Bonding at copper-alumina interfaces established by different surface treatments:: a critical review

The present study summarizes the effects of processing conditions and substrate cleaning procedures on the structure, chemistry and bonding of Cu/(0001)Al2O3 interfaces as determined by advanced transmission electron microscopy techniques. The Cu/(0001)Al2O3 samples were prepared by molecular beam epitaxy (MBE) and solid-state diffusion bonding. Investigations of the MBE samples showed that the Al2O3 cleaning procedure alters the interfacial bonding. Metallic bonds occurred for an Ar+ ion sputtering and subsequent ultra-high vacuum (UHV) annealing treatment. Strongly ionic-covalent bonds were found for a wet chemical cleaning process followed by UHV annealing. The interfacial electronic structure did not reveal any significant changes compared to the bulk electronic structure for samples where the substrate surface was annealed in an oxygen-containing atmosphere after Ar+ ion pre-sputtering and UHV annealing. The results obtained at the solid-state diffusion-bonded Cu/Al2O3 samples indicated that the processing parameters such as temperature and load do not change the bonding behavior. Post-annealing of the solid-state diffusion-bonded Cu/Al2O3 samples in a well-defined oxygen partial pressure led to the formation of CuAlO2 at the interface between Cu and Al2O3, which improved the adhesion.