EUV Mirrors Research Articles

Development of a narrowband multilayer mirror for extracting a single order component from high harmonics in the 63–70 eV region

The Y/AlSi multilayer with narrowband performance was developed for extracting a single order component from high harmonics. We compared the performance and thermal stability of Y/AlSi with another EUV mirror, Zr/AlSi multilayer, which has shown promising performance in the EUV region. At near-normal incidence, the measured peak reflectances of Y/AlSi and Zr/AlSi multilayers are 43% and 52% at ∼63.8eV, and the energy bandwidths (FWHM) are 1.7 eV and 2.5 eV, respectively. The narrower bandwidth feature obtained in the Y/AlSi multilayer allows better suppression on adjacent high harmonic lines. After heating up to 200°C, there is no significant decrease in the reflectance of either multilayer. No decrease in reflectivity was observed after storage in air for one year.

Application of Novel Multilayer Normal-Incidence Mirrors for EUV Solar Spectroscopy

Recent significant progress in the development of multilayer normal-incidence EUV mirrors is primarily related to an increase in the reflection coefficient at the working wavelength, a decrease in the spectral width of the reflection curve, and application of high-efficiency multilayer coatings for short-wavelength (3–9 nm) and long wavelength (greater than 50 nm) spectral ranges. Such mirrors allow astrophysical study of the Sun, since relatively narrow spectral widths of the mirrors and high reflection coefficients make it possible to image corona in monochromatic lines. Telescopes based on the mirrors are promising for dynamic spectral diagnostics of the solar disk with the aid of imaging spectroscopy. The method is based on the detection of monochromatic images of the Sun using the EUV spectral lines with relatively high spatial and temporal resolution. Possible progress in the solar study related to application of specified optical elements is discussed.

CrB2 diffusion barriers for Mo/Si multilayer structures

Properties of CrB2 thin diffusion layers sputtered in Mo/Si multilayer structures and their effect on the x-ray/EUV reflectance are reported. A special attention is paid to determine a critical thickness t* for barrier layers where they isolate parent layers of Mo and Si completely and to reveal the degree of interaction of the barrier material with parent materials. A dependence of t* on the deposition rate of CrB2 is found. CrSi2 and B3Si are established as the most probable phases formed in the barrier layers. The diffusion coefficients and temperature during formation of CrB2 are estimated. The use of thin CrB2 barriers is shown to be profitable in a fabrication of the EUV mirrors with enhanced reflectivity.

Robust design of broadband EUV multilayer using multi-objective evolutionary algorithm

Considering the random fluctuations of the layer thickness, a method of robust design of broadband EUV multilayers based on multi-objective evolutionary algorithm is presented. Owing to the optimization of multi-objective evolutionary algorithm, the optical performance and robust quality of broadband Mo/Si multilayer can be optimized simultaneously, and then a set of robust designs of aperiodic EUV multilayers, which are insensitive to the thickness errors can be obtained in one single simulation run. The robust designs of broadband Mo/Si multilayers could be used to reduce the production risks of EUV mirrors, and this research demonstrates a great potential of application of multi-objective evolutionary algorithm on the design of optical thin film.

