Novel Sources for Focused-ion Beams

Focused beams of ions have a wide range of uses, from nanoscale imaging to the fabrication of nanomaterials.  At the CNST, researchers are developing a novel magneto-optical trap based ion source (MOTIS) to overcome the shortcomings of previous beam production techniques.  This new technique uses a variety of atomic sources to produce highly focused ion beams, with tightly controlled energy.  Project researchers are working with industrial partners to develop commercial applications from these novel ion sources.

Commercial focused ion beams (FIBs) are used as diagnostic tools, slicing through a silicon wafer to ensure that it was fabricated correctly.   They can also shape nanoscale materials either by adding atoms to a structure or by shaving them off.   And they can be used in microscopes that create images of nanoscale surfaces.   Collectively, tools that use focused ion beams make up a $300 million to $600 million industry.

Although focused-ion beams have been in use since the 1980s, the technology has several limitations. The most common way to produce ions, the liquid metal ion source (LMIS), uses ions from the metal gallium because of its low melting point and low vapor pressure. Because gallium is a relatively heavy atom, the force of gallium ions hitting a surface can easily cause erosion. Such beams can be useful for milling and deposition, but gallium can contaminate the surface. Erosion and contamination also make gallium a poor ion for imaging techniques. 

MOTIS overcomes these limitations.  Building on NIST researcher William D. Phillips’ Nobel Prize-winning work using lasers to trap and cool atoms, the system’s magneto-optical traps use a criss-crossing network of laser beams and magnetic fields to confine a group of atoms and cool them to temperatures just hundreds of millionths of a degree above absolute zero.  An additional laser can then remove single electrons from neutral atoms in this cloud to create ions that can be accelerated and focused into a beam.  Project researchers have demonstrated that these beams, because they are produced at extremely low temperatures, have low emittance, meaning that the ion beam doesn’t spread laterally.  Initial experiments indicate that the beam can be focused to spots just nanometers across and used for imaging, milling, and deposition with comparable or better resolution than a traditional Ga LMIS beam.

Magneto-optic traps can produce ion beams from a variety of different atomic sources, and the ability to use lighter elements such as helium or lithium could be particularly useful for imaging applications, such as ion microscopy. Ion microscopes have the potential to provide sharper images than electron microscopes, as long as the ions are not so heavy that they damage the surface.  Working in partnership with the nanoscale imaging company FEI Co., CNST researchers have developed a microscope based on this technology. 

Magneto-optical trap ion sources can also produce beams of ions with very low energy, as much as 10,000 times lower than the energy of LMIS beams -- 1 eV to 100 eV compared to 10 keV to 20 keV. Highly focused ion beams with energy this low have not previously been available, and could make possible new types of microscopy measurements.

MOTIS could also help solve a crucial problem at the frontier of nanofabrication. As circuits get smaller it becomes increasingly important to control the precise placement of single metal atoms within semiconductor materials to give them the desired electrical properties.  This project has developed specialized single-atom magneto-optical trap techniques with the potential to produce single ions on demand and to deliver them to a precise point in space.  Such technology would allow unprecedented control over next-generation nanodevices.

• Nanoscale focused ion beam from laser-cooled lithium atoms, B. Knuffman, A. V. Steele, J. Orloff, and J. J. McClelland, New Journal of Physics 13, 103035 (2011).
  NIST Publication Database        Journal Web Site
• Inter-ion coulomb interactions in a magneto-optical trap ion source, A. V. Steele, B. Knuffman, and J. J. McClelland, Journal of Applied Physics 109, 104308-104308-8 (2011).
  NIST Publication Database          Journal Web Site
• Focused chromium ion beam, A. V. Steele, B. Knuffman, J. J. McClelland, and J. Orloff, Journal of Vacuum Science and Technology B 28, C6F1-C6F5 (2010).
  NIST Publication Database          Journal Web Site
• Magneto-optical-trap-based, high brightness ion source for use as a nanoscale probe, J. L. Hanssen, S. B. Hill, J. Orloff, and J. J. McClelland, Nano Letters 8, 2844-2850 (2008).
  NIST Publication Database          Journal Web Site