Abstract:
Sputter target assemblies ( 10 ) and methods of making the sputter target assemblies in which the HIP processes conventionally used are minimized, or eliminated, while producing higher yields of sputter target assemblies in less time. In one instance the sputter target assemblies include a single, or multiple, layered interlayer ( 14, 16 ) between the target and backing plate ( 18 ) in order to achieve intermetallic diffusion bonds between adjacent layers during a single HIP process. A mechanical interlock between the target ( 12 ) and backing plate is also achieved preferably during a single HIP process. In another instance, the target and backing plate are welded directly together by electron beam welding, and the interlayer and HIP process are omitted. In either case, the process for making the sputter target assembly is shortened, rendering it less expensive and subject to less failures, while achieving assemblies having robust strength.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/388,780, filed Jun. 14, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     This invention relates to sputter target assemblies and methods of making the same.  
         [0004]     2. Description of Related Art  
         [0005]     Sputter targets of high purity metals or metal alloys attached to backing plates are typically used to deposit thin films on substrates such as, for example, semiconductor devices. In some methods, high purity metal and metal alloy sputter targets historically have been bonded to backing plates by a two step diffusion bonding process. The two step operation requires, for example, diffusion bonding a foil to the target by subjecting the foil/target combination to hot isostatic pressing (HIP). Thereafter, the diffusion bonded foil/target is machined, if desired, and diffusion bonded to the backing plate by another HIP process. Other techniques include separately soldering the foil/target combination to the backing plate.  
         [0006]     A variety of bond types and structures are shown for example in U.S. Pat. No. 6,376,281; WO 98/41669; U.S. Pat. Nos. 5,693,203; and 5,224,556.  
         [0007]     It is preferable to minimize the amount of processing a sputter assembly is subjected to. It is similarly preferable to produce sputter target assemblies in less time than is achieved using conventional methods. Even further still, it is preferable to provide sputter target assemblies having robust bond strength while minimizing assembly production time and effort.  
       SUMMARY OF THE INVENTION  
       [0008]     One aspect of the invention pertains to a sputter target assembly comprised of a target, an interlayer, and a backing plate that, in one aspect of the invention, are bonded together during a single HIP process. The interlayer is thus placed between the target and backing plate and diffusion bonded to the adjacent target and backing plate materials. The interlayer may be a single layer comprised of a metal alloy, for example, or may be multiple layers each comprised of a distinctly different material. The target and backing plate interface at a substantially single level, or may interface at multiple levels, depending on the formations of the target and backing plate. In either case, the interlayer forms intermetallic diffusion bonds between adjacent layers.  
         [0009]     In an especially preferred embodiment, the invention separately provides a target comprised of tantalum, a first interlayer comprised of aluminum adjacent the target, a second interlayer comprised of titanium adjacent the first interlayer, and a backing plate comprised of copper, or alloy thereof, adjacent the second interlayer. The adjacent layers are subjected to a single HIP process, whereby the adjacent layers diffusion bond to one another to form a robust sputter target assembly.  
         [0010]     This invention separately provides a sputter target assembly comprising a mechanical bond formed between the target and backing plate, in addition to the diffusion bonds between adjacent layers, to further secure the sputter target assembly together. A central stud is provided on one of the target and backing plate and fits into a corresponding recess provided in the other of the target and backing plate. The recess form a negative or re-entrant angle due to outwardly flaring side walls of the recess extending through the thickness of the target or backing plate the recess is provided in. The negative angle is filled with material during HIP processing to form the mechanical interlock between the target and backing plate. Similar negative angles are provided along a perimeter of each level of the target or backing plate that similarly fill with material to form additional mechanical interlocks between the target and backing plate during HIP processing. The resulting sputter target assembly thus comprises intermetallic diffusion bonds between the target, the interlayer, and the backing plate, as well as mechanical interlocks between the target and the backing plate. In various exemplary embodiments of the invention, the target or backing plate having the negative angles formed therein is a single level, whereas in other exemplary embodiments of the invention the target or backing plate having the negative angles formed therein is comprised of multiple levels.  
         [0011]     In still other exemplary embodiments of the invention, the sputter target assemblies formed by the single HIP processing may comprise targets and backing plates having corresponding grooves providing increased contact surface area between adjacent layers. An increased amount of intermetallic diffusion bonds thus form between adjacent layers due to the increased contact surface area.  
