Abstract:
A sputtering target, particularly for sputter depositing a target material onto large rectangular panels, in which a plurality of target tiles are bonded to a backing plate in a two-dimensional non-rectangular array such that the tiles meet at interstices of no more than three tile, thus locking the tiles against excessive misalignment during bonding. The rectangular tiles may be arranged in staggered rows or in a herringbone or zig-zag pattern. Hexagonal and triangular tiles also provide many of the advantages of the invention.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to sputtering of materials. In particular, the invention relates to the a target containing multiple tiles of target material.  
       BACKGROUND ART  
       [0002]     Sputtering, alternatively called physical vapor deposition (PVD), is widely used in the commercial fraction of semiconductor integrated circuits for depositing layers of metals and related materials. A typical DC magnetron plasma reactor  10  illustrated in cross section in  FIG. 1  includes an electrically grounded vacuum chamber  12  to which a target  14  is vacuum sealed through an electrical isolator  16 . A DC power supply  18  negatively biases the target  14  with respect to the chamber  12  or a grounded sputter shield within the chamber  12  to excite an argon sputter working gas into a plasma. However, it is noted that RF sputtering is also known. The positively charged argon ions are attracted to the biased target  14  and sputter material from the target  14  onto a substrate  20  supported on a pedestal in opposition to the target  14 . A magnetron  24  positioned in back of the target projects a magnetic field parallel to the front face of the target  14  to trap electrons, thereby increasing the density of the plasma and increasing the sputtering rate. In modern sputter reactors, the magnetron may be small and be scanned about the back of the target  14 . Even a large magnetron may be scanned in order to improve the uniformity of erosion and deposition.  
         [0003]     Although aluminum, titanium, and copper targets may be formed as a single integral member, targets for sputtering other materials such as molybdenum, chromium, and indium tin oxide (ITO) are more typically formed of a sputtering layer of the material to be sputtered coated onto or bonded to a target backing plate of less expensive and more readily machinable material.  
         [0004]     Sputter reactors were largely developed for sputtering onto substantially circular silicon wafers. Over the years, the size of silicon wafers has increased from 50 mm diameters to 300 mm. Sputtering targets or even their layers of sputtering material need to be somewhat larger to provide more uniform deposition across the wafer. Typically, wafer sputter targets are formed of a single circular member for some materials such as aluminum and copper or a single continuous sputter layer formed on a backing plate for more difficult materials.  
         [0005]     In the early 1990&#39;s, sputter reactors were developed for thin film transistor (TFT) circuits formed on glass panels to be used for large displays, such as liquid crystal displays (LCDs) for use as computer monitors or television screens. The technology was later applied to other types of displays, such as plasma displays and organic semiconductors, and on other panel compositions, such as plastic and polymer. Some of the early reactors were designed for panels having a size of about 400 mm×600 mm. It was generally considered infeasible to form such large targets with a single continuous sputter layer. Instead, multiple tiles of sputtering materials are separately bonded to a single target backing plate. In the original sizes of flat panel targets, the tiles could be made big enough to extend across the short direction of the target so that the tiles form a one-dimensional array on the backing plate.  
         [0006]     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. 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. For such large targets, a two-dimensional tile arrangement illustrated in plan view in  FIG. 2  may become necessary. Rectangular target tiles  30  are arranged in a rectangular array and bonded to a target backing plate  32 .  
         [0007]     As shown in the plan view of  FIG. 2 , a substantially rectangular target  30  includes rectangular target tiles  32  arranged in a rectangular array and bonded to a target backing plate  34 . The tile size depends on a number of factors including ease of fabricating the tiles and they may number 4×5, but the tiles  30  may be of substantial size, for example 75 mm×90 mm, such that a 3×3 array is required for a larger panel. The number of tiles in the tile array may be even greater if the target material is difficult to work with, such as chromium or molybdenum. The illustrated target backing plate  34  is generally rectangularly shaped to conform to the shape and size of the panel being sputter coated but its corners  36  are rounded to conform to the chamber body supporting it and it includes an extension  38  from the chamber body containing an electrical terminal for powering the target and pipe couplings for the cooling fluid used to cool the target  30 . As illustrated in cross section in  FIG. 3 , the target backing plate  34  for flat panel sputtering is typically formed from two metal plates  42 ,  44 , for example, of titanium welded or otherwise bonded together. One of the plates  42 ,  44  is formed with linear cooling channels  46  through which the cooling fluid circulates. This backing plate  34  is more complex than the usual backing plate for wafer processing since, for the very large panel sizes, it is desired to provide a backside vacuum chamber rather than the usual cooling bath so as to minimize the differential pressure across the very large target  30 .  
