High-performance Mo-Si multilayer coatings for extreme-ultraviolet lithography by ion-beam deposition

An ion-beam deposition system has been used to fabricate Mo-Si multilayer coatings for masks and imaging optics to be used for extreme-ultraviolet lithography. In addition to high reflectivity and excellent profile control, ion-beam deposition has the capability to smooth rough substrates. For example, we achieved reflectivity of 66.8% on a substrate with 0.39-nm roughness. Smoothing can be further enhanced with a second ion source directed at the multilayer coating. The smoothing capabilities relax the requirement on the finish of the mirror and the mask substrates and could dramatically reduce the cost of these components. Thickness profile control is in the +/-0.01% range, and the figure error added to the mirror substrate by errors in the multilayer thickness is less than 0.1 nm. Peak reflectivities obtained on smooth substrates are 67.5-68.6%.     

Effects of mo/si multilayer coatings on the imaging characteristics of an extreme-ultraviolet lithography system

The role of Mo/Si reflective coatings in the imaging performance of an extreme-ultraviolet projection lithography system under polychromatic illumination has been theoretically examined. Using a thin-film computer model, we have explored various multilayer design criteria. Optimum operating conditions, leading to the maximum system transmittance, were found for a tuned multilayer system operating at lambda = 12.7 nm. In this configuration, Mo/Si coatings have been shown to be nondetrimental to the imaging performance of our system with the introduction of only minor modifications to the propagating wave front, which can be adequately described by a simple tilt and defocus term.     

Optical constants of in situ-deposited films of important extreme-ultraviolet multilayer mirror materials

We have performed angle-dependent reflectance measurements of in situ magnetron sputtered films of B(4)C, C, Mo, Si, and W. The Fresnel relations were used to determine the complex index of refraction from the reflectance data in the region of approximately 35-150 eV. In the cases of Si, C, and B(4)C we found excellent agreement with published data. However, for Mo and W we found that the optical properties from 35 to 60 eV differed significantly from those in the literature.     

Localized defects in multilayer coatings

We present a non-linear continuum model of the growth of localized defects in multilayer coatings nucleated by particles on the substrate. The model is valid when the deposition and etch fluxes are near normal incidence so that shadowing effects are negligible. Three-dimensional simulations of defects in Mo/Si multilayer films nucleated by arrays of lithographically patterned particles are shown to be in good agreement with experimental measurements. Our results confirm that incorporating ion beam etching in the multilayer deposition process significantly suppresses the defect growth. This has a potentially important application in the fabrication of defect-free masks for extreme ultraviolet lithography.     

Multilayer coatings with high reflectance in the extreme-ultraviolet spectral range of 50 to 121.6 nm

Multilayer coatings with three layers were designed to yield an increase in normal-incidence reflectance in the extreme ultraviolet over that of the available single-layer coatings. Multilayer coatings based on Al, MgF(2), and either SiC or B(4)C were demonstrated to have higher reflectance than single layers of SiC and B(4)C in the spectral region from 57.9 nm to the H Lyman-alpha line (121.6 nm) and above. The increase in reflectance was higher at wavelengths close to 121.6 nm. Reflectance degraded slightly over time in the same way as for single layers. After a few months, multilayer coatings maintained higher reflectance than their single-layer counterparts.     

Uncovering changes in spider orb-web topology owing to aerodynamic effects

An orb-weaving spider''s likelihood of survival is influenced by its ability to retain prey with minimum damage to its web and at the lowest manufacturing cost. This set of requirements has forced the spider silk to evolve towards extreme strength and ductility to a degree that is rare among materials. Previous studies reveal that the performance of the web upon impact may not be based on the mechanical properties of silk alone, aerodynamic drag could play a role in the dissipation of the prey''s energy. Here, we present a thorough analysis of the effect of the aerodynamic drag on wind load and prey impact. The hypothesis considered by previous authors for the evaluation of the drag force per unit length of thread has been revisited according to well-established principles of fluid mechanics, highlighting the functional dependence on thread diameter that was formerly ignored. Theoretical analysis and finite-element simulations permitted us to identify air drag as a relevant factor in reducing deterioration of the orb web, and to reveal how the spider can take greater—and not negligible—advantage of drag dissipation. The study shows the beneficial air drag effects of building smaller and less dense webs under wind load, and larger and denser webs under prey impact loads. In essence, it points out why the aerodynamics need to be considered as an additional driving force in the evolution of silk threads and orb webs.     

