Ion flux–energy distributions across grounded grids in an RF plasma source with DC-grounded electrodes

Abstract

We present an experimental investigation of the ion flux-energy distribution functions (IFEDF) obtained across grounded grids in an asymmetric capacitively coupled RF source using helium discharge. The powered electrode in the RF source is DC-grounded via a λ/4 filter, which lifts its DC potential to zero. Grids of different dimensions (hole width, thickness and geometric transparency) were used to confine the plasma, while the IFEDF of the ion beam departing the grid and reaching the reactor walls was studied using a retarding field energy analyzer. The IFEDF obtained was double-peaked, indicating the presence of fast ions arriving from the plasma source, and cold ions generated upon charge exchange collisions between the fast ions and neutrals. The flux, as well as the peak energies of the two ion groups, depended significantly on the process parameters: RF power, He pressure, the distance between grids and walls, as well as the dimensions of the grids. The results indicate that confining plasma with grids can reduce the ion flux at the walls by over 60%, significantly lowering the wall sputtering rate. This was confirmed with a dedicated long exposure plasma discharge with a gridded plasma reactor, wherein less than 1nm of Cu deposition was found on the DC-grounded powered electrode, and the surface reflectivity was preserved to pristine values. In contrast, a similar experiment in a gridless reactor resulted in Cu deposition of 35nm with a drastic drop in surface reflectivity. These studies are of great importance for the application of similar RF plasma sources with in-situ cleaning of diagnostic mirrors in fusion devices, as well as in a variety of plasma processing applications.

Effect of 3 T magnetic field on RF plasma sputtering in an ITER-relevant first mirror unit

“…However, at lower α the overall A g increases and so does the deposition on M2. This can however be reduced as well by using mitigation strategies like floating wall components at the wet surface of the FMU [16], or grids between the mirrors and the FMU wall [17].…”

“…The sputtered wall material in turn gets deposited on the FMs making the plasma cleaning process less effective [16]. Hence CCRF plasma cleaning with a λ/4 filter requires additional mitigation strategies to achieve FM cleaning [16,17].…”