Etching mechanism of amorphous hydrogenated silicon nitride by hydrogen fluoride

We report the etching mechanism of amorphous hydrogenated silicon nitride by hydrogen fluoride (HF) gas using density functional theory (DFT) calculations. Since silicon nitride films are deposited as amorphous with a significant amount of hydrogen, we constructed an amorphous substrate model with a hydrogen concentration of 25 at.% using molecular dynamics simulation and DFT calculations. We then created slab models with different degrees of fluorination and simulated all possible fluorination pathways. The pathways involving cleavage of Si–N or Si–Si bonds showed low activation energies of 0.90 eV or lower, while the pathways involving cleavage of a Si–H bond showed high activation energies of 1.54 eV or higher·NH3, SiF4, SiH2F2, and SiHF3 were released with low activation energies, indicating that etching would be favorable. Next, we modeled the formation and desorption of the (NH4)2SiF6 salt on the fluorinated surface. The salt formation was exothermic with low activation energies, consistent with self-limited etching at low temperatures. At temperatures of 152 °C or higher, (NH4)2SiF6 would desorb, leaving no solid residue, consistent with the high etch rate at elevated temperatures. Our DFT calculations using the amorphous hydrogenated slab model successfully explained the silicon nitride etching process, which could not be explained by a crystalline Si3N4 slab model.