Photoselective Catalysis of Electroless Copper Solutions for the Formation of Adherent Copper Films onto Polyimide

Abstract

The photoreduction of potassium iron(3+) oxalate in the presence of palladium(2+) chloride results in the selective formation of palladium metal, which can catalyze copper plating from an electroless bath. This process is characterized by a rapid photospeed at several ultraviolet wavelengths, and selective electroless deposition of copper occurs only in the irradiated regions. A chemical treatment of the polyimide film, prior to treatment with the photocatalytic mixture results in excellent adhesion of the copper-plated pattern to polyimide. In an attempt to understand the mechanism for both adhesion and the photoformation of the active catalyst, an in situ X-ray photoelectron spectroscopic study was initiated. The results indicate base hydrolysis of the Kapton to form the disodium salt of polyamic acid, cation exchange at the polyamic acid groups, the rapid photoreduction of Fe3+ to Fe2+ and the slower reduction of Pd2+ to Pd°. Mechanisms for copper–Kapton adhesion and for the photoformation of palladium metal are proposed based upon the results of this surface study. The described process utilizes aqueous solutions and thus is attractive for both manufacturing and environmental reasons. The simplicity of this method for creating selective copper circuit patterns and for producing highly adherent films of copper onto polyimide is highlighted. © 1991, American Chemical Society. All rights reserved.