SiC Mirror Research Articles

X-ray instrumentation for a focused wiggler beamline at the EMBL Outstation Hamburg

A flexible setup consisting of two independent water-cooled X-ray monochromators providing radiation for two separate beamlines (A and B) has been installed on the multipole wiggler BW7 at the DORIS III storage ring. The first optical element of both lines is a Rh-coated SiC mirror that reduces the heat load on subsequent energy defining elements. Line B, which will be predominantly used for protein crystallography, is optimized for flux and employs a triangular monochromator and segmented glass mirror for horizontal and vertical focusing, respectively; it can be operated in a wavelength range from 0.5 to 1.8 Å. Line A features a high-energy resolution fixed exit, nondispersive double monochromator, optimized for accurate and fast energy scans. It will mainly be used for anomalous scattering, EXAFS, and small-angle scattering. The use of encoders, intelligent stepper/dc-motor controllers, and position sensitive beam monitors allow a comfortable and fast alignment of all axes. Both experimental hutches are equipped with large motorized optical tables, offering a versatile and convenient mounting surface for experiments.

Reflectivities of chemical vapor deposition-diamond mirrors in the vacuum ultraviolet region

Diamond mirrors have been fabricated by depositing a diamond thin film on a Si3N4 substrate by means of chemical vapor deposition (CVD). The reflectivities of the CVD diamond mirrors in the vacuum ultraviolet region from 30 to 2000 Å are reported and compared with those of SiC and gold-coated SiC mirrors measured in this work. The optical constants of the CVD diamond film are obtained from the reflectivities measured and also compared with those of a bulk crystal reported in the literature.

High heat load vacuum ultraviolet mirror development in Japan

Developments of vacuum ultraviolet (VUV) mirrors and optical elements for intense synchrotron radiation in Japan are reported. High heat load tests of chemical-vapor-deposited silicon carbide (CVD SiC) mirrors, 500 x 50 x 40 mm 3 , were successfully made for undulator radiation with heat flux of 0.4 W/mm 2 . As extensions of various CVD SiC mirrors developed at the Photon Factory, the largest CVD SiC mirror with sintered SiC substrate, 1000 x 140 x 25 mm 3 , and the thin CVD SiC mirror, 300 x 50 x 5 mm 3 , were fabricated by the SP-ring 8 project. Heat load tests of a CVD-SiC-based grating and a multilayer mirror exhibit possibilities of making high heat load optical elements in the VUV and soft x-ray (SX) regions. Measurements of optical properties of CVD SiC mirrors produced by different methods showed that the optical constants in the VUV and SX regions were almost equal to one another. A profilometer for measuring surface figures of large-size aspheric mirrors was constructed on the basis of the Twyman-Green interferometer with heterodyne phase detection method.

Development and Surface Evaluation of Large SiC X-Ray Mirrors for High-Brilliance Synchrotron Radiation

Flat SiC mirrors coated with Pt, of which size are 800?50 mm2 and 1000?140 mm2 in area, are developed for high-brilliance synchrotron radiation. The large mirrors are well polished over the entire area from the viewpoint of surface roughness for application to the use for the synchrotron radiation beamline. The performance of the large mirror surface is evaluated by the measurements of not only reflectivity near the critical angle but also of the angle-resolved scattering (ARS) curve as a function of mirror surface position at several glancing angles with CuK? X-rays from a rotating target at the 30-m-long X-ray beamline facility of ISAS. By analyzing the ARS curves the power spectral density functions are derived as a function of the surface wave number. It is suggested that the large mirror may have a fractal surface finish like that of small mirrors. Evaluation of the longitudinal mirror curvature on the order of 10 km from the reflected beam profile width is also described. The results are compared with the measurements of the laser interferometers.

Fixed abrasive grinding of CVD SiC mirrors

A strategy for machining ceramic (silicon carbide) mirrors is described. By taking advantage of the relative ductility of ceramics compared with that of glass, it is shown that fixed-abrasive, ultraprecision grinding is a viable fabrication process for high-performance aspheric mirrors. Also, it is shown that ductile grinding of ceramic substrates can produce optical quality surfaces, particularly in infrared applications. Laboratory-scale ductile-regime grinding of chemically vapor deposited silicon carbide is described, as is a scale-up of the technology to a commercially available ultraprecision machine tool that has been retrofitted for grinding aspheric ceramic components.

