Abstract:
A sputtering target and a backing plate are diffusion-bonded with or without an insert or inserts interposed there-between so as to have a solid phase diffusion-bonded interface. The sputtering target substantially maintains its metallurgical characteristic and properties even though it has been diffusion-bonded to the backing plate. The solid-diffusion bonding of the target and backing plate, is achieved at a low temperature and pressure and results in interdiffusion of constituent atoms to attain high adhesion and bond strength without attendant deterioration or large deformation of the target material, while inhibiting the crystal growth in the target material. The bond undergoes no abrupt decrease in bond strength upon elevation of the service temperature. One hundred percent bonding is achieved with non-bonded portions such as pores left along the interface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   Priority filing benefit of (1) International PCT application PCT/USO1/49126 filed Dec. 17, 2001, and published under PCT 21(2) in the English language and (2) U.S. provisional application Ser. No. 60/255,873 filed Dec. 18, 2000 is hereby claimed. 

   FIELD OF THE INVENTION 
   The present invention relates to sputter target assemblies for use within sputtering systems. More specifically, the present invention pertains to sputter target assemblies which include a target material bonded by low temperature pressure consolidation to a backing plate material with a mechanical bond around the periphery thereof. 
   BACKGROUND OF THE INVENTION 
   Cathodic sputtering is widely used for the deposition of a thin film, or layer, of material onto desired substrates. The sputtering process employs gas ion bombardment of a target material having a face formed of a material that is to be deposited as a thin film, or layer, on the given substrate. Ion bombardment of the target material not only causes atoms or molecules of the target material to be sputtered, it imparts considerable thermal energy to the sputter target assembly. This heat is typically dissipated by use of a cooling fluid circulated beneath, through or around a thermally conductive, non-magnetic, backing plate material that is positioned in heat exchange relation with the target material. 
   The target material and backing plate material form a part of a cathode assembly which, together with an anode, is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target material surface, dislodge the target material. The dislodged target material traverses the evacuated enclosure and deposits, as a thin film, or layer, on the given substrate. The substrate is normally located proximate the anode within the evacuated chamber. 
   In addition to the use of an electrical field, increased sputtering rates have been achieved by concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target material surface thereby increasing the number of electron-gas atom collisions in the annular region to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e., consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target raceway. Magnetron sputtering imposes considerable thermal energy upon the sputter target assembly, especially within the concentrated annular region of the target raceway. The corresponding temperature variations within the target assembly induce forces which encourage separation of the target material from the backing plate material; this is especially true with regard to the peripheral zone of the target assembly. 
   In order to achieve good thermal and electrical contact between the target material and backing plate material, these members are commonly bonded to one another by means of soldering, brazing, diffusion bonding, clamping and by epoxy cement and the like. These bonding methods typically involve imposition of high temperatures. Sputter target assemblies bonded by these methods can bow or bend at high sputtering rates, especially when a large difference exists between the coefficients of thermal expansion for the target material and backing plate material. In many cases, in order to minimize bowing and bending at high sputtering conditions, target materials and backing plate materials having similar thermal expansion coefficients are utilized. In sputter target assemblies with internal cooling channels, bowing and bending induces leakage; the typical bonding methods, described above, tend to deform, or otherwise partially constrict, the cooling channels. Additionally, known bonding techniques, as discussed above, result in undesirable grain growth in the target material or the resulting bond cannot withstand the stresses imposed by high sputtering rates. 
   In some prior art target assemblies, projections formed in either the interfacial surfaces of the target material or backing plate material are received in corresponding recesses or grooves of the other member to improve thermal conductivity. For example in Hillman U.S. Pat. No. 4,885,075, annularly arranged members of high thermal conductivity protrude from either the target material or backing plate material into a corresponding recess formed in the other member. Upon heating of the target assembly during sputtering, the protruding members expand radially to make contact with the recess wall to thereby improve heat transfer between the target material and the backing plate material. 
   Elastic and plastic deformation of backing plate members in target assemblies is minimized in accordance with U.S. Pat. No. 5,269,899 (Fan), of common assignment herewith, by the provision of mating teeth like projections disposed along the target material/backing plate material interfacial surfaces, with the mating surface of the backing plate material having a concave surface. Here again, upon heating of the target assembly, the target material teeth expand radially and make snug contact with the mating teeth on the backing plate material to enhance thermal conductivity. 
