Ionic Liquid Ion Sources: the Flexible Ion Beam

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The use of ion beams for etching is a widely applied technique, providing a valuable tool for completing in situ lithography of micro manufactured components. Today conventional gallium Liquid Metal Ion Sources (LMIS) is the most well appleid focused ion beam etching method, in which gallium ions are extracted from a liquid metal surface through the application of a strong electrostatic field. LMIS offer direct, resistless, three dimensional patterning, but the choice of one ion type is limiting, both in terms of etching rate, etching resolution, and application. The properties of the ion influences significantly the physical and chemical nature of the resulting etched nano and microstructures, limiting the potential application areas. In this project, we propose to investigate the application of an Ionic Liquid Ion Source (ILIS) as an alternative ion beam source for etching. These fundamentally circumvent the limitation of LMIS - the very limited number of ion types available - through using an organic 'ionic liquid' molecule to create the ions. An ionic liquid consists of molecular cations and anions bonded ionically in solution, from which a high electric field can extract a very collimated beam of highly monodisperse molecular ions. Currently over 5000 room temperature variants of ionic liquids are known, with widely varying properties and size of ions, which offer a vast variation in the charged species being emitted, allowing for a bespoke selection of ions (big or small, chemically reactive or inert, capable of ion implantation or not, etc.). This leads to much greater flexibility of the ion beam source, for example allowing for high etching rates using a large ionic liquid molecrular ion, and then switching to a smaller molecular ion (by simple ionic liquid variation), resulting in more accurate final etching.We aim to investigate the potential of applying an ionic liquid ion source to the etching of material substrates. We have developed a novel ionic liquid ion source that produces a high-energy monodisperse ion beam, using an ionic liquid demonstrated to produce good etching capability. We aim to demonstrate that it can offer an alternative to conventional liquid metal ion sources, whilst investigating the fundamental phenomena that enable ILIS devices to operate.There are three main subjects that will be investigated in this project. Firstly a systematic analysis of ionic liquid ion sources for micro fabrication will be completed. Studies have demonstrated the promising nature of the technique, but to a limited aspect and not fully investigating its wide potential. We will complete a systematic study of the technique, providing a comprehensive data set which will highlight where the technique can be particularly advantageous. Secondly there is a need for greater understanding of how the ionic liquid ion source operates. The source can produce a highly monodisperse beam, but the parameters that affect this beam output (liquid properties, source design) are incompletely understood. We will apply theoretical and simulation techniques complimented by experimental analysis of the plume to greater understand the processes that are occurring. This will feed into the design of the source.Thirdly the lessons learnt from the above analysis will allow us to produce a ionic liquid ion source designed specifically for etching processes, rather than an adaptation of spacecraft propulsion ion thrusters that will be used at the start of the project.