Abstract:
A target assembly including a plurality of target tiles bonded to a backing plate by adhesive, for example of indium or conductive polymer, filled into recesses in the backing plate formed beneath each of the target tiles. A sole peripheral recess formed as a rectangular close band may be formed inside the tile periphery. Additional recesses may be formed inside the peripheral recess, preferably symmetrically arranged about perpendicular bisectors of rectangular tiles. The depth and width of the recesses may be varied to control the amount of stress and the stress direction.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to sputtering of materials. In particular, the invention relates to sputtering targets composed of multiple tiles.  
       BACKGROUND ART  
       [0002]     Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. Sputtering is now being applied to the fabrication of flat panel displays (FPDs) based upon thin film transistors (TFTs) in combination with liquid crystal devices (LCDs). Demaray et al. describe such a flat panel sputter reactor in U.S. Pat. No. 5,565,071, incorporated herein by reference in its entirety. However, other panel technologies are being pursued, such as plasma display, electron emission displays, and organic light emitting diodes (OLEDs). FPDs are typically fabricated on thin rectangular sheets of glass but other substrates are possible, such as polymeric sheets. A layer of silicon is deposited on the glass panel or other substrate and silicon transistors are formed in and around the silicon layer by techniques well known in the fabrication of electronic integrated circuits. The electronic circuitry formed on the glass panel is used to drive optical circuitry, such as LCDs subsequently mounted on the glass panel or plasma emitters formed in layers deposited on the glass panel.  
         [0003]     Size constitutes one of the most apparent differences between electronic integrated circuits and flat panel display and in the equipment used to fabricate them. Demaray et al. disclose many of the distinctive features of flat panel sputtering apparatus in U.S. Pat. No. 6,199,259, incorporated herein by reference. That equipment was originally designed for panels having a size of approximately 400 mm×600 mm. Because of the increasing sizes of flat panel displays being produced and the economy of scale realized when multiple displays are fabricated on a single glass panel and thereafter diced, the size of the glass panels being processed has been continually increasing. Flat panel fabrication equipment is commercially available for sputtering onto panels having a minimum size of 1.8 m and equipment is being contemplated for panels having sizes of 2 m×2 m and even larger.  
         [0004]     For many reasons, the target for flat panel sputtering is often formed of a sheet of the target material bonded to a target backing plate, typically formed of titanium. In the conventional method of bonding a target layer to a backing plate, a bonding layer of indium is coated on one of the two sheet-like members and the two members are pressed together at a temperature above indium&#39;s melting point of 156° C. In a more recently developed method of bonding, a conductive elastomer or other organic adhesive is applied at much lower temperature and typically cured at an elevated but relatively low temperature. Such elastomeric bonding services are available from Thermal Conductive Bonding, Inc. of San Jose, Calif. Demaray et al. in the aforecited patent disclose autoclave bonding.  
         [0005]     It is difficult to form sputtering targets of some materials, such as refractory metals, in the large sizes commensurate with the large-size panels. Accordingly, as disclosed by Demaray et al., multiple target tiles of smaller size can be bonded to the backing plate.  
         [0006]     It is desired to improve upon the method of bonding one or more sputtering tiles to a backing plate.  
       SUMMARY OF THE INVENTION  
       [0007]     Multiple target tiles are bonded to a backing plate by adhesive filled into recesses formed in the backing plate inside of areas overlain by the bonded tiles. Preferably, the recesses do not extend to the edges of the tile areas.  
         [0008]     The tiles may be rectangularly shaped and arranged in one- or two-dimensional arrays with gaps therebetween. However, other tile shapes are possible.  
         [0009]     The adhesive may be a conducting organic polymeric adhesive with a relatively large thickness when filled into the recesses.  
