Patent Number: 050864432
Section: description

DETAILED DESCRIPTION OF THE INVENTION Briefly, the present invention includes the use of a multiple-layer "wavetrap" deposited over the surface of a layered, synthetic-microstructure soft x-ray mirror optimized for reflectivity at chosen wavelengths for reducing the reflectivity of undesired, longer wavelength incident radiation thereon. In three separate mirror designs employing an alternating molybdenum and silicon layered mirror structure overlaid by two layers of a molybdenum/silicon pair antireflection coating, reflectivities at the wavelengths 133, 171, and 186 .ANG. have been optimized, while that at 304 .ANG. has been minimized. The optimization process involves the choice of materials, the composition of the layer/pairs as well as the number thereof, and the distance therebetween for the mirror, and the simultaneous choice of materials, the composition of the layer/pairs, their number and distance for the "wavetrap." Many details of the present invention are disclosed in the journal article entitled "Metal Multilayer Mirrors For EUV/Ultrasoft X-Ray Wide-Field Telescopes," by Barnham W. Smith, Jeffrey J. Bloch, and Diane Roussel-Dupre, Optical Engineering 29. 592 (1990), the teachings of which are hereby incorporated by reference herein. Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Turning now to FIG. 1 hereof, there is plotted the performance of layered, synthetic-microstructure mirrors showing the effects of employing the "wavetrap" of the present invention versus varying the composition of the layers of the mirror. A*.OMEGA. represents a telescope's total area-solid-angle product, and is a measure of a multilayer mirror's performance as it operates within a telescope. To determine A*.OMEGA. of a telescope at a given wavelength, the differential contribution of incident rays reflected from the mirror multiplied by the multilayer reflectivity curve for the mirror must be integrated as a function of incidence angle for that wavelength. The goal is to maximize the reflectivity at 171 .ANG., but minimize the reflectivity for the background radiation at 304 .ANG.. Therefore, improved mirror designs are to be found toward the left and top of the plot. FIG. 1a shows the theoretical performance of a molybdenum/silicon multilayer mirror without the "wavetrap" of the present invention as the thickness of the molybdenum layer is varied. .GAMMA. is given by the relationship .GAMMA.=Mo layer thickness/(Mo layer thickness+Si layer thickness). FIG. lb represents the theoretical performance of the 171 .ANG. optimized "wavetrap," while FIG. 1c represents a conservative empirical estimate of how well the "wavetrap" of the present invention will work based on fabricated samples. Optimizations were performed to determine the layer thicknesses yielding the best compromise of high reflectivity for the chosen soft x-ray wavelengths and low response for selected background radiation having longer wavelength. Multilayer reflectivity models were computed with a computer code uses the complex matrix solution method of M. Born and E. Wolf, Principles Of Ootics, Pergammon Press, London (1959), while optical constants employed for molybdenum and silicon were obtained from D. L. Windt, Appl. Opt. 27. 246 (1988), and E. B. Palik, Handbook Of Optical Constants. Academic Press, New York (1985), respectively. Peak reflectivity for the desired angle .theta. is obtained using the Bragg condition for the working wavelength to initially set the spacing of the Mo/Si layers (Bragg condition: 2dsin.theta.=n.lambda., where d is the total thickness of the Mo and the Si layers in each layer pair, .theta. is the angle of reflection from the surface, n is a positive integer and equals unity in this case, and .lambda. is the wavelength). Further fine tuning is necessary because of refraction, absorption, and atom migration in the interface between the surfaces when the layers are set down, as will be set forth below. While maximizing the reflectivity in each mirror's bandpass, it is necessary to minimize sensitivity to background emissions. As stated above, the most serious background for the ALEXIS telescope system in low earth orbit is the geocoronal emission of ionized helium at 304 .ANG.. This radiation is quite intense, perhaps 10.sup.5 times the signal for which measurements are desired in the soft x-ray region from hot, interstellar plasma and other cosmic sources. Therefore, it is necessary to achieve a rejection ratio of at least 10.sup.6 between 304 .ANG. and the peak wavelength for each mirror. A "wavetrap" consisting of two-layer pairs with a different d spacing from that of the multilayered mirror is deposited on top of the other layers. Generally, mirrors are fabricated with a silicon (low-Z) layer farthest from the incident radiation, and a molybdenum (high-Z) layer facing the source of incident radiation, so that the first layer of the "wavetrap" is deposited directly on the high-Z material of the mirror, and is itself a low-Z material. To suppress reflection at 304 .ANG. , the spacing of these extra two-layer pairs are such that standing-wave patterns are set up which destructively interfere with the reflected wave. The 304 .ANG. radiation is then absorbed within the multilayer. It should be mentioned that experiments using one layer pair and three layer pairs were performed with the result that the former did not yield sufficient rejection of the 304 .ANG. radiation, while the latter exhibited excessive absorption of the wavelength for which the mirror reflectivity was optimized. Therefore, two layer pairs were found to be optimal. Since the destructive standing waves of 304 .ANG. radiation in the top two layers interact with the structure of the multilayer mirror below them as a boundary condition, the exact d spacing of the "wavetrap" and the mirror must simultaneously be optimized. That is, the exact d spacing of the "wavetrap" will be different for each type of mirror. Calculations predict the 186 .ANG. design will have a peak reflectivity of 35% for the 186 .ANG. radiation, and a 304 .ANG. reflectivity of less than 10.sup.