Abstract:
A CMP polishing head having multiple concentric pressure zones for selectively increasing polishing pressure against selected regions of a semiconductor wafer in order to compensate for variations in polishing rates on the wafer surface otherwise caused by ridges or other non-uniformities in the wafer surface. The polishing head of the present invention comprises multiple, concentric, inflatable pressure rings each of which may be selectively inflated to increase the polishing pressure against a concentric ridge or material elevation on the corresponding concentric region of the wafer surface and increase the polishing rate of the concentric ridge or elevation between the rotating polishing head and a stationary polishing pad. A channel selector may be included in the polishing head for selectively aligning an air/pressure vacuum source with a selected one of multiple pressure tubes that connect to the respective pressure rings.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to chemical mechanical polishing apparatus used in the polishing of semiconductor wafers. More particularly, the present invention relates to a CMP apparatus polishing head which includes multiple concentric pressure zones for applying variable polishing pressure against various regions on a semiconductor wafer.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.  
           [0003]    A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.  
           [0004]    A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen, with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing of a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing of a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear; however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.  
           [0005]    A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.  
           [0006]    Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.  
           [0007]    As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.  
           [0008]    A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus  20  for chemical mechanical polishing includes a polishing head  8  which includes a rotating wafer holder  14  that holds the wafer  10 , the appropriate slurry  24 , and a polishing pad  12  which is normally mounted to a rotating table  26  by adhesive means. The polishing pad  12  is applied to the wafer surface  22  at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films.  
           [0009]    A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel.  
           [0010]    In a CMP head, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a major cause for wafer non-uniformity. In the improved CMP head design, even though a pneumatic system for forcing the wafer surface onto a polishing pad is used, the system cannot selectively apply different pressures at different locations on the surface of the wafer. Accordingly, while the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.  
           [0011]    As shown in FIG. 1B, the surface profile of unpolished wafers  10  typically includes one or more annular, flat-topped ridges  23  which extend from the wafer surface  22 . Because the wafer holder  14  of the polishing head  8  typically exerts uniform polishing pressure against all regions on the backside  28  of the wafer  10 , this non-uniformity in the wafer surface profile causes difficulty in uniform polishing of the wafer surface  22  at the interface of the wafer surface  22  and the polishing pad  12 . Some wafer holders  14  utilize a pressure membrane (not shown) at the center of the wafer holder  14  to exert extra pressure against the center region of the wafer  10  and thus, increase the polishing rate at the center relative to the peripheral regions of the wafer surface  22 . While this ameliorates the non-uniform polishing rates between the central and peripheral regions of the wafer surface  22 , non-uniformity in the polishing rates between the central and peripheral regions of the wafer surface  22 , caused by the ridge or ridges  23 , remains. Accordingly, a polishing head is needed which includes multiple pressure zones for applying pressure against various regions of a wafer in order to facilitate more uniform polishing rates among all regions on the wafer surface due to ridge or basin profiles in the wafer surface.  
           [0012]    An object of the present invention is to provide a new and improved polishing head for a chemical mechanical polisher.  
           [0013]    Another object of the present invention is to provide a new and improved polishing head which facilitates uniform polishing rates among multiple regions on a wafer surface during a chemical mechanical polishing process.  
           [0014]    Still another object of the present invention is to provide a new and improved polishing head which includes multiple, independently-controlled pressure zones for increasing pressure against various regions of a wafer for uniform polishing of the wafer surface.  
           [0015]    Yet another object of the present invention is to provide a new and improved CMP polishing head which facilitates improved polishing rates in the polishing of semiconductor wafers having a ridge or basin wafer surface profile.  
           [0016]    A still further object of the present invention is to provide a CMP polishing head which utilizes a channel selector to select among one or more of multiple pressure zones which exert pressure against a wafer to facilitate substantially uniform polishing rates among all regions on the surface of the wafer.  
           [0017]    Yet another object of the present invention is to provide a CMP polishing head which includes multiple concentric pressure rings that may be independently inflated and pressurized against selected concentric regions on a wafer interposed between the polishing head and a polishing pad in order to increase the polishing rate of the regions on the wafer pressurized against the polishing pad by the pressure ring or rings.  
