Abstract:
Employing a source of radiation, such as an electric discharge source, that is equipped with a capillary region configured into some predetermined shape, such as an arc or slit, can significantly improve the amount of flux delivered to the lithographic wafers while maintaining high efficiency. The source is particularly suited for photolithography systems that employs a ringfield camera. The invention permits the condenser which delivers critical illumination to the reticle to be simplified from five or more reflective elements to a total of three or four reflective elements thereby increasing condenser efficiency. It maximizes the flux delivered and maintains a high coupling efficiency. This architecture couples EUV radiation from the discharge source into a ring field lithography camera.

Description:
This invention was made with Government support under Contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights to the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the production of radiation, particularly extreme ultraviolet and soft x-rays, with a shaped, extended capillary electric discharge source for projection lithography. 
     BACKGROUND OF THE INVENTION 
     Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. “Long” or “soft” x-rays (a.k.a. Extreme UV) (wavelength range of λ=100 to 200 Å (“Angstrom”)) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces. 
     Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced because the mask does not have to be positioned within microns of the wafer as is the case for proximity printing. The cost of mask fabrication is considerably less because the features are larger. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of EUV radiation in bands at which multilayer coatings have been developed (i.e., λ=13.4 nm, λ=11.4 nm) allows the use of near-normal reflective optics. This in turn has lead to the development of lithography camera designs that are nearly diffraction limited over useable image fields. The resulting system is known as extreme UV (“EUVL”) lithography (a.k.a., soft x-ray projection lithography (“SXPL”)). 
     A favored form of EUVL projection optics is the ringfield camera. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow arcuate fields of aberration correction located at a fixed radius as measured from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of Jewell et al., U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width defines a region in which features to be printed are sharply imaged. Outside this region, increasing residual astigmatism, distortion, and Petzval curvature at radii greater or smaller than the design radius reduce the image quality to an unacceptable level. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image and use of object:image size reduction of, for example, 4:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask. 
     Sweatt al. al. U.S. Pat. No. 6,118,577 discloses a condenser system that couples radiation from a small diameter source to a ringfield camera. The condenser system typically includes six substantially equal radial segments of a parent aspheric mirror, each having one focus at the radiation source and line focus filling the object field of the camera at the radius of the ringfield and each producing a beam of radiation. The condenser system also includes a corresponding number of sets of correcting mirror means which are capable of translation or rotation, or both, such that all of the beams of radiation pass through the real entrance pupil of the camera and form a coincident arc image at the ringfield radius. 
     The overall layout of an EUV lithography system used with the Sweatt condenser is shown in FIG.  4 . The radiation is collected from the source  22  by mirror segments  30  (referred to collectively as the “C 1 ” mirrors) which create arc images that are in turn are rotated by roof mirror pairs illustrated collectively as mirrors  40  and  50  (referred to as the “C 2 ” and “C 3 ” mirrors, respectively). Beams of radiation reflected from mirrors  50  are reflected by a toric mirror  60  (or C 4  mirror) to deliver six overlapped ringfield segments onto reflective mask  70 . Mirror  31  creates an arc image and roof mirror pair  41  and  51  rotates the arc image to fit the slit image and translate it to the proper position. Similar arc images are created and processed by mirror combinations  32 ,  42 , and  52 , and so on. Mirrors  41 ,  42 , and  43  are parts of different and unique channels; and the group of mirrors  44 ,  45 , and  46  is a mirror image of the group of mirrors  41 ,  42 , and  43 , respectively. An illustrative arc  71  is shown on mask  70 . The EUV lithography system further includes a ringfield camera  77  having a set of mirrors which images the mask using the radiation onto wafer  78 . 
     Despite the advantages of the Sweatt condenser system, the art is still searching for improved efficiency. Achieving sufficient EUV flux at the wafer to support a high wafer throughput commercial EUV lithography “step-and-scan” exposure tool is a significant challenge. Of the many elements that impact tool throughput, EUV source power and condenser efficiency both have tremendous leverage. For example, eliminating a single mirror in a condenser can increase flux at the wafer by a factor of (R mirror ) −1  or approximately 1.5x. 
