LASER-ASSISTED PARTICLE REMOVAL FROM SILICON SURFACES

Laser-assisted particle removal (LAPR) is a new technique capable of removing micron and submicron scale particles from solid surfaces. In LAPR, the contaminated substrates are dosed with water vapor or other condensible gases as an energy transfer medium which preferentially adsorbs in the capillary spaces under and around the particles. The dosed substrate is then irradiated with a pulsed laser causing explosive evaporation of the energy transfer medium and propelling the particles off the substrate surface. In our experiments, LAPR was used to remove 9.5 mum Al2O3, 5 mum Al2O3, 1 mum Al2O3, and 1 mum polystyrene (PS) particles from Si substrates. Removal threshold measurements were obtained using a TEA CO2 laser (TEM00 mode) at wavelengths of 9.6 mum and 10.6 mum. The temperature rise in the energy transfer medium, water, was estimated at the LAPR threshold. The results suggest that superheating of the water droplet is a reasonable mechanism for LAPR. Reflection and scattering for a cw Ar+ laser parallel to the substrate surface at various displacements indicated the presence of a shock wave, water vapor and ejected particles. These results are similar to time-resolved measurements of polymer ablation in which shock wave generation, propagation away from the surface at supersonic speeds and ablated particulate materials travelling at slower speeds were observed. To determine the conversion efficiency and the threshold of the shock wave, we used a self-similar approximation. The results show that we can remove particles from the surface at laser fluences significantly below the shock wave threshold.