         SUMMARY OF THE INVENTION  
         [0018]    In accordance with these and other objects and advantages, the present invention is directed to a CMP polishing head having multiple concentric pressure zones for selectively increasing polishing pressure against selected regions of a semiconductor wafer in order to compensate for variations in polishing rates on the wafer surface otherwise caused by ridges or other non-uniformities in the wafer surface. The polishing head of the present invention comprises multiple, concentric, inflatable pressure rings each of which may be selectively inflated to increase the polishing pressure against a concentric ridge or material elevation on the corresponding concentric region of the wafer surface and increase the polishing rate of the concentric ridge or elevation between the rotating polishing head and a stationary polishing pad. A channel selector is typically included in the polishing head for selectively aligning an air/pressure vacuum source with a selected one of multiple pressure tubes that connect to the respective pressure rings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1A is a cross-sectional view of a typical conventional CMP apparatus during a CMP wafer polishing process;  
         [0021]    [0021]FIG. 1B is a cross-sectional view of a typical conventional CMP apparatus during a CMP wafer polishing process, wherein the unpolished wafer includes an annular ridge or material elevation in the polishing surface thereof;  
         [0022]    [0022]FIG. 2 is a cross-sectional view of an illustrative embodiment of the polishing head with concentric pressure zones of the present invention;  
         [0023]    [0023]FIG. 3 is a cross-sectional view of a typical channel selector component of the polishing head of the present invention;  
         [0024]    [0024]FIG. 4 is a cross-sectional view, taken along section lines  4 - 4  in FIG. 2, of the polishing head;  
         [0025]    [0025]FIG. 5 is a cross-sectional view of a pressure ring component of the polishing head of the present invention;  
         [0026]    FIGS.  6 A- 6 D are schematic cross-sectional views of the channel selector, illustrating successive positions of the channel selector interior components during switching from one pressure ring to another pressure ring in the polishing head;  
         [0027]    FIGS.  7 A- 7 D correspond to FIGS.  6 A- 6 D, respectively, and are schematic views of a duct roller component of the channel selector, illustrating successive positions of the duct roller during switching from one pressure ring to another pressure ring in the polishing head;  
         [0028]    [0028]FIG. 8 is a cross-sectional view of the polishing head, illustrating inflation of one of the pressure rings in the polishing of a semiconductor wafer; and  
         [0029]    [0029]FIG. 8A is a cross-sectional view of the inflated pressure ring of FIG. 8.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    The present invention has particularly beneficial utility in the uniform polishing of semiconductor wafers having a non-uniform surface in the semiconductor fabrication industry. However, the invention is not so limited in application, and while references may be made to such semiconductor wafers, the present invention is more generally applicable to polishing substrates in a variety of mechanical and industrial applications.  
         [0031]    Referring initially to FIG. 2, a polishing head  32  of the present invention includes a housing  39  which is connected to a hub  33  supported on a drive shaft (not shown) to rotate therewith during polishing about an axis of rotation which is substantially perpendicular to the surface of a polishing pad (not shown) during polishing, as hereinafter described. The housing  39  may be circular in shape to correspond to the circular configuration of the substrate to be polished. A cylindrical bushing  48  may fit into a vertical bore extending through the hub  33 . A frame  40  may be mounted on the hub  33  inside the housing  39 . A base  41  is mounted inside the housing  39  beneath the frame  40 . The frame  40  may be connected to the base  41  by a rolling diaphragm  45 . The rolling diaphragm  45  seals the space between the frame  40  and the base  41  to define a loading chamber  43  between the frame  40  and the base  41 . By delivery of air or nitrogen into the loading chamber  43  through a loading chamber passage  34  extending through the hub  33  and the frame  40 , air or nitrogen pressure in the loading chamber  43  applies a downward pressure to the base  41  to control the vertical position of the base  41  relative to the polishing pad. A retainer ring  44  is mounted on the bottom of the base  41 . A gimbel mechanism  42  mounted on the base  41  permits the base  41  to pivot with respect to the housing  39  such that the base  41  may remain substantially parallel with the surface of the polishing pad. The gimbel mechanism  42  includes a gimbel rod  38  which fits into a gimbel rod bore  48  extending through the hub  33  and the frame  40 . The gimbel rod  38  may slide vertically along the gimbel rod bore  48  to impart vertical motion to the base  41 , and prevents lateral motion of the base  41  with respect to the housing  39 . A membrane duct passage  36  may extend through the gimbel rod  38  and the gimbel mechanism  42  for purposes which will be hereinafter described.  
