Visualization of beams from ionic liquid ion sources for focused ion beam applications

Abstract

Ionic liquid ion sources (ILIS) share qualities with liquid metal ion sources necessary for focused ion beams (FIB) operation, such as pure ion emission and low energy spreads. These sources work at lower currents, room temperature, and are able to produce both positive and negative ions from a large number of ionic liquids. The influence of the applied voltage on the beam emitted from the ILIS based on the liquid EMI-BF4 (1-ethyl 3-methyl imidazolium tetrafluoroborate) is characterized by using a beam visualization system. As the applied voltage is increased, the source transitions from single to multiple beam emission. The visualization tool is also used to determine the spatial distribution of the neutral particle population contained within the beam emitted from ILIS. Cluster ions can break during flight, yielding a new ion and neutral particles. This neutral population can be highly energetic, and might be of interest for material treatment applications where charging of targets is undesirable. For FIB applications, these neutrals could lead to undesired effects in the sample, and so it is necessary to know the distribution of the neutrals within the beam in order to specify the filtering and optics required to implement an ILIS-based FIB. It is found that 20% of the ion beam results from fragmentation events, and that the neutral population is concentrated at the center of the beam.