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 .ANG. for these applications, the surface must be smooth on the atomic scale. To provide such a smooth surface is an exceedingly difficult and time-consuming procedure.
In 1952, Wolter 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 .ANG. 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 .ANG. 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 reflecting surface. The decrease in efficiency and attainable imaging resolution resulting from a surface roughness of 12-15 .ANG. rms is unacceptable.
Thus, there is a need for a method to make x-ray optics with a surface roughness less than 12 .ANG. rms. The present 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 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.