Abstract:
A tangential blower for use in a gas discharge laser is provided provides improved homogeneity of laser gas flow through the discharge region of the laser. A flange which supports adjacent blower sections has an aerodynamic shape and occupies a minimal portion of the space in the inlet region of the blowers. The ends of the blower&#39;s shafts may be formed as a twice-profiled polygon which is has a non-uniform and preferably rounded geometry along its longitudinal axis where it fits into end flanges. The blades of the blower may be formed with varying thickness and radii of curvature. The blower&#39;s blades and hubs may be cast as a single piece of steel, titanium alloy, or other suitable material.

Description:
PRIORITY 
   This invention claims the benefit of priority to U.S. provisional patent application No. 60/193,048, filed Mar. 29, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to tangential blowers (also known as cross-flow blowers) generally and more particularly to tangential blowers used for circulating gas within a gas discharge laser. 
   2. Discussion of the Related Art 
   Tangential blowers have been used in various applications for several decades. Interest in tangential blowers has been heightened in recent years because such blowers are well suited for use in pulsed gas discharge lasers. 
   The operation of a typical electrical discharge system for a gas discharge laser is shown in FIG.  1 . Electrodes  101  and  102  are separated by a gap where the gas discharge occurs. This discharge occurs quickly and is typically repeated many times per second. It is recognized herein that for various applications including microlithography, repetition rates of 1000 Hz and more may be used. 
   Laser gas is circulated around a chamber and through the discharge gap. The gas is circulated within the chamber by a cross-flow or tangential blower, as shown in FIG.  1 . Cross-flow blower  103  comprises shaft  104 , which is normally parallel to blades  105 . The housing  106  contains the laser gas. When cross-flow blower  103  rotates, gas is circulated between electrodes  101  and  102 .  FIG. 2  illustrates the air flow through a cross-flow blower. 
   Operating in a gas discharge chamber places numerous demands on the blower. Normally, the laser gas is strongly electronegative and therefore corrosive. In addition, a pulse rate of 1000 Hz means that a blower may revolve at, e.g., 3,300 r.p.m. in order to clear the gas from the discharge region between pulses. At such speeds, the bearings, shaft and other structural elements are subjected to stresses and vibrations. At still it) further higher repetition rates such as 2 kHz or more, at which repetition rates it is recognized herein that it is desired to have excimer and molecular fluorine lasers capable of operating at, there is still greater demands on the gas flow speed. That is, to fully clear the gas through the discharge volume, or volume of space between the electrodes that participates in the discharge, from one pulse to a succeeding pulse, either the gas flow speed is to be increased or the electrode width is to be reduced, or a combination of these two, to ensure that the gas mixture clears the discharge region from pulse to pulse at these higher repetition rates. 
   In part because of the demands of operating in a gas discharge chamber, there have been recent attempts to strengthen tangential blowers and make their components more durable. One approach is described in U.S. Pat. No. 5,870,420, which is hereby incorporated by reference and which teaches the use of truss elements welded to the inside of the frame of a cross-flow blower, as illustrated in FIG.  3 . This patent describes blades with a single radius of curvature. The blades are fitted into slots in the frame and welded to the frame. 
   However, the braced tangential blower described in the &#39;420 patent increased the gas flow rate by only a few percent as compared to conventional tangential blowers operated at the same speed. In order to obtain this modest increase in flow rate, the trussed blower required an increase in current of 27% to 28% as compared to a non-trussed tangential blower. A blower which is stiffer than the trussed blower of the &#39;420 patent would be desirable, so that the blower could be rotated faster without excessive vibrations. 
