Abstract:
A sputter target assembly particularly useful for a large panel plasma sputter reactor having a target assembly sealed both to the main processing chamber and a vacuum pumped chamber housing a moving magnetron. The target assembly to which target tiles are bonded includes an integral plate with parallel cooling holes drilled parallel to the principal faces. The ends of the holes may be sealed and vertically extending slots arranged in two staggered groups on each side and machined down to respective pairs of cooling holes on opposite sides of the backing plate in pairs. Four manifolds tubes are sealed to the four groups of slots and provide counter-flowing coolant paths.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to sputtering apparatus. In particular, the invention relates to cooling of the sputtering target. 
     BACKGROUND ART 
     Sputtering is a well established technology in the fabrication of silicon integrated circuits, in which a metal target is sputtered to deposit target material onto the silicon wafer. Sputtering has also been applied to other uses, such as window coatings. In recent years, sputtering has also been applied for similar purposes as for silicon integrated circuits in the fabrication of flat panel displays, such as flat computer displays and large flat televisions and the like. Various types of flat panel displays may be fabricated typically including thin film transistors (TFTs) formed on large thin insulating rectangular substrates, often called panels, and including liquid crystal displays (LCDs), plasma displays, field emitters, and organic light emitting diodes (OLEDs). 
     A conventional flat panel sputter reactor  10  is schematically illustrated in the cross-sectional view of  FIG. 1 . Demaray et al. (hereafter Demaray) disclose more details of such a reactor in U.S. Pat. No. 5,565,071, incorporated herein by reference. A pedestal  12  within a main vacuum chamber  14  supports a rectangular panel  16  to be sputter coated in opposition to a generally rectangular target tile  18  bonded to a backing plate  20  sealed to but electrically isolated from the main chamber  14  by an isolator  22 . The panel  16  may be composed of a glass, a polymeric material, or other material. The target material is most typically a metal such as aluminum, molybdenum, or indium tin oxide (ITO) although other metals may be freely substituted depending on the type of layer desired to be formed on the panel  16 . Larger targets may require the bonding of multiple target tiles to the backing plate in one- or two-dimensional arrays. An unillustrated vacuum pump system pumps the interior of the main chamber  14  to a base pressure of 10 −6  to 10 −7  Torr or below. A gas source  24  supplies a sputter working gas such as argon into the chamber  14  through a mass flow controller  26  and the main chamber pressure is kept typically at no more than a few milliTorr during sputtering. A DC power supply  28  applies a negative DC bias of several hundred volts to the target  18  in opposition to the grounded pedestal  12  and unillustrated chamber shield to cause the argon to be excited into a plasma. The positively charged argon ions are attracted and accelerated by the negatively biased target  18  with sufficient energy to sputter atoms of the target material from it. Some of the sputtered material strikes the panel  16  and coat it with a thin layer of the target material. Optionally, a reactive gas such as nitrogen, may be additionally admitted to the chamber to cause the sputtered metal to react with it and form a metal compound such as a metal nitride on the panel surface. 
     Sputtering is greatly enhanced if a magnetron  30  having opposed magnetic poles is placed in back of the backing plate  20  to project a magnetic field B into the main chamber in front of the target  18 . The magnetic field traps electrons and thus increases the density of the plasma adjacent the target  18 , greatly increasing the sputtering rate. To achieve uniform erosion of the target  18  and uniform deposition on the panel  16 , the magnetron  30  is scanned in a one- or two-dimensional pattern across the back of the backing plate  20 . The form of the magnetron  30  may be much more complex than that illustrated. 
     Almost all panel fabrication equipment is distinguished by its large size. The original generation was based on panels having lateral dimensions of the order of 500 mm. Various economic and product factors have prompted successive generations of flat panel fabrication equipment of ever increasing sizes. The next generation is being developed to sputter deposit on panels having sides of greater than 2 m. This large size has introduced several problems not experienced in wafer fabrication equipment limited to sizes of about 300 mm in the most recent equipment. 
