Abstract:
A method and apparatus for sputter deposition. The method including: providing a sputter target having a back surface and an exposed front surface; providing a source of magnetic field lines, the magnetic field lines extending through the sputter target from the back surface to the exposed front surface of the sputter target; providing one or more pole extenders between magnetic poles of the source of the magnetic field lines and the exposed front surface of the sputter target.

Description:
This Application is a continuation of U.S. patent application Ser. No. 10/711,818 filed on Oct. 7, 2004, now U.S. Pat. No. 7,485,210, issued Feb. 3, 2009. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of sputter deposition; more specifically, it relates to a cathode assembly including a sputtering target and a method for improved efficiency of use of the target. 
     BACKGROUND OF THE INVENTION 
     Sputter deposition or physical vapor deposition (PVD) systems are well known and typically include a magnetron cathode assembly and a target. Typically the cathode assembly is placed in a vacuum chamber into which a gas plasma is generated between the target and a substrate, in one example a semiconductor wafer. The magnetic field generated by the magnets in the cathode enhances the ability of ionized atoms to strike the target thus sputtering off target material which is deposited as a layer on the substrate. This process uses up the target, forming sputtering grooves in the target. When the grooves become too deep or deposited film quality starts to degrade, the target must be replaced even though a very large percentage of the target material still remains on the target. Considering the exotic and expensive target materials used in the semiconductor and other industries, target replacement is a very costly. Therefore, there is a need for a efficient magnetron sputtering cathode. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a method of sputter deposition, comprising: providing a sputter target having a back surface and an exposed front surface; providing a source of magnetic field lines, the magnetic field lines extending through the sputter target from the back surface to the exposed front surface of the sputter target; providing one or more pole extenders between magnetic poles of the source of the magnetic field lines and the exposed front surface of the sputter target. 
     A second aspect of the present invention is an apparatus, comprising: a sputter target having a back surface and an exposed front surface; a source of magnetic field lines, the magnetic field lines extending through the sputter target from the back surface to the exposed front surface of the sputter target; one or more pole extenders between magnetic poles of the source of the magnetic field lines and the exposed front surface of the sputter target. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1A  is a conceptual sectional view and  FIG. 1B  is a top conceptual view illustrating target erosion in a magnetron cathode assembly; 
         FIGS. 2 through 10  are conceptual diagrams illustrating the method of increasing target use efficiency in a magnetron cathode assembly according to the present invention 
         FIG. 11A  is a plan view and  FIGS. 11B through 11E  are cross-sectional views through line  11 B/C/D/E- 11 B/C/D/E of  FIG. 11A  illustrating various locations for placing pole extenders in a target of a magnetron cathode assembly according to the present invention; 
         FIG. 11F  is a cross-sectional view of a cathode only cathode assembly according to the present invention; 
         FIGS. 12A through 12I  are cross-sectional views illustrating various cross-sectional geometries for pole extenders according to the present invention; 
         FIGS. 13A through 13C  are partial side sectional views of magnetron cathode assemblies illustrating the general placement of various magnet shapes in relationship to the pole extenders of the present invention; 
         FIG. 14A  is a top plan view and  FIG. 14B  is cross sectional side view through line  14 B- 14 B of  FIG. 14A  illustrating a first type of magnet assembly suitable for use in the present invention; 
         FIG. 15A  is a top plan view and  FIG. 15B  is cross sectional side view through line  15 B- 15 B of  FIG. 15A  illustrating a second type of magnet assembly suitable for use in the present invention; 
         FIG. 16A  is a top plan view and  FIG. 16B  is cross sectional side view through line  16 B- 16 B of  FIG. 16A  illustrating a third type of magnet assembly suitable for use in the present invention; and 
