Because of their high photon flux, x-ray free-electron lasers (FEL) allow to resolve the structure of individual nanoparticles via coherent diffractive imaging (CDI) within a single x-ray pulse. Since the inevitable rapid destruction of the sample limits the achievable resolution, a thorough understanding of the spatiotemporal evolution of matter on the nanoscale following the irradiation is crucial. We present a technique to track x-ray induced structural changes in time and space by recording two consecutive diffraction patterns of the same single, free-flying nanoparticle, acquired separately on two large-area detectors opposite to each other, thus examining both the initial and evolved particle structure. We demonstrate the method at the extreme ultraviolet (XUV) and soft x-ray Free-electron LASer in Hamburg (FLASH), investigating xenon clusters as model systems. By splitting a single XUV pulse, two diffraction patterns from the same particle can be obtained. For focus intensities of about 2x1012 W cm-2 we observe still largely intact clusters even at the longest delays of up to 650 picoseconds of the second pulse, indicating that in the highly absorbing systems the damage remains confined to one side of the cluster. Instead, in case of five times higher flux, the diffraction patterns show clear signatures of disintegration, namely increased diameters and density fluctuations in the fragmenting clusters. Future improvements to the accessible range of dynamics and time resolution of the approach are discussed.

We present the design, integration, and operation of the novel vacuum ultraviolet (VUV) beamline installed at the free-electron laser (FEL) FLASH. The VUV source is based on high-order harmonic generation (HHG) in gas and is driven by an optical laser system synchronized with the timing structure of the FEL. Ultrashort pulses in the spectral range from 10 to 40 eV are coupled with the FEL in the beamline FL26, which features a reaction microscope (REMI) permanent endstation for time-resolved studies of ultrafast dynamics in atomic and molecular targets. The connection of the high-pressure gas HHG source to the ultra-high vacuum FEL beamline requires a compact and reliable system, able to encounter the challenging vacuum requirements and coupling conditions. First commissioning results show the successful operation of the beamline, reaching a VUV focused beam size of about 20 µm at the REMI endstation. Proof-of-principle photo-electron momentum measurements in argon indicate the source capabilities for future two-color pump-probe experiments.

Novel reticle absorber materials are required for high-NA EUV lithography. TNO and ASML developed an assessment for the compatibility of novel high-NA reticle absorber materials with conditions that mimic the EUV scanner environment[1]. Four candidate reticle absorber materials were evaluated, TaCo, RuTa, PtMo and Pt2Mo alloys, in a joint research program. For the compatibility tests, dedicated samples with silicon wafer substrates were fabricated. The silicon wafers were coated with a Mo-Si multilayer coating, followed by a Ru capping layer and finally the absorber material. Chemical outgassing tests, in presence of hydrogen radicals, did not show chemical outgassing for the TaCo and PtMo alloys. RuTa and Pt2Mo alloys were not tested, based upon their elemental composition chemical outgassing is not expected. Next, all four materials were exposed in a hydrogen plasma resistance test equivalent to an EUV exposure of at least 250 thousand wafers** in the EUV scanner. No plasma-induced defects, like blistering or delamination, were found that were related to the intrinsic absorber properties. The RuTa and PtMo alloys were selected for EUV exposure in the EBL2 facility at TNO. Both materials were exposed to an 9.6 kJ/mm2 EUV peak dose at an EUV peak intensity of 450 mW/mm2 in a hydrogen environment. This EBL2 EUV exposure of 6 hours represents about 1-2 months of EUV dose (at least 150 thousand wafers) in a NXE or EXE scanner with a 300 W source. Both materials showed good performance during EUV exposure. Relevant surface defects and chemical outgassing were not observed. The few observed blisters in the low EUV intensity areas are likely provoked by particle contamination or coating defects. All four absorber materials, TaCo, RuTa, PtMo and Pt2Mo alloys performed well in the compatibility tests that were executed. Not all compatibility tests could be performed on each absorber material within the scope of our research program. Further testing would be needed to complete the compatibility assessment, including an EUV exposure on a patterned reticle.

As it is rich in spectral lines emitted by plasma between 10000 K and 20 MK, the vacuum ultraviolet (VUV - 17 to 200 nm) solar spectrum is extremely valuable for instruments that study the physics of the solar atmosphere. We present multilayer coatings with simultaneous broadband reflectance in the two spectral ranges of 16.9 nm to 21.5 nm and 46.3 nm to 127.5 nm. The coatings are based on Mo/Si multilayers with a thin capping layer of boron carbide (B4C). Samples were produced and their reflectance measured. Their performance in terms of resistance to high temperatures and low micro-roughness was also assessed by measurement. Our study shows that a coating with the characteristics required by next generation spectrometers for studies of the solar atmosphere is feasible.

