We present a comparative study of three metal Si multilayer coatings, Mo-Si, Ru-Si, and Rh-Si, designed for optimum normal incidence reflectivity at x-ray wavelengths near 130 angstroms. The ML structures are characterized using x- ray diffraction and high-resolution electron microscopy, and the results are correlated with the normal incidence reflectivity measured using synchrotron radiation. It is found that interlayers of mixed composition are formed to various degrees in the as-deposited structures for all three material compositions. The thinnest interlayers are observed in Mo-Si and the most intermixing occurs in Rh-Si, where the pure Rh layers are completely consumed. The stoichiometry of the interlayers in all cases is most consistent with the formation of the Si rich silicide. The highest peak normal incidence reflectivity of 61% at approximately 130 angstroms is achieved with Mo-Si, whereas the highest integrated reflectivity of 4.4 angstroms is achieved with Ru-Si. In a separate study, the formation of the interlayers in as-deposited Mo-Si multilayer coatings is investigated by systematically varying the thickness of either the Mo or Si layers. We observe a contraction of the multilayer period which is consistent with formation of MoSi2 at the interfaces. The presence of the MoSi2 interlayers decreases the normal incidence reflectivity, thereby limiting the x-ray throughput in soft x-ray projection lithography applications.

A process for direct application of multilayered coatings to large area (180 cm2) planar or curved optical surfaces has been developed. Small diameter (2.54 cm) sputter sources are appropriately configured in an array for the deposition of multilayered coatings on virtually any optical surface. Potential application in soft x-ray projection lithography is examined for the deposition of Mo/Si multilayers with a uniform, 6.5 - 7 nm, layer pair spacing onto 15.2 cm diameter silicon wafers. Layer pair spacing variations less than 1.5%, over surface areas exceeding 120 cm2, are measured using Cu (Kappa) (alpha) grazing incidence diffraction.

Precise structural analysis of Mo/Si multilayers deposited by a diode rf-sputtering system has been made using in-situ kinetic ellipsometry, grazing x-ray reflection, x-ray diffraction, Auger profile analysis, Rutherford backscattering, and high-resolution electron microscopy. The main structural imperfections (interface roughness and interdiffusion) have been related to the Mo crystallization and to the molybdenum silicide formation at the interfaces. The comparison to Co/Si multilayers deposited in the same conditions was useful to deduce the influence of the intrinsic properties of these systems on their structural behavior. Silicide layers are formed in real-time during the growth of the samples. They are completely amorphous and their composition is not far from defined compounds (MoSi2 and CoSi2). In Mo/Si multilayers the Mo on Si interface is always thicker than the other interface (approximately equals 15 angstrom compared to 8 angstrom). It is not due to the deposition conditions but to the crystallization of the molybdenum layers which reduces the silicon diffusion at the Si on Mo interface. The higher reactivity of cobalt with silicon produces thicker quasi-symmetric silicide layers (approximately equals 25 angstrom). The thermal behavior of the two systems is also controlled by the interdiffusion and the crystallization of the silicide layers. Absolute soft x-ray reflection was measured on different Mo/Si x-ray mirrors by synchrotron radiation at various wavelengths above the Si L-(alpha) line and related to the structural characteristics. In spite of the occurrence of thin silicide layers at each interface, reflectivities as high as 55% in normal incidence have been obtained at 130 angstrom.

Soft x-ray projection lithography (SXPL) requires uniform, high reflectivity multilayer (ML) coatings on figured optical surfaces with lens speeds (f) between 3 and 6 and diameters of 10 to 15 cm. High reflectivity Mo-Si ML coatings for operation near 13 nm were deposited on f 3.4 and f 6 optics 5 and 7.5 cm in diameter using planar dc magnetron sputtering. Measurements of the normal incidence reflectivity (NIR) of 63% at 13 nm uniform over the central 5 cm of the figured surface were obtained. Comparison of the measured values to model calculations of the wavelength dependent reflectivity indicate that the ML period is uniform to better than 0.04 nm over this region.

We have deposited, via magnetron sputter deposition, multilayer structures comprised of alternating layers of low atomic number materials such as (1) carbon and boron carbide, (2) silicon and boron carbide, (3) silicon and carbon, and (4) aluminum and boron carbide. Layer periods for these materials combinations range from 63.5 to 75 angstrom. These low atomic number multilayers exhibit significant first-order Bragg diffraction of Cu k-alpha radiation. Calculations of the reflectivity performance for multilayers of this composition have been made using a computer code based on the modified Darwin-Prinz theory. Experimental measurements and code predictions are in close agreement. Multilayers of this type may find application in devices requiring ultralow dispersion focusing x-ray optics, such as long focal length focussed beam lines, x-ray microscopes, and x-ray telescopes. Diagnostics for plasma characterization in fusion experiments that are free from L-edge absorption, high transmittance/high resolution beam splitting x-ray optics, and output couplers soft x-ray laser cavities are other possible applications for low-Z1/ low-Z2 multilayers.

