Abstract:
A rotary union is provided for chemical/mechanical polishing of silicon wafers, especially silicon wafers containing chemically sensitive integrated circuit structures. A rotary union is provided with a union rotor and union stator, coupled at the free end of a support spindle carrying a CMP polishing table. In the preferred embodiment the rotary union is joined to a coolant union forming the lower end of the support spindle. A passageway through the union rotor extends past the bottom end of the coolant union rotating part to avoid contact of fluid transmitted through the passageway formed in the union rotor, with the rotating part of the coolant union.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention pertains to rotary unions for semiconductor wafer applications, such as chemical/mechanical polishing. In particular, the present invention pertains to such rotary unions which conduct a supply of “ultra pure” water which contacts the semiconductor wafer and must therefore be chemically compatible with the semiconductor wafer.  
           [0003]    2. Description of the Related Art  
           [0004]    Silicon wafers are typically employed for the mass production of commercially important integrated circuits. A plurality of integrated circuit devices are formed on a silicon wafer substrate, layer by layer, and chemical/mechanical polishing (CMP) must be performed on the wafer,. between layering steps. Layering is typically carried out using photolithographic techniques which require an accurately flat surface.  
           [0005]    Planarization of silicon wafers provides the high degree of flatness required for integrated circuit fabrication using photolithographic techniques. The active surface of the wafer substrate is placed in contact with a rotating polishing pad in the presence of various chemical agents which can include deionized water, etchants and polishing slurries. The polishing of commercially significant silicon wafers can include a more aggressive material removal process in which a slurry of polishing particles includes a chemically reactive agent. While it is desirable to polish a semiconductor wafer as quickly as possible in order to obtain the desired flatness or planarization, it is important that the over polishing be avoided. This requires a constant or near constant monitoring of the polishing process.  
           [0006]    One type of polish monitoring employed today uses optical and other types of sensors embedded in a polish table. As mentioned, slurries and other types of chemical mixtures are employed in chemical/mechanical polishing and other types of wafer treatments. Typically, the polish table is flooded with slurry which also covers or otherwise interferes with the monitoring instrumentation. Accordingly, it is customary to wash the active face of the monitoring instrumentation which may comprise, for example, the free ends of optical wave guides embedded in the polish table. A flushing medium is employed to displace slurry or other wafer treatment chemicals from the active surface of the monitoring instrumentation.  
           [0007]    In addition to flushing away material from the active face of the monitoring instrumentation, the flushing media must be compatible with the semiconductor wafer in all respects, especially in the sense of being chemically compatible with the wafer substrate and the integrated circuit structures built on the water substrate. Water is frequently chosen as the flushing medium since it is relatively inert in many respects. However, even “pure” water must be treated to attain very high levels of chemical inertness with regard to the semiconductor wafer and the term “ultra pure” has been applied to describe these special requirements. In order to maintain its ultra pure qualities, even brief incidental contact with metallic components must be avoided.  
           [0008]    As mentioned, chemical/mechanical polishing is carried out using a polishing table and accordingly a rotating support shaft is customarily employed. In addition, devices used to contact the wafer with the polishing pad are also rotationally driven. These types of wafer-holding devices, usually termed wafer carriers, oftentimes are called upon to supply a fluid as part of the wafer treatment process. Thus, fluid communication must be maintained between the rotating wafer carrier and an external, non-rotating source.  
           [0009]    Rotary unions, such as those described in U.S. Pat. No. 5,443,416 provides continuous fluid communication between a fluid source and a fluid chamber associated with the rotating wafer carrier. Although similar in some respects, a rotating polish table is much more massive than typical wafer carriers, and is subjected to much greater forces. If a rotary union is to be provided with fluid communication, a different type of arrangement from those employed in wafer carriers is needed. And, if a rotary union of a polish table is required to provide continuous fluid communication, different arrangements, other than those employed with wafer carriers, must be provided. Solutions to these and other problems attendant with polish table used in chemical/mechanical polishing are continually being sought.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide a rotating union for a polish table used in chemical/mechanical polishing of semiconductor wafers.  
           [0011]    Another object of the present invention is to provide a rotary union of the above type which provides a continuous fluid communication from a remote stationary fluid source to the polishing table and semiconductor wafers.  