         [0008]     The tiles  32  are bonded to the backing plate  34  on its chamber side with a gap  48  possibly formed between the tiles  32 . Typically, the tiles  32  have a parallelopiped shape with perpendicular corners with the possible exception of beveled edges at the periphery of the tile array. The gap  32  is intended to satisfy fabricational variations and may be between 0 and 0.5 mm. Neighboring tiles  32  may directly abut but should not force each other. On the other hand, the width of the gap  48  should be no more than the plasma dark space, which generally corresponds to the plasma sheath thickness and is generally somewhat greater than about 0.5 mm for the usual pressures of argon working gas. Plasmas cannot form in spaces having minimum distances of less than the plasma dark space. As a result, the underlying titanium backing plate  34  is not sputtered while the tiles  32  are being sputtered.  
         [0009]     Returning to  FIG. 2 , the tiles  32  are arranged within a rectangular outline  40  conforming approximately to the area of the target  30  desired to be sputtered or somewhat greater. The magnetron  24  of  FIG. 1  is scanned with this outline  40 . Shields or other means are used to prevent the untiled surface of the backing plate  34  from being exposed to high-density plasma and be thereby sputtered. Clearly sputtering a titanium backing plate  34  supporting molydenum or other tiles is not desired. Even if the backing plate  34  is composed of the same material as the target tiles  32 , sputtering of the backing plate  34  is not desired. The backing plate  34  is a complex structure and it is desired to refurbish it after one set of tiles  32  have been exhausted and to use it for a fresh set of tiles  32 . Any sputtering of the backing plate  34  should be avoided.  
         [0010]     The rectangular tile arrangement of  FIG. 2  presents difficulties with increases in the panel size. There are several processes available for bonding target tiles to backing plates. One popular process illustrated in  FIG. 4  includes an apparatus comprising two heating tables  60 ,  62 . The tiles  32  are placed on one table  60  with their sputtering face oriented downwards. Each tile  32  is painted on its backside with a coating  64  of indium. The heating table  60  heats the coated tiles  32  to about 200° C., far above indium&#39;s melting point of 156° C. so that indium wets to the tiles  32  and forms a uniform molten layer. Similarly, the backing plate  34  is placed on the other heating table  62  and is painted with an indium coating  66  and is heated to about 200° C. With all indium coatings  64 ,  66  in their molten state, the tiles  32  are removed from the first table  60 , inverted, and placed on top of the backing plate  34  with the melted indium coatings  64 ,  66  facing each other and the sputtering faces oriented upwards. Upon cooling, the indium solidifies and bonds the tiles  32  to the backing plate  34 .  
         [0011]     The transfer operation must be performed quickly enough that the indium coating  64  on the tiles  32  does not solidify during transfer. For smaller targets, the transferring could be done manually. However, with the target and tiles becoming increasingly larger, a transfer fixture grasps the edges of the tiles, and a crane lifts the fixture and moves it to the second table.  
         [0012]     Such large mechanical structures are not easily manipulated to provide the desired degree of alignment, specifically, the bonded tiles being separated by no more than 0.5 mm. Instead, as illustrated for a corner area  40  between four tiles  32  in the plan view of  FIG. 5 , the four tiles  32  arranged in a rectangular array tend to slide along each other and be misaligned with different sizes for the inter-tile gaps  48 . More importantly, an interstix  72  between the corners of the four tiles may become much larger than intended. By an interstix is meant a point or space at the interfaces between three or more tiles so that the term does not include the line between two tiles. Even a well defined interstix  72  presents the greatest gap between tiles  32 . As a result, the widest point of the interstix  72  for misaligned tiles  32  may become larger than the plasma dark space, e.g., 1 mm, so that the plasma may propagate towards the backing plate  34 . If the gap is only slight lrager than the plasma dark space, the plasma state in the gap may be unsteady and result in intermittent arcing. Even if the arcing is confined to tile material, the arc is likely to ablate particles of the target material rather than atoms and create contaminant particles. If the plasma reaches the backing plate, it will be sputtered. Plate sputtering will introduce material contamination if the tiles and backing plate are of different materials. Furthermore, plate sputtering will make it difficult to reuse the backing plate for a refurbished target. Even if the plasma does not immediately reach the backing plate, an oversized interstix  72  allows the plasma to sputter the sides of the tiles  32  facing the interstix  72 . The side sputtering will further enlarge the interstix  72  and worsen the situation of plate sputtering.  
