Etching suppression of silicon nitride by ammonium fluorosilicate formation during cryogenic NF3/NH3 plasma reactive ion etching

During the fabrication of 3D semiconductor devices, high-aspect-ratio etching is a critical process involving key materials such as silicon nitride (SiN) and silicon dioxide (SiO2). As device scaling continues, cryogenic high-aspect-ratio etching technology has been widely explored to improve profile control and mask selectivity. However, under cryogenic conditions, the etch rate of SiO2 increases while that of SiN decreases, posing a significant challenge. This study investigates the mechanism underlying the reduced SiN etch rate associated with ammonium fluorosilicate [AFS, (NH4)2SiF6] formation on the SiN surface by combining cryogenic NF3/NH3 plasma reactive ion etching with plasma diagnostics and surface analysis. At 20 °C, increasing the NH3 concentration leads to a pronounced decrease in the SiN etch rate, attributed to the formation of AFS. In addition, as the temperature is lowered to −60 °C, the AFS layer appears to show a more compact morphology, which may act as a more effective passivation layer by limiting the transport of HF and other reactive species to the SiN surface. These findings suggest that the thermal stability and morphology of the AFS layer are important factors governing SiN etching behavior under cryogenic conditions, offering valuable insights for optimizing SiN etching processes in next-generation 3D device fabrication.