Performance limits of micromachined piezoelectric microphones are examined theoretically and experimentally. The various sources of microphone noise are combined to form a model for the signal-to-noise ratio, and this model is used to generate design rules for piezoelectric microphones. The piezoelectric constitutive equations, classical plate equations, and a model of residual-stress effects are combined to describe microphone sensitivity. An acoustic model for the packaged piezoelectric microphones is also presented.
The control of residual stress in these clamped-clamped, laminated micromachined microphones by using compensating layers of compressive and tensile residual stress is investigated. In order to maximize microphone sensitivity, highly compressively stressed (~1 GPa) piezoelectric zinc oxide is compensated by a supporting layer of tensile silicon-rich silicon nitride. The vibrational mode shape and resonant frequency of the stress-compensated plates are measured using an array of piezoelectric electrodes. A model for the sensitivity dependence of device operation to residual stress is confirmed by applying external strain. For the microphone structure considered, the best performance is obtained with a slightly buckled plate. Finite-element simulations corroborate experimental results.
Using residual-stress compensation, an IC-processed piezoelectric microphone with on-chip, large-scale-integrated (LSI) CMOS circuits has been designed, fabricated, and tested in a joint, interactive process between a commercial CMOS foundry and a university micromachining facility. The 2500 x 2500 x 3.5 um^3 microphone has a piezoelectric ZnO layer on a sup- porting low-pressure chemical-vapor-deposited (LPCVD), silicon-rich, silicon nitride layer. The packaged microphone has a resonant frequency of 18 kHz, a quality-factor Q = 40, and an unamplified sensitivity of 0.92 mV/Pa, which agrees well with the calculated sensitivity. Differential amplifiers provide 49 dB gain with 13 uV A-weighted noise at the input.
A micromachined piezoelectric microphone with an electrothermally tunable resonant frequency fm has been built and tested. Resonant-frequency modulation is achieved via the thermal expansion induced by on-diaphragm, polysilicon heaters. A near linear decrease in fm with increasing resistor power at the rate of -127 Hz/mW is measured between 20.8 and 15.1 kHz. The mechanical quality-factors Qm are approximately 100. The temperature increases linearly by 0.3degC/mW at the hottest sense element on the diaphragm. The variable resonant frequency and high quality-factor provide an acoustic filtering capability which may have applications to ultrasonic range finders, velocity sensors, and signaling elements. These piezoelectric microphone designs are compared and contrasted with concurrent micromachined-microphone research around the world in order to suggest directions for future research.
May 31, 1994
Ried, R. P. (1994). Micromachined Piezoelectric Microphones. United States: University of California, Berkeley.