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Title:Modeling of generation and transport of particles in low pressure glow discharges and contamination of wafers
Author(s):Choi, Seung Jin
Doctoral Committee Chair(s):Kushner, Mark J.
Department / Program:Electrical and Computer Engineering
Discipline:Electrical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Electronics and Electrical
Physics, General
Physics, Fluid and Plasma
Abstract:Large particles (tens of nm to tens of $\mu$m in diameter) are problematic in low pressure ($<$1 Torr) plasma processing (etching, deposition) discharges because they can contaminate the product and can perturb electron transport. The role of negative ions in the formation of large clusters, the precursors to particles, in low pressure plasmas is theoretically investigated. We find that the formation of particles requires a critically large cluster. We propose that negatively charged intermediates, which are trapped in electropositive plasmas, increase the average residence time of clusters to allow the growth of critically large clusters.
Particulates (or "dust") preferentially accumulate near the cathode sheath-plasma boundary where energetic electrons accelerated in the cathode fall emanate into the negative glow. We theoretically investigate the penetration of the electron flux generated in dc cathode falls through the particulate "barriers" formed by dust contamination. We find that at constant current densities, the plasma responds to the reduction in ionization rate coefficients caused by the particulates by increasing the electric field in the cathode fall.
The dynamics of the shielding of particulates in low pressure glow discharge has also been investigated with a pseudoparticle-in-cell simulation (PICs) for electrons and ions in the vicinity of a dust particle. We find that the shielding distance around the dust particle is well-characterized by the ion Debye length. Collisions of orbiting ions effectively decrease the ion temperature, thereby increasing its potential to more negative values. Electron and ion momentum transfer and collection cross sections for scattering from the dust particle are calculated. We also report on results of PICs of the mutual shielding of two adjacent dust particles. We found that two closely spaced particles not only shield each other but can shadow their partner, thereby resulting in asymmetric charging of otherwise identical particles.
The distribution of dust particles in plasma processing reactors is determined by a variety of forces, the most important being electrostatic, viscous ion drag, gravitational, thermophoretic and neutral fluid drag. We find that the spatial distribution of dust depends on the spatial dependence of the sheaths and plasma potential in bulk plasma which in turn depend upon the electrical topography of the surfaces. Experimentally observed "dome" and "ring" distributions of dust particles are computationally reproduced for specific combinations of discharge power particle size and substrate topography.
Issue Date:1993
Rights Information:Copyright 1993 Choi, Seung Jin
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9411590
OCLC Identifier:(UMI)AAI9411590

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