Combined EUV reflectance and X-ray reflectivity data analysis of periodic multilayer structures

We present a way to analyze the chemical composition of periodical multilayer structures using the simultaneous analysis of grazing incidence hard X-Ray reflectivity (GIXR) and normal incidence extreme ultraviolet reflectance (EUVR). This allows to combine the high sensitivity of GIXR data to layer and interface thicknesses with the sensitivity of EUVR to the layer densities and atomic compositions. This method was applied to the reconstruction of the layered structure of a LaN/B multilayer mirror with 3.5 nm periodicity. We have compared profiles obtained by simultaneous EUVR and GIXR and GIXR-only data analysis, both reconstructed profiles result in a similar description of the layered structure. However, the simultaneous analysis of both EUVR and GIXR by a single algorithm lead to a ∼2x increased accuracy of the reconstructed layered model, or a more narrow range of solutions, as compared to the GIXR analysis only. It also explains the inherent difficulty of accurately predicting EUV reflectivity from a GIXR-only analysis

Characterization of megahertz X ray laser beams by multishot desorption imprints in PMMA

Proper diagnostics of intense free electron laser FEL X ray pulses is indisputably important for experimental data analysis as well as for the protection of beamline optical elements. New challenges for beam diagnostic methods are introduced by modern FEL facilities capable of delivering powerful pulses at megahertz MHz repetition rates. In this paper, we report the first characterization of a defocused MHz 13.5 nm beam generated by the free electron laser in Hamburg FLASH using the method of multi pulse desorption imprints in poly methyl methacrylate PMMA . The beam fluence profile is reconstructed in a novel and highly accurate way that takes into account the nonlinear response of material removal to total dose delivered by multiple pulses. The algorithm is applied to experimental data of single shot ablation imprints and multi shot desorption imprints at both low 10 Hz and high 1 MHz repetition rates. Reconstructed response functions show a great agreement with the theoretical desorption response function mode

Experimental study of EUV mirror radiation damage resistance under long term free electron laser exposures below the single shot damage threshold

The durability of grazing and normal incidence optical coatings has been experimentally assessed under free electron laser irradiation at various numbers of pulses up to 16 million shots and various fluence levels below 10 of the single shot damage threshold. The experiment was performed at FLASH, the Free electron LASer in Hamburg, using 13.5 nm extreme UV EUV radiation with 100 fs pulse duration. Polycrystalline ruthenium and amorphous carbon 50 nm thin films on silicon substrates were tested at total external reflection angles of 20 and 10 grazing incidence, respectively. Mo Si periodical multilayer structures were tested in the Bragg reflection condition at 16 off normal angle of incidence. The exposed areas were analysed post mortem using differential contrast visible light microscopy, EUV reflectivity mapping and scanning X ray photoelectron spectroscopy. The analysis revealed that Ru and Mo Si coatings exposed to the highest dose and fluence level show a few per cent drop in their EUV reflectivity, which is explained by EUV induced oxidation of the surfac

Mechanism of single shot damage of Ru thin films irradiated by femtosecond extreme UV free electron laser

Ruthenium is a perspective material to be used for XUV mirrors at free electron laser facilities. Yet, it is still poorly studied in the context of ultrafast laser matter interaction. In this work, we present single shot damage studies of thin Ru films irradiated by femtosecond XUV free electron laser pulses at FLASH. Ex situ analysis of the damaged spots, performed by different types of microscopy, shows that the weakest detected damage is surface roughening. For higher fluences we observe ablation of Ru. Combined simulations using Monte Carlo code XCASCADE 3D and the two temperature model reveal that the damage mechanism is photomechanical spallation, similar to the case of irradiating the target with optical lasers. The analogy with the optical damage studies enables us to explain the observed damage morphologie

Spectral properties of La/B - Based multilayer mirrors near the boron K absorption edge

