Recent Progress on the Modeling of Laser Surface Cleaning
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Recent Progress on the Modeling of Laser Surface Cleaning Y.F. Lu, W. D. Song, B.S. Lukyanchuk, M.H. Hong and W.Y. Zheng Laser Microprocessing Laboratory, Department of Electrical Engineering and Data Storage Institute, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, ABSTRACT Laser cleaning has emerged to effectively remove contaminants from solid surfaces. In this paper, recent progress on laser cleaning has been studied. First, a cleaning model is established for removal of particles from substrate surfaces. The model not only explains the influence of fluence on cleaning efficiency, but also predicts the cleaning thresholds. Following that, the optical resonance and near field effect are discussed for transparent particles with a size of a - 2 (radiation wavelength) which strongly influences the intensity distribution in the contacted area (substrate surface). The characterization of ejected particles during laser cleaning is finally investigated. It is found that the particle distribution curves closely fit to Gaussian curve.
INTRODUCTION Recently, laser cleaning was demonstrated to be an efficient cleaning method for removal of particles from solid surfaces [1-21]. Two types of laser cleaning have been reported in the literature, relying on pulsed laser heating of the solid surfaces without or with the presence of a thin liquid coating. We shall refer to these two types as dry laser cleaning and steam laser cleaning, respectively. For dry laser cleaning, particles can be ejected from particulatecontaminated surfaces by short-pulse laser irradiation. The proposed mechanism of the ejection is fast thermal expansion of the particle and/or solid surfaces, which induces large cleaning force to overcome the adhesion force between particles and solid surfaces. For steam laser cleaning, the proposed mechanism is the momentum transfer from the laser-heated and suddenly
evaporating liquid film to the particles on the solid surfaces. The following discussion will focus on recent progress on laser cleaning including cleaning model for laser-induced removal of particles from solid surfaces, the optical resonance and near field effect for transparent particles and characterization of ejected particles during laser cleaning. CLEANING MODEL Van der Waals force is one of the major forces between a particle and a rigid solid surface. If the shortest distance between the particle and the substrate surface is H0 and the particle has a radius of R, the adhesion energy Ws in absence of deformation should be [22] AR W, (HO) =-H (1) 6H 0
where A is the Hamaker constant which is determined by the material properties of both particle and substrate. H0 is the separation distance between particle and solid surface.
Supposing that the particle is near the surface to a point where H0 = e- (-4 A), the particle begins deforming due to the strong pressure on the contacting area. This status is called "point
J1.4.1
contact". Derjaguin and co-works [22-24] have investigated the deformation induced by the adhesi
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