A 13.56 MHz multicusp ion source for high intensity Ar beam

Abstract

A 13.56 MHz radio frequency (rf) inductively coupled multicusp ion source has been developed for producing an intense argon beam with current density in excess of 30 mA cm−2. The source chamber is a 10 cm diam aluminum cylinder surrounded by 20 rows of 3.5 kG Sm–Co5 magnets which form a longitudinal line-cusp field configuration. The rf antenna coil, placed inside the source chamber, is made of a braided wire threaded through a two turn pyrex tube 6 cm in diameter to prevent electrical leakage and ion sputtering. A 10:1 turns-ratio matching transformer is used to match the 50 Ω output impedance of the rf generator to the impedance of the plasma load. Preliminary measurements were carried out with a single- and four-hole aperture extracting system using argon gas. For single-hole extraction with pressure at 5 mTorr and rf power at 500 W, the ion current density was 27 mA cm−2. Under the same operating condition, a four-hole extracting system could achieve almost the same amount of current density of 25 mA cm−2.