         FIG. 17  is a cross-sectional view through an exemplary magnetron cathode assembly according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a conceptual sectional view and  FIG. 1B  is a top conceptual view illustrating target erosion in a magnetron cathode assembly. In  FIG. 1A , magnetron cathode assembly  100  includes a rotatable magnet assembly  105  and a cathode assembly  110 . Magnet assembly  105  includes magnetic north pole regions  115 A and magnetic south pole regions  115 B and is rotatable about an axis  120 . Generally, axis  120  is off-center from the geometric axis of magnetic assembly  105  as illustrated in  FIG. 17  and described infra. It should be understood that north pole and south pole regions  115 A and  115 B are not the actual magnets, but are intended to show in cross-section, the magnetic fields swept out by the magnets as magnet assembly  105  rotates. As illustrated in  FIG. 1B , north and south pole regions  115 A and  115 B are thus rings within which magnetic flux at any given point along the ring varies as the magnets rotate. Cathode assembly  110  includes a backing plate  125  and a target  130 . North and south pole regions  115 A and  115 B induce magnetic field lines  135  that extend through backing plate  125  and target  130 . A circular groove  140  is formed in target  130  where magnetic field lines  135  are approximately parallel to surface  145  of target  130  as the magnetic field enhances sputtering (removal of target material by ion bombardment) in these regions of the target. Groove  140  is smoothed as the magnetic poles are rotated. Groove  140  is formed between north magnetic poles  115 A and south magnetic poles  115 B and is roughly centered between the poles. There are smaller (both in depth and width) higher order grooves formed inside of groove  140  but they have not been illustrated. 
     The net magnetic field at target surface  145  is a function of (1) the thickness of backing plate  125  and target  130 , (2) a distance G between magnet assembly  105  and cathode assembly  110 , (3) the magnetic properties of the target and (4) the electric conductivity of backing plate  125  as far as it effects eddy currents shunted within backing plate  125 . These factors result in a reduction in the net magnetic field at target surface  145  as well as limiting the length of the portion of each of magnetic field lines  135  that is parallel to surface  145  of target  130 . Backing plate  125  is electrically conductive and non-magnetic. Target  130  is electrically conductive. Decreasing the distance G between magnet assembly  105  and cathode assembly  110  increases the sputtering rate of target  130 . Increasing the distance G between magnet assembly  105  and cathode  110  decreases the sputtering rate of target  130 . Certain target materials, for example, cobalt, will shunt the magnetic fields and therefore are kept thin. 
       FIGS. 2 through 10  are conceptual diagrams illustrating the method of increasing target use efficiency in a magnetron cathode assembly using pole extenders according to the present invention. The horizontal direction is defined as the direction parallel to bottom surface  145  of target  140  before any grooves  140  have been formed (see  FIG. 1A ). The vertical direction is defined as the direction perpendicular to bottom surface  145  of target  140 . Axis  120  (see  FIG. 1A ) extends in the vertical direction. 
     In  FIG. 2 , magnetron cathode assembly  100 A is similar to magnetron assembly  100  of  FIG. 1A , except pole extenders  150 A and  150 B have been embedded in backing plate  125 . Pole extenders  150 A and  150 B are rings as illustrated in  FIGS. 11A and 11B  (as pole extenders  205  and  210  respectively) and described infra. On one side of axis  120 , an outer edge  155 A of north pole extender  150 A is aligned to an outer edge  160 A of north pole region  115 A. An inner edge  155 B of north pole extender  150 A is aligned to an outer edge  160 B of north pole region  115 A. An outer edge  165 A of south pole extender  150 B is aligned to an outer edge  170 A of south pole region  115 B. An inner edge  165 B of south pole extender  150 B is aligned to an inner edge  170 B of south pole region  115 B. Pole extenders are formed of a magnetic material. This alignment is mirrored on the opposite side of axis  120 . 
     The presence of pole extenders  150 A and  150 B result in an increase in the net magnetic field at target surface  145  as well as an increase in the length of the portion of each of magnetic field lines  135 A that is parallel to surface  145  of target  130 . Increasing the length of the portion of each of magnetic field lines  135 A that is parallel to surface  145  of target  130  causes groove  140 A of  FIG. 2  to be wider than groove  140  of  FIG. 1 , thus increasing efficiency of target material use. Additionally, the higher order grooves discussed supra, are widened and deepened thus increasing target material utilization efficiency still more. 
