Mutual shielding of closely spaced dust particles in low pressure plasmas

Abstract

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.