Abstract:
A sputtering target includes an outer target tube, an inner support tube of rectangular cross-sectional shape supporting a magnet carrier extending along substantially the entire length of the inner support tube; and a water cooling circuit including at least one passageway within said inner support tube with an inlet at one end thereof adapted to receive cooling water from an external source, at least one outlet aperture at an opposite end thereof opening to a chamber radially between the inner support tube and the outer target tube; and at least one cooling water outlet at the one end of the inner support tube.

Description:
[0001]     This is a continuation-in-part of application Ser. No. ______ (Atty. Dkt. No. 3691.881), entitled: SPUTTERING TARGET AND METHOD/APPARATUS FOR COOLING THE TARGET, and application Ser. No. ______ (Atty. Dkt. No. 3691.883), entitled: SPUTTERING TARGET AND METHOD/APPARATUS FOR COOLING THE TARGET.  
       BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to a sputtering target and, more specifically, to an internal magnet support tube configuration with cooling enhancement features.  
         [0003]     The use of sputtering in order to deposit coatings on substrates is known in the art. For example, and without limitation, see U.S. Pat. Nos. 5,403,458; 5,317,006; 5,527,439; 5,591,314; 5,262,032; and 5,284,564. Briefly, sputter coating is an electric-discharge-type process which is conducted in a vacuum chamber in the presence of at least one gas. Typically, a sputtering apparatus includes a vacuum chamber, a power source, an anode, and one or more cathode targets which include material used to create a coating on an adjacent substrate. The target may include an outer tube enclosing a magnet bar assembly including an associated inner magnet bar support tube and a magnet carrier. More specifically, in certain known arrangements, the magnet bar is secured to the underside of the support tube along substantially the length of the support tube.  
         [0004]     When an electrical potential is applied to the cathode target, the gas forms a plasma which bombards the target causing particles of the material from the target to leave the exterior surface of the outer target tube. These particles fall onto the substrate to thereby form the coating thereon. The outer target tube typically rotates about the stationary magnets supported by the inner support tube so that particles are “sputtered” uniformly from the entire periphery of the target tube as it rotates past the fixed magnet bar.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     It has been found that non-uniform sputtering, i.e., non-uniform eroding of the target surface, is caused at least in part by non-uniform cooling of the interior surface of the outer target tube. Specifically, in a typical target water-cooling arrangement, cooling water flows through the inner magnet bar support tube and reverses direction to flow between the inner support tube and outer target tubes, it has been discovered that air bubbles and “dead water” zones are likely to form at or near the inner tube outlet jets where the cooling water reverses direction (sometimes referred to herein as the “flow-reversal zone”). It has also been discovered that insufficient circulation of cooling water along the inner magnet support bar (i.e., in the radial space or region between the magnet bar and target tube), further degrades the cooling of the target tube in this area. Resulting localized hot spots on the target tube surface can lead to non-uniform sputtering and decreased target component life.  
         [0006]     In accordance with an exemplary embodiment of this invention, mechanical flow enhancement devices are attached to the external surface of the inner magnet bar support tube to promote and enhance cooling of the target tube. More specifically, two enhancement devices are provided that may be used alone or in combination to enhance cooling flow and thus promote more uniform sputtering.  
         [0007]     The first device is a baffle in the form of a flat plate with an array of apertures formed therein. The baffle is secured to the exterior of the inner magnet bar support tube by any suitable fastening bracket adjacent that end of the inner support tube where the cooling water exits into the space between the inner and outer tubes. In this regard, the cooling water exits the inner tube in the form of a plurality of jets and almost immediately reverses direction to flow between the inner support tube and the outer target tube. By forcing the water to flow through the baffle, sufficient turbulence is created to avoid or at least reduce the creation of air bubbles and/or dead water zones at or near the cooling water flow-reversal zone.  
         [0008]     The second device is a flow member in the form of spiral vane segments that are attached to the inner support tube and extend substantially the entire distance between the baffle and the opposite end of the inner and outer tubes. The series of discontinuous vane segments provide an axially extending space on the underside of the inner support tube to accommodate the axially extending magnet bar. The spiral vane segments cause the cooling water that otherwise might stagnate under the magnet bars to continuously circulate into and out of this region to thereby more uniformly cool the target or outer tube, and thus enhance sputter coating uniformity.  
         [0009]     In addition, it has been found that an inner magnet bar support tube of rectangular cross-sectional shape reduces undesirable bending along the length of the support tube.  
         [0010]     Both the baffle and spiral vane segments as described above may also be used with the rectangularly shaped magnet bar support tube with appropriate modifications.  
         [0011]     Accordingly, in one aspect, the invention relates to a sputtering target comprising an outer target tube, an inner support tube of rectangular cross-sectional shape supporting a magnet carrier extending along substantially the entire length of the inner support tube; and a water cooling circuit including at least one passageway within the inner support tube with an inlet at one end thereof adapted to receive cooling water from an external source, at least one outlet aperture at an opposite end thereof opening to a chamber radially between the inner support tube and the outer target tube; and at least one cooling water outlet at the one end of the inner support tube.  
