Motto “Science is fun. I am still having fun” Charles Townes (2010 – 95 years old) - Nobel Prize in Physics (1964) for maser-laser XPS(ESCA) –Kai Siegbahn –Nobel Prize in Physics (1981) Acronyms XPS = X-ray Photoelectron Spectroscopy ESCA = Electron Spectroscopy for Chemical Analysis XPS / ESCA is one of the most powerful technique used in the surface, interface and thin film analysis according to ISO(TC-201: “Surface Chemical Analysis”). In an XPS experiment, the sample is irradiated by low energy (<1.5 keV) X-rays in an ultra high vacuum environment. This causes photo-ionisation (photoelectric effect) of the atoms at the specimen's surface: photoelectrons are emitted from the atomic energy levels with very specific Binding Energies(BEs) and , consequently, with a very accurate spectral signature/fingerprint for all the elements from the Periodic Table and their chemical compounds. The quantitative sensitivity is in the range of (10 -2 – 10 -4 ) of a monolayer and the surface sensitivity is in the range of (2-100) monolayers (<0.5 – 20nm) Schematic diagram of the photoelectric effect. [The photon energy hν is well known (1486.6eV for AlKα), the Kinetic energy(E K ) is measured by the analyzer. Thus, we calculate the Binding Energy(BE) = hν + E K. This is very specific to every element from the Periodic Table and - by chemical shift and other features – to every associated chemical species]. By simplifying: XPS = the photoelectric effect + the shell model
Transcript
Motto “Science is fun. I am still having fun” Charles Townes (2010 – 95 years old) - Nobel Prize in Physics (1964) for maser-laser XPS(ESCA) –Kai Siegbahn –Nobel Prize in Physics (1981) Acronyms XPS = X-ray Photoelectron Spectroscopy ESCA = Electron Spectroscopy for Chemical Analysis XPS / ESCA is one of the most powerful technique used in the surface, interface and thin film analysis according to ISO(TC-201: “Surface Chemical Analysis”). In an XPS experiment, the sample is irradiated by low energy (<1.5 keV) X-rays in an ultra high vacuum environment. This causes photo-ionisation (photoelectric effect) of the atoms at the specimen's surface: photoelectrons are emitted from the atomic energy levels with very specific Binding Energies(BEs) and , consequently, with a very accurate spectral signature/fingerprint for all the elements from the Periodic Table and their chemical compounds. The quantitative sensitivity is in the range of (10-2 – 10-4) of a monolayer and the surface sensitivity is in the range of (2-100) monolayers (<0.5 – 20nm)
Schematic diagram of the photoelectric effect. [The photon energy hν is well known (1486.6eV for AlKα), the Kinetic energy(EK) is measured by the analyzer. Thus, we calculate the Binding Energy(BE) = hν + EK. This is very specific to every element from the Periodic Table and - by chemical shift and other features – to every associated chemical species]. By simplifying: XPS = the photoelectric effect + the shell model
The XPS capabilities.
1. Element identification by photoelectron and Auger lines both qualitatively and quantitatively:
by recording Survey (wide scan) spectrum : 0-1400 eV , in steps of (0.5 – 1.0) eV
2. Surface, interface, thin films chemistry and, for powders, even the bulk chemistry:
by recording High-resolution (narrow scan) spectra : (0-10) eV up to (0-70eV) for complex spectra in steps of ( 0.05 – 0.5) eV.
We find out the chemical environment of the detected elements by their chemical(spectral) fingerprint / signature:
- the chemical shift (the most imp. source) in both photoel. and/or Auger lines - the shake-up / shake – off sattelites - surface / bulk plasmons - multiplet splitting - electron – hole pairs
Tabel 1 The spin –orbit splitting parameters. ____________________________________________________________ Atomic level j = l ± s Area ratios ____________________________________________________________ s 1/2 - p 1/2, 3/2 1/2 d 3/2, 5/2 2/3 f 5/2, 7/2 3/4 ___________________________________________________________ Except s-lines , all the other photoelectron lines are doublets ( p,d,f states). The above information help us to perform very useful constraints during the deconvolution process.
