ISFET´s with ion-sensitive membranes fabricated by ion implantation

Ion sensitive FETs (ISFETs) for sodium ions (Na⁺) fabricated by ion-implantation are investigated. The sensing layers are produced by implanting Na⁺ ions into the surface of an oxidized Si₃N₄ layer through an Al buffer layer deposited beforehand, in order to reduce the damage to the gate insulator of the ISFET during ion implantation. The Na⁺ sensitivity, selectivity, repeatability, thermal characteristics, and long-term stability are evaluated. The ISFET responds to Na⁺ ions independent of pH within the range of pH 7-10, and the Na⁺ sensitivity is nearly Nernstian. The ISFETs repeatability and long-term stability (about 1300 h), suggests that the ion-implantation technique is a suitable method for fabricating a stable ion-sensing layer