Electrochemical material synthesis in ultra-high vacuum: elementary steps of electrochemical deposition of tantalum and niobium in ionic liquids

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Refractory metals such as tantalum and niobium are used in a variety of technical applications.For example, tantalum electrolytic capacitors are particularly important for today'smicroelectronics. The physico-chemical properties of tantalum allows miniaturised capacitors with high capacitance to be produced. Due to the lower occurrence of tantalum, niobium is increasingly used as a replacement material for capacitors. In addition, tantalum is widely used in medicine for implants due to its biocompatibility. Electrochemically, refractory metals cannot be deposited from aqueous and organic solutions due to their relatively negative standard electrode potentials. Hitherto, only high-temperature molten salts have proven to be efficient baths for the electrochemical deposition of several μm thick refractory metals. However, these electrolytes suffer from many technical and economic problems, such as the loss of current efficiency of the electrolysis process due to the partial dissolution of the metal after its deposition. Furthermore, corrosion problems occur at the necessary high temperatures. Due to their wide electrochemical windows (up to 6 V), ionic liquids (ILs) offer themselves aselectrolytes for the electrodeposition of these metals at low temperatures as an alternative. Ithas been shown that Ta can be electrodeposited in thin layers from TaF 5 in a range of air and water stable ionic liquids. The electrodeposition of Ta and of Nb is quite complex and not understood in detail. In addition, the high reactivity of the metals makes it difficult to analyse the deposited layers with ex-situ methods. This project now takes advantage of the fact that ionic liquids generally have a very low vapour pressure and can therefore be investigated with vacuum-based methods such as X-ray photoelectron spectroscopy (XPS) in ultra-high vacuum (UHV). We want to investigate the electrochemical processes during refractory metal deposition in ILs step by step directly in UHV with monochromatic XPS in-situ using tantalum and niobium precursors. A specially designed spectroscopy unit is available for this type of material synthesis. Starting with the analysis of the UHV electrochemistry of the pure ILs, we will then specifically investigate the influence of the precusors on the deposition. This will alsoprovide us with information about the stability of ILs and solutions due to the influence ofelectrochemistry in a very controlled atmosphere. With this approach, we will fundamentally clarify how tantalum and niobium are deposited electrochemically. In the medium-term we would like to show to electrochemical surface engineering ways to deposit refractory metals under mild conditions.