Development of MEMS MOS gas sensors with CMOS compatible PECVD inter-metal passivation

Metal oxide semiconductor (MOS) based micro-electromechanical sensors (MEMS) for gas measurement were developed using back-end-of-line (BEOL) compatible inter-metal dielectric films deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD). As MOS MEMS require sintering above 600°C, a set of PECVD silicon oxide and nitride films was characterized before and after annealing up to 700°C, in order to identify the best candidate inter-metal dielectric materials. The effect of thermal load on the films was determined by measuring stress, thickness and refractive index after annealing both in dry air and nitrogen atmosphere. Nano-indentation was used to estimate the elastic modulus after annealing. The best performing films were employed as inter-metal dielectric in the full fabrication of microheater MEMS gas sensors. Among the selected films, only silicon oxide deposited with high frequency plasma generator allowed for the complete gas sensor fabrication with no damage. SnO2 paste was successfully screen printed on the fabricated membranes and sintered to enable gas sensing. Electrical and gas sensing tests were performed with the fabricated microheaters, demonstrating full functionality. Thermal stress endurance tests were also conducted, with no membrane deformation after more than 500,000 duty cycles. Results show that the proposed PECVD passivation is compliant up to 650°C firing temperature and 450°C operating temperature, and demonstrate the use of CMOS-BEOL PECVD inter-metal passivation for MOS MEMS gas sensor fabrication.