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Cu-Cu Thermocompression Bonding using Ultra Precision Cutting of Cu Bumps for 3D-SIC
Keywords: Cu-Cu bonding, ultra precision cutting, low temperature
We have developed a new Cu-Cu thermocompression bonding technique by using cut Cu bumps in order to achieve high density 3D-stacked IC (3D-SIC). Currently, Sn layer is formed between Cu bumps, and then solid-liquid bonding is made by tin melting to connect Cu bumps. But using Sn layer can cause undesirable issues, such as electro-migration and Kirkendall voids formation between Cu/Sn interfaces, which could decrease bonding reliability. Therefore we believe that Cu-Cu thermocompression bonding is an essential technology especially in 3D interconnection. In the present study, cut Cu bumps were obtained by ultra-precision cutting using a single crystal diamond bite that would give a highly smooth Cu surface (Ra:7nm). A major advantage of cut Cu bumps is that they have an amorphous-like layer on the surface. In TEM observation, it was found that about 120nm thick amorphous-like layer was formed after cutting of Cu bumps. This layer has a potential to connect bumps each other at a low temperature similar to solder bonding because amorphous-like layer accelerates a recrystallization reaction of Cu crystal. Cut Cu bumps on both sides of LSI and substrate have been successfully bonded at 250 degrees C condition. From the analysis of crystal orientation by EBSD, it was found that the bonding interface had disappeared, which means solid diffusion was occurred and crystal grain grew across the interface.
Taiji Sakai,
Fujitsu Laboratories Ltd.
Atsugi, Kanagawa 243-0197,
Japan


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