Early electromigration failure has been recognized as a generic phenomenon in W-plug via structures. Previous studies have attributed early failure to the properties of the Al layers in contact with the plug, via delamination or to current density distributions in the structure. In this paper, we investigate early failure phenomena in W-plug vias and report a novel early via failure induced by the presence of a crack crossing the lower metal level TiN anti-reflection coating layer in the vicinity of W-plug. The cracks in the TIN shunting layer may cause large resistance jumps of 250-1000 Omega. Resistance simulation results show that the magnitude of resistance jump increases with the size of the crack. The source of the TiN cracks is discussed.