These results additionally show that Cu preferentially migrates towards the areas of higher Co concentration in the surface, which is consistent with Cu 2 and Cu 4 adsorption on Co-doped TaN. In our previous work, we found that Ru also acted as a nucleation site for Cu atoms, however we did not observe such preferential migration during the aiMD runs on the equivalent Ru-doped surface in ref . While a smooth film can likely be achieved with 50% Co doping, for interconnect applications a surface like Co 25 , where the dopants are more distributed should have an advantage in promoting growth of a 2D Cu film. However, it is possible that area selective growth of Cu on CoN could be achieved by combining CoN and TaN. Based on our results, atoms should selectively migrate towards the Co-rich areas. The stronger adhesion of Cu on CoN compared to TaN also means that it could be possible to selectively remove any unwanted Cu atoms in the Ta-rich areas of the surface. The literature around selective atom removal tends to focus on using irradiation techniques to change the composition of polyatomic materials. Other work on selective removal targets specific surface sites based on localised electronic excitations in the surface with the aim to smooth rough metal surfaces. Meanwhile, processes for selective atomic layer etching are still in development and tend to be material-selective rather than truly area-selective. Although the rhombus rearranged into a tetrahedron during the Cu 4 aiMD simulation in Section 3.3, the results of the Cu 29 aiMD simulation and the fact that the Cu 4 rhombus structure was preferred during the geometry relaxation suggest that W 100 acts as a liner. As we see no exchange of Cu and surface atoms combining WN and TaN like this should yield a thermally stable material that can act as a barrier, despite the possible inferior barrier reliability of W x N y compared to TaN. On W 25 , the Cu atoms rearrange into a wire-like structure. Additionally, a single Cu atom breaks away from the cluster and migrates across the surface, indicating enhanced atom mobility. We also observe several transient states throughout the aiMD run-time where the Cu 29 structure separates into smaller clusters, before rearranging into the wire-like structure. This is shown in Figure . As the W dopants are uniformly distributed throughout the surface, meaning that there are no W-rich areas, this rearrangement is clearly not caused by enhanced interaction between Cu and W. Given that the only difference between the interactions of Cu with W 25 compared to the other W-doped surfaces is an increased Cu-substrate energy, it is possible that the distribution of W-dopants in TaN can strengthen the interaction with Cu. Even though there are no clear indications in our analysis why W 25 behaves differently in this respect, provided that these wire-like structures would join into a smooth thin-film, 25% W-doped TaN may be a candidate for a combined barrier/liner material. While the weaker overall adhesion of Cu atoms is of some concern, due to the lack of rearrangements and surface gaps in W 25 this composition may have superior ), Cu-substrate interaction energies (Equation ) and Cu-Cu interaction energies (Equation ) for Cu 29 interacting on doped TaN surfaces. stability compared to Ru-and Co-doped TaN.