Optimization of the Copper Microstructure to Improve Copper-to-Copper Direct Bonding

Abstract

Advanced packaging solutions and heterogeneous integration are key technologies to enable devices with improved operating characteristics, including higher performance, increasing power efficiency, and decreasing form factor. Packages with high interconnect densities are required to efficiently combine, e.g., processing and memory units but impose restrictions to the pitch of the interconnects. Conventional technologies, including wire bonds and flip chip bonds are limited to larger pitches and, therefore, not suitable to meet the requirements of upcoming packaging technologies with respect to interconnect densities. Direct copper-to-copper interconnects are supposed to allow such small pitches of 10 µm or even below. However, formation of such bonds usually requires high temperatures and pressures. Temperature-sensitive devices like DRAM components restrict the maximum temperature that can be applied to the package. Thus, copper material is required, which allows bond formation at relatively low temperatures. In this context, hybrid bonding processes were discussed that involve initial bond formation via the usually oxide-based dielectric at room temperature followed by copper-to-copper bonding at elevated temperatures. The copper material is usually prepared by electrolytic deposition and the properties of the respective deposits may be modified by properly designed organic additives as well as process parameters. Strong bond formation of the copper should be obtained upon grain growth over the interface of the two deposits, which are brought into contact during the bonding step. In order to facilitate such growth at relatively low temperatures, suitable microstructures need to be prepared. Ideally, morphologies should be chosen in a way that they can be maintained throughout all process steps after the electrolytic deposition but, at the same time, allow grain growth over the interface during copper-to-copper bonding. Various strategies to enable improved seamless grain growth but maintain a metastable microstructure throughout the preceding process steps and the corresponding unique copper microstructures will be compared. In this context, different electrolytic copper deposition processes, the resulting microstructures, as well as their respective advantages and challenges with regards to copper-to-copper bond formation will be discussed.