A method for aligning a wafer stack includes providing a wafer stack including a top wafer with a top mark and a bottom wafer with a bottom mark in particular the top mark and the bottom mark capable of corresponding to each other; adjusting a relative position between the top wafer and the bottom wafer so that the top mark and the bottom mark are in contact with each other; applying an electrical signal on the top mark to obtain an electrical reading and optimizing the electrical reading to substantially align the wafer stack.