Special Sensor Ultraviolet Limb Imager: an ionospheric and neutral density profiler for the Defense Meteorological Satellite Program satellites

The Naval Research Laboratory is developing a series of far- and extreme-ultraviolet spectrographs (800 to 1700 Å) to measure altitude profiles of the ionospheric and thermospheric airglow from the U.S. Air Force Defense Meteorological Satellite Program's Block 5D3 satellites. These spectrographs, which comprise the Special Sensor Ultraviolet Limb Imager (SSULI), use a near-Wadsworth optical configuration with a mechanical grid collimator, concave grating, and linear array detector. To image the limb, SSULI employs a rotating planar SiC mirror that sweeps the field of view perpendicular to the limb of the Earth. In the primary operating mode, the mirror sweeps the instrument field of view through 17 deg to view tangent heights from about 50 to 750 km. The SSULI detectors use microchannel plate intensification and wedge-and-strip decoding anodes to resolve 256 pixels in wavelength dispersion. The detector is windowless and uses an o-ring sealed door to protect the Csl photocathode from exposure prior to insertion in orbit. The altitude distributions of the airglow measured by the SSULI sensors will be used to infer the altitude distributions of electrons and neutral species. At night, electron densities will be determined by measurement of ion recombination nightglow. Daytime electron densities will be obtained from measurements of multiple resonant scattering of O⁺ 834-Å radiation produced primarily by photoionization excitation of atomic oxygen. Dayside neutral densities and temperatures will be inferred from the measurement of dayglow emissions from N₂ and O produced by photoelectron impact excitation.

Generation of Parabolic and Toroidal Surfaces on Silicon and Silicon-Based Compounds Using Diamond Cup Grinding Wheels

This paper presents the methods and experimental results using diamond cup wheels and CNC machining centers for grinding parabolic and toroidal surfaces made of Si and SiC, the former for use as a thermal imaging lens and the latter for synchrotron radiation facilities. Generation of aspheric surfaces, by zonal material removal, on Si using cup grinding wheels and a 5-axis machining center was investigated. Theoretical and experimental investigation of the grinding and material parameters that influence ductile grinding are discussed. Ductile grinding is possible with wheels having small grit size and with proper combination of feed and unit normal load; a resinoid bonded wheel promotes this more easily than a metal bonded wheel, though at the expense of profile accuracy. Profile analysis, surface integrity, and examination of grinding swarf using digital imaging are discussed. Also, generation of toroidal surfaces on SiC using metal-bonded cup wheels, a 3-axis machining center and a micro-displacement table with piezoelectric actuators was studied. Toroidal SiC mirrors were ground with good shape accuracy, mirror finish, and low surface roughness. The time consumed in the process is very short. The machines used are inexpensive and not specially designed for ultra-precision grinding.

High-power operation of an argon excimer laser with a MgF/sub 2/ and SiC cavity

An intense coherent light pulse whose energy, height and width (FWHM) were 80 mJ, 16 MW, and 5 ns, respectively, was obtained as 126 nm from an argon excimer laser. A superpoished SiC mirror was used in combination with a MgF/sub 2/ output coupler for an electron-beam-pumped argon excimer laser. The SiC mirror had a reflectivity at 126 nm as high as 47% and was highly damage resistant. Laser mirror materials for high-power short-wavelength lasers are discussed. Surface damage on the MgF/sub 2/ output mirror could not be avoided; this prevented stable operation of the argon excimer laser. In order to obtain such high power from the argon excimer laser, it is necessary to construct the resonator without a MgF/sub 2/ coupler. An unstable resonator is considered as the best candidate for the stable operation of the argon excimer laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical coating technology for the EUV

Advances in optical coating and materials technology are one of the key motivators for the development of missions such as the Far Ultraviolet Spectroscopic Explorer recently selected by NASA for an Explorer class mission in the mid 1990's. The performance of a range of candidate coatings are reviewed for normal-incidence and glancing-incidence applications, and attention is given to strengths and problem areas for their use in space. The importance of recent developments in multilayer films, chemical-vapor deposited SiC (CVD-SiC) mirrors, and SiC films are discussed in the context of EUV instrumentation design. For example, the choice of optical coatings is a design driver for the selection of the average glancing angle for the FUSE telescope, and impacts efficiency, short-wavelength cut-off, and physical size.

