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 C F4 plasma was studied using three types of Faraday cages located in a trans former coupled plasma etcher. The SiO2 substrates were fixed on sample hold er slopes that have different angles to the cathode. The substrate was subj ected to one of three processes depending on the design of the Faraday cage , i.e., redeposition of sputtered particles from the SiO2 bottom surface (c ase I), substrate etching by incident ions (case II), or simultaneous etchi ng and redeposition (case III). Both the redeposition and the etch rates we re measured by changing the substrate-surface angle and the self-bias volta ge 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 t he rates at -100 V followed the cubic curve, indicating different mechanism s of the bottom SiO2 etching depending on the energy regimes. The steep inc rease of the redeposition rate with the angle was attributed to three facto rs: the substrate-bottom distance, the angular distribution of emitted part icles from the bottom surface, and the particle incident angle on the subst rate surface. The etch-only rate curves (case II) closely followed the cosi ne 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 d egrees, showing a negative value, i.e., a net redeposition, beyond 75 degre es. The drastic decrease in the net etch rate coincided with the steep incr ease in the redeposition rate, implying the significant effect of redeposit ion. (C) 2001 American Vacuum Society.