         [0013]     A similar problem arises from the differential thermal expansion between the materials of the target tiles and the backing plate. When the bonded assembly is cooled to room temperature, the differential thermal expansion is likely to cause the assembly to bow. Because of the softness of solid indium, the bow can be pressed out of the bonded assembly. However, the pressing is a generally uncontrolled process and the tiles may slide relative to each other during the pressing to create the undesired tile arrangement of  FIG. 5 .  
         [0014]     Techniques have been developed to bond tiles to backing plates with a conductive elastomer that can be applied at a much lower temperature. Such bonding services are available from Thermal Conductive Bonding, Inc. of San Jose, Calif. Nonetheless, elastomeric bonding does not completely eliminate the misalignment problem with larger array of target tiles.  
       SUMMARY OF THE INVENTION  
       [0015]     A target, particularly useful as a rectangular target, includes rectangular target tiles which are bonded to a target backing plate in a non-rectangular two-dimension array.  
         [0016]     The rectangular tiles may be arranged in staggered rows such that only three tiles meet at an interstix and only two of those tiles have acute corners adjacent to the interstix. In one embodiment of the row arrangement, one row may include only plural whole tiles while a neighboring row has one less whole tile and two half tiles on the ends. In another embodiment of the row arrangement, all rows include the same number of whole tiles and one partial tile with the partial tiles being disposed on opposed ends of neighboring rows.  
         [0017]     The rectangular tiles may alternatively be arranged in a herringbone or zig-zag pattern of whole rectangular tiles having a 1:2 size ratio and square tiles disposed on the periphery of the rectangular outline.  
         [0018]     Alternatively, the tiles may be hexagonally shaped and arranged in a close-packed structure.  
         [0019]     Yet further alternatively, the tiles may be triangularly shaped, preferably having isosceles shapes within the interior of the rectangular outline.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a schematic cross-sectional view of a conventional plasma sputter reactor.  
         [0021]      FIG. 2  is a plan view of rectangular target of the prior art formed from a two-dimensional array of target tiles.  
         [0022]      FIG. 3  is a cross-sectional view of a conventional configuration of target tiles bonded to a target backing plate including cooling channels.  
         [0023]      FIG. 4  is a schematic view illustrating a conventional method of bonding target tiles to a backing plate.  
         [0024]      FIG. 5  is a plan view illustrating a problem with the conventional rectangular arrangement of target tiles.  
         [0025]      FIG. 6  is a plan view of a first embodiment of the invention including rectangular target tiles arranged in staggered rows.  
         [0026]      FIG. 7  is a plan view of a second embodiment including rectangular tiles arranged in staggered rows with partial end tiles of the same size arranged on opposing ends of neighboring rows.  
         [0027]      FIG. 8  is a plan view of a third embodiment including rectangular tiles arranged in a herringbone or zig-zag pattern.  
         [0028]      FIG. 9  is a plan view of a fourth embodiment including hexagonal tiles.  
         [0029]      FIG. 10  is a plan view of a fifth embodiment including triangular tiles. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     Targets made according to the invention avoid many of the problems of conventional targets composed of tiles arranged in a rectangular array. Instead, as illustrated in the plan view of  FIG. 6 , a target  80  of one embodiment of the invention includes rectangular tiles  32  each of substantially the same composition at least on its sputtering face and arranged in staggered rows and bonded to the target backing plate  34 . In this embodiment, the tiles  32  of one row are offset in the row direction from the tiles  32  of the neighboring rows. In some of the rows, end tiles  82  have a length in the row direction that is only a fraction of the corresponding length of full tiles  32 . In this embodiment, it is preferred that the length of the end tiles  82  be one-half the full length less the desired size of the gap between tiles so that only two sizes of tiles  32 ,  82  are needed. While the tiles  32 ,  82  can still slide in the row direction during their transfer to and bonding with the backing plate  34 , movement in the perpendicular direction is quite limited. As a result, interstices  84  at the corners between tiles  32 ,  82  are much less likely to grow to abnormally large sizes. Furthermore, each interstix  84  forms between three tiles  32 ,  82  and only two of the tiles  32 ,  82  present acute angles to the interstix  84 . Accordingly, plasma arcing is less severe than with four tiles presenting four acute angles, as in the prior art target  30  of  FIG. 1 .  
