Simulated performance of an ultracold ion source

Abstract

At present, the smallest spot size which can be achieved with state-of-the-art focused ion beam ͑FIB͒ technology is mainly limited by the chromatic aberrations associated with the 4.5 eV energy spread of the liquid-metal ion source. Here we numerically investigate the performance of an ultracold ion source which has the potential for generating ion beams which combine high brightness with small energy spread. The source is based on creating very cold ion beams by near-threshold photoionization of a laser-cooled and trapped atomic gas. We present ab initio numerical calculations of the generation of ultracold beams in a realistic acceleration field and including all Coulomb interactions, i.e., both space charge effects and statistical Coulomb effects. These simulations demonstrate that with existing technology reduced brightness values exceeding 10 5 A m −2 sr −1 V −1 are feasible at an energy spread as low as 0.1 eV. The estimated spot size of the ultracold ion source in a FIB instrument ranges from 10 nm at a current of 100 pA to 0.8 nm at 1 pA.

Low-Energy-Spread Ion Bunches from a Trapped Atomic Gas

“…This result will certainly hold for continuous operation of the source, because space charge effects will be much less important at demonstrated currents of 1.4 pA [9] and achievable currents of 100 pA [4]. The lowest energy spread observed here, U ¼ 0:02 eV, constitutes a 2 orders of magnitude improvement over the state-of-the-art liquid metal ion source.…”

“…An additional longitudinal energy spread is introduced due to the initial longitudinal size of the ionization volume. A more detailed description of the source and its expected performance is given in our previous paper [4].…”

“…Recently a new ion source was proposed, the Ultra Cold Ion Source (UCIS), which is based on extraction of ions from a laser cooled atomic gas [4][5][6]. The UCIS promises a much smaller energy spread than the LMIS at comparable brightness, enabling 1 nm focal spot sizes.…”

Performance predictions for a laser-intensified thermal beam for use in high-resolution focused-ion-beam instruments

“…Apart from promising high-brightness ion beams, laser cooling can be applied to a variety of atomic species ranging from the alkali and alkaline metals, several transition and rare-earth metals and some p-block materials which would open up new possibilities for FIB users. The Ultra Cold Ion Source (UCIS) [9] and Magneto-Optical Trap Ion Source (MO-TIS) [10] both use laser cooling and trapping in 3D followed by in-field photo-ionization to create ion bunches or beams. Although longitudinal energy spreads down to 20 meV have been demonstrated [11] the target brightness could not be achieved with these sources [12].…”

Performance predictions of a focused ion beam from a laser cooled and compressed atomic beam

“…Due to the low diffusion rate in a magneto optical trap (MOT), the current density that can be extracted from the ionization volume is limited, which limits the brightness and the extractable current as well. 15 Here, we report on calculations of the expected performance of a focused ion beam based on photo-ionization of a laser cooled and compressed atomic beam of rubidium. By first creating an atomic beam and then laser cooling and compressing it in two dimensions, the fundamental limitations of the UCIS and MOTIS are overcome.…”