Low temperature Cu–Cu direct bonding in air ambient by ultrafast surface grain growth

Fine-grain copper (Cu) films (grain size: 100.36 nm) with a near-atomic-scale surface (0.39 nm) were electroplated. Without advanced post-surface treatment, Cu–Cu direct bonding can be achieved with present-day fine-grain Cu films at 130℃ in air ambient with a minimum pressure of 1 MPa. The instantaneous growth rate on the first day is 164.29 nm d^-1. Also, the average growth rate (∆R/∆t) is evaluated by the present experimental results: (i) 218.185 nm d^-1 for the first-day period and (ii) 105.58 nm d^-1 during the first 14-day period. Ultrafast grain growth and near-atomic-scale surface facilitate grain boundary motion across the bonding interface, which is the key to achieve Cu–Cu direct bonding at 130℃ in air ambient.