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Physical Review A

Laser-cooled atoms as a focused ion-beam source

J. L. Hanssen, Jabez J. McClelland, E. A. Dakin, Marcus Jacka

Published Dec 21, 2006• 48 citations
Publisher

Abstract

The evolving field of nanofabrication demands that more precise fabrication and evaluation tools be developed. We describe a method for creating a high quality focused ion beam with enhanced capabilities using an ion source based on laser-cooled neutral atoms in a magneto-optical trap. This technique will improve resolution and brightness beyond the current state of the art, can be used with the full range of atomic species that can be laser cooled and trapped, and will allow unprecedented control over the ion emission, allowing, for example, the production of single ions ``on demand.'' We give estimates for the emittance and present a realistic ray tracing analysis of a basic focusing system demonstrating the feasibility of focusing the beam to a spot size of less than $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

DOI
10.1103/physreva.74.063416

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The authors describe a new method for creating a high quality focused ion beam using laser-cooled neutral atoms in a magneto-optical trap as an ion source. They show that this new technique can provide spot resolutions and brightness values that are better than the state of the art in focused ion beams. The source can be used with a range of different ionic species and can be combined with laser cooling techniques to exert unprecedented control over the ion emission, for example, producing single ions “on demand.” The beam quality is a result of a high brightness and a narrow energy distribution, both of which stem from the cold temperature (≈100μK) of the atoms. The ions are produced by subjecting the cold neutral atoms to a photoionization laser, after which they become a compact source of nearly monoenergetic ions. With the application of a potential gradient, the ions form a beam that can be focused via standard ion optical techniques. They discuss estimations based on the initial size of the ion cloud and the energy distribution and show that the resulting beam has a low emittance.
We report on the demonstration of a low emittance, high brightness ion source based on magneto-optically trapped neutral atoms. Our source has ion optical properties comparable to or better than those of the commonly used liquid metal ion source. In addition, it has several advantages that offer new possibilities, including high resolution ion microscopy with ion species tailored for specific applications, contamination-free ion milling, and nanoscale implantation of a variety of elements, either in large quantities, or one at a time, deterministically. Using laser-cooled Cr atoms, we create an ion beam with a normalized rms (root-mean-square) emittance of 6.0 x 10 (-7) mm mrad M e V and approximately 0.25 pA of current, yielding a brightness as high as 2.25 A cm (-2) sr (-1) eV (-1). These values of emittance and brightness show that, with suitable ion optics, an ion beam with a useful amount of current can be produced and focused to spot sizes of less than 1 nm.
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Photoionizing a beam of ultracold atoms can provide a high-brightness ion beam, to improve resolution in focused-ion-beam nanofabrication. With this aim, the authors use laser cooling and compression to give a 85{}^{85}85Rb atomic beam an equivalent brightness better than the current standard for the semiconductor industry. This could represent an important step toward smaller feature sizes in integrated circuits.
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