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|Instrument and Analysis Details Used to Make Spectral Data Contained in the |
XPS International Library of XPS Spectra
A. Instrument Details
Manufacturer: Surface Science Instruments (SSI)
S-Probe (upgraded from M-Probe model 2703)
Software Version; 1.36. (Compiled in MS-DOS "C" version 6.0)
Analyzer Type: Fixed Analyzer Transmission (FAT)
Fixed (Constant) Pass Energy
180° Hemi-spherical (truncated)
Input Lens Field of View: 30° for sample normal to lens axis (1" diam port)
(always larger than X-ray beam; retarding potential scanned)
X-ray Type: Al° monochromatic (one 2 " diameter thin natural SiO2
crystal wafer glued onto Zerodur substrate heated to 65° C)
X-ray kV and mA Emission: 10 KV, 1.5-22.0 mA (depending on spot size used)
X-ray Energy Defined as: 1486.7 eV (8.3393 Å), Bragg Angle=78.5°
Excitation Source Window: 0.6 µ aluminum in S-Probes (1-5µ mylar in X-Probe)
Angle of X-ray Incidence: a = 71° (relative to sample normal)
Electron Emission Angle: b = 0° (relative to sample normal)
Angle Between X-ray Axis
and Electron Analyzer Axis: f =71° (fixed, non-variable)
Pass Energy of Analyzer: 150 V for Resolution 4 setting
100 V for Resolution 3 setting
50 V for Resolution 2 setting
25 V for Resolution 1 setting
Type & Size of Input Slit: Fixed (2 mm X 35 mm); magnetic compression
Type & Size of Output Slit: None (dispersion limited by hemisphere voltages)
Electron Collection Lens Field of View : ~ 1 mm2 for b = 0° at 1000 eV KE
Electron Collection Lens Efficiency : 7% over 2p steradians
Sample Surface to Tip of Electron Collection Lens Distance: ~33 mm
X-ray Crystal to Sample Surface Distance: ~190 mm
X-ray Crystal to X-ray Anode Distance: ~190 mm
True Background Count of Noise: <10 electrons/second at -50 eV (shot noise limited)
Detector Type: SSI Position Sensitive Detector, resistive anode, 40 mm X 40 mm
electronically defined as 128 active channels, 1,000,000 cps
Dead Time: Zero (unless ion etching sample while collecting XPS data)
Base Pressure: 4. x 10-10 torr
Normal Operating Pressure: 1.6 x 10-9 torr
FWHM of X-rays Diffracted by natural SiO2: ~0.25 eV
Power Settings: 200 Watts in a 250 x1100 µ X-ray beam
100 Watts in a 150 x 800 µ X-ray beam
45 Watts in a 80 x 350 µ X-ray beam
15 Watts in a 40 x 250 µ X-ray Beam
X-ray Induced Current: 1.1 x 10-9 amps for a 600 µ spot in X-Probe
Converted from amps to watts:
Approximate True X-ray Power : ~6 x 10-6 W in a 600 µ spot
Approximate True X-ray Irradiance: ~8 W/m2
Approximate True X-ray Photon Flux: ~7 x 109 photons/sec
B. Experimental Details:
Electron Take-Off-Angle: 90° relative to sample surface (unless otherwise reported)
Pass Energies Used: Wide scans were done at PE = 150 eV
Narrow scans were normally done at PE = 50 eV
Valence band scans were done at PE=150 eV
X-ray Beam Size Used: Wide scans: 250 x 1500 µ ellipse (at 90° TOA)
(for S-Probe) 250 x 1100 µ ellipse (at 35° TOA)
Narrow Scans: 250 x 1500 µ ellipse (at 90° TOA)
150 x 1000 µ ellipse (at 90° TOA)
SSI Mesh-Screen: A 90% transmission (20 µ diameter wire with 200 µ
spacing) nickel metal mesh screen was adhered to
a small 25 mm x 25 mm x 1.5 mm (W x L x H)
aluminum plate over a 20 mm x 20 mm aperture. This
mesh-screen was placed over all oxide samples
so that the distance between the sample surface
and the mesh-screen was <1.0 mm but >0.3 mm.
