Toward 7 Bits per Cell: Synergistic Improvement of 3D Flash Memory by Combination of Single-crystal Channel and Cryogenic Operation

In this paper, it is shown that the combination of single-crystal channel and cryogenic operation at 77 K using liquid nitrogen improves the cell transistor characteristics and the storage performance of 3D Flash memory. Compared to the cryogenic operation with poly-Si channels, that we have already reported, the cryogenic operation with single-crystal channels results in a steepening of the cell transistor subthreshold slope and reduced read noise. In particular, read noise is significantly suppressed to one-third due to the synergistic effect of the improvement by single-crystal and the cryogenic operation, compared with poly-Si channel in room temperature. Furthermore, data retention is improved at cryogenic temperature compared to room temperature. These improvements lead to a narrower Vth distribution of the cell, which enables bit-cost scaling through a multi-level cell. An ultra-multi-level cell of 7 bits per cell is successfully demonstrated for the first time, and its feasibleness in future storage products is shown.