   U.S. Pat. No. 5,230,459 (Mueller et al.) of common assignment herewith teaches the use of diffusion bonding methods wherein one of the target material/backing plate material mating surfaces is first prepared with grooves or the like. During the diffusion bonding process, jagged portions of the grooves penetrate into the metal from the opposed interfacial surface and disrupt the formation of oxide and other bond inhibiting layers that may otherwise form along the target material/backing plate material interface to thereby improve bonding efficacy. The method results in a strong bond with increased shear resistance such that the target assembly can withstand thermal stress that otherwise may tend to result in debonding or target assembly warpage during use. 
   As stated above, the induced separation forces tend to concentrate adjacent a peripheral zone of the target assembly. Prior art joining techniques employ a conventional weld around the periphery of the interface of the target material and backing plate material. For the reasons mentioned above, conventional welding techniques, even along only the periphery zone, are undesirable. 
   Therefore, there remains a need in the art of sputter target assemblies for a method of bonding the target material to the backing plate material which will withstand the stresses imposed by high sputtering rates, will allow for use of materials with dissimilar thermal expansion characteristics, and will not induce grain growth or cooling channel deformation. Specifically, improvement in the bond strength at the periphery is desirable. 
   SUMMARY OF THE INVENTION 
   The primary objective of the present invention is to provide low temperature pressure consolidation methods for bonding target material to backing plate material wherein the methods produce bonds capable of withstanding the stresses imposed by high sputtering rates. The preferred sputter target assemblies in accordance with the present invention are comprised of target materials and backing plate materials having dissimilar thermal expansion coefficients and incorporate a peripheral mechanical bond joining the target material and backing plate material. 
   Although the preferred target material and backing plate material have dissimilar thermal expansion coefficients, the bonding methods of the present invention allow these unconventional assemblies to be bonded together and used at high sputtering rates without significant bowing or bending and without separation adjacent the periphery. Additionally, the low temperature pressure consolidation methods of the present invention do not cause undesirable grain growth of the target material during bonding of the target material with the backing plate material. The bonding methods of the present invention have been found to be particularly useful with regard to joining titanium (Ti) materials with aluminum (Al) materials; welding of such materials to one another imparts formation of brittle intermetallic compounds. 
   A second objective of the present invention provides for simplification of joining the target material and backing plate material free of conventional welding or high temperature bonding techniques. In most cases, the circumferentially extending mechanical bond in accordance with the present invention along the periphery zone provides sufficient resistance to separation. In cases where increased bond strength is desired, a series of interlocking protruding portions are formed in the target material or backing plate material mating surface and become imbedded in the mating material subsequent to being low temperature pressure consolidated. In an alternate embodiment, grooves can be formed in the opposing mating surface, designed to receive the protruding portions and provide additional mechanical interlocking, with additional bonding strength, between the target and backing plate material. 
   After the target material and backing plate material are bonded together by the low temperature consolidation methods as described above, the target assembly is machined to the desired dimensions. 
   Other advantages and benefits of the present invention will become apparent with further reference to the appended drawings, the following detail description and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a cross sectional view of the preferred target assembly in accordance with the present invention prior to the low temperature pressure consolidation step; 
       FIG. 2  depicts a cross sectional view of the target assembly of  FIG. 1  subsequent to the low temperature pressure consolidation step; 
       FIG. 3  depicts a cross sectional view of a target assembly in accordance with the present invention with a substantially rectangular shaped circumferentially extending lip subsequent to the low temperature pressure consolidation and machining steps; 
       FIG. 4  depicts a cross sectional view of the preferred get assembly in accordance with the present invention with a substantially triangular shaped circumferentially extending lip subsequent to the low temperature pressure consolidation and machining steps; 
       FIG. 5  depicts a cross sectional view of a target assembly in accordance with the present invention showing mechanical interlocking protruding portions and grooves prior to the low temperature pressure consolidation step; 
       FIG. 6  depicts a cross sectional view of the target assembly of  FIG. 3  subsequent to the low temperature pressure consolidation step; 
       FIG. 7  depicts a cross sectional view of a target assembly in accordance with the present invention, having “M” shaped protrusions and addition of interposing material adjacent the periphery, prior to the low temperature pressure consolidation step; and 
       FIG. 8  depicts a cross sectional view of the target assembly of  FIG. 7  subsequent to the low temperature pressure consolidation step. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring initially to  FIG. 1 , target assembly  5  in accordance with the present invention is depicted prior to the target material  10  being low temperature pressure consolidated with backing plate material  15 . As shown, the target material is first formed to be substantially disc shape and the backing plate material is formed to receive the target material within. Exemplary target materials comprise, Al, Cu, Ti, Co, and their alloys. Exemplary backing plates may comprise Al, stainless steel, Cu, Ti, and their alloys. 
