Patent Number: 055127592
Section: description

V. DETAILED DESCRIPTION OF THE INVENTION The following terms of art are defined before providing a description and discussion of the present invention. A. Terms of Art Synchrotron Source: X-ray radiation source for accelerating electrons or protons in closed orbits in which the frequency of the accelerating voltage is varied (or held constant in the case of electrons) and the strength of the magnetic field is varied so as to keep the orbit radius constant. Synchrotron Radiation: The delineating electromagnetic radiation generated by the acceleration of charged relativistic particles, usually electrons, in a magnetic field as incident on and producing an illumination field on a mask. The illumination field is characterized by its intensity, direction, divergence, and spectral width. EUV: Extreme Ultra-Violet Radiation, also known as soft x-rays, with wavelength in the range of 50 to 700 .ANG.. 1.times. Camera: A camera of the class disclosed in U.S. Pat. No. 3,748,015. 5.times. Camera: A camera of the class disclosed in U.S. Pat. No. 5,315,629. Spherical Mirror (Powered Mirror): A mirror, either concave or convex, whose surface forms part of a sphere. Although the present invention employs the use of spherical mirrors for convenience and economical concerns, it is intended that other mirrors be covered by the present invention, such as toroidal, conic sections (e.g., parabolic, hyperbolic, elliptical, etc.), mirrors that may be substituted for spherical mirrors within tolerable industry standards (including those with minor flaws or aberrations), etc. Flat Mirror: A mirror whose surface is nearly flat within manufacturing tolerances. Although the present invention employs the use of flat mirrors, it is intended that the present invention be easily modified by those of ordinary skill in the art to employ the use of spherical mirrors where flat mirrors are disclosed in the following discussion. Divergence: As used by itself, the term refers to mask divergence, i.e., the largest angle about the axis of the cone of radiation as incident on a mask. In projection lithography, the axis is generally a few degrees off normal incidence as required for reflection masking. The magnitude of divergence required in projection lithography is that needed to reduce ringing at feature edges to the extent necessary for desired resolution and contrast. In full-field exposure mode, divergence should be similar at every illumination point. In scanning mode, some nonuniformity in the scanning direction may be averaged out. Convergence: As used by itself, the term refers to mask convergence, i.e., the smallest angle about the axis of the cone of radiation as incident on a mask. Condenser: Optical system for collecting the synchrotron radiation, for processing the synchrotron radiation to produce a ringfield illumination field and for illuminating the mask. Collecting Optics (or Collector): The optics within the condenser responsible for collecting the synchrotron radiation. The collector has a focus. Processing Optics: The optics within the condenser, in addition to the collecting optics, responsible for processing collected radiation for delivery to the mask. Imaging Optics (or Camera Optics): The optics following the condenser, in addition to the collecting and processing optics, responsible for delivering mask-modulated radiation to the wafer, i.e., the camera optics. Camera Pupil: Real or virtual aperture that defines the position through which synchrotron radiation must enter the camera, of angular size defining the diffraction limit of the camera. Its physical size is that of an image of the real limiting aperture of the camera. Aperture Stop: The point at which the principal rays cross; the stop serves to fold the ray bundles, i.e., to move the image to the other side of the optics. Lens: The term is used in this description to define any optical element which causes x-ray radiation to converge or diverge. "Lenses," in x-ray systems, are generally reflecting and are sometimes referred to as "mirrors." Contemplated lenses may be multi-faceted or may be non-faceted, i.e., continuous, e.g., of ellipsoidal or other curvature. The convergence or divergence is a result of action analogous to that of a transmission optical lens. Full-field Exposure: Simultaneous (rather than sequential) exposure of all subareas of an image field. In its derivation, the term refers generally to a complete circuit pattern such as that of an entire chip. In this description, it is used to refer to any low-aspect ratio rectilinear pattern region, whether of an entire or partial pattern. Contemplated partial patterns may be stitched together by step-and-repeat to constitute the entire pattern. B. The