Fluxless wafer-to-wafer bonding in vacuum using electroplated Sn-rich Sn-Ag dual-layer structure

We report a wafer-to-wafer bonding technique with solder as the bonding layer but without the use of any flux. This fluxless or flux-free feature makes void-free and uniform bonding layer possible. If flux were used as in the case of typical soldering processes, the flux or flux residues could be easily trapped in the joint, resulting in voids and uneven solder layer. Flux residues, if not completely removed, could also cause potential reliability problem. The bonding process is performed in a developed vacuum chamber to inhibit solder oxidation. To prevent oxidation during solder manufacturing, 20mum of Sn is electroplated over the entire 2-inch silicon wafer, followed immediately by a thin capping layer of Ag. This outer Ag layer prevents the inner tin from oxidation when the wafer is removed from the plating bath and exposed to air. To evaluate the joint quality and study the microstructure and composition, scanning acoustic microscopy (SAM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) are used. The success of this development shows that it is indeed possible to bond entire wafers together with a thin metallic joint of high quality. This fluxless bonding technique can be extended to bonding wafers of different materials for new device and packaging applications