Abstract:
The apparatus is an x-ray tube which generates collimated x-rays. The x-ray tube anode has an x-ray generating structure which is a single crystal, so that regardless of their locations of origin all the x-ray beams leave the structure at the same limited few angles. With the structure formed as a curve, one set of beams converges at the focal point of the curve, and with the structure flat, the beams illuminate an area with parallel, collimated, x-ray beams.

Description:
BACKGROUND OF THE INVENTION 
     This invention deals generally with x-ray tubes and more specifically with an x-ray tube which generates highly collimated radiation. 
     X-ray tubes function on the basis of an electron beam being generated by a cathode within the tube, and the electron beam bombarding a very small spot on an anode which is also within the tube. The bombardment of the anode, which is constructed of a suitable x-ray generating material, creates the x-rays along with a great deal of heat. 
     Until now most x-ray tubes have generated radiation which is poorly focused and have required secondary structures or devices to focus the beam on an object to be studied. Typical focusing structures external to the x-ray source have been spherical mirrors (U.S. Pat. No. 5,604,782 by Cash), curved crystals (U.S. Pat. 5,008,910 by Van Egeraat), capillary tubes (U.S. Pat. No. 5,001,737 by Lewis et al), and bent crystals on the inside surface of tubular structures (U.S. Pat. No. 3,898,455 by Furnas, Jr.). 
     A few efforts have also been made to generate a more focussed beam within the x-ray tube itself. In U.S. Pat. No. 4,352,021 by Boyd et al, multiple curvelinear anodes are disclosed, but they are also followed by a collimator structure to improve the focus. In U.S. Pat. No. 3,821,574, Burns discloses a single crystal anode of elongated channel shape which is used to generate a more intense x-ray beam because the beam is diffracted from the single crystal structure many times as it travels along the channel. 
     Despite this prior art, a simple structure for an x-ray tube which produces a collimated beam is not available. It would be very beneficial for both industrial and medical applications to have available an x-ray tube which is essentially interchangeable with x-ray tubes in common use but which produces a highly collimated beam which requires minimal external focusing devices. 
     SUMMARY OF THE INVENTION 
     The present invention is an x-ray tube which generates a highly collimated beam within the x-ray tube itself. To accomplish this a single crystal or a highly oriented coating is used for the x-ray generating anode (or target) of the tube. To generate a focused beam, this single crystal structure is attached to a spherical or parabolic surface. Thus, x-ray photons which leave the structure on a path perpendicular to the surface are focused at a specific focal point determined by the curvature of the single crystal. 
     For some applications it may be desirable to produce a collimated beam which is not focused, that is, a beam which actually is comprised of multiple parallel individual beams. Such a beam, which can, for instance, be used in large area illumination of photolithographic masks, can be generated by the use of a single crystal attached to or comprising a flat anode surface. 
     The x-ray photons are generated in a conventional manner by bombarding the anode with electrons from an electron source within the x-ray tube. The electrons emitted from the source are accelerated to a high velocity before striking the anode by the use of a voltage gradient between the electron source and the anode. The voltage gradient is established by the application of appropriate voltages to the electrodes from an external power supply. 
     The electron beam can also be scanned by a magnetic deflection coil, similar to that used in television picture tubes. Such scanning permits the generation of x-rays from multiple points on a large surface as opposed to the more traditional manner of directing the electron beam to a single location on the anode, and, in some x-ray tubes, rotating the anode so that no single location on the anode overheats. 
     The benefit derived from the single crystal structure is the limited number of paths followed by photons generated within the crystal lattice and the parallelism of all the photons emitted in any one of the limited directions. Photons which try to leave the crystal lattice in directions other than the several preferred paths are refracted into the preferred paths or absorbed by the crystal lattice and re-emitted in one of the preferred paths. Thus, if the anode surface is perfectly flat, although photons are emitted at several specific angles to the surface, all the photons leaving the surface at each of the specific beam angles will be parallel to all the other beams of photons departing from the surface, even though the photons are generated at multiple locations within the crystal lattice. 