         [0012]     Another aspect of the invention relates to a sputter target assembly comprised of a target and a backing plate welded directly to one another by electron beam welding. The electron beam welding causes a weld bond to occur between the materials of the target and the backing plate. The weld bond may occur, for example, at the outer perimeter of the target and backing plate. The otherwise immiscible materials comprising the target and backing plate become miscible in a liquid state when subjected to the electron beam welding, thereby permitting the weld bond to form between the target and backing plate. In addition, grooves provided on the target and backing plate are pressed together and help to further secure the target and backing plate to one another as well.  
         [0013]     These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Various exemplary embodiments of the systems and methods of this invention will be described in detail with reference to the following figures, wherein:  
         [0015]      FIG. 1  illustrates a first exemplary embodiment of a sputter target assembly made in accordance the invention;  
         [0016]      FIG. 2  illustrates an exploded view of the sputter target assembly of  FIG. 1 ;  
         [0017]      FIG. 3  illustrates the sputter target assembly of  FIG. 1  wherein the sides of the interlayer foils are not exposed;  
         [0018]      FIG. 4  illustrates a second exemplary embodiment of a sputter target assembly according to the invention having diffusion bonding between interlayers and a mechanical interlock between multiple levels;  
         [0019]      FIG. 5  is a third exemplary embodiment of a substantially single level sputter target assembly having diffusion bonds and mechanical interlocks;  
         [0020]      FIG. 6  illustrates an exemplary target having grooves and ridges for making a sputter target assembly in accordance with the invention;  
         [0021]      FIG. 7  illustrates an exemplary backing plate having grooves and ridges corresponding to the grooves and ridges of the target of  FIG. 7 ; and  
         [0022]      FIG. 8  illustrates a fourth exemplary embodiment of a sputter target assembly made with a weld bond in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0023]      FIG. 1  shows a sputter target assembly  10  according to a first exemplary embodiment of the invention. The sputter target  10  is formed by a target  12 , a first interlayer  14 , a second interlayer  16 , and a backing plate  18 . As shown in  FIG. 1 , the first layer is adjacent the target and the second interlayer, and the second interlayer is adjacent the backing plate and the first interlayer. The first interlayer is comprised of a material that diffusion bonds to the target and the second interlayer, whereas the second interlayer is comprised of a material that diffusion bonds to the first interlayer and the backing plate. The sides of the various layers are exposed in  FIG. 1  to illustrate more clearly the various layers of materials comprising this embodiment of the invention. When fully assembled, the first and second interlayers  14 ,  16  of the sputter target assembly are not exposed, as shown in  FIG. 3 .  
         [0024]      FIG. 2  is an exploded view of the components of the target assembly  10  shown in  FIG. 1 . More particularly,  FIG. 2  shows that the diameters d 1  of the backing plate and the diameter d 2  of the target are approximately the same, whereas the diameters d 3  and d 4  of the first and second interlayers, respectively, are less than the diameters d 1  and d 2  of the target and backing plate. Thus, the relationship of the various layers of the embodiment shown in  FIG. 2  is (d 1 =d 2 )&gt;(d 3 =d 4 ). Although the thickness of the interlayer in  FIGS. 1 and 2  may vary, a preferred thickness of the first interlayer is 0.015 inches, and a preferred thickness of the second interlayer is 0.001 inches.  
         [0025]     The target and backing plate shown in  FIGS. 1 and 2  are each shown as a substantially single level such that the interface between the target and backing plate occurs at the plane formed by the mating surfaces and the interlayer when the target and backing plate are joined. Thus the first and second interlayers generally overlie substantially the entire mating surface of the respective target and backing plate. The adjacent layers thus assembled, are placed in a HIP can and subjected to a single HIP process. As a result of the HIP process, diffusion bonds form between the adjacent layers in order to form one exemplary embodiment of the sputter target assembly according to the invention.  