         [0032]    A substrate backing assembly  50  of the polishing head  32  includes a support plate  51  which is mounted to an annular support structure  46 . The support structure  46  is connected to the base  41  by an annular flexure  57 . An annular inner tube  47  may be provided in the base  41  and inflated to apply downward air or nitrogen pressure against the support structure  46 , as hereinafter described. An outer pressure ring  52 , a middle pressure ring  53  and an inner pressure ring  54  are supported by the support plate  51  in concentric relationship to each other. A pair of concentric inside pressure rings  56  may further be supported by the support plate  51 , inside the inner pressure ring  54 . An air- or nitrogen-actuated central membrane  58  may be further included in the center of the support plate  51 . A channel selector  65  is mounted in the loading chamber  43 , typically on the bottom surface of the frame  40 , and is confluently connected to the outer pressure ring  52 , the middle pressure ring  53 , the inner pressure ring  54 , the inside pressure rings  56  and the central membrane  58 . The channel selector  65  inflates and deflates a selected one of the outer pressure ring  52 , the middle pressure ring  53 , the inner pressure ring  54 , the inside pressure rings  56  and the central membrane  58 , as hereinafter described. A flexible membrane  55  is mounted on the retainer ring  44  beneath the support plate  51 .  
         [0033]    As shown in FIG. 4, in accordance with the present invention, the outer pressure ring  52 , the middle pressure ring  53  and the inner pressure ring  54  are mounted on the support plate  51  in concentric relationship to each other. As shown in FIG. 5, each of the pressure rings  52 - 54  typically includes a ring support  60  which is mounted to the support plate  51 ; an air passage  61  which extends through the ring support  60 ; and a flexible, typically rubber ring membrane  62  which is pneumatically sealed against the ring support  60  to define a bladder  63 . The channel selector  65  is confluently connected to the outer pressure ring  52 , the middle pressure ring  53 , the inner pressure ring  54 , the inside pressure rings  56  and the central membrane  58  through respective proximal tubes  3 , as shown in FIGS.  7 A- 7 D, and distal tubes  1  which are connected to the proximal tubes  3  by respective tube connectors  2  that extend through the gimbel mechanism  42 . The channel selector  65  is further confluently connected to the inner tube  47  through a proximal tube  3 . The channel selector  65  is actuated by pressurized air or nitrogen and vacuum pressure alternately distributed through a channel selector air passage  35  extending through the hub  33  and through a channel selector tube  4  that connects the channel selector passage  35  to the channel selector  65 . The channel selector  65  distributes pressurized air or nitrogen and vacuum pressure to a selected one of the outer pressure ring  52 , the middle pressure ring  53 , the inner pressure ring  54 , the inside pressure rings  56 , the central membrane  58  and the inner tube  47  by receiving the air, nitrogen or vacuum pressure through a pressure ring passage  37  extending through the hub  33  and the frame  40 , respectively. The pressurized air or nitrogen or the vacuum pressure is distributed to the pressure rings  52 - 54 , inside pressure ring  56 , central membrane  58  or inner tube  47  through a corresponding one of the multiple proxmial tubes  3  and distal tubes  1 .  
         [0034]    As shown in FIG. 3, the channel selector  65  typically includes a casing  66  which defines a casing interior  67 . The channel selector tube  4  is disposed in fluid communication with the casing interior  67  through a casing opening  66   a.  A disc-shaped active ratchet wheel  68 , having multiple ratchet fingers  69  extending upwardly therefrom in a circular pattern, is slidably disposed in the bottom portion of the casing interior  67 . The upper, extending end of each ratchet finger  69  is terminated by a pair of bevels  70 , which define a pointed configuration. A fixed ratchet wheel  72  is fixedly mounted to the casing  66 , in the casing interior  67  above the active ratchet wheel  68 . Multiple finger openings  73  extend through the fixed ratchet wheel  72  in a circular pattern for receiving the respective ratchet fingers  69  of the active ratchet wheel  68 . Bevels  74  are provided in the upper surface of the fixed ratchet wheel  72 , between the respective finger openings  73 . A passive ratchet wheel  76  is slidably disposed in the casing interior  67  above the fixed ratchet wheel  72 , and includes multiple downwardly-extending ratchet fingers  77  that are arranged in a circular pattern and are capable of removable insertion into the respective finger openings  73  of the fixed ratchet wheel  72  and engaging the ratchet fingers  69  of the active ratchet wheel  68  and the bevels  74  of the fixed ratchet wheel  72  to rotate the passive ratchet wheel  76 , as hereinafter described. A bevel  78  is provided in the lower, extending end of each ratchet finger  77 . A base collar  79  extends upwardly from the passive ratchet wheel  76  and includes tab slots  80 . A duct roller  82 , having a duct roller collar  83  extending downwardly therefrom, is rotatably disposed in the casing interior  67 , above the passive ratchet wheel  76 . The duct roller collar  83  is fitted with a pair of tabs  84  that slidably engage the respective tab slots  80  in the base collar  79  of the passive ratchet wheel  76 . A spring  85  interposed between the duct roller  82  and the passive ratchet wheel  76  normally biases the passive ratchet wheel  76  downwardly, away from the duct roller  82 . At least one L-shaped duct  86  extends through the duct roller  82 , one end of which duct  86  is provided at the center of the duct roller  82 , at an opening  66   b  in the casing  66 , in confluent communication with the pressure ring air passage  37  (FIG. 2) which extends through the hub  33 . The opposite end of the duct  86  is disposed in confluent communication with a selected one of the proximal tubes  3  (FIG. 2) leading to the outer pressure ring  52 , the middle pressure ring  53 , the inner pressure ring  54 , the inside pressure rings  56  or the central membrane  58 , respectively, depending on the position of the duct roller  82  in the casing interior  67 . As shown in FIGS.  7 A- 7 D, two or more of the ducts  86  may be provided in the duct roller  82  for simultaneous alignment with two or more of the proximal tubes  3 . In that case, two or more of the outer pressure ring  52 , the middle pressure ring  53 , the inner pressure ring  54 , the inside pressure rings  56  or the central membrane  58  may be pressurized simultaneously.  
