MEMS Infrared Emitter and Detector for Capnography Applications

Non-dispersive Infrared (NDIR) is one of the most reliable methods of measuring carbon dioxide (CO2) level in exhaled human breath. In this paper, we describe the application of infrared (IR) emitters and detectors manufactured on standard low cost CMOS silicon substrate with Deep Reactive Ion Etching (DRIE) process. Using these devices we have shown that due to fast thermal response we are able to optically modulate the IR signal without the need for a mechanical chopper to minimise the 1/f noise. Our measurements show that with emitter electronically chopped at 8 Hz, we are able to measure real-time breath with signal-to-noise ratio between 40-60. Furthermore, by using the SMD packaged IR devices we have also managed to shrink the size of the CO2 sensing module to 10 mm x 10 mm x 16 mm. Using our experimental system we have successfully demonstrated the suitability of MEMS IR devices for capnography applications.