The chemical mechanical polishing (CMP) process is indispensable to the fabrication of Cu wiring layers in the large-scale integration (LSI). Recently, a direct bonding method with low bonding temperature is required for the CMP-Cu surface in order to obtain a narrow bonding pitch. In this study, we realized a direct bonding between CMP-Cu films by means of the surface activated bonding (SAB) method at room temperature. The critical vacuum pressure to obtain large bonding strength was estimated at about 4 x 10(-3) stop Pa from the growth rate of oxide on an active surface measured by the X-ray photoelectron spectroscope (XPS). The films were bonded successfully at the vacuum pressure better than around 3 x 10(-3) stop Pa with the shear strength larger than 50 MPa. The transmission electron microscope (TEM) observation showed that the polycrystalline films with the mean surface roughness of 0.3 nm were bonded directly between Cu grains in atomic level. Moreover, the adhesion between the films was improved due to the stress relaxation at the interface during the thermal aging test conducted at 200 and 300 degrees C in the vacuum condition. (c) 2005 Springer Science + Business Media, Inc.