(Dr. Levent Degertekin, advisor)
"Micromachined Broadband Acoustic Transducers with Integrated Optical Displacement Detection"
Micromachined broadband acoustic transducers with diffraction-based optical displacement displacement detection are introduced. A compliant membrane is made part of a phase sensitive diffraction grating, and the deflection resulting from external acoustic pressure alters the intensities of the diffracted orders which are monitored with integrated photodiodes. The scheme provides the displacement sensitivity of a Michelson interferometer, and is easily integrated into a robust, microscale package. Due to the enhanced displacement detection capabilities, mechanical sensitivity requirements for micromachined microphones can be relaxed as much smaller, stiffer membranes with diameters on the order of 200µm can be used for enhanced bandwidth, robustness, and fabrication yield. For capacitive micromachined ultrasonic transducers (cMUTs), optical detection offers a receive sensitivity independent of frequency, gap height, and capacitance, making the method ideal for ultrasound applications in air. Capacitive actuation of the membrane is used to provide a self-calibration feature and, for ultrasonic transducers, sound generation capabilities. In this proposal, the method is analyzed and demonstrated with integrated cMUTs fabricated on quartz substrates operating in air. Future work includes the integration and characterization of broadband microphones designed to resonate near 100 kHz. These devices should have detection performance similar to high quality measurement microphones with much smaller dimensions.