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Sub-100 nanometer
Ion Beam Probe for Applications in Biology and Many Other
Disciplines+ 1FM Technologies, Inc., 4431-H Brookfield Corporate Dr., Chantilly, VA 20151
2Institute
for Research in Electronics and Applied Physics, University of
Maryland, College Park, MD 20742.
Focused ion beams (FIB), at sub-100 nanometer scale, have utility for a wide range of applications in both basic and applied science covering geology, materials science, microelectronics and biology. An example of current interests is imaging by FIB-SIMS (Secondary Ion Mass Spectrometry). In biology and medicine its utility is recognized [1-2] due to the relevance of SIMS to study fundamental problems, for example mapping distributions of targeted chemical species in tissues. The critical role of ion beams is realized in ion beam milling and diagnostics instruments including in-line inspection of semiconductor devices for defect management. While significant progress has been made to design an efficient ion-optical column, the critical issues to achieve a high-resolution ion beam probe with target beam intensity of about 1 A/cm2 are governed by beam characteristics from the ion source, namely, virtual source size, beam brightness, and energy spread. An acute need prevails to advance the state-of-the art of sources so that desirable beam characteristics for a variety of beam species, especially, for hydrogen, oxygen, argon, and many complex molecular ions, can be achieved. This article especially focuses on developing high-brightness negative ion beams and coupling the beams to an ion-optical column. The goal is to achieve a sub-100 nm ion beam probe. Negative ions have unique merits for applications involving beam-materials interactions. When negative ions strike a surface, especially an electrically isolated surface, the surface charging voltage does not exceed a few volts, and the charging voltage is independent of the beam energy [3]. Figure 1 shows an example of the charging voltage of an isolated alumina surface due to irradiation by H- beams [4]. This property can be effectively utilized to circumvent problems due to surface charging that can cause device or sample damage and ion-beam defocusing. A compact plasma source, with the capability to produce negative as well as positive ion beams, is developed. Intense ion beams, including H-, O- and Ar+, have been generated. The unnormalized beam brightness for 10 keV H- beams is ~105Acm-2sr-1[5]. Figure 2 shows an example of H- beam spot when the beam is focused using a simple einzel lens with magnification of about 0.1. This spot size of about 2 µm suggests that a focused spot size of about 20 nm is achievable by an additional magnification of 0.01. The results of ion-optical calculations, in Fig. 3, for a three-lens column show the required beam focusing for sub-100 nm spot size[4]. Experiments on measurements of beam characteristics, design and development of ion-optical column and application goals will be discussed.
+Supported by SBIR Grant from National Cancer Institute, National Institute of Health.
[1] P. J. Todd, T. G. Schaaff, P. Chaurand, R. M. Caprioli, J. Mass. Spectrom. 36, 355(2001). [2] S. Chandra and D. R. Lorey, Cell. Mol. Biol. 47, 503 (2001). [3] P. N. Guzdar, A. S. Sharma, S. K. Guharay, “Charging of substrates irradiated by particle beams” Appl. Phys. Lett. 71, 3302 (1997). [4] S. K. Guharay, S. Douglass and J. Orloff, “High resolution primary ion beam probe for SIMS”, Applied Surface Science 231-232, 926 (2004). [5] S. K. Guharay, E. Sokolovsky, J. Orloff, “Characteristics of ion beams from a Penning source for focused ion beam applications” J. Vac. Sci Technol. B17, 2779 (1999).
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Presented at the
International Congress of Nanotechnology, November 7-10, 2004
San Francisco, USA
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