Open Block Characterization and Read Voltage Calibration of 3D QLC NAND Flash

Abstract

3D QLC NAND has recently entered the SSD market offering capacity increase and cost reduction compared to 3D TLC NAND. However, the endurance of QLC NAND is limited. Moreover, due to reduction of the available margin between the programmed threshold voltage distributions, QLC NAND is more susceptible to bit errors. Read voltage calibration is a key element of modern NAND flash memory controllers to improve the overall bit-error rate and maintain enterprise level reliability. To reduce the calibration overhead associated with the increased number of pages and read voltages in QLC NAND, page grouping is an effective approach. This paper presents open block characterization and read voltage calibration results of state-of-the-art 3D QLC NAND. We present experimental measurements of the bit-error characteristics and threshold voltage distributions based on closed and open block test patterns. We discuss the reliability issues with open blocks in preserving uniform characteristics within a page group at the boundary programmed layer and analyze the performance of different calibration algorithms.