Highly sensitive CMUT-based humidity sensors built with nitride-to-oxide wafer bonding technology

摘要

In this paper, we present highly sensitive capacitive micromachined ultrasonic transducer (CMUT) based gravimetric humidity sensors that are fabricated with nitride-to-oxide wafer bonding technology. In contrast to conventional wafer bonding CMUT processes that use expensive silicon-on-insulator (SOI) wafers to produce resonating membranes, the proposed process employs low-pressure chemical vapor deposition (LPCVD) silicon nitride as the membrane material. The proposed process provides thinner and lighter membranes, and thus more sensitive CMUT resonators. Furthermore, additional benefits such as reduced fabrication complexity and more controllable membrane thickness are possible. Ten MHz CMUTs were designed, fabricated and functionalized by graphene oxide (GO) with three different concentrations. The device uniformities and resonance frequencies were characterized by a laser vibrometer and a vector network analyzer. Humidity sensing performance of the sensors was analyzed through the resonance shifts in a relative humidity (RH) range of 11.3%-93.6%. The CMUT coated with the highest concentration of GO (0.5 mg/ml) exhibited the highest sensitivity of up to 15.3 kHz/% RH. Decent repeatability and a short response/recovery time were achieved by all the CMUT sensors. This study demonstrates that the CMUT humidity sensors that are realized with nitride-to-oxide bonding technology can be an excellent candidate for humidity sensing due to their high sensitivity and CMOS-compatible fabrication process.