Dwell Time (counting time): 200 milliseconds/channel (usual setting)
Data Transfer Time: 4 milliseconds
Max. Number of Channels: 5000 (channels = data points)
Scan Time for One Wide Scan: ~ 3.5 minutes (using 1024 data points)
Scan Time for One Narrow Scan: ~100 seconds (using 256 data points)
Energy Range: -100 to +1400 eV (BE range)
Typical Step Size: 0.1 eV/step (i.e. 0.1 eV/data point)
C. Data Processing Details:
Baseline Subtraction: None, unless S/BG gave a small display. When the
baseline was removed, the intensity of the lowest
point was subtracted from all points.
Data Smoothing: None
Energy Shifting: None
Intensity Scaling: None
D. Sample Details:
The "Description" given on each XPS spectrum reports the empirical elemental formula for the oxide, purity, source, production lot number, a note, if appropriate, about being conductive or semi-conductive, the abbreviation "scrn" which means that the SSI mesh-screen was used, and a number, e.g. 90 which reports the electron take-off-angle used to collect the data for that sample.
Abbreviations used in the description and their full meaning include: Aldr = Aldrich Chemical Co., RMC = Rare Metallics Co., semi-con = semi-conductive behavior, scrn = SSI mesh-screen used, TOA = electron Take-Off-Angle, Tech = technical grade purity, pellet = sample pressed into pellet form, plt = pellet, pel = pellet, MS Co. = Metal Samples Company in Munford, Alabama USA (Tel 205-358-4202), SPP = Scientific Polymer Products Inc. in Ontario, New York state, USA (Tel 716-265-0413)
Sources of Elements and Chemical Compounds Used for Element Series
The pure element samples were obtained from various sources without any specific information about sample purity so pure element samples must be assumed to be pure at the 99% level. The "halide" salts used to produce spectra from gaseous or highly reactive elements were also obtained from various sources. These halide samples were obtained as crystalline "windows" which are normally used in Infrared spectroscopy and have purities at the 99% level.
The Boron Nitride (BN) sample was a white ceramic standoff which was fractured in air. The copper foil material, which was always used to determine reference energies, were obtained as 99% pure foil which was designed as a multiple purpose foil for use around the home. The gold ingot material, which was also used to determine reference energies was obtained as a 99.999% pure sample from Aldrich Chem. Co..
Source of Polymer Materials
A special kit (#205) of the 100 polymer materials was obtained from Scientific Polymer Products, Inc. which is located at 6265 Dean Parkway, Ontario, New York, USA 13519 (Tel 716-265-0413).
Source of Alloys
A special kit of 54 metallic alloys was obtained from the Metal Samples Co., which is located at Route #1, Box 152, Munford, Alabama, USA, 36268 (Tel 205-358-4202). This kit includes a materials analysis report on each alloy in weight percents. The National Research Institute for Metals in Tsukuba Japan has provided a series of various binary alloys made of AuCu and CoNi alloys.
Sources of Semi-Conductor Materials
Over the course of many years, many people in the Japanese semi-conductor business have given samples of various semi-conductor materials in crystalline wafer form. Various samples were donated by the Oki Electric Company, Mitsubishi Materials, Canon, and various universities. The source of each material is included with the individual sample descriptions whenever that information was provided.
Sources of Binary Oxide Samples
Most of the commercially pure binary oxides were purchased from the Aldrich Chem. Co.. Many packages from the Aldrich Chemical Co. included an "Analytical Information" sheet which described an ICP or AA analysis summary, a production lot number, the Aldrich product number, sample purity number (e.g. 99+%), sample appearance (color and physical form), date of chemical analysis, formula weight and a label on the bottle that reports the melting point, toxicity, Chemical Abstracts registry number and density. The samples from Aldrich were generally quite pure at the surface.
