A new microstructured silicon substrate for ultrathin gas-sensitive films

Abstract

Microstructured substrates featuring interdigitated electrodes with a high length-to-width ratio allow the evaluation of thin films with very high sheet resistivity even in environments with large electromagnetic interference. However, thin phthalocyanine (Pc) films vacuum evaporated on substrates with conventional interdigitated (ID) electrodes deposited 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 magnitude higher than expected for the chosen length-to-width ratio. A new microstructured Si substrate with in-plane interdigitated (IPID) electrodes is designed to achieve minimum height differences on the surface. This substrate features interdigitated electrodes with a length-to-width ratio of 52 000:1 and, additionally, an integrated heating element and a temperature-sensitive resistor. Thin Pc films with various thicknesses between 50 ML ( = 20 nm) and 500 ML ( = 200 nm) exhibit sheet resistivities as were expected for the chosen length-to-width ratio. These sensing elements were successfully employed in the detection of diesel exhaust gases in ambient air and they have proven their stability and sensitivity in polluted air during several months.