A fabrication process of flexible IDE capacitive chemical sensors using a two step lift-off method based on PVA patterning

A versatile, two-step, lift-off method, for the fabrication of flexible capacitive chemical sensors based on interdigitated electrodes (IDEs) is presented. A Polyimide (PI) foil whose one side is coated with a thin Aluminum (Al) film is used as the sensor substrate. The IDEs are then formed on the Al film using conventional semiconductor processes. Precise patterning of the sensor sensing layer, on the surface of the Al IDEs, is achieved using a two-step lift-off process which involves a first lift-off during which a Polyvinyl alcohol (PVA) film is spin-coated and patterned, followed by the deposition or spin coating of the sensing layer and a second lift-off to pattern it on the IDEs area and form the final sensor. The process allows for upscaling when a single sensing layer is deposited and patterned in batch over the whole wafer or for the easy fabrication of chemical sensor arrays when depositing and precisely patterning different sensing layers on an array of IDEs. Showcasing the first approach, PHITA copolymer (a methacrylate tertrapolymer) is deposited and patterned over sensors with varying electrode gaps in order to fabricate simple humidity sensors, while in a following experiment, Poly(2-hydroxyethyl methacrylate) (PHEMA), Epoxy novolac resin (EPN), Poly(methyl methacrylate) (PMMA) and Poly (hydroxy styrene) PHS are patterned on the IDEs in order to form a sensor array. The different sensor configurations realized are evaluated when exposed to water, methanol and ethanol vapors with PHS sensor exhibiting a capacitance change of 44 fF (sensitivity: 1.87 fF / %RH), 7.4 fF and 4.6 fF at 10,000 ppm respectively.