Micromachined polymer-based chemical gas sensor array

We have developed a miniature polymer-based chemical gas sensor array on silicon using micromachining technology. The sensors are polymer–carbon black composite films, which swell reversibly and cause a resistance change upon exposure to a wide variety of gases. We have fabricated two types of devices which can measure this resistance change using a “well” design. These “wells” contain the polymer–carbon black–solvent liquid volume present during deposition and allow the sensor film to be placed reproducibly in a specific and well-constrained area. After deposition, the solvents evaporate and leave behind a polymer–carbon black residue crust between metal leads on each side of the “well”. Two types of devices, a bulk micromachined sensor and a patterned thick-film sensor, have been fabricated, ranging in size from 500 μm×600 μm to 100 μm×100 μm. Since the composite film sensors are not specific to any one gas, an array of these sensors, each with a different sensing film, is used to identify gases and gas mixtures through the pattern response of the array. Six polymer–carbon black composite films were deposited into the sensor array and exposed to three chemical gases at five different concentration levels. The sensors were able to uniquely detect these gas vapors and demonstrated a linear response to concentration levels between 2000 and 10,000 ppm. Results also indicate that a reduction in sensor area by an order of magnitude (from 4.32 to 0.30 mm2) does not reduce sensor response. This design allows the integration of circuits to process the changes in resistance which will permit the realization of a completely integrated miniature gas sensor.