Characteristics of secondary etching of SiO2 by ions reflected from a primary SiO2 target in a CHF3 plasma

The etching of a secondary SiO2 target, target (2), by ions reflected from a primary SiO2 target, target (1), in a CHF3 plasma using various angles for the ions incident on target (1), θi, was examined. The etch rate of target (2) was enhanced by collision with reflected ions and the extent of etch-rate enhancement was significantly affected by the surface roughness of target (1). The extent and range of secondary etching increased when θi was increased from 60° to 80°. Under the conditions used in this study, the maximum scattering angle for initiating secondary etching was nearly constant, at about 60°, irrespective of θi, when the latter was between 60° and 80°. When θi was increased to 85°, the surface of target (1) was covered with a fluorocarbon polymer layer, which drastically decreased the extent of secondary etching. At θi=85°, the maximum scattering angle was lowered to about 40° and, as a result, the range of secondary etching was reduced.