Dry etching in the presence of physisorption of neutrals at lower temperatures

In this article, we give an overview about the chemical and physical processes that play a role in etching at lower wafer temperatures. Conventionally, plasma etching processes rely on the formation of radicals, which readily chemisorb at the surface. Molecules adsorb via physisorption at low temperatures, but they lack enough energy to overcome the energy barrier for a chemical reaction. The density of radicals in a typical plasma used in semiconductor manufacturing is one to two orders of magnitude lower than the concentration of the neutrals. Physisorption of neutrals at low temperatures, therefore, increases the neutral concentration on the surface meaningfully and contributes to etching if they are chemically activated. The transport of neutrals in high aspect ratio features is enhanced at low temperatures because physisorbed species are mobile. The temperature window of low temperature etching is bracketed at the low end by condensation including capillary effects and diminished physisorption at the high end. The useful temperature window is chemistry dependent. Besides illuminating the fundamental effects, which make low temperature processing unique, this article illustrates its utility for semiconductor etching applications.