Fluctuation Mechanism of Mechanical Properties of Electroplated-Copper Thin Films Used for Three Dimensional Electronic Modules

The mechanical properties of copper thin films formed by cold-rolling and electroplating were compared using tensile test and nano-indentation. Both the Young´s modulus and tensile strength of the films were found to vary drastically depending on the microstructure in the films. The Young´s modulus of the cold-rolled film was almost same as that of bulk material. However, the Young´s modulus of the electroplated thin film was about a fourth of that of bulk material. The micro structure of the electroplated film was polycrystalline and a columnar structure with a diameter of a few hundred-micron. The strength of the grain boundaries of the columnar grains seemed to be rather week. Such a columnar structure causes the cooperative grain boundary sliding and the film shows low elasticity and superplastic deformation. In addition, there was a sharp distribution of Young´s modulus along the thickness direction of the film. Though the modulus near the surface of the film was close to that of bulk material, it decreased drastically to about a half within the depth of about 1 mum. There was also a plane distribution of Young´s modulus near the surface of the film.