     Pole extenders  150 A and  150 B have the following impacts: (1) they reduce the effective thickness of backing plate  125  by channeling magnetic flux through the backing pole extender material rather than backing plate material, (2) magnetic poles  115  will appear closer to target  130  than is physically possible (which allows optimal magnet placement for other considerations such as cooling of magnetron cathode assembly  100 A), (3) the effect of magnetic target materials on net magnetic field is reduced because of number (1) and (4) the eddy current effect is substantially reduced as the magnetic field is shunted through backing plate  125  rather than within the backing plate. 
     It should be noted that in  FIG. 2 , pole extender  150 A is about the same width W 1  as the magnets (not shown) generating north pole region  115 A and co-aligned to those magnets. Also, pole extender  150 B is about the same width W 2  as the magnets (not shown) generating south pole region  115 B and co-aligned to those magnets.  FIG. 2  (as do  FIGS. 3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9  and  10 ) assumes vertical bar or horseshoe magnets (see  FIGS. 13A and 13B ). With vertical or horseshoe magnets, the north and south pole regions illustrated have a width (or diameter in the case of cylindrical magnets) about equal to the width (or diameter) of the magnet itself. The W 1  and W 2  dimension of pole extenders  150 A and  150 B will be different for the W 1  and W 2  dimensions of horizontal bar magnets (see  FIG. 13C ). 
     In  FIG. 3 , magnetron cathode assembly  100 B is similar to magnetron assembly  100 A of  FIG. 2 , except pole extender  150 A of  FIG. 2  is replaced with pole extender  150 C. Pole extender  150 C is a disk rather than a ring. Pole extender  150 C has a width W 3  about equal to the outer diameter of an inner magnet assembly (not shown) generating south pole regions  1115 B. On one side of axis  120 , an outer edge  175  of pole extender  150 C is aligned to outer edge  170 A of south pole region  115 B. This alignment is mirrored on the opposite side of axis  120 . 
     In  FIG. 4 , magnetron cathode assembly  100 C is similar to magnetron assembly  100 A of  FIG. 2 , except pole extender  150 B of  FIG. 2  is not present in magnetron assembly  100 C of  FIG. 4 . 
     In  FIG. 5 , magnetron cathode assembly  100 D is similar to magnetron assembly  100 A of  FIG. 2 , except on one side of axis  120 , an outer edge  180 A of a pole extender  150 D extends past outer edge  160 A of north pole region  115 A and an inner edge  180 B of pole extender  150 D is aligned between outer edge  160 A and an inner edge  160 B of north pole region  115 A. An outer edge  185 A of a pole extender  150 E is aligned between outer edge  170 A and an inner edge  170 B of south pole region  115 B and an inner edge  185 B of pole extender  150 E extends past inner edge  170 B of south pole region  115 B. This has the effect of extending the portion of each magnetic field line  135 D that is parallel to surface  145  further, and thus producing a wide groove  140 D. This alignment is mirrored on the opposite side of axis  120 . 
     In  FIG. 6 , magnetron cathode assembly  100 E is similar to magnetron assembly  100 D of  FIG. 5 , except pole extender  150 E of  FIG. 5  is replaced with pole extender  150 F. Pole extender  150 F is a disk rather than a ring. On one side of axis  120 , an outer edge  195  of a pole extender  150 F is aligned between outer edge  170 A and inner edge  170 B of south pole region  115 B. This alignment is mirrored on the opposite side of axis  120 . 
     In  FIG. 7 , magnetron cathode assembly  100 F is similar to magnetron assembly  100 D of  FIG. 5 , except pole extender  150 G replaces pole extender  150 D of  FIG. 5 . On one side of axis  120 , an outer edge  195 A of pole extender  150 G extends past outer edge  160 A of north pole region  115 A and an inner edge  195 B is aligned to inner edge  160 B of north pole region  115 A. This alignment is mirrored on the opposite side of axis  120 . 
