Improvement of Read Disturb, Program Disturb and Data Retention by Memory Cell VTH Optimization of Ferroelectric (Fe)-NAND Flash Memories for Highly Reliable and Low Power Enterprise Solid-State Drives (SSDs)

This paper presents an improvement of the memory cell reliability by the memory cell V-TH optimization of the ferroelectric (Fe)-NAND flash memory. The effects of the memory cell V-TH on the reliability of the Fe-NAND flash memory are experimentally analyzed for the first time. The reliability is evaluated by the measured V-TH shift due to the read disturb, program disturb and data retention. Three types of Fe-NAND flash memory cells, a positive, zero and negative V-TH memory cell, are defined on the basis of the memory cell V-TH. The middle of V-TH of programmed and erased states is I V, 0 V and -0.3 V in a positive, zero and negative V-TH memory cell, respectively. The V-TH shift of the positive, zero and negative V-TH memory cells show similar characteristics in the program/erase and the V-PASS and V-PGM disturbs because the external electric field is so high that the internal depolarization field does not affect the V-TH shift. On the other hand, in the data retention, the V-TH shift of the three types of V-TH memory cells show different characteristics. The reliability of the Fe-NAND flash memory is best optimized in the zeroV(TH) memory cell. In the proposed zero V-TH Fe-NAND flash memory cell scheme, the measured V-TH shift due to the read disturb, program disturb and data retention decreases by 32%, 24% and 10%, respectively, compared with conventional positive V-TH Fe-NAND flash memory cell scheme. Contrarily, in the negative VTH memory cell, the V-TH shift during the data retention is 0.49 V and unacceptably large because of the depolarization field. The conventional positive V-TH memory cell suffers from a sever read and program disturb. The measured results are drastically different from those of the conventional floating-gate NAND flash memory cell where the negative V-TH memory cell is most suitable in terms of the reliability.