We have previously reported that columnar-structures were formed in electron beam (EB) deposited and DCmagnetron sputtered Mo-Si layered synthetic microstructures (LSMs). The columnar structures reduced x-ray reflectivity by roughing layer interfaces of the LSMs. We here investigated the conditions to suppress columnar structure formation, by varying the substrate temperature (T9) in EB deposition and the argon pressure (PAT ) in D C- and RF-sputtering. In the EB deposited LSMs, the columnar structure disappeared and almost uniform LSMs were obtamed both at T□ 400°C and T9< -155°C. In the DC- and RF-sputtered LSMs, the columnar structure formation was suppressed by lowering PAr. The measured x-ray reflectivity of the LSMs increased according to the suppression of the columnar structure formation.

We have investigated the optimum deposition conditions for fabricating Mo/Si multilayers on Si wafers by means of electron-beam deposition, taking the substrate temperature and deposition rate as parameters. Mo/Si multilayers made at substrate temperatures of room temperature to 420 °C and at deposition rates of 0.2-10 A/sec were evaluated by transmission electron micrographs of their cross sections, x-ray diffraction patterns, and their spectral reflectances for soft x-rays of 110-170 A. The optimum deposition parameters were found to be 100-150 °C and 0.2-2 A/sec, respectively, for substrate temperature and deposition rate. The Mo/Si multilayer made under the optimum condition showed fairly smooth layered structure and a reflectance of '3O for unpolarized soft x-ray of 146 A at an incident angle of 200.

Molecular Beam Epitaxy (MBE) is capable of producing a variety of high purity, epitaxial multilayer films with well controlled interfaces. However, the conditions necessary for MBE growth somewhat restricts the choice of useful materials. We discuss our material selection procedure for MBE-deposited x-ray optical multilayer materials. This procedure takes into account factors such as chemical reactivity, thermal stability, and lattice match,. as well as the maximum theoretical reflectivity obtainable with a given material pair. The present work consists of a comprehensive study of elemental films, and a more detailed consideration of a few select compound systems. Both the precise deposition contrOl possible with MBE, as well as the many in situ characterization methods, combine to allow a high degree of control over the formation of interfaces. Our principle MBE system for x-ray optics contains in situ Reflection High Energy Electron Diffraction (RHEED), Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spectroscopy (ISS). An overview of the techniques used to analyze our films be given, including data from our growth of epitaxial CoSi2 absorbing layers on Si (111) using a solid state reaction technique.

The widely used physical vapor deposition techniques to produce multilayer x-ray optics with uniform layer pair spacings (<1% variation) over large areas (> 10cm x 10 cm) have all been limited by the geometry of the vapor source. Magnetron sputtering sources, geometrically a convolution of point sources in a circular or rectangular array, provide uniformly thick regions of coating only within the boundaries of the erosion track. To maximize uniformity over large regins requires target materials equally as large, proving a costly proposition. Electron beam or molecular beam sources are similarly limited by the size of the melt pooi or effusion cell diameter. For ion beam deposition, spatial divergence from typical ion sources results in coating thickness variations of 5%or more for large areas as previously described. To minimize the ultimate expense of designing a necessarily large, single deposition source to provide a small thickness variation (without the use of compensating substrate motion or elaborate shielding over the deposition sources), several small sources arranged in an appropiate array may provide a viable alternative. To this end, the use of a linear array of one-inch magnetron sources has proven effective. Material has been deposited within the limitations of 1.5% thickness variation, along the axis of a linear gun array, over 15cm in length. The feasibility of using two linear arrays of magnetron sources is investigated to prepare large area multilayer mirrors with minimal layer pair spacing variations. Such a deposition system also allows for gradually varying the layer pair spacings across the surface of an optic, in a designed manner, which proves useful for focusing applications.

Si/Si02 and Si/Si3N4 multilayers have been fabricated using a locally made reactive diode if-sputtering systern. 'Ihe layer alternation is obtained by modulating a partial pressure of oxygen or nitrogen near the sample using a silicon target and argon as sputtering gas. 02 and N2 partial pressure conditions were optimized to deposit stoechiornetric Si02 and Si3TV4 films without significant reaction with the silicon target. In-situ kinetic ellipsometry was used to monitore both thick film and multilayer deposition. 'I'he different interfaces appear very sharp with a little contamination of the silicon layers especially using oxygen. The multilayers were characterized by gazing X-ray reflection ( Cu - K line ), and the reflectivity was measured in the soft X-ray range (120 - 350 A ) by synchrolroii radiation. BotI1 Si/Si02 and SiIS13N4 multilayers exhibit well defmed Bragg peaks with very narrow bandpasses ( two to three times lower than the conventional M'/Si multilayer ),and high absolute reflectivities (up to 22% at 1 30 A ). The soft X-ray performances of these mirrors are explained using the physical characteristics deduced from kinetic ellipsometry, grazing X-ray reflection, infrared absorption and transmission electron microscopy measurements.

