Copper-copper direct bonding: Impact of grain size

In recent years, a great interest has emerged in the development of new wafer-scale assembly processes. Beside the mechanical strength required, some applications need a vertical conductivity leading to implement metal thin films as bonding layers. For its interesting properties in terms of resistivity and reliability, copper has been already used in metal-metal direct bonding configuration. Initially developed on amorphous silicon dioxide layers, the polycristallinity character of metal films has a direct impact on the direct bonding mechanisms. In this paper, we will study the effect of grain size on direct bonding of polycrystalline copper thin films. More specifically at temperature below 150°C, a fine-grain copper microstructure demonstrates a fast sealing strengthening. For higher temperature application, a larger grain size enables limiting the copper-barrier interface damage and preserves a strong mechanical link between substrates.