Zero-bias resonant sensor with an oxide-nitride layer as charge trap

We report on a capacitive resonant sensor with an oxide-nitride (ON) layer used as charge trap. The main idea is that we intentionally inject charges into the ON layer by biasing the device for 30 s with 160% of the pull-in voltage. We use a capacitive micromachined ultrasonic transducer (CMUT) to demonstrate this idea. The CMUT is fabricated via high temperature assisted direct wafer bonding after a local oxidation of silicon (LOCOS) to form evacuated cavities (vacuum gaps), and, thus, the device inherently has the ON layer beneath single crystal silicon plates and vacuum gaps. Therefore, this device is ideal for this work. It allows us to test an elegant charge injection mechanism. By simply pulling in the plate a high electric field strength (~ 8.9 MV/cm) is created in the ON layer for a designated time, which results in charge injection. These charges stay trapped in the ON layer and create an intrinsic electric field in the vacuum gaps, which would otherwise require an external dc bias voltage of 44% of the pull-in voltage. We successfully implemented a 5.3-MHz oscillator with this zero bias resonator and achieved excellent noise performance of 0.06 Hz of Allan deviation.