Ablation of polymers with pairs of ultraviolet laser pulses with controlled temporal separation

Abstract

In our dynamic model1 for the interaction of ultraviolet laser pulses with condensed organic matter, it was proposed that successive absorption of two or more UV photons by the chromophores in the solid resulted in excitation to high electronic excited states and was followed by decomposition and ablation. Since the population of the upper excited states would depend on the power density of the radiation at the surface of the polymer, the width of the laser pulse is emphasized in this model. In earlier work2 the success of this model in fitting data which were obtained at one laser pulse width was realized. In the present work, the width of the laser pulse was varied from 40ns to 100ns by using pairs of identical laser pulses (each of FWHM = 40ns) which were separated by a set time interval. The response of PMMA to etching by pairs of pulses suggests that short-lived species which may be electronic states (e.g. triplets) and/or radicals play an important part in polymer ablation. The etching of polyimide by these extended pulses shows trends that are opposite to those observed in PMMA. In this instance, the shielding of the latter portion of the incoming pulse by the products that are ablated by the front portion is probably a serious effect. © 1989 SPIE.