The present candidates for low loss dielectric optical coatings at VUV excimer laser wavelengths are fluorides. Within this group, only one material - namely lanthanum fluoride - is used almost exclusively as high index film material. In search of additional high index film materials for use in VUV we investigated a broader spectrum of lanthanide tri-fluorides since little is known about their properties and the advantages or disadvantages with regards of their use in DUV- and VUV - optical stacks. Fluorides of lanthanum, neodymium, samarium, gadolinium, ytterbium and also yttrium were evaporated thermally. Precision VUV-measurement were initiated to give an overview of the ranges of UV-transparency up to the absorption edges and to determine the optical indices of these coating materials.
Supplementary, also stress measurements, atomic force microscopy and XRD measurements were performed to scrutinize the properties of the films.
Since excimer laser applications extend to deep and vacuum UV wavelengths at 248 nm, 193 nm and 157 nm, renewed research interest has recently arisen on fluoride thin films due to their unrivaled position as wide-band-gap material for the vacuum UV (VUV). Among these materials, only a very limited number can act as the high refractive index component in multiplayer interference stacks. Besides LaF3, gadolinium tri-fluoride is a potential candidate especially for wavelengths at about and below 200nm. We report on the evaluation of the structural properties, the optical properties with emphasis to the DUV - spectral range, and the mechanical properties of GdF3 single layer by means of XRD, GIXR, AFM measurements, spectral photometry and by ex - situ mechanical stress analysis using the laser beam deflection method to measure the substrate deformation. The samples were deposited onto fused silica and silicon substrates by a low-loss evaporation technology in a BAK 640 coating plant applying various deposition conditions.
Evaporated CeO2 films are known to show a pronounced optical inhomogeneity in dependence on the chosen deposition parameters, especially in dependence on the substrate temperature during deposition. Samples were manufactured by reactive electron beam evaporation and by ion assisted deposition onto substrates held at different temperatures. Optical analysis of spectral transmittance and reflectance data supplemented by x-ray diffraction and infrared measurements were carried out on the basis of an inhomogeneous refractive index layer model. Using the Wemple - DiDomenico single oscillator dispersion formula together with an effective medium approach for porous materials the inner part of the films at the substrate interface was found to show "void-like" behaviour, while most of the film show "crystallite - like" behaviour in accordance with the columnar growth observed with evaporated CeO2 - films. By IAD the optical as well as the crystalline properties of the deposited films were modified.
Since excimer laser applications extend to deep and vacuum UV wavelengths at 193 nm and 157 nm, renewed research interest has recently arisen on fluoride thin films due to their unrivaled position as wide-band-gap material for the vacuum UV (VUV). In order to evaluate the development of mechanical stress in all dielectric fluoride mirrors which causes difficulties to grow the layer stacks on fused silica substrates with a sufficient large number of quarter-wave pairs of a low (L) and of a high index (H) fluoride material, a systematic study was performed on evaporated quarter-wave stacks of LaF3/MgF2 and LaF3/AlF3 with a growing number of LH-pairs. The samples deposited onto fused silica and silicon substrates by a low-loss evaporation technology in a BAK 640 coating plant were investigated by means of complex ex - situ mechanical stress analysis including temperature dependence of stress, optical measurements, infrared measurements, evaluation of structural and morphological parameters by AFM and XRD. When deposited at high substrate temperature of about 300°C, the LaF3/MgF2 tends show high tensile stress due to the thermal stress component arise from the large thermal expansion coefficient difference between the substrate and the film materials resulting in micro crack formation already starting after deposition of about 10 layer pairs. LaF3/AlF3 appear to have a larger crack resistance due to lower stress which can be correlated to the higher water content in these kind of stacks. By adjusting the deposition temperature, mirror stacks with high reflectance at 193nm can be grown.