         [0012]     In another aspect, the invention relates to a magnet bar support tube for use in a sputtering apparatus, the magnet bar support tube having a rectangular cross-sectional shape and supporting a magnet carrier extending substantially the entire length of the inner support tube.  
         [0013]     In still another aspect, the invention relates to a sputtering target comprising an outer target tube, an inner support tube of rectangular cross-sectional shape supporting a magnet carrier extending along substantially the entire length of the inner support tube; a water cooling circuit including at least one passageway within the inner support tube with an inlet at one end thereof adapted to receive cooling water from an external source, at least one outlet aperture at an opposite end thereof opening to a chamber radially between the inner support tube and the outer target tube; a baffle comprising a substantially flat plate attached to the inner support tube adjacent the opposite end, the plate extending radially within the chamber between the inner support tube and the outer target tube and having an array of flow apertures therein; and a plurality of spiral vane segments attached to an outer surface of the inner support tube, downstream of the baffle in a direction of flow of water through the baffle.  
         [0014]     The invention will now be described in detail, in connection with the drawings identified below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a simplified and partially schematic side elevation of a conventional sputtering apparatus;  
         [0016]      FIG. 2  is an end view of an inner magnet support bar and cooling flow baffle in accordance with an exemplary embodiment of the invention;  
         [0017]      FIG. 3  is a partial side elevation of the inner magnet support bar shown in  FIG. 2 ;  
         [0018]      FIG. 4  is a section taken along the lines  4 - 4  in  FIG. 3 ;  
         [0019]      FIG. 5  is an end view of an inner magnet support bar in accordance with another exemplary embodiment of the invention;  
         [0020]      FIG. 6  is a plan view of the inner magnet support bar shown in  FIG. 5 ;  
         [0021]      FIG. 7  is an end view of the inner magnet support bar shown in  FIG. 5  but with a buffer plate added; and  
         [0022]      FIG. 8  is an end view of the inner magnet support bar shown in  FIG. 5  but with spiral vane segments added. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIG. 1  illustrates in simplified form a conventional magnetron sputtering apparatus  10 . The apparatus includes metal walls  12  of the vacuum chamber in which sputtering is performed; a cylindrical rotating target (or outer target tube)  14  that is supported at opposite ends by end blocks, i.e., a bearing block  16  and a drive block  18  so that the target is rotatable about axis  20 ; and an inner support tube  22  that supports the magnet carrier represented simplistically by the single block  24  in  FIGS. 1-4 ) that extends along the underside of the inner support tube  22 , substantially the entire length of both the inner support tube and the outer target tube. Gas is supplied to the vacuum tube via an external gas supply  26  while power is supplied via external power supply  28 . The vacuum tube is evacuated by a vacuum pump  30 .  
         [0024]     In a typical sputtering process, the plasma formed when an electrical potential is applied to the cathode target bombards the target and the dislodged particles fall on the substrate  32 , forming a coating thereon. It is important that throughout the process, the target tube be cooled to a specified temperature. Accordingly, cooling water from a source  34  is introduced into the interior of the hollow inner support tube  22  at one end thereof through the bearing block  16 , and exits the opposite end of the tube through a plurality of nozzle or jet apertures  36  provided in an end plate  38  (see  FIG. 2 ). The apertures  36  may be arranged to emit streams parallel to the longitudinal axis of the target tube and/or at an acute angle thereto. The cooling water then reverses direction and flows back along the exterior of the inner tube  22 , in the chamber or space  40  radially between the inner support tube  22  and the outer target tube  14 , exiting the target tube via the same bearing block  16 . Note that while the support tube  22  is terminated short of the drive block  18  to permit the cooling flow to exit the tube and reverse flow through the chamber  40 , an inner spindle  42  that may be fixed to the end plate  38 , supports the inner tube in the drive block  18 . The particular manner in which the inner support tube  22  is mounted vis-à-vis the rotatable target tube  14  is not of particular significance to the invention described herein.  
         [0025]     This invention relates to devices employed to enhance the cooling of the target tube  14 . With reference to  FIG. 2 , one example of such a device is in the form of a baffle  44  attached by tube clamp-type bracket components  46 ,  48  and suitable fasteners (or equivalents) to the exterior surface of the inner support tube  22 . The baffle  44  is located at the end of the inner support tube  22  where the cooling water exits the inner support tube and is thus adjacent and downstream of the flow-reversal zone.  
         [0026]     Baffle  44  in the example disclosed is composed of a flat, generally circular metal plate  50  that is designed to generally surround the inner tube  22 , but with a generally rectangular cut-out  52  that accommodates the magnet bar array  24 . Note also that the baffle plate is solid about an inner peripheral band portion  54 , thus blocking flow at the radially innermost portion of the cooling flow path between the inner and outer tubes. The remaining area of the baffle plate is provided with a dense array of flow apertures  56 . The array of apertures extends approximately 270° about the inner tube, from one side of the cut out  52  to the opposite side of the cut-out. Thus, after the streams of water exit the nozzle or jet apertures  36  in the end plate  38 , the cooling water reverses direction and flows immediately through the array of apertures  56  in the baffle  44  and into the chamber or space  40 . The presence of band portion  54  forces the cooling water to flow radially away from the inner support tube  22 , ensuring more cooling flow closer to the target tube  14 . The baffle  44  also increases the flow velocity above and below the magnet carrier  24  and creates a significant degree of turbulence adjacent the drive block  18 , thereby establishing good mixture of the radially inner and outer flows and eliminating dead water zones at or adjacent the flow-reversal zone. At the same time, rotation of the target tube  14  reduces bubble adhesion on the inner support tube wall.  
