This PDF file contains the front matter associated with SPIE Proceedings Volume 6586, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The proposed 4GLS is a suite of temporally synchronised accelerator based light sources, including a low-Q cavity free electron laser operating in the energy range 3 - 10 eV (the VUV-FEL), a seeded XUV-FEL designed to operate in the range 8 - 100 eV, an infra-red cavity FEL (the IR-FEL), as well as spontaneous radiation sources. The output from these sources is summarized. With respect to radiation damage, the two main areas of concern are the high average power from the VUV-FEL and the high peak power from the XUV-FEL. The suitability of different materials for the cavity mirror for the VUV-FEL and beamline optics is investigated. Beamline design strategies to militate against radiation damage are described and an example of a high throughput beamline in the XUV-FEL is given. The need for accurate optical constants in the VUV region, as well as damage threshold measurements, is highlighted and we outline a program of experimental measurements to be undertaken in the near future.

Free electron lasers (FEL) for the hard- and soft X-ray regime are promoted in many projects around the world and the first VUV-FEL with wavelengths down to 20 nm is already in operation at DESY in Hamburg. They open up new domains in coherent radiation, time resolution and intensity and extend current X-ray science. The BESSY project of a 2^nd generation FEL facility proposes the implementation of an externally seeded FEL with significantly improved beam quality compared to the conventional self seeded SASE-FELs. The BESSY design as described in the technical design report¹ provides full tunability of photon energies from 24 eV to 1 keV, complete polarisation control and reproducible pulse structures with pulse lengths in the femtosecond range. The external seeding ensures the synchronisation to other laser sources necessary for pump-probe experiments and provides shot-to-shot reproducible pulse properties.

X-ray free-electron laser (XFEL) is being constructed at the SPring-8 site by RIKEN and JASRI. The basic configuration of the XFEL beamline is similar to the design of a standard x-ray undulator beamline. Qualities of the optical components such as a diamond crystals, a beryllium window, and a mirror have been improved for XFEL applications. From the doses evaluation of the optical components and the availability, the guideline for the detailed design is presented.

Selected aspects of simulation, preparation and characterization of total reflection and multilayer X-ray optics will be discussed. The best multilayer is found by calculating the optical properties of the coating. Sophisticated improvements in deposition technology allow the precise realization of the specified parameters when manufacturing the X-ray optics. The quality of the shape of the substrate for the optics is measured with the aid of profilometry. X-ray reflectometry measures both film thickness as well as their lateral gradient. Last but not least we will be showing results of the development of carbon coatings as total reflection mirrors for FEL (free electron laser) sources. Over the past years we have developed optimized optics for the XUV range up to 200 eV. First FEL irradiation tests have shown that carbon coatings offer high reflectivity > 95%, high radiation stability, good uniformity in thickness and roughness. An optimized coating of two stripes for different beam energies was produced especially for a tomography beamline, where a Ru/C multilayer was chosen for energies between 10 and 22 keV and a W/Si multilayer for energies between 22 and 45 keV.

We have investigated ablation of inorganic materials using pulsed and focused soft X-rays. Soft X-rays in a range of 1-500 eV were produced by irradiation of Ta targets with Q-switched Nd:YAG laser light (532 nm, 0.5-0.8 J/pulse) with a pulse duration of 10 ns in a vacuum chamber. The soft X-rays were focused on the surfaces of inorganic materials using an ellipsoidal mirror at approximately 0.1 J/cm². The ellipsoidal mirror is designed so as to focus soft X-rays at about 100 eV efficiently, while it can not reflect soft X-rays above 200 eV. We found that synthetic quartz glass, Pyrex, LiF, CaF₂, Al₂O₃ and LiNbO₃, can be ablated by focused and pulsed soft X-rays. Typically, synthetic quartz glass is ablated at 50 nm/shot. We found that ablation occurs at X-ray fluences beyond the ablation threshold. Using a nano-scaled contact mask, trenches with a width of 50 nm and an aspect ratio of 1 are formed. The result indicates the diffusion length of absorbed energy during irradiation is less than 50 nm and that the accumulation results in ablation. The technique can also be applied to basic research of the interaction of intense soft X-rays with materials and resulting damage to the materials.

