Prototype sodium and potassium sensitive micro ISFETS

Design and fabrication methods, along with the results of a preliminary evaluation, are described for microprobe ISFET sensors intended for extracellular ion-activity measurements. The devices are 5 mm long with a gradual taper to the tip where the cross-sectional area is 30 × 30 μm². Since the ion-sensing area is located at the end of the tip, a very small sensing region is achieved. Photo-induced noise generated in the depletion zone of the long drain diffusion layer was found to be particularly troublesome. This effect was greatly reduced by the use of an opaque tantalum thin film deposited on the probe surface. Preliminary tests of micro pH, pNa and pK ISFETs have shown that it is possible to measure extracellular ion activities using ISFETs.