By Applied Microengineering Ltd.
The eutectic temperature of a two-component system corresponds to the lowest melting point composition of the two components. This property can be exploited for bonding of wafers by coating one of the wafers with one component of the system and the other wafer with the second component. When the wafers are heated and brought into contact, diffusion occurs at the interface and alloys are formed. The eutectic composition alloy at the interface has a lower melting point than the materials either side of it, and hence the melting is restricted to a thin layer. It is this melted eutectic layer that forms the bond. The most commonly used eutectic in MEMS is the Au-Si system with a eutectic temperature of 363°C, and eutectic composition of 97.1 Wt % Au: 2.9 Wt % Si. Other common material combinations are Au-Sn, Al-Ge and Au-In. In the case of Au-In, it may be desirable to remove the oxide from the indium and the AWB systems provide for delivery of a saturated vapour pressure of formic acid vapour into the bond chamber, for efficient in-situ removal of the oxide. Pumping down to high vacuum immediately after this step, and then aligning and contacting the wafers, ensures that the wafers are bonded without any opportunity for the oxide to regrow. Advantages of Eutectic Bonding: - Hermetic sealing – low outgassing - Strong bond - Can accommodate surface topography and contamination - Can be used for electrical connections between wafers - Bond reflow temperature is significantly higher than the original bonding temperature Disadvantages: - Requires accurate deposition of eutectic components (apart from Au-Si) - Possible spillage of the material in liquid phase - Diffusion barrier required - Al-Ge may require high force to break through the aluminium oxide Applications: - MEMS - WLP - vacuum or pressure encapsulation