   In order to add stiffness to the blower and reduce vibration, the blower may be divided into two or more sections in an axial direction, as illustrated in FIG.  4 . However, one consequence of dividing the blower into sections is that a region of non-homogenous laser gas flow is created in the discharge gap between the two electrodes. As shown in the cross-sectional view of  FIG. 5 , flange  201  divides the blower cavity into two axial compartments. The laser gas from both compartments is not allowed to interflow until after being discharged from the blowers and directly before entering the discharge gap between the electrodes. In the short distance between the end of the top portion of flange  201  and the discharge gap, the laser gas volumes from either side of the flange have not had an opportunity to properly interflow, and there is a volume of inhomogeneous gas as shown at  FIG. 4  between the two laser gas volumes that are directed to the discharge gap. This inhomogeneous laser gas volume lowers the productivity of the laser. It would be desirable to provide a device for supporting the interior ends of the blowers without creating such a region of inhomogeneous laser gas. 
   SUMMARY OF THE INVENTION 
   In view of the above, it is an object of the present invention to provide a tangential blower with a smaller region of inhomogeneous laser gas, especially in the discharge area of the blower. 
   It is a further object of the present invention to provide a more efficient tangential blower. 
   It is a further object of the present invention to provide a tangential blower which is stronger and stiffer than conventional blowers. 
   It is a feature of the present invention to provide a tangential blower with blades which have an aerodynamic shape. 
   It is another feature of the present invention to provide a tangential blower, the blades and hubs of which have been cast as a single piece. 
   It is a feature of the present invention to provide a tangential blower in which rounded portions of the shaft are fitted to the end hubs of the blowers. 
   It is an advantage of the present invention that the tangential blower provided is more durable. 
   It is a further advantage of the present invention that the tangential blower provided creates less turbulence and reduces the region of inhomogeneous flow between blower sections. 
   According to one embodiment, a tangential blower is provided in which the thickness and radii of curvature of the blades are varied. 
   According to another embodiment, the ends of a tangential blower&#39;s shaft are formed as a twice-profiled polygon which is rounded along its longitudinal axis where it fits into end flanges. 
   According to yet another embodiment, the blades and hubs of a tangential blower are cast as a single piece. 
   According to still another embodiment, an improved flange supports the tandem blower sections while occupying less space than related art flanges, thereby reducing the region of inhomogeneous flow in the discharge area of the blower. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a gas discharge chamber of a known gas discharge laser. 
       FIG. 2  is a schematic view which illustrates how gas is moved through a blower. 
       FIG. 3  illustrates a tangential blower described in U.S. Pat. No. 5,870,420. 
       FIG. 4  is a cross-section of a gas discharge laser along the axis of the blowers comprising two adjacent blower sections joined by a flange  201 . 
       FIG. 5  is an enlarged view of a cross-section of a gas discharge laser along plane B—B shown on  FIG. 4 , illustrating a side view of a flange  201 . 
       FIG. 6  is a cross-section of a gas discharge laser along the axis of the blowers illustrating the embodiment of flange  301 . 
       FIG. 7  is a cross-section through plane C—C of  FIG. 6  of a gas discharge laser. 
       FIG. 8  is a cross-section of the embodiment of flange  301  through plane D—D of FIG.  7 . 
       FIG. 9  is a cross-section with the same orientation as that of  FIG. 7 , illustrating a side view of an other embodiment of flange  401 . 
       FIG. 10A  is a cross-section of the end of the shaft of a conventional tangential blower, in which the plane of the paper is parallel to the shaft&#39;s long axis. 
       FIG. 10B  is a cross-section of the shaft through plane E—E of FIG.  10 A. 
       FIG. 11A  is a cross-section of the end of the shaft of a tangential blower according to a preferred embodiment, in which the plane of the paper is parallel to the shaft&#39;s long axis. 
       FIG. 11B  is a cross-section through plane F—F of  FIG. 11A  showing the end of the shaft of a tangential blower according to a preferred embodiment. 
       FIG. 12  is a longitudinal cross-section of one embodiment of the blower of a gas discharge laser. 
       FIG. 13  is a longitudinal cross-section of one embodiment of the blower of a gas discharge laser. 
       FIG. 14  is a longitudinal cross-section of one embodiment of the blower of a gas discharge laser. 
       FIG. 15  is a longitudinal cross-section of a gas discharge laser. 
