X-ray Metrology, wafer

Micro X-ray Fluorescence (µ-XRF)

The technology

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It is basically the same technique as “classical” XRF. The intensity of the fluorescent radiation of an element is a measure for the number of atoms of that element. Underneath the limit thickness (which is dependent on several parameters, such as the element, the irradiating beam, the density and matrix), the intensity is a measure for the layer thickness and concentration. Above this limit thickness, the intensity is – linearly – dependent on the concentration. Classical XRF is in most of the cases probing bulk samples with infinite thickness. It is therefore a technique to analyze element concentrations.

X-ray Metrology customers often have thin layered samples (wafers, solar cells) with thicknesses inferior to the saturation thickness. XRF is in this case used to analyze the chemical composition and/or the thickness of films and layers. A small beam size is usually required, which is obtained by using a collimator or a polycapillary lens. The fluorescent radiation is detected with an energy dispersive silicon drift detector (SDD).

The used software is based on fundamental parameters. It takes the experimental setup, matrix effects, absorption and secondary enhancement by neighbouring atoms into account, so that only one standard is required to calibrate the system to obtain quantitative results. XRR and XRF are considered as complementary techniques.