Abstract:
Continuous casting of copper to produce sputter targets ( 20 ). The resulting targets comprise a relatively low amount of inclusions therein and will result in reduced particulate sputtering leading to a more uniform coating that is sputtered onto the desired substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     Priority benefit of U.S. Provisional Application Ser. No. 60/579,746 filed Jun. 15, 2004 is hereby claimed. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention relates to continuous casting of sputtering targets. More particularly, the present invention relates to a continuous casting method for producing sputter targets that will inhibit particle contamination on the desired substrate during the sputtering process.  
       BACKGROUND OF THE INVENTION  
       [0003]     The quality of a thin film formed on a semiconductor wafer or any substrate by a sputtering method can be influenced by the purity level and microstructure of the target material. When contaminants and protrusions in the crystal grain microstructure having a larger size than a certain level are present on the surface of the target material, an abnormal discharge, or arcing can be caused during the sputtering process. This can result in macroparticles being scattered out from the surface of the target material and being deposited onto the substrate. The deposited macroparticles can cause blobs on the thin film, resulting in undesired short circuiting of the semiconductor thin film circuits.  
         [0004]     Several conventional methods are typically used for forming sputtering targets. For example, targets can be formed from powder metallurgy, in which precious metal powder is shaped and pressure consolidated via hot isostatic pressing (HIP) or other methods. HIPing has been shown to be a practical method for producing a target construction; however, due to the fact that the raw material powder easily becomes contaminated or easily absorbs impurities, it is difficult to produce a target material of uniform structure and purity through powder metallurgy unless production conditions, including storage of raw material powder, are very strictly controlled. These additional steps are cumbersome and time consuming, thereby increasing production costs and disadvantageously elevating product pricing.  
         [0005]     Another conventional method that has been proposed for forming a target construction is vacuum induction melting (VIM). VIM was originally developed for processing of specialized and exotic alloys and is consequently becoming more commonplace as these advanced materials are increasingly employed in the sputtering process. The process involves melting of a material under vacuum conditions, usually by means of electron beam bombardment or electromagnetic conduction. The molten material may then be poured or cast under vacuum or inert gas environments. A problem with VIM is that impurities from the refractory material of the crucible make it difficult to maintain the purity of the original material. In this way, conventional VIM technology is problematic in that it is difficult to achieve a high degree of purity, and it is apt to increase the concentration of impurities in the metal. In addition, in some cases, VIM is known to provide undesired cast microstructure and allows higher gaseous concentration in the target material. Moreover, much energy is required for melting a precious metal having a high melting point, and the number of production steps is high, which decreases product throughput and elevates production costs.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with the invention, a copper sputter target is produced that is made by a continuous casting method in which molten copper is continuously cast into a solidified mass. A target blank is formed from the mass and the blank is worked and formed into the desired shape or configuration for use as a sputter target in PVD systems. The artisan will appreciate that references to copper as used throughout the specification and claims refer to copper and all alloyed forms thereof.  
         [0007]     The invention will be further described in conjunction with the appended drawings wherein:  
     
    
     DRAWINGS  
       [0008]      FIG. 1  is a schematic process diagram of the continuous casting method of the invention;  
         [0009]      FIG. 2  is a schematic perspective view of a copper billet in accordance with the invention and target blank separated from the billet via a cutting or other separation technique; and  
         [0010]      FIG. 3  is magnified cross sectional schematic of a target blank produced in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0011]      FIG. 1  is a schematic illustrating process line  2  for forming continuously cast copper. Ladle  4  is fed with molten copper heated to exceed its melting temperature in a vacuum furnace or the like. A reservoir of molten copper is maintained in a tundish  6  having a submerged nozzle  38  for communication with chilled copper or graphite mold  8  as shown. The nozzle  38  is preferably formed from ceramic or graphite.  
         [0012]     As is conventional in the art, the mold  8  may be water cooled so that heat is extracted from the melt so as to solidify in the form of a skin or shell as shown at  36 . This shell is withdrawn from the bottom of the mold at a speed that ultimately correlates to the feed of molten copper through nozzle  38 . Also as is conventional in the art, the mold may be vibrated or oscillated to minimize friction and sticking of the solidifying shell. Lubricants may also be utilized along the interior of the mold to reduce friction. A temporary false bottom (not shown) can be disposed at the downstream end of the mold  8  at process startup to prevent molten copper from escaping before it has a chance to form the required shell or skin.  
         [0013]     Although not illustrated in the drawings, water and/or air sprayers can be conventionally located downstream from the chilled mold  8  to spray cool the solidifying melt as it continues its downstream travel guided by rollers  10  or the like which also may be chilled. Solidified copper  16  is then cut via torch  12  or other conventional means into billets  18  such as those shown in  FIG. 2 . Individual disks or target blanks  20  are then cut from the billet and formed into the desired target shapes by conventional processes such as hydroforming, spinning, deep drawing etc. The artisan will appreciate that in addition to billets, the continuous casting process can also be readily adapted to form blooms, plates, slabs etc. In one exemplary embodiment, cylindrical billets will be formed by the process substantially as shown in  FIG. 1 . The resulting target blank will then be heat treated and rolled. It will then be hydroformed into a substantially cup shaped hollow cathode magnetron target substantially as shown in U.S. Pat. No. 6,419,806 incorporated by reference herein.  
         [0014]     As schematically shown in  FIG. 3 , the target blanks are characterized by their relatively low number of inclusions  22  therein in contrast to conventional stationary casting techniques. These inclusions typically have a maximum size of about 10 microns and the blanks  20  will have a maximum of about 2,000 inclusions of 0.4 microns and above per 2890679.7 um 2 . Sputter targets made from the blanks will exhibit improvement in particulate count found on substrates coated by the targets. Copper layers coated by these targets will exhibit reduced particulate counts.  
         [0015]     The foregoing description of specific embodiments has been made for purposes of illustration only. Those skilled in the art will appreciate that numerous alterations and modifications may be made without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or equivalent thereof.