Abstract:
In a sputtering target assembly comprising a plurality of tiles bonded to a target backing plate with gaps formed between the tiles, centering mechanisms for aligning and centering each of the tiles to the backing plate. The centering mechanism for each tiles can comprise a two or three grooves formed in the backing plate along axes intersecting near the tile center and slidably accommodating corresponding pins extending from the tile. Alternately, a pin and groove can be combined with another tile pin and a circular hole in the backing plate near the tile center.

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). FPDs may assume several forms based upon liquid crystal devices (LCDs), plasma displays, field emission displays, and organic light emitting diodes (OLEDs) FPDs are typically fabricated on thin rectangular sheets of glass although the technology is being developed for polymer and other types of substrates. A layer of silicon is deposited on the glass panel or other substrates 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 substrate is used to drive optical elements, such as LCDs, OLEDs, or other elements, developed in or subsequently mounted on the substrate.  
         [0003]     Size constitutes one most apparent difference between electronic integrated circuits and flat panel displays 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 panels has been continually increasing. The increase applies also to other types of substrates. 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 usually formed of a sputtering layer of the target material bonded to a target backing plate, typically formed of titanium. 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 to fabricate in large sizes. The size problem has been addressed by forming the target sputtering layer from multiple target tiles. Targets formed from multiple tiles each occupying less than the total area of the substrate to be sputter coated have introduced several problems not experienced with laterally homogeneous targets.  
       SUMMARY OF THE INVENTION  
       [0005]     A centering mechanism for aligning a plurality of sputtering tiles bonded to a target backing plate in a one- or two-dimensional array with gaps therebetween. The resultant target assembly may be used in a magnetron sputter reactor, particularly one intended for flat panel displays.  
         [0006]     The centering mechanism for each tile may comprise at least one pin extending from the tile toward the backing plate and a corresponding groove formed along a centering axis in the backing plate slidably accommodating the pin.  
         [0007]     There may be two, three, or possibly more pairs of pins and grooves with the groove axes preferably intersecting near the target center.  
         [0008]     Alternately, one pair of pin and groove may cooperate with another pin in the tile and a circular recess in the backing plate pivotally capturing the added pin and located along the axis of the groove, preferably at the tile center. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic cross-sectional view of a conventional plasma sputter reactor.  
         [0010]      FIG. 2  is bottom plan view of a target assembly including target tiles bonded to backing plate.  
         [0011]      FIG. 3  is a schematic plan view of a first embodiment of the invention including centering mechanisms for centering target tiles on a backing plate.  
         [0012]      FIG. 4  is a cross-sectional view of part of the centering mechanism of the first embodiment.  
         [0013]      FIG. 5  is a cross-sectional view of a variant of the first embodiment.  
         [0014]      FIG. 6  is a schematic plan view of a second embodiment of the invention including a different type of centering mechanism.  
         [0015]      FIG. 7  is a cross-sectional view of part of the second embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     The invention may be practiced in sputtering apparatus such as a sputtering chamber  10 , schematically illustrated in the cross-sectional view of  FIG. 1 , which 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 or other substrate  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 over 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  needs to be somewhat larger than the panel  18  being sputter coated so that its size as well has been increasing with more recent equipment. 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.  
         [0017]     The configuration of tiles assembled to form a target will now be described. As schematically illustrated in the plan view of  FIG. 2 , multiple target tiles  24  are set on a backing plate  26  with a predetermined gap  28  between them. The tiles  24  are thereafter bonded to the backing plate  26 . The large peripheral area of the backing plate  26  outside the tiles  24  is used to support the target  14  on the vacuum chamber  12  and an extension  29  of the backing plate  26  falls outside of the outline of the vacuum chamber  12  to provide electrical terminals and plumbing ports for the water cooling channels formed in the backing plate  26 .  
         [0018]     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. 10/863,152, filed Jun. 7, 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.  
         [0019]     The gap  28  between the tiles  24  must be carefully designed and maintained. Typically, the gap  28  is not filled with other material and the backing plate or material other than the target material is exposed at the bottom of the gap  28 . However, if the gap  28  (or at least part of it) is maintained at no more than about 1 mm, the sputtering plasma cannot propagate into the gap because the gap is less than the plasma dark space. Because the plasma does not propagate to the bottom of the gap  28 , the backing plate  26  is not sputtered. It is possible, although not preferred, that some of the gaps at some temperatures have a zero thickness as the neighboring tiles touch or press against each other.  
