Miniaturized capacitive micromachined ultrasonic transducer (CMUT) chemical sensor with low operating voltage

Abstract

This thesis reports on a miniaturized resonant chemical sensor based on Capacitive Micromachined Ultrasonic Transducer (CMUT) developed for portable gas sensor applications. Previously reported CMUT chemical sensors require a DC operating voltage (~ 46 V) higher than the supply voltages of common circuits (e.g. 1.8 ~ 5 V). Thus, additional circuitry such as a charge pump circuit is often required to supply the DC voltage to CMUT when implementing sensor interface circuits, which results in additional power consumption and larger footprint. In this work, the vacuum gap which determines the operating voltage was reduced to 50 nm through double oxidation. By utilizing thermal oxidation to control the gap height, the uniformity of the vacuum gap across the wafer was tightly controlled. In addition, by defining the vacuum cavity through isotropic wet etching, the protrusion during the second thermal oxidation was minimized to ensure successful direct wafer bonding. Through this double oxidation process, not only we achieved a high yield across the 4-inch wafer, but also we achieved a much smaller pull-in voltage (~ 7 V) for a 500-nm-thick CMUT resonant sensor. The CMUT is operated at approximately 80% of the pull-in voltage and thus the same power supply as the ones for discrete opamp circuits can be used to operate the CMUT sensor. Based on the resonant frequency of 7 MHz, the theoretical sensitivity of the fabricated CMUT resonant sensor is 1.74 fg/Hz. In addition, an oscillator circuit was designed and implemented to track the resonant frequency shifts of the CMUT sensor in real time. Lastly, by adding a functionalization layer on the resonant sensor, frequency shifts due to the chemical reaction of volatile organic compounds, such as Toluene, with the functionalization layer were observed. Thus, this work demonstrates the potential of the developed CMUT with low bias voltage as key components for a portable chemical sensor system.