The spectral properties of La/B, La/B 4C, and LaN/B, LaN/B 4C multilayer mirrors have been investigated in the 6.5-6.9 nm wavelength range based on measured B and B 4C optical constants. Experimentally it is verified to what extent measured and tabulated optical constants are applicable for simulations of the reflectivity of these short period multilayer mirrors. The measured maximum reflectance at various wavelength values around the boron-K absorption edge is compared to calculated values from model systems. The measured reflectance profiles of La/B and La/B 4C show a maximum at a slightly larger wavelength than calculations would predict based on the measured B and B 4C optical constants. This is explained by the influence of a formed boron-lanthanum compound on the wavelength where the multilayer shows maximum reflectance. The maximum reflectance profiles of LaN/B and LaN/B 4C multilayers can be described accurately by using the same boron atomic scattering factors, indicating boron in the LaN/B 4C multilayer to be in a similar chemical state as boron in the LaN/B multilayer. It also indicates that nitridation of the La layer in the multilayer prevents the formation of La-B compounds. We show that the optimal wavelength for boron based optics is about 6.65 nm and depends on the B chemical state. Finally, using the measured B optical constants we are able to calculate the spectral response of the multilayers, enabling the prediction of the optimal parameters for the above mentioned multilayers © 2012 Optical Society of America

Spectral properties of La/B - based multilayer mirrors near the boron K absorption edge

The spectral properties of La/B, La/B4C, and LaN/B, LaN/B4C multilayer mirrors have been investigated in the 6.5-6.9 nm wavelength range based on measured B and B4C optical constants. Experimentally it is verified to what extent measured and tabulated optical constants are applicable for simulations of the reflectivity of these short period multilayer mirrors. The measured maximum reflectance at various wavelength values around the boron-K absorption edge is compared to calculated values from model systems. The measured reflectance profiles of La/B and La/B4C show a maximum at a slightly larger wavelength than calculations would predict based on the measured B and B4C optical constants. This is explained by the influence of a formed boron-lanthanum compound on the wavelength where the multilayer shows maximum reflectance. The maximum reflectance profiles of LaN/B and LaN/B4C multilayers can be described accurately by using the same boron atomic scattering factors, indicating boron in the LaN/B4C multilayer to be in a similar chemical state as boron in the LaN/B multilayer. It also indicates that nitridation of the La layer in the multilayer prevents the formation of La-B compounds. We show that the optimal wavelength for boron based optics is about 6.65 nm and depends on the B chemical state. Finally, using the measured B optical constants we are able to calculate the spectral response of the multilayers, enabling the prediction of the optimal parameters for the above mentioned multilayers (C) 2012 Optical Society of Americ

Optimization of LaN/B multilayer mirrors for 6.x nm wavelength

In this article we present an overview of the optimization of LaN/B multilayers that enabled the deposition of a multilayer with a normal incidence reflectance of 57.3 % at 6.6 nm wavelength, the highest value reported to date. Two different ways of nitridation of the La layers were investigated: firstly N-ion post treatment of the La layer and secondly reactive magnetron sputtering of La in N2 atmosphere. Initially the optimization of the multilayers was performed for 50 period test multilayers, followed by the selection of the best process to study the stability of the full stack deposition and the optical performance of the mirrors. The scaling of reflectivity with increasing number of periods for LaN/B multilayer mirrors will also be discussed

Influence of noble gas ion polishing species on extreme ultraviolet mirrors

Low energy ion polishing is attractive in thin films because of the small interaction zone with the treated material. In this context, various noble gases (Ne, Ar, Kr, and Xe) have been applied for low energy ion polishing of interfaces in nanoscale optical Mo/Si multilayers in order to mitigate the evolving roughness during the deposition process. The interface morphology has been studied by grazing incidence small angle x-ray scattering, the multilayer composition by x-ray photoelectron spectroscopy, and the general performance by extreme ultraviolet (EUV) reflectometry. Both the average roughness level and the vertical correlation length of the roughness can be reduced significantly by increasing the atomic mass of the ion species applied for polishing. Maximum EUV reflectance is observed for Krþ-polishing, while Xeþ-polishing shows a superior structure. This apparent contradiction is explained by taking into account the optical absorption from noble gas residuals in the amorphous silicon layers

Wavelength selection for multilayer coatings for the lithography generation beyond EUVL

The spectral properties of LaN/B and LaN/B4C multilayer mirrors have been investigated in the 6.5-6.9 nm wavelength range, based on measured B and B4C optical constants. We show that the wavelength of optimal single mirror reflectance for boron based optics is between 6.63 and 6.65 nm, depending on the boron chemical state. The wavelength of the maximum reflectance of the LaN/B4C multilayer system is confirmed experimentally. Calculations of the wavelengthintegrated reflectance for ideal 10-multilayer-mirror stacks show that a B-based optical column can be optimized for a wavelength larger than 6.65 nm