A single photon of EUV radiation carries enough energy to break any chemical bond or excite electrons from inner atomic shells. It means that the radiation regardless of its intensity can modify chemical structure of molecules. It is the reason that the radiation even with low intensity can cause fragmentation of long chains of organic materials and desorption of small parts from their surface. In this work interaction of EUV radiation with inorganic materials was investigated. Different inorganic samples were irradiated with a 10 Hz laser - plasma EUV source based on a gas puff target. The radiation was focused on a sample surface using a lobster eye collector. Radiation fluence at the surface reached 30 mJ/cm² within a wavelength range 7 - 20 nm. In most cases there was no surface damage even after several minutes of irradiation. In some cases there could be noticed discolouration of an irradiated surface or evidences of thermal effects. In most cases however luminescent and scattered radiation was observed. The luminescent radiation was emitted in different wavelength ranges. It was recorded in a visible range of radiation and also in a wide wavelength range including UV, VUV and EUV. The radiation was especially intense in a case of non-metallic chemical compounds.

In recent years technological developments in the area of extreme ultraviolet lithography (EUVL) have experienced great improvements. So far, intense light sources based on discharge or laser plasmas, light guiding and imaging optics, as well as detection devices are already available. Currently, the application of EUV radiation apart from microlithography, such as metrology, high-resolution microscopy, or surface analysis comes more and more into focus. The aim is to make use of the strong interaction between soft x-ray radiation and matter for surface-near probing, modification or structuring techniques. In this contribution, we demonstrate the surface-near direct structuring of different polymeric materials as well as lithium fluoride crystals using EUV radiation with a wavelength of 13.5 nm. The setup consists of a table-top EUV source based on a laser-induced plasma and a modified Schwarzschild objective with a resolution down to 130 nm. The mirrors of the employed objective were coated with Mo/Si multilayers, providing a transmittance of around 42 % (reflectivity ~65 % @ 13.5 nm per mirror). With a demagnification factor of 10 small foci are generated, leading to spot diameters of 30 &mgr;m in plasma imaging mode and down to 1 &mgr;m in mask imaging mode, respectively. The EUV energy density of ~100 mJ/cm² obtained in the focus is sufficient to observe direct photo-etching of polymers, e.g. PMMA. Thus, material interaction studies are currently in progress. The investigations revealed already that in contrast to common excimer laser ablation there are no incubation pulses when using EUV radiation. For lower energies the ablation rate is found to be linear with respect to the applied dose, whereas for higher energies a saturation behavior is observed. By EUV irradiation of LiF samples surface-near defects within the crystal lattice are formed. These color-centers (mainly F₂- and F₃ ⁺-color centers) are known to be stable at room temperature. They are able to emit characteristic radiation in the visible range after optical excitation with a wavelength around 450 nm. In the future structured areas of such color centers could be used as laser-active gain medium in distributed feedback lasers. For measuring the radiation resistence of Mo/Si mirrors, the setup was used as a top-illuminated microscope at 13.5 nm. By placing the analyzing Mo/Si mirror into the image plane of the objective, the change in reflectivity due to irradiation at a fluence of 20mJ/cm² could be observed.

Effects of carbon 1s core excitations on the structure of tetrahedral amorphous carbon films were investigated by illuminating samples with intense soft X-rays from a synchrotron radiation source in the photon energy range of 240 eV to 310 eV. The structural changes detected in the X-ray absorption spectra obtained before and after the intense illumination are characterized by graphitic ordering, similar to the effect of high-energy electron irradiation. The excitation spectrum for the photo-stimulated structural changes was found to consist of a non-resonant component and a resonant component peaking at 289 eV close to the 288 eV XAS peak characteristic of ta-C films. The microscopic mechanisms are discussed for the non-resonant and the resonant effects.

