In this work, we analyze the effect of CF4/O-2 plasma treatment on the contact interface between the amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) semiconductor and Titanium-Gold electrodes. First, the influence of CF4/O-2 plasma treatment is evaluated using transmission line structures and compared to pure O-2 and CF4 plasma, resulting in a reduction of the contact resistance RC by a factor of 24.2 compared to untreated interfaces. Subsequently, the CF4/O-2 plasma treatment is integrated in the a-IGZO thin-film transistor (TFT) fabrication process flow. We achieve a reduction of the gate bias dependent RC by a factor up to 13.4, which results in an increased current drive capability. Combined with an associated channel length reduction, the effective linear field-effect mobility mu(lin,FE,eff) is increased by up to 74.6% for the CF4/O-2 plasma treated TFTs compared to untreated reference devices.