Abstract:
A cylindrical magnetron having a cylindrical cathode surrounding a cylindrical target. The cathode is without features extending inwards thereof at either end such that the target may be axially removed from or installed into the cathode from either end. The target is positioned and axially retained within the cathode by resilient means operative between the outer surface of the target and the inner surface of the cathode. The invention is especially useful in magnetron assemblies having a plurality of abutting, coaxially disposed magnetrons, wherein all the targets may be removed and replaced from the cathode assembly without requiring disassembly and reassembly of the cathodes.

Description:
RELATIONSHIP TO OTHER APPLICATIONS 
     The present application draws priority from the filing date of U.S. Provisional Application Serial No. 60/235,276, filed Sep. 25, 2000 by David A. Glocker and Mark Romach. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to apparatus for sputter coating of substrates; more particularly, to such apparatus wherein the sputtering target is cylindrical; and most particularly to such apparatus wherein cylindrical targets may be inserted into, operationally retained in, and removed from the surrounding cylindrical cathodes without disassembly of the cathodes. 
     BACKGROUND OF THE INVENTION 
     Cylindrical magnetron sputtering is a useful method for coating three-dimensional complex shapes, such as those used for cutting and forming tools, biomedical devices, optical fibers and so on. Cylindrical magnetron sputtering devices have been described in a series of patents, including U.S. Pat. Nos. 3,884,793; 3,995,187; 4,030,986; 4,031,424; 4,041,353; 4,111,782; 4,116,793; 4,116,794; 4,132,612; 4,132,613; 5,178,743; 5,228,963; 5,437,778; and 5,529,674; all of which are incorporated herein by reference. 
     Prior art cylindrical magnetrons use a variety of means to create traps for the secondary electrons produced by ion bombardment that are responsible for maintaining the sputtering plasma. Some plasma traps are formed by axial magnetic fields working together with electrostatic wings on the cathode and/or target. Alternatively, the magnetic field and each individual electrode may define other kinds of traps. A variety of examples of such traps are described in the reference patents. 
     One important advantage of cylindrical magnetrons is that they can use targets that are simple cylinders that slide into a cylindrical cathode body. Therefore, no clamping or bolting is needed to assure that good thermal contact is made between the target and cathode body. As the target temperature rises, the target increases in diameter and clamps more tightly to the water-cooled cathode jacket that surrounds it. In this way a self-limiting temperature is reached as long as an adequate flow of cooling water is maintained. In prior art cylindrical magnetrons, the target is located with respect to the cathode body and held in place by clamps in the ends of the cathode or by flange(s). 
     It is sometimes advantageous to operate multiple individual cylindrical magnetron cathodes in a single chamber. For example, it may be desirable to have targets of different materials that can be sputtered independently using separate power supplies. Or, in some cases, it is useful to operate two electrically isolated targets with an alternating current power supply in the mid-frequency or radio frequency range. See, for example, the 43 rd  Annual Technical Conference Proceedings of the Society of Vacuum Coaters, “AC Reactive Sputtering with Inverted Cylindrical Magnetrons,” pp. 81-85. 
     In prior art cylindrical magnetron assemblies having two or more electrically independent cathodes and targets, flanges and clamps typically are used to locate and hold the targets axially within the cathodes. Prior art configurations require that when multiple targets are used that they be installed and removed from both ends of the cathode body. This is very inconvenient, particularly if one end of a cathode body is sealed directly to the vacuum system, as is sometimes the case. 
     It is a primary object of the invention to provide cylindrical targets that mate with corresponding cylindrical magnetron cathodes, wherein multiple targets may be easily installed from one end of a cathode assembly. 
     It is a further object of the invention to allow easy removal and installation of targets within a single cylindrical magnetron cathode without the need to remove any fasteners. 
     It is a still further object of the invention to provide mating cylindrical magnetron targets and cathodes that minimize the area of surfaces subject to the unwanted buildup of sputtered material during the coating process. 