Spin-on-glass coatings for the generation of superpolished substrates for use in the extreme-ultraviolet region

Substrates intended for use as extreme-ultraviolet (EUV) optics have extremely stringent requirements in terms of finish. These requirements can dramatically increase the cost and fabrication time, especially when nonconventional shapes, such as toroids, are required. Here we present a spin-on-glass resist process capable of generating superpolished parts from inexpensive substrates. The method has been used to render diamond-turned substrates compatible for use as EUV optics. Toroidal diamond-turned optics with starting rms roughness in the 3.3-3.7 nm range have been smoothed to the 0.4-0.6 nm range. EUV reflectometry characterization of these optics has demonstrated reflectivities of approximately 65%.     

The effects of nitrogen bonding on hardness of AIN/CrN multilayer hard coatings

AIN/CrN multilayer hard coatings with various bilayer thicknesses were fabricated by a reactive sputtering process. The microstructural and mechanical characterizations of multilayer coatings were investigated through transmission electron microscope (TEM) observations and the hardness measurements by nano indentation. In particular, the variation of chemical bonding states of the bilayer nitrides was elucidated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Many broken nitrogen bonds were formed by decreasing the bilayer thickness of AIN/CrN multilayer coatings. Existence of optimum AIN/CrN multilayer coatings thickness for maximum hardness could be explained by the competition of softening by the formation of broken nitrogen bonds and strengthening induced by decreasing bilayer thickness.     

Photothermal deflection in multilayer coatings: modeling and experiment

A model of the photothermal deflection signal in multilayer coatings is presented that takes into account optical interference effects and heat flow within the stack. Measurements are then taken of high-reflectivity HfO2/SiO2 ultraviolet mirrors made by plasma ion assisted deposition and compared to calculations. Good agreement is found between the experimental results and the model. Using this model for the calibration and the setup described, one can measure absorption in multilayer coatings accurately down to 10(-7) of the incident power.     

Markov-chain Monte Carlo approach to the design of multilayer thin-film optical coatings

We reconsider the problem of locating the globally optimal solution of a multilayer-optical-coating design problem, within some predetermined space of parameters, with the aim of obtaining a robust technique that requires a minimum of user intervention. The approach we adopt centers on exploring the space of the parameters of interest by using a Markov-chain Monte Carlo sampling algorithm. This technique enables one to locate the global optimum automatically with high confidence and without the need for a good starting design. It also allows the trivial inclusion of prior constraints on the variables and provides a natural means for investigating the robustness of the optimal solution.     

Sub-quarter-wave multilayer coatings with high reflectance in the extreme ultraviolet

Multilayer coatings with a small number of layers were designed and prepared to provide an increase in normal-incidence reflectance in the extreme ultraviolet compared with the reflectance of available single-layer coatings, namely, SiC, B4C, and Ir. Multilayers were designed to produce coatings with the highest possible reflectance at 91.2 and at 58.4 nm. At these wavelengths all the materials absorb radiation strongly, but still a reflectance enhancement can be obtained by means of sub-quarter-wave multilayer coatings with two or more different materials. Sub-quarter-wave multilayer coatings based on Al, MgF2, diamondlike carbon, B4C, SiC, and Ir showed higher reflectance than single-layer coatings of SiC and B4C not only at the target wavelength but in a wide band ranging from 50 nm to the 121.6-nm H Lyman-alpha line. Multilayer coatings suffered some reflectance degradation over time. However, after approximately 80-100 days of aging in a desiccator, the reflectance for the multilayer coatings was greater than for the single-layer coatings.     

Third-order coma-free point in two-mirror telescopes by a vector approach

In this paper, two-mirror telescopes having the secondary mirror decentered and/or tilted are considered. Equations for third-order coma are derived by a vector approach. Coma-free condition to remove misalignment-induced coma was obtained. The coma-free point in two-mirror telescopes is found as a conclusion of our coma-free condition, which is in better agreement with the result solved by Wilson using Schiefspiegler theory.     

