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The Characteristics of Cu/Sn/Cu and Ni/Sn/Ni Sandwich Solder Systems for Gold-free Wafer Bonding Technology
Keywords: Soldering, Intermetallic compounds, Wafer bonding
The recent increase in demand for high-quality wafer bonding at low temperatures has created a need to overcome the challenges posed in producing void-free and uniform solder joints over an entire wafer. Although Au-Sn solder has proven to be an excellent candidate material for wafer bonding, there is a need to find a suitable substitute owing to the high cost of gold and the high melting temperature (280C) of the eutectic Au-Sn alloy. Copper and nickel both offer a significant economic advantage over gold, with the melting temperatures of their respective eutectic alloys with tin being 227C and 221C, which reduces the bonding temperature required. Moreover, costs can be further reduced by producing a liquid phase (TLP) bond that combines Sn with cheaper side-layers of Cu and Ni. In this study, two sandwich-structured solder systems (Cu/Sn/Cu, Ni/Sn/Ni) are suggested for low temperature wafer bonding with a thermally stable joint. The influence of the thickness ratio of the metal layers on the microstructure and mechanical bonding strength of the solder joints was investigated by varying the thicknesses of the Cu or Ni side-layer. During the bonding process, the side-layer metal (Cu or Ni) reacts so rapidly with Sn that it is completely transformed into high melting point IMCs (Cu/Sn/Cu: Cu6Sn5, Cu3Sn or Ni/Sn/Ni: Ni3Sn4, Ni3Sn). Although the microstructures of sandwich joint samples are very similar to each other, the bond strength of the Cu-Sn samples is notably higher than that of Ni-Sn samples. It is therefore inferred that both the irregular morphology and brittleness of the Ni-Sn IMCs could strongly affect the strength of Ni-Sn sample, resulting in a reduced bond strength compared to Cu-Sn. However, the bond strengths of the joints formed by both sandwich structures were higher than the fracture strength of the Si wafer, making it sufficient for application to conventional wafer bonding processes. The sandwich-structured solder systems could therefore be widely applied to low temperature wafer bonding of various electronic devices and packaging requirements.
Kunmo Chu,
Samsung Advanced Institute of Technology

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