Abstract:
Embodiments of the present invention generally relate to sputtering targets used in semiconductor manufacturing. In particular, the invention relates to bonding the sputtering target to a backing plate that supports the sputtering target in a deposition chamber. In one embodiment, a method of bonding at least one sputtering target tile to a backing plate comprises providing an elastomeric adhesive layer between the at least one sputtering target tile and the backing plate, and providing at least one metal mesh within the elastomeric adhesive layer, wherein at least a portion of the at least one metal mesh contacts both the at least one sputtering target tile and the backing plate, and the at least a portion of the at least one metal mesh is made of metal wire with diameter greater than 0.5 mm.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application Ser. No. 60/686,095, filed May 31, 2005, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to sputtering targets used in electronic device manufacturing. In particular, the invention relates to bonding the sputtering target to a backing plate that supports the sputtering target in a deposition chamber. 
     2. Description of the Related Art 
     Physical vapor deposition (PVD) is one of the most commonly used processes in fabrication of electronic devices. PVD is a plasma process performed in a vacuum chamber where a negatively biased target is exposed to a plasma of an inert gas having relatively heavy atoms (e.g., argon) or a gas mixture comprising such inert gas. Bombardment (or sputtering) of the target by ions of the inert gas results in ejection of atoms of the target material. The ejected atoms accumulate as a deposited film on a substrate placed on a substrate pedestal disposed underneath the target within the chamber. Flat panel display sputtering is principally distinguished from the long developed technology of wafer sputtering by the large size of the substrates and their rectangular shape. 
       FIG. 1  (prior art) shows a typical DC magnetron PVD reactor  10 , which includes an electrically grounded vacuum chamber  12  to which a target  14  is vacuum sealed through an electrical isolator  16 . A DC power supply  18  negatively biases the target  14  with respect to the chamber  12  to excite an argon sputter working gas into a plasma. However, it is noted that RF sputtering is also known. The positively charged argon ions are attracted to the biased target  14  and sputter material from the target  14  onto a substrate  20  supported on a pedestal in opposition to the target  14 . A magnetron  24  positioned in back of the target projects a magnetic field parallel to the front face of the target  14  to trap electrons, thereby increasing the density of the plasma and increasing the sputtering rate. In modern sputter reactors, the magnetron may be smaller and be scanned about the back of the target  14 . Even a large magnetron may be scanned in order to improve the uniformity of erosion and deposition. Aluminum, titanium, and copper targets are typically formed of a sputtering layer of the material to be sputtered coated onto or bonded to a target backing plate of less expensive and more readily machineable material. 
     Sputter reactors were largely developed for sputtering onto substantially circular silicon wafers. Over the years, the size of silicon wafers has increased from 50 mm diameters to 300 mm. Sputtering targets need to be somewhat larger than the size of the wafers to provide more uniform deposition across the wafers. Typically, wafer sputter targets are formed of a single circular member for some materials such as aluminum and copper or a single continuous sputter layer formed on a backing plate for more difficult materials. 
     In the early 1990&#39;s, sputter reactors were developed for thin film transistor (TFT) circuits formed on glass panels to be used for large displays, such as liquid crystal displays (LCDs) for use as computer monitors or television screens. The technology was later applied to other types of displays, such as plasma displays, and solar panels, and on other panel compositions, such as plastic and polymer. Some of the early reactors were designed for panels having a size of about 400 mm×600 mm. Because of the increasing sizes of flat panel displays being produced and the economy of scale realized when multiple displays are fabricated on a single glass panel and thereafter diced, the size of the panels has been continually increasing. Flat panel fabrication equipment is commercially available for sputtering onto panels having a minimum size of 1.8 meter and equipment is being contemplated for panels having sizes of 2 m×2 m and even larger. 
