DOPANT-INDUCED LASER-ABLATION OF PMMA AT 308-NM - INFLUENCE OF THE MOLECULAR-WEIGHT OF PMMA AND OF THE PHOTOCHEMICAL ACTIVITY OF ADDED CHROMOPHORES

Polymers which are not absorbing at the wavelength of irradiation may be sensitized by doping with low concentrations of suitable compounds for laser-induced surface modification and ablation. In the present study this approach is applied to the ablation of PMMA by 308 nm irradiation (XeCl* excimer laser). Substituted phenyltriazene and diphenyltriazene compounds, two pentazadienes and a hexazadiene are tested as ablation promoters in concentrations of 1, 2, and 5 wt.%, respectively. From all of these compounds, nitrogen is released upon photochemical decomposition. A significant influence of the PMMA molecular weight on the ablation characteristics is found: higher molecular weights result in lower ablated depths per pulse. The etch rates achieved for the various dopants are correlated with the photophysical and photochemical parameters (i.e., absorption cross section and photolysis quantum yield) in solution. Above a characteristic minimum concentration of the additive, the ablated depth is approximately inversely proportional to the dopant concentration. In the regime of low laser fluence, the ablated depth per pulse scales with the logarithm of fluence, and is proportional to the quantum yield of photolysis in solution. For the limiting etch rate at high fluence, no correlation with the solution absorption cross section was found.