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HIGH EFFICIENCY REPLICATED X-RAY
OPTICS AND FABRICATION METHOD
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to x-ray optical devices, particularly devices having a tubular shape, and more particularly to high efficiency replicated x-ray optics and a method of fabrication using a super-polished mandrel.
2. Description of Related Art
X-ray optical devices are used to change the propagation path of travel of x-rays. These devices can also serve to preferentially select x-rays of a desired wavelength range from a broader band of wavelengths. X-ray optical elements primarily use the mechanism of reflection, in contrast to visible light optics that commonly use refraction. To be efficient, x-ray mirrors must have a surface smoothness on the scale of the x-ray wavelength. Since typical x-ray wavelengths are 1-100 A for these applications, the surface must be smooth on the atomic scale. To provide such a smooth surface is an exceedingly difficult and timeconsuming procedure.
In 1952, Wo Iter proposed the application of a double specular reflection mirror system having a closed surface for focusing of x-rays. This structure was substantially more complex than previous optics and presented serious fabrication difficulties. First attempts to produce Wolter optics were initiated in the 1960's using electrodeposition on negative forms due to the closed surface of these optics. These replication attempts were unsuccessful as very poor figure and surface quality were achieved. In the 1980's, efforts were reinitiated for the development of thin shell structures for space telescopes. These negative form electrodeposition replication efforts have been used in the Czech Republic, Italy, and the United States. Several replication fabricated Wolter structures have been flown in space. These mirrored surfaces achieved the figure and roughness values approaching 15 A rms that are adequate for those applications, but not for applications requiring greater resolution and using shorter x-ray wavelengths.
The replication technique has the potential of lower cost and ease of manufacture. The cost of internally polishing and coating the surface of a tubular optic (typical length 10 cm, average diameter 2 cm) and achieving the smooth internal surface finish required is on the order of $500,000 and requires about one year to fabricate. Each optic device produced would have similar cost and time considerations. By comparison, the use of a negative form mandrel reduces the cost by a factor of 10-100 per mandrel for substrate preparation during development, with further significant cost reductions in the manufacturing stage. In view of the demonstrated effectiveness of the replication approach in the fabrication of moderate resolution Wolter space telescopes, research was directed towards the use of replicated optics for x-ray microscopes used in inertial confinement fusion studies and collimators for x-ray proximity lithography.
A primary problem with replicated optics has been achieving smoothness on the replicated part. Past efforts have not been able to achieve a roughness less than 12-15 A rms. This resulted from the low strength of the layer
directly in contact with the mandrel and the lack of control of the adhesion of this layer to the mandrel. Parting of the optic from the mandrel causes plastic deformation of the reflecting layer and degradation of the smoothness of the
5 reflecting surface. The decrease in efficiency and attainable imaging^ resolution resulting from a surface roughness of 12-15 A rms is unacceptable.
Thus, there is a need for a method to make x-ray optics with a surface roughness less than 12 A rms. The present
1° invention is based on the recognition that magnetron sputtering deposition can be used, even though previously sputter deposited replicated optics have been of poor quality. The fabrication method of the present invention, based on supporting multilayer structures and a special parting layer, has
15 been developed to produce strong stress-relieved reflecting surfaces with supporting shells that do not deform during the separation process and consequently produce super-smooth surfaces comparable to that of the initial mandrel.
20 SUMMARY OF THE INVENTION
It is an object of the present invention to provide replicated x-ray optics having a surface roughness of less than 12 angstroms rms and a method for reproducibly fabricating these x-ray optics with a super-smooth surface. A further 25 object of the invention is to provide x-ray optical devices that have a tubular shape, open at both ends, and an interior surface highly reflective to x-rays within a specified wavelength band.
Another object of the invention is to provide x-ray optical 30 devices having shapes that are truncated paraboloidal, ellipsoidal, hyperboloidal, or polynomial shells of revolution.
Another object of the invention is to provide a method of 35 fabricating tubular shaped x-ray optics by dc or rf sputter deposition of reflecting layers onto a super-polished reusable mandrel, strengthening the reflecting layers by a sputter deposited multilayer, then further supporting this structure with a low residual stress electrodeposited layer, and separating the layered optical device from the mandrel, resulting in a tubular shell with an interior surface having the shape and surface smoothness of the mandrel.
A further object of the invention is to provide increased strength to the reflecting layer resulting from a supporting 45 multilayer, which enhances the ability to part the replica from the mandrel without degradation in surface roughness and performance.
Another object of the invention is to provide a parting layer that maintains or enhances the smoothness of the 50 mandrel, provides uniform adhesion, and substantially decreases the adhesion of the reflecting surface material to the mandrel, and reduces the forces required to part the replica structure and thus the potential for increased surface roughness.
55 Yet another object of the invention is to provide a tubular shaped optic wherein the inner reflecting surface can be composed of either a single layer grazing reflection mirror or a resonant multilayer mirror, where the wavelength bandpass of the multilayer mirror can be used to select a specific
60 band of x-ray energies.
The invention involves high efficiency replicated x-ray optics and the method of fabrication. The x-ray optical device has a tubular shape that is open at both ends, with the interior surface being highly reflective to x-rays within a
65 wavelength band of interest. A beam of x-rays enters one end, undergoes a single reflection at the interior surface, and exits from the other end with a different direction of travel.
The shapes of the optics are truncated paraboloidal, ellipsoidal, hyperboloidal, or polynomial shells of revolution. Optics having a combination of these shapes can also be fabricated from a single mandrel.
