ISFETS WITH SPUTTERED SODIUM ALUMINOSILICATE GLASS MEMBRANES

Sodium ion sensitive field-effect transistors (ISFETs) based on reacti vely sputtered sodium alumino-silicate (NAS) glass membranes are inves tigated. Using an inverted cylindrical magnetron, NAS films with defin ed and reproducible composition are obtained. Radiation damage in the gate dielectric is largely eliminated by annealing at 450 degrees C. T he Na+ sensitivity, the selectivity with respect to H+ and K+ ions, th e response time, the conditioning period, the long-term drift, and the lifetime are evaluated. The ISFETs exhibit a near-Nernstian Na+ respo nse down to 10(-4) mol/l in solutions of pH greater than or equal to 7 . The NAS films are physically very robust and have excellent chemical durability, e.g., a sensor with a 110 nm thick membrane lasts for abo ut 8 months. The overall sensor properties compare well with those of a commercial Na+ selective glass electrode.