HEAT-TRANSFER AND MATERIAL REMOVAL IN PULSED EXCIMER-LASER-INDUCED ABLATION - PULSEWIDTH DEPENDENCE

The pulsewidth-dependent ablation of polyimide, poly(methylmethacrylat e), poly (etheretherketone), poly(ethylene terepthalate), and poly(eth ersulphone) exposed to 248 and 308 nm, nanosecond UV laser pulses was modeled assuming one-dimensional heat transfer and a published model [ G. H. Pettit and R. Sauerbrey, Appl. Phys. A 56, 51 (1993)] for photon absorption. The polymers were assumed to degrade/ablate after reachin g a threshold temperature determined either from published temperature calculations of the ablating surface or the ceiling temperature. Sinc e heat transfer calculations suggest that this temperature is reached before the end of the laser pulse, it was assumed that the degraded/ab lated material continues to attenuate the incoming laser energy for th e remaining duration of the laser pulse. Since the fluence-dependent a bsorption coefficient of this degraded material is unknown, it was obt ained by fitting the experimental pulsewidth dependent ablation rate d ata of Schmidt, Ihlemann, and Wolff-Rottke (unpublished). The resultin g values are consistent with mass spectrometric analysis of the ablati on products and with the absence or occurrence of incubation. The thre shold fluences and ablation rates predicted by this model are in good agreement with reported literature values; however, the use of a well- defined threshold temperature in the model leads to a different limiti ng etch rate dependence on the fluence at the threshold.