Nanometer flat blazed x-ray gratings using ion beam figure correction

With the development of nanometer accuracy stitching interferometry, ion beam figuring (IBF) of x-ray mirrors can now be achieved with unprecedented performance. However, the process of producing x-ray diffraction gratings on these surfaces may degrade the figure quality due to process errors introduced during the ruling of the grating grooves. To address this challenge, we have investigated the post-production correction of gratings using IBF, where stitching interferometry is used to provide in-process feedback. A concern with ion beam correction in this case is that ions will induce enough surface mobility of atoms to cause smoothing of the grating structure and degradation of diffraction efficiency. In this study we found however that it is possible to achieve a nanometer-level planarity of the global grating surface with IBF, while preserving the grating structure. The preservation was so good, that we could not detect a change in the diffraction efficiency after ion beam correction. This is of major importance in achieving ultra-high spectral resolution, and the preservation of brightness for coherent x-ray beams.

6000 lines/mm blazed grating for a high-resolution x-ray spectrometer

We have designed and fabricated a high groove density blazed grating for a Resonant Inelastic X-ray Scattering spectrometer for the new Qerlin beamline at the Advanced Light Source (ALS) synchrotron facility. The gratings were fabricated using a set of nanofabrication techniques including e-beam lithography, nanoimprint, plasma etch, and anisotropic wet etching. Two gratings with groove density of 6000 lines/mm and 3000 lines/mm and optimized for operation in the 1^st and 2^nd negative diffraction order respectively were fabricated and tested. We report on fabrication details and characterization of the gratings at beamline 6.3.2 of the ALS.

Highly efficient ultra-low blaze angle multilayer grating

We have developed an advanced process for blaze angle reduction of x-ray gratings for the soft, tender, and EUV spectral ranges. The process is based on planarization of an anisotropically etched Si blazed grating followed by a chemically selective plasma etch. This provides a way to adjust the blaze angle to any lower value with high accuracy. Here we demonstrate the reduction of the blaze angle to an extremely low value of 0.04°±0.004°. For a 100 lines/mm grating with a Mo/Si multilayer coating, the grating exhibits diffraction efficiency of 58% in the 1^st diffraction order at a wavelength of 13.3 nm. This technique will be applicable to a wide range of uses of high efficiency gratings for synchrotron sources, as well as for Free Electron Lasers (FEL).

Ultra-low blaze angle gratings for synchrotron and free electron laser applications

We have developed a method for the manufacture of x-ray diffraction gratings with arbitrarily small blaze angles. These gratings are made by a process in which a high blaze angle grating made by anisotropic etching of Si (111) is subjected to planarization and reactive ion etching. Differential etching of the planarization medium and silicon ensures reduction of the blaze angle. Repeated application of this process leads to gratings of increasing perfection with an arbitrarily small blaze angle. This opens the way to highly efficient low line density gratings, to damage resistant gratings for ultra-high power applications such as free electron lasers, and for extension of the use of gratings into the hard x-ray energy range for dispersive spectroscopy.

Large area nanoimprint enables ultra-precise x-ray diffraction gratings

A process for fabrication of ultra-precise diffraction gratings for high resolution x-ray spectroscopy was developed. A grating pattern with constant or variable line spacing (VLS) is recorded on a quartz plate by use of e-beam lithography with nanometer scale accuracy of the groove placement. The pattern is transferred to a massive grating blank by large area nanoimprint followed by dry or/and wet etching for groove shaping. High fidelity of the nanoimprint transfer step was confirmed by differential wavefront measurements. Successful implementation of the suggested fabrication approach was demonstrated by fabrication of a lamellar 900 lines/mm VLS grating for a soft x-ray fluorescence spectrometer.

Refraction effects in soft x-ray multilayer blazed gratings

A 2500 lines/mm Multilayer Blazed Grating (MBG) optimized for the soft x-ray wavelength range was fabricated and tested. The grating coated with a W/B₄C multilayer demonstrated a record diffraction efficiency in the 2nd blazed diffraction order in the energy range from 500 to 1200 eV. Detailed investigation of the diffraction properties of the grating demonstrated that the diffraction efficiency of high groove density MBGs is not limited by the normal shadowing effects that limits grazing incidence x-ray grating performance. Refraction effects inherent in asymmetrical Bragg diffraction were experimentally confirmed for MBGs. The refraction affects the blazing properties of the MBGs and results in a shift of the resonance wavelength of the gratings and broadening or narrowing of the grating bandwidth depending on diffraction geometry. The true blaze angle of the MBGs is defined by both the real structure of the multilayer stack and by asymmetrical refraction effects. Refraction effects can be used as a powerful tool in providing highly efficient suppression of high order harmonics.

