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In this thesis, we present the development and study a numerical model of EUV-induced plasma. Understanding of behavior of low pressure low density plasmas is of industrial relevance, because of their potential use for on-line removal of different forms of contaminations from multilayer mirrors, which will help increase the throughput of EUV lithography. The model is 2D axially symmetric particle-in-cell code, hence it allows the full geometry of an axially symmetric chamber to be taken into account. Therefore, a quantitative connection between different experimental data, such as discharge characteristic measurements, and plasma parameters could be established. In order to ensure that the simulations could be relied on for quantitative comparisons, special attention was paid to validating the model. First, the mplementation of the model was tested using the accumulated large body of the swarm data to check the values of cross-sections (see chapter 2, section 2.8). In a second step (see chapter 5), direct simulation of the dynamics of a low-density plasma, ignited by an electron avalanche, in the presence of a diagnostic probe was used as both a validation step and as a first direct comparison between experiment and model. In this study, the plasma, the probe response to the plasma, and the probe’s influence on the plasma were all included in the model. Besides validation, the simulated plasma parameters were compared with that estimated from the probe IV curves using a variety of techniques. It was shown that different probe analysis techniques lead to significant under and over estimates of plasma densities. From these results, we suggested a useful criterion for estimating the error margin in experiments. The introduction of EUV radiation into the experimental chamber adds a new layer of unknown and/or poorly known parameters, such as the power spectral density and intensity. These new unknowns were treated, within limits, as free parameters that were varied to obtain close agreement between the numerical model and experimentally measured parameters. This approach was only possible because of the extensive testing and validation of the model, which allowed us to freeze the model state, and concentrate on the effects of added the parameters and new EUV plasma dynamics. The influence of different parameters on the ignition and dynamics of EUV induced plasmas was studied (see chapter 6). It was found that even low (i.e. 1%) transmission in the spectrum purity filters in vacuum-ultraviolet wavelength range can have a significant role for EUV-induced plasma formation. Finally, an experiment with carbon etching, due to EUV induced plasma was considered (see chapter 7). It was found that the predicted yield of carbon removal under EUV radiation in a hydrogen atmosphere corresponds well to etching experiments without EUV. This suggests that the underlying physical mechanism is the same in both cases, and that direct EUV processes play an insignificant role in carbon etching. The developed model is applicable to pulsed low pressure low density EUV generated plasmas. It has proven to be a convenient instrument to help experimentalists understand EUV plasma dynamics. Although the model can be extended in various ways, the most promising, from the perspective of understanding EUV plasma, is the extension of the model to a hybrid plasma model (see Chapter 8).

In this thesis we describe the development of a new class of optical components to enhance the imaging performance by enabling adaptations of the optics. When used at extreme ultraviolet (EUV) wavelengths, such ‘adaptive optics’ offers the potential to achieve the highest spatial resolution in imaging known to date, i.e., a resolution down to several nanometers. These optics are discussed, e.g., in the field of photolithography for the fabrication of more energy efficient and faster electronic equipment. The topics in this thesis can be subdivided into two parts. The first part includes a mirror design that can be used to enhance the spectral quality of EUV light sources. Our design consists of a diffractive Fresnel zone plate patterned in the surface of a curved mirror to achieve spectral purification, i.e., removing undesired wavelengths of the EUV light sources. The removed light is recycled to produce more EUV light. This design achieves four orders of magnitude purification and provides a 35% improvement in efficiency. The second part of the thesis describes a new class of adaptive multilayer mirrors that can be used to correct wavefront distortions, reflectance degradations or wavelength deviations. These adaptive optics contain integrated crystalline piezoelectric thin films of which the film thickness can be adjusted by an externally applied voltage. Because the optics substrates typically have an amorphous structure, so far it was not possible to grow high quality crystalline piezoelectric films on them. We utilized crystalline nanosheets on these substrates to grow the highest quality, i.e., epitaxial, piezoelectric films (PbZr0.52Ti0.48O3, PZT). The PZT films show a record high piezoelectric coefficient of 98 pm/V, although the piezoelectric response in this case is still reduced due to clamping by the substrate. We minimized the clamping effect by depositing films with a columnar structure, which enhances the voltage-controlled change in film thickness. In these films we measured a piezoelectric response of 280 pm/V, the highest value reported on glass substrates. Importantly, the developed films can produce a stroke of 25 nm. This order of magnitude is useful when considering to correct EUV wavefront distortions.

