Methods for forming a low resistivity ruthenium (Ru) thin film that include depositing ruthenium onto a substrate via ion beam deposition with assist ion beam in a process chamber having reactive and noble gas species therein. The substrate is at at least 250° C. A resulting thin ruthenium film has a thickness of no more than 30 nm, a resistivity less than 12 μΩ-cm and a crystalline structure comprising grains having a (0001) orientation. The resistivity will differ at different thickness; for example, less than 9 μΩ-cm for films of 50 nm and thicker, less than 9.5 μΩ-cm for films of 35 nm and thicker, less than 11 μΩ-cm for films of 20 nm and thicker, less than 15 μΩ-cm for films of 10 nm and thicker or less than 20 μΩ-cm for films of 2 nm and thicker. The grains have a mean grain size at least three times the film thickness.