The sidewall passivation films produced during Si trench etching with photoresist masks were examined in two etching systems, helicon and inductively coupled plasma (ICP). The trench profiles obtained from the ICP system were more tapered with thicker sidewall films compared to those from the helicon system. The trench etch profiles with Cl-2/HBr/O-2 plasma in the ICP system appeared to be double-sloped. X-ray photoelectron spectroscopy (XPS) analysis indicated that N-2 addition to Cl-2/HBr/O-2 plasma induced the formation of Si-N bonds in the sidewall films, in addition to the Si-O and Si-Br bonds observed with Cl-2/HBr/O-2. The sidewall films formed in Cl-2/HBr/O-2 plasma showed higher oxygen intensities and chemical binding energies compared to those formed in Cl-2/HBr/N-2/O-2 plasma.