     The target  18  and more particularly its backing plate  20  must be relatively thin so that the magnetron  30  can project a substantial magnetic field through it. However, absent other means, the backing plate  20  needs to stand off a considerable force (differential pressure times the area) between its back and the high vacuum of the main chamber  14  and further the backing plate  20  should not significantly bow under these pressure differentials. To provide such large thin targets, Demaray proposed placing the magnetron  30  inside a magnetron chamber  32  sealed to the back of the backing plate  20  and pumped to a relatively low pressure in the sub-Torr range, the limit of a mechanical vacuum pump. Such back pumping reduces the force exerted on the backing plate  20  by a factor of about a thousand. 
     Such a structure contrasts with a conventional wafer sputter reactor in which a corresponding chamber at the back of the target backing plate  20  is filled with chilling water to cool the target during sputtering. Demaray, instead, recirculates cooling liquid from a chiller  34  through cooling channels formed within the backing plate  32 . As shown in the cross-sectional view of  FIG. 2 , a substantially rectangular conventional target  40  includes a backing plate  42  formed of top and bottom plates  44 ,  46 . Cooling channels  48  of generally rectangular cross section are machined into the surface of the top plate  44  to extend generally between the two sides of the backing plate  42  although larger horizontal distribution manifolds may be formed nearer the two sides to connect the cooling channels  48  to a common cooling liquid inlet and a common cooling liquid outlet. The bottom plate  46  is then bonded to the top plate  44  to enclose and seal the cooling channels  48  and manifolds. A target tile  50  is then bonded to the backing plate  42 . In the past, indium bonding was most often used but conductive polymeric adhesive bonding is gaining favor. 
     The bonding of the two plates  46 ,  48  of the backing plate  42  has presented technical challenges, particularly at the larger panel sizes. It is desired to reuse the backing plate  42  when sputtering has effectively eroded through the target tile  50 . That is, it is desired to remove the old target tile  50  and replace it with a new one. The backing plate  42  needs to be rugged to survive refurbishment when the used target tile is delaminated from the backing plate and a new target tile is laminated. Targets and their backing plates have become increasingly expensive for the larger sizes of panels. Thus, their cost should be reduced while their ruggedness should be maintained and preferably increased. The two plates  44 ,  46  can be welded together, but welding tends to deform thin plates. The two plates  44 ,  46  can be screwed together with a sealant placed in the interface. However, the number of screws required for a 2.5 m×2.5 m target becomes very large. Indium bonding can be used, but its ruggedness is questionable. Autoclaving has been suggested, but this is a complex and expensive process. 
     The larger target sizes have also presented a challenge in uniformly cooling a larger area without unduly increasing the thickness of the target assembly. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention includes a sputtering target backing plate to which one or more target tiles are bonded and which has parallel laterally extending cooling holes formed parallel to the principal surface of the backing plate for the flow of cooling water or other liquid. The backing plate is preferably integral and cylindrical cooling holes may be bored across its lateral dimension, for example, by gun drilling. 
     Another aspect of the invention includes dividing the cooling holes into two interleaved groups and counter-flowing cooling liquid in the two groups of cooling holes, that is, in anti-parallel directions to thereby reduce the temperature differential across the target and its backing plate. 
     A further aspect of the invention includes vertical inlet and outlet holes or slots formed from a principal surface of the backing plate on two opposed peripheral sides and each joined to one or more of the cooling holes to supply and drain cooling liquid from the horizontally extending cooling holes. The slots advantageously join two to six adjacent cooling holes The ends of the cooling holes outside of the vertical and outlet holes are plugged. Advantageously, the holes or slots on each peripheral side alternate in offset along the axial direction of the cooling holes to provide alternating inlet and outlet holes or slots. Supply and drain manifolds may then be arranged in parallel and sealed to the respective inlet and outlet holes or slots. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a conventional flat panel sputtering chamber. 
         FIG. 2  is a cross-sectional view of a conventional target including a backing plate with cooling channels and a target tile bonded to it. 
         FIG. 3  is a schematic orthographic view of a simplified embodiment of a backing plate of the invention. 
