A COMPARISON OF THICK-FILM AND THIN-FILM GAS-SENSITIVE ORGANIC SEMICONDUCTOR COMPOUNDS

The use of metal-based phthalocyanines in the construction of an array of gas-sensitive elements has been explored. A five-element array has been developed in which each of the sensor sites has an individual platinum heating element for independent temperature control. In this way the array can consist of different phthalocyanines and/or operating temperatures, allowing pattern-recognition techniques to be used in the detection of specific gases. The sensor array has been realized as a 28 pin dual in-line package based on an alumina substrate with laser scribed slots to give thermal isolation of adjacent sites. Two methods of phthalocyanine deposition have been investigated: a thin-film method utilizing low-pressure vapour deposition to give a sensor thickness of typically 1-mu-m, and a thick-film method whereby the phthalocyanine is made into a screen-printable ink, producing a typical sensor thickness of between 15 and 20-mu-m. The sensors produced by the two methods exhibit distinct morphological differences which significantly affect their respective sensitivities. It has been found that the more porous thick-film sensors have sensitivities comparable to that of their thin-film counterparts. These results support the theory that the conduction mechanisms in organic semiconductor gas sensors are primarily diffusion limited. Of the two fabrication methods described, the thick-film screen-printing technique is far more conducive to volume manufacture. Hence the comparability of the sensitivities of the two types of sensor suggests that screen printing of organic semiconductors is likely to prove a more viable method for the commercial production of gas-sensor arrays.