Fabrication and model validation for CMUTs operated in permanent contact mode

We present the successful fabrication and finite element analysis (FEA) validation of capacitive micromachined ultrasonic transducers (CMUTs) targeting applications under a wide and varying pressure range (~ 1 - 20 atm), such as ultrasonic flow metering (UFM). In our devices each plate is in permanent contact with the bottom of the cavity, even at zero d.c. bias voltage condition. The fabrication is based on a direct wafer bonding process (thick buried oxide layer process), which allows the realization of only partially connected bottom electrodes. The fabricated devices show measured performance in good agreement with FEA. The results for the measured plate profiles confirm the permanent contact and are within 3.5% of the static FEA results. Further, in comparison to laser vibrometer measurements, the FEA modal analysis predicts the 1^st and 2^nd mode frequencies for permanent contact devices within 3% and 1.5%, respectively. In addition, the overall trends of resonant frequency and contact radii as a function of d.c. bias voltage are both consistent with FEA. Our results show that the fabrication of CMUTs in permanent contact mode is feasible, and that FEA serves as an excellent tool for predicting and designing both the static and dynamic behavior of CMUTs operated in the permanent contact mode.