         [0010]     The recesses may include a peripheral recess formed as a closed band inside the periphery of the tile areas. For rectangular tiles, the peripheral recess may include four straight and connected portions. Additional recesses may be formed inside of the peripheral recesses and preferably not connected to the peripheral recesses. Preferably also, all recesses are symmetric about the perpendicular bisectors of the target areas. The additional recesses may be one or more straight recesses. They may be intersecting straight recesses, for example, formed along the perpendicular bisectors or along the diagonals of rectangular tile areas. They may be plural straight recesses arranged in parallel. They may be formed as one or more additional closed bands.  
         [0011]     It is possible that there be no peripheral closed-band recess. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic cross-sectional view of a conventional magnetron sputter reactor.  
         [0013]      FIG. 2  is a cross-sectional view of a conventional sputter target assembly including multiple target tiles bonded to a backing plate.  
         [0014]      FIG. 3  is a cross-sectional view of a deleterious crowning of tiles with the conventional sputter target assembly of  FIG. 2 .  
         [0015]      FIG. 4  is a cross-sectional view of an embodiment of a sputter target assembly of the invention including recesses for adhesive bonding the tiles to the backing plate.  
         [0016]      FIG. 5  is a bottom plan view taken along view line  5 - 5  of  FIG. 4  of a first embodiment of an arrangement of a peripheral adhesive recess.  
         [0017]      FIG. 6  is a bottom plan view of a second embodiment of an arrangement of a peripheral adhesive recess and one interior adhesive recess.  
         [0018]      FIG. 7  is a bottom plan view of a third embodiment of an arrangement of a peripheral adhesive recess and multiple parallel interior adhesive recesses.  
         [0019]      FIG. 8  is a bottom plan view of a fourth embodiment of an arrangement of a peripheral adhesive recess and two Cartesian crossing interior adhesive recesses.  
         [0020]      FIG. 9  is a bottom plan view of a fifth embodiment of an arrangement of a peripheral adhesive recess and two obliquely arrangement crossing interior adhesive recesses. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     A sputtering chamber  10 , schematically illustrated in the cross-sectional view of  FIG. 1 , includes a vacuum chamber  12 , a target  14  sealed to but isolated from the electrically grounded chamber  12 , and a pedestal  16  supporting a panel  18  to be sputter coated. The target  14  includes a surface layer of the material to be sputtered onto the panel  18 . An argon working gas is admitted into the chamber with a pressure in the milliTorr range. A power supply  20  electrically biases the target  14  to a negative voltage of a few hundred volts, causing the argon gas to discharge into a plasma. The positive argon ions are attracted to the negatively biased target  14  and sputter target atoms from it. A magnetron  22  is scanned along the back of the target  14  to intensify the plasma and increase the sputtering rate. Some of the target atoms strike the panel  18  and form a thin film of the target atoms on its surface. The target  14  is often somewhat larger than the panel  18  being sputter coated. Sputtering has been applied to a large number of target materials including aluminum, copper, titanium, tantalum, chromium, and indium tin oxide (ITO) as well as other materials.  
         [0022]     One problem arising from the increased panel sizes and hence increased target sizes is the difficulty of obtaining target material of proper quality in the larger sizes. Refractory materials such as chromium are particularly difficult materials. The size problem has been addressed by forming the target sputtering layer from multiple target tiles. As schematically illustrated in the plan view of  FIG. 2 , multiple target tiles  22  are set on a backing plate  24  with a predetermined gap  26  of size ΔL 1  between them. A continuous adhesive layer  28  bonds the tiles  22  to the backing plate  24 . The large peripheral area of the backing plate  24  outside the tiles  22  is used to support the target  14  on the vacuum chamber  12  and to provide plumbing ports for the water cooling channels formed in the backing plate  24 .  
