Patent Number: 
Section: description

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 xe2x80x9cborexe2x80x9d 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 xe2x80x9carcxe2x80x9d is meant a continuous portion (as of a circle or ellipse) of a curved segment and by xe2x80x9cslitxe2x80x9d 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 xe2x80x9cslitxe2x80x9d 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""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""s length is modified by the condenser optics to match a desired ratio at the camera""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""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 1xc3x9710xe2x88x923 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 xcexcsec. 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. 3A. 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. 3A). 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 xe2x80x9cHigh-power extreme ultraviolet source based on gas jet,xe2x80x9d Proceedings of SPIE 3331, 81-89 (1998), Klosner and Silfvast xe2x80x9cIntense xenon capillary discharge extreme-ultraviolet source in the 10-16-nm-wavelength region.xe2x80x9d Optical Letters 23, 20 1609-1611 (1998), Kubiak et al U.S. Pat. No. 5,577,092 xe2x80x9cCluster Beam Targets for Laser Plasma Extreme ultraviolet and Soft X-Ray Sourcexe2x80x9d, Silfvast U.S. Pat. No. 5,499,282 xe2x80x9cEfficient Narrow Spectral Width Soft-X-Ray Discharge Sources, and Silfvast et al. U.S. Pat. No. 5,963,616 xe2x80x9cConfiguration, Materials, and Wavelengths for EUV Lithium Plasma Discharge Lampsxe2x80x9d, and Silfvast et al. U.S. Pat. No. 6,031,241 xe2x80x9cCapillary Discharge Extreme Ultraviolet Lamp Source for EUV Microlithography and Other Related Applications,xe2x80x9d 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.