We report on the demonstration of an ion microscopy system based on a magneto-optical trap ion source (MOTIS). Laser-cooled neutral atoms are trapped in a magneto-optical trap and cooled to a temperature of a few hundred microkelvin. An ionization laser tuned very close to threshold creates ions, which are accelerated by an electric field into a beam. The emittance of the beam, determined by the size of the source and the temperature of the ions, is extremely small, as a result of the ions' low temperature. This small emittance, together with a very low inherent energy spread in the beam, opens the possibility for creation of nanometer-scale focal spots.
Our microscopy system uses lithium atoms, which are cooled and trapped with diode laser light at 671 nm and ionized at 350 nm. The ion source is mated to a commercial focused ion beam system originally designed for use with a gallium liquid metal ion source. We have successfully demonstrated 2 keV ion beams with currents ranging from a few picoamperes to as high as 60 pA, and focal spot sizes as small as 27 nm (25%-75% width).1 We will show examples of images taken with the microscope, and also discuss a number of interesting applications of nanoscale lithium implantation.
The demonstration of a lithium MOTIS as a viable microscopy tool represents a major step forward in the implementation of cold ion sources for focused ion beam applications. The ever-increasing number of atomic species that can be laser cooled and trapped (over 22 at present) suggests a great many new applications, including heavy-ion milling, low-damage light-ion microscopy, and species-specific nanoscale implantation. The low energy spreads inherent to the MOTIS make possible focused ion beams with unprecedented low energy, opening up new contrast mechanisms for imaging, as well as applications in precision milling depth control and ultra-low-damage milling.
1B. Knuffman, A. V. Steele, J. Orloff, and J. J. McClelland, New Journal of Physics 13, 103035 (2011).
