Analysis of the excimer laser ablation characteristics of bisphenol a polycarbonate in ambient air and under thin film laminar flow water immersion

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 in comparison with a common ambient air interaction. To investigate and quantify the possible gains and losses, equipment has been developed to allow feature machining under a controllable liquid film. The results showed a decrease in surface roughness at the bottom of the feature when using liquid immersion. To observe comparative ablation threshold and ablation rate measurements have been conducted on samples ablated in ambient air and immersed in a thin film laminar flow of water. In addition, to identify the cause of any differences the attenuation coefficient of the immersion liquid has also been measured. A change in ablation rate and ablation threshold was observed that is not directly explained by the attenuation coefficient of the water medium. It is believed that the change in ablation rate is generated by the increased confinement of the vapour plume by a liquid medium during immersion ablation, which in turn generates higher Bremsstrahlung attenuation of the beam; hence a greater proportion of material removal is conducted by the vapour plume during immersion ablation than in a dry interaction. On account of this vapour plume confinement the observed decrease in surface roughness resulted. It is believed this was due to the plume of the compression caused by immersion generated high vapour turbulence inside the plume causing a more homogenous removal rate of material than a homogenized excimer beam profile.