Residues appearing on the surface of the TIN antireflective coating (ARC) layer after a resist ashing process were investigated by transmission electron microscope and field emission Auger electron spectroscopy. These residues tend to appear linearly in the center of Al lines covered with the TiN ARC layer, and are generally called the "backbone." We found that a triangular product mainly composed of Al atoms appears in the center of the resist surface in the final stage of the ashing process and that a thin layer, about 10 nm thick, including Ti atoms, is formed to cover the triangular product and the resist surface. Due to the small solubility of the thin layer including Ti atoms to the alkaline stripper, the triangular product under the thin layer remains as a backbone residue. We demonstrate that the formation of the backbone depends on the distribution of Al and Ti atoms in the sidewall polymer.