Effects of insulator thickness fluctuations on MNOS charge storage characteristics

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 Si
3
N
4
layers on thermally grown SiO
2
films. The oxide films were thin enough (∼27 Å) for the electron transfer due to electrical pulsing to occur via direct tunneling through them (the SiO
2
films). Both the Si
3
N
4
and SiO
2
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 flat-band potential spread (or an equivalent threshold voltage spread in actual MNOSFET structures) in the pulsed devices. In the range of practical interest for charge storage devices, this spread can be substantial if the thickness deviations exceed ±1 Å for the SiO
2
films or, equivalently, ±40 Å for the Si
3
N
4
films. For a typical Al-Si
3
N
4
(400 Å)-SiO
2
(20-25 Å)-Si structure, for example, a ±2 Å fluctuation can result in a 2- to 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 so- called flat-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.