Necessary Thickness of Au Capping Layers for Room Temperature Bonding of Wafers in Air using Thin Metal Films with Au Capping Layers

This study elucidated room temperature bonding of wafers in air using Ag, Al, and Cu films with thin Au capping layers. To avoid oxidation of 20-nm-thick of Ag and Cu films, a 2-nm-thick Au layer was found to be effective. Bonding was achieved in air over the entire area without a loading pressure. However, the necessary thickness of the Au capping layer for 20-nm-thick Al films was 6 nm or more, which is three times thicker than those for Ag and Cu films. The necessary Au layer thickness increased as the thicknesses of Al and Ag layers increased. Moreover, a loading pressure was necessary for bonding using thick films. These results were attributed to increased the surface roughness. However, the necessary Au capping layer thickness for bonding was 1/10 or less of the thickness of Al or Ag layers in the present thickness range of 20-500 nm.