METHOD FOR PROVIDING IMPROVED STEP COVERAGE OF DEEP TRENCHES AND USE THEREOF

Disclosed is a method for providing improved step coverage of deep trenches. A hard mask which constitutes a bottom silicon oxide layer and a top silicon nitride layer is formed on a substrate and patterned to form a opening. A deep trench extending into the substrate is formed through the opening. After both hard mask and substrate are patterned, HF vapor is performed to selectively etch away portions of hard mask. Then a deep trench with a ladder-like hard mask which has improved step coverage is obtained.