Impact of Top-Surface Tunnel-Oxide Nitridation on Flash Memory Performance and Reliability

Two approaches to top-surface nitridation of tunnel oxide, i.e., rapid thermal nitridation using NH₃ anneal and decoupled plasma nitridation, are compared. Floating-gate MOS capacitors with source/drain were used to evaluate Flash memory performance and reliability. Tunnel-oxide NH₃ anneal degrades postcycling retention performance compared to plasma nitridation for the same equivalent oxide thickness reduction. The poorer performance of NH₃ anneal is related to higher N incorporation into SiO₂ bulk rather than top surface. Postcycling memory erase-level shift and memory window (MW) closure is lower for plasma nitridation compared to NH₃ anneal. A new integration scheme using plasma nitridation followed by NO anneal produces the lowest MW closure with cycling.