In-plane interdigitated (IPID) electrodes for thin film applications

Abstract

Microstructured substrates featuring interdigitated electrodes with a large 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 that expected for the chosen length to width ratio. A new microstructured Si-substrate with in-plane interdigitated (IPID) electrodes has been 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 NOx emissions in cars and their stability and sensitivity have been proved over several months in polluted air.