A self-aligned surface MgO layer was used as a mask for dry etching the Cu(Mg) alloy films using an O2 plasma and H(hfac) chemistry. The surface MgO layer was formed by diffusion of Mg from Cu film to the free surface. Cu(4.5at%Mg) film having thickness of 300nm was annealed in O2 ambient at 10 mTorr, 500°C for 30min, followed by the patterning of the MgO layer using photolithography and HF wet etch process. The patterned MgO layer successfully served as a hard mask for dry etching the Cu(Mg) alloy films with a taper slope. In addition, the high quality self-aligned MgO layer was formed upon annealing the Cu(4.5 at.% Mg) alloy films at the low temperature of 300°C. Furthermore, the surface MgO layer grown on Cu(2.3at.% Mg) alloy films at 500°C withstood as a hard mask for dry etching, and thus achieving the patterned copper alloy lines with the low resistivity of 2.2 μΩ-cm. Consequently, this novel etch process using a self-aligned MgO mask can be used for patterning the low resistivity copper alloy lines with a low thermal budget, which is suitable for large-size TFT-LCDs.