Optimizing De-trap Pulses in Gate-injection Type Ferroelectric NAND Cells to Minimize Read After Write Delay Issue

초록

The ferroelectric (FE) NAND flash, featuring metal-interlayer-FE-interlayer-silicon (MIFIS) gate stacks, leverages both charge trapping and polarization (P) switching to achieve a broad memory window (MW) and low operation voltage. These remarkable advancements establish it as a viable contender for future NAND flash technologies. However, the read-after-write-delay (RAWD) problem during program/erase (PGM/ERS), caused by channel-injected interface trapped charges (Q(it)) between the FE layer and the channel interlayer (Ch.IL), leading to short-term V-th variations, remains unexplored in MIFIS FE-NAND cells. This letter presents the first analysis of RAWD in FE-NAND cells, including the experimental optimization of a de-trap pulse that effectively eliminates Qit whereas preserving both gate-injected interface trapped charges (Q(it)') and P. Consequently, the FE-NAND cell exhibits a narrow MW of 3.45 V at a delay time (t(Delay)) of 1 mu s between PGM/ERS and read operations, expending to 7.40 V at a t(Delay) of 1 s. This variation is attributed to the generation of Q(it) and the subsequent de-trap process, affecting channel conductivity. To thoroughly address the RAWD, various pulse widths and amplitudes are experimentally explored immediately post-PGM/ERS to optimize the de-trap pulse for selective Q(it) removal. Upon applying the optimized de-trap pulse, the stable wide MW (7.40 V) is consistently maintained regardless of t(Delay). This work is meaningful as it brings attention to previously unexplored issues in next-generation ferroelectric (FE) NAND cells and suggests practical operational solutions.

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