Patent Publication Number: US-6666756-B1

Title: Wafer carrier head assembly

Description:
FIELD OF THE INVENTION 
     The present invention relates to a carrier assembly for releasably holding a thin material. More particularly, the present invention relates to a wafer carrier assembly for use in chemical mechanical polishing/planarization of semiconductor wafers. 
     BACKGROUND 
     Semiconductor wafers are commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are made to connect up to the next level of the circuit. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer that must be smoothed out before generating the next circuit layer. 
     Chemical mechanical planarization (CMP) techniques are used to planarize the raw wafer and each layer of circuitry added. Available CMP systems, commonly called wafer polishers, often use a rotating wafer carrier head that brings the wafer into contact with a polishing pad rotating in the plane of the wafer surface to be planarized. A chemical polishing agent or slurry containing microabrasives is applied to the polishing pad to polish the wafer. The wafer carrier head then presses the wafer against the rotating polishing pad and is rotated to polish and planarize the wafer. The mechanical force for polishing is derived from the rotating table speed and the downward force on the wafer carrier head. The chemical slurry is constantly transferred under the wafer carrier head. Rotation of the wafer carrier head helps in the slurry delivery as well in averaging the polishing rates across the substrate surface. 
     Another technique for performing CMP to obtain a more uniform polishing rate is the use of a linear polisher. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated to average out the local variations. An example of a linear polisher is the TERES™ polisher available from Lam Research Corporation of Fremont, Calif. 
     With either type of polisher (linear or rotary), the wafer carrier head is an important component of the polishing tool. The wafer carrier head provides means for holding and supporting the wafer, rotating the wafer, and transmitting the polishing force to engage the wafer against the pad. The wafer carrier head is coupled to a rotating mechanism that also applies a pressure to the wafer so that the wafer can rotate while being pressed against a polishing surface. 
     In conventional wafer carrier head designs, it is customary to employ the use of a wafer mounting pad or carrier film that is adhesively bonded to a wafer mounting plate. This film serves to absorb or conform to surface irregularities on the back side of the wafer and, due to its high coefficient of friction, prevent the wafer from rotating inside the wafer carrier head as the wafer carrier head is being rotated during the polishing process. However, these designs also require that the film be replaced following a set number of polishing cycles. 
     In designs that employ a wafer mounting plate and a wafer mounting pad or film, the wafer is held by the wafer carrier head via a series of holes in the mounting pad. The holes allow passage of vacuum forces to the side of the wafer that is in contact with the mounting pad. However, this design has the disadvantage of drawing polishing slurry back through the holes and up into the vacuum lines, necessitating a flush system to periodically flush out the slurry. 
     Other designs employ the use of an inflatable elastomeric membrane to hold the wafer as it is being transferred to a polishing surface. Once the wafer carrier head is lowered to a polishing surface, the membrane inflates and applies a downward force onto the wafer so that the wafer contacts the polishing surface. These designs also employ a fixed non-adjustable retaining ring. The carrier head is lowered so that the retaining ring contacts the polishing surface. The retaining ring then prevents the wafer from slipping out from under the carrier head as the membrane is being inflated so that the wafer contacts the polishing surface. However, this design has the disadvantage of requiring precise timing between contacting the ring to the polishing surface and inflating the membrane. If the membrane inflates before the retaining ring contacts the polishing surface, the wafer may extend beyond the retaining ring. The wafer will then lose its peripheral containment and will slip out from under the carrier head when it reacts to the frictional forced introduced by contacting the moving polishing surface. 
     Edge exclusion is another disadvantage of wafer carrier head designs that employ an inflatable membrane. Edge exclusion categorically is a portion of the wafer edge that does not receive the same degree of polishing action as the balance of the wafer. The result is a reduction of usable area for product production. 
     Wafer carrier heads should be capable of gimballing in order to accommodate changes in parallelism between the carrier head and the polishing surface. Many wafer carrier heads gimbal through the use of a mechanical gimbal. However, mechanical gimbals have the disadvantage of causing a moment arm to form whose length is equal to the distance between the mechanical gimbal point and the polishing surface. This moment arm in turn aggravates a problem known as “dig in”, a problem common to carrier heads that gimbal. Dig in occurs when the wafer mounting surface digs into the leading edge of the wafer and causes a higher removal rate at the wafer edge than the remainder of the wafer. The moment arm associated with mechanical gimbals multiplies this tendency, and the resultant “dig in” is directly proportionate to the length of the moment arm. 