Invention The present invention makes effective use of x-ray synchrotron radiation, collected over a large emission arc for use in illuminating a pattern mask. The arc is at least 100 mrad, or preferably 200 mrad, to a full radian or more. Pattern delineation to which the radiation is to be applied may take a variety of forms. It may take the form of full-field exposure or of a scanning (e.g., ringfield) region. Exposure may be by proximity printing or by projection lithography. A favored form of projection lithography, known as ringfield projection lithography, makes use of a scanning region of arcuate shape, likely with object:image size reduction, perhaps by a ratio of 5:1, to permit use of more economical, larger-feature masks. Synchrotron radiation is not well adapted to satisfy either ringfield scanning or full-field exposure needs. Synchrotron radiation is shown in FIG. 1. As the high speed electrons within beam (10) follow a curved path (11), they emit a fan shape of electromagnetic radiation (12) in a horizontal plane, also referred to as synchrotron emission light. The photon energy is determined by the electron energy and by the curvature of the electron path. Electron energies of 5.times.10.sup.8 to 1.times.10.sup.9 are useful for x-ray radiation at the 5 to 150 .ANG. wavelength range of interest (for synchrotron devices in present use). The emitted radiation fan is very thin, perhaps 1 mm thick, spreading to a thickness of a few millimeters at a distance of several meters from the synchrotron source. The angle of the emission fan is the same as that of the bent emitting path. Because synchrotron radiation has a high degree of coherence, it is possible to capture all the radiation emitted, with any losses coming only from the finite reflectivity of the mirrors employed in the condenser design. C. The Condenser The condenser of the present invention provides for collection of a large arc of the synchrotron radiation by use of a plurality of spherical mirrors arranged in a series (Six beams of synchrotron light and six beams of light and six sets of mirrors are shown in FIGS. 3 and 5 for convenience in introducing the invention. The figures will be discussed infra with respect to the reference numerals.) to collect the synchrotron light, and a plurality of flat mirrors follow the spherical mirrors to process the synchrotron light. Following the flat mirrors are optics comprising flat mirrors located in the same plane as the real entrance pupil of the camera and directs the beams toward the ringfield of the camera. Following the flat mirrors is a spherical mirror that projects the image formed at the real entrance pupil through the resistive mask and into the virtual entrance pupil of the camera. The condenser optics are located intermediate to the synchrotron source and the ringfield camera. For convenience in introducing the invention, the following discussion is generally in terms of the single elements that collect and process a single beam of synchrotron light. Even though depicted as single elements, however, two or more elements may be combined for a given change in beam direction. Also, any number of beams may be collected depending on the power to be collected from the synchrotron source, which would require a corresponding set of (six) mirrors for each beam to be collected, processed, and imaged. In a preferred embodiment, the condenser system comprises, from synchrotron source plane to image (the wafer) plane, at least two spherical mirrors for collecting and shaping a single light beam, wherein the spherical mirrors comprises a first mirror that is concave and a second mirror that is convex. The first and second spherical mirrors are tilted for collecting and transforming the light beam into an arc-shaped light beam. The resulting arc-shaped light beam fits the ringfield of the camera. A third mirror, following the second mirror and shown in FIG. 4 (discussed in detail infra), is a flat mirror for rotating and directing the light into a real entrance pupil of a ringfield camera. A fourth mirror, following the third mirror, is a flat mirror also for rotating and directing the light into a real entrance pupil of a ringfield camera. A fifth mirror, following the fourth mirror, is a flat mirror. It is located at the real entrance pupil of the camera and is individually tilted to make all the light beams substantially parallel to each other when more than one beam is collected. The fifth mirror directs the beam(s) toward the ringfield of a camera. A sixth mirror, following the fifth mirror, is common to all of the beam(s) emitted from the preceding mirror sets and is a spherical mirror. The sixth mirror images the beams located at the real entrance pupil through the resistive mask and