         [0027]     A second structural member promotes good circulation along the entire length of the inner support tube  22 . With reference especially to  FIG. 3 , a plurality of spiral vane segments  50 ,  52 , etc. are attached by welding or other suitable means to the exterior surface of the inner tube  22 . The segments are aligned or oriented so as to establish a substantially continuous spiral flow path extending from a location adjacent the baffle  44  to the opposite end of the inner tube  22 . The spaces between the segments  50 ,  52 , etc., are aligned along the underside of the inner support tube  22  to provide the space necessary to accommodate the magnet carrier  24 . The spiral vane segments  50 ,  52 , etc., extend radially substantially the same distance as the baffle, with only a generous tolerance between the baffle  44  and vane segments  50 ,  52 , etc., and the ID of the target tube  14  to enhance circulation but also to preclude any abrasion of the target tube inner wall. Thus, after passing through the apertures  56 , the cooling water is forced to flow along the spiral path established by the vane segments  52 .  
         [0028]     The spiral vane segments  52  promote good circulation of the cooling water along the entire length of the target tube, and insure continuous flow of cooling water into and out of the region of the plenum below the magnet bar(s). This arrangement also reduces bubble adhesion all along the length of the inner support tube and magnet bar(s).  
         [0029]     Another aspect of the present invention is the re-shaping of the inner support tube and the “decoupling” of the magnet carrier from the inner support tube. Specifically, differently-shaped support bars result in varying mechanical stability and, in this instance, bending along the longitudinal axis is of principal concern. Comparisons of various inner support tube shapes are set out in Table I below in terms of maximum bending and weight for inner support bars of similar wall thickness and overall length.  
                                                       structural shape   bending   weight                           circular tube   14.8 mm   58 kg           square tube   10.8 mm   58 kg           Square tube with angle plates    6.7 mm   74 kg           rectangular tube    3.8 mm   63 kg                      
 
         [0030]     A noticeable improvement in bending resistance is apparent for rectangular tubes, where bending along the entire length of the inner support tube is reduced to 3.8 mm.  
         [0031]      FIGS. 5 and 6  illustrate an inner support tube  54  of rectangular cross-sectional shape, with the height dimension greater than the width dimension (the exact dimensions may vary to suit particular applications).  
         [0032]     The inner support tube  54  in this example is supported within end blocks by spindles (one shown at  55 ), and rigidified by a pair of angle braces  56 ,  58  welded to the opposite sides  60 ,  62 , respectively, of the tube  54 . The braces  56 ,  58  extend along substantially the entire length of the tube  54 . The magnet bar assembly  64 , including carrier  66 , attachment flanges  68 ,  70  and magnet array  72 , is suspended from the underside of the tube  54  by bolts or other suitable fasteners  74  that pass through the angle brace flange portions  76 ,  78  and the attachment flanges  68 ,  70  at axially-spaced locations along the length of the carrier  66 . By “decoupling” of the magnet carrier  66  from the inner support tube  54 , any bending of the inner support bar  54 , due to gravity, is minimized if not eliminated in the magnet carrier  66 . Similarly, any tension caused by tuning of the magnets (typically done by inserting metal pieces beneath the magnets) is not transferred to the support bar as a result of this “decoupling”.  
         [0033]     A pair of rollers  80 ,  82  may be located near the center of the tube  54 , each supported by a pair of angle plates  84 ,  86 . These rollers are designed to prevent the magnet bar array from contacting the interior surface of the round target tube in the event of any bending, the maximum degree of which would occur at this location. In alternative arrangements, particularly in the case of extended length targets, additional roller pairs may be utilized at desired intervals.  
         [0034]     It will be appreciated that the rectangularly-shaped support tube could also be rigidified in other ways, for example, by increasing the thickness of the tube or by other added reinforcements.  
         [0035]      FIG. 7  illustrates the rectangular support tube  54  with a baffle plate  88  fixed thereto and assembled within a target tube  90  that is similar in all respects to the baffle plate  44  described hereinabove, with the exception that the plate  88  is configured to surround a rectangular rather than round tube. The location of the baffle pattern of apertures  92 , and its functional aspects remain substantially as described above.  
         [0036]      FIG. 8  illustrates the rectangular support tube  54  with a plurality of spiral vane segments  94  secured thereto, and assembled within a target tube  96 . Here again, the configuration and function of the vane segments remain as stated hereinabove. The principal difference lies in the rectangular cut-out shape where the vane segments are joined to the tube  54  and angle braces  56 ,  58 . As in the earlier described embodiment, the baffle plate  88  and spiral vane segments  96  may be utilized together on the tube  54 , in substantially the same spatial arrangement as shown in  FIG. 3 .  
         [0037]     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.