Nanoscale multilayered structures are employed as reflective coatings for short wavelengths in a wide range of analytical and imaging technologies. Thermal stability is vital for their functionality. This thesis addresses the thermally induced internal structural changes in Mo/Si-based and La/B-based multilayers, designed for the extreme ultraviolet wavelength range. The focus of the work is to study the diffusion phenomena and the interactions between nanoscale layers, with particular emphasis on the formation of compound interlayers during thermal loading.

The physics investigations described in this thesis have been inspired by the perspective of the application of La/B-based multilayer mirrors in photolithography. This perspective however, demands the highest possible reflectance of the mirrors and full control over the multilayer growth process. The corresponding research in this thesis therefore covers the following topics: a theoretical and experimental analysis of the spectral properties of La/B-based multilayer stacks, an experimental layer composition and growth study of the candidate La/B-based multilayer optics, and the development of techniques to enable characterization of the internal structure of such multilayers.

Being of a versatile and diverse nature, the field of particle-material interaction physics has traditionally been driven by applications. Two important current drivers are materials research for thermonuclear reactors and photolithography at Extreme UV wavelength. These topics have in common that they both represent extreme conditions: high photon, particle and plasma fluxes lead to non-linear processes and complex interaction phenomena. Both focus on unravelling the material damage mechanisms involving the lightest atom, hydrogen, as the active particle. The use of a nano-scale layered material, mimicking the lithography application, afforded an opportunity to perform depth and material sensitive studies. The challenge to identify the respective processes has been undertaken in the current thesis.

A major challenge in the soft x-ray (SXR) and eXtreme UltraViolet (XUV) spectral ranges is the ability to manipulate the incident radiation using optical elements. By patterning conventional multilayer mirrors with nanoscale structures, novel optical elements with a variety of optical properties can be obtained. In this work, the design, fabrication and characterization of Lamellar Multilayer Gratings (LMG) was investigated. Such LMGs are a particular form of the general class of Bragg-Fresnel optics which combine Fresnel optics with Bragg reflection to provide unique dispersive and focusing optics. In particular, LMGs can be used to improve the spectral resolution of x-ray fluorescence techniques. A Coupled Waves Approach (CWA) was derived to simulate the optical performance, in terms of resolution and reflectivity, of LMGs. This CWA allowed to study the physical principles behind LMGs and resulted in the identification of an optimal LMG operating regime. In this regime, the incident beam is reflected in a single diffraction order and is hence referred to as the singleorder regime. Such single-order LMGs were fabricated using UV-NanoImprint Lithography and Bosch Deep Reactive Ion Etching, a process chosen to enable rapid practical development. Single-order operation was experimentally demonstrated and showed an improvement in spectral resolution of a factor of 3.8 with regard to convenventional multilayer mirrors. Singe-order excitation of higher diffraction orders was also measured and analyzed. Single-order operation often requires multilayer stacks and grating structures that cannot be fabricated with sufficient accuracy using current technologies. As this invalidates the semi-infinite multilayer approximation, we investigated the optical performance of LMGs with finite multilayer stacks. We determined the ratio between the absorber thickness and bi-layer period of the multilayer stack can be used to further tailor LMG optical performance to maximize bandwidth reduction or minimize peak reflectivity loss. We also investigated various degradation processes that could limit the lifetime of LMGs, which is important for the applicability of such elements. Oxidation of tungsten and silicon as well as changes to the sidewall composition were clearly seen. However, SXR reflectivity remained stable to within measurement accuracy for an extended storage period of 18 months in a 1 atm air environment. The possibility of applying a capping layer on LMG structures to reduce degradation was also studied.

The research described in this thesis is on Mo/Si based multilayer mirror structures. Two important topics in this field are increase of reflection and improvement of thermal stability. The "standard" Mo/Si multilayer structure in theory can reflect about 75% of the incoming 13.5 nm radiation. However, interlayer formation during deposition and roughness of the interfaces reduce the reflection of deposited multilayer structures. To further improve the multilayer structures, a better understanding of interdiffusion effects at the interfaces is required.