     BRIEF SUMMARY 
     To alleviate the disadvantages of the prior art, a carrier assembly for releasably holding a wafer is provided herein. According to a first aspect of the invention, the carrier assembly includes a primary housing having an adjustable retaining ring that protrudes downwardly from the primary housing. A secondary housing, fixed to the primary housing, has a wafer holding mechanism positioned in an area surrounding the circumference of the retaining ring. The retaining ring is movable with respect to the primary housing. The retaining ring moves independently of the wafer holding mechanism, and retains an edge of the wafer on the polishing surface when the wafer is lowered onto the polishing surface. 
     In another aspect of the invention the carrier assembly includes a primary housing. The primary housing has an adjustable wafer retaining mechanism that is configured to retain an edge of a wafer on a polishing surface when the wafer is being lowered onto the polishing surface. A secondary housing is fixed to the primary housing and has an adjustable wafer holding mechanism. The wafer holding mechanism is configured to apply one of a downward force and an upward force to the wafer to retain and transport the wafer to and from the polishing surface and to retain the wafer on the polishing surface. The wafer holding mechanism provides an adjustable and controllable downward force on the wafer so that the wafer is uniformly polished when the wafer holding mechanism is retaining the wafer on the polishing surface. 
     In another aspect of the invention the carrier assembly includes a primary housing having a vertically adjustable wafer retaining mechanism. The wafer retaining mechanism retains the edge of a wafer on a polishing surface when the wafer is being lowered onto the polishing surface. A secondary housing is fixed to the primary housing and has a vertically adjustable wafer holding mechanism that retains and transports a wafer to and from a polishing surface and retains the wafer on the polishing surface. The wafer retaining mechanism and the wafer holding mechanism are configured to pivotally accommodate changes in parallelism between the wafer and the polishing surface when the wafer is being polished by the polishing surface. 
     According to another aspect of the invention, a method for handling a wafer to be polished includes the steps of receiving the wafer at a wafer carrier head so that the wafer contacts the wafer carrier head. A uniform upward force is applied from the wafer carrier head to retain the wafer. The wafer carrier head is then transported to a polishing surface, and a protruding retaining mechanism is lowered from the wafer carrier head onto the polishing surface so that the retaining mechanism contacts the polishing surface. A uniform downward force is then applied onto the wafer. The wafer retaining mechanism is then raised, and the wafer is polished. 
    
    
     The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a perspective view of a preferred embodiment of the present invention; 
     FIG. 2 is a front sectional view of a preferred embodiment of FIG. 1 showing a wafer being supported by the wafer handling mechanism below the wafer carrier head; 
     FIG. 3 is a front sectional view of a preferred embodiment of FIG. 1 showing the wafer being supported by the wafer handling mechanism and contacting the wafer carrier head; 
     FIG. 4 is a front sectional view of a preferred embodiment of FIG. 1 showing the wafer carrier head lowered onto a polishing surface; 
     FIG. 5 is a front sectional view of a preferred embodiment of FIG. 1 showing a wafer being polished by the polishing surface and being retained by the wafer carrier head. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     FIGS. 1 &amp; 2 illustrate a preferred embodiment of the wafer carrier head  2 . A novel wafer carrier head  2  to perform chemical-mechanical polishing (CMP) on a wafer  4  that addresses the drawbacks of the prior art discussed above is described below. The wafer  4  has a downward side  6  having an outer part  8  and an upward side  10  having an outer part  12  that opposes the outer part  8 . As will be more fully described below, in order for the wafer  4  to be held on a polishing surface  18 , the outer parts  8 ,  12  of the wafer  4  receive pressure from an inner bellows assembly  80 , as opposed to the remainder of the wafer  4  that does not receive pressure from the inner bellows assembly  80 . A wafer handling mechanism  16 , which may be any of a number of commercially available wafer handling robots or other mechanical device suitable for use in transporting wafers, will support the downward side  6  of the wafer  4  and bring it over to the wafer carrier head  2 . The wafer carrier head  2  will retrieve the wafer  4  from the wafer handling mechanism  16  via the upward side  10  and will transport the wafer  4  to a polishing surface  18  (FIG.  4 ). 
