Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Electrical Engineering


Measurement ofNO2, CO, and H2O combustion gases in automotive exhaust would allow significant improvement of engine control system abilities. Existing sensors cannot handle high exhaust temperatures and are expensive. Therefore, there is a need for small, low cost sensors with low power consumption that can measure CO, NO2, and H2O. Microelectromechanical systems (MEMS) offer a small, low cost, and low power platform that, combined with appropriate sensing films, could yield a gas sensor array for improved automotive emissions sensing. The goal of this work was to study a micromachined sensor array for measuring CO, NO2, and H2O concentrations. Four test platforms were examined: I) a MEMS microhotplate gas sensor which was contract fabricated and etched; 2) a pseudo-microhotplate that used a resistive temperature detector (RTD) as a heater and electrodes with geometries similar to that of a MEMS microhotplate for preliminary film characterization; 3) a surface micromachined microhotplate that was designed but not developed further due to design limitations; and 4) a diaphragm based microhotplate array that was micromachined at SDSU. This platform was not available for sensor film studies due long fabrication lead time. The metal-oxide sensing films and operating temperatures were chosen using the criteria of maximum sensitivity and selectivity for the three gases of interest. WO3 and SnO2/Al were used to sense NO2, SnO2/Pt was used to sense CO, and AliO3 and SnO2 were used to sense H2O. The films were conditioned and a gas delivery system was used to expose the sensors to the gases of interest and three interferent gases (NH3, CI-Li, and H2S). The SnO2/ Al film was sensitive and selective to NO2 but poisoned by the interferent gases. The SnO2/Pt film was sensitive but not selective to CO and was also poisoned by the interferent gases. The undoped SnO2 film was sensitive but not selective to H2O. The sensor array responses showed that each gas had a unique array response indicating that pattern recognition may be used to improve selectivity. Microhotplate arrays offer a promising platform for the next generation of gas sensors because numerous sensing films can be combined on one low cost, small size, and low power consumption platform. However, several areas need to be addressed to make these sensors practical including a redesign of the micro hotplates to decrease sensing film resistance, film optimization to reduce poisoning by interferent gases, and pattern recognition to improve selectivity.

Library of Congress Subject Headings

Gas detectors
Automobiles -- Motors -- Exhaust gas
Microelectromechanical systems



Number of Pages



South Dakota State University