This thesis describes the development of molybdenum/silicon based multilayer reflective elements for the Extreme UV wavelength range, as motivated by their application in photolithography for semiconductor manufacturing. The thesis reflects the basic thin film physics, technological developments, and valorisation activities of the last two decades of research, a period in which the author was involved in more than 20 research projects, carried out at FOM with numerous academic and industrial partners. This thesis contains three major aspects: basic thin film growth and analysis studies, the development of deposition processes and associated instrumentation, and the demonstration of the knowledge by producing prototype industrial optics coatings for lithography. The thin film studies described contribute to answers to basic questions such as: what are the leading film growth processes for layers with nanoscale thicknesses, which mechanisms determine layer smoothening and interlayer formation, what determines the amorphous, crystalline or chemical state, how can one control atomic diffusion and arrive at temporally and thermally more stable multilayer structures? Also described are answers to the more applied aspects, like: how can multilayer induced stress be controlled, and what optical response can result from non-periodic or laterally structured multilayer systems? Dedicated process and instrumentation development has made many of these detailed studies possible. The various generations of deposition facilities that we designed, developed, and continuously improved during the last two decades have been based on e-beam evaporation, magnetron sputtering, as well as plasma and ion-beam surface treatments. Two new research facilities are being designed and built: the Atomic Growth and Analysis facility (AG/A) for fundamental thin film research and a large and versatile deposition system for multilayer development. The third aspect of the thesis work, its valorisation, concerns the production of proof-of-principle prototype multilayer depositions on industrial lithography optics. This includes the successful coating of optical components for ASML and Zeiss’ first two generations of EUV lithography machines. Demonstrations are described on achieving the lateral control of the deposition process over large area surfaces to meet all aspects of the optics design specifications. In short, in the thesis the author attempts to summarize his knowledge on stateof- the-art multilayer EUV deposition know-how and technology, in order to support the EUV activities and to form a basis for further, industrially-inspired thin film physics and development programmes.

Nanoscale molybdenum/silicon (Mo/Si) multilayer structures are employed as reflective optical elements for wavelengths in the extreme ultraviolet (EUV, =13.5 nm). When using appropriate ion-beam parameters, Kr+ bombardment and sputtering of a-Si layers is known to suppress layer growth intrinsic roughness in Mo/Si multilayers. In chapter 2 the scalability of the smoothing process is investigated, aiming for optimization suppression of substrate roughness. In chapter 3 is reported on modifications of the in-layer structure of a-Si layers by implantation of H+ ions. The resulting increased layer porosity is found to enhance Kr+-induced smoothing, towards ultrasmooth surfaces. In chapter 4 the interface roughness in Mo/Si multilayers as a function of the noble gas ion polishing species (Ne, Ar, Kr, and Xe) is investigated by grazing incidence small angle x-ray scattering (GISAXS). Optimal interface smoothness is obtained by Xe+ polishing, however, Kr+ polishing yields highest EUV reflectance. This is clarified by the residual amounts of polishing gas in the Si layers, affecting the optical constants. Chapter 5 discusses the application of an EUV reflective multilayer on a non-flat, grating substrate, which is appealing as a high EUV throughput optical filtering device. The deposition of multilayers onto non-flat substrate topographies as a model system for real gratings, is described in chapter 6. With these findings, multilayer deposition on a grating is shown feasible. However, the tolerances on grating roughness and flatness of the facets are extremely tight. In chapter 7 a deposition scheme of a class of rectangular grating phaseshift reflectors on flat substrates is reported.

Extreme ultraviolet lithography (EUVL) is a next generation photolithographic technique that uses 13.5 nm or Extreme UV radiation and multilayer coated reflective optics. The reflectance of these optical elements can be strongly reduced when, as a consequence of exposing the optics to EUV photons, a contamination layer is built up on the mirrors. Since this will reduce the throughput of EUV lithography machines, contamination monitoring is considered to be necessary. Direct observation of the EUV reflectance of the mirrors is hardly possible since the required accuracy can only be achieved in very sophisticated lab reflectometers. This thesis describes experimental research on the topic of EUV induced contamination and its monitoring using alternative, in situ techniques. Occasional techniques for such a task have been mentioned, but no real investigations were carried out. This thesis reviews the suggested and new techniques and describes experimental work on the three most promising: laser-generated surface acoustic waves (LG-SAWs), secondary electron yield (SEY), and spectroscopic ellipsometry (SE). The goal was not only to develop an appropriate monitoring method, but also to get insight in the material properties, both mechanical and optical, of the contamination layer in order to predict the EUV reflectance loss and the possibilities to clean the optics.