We present an extreme ultraviolet (EUV) microscope using a Schwarzschild objective which is optimized for single-shot sub-micrometer imaging of laser-plasma targets. The microscope has been designed and constructed for imaging the scattering from an EUV-heated solid-density hydrogen jet. Imaging of a cryogenic hydrogen target was demonstrated using single pulses of the free-electron laser in Hamburg (FLASH) free-electron laser at a wavelength of 13.5 nm. In a single exposure, we observe a hydrogen jet with ice fragments with a spatial resolution in the sub-micrometer range. In situ EUV imaging is expected to enable novel experimental capabilities for warm dense matter studies of micrometer-sized samples in laser-plasma experiments.

With the introduction of the NXE:3400B scanner, ASML has brought EUV to High-Volume Manufacturing for sub-10nm node lithography. And work has already been started on a successor high-NA system with NA=0.55. For both these systems, node resolution will go down faster than NA increases, resulting in decreasing k(1)-factors and tightening of aberration requirements. A crucial component for measuring and controlling aberrations in-situ is a diffuser to fill the full pupil of the projection optics appropriately. This paper presents several new diffuser concepts, both reflective as well as transmissive, with their respective key performance metrics for both NA= 0.33 and NA= 0.55 EUV projection optics. These concepts can be used for measuring wavefront quality from dedicated fiducial plates, or for measuring directly from the imaging reticle. The latter would enable a combination of reticle alignment with lens aberration control without throughput penalty. It will be shown that with these diffuser concepts, we have a solution for in-situ aberration control for 5nm nodes and below.

Extreme ultraviolet (XUV) and X-ray free-electron lasers enable new scientific opportunities. Their ultra-intense coherent femtosecond pulses give unprecedented access to the structure of undepositable nanoscale objects and to transient states of highly excited matter. In order to probe the ultrafast complex light-induced dynamics on the relevant time scales, the multi-purpose end-station CAMP at the free-electron laser FLASH has been complemented by the novel multilayer-mirror-based split-and-delay unit DESC (DElay Stage for CAMP) for time-resolved experiments. XUV double-pulses with delays adjustable from zero femtoseconds up to 650picoseconds are generated by reflecting under near-normal incidence, exceeding the time range accessible with existing XUV split-and-delay units. Procedures to establish temporal and spatial overlap of the two pulses in CAMP are presented, with emphasis on the optimization of the spatial overlap at long time-delays via time-dependent features, for example in ion spectra of atomic clusters.

s ................................................................................................................................... 11 List of participants .................................................................................................................... 67

s by Paper Number ________________________________________ 5 2016 International Workshop on EUV Lithography www.euvlitho.com 4 Welcome

The combination of a 10.6 μm main pulse CO2 laser and a 1064 nm pre-pulse Nd:YAG laser in EUV source concepts for HVM would require collector mirrors with an integrated spectral purity filter that suppresses both laser wavelengths. This paper discusses a new approach of a dual-wavelength spectral purity filter to suppress 10.6 μm and 1064 nm IR radiation at the same time. The dual-wavelength spectral purity filter combines two binary phase gratings that are optimized for 10.6 μm and 1064 nm, respectively. The dual phase grating structure has been realized on spherical sub-aperture EUV collector mirrors having an outer diameter of 150 mm. IR suppression factors of 260 at 10.6 μm and 620 at 1064 nm have been measured on the sub-aperture EUV collector while its EUV reflectance exceeded 64 % at 13.5 nm.

Laser-plasma gas-puff target sources suitable for EUV and SXR microscopy have been presented. Microscopy with Fresnel zone plates, due to their high dispersion, requires monochromatic radiation, which to a good approximation is the quasi-monochromatic emission from an argon based EUV source at 13.84 nm. This source was used for microscopy experiments and provided a spatial resolution of about 50 nm. Similarly a nitrogen based source can be used for SXR microscopy with the same type of optic. An argon based SXR source offers much higher photon flux through the broadband emission and, although not suitable for zone plate objectives, were employed in SXR microscopes with reflective, dispersion-free optics such as Wolter type I objectives.

Today’s EUV source concepts for EUV Lithography focus on laser-induced plasma generation using CO2 lasers in combination with Sn droplets. Different approaches of CO2 laser suppression have been discussed and realized in the past such as binary phase gratings and CO2 AR coatings. While CO2 AR coatings suffer from a significant EUV reflectance loss at 13.5 nm wavelength, binary phase gratings for 10.6 μm show great advantages in terms of EUV reflectance, IR suppression factors and mechanical stability. Binary grating structures for 10.6 μm are implemented in today’s LPP collector mirrors. They significantly suppress the CO2 laser wavelength of 10.6 μm and contribute to clean EUV photons in the intermediate focus.

Schwarzschild objectives are used in the EUV spectral range because of their large aperture, high mechanical stability and excellent achromaticity. The large aperture results in a small, theoretically diffraction limited focus diameter with ideal values of below 200 nm with the current configuration. We employed a zone plate with matched numerical aperture (0.19) to image the focus onto an X-ray CCD camera. Emission from high harmonic generation and a liquid-jet laser-plasma were used as light sources. Images at magnifications of about 150-fold were acquired and focus diameters of 300 nm (FWHM) were observed.