     SUMMARY OF THE INVENTION 
     Briefly described, a cylindrical magnetron has a cylindrical cathode surrounding a cylindrical target. The cathode is unfeatured at each end, as by a target-supportive flange, such that the target may be removed from or installed into the cathode from either end. The target is positioned and axially retained within the cathode by resilient means operative between the outer surface of the target and the inner surface of the cathode. The invention is especially useful in magnetron assemblies having a plurality of abutting, coaxially disposed magnetrons, wherein all the targets may be removed and replaced from the cathode assembly without requiring disassembly and reassembly of the cathodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention, as well as presently preferred embodiments thereof, will become more apparent from a reading of the following description in connection with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of a prior art cylindrical magnetron wherein the cylindrical target is held in place within the cylindrical cathode by removable flanges that extend inboard of the target-surface at either end; 
     FIG. 2 is a cross-sectional view of a prior art cylindrical magnetron wherein the cylindrical target is held in place within the cylindrical cathode by a flange integral with the cathode, the target also having inwardly extending electrostatic flanges, or “wings,” for confining the plasma over the surface of the target; 
     FIG. 3 is a cross-sectional view of a prior art cylindrical magnetron wherein the cylindrical target is absent the confining “wings” shown in FIG. 2; 
     FIG. 4 is a cross-sectional view of a prior art cylindrical magnetron having a plurality of cathodes and targets axially assembled; 
     FIG. 5 is a cross-sectional view of a cylindrical magnetron in accordance with the invention having a plurality of cathodes and targets axially assembled; 
     FIG. 6 is an enlarged view taken within circle  6  in FIG. 5; and 
     FIGS. 7,  8 ,  9 , and  10  show other embodiments of means for positioning and retaining a cylindrical target within a cylindrical cathode in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cylindrical magnetron  10  as disclosed in the prior art. The cathode body  12  is a cylinder, typically having a water cooling channel or other cooling means (not shown). A sputtering target  14  is cylindrical in shape and has an outer diameter slightly less than the inner diameter of body  12  such that target  14  can be easily inserted into the cathode body  12  by simply sliding it into place. An axial magnetic field  16  is produced by conventional means (not shown) external to the cathode body, as is well known in the art. Wings  18  are flanges which serve two purposes: they locate and support the target within the cathode, and they form part of a plasma trap that permits sputtering. Wings  18  are at the target voltage and cooperate with axial magnetic field  16  to produce the plasma trap in what is well known in the art as hollow cathode confinement. The wings may be separate flanges  18  which are attached to body  12  as by bolts  20 . The objects  23  to be coated, for example, wires, fibers, cutting tools, optical elements, and the like, are placed within the interior  24  of the cathode body/target assembly  10 . By applying a voltage to the cathode body/target  10  in the presence of a sputtering gas at the proper gas pressure, a plasma is produced that bombards the inner surface  26  of target  14  and produces a sputtered coating of target material on objects  23 . Also not shown are the vacuum pumps, vacuum chamber, gas flow equipment and other means of producing a vacuum coating environment around the cathode, which will be easily inferred by those skilled in the art. The configuration of wings and the means for connecting them to the cathode body may take various forms, but they always extend inboard of the inner target surface in order to confine the plasma. Therefore, in this or related prior art configurations, the wings must be removed to replace or change the target material. This step requires time and the fabrication of the wings and the mounting method, which adds to the cost and complexity of the device. 
     Referring to FIG. 2, an alternative prior art configuration  21  has wings  18 ′ incorporated at the ends of the target  14 ′ that serve as the means for plasma confinement. In this way there is no need to have wings attached to the cathode body  12 ′ and the target  14 ′ can simply slide in and out of the cathode body without removing any parts. However, a small flange  28  on the cathode body is needed to support the target against gravity and/or positively locate the target in this configuration. Furthermore, the wings  18 ′ can increase the cost of fabricating the target significantly. They require complicated shaping, often of materials that are difficult to machine. 
     FIG. 3 shows another configuration  30  of a prior art cylindrical magnetron that eliminates the need for wings in a cylindrical magnetron sputtering device. As is well known in the art, cylindrical magnetrons do not require hollow cathode confinement with wings to operate. The cathode body  12 ′ has a flange  28  similar to flange  28  shown in FIG.  2 . However, the magnetic field lines  16 ′ form a closed plasma trap that is produced by magnets (not shown) external to the cathode body. This type of trap is well known in planar magnetron sputtering and is described in the prior art of cylindrical magnetrons as well. This field configuration requires no wings at the ends of the target for plasma confinement, although erosion of the target surface is axially non-uniform. Therefore, the target can be a simple cylinder, like target  14  in FIG. 1, that simply slides into place within the cathode and is thus easy and inexpensive to fabricate. However, some means of locating and supporting the target is still necessary, such as flange  28 . 
     In some cases, such as when co-sputtering two materials or when reactively sputtering with mid-frequency AC power, it is necessary to use an assembly  32  comprising two coaxial cylindrical magnetrons, for example, two such magnetrons  30 , as shown for the prior art in FIG.  4 . Furthermore, it may be desirable to make the two independent cathodes part of a single vacuum-tight assembly. Two cathode bodies  12 ′ are electrically isolated from each other by an insulating material  34  that is also vacuum tight. Preferably, the cathode bodies are sealed to the insulating material  34  with O-rings  36 . Furthermore, in some prior art designs the cathode assembly is mounted directly on a pumping system  38 , making the cathodes the vacuum vessel itself, rather than placing the cathodes inside another vacuum chamber. When the cathodes  12 ′ are assembled as shown in FIG. 4, target  14   a  can be easily removed but target  14   b  cannot be removed because of flange  28  on the upper cathode. In such cases, in order to change the targets the cathode bodies must be separated from one another or removed from the pumping system  38 . Often these cathodes are large, up to more than a foot in diameter, making them heavy and awkward to move. Furthermore, the process of disassembling and reassembling them is time-consuming and requires breaking vacuum seals, which creates the possibility of subsequent vacuum leaks. 
     One prior art way of dealing with this disadvantage is to make the cathodes of different diameters, with correspondingly different target diameters, so that the small target is installed by being passed through the larger cathode. However, this increases the complexity of the multiple cathode system and eliminates the advantage of using common parts for the different cathodes and targets. 
