| AVS 56th International Symposium & Exhibition | |
| Applied Surface Science | Wednesday Sessions |
| Session AS-WeA |
| Session: | Angle-resolved X-ray Photoelectron Spectroscopy |
| Presenter: | P. Cumpson, Newcastle University, UK |
| Correspondent: | Click to Email |
We present new, fast algorithms for automated depth-profiling by Angle-Resolved XPS and AES, and first results from a microfabricated electron energy analyzer array optimised for use with them.
The period to 1995 saw the development of good models for simulating XPS spectra, often based on Monte Carlo simulation[1]. These helped in improving understanding, but were too slow to help analysts interpret particular spectra directly. Also these calculations were in the opposite direction to that desired - spectra were calculated from composition, whereas what we need is the reverse. Nevertheless, progress was made in speeding these calculations substantially[2].
In addition to the usual improvement in CPU speed since then, more of a surprise perhaps is the emergence of PC Graphics Processing Units ("GPUs"). No longer simple arrays of screen memory, these contain tens - often hundreds - of separate processors on one chip. Typically they are used for ray-tracing, following single rays of light back from the detector (a human eye) to their origin, including various types of scattering, reflection and refraction. They do so in parallel, and are used intensively in computer games. We can capitalise on the similarity to electron transport. We have developed algorithms to use these GPUs to simulate XPS and AES emission processes - and in the direction we need - that is, from measured spectrum back to determine the original concentration depth-profile. Most medium-power PCs typically have a suitable graphics card already installed. We have demonstrated a speed-up of roughly 20 times on a 240 processor GPU card, compared to the PC CPU alone, making the time taken for calculation of the composition-depth profile similar to the that of acquiring the spectra in many cases - so it can potentially be done in "real time".
The flexibility of these algorithms mean that the chemical information in spectra can be extracted even if analyzer performance (transmission, resolution, scattering) is poorer than that of modern analyzers, but one needs at least three emission angles[3]. Therefore we have fabricated an array of three silicon MEMS analyzers, similar to a type previously proposed for plasma measurements from spacecraft. We present initial results, though there are significant issues of low transmission and scattering compared to macroscopic hemispherical sector analyzers. We expect to improve their performance to the point where they can be used in conjunction with our GPU algorithms.
[1] A Jablonski and J Zemek, Phys Rev B 48, 4799-4805 (1993)
[2] P J Cumpson, Surf. and Interface Anal. 20, 727 (1993)
[3] P J Cumpson, J. of Elec. Spectrosc., 73 25 (1995)