Micromachined Ultrasonic Transducer Arrays with Embedded MEMS T/R Switches

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 DESCRIPTION (provided by applicant): Ultrasound imaging systems require high-voltage (> 100 Vpp) excitation of transducer arrays in the trans- mission mode and low-noise amplification of echo signals in the reception mode. The system is switched back and forth between the transmission and reception modes during imaging. The currently available T/R switch implementations based on electronic components such as diodes and field-effect transistors deviate from the ideal on/off switching behavior. The common implementations of the T/R switch in ultrasound frontends are based on using a diode bridge or cross-coupled diode pair, which are not ideal as they introduces noise and distortion in the receive path and fail to completely isolate the receiver input from the transmit pulse causing some extended dead zone in the near field. The use of a MEMS switch can significantly help as MEMS switches provide a high isolation in the open state and low insertion loss in the closed state. However, these MEMS switches are often designed for specific RF applications, in which fast switching speed is not a requirement. For ultrasound frontends switching times of a few hundreds of nanoseconds are desired. We are proposing to develop a fast-switching (<1 µs), low control voltage (<10 V), DC-contact mode micro-electro-mechanical switch integrated and co-fabricated with a 1D capacitive micromachined ultrasonic transducer (CMUT) array. Having the ability to integrate the micro-electro-mechanical switches on the same substrate with the transducer array will also enable implementation of the low-noise preamplifier circuitry in a standard low-voltage CMOS process. This frontend signal amplification is essential to preserve the bandwidth and signal integrity, especially when small transducer elements are loaded by a long cable. Our two specific aims in this study are: Specific Aim 1: Develop a detailed 3D finite element model of the proposed micro-electro-mechanical switch structure and based on this model finalize the dimensions of the switch structure to achieve a switching speed of < 1 µs and a control voltage of < 10 V. Specific Aim 2: Fabricate and test the design developed in Specific Aim 1 as a standalone testable unit as well as in combination with a 1D CMUT array using the wafer bonding based fabrication process we developed at the NCSU Nanofabrication Facility.