CDS: Coupled Data Storage to Enhance Read Performance of 3D TLC NAND Flash Memory

Due to the strong demand of massive storage capacity, the density of flash memory has been improved in terms of technology node scaling, multi-bit per cell technique, and 3D stacking. However, these techniques also degrade read performance and reliability. The long read latency comes from increased data sensing time and time-consuming ECC decoding time. Storing multiple bits per cell results in more read reference voltages and increased latency of identifying appropriate threshold voltages. To deal with error correction, LDPC is widely used in flash memory to provide stronger ECC capability. However, LDPC incurs a long decoding latency when bit errors are numerous. In this work, we propose coupled data storage (CDS) to improve the read performance of 3D NAND flash-memory storage devices. CDS supports two modes to improve read latency: The high read-speed mode is designed to improve data sensing time with reduced voltage states, while the data correction mode is designed to mitigate bit errors and LDPC overhead. Experiment results showed that CDS could reduce 50~66.6% read latency and 25.7~27.5% write latency under the high read-speed mode. For the data correction mode, RBER could be decreased by 37~52% and the lifetime could be prolonged to 1.6 to 3 times.