Laser-assisted cleaning of thin film coatings of organic materials

The samples used in the experiments were PMMA and epoxidic resin thin films, deposited on silicon, glass, and Al₂O₃-TiC substrates by spin coating. The film thickness varies between 0.5 and 1.5 μm. The samples were processed with KrF and ArF excimer laser radiation (248 and 193 nm, respectively). High cleaning efficiency was obtained for both contaminant materials, but the respective cleaning mechanisms are entirely different. The epoxidic resin is a strong radiation absorber at both wavelengths used. As a result, radiation is absorbed in the upper layers of the thin film and stripping of the coating occurs by progressive material removal from top to bottom, as new layers are ablated at each laser pulse. Conversely, PMMA is a weak absorber. A simple model based on the Beer-Lambert law is proposed to explain the stripping behaviour of epoxidic resin films and other strongly absorbing materials. Another semiempirical model was developed to explain the stripping of PMMA thin films.