Novel Sources for Focused-ion Beams

Focused beams of ions have a wide range of uses, from nanoscale imaging to the fabrication of nanomaterials. In this project, researchers are developing novel ion-beam technology using magneto-optical traps to overcome the shortcomings of previous beam production techniques. The new technique produces highly focused ion beams, with tightly controlled energy, from a variety of atomic sources. Project researchers are working with industrial partners to develop commercial applications from these novel ion sources.

Focused ion beams can act 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 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.

To overcome these limitations, project researchers are developing a magneto-optical trap based ion source (MOTIS) for highly focused ion beams. Magneto-optical traps, which extend from work for which NIST researcher William D. Phillips shared the 1997 Nobel Prize in physics, 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 could be focused to spots smaller than 1 nanometer across, and could be used for imaging, milling, and deposition with comparable or better resolution than an 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. NIST researchers are already developing a microscope based on this technology in partnership with the nanoscale imaging company FEI Co.

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 to 100 electronvolts (eV) compared to 10 to 20 keV. Highly focused ion beams with energy this low have not previously been available, and could make possible to 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.