Rational Design of Metalorganic Complexes for the Deposition of Solid Films: Growth of Metallic Copper with Amidinate Precursors

A fourth-generation copper metalorganic compound, Cu(I)-2-(tert-butylimino)-5,5-dimethyl-pyrrolidinate, was designed, and its chemistry on nickel surfaces was characterized, for use as a precursor for atomic layer deposition (ALD) of thin solid metal films. On the basis of surface science studies with similar acetamidinate, guanidinate, and iminopyrrolidinate complexes, it was concluded that the high (and undesirable) reactivity of these when adsorbed on metal surfaces is due to the lability of their C-N bonds, which can be triggered by beta-hydrogen elimination steps. Accordingly, a ligand was designed without any available hydrogen atoms at these positions. The result is a much more stable reactant. Temperature-programmed desorption (TPD) experiments indicated that dehydrogenation from the new compound on Ni(110) starts only at 450 K, an increase of about 200 K in comparison with any of the earlier-generations ALD precursors. TPD and X-ray photoelectron spectroscopy (XPS) data were used to establish the details of the decomposition mechanism of the ligands, which appears to be initiated by the scission of the iminopyrrolidine C-N bond. Many byproducts are produced, including HCN, N-2, iso-butene, and possibly pyrroline and other olefins such as pentenes. However, all of that occurs at relatively high temperatures, leaving an acceptable temperature window for the use of this complex for the deposition of copper films. An increased stability of the new ligands in our new copper ALD precursor was also observed on SiO2 thin films, attesting to the generality of our conclusions. We suggest that our methodology for the rational design of this ALD precursor, based on studies of its surface chemistry, can be easily extended to other cases.