Patent Number: 050777742
Section: summary

TECHNICAL FIELD This invention relates to an apparatus which ses transition radiation, X-ray optics and special radiation geometries for the production of high intensity, uniform X rays for lithography in the production of integrated microcircuits. BACKGROUND OF THE INVENTION 1. Prior Art: X-ray Lithography Current commercial lithography techniques utilize optical and ultraviolet light to expose photoresist through a mask which has the circuit pattern imprinted on it. Present geometries of the circuit elements are limited by the wavelength of the radiation to greater than 0.5 microns in size. To produce smaller geometries, shorter wavelengths are needed. The use of soft X-rays for the exposure of photoresist will produce geometries of less than 0.25 microns; lines as small as 0.16 microns have already been produced. Four X-ray sources have been considered for lithography. These are the conventional X-ray tube (or electron impact sources), synchrotron storage ring, laser plasma sources and transition radiators. To date synchrotron and plasma sources have been the most widely discussed and used. Electron-impact sources have intrinsically low X-ray power, requiring 20 minutes or more for small area exposures, and are uncollimated, requiring stringent mask-to-wafer spacing. Synchrotron radiation has received the most interest as a commercial source for the high-volume production of integrated circuits, where laser-plasma sources have been touted as a relatively inexpensive source of soft X-rays for low-volume integrated-circuit production. The properties of synchrotron radiation which make it useful for X-ray lithography are its high intensity and collimation. See for example E. Spiller and R. Feder, "X-Ray Lithography," Topics in Applied Physics, vol. 22 (ed. H. J. Queisser; Springer Verlag, Berlin, Heidelberg, New York, 1977.) Unfortunately, synchrotron radiation requires massive machines costing $25 million for the storage ring and $2.5 million per lithographic station for a total of 16 to 24 stations. A 16 station synchrotron based lithography system is projected to have a per station cost of $4M to $5M and a total system cost of $64M to $80M. This exorbitant initial cost has prevented most U.S. companies from entering into X-ray lithography. Thus there is considerable interest in a relatively inexpensive X-ray lithography source which can have an initial cost of 4 to 6 million dollars and have one or several stations with the same cost of 4 to 6 million dollars per station. Laser-plasma sources are a potential single-station source of X rays in this price range. Hampshire Laboratories is producing an experimental system but the system has demonstrated only 2 milliwatts of X-ray power. Laser-plasma sources have the intrinsic technical problems of low power and lack of collimation. Low X-ray power limits this source to an experimental research tool or, to low volume integrated-circuit production. Lack of collimation requires it to be extremely close to the mask and wafer (10 to 20 cm) to maintain flux density, which then requires the spacing between the mask and wafer to not more than .+-.0.5 .mu.m variation to minimize shadowing and blurring of the circuit image. This required tolerance of .+-.0.5 .mu.m is difficult to achieve and may be prohibitive. The synchrotron source requires a maximum variation of only .+-.5 .mu.m. A new X-ray source is needed with the synchrotron's excellent technical characteristics and laser-plasma's moderate cost. 2. Prior Art: Transition Radiation as an X-ray Lithography Source In the prior art, transition radiation has been considered as an alternative source of soft X rays by M. A. Piestrup, J. O. Kephart, H. Park, R. K. Klein, R. H. Pantell, P. J. Ebert, M. J. Moran, B. A. Dahling, and B. L. Berman, "Measurement of transition radiation from medium-energy electrons," Phys. Rev. A vol. 32, pp. 917-927, Aug. 1985. and M. A. Piestrup, M. J. Moran, B. L. Berman, P. Pianetta, D. Seligson, "Transition radiation as an X-ray source for lithography," SPIE vol. 773, Electron-Beam, X-ray, and Ion-Beam Lithographies, pp. 37-44, 1987. Transition radiation has a number of advantages which it shares with synchrotron emitters. Both are high brightness, collimated source. The high degree of radiation parallelism (collimation) decreases geometrical distortions such as run-out and blurring of the circuit elements in a lithograph (See FIG. 1). Unique advantages of transition radiation are its relative low cost, moderate vacuum requirements and excellent spectral characteristics. One advantage synchrotron radiation now has over transition radiation is that of total X-ray power. Although the X-ray production by transition radiation is at least three orders of magnitude brighter on a per electron basis, storage rings have a