Abstract:
A variable thickness sputtering target which increases the target material thickness at strategic locations to greatly improve the yield of usable wafers per target, and a method of manufacturing such target comprising forming a generally flat and circularly shaped target blank so that a thickness dimension between the top and bottom surfaces decreases as a function of radius of the target blank. The variable thickness target blank is then formed into a variable thickness dome shaped target member having a bottom portion and a sidewall portion, wherein a wall thickness of said variable thickness dome-shaped target member is thickest proximate a center portion of said bottom portion. In one embodiment of the invention, the variable thickness target blank is formed by clock rolling (or compression rolling) the target blank with crowned rolls to obtain a variable thickness target blank.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/697,501, filed Jul. 8, 2005. 

   FIELD OF THE INVENTION 
   The present invention relates generally to sputtering systems and, more particularly, to a system and method for producing a variable thickness sputtering target for use in a sputtering system. 
   BACKGROUND OF THE INVENTION 
   Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto substrates. Basically, a cathode assembly including the sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. A substrate is positioned in the chamber near the anode with a receiving surface oriented normal to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode causing electrons to eject from the cathode assembly and ionize the inert gas. The positively charged ions of the inert gas are then propelled against a sputtering surface of the sputter target due to the electric field. The ion bombardment against the sputtering surface of the target causes portions of the material of the sputtering surface to dislodge from the sputter target surface and deposit as a thin film or layer on the receiving surface of the substrate at an opposite end of the chamber. 
   Sputtering targets are typically formed as a generally circular disk of target material, such as aluminum, gold, silver, tantalum, copper, titanium, tungsten or platinum and alloys thereof. The disk of target material may be soldered or otherwise bonded to a supporting target backplate to form a replaceable sputtering target assembly. During the sputtering operation, material is sputtered from the top surface of the target and deposited on the wafer. The sputtering material typically erodes unevenly across the width or face of the target exposed to the wafer, with some areas of the target eroding more quickly than other areas. 
   Recently, non-planar sputter targets have been developed to provide improved sputtering and deposition uniformity. For example, open-ended cup-shaped hollow cathode magnetron (HCM) sputter targets have been developed, as disclosed in U.S. Pat. No. 6,419,806 of common assignment herewith, the disclosure of which is incorporated herein by reference. These cup or dome shaped targets are generally comprised of a high purity metallic material as the target surface and they are typically formed from well-known metal-working operations such as hydroforming. The closed end of the cup-shaped target comprises a dome. Sidewalls extend from the dome to an open end of the target. 
   Typically, these cup or dome shaped targets experience the most target material consumption (i.e., erosion) in the dome area with the corner areas that connect the dome to the sidewalls also experiencing considerable erosion but less than that in the dome area. The sidewall areas of the cup shaped target usually experience the least amount of target erosion. 
   SUMMARY OF THE INVENTION 
   The present invention provides a variable thickness cup shape sputtering target which increases the target material thickness at strategic high erosion locations to greatly improve the yield of usable wafers per target. A method of manufacturing a variable wall thickness sputtering target comprises forming a target blank of sputtering material having a top surface, a bottom surface, and a substantially circular outer peripheral surface, and forming the top and bottom surfaces so that a thickness dimension between the top and bottom surfaces decreases as a function of radius of the target blank, wherein the blank is thinner proximate the edge and thicker proximate the center. The variable thickness target blank is then formed into a variable thickness dome shaped target member having a top or dome portion and a sidewall portion, wherein the wall thickness of the dome-shaped target member is thickest proximate the center of the dome portion. 