         [0031]     The target  80  contains some rows containing a number N of whole tiles  32  alternating with rows containing N−1 whole tiles  32  and two half tiles  82 . Within a factor that is a ratio of the number of rows and number of columns, the aspect ratio of the whole tiles  32  determines the aspect ratio of the useful target area covered by the tile  32 ,  82 .  
         [0032]     A closely related target  90  illustrated in plan view in  FIG. 7  has rectangular tiles  92  arranged in rows all containing N full tiles  92  and one partial rectangular tile  94 . The partial tiles  94  are arranged on opposite ends of neighboring rows and may have the same length in the row direction so that only two sizes of tiles are required. The length of the partial tiles  94  in the row direction is not limited to one-half the corresponding length of the full tiles  92 . Even if the full tiles  92  are square, the aspect ratio of the useful area of the target can be nearly freely chosen by varying the row dimension of the partial tiles  94 .  
         [0033]     In both the targets  80 ,  90 , the full tiles  32  are arranged in a parallelogram arrangement of similarly oriented tiles  32 .  
         [0034]     A target  100  of a third embodiment of the invention is illustrated in  FIG. 8  has rectangular tiles arranged in a herringbone arrangement, alternatively called a zig-zag arrangement. Viewed in the orientation of  FIG. 8 , the herringbone pattern includes tiles  102  having an 1:2 aspect ratio, taking into account any desired gap between the tiles  102 . In the herringbone pattern, the tiles  102  are arranged in both the vertical and horizontal directions with paths passing through the short dimension of a first tile on a first end, through the long dimension of a second tile, and then through the short dimension of a third tile on a second end opposite the first end of the second tile. Thereafter, the pattern repeats. Viewed along the direction of the diagonal passing from lower left to upper right, there are parallel chevron patterns along the diagonal of pairs of orthogonally arranged tiles  102 . The edges around the rectangular pattern require several half tiles  104 . Note that a whole tile  106  at the upper right corner replaces two half tiles of the precise herringbone pattern.  
         [0035]     The herringbone pattern provides many interlocking corners and thus allows little slippage to accumulate. This rigidity is accomplished with only two sizes of tiles. However, there is very little flexibility in the aspect ratio of the tiles in the simple illustrated herringbone pattern so that the overall aspect ratio of the useful area of the target is constrained to ratios of small integers. The target aspect ratio can be more freely chosen if rectangularly shaped target tiles of nearly arbitrary aspect ratio are lined up on one of the edges of the herringbone pattern. (A similar edge row of differently sized tiles may be used with the other rectangular arrangements to more easily attain an arbitrary aspect ratio.) The herringbone pattern can be characterized as pairs of perpendicularly oriented 1:2 tiles arranged in an parallelogram pattern.  
         [0036]     In all the rectangular embodiments described above with reference to  FIGS. 6, 7 , and  8 , a tile in the interior of the two-dimensional array away from the periphery abuts along a line six other tiles, whether they be full or partial tiles in contrast to the four tiles abutted in the prior art rectangular arrangement of  FIG. 1 .  
         [0037]     All the previously described patterns involve rectangular tiles. In contrast, a target  110  illustrated in plan view in  FIG. 9  includes regular hexagonal tiles  112  arranged in a hexagonal close packed structure, alternatively characterized as a rhombohedral pattern with one pair of sides aligned with the rectangular outline. It is not conventional to fabricate tiles in non-rectangular shapes. However, targets of many high-temperature metals are formed by sintering powders in a mold. The mold can be shaped in the required non-rectangular shape, in this embodiment a hexagonal shape. Fitting the hexagonal tiles  122  into a rectangular shape requires extra edge pieces. However, in the design of  FIG. 9 , the edge pieces can be restricted to tiles of two shapes, trapezoidal tiles  114  along set of opposed edges, which are half hexagons, and pentagonal tiles  116  along the other set of opposed edges. Although the illustrated hexagons are regular, they may be stretched or shrunk along one opposed pair of sides with all interior corners maintained at 60°. Even with regular hexagons having a fixed aspect ratio, the length of the parallel sides of the pentagonal tiles  106  may be varied to provide more freedom in the overall target aspect ratio. The limitation to three sizes of tiles  112 ,  114 ,  116  is obtained when there are an odd number of rows in the illustrated orientation of an odd number of abutting hexagon tiles  112 , one of which may be split into two trapezoidal tiles  114  for the edges. The hexagonal arrangement produces interstices  118  abutting three tiles  112  (including edge tiles  114 ,  116  as appropriate). Each of the abutting tiles abuts at corners having an exterior obtuse angle of 120°. Similarly to the rectangular patterns of the invention, each hexagonal tile  112  in the interior of the arrangement abuts along a line six other tiles, whether they be full or partial tiles.  