         [0026]     Although the sputter target assembly described with respect to  FIGS. 1 and 2  show first and second interlayers, the assembly may also be formed using a single interlayer. If the interlayer is a single layer, it is preferably comprised of a metal alloy that will form diffusion bonds, ideally equally well, with the adjacent target and backing plate materials. This contrasts from the first and second interlayers shown in  FIGS. 1 and 2  that are each comprised of distinctly different materials such that the first interlayer forms diffusion bonds with the target and the second interlayer, and the second interlayer forms diffusion bonds with the backing plate and the first interlayer. Stated differently, in the embodiment shown in  FIGS. 1 and 2 , neither of the first and second interlayers form diffusion bonds directly with both the target and the backing plate, whereas when a single interlayer is used, the single interlayer must diffusion bond directly to both the target and the backing plate. In either case, however, the single or multiple layered interlayer forms intermetallic diffusion bonds between adjacent layers.  
         [0027]     In a preferred embodiment of the invention, the target is comprised of tantalum, the first interlayer is comprised of aluminum, the second interlayer is comprised of titanium, and the backing plate is comprised of copper, or an alloy thereof, for example copper-1% chromium or copper-zinc. Previous experience has shown that tantalum is separately successfully diffusion bonded to aluminum, aluminum is separately successfully diffusion bonded to titanium, and titanium is separately successfully diffusion bonded to copper-1% chromium. Thus, the preferred embodiment of the invention combines these materials in adjacent layers to diffusion bond a tantalum target to a copper-1% chromium backing plate in one step. The standard HIP process for Ti/Al6061 diffusion bonding, for example, may be used.  
         [0028]     As a result of diffusion bonding the adjacent layers comprised of tantalum-aluminum-titanium-(copper-1% chromium) as in the preferred embodiment, brittle Al/Cu compounds between the aluminum first interlayer and the backing plate are less likely to occur even if ductile fractures in the aluminum interlayer were to occur, for instance. Rather, as shown in  FIG. 4 , the titanium second interlayer  16  remains in tact, minimizing the likelihood of producing such Al/Cu compounds between the aluminum interlayer  14  and backing plate  18 .  
         [0029]     Maintaining the integrity of the titanium interlayer is important to minimize, or ideally to prevent, contacting the aluminum interlayer, for example, with the copper backing plate. Contact of the aluminum interlayer with the copper backing plate would weaken the bond strength of the sputter assembly as a result of the brittle Al/Cu compounds that would form in the absence of the titanium interlayer, for example. The preferred embodiment of the invention therefore provides a sputter target assembly with increased strength and stability using a single HIP process as a result of the adjacent tantalum-aluminum-titanium-copper backing plate layers.  
         [0030]      FIG. 4  illustrates another exemplary embodiment of the sputter target assembly according to the invention. The assembly shown in  FIG. 4  is comprised of a multi-level target  12  diffusion bonded to a multi-level backing plate  18  having first and second interlayers  14  and  16  therebetween. As shown in  FIG. 4 , the diameter d 1  of the backing plate is slightly larger than the diameter d 2  of the target, the diameter d 2  of the target is slightly larger than the diameter d 3  of the first interlayer, and the diameter d 3  of the first interlayer is slightly larger than the diameter d 4  of the second interlayer. Thus the relationship of the various layers in the sputter target assembly of  FIG. 4  is d 4 &lt;d 3 &lt;d 2 &lt;d 1 .  
         [0031]     The backing plate  18  of  FIG. 4  is provided, for example, with three layers  20 ,  21 ,  22  and the target is provided, for example, with three levels  30 ,  31 ,  32 . Level  21  of the backing plate is recessed from the mating surface of the backing plate to form a cavity in which the first interlayer  14  and level  31  of the target is received. Level  22  of the backing plate is recessed even further from the mating surface of the backing plate to form a cavity in which the second interlayer  16  and level  32  is received. The backing plate is provided with a central stud  25  projecting through holes in each of the first and second interlayers and into a recess  35  extending into the target through level  32 . Side walls  36  of the recess  35  flare outwardly to form a negative angle into which backing plate materials will flow during HIP processing. The perimeters of each level  21  and  22  of the target are similarly negatively or re-entrantly angled, and will be similarly filled with molten materials during HIP processing. The filling of the negative angles formed in the recess and perimeters of the target levels during HIP processing form mechanical interlocks between the target and backing plate. When fully assembled, the various levels fit flush with one another such that the assembly appears as shown in  FIG. 3 .  
