Copper wafer bonding in three-dimensional integration

Abstract

Three-dimensional (3D) integration, in which multiple layers of devices are stacked with high density of interconnects between the layers, offers solutions for problems when the critical dimensions in integrated circuits keep shrinking. Copper wafer bonding has been considered as a strong candidate for fabrication of three-dimensional integrated circuits (3-D IC). This thesis work involves fundamental studies of copper wafer bonding and bonding performance of bonded interconnects. Copper bonded wafers exhibit good bonding qualities and present no original bonding interfaces when the bonding process occurs at 400⁰C/4000 mbar for 30 min, followed by nitrogen anneal at 400⁰C for 30 min. Oxide distribution in the bonded layer is uniform and sparse. Evolution of microstructure morphologies and grain orientations of copper bonded wafers during bonding and annealing were studied. The bonded layer reaches steady state after post-bonding anneal. The microstructure morphologies and bond strengths of copper bonded wafers under different bonding conditions were investigated.A map summarizing these results provides a useful reference on process conditions suitable for three-dimensional integration based on copper wafer bonding. Similar microstructure morphology of copper bonded interconnects was observed to that of copper bonded wafers. Specific contact resistances of bonded interconnects of approximately 10⁻⁸ [ohms]-cm² were measured by using a novel test structure which can eliminate the errors from misalignment during bonding. The bonding qualities of different interconnect sizes and densities have been investigated. In addition to increasing the bonding temperature and duration, options such as larger interconnect sizes, total bonding area, or use of dummy pads for bonding in the unused area improve the quality of bonded interconnects. Process development of silicon layer stacking based on Cu wafer bonding was successfully applied to demonstrate a strong four-layer-stack structure.Bonded Cu layers in this structure become homogeneous layers and do not show original bonding interfaces. This process can be reliably applied in three-dimensional integration applications.