Patent Document

BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a chemical and mechanical polishing apparatus, and more particularly to a polishing head of a chemical and mechanical polishing apparatus capable of uniformly polishing a wafer.  
           [0003]    2. Description of the Related Art  
           [0004]    In a semiconductor device manufacture process, a surface of a semiconductor wafer is polished, using chemical and mechanical components such as abrasive supplied between the wafer and a polishing pad. This process is commonly known as a chemical and mechanical polishing (CMP).  
           [0005]    CMP has developed into an integral component of the manufacture and yield of cost effective semiconductor products. In general, a CMP apparatus for planarizing surface of the wafer includes a polishing head for supporting and pressing the wafer, a polishing platen rotatively operating and having a polishing pad, a slurry supplying device, and a conditioner for conditioning the polishing pad.  
           [0006]    When the CMP apparatus performs the polishing process, the surface of the wafer being polished must be uniformly polished throughout. Therefore, the polishing pressure applied to a respective region of the wafer, the amount of slurry, and the condition of the polishing pad which come in contact with the wafer must be regularly controlled.  
           [0007]    [0007]FIG. 1 is a cross sectional view of a conventional polishing head of a CMP apparatus.  
           [0008]    Referring to FIG. 1, when a wafer is polished, a surface of the wafer W contacts with a polishing pad  10  and a polishing head  12  contacts with the other surface of the wafer for applying a pressure to the wafer W. The polishing pad  10  and the polishing head  12  rotate in the same direction or opposite direction, respectively.  
           [0009]    The polishing head  12  includes a body  14  capable of moving upward and downward. The body  14  includes air passages  16  for receiving and discharging air and a retainer ring  18  for securing the wafer W and preventing it from separating from the body  14 .  
           [0010]    A perforated plate  20  is disposed on a lower portion of the body  14  for distributing a pressure of air supplied through the air passages  16 . A plurality of thru-holes  20   a  are formed in the perforated plate  20  for uniformly distributing the air pressure.  
           [0011]    A membrane  22  is attached to the body  14  to enclose a lower portion of the perforated plate  20 .  
           [0012]    [0012]FIG. 2 is a perspective view showing the membrane  22  mounted to the polishing head  12  shown in FIG. 1.  
           [0013]    The membrane  22  is resiliently expanded and shrunk by the air pressure supplied through the plurality of thru-holes  20   a  (in FIG. 1).  
           [0014]    Referring again to FIG. 1 for describing the polishing process of the wafer, the air is exhausted through the air passages  16  from the body  14 , so that the body  14  is in a vacuum state at a certain portion thereof. Due to the vacuum state in the body  14 , the wafer W adheres to the lower surface of the membrane  22 , which contacts with the polishing pad  10 . Air is then introduced through the air passages  16  into the body  14  to expand the membrane  22 . The expanded membrane  22  presses the wafer W. A problem with this process is that the membrane  22  is non-uniformly expanded at a center portion and an edge portion thereof.  
           [0015]    [0015]FIG. 3 is a partially enlarged cross sectional view showing the edge portion A of the expanded membrane of the polishing apparatus shown in FIG. 1.  
           [0016]    Referring to FIG. 3, the membrane  22  is inclined at the edge portion during the expansion. When the membrane  22  is expanded, a gap  22   a  is formed between the edge portion of the membrane  22  and the edge portion of the wafer W so that the membrane  22  does not contact with the wafer W at the edge portion thereof. As a result, the pressure is not applied to the edge portion of the wafer W.  
           [0017]    Accordingly, the wafer W has a different polishing rate between the edge portion and portions other than the edge portion during the polishing. Due to the different polishing rate, the wafer W has a non-uniform profile at the edge portion thereof.  
           [0018]    U.S. Pat. No. 6,116,992 (issued to Prince on Sep. 12, 2000) discloses a CMP apparatus having a retainer ring of which a bottom surface is projected to help the polishing of the wafer W. However, nonuniformity in a polished wafer still exists in the CMP apparatus, if the membrane has uneven expansion characteristics.  
           [0019]    Accordingly, there is a problem in that a process error is found in chips formed on the edge portion of the wafer as the edge portion of the wafer is non-uniformly polished, resulting in decreased yield and reliability of the semiconductor devices.  
