CO2 Laser Cleaning of Hydrophilic Oxidized Silicon Surfaces

Abstract

ABSTRACTThe removal of particles as small as 0.1 μm was achieved using a pulsed CO2 laser to induce the explosive vaporization of condensed water. The surfaces used were hydrophilic oxidized silicon. The contaminant particles were 0.1 μm alumina, 0.1-0.2 μm fumed silica, and 0.1 μm polystyrene latex; their zeta potentials in water vary from positive to negative. The effect of the laser beam energy flux on the cleaning efficiency was thoroughly investigated. It was varied between 0.5 and 3 J/cm2. Whatever the nature of the contaminants, the cleaning process was characterized by an upper limit, the surface damage threshold energy density, determined to be 1.5 J/cm2. If the removal efficiency drops for the lowest beam energy flux, i.e., 0.5 J/cm2, and 0.1 μmAl2O3 particles, no single, sharp removal threshold was observed. Another parameter of importance investigated was the thickness of the condensed water. It was varied by changing the time of exposure of the substrate surface to water vapor before laser irradiation, the vapor flow being fixed to 4700 ml/min. Exposure times ranging from 1.5 to 2.5 s were evaluated to be the most effective.