Luminescence measurements have been set up in order to study the interaction of UV-laser radiation with dielectric thin films. The pulsed laser excitation was carried out at 193-nm (6.4eV), the coating materials comprised wide-band-gap oxides and fluorides. Experiments show the significant optical response of single- and multilayer coatings on the low fluence excitation at sub-band-gap energy. Time- and spectrally-resolved measurements indicate characteristic emission bands of color centers in the deep-UV and vacuum-UV coating materials. An assignment of these optical transitions can be derived from the comparison with known bulk-material studies.
Fabrication of dielectric optical masks by ablation patterning using a 193 nm laser is demonstrated. Ablation of layer systems that are highly reflective at 193 nm (HR 193nm) is accomplished by using an absorbing layer to initiate ablation. This absorbing layer is deposited on the substrate under the HR system. Thus it is possible to induce spatially defined ablation of the whole stack by irradiating the absorbing layer through the substrate (rear side ablation). Using the system as a mask, i.e. irradiating it from the front side, the reflective layer system prevents penetration of high fluence radiation to the absorbing layer, so that the function of the reflective mask is not restricted. A HR 193 nm system, composed of 42 layers, alternating Al2)3 and SiO2 was prepared. The substrate was fused silica. For the absorbing layer (a) HfO2 and (b) SiOx (x<2) of about 30 nm thickness were used. In the case of HfO2, residual fragments of the absorbing layer diminish the transmission in the ablated areas. In the case of SiOx, which is absorbing at 193 nm for x<2, these residuals can be oxidized to SiO2 has high transmission at 193 nm improving the mask performance. For the ablation process single pulse irradiation with a fluence of about 800 mJ/cm2 is sufficient. The mask has a damage threshold of >1 J/cm2.
Storage Ring Free Electron Laser (FEL) are attractive, full of promise, tuneable and powerful laser sources for the UV range. High reflectivity dielectric mirrors should be produced in order to allow lasing at very short wavelength, with a long stability in a strongly harsh environment and to optimize the extracted FEL power required for most of the newest applications. The front mirror of the laser cavity receives all the synchrotron radiation (SR) emitted by the wiggler, which is responsible for the mirror degradation, combined with the contamination by the vacuum residuals. We are tackling the problem of tests and manufactures of reliable robust mirrors and explore themes such as resistance analysis of UV mirrors to FEL multiscale power, broadband (X-UV) mirror robustness. Under drastic SR conditions, multiscale wavelength damages could be observed. Specific measurement techniques, able to investigate localized spatial modification induced by the non-uniform synchrotron radiation are presented. A local crystalline structure modification of the high index material appears together with a severe increase of the roughness.
In order to improve the degradation stability of dielectric mirrors for the use in UV-Free Electron Laser optical cavities a comparative study of the properties of SiO2, Al2O3, and HfO2 single layers was performed which was addressed to grow very dense films with minimum absorption in the spectral range from 200 nm to 300 nm. The films have been deposited by low loss reactive electron beam evaporation, by ion assisted deposition using a Mark II ion source, and by plasma ion assisted deposition using the APS source. Optical and structural properties of the samples have been studied by spectral photometry, infrared spectroscopy, x-ray diffraction and - reflectometry, as well as by investigation of the surface morphology. The interaction of UV radiation with photon energies close to the band gap was studied. For HfO2 single layer, LIDT at 248 nm were determined in the 1-on-1 and the 1000-to-1 test mode in dependence on the deposition technology and the film thickness.
Storage Ring Free Electron Laser are attractive and full of promise tunable and powerful laser sources for the UV range. Concerning the optical cavity, the relatively small gain obtained in the UV calls for the necessity to use high reflectivity multilayer mirrors with reliable longevity in synchrotron environment. It is also crucial to limit their absorption in order to optimize the extracted power required for most of the applications. Indeed, the front mirror of the laser cavity receives not only the first harmonic where the lasers operates but all the synchrotron radiation emitted by the undulator: a wide spectrum extending towards X rays. These short wavelengths are responsible for the mirror degradation which results from changes in the coating materials (high induced absorption, color centers, heating...) as well as from carbon contamination due to cracked hydrocarbons originating from the residual vacuum atmosphere. Deposition technologies which allow the manufacture of very dense oxide coatings with low absorption and high reflectivity in UV spectral region were optimized and characterized for this purpose. We report here degradation studies performed on UV mirrors for Storage Ring Free Electron Lasers down to wavelengths as short as 200 nm.
