Growth of Metal Nucleus on Polyimide Surface with Photochemical Reaction
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ABSTRACT Circuit patterned nucleation of copper or nickel atoms onto polyimide surface was demonstrated by using ArF excimer laser (A=193nm) and copper sulfate aqueous solution (CuSO 4.5H20+nH20), or nickel sulfate aqueous solution (NiSO 4 -6H2 0+nH 2O) in an atmospheric pressure. Photo-excited C-H bonds of the polyimide surface were effectively dehydrated with hydrogen atoms which were photodissociated from the water. The dangling bonds of the dehydrated hydrogen atoms were combined with the oxygen and the copper or nickel atoms that were photodissociated by CuSO 4 solution or NiSO 4 solution. Thus, C-O-Cu bonds or C-O-Ni bonds were formed on the surface. After this copper and nickel substitution, it was carried out onto the nucleated parts by usual electroless plating at 70 0C. In this process, the conductive circuit patterned copper or nickel thin film was deposited on polyimide surface.
INTRODUCTION All aromatic group polyimides have advantages, which are higher mechanical strength, flame resistance, chemical resistance and heat resistance than other plastics. Especially, this material has over 500 'C heat resistance, and shows good dimensional stability between -269 °C to +400 'C. Because of these advanced characteristics, this material is used for bearing material and as electrical insulating material and X-ray radiation resistance film in aerospace and nuclear engineering applications. Unfortunately, it is very difficult to have surface modification on the surface of all aromatic group polyimides, because of these advanced properties. There are known methods to modify the surface of the plastics. Laser-ablation and photochemical methods to modify the surface of polyimide have been reported [1-5]. Our group has been focused on photochemical methods using a UV light source [6-8]. Our group is interested in using an all aromatic group polyimide which is advanced in heat resistance rather than a thermo-hardening polyimide. We found a method to have Cu or Ni thin-films on the surface of all aromatic group polyimide with photochemical surface modification. We are interested in water-absorbency of polyimide. To use CuSO 4 or NiSO 4 water solution for material solutions, we could run a metal substitution reaction effectively. On the other hand, polyimide has a high adsorption band at 193nm. Therefore we can substitute metal nuclei. In this paper, we report optimization of laser fluence and concentration of material solution, to decrease the number of required laser pulses down to 1 pulse. We could confirm effective substitution reactions of Cu or Ni nuclei onto polyimide surface. This process is a purely chemical binding of Cu or Ni and the polyimide surface. We expect all aromatic group polyimide printed circuit boards, treated by this method, could show improved heat resistance and less noise in high frequency use.
PHOTOCHEMICAL REACTION Fig. 1 shows the principle of photochemical reaction. An ArF laser having photon energy 197
Mat. Res. Soc. Symp. Proc. Vol. 585 © 2000 Materials Research Society
of 147 kcal/
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