High-surface-quality amorphous carbon (a-C) optical coatings with a thickness of 45 nm, deposited by magnetron sputtering on a silicon substrate were irradiated by the focused beam of capillary-discharge Ne-like Ar XUV laser (CDL). Laser wavelength and pulse duration were of 46.9 nm and 1.7 ns, respectively. The laser beam was focused onto the sample surface by a spherical Sc/Si multilayer mirror with a total reflectivity of about 30%. Laser pulse energy was on the sample surface varied from 0.4 &mgr;J to 40 &mgr;J. The irradiation was carried out at five fluence levels between 0.1 J/cm² and 10 J/cm², accumulating five different series of shots, i.e., 1, 5, 10, 20, and 40. The damage to a-C thin layer was investigated by atomic force microscopy (AFM) and Nomarski differential interference contrast (DIC) optical microscopy. Obtaining the dependence of single-shot-damaged area on pulse energy makes it possible to determine a beam spot diameter in the focus. Its value was found to be equal to (23.3±3.0) &mgr;m using AFM data and considering the beam to have a gaussian profile. Calculations based on a more realistic assumption about the beam profile are in progress. Such a plot can also be used for a determination of single-shot damage threshold in a-C. Single-shot threshold value of 1.1 J/cm² was found by plotting the damaged areas determined by means of AFM. Investigating consequences of the multiple-shot exposure it has been found that an accumulation of 10, 20 and 40 shots at a fluence of 0.5 J/cm², i.e., below the single-shot damage threshold, causes irreversible changes of a-C thin layer which can be registered by both the AFM and the DIC microscopy. In the center of the damaged area, AFM shows a-C removal to a maximum depth of 0.3, 1.2 and 1.5 nm for 10-, 20- and 40-shot exposure, respectively. Raman micro-probe does not indicate any change in the structure of the remaining a-C material. The erosive behavior, reported here, contrasts with the material expansion observed on the a-C sample irradiated by a large number of femtosecond pulses of XUV high-order harmonics (HHs).

Gamma radiation induced absorption processes of various scintillating materials were studied at room temperature. Single crystal of PbWO₄, Ce-doped YAlO₃ and Ce(Pr)-doped Y(Lu)₃Al₅O₁₂ were irradiated by ⁶⁰Co and doses ranging between 1-500 Gy. Broad induced absorption spectra obtained were decomposed into separate Gaussian components and tentatively ascribed to specific color centres. Supporting thermoluminescence and electron paramagnetic resonance experiments were performed to reveal the nature of charge carrier traps. The influence of codoping by aliovalent ions is also shown and discussed.

Light sources capable to deliver intense and ultrashort pulses in the VUV domain, based on free electron lasers or on the high order harmonic generation have appeared recently. They bring the possibility to explore a new domain in the field of laser matter interaction. Such sources are available in the visible or near IR range -specially at 800 nm, thanks to Ti-Sa lasers - since more than ten years, and the interaction of femtosecond pulses with solids has been studied in great details. In this paper we will discuss how the knowledge which has been acquired in the visible domain can be used for the VUV studies. I will concentrate on the case of wide band gap dielectric materials (SiO₂, MgO, Al₂O₃), and on the intensity domain around breakdown and ablation threshold. This type of material is interesting not only because they are involved in numerous applications, but above all because their band gap (Eg) lying in the range 6 to 10 eV, a clear distinction can be made for what concern their interaction with visible (hνEg). We discuss here two important aspects that must taken into account to understand the energy balance of the interaction. The first is the energy distribution of photoexcited carriers, which are clearly different in the case of visible or VUV light. Photoemission spectroscopy demonstrate that the distribution highly depends upon the incident intensity in the visible and near IR, and can be "warmer" than the one observed by irradiation with VUV photon, despite their much larger energies. The second important parameter is the excitation density achieved during the excitation. Experiments carried out in the IR using the technique of time resolved interferometry allow to measure the density of electrons excited in the conduction band at intensities above and below the optical breakdown threshold. The results show that in the process of laser breakdown multiphoton excitation dominates the avalanche process for picosecond and subpicosecond pulses. The simulations performed to interpret these measurements can be used to predict the damaging mechanism of wide band gap dielectrics submitted to ultra intense VUV pulses.

