Fabrication of Capacitive Micromachined Ultrasonic Transducers (CMUTs) using wafer bonding technology for low frequency (10 kHz-150 kHz) sonar applications

This paper introduces a new method for fabricating Capacitive Micromachined Ultrasonic Transducers (CMUT) that uses a wafer-bonding technique. The transducer membrane and cavity are defined separately on a Silicon-On-Insulator (SOI) wafer and on a prime quality silicon wafer, respectively. Using silicon direct bonding in a vacuum environment, the two wafers are bonded forming the transducer. Among the many advantages this wafer-bonding technique, and probably the most important for low frequency transducer applications, is the ability to define relatively large membranes and large gaps easily. The particular device reported in this paper is designed to operate in the 10 kHz - 150 kHz range as a transmitter only for a sonar application. In this paper, we describe the new fabrication process to build CMUTs, and present the first experimental results obtained from this particular device that demonstrate wide-band operation in the above mentioned frequency range.