Nature of "incubation pulses" in the ultraviolet laser ablation of polymethyl methacrylate

Abstract

The term "incubation pulses" is applied to the pulses of ultraviolet laser radiation which are directed initially at a polymer surface and which etch less depth per pulse than subsequent pulses which remove identical depths of material at each pulse. This incubation effect which is particularly strong when the polymer absorbs moderately at the laser wavelength has been studied in polymethyl methacrylate at 193 and 248 nm. The transformation of the surface during incubation pulses has been followed by 1) photographing the blast wave that is produced by the products that emerge from the surface, using a fast (<1 ns) dye laser that is electronically triggered to light the ablation plume after a set delay, and 2) by treating the exposed surface with a solvent to dissolve out any photolyzed material that is left in the exposed area. The results show that ablation, as shown by the ejection of the products at high velocity, occurs even during the incubation period but the surface is not etched at all or only partially. The etch pit is filled with chemically transformed material which is removed by dipping the sample in a solvent. In the photochemical model of Sutcliffe and Srinivasan [J. Appl. Phys. 60, 3315 (1986)], if UV laser ablation is seen as a volume explosion, then incubation can be attributed to insufficient production of the gaseous products that are needed to build up sufficient pressure to eject the large fragments into which the solid is largely broken up.