Plasma–surface interactions occurring during overetch of polycrystalline silicon (poly-Si) gate etching with high-density HBr/O2 plasmas have been investigated by x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The temporal variation of the gate oxide thickness measured by XPS indicates that both deposition of etch by-products SiBrxOy and oxidation of Si substrate underlying thin SiO2 occur during exposure to HBr/O2 plasmas. In particular, significant deposition of SiBrxOy, presumably coming from reactor walls, was observed at the beginning of the overetch step. Furthermore, TEM observations revealed that the profile evolution of oxide-masked poly-Si gates during overetch is limited by a sidewall deposition layer, which acts as an oxide mask for etching of poly-Si. © 2001 American Institute of Physics.