Vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry for metallic contamination analysis on Ge wafers
Ge substrates are recently being reconsidered as a candidate material for the replacement of Si substrates in advanced semiconductor devices. The reintroduction of this material requires reengineering of the standard IC processing steps. In this paper, we present the extension of the methodology of vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry (VPD–DC–TXRF) for metallic contamination analysis towards Ge substrates. A first step that asked for adaptation was the collection chemistry as the Ge wafers surface is not hydrophobic after the VPD treatment. The contact angle could be significantly increased using a concentrated HCl solution. This chemistry has been proved to perform well in the collection of metals from intentionally contaminated Ge wafers. A second step that needed optimization was the matrix removal method as a sample preparation step prior to the TXRF analysis. First, the upper limits of TXRF on Ge containing solutions have been characterized. The accuracy of TXRF is found to be acceptable for Ge contents lower than 1×1014 atoms (250 ppb in 50 μL) but decreases systematically with higher Ge contents. Fortunately, Ge can be volatilized at low temperatures as GeCl4 by the addition of HCl. The parameters within this method have been investigated with respect to the removal of Ge and the recovery of metal traces. Finally, the full VPD–DC–TXRF method has been applied on intentionally contaminated Ge wafers and proved to be very accurate.