Ever since a 3b/cell (TLC) NAND Flash memory became the mainstream in nonvolatile memory market, a new demand for a 4b/cell (QLC) NAND flash memory has been emerging for low-cost applications. However, QLC has inherently much longer page program time than TLC because of 16-state programming within a limited program and erase (PE) window, as well as narrower V th distributions. The longer page-program time, subsequently, degrades sequential write performance. Thus it is not possible to meet the required sequential-write performance in applications such as mobile devices and solid state drives (SSDs).
