Free-standing Transmission Gratings Research Articles

Effect of sidewall roughness on the diffraction efficiency of EUV high aspect ratio freestanding transmission gratings.

Manufacturing-induced sidewall roughness has a significant impact on the diffraction efficiency of extreme ultraviolet (EUV) gratings and masks, which could be evaluated by a Debye-Waller damping factor. The rough profile models of line structures are always parallel to the surface for the reflective elements. In this manuscript, a model of rough lines along the thickness direction is established, which cannot be ignored for high aspect ratio transmission gratings. Numerical calculations are carried out using both a rigorous model and a Fraunhofer approximation model. The two models agree with each other on the low-order transmission efficiencies, and the fitted Debye-Waller factor indicates a larger roughness value than that of the model due to the absorption of EUV irradiation for 90° sidewall angle. When the sidewall angle is smaller than 88°, an extra degree of freedom is introduced to the traditional Debye-Waller factor-based formula. The +1-order transmission efficiency and absorptivity with smooth and rough sidewalls are also analyzed, as well as the effect of incidence angle, wavelength and grating thickness.

Transmission grating based imaging spectrometers in the XUV and VUV for various plasmas

This report describes the design of imaging survey spectrometers in the XUV and the VUV ranges respectively. These survey spectrometers provide space resolved spectra using free standing transmission gratings and CCD based direct photon detection. The spectrometers were tested on a table top reflex discharge. Spectra shown in this report highlight the space and spectral resolving capabilities of the spectrometers.

Transmission-grating spectrometer for highly efficient and high-resolution soft X-ray emission studies

A high-resolution soft X-ray emission spectrometer has been developed for site-specific electronic structure analysis of functional materials in the photon energy range of 50–600eV on the in-vacuum undulator beamline BL3U of the UVSOR facility. In order to realize the high detection efficiency without sacrificing the energy resolution to the signal intensity, the present X-ray emission spectrometer adopts an optical design, which was originally adopted in the field of astrophysics such as Chandra X-ray observatory. This new-type X-ray emission spectrometer consists of a Wolter type I mirror, a freestanding transmission grating, and a charge coupled device (CCD) detector, allowing the focusing with 20 times larger acceptance angle (1.5×10−3sr) than commercially available compact spectrometers with reflection-type gratings such as VG-SCIENTA XES350. The X-ray emission energy resolution of 15–34meV has successfully been achieved in the photon energy of 60–100eV by using a transmission grating with the groove density of 5555lines/mm.

Design and fabrication of 1000 line/mm soft X-ray freestanding blazed transmission gratings

The performance prediction of freestanding blazed transmission grating is discussed using scalar diffraction theory, and grating design parameters are accodingly given. A freestanding transmission grating with a period of 1 m, duty cycle of 0.10.2, aspect ratio of about 100, thickness of 10 m and fractional area of 65% is successfully fabricated by holographic lithography and anisotropic wet etching. The size of a single die is 15 mm 15 mm divided into four 5 mm 5 mm windows. The diffraction efficiency of the fabricated grating is tested at the National Synchrotron Radiation Laboratory in a wavelength region of 550 nm. The wavelength scanning results show a strong blazing effect in the direction of specular reflection from mirror-like grating sidewalls, as expected. The measured and normalized diffraction efficiency is consistent with the theoretical prediction within 38%49% due to grating structural imperfections. These experimental results prove not only the concept of blazed transmission grating but also practicability of the fabrication process.

Stand-alone diamond binary phase transmission gratings for the EUV band

We report on the development of true free-standing phase transmission gratings for the extreme ultraviolet band. An ultra-nanocrystalline, 300 nm thin diamond film on a backside etched silicon wafer is structured by electron-beam lithography to periods of 1 μm. In this way, flat and stable gratings of 400 μm in diameter are fabricated. First-order net efficiencies up to 28% are obtained from measurements at a synchrotron beamline within a wavelength range from 5.0 nm to 8.3 nm, whereas the 0th order is suppressed to 1% near 6.8 nm. Higher diffraction orders up to the 3rd one contribute less than 7% in sum to the far-field pattern.

