Oscillatory Behavior in Cobalt Electrodeposition with 3-Mercapto-1-Propanesulfonate
Authors: Hu Yang , Huang Qiang 
Published: December 2020

As the copper interconnects used in integrated circuits1 are facing a physical limitation of resistivity increase in dimensions below the electron mean free path of Cu, alternative metals with a shorter electron mean free path such as cobalt (Co) are being explored as alternative conductors2. The damascene fabrication of Cu interconnects relies on the synergistic interactions between organic additives3. Such interactions result in bi- or even multi- steady states of deposition, which not only enables defect-free filling of Cu into small trenches but also often results in oscillatory behavior between these steady states during thin film deposition4. For example, the rapid self-breakdown of a suppressor-accelerator complex molecule mediated by a cuprous ion, together with the slow formation and accumulation of such a complex on the electrode surface, results in potential oscillations during galvanostatic deposition of Cu using 3-mercapto-1-propanesulfonate (MPS) as accelerator and a copolymer of imidazole and epichlorohydrin (Imep) as leveler4. While similar oscillations have also been reported with other commercial Cu chemistries, MPS or its dimer remains as the key component of the chemistry, the accelerator. Here, we are reporting a study of Co deposition with MPS as a single-component additive, where potential oscillations are also observed. A systematic study on the effects of several influencing factors (e.g. pH, Co2+/ MPS concentrations, current density and agitation) are thoroughly investigated and will be presented. A competitive adsorption mechanism is proposed to explain these observations. In addition, the effect of potential oscillation on the morphology of electrodeposited Co films will also be discussed.

Figure 1 (a) shows the cyclic voltammograms of Co deposition on platinum rotating disk electrode with the absence and presence of 300 ppm MPS at pH = 4. The suppression effect of MPS on Co deposition is observed regardless of agitation. Figure 1 (b) shows the potential oscillation at different current densities with 300 ppm MPS at pH = 4 with the magnified oscillation curves included as insets. Stable oscillations are observed when the current is increased to -6.12 mA/cm2, which indicates that a threshold current or potential must be reached to enable such oscillation phenomenon.

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Figure 1