           [0012]    A further object of the present invention is to provide rotating union of the above-described type which maintains the desired condition of ultra pure fluids, such as ultra pure water, as it flows through the rotary union to eventually contact directly or indirectly, a semiconductor wafer being treated on the polishing table.  
           [0013]    These and other objects according to principles of the present invention are provided in a rotary union for mounting to a rotating element which has an element bore wall defining an element bore of preselected size. The rotary union maintains semiconductor wafer treatment fluids in an ultra pure condition and comprises a union stator having a support face, a union rotor having a support face and an opposed mounting face adjacent the rotating element and at least one mount for movably mounting the union rotor toward and away from the rotating element. The union rotor defines a union bore of smaller size than said element bore. a spring bias between said union rotor mounting face and said rotating element, biasing said union rotor away from said rotating element, and a face seal between said union stator support face and said the union rotor support face, said face seal in the form of a flat washer and comprised of expanded TEFLON material. The union rotor also defines a passageway for the semiconductor wafer treatment fluids, said passageway extending from said union rotor support face to a portion of said union rotor mounting face radially interiorly of said element bore wall.  
           [0014]    Other objects according to principles of the present invention are attained in a rotary union for mounting to a metallic rotating element which has an element bore wall defining an element bore of preselected size. The rotary union maintains semiconductor wafer treatment fluids in an ultra pure condition and comprises a union stator having a support face, a union rotor having a support face and an opposed stepped mounting face adjacent the rotating element, the union stator and the union rotor of nonmetallic composition which maintains semiconductor wafer fluids in an ultra pure condition, and a plurality of elongated fasteners movably mounting the union rotor toward and away from the rotating element. The union rotor defines a union bore of smaller size than said element bore, a spring bias between said union rotor mounting face and said rotating element, biasing said union rotor away from said rotating element, and a face seal between said union stator support face and said the union rotor support face, said face seal in the form of a flat washer and comprised of expanded TEFLON material. The union rotor defines a passageway for the semiconductor wafer treatment fluids, said passageway extending from said union rotor support face to a portion of said union rotor mounting face radially interiorly of said element bore wall. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is an exploded perspective view of a rotary union according to principles of the present invention;  
         [0016]    [0016]FIG. 2 is a cross-sectional view taken along the line  2 - 2  of FIG. 1;  
         [0017]    [0017]FIG. 3 is a cross-sectional view of a rotating polish table and the rotary union of FIG. 2;  
         [0018]    [0018]FIG. 4 is a fragmentary view of the bottom portion of FIG. 3, taken on an enlarged scale;  
         [0019]    [0019]FIG. 5 is a cross-sectional view similar to that of FIG. 3, but showing additional features of a monitoring instrumentation system; and  
         [0020]    [0020]FIG. 6 is a cross-sectional view taken along the line  6 - 6  of FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring now to the drawings, and initially to FIGS. 1 and 2, a rotary union  10  according to principles of the present invention is shown in combination with a coolant union  12  having a rotor portion  14  and a stator portion  16 . The coolant union  12  is supported from above by a polish table supporting spindle assembly  20  which includes a rotating spindle portion  22  and a stationary spindle portion  24  (see FIG. 3). As will be described below, it has been found convenient to secure rotary union  10  to coolant union  12 . The rotary union  10  could also be connected directly to spindle assembly  20 , if desired, and such is contemplated by the present invention. When the rotary union  10  is fit to the spindle shaft, support bearings could be provided between the rotor and stator members  30 ,  32  of rotary union  10 . If desired, rotary union  10  could also be fit to a fluid distribution manifold.  
         [0022]    Turning now to FIGS.  1 - 3 , rotary union  10  comprises a rotating part or union rotor  30  and a stationary part or union stator  32 . As shown in FIG. 3, threaded fasteners  36  extend through holes  38  (see FIG. 2) to secure union stator  32  to the stationary part  16  of coolant union  12 . Referring to FIGS. 1 and 2, threaded fasteners  42  having threaded portions  42   b  and shoulder portions  52   a  secure union rotor  30  to the rotating part  14  of coolant union  12 . Preferably, fasteners  42  comprise shoulder bolts with shoulders dimensioned sufficiently long as to permit union rotor  30  to reciprocate back and forth along shoulder portions  42   a , as schematically indicated in FIG. 2 by a spacing  46  between shoulder portion  42   a  and enlarged head  42   c  of fastener  42 . Fasteners  42  function as a mount for movably mounting union rotor  30  toward and away from rotating part  14  of coolant union  12 . A spring member  48  is located between rotating part  14  of coolant union  12  and union rotor  30 , so as to bias union rotor  30  in a direction away from rotating part  14  urging a lower face  30   a  of union rotary  30  toward an opposing inner face  32   a  of union stator  32 .  
