Abstract:
A sputtering chamber contains a plurality of substantially triangular targets supported by a top wall. The targets have narrow ends pointing toward a center of the top wall. Above each target is a relatively small substantially triangular magnet. Each magnet is connected to a single central actuator that scans all magnets back and forth through an arc across its associated target. Each magnet is also movably connected to an arm connected to the central scanning actuator. A linear actuator moves each magnet up and down the arm simultaneously with the angular scanning movement. The combination of the simultaneous angular movement and linear movement (perpendicular to the arc) of the magnet causes each magnet to move only over a substantially triangular area corresponding to an area of an associated target. In one embodiment, the linear speed of the magnets is varied to achieve uniform erosion of the target.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to sputtering systems and, in particular, to a magnetron for use in a sputtering system. 
       BACKGROUND 
       [0002]    Sputtering systems are widely used in the semiconductor manufacturing industry for depositing materials on semiconductor wafers. Sputtering is sometimes referred to as physical vapor deposition, or PVD. In a sputtering operation, thin films comprising materials such as Al, Au, Cu, Ta are deposited in a vacuum on silicon wafers or other substrates. 
         [0003]    The present assignee has obtained a U.S. Pat. No. 7,479,210 on a sputtering tool, which is shown in the prior art  FIGS. 1 and 2 . Related applications using the same specification are pending. U.S. Pat. No. 7,479,210 is incorporated herein by reference. 
         [0004]    Prior art  FIG. 1  is a cutaway view of a sputtering system  12  for workpieces such as semiconductor wafers, LCD panels, and other workpieces requiring the deposition of thin films. Examples of thin films include Al, Cu, Ta, Au, Ti, Ag, Sn, NiV, Cr, TaNx, Hf, Zr, W, TiW, TiNx, AlNx, AlOx, HfOx, ZrOx, TiOx, magnetic films, and various alloys of these materials. The system  12  is completely described in the present assignee&#39;s U.S. Pat. No. 7,479,210 and only a brief summary is provided. 
         [0005]    Since the present invention is a new magnetron assembly, shown in  FIGS. 3 and 4 , that replaces the magnetron in prior art  FIG. 1 , the resulting system is the system of  FIG. 1  but employing the magnetron of  FIGS. 3 and 4 . Therefore, the below description of the system  12  is provided to describe a preferred environment of the new magnetron. 
         [0006]    The top cover of the sputtering system  12  has been removed. A robotic arm (not shown) in a wafer transport module inserts and removes wafers  41  via the access port  14 . Typical wafer sizes are 6, 8, and 12 inches, and the system is customized for the particular workpieces for processing. 
         [0007]    In one embodiment, the system  12  simultaneously processes three or more wafers  41  (preferably five or six) using three or more sputtering targets  43 . 
         [0008]    A pallet  36  rotates to align a wafer  41  below an appropriate target  43 . Each target  43  may be a different material for forming successive thin films of different materials on a wafer  41 . The wafers  41  are supported on wafer support areas  32 . A wafer support area  32  is an indented area in pallet  36  sized to accommodate the particular wafers being processed. 
         [0009]    Four pins (not shown) below pallet  36  are raised, using pin bellows  39 , to extend through four holes in the wafer support area  32  to temporarily lift the wafer  41  during insertion of the wafer  41  into the chamber and removal of the wafer  41  from the chamber. 
         [0010]    The pallet  36  is mounted on a rotatable table  40 . Pallet  36  and table  40  may be formed of aluminum. Pallet  36  may be continuously rotated at any speed or may be temporarily stopped to control the deposition of a sputtered material from a target  43  overlying a wafer. 
         [0011]    A chamber shield  35  prevents contaminants from accumulating on the vacuum chamber wall. 
         [0012]      FIG. 2  is a cross-sectional view of pallet  36  and table  40 . Pallet  36  is a single piece that is fixed to table  40  by a countersunk screw  42  at the indentation in each wafer support area  32  so that the wafers block the sputtered materials being deposited on screws  42 . Pallet  36  may be removed for cleaning by unscrewing screws  42 . 
