Deposition and etching of diaphragm and sacrificial layer in novel MEMS capacitive microphone structure

Abstract

This paper will describe how a novel single-chip capacitive microphone was successfully fabricated on silicon wafer using MEMS process. The capacitive microphones generally consist of a diaphragm, back plate and air gap. The novelties of the method relies on the moveable aluminum (Al) diaphragm positioned over the backplate electrode, where the diaphragm includes a plurality of holes to allow the air in the gap between the electrode and the diaphragm to escape and thus reducing acoustical damping in the microphone. Spin-on-glass (SOG) was used as a sacrificial and isolating layer. Back plate is mono crystalline silicon wafer, that it is more stiff. Compared with the previously works, this microphone has several advantages: By making acoustic holes in diaphragm, no KOH etching is needed to make back chamber, thus the chip size of microphone is reduced. The fabrication process use minimal number of layers and masks to reduce the fabrication cost. SOG sacrificial layer is easy to deposition by spin coater and also easy to release by PAD etch solution. The temperature used for fabrication is less than 200degC, thus reducing temperature induced damages. The diaphragm size is 0.5times0.5 mm2 which is smaller than conventional MEMS capacitive microphone. Since Al diaphragm is electrically conductive, thus no extra electrode is needed.