Efficient extreme ultraviolet transmission gratings for plasma diagnostics

We report on a theoretical study of free-standing phase transmission gratings for high-resolution extreme ultraviolet (EUV) and soft x-ray spectroscopy and investigate their properties. Designed for wavelengths between about 2 and 40 nm, the devices may provide a first order diffraction efficiency beyond 30%. We use rigorous coupled wave analysis methods in order to optimize the grating design parameters and discuss features of segmented grating arrays. Elemental, as well as compound, materials such as Be, Mo, LiF, and poly-(methylmethacrylate) are considered with respect to their potential and practical limitations in terms of feasibility and sensitivity to radiation damage. Simulations are performed for several samples on the radiation produced by a table-top EUV plasma source and applications to astrophysical problems are considered.

Diffraction efficiency of 200-nm-period critical-angle transmission gratings in the soft x-ray and extreme ultraviolet wavelength bands

We report on measurements of the diffraction efficiency of 200-nm-period freestanding blazed transmission gratings for wavelengths in the 0.96 to 19.4 nm range. These critical-angle transmission (CAT) gratings achieve highly efficient blazing over a broad band via total external reflection off the sidewalls of smooth, tens of nanometer thin ultrahigh aspect-ratio silicon grating bars and thus combine the advantages of blazed x-ray reflection gratings with those of more conventional x-ray transmission gratings. Prototype gratings with maximum depths of 3.2 and 6 μm were investigated at two different blaze angles. In these initial CAT gratings the grating bars are monolithically connected to a cross support mesh that only leaves less than half of the grating area unobstructed. Because of our initial fabrication approach, the support mesh bars feature a strongly trapezoidal cross section that leads to varying CAT grating depths and partial absorption of diffracted orders. While theory predicts broadband absolute diffraction efficiencies as high as 60% for ideal CAT gratings without a support mesh, experimental results show efficiencies in the range of ∼50-100% of theoretical predictions when taking the effects of the support mesh into account. Future minimization of the support mesh therefore promises broadband CAT grating absolute diffraction efficiencies of 50% or higher.

Design, fabrication and test of X-ray freestanding transmission gratings

The diffraction efficiency of freestanding x ray transmission gratings is discussed in relation to the wavelength and grating period using both scalar diffraction theory and rigorous coupled wave analysis, and the grating design is accordingly optimized. A gold freestanding x ray transmission grating with period of 300 nm, line/period ratio of 055, thickness of 200 nm a size of 1 mm ×1 mm and fractional grating area of 65% is fabricated. The absolute diffraction efficiency of the fabricated grating was tested at the National Synchrotron Radiation Laboratory in the wavelength region of 55— 38 nm. The test result shows that the grating has a maximum efficiency of nearly 10%. The grating also shows a stable diffraction efficiency in the wavelength region from 15 nm to 35 nm.

Freestanding diffractive optical elements as light extractors for burning plasma experiments

Optical diagnostics will be critical for the operation and performance assessment of burning plasma experiments, such as ITER. At the same time, extracting light for these diagnostics with reflective mirrors becomes difficult in the burning plasma environment due to the deleterious effects of the prolonged exposure on plasma and nuclear radiations. As an alternative, we explore the possibility to use freestanding diffractive optical elements, such as transmission gratings and zone plates, as light extractors. Since in diffractive systems, light is deflected by periodic slits rather than by a surface, these may withstand plasma exposure with less degradation of their optical properties. To investigate this possibility, we developed freestanding transmission gratings for the visible range and exposed them to conditions resembling (or even exceeding) those expected for the ITER “first mirrors.” The results of this study indicate that the gratings can withstand high heat fluxes and plasma and energetic radiation bombardment. Additionally, in contrast to the reflective elements, the extraction efficiency of diffractive elements may even improve with plasma exposure, which is possibly due to the shaping and thinning of the grating bars by plasma erosion. Moreover, in tightly collimated configurations, even very thin gratings can be used to extract light from hot fusion plasmas, as demonstrated by our tests of an extreme ultraviolet extractor at the National Spherical Torus Experiment.