-5, compared with a peak reflectivity of 40% and a 304 .ANG. reflectivity of 10.sup.-3 without a "wavetrap." Having generally described the present invention, the following example is provided to more particularly set forth the details of apparatus hereof. EXAMPLE Optimized mirrors for three soft x-ray wavelengths have been designed and fabricated as is illustrated in the Table. Therein, the thicknesses of the high-Z material and low-Z material for both the mirrors and their associated "wave traps are provided". Mirrors were constructed having between sixty and one hundred layers and, as stated above, "wavetraps" were found to optimize at two layer pairs. These mirror designs effectively reject the reflections from 304 .ANG. radiation. TABLE ______________________________________ Mirror: "Wavetrap" Wavelength Mo Si Mo Si ______________________________________ 186 .ANG. 31 .ANG. 70 .ANG. 11 .ANG. 47 .ANG. 170 .ANG. 35 .ANG. 58 .ANG. 11 .ANG. 45 .ANG. 130 .ANG. 28 .ANG. 42 .ANG. 10 .ANG. 45 .ANG. ______________________________________ Calculations for the 186 .ANG. wavelength situation suggested that the molybdenum and silicon thicknesses for the mirror and "wavetrap" should be 38 and 74 .ANG., and 10 and 55 .ANG., respectively, as opposed to the values quoted in the Table. When fabricated, although the mirror having these dimensions had a wavelength for peak reflectivity at 186 .ANG., that for the peak rejection efficiency was not at 304 .ANG.. This is thought to be due to migration of the atoms of one layer into another during the deposition process or surface contamination. That is, the layer thicknesses are not precisely defined. Empirical studies and modification of the calculations provided a route to prediction of the optimized values. FIGS. 2a and 2b show the reflectivity for 130 .ANG. radiation and that for 304 .ANG. radiation, respectively, as a function of incidence angle for a typical mirror. As stated, a major problem in the fabrication of multilayer mirrors according to the teachings of the present invention is the layer-to-layer uniformity of the sputtered layers, since only a well-defined layered structure will provide the constructive interference required for maximum reflectivity. The boundary definition can be determined from the number of satellite peaks observed at Cu--K.sub..alpha.. A typical fabricated mirror may have as many as sixteen higher orders visible in a diffractometry measurement. Therefore, attempts to model the Cu--K.sub..alpha. measurements with more than 0.5 .ANG. or .+-.0.5% deviation of the thickness fails to reproduce the observations, indicating that the mirrors are uniform to within this diagnostic's capabilities. However, the empirical fine-tuning required to optimize the "wavetrap" specifications indicate, that the thicknesses may not be exact. Another possibility is that the optical constants derived from the literature are slightly incorrect. Other issues include the two-dimensional uniformity over the surface of the mirror. Nonuniform distances between the substrate and the sputtering system for curved pieces are the source of nonuniformities in layer thickness. Tests on fabricated mirrors show .+-.1% uniformity in the d spacings over the surface over several centimeters diameter piece, and .+-.1.5% over a 15 cm diameter circle. The foregoing description of several preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, although x-ray mirrors for telescopes have been described herein, it would be apparent to one having ordinary skill in the art of x-ray optics that the teachings of the present invention are applicable to focusing mirrors for x-ray lithography procedures using a free-electron laser where the absence of suitable optics currently requires the use of masks having the same dimensions as the circuit dimensions desired on the final integrated circuit chips. The free-electron laser source presents special problems that are solvable using our invention, since a number of harmonics (longer wavelength) of the soft x-ray wavelength to be generated are also present in the laser output. Removal of these harmonics is essential to provide the high resolution required for current lithography processes, since diffraction problems increase as the wavelength increases. Moreover, materials such as tungsten and carbon are known to have good optical properties in the soft x-ray region of the electromagnetic spectrum and are suitable for fabrication of the multilayer mirrors and "wavetraps" of the subject claimed invention. The embodiments were chosen and described to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.