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A key to a high-wiring-density build-up substrate is fine circuitry formation technology to satisfy the ever-increasing demands for miniaturization of electronics products. The surface roughness of a dielectric layer needs to be in the submicrometer scale for fine circuitry such as a line less than 10 μm wide. However, the Cu to dielectric adhesion strength of such a line would not be sufficient to prevent peeling off during manufacture and after heat treatment. Consequently, it is essential to have good adhesion between Cu and dielectric layer with chemical bonds between the metal and the dielectric layer. A polyimide film was introduced as a dielectric layer in a build-up substrate. Argon plasma-modified polyimide surfaces were sputtered with NiCr and then subjected to Cu electroplating. While unmodified polyimide had a weak adhesion strength of 0.12 kN m-1, Ar plasma-modified polyimide showed good adhesion strength of more than 0.5 kN m-1 even after 10 days of heat treatment at 172°C. X-ray photoelectron spectroscopy studies revealed that the adhesion strength was attributable to chemical bonds between Cr and the polyimide. Ar plasma-treated polyimide produced a large quantity of oxygen functional groups containing C=O bonds on the surface of polyimide, and subsequent NiCr sputtering produced C-O-Cr or C=O⋯Cr bonds to the polyimide. In addition, NiCr sputtering also attacked some of N-C=O and N-C bonds, and modified them to produce C-N-Cr or C-N⋯Cr bonds to the polyimide. These two types of mechanism produced sufficiently high Cu to polyimide adhesion to achieve fine line circuitry.