A low stress bond pad design optimization of low temperature solder interconnections on TSVs for MEMS applications

In new applications (such as MEMS, bio-MEMS), vertical integration requires a low processing temperature below 200°C to bond these devices without degrading their performance. Low temperature bonds are thin intermetallic compounds (IMCs) bonds that are formed between devices when plated layers of different metals on each side of the component come into contact under relatively low temperature and high pressure. These joints comprised completely of IMCs, will fail in a sudden unexpected manner, as compared to normal solder joints, which fail in a ductile manner, where cracks grow more slowly. This problem of weak interconnects is further exacerbated when these thin interconnections are formed on pads located above TSVs. In this paper, a novel bond pad (which decouples the interconnection from the TSV) has been designed and simulated to reduce the joint stress. Low temperature bonding technology was demonstrated using Au/In/Sn alloy solder at a low temperature below 200°C forming robust IMC joint with high re-melting temperature (>; 400°C), so that after bonding, the IMC joints can withstand the subsequent processes without any degradation. Actual bonded samples were built in order to compare the proposed low stress pad design with the conventional pad design. Shear test shows improvement in shear strength with the proposed pad design. Finally, finite element modeling has also been carried out again to understand how the proposed low stress pad design is extensible to the small diameter TSVs. The results show that the proposed pad design with TSV diameter less than 4 μm does not show much stress reduction and therefore may not be required.