Plasma etching, a purely chemical dry etching technique,
basically consists of the following steps:  1)  generation
of reactive species in a plasma; 2)  diffusion of these
species to the surface of the material being etched; 3)
adsorption of these species on the surface; 4) occurrence
of chemical reactions between the species and the material
being etched, forming volatile byproducts; 5) desorption of
the byproducts from the surface; and 6) diffusion of the
desorbed byproducts into the bulk of the gas.