SiO2 and Si3N4 etch mechanisms in NF3/hydrocarbon plasma

Abstract

Low-pressure inductive plasma was used to study SiO2 and Si3N4 etching with NF3/hydrocarbon chemistry. NF3 and a hydrocarbon were used so that fluorine and carbon could be supplied from feed gases other than global warming fluorocarbons. Etch rates of SiO2 are less than the Si3N4 etch rates over a wide range of conditions. With 50 W of wafer power, the SiO2 etch rate was of the order of 25 nm min−1 while the Si3N4 etch rate was of the order of 50 nm min−1. Similarly to a fluorocarbon chemistry, the etch process yields a very thin carbon-based steady-state film whose characteristics were determined with ex situ x-ray photoelectron spectroscopy (XPS). From mass spectrometry and XPS, CHxF, CF2 and CF3 were produced but in small concentrations compared with CHx and CN. Comparisons of normalized F 1s spectra of nitride and oxide show that relative concentrations of CF2 and CF3 on SiO2 are much lower than the concentrations on Si3N4. It appears that SiO2 preferentially reacts with CF2 and CF3 only but not with C–C or CHxF. Differences in the abilities of SiO2 and Si3N4 to react with C–C structures contributed to higher etch rates of Si3N4.