Interfacial and adhesional aspects in polyurethane (PUR) membrane coating on Si3N4 surface of ISFET gate for REFET fabrication

Polyurethane membrane coatings on: (i) packaged ISFET (ion-sensitive field-effect transistor) devices and (ii) those in the form of chips have been investigated. The study has revealed that for reliable membrane deposition, proper film adhesion at polyurethane-silicon nitride and polyurethane-cured epoxy interfaces of ISFET was necessary. Membranes formed on packaged ISFETs were found to lift off after a short time period (<1 h) when immersed in pH 6.86 buffer solution. This was because the adhesion between polyurethane and surrounding cured epoxy was insufficient. If the membrane coating was performed on the chip, and the chip was subsequently packaged (protecting the coating on the sides with epoxy and curing the epoxy to fasten the membrane), the coating was not attacked in buffer solution. Based on these studies, a process for depositing reliable, long-lasting polyurethane membranes on ISFET gate surfaces has been proposed. Membrane reliability has been confirmed by storing the membrane-coated ISFET in buffer solution over seven days and periodically observing it microscopically. Membranes have been characterized through pH sensitivity and gate-source impedance measurements. pH sensitivities of ISFETs with polyurethane membranes were measured to be ≤4.2 mV/pH.