Very low pressure RF glow discharge plasma using multi-dipole surface magnetic fields

The authors show that by taking advantage of surface multi-dipole fields, rf glow discharges can be maintained down to pressures at least as low as 2 x 10-5 torr. Large effective electrode separation and good plasma uniformity can be achieved by making use of surface multi-dipole magnetic fields. Only one conventional electrode was used. Two different types of rf powered electrodes were explored---a conventional plate (diam. = 15 cm) and a rod (diam. = 1.3 cm). The powered electrode was located sufficiently far from the magnets that the magnetic field could be ignored. The line cusps replace the other electrode. Line cusps fully surround a 20 liter plasma. Experiments employed argon plasmas with 13.56 MHz rf. A second grounded electrode was also employed. This plate differed from the powered electrode in that it was located within the surface magnetic field. Adjustment of the separation of this plate from the magnet plane permits continuous variation of the maximum surface magnetic field (Bm) seen by plasma and the effective loss area at the electrode. The authors show that the plasma potential can be continuously varied by changing the position of the grounded plate. They also directly demonstrate that the surface magnetic field is effective in improving plasma confinement at pressures at least as high as 10-3 torr. Comparison is given with theory of lowest pressure at which the plasma can be maintained and the power balance for the plasma