Use of thin laminar liquid flows above ablation area for control of ejected material during excimer machining

Abstract

To observe excimer laser machining through thin liquid films and the effects thereof on debris control, equipment was designed to contain a small control volume that can be supplied with a laminar thin film of DI water to flow over the workpiece. Using the same equipment, comparison with non-liquid ablation was possible. Reliable calculations of the debris size and density with respect to the distance from the centre of the shot, as well as the identification of modal trends in the dispersion of the debris were obtained from analysis of microscope images with graphical analysis software. The results suggest that the positional debris deposition of samples machined in ambient air show modal tendency reliant on the feature shape machined and according to species size. This is proposed to be due to the interaction of multiple shockwaves at the extent of ablation plumes generated at geometry specific locations in the feature. Debris is deposited where the shockwaves collide. Large debris did not typically travel much further than the boundary of the machined feature, whereas intermediate debris was found in radial streaks at a normal to the circular feature’s perimeter. Laminar flows of liquid have shown potential to modify the end position and typical size of the debris produced, as well as increased homogeneity of deposition density. The use of immersion has reduced typical range by 17%, and the deposition within the boundary of the ablation plume has a comparatively even population density with respect to the sample machined in ambient air. Outside the ablation plume extents, evidence of positional control of deposited debris species by laminar flow DI water immersion is shown by debris deposition in rippled flow line patterns for both circular and square features, indicating the action of transport by fluid flow. A typical increase in debris size by an order of magnitude when using DI water as an immersing liquid has been measured, a result that is in line with a colloidal interaction response

Ablation debris control by means of closed thick film filtered water immersion

“…This work shows that minor modification to the beam focal point can cause a marked change in the cross sectional geometry of the machined feature. This is an outcome that is confirmed by the poor geometry of the features machined using open thin film flowing liquid immersed KrF excimer laser ablation [28], which had a rippled and variable surface. Furthermore, there is the ability to control (and increase) the flow velocity of the fluid though the duct; this will increase the drag force imparted on a debris particle by the flow.…”

“…In an open flow, turbulence commences following a characteristic distance measured between the contact point of a fluid on a flat plate and the point at which the flow becomes turbulent [33]; however, this case requires all dimensions of the flow, including film depth, to be large compared to the characteristic distance. In the case of this flow, the meniscus defines the flow geometry; thus, the effect of rolling eddies alone are not responsible for the deposition patterns evidenced by Dowding and Lawrence [28], where debris distribution appeared to lie in ripple patterns downstream of the feature machined. Instead, large inertial, capillary and viscous contributions complicated the flow path.…”

“…The production of nano and micro particles has also caused immersion of the sample during lasing to be experimented with, proving to entirely capture all ablation generated debris [18,19], so use of a liquid medium to control ablation generated debris during machining appears achievable. Sattari et al [27] used ablation of ceramics immersed in 4 to 6 mm of distilled water running at a constant flow rate of 116 ml/min to produce a controlled collection of nanoparticles at the surface of the immersing liquid, Dowding and Lawrence [28] cite the action of surface tension at the perimeter of microbubbles formed by ablation plume gas trapped in the immersing liquid as a cause for larger colloidal debris products that are resultant of liquid immersed ablation [21,29]. Particle productivity has been shown to be proportional to liquid flow velocity [27].…”

“…Katto et al [31] found that particle size was governed by the wavelength of the laser (a result common with traditional laser machining in gaseous mediums) and was proportional to the fluence of the beam. Dowding and Lawrence [28] used open thin film immersion to successfully demonstrate control of debris from the ablation spot, with 90% of debris found deposited downstream of the ablation spot whilst using only a low flow velocity. The use of open immersion has significant limitations: surface ripple, rolling turbulence and limited flow velocity.…”

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.…”