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 (<2×10−10Torr). Works of adhesion ∼3J∕m2 were obtained at room temperature, the ideal value expected for bulklike bonding. The bond toughness was degraded to ∼0.1J∕m2 when surfaces were exposed to 10−6Torr 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.