Document Type

Thesis - University Access Only

Award Date

2001

Degree Name

Master of Science (MS)

Department / School

Electrical Engineering

Abstract

Gas sensors have been used for many years to warn of workplace hazards and to monitor chemical processes in industry. More recently, air quality in agricultural settings has become a concern, particularly in animal confinement facilities, where hydrogen sulfide (H2S), ammonia (NH3), and methane (CH4) are present. Individual sensors for these gases exist, but they are relatively large and expensive, have high power consumption, and suffer from cross-sensitivity to other gases. There is therefore a strong need for a low-cost, portable sensor array that can selectively measure these gases. Microelectromechanical systems (MEMS) offer a potential solution for low power, low cost sensor arrays since they are made by micromachining silicon wafers using standard microelectronic fabrication techniques. The objective of this work was to develop a hazardous gas sensor array using thin metal oxide films selected from the literature that could be sputtered on a MEMS platform for sensing NH3, CH4, and H2S. MEMS platforms that can be both fabricated in-house as well as out-sourced to a foundry were examined. Microhotplate arrays were successfully manufactured via outside sources using low cost CMOS fabrication. The custom platform designed to be fabricated at SDSU was made with LPCVD SiJN4 that was deposited and patterned via an outside source. No further work was done on this platform since the CMOS platforms were available. Studies of metal oxide films indicated that deposition in an Ar/02 plasma and heating the sensing platform during deposition provided some control of the final operating resistance, and that film conditioning prior to use improved stability. The Sn02/Pt, W03/Au, and ZnO sensing films were sensitive to their target gases (NH3, H2S, and CH4 respectively) but had poor selectivity and also responded to CO and H20. Other limitations of these sensing films were baseline drift, high resistance, and long recovery time. The array responses of the test platforms appeared to be linearly independent which should allow poor selectivity to be addressed since a "fingerprint" was produced for each gas. CMOS-based microhotplates were tested but not used in an array since some of them were damaged or had to be re-etched. An array of microhotplates, combined with pattern recognition or neural network systems, should be able to identify and quantify these gases in the presence of interferents. Therefore, micromachined microhotplates are a promising platform for low power sensor arrays for hazardous gas sensing.

Library of Congress Subject Headings

Gas detectors
Microelectromechanical systems

Format

application/pdf

Number of Pages

135

Publisher

South Dakota State University

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