         FIG. 4  is a cross-sectional view of a vertically extending cooling inlet or inlet to a horizontally extending cooling hole. 
         FIG. 5  is a cross-sectional view of a plurality of horizontally extending cooling holes formed in a target backing plate. 
         FIG. 6  is a bottom plan view of a multi-tile target and backing plate of the invention including four columns of cooling inlets and outlets. 
         FIG. 7  is an exploded orthographic view of a corner of the target backing plate of  FIG. 6  including 
         FIG. 8  is an orthographic view of an embodiment of one of two cooling manifolds to be attached to the backing plate of  FIG. 6 . 
         FIG. 9  is a plan view of a planar side of a manifold plate forming part of the manifold of  FIG. 8 . 
         FIG. 10  is an orthographic view of the backing plate of  FIG. 6  to which are attached two manifolds of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A backing plate  60  of one embodiment of the invention, very schematically illustrated in the orthographic view of  FIG. 3  from the bottom, is formed in an integral metal plate  62  having lateral dimensions corresponding to the desired size of the backing plate  60 , for example, greater than 2 m on a side for the planned next generation. A series of parallel cylindrical cooling holes  64  are bored to extend from one lateral side to the other of the metal plate  62  and parallel to the principal surfaces of the metal plate  62 . Exemplary dimensions are a thickness for an aluminum plate of 33 mm and hole diameter of 12 mm. The hole boring over such a great distance may be achieved by gun drilling, that is, using a very long drill bit. In view of the long lengths, it is advantageous to drill holes from both sides which join in the middle. The cooling water or other liquid flows through the holes  64  to cool the backing plate  60  and hence the target tile affixed to the backing plate  60 . 
     In the illustrated embodiment, cooling water is supplied and drained from elongated or oblong holes or slots  66 ,  68 ,  70 ,  72  milled from a principal surface of the metal plate  62  to preferably at least the median depth of the holes  64  but not to the opposite side of the metal plate  62 . As a result, the cooling holes  64  are exposed to respective pairs of the slots  66 ,  68 ,  70 ,  72 . The slots  66 ,  68 ,  70 ,  72  are located in two sets on opposed lateral sides of the metal plate  62  at positions outside of the vacuum chamber  14  and the magnetron chamber  32  to which the backing plate  60  will be sealed. For convenience of plumbing connections, the slots are preferably located on the illustrated bottom side of the backing plate  60  to which the target tile will be bonded. Machining and sealing are simplified if the slots  66 ,  68 ,  70 ,  72  expose pairs of the cooling holes  64 . The slots may be formed as circular holes, especially if they expose only one respective cooling hole but elongated slots linked to multiple cooling holes  64  are advantageous. More than two cooling holes  64  per slot would further simplify the machining and sealing but at the cost of degraded cooling uniformity. Generally, six cooling holes  64  per slot are a reasonable upper limit. As illustrated in the cross-sectional view of  FIG. 4 , the ends of cooling holes  64  laterally outside of the slots  66 ,  68 ,  70 ,  72  are water sealed with plugs  74  so that the water flows from slot to slot on opposed sides of the backing plate  60  through the middle portion of the cooling holes  64 . 
     The material of the backing plate  60  is not limited to aluminum or aluminum alloys but, in view of the gun drilling, it is preferred that the material be easily machinable, such as aluminum or brass. 
     It is preferred that the cooling water or other liquid coolant be supplied to and drained from the slots to set up counter flowing coolant. For example, slot  66  can serve as in inlet and slot  68  as an outlet for coolant flowing to the right and slot  72  can serve as an inlet and slot  70  as an outlet for coolant flowing to the left. The counter flow greatly reduces the temperature differential across the backing plate  60  when there are many more anti-parallel flowing groups of cooling holes  64 . It is typical for cooling water to heat up from about 20° C. to 25° C. in one pass across the backing plate  60  under normal sputtering conditions. For single directional flow, the backing plate  60  would have a similar 5° C. temperature differential from one side to the other, which amounts to a differential thermal expansion of about 1 mm in aluminum, a value which should be reduced. On the other hand, for counter flowing coolant neighboring pairs of cooling holes  64  have an opposite temperature gradient and they are close enough that the backing plate  60  is substantially cooled to the average of the two flows, that is, a nearly constant 22.5° C. as averaged over the area between the counter-flowing holes although more localized but compensating temperature variations will occur. 
