Fabrication and testing of polymer-based capacitive micromachined ultrasound transducers for medical imaging

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.Ultrasound imaging: plastic transducersA procedure to fabricate capacitive micromachined ultrasound transducers (CMUTs) from plastic offers low cost and high performance. Ultrasound imaging typically relies on piezoelectric materials for transducers. However, their performance is inhibited by limited bandwidth, and they are challenging to implement in large arrays. Now, a team from University of British Columbia, Canada, led by Robert Rohling and Edmond Cretu reports a simplified and low-cost process for fabricating polymer-based CMUTs (polyCMUTs), which are attractive alternatives to their piezoelectric-based counterparts. Key to their process is encapsulating the electrode inside a membrane. The device works in a liquid medium, at low operating voltages, and at frequencies comparable to CMUTs fabricated from silicon nitride. PolyCMUTs could help to expand the use of ultrasound with flexible electronics.