ISFETs with sputtered sodium alumino-silicate glass membranes

Sodium ion sensitive field-effect transistors (ISFETs) based on reactively sputtered sodium aluminosilicate (NAS) glass membranes are investigated. Using an inverted cylindrical magnetron, NAS films with defined and reproducible composition are obtained. Radiation damage in the gate dielectric is largely eliminated by annealing at 450 degrees C. The Na(+) sensitivity, the selectivity with respect to H(+) and K(+) ions, the response time, the conditioning period, the long-term drift, and the lifetime are evaluated. The ISFETs exhibit a near-Nernstian Na(+) response down to 10(-4) mol/l in solutions of pH>/=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 about 8 months. The overall sensor properties compare well with those of a commercial Na(+) selective glass electrode.