A compact electron-based extreme ultraviolet (EUV) source for advanced at-wavelength mirror metrology is developed. The source concept is based on the transfer of advanced microfocus x-ray tube technology into the EUV spectral range. This allows the realization of a flexible, debris-free, and long-term stable EUV source. In the EUV tube, silicon targets are used to generate radiation at 13.5 nm. Detailed characteristics of the source performance are reported and different applications of the EUV tube in the field of at-wavelength mirror metrology are presented.
Next generation lithography is likely to deploy extreme UV (EUV) light at 13.5 nm wavelength for key manufacturing processes. Currently, all promising EUV light source concepts require efficient light collection optics in order to deliver sufficeintly high light power for profitable chip production. With densely nested Wolter-Type 1 reflective optics we designed, and fabricated such optics. In this paper we report on the latest achievements in design, development and on our first at wavelength testing results of such collection optics.
We developed a high-numerical-aperture EUV exposure tool (HiNA). HiNA is equipped with an illumination system, projection optics, a mask stage and a wafer stage in the vacuum chamber. The projection optics consist of two aspherical mirrors (M1 and M2). The numerical aperture of the optics is 0.3. Thus far, we fabricated two sets of projection optics (set-1 and set-2). The wavefront errors of set-1 and set-2 were 7.5nm rms and 1.9nm rms, respectively. We developed the third set of projection optics (set-3), the target wavefront error of which was less than 1nm rms. In set-3, we also attempted to reduce flare. We completed the mirror polishing, coating and mirror adjustment of set-3. Using a new polishing method, we successfully reduced low-spatial-frequency roughness (LSFR), mid-spatial-frequency roughness (MSFR) and high-spatial-frequency roughness (HSFR) simultaneously. The predicted wavefront error calculated from the LSFR number was 0.69nm rms. MSFR, which strongly affects the flare of the optics, was significantly reduced to less than 0.2nm rms. The estimated flare was 7%, which is significantly reduced to one-fourth that of set-2. The wavefront error of set-3 was measured with the visible-light point diffraction interferometer (PDI) after coating and assembly. The wavefront error measured after adjustment and cramping of the adjustment system was 0.90nm rms, which is less than one-half the wavefront error of set-2.
EUVL (extreme ultraviolet lithography), utilizing an actinic wavelength of about 13 nm , appears to be the most promising technology approach to reach the 30 nm node. Calling for diffraction limited imaging performance, EUV demand unprecedented requirements for figure metrology and fabrication technology. This paper gives an overview over problems rising from the interferometric measurement of aspheric EUV mirrors.
The recent experimental results of EUV wavefront metrology in EUVA are reported. EUV Experimental Interferometer (EEI) was built at the NewSUBARU synchrotron facility of University of Hyogo to develop the most suitable wavefront measuring method for EUV projection optics. The result is to be reflected on EWMS (EUV Wavefront Metrology System) that measures wavefront aberrations of a six-aspherical mirror projection optics of NA0.25, of a mass-production EUV lithography tool. The experimental results of Point Diffraction Interferometer (PDI) and Lateral Shearing Interferometer (LSI) are shown and the error factors and the sensitivity of astigmatism measurements of these methods are discussed. Furthermore, for reducing these kinds of errors, another type of shearing interferometer called DTI (Digital Talbot interferometer) is newly introduced.
In the paper we describe the development of a reflective optical system for EUV-microscopy containing an ellipsoidal formed collector optics and a Schwarzschild objective (magnification M=21, numerical aperture NA=0.2) for EUV radiation of a wavelength λ=13.5nm. In order to collect the maximum intensity of an EUV gas discharge plasma source, the grazing incidence collector has been inside-coated with molybdenum by Pulsed Laser Deposition (PLD). This method enables the deposition of uniform and highly reflective molybdenum layers, which have been protected against oxydation by using thin carbon top layers. The two mirrors of the Schwarzschild objective consist of highly reflective Mo/Si- multilayers produced by Magnetron Sputter Deposition (MSD). In order to obtain the best optical performance, laterally graded multilayers with rotational symmetry have been deposited by using a new mask-deposition technique. Thus the multilayer thickness corresponds at each point of the curved mirrors to wavelength and incidence angle of the EUV beam. Ray tracing simulations were performed for the two optical elements, collector optics and Schwarzschild objective. The results of these calculations are shown and compared with the results obtained by the EUV-microscope.
