Ion energy distributions in dual- and single-mode microwave/radio-frequency plasma

Abstract

Ion energy distribution functions (IEDF) were measured at the surface of a grounded or a rf-powered electrode exposed to a microwave (MW, 2.45 GHz) or a rf (13.56 MHz) discharge in argon. The IEDFs measured on the grounded electrode in both the rf and the MW modes show a higher contribution of low-energy ions when the pressure is raised. The maximum ion energy Em decreased from 12.0 to 8.5 eV in the MW plasma, but remained at 26±0.5 eV in the rf plasma. The mean ion energy Ē decreased from 6.1 to 3.8 eV (MW) and from 16.0 to 12.0 eV (rf) when the pressure was raised from 30 to 210 mTorr. The IEDFs in the rf mode exhibit a single peak, while a bimodal structure is observed in the MW mode. When the electrode is rf powered to achieve a negative dc bias voltage VB, the Em values reach ∼1.2eVB (rf) and ∼1.1eVB (MW), respectively. The IEDFs in the rf mode display a multiple peaked structure which is associated with a rf-modulated sheath. In the dual-frequency MW/rf mode the IEDF is single peaked, narrow, and centered around eVB. A nearly tenfold increase in the ion flux is observed when increasing the MW input power, while the ion energy remains unchanged for a constant VB value. This allows an independent control of ion energy and ion flux, suitable for the control of material properties in plasma processing.