     In  FIG. 8 , magnetron cathode assembly  100 G is similar to magnetron assembly  100 F of  FIG. 7 , except pole extender  150 F (the same as in  FIG. 6 ) replaces pole extender  150 E of  FIG. 7 . On one side of axis  120 , outer edge  190  of a pole extender  150 F is aligned between outer edge  170 A and inner edge  170 B of south pole region  115 B. This alignment is mirrored on the opposite side of axis  120 . 
     In  FIG. 9 , magnetron cathode assembly  100 H is similar to magnetron assembly  100 D of  FIG. 5 , except pole extender  150 E of  FIG. 5  is not present in magnetron assembly  100 H of  FIG. 9 . 
     In  FIG. 10 , magnetron cathode assembly  100 I is similar to magnetron assembly  100 F of  FIG. 7 , except pole extender  150 E of  FIG. 7  is not present in magnetron assembly  100  I of  FIG. 10 . 
     While  FIGS. 2 through 10  have been concerned with options for horizontal locations of the pole extenders the present invention relative to north and south pole regions of the magnet assembly, there are options for the vertical location as well, which are illustrated in  FIGS. 11A through 11E  and described infra. 
       FIG. 11A  is a plan view and  FIGS. 11B through 11E  are cross-sectional views through line  11 B/C/D/E- 11 B/C/D/E of  FIG. 11A  illustrating various locations for placing pole extenders in a target of a magnetron cathode assembly according to the present invention. In  FIG. 11A , cathode assembly  200  includes an inner pole extender  205  and an outer pole extender  210 . Both inner and outer pole extenders  205  and  210  are illustrated as rings, but inner pole extender  205  may be a disk. In  FIG. 11B , cathode assembly  200  includes a backing plate  215  having a top surface  220  and a bottom surface  225  and a target  230  having a top surface  235  and a bottom surface  240 . Bottom surface  225  of backing plate  215  is in physical and electrical contact with top surface  235  of target  230 . Inner and outer pole extenders  205  and  210  are fitted respectively in annular slots  245  and  250  formed in and open to bottom surface  225  of backing plate  215 . Pole extenders  205  and  210  are coated with an optional galvanic corrosion or chemical corrosion protection coating  255  for cases where the material of either backing plate  215 , target  230  or both would galvanically or chemically react with the material of the pole extenders. Pole extenders  205  and  210  contact top surface  235  of target  230 . Inner and outer pole extenders  205  and  210  need not fit tightly in slots  245  and  250  respectively. 
     Suitable materials for pole extenders  205  and  210  is not limited to iron, iron alloys, cobalt and other electrically conductive magnetic materials, a magnetic material being defined as a material that may be strongly magnetized or that can channel magnetic fields. Both inner and outer pole extenders  205  and  210  need not be the same material. Suitable materials for backing plate  215  includes but is not limited to aluminum, copper, brass, stainless steel and other electrically conductive non-magnetic materials. A non-magnetic material is defined as a material that cannot not be strongly magnetized or is poor at channeling magnetic fields. Suitable target materials for target  230  include, but are not limited to copper, tantalum, aluminum, platinum, cobalt, gold, titanium, tungsten, other refractory metals, other electrically conductive materials and alloys thereof. The invention may be applied to non-metallic sputtering. In the case of non-metallic sputtering suitable target materials for target  230  include, but are not limited to carbon, silicon oxide, tantalum oxide, aluminum oxide, other metal oxides and other dielectric materials. Suitable materials for optional galvanic or chemical corrosion protection coating  255  include but are not limited to metal nitrides, plated metals and organic or ceramic coatings. 