Boron is potentially useful as a spacer in multilayer x-ray optics for wavelengths between the boron K-edge at 67 angstrom and approximately 125 angstrom. To investigate the usefulness of this element as a spacer, we have fabricated multilayers incorporating boron by alternately depositing several material pairs, including Ag/B, Pd/B, and B/Si. We are interested in the Ag/B system as a possible crystalline superlattice x-ray mirror. Unfortunately, the tendency of Ag to form islands has frustrated our efforts to date to grow both single crystal B overlayers and quality nonepitaxial multilayers. The Pd/B system shows promise as an amorphous multilayer system. Our work shows that the palladium reacts with the boron to form an amorphous, palladium-rich boride. However, calculations indicate that the ideal reflectivity of the reacted multilayers may be as high as 51% at 80 angstrom. In the B/Si system, boron functions as the absorber layer for wavelengths greater than approximately 125 angstrom, resulting in a narrow bandpass mirror. We present evidence that the multilayers formed are layered and indirect evidence that the interfaces formed are relatively sharp. All depositions for the present work were performed under ultrahigh vacuum conditions in a Perkin-Elmer 433-S Molecular Beam Epitaxy system. All of the materials were electron-beam deposited except palladium, which was evaporated from a high temperature Knudsen cell. These multilayers were characterized in situ by reflection high energy electron diffraction (RHEED), and ex situ by low-angle x-ray diffraction.

Boron-based multilayer mirrors have theoretical reflectivities > 50% for wavelengths between 65 and approximately 120 angstrom. Such multilayers would be quite useful, since this wavelength region lies above that for which W/C functions, and below that for which Mo/Si works well. We have studied the growth of several candidate materials on boron in order to determine their growth modes, the chemical sharpness of the interfaces, and the structure of the layers formed. The films were deposited in ultrahigh vacuum and studied with in situ Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and reflection high-energy electron diffraction. Scanning tunneling microscopy characterization and Rutherford backscattering calibrations were performed after removing the samples from the vacuum system. For all of these studies amorphous B films were first formed by depositing at room temperature onto oxidized Si wafers. Following deposition of each B film, overlayers of Pd, Ag, or Si were deposited at substrate temperatures of approximately 50 degree(s)C and immediately studied with AES and XPS. In the Pd case, we find that it reacts with the B to form a smooth, amorphous PdxB1-x layer, with x approximately equals 0.9. This reaction occurred for all Pd coverages studied, from 2.3 to 230 angstrom. However, due to high Pd content of this reacted compound, our calculations show that the PdxB1-x should perform as a good absorber layer as part of PdxB1-x/B multilayer mirrors. We find that Si deposited onto B forms a sharp interface and an amorphous overlayer of pure Si. Depositing Ag on B results in a polycrystalline layer, composed of large Ag islands.

A baseline design and wafer cost analysis for a soft x-ray projection lithography (SXPL) system are presented. The design study provides an overview of the likely form and structure of a SXPL prototype. The wafer cost analysis provides an assessment of the relative importance of different cost factors in the SXPL system.

Soft x-ray projection lithography (SXPL) may be used to fabricate high-resolution structures for future integrated circuits. This technique uses a reflection mask which is a substrate coated with an x-ray multilayer mirror and patterned with a thin (approximately 50 nm) layer of x-ray absorber. Mask patterning processes must not degrade the reflectivity of the x-ray mirror and mask repair techniques must be developed. The technical challenges of conventional reflecting optical imaging system designs are severe and mask technology can have a significant impact on this issue. Specifically, innovative mask designs can reduce the complexity of the optical system by decreasing the number of mirrors and replacing aspheric optical surfaces with spherical surfaces. We have developed a technique, called Encoded Mask Lithography, with which we have designed an optical system which uses only two (spherical) imaging mirrors and has < 100 nm spatial resolution, negligible distortion, and > 30 mm diameter field of view.

Based on Korsch's method for design of high performance telescopes, a differential equation design method (DEDM) for soft x-ray projection lithography has been developed. This method enables one to solve numerically for the shapes of two mirrors in a system such that the entire system rigorously satisfies the conditions of aplanatism. The shape of the other mirror(s) in the system are adjusted to minimize the residual aberrations. Three-mirror and four-mirror projection systems have been designed by using DEDM. Analysis of results is presented.