     Due to the increasing sizes of panels, conventional way of bonding the sputtering targets to the backing plates by indium bonding is getting very expensive as the sputtering targets get larger and larger. Therefore, there is a need for a method of bonding sputtering targets to backing plates for large area substrate processing system that is economical and robust. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention provide a method of bonding sputtering target tiles to the backing plate. Embodiments of the invention further provide a sputtering target assembly comprising an elastomeric adhesive layer between a sputtering target tile and a backing plate. In one embodiment, a method of bonding at least one sputtering target tile to a backing plate comprises providing an elastomeric adhesive layer between the at least one sputtering target tile and the backing plate, and providing at least one metal mesh within the elastomeric adhesive layer, wherein at least a portion of the at least one metal mesh contacts both the at least one sputtering target tile and the backing plate, and the at least a portion of the at least one metal mesh is made of metal wire with diameter greater than 0.5 mm. 
     In another embodiment, a method of bonding at least one sputtering target tile to a backing plate comprises providing at least one elastomeric adhesive layer between the at least one sputtering target tile and the backing plate, and providing at least one metal mesh within the at least one elastomeric adhesive layer, wherein at least a portion of the at least one metal mesh contacts both the at least one sputtering target tile and the backing plate and the total surface area of the openings of the at least one metal mesh is between about 40% to about 70% of the total bonding surface area of the at least one sputtering target tile. 
     In another embodiment, a sputtering target assembly comprises at least one sputtering target tile, a backing plate for the at least one sputtering target tile, an elastomeric adhesive layer between the at least one sputtering target tile and the backing plate, wherein the elastomeric adhesive layer bonds the at least one sputtering target and the backing plate together, and at least one metal mesh within the elastomeric adhesive layer, wherein at least a portion of the at least one metal mesh contacts both the at least one sputtering target tile and the backing plate, and the at least a portion of the at least one metal mesh is made of metal wire with diameter greater than 0.5 mm. 
     In yet another embodiment, a sputtering target assembly comprises at least one sputtering target tile, a backing plate for the at least one sputtering target tile, at least one elastomeric adhesive layer between the at least one sputtering target tile and the backing plate, wherein the at least one elastomeric adhesive layer bonds the at least one sputtering target and the backing plate together, and at least one metal mesh within the at least one elastomeric adhesive layer, wherein at least a portion of the at least one metal mesh contacts both the at least one sputtering target tile and the backing plate, and the total surface area of the openings of the at least one metal mesh is between about 40% to about 70% of the total bonding surface area of the at least one sputtering target tile. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  (prior art) is a schematic cross-sectional view of a conventional plasma sputter reactor. 
         FIG. 2  is a plan view of a rectangular target formed from a two-dimensional array of target tiles. 
         FIG. 3A  is a cross-sectional view of a conventional configuration of target tiles bonded to a target backing including cooling channels. 
         FIG. 3B  is a plan view of a corner area between four tiles. 
         FIG. 4  (prior art) is a schematic view illustrating a conventional method of bonding target tiles to a backing plate. 
         FIG. 5  is a cross-sectional view of one embodiment of the current invention of sputtering target bonded to a backing plate. 
         FIG. 6  is a perspective view of one embodiment of a metal mesh used in the current invention. 
         FIG. 7  shows a process flow of bonding a sputtering target to a backing plate using the metal mesh in  FIG. 6 . 
         FIG. 8A  shows a plan view of a rectangular target formed from a 3×2 array of target tiles. 
         FIG. 8B  shows a cross-sectional view of target tiles  830 C,  830 E being bonded to the backing plate of  FIG. 8A . 
         FIG. 8C  shows a plan view of a rectangular target formed from a 3×1 array of target tiles. 
         FIG. 8D  shows a plan view of a rectangular target formed from an one-piece target. 
