Towards Copper-Copper Direct Bonding: Controlled Crystal Growth of Copper Deposits for Minimization of Interface Formation During Bonding

Abstract

Electrochemical deposition of copper usually results in certain crystalline layers at first, followed by growth of the individual crystals to the final microstructure. The deposit properties that determine the extent to which this growth occurs, the corresponding timeframe, and the required temperature strongly depend on the respective electrochemical deposition process. In turn, the result of the deposition process may be purposefully controlled by organic additives, their mutual concentrations, as well as process parameters such as current density, convection, or temperature. By this means, the behavior of the respective deposits may be tuned, e.g. as a function of the co-deposited impurities and the initial microstructure. Both, purity and microstructure usually depend on the electrochemical characteristics of the plating additives and the applied current density. By purposefully changing the postprocessing behavior of the initially formed layer after deposition and throughout subsequent processes like, for example CMP, the formation of the final microstructure and properties of the deposits may be directed to a post-processing step at elevated temperatures. Depending on the respective application and integration scheme, such post-processing steps requires a certain thermal load. The temperature that allows to obtain the final deposit properties may also be controlled by the parameters of the electrochemical deposition process. Ideally, this temperature should be adjusted to the thermal load, which is required for the respective following process step of the particular application under consideration. The results of this study of the time- and temperature-dependent behavior of electrodeposited copper could potentially be used to optimize the process for upcoming copper-to-copper direct bonding technologies.