Fluxless wafer bonding with Sn-rich Sn–Au dual-layer structure

We report our initial result of bonding two 2 in. silicon wafers using Sn-rich Sn–Au dual-layer structure that is produced by electroplating process. No flux is used in the bonding process. Comparing to Au-rich Au80Sn20 eutectic alloy, it is more difficult to achieve fluxless feature using Sn-rich Sn–Au alloys due to tin oxidation. In this initial effort, two samples are produced. The resulting Sn-rich solder joint layer, about 30 μm in thickness, is very uniform over the entire 2-in. sample. The quality of the joint is examined using scanning acoustic microscope (C-SAM) and X-ray micro-imaging technique. Images of these two techniques indicate that the joints are of high quality. Of the two samples, the better one shows nearly perfect joint with only 2% of possible void area. This initial success shows that it is indeed possible to bond entire wafers together with a thin metallic joint of high quality. This fluxless technique can be extended to bonding wafers of different materials for new device and packaging applications. The use of metallic alloy layers of high melting temperature is also possible and is being considered.