Many applications of multilayers in the EUV spectral region require not only high normal incidence reflectivity but also high thermal stability. We instigated the thermal stability of Mo/Si multilayers in comparison with the new material combination Mo2C/Si in the temperature range from 200 degrees Celsius to 700 degrees Celsius. Additionally, we deposited and studied Mo/Si multilayers having Mo2C diffusion barriers with 0.6 nm single layer thickness. The multilayer mirrors were designed for normal incidence reflectivity at about 13 nm wavelength and were deposited by dc magnetron sputtering. X-ray scattering, transmission electron microscopy and atomic force microscopy were used for characterization of the multilayer structures. The results are correlated to the measured normal incidence reflectivity using synchrotron radiation. We achieved maximal normal incidence reflectivities of 61.8% 13.0 nm wavelength for Mo2C/Si and 59.9% 13.3 nm for Mo/Si multilayers having Mo2C diffusion barriers. While the reflectivity of Mo/Si multilayers decreased considerably after annealing above 300 degrees Celsius the Mo2C/Si multilayers showed a superior thermal stability up to 600 degrees Celsius.
The key technologies for modern production processes with enhanced spatial resolution, require high performance DUV- excimer laser optics with enhanced optical properties. Major challenges imposed onto the requested new generation of optical elements are concentrated on lowest absorption and scattering as well as stability against highest pulse number throughput. These targets are the driving force within the German Joint Research Project 'OPUS II', which is dedicated to the development of high quality optical components for the DUV spectral range. As a major contribution to these investigations, sets of reflecting stacks with four different numbers of layer pairs of LaF3/MgF2 were produced by 6 partners of the consortium and characterized in respect to their optical performance and structural properties. The characterization includes spectrophotometric measurements from the VUV up tot eh mid RI range. calorimetric absorption measurements at 193 nm, and a comparative study in total scatter behavior at 193 nm, which was performed by three laboratories within the project. Also, besides the intrinsic stress and the surface topography of the layers, the non-linear absorption behavior of selected samples have been determined. The results are presented and discussed with respect to possible applications.
Recent developments of DUV-excimer laser applications have gained in demands for radiation resistant coated components at interesting wavelengths. To meet the requirements of long term reliability and high pulse number throughput superior performance of the optical components with lowest absorption and scattering losses are necessary. In the framework of the German Joint Research Project OPUS II efforts are made to investigate the optical properties, the radiation resistance and long term stability of single layers and layer system of interest in the DUV. The evaluation of optical coatings and coating system on different substrate materials was carried out by scattering experiments, atomic force microscopy, IR spectroscopy, calorimetric absorption measurements, and determination of laser induced damage threshold. Additionally, from the spectralphotometric measurements the optical behavior of the films was examined.
HfO2 is one of the most important high index thin film materials for the manufacture of interference coatings in the DUV spectral region down to 248 nm. High quality coatings and multilayer interference systems in conjunction with SiO2 as low index material can be deposited by various PVD technologies including reactive e-beam evaporation (RE), ion assisted deposition (IAD) and plasma ion assisted deposition (PIAD). Thin HfO2 films with optical thickness up to 3(lambda) /4 were deposited by RE, IAD and PIAD onto fused silica. The optical and structural properties of these films were investigated. The optical properties are related to the film structure and film density. The interaction of UV radiation with photon energies close to the band gap of HfO2 with different films was studied. LIDT at 248 nm were determined in the 1- on-1 and the 1000-on-1 test mode in dependence on the deposition technology and the film thickness. LIDT values of all investigated films decrease with increasing thickness due to the higher absorption and defect density. Additionally, data on the radiation resistance of interference coatings containing HfO2 will be presented.
