SD-PCM: Constructing Reliable Super Dense Phase Change Memory under Write Disturbance

摘要

Phase Change Memory (PCM) has better scalability and smaller cell size comparing to DRAM. However, further scaling PCM cell in deep sub-micron regime results in significant thermal based write disturbance (WD). Naively allocating large inter-cell space increases cell size from 4F~2 ideal to 12F~2. While a recent work mitigates WD along word-lines through disturbance resilient data encoding, it is ineffective for WD along bit-lines, which is more severe due to widely adopted μTrench structure in constructing PCM cell arrays. Without mitigating WD along bit-lines, a PCM cell still has 8F~2, which is 100% larger than the ideal. In this paper, we propose SD-PCM for achieving reliable write operations in super dense PCM. In particular, we focus on mitigating WD along bit-lines such that we can construct super dense PCM chips with 4F~2 cell size, i.e., the minimal for diode-switch based PCM. Based on simple verification-n-correction (VnC), we propose LazyCorrection and Pre-Read to effectively reduce VnC overhead and minimize cascading verification during write. We further propose (n:m)-Alloc for achieving good tradeoff between VnC overhead minimization and memory capacity loss. Our experimental results show that, comparing to a WD-free low density PCM, SD-PCM achieves 80% capacity improvement in cell arrays while incurring around 0-10% performance degradation when using different (n:m) allocators.