Dynamic analysis of particle adhesion and removal in the micro/nano scale

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2004-01-01
Authors
Seetharaman, Satya
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Mechanical Engineering
Abstract

Adhesion of particles to surfaces is one of the major problems encountered in Micro Electro Mechanical Systems (MEMS). Typically, this problem affects gripping of micro/nano particles. The particle gets stuck to the surface and removal becomes a crucial issue. It is also encountered in Chemical Mechanical Polishing (CMP), wherein sub-micron particles adhere to the silicon wafers after polishing, causing circuit defects and other possible damages to the electronic networks. The major contributors to adhesion force are the Van Der Waal and capillary forces, formed due to polarization between molecules and fluid film condensation due to humidity. The aim of this thesis is to investigate the issue of particle adhesion and propose effective means of particle removal. The first method proposed utilizes longitudinal vibrations to separate the particle from the substrate. Vibrations of any form (thermal/mechanical) have been found to be an effective method of particle removal. The longitudinal model determines a stability criterion based on eigenvalue analysis and Lyapunov's stability theorems. Simulations to verify the stability conditions are carried out using Matlab. The dynamic system is formulated and simulations are carried out on the linearized system and also on the fully non-linear system. The phenomenon of negative damping, which causes an unstable limit cycle behavior, is encountered during the numerical simulations. The second method, which is a more effective one, proposes the use of lateral removal moments to separate the particle from the surface. Different lateral removal techniques are discussed and the criterion for separation is established. Separation is induced once the lateral removal moments overcome the adhesion resisting moment. It is found that friction plays an important role during separation. Contrary to existing beliefs that friction hinders motion, friction is actually found to aid particle separation during the lateral removal moment technique. Comparisons of the lateral model are made with the longitudinal removal technique and the conclusions prove the effectiveness of the lateral removal model, over the previous technique, in that, a lower removal force is required when particle separation is brought about by the lateral removal method. The third method studies the effect of coupled vibrations. Coupled vibrations refer to the coupling enforced between lateral and longitudinal directions of motion, during the formulation of the dynamic system, through the relationship between the adhesion and friction forces. A characteristic matrix is postulated, which comes up with a stability criterion for the equilibrium points and lateral stiffness. Enforcing the stability criteria makes the system stable. The numerical simulations carried out on solving the longitudinal system analytically and inserting the solution into the lateral system prove that the stability conditions are true. During the simulation, the interesting phenomenon of beating frequency is observed and important conclusions are made on exciting the dynamic system with a combination of the excitation and beating frequency. Separation is found to be enhanced by a combination of the beating and excitation frequency along with an increase in the amplitude of vibration.

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Thu Jan 01 00:00:00 UTC 2004