Magneto-optical trap-based ion source (MOTIS)

Figure 1. Schematic of a magneto-optical trap-based ion source. Neutral atoms are trapped in a magneto-optical trap (MOT), ionized by an ionization laser, and extracted by an electrode with voltage -V.

We are developing a novel type of ion source for focused-ion beam applications. The source uses magneto-optically trapped neutral atoms, which are photoionized and extracted to form an ion beam with unique properties. The source offers several advantages over conventional ion sources, including

• a very low emittance, and a correspondingly high brightness, due to the extremely cold temperature of the trapped atoms (≈100 µK)
• the ability to produce a range of ionic species, including light and heavy noble gases
• a narrow energy spread, of order 0.1 eV
• the ability to deterministically produce single ions on demand.

Focused ion beams are one of the most useful tools of modern nanotechnology. Using sophisticated charged-particle optics systems similar to what is found in electron microscopes, an ion beam can be focused to the nanometer scale, where it can be used not only to image nanostructures, but also to create them by ion milling.

Present day ion sources, such as the liquid metal ion source (LMIS), have seen widespread implementation, despite a number of shortcomings. For example, the ion species is for the most part limited to gallium for practical reasons, even though gallium can cause contamination of the specimen during ion milling, and erosion during imaging. Also, the brightness of the source comes from a very small effective source size (≈50 nm), which makes it subject to space charge and stability concerns. Furthermore, the energy spread can be very large – up to several eV – necessitating high energy operation to avoid chromatic aberration.

The MOTIS has the potential to address these shortcomings. We have numerically analyzed the performance of a MOTIS and found that emittances as low as 3.3.x10^-7 π mm mrad MeV^½ and brightnesses as high as 1.5x10¹¹ A cm^-2 sr^-1MeV^-1 are possible. These compare favorably with LMIS values of 1.1x10^-6 π mm mrad MeV^½ and 5.8x10⁷ A cm^-2 sr^-1MeV^-1.  We have also performed ray tracing analysis on a prototype focusing system and have found that spot sizes of order 7 nm can be easily achieved with a 1.2 keV beam. While these estimates do not include space charge effects, there is reason to believe that space charge will not affect the MOTIS in the same way it does the LMIS. This is because the current density can be maintained at a very low value throughout the beam, since the low emittance is obtained through low angular spread, not small source size.

We have also constructed a prototype MOTIS and carried out emittance measurements. Using a Cr magneto-optical trap, we created an ion beam with variable energy from 25 eV to 5 kV in a simple extraction geometry consisting of two parallel plates. Measuring the beam width after a free flight of 1.2 m as a function of beam energy resulted an emittance estimate of 6 x10^-7 mm mrad MeV^½. Despite a low MOT density resulting from collisional effects specific to Cr, we were able to extract currents as high as 1.4 pA and measure a brightness as high as 2.25 A cm^-2 sr^-1 eV^-1.

These measurements demonstrate that the MOTIS is capable of producing an ion beam with a useful amount of current that can be focused to a spot size less than 1 nm. Work is ongoing to integrate a MOTIS with a focusing column to demonstrate this capability.

Figure 2. MOTIS realization for emittance measurements. MOT laser beams pass through a window with a conductive indium tin oxide (ITO) coating that forms an electrode held at a high voltage +HV. Another electrode with a mirror surface reflects the laser beams to form the MOT. Ions are extracted through a small hole and strike a multichannel plate and phosphor screen, where they are observed with a charge-coupled device (CCD) camera.

Magneto-Optical Trap-Based, High Brightness Ion Source for Use as a Nanoscale Probe
Laser Cooled Atoms as a Focused Ion Beam Source
Using Laser-Cooled Atoms as a Focused Ion Beam Source

Jabez J. McClelland - NIST
Brenton Knuffman - NIST
Adam Steele - NIST

Jon Orloff - FEI Co.

Elizabeth Dakin - OADS, Inc.
James Hanssen - US Naval Observatory
Shannon Hill - NIST, Physics Laboratory

Marcus Jacka - University of York, UK

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Online: February 2008
Last Updated: March 2009

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