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rica nt g device characteristics and reliabilities are delivered. 2008 Elsevier Ltd. All rights reserved. nvolat As an alternative, NVM devices with multi-bit operation have received a great interest to increase the bit-density without com-promising area [4–7]. For this purpose, non-conductive materials such as silicon nitride, high-k dielectrics and nanocrystals are used instead of the conventional poly-Si to localize the injected charges [8–14]. Using these kinds of materials as a charge storage medium, so called stress induced leakage current (SILC) related retention problem is eliminated and thus further tunnel oxide scaling is investigated because of the limitations for increasing memory den-sity through device scaling. It can be realized simply by adding one more bit-line contact for NOR flash memory. However, just one of charge localization concept or physical separation of charge storage nodes is mainly utilized to achieve 2-bit/cell operation up to now. Recently, devices with 4-bit/cell operation have been developed for further increment of the memory density. For realization of 4-bits in one device, multi-bit and multi-level operations are adopted at the same time. Only multi-bit operation can also be used to achieve the 4-bit/cell operation like the double SONOS memory (DSM) [4]. However, it is hardly possible to make 4-bit/cell device