     The wafer carrier head  2  will then lower the wafer  4  onto the polishing surface  18  so that the downward side  6  can be polished. Upon completion of the polishing, the wafer carrier head  2  will remove the wafer  4  from the polishing surface  18  and transport it to be unloaded. The wafer carrier head  2  will then release the polished wafer  4  back onto the wafer handling mechanism  16 . Further detail about the wafer carrier head  2  and its operation is given below. An example of a suitable wafer polisher, having a suitable wafer polishing surface, is the TERES™ CMP System available from Lam Research Corporation of Fremont, Calif. 
     Referring to FIGS. 2 &amp; 3, an inner housing  22  having a downwardly facing wall  24  and an outer housing  20  are included. The outer housing  20  includes an outer housing flange  26  and the inner housing  22  includes a lower flange  28  and an upper flange  30 . Fasteners  32 , preferably bolts, hold the inner housing  22 , the outer housing  20 , and an outer bellows plate  34  stationary with respect to each other. The fasteners  32  also attach an inner bellows plate  36  to the outer bellows plate  34 . In the preferred embodiment there are at least eight fasteners  32 , but a different number of fasteners can be used in other configurations. Seals  38  are provided to seal the areas where the fasteners  32  enter the inner housing  22 , the outer housing  20 , the outer bellows plate  34 , and the inner bellows plate  36 . In a preferred embodiment the seals  38  used are o-rings. 
     The outer housing  20  also includes a receiving mechanism  40 , a cavity  42 , an outer port  44 , and an inner port  46 . The receiving mechanism  40  receives a connection mechanism  48  for attaching to a spindle (not shown). The spindle applies downward and rotational forces to the wafer carrier head  2  during operation. The spindle and wafer carrier head  2  releasably attach to one another with respective male and female tool changer mechanisms. 
     The cavity  42  is preferably a recess in the outer housing  20  that allows positive pressure and vacuum forces to pass through to the outer port  44 . The cavity  42  also provides space for the fasteners  32  that affix the inner bellows plate  36  to the outer bellows plate  34 . 
     The inner port  46  and the outer port  44  each receive positive pressure and vacuum forces. Preferably, the positive pressure and vacuum forces are pneumatic. The inner port  46  and the outer port  44  operate independently of each other. One port can receive a positive pressure and at the same time a vacuum can be applied to the other port. The positive pressure and vacuum forces are both variable and may be provided by any of a number of pressure or vacuum generating devices. 
     An outer bellows assembly  52  is attached to the outer port  44  and receives the positive pressure and vacuum from the outer port  44 . Preferably, the outer bellows assembly  52  is made up of a small bellows  51  and a large bellows  53  that are concentric with each other. Small bellows  51  and large bellows  53  are each made by welding together formed rings  55  until the length desired for the small bellows  51  and large bellows  53  is achieved. An upper portion  57  of small bellows  51  and an upper portion  54  of large bellows  53  are fixed to outer bellows plate  34  by being welded to outer bellows plate  34 . Similarly, a bottom portion  59  of small bellows  51  and a bottom portion  58  of large bellows  53  are fixed to a lower ring  56  by being welded to lower ring  56 . The outer bellows assembly  52  is preferably flexible and vertically extends and contracts in a direction substantially perpendicular to the upward side  10  of the wafer  4  when the positive pressure and vacuum, respectively, are applied to the outer port  44 . 
     The lower ring  56  moves in a substantially vertical direction with the outer bellows assembly  52  when the outer bellows assembly  52  is extending and contracting. A lower ring flange  60  is included with the lower ring  56  and enters into and out of contact with the outer housing flange  26 . In a preferred embodiment, the outer housing flange  26  limits the vertical distance that the outer bellows assembly  52  can extend. When the lower ring flange  60 , which along with the lower ring  56  moves with the outer bellows assembly  52 , comes into contact with the outer housing flange  26  the outer bellows assembly  52  cannot extend any further. 
     A wafer retaining ring  62  is attached to the lower ring  56  and protrudes from the lower ring  56  in a downward direction substantially perpendicular to the upward side  10  of the wafer  4 . The retaining ring  62  is preferably made from a plastic material and prevents the wafer  4  from separating from the wafer carrier head  2  as it is being lowered to contact the polishing surface  18 . The initial amount that the retaining ring  62  protrudes downwardly from the lower ring  56  is adjustable. In one embodiment, shims  64  may be placed between the lower ring  56  and the retaining ring  62  until the desired amount of protrusion is attained. Preferably, the shims are constructed from a Mylar material. The retaining ring  62  moves with the outer bellows assembly  52  and the lower ring  56 , and the outer housing flange  26  and lower ring flange  60  also limit the vertical distance the retaining ring  62  can travel. 