Radiation damage to multilayer mirrors has been intently studied in the view of the EUV lithography (EUVL) application in recent years.
To investigate the radiation damage, a reflectance measurement system for EUVL mirrors was developed at beam line 9 at the NewSUBARU SR facility. This system can irradiate the mirror using EUV radiation from a long undulator (10.8 m) and simultaneously measure changes in reflectance caused by radiation damage. The actual measurement of the power density of the EUV radiation at the sample mirror was about 500 mW/mm2, which is sufficiently intense for quickly investigating radiation damage. The EUV wavelength, 13.5 nm, was selected from the undulator radiations by using a planar multilayer mirror with a maximum reflectance of 13.5 nm. The θ and 2 θ stages were adopted for reflectance measurements, making the system more valuable and flexible. Because the system is equipped with a removable pinhole to restrict the incident beam size and x-z automatic stages, it can also be used to measure the spatial distribution of the reflectance and photoemission current.
The ultimate vacuum was in the order of 10-5 Pa even though the automatic stages were moving. Some aspects, which depend on the atmospheres, capping layers on mirrors, and flux density of the irradiation beam, were measured. The photoemission current was also measured. These measurements provide important information about the extent of the radiation damage and whether or not it is proportional to the flux density.
A free electron laser (FEL) is being set up at DESY (Deutsches Elektronen Synchrotron, Hamburg, Germany). In the current XUV range of the FEL, total-reflection X-ray mirrors are needed for beam guidance, beam alignment, and monochromatisation. Such X-ray optics are used at a grazing incidence angle of about 2°; thus a maximum length of about 500 mm is required. Due to the working range of the FEL (50 - 200 eV), carbon has been selected as a suitable material with an absorption edge at 284 eV. The amorphous carbon coatings were manufactured by magnetron sputtering in a special UHV system for large deposition at GKSS research centre (Geesthacht, Germany). The variation in film thickness over the whole length has been investigated by X-ray reflectometry (XRR). Good uniformity (better than 2 %) and low roughness (< 0.5 nm) have been observed.
A number of X-ray astronomical missions of near future (Constellation-X, XEUS, Simbol-X) will make use of hard X-rays (10-100 keV) optics with broad-band multilayer coatings. A possible technique under development is based on an extension of the already tested replication of a coated mandrel by e-beam deposition and nickel electroforming already successfully used for the soft (0.1 - 10 keV) X-ray mirrors of the Beppo-SAX, XMM, JET-X/Swift missions. In this case graded multilayers are deposited and replicated from the mandrel replicated instead of a single layer. The roughness reduction in order to improve the coating reflectivity could be achieved by an ion assistance during the e-beam deposition. The e-beam deposition with ion assistance is a technique that allows to reach comparable (if not better) smoothness levels with respect to other methods (e.g. ion sputtering), taking the advantage of a stress mitigation between the layers and of a further improvement in reflectivity due to the low density of the e-beam evaporated Carbon, which is used as bilayer spacer. In this paper we discuss the adopted deposition technique and its implementation: we present topographic (AFM) tests and X-ray reflectivity tests performed on preliminary samples.
In order to obtain high reflectance of EUV and X-ray multilayer mirrors, highly polished substrate surfaces with rms roughness σrms = 0,1-0.2 nm are necessary. However, the simultaneous achievement of low micro-roughness and precise surface figure is very challenging and often not accomplished. Therefore deposition techniques capable to deposit layers with smoothing properties are very desirable. One potential method that enables the formation of such layers is the pulsed laser deposition (PLD). This technique generates particles with high kinetic energies of up to several 100 eV. We investigated the deposition of carbon based smoothing layers by PLD on numerous substrates with roughness between σrms = 0.15 and 0.75 nm using different laser power densities and film thicknesses. Besides pure carbon layers we also used metal/carbon (metal = Ni, W, Pt) multilayers with respect to their capabilities to smooth surface roughness. As a general trend it turns out that a better smoothing can be obtained with higher laser power densities, whereby diamond-like carbon films are created. Furthermore, the intrinsic stress of the smoothing layers has been investigated. Due to the high kinetic energy of the impinging particles during the film growth, the layers show compressive stress. The degree of the stress depends on the concrete metal that is combined with carbon in the multilayer stack. Up to now the lowest compressive stress is obtained with Ni/C multilayers.