     In  FIGS. 11C through 11E , optional galvanic corrosion protection coating  255  is not illustrated, but may be present.  FIG. 11C  is similar to  FIG. 11B  except in  FIG. 11C , an inner pole extender  205 A and an outer pole extender  210 A are embedded in backing plate  215  and extend a distance D 1  into a target  230 A. Target  230 A has a thickness D 2  and, in one example, the ratio D 1 /D 2  does not exceed about 0.8.  FIG. 11D  is similar to  FIG. 11C  except in  FIG. 11D , an inner pole extender  205 B and an outer pole extender  210 B are embedded just in target  230 A.  FIG. 11D  is similar to  FIG. 11D  except in  FIG. 11E , inner pole extender  205 C and an inner pole extender  205 C is not embedded as deeply into a target  200 C as outer pole extender  210 B is embedded. 
       FIG. 11F  is a cross-sectional view of a cathode only cathode assembly according to the present invention. A cathode only cathode assembly is defined as a cathode assembly that does not have a backing plate. In  FIG. 11F , there is no backing plate and an inner pole extender  205 D and an outer pole extender  210 D are embedded in target  230 D. Target  230 D is mounted directly into a magnetron cathode assembly (see  FIG. 17 ). All embodiments of the present invention (except those where pole extenders are illustrated and described as extending into a backing plate) may be applied to cathode only magnetron assemblies. 
     Though not illustrated, the inner pole extender may be embedded deeper into the target than outer the outer pole extender and the inner and outer pole extenders of unequal extension into the target may also extend into the backing plate. Inner and outer pole extenders need not extend the same distance into the backing plate. 
     Both horizontal and vertical locations of pole extenders in cathode assemblies have been discussed. Attention will know be turned to the cross-sectional geometry of pole extenders. All pole extenders described or illustrated su pra have had rectangular or square cross-sectional geometries.  FIGS. 12A through 12I  are cross-sectional views illustrating various other cross-sectional geometries and combinations for pole extenders according to the present invention. The pole extender shapes illustrated in  FIGS. 12A through 12I  allow manipulation of the geometry of the magnetic field lines of a magnetron cathode assembly (see  FIG. 17  for an example of a magnetron cathode assembly). Optional galvanic corrosion or chemical corrosion protection coatings are not illustrated in  FIGS. 12A through 12I , but may be present. Also only the case of pole extenders contained completely within the target are illustrated, but it should be understood the pole extender cross-sectional geometries described infra are applicable that any of the vertical location options illustrated in  FIGS. 11B through 11E  and/or discussed supra. Unless otherwise note, the inner and outer pole extenders illustrated in  FIGS. 12A through 12I  are rings. 
     In  FIG. 12A , a cathode assembly  300 A includes a backing plate  305 A and a target  310 A. Target  310 A includes an inner pole extender  315 A and an outer pole extender  320 A. Inner and outer pole extenders  315 A and  320 A have a cross-sectional geometry of a rectangle with two adjacent corners chamfered to meet in a point. The points of inner and outer pole extenders  315 A and  320 A facing an exposed surface  325 A of target  310 A. 
     In  FIG. 12B , a cathode assembly  300 B includes a backing plate  305 B and a target  310 B. Target  310 B includes an inner pole extender  315 B and an outer pole extender  320 B. Inner and outer pole extenders  315 B and  320 B have a cross-sectional geometry of a rectangle with two adjacent corners filleted to form a semicircle or curve. The semicircular surface of inner and outer pole extenders  315 B and  320 B facing an exposed surface  325 B of target  310 B. 
     In  FIG. 12C , a cathode assembly  300 C includes a backing plate  305 C and a target  310 C. Target  310 C includes an inner pole extender  315 C and an outer pole extender  320 C. Inner and outer pole extenders  315 C and  320 C have a cross-sectional geometry of a rectangle with two adjacent corners chamfered but still retaining a flat surface between the chamfered corners. The flat surface of inner and outer pole extenders  315 C and  320 C facing an exposed surface  325 C of target  310 C. 
