Study of the Au/In Reaction for Transient Liquid-Phase Bonding and 3D Chip Stacking |
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Authors: | W Zhang W Ruythooren |
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Affiliation: | (1) IMEC, Kapeldreef 75, 3001 Leuven, Belgium |
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Abstract: | The latest three-dimensional (3D) chip-stacking technology requires the repeated stacking of additional layers without remelting
the joints that have been formed at lower levels of the stack. This can be achieved by transient liquid-phase (TLP) bonding
whereby intermetallic joints can be formed at a lower temperature and withstand subsequent higher-temperature processes. In
order to develop a robust low-temperature Au/In TLP bonding process during which all solder is transformed into intermetallic
compounds, we studied the Au/In reaction at different temperatures. It was shown that the formation kinetics of intermetallic
compounds is diffusion controlled, and that the activation energy of Au/In reaction is temperature dependent, being 0.46 eV
and 0.23 eV for temperatures above and below 150°C, respectively. Moreover, a thin Ti layer between Au and In was found to
be an effective diffusion barrier at low temperature, while it did not inhibit joint formation at elevated temperatures during
flip-chip bonding. This allowed us to control the intermetallic formation during the distinct stages of the TLP bonding process.
In addition, a minimal indium thickness of 0.5 μm is required in order to enable TLP bonding. Finally, Au/In TLP joints of ∅40 μm to 60 μm were successfully fabricated at 180°C with very small solder volume (1 μm thickness). |
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Keywords: | Au/In interdiffusion transient liquid-phase bonding flip chip 3D chip stacking |
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