into the virtual entrance pupil of the camera. The sixth mirror also serves to converge the six beams at the mask plane. Thus, the condenser is comprised of a plurality of beams with five mirrors corresponding to a single beam plus one mirror that is common to the plurality of beams. In an alternate embodiment, the third and fourth mirrors could be combined to use only one flat mirror, provided the first and second mirror are positioned accordingly as shown in FIGS. 8 and 9. The design and function of this alternate embodiment is equivalent to the design and function of the preferred embodiment discussed supra with the exception of the use of fewer processing mirrors. Thus, the discussion of the preferred embodiment applies to this alternate embodiment. Referring to FIG. 8, as in the preferred embodiment, the first mirror (84) is a concave spherical mirror, the second mirror (85) is a convex spherical mirror. The third mirror (86) is substituted for the combined function of the third and fourth mirrors in the preferred embodiment. Referring now to FIG. 9, the first mirror (94), which is concave, is positioned below the second mirror (95), which is convex. The first (94) and second (95) mirrors collect the light beams and translate them into arc-shaped beams. The third mirror (96), which is flat, receives a converging beam from the second mirror and processes the beams. The condenser is then comprised of a plurality of beams with four mirrors corresponding to a single beam plus one mirror that is common to the plurality of beams. Optional, airtight valves (89) preserve vacuum in the synchrotron. All of the optics discussed are flat mirrors or long-F/no. spherical mirrors. The plurality of arc images all have a common orientation at the curved slit of the camera. The third mirror in the set can be moved axially to focus the arc image in the camera's entrance slit. The tilt of the fifth mirror allows the arc image to be pointed into the slit. The magnification of the arc image could be changed by a small amount by playing off the individual beam image positions and the distance from the camera and the fifth mirror. The image quality realized by the present invention is adequate to illuminate any ringfield with a width of W.gtoreq.100 .mu.m. The transmission efficiency .eta. of the complete system is equal to the product of the six mirror reflectivities in each set of mirrors. There are three mirrors that are tilted (approximately 45.degree.) in orientation (S-polarization), two near-normal mirrors, and the fourth mirror could be set at grazing incidence (not shown in the figures). The radiation from the synchrotron is horizontally polarized, which is the "S-polarization" for all of the tilted (e.g., 45.degree.) mirrors. Partial coherence in the illumination affects the image quality. For the small design features sought by the EUVL system, it is important that the condenser provide uniform, partial coherence illumination properties along the ringfield. In an incoherently illuminated optical system, small features are attenuated due to the fall-off of the modulation transfer function ("MTF"). Partial coherence can be introduced into the illumination to counter this attenuation. This is normally accomplished by underfilling the entrance pupil in a system with Kohler illumination. In other words, the source (which is usually a disk) is imaged into the entrance pupil, and this image is smaller than the pupil by a partial coherence factor of .sigma..apprxeq.0.6. This value of .sigma. is a reasonable compromise, which amplifies the smaller features and does not add too much "ringing" to the larger features. The partial coherence factor .sigma. could be in the range of 0.5&gt;.sigma.&gt;0.65. The entrance pupil illumination for this embodiment is shown in FIG. 7, which shows an end view of the six light beams (73) in the same plane as that of the real entrance pupil (70) of the ringfield camera. The flat mirrors (71) and (72) serve to focus the light beams coming from the fourth mirrors into the real entrance pupil (70) of the camera. The six flat mirrors (71) and (72) are arbitrarily located, for example, 1 m in front of the real entrance pupil of the camera. Five light beams (73) are received by five flat mirrors (71) that are arranged in a symmetrical pattern about a single flat mirror (72) that receives a sixth beam (74) at the real entrance pupil (70) of the camera. Illustrative work discussed in detail provides for collection over a full radian (.about.57.degree.) of synchrotron radiation. This two-order-of-magnitude increase in collection angle increases wafer throughput or productivity. Specific needs are met by a variety of arrangements. One of the primary advances herein is the ability to illuminate