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 μm. There was also a plane distribution of Young's modulus near the surface of the film.