A Comparison of CF4, CHF3 and C4F8 + Ar/O2 Inductively Coupled Plasmas for Dry Etching Applications

In this work, we performed the comparative study of plasma parameters, steady-state gas phase compositions and Si reactive-ion etching kinetics in CF4 + O-2 + Ar, CHF3 + O-2 + Ar and C4F8 + O-2 + Ar gas mixtures with variable O-2/Ar component ratios. It was found that the substitution of Ar for O-2 (a) did not disturb the well-known correlation between the polymerizing ability and the F/C ratio in the original fluorocarbon molecule; (b) causes similar changes in electrons- and ions-related plasma parameters (electron temperature, plasma density, ion bombardment energy); and (c) always suppresses densities of polymerizing radicals and reduces the polymer film thickness. At the same time, the specific effects of oxygen on F atom kinetics result in sufficient differences in their densities and fluxes. It was shown that the dominant etching mechanism for Si in all three gas systems is the chemical etching pathway provided by F atoms (since the contribution of physical sputtering is below 10%) while measured etching rates do not follow the behavior of F atom flux. The phenomenological analysis of heterogeneous process kinetics allowed one to suggest factors influencing the effective reaction probability. These are either the transport of F atoms through thick polymer film (in the case of high-polymerizing C4F8 + O-2 + Ar plasma) or heterogeneous reactions with a participation of oxygen atoms under the condition of thin or non-continuous polymer film (in the case of low-polymerizing CF4 + O-2 + Ar plasma).