Using state-of-the-art instruments and electron microscopy, the interfaces in Mo/Si multilayer mirrors fabricated by UHV electron-beam evaporation and by sputtering were examined. Peak shifts were observed in the XPS spectra at growth temperatures as low as 50 C, indicating the formation of silicide at the interface. The TEM micrographs showed that, in multilayers, the majority of the silicide formed at Mo on Si interfaces, rather than at Si on Mo interfaces. At 300 C, the multilayer structure is almost completely destroyed by the reaction. Mirrors with d(Mo) of about 40 A performed much better than those with d(Mo) of about 55 A. The Auger depth profiles showed that oxygen uptake in the sputtered samples and in samples evaporated at 200 C is limited to the top Mo layer, while samples evaporated at temperatures below 200 C had oxygen penetration through at least the first few bilayers, indicating that the silicide layers act as diffusion barriers to the oxygen.

The fabrication, by electron beam evaporation, of Mo/Si and Ta/Si mutilayers designed as soft-X-ray mirrors is described. The mirrors were characterized using surface analytical methods (RBS and sputtering in combination with AES), Cu-k(alpha) reflection, and soft-X-ray optical methods, and their soft-X-ray optical properties were correlated with microstructural characteristics. A comparison of in situ C-k reflectivity curves with calculations disclosed the existence of roughnesses at the interfaces, which can not be completely described by multiplying the reflected amplitude at each interface by a Debye-Waller factor. It was found that heating of Ta/Si samples induces a considerable change (up to and above 10 percent) in the d-spacing of multilayers, while the reflected amplitude is only reduced to two thirds of its original value.

The performance of multilayer-coated Schwarzschild objectives, and similar near normal incidence soft xray focussing systems, can be affected by the changing angles of incidence for different rays and possible variations in multilayer period across the curved mirror surfaces. A design analysis which considers these issues is presented, using as an example a 20 times demagnifying Schwarzschild objective coated with molybdenum/silicon multilayers for operation at roughly 76 eV. The large bandwidth of these multi].ayers eases requirements on control of the variation of the d-spacing for the system considered. Implications for extension to similar systems operating with different magnifications, sizes, and photon energies are discussed.

The imaging performance of a spherical mirror was measured at 14nm. A Foucault knife-edge test was used to determine the focal spot size formed by a 400 mm radius mirror at a numerical aperture of 0.04. The image formed by the mirror is compared with a single-layer AuPd coating is compared with that formed when coated with a Mo/Si multilayer. By fitting the results with a diffraction model, it is possible to quantify the change in effective surface figure caused by the multilayer. It is concluded that a multilayer-coated mirror can perform at the diffraction limit under certain conditions.

Near normal incidence imaging optics for use at wavelengths A in the 'water window' (2.4X4.4nm) utilise the in-phase reflections from a multilayer stack deposited on a figured substrate. The reflectivity of such a multilayer depends upon the number N of contributing interfaces and the amplitude reflectivity r at each interface where r M + ituz is determined by the difference in the real and imaginary parts of the refractive indices of the media defining the interface. Thus r may be predominantly determined by (Class I) or & (Class II) or a combination of both (Class III). The spectral variation of r (2 .3X4 .8nm) has been computed for interfaces formed by a-C and fifty four different elements which are thereby classified as I, II or III. The normal incidence reflectivity R versus A of representative multilayers from each class has been calculated as a function of N and layer thickness ratio 'y and criteria established for the most suitable choice of layer materials for the 'water window' region. In practice the particulate nature of the thin metal film components of the multilayer results in the experimental values of R being much less than the theoretical values.

Considerable effort has been devoted recently to the design, analysis, fabrication, and testing of spherical Schwarzschild microscopes for soft x-ray applications in microscopy and projection lithography. The spherical Schwarzschild microscope consist of two concentric spherical mirrors configured such that third order spherical aberration and coma are zero. Since multilayers are used on the mirror substrates for x-ray applications, it is desirable to have only two reflecting surfaces in a microscope. In order to reduce microscope aberrations and increase the field of view, generalized mirror surface profiles have been considered in this study. Based on incoherent, sine wave modulation transfer function (MTF) calculations, the object plane resolution of a microscope has been analyzed as a function of the object height and numerical aperture (NA) of the primary for several spherical Schwarzschild, conic, and aspherical reflecting two-mirror microscope configurations. The ultimate resolution of an aspherical two-mirror microscope appears to be about 200 A when using 100 A radiation. Better resolution can be produced when shorter wavelength radiation is used.

Photos obtained during 5 mm. of observation time from the flight of our 10 inch normal incidence soft x-ray (1 = 63.5A) telescope on September 11, 1989 are analyzed and the data are compared to the results expected from tests of the mirror surfaces. These tests cover a range of spatial periods from 25 cm to lÅ. The photos demonstrate a reduction in the scattering of the multilayer mirror compared to a single surface for scattering angles above 1 arcmin, corresponding to surface irregularities with spatial periods below 10 tim. Our results are used to predict the possible performance of future flights. Sounding rocket observations might be able to reach a resolution around 0. 1 arcsec. Higher resolutions will require flights of longer durations and improvements in mirror testing for the largest spatial penods.

A development model of the X-ray imaging concentrators designed for the Italian satellite for X-ray astronomy (SAX) was used to verify the thermomechanical and imaging properties of the optical system. The development model has a set of 29 mirrors (of the total of 30 mirrors of SAX), which are representative of the final mirrors except for the microroughness, which is about 3 nm instead of less than 1 nm. Results are presented of the optical system tests with low-energy X-ray beam (E = 0.27 KeV) and with visible light.

