C-F INCORPORATED OVERLAYER GROWTH ON SILICON EXPOSED TO A DC-EXCITED CF4 PLASMA

The effects of d.c. biasing on the etching characteristics of silicon exposed to a d.c.-excited CF4 plasma have been analysed to provide further insight into the energy dependence of surface ion-assisted chemistry in fluorocarbon plasmas. Time dependence variations of the desorbed SiF4 partial pressure are reported and discussed in relation to the surface characterization with Auger sputter profiling. It is shown that, beyond a threshold ion energy W(T), a C-F blocking overlayer develops of which the steady state thickness increases with increasing ion energy, therefore involving an increasing attenuation of the silicon removal. Auger depth profiles demonstrate the difference between this "C-F incorporated overlayer" and the "C-F mixed layer" which is set up at ion energies lower than W(T). The threshold ion energy decreases as the neutral pressure decreases, ion current density increases, surface temperature increases, and F:C ratio of the active species decreases. These results are discussed on the basis of an "incorporation-combination-sputtering model" accounting for the carbon surface and volume mass balance.