On mechanisms to control SiO2 etching kinetics in low-power reactive-ion etching process using CF4 + C4F8 + Ar + He plasma

This work discusses effects of gas mixing ratios and bias power (at 13.56 MHz and 2 MHz) on both gas-phase characteristics and etching kinetics of SiO2 in the CF4 + C4F8 + Ar + He plasma. The subject was the low input power regime (∼0.05 W/cm3). The substitution of C4F8 for CF4 exhibits no principal influence on electrons- and ions-related plasma parameters, produces weakly increasing F atom density as well as causes a comparable increase in the etching rate. The latter is provided by the chemical etching pathway with the nearly constant reaction probability. The substitution of Ar for He sufficiently affects plasma parameters, reduces the F atom density, but accelerates the etching process due to increasing reaction probability. Probably, the latter traces decreasing surface passivation by fluorocarbon radicals. Bias parameters do not disturb bulk plasma, but do influence the etching kinetics through the ion bombardment energy. The growth of etching rate vs. bias power mainly appears due to increasing efficiency of chemical etching pathway. The 2 MHz bias frequency produces lower etching rates in spite of higher ion bombardment energies. This effect is probably due to higher desorption rate and lower sticking probability for F atoms. It was found that most effective parameters to control etching profile are Ar/He mixing ratio and bias frequency.