PLASMA PARAMETERS AND SiO2 ETCHING KINETICS IN C4F8

The effect of Ar/O-2 mixing ratio on plasma parameters, steady-state densities of active species and SiO2 etching kinetics in the three-component C4F8+Ar+O-2 gas mixture was studied under typical conditions of reactive ion etching process (inductive 13.56 MHz RF discharge, total gas pressure of 6 mTorr, input power of 700 W and bias power of 200 W). The investigation combined etching rate measurements, plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling in order to determine steady-state densities and fluxes of plasma active species. It was found that the full substitution of Ar for O-2 at constant fraction of fluorocarbon gas (in fact, the transition from 50% C4F8 + 50% Ar to 50% C4F8 + 50% O-2 gas system): 1) results in weakly non-monotonic (with a maximum) SiO2 etching rate with close values for both O-2-free and Ar-free plasmas; 2) causes the monotonic decrease in both F atom flux and ion energy flux; and 3) suppresses the formation of the fluorocarbon polymer film on the etched surface through its oxidative destruction pathway. The model-based analysis of SiO2 etching kinetics allowed one to conclude that an increase in effective probability for SiO2 + F reaction contradicts with the behavior ion energy flux as well as demonstrate the agreement with the change in gas-phase parameters characterizing the fluorocarbon film thickness. Therefore, an increase in O-2 content in a feed gas influences the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.