A Wolter type-I grazing incidence telescope is currently under construction at the University of Colorado, Boulder. The telescope inirrors were fabricated by machining on a lathe to approximately 1-2 arcminutes quality and ground and polished to improve their figures. Current measurement of the telescope's image quality gives a FWHM measurement of 44 arcseconds. We hope to achieve 5-10 arcsecond quality when completed.

Progress in the development of high quality X-ray reflecting foils for high throughput X-ray imaging experiments is briefly given and discussed.

The paper describes the main features and selected results of the calibration of the scientific instruments to be flown on the Extreme Ultraviolet Explorer in 1991. The instrument payload includes three grazing incidence scanning telescopes and an EUV spectrometer/deep survey instrument covering the spectral region 70-800 A. The measured imaging characteristics, the effective areas, and the details of spectral responses of the instruments are presented. Diagrams of the cross-sectional views of the scanning telescope and the deep-survey/spectrometer telescope are included.

The Multi-Spectral Solar Telescope Array (MSSTA) is a sounding rocket-borne observatory designed to produce ultra-high resolution full-disk images of the Sun. The MSSTA utilizes an array of Ritchey-Chretien, Cassegrain, and Herschelian normal incidence multilayer x-ray/EUV telescopes and thin film interference-coated FUV telescopes to produce narrow band solar images at selected wavelengths in this broad region of the electromagnetic spectrum. This mission places extremely strenuous requirements upon the imaging detector. The desire for ultra-high resolution images of the solar disk and corona out to 1.5 R demands an information storage capacity which can at the present time be met only by the highest quality photographic emulsions. Furthermore, there exists a tremendous range in intensity levels and contrast of solar x-ray/EUV/FUV emission features. The MSSTA imaging detectors must have the ability to record extremely bright, high contrast features associated with flares and active regions without saturating, whicle still maintaining the capability of capturing very faint, low contrast structures in coronal holes, polar coronal plumes, network structures and faint ioops in the corona. This very difficult requirement established the need for very wide latitude photographic emulsions so that these diverse features can be imaged at appropriate density levels within the range of exposure times that can be accommodated on a sounding rocket mission. Furthermore, the films selected for the MSSTA flight must be sensitive over a very broad and difficult portion of the electromagnetic spectrum. The payload must be evacuated prior to launch to preserve the integrity of the delicate EUV filters. Hence, the films chosen must also have low-outgassing rates and possess the ability to be unspooled from the reel and transported through conventional film cameras while dry, after exposure to high vacuum, without experiencing the degradation to the images which can result from electrostatic discharges. In this paper we describe the performance and characteristics required of photographic films for solar observations in the soft x-ray/EUV and FUV wavelength regimes. We discuss the properties of the important new emulsions that have been selected for flight on the Multi-Spectral Solar Telescope Array. We also present exciting and important new data on the response characteristics of a tabular grain Experimental XUV 100 film and an uncoated Experimental Spectroscopic 649 emulsion based upon measurements of these films at the SURF II synchrotron facility of the National Institute of Standards and Technology and at the Space Sciences Laboratory of the University of California, Berkeley.

The Multi-Spectral Solar Telescope Array (MSSTA) is a sounding rocket borne observatory for investigations of the Sun in the soft x-ray/EUV and FUV regimes of the electromagnetic spectrum. At soft x-ray wavelengths (),\ < 100 A), the MSSTA utilizes single reflection multilayer coated Herschelian telescopes. For selected wavelengths in the EUV (100 - 1000 A) the MSSTA employs five doubly reflecting, multilayer coated Ritchey-Chretien and two Cassegrain telescopes. In the FUV ()\ > 1000 A) the MSSTA utilizes two Ritchey-Chretien telescopes, with optics coated with thin film interference coatings. In this paper, we describe the interferometric alignment, testing, focusing, visible light testing, and optical performance characteristics of the Ritchey-Chretien and Cassegrain telescopes.

The feasibility of using X-ray projection cameras as a practical lithography tool for making integrated circuits with tenth-micron features was investigated in experiments performed with a 20-fold reduction Schwarzschild camera operating at 36 nm and 14 nm, and with a 1:1 magnification Offner ring field system at 40 nm. The paper examines the requirements on the resist, the source, the camera design and the fabrication of its mirrors, the mask, and the alignment system. The experiments proved that high-reflectance multilayer mirrors are capable of diffraction limited imaging. Some problems exposed by the experiments, such as the deposition of carbon on surfaces exposed to X-rays, are discussed.

Free-electron laser sources, driven by ri-linear accelerators, have the potential to operate in the extreme ultraviolet (XUV) spectral range with more thansutficient average power for high-volume projection lithography. For XUV wavelengths from 100 nm to 4 nm, such sources will enable the resolution limit of optical projection lithography to be extended from 0.25 pm to 0.05 im and with an adequate total depth of focus (1 to 2 jtm). Recent developments of a photoinjector of very bright electron beams, high-precision magnetic undulators, and ring-resonator cavities raise our confidence that FEL operation below I 00 nm is ready for prototype demonstration. We address the motivation for an XUV FEL source for commercial microcircuit production and its integration into a lithographic system, including reflecting reduction masks, reflecting XUV projection optics and alignment systems, and surface-imaging photoresists.

