Low-temperature direct copper-to-copper bonding enabled by creep on highly (1 1 1)-oriented Cu surfaces

We achieve low-temperature Cu-to-Cu direct bonding using highly (1 1 1)-orientated Cu films. The bonding temperature can be lowered to 200 °C at a stress of 114 psi for 30 min at 10−3 torr. The temperature is lower than the reflow temperature of 250 °C for Pb-free solders. Our breakthrough is based on the finding that the Cu (1 1 1) surface diffusivity is the fastest among all the planes of Cu and the bonding process can occur through surface diffusion creep on the (1 1 1) surfaces.