     
    
    
     DETAILED DESCRIPTION 
     For large sputtering targets, a two-dimensional tile arrangement, such as the one illustrated in  FIG. 2 , is sometimes necessary. Rectangular target tiles  32  are arranged in a rectangular array and bonded to a target backing plate  34 . As shown in the plan view of  FIG. 2 , a substantially rectangular target  30  includes rectangular target tiles  32  arranged in a rectangular array and bonded to a target backing plate  34 . The tile size depends on a number of factors including ease of fabricating the tiles and they may number 3×1, 3×3, 4×5 (as shown in  FIG. 2 ), or other applicable combinations. The tiles  32  may be of substantial size, for example 750 mm×900 mm, such that a 3×3 array is required for a larger panel. The number of tiles in the tile array may be even greater if the target material is difficult to work with, such as chromium or molybdenum. The illustrated target backing plate  34  is generally rectangularly shaped to conform to the shape and size of the panel being sputter coated but its corners  36  are typically rounded to conform to the chamber body supporting it and it could include an extension  38  from the chamber body containing an electrical terminal for powering the target and pipe couplings for the cooling fluid used to cool the target  30 . As illustrated in an exemplary cross section in  FIG. 3A , the target backing plate  34  for flat panel sputtering is typically formed from two metal plates  42 ,  44 , for example, of titanium welded or otherwise bonded together. One of the plates  42 ,  44  is formed with linear cooling channels  46  through which the cooling fluid circulates. This backing plate  34  is more complex than the usual backing plate for wafer processing since, for the very large panel size, it is desired to provide a backside vacuum chamber rather than the usual cooling bath so as to minimize the differential pressure across the very large target  30 . 
     The tiles  32  are bonded to the backing plate  34  on its chamber side with a gap  48  possibly formed between the tiles  32 . Typically, the tiles  32  have a parallelopiped shape with perpendicular corners with the possible exception of beveled edges at the periphery of the tile array. The gap  48  is intended to satisfy fabricational variations and thermal expansion of tiles and may be between 0 and 0.5 mm. Neighboring tiles  32  may directly abut but should not force each other. On the other hand, the width of the gap  48  should be no more than the plasma dark space, which generally corresponds to the plasma sheath thickness and is generally slightly greater than about 0.5 mm for the usual pressures of argon working gas. Plasmas cannot form in spaces having minimum distances of less than the plasma dark space. In addition, as illustrated for a corner area  70  between four tiles  32  in the plan view of  FIG. 3B , the four tiles  32  arranged in a rectangular array tend to slide along each other and be misaligned with different sizes for the inter-tile gaps  48 . An interstix is meant a point or space at the interfaces between three or more tiles so that the term does not include the line between two tiles. Even a well defined interstix  72  presents the greatest gap between tiles  32 . It&#39;s important to keep with greatest width of interstix  72  to be blow the plasma dark space to prevent formation of plasma in the interstix  72 . As a result, the underlying titanium backing plate  34  is not sputtered while the tiles  32  are being sputtered. 
     Returning to  FIG. 2 , the tiles  32  are arranged within a rectangular outline  40  conforming approximately to the area of the target  30  desired to be sputtered or somewhat greater. The magnetron  24  of  FIG. 1  is scanned within this outline  40 . Shields or other means are used to prevent the un-tiled surface of the backing plate  34  from being exposed to high-density plasma and be thereby sputtered. Clearly sputtering a backing plate  34 , made of material such as titanium, supporting molybdenum or other tiles is not desired. Even if the backing plate  34  is composed of the same material as the target tiles  32 , sputtering of the backing plate  34  is not desired. The backing plate  34  is a complex structure and it is desired to refurbish it after one set of tiles  32  have been exhausted and to use it for a fresh set of tiles  32 . Any sputtering of the backing plate  34  should be avoided. 
     There are several processes and bonding materials available for bonding target tiles to the backing plates. One popular process illustrated in  FIG. 4  (prior art) includes an apparatus comprising two heating tables  60 ,  62 . The tiles  32  are placed on one table  60  with their sputtering face oriented downwards. Each tile  32  is painted on its backside with an adhesive coating  64 , such as indium. The heating table  60  heats the coated tiles  32  to about 200° C., far above indium&#39;s melting point of 156° C. so that indium wets to the tiles  32  and forms a uniform molten layer. Similarly, the backing plate  34  is placed on the other heating table  62  and is painted with an indium coating  66  and is heated to about 200° C. With all indium coatings  64 ,  66  in their molten state, the tiles  32  are removed from the first table  60 , inverted, and placed on top of the backing plate  34  with the melted indium coatings  64 ,  66  facing each other and the sputtering faces oriented upwards. Upon cooling, the indium solidifies and bonds the tiles  32  to the backing plate  34 . 
