Abstract:
A device for holding wafer-shaped articles, such as semiconductor wafers, is equipped with a series of pins that are brought into contact with a peripheral edge of the wafer-shaped article, under control of a common gear ring or a series of conjointly operated gear sectors. In the regions of the gear ring or gear sectors engaging the pin assemblies, those elements are designed to yield more readily than other regions of the gear ring or gear sectors, to accommodate differential thermal expansion of the chuck components in the vicinity of the pin assemblies.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to devices for holding wafer-shaped articles, such as semiconductor wafers. 
         [0002]    Semiconductor wafers are subjected to various surface treatment processes such as etching, cleaning, polishing and material deposition. To accommodate such processes, a single wafer may be supported in relation to one or more treatment fluid nozzles by a chuck associated with a rotatable carrier, as is described for example in U.S. Pat. Nos. 4,903,717 and 5,513,668. 
         [0003]    The patents referenced above operate on the Bernoulli principle, such that the wafer receives subjacent support from a gas cushion rather than by contact with the chuck. Such chucks nevertheless ordinarily include a circular series of pins that are positioned radially outwardly of a wafer positioned on the chuck. Those pins prevent lateral displacement of the wafer relative to the chuck. 
         [0004]    As described in U.S. Pat. No. 4,903,717, each pin projects upwardly from a respective pivotal base. The axes of the pin and base are vertical but offset from one another such that pivoting of the base causes the associated pin to travel along a circular arc and hence be adjustable in its radial position. The pivot bases are each provided with gear teeth, which mesh with the teeth of a common gear ring that is coaxial with the axis of rotation of the chuck. Rotation of the gear ring relative to the chuck thus causes all of the pins to move conjointly and to the same extent. 
         [0005]    That construction permits the pins to be moved radially outwardly for placement or removal of a wafer, and then to be moved radially inwardly to be brought into contact with the peripheral edge of a wafer. Such contact prevents not only lateral displacement of the wafer relative to the chuck, but also relative rotation between the wafer and chuck as the chuck is spun. 
         [0006]    Other spin chucks operate under control of magnetic fields, by fashioning the chuck body as a magnetic rotor positioned coaxially within a surrounding annular magnetic stator, as described for example in U.S. Pat. No. 6,485,531. In such chucks the rotary head supports the wafer. The present assignee has designed such chucks utilizing pins having characteristics in common with those described above, but which serve in this context also to support the weight of the wafer. 
         [0007]    Spin chucks are subjected to extreme temperatures and highly acidic etching solutions in use. Designs that perform without incident under one set of processing conditions might perform with less consistency under different processing conditions. 
       SUMMARY OF THE INVENTION 
       [0008]    The present inventors have discovered that, in chucks of the type described above, the pins do not always open and close reliably under extreme processing conditions. According to the present invention, a device for holding wafer-shaped articles, such as semiconductor wafers, is equipped with a series of pins that are brought into contact with a peripheral edge of the wafer-shaped article, under control of a common gear ring or a series of conjointly operated gear sectors. In the regions of the gear ring or gear sectors engaging the pin assemblies, those elements are designed to yield more readily than other regions of the gear ring or gear sectors, to accommodate differential thermal expansion of the chuck components in the vicinity of the pin assemblies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of preferred embodiments of the invention, given with reference to the accompanying drawings, in which: 
           [0010]      FIG. 1   a  is a perspective view, partly in section, illustrating a device according to one embodiment of the invention; 
           [0011]      FIG. 1   b  is a perspective view, also partly in section, of the detail D 1   b  of  FIG. 1   a;    
           [0012]      FIG. 1   c  is a view corresponding to that of  FIG. 1   b,  in which the stator and hence also the chuck have been elevated relative to the cylindrical wall of the process chamber; 
           [0013]      FIG. 1   d  is a view corresponding to that of  FIG. 1   e,  in which the chuck is in a different angular orientation to expose a pin assembly; 
           [0014]      FIG. 2  is a plan view, partly broken away, illustrating the connection between the gear ring, pin assemblies and the chuck, and the interaction of the gear ring and its associated control mechanism; 
           [0015]      FIG. 3  is a radial sectional view of a first example of a gear ring or gear sector according to the invention; 
           [0016]      FIG. 4  is a radial sectional view of a second example of a gear ring or gear sector according to the invention; 
           [0017]      FIG. 5  is a radial sectional view of a third example of a gear ring or gear sector according to the invention; 
           [0018]      FIG. 6  is a radial sectional view of a fourth example of a gear ring or gear sector according to the invention; 
           [0019]      FIG. 7  is a radial sectional view of a fifth example of a gear ring or gear sector according to the invention; 
           [0020]      FIG. 8  is a radial sectional view of a sixth example of a gear ring or gear sector according to the invention. 