         [0013]    The entire back surface of each wafer is thus in electrical and thermal contact with pallet  36 , which is in turn in electrical and thermal contact with table  40 . 
         [0014]    The temperature of the wafers is controlled by flowing a coolant  44  ( FIG. 2 ) through a copper tube  46  in direct contact with table  40 . The copper tube  46  runs in a groove  48  around the table  40 . The copper tube  46  extends up through a rotating shaft  49  attached to table  40 . 
         [0015]    An external cooling source  50  cools the coolant (e.g., water) and recycles the coolant back to table  40 . Flexible tubing  51  from the cooling source  50  attaches to a rotatable coupler  52  for providing a sealed coupling between the rotating copper tubes  46  (input and output) and the stationary tubing  51  to/from the cooling source  50 . 
         [0016]    An RF and DC bias source  54  is electrically coupled to the copper tube  46  by the rotatable coupling  52  to energize table  40  and thus energize pallet  36  and the wafers for the sputtering process. In another embodiment, table  40  is grounded, floated, or biased with only a DC voltage source. 
         [0017]    When the chamber is evacuated and back filled with a certain amount of Ar gas at a certain pressure (for example, 20 milli-torr) and the gas is energized with a DC source, an RF source, or a combination of the two sources, an electromagnetic field is coupled inside the chamber to excite a sustained high density plasma near the target surface. The plasma confined near the target surface contains positive ions (such as Ar+) and free electrons. The ions in the plasma strike the target surface and sputter material off the target. The wafers receive the sputtered material to form a deposited layer on the surface of the wafers. In one instance, up to twenty kilowatts of DC power can be provided on each target. In such a case, each target can deposit approximately 1 micron of metal per minute on an underlying work piece. 
         [0018]    The chamber wall is typically electrically grounded in processing operations. 
         [0019]    A bias voltage on the wafers can drive a flux of an electrically charged species (Ar+ and/or atomic vapor sputtered off the target) to the wafers. The flux can modify the properties (for example, density) of the sputtered material to the wafers. 
         [0020]    Generating a plasma for sputtering and the various biasing schemes are well known, and any of the known techniques may be implemented with the described sputtering system. 
         [0021]    In a preferred embodiment, the chamber gas is provided by a distribution channel at the bottom of the chamber, rather than from the top, which reduces particle contamination during the sputtering process and allows optimization of the magnetron assembly. 
         [0022]      FIG. 1  illustrates a motor  58  for rotating shaft  49 . Shaft  49  is directly coupled to the motor  58  so that pallet  36  is directly driven by motor  58 . The motor  58  surrounds shaft  49  and has a central rotating sleeve fixed to shaft  49 . Motor  58  may be a servo or stepper motor. In one embodiment, the motor is a servo motor that uses an absolute encoder attached to shaft  49  to determine the angular position of shaft  49 . A typical RPM of pallet  36  during the deposition process is 5-30 RPM. 
         [0023]    A seal  57  provides a seal around shaft  49  in order to maintain a low pressure in the chamber. 
         [0024]    A cross-contamination shield  96  helps confine sputtered material to an area under the target. 
         [0025]    The sputtering system  12  uses a magnetron assembly, outside the vacuum, to further control the bombardment of the target by the plasma. A magnet  60  is located behind each target  43  so that the plasma is confined to the target area. The resulting magnetic field forms a closed-loop annular path acting as an electron trap that reshapes the trajectories of the secondary electrons ejected from target into a cycloidal path, greatly increasing the probability of ionization of the sputtering gas within the confinement zone. Inert gases, specifically argon, are usually employed as the sputtering gas because they tend not to react with the target material or combine with any process gases and because they produce higher sputtering and deposition rates due to their high molecular weight. Positively charged argon ions from the plasma are accelerated toward the negatively biased target and impact the target, resulting in material being sputtered from the target surface. 
