Nitride framed shallow trench isolation (NFSTI) for self-aligned buried strap in high performance trench capacitor DRAM/eDRAM

A self-aligned buried strap process is developed, using nitride frame with oxide hard mask in shallow trench isolation (STI). The connection between cell access transistor and storage node electrode is a key process in trench type DRAM fabrication. Typical trench cell capacitor DRAM technology forms the strap connection under Si substrate (Buried Strap) for better surface planarity. Trench based e-DRAM has significant advantages due to wafer planarity. As the ground rule shrinks beyond 150 nm, the strap resistance variation is critical due to the overlay sensitivity. A new overlay independent strap formation method is introduced, using nitride framed self-aligned trench isolation process which eliminates any possible parasitic connection between the strap and substrate. Masking material and Si RIE process used in NFSTI formation improves array device characteristics. In addition, NFSTI process improves trench level alignment signal contrast due to a phase shift effect.