Fabrication, characterization and application of a microelectromechanical system (MEMS) thermopile for non-dispersive infrared gas sensors

Abstract

We report on the design, fabrication, and characterization of a non-dispersive infrared (NDIR) gas sensor using an integrated thermopile on a micromachined silicon nitride membrane. The NDIR sensor consists of an optical cavity with new specular reflectors around the light bulb. The multi-layer absorber showed an absorptance of over 90% at 3.3-4.9 mu m. The thermopile with this absorber has an output voltage of 144.83 mV at a 5 mW incident power and a sensitivity of 30 V W-1. The sensitivity of the thermopile packaged with a Fresnel lens was 51 V W-1, approximately 1.7 times higher than that of a thermopile with only an absorber. This is due to the decrease in thermal mass and heat loss from a hot junction, and due to the increase in absorptance. Using this newly fabricated thermopile, we developed a small and sensitive NDIR gas detector module for accurate air quality monitoring systems for energy-saving buildings and automotive applications. Our novel sample cavity design is configured to uniformly emit collimated light into the entrance aperture of the cavity to enhance the sensitivity of the NDIR gas detector.