Micromachined Capacitor Microphones
The demand for inexpensive microphones, coupled with advancements in silicon micromachining technology, has led to the development of many miniature acoustic pressure sensors. The development of miniature microphones primarily has focused on hearing aid applications, and therefore concentrates on acoustic detection only in a limited audio range. However, some scientific, industrial, and military applications require acoustic data collection over a broader bandwidth for proper signal identification. Frequencies above 25 kHz contain useful information for condition monitoring applications, particularly for machining and welding processes. Acoustic sensing below the audio range is useful for target tracking and monitoring of heavy equipment and engines, as well as for studying infrasonic geophysical phenomena. In addition, unattended operation in harsh outdoor environments requires sensors that are impervious to dust and moisture. While certain measurement microphones have the necessary bandwidth and durability for such applications, the high cost of precision instruments can be a barrier to their widespread use. A micromachined silicon sensor is therefore desirable, as micromachining and lithography techniques offer the prospect of inexpensive, mass produced acoustic sensors and sensor arrays. A durable, wideband micromachined microphone is possible using ultrasonic membranes that are vacuum-sealed. To maintain sensitivity, a radio frequency (RF) detection technique senses the changes in membrane capacitance in order to recover the acoustic signal. RF techniques can be applied to measure changes in microphone capacitance (Fig. 1) [1].
Silicon microphones based on capacitive micromachined ultrasonic transducer membranes and radio frequency detection overcome many of the limitations in bandwidth, uniformity of response, and durability associated with micromachined condenser microphones. These membranes are vacuum-sealed to withstand submersion in water and have a flat mechanical response from dc up to ultrasonic frequencies. However, a sensitive radio frequency detection scheme is necessary to detect the small changes in membrane displacement that result from utilizing small membranes.
We have developed a mathematical model for calculating the expected output signal and noise level and verified the model with measurements on a fabricated microphone. Measurements on a sensor with 1.3 mm2 area demonstrated less than 0.5 dB variation in the output response between 0.1 Hz to 100 kHz under electrostatic actuation and an A-weighted equivalent noise level of 63.6 dB(A) SPL in the audio band. Because the vacuum-sealed membrane structure has a low mechanical noise floor, there is the potential for improved sensitivity using higher carrier frequencies and more sophisticated detection circuitry. More details on this project can be found in [2].
FIGURE 1. (a) As membrane capacitance varies with the acoustic signal, the propagation constant of the line also varies. This phase-modulates the RF carrier. (b) A phase-detection circuit demodulates the RF carrier and recovers the acoustic signal.
FIGURE 2. (a) (a) Both microstrip and coplanar waveguide (CPW) transmission line microphones have been fabricated. CPW is preferable since line impedance is determined independently of capacitor gap. (b) Signal metal and doped polysilicon ground bridges form capacitor electrodes. Metal cannot be used for ground bridges due to high processing temperatures for thin films.
FIGURE 3. Mask layout of the same 1-mm2 device (left). Microscope photograph of a finished 1-mm2 device (right).
FIGURE 4. Packaged prototype microphone.
References
[1] A. S. Ergun, B. Temelkuran, E. Ozbay, and A. Atalar, ‘‘A new detection method for capacitive micromachined ultrasonic transducers,’’ IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 48, 932–942, 2001.
[2] S. Hansen, A. S. Ergun, W. Liou, B. A. Auld, B. T. Khuri-Yakub, “Wideband Micromachined Capacitive Microphones with Radio Frequency Detection,” J. Acst. Soc. Am., vol. 116 (2), Aug. 2004.
Acknowledgements
This work is supported by the Defense Advanced Research Projects Agency’s Microsystems Technology Office. We also wish to thank Nino Srour, Michael Scanlon, and their colleagues at the Army Research Laboratory for their assistance in testing the microphone. The microphone was fabricated at the Stanford Nanofabrication Facility, which is supported in part by the National Science Foundation.