higher average beam current, and this advantage is lost. The power density required for lithography is between 10 to 100 mW/cm.sup.2. Larger values than 100 mW/cm.sup.2 will result in excessive heating and possible damage to the X-ray mask. Lower values than 10 mW/cm.sup.2 will result in prohibitively long exposure times. Exposure times depend upon resist sensitivity that can vary between 10 mj/cm.sup.2 to 1000 mj/cm.sup.2. Higher resolution resist usually requires higher energy deposition. For example, polymethylmethacrylate (PGMA) photoresist with 0.25 .mu.m resolution or better requires 230 mj/cm.sup.2. This experimental resist would require 15 mW/cm.sup.2 for 15 second exposures. In the prior art, transition radiators have produced only fluxes of 0.7 mW (see M. A. Piestrup, M. J. Moran, B. L. Berman, P. Pianetta, D. Seligson, "Transition radiation as an X-ray source for lithography," SPIE vol. 773, Electron-Beam, X-ray, and Ion-Beam Lithographies, pp. 37-44, 1987). The Lawrence Livermore National Laboratory's (LLNL) electron-position accelerator was utilized to produced a 104-MeV (million electron volts), 44-.mu.A (amperes) electron beam which penetrated a stack of fifteen 1.5-.mu.m-thick aluminum foils generating only 0.7 mW of soft X rays at a peak photon energy of 1.4 keV (kilo-electron volts) with an approximate bandwidth of 100%. Exposure times for the mask and wafer (here in after called the mask/wafer) were long due to the large area of the X-ray beam and the large distance between the radiator and the mask/wafer (6 m). Since the X ray annulus diverges at 1/.gamma., after 6 m the area of the annulus is much larger than 6 to 12 cm.sup.2 for electron beam energies of 150 MeV and larger. Fluxes of 60 mW exposing area of 6 cm.sup.2 or less are needed for a 10 mW/cm.sup.2. The conical X-ray pattern emitted from the transition radiator used in this experiment was not uniform and exposed only an annular ring on the photoresist. Thus methods for making the radiation pattern uniform and increasing the X-ray power are needed. In the prior art it was believed that transition radiation was not adequate for high throughout (high production) of integrated-circuit chips since the exposed area was small and the total power of the transition radiator was small. Wafers of 4 to 6 inches in diameter are used in optical lithography, and it was assumed that transition radiation had to expose equivalent size areas: the total X-ray flux had to cover the entire area of the 4 to 6 inch wafers and still maintain a minimum of 10 mW/cm.sup.2 average power. Since the total from the transition radiator is limited, the number of chips per hour that this source could produce would be small. Thus transition radiation has not been seen as a competitor for X-ray lithography. In the prior art Holman and others assumed that extremely high currents were needed to generate adequate soft X-ray flux from transition radiators needed for X-ray lithography. Holman suggests a 1 mA, 100 MeV electron beam. This is 100 kW (kilowatts) of power, a difficult electron-beam power to achieve. In addition, the foils will melt as such a high current and the radiation hazard generated by such a high-current electron beam is appreciable. As Holman, himself says, "The assumption of beam uniformity is optimistic, as is the 1 mA current." Richard Holman, Intel Corporation, "X-ray lithography using broadband sources" (1986). 3. Prior Art: Transition Radiation Design X-rays are produced by transition radiation when high-energy electrons cross the interface between two media or between vacuum and a medium. The photon production for a single interface is small; however, by stacking a number of foils, the yield can be greatly increased. In most applications, individual foils separated by vacuum are used to reduce re-absorption of the X rays in the interventing medium. The photon production from transition radiation is intimately related to the thickness of an individual foil, not only due to re-absorption of the emitted radiation in the foils themselves, but also because a minimum thickness (known as the formation length) is needed for photon production. Re-absorption can be minimized by making the foils as thin as possible; however, if they are made thinner than the formation length, the photon production will drop. Thus, there is an optimum foil thickness that balances production with re-absorption, giving a maximum photon yield. For soft X rays, the thicknesses used in previous studies were between 0.5 and 5 .mu.m. In the prior art, the radiator would be constructed of thin foils of thickness l.sub.2 and plasma frequency .omega..sub.2 separated by either a gas or vacuum of thickness l.sub.1 and plasma frequency .omega..sub.1 (for the gas). See M. A. Piestrup, J. O. Kephart, H. Park, R. K. Klein, R. H. Pantell, P. J. Ebert, M. J. Moran, B. A. Dahling, and