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 st udy this approach is applied to the ablation of PMMA by 308 nm irradia tion (XeCl excimer laser). Substituted phenyltriazene and diphenyltri azene compounds, two pentazadienes and a hexazadiene are tested as abl ation promoters in concentrations of 1, 2, and 5 wt.%, respectively. F rom all of these compounds, nitrogen is released upon photochemical de composition. A significant influence of the PMMA molecular weight on t he ablation characteristics is found: higher molecular weights result in lower ablated depths per pulse. The etch rates achieved for the var ious dopants are correlated with the photophysical and photochemical p arameters (i.e., absorption cross section and photolysis quantum yield ) in solution. Above a characteristic minimum concentration of the add itive, the ablated depth is approximately inversely proportional to th e dopant concentration. In the regime of low laser fluence, the ablate d depth per pulse scales with the logarithm of fluence, and is proport ional to the quantum yield of photolysis in solution. For the limiting etch rate at high fluence, no correlation with the solution absorptio n cross section was found.