Ultrasound barrier microsystem for object detection based on micromachined transducer elements

This work reports on the fabrication, characterization, and encapsulation of micromachined membrane resonators for ultrasound generation and detection. Based on the encapsulated 1/spl times/1-mm membrane resonators, an ultrasound barrier microsystem for short-distance object detection including the necessary driving circuitry has been developed. Using the ultrasound principle, the barrier microsystem is able to detect, e.g., optically transparent objects. The membrane resonators are fabricated using an industrial silicon technology compatible with bipolar integrated circuit (IC) technology in combination with a postprocessing etching step. Because of the compatibility with IC fabrication technology, the membranes feature electrothermal excitation and piezoresistive detection of transverse vibrations. For the generation of ultrasound, membranes, which exhibit a slight initial buckling due to fabrication-induced compressive stresses, are best suited. At the fundamental resonance frequency f=87 kHz, maximum sound-pressure amplitudes up to 0.25 Pa can be achieved at 50-mm distance. With the piezoresistors arranged in a Wheatstone bridge, a sensitivity for the detection of sound at the resonance S/sub sound/=2 /spl mu/V/mPa (for an applied bias voltage of 5 V) is obtained. To protect the membranes from the environment, they are covered by a metal cap with a periodic grid of holes and a thin, porous membrane. With this encapsulation approach, the generated sound pressure in forward direction is only slightly lowered, while the total beam angle (/spl ap/80/spl deg/) of the generated sound field is distinctly reduced.