B. L. Berman, "Measurement of transition radiation from medium-energy electrons," Phys. Rev. A vol. 32, pp. 917-927, Aug. 1985. For the usual case, when l.sub.1 &gt;&gt;l.sub.2 and .omega..sub.1 &gt;&gt;.omega..sub.2, then the radiation is emitted at frequencies &lt;.gamma..omega..sub.2. This frequency represents a cutoff frequency above which the radiation falls dramatically. Since the plasma frequency of a material is proportional to the square root of its density, this cutoff frequency is proportional to the square root of the foil density. For beryllium foils, .omega..sub.2 =24.5 eV, and a .gamma. of 50 to 100 is needed for adequate photon production at 1.5 keV. The foil thickness is obtained from M. L. Cherry, D. Muller, and T. A. Prince, "Transition Radiation from relativistic electrons in periodic radiators," Phys. Rev. D., vol. 10, pp. 3594-3607, Dec. 1974. For photons to be generated the foil thickness, l.sub.2, and foil spacing, l.sub.1, must be on the same order of magnitude or greater than their "formation lengths" which are given approximately by: ##EQU1## the foil spacing: ##EQU2## where .omega. is the angular frequency of the radiation, .lambda.=c/.omega., .omega..sub.i (i=1,2) are the plasma frequencies of the interfoil gas and the foil material, respectively. In most cases, the foils are in a vacuum and .omega..sub.1 =0. In the prior art, the number of foils, M, that can be used is limited by absorption in the foils; the maximum number is: ##EQU3## where .mu. is the X-ray absorption coefficient of the foil material and l.sub.2 is the thickness of the foil. The photons produced at the first few foils will be absorbed by the succeeding foils. This is a fundamental limitation of the number of foils of a prior art transition radiator. For example, using this criterion only 35 foils of 1.5 .mu.m beryllium can be used to produce 1.5 keV X-rays. The soft X rays from a number of transition radiators using a moderate energy electron beam (17 to 300 MeV) are in the region of the spectrum needed for lithography. Several spectra are compared in FIG. 2. The spectra match the desired photon energy range for the mask/resist absorption band between 800 eV and 3.5 keV. The beryllium foil stack is by far the most intense soft X-ray source, followed by aluminum. Unfortunately, these foils are extremely toxic and if atomized by heating, would contaminate the accelerator. Hence, no beryllium foils have been used at high currents. In the prior art, only extremely low currents have been used to measure the spectra of beryllium stacks (see for example, M. A. Piestrup, J. O. Kephart, H. Park, R. K. Klein, R. H. Pantell, P. J. Ebert, M. J. Moran, B. A. Dahling, and B. L. Berman, "Measurement of transition radiation from medium-energy electrons," Phys. Rev. A vol. 32, pp. 917-927, Aug. 1985). Previously the peak X-ray flux, photons/(electron-eV), was optimized at a particular photon energy, .omega..sub.0, by setting: EQU l.sub.2 =1.32 Z.sub.2 (.omega..sub.0) (4) ##EQU4## This is demonstrated in FIG. 3. For the cases shown, the spectra are optimized at 700, 1200, and 1800 eV. The electron beam energy is 150 MeV. The peak flux, number of foils, and foil thickness increase with increasing .omega..sub.0. The spectrum grows and slides to the right toward the harder X-ray region of the spectrum. Unfortunately, the less desirable, harder X-rays above 3 keV increase, while the more desirable, soft X rays decrease. The soft X rays are more absorbed in the photoresist while the harder X rays pass through both the mask absorber and substrate decreasing the contrast of the circuit image. 4. Prior Art: Methods of Increasing the X-ray Power In the prior art the maximum power produced by the transition radiator is limited by the maximum number of foils and the maximum electron-beam power that foils can withstand without melting or distorting. For example as suggested theoretically in M. A. Piestrup, J. O. Kephart, H. Park, R. K. Klein, R. H. Pantell, P. J. Ebert, M. J. Moran, B. A. Dahling, and B. L. Berman, "Measurement of transition radiation from medium-energy electrons," Phys. Rev. A vol. 32, pp. 917-927, Aug. 1985, a maximum number of 60 beryllium foils would be necessary to produce 12.5 mW/cm.sup.2 at a distance of 1 m from the foil stack. This would require a 100-MeV, 600-.mu.A electron beam. The stack would heat to 960.degree. C. for a 2-mm diameter electron beam. This is an extremely high-power electron beam and the foils are unlikely to survive such high temperatures. Increasing the electron-beam power would result in the melting of the foils. Other methods of increasing the total output power are needed to give adequate X-ray flux for lithography and prevent the melting of the foils. A prior art method of increasing the total X-ray power uses the concept of