Study on transduction based on electret and field effect transistor and its application to micro-machined microphones with high performance

Abstract

Capacitive-type transduction is now widely used in MEMS microphones. However, its sensitivity decreases with reducing size, due to decreasing air gap capacitance. In the present study, we proposed and developed the Electret Gate of Field Effect Transistor (ElGoFET) transduction based on an electret and FET (field-effect-transistor) as a novel mechanism of MEMS microphone transduction. The ElGoFET transduction has the advantage that the sensitivity is dependent on the ratio of capacitance components in the transduction structure. Hence, ElGoFET transduction has high sensitivity even with a smaller air gap capacitance, due to a miniaturization of the transducer. And ElGoFET transduction is displacement based transduction. Therefore, microphone application using ElGoFET transduction does not need additional large volume for back chamber to endure membrane’s velocity. Theoretical approach of ElGoFET transduction was conducted. And electro-mechano-acoustic transduction model was developed and designed for application to microphone. As feasibility study, a FET with a floating-gate electrode embedded on a membrane was designed and fabricated and an electret was fabricated by ion implantation with Ga+ ions. During the assembly process between the FET and the electret, the operating point of the FET was characterized using the static response of the FET induced by the electric field due to the trapped positive charge at the electret. And we evaluated the microphone performance of the ElGoFET by measuring the acoustic response in air. The results confirmed that the proposed transduction mechanism has potential for microphone applications. Furthermore, the ElGoFET microphones are batch-fabricated on 6-inch wafer. The FET with a floating-gate electrode embedded on a backing structure and electret embedded on the oxide/nitride thin membrane were designed and fabricated on separate wafers. The electret layer was fabricated ion-implantation with B+ ions. The bonding integration was employed for combining separate wafers for electrets and FETs, which are combined via eutectic wafer bonding process. The fabricated ElGoFET microphone was characterized with a measured sensitivity of -31dB ±1dB (Ref. 1V/Pa) for frequencies in the range 2-1000 Hz. These result shows the potential for high-performance micromachined microphone for wide-band and low frequency application, as well as system-on chip functionality.