Abstract:
Method of forming a two-piece hollow cathode sputter target assembly and the assembly formed thereby. The sputter target assembly includes an outer shell having a substantially cylindrical side wall and is composed of a relatively low purity metallic material. A sputtering insert includes a substantially cylindrical side wall and is concentrically received within, and bonded to, the outer shell. The sputtering insert is composed of a relatively high purity metallic material as used for depositing a thin layer or film onto a desired substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 PCT/US88/28723 filed Dec. 3, 1999, and published under PCT 21(2) in the English language which claims the benefit of U.S. provisional application Serial No. 60/110,765 filed Dec. 3, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods for preparing sputter target assemblies, and to the sputter target assemblies prepared by these methods. In particular, the invention relates to pot-shaped, or hollow cathode, sputter target assemblies, and methods for making such assemblies. 
     2. Description of the Prior Art 
     Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired 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. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode. 
     Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits to form the thin layer or film on the receiving surface of the substrate. 
     The sputter target is heated during the sputtering process by the thermal energy of the bombarding gas ions. In conventional cathode target assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is typically water-cooled to carry away the heat generated by the ion bombardment of the target. 
     One type of knows system for coating substrates by cathodic sputtering includes a cathode having a hollow body with an open end facing the substrate and anode. A series of magnets may be provided around the hollow cathode body for optimizing the ionization through magnetic fields. As such, coating coverage and uniformity on the substrate is improved. Examples of such hollow cathode assemblies are illustrated in U.S. Pat. No. 5,985,115 to Hartsough et al. U.S. Pat. No. 4,966,677 to Aichert et al. and U.S. Pat. No. 5,728,280 to Scherer, both of which are incorporated herein by reference. 
     High purity metal and metal alloys having a purity level greater than that available from commercial grade materials and typically selected form the group including titanium, copper, tantalum, cobalt, tungsten and aluminum, are utilized as the sputtering material in conventional target cathode assemblies. As may be appreciated, such high purity metals and metal alloys are often not readily available and are costly to obtain. Additionally, typical target cathode assembly sputtering materials have relatively high specific gravities resulting in difficulty in their manipulation due to excessive weight. 
     The above described traditional hollow cathode target assemblies are monolithic in design such that the assembly is formed entirely of the costly and often relatively heavy high purity metals and metal alloys. Additionally, such hollow cathode target assemblies are generally inefficient in that a significant portion of the sputtering material remains unused when the assembly requires replacement. 
     Accordingly, there remains a need for a hollow cathode target assembly which increases the efficient use of the expensive high-purity sputtering material and generally reduces the assembly&#39;s overall weight. Additionally. there is a need for a method of producing such a hollow cathode target assembly. 
     SUMMARY OF THE INVENTION 
     The present invention provides a two-piece pot-shaped, or hollow cathode, sputter target assembly including a sputtering insert of high purity sputtering material which is concentrically received with an outer shell of less expensive, lower purity and preferably, lighter weight material. 
     More particularly, the invention contemplates a sputter target assembly comprising a sputtering insert including a substantially cylindrical side wall defining a longitudinal axis and having inner and outer surfaces. One end of the  30  sputtering insert is closed by a substantially planar end wall connected to the side wall and having an inner surface connected to the inner surface of the side wall The opposing end of the sputtering insert is open such that the inner surfaces of the side wall and end wall define a cup-shaped sputtering surface. 
     The sputtering insert is concentrically received within an outer shell. The outer shell includes a substantially cylindrical side wall having inner and outer surfaces. A substantially planar end wall is connected to the side wall at one end of the outer shell and includes an inner surface. The inner surfaces of the side wall and end wall of the outer shell are adapted to mate with the outer surfaces of the side wall and end wall of the sputtering insert along an interfacial area located between the sputtering insert and the outer shell. The interfacial area includes a substantially cylindrical portion extending coaxial to the longitudinal axis. 
     The sputtering insert is preferably composed of a first metallic material selected from the group consisting of high purity titanium, copper, tantalum, cobalt, tungsten, aluminum and alloys thereof. The outer shell is preferably composed of a second metallic material selected from the group consisting of low purity aluminum, copper, steel, titanium and alloys thereof. The first metallic material has a purity significantly greater than that of a respective second metallic material. 
     One embodiment of the method of the present invention for forming the above-described two-piece hollow cathode sputter target assembly includes the steps of forming a blank of a first metallic material into a sputtering insert including a substantially cylindrical side wall and an end wall, the side wall and the end wall defining an outer mating surface and an inner sputtering surface. A blank of a second metallic material is formed into an outer shell including a substantially cylindrical side wall and an end wall, the side wall and end wall defining an inner mating surface. 
