Low power highly sensitive platform for gas sensing application

This paper reports a low power miniaturized MEMS based integrated gas sensor with 36.84 % sensitivity (Delta R/R-0) for as low as 4 ppm (NH3) gas concentration. Micro-heater based gas sensor device presented here consumes very low power (360 degrees C at 98 mW/mm(2)) with platinum (Pt) micro-heater. Low powered micro-heater is an essential component of the metal oxide based gas sensors which are portable and battery operated. These microheaters usually cover less than 5 % of the gas sensor chip area but they need to be thermally isolated from substrate, to reduce thermal losses. This paper elaborates on design aspects of micro fabricated low power gas sensor which includes 'membrane design' below the microheater; the 'cavity-to-active area ratio'; effect of silicon thickness below the silicon dioxide membrane; etc. using FEM simulations and experimentation. The key issues pertaining to process modules like fragile wafer handling after bulk micro-machining; lift-off of platinum and sensing films for the realization of heater, inter-digitated-electrodes (IDE) and sensing film are dealt with in detail. Low power platinum microheater achieving 700 degrees C at 267 mW/mm(2) are fabricated. Temperature calculations are based on experimentally calculated thermal coefficient of resistance (TCR) and IR imaging. Temperature uniformity and localized heating is verified with infrared imaging. Reliability tests of the heater device show their ruggedness and repeatability. Stable heater temperature with standard deviation (sigma) of 0.015 obtained during continuous powering for an hour. Cyclic ON-OFF test on the device indicate the ruggedness of the micro-heater. High sensitivity of the device for was observed for ammonia (NH3), resulting in 40 % response for similar to 4 ppm gas concentration at 230 degrees C operating temperature.