The tubular optical devices are fabricated using a reusable 5 mandrel with a super-polished surface. The replicated optic is deposited by dc or rf sputter deposition of a reflecting layer or layers onto the mandrel surface, and thereafter the reflecting layers are strengthened by a sputter deposited multilayer, and then this structure is further supported with 1° a low residual stress electrodeposited layer. Aspecial parting layer of sputter deposited amorphous carbon may be deposited on the mandrel surface prior to deposition of the reflecting structure.
When the layered device is removed from the mandrel, 15 the tubular shell has an inner surface having the shape and surface smoothness of the master form mandrel. Surfaces having a roughness of less than 10 A rms, and as low as 3-5 A rms, have been fabricated. The low stress required to part the replica from the mandrel has made possible the main- 20 tenance of the surface figure of the mandrel in the replicated part and has also minimized the potential for damage to the mandrel during parting so that multiple replicas can be manufactured from a single mandrel.
The optic elements resulting from the present invention can form single element devices, or combinations of elements can be assembled to form multi-element (compound) devices. The optical elements can be used for applications including x-ray proximity and projection lithography, x-ray 3Q crystallography, x-ray microscopy, x-ray radiography, tomography, and x-ray fluorescence analysis. These reflective optics can also be used at longer ultraviolet wavelengths where conventional refractive optics do not exist.
Other objects and advantages of the present invention will 35 become apparent from the following description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated into 40 and form a part of the disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
FIG. 1 illustrates in partial cross-section a collimator that incorporates a tubular x-ray optic made in accordance with the present invention.
FIG. 2 illustrates the end-view cross-section of a mandrel having a parting layer, reflective coating, a supporting multilayer, and an electroplated outer coating. 50
FIG. 3 shows a cross-section of a tubular optic having a grazing reflection inner mirrored surface, a circular crosssection, and a polynomial shape.
FIG. 4 illustrates an experimental arrangement for collimator characterization. 55
FIG. 5 is a schematic showing a collimated ring field scanned across a lithographic print field.
FIG. 6 illustrates a schematic of a scanning collimator system.
FIG. 7 graphically illustrates an example of a truncated paraboloidal collimator that produces a parallel beam ring field.
FIG. 8 graphically illustrates an axicon scanning collimator. 65
FIG. 9 illustrates an embodiment of a two-element multilayer axicon system for focusing applications.
FIG. 10 illustrates ellipsoidal concentrating mirrors.
FIG. 11 illustrates an embodiment of a two-element collector/analyzer system.
FIG. 12 illustrates an x-ray microscope system having a two-element paraboloidal condenser and a two-element hyperboloidal-ellipsoidal Wolter objective.
DETAILED DESCRIPTION OF THE
The present invention involves high-efficiency replicated x-ray optics and a method of fabrication. A replicated optic having a tubular shape open at both ends with an interior surface that is highly reflective to x-rays within a wavelength band of interest is fabricated by dc or rf sputter deposition of reflecting layers onto a super-polished reusable mandrel. The reflecting layer or layers are strengthened by a supporting multilayer deposited thereon, and an outer mechanical supporting layer.
The supporting multilayer structure results in stronger stress-relieved reflecting surfaces that do not deform during separation from the mandrel, and consequently produce super-smooth surfaces comparable in smoothness to the initial mandrel surface (i.e., <12 A, and as low as 3-5 A rms). The increased strength of the reflecting layer enhances the ability to part the replica from the mandrel without degradation in surface roughness and performance. In addition, a parting layer is typically first sputter deposited on the mandrel to ease removal of the optic from the mandrel and to maintain its surface smoothness.
Upon separation of the layered device from the mandrel, the formed tubular shell has an inner surface with the shape and surface smoothness of the master mandrel. In operation, a beam of x-rays enters one end of the tubular shell, undergoes a single reflection at the interior surface, and exits from the other end with a different direction of travel. The shape of the optics is unlimited since the optical device mimics or replicates the shape of the mandrel. The optics may be truncated paraboloidal, ellipsoidal, hyperboloidal, or polynomial shells of revolution. The optics can be used singly or in combinations to form a range of x-ray optical systems.
Depending on the shape, x-rays are focused, collimated, or otherwise manipulated when they are allowed to enter one end of the shell, reflect from the inner mirrored surface, and then exit in a new direction. The inner reflecting layer may be either a single layer grazing reflection mirrored surface or, alternatively, a multilayer resonance reflection mirrored surface. The wavelength bandpass of the multilayer mirror can be used to select a specific range of x-ray energies.
High efficiency results from the high quality of the mirror, large reflection angles (especially for resonant mirrors), and the large collection solid angle of the tubular structure compared to more conventional open geometry x-ray optics. These optics have a number of applications, including static and scanning collimators for x-ray lithography, one and two element collection and focusing optics for x-ray crystallography, collection and concentration optics for x-ray fluorescence analysis, Wolter and zone-plate x-ray microscopes; x-ray radiographic systems and tomography.
FIG. 1 illustrates an embodiment of a collimator generally indicated at 10 comprising a collimator housing 11, an opening 12 extending therethrough, in which is located a cone 13 having a tapered inner surface in which is positioned a tapering tubular member 14 having a multilayer reflector shell 15 on the inner surface thereof. A beam block 16 may be located in an open end 17 of tubular shell 14. Thus, a