Instrument Details, Parameters and Design
Philosophy for Data Collection
Our philosophy is to collect spectra under analysis conditions that are practical, readily reproduced, and typically used in laboratories that use monochromatic X-ray sources and work under real world practical analysis conditions. We have assumed that the most XPS laboratories need practical reference spectra and will not spend the time or money to produce and to analyze pure, clean surfaces under ultimate energy resolution conditions.
Instrument Capabilities and Specifications
Capabilities, Geometry, and Design Specifications for SSI S-Probe XPS Instrument
Energy Resolution Performance
Experimentally Observed Relation Between Energy Resolution and Measured FWHM from Reference Materials
| Element (XPS signal analyzed) | Resulting FWHM | Resolution Setting | Pass Energy | X-ray Spot Size |
| Silicon Wafer – Freshly Fractured Edge | ||||
| Si (2p3/2) crystal – fractured edge | 0.38 eV | 5 | 10 eV | 40 x 250 µ |
| Si (2p3/2) crystal – fractured edge | 0.43 eV | 1 | 25 eV | 80 x 350 µ |
| Gold Piece – Freshly Ion Etched – One Minute | ||||
| Au (4f7/2) foil – ion etched clean | 0.64 eV | 5 | 10 eV | 250 x 1000 µ |
| Au (4f7/2) foil – ion etched clean | 0.79 eV | 1 | 25 eV | 250 x 1000 µ |
| Au (4f7/2) foil – ion etched clean | 0.86 eV | 2 | 50 eV | 250 x 1000 µ |
| Au (4f7/2) foil – ion etched clean | 1.40 eV | 4 | 150 eV | 250 x 1000 µ |
| Silver Piece – Freshly Ion Etched – One Minute | ||||
| Ag (3d5/2) foil – ion etched clean | 0.42 eV | 5 | 10 eV | 40 x 250 µ |
| Ag (3d5/2) foil – ion etched clean | 0.64 eV | 1 | 25 eV | 40 x 250 µ |
| Ag (3d5/2) foil – ion etched clean | 0.75 eV | 2 | 50 eV | 40 x 250 µ |
| Ag (3d5/2) foil – ion etched clean | 1.00 eV | 3 | 100 eV | 40 x 250 µ |
| Ag (3d5/2) foil – ion etched clean | 1.30 eV | 4 | 150 eV | 40 x 250 µ |
| Copper Piece – Freshly Ion Etched – Two Minutes | ||||
| Cu (2p3/2) foil – ion etched clean | 0.85 eV | 5 | 10 eV | 250 x 1000 µ |
| Cu (2p3/2) foil – ion etched clean | 0.94 eV | 1 | 25 eV | 250 x 1000 µ |
| Cu (2p3/2) foil – ion etched clean | 1.06 eV | 2 | 50 eV | 250 x 1000 µ |
| Cu (2p3/2) foil – ion etched clean | 1.60 eV | 4 | 150 eV | 250 x 1000 µ |
| Smaller Spot Size | ||||
| Cu (2p3/2) foil – ion etched clean | 0.85 eV | 5 | 10 eV | 150 x 800 µ |
| Cu (2p3/2) foil – ion etched clean | 0.96 eV | 1 | 25 eV | 150 x 800 µ |
| Cu (2p3/2) foil – ion etched clean | 1.05 eV | 2 | 50 eV | 150 x 800 µ |
| Cu (3s) Check | ||||
| Cu (3s) foil – ion etched clean | 2.35 eV | 2 | 50 eV | 250 x 1000 µ |
Theoretical Analyzer Resolution versus Pass Energy Settings
| Theoretical Analyzer Resolution |
Pass Energy Setting |
Effective Detector Width |
| 0.25 eV | 25 eV | 3.5 eV |
| 0.50 eV | 50 eV | 7.0 eV |
| 1.00 eV | 100 eV | 14.0 eV |
| 1.50 eV | 150 eV | 21.0 eV |
Energy Scale Calibration & Reference Energies
From May 1986 to January 1993 for Surface Science Instruments (SSI) X-Probe XPS Instrument
Energy Scale Reference Energies: 932.47 eV for Cu (2p3/2) signal, 122.39 eV for Cu (3s) signal, 83.96 eV for Au (4f7/2) signal
Binding Energy Uncertainty: less than ±0.08 eV
Digital-to-Analog (DAC) Conversion Setting: 163.88
The SSI XPS instruments are based on the original HP monochromatic XPS system that Siegbahn helped to design.
