Viscosity of transient melt layer on polymer surface under conditions of KrF laser ablation F. Weisbuch a , V.N. Tokarev a,1 , S. Lazare a,* , D. De Âbarre b a Laboratoire de Physicochimie Mole Âculaire LPCM), UMR 5803 du CNRS, Universite  de Bordeaux 1, 351 Cours de la Libe Âration, F-33405 Talence, France b IEF UMR 8622, Universite  Paris XI, 91405 Orsay, France Abstract An important parameter controlling the mobility of melt layer in laser ablation of polymers is the viscosity. We report the ®rst 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-solidi®ed 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 ¯ow 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. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polymers; Melt layer; Ablation; Excimer laser; Viscosity; Ablation pressure 1. Introduction Understanding and modelling the mechanisms of laser ablation of polymers and search for ways to improve the quality and precision of laser processing demand to recognise quantitatively accompanying melting phenomena and to have an information on their controlling parameters. An important parameter controlling the mobility of melt and the stability of irradiated surface is the polymer viscosity at the stage of laser ablation. It has been shown recently that high enough viscosity and/or high absorption coef®cient) strongly stabilise the surface of polymers with respect to the excitation of wavy disturbances in the laser-irradiated zone [1] and suppress the lateral melt ¯ows from the centre to the periphery of the spot [2] thus providing a clean sharp spot border free of re-solidi®ed melt known in the literature as clean precise laser ablation). On the other hand, a low viscosity and/or low absorption coef®cient) of the polymer gives an opposite effectÐit provides high mobility of the melt thus destabilising the surface relief and giving signi®cant amount of speci®c melt bump expelled to the border of the spot and re-solidi®ed here. The available experimental and modelling data describe viscosity of polymers in temperature range only slightly exceeding the melting point, i.e. at about 300±350 8C see [3]). However, there is no data on viscosity at the extreme conditions of laser ablation Applied Surface Science 186 2002) 95±99 * Corresponding author. Tel.: 33-5-5684-6975; fax: 33-5-5684-6645. E-mail addresses: tokarev@kapella.gpi.ru V.N. Tokarev), lazare@frbdx11.cribx1.u-bordeaux.fr S. Lazare). 1 On leave from General Physics Institute, ul. Vavilova 38, Moscow 117942, Russia. 0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0169-433201)00595-5