Atomic Interdiffusion and Diffusive Stabilization of Cobalt by Copper During Atomic Layer Deposition from Bis(N-tert- butyl‑N′‑ethylpropionamidinato) Cobalt(II) Tyler D.-M. Elko-Hansen, † Andrei Dolocan, ‡ and John G. Ekerdt* ,† † Department of Chemical Engineering, and ‡ Texas Materials Institute, University of Texas at Austin, Austin, Texas 78712-1589, United States * S Supporting Information ABSTRACT: Electromigration of copper in integrated circuits leads to device failure. Potential solutions involve capping the copper with ultrathin cobalt films. We report the properties of cobalt films after deposition on polycrystalline Cu at 265 °C by atomic layer deposition from H 2 and bis(N-tert-butyl-N′-ethylpropionamidinato) cobalt(II) (CoAMD). We find intermixing of Co and Cu producing a transition layer on the Cu nearly as thick as the Co-rich overlayer. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry depth profiling reveal that a finite amount of Cu continuously segregates to the progressing Co surface, minimizing the free surface energy, throughout deposition up to at least 16 nm. The Cu-stabilized Co film initially follows 2D growth and strain-relieving 3D crystal formation is apparent beyond 2 nm of film growth. Depth profiling indicates that Cu likely diffuses within the Co film and along the polycrystalline Co grain boundaries. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis K nown generally to exhibit good adhesion on Cu surfaces, Co and Co alloy films have recently been applied as Cu capping layers to mitigate premature microelectronic device failure due to Cu interconnect electromigration (EM). 1-3 Decreasing microelectronic device dimensions continue to exacerbate the EM-induced self-diffusion of Cu. Therefore, reducing Cu EM-induced failures in back end of line (BEOL) interconnects without increasing resistance-capacitive delay is an ongoing concern. 4 Cu alloys and EM-resistant metal capping layers are possible solutions to the EM challenge; however, capping is preferred to alloying due to resistivity increases associated with alloying Cu interconnects. 5 Co/Cu multilayers have been of great interest for many years for their applications in magnetic and microelectronic materials. The Co/Cu interface and structure has garnered particular interest, and efforts have demonstrated lattice matching of Co on Cu (111) surfaces. Further, it was suggested that Co overlayers may be stabilized by monolayer-thick Cu segregating at the surface and by alloying at the interface. 6,7 While alloying is generally unexpected in Co/Cu (111) systems given their low bulk solubility, it is energetically favorable for monolayers up to ∼50/50 mixtures. 8 Further, Co capping layers have been demonstrated to reduce Cu EM more effectively than silicidation of the Cu surface. Co caps result in less resistance-capacitance increase in the metallization structure than SiCN caps and adhere better to Cu. 1-3,5,9 Currently selective, electrolessly deposited CoWP is a benchmark Cu EM barrier. 1,3,5 Nevertheless, contamination of the adjacent dielectric materials from the plating bath is a concern, and much interest exists to develop alternative deposition methods. 10 Chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes are of particular interest due to their ability to deposit ultrathin and conformal films and their potential application for surface selective deposition of barriers. 11,12 Previous work from Lim et al. demonstrated the deposition of Cu, Ni, and Co transition metals on a variety of substrates from a series of chelating amidinate precursors under ALD and CVD conditions. 13 Bis(N-tert-butyl-N′-ethylpropionamidinato) cobalt(II) (CoAMD) is a proven ALD precursor for Co deposition that is amenable to carrier-gas-based vapor delivery and can be deposited using reducing agents like H 2 or NH 3 rather than O 2 . 14 Avoiding oxidation is important for BEOL components, especially the Cu metallization lines. We have separately demonstrated that CoAMD exhibits self-limiting adsorption on Cu and a preference to deposit on Cu rather than on SiO 2 or carbon-doped oxides that might comprise the BEOL interlayer dielectric materials (unpublished work). The inherent selectivity of CoAMD for Cu over Si-based dielectric materials makes it an interesting candidate for selective-ALD processes. In this study, we deposit sub-20 nm Co films by ALD from CoAMD on Cu substrates to better understand the Co/Cu interface and Co film properties. Understanding the properties Received: February 10, 2014 Accepted: March 13, 2014 Published: March 13, 2014 Letter pubs.acs.org/JPCL © 2014 American Chemical Society 1091 dx.doi.org/10.1021/jz500281k | J. Phys. Chem. Lett. 2014, 5, 1091-1095