a narrow ringfield of a camera by forming and focusing arc-shaped light beams into the entrance pupil of the camera, thus maximizing the collection efficiency of the condenser. Collected radiation may be reassembled in proximity printing to yield a scanning slit; or alternatively, to yield an illumination region of small aspect ratio for full-field patterning. The present invention provides for scanning (e.g., ringfield) or full-field projection. The specific description that follows emphasizes ringfield projection lithography. Where fall-field exposure, either proximity printing or projection lithography, requires different optics, notation is made in the final discussion in each section. The significant case of ringfield projection lithography is represented by FIG. 2. In FIG. 2, the resistive mask (20) includes a rectilinear patterned region (21), which is being swept horizontally in direction (23) by an arc-shaped illumination region (22) which may be 1 to 8 mm wide by about 130 mm long. The energy from the condenser must illuminate only region (22) and no other part of region (21). In full-field exposure (as distinguished from the scanning shown) pattern region (22) and region (21) must be simultaneously illuminated. 1. The Collecting Optics The collecting optics, or collector, of the condenser system are comprised of at least two spherical mirrors, a first mirror that is concave and a second mirror that is convex, positioned symmetrically about the periphery of the synchrotron source. It is expected that at least arc of collection will be 100 mrad, likely from 200 mrad to 1.5 rad. Collection over a radian for a synchrotron of radius of 1 to 2 m may require a collector length of the order of a meter. The collector length may be accommodated by using a plurality of parallel channel systems comprised of approximately 15 cm class mirrors. The distance from the collector to the synchrotron orbit is typically 1 to 3 m, but could be varied. A shorter distance may require a greater angle of incidence on the first mirror of the condenser, which would reduce the flux on the mirror. A longer distance may require collector lenses of excessive size. The spectrum of the synchrotron emission light is broad. It is desirably tailored to meet particular needs. In projection lithography, a wavelength range of .lambda.=120 to 140 .ANG. takes advantage of most efficient reflectivity (of both lenses and mask). In proximity printing, a shorter wavelength in the range of .lambda. is=8 to 16 .ANG. is required for resolution and meets characteristics of available resists. Efficient operation of the condenser is aided by spectral narrowing, minimizing unwanted heating caused by radiation which is relatively ineffective for resist exposure. The use of multi-layer mirrors in the condenser of the present invention accomplishes the spectral narrowing. In a preferred embodiment, all of the mirrors of the condenser are multi-layer mirrors. The relatively long wavelength radiation of EUVL also permits use of glancing-angle lenses with relatively large angles of incidence. Glancing-angles lenses may inherently produce some spectral narrowing. When operating at or near the critical angle for the desired radiation wavelength, shorter wavelength radiation is not reflected. A condenser should be able to capture several watts of radiation in the pass band of a ringfield camera and deliver over a watt to the mask, which is enough power to expose resist coated wafers at a rate of several square centimeters per second. A typical ringfield camera pass band may be 130.+-.1.3 .ANG.. This pass band is determined by present multi-layer mirror technology. U.S. Pat. No. 5,315,629 is illustrative of a state-of-the-art ringfield projection camera. A reflectivity of 60 to 65% results from use of 40 successive Mo--Si layer pairs. Soft x-ray is also favored for surface reflection off certain metal mirrors. Angles of incidence of 5.degree. to 10.degree. from grazing incidence may result in reflectivity of 80 to 90%. A plurality of pairs of spherical mirrors, a first mirror that is concave and a second mirror that is convex, are sequentially placed about the synchrotron orbit as shown in FIG. 3 to produce an illumination field. The Etendu or Lagrange Optical Invariant requires that the product of convergence angle, .THETA., and the corresponding focus dimension equal or exceed the same product at the mask if a dispersing element (e.g., a scatter plate) is to be avoided as in the present invention. FIG. 3 is a top view of the optics of the complete condenser system near the synchrotron source and is illustrative in providing for a plurality of collector lenses (34) and (35), which are nominally spherical mirrors. Each mirror receives x-rays from a