The Impact of Medium Chemistry on Flowing Liquid Closed Immersion Ablation of Bisphenol A Polycarbonate

C. F. Dowding and J. Lawrence

Equipment has been developed to enable KrF excimer laser ablation machining of a bisphenol A polycarbonate sample under closed thick film flowing liquid immersion. The liquid medium is easily changeable, offering the possibility of chemical modification of the material etching mechanism. Previous work using a medium of filtered tap water has proven the ability of this equipment to control debris; however, this medium had the simultaneous consequence of modifying the primary ablation characteristics from those achieved in ambient air. De-ionized (DI) water is a chemically similar medium that displays contrasting electrostatic properties and was used in this work with the intention of modifying the adhesion mechanisms active. Use of DI water resulted in close agreement of ablation characteristics observed using filtered tap water. Etch rate and threshold displayed a marginal loss in machining efficiency by magnitudes of 11.7% and 4.3% respectively when using DI water for immersion of laser ablation compared to filtered water. This loss is proposed to be caused by increased colloidal grouping of small debris particles to form medium sized items that more completely attenuate the laser beam. As with filtered water, the etch efficiency was also found to be flow velocity dependent due to changing fluid flow-plume interaction states. The mode of debris control afforded by the use of DI water as a laser ablation immersion medium was similar to that of filtered water. But, the volume of debris deposited was significantly greater and was deposited in closer proximity to the feature. Electrostatic insulation by DI water allows greater attraction of particles to the surface due to the suppression of Yukawa forces. Moreover, the action of colloidal aggregation of particles caused DI water to deposit a proportionally large volume of medium sized debris when compared to the proportional population of medium sized debris deposited by filtered water. This work demonstrates that choice of medium offers the immersed laser ablation user control of ablation characteristics without modification of laser parameters.

Keywords: excimer, laser ablation, liquid immersion, medium, plume etching.