Drift-Invariant Detection for Multilevel Phase-Change Memory

Next-generation memory (NGM) technologies present a major opportunity but also a significant challenge, due to their intricate reliability issues. In particular, multilevel-cell (MLC) storage is highly desirable for increasing storage capacity and lowering total cost-per-bit. In phase-change memory (PCM), MLC storage is hampered by sensitivity to temperature variations and resistance drift. A novel drift-invariant detection (DID) scheme that estimates variable read thresholds based on ordered statistics and clustering of the soft read-back signals from a small block of 32 cells has been developed and implemented in hardware to improve reliability and prolong data retention. A low-complexity implementation of the DID on a FPGA platform comprises 20'000 LUTs and 6'000 flip-flops and has a latency of 90ns. We present results from an extensive performance verification that ascertains highly reliable data retrieval up to 13 orders of magnitude in time after programming. Such elevated reliability is necessary for the most anticipated application of NGM, namely persistent far-memory, where the NGM is used as a large memory pool, possibly together with DRAM.