Bonding process for sensitive micro- and nano-systems

A metal inter-diffusion bonding method comprises providing a stack of a first metal on a surface of both a first wafer and second wafer to be joined together. Said first metal is susceptible to oxidation in air. A layer of a second metal is provided on the first metal and has a melting point lower than that of the first metal. On the top surface of each stack of first metal is the layer of second metal sufficiently thick to both inhibit oxidation of the top surface of the first metal and to ensure that unreacted Sn remains when partners are brought together. A bond is initiated between the first and second wafers by bringing the layers of the second metal on each wafer into contact at a bonding temperature that is below the melting point of the second metal, by applying a bonding force that is sufficient to initiate inter-diffusion at an interface between the layers of the second metal. The bonding temperature is increased whilst applying the bonding force to achieve a desired inter-diffusion at the bond interface, thereby bonding the first and second wafers together, and no pre-treatment of the wafers is required prior to bonding.