Non-contact thermoacoustic detection of embedded targets using airborne-capacitive micromachined ultrasonic transducers

Abstract

A radio frequency (RF)/ultrasound hybrid imaging system using airborne capacitive micromachined ultrasonic transducers (CMUTs) is proposed for the remote detection of embedded objects in highly dispersive media (e.g., water, soil, and tissue). RF excitation provides permittivity contrast, and ultrasensitive airborne-ultrasound detection measures thermoacoustic-generated acoustic waves that initiate at the boundaries of the embedded target, go through the medium-air interface, and finally reach the transducer. Vented wideband CMUTs interface to 0.18 lm CMOS low-noise amplifiers to provide displacement detection sensitivity of 1.3 pm at the transducer surface. The carefully designed vented CMUT structure provides a fractional bandwidth of 3.5% utilizing the squeeze-film damping of the air in the cavity. V

The Dynamic Performance of Flexural Ultrasonic Transducers

“…Due to the significant impedance mismatch between many piezoelectric ceramics (with acoustic impedances in the region of approximately 35 MRayl) and air (the acoustic impedance of which is around 400 Rayl), impedance matching layers are generally required, which can decrease the bandwidth of the transducer [ 6 ]. Micro-machined transducers possess improved bandwidth and coupling with air [ 6 ], and two configurations which have been successfully applied for air-coupled ultrasound include the piezoelectric micro-machined ultrasound transducer (PMUT) [ 7 , 8 ], and the capacitive micro-machined ultrasound transducer (CMUT), which is characterised by a wide bandwidth and excellent air-coupled performance [ 9 , 10 , 11 ]. The PMUT and CMUT contain very small and thin membranes, and are effectively flexural transducers that are driven off membrane resonance.…”

Squeeze film damping and stiffening in circular CMUT with air-filled cavity: Influence of the lateral venting boundary conditions and the bias voltage

“…1). When the top of the membrane is loaded by a fluid or a gas, the membrane vibration can be used for ultrasound waves transmitting or receiving in immersion [1][2][3] and air-coupled [4][5][6][7][8][9] applications.…”

Air-coupled ultrasound detection using capillary-based optical ring resonators

“…Since the acoustic transmission coefficient of water/air interface is −65.4 dB 13 , the photoaoustic pressure coupled into air is estimated to be ~520 Pa. Assuming that the laser spot size on the sample is 10 µm, and the ring resonator is 5 mm away from the photoacoustic source, the final acoustic pressure reaching the sensor is expected to be ~520 mPa.…”