As the miniaturization trend in integrated circuits fabrication continues, alternative liner materials in copper interconnects have been actively explored. Cobalt has been widely investigated and employed due to its low resistance at nanoscales, integration compatibility, as well as benefit of improving copper electromigration resistance. However, direct electrodeposition of copper onto cobalt remains less explored due to the stability issue of cobalt in acidic copper damascene process. Subsequent processes of copper seeding and copper electroplating face increasing technical difficulties when widths and aspect ratios of electro-fill features become more challenging.

Direct electrodeposition of copper on cobalt has been explored in acidic and alkaline chemistries, both of which were formulated by MacDermid Alpha Semiconductor Solutions. In both cases, well-adherent, nearly-pinhole-free, and ultrathin Cu has been successfully electrodeposited onto cobalt substrate with less than 10% cobalt loss. SEM images in Figure 1 imply uniform dispersion of copper on cobalt. XRF and ICP analysis confirm that the thinnest copper films deposited could be no thicker than 2 nm. Corrosion test in acidic copper plating bath shows that an ultrathin copper layer of thickness less than 2 nm can effectively suppress cobalt dissolution by a factor of three in comparison to non-coated bare cobalt.

This work could pave the avenue towards direct copper electrodeposition onto cobalt liner in semiconductor manufacturing for advanced technology nodes.

Figure 1. Top-down SEM images. (left) As-received cobalt substrate; (center) Cu of 2 nm nominal thickness electrodeposited on cobalt from an acidic solution; (right) Cu of 2 nm nominal thickness electrodeposited on cobalt from an alkaline solution. All three images have the same scale and scale bars are 200 nm.