     An inner port chamber  66  is preferably connected to, and in fluid communication with, the inner port  46  and receives the positive pressure and vacuum forces from the inner port  46 . The inner port chamber  66  includes a top member  68 , side members  70 , and a bottom member  72 . The inner port  46  is preferably in communication with the inner port chamber  66  via a passage  74  defined by the top member  68  of the inner port chamber  66 . In a preferred embodiment, a side passage  76  extends from the side member  70  and at least two lower passages  78  extend from the bottom member  72 , though in other embodiments a different number of side passages  76  and lower passages  78  can also be used. 
     The side passage  76  connects the inner port chamber  66  to an inner bellows assembly  80  and allows the positive pressure or vacuum forces to travel from the inner port chamber  66  to the inner bellows assembly  80 . In a preferred embodiment, the inner bellows assembly  80  is oriented and substantially perpendicular to plane of the wafer  4 . The inner bellows assembly  80  is made by welding together formed rings  55  until the length desired for the inner bellows assembly  80  is achieved. An upper end  82  of the inner bellows assembly  80  is welded to the inner bellows plate  36  and a bottom portion  86  of the inner bellows assembly  80  is welded to an inner ring  84 . The inner bellows assembly  80  is flexible, and vertically extends and contracts in a direction substantially perpendicular to the upward side  10  of the wafer  4  when the positive pressure and vacuum, respectively, travel from the inner port  46  and through the inner port chamber  66  and side passage  76 . 
     The inner ring  84  preferably includes a downwardly facing bottom surface  87  and an inner ring flange  88 . The inner ring  84  extends and contracts with the inner bellows assembly  80 . In a preferred embodiment, the upper flange  30  and lower flange  28  limit the vertical distance that the inner bellows assembly  80  and the inner ring  84  can extend or contract. When the inner ring flange  88  comes into contact with the lower flange  28 , the inner bellows assembly  80  and inner ring  84  cannot extend any further. Likewise, when the inner ring flange  88  comes into contact with the upper flange  30 , the inner bellows assembly  80  and inner ring  84  cannot contract any further. 
     Alternatives to the interaction between the inner ring flange  88  and the upper flange  30  and the lower flange  28  discussed above are also contemplated. For example, the inner housing  22  can include an inner housing flange and the inner ring  84  can have an upper flange and a lower flange. Contact by the upper flange on the lower ring with the inner housing flange would limit the distance the inner ring  84  and inner bellows assembly  80  could vertically extend. Likewise, contact by the lower flange on the lower ring with the inner housing flange would limit the distance the inner ring  84  and inner bellows assembly  80  could vertically contract. In other embodiments, the flanges could be discontinuous or staggered around the circumference of inner ring  84  and inner housing  22 . 
     The lower passages  78  of the inner port chamber  66  allow the positive pressure and vacuum forces from the inner port  46  to be applied to a membrane  90  and a gap  92 . The membrane  90  includes a top face  94  having an outer side  96  and a bottom face  98  having an outer side  100  that opposes the outer side  96 . The outer side  96  of the top face  94  is attached to and covers the downwardly facing bottom surface  87  of the inner ring  84 . In a preferred embodiment, the outer side  96  is adhesively bonded to the bottom surface  87  of the inner ring  84  and extends and contracts with the inner ring  84  and inner bellows assembly  80 . The membrane is preferably elastomeric and deformable. The gap  92  is formed between the top face  94  of the membrane and the downwardly facing wall  24  of the inner housing  22  when the membrane  90  is in a non-deformed state. The gap  92  is eliminated when the vacuum force is applied to the inner port  46  and the top face  94  of the membrane  90  seals up against the downwardly facing wall  24  of the inner housing  22 . 
     The operation of the wafer carrier head  2  embodiments set forth above is now described. To first load a wafer  4  onto the wafer carrier head  2  the wafer handling mechanism  16 , supporting the downward side  6  of the wafer  4 , transports the wafer  4  and aligns it with the inside diameter of the retaining ring  62 . A positive pressure is applied to the outer bellows assembly  52  and causes the outer bellows assembly  52  to extend until the lower ring flange  60  on the lower ring  56  contacts the outer housing flange  26 . No positive pressure or vacuum is applied from the inner port  46  to the inner bellows assembly  80 . 
