Investigating the Viability of MEMS Vapor Sensors for Detecting Land Mines

This paper reports the design specification and process for constructing a gas identification instrument using MEMS technology for the purpose of detecting buried land mines. The aim is to design an instrument to detect and identify gases that leak from buried land mines using an array of sensors and a pattern recognition/signature identification system. This report focuses on the design of the sensor array. The principal component of the sensors is a conducting polymer that is reversibly physically altered when exposed to different chemical compounds. Physical changes experienced by the polymer in the presence of the gas molecules are detected and converted into electrical signals by two kinds of transducers. Crystal oscillator-microbalances with MHz oscillating frequencies detect changes in the polymer’s mass via the shift in frequency in one type of sensor. In the other sensing configuration, current traveling through the polymer for a fixed voltage source is measured to detect the changes in the polymer’s resistance. The first type of sensor is a miniaturized version of the quartz crystal microbalance (QCM) which is based on the mechanical principle that resonant frequency decreases with increased mass. The second type of sensor uses carbon black-polymer composite films, which swell reversibly when exposed to a variety of gases and thus induce a resistance change. In order to allow this sensor to be reproduced accurately, high aspect ratio wells are constructed using an SU-8 photoresist, and the polymer solvent liquid is injected into these wells. The response of the sensor array to different gases, and also different concentrations of the gases, provides a signature by means of which the identity and concentration of a variety of gases can be recognized.