This study improved the thermal processing stability of Si-based coplanar waveguides (CPW) through Ar ion implantation with rapid thermal annealing (RTA). Ar ion implantation damaged the surface layer of the high-resistivity silicon substrate, which decreased attenuation of the CPW line. However, the damaged layer recrystallized during a high temperature process, which caused thermal processing instability. A RTA process was performed to retard the epitaxial regrowth from the substrate, this improved thermal processing stability and decreased dc-voltage dependence of the attenuation of the CPW line. These improved properties were found to be due to the RTA process retaining more bulk traps within the damaged layer.Index Terms-Coplanar waveguide (CPW), microwave loss, surface passivation, thermal processing stability, thermally-induced electrical instability.