Cu direct bonding has been achieved at 150 C by using (111)-oriented nanotwinned Cu (nt-Cu) because it has the fastest surface diffusivity. In this study, nt-Cu was bonded at even lower temperature of 120 C by roughening one of the nt-Cu surfaces. However, the flat-to-flat nt-Cu could not be bonded at the same temperature. This result violates a bonding concept that "for good wafer bonding, the contact area must be as large as possible". This paper discusses in detail two possible bonding mechanisms: diffusion and creep/plastic deformation.