     As illustrated in the cross-sectional view of  FIG. 5 , one or more target tiles  76  are bonded to the bottom side of the backing plate  60  (top as illustrated) in a target area  78  of  FIG. 3  adjacent the cooling holes  64  and between the slots  66 ,  68 ,  72 ,  72  providing coolant to them. 
     The illustration of the backing plate  60  of  FIG. 3  is very simplified. A more realistic target and backing plate assembly  80 , illustrated in the bottom plan view of  FIG. 6 , includes an integral backing plate  82  with angled corners  84 , which are illustrated in more detail in the exploded orthographic view of  FIG. 7 . It includes  42  parallel cooling holes  86  in alternating pairs for the counterflow. The cooling holes  86  may be are gun drilled from the opposed edges of the backing plate  82  including the angled corners  84 . Exemplary dimensions are a thickness for an aluminum or aluminum alloy plate  82  of 33 mm and a hole diameter of 12 mm, that is, a hole diameter of preferably greater than 25% and less than 75% and preferably less than 50% of the plate thickness. The plate thickness may be varied, for example, between 20 and 60 mm. Slots  88 ,  90  are machined from the bottom operational surface of the backing plate  82  in two staggered columns on each side to expose pairs of the cooling holes  88 . Plural, for example, 10 sets of coupled slots  88  and plural, for example, 11 sets of coupled slots  90  provide the pair-wise coupling of the slots  88 ,  90  to the cooling holes  86 . As mentioned above, number of cooling holes  88  exposed by a single slot  88 ,  90  may vary. Also, the number of slot sets may be varied but cooling uniformity is improved by increasing the number of sets. Plugs  92  are screwed into or otherwise sealed to both ends of the holes  88 ,  90  so that all coolant flows through the slots  88 ,  90 . The plugs  92  may chosen from various commercially available types, for example, Swageloks, Farmington plugs, SAE plugs or they may be specially fabricated. A welded rod plug is also possible although warpage should be avoided. 
     The described embodiment evenly spaces the cooling holes  86  and slots  88 ,  90  across the backing plate  82 . However, non-uniform distributions may be used to tailor the cooling, for example, more cooling holes and hence more cooling in the center of the backing plate  82 . 
     The described fabrication technique for an integral backing plate with cooling holes bored laterally therethrough provides several advantages. The fabrication based mostly on machining is much less expensive than the previously practiced bonding of multiple plates. Even if the diameter of the holes is a sizable fraction of the plate thickness, they do not greatly reduce the plate&#39;s rigidity. Furthermore, the resultant backing plate is not subjected to delamination during usage or target refurbishment. 
     After fabrication of the backing plate  82 , target tiles  94  are bonded to the backing plate  82 , preferably with a conductive polymeric adhesive in a process available from TCB of San Jose, Calif. although conventional indium bonding or other method may be used. The illustration shows multiple tiles  94  in a two-dimensional array with predetermined gaps of about 0.5 mm between them, a useful arrangement if large target tiles are not readily available. However, other tile arrangements may be used such as a one-dimension array of multiple tiles or a single large tile. 
     Two manifolds  100 , one of which is illustrated in the orthographic view of  FIG. 8  generally from the bottom in their operational position, are attached to the opposed sides of the backing plate  82  on its operational bottom side to cover and couple to the offset rows of slots  88 ,  90 . Advantageously, they can easily formed of stainless steel without affecting the cleanliness within the sputtering chamber. Each manifold  100  includes a manifold plate  102  and a short rectangular manifold tube  104  and a long rectangular manifold tube  106 , each having respective pairs of hose fittings  108 ,  110  for the supply and draining of cooling water or other liquid coolant through unillustrated hoses to the chiller  34 . Multiple holes fittings  108 ,  110  mounted on and coupled to the interiors of each manifold tube  104 ,  106  provide a more uniform flow of coolant to each of the large number of slots  88 ,  90  and associated cooling holes  86 . The two manifold tubes  104 ,  106  are welded from within each of the manifold plate slots  112 ,  114  between the slot periphery and the manifold plate  106 . When welded, the two manifold tubes  104 ,  106  are separated by about 1 cm between them to allow screwing of fasteners between the manifold plate  106  and the backing plate  82  in the area between the manifold tubes  104 ,  106 . 
