Investigation of dilute SF6 discharges for application to SiC reactive ion etching

Abstract

Impedance matching and power coupling efficiency optimization of SF6 gas discharges diluted with Ar and He have been compared in relation to reactive ion etch rates and etched feature anisotropy of hexagonal (6H) silicon carbide (SiC). In contrast to the measured radio frequency (rf) power coupling efficiency, He diluted mixtures resulted in greater etch rates by up to 50%, with superior anisotropy and surface morphology than comparable Ar:SF6 mixtures. The superior etched surface finish for the He diluted mixtures possibly arises from the less severe sputtering damage of SiC for He+ and SFX+, compared to Ar+ with the same energy. Etch rates over 3300 Å/min have been achieved with excellent surface morphologies and anisotropy. These results conflict with the notion that Ar+ ions are expected to enhance the ion assisted etch mechanism in technical gas mixtures. We observed superior, SiC etch performance for He:SF6 mixtures, compared to Ar:SF6, over the entire 10%–90% fractional ratios investigated. This result appears to be due to differing bulk discharge chemistries which control the flux of radicals and ions to the substrate, resulting in optimal surface polymerization conditions. This suggestion is based on our estimated bulk E/n values, known partial ionization cross sections, production rate coefficients for SF3+ and SF5+, and higher electron energy distributions for He dilution. In addition, we provide further evidence for the generic nature of utilizing the complex electrical characteristics of the rf discharge to optimize plasma etch conditions. On the other hand, we have demonstrated that understanding the details of gas phase kinetics is also necessary to fully exploit the power coupling optimization scheme for rf discharges, which are specific to the gas mixtures being utilized.