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

Impedance matching and power coupling efficiency optimization of ${\mathrm{SF}}_{\mathrm{6}}$ 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 ${\mathrm{Ar:SF}}_{\mathrm{6}}$ mixtures. The superior etched surface finish for the He diluted mixtures possibly arises from the less severe sputtering damage of SiC for ${\mathrm{He}}^{\mathrm{+}}$ and ${\mathrm{SF}}_X^{+},$ compared to ${\mathrm{Ar}}^{\mathrm{+}}$ 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 ${\mathrm{Ar}}^{\mathrm{+}}$ ions are expected to enhance the ion assisted etch mechanism in technical gas mixtures. We observed superior, SiC etch performance for ${\mathrm{He:SF}}_{\mathrm{6}}$ mixtures, compared to ${\mathrm{Ar:SF}}_{\mathrm{6}},$ 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 $\mathrm{E/n}$ values, known partial ionization cross sections, production rate coefficients for ${\mathrm{SF}}_{\mathrm{3}}^{\mathrm{+}}$ and ${\mathrm{SF}}_{\mathrm{5}}^{\mathrm{+}},$ 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.