Patent Publication Number: US-6712673-B2

Title: Polishing apparatus, polishing head and method

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to apparatus for polishing semiconductor or similar type materials, and more specifically to such apparatus which facilitates equalization of the downward pressure over the polished wafer surface and the polishing head of the apparatus. 
     Polishing an article to produce a surface which is highly reflective and damage free has application in many fields. A particularly good finish is required when polishing an article such as a wafer of semiconductor material in preparation for printing circuits on the wafer by an electron beam-lithographic or photolithographic process (hereinafter “lithography”). Flatness of the wafer surface on which circuits are to be printed is critical to maintain resolution of the lines, which can be as thin as 0.13 microns (5.1 microinches) or less. The need for a flat wafer surface, and in particular local flatness in discrete areas on the surface, is heightened when stepper lithographic processing is employed. 
     Flatness is quantified in terms of a global flatness variation parameter (for example, total thickness variation (“TTV”)) or in terms of a local site flatness variation parameter (e.g., Site Total Indicated Reading (“STIR”) or Site Focal Plane Deviation (“SFPD”)) as measured against a reference plane of the wafer (e.g., Site Best Fit Reference Plane). STIR is the sum of the maximum positive and negative deviations of the surface in a small area of the wafer from a reference plane, referred to as the “focal” plane. SFQR is a specific type of STIR measurement, as measured from the front side best fit reference plane. A more detailed discussion of the characterization of wafer flatness can be found in F. Shimura, Semiconductor Silicon Crystal Technology 191-195 (Academic Press 1989). Presently, flatness parameters of the polish surfaces of single side polished wafers are typically acceptable within a central portion of most wafers, but the flatness parameters become unacceptable near the edges of the wafers, as described below. 
     Polishing machines typically include an annular polishing pad mounted on a turntable for driven rotation about a vertical axis passing through the center of the pad. The wafers are fixedly mounted on pressure plates above the polishing pad and lowered into polishing engagement with the rotating polishing pad. A polishing slurry, typically including chemical polishing agents and abrasive particles, is applied to the pad for greater polishing interaction between the polishing pad and the wafer. 
     In order to achieve the degree of polishing needed, a substantial normal force presses the wafer into engagement with the pad. The coefficient of friction between the pad and wafer creates a significant lateral force on the wafer. This lateral force can give rise to certain distortions in the polish, such as by creating a vertical component of the frictional force at the leading edge of a wafer. The vertical component of the frictional force is created because the wafer is mounted to pivot about a gimbal point under influences of the lateral friction forces. A change in the net vertical force applied to the wafer locally changes the polishing pressure and the polishing rate of the wafer, giving rise to distortions in the polish. Often the uneven forces cause the wafer&#39;s peripheral edge margin to be slightly thinner than the majority of the wafer, rendering the edge margin of the wafer unusable for lithographic processing. This condition is a sub-species of the more general problems associated with wafer flatness, and will be referred to hereinafter as edge roll-off. 
     Improvements in wafer polishers have helped reduce edge roll-off. Recent configurations have incorporated conic bearing assemblies between the wafer and the mechanism applying the polishing force, while permitting free rotation of the wafer. Conic bearing assemblies are an improvement over traditional ball and socket configurations because the gimbal point of the mechanism is at a point below the bearing, nearer the interface between the wafer and the polishing pad. Wafers polished with a gimbal point near the work surface exhibit superior flatness characteristics, particularly near the outer edge of the wafer where conventional polishing processes exhibit characteristic “roll-off” and near the center of the wafer where slurry starvation may occur. 