Other oxide samples were obtained from either Cerac Inc. (USA) or Rare Metallics Co., Ltd. (Japan). The packages from Cerac Inc. included a "Certificate of Analysis" with an ICP or AA analysis summary, a production lot number, a product number, purity (e.g. 99+%),and mesh size. The packages from Rare Metallics Co. did not include analytical data reports, but instead had stock numbers and a purity statement.
Two samples (i.e. SiO2 natural crystal and Al2O3 fused plate) were obtained from in-house sources and do not have any purity reports.
Powdered Samples Pressed into 3mm Diameter Pellet
Until analyzed, all finely powdered samples were kept stored in their original glass or plastic containers, which were packaged inside of plastic-lined aluminum bags. Just prior to XPS analysis, each bottle was opened in the normal air of the room where the XPS system was kept, and a small 50-100 mg portion of the sample was removed via a clean nichrome spatula and placed in the compression chamber of a hand-operated, stainless steel pellet press. All finely powdered samples were compressed without any chemical treatments, which, if done, may have introduced unusual contamination or produced some change in the samples. The resulting pellets varied in thickness from 0.3 - 0.8 mm. To avoid iron and /or chromium contamination from the anvil, a thin sheet of paper was placed over the sample in the compression chamber.
Any powders, which were clumped together, were very gently pressed into a powder just prior to compression. To avoid unnecessary heat-induced oxidation, those samples which were hard and granular were very gently ground into a fine powder in a agate marble mortar and pestle.
As soon as each sample was removed from the compression chamber, it was mounted onto silver (Ag°) paint inside of a 5mm wide round brass boat which was 1.3 mm in height. Silver paint was used so that conductive oxides could behave as true conductors thereby providing true electron binding energies for those oxides that were indeed conductive. In general, each oxide was exposed to room air for <15 min..
Benefits of Pressing Powders into Pellets (increased counts and simple charge control)
A comparison of the electron counts obtained from powdered samples pressed onto double-sided adhesive tape and positioned at a 35° electron take-off-angle with the electron counts obtained from hand-pressed glossy or semi-glossy pellets positioned at a 90° electron take-off-angle (TOA) revealed that a pellet at a 90° electron TOA produces 3-5 times higher electron counts than a powdered sample pressed onto double-sided tape at a 35° electron TOA.
By pressing the finely powdered oxides into pellets, it was also found the surface charging behavior of these glossy or semi-glossy samples was very easy to control by using the mesh-screen electron flood-gun combination with the flood gun set to 4-6 eV acceleration energy and approximately 0.5 mA filament current.
Problems Caused by Pressing Samples into Pellets
By pressing the finely powdered oxides into pellets, the surface of the resulting samples were usually smooth enough to appear glossy or semi-glossy, but some samples had iron or chromium contamination which indicated that the oxide had undergone a pressure induced reaction with the stainless steel anvil.
Very strong hand pressure caused some oxides to react with the stainless steel anvil, but medium hand pressure usually did not produce undesired iron and chromium contamination. All analyses that showed any unexpected contamination were repeated.
Other forms of accidental contamination (chlorine or previously analyzed oxides) were caused by insufficient cleaning of the stainless steel anvil, which was normally cleaned with a metal polishing solution (Pikal) and rinsed with distilled water and isopropanol. All analyses that showed any unexpected contamination were repeated.
Solution to Pressure Induced Contamination of Pellets
Experiments on ways to avoid the pressure-induced iron or chromium contamination, produced pellets with semi-smooth non-glossy surfaces which required more effort to produce good charge control. These non-glossy surfaces also gave electron count rates that were about 10-50% lower than the glossy or semi-glossy surfaces. As a result, it appears that very smooth surfaces, which appear glossy or semi-glossy, greatly simplify efforts to control surface charging under the charge-control mesh-screen and also enhance the electron count rate by 10-50% more than a pellet that has a semi-rough non-glossy appearance.
