Space charge distribution limitation on scale down of MNOS memory devices

The space charge distribution due to the trapped charge in the nitride of a MNOS memory device, produces a significant modulation of the nitride field outside the space charge region, that can appreciably effect the limit to which these devices can be scaled. Trapped hole and electron space charge distribution in silicon nitride have been derived from steady-state measurements of the change in flat-band voltage vs the maximum field in the nitride. Here, the steady-state density of trapped charge near the nitride-oxide interface nt^st(0), trapping length, Xo, and capture cross section, σt, were determined for nitrides deposited with a LPCVD system and an APCVD system. It was observed that the hole and electron trapped charge density increased and the trappings length decreased as the NH3to SiH4ratio is decreased. The trapping length for holes, however, is much greater than for electrons and thus the hole distribution limits the extent to which the nitride can be reduced in thickness. As temperature was increased, the trapping length of electrons increased more rapidly and approached the hole trapping length at 125°C. An approximate scaling limit is defined as the nitride thickness which is twice the charge centroid for a stored charge of 1µC/cm². By changing the NH3to SiH4ratio from 450:1 to 28:1, the nitride thickness scaling limit at 25°C was decreased from 320Å to 190Å. Both the threshold shift and voltage decay rate of the MNOS memory device scale linearly with the reduced nitride thickness over a limited range of stored charge.