Thin Film Transmission Lines on Low-k Polymer Films for Sub-THz Applications

Low-k (εr ≤ 2.5) polymers are gaining significant attention for their inherent ability to reduce parasitic capacitance and enhance the efficiency of passives and interconnects in millimeter-wave (mmWave) and sub-THz frequency applications. This paper investigates the performance of transmission lines on various low-k polymer films (≤5 µm) upto 220 GHz. Conductor backed coplanar waveguide (CB-CPW) and microstrip (MS) lines were fabricated on a substrate to benchmark the performance of the different materials. The stackup consists of copper signal layer on the top, target low-k polymer dielectric on the inter-layer and copper ground plane on a semiconductor substrate. The samples were fabricated using semi-additive processes (SAP). De-embedded measurement results show strong correlation with simulations, with measured insertion losses ranging from 0.7 to 1.5 dB/mm at 220 GHz. The performance comparison of these low-k samples is compared with other materials in literature.