The role of diluents in electronegative fluorinated gas discharges

To study the role of diluents in NF3 plasma processing we have correlated SiO2 and plasma chemical vapor deposition silicon nitride (SiN) etch rate measurements with rf electrical impedance analysis. A series of rare gas (He, Ar) and molecular (N2, O2, N2O) mixing gases were added to NF3 plasmas at different pressures to understand the effect of diluents on the chemical and physical properties of NF3 discharges. The etch rate experiments show that for NF3 plasmas the choice of mixing gas can have a profound effect on the etch rates of SiO2 and SiN with 25 mol % NF3 in Ar yielding the highest rates and 25 mol % NF3 in N2O the lowest. The electrical measurements revealed that the diluents have a profound effect on the plasma impedance and actual power dissipated in the discharge. NF3 plasmas diluted with Ar exhibited the lowest impedances and highest real power dissipation at higher pressures while N2O diluted plasmas had the highest impedances and lowest power dissipation levels. These results indicate that the diluents which result in the highest power dissipation in the discharge, at high pressures, result in the highest etch rates. We propose that the dominant role of the diluent in NF3 plasmas is to control the electronegativity of the discharge, and thus to control real power dissipation. This function is in contrast to the role of diluents in plasmas based on other fluorinated gases, where the diluents are seen as primarily affecting the concentrations of reactive species which deposit or remove materials from the surface of the thin film being processed.