Self-Aligned SiGe HBT Based on Combined Dry and Wet Etching

In this paper, a self-aligned SiGe/Si HBT was fabricated based on dry & wet etching, in which Si and SiGe materials were etched by KOH (potassium hydroxide) solution and SF6 (sulfur hexafluoride). The measured results are: cutoff frequency fT=103.3GHz, maximum oscillation frequency fmax=124.2GHz. 1. Mechanism of chemical reactions for etch The main chemical composition of etch solution is KOH which reacts with silicon as follows [1][2]: Si +2KOH+H2O=K2SiO3 +H2↑ (1) Germanium is not as active as silicon, and doesn’t react with KOH solution at room temperature. When SiGe is etched in the etching solution, germanium plays an obstructive role in etching, decreasing the etch rate greatly. The hydrolysis in water of H2GeO3 formed by reaction of Ge and etch solution is pasty deposit. The reaction is as follows. Ge + 4OH=GeO2.2H2O (2) Adsorptive power of the substrate is so high that SiGe alloy surface is protected. Therefore, KOH contributes little to etching SiGe alloy. Roughly speaking, KOH doesn’t etch SiGe alloy as an etch endpoint. Silicon was etched by SF6 and oxygen. At RF power, SF6 is dissociated into F groups that have high chemical activity and reacts easily with silicon, germanium, etc. The chemistry is as follows.