Process-variation Effects on 3D TLC Flash Reliability: Characterization and Mitigation Scheme

In Solid State Drives, flash management techniques such as wear-leveling and refresh usually assume NAND flash memories have the same endurance value. However, the actual endurance values differ from blocks to blocks. This reliability difference is introduced by process-variation during flash fabrication. In recent years, for improving flash management techniques, various works have been done on the reliability variation of 2D flash memory. As 2D NAND transmitted to 3D NAND flash, the vertical structure and multi-layer stacking changed the effect of previously known reliability problems. In this paper, we are first to characterize the process-variation effects on 3D TLC flash reliability. The characterization includes two parts: endurance variation and error feature variation. Second, we propose an adaptive error prediction scheme to mitigate the process-variation effects. This scheme uses the machine-learning model to realize the error prediction operation. We also discuss the implications of this scheme on main flash management techniques.