A mode-mismatched surface thermal lens technique with pulsed top-hat beam excitation and near field detection scheme is developed to measure in situ the thermoelastic response of UV dielectric coatings and bulk materials under excimer laser irradiation. The thermal lens technique is demonstrated to be not only convenient for an accurate determination of the laser-induced damage threshold (LIDT), but also sensitive to measure the thermoelastic response of dielectric coatings irradiated with fluences much below the LIDT, and hence, to carry out time resolved predamage investigation. The minimum detectable surface displacement of approximately 0.002nm is achieved with a simple experimental configuration. Nonlinear absorption of UV dielectric materials and coatings are demonstrated. The surface thermal lens technique is also a convenient technique to distinguish different damage mechanisms: thermal stress induced damage or melting induced damage, depending on the thermo-elastic properties of the substrate. Hence, this technique allows to indicate qualitatively the relative contribution of linear and nonlinear absorption as possible causes for laser damage. Moreover, the nonlinear effect in laser conditioning of a LaF3/MgF2 highly reflective dielectric coating has been observed experimentally.
New absorption measurements for aluminum oxide optical coatings at 193nm are presented. Apart from the strong linear absorption at this wavelength the data indicate a nonlinear absorption within the thin dielectric layer. By varying the laser thickness, the intrinsic contribution of the layer material to the overall absorption was separated from the contribution of the substrate and the interface. In addition, the conditioning behavior of the coatings was examined. A strong long term conditioning in the linear absorption was found for Al2O3 containing systems. Comparing the absorption and conditioning behavior of the single layers and a high-reflective system, we can show that the absorption properties of the HR-system are determined by its Al2O3 layers.
To grow dense and hard MgF2 films substrate temperatures of about 300 degrees C are required, which unfortunately leads to high tensile film stress and the ability of crack formation. Lowering tensile stress in MgF2 films can be achieved by admixture a second fluoride material of higher cation radius than Mg2+. While former investigation were performed with non-heated films the purpose of the present work was to verify the behavior of mixed films when deposited at elevated substrate temperatures. One of the promising add material is BaF2 which enables evaporation of appropriate pre-mixed materials from a single source. The BaF2 content in the mixed films was varied from 3 to 55 mol percent in the MgF2 host. Optical, mechanical, and structural properties of samples deposited at different substrate temperatures have been studied by spectral photometry, IR spectroscopy, ex situ measurement of mechanical stress, x-ray diffraction, and -reflectometry, RBS, as well as investigation of surface morphology.
In this paper, we report on our investigations of radiation induced processes in optical interference coatings for 193 nm applications with respect to the microstructure of the coating. Experimental studies revealed that fluoride coatings contribute the main source for radiation induced optical changes during its exposure to 193 nm laser irradiation due to their porous microstructure. NIR spectroscopy could identify the origin of optical changes in interference coatings as a reversible hydrocarbon contamination which occurs within the coatings from storage in air atmosphere. Additionally, Laser Induced Damage Threshold measurements show a direct influence of the hydrocarbon contamination on the radiation durability of the multilayer systems during laser exposure. Experiments were carried out by using several characterization techniques including DUV spectrophotometry, ATR-IR-spectroscopy, x-ray diffractometry, and the determination of the '1-on-1' laser induced damage threshold. Test methods were applied to DUV coatings before and after exposure to 193 nm radiation with irradiation doses of up to 108 laser pulses at a fluence of 70mJ/cm2. Test samples consisted of several coating designs, primarily of high reflective multilayer systems.
The mechanical behavior of fluoride single layers and thick model layer systems were carried out ex-situ after deposition at laboratory conditions. The samples on substrates with different thermal expansion coefficients have been prepared by a low loss evaporation process. Investigation were performed for single layers deposited at different film thickness, substrate temperature and storage time. All investigated fluorides posses tensile stress of various amounts. The thermal stress component seems to be the major contribution to the total stress MgF2 films of about 100 nm thickness deposited onto silicon substrate. The stress and force per unit width of model multilayer systems formed from different fluorides were examined and correlated to the ability of crack formation on thick rigid fused silica substrates. Optimization of the stress values, the bending force and the crack formation was performed by adjusting the deposition temperature and by introducing a third stress reducing material in the stack. An example of a stress optimized high reflector will be shown.