Several configurations of two- to four-multilayer mirror systems that have been proposed for use in soft-X-ray projection lithography are examined. The performance capabilities of spherical and aspherical two-mirror projection systems are compared, and a two-spherical-mirror four-reflection system that can resolve 0.1-micron features over a 10 x 10 mm field is described. It is emphasized that three-mirror systems show promise of high resolution in telescope applications, but have not been fully analyzed for projection lithography applications. It has been shown that a four-mirror aspheric system can be designed to meet the resolution requirements, but a trade-off must be made between reducing distortion below 10 microns over the field of view and increasing the modulation transfer function greater than 50 percent at spatial frequency of 5000 cycles/mm.

The extension of present step and scan technology to EUV projection techniques to print patterns with .ljim feature sizes is examined.

Inter-mask-level alignment and overlay measurements are becoming more challenging as new generations of lithography progress to critical dimensions well below a micrometer. Registration requirements are typically a small fraction (. 20%) of the critical dimension. Despite the importance of alignment to the realization of future generations of lithography, relatively little effort has been expended in contrast to the large body of work on new source and photoresist technologies. Moire techniques are shown to be an effective technique for alignment and overlay registration to well below a micrometer. Initial experiments are presented for several promising moire metrology techniques which are shown to be sufficiently precise to resolve two gratings with periods differing by less than 0.1 nm. A high-resolution Si-based optical position sensor, which relies on electronic transport mechanisms, rather than optical phase information, is also discussed in the context of submicrometer alignment.

Soft x-ray projection lithography in the 60-400 A wavelength range has recently demonstrated the ability to print 0.1 im resolution features in thin single layer and thicker trilayer resist films.7'8 However, for use on topographic substrates thicker resist layers with planarizing properties probably will be required. This paper analyzes resist absorption and planarization aspects of soft x-ray projection lithography, couples them with projections of potential multilayer and other types of reflectors and reviews resist approaches which are compatible with them. Primarily due to absorption issues and the strong processing tendency to retain a single layer resist technology, dry-developed resist schemes incorporating planarization are favored. Results for a variety of such schemes developed for deep-UV use are surveyed and recent results for soft x-ray exposure at 140 A are discussed.

The reflectances of seven Mo/Si multilayer coatings have been measured using three different reflectometers in order to determine whether reflectance measurements made using different reflectometers yield consistent results. By comparing the deduced adjustable parameters used to fit the measured reflectances with those calculated from a model based on recursive application of the modified Fresnel equations, it is concluded that the measurements made with the three reflectometers are inconsistent. The discrepancies are attributed to systematic measurement errors, including those associated with the spectral purity of the incident radiation.

A monochromatized laser-produced plasma source of XUV radiation has been used to study resists for use in projection x-ray lithography. We report the characterization of exposure sensitivity and contrast of poly(cyclohexylmethyl-co-trimethylsilylmethylsilane) (CMTMS) at photon energies near 100 eV, where projection x-ray lithography is being developed. Using monochromatized XUV exposures on the Si-Si a resonance at 105 eV, followed by solvent dissolution development, the polysilane yields positive tone, exhibiting a sensitivity of 600 mJ/cm2 to achieve a saturation depth of 0.17 p.m and a contrast of 1.4. Beyond this saturation dose, the tone is observed to reverse (i.e. greater doses yield smaller developed depths). Exposure sensitivity measurements have also been performed below the edge at 92 eV where we find only minor differences in contrast and saturation dose from the results at 105 eV. These can be accounted for by a simple decrease in film absorption. The exposure sensitivities of selected commercially-available electron beam resists have also been characterized. Two PMMA resists (M =74 K and 500 K) and the novolac-based SYSThM-9 (Shipley Co.) have been exposed to monochromatized radiation at 140 A. PMMA exhibits a sensitivity of 600 mJ/cm2 (0.58 m developed depth) and a contrast of 1.8 while the SYSTEMTh-9 resist requires only 22 mJ/cm2 (0.43 j.tm depth) and exhibits a contrast of 4. 1.

The structure of XUV multilayer coatings has been examined using high-resolution transmission electron microscopy. A variety of techniques have been used to measure the interface widths and the interface topography from the digitized TEM images and the results have been compared with X-ray and XUV reflectance measurements. It is found that the structural parameters measured from the TEM images and those deduced from reflectance are consistent in light of the probable systematic errors associated with the measurement and interpretation techniques.

The UV and EUV instruments designed for the ESA/NASA Solar and Heliospheric Observatory (SOHO) are described, and their characteristics are presented. The SOHO instruments include the SUMER telescope for measuring line profiles and images in the wavelength range 500-1600 A, with a 1.5-arcsec resolution; the Coronal Diagnostics Spectrometer for measuring line ratios and images in the range 170-800 A, with a 2 arcsec resolution; a EUV imaging telescope for producing narrow band pass images in the spectral lines at 171 A, 195 A, 195 A, 284 A, and 304 A, with a 3-arcsec resolution; a UV coronagraph spectrometer for measuring line profiles and images of EUV lines from about 500 A to about 1200 A, with a several arcsec rosolution; and the SWAN, a lens with hydrogen absorption cell for measuring Ly-alpha profiles in the far corona and heliosphere.

