Design and Implementation of CMOS-MEMS Schottky Gas Sensor With Asymmetric Metal Contacts for Performance Enhancement at Room Temperature

The CMOS-MEMS Schottky gas sensor is designed and implemented using a standard commercially available CMOS process, along with post-CMOS processes. The proposed gas sensor consists of open interdigitated electrodes (open IDEs) for signal acquisition and a microcavity for the sensing area. The metal and dielectric layers from the standard CMOS process are exploited to realize the structures. During the post-CMOS processes, two additional materials (noble metal and sensing material) are deposited onto the electrodes to improve sensing performance. One side of the electrodes is formed by the inherent metal layer from the CMOS process (AlCu), while the opposite side is deposited with the Au metal layer to create asymmetric metal contacts for the sensor. The related Schottky effects are achieved after dispensing the compound of titanate nanotubes Na0.23TiO2/TiO2 (NTT) onto the sensing area. Results reveal that the proposed design with asymmetric metal contacts demonstrates a sensitivity of 0.22 to NO2 gas in the range of 0–10 ppm at room temperature, with response and recovery times of 17 and 40 s, respectively. Compared to the reference design with symmetric metal contacts, the sensor with asymmetric metal contacts exhibits better linearity and a twofold enhancement in sensitivity to NO2 gas within the same footprint. The sensitivity improvement can be ascribed to the electrodes having a higher Schottky barrier at the junction (Au/NTT). Furthermore, the sensor demonstrates selectivity among NO2, NO, HCHO, ethanol, and NH3 gases at room temperature. In summary, the CMOS-MEMS Schottky gas sensor with asymmetric metal contacts shows good sensitivity, fast response time, and high selectivity to NO2 gas at room temperature.