Diffraction of 0.5 keV electrons from free-standing transmission gratings

A nanostructured grating was used to diffract a low-energy (500 eV) electron beam, and the current transmitted into the zeroth diffraction order was greater than 5% of the incident beam current. This diffraction efficiency indicates that the 55-nm-wide grating bars absorb electrons but the 45-nm-wide slots between bars transmit electron de Broglie waves coherently. The diffraction patterns can be asymmetric, and can be explained by a model that incorporates an electrostatic potential energy for electrons within 20 nm of the grating structure calculated by the method of images.

Design of a novel transmission-grating spectrometer for soft X-ray emission studies

The design of a transmission-grating spectrometer for high-resolution soft X-ray emission studies has been proposed. It is different from conventional types of soft X-ray emission spectrometers; that is, the spectrometer has a Wolter type I mirror, a free-standing transmission grating, and a back-illuminated CCD. A high collection angle up to 1.5 × 10 −3 sr is achieved by utilizing the Wolter mirror as a prefocusing system. The CCD is mounted at 1400 mm downstream of the grating on a Rowland torus mount. Diffracted X-rays are detected by the CCD in the normal incidence geometry, resulting in high detection efficiency. The energy resolution is limited by the figure errors of the optical elements and the spatial resolution of the detector. The ray-tracing results confirm that the aberrations do not practically degrade the energy resolution.

Extreme-ultraviolet efficiency measurements of freestanding transmission gratings.

We report new, near-normal-incidence, transmission grating efficiency results at selected extreme-ultraviolet wavelengths between 4.5 and 30.5 nm for two transmission gratings, one with a period of 200 nm and the other with a period of 400 nm. These gratings consist of opaque gold bars separated by open spaces that have been produced by photolithography techniques commonly used to produce electronic components. The gold bars and the open spaces are nominally of the same width. Both gratings have a thickness of 470 nm. The transmission efficiency at the central, first, and, when possible, second order of diffraction was measured. In addition, guided-wave phenomena at nonnormal angles of incidence, as well as transmission differences depending on which side of the grating was illuminated, were investigated. The observed guided-wave effects allow one to selectively enhance the transmission of the grating at desired wavelengths, as is realized with a blazed reflection grating.

Probing attosecond pulse trains using “phase-control” techniques

We propose a method for the measurement of attosecond beating structures that result from the superposition of harmonics of laser radiation. It utilizes the phase control of the excitation probability of atoms through different interfering channels. The channels consist of multiphoton excitation by the fundamental frequency and single-photon excitation by the superposition of the harmonic fields, or alternatively, by each harmonic or frequency interval within the bandwidth of each harmonic. The method is sensitive to the relative phase of the harmonics, including phases introduced by chirp, and thus allows evaluation of the temporal structure of the superimposed harmonic fields. An experimental setup for the implementation of the method based on freestanding transmission grating is suggested as well.

A liquid-xenon-jet laser-plasma x-ray and EUV source

We describe a laser-plasma soft-x-ray source based on a cryogenic-xenon liquid-jet target. The source is suitable for extreme ultraviolet (EUV) projection lithography and proximity x-ray lithography (PXL). Absolute calibrated spectra in the 1–2 nm range and uncalibrated spectra in the 9–15 nm range are obtained using a free-standing transmission grating and a CCD-detector.

Effects of gap width on vacuum-ultraviolet transmission through submicrometer-period, freestanding transmission gratings

The effects of gap width on the transmission coefficient of vacuum-ultraviolet light through submicrometer-period, freestanding transmission gratings are reported. Results from computations and an analytical waveguide model are shown to be consistent with experimental measurements. These results show that thin gratings with narrow gaps and thick gratings with wide gaps are equally effective at eliminating 121.6-nm radiation. The thin gratings with the narrow gaps have the advantage of better attenuation of shorter-wavelength radiation than the thick gratings with the larger gaps.