related spot in the orbital path of the synchrotron beam (30), and each mirror directs its reflected x-ray into focus. Synchrotron beam (30) in following curved path (31) emits a fan of radiation (32), considered as the composite of x-rays (33) produced by point sources within the arc of the synchrotron source. The fan of radiation (32) is collected by collector mirrors (34) and (35), which directs the light beams in a configuration to uniformly illuminate a ringfield of a projection lithography camera. The fan of x-rays (32) is shaped into arc-shaped beams by second mirror (35) of the collecting optics. A preferred structure for ringfield reduction projection may use six or more sets of mirrors for six light beams as shown in FIG. 3, depending upon the power to be collected from the synchrotron source. Optional, airtight valves (39) preserve vacuum in the synchrotron. In FIG. 3, the first mirror (34) in each set (corresponding to a single beam) collects a 3.5.degree. section of the synchrotron emission light beam with its coma-like aberration and translates it into a round spot. The first mirror (34) is nominally spherical and is tilted, for example, 9.5.degree. in the horizontal plane. It is arbitrarily located, for example, at a distance of 3 m from the ringfield along a tangent. The ends of the collected arc of radiation are slightly defocused, making the round spot 9% larger than would be expected. This small variation is negligible because the light beam is probably smaller than 1 mm. The round spot image radiates into a solid angle described by a small vertical angle and a large horizontal angle, similar to the 3.5.degree. section of the synchrotron emission light. The power radiated (from the spot) per unit of horizontal angle is constant, just as is the radiation exiting the synchrotron. Hence, at a distance from the spot it will form a line constant power along its length. The second mirror (35) translates the beam's straight line cross-section into an arc cross-section so that it will fit into the ringfield of the camera. FIG. 4 is a side view of the first four mirrors in the set of mirrors showing that second mirror (45) directs the beam upwards (e.g., angle of incidence i=48.4.degree.); a result that is corrected by the following third mirror (46), which is flat (discussed below under The Processing Optics). Second mirror (45) is arbitrarily located, for example, at 500 mm from first mirror (44) but must be placed a few millimeters lower than the fan of radiation (32) emitted from the synchrotron. In a preferred embodiment, the second mirror (45) is designed to have a back focal distance of BFD=10 m, for this particular geometry, to enable the projection of an image of the spot into the real entrance pupil of the camera. The focus of the collector may correspond with a real aperture of the camera, or it may itself define a virtual aperture of the camera. Adjustability of a real aperture is useful in obtaining a desired pupil fill. 2. The Processing Optics The condenser further comprises processing optics for matching the characteristics of the ringfield camera. Characteristically, a projection reduction camera operates with a divergence of 5 to 15 mrad. Shape and size of the imaging region, again the responsibility of the processing optics, varies with the camera design. Referring to FIG. 4, the correcting, third mirror (46) turns the beam back into the horizontal plane (e.g., i=41.6.degree.) and almost parallel to the beam between the first mirror (44) and the second mirror (45). Thus, the third mirror (46) orients the cross-section of the beam's arc so the center is horizontal. All of the beams collected from the synchrotron are subject to the same manipulation, therefore, their arcs' cross-sections can be overlapped at the ringfield. The fourth mirror (47) is near-normal and flat. The fourth mirror (47) directs the beams toward the fifth mirror (58) as shown in FIG. 5. The fifth mirror (58) is located at the real entrance pupil (55) of the camera (51). The fifth mirror (58) is tilted to direct the beams toward spherical mirror (59). The fourth mirror (47) of FIG. 4 turns the beam in a horizontal plane so that the image remains "horizontal." The spacing between the third mirror (46) and the fourth mirror (47) may be selected so as to "focus" the round image at the real entrance pupil. If the fourth mirror (47) could be oriented at grazing incidence, then the grazing angle would vary by at least 10.degree., beam to beam. Therefore, the reflectances would vary by 15 to 20%. In a configuration where a plurality of beams are to be processed, the fifth mirrors (58) could be arranged in a symmetrical pattern, such as the pentagonal pattern shown in FIG. 7, at the real entrance pupil of the