Transparent capacitive micromachined ultrasonic transducer (CMUT) arrays for real-time photoacoustic applications

Abstract

Photoacoustic imaging has shown great potential for non-invasive high-resolution deep-tissue imaging. Minimizing the optical and acoustic paths for excitation and detection could significantly increase the signal-to-noise ratio. This could be accomplished by transparent transducers permitting through-transducer illumination. However, most ultrasound transducers are not optically transparent. Capacitive micromachined ultrasound transducer (CMUT) technology has compelling properties compared to piezoelectric transducers such as wide bandwidth and high receive sensitivity. Here, we introduce transparent CMUT linear arrays with high transparency in the visible and near-infrared range. To fabricate the devices, we used an adhesive wafer bonding technique using photosensitive benzocyclobutene (BCB) as both a structural and adhesive layer with a glass-indium-tin-oxide (ITO) substrate. Silicon nitride is used as the membrane material ensuring hermiticity and optical transparency. Our fabricated transducer arrays consist of 64 and 128 elements with immersion operation frequency of 8 MHz, enabling high-resolution imaging. ITO, along with thin metal strips, are used as a conductive layer for the top electrodes with minimal impact on device transparency. Fabricated devices have shown average transparency of 70% in the visible wavelength range that goes up to 90% in the near-infrared range. Arrays are wire-bonded to interfacing electronics and connected to a research ultrasound platform for phantom imaging. Arrays exhibited signal-to-noise (SNR) of 40 dB with 30V bias voltage and laser fluence of 13.5 mJ/cm2. Arrays with 128 channels provided lateral and axial resolutions of 234 µm and 220 µm, respectively.

MEMS Ultrasound Transducers for Endoscopic Photoacoustic Imaging Applications

“…Visible-light-transparent cMUTs were also developed [ 170 , 174 ]. By using a glass wafer as the substrate, a photo BCB polymer as the bonding agent, silicon nitride as the membrane material, and indium-tin-oxide (ITO) as the transparent electrodes, in 2019, Li et al presented cMUTs with a transparency of up to 82% in the visible range for PAI applications [ 174 ].…”

“…By using a glass wafer as the substrate, a photo BCB polymer as the bonding agent, silicon nitride as the membrane material, and indium-tin-oxide (ITO) as the transparent electrodes, in 2019, Li et al presented cMUTs with a transparency of up to 82% in the visible range for PAI applications [ 174 ]. With this method, in 2020, Ilkhechi et al also developed transparent cMUT arrays for real-time PAI applications, showing an average transparency of 70% in the visible wavelength and up to 90% in the NIR range [ 170 ]. The reported NEP are 67 and 10.4 with the bias voltages of 40 V and 250 V, respectively.…”

“…Unlike TEM pMUTs that are mostly single-element devices, FVM pMUT arrays and cMUT arrays have been developed with large numbers of pixels. Each pixel can also be composed of multiple pMUT or cMUT cells connected in parallel to increase the sensing area or bandwidth [ 140 , 141 , 164 , 170 ].…”

Wafer-Bonding Fabricated CMUT Device with Parylene Coating

“…In the past 3 decades, extensive research has been conducted on modeling [ 15 , 16 ], performance improvement [ 17 , 18 , 19 ], fabrication methods [ 9 , 20 , 21 , 22 , 23 , 24 ], front-end circuitry [ 4 , 8 , 10 , 25 , 26 , 27 , 28 ], and applications [ 4 , 5 , 11 , 12 , 29 , 30 , 31 , 32 ] of the CMUT technology. The pioneering research on the CMUT mostly focused on medical ultrasound imaging applications and less consideration was given to underwater ultrasound imaging applications.…”

Miniature all-optical flexible forward-viewing photoacoustic endoscopy probe for surgical guidance

“…Furthermore, as with piezoelectric detection technology, meeting the very broad bandwidth and fine spatial sampling requirements required for high resolution endoscopic PA imaging using CMUTs remains a challenge. Moreover, with a few exceptions [ 21 , 22 ], most piezoelectric and CMUT arrays are non-transparent. This makes it challenging to deliver a widefield excitation laser beam to the target tissue in a forward-viewing probe.…”