Advanced Packaging

Metallic Sealing

Like in soldering with tin-based alloys, waferbonding equally offers the possibility to create sealing frames or vertical contact structures for interconnecting opposed surfaces through metal contacts. In contrast to the carefully optimised composition of solder alloys from generally two or more metals, the wafer bonding materials are applied by electroplating and thus usually more limited to pure gold, copper or tin layers of only a few micrometer thickness. The precise control of the layer thickness and the temperature profiles are therefore decisive for the bondline quality, i.e. its composition, but also its shape and cross-section. Like in gold-silicon bonding, a large amount of liquid metal may be released, which again means that additional design measures and well-managed process conditions are required.

The advantage of the gold-tin system compared to the gold-silicon bonding is its ability to compensate substrate surface roughness, but also its low bonding temperature (eutectic point at 280°C), which makes this technique most suitable for use on CMOS processed wafers.