Read and Write Voltage Signal Optimization for Multi-Level-Cell (MLC) NAND Flash Memory

Abstract

The multi-level-cell (MLC) NAND flash channel exhibits nonstationary behavior over increasing program and erase (PE) cycles and data retention time. In this paper, an optimization scheme for adjusting the read (quantized) and write (verify) voltage levels to adapt to the nonstationary flash channel is presented. Using a model-based approach to represent the flash channel, incorporating the programming noise, random telegraph noise (RTN), data retention noise and cell-to-cell interference as major signal degradation components, the write-voltage levels are optimized by minimizing the channel error probability. Moreover, for selecting the quantization levels for the read-voltage to facilitate soft LDPC decoding, an entropy-based function is introduced by which the voltage erasure regions (error dominating regions) are controlled to produce the lowest bit/frame error probability. The proposed write and read voltage optimization schemes not only minimize the error probability throughout the operational lifetime of flash memory, but also improve the decoding convergence speed. Finally, to minimize the number of read-voltage quantization levels while ensuring LDPC decoder convergence, the extrinsic information transfer (EXIT) analysis is performed over the MLC flash channel.