   The target material  10  is formed to include a mating surface  20  with tapered grooves  30  and a side portion  24 . The target material is further formed to include a lip  25  extending from the side portion  24 . The lip  25  is substantially rectangular in cross section with tip  26  defining the outer most edge of the target material in a radial direction. 
   The backing plate material  15  is formed to include a mating surface  35  with protruding portions  45  extending therefrom. The backing plate material  15  further includes a clamp  40  with corresponding retaining surface  60 . The clamp  40  includes a base portion  70  with wall  71  and steps  55 . 
   As shown with dashed lines  65 , the target material  10  and backing plate material  15  are formed such that the tip  26  aligns with wall  71  and the protruding portions  45  align with corresponding grooves  30 . The protruding portions  45  and grooves  30  can extend parallel to the corresponding mating surface  20 ,  35  to form circular patterns, rows or combinations thereof. The preferred steps  55  facilitate formation of the mechanical bond, in accordance with the present invention as described herein. 
   The preferred mechanical bond in accordance with the present invention is formed with application of force parallel to wall  71  only. The deformed steps  55   a , as shown in phantom in  FIG. 1 , engage the retaining surface  60  such that force exerted parallel to wall  71  will induce deflection of wall  71  toward side portion  24 . Optionally, in accordance with the present invention, a secondary force may be exerted perpendicular to wall  71  to impart deflection of wall  71  toward side portion  24 . Various shaped clamps can be employed in accordance with the present invention, most preferably, the clamp is designed to induce deflection of wall  71  toward side portion  24  with application of force parallel to wall  71  only. Formation of the mechanical bond in accordance with the present invention, utilizing application of force parallel to wall  71  only, simplifies the required manufacturing process and limits the required equipment. 
   Turning now to  FIG. 2 , the clamp  40  is depicted to have cooperated with the lip  25  to form a mechanical bond around the periphery of the target material  10 . The target and backing plate materials have been subjected to low temperature pressure consolidation such that the deformed steps  55   a  of deformed clamp  40   a  engage retaining surface  60 . Upon being subjected to low temperature pressure consolidation, the deformed base portion  70   a  deflects such that deformed wall  71   a  engages side portion  24 , trapping the deformed lip  25   a  stherebeneath. 
   The deformed tip  26   a  of the deformed lip  25   a  is forced, slightly, into the backing plate material. The combination of the deformed wall  71   a  engaging side portion  24  and the deformed base portion  70   a  trapping the deformed lip  25   a  forms the desired mechanical bond around the periphery of the target material  10  and the backing plate material  15 . The substantially rectangular lip  25 , as shown in  FIG. 3 , becomes deformed such that tip  26   a  of the deformed lip  25   a  is substantially embedded into the backing plate material. 
   Turning now to  FIG. 4 , target assembly  105  is a preferred embodiment in accordance with the present invention with a deformed lip  125   a  which is substantially triangular in cross section. With further reference to  FIG. 4 , it can be that the target material  110  with mating surface  120  and backing plate material  115  with mating surface  135  have been subjected to low temperature pressure consolidation such that deformed protruding portion  145   a  has been received within the corresponding groove such that material  145   b  is trapped within. The deformed substantially triangular lip  125   a  has been trapped beneath the deformed base portion  170   a  such that the tip  126   a  is not embedded within backing plate material  115  to the degree that tip  26   a  is embedded into backing plate material  15 . 
   The lip can be formed to include a number of various cross sectional shapes in accordance with the present invention. Various shaped lips have been found to result in more or less deflection in the tip and embedment within the backing plate material. It has been found that excessive deflection may cause cracking of the backing plate material adjacent the tip, therefore, the optimum lip cross sectional shape will depend upon the given materials. 