         SUMMARY OF THE INVENTION  
         [0020]    The present invention has been made to solve the above-mentioned problem, and accordingly it is an object of the present invention to provide a CMP apparatus capable of uniformly polishing a wafer.  
           [0021]    A polishing head of a chemical and mechanical polishing apparatus for polishing a wafer is provided, the polishing head includes: a body having at least one air passage therein, through which air is introduced into the body and exhausted from the body; an air pressure distributing unit mounted to a lower portion of the body for distributing air pressure supplied through the air passage; a membrane for enclosing a lower surface of the air pressure distributing unit, the membrane having a lower surface for making contact with a back surface of the wafer, the membrane being expanded and shrunk by the pressure of the air supplied through the air pressure distributing unit; and an air pressure compensating unit for distributing pressure to selected portions of the surface of the wafer connecting the membrane.  
           [0022]    According to an embodiment of the present invention, the selected portions of the surface of the wafer include a center portion and an edge portion of the wafer. The air pressure compensating unit is disposed between the air pressure distributing unit and the membrane and an upper surface and a lower surface of the air pressure compensating unit are respectively fixed to edge of the lower surface of the air pressure distributing unit and edge of an upper surface of the membrane. The air pressure compensating unit has a ring shape, which is adhered to edges of the air pressure distributing unit and the membrane. The air pressure compensating unit has a cross-sectional shape of which an upper side and a lower side are parallel with each other, an inner side is inclined and an outer side is vertical to the upper and lower sides. The inner side of the air pressure compensating unit is inclined in a direction to a center of the membrane so as to compensate a space between the membrane and an edge of the wafer when the membrane is expanded by the air pressure. The air pressure compensating unit has a thickness enough to compensate an expanding difference between the edge portion and the center portion of the membrane when the membrane is expanded by the air pressure. The air pressure compensating unit has a ring shape, disposing under the membrane and an edge of the membrane. The air pressure compensating unit is inclined in a direction to a center of the membrane. The air pressure compensating unit is comprised of resilient material. The resilient material is a rubber or a silicon resin. The air pressure distributing unit is formed as a disc shape having a predetermined diameter, in which a plurality of thru-holes are formed at portions within a range of a predetermined radius from a center of the disc shape.  
           [0023]    A polishing head of a chemical and mechanical polishing apparatus for polishing a wafer is also provided, the polishing head includes: a body having a plurality of air passages therein, through which air is introduced into and exhausted from the polishing head, and a retainer ring for securing the wafer, said body being movable upwardly and downwardly; an air pressure distributing plate mounted to a lower portion of the body for distributing air pressure supplied through the air passages; an air cushion connected with an end portion of one of the air passages so as to close the end portion and make contact with an edge portion of an upper surface of the air pressure distributing plate, the air cushion being resiliently expanded and shrunk during introducing and venting of the air through the air passages for applying pressure to the edge portion of the upper surface of the air pressure distributing plate; a membrane mounted to enclose a lower surface of the air pressure distributing unit for being expanded and shrunk by the pressure of the air supplied through the air pressure distributing plate, a lower surface of the membrane making contact with a back side of the wafer; and an air pressure compensating member for uniformly distributing the pressure that is applied at central and edge portions of the wafer which makes contact with the membrane. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The above and other objects and advantages of the present invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:  
         [0025]    [0025]FIG. 1 is a cross sectional view showing a conventional polishing head of a CMP apparatus;  
         [0026]    [0026]FIG. 2 is a perspective view showing a membrane mounted on the CMP apparatus shown in FIG. 1;  
         [0027]    [0027]FIG. 3 is a partially enlarged cross sectional view showing an edge of the membrane of the CMP apparatus, in which the membrane is expanded to apply pressure to a wafer;  
         [0028]    [0028]FIG. 4 is a cross sectional view showing a polishing head of a CMP apparatus according to a preferred embodiment of the present invention;  
         [0029]    [0029]FIG. 5 is a perspective view showing a membrane mounted on the CMP apparatus as shown in FIG. 4;  
         [0030]    [0030]FIG. 6 is a plan view showing the membrane mounted on the CMP apparatus shown in FIG. 4; and  
         [0031]    [0031]FIG. 7 is a partially enlarged cross sectional view showing an edge of the membrane of the CMP apparatus shown in FIG. 4, in which the membrane is expanded to apply pressure to a wafer. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0032]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0033]    [0033]FIG. 4 is a cross sectional view showing a polishing head of a CMP apparatus according to an embodiment of the present invention.  
