Reliable via formation requires complete polymer removal in vias prior to barrier deposition to obtain low electrical resistance contacts between metal conductor layers. A failure mechanism of high resistance and open vias was observed due to aluminum polymer in the via either partially or fully blocking barrier deposition onto the metal layer at the via bottom. This failure mechanism involved a breach of the titanium-nitride (TiN) anti-reflective coating (ARC) layer over the aluminum in the underlying conductor metal layer, allowing via clean chemistry to react with the aluminum. An aluminum polymer is formed, partially or fully blocking barrier deposition. Experimental data show that open via occurrences worsened with increased sit-time between via clean and the via barrier deposition step. In this paper we will discuss the mechanism causing high resistance or open via failures due to incomplete hydroxylamine (HDA) chemistry removal.