Abstract:3D NAND flash memory has been widely used in many fields due to its high storage density and high performance. However, when the 3D NAND flash memory blocks are not fully programmed, there is a significant difference in the retention error characteristics between the internal word lines and the edge word lines, which poses a serious threat to the reliability of data storage. To address this challenge, a 4-8LC method suitable for edge word lines has been proposed. This method evenly divides the 8 threshold voltage states of TLC flash memory into 4 groups, with the information encoding of the two states within each group having a Hamming distance of 1. After grouping, flash memory cells with another threshold voltage state in the same group as the erased state are programmed to the erased state, and the flash memory cells in the other 3 groups that are in a lower threshold voltage state are programmed to the other state in the same group. Data retention experiments were then conducted, and error information was read. The experimental results show that, under the condition of the flash memory block going through 3000 P/E cycles and data retention at room temperature for one year, the average number of retention errors of the edge word lines reached 2.97 times that of the internal word lines. The proposed 4-8LC method reduced the error code rate of the low page and high page of the edge word lines by 99.7% and 99.9% respectively, and the effect of improving reliability is 1.74 times that of the EP method.