         [0034]    Referring to FIG. 4, when a wafer W is polished, a surface of the wafer W is in contact with a polishing pad  30  and the other surface of the wafer W is in contact with a polishing head  32  so that pressure is applied to the wafer W. A polishing pad  30  and the polishing head  32  rotates in the same direction or opposite direction.  
         [0035]    The polishing head  32  includes a body  34  that is capable of being rotated and moving upwardly and downwardly. The body  34  includes a housing  36 , a base  38  for supporting the housing  36 , and a retainer ring  40  mounted on edges of a lower portion of the base  38  for securing the wafer W and preventing it from being separated from the body  34 .  
         [0036]    The housing  36  is connected with a driving section (not shown), which is upwardly and downwardly moved in a vertical direction to a surface of the polishing pad  30  by the driving section. Further, the housing  36  rotates about a rotation axis that is in a vertical direction to the surface of the polishing pad  30 . The housing  36  having upper and lower surfaces in a disc shape includes a plurality of tubes  42   a ,  42   b , and  42   c  for supplying and exhausting air to/from the CMP apparatus first tube  42   a . The first tube  42   a  passes through a center portion from the upper surface to the lower surface. Second and third tubes  42   b  and  42   c  spaced apart from the first tube  42   a  in the radial direction of the housing  36  pass through from the upper surface of the housing  36  to the lower surface of the housing  36 . The second and third tubes  42   b  and  42   c  are preferably disposed symmetricly about the first tube  42   a.    
         [0037]    The base  38  is connected with a certain portion of the lower surface of the housing  36  for supporting the housing  36 . The base  38  has upper and lower surfaces in a disc shape. The first tube  42   a  of the housing  36  is extended into the base  38  so that the first tube  42   a  passes through a center portion of the base  38 . Further, the second tube  42   b  is also extended to pass through the base  38 .  
         [0038]    The housing  36  and the base  38  are connected with each other by means of a connecting member  60 . The connecting member  60  includes an outer clamp (not shown) to be connected to the lower surface of the housing  36 , an inner clamp (not shown) fixed to an upper surface of the base  38  by means of screws (not shown) and an annular resilient sheet (not shown) connecting the housing  36  to the base  38 .  
         [0039]    When the housing  36  is connected with the base  38  by means of the connecting member  60 , a first chamber  44  is formed between the housing  36  and the base  38 . The first chamber  44  is defined by the housing  36 , the base  38 , and the connecting member. The third tube  42   c  passing through the housing  36  connects with the first chamber  44  and defines a passage for introducing and exhausting air to and from the first chamber  44 . Therefore, when the air is introduced into the first chamber  44  through the third tube  42   c , the air pressure increases in the first chamber  44  to downwardly push the base  38 . Conversely, when the air is exhausted through the third tube  42   c  from the first chamber  44 , the first chamber  44  is vacuumed to generate a vacuum pressure. As a result, the base  38  is upwardly moved to the housing  36  by the vacuum pressure.  
         [0040]    The retainer ring  40  has a ring shape, which is fixed along an edge of the lower surface of the base  38 . The retainer ring  40  is preferably fixed to the base  38  by screws (not shown) for preventing the wafer W from being departed from the polishing head  32  during the polishing process.  
         [0041]    A perforated plate  46  is mounted on a lower portion of the body  34  for evenly distributing air pressure supplied through the first tube  42   a . Preferably, the perforated plate  46  is mounted on the lower surface of the base  38  and inside of the retainer ring  40 . The perforated plate  46  is formed as a disc shape having a predetermined diameter, in which a plurality of thru-holes  46   a  are formed in a predetermined range of radius from a center of the perforated plate  46 . The air pressure is distributed through the thru-holes  46   a  of the perforated plate  46  toward the lower direction. That is, the air is introduced in and exhausted from a space between the base  38  and the perforated plate  46  through the first tube  42   a , and the air is distributed through the thru-holes  46   a  of the perforated plate  46  toward the lower direction.  
         [0042]    A dividing plate  48  is further disposed between the base  38  and the perforated plate  46  to support the lower portion of the base  38 . The dividing plate  48  is formed as a disc shape having a predetermined thickness, in which a pipe  48   a  passes through the center portion of the dividing plate  48  to connect with the perforated plated  46 . The pipe  48   a  is disposed substantially at the center of the dividing plate  48  and is extended upward.  