Multilayered thin film structures are widely applied as reflective coatings for optical elements in the extreme ultraviolet wavelength regime. In this thesis we investigate the structural and chemical changes that occur in Mo/Si based multilayers as a result of radiation induced thermal loads and other heating schemes. This thesis addresses thermally induced diffusion in such multilayers, focussing on reaction mechanisms at the interfaces and how these are modified in the presence of a diffusion barrier layer. To allow these studies, a new, in-situ X-ray diffraction method is introduced to analyse diffusion induced interface growth and measure diffusion speeds in Mo/Si multilayers during thermal annealing. This method can determine the change in the interface thickness at picometer accuracy. Because of this high accuracy it is possible to study diffusion at relatively low temperatures. A diffusion-reaction model was developed to describe the interface growth. The diffusion in multilayers is shown to be dependent on the structure of all the layers as well as on the chemical interactions with barrier layer materials. In particular, the crystallinity of the Mo layer (crystalline or quasi-amorphous) has a large influence on the diffusion speed. The structure and density of the B4C diffusion barrier layers have a large influence on the diffusion coefficient. Furthermore it is shown that B4C also forms molybdenum boride compounds during annealing, which reduce the diffusion rate. In conclusion, the diffusion properties of thin film multilayer structures are determined by both the structure and the chemical interactions of the individual (barrier-)layers. The damage mechanisms of these layered structures by intense femtosecond pulses are also investigated, to find a possible difference in damage mechanism between continuous heat loads and ultrafast pulsed heat loads. This investigation was performed on MoN/SiN multilayers. We find that the damage mechanisms for annealing and pulsed irradiation are fundamentally the same. In both cases, the MoN layer dissociates and N2 gas is released, which subsequently forms bubbles in the MoN layer which may lead to delamination.

This thesis addresses the physical and chemical phenomena in Mo/Si multilayer structures with and without B4C diffusion barrier layers at the interfaces, which are applied as extreme ultraviolet (EUV) / soft X-ray optics. Since interdiffusion and interlayer formation limit both the performance and the lifetime of the optics, the interlayers and their evolution in time have been investigated in detail. To this end, a procedure was developed to utilize the depth-resolved information in a LEIS spectrum in order to non-destructively study interdiffusion in ultrathin films with sub-nanometer resolution. Using this method, it was found that the diffusion in Mo/B4C/Si layered structures initially obeys Fick’s second law. However, after a certain time, the diffusion rate instantaneously increased by up to one order of magnitude. The cause of this acceleration was found to be the amorphous-to-nanocrystalline transition of the MoSi2 interface. Besides diffusion and crystallization, the chemical processes upon annealing of Mo/B4C/Si layered structures have been identified. Mo/B/Si and Mo/C/Si samples were studied as reference systems. Initially, predominantly MoBxCy (resp. MoBx, MoCx) formed, plus small amounts of SiBxCy (resp. SiBx, SiCx). Subsequently, MoSi2 formed, while the already formed MoBxCy (resp. MoBx, MoCx) diffused further into the Mo layer. Stable silicides, through which Si diffused to form MoSi2 in the second stage, only formed in the samples with C and B4C interlayers. Furthermore, it was investigated whether the interlayer thickness can be reduced by depositing the multilayer mirror at a low temperature. Even after warming up to room temperature, the interlayers that formed upon cryogenic deposition were found to be nearly 60 % thinner than after room temperature deposition. Finally, since rough interfaces between the layers of a multilayer mirror are equivalent to thick interlayers, it is important to control the roughness during the deposition. The development of roughness is usually mitigated through ion bombardment of the Si layers. Therefore, in order to gain understanding, the roughness evolution of Si surfaces upon Ar ion erosion was studied in real-time. It was demonstrated that ion treatment can cause roughening as well as smoothening, depending on the initial roughness of the substrate.