         [0026]      FIG. 1  illustrates one of the three prior art magnets  60  overlying a target backing plate  59 , where the target backing plate  59  is supported by and electrically insulated from a grounded top plate  62  in the sputtering system  12 . An insulating bracket  67  secures each magnet  60  to a scanning actuator  66  (e.g., a reciprocating motor) so that there is a minimum gap between the oscillating magnet  60  and the target backing plate  59 . Magnet  60  has a substantially triangular or delta shape with rounded corners and has about the same length as the substantially triangular target  43  but narrower. Two other identical magnets (not shown) are located above two other targets centered at 120 degree intervals. The actuator  66  is controlled by a controller to oscillate the three magnets  60  back and forth in unison over their associated targets at an oscillating period of between 0.5-10 seconds. The magnets  60  are oscillated so that the magnetic fields are not always at the same position relative to the target. By distributing the magnetic fields evenly over the target, target erosion is more uniform. 
         [0027]    The size of magnets  60  depends on the size of the wafers, which determines the size of the targets. In one embodiment, a magnet  60  is about 10.7 inches (27 cm) long and about 3 inches (7.6 cm) wide at its widest part. An eight inch wafer may use a target that is from 10-13 inches long in the radial direction. A twelve inch wafer may use a target that is from 13-18 inches long in the radial direction. 
         [0028]    Since the plasma makes the targets hot, a coolant channel is provided in each target support plate  59  through which a coolant flows. The highest heat is generated under the magnet  60 . Adequate and uniform cooling becomes a problem for high density plasmas. 
         [0029]    The structure of  FIG. 1  has been improved by the assignee by providing a vertical wall of confining magnets surrounding the target  43  and extending downward toward the pallet. The wall of confining magnets confines the sputtered ions to an area over the wafer  41  and directs the sputtered ions in a more normal path relative to the wafer surface. The improved sputtering system is described in U.S. application Ser. No. 12/239,644, filed Sep. 26, 2008, entitled Confining Magnets in Sputtering Chamber, incorporated herein by reference. 
         [0030]    Although the sputtering system  12  is very good, there is a practical limit on the ionization power that is supplied to maintain a high sputtering rate. The scanning magnet  60  covers approximately one-half the target at any instant, and the ionization power must be sufficient to create a high density plasma in the area of each target being influenced by the relatively large area magnetic field for a desired high sputtering rate. Such high power requires a large amount of cooling of the target backing plate  59  via the coolant channel in the plate  59 . Uneven cooling of the target results in nonuniform sputtering and erosion. 
         [0031]    Additionally, the large magnet causes some nonuniform erosion/sputtering of the target due to some target areas being subjected to different average magnetic fluxes over time. 
         [0032]    What is needed is an improved sputtering system  12 , using mostly its existing components, that achieves a higher sputtering rate with the same ionization power and cooling of the target, and which provides more uniform erosion of the target. 
       SUMMARY 
       [0033]    In a preferred embodiment, each of the large scanning magnets in  FIG. 1  is replaced with a much smaller substantially triangular magnet that is controlled to move linearly along the length of the substantially triangular target as well as simultaneously scanned in an arc by a scanning actuator. After a complete cycle, the small magnet has moved over a substantially triangular area substantially corresponding to the shape of the target. The scanning magnet will typically be one-quarter to one-half the size of the magnet in  FIG. 1 . 
         [0034]    In one embodiment, the same central scanning actuator of  FIG. 1  has connected to it three arms, each connected to a relatively small magnet. Each arm includes a motor that rotates a screw gear connected to a back surface of the magnet to move the magnet linearly up and down the arm while the arm is simultaneously scanning back and forth in a small arc across the target. The motor reverses direction when the magnet reaches the end of the target. After one or more cycles of the linear and scanning movements of the magnet, the magnet covers a substantially triangular area corresponding to the area of the target. 
