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Proton Beam Writing



Panoramic view of Ion Source Test Bench at CIBA, NUS



Key features

  • Ion Energy:1 - 17 keV (planned to optimize upto 30 keV)

  • Ion species: He, N, Ar (H in near future)

  • Beam current (on target): ~20 nA (through 0.5 mm diameter aperture)

  • Energy separation: Wien filter (home built)

  • Control system: Wireless control through PC

  • Vacuum level: 1x10-6 mbar

  • Sample manipulation: XY nano-positioner with 12 mm travel (to be implemented)
  • Possible applications: In the field of Ion sputtering, damage of material, material modification and sub-surface ion implantation etc…

    Person In-charge: A/P. Jeroen van Kan, Dr. P. Santhana Raman, Mr. Liu Nannan and Mr. Xu Xinxin.

    In the recent past we have demonstrated the potential of proton beam writing (PBW) as a leading candidate for the next generation lithography technique [1-3]. We are now progressing towards sub-10 nm lithography in nuclear microprobe experiments [4]. To achieve this goal, plans are being rolled out to improve the performance of the existing low brightness (~15 – 70 pA/µm2 mrad2 MeV) radio frequency (RF) ion source, used for the production of proton beams at CIBA, NUS. This RF ion source has potential to deliver higher brightness [5]. An Ion Source Test Bench (ISTB) set-up has been designed and commissioned in-house to extract the full potential of the existing RF ion-source by improving its reduced brightness. The figure shows the panoramic view of the ISTB set-up, coupled with a RF ion source. An oscillating RF voltage (~100 MHz) is capacitatively coupled onto a quartz tube, which is filled with the gas of interest, produces a stable plasma. The positively biased plasma is then extracted through a 2 mm diameter canal, with a variable extraction voltage of 0 to -3 kV. The extracted beam is accelerated (upto a maximum energy of about 200 keV) by passing through a potential divider array. The accelerated beam is collimated using a Ni object aperture before entering a Wien filter assembly, where ions of selected mass and charge state are transmitted un-deflected. The beam of interest will then pass through a second Ni collimator aperture before reaching the target. The aperture assemblies will be used, in combination with the ion current measurements carried out on the target, to evaluate the reduced brightness of the ion beam (Br). Meanwhile we have also fabricated and tested a novel ion source called electron impact gas ion source, whose reduced brightness is expected to reach up to 107 pA/µm2 mrad2 MeV. Plans for integrating it into ISTB are actively being explored. This electron impact gas ion source has the potential to form part of a compact proton beam writing system, wherein sub-10 nm beam spot size can be obtained. Currently this ISTB is also used as a Low Energy Ion Implanter (LEII). Using LEII we are routinely irradiating samples, mainly for material modifications. A few examples of such low energy ion beams (of helium, argon and nitrogen within about 20 keV) irradiations were carried out on recording magnetic media, graphene and to produce ordered sputtering on stainless samples over nanoscale regime.

    References:

    1. J. A. van Kan, A. A. Bettiol, and F. Watt, Nano Lett. 6 (2006) 579.

    2. F. Watt, M.B.H. Breese, A.A. Bettiol, and J.A. van Kan, Mater. Today 10 (2007) 20.

    3. Y. Yao, M. W. van Mourik, P. S. Raman, J. A. van Kan, Nucl. Instr. Meth. Phys. Res. B, 306 (2013) 265.

    4. Y. Yao, P. Santhana Raman, and J.A. van Kan, Microsyst Technol, 2014 DOI 10.1007/s00542-014-2066-2.

    5. C.D. Moak, H. Reese, and W.M. Good, Nucleonics 9 (1951) 18.