Variable program and program verification methods for a virtual ground memory in easing buried drain contacts

Abstract

Methods for programming and program verification of a flash memory are described that ease the buried drain contact induced operation and increase the retention window. In a first aspect of the invention, a program operation method provides varying program biases which are applied to different groups of memory cells. The program biases can be supplied as drain bias voltages or gate bias voltages. The program biases vary depending on which group of memory cells is programmed. In one embodiment, a first drain voltage Vis applied to the first group of memory cells Mand M. A second drain voltage Vis applied to the second group of memory cells Mand M, where V=V+ΔV. In a second aspect of the invention, a plurality of program verification voltage levels are selected to verify that the memory cells pass the program voltage levels.