Eutectic bonding, also referred to as eutectic soldering, describes a wafer bonding technique with an intermediate metal layer that can produce a eutectic system. Those eutectic metals are alloys that transform directly from solid to liquid state, or vice versa from liquid to solid state, at a specific composition and temperature without passing a two-phase equilibrium, i.e. liquid and solid state. The fact that the eutectic temperature can be much lower than the melting temperature of the two or more pure elements can be important in eutectic bonding.
Eutectic alloys are deposited by sputtering, dual source evaporation or electroplating. It also can be formed by diffusion reactions of pure materials and subsequently melting of the eutectic composition.
Eutectic bonding is able to produce hermetically sealed packages and electrical interconnection within a single process (compare ultrasonic images). In addition this procedure is conducting at low processing temperatures, low resultant stress induced in final assembly, high bonding strength, large fabrication yield and a good reliability. Those attributes are dependent on the coefficient of thermal expansion between the substrates.