Micromachined capacitive transducers with improved optical detection for ultrasound applications in air

Diffraction based integrated optical detection method for capacitive micromachined ultrasonic transducers (cMUTs) can provide better receiver sensitivity without compromising transmit power as compared to conventional capacitance detection schemes especially for transducers operating below 1 MHz. This makes the method suitable for ultrasound applications and implementation of transducer arrays operating in air. The detection sensitivity of the method is improved by 11 dB forming a Fabry-Perot cavity structure in the transducer membrane-substrate gap, as predicted by calculations. The cavity is fabricated by depositing approximately 20 nm thick semi transparent silver layer under the grated bottom electrode. The optical detection scheme also allows one to obtain signals from individual elements of 1-D and 2-D cMUT membrane arrays for ultrasonic array imaging. This capability is demonstrated by imaging wire targets at 750 kHz using receive only synthetic aperture reconstruction.