A layer structure, chemical composition and cleaning process of oxide dry etch residue on silicon substrate were studied. It was observed that the structure of the etch residue consisted of CF_x-polymer/ SiO_yC_z. The ratio of y and z in SiO_yC_z layer is monotonously changing with depth : y/z is maximum at the interface of CF_x-polymer and SiO_xC_z. and minimum at the interface of SiO_yC_z and silicon substrate. Two step cleaning was proposed : dry and wet cleaning. The most effective process was silicon light etch(CF₄/O₂ REMOTE PLASMA), followed by NH₄OH-H₂O₂ mixture and HF dip. From X-Ray Photoelectron Spectroscopy(XPS) data, it was found that oxide etch residue was completely removed by dry and wet cleaning.