Improved Copper Damascene Wires Using Direct Plate on Cobalt Process
Authors: Brogan Lee J , Liu Yihua , Huie Matthew M. , Reid Jonathan David , Kelly James , Shobha Hosadurga k , Huang Huai , Motoyama Koichi , Hu Chao-kun 
Published: February 2020

We preset an electrochemical damascene fill process for copper which is capable of direct nucleation and fill on a cobalt substrate. This process preserves the benefits of a cobalt liner while simultaneously reducing problems inherent in the use of a PVD copper seed on small interconnect features. The process depends upon the use of an additive which enhances the action of traditional damascene suppressors. This enables improved nucleation on the Co substrate without sacrificing the ability of the bath to produce the bottom-up fill typical of damascene Cu plating. We demonstrate good fill across a wafer on a thin, resistive cobalt substrate, producing lines with better electrical properties than a control created using PVD seed followed by a traditional damascene process.
As integrated circuit features get smaller, the use of electrochemical copper metallization for interconnects has become more challenging. A major reason for this is the requirement for a PVD copper seed layer on top of a cobalt liner to enable traditional damascene plating. In very small features, the seed layer takes up a substantial portion of the feature volume. This produces a serious risk of voids as either the PVD itself or subsequent electrodeposition can close off the feature top before it has been filled with metal. One way to extend the manufacturability of copper interconnects is to eliminate the PVD copper deposition step entirely and instead plate directly on the cobalt liner.
Direct plating of copper onto cobalt is a challenging problem. Cobalt dissolves readily in acidic solutions used for damascene Cu plating. Further, the reduction potential of Cu(II) is such that it will galvanically corrode Co substrates exposed to it. Copper has been successfully plated on cobalt, but previous attempts use heavily complexed alkaline baths to preserve the cobalt substrate. Unfortunately, both high pH and heavily complexed copper inhibit the ability of traditional damascene additives to produce good bottom up fill.
Our approach to this problem has been to use an additive which improves the nucleation of Cu on Co not by complexing the Cu in the bulk solution, but by improving the ability of damascene suppressor molecules to quickly and effectively inhibit Cu plating on newly created Cu surfaces. This fast inhibition makes it more favorable for Cu nuclei to expand across the Co surface rather than growing in a more perpendicular direction. The faster passivation of the surface with Cu in turn dramatically reduces Co corrosion loss to the bath. Because this behavior is produced using traditional damascene additives, bottom up fill can be established after nucleation occurs if plating conditions are properly chosen.
Good nucleation and fill on a 36nm pitch structure can be seen in the benchtop scale fill progression tests shown in Figure 1. Nucleation is slightly rougher than is observed when plating on a Cu substrate, but fill ability is sufficient to compensate. A Sabre electrodeposition tool from Lam Research was used to scale this process up to 300mm wafer plating. In Figure 2 electrical data statistics from these wafers are compared with similar results form a pair of control wafers plated using traditional damascene process on a PVD Cu substrate.

A portion of this work was performed by the Research Alliance Teams at various IBM Research and Development facilities.

Figure 1