           [0021]      FIG. 9  is a perspective view from above of a second embodiment of a chuck according to the invention; 
           [0022]      FIG. 10   a  is a perspective view from below of the  FIG. 9  embodiment; and 
           [0023]      FIG. 10   b  is a schematic perspective view of one example of the gear ring  30 ′ within detail D 10   b  of  FIG. 10   a.    
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    The device  100  of  FIG. 1   a  comprises a chamber, an annular chuck  20  for gripping and rotating a wafer (disc-like article) and a stator  80 . The chamber comprises a cylindrical wall  60 , a bottom plate  65  and a top plate (not shown). An upper dispensing tube  63  is led through the top plate and a lower dispensing tube  67  through the bottom plate  65 . 
         [0025]    Stator  80  is mounted to a stator base plate  5  and is concentric with the cylindrical wall  60 . The stator base plate  5  can be moved axially along the axis of the cylindrical wall  60 , e.g. with pneumatic lifting devices. The stator base plate  5  and the stator  80  mounted thereto have central openings, whose diameter is greater than the outer diameter of the cylindrical wall  60 . The top plate can also be moved axially to open the chamber. In its closed position the top plate is sealed against the cylindrical wall  60 . 
         [0026]    The stator  80  comprises several coils  84  for axial and radial orientation and for driving the rotor  85 , which is part of the annular chuck. Such arrangement is further described in U.S. Pat. No. 6,485,531. The diameter of the annular chuck  20  is less than the inner diameter of the cylindrical wall so that it can freely levitate and rotate within the cylindrical wall  60 . The annular chuck  20  comprises an inner chuck base body  21  with an annular groove circumferentially surrounding the outside of the inner chuck base body  21 , with the annular groove receiving the gear ring  30 . The gear ring  30  is preferably made of PEEK, aluminum, or stainless steel. 
         [0027]    Gear ring  30  comprises inwardly facing teeth  31 . The inwardly facing teeth  31  in turn drive the teeth of a pin shaft  27  (see  FIG. 1   c ). 
         [0028]    This embodiment has six downwardly oriented pin shafts  27 , each of which has a small gear, which is driven by the gear ring  30 . The pin shafts  27  are mounted so that they can turn about an axis A, which is parallel to the rotation axis of the annular chuck. 
         [0029]    A pin  28  is mounted to or formed integrally with each pin shaft  27 , at a position that is eccentric with respect to the axis of rotation A of the pin shaft  27 . Consequently, the pins  28  are displaced radially of the chuck when the pin shafts  27  are turned by the gear ring  30 . As the pins and the gear ring  30  are both carried by the chuck base body  21 , the pins shafts  27  are rotated by the gear ring  30  only when the gear ring  30  rotates relative to the chuck base body. 
         [0030]    Pins  28  are positioned so as to contact a wafer W on its peripheral edge. As the pins  28  also support the weight of the wafer W, the pins  28  may either be cylindrical in shape or have recessed portions on their radially inwardly facing sides contacting the wafer edge, to prevent axial displacement of the wafer W relative to the pins  28  when the wafer is being gripped. 
         [0031]    In order to mount the gear ring  30  into the annular groove of the chuck base body  21  the gear ring  30  consists of two separate segments, which are fixed together when inserted into the annular groove. The chuck base body  21  and the gear ring  30  are connected via one or more helical springs  40  (see  FIG. 2 ) so that the gear ring  30  urges the pins  28  into their radially innermost positions, corresponding to the gripping of a wafer. 
         [0032]    Two permanent magnets  33  (see  FIG. 1   b ) are mounted to the tooth gear ring  30 . A plurality of at least twenty-four rotor magnets  85 , which are permanent magnets, are evenly arranged around the chuck base body  21 . These rotor magnets  85  are part of the drive and positioning unit, namely, part of the rotor (elements of the active bearing), which is mounted to the chuck base body  21 . 