Laser produced plasma sources are considered attractive for high-volume extreme-ultraviolet (EUV) lithography because of their high power at the target wavelength 13.5 nm. However, besides the required EUV light, a large amount of infrared (IR) light from the CO2 drive laser is scattered and reflected from the plasma as well as from the EUV mirrors in the optical system. Since these mirrors typically consist of molybdenum and silicon, the reflectance at IR wavelengths is even higher than in the EUV, which leads to high energy loads in the optical system. One option to reduce this is to structure the EUV multilayer, in particular the collector mirror, with an IR grating that has a high IR-suppression in the zeroth order. In this paper, the characterization of such an optical element is reported, including the IR-diffraction efficiency, the EUV performance (reflectance and scattering), and the relevant surface roughness. The measurement results are directly linked to the individual manufacturing steps.

We studied 1-on-1 and 10-on-1 damage threshold investigations on Mo/Si multilayers with EUV radiation of 13.5 nm wavelength, using a table-top laser produced plasma source based on solid gold as target material. The experiments were performed on different types of Mo/Si mirrors, showing no significant difference in single pulse damage thresholds. However, the damage threshold for ten pulses is approximate to 60 % lower than the single pulse threshold, implying a defect dominated damage process. Using Nomarski (DIC) and atomic force microscopy (AFM) we analysed the damage morphologies, indicating a primarily thermally induced damage mechanism. Furthermore, we studied the radiation-induced change of reflectivity upon damage of a multilayer mirror. Additionally, we characterised transmission and reflection properties of novel Mo/Si multilayer beam splitters performing wavefront measurements with a Hartmann sensor at 13.5 nm wavelength. Such wavefront measurements allow also actinic investigations of thermal lens effects on EUV optics.

The usable power of high-power EUV light sources at 13.5 nm and also the lifetime of source and collector optics are currently considered to be the largest challenges encountered during the transition of EUV lithography from the current beta-tool status to high-volume manufacturing. Fraunhofer IOF Jena has developed cost-effective refurbishment technologies of multilayer-based near normal incidence collector mirrors for high-power laser-produced plasma sources. Presently, the collector mirror lifetime exceeds 80 billion laser pulses which correspond to a lifetime of several months during continuous use of the source. Together with their partners Cymer is currently carrying out a focused program to improve the collector lifetime. New multilayer coatings together with new in-situ cleaning strategies during source operation are key technology development strategies to get closer to the ultimate target of about one year collector lifetime. The paper discusses different LPP collector refurbishment strategies and presents the recent status on collector refurbishment techniques.

We present a Michelson interferometer for 13.5 nm soft x-ray radiation. It is characterized in a proof-of-principle experiment using synchrotron radiation, where the temporal coherence is measured to be 13 fs. The curvature of the thin-film beam splitter membrane is derived from the observed fringe pattern. The applicability of this Michelson interferometer at intense free-electron lasers is investigated, particularly with respect to radiation damage. This study highlights the potential role of such Michelson interferometers in solid density plasma investigations using, for instance, extreme soft x-ray free-electron lasers. A setup using the Michelson interferometer for pseudo-Nomarski-interferometry is proposed.

We present a laser plasma based x-ray microscope for the water window employing a high-average power laser system for plasma generation. At 90 W laser power a brightness of 7.4 x 10(11) photons/(s x sr x μm(2)) was measured for the nitrogen Lyα line emission at 2.478 nm. Using a multilayer condenser mirror with 0.3 % reflectivity 10(6) photons/(μm(2) x s) were obtained in the object plane. Microscopy performed at a laser power of 60 W resolves 40 nm lines with an exposure time of 60 s. The exposure time can be further reduced to 20 s by the use of new multilayer condenser optics and operating the laser at its full power of 130 W.

We present 1-on-1 and 10-on-1 damage threshold investigations on Mo/Si multilayers with EUV radiation of 13.5 nm wavelength, using a table-top laser produced plasma source based on solid gold as target material. The experiments were performed on different types of Mo/Si mirror, showing no significant difference in single pulse damage thresholds. However, the damage threshold for ten pulses is ≈60 % lower than the single pulse threshold, implying a defect dominated damage process. Using Nomarski (DIC) and atomic force microscopy (AFM) we analysed the damage morphologies, indicating a primarily thermally induced damage mechanism. Additionally, we studied the radiation-induced change of reflectivity upon damage of a multilayer mirror.

Scattering experiments on xenon nanoclusters with high-intensity soft x-ray laser pulses from the Free-Electron LASer in Hamburg (FLASH) are performed to investigate different cluster morphologies in the gas phase. Three different types of scattering patterns can be identified. The most frequent pattern of concentric rings reflects the event of a single spherical cluster in focus. Fine interference rings similar to Newton rings appear when two clusters are illuminated at μm distance, revealing three-dimensional information about the location of the clusters. Between 10 and 30% of all hits show a previously unknown twin cluster configuration with two clusters in direct contact. Simulations of scattering patterns for twin clusters with different sizes of the two particles, degree of fusion and orientation in space allow us to explain all the observed patterns.