     A problem arises with indium bonding of target tiles to the backing plate resulting from the differential thermal expansion between the materials of the target tiles and the backing plate, such as molybdenum target bonded to Al backing plate. When the bonded assembly is cooled from bonding temperature of about 200° C. for indium to room temperature, the differential thermal expansion is likely to cause the assembly to bow. Because of the softness of solid indium, the bow can be pressed out of the bonded assembly. However, the pressing is a generally uncontrolled process and the tiles may slide relative to each other during the pressing to create undesired tile misalignment such as a gap greater than the plasma dark space, e.g. 1 mm. 
     In addition, such large mechanical structures are not easily manipulated to provide the desired degree of alignment, specifically, the bonded tiles being separated by no more than 0.5 mm. As a result, the widest point of the misaligned tiles  32  may become larger than the plasma dark space, e.g., 1 mm, so that the plasma may propagate towards the backing plate  34 . If the gap is only slightly larger than the plasma dark space, the plasma state in the gap may be unsteady and result in intermittent arcing. Even if the arcing is confined to tile material, the arc is likely to ablate particles of the target material rather than atoms and create contaminant particles. If the plasma reaches the backing plate, it will be sputtered. Plate sputtering will introduce material contamination if the tiles and backing plate are of different materials. Furthermore, plate sputtering will make it difficult to reuse the backing plate for a refurbished target. 
     Embodiments of the invention provide a method of bonding sputtering targets or target tiles to the backing plate by using an elastomeric adhesive. The backing plate with bonded target tiles can be cured at a relatively low temperature, such as less than 100-150° C., to reduce the risk of bowing of the backing plate and the bonded target tiles. The concept of the invention can be applied to bond all sizes of sputtering targets or target tiles used for processing any type of substrates. The concept of the invention can be applied to targets greater than 750 cm 2 , preferably to targets greater than 2000 cm 2 , more preferably to targets greater than 15000 cm 2 , and most preferably to targets greater than 40000 cm 2 .  FIG. 5  shows a cross section of a target (or target tile)  130  being bonded to a backing plate  110  by an elastomeric adhesive  120  with at least one metal mesh  125 . The target  130  could be single piece target, target tiles welded into one piece, target tiles placed closed to one another, or welded target tiles placed close to one another. The target could be made of any type of sputtering materials that contain elements such as aluminum, copper, gold, nickel, tin, molybdenum, chromium, zinc, palladium, stainless steel, palladium, tin, indium, titanium, tungsten, neodymium and combinations thereof. Examples of films deposited by sputtering targets include: aluminum, aluminum neodymium, aluminum alloy, copper, copper alloy, gold, nickel, tin, tin alloy, molybdenum, molybdenum nitride, titanium, titanium nitride, tantalum, tantalum nitride, tungsten, chromium, zinc, zinc oxide, palladium, palladium alloys stainless steel, and indium tin oxide (ITO). 
     An example of target material for manufacturing of flat panel displays is molybdenum. The backing plate provides support for the sputtering target and also provides electrical connection to the power source. The backing plate  110  could be made of conductive materials, such as aluminum, copper, gold, nickel, tin, Molybdenum, chromium, zinc, palladium, stainless steel, palladium alloys, tin alloy, aluminum alloy and copper alloy. An example of backing plate material for a PVD chamber for manufacturing of flat panel displays is aluminum. 
     The elastomeric adhesive  120  could be any adhesives made of elastomers. The elastomers suitable to bond the target (or target tiles) and the backing plate should have low stress before and after curing process, which is performed after the target and the backing plate are pressed together with the elastomeric adhesive  120  between them. The elastomers  120  used to bond the target  130  and the backing plate  110  should also have thermal coefficient close to the target  130  and the backing plate  110 . 3M of St. Paul, Minn., Dow Chemicals of Midland, Mich., and GE Consumer Electrics of Oklahoma City, Okla., offer many types of commercial elastomers for bonding between metals. An example of elastomer adhesive that can be used to bond metal to metal is AF 30/EC 1660 adhesive, available from 3M. 
