Optimization of the Cu Microstructure to Improve Copper-to-Copper Direct Bonding for 3D Integration

Advanced packaging solutions and heterogeneous integration are key technologies to enable devices with higher performance and decreasing form factor. In this context, packages with high I/O densities are required, especially for high performance computing, which imposes restrictions to the pitches of the interconnects. Conventional bonding technologies, including wire bonds and flip chip are limited to larger pitches and, therefore, not suitable to meet the requirements of upcoming packaging technologies with respect to I/O densities. Direct copper-to-copper interconnects are supposed to fill this gap and to allow such small pitches of 5μm or even below. However, formation of such bonds usually requires high temperatures and pressures. Temperature-sensitive devices like DRAM restrict the maximum temperature that can be applied to the packages. Thus, processes and copper material are required, which allows proper bond formation at relatively low temperatures. In this context, hybrid bonding was discussed and involves initial bond formation via the dielectric at room temperature followed by copper-to-copper bonding at moderately elevated temperatures. The copper material is usually prepared by electrodeposition and the properties of the respective deposits may be modified by properly designed organic additives as well as process parameters. In Literature, different electrolytic deposition processes were discussed with respect to suitability of their respective microstructures to typical hybrid bonding process flows. Excellent bond formation of the copper by grain growth through the interface of the two deposits, which were brought into contact during the bonding step, was obtained employing metastable copper. This particular microstructure was designed to exhibit filling capabilities, high purity, small grains after deposition that allow for further growth, and eventually large grains after typical thermal budgets of copper-to-copper bond formations. Initial strategies were proposed to extent the persistence of the metastable microstructure. Such extension was required for compatibility to the complex hybrid bonding process flow and to be able to perform all process steps after electrolytic copper deposition until the final copper-to-copper bond formation. In this work, strategies will be provided to further improve uniformity and lifetime of the metastable microstructure. In addition, different production sequences will be discussed to allow for more flexibility and prolonged holding times during wafer-to-wafer and die-to-wafer production processes.