     In more familiar terms, the emission of x-rays from each spot on a single crystal anode structure is similar to the illumination from the narrow beams of several spotlights positioned at a single location, so that they form a limited number of narrow beams of light from that location. Furthermore, all other locations on the anode generate only light beams which are parallel to those from the first location. 
     In a similar example, each x-ray generating spot of a typical prior art x-ray anode can be represented by a single simple incandescent light bulb which sends out photons in a full semi-spherical pattern. Just as we regularly do with flashlights and search lights, the x-rays from conventional anodes must then be focused with reflectors and lenses. 
     However, the focus of x-rays from a single crystal structure can be determined, not by external focusing devices, but by the curvature of the anode surface itself. When the surface is parabolic, the x-rays will be focused at the focal point of the parabola, and if the surface is perfectly flat the x-rays will simply generate a shaft of parallel collimated x-ray beams. 
     This pattern of collimated beams is particularly useful in the photolithography process used in the semiconductor industry. The number of circuit elements which can be squeezed into a specific area is now approaching a new limit, the resolution available with the light used for illuminating the photolithography mask. The minimum spacing between individual elements is limited by the wavelength and collimation of the light used for transferring the image from the mask to the semiconductor material. An x-ray beam generated by a single crystal can take this process to the next level because the wavelengths of x-rays are not only much shorter than those of visible light but they are also collimated. 
     Thus, the present invention can not only furnish better focused x-rays for use in conventional medical and industrial uses, but can also yield shorter wavelength collimated beams for improving the integrated circuit manufacturing process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a partial cross section side view of the preferred embodiment of the invention. 
     FIG. 2 is a side view of an alternate embodiment of the x-ray generating anode of the invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     FIG. 1 is a schematic representation of a partial cross section side view of x-ray tube  10  within which electron bombarded and x-ray generating structure  12  of anode  14  is attached to and cooled by a base structure, which is heat pipe  16 , while generating x-ray beam  18 . Such a tube is constructed with cathode  20  mounted within evacuated envelope  24  and interconnected to suitable power supplies (not shown) by cathode connections  26  which penetrate envelope  24 . Electron beam  22  originates at cathode  20  and bombards x-ray generating structure  12 . 
     FIG. 1 also schematically depicts a structure which can be used to control electron beam  22 . Magnetic coil  28  is a device which can deflect electron beam  22  in any direction along bombarded structure  12 , as indicated by beam lines  22 A and  22 B. However, it should be appreciated that there are other devices in the art, such as electrostatic plates, which can also be used to deflect electron beam  22  and scan it across structure  12 . 
     Heat pipe  16  penetrates envelope  24  and is sealed to it at vacuum seals  30  by conventional means. Heat pipe  16  eliminates the need to rotate anode  14  because heat pipe  16  is capable of cooling bomdarded structure  12  well enough to prevent thermal damage to structure  12  by the electron beam. 
     In this embodiment, in order to sufficiently cool bombarded structure  12 , heat pipe  16  is constructed with a tungsten casing, lithium fluid, and a niobium powder wick for high power density operation. Heat pipe  16  removes the heat generated at the spots at which electron beam  22  bombards structure  12 . Cooling coil  32 , located at the condenser end of heat pipe  16 , and through which a cooling fluid is pumped, then moves the heat from heat pipe  16  to a remote heat exchanger (not shown). 
     Elimination of the need to rotate anode  14  complements the ability to deflect electron beam  22  because it permits full electronic control of the location of the spots which generate x-ray beam  34 . With the structure shown in FIG. 1, the electron beam can be moved around structure  12  instead of requiring the rotation of anode  14 . Furthermore, with the rotation of the anode eliminated, the invention is not restricted to circular layouts for x-ray generating structure  12 . Thus, it is quite practical to construct anode  14  and heat pipe  16  with rectangular plan views, and with the concave cross section of structure  12  as shown in FIG. 1, to generate x-rays which yield a linear configuration on the illuminated surface. 
