I A /N A /I B /N B ) 33

AES Imaging and Spectroscopy dN(E)/E 34 C Map Cr Map

AES Imaging and Depth Profiling Electron Beam in combination with an SED detector allows for imaging of the sample to select the area for analysis. Fracture surface of Carbon fibers in BN matrix - analysis area outlined in black 35

Sample requirements Sample size limits analysis spot -1x2 mm ellipse. Largest size for a monochromatic beam of X-rays is 1–5 mm. Non- monochromatic - 10–50 mm in diameter. For AES – 0,5 μm Area typically ~ 10 mm x 10 mm, thickness ~ 1-2 mm 1cm 2 sample is easy to analyze, but samples can be smaller, larger, up to 80mm diameter. N of samples - as many as will fit on an 80mm diameter area. Typically analysis depth is ~5nm for metals and ~10nm for polymers. Analysis time 1–20 minutes - a scan that measures the amount of all detectable elements, 1–15 minutes - scan that reveal chemical state differences, 1–4 hours - depth profile that measures 4–5 elements as a function of etched depth 36

Comparison to other technics main characteristics of XPS, AES and SIMS (secondary ion mass spectrometry) CharacteristicsXPSAESSIMS Primary beamX-rayselectronsions Analyzed beamElectrons (energy) Ions (mass) Type of sampleAll, charging possibleConductiveAll, charging possible Area of analysis10 μm10 nm100 nm Surface selectivity1 to 5 nm 0.1 to 1 nm elementalAll, except H, He all Identification Sensitivity 0.1 % <1 ppm (dynamic) 100 ppm (static) QuantificationRequiring standards Requiring close standards Molecular identification Not possible Mostly possible Nature of chemical bonding Shift, straightforwardShift and shape requiring data analysis Not possible Depth profilingElemental, chemical Elemental Destructive natureNone if not sputtered always 37

Advantages / disadvantages 38

Exemple of experiment from a publication 39

Bibliography 1.Siegfried Hofmann Auger-and X-Ray Photoelectron Spectroscopy in Materials Science a user oriented guide. 2.Surface and thin film analysis: principles, instrumentation, applications. Edited by H. Bubert and H. Jenett. 3.Surface Analysis Methods in Materials Science J. O'Connor, B.A. Sexton, R.St.C. Smart, Series Editors: G. Ertl, H. Liith and D.L. Mills 4.Preparation and Application of Surface Science Model Catalysts in Realistic Conditions, Dissertation zur Erlangung des akademischen Grades, Doktor der Naturwissenschaften (Dr. rer. nat.) im Fach Physik eingereicht im Fachbereich Physik der Freien Universität Berlin, vorgelegt am 20. September 2013 von Franziska Ringleb 5.X ‐ ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) Rick Haasch, Ph.D. Supported by the U.S. Department of Energy under grants DEFG02-07-ER46453 and DEFG ER University of Illinois Board of Trustees. 40

Thank you for your attention! 41

Nomenclature of absorption edges 42