Resonant-infrared laser ablation of polymers: mechanisms and applications

Abstract

Resonant infrared (RIR) laser ablation of two model polymer systems is studied. A tunable free-electron laser (FEL) operating in the mid-infrared is used to resonantly excite vibrational modes of polystyrene and poly(ethylene glycol) to initiate ablation. Time-resolved shadowgraph images, coupled with etch-depth measurements and temperature-rise calculations indicate that ablation begins after a superheated surface layer reaches a temperature of ~1000 C and undergoes spinodal decomposition. The majority of the ablated material is then expelled by way of recoil-induced ejection as the pressure of the expanding vapor plume compresses a laser-melted area. For the first time, a consistent argument is presented to describe RIR polymer ablation from beginning to end. Applications of RIR ablation are also emonstrated through thin-film growth of electronic and optoelectronic polymers. Conductive coatings of a commercially available thiophene polymer are made through a vapor-phase growth process, and polymer light-emitting diodes (PLEDs) are made by laser-transfer of a poly(phenylene vinylene) light-emitting polymer in vacuum. The functionality of the deposited films indicates that the RIR laser ablation process does not entirely decompose the con- jugation of the polymers. The mechanisms derived from ablation studies on model polymer systems are used to explain various observations relating to film quality and device performance.