Ambient Copper-Copper Thermocompression Bonding using Self Assembled Monolayers
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1112-E02-03
Ambient Copper-Copper Thermocompression Bonding using Self-Assembled Monolayers Xiao. F. Ang1, J. Wei2, Z. Chen1 and Chee. C. Wong1 1 School of Materials Science & Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. 2 Joining Technology Group, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive Singapore 638075. ABSTRACT A typical copper-copper thermocompression bonding process is carried out in an ultrahigh vacuum (UHV) or inert environment at a bonding temperature >300°C. The ultraclean environment serves a single purpose – to maintain oxide-free copper surfaces, allowing intimate physical contact between copper atoms. This study investigates the temperature dependence of direct copper bonding from room temperature to 300°C under ambient condition. An anomalous thermal dependence of bond strength occurs between 80°C to 140°C where an increase in bonding temperature within this regime is in fact, detrimental to joint strength. This is interpreted as a thermal competition between oxidation and bond formation. This study also demonstrates that by simply coating the copper surface with a self assembled monolayer of 1-undecanethiol prior to bonding, Cu joints can be successfully formed at close to ambient temperature without a vacuum, yielding joint shear strengths on the order of 70MPa. The densely packed monolayer serves to passivate the copper surface against oxidation under ambient conditions. The ultrathin organic monolayer structure, as compared to a bulk oxide layer, could be easily displaced during the mechanical deformation at the bonding interface which accompanies thermocompression. This method could be an effective simple bonding solution for three-dimensional integrated chips. INTRODUCTION Due to its lower electrical resistivity and higher melting point, copper, instead of aluminum, has become the mainstream material for today’s interconnect technology, yielding benefits such as a reduction in interconnect-related RC propagation delay and improved electromigration resistance [1-3]. However forming good quality Cu bonds is not as easy as compared to other interconnect materials such as gold. The ease of oxidation of copper often entails a need for high bonding temperature (> 300°C) and/or ultrahigh vacuum conditions. A post-annealing process at temperature equivalent to or higher to the bonding temperature under inert atmosphere over an adequate amount of time is also reported to help in enhancing bond integrity [4, 5]. Unfortunately, current harsh bonding conditions pose a severe challenge to the reliability to the numerous underlying device layers in 3-D stacked ICs. It has been noted that junction temperature for any device technology should not exceed 200°C [6], otherwise a highly stressed system as a result of huge thermal stress build-up during fabrication process would lead to a drastic drop in overall performance. Immediate effort in sourcing solutions to thermal issues faced in 3-D integration is crucial.
Bond quality of the copper joints
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