Ultraviolet out-of-band radiation studies in laser tin plasma sources

Out-of-band long wavelength emission measurements from high power, high-repetition-rate extreme-ultra-violet lithography (EUVL) laser plasma sources are imperative to estimating heat deposition in EUV mirrors, and the impact of short wavelength light transported through the imaging system to the wafer surface. This paper reports a series of experiments conducted to measure the absolute spectral irradiances of laser-plasmas produced from planar tin targets over the wavelength region of 124 to 164 nm by 1.06 μm wavelength, 10 ns full-width-at-half-maximum Gaussian laser pulses. The use of spherical targets is relevant to the EUVL source scenario. Although plasmas produced from planar surfaces evolve differently, there is a close similarity to the evolution of current from 10.6 μm CO2 laser EUVL sources, which use a pre-pulse from a lower energy solid-state laser to melt and reform an initial spherical droplet into a thin planar disc target. The maximum of radiation conversion efficiency in the 124–164 nm wavelength band (1%/2πsr) occurs at the laser intensity of 1010 W cm−2. A developed collisional-radiative model reveals the strong experimental spectra that originate mainly from the 4d105p2-4d105s5p, 4d105p-4d105s resonance lines, and 4d95p-4d95s unresolved transition arrays from Sn III, Sn IV, and Sn V ions, respectively. The calculated conversion efficiencies using a 2D radiation-hydrodynamics model are in agreement with the measurements. The model predicts the out-of-band (100–400 nm) radiation conversion efficiencies generated by both 1.06 and 10.6 μm pulses. The 10.6 μm laser pulse produces a higher conversion efficiency (12%/2πsr) at the lower laser intensity of 109 W cm−2.

Damage threshold measurements on EUV optics using focused radiation from a table-top laser produced plasma source

We present first damage threshold investigations on EUV mirrors and substrate materials using a table-top laser produced plasma source. A Schwarzschild objective with Mo/Si multilayer coatings for the wavelength of 13.5 nm was adapted to the source, generating an EUV spot of 5 microm diameter with a maximum energy density of approximately 6.6 J/cm(2). Single-pulse damage tests were performed on grazing incidence gold mirrors, Mo/Si multilayer mirrors and mirror substrates, respectively. For gold mirrors, a film thickness dependent damage threshold is observed, which can be partially explained by a thermal interaction process. For Mo/Si multilayer mirrors two damage regimes (spot-like, crater) were identified. Fused silica exhibits very smooth ablation craters, indicating a direct photon-induced bond breaking process. Silicon shows the highest damage threshold of all investigated substrate and coating materials. The damage experiments on substrates (fused silica, silicon, CaF(2)) were compared to excimer laser ablation studies at 157 nm.

Materials for Optical Lithography Tool Application

This review of materials development for optical lithography covers two areas: materials for optical steppers and scanners, and materials for EUV lithography. In the former, materials development for advanced immersion lenses and for high-index immersion lens is discussed, whereas in the latter, materials used in multilayer EUV mirrors and to generate EUV photons are discussed.

Manufacturing of XEUV mirrors with a sub-nanometer surface shape accuracy

In this paper, first results of finishing EUV mirror substrate surface shape to given parameters using correction methods such as vacuum thin films deposition and local ion-beam etching through the mask are presented. For spherical mirror substrate with radius of curvature R=260 mm and a diameter of 130 mm, obtained values of PV=4.7 nm and RMS=0.6 nm.

Source for extreme ultraviolet lithography by the tabletop storage ring MIRRORCLE

Advances of electron storage rings to beam currents of above 1 A and tabletop sizes make possible the development of a synchrotron-based source for EUV lithography (EUVL) at ~13.5-nm wavelength. The MIRRORCLE storage rings can provide on average 3-A electron beam current, 1-min lifetime, 15-ms radiation damping, and beam size ~3*3 mm2. MIRRORCLE-20SX, MIRRORCLE-6X, and MIRRORCLE-CV4 store electrons with energies of 20 MeV, 6 MeV, and 4 MeV, respectively. These machines can emit EUV from a tiny target, hit by the circulating beam, via transition radiation or diffusive radiation. Using a multilayer microelectromechanical system (MEMS) target allows enhancement and spectral purification of the emitted EUV. Aligning many such MEMS along the electron beam orbit and radiation collection by only one quasi-elliptical EUV mirror can provide EUV satisfying the joint requirements for an EUVL source.

Optimization of extreme ultraviolet mirror systems comprising high-order aspheric surfaces

When extreme ultraviolet EUV mirror systems having sev- eral high-order aspheric surfaces are optimized, the configurations sometimes enter into highly unstable regions of the parameter space. Small changes of system parameters lead then to large changes in ray paths and optimization algorithms fail. A technique applicable for any rotationally symmetric optical system that keeps the configuration away from unstable regions during optimization is described. Afinite-aberration quantity is computed for several rays, and its average change per sur- face is determined for all surfaces. For not too large values of these average changes, optimization remains stable. A design for EUV litho- graphy is discussed. © 2008 Society of Photo-Optical Instrumentation Engineers.

