Patent Number: 
Section: description

As shown in FIG. 1, an optical element 2 includes a substrate 4 with a multilayer structure 6 applied thereto. The multilayer structure 6 includes a plurality of layer sets 8 where each layer set 8 is made up of two separate layers of different materials: one with relatively high atomic number, or Z, and a second with relatively low atomic number. As shown in FIG. 1, the layer sets 8 are uniform, meaning that the d spacing does not vary either laterally or through the depth of the multilayer structure. The thickness d of each layer set 8 has a value ranging from approximately 1 nm to approximately 100 nm depending on the energies of the radiation to be reflected. From about 10 to 1000 layer sets may be deposited on a substrate, depending on the desire qualities of the multilayer structure 6. The layer sets 8 preferably are composed of two material layers 10, 12 with diverse electron densities. Each of the material layers 10 and 12 preferably have a substantially identical thickness ranging from approximately 0.5 nm to approximately 50 nm. The absorber or high electron density layer 10 behaves like the plane of atoms in a crystal, while the low electron density layer 12 is analogous to the space between the planes. The high electron density layers 10 can include either W, Ni, Mo, Fe, Cr, Co, V, Mn, Nb, Ru, Rh, Pd, La, Ta, Re or Pt. The low electron density layer can include either silicon, carbon, B4C, Be, Li, B, Al or Sc. Thus, examples of layer sets 8 are WB4C, NiB4C, NiC, WSi, MoSi or MoB4C. Note that while the layer sets 8 of FIG. 1 show the low Z/low electron density layers 10 above the high Z/high electron density layers 12, it is possible to reverse their order without departing from the spirit of the invention so that the high Z layers 12 are above the low Z layers 10. If such reversal is performed, then the thicknesses of the various layers 10, 12 and the protective layer 14 are kept the same in the case where they are not reversed. The substrate 4 upon which the layer sets 8 are produced must meet precise specifications. The surface of substrate 4 must be capable of being polished to roughness that is precise on an atomic level. The root mean squared surface roughness of the substrate of the preferred embodiment will range from 0.5 to 20 angstroms, measured at intervals of about 10 angstroms. Examples of material used for substrate 4 are silicon wafers, mica, quartz, zerodur, sapphire, germanium, pyrex, silicon carbide or other like substances as are described in U.S. Pat. No. 5,646,976, the entire contents of which are incorporated herein by reference. In order to reduce the risk of radiation enhanced damage of the multilayer structure 6, a protective coating 14 is applied on the exterior surface 16 of the multilayer structure by such well known techniques as sputter down techniques, magnetron sputtering, e-beam evaporation, ion-beam sputtering, evaporation, electron beam implantation/plating. The protective coating 14 is generally made of a stable element like Silicon or a stable compound like SiC. The protective coating 14 has a thickness having a value ranging from approximately 60 xc3x85 to approximately 500 xc3x85, which corresponds to a loss in reflectivity of about 0.5% to 1% for Cu Kxcex1 radiation depending on the material used and the thickness of the protective coating 14. In addition, the thickness of the protective coating 14 can vary depending on the wavelength of the incident radiation. In the case of the protective coating 14 being silicon and the layer sets 8 being NiB4C, air interacts with the silicon to form a very thin layer of the oxide SiO2, which is known to be very resistant to radiation enhanced damage. The oxide reduces the possibility of the absorber layer 10 of Ni reacting with ionizing oxygen thereby extending its lifetime and the lifetime of the multilayer structure 6. The embodiment of the optical element described above with respect to FIG. 1 uses a single layer for the protective coating. As will be explained below with respect to FIGS. 2 and 3, it is possible to embody the protective coating as a multi-layer structure. In particular, an optical element 2xe2x80x2 shown in FIG. 2 includes a multilayer protective coating 14xe2x80x2 deposited on the multilayer structure 6 and substrate 4 discussed previously with respect to FIG. 1. The multilayer protective coating 14xe2x80x2 includes a plurality of layer sets 18 where each layer set 18 is made up of two separate layers of different materials: one with relatively high atomic number, or Z, and a second with relatively low atomic number. The multilayer protective coating 14xe2x80x2 has two roles to perform: 1) act as a protective coating and 2) contribute to reflection. The second role implies that the multilayer protective coating 14xe2x80x2 will have a spacing dxe2x80x2 that is the same as the spacing d of the multilayer structure 6. In particular, the layer sets 18 are uniform, wherein each layer set 18 has a spacing dxe2x80x2 that has a value ranging from approximately 1 nm to approximately 100 n depending on the energies of the radiation to be reflected. The layer sets 18 preferably are composed of two material layers 20, 22 with diverse electron densities. Each of the material layers 20 and 22 preferably has a substantially identical thickness having a value ranging from approximately 0.5 nm to approximately 50 nm. The absorber or high electron density layer 20 can include either W, Ni, Mo, Fe, Cr, Co, V, Mn, Nb, Ru, Rh, Pd, La, Ta, Re or Pt. While the high electron density layer 20 preferably is made of the same material as the high electron density layer 12 of the multilayer structure 6, it can be made of a different material. The low electron density layer 22 can include either silicon, carbon, B4C, Be, Li, B, Al or Sc, wherein the low electron density layer 22 is made of a material that is different than the material of the low electron density layer 10 of the multilayer structure 6. The low electron density layer 22 should be more resistant to radiation damage than the low electron density layer 10 and should have a density that is as close as possible to the density of the low electron density layer 10. Note that while the layer sets 8 and 18 of FIG. 2 show the low Z/low electron density layers 10, 22 above the high Z/high electron density layers 12, 20, respectively, it