High-Efficiency Low Energy Neutral Beam Generation Using Negative Ions in Pulsed Plasma

Abstract

To prevent several kinds of radiation damage caused by charge build-up and by ultraviolet and X-ray photons during etching processes, we have developed a high-performance, neutral-beam etching system. The neutral-beam source consists of an inductively coupled plasma (ICP) source and top and bottom carbon parallel plates. The bottom carbon plate includes many apertures for extracting neutral beams from the plasma. By supplying a positive or negative direct current (DC) bias to the top and bottom carbon plates in the pulsed SF6 plasma, the generated positive or negative ions are respectively accelerated towards the bottom plate. The negative ions are more efficiently converted into neutral atoms in comparison with the positive ions, either by neutralization in charge-transfer collisions with gas molecules during the ion transport or with aperture sidewalls in the bottom plate. The neutralization efficiency of negative ions was more than 98% and the neutral flux density was equivalent to 4 mA/cm2.

200-mm-diameter neutral beam source based on inductively coupled plasma etcher and silicon etching

“…The etcher has a vacuum chamber with a turbomolecular pump, a 200-mm stage with an electrostatic chuck and temperature control system, and an ICP antenna on top of the chamber with a dielectric window. A graphite plate with numerous aperture holes, each with a diameter of 1 mm and an open ratio of 50% ͑neutralization aperture plate͒, 12,13 was installed between the quartz window for the ICP antenna and the sample stage to neutralize ions in the plasma and to generate a beam of neutral particles. The distance between the neutralization aperture plate and the sample stage was fixed at 2 cm.…”

“…We fixed the aperture diameter, aperture plate thickness, and open ratio at 1 mm, 10 mm, and 50%, respectively, which are same as in the previous studies. 12,13 Fluorine and Ar gases were supplied to the plasma chamber via MFCs. We used F Gene ͑Showa Denko͒ to supply high-purity F 2 gas.…”

“…3,4 Neutral beam systems have been developed to prevent such plasma-induced damage. [5][6][7][8][9][10][11][12][13] Among the reported neutral beams, plasmabased systems are expected to be scalable and practical for achieving large-diameter neutral beam source.…”

“…Samukawa et al 12,13 developed a neutral beam source using inductively coupled plasma ͑ICP͒ as an ion source and surface neutralization using a graphite aperture. They achieved high neutralization efficiency by using negative ions from pulse-time modulation plasma, 13 and UV irradiation was drastically reduced.…”

“…They achieved high neutralization efficiency by using negative ions from pulse-time modulation plasma, 13 and UV irradiation was drastically reduced. 14 The high neutralization efficiency and the removal of UV irradiation enabled for various ultralow-damage processes.…”