     In  FIG. 12D , a cathode assembly  300 D includes a backing plate  305 D and a target  310 D. Target  310 D includes an inner pole extender  315 D and an outer pole extender  320 D. Inner pole extender  315 D has a cross-sectional geometry of a rectangle. Outer pole extender  320 D has a cross-sectional geometry of a rectangle with two adjacent corners chamfered to meet in a point. The points of inner and outer pole extenders  315 D and  320 D facing an exposed surface  325 D of target  310 D. 
     In  FIG. 12E , a cathode assembly  300 E includes a backing plate  305 E and a target  310 E. Target  310 E includes an inner pole extender  315 E and an outer pole extender  320 E. Inner pole extender  315 E has a cross-sectional geometry of a rectangle. Outer pole extender  320 E has a cross-sectional geometry of a rectangle with two adjacent corners filleted to form a semicircle or curve. The curved or semi-circular surfaces of inner and outer pole extenders  315 E and  320 E facing an exposed surface  325 E of target  310 E. 
     In  FIG. 12F , a cathode assembly  300 F includes a backing plate  305 F and a target  310 F. Target  310 F includes an inner pole extender  315 F and an outer pole extender  320 F. Inner pole extender  315 F has a cross-sectional geometry of a rectangle. Outer pole extender  320 F has a cross-sectional geometry of a rectangle with two adjacent corners chamfered but still retaining a flat surface between the chamfered corners. The flat surfaces between chamfered corners of inner and outer pole extenders  315 F and  320 F facing an exposed surface  325 F of target  310 F. 
     In  FIG. 12G , a cathode assembly  300 G includes a backing plate  305 G and a target  310 G. Target  310 G includes an inner pole extender  315 G and an outer pole extender  320 G. Inner pole extender  315 G has a cross-sectional geometry of a rectangle. Outer pole extender  320 G has a cross-sectional geometry of a rectangle having two opposite sides of unequal length joined by a sloped surface, the sloped surface facing an exposed surface  325 G of target  310 G, and the sloped surface of outer pole extender  320 G facing away from inner pole extender  315 G. 
     In  FIG. 12H , a cathode assembly  300 H includes a backing plate  305 H and a target  310 H. Target  310 H includes an inner pole extender  315 H and an outer pole extender  320 H. Inner pole extender  315 H has a cross-sectional geometry of a rectangle. Inner and outer pole extenders  315 H and  320 H have a cross-sectional geometry of a rectangle having two opposite sides of unequal length joined by a sloped surface, the sloped surface facing an exposed surface  325 H of target  310 H, and the longest side of inner pole extender  315 H facing the longest side of outer pole extender  320 H. 
     In  FIG. 12I , a cathode assembly  300 I includes a backing plate  305 I and a target  310 I. Target  310 I includes a disk inner pole extender  315 I in the form of a and an outer pole extender  320 I. Inner pole extender  315 I has a cross-sectional geometry of a rectangle. Outer pole extender  320 I has a cross-sectional geometry of a rectangle having two opposite sides of unequal length joined by a sloped surface, the sloped surface facing an exposed surface  325 I of target  310 I, and the sloped surface of outer pole extender  320 I facing away from inner pole extender  315 I. 
       FIGS. 12 through 12I  should be exemplary, as many other pole extender cross-sectional geometries and combinations of different inner and outer pole extender cross-sectional geometries are possible. 
       FIGS. 13A through 13C  are partial side sectional views of magnetron cathode assemblies illustrating the general placement of various magnet shapes in relationship to the pole extenders of the present invention. In  FIG. 13A , a magnetron cathode assembly  400  includes vertical bar magnets  405 , a backing plate  410  and a target  415  including pole extenders  420 A and  420 B. Each vertical bar magnet  405  is shown aligned to either pole extender  420 A or  420 B. Real time positioning varies for inner pole extenders and depends on where in their circle of rotation the magnets are relative to backing plate  410  and target  415 . Magnetic field lines  425  form a closed loop through magnets  405  and pole extenders  420 A and  420 B. In the case of vertical bar magnets, the pole extenders are aligned to the pole ends of the vertical bar magnets, at least a portion of each pole extender positioned under a corresponding pole of a vertical bar magnet (in at least a portion of the rotation cycle of the magnet). 
