An ISFET-based calcium sensor using a photopolymerized polysiloxane membrane

In this paper an on-wafer fabrication technique for membrane-covered calcium-sensing ISFETs is presented. The complete structure consists of a standard Al₂O₃ pH-ISFET, covered with a hydrogel membrane and a second polysiloxane-type calcium-sensing membrane. Both membranes are deposited and photolithographically structured on the wafer. The adhesion of the membranes to the ISFET surface is ensured by presilylating the surface prior to the deposition of the membranes, and has been confirmed by tests in an ultrasonic bath. The on-wafer fabrication of calcium-sensitive ISFETs with a 25 μm thick poly (hydroxyethyl methacrylate) (polyHEMA) membrane covered with a 80 μm thick polysiloxane membrane is described and illustrated. The structural membrane definition for both membranes is better than 20 μm. The calcium-sensing properties of three completed devices have been investigated. A reproducible sensitivity of 28.7 mV/pCa and a detection limit of 10^-6.4 M calcium are found upon titration of CaCl₂ in demineralized water. The selectivity coefficients with respect to magnesium, sodium, and potassium are found to be log K_CaMg ^Pot = -4.9, log K_CaNa ^Pot = -4.5, and log K_Cak ^Pot = -4.1 respectively, which are comparable with the literature values.