Mo/Si multilayer mirrors with a high reflectance at normal incidence in the 232 - 236 angstrom spectral region have been deposited by rf magnetron sputtering for use in a XUV Ge-laser. The mirrors had a peak reflectance of 26% in this wavelength region. Characterization by TEM and XRD indicates good thickness control in the deposition process and low interface roughness, although interdiffusion is present at the interfaces. Preliminary experiments indicate that the XUV laser output intensity was increased when a multilayer mirror was added to allow a double pass through the gain medium.

In the PTB laboratory at the electron storage ring BESSY flat multilayer mirrors as well as curved multilayer optics are characterized in the soft x-ray region using synchrotron radiation. Precision reflectometry is performed in the spectral range between 5 and 30 nm where the content of higher orders in the monochromatized radiation is below 2%. The existing facilities and current improvements of the instrumentation are described. Examples for spatially resolved measurements of the reflectance of curved multilayer optics are presented.

A new high-resolution soft x-ray reflectometer system utilizing a laser-plasma light source is described. This reflectometer is used to measure the reflectance of multilayer-coated optics for projection lithography. A detailed description of the instrument is presented, along with recent results obtained for a variety of soft x-ray multilayer coatings, multilayer-coated figured optics, and patterned x-ray reflection masks

Two different electron microscopy techniques were used to study a Ru/Si multilayer (ML) film. The structure of the multilayer was characterized by high-resolution electron microscopy (HREM). The multilayer compositional profile and its thermal stability were studied by using high-angle annular dark-field (HAADF) microscopy. Initially, the Ru/Si ML was found to have a well-defined multilayer structure. After annealing of the sample at 150 degree(s)C for 30 minutes, we observed that the Ru and Si layers were highly interdiffused with large Ru silicide crystals being formed in the multilayer film.

Beryllium has extremely low absorption (4/cm) for wavelengths below the carbon K absorption edge at 44 angstrom. This property makes Be a potentially useful spacer material for x-ray multilayer mirrors in the water window region of the spectrum (24 - 44 angstrom). In addition, Be based mirrors would have high reflectivities for wavelengths above the Be K edge ((lambda) > 114 angstrom), making them useful for applications in the spectral region around the Si L edge (at 124 angstrom). However, because thin films of this material have not been previously studied in detail, relatively little is known about the optimum conditions for Be growth. We have adapted a Riber 1000 Molecular Beam Epitaxy system, formerly used for epitaxy of GaAs, for deposition of beryllium films. We have grown hcp beryllium epitaxially on (001) sapphire substrates and have fabricated several beryllium/germanium and beryllium/bismuth multilayers. Ge and Bi were chosen as candidates for the absorber layers in our initial growth studies because of their expected low reactivity with Be as well as high predicted reflectivities (up to 53%) in the wavelength regions of interest. Characterization of our beryllium films and multilayers includes studies with reflection high energy electron diffraction, low and high-angle 0 - 20 x-ray diffraction, scanning electron microscopy, scanning tunneling microscopy, Auger depth profile analysis, Rutherford backscattering spectrometry, and ion beam channeling.

Tungsten/carbon (W/C) multilayer thin films with a nominal d spacing varying from 2.5 to 14 nm were prepared by magnetron sputtering technique. The thicknesses of the W and C layers were varied from 0.5 to 12 nm. The multilayers were subjected to isochronal anneals in a quartz tube furnace in the range of 300 to 1000 C under high purity Ar flow conditions. X-ray diffraction, Raman scattering and Auger depth profiling were used to characterize the structure of the as-prepared and annealed films. It is found that an overcoat layer of silicon nitride (30-50 nm) prevents the multilayers from oxidation during the 1 hr heat treatment at temperatures as high as 1000 C in Ar flow. In all studied W/C multilayers, the carbon layers are amorphous (up to 12 nm). The tungsten layers are also amorphous when their thicknesses are less than 5 nm. Tungsten layers thicker than 5 nm show crystalline W peaks in addition to the amorphous W feature. Annealing of samples with a silicon nitride protective layer results in several structural changes which depend on annealing temperature, d spacing, the as-deposited W layer structure and the layer thickness ratio of W to C. For W layer thicker than C layer and W layer thickness > 4 nm and/or C layer thickness < 1 nm, the multilayers show the initial crystal formation of microcrystalline W2C occurring at C-W interfaces (that interface in which C was deposited on W) after 600 C anneal, followed by a second crystallization of a-W or a-W and WC at W-C interfaces (W was deposited on C) at the annealing temperature of 900 C. They reveal a relatively small (< 5 %) or essentially no layer expansion. For those multilayers having thin W layers (2 nm) and the same or thicker C layer thicknesses, the initial crystallization takes place at both W-C and C-W interfaces at 900 C or higher. The crystal formed is a-W or a-W and WC. The layer pair period of the multilayers in this group increases monotonically with increasing annealing temperature. Expansion is up to 16 % of the original d spacing and occurs in both W and C layers at approximately equal rates. The expansion in all multilayers is interpreted to be associated mainly with the structural ordering processes in the amorphous W and C layers.

