Preparation of High-Speed Signal Transmission Line on Thin Polyimide Layer By All-Wet Process
Authors: Yokoshima Tokihiko , Kagawa Hiroki , Hachisu Takuma , Sugiyama Atsushi , Osaka Tetsuya , Xu Lei , Watariguchi Shigeru , Tajima Kazutaka , Kikuchi Katsuya , Aoyagi Masahiro 
Published: February 2020

Polyimide materials are widely used for printed circuit board as substrate/insulation layer because of its high mechanical stability, high electrical insulation properties, high heat resistance, etc. Low dielectric constant and superior flat surface are requested for applying to high speed signal circuit. Applying the polyimide film to high-speed circuit board, high adhesion length and superior flat interface between metal wiring and polyimide is needed. Our group proposed an all-wet process to fabricate the diffusion barrier layer and Cu wiring by using an organosilane layer as an adhesive/catalyst layer [1, 2]. Additionally, wet processes offer relatively simple procedures and tools, and usually are cost effective. In previous study, an immobilized aminosilane film on polyimide works as an adhesion layer and a catalyst supporting layer for electroless deposition of NiB. This process showed superior smooth surface of catalyzed polyimide because of without etching processes. Adhesion of electroless-deposited NiB film on superior smooth polyimide is a particular need for realizing a high-density printed circuit board [3,4]. In this investigation, this technique was applied for preparation of the silicone chip substrate called interposer. The high-speed signal transmission line for interposer was developed by our proposed method.
Soluble block copolymerization polyimide (Q-VR-X1014, PI R&D CO.,LTD) was used for polyimide film source, and thin polyimide layer with and superior flat surface was prepared on glass substrate by spin-coating. After drying, hydrophilic surface of polyimide was developed by UV-irradiation in 10 min. Aminosilane layer in the polyimide was formed in toluene solution containing aminocilane molecule at 60 oC, 10 min. Then, modified surface was catalyzed by palladium, and thin metal layer as seedlayer/adhesion layer and conductive layer were formed by electroless NiB deposition and Cu electrodeposition, respectively. Cu electrodeposition was carried out using alkaline electrolytes bath (Meltex Inc).
First, Cu electrodeposition on polyimide was investigated. Figure 1 shows schematic illustration of electrodeposited Cu film on aminocilane-modified polyimide film. To form Cu conductive layer using electrodeposition with conventional acidic bath, film peeling between Pd/aminosilane interfaces was observed. Acid condition was affected for Pd catalyst layer, and Pd was dissolved from the aminocilane supporting layer. We therefore used the alkaline electrolytes bath for Cu electrodeposiotion. We optimized operating condition of electrodeposition and thermal annealing to decrease electronic conductivity, adhesion strength. Using this optimized condition, Cu film with 2 μΩcm could be obtained.
Second, roughness of the surface and its interface was measured. Figure 2 shows cross sectional TEM image of electrodeposited Cu film on aminosilane-modified polyimide film after annealing. Roughness of interface between polyimide and NiB was not observed and superior smooth interface was obtained using this technique. Moreover, surface roughness of deposited Cu film was very smooth of Rq = 14 nm.
Final, a coplanar waveguide as high-speed signal transmission line was fabricated by this technique. Cu wiring was prepared by semi-additive method. Line width was 100 μm, and film thickness and space between grand and signal line was varied. The characteristic impedance of the fabricated coplanar waveguide showed almost the same value of the theoretical designed one that was 50 Ω.
References
[1] T. Osaka, M. Yoshino, Electrochim. Acta, 53, 271 (2007).
[2] M. Yoshino, T. Masuda, T. Yokoshima, J. Sasano, Y. Shacham-Dimand, I.Matsuda, T. Osaka, A. Hashimoto, Y. Hagiwara, I. Sato, J. Electrochem. Soc, 154, D122 (2007)
[3] T. Osaka, S. Wakatsuki, T. Masuda, M. Yoshino, N. Yamachika, J. Sasano, I. Matsuda, Y. Okinaka, Electrochemistry, 76, 191 (2008).
[4] T. Osaka, T. Yokoshima, H. Kagawa, T. Hachisu, A. Sugiyama, Meeting abstract of 227th ECS Meeting, No. 1133 (2015).

Figure 1