allowing X-rays to escape without striking the foils. This has been tried by M. A. Piestrup, J. O. Kephart, H. Park, R. K. Klein, R. H. Pantell, P. J. Ebert, M. J. Moran, B. A. Dahling, and B. L. Berman, "Measurement of transition radiation from medium-energy electrons," Phys. Rev. A vol. 32, pp. 917-927, Aug. 1985. In this scheme, the stack is "split in half" allowing half of the X-ray cone to escape. The electron beam is steered by approximately one beam diameter into the transverse dimension of the foil stack. The X-rays generated in the upper half of the radiation cone will leave the stack, while X-rays in the lower half will be emitted largely from the last few foils (of total thickness 2/.mu.l.sub.2). The spacing between the foils is adjusted, limiting the number of foils per unit length, so that the number of foils encountered by a photon in the upper half plane is less than M=2/.mu..sub.2 l.sub.2. From simple geometric considerations, the spacing between the foils should be L.perspectiveto.d/M.theta..perspectiveto..mu..sub.2 l.sub.2 .gamma.d/2, where d is the diameter of the electron beam and l.sub.1 &gt;&gt;l.sub.2. The split stack has the disadvantage that approximately one half of the radiation is lost in the foils and the resulting pattern is not symmetrical. 5. Prior Art: X-ray Beam Uniformity The transition X rays are emitted in a narrow cone with an apex angle of .theta.=1/.gamma. where .gamma.=E/E.sub.0, E is the electron energy and E.sub.0 is its rest energy. The resulting X-ray pattern is a ring or annulus when it strikes the wafer and mask. This is shown graphically in FIG. 1. For example at 50 MeV the apex angle is 10 mr (0.6.degree.). The diameter of the annulus would be only 1.2 cm at a distance of 1.3 meters from the stack. Thus the radiation is almost laser-like at these electron-beam energies, but the radiation pattern is not uniform, having a hole in the center of the beam. For lithography the X-ray beam must have a uniform cross-sectional intensity at the mask and wafer for the exposure to be uniform. Variations across the X-ray beam should be less than .+-.5%. Since the radiation pattern from transition radiation is an annulus, some method of collimation and radiation re-distribution must be achieved. As discussed by J. Maldonado in his paper entitled "X-ray Lithography, Where It Is Now, and Where Is It Going" (paper presented at the 2nd Workshop on Radiation Induced and on Processing Related Electrically Active Defects on Semiconductor-Insulator Systems, September 1989, MCNC, N.C. to be published), "Due to this geometry (the transition radiation annulus), there is concern on whether the required (X-ray) beam uniformity for XRL (X-ray Lithography) could be achieved." In patent application Ser. No. 7/378,907, filed July 12, 1989, entitled "A Focused X-ray Source," by M. A. Piestrup, D. G. Boyers, C. I. Pincus, and P. Maccagno cylindrical optics are used to collect the annular X rays into a single millimeter spot. The apparatus has a transition radiation source which generates X-rays in a conical radiation pattern. An electron beam usually housed in a vacuum pipe strikes the thin foils, thus generating the X-rays, which are then collected by the optics which focus the radiation at an appreciable distance from the radiator. In one embodiment of patent application Ser. No. 7/378,907, the optic consists of a smooth-bore tube composed of a solid such as metal, glass, or quartz. X-ray focusing is achieved by having the X-rays strike the surface of the tube at a grazing angle such that the X-rays are almost entirely reflected. The nature of a diverging cone traversing down the axis of a cylinder of revolution and intersecting the cylinder is such that an appreciable amount of the radiation will be reflected and focused. The focal spot dimensions are on the order of the emitting electron-beam dimensions at the transition radiation foil stack. Spots of 0.3 to 2 mm diameter have been obtained at a distance of 1.35 meters from a transition radiator. Smaller spot sizes are possible. These sizes result in a large increase in the intensity at the focus. In the prior art, the X rays produced by a synchrotron source are emitted in a vertically narrow beam smeared in the horizontal plane. The analogy between a search light and this source can be made: the optical photons emitted from a search light come out in a narrow beam that is rotated horizontally, resulting the beam smeared in the horizontal plane but collimated in the vertical plane. The same occurs for the synchrotron source. In the synchrotron source the X rays are picked up by a small mirror which deflects them out to the mask and wafer. The area illuminated is a narrow rectangle measuring, for example, 2 mm vertically, and 2 cm horizontally. Thus the entire wafer is not illuminated. To accomplish this the X-ray mirror must be mechanically scanned across the mask and wafer. This adds to the angular emittance of the X-ray beam and increases the shadowing of the mask circuit elements on the wafer, resulting in increased blurring. The small rectangular, high-intensity X-ray beam can cause localized heating of the mask, resulting in the mask distorting as the beam is scanned. 