High throughput microfabrication using laser induced plasma in saline aqueous medium

We report a novel micro-fabrication process that achieves high material removal rate by utilizing thermal ablation by a laser-induced plasma inside an aqueous medium with added salinity. The process involves focusing a pulsed laser beam of visible wavelength within the saline solution, leading to optical breakdown and subsequent formation of a localized plasma plume. Therefore higher material removal rates and greater aspect ratios of machined features can be obtained as compared to laser ablation, while preserving its resolution and precision of machining. The underlying phenomenon behind the process is the enhancement of plasma energy density due to availability of dissociated ions from salt, combined with an increase in optical attenuation coefficient of the medium, leading to an optimum salinity level for maximum material removal. Micro-channels were machined on 6061 Aluminum mirror-finish flat samples by this process, and the material removal rate was found to be higher by up to 97% for an optimized salinity level of 2-4 g/100 ml, as compared to that obtained by pure aqueous media (distilled water). Moreover, the machining characteristics correlated with the experimentally determined plasma generation thresholds for the corresponding salinity levels. In this paper, the experimental results for plasma generation thresholds, plasma fluence and material removal parameters (depth, width, and material removal rates) are presented.