Miniaturized magnetic nitrogen DC microplasmas

This paper explores the use of miniaturized magnets to enhance the parameters of dc microplasmas. The microplasmas are powered by thin coaxial electrodes and are enhanced by a coaxial magnetron configuration machined from niobium composite magnets. At operating pressures of 1-4 torr, a glow region that is confined to the volume directly over the cathode, forms a traditional magnetron-type annular ring. Three coaxial magnets, ranging in total size from 3.2 to 7.2 mm in outside diameter generate measured magnetic fields up to 3030 G. The magnetic field structure is profiled with a small Hall probe and is modeled by finite-element analysis. The plasma currents for various applied voltages are measured, and the plasma breakdown/termination voltages are determined. In nitrogen ambients at 1.2 torr and 370-V bias, the current changes from 9.3 mA in the absence of a magnetic field to 17.6 mA with the addition of the largest magnet. The sheath region decreases with the addition of the magnetic structures, illustrating an effect on the Debye length and, therefore, the local plasma density. The dimensions of the sheath are found to vary radially within the annular microplasma. The smallest sheath corresponds to the region of highest magnetic field over the south pole for the largest magnetic configuration. This effect is used to generate a microplasma in SF₆ on a silicon wafer producing a localized etch. The etch rate in the region of the brightest glow is three times greater than the weakest etch rate, allowing spatially localized etch selectivity without masking.