Studies on surface texturing by chemically enhanced laser ablation in a variety of materials, particularly silicon and germanium are reported. The materials are exposed either to femtosecond or nanosecond laser irradiation in a variety of vacuum or gaseous environments including air, He, sulfur hexafluoride (SF₆) or hydrogen chloride (HCl). The dynamics of pillar formation are elucidated and it is shown that the mechanisms are very different in these two pulse length regimes. Surface texturing responds to the combined effects of laser assisted chemical etching and laser ablation. Various processing steps either before or after laser irradiation allow us to modify the nature of the pillars that are formed. In this way we can make ordered arrays that extend over ≥1 cm² in just a few minutes of laser exposure. Post-laser processing wet etching can produce Si pillars that are over 50 &mgr;m long with tips that are only 10 nm across as well as macroporous silicon with crystallographically defined pores. A process we call solidification driven extrusion creates nanoscale spikes atop the pillars under certain circumstances - a process that is more prevalent for Ge than Si. Pillar-covered surfaces of Si and Ge are black; that is, they exhibit very low reflectivity. For Si this low reflectivity extends to wavelengths far below the band gap raising the possibility that we may be able to make other transparent materials highly absorptive by laser texturing.

The new XUV sources, which deliver spatially coherent pulses of high peak power, allow to study elementary processes in the light/solid interaction in the high intensity regime (⩾10¹¹W/cm²). Here, we report two studies which have used high-order laser harmonics (HH) generated in gas as the excitation source. Firstly, we have investigated the dynamics of electron relaxation in the wide gap CdWO₄ dielectric crystal, an efficient scintillator material, using time-resolved luminescence spectroscopy. The kinetics decay of luminescence shows evidence of non radiative relaxation of the self-trapped excitons at the &mgr;s damage to surfaces of poly(methyl methacrylate) - PMMA, induced by a multi-shot XUV-irradiation (1 kHz reprate) for given fluence, below damage threshold range of ≈mJ/cm². The main processes participating in the surface modification, polymer chain scission followed by the blow up of the volatile, molecular fragments and cross-linking in the near-surface layer of remaining material, are tentatively identified and associated to, crater formation for short-time exposure (< 1min) and surface hardening for long-time exposure (⩾1min).

Multilayers are artificially layered structures that can be used to create optics and optical elements for a broad range of x-ray wavelengths, or can be optimized for other applications. The development of next generation x-ray sources (high brightness synchrotrons and x-ray free electron lasers) requires advances in x-ray optics. Newly developed multilayer-based mirrors and optical elements enabled efficient band-pass filtering, focusing and time resolved measurements in recent FLASH (Free Electron LASer in Hamburg) experiments. These experiments are providing invaluable feedback on the response of the multilayer structures to high intensity, short pulsed x-ray sources. This information is crucial to design optics for future x-ray free electron lasers and to benchmark computer codes that simulate damage processes.

A Particle-in-Cell Monte Carlo model is used to simulate extreme ultraviolet driven plasma. In an extreme ultraviolet lithography tool, photons of a pulsed discharge source will ionize a low pressure argon gas by photoionization. Together with the photoelectric effect, this results in a strongly time dependent and low density plasma, which is potentially dangerous to the optical elements, the collector in particular. Plasma sheaths will develop and ions are accelerated towards the collector, which might lead to sputtering. A spherical geometry is used to study the plasma between the point source and collector. Simulations are performed to study the in.uence of background pressure and source intensity on the damage to the collector by sputtering.

Optical fibers for propagating deep ultraviolet light were developed using fluorine doped silica glasses called modified fused silica. Optimizing the fiber drawing conditions improved the transmission of the fiber in the deep UV region. The transmittance of the fiber at 193 nm reached more than 65% per 1 m long without reflection loss. Significant absorption bands from defects were not observed throughout the wavelengths of the deep UV-visible-infrared region. Hydrogen-impregnation into the fibers suppressed the degradation of the transmission induced by irradiating with an ArF excimer laser and 4th harmonic generation of Nd:YAG laser. Transmission in the DUV region and resistance to laser irradiation were drastically improved compared to high OH silica fibers.