     SUMMARY OF THE INVENTION 
     The present invention is based in part on the recognition that employing a source of radiation, such as an electric discharge source, which is equipped with a capillary region that is configured into some predetermined shape, such as an arc or slit, can significantly improve EUV flux. One reason is that the condenser which delivers critical illumination to the reticle can be simplified from five or more reflective elements to a total of three or four reflective elements thereby increasing condenser efficiency. In this regard, preferably the dimensions of the non-circular shaped capillary bore correspond to that of the desired image that is focused by the camera. In the case where the inventive capillary discharge source is used in an EUV lithography system where the camera focuses arc or slit shaped images, the capillary discharge source has a bore having a length to width ratio that substantially matches that of the arc or slit shaped image that is focused by the camera. This enables the employment of a simpler condenser with fewer mirrors since the magnification parallel and perpendicular to the arc or slit can be approximately equal. 
     Accordingly, in one embodiment the invention is directed to a capillary discharge source that includes: 
     a body constructed from a dielectric material that defines a capillary with a bore having a non-circular shaped cross section; and 
     a gaseous species inserted into the capillary, wherein the capillary is used to generate radiation discharges. 
     In a preferred embodiment, the bore has a proximal end and a distal end and the source further includes: 
     (i) a source of gas that is in communication with the capillary bore; 
     (ii) a first electrode positioned at the distal end of the bore; 
     (iii) a second electrode at a reference potential positioned at the proximal end of the bore; and 
     (iv) a source of electric potential that is selectively connectable to the first electrode. 
     In another embodiment, the invention is directed to a source of radiation that includes: 
     means for generating radiation; and 
     a channel having a non-circular cross section that is coupled to the means for generating radiation so that radiation emanating from the source comprises a beam of radiation having a non-circular cross section. 
     In a further embodiment, the invention is directed to a photolithography system for projecting a mask image onto a wafer that comprises: 
     a ringfield camera; 
     a capillary discharge source that includes: 
     (i) a body constructed from a dielectric material that defines a capillary with a bore having a non-circular shaped cross section; and 
     (ii) a gaseous species inserted into the capillary, wherein the capillary is used to generate radiation discharges; 
     a condenser for processing the source radiation to produce a ringfield illumination field and for illuminating a mask; 
     a mask that is positioned at the ringfield camera&#39;s object plane and from which the mask image in the form of an intensity profile is reflected into the entrance pupil of the ringfield camera; and 
     a wafer onto which the mask imaged is projected from the ringfield camera. 
     In yet another embodiment, the invention is directed to a photolithography system for projecting a mask image onto a wafer that includes: 
     a ringfield camera; 
     a source of radiation that includes: 
     (i) means for generating radiation; and 
     (ii) a channel having a non-circular cross section that is coupled to the means for generating radiation so that radiation emanating from the source comprises a beam of radiation having a non-circular cross section; 
     a condenser for processing the source radiation to produce a ringfield illumination field and for illuminating a mask; 
     a mask that is positioned at the ringfield camera&#39;s object plane and from which the mask image in the form of an intensity profile is reflected into the entrance pupil of the ringfield camera; and 
     a wafer onto which the mask imaged is projected from the ringfield camera. 
     In an additional embodiment, the invention is directed to a method of producing radiation that includes the steps of: 
     (a) providing a capillary discharge plasma source that comprises a body that defines a capillary bore having a non-circular shaped cross section; 
     (b) introducing gaseous species into the capillary bore; and 
     (c) creating a plasma within the capillary bore thereby producing radiation of a selected wavelength that is emitted from the capillary bore whereby the emitted radiation has a non-circular shaped cross section which matches that of the capillary bore. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates an EUV lithography system. 
     FIG. 1B illustrates a radiation source. 
     FIG. 2A illustrates an electric discharge source. 
     FIG. 2B illustrates the cross-sectional view of a shaped, extended capillary arc-shaped bore in a dielectric medium. 
     FIG. 3A illustrates a three mirror condenser system. 
     FIG. 3B illustrates the shape of an illumination pattern on the mask plane. 
     FIG. 4 illustrates a prior art lithography system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The capillary discharge source of the present invention will be illustrated as an electric discharge source that is particularly suited for generating EUV and soft x-rays for photolithography, however, it is understood that the invention can be employed to generated shaped emissions of radiation at other wavelengths as well. FIG. 1A schematically depicts an apparatus for EUV lithography that comprises a radiation source  11  that has a shaped emission and that emits soft x-rays  12  into condenser  13  which in turn emits beam  14  that illuminates a portion of reflective reticle or mask  15 . Suitable condensers are described for example in U.S. Pat. Nos. 6,118,577 and 6,210,865, which are incorporated herein by reference. The emerging patterned beam is introduced into the imaging optics  16  which projects an image of mask  15 , shown mounted on mask stage  17 , onto wafer  18  which is mounted on stage  19 . Element  20 , an x-y scanner, scans mask  15  and wafer  18  in such direction and at such relative speed as to accommodate the desired mask-to-image reduction. The imaging optics  16  can comprise a ringfield camera which is described for example in U.S. Pat. Nos. 6,072,852, 6,183,095 and 6,188,513, which are incorporated herein by reference. 
