Mechanistic studies of the initial stages of etching of Si and SiO 2 in a CHF 3 plasma

Abstract

The objective of this paper is do to a comparative study of the mechanism of reactive ion etching of both Si and SiO 2 substrates using CHF 3. XPS and ellipsometry in the visible are used to follow the surface modifications in relation to the plasma exposure time. For Si, a transient stage is observed before a steady-state etching regime is attained. This transient stage is separated into two distinct phases corresponding to the growth of a fluorocarbon overlayer and to the initiation of the Si etching, respectively. For SiO 2 no transient stage is noted so a stationary etching situation is obtained as soon as the plasma is created. These results suggest that the fluorocarbon overlayer is not a simple blocking overlayer, but is part and parcel of the Si etching mechanism: this overlayer provides fluorine atoms necessary for the etching. On the contrary, the plasma species react directly with an SiO 2 surface to form precursors of etch products. Consequently, no fluorocarbon overlayer is needed in the SiO 2 etching mechanism.