Inelastic mean free path (imfp) (nm) vs. kinetic energy (KE)
Samples All kind of solid samples can be accommodated on specific platens (standard, angle resolved, powders). Some requirements:
- Volatile substances must be removed prior the sample loading - Some organic contaminants must be removed with appropriate organic
solvents - Neutralization (for insulating samples) and Argon ion sputtering is highly
recommended - Argon ion etching is commonly used to obtain information on composition vs.
sampling depth - Some materials can be processed , fractured or scraped in a glove box - Some powders can be ground in a mortar to expose a fresh surface or the
bulk. The mortar must be very careful cleaned before reuse - Abrasion can be done in a glove box in an inert atmosphere - Sample transportation require a chamber sealed under vacuum or a glove
box under inert atmosphere - Pressing a powder into Indium foil or spreading onto a double sided scotch
tape is often used.
Intraocular lens
Teflon Polyethylene
Vascular Implant ( Dacron Fabric)
Polyethylene terephthalate (PET)
Filter paper
Types of XPS data 1. Survey (wide - scan spectrum) A Survey is one spectrum acquired from a quick, high-sensitivity scan of a wide energy range (typically 0 to 1100 eV in 1 eV steps) to survey the elements present at a point on the sample or over an area. In point analysis, a stationary x-ray beam is positioned on a specific point. In area analysis, the beam is rapidly scanned, or rastered, over an area or multiple areas of the surface.
SEM-like Ease of Use
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Binding Energy (eV)
-F KLL
-F 1s
-O 1s -C 1s
Si 2s,2p
-O KLL
Atomic %F 50.9 C 41.2 O 4.4 Si 3.5
Atomic %C 73.7 O 24.8 Si 1.5
100 µm
100 µm
Area 2 Area 1
Secondary Electron Image
Area 1
Area 2
Spectra obtained using 20 µm diameter x-ray beam in less than 5 minutes
Confident point and click analysis area definition
Survey spectra from selected areas identify an unknown contaminant
SEM-like Ease of Use
02004006008001000Binding Energy (eV)
-Zn 2p1
-Zn 2p3
-O KLL
-O 1s
-Zn LMM
-N 1s
-C 1s
-Si2s -Si2p
-Zn3p3
-O2s
Atomic % C 1s 74.5 O 1s 22.0 N 1s 1.7 Si 2p 1.1 Zn 2p3 0.7
100 µm
100 µm
Point 4
Point 4
Spectrum obtained using 10 µm diameter x-ray beam detected
Zn, possibly Zn stearate
Secondary electron images, maps, and 10 µm spectra quickly identified a contaminant that would go undetected in a non-microprobe system
Secondary Electron Image
Large Area Spectroscopy
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-O1s
-C1s
-Cl2s -S2s
-Cl2p -S2p
-Pt4f
Surface composition (atom %) C 95.1 O 3.3 Cl 0.9 Pt 0.6 S 0.2
High Sensitivity (high speed) Large Area Spectroscopy
Pt Catalyst on Graphite Support
Excellent detection limits for heavy and light elements
1400 x 300 um analysis area 100 watts, Al Kα 280 eV pass energy 3 minute analysis time
Pt/V samples : survey spectra
5V fresh 5V used Pt 5V fresh Pt5V used
O KLL
O 1s
C 1s
Al 2p Al 2s
V 2s V 2p
Al KLL
2. Multiplex: High- resolution (narrow) spectra
Pt 4f (7/2,5/2) deconvolution constraints: Area5/2 : Area 7/2 = 3 / 4 and ∆ (spin - orbit splitting parameter) = 3.33 eV
Large Area Spectroscopy
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C 1s 95.1 atom%
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Pt 4f 0.6 atom %
3 minutes 26 eV pass energy 15 minutes 26 eV pass energy
PtCl4
Pt(OH)2 PtCl2
High Sensitivity - High Resolution Large Area Spectroscopy
Pt Catalyst on Graphite Support
Graphite
SEM-like Ease of Use
100 µm
Area 2 Area 1
Secondary Electron Image
275 280 285290295300Binding Energy (eV)
Carbon 1s Spectra
Area 1 Fluorocarbon Contaminant
Area 2 PET
CF2 CH
O=C-OC-O
Spectra obtained using 20 µm diameter x-ray beam in less than 5 minutes
Detailed ‘high resolution’ spectra from selected areas provide chemical state information about the contaminant
Other spectra deconvolutions
Complex XPS spectrum of Ce 3d showing the deconvolution process performed by imposing constraints both on area ratios of the doublets (see Table 1) and on the spin - orbit splitting parameter (see the PHI-Handbook of XPS)
Normalized, superimposed XPS spectra. By using CeO2 (Ce4+) as reference we can detect the contribution of the 3+ oxidation states in the other samples (corresponding to the doublet number 2 in the above spectrum) Thus, the ratio Ce3+ / [Ce(3+) + Ce (4+)] can be determined straightforward .
