Future extreme ultraviolet (EUV) lithography will require high radiation intensities at a wavelength around 13.5 nm. The limits of emission in this spectral range from discharge based plasmas are discussed theoretically. The discussion is based on a simple MHD approach for a xenon plasma discharge and atomic data from the ADAS software package for radiative transitions, excitation and ionization of different ionization levels. Discharge parameters are chosen for the Philips' hollow cathode triggered pinch plasma. The calculations show that the 13.5 nm emission originates only from of Xe10+ ions and is optically thin. Ideally, the conversion efficiency is expected to scale linearly with the electron density in this case. The MHD calculations, however, show a lower increase with density. The loss channels leading to this behaviour, like leakage currents, will be discussed in detail. The identification of these losses allow, on the other hand, for a systematic improvement of the electrode system and the electrical circuit. In addition, theoretical emission spectra of xenon and tin as the most promising emitters around 13.5 nm will be compared with respect to the possible optimization potential of spectral emission characteristics.

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We suggest a reflectometer for thin film analysis based on a plasma-discharge source utilizing extreme ultraviolet (XUV) radiation in a wavelength region of 4–40 nm. In contrast to other laboratory based reflectometers, which are designed for the near normal incidence case to characterize XUV multilayer optics and maskblanks, in our approach we move to a selectable fixed grazing incidence angle that enables surface sensitive analysis of almost arbitrary ultrathin film systems providing high elemental contrast due to the characteristic absorption of XUV by matter. Most materials (e.g., Si, Al, Gd, and Ag) exhibit characteristic absorption edges allowing not only to determine layer thicknesses, surface or interlayer roughnesses in a stack, but also elemental composition and even analyzing the absorption fine structures. Together with our simulations we show that our polychromatic approach makes it possible to provide all these parameters in one measurement.

In this work, we report about the optimization of the spectral emission characteristic of a gas discharge plasma source for high-resolution extreme ultraviolet (EUV) interference lithography based on achromatic Talbot self-imaging. The working parameters of the source are optimized to achieve a required narrowband emission spectrum and to fulfill the necessary coherence and intensity requirements. The intense 4f-4d transitions around 11 nm in a highly ionized (Xe8+-Xe12+) xenon plasma are chosen to provide the working wavelength. This allows us to increase the available radiation intensity in comparison with an in-band EUV xenon emission at 13.5 nm and opens up the possibility to strongly suppress the influence of the 5p-4d transitions at wavelengths between 12 and 16 nm utilizing a significant difference in conditions for optical thickness between 4f-4d and 5p-4d transitions. The effect is achieved by using the admixture of argon to the pinch plasma, which allows keeping the plasma parameters approximately constant while, at the same time, reducing the density of xenon emitters. It is demonstrated that with this approach it is possible to achieve a high intensity 11 nm EUV radiation with a bandwidth of 3%-4% without the use of multilayer mirrors or other additional spectral filters in the vicinity of the working wavelength. The achieved radiation parameters are sufficient for high-performance interference lithography based on the achromatic Talbot effect

We report the demonstration of scanning-probe coherent diffractive imaging method (also known as ptychographic CDI) using a compact and partially-coherent gas-discharge plasma source of extreme ultraviolet (EUV) radiation at 17.3 nm wavelength. Until now, CDI has been mainly carried out with coherent, highbrightness light sources, such as 3rd generation synchrotrons, X-ray free-electron lasers and high harmonic generation. Here we performed ptychographic lensless imaging of an extended sample using a compact, labscale source. The CDI reconstructions were achieved by applying constraint relaxation to the CDI algorithm. Experimental results indicate that our method can handle the low spatial coherence, broadband nature of the EUV illumination as well as the residual background due to visible light emitted by the gas-discharge source. The ability to conduct ptychographic imaging with labscale and partially coherent EUV sources is expected to significantly expand the applications of this powerful CDI method. © Coherent diffractive imaging (CDI) is a rapidly emerging imaging technique to achieve diffraction-limited resolution without using imaging optics [1][2][3]. This makes CDI very attractive for imaging in the extreme ultraviolet (EUV) and X-ray spectral range, where the use of focusing optics is limited. In CDI, a coherent wave illuminating a sample produces a diffraction pattern related to the Fourier transform of the sample structure. While the magnitude of the Fourier transform (i.e. the square root of the diffraction intensity) can be collected by a detector, the phase information is lost, which constitutes the wellknown phase problem. If the diffraction intensity is properly measured, the phase information can be retrieved with an iterative algorithm and the sample structure can then be reconstructed [4]. With the rapid development of coherent X-ray sources worldwide, various CDI methods have been demonstrated and have found broad applications in both physical and biological sciences.One of the powerful CDI methods is termed ptychography (also known as scanning probe CDI) [5], in which an object is scanned relative to a structured illumination probe and a sequence of diffraction patterns is collected with an overlap between adjacent illuminated areas. In contrast to conventional CDI [1][2][3], ptychography uses the overlapping areas as a real space constraint, allowing the reconstruction of extended objects [5]. For high-resolution imaging, CDI and ptychography experiments typically employ large scale X-ray facilities, such as 3rd generation synchrotrons and X-ray free-electron lasers (XFELs) [1][2][3]. In the last decade, CDI and ptychography have also been successfully implemented with highly coherent tabletop femtosecond lasers generating EUV high harmonics [6][7][8][9][10].In this letter, we present an example of ptychographic imaging with a partially coherent compact gas-discharge EUV light source operating at 17.3 nm wavelength (Li-like oxygen, 1s 2 2p-1s 2 3d transition). In our gas-discharge light...

The development of suitable radiation sources is a major challenge for extreme ultraviolet lithography (EUVL). For the optimization of these sources and for the determination of the parameters needed for the system design and the system integration these sources have to be characterized in terms of the absolute in-band power, the spectral distribution in the EUV spectral region and the out-band spectral regions, the spatial distribution of the emitting volume and the angular distribution of the emission. Also the source debris has to be investigated. Therefore, JENOPTIK Mikrotechnik GmbH is co-operating with the Laser Laboratorium Göttingen, the Physikalisch-Technische Bundesanstalt (PTB) and the AIXUV GmbH in developing ready-for-use metrology tools for EUVL source characterization and optimization. The set of the tools employed for EUV-source characterization is presented in detail as well as concepts of for calibration and measurement procedures.

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