In the broad scientific field of thin films, applications have rapidly expanded since the second half of the 20th century, as has been described in the valorization chapter. This thesis describes research at the interface between surface chemistry and physics. The characterization of d-metal surface nanolayer caps in chapter 3 has revealed that they can actually promote oxidation of the Mo layer on which they are deposited. Vise versa, Ni and Co caps themselves oxidize, but in monolayer quantities already prevent Mo oxidation. We attribute these observations to sacrificial e- donation, and in the case of Au and Cu, also to roughening of the surface. Chapter 4 shows that all cap oxide and part of the MoO3 can be reduced by atomic H exposure, which also removes part of the carbonaceous contaminants that absorb on the surface both in ambient air and during use in lithography tools. The carbonaceous contaminants are hydrogenated to volatile hydrocarbons that desorp from the surface. Atomic O exposure removes remaining C and leaves a clean metal oxide surface without erosion via volatile metal oxides. With subsequent atomic H treatment, this offers an attractive technique for non-destructive surface cleaning of the optical elements in lithography. The in-depth chemical and vacancy mediated diffusion have further been investigated in chapter 5 and 6 with XPS, AES, CS-EELS, HAADF-STEM and EDX. Several capping layers and boron are observed to diffuse into and through the underlying Mo, B4C and Si layers without significant chemical interaction, towards the subsurface Si-on-Mo interface, where d-metal silicide formation and agglomeration occurs. Rather than in the mid-Si layer, kinetically favorable silicides and borides are formed at the Si-on-Mo interface front, notably RuSix and MoBx. The MoBx appeared to be an attractive barrier material and has been patented. Reversed “substrate-on-adlayer” interfaces can yield entirely suppressed reactivity and diffusion, stressing the influence of surface free energy and the supply of atoms to the interface via segregation on thin layer growth. Chemical diffusion and LaB6 and LaC2 interlayer formation in La/B4C multilayers for reflection of wavelengths just above 6.65 nm have been studied in chapter 7 and 8. The research in chapter 8 reveals surfactant mediated growth and chemical stabilization of the La/B4C interface by nitridation. We have shown in chapter 8 that B4C is more readily nitridated by N2 + bombardment during or after growth than La, which apparently yields an equilibrium that involves dinitrogen complexes. The loosely bound N or N2 in the La/B4C interface substrate partially diffuses into the adlayer, resulting in surfactant mediated growth. Subsequent nitridation of the adlayer is observed, yielding nitridated interfaces that are chemically inactive to LaB6 and LaC2 interlayer formation. These processes occur without significantly affecting interface diffuseness when N2 + bombardment is applied after individual layer growth, yielding local BN and more diffuse LaN formation. Complete interdiffusion is observed when N2 + bombardment is applied during B4C growth. Nitridation of the interfaces has greatly improved the reflective properties of La/B4C multilayers. This makes application in reflective optics highly advantageous for short-wavelength light sources, e.g. the FLASH free electron laser, next generation photolithography beyond EUVL, soft x-ray spectroscopy, Rontgen fluorescence analysis and imaging.

This thesis describes the design, production, and characterization of multilayer mirrors intended as wavelength dispersive elements for the soft x-ray range. The central theme is the analysis and control of the different processes occurring at the materials interfaces between the layers.

The thesis addresses several methods to characterize as well as enhance the Extreme UV emission of laser produced plasmas generated from gaseous targets. Xenon was selected as the target material due to the presence of a spectral peak near 13.5 nm in Xe plasma radiation and the absence of debris particles and contamination by the plasma EUV source. In principle some of the methods developed are also valid for plasmas generated from other gaseous systems including discharge produced plasmas. Generally, the EUV yield of plasmas generated from gaseous targets dramatically depends on the target density profile, i.e. the geometry of the target system including the expansion of target material. Several methods have been applied to increase the EUV yield. These included the increase of the target gas density, the use of (super)sonic nozzles with collimated jets, the application of a concentric flow of transparent buffer gas around the target, and the generation of shock-waves in the plasma. Increased target densities did increase the EUV production but also led to stronger EUV absorption in the outer parts of the gaseous target. Applying an EUV transparent buffer gas from an annular, concentric nozzle did reduce that EUV absorption significantly. Thirdly, shockwaves were shown to lead to a local increase of the gas density resulting in a considerable (up to a factor of five) increase of the EUV yield. The different mechanisms causing these effects, are discussed. Finally, the relation between EUV emission and the duration of the laser pulse was investigated, indicating that critical processes to create the relevant ionization stages take place on a time scale of the order of relatively short laser pulses (few ns). A number of XUV diagnostics were constructed and applied to characterize the plasma and it’s emission properties. X-ray backlighting was used to record X-ray shadowgraphs of the gas jet. By using Abel inversion, the density profiles were reconstructed. Raleigh scattering was used to determine the change in average particle size, which was used to determine the degree of condensation. Characterization of the XUV emission properties of the plasmas was performed using an XUV spectrograph based on a transmission grating, a narrow-band XUV diagnostic based on the use of a multilayer mirror, and an XUV pinhole camera. In addition to the selection of different target geometries, several debris mitigation techniques have been applied in order to mitigate any residual contamination from the plasmas. The foil trap technique, which is based on adsorption of thermalized atoms from the source at the surface of radially positioned foils, was characterized, using a fast rotating target as a source of debris of different composition. The diagnostics included collection of debris on witness plates studied by optical microscopy and SEM, oscillating quartz crystals to measure the deposited mass, and monitoring of the reflectivity of exposed multilayer mirrors. The mechanisms active in the mitigation of the debris were analyzed, and relations were found between debris composition and suppression techniques.