# EUV-Induced Plasma: A Peculiar Phenomenon of a Modern Lithographic Technology
## 2 EUV-Induced Plasma
### Recent Experimental Work on EUV-Induced Bulk Plasmas  
One of the most obvious ways to study light-emitting plasmas is by making photographs of it. This task is best performed using intensified charge-coupled device (iCCD) cameras, since the relevant time scales are short (ns-\(\upmu\)s) and light emission levels are low. Figure 4 shows iCCD photographs visualizing the region where the EUV-induced plasma emits light in the UV and visible spectrum, together with time-resolved measurements of the total and wavelength-integrated intensity and the EUV-irradiation intensity. Figure 3: Examples of numerical PIC simulations matching reasonably well measurement of the electron density (measured with Microwave Cavity Resonance Spectroscopy, see next section) in EUV-induced plasmas after irradiation of low pressure hydrogen (**a**) and argon (**b**) gas by one single pulse of EUV radiation. Figure produced using data from [21]. Fundamental processes in EUV-induced plasmas have been investigated by cleverly using the fact that EUV-induced plasmas radiate in the EUV and UV/visible range. At considerably higher pressures (close to atmospheric pressure) than expected in EUV lithography tools (a few Pa), [PERSON] et al. [27; 28; 29; 30] used spectral measurements to study ionic states in EUV-induced plasmas in various gases, such as xenon, argon, neon, and helium. In photoionized plasmas in argon, they used laser interferometry to measure electron densities up to 10\({}^{24}\) m\({}^{-3}\). [PERSON] et al. [31] used spectroscopic measurements in the ultraviolet/visible range to obtain local information regarding the temperature and density of electrons in EUV-induced plasmas in a mixture of Kr/Ne/H\({}_{2}\) gas. Values of 0.9 to 1.7 eV and 10\({}^{21}\) to 10\({}^{22}\) m\({}^{-3}\) were found for the temperature and density of electrons in that research, respectively. Although these investigations provide valuable information with respect to understanding the fundamental behavior of photoionized gases, we remain close to the pressure range used in EUVL in the following of this manuscript (i.e., 1-10 Pa). As mentioned before, plasma bulk dynamics are most dominantly described by the temperature (\(T_{e}\)) and density (\(n_{e}\)) of free electrons in this pressure range. From an experimental point of view, classical plasma diagnostics provide several ways to probe the presence of electrons. For instance, Thomson scattering [32]--based on the scattering of laser light by free electrons--enables the retrieval information about the energy and density of free electrons in plasma. However, this technique only delivers sufficient signal at electron densities of 10\({}^{17}\) m\({}^{-3}\) and higher [32], meaning that this technique is not suitable to probe electrons in EUV-induced plasmas under scanner-like conditions. Another technique that is commonly used for this purpose includes the utilization of traditional Langmuir probes from which the voltage-current characteristics provide information regarding \(n_{e}\) and \(T_{e}\). Although this technique may be able to probe electrons in the here relevant density range, [PERSON] et al. [33] concluded that it was not feasible to apply probes to EUV-induced plasmas, with the main reason being that the photo-electric currents obscured the measurements of the I-V curves. Later,  
Figure 4: **(b)** Total emission (in the UV and visible spectrum range) from an EUV-induced plasma in 5 Pa argon created inside a cylindrical measurement volume (**a**). The numbers displayed in this figure correspond to the numbered images in (**c**). The horizontal error bars represent the set intensified charge-coupled device (iCCD) camera’s gate width. **(c)** Gated iCCD images at various moments in time (indicated in each picture individually) after one pulse of EUV-irradiation. The beam of EUV photons is directed through a cylindrical aluminum measurement chamber from the left to the right (the side walls are indicated with white lines).