Development of a small Wolter type-I mirror that is mainly used as an objective for the X-ray microscope is described. Small Wolter mirrors for X-ray microscopes are fabricated by the vacuum replication method because of their long aspherical shape. Master mandrel is ground and polished by an ultra-precision NC lathe. Tungsten carbide was selected as a material because its thermal expansion coefficient is a little larger than the replica glass. It was ground by ELID (Electrolytic In-process Dressing) grinding technique that is appropriate for the efficient mirror surface grinding. After ultra-precision grinding, the figure error of master mandrel was better than 0.5μm except the boundary between the hyperboloid and the ellipsoid. Before vacuum replication, the mandrel was coated with Au (thickness 50nm) as the parting layer. Pyrex glass was empirically selected as mirror material. The master mandrel was inserted into the Pyrex glass tube and heated up to 675°C in the electric furnace. Although vacuum replication is a proper technique in terms of its high replication accuracy, the surface roughness characterized by the high spatial frequency of the mandrel was replicated less accurate than the figure error characterized by the low spatial frequency. This indicates that the surface roughness and the figure error depend on the glass surface and the figure error of the master mandrel, respectively. A fabricated mirror was evaluated by the imaging performance with a laser plasma X-ray source (λ=3.2nm).
The next generation of X-ray observatories requires large area optics, with optimal angular resolution, minimal mass, and affordable fabrication techniques. Furthermore, for survey applications, a Ritchey-Chretien or polynomial design is called for, which precludes the use of foil or glass segment cone approximations. In order to meet these requirements, we have been exploring the use of plasma spraying as a replication technology to improve shape control and stiffness with a minimal mass penalty. Our main improvements to the basic concept is the lamination of the sprayed material with electroformed Ni on the outer surface along with the electroformed Ni inner surface of the mirror. We have also used metal-coated ceramic micro-spheres for the sprayed material and controlled the substrate temperature during spraying. These enhancements show the promise of making the technology viable. An up-to-date characterization of the properties of test pieces are presented.
Magnetorheological finishing (MRF) is a production proven, sub-aperture polishing process for flat, spherical, aspherical, and cylindrical optics in the size range of 10 - 400 mm. Surface figure accuracy of better than 30 nm peak-to-valley (better than 5 nm rms), and microroughness better than 1 nm rms is routinely achieved on a variety of glasses, glass ceramics and single crystal materials. Recent work has demonstrated the applicability of MRF for larger apertures and lightweight optics. A platform capable of finishing 1000 mm apertures has already been built. Engineering studies for extending the aperture size further are underway. Finishing of large, lightweight mirrors has additional challenges because the non-uniform support of the face-sheet requires special efforts to avoid quilting errors caused by print-through of the cell structure due to fabrication processes, gravity and/or temperature effects. Unique characteristics of MRF such as a competitively high, stable removal rate, the conformal nature of the sub-aperture tool and a shear mode of material removal give it advantages in finishing this class of optics. Specifically, MRF avoids generating print-through errors and has a high rate of convergence in correcting quilting errors created by other processes, gravity or temperature effects. An additional important quality is that it has been shown that inserting MRF into a manufacturing process can substantially reduce the subsurface damage (SSD), increasing the laser damage threshold of a surface, providing advantages for use in mirror fabrication for high-energy applications. Supporting results will be given in this paper.
A hardware demonstration has been performed in which a nominally flat, complex aspheric mirror is used to correct the high-order aberrated wavefront error of an off-axis parabolic mirror to 0.5 nm rms. The purpose of the project is to demonstrate the viability of using a static, aspheric optic to correct a telescope wavefront to the degree needed for detection of extra-solar Jovian planets. The demonstration procedure and test results are presented.
In our previous study, we realized a hard X-ray focused beam having a 180nm×90nm focal size using fabricated ellip-tical mirrors. In this study, to realize a smaller focal size, more steeply curved elliptical mirrors with platinum-coated surfaces were fabricated. We showed that aspheric quality mirrors can be manufactured with recently developed ma-chining methods. We carried out line focusing tests on the elliptical mirror at the 1-km-long beamline of SPring-8. A full width at half maximum of 40 nm was achieved in the focal beam profile under the best focusing conditions.