is possible to reverse their order without departing from the spirit of the invention so that the high Z layers 12, 20 are above the low Z layers 10, 22. If such reversal is performed, then the thicknesses of the various layers 10, 20, 12 and 22 are kept the same in the case where they are not reversed. Normally, the number of layer sets 18 of the protective coating 14xe2x80x2 is much smaller than the number of layer sets 8 of the multilayer structure 6. For example, if the number of layer sets 8 deposited for the multilayer structure 6 is, say, 80 then, the number of layer sets 18 deposited for the multilayer protective coating 14xe2x80x2 is around 10. There are several advantages for using a multilayer protective coating 14xe2x80x2 instead of a single layer protective coating 14. For example, a single layer protective coating acts primarily as an absorber. That is, while it shields the multilayer structure from direct exposure to atmosphere, it also absorbs x-rays thereby reducing the reflectivity. On the other hand, deposition of a multilayer protective coating, while shielding the multilayer structure from direct exposure to atmosphere, also contributes to reflection. This implies that the loss of reflectivity is minimized. Another advantage of using a multilayer protective coating is that since such a coating contributes to reflection, many layers can be deposited which, in turn, increases the thickness of the protective coating thereby affording better protection to the multilayer structure. A simple example of the above scheme is to deposit 80 layer sets 8 of Ni/C, which will act as the multilayer structure 6 and then deposit 10 layer sets 18 of Ni/Si on top of the multilayer structure 6, which will act as the multilayer protective coating 14xe2x80x2. A second embodiment of an optical element that includes a multilayer protective is shown in FIG. 3. The optical element 6xe2x80x3 includes a multilayer protective coating 14xe2x80x3 deposited on the multilayer structure 6 and substrate 4 discussed previously with respect to FIG. 1. The multilayer protective coating 14xe2x80x3 includes a plurality of layer sets 18xe2x80x2 where each layer set 18xe2x80x2 is made up of two separate layers of different materials, each with a relatively low atomic number. In contrast with the multilayer protective coating 14xe2x80x2 of FIG. 2, the multilayer protective coating 14xe2x80x3 does not significantly contribute to reflection. Instead, the multilayer protective coating 14xe2x80x3 is designed to increase resistance to radiation damage by exploiting resistive properties of the layer sets 18xe2x80x2. Since the multilayer protective coating 14xe2x80x3 does not significantly contribute to reflection, the spacing dxe2x80x2 of the layer sets 18xe2x80x2 may or may not be equal and need not be the same as the spacing d of the multilayer structure 6. In this scheme, a single layer topcoat is replaced with multiple layers of light elements. Consider, for example, a multilayer of NiC. A single Si layer of thickness, say 100 xc3x85, can be deposited in the embodiment of FIG. 1 or two layer sets 18xe2x80x2 can be deposited, wherein each layer set 18xe2x80x2 contains alternating 25 xc3x85 thick layers of Si and C, as shown in FIG. 3. In this scenario, the top layers do not contribute to reflection but afford the possibility of making the protective topcoat 14xe2x80x2 more resistant to radiation damage by exploiting resistive properties of their combination. In this scenario, the thickness dxe2x80x3 of the layer sets 18xe2x80x2 mayor may not be equal and need not be equal to the d spacing of the multilayer 6. The thicknesses of the layer sets 18xe2x80x2 are primarily determined by a combination that affords the best protection. The layer sets 18xe2x80x2 preferably are composed of two material layers 20xe2x80x2, 22xe2x80x2 with similar electron densities. The low electron density layers 20xe2x80x2, 22xe2x80x2 each include either silicon, carbon, B4C, Be, Li, B, Al or Sc, wherein the material of layer 20xe2x80x2 differs from the material of layer 22xe2x80x2. The thicknesses and numbers of the layer sets 18xe2x80x2 and layers 20xe2x80x2 and 22xe2x80x2 are chosen so as to maximize radiation protection for the multilayer structure 6. For example, two layer sets 18xe2x80x2 having a spacing dxe2x80x3 of 50 xc3x85 can be used, wherein each layer set 18xe2x80x2 includes alternating layers 20xe2x80x2 and 22xe2x80x2 of Si and C. The layers 20xe2x80x2 and 22xe2x80x2 have equal thicknesses of 25 xc3x85. The multilayer structures 6 and protective coatings 14, 14xe2x80x2, 14xe2x80x3 described above with respect to FIGS. 1-3 can be applied in another of ways. For example, the multilayer structure 6 and protective coatings 14, 14xe2x80x2, 14xe2x80x3 can be used to form either flat or curved optical elements 2, 2xe2x80x2, 2xe2x80x3. An example of a method of forming flat and curved optical elements is described in U.S. Pat. No. 5,646,976, the entire contents of which are incorporated herein by reference. Proposed applications of such optical elements 2, 2xe2x80x2, 2xe2x80x3 include spectroscopy and diffractometry. As shown in FIG. 4, such systems include an x-ray source 16 that emits a set of x-rays that are directed to the optical elements 2, 2xe2x80x2 or 2xe2x80x3. X-rays pass through the protective layer 14, 14xe2x80x2, 14xe2x80x3 and are diffracted by the multilayer structure 6. The optical elements 2, 2xe2x80x2, 2xe2x80x3 could also be applied to focusing optics, for x-ray lithography and microscopy, in particular, optics for high resolution scanning x-ray microscopy, point-to-point imaging optics including multi-element systems, an optic for monochromatization of broad-band radiation, synchrotron radiation in particular. Many medical applications are also contemplated, in particular, as power filters to eliminate undesired energy or use in radiography where a high contrast image is desired. While the above description constitutes the preferred embodiments of the present invention, it will be appreciated that the invention is susceptible of modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims. For example, the optical elements can also be used for transformation beams of cold and thermal neutrons. In particular, they can be used for increasing density and uniformity of neutron flux or separation of the neutrons with different spin.