         [0023]    Disposed between opposed faces  30   a  and  32   a  of union rotor  30  and union stator  32  is a face seal  50  having the form of a flat washer or disk with a central aperture  52  generally co-extensive with a central aperture  54  of union stator  32 . As can be seen in FIG. 2, these apertures are generally co-extensive with an aperture  56  defined by rotating part  14  of coolant union  12 . Referring to FIG. 1, guide pins  60  in rotating part  14  of coolant union  12  align union rotor  30  to coolant union  12 . Alignment between the rotating part of coolant union  12  and union rotary  30  is also provided by a step portion  64  formed at the upper end of union rotor  30  dimensioned so as to be received in a central bore  66  defined by rotating part  14  of the coolant union.  
         [0024]    As can be seen, for example, in FIGS. 2 and 4, step portion  64  includes a recess or socket portion  70  for receiving a quick connect fitting  72  of a flexible water tube  74  (see FIG. 4). With reference to FIG. 2, recess  70  forms part of a passageway  80  which extends through union rotor  30 , having an enlarged portion  80   a  at one end and an annular channel  80   b  at the other end. As can be seen in FIG. 2, passageway  80  is aligned with a passageway  82  in face seal  50  and a passageway  84  in union stator  32 . A groove  86  formed in the interior surface of union stator  32  communicates with passageway  84 . With reference to FIG. 4, a quick connect fitting  88  provides connection between an external fluid supply  90  and a connection to passageway  80 . It is generally preferred that face seal  50  be secured to union stator  32  using adhesives or other conventional securement arrangements. Thus, the surface of union rotary  30  defining annular channel  80   b  wipes across the upper face of seal  50 . If desired, the arrangement could be reversed, with face seal  50  secured to union rotor  30  and an annular groove formed in union stator  32 , communicating with passageway  84 . In either event, it is generally preferred that wear on face seal  50  be limited to one of its two major surfaces. As a less preferred alternative, face seal  50  could be made to move freely about both of its major surfaces.  
         [0025]    With reference to FIGS. 3 and 5, a polish table assembly  102  is mounted atop rotating spindle portion  22  and is rotatably driven therewith, by a drive belt connected to drive sprocket  104 . Included in polish table  102  is polish monitoring instrumentation  108  which comprises conventional polish monitoring instrumentation, such as optical end point determination equipment. As indicated in FIG. 5, a face  110  of polish monitoring instrumentation  108  is aligned with face  112  of polish table assembly  102 . Included on face  112  is a conventional polish pad (not shown).  
         [0026]    In use, polish table face  112  is covered with slurry or other CMP polishing media. In order to maintain the face  110  of instrumentation  108  in an operational condition, face  110  is flushed with suitable flushing media, such as ultra pure water which is fed to surface  110  by flexible tube  74 , which is connected to union rotor  30  as explained above with reference to FIG. 4. A supply of flushing medium is transported from an external source  90 , thorough channel  80  in union rotor  30  so as to be received in flexible tube  74 . Thus, during rotation of table assembly  194 , face  110  of instrumentation  108  is maintained in an operational (that is, optically unobstructed) condition, with a continuous or intermittent flow of flushing agent. With reference to FIG. 6, the underside of the polish table is indicated at  120 . A manifold arrangement  122  and tubing  124  distributes flushing media to selected points about the polish table to provide flushing for multiple instrumentation locations.  