Single-crystal silicon is one of the substrate materials often used for x-ray optical components, such as mirrors and monochromators. Silicon crystal is elastically anisotropic, mechanical properties are direction dependent. Because anisotropic analysis is complicated, isotropic approximation is commonly used in the design of optical substrates. This approximation is satisfactory in most cases. However, a full anisotropic analysis is required to precisely characterize the performance of optical substrates or determine the effect of anisotropy. In this paper, single-crystal-silicon anisotropy and its effects on deformation of bendable optics are discussed. The resulting anticlastic bending is described, and a complete numerical solution as well as approximate analytical formulations, is provided. Anisotropy can be used advantageously to accentuate anticlastic bending.
The Advanced Photon Source (APS) x-ray optics Metrology Laboratory currently operates a small-aperture Wyko laser interferometer in a stitching configuration. While the stitching configuration allows for easier surface characterization of long x-ray substrates and mirrors, the addition of mechanical components for optic element translation can compromise the ultimate measurement performance of the interferometer. A program of experimental vibration measurements, quantifying the laboratory vibration environment and identifying interferometer support-system behavior, has been conducted. Insight gained from the ambient vibration assessment and modal analysis has guided the development of a remediation technique. Discussion of the problem diagnosis and possible solutions are presented in this paper.
We derive mathematical relations for hard X-ray moire wavefront analysis with a grating interferometer. In particular, the first derivative of the wavefront phase profile and the local radius of curvature of the wavefront are related to the position and inclination of the observed moiré fringes.
The temperature gradients in a side-cooled mirror would create a thermal bending moment along the mirror length. For a slender side-cooled mirror with longitudinally uniform incident beam, the tangential slope error is primarily due to the bowing deformation caused by this thermal bending moment. The thermal bending moment depends on the temperature distribution, which is a function of the mirror geometry, heat load, and cooling design. Optimal design of a side-cooled mirror can achieve a “favorable” temperature profile to make the thermal bending moment, with respect to the substrate neutral plane, approach zero, so that the bowing deformation of the mirror is minimized. To understand the deformation of a side-cooled mirror and achieve an optimal design, a theoretical formulation is developed.
A multilayer coating of Mo/Si is usually used as an EUV optics for space science, especially for He-II (30.4nm) radiation, because it is highly stable under vacuum and atmosphere. The fairly high reflectivity of 15-20% was achieved. But space science community will need higher reflective coating at 30.4 nm radiation for the future satellite missions. In this work, for developing new multilayer mirror of He-II radiations, we report the design of a multilayer, consisting of a pair of Mg and SiC, and its fabrication, and result of the reflectance with the monthly degradation under the atmosphere circumstance.
Metrology plays more important role than machining in surface figuring at sub-nanometer accuracy. The microstitch-ing interferometry based on a microscopic interferometer having peak-to-valley (p-v) height accuracy of sub-nanometer order and lateral resolution higher than 20 mm was developed to measure surface figures of X-ray mir-ror optics. In addition, the relative angle determinable stitching interferometry was also developed to measure surface profiles of elliptical mirrors to realize hard X-ray nanofocusing. By combining the two interferometies, the absolute measurement accuracy of approximately 3 nm (peak-to-valley) was achieved in the measurement of a cylindrical sur-face having the same curvature as the elliptically designed shape to enable nanofocusing.
The spatial resolution of the scanning X-ray microscopy apparently depends on the beam size of the focused X-ray. Recently, highly accurate elliptical mirrors were reported to be fabricated, and nearly diffraction-limited line focusing was achieved. In this study, to realize diffraction-limited and 2-dimentional focusing with such highly accurate mirrors, accuracies to be realized in mirror alignings, for example, adjusting the glancing angle and the in-plane rotation, were estimated by employing two types of simulators. They are appropriately based on geometrical or wave-optical theories. They are alternatively employed according to the degree of accuracy required in the mirror alignment. A focusing unit with the adjusting mechanism fulfilling the required alignment accuracies was constructed, and the relationships between the alignment errors and focused beam profiles were quantitatively examined at the 1km-long beamline (BL29XUL) of SPring-8. Obtained results were in good agreement with the calculated results. Additionally, the alignment accuracy to be realized in the K-B unit equipping mirrors of larger NA (numerical aperture) was calculated to realize sufficient performances in focusing.