Bias Instability in Two-Layer MIS Structures

MIS structures consisting of a deposited film of A12O3 or Si3N4 on thermally grown SiO2 were studied to determine the effect that charge accumulation at the deposited film-SiO2 interface has on the threshold voltage stability. In order to study the effects of metallization, aluminum, gold and titanium field plates were evaporated on the deposited film. The experimental data indicate that Al forms a lower potential barrier on the deposited film than any of the other metallizations tested. For a given applied bias the shift in the flatband voltage is always greater for samples contacted with aluminum. Ti contacted and Au contacted slices show similar but much smaller shifts in the flatband voltage. It was determined that the charge accumulation is a result of Schottky emission from the metal-deposited film interface. This was determined by the field dependence and supported by the bias asymmetry of the accumulated charge. As a result of this investigation it was determined that the metallization plays an important role in determing the stability of MIS devices under applied bias. However, the use of proper metallization and the selection of low operating field will reduce the accumulation to a point where it will not present a serious reliability problem.