QLC NAND study and enhanced Gray coding methods for sixteen-level-based program algorithms

After the development of TLC (Triple level cell) NAND, QLC (Quad level cell) technology will be one of the solutions for next generation NAND flash product. QLC (4 bits/cell) NAND's data levels are up to 16 levels which bring the narrower and tighter voltage ranges and worse performance. This paper studies QLC program and read mechanisms, and presents five types of program algorithms for writing data in QLC cells. Correspondingly, the enhanced Gray coding methods for data mapping based on these different program algorithms are researched. They will be beneficial to data reading. After simulating with changing cell voltages to generate the data errors in the QLC model, the paper presents four enhanced Gray codings to match 16 voltage levels for different program algorithms and read operations. This study will improve the efficiency and keep balance in system-level application for the data error correction of QLC 4 pages which are mapped in the same wordline (WL).