Radiative Striations of Spin-Coating Films: Surface Roughness Measurement and in-situ Observation

Silica gel films were prepared by spin-coating from Si(OC₂H₅)₄-HNO₃-H₂O-ROH solutions and heated at 200°C where ROH = CH₃OH, C₂H₅OH, CH₃OC₂H₄OH and CH₃CH(OH)CH₂OH. Striations were observed with an optical microscope, and quantitatively evaluated by surface roughness measurement. A decrease in height and an increase in spacing of the striations were observed when alcohols of high boiling points were used. However, even when the boiling point of the alcohols was high, an evolution of the striations did occur on spin-coating films, which disappeared while keeping the film in the ambient atmosphere. Convections, which might be the source of the striations, were observed in sol layers placed on a stationary substrate irrespective of the boiling point of the alcohols.