ULTRAVIOLET-LASER ABLATION OF POLYMERS - SPOT SIZE, PULSE DURATION, AND PLUME ATTENUATION EFFECTS EXPLAINED

A versatile model for ultraviolet (UV) laser ablation of polymers is p resented, which is very successfully applied to the calculation of a v ariety of different properties of this process, including the influenc e of plume attenuation dynamics. The polymer is described as a system of chromophores with two possible electronic states. The model is base d on the combination of photothermal decomposition and photodissociati ve bond breaking in the electronically excited state. Laser induced ch emical modifications are incorporated via different absorption coeffic ients for the initial and for the modified polymer after absorption of UV light. Dynamic attenuation of the expanding ablation plume and hea t conduction are taken into account. The results of the theoretical ca lculations are compared with the results of three different series of experiments performed with polyimide (PI) and polymethylmethacrylate a t the excimer laser wavelength 248 nm and with PI also at 308 nm: (1) Measurement of the ablation rate as a function of fluence for four dif ferent pulse durations between 20 and 250 ns; (2) Measurements of the ablation rate as a function of fluence for five different laser irradi ation spot radii between 10 and 150 mu m, and (3) Time resolved measur ement of the dynamic plume attenuation at the ablating laser wavelengt h as a function of fluence for four different pulse durations between 20 and 250 ns. The model leads to a prediction of etch rates, ablation thresholds, plume attenuation, and surface temperatures during the ab lation process, which is in good agreement with the experimental resul ts. The observed increase of the ablation rate with increasing pulse l ength and with decreasing laser spot size can be explained by the mode l as a consequence of laser induced modified absorption in combination with the dynamic shielding of the expanding plume. (C) 1998 American Institute of Physics.