         [0020]     A problem arises, however, from the not insignificant fraction of atoms that are sputtered from the front face of the target and redeposit upon the target rather than upon the deposition substrate. The sputter atoms redeposited on the planar target face are typically sputtered at a faster rate than they are redeposited so the redeposited material does not build up. On the other hand, sputter atoms may also redeposit in the gap away from the sputter plasma. Hence, the deposited material tends to develop a growing layer on the sidewalls and bottom of the gap between the tiles. The redeposited material tends to not stick well to the underlying target or backing plate. An excessive thickness of redeposited sputter material tends to flake off in sizable particles that then fall upon the substrate being sputter deposited. The particles may have size greater than features being formed in the substrate so that a single particle may cause a fatal defect in the entire large flat panel display. Clearly, the number of such particles needs to be reduced or eliminated to increase the yield of flat panel displays or other circuitry being developed in the substrate.  
         [0021]     The number of such particles is lessened by reducing the width of the gap to less than the plasma dark space, but a finite gap is required because of the differential thermal expansion between the target tiles and the backing plate during thermal cycling during substrate processing or during tile bonding. A gap width of about 0.5 mm represents a current design thickness.  
         [0022]     There are several methods of bonding tiles to a backing plate. Indium solder bonding is typically used for many large targets. Indium&#39;s melting point is 156° C. so the soldering process needs to be performed with both the target and backing plate held at somewhat higher temperatures. As a result, differential thermal expansion is significant during bonding. If the indium is not applied in a symmetric pattern, the non-symmetrically bonded tiles may shift during cooling so that one or more gaps may be larger than desired.  
         [0023]     A more recently developed bonding process places a conductive elastomer or other organic adhesive between the target and backing plate. The elastomer can be cured at relatively low temperatures so that differential thermal expansion during bonding presents much less a problem and the design gap thickness may be reduced. Such elastomeric bonding services are available from Thermal Conductive Bonding, Inc. of San Jose, Calif. Nonetheless, the target assembly is still subject to some differential thermal expansion, either during the bonding process or during the operational life of the target as the target temperature rises during sputtering when power is applied to the target and falls during quiescent periods when no power is applied. Cured elastomeric adhesives are perceived to be much more pliable and deformable than indium solder joints and it is possible for tiles to walk during thermal cycling, that is, their positions at the same temperature before and after thermal cycling may change with an accompanying change in gap thicknesses.  
         [0024]     Demaray in the aforecited patent suggests autoclaving at high temperature and pressure so the tiles and backing plate diffuse together. While autoclaving produces a very strong bond, the required high temperatures necessitates that the design gap thickness is somewhat large.  
         [0025]     Whatever the bonding method, it is thus believed that extra precaution should be exercised in maintaining the gap widths.  
         [0026]     To improve the relative orientation of multiple tiles, mechanical guiding means may be developed between the target tiles and the backing plate or other support structure that tend to return the tiles to mechanically defined positions on the backing plate as the tiles expand and contract relative to the backing plate.  
         [0027]     A first embodiment of the invention is illustrated schematically in plan view in  FIG. 3 . A target assembly  30  includes seven rectangular tiles  32  bonded to a target backing plate  34  in a two-dimensional array, which is staggered in the illustration but other one- and two-dimensional arrangements may be used. It is appreciated that for flat-panel sputtering, the tiles fill a rectangular outline of the backing plate  34  overlying the panel to be sputter coated. For the staggered configuration, half tiles  32   a  fill the corners or other vacancies of the full tiles  32  within a rectangular outline. Each full tile  32  includes three alignment pins  36   a ,  36   b ,  36   c  formed on the tile side facing the backing plate  34  and disposed on respective centering axes  38   a ,  38   b ,  38   c . The first axis  38   a  preferably bisects the rectangular shape of the tile  32  and extends along the shorter direction of the tile  32  if the tile  32  is non-square while the other two axes  36   b ,  36   c  preferably fall on the two diagonals of the tile  32 . More importantly than their location within the tile  32 , the three axes  38   a ,  38   b ,  38   c  are inclined with respect to one another and intersect at a common point  40 , preferably at or near the center of the tile  32 . The half tiles  32   a  include the same elements but spaced on a shrunken scale in one dimension.  
