Low bias voltage and high sensitivity CMOS condenser microphone using combined stress relaxation design

During the past decade, many companies have applied MEMS micromachining technique to the fabrication of miniature microphone such as Knowles SiSonicTM, Yamaha, Akustica, and Omron. In addition to the miniature volume, one major advantage that MEMS microphones is the capability of handling the heat produced during the reflow soldering processes on typical surface mount fabrication lines. However, most MEMS microphones are connected to ASIC chips through wire bonding that will induce undesired parasitical capacitance. The requirements of monolithic integration of sensors with electronic part comes form lowing the manufacturing cost, simplifying total process, solving interconnect bottleneck, miniaturizing total size, and reducing the parasitic capacitance. One example is the accelerometer made by Analog Device. A piezoelectric microphone with on-chip CMOS amplifier has been demonstrated. The square diaphragm with the size 2 mm on a side was fabricated by combining a silicon-nitride diaphragm with ZnO thin film layers. The measured sensitivity is 80 uV/Pa at 3 KHz and the frequency response range is form 800 Hz to 30 KHz. The capacitive sensing is more widely used to detected membrane vibration. Condenser microphone has benefits of high sensitivity, flat frequency response, and low noise level. A FET condenser microphone with sensitivity up to 38 mV/Pa had been reported. Basically, the sensitivity of microphone can be boosted by increasing membrane size and bias voltage, reducing the air gap, and relieving the residual stress. To acquire enough bias voltage, the additional charge pump circuit is needed; thus, this additional charge pump circuit will increase the cost and the overall volume of microphone. To this consideration, a low bias voltage with high sensitivity CMOS microphone using combined stress relaxation is developed. The design concept, fabrication processes, and measurement result are proposed in this study.