Demonstration of the high collection efficiency of a broadband Mo/Si multilayer mirror with a graded multilayer coating on an ellipsoidal substrate

A graded and broadband Mo/Si multilayer mirror for EUV spectroscopy is demonstrated. This mirror has an average reflectivity profile of 16% in the wavelength region from 15 nm to 17 nm and an effective area of 1100-1500 mm(2). This reflectivity is about 4 times larger than that of a standard Mo/Si multilayer mirror on a 1 in. diameter substrate, showing that the mirror can be used for measuring EUV fluorescence at wavelengths in the region around 15 nm to 17 nm.

Aqueous solution/metal interfaces investigated in operando by photoelectron spectroscopy

We describe a new in operando approach for the investigation of heterogeneous processes at solid/liquid interfaces with elemental and chemical specificity which combines the preparation of thin liquid films using the meniscus method with standing wave ambient pressure X-ray photoelectron spectroscopy [Nemšák et al., Nat. Commun., 5, 5441 (2014)]. This technique provides information about the chemical composition across liquid/solid interfaces with sub-nanometer depth resolution and under realistic conditions of solution composition and concentration, pH, as well as electrical bias. In this article, we discuss the basics of the technique and present the first results of measurements on KOH/Ni interfaces.

Enhancement of diffraction efficiency via higher-order operation of a multilayer blazed grating

Imperfections in the multilayer stack deposited on a saw-tooth substrate are the main factor limiting the diffraction efficiency of extreme ultraviolet and soft x-ray multilayer-coated blazed gratings (MBGs). Since the multilayer perturbations occur in the vicinity of antiblazed facets of the substrates, reduction of the groove density of MBGs is expected to enlarge the area of unperturbed multilayer and result in higher diffraction efficiency. At the same time the grating should be optimized for higher-order operation in order to keep high dispersion and spectral resolution. In this work we show the validity of this approach and demonstrate significant enhancement of diffraction efficiency of MBGs using higher-order diffraction. A new record for diffraction efficiency of 52% in the second diffraction order was achieved for an optimized MBG with groove density of 2525  lines/mm at the wavelength of 13.4 nm.

Ultra-high efficiency multilayer blazed gratings through deposition kinetic control

Diffraction efficiency of multilayer-coated blazed gratings (MBG) strongly depends on the perfection of the sawtooth-shaped layers in the overall composite structure. Growth of multilayers on sawtooth substrates should be carefully optimized to reduce groove profile distortion and, at the same time, to avoid significant roughening of multilayer interfaces. In this work, we report on a way to optimize growth of sputter-deposited Mo/Si multilayers on sawtooth substrates through variation of the sputtering gas pressure. We believe a new record for diffraction efficiency of 44% was achieved for an optimized MBG with groove density of 5250 lines/mm at the wavelength of 13.1 nm.

Suppression of near-Fermi level electronic states at the interface in a LaNiO3/SrTiO3 superlattice

Standing-wave-excited photoemission is used to study a SrTiO3/LaNiO3 superlattice. Rocking curves of core-level and valence band spectra are used to derive layer-resolved spectral functions, revealing a suppression of electronic states near the Fermi level in the multilayer as compared to bulk LaNiO3. Further analysis shows that the suppression of these states is not homogeneously distributed over the LaNiO3 layers but is more pronounced near the interfaces. Possible origins of this effect and its relationship to a previously observed metal-insulator-transition in ultrathin LaNiO3 films are discussed.

