XPS / ESCA

Applications

• The major part of studies with that technique is about surface. For interfaces, the angular analysis, for up to 10 nm depth, or ion etching, for around a micrometer depth, are used.
• Each material compatible with ultra high vacuum : organics, polymers, powders, metals, glass, …
• Elementary compositions (except H and He)
• Quantitative + stœchiometry analysis (detection limits: around 0.3 % atomic)
• Chemical functions, degrees of oxidation
• Non destructive measurement of thin layers depth under 10 nm by angular analysis
• Profiles of distribution in depth by angular analysis and ionic etching
• Chemical imaging (resolution ‹ 3 µm)
• Analysis size : 0.5 x 4 mm² or 300 x 700 µm2 (µ-spot : 27 x 27 µm²)

Principle

The sample is irradiated with X-Rays of hν energy causing the ionisation of its atoms through photoelectrical effect. During the interaction of photons with the atom, a part of their energy is used to break the link, its binding energy (E_B) ; the rest is transfered to the electron with kinetic energy (E_C). Propulsed in the material, they travel a certain distance, function of that kinetic energy and of the material. If their kinetic energy is sufficient, they are extracted from the material and go through emptiness. The kinetic energy of photoelectrons emitted allows to determinate their binding energy (E_B = hν– E_C), characterised for emitter atoms  and the nature of chemical shapes.
 

Spectrums

XPS spectra represent the intensity (number of photoelectrons)  according to their binding energy. The identification of elements present on the surface of a material is made of photoelectrical peaks which appear on the spectrum.

Angular analysis

The analysis depth is function of the inelastic mean free paths of photo-electrons in the material. When the electrons sensor is standard, the analysis depth is about 10 nm. Making the angle change between the sensor and the standard on surface from 0° to 75°, the analysis depth can change from 10 nm to 3 nm. This mode allows to realise non destructive profiles on a depth of 10 nm from the sample surface.

Concentration profiles

In order to obtain concentration profiles, the sample is etched by a ion beam (Argon) leading to an abrasion starting from the surface of sample. The alternating of analysis cycles and abrasion allows to perform profiles of concentrations of elements throught multi-layers. In order to determine with precision the etching speed, we need to do a calibration thanks to yardsticks or measurement of depth of the abrasion crater throught optical methods (interferometry) or mechanical methods (Talystep).

Imaging

Chemical images (2D) can also be realised with a lateral resolution under 5 µm. We can then study the homogeneity of a surface treatment or the chemical nature of localised defaults.