     With the positive pressure still applied to the outer bellows assembly  52 , the wafer handling mechanism  16  raises the wafer  4  upward until the upward side  10  of the wafer  4  contacts the membrane  90 . The wafer handling mechanism  16  pushes the wafer  4  in an upwardly direction until the inner ring flange  88  contacts the upper flange  30  on the inner housing  22 . The upward force exerted by the wafer handling mechanism  16  against the wafer  4  results in a sealing action between the outer part  12  of the upward side  10  of the wafer  4  and the outer side  100  of the bottom face  98  of the membrane  90 . 
     While the positive pressure is still being applied to the outer bellows assembly  52  and with the wafer handling mechanism  16  exerting an upward force, a vacuum is introduced at the inner port  46 . The vacuum enters the inner port chamber  66  and, via the lower passages  78 , travels into the gap  92 . The vacuum causes the inner bellows assembly  80  to contract and exerts an upward force on the inner ring  84 . This causes the inner ring flange  88  to maintain contact with the upper flange  30  on the inner housing  22 . 
     As shown in FIG. 4, the vacuum in the gap  92  (FIG. 3) deforms the membrane  90  and draws it up and against the downwardly facing wall  24  of the inner housing  22 , eliminating the gap  92 . Since the outer part  12  of the wafer  4  is sealed against the membrane  90 , a secondary vacuum  102  results between the remainder of the membrane  90  and the remainder of the wafer  4 , holding the wafer  4  in place. The wafer handling mechanism  16  is then removed and the wafer carrier head  2  is moved to a polishing area and is positioned above the polishing surface  18 . 
     The wafer carrier head  2  is lowered to a pre-determined position that brings the retaining ring  62  into contact with the polishing surface  18 . With the retaining ring  62  bearing against the polishing surface  18 , the vacuum is removed and a positive pressure is instead applied to the inner bellows assembly  80 . As shown in FIG. 5, the inner bellows assembly  80  extends and causes the upper flange  30  and inner ring flange  88  to come out of contact with each other. The positive pressure eliminates the secondary vacuum  102  and causes the membrane  90  to return to its non-deformed state. The upward side  10  of the wafer  4  is no longer in contact with the downwardly facing wall  24 . The wafer  4  is moved in a downwardly direction until the downward side  6  contacts the polishing surface  18 . The positive pressure causes the membrane  90  to exert a uniform downward force onto the wafer  4 . 
     Once the positive pressure causes the downward side  6  of the wafer to contact the polishing surface  18 , the positive pressure is relieved from the outer bellows assembly  52  and a vacuum is instead applied. The outer housing flange  26  and the lower ring flange  60  come out of contact with each other and the retaining ring  62  is raised so that the downward side  6  of the wafer  4  protrudes downwardly past the retaining ring  62 . 
     The amount the downward side  6  of the wafer protrudes past the retaining ring  64  is also known as wafer reveal. The maximum amount of wafer reveal is preferably defined by the amount the downward side  6  of the wafer  4  protrudes past the retaining ring  62  when the retaining ring  62  is in the fully raised position, i.e., when the outer bellows assembly  52  is fully contracted. Placing shims  64  such as Mylar shims above the retaining ring  62  will vary the maximum amount of wafer reveal. Applying positive pressure and/or vacuum forces to the outer bellows assembly  52  during the polishing process changes the position of the retaining ring  62 , which in turn allows the amount of wafer reveal to be variable. 
     By applying a positive pressure to inner bellows assembly  80 , a uniform downward force is applied to the entire wafer  4  during the polishing cycle because the mean diameter of the inner bellows assembly  80  is substantially the same as the diameter of the wafer  4 . The mean diameter of the inner bellows assembly  80  is the average between the outer diameter of the inner bellows assembly  80  and the inner diameter of inner bellows assembly  80 , and the mean diameter is the effective area on which the positive pressure acts. The bottom surface  87  of the inner ring  84 , with the outer side  96  of the membrane  90  covering it, applies the same downward force to the outer part  12  of the upward side  10  of the wafer  4  as is applied to the balance of the wafer  4  by the balance of the membrane  90 . Because the same downward force is applied to the entire wafer  4 , a uniform polishing action is applied to the entire downward side  6  of the wafer  4 . 
     During the polishing cycle the wafer carrier head  2  is capable of gimballing. Because of their flexible nature, the inner bellows assembly  80  and the outer bellows assembly  52  can accommodate changes in parallelism between the wafer carrier head  2  and the polishing surface  18 . The distance between the upper flange  30  and the inner ring flange  88  during the polishing cycle and the distance between the outer housing flange  26  and the lower ring flange  60  during the polishing cycle define the wafer carrier head gimbal allowance. 