The detection of damages at optics for the extreme ultraviolett (EUV) requires precise tools for at-wavelength-metrology. Excellent stability of the probe radiation is a precondition for precise measurements. As an EUV-source we use an electron-based microfocus EUV-tube. This EUV-source is debris-free, and it provides a output of up to 300&mgr;W at 13.5 nm. The metrology setup benefits from the very good long-time stability and spatial stability of this tube. Optical samples were characterized in reflectivity and transmission. Optical defects of EUV-optics were analyzed at-wavelength. The incidence angle of the EUV-radiation was varied from grazing incidence to nearly normal incidence. Our reflectivity measurements were compared with a calibrated synchrotron measurement at the German national metrology institute (PTB). The absolute accuracy of the reflectivity measurement was found to be better than 3% for any incidence angle. The reproducibility of the measurement was found to be better than 0.5%. Investigations are performed to further improve the reproducibility and absolute accuracy. The metrology setup is flexible, thus it can be fit to different types of measurement for different applications. The concept of the metrology setup is discussed and recent results are presented. The devices can be purchased from the Laser Zentrum Hannover e.V.

X-ray free-electron lasers generate ultrashort and very intense x-ray radiation in the wavelength domain reaching from the VUV (100 nm and shorter) all the way to the hard x-ray domain (typically 0.1 nm). FEL radiation features extreme brilliance, ultrashort pulse duration, and high peak power. Superconducting accelerators provide furthermore the possibility to accelerate a large number of electron bunches during a single radio-frequency pulse. Likewise the total number of x-ray pulses available for the experiments is increased leading to a significantly higher average brilliance. FEL light sources, and those based on super-conducting accelerator technology, are therefore considered to provide a new quality of short wavelength radiation if compared to existing x-ray sources. The high intensity and the high repetition rate lead to new requirements for x-ray optics in terms of peak and average power. Values for peak and average power are presented in relation to the proposed realization of the photon beamlines at the European XFEL facility.

An advanced time integrated method has been developed for soft X-ray pulsed laser beam characterization. A technique based on poly (methyl methacrylate) - PMMA laser induced ablation has been used for beam investigations of soft X-ray laser sources like FLASH (Free-electron LASer in Hamburg; formerly known as VUV FEL and/or TTF2 FEL) and plasma-based Ne-like Zn laser performed at PALS (Prague Asterix Laser System). For the interaction experiments reported here, the FLASH system provided ultra-short pulses (~10-fs) of 21.7-nm radiation. The PMMA ablation was also induced by plasma-based Ne-like Zn soft X-ray laser pumped by NIR beams at the PALS facility. This quasi-steady-state (QSS) soft X-ray laser provides 100-ps pulses of 21.2-nm radiation, i.e. at a wavelength very close to that of FLASH but with about 5,000 times longer pulses. In both cases, the PMMA samples were irradiated by a single shot with a focused beam under normal incidence conditions. Characteristics of ablated craters obtained with AFM (Atomic Force Microscope) and Nomarski microscopes were utilized for profile reconstruction and diameter determination of the focused laser beams ablating the PMMA surface.

Short-pulse ultraviolet and x-ray free electron lasers of unprecedented peak brightness are in the process of revolutionizing physics, chemistry, and biology. Optical components for these new light sources have to be able to withstand exposure to the extremely high-fluence photon pulses. Whereas most optics have been designed to stay intact for many pulses, it has also been suggested that single-pulse optics that function during the pulse but disintegrate on a longer timescale, may be useful at higher fluences than multiple-pulse optics. In this paper we will review damage-resistant single-pulse optics that recently have been demonstrated at the FLASH soft-x-ray laser facility at DESY, including mirrors, apertures, and nanolenses. It was found that these objects stay intact for the duration of the 25-fs FLASH pulse, even when exposed to fluences that exceed the melt damage threshold by fifty times or more. We present a computational model for the FLASH laser-material interaction to analyze the extent to which the optics still function during the pulse. Comparison to experimental results obtained at FLASH shows good quantitative agreement.

