Ru thin films were etched using CF/$_4$O$_2$ plasma in an ICP (inductively coupled plasma etching) system. The maximum etch rate of Ru thin films was 168 nm/min at a CF$_4$/O$_2$ gas mixing ratio of 10 %. The selectivity of SiO$_2$ over Ru was 1.3. From the OES (optical emission spectroscopy) analysis, the optical emission intensity of the O radical had a maximum value at 10% CF$_4$ gas concentration and drcrease with further addition of CF4 gas, but etch slope was enhanced. From XPS (x-ray photoelectron spectroscopy) analysis, the surface of the etched Ru thin film in CF$_4$/O$_2$ chemistry shows Ru-F bonds by the chemical reaction of Ru and F. RuF$_{x}$ compounds were suggested as a surface passivation layer that reduces the chemical reactions between Ru and O radicals. From a FE-SEM (field emission scanning electron microscope) micrograph, we had an almost perpendicular taper angle of 89$^{circ}$.>.