A NEW MICROSTRUCTURED SILICON SUBSTRATE FOR ULTRATHIN GAS-SENSITIVE FILMS

Microstructured substrates featuring interdigitated electrodes with a high length-to-width ratio allow the evaluation of thin films with ver y high sheet resistivity even in environments with large electromagnet ic interference. However, thin phthalocyanine (Pc) films vacuum evapor ated on substrates with conventional interdigitated (ID) electrodes de posited on top of a planar surface, develop cracks at the edges of the contact stripes. For samples with a film thickness of 200 monolayers (ML) corresponding to 80 nm this causes a resistance two orders of mag nitude higher than expected for the chosen length-to-width ratio. A ne w microstructured Si substrate with in-plane interdigitated (IPID) ele ctrodes is designed to achieve minimum height differences on the surfa ce. This substrate features interdigitated electrodes with a length-to -width ratio of 52000:1 and, additionally, an integrated heating eleme nt and a temperature-sensitive resistor. Thin Pc films with various th icknesses between 50 ML (= 20 nm) and 500 ML (= 200 nm) exhibit sheet resistivities as were expected for the chosen length-to-width ratio. T hese sensing elements were successfully employed in the detection of d iesel exhaust gases in ambient air and they have proven their stabilit y and sensitivity in polluted air during several months.