Mechanism of low-temperature copper-to-copper direct bonding for 3D TSV package interconnection

Cu-Cu direct bonding is the leading method for fine pitch (10 μm) chip-to-chip interconnects. We performed several measurements on blanket Cu film samples in an effort to determine the impact of the Cu film properties on wafer-to-wafer Cu-Cu direct bonding. X-ray photoelectron spectroscopy (XPS) measurements were performed on uncleaned and cleaned samples to evaluate the effectiveness of three surface cleaning methods, Ar sputtering in vacuum, forming gas annealing and N₂ annealing (NA). The XPS results were correlated with the bonding quality of the wafers cleaned by above mentioned methods using a C-mode scanning acoustic microscope (CSAM) and four-point bending test. The grain structure and texture information of the Cu surface were studied by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) and the residual stress of Cu films was measured by X-ray diffraction (XRD). The roles that these microstructural variables may play in Cu-Cu direct bonding were also discussed.