The process flow utilizes a multi-layer masking scheme to pattern the Shallow Trench Isolation (STI) regions and define the boundaries of the transistors T1.Following photolithography, the "Oxide Etch" step transfers the resist pattern into the underlying deposited SiO hard mask P1.This thick oxide layer is required because the photoresist alone cannot withstand the prolonged plasma exposure needed for the subsequent deep silicon trench etch P2.By opening the oxide hard mask first, the sequence