Performance characteristics of a capacitively coupled 3 cm rf ion source^a)

A novel 13.56 MHz radio frequency capacitively coupled ion source has been developed and characterized with special emphasis on low-energy (<1 keV) broad ion beam extraction. When operating the source with chamber pressures ranging from 1 × 10^-4 to 1.7 × 10^-3 mbar (O₂ and Ar), rf-power levels from 5 to 120 W, axial air coil magnetic fields up to 70 mT, and maximum extraction voltages of 650 V extractable ion currents were as high as 50 mA. With a total grid area of 7 cm² and a transparency of 46%, this amounts to a grid current density of more than 7 mA/cm² while the ion saturation current density is calculated as 9.3 mA/cm². The potential oscillation amplitude of the rf excitation electrode can be as high as 650 V. Due to an external axial magnetic field, a dc bias up to 70 V is induced between the grounded accelerator grid and cylindrical anode. The performance data are traced back to constructional source details such as plasma excitation geometry, axial magnetic field, and floating electrostatic end confinement. Under all operating conditions, the source shows an excellent long-time stability even with reactive gases (e.g., more than 100 h without maintenance with oxygen).