Method for bonding wafers

A method for bonding wafers, the method comprising:

• preparing a first wafer having a first metal layer with a first rigidity modulus, the first metal being formed of an aluminum-copper alloy, the percentage of copper by weight in the aluminum-copper alloy by weight being 1% or less, aluminum being the main constituent of the first metal layer, the first metal being one of an alloy of aluminum and copper and an alloy of aluminum, silicon and copper;
• preparing a second wafer having a second metal layer with a second rigidity modulus higher than the first rigidity modulus, the second metal layer is formed of a metal containing germanium;
• removing a first oxide film on a first surface of the second metal layer while not removing a second oxide film on a second surface of the first metal layer;
• and
• bonding the first surface of the second metal layer to the second surface of the first metal layer in such a manner that the oxide film on the second surface of the first metal layer is broken due to mechanical shock.