Surface processes in plasma-assisted etching environments

Plasma-assisted etching involves the interaction of radicals generated by a glow discharge (e.g., fluorine atoms) with solid material (e.g., silicon) to produce volatile products (e.g., SiF4). Moreover, this type of reaction is frequently enhanced by bombardment with ions, electrons, and photons. The purpose of the present paper is to review surface processes which lead from the adsorption of radicals to the evolution of products and to formulate a conceptual framework for the interpretation of experimental data obtained in plasma environments. It will be suggested that etching reactions are quite analogous to similar reactions which lead to oxide formation. In particular, it is suggested that field-assisted mechanisms of the Mott–Cabrera type involving place exchange and motion of cations and/or anions are likely to influence or dominate etching reactions. In addition, it will be shown that surface reconstruction, precursor states, and the surface concentration of adsorbed species are likely to be important factors which influence the kinetics. Furthermore, emphasis will be placed on mechanisms which cause radiation (ions, electrons, photons) to enhance the reaction rate. Ion-enhanced etching could be dominated by physical sputtering, chemical sputtering, surface damage, or a combination of these phenomena. Photon-enhanced etching could be initiated by excitation of gas phase molecules, solid atoms or chemical species adsorbed on the surface. The characteristics of each type of reaction will be discussed with experimental illustrations.