Defect induced laser damage in oxide multilayer coatings for 248 nm

Photothermal displacement microscopy is used for the detection of μm-sized defects in Al₂O₃/SiO₂ multilayer coatings highly reflective for 248 nm. It is shown that for high quality coatings the global (averaged over several cm²) laser-induced damage threshold for coatings of different quality is determined by the density and absorption strength of the light absorbing defects. This is confirmed by a measurement of local (averaged over 0.01 mm²) damage thresholds with the pulsed photoacoustic mirage technique allowing a direct correlation of local damage with photothermally detected thin film defects. An analysis of laser-damaged spots reveals damage-craters of 13 μm diameter at the onset of detrimental irradiation effects. The formation of these craters is explained by a thermoelastic model describing the tensile stress in the film system resulting from evaporation of a defect located at the interface between thin film and substrate.     

Phase transformation of yttria-stabilized zirconia plasma-sprayed coatings in a humid atmosphere

The tetragonal to monoclinic phase transformation resulting from the hydrothermal ageing of 4–20 mass % yttria-stabilized zirconia plasma-sprayed coatings formed from commercial powders has been studied with respect to the Y2O3 distribution inside each coating. The phase transformation was prevented when the nominal Y2O3 concentration was greater than 8 mass % with its point to point distribution ranging between 6.82 to 9.90 mass %. It is suggested that a close correlation exists between the durability of the tetragonal phase in a humid atmosphere and the Y2O3 distribution in the tetragonal zirconia coatings. A content of 6.3–6.8 mass % Y2O3 was calculated to be necessary to prevent the transformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Rigorous analysis of the spatial dispersion effect in multilayer thin-film coatings

We demonstrate a wavelength-dependent spatial lateral beam shift of a reflective Gaussian beam on the surface of multilayer thin-film coatings. Numerical analysis of the behavior of a Gaussian-form light beam incident on the surface of the coatings is performed. The theoretical analysis is for a two-dimensional light sheet. The spatial lateral beam dispersion can be determined rigorously from the numerical wave field in the region of the incident medium. The coatings present a spatial dispersion effect much larger than in previous reports. The thin-film coatings can act not only as positive spatial dispersion devices but in a proper case, also as negative spatial dispersion devices. As such, the devices produce a wavelength-dependent negative lateral shift of the reflective beam.     

Sputtering simulation of multilayer coatings in industrial PVD system with three-fold rotation

Multilayer coatings in physical vapour deposition (PVD) systems are usually prepared by rotation of substrates along different targets. In industrial PVD systems two- or three-fold rotations are typically applied. When substrates rotate around the targets, deposition rate of material varies. Rotation therefore influences the thickness and sequence of layers in the multilayer coating. Structure of the multilayer coating depends on the parameters of rotation, initial position and orientation of the substrates. A computer simulation of deposition process with four unbalanced magnetron sources was made for two- and three-fold rotation. Time dependence of growth rate and thickness was calculated. Calculations for three-fold rotation show that additional modulation is super-imposed on the basic period. Results of simulation are in accordance with experimental data from scanning electron microscope (SEM) and Auger electron spectroscopy (AES).     

Near-field effects in multilayer inductive metal meshes

The transmittance of inductive single-layer and multilayer cross-shaped metal meshes has been calculated with the Micro-Stripes software program. The effect of symmetric and asymmetric alignment of the crosses of one mesh with respect to another was studied and compared with transmission line theory, which presents the nonaligned case. Significant differences are found for small spacing at approximately 1/5 the periodicity constant, whereas the differences disappear for large spacing. A pair of coupled surface waves is used to represent the mode of a single mesh. The resulting modes corresponding to the transmittance of multilayer metal meshes are interpreted by modes composed of resonance modes of a single mesh coupled by Fabry-Perot modes depending on the separation.     

Magnetostatic wall interaction effects in multilayer films

The critical film thickness for Bloch-Néel (B-N) wall-type transition has previously been determined experimentally for the configuration of two parallel and equally thick 81 percent Ni-19 percent Fe films separated by a nonferromagnetic layer of SiO as a function of the thickness of the separating layer. The experimental results are interpreted assuming a simple coupled-wall model and using the Néel approximation for wall width and wall energy calculations.