     One critical feature of the present invention is that the radiation emanating from the radiation source, e.g., the cross section of soft x-rays  12  in FIG. 1A, has a predetermined or desired non-circular shape such as, for example, a cross-section that is shaped as an arc or slit. The preferred embodiment of the invention will be illustrated with an electric discharge source that has a novel capillary design wherein the capillary bore is arc-shaped. As is apparent, it is not necessary for the entire length of the capillary bore to have the non-circular, e.g., arc, shape. Rather, it is only necessary that the bore at the capillary exit exhibit a non-circular configuration sufficient to cause the emitted radiation to have a matching non-circular cross section. Thus, the term “bore” when used in reference to the non-circular cross-section refers to (i) the bore exit or (ii) the bore exit and at least portions of the capillary bore along its length. By “arc” is meant a continuous portion (as of a circle or ellipse) of a curved segment and by “slit” is meant an elongated, usually a long, narrow rectangular opening. The shape of the arc is defined by its width, length, and radius of curvature and the slit is defined by the width and length. As is apparent, a “slit” can be viewed as an arc with an infinite radius of curvature. Preferably when the capillary discharge source is used in an EUV lithography system where the camera images arc or slit shaped images, the capillary discharge source has a bore that has a length to width ratio that substantially matches the length to width ratio of the arc or slit shaped mask area that is imaged by the camera. In this fashion, the condenser can be more readily design with simpler optics, i.e., fewer mirrors, because of the magnification parallel and perpendicular to the arc or slit can be approximately equal. In the case where the bore of the discharge source has a slit configuration, i.e., the bore exit is a narrow elongated aperture, so that the discharge emissions have a matching cross section, a suitably designed condenser can be employed to process or modify the rectangular contour of the discharge emissions to produce an illumination that has a contour that substantially matches the ringfield camera&#39;s arcuate slit field. In particular, the condenser can be designed so that as it maps the arc or slit shaped discharges, the ratio of (i) the radius of curvature of the arc or slit to (ii) the arc or slit&#39;s length is modified by the condenser optics to match a desired ratio at the camera&#39;s slit, that is, to substantially match the same measured ratio of the mask image (or an intensity profile) or area of illumination at the mask plane. For example, when the bore has a slit cross section, the condenser can image the radiation onto the desired arcuate field at the camera&#39;s mask plane by an appropriately designed condenser. 
     The radiation source is preferably an electric discharge source. Conventional electric discharge sources can be modified with the novel capillary design. For example, the axisymmetric capillary of a conventional source, which is typically has a 1-1.5 mm diameter circular bore is replaced with the novel capillary. A preferred embodiment is illustrated in FIG. 2A which shows the cross-section of an electric capillary discharge source  110  which preferably comprises an insulating disk  112  that has a capillary bore  114 . The disk  112  is mounted between two electrodes  120  and  130  which are in proximity to the front and back surfaces of the disk, respectively. The disk is made of any suitable ceramic material, such as diamond or boron nitride, and more preferably of pyrolytic boron nitride, compression annealed pyrolytic boron nitride, or cubic boron nitride. FIG. 2B is the cross section of capillary bore  114  showing the arc-shaped opening formed within the dielectric medium. 
     Front electrode  120  is typically grounded and has an aperture  122  having a center that is aligned with the center of the capillary bore  114 . Rear electrode  130  has a channel  132  with an inlet and an outlet  134 . The outlet  134  is connected to the capillary bore at the back end of disk  112  while the inlet is connected to a gas source  170 . Rear electrode  130  is also connected to a source of electric potential  160  which includes a switch mechanism  162  to generate electric pulses. To facilitate the removal of heat, front and rear electrodes and capillaries are preferably encased in a thermally conductive housing  150  which in turn can be surrounded by coils  152  through which a coolant, e.g., water, is circulated. Flange  140  is secured to an outer edge of the conductive housing  150 . Front and rear electrodes are made of any suitable electrically conductive and erosion resistant material such as refractory metals, e.g., tantalum or tungsten. 
     The electric capillary discharge source  110  can employ a pulsed electric discharge in a low-pressure gas to excite a plasma confined within a capillary bore region. A high-voltage, high-current pulse is employed to initiate the discharge thereby creating a plasma, e.g., 2-60 eV, that radiates radiation in the EUV region. The source of gas  170  contains any suitable gas that can be ionized to generate a plasma from which radiation of the desired wavelength occurs. For generating extreme ultraviolet radiation and soft x-rays, xenon is preferred. 
