8-Level NAND flash memories with 51 nm design rule and 44-cell string floating gate technology have been successfully developed for the first time. 44-Cell string with floating poly silicon and tungsten silicide (WSi) gate structure reduced the cell area per bit and improved chip cost efficiency. 44-Cell string structure shows acceptable cell current and the results of endurance and disturbance characteristics are quite comparable to the conventional 32-cell string structure. Importance of pass voltage disturbance regarding 8-level and extended cell string was analyzed and improved. Controlled cell dimension uniformity and crystalline property achieved tight distribution of cell V-th. (C) 2009 Elsevier Ltd. All rights reserved.