X‐ray photoelectron study of the reactive ion etching of Si_xGe_1−x alloys in SF₆ plasmas

Reactive ion etching of Si_xGe_1−x alloys (0≤x≤100%) deposited on silicon wafers using the electron‐beam evaporation technique was investigated in a low pressure fluorine‐based plasma (SF₆). The etch rates of the Si_xGe_1−x alloys increase with the Ge content of the alloy but are constantly lower than the etch rates expected from independent etching mechanisms of Ge and Si atoms in the alloys. Analysis of the reactive layer on the surface of the alloys before and after etching was performed in situ by x‐ray photoelectron spectroscopy (XPS). The reactive layer on the etched alloys consists of silicon, germanium, fluorine, and sulfur atoms. Fluorinated Ge (GeF_n) and Si (SiF_n) species, known to be precursors in the formation of the GeF₄ and SiF₄ etch products, are identified. A precise analysis of the sulfides of Ge (GeS_n) and of Si (SiS_n) present on the surface revealed a strong difference between sulfur–germanium and sulfur–silicon interactions by the detection of mostly GeS₂ and SiS species. A sulfur‐rich reactive layer is measured on the surface of the germanium‐rich alloys, whereas a fluorine‐rich layer exists on the surface of the silicon‐rich alloys. Still, the sulfur‐rich reactive layer exists over a large range of alloy stoichiometry (0≤x≤80%) with an increase of GeS₂ density as a function of Si content in the alloy. The XPS analysis showed no enrichment of Ge or Si in the reactive layer compared to the substrate, suggesting equal etch rates of Ge and Si atoms of the alloy.