X-ray Photoelectron Spectroscopy (XPS)


XPS Instruments at Surface Science Western

Kratos AXIS Ultra (front) and AXIS Nova (back) at Surface Science Western

X-ray photoelectron spectroscopy (XPS or ESCA), is an analytical technique that depends upon the measurement of the energies of photoelectrons that are emitted from atoms when they are irradiated by soft X-ray photons (1 – 2 keV). When used to study solids, XPS analysis has a number of powerful attributes, including a high (and variable) range of sensitivities to structures on the outermost surface of the solid (outer few nanometres), an ability to identify such structures chemically, a reasonable capacity for elemental quantification, as well as the ability to determine structure thickness. As a method for characterizing surface composition, there is no single other technique that can compare with XPS, in terms of the wealth of useful information, reliability of the data, and ease of interpretation.

xps cr 2p spectrum

Cr 2p high resolution XPS spectrum.

Advancements in XPS spectrometer technology have resulted in major improvements in spectral resolution and counting efficiency over the past 20 years. This has dramatically improved the level of confidence in spectral positions, and the ability to carry out analyses in numbers that have much better statistical significance. The exploitation of the imaging developments is likely the most exciting prospect, because, historically, little research has been done using highly-resolved XPS images. The recognition of co-localization of different species (elemental or chemical) will be one of the most powerful elements shaping XPS analysis in the future.

The principal XPS at SSW is a Kratos AXIS Ultra spectrometer. The AXIS Ultra spectrometer integrates a Magnetic Immersion Lens and Charge Neutralization System with a Spherical Mirror Analyser. The patented charge neutralization system means that electrically non-conductive samples can be easily analyzed, and high quality XPS spectra can be obtained. The Spherical Mirror Analyser provides real-time chemical state and elemental imaging using a full range of pass energies. A micro-channel plate and phosphor detection system are incorporated into the system to provide parallel imaging capability with high spatial resolution and high sensitivity. Advanced zoom optics enable a variable viewing area for the identification of macro or micro features. The instrument’s exceptional small spot capabilities (220, 110, 55, and 27 microns) are achieved via a combination of the magnetic lens and selected area apertures. Under normal analysis conditions, the analysis spot is approximately 400 x 700 microns. Two different reaction chambers are attached to the spectrometer itself.

The second XPS at SSW is a Kratos AXIS Nova spectrometer. The AXIS Nova has analysis capabilities similar to that of the AXIS Ultra, employing a monochomatic Al Kα X-ray source with a state-of-the-art delay line detector (DLD) system. This detector allows for faster image acquisition, enhanced small spot analysis sensitivity, and true quantitative XPS imaging. The system employs an automated multiple sample platen system that allows for increased sample throughput.

Kratos AXIS Ultra Spectrometer

System Capabilities:

  • In-situ fracturing at low temperature
  • Sample heating (600° C) and cooling (-120° C)
  • Variable angle sample orientation (angle resolved analysis)
  • High precision automated stage for remote or non-attended operation
  • Depth profiling (low energy beam)
  • Dosing of sample with gases (both benign and more reactive gases in separate chambers)
  • Interfaced to a glove box for chemical preparation under inert atmosphere
  • Analysis of sample sizes up to 2 cm in diameter and up to 10 mm thick
  • Elemental and chemical imaging, small spot analysis
  • Multiple X-ray sources including monochromatic Al Kα, Mg Kα, Zr L(α,β)

Kratos AXIS Nova Spectrometer

System Capabilities:

  • Variable angle sample orientation (angle resolved analysis)
  • High precision automated stage for remote or non-attended operation
  • Depth profiling (low energy beam)
  • Sample platens can hold numerous samples of varying sizes (up to 10 mm thick). Eight inch wafer analysis possible (some restrictions apply)
  • Elemental and chemical imaging, small spot analysis
  • Monochromatic Al Kα X-ray source
  • MCP delay-line detector allowing for scanned and snapshot data collection as well as elemental and chemical imaging, small spot analysis
  • Equipped with a specially designed inert transfer vessel allowing for mounting of air sensitive samples in a glove box, and transferring to the instrument without air exposure
  • Transfer vessel can also be used to transfer samples in vacuum to our ToF-SIMS instrument
  • He(I) and He(II) ultraviolet source (UPS)
X-ray Photoelectron Spectroscopy (XPS) Reference Pages

The XPS team at SSW also maintains the X-ray Photoelectron Spectroscopy (XPS) Reference Pages website. This site is a collection of curve-fitting procedures, reference materials, and useful tips on XPS analyses and spectral interpretation.

Selected Applications in Industry:

  • Reactions of water vapour with polycrystalline iron: near surface compositions revealed using QUASES™ evaluation of XPS spectra
  • XPS analysis of an adhesive failure
  • Estimation of oxide thickness
  • High resolution spectrum from magnesium oxide on glass substrate
  • Angle resolved study of pyrite surfaces
  • Imaging XPS study of a Monel 400 corrosion pit
  • Studies of chromium compounds
  • Analysis of surface stains on mirrored surfaces