Nanoparticle adhesion and removal studied by pulsed laser irradiation

The contactless removal of small particles from surfaces by irradiation with intense laser pulses dubbed laser cleaning has been used and studied for nearly two decades. Nevertheless, its applicability and the mechanisms involved are still under debate. Here we give first a brief overview on relevant processes, and then present measurements of the velocities of colloidal model particles after detachment under vacuum conditions. We also demonstrate a new Laser Cleaning approach, by which submicrometer particles are removed by laser irradiation of the rear side of the wafers. The particles are detached by an acoustic shock wave traveling to the wafer front side after laser ablation of the rear side. Not only is this promising approach capable of defect free surface cleaning, detailed studies of particle velocities versus laser fluence also allow insight into the different cleaning mechanisms involved. Furthermore, this technique could be applied to determine adhesion energies of particles in the future

Jumping nanodroplets

Flat Au nanostructures on inert substrates like glass or graphite are illuminated by single intensive laser pulses with fluences above the Au melting threshold. The liquid structures produced in this way are far from their equilibrium shape, and as a consequence a dewetting process sets in: On a timescale of a few nano seconds the liquid contracts towards a sphere. During this contraction the center of mass moves upwards, which can lead to a detachment of the droplets from the surface due to inertia, resulting in velocities on the order of 10m/s for droplets with radii in the range of 100 nm