MEMS Micro-Sensor for Sensitive Low Power Methane Detection
Authors: Findlay Melvin W , Stetter Joseph R , Carter Michael T , Ploense Lloyd 
Published: February 2020

This work, funded in part by the US EPA and NASA, is focused on creating a novel, ultra-low power and low cost microfabricated methane sensor. The focus of this work will develop a tiny low cost and low power platform sensor technology for identification of fugitive methane emissions, and subsequently serve the expanding methane/natural gas leak and safety market due to increased exploration, production and use of natural gas. Current work is dedicated to design, fabrication and testing of the detector system consisting of a unique MEMS sensor, discreet electronic components and proprietary “smart” algorithm. This program is important to overall greenhouse gas monitoring and contributing to the effort around the world to curb man-made contribution to climate change. Further, the sensor itself is “green” because it uses far less materials, and provides substantial power savings because it uses 10-100 times less power than current lowest power CH4 sensors. Because of the small size of the sensing element, it has a very low thermal mass and therefore improved sensitivity and a very fast response time measured in microseconds. Additional applications for this platform will be possible and extend the applications beyond methane to other atmospheric and fuel gases and vapors
Micro-thermal conductivity detector (μTCD) gas sensors work by detecting changes in the thermal conductivity of the surrounding medium and are used as detectors in many applications such as gas chromatography systems, leak detectors, and pressure gauges. Conventional TCDs use steady-state resistance (i.e., temperature) measurements of a micro-heater. In this work, we are developing a new measurement method and hardware configuration based on the processing of the transient response of a low thermal mass TCD to an electric current step. The chip with three different design elements capable of 2 and 4 terminal measurements is illustrated at right. The method was implemented for a 100-μm-long and 1-μm-thick micro-fabricated bridge that consisted of doped polysilicon conductive film passivated with a 200-nm silicon nitride layer. Transient resistance variations of the μTCD in response to a square current pulse were studied in multiple mixtures of dilute gases in nitrogen. Simulations and experimental results are presented and compared for the time resolved and steady-state sensor response. Thermal analysis and simulation show that the sensor response is exponential in the transient state, that the time constant of this exponential variation was a linear function of the thermal conductivity of the gas ambient, and that the sensor was able to quantify the gas composition. The level of detection was estimated at 25 ppm for helium in N2 to 178 ppm carbon dioxide in N2. With this novel approach, the sensor requires approximately 3.6 nJ for a single measurement and needs only 300 μs of sampling time. This is significantly less than the energy and time required for steady-state DC measurements.

Reference: US Patent 8,884,382: Stetter et al. Microfabricated Multi-dimensional Sensors and Sensing Systems

Figure 1