Ti compounds in a glass sample
Ti and TiO2 standard XPS spectra together with a complex Ti spectrum recorded after Ion Beam Oxidation (IBO) experiment
Ti 2p photoelectron lines of the element, full oxide and suboxides
Ti 2p “band –like” XPS spectrum
The Silicon deconvoluted spectrum.
The suboxides (1+, 2+, 3+ oxidation states) can be observed together with elemental Si (2p3/2, 1/2) and the native full oxide (4+ oxidation state).
High resolution Appearance Potential spectra of Cr, Fe and Ni from the bulk of the “304” stainless steel.
The surface is coated with Cr2O3 which is responsible for its excellent properties against corrosion. The unoccupied DoS from the Conduction Band can be calculated after data processing while the energy resolution could be as high as in the UPS technique. (Dr. Petre Osiceanu – PhD thesis ; the spectra were recorded using a home-made, artisan APS instrument).
3. Depth Profiles An XPS Depth Profile provides compositional data as a function of depth. A sputter depth profile consists of a series of spectra that have been collected at different depths as material was removed from the sample. An XPS depth profile is generally made by alternating sputtering with data acquisition.
High Performance XPS Depth Profiling
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Atomic Concentration (%)
O
Sn (oxide)
Sn (metal)
C
Pb (oxide)
Pb (metal)
Point #1 – On Solder Bump
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Point #2 – Off Solder Bump
20 µm X-ray Beam - 500V Ar Ion Beam – Etch Rate ~0.5 nm/min (SiO2)
Solder Mask Material
Pb Residue on Solder Mask
High Performance XPS Depth Profiling
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metal
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Point #1 – On Solder Bump Point #2 – Off Solder Bump
20 µm X-ray Beam – 78 seconds/spectrum
4. Angle Resolved XPS (ARXPS)
An XPS Angle Profile provides compositional data as a function of depth. Angle-Resolved (angle-dependent) profiles are used to probe the near surface region of a sample in a nondestructive manner. Angle Profiles are possible, because the input lens of the analyzer (SCA) can operate with a small acceptance angle, and it is possible to tilt the sample in front of the analyzer. The analysis depth at a given angle is defined by the equation: d = λ sin θ where: d is the effective analysis depth, λ is inelastic mean free path (escape depth) and θ is the angle between the sample surface and the analyzer input lens. Angle Profiles are used typically to measure the thickness or explore the chemistry of very thin layers. Applications include the study of chemically modified surfaces, lubrication, cleaning processes, and such.
5. Mapping XPS A Map is a set of intensity-value arrays acquired over the area of the sample to show the surface distribution of specific elements. Each array of intensity values corresponds to an element, and each value in the array corresponds to a point in the map area. The intensity value is obtained by measuring the intensity at a specific XPS peak energy, then subtracting the background intensity. Acquiring map data for every element identified in the survey will completely characterize the distribution of elements in the analysis area. These maps are then compared to the SXI image or platen view
Fig. 1: X-ray beam induced secondary electron image and survey spectra from selected areas on a contaminated polymer surface that show the presence of fluorine in the contaminated area.
Fig. 2: High resolution carbon 1s spectra from the same selected areas that show the presence of a fluorocarbon contamination in localized areas on the polymer surface.
