Hearing aids are useful tools an aging population can use to improve their quality of life. Miniature microphones are a key component in hearing aids and their acoustic performance determines the effectiveness of hearing aids in helping the hearing impaired communicate in noisy environments. Micro-Electro-Mechanical Systems (MEMS) microphones have grown to dominate the market for miniature microphones in portable electronic devices such as cell phones due to their small form factor as well as their compatibility with high-volume manufacturing processes. While MEMS microphones have the potential of providing significant performance improvements in hearing aids, they have generally not yet demonstrated sufficient performance for this demanding application. A goal of the proposed research is to take better advantage of MEMS technology to enable a revolutionary change in the design of high-performance MEMS microphones. If successful, this research will lead to more sensitive MEMS microphones for hearing aids and will have a tremendous impact on the lives of hearing impaired. Noise performance of the MEMS microphone will be the major improvement that can help the aging population hear better.
Technical Capacitive sensing using electrostatic repulsive fingers is a novel concept that can transform many applications suffering from limited functionality due to the pull-in instability of conventional parallel plate sensors. Since the invention of capacitive microphones in 1916, the limitation of pull-in voltage has significantly diminished the functionality of these devices in terms of noise performance and sensitivity. We propose using a Micro-Electro-Mechanical Systems (MEMS) capacitive sensing mechanism based on the repulsive force that, unlike conventional capacitive sensors, does not suffer from pull-in voltage. This approach allows significantly increased bias voltages that will substantially improve the signal-to-noise ratio and electrical sensitivity of microphones. The MEMS microphone examined here is composed of a silicon diaphragm that rotates about an axis upon exposure to sound pressure. The diaphragm incorporates a number of capacitive sensor unit cells, and their change in capacitance converts to a voltage in the preamplifier. A unit sensor cell consists of a movable finger above a vertically aligned fixed finger next to an unaligned fixed finger. When potential differences are made in the aligned and unaligned fingers, an asymmetric electric field is created on the movable finger, generating a repulsive force that pushes it away from the substrate. Our preliminary studies show that the initial gap of the unit sensor cell can be designed so the force on the movable finger remains repulsive, which prevents the collapse of the movable fingers on the substrate (pull-in). The asymmetric electric field distribution on the movable finger causes a complicated coupled relationship between the capacitance and mechanical displacement. Therefore, to gain a fundamental understanding of the coupled system behavior, we will create analytical and numerical models of the capacitive MEMS sensor. Using the model simulations, we will design the sensor for desired sensitivity. Using microfabrication techniques, we will build the sensor and characterize its behavior using a Laser Doppler Vibrometer. After integrating the sensor with preamplifier circuitry, we will test the system in an anechoic chamber to verify the sensor improves the performance and sensitivity of microphones.
