Deposition of small organic molecules by the displacement of two immiscible supercritical phases

A new coating process is described (deposition from two immiscible supercritical phases, or DISP) in which a solution of supercritical carbon dioxide (scCO ₂) with a desired solute is displaced by supercritical helium (scHe). After depressurization, the solute is deposited on substrates initially submerged in the coating solvent. Micron-sized particles and thin films of sucrose octaacetate (SOA) were formed on silicon wafer substrate coupons from DISP at relatively low temperatures and pressures (<6500 psi and <60 °C). The particle size, film thickness, and morphology of SOA were characterized as a function of coating conditions-solution concentrations, withdrawal velocities, and pressures. Particles in the range of 1-14 μm in diameter were deposited at low solute concentrations (<0.2 wt % at 4500 psi), whereas films in the range of 0.1-0.5 μm in thickness were deposited at higher solute concentrations (> 1.5 wt % at 4500 psi). Particle sizes decreased with increasing displacement velocity and increasing pressure. Estimates of characteristic times for diffusion and nucleation indicate that DISP is a diffusion-limited process. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology, including defect formations and film roughness. Highly uniform films with low root-mean-square (RMS) roughness (∼10 Å) were obtained at a low displacement velocity of 0.0035 cm/s, while ring-like defect structures were observed in films deposited at a higher displacement velocity of 0.035 cm/s. The film thickness and morphology of the films deposited from DISP were compared with films from normal dip coating with typical organic solvents (acetone and toluene). Films deposited from scCO ₂ by DISP were much thicker, more uniform, and exhibited much fewer drying defects and lower RMS roughness compared with films from the organic solvents.