Angular dependence of the redeposition rates during $SiO2$ etching in a $CF4$ plasma

The angular dependence of the redeposition rates during $SiO2$ etching in a $CF4$ plasma was studied using three types of Faraday cages located in a transformer coupled plasma etcher. The $SiO2$ substrates were fixed on sample holder slopes that have different angles to the cathode. The substrate was subjected to one of three processes depending on the design of the Faraday cage, i.e., redeposition of sputtered particles from the $SiO2$ bottom surface (case I), substrate etching by incident ions (case II), or simultaneous etching and redeposition (case III). Both the redeposition and the etch rates were measured by changing the substrate–surface angle and the self-bias voltage in the range of −100 to −800 V. The redeposition-only rates (case I) at −450 and −800 V closely followed the quadratic curve of the angle whereas the rates at −100 V followed the cubic curve, indicating different mechanisms of the bottom $SiO2$ etching depending on the energy regimes. The steep increase of the redeposition rate with the angle was attributed to three factors: the substrate–bottom distance, the angular distribution of emitted particles from the bottom surface, and the particle incident angle on the substrate surface. The etch-only rate curves (case II) closely followed the cosine of the surface angle. The etch-rate curve changed into a reverse-S shape when the substrate was subjected to simultaneous etching and redeposition (case III). The net etch rate for case III decreased drastically above 60°, showing a negative value, i.e., a net redeposition, beyond 75°. The drastic decrease in the net etch rate coincided with the steep increase in the redeposition rate, implying the significant effect of redeposition.