Interference inspection of the aspherical components of an objective for nanolithography

Methods and schemes to inspect the EUV mirror shape are developed on the basis of a point diffraction interferometer with computer processing of interferograms. A measurement accuracy to within 0.001 of a wavelength in the visual range is achieved.

Effects of ion bombardment and gas incorporation on the properties of Mo/a-Si:H multilayers for EUV applications

A study of Mo/a-Si:H multilayer for EUV mirrors produced by rf-magnetron sputtering in pure Ar, Xe and Ar/H 2 gas mixtures is presented. The high reflectance mirrors are designed for solar space experiments at 17, 19 and 30.4 nm and for the lithography applications at 13 nm. The multilayers are grown in different ion bombardment conditions determined by different bias levels of the substrate. Plasma diagnostics are performed to correctly evaluate flux and energy of plasma ions bombardment. The a-Si:H layers have been obtained by introducing H 2 gas in the sputtering chamber at different partial pressures during silicon deposition. The deposition rates as well as the composition of Mo and Si layers have been investigated by Ion Beam Analysis (RBS and ERDA). The coating microstructure has been characterised primarily by X-ray micro-diffraction (micro-XRD). X-Ray Reflection analysis (XRR) has been carried out to investigate the layer density and the multilayer structure. RBS analyses show that noble gas (Ar, Xe) incorporation is limited to the Si layers and depends on the sample bias. Noble gas concentration is correlated to the different ion bombardment growth conditions that influence also the (110) orientation of Mo nano-crystals. ERD analyses of Si layers show hydrogen incorporation up to 30 at.% as a function of the H 2 partial pressure. Hydrogen incorporation leads to a decrease of the a-Si layer density. Hydrogen content in Mo layers is less than 0.5 at.% even at the highest H 2 partial pressure. The optical properties of these mirrors have been characterised by EUV reflection measurements.

光ファイバからの点光源回折球面波を計測の絶対基準とした位相シフト光干渉計の開発

This paper describes a novel phase-shifting point diffraction interferometer (PS-PDI) that employs 2 fiber-optic point sources, as a tool for measuring surfaces for SOR and EUV mirrors with sub-nanometer accuracy using visible light optics. In conventional interferometry, the figure accuracy of the reference surface determines the measurement accuracy. We therefore need an absolute reference surface with sub-nanometer accuracy. One of the qualified surfraces is a spherical wavefront which comes from point light source. The proposed PS-PDI system uses 2 single-mode optical fibers as point light sources. The system is applied to the measurement of a commercial spherical concave mirror with 500 mm in radius of curvature and 30 mm in diameter. The repeatability of measurement for the mirror is estimated to be about 3 nm in peak-to-valley value at the wavelength of 632.8 nm in He-Ne laser.

Aberration retrieval using the extended Nijboer-Zernike approach

We give the proof of principle of a new experimental method to determine the aberrations of an optical system in the field. The measurement is based on the observation of the intensity point-spread function of the lens. To analyze and interpret the measurement, use is made of an analytical method, the so-called extended Nijboer-Zernike approach. The new method is applicable to lithographic projection lenses, but also to EUV mirror systems or microscopes such as the objective lens of an optical mask inspection tool. Phase retrieval is demonstrated both analytically and experimentally. The extension of the method to the case of a medium-to-large hole sized test object is presented. Theory and experimental results are given. In addition we present the extension to the case of aberrations comprising both phase and amplitude errors.

Multilayer dispersion optics for X-ray radiation

We report the experimental results on production of multilayer soft X-ray and EUV mirrors and their application in X-ray spectroscopy of the high-temperature plasma and used as polarizers and polarimeters. The problem of the producing and investigation of the short-period X-ray multilayers is discussed.