Gas and humidity sensors based on organic active thin films

Abstract

For more than 10 years we have investigated the fundamental electronic properties of lead phthalocyanine (PbPc) thin films, including quasi-one-dimensional metallic and switching behaviour depending on deposition parameters, crystallographic structure and film morphology. By close connections with microelectronic technology we have prepared thin-film gas sensors from high purity PbPc with slit and comb-like gold electrodes down to 1 μm slit width, the highest miniaturization possible with electron beam lithography. Results are given on the high sensitivity of PbPc sensors to NO 2 in air as a function of temperature, concentration of NO 2 in the region of 100 ppb to 100 ppm and exposure time. The sensors show no interference with gases like NH 3, CO, CO 2, CH 4, H 2O, H 2S, SO 2, NO and C 6H 14, but they are weakly influenced by ozone. Another long-lasting topic of our research group is the deposition of glow discharge polymerization films. Like nearly all polymers, polymeric materials grown from malonic acid dinitrile, acetonitrile and chlorobenzene also show defined absorption of water from humid surroundings. Such thin films are used as the active part in humidity sensors. They have high linearity of the capacitance/humidity dependence. For evaluation we prefer the capacitive measurement because of its lower sensitivity to impurities on the film surface and higher linearity compared with the resistivity measurement, especially with a pulse-charging method.