Viscous drag force effect on transportation of submicron particle contaminants during dry and steam laser cleaning

Abstract

Viscous air drag and diffusive Brownian motion result in unfavorable re-deposition of sub-micron spherical particle contaminants after their dry laser-assisted detachment from critical surfaces. Theoretical modeling and experimental results on particle transportation in air and thin variable liquid layers point out particle size and lift-off velocity as the most important parameters for efficient particle removal from critical substrates, while for smaller particles with lower inertia, lower lift-off distances and higher diffusion rates dry laser cleaning is less effective due to their fast diffusive redeposition back on these substrates. Under these circumstances one excellent option is the steam laser cleaning technique, when contaminating particles lift-off together with explosively boiling pre-deposited layer or separate micronsized droplets of a low-boiling liquid energy transfer medium and travel in the hydro- and gas-dynamic flow produced to much higher distances from the substrate irrespectively on particle size. Mechanical coupling of particles to the lifting-off liquid layer or separate droplets occurs via the known "inertial" mechanism and/or new mechanism of "dragging" contaminating particles off the substrate by the liquid environment, demonstrated for the first time in this work. Nearly 100% cleaning efficiencies and no any indication of re-deposition were observed for different particles in single-shot steam laser cleaning experiments. Another disadvantageous aspect of DLC is nearly linear increase of cleaning laser fluence with increasing inverse radius of contaminating particles. This circumstance may result in damage (melt-ing, ripples, ablation) of the critical surface at high laser fluences necessary for removal of smaller (nanometer-size) particles and, thus, imposes serious limitation on the operation range of DLC. Fortunately, SLC technique may be applied in such instances, providing cleaning at quite low cleaning laser fluences, which are shown to be universal over broad particle size range.