Fast Sensitive Ultra-Low Power Micro Thermal Conductivity Gas Sensor

Abstract

A miniature, ultra-low power, sensitive, microbridge-based thermal conductivity gas sensor has been developed. The batch fabrication of the sensors was realized by CMOS compatible processes and surface micromachining techniques. Doped polysilicon was used as the structural material of the suspended sensing element with critical dimension of 500 nm. To understand the design of the sensors, a model of the suspended element was simulated in COMSOL that couples electrical and thermal physics together and includes minimal simplifications. Modeling results coincide with experimental data in predicting resistance changes. The sensor was tested with nitrogen, carbon dioxide, helium and methane.An electronic circuit was developed to measure very fine changes in the signal output and to keep the signal to noise ratio such that ppm detection is possible, while exciting the sensor at minimal power levels. The sensor was able to detect ppm levels of helium, methane, or carbon dioxide in nitrogen at room temperature. The sensor can make a measurement in less than 2 ms consuming only 5 nJ or power per reading.The transient as well as steady state response was investigated as potential indicator of gas thermal capacity; as thermal conductivity and thermal capacity are independent properties of a gas, this new approach may enable MicroTCD sensors to make detections in gas mixtures by thermal analysis [1].[1]U.S. Patent 8,426,932 B2, April 23, 2013, Apparatus and Method for Microfabricated Multidimensional Sensors and Sensing Systems, Joseph R. Stetter.