Effects of Insulator Thickness Fluctuations on MNOS Charge Storage Characteristics

Abstract

The effects of insulator thickness fluctuations on the charge storage characteristics of MNOS direct tunneling devices were investigated, using capacitor structures carefully fabricated by depositing Si3N4 layers on thermally grown SiO2 films. The oxide films were thin enough (Ä 27 Å) for the electron transfer due to electrical pulsing to occur via direct tunneling through them (the SiO2 films). Both the Si3N4 and SiO2 layers were prepared deliberately with a thickness gradient across the silicon substrates, and the thickness of each dielectric was ellipsometrically profiled to within ±1 percent accuracy. In this way the effect of insulator thickness deviations from nominal values could be experimentally evaluated under virtually identical processing conditions. Experimental results in these capacitor specimens indicate that other things being equal, a small thickness fluctuation causes a significant fiat-band potential spread (or an equivalent threshold voltage spread in actual MNOSFET structures) in the pulsed devices. In the range of pracical interest for charge storage devices, this spread can be substantial if the thickness deviations exceed ±1 A for the Si02 films cr3 equivalently, ±40A for the Si3N4 films. For a typical Al-Si3N4 (400 A.)-SiO2 (20–25 A)-Si structure, for example, a ± 2 A fluctuction can result in a 2to 4-V spread of the Si flat-band potential after charging, depending on the pulse height used. In the microsecond range of switching time, such threshold spreads will reduce the sc-called fiat-band hysteresis window from 6 to 2 V typically-which is detrimental to device performance. A simple analysis is given to show that the thickness fluctuation effect in MNOSFET devices can be predicted semi-empirically, using the charging characteristics of a device having the nominal insulator thicknesses. Copyright © 1972 by The Institute of Electrical and Electronics Engineers, Inc.