The characteristics and feasibility of an in-line debris control technique for KrF excimer laser ablative micromachining

To observe KrF excimer laser ablation through thin liquid film of de-ionized (DI) water and the effects thereof on debris control, equipment was designed to contain a small control volume that could be supplied with a fixed flow velocity thin film of DI water to immerse a bisphenol A polycarbonate workpiece. Using the same equipment comparison with ablation in ambient air was possible. The positional debris deposition of samples machined in ambient air was found to 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 was deposited where the shockwaves collide. Ablating under a flowing thin film of DI water showed potential to modify the end position and typical size of the debris produced, as well as increased homogeneity of deposition density. Compared with a sample machined in ambient air, the use of immersion has reduced the range of debris deposition by 17% and the deposition within the boundary of the ablation plume has a comparatively even population density. Unlike samples machined in ambient air, outside the ablation plume extents positional control of deposited debris by thin film flowing DI water immersion was evidenced by rippled flow line patterns, 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 was measured, a result that is in line with a colloidal interaction response... cont'd.