       FIG. 16  is a cross-section of a radial blade used in a conventional tangential blower. 
       FIG. 17  is a cross-section of a radial blade for a tangential blower according to one embodiment of the present invention. 
       FIG. 18  is a cross-section through the circumference of blower  902  of a gas discharge laser. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Improved Flange for Joining Blower Sections 
   A first embodiment for joining tandem sections of a blower is illustrated in  FIGS. 4 and 5 . In  FIG. 4 , gas discharge laser  200  contains flange  201  which joins first blower section  202  and second blower section  203  at bearing  204 . Bearing  205  and bearing  204  support blower section  202 , and bearing  204  and bearing  206  support blower section  203 . The laser gas is discharged into the gap between upper electrode  207  and lower electrode  208 . Lower electrode support  209  supports lower electrode  208 .  FIG. 5  shows a cross-section of the gas discharge laser  200  through plane B—B of FIG.  4 . Housing  211  and lower electrode support  209  illustrate how the laser gas is guided towards the gap between upper electrode  207  and lower electrode  208 . Bearing  204  is shown supporting blower  202 . Blower  202  is indicated with dotted lines because it is on an opposite side of flange  201  and thus would not be visible through plane B—B of FIG.  4 . 
   As seen in  FIG. 5 , flange  201  separates the laser gas roughly into two volumes, one volume on either side of the flange. There are thus two volumes of laser gas being discharged by the tandem blowers towards the electrode discharge gap separated by a region of inhomogeneity between them. 
   In order to reduce or substantially eliminate the volume of inhomogeneous flow in the discharge region between the two volumes of laser gas, a present embodiment of the tandem blowers includes flange  301  depicted in  FIGS. 6 and 7 . In  FIG. 6 , flange  301  is shown supporting bearing  204 .  FIG. 7  is an enlarged view of a cross-section through plane C—C of FIG.  6 .  FIG. 7  shows flange  301  mounted on the housing  211  only in the region where the outer circumference of the blower nears housing  211  and opposite electrode  207 . In contrast to flange  201 , this preferred embodiment including the improved flange  301  is not further affixed to housing  211  nor is it affixed to lower electrode support  209 . The flange  301  is shared so that it does not interfere with mixing of the gases discharged by adjacent blower sections. Part of a trailing edge  301   b  of the flange  301  is located inside a cylindrical volume defined by the combined cylindrical forms of the blower sections, as shown in FIG.  7 . The left and upper portions of the cylindrical blower  203  are visible in  FIG. 7  behind the trailing edge  301   b  of the flange  301 . The discharges of the blower sections are not separated by the flange, which allows the discharged gases to intermix. Once the gas has flowed beyond the trailing edge  301   b  of the flange  301 , it is able to mix with the gas being discharged by the adjacent blower. Thus, the laser gas is allowed to interflow soon after it enters the inlet portion of the blowers, and prior to entering the discharge gap between the electrodes  207  and  208 . 
   Flange  301  preferably comprises a leading edge  301   a  and a trailing edge  301   b  as depicted in  FIGS. 7 and 8 .  FIG. 8  is a cross-section of the blowers and flange through plane D—D of FIG.  7 . In  FIG. 8  blower  202  and blower  203  are supported by bearing (may be a double bearing)  204 . Leading edge  301   a  and trailing edge  301   b  are shaped in an aerodynamic fashion in a preferred embodiment. The aerodynamic shape of the edges  301   a  and  301   b  of flange  301  further decreases the volume of the inhomogeneous laser gas flow in the discharge region of the blowers as ell as increase the efficiency of the tandem blowers, e.g., by reducing turbulence created by air flow around the flange. 