Transmission grating filtering of 52–140-nm radiation

Filtering extreme UV radiation by gold freestanding transmission gratings was studied experimentally in the 52-140-nm wavelength range. Computer simulations are in good agreement with experimental results.

Extreme-ultraviolet radiation filtering by freestanding transmission gratings

Measurement of energetic neutral atoms fluxes in space requires efficient suppression of exceptionally strong background extreme-ultraviolet (EUV) and UV radiation. Diffraction filters make it possible to separate (transmit) charged and neutral particles from the background radiation (which would be suppressed). Recently developed freestanding transmission gratings look especially promising for implementation in a new family of diffraction EUV/UV filters. The first results of our experimental study of filtering properties of freestanding transmission gratings with a period of 200 nm are presented. The grating transmission was measured in the 52-131-nm wavelength range, and grating polarization properties were determined at 58.4 nm. It is shown that transmission gratings can be used efficiently as filters and polarizers in the EUV/UV spectral range.

Extreme-ultraviolet polarization and filtering with gold transmission gratings

The polarization and transmission characteristics of freestanding gold transmission gratings, with 200-nm periods, for extreme-ultraviolet (EUV) radiation (1 < 200 nm) have been measured. We find that EUV transmission through the gratings is dominated by the waveguide characteristics of the gratings and that polarization efficiencies of 90% for wavelengths of 121.6 nm are achievable. Both the EUV polarization and transmission properties are in good agreement with a complete vector, numerical solution of Maxwell's equations. The fraction of open area to total area of the grating has been measured using a 10-keV proton beam and was found to be in good agreement with the microscopic slit and wire dimensions that were obtained by scanning electron microscopy. The use of these gratings for particle measurements in the presence of intense EUV radiation is briefly discussed.

Novel low-energy neutral atom imaging technique

Recently proposed low-energy neutral atom (LENA) imaging techniques rely on collisional processes to convert LENAs into ions to separate the neutrals from the intense UV radiation background. At low energies, these collisional processes have poor conversion efficiencies and limit the angular resolution of these devices. However, if the intense UV light background can be suppressed, direct LENA detection is possible. We present results from a series of experiments designed to develop a novel filtering structure based on free-standing gold transmission gratings. If the grating period is sufficiently small, the gratings can substantially polarize UV light in the wavelength range 300 to 1500 Å. If a second grating is placed behind the first grating with its axis of polarization oriented perpendicular to that of the first, considerable attenuation of the UV radiation is achievable. The neutrals pass through the remaining open area of two gratings and are directly detected. We have obtained nominal 2000-Å-period (1000-Å bars with 1000-Å slits) gratings and measured their UV and atomic transmission characteristics. The geometric factor of a LENA imager based on this technology is comparable to that of other proposed LENA imagers, with a significantly better angular resolution.

Normal incidence spectrophotometer with high-density transmission grating technology and high-efficiency silicon photodiodes for absolute solar extreme-ultraviolet irradiance measuremen

New developments in transmission grating and photodiode technology now make it possible to realize spectrometers in the extreme ultraviolet (EUV) spectral region (λ<1000Å), which are expected to be virtually constant in their diffraction and detector properties. Time-dependent effects associated with reflection gratings are eliminated through the use of free-standing transmission gratings. These gratings together with recently developed and highly stable EUV photodiodes have been utilized to construct a highly stable normal incidence spectrophotometer to monitor the variability and absolute intensity of the solar 304-Å line. Owing to its low weight and compactness, such a spectrometer will be a valuable tool for providing absolute solar irradiance throughout the EUV. This novel instrument will also be useful for cross-calibrating other EUV flight instruments and will be flown on a series of Hitchhiker shuttle flights and on SOHO. A preliminary version of this instrument has been fabricated and characterized, and the results are described.