Effects of patterning on the interface toughness of wafer-level Cu–Cu bonds

The chevron test has been employed to characterize the toughness of patterned wafer-level Cu–Cu thermocompression bonds created at 300 °C, for pattern sizes ranging from 2 to 500 μm. Features oriented perpendicular to the debond propagation direction (lines and pads) exhibited a significant increase in toughness (from 3 to 30 J m−2 under mode I) with decreasing feature size (from 250 to 25 μm) for both mode I and mixed-mode loading, while no size-dependence was observed for debond propagation in the direction parallel to the bonded lines. The bond toughness was found to scale with the degree of discontinuity along the debond growth direction, due to greater energetic cost for multiple crack initiation events in the ductile bonded stack. Fractured surfaces of the discontinuous bonded interfaces exhibited ductile cohesive failure through the Cu film stack, signifying enhanced plastic energy dissipation leading to higher bond toughness.