Thermal activation in KrF laser ablation of CuCl

Abstract

248 nm excimer laser ablation of carefully prepared CuCl samples is reported, and shown to occur by a predominantly thermal mechanism. Using a quartz-crystal microbalance (QCM) to monitor ablation, a precise and detailed plot of single-pulse mass removal versus incident fluence was obtained for fluences up to 150 mJ/cm2. A two-parameter Arrhenius exponential function was found to fit the experimental ablation data. Calculations of laser-induced surface heating were carried out by use of a finite-difference heating code, formulated in terms of enthalpy. Ablation was observed to commence at a fluence of 25 mJ/cm2, where the calculated surface temperature is approximately 910 K - some 200 K above the melting point. Dynamic ablation was included in the finite-difference calculation by allowing the position of the CuCl surface ξ to vary in time. The best data fit is provided by the zeroth-order kinetic equation: dξ(t)/dt=(16 Å/ns)exp[(-38 kJ/mole)/RTξ], where Tξ is the surface temperature. A thermodynamic calculation shows the average heat of CuCl vaporization in the temperature range from 900 to 2000 K to be near the fit value of 38 kJ/mole. From plots of the ablation depth versus time, the CuCl surface was estimated to recede during the ablation at rates up to 10 cm/s.