Effect of O2 dilution and substrate temperature on the etching of SiNx in a C4F6/Ar plasma

The reactive ion etching (RIE) of SiNx is commonly realized with fluorocarbon plasmas. During RIE, a carbon-rich CFx film typically accumulates on the surface, which controls the etch rate. Using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that O2 dilution of a C4F6/Ar plasma prevents the continuous built-up of a CFx film during RIE, resulting in linear etching of SiNx. Using in situ ellipsometry and ex situ x-ray photoelectron spectroscopy, we established a relationship between the surface bonding on SiNx and the steady-state etch rate in C4F6/O2/Ar plasmas with varying O2 to C4F6 flow rate ratios at substrate temperatures of 70 and 120 °C. Two distinct regimes for O2 dilution were identified: at low O2 to C4F6 ratios, the etch rate is governed by the thickness of the surface CFx layer, while the mixing layer composition dominates etching at higher O2 to C4F6 ratios. At both substrate temperatures, the SiNx etch rate increases dramatically with increasing O2 to C4F6 flow rate ratios, but beyond a ratio of 0.75, there is a much more gradual increase in the SiNx etch rate with increasing O2 dilution as the CFx layer becomes much thinner. We also observed different temperature dependence for the etch rate of SiNx in the low and high O2 dilution regimes: the etch rate was lower at 70 °C than at 120 °C for low O2 dilutions, but the trend reversed for higher O2 dilutions. A possible explanation for this phenomenon is that a higher sticking coefficient for CxFy radicals at 70 °C leads to the formation of a thicker CFx film on the SiNx surface at lower O2 dilutions, resulting in a slower etching in the CFx layer-dominated regime. In the high O2 dilution regime where the etch behavior is governed by the supply of etchants, a higher sticking coefficient for the CxFy radicals at 70 °C leads to a higher etch rate than at 120 °C for the same O2 to C4F6 flow rate ratio.