         [0035]    The smaller magnet exhibits about the same magnetic flux density (gauss) as the larger magnet of  FIG. 1 . In one embodiment, the smaller magnet is moved over the entire surface of the target within a cycle of about 1-5 seconds to achieve the same sputtering rate as the larger magnet scanning over the surface of the target during the same period. The inventors have found that higher ionization (power density) was achieved at the target compared to the ionization using the larger magnet of  FIG. 1 , with the same input power, due to a surprising phenomenon. The particular movement of the smaller magnet over the target also resulted in more uniform erosion of the target. 
         [0036]    The relative cycle periods of the linear motor and scanning actuator are set so that, over a suitable time period, the magnet scans over substantially the entire surface of the target. Such suitable cycles depend on the particular shapes of the target and magnet. In one embodiment, the linear motor varies its speed depending on the position of the magnet along the screw gear, such as to slow the linear movement of the magnet near the wide end of the target, while the magnet scans through arcs at a constant rate, to achieve uniform coverage of the target by the magnet. In other words, the linear movement of the magnet is varied so that the magnet overlies all portions of the target about equal times during a complete cycle. In one embodiment, the magnet moving along the linear path actually stops (dwells) at one or more positions along the path, such as at the widest portion of the target, while the magnet continuously scans through arcs back and forth across the target. Such dwells times at various positions are for the purpose of achieving a desired erosion over the entire surface of the target. 
         [0037]    In one embodiment, the target is 15-17 inches long, requiring the arm to be a similar length. 
         [0038]    The new magnetron may be used in conjunction with the system of  FIG. 1  or any other suitable system using substantially triangular targets. The triangular targets enable multiple targets to be used in a circular chamber in conjunction with a rotating pallet that positions wafers beneath suitable ones of the targets for a compact and versatile sputtering system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1  is a cutaway view of the present assignee&#39;s prior art sputtering tool. 
           [0040]      FIG. 2  is a cross-sectional view of the rotating shaft, table, and pallet in the prior art sputtering tool of  FIG. 1 . 
           [0041]      FIG. 3  is a bottom up view of a portion of an inventive magnetron to replace the prior art magnetron of  FIG. 1 . 
           [0042]      FIG. 4  is a top down view of the complete magnetron simultaneously scanning three substantially triangular magnets over three larger substantially triangular targets. 
           [0043]      FIG. 5  is a bottom up view of a single magnet illustrating the layout of small magnets mounted on a plate in a substantially triangular pattern. 
       
    
    
       [0044]    Elements with the same numbers in the various figures are the same. 
       DETAILED DESCRIPTION 
       [0045]      FIG. 3  is a bottom up view of the magnetron, looking up through the top wall of the vacuum chamber. 
         [0046]    A scanning actuator  70 , located at the center of the top wall of the chamber, rotates through an arc about axis  72  and then reverses. The actuator  70  may the same as actuator  66  in  FIG. 1 . 
         [0047]    A scanning controller  73  controls the actuator  70  to rotate through a predetermined angle (less than 120 degrees) and then reverse direction. 
         [0048]    Attached to the actuator  70  and extending radially is an arm  74 . The arm  74  comprises a support beam with a slot. Within the slot is a screw gear  78 . A motor  80  at the base of the arm  74  is connected to the screw gear  78  for turning the screw gear  78  in one direction through a predetermined angular rotation and then turning the screw gear  78  in the opposite direction through a predetermined angular rotation. The motor  80  may be a servo motor or a stepper motor. 
         [0049]    A linear movement controller  82  controls the motor  80  speed and direction. Since the speed can be variable during a single cycle, the controller  82  is programmable. 
         [0050]    A permanent magnet  84  has a substantially triangular shape, being formed of three straight edges and rounded corners. The magnet  84  comprises a ferrous backing plate populated with a pattern of relatively small magnets affixed to the plate. The magnet  84  will be described in more detail later with respect to  FIG. 5 . Many different embodiments of magnets may be suitable. 
