XPS (X-Ray Photoelectron Spectroscopy)

ESCA(Electron Spectroscopy for Chemical Analysis) or XPS(X-Ray Photoelectron Spectroscopy)

Specifications

qualitative elemental analysis: all elements can be detcted except H and He,
average detection level: 0.1 - 0.5 atomic %,
quantitative analysis: precision 2-5 % - accuracy 20 %,
surface analysis: analysis depth < 100 Å (about 30 to 50 Å),
chemical analysis: type of bonds, oxide/metal rate...,
spot size (for our device): 150 x 150 µm² to 250 x 1000 µm²,
non destructive... except in case of concentration profiles (up to a few thousands Å),
conductive and insulating samples,
analysis under ultrahigh vacuum (10-9 - 10-10 torr).

Principle

The surface of the sample is bombarded by a monochromatic X-ray beam. X-ray photons are absorbed by the material, the energy transfered, Ex, is partly used to excite core levels electrons: Ex = El + Ec (El binding energy and Ec kinetic energy of the excited electron).
El is characteristic of a given electron level for a given element. Ex fixed and Ec measured, it is possible to determine El thus the type of excited atom.

XPS image

The detection consists of a kinetic energy filtering of emitted electrons. The ESCA spectrum is described by a succession of peaks which correspond to a given El (for exemple, C 1s peak = excitation of carbon 1s level electrons), that's why it is possible to perform elemental analyses.

There are few interferences between peaks of the different elements. Even when a peak can correspond to several elements, there is, in general, a way to satisfy doubts by studying the whole elemental spectrum: presence/absence of peaks, relative intensity of characteristic peaks of the suspected elements, study of the Auger peaks...

The signal under each peak of element A is proportional to the number of A atoms, so this analysis can be quantitative.

Finally, the signal intensity as a function of sample thickness t is damped by exp(-t/l) with l, the mean free path of electrons in material: the deepest you go in the sample, the less the ejected electrons have a probability to be detected, i.e., the weakest is their contribution to total signal. Thus 70 % of detected signal comes from the first l Å, and beyond 3l, the contribution is negligeable. l being about 10 to 20 Å, the thickness analyzed is about 30 to 50 Å, that's why ESCA is a surface analysis technique.

Probion Analysis
37, rue de Fontenay - 92220
Bagneux - France
Tel : 33 (0)1 46 12 05 00 - Fax : 33 (0)1 46 54 10 33
info probion.fr