Surface Chemistry of Copper(I) Acetamidinates in Connection with Atomic Layer Deposition (ALD) Processes

Given the high stability of amidines, they have long been considered good choices as bidentate ligands in organometallic compounds. Early uses of metal amidinates have been reported in catalysis to promote polymerizations and other related reactions. 3 More recently, metal amidinates have been developed as promising precursors for the deposition of solid thin films. 8 They have proven particularly useful in atomic layer deposition (ALD) processes, where the surface chemistry of film growth is split into two self-limiting and complementary halfreactions in order to control the deposition at a monolayer level. 11 Ideally, the metal-based precursor in metal ALD processes should adsorb on the substrate until saturation of a monolayer, preferably retaining the structure of most if not all of its ligands intact, after which a second reactant is introduced to remove those ligands and to activate the surface for the next ALD cycle. Unfortunately, most organometallic precursors used in ALD do show some secondary thermal chemistry involving the conversion of the ligands during adsorption into new surface species. This is often undesirable because the new adsorbates may bind strongly to the surface and remain there even after exposure to the second reactant. Such side reactions are the most common source of impurities in the films grown by chemical means and the reason why stable ligands are sought for these applications. In the case of amidinates ligands, for instance, the hope is that those moieties survive the first half of the cycle so they can be hydrogenated to the corresponding amidine in the second half. To date, though, not much is known about the chemistry of either metal amidinates or amidines on solid surfaces. An infrared absorption study on the chemistry of lanthanum(III)-tris-N,N0di-iso-propylacetamidinate on a hydrogen-terminated Si(111) surface indicated the incorporation of acetate/carbonate and hydroxyl impurities in the growing films. It was also established in that work that deposition at 573 K, a temperature low enough to prevent the formation of interfacial SiO2, leads to decomposition of the adsorbed ligands and the formation of cyanamide or carbodiimide surface species. Analogous chemistry has also been proposed for the gas-phase chemistry of a related copper guanidimate compound. Amore recent report on the surface chemistry of copper(I)-N, N0-di-sec-butylacetamidinate on SiO2 shows a cleaner surface chemistry, with initial adsorption occurring via displacement of one of the ligands at a surface hydroxide site. According to that work, subsequent exposure to molecular hydrogen leads to the hydrogenation of most of the remaining N,N0-di-sec-butylacetamidinates to free N,N0-di-sec-butylacetamidine vapor. Reattachment of some of the released acetamidine to the SiO2 surface was suggested as the source of the carbon contamination seen during the initial cycles of growth. Finally, our initial work on the deposition of the same copper precursor on a Ni(110) singlecrystal surface identified a temperature window between