Dielectric mirrors are key optical components in ArF excimer laser based devices for applications in DUV photolithography as well as in material processing. In all these applications different requirements of laser radiation resistance have to be met in relation to fluence, repetition rate and pulse number lifetime. Investigations have been performed into the radiation resistance of dielectric mirrors consisting of fluorides and oxides with emphasis to the properties of bending point mirrors used in beam delivery systems of wafer steppers. Problems and limitations for the improvements of the laser-induced-damage-thresholds of the coatings are discussed.
Scanning electron microscope observations off damaged coatings from 355nm to 193nm are reported. From these results it appears that oxide based coating are strongly limited at 193nm while they performed very well at the other wavelength. Damages occurs at the coating surface for oxides while they concern both surface and substrate interfaces with fluoride base coatings. While fluorides have a better threshold than oxide at 193nm they are limited by localized defects. The ability to overcome this limitation with ion beam sputtering fluoride material at 193nm starting from results at 355nm.
CaF2 has received increasing attention as a promising substrate for coatings in the VUV range. Optimization of the optical properties of these optical components requires the study of basic characteristics of the coated and uncoated CaF2 substrates such as surface roughness, optical performance, absorption and scatter losses, and laser induced damage threshold. The investigations have revealed the influence of different substrate polishing grades on the quality of AR-193nm -and HR-193nm/0 degrees coated samples. LIDT values at the ArF-excimer laser wavelength were measured as high as 5.6 J/cm2 and 4.6 J/cm2 for the best AR- and HR-coated samples, respectively.
Recent developments of DUV-excimer laser applications have gained in demands for radiation resistant coated components at interesting wavelengths. To meet the requirements of long term reliability and high pulse number throughput a superior performance of the optical components with lowest absorption and scattering losses is necessary. Within the framework of the German Joint Research Project "OPUS II" efforts are made to investigate the optical properties, the radiation resistance, and long term stability of single layers and layer systems of interest in the DUV. The evaluation of optical coatings and coating systems (AR and H R) on different substrate materials was carried out by scattering experiments, atomic force microscopy, infrared spectroscopy, calorimetric absorption measurements, determination of laser induced damage threshold (1-on-i, 1 000-on-i) and scratch tests. Additionally from spectrophotometric measurements the optical behaviour of the films was examined.
The thermal decay of surface temperature has been calculated and measured on Al2O3/SiO2 coatings by using photothermal technique near damage threshold fluence. It could be shown that under certain conditions the decay times (tau) has been found to be in the order of some microseconds. This fact gives no explanation of the measured decrease of threshold fluence in some cases for higher repetition rates and shot numbers on Al2O3/SiO2 and HfO2/SiO2 multilayers for (lambda) equals 248nm. Furthermore, it could not be found any influence of the substrate materials. Thus, other than thermal accumulation is responsible for the lowered damage threshold by increasing repetition rate. Additionally, performed calculations of the thermal decay using 20ns, 248nm laser excitation confirm the experimental results. Even in the case of Al2O3/SiO2 coatings on copper no effect of the substrate as a heat sink could be measured. For HfO2/SiO2 coatings the behavior is in accordance with the fact that the thermal conductivity of HfO2 films is markedly lowered compared to the bulk value.
The exposure of optical interference coatings to low-fluence DUV-radiation reveals changes of thin layer properties due to interactions between radiation field and thin film structure. An experimental set up for irradiating antireflective as well a high reflective coatings with 193nm excimer laser was used in order to study permanent cumulative changes in optical coatings at fluences ranging from 20mJ/cm2 with up to 240 106 laser pulses. The optical ex-situ monitoring of radiation induced modifications enabled the differentiation of coating specific and substrate inherent alteration effects. The identification of conditions as well as degradation processes during the exposure could be achieved for several types of DUV-coating materials. They were deposited with an ultra low loss evaporation process onto calcium fluoride and fused silica substrates. Fluoride coating included LaF3, Na3AlF6, MgF2, AlF3 oxide coatings consisted of SiO2 and Al2O3 exclusively.
