Heat transfer and material removal in pulsed excimer-laser-induced ablation: Pulsewidth dependence

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Abstract
The pulsewidth-dependent ablation of polyimide, poly(methylmethacrylate), poly (etheretherketone), poly(ethylene terepthalate), and poly(ethersulphone) 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 reaching a threshold temperature determined either from published temperature calculations of the ablating surface or the ceiling temperature. Since heat transfer calculations suggest that this temperature is reached before the end of the laser pulse, it was assumed that the degraded/ablated material continues to attenuate the incoming laser energy for the remaining duration of the laser pulse. Since the fluence-dependent absorption coefficient of this degraded material is unknown, it was obtained by fitting the experimental pulsewidth dependent ablation rate data of Schmidt, Ihlemann, and Wolff-Rottke (unpublished). The resulting values are consistent with mass spectrometric analysis of the ablation products and with the absence or occurrence of incubation. The threshold 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 limiting etch rate dependence on the fluence at the threshold.