The next generation of lithography machines uses extreme ultraviolet (EUV) light originating from laser-produced plasma (LPP) sources, where a small tin droplet is ionized by an intense laser pulse to emit the requested light at 13.5 nm. Numerous irradiation schemes have been explored to increase conversion efficiency (CE), out of which a double-pulse approach comprising a weak picosecond Nd:YAG pre-pulse followed by a powerful pulse is considered to be very promising [1]. Nevertheless, even for such CE-optimized schemes, ion debris ejected from the plasma with kinetic energies up to several keV remain a factor that hampers the maximum performance of LPP sources. In this letter we propose a novel pre-pulse scheme consisting of a picosecond pulse pair at 1064 nm, which decreases the amount of undesirable fast ions, avoids back-reflections to the lasers and enables one to tailor the target shape.In the past two decades a large number of theoretical and experimental studies have been conducted on possible light sources for EUV lithography, including synchrotron radiation [2, 3], free-electron lasers [4,5], plasma sources [6-10] and high-harmonic generation [11]. From the aforementioned solutions, a tin-based laser-produced plasma source received the most attention due to its high conversion efficiency, robustness and scalability [12,13], resulting in a first commercial machine launched in 2010. In such an LPP source, narrowband radiation around 13.5 nm comes from multiple ionic states, Sn 8+ to Sn 14+ [14,15], collisionally excited by plasma electrons heated through interaction with a powerful CO2 laser. An effective coupling between laser light and plasma occurs near the critical density, which for CO2-laser-driven plasma is around 10 19 cm -3 , meaning that a mass-limited tin target should be expanded to reduce its density. At the same time the size of the EUV source cannot be too large to match the requirements for the maximum etendue [16]. The precise control of the target shape is thus crucial for the production of EUV light in an industrial setting.

A method in described for the simultaneous determination of trace concentrations of Ca, Cr, Cu, Fe, Mn, Ni, Mo, As, Zn, Co, Th, V and Ti in nuclear‐grade uranium oxides or other uranium compounds by x‐ray fluorescence spectrometry without the use of chemical treatment. The limits of detection are Mo 25, As 40, Cu 5, Ni 29, Co 17, Fe 5, Mn 30, Cr 8, Th 7, Zn 5, V 52, Ti 10 and Ca 80 ppm. Samples are prepared in the form of double‐layer pellets with starch as a binding agent. Standards are prepared in a U3O8 matrix, which is more chemically stable than other uranium oxides. The dynamic ranges of the calibration graphs are very large because, for all impurity elements, they are linear over three orders of magnitude of concentration. The method is fast, since it requires no chemical treatment and only limited manipulations. The method has good precision for the impurity elements.