In this work, the use of silicon rich oxide (SRO) and chemical vapor deposition SiO2 double layers as passivation films of coplanar waveguides (CPW) on high resistivity silicon (HR-Si) with an N+ backside is studied. The microwave performance of the fabricated CPWs is evaluated by computing the attenuation loss of the devices in the 0.045–50 GHz frequency range. Experimental results show that the N+ layer can be used without affecting CPW performance. Also, using a combined dielectric layer (SRO20/SiO2), the attenuation losses are reduced compared to single dielectric layers.