         [0033]    The plurality of rotor magnets  85  and the gear ring  30  carrying the permanent magnets  33  are encapsulated in a hollow annular space provided by the chuck base body  21 , outer lower chuck cover  22 , and the rotor magnet cover  29 . Such rotor magnet cover  29  can be a stainless steal jacket. The covers  22  and  29  are annular and concentric with the chuck base body  21 . 
         [0034]    When assembling the chuck  20  the pin shafts  27  are inserted from above into their respective seats so that the pin shafts tightly seal against the chuck base body  21  as shown in  FIG. 1   c.  Each pin shaft  27  is fixed in position with a screw  24 . Additionally, each pin shaft may be pressed into its seat by a helical spring between the pin shaft and the screw. 
         [0035]    Attached to the stator base plate  5  is the stator and active positioning unit  80  which is concentrically arranged with respect to the cylindrical wall  60 . The positioning unit  80  corresponds with the rotor magnets  85  therefore levitating, positioning and rotating the chuck  20 . 
         [0036]    Below the active positioning unit  80  there are two pneumatic cylinders  50  mounted to the stator base plate  5 . On the distal ends of the rods of the pneumatic cylinders  50  locking magnets  55  (permanent magnets) are arranged. The locking magnets correspond to the permanent magnets  33  of the gear ring  30 . The pneumatic cylinders  50  are arranged so that the locking magnets  55  can be radially moved with respect to the axis of the cylindrical wall  60 . 
         [0037]    When the pins are to be opened e.g. to release a wafer the following procedure is conducted: the stator base plate  5  is lifted and therewith the levitating chuck  20  so that the cylindrical wall  60  is no longer in the gap between the locking magnets  55  and the chuck  20  (see  FIG. 1   c ). Thereafter the pneumatic cylinders  50  move the locking magnets  55  in close proximity to the chuck  20  and the chuck is turned so that the permanent magnets  33  and therewith the gear ring  30  is locked by the locking magnets. Now the chuck is turned while the gear ring stands still and thus the pins  28  open. Alternatively the chuck base body might stand still while the pneumatic cylinders are moved so that the locking magnets tangentially turn (along the circumference of the chuck), whereby the gear ring is turned. 
         [0038]    However, as described above, the present inventors have discovered that, at higher process temperatures, the pins of a chuck such as that described could fail to open upon completion of processing, or when time to effect a wafer shift during processing. The pins and their associated activating mechanism are not observable when the chuck is in use, as the processing chamber might contain hot sulfuric acid or water superheated to temperatures on the order of 120° C. Moreover, the gear ring  30  of this embodiment is seated in a groove in the chuck body  21 . 
         [0039]    The present inventors nevertheless identified the source of this problem as being that the ring gear  230 , which is preferably made of stainless steel, undergoes thermal expansion at high temperatures to a lesser extent than does the chuck base body  21 , which is preferably made of plastic, such as PVDF (polyvinylidene fluoride), or ECTFE (ethylene-chlorotrifluoroethylene). As the pin shafts  27  are fitted in the chuck base body  21 , the relatively greater expansion of the chuck base body  21  would displace the pin assemblies radially outwardly, without a corresponding displacement of the gear ring  30 , such that the pins  28  would be pressed against the teeth of gear ring  30  with sufficient force to cause the gear ring  30  to jam. The phenomenon can also be understood as the gear ring  30  urging the pin assemblies radially inwardly such that the pin shafts  27  would seize within the expanded chuck base body  21 . As the pin assemblies are in meshing engagement with the gear ring  30 , the result is the same, i.e., seizing of the gear ring  30 . 
         [0040]    Having identified the source of the problem, the inventors devised various solutions thereto involving weakening the gear ring in at least those regions where the gear ring contacts the respective pin assemblies, to accommodate the high temperature displacement of the pin assemblies arising from the thermal expansion of the chuck base body. 
         [0041]      FIGS. 3-8  illustrate several examples of structures designed to implement the present invention. Each of those figures shows in radial section a portion of ring gear  30  that includes the region contacting a respective pin assembly. Thus, in the present embodiment, the structure depicted in each of these figures would be repeated six times over the circumference of the gear ring  30 , with the six sectors being interconnected by solid or unmodified regions of the gear ring  30  of approximately the same width as shown in those figures. 
