Binding Energy (BE) Scale Calibration
& Binding Energy Reliability

 

 

Philosophy for Binding Energy (BE) Reliability

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.

The spectral data contained within this website are designed to assist engineers, scientists, analysts, theoreticians, and teachers who use XPS on an everyday basis under practical working conditions. We believe that these spectra will help XPS users to analyze industrial problems, gather reference data, perform basic research, test theories, and teach others. These spectra are designed to be practical tools for everyday use and were obtained under practical working conditions. No attempt was made to produce research grade spectra, but many of the spectra are actually research grade spectra because of the self-consistent methods used.

The elements have been analyzed under conditions that have maximized the accuracy of the binding energies. The binding energies for the pure elements are referenced to the reference energies recommended by the ISO in Switzerland and National Physical Laboratory in the UK. Please refer to section “Energy Scale Reference Energies and Calibration Details” for more details about calibration.

When the sample is an insulating chemical compound, then a C (1s) spectrum is provided to allow the user to correct for sample charging by using a method that you prefer.  We have assigned the BE of C (1s) of the hydrocarbon peak to be at 285.0 eV. 

All narrow scan spectra are peak-fitted to reveal FWHM, and peak separation for spin-orbit pairs. The strong signals observed in the wide scan survey spectra are labeled and tabulated together with rough BE values of those strong signals. The details of the experimental protocol used to produce each these spectra are provided in the “Instrument and Analysis Details” section. All pure elements, except for Silicon and Selenium, were ion etched prior to analysis.

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Binding Energy Scale
Reference Energies and Calibration Details

SSI XPS Instruments (X-Probe and S-Probe)

 

 

From May 1986 to January 1993

XPS Instrument used:  Surface Science Instruments (SSI) X-Probe
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
In 1994, the BE scale of these early spectra was incrementally increased to produce a Cu (2p3/2) peak from 932.47 to 932.67 eV.  The Au (4f7/2) BE remained at 83.96 eV

 

After January 1993 to 1999 

XPS Instrument used:  Surface Science Instruments (SSI) S-Probe
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

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Instrument Stability and Long Term Calibration of the X-Probe and S-Probe XPS Instruments

Initially the BE scale of each of the two SSI systems (X-Probe and S-Probe), that we have used, was measured 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.

 

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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/ISO 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.

 

 

Binding Energy Scale
Reference Energies and Calibration Details

Thermo Scientific K-alpha + XPS Instrument

 

 

After June 2009 to 2021  (in the USA) using a Thermo Scientific K-alpha (+) XPS Instrument

XPS Instruments used:  Thermo K-Alpha (Probe) and Thermo K-Alpha (+) Plus
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

The Thermo K-Alpha XPS instrument is a direct derivation and uses many of the key components of the SSI S-Probe XPS instrument.  The 128 channel PSD detector is the same.  The X-ray focusing beam design is very similar. The X-ray focusing crystal (Johannsson design) is based on the SSI design, and has been improved. The electron collection lens is 60 degrees wide which is 2X larger than the 30 degree wide angle of the S-Probe.  The electron flood gun of the S-Probe has been replaced by a dual beam gun that uses a low voltage beam of Argon ion coaxial with a low voltage beam of electrons.

 

Instrument Stability and Long Term Calibration of the Thermo K-alpha and K-Alpha Plus XPS Instruments

Initially each Thermo K-alpha the BE scale was checked 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 >3 months, it was decided that good calibration could be maintained by checking and, if necessary, using the on-board automated BE scale calibration routine to check and adjust the BE scale to the ISO values.  The K-alpha instruments are equipped with a set of calibration metals, a ZnS phosphor, and various ion beam test hole sizes attached to the inside of the main analysis chamber which are used to check and align the instrument by computer program control.

Over several years time Thermo has accumulated a run trend that shows the excellent long term stability of the electronics of the K-alpha. It was found that the maximum uncertainty (error in pass energies) was normally <±0.10 eV.  In a very rare case, the uncertainty rose to 0.20 eV. Long term use of the K-alpha has demonstrated the excellent BE scale stability in a room with reasonable air conditioning and temperature control.

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Binding Energy (BE) Scale Calibration

 

 

Binding Energy (BE) Scale Calibration

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Calibration (reference) energies, developed by an international team of XPS experts, were published in 2001 by the International Standards Organization (ISO) Technical Committee (TC/201) for Surface Chemical Analysis as ISO document #15472 (titled: Surface chemical analysis – X-ray photoelectron spectrometers –Calibration of energy scales) [73].

The ISO BE calibration energies for a monochromatic Al-k-alpha source are:

 

Cu (2p3/2),  Cu (3p3/2),  Ag (3d5/2)  and  Au (4f7/2)  signals are:   

932.62,   75.13,   368.21   and   83.96 eV,  respectively.

