Large particles (tens of nm to tens of ,um 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. Although the source of these particles has been studied by a number of groups, a definitive explanation is still lacking. In this paper, we theoretically investigate the role of negative ions in the formation of large clusters, the precursors to particles, in low-pressure plasmas. We find that the formation of particles requires a critically large cluster. Forming the critically large cluster requires longer residence times in the plasma than is usually possible if clustering involves only neutral particles. 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.

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The transport of particles ('dust') in low-pressure electrical glow discharges is of interest with respect to contamination of semiconductor wafers during plasma etching and deposition. The distribution of dust particles in these reactors is determined by a variety of forces, the most important being electrostatic, viscous ion drag, gravitational, thermophoretic and neutral fluid drag. In this paper we present results from a series of computer models to predict the spatial distribution of dust particles in capacitively coupled electrical glow discharges considering these forces. The results are parametrized over power deposition, gas flow and particle size. 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.

The transport of particles (‘‘dust’’) in low pressure electrical glow discharges is of interest as a result of their role in contaminating wafers during plasma etching and deposition of semiconductors. Particles (10s nm to many micrometers) negatively charge in glow discharges and, to first order, appear to be massively large negative ions around which sheaths develop. The electrical and fluid forces acting on dust particles in plasma processing discharges may cause the interparticle spacing to be less than the shielding distance around particles. The mutual shielding of dust particles is therefore of interest. In this article, we report on results from a pseudoparticle-in-cell simulation of the mutual shielding of two adjacent dust particles. Results will be discussed for charge, potential, and electrostatic forces on dust particles as a function of particle size and separation distance between two 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.