A 10,000 groove/mm multilayer coated grating for EUV spectroscopy

Ultra-high spectral resolution in the EUV and soft x-ray energy ranges requires the use of very high line density gratings with optimal design resulting in use of a Blazed Multilayer Grating (BMG) structure. Here we demonstrate the production of near-atomically perfect Si blazed substrates with an ultra-high groove density (10,000 l/mm) together with the measured and theoretical performance of an Al/Zr multilayer coating on the grating. A 1st order absolute efficiency of 13% and 24.6% was achieved at incidence angles of 11° and 36° respectively. Cross-sectional TEM shows the effect of smoothing caused by the surface mobility of deposited atoms and we correlate this effect with a reduction in peak diffraction efficiency. This work shows the high performance that can be achieved with BMGs based on small-period anisotropic etched Si substrates, but also the constraints imposed by the surface mobility of deposited species.

Damage mechanisms of MoN/SiN multilayer optics for next-generation pulsed XUV light sources

We investigated the damage mechanism of MoN/SiN multilayer XUV optics under two extreme conditions: thermal annealing and irradiation with single shot intense XUV pulses from the free-electron laser facility in Hamburg - FLASH. The damage was studied "post-mortem" by means of X-ray diffraction, interference-polarizing optical microscopy, atomic force microscopy, and scanning transmission electron microscopy. Although the timescale of the damage processes and the damage threshold temperatures were different (in the case of annealing it was the dissociation temperature of Mo2N and in the case of XUV irradiation it was the melting temperature of MoN) the main damage mechanism is very similar: molecular dissociation and the formation of N2, leading to bubbles inside the multilayer structure.

Single shot damage mechanism of Mo/Si multilayer optics under intense pulsed XUV-exposure

We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference-polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources.

Tri-material multilayer coatings with high reflectivity and wide bandwidth for 25 to 50 nm extreme ultraviolet light

Magnesium/silicon carbide (Mg/SiC) multilayers have been fabricated with normal incidence reflectivity in the vicinity of 40% to 50% for wavelengths in the 25 to 50 nm wavelength range. However many applications, for example solar telescopes and ultrafast studies using high harmonic generation sources, desire larger bandwidths than provided by high reflectivity Mg/SiC multilayers. We investigate introducing a third material, Scandium, to create a tri-material Mg/Sc/SiC multilayer allowing an increase the bandwidth while maintaining high reflectivity.

Efficient high-order diffraction of extreme-ultraviolet light and soft x-rays by nanostructured volume gratings

We report what is believed to be the first demonstration that volume gratings diffract extreme-ultraviolet light (EUV) or soft x-rays into high orders approximately an order of magnitude more efficiently than predicted by classical thin-grating theory. At the 13-nm wavelength, copolymer grating structures with 200-nm period and aspect ratios of ~10:1 achieved diffraction efficiencies of 11.2%, 15.3%, 11.5%, and 9.1% in the orders m of 2, 3, 4, and 5, respectively. In addition, the measured transmission spectra are consistent with electrodynamic calculations by coupled-wave theory. High-order diffraction can now be employed for substantially improved diffractive EUV and x-ray optics, e.g., highly resolving diffractive lenses and large-aperture condensers.

Experimental technique for radiative-process-resolved X-ray absorption spectroscopy at the inner-shell excitation thresholds

Partial-fluorescence-yield (PFY) x-ray absorption measurements, rising the optimized window widths of position sensitive detectors in wave-length dispersive x-ray spectrometers, have been applied for radiative process-resolved (RPR) x-ray absorption spectroscopy. We have measured PFY-absorption spectra of graphite and diamond at the C K threshold and of h-BN and c-BN at the B K threshold. Resonant elastic x-ray scattering was observed in graphite and h-BN on their PFY-absorption spectra, and excitonic x-ray scattering was observed in diamond and c-BN. These results show that PFY-absorption measurements for RPR x-ray absorption spectroscopy can provide the information about the electronic structures and the radiative-decay process in inner-shell excitation.

Transmission phase gratings for EUV interferometry

The performance of the recently developed EUV phase-shifting point diffraction interferometer (PS/PDI) depends heavily on the characteristics of the grating beamsplitter used in the implementation. Ideally, such a grating should provide throughput of better than 25% and diffraction efficiency, defined as the ratio of the first-diffracted-order power to the zero-order power, variable in the range from approximately 10 to 500. The optimal method for achieving these goals is by way of a phase grating. Also, PS/PDI system implementation issues favor the use of transmission gratings over reflection gratings. Here, the design, fabrication, and characterization of a recently developed transmission phase grating developed for use in EUV interferometry is described.

