In Situ X-ray Diffraction and Spectro-Microscopic Study of ALD Protected Copper Films

In many applications of copper in industry and research, copper migration and degradation of metallic copper to its oxides is a common problem. There are numerous ways to overcome this degradation with varying success. Atomic layer deposition (ALD) based encapsulation and passivation of the metallic copper recently emerged as a serious route to success owing to the conformality and density of the ALD films. So far, the majority of the studies have been focused on corrosion protection of copper in a variety of chemical environments, mostly at ambient temperature. An investigation of the stability of the ALD film stacks and copper's interaction with them at elevated temperatures has been lacking. Here, we study the mitigation of copper oxidation and migration in 50 nm thick Al₂O₃/TiO₂ and Al₂O₃/SiO₂ bilayer ALD stacks. First, the corrosion dynamics were investigated via in situ X-ray diffraction (XRD) at 350 °C under atmospheric conditions, and second, the interaction of copper with the passivation layers have been examined post factum using detailed spectro-microscopic investigations. According to the XRD results, both ALD films exhibited excellent oxidation protection. In contrast, bare Cu immediately started to oxidize at 350 °C and transformed entirely to its known oxide phases in 4 h. Spectro-microscopic studies revealed that there are structural and chemical changes on the top surface and within the film stacks. The TiO₂ layer was crystallized during annealing, while the SiO₂ layer stayed in the amorphous phase, which was analyzed by grazing incidence XRD and transmission electron microscopy. According to scanning electron microscopy and X-ray photoelectron spectroscopy analysis, copper was detected on the surface with a higher amount in Al₂O₃/TiO₂ than Al₂O₃/SiO₂, 5.2 at.% and 0.7 at.%, respectively. Based on the surface and cross-sectional analysis, copper migration was observed on both layers, albeit more substantially in Al₂O₃/TiO₂. In the case of Al₂O₃/SiO₂, the bulk of the copper was captured at the interface of the two oxides.