Deposition of poly[2-(perfluorooctyl)ethyl acrylate] from liquid CO2 high-pressure free meniscus coating—Uniformity and morphology

Ultrathin fluoropolymer films were prepared by depositing poly[2-(perfluorooctyl)ethyl acrylate] (PFOEA) on 12.5 cm diameter silicon wafer substrates using high-pressure free meniscus coating (hFMC) with liquid CO2 (l-CO2) as a coating solvent. Dry film thickness across the wafer substrate and the morphology of deposited films were characterized as a function of coating conditions—withdrawal velocities, solution concentrations and evaporation driving forces (ΔP). Thickness measurements by ellipsometry revealed that at zero or low evaporation driving forces (ΔP = 0–0.0138 MPa), highly uniform films with thicknesses in the range of 7–30 nm were deposited over the entire concentration range (1–7 wt.%). However, films deposited at high evaporation driving forces (ΔP = 0.0414–0.0552 MPa) or larger concentrations (5–7 wt.%) with a ΔP of 0.0276 MPa were thicker (35–70 nm) and less uniform. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology including drying defects and film roughness. Films deposited at zero or low ΔP of 0.0138 MPa and low concentrations of 1–3 wt.% exhibited few drying defects and a low root mean square (RMS) roughness (∼2 nm). At higher evaporation driving forces and higher concentrations, ring-like drying defects were observed with diameters ranging 5–30 μm. The film thickness and morphology of PFOEA films deposited from l-CO2 hFMC were compared to those deposited from 1,1,2-trichlorotrifluoroethane (Freon113) by normal atmospheric dip coating. Films deposited from l-CO2 hFMC were much thinner and more uniform, and exhibit much fewer drying defects and lower RMS roughness.