Ion-assisted e-beam evaporated gas sensor for environmental monitoring

With increasing air pollution problems, gas sensors for toxic gas detection have recently gained much attention. Among different types of gas sensors, thin film devices have been much of interest because of microelectronic batch-fabricated compatibility, reproducibility, and the ability to form multilayer device structures. In this work, thin film based gas sensors fabricated by electron beam evaporation with ion-assisted deposition (IAD) are developed for immediate applications of CO detection for environmental monitoring and Alcohol testing for drivers. The IAD process offers several advantages for gas sensor fabrication, including reactive deposition for gas-sensitive metal-oxide material optimization and improved thin film adhesion for better device reliability. Gas-sensing performance of ion-assisted electron beam evaporated metal oxide thin film materials, including SnO2 and WO3, has been characterized as a function of IAD deposition parameters. The metal oxide layer was deposited on Au/Al interdigitated electrodes on alumina or glass substrates with an unpatterned NiCr thin film heater on the backside. The sensors were tested with reducing gases, including alcohol vapour and CO, in the temperature range between 200 and 350 °C. Experimental results indicate that the SnO2 thin film has higher sensitivity to both alcohol and CO than the WO3 thin film under the same IAD deposition parameters. In addition, alcohol and CO sensitivity tends to improve with increased oxygen-ion addition during e-beam evaporation. Our results indicate that ion-assisted e-beam evaporation is a useful and well-controlled process suitable for gas sensor fabrication.