The phenomena to be studied with the Ultra-High Resolution XUV Spectroheliograph (UHRXS) which will be included into the instrumental load of the Space Station Freedom are discussed together with the characteristics of the nine multilayer Ritchey-Chretien telescopes constituting the UHRXS system. The telescopes will cover a spectral range from about 70 A to about 300 A. Each telescope will be able to isolate emission line multiplets excited over a narrow temperature range, providing images of diagnostic quality covering structures in the solar atmosphere ranging from T of about 50,000 K to 30,000,000 K. One of the nine UHRXS telescopes is configured as a coronagraph to allow observations to 6 solar radii beyond the solar limb. The paper presents an analysis of the expected sensitivity and resolving power of the UHRXS telescopes, and the diagnostic response of the various UHRXS instruments to structures in the solar atmosphere between 10,000 K and 100,000,000 K.

As part of the planning process for the Orbiting Solar Laboratory (OSL), a proposed NASA space-borne solar observatory, the authors have developed an instrument concept, the "Active Sun Telescope Array" (ASTA), for a high resolution soft x-ray/Extreme Ultraviolet (XUV) experiment designed to investigate the phenomenology of the solar chromosphere and corona. ASTA is an ensemble of six EUV/XUV solar telescopes designed to provide: (i) wide field (i.e. full disk) solar images in narrow wavelength bands, (ii) high resolution spectroheliograms, and (iii) line profiles, of the solar plasma in the temperature range from 10,000 K to 30,000,000 K. The ASTA also incorporates specific operational modes to allow the precise registration of its EUV and XUV images and the visible/ultraviolet and far ultraviolet images of the other primary OSL instruments. The simplest (baseline) ASTA configuration incorporates six identical telescopes, which differ only in the reflective coatings used normal incidence multilayer coated optics are used for XUV observations, principally of the solar plasma from 900,000 K to 30,000,000 K, and SiC optics are used for extreme-ultraviolet observations of the solar plasma from 20,000 K to 2,000,000 K. An attractive option, which allows the ASTA to extend its coverage of the solar plasma to material as hot as 100,000,000 K, is the replacement of one or more of the XUV telescopes with Wolter I grazing incidence telescopes to allow soft x-ray observations down to -1.5 A. The detector used in the ASTA is the MAMA photon counting array detector.

Several approaches to imaging hard x-rays emitted from solar flares have been proposed for the nineties. These include the fixed modulation collimator, the rotating modulation collimator (RMC), the spiral fresnel zone pattern, and the redundantly coded aperture. These techniques are under consideration for use in the Solar Maximum '91 balloon program, the Japanese Solar-A satellite, the Controls, Astrophysics, and Structures Experiment in Space (CASES), and the Pinhole/Occulter Facility (P/OF) and are outlined and discussed in the context of preliminary results from numerical modeling done at MSFC and the requirements derived from current ideas as to the expected hard x-ray structures in the impulsive phase of solar flares. Preliminary indications are that all of the approaches are promising, but each has its own unique set of limitations.

Several mechanisms can induce a detectable amount of linear polarization (□ 1%) in spectral lines emitted by the outer solar atmosphere at EUV/FUV wavelengths (100 A< A < 1500 A): (1) Polarization in FUV lines (up to 20%) can be originated by resonance scattering of radiation anisotropically illuminating the emitting atoms. Modifications of this polarization can then result from the presence of a magnetic field (Hanle effect); (2) Impact line polarization can arise from anisotropic collisional excitation of the EUV emitting atoms by particles (electrons, protons) with non-Maxwellian velocity distributions. We suggest how new technological developments associated with the production of ultra-smooth, low scatter flow-polished mirror substrates and high quality multilayer and interference film coatings can make possible some exciting new optical instruments which should permit observations of these polarization effects. Polarization measurements have not previously been obtained in these wavelength regimes. We give the observational parameters for the development of all refledive FUV coronagraph/polarimeter and EUV imaging polarimeter instruments. A coronagraph/polarimeter, operating at hydrogen Lyman a, could provide - via the Hanle effect - the first direct measurements of coronal vector magnetic fields, of relevance to numerous fundamental questions in modern solar physics. A multilayer EUV imaging polarimeter, operating at EUV helium lines (e.g., 304 A, 584 A), could provide information on impact polarization phenomena, relevant to questions concerning the relative importance of thermal and non-thermal processes in solar flares. The reflecting polarization analyzers for these instruments will operate at the Brewster angle and will be coated with advanced thin film interference coatings or EUV multilayer coatings. We describe some new types of EUV/FUV polarimeters based on these polarization optics.