Chemically vapor deposited silicon carbide (SiC) for optical applications

In this paper we present important physical, thermal, mechanical, and optical properties of cubic (β) silicon carbide produced via a bulk chemical vapor deposition (CVD) process developed at CVD Incorporated. This CVD SiC has been identified as the leading mirror material for high energy synchrotron radiation because of its high thermal conductivity, low thermal expansion, high polishability, and high reflectance in the vacuum UV. However, it has been difficult to obtain high quality, monolithic, CVD SiC mirrors in large sizes, i.e., greater than 10–20 cm. Recently, CVD Incorporated has been successful in scaling an SiC CVD process to produce large monolithic pieces of SiC up to 60 cm (24 in.) in diameter and plates up to 76 cm (30 in.) long by 46 cm (18 in.) wide with thicknesses up to 13 mm (0.5 in.). The properties of this material that make it attractive for optical applications, such as synchrotron optics, will be discussed.

Angular resolved soft X-ray scattering from optical surfaces

We present angular resolved scattering (ARS) distributions of various glass mirrors and of SiC mirrors at glancing incidence. The measurements have been performed at the XUV reflectometer station at the DORIS2 storage ring (HASYLAB) between 25 and 1.2 nm wavelength. The data are compared with different scattering theories by using least squares fitting procedures. Of all the available theories the Rayleigh-Rice vector perturbation theory is based on the most realistic model and gives by far the best agreement between experiment and theory. Indeed, even near the critical angle, where anomalous scattering arises, excellent agreement was obtained. Also, convincing evidence for scattering from dielectric fluctuations inside some of the glass samples was found. The rms-roughness, autocorrelation length (measure of the mean lateral separation of the surface irregularities), and the type of correlation function could be determined. Furthermore, we show that ARS-measurements are sensitive to detect irregularities with a mean lateral separation from 50 to 1500 nm. Therefore ARS-measurements in the soft X-ray range could become an important tool to supplement other methods characterizing optical surfaces. Practically all other familiar methods are restricted in spatial frequency corresponding to a lateral resolution above a few microns.

Super-polished silicon carbide mirror for high-power operation of excimer lasers in a vacuum ultraviolet spectral range

We demonstrate the usefulness of a high-quality SiC mirror for the high-power operation of vacuum ultraviolet lasers. A void free SiC film of 400-μm thickness was deposited onto a high-purity graphite substrate by chemical vapor deposition. The super-polished SiC mirror, whose surface roughness was 0.2 nm rms, was used as a cavity reflector of a high-power Ar excimer laser, which operates at 126 nm. The highest output energy of 400 mJ/cm2 per pulse, which corresponds to the energy density of 1 J/cm2 on the SiC mirror surface, has been obtained without any damage to the SiC mirrors.

Optical constants of SiC mirrors produced by different methods for photons of 60–1000 eV

The optical constants of seven CVD-SiC mirrors produced by different processes and a sintered SiC mirror have been measured in the soft x-ray region by means of the reflectance method. Although they are different in the crystal structure, the optical constants of the CVD-SiC mirrors are almost independent of the samples over the measured 200–1000 eV range. But the optical constants of the sintered SiC differ from those of the CVD-SiC mirrors over the measured range of 60–1000 eV. The photon-energy dependence of the optical constants of CVD-SiC is represented in terms of power formulas above 300 eV.

Experimental evaluation of thermal deformation effects of SiC mirror surfaces irradiated by undulator radiation

Exposure tests were made on newly developed large-sized SiC mirrors made of recrystallized and sintered SiC substrates with a surface roughness of ≊5 Å (rms) using undulator radiation with a total power of up to 1 kW and central peak-power density of 400 mW/mm2. The mirrors withstood the long-term exposure even in a horizontal deflection arrangement. The maximum slope and swell of thermal deformation were observed as 13 mrad and ≊39 μm, respectively, and the surface temperature at the mirror center was raised up to ≊240 °C. No deterioration in quality was observed in the intensely irradiated area on the mirror surfaces.

