MOTIS: A Focused Ion Beam Source Based On Laser- Cooled Atoms

Focused ion beams (FIBs) are an invaluable tool for the creation and observation of materials with nanoscale feature sizes, providing key capabilities in integrated circuit sectioning, critical dimension measurement, and circuit repair. However, ion sources capable of both high brightness and low emittance operation are difficult to realize, with practical sources limited to very few atomic species. Recently, we have developed a new type of low-emittance, high-brightness ion source based on the ionization of magnetooptically trapped atoms[1]. Using this source, known as a magneto-optical trip ion source (MOTIS), we have demonstrated nanoscale-focused ion beams of both lithium and chromium. The MOTIS uses laser-cooled neutral atoms that are photoionized to create an isotopically pure beam of ions whose ion optical properties are comparable to or better than those of liquid metal ion sources. The MOTIS paradigm can be extended to create ion sources from any atomic species that can be laser-cooled, i.e., Li, Na, K, Rb, Cs, Fr, Mg, Ca, Sr, He, Ne, Ar, Kr, Xe, Al, Ag, Cr, Er, Cd, Hg, and Yb. This flexibility allows the atomic species of the source to be tailored to the specific application, e.g., microscopy with light ions, milling with heavy ions, and nanoscale implantation of a variety of elements. In addition, the inherently low energy spread enables the creation of high-resolution focused ion beams at low energy. We report on the construction and performance of MOTIS-technology-based lithium and chromium FIBs. Each FIB platform consists of a chromium or lithium MOTIS that is integrated with a conventional ion optical column. We will present results of microscopy using chromium and lithium beams and a characterization of focal spot sizes. We will also examine applications of this technology, such as beamactivated surface chemistry, direct deposition and removal of materials, and deterministic single ion implantation. FIGURE 1. (a) Schematic of Magneto-Optical Trap Ion Source. (b) Image of microchannelplate pores acquired with a 2 kV, 2 pA Li beam.