Wear relief for high-density phase change memory through cell morphing considering process variation

Due to the scalability and large leakage power, dynamic random-access memory (DRAM) has a lot of challenges in scaling. As an alternative, phase change memory (PCM) has demonstrated promising potential to serve as the main memory in deep submicrometer regime. The broad resistance range of PCM cells enables several cell modes with various densities, pertaining to multiple level cell (MLC), triple state cell (TSC), and single level cell (SLC). High-density mode outperforms low-density ones in terms of capacity and cost-per-bit, but suffers from a weaker cell endurance. Wear leveling strategies are proposed to enhance the memory endurance but encounter more challenges with the aggravating process variation. Due to endurance variations, physical domains are fabricated with irregular tenacity. As a result, balanced write traffic, which is the objective of traditional wear leveling, cannot fully exploit the PCM endurance since the weak parts will be worn out sooner than others. In this paper, considering process variation, we propose a cell morphing based wear leveling scheme. Cell morphing refers to the cell mode transformation between high density (e.g., MLC) and low densities (e.g., TSC and SLC). Instead of redistributing write operations, the proposed wear leveling scheme dynamically transforms weak and frequently written portions into low-density mode for endurance benefits. Multitier cell morphing schemes are proposed to support mode transformation among multiple density levels. The experimental results show 236% endurance improvement for single-tier cell morphing and 209% for two-tier cell morphing with 2% low-density page percentage, when compared with the most related work.