     However, the present invention also uses special material for x-ray generating structure  12  which gives x-ray beam  34  special characteristics and increased versatility. Structure  12  is constructed as a single crystal or a highly oriented coating of a material such as tungsten. Such a highly oriented coating can be produced by chemical vapor deposition, a process well understood in the art of material coating. 
     For the preferred embodiment, structure  12  is a single crystal structure of tungsten with a thickness of 0.001 to 0.010 inch. However, many other materials can be produced as single crystal structures, and each material has different x-ray generating characteristics such as wavelength and beam orientation. These characteristics of materials are well documented in the literature dealing with x-rays. 
     The characteristic of such a single crystal structure is that there are a limited number of exit paths available to the photons generated within the crystal lattice of the material, and that all the photon emission paths originating from any location on the structure are parallel to the emission paths originating at all the other locations. Thus, for a flat structure, although photons are emitted at several specific angles to the surface, all locations on the structure will emit photons at only the same few limited angles at which every other location emits photons, and the result will be many parallel beams of photons leaving the structure at each of the limited number of angles. 
     In the simplest case which is illustrated in FIG. 1, if one of the exit path angles for a particular material is perpendicular to structure  12 , any spot of structure  12  which is bombarded by electron beam  22  will generate, along with a limited number of other x-ray beams, an x-ray beam  18  exiting perpendicular to structure  12 . Therefore, when structure  12  is shaped as a parabola or a small radius sphere approximating a parabola, the x-ray beams from all locations of structure  12  exit perpendicular to parabolic structure  12 . Those beams, such as beams  18 A and  18 B, then meet at focal point  34 , after exiting tube  10  through window  36 , regardless of where on structure  12  they originated. 
     It should be appreciated that parabolic structure  12  is not functioning as a reflector as might be first supposed, but rather as a parabolic radiation generator. Moreover, structure  12  need not necessarily be a parabola, but can be any curved structure to focus a beam at a particular location or locations. A deviation in the curved structure is particularly helpful when the exit angles of the beams from structure  12  which are being used is other than perpendicular. 
     One such variation of the electron bombarded and x-ray generating structure of an anode is depicted in FIG.  2 . FIG. 2 is a side view of an alternate embodiment of the x-ray generating anode  40  of the invention in which structure  42  is flat and, as in many x-ray tubes, angled to deliver x-ray beam  44  out the side of the tube wall  46 . As in FIG. 1, an electron beam  48  bombards x-ray generating structure  42 , and electron beam  48  can be moved over entire structure  42  as is indicated by beam lines  48 A and  48 B by a deflection coil (not shown). 
     However, anode  40  in FIG. 2 differs from anode  14  in FIG. 1 because x-ray generating structure  42  is flat so there is no focusing action and also because the angles of exit of x-ray beams  44 ,  44 A, and  44 B from structure  42  are not perpendicular to structure  42 . Nevertheless, when x-ray beams  44 ,  44 A, and  44 B originate from single crystal structure  42 , or any highly oriented coating, they are all collimated and parallel to each other regardless of the origin points of the beams. The structure of FIG. 2 therefore makes it possible to illuminate areas equivalent in size to structure  42  itself with x-rays. As previously discussed, such illumination is useful in exposing masked areas in photolithography to x-rays. 
     FIG. 2 also shows an alternate structure for cooling the x-ray generating structure of the anode. In FIG. 2, x-ray generating structure  42  is attached to hollow casing  50 , and high velocity, high turbulence liquid is pumped into casing  50  through input pipe  52  which extends into casing  50  until near structure  42 . Output pipe  54  removes the heated liquid from casing  50  and is interconnected to an external heat exchanger (not shown) where the liquid is cooled for return to input pipe  52  by a pump (not shown). 
     It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims. 
     For example, various materials can be used in single crystal form to generate different wavelengths of x-rays, and to yield x-ray beams with different exit angles from the single crystal. Furthermore, as previously discussed, materials can be coated onto the anode for the x-ray emitting structure by means of chemical vapor deposition. Such coated materials are also capable of generating highly collimated x-rays.