6. Prior Art: Transporting and Condensing X-rays for Lithography As stated by A. Heuberger in his paper "X-ray Lithography," Solid State Technology, Feb. 1986, "There are no imaging optics available for X rays which possess a useful efficiency, which means that a condenser for homogeneous illumination of the wafer is not realizable . . . the lack of a useful optics such as lenses, in this wavelength range means that the radiation must be used in the same form (i.e., wavelength distribution and geometrical characteristics) as that emitted from a given X-ray source." Laser plasmas are highly divergent point sources while synchrotron sources are collimated in only one plane and require aperturing in the other plane. The lack of collimation and collection means that all sources with finite divergence must be close to the source. In the case of plasma sources this means that they must be within 10 to 20 cm of the radiator. For synchrotron sources the distance can be much greater 3 to 10 meters depending upon the electron-beam energy). In the prior art no one has realized a source that is of small divergence (partially collimated). The prior art sources of laser plasma, X-ray tubes (impact sources), and synchrotron sources because of their divergence do not lend themselves to existing X-ray optics which could be used to collect and transport the X-rays. In patent application Ser. No. 7/378,907, filed July 12, 1989, entitled "A Focused X-ray Source," by M. A. Piestrup, D. G. Boyers, C. I. Pincus, and P. Maccagno it was recognized for the first time that transition radiation was an ideal source for cylindrical X-ray optics. The source has a low divergence radiation pattern that is almost laser-like. Its radial axial mode (annulus) is ideal for surface-of-revolution optics. For synchrotron and transition radiation sources transportation of the X rays to the mask target area is important for several reasons. The electron beam generates other ionizing radiation which can be harmful to both the lithography process and to the operators. Thus there must be adequate shielding between the electron beam and the mask target. Adequate shielding may require several meters of concrete and neutron shielding (e.g. parafin wax). The power density of the X rays must not drop appreciably as it is being transported and additional geometrical distortion due to high divergence of the X rays must not be introduced in the process of collection and transportation. Ideally the optic would not only transport the beam but would collect most of the radiation being generated. 7. Prior Art: Multiple Stations In the prior art synchrotron sources have been made less expensive by increasing the number of X-ray beamlines or "stations" around the storage ring. Synchrotron radiation is emitted in the plane of the electron-beams orbit; thus, photon beamlines oriented tangentially to the curve of the electron beam and in the plane of the orbit will intercept much of the X-ray emission. In compact-ring designs, 10 to 20 stations are allowed per ring. For transition radiators only one radiator has been used. A 16-station synchrotron-based system is projected to have a per station cost of $4-5M and a total system cost of $64M to $80M. However, IBM and others have identified the need for a much smaller system (comprising one to several stations) with the same per station cost but a much lower total system cost. This would enable companies to enter the X-ray lithography business with an initial investment of $4-6M and add additional capacity in discrete increments. Many U.S. companies are discouraged from entry by the extraordinarily high cost of X-ray lithography systems. Currently, laser-plasma source X-ray lithography systems selling for $4-5M are considered to be serious contenders for this segment of the market. Laser plasma and electron impact sources are inherent single-station sources and cannot be expanded. An optimum source would be one that was approximately the same cost as a laser-plasma system but the number of stations could be increased as the need for higher integrated-circuit production increases. SUMMARY OF THE INVENTION The present invention provides an X-ray source which is brighter than previous transition radiators and has a uniform X-ray pattern. The electron-beam current and energy needed for this present invention is available from current linacs and is not excessively large, unlike previous predictions in the prior art. In addition