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 2p_3/2 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 Si_0.85Ge_0.15 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 Si_0.85Ge_0.15 to at least 0.14 mM on Si _0.70Ge_0.30 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 CO ₂ 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.