Excimer Laser Machining of Bisphenol a Polycarbonate under Closed Immersion Filtered Water with Varying Flow Velocities and the Effects on the Etch Rate

Abstract

Until now, progress in laser ablation micromachining has been significantly limited with respect to feature miniaturization and output yield by ablation-generated debris. Gas-jetting techniques have proven to be inadequate and vacuum environments are unwieldy in an industrial setting. To this end, a controlled geometry for both the optical interfaces of a flowing liquid film can be provided by a closed flowing thick-film filtered water immersion technique. This ensures repeatable machining conditions and allows control of liquid flow velocity. To investigate the impact of this technique on etch rate, bisphenol A polycarbonate samples have been machined using KrF excimer laser radiation passing through a medium of filtered water flowing at a number of flow velocities that are controllable by modifying liquid flowrate. A mean increase in etch rate of 8.5 per cent when using a turbulent flow velocity regime immersed ablation over ablation in ambient air was recorded. However, use of laminar flow velocities resulted in a mean loss of 26.6 per cent in etch rate compared to ablation in ambient air. Plotting the recorded etch rate achieved with respect to flow velocity gives support for previously proposed flow—plume interactions: the primary cause of a 37 per cent variance in etch rate over a 72 per cent change in laminar flow velocity was a shift in the ratio between the refresh rate of liquid volume over the feature and laser repetition rate. The small variance of etch rate achieved by modification of turbulent regime flow velocity indicates that laser etching provided the dominating contribution to the total etch rate measured. This work demonstrates that this technique developed for ablation debris control does not reduce the efficiency of laser etching with respect to that achieved with established gas media laser ablation machining. Therefore, this process shows great promise for industrial implementation development.