Testing Dielectric Slab Mode Excitation, Non-Rectangular Conductor Profiles and Edge Roughness as Sources of Additional Loss in mmWave Transmission Lines

Losses in mmWave transmission lines often exceed first-principles predictions based on measurements of dc-resistivity and a rectangular conductor geometry. In our case, we observed an additional resistance of coplanar waveguides on DyScO₃ substrates. Here, we test three hypotheses concerning the source of the additional resistance: dielectric slab mode excitation, non-rectangular conductor profiles, and conductor edge roughness. We compare the ac-resistance and inductance of coplanar waveguides on DyScO₃ substrates mounted on metallic and dielectric chucks. To test the non-rectangular conductor profiles and conductor edge roughness, we implement results from atomic force microscopy of the conductor edge in simulations. Our experiments showed no evidence that the slab mode impacted the additional resistance. Rather, we found that conductor edge profile and roughness may account for some observed additional resistance. We expect these findings to impact the choice between stepper lithography and microlens array lithography for rapid prototyping and multiproject wafers in industrial processes.