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Nanoporous gold bumps for thermocompression bonding

: Oppermann, Hermann; Dietrich, Lothar; Weber, Constanze; Ziedorn, Morten; Aschenbrenenr, Rolf

Suga, T. ; Institute of Electrical and Electronics Engineers -IEEE-; IEEE Components, Packaging, and Manufacturing Technology Society:
4th IEEE International Workshop on Low Temperature Bonding for 3D Integration, LTB-3D 2014. Proceedings : 15-16 July 2014, Tokyo, Japan
Piscataway, NJ: IEEE, 2014
ISBN: 978-1-4799-5260-1
ISBN: 978-1-4799-5261-8
ISBN: 978-1-4799-5262-5
International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D) <4, 2014, Tokyo>
Fraunhofer IZM ()

In 3D integration components are sacked on each other by flip chip bonding. For gold-gold thermo-compression bonding tiny bumps with 20 to 50 µm diameter are deposited on a silicon wafer. The wafer is diced and chips are bonded typically with 200 MPa bonding pressure at 300°C to form all the electrical and mechanical interconnects in one step. We have developed a method to fabricate gold bumps with an open-porous cellular structure on wafer level. First a Au-Ag alloy of 10 to 15 µm thickness was electroplated into a lithographic structured resist mask which defines the bump pattern on the wafer. We developed a stable plating bath consisting of electrolytes for co-deposition of silver and gold. The open-porous gold nano-sponge was formed in an etching step by the removal of the silver. During de-alloying micro-cracks were found for higher silver content around 80%, whereas no cracks were detected with compositions in the range of 70% silver. Thermocompression bond experiments of nano-sponge on thin electroplated Au show, that a well formed bond interface can be achieved at low temperatures and with low pressure loads down to 150°C and 10 MPa. A compression of 25 to 30% was observed for successful flip chip bonds. The open porous gold bumps are highly compressible and can tolerate implanarities. They can be densified locally to enclose particles or to incorporate low passivation steps on the wafer. As the porosity occurs at nanoscale a highly reactive surface is provided and leads to the reduction of bonding temperature and or bonding pressure.