# In Situ Annealing Behavior of Cu Thin Films Deposited over Co-W Diffusion Barrier Layers
## 4 Discussion  
Quantitative analysis of the evolution of the Cu surface coverage at the end of each isothermal stage is represented in Figure 7. Results show that both the lower and the higher heating rates induced Cu film dewetting at temperatures above 450 \({}^{\circ}\)C. As the temperature value for the onset of dewetting remained constant through different substrate materials and heating rates, it indicated Cu-film thickness to be the overall most influencing parameter that defined the dewetting behavior [1]. The Cu/Co-W surface remained relatively unaltered below the said stage, above which dewetting of the Cu film progressed very rapidly in the case of the 30 \({}^{\circ}\)C/min. heating rate. This behavior could be attributed to the energy release and different grain growth rates associated with different heating rates, which translate to different recovery kinetics for each condition [20]. This difference in dewetting progression rates suggests that the interfacial energy between the two films should not be the most determinant factor concerning thin-film dewetting behavior during thermal processing, as this initial value should be identical for both heating rates applied to each of the systems. The strain energy associated with different thermal expansion coefficients for the materials involved may also significantly contribute to the dewetting behavior of the Cu films [1]. The results regarding increased dewetting at high temperatures are in line with the findings of previous works [21]. The Cu/Ta results at 10 \({}^{\circ}\)C/min. heating rate indicate a considerably lower surface tension of the Cu film than the one observed for the Co-W alternative under equivalent annealing conditions, as evidenced by the earlier status of Cu agglomeration registered at 550 \({}^{\circ}\)C. Hillock formation can also lead to dewetting  
Figure 6: Surface of the Cu film deposited over a Ta layer from 150 \({}^{\circ}\)C (**a**) to 550 \({}^{\circ}\)C (**i**), using a 30 \({}^{\circ}\)C/min. heating rate. through thinning of the film near the hillock until the substrate is exposed [17]. The grain growth morphology registered in this work deviates slightly from the definition of hillock presented in other works, however, seeing as all Cu islands formed after the longer annealing times were round shaped, as opposed to exhibiting faceted surfaces [10]. Said work associated hillock formation with void presence near grain boundaries, with the present findings showing that hillock formation does not seem to necessarily occur before Cu-film dewetting and grain growth, assuming voids are inherently forming to foster grain growth. A previous work claims that strain relaxation during annealing may be responsible for the first stages of dewetting and Cu agglomeration, along with lattice mismatches between the different materials present in the stack [22]. The behavior displayed by the Cu film during this study is consistent with the Ostwald ripening mechanism, where abnormally large grains grow through consumption of the atoms of smaller grains of neighboring regions [9]. It is worthy of note that Cu atoms can seemingly diffuse along considerably long distances, i.e., some micrometers, to feed said abnormal grain growth, as opposed to being restricted to their immediate vicinity. Comparison of the behavior of both types of substrates used showed that the Cu film deposited over the Co-W substrate was more resistant to the onset of dewetting, but after the phenomenon had started it exhibited a more aggressive progression than over the Ta film. The dimensional stability of the pinholes present in the Co-W stack across a broad temperature range, when annealed at 30 \({}^{\circ}\)C/min, is an interesting result considering this type of defect is often depicted as being the precursor to solid-film dewetting. This behavior suggests the driving force for Cu-film dewetting is influenced by the defects at the surface of the material and, likely more prominently, by total energy minimization mechanisms as mentioned in previous works [1].