The interface between bottom anti-reflective coatings (BARCs) and a model deprotected photoresist, poly (4-hydroxystyrene) (PHS), was investigated using near edge x-ray absorption fine structure spectroscopy to identify mechanisms responsible for pattern degradation at the BARC/ photoresist interface. Interactions at this interface can lead to pattern deviations such as footing, undercut, and pattern collapse. It was found that a residual layer is only formed when the bilayer is subject to ultraviolet exposure. The spectra of the BARC surfaces after photoresist processing and development show a combination of spectral features from both PHS and the BARC formulations. The data suggest that the residual layer results from interactions between crosslinker and photoresist that occur during normal photoresist processing. (C) 2003 American Vacuum Society.