         [0032]     As in earlier described embodiments, preferably the target is comprised of tantalum, the first interlayer is comprised of aluminum, the second interlayer is comprised of titanium, and the backing plate is comprised of copper, or an alloy thereof, preferably copper-1% chromium or copper-zinc. As a result, a multi-level sputter target assembly may be achieved as shown in  FIG. 4 , wherein diffusion bonds form between the adjacent layers during HIP processing in order to form the desired sputter target assembly. As also described earlier, the first and second interlayers may instead be comprised of a single layer comprised of a metal alloy that forms bonds directly with the various levels of the target and backing plate, ideally equally well. In either case, the assembly includes both diffusion bonds and mechanical interlocks between adjacent layers.  
         [0033]      FIG. 5  shows another exemplary embodiment of a sputter target assembly according to the invention. The embodiment shown in  FIG. 5  is comparable to that shown in  FIG. 4 , except that the target  12  is a single level, level  31 , in  FIG. 5 , rather than multiple levels, levels  31  and  32 , as in  FIG. 4  and only a single interlayer  14  is used in the embodiment shown in  FIG. 5 . The negative angled recess and negative angled perimeter at the single level (level  31 ) of the target in  FIG. 5  are similarly filled with material during HIP processing to achieve mechanical interlocks in addition to the diffusion bonds formed between the adjacent layers as in the earlier described embodiment shown in  FIG. 4 .  
         [0034]     The diffusion bonding that occurs between the target and backing plate materials in all of the exemplary embodiments described thus far is achieved due to the materials used to comprise the various layers, and due to the time, temperature and pressure conditions of the HIP process and materials to join together through diffusion bonding and mechanical interlocking. In addition to the chemical nature of the intermetallic diffusion bonds formed exclusively between the adjacent target, first interlayer, second interlayer, and backing plate layers in some embodiments of the invention, mechanical interlocks between the target and backing plate also occurs in other embodiments of the invention as the heated plastic materials cool and harden around the negative angles. The combination of the intermetallic diffusion bonding and mechanical interlocking provides a robust strength to the sputter target assembly that is accomplished relatively quickly with a single HIP process.  
         [0035]      FIGS. 6 and 7  show a modification to the exemplary embodiments shown in  FIGS. 1-5 , wherein the target  12  ( FIG. 6 ) and backing plate  18  ( FIG. 7 ) are provided with corresponding grooves  40  and ridges  41  on those respective sides of the target and backing plate that are adapted to mate with the interlayer(s). The ridges  41  may be slightly larger than the width of the grooves  40  so as to provide an interference fit when the target and backing plate are pressed together, if desired. More importantly, however, the grooves and ridges increase the contact surface area between the adjacent layers of the sputter target assembly. Thus, in the embodiment shown in  FIGS. 1-5 , the sputter target assembly tends to have more intermetallic diffusion bonds due to the increased surface area provided by the grooves and ridges, thus rendering a still stronger assembly using a single HIP process. The grooves and ridges may be concentric as shown in  FIGS. 6 and 7 , however, the artisan will appreciate that the grooves and ridges need not be concentric. Rather, any pattern increasing the contact surface area between the adjacent layers that does not inhibit the diffusion bonding and mechanical bonding desired in the various exemplary embodiments discussed is contemplated.  
         [0036]     Although the artisan will appreciate that the target, first and second interlayers, and backing plate may be comprised of many alternative combinations of materials to achieve the intermetallic diffusion bonds between the adjacent layers, the exemplary materials discussed herein with respect to the first and second exemplary embodiments of the invention comprise a Ta target, an Al first interlayer, a Ti second interlayer, and a Cu-1% Cr backing plate. Of course, the artisan will appreciate that the first and second interlayers comprised of distinctly different materials, may instead be a single interlayer comprised of a metal alloy, such as, for example, silver-copper-tin or silver-copper-tin-zinc. The single metal alloy interlayer would thus lie between the target and backing plate. The artisan will also appreciate, with respect to those embodiments having a mechanical interlock, that the stud and recess are a corresponding pair that may instead be provided in inverse order on the target and backing plate provided the corresponding pair exists between the target and backing plate, and the various layers may be inversely oriented on the other of the target and backing plate provided corresponding cavities are provided to accommodate the different adjacent layers is provided to form the assembly with the intended mechanical interlocks in those embodiments.  