Extensive experiments on different methods to avoid contamination of the pellets revealed that contamination is minimized or avoided by using freshly cleaned aluminum foil as a "buffer" between the oxide powders and the metals in the steel anvil components. The aluminum foil, which is sold as a kitchen wrap material, is cleaned with 100% isopropanol (isopropyl alcohol) just prior to use. The foil is cut to a size that is readily useful with the pellet press device after it is cleaned. Alternately, we have also used a type of "glycine" paper which is commonly used to as a paper to hold powders when weighing a powdered sample. This "weighing" paper is common in many chemical laboratories and can be substituted for the aluminum foil whenever the pressing results with the aluminum foil produce undesired binding results. The glycine paper method sometimes introduces very small amounts of contaminants which produce a N (1s) and C (1s) signals. The amount of these contaminants is much smaller than the amount of contaminants that occur by simply pressing the powder without any sort of paper or aluminum foil buffers.
Source of Pellet Press Equipment
"Qwik Handi-Press" from Barnes Analytical Division, Spectra-Tech, Inc.652 Glenbrook Road, Stamford, Connecticut, 06906 (FAX 203-357-0609)
Kit: Part # 0016-111 to 0016-121 contains 1,3, and 7 mm die sets. Originally purchased through Aldrich Chem. Co. in 1989.
E. Energy Resolution Details:
Analyzer Resolution vs Pass Energy:
Theoretical Analyzer Resolution Pass Energy Resulting Detector Width
0.25 eV 25 eV 3.5 eV window
0.50 50 7.0
1.00 100 14.0
1.50 150 21.0
Experimentally Observed Resolution vs Pass Energy (scanning mode):
Pass X-ray Resulting Resolution
Element (XPS signal) Energy Spot Size FWHM Number (1-5)
Si (2p3) crystal fractured edge 10 eV 40x250 µ 0.38 eV 5
Si (2p3) crystal fractured edge 25 eV 80x350 µ 0.43 eV 1
Au (4f7) ion etched clean 10 eV 250x1000 µ 0.64 eV 5
Au (4f7) ion etched clean 25 eV 250x1000 µ 0.79 eV 1
Au (4f7) ion etched clean 50 eV 250x1000 µ 0.86 eV 2
Au (4f7) ion etched clean 150 eV 250x1000 µ 1.40 eV 4
Ag (3d5) ion etched clean 10 eV 40x250 µ 0.42 eV 5
Ag (3d5) ion etched clean 25 eV 40x250 µ 0.64 eV 1
Ag (3d5) ion etched clean 50 eV 40x250 µ 0.75 eV 2
Ag (3d5) ion etched clean 100 eV 40x250 µ 1.00 eV 3
Ag (3d5) ion etched clean 150 eV 40x250 µ 1.30 eV 4
Cu (2p3) ion etched clean 10 eV 250x1000 µ 0.85 eV 5
Cu (2p3) ion etched clean 25 eV 250x1000 µ 0.94 eV 1
Cu (2p3) ion etched clean 50 eV 250x1000 µ 1.06 eV 2
Cu (2p3) ion etched clean 150 eV 250x1000 µ 1.60 eV 4
Cu (2p3) ion etched clean 10 eV 150x800 µ 0.85 eV 5
Cu (2p3) ion etched clean 25 eV 150x800 µ 0.96 eV 1
Cu (2p3) ion etched clean 50 eV 150x800 µ 1.05 eV 2
Cu (3s) ion etched clean 50 eV 250x1000 µ 2.35 eV 2
F. Energy Scale Reference Energies and Calibration Details
From May 1986 to January 1993
Energy Scale Reference Energies: 932.47 eV for Cu (2p3) signal
122.39 eV for Cu (3s) signal
83.96 eV for Au (4f7) signal
Binding Energy Uncertainty: <±0.08 eV
Digital-to-Analog (DAC) Conversion Setting: 163.88
After January 1993
Energy Scale Reference Energies: 932.67 <±0.05 eV for Cu (2p3) signal
122.45 <±0.05 eV for Cu (3s) signal
83.98 <±0.05 eV for Au (4f7) signal
Observed Reference Energy: 75.01 <±0.05 eV for Cu (3p3) signal
Binding Energy Uncertainty: <±0.08 eV
Digital-to-Analog (DAC) Conversion Setting: 163.87
Reference Energies of Adventitious Hydrocarbon Contaminants
