Jabez J. McClelland
MOTIS: a magneto-optical trap based ion source for focused ion beam applications
J. J. McClelland(1), A. V. Steele(1), B. J. Knuffman(1), and J. Orloff(2)
(1) Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD, USA
(2) FEI Company, Hillsboro, OR, USA
Focused ion beams have seen wide application in nanotechnology for both high-resolution milling and microscopy. Commercial systems have been available for several decades, and a strong and growing market continues today for this technology. Nearly all commercial systems rely on the gallium liquid metal ion source (LMIS) to produce an ion beam with the requisite brightness and emittance to make possible nanometer-scale focusing with a suitable current (at least several pA). However, a great deal of increased functionality could be realized if a source that produces ions other than gallium and also has a narrow energy spread were available. Other ions would make possible microscopy that exceeds the performance of electron microscopy without sample damage (using light ions) or ion milling without contamination (using inert, heavy ions). A narrow energy spread would open the possibility of low-energy ion microscopy, allowing access to new contrast mechanisms. To address these issues we have developed an alternative source of ions based on photoionization of laser cooled and trapped neutral atoms. Due to the cold temperature of the trapped atoms, the ion beam produced is highly collimated and has a very narrow energy spread. Our measurements show that the collimation of the beam translates into an emittance equivalent to or better than that found in a typical LMIS. Moreover, this emittance is obtained without any of the high-current density regions (point emitter or crossover) found in other systems, significantly reducing space-charge effects. We have also demonstrated a current of 1.4 pA, and expect to be able to produce currents exceeding 100 pA with an optimized system. Our source opens major new opportunities for focused ion beams in both microscopy and milling, making use of the wide range of atomic species that can be laser cooled and trapped, as well as the narrow energy spread. Further applications such as deterministic doping with single ions are also enabled.