         [0037]     The general method for forming the sputter target assembly of the first and second embodiments is generally as follows: 
        a. provide a backing plate comprised of a first material and a mating surface;     b. provide a target comprised of a second material and a mating surface;     c. provide an interlayer between the target and backing plate, the interlayer being comprised of a material different than the first and second material;     d. place the target, interlayer, and backing plate as adjacent layers into a HIP can and subject the adjacent layers preferably to a single HIP processing step to form an assembly;     e. form intermetallic diffusion bonds between the adjacent layers; and     f. remove the assembly from the HIP can.        
 
         [0044]     Of course, the target and backing plate provided in steps a and b may be a multi-level combination wherein the variously diametered adjacent layers are accommodated in corresponding levels of one of the target and backing plate. In those embodiments requiring the mechanical interlock, a central stud and corresponding recess is provided on the target and backing plate, and the interlayer(s) is provided with the necessary hole(s) to accommodate the central stud passing therethrough to seat into the recess. The perimeter of each layer of the target, for example, is also negative angled. The mechanical interlock is thus formed between steps e and f above. The interlayer provided in step c may be comprised of multiple layers of different materials. In addition, the target and backing plate may be provided with grooves and ridges to increase the surface area whereat intermetallic diffusion bonds are formed between the various layers during the HIP processing.  
         [0045]      FIG. 8  shows another exemplary embodiment of a sputter target assembly  100  according to the invention. The sputter target assembly  100  is comprised of a target  112  and a backing plate  118 . Thus, the interlayer between the target and backing plate, as in the earlier-described embodiments, is omitted in the embodiment shown in  FIG. 8 .  
         [0046]     The target  112  and backing plate  118  may be provided with corresponding grooves and ridges similar to those shown in  FIGS. 5 and 6 . However, the target  112  and backing plate  118  of the third embodiment are not provided with the central stud and recess described in the first and second embodiments.  
         [0047]     The target  112  and backing plate  118  of the third embodiment are bonded together by electron beam welding. Preferably, the weld bonding occurs such that the outer perimeters of the target and backing plate are welded together. The electron beam welding liquefies the otherwise immiscible materials comprising the target and backing plate, and welds he target and backing plate together.  
         [0048]     In addition, the corresponding grooves and ridges provided on the target and backing plate are pressed together to form an interference fit between the target and backing plate when the target and backing plate are pressed together. As discussed before, the artisan will appreciate that the grooves and ridges may be, but need not be, concentric about the mating surface of the target and backing plate. Rather, the grooves and ridges may be any pattern corresponding to one another so as to achieve the desired interference fit between the target and backing plate in addition to the weld bonding of the third exemplary embodiment. Thus, the third exemplary embodiment omits the HIP processing and the interlayer(s), while still yielding a sputter target assembly of robust strength as a result of the weld bonding and interference fit that occurs.  
         [0049]     The method for forming the sputter target assembly of the third embodiment is generally as follows: 
        a. provide a target comprised of a first material and a mating surface;     b. provide a backing plate adjacent the target, the backing plate being comprised of a second material having a mating surface;     c. press the mating surfaces of the target and backing plate together; and     d. subject the target and backing plate assembly to electron beam welding to weld the first and second materials of the target and backing plate.        
 
         [0054]     As stated earlier with respect to the first and second embodiments, the artisan will appreciate that the target and backing plate may be comprised of many alternative combinations of materials to achieve the diffusion bonds and interference fit between the target and backing plate, although the description of the third exemplary embodiment contemplates, for illustrative purposes only, that a Ta target  112  and a Cu-1% Cr backing plate  118  are used. Grooves and ridges, or other patterned mating surfaces, may be provided on the target and backing plate to achieve interference fit between the target and backing plate in addition to the weld bonding of step d. Further, step d preferably welds the target and backing plate along an outer perimeter thereof.  
         [0055]     While this invention has been described in conjunction with the specific embodiments described above, it is evident that many alternatives, combinations, modifications, and variations are apparent to those skilled in the art. Accordingly, the preferred embodiments of this invention, as set forth above are intended to be illustrative, and not limiting. Various changes can be made without departing from the spirit and scope of this invention.