     In  FIG. 13B , a magnetron cathode assembly  430  includes a vertical horseshoe magnet  435 , backing plate  410  and target  415  including pole extenders  420 A and  420 B. Each pole of vertical horseshoe magnet  435  is shown aligned to a pole extender  420 A or  420 B. Real time positioning varies for inner pole extenders and depends on where in their circle of rotation the vertical horseshoe magnet is relative to backing plate  410  and target  415 . Magnetic field lines  440  form a closed loop through vertical horseshoe magnet  435  and pole extenders  420 A and  420 B. In the case of a vertical horseshoe magnets, the pole extenders are aligned to the pole ends of the magnet, at least a portion of each pole extender positioned under a corresponding pole of the magnet (in at least a portion of the rotation cycle of the magnet). 
     In  FIG. 13C  a magnetron cathode assembly  445  includes horizontal bar magnets  450 , backing plate  410  and a target  455  including pole extenders  460 A and  460 B. Pole extenders  460 A and  460 B are positioned in target  455  where magnetic field  465  is most perpendicular to exposed surface  470  of target  455 . Both pole extenders  460 A and  460 B must intercept at least a portion of magnetic field  450  during at least a portion of the rotation cycle of horizontal bar magnet  450 . Real time positioning varies for inner pole extenders and depends on where in their circle of rotation the magnet is relative to backing plate  410  and target  415 . Magnetic field lines  465  form a closed loop through horizontal bar magnet  450  and pole extenders  460 A and  460 B. 
       FIG. 14A  is a top plan view and  FIG. 14B  is cross sectional side view through line  14 B- 14 B of  FIG. 14A  illustrating a first type of magnet assembly suitable for use in the present invention. In  FIG. 14A  a rotatable magnet assembly  500  includes a ring of outer of vertical bar magnets  505  and a ring of inner vertical bar magnets  510 . Outer vertical bar magnets  505  are arranged equidistant from one another and the same distance from a geometric center  515  of magnet assembly  500 . Inner vertical bar magnets  505  are arranged equidistant from one another and the same distance from a geometric center  515  of magnet assembly  500 , but closer to geometric center  515  than outer vertical bar magnets  505 . Magnet assembly  500  is rotatable about a rotation axis  520  located between inner vertical bar magnets  510  and outer vertical bar magnets  505 . 
     Turning to  FIG. 14B , outer vertical bar magnets  505  are contained between an upper ring  525  and a lower ring  530 . Inner vertical bar magnets  510  are contained between and upper disk  535  and a lower disk  540 . Upper ring  525 , lower ring  530 , upper disk  535  and lower disk  540  are fabricated from electrically conductive magnetic materials. The subassembly formed of upper ring  525 , lower ring  530 , upper disk  535  and lower disk  540 , inner vertical bar magnets  510  and outer vertical bar magnets  505  is fastened to a support plate  545  fabricated from electrically conductive but non-magnetic material. The magnetic poles of all inner vertical bar magnets  510  are orientated in the a first direction and the magnetic poles of all outer vertical bar magnets are orientated in an opposite second direction. 
       FIG. 15A  is a top plan view and  FIG. 15B  is cross sectional side view through line  15 B- 15 B of  FIG. 15A  illustrating a second type of magnet assembly suitable for use in the present invention. In  FIG. 15A , a rotatable magnet assembly  600  includes an circular array of ring of horseshoe magnets  605  are arranged like the spokes of a wheel. The same first magnetic poles  610 A of each horseshoe magnet  605  are arranged in a circle a uniform distance from a geometric center  615  of magnet assembly  600 . The same second magnetic poles  610 B of each horseshoe magnet  605  arranged in a circle a uniform distance from a geometric center  615  of magnet assembly  600 , but closer to geometric center  615  than first magnetic poles  610 A. Magnet assembly  600  is rotatable about a rotation axis  620  located between the circle of first magnetic poles  610 A and the circle of second magnetic poles  610 B. 