The thermal stability of Mo/Si multilayers for x-ray mirror applications was investigated by annealing studies at relatively low temperatures for various times. The as-deposited and annealed multilayers were examined using conventional small and large angle x-ray diffraction, normal incidence x-ray reflectance measurements using a synchrotron source, selected area electron diffraction, and high-resolution electron microscopy. The as-deposited structure consists of pure layers of crystalline Mo and amorphous Si separated by thin regions of amorphous Mo-Si. At temperatures between 200 - 400 degree(s)C the amorphous Mo-Si interlayers grow and hexagonal MoSi2 forms by a thermally activated process(es) and the bilayer spacing and x-ray reflectivity decrease. A determination of the effective activation energy of the process(es) suggests long-term stability at the mirror operating temperature, although additional low temperature testing is warranted.

We report on two methods for producing multilayer optics with considerably better reflectivities than `state of the art' coatings: deposition at optimized substrate temperatures and application of ion beam bombardment. In the first case we show that deposition of Ni - C multilayer coatings at optimized substrate temperatures results in increases in reflectivity at (lambda) equals 6.76 nm with a factor of 1.4 (Toptimum approximately equals 100 degree(s)C, d approximately equals 5.0 nm) compared to those deposited at room temperature. In the case of ion bombardment (at room temperature) the increase in reflectivity is more dramatic: a factor of 4 was observed for ion etched Ni - C coatings compared to nontreated ones (ion beam of 200 eV Ar+ at an angle of incidence of 45 degree(s), d approximately equals 2.8 nm). We note that due to the difference in d-spacing of the coatings the increases can not be compared directly. Finally, we discuss thermal treatment of Ni - Si multilayer coatings: contrary to Ni - C we observed an increase in reflectivity at temperatures below room temperature.

Fabrication and environmental damage issues may require that the multilayer x-ray reflection coatings used in soft x-ray projection lithography be replaced or repaired. Two repair strategies were investigated. The first was to overcoat defective multilayers with a new multilayer. The feasibility of this approach was demonstrated by depositing high reflectivity (61% at 130 A) molybdenum silicon (Mo/Si) multilayers onto fused silica figured optics that had already been coated with a Mo/Si multilayer. Because some types of damage mechanisms and fabrication errors are not repairable by this method, a second method of repair was investigated. The multilayer was stripped from the optical substrate by etching a release layer which was deposited onto the substrate beneath the multilayer. The release layer consisted of a 1000 A aluminum film deposited by ion beam sputtering or by electron beam evaporation, with a 300 A SiO protective overcoat. The substrates were superpolished zerodur optical flats. The normal incidence x-ray reflectivity of multilayers deposited on these aluminized substrates was degraded, presumably due to roughness of the aluminum films. Multilayers, and the underlying release layers, have been removed without damaging the substrates.

In response to the needs of the emerging field of normal incidence soft x-ray optics, a field with applications ranging from extreme ultraviolet (XUV) solar imaging to x-ray lithography, the National Institute of Standards and Technology (NIST) has initiated an XUV multilayer and optical substrate characterization program. In this paper, we give an overview of the present capabilities of the NIST facility and discuss some of the proposed improvements, concentrating on the new soft x-ray reflectometry facility being built at SURF.

The successful development of soft x-ray projection lithography will depend heavily on the production of efficient, durable optical components. A leading candidate for x-ray mirrors near 130 angstrom is multilayer structures made of alternating thin layers of Mo and Si. High- resolution electron microscopy reveals that the interface created by deposition of Mo on Si is much more diffuse than that produced by depositing Si on Mo. We have performed molecular dynamics simulations of the deposition process and observed significant penetration of the Si substrates by incident Mo atoms while Si atoms remain on the surface of the Mo substrates.

Fe/C multilayers were rf-sputtered on two silicon substrates with substantial difference of surface quality. Microroughness and figure were measured for each substrate before and after layer deposition by using a scanning tunnelling microscope, an optical microscope interferometer, and a Fizeau interferometer. Although the optical quality of both substrates is better than that of current wafers, differences of x-ray reflectivity and peak shape are still observed. The better sample was exposed to an x-ray wiggler beam of 4.6 keV critical energy, so that the multilayer surface received a power density of 5W/mm2 during a 4 hour period, the sample being cooled from the backside. The reflection properties of the sample before and after x ray exposure are compared. At first glance we did not see any significant changes in reflectivity or bandpass, which is very promising for the use of multilayers on high-power synchrotron x-ray beamlines.