         [0027]    In order to maintain the fluid traveling through rotary union  10  in an ultra pure (fully wafer-compatible) condition, as described, the fluid passageway is maintained separate from contaminating materials such as the rotation part  14  of coolant union  12 , which is preferably made of a metallic composition. It has been found, for example, that even if the rotating part  14  is made of traditionally “pure” materials such as various stainless steel compositions, some silicon wafer chemistries will be negatively impacted if contacted by ultra pure water which even briefly touches metallic rotating part  14  on its path toward the surface of polish table assembly  102 . Accordingly, as can be seen for example in FIGS. 2 and 4, care is taken to maintain passageway  80  entirely within union rotary  30  and to extend passageway  80  beyond (i.e., downstream of) the lower face of rotating part  14  of coolant union  12 . As mentioned above, recess  70  (see FIG. 2) is provided to receive a quick connector for flexible tube  74 , shown installed in FIG. 4. As can be seen for example in FIG. 4, a wall portion  130  of union rotor  30  separates the fluid passageway from the lower surface  132  of rotating part  14 .  
         [0028]    Chemicals, such as a flushing media, coming into contact with the wafer circuits pass through the union rotor  30  before coming into contact with its surfaces. As seen above, the fluid pathway extends through union rotor  30  which provides shielding from potentially incompatible materials conventionally employed in polish table spindle arrangements. Union rotor  30  can be readily manufactured with a minimum number of conventional machining steps which can be employed with a wide variety of materials. A particular advantage of the present invention is that different materials can be readily substituted for the union rotor  30  without a substantial increase in manufacturing costs. Thus, the present invention contemplates that different materials may be used for the fluid passageway, as may be dictated by so-called “wafer chemistries” (a term which refers, for example, not only to chemical interactions with the silicon wafer substrates, but also the integrated circuit structures deposited thereon). Recently, metallic circuits have been formed using copper alloys and other materials which require a strict chemical regimen in order to avoid undesirable effects, such as corrosion.  
         [0029]    If subsequent operational changes raise issues of chemical compatibility, union rotor  30  can be quickly and easily fabricated from a different candidate material, thus expediting further testing and evaluation. In the preferred embodiment, union rotor  30  is of monolithic construction, made from PET (polyethylene terephthalate) also known as ERTALYTE. This union rotor material has been chosen for its compatibility with chemical/mechanical polishing of wafer compositions of current commercial interest. While it is generally preferred that union rotor  30  be made of non-metallic materials, it will be appreciated that a wide variety of materials chosen according to their chemical compatibility with wafer substrate and associated integrated circuit structures.  
         [0030]    As can be seen for example in FIGS. 2 and 4, it is also important that face seal  50  be compatible with fluids contacting the wafer substrate and its structures. Additionally, face seal  50  must provide the wear characteristics and low friction qualities necessary for rotational sealing of the union rotor with respect to the union stator. In the preferred embodiment, face seal  50  is made of expanded PTFE material and most preferably is made of type GR PTFE material commercially available from GORE-TEX Corporation. The GORE-TEX type GR material has been found to be sufficiently inert for current commercial “ultra pure” water applications. That is, this material was found to be chemically compatible and non-contaminating with respect to commercially significant silicon wafers and integrated circuit structures in use today. Further, the GORE-TEX type GR material was found to be sufficiently hydrophobic, contributing also to the heightened level of cleanliness required for ultra pure applications. The chosen material was also found to be very lubricious when employed in the manner indicated.  
         [0031]    Over extended use, face seal  50  is prone to wear, so as to take on a reduced thickness. Springs  58  urge union rotor  30  to apply pressure against the face seal  50 , so as to maintain its desired operating characteristics, despite wear. Referring to FIG. 4, spring  48  preferably comprises a conventional wave spring made of medium steel material. If desired, the wave spring  48  could be replaced by conventional compression springs disposed about the fasteners  42 . It has been found preferable to “back up” spring  48  with a washer  140 . Although the rotary part of fluid union  12  and the union rotor  30  could be dimensioned for an accurate fit with regard to a selected spring  48 , it has been found convenient to enlarge the spacing provided for spring  48  and to fill the space with one or more washers  140 , thereby providing a convenient arrangement for controlling the end play and related forces exerted on face seal  50 . The aforementioned GORE-TEX type GR material held its desired shape and lubricious properties under extended wear conditions associated with relatively massive turntable operations.  
         [0032]    The drawings and the foregoing descriptions are not intended to represent the only forms of the invention in regard to the details of its construction and manner of operation. Changes in form and in the proportion of parts, as well as the substitution of equivalents, are contemplated as circumstances may suggest or render expedient; and although specific terms have been employed, they are intended in a generic and descriptive sense only and not for the purposes of limitation, the scope of the invention being delineated by the following claims.