Copper was implemented as a conductor for advanced back-end-of-line (BEOL) interconnects based on its low electrical resistivity and improved electromigration life relative to the aluminum baseline.^{1, 2} Over the past two decades, the smallest dimension of Cu interconnects has decreased from about 300 to the range of 10-20 nanometers.³ Until recently, damascene Cu electroplating on PVD Cu seed has been optimized to provide void-free Cu metallization using a process relying on inorganic and organic additives to enable bottom-up growth from the base of vias and trenches.^1,2,4 With ever-decreasing feature size, the conventional damascene process is becoming increasingly limited by the inability to deposit a sufficiently continuous Cu seed on a barrier/Co liner stack without resulting in pinch-off of the PVD seed.

In this talk we will report on the development of direct Cu plating on Co using a range of deposition processes to illustrate the challenges and possible direction for improvement of the nucleation and subsequent growth of Cu on the Co seed. One challenge is to create a Cu layer without significantly etching the Co in the plating bath. This can be achieved by reducing the electrode potential difference between Cu electrodeposition and Co corrosion using proper additives in the plating bath to increase suppression (Figure 1). Such a large overpotential translates into improved nucleation density, which effectively inhibits the complete dissolution of the ultrathin Co layers. This effect is also illustrated by measuring the galvanic displacement reaction between Cu ions and Co at open circuit potential (OCP). Figure 2 reveals quick deposition of a monolayer equivalent of Cu followed by a slight increase in Cu thickness over time.

A second challenge involves generation of dense nucleation which results in a smooth initially plated Cu film. The number of protrusions is reduced noticeably when a potential-controlled entry is replaced by leaving the electrode at OCP for a short time before any current is applied (Figures 3a and 3b). A short OCP time allows Co oxides to first dissolve, thereby making more surface sites available for nucleation. The smoothness can be further enhanced by increasing the acidity of the bath (Figure 3c). Higher acid could have modulated the reactivities of additive(s) to promote lateral growth. Improved nucleation can also be attained through substrate engineering. This is shown in Figure 4(a-c), where the morphology/continuity of the electroless film exhibits a strong dependency on Co thickness. Furthermore, agglomeration caused by diffusion of Cu across the substrate during deposition can have a large effect on the final film morphology. This can be seen in figure 4 (d-f), in which differences in deposition morphology across a range of conditions are captured in a simple lateral diffusion simulation.

We will discuss the mechanisms leading to these effects and their impact on the design of systems to produce improved film morphology.

References

(1) P.C. Andricacos, C. Uzoh, J. O. Dukovic, J. Horkans, H. Deligianni, IBM J. Res. Develop. 42, 567 (1998)

(2) J. Reid, Jpn. J. Appl. Phys. 40, 2650 (2001)

(3) I. Ciofi, P. J. Roussel, R. Baert, A. Contino, A. Gupta, K. Croes, C. J. Wilson, D. Mocuta, Z. Tokei, IEEE Trans. Electron. Devices, 66, 2339 (2019).

(4) T.P. Moffat, D. Wheeler, S. K. Kim, D. Josell, J. Electrochem. Soc. 153, C127, (2006)