From May 1986 to January 1993 the electron binding energy of adventitious hydrocarbons was assumed to occur at 284.6 eV based on SSI and C. D. Wagner's research and recommendations. Publications by P.Swift (Surface and Interface Analysis 4, 47 (1982), S. Kohiki and K. Oki (J. Electron Spectrosc. Related Phenom. 33, 375-380 (1984), and G. Barth, R. Linder and C. E. Bryson, III (Surface and Interface Analysis 11, 307-311 (1988) have shown that the electron binding energy for various hydrocarbon contaminants and polymers is not necessarily a constant number. Research by this author indicates that the electron binding energy for adventitious hydrocarbons lies somewhere between 284.4 and 287.0 eV depending on the underlying oxide materials. By taking a simple average of all available binding energies, the author has found that 284.9 eV is preferred for hydrocarbons on ion etched metals where the hydrocarbon is many hours old. For naturally-formed native oxides the preferred binding energy is 285.2 eV. Oxide based materials at the far left of the periodic element table (columns 1-4) tend to have higher values (285.2-287.0 eV, while most of the transition metal oxides center around 285.0 eV. Near the far right of the periodic table, the binding energy again rises to a 285.2-286.5 eV range (columns 12-14). In routine practice, this author prefers to use the 285.0 eV number.
Some potential factors that may cause this rather large range of electron binding energies for adventitious hydrocarbon contamination includes the dipole moment at the surface of the oxide material, which is expected to be much stronger than the dipole moment of a pure metal, and also, in the case of naturally formed native oxide films, the thickness of the native oxide, any physical or chemical treatments, the thickness of the adventitious hydrocarbon layer, and the type of instrument used to analyze the sample. The type of instrument being used may cause different shifts in the observed binding energy of the adventitious hydrocarbon contamination because the source may or may not generate different amounts of low energy secondary electrons from the window that protects the X-ray source. The heat from the source and contamination that degases from a just turned on source may also influence the observed binding energy. Electron flood guns may or may not influence the binding energy as well.
Instrument Stability and Long Term Calibration
Initially each of the three SSI systems, that we have used, was calibrated 2-3 times per week because its ability to maintain accurate voltage settings was unknown. Once it was determined that the systems could maintain reliable voltage settings for 1-3 months, it was decided that good calibration could be maintained by checking and, if necessary, correcting the pass energies of the system on a 2-4 week basis. Each of the three SSI XPS instruments, that we have used, have been calibrated on a routine basis every 2-4 weeks by using SSI's reference energies. By using this method over several years time, it was found that the maximum uncertainty (error in pass energies) was normally <±0.10 eV, but a few times rose to ±0.15 eV or less. In a very rare case, the uncertainty rose to 0.20 eV.
Long term use of the SSI systems has shown that the DAC circuit does not change enough to be observed unless the room temperature changes by more than 10 deg Centigrade. If the room temperature changes within a few hours time by more than 10 deg or the temperature of the DAC chip is changed by more than 10 deg, then a >0.1 eV shift, which is much smaller than the reliability of almost all literature BEs, can be observed.
Variables, which seem to cause pass energy settings to change slightly, include building line-voltages, ion etching conditions, and the addition or removal of some electrical device.
