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 ∕ m 2 were obtained at room temperature, the ideal value expected for bulklike bonding. The bond toughness was degraded to ∼ 0.1 J ∕ m 2 when surfaces were exposed to 10 − 6 Torr O 2 before bonding. Cu layers exposed to O 2 must be bonded at 300 ° C or above to achieve the same toughness achieved at room temperature with clean surfaces.