FEATURES OF SiO2 REACTIVE-ION ETCHING KINETICS IN CF4

Abstract

The effect of Ar/O-2 mixing ratio on gas-phase characteristics and SiO2 etching kinetics in CF4 + Ar + O-2 and C4F8 + Ar + O-2 plasmas was studied under conditions of 13.56 MHz inductive RF discharge. The constant processing parameters were fraction of fluorocarbon component in a feed gas (50%) total gas pressure (6 mTorr), input power (700 W) and bias power (200 W). It was found that the full substitution of Ar for O-2 in both gas systems results in non-monotonic (with a maximum at similar to 25% Ar + 25% O-2) SiO2 etching rates as well as in monotonically increasing photoresist etching rate with higher absolute values for CF4-containing mixture. The steady-state densities of active species were determined using a combination of plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling. Corresponding results indicated that both gas systems are characterized by quite close parameters of electron and ion components while exhibit sufficient differences in the kinetics of neutral species, especially in the presence of O-2. The latter produces opposite changes in F atom density as well as in effective probability of ion-assisted chemical reaction vs. Ar/O-2 mixing ratio. Relationships between type of fluorocarbon component and heterogeneous process kinetics were analyzed through the set of gas-phase-related parameters (fluxes, flux-to-flux ratios) characterizing chemical etching pathways for SiO2 and formation/destruction balance for the fluorocarbon polymer film. It was suggested that the transition toward O-2-rich plasma in the low-polymerizing CF4 + Ar + O-2 plasma suppresses the effective probability for SiO2 + F reaction through decreasing efficiency for oxide bond breaking and desorption of etching products due to decreasing ion energy flux. Oppositely, an increase in O-2 content in the high-polymerizing C4F8 + Ar + O-2 mixture lifts up the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.