Modeling of velocity and surface temperature of the moving interface during laser ablation of polyimide and poly(methyl methacrylate)

A recently developed model, which is very successful in explaining pulse du ration and spot size dependences of nanosecond laser ablation rates of poly imide (PI) and poly(methyl methacrylate) (PMMA) [H. Schmidt, J. Ihlemann, B . Wolff-Rottke. K. Luther, J. Troe, J. Appl. Phys.. 83 (1998) 5458.], is ap plied to calculate the temporal behavior of position and temperature of the moving interface between the condensed phase of the polymer sample and the ablation plume, i.e., the receding sample surface, during the ablation pro cess. The model describes the polymer as a system of chromophores with two possible electronic states. It is based on the combination of photothermal decomposition and photodissociative bond breaking in the electronically exc ited state. Laser-induced chemical modifications, are incorporated via diff erent absorption coefficients for the initial and fur the UV-modified polym er. Dynamic attenuation of the incoming radiation by the expanding ablation plume and heat conduction are taken into account. The model predicts maxim um surface temperatures during ablation of about 3000 K in the case of PI a nd about 700 K in the case of PMMA. Typical maximum velocities of the movin g interface range from 20 ms (PI) to 100 ms (PM,I IA) for a pulse duration of 20 ns at a fluence of a J/cm(2). The simulations show that the attenuati on of the laser pulse by the Flume of ejected material, which reaches a fac tor of up to 5 shortly after the peak of the pulse, starts to decrease towa rds the end of the laser pulse. These calculations support the significance of three dimensional plume expansion for the increase of ablation rates wi th growing pulse duration and decreasing laser spot size. (C) 1999 Elsevier Science B.V. All rights reserved.