         [0028]     Correspondingly, the side of the backing plate  34  facing the tiles  32  is formed with sets of grooves  42   a ,  42   b ,  42   c  having lengths extending along the axes  38   a ,  38   b ,  38   c  sufficient to capture the corresponding pins  36   a ,  36   b ,  36   c  during movement for any temperature experienced by the target assembly  30  during fabrication or use. The grooves  42   a ,  42   b ,  42   c  have widths that closely accommodate the widths of the tile pins  36   a ,  36   b ,  36   c  so as to guide the pins  36   a ,  36   b ,  36   c  during differential thermal expansion. In  FIG. 4  is illustrated a cross-sectional view of a target assembly  50  taken along one centering axis  38 . The target tile  32  is bonded to the target backing plate  34  with a thin bonding layer  56 , which may be composed of indium, a conductive elastomer, or other suitable thermally and electrically conductive bonding material. The invention is also applicable to autoclaved targets in which no bonding material is required and is useful to guaranteeing alignment during the rigors of autoclave bonding. The backing plate  34  may be composed of multiple layers of a suitable material such as titanium and include cooling channels  58  for a cooling fluid such as chilled water to circulate through to maintain the target assembly  50  at a reasonably low temperature during sputtering.  
         [0029]     The target tile  32  is generally planar in its central region but, according to the invention and as illustrated in  FIG. 4 , it includes a pin  36  extending above its bonded surface and having sufficient length to extend through the bonding layer  56  into a groove  42  formed on the bonded side of backing plate  34 . However, a thin clearance  60  exists between the top of the pin  36  and the roof of the groove  42  recess so that the top of the pin  36  does not contact the backing plate  34  and impede the movement of the pin  36  within the groove  42 . The height of the pin  36  and the depth of the groove  42  should be relatively small so that the thickness of the backing plate  34  is not unduly increased since an increased thickness disadvantageously attenuates the magnetic field from the magnetron as it penetrates the backing plate  34 . Although the target tile  34  is bonded to the backing plate  34 , differential thermal expansion and possibly other effects cause some differential movement between the two but the pin  36  is confined to the groove  42  during this motion and thus is guided to move along the illustrated axial direction of the groove  42 . In the unillustrated transverse direction, the pin  36  is closely fit within the groove  42  but is not fit tight enough at any temperature experienced by the target assembly  50  to bind and impede movement along the groove&#39;s axial direction.  
         [0030]     It is thus clear that, with reference to  FIG. 3 , as the target tiles  32 ,  32   a  expand or contact with respect to the backing plate  34 , the pins  36  of the tile are confined within the grooves  42  and guided by them to move to or away from the center  40 . As a result, the tile  32  is aligned with and centered on the backing plate  34  despite the thermal cycling. The centering and alignment between the tiles  32  and backing plate  34  are also useful during the bonding process. The bonding layer  56  accommodates the relative movement although some stress may build up in the tile  32  or backing plate  34  to absorb some of the relative movement. The centering does not depend upon the bonding layer but upon the mechanical guiding of the pins  36  by the grooves  42 . The axial extent of the groove  42  needs to accommodate only the anticipated relative movement of the pin  36  within the groove  42  so the axial length typically needs not be as long as illustrated.  
         [0031]     The multiple sets of pins and grooves constrains the sides of the tiles  32  to remain parallel to their original orientations. Further, since the tile center  40  or other point fixed to the intersection point is maintained to a fixed point on the backing plate, the gaps on opposed sides of the tiles  32  do not walk during thermal cycling.  
         [0032]     The three sets of pins  36  and grooves  42  provide a mechanically rigid interface between the tiles  32  and the backing plate  34  to thereby minimize tolerances. However, the three sets overly define the center  40  so that, under differential thermal expansion, the center  40  of the tile  32  with respect to the pins  36  may deviate by a small distance from the corresponding center position  40  of the backing plate  34  with respect to the grooves  42 . Assuming that both the tiles  32  and backing plate  34  have isotropic coefficients of thermal expansion in the plane of the target, the separation of the centers can be eliminated by requiring the three pins  36   a ,  36   b ,  36   c  to be equidistant from the center  40 .  
