Multi-electrode substrate for selectivity enhancement in air monitoring

A new microstructured silicon substrate for thin gas sensitive layers has been developed composing a thermoresistor at the chip surface and a buried polysilicon heater. The process flow of the largely MOS-compatible fabrication is described. The two pairs of planarized interdigitated electrodes with both a width and spacing of 2 microns were manufactured by a special lift-off process. A second interdigitated electrode is placed above the sensing tin oxide layer and can also serve as a measuring electrode. As a further possibility, we suggest the application of a transverse electric field to the sensing layer by using the upper interdigitated electrode as a gate electrode and the heater as a counter electrode. This additional electric field can influence the effective signal values caused by desorption and adsorption of gas specimen thus enhancing the selectivity of resistive gas sensors. In order to detect oxidizing gases like nitrogen oxides with a metal oxide layer we have developed a reactive sputter process for tin oxide with a homogenous unstoichiometry across the complete grains. Measuring the d.c. resistance of these strongly reduced layers at relatively low temperatures of about 175 degrees C we have achieved a sensitivity in the sub-ppm range with regard to nitrogen dioxide in dry air. (C) 1997 Elsevier Science S.A.