Data-Retention Characteristics Comparison of 2D and 3D TLC NAND Flash Memories

Data-retention characteristics of 3-dimensional (3D) NAND flash memory have been evaluated with the optimal VREF (read reference voltage) shift in comparison with 2-dimentional (2D) (1Xnm) NAND flash memory. Bit-error rate (BER) of dataretention and write/erase (W/E) cycling in 3D NAND flash are much smaller than that in 2D NAND flash. Also, in 3D NAND flash, BER of Bottom Word-line is 1.9-times higher than Top Word-line, due to wider VTH distribution width. The dataretention lifetime of 3D NAND flash is estimated to about 10 years at 85degC, W/E= 1 similar to 300 cycles by the VTH distribution margin evaluation without considering the inter-block, interwafer and inter-lot variations and process/voltage/temperature (PVT) variations of the read reference voltage circuit. On the other hand, the data-retention lifetime of 2D NAND flash is estimated 1000 years in W/E= 1 and is rapidly degraded to 1 year in W/E= 300. Therefore, 3D NAND flash is suitable to apply to Cold Flash with a few hundred times update and data center SSD with many times update. 2D NAND flash is suitable for digital archive: millennium memory with only one time write.