G. Electron Counting and Instrument Response Function Details (for X-Probe System only)
Instrument Response Function: Q(E)=E+0.27 for 150 eV PE (ref.3)
Instrument Response Function: Q(E)=E+1.0 for 50 eV PE (ref.3)
Lens Voltage Settings Available via Software under Instrument Calibration
Pass Energy* 29.6-29.8 54.7-54.9 105.1-105.3 155.9-156.2
Detector Widths 3.743 7.486 14.954 22.297
Sensitivity Exponent 0.7 1.1 1.3 1.5
V1 Offset 30 55 105 155
V1 Slope 0.600 0.611 0.676 0.709
* These pass energies include corrections for instrument work function.
True pass energies were set to 25, 50, 100, and 150 eV ±0.1 eV.
Signal/Background Ratios for Ion Etched Silver using a 250x1000 µ Spot*
Pass Energy 25 eV 50 eV 100 eV 150 eV
S/BG ratio** >140 >110 >70 >50
* Using a 90° electron take-off-angle and a smooth Ag°/mylar film.
** The S/BG ratio is a simple numerical ratio of electrons counts at the peak maximum
relative to the average electron counts observed at approximately 10 eV lower BE.
H. Effects of Poorly Focussing the Distance between the Sample and the Electron Lens
If the focus distance between the sample surface and the electron collection lens is poorly adjusted, then the number of electron counts drops very quickly. A 0.5mm error in focus produces a >300% decrease in counts, but does not produce any observable error in binding energies, which is a common problem with many other instruments. A 0.1mm error in focus produces a 15% decrease in peak area counts and is easily observed as a horizontal displacement in the static (un-scanned mode) XPS signal as observed on the standard CRT display of the detector response. Such a decrease in signal intensity generally causes the operator to correct the focus error so as to maximize the electron count rate. In this manner, the operator has avoided any chance of obtaining false BE readings and has accurately reproduced a nearly absolute focus point which greatly increases the quantitative accuracy of any unknown sample.
Experiments with the Bragg angle alignment of the crystal indicated that the maximum error due to an unusual bad alignment of the crystal would be <0.1 eV. To observe an error greater than 0.1 eV, the electron counts were found to decrease by >50%.
J. Quantitation Details and Choice of "Sensitivity Exponents"
By default, the SSI software uses a 0.7 number as the sensitivity exponent factor for each pass energy setting which are used in an equation that modifies theoretically calculated atomic photo-ionization cross-sections (John H. Scofield) to generate relative sensitivity factors that are valid for this XPS systems and which can be used to generate valid atomic percentages. The 0.7 value produces a ±10% accuracy in quantitative results for XPS signals obtained by using a 150 eV pass energy and occur within the 0-700 eV BE range. For signals that occur at higher BEs, it is generally necessary to change the sensitivity exponent factor to a 1.1 or higher value. To measure signals obtained by using other pass energies for quantitation, it is necessary to use other sensitivity exponent factors, if the user desires to maximize quantitative accuracy.
To determine useful sensitivity exponents, it is possible to use freshly ion etched poly-crystalline copper foil to test the validity of the sensitivity exponent for larger BE ranges and different pass energies. By integrating the peak areas of the Cu (2p1), Cu (2p3), Cu (3s), Cu (3p) and Cu (3d) signals with a modest amount of attention to baseline end points it is possible to perform trial and error choices of the sensitivity exponents until a useful number is determined. Once a useful number has been entered into the computer software routine, then the software can generate fictional atomic percentages for each of the integrated copper signals which will generate 20 atom % values with a uncertainty of ±1-2 atom %. If the exponent factor is severely wrong then the atomic percentages will generate numbers such as 10%, 11%, 26%, 24%, and 29% or perhaps 31%, 28%, 14%, 13%, and 14%. This trial-and-error approach may require 1-2 hours time and can be done on either wide scan data or more preferably narrow scan data for each of the 4-5 pass energies. This method, in effect, assumes that all five of the relative sensitivity factors for copper are reasonably correct. If wide scan data are used, this method requires a little extra effort to avoid the satellites associated with the Cu (2p) signals. This method, in effect, pretends that the pure copper sample is a standard material that is composed of 5 components which are present in 20 atomic % concentration. The objective is to change the sensitivity exponent until the software generates a 20 atom % result for each of the five copper signals.
