Comparison of High-Frequency Transmission Characteristics of Transmission Lines on Copper-Plated Cycloolefin Polymer Using Directly Sputtered Copper Seed Layer and Electroless Copper Plating Seed Layer

We have developed a technology to form a copper plating layer using a directly sputtered copper seed layer without an adhesion layer on cycloolefin polymer (COP) film for application in antenna circuits for 6G communications. This study compared the high-frequency transmission characteristics up to 100 GHz in coplanar transmission lines on copper-plated COP films using either a directly sputtered copper seed layer (sputtered type) or an electroless copper plating seed layer (electroless type). Our findings indicate that the sputtered type exhibits superior transmission loss characteristics compared to the electroless type, particularly beyond 60 GHz. This difference is attributed primarily to the skin effect at high frequencies, where current primarily flows through the copper plating layer for the sputtered type, compared to the electroless type, where it flows through the electroless copper plating layer, which has significantly lower conductivity.