Pulse laser particulate separation from polycarbonate: surface acoustic wave and thermomechanical mechanisms

Abstract

The mechanisms of graphite particle and polystyrene-co-divinyl benzene microsphere removal from flat polycarbonate substrates by nanosecond pulse laser interaction at 532 nm were studied both experimentally and theoretically. These model contaminants exhibited an extremely contrasting behavior in respect to phase separation and collateral damage to the polycarbonate substrate. Opaque graphite particles within the irradiated spot area either desorbed due to their thermal expansion or undertook vaporization/ablation. The transparent polystyrene microspheres caused local ablation of the substrate in their optical near-field. This process led to the removal of the particles, but eradicated the available cleaning fluence window. The opaque graphite particles, on the other hand, showed efficient clearance, particularly in a practicable cleaning window above $0.5~\mathrm{J/cm}^{2}$ and low pulse numbers of about two. Besides the mechanisms occurring within the irradiated spot, a separation process in the proximity beyond the laser spot (more than double the Gaussian radius) could be related to the action of high-amplitude surface acoustic waves (SAW). A minimum surface acceleration of $10^{9}~\mathrm{cm/s}^{2}$ was calculated to be sufficient to overcome the adhesion forces in this particle separation model.