     Turning to  FIG. 15B , horseshoe magnets  605  are attached to an outer ring  625  and an inner disk  630 . First magnetic poles  610 A of each horseshoe magnet  605  are in electrical contact with outer ring  625 . Second magnetic poles  610 B of each horseshoe magnet  605  are in electrical contact with inner disk  630 . Outer ring  625  and inner disk  630  are fabricated from electrically conductive magnetic materials. The subassembly formed of inner ring  630 , outer ring  625  and horseshoe magnets  605  are fastened to a support plate  635  fabricated from electrically conductive but non-magnetic material. 
       FIG. 16A  is a top plan view and  FIG. 16B  is cross sectional side view through line  16 B- 16 B of  FIG. 16A  illustrating a third type of magnet assembly suitable for use in the present invention. In  FIG. 16A  a rotatable magnet assembly  700  includes an circular array of horizontal bar magnets  705  arranged like the spokes of a wheel. The same first magnetic poles  710 A of each horizontal bar magnet  705  are arranged in a circle a uniform distance from a geometric center  715  of magnet assembly  700 . The same second magnetic poles  710 B of each horizontal bar magnet  705  arranged in a circle a uniform distance from a geometric center  715  of magnet assembly  700 , but closer to geometric center  715  than first magnetic poles  710 A. Magnet assembly  700  is rotatable about a rotation axis  720  located between the circle of first magnetic poles  710 A and the circle of second magnetic poles  710 B. First magnetic poles  710 A of each horizontal bar magnet  705  are in electrical contact with an outer ring  725  and second magnetic poles  710 B of each horizontal bar magnet  705  are in electrical contact with an inner ring  730 . Outer ring  725  and inner disk  730  are fabricated from electrically conductive magnetic materials. 
     Turning to  FIG. 16B , horizontal bar magnets  705  are attached to an outer ring  725  and an inner disk  730 . The subassembly formed of inner ring  730 , outer ring  725  and horizontal bar magnets  705  is fastened to a support plate  735  fabricated from electrically conductive but non-magnetic material. 
       FIG. 17  is a cross-sectional view through an exemplary magnetron cathode assembly according to the present invention. In  FIG. 17 , a magnetron cathode assembly  800  includes magnet assembly  500  (the same as illustrated in  FIGS. 14A and 14B  and described supra) and cathode assembly  200 B (the same as illustrated in  FIG. 11D  and described supra) fitted into a housing  805  having a moveable plate  810  free to move up and down relative to housing  805 . Cathode assembly  200 B is fixed to housing  805  by seals  815 . Magnet assembly  500  is rotatably fixed to moveable plate  8105  by a bearing/seal  820  and gap distance G between lower ring  530  and lower disk  540  and backing plate  215 B is adjustable by moving moveable plate  810  up and down in housing  805 . A void  830  is defined by moveable plate  810 , top surface  220 B of housing  215 B and backing plate  215 B. Void  830  may be filled with a coolant such as water or an inert gas. 
     Center of rotation  520  of magnet assembly  500  is co-incident with an axis perpendicular to top surface  220 B of backing plate  215 B and running through the geometric center of cathode assembly  200 B. Outer magnets  505  are aligned to inner and outer pole extenders  205 B and  210 B. Alignment of inner magnets  510  to inner pole extender  205 B is variable. 
     While magnet assembly  500  and cathode assembly  210 B (using rectangular cross section pole extenders contained totally with the backing plate) have been illustrated in  FIG. 17 , any combination of magnet assemblies, cathode assemblies and cross sectional pole extender geometries and vertical locations of pole extenders described supra may be substituted. Not all pole extenders of a given apparatus need be comprised of the same material, be the same length (in the vertical direction) or be embedded in the backing plate and/or sputter target to the same distances. 
     Thus the present invention provides an efficient magnetron sputtering cathode. 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. For example, while the present invention was described using permanent magnets, electro-magnets may be substituted. Further, additional non-rotating magnets may be provided. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.