Development and characterization of cyclic olefin copolymer thin films and their dielectric characteristics as CPW substrate by means of Terahertz Time Domain Spectroscopy

We describe recent advances in the fabrication of cyclic olefin copolymer (COC) thin films and their electrical characteristics between 0.3 and 2 THz (terahertz). COC films were fabricated by dissolving the raw material in toluene at different concentrations. Three different techniques were used to fabricate thin solid films of COC: Spin Coating (SC), in-house developed technique Vacuum Solvent Desorption (VSD), and Thermic Solvent Desorption (TSD). Film characteristics such as roughness, thicknesses, and homogeneity were compared. These studies were performed to establish the technique that ensures the lowest roughness, highest homogeneity, and best control over thin film thickness. We also investigated the effect of film roughness and homogeneity on dispersion and losses related to the complex dielectric function ε(f). Then, we used Terahertz Time-Domain Spectroscopy (THz-TDS) characterization up to 2 THz to obtain the dielectric function of COC thin films fabricated by the three methods, and we compare these values with values in literature and state-of-the-art polymeric material BCB. We found that COC has low dispersion (the real part of the dielectric function is almost constant with frequency up to 2 THz), low losses regardless of the fabrication technique, and negligible losses due to roughness and non-homogeneity. Finally, we report the design of a coplanar waveguide, its loss characterization (0.1–0.6 THz), and the microfabrication process on a COC substrate. These results clearly demonstrate that COC films are serious contenders for developing low-loss and low-cost THz electronic devices and optoelectronics.