         [0043]    Accordingly, the air is introduced in and exhausted from the space between the dividing plate  48  and the perforated plate  46 . Since the dividing plate  48  is disposed between the base  38  and the perforated plate  46 , the space for distributing the air which is supplied through the first tube  42   a  can be reduced and the air pressure increases in the space.  
         [0044]    At an edge of an upper surface of the perforated plate  46 , an air cushion  50  is connected with an end position of the second tube  42   b  so as to close an end part of the second tube  42   b . The air is introduced into or exhausted from the air cushion  50  through the second tube  42   b , therefore the air cushion  50  is resiliently expanded or shrunk. Accordingly, when air is introduced into the air cushion  50 , the air cushion  50  is expanded to apply the pressure to the edge portion of the perforated plate  46 .  
         [0045]    The polishing head  32  further includes a membrane  52  that encloses a substantially lower portion of the perforated plate  46 .  
         [0046]    [0046]FIG. 5 is a perspective view showing the membrane  52  mounted on the CMP apparatus as shown in FIG. 4. FIG. 6 is a plan view showing the membrane  52  mounted on the CMP apparatus as shown in FIG. 4.  
         [0047]    Referring to FIGS. 5 and 6, the membrane  52  is comprised of a resilient material, an elastic material, or a rubbery material, which can be resiliently expanded and shrunk by the air pressure supplied through the perforated plated  46 . According to an embodiment of the present invention, the membrane  52  includes a pressure compensating member  54  at an inside edge of the membrane  52 . The details related to the pressure compensating member  54  will be described below.  
         [0048]    The membrane  52  is in contact with a back surface of the wafer W. As the membrane  52  is expanded, the pressure is applied to the back surface of the wafer W during polishing of the wafer W. The membrane  52  continuously encloses the lower surface, the side and the edge of the upper surface of the perforated plate  46  (in FIG. 4) having a disk shape, and the membrane  52  is adhesively fixed to the side of the perforated plate  46  and the edge of the upper surface of the perforated plate  46 .  
         [0049]    When air is introduced through the perforated plate  46 , the membrane  52  is expanded downwardly due to the air pressure. Furthermore, when air is exhausted through the perforated plate  46 , the membrane  52  is in close contact with the perforated plate  46 , and some portions of the membrane  52  are sucked into the thru-holes  46   a  of the perforated plate  46 . During polishing of the wafer W, the membrane  52  is in contact with a back surface of the wafer W.  
         [0050]    Hereinafter, an operation of the polishing head according to a preferred embodiment of the present invention will be described in detail.  
         [0051]    Referring again to FIG. 4, air is exhausted through the first tube  42   a  from the space between the dividing plate  48  and the perforated plate  46  to provide vacuum. When the space between the perforated plate and the dividing plate  48  is under vacuum pressure, the back surface of the wafer W is stuck to the lower surface of the membrane  52 . Then, the body  34  is moved downwardly so as to allow the front surface of the wafer W to make contact with the polishing pad  30 . When the front surface of the wafer W is in contact with the polishing pad  30 , the air is introduced between the perforated plate  46  and the dividing plate  48  to expand the membrane  52 . The lower surface of the membrane  52  applies the pressure to the wafer W as the membrane  52  is expanded. The polishing pad  30  and the polishing head  32  are respectively rotated to polish the wafer W.  
         [0052]    [0052]FIG. 7 is a partially enlarged cross sectional view showing an edge B of the membrane  52 , in which the membrane  52  as shown in FIG. 4 is expanded to apply the pressure to the wafer W.  
         [0053]    Air introduced through the first tube  42   a  between the perforated plate  46  and the dividing plate  48  applies air pressure to the membrane  52  through the perforated plate  46 . Since the perforated plate  46  has the disc shape, in which the plurality of the thru-holes  46   a  are formed in the range of the predetermined radius from the center axis thereof, air pressure cannot be uniformly applied to the whole surface of the membrane  52  through the perforated plate  46 . That is, the edge portion of the perforated plate  46  has a few thru-holes so that a small amount of air is introduced into the edge portion of the perforated plate  46 . Therefore, the membrane  52  is greatly expanded at the center portion rather than the edge portion thereof. In addition, the edge portion of the membrane  52  that is expanded is inclined toward the center portion so that the edge portion of the membrane  52  does not make contact with the wafer W. As a result, the pressure is not applied to the edge portion of the wafer W.  
