Development and characterization of an iodine field emission ion source for focused ion beam applications

Emission of positive and negative ions is possible when a room-temperature molten salt, or ionic liquid, is exposed to a sufficiently high electric field. Ionic liquid ion sources (ILISs) have shown potential to be used in various focused ion beam (FIB) applications, since their operation and characteristics are similar to those of liquid metal ion sources, with the advantage that ILIS work at low temperatures in comparison and a large number of ionic liquids with many different compositions are available. In this article, the authors present results on the emission characteristics of negative ions extracted from an iodine-based ionic liquid using a time-of-flight mass spectrometer and a retarding potential analyzer. The ionic liquid 1-butyl-3-methylimidazolium iodide (BMI-I) is used as source media, producing a droplet free beam with multiple solvated ion species. Attention is given to BMI-I, in particular, due to the potential of creating a beam of pure and clustered I-ions, which are expected to improve the performance in applications based on secondary emission and reactive species. Properties important to the focusing of the ion beam such as mass and energy distributions are obtained. The full width at half maximum of the energy distribution is measured to be on the order of 10 eV, with an estimated angular intensity of 0.65x10(-6) A sr(-1). Estimated performance of the ILIS in a hypothetical FIB column gives current densities of 0.15 A cm(-2).