X-RAY PHOTOELECTRON STUDY OF THE REACTIVE ION ETCHING OF SIXGE1-X ALLOYS IN SF6 PLASMAS

Reactive ion etching of SixGe1-x alloys (0 less than or equal to x les s than or equal to 100%) deposited on silicon wafers using the electro n-beam evaporation technique was investigated in a low pressure fluori ne-based plasma (SF6). The etch rates of the SixGe1-x alloys increase with the Ge content of the alloy but are constantly lower than the etc h rates expected from independent etching mechanisms of Ge and Si atom s 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 photoel ectron spectroscopy (XPS). The reactive layer on the etched alloys con sists of silicon, germanium, fluorine, and sulfur atoms. Fluorinated G e (GeFn) and Si (SiFn) species, known to be precursors in the formatio n of the GeF4 and SiF4 etch products, are identified. A precise analys is of the sulfides of Ge (GeSn) and of Si (SiSn) present on the surfac e revealed a strong difference between sulfur-germanium and sulfur-sil icon interactions by the detection of mostly GeS2 and SiS species. A s ulfur-rich reactive layer is measured on the surface of the germanium- rich alloys, whereas a fluorine-rich layer exists on the surface of th e silicon-rich alloys. Still, the sulfur-rich reactive layer exists ov er a large range of alloy stoichiometry (0 less than or equal to x les s than or equal to 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 e qual etch rates of Ge and Si atoms of the alloy. (C) 1996 American Vac uum Society.