During silicon gate etching in low pressure high density HBr/Cl-2/O-2 plasma, SiOClx layers are deposited on the reactor walls. These layers are at the origin of process drifts. However their chemical composition, deposition mechanism and their influence on the plasma chemistry remains poorly understood. In this study, the chemical composition of this layer has been investigated by a "plasma etching-sputtering" technique: the silicon oxychloride layer deposited on the reactor walls during the etching of a 200 mm diam silicon wafer has been subsequently submitted to an Ar plasma with the addition of a few % SF6. During the slow etch process of this layer, time-resolved optical emission spectroscopy and mass spectrometry have been used to follow the time evolution of the gas phase concentration of different atoms and radicals, the etch products of the SiOClx layer. The results give insight into the chemical nature of the deposited layer and of its variation as a function of the depth. In particular, it will be shown that these layers are chlorine-rich and not oxidelike. Also, their composition is not homogeneous through their depth. In a second set of experiments, the SiOClx, layer has been exposed to an Ar/O-2 plasma providing information on the oxidation mechanism of the Si-Cl bonds and thus on the SiOClx film deposition mechanism in HBr/Cl-2/O-2 plasmas. This oxidation mechanism is acting through the entire volume of the 10 nm thick layers and proceeds by substitution of Cl atoms of SiCl bonds by O atoms, resulting in desorption of a large amount of Cl atoms from the chamber walls during the layer oxidation. Finally, the layer on the chamber walls has been exposed to an Ar/Cl-2 plasma, demonstrating that SiOClx layers are not etched significantly by Cl atoms. (C) 2004 American Vacuum Society.