   Target assemblies in accordance with the present invention may be produced free of protruding portions and free of grooves; however, incorporation of protruding portions and, or, grooves imparts improved bond strength, electrical conductivity and thermal conductivity between the target material and backing plate material. 
   With further reference to  FIG. 2 , target assembly  5  is shown subsequent to the target material  10  and backing plate material  15  being low temperature pressure consolidated. As can be seen, mating surfaces  20 ,  35  engage one another with mechanical interlocks  45   a  being formed from the reception, and deformation, of protruding portions  45  within tapered grooves  30 . 
   As can be seen additionally referring to  FIGS. 5 and 6 , the preferred protruding portion  45  shape has a substantially triangular shaped tip  46 . The preferred tapered groove  30  shape, as shown in  FIGS. 1-6 , is trapezoidal. With the preferred shaped protruding portions  45  and grooves  30 , the deformed protruding portion  45   a  will mate with the corresponding grooves  30  such that trapped material  45   b ,  45   c  is forced within groove voids  31 ,  32  forming a mechanical interlock between the target material  10  and backing plate material  15 . The protruding portions  45  and grooves  30  may take on a number of mating shapes in accordance with the present invention, however, most preferably the volume of material associated with a given protruding portion  45  is matched with the volume associated with the corresponding groove  30  such that the deformed protruding portion  45   a  substantially fills the entire groove. 
   With further reference to  FIGS. 3 and 4 , the respective target assembly  5 ,  105  is depicted subsequent to being subjected to a machining operation. As can be seen, the given target assembly  5 ,  105  is machined such that only a section of the deformed base portion  70   a ,  170   a  remains; a section of the target material  10 ,  110  is removed as well. The corresponding mechanical bond remains with the deformed lip  25   a ,  125   a  trapped beneath the remaining section of the deformed base  70   a ,  170   a . Various amounts of material may be machined away from the target material and backing plate material within the scope of the present invention. 
   The phrase “low temperature pressure consolidation” refers to pressure consolidation that may occur at temperatures of less than about 50% of the melting temperature of the lower melting point of either the target material  10  or backing plate material  15 . Preferably, the application of pressure of about 50-5,000 tons is performed; most preferably less than about 1,000 tons, at about room temperature to about 150° C.; more preferably below 100° C.; even more preferably at less than 38° C. The protruding portions  45  are preferably formed in the softer material of either the target material  10  or backing plate material  15  with the tapered grooves being formed in the opposing mating material. 
   The target material and backing plate material may be low temperature pressure consolidated such that the deformation of the protruding portions and formation of the mechanical bond occur simultaneously or, most preferably, the protruding portions are deformed in an initial low temperature pressure consolidation step followed by a second low temperature pressure consolidation step forming the mechanical bond. In accordance with the present invention, deformation of the protruding portions may occur at a pressure, and or, temperature different from the corresponding formation of the mechanical bond. 
   Turning now to  FIGS. 7 and 8 , an alternate embodiment in accordance with the present invention is shown as target assembly  205 . As can be seen, target assembly  205  include interposing material  275  positioned adjacent the intersection of wall  271  and mating surface  235 . Incorporation of interposing material  275  assists in inhibiting oxidation of mating surfaces  220 ,  235 . 
   Target material  210  is formed to include mating surface  220  and side portion  224  with lip  225  extending therefrom. The tip  226  of lip  225  defines the outer most edge of the target material in the radial direction. 
   Backing plate material  215  is formed with “M” shaped protruding portions extending from mating surface  235  and clamp  240 . Clamp  240  is formed to include steps  255  and base portion  270  having wall  271 . 
   Upon imposition of low temperature pressure consolidation, the protruding portions  245  become embedded within the target material defining deformed “M” shaped protruding portions  245   a  producing a mechanical interlock between the target material and the backing plate material. The deformed steps  255   a  of the deformed clamp  240   a  engage retaining surface  260  such that deformed base portion  270   a  deflects toward side portion  224 . The deformed wall  271   a  is forced against side portion  224  and traps the deformed lip  225   a  and deformed interposing material  275   a  therebeneath. Preferably, the lip is shaped such that the deformed tip  226   a  becomes slightly embedded into the backing plate material.