Impact of open de-ionized water thin film laminar immersion on the liquid-immersed ablation threshold and ablation rate of features machined by KrF excimer laser ablation of bisphenol A polycarbonate

Abstract

Debris control and surface quality are potential major benefits of sample liquid immersion when laser micromachining; however, the use of an immersion technique potentially modifies the ablation mechanism when compared to an ambient air interaction. To investigate the machining characteristics, bisphenol A polycarbonate has been laser machined in air and under a controllable open liquid film. To provide quantitative analysis, ablation threshold, ablation rate and the attenuation coefficient of the immersing DI water fluid were measured. In ambient air the threshold fluence was measured to be 37 mJ.cm -2 . Thin film immersion displayed two trends: threshold fluences of 58.6 mJcm -2 and 83.9 mJcm -2 . The attenuation of DI water was found to be negligible; thus, the change in ablation rate resulted from increased confinement of the vapour plume by the liquid medium, generating higher Bremsstrahlung attenuation of the beam, lowering the laser etch rate.Simultaneously, splashing motivated by the confined ablation plume allowed release of plume pressure before plume etching commenced. This contributed to the loss of total etching efficiency.Two interaction scenarios were obsereved as a result of splashing: (i) intermediate threshold fluence, where splashing occured after every pulse in a mode that interrupted the flow entirely, leaving an ambient air interaction for the following pulse; (ii) high threshold fluence, where splashing occured for every pulse in a mode that allowed the flow to recommence over the image before the next pulse causing every pulse to experience Bremsstrahlung attenuation. Since attenuation of the immersion liquid was negligible, it is the action of the constrained ablation plume within a thin flowing immersion liquid, the resultant Bremsstrahlung attenuation and splashing events that are the critical mechanisms that modify the primary ablation characteristics.

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

“…This was attributed to irregular but broadly increased plume pressure, as identified by Berthe et al [17], which significantly attenuates the laser beam en route to the sample surface [18]. When using open thin-film immersion, the necessary volume of liquid is not available to confine the ablation plume pressure; thus, a threshold pressure exists in this condition; once exceeded, rupture occurs and plume gas escapes violently from the covering liquid film into the ambient air above, producing liquid splashing as a by-product [19]. Furthermore, surface ripple occurred and hence the depth of liquid above the plume varied with respect to time and position above the sample, resulting in an irregular and nonrepeatable plume etch-rate.…”

“…Furthermore, surface ripple occurred and hence the depth of liquid above the plume varied with respect to time and position above the sample, resulting in an irregular and nonrepeatable plume etch-rate. This was demonstrated by the ablation depth with respect to pulse number ratio fluctuations from the expected linear trend [18,19], given the regular, repeatable surrounding medium used. This technique aims to remove this problem by enclosing the flow in a sealed duct.…”

Effects of closed immersion filtered water flow velocity on the ablation threshold of bisphenol A polycarbonate during excimer laser machining

“…To compound matters, debris can coat machinery, requiring costly downtime for cleaning and servicing [7], or the debris can become airborne in the working environment of tool users, posing potential respiratory health issues [8]. The use a technique involving closed flowing thick film filtered water immersion of the sample during laser ablation has shown promise as a plausible solution for such problems [9,10]; however, the impact of such techniques on the basic laser machining characteristics are not extensively documented; only the effect of thin film open immersion on ablation rate and threshold having been previously detailed [11,12]. This work will explore the impact of closed flowing thick film filtered water immersion laser ablation machining using KrF excimer laser radiation on the ablation threshold of bisphenol A polycarbonate in comparison to the machining properties of the same material in ambient air and, furthermore, the importance of liquid flow velocity, V, to the ablation threshold will also be explored in detail in this work.…”

“…This was attributed to irregular but broadly increased plume pressure, which significantly attenuates the laser beam en-route to the sample surface [12]. In a thick film regime, the volume of liquid above the ablation plume confines the plume expansion and prevents it from free expansion in the manner allowed by the less viscous medium of ambient air, thus the compressed, high pressure ablation plume attacks the surface of the sample to be machined causing a high etch rate due to the plume which more than compensates for the loss of laser etching due to inverse Bremstrahlung attenuation [23,24,25,12].…”

“…In a thick film regime, the volume of liquid above the ablation plume confines the plume expansion and prevents it from free expansion in the manner allowed by the less viscous medium of ambient air, thus the compressed, high pressure ablation plume attacks the surface of the sample to be machined causing a high etch rate due to the plume which more than compensates for the loss of laser etching due to inverse Bremstrahlung attenuation [23,24,25,12]. When using open thin film immersion, the same volume of liquid is not available to confine the ablation plume pressure, causing a threshold pressure exists in this condition; once exceeded, rupture occurs and plume gas escapes violently from the covering liquid film into the ambient air above, producing liquid splashing as a by-product [12]. The action of turbulence, instigated by the non-symmetrical drag profile, combined with the strong contribution of meniscus instability such as inertial, capillary and viscous effects, described schematically in Figure 1 …”