     It is very desirable to be able to remove all of the targets by simply passing them through cathodes that are of the same diameter and aligned with one another, without the need to disassemble the cathodes. In accordance with the invention, FIGS. 5 and 6 show a dual magnetron assembly  40  having first means  42  for allowing the targets  14   a ,  14   b  to be removed from both cathode bodies in a simple way without requiring the separation of the cathode bodies or removing them from the pumping system. A retaining element  44   a,b , which may be an internal design retaining ring or internal housing ring, and which can be easily removed, fits into an annular groove  46  formed in the inner wall  48  of each cathode body  12 ″, which cathode bodies are otherwise smooth on their inner surfaces and lack flanges  28  found on prior art cathodes. A mating step  50  in target  14   b  locates the target axially with respect to the cathode body and supports it against gravity. This design is repeated in upper cathode body  12 ″, upper target  14   a , and upper retaining element  44   a . To remove the lower target  14   b , the upper target  14   a  is first removed and the upper retaining element  44   a  in the upper cathode body  12 ″ is also removed. Lower target  14   b  can then be lifted out of the assembly without the need to disconnect the cathode bodies or break any vacuum seals. In this embodiment, the upper retaining element functions analogously to flange  28  in the prior art but is readily removable without disassembly of the cathode structure. 
     Instead of a snap ring or spiral retaining ring, retaining elements  44   a,b  may be elastomeric 0-rings that compress in grooves  46  sufficiently for the targets to slide past them and yet provide enough radial force to positively locate the targets and support them. Alternatively, elements  44   a,b  may be annular springs made from stainless steel or other suitable material. 
     Of course, it should be understood that groove  46  for retaining elements  44   a,b  may be formed just as well in the outer wall of the target rather than the inner wall of the cathode, to the same effect. 
     Furthermore, retaining elements  44   a,b  need not be circular in cross-section or in profile. For example, FIG. 7 illustrates another embodiment  44   c . In this top view of cathode body  12 ″ and target  14   a  or  14   b , a multi-sided wire hoop  44   c  (shown as a solid line for clarity) rests in a groove  46  in the cathode body and supports target  14   a  or  14   b . Hoop  44   c  may be more easily removed than a conventional snap ring or O-ring. 
     FIGS. 8 and 9 show yet another means  53  for positioning and retaining the target within the cathode body as taught by the invention. Cathode body  12 ″′ is provided with at least one and preferably a plurality of jig buttons  50  that extend a short distance inwardly of cathode inner surface  48 . Target  14 ′ (shown in elevation in FIG. 9) is provided with at least one groove  52 , and preferably the same number of grooves  52  as buttons  50 , formed in the outer surface  54  of target  14 ′ for matably engaging with buttons  50  when the target is inserted into the cathode body. Each of grooves  52  is formed with a circumferential slot  56  on its side, such that when the target is passed through the cathode body buttons  50  line up with slots  56 . The target may then be rotated in bayonet fashion to force buttons  50  into slots  56  to lock the target axially and radially in place within the cathode body. The buttons and corresponding grooves can be made very small so that the reduction in useful target thickness is minimized. Furthermore, the impact on target cooling of such small grooves is negligible. 
     Of course, it is to be understood that the buttons may be formed on the outer surface of the target and the grooves formed on the inner surface of the cathode, to equivalent effect. 
     Still another possible means  58  for positioning and releasably retaining a target within a cathode body in accordance with the invention includes an arrangement of spring-loaded plungers or balls. FIG. 10 shows such an arrangement. A compression spring  60  and ball  62  are recessed and captured in a radial bore  64  formed in the inner wall  48  of the cathode body  12 . Preferably, identical arrangements are provided at a plurality of radial locations. Mating dimples  66  in the outer surface  54  of the target provide the locating and support mechanisms. A vent hole  68  allows gas to enter or escape from the bore as the ball moves radially. This same method can be used with commercial devices, commonly known as Vlier plungers. 
     From the foregoing description it will be apparent that there has been provided an improved cylindrical magnetron assembly for sputter deposition of target material on a substrate, wherein a cylindrical target or targets may be installed into or removed from a cylindrical cathode body or bodies without disassembly of the cathode assembly. Variations and modifications of the herein described cylindrical magnetron apparatus, in accordance with the invention, will undoubtedly suggest themselves to those skilled in this art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense. 
     PARTS LIST 
       10  first prior art cylindrical magnetron 
       12  cathode body 
       14  target 
       16  magnetic field 
       18  wings, flanges 
       20  bolts 
       21  second prior art cylindrical magnetron 
       22  flanges 
       23  objects to be coated 
       24  interior of  10   
       26  inner surface of  14   
       28  flange 
       30  third prior art cylindrical magnetron 
       32  prior art dual magnetron assembly 
       34  insulator 
       36  O-rings 
       38  pumping system 
       40  improved dual magnetron assembly 
       42  firt means 
       44  retaining elements 
       46  grooves 
       48  cathode inner surface 
       50  jig buttons 
       52  groove in  54   
       53  second means 
       54  outer surface of  14 ′ 
       56  slot 
       58  third means 
       60  spring 
       62  ball 
       64  bore 
       66  dimples 
       68  vent