Innovative Programming Approaches to Address Z-Interference in High-Density 3D NAND Flash Memory

Abstract

Increasing the bit density in 3D NAND flash memory involves reducing the pitch of ON (Oxide-Nitride) molds in the Z-direction. However, this reduction drastically increases Z-interference, adversely affecting cell distribution and accelerating degradation of reliability limits. Previous studies have shown that programming from the top word line (WL) to the bottom WL, instead of the traditional bottom-to-top approach, alleviates Z-interference. Nevertheless, detailed analysis of how Z-interference varies at each WL depending on the programming sequence remains insufficient. This paper investigates the causes of Z-interference variations at Top, Middle, and Bottom WLs through TCAD analysis. It was found that as more electrons are programmed into WLs within the string, Z-interference variations increase due to increased resistance in the poly-Si channel. These variations are exacerbated by tapered vertical channel profiles resulting from high aspect ratio etching. To address these issues, a method is proposed to adjust bitline biases during verification operations of each WL. This method has been validated to enhance the performance and reliability of 3D NAND flash memory.