   Another alternative embodiment of the improved flange is shown in FIG.  9 . In this embodiment, the base of improved flange  401  is attached to housing  211 , opposite upper electrode  207 , and the upper portion of flange  401  is attached to the lower electrode support  209 . In this embodiment, leading edge portion  401   a  and trailing edge portion  401   b  are shaped in an aerodynamic fashion, e.g., as shown in FIG.  8 . In addition, the flange  401  is cut off on the downstream end of the blower compared with flange  201  shown in FIG.  5 . As in the  FIG. 7  embodiment, the  FIG. 9  embodiment has part of a trailing edge  401   b  of the flange located within a cylindrical volume defined by the combined cylindrical forms of the blower sections. The discharges of the blower sections are not separated by the flange, which allows the discharged gases to intermix. This allows for a reduction of the volume of the inhomogeneous region as it enters the discharge gap between the upper electrode  207  and lower electrode  208 , in accord with the alternative embodiment that is shown in  FIGS. 7 and 8 . An advantage of the embodiment shown at  FIG. 9  is improved mechanical stability and reduced vibration sensitivity. 
   The Shaft and Hub 
   As shown in  FIGS. 10A and 10B , the ends of the shaft of a cross-flow blower for use with an gas discharge laser are formed with a constant thickness in the longitudinal direction.  FIG. 10B  is a cross-section through plane E—E of FIG.  10 A. The shaft of  FIGS. 10A-10B  has constant thickness and can tend to wear at the ends where the shaft is fixed to the end hub of a blower. Wear is also caused to the end hub due to the vibration of the blower. A reason for this is that a bending mode vibration tends to occur when the blower is in operation. As the blower/shaft assembly rotates and vibrates in a bending mode, a rocking action takes place where the shaft is constricted by the end hub of the blower, this rocking motion thus causing wear to the end hub and the shaft and/or bending shaft and hub respectively 204, 203. 
   Therefore, according to a preferred embodiment, the end portion  501  of shaft  502  may be formed as shown in  FIGS. 11A and 11B  (which are not drawn to scale).  FIG. 11A  shows the shaft  502 , and the shaft end  501 . Shaft end  501  has a maximum diameter D max  at its center  503  and a minimum diameter D min1  at its first end  504  and a minimum diameter D min2  at its second end  505 . The end portion of the shaft  502  may also have a radius of curvature R, and the end portion of the shaft may be preferably manufactured with a constant radius. The reduction of the end portion  501  diameter from the center to the ends, allows the shaft to rock smoothly in the end hub, thus reducing wear to the shaft and end hub. In a preferred embodiment, the radius of curvature R may be made the same for the top and bottom of the shafts. The larger the radius of curvature, which results in a less severe curve to the shaft, the greater the area of the shaft in contact with the bearing, thus allowing a greater pressure between the bearing and the shaft. Thus R is preferably large compared to the diameter D of the end portion of the shaft. In one embodiment, D max =8 mm and D min =7.95 mm, thus D max −D min =0.05 mm. In a preferred embodiment, D max −D min =0.02 mm. 
   Note that the radius of curvature indicated in  FIG. 11A  appears to be much greater for illustration than would be indicated by the preferred change of diameter. Also, the change in thickness between shaft  502  and end portion  501  would typically be much less than indicated by FIG.  11 A. In one embodiment, there is a smooth transition between shaft  502  and end portion  501  with little or no thickness change at the interface. 
   Also note that in order to reduce wear and/or bending due to the bending mode of the shafts, it is not required that the end portions of the shaft  501  have a constant radius of curvature along its axis, in FIG.  11 A. The shaft may have an elliptical shape, a parabolic shape, any smooth shape, or even a step shape with no radius of curvature whatsoever along its axis. As long as the end portion of the shaft  501  has a D max −D min  in the range of preferably 0.005 mm to 0.05 mm, and generally less than around 1 mm, the wear to the shaft and end hub will be reduced. 
   According to this aspect of the present invention, the blower end hub is made to receive the cross-sectional shape of shaft end  501  illustrated in FIG.  11 B.  FIG. 11B  is a cross-section view of shaft end  501  through plane F—F. Although polygons are generally considered to be bounded by straight lines, for the sake of convenience the improved blower hub will be referred to as end hub  601 , which will be discussed in more detail below.  FIG. 11B  illustrates that the shaft end  501  is thicker in the middle as defined by D max  than at the ends, as defined by D min1  and D min2 . 
