Evaluation of the High-Frequency Transmission Characteristics of Coplanar Transmission Lines Fabricated on a Copper-Plated Cycloolefin Polymer Using a Direct Sputtering Copper Seed Layer
Akihiro Shimizu 1, 2, Kazuhiro Fukada 3, Mitsunori Abe 4, Akiko Matsui 4, and Shinichi Endo 1
1 Ushio Inc. (Japan)
2 Gifu University (Japan)
3 Shibaura Machine Co., Ltd. (Japan)
4 NTT DevIces Cross Technologies Corporation (Japan)
This study investigated how seed layer formation methods—direct copper sputtering and electroless copper plating—affect the high-frequency transmission characteristics of coplanar transmission lines fabricated on copper-plated cycloolefin polymer (COP) films, up to 100 GHz. The direct copper sputtering method exhibited slightly lower transmission loss than the electroless copper plating method below 60 GHz. However, the transmission loss was markedly lower with the direct copper sputtering method above 60 GHz. At 100 GHz, the transmission loss was measured to be −0.817 dB/cm for direct copper sputtering method and −0.977 dB/cm for electroless copper plating method. This performance disparity is primarily attributed to the skin effect at high frequencies, where current tends to concentrate near the conductor surface. Above 60 GHz, in the case of direct copper sputtering method, the current flows predominantly through the electroplated copper layer, which offers superior conductivity. Conversely, with electroless copper plating method, the current is confined largely to the low-conductivity seed layer, resulting in greater transmission loss.
