Plasma uniformity enhancement in a single-core ferromagnetic inductor coupled plasma source

Abstract

We describe the phenomenon of plasma uniformity enhancement in the recently developed simple version of the ferromagnetic inductor coupled plasma source in which a single thin ferromagnetic core having a large diameter (ferroinductor) was fully immersed in plasma. The plasma appeared due to the gas discharge driven by a comparatively low ac voltage ( 350 V) applied to the primary winding of this core. Under certain circumstances the plasma nonuniformity did not exceed a few per cent in the radial direction as well as in the axial direction while the plasma density reached almost 10 13 cm −3 . A qualitative explanation of such uniformity enhancement is suggested.

Physical limitations in ferromagnetic inductively coupled plasma sources

“…Comparing the voltage required to get the saturation and the evaluated values of T eff (tens of volt vs volts), one might expect that intensive electron beams exist in the "cold" bulk plasma. The evaluated T eff was about 2-3 times smaller than the one obtained formerly in these plasma sources [4][5][6][7][8][9] . To verify this point we compared the decay time of the afterglow plasma t dec at high and low plasma current I pl .…”

“…They are comparatively smooth and very similar to those obtained in Refs. 4,7 . Above the threshold, when I pl > I th , a narrow "filament" of I p appears in parallel to the core axis somewhat off center (Fig.…”

“…To connect the measured ion probe currents to the plasma densities we used the microwave cut-off method (see e.g. [6][7][8][9] and refs. therein).…”

“…This indicated a minimal pressure p for each sort of gas, namely 0.1 mTorr for Xe, 0.3 mTorr for Ar, and 5 mTorr for He. In all these cases the measurements were started from plasma current I pl = 200−300A, which was actually the maximal current for all former experiments [4][5][6][7][8][9] .…”

“…The differences between these two FICP versions are the following: the multi-core version 5 is able to produce a more uniform plasma (the non-uniformity could be below 3-4%) as well as it is able to form a required plasma density gradient, but the maximal plasma density usually did not exceed 2 • 10 12 cm −3 . The single-core FICP 4,7 forms less uniform plasmas (the non-uniformity is about 7-10%) for the same plasma size (20-30 cm), and there is no option to vary the spatial distribution of the produced plasmas, but it is able to produce denser plasmas, up to 10 13 cm −3 , with ionization rate above 90%. Another advantage of the single-core device is that its input resistance is often independent of the plasma density 8 .…”