         [0035]    FIGS.  6 A- 7 D illustrate operation of the channel selector  65  to facilitate flow of pressurizing air or nitrogen or de-pressurizing vacuum pressure from the channel selector passage  35  (FIG. 2) to a selected one of the outer pressure ring  52 , the middle pressure ring  53 , the inner pressure ring  54 , the inside pressure rings  56 , the central membrane  58  and the inner tube  47 . In FIGS. 6A and 7A, the air duct  86  in the duct roller  82  is initially disposed in confluent communication with a proximal tube  3 a which establishes confluent communication between the pressure ring passage  37  and the distal tube  1  connected to the outer pressure ring  52 , for example. Accordingly, pressurized air or nitrogen, typically at a pressure of up to about 10 psi, is capable of flowing through the pressure ring passage  37 , the duct  86 , the proximal tube  3   a,  the corresponding distal tube  1 , and finally, into the bladder  63  (FIG. 5) of the outer pressure ring  52 . The ring membrane  62  of the outer pressure ring  52  therefore expands, as shown by the dotted line in FIG. 5, and presses against the flexible membrane  55 , as shown in FIG. 8. As the polishing head  32  is rotated in conventional fashion with a wafer  90  interposed between the flexible membrane  55  and the polishing pad  92 , the flexible membrane  55  thus presses against the corresponding portion of the wafer  90  to enhance the polishing rate against that portion of the wafer  90 , as hereinafter described.  
         [0036]    The outer pressure ring  52  may be deflated and one of the other pressure rings  53 , 54 , inside pressure rings  56 , central membrane  58  or inner tube  47  inflated, as needed to achieve the desired relative polishing rates on the wafer  90 , as follows. For purposes of explanation, the proximal tube  3 b shown in FIGS.  6 A- 7 D connects the channel selector  65  to the distal tube  1  which is connected to the middle pressure ring  53 . Accordingly, the outer pressure ring  52  may deflated and the middle pressure ring  52  inflated to increase the polishing rate of a second annular region on the wafer  90 , as needed, by initially applying vacuum pressure to the pressure ring passage  37  in the hub  33  (FIG. 2). Because the duct  86  is still aligned with the proximal tube  3   a  that communicates with the outer pressure ring  52 , as shown in FIG. 7A, the vacuum pressure draws the pressurizing air or nitrogen in the outer pressure ring  52  from the bladder  63  (FIG. 5), through the distal tube  1 , the proximal tube  3   a,  the duct  86  of the duct roller  82 , and the pressure ring passage  37  in the hub  33 , respectively. The channel selector  65  is then actuated to provide confluent communication between the pressure ring passage  37  and the middle pressure ring  53 , as follows.  