         [0054]    According to an embodiment of the present invention, the pressure compensating member  54  in the polishing head  32  compensates the air pressure at the edge of the membrane  52  so as to make uniform the pressure applied to the wafer W in contact with the membrane  52 . The pressure compensating member  54  is provided between the membrane  52  and the perforated plate  46 . Preferably, an upper surface of the pressure compensating member  54  is fixedly adhered to the edge of the lower surface of the perforated plate  46  and the lower surface of the pressure compensating member  54  is fixedly adhered to the edge of the inner surface of the membrane  52  that faces the edge of the lower surface of the perforated plate  46 . The pressure compensating member  54  has the ring shape, the upper surface and the lower surface of the pressure compensating member  54  are respectively and continuously adhered to the edges of the perforated plate  46  and the membrane  52 .  
         [0055]    In the cross-sectional view, an upper side and a lower side of the pressure compensating member  54  are in parallel with each other. Furthermore, an inner side of the pressure compensating member  54  is inclined in a direction to the center of the membrane  46  and an outer side of the pressure compensating member  54  is vertical to the upper side and the lower side of the pressure compensating member  54 .  
         [0056]    The inner side of the compensating member  54  has a desired gradient enough to compensate the space between the edge of the wafer W and the membrane  52  when the membrane  52  is expanded due to the air pressure. Preferably, the membrane  52  is inclined at a predetermined angle from the edge to the center axis thereof while being expanded by the pressure of the air supplied through the thru-holes  46   a  of the perforated plate  46 . Therefore, when the membrane  52  is expanded, the pressure compensating member  54  functions as filling material between the wafer W and the membrane  52 , depending on the gradient of the edge of the membrane  51  to uniformly polish the center portion and the edge portion of the wafer W during the polishing of the wafer W. Accordingly, the inner side of the pressure compensating member  52  must have the gradient enough to allow the membrane  52  to make close contact with the wafer W. Further, the pressure compensating member  54  has a thickness enough to compensate for any expansion due to air pressure between the edge portion and the center portion of the membrane  52 .  
         [0057]    According to an embodiment of the present invention, the pressure compensating member  54  is also disposed under the membrane  52  and an edge of the membrane  52 , and is inclined in a direction to a center of the membrane  52 .  
         [0058]    According to an embodiment of the present invention, the pressure compensating member  54  is preferably comprised of resilient material such as a rubber and a silicon resin, of which the edge portion is resiliently bent by the air pressure.  
         [0059]    When the membrane  52  is expanded and presses the wafer W, the pressure compensating member  54  pushes the edge portion of the membrane  52  to remove the space between the edge portions of the membrane  52  and the wafer W. Therefore, the pressure that is supplied for the edge portion and the center portion of the wafer W can be uniform. Accordingly, the edge portion and the center portion of the wafer W are uniformly polished and a polishing profile of the wafer W is improved. Furthermore, since the process failure caused at the edge portion of the wafer W is reduced, the yield and the reliability of the semiconductor device is improved.  
         [0060]    Further, the pressure compensating member  54  uniformly applied pressure to the edge portion and the center portion of the wafer W while preventing the wafer from being dropped downwardly when the wafer W is stuck to the membrane  52 . Particularly, when the polishing of the wafer W is finished or temporarily stopped, the wafer W is stuck to the membrane  52  as the air is exhausted through the first tube  42   a  from the space between the dividing plate  48  and the perforated plate  46 . When the wafer W is stuck to the membrane  52 , the wafer W is somewhat bent in a state that the center portion of the wafer W is concaved downwardly as the pressure compensating member  54  is disposed at the edge of the membrane  52 . When the wafer W is stuck to the membrane  52  in the state of being bent, the membrane  52  is sucked into the thru-holes  46   a  of the perforated plate  46  while the wafer W maintains a close contact with the membrane  52 . As a result, the vacuum pressure increases in the space between the membrane  52  and the wafer W so as to prevent the wafer W from being dropped downwardly.  
         [0061]    Although preferred embodiments of the present invention have been described, it is understood that the present invention should not be limited to these preferred embodiments but various changes and modifications can be made by one skilled in the art within the spirit and scope of the present invention as hereinafter claimed.

Technology Category: 7