Improving multilevel PCM reliability through age-aware reading and writing strategies

Given its low power consumption and high density, Phase Change Memory (PCM) has been treated as a promising alternative to DRAM for main memory storage. Multilevel Cell (MLC) PCM outperforms regular single level cell (SLC) PCM with even higher information density, yet requires more accurate control for cell reading and writing. More crucially, the resistance of a MLC PCM cell may drift over time, thus introducing high error rate during cell reading if the quantization thresholds are constant. While previous work tries to adjust the quantization method when reading a cell, its accuracy is still limited due to the inter-cell variations in data age. In this paper, we propose various PCM writing and quantization strategies to improve MLC PCM reliability. Cell quantization accuracy is improved by taking into consideration not only the time information but also inter-cell age variations. Moreover, by making the write strategy be aware of time, the inter-level quantization margin can be guaranteed when the cells exhibit large age variations. This time-aware writing scheme is adaptively applied to maximize achievable benefits. The experimental results show that the proposed writing and reading approaches can effectively reduce the quantization error rate in MLC PCM by 95%.