Emission spectra and etching of polymers and graphite irradiated by excimer lasers

Emission spectra in the visible and ultraviolet range, scanning electron microscopy of various polymers [polyimide, Mylar, and polymethyl-methacrylate (PMMA)], and graphite exposed to 193-, 248-, and 351-nm laser radiation were used to investigate the laser etching process. Measurements were performed under vacuum and in air and He environments. At low laser fluence, which depends on the sample and laser wavelength, an unresolved continuum emission was observed. At higher fluences clear C2, C, and in some cases also CN emissions were found in addition to the continuum. At still higher fluences ionic carbon emissions became dominant. It was found that laser-produced plasma processes are responsible for most of the observed emissions. Rough and smooth etched surfaces were obtained at different laser fluences depending on the linear absorption coefficient of the sample at the laser wavelength. In polyimide, a rough-to-smooth surface transition versus the laser fluence was found to occur exactly when the C2 and C emissions appeared on top of the continuum. It is suggested that the laser etching mechanism is mostly a statistical thermodynamic process, but without complete energy randomization.