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

Abstract

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 β-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, N2, 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.

Role of oligomer structures in the surface chemistry of amidinate metal complexes used for atomic layer deposition of thin films

“…There, it is clear that, upon adsorption and dissociation of (1), the copper atoms in the ALD precursor are reduced to a metallic state. This is in good agreement with experimental XPS results [14,15,17,18,20,21,33]. We have in the past argued that the initial dissociation or decomposition of the adsorbed ligands is rate limiting, and that that first step is accompanied by the reduction of the metal ions of the ALD precursor.…”

“…It is worth recalling that the protonated ligand is a stable molecule by itself, and perhaps not prone to easy deprotonation/ dehydrogenation (at the amine group), the step that presumably could trigger further surface decomposition. Some decomposition does take place on the Ni(110) surface, as reported before [21], mainly to produce not only H 2 , HCN, N 2 , and isobutene but also 5,5-dimethyl-pyrrolidine. Here, we can highlight the weak peaks seen in the TPD data in Fig.…”

“…This assignment is tentative, however, because the TPD data are by themselves difficult to interpret, given the extensive overlap with cracking fragments from other species and the possibility of more than one species being formed at this temperature. One additional observation in favor of our proposal is the fact that this dimer desorbs as the Cu centers become reduced to their metallic state [21]. It could be argued that the metal reduction is achieved via a reductive elimination step such as the ligand coupling proposed here; similar chemistry has been seen with other ALD precursors [6,25,27,28].…”

“…The general aspects of the thermal chemistry of (1) on Ni(110) surfaces were already characterized by TPD and X-ray photoelectron spectra (XPS) in the past, and the results reported in a previous publication [21]. The adsorbed molecules were determined to decompose in a stepwise fashion, starting at approximately 450 K. One characteristic feature of this type of decomposition is the emission of H 2 in TPD experiments, which can be used as a proxy to determine the reactivity temperature ranges as well as the yields of the products resulting from the decomposition.…”

“…To address this issue, we in our laboratory have focused on the characterization of copper precursors made with amidinate and related ligands used for atomic layer deposition (ALD) applications [12,13,14,15,16,17,18,19,20,21,22]. We rely on our extensive experience in the study of reaction mechanisms on solid surfaces using modern surface-sensitive techniques [13,23,24,25].…”