Etching Mechanisms of SiO2 and SiNx:H Thin Films in HF/Ethanol Vapor Phase: Toward High Selectivity Batch Release Processes

The etch dynamics of low-density undoped silica glass oxide (USG) and Si-rich SiN
_x
:H thin films in a 25-wafer batch vapor HF tool operating at low etching pressure were studied. Up to etching temperatures approaching the water boiling point, as defined by the etching pressure, the kinetics of the etch as a function of the etching temperature is found to be driven by adsorption phenomena for both oxide and nitride layers. The etch rate increases with etching time before reaching a stable regime, according to adsorption laws in the case of the USG layers and to more complex phenomena that might include a partial oxidation of the layer and/or the creation of solids by-product residues sticking to the surface, in the case of nitride layers. Selectivities in the etch rates of oxide versus nitride layers higher than 60 were achieved, together with a high within-wafer and wafer-to-wafer uniformity. The etching mechanisms are proposed to be a similar to liquid phase etching: low-density USG layers are preferentially etched through nucleophilic substitutions, while monofluorides are tentatively identified as the most reactive agent in the case of Si-rich SiNx:H layers where Si-N or, depending on the composition of the etching media, Si-Si bonds have to be broken.