         [0051]    The view of  FIG. 3  is of the front of the magnet  84  that faces the target  86 . The target  86  is shown in dashed outline since the view of  FIG. 3  is through the target  86 , through the top wall of the chamber, and through the target backing plate. 
         [0052]    Affixed to the back of the magnet&#39;s backing plate is a standard screw gear threaded sleeve that receives the screw gear  78  so that rotation of the screw gear  78  causes the magnet  84  to move linearly with respect to the arm  74  while remaining in a single plane. 
         [0053]    As the scanning actuator  70  scans at a constant rate, the motor  80  moves the magnet  84  at either a constant speed or a varying speed up and down the length of the target  86  so that ultimately the magnet  84  covers substantially the entire area of the triangular target  86  after one or more cycles of the repetitive angular and linear movement of the magnet. In one embodiment, the actuator  70  completes a back and forth scan in between 1-3 seconds, and the motor  80  completes and up and down movement of the magnet between 1-5 seconds. The cycle times may be greater or less than these periods, depending on the sizes of the target and magnet. 
         [0054]    Due to the triangular shape of the target  86 , to achieve uniform coverage of the target  86  by the magnet  84 , the linear motor  80  varies its speed depending on the position of the magnet  84  along the screw gear  78 . For example, the linear movement of the magnet  84  is slowed near the wide end of the target, and may even be dwelled, while the magnet scans through arcs at a constant rate. In other words, the linear movement of the magnet may be varied so that the magnet overlies all portions of the target about equal times during a complete cycle. Additionally, due to edge effects, the plasma may not be uniformly created across the surface of the target  86 , and the linear speed of the magnet  84  may be controlled to vary to cause substantially uniform erosion of the target even though the magnet  84  does not overlie all portions of the target  86  for equal times during a complete cycle. For example, the speed of the magnet  84  may need to be slowed or dwelled at the narrow end of the target  86 . The variation in speed may be programmed into the controller  82  based upon empirical data after long periods of testing and examining the erosion of the target  86 . 
         [0055]    In one embodiment, the variation in linear speed may not repeat for each linear scan of the magnet  84  in order to achieve full coverage of the target. 
         [0056]    In one embodiment, the magnet moving along the linear path actually stops (dwells) at one or more positions along the screw gear, such as at the widest portion of the target  86 , while the magnet continuously scans through arcs back and forth across the target. Such dwells times at various positions are for the purpose of achieving a desired erosion (e.g., uniform erosion) over the entire surface of the target. Without such dwelling, certain areas of the target may be overlapped by the magnet more than others due to the multi-direction scanning of the magnet, or some areas may be not covered due to the interaction of the multi-direction scanning. In one embodiment, the magnet dwells at a position along the screw gear for up to five seconds by stopping the linear motor  80 . Such control by the linear motor  80  is programmed into the motor&#39;s controller and may be based on computer simulation and actual testing results. When a dwell time is used, a period for the motor  80  to move the magnet up and down the arm  74  may exceed 20 seconds. 
         [0057]    In one embodiment, the magnet  84  has a shape generally corresponding to the shape of the target but smaller in all dimensions. The magnet will typically be between one-quarter to one-half the size of the target. Since the magnet  84  is smaller than a magnet having the same length as the target, it must be scanned along the length of the target to fully cover the target over time. The smaller magnet will create a higher power density, compared to a full-length bigger magnet, for the same input power into the system because all the power is concentrated in a smaller footprint. This increases the ion concentration at the target, which increases the deposition rate. 
         [0058]      FIG. 4  is a top down view illustrating three magnets  84 ,  88 , and  90  in a sputtering system  92 . Target backing plates  94 ,  96 , and  98  are shown, where the targets inside the vacuum chamber are affixed to the undersides of the target backing plates  94 ,  96 , and  98  and have the same general shape and size as the plates  94 ,  96 , and  98 . Each magnet is connected to an arm identical to the arm  74  in  FIG. 3 . All the motors for the screw gears  78  ( FIG. 3 ) may be controlled identically by the single linear movement controller  82 . The magnets may all be identical and the sizes of the targets may all be identical but may be composed of different materials. The amount of sputtering is dependent on the time that the wafer is below the target, the plasma density, the biasing voltages, and other factors. 
