Synthesis and Reactivity of Chromium and Vanadium Complexes for Atomic Layer Deposition

Abstract

New vapour phase thin film deposition methods to enable manufacturing of increasingly smaller transistors are in high demand. Atomic layer deposition (ALD) is a thin film deposition method that yields uniquely conformal and uniform thin films through the sequential exposure of vapour-phase reagents to a surface. Typically, two reagents are used: (1) a metal-containing precursor complex and (2) a co-reagent which reacts with the precursor complex to afford the target material. Any by-products of this surface-based reactivity must be volatile, such that they can be removed by interpolated inert gas purges. One ALD cycle consists of exposure of the surface to a precursor, followed by an inert gas purge, followed by a pulse of co-reagent, and then a final inert gas purge. The thin film thickness in an ideal ALD method is controlled solely by the number of ALD cycles utilized. Developing an ALD method requires accurate control of precursor design, so as to achieve the required thermal stability and volatility, as well as the desired reactivity with a chosen co-reagent. This work investigates the feasibility of using homoleptic chromium and vanadium alkoxides as precursors for metal ALD when combined with hydrosilanes or hydroboranes as the co-reagents. The synthesis of [Cr(OtBu)4] (1) and [V(OtBu)4] (2) was carried out according to literature procedures, with some modifications. Complex 1 sublimed at 25 °C (10 mTorr) and decomposed when a neat sample was heated to 110 °C for 24 hours. Complex 2 distilled at 25 °C (10 mTorr) and decomposed when a neat sample was heated to 110 °C (in the dark) for 24 hours. Pinacolborane (HBpin) and phenylsilane (PhSiH3) commercial reagents were explored as potential co-reagents through solution reactivity studies. Decamethylsilicocene (3), and 1,4-bis(trimethylsilyl)-2-methyl-2,5-cyclohexadiene (4), which were synthesized according to literature procedures, and were also explored as potential co-reagents. The reactions of 1 and 2 with HBpin were particularly promising, and suggested the possibility of utilizing these reagents for the development of a new method for chromium and vanadium metal ALD. Additionally, chromium and vanadium complexes containing fluorinated alkoxide ligands, perfluoro-tert-butoxide (pftb), were synthesized in an attempt to increase the thermal stability of the precursors. [CrCl(pftb)2(THF)3] (5) and K[V(pftb)4] (6) were crystallized from these reactions. Complex 5 could not be isolated in pure form, whereas pure 6 was isolated in 59% yield. Complex 6 sublimed at 75-85 °C (10 mTorr) and exhibited some decomposition when a neat sample was heated at 130 °C for 24 hours. Complexes 5 and 6 proved to be far less reactive towards HBpin and PhSiH3 than 1 and 2. In order to determine a suitable oxidizing agent for the synthesis of the target complex, [V(pftb)4], cyclic voltammetry was utilized to determine the oxidation potential of 6. The E1/2 value for 60/+ was determined to be approximately 0.64 V versus [FeCp2]+/0 in 1,2- difluorobenzene.