At-wavelength, system-level flare characterization of extreme-ultraviolet optical systems

The extreme-ultraviolet (EUV) phase-shifting point-diffraction interferometer (PS/PDI) has recently been developed to provide high-accuracy wave-front characterization critical to the development of EUV lithography systems. Here we describe an enhanced implementation of the PS/PDI that significantly extends its measurement bandwidth. The enhanced PS/PDI is capable of simultaneously characterizing both wave front and flare. PS/PDI-based flare characterization of two recently fabricated EUV 10x-reduction lithographic optical systems is presented.

Absolute Photoabsorption Measurements of Molybdenum in the Range 60-930 eV for Optical Constant Determination

Transmission measurements for the optical constants delta, beta of thecomplex refractive index n = 1 - delta + ibeta of molybdenum are performed in the energy range 60-930eV. Free-standing C/Mo/C foils of five different thicknesses are used, and the results are normalized for the presence of the carbon layers in the samples. These absorption results are combined with previous experimental data in the lower energy range and values fromthe atomic tables to obtain the imaginary (absorptive) part of the refractive index for molybdenum in the range 1-30,000 eV. The real(dispersive) part of n was calculated from Kramers-Kronig analysis with the above absorption data. An evaluation with the partial sum rules demonstrates that this new compilation provides an improved set of values for n covering a wider energy range compared with the current tabulated values. The new results are applied so as to calculate thenormal-incidence reflectivities of Mo/Si and Mo/Be multilayer mirrors.

Reflectance measurements on clean surfaces for the determination of optical constants of silicon in the extreme ultraviolet-soft-x-ray region

The refractive index n = 1 - delta + ibeta of Si in the energy range 50-180 eV is investigated with angle-dependent reflectance measurements. The optical constants delta and beta are both determined by fitting to the Fresnel equations. The results of this method are compared with the values in the atomic tables derived from experimental data for beta and implementation of the Kramers-Kronig relations for delta. The samples were prepared by UV irradiation and HF:ethanol dipping to H passivate the surface. It is found that the values of delta in the atomic tables are 8-15% too high in the region 50-90 eV. This is attributed to missing oscillator strength in the tabulated absorption coefficient for Si. The measured values of beta for crystalline Si exhibit structure below the L (2,3) edge (99.8 eV), as was previously observed in transmission measurements of Si(111). It is also found that the method of least-squares fitting reflectance data to obtain optical constants is most effective for energies well below the edge, where delta > beta, while for a range of energies around and above the edge, where delta < beta, the optical constants are determined with large uncertainties. This behavior is not unique to the Si L(2,3) edge.

Numerical and experimental study of disordered multilayers for broadband x-ray reflection

The effect of layer thickness disorder in periodic multilayers on x-ray reflectivity is investigated numerically and experimentally. We present ensemble calculations, taking into account absorption and interfacial roughness. It is demonstrated that layer thickness disorder yields band broadening and increased integrated reflectivity For applications we concentrate on extrema of the ensembles, giving the highest integrated reflectivity We develop global optimization methods that can also be used to generate specified reflection band structures. In a few examples, applications of the optimization methods are discussed. To illustrate the practical applicability of the methods, we compare experimental realizations to the calculation. In one case we achieve a 42% increase in integrated reflectivity in the 130 Å < λ < 190 Å spectral range with respect to a periodic multilayer with its first-order Bragg peak in the center of that range. Accurate control of layer thicknesses is our main experimental obstacle.

Large-area avalanche photodiodes for the detection of soft x rays

The charge-collection efficiency of beveled-edge-type silicon avalanche photodiodes has been determined for soft x rays in the 50-300-eV range. An efficiency of greater than 80% is measured for energies below the Si L absorption edge. The measured efficiency is well described by a model that accounts for absorption in an oxide overlayer and recombination at the front surface of the diode. The avalanche photodiodes are shown to be significantly more sensitive compared with other detectors for pulsed sources such as a laser-produced plasma source. These results are also very encouraging for soft-xray/ extreme-UV applications involving synchrotron radiation.

Molybdenum/beryllium multilayer mirrors for normal incidence in the extreme ultraviolet

We report on a series of normal-incidence reflectance measurements at wavelengths just longer than the beryllium K-edge (11.1 nm) from molybdenum/beryllium multilayer mirrors. The highest peak reflectance was 68.7 ± 0.2% at λ = 11.3 nm obtained from a mirror with 70 bilayers ending in beryllium. To our knowledge, this is the highest normal-incidence reflectance that has been demonstrated in the 1-80-nm spectral range.