In this paper we describe new EUV/FUV (100 A < ) < 1500 A) polarimeter instrument concepts for solar research. These instruments are designed to observe linear polarization in EUV/FUV spectral lines originating in the outer solar atmosphere. Specifically: ( i) a new coronagraph/polarimeter operating at 1215.6 A (neutral hydrogen Lyman a), which could observe this line in the near solar corona and lead to the first direct measurements ofboth strength and direction of coronal magnetic fields; (ii) a new multilayer EUV imaging polarimeter, operating at the wavelengths of strong helium emission lines (e.g., 304 A, 584 A), which could observe impact polarization phenomena and provide information concerning the relative im'portance of thermal and non-thermal processes in solar flares. The emission mechanisms we will address with these instruments include resonance scattering and impact polarization. Resonance scattering of chromospheric radiation anisotropically illuminating the emitting atoms in the corona can produce up to 20% linear polarization in FUV coronal lines. Modifications, via the Hanle effect, of this polarization would result from the presence of a magnetic field. In the EUV, detectable polarization may be produced by impact polarization, which results from anisotropic collisional excitation of the emitting atoms by particles (electrons, protons) with non-Maxwellian velocity distributions produced during flares. These coronagraph/polarimeter instruments employ all-reflective optical systems utilizing ultra-smooth, low-scatter normal incidence mirrors and reflective polarization analyzers comprised of advanced flowpolished substrates with state-of-the-art thin film FUV interference and EUV multilayer coatings. The reflecting polarization analyzers operate at the Brewster angle. We discuss several instrument configurations and provide theoretical calculations and performance predictions for coronagraph/polarimeter instruments utilizing an optical design similar to the Ritchey-Chrétien EUV/FUV telescopes currently being developed for flight on the Stanford/MSFC/LLNL Multi-Spectral Solar Telescope Array (MSSTA).

We have developed compact soft x-ray, extreme ultraviolet (BUY), and far-ultraviolet (FUV) multilayer coated telescopes for the study of the solar chromosphere, corona, and corona/solar wind interface. Because these systems operate at short wavelengths (-40 A <2< 1550 A), the modest apertures of4O mm - 127 mm allow observations at very high angular resolution (0. 1 - 0.7 arc second). In addition to permitting traditional normal incidence optical configurations (such as Cassegrain, Ritchey-Chrétien, and Herschelian configurations) to be used at soft x-ray/EUV wavelengths, multilayer coatings also allow a narrow wavelength band (/& - 15 - 100) to be selected for imaging. The resulting telescopes provide a very powerful and flexible diagnostic instrument for the study of both the fine scale structure of the chromosphere/corona interface and the large scale structure of the corona and corona/solar wind interface. In previous papers we have described a new solar rocket payload, the Multi-Spectral Solar Telescope Array (MSSTA), which is composed of 17 of these compact telescopes. In the present paper, we report on the ability of the MSSTA payload to obtain temperature diagnostic information about the optically thin solar plasma. We also discuss applications of this information to studies of coronal structure.

The traditional coronagraph is a low scatter visible light optical system designed so that the solar image is formed on an occulting disk which blocks the intense light from the photosphere of the sun, allowing the faint coronal emissions to be recorded in the absence of the nawral occultation of a lunar eclipse. The visible coronal radiation imaged comes from three sources: photospheric radiation scattered by coronal electrons (the K corona), photospheric light scattered by dust (the F corona) and the emission from forbidden tranaltions in highly ionized ions (the E corona) such as the "green line" (Fe XIV X 5303 A) and the "red line" (Fe X . 6374 A). These sources are in general weak, and photospheric light scattered in the insirument creates a serious background problem. The advantage of observations at xtJv and EUV wavelengths for studies of the corona and corona/solar wind interface are obvious. Most of the energy emitted by the corona appears in the resonance lines of ions such as Fe VIII -FeXVI in the wavelength interval 170 A < . < 350 A, where the contrast between the brightness of the solar disk and the inner corona is much less extreme (typically a factor of less than 100, compared to iO -108 for the coronal forbidden lines), placing less extreme demands on an occulting disk. Coronal XUV and EUV observations offer a number of advantages including (i) improved temperature and density diagnostics compared to E coronal observations, (ii) the ability to observe the corona and solar wind to a greater height than is possible with the E corona, and (iii) the ability to follow coronal mass ejections and other transient phenomena from the low corona to the far corona with a single instrument. We describe the design and anticipated performance of XUV and EUV coronagraphs using normal incidence multilayer optics, and comment on farultraviolet (FUV) coronal observations.

We present further data on the linear polarisation sensitivity of CsI X-ray/UV photocathodes. A radical physical origin- "non local" photoemission- is proposed for the X-ray vectorial effect. Predictions from a uniaxial photocathode model are compared with measurements of (i) relative pulse quantum yields, relative current quantum yields and electron number distributions for both s- (electric vector perpendicular to the plane of incidence) and p- ( parallel to the plane of incidence) polarised soft X-rays in the wavelength range 4.6-125A and (ii) relative current quanm yields from illumination by partially-polarised, broad-band UV radiation with a modal wavelength of 1550A. An origin seated in the polarimeter geometry is also proposed for the off-axis "phase shift" phenomenon reported in previous papers. The impact of these measurements on (i) an understanding of soft X-ray Interactions with materials and (ii) the design and sensitivity of the Spectrum X-Gamma Photoemission Polarimeter is discussed ; our future plans for the investigation of photocathode physics are finally outlined.

X-ray scattering from a lithium disc and Bragg reflection from a mosaic graphite crystal can be exploited to measure the linear polarization of radiation emitted from cosmic x-ray sources. The sensitivity is greatly enhanced if these polarimeters are placed at the focus of an x-ray telescope. Such devices form two of the three components of the Stellar X-Ray Polarimeter experiment scheduled to fly on the SPECTRUM-X-Gamma mission. The experiment will reside at the focus of one of the SODART x-ray telescopes. We describe the expected on-axis performance of these two components of the Stellar X-Ray Polarimeter experiment based on detailed Monte-Carlo simulation:;. We also discuss various systematic effects, both external and internal to the experiment, that must be considered in order to properly design and utilize the experiment.

