Studies of the reactive ion etching of silicon-germanium alloys

Reactive-ion etching (RIE) of epitaxial, strained Si1-xGex alloys, x=0.20, in fluorine-chlorine-, and bromine-based low-pressure plasmas has been investigated. The reactive-ion etch rates of the Si1-xGex alloys increase with the Ge content of the alloy for most etching gases. This effect is most pronounced for fluorine (CF4 and SF6) plasmas where the etch rate of a Si80Ge20 alloy is increased by a factor of ¿2 over that of Si. The etch rate enhancement is of reduced importance for chlorine (CF2Cl2) and bromine (HBr) plasmas, where the etch rate increases by less than 50% for a Si80Ge20 alloy relative to that of elemental Si. Analysis shows that the etch rate increase is not accounted for by the greater rate of gasification of Ge atoms alone but that the presence of Ge atoms in the SiGe alloy increases the Si etch product formation rate. Directional SiGe profiles are observed for CF2Cl2 and HBr plasmas which are identical to those obtained with Si. After etching in CF4 or SF6 plasmas the Ge surface concentration of the SiGe alloy is increased relative to its bulk value and both fluorinated Si and Ge are observed by in situ x-ray photoemission spectroscopy. On the other hand, for CF2Cl2 and HBr plasmas the SiGe surface is Si rich. The effect of band-gap narrowing, strain and plasma-induced surface modifications on the etching of the SiGe alloys is discussed.