H.Matsui, M.Yoda, T.Toriyama*, S.Konishi and S.Sugiyama
Faculty of science and engineering, Ritsumeikan University
1-1-1Noji-Higashi, Shiga, Kusatsu 525-8577, Japan
*New Energy and Industrial Technology development Organization
Higashi-ikebukuro, Toshima-ku, Tokyo 170-6028, Japan

Keyword : condenser microphone, polysilicon membrane, MICS

A condenser microphone has been fabricated by MICS (Multi-User Integrated Chip Service: three layer polysilicon structure).  The condenser microphone having a 2 mm - wide polysilicon membrane has been designed for a sensitivity range of more than -60 dB (ref. to 1V/Pa) and frequency range 100 〜 2kHz.　Fabrication of a condenser microphone is attempted using only surface micromachining. Figure 1 shows the schematic view of a condenser microphone.  The condenser microphone consists of a square vibration membrane and a square fixed electrode.  Acoustic holes to reduce air dumping are arranged in the support part of the vibration membrane, whose longitudinal directions are parallel to the substrate.

Sensitivity of a condenser microphone is expressed as:

where V_b is bias voltage, a is membrane width, d is gap between the vibration membrane and the fixed electrode and Zt is total equivalent impedance of the circuit.  In the analysis, 2mm - wide of square shaped membrane and 2.75μm - gap between the vibration membrane and the fixed electrode are used. The bias voltages are selected for 5V, 10V and 15V.

Figure 3 shows SEM image of capacitance microphone before the sacrifice layer etching.  Behind the sacrifice layer etching, A bend occurred to the vibration membrane is visible. This was regarded as a compression stress occurring at the time of LPCVD.

To confirm the static response of the microphone, the change of the capacitance in the case of bias voltage load, using a LCR meter (HP4284A), was measured.  The capacitance in the case of a 0V bias voltage load was 9 pF and agreed approximately with the 12 pF of the theory value. Also, the increase of the capacitance with an increase of bias, could be confirmed as shown in Figure 4.