Spatial distributions of hydrogen RF discharge plasma using a hollow cathode with double toroidal grooves combined with magnets*

A RF capacitively coupled plasma is an attractive plasma source, because it has a simple structure and maintenance can be done easily. However, such RF discharges provide only a low plasma density. In our previous work[1], a hydrogen RF plasma has been developed to solve this problem by using a hollow cathode with double hollow grooves and neodymium magnets consisting of a ring-shaped one at the center and 42 cylindrical ones fixed on an iron yoke disk. However, the surface loss of electrons and ions in the hollow grooves increased, because the magnetic flux lines crossed the groove bottom by using the iron yoke disk. In this work, spatial distributions of the ion flux have been measured at various hydrogen gas pressures from 1 to 20 Pa under conditions of improved magnet designs (reducing the cylindrically-shaped magnets without the iron yoke disk) to decrease the surface loss and the magnet cost. It is found that the hybrid discharge between a hollow cathode effect and magnetic confinement of electrons is attained for p ≥ 5 Pa, while for p ≤ 3 Pa, a conventional capacitive discharge is generated. The radial profile of the ion flux becomes uniform with increasing axial position for the hollow cathode discharge.