         [0023]     The arrangement of two tiles illustrated in  FIG. 2  represents the simplest tile arrangement, two tiles in a linear array with a single gap between them. Demaray in the aforecited patent discloses a larger number N&gt;2 of tiles in a linear array with (N−1) gaps between them. Tepman in U.S. patent application Ser. No. 10/888,383, filed Jul. 9, 2004 discloses a two-dimensional array of tiles with vertical and horizontally extending gaps intersecting each other. The array may be a rectangular array, a staggered array as in simple brick wall, or more complicated two-dimensional arrays including herringbone patterns. Although rectangular tiles present the simplest geometry, other tile shapes are possible, such as triangular and hexagonal tile shapes with correspondingly more complex gap arrangements.  
         [0024]     The gap  26  between tiles must be carefully designed and maintained. Typically, the gap is not filled with other material and the adhesive  28  or material other than the target material is exposed at the bottom of the gap  26 . However, if the gap (or at least part of it) is maintained at about 0.3 mm, then the sputtering plasma cannot propagate into the gap because the gap is less than the plasma dark space. With no plasma propagating to the bottom of the gap, the backing plate or adhesive exposed at the bottom of the gap is not sputtered. However, after burn in of the target at high power and high temperature, the tiles are likely to deform and develop a crown-like shape illustrated in  FIG. 3  and the gap is reduced to ΔL 2 , which is less than ΔL 1  and may be zero. Uncontrolled gap size may cause arcing during deposition, which is likely to produce large particles creating defects in the display. The crown shape means that the distance between the target tiles and the panel being coated is not uniform, thus introducing non-uniformities in deposition. It is believed that the crown shape represents partial delamination of the target tile from the backing plate results in lack of direct thermal contact between the central portions of the target tiles from the backing plate being used to cool the tiles, further increasing the average temperature and temperature differentials.  
         [0025]     The crown shape of the target tiles is believed to arise from the thermal expansion mismatch between the target material, for example, molybdenum, the adhesive, particularly a polymeric adhesive, and the backing plate, for example, titanium. The thermal mismatch produces stress in the adhesive, which is a function of the bonded area and the thickness of the adhesive. According to the well known theory of static deformation, stress S in the adhesive layer is related to the strain ε through the modulus of elasticity G by the relation 
 
S=Gε. 
 
 The strain ε is related to the dimensional change ΔL over a length L and to the differential coefficient of thermal expansion Δα and temperature change ΔT according to  
       ɛ   =         Δ   ⁢           ⁢   L     L     =     Δ   ⁢           ⁢     T   ·     Δα   .               
 
 It is believed that the greatest stress arises between the adhesive and the target tile. The invention allows the reduction of stress for number of reasons. The width of the adhesive layer may be reduced. The thickness t of the adhesive layer may be increased while the width of the bonded area is reduced since the stress also varies with the thickness of the layer over which the strain must be accommodated according to  
       ɛ   ∝       Δ   ⁢           ⁢   t     t         
 
 so that increasing the thickness decreases the stress. However, increased thickness t in a planar adhesive layer is not desired because of the increased thermal impedance. The direction of stress may also be controlled. 
 
         [0026]     In one embodiment of the invention illustrated in the cross-sectional view of  FIG. 4 , one or more grooves  30  are formed in the backing plate  24  in areas underlying the respective tiles  28 . The grooves  30  are filled with adhesive, for example, a conductive polymeric adhesive and the target tiles  28  are bonded to the backing plate  24  through the adhesive in a bonding technique including pressure and typically an elevated temperature. Non-bonded areas may contact the backing plate  24  for effective thermal coupling. The uncertain electrical contact across the two non-bonded members  24 ,  28  is guaranteed by the conduction paths through the conductive polymeric adhesive.  
         [0027]     Although there is great freedom is the parameters of the grooves  30  of this embodiment and others, it has been found advantageous to form the grooves with depth of between 0.1 to 0.5 mm and the total area of the grooves is between about 30 and 45% of the total area of the respective tile they is bonding to the backing plate  24 .  
