Sensors and Actuators B, 2 (1990) 103-110 Hysteresis in A1203 -gate ISFETs LUC BOUSSE* Integrated Circuits Laborarory, Stanford University, Stanford, CA 94305 (U.S.A.) H. H. VAN DEN VLEKKERT and N. F. DE ROOIJ Insriture of Microtechnology, University of Neuchdtel, Neuchcirel (Switzerland) (Received June 1, 1989; in revised form November 20, 1989; accepted November 27, 1989) 103 Abstract When used as pH-sensing surfaces, insulators such as SiOz, Si3N4 and A1203 are subject to non-idealities such as hysteresis and drift. These effects limit the accuracy obtainable from pH-sensitive ISFETs. This paper studies the hysteresis effects in Al*Os-gate ISFETs by exposing the devices to a cycle of pH values. When the pH is cycled between 3 and 11 in a 0.1 M NaCl solution, hysteresis of 3 to 6 mV is seen, which corresponds to 0.8 to 1.6% of the total response. The hysteresis is somewhat less when back- ground phosphate buffers are used rather than a Tris buffer. Oxalate and aluminate ions have no significant effect. The presence of fluoride ions in acid solutions causes a strong irreversible drift. Alkaline solutions also induce such.a drift, but to a much smaller extent. The observations can be explained by the presence of tw0 different effects: first, part of the pH response is delayed, resulting in hysteresis. Secondly, OH- and F- ions cause an irreversible drift towards more negative threshold voltages, which will also affect measured hysteresis values. Introduction Although ISFET pH sensors were first made in 1970 [ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 11, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA they have not replaced conventional pH sensors such as glass electrodes in most applications. A possible exception is in vivo use, where glass elec- trodes cannot be used, and where catheter-tip ISFETs have been successfully demonstrated [2]. Although other such examples may be forthcoming, glass elec- trodes are still the dominant pH sensors. The basic reason for this is that they are more accurate than ISFETs. Two mechanisms limit the accuracy with which an ISFET can be used to measure pH. The first is drift, which has been reported by several authors. For AlzOs-gate ISFETs, this has been shown to be a *Present address: Molecular Devices Corporation, 4700 Bohannon Drive, Menlo Park CA 94025, U.S.A. 39254005/90/$3.50 logarithmic function of time [3]. This means that by waiting for a sufficiently long time, drift can be reduced to very low values. Furthermore, it is predict- able, which means that correction is possible. Another limitation on accuracy is the hysteresis observed when an ISFET is exposed to consecutive upward and downward pH sweeps. This effect was found to be quite severe in SiO*-gate ISFETs [4,5], and also exists in ISFETs with Si3N4 and A1203 gates [6,7], although to a much lesser extent. In this paper, we shall present experimental results about the hysteresis behavior of AlzO,-gate ISFETs in various solutions. It is known that the drift behavior of A&O,-gate ISFETs originates from the bulk of the A120s laier, and not from the A&Os-electrolyte interface [3]. In particular, MISFETs with aluminum gates have a drift behavior similar to that of ISFETs [3]. On the other hand, Turfa [8] has reported that the hysteresis behavior depends on the nature of the electrolyte solution. This indicates that the mecha- nisms of drift and hysteresis in an ISFET are differ- ent, and that hysteresis is related to the chemical interactions between the A&O3 surface and the electrolyte solution. Turfa reports a strong increase in hysteresis in the presence of fluoride ions [8]. This suggests a possible link with the dissolution of A1203. We have therefore included oxalic acid in the list of electrolytes tested, since it is known that oxalic acid greatly accelerates the dissolution of A120s [9-3. Experimental Conditions The ISFET Devices The ISFETs used in this study were fabricated at the University of Neuchltel, and are identical to those reported ln refs. 3 and 10. The gate insulator is composed of a 80 nm layer of thermally grown SiOz, covered with the ion-sensitive A1203 layer deposited by chemical vapor deposition using the following reactions [lo] : 2N0 + 2H2 - Nz + 2Hz0 2AlBrs + 3Hz0 - A1203 + 6HBr 0 Elsevier Sequoia/Printed in The Netherlands