A method for bonding includes providing a first die and a second die; scanning at least one of the first die and the second die to determine thickness variations of the at least one of the first die and the second die; placing the second die facing the first die with a first surface of the first die facing a second surface of the second die; aligning the first surface and the second surface parallel to each other using the thickness variations; and bonding the second die onto the first die. The step of aligning the first surface and the second surface includes tilting one of the first die and the second die.