Valid Window: A New Metric to Measure the Reliability of NAND Flash Memory

NAND flash memory has been widely adopted in storage systems today. The most important issue in flash memory is its reliability, especially for 3D NAND, which suffers from several types of errors. The raw bit error rate (RBER) when applying default read reference voltages is usually adopted as the reliability metric for NAND flash memory. However, RBER is closely related to the way how data is read, and varies greatly if read retry operations are conducted with tuned read reference voltages. In this work, a new metric, valid window is proposed to measure the reliability, which is stable and accurate. A valid window expresses the size of error regions between two neighboring levels and determines if the data can be correctly read with further read retry. Taking advantage of these features, we design a method to reduce the number of read retry operations. This is achieved by adjusting program operations of 3D NAND flash memories. Experiments on a real 3D NAND flash chip verify the effectiveness of the proposed method.