MEMS thin film teflon electret condenser microphones

Citation

Hsieh, Wen Hsuan (2001) MEMS thin film teflon electret condenser microphones. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/sk4y-ws65. https://resolver.caltech.edu/CaltechETD:etd-08302005-135533

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The goal of this thesis is to develop miniature, inexpensive, high-quality, self-biasing electret condenser microphones that are fabricated using Micro Electro Mechanical Systems (MEMS) technology. These MEMS electret microphones are to be used in any application where a conventional electret microphone can be used (such as in cell phones, and hearing aids) and in new acoustic sensing applications where current microphone technology cannot be applied (such as in smart cards of the future or in new applications where it is advantageous to integrate microelectronics with the microphone). To accomplish this, a MEMS-compatible Teflon electret technology has been developed. The electret material used is thin film spin-on Teflon AF. A custom-built pulsed electron gun, called the Back-Lighted Thyratron (BLT), is used for charge implantation. Thermal annealing is used to stabilize (age) the implanted charge. An electric field compensation method is used to measure the charge density of the electrets. The electrets obtained have stable charge densities on the order of [...] to [...]. Two main types of MEMS thin film Teflon electret condenser microphones have been successfully fabricated and tested. Both microphones use silicon substrates and are fabricated using bulk-micromachining techniques. Each microphone is manufactured as a two piece structure, comprising a microphone membrane unit having an extremely thin diaphragm and a perforated microphone backplate unit. When one is placed on top of the other, the two units form a highly reliable, inexpensive microphone that can produce a signal without the need for external biasing. This reduces system volume and complexity. One type of microphone uses a silicon nitride/Teflon AF composite diaphragm, while the other type uses a Parylene C/Teflon AF composite diaphragm. Both microphones use the same perforated silicon nitride/Parylene C composite backplate. Both types of millimeter-scale electret microphones have very low stray capacitance, are self-biasing, mass producible, arrayable, integratable with on-chip electronics, structurally simple and extremely stable over time in the ordinary environment. The dynamic range is from less than 30 dB to above 110 dB SPL (re. 20 [...]) and the open-circuit sensitivities obtained range from 3.5 - 44 mV/Pa over the frequency range 100 Hz - 13 kHz. The total harmonic distortion of both devices is less than 2% at 110 dB SPL, 1 kHz.

Thesis Files