Influence of Water on Chemical Vapor Deposition of Ni and Co thin films from ethanol solutions of acetylacetonate precursors

Abstract In chemical vapor deposition experiments with pulsed spray evaporation (PSE-CVD) of liquid solutions of Ni and Co acetylacetonate in ethanol as precursors, the influence of water in the feedstock on the composition and growth kinetics of deposited Ni and Co metal films was systematically studied. Varying the water concentration in the precursor solutions, beneficial as well as detrimental effects of water on the metal film growth, strongly depending on the concentration of water and the β-diketonate in the precursor, were identified. For 2.5 mM Ni(acac)2 precursor solutions, addition of 0.5 vol% water improves growth of a metallic Ni film and reduces carbon contamination, while addition of 1.0 vol% water and more leads to significant oxidation of deposited Ni. By tuning the concentration of both, Ni(acac)2 and water in the precursor solution, the fraction of Ni metal and Ni oxide in the film or the film morphology can be adjusted. In the case of Co(acac)2, even smallest amounts of water promote complete oxidation of the deposited film. All deposited films were analyzed with respect to chemical composition quasi in situ by XPS, their morphology was evaluated after deposition by SEM. Introduction Chemical Vapor Deposition (CVD) and its variants such as Atomic Layer Deposition (ALD) are key technologies in the industrial production of thin metal films for a broad range of applications1. In the CVD family of processes a solid film is deposited from a vapor of precursor molecules by chemical reactions occurring on or in the vicinity of a substrate surface, which is usually heated to thermally stimulate the reaction. Metal CVD can be used to produce fibers, monoliths, foams, and powders of pure metals or alloys with free variation of the composition for catalysis, microelectronic, and optical devices2,3. There are two classes of organic precursors which are routinely used in CVD: organometallic (OM) precursors with metal centers mostly bonded to carbon and metal-organic (MO) precursors such as the class of β-diketonate complexes including the acetylacetonates (acac), hexafluoroacetylacetonates (hfac), dipivaloylmethanates (dpm), and alkoxides of different metals, i.e., with metal centers bonded to typically oxygen, nitrogen or sulfur3,4. Elucidating the CVD reaction mechanisms for these precursors comprehensively and in detail is prerequisite for process optimization and beneficial for the selection and synthesis of new precursors. In this respect, also boundary conditions may play important roles but often lack systematic studies. One of the big problems using solid organic precursors in metal CVD is their sensitivity to air and moisture5,6,7. Generally, the hygroscopic nature of many OM and MO precursors promotes the formation of hydrates which leads to significant configuration changes within the precursor depending on the water content. For example, anhydrous Ni(acac)2 forms trimers in order to achieve an octahedral coordination around each nickel atom. When water is present, each Ni(acac)2 prefers to take up position in an octahedral complex with two water molecules for that purpose7,8,9,10. Such changes may induce strong dependencies of the precursor volatility and thermal stability on the water concentration7,10,11. Additionally, water may influence the precursor adsorption selectivity by formation of OH groups on the substrate surface and affect reaction mechanisms thereon3,9,12,13,14. Overall, the sensitivity of the precursor to moisture may render the control of parameters such as nucleation rate, growth rate, or precursor fragmentation difficult and thus affect quality (morphology) and purity (carbon incorporation) of the CV deposits. Several studies have reported that water affects the reduction mechanisms of β-diketonates, in particular metal (M) (acac)x precursors, employed in classical CVD and ALD regimes. M(acac)x as CVD precursors have been in the focus of interest because they are commercially available low-cost products of minor toxicity and exhibit low evaporation temperatures and easily controllable purity7,15. Only few systematic studies on the influence of water on β-diketonates under CVD conditions are available and their results often appear to be contradictory. For instance, it has been reported that CVD with a β-diketonate such as Cu(hfac)2 as precursor in combination with H2 as reducing agent can be area-selective, i.e., copper, in this case, is deposited on metal substrates but not on oxide areas16,17,18,19. When adding water to the reaction gas mixture, some authors found a loss of the precursor’s selectivity to metal substrates19 while others reported, for apparently similar experiments, that the selectivity is unaffected20. Furthermore, there are several reports on an increase of carbon contamination due to the presence of water in the gas phase during metaloxide (MOx) deposition on soda lime glass substrates from Cu and Ni β-diketonates such as M(acac)2, M(dmg)2, and M(hfac)23,7,9,21. Using the same β-diketonate precursors for obtaining M, MOx, and metalnitride films on SiO2, α-Al2O3 and polyimide substrates, others found, however, a decrease of carbon content when water was added to the CVD reactants mixture22,23,24. Finally, Utraininen et al. found as a general trend that H2O in combination with β-diketonate complexes like Cu/Ni/Pt (acac)x (deposited on glass, SiOx, AlOx, and TiOx) leads to the deposition of metal oxide films during ALD9,11. In accordance with this trend, Borgharkar et al. showed for CVD from Cu(hfac)2 on TiN-coated Si(100) substrates that the metal film growth rate and the electrical conductivity of the deposit can be improved by eliminating H2O from the starting precursor25. In contrast, several other CVD studies (on many different types of substrates including Kapton) reported that addition of water enhanced the metal film nucleation and increased the rate of β-diketonate reduction by H216,17,22,26–28. In practice, controlling water contamination in the entire CVD process is harder than just to care for pure precursor feedstock. As pointed out by Pierson2, a pure reactant can become contaminated in the distribution system to the reactor by, amongst others, moisture even if gas-tight metal lines are used. Therefore, in order to limit costs it is essential to know what grades of purity of precursors, feed gases and reactor lines have to be maintained for good results. Also, as demonstrated above, water may even have beneficial effects on the composition or growth rate of the deposit. Consequently, in order to make use of these effects purposefully and reach optimal growth conditions with reasonable efforts, systematic studies on the influence of water on CVD processes are, as also noted by others21, a prerequisite. In the following we will present a comprehensive study of the influence of water on metal film formation exemplarily in a special field of CVD: the pulsed-spray evaporation (PSE)-CVD from a (...truncated)