     The manifold plate  102 , as shown in the top plan view of  FIG. 9  includes two staggered rows of manifold slots  112 ,  114  in correspondence to the slots  88 ,  90  in the backing plate  82 . O-ring grooves  116  surround each of the manifold slots  112 ,  114  to accept respective O-rings used to seal the manifold  100  and its slots  112 ,  114  to the backing plate  82  around its slots  88 ,  90 . The bases of the manifold tubes  104 ,  106  have corresponding slots machined into them to allow cooling liquid to freely circulate between the manifold tubes  104 ,  106  and the corresponding groups of the cooling holes  86 . Three rows of unillustrated through holes bored through the manifold plate  102  match corresponding unillustrated tapped holes in the backing plate  82  for screw attachment and sealing of the manifold  100  to the backing plate  82 . The through and tapped holes are arranged such that four screws are fastened in a rectangular pattern around each of the manifold slots  112 ,  114  to uniformly seal the O-rings  116 . 
     An operational target assembly  120  is illustrated in the partial orthographic view of  FIG. 10  generally from the bottom in it operational orientation. The operational target assembly  120  includes the target and backing plate assembly  80  of  FIG. 8  and two manifolds  100  of  FIG. 8  (only one of which is illustrated) fixed and sealed to two opposed peripheral sides of the backing plate  82  outside of its vacuum seals to the main and magnetron chambers  14 ,  32 . The operational target assembly  120  additionally includes a multi-branch supply hose  122  and a multi-branch drain hose  124  connected between a chiller  118  and the hose fittings  108 ,  110  on both lateral sides of the backing plate  82 . On the illustrated manifold  100 , the supply hose  122  supplies chilled coolant to the short manifold tube  106  while the drain hose  124  drains coolant warmed by the target from the long manifold tube  106 . The double hose connection to each manifold tube  104 ,  106  evens the flow between the large number cooling holes. In contrast, on the unillustrated manifold  100  fixed to the other unillustrated lateral side of the target  80  with similar hose fittings  108 ,  110 , the supply hose  122  supplies chilled coolant to the long tube manifold tube  106  through the two hose fittings  110  and the drain hose  124  drains warmed coolant from the short manifold tube  104  through the two hose fittings  108 . As a result, a first coolant flow is set up in one direction between the two short manifold tubes  104  and a second coolant flow is set up in the opposite direction between the two long manifold tubes  106 . 
     The external manifolds provide several advantages of their own. They can be manufactured separately from the target assembly and can be easily reused. Furthermore, in combination with the large number of parallel cooling holes, they enable a more uniform cooling of the target. 
     An alternative embodiment includes a single row of backing plate slots  88 ,  90  on both principal surfaces of the backing plate  82  and on both its lateral sides connecting to different ones of the cooling holes  64 . Separate liquid manifolds may be affixed to the top and bottom of the backing plate  82 . This configuration reduces the length of the backing plate. Yet other forms of the manifolds are included within the invention. 
     Although the above embodiments have been described with respect to the orientations of the sputter chamber of  FIG. 1 , it is clear that the orientation may be inverted, put on its side, or arranged at another angle without departing the spirit of the invention. The directions recited in the claims are for convenience only and may be changed to other orientations with respect to gravitational force. 
     The invention is not limited to sputtering onto panels intended for displays but may be applied to other applications. 
     The several features of the invention may be practiced separately or in combination and with limitations restricted only by the claims. 
     The invention thus provides a less expensive, more rugged target assembly and reusable backing plate providing improved thermal control.