Laser-induced explosive boiling during nanosecond laser ablation of silicon V. Craciun a,* , N. Bassim a , R.K. Singh a , D. Craciun b , J. Hermann c , C. Boulmer-Leborgne c a Engineering Research Center for Particle Science and Technology, Materials Science and Engineering, University of Florida, P.O. Box 116400, MAE 205, Gainesville, FL 32611-6400, USA b National Institute for Laser, Plasma, and Radiation Physics, Bucharest, Romania c Laboratoire GREMI, Universite d'Orleans, Orleans, France Abstract The surface morphology of single crystal 1 0 0) Si wafers irradiated by 266 and 1064 nm laser pulses emitted by a Nd:YAG laser has been investigated. The morphology of the bottom of craters formed as a result of single or multipulse laser irradiation by the 266 nm wavelength, which is well absorbed by Si optical absorption coef®cient, a 10 6 cm 1 ), remained ¯at and almost featureless up to the maximum ¯uence of 18 J/cm 2 used in this study. The rims of the craters showed signs of radial liquid ¯ow but it is apparent that the vaporization process was con®ned to the surface region. A different morphology was observed at the bottom of the craters formed by the 1064 nm wavelength laser pulses. Because this wavelength is absorbed in volume, a < 10 4 cm 1 , a rather thick liquid Si pool formed at the surface. For laser ¯uences higher than 3 J/cm 2 evidence of boiling sites were observed at the bottom of the crater. By analyzing their formation mechanisms, density and shape, we suggest that they were induced by heterogeneous boiling and not homogeneous boiling, also known as phase explosion. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Nd:YAG laser; Phase explosion; Droplets 1. Introduction The pulsed laser deposition technique has become one of the most versatile for thin ®lm growth. Despite many advantages that explain its widespread use, the presence of micrometer-sized droplets on the surface of the ®lms grown by PLD remains one of the most serious limitations of this new technique. A simple problem that has yet to be explained satisfactorily is the presence of more droplets on the surface of ®lms grown by PLD when using a laser wavelength poorly absorbed by the target than when using a wavelength which is well absorbed. Results obtained using the same laser wavelength for ablation but increasing by doping the optical absorption coef®cient, a, of the target, from transparent, a < 2  10 4 cm 1 , to absor- bant, a > 2 10 5 cm 1 , showed a signi®cant reduction of droplet density [1]. We suggested that this high density of droplets observed should be related to laser- induced explosive volume boiling. Results regarding the morphology of the craters formed on single crystal Si wafers by single or multipulse laser ablation with wavelengths that are poorly or well absorbed by Si are Applied Surface Science 186 2002) 288±292 * Corresponding author. Permanent address: National Institute for Laser, Plasma, and Radiation Physics, Bucharest, Romania. Tel.: 1-352-392-5714; fax: 1-352-846-0326. E-mail address: vcrac@mse.ufl.edu V. Craciun). 0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0169-433201)00766-8