An Ultra-Low Power Microbridge Gas Sensor

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 bridge with critical dimension of 1 m. The sensor has been tested with nitrogen, carbon dioxide, and helium. Heat loss from constant voltage application was observed to be a function of the thermal conductivity of the gas ambient, resulting in different magnitude of resistance change. The response time to a voltage pulse was found to be less than 0.5 mu s and stability of the sensor excellent. We demonstrated that the sensitivity for helium in nitrogen was 2.05m Omega/ppm when operated at 3.6V do supplies (a power level in the order of 4 milliwatts). With a Wheatstone bridge with ac excitation and a lock-in amplifier the sensor limit of detection was similar to 100ppm helium in nitrogen.