Notching in un-doped amorphous silicon gate etching, a profile distortion phenomenon, has been studied and analyzed with low pressure, high density plasmas using HBr/Cl2/O2 chemistries. In contrast with the notching phenomenon observed in doped polysilicon etching, a notch was evolved at the outer sidewall foot of the edge line, where the outer sidewall of the edge line was exposed to a microscopic open area. A considerable recess was observed at the upper corner of a trench etched silicon substrate in photoresist-masked gate etching, while an oxynitride-masked gate structure showed little silicon recess at the same region. The subsequent accumulation of ions caused by deflected ions on the oxide surface adjacent to the outermost gate line leads to the formation of a local electric field near the outermost gate line, intensifying the ion trajectory distortion toward the sidewall foot of the outermost gate line.