   In one embodiment of the invention, the variable thickness target blank is formed by clock rolling (or compression rolling) the target blank with crowned rolls to obtain a variable thickness target blank. In another embodiment of the invention, the variable thickness target blank is formed by pressing an outer edge portion of the target blank to make it thinner than an inner, central portion of the target blank. In yet another embodiment of the invention, the target blank is formed by welding a thicker center region of sputtering material into a thinner generally annular shaped outer region of sputtering material, thereby forming a variable thickness blank. The variable thickness target blank is then formed into a variable thickness dome-shaped target member for example by hydroforming, wherein the dome portion of the target member is thicker than the side wall portions. 
   While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a sputtering target made in accordance with the present invention; 
       FIG. 2  is a perspective view of the sputtering target of  FIG. 1 ; 
       FIG. 3A  is a cross-sectional view of a variable thickness target blank made in accordance with an exemplary embodiment of the present invention; 
       FIG. 3B  is a cross-sectional view of a variable thickness target blank made in accordance with another exemplary embodiment of the present invention; 
       FIG. 3C  is a cross-sectional view of a variable thickness target blank made in accordance with yet another exemplary embodiment of the present invention; 
       FIG. 4  is a schematic top plan representation of the step of clock rolling a target blank according to a method of the present invention; 
       FIG. 5  is a cross sectional view of the schematic of  FIG. 4  taken along the plane indicated by the lines and arrows  5 - 5  of  FIG. 4 ; and 
       FIG. 6  illustrates a schematic diagram of a hydroforming press that may be used in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides a method of making a sputtering target. The method includes providing a sputtering target workpiece blank comprising a high purity metallic material at the target surface. Preferably the target is a dome-shaped (or cup-shaped) hollow cathode magnetron (HCM) sputter targets, as disclosed in U.S. Pat. No. 6,419,806 of common assignment herewith, the disclosure of which is incorporated herein by reference. 
   Referring to  FIG. 1 , there is shown a cross-sectional view of an exemplary variable thickness sputter target  10  according to the present invention. For clarity,  FIG. 2  illustrates a perspective view of the dome-shaped variable thickness target of  FIG. 1  As shown in  FIGS. 1 and 2 , the variable thickness sputter target  10  is in the form of a hollow dome or cup comprising sidewalls  6 , corners  4 , mounting flanges  8 , and a top or dome portion  2 . As stated above, quite typically, the life of the target is limited by the target area that experiences the most erosion. In these cup shaped configurations, the most erosion is seen in the area of the dome  2  with the corners  4  that connect the dome to the sidewalls  6  eroding more than the sidewalls but less than the dome. The sputter target  10  is typically formed from a generally flat and circularly shaped target blank  12  as best shown in  FIGS. 4 and 5 . 
   In order to enhance the yield of a variable wall thickness dome-shaped sputtering target  10  having a thicker dome section  2 , it is desirable to form the target from a target blank  14  having a variable wall thickness as shown in  FIG. 3A . This can be accomplished by clock rolling (sometimes referred to as compression rolling) a flat target blank  12  on a mill with crowned rolls  20 ,  22  as shown in  FIGS. 4 and 5 . As shown, the surfaces of the rollers are crowned so that the distance between the roller surfaces at the center  36  of the nip is greater than that at the lateral borders of the rollers  38 ,  40 . In one embodiment, a generally circular shaped target blank  12  is provided. Thereafter, the target blank  12  is clock rolled at a variety of different angular orientations to obtain a variable thickness target blank. The rolled target blank is then water jet cut around the periphery to obtain a generally circularly shaped target blank  12 . 
   In one exemplary clock rolling process, the finished blank ingot size is approximately 34 inches diameter by 0.410 inches thick at the edge  6  and 0.430 inches thick toward the center region  2 . The starting ingot size is approximately six inches diameter by 13.250 inches. A sequence of sixteen roller passes are scheduled at different angular orientations with about an 11.63 percentage reduction per pass. In addition, three small passes (numbered 17-19 in the Table 1 below) are used to obtain the desired diameter. The sequence orientation and thickness parameters for each pass are summarized in Table 1 below. 
   
