Chemical Fluid Deposition: Reactive Deposition of Platinum Metal from Carbon Dioxide Solution

Chemical vapor deposition (CVD) is an established, versatile technique for the preparation of high-quality metal and semiconductor thin films on solid surfaces.1,2 Despite its utility, constraints inherent to the process, including the requirement of high precursor vapor pressure and thermally coupled transport and deposition steps, generally preclude its use at low temperature and for non-line-of-sight applications such as the metallization of microporous and mesoporous supports. In this report, we describe chemical fluid deposition, a fundamentally new approach to metal deposition that involves the chemical reduction of soluble organometallic compounds in supercritical carbon dioxide at low temperature.3 The process circumvents the limitations described above and yields CVD-quality deposits on polymer and inorganic solid surfaces and within porous inorganic supports at modest temperature (80 °C). In thermal CVD, a volatile precursor is transported into a deposition chamber by means of a carrier gas. The precursor adsorbs to a heated surface and reacts to yield a metal atom and surface-bound ligand decomposition products, which subsequently desorb from the nascent surface and are removed from the reactor. Inorganic precursors such as metal halides produce pure metal films, but deposition temperatures are prohibitively high, often in excess of 600 °C. Organometallic or metal-organic compounds can yield metal films at less severe conditions, but temperatures above 250 °C are usually required to maintain acceptable purity and deposition rates. Under these conditions, film purity and precursor decomposition rates are often enhanced by the addition of a reducing agent such as hydrogen gas. Current research in metal CVD is directed at achieving still lower deposition temperatures through precursor design.4 Reduced temperatures would suppress the development of thermal-mechanical stress during device fabrication,5 minimize interdiffusion and reaction between adjacent layers, and accelerate the development of polymer-based dielectrics. Unfortunately, since desorption of the ligand decomposition products is thermally activated, reducing CVD deposition temperature typically increases contamination of the film. Moreover, low-temperatures exacerbate precursor volatility constraints: low concentrations of precursor in the vapor can result in mass-transfer limitations to film deposition, position dependent growth rates, and thus nonuniform films. Therefore, at low temperature, the combination of low precursor volatility, unfavorable adsorption equilibrium constants for ligand desorption, and sluggish decomposition kinetics are at odds with the nature of the thermal CVD. Chemical fluid deposition (CFD) offers a flexible alternative to CVD. The key to CFD is the physicochemical properties of the solvent, which lie intermediate to those of liquids and gases. Table 1 compares process parameters for metal deposition from vapor, liquid, and supercritical fluid (SCF) media. The density of supercritical CO2 can approach or exceed that of liquids, and thus it can be a good solvent for organometallic compounds and their organic decomposition products. Consequently, precursor transport occurs in solution and reduction occurs at the solution/solid interface at significantly lower temperatures and higher reagent concentrations than those of vapor-phase techniques such as CVD. Moreover, while the presence of SCF CO2 as solvent facilitates desorption of ligand decomposition products, it adsorbs only weakly to metal surfaces and is unlikely to cause contamination or compete with precursor for active sites. Finally, although CFD is solution-based, the transport properties of the SCF5 (low viscosity and high diffusivity relative to liquids), the absence of surface tension, and its miscibility with gaseous reducing agents, such as H2, render the process unencumbered by issues of poor mass transfer and poor deposition rates associated with liquid phase reductions. Thus, CFD uniquely combines the advantages of CVD and liquid-phase epitaxy while minimizing the disadvantages of each. Here we validate CFD for the deposition of CVDquality platinum metal films on silicon wafers and polymer substrates via hydrogenolysis of dimethyl(cyclooctadiene)platinum(II) (CODPtMe2) at 80 °C. Pt films have a number of uses, such as corrosion resistant contacts for microelectronic devices. We also demonstrate that independent control of the transport (via solution) and deposition mechanisms (via chemical reducing agent) renders CFD effective for the metallization of porous solids. This deposition scheme is particularly well-suited for the preparation of nonacidic supported Pt catalysts. We choose CODPtMe2 as the † Department of Chemical Engineering. ‡ Department of Polymer Science and Engineering. (1) Hitchman, M. L.; Jensen, K. F., Eds. Chemical Vapor Deposition Principles and Applications; Academic Press: London, 1993. (2) Hampden-Smith, M, J.; Kodas, T. T., Eds. The Chemistry of Metal CVD; VCH: Weinheim, 1994. (3) Watkins, J. J.; McCarthy, T. J. Method of Chemically Depositing Material on a Substrate. U.S. Patent 5,789,027, 1998. (4) Hampden-Smith, M, J.; Kodas, T. T. Chem. Vap. Depos. 1995, 1, 8. (5) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice; Butterworth: Boston, 1986. Table 1. Comparison of Reduction Media for the Deposition of Metal Films