CMUT-based resonant gas sensor array for VOC detection with low operating voltage

Abstract

With the anticipation for a more connected world through the Internet of Things, there is still a strong demand for miniaturized chemical sensors. Here, we report on a miniaturized resonant chemical sensor based on a Capacitive Micromachined Ultrasonic Transducer (CMUT) with a low operating voltage suitable for portable gas sensor applications. Previously reported CMUT chemical sensors required a DC operating voltage (16∼50 V) higher than the supply voltages of common circuits (e.g., 1.8–5 V). Thus, additional circuitry such as a charge pump circuit often was required as a part of the sensor interface circuits to supply the DC voltage to CMUT. This resulted in additional power consumption and a larger footprint. In this work, the vacuum gap of the CMUT which determines the operating voltage was reduced to 50 nm through the development of a double oxidation process with a high wafer-level yield. We achieved a significantly smaller pull-in voltage (<10 V) for a 500-nm-thick CMUT resonant sensor. The CMUT was operated at approximately 80% of the pull-in voltage (∼8 V) which does not require additional voltage supply to that of the real-time read-out circuitry. Based on the resonant frequency of 6.7 MHz, the theoretical sensitivity of the fabricated CMUT resonant sensor was 0.8 Hz/fg. By coating four different chemically-sensitive polymer layers, we confirmed the operation of the CMUT with low operating voltage as a chemical sensor. Frequency shifts due to the chemical reaction of volatile organic compounds were observed and analyzed through principal component analysis. This work demonstrates the potential of the developed CMUT with low bias voltage as a key component of a portable chemical sensor system.