         [0037]    First, pressurized air or nitrogen is distributed through the channel selector passage  35  in the hub  33 , through the channel selector tube  4  and into the casing interior  67  of the channel selector  65 , respectively. As shown in FIG. 6B, the pressurized air or nitrogen impinges against the active ratchet wheel  68 , slidably displacing it in the casing interior  67  such that the ratchet fingers  69  of the active ratchet wheel  68  extend through the respective finger openings  73  (FIG. 3) of the fixed ratchet wheel  72 . The moving ratchet fingers  69  engage and push against the respective ratchet fingers  77  of the passive ratchet wheel  76 , against the bias imparted by the spring  85 , beyond the respective bevels  74  of the fixed ratchet wheel  72 . Due to the sloped configuration of the bevels  74  of the fixed ratchet wheel  72 , the bevels  78  of the ratchet fingers  77  of the passive ratchet wheel  76  slide on the bevels  74  of the fixed ratchet wheel  72  as the spring  85  simultaneously pushes the passive ratchet wheel  76  against the fixed ratchet wheel  72 . This causes the passive ratchet wheel  76  to rotate in the counterclockwise direction, as shown in FIG. 6C, as the bevels  78  of the passive ratchet wheel  76  slide against the respective bevels  74  of the fixed ratchet wheel  72 . Simultaneously, the tabs  84  on the duct roller collar  83  are engaged by the tab slots  80  on the base collar  79  of the passive ratchet wheel  78 , such that the duct roller  82  rotates with the passive ratchet wheel  78 , as shown in FIG. 7C. The spring  85 , combined with vacuum pressure applied to the casing interior  67  through the channel selector air tube  4 , as shown in FIG. 6D, finally displaces the passive ratchet wheel  76  in the casing interior  67  such that the ratchet fingers  77  of the passive ratchet wheel  76  are again inserted in the respective finger openings  73  of the fixed ratchet wheel  72 . At this point, the duct  86  is disposed in fluid communication with the proximal tube  3   b,  as shown in FIG. 7D. Accordingly, the middle pressure ring  53  is inflated by introducing pressurized air or nitrogen through the pressure ring passage  37 , the duct  86 , the proximal tube  3   b,  the corresponding distal tube  1  and into the middle pressure ring  53 , respectively. The middle pressure ring  53  is deflated and one or more of the inner pressure ring  54 , the inside pressure rings  56 , the central membrane  58  or the inner tube  47  pressurized with air or nitrogen, typically at a pressure of up to about 10 psi, by operating the channel selector  65  to incrementally establish confluent communication between the pressure ring passage  37  and the appropriate proximal tube  3  which corresponds to the inner pressure ring  54 , the inside pressure rings  56 , the central membrane  58  or the inner tube  47 , in the same manner as heretofore described with respect to the transition between the proximal tube  3   a  and the proximal tube  3   b.    
         [0038]    Referring next to FIGS. 8 and 8A, in application of the polishing head  32 , a wafer  90  is mounted in a face-down position on the flexible membrane  55 , typically according to conventional methods for mounting the wafer  90  on CMP polishing heads. The wafer  90  typically includes one or more annular ridges  91  protruding from the face thereof, as shown in FIG. 8A, and the pressure rings  52 - 54 , as well as the inside pressure rings  56 , may be selectively pressurized with air or nitrogen to facilitate enhanced polishing uniformity of all areas on the surface of the wafer  90 , including the ridges  91 . Accordingly, as the polishing head  32  is rotated, the flexible membrane  55  presses the wafer  90  against a polishing pad  92  of a CMP apparatus. The polishing pad  92  removes wafer material from the surface of the wafer  90  to provide a substantially uniform surface for the subsequent fabrication of integrated circuit devices on the wafer  90 . As shown in FIG. 8A, in the event that a ridge or other elevation  91  on the surface of the wafer  90  is located beneath the outer pressure ring  52  of the polishing head  32 , the outer pressure ring  52  is pressurized with air or nitrogen at a pressure of up to typically about 10 psi in the manner heretofore described with respect to FIGS. 2 and 6A- 7 D. Accordingly, the pressurized outer pressure ring  52  applies extra downward pressure against the flexible membrane  55  which, in turn, applies the pressure against the backside  89  of the wafer  90 , directly above the ridge  91 . This extra pressure applied to the ridge  91  against the polishing pad  92  causes polishing of the ridge  91  at a faster rate than polishing of the flat areas on the wafer  90 , resulting in a more uniform polishing rate among all regions on the wafer  90 . The outer pressure ring  52  may be deflated and one of the other pressure rings  53 ,  54 , inside pressure rings  56 , or central membrane  58  inflated by actuation of the channel selector  65 , as heretofore described, to apply increased pressure at the respective regions of the wafer  90  which correspond to the locations of the pressure rings  53 ,  54 , inside pressure rings  56 , or central membrane  58  above the wafer  90 , as needed to increase the polishing rate at those locations on the wafer  90 . Pressurized air or nitrogen may be introduced into the loading chamber  43  through the loading chamber passage  34  to pressurize the loading chamber  43 . The inner tube  47  may be pressurized by introducing pressurized air or nitrogen through the appropriate proximal tube  3  and into the inner tube  47  by operation of the channel selector  65 , as heretofore described. Accordingly, the inner tube  47  inflates and exerts downward pressure against the support plate  51  through the support structure  46  to apply extra polishing pressure, as needed, to the support plate  51 .  
         [0039]    While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.