     Referring to FIG. 5, once the polishing cycle is completed, the wafer unload sequence begins. A vacuum is introduced to the inner port  46 . The vacuum is introduced into the inner port chamber  66  and via the lower passages  78  travels into the gap  92 . The vacuum causes the inner bellows assembly  80  to contract and exerts an upward force on the inner ring  84 . As shown in FIG. 4, this causes the inner ring flange  88  to contact the upper flange  30  on the inner housing  22 . The vacuum in the gap  92  deforms the membrane  90  and draws it up and against the downwardly facing wall  24  of the inner housing, thus eliminating the gap  92 . 
     Since the outer part  12  of the wafer  4  is sealed against the membrane  90 , the secondary vacuum  102  results between the remainder of the membrane  90  and the remainder of the wafer  4 , holding the wafer  4  in place. The secondary vacuum  102  draws up the wafer  4  so that the retaining ring  62  protrudes downwardly past the wafer  4 . The wafer carrier head  2  is then transported to an unload station. 
     As shown in FIG. 3, the wafer handling mechanism  16  is brought up adjacent to the retaining ring  62  and is raised to contact and apply an upward force to the wafer  4 . A positive pressure is then introduced at the inner port  46  and into the inner port chamber  66 . Via the side passage  76 , the positive pressure travels into the inner bellows assembly  80 . The positive pressure also travels through the lower passages  78  and eliminates the secondary vacuum  102 . The upward force applied by the wafer handling mechanism prevents the membrane  90  from returning to its non-deformed state. 
     Referring to FIG. 2, the wafer handling mechanism  16  is lowered away from the wafer  4 . As the wafer handling mechanism  16  is lowered, the now unrestrained membrane  90  is able to return to its non-deformed state and expels the wafer  4  from the wafer carrier head  2  and onto the wafer handling mechanism  16 . The wafer carrier head  2  is now ready to repeat the above-described method to accept a new wafer for polishing. 
     The advantages of the above-described embodiments of the invention are numerous. For example, having a uniform downward force applied to the entire wafer  4  minimizes the problem of edge exclusion. Edge exclusion occurs when the outer part  8  of the wafer  4  does not receive the same degree of polishing action as the balance of the wafer  4 . The result is a reduction of usable area for production that is made available. The wafer carrier head herein described minimizes edge exclusion by applying the same down force to the outer part  8  of the wafer  4  as is being applied to the remainder of the wafer  4 . 
     Another advantage is the elimination of polishing slurry entering into the lower passages  78  of the carrier head  4 . Because the membrane  90  seals the lower passages  78  from the slurry, the slurry is unable to enter into the wafer carrier head  2 . 
     Employing an adjustable wafer retaining ring  62  also minimizes the chances of having the wafer  4  slip out from under the wafer carrier head  2 . The retaining ring  62  is adjusted by applying positive pressure and vacuum forces to the outer bellows assembly  52 . Adjusting the amount the retaining ring  62  protrudes from the lower ring  56  prevents the wafer  4  from extending beyond the retaining ring  62  when the inner bellows assembly  80  is applying a positive pressure to the wafer  4  to lower it onto the polishing surface  18 . By employing an adjustable wafer retaining ring, the timing between lowering the retaining ring  62  to the polishing surface  18  and applying pressure at the inner bellows assembly  80  to the membrane  90  to lower the wafer  4  onto the polishing surface  18  is not critical. 
     Another advantage of the present embodiments is the elimination of a mechanical gimbal point, which in turn will eliminate any resultant moment arm because the effective gimbal point will be located at the wafer  4 . Eliminating the moment arm will reduce the amount the carrier head  2  will tend to dig into the outer part  12  of the wafer  4 . This problem is common to wafer carrier heads  2  and causes a higher rate of wafer removal to occur at the outer part  8  of the wafer  4  than the remainder of the wafer  4 . The amount the carrier head  2  digs into the wafer  4  is directly proportionate to the length of the moment arm. The present embodiments minimize this amount because the lack of a mechanical gimbal point means that there is no moment arm associated with the present invention. 
     The embodiments of the wafer carrier head herein described also employ components made up of high-strength plastics. In preferred embodiments of the invention, the outer housing, inner housing, inner housing chamber and the retaining ring can be made from high-strength plastics. This provides the added advantage of reducing the overall weight of the wafer carrier head. 
     The embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.