Multimode analysis of transmission lines and substrates for (sub)mm-wave calibration

In this contribution we present a numerical and experimental analysis of the multimode propagation over coplanar transmission lines, in order to define guidelines for substrate selection to achieve accurate wafer-level (sub)mm-wave calibrations. Losses and coupling effects resulting from the multiple propagating modes on transmission lines conventionally employed for probe-level calibrations, are analysed by means of 3D electro-magnetic simulations. A low loss-tangent, low dielectric constant material (i.e., fused silica), providing low radiation losses due to mode coupling, is proposed as substrate to achieve accurate (sub)mm-wave calibrations. The structures required to perform a TRL calibration are integrated on the proposed fused silica substrate and the experimental data of probe-level calibration performed in the 220-325GHz band are presented.