# In Situ Annealing Behavior of Cu Thin Films Deposited over Co-W Diffusion Barrier Layers
###### Abstract  
The development of new materials for the electronics industry has been in focus in recent years, as circuit miniaturization poses challenges for conventional solutions. Dewetting of Cu films over diffusion-barrier layers has fostered an interest in developing new solutions with lower interfacial energies, to withstand processing and service life. Co-W is a candidate material for seedless Cu-interconnect deposition, but its behavior during annealing is still not properly addressed. This study used an in situ scanning-electron-microscopy (SEM) approach to assess how heating rates affect dewetting behavior, as well as to determine the limits of annealing of 40 nm-thick Cu films deposited over this substrate. The 10 \({}^{\circ}\)C/min heating rate used showed copper dewetting starting at 450 \({}^{\circ}\)C, whereas the higher 30 \({}^{\circ}\)C/min rate induced dewetting at 400 \({}^{\circ}\)C. The Cu film deposited over Ta exhibited slightly different dewetting, with its onset starting earlier, but developing a slower progression throughout the temperature range analyzed in the annealing treatments.  
in-situ dewetting; cobalt tungsten; diffusion barrier; copper interconnects; thin film +
Footnote †: journal: Applied Physics Letters  
to use a Cu-seed layer before Cu electroplating for via filling, which limits the minimum thickness attainable by the most commonly used technology. [PERSON] et al. [9] claim that since Ta has a higher melting temperature than Cu, its surface energy should be higher. By this reasoning, both Co and W should exhibit a higher surface tension value than Cu, as both their melting temperatures are higher than Cu. Although several studies have been carried out regarding the properties of these materials, thin film dewetting or dewetting of Cu films, there is not currently any study concerning the phenomenology of the dewetting phenomenon of the Cu films deposited over these new alternative-barrier layer materials [10; 11; 12; 13].  
Using in situ microscopy enables investigation of the behavior involved in the thin film dewetting process. This type of study can prove instrumental in identifying the structural evolution of the Cu film over each of the materials under study during annealing, which in turn can help assess substrate viability as Cu-adhesion barrier layers. By using this technique, observation of the evolution of the surface of the films can be registered as the materials are submitted to high temperatures in real time. A recent in situ study of Cu films deposited over Si determined that Cu-film thickness was related to the shape in which dewetting occurs, with 20 nm thick films exposing the substrate through the expansion of fractal shapes. The same study found that hilllocking of grains was the behavior registered for films of thickness higher than 55 nm [10; 14]. In this work, we explore how a thin Cu film deposited over an equimolar Co-W substrate evolves at progressively higher temperatures via in situ scanning-electron-microscopy (SEM) analysis. A Ta substrate was also studied to compare its Cu-adhesion performance under identical processing conditions. By using this approach, we obtained real-time data of the surface of the stacks during annealing, and in this way were able to assess the associated surface stability of the systems as well as evaluate how Cu surface dewetting develops.