   Casting the Blades, the Hubs and the Polygonal Hub 
   The preferred method of fabricating the blower  600 , blower blades, end hubs  601 , Internal hubs  602  is by casting them as one piece, as shown in  FIG. 12  (blower blades not shown in order to simplify drawing). However, one or more of these components may be formed separately. For example,  FIG. 13  illustrates the blower blades, internal hubs  602 , and blower  600  cast separately from end hub  601 .  FIG. 14  shows a separately-cast end hub  601  coupled to a blower like the one illustrated in FIG.  12 . 
   Investment casting, also known as “lost wax” casting, is the preferred method of casting blower  600 . However, any form of precision casting, e.g., die casting, may be used. The discussion in Chapter 8 of Davies,  Solidification and Casting  (Applied Science Publishers, Ltd. 1973) describes these well-known processes and is hereby incorporated by reference; copies of the relevant pages are filed herewith. 
   The first step in the investment casting process is to produce an expendable pattern of the desired blade and hub shape in wax, plastic (e.g., polystyrene) or other pattern material which is easily worked and has a relatively low melting temperature. 
   The pattern is made by pouring or injecting the pattern material into a mold, generally a metal mold. Although a pattern for a tangential blower could be made in one step using an integral gating system, the pattern is easier to make by assembling separate components (e.g., of the blade and end hub sections) which are formed individually. However, a pattern formed by assembling separate components can be used to make a one-piece casting. 
   After the pattern is formed, it is dip-coated with a slurry coat of fine particles to give it a smooth surface, “stuccoed” with coarser refractory material and then dried and fired. The pattern material either melts away or will be burned away during the process of firing, whereas the refractory material will harden. 
   Then, metal is cast into the resulting hollow mold. A tangential blower may be formed of any suitable metal, but alloys of aluminum, magnesium, titanium or steel are preferred. Suitable metal is preferably substantially free of silicon. 
   Referring to  FIG. 15 , blower  700  is supported at each end by shafts  701 , which are supported for rotation by bearings  702 . End portions  703  of shafts  701  engage with blower end hubs  704 . Blower  700  circulates laser gas between upper electrode  705  and lower electrode  706 . Insulator  707  separates upper electrode  705  and cover  708 . Laser windows  709  are formed in tube wall  710 . 
   Aerodynamic Blades 
   In one embodiment of a tangential blower, the blades have inner and outer surfaces with roughly the same radius of curvature and have a relatively constant cross-sectional thickness from the leading edge to the trailing edge of the blade.  FIG. 16  illustrates an enlarged cross-section of a single blade, according to this embodiment, used in a cross-flow blower. The inner and outer surfaces of the blade shown in  FIG. 16  have the same radius of curvature and the blade&#39;s thickness is roughly constant. 
   According to a preferred embodiment, the blades of an improved tangential blower are formed with differing radii of curvature for the inner and outer surfaces of the blade. One such embodiment is shown in FIG.  17 . The shape of blade  801  in  FIG. 17  yields a superior aerodynamic performance. A blower with aerodynamically improved blades can move laser gas faster than the same blower rotating at the same speed with conventional blades. In addition, blades  801  add “stiffness” to the blower. Aerodynamic blades may be made by extrusion or any other possible technique as understood by those skilled in the art. 
   The width of blade  801  may vary, but is typically on the order of 10 mm wide, with 20 to 40 blades per blower. Blades  801  run substantially the length of the blower, which is on the order of half a meter. If the blower is divided into sections, the blades run substantially the length of the blower sections.  FIG. 18  is a view perpendicular to that of FIG.  15  and illustrates one way that blades  901  may be arranged in blower  902 . 
   It will be readily understood by those persons skilled in the art that the present invention is susceptible of a broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiments, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiment, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and structural and functional equivalents thereof.