Masked deposition techniques for achieving multilayer period variations required for short-wavelength (68-A) soft-x-ray imaging optics

Practical issues in the development of multilayer coatings for reflective imaging systems operating at λ ≈ 68 Å are discussed. The 1% bandpass of Ru/B(4) C multilayers at this short wavelength imposes stringent tolerances with which the actual multilayer period variation across the curved surfaces must match the ideal period variation for a 20× demagnifying Schwarzschild objective. New deposition techniques that use masks to correct the period variation across the curved surfaces of each optic have been developed to ensure reflectance over the entire clear aperture. The narrow bandpass together with steep lateralperiod gradients and steeply curved surfaces requires improved metrology for an acceptable period variation to be obtained and the overlap of the reflectance peaks on the two mirrors to be verified.

Tarnishing of Mo/Si multilayer x-ray mirrors

Multilayer x-ray mirrors of molybdenum and silicon operating at normal incidence at energies just below the Si L(II,III) absorption edges are a key component in the development of soft-x-ray projection lithography. In this application high reflectivity is essential. Aging tests on such reflectors, with Mo as the last layer deposited, show that the structures decline in reflectivity with time when stored in air. Chemical analysis of a well-aged surface by photoelectron spectroscopy techniques reveals that the uppermost Mo layer eventually becomes completely oxidized to MoO(3) and MoO(2) and contaminated with carbonaceous materials. The oxidation can be prevented by storing the mirrors in an oxygen-free atmosphere or by depositing the silicon as the top layer. The reflectivity of tarnished mirrors can be restored by removing the oxides by argon-ion etching or wet chemical methods.

A Soft X-Ray/EUV Reflectometer Based on a Laser Produced Plasma Source

A soft x-ray reflectometer is described which is based on a laser-produced plasma source and is continuously tunable over the range 40 Å < λ < 400 Å. The source is produced by focusing 0.532-μm light from a Q-switched Nd:YAG laser on a solid target. The x-ray wavelength is defined using a high throughput spherical grating monochromator with moderate resolving power (λ/Δλ ≈ 100 to 500). A time-averaged monochromatized flux of more than 109 photons/s in a 1% bandwidth at 100 eV is obtained. Photon "shot noise" limited measurements are obtained by the use of an I0 detector to normalize out the shot-to-shot variations in source intensity. Measurements with submillimeter spot sizes are readily obtainable. Various detectors have been used and the advantages and disadvantages of each are discussed. The higher order contamination of the monochromator output has been analyzed using a second grating for the purpose of making measurement corrections. The reflectometer thus provides the capability for precision absolute measurements of the reflectance of gratings and multilayer mirrors, the transmittance of thin film filters, or other properties of x-ray optical elements.

Design and Characterization of X-Ray Multilayer Analyzers for the 50-1000 eV Region

This report describes a synthesis of more than 10 years of this program's development and application of multilayer analyzers for absolute Bragg spectrometry in the low-energy x-ray region of 50-1000 eV. Multilayers, denned here as systems of periodic layered structures parallel to the analyzer surface, have been applied principally in the diagnostics and application of the new, intense sources of synchrotron and high-temperature plasma x radiation. Detailed absolute reflectivity characterizations are presented for selected examples of these multilayers which have been semiempirically determined for mica, potassium acid phthalate, and the fabricated Langmuir-Blodgett and sputtered multilayer analyzers with d-spacings in the 10-200 Å range. Design requirements for absolute spectrometry are established. Efficient analytical multilayer reflectivity models are derived and parameterized (based upon a modification of the Darwin-Prins model for the low-energy x-ray region), including, for the sputtered multilayers, parameters for defining interface structure. The dependence of the reflectivity characteristics, high-order Bragg diffraction suppression, and overall efficiency upon the model parameters is analyzed. A special spectrograph and procedure for the absolute measurement of the relevant reflectivity characteristics are described. Detailed measurements and semiempirical characterizations are presented. Programs for small laboratory computers have been developed that allow rapid and flexible spectral analysis, transforming measured spectra to absolute Spectra.