Fabrication of an 8-Ring Annular Array from 2D Capacitive Micromachined Ultrasonic Transducer (CMUT) Array Elements Using a Metal Redistribution Layer.

The emergence of capacitive micromachined ultrasonic transducers (CMUTs) has enabled the possibility of different applications because of the inherent advantages associated with this technology. One of the notable advantages of this technology is the flexibility to implement complex transducer geometries with the established silicon microfabrication techniques. With this capability, application-specific CMUTs can be implemented, and with the possibility of CMUT integration with supporting electronics, compact form-factor ultrasound systems can be developed. One approach to develop different CMUT array geometries is by fixed lithographic definition. This study presents another approach for implementing different CMUT geometries on a 2D CMUT array canvas with the use of an additional metal layer as needed. This approach further simplifies the implementation of different complex CMUT geometries with a short turnaround time. To demonstrate this approach using a sacrificial release-based fabrication approach, an 8-ring concentric annular CMUT array was fabricated on a borosilicate glass substrate by connecting a group of elements on a 2D CMUT array canvas. The fabricated rings show an average resonant frequency of 4.8 MHz in air. In immersion, the 8-ring annular array, 7.98 mm in diameter, shows a focused pressure of 2.78 MPa peak-to-peak in oil at an optimum frequency of 3.46 MHz, and this pressure was measured at 8 mm from the array surface (F#1).