The traditional nitride-only charge storage layer of a SONOS device is replaced
by a multifilm charge storage layer comprising more than one dielectric material.
Examples of such a multifilm charge storage layer are alternating layers of silicon
nitride and silicon dioxide, or alternating layers of silicon nitride and aluminum
oxide. The use of more than one material introduces additional barriers to migration
of charge carriers within the charge storage layer, and improves both endurance
and retention of a SONOS-type memory cell comprising such a charge storage layer.