     The capillary discharge source is typically employed so that at least the front electrode is positioned within a housing that is maintained at a sub-atmospheric pressure, typically, at a pressure of approximately 1×10 −3  Torr or less. The rear electrode can be coupled to a high-voltage source such as a pulser capable of producing sufficient discharge current for a duration that ranges, for example, from about 0.5 to 4 μsec. 
     Because of the arc-shaped cross-section of the capillary bore in electric discharge source, the radiation beam emanating from the electric capillary discharge source will have a cross-section matching that of the arc-shaped cross-section of the capillary bore. As a result, condenser  13  as depicted in FIG. 1 can be modified to require fewer reflective surfaces to focus an arc image to the reflective reticle or mask. It is expected that the number of reflective elements in the condenser can be as few as two although three or four mirrors may be more practical when the inventive radiation source is employed. 
     A modified condenser using only 3 mirrors (or 3 sets of mirrors) suitable for use with the radiation source of FIG. 2A that has an arc-shaped bore shown in FIG. 2B is illustrated in FIG.  3 A. The radiation is collected from the source  80  by a mirror  82  which reflects the arc-shaped image from the radiation source to mirrors  84  and  86  and onto mask  88 . Mirrors  82  and  84  are illustrated as off-axis conic sections. Mirror  88  is preferably a toroidal mirror. FIG. 3B shows the shape of the imaged area on the surface of mask  88  (FIG.  3 A). 
     The use of critical illumination places tight tolerances on the intensity uniformity of the shaped capillary source emission since an image of the source intensity distribution is projected directly on the reticle. It may be possible to make the effective scan-averaged intensity distribution more uniform by modulating the capillary arc width along its length, for example, by adding serifs near the ends of the arc. Another possibility for improving source uniformity is to use guiding magnetic fields to tailor the plasma current density along the capillary arc. 
     A scheme that would smooth out small bright or dark spots in the radiation source is to defocus the source image at the mask. This would require that the source be somewhat oversized, so it would waste some power. The defocus could be introduced in one direction (astigmatism) or in both directions. 
     There are a number of other possible advantages of the inventive shaped, extended capillary source. First, it is likely that its EUV emission pulse energy can be made larger than that from the conventional axisymmetric capillary due to the significant increase in its source emission area. To achieve increased EUV pulse energy and to approximately maintain the present axisymmetric capillary source brightness, it is very likely that the peak current necessary to drive the discharge would have to be increased to conserve peak current density within the extended capillary region. This will place additional demands on the high-voltage pulsed power supply and also on capillary cooling requirements (if the pulse duration is not shortened). Alternatively, it may be better to keep the peak current at its present level and take advantage of the increased area of the extended capillary to reduce the incident power density on the inner wall surface of the capillary. This will reduce the transient temperature rise at the plasma/capillary interface and will also result in more efficient heat extraction from the capillary body, allowing the repetition rate to be increased. 
     It is understood that the inventive radiation source can comprise any suitable device that generates radiation, e.g., x-rays or EUV; the only requirement is that it includes means for shaping the light beam that enters the condenser. Any conventional capillary discharge source can be modified by employing the novel capillary design described above. Conventional radiation sources that can be employed with the appropriate novel channel or modified with the novel capillary include, for example, a synchrotron and laser-generated plasma sources. Suitable radiation sources are further described, for example., in Kubiak et al “High-power extreme ultraviolet source based on gas jet,” Proceedings of SPIE 3331, 81-89 (1998), Klosner and Silfvast “Intense xenon capillary discharge extreme-ultraviolet source in the 10-16-nm-wavelength region.” Optical Letters 23, 20 1609-1611 (1998), Kubiak et al U.S. Pat. No. 5,577,092 “Cluster Beam Targets for Laser Plasma Extreme ultraviolet and Soft X-Ray Source”, Silfvast U.S. Pat. No. 5,499,282 “Efficient Narrow Spectral Width Soft-X-Ray Discharge Sources, and Silfvast et al. U.S. Pat. No. 5,963,616 “Configuration, Materials, and Wavelengths for EUV Lithium Plasma Discharge Lamps”, and Silfvast et al. U.S. Pat. No. 6,031,241 “Capillary Discharge Extreme Ultraviolet Lamp Source for EUV Microlithography and Other Related Applications,” which are all incorporated herein by reference. 
     Although only preferred embodiments of the invention are specifically disclosed and described above, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.