The deposition and adhesion of copper films on smooth polymer surfaces

Abstract

Electroless copper plating (ECP) of polymeric surfaces has found several important applications, such as the metallization of epoxy glass laminates (EGL) employed in the production of printed circuits. In this process EGL boards are degreased and then prepared for metallization by preliminary oxidative etching with chromic acid and then surface activation for ECP. Such activation is typically achieved with catalysts derived from palladium and stannous chlorides or by the use of different photochemical processes [1-4]. The activated boards are then metallized in an electroless copper bath, which contains a copper source, a complexing agent, a reducing agent, a stabilizer and a pH buffer [5]. In order to produce printed circuits on microscopically smooth polymer surfaces, capable of high-frequency signal transmission, we have investigated the possibility of forming adherent copper films on polymer surfaces, which have not been subjected to chemical or mechanical roughening. In this study we have evaluated the suitability of several commercial polymers and blends of polymers for this purpose. We have been able to demonstrate that chemical interaction between the copper film and certain polymer surfaces contributes to the adhesion exhibited by the copper. The substrates chosen for electroless copper plating were thin glossy polymeric films cured on EGL boards. Brominated epoxy and Novolac resins (DER-531 from DOW Chemical Co. and EPIPHEN ER-825-A from Monomer Polymer Laboratories) were used as unblended polymer coatings. These epoxy resins were also blended with poly(4-vinylpyridine) (P4VP) (molecular mass 20 000) at weight ratios of epoxy to P4VP of 10:1, 5:1, 2:1, 1:1, 1:2 and 1:4 and these blends were then spin-coated on EGL boards and cured. The addition of P4VP to the epoxy resin aimed at increasing the copper adhesion to the smooth surface, since the pyridine group was expected to be the preferred binding site for copper [6, 7]. The resulting coatings were glossy and without microdefects as observed by optical and scanning electron microscopy (SEM). The thickness of the layers were estimated by SEM analysis to be about 5 gm. Freshly cured coatings were activated for electroless plating by a photochemical process [3, 4]: a solution of the metal salt (e.g. copper acetylacetonate), photosensitizer and solvent was applied to the polymer surface, which was then irradiated with