Effects of halogens, CF4, Cl-2, or HBr, on the photoresist trimming in high-density oxygen plasmas for sub-0.1-mum device fabrication were studied in an inductively coupled plasma (ICP) etcher. The trim rates were measured as a function of halogen gas percentages. The activation energy and the resulting resist profiles were investigated as well, showing that the CF4/O-2 gives the highest trim rate, followed by HBr/O-2 and then Cl-2/O-2 at the same amount of additive gas in the mixture. Effects of different plasma chemistry on the chemical constituents of the resist sidewall films were also examined with the angle-resolved X-ray photoelectron spectroscopy (XPS). XPS analysis reveals that all halogen gases are useful for resist sidewall protection because of the passivation by the halogen-containing polymer. The effects of halogen addition include the reaction enhancement, reaction-site competition between oxygen and halogen, changes in the plasma gas chemistry, and the resist passivation. (C) 2004 American Institute of Chemical Engineers AIChE J, 50: 1578-1588, 2004