R&D: Resolving Reliability Issues of Open Blocks for 3D NAND Flash, Observations and Strategies

IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems has published an article written by Qiao Li, School of Informatics, Xiamen University, Xiamen, China, Min Ye, Department of Computer Science, City University of Hong Kong, Kowloon Tong, Hong Kong, Yufei Cui, School of Computer Science, McGill University, Montreal, Canada, Tianyu Ren, Department of Computer Science, City University of Hong Kong, Kowloon Tong, Hong Kong, Tei-Wei Kuo, Department of Computer Science and Information Engineering, National Taiwan University, Taipei, Taiwan, and Chun Jason Xue, Department of Computer Science, City University of Hong Kong, Kowloon Tong, Hong Kong.

Abstract: “While the block size of 3-D NAND flash memory increases with the density and capacity, the raw bit-error rates (RBER) of open blocks could be significantly increased. This article conducts a systematic study over reliability issues caused by open blocks, and reports several new observations. We found that the reliability degradation, due to long open time in writing a block, could happen over all layers in a 3-D NAND block, even after the block is closed. To address the reliability issues of open blocks, this article first proposes to adaptively allocate active blocks to serve write requests based on the workload characteristics for open time reduction. We then propose a partial-block refreshing strategy to alleviate the amplified RBER variations in open blocks and, thus, avoid unnecessary refreshing operations in low-RBER layers. Experimental results show that the proposed method can reduce the RBER by 43% through the reduction of the open time by 28% on average, and reduce the extra write operations for refreshing by 23% on average.“