Chamber with Inverted Electrode Geometry for Measuring and Control of Ion Flux-Energy Distribution Functions

Abstract

Measurements of ion flux-energy distribution functions at the high sheath potential of the driven electrode in a classical low-pressure asymmetric capacitively coupled plasma are technically difficult as the diagnostic device needs to float with the applied radio frequency voltage. Otherwise, the ion sampling is disturbed by the varying electric field between the grounded device and the driven electrode. To circumvent such distortions, a low-pressure plasma chamber with inverted electrode geometry, where the larger electrode is driven and the smaller electrode is grounded, has been constructed and characterized. Measurements of the ion flux-energy distribution functions with an energy-selective mass spectrometer at the high sheath potential of the grounded electrode are presented for a variety of conditions and ions. The potential for suppressing low-energy ions from resonant charge transfer collisions in the sheath by the dilution of the working gas is demonstrated. Additionally, the setup is supplemented by an inductively coupled plasma that controls the plasma density and consequently the ion flux to the substrate while the radio frequency bias controls the ion energy. At high ion energies, metal ions are detected as a consequence of the ionization of sputtered electrode material. The proposed setup opens a way to study precisely the effects of ion treatment for a variety of substrates such as catalysts, polymers, or thin films.