Optical field enhancement effects in laser-assisted particle removal

We report on the role of local optical field enhancement in the neighborhood of particles during dry laser cleaning (DLC) of silicon wafer surfaces, Samples covered with spherical colloidal particles (PS, SiO2) and arbitrarily shaped Al2O3 particles with diameters from 320-1700 nm were cleaned using laser pulses with durations from 150 fs to 6.5 ns and wavelengths ranging from 400-800 nm. Cleaned areas were investigated with scanning electron and atomic force microscopy. Holes in the substrate with diameters of 200-400 nm and depths of 10-80 nm, depending on the irradiation conditions, were found at the former positions of the particles. For all pulse durations analyzed (fs, ps, ns), holes are created at laser fluences as small as the threshold fluence. Calculations of the optical field intensities in the particles' neighbourhood by applying Mie theory suggest that enhancement of the incident laser intensity in the near field of the particles is responsible for these effects. DLC for sub-ns pulses seems to be governed by the local ablation of the substrate rather than by surface acceleration.