Relation between the ion flux, gas phase composition, and wall conditions in chlorine plasma etching of silicon

Abstract

Transients in plasma composition and positive ion flux due to changing chamber wall conditions during Cl2 plasma etching of Si were studied using multiple plasma and surface diagnostics. In presence of Si and O containing species in the gas phase a glassy silicon oxychloride film coats the chamber walls over a time scale determined by the concentrations of the Si and O containing deposition precursors. This time scale can be a few minutes as in the case of Si etching with Cl2 plasma, where the concentration of silicon chloride etching products can be high, or hours as in the case of a Cl2 plasma maintained in absence of Si wafer, where the Si and O can only come from very slow etching of a quartz window. In either case, SiClx (1⩽x⩽4) and Cl concentrations in the gas phase and the total ion flux impinging on the wafer surface increase as the chamber walls are coated with this glassy film. The increase in SiClx and Cl concentrations are primarily due to lower loss probability of these species by recombination on the chamber walls. The ion flux increases primarily due to higher SiClx concentration in the discharge. During etching of Si, increases in Cl concentration and ion flux through the mechanism described above increases the etching and SiClx production rates. This strong coupling among the discharge properties, the wall conditions, and etching rate lead to transients in plasma operation.