Single-chip condenser microphone using porous silicon as sacrificial layer for the air gap

A single-chip IC-compatible silicon condenser microphone with a highly sensitive silicon nitride membrane and a rigid monocrystalline silicon counterelectrode with acoustic holes was designed and built. Porous silicon with its high dissolution rate in 1% KOH was used as an auxiliary sacrificial layer in combination with sputtered SiO₂ to define the air gap. This results in low parasitic capacitances and a microphone structure where the membrane is coplanar with its suspensions. The rigid backelectrode is undistorted, the membrane under low tensile stress, a prerequisite for high sensitivity. Microphones of different dimensions of round and square electrodes with single membranes and membrane arrays were built and packaged in round chip carriers. The open loop sensitivity is in the mV/Pa range depending on the type of microphone. The frequency response goes beyond 25 kHz for an air gap of 1.3 μm