Removing nanometer scale particles from patterned substrates is a critical, yet challenging, technical task with numerous applications. Due to environmental concerns, there is a drive to reduce chemical usage and waste in semiconductor and other nano-technology industries. The laser plasma method is a novel removal method for nanoparticles. For the work reported in this study, the method is applied to substrates with features, i.e. trenches and pinholes. The technique, which is dry and non-contact, takes advantage of shock wavefronts initiated by plasma formation under a focused laser beam pulse and their interaction with the substrate. In the reported experiments, a Q-switched Nd : YAG pulsed laser is employed as a plasma and shockwave generation source. Various mechanisms are responsible for the removal effect. The strong shock wave in air generates complex pressure wave elds resulting in both drag and lift on the particle and acceleration of the substrate. However, shock waves are not transmitted into the solid substrate as discontinuous shock fronts due to a large difference between the relevant wave phase speeds in the two media. Also, damage concerns due to cavitation, which is a common effect in megasonic cleaning, are avoided. However, damage due to the high temperatures associated with the plasma formation is found to be an issue. Patterned silicon wafers and micrometer level pinholes were used for the particle removal experiments. The effects of the distance between the surface and the plasma boundary on the removal ef ciency are reported. With this method, we were able to remove particles from the wall of a micrometer level pinhole and patterned silicon wafers.
Damage-free removal of nanoparticles and thin lms from substrates is a critical requirement in micro-and nano-manufacturing. In laser cleaning, particles are removed by application of inertial forces at the particles attached to a substrate by means of surface acceleration. While the technique is promising, experimental results indicate that damage can occur in the process due to high levels of uence. Potential damage mechanisms include surface breakage, interferometric interactions due to bumps/ surface features, diffraction related focusing, micro-cracks, and peeling of top layers. To reduce and/ or eliminate damage risk by avoiding excessive heat deposition, the absolute acceleration requirement must be well understood and accurately modeled. In the current work, a set of transient simulations for the thermoelastic response has been conducted to determine the surface acceleration vector and temperature elevation under a nanosecond pulsed-laser. It is known that in nanoparticle removal rolling motion-based removal requires the least amount of acceleration. The distribution of the acceleration eld on the surface of a half-space is obtained. The magnitude of a jump in the radial acceleration component at the boundary of the irradiated area and the dark zone is determined, and its use in nanoparticle removal has been discussed and demonstrated. Preliminary experimental data for a novel removal method based on this acceleration localization is presented and discussed.
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