Program schemes for multilevel flash memories

This paper presents a synthetic overview of multilevel (ML) flash memory program methods. The problem of increasing program time with the number of bits stored in each cell is discussed and methods based on both channel hot electrons (CHE) and Fowler-Nordheim tunneling (FNT) are discussed. In the case of CHE, the use of an increasing voltage rather than a constant one on the control gate (CG) leads to narrower threshold voltage distributions and smaller current absorption, with positive effects on the degree of parallelism and program throughput. As for FNT, much faster programming than that commonly used today can be done using high CG voltages without producing intolerable degradation of cell reliability.