camera. The fifth mirrors shown in FIG. 7 are arranged in a symmetrical, pentagonal pattern with five of the mirrors (71) arranged around a centrally disposed single mirror (72). Referring to FIG. 5, the fifth mirrors (58) operate to turn the beams downward (e.g., i.apprxeq.55.degree.) and point them toward the image of the mask as seen behind the sixth mirror which is located at the ringfield (52). In the horizontal plane, the centerlines of the beams emitted from the fifth mirrors all converge toward a common point at the ringfield (52) on the mask plane due to the individual tilting of the fifth mirrors. Also, the input beams to the sixth mirror are nearly horizontal. FIG. 6 illustrates an end view of one proposed beam configuration at the real entrance pupil (60) of the camera. Five of the beams (61) are arranged in a symmetrical pattern about a single beam (62). The illumination region must be directed into the virtual entrance pupil of the camera to the proper degree of fill. Fractional filling, e.g., pupil fill, optimizes contrast for a range of feature sizes. 3. The Imaging Optics The sixth mirror (59) shown in FIG. 5, which is spherical, is common to all beam lines. It images the light beams from the real entrance pupil (54) through the resistive mask (52) and into the virtual entrance pupil of the camera (not shown). The real entrance pupil (54) is created by the fifth mirrors (58), which is an image of the actual real pupil (57) of the camera (51). The real entrance pupil is located one focal length away, so the virtual entrance pupil is projected to infinity. The distance between the sixth mirror (59) and the resistive mask is selected such that the arc image departing second mirror (45) is imaged into the ringfield of the camera. All of the arc-shaped beams collected are overlapped onto the mask by the imaging optics. The collecting and processing optics of the condenser collects a plurality of x-ray beams that are emitted from the synchrotron source and combines them in, for example, a symmetrical, circular pattern at the real entrance pupil of a ringfield camera as shown in FIG. 7. (In the interest of clarifying the present invention, FIG. 7 depicts six light beams, but any number of light beams are possible, depending upon the power to be collected and the image quality desired.) One of the six beams could be positioned in the center with the remaining five beams symmetrically located about the centered, beam. The present invention provides nearly uniform coherence properties for features on the mask oriented at any angle (angles measured in the r-.THETA. plane). All six light beams are received by flat mirrors positioned at the real entrance pupil and imaged through the resistive mask and into the virtual entrance pupil of the camera. The entire arc of the camera is illuminated with each of the six beams to ensure uniform illumination and coherence along the length of the arc. With efficient design there is no clean line of separation between the collecting optics and the processing optics. The collector itself functions as processing optics to the extent that collectors are designed to direct, shape, or otherwise define the illumination region and to the extent that the collector goes beyond focusing collected radiation. The collector may increase divergence and may shape the illumination field. In most projection systems, separate processing optics is preferred, if only to avoid undue complexity in the collector design. Processing lenses may be tilted (e.g., 45.degree.) mirrors. In an alternative embodiment of the present invention, the substituting a single flat mirror for the third and fourth flat mirrors (described above) provided the first and second mirrors are positioned accordingly as illustrated in FIGS. 8 and 9. This embodiment accomplished the same result with less mirrors in the condenser system. Minimal processing is required in proximity printing. FIG. 3 illustrates a condenser for use in full-field proximity printing. U.S. Pat. No. 5,315,629 is illustrative of state-of-the-art ringfield projection lithography. It is contemplated that the system may experience temporal coherence between the six beams. If so, then the coherence could be eliminated by moving a mirror in each set of mirrors, corresponding to a single beam, perpendicular to its normal. This could be accomplished with the use of piezoelectric drivers, each oscillating at a different frequency. a. Example 1 The particular values and configurations discussed in this Example 1 can be varied and are cited merely to illustrate a particular embodiment, and are not intended to limit the scope of the invention. In this Example 1, the condenser coupled light from a 24.degree. sector of the synchrotron emission light into the ringfield of a 1.times. Offner lithography camera positioned in a vertical orientation. The condenser was configured to capture six beams of synchrotron emission light as exemplary shown in FIG. 3. The condenser system was configured to collect synchrotron emission light at a preferred wavelength of .lambda.