After useful sensitivity exponents are found, they are tested by analyzing freshly exposed bulk regions of crystalline materials such as SiO2, Al2O3, and NaCl.
The high and low BE signals of the NaCl crystal are especially useful to test the validity of the sensitivity exponents. As further checks, the freshly exposed bulk of common polymers (e.g. mylar or PMMA) or a thin film of high purity silicone oil can also be analyzed. Teflon has repeatedly given slightly larger than desirable error by comparison to the other materials listed above. For that reason Teflon is a less desirable material to test the sensitivity exponents.
K. Crude Tests of the Reliability of Relative Sensitivity Factors
Crude testing of Scofield's numbers are included in atomic percentage composition tables that give atomic percentages for only one element. This testing used the software's automatic peak area integration software that is reasonably accurate.
The results indicate that some of the relative sensitivity factors for some of the weaker signals are less reliable. If, however, all factors are taken into account, then Scofield's numbers are reliable to a 95% accuracy level for truly homogeneous materials.
L. Traceability Details
The definition of traceability reported by Martin P. Seah and Cedric J. Powell in the J. Vac. Soc. Technol. Vol 8, p.736 (1990) publication is: "The property of a result of a measurement whereby it can be related to appropriate standards, generally international or national standards, through an unbroken chain of comparisons."
Traceability of Reference Binding Energies (Calibration)
At this time, there are no international standards for binding energies or reference energies. Numbers which are considered to be standard binding energies (BE), which would lead to traceability in BEs, include (a) those provided by Martin P. Seah at the National Physical Laboratory (NPL) in the United Kingdom (England), and (b) those provided by the ASTM in the USA "Standard Practice for Checking the Operating Characteristics of XPS Spectrometers" designated as "E 902-88". Other nations also have similar national standards, which tend to imitate those set by the USA and the UK.
Recently, many people in the world have been using NPL's reference energies, which have become "de facto" standards but have not yet been accepted by the International Standards Organization (ISO). There are still many workers and researchers using various numbers provided by the instrument makers. The author of this book was using Surface Science Instruments (SSI) Co. reference energies until December 1992 and then switched to NPL BEs in January 1993. SSI reference energies came from Hewlett-Packard (HP). SSI and HP both used high precision voltage meters from HP to calibrate their ESCA machines (i.e. X, M, and S-Probe and HP-5950 A-type and B-type, resp.). Hewlett Packard was the first company to offer a commercial ESCA system, which used reference energies developed in cooperation with Kai Siegbahn at Uppsala, who effectively developed ESCA into a useful science and received the Nobel Prize.
In a recent effort to improve the accuracy of BEs obtained from pure elements, the S-Probe pass energies were checked and corrected, if needed, almost every work-day for two months to obtain high precision and high accuracy BEs for the pure elements that are metals. This study used the NPL reference energies with Cu (2p3) at 932.67 eV with +/- 0.02 uncertainty and Au (4f7) 83.98 eV with +/-0.02 uncertainty by using 0.02 eV/pt. steps for the calibrations. To determine the "true" BE of each of the pure elements, which were scraped clean in air and then ion etched in vacuum, a 0.05 eV/pt. step was used. A repetitive ion etching (depth profile) style was used to collect wide scan, valence (Fermi edge) band, and narrow scans of the main signals for each metal at 50, 25 and 10 eV pass energies. Each repetitive experiment run lasted about 4 hours. Therefore, if NPL's BE numbers are accepted as "de facto" international standards, then the ultimate traceability of BEs in this data collection can be related to NPL BE numbers for Cu (2p3) and Au (4f7). In a different, but similar manner, the BEs used to calibrate the S-Probe are traceable to Siegbahn's work and HP's high precision, high voltage voltage meters.
