Integration of Gas Sensors with CMOS Technology

For the last several decades, the primary concern of the semiconductor industry was transistor scaling along Moores Law. However, as scaling on silicon approaches its physical limits, several research avenues have been directed towards functional integration of different devices using the More-than-Moore approach. The main goal of this approach is the integration of sensors, RF circuits, and other functionalities in CMOS technology in order to enable non-digital system-in-package and digital system-on-chip integration for a higher value system. This can be achieved by innovation in packaging, three-dimensional integration, and fabrication using mature CMOS technology on silicon. When it comes to gas sensors, semiconductor metal oxide (SMO) thin film based sensors have recently shown their potential for integration with digital electronics, which is an important step to their integration in wearable and portable electronics. The recent achievements in the integration of SMO sensors with CMOS fabrication are very promising. The complexity therein lies in the need for an integrated microheater, since SMO films operate as sensors only at quite high temperature. The microheater, therefore, must be isolated from other electronics, which is achieved by using a suspended membrane with an air cavity underneath, a feature common in microelectromechanical systems (MEMS). The power- and performance-optimized integration of MEMS and CMOS requires many design tradeoffs. Among recent achievements is the fabrication of perforated membranes, which combine the benefits of their suspended and closed alternatives. Finally, several outstanding concerns which are still hindering a full integration of SMO sensors in wearable and hand-held electronics are described and current approaches to engineer solutions are summarized.