Fluorocarbon-based plasma etching of SiO2: Comparison of C4F6/Ar and C4F8/Ar discharges

A gas phase and surface chemistry study of inductively coupled plasmas fed with C4F6/Ar and C4F8/Ar intended for SiO2 etching processes was performed. Adding Ar to those fluorocarbon gases results in a strong increase of the ion current, by up to a factor of 5 at 90% Ar relative to the pure fluorocarbon gases. The fluorocarbon deposition rate is higher for C4F6/Ar than for C4F8/Ar, whereas the fluorocarbon etching rate is lower, and both quantities decrease as the amount of Ar is increased. For both C4F6/Ar and C4F8/Ar, the CF2 density is more than an order of magnitude greater than the CF density. The CF2 partial pressure decreases as more Ar is added to the C4F6/Ar plasmas. A comparison of these data with corresponding results obtained with C4F8/Ar shows that the CF2 partial pressure in C4F6 is higher for Ar-lean gas mixture than for C4F8/Ar. This remains true up to 40% Ar. Above 40% Ar the CF2 partial pressure in C4F8 is higher than for C4F6. The CF and COF2 partial pressures in C4F8 are higher than for C4F6. The SiO2 etch rate is higher for C4F8/Ar than for C4F6/Ar. This may be attributed in part to the higher F/C ratio of the steady-state fluorocarbon film formed on SiO2 surfaces for C4F8/Ar which was determined by x-ray photoemission spectroscopy (XPS). The etching selectivity of SiO2 over resist and silicon is increased by the addition of Ar to the fluorocarbon gases. Overall, the SiO2/resist and SiO2/Si etching selectivity are higher for C4F6/Ar (i.e., 4 and 9, respectively) at 90% Ar than for C4F8/Ar (i.e., 2 and 5, respectively) at 90% Ar and otherwise identical conditions. Both ellipsometry and XPS measurements show that the steady-state fluorocarbon layer thickness is greater for C4F6/Ar (∼4 nm) than for C4F8/Ar (∼2.8 nm). Argon addition leads to a strong decrease of the fluorine content of the steady-state fluorocarbon layers on both Si and SiO2 surfaces relative to films produced in pure fluorocarbon discharges, and this effect is related to the increase of the SiO2/Si and SiO2/resist etching selectivity.