Fig. 3: Secondary electron image showing a selected area for XPS imaging and a color overlay image of C and F 1s maps confirming the presence of a localized fluorocarbon surf. contaminant
Quantification :from spectra to numbers (relative concentrations) The following quantitative results are obtained with errors <10% and <5% for using well known standards: -element relative concentrations (atom%) -oxidation states relative concentrations (%) -chemical states relative concentrations (%)
I ~ n S, S = Sensitivity factor (scaling factor) – SF • I – line intensity ( peak area) ( counts /sec ) • n = number of atoms of the element per cm3 for “x” element (cm-3)
SF = f (σ, λM , T) • σ = the photoelectric cross section for the atomic orbital of interest (cm2) • T = the transmission function of the spectrometer • λM = imfp (escape depth) of the photoelectrons in the sample - a function of the matrix M and the KE of the photoelectrons. Example1: Silicon oxide contaminated with adsorbed hydrocarbon “x”= Si, “y “ = O, “z” = C ( homogeneous distributed) Element rel. conc. (atom%) (NOT absolute density of atoms, n)
nx Ix / Sx • Cx = ------------------ = -------------------------------------- , Cy, Cz nx + ny + nz Ix / Sx + Iy / Sy + Iz / Sz
Atom ratios:
• nx / ny = Cx / Cy ……. We have to emphasize that the microscopic character of an heterogeneous sample influences the quantitative results. Moreover , an overlying contamination layer has the effect of diminishing the intensity of high BEs (low KEs) more that of the low BEs (high KEs) peaks. A useful test for quantitative work is the recording of the Cu three widely spaced spectral lines: Cu 2p3/2 = 932.67 eV, Cu L3MM = 567.96 eV, Cu 3p = 75.14 eV. These lines are also useful for BE scale calibration.
Si 2p
O 1s
C 1s
Surface Composition Table (from the above narrow spactra)
• Si O C • Sensitivity Factor(SF) 0.22 0.66 0.25 • Area 48586 96243 28196 • Area / scan 5398 19249 5639 • Area / scan / SF 5978 7718 5639 • Total sum 5978 + 7718 + 5639 = 19.335 • Rel. conc. (atom %) 5978 /19335 = 30.9 39.9 29.2 • Conclusion:Sample content: Si -30.9%, O -39.9 % and C - 29.2%. • Attn! Σ Rel. conc. = 100% ( 30.9 + 39.9 + 29.2 = 100%).
Example 2: Quantitative analysis from Survay spectra. Bayer aspirin and Romanian aspirin – a comparison Bayern Aspirin Interior
Romanian Aspirin Interior
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-O2s
-Na1s -O KLL
-O1s
-Na KLL
-C1s
-Si2s -Si2p
Atomic % C1s 56.7 O1s 35.1 Si2p 7.9 Na1s 0.4
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-O KLL
-O1s
-C1s
Atomic% C1s 64.0 O1s 36.0
Bayer Aspirin Exterior
Romanian Aspirin Exterior
0204060801000
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Binding Energy (eV)
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-C
-O K
-O
-O
-Si
-Si
-Mg
-M
-M
Atomic % O1s 52.9 C1s 28.6 Si2p 11.1 Mg2s 7.4
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-Mg2s -Mg2p
-O KLL
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-O1s
-C1s -Si2s -Si2p
Atomic % O1s 54.4 C1s 24.8 Si2p 12.6 Mg2s 8.1
Example3 Typical XPS Table showing both the surface chemistry(BEs) and the quantification (relative concentrations leading to stoichiometries). Table 2. T10YZ [ 10(TiO2)8(Y2O3)82(ZrO2)]; Element O(1s) Ti(2p3/2,1/2) Y(3d5/2, 3/2 Zr(3d5/2,3/2) Binding Energies(BE’s) (eV)
529.8 (O2
- species) 531.9 (OH groups)
458.3 463.6
157.3 159.4
182.2 184.6
Element rel. conc. (atom%).
62.8 1.6 6.1 29.5
Element rel. conc. of metal ions (atom%)
5.4 15.1 79.5
Nominal (intended) bulk stoichiometry : Ti0.092Y0.148Zr0.759 OY Experimental surface stoichiometry: Ti0.054Y0.151Zr0.795 OY We can notice a trend of Ti difussion from the surface to the bulk and the segregation of Zr to the surface, suggesting a process of substitution of Zr4+ ions by Ti 4+ ions in the lattice of the sample, while Yttrium remain constant within the experimental errors.