an X-ray-beam area is achieved which matches the mask/wafer target area. The present invention is designed to provide uniform, high-intensity, soft X-ray radiation for the exposure of photoresist on silicon wafers. This invention provides a less expensive alternate to synchrotron radiation and has better technical characteristics when compared to the laser plasma X-ray lithography system. Two embodiments are given which correspond to two regions of electron beam energies: moderate electron-beam energies (100 MeV to 250 MeV), and low energies (25 MeV to 100 MeV). The distinction between these two cases is primarily that the lower electron-beam energies may need optics to collect and translate the X rays to the mask/wafer target. If the electron beam energy is sufficiently high (E&gt;100 MeV) the conical X-ray annulus will match or be smaller than the mask/wafer target area, and the distance between the foil stack and the mask/wafer target will be sufficiently large for adequate radiation shielding and magnetic deflection of the electron beam (for separation of the electrons from the X rays). Lower electron beam energies (E&lt;100 MeV) will require collection optics to collect and translate the X rays to the mask/wafer target in order to have adequate spacing for radiation shielding and magnetic deflection of the electron beam. Optics may also be used in the high energy case for more efficient collection of the entire conical X-ray annulus, and for uniform illumination of the mask/wafer target area. The present invention has demonstrated a performance of 15.2 milliwatts using only a 7-kW electron-beam power and has produced experimental lithographs with a circuit element resolution of 0.5 micron. This higher flux was obtained by using beryllium foils, increasing the electron-beam energy (105 to 245 MeV), and bringing the foil stack and mask/wafer closer together (from 6 m to 3 m). An additional factor of 3 can be achieved by using multiple stacks as described in this patent. A total of 100 mW can be achieved with a power density of 10 to 20 mW/cm.sup.2 using electron-beam powers of 10 to 20 kW. With the present invention the following improvements make the transition radiation a viable source for X-ray lithography: (1) foil parameters, foil to mask spacing and electron-beam energy are selected for increased total power and for optimum spectral content to achieve high-contrast ratio for exposed pattern on the photoresist (2) higher X-ray power density is achieved by increasing the electron-beam energy, shortening the radiator-to-mask distance, and increasing the electron-beam current coupled with cooling the foil stack (3) higher total X-ray power is achieved by increasing the number of foils in the electron beam utilizing novel foil stack geometries, stack spacing or magnetic deflection between foil stacks; (4) X-ray beam uniformity is achieved by using changes in the electron-beam angular spread and area at the radiator, novel X-ray optics, X-ray beam scanning, and electron-beam scanning, (5) multiple station operation is achieved by magnetic deflection, special foil stack geometries, and additional foil stacks. 1. Parameter Selection In accordance with preferred embodiments of the invention the parameters of foil thickness, foil material, electron-beam energy and radiator-to-mask distance, Z.sub.0, are selected (1) to achieve maximum total X-ray power in the X-ray frequency range of optimum resist sensitivity and maximum image contrast; (2) to achieve the optimum spectral content for high-contrast ratio for the exposed pattern on the photoresist; and (3) to achieve an area of exposure of approximately 6 to 12 cm.sup.2. 1.1 Foil material, thickness and number selection The foil thickness, number, and material are selected to achieve maximum photon production and optimum spectral content for high-contrast ratio for the exposed pattern on the photoresist. The foil thickness, foil number, and material are selected to maximize photon production; however, these parameters, along with electron-beam energy, also determine spectral shape, and distribution. In the present invention the spectrum is optimized by selecting foil thicknesses optimum for photon energies on the low photon-energy range of the desired spectrum unlike the prior art which was optimized at the center of the desired spectrum. The number of photons is then increased above M=2/.mu..sub.2 l.sub.2, causing the spectrum to increase in total photons. As the number of foils is increased the portion of the hard X rays also increase. We therefore limit the number of foils to the point where the increased number of hard X-rays will degrade the contrast of the circuit image on the wafer. The number of foils is limited to approximately 50 foils of 1-.mu.m beryllium. The result is an increase in total power over the prior