A hollow cylindrical proportional counter is the best choice for an X-ray polarimeter based on the Thomson scattering. Here we report on the results of Montecarlo simulations performed to optimize the geometrical configuration of such a polarimeter, conceived as a focal plane instrument of an X-ray telescope. We also present the design characteristics of a prototype of a cylindrical proportional counter, presently under testing in our laboratories.

We have developed a low density (30-670 kg/rn3), high porosity (up to 96%) foam containing lithium hydride and amorphous beryllium hydride. This foam may be attractive as the X-ray scauering element in experiments such as the Thomson Scattering Polarimeter as proposed for inclusion in the High Throughput X-Ray Spectroscopy Space Mission, XMM, sponsored by the European Scientific Agency. We discuss ways to synthesize the foam, and report on physical properties such as bulk density and porosity, electron density, uniformity of foam density, concentration of impurities, pore size distribution, and sensitivity to moisture. The composition of the foam can be varied, but equimolar mixtures of LiH and BeH2 give the most vigorous foaming reaction. The force needed to expand the plastic reaction mass into a foam is provided by hydrogen and organic contaminants in the beryllium hydride that are released during the foaming reaction. Simultaneous differential thermal analysis and thermogravimetric analysis show that the reaction is endothermic with very little mass loss. The presence of LiBeH3 as either a reaction intermediate or final product is suggested by the fact that the most vigorous foaming occurs with equimolar mixtures oflithium hydride and beryllium hydride.

Further studies are reported on the polarization dependence of the photoelectric effect produced by soft X-rays from CsI. A major difficulty in these experiments is the geometrical effects which mimic the polarization signature. We present a detailed calculation of these geometrical effects that are produced when the X-ray beam is not precisely aligned on a rotatable plane photocathode. These effects were observed experimentally and were used in turn to precisely determine the alignment of the incident beam of polarized X-rays on a rotatable photocathode. From these studies, we are able to uniquely determine the true polarization dependence of the photoemission from CsI. We confirm that the photoelectric effect in CsI is dependent on the polarization state of the X-rays. The "phase shift" which was reported previously has now been explained as a result of these off-axis effects. This shows that there is no intrinsic "phase shift" in the polarization dependence of the photoemission from CsI. Preliminary surface analysis of the CsI photocathode was pe'formed to determine the surface quality.

We are designing a Bragg crystal polarimeter for the focal plane of the SODART telescope on the Spectrum-XGamma mission. A mosaic graphite crystal will be oriented at 45 0 to the optic axis of the telescope, thereby preferentially reflecting those x-rays which satisfy the Bragg condition and have electric vectors that are perpendicular to the plane defmed by the incident and reflected photons. The reflected x-rays will be detected by an imaging proportional counter with the image providing direct x-ray aspect information. The crystal will be 50 jtm thick to allow x-rays with energies &square; 4 keV to be transmitted to a lithium block mounted below the graphite. The lithium is used to measure the polarization of these high energy x-rays by exploiting the polarization dependence of Thomson scattering. The development of thin mosaic graphite crystals is discussed and recent reflectivity, transmission, and uniformity measurements are presented.

The multilayer mirrors used in the nonnal incidence optical systems of the Multi-Spectral Solar Telescope Array (MSSTA) are efficient reflectors for soft x-ray/extreme ultraviolet (EUV) radiation at wavelengths that satisfy the Bragg condition, thus allowing a narrow band of the soft x-rayfEUV spectrum to be isolated. However, these same mirrors are also excellent reflectors in the visible, ultraviolet, and far ultraviolet (FUV) part of the spectrum, where normal incidence reflectivities can exceed 50%. Furthermore, the sun emits far more radiation in the ultraviolet and visible part of the spectrum than it does in the soft x-rayIEUV. For this reason, thin foil filters are employed to eliminated the unwanted longer wavelength solar emission. With the proper choice of filter materials, the filters can also be used to eliminate EUV radiation at longer wavelengths, where the increasing specular reflectivity of multilayer mirrors and the high intensity of solar emissions can cause "contamination" of the image in the narrow band defined by the Bragg condition. In addition, filters can eliminate higher order multilayer reflections. Finally, filter absorption edges can sometimes be utilized to reduce the width of the primary bandpass. The MSSTA instrument uses various combinations of thin foil filters composed of aluminum, carbon, tellurium, potassium bromide, beryllium, molybdenum, rhodium, lexan, and phthalocyanine to achieve the desired radiation rejection characteristics. The filters are currently being manufactured by Luxel Corporation of Friday Harbor, Washington, and Penn-Spectra Incorporated of Wallingford, Pennsylvania. This paper discusses issues concerning the design, manufacture, and performance of the MSSTA filters.

The ORFEUS instrument is the first of a few missions which have to be flown with the ASTRO-SPAS satellite. The instrument consists of an on-axis telescope with 1 m primary mirror together with two focal plane spectrometers. The main scientific objectives are spectroscopic easureents of cosmic radiation sources in the temperature region between 10 to 10 K. The Rowland spectrometer which operates in the spectral region between 40 nm to 120 rim is supplied by the Space Astronomy Group (SAC) of the University of Berkeley, the Echelle spectrometer was designed by the Landessternwarte Heidelberg (LSW, FRG) and covers the spectral region between 90 nm to 125 nm. The overall scientific responsibility is at the Astronomisches Institut TUbingen (AlT, FRG).