The aim of our investigation was to explain the causes and kinds of the destruction of optical coatings during laser radiation at the wavelength of ArF excimer laser. Therefore, HR layer stacks with an increasing number of HL-pairs were deposited on different substrates CaF2 and fused silica, respectively. SiO2/Al2O3-, LaF3/MgF2- and AlF3/LaF3-combinations were used as coating materials. While fluoride coatings have been deposited by conventional evaporation, the oxide coatings were deposited by reactive e-beam evaporation with or without plasma ion assistance. The interaction of UV laser radiation with optical coatings as mentioned above was investigated by a pulsed two probe beam photothermal technique as well as optical microscopy, respectively. In the case of fluoride layers the single shot damage threshold increases with higher number of HL-pairs. Additionally, an aging effect could be observed.
CaF2 has received increasing attention as a promising substrate for coatings in the VUV range. Optimization of the optical properties of these optical components requires the study of basic characteristics of the coated and uncoated CaF2 substrates such as surface roughness, optical performance, absorption and scatter losses, and laser induced damage threshold. The investigations reveal the influence of different substrate polishing grades on the quality of the coated components.
We report on our investigations on the long-term behavior of optical coatings under 193 nm laser irradiation in dependence on coating materials, radiation conditions, and substrate properties. A wide variety of different highly reflective dielectric mirrors and antireflective coatings, deposited by an ultra low loss evaporation process onto calcium fluoride and fused silica, has been tested. Irradiation experiments with highly reflective coatings show that fluoride coatings exhibit nearly no changes of their optical function in air as well as in argon atmosphere due to low initial absorption levels. Temporal atmospheric contaminations can be removed by using appropriate irradiation conditions. Oxide layers tend to post-oxidize during 193 nm exposure in air and the DUV absorption level will be reduced. Effectively, reflectance of multilayer coatings on the basis of oxide materials can be improved through laser irradiation. Irradiation experiments with antireflective coatings point out the dominant role of bulk and surface properties of the substrate for prolonged laser irradiation. In addition, we present laser induced damage thresholds to demonstrate upper limits of laser radiation resistance that can be achieved nowadays with several types of coatings.
Properties of single-layer and multilayer Al2O3/SiO2 coatings deposited by Plasma Ion Assisted Deposition (PIAD) and Low Loss Reactive Evaporation (LL-RE) have been studied with emphasis on their use in the UV and VUV spectral region. The influence of significant deposition parameters, mainly the bias voltage in the case of PIAD and the substrate temperature in the case of LL-RE, on the optical and structural properties as well as on the film stress is investigated by spectrophotometry, IR- spectroscopy, light scattering, atomic force microscopy, and laser beam deflection stress measurements. Laser photon interaction with single-layer films and multilayer coatings was studied for the different wavelengths of excimer lasers (ArF (193 nm), KrF (248 nm), XeCl (308 nm) and the 3rd harmonic (355 nm) of the Nd:YAG solid state laser. High laser damage resistance and environmentally stable optical characteristics have been accomplished for multilayer coatings, especially for KrF (248 nm) excimer laser. The influence of the surface roughness of the substrates on the surface topography and the related scatter losses of the coatings has been investigated by integrated light scattering and atomic force microscopy measurements.
Stable and efficient UV-laser systems require stable and efficient optics which can only be realized by applying high performance, low loss and highly damage resistant thin film interference coatings. The interaction between excimer laser photons and optical coatings is determined by combined effects of high repetition rates and high energy densities. Optical stability investigations on oxide and fluoride thin films have been performed to estimate ns-laser-induced- damage-thresholds. By using thermal mirage technique both the detection of the damage onset after illumination with KrF excimer laser (248 nm, 20 ns) and the exploitation of the increase of the signal per unit fluence for the interpretation of the origins of radiation damage were possible. Damage sensitive defects were identified to origin both from residual gas and evaporation sources. By optimizing electron beam evaporation technology the defect density was drastically reduced.
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