     The metal mesh  125  could be made of any metal with good electrical and/or thermal conductivities, such as copper, aluminum, copper alloy, aluminum alloy. There could be one or more metal meshes  125  between the target  130  and the backing plate  110 . The metal mesh  125  provides electrical and/or thermal conductive paths between the target  130  and the backing plate  110 , which is connected to the power source.  FIG. 6  shows a perspective view of one embodiment of the metal mesh  125 , which is made of overlapping metal wires. The metal wires of the metal mesh  125  could be arranged in various fashions. The opening  126  in the metal mesh  125  should be large enough to let the elastomeric adhesive  120  pass through, which ensures the metal mesh has good contact with the target  130  and the backing plate  110 , and prevents the elastomeric adhesive  120  from building up in certain parts between the target  130  and the backing plate  110 , such as near the central region of the target. The percentage of the total surface area of openings  126  to the total bonding surface area is between about 35% to about 90%, preferably between about 40% to about 70%, and most preferably between about 55% to about 65%. If the percentage of the total surface area of the openings  126  is too low, the elastomeric adhesive cannot go through the metal mesh and prevents the metal mesh from contacting the target and/or backing plate. In addition, to ensure the metal mesh  125  to provide good contact between the target  130  and the backing plate  110  for sputtering target for PVD chamber to process large area substrates, the diameters of the metal wires used to make the metal mesh should be at least 0.5 mm, preferably greater than 0.8 mm, to provide sufficient electrical and/or thermal contact. 
     The elastomeric adhesive  120  is combined with the metal mesh  125  to provide good thermal conductivity between the target  130  and the backing plate  110  and ensures the target  130  can be properly cooled by the cooling medium (not shown) flowing through the backing plate  110  during sputtering process. The thermal conductivity can come from the metal mesh  125 , or the elastomeric adhesive could optionally include metallic powder, such as silver powder, in the weight range between about 0% to about 80% to increase thermal conductivity. Since the sputtering process is operated under high vacuum, such as &lt;10 −4  Torr, the elastomer between the target  130  and the backing plate  110  needs to be cured prior to usage in the process chamber to prevent out-gassing. 
       FIG. 7  shows an embodiment of a process for bonding the target  130  and the backing plate  110  with elastomeric adhesive  120  and at least one metal mesh  125 . At step ( 1 ), the target  130  and backing plate go through surface preparation sub-steps, such as sanding the edges of the target and washing the surfaces of target and backing plate that will receive the bonding material (or adhesive) and are placed on supporting structure (not shown) after the surface preparation sub-steps. The supporting structure for the target  130  could include mechanical devices that would hold or grab the target  130  and move it at a later step. The preparation sub-steps are performed to ensure that the target  130  and the backing plate  110  be prepared and secured to the supporting structures. The target  130  could be made of tiles welded together into one piece or several pieces, or could be a single un-welded piece. The target  130  could also be used for semiconductor wafer processing for &gt;300 mm substrates. In such a case, the target could be circular. 
     At step ( 2 ), a layer of elastomer (or elastomeric adhesive)  120 A is applied on the target  130  surface that is away from the supporting structure (or the surface that will face the backing plate). A layer of elastomer (or elastomeric adhesive)  120 B is also applied on the backing plate  110  surface that is away from the supporting structure (or the surface that will face the target). 
     At step ( 3 ), a metal mesh  125  (or an electrical and/or thermal conductor) is placed on the elastomer layer applied on the backing plate in step ( 2 ), the target  130  with the elastomer layer  120 A applied during step ( 2 ) is flipped over to allow the elastomer layer  120 A to be faced downward towards the backing plate  110 . The target  130  is then positioned over the backing plate  110  for bonding the target  130  and the backing plate  110  together. The metal mesh  125  should be placed in pre-determined bonding location to allow the metal mesh to be placed between the target  130  and the backing plate  110  after the target  130  and the backing plate  110  are pressed together. The target  130  can be flipped over and positioned above the backing plate  110  manually or by mechanical devices. For large targets, such as surface area greater than 20 m 2 , manual flipping and carrying the target could be difficult due to the size and weight of the target, and mechanical devices might be needed to perform the tasks. 