=134 .ANG.. Collection of the light was made difficult due to the fact that the six beams to be collected spread out in a fan shape. Furthermore, collection was difficult because the Lagrange Optical Invariant for the 1.times. camera is relatively small (in comparison to, for example, a 5.times. camera). The ringfield width of the 1.times. camera is relatively narrow (again, in comparison to, for example, the ringfield width of a 5.times. camera (W.sub.1x =100 .mu.m while W.sub.5x =1 mm)). The camera's numerical aperture was assumed to be n.a..sub.1x =0.08, and a 1 cm length of the arc was illuminated, on a 50 mm radius. The entrance pupil of the camera had a diameter of 133 mm. The collecting optics collected a 3.5.degree. segment of the fan of synchrotron emission light and converted each of the six segments into six arc-shaped beams of light. The first (concave) mirror was spaced apart 3 m from the tangent of the source. It had an angle of incidence i=9.8.degree.. The first (concave) mirrors and second (convex) mirrors were arbitrarily spaced apart by 500 mm. The second (convex) mirrors had an angle of incidence i=48.degree.. Following each of the second mirrors in each of the six pairs, were six third (flat) mirrors and fourth (flat) mirrors comprised of six near normal flat mirrors for rotating and directing each of the six beams toward the real entrance pupil of the camera where they were received by six flat mirrors arranged in a symmetrical, circular pattern (as shown in FIG. 7). The distance between the second (convex) mirrors and the third (flat) mirrors was about 500 mm. The vertical focus was located 2 meters downstream, and the horizontal focus was located 10 meters downstream. The curved vertical "focus" was imaged into the camera's real entrance pupil. The fifth (flat) mirrors, which were .apprxeq.45.degree. mirrors (.apprxeq.17.times.10 mm), were located adjacent to the real entrance pupil of the camera, about 1 m away from the real entrance pupil, and were individually tilted to make all six beams substantially parallel to each other and to direct all six light beams up into the camera, which was vertically-oriented. The six beams received at the real entrance pupil of the camera each had a diameter of about 5 mm. Finally, the beams were received by a sixth, spherical (concave) mirror that imaged the arc images through the mask and into the camera's curved entrance slit. As discussed earlier, the system's transmission efficiency .eta. is a function of the reflectivity of the mirrors. The three near-normal mirrors had reflectivities of R.apprxeq.63%. The two 45.degree. mirrors had reflectivities of R.gtoreq.67%. The condenser system also included a grazing-incidence mirror (not shown) with a reflectivity of R.apprxeq.90%. The efficiency for the condenser system, therefore, is equal to the product of the six mirror reflectivities in each series of mirrors as follows: .eta.=(0.67).sup.3 *(0.63).sup.2 *(0.9)=10.74%. Other variations and modifications of the present invention will be apparent to those of ordinary skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. For example, a one-beam system could be used to simulate the six-beam final configurations illustrated by tilting the third (flat) mirror (36) in the set of mirrors of FIG. 3 to move the beam around in the entrance pupil and by tilting one of the subsequent mirrors in the chain to compensate for the tilt introduced by the third mirror. The tilt angles would be approximately 3 mrad. Also, scan simulation would have to be performed to provide uniform illumination across the 100 .mu.m wide entrance slit. Furthermore, the configuration could be modified to collect a 50.degree. to 60.degree. fan of radiation. The particular values and configurations discussed above can be varied and are cited merely to illustrate a particular embodiment of the present invention and are not intended to limit the scope of the present invention. It is contemplated that the use of the present invention may involve components having different characteristics as long as the principle, the presentation of a condenser that collects light from a synchrotron source and directs the light into the ringfield of a camera, is followed. It is intended that the scope of the present invention be defined by the claims appended hereto. The entire disclosures of all references, patents, and publications cited herein are hereby incorporated by reference.