SURFACE-SCIENCE ASPECTS OF PLASMA-ASSISTED ETCHING

Plasma-assisted etching methods have been used in the manufacture of i ntegrated circuits for more than 10 years and yet the surface-science aspects of this technology are poorly understood. The chemistry must b e such that the reactive species generated in the plasma react with th e surface being etched to form a volatile product. The chemistry is us ually dominated by atoms, molecular radicals and low-energy (20-500 eV ) positive ions. In microstructure fabrication, the positive ions are accelerated from the plasma towards the etched surface arriving essent ially at normal incidence. Thus, the bottom surface of a very small fe ature being etched is subjected to both energetic ions and reactive ne utral species, whereas the sidewalls of the feature are exposed to rea ctive neutral species only. The role of the energetic ions is primaril y to accelerate the reaction between the neutral species and the etche d surface (i.e., accelerate the etch rate), thereby reducing the stead y-state top-monolayer coverage of the etching species on the etched su rface. On the sidewalls, however, the reacting-species coverage is a s aturation coverage. The present understanding of some of the surface-s cience aspects of this complex environment will be summarized, often u sing the Si-F system as an example, and some phenomena which are not w ell understood will be described.