Multiple Moving Membrane Capacitive Micromachined Ultrasonic Transducer With Dynamic Control Provision of Effective Cavity Height

A novel biasing scheme having the potential to dynamically control the effective cavity heights of two fabricated, multiple moving membrane capacitive micromachined ultrasonic transducers ( \text{M}^{{3}} -CMUTs) has been developed. With this unique approach, it may be possible to avoid design tradeoff requirements of the ultrasonic transducers that affect both receiving and transmitting operating modes. Each of these \text{M}^{{3}} -CMUTs has two vibrating membranes suspended over a fixed bottom electrode. The two air-coupled \text{M}^{{3}} -CMUT devices with the resonant frequencies of 1 MHz single cell and 1.5 MHz array device were fabricated. A finite element analysis (FEA) was carried out to investigate the effect of an additional middle membrane on the effective cavity height and device performance. Experimental validation was then completed with the transducers operating under both positive and negative biasing conditions of middle membrane while keeping the top membrane at selected positive biasing voltages to realize a dynamically controlled range of effective cavity heights. The new biasing scheme may facilitate dynamic tunning of resonant frequencies of transmitter and receiver transducers through the variation of effective cavity height for improved performance.