Reference-free XRR-GIXRF combined analysis for thin layers

X-ray fluorescence is the emission of photon radiation from a vacancy created in an atom's electron pattern by higher-energy photon excitation. This way of interrogating matter is potentially non-destructive for the sample to be measured, as it requires no special preparation. The quantification possible with this technique concerns the quantity of matter given in mass or surface mass, or the number of atoms, and so on. Nevertheless, in the majority of current applications, sample preparation is an essential step, the reason being the need to calibrate the measuring device for the mass ranges of interest. Here, we place ourselves within the general framework of the fundamental equations of this interaction in order to present quantification without reference, i.e. based solely on fundamental principles, with its advantages and disadvantages.

We will present the equations for this interaction, followed by an application to the quantification of nanometer-thick thin films, for which it is unrealistic to use multiple reference samples to calibrate a measuring device. The analysis technique combining X-ray reflectivity (XRR) and grazing incidence X-ray fluorescence (GIXRF) is designed without reference to standards, and uses calculation of the stationary electric field in the layer(s) to deduce the X-ray fluorescence emission profile. This method combines the electron density sensitivity of X-ray reflectivity with the atomic sensitivity of X-ray fluorescence to deduce the density, thickness, interface roughness and atomic composition of thin films.

We will present the tools needed to implement this approach, as well as the fundamental data on which the calculation is based. The tools concern the X-ray source, which must be monochromatic, and whose beam must have as little divergence as possible, given the angular accuracy required. The detectors are of two types:

• photodiode-type photon integrators which measure a current proportional to the quantity of photons entering, but do not provide information on their spectral distribution;

• spectrometers, which measure the energy of individual photons to produce a line spectrum representative of the various emitting atoms.