Model prediction of radical composition in C4F8 plasmas and correlation with measured etch characteristics of silicon dioxide

A computational model for plasma chemical reactions has been developed. An ab initio molecular orbital method was used to determine dissociation paths and their threshold energies. Plasma characteristics were calculated by a plasma kinetic method. The radical compositions in C4F8, with additional gases such as Ar, He and CH2F2 were calculated. Radicals influencing the selective etching of SiO2 over Si3N4 were analysed. With increased microwave power or decreased flow rate, CF2 density decreased and CF, C and F densities increased. The increase of radicals with abundant carbon relative to fluorine would result in high etch selectivity of SiO2 over Si3N4 and a low etch rate. Increases in the concentration of F radicals are correlated with increases in SiO2 etch rates. Electron temperature was high with He addition, and dropped with C4F8 alone and Ar addition discharges. On the contrary, the electron density was high in the reverse order. The highest etch selectivity was obtained with He addition. A high electron temperature discharge would be one solution to obtain high etch selectivity of SiO2 over Si3N4.