Traceability Transfer from Pure Metals to Non-conductive Binary Oxides
A question that should be posed is traceability to the oxide BEs. Traceability begins with NPL's BEs for pure copper and gold as state above. Traceability then transfers to pure element BEs which are based on NPL reference BEs. Traceability then transfers to pure element BEs based on SSI's reference BEs, and then the naturally formed native oxide data published in Volume 2 of our XPS Spectral Handbook series where BEs were measured from pure element signals and also the naturally formed native oxide signals. Naturally formed native oxides typically have thin oxide films (10-80Å) which, in general, behave as good or true electrical conductors, which allows a direct measure of the true binding energy of many, but not all, binary oxides. To determine if traceability can indeed be transferred to true binary oxides, it was necessary to study the behavior of the naturally
formed native oxides by applying various flood gun settings with the samples grounded and insulated. The results from that study can be used to transfer traceability to the experimentally observed BEs of pure binary oxides. The most difficult transfer of traceability occurs for the naturally formed native oxide systems. If the flood gun study was not done, then it is difficult to transfer traceability in a reliable manner from a conductive metal to one of its corresponding non-conductive binary oxides.
Traceability of Instrument Response (Throughput) Function
Copper, gold and silver data obtained from the M-Probe system were submitted to Martin P. Seah at the NPL for a round robin test on transmission function; the results of which were published in Surface and Interface Analysis, p.243 (1993). In that publication, M-Probe data, which we contributed, were attributed to group #35. That paper reported that instrument has a Q(E) =E0.27 for Rex 4 pass energy (PE=150 V)and a Q(E) =E1.0 for the Res 2 pass energy (PE=50 V). If the NPL method is accepted as a "de-facto" standard, even though it is not an internationally recognized standard, then the transmission function and quantitation results of the S-Probe system are traceable to the "metrology spectrometer" at NPL.
Traceability of Relative Sensitivity Factors used for Quantitation
Scofield's theoretically calculated photo-ionization cross-sections are internationally used as the "de-facto" standard theoretical numbers, except in Russia and a few other places, where Band's numbers are preferred but are almost identical to Scofields. The SSI system uses a very simple equation that modifies Scofield's numbers to generate relative sensitivity factors that are used by the SSI software to calculate atom %s. That equation corrects for pass energy differences, transmission function differences, and inelastic mean free path versus kinetic energy dependency. The SSI system relies on Scofield numbers and that simple equation. Other instrument makers prefer to blend Scofield's numbers and experimentally determined numbers.
Traceability of Sample Purity.
The purity of the commercially pure (99+%) binary oxides can be traced to Aldrich's ICP or AA analyses performed by Aldrich. Copies of their results are included in the handbook at the beginning of each group of spectra. Similar data sheets were also obtained for samples bought from Cerac.
A set of gold, copper, and silver samples, i.e. "Reference Metal Samples SCAA90" set, kit #367, was obtained from the NPL and used to test the instrument response function of the M-Probe system. Binding energies obtained from those gold, copper, and silver samples were identical to binding energies obtained from our commonplace gold, copper, and silver samples within the expected uncertainty of ±0.08 eV used for routine instrument calibration.
M. Reference Papers Describing Capabilities of X-Probe and M-Probe XPS systems:
1. Robert L. Chaney, Surface and Interface Analysis, 10, 36-47 (1987) [re: X-Probe]
2. Noel H. Turner, Surface and Interface Analysis, 18, 47-51 (1992) [re: Quantitation]
3. M. P. Seah, Surface and Interface Analysis, 20, 243-266 (1993) [re: Response Function]
4. L.T. Weng et al, Surface and Interface Analysis, 20, 179-192 (1993) [re: Response Function]
5. L.T. Weng et al, Surface and Interface Analysis, 20, 193-205 (1993) [re: Response Function]
6. B. Vincent Crist, Surface Science Spectra, 1, 292-296 (1993) [re: KBr spectra]
7. B. Vincent Crist, Surface Science Spectra, 1, 376-380 (1993) [re: Ar/C spectra]
© 1997 XPS International Instrument & Analysis Details