 

These ISO values are reported with ±0.02 eV uncertainty, and they represent the first international effort to standardize the calibration energies used to calibrate the energy scales of XPS instruments worldwide.

Table 1 shows the large variation in BE Calibration Values that were previously promoted by many XPS instrument makers.  This large variation is BE Calibration values caused many scientists and researchers to publish BEs from research materials that can not be reproduced by other researchers or scientists.  These large variations caused large variations in the BEs published in the literature, many handbooks and the NIST SRD20 database.

As a result we need to be careful when we use literature BE values from insulators and conductors before 2010.  We need to make fresh BEs from pure materials.

 

 

 

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Can we Calibrate the Energy Scale using ONLY Copper (Cu) ?

YES !

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Routinely Check BE Calibration Values – Monthly

  • Check BE Calibration by using the routine conditions used to separate and identify different chemical states.  If more than one pass energy and step size (eV/step) is routinely used, then consider checking those settings at least one time.
  • Check the Energy Scale by measuring the Cu 2p3 peak (932.62 eV) and either the Cu 3s peak (122.45 eV) or Cu 3p3 peak (75.13 eV) from pure (>99.9%) Copper metal that was freshly cleaned (e.g. scraped) and strongly ion-etched.
  • If any BE is wrong by more than >0.15 eV (practical conditions), we can use the measured BEs to adjust the work function (energy offset), energy scale factor (DAC) or pass energies of that XPS instrument.
  • Each time one instrument voltage is adjusted, the Cu 2p3 BE and Cu 3p3 (or Cu 3s) must be re-measured.

Various handbooks and standards on XPS recommend the use of two or more pure elements to check, and if necessary, adjust the reference energy scale of an XPS system.  Various methods to check the accuracy of atom % results usually involve complicated checks of the transmission function or instrument response function of an XPS instrument, and often involve the use of additional materials.

While existing methods are valid, most are tedious, time consuming, expensive and seldom used.  The end result of this conundrum is that published BEs and atom % values are unnecessarily inaccurate and, too often, overly deviant.  As evidence of this still ongoing problem, please review the BE data, derived from the most recent version of the NIST XPS BE Database SRD20, v3.4 – shown as Figure 1.

This poster will present fast and simple methods to, not only check BE calibration, but also at the same time, check atom % accuracy by using a single piece of ~99.9% pure copper foil. The pure copper piece, just like all reference materials, must have minimal amount of surface contamination on both the top and bottom surfaces that ensures good electrical contact.

The sample mount, sample stage, mounting screws, mounting clips and those parts that connect the copper sample to the spectrometer, and to ground, must provide good electrical conductivity.  This is the standard requirement for everyday analytical work, as well as the complicated calibration checks.  Good electrical conductivity, that extends from the surface of a clean piece of copper on a sample mount all the way to the spectrometer, and to ground, sounds easy to achieve, but, from time to time, can be complicated by unexpected surface contamination/corrosion of sample mounts, sample stages and pressure-based electrical contacts inside the main analytical chamber.

This method expects that the copper foil sample has a Fermi level contact to a conductive sample mount and that the sample stage inside the main UHV chamber has a Fermi level contact to the instrument.

Look at the top and bottom of the copper sample and decide if it is or is not clean enough.  If either looks brown, green or blue, then clean it using fine sandpaper.   Don’t use metal polish as these have various chlorine agents that allow the copper to re-oxidize quickly.  Surface cleanliness is more important than smoothness, although smoothness will affect the counts in the signals to be measured.

The time needed to remove a modest layer of oxidized copper is typically 2-3 minutes when using Argon ions and a 2-4 kV argon beam with a 5-10 mA current setting. Ion etch a region of the copper sample until the oxygen signal has dropped to near zero or zero.  If possible, use a piece of copper foil that provides regions that will not be repeatedly ion etched clean.  Store this sample in a closed box or keep it wrapped in aluminum foil to minimize the time needed to remove the oxidized copper layer.  If you are able to store the sample in the prep lock or the main chamber then do that.  If you only have an old piece of copper that is heavily oxidized and looks brown in color, then grab a knife or a razor blade and scrape the surface at different angles until you produce a bright copper surface.  The roughness that will result has no effect on the BEs, but will the sample can more easily oxidize due to the roughness of the surface.

Adjust the sample to the optimum position for the instrument and begin measurements.  This position may or may not be the optimum position for the instrument.  If you suspect there may be some variation in BE as a function of sample height or pass energy, then you might want to run a series of measurements to decide if the instrument needs adjustment by a service engineer.

 

Variables that may Affect BE Calibration Checks

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Reference Papers Describing Capabilities and Designs 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)
L.T. Weng et al, Surface and Interface Analysis, 20, 193-205 (1993)
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

 

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