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 de-ionized (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 a modal tendency that is reliant on the machined feature shape and is a function of 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 per cent, 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.