KRF-LASER ABLATION OF POLYURETHANE

Polyurethane can be effectively and cleanly ablated with 248 nm excimer-laser radiation. For fluences above 200 mJ/cm2, very little post-ablation debris is observed - a fact indicative of a polymer that decomposes readily into volatile, small molecular-weight compounds. Ablation-rate data have been obtained both by stylus profilometry and the quartz-crystal microbalance (QCM) technique, and the results of both methods are in good agreement. The more sensitive QCM technique first detects material removal near 20 mJ/cm2, which is likely due to out-gassing, surface chemistry, or low quantum-yield processes. At 37 mJ/cm2, an ablation ''threshold'' with a sharp increase of the ablation rate is observed and marks the onset of efficient, explosive ablation. The densely sampled rate curve provided by the QCM permits the conclusion that an Arrhenius-like exponential does not give a satisfactory fit to the data. This demonstrates that the ablation process is not solely governed by thermal processes. Applying a Beer's-law analysis of rate versus the natural logarithm of the fluence yields excellent agreement with the data up to 300 mJ/cm2. The absorption coefficent derived from this analysis agrees well (within 4%) with the value obtained from the low-intensity absorption spectrum.