We have used x-ray photoelectron spectroscopy (XPS) to investigate the resist-patterns' transformations after exposure to resist trimming plasmas. The influence of the chemistry, the feed gas ratio, the bias power and the pressure have been studied for a Cl-2/O-2 and a HBr/O-2 trim chemistry. An experimental procedure based on the chemical-topography analysis of resist patterns using XPS has been set up to determine the chemical composition and thickness of the reactive layers that are formed both on tops and sidewalls of the resist features during the resist trimming process. The resist modifications obtained by XPS have been correlated with the process performance (trim rate) obtained by SEM observations. In this study, we show that the main difference between HBr/O-2 and Cl-2/O-2 trimming processes is attributed to the difference in reactivity between Br and Cl radicals with the resist during plasma exposure. We succeed in explaining the impact of the plasma parameters (oxygen percentage in the gas phase, bias power, pressure) on the Cl-2/O-2 trimming process. A decrease in trim rate is well correlated with the formation of a thick and chlorine-rich layer formed on the resist sidewalls. (C) 2004 American Vacuum Society.