     At step ( 4 ), the target  130  is slowly lowered to the backing plate  110  to allow the elastomer layer  120 A on the target  130  to come in contact with the metal mesh  125  and elastomer layer  120 B on the backing plate  110 . The elastomer layer  120  is formed by pressing elastomer layer  120 A and elastomer layer  120 B together, and is then cured. The elastomer layer  120 A on the target  130  and the elastomer layer  120 B on the backing plate  110  fill the space in the metal mesh and become elastomer layer  120  and the metal mesh  125  comes in contact with the target  130  and the backing plate  110  to provide electrical and thermal contact. In one embodiment, static weight is placed on top of the sputtering target to press the sputtering target against the backing plate. In another embodiment, the sputtering target and the backing plate can be pressed together by clamps. Heated liquid, such as water, or gas, such as N 2 , is then passed through the heating channel(s) in the backing plate  110  to cure the elastomer. The elastomer is cured to prevent outgassing during PVD processing, which operates under high vacuum, e.g. 10 −4  Torr. To prevent target and/or backing plate bowing, a low temperature curing is desired. The temperature range could be between about 25° C. to about 100° C., and preferably between about 45° C. to about 70° C. and most preferably between about 45° C. to about 55° C. Curing of elastomer can be performed at room temperature, which will take a relatively long time, e.g. 1 day. Curing at higher temperature will shorten the curing time, e.g. 8 hours at 50° C. Elastomer bonding has the advantage of low processing temperature (30° C.-100° C.), compared to the processing temperature of indium bonding (about 200° C.). This low processing temperature reduces the risk of bowing of target and bonding plate after bonding process. 
     The time between step ( 2 ) after the elastomer has been applied on the surface of target and backing plate and step ( 4 ) when the target is pressed against the metal mesh and backing plate should not be too long to prevent the elastomer from being dried up and lose its fluidity. The duration should be kept less than 12 hours, preferably less than 6 hours, most preferably less than 2 hours. 
     The embodiment of process described in  FIG. 7  shows that the sputtering target is positioned above the backing plate to be pressed against the backing plate. The positions of the sputtering target and the backing plate can be flipped with the sputtering target being positioned below the backing plate. The metal mesh in this case is placed on the elastomer layer on the sputtering target before the backing plate is lowered to be pressed against the metal mesh and the sputtering target. 
     The target  130  could be one piece or constructed from multiple pieces of target material. In one embodiment, multiple target tiles are bonded to the backing plate. The tiles are placed closely together and the spacing between the tiles is preferably kept below the plasma dark space, e.g. 1 mm.  FIG. 8A  shows a top schematic view of 6 tiles  830 A,  830 B,  830 C,  830 D,  830 E,  830 F being bonded to a backing plate  810 . The 6 tiles  830 A,  830 B,  830 C,  830 D,  830 E, and  830 F could be all in one size or have different sizes.  FIG. 8B  shows a cross-sectional schematic view of part of tiles  830 C and  830 E over the backing plate  810 . Between the backing plate  810  and target tiles  830 C,  830 E, there is a bonding layer  820  with metal mesh in the layer and the metal mesh contacts the target tiles  830 C,  830 E and the backing plate  810 . The widest spacing  850  between the target tiles  830 C,  830 E should be kept below plasma dark space.  FIG. 8C  shows another example of multiple tiles being bonded to the backing plate. The three tiles  830 G,  830 H,  8301  are bonded to the backing plate  810 . The tiles can also be welded together into one large piece before being bonded to the backing plate. The target tiles can be bonded by laser welding, e-beam welding, or FSW before being bonded to the backing plate.  FIG. 8D  show a single piece target  830 J being bonded to backing plate  810 . 
     The concept of the using a metal mesh as an electrical and/or thermal conductor can also be applied to other types of bonding. For example, a metal mesh can also be placed between two conductive layers using indium as bonding materials. The concept of using metal mesh made from metal wires with diameters greater than 0.5 mm can also be used for target bonding for semiconductor wafer processing in PVD chambers. The concept of the invention can be applied to bond sputtering targets to manufacture flat panel displays, solar panels, or electronic devices. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.