Native oxide removal from SiGe using mixtures of HF and water delivered by aqueous, gas, and supercritical CO2 processes

HF and water delivered using liquid, gas, and supercritical CO2 phases were used to remove native oxides and suboxides from SiGe surfaces containing 15-30% Ge. X-ray photoelectron spectroscopy was used to monitor the Si 2p and Ge 2p3/2 peaks as a function of the process type, HF concentration, and Ge fraction. A larger percentage of Si oxides were removed compared to Ge oxides by all three processes. The threshold HF concentration in aqueous-phase solutions for complete removal of Si and Ge oxides was approximately 28 millimole/l (mM) on Si0.85Ge0.15 and increased by approximately an order of magnitude on the surfaces with the higher Ge concentrations. The threshold concentration increased from 0.007 mM on Si0.85Ge0.15 to at least 0.14 mM on Si0.70Ge0.30 to obtain significant removal of both Si and Ge oxides using supercritical CO2 to carry the HF and water etching solution. Although air exposure of the liquid water and supercritical CO2 etched samples provided an opportunity for reoxidization, the trends in the metal and oxide XPS peak areas with HF concentration suggest that chemical treatment had the largest effect. Gas-phase HF/vapor processes removed all of the SiO2, Si suboxides, and GeO2 in less than 30 s offering the best performance of the processes studied without roughening the surface.