Nanoparticle Removal Using Laser Induced Plasma Shockwaves

Nano-scale substrate cleanliness is an essential requirement in variety of nanotechnology applications. Currently, the detachment and removal of sub-100nm particles is of a particular interest and challenge in semiconductor manufacture, lithography, and nanotechnology. The proposed particle removal technique based on pressure shock waves generated by a laser induced plasma (LIP) core is of interest in various nano/micro fabrication applications in which the minimum feature size has been reducing rapidly. Any removal technique adopted in a fabrication process must be on the same shrinking feature reduction curve since, for device reliability, the minimum tolerable foreign particle size on a substrate depends on the minimum feature size on a nano/micro-system or device. In recent years, we have demonstrated that nanoparticles can be detached and removed from substrates using LIP shock wavefronts. While we have experimentally established the effectiveness of the LIP technique for removing nanoparticles in the sub-100nm range, the removal mechanisms were not well-understood. In the current work, we introduce a set of novel removal mechanisms based on moment resistance of the particle-substrate bond and discuss their effectiveness and applicability in laser-induced plasma shock nanoparticle removal. To gain better understanding for the detachment mechanisms, the resultant force and rolling moment induced on the nanoparticle by the LIP shockwave front need to be determined. Since, for sub-100nm nanoparticles, the Knudsen number K_n exceeds 0.1, the applicability of the Navier-Stokes equations for the gas motion becomes questionable as the continuum assumption for the medium breaks down due to the invalidity of the transport terms in these equations. Detachment and detachment mechanisms of nanoparticles from flat surfaces subjected to shockwaves are investigated by employing molecular gas dynamic simulations using the direct simulation Monte Carlo method and experimental transient pressure data. Two new mechanisms for nanoparticle detachment based on rolling moment resistance of the adhesion bond and the elastic restitution effect are introduced. As a result of present simulations, it is computationally demonstrated that the pulsed laser-induced shockwaves can generate sufficient rolling moments to detach sub-100nm particles and initiate removal. The transient moment exerted on a 60nm polystyrene latex (PSL) particle on a silicon substrate are presented and discussed.