Realizing Unequal Error Correction for nand Flash Memory at Minimal Read Latency Overhead

Abstract

In nand Flash memory, all pages have the same storage capacity and hence accommodate the same amount of redundancy in support of error correction. In current practice, user data in all the pages are protected by the same error correction code. However, different types of pages in multibit per cell memory have largely different bit error rates, for which appropriate unequal error correction can achieve a better utilization of memory redundancy and hence improve program/erase (P/E) cycling endurance. Nevertheless, a straightforward realization of unequal error correction suffers from severe memory read latency penalty. This brief presents a design strategy to implement unequal error correction through concatenated coding, which can well match the unequal error rates among different types of pages at minimal memory read latency penalty. Based on measurement results from commercial sub-22-nm 2 bits/cell nand Flash memory chips, we carried out simulations from both the coding and storage system perspectives, and the results show that this design strategy can improve the P/E cycling endurance by 20% and only incur less than 7% increase of storage system read response time at the end of Flash memory lifetime with the P/E cycling of around 1800.