A Novel Structure Between WL Spaces to Improve the Retention Characteristics in 3D NAND Flash

Abstract

As NAND flash evolved from two-dimensional (2D) to three-dimensional (3D), all cells have been changed to share a charge trap layer (CTL). This change has a lateral charge spreading effect, which is the trapped charge spreading laterally. This lateral charge spreading effect causes a major problem in NAND flash reliability. In this study, we introduce a new structure that can improve lateral charge spreading by defining a new parameter called ‘intercell CTL thickness’ and modifying the CTL structure in the WL spaces. When the intercell CTL thickness decreases, the ISPP slope remains relatively constant up to a certain thickness, indicating that program efficiency does not decrease until that critical point. However, as the intercell CTL thickness decreases, the current also decreases, which can be explained by the screen effect and dielectric constant reduction. As for the thickness of the CTL, which is the trap nitride thickness, it decreases while the oxide thickness increases. As a result, it causes a decrease in the total dielectric constant, resulting a decrease in cell current. In addition, as the physical CTL thickness decreases, more charges will be trapped in the same $\text{V}_{\mathrm {TH}}$ condition. More charges strengthen the screen effect on the electric field, causing a decrease in cell current. In this study, we will discuss the retention characteristics of this novel structure, investigate the window characteristics between lateral charge spreading with cell current, and propose the optimal point.

Keywords

• 3D NAND
• lateral charge spreading
• charge trap layer
• intercell CTL thickness
• ISPP slope
• cell current