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)

Abstract

This paper presents an improvement of the memory cell reliability by the memory cell VTH optimization of the ferroelectric (Fe)-NAND flash memory. The effects of the memory cell VTH on the reliability of the Fe-NAND flash memory are experimentally analyzed for the first time. The reliability is evaluated by the measured VTH shift due to the read disturb, program disturb and data retention. Three types of Fe-NAND flash memory cells, a positive, zero and negative VTH memory cell, are defined on the basis of the memory cell VTH. The middle of VTH of programmed and erased states is 1V, 0V and -0.3V in a positive, zero and negative VTH memory cell, respectively. The VTH shift of the positive, zero and negative VTH memory cells show similar characteristics in the program/erase and the VPASS and VPGM disturbs because the external electric field is so high that the internal depolarization field does not affect the VTH shift. On the other hand, in the data retention, the VTH shift of the three types of VTH memory cells show different characteristics. The reliability of the Fe-NAND flash memory is best optimized in the zeroVTH memory cell. In the proposed zero VTH Fe-NAND flash memory cell scheme, the measured VTH shift due to the read disturb, program disturb and data retention decreases by 32%, 24% and 10%, respectively, compared with conventional positive VTH Fe-NAND flash memory cell scheme. Contrarily, in the negative VTH memory cell, the VTH shift during the data retention is 0.49V and unacceptably large because of the depolarization field. The conventional positive VTH 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 VTH memory cell is most suitable in terms of the reliability.