Integrated Micro Multi Ion Sensor Using Field Effect of Semiconductor

The fabrication of a microprobe for simultaneous, independent and in-vivo measurements of H+ and Na+ ion activities is described. By means of monolithic integrated circuit techniques, ion sensitive field effect transistor (ISFET) structures can be incorporated in the tip of a microprobe. In the ISFET, the role of the usual metallic gate of the insulated gate field effect transistor (IGFET) is played by an electrolyte. The interface potential between the gate insulator and the electrolyte varies selectively with the specific ion activity in the electrolyte and the channel conduction is then controlled by the ion activity in the electrolyte. The pH and pNa selectivities of several materials were investigated, and it was found that silicon nitride was adequate for pH selection, and aluminosilicate for pNa. In both cases it was found that the sensitivity is close to the value predicted by the Nernst equation. The interfacial electrochemical equilibrium was investigated by Auger electron spectroscopy. It was confirmed that the selectivity mechanism of ISFET is similar to that of glass electrodes.