We describe the design of a high-resolution stigmatic extreme-ultraviolet (EUV) spectroheliometer, configured for flight on a Black Brant sounding rocket, which consists of a 45-cm Gregory telescope coupled to a spectrometer employing a single toroidal diffraction grating in a normal-incidence Rowland circle mounting and an imaging pulse-counting Multi-Anode Microchannel Array (MAMA) detector system. The toroidal diffraction grating is fabricated by a new technique employing an elastically-deformable sub-master grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The spectroheliometer will produce spatially-resolved spectra of the chromosphere, transition-region and corona with an angular resolution of 0.4 arc sec or better, a spectral resolution AII of about 1O in first order, and a temporal resolution of the order of seconds. Because of the geometric fidelity of the MAMA detector system, the speciroheliometer will be able to determine Doppler shifts to a resolution of at least 2 mA at wavelengths near 600 A (-1.0 km s1), depending on the level of the accumulated signal. The unique characteristics of the spectroheliometer will be used in combination with plasma-diagnostic techniques to study the temperature, density and velocity structures of specific features in the solar outer atmosphere.

A prototype polarimeter with a 50 mm diameter has been constructed. With polarized x-rays from a simple polarizer, the detection efficiency and modulation factor of the polarimeter with a 40 mm thick scatterer were 3.2% and 0.57%, respectively, at 60 keV. The fabrication and performance of the polarimeter and polarizer are presented.

The Reflection Grating Spectrometer Experiment (RGS), which has been selected for flight on the European Space Agency's X-Ray Multi-Mirror Mission (XMM), includes two arrays of reflection gratings that are placed in the X-ray optical path behind two separate grazing incidence X-ray telescopes. Each of the grating arrays picks off roughly half the X-ray light emanating from its telescope and diffracts it to a dedicated strip of charge-coupled device (CCD) detectors offset from the telescope focal plane. The arrays contain 224 100 mm X 200 mm gratings, each mounted at a graze angle of 1.58° to the incident beam. The gratings are produced by epoxy replication of a common master onto very thin substrates. Both the gratings and the detectors are mounted on a Rowland circle which also includes the telescope focus. In this paper, we review the current state of both the engineering and the optical designs for the grating arrays.

An approach and development plan for the production of replicated variable line spacing X-ray reflection gratings on lightweight substrates is described. The gratings will be arrayed in spectrometers to analyze the soft X-rays gathered by the telescopes flown aboard the European Space Agency (ESA) "X-ray Multi- Mirror Mission" (XMM). The mission requires about 750 gratings to form 3 arrays. Each array provides a grating area of approximately 5 square meters. From relatively few master gratings, the low blaze angle gratings will be replicated onto 55 gram substrates having an equivalent length/thickness aspect ratio &square;40. We will describe the criteria used for selecting the substrate design and material. A multi-phased study to solve identified technical problems associated with the replication of these gratings is also described. We will report on the present status of the substrates and replication process.

We report on the fabrication of a multilayer linear grating, characterization in the soft x-ray region and modeling of its perfonnance. Holographic lithography was used to produce a 0.24 p.m spatial period grating on a triode sputtered Mo/C multilayer mirror. The pattern was transferred into the multilayer mirror by reactive ion etching in an SF6 plasma after an intermediate lift-off step. The position and relative efficiency of the different orders of a grating etched down to the silicon substrate were measured at the Cu Lcz line (1.33 nm). The results were compared to the values calculated within the framework of a scalar kinematic diffraction theory of relief gratings.

We report on the fabrication of linear and circular reflective Bragg-Fresnel zone plates made by a multistep process using microfabrication technologies. The zone plate patterns were generated by electron beam lithography on a Mo/C triode sputtered multilayer interference mirror. The minimum zone size was 0.3 p.m wide. The pattern was transferred anisotropically into the multilayer by reactive ion etching in a fluorinated plasma. An intermediate metallic mask made by lift-off was used for the transfer process. The groove depth was monitored by following the reflectivity of the structure with a helium-neon laser during the etching process. The groove profile and dimensional control were considered.

We have investigated the properties of small period multilayers as possible diffracting elements in a monochromator for synchrotron radiation angiography. This type of angiography uses two photon beams, tuned above and below the iodine K-edge (33 KeV) resp., to image an iodine contrast agent injected into the coronary circulation. The fine energy resolution of the perfect Si(1 1 1) Bragg reflectors now used is not necessary and severely limits the useful flux from the synchrotron. We report measurements showing that an increase in flux of nearly 2 orders of magnitude times that of Si may be obtained by using small period (20 to 27 A) W/C or W/B C multilayers. However, the background in the angiogram due to bone and tissue increases as the energy difference between the two beams goes up, and therefore as the bandpass increases. Additional work is needed to determine the maximum allowable bandpass.

Performance of a multilayer coated diffraction grating has been evaluated in the EUV. The application of a multilayer coating to a blazed toroidal grating of high ruling density has produced a significant enhancement in grating efficiency in the 30 run spectral region in first order and has maintained the excellent quasi stigmatic spectral resolution of the grating.