After January 1993 to 1999 for SSI S-Probe XPS Instrument
Energy Scale Reference Energies: 932.67 <±0.05 eV for Cu (2p3/2) signal, 122.45 <±0.05 eV for Cu (3s) signal, 83.98 <±0.05 eV for Au (4f7/2) signal
Observed Reference Energy: 75.01 <±0.05 eV for Cu (3p3.2) signal Binding Energy Uncertainty: less than ±0.08 eV
Digital-to-Analog (DAC) Conversion Setting: 163.87
Note: NPL has recently revised reference energies to be 932.62 eV for Cu (2p3/2) and 83.96 eV for Au (4f7/2) for monochromatic systems using an electron take-off-angles of 45o
Instrument Stability and Long Term Calibration for SSI XPS Instruements
Initially each of the two 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 two 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 degrees Centigrade. If the room temperature changes within a few hours time by more than 10 degrees or the temperature of the DAC chip is changed by more than 10 degrees, 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.
From June 2009 to June 2019 (in USA)
Used Thermo K-Alpha+ XPS system
Instrument Response Function and Electron Counting Details
Instrument Response Function (Transmission Function and Traceability)
Copper, gold and silver data obtained from the X-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, X-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 (RSFs) 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 Scofield’s. 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.
Instrument Response Functions for the X-Probe XPS based on NPL Measurements
| Instrument Response Function: | Q(E)=E+0.27 for 150 eV PE |
| Instrument Response Function: | Q(E)=E+1.0 for 50 eV PE |
Signal/Background (S/BG) Ratios for Ion Etched Silver using a 250×1000 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.
Electron Collection Lens Voltage Settings Available via Software under Instrument Calibration
| Pass Energy (eV)* | 29.6-29.8 | 54.7-54.9 | 105.1-105.3 | 155.9-156.2 |
| Detector Widths (eV) | 3.743 | 7.486 | 14.954 | 22.297 |
| Sensitivity Exponent | -0.1 | 0.3 | 0.7 | 1.1 |
| V1 Offset (eV) | 30 | 55 | 105 | 155 |
| V1 Slope (eV) | 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.
Quantitation Details and Choice of “Sensitivity Exponents” for The SSI XPS Instruments
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, Ph.D.) 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 (1.4). 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 useful 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 (2p1/2), Cu (2p3/2), 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, then this method requires a little extra effort to avoid the satellites of 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 a slightly larger than desirable error by comparison to the other materials listed above. For that reason Teflon seems to be a less desirable material to determine useful sensitivity exponent values.
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. These results are available when using the Spectral Data Processor software.
Effects of Poorly Focusing 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 0.5 mm 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 XPS instruments. A 0.1 mm 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 urges 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%.
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.” Based on this definition, the following correlations were envisioned.
Traceability of Reference Binding Energies (Energy Scale 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 the high precision, high voltage meters produced by the Hewlett Packard Company.
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 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.
Reference Papers Describing Capabilities of X-Probe, M-Probe, and S-Probe XPS Systems
Robert L. Chaney, Surface and Interface Analysis, 10, 36-47 (1987)
Noel H. Turner, Surface and Interface Analysis, 18, 47-51 (1992)
M. P. Seah, Surface and Interface Analysis, 20, 243-266 (1993) [re: Response Function]
L.T. Weng et al, Surface and Interface Analysis, 20, 179-192 (1993) [re: Response Function]
L.T. Weng et al, Surface and Interface Analysis, 20, 193-205 (1993) [re: Response Function]
B. Vincent Crist, Surface Science Spectra, 1, 292-296 (1993)
B. Vincent Crist, Surface Science Spectra, 1, 376-380 (1993)
M. P. Seah, I.S. Gilmore, and G. Beamson, Surface and Interface Analysis, 26, 642-649 (1998)
Abbreviations Used
Due to the limited space provided to describe each sample in each electronic data-file, it was necessary to use various abbreviations. The abbreviations are:
scr = screen used for charge compensation
scrn = screen used for charge compensation
TOA = take-off-angle for the electrons
Aldr = Aldrich Chemical Co.
RMC = Rare Metallics Co.
SPP = Scientific Polymer Products Co. MS Co. = Metal Samples Company
FG = flood gun,
mesh = mesh-screen used for charge control,
1 mm=1 mm height used for the mesh-screen,
semi-con = semi-conductive behavior
conduc, = conductive behavior
Tech = technical grade purity,
pellet = sample pressed into pellet form by pellet press used to make Infrared KBr pellets,
plt = pellet
pel = pellet