     The sputtering insert is positioned concentrically within the outer shell and a substantially cylindrical plug is next positioned concentrically within the sputtering insert. The sputtering insert and plug are then placed within a hot isostatic press can. A closure plate is then secured to the can to form a vacuum tight can assembly wherein residual air is evacuated from the can assembly. The can assembly is then subjected to a predetermined temperature at a predetermined pressure for a predetermined period of time, thereby diffusion bonding the sputtering insert to the outer shell. 
     A first alternative embodiment of the method of the present invention includes the steps of forming the outer shell and sputtering insert as detailed above but in a manner providing for an interference fit at room temperature. More particularly, the inner diameter of the side wall of the outer shell is selected to be less than the outer diameter of the sputtering insert at ambient room temperature. The sputtering insert is preferably provided at a first predetermined temperature no greater than ambient room temperature and the outer shell is heated to a second predetermined temperature above ambient room temperature. Next, the sputtering insert is slidably received within the outer shell. The sputtering insert is then heated, if necessary, and the outer shell cooled, to ambient room temperature wherein the outer shell contracts, thereby providing an interference fit between the side walls of the sputtering insert and the outer shell at room temperature. 
     In a second alternative embodiment of the method of the present invention, a substantially planar sputtering blank composed of a first metallic material having a first mating surface is provided. Likewise, a substantially planar shell blank composed of a second metallic material having a second mating surface is provided. The shell blank is bonded with the sputtering blank to form a blank assembly. 
     Bonding is preferably accomplished by pressing the first mating surface together with the second mating surface at a predetermined temperature below melting points of the first and second metallic materials such that a diffusion bond is formed along the first and second mating surfaces to define a blank assembly. The blank assembly is next formed using traditional metal working methods into a cup-shaped sputter target assembly including an outer shell composed of the shell blank and a sputtering insert composed of the sputtering blank wherein the sputtering insert is concentrically disposed within the outer shell. The sputter target assembly includes an outer cylindrical wall defined by the outer shell and an inner cylindrical wall defined by the sputtering insert wherein the inner cylindrical wall is bonded to the outer cylindrical wall. 
     Therefore, it is an object of the present invention to provide a hollow cathode target assembly which increases material efficiency by reducing the amount of expensive sputtering material which is not fully processed. 
     It is a further object of the present invention to provide a hollow cathode target assembly of a reduced weight, thereby facilitating manipulation thereof. 
     It is another object of the present invention to provide a hollow cathode target assembly which is inexpensive. 
     It is yet another object of the present invention to provide a method of forming such a hollow cathode target assembly by providing a relatively inexpensive outer shell which concentrically receives a high purity sputtering insert. 
     It is a further object of the present invention to provide such a method of forming a hollow cathode target assembly which provides adequate bonding strength. 
     Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the sputtering target assembly made in accordance with the present invention; 
     FIG. 2 is a cross-sectional view of the sputtering target assembly of FIG. 1; 
     FIG. 3 is an exploded perspective view of the sputtering target assembly of FIG. 1; 
     FIG. 4 is a diagrammatical view illustrating a series of steps performed in accordance with a first preferred method of the present invention; 
     FIG. 5 is a diagrammatical view of a hot isostatic press can placed within a hot isostatic pressure chamber, and 
     FIG. 6 is a diagrammatical view illustrating a series of steps performed in accordance with a second preferred method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIGS. 1-3. the sputter target assembly  10  of the present invention includes an outer shell  12  which is bonded to a sputtering insert  14 . The sputtering insert  14  is preferably composed of a first metallic material selected from the group consisting of high purity titanium, copper, tantalum, cobalt, tungsten, aluminum and alloys thereof. This group of first metallic materials have relatively high purity as defined to be greater than that available from commercial grade materials. The particular material utilized for the sputtering insert  14  may be selected from this group based upon a variety of criteria including, but not limited to, etching ability, ease of manufacture, and resistivity. The outer shell  12  is preferably composed of a second metallic material which is selected from the group consisting of relatively low purity aluminum, copper, steel, titanium and alloys thereof. The second metallic material is generally commercial grade and has a purity significantly lower than that of the first metallic material. 
     The outer shell  12  is cup-shaped and includes a substantially cylindrical side wall  16  defining a longitudinal center axis  18 . A first end  20  of the outer shell  12  is closed by a substantially planar end wall  22  connected to the side wall  16  and extending substantially transversely to the longitudinal axis  18 . The side wall  16  includes an inner or fast mating surface  24  and an outer surface  26 . Likewise, the end wall  22  includes an includes or first mating surface  28  and an outer surface  30 . A second end  32  of the outer shell  12  remains open to define an open chamber  33 . 
