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The mechanisms of programming/erasing (P/E) and endurance degradation have been investigated for multilevel-cell (MLC) Flash memories using a (NROM) or a dual charge storage layer (DCSL). Threshold-voltage -level disturbance is found to be the major endurance degradation factor of NROM-type MLCs, whereas separated charge storage and step-up potential wells give rise to a superior -level controllability for DCSL MLCs. The programmed levels of DCSL MLCs are controlled by the spatial charge distribution, as well as the charge storage capacity of each storage layer, rather than the charge injection. As a result, DCSL MLCs show negligible -level offsets (< 0.2 V) that are maintained throughout the P/E cycles, demonstrating significantly improved endurance reliability compared to NROM-type MLCs.