Copper-copper direct bonding is a fundamental procedure in three-dimensional integration. It has been reported that voiding occurs in bonded copper layers if process temperatures exceed 300A degrees C; this leads to serious reliability issues. However, voiding nucleation and growth mechanisms have not been clearly established. Void characteristics were compared for different bonded structures specifically designed to identify the origin of void formation and its contribution. It seems that mechanical stress caused by different dilatation of silicon substrates and metal thin films leads to metallurgical creep. This stress-driven vacancy diffusion makes a major contribution to the reliability problem. This study provides better understanding of these physical phenomena and can be used as guideline for metal integration.