SiO$_{2}$ and Si$_{3}$N$_{4}$ Etch Mechanisms in NF$_{3}$/C$_{2}$H$_{4}$ Plasma*

Low-pressure inductive plasma was used to study SiO$_{2}$ and Si$_{3}$N$_{4}$ etching with the NF$_{3}$/C$_{2}$H$_{4}$ chemistry. NF$_{3}$ and C$_{2}$H$_{4}$ were used so that fluorine and carbon could be supplied from feed gases other than global warming fluorocarbons. Etch rates of SiO$_{2}$ over a wide range of conditions are less than 0.8 times the Si$_{3}$N$_{4}$ etch rates. Ex-situ XPS was used to determine the characteristics of a very thin steady-state film, to establish etch mechanisms. XPS results show that CH$_{x}$F, CF$_{2}$, and CF$_ {3}$ were produced but in small concentrations compared to CH$_{x}$ and CN. Mass spectrometry and optical emission gave consistent results. C1s spectra from etched oxide samples show a large percentage of H$_{x}$C-CH$_{x} $ structures. Si$_{3}$N$_{4}$ appears to react easily with H$_{x} $C-CH$_{x}$ structures, yielding CN-bearing products and SiC. Etch rate and selectivity results of NF$_{3}$-based discharges fed with C$_{2}$H$_{2}$, C$_{4}$H$_{10}$, and CH$_{3}$F are similar to the NF$_{3}$/C$_ {2}$H$_{4}$ plasma. Comparisons of normalized F1s spectra of nitride and oxide etched under the same conditions show that relative concentrations of CF$_{2}$ and CF$_{3}$ on SiO$_{2}$ are much lower than the concentrations on Si$_{3}$N$_{4}$. It appears that SiO$_{2}$ preferentially reacts with only CF$_{2}$ and CF$_{3}$ but not with H$_{x}$C-CH$_{x}$ or CH$_{x} $F. Differences in the abilities of SiO$_{2}$ and Si$_{3}$N$_{4}$ to react with H$_{x}$C-CH$_{x}$ contributed to higher etch rates of Si$_{3} $N$_{4}$. Effects of bias frequency are presented.