Alternative Metals for Advanced Interconnects: Cobalt and Beyond

Concerns about the extendibility of copper as an interconnect conductor for future technology nodes have resulted in increasing interest in alternative conductor metals. 1 With ever-decreasing interconnect dimensions, alternative metals bring hope of improved resistivity and reliability through more favorable electron scattering and cohesive energy and the possibility of improved scaling through barrier layer removal. 2,3 Cobalt, in particular, has been the subject of several recent studies and has already found its way into at least one major semiconductor manufacturer’s scheme as a replacement for tungsten in trench contacts. 4 Considerable work has focused on understanding the cobalt electrofill process to enable its implementation in current interconnect technology. 5–8 While conventional copper damascene plating uses a three-additive system, cobalt superfill can be achieved while using only one organic component, with the hydrogen evolution reaction having been shown to also play a role. 5,7,8 Recently, we have demonstrated that boric acid is also an important component of cobalt plating baths. 6 As shown in Figure 1, boric acid forms a series of polyborate species in solution as a function of both concentration and pH. Presence of the polyborates is correlated with pH buffering under typical cobalt electroplating conditions, preventing precipitation of cobalt hydroxide and improving nucleation within the features. Just as the cobalt electrofill process represents a departure from the established damascene copper plating techniques, development of processes for other alternative metals for future technology nodes will require new and creative formulations and methods to overcome electroplating challenges for these metals. K. Lin, M. Chandhok, and M. Reshotko, in 2018 IEEE International Interconnect Technology Conference (IITC) , p. 2–3, IEEE, Santa Clara, CA, USA (2018) https://ieeexplore.ieee.org/document/8430404/. D. Josell, S. H. Brongersma, and Z. Tőkei, Annu. Rev. Mater. Res. , 39 , 231–254 (2009). K. Sankaran, S. Clima, M. Mees, and G. Pourtois, ECS J. Solid State Sci. Technol. , 4 , N3127–N3133 (2015). J. Yoshida, EETimes (2020) https://www.eetimes.com/apple-huawei-use-tsmc-but-their-7nm-socs-are-different/. M. A. Rigsby et al., J. Electrochem. Soc. , 166 , D3167–D3174 (2019). M. A. Rigsby, T. A. Spurlin, and J. D. Reid, J. Electrochem. Soc. , 167 , 112507 (2020). J. Wu, F. Wafula, S. Branagan, H. Suzuki, and J. van Eisden, J. Electrochem. Soc. , 166 , D3136–D3141 (2019). C. H. Lee, J. E. Bonevich, J. E. Davies, and T. P. Moffat, J. Electrochem. Soc. , 156 , D301 (2009). Figure 1: Raman spectra showing speciation of (a) 10 g/L and (b) 40 g/L boric acid in water as a function of pH. 6 Figure 1