         [0028]     The grooves may be arranged in a number of different ways. In a first embodiment illustrated in the plan view of  FIG. 5 , a peripheral groove  40  is associated with each tile, represented by dotted lines  42 . The groove  40  is formed in the backing plate  24  as a closed rectangular band generally at the periphery or inside the periphery of the tile  42 . The grooves  40  are filled with adhesive and the tiles  42  are placed over the backing plate  24  in the indicated positions and bonded to the backing plate  24  with predetermined gaps  44  between them. It is possible to vary the width and depth of each of the grooves, for example, between perpendicular sides of a non-square target tile, to control the stress in the tile. The large area of the backing plate  24  outside the outline of the tiles  42  forms part of the vacuum-sealed chamber wall, and the backing plate  24  includes one or more extensions outside the outline of the chamber wall to accommodate an electrical connection and plumbing ports for the cooling water circulating in the backing plate  24  to cool it and the tiles  42  during sputtering.  
         [0029]     Note that the number of tiles  42  is not limited to the illustrated 2×2 array. The tiles  22  can form a simple linear array or can be arranged in a larger two-dimensional array. The individual tiles can have a more even aspect ratio, that is, more square, or a larger aspect ratio. The invention can be applied to less regularly shaped and arranged tiles with gaps between them. Indeed, some aspects of the invention are applicable to a single tile more common with smaller sized targets.  
         [0030]     In another embodiment illustrated in the plan view of  FIG. 6 , a central groove  50  is formed inside the peripheral groove  40  parallel to one of the linear portions of the peripheral groove  40 . Generally, the central groove  50  may be connected or not connected to the peripheral groove  40 . The widths and depths of the grooves  40 ,  50  may be the same or may be different, thereby allowing further control of the stress within the tile  44 . This embodiment can be extended to multiple parallel central grooves  52 , illustrated in the plan view of  FIG. 7 . The multiple parallel grooves  52 , are separated from each other and from the peripheral groove  40 . They may have the same or different widths and depths compared to the peripheral groove, and they may have the same or different widths, depths, and lengths among themselves. Three central grooves  52  are illustrated in  FIG. 7 , but the number is not so limited.  
         [0031]     In yet a further embodiment illustrated in the bottom plan view of  FIG. 8 , the peripheral groove  40  surrounds an internal cross-shaped groove  54  with linear portions extending in Cartesian coordinates parallel to the edges of the tile  42  and preferably crossing at the center of the tile  42 . The cross-shaped groove  54  may have the same depth and width as the peripheral groove  40  or the depths and widths may be different. The perpendicular arms of the cross-shaped groove  54  may have the same or different depths, widths, and lengths between them. In a variant illustrated in the bottom plan view of  FIG. 9 , an X-shaped groove  58  has arms extending generally along diagonals of the tile  42  and crossing at the center of the tile  42 .  
         [0032]     More complex combinations are possible. For example, as illustrated in the bottom plan view of  FIG. 10 , the peripheral groove  40  surrounds a closed internal groove  56  of similar but smaller rectangularlized annular shape. The depths of the two concentric grooves  40 ,  56  may differ in order to control the stress over the distance from the centers of the tiles  42 .  
         [0033]     In yet another embodiment illustrated in the plan view of  FIG. 7 , a second central groove  48  is added which is perpendicular to the first central groove  46  and crosses it preferably at the center of the tile  42 .  
         [0034]     The adhesive or bonding method is not limited to a polymeric or organic adhesive. Other adhesives such as indium may be used.  
         [0035]     Although the embodiments described above have been applied to multi-tile targets, recesses in the backing plate filled with adhesive may be used with single-tile rectangular targets and with circular targets whether composed of one or more tiles.  
         [0036]     Although the above embodiments includes a closed peripheral recess, the invention is not limited to such. For example, the target of  FIG. 7  may include the multiple parallel recesses  52  for each tile  42  but no peripheral recess  40 .  
         [0037]     The invention thus provides a simple method of controlling and reducing stress in the bonding of one or more sputtering target tiles to a backing plate and thus enhancing sputtering performance.