Validation of vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry for metallic contamination analysis of silicon wafers

The combination of vapor phase decomposition–droplet collection (VPD–DC) with total reflection-X-ray fluorescence spectrometry (TXRF) is a well-established method for metallic contamination analysis of Si wafers. However, the efficiency of the methodology is not fully quantitatively understood. This study aims for the identification of the sources of error in the VPD–DC–TXRF process. From a study of systematic recovery rate (RR) on standard wafers, it is concluded that the VPD–DC method is very efficient in collecting impurities from Si wafers. TXRF saturation effect on the micro-droplet residue limits the accuracy of quantification to the levels below 3×1013 atoms of metallic contamination. This represents a homogenous single element surface contamination of 1×1011 at/cm2 on a 200-mm wafer. Implications of this finding are more serious for multi-element contaminations and for larger wafer dimensions. The origin of this saturation effect is discussed and solutions are evaluated. Further, we suggest a monitoring of the scattered X-rays from VPD–DC residues as an indicator for TXRF accuracy.