     
       
             
             
             
           
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Pass No. 
               Dimension 
               Angular Orientation 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               1 
               2.747 
               0° 
             
             
               2 
               2.435 
               90° 
             
             
               3 
               2.159 
               180° 
             
             
               4 
               1.913 
               270° 
             
             
               5 
               1.696 
               45° 
             
             
               6 
               1.503 
               135° 
             
             
               7 
               1.332 
               225° 
             
             
               8 
               1.181 
               315° 
             
             
               9 
               1.047 
               30° 
             
             
               10 
               0.928 
               120° 
             
             
               11 
               0.822 
               210° 
             
             
               12 
               0.729 
               300° 
             
             
               13 
               0.646 
               75° 
             
             
               14 
               0.573 
               165° 
             
             
               15 
               0.508 
               255° 
             
             
               16 
               0.450 
               345° 
             
             
               17 
               0.400 
             
             
               18 
               0.380 
             
             
               19 
               0.370 
             
             
                 
             
           
        
       
     
   
   Thereafter, in one exemplary embodiment the variable thickness round target blank is placed on a mandrel of the type shown in  FIG. 6  and hydroformed to make a dome-shaped target of the type shown in  FIG. 2 . Minimal wall thinning occurs. The thickest regions of the original flat plate remain thickest after the forming operation. Some wall thickening or thinning can be managed by the forming process. As described above, the finished part is radially symmetric, but the thickness is radius dependent; that is, the plate is thickest proximate the center and thinnest proximate the edge. 
   Turning to the  FIG. 6  of the drawings, there is shown a hydroforming press of the type that may be used to form the variable wall thickness cup shaped target of the invention. The variable wall thickness blank  14  such as that shown in  FIG. 3A  is placed over a suitable configured mandrel or punch  130  surrounded by platen  110  in the hydroforming press  100 . A bladder  120  is filled with hydraulic fluid and is positioned in the housing above the variable wall thickness blank  14 . As the artisan will appreciate, the mandrel or punch  130  is configured so as to be congruent with and to form the desired cup shaped target. 
   In practice, the housing is lowered to contact the bladder with the upper surface of the blank. Thereafter, the mandrel is urged upwards through the opening in the annular platen and against a lower surface of the blank. As the mandrel continues to press upwards against the blank, the upper surface of the blank is urged against the bladder. At the same time, the pressure in the bladder increases up to as much as about 10,000 psi. Thus, as the mandrel urges the blank upwards, the pressurized bladder provides resistance until the blank is made to conform to the shape of the mandrel. The mandrel and bladder are retracted and the target is removed from the hydroforming press, for example. The entire hydroforming process may take about 1.5 minutes and is preferably performed at room temperature. Because the process is performed at room temperature, heat induced crystallographic orientation changes of the target are minimized. 
   Other methods that may be used to make sputter targets from the variable thickness blanks such as to those shown in  FIG. 3A  include deep drawing or spinning to obtain a generally circularly symmetric target with a variable wall thickness, wherein the part is thickest near the center of the dome portion  2 . By way of example and not by way of limitation, in one exemplary embodiment the wall thickness at the dome portion  2  can be between about 1.25 and about 4 times the thickness of the upper wall  6  in the finished dome-shaped target  10 . There is some variability from source-to-source and target-to-target, but it is possible to calculate expected target life based on worse case erosion data at each of the areas of interest according to the equation:
 
current target life (kWh)=erodible material (mils)÷erosion rate (mils/kWh).
 
   With hollow dome-shaped targets, we are primarily interested in calculating target life based on the erosion rates at three areas of interest: 
   1. the dome portion of the target; 
   2. the corner portion of the target; and 
   3. the sidewall portion of the target. 
   Exemplary maximum expected erosion rates have been found to be approximately 0.03 mils/kWh near the dome portion of the target, and approximately 0.02 mils/kWh near the corners of the target. Therefore, based on differing erosion rates at each location, we can calculate the optimum thickness required at each location to provide the desired target life in accordance with the above equation. Exemplary target life calculations are summarized in Table 2 below. 
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 2 
             
           
           
             
                 
                 
             
             
                 
               Target Life (kWh) 
                 
             
           
        
         
             
                 
               5000 
               5800 
               7500 
               10,000 
             
             
                 
                 
             
           
        
         
             
                 
               DOME 
               250 
               274 
               325 
               400 
             
             
                 
               CORNER 
               225 
               241 
               275 
               325 
             
             
                 
               SIDEWALL 
               200 
               216 
               250 
               300 
             
             
                 
                 
             
             
                 
               Required thickness in each zone (mils). 
             
           
        
       
     
   
   As described above, hydroforming may also be used to form the desired cup shape target from the variable wall thickness precursor plate. Additionally, in that process a higher pressure could be provided around the sidewall of the target to therefore make it thinner relative to the base or dome portion of the target. Thus, the thinning of the sidewalls could be accomplished during the target shaping or forming step. 
   As shown in  FIG. 3B , the present invention also contemplates forging or pressing the outer edge of the target blank  14 B to make it thinner than the body of the target blank itself. In this way another variable thickness target blank  14 A may be provided. 
   As another option,  FIG. 3C  illustrates a target blank  14 C comprising a thick center region welded into a thinner annular region prior to the rolling operation so as to form a variable thickness welded target blank. 
   In an exemplary embodiment of the invention, the variable thickness target blanks are used to form a sputter target comprising a substantially cylindrical side wall portion, a generally annular dome portion, and an arcuate corner portion adjoining the side wall portion and the dome portion, thereby forming a substantially hollow dome-shaped target member, wherein the wall thickness of the dome portion is thicker than the wall thickness of the corner portion, and wherein the wall thickness of the corner portion is thicker than the wall thickness of the side wall portion. Such an exemplary variable thickness dome-shaped target member may also comprise a flange portion connected to the side wall portion, wherein the flange portion comprises a plurality of apertures for mounting the target member to the sputter reactor. In another exemplary embodiment, a variable thickness dome-shaped target member is provided wherein the thickness of the dome portion is between about 1.25 and about 4 times the thickness of the side wall portion. 
   While the methods herein described and the products produced by these methods constitute exemplary embodiments of this invention, it is to be understood that the invention is not limited to these precise methods and products, and that changes may be made in either without departing from the scope of the invention which is defined in the appended claims.