TRIAZENE POLYMERS DESIGNED FOR EXCIMER LASER ABLATION

The excimer laser-induced ablation of triazene polymers at a wavelength of 308 nm has been investigated as a function of laser fluence deposited on the surface in one or several pulses. PMMA, which as a homopolymer can not be ablated at 308 nm, has been sensitized for ablation at this wavelength by synthesizing a copolymer in which 0.3 mol-% of the side chains contain the triazene functional groups. The high ablated depths per pulse achieved in this manner are similar to the ones observed upon physical doping of PMMA with monomeric triazene compounds. In an alternative approach, the triazene functional group is introduced once (polymer TP 1) or twice (TP 2) into each repeating unit of the polymer backbone by a polycondensation reaction. Irradiation of TP 1 gives rise to ill-defined ablation profiles with sloping wall. In contrast, clean circular ablation profiles are obtained with polymer TP 2, which are characterized by circular contours, steep edges, and flat bottoms of the ablated craters. The origins of these differences are investigated. A comparative study of excimer laser ablation of TP 2 at 248 and 308 nm shows that the latter wavelength is more effective; the plateau value of the ablated depth per pulse corresponds to almost-equal-to 1 mum at 248 nm and almost-equal-to 3 mum at 308 nm. This dependence is attributed to the photolysis behaviour of the triazene compounds: nitrogen released upon photolytic bond cleavage acts as a driving gas which promotes the ablation. As a consequence, no 'incubation pulses' are required for triazene polymer ablation in the investigated fluence range.