Theory and Application of Zero Bias CMOS MEMS Capacitive Micromachined Ultrasound Transducer

According to the fundamental principle of CMUT (Capacitive Micromachined Ultrasound), DC bias must be applied on CMUT for better energy transfer efficiency. In 2013, our team fabricated low collapse voltage CMUTs device and found out its sensitivity was larger than expected. Even without applied DC bias, the CMUTs could receive pulse-echo signal with acceptable sensitivity. Our team deduced there was charges accumulated in CMUTs device. Thus, the build-in electric field enhanced the performance of the CMUTs device. There are also empirical model of CMUT charge injection in other teams’ research. However, all those researches lacks the explanation of the charging mechanism, or they only discuss charge injection via Fowler-Nordheim tunneling in high voltage region. Most charging models cannot explain the charging effect in our CMUTs which were operated under relatively low gate voltage. Based on the theory of MIM (Metal-Insulator-Metal), we strive to build a more all-around CMUTs charging model under various bias. And we analyze the I-V characteristic change of the CMUTs device to verify our charging model. Our research utilizes low collapse voltage CMUTs fabricated by TSMC CMOS 0.35 process and etching post-process. The former part focused on the fabrication of CMUTs and the yield rate improvements. The latter part focused on the modeling of CMUT charging effect, the analysis of I-V characteristic and the verification of pulse-echo operation of the pre-charged CMUTs.