A reduction in contact resistance due to reactive ion etching using an O-2 plasma is reported. This treatment-can reduce the contact resistance to the same level as when chemical dry etching is used, which is an isotropic etching process using fluorine-containing gases such as CF4. The mechanism of this reduction was investigated using X-ray photoelectron spectroscopy, transmission electron microscopy, and thermal wave modulated reflectance. Removal of a thick modified layer which contains carbon, fluorine, and oxygen reduces the contact resistance. Further; the influence of O-2 plasma treatment time and bias radio frequency power on contact resistance is reported. An increase in contact resistance at high radio frequency power shows that there is reformation of the damaged layer by O-2 plasma treatment, and that it is important to control the removal and reformation of the damaged layer.