Abstract :
A recently developed model, which is very successful in explaining pulse duration and spot size dependences of
nanosecond laser ablation rates of polyimide PI. and poly methyl methacrylate. PMMA. wH. Schmidt, J. Ihlemann, B.
Wolff-Rottke, K. Luther, J. Troe, J. Appl. Phys., 83 1998.5458.x, is applied 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 process. 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 excited state. Laser-induced chemical modifications are incorporated via different
absorption coefficients for the initial and for the UV-modified polymer. Dynamic attenuation of the incoming radiation by
the expanding ablation plume and heat conduction are taken into account. The model predicts maximum surface
temperatures during ablation of about 3000 K in the case of PI and about 700 K in the case of PMMA. Typical maximum
velocities of the moving interface range from 20 mrs PI.to 100 mrs PMMA.for a pulse duration of 20 ns at a fluence of
4 Jrcm2. The simulations show that the attenuation of the laser pulse by the plume of ejected material, which reaches a
factor of up to 5 shortly after the peak of the pulse, starts to decrease towards the end of the laser pulse. These calculations
support the significance of three dimensional plume expansion for the increase of ablation rates with growing pulse duration
and decreasing laser spot size. q1999 Elsevier Science B.V. All rights reserved
