The etching behavior of APCVD PSG thin films used as sacrificial layers for surface micromachined resonant microstructures

The deposition and the configuration of the sacrificial (phosphosilicate glass (PSG)) and active layers (polysilicon) are one of the most sensitive processing steps in surface micromachining technique for resonant microstructures. Starting from the technological steps for the resonant microstructures manufacturing, a new solution in order to optimise these technological steps are proposed. During the doping processes of polysilicon (∼ 1000 °C) a significant densification of the PSG layer appears. This leads to a dramatical decrease of the PSG etch rate in buffered HF solution and a strong limitation in surface micromachining technique (with unwanted effects on resist mask and metallic layer integrity) is resulting. The decrease of the PSG etch rate is mainly due to the ex-diffusion of the phosphorus from PSG film. An original solution is proposed in order to minimize the decrease of the PSG etch rate: the replacement of the PSG film with a Si₃N₄/PSG/SiO₂/Si₃N₄ multilayer. The using of a thin Si₃N₄ layer has the advantage of suppressing the ex-diffusion of P atoms and avoids the attack of HF on the polysilicon layer.