A micromachined piezoelectric flexural mode hydrophone for audible frequency application. Free

A micromachined hydrophone is designed for audible frequency application (20 Hz–20 kHz). The basic structure is a flexural unimorph consisting of a clamped silicon diaphragm and a piezoelectric film. Two design approaches can enhance its sensitivity. One is a unimorph with unequal radii between the piezoelectric film and the diaphragm, which increases electromechanical coupling. The other is an air‐backed chamber, which reduces mechanical impedance in the flexural vibration and, consequently, enhances sensitivity. To predict the effects of these approaches on sensitivity, an equivalent transduction model is composed and used for calculations. In the case of oil backing, compared to air backing, a reduction in sensitivity of about 40 dB is expected. The dimensions of the micromachined diaphragm (piezoelectric film) are 630 μm (400 μm) radius and 15 μm (3 μm) thick. A fabricated microsensor is constructed as a hydrophone by using a rubber housing filled with castor oil. The measured sensitivity is −228.3 dB at 1V/μPa without a preamplifier. A flat frequency response within ±1 dB deviations were measured in the 100 Hz–5 kHz band. Based on the simulation, it can resist the hydrostatic pressure of 100 m depth, without pressure balancing, while decreasing in sensitivity. The results of a larger model having a higher sensitivity of −207.6 dB will also be presented. [Research supported by MRCND and R0A‐2007‐000‐20042‐0.]