     The side wall  16  proximate the open second end  32  supports a mounting element comprising an annular flange  34  extending radially outwardly from the side wall  16 . A plurality of mounting bores  36  are circumferentially equally spaced around the mounting flange  34 . An annular groove  38  extends within the mounting flange  34  from proximate the open second end  32  towards the closed first end  20 . A sealing member, preferably an O-ring  40  is received within the annular groove  38  for providing a seal between the outer shell  12  and a wall (not shown) of the sputtering system. 
     An annular recess  42  is likewise formed within the mounting flange  34  and is in communication with the annular groove  38 . The annular recess  42  is adapted to receive an insulating member (not shown) for providing insulation between the outer shell  12  and the wall of the sputtering system. The side wall  16  includes a receiving notch  44  proximate the second open end  32  and extending radially outwardly from the longitudinal axis  18 . 
     The sputtering insert  14  is cup-shaped and concentrically received within the outer shell  12 . The sputtering insert  14  comprises a substantially cylindrical side wall  46  having an inner surface  48  and an outer or second mating surface  50 . A first end  52  of the sputtering insert  14  is closed by a substantially planar end wall  54  connected to the side wall  46  and extending substantially transverse to the longitudinal axis  18 . The end wall  54  includes an inner surface  56  and an outer or second mating surface  58 . The inner surfaces  48  and  56  of the side wall  46  and end wall  54  form a sputtering surface which defines an open chamber  59 . The side wall  46  proximate an open end  60  of the sputtering insert  14  includes a radially outwardly extending lip  62  for engaging the receiving notch  44  of the outer shell  12 . As may be appreciated, the lip  62  provides an extremity for the sputter target assembly  10  consisting solely of the high purity first metallic material of the sputtering insert  14 . 
     The inner or first mating surfaces  24  and  28  of the outer shell  12  are adapted to mate with the inner or second mating surfaces  48  and  56  of the sputtering insert  14  along an interfacial area  64  between the sputtering insert  14  and the outer shell  12 . The interfacial area  64  includes a substantially cylindrical portion  66  extending coaxially with the longitudinal axis  18  (FIG.  2 ). 
     Turning now to FIGS. 4 and 5. a first preferred embodiment of forming the sputter target assembly  10  of the present invention is illustrated. Initially. the sputtering insert  14  is formed from a blank of the first metallic material through traditional metal working methods. Likewise, a blank of the second metallic material is formed into the outer shell  12  by traditional metal working techniques. These traditional metal working techniques preferably comprise a spinning or deep drawing operation. Once the outer shell  12  and sputtering insert  14  are thus formed, they may be machined to appropriate dimensions as required. It should be noted that through experimentation it has been discovered that the final bonded target assembly  10  may shrink during diffusion bonding such that dimensions of the sputtering insert  14  should be adjusted accordingly to allow for such shrinkage. 
     Next, a substantially cylindrical solid plug  68  is inserted within the sputtering insert  14 . The plug  68  is utilized to prevent the outer shell  12  and sputtering insert  14  from deforming or collapsing during subsequent bonding operations. The plug  68  is preferably composed of aluminum, steel or graphite, wherein aluminum is the preferred material due to its thermal properties, ease of machining and expense. Further, the plug  68  is preferably coated with boron nitride to facilitate its removal following subsequent bonding operations. A 0.005 inch gap is preferably provided between the side walls  16  and  46  of the outer shell  12  and the sputtering insert  14 . Likewise, a 0.005 inch gap is preferably provided between the side wall  46  of the sputtering insert  14  and the outer surface  70  of the plug  68 . 
     After the outer shell  12 , sputtering insert  14  and plug  68  are all concentrically disposed, a top closure plate  72  is welded to the outer shell  12  to form a can assembly  74  defining a vacuum tight closure, as shown in FIG.  5 . The closure plate  72  is preferably welded to the outer shell  12  through electron beam welding in a vacuum atmosphere. This process of electron beam welding in vacuum is well known in the art and there are numerous electron beam welders available on the market which may be utilized for this purpose. 
     Next, the can assembly  74  is placed within a hot isostatic press (HIP) chamber  76  and is subjected to an HIP process at a predetermined temperature and pressure for a selected time. The can assembly  74  is typically subjected to equal pressure from all sides by means of a pressurizing gas, usually argon. The particular conditions used for the HIP process are selected to meet the requirements necessary to achieve sound bonds between the outer shell  12  and sputtering insert  14 . 
