Enhanced quality of tunnel oxide by in-situ screen oxide for embedded flash memory application

Abstract

We have developed flash-memory-integrated VLSI logic chips. The base technology is logic process and logic design system is fully utilized for this chip design. In the flash memory, tunnel oxide film plays very important roles for data retention, write, and erase operations. Especially data retention is one of the most important parameters. Flash memory data is stored as electrons in the conductor isolated by tunnel oxide. Therefore, data retention performance is mainly dominated by electron leakage property of the tunnel oxide film. In this paper, tunnel oxide electron leakage dependency on the following two process conditions is described One is the wafer position in the tunnel oxide growth furnace tube. Wafers loaded at the bottom of the furnace boat showed lower electron leakage. The other is in-situ screen oxide formation at tunnel oxide formation. In-situ oxidation just prior to the tunnel oxide growth forms 2-3 nm-screen oxide. This screen oxide reduces electron leakage current through tunnel oxide. Tunnel oxide surface roughness was measured by Atomic Force Microscope (AFM) to evaluate electron leakage through tunnel oxide. In-situ screened tunnel oxide showed smoother surface than that of non-screened tunnel oxide. Data retention performance is improved as the tunnel oxide surface get smoother.