Electronically controlled etch-mask for silicon bulk micromachining

Wafers that are to be submitted to anisotropic etching in aqueous KOH are conventionally passivated with a silicon dioxide or nitride layer in which backside windows are etched to define the microstructures. A different method to mask the backside of a silicon wafer for this purpose is presented. The method makes use of the phenomenon that silicon is not etched in KOH when biased above the passivation potential. The mask is defined by applying a set of bias voltages to the front of the wafer instead of patterning a deposited passivation layer at the backside, for which an accurate double-sided alignment is required. The feasibility of the method was demonstrated with the fabrication of membranes and suspended masses of various sizes