# EUV-Induced Plasma: A Peculiar Phenomenon of a Modern Lithographic Technology
## 4 Discussion, Conclusions, and Outlook  
EUV-induced plasma is a highly-transient and peculiar type of plasma with significant impact on EUVL equipment and processes, as may have become clear from this contribution. Although numerical and experimental research campaigns have enlightened several of this plasma's secrets, several aspects of it remain unexplored. Below, we define some of the most urgent research areas as being interesting from both a fundamental and EUVL application point of view. Note that new research project in some of these directions have already been recently initiated by the authors. * Radical production  
With regards to bulk plasma and from an experimental point of view, electrons and ions have already been characterized. However, in the full picture the characterization of radicals (e.g., atomic H) is lacking, despite their potential impact on cleaning and lifetime issues. More knowledge with respect to radical production could be gained through plasma-chemical (numerical) models and experiments utilizing techniques, such as UV absorption or Two-photon Absorption Laser Induced Fluorescence (TALIF). * Gas admixtures (e.g., N\({}_{2}\))  
Up to now, most of the studies have only been performed in hydrogen (or argon only or helium only). However, the addition of small amounts of other gases can have an important influence on the composition of the EUV-induced plasma and it might significantly alter plasma chemistry and plasma physical processes, and hence representing real scanner conditions more accurately [62]. Although the first explorative study on N\({}_{2}\)-dilution of EUV-induced plasmas in H\({}_{2}\) has recently been published [63], there remains a strong demand from industry to understand the effect of other gases/vapors (e.g., H\({}_{2}\)O, O\({}_{2}\), and N\({}_{2}\)) on the whole model of EUV-induced plasma dynamics. * Interaction with nano- to micrometer sized particles  
Although EUV-induced plasma dynamics and the impact thereof are much better understood than 10 years ago, the interaction of nano- to micrometer sized particles with the complex atmosphere of an EUV lithography tool (including plasma, electric fields, energetic photons, etc.) is a blank spot that should be enlightened. The reason for the importance of plasma-particle interaction is the fact that bulk plasma normally has higher potential when compared to its surroundings [60]. This, in combination with the ability of plasma to deliver high surface charges to the particles [100], gives rise to concerns regarding particle transport from the location where the particles are released to basically every other location in the tool where the particles may harm. Whereas, in normal low pressure plasmas, the charge on micrometer-sized particles is permanently negative and it ranges in magnitude from a few 1000 times the elementary charge in the space charge regions at the plasma's border [48; 101; 102] up to a few \(10^{5}\) times the elementary charge in the plasma bulk [100], the charge on particles under EUVL tool conditions may significantly vary and may even become positive for certain fractions of the time. It is especially the transient behavior of the kind of plasmas occurring in EUVL tools and the presence of ionizing radiation that can have considerable influence on the dynamic particle charging from both a temporal [103] and a spatial [104] point of view. We expect in the coming decade enhanced research efforts in the field of Complex Ionized Media (CIM) with nano-contamination control becoming more dominantly important for EUVL and with (EUV-induced) plasma being a strong candidate with respect to achieving cleanliness targets, i.e., the field studying the interaction between nano-contaminants and ionized media. With respect to the interaction of EUV-induced plasma with plasma-facing materials, in general, the understanding does not extend much beyond what is known from classic (discharge) plasma physics.