         [0038]     The rectangular and hexagonal tiles described above have interior angles of 90° and 60° respectively. It is possible to modify these shapes to more oblique shapes. As long as the opposed sides of the tiles are parallel, they can be close packed. However, such oblique shapes require additional edge pieces.  
         [0039]     Another target  120  illustrated in plan view in  FIG. 10  includes triangular tiles. In the illustrated embodiment, each row includes alternating triangular tiles  122 ,  124  of the same shape of an isosceles triangle but with inverted orientations with respect to the perpendicular of the horizontally illustrated row direction. Two right triangular tiles  126  are disposed at the end of the rows to provide the desired overall rectangular shape. If there are matched pairs of tiles  122 ,  124  in each row, that is, N of each, then the right triangular end tiles  128  have the same shape even if their tops and bottoms need to be differentiated. As a result, only two sizes of tiles  122 ,  124  and  126  are required. The vertically oriented vertex of one isosceles tile  124 ,  126  abuts the base of another similar oriented isosceles tile  124 ,  126  so that interior interstices  128  are bordered by three acute apexes and one flat side of four respective tiles  124 ,  126 . If the isosceles triangles of the tiles  124 ,  126  are equilateral triangles, the minimum apex angle is increased and the perimeter-to-area ratio decreased. However, an equilateral design provides little flexibility in overall aspect ratio of the target while a more general isosceles design allows different base-to-side ratios in the triangles. In the illustrated triangular arrangement, each tile  122  or  124  at the interior of the pattern abuts along a line four other triangular tiles, whether they be full or partial. It may be desirable to line one edge of the triangular array, whether isosceles or equilateral, with rectangular tiles of arbitrary aspect ratio to thereby allow an arbitrary target aspect ratio.  
         [0040]     The illustrated triangular arrangement can be characterized as a rectangular arrangement of non-rectangular elements although non-rectangular arrangements are possible. In any case, all the embodiments described above include a two-dimensional array of tiles arranged and bonded to the backing plate such that the edges of the tiles do not conform to a rectangular two-dimensional grid, as do the tiles of the prior art arrangement of  FIG. 2 .  
         [0041]     Other triangular shapes and staggering patterns are possible, but the isoceles design of  FIG. 10  provides a large minimum apex angle and a small number of extra edge pieces.  
         [0042]     The invention is useful not only for refractory metal targets such as molybdenum, chromium, and tungsten as well as silicon, targets of which are difficult to fabricate in large sizes. Similarly, the invention is also useful for targets of more complex composition, such as indium tin oxide (ITO), which is typically sputtered from a target of a mixture of indium oxide and tin oxide in the presence of an oxygen ambient. Nonetheless, the invention is also useful for more common metals such as aluminum, copper, and titanium, particularly when a target backing plate is used which is intended to be refurbished. That is, the invention is not limited to the composition of the target The invention may further be applied to targets used in RF sputtering, such as insulating targets, as may be used for sputtering metal oxides. A magnetron is not essential for the invention. Furthermore, the invention can be applied to round targets although a large variety of edge pieces are required.  
         [0043]     Although the invention has been described on the basis of planar bodies having straight sides, it is understood that the edges may have cross-sections of more complexity, such as steps, as long as the overall shape is describable as rectangular, etc. Similarly, the corners of the shape may be somewhat rounded, either intentionally or unintentionally.  
         [0044]     The invention is most useful for large rectangular targets having minimum dimensions of greater than 1.8 m. However, the invention is applicable to smaller targets for which tiling is still desired. Especially for smaller targets, the target backing plate may be simpler than the one illustrated and not include the cooling channels.  
         [0045]     The invention thus provides less tile misalignment and improved sputtering performance with only a small increase in the complexity of the tiled target and its fabrication.