V.N. Tokarev

Modelling of high-aspect ratio microdrilling of polymers with UV laser ablation

Abstract Based on experimental results, an analytical model of multipulse excimer laser drilling in polymers is developed in this paper. It is shown that the adequate account of the mechanism of radiation propagation and absorption inside the keyhole is important for the good agreement of theory and experiment. The controlling factors of drilling are revealed. Keyhole profile and depth versus incident fluence are calculated for top-hat beam. The matching conditions for laser fluence, parameters of optical scheme and material parameters are derived in an explicit analytical form, allowing to produce deep narrow keyholes with practically parallel side walls and aspect ratios as high as 300–600.

Negative pressure model for surface foaming of collagen and other biopolymer films by KrF laser ablation

A single KrF laser pulse of energy larger than 0.5 J/cm2 is enough to create a microfoam layer on the surface of a collagen film and other related biopolymers. This is a new result that can be of interest for many new applications. The target material is excited in the radiation absorption depth of ~17 µm and expands into a foam layer whose new surface is ~5 µm above the original one. The estimated surface transient temperature of ~83°C at threshold fluence does not account satisfactorily for this fast foaming process but consideration of the bipolar pressure variation ~±200 bar, i.e. laser induced acoustic wave suggests that a cold homogeneous boiling is induced by the tensile part of the pressure wave in the laser excited volume. The classical nucleation theory predicts a spontaneous dense and homogeneous bubble formation when the pressure is negative in the inviscid liquid. These results constitute new examples of laser induced fast expulsion of liquid due to the hydrodynamic pressure wave which can also be considered as resulting from the surface acceleration/deceleration sequence.

Viscosity of transient melt layer on polymer surface under conditions of KrF laser ablation

Abstract An important parameter controlling the mobility of melt layer in laser ablation of polymers is the viscosity. We report the first experiments to measure it at the extreme conditions of ultraviolet (UV) KrF laser ablation for poly(ethylene terephthalate) (PET). The volume of laser-induced bumps (re-solidified molten material expelled from the irradiated area to its periphery) is measured with AFM microscopy. The comparison of this value with an analytical modelling (based on the solution of Navier–Stokes equations for viscous melt flow induced by the gradient of ablation plume pressure with the use of experimentally measured melt depth) allows to calculate the kinematic viscosity of laser melt layer on the polymer surface. It appears to be about an order of magnitude greater than the kinematic viscosity of water at room temperature.