Optimized cell programming for flash memories

Flash memory cells use the charge they store to represent data. The amount of charge injected into a cell is called the cell´s level. Programming a cell is the process of increasing a cell´s level to the target value via charge injection, and the storage capacity of flash memories is limited by the precision of cell programming. To optimize the precision of the final cell level, a cell is programmed adaptively with multiple rounds of charge injection. Due to the high cost of block erasure, when cells are programmed, their levels are only allowed to increase. Such a storage medium can be modelled by a Write Asymmetric Memory model. It is interesting to study how well such storage media can be programmed. In this paper, we focus on the programming strategy that optimizes the expected precision. The performance criteria considered here include two metrics that are suitable for the multi-level cell technology and the rank modulation technology, respectively. Assuming that the charge-injection noise has a uniform random distribution, we present an effective algorithm for finding the optimal programming strategy. The optimal strategy can be used to program cells efficiently.