RADIO-FREQUENCY PLASMA SPUTTER TYPE HEAVY NEGATIVE-ION SOURCE

A rf plasma sputter type heavy negative ion source with mA-class ion currents in dc operation has been developed. Although maximum negative ion production efficiencies by sputtering on a cesiated sputtering target surface were high (about 10% or more for Cu, C, Si, Ge and W), electron-detachment cross-sections were also considerably high (about 10(-15) cm2). Therefore, in order to extract an intense negative ion beam through a generated plasma region from a sputtering target it is essential that the plasma should be discharged at low gas pressures in the order of 10(-4) torr. By using a rf (13.56 MHz) discharge with a three- turn rf coil 50 mm in diameter, a dense plasma of 10(11) cm-3 order was obtained in the xenon gas pressure range of 10(-5)-10(-4) torr. In this source a sputtering target 42 mm in diameter whose surface was part of a sphere was used, and negatively biased by 600 V against the plasma. Extracted total negative ion currents were 6.5 mA for a copper target and 4.2 mA for a graphite target, in which electrons were eliminated by a magnetic field near the extraction hole. The concentration of Cu-current in the total copper negative ion current was 96.6%. The percentages of C- and C2- currents in the carbon negative ion current was 38.1 and 54.5%, respectively.