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Topography of Features Machined into Bisphenol A Polycarbonate Using Closed Thick Film Flowing Filtered Water Immersed KrF Excimer Laser Ablation
C.F. Dowding and J. Lawrence

Laser micromachining with the use of liquid immersion allows debris control. Use of an immersion technique potentially modifies the ablation mechanism when compared to laser material interactions in a medium of ambient air. Equipment has been developed to allow feature machining under a controllable liquid film and thereby elucidate and quantify and changes in feature topography caused. The findings of this study revealed that immersion of bisphenol A polycarbonate samples in closed thick film flowing filtered water during KrF excimer laser ablation markedly altered feature geometry, waviness and roughness laser machining in ambient air. Feature geometry definition improved with flow velocity. Waviness was found to be less regular and less predictable and roughness became fluence dependant. Variation of flow velocity during immersion resulted in modification of the surface waviness: an optimum flow velocity exists, producing maximum waviness. Surface roughness displayed a power law relationship with flow velocity. These observed effects are explained through examination of the flow – plume interaction: closed thick film flowing filtered water immersion caused a combination of photomechanical etching promoted by plume confinement, laser etching and plume distortion by rapid flow velocity. Furthermore, the influence each of these interactions varied depending on the flow velocity. This is not an eventuality possible when using an open thin film immersed laser ablation technique: film rupture and splashing limited the plume etching contribution to the confined laser ablation process. It is apparent that the changes to feature geometry, waviness and roughness observed when KrF excimer laser machining under closed thick film flowing filtered water were brought about directly by the immersion, rather than by variations in fluence level.

Keywords: KrF excimer laser, bisphenol A polycarbonate, micromachining, water immersion, topography

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