Fabrication and characterization of 1-dimensional and 2-dimensional Capacitive Micromachined Ultrasonic Transducer (CMUT) arrays for 2-dimensional and volumetric ultrasonic imaging

Capacitive Micromachined Ultrasonic Transducers (CMUTs) were introduced about a decade ago as an alternate method of generating and detecting ultrasound. Since their introduction, considerable research has been done to characterize CMUTs. They have been shown to have broad frequency bandwidth and very good sensitivity. Besides, CMUTs are built on silicon using standard surface micromachining techniques, and therefore have all the advantages of 1C processing, such as parallel production, batch fabrication and very high level of integration. All these qualities made CMUTs and CMUT arrays an alternative to their piezoelectric counterparts. In this paper, we focus on the CMUT fabrication process and present recent advances which made it possible to achieve very high process yields (practically 100%) leading to the fabrication of fully functional one-dimensional (ID) and two-dimensional (2D) CMUT arrays. Because of limitations on the element size, the fabrication of 2D CMUT arrays involves the use of electrical through-wafer interconnects (ETWI) which brings the electrical connection of each element from the transducer side to the backside of the wafer. In this paper, we also present ETWIs that have parasitic capacitance as low as 0.25 pF integrated with a 2D CMUT array of 128 by 128 elements. These arrays are characterized and tested in real imaging cases. The paper concludes with the presentation of the sample imaging results that demonstrate the viability of the CMUT process for array fabrication.