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Langmuir 2006, 22, 642-657
Ultrathin Film Deposition by Liquid CO2 Free Meniscus CoatingsUniformity and Morphology Jaehoon Kim,† Brian J. Novick,†,# Joseph M. DeSimone,†,‡ and Ruben G. Carbonell*,† Department of Chemical and Biomolecular Engineering, North Carolina State UniVersity, Raleigh, North Carolina 27606-7905, and Department of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed August 8, 2005. In Final Form: October 25, 2005 Ultrathin organic films of sucrose octaacetate (SOA) were deposited on 12.5 cm diameter silicon wafer substrates using high-pressure free meniscus coating (hFMC) with liquid CO2 (l-CO2) as a coating solvent. The dry film thickness across the wafer and the morphology of deposited films were characterized as a function of coating conditionss withdrawal velocity, solution concentration, and evaporation driving force (∆P). When no evaporation driving force was applied (∆P ) 0), highly uniform films were deposited with thickness in the range of 8-105 Å over the entire concentration range (3-11 wt%). Uniform films were also obtained at low concentrations (3-5 wt%) with a low evaporation driving force (∆P ) 0.0138 MPa). However, films deposited at medium to high concentrations (7-11 wt%) were thicker (110-570 Å) and less uniform, with larger nonuniformities at higher applied evaporation driving forces. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology including drying defects and film roughness. Films deposited without evaporation had no apparent drying defects and very low root-mean-square (RMS) roughness (1.4-3.8 Å). Spinodal-like dewetting morphologies including holes with diameters in the range of 100-300 nm, and surface undulations were observed in films deposited at medium concentration (7 wt%) and low ∆P (0.0138-0.0276 MPa). At higher concentrations and higher evaporative driving forces, spinodallike dewetting morphologies disappeared but concentric ring defect structures were observed with diameters in the range 20-125 µm. The film thickness and morphology of SOA films deposited from 1-CO2 hFMC were compared to those deposited from toluene and acetone under normal dip coating. Films deposited from l-CO2 hFMC were much thinner, more uniform, and exhibited much fewer drying defects and lower RMS roughness.
1. Introduction devices,1-3
Microelectronic chemical and biological sensors,4-7 light-emitting devices and displays,8-10 molecular-level lubricants,11-13 and many other technologies rely on the deposition of uniform, thin (1000-10 000 Å) or ultrathin (
