H." Sanuki

Y.N. "Nejoh

National Institute for Fusion Science (NIFS-Repository)

§37. Double-Sheath Structures and Trapping of Dust Grains in a Plasma Nejoh, Y.N. (Hachinohe Inst. of Tech.), Sanuki, H. The presence of particulate contaminants in etching, sputtering and deposition processors re-mains a major problem in engineering plasma. The particulates occur in the range of sizes, 0.01-10J.Lm being the most common although grains of 10J.Lm or larger are also possible. They tend to congregate into particle traps due to their electri-cal nature, sometimes referred to as electrostatic traps. It is shown that these traps have long-lived structures (lasting many tens of minutes) at the edge of the plasma sheath boundary by the laser scattering measurements. These phenom-ena can have the appearance of domes , rings or filaments associated with particulates suspended above the experimental devices. The density of electrons ne forms the Boltz-mann distribution. The secondary electron den-sity n 8 produced by the secondary emission. In order to investigate the dust grain density includ-ing trapped particles, we introduce the distribu-tion function; where the curve ve ( Vr) has to be found, depend-ing on the trajectories of particles, and Q = eZ. Poisson's equation in the sheath ( tP <0) is given as where p is the space charge density. Quasineu-trality condition holds at the sheath edge. The boundary conditions are also considered. In this case, integration of Poisson's equation gives rise to the electric field in the sheath. Fig.1 shows the sheath structure of the cases of secondary electrons and no secondary electrons. The elec-tric fields are shown in Fig.2. It turns out that trapped dusts form the multiple-sheath, and the potential and electric field become mild due to the secondary electrons. These facts connect with the study of the plasma-wall interaction. 0 - 0.2 - 0.4 ¢ - 0.6 -0.8 - 1 ... -------secondary electrons no secondary electrons 0 0.25 0.5 0.75 1 1.25 1.5 xjAD Fig.l Sheaths with secondary electrons and no secondary electrons. E 10 0.1 0.01 0.001 0.0001 secondary electrons no secondary electrons ""'.-- .... ' .. ' \ \ \ \ \ 0.00001 L-~-~~~~~~...__.__._., 0 0.25 0.5 0.75 1 1.25 1.5 x/A0 Fig. 2 Electric fields with secondary electrons and no secondary electrons. electrode experimental result our ·m.'-'<>:>·~ ........ resul t I .---- - 7.7mm ~~ ~5.9mm---Fig.3 Comparison of our result with the experi-mental one on the trapping of dust grains. References Y.N.Nejoh, Phys. Plasmas, 8 (2001) to be pub-lished; Nonlin. Proc. Geophys., 5, 53, (1998); Phys. Plasmas, 6, 1048, (1999). 369 

Double-Sheath Structures and Trapping of Dust Grains in a Plasma

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)