The key step in producing a movable microstructure is the controllable sacrificial layer etching process. For the current study, a 1 μm silicon-rich silicon nitride structural layer was deposited on a 2 μm phosphosilicate glass (PSG) sacrificial layer that had been deposited on a single-crystal silicon wafer. Silicon nitride, unlike the more common polycrystalline silicon (poly-Si) structural material, is transparent and, therefore, allows the observation of an etch front in the underlying PSG sacrificial layer. The PSG is an amorphous silicon dioxide, with phosphorus added as a controlled impurity, that can be selectively etched in hydrofluoric acid. Several experiments have been performed with four HF concentrations using these test structures. Diffusion limited etching is observed at long etching times for each concentration. Additional experimental work has been done to determine appropriate kinetic expressions for the PSG/HF reactions.
The sacrificial layer etching process has been modeled as a chemical reaction/diffusion system. Constants for the kinetic expression have been found from independent experimental work. This Deal-Grove type model assumes isothermal conditions, constant density solution, onedimensional etching, and the equality of the diffusion and reaction fluxes. The model calculates the HF surface concentration and the length of PSG underetched as a function of time. It fits the data well but requires the use of an unphysical diffusion coefficient. An extension of the Deal-Grove model to nonfirst order kinetics allows for a reasonable estimate of the reactant/product mixture diffusivity in water to be used when fitting the model to experimental data.