Formation of Cu–Cu interfaces with ideal adhesive strengths via room temperature pressure bonding in ultrahigh vacuum

Low-temperature metal-metal bonding is needed for three-dimensional circuit fabrication and other technologies. Atomic force microscope pull-off measurements were used to characterize the tensile toughness of bonds between Cu layers deposited, pressure bonded, and tested under ultrahigh vacuum (≪2×10^-10 Torr). Works of adhesion ∼3 J/m² were obtained at room temperature, the ideal value expected for bulklike bonding. The bond toughness was degraded to ∼0.1 J/m² when surfaces were exposed to 10^-6 Torr O₂ before bonding. Cu layers exposed to O₂ must be bonded at 300 °C or above to achieve the same toughness achieved at room temperature with clean surfaces.