Retention Correlated Read Disturb Errors in 3-D Charge Trap NAND Flash Memory: Observations, Analysis, and Solutions

3-D NAND flash memory has been attracting much attention owing to its ultrahigh storage density and low bit cost, and it has been widely applied in data centers and mobiles. 3-D triple-level-cell (TLC) NAND flash memory can achieve much larger storage capacity by storing 3 bits in each cell. However, the data reliability issues induced by data retention (DR) and read disturb (RD) greatly limit 3-D TLC NAND applications in hot data storage where the stored data are frequently accessed and RD is more serious. In this article, we first systematically study the retention correlated RD (RCRD) errors in 3-D charge trap (CT) TLC NAND flash memory under various conditions. Error characteristics and underlying mechanisms much different from 2-D NAND flash memory are observed: 1) for RCRD with short retention-after data program, error bits increase due to the negative-shift of program states and 2) for RCRD with long retention-after retention 12 h, error bits can be partially recovered on the contrary due to the charge compensation. We propose schemes of precharge the storage layer (PCSL) and thermally stabilize the storage layer (TSSL) to improve the reliability of 3-D NAND flash memory. By using these two methods, experimental results show that raw bit error rates (RBERs) can be significantly reduced by 30% and 20%, respectively.