art and a more optimum spectrum for best contrast. In the present invention beryllium is found experimentally to give the most soft X rays for lithography. The toxic hazards of beryllium are reduced by the small amount of beryllium used. 1.2 Electron Beam Energy The maximum field size with the present mask technology is limited due to mask distortion, and handling considerations to be about 6 to 12 cm.sup.2. The current maximum exposed area is limited by the registration of the mask with the various levels of previous resist patterning to be approximately 2.54 cm (or 1 inch) diameter. This limit is primarily due to existing methods of mask alignment whose registration is no better than 0.25 .mu.m. As discussed in the previously cited Maldonado paper, "The maximum field size . . . is . . . still limited due to mask distortion, and handling considerations in the relatively thin membranes utilized for mask substrates." Thus, unlike what was thought in the prior art, the small X-ray-beam spot size produced by transition radiation is adequate for achieving moderate throughput (chips/hour) and is not a hindrance in its use as a source for X-ray lithography. In the present invention the electron-beam energy and distance from the mask to the source is adjusted so that the exposed area is roughly 6 to 12 cm.sup.2. The knowledge that the required exposure area was limited was not discussed or described in the prior art concerning transition radiation as an X-ray source for lithography. This resulted in transition radiation being rejected as a possible source because of the small area of illumination. If the total transition radiation X-ray power is distributed over the entire wafer (3" to 6" in diameter) the power density would be too low and would result in a low rate of production of integrated circuit. 2. X-ray Power Increase by Novel Foil Cooling Techniques In accordance with the present invention the X-ray flux can be increased by increasing the electron-beam current and cooling the foils by either convection or conduction methods or redistribution of the heat by foil-stack rotation or electron-beam scanning. In the prior art no such methods were suggested. One embodiment for increasing the total output power of the transition radiator is to increase the total electron-beam current through the foil stack and to distribute the heat by rotating the foils in the electron beam. This increases the area that the electron beam strikes, thus distributing the total thermal losses, and allowing larger electron-beam currents to be used. 3. X-ray Power Increase by Novel Geometries In the present invention, the total power of a transition radiator can be dramatically increased by arranging a number of foil stacks or individual foils such that the entire conical radiation pattern of transition radiator misses succeeding foil stack or foils. There are two embodiments for increasing the X-ray power by increasing the effective number of foils: (1) increase the number of foils by spacing the foils or foil stacks such that the naturally occurring radiation cone from the foils upstream will miss the succeeding foils and (2) magnetically adjust the angle of the electron beam in each foil stack such that the X-ray cone misses succeeding stacks. Unlike the prior art, the entire conical X-ray annulus is used. In the first embodiment of this idea spacing the foils or foil stacks at large enough distances, the cone of radiation will miss succeeding stacks and the total output power will increase linearly with the number of foils. The diameter of succeeding stacks will be limited to approximately twice the electron-beam diameter. Each stack has a maximum number of foils based on the absorption criterion of M.perspectiveto.2/.mu..sub.2 l.sub.2. The resulting annuli will add in a "bull's-eye" like pattern. This bull's-eye pattern is also useful for making the radiation pattern more uniform. In the second embodiment of this idea the electron beam is bent at each foil stack such that the radiation pattern misses the succeeding foil stacks. The angle that the electron beam makes at each stack can be adjusted such that the radiation cones all add concentrically. This results in an N-fold increase in X-ray power, where N is the number of stacks. As in the first embodiment, the output pattern can be designed with the added benefit that the power density is more uniform across the mask and wafer. 