Experimental approach to estimate diffusivity of metal organics in supercritical CO2 at high temperatures

Abstract A new experimental method to measure the diffusivity of reactive metal organics in high-temperature (>150 °C) supercritical carbon dioxide (scCO 2 ) is reported. The technique used supercritical fluid deposition (SCFD) to deposit films on planar and high-aspect-ratio (high-AR) trench substrates. SCFD consists of carrying out chemical reactions in scCO 2 and is a commonly used method to form inorganic films. Diffusivity can be estimated by fitting the profile using surface reaction kinetics measured on a planar substrate, as the film thickness profile within a trench is a consequence of the diffusion and surface reactions of the source precursor. The method was demonstrated for titanium-di(isopropoxide)bis(tetramethylhexanedionate) at 250–325 °C and 10–20 MPa. The obtained diffusivities were correlated by the Stokes–Einstein and Chapman–Enskog equations, derived for liquids and gases, respectively. The diffusivity was well fit by the modified Chapman–Enskog equation, incorporating a collision parameter to express the behavior of a supercritical fluid. This modeling indicated that the mass transfer was similar to that of gases, which was consistent with our experimental conditions that were close to gaseous.