Applications in the polymers field (Carbopol)
Fig.1 The XPS wide – scan spectrum of the Carbopol sample
Fig2. The C 1s photoelectron spectrum after deconvolution
0.00E+00
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nts
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Binding Energy (eV)
Survey5 Scans, 5 m 40.3 s, 400µm, CAE 200.0, 1.00 eV
O1s
C1s
O-Auger C-Auger
O 2s
Table 1. The C 1s chemical states relative concentrations - CH2 – CH < and /or> CH – CH < 58.4% > CH – COOH 10.3% > CH – O - 3.4% - COOH 27.9%
Fig 2. The O 1s photoelectron spectrum after deconvolution Table 2. The O1s chemical states relative concentrations O = C < 47.0% HO – C 53.0% Table 3. Element rel. conc.(atom%) from high resolution spectra
Name At. % C 1s 65.8O 1s 34.2
XPS Valence Band Some VB (0-20 eV) features are shown in Figs.1-2 In this energy range the following XPS lines can be found: Zn3d(5/2 and 3/2) in : Zn0 / SiO2, ZnSe, (Zn5)CdSe and (Zn10)CdSe Cd4d(5/2 and 3/2) in : Cd0 /SiO2 , CdSe, (Zn5)CdSe and (Zn10)CdSe (Zn5) and (Zn 10) stand for 5 sec and 10 sec MBE deposition of Zn on the top of CdSe substrate. A good statistics are obtained by taking longer acquisition time .
Zn0 3d photoelectron spectrum superimposed on ZnSe, Zn5 and Zn10 BV
Cd0 4d photoelectron spectrum superimposed on CdSe, Zn5 and Zn10 BV.
Auger Parameter(AP) Some differences between Auger and photoelectron chemical shifts could appear as a result of final states effects involving two vacancies in the inner shells and one hole, respectively. This gives rise to differences in the polarization energies and, consequently in the ionization and kinetic photoelectron energies. The AP is defined as: α = KEA – KEP = BEP – BEA where P stands for “photoelectron” and A for “Auger”. The last difference can be accurately determined as the charging effect is canceled. We have used the modified Auger Parameter(AP) defined as: α' =AP = α + hν = KEA + BEP = (1253.6 eV - BEA ) + BEP .
Table1. Binding energies(BEs)(eV) and Auger Parameters(AP) of Zn XPS features in(Zn5)CdSe and(Zn10)CdSe - samples prepared for calibration of MBE. The values for Zn0 /SiO2 and ZnSe were added for comparison. Bremen selenides
Conclusion One can notice that Zn on the top surface layer of the ternary compounds is found in a chemical bonding with Se (like ZnSe ) according to Auger Parameter values. We have to emphasize that the BEs of the photoelectron lines offer no information on the presence of these chemical bonding because the chemical shift is insignificant.
(AP)_Zn0 = 2011.84 (AP)_ZnSe = 2013.75 Δ ~ 2 eV
Example : “Bremen Selenides” - “ in situ “ MBE +XPS experiments – ZnSe, CdSe, (Zn5)CdSe, (Zn10)CdSe + elemental: Zn0, Cd0, Se0 / Si Zn0 = elemental Zn…. (Zn5), (Zn10) stand for Zn deposition on the top of CdSe surface for 5 and 10 sec , respectively by MBE. -
STANDARDIZATION
ISO - 201 TC “Surface Chemical Analysis” SC1: Terminology WG1: Definitions of terms SC2: General Procedures WG1: Specimen Handling WG2: Reference Materials WG3: Reporting Results SC3: Data Management and Treatment WG1: Data Transfer and Storage WG2: Surface Science Data Models SC4: Depth Profiling WG1: Definitions and Procedures WG2: Reference Materials SC5: Auger Electron Spectroscopy WG1: Procedures for Quantification (joint with SC7) SC6: Secondary Ion Mass Spectrometry WG1: Quantification of B in Si WG2: Documentary Standards SC7: X - Ray Photoelectron Spectroscopy WG1: Instrument Specification and Operation (joint withSC5) WG2: Energy Scale Calibration (joint with SC5).
Acknowledgments The ULVAC – PHI team, in particular Dr. John Moulder and Dr. John Hammond, are gratefully acknowledged for helpful discussions during demo-session and seminars related to PHI Quantera instrument acquisition. Thus, the Sample, Depth profiling and Mapping sections use data provided by PHI together with SEM-like pictures and the spectra attached.
