Airborne Capacitive Micromachined Ultrasonic Transducer Cell Design

date March 20, 2016
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Friday, March 25, 2016 4:00 p.m. in ETC 4.150

Dr. Asli Yilmaz
Electrical Engineering Department
Bilkent University
Ankara, Turkey

All transducers used in airborne ultrasonic applications, including capacitive micromachined ultrasonic transducers (CMUTs), incorporate loss mechanisms to increase the frequency bandwidth. However, CMUTs can yield high efficiency in airborne applications and unlike other technologies, they offer increased bandwidth due to their low characteristic impedance, even for efficient designs. Despite these advantages, achieving the full potential is challenging due to the lack of a systematic method for design. In this presentation, a method for airborne CMUT design will be explained. A lumped element circuit model and a harmonic balance (HB) approach to optimize CMUTs for maximum transmitted power is proposed. Airborne CMUTs have a narrowband characteristic behavior mechanically, due to low radiation impedance. In this presentation, the analysis is restricted to a single frequency and the transducer is driven by a sinusoidal voltage with half of the frequency of operation frequency, without any dc bias. A new mode of airborne operation for CMUTs is proposed, where the plate motion spans the entire gap. This maximum swing is achieved at a specific frequency applying the lowest drive voltage and this mode of operation is called Minimum Voltage Drive Mode (MVDM). An equivalent circuit-based design for airborne CMUT cells is presented and verified by the testing of fabricated CMUTs. The performance limits for silicon membranes for airborne applications are derived. Experimental results show that 78.9 dB re 20 μPa @ 1 m source level at 73.7 kHz is obtained, with a CMUT cell of radius 2.05 mm driven by 71 V sinusoidal drive voltage at half the frequency. The measured quality factor is 120.