         [0033]     Only two sets of pins and grooves are required to provide the centering mechanism for the tiles  32  although with reduced mechanical tolerances for the parts. For example, the two sets of pins  36   a ,  36   b  and grooves  42   a ,  42   b  would suffice. Also, the two sets of pins  36   b ,  36   c  and grooves  42   b ,  42   c  would suffice. In a more preferred arrangement, schematically illustrated in the plan view of  FIG. 5 , each tile  32  includes the first pin  36   a  arranged along the centering axis  38   a  bisecting one dimension of the rectangle and another pin  36   d  arranged along a centering axis  38   d  perpendicular to the first center axis  38   a  and bisecting the second dimension of the rectangle. The first pin  36   a  fits within the first centering groove  42   a  extending along the first centering axis  38   a  and the other pin  36   d  fits within another centering groove  42   d  extending along the perpendicular other centering axis  38   d . With the arrangement of perpendicular centering axes  38   a ,  38   d , there is substantially no problems with the centers  40  of the tiles  32  and the corresponding points on backing plate  36  deviating from each other. It is not necessary that the two centering axes  38   a ,  38   d  bisect the tile  32 , but such a restriction reduces the deformation of the bonding layer during thermal cycling.  
         [0034]     More than three sets of pins and grooves may be used but they are not considered to be necessary. It is also possible to place two sets of pins and grooves along one centering axis, which would provide increased mechanical rigidity.  
         [0035]     It is not necessary that the centering pins  36  be circular. Instead, they may have straight lateral sides extending in parallel to the lateral sides of the grooves  42 .  
         [0036]     In a second embodiment of the invention schematically illustrated in plan view in  FIG. 6 , a target assembly  70  includes the multiple tiles  32  bonded to the backing plate  34 . Each tile  32  includes the centering pin  36  guided by the centering groove  42   c  in the backing plate  34 , both arranged along the diagonal centering axis  38   c . Each tile  32  additionally includes, as further illustrated in the cross-sectional view of  FIG. 7 , a central pivoting pin  72  slidably fit within a centering hole  74 . The pivoting pin  72  is located on the diagonal centering axis  38   d  and preferably located at the center of the rectangular tile  32 , and the centering hole  74  is formed at a corresponding position in the backing plate  34 . At least one and preferably both of the centering pin  72  and the centering hole  74  should be circular so that the tile  32  is pivotally guided about the center of the centering hole  74 , preferably located at the center of the tile  32 . Although  FIG. 7  illustrates a distinct clearance  76  between the sides of the pivoting pin  36  and the centering hole  74 , the clearance  76  is preferably made as small as possible to minimize the radial movement between the pivoting pin  72  and the centering hole  74  at all temperatures experienced by the assembly but to nonetheless allow free rotation of the pivoting pin  72  within the centering hole  74 . The centering operation is still possible if the pivoting pin  72  binds within the centering hole  74  as long as excessive stress is avoided and the sides of the tile  32  are aligned with the perpendicular coordinates of the backing plate  34  when the binding occurs.  
         [0037]     The location of the pivoting pin  72  at the tile center on the tile diagonal axis  38   c  and the location of the centering pin  36   c  near the end of diagonal axis  38   c  provides symmetric centering of the sides of the tile  32  and the greatest tolerance for the groove  42   c . However, such positions are not necessary as long as the pivoting pin  72  lies on or near the axis of the groove  42   c.    
         [0038]     Although the invention has been described with reference to rectangular target tiles, other tiles shapes can be utilized in conjunction with the invention.  
         [0039]     Although the centering pins are most conveniently composed of target material and formed together with the target tile in its fabrication, it is possible that the centering pins be composed of different material or be fixed to a pre-existing target tile.  
         [0040]     Although the invention was developed for sputtering onto glass substrates for flat panel displays, it may be applied to sputtering onto other types of substrates, for example, for solar cells and may also be applied to sputter targets for large circular wafers.  
         [0041]     The invention thus assures the centering or alignment of the target tiles on the backing plate and prevents the gap between tiles from growing too large during thermal cycling or incompletely controlled bonding.