Scaling challenge of Self-Aligned STI cell (SA-STI cell) for NAND flash memories

This paper describes scaling limitations and challenges of Self-Aligned STI cell (SA-STI cell) over 2X–0X nm generations for NAND flash memories. The scaling challenges are categorized to (1) narrow Read Window Margin (RWM) problem, (2) structural challenge, and (3) high field (5–10 MV/cm) problem. First, (1) the narrow RWM is investigated by extrapolating physical phenomena of FG–FG coupling interference, Electron Injection Spread (EIS), Back Pattern Dependence (BPD), and Random Telegraph Noise (RTN). The RWM is degraded not only by increasing programmed Vt distribution width, but also by increasing Vt of erase state mainly due to large FG–FG coupling interference. However, RWM is still positive in 1Z nm (10 nm) generation with 60% reduction of FG–FG coupling by air-gap process. For (2) structural challenge, the Control Gate (CG) fabrication margin between Floating Gates (FGs) is becoming much severer beyond 1X nm generation. Very narrow 5 nm FG width/space has to be controlled. For (3) high field problem, high field between CGs (word lines; WLs) is critical during program. By using WL air-gap, high field problem can be mitigated, and 1Y/1Z nm generation seems to be realized. Therefore, the SA-STI cell is expected to be able to scale down to 1Z nm (10 nm) generation, with the air gap of 60% reduced FG–FG coupling interference and an accurate control of FG/CG formation process.

► Scaling limitations of SA-STI cell is studied for 2X–0X nm NAND flash.
► Read Window Margin, structural challenge, and high field problem are investigated.
► The SA-STI cell is expected to be able to scale down to 1Z nm (10 nm) generation.
► Air gap of 60% FG–FG coupling reduction and ∼5 nm width FG/CG process are needed.