Cryogenic Operation of 3-D Flash Memory for Storage Performance Improvement and Bit Cost Scaling

This report introduces the cryogenic operation and storage performance of 3-D flash memory. The cell transistor characteristics and the basic functionalities, including read and program and erase (P/E) operations, are investigated at an extremely low temperature of 77 K cooled by the liquid nitrogen. The cell transistor has a steep subthreshold slope at 77 K compared with at 300 K, and the read operation is fully functional even though the saturation current becomes relatively small. P/E operations at 77 K are not much different from those at 300 K, and the same incremental-step pulse programming (ISPP) and incremental-step pulse erase (ISPE) methods are applicable. In addition, the storage performance and the reliability characteristics of 3-D flash memory, such as the read noise, the disturb characteristics, the data retention, and the cycle endurance, are also investigated. While there is no degradation in the disturb characteristics, the read noise at 77 K was significantly minimized compared with that at 300 K. The data retention characteristics are about three times improved, and the cycle endurance is about ten times improved at 77 K compared with at 300 K. These storage performance improvements and high-reliability characteristics at cryogenic operation enable us to achieve the ultramultilevel cell. We show the successful demonstration of 6-bit per cell (HLC) and discuss the impact of additional cooling cost and the possibility of future storage devices beyond HLC. The cryogenic operation of 3-D flash memory can greatly improve the storage performance and opens the door for potential new applications and the bit cost scaling for future storage devices.