The performance of multilayer optics depends on the quality of interfaces between spacer and absorber materials. Intermixing at the interfaces affects the optical behavior. An experimental method is presented here to obtain the amount of intermixing at the buried interfaces of multilayer structures. The method is based on the combined use of photoemission and a rocking scan through the Bragg peak. The possibility of obtaining quantitative information - through a phenomenological model - on the width of the intermixing region is presented and discussed.

Exposure of collector mirrors facing the hot, dense pinch plasma in plasma-based EUV light sources to debris (fast ions, neutrals, off-band radiation, droplets) remains one of the highest critical issues of source component lifetime and commercial feasibility of nanolithography at 13.5-nm. Typical radiators used at 13.5-nm include Xe and Sn. Fast particles emerging from the pinch region of the lamp are known to induce serious damage to nearby collector mirrors. Candidate collector configurations include either multi-layer mirrors (MLM) or single-layer mirrors (SLM) used at grazing incidence. Studies at Argonne have focused on understanding the underlying mechanisms that hinder collector mirror performance at 13.5-nm under fast Sn or Xe exposure. This is possible by a new state-of-the-art in-situ EUV reflectometry system that measures real time relative EUV reflectivity (15-degree incidence and 13.5-nm) variation during fast particle exposure. Intense EUV light and off-band radiation is also known to contribute to mirror damage. For example offband radiation can couple to the mirror and induce heating affecting the mirror's surface properties. In addition, intense EUV light can partially photo-ionize background gas (e.g., Ar or He) used for mitigation in the source device. This can lead to local weakly ionized plasma creating a sheath and accelerating charged gas particles to the mirror surface and inducing sputtering. In this paper we study several aspects of debris and radiation-induced damage to candidate EUVL source collector optics materials. The first study concerns the use of IMD simulations to study the effect of surface roughness on EUV reflectivity. The second studies the effect of fast particles on MLM reflectivity at 13.5-nm. And lastly the third studies the effect of multiple energetic sources with thermal Sn on 13.5-nm reflectivity. These studies focus on conditions that simulate the EUVL source environment in a controlled way.

The development of new high power EUV sources and EUV space imaging requires optics having specific properties which depend on applications and operating conditions. These both applications are very different in the working multilayers environment. For the high power sources, multilayers are submitted to short pulses with high energy peak whereas, for the space imaging, multilayers are submitted to continuous flux with low level. Moreover photon energy and environment for both applications may be different. The environment may affect structure and top layer contamination when optics are stored, handled, mounted on the final device and finally operating. Main environmental parameters investigated are temperature and humidity variation. One objective is the optimisation of multilayer coatings to offer the highest resistance under photonic, ionic fluxes and temperature cycle. This means that interfacial diffusion between thin layers and degradation of the capping layers have to be avoided or reduced. The present study relies with designing, depositing and testing different structures of multilayer coatings in order to minimise the influence of the environment. Multilayer coatings based on molybdenum, silicon and silicon carbide materials have been deposited by magnetron sputtering on silicon and zerodur substrates. Samples were submitted to radiations emitted by an EUV source at wavelength closed to 13.5 nm. Furthermore they were also submitted to thermal cycles and annealing under warm humidity in the aim to simulate extremes storage or handling conditions as space mission's conditions. The damages and the performance of the multilayers were evaluated by using grazing incidence reflectometry at 0.154 nm and EUV reflectometry at the operating wavelength. After a presentation of the multilayer design, deposition and metrology tools, we will describe the different environmental effects on the coatings to take in care during EUV source exposure, handling and storage conditions. First results on multilayers performances to EUV source exposure and space specification tests are presented. Main damages studies were on annealing, thermal cycling and warm humidity.