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Coplanar waveguides (CPWs) with extremely low loss have been successfully developed on high-resistivity silicon (HR-Si) substrates as interposers of multichip modules for microwave and millimeter-wave bands. The attenuation constant of these CPWs on HR-Si is less than 1 dB/mm for frequencies up to 100 GHz, which is comparable with that of CPWs on semi-insulating compound semiconductor substrates. Conventional CPW structures show a larger attenuation constant due to the effects of the low-resistivity layer generated at the interface between the insulating layer of SiN and the HR-Si substrate, which has been detected through both experimental investigations and numerical calculations. A CPW structure that suppresses the effects of the low-resistivity layer is presented in this paper. The fabrication process is rather simple and can be smoothly integrated in conventional semiconductor device fabrication processes.