     In a preferred HIP process, the sputtering insert  14  comprises tantalum and the low purity outer shell  12  comprises aluminum. The can assembly  74  may be subjected to a temperature of approximately 565° C. within a range of ±4° C. at a pressure between 10.17 Mega Pascals (MPa) and 10.35 MPa. It is preferred that these parameters be maintained for ±3 hours 0.1 hours. Hot isostatic pressing methods are described in more detail in U.S. Pat. No. 5,234,487, to Wickersham et al. and U.S. Pat. No. 5,230,459, to Mueller et al., the disclosures of which are incorporated herein by reference. 
     After the HIP process is completed, the closure plate  72  is machined off and the plug  68  removed from within the sputtering insert  14 . The assembly may then be machined, if desired, by conventional means to predetermined dimensions for the final sputter target assembly  10 . 
     A second preferred method of bonding the sputtering insert  14  within the outer shell  12  comprises a shrink fit bonding method wherein an interference is provided between the side walls  16  and  46  of the outer shell  12  and sputtering insert  14  at room temperature. More particularly, the steps for providing the shrink fit bond include providing the sputtering insert  14  at a first predetermined temperature no greater than ambient room temperature. The sputtering insert  14  may be cooled to a temperature below ambient room temperature. The outer shell  12  is heated to a second predetermined temperature above ambient room temperature. The sputtering insert  14  at the first temperature is then concentrically placed within the outer shell  12  at the second elevated temperature. Next, the sputtering insert  14  and the outer shell  12  are returned to ambient room temperature by heating and cooling, as required. The outer shell  12  contracts thereby providing an interference fit between the side walls  16  and  46  of the outer shell  12  and the sputtering insert  14  at room temperature. If the sputtering insert  14  is heated from a first temperature below ambient room temperature, it will expand thereby providing additional interference between the side walls  16  and  46 . 
     In a preferred shrink fit method of bonding, the sputtering insert  14  comprises tantalum and the outer shell  12  comprises low purity aluminum. The sputtering insert  14  and outer shell  12  are formed to define an approximate 0.060 inch interference between the outer diameter of the side wall  46  and the inner diameter of the side wall  16  at ambient room temperature. The sputtering insert  14  remains at ambient room temperature of approximately 24° C., while the outer shell  12  is simultaneously heated to approximately 482° C. for about 1 hour. The sputtering insert  14  is then slidably received within the heated outer shell  12 . The assembly is then air cooled to ambient room temperature. 
     Referring now to FIG. 6, a third preferred embodiment of the method of the present invention comprises providing a substantially planar sputtering blank  78  composed of the first metallic material having a first mating surface  80  and providing a substantially planar shell blank  82  composed of the second metallic material having a second mating surface  84 . The planar shell blank  82  and planar sputtering blank  78  are bonded together to form a blank assembly  86 . 
     In a diffusion bonding method, the first and second mating surfaces  80  and  84  are pressed together at a predetermined temperature below the melting points of the first and second metallic materials such that a diffusion bond is formed along the first and second mating surfaces  80  and  84 . This conventional diffusion bonding method is well known in the art. 
     In a weld bonding method, a weld is formed along the edge of the shell blank  82  and sputtering blank  78  under vacuum conditions. The weld seals a vacuum between the first and second mating surfaces  80  and  84  thereby bonding the blanks  78  and  82  together. It is important that the first and second metallic materials be weld compatible, i.e., exhibit similar material properties. As such, it is believed that a preferred combination comprises a first metallic material of high purity copper (about 5-6 N purity) bonded to a second metallic material of low purity copper (about 4 N purity). 
     The blank assembly  86  so bonded is then formed using conventional metal working techniques into the pot-shaped formed blank assembly  88 . The working technique may comprise spinning and/or deep drawing. The sputtering insert  14  is thus concentrically disposed within the outer shell  12  such that the formed blank assembly  88  includes an outer cylindrical wall  16  defined by the outer shell  12  and an inner cylindrical wall  46  defined by the sputtering insert  14  wherein the inner and outer walls  46  and  16  are bonded together. 
     Next, a ring defining the mounting flange  34  is concentrically disposed over the outer shell  12 . The mounting flange  34  is then welded in place to a projecting portion  90  of the side wall  16 . 
     In this alternative embodiment of the method of the present invention, it is preferred that the first metallic material and the second metallic material possess similar metal forming, particularly work hardening, properties to help maintain the bond between the shell blank  82  and sputtering blank  78  during the forming operation. 
     While the methods herein described and the products produced by these methods constitute preferred 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.