First-principles study of adhesion at Cu/SiO₂ interfaces

The structural, electronic, and adhesive properties of Cu/Cu/SiO₂ interfaces are investigated using first-principles density-functional theory within the local density approximation. Interfaces between fcc Cu (001) and α-cristobalite (001) slabs with different surface stoichiometries are considered. Interfacial properties are found to be sensitive to the choice of the termination and the interfacial oxygen density is the most important factor influencing the strength of adhesion. For oxygen-rich interfaces, the O atoms at the interface substantially rearrange after the deposition of Cu layers, suggesting the formation of Cu-O bonds. The large structural rearrangement, site-projected local densities of states, and changes in electron density indicate hybridization between Cu − d and O − p states at the interface. As oxygen is systematically removed from the interface, less rearrangement is observed, reflecting less hybridization and weaker adhesion. Computed adhesion energies for each of the interfaces are consistent with the observed structural and bonding trends, leading to the largest adhesion energy in the oxygen rich cases. The adhesion energy is also calculated between Cu and SiO₂ substrates terminated with hydroxyl groups, and adhesion of Cu to these substrates is found to be considerably reduced. This work supports the notion that Cu films can adhere well to hydroxyl-free SiO₂ substrates should oxygen be present in sufficient amounts at the interface.