Influence of the excitation energy on absorption effects in Total Reflection X-ray Fluorescence analysis

Abstract

Total Reflection X-ray Fluorescence (TXRF) analysis is a well-established analytical method in the semiconductor industry for the analysis of silicon wafer surfaces. To improve the detection limits of TXRF (given in at cm⁻²) for wafer surface analysis vapor phase decomposition-droplet collection (VPD-DC) is used to collect the impurities of the total surface in one droplet. This leads to higher sample masses to be analyzed than in straight TXRF. In TXRF, absorption effects concerning the excitation and the detected radiation are usually disregarded. This is justified because mostly small sample amounts (pg to ng range) are used and the thin film approximation is valid. For higher total amounts of sample deviations from the linear relation between the fluorescence intensity and sample amount have been observed (saturation effect). These lead to difficulties in quantification with external standard, which is the calibration method used in TXRF wafer surface analysis. The content of the presented work is an investigation of the absorption phenomenon and hence the fluorescence intensity to improve the quantification of TXRF using VPD-DC. Samples with different total amounts of nickel were prepared and the emitted fluorescence intensities were measured at two different excitation energies to estimate the upper limit of sample mass where the relation between the fluorescence intensity and sample amount diverges from linearity depending on the excitation energy. The measurement results were compared to calculations performed with a self-developed simulation model. It could be verified that if the excitation energy is closer to the absorption edge of the excited element (which means better excitation) the saturation effect appears at a lower sample mass.