         [0042]    The solution to that problem was that the area ( 31 ), where the tooth gear ( 30 ) touches the pin shaft ( 27 ), was embodied in a weaker form meaning having an area with a decreased young module. 
         [0043]    In  FIG. 3 , behind the teeth  31  there is an enclosed cut-out  32  that defines narrower region  33 ,  34  that are thereby significantly weakened. The material of the ring gear  30  has sufficient elasticity that the narrower region  33  will deflect radially outwardly upon thermal expansion of chuck base body  21  and associated displacement of pin assembly  27 , yet will return to its initial position as the chuck base body  21  cools and contracts. 
         [0044]    In  FIG. 4  the cut-out  32 ′ is not fully enclosed, so that the weakened region  33 ′ is in a cantilever configuration. 
         [0045]    In  FIG. 5  the teeth  31  are formed in a separate block  35  that is fitted within a cavity  36  formed in the gear ring  30 . Coil springs  37  urge the block  35  radially inwardly into meshing engagement with pin assembly  27 , yet accommodate the radially outward displacement of the pin assemblies  27  upon thermal expansion of chuck base body  21 . 
         [0046]    The examples of  FIGS. 6-8  operate on essentially the same principle as that of  FIG. 5 . In  FIG. 6 , the coil springs  37  of  FIG. 5  are replaced by leaf spring  38 . In  FIG. 7 , the springs of he previous examples are replaced by a body of elastomeric material  39  whose elasticity in compression accommodates outward displacement of the pin assemblies as would the springs of the previous examples. Lastly,  FIG. 8  shows block  35 ′ not fitted within cavity  36 ′ but rather connected to the edges of that cavity via strip springs  41 . 
         [0047]    Common to each of the examples of  FIGS. 3-8  is that the teeth area that drives the pin shaft when opening and closing the pins is significantly weakened relative to the full radial width of the ring gear in the adjacent regions. Thus, the weakened regions can be considered to have a lower effective Young&#39;s modulus than the adjacent non-weakened regions. 
         [0048]    With all these embodiments the maximum force that is applied to each small gear of the pin shaft is limited by these “weakened” areas, and is preferably limited to a value in the range of 5 to 20 N. 
         [0049]      FIG. 9  shows a spin chuck  10 ′ that operates on the Bernoulli principle, with a wafer W in position. Pin assemblies  27 ′ are mounted in the upper chuck body  11 , and the eccentric pins  28 ′ that engage the wafer edge project upwardly from the upper chuck body  11 . Upper chuck body  11  is joined to lower chuck body  12 , which in turn is mechanically driven via a shaft (not shown). 
         [0050]    In  FIG. 10   a  the chuck  10 ′ is shown from below, with the lower base body  12  having been removed. This reveals the somewhat different gear ring  30 ′ of this embodiment, in which teeth  31  are provided on the radially outer edge of the gear ring, in contrast to the previous embodiments. The circular arrow depicts the clockwise rotation of the chuck, although the arrow itself is directed counterclockwise due to the chuck being shown from below in this figure. 
         [0051]    Teeth  31  mesh with the corresponding teeth on pin assemblies  27 ′, while the body of the pin assembly  27  is seated within the upper base body  11 . Coil springs  43  connected between the gear ring  30 ′ and the upper base body  11  urge the gear ring toward an angular orientation relative to the upper base body  11  in which the pins  28 ′ projecting from the upper surface of the upper base body  11  are in their radially innermost position, corresponding to gripping a wafer W during use of the chuck. 
         [0052]    The detail D 10   b  of  FIG. 10   a  is show in schematic cross-section in  FIG. 10   b , in which only the gear ring is depicted for ease of understanding.  FIG. 10   b  is similar to  FIG. 3 , but shows the teeth  31  on the radially outward narrow region  44 , as the pin assemblies  27  of this embodiment lie radially outward of the gear ring  30 ′. Without further depiction it will be recognized that the examples of  FIGS. 4-8  can be similarly adapted for this embodiment. 
         [0053]    Although the present invention has been described in connection with spin chucks it may also be used in a non-rotating chuck. Furthermore, although the invention has described in connection with a chuck used for wet chemical processing, it could also be used for dry processes. 
         [0054]    While the present invention has been described in connection with various preferred embodiments thereof, it is to be understood that those embodiments are provided merely to illustrate the invention, and should not be used as a pretext to limit the scope of protection conferred by the true scope and spirit of the appended claims.