Scaling challenges of NAND flash memory and hybrid memory system with storage class memory & NAND flash memory

Abstract

This paper summarizes the scaling challenges of the conventional 2D floating-gate cell NAND flash memories [1, 2]. The scaling trends and limits of the bulk and SOI NAND flash memories are investigated in terms of short channel effects and channel boosting leakage from 20nm to below 10nm generation using 3D-device simulation. In the bulk NAND cell, 13nm generation is the scaling limit for realizing both channel boosting during program-inhibit and SCE suppression. The SOI NAND cell scaling limit is decreased to 8nm generation. Then, scaling problems and device design for 3D-stackable NAND flash memory are investigated [3]. Control gate length (Lg) and spacing (Lspace) are paid attention since they can be separately varied in 3D NAND and significantly affect the cell area of the 3D NAND as well as the electrical characteristics. Lg and Lspace should be the same to cope with the tradeoff between memory window and disturbance. If the number of stacked layers is 18 with the layer pitch of 40nm, the effective cell size of the 3D NAND corresponds to that of 15nm planar NAND technology. Then, this paper discusses an error prediction (EP) low density parity check (LDPC) error correcting code (ECC) which realizes an over 10-times extended lifetime [4, 5]. As the design rule shrinks, the floating gate (FG)-FG capacitive coupling among neighboring memory cells seriously degrades the memory cell reliability. The EP-LDPC ECC calibrates the inter-cell coupling without access time penalty. Finally, this paper overviews a state-of-the-art hybrid memory solution with storage class memory (SCM) and NAND flash memory for the big data solid-state storage system [5, 6]. Data fragmentation of MLC NAND flash memory is suppressed and efficient MLC NAND flash usage is realized by storing small hot data to SCM. The 3D TSV hybrid SSD realizes 11 times performance increase, 6.9 times endurance enhancement and 93% write energy reduction.