Abstract
The ultrasonic transducer industry is dominated by piezoelectric materials. As an emerging alternative, capacitive micromachined ultrasound transducers (CMUTs) offer wider bandwidth, better integration with electronics, and ease of fabricating large arrays. CMUTs have a sealed cavity between a fixed electrode and a suspended metalized membrane. Manufacturing cost and sensitivity are limiting factors in current CMUTs that depend on the fabrication equipment and, especially, on the materials used. For widespread use of CMUTs, a much lower fabrication cost that uses inexpensive materials, which maintain or improve upon existing sensitivity, is needed. Herein, a new fabrication process is described for polymer-based CMUTs (polyCMUTs) using the photopolymer SU-8 and Omnicoat. The first ultrasound B-mode image of a wire phantom created with a 64-element linear array using synthetic aperture beamforming techniques is presented. A 12 VAC signal superimposed on a 10 VDC signal was used on the transmission side, and only a bias-tee, with no amplifiers, was used on the receiving side. The low operational voltage and high sensitivity of this device can be partially attributed to a pre-biasing condition on the membrane. By using a novel sacrificial layer combined with a top electrode embedded inside the membrane, we demonstrated that SU-8 can be used to manufacture CMUTs inexpensively. Moreover, the fabrication used relatively simple equipment, and the number of fabrication steps was reduced compared to traditional CMUT fabrication. This new fabrication process has the potential to increase the use of CMUTs in the ultrasound market, including the market for wearable transducers.
Highlights
Ultrasound imaging is the most widely used medical imaging modality in the world when considering the number of images created annually
The topography of the final device was characterized using a white light interferometer (Polytec, Irvine, CA, USA) and is shown in Fig. 3b with the thick SU-8 membranes of the Capacitive micromachined ultrasound transducers (CMUTs) cells displayed in blue
This work provides a proof of concept that polymeric materials can be used to fabricate CMUTs for biomedical imaging
Summary
Ultrasound imaging is the most widely used medical imaging modality in the world when considering the number of images created annually. 1) has boosted research in medical ultrasound fields, especially with regards to transducer design. The ultrasonic transducer industry is dominated by piezoelectric materials, with incremental development of the same basic transduction mechanism detailed almost a Capacitive micromachined ultrasound transducers (CMUTs) have emerged as alternatives to piezoelectric. Gerardo et al Microsystems & Nanoengineering (2018)4:19 imaging transducers[3]. A CMUT is essentially a parallelplate capacitor with a fixed electrode at the bottom and a clamped metalized membrane suspended above a cavity. Ultrasound waves are generated when an AC signal, usually superimposed on a DC voltage, is applied between both electrodes; ultrasound waves can be detected by measuring the variation in capacitance of the device while a DC voltage is applied in the presence of incoming ultrasound waves.
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