SiC mirror development at the Photon Factory (invited)

Several large-size mirrors coated with a thin layer of CVD-SiC for intense synchrotron radiation have been fabricated and commissioned at the Photon Factory. Substrates of these mirrors are high-purity graphite, recrystallized SiC and sintered SiC. Detailed properties of mirror-materials are described. First test of exposure of high-power mirrors to intense radiation from the 53-pole permanent-magnet wiggler are briefly reported.

New multipole wiggler/undulator beamline (BL-16) at the Photon Factory

Design and experience of the initial operation of a new beamline (BL-16) for a 3.6-m-long, 53-pole wiggler/undulator constructed at the Photon Factory are described. The insertion device has hybrid permanent magnets with a period of 12 cm and a maximum magnetic field of 1.47 T. In the undulator mode, the energy of the fundamental peak can be varied from 40 to 400 eV. In the wiggler mode, an x-ray beam (critical energy of 6 keV) 50–100 times more intense than that from the bending magnets is obtained. The beamline consists of two time-shared branch lines: a side beamline for soft x-rays in the undulator operation, to which photon beams are deflected sideways by a cylindrical SiC mirror, and a straight line for hard x rays under the wiggler operation. On the hard x-ray line, a fixed-exit sagittal-focusing double-crystal monochromator has been installed and commissioned. Collimating and refocusing mirrors will be installed in 1989. On the undulator beamline, a soft x-ray monochromator utilizing sophisticatedly devised aberration correction optics is commissioned.

Holographic Pattern Etching of Silicon—Carbide by Excimer Laser

ABSTRACTThe diffraction grating on SiC mirror was performed by a laser holographic method. In the present method, KrF laser and CIF3 was used for etchant gas. The ClF3 gas has an absorption band in the range between 200 and 400 nm. Therefore, CIF3 gas is effectively decomposed by the XeF, KrF and ArF excimer lasers' radiation. It is found that absorption of Si—C is about 50% in the range of between200 and 400 nm, and that the bonding energy of Si—C is lower than the photon energy of KrF laser beam. The above results indicate the direct decomposition of Si—Cbond. On the other hand, the threshold fluence energy for etching was 800 mJ/cm2 in 249 nm and in 193 nm as high as 7 J/cm2. In these results, the KrF laser is more effective than ArF laser. Then we applied KrF laser to crystalline SiC in an atmosphere of C1F3 gas. The divided two polarized KrF laser beams were interfered on the substrate. And the beams were used to photodissociated CIF3 gas in the proximity of substrate. Fluence of KrF laser beam was 1 J/cm2. The incidential angle of KrF laser beams was 20º and the grating gaps were 7170 Å, etching depth 1000 Å, and etching rate was 5 Å/pulse.

Finite element analysis of a SiC mirror receiving synchrotron radiation

The finite element method is used to answer some fundamental questions regarding maximum temperature, thermal stresses, and thermal distortions of a SiC mirror under thermal load. Absorption of synchrotron radiation, generated from sources such as insertion devices and bending magnets, produces the thermal load. Results of several analyses are presented to show typical behavior of a mirror with simple geometry under different conditions and designs. Both steady state and transient cases with radiation cooling alone and with a combination of radiation and convection cooling are considered.

Soft x-ray reflection from SiC, TiC, and WC mirrors

The optical constants of SiC, TiC, and WC in the soft x-ray region (80–1200 eV) derived from the incidence-angle dependence of the specular reflectance are reported. The angular dependence of the specular intensity can be explained by simple scattering theory with a Gaussian distribution of the surface roughness in the soft x-ray wavelength range.

X-ray beam line at the NSLS for X-ray absorption studies in material science

The development of the Material Science EXAFS line which covers the energy range of 4 keV to > 25 keV is described. The general design parameters of the line have been developed specifically for the application of X-ray absorption spectroscopy to problems in material science. The optics systems consisting of a vertically collimating SiC mirror and a unique four crystal monochromator are described along with the results of extensive ray tracing studies. The two backend configurations to be used on this line will also be discussed.