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

Abstract

• This is an article from the journal, Proceedings of the IMechE, Part B: Abstract: Until now, progress in laser ablation micromachining has been significantly limited with respect to feature miniaturization and output yield by ablation-generated debris. Gasjetting 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.

Analysis of KrF excimer laser beam modification resulting from ablation under closed thick film flowing filtered water

“…Additionally, the modification of the ablation mechanism, to feature a significant plume etching contribution as alluded to in previous contributions [18,19,30], where the drag of immersion fluid flowing past the ablation plume distorted the plume geometry. Varying the flow velocity resulted in modified ablation threshold [18], ablation rate [30] and surface topography [19].…”

“…Varying the flow velocity resulted in modified ablation threshold [18], ablation rate [30] and surface topography [19]. The recorded modification of surface topography demonstrated that the surface roughness was proportional to flow velocity and laser fluence.…”

“…The depth of focus is defined primarily by the characteristics of the demagnifying lens; but, the application of further optics may modify this dimension. Moreover, modifying the ablation mechanism from one that is dependant upon the laser etching provided by a beam in focus, to one that can operate using the etching contribution of a compressed plume by use of a compressed ablation plume, as has been proposed previously [17][18][19]30], could present the possibility of an increased range of focal length that produces an acceptable image that can be deemed to be "in focus". Alternatively, the implication of such a mechanism could equally reduce the range of focal length that produces an equivalent feature definition to that in ambient air.…”

Cavity formation and surface modeling of laser milling process under a thin-flowing water layer

“…As per the model developed in Eq. (21), the comparisons between the experiments and computations on cavity width, depth and roughness were subsequently presented.…”

“…This is due to the water flow that can carry away the cut debris and bubbles from the ablation zone, thereby maintaining the beam quality throughout the process. Lastly, the plasma formation in still water is more significant and stable than in flowing water [21]. A larger plasma size is possibly apparent in the still water, where plasma can grow and exist for a longer period in the confined volume of water, thus attenuating the laser beam to reach and ablate the work material accordingly.…”