4. Collection and Transportation of the Transition X-rays Transition radiation's unique radial mode is ideally suited for X-ray surface-of-revolution optics. The low divergence cone of transition radiation can be easily focused by grazing-angle optics. No other known X-ray source allows almost the entire emitted radiation pattern to be collected. Synchrotron radiators, conventional X-ray tube bremsstrahlung, and laser-plasma sources cannot have their entire radiation patterns focused. The surface-of-revolution optics invented for transition radiation discussed in patent application Ser. No. 7/378,907 can do three things when used in an X-ray lithography system: (1) it permits the collimation and concentration of the X-rays at the mask target area; (2) it can achieve X-ray beam uniformity; and (3) it permits the X-rays to be utilized at a long distance from the transition radiator. Since the radiation cone is diverging at an angle of approximately .theta.=1/.gamma., after a short distance, depending upon the electron-beam energy, the radiation annulus will be unacceptably large (greater than 6 to 12 cm.sup.2). although it is possible to use the source as it comes directly off the radiator by bringing the mask and wafer close to the radiator (1 to 3 meters). An optic which would collimate and redirect the radiation cone over a long distance (3 to 6 meters) would permit room for radiation shielding and electron optics for directing the electron beam away from the X-ray beam. In addition, much of the X-ray power is at larger cone angles than .theta.=1/.gamma.; thus X-rays outside the 1/.gamma. cone will not be used with an uncollimated S-ray beam because the power density is lower outside the 1/.gamma. cone. Although the power density drops dramatically after 2/.gamma., there is radiation emitted out to: EQU .theta..sub.max =(1/.gamma..sup.2 =(.omega..sub.2 /.omega.).sup.2).sup.1/2( 6) For large .gamma., then .theta..sub.max .perspectiveto..omega..sub.2 /.omega.. For beryllium foils .omega..sub.2 =26.1 eV, for .omega.=1500 eV, .theta..sub.max =18 mR. As we shall see, we need to collect as much of the radiation cone out to this angle as possible for electron beams of energies greater than 50 MeV. In the new art a specially designed optic of either a surface-of-revolution or Fresnel lens is used to collect, collimate, and transport the X-rays. The optics are designed to capture almost the entire radiation cone and direct it toward the mask target area. The benefit is that a relatively moderate power source of X rays with an unusual conical radiation pattern can be made to deliver intense, collimated X-rays to a designated wafer target area. 5. X-ray Beam Uniformity Conventional thinking would suggest that such a source is highly inappropriate for lithographic applications, which demand very uniform illumination (.+-.5% over a 6 cm.sup.2 to 12 cm.sup.2 disk). However, in the present invention the conical X-ray annulus is redistributed by five methods: (1) the electron-beam divergence is changed so that the annulus is smeared and the hole is filled, (2) the time average annular radiation pattern is smeared and made uniform by scanning the X-ray beam by grazing incidence mirror, (3) the time average annular radiation pattern is smeared and made uniform by scanning the electron beam across the radiator, (4) the mask and wafer are moved such that the time average power across the mask and wafer target areas are uniform (5) a surface-of-revolution lens is constructed using a computer-aided design such that the radiation arriving at the mask plane is uniform. We have used electron-beam optics to fill in the conical X-ray annulus resulting in a more uniform power density across the X-ray beam. This was accomplished by changing the direction size and shape of the electron beam where it struck the foil stack. Prior art transition radiators have not used this concept. In another embodiment of the present invention the electron beam is also used to alter the direction of the radiation pattern. The electron beam is steered through the foil stack so that the direction of the emitted X rays can be much different than that of the original direction of the electron beam and off axis to the axis of the foil stack. This can be used to scan the electron beam to alter the time-average power striking the mask/wafer target area. In another embodiment, the average area covered by the conical X-ray annulus is changed by oscillating a grazing-angle output mirror. This smears the X rays across the mask and silicon wafer so that on a time average basis a uniform X-ray intensity is achieved. None of these embodiments have been attempted using transition radiation in the prior art. 6. Multiple Station Operation To decrease the overall production costs, additional foil stacks are added to the electron beam, each with an X-ray beamline and a mask and wafer alignment system. Thus the cost per station is reduced by sharing the linac and the electron beam. This can be done because the electron beam is only slightly perturbed as it passes through the foil stacks. The electrons are elastic-scattered and experience some energy loss as they pass through the foils. However this scattering and loss are minimal for a finite number of foils.