         [0059]    In  FIG. 4 , ghost images of the magnet  90  are shown in dashed outline to show two positions of the magnet  90  during a scanning cycle at two instances in time, and the magnet  90  will ultimately overlap all areas of the target after a certain period, then repeat the cycle so that the moving magnetic field generated by the magnet over time will have covered a substantially triangular shape and provide a substantially uniform erosion of the target, resulting in uniform sputtering onto the underlying wafer  41  ( FIG. 1 ). 
         [0060]      FIG. 4  also shows a support structure  100  that affixes the scanning actuator  70  on the top wall  102  of the chamber. An outer wall  104  of the chamber is shown, which defines the outer perimeter of the substantially circular top wall  102  of the vacuum chamber that supports the electrically insulated target backing plates  94 ,  96 , and  98 . The top wall insulation is described in detail in U.S. Pat. No. 7,479,210 discussed above. By making the targets and target backing plates substantially triangular, at least three targets can be arranged around the circular chamber, resulting in a very compact sputtering system. 
         [0061]      FIG. 5  illustrates one possible arrangement of magnets  106  on a ferrous backing plate  108 . There are three rings (nested patterns) of individual magnets  106 , where adjacent rings have opposite poles so that a magnetic field spans across one ring to the next. Some magnetic field lines  110  are shown. Since there are three rings of magnets, there are two racetracks of field lines. These magnetic fields pass through the target backing plate  59  ( FIG. 1 ) and intersect the target  43  (or  94 ,  96 ,  98  in  FIG. 4 ) attached to the underside of the target backing plate  59 . The plasma density at the target (and thus the erosion rate) is greatest at the highest magnetic field intensity. The sizes, shapes, and distribution of the individual magnets  106  are selected to create a uniform erosion of the target as the magnet is scanned over the target. 
         [0062]    The individual magnets  106  along the edge of magnet  84  are smaller that the inner magnets so that the magnetic field extends close to the edge of the magnet. The span of a magnetic field can be approximated by the distance between the centers of the two opposite poles. Hence, the diameters of the outer magnets  106  are made small (e.g., 0.5-1 cm). The inner rings of magnets  106  may be larger. In the example, the magnets  106  may be rectangular or circular. 
         [0063]    The magnetron assembly of  FIG. 4  simply replaces the magnetron assembly of  FIG. 1  with no other changes to the system. 
         [0064]    The described sputtering system allows for all three targets to concurrently sputter the same or different materials on the wafers during a batch process. This increases throughput and allows the sputtering of alloys or layers on the wafers without breaking a vacuum. To select an alloy composition, one target may be one material, and the other two targets may be a second or third material. For depositing stacked layers of distinct materials, then only one material may be deposited at a time (e.g., one target energized at a time or multiple targets of the same material energized at a time). For depositing mixed layers (e.g. alloys of distinct materials), then all targets may be energized at the same time, assuming the targets are of different materials. 
         [0065]    More targets and wafers than shown in the examples may be employed in the system. For example, there may be eight targets. The number of such targets is limited only by the ability to build increasingly narrow magnets, which deliver a suitable magnetic flux on the target surface. 
         [0066]    Conventional aspects of the system that have not been described in detail would be well known to those skilled in the art. U.S. Pat. No. 6,630,201 and U.S. Patent Application Publication 2002/0160125 A1 are incorporated herein by reference for certain conventional aspects primarily related to creating a plasma and supplying gas to a process chamber. 
         [0067]    Although the system has been described with respect to forming a metal film on semiconductor wafers, the system may deposit any material, including dielectrics, and may process any workpiece such as LCD panels and other flat panel displays. In one embodiment, the system is used to deposit materials on multiple thin film transistor arrays for LCD panels. 
         [0068]    Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.