Thin Film Analysis with SEM micro-XRF

As X-rays may pass through matter, X-ray Fluorescence (XRF) allows the determination of layer thickness. Using micro-XRF on SEM, the layer analysis (thickness and composition) is rendered feasible with spatial resolution at the micrometer scale. Layer analysis is strongly based on quantification using atomic fundamental parameter (FP). It can be improved with the use of standard samples, so various types of layer systems can be investigated by FP, such as metallization on wafer, multi-metal pretreatment coatings and solar cells. When standards are available, they can be used to enhance the accuracy. But since FP does not require standards, novel layer systems in an R&D environment can also be tested. Layer structures, such as solar cells (Fig. 1), are important in numerous industries.

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Fig. 1: Example of thin film analysis on solar cells. In most cases, the absorber is produced from CIGS-structures, which are Cu-In-Ga-Se or Cu-In-Ga-S compounds. CIGS-solar cells are typically manufactured by deposition on glass substrates which are coated with Mo-layer. Compared to traditional film thickness analysis with SEM, where layers are measured in cross section mode, XTrace measures samples with SEM directly on the planar surface, it is a fast and non-destructive technique that requires no sample preparation.

Fig. 2: Micro-XRF spectra of the solar cell showing all relevant element lines coming from different depth, even Mo which is located ~ 2 µm below the sample surface. Note, the high energy Z-elements lines (e.g. Mo-K and In-K) which can be easily excited by using a XRF source. The larger depth of X-ray excitation allows a deeper look inside the material, enabling the characterization of relatively thick layers or even multilayer systems starting from 1 nm and up to 40 µm. Furthermore, the composition of the CIGS structure will also be determined. This sample was analyzed by using a standardless fundamental parameter approach. Standards can be applied to enhance precision.