A PHOTOTHERMAL MODEL FOR POLYMER ABLATION - CHEMICAL MODIFICATION

Ablation of strongly absorbing polymers, exemplified by polyimide, and weakly absorbing polymers, exemplified by poly(methyl methacrylate), is discussed in terms of a photothermal model. Etching was modeled by an absorption step in which the impinging laser light deposits its energy into an assembly of three-state chromophores, resulting in a temperature rise. The ensuing degradation step allowed for pyrolysis as well as chemical alteration of the chromophores. Chromophore rearrangement and pyrolysis were assumed to follow first-order kinetics, the rate constants being calculated according to Eyring theory. The chromophores of polyimide were assumed to rearrange into more weakly absorbing species, whereas the chromophores of poly(methyl methacrylate) were assumed to rearrange into more strongly absorbing species. In this way, incubation effects could be explained. Optically, the second absorption was assumed to be nonsaturable, and the cross section was assumed to be 10 times that of the first absorption. Thus, short temporal pulses exhibited substantial two-photon absorptions. As a result, the ablation threshold and the slope of the depth versus fluence curve were smaller for short laser pulses (10 ps) than for long laser pulses(15 ns), as is observed experimentally. Direct comparison to experimental etch curves was made. The photothermal model afforded a reasonably good semiquantitative description.