     Another improvement directed toward more uniform wafer polishing is the use of a membrane to apply pressure to the rear surface of the wafer. Because membranes rely on air pressure to exert force upon the wafer, the pressure is thought to be more uniform over the wafer surface throughout the polishing process. Membranes, however, suffer from drawbacks. First, membranes must stretch during inflation to apply pressure over the wafer. Because the entire membrane must stretch as it attempts to engage the wafer, a portion of the pressure is used to stretch the wafer, instead of applying pressure to the wafer. Moreover, as the central portion of the membrane stretches toward the wafer, the lateral edges of the membrane are held tightly and cannot stretch enough to fully engage the wafer. By stretching the central portion only, while inhibiting the lateral edges of the membrane from engaging the wafer, the membrane provides inadequate support at the wafer&#39;s edge. Thus, the pressure applied at the edge of the wafer is due to the stiffness of the wafer itself, rather than from engagement with the membrane, causing the wafer edge to be underpolished. Secondly, if the rotational speed of the wafer and polishing pad become unsynchronized, torque is created on the wafer. Such torque can wrinkle the membrane, leading to uneven polishing or catastrophic failure, as the wafer may slip out of the polishing head during polishing. Thus, a configuration is needed incorporating further features for facilitating wafer flatness due to more uniform polishing, while overcoming the drawbacks mentioned above. 
     SUMMARY OF THE INVENTION 
     Among the several objects and features of the present invention may be noted the provision of a semiconductor wafer polishing apparatus, method and polishing head which apply uniform polishing pressure over the surface of the wafer; the provision of such an apparatus, method and head which facilitate better polishing pressure near the lateral edge of the wafer; and the provision of such an apparatus, method and head which provide efficient pick-up and release of the wafer from the polishing head. 
     Generally, a wafer polishing apparatus of the present invention for polishing a front surface of a wafer comprises a base for supporting elements of the polishing apparatus. A turntable mounts on the base for rotation about an axis on the base and is adapted to support a polishing pad for conjoint rotation with the turntable. The polishing pad has a work surface engageable with the front surface of the wafer for use in polishing the front surface of the wafer. A turntable drive mechanism operatively connects to the turntable for selectively driving rotation of the turntable about the axis of rotation. A polishing head mounts for holding the wafer in generally opposed relation with the turntable and for rotation about an axis generally parallel to the axis of rotation of the turntable. The polishing head includes a back plate having at least a central region in opposed relation with a rear surface of the wafer when the wafer is received by the polishing head. An annular sealing ring of flexible material has a thickness and is disposed around the central region of the back plate. The sealing ring has a central opening extending through the complete thickness of the sealing ring and is disposed for engaging a peripheral edge margin of the wafer, such that the rear surface of the wafer, the sealing ring and the back plate define a substantially fluid-tight cavity for controlling fluid pressure in the cavity. 
     In yet another embodiment of the present invention, a method of polishing a semiconductor wafer comprises placing a rear surface of the semiconductor wafer in engagement with a seal of the polishing head of a wafer polishing apparatus to form a fluid pressure cavity defined by the rear surface of the wafer, the seal and the polishing head. The wafer is mounted on the polishing head by evacuating the fluid pressure cavity to draw the wafer to the polishing head and hold the wafer. The method further comprises engaging a front surface of the wafer on the polishing head with a polishing pad on a turntable and urging the front surface of the wafer against the polishing pad by selectively applying air pressure within the cavity for pressing the wafer surface uniformly against the polishing pad. Air within the cavity directly engages a majority of the rear surface of the wafer. The wafer is disengaged from the turntable and removed from the polishing head. 
     The present invention is also directed to a polishing head generally set forth as above. 
     The present invention is also directed to a method of processing a semiconductor wafer. An oxide layer is formed on a rear surface of the semiconductor wafer. The semiconductor wafer is then free-mounted on a polishing head of a wafer polishing apparatus. A front surface of the wafer on the polishing head engages a polishing pad on a turntable. Relative motion between the wafer and the polishing pad is obtained, and the front surface of the wafer is urged against the work surface. The wafer is removed from the polishing head. 
     Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side elevation of the wafer polishing apparatus inside a non-contamination booth; 
     FIG. 2 is a section of the polishing head of the present invention; 
     FIG. 3 is an enlarged, fragmentary portion of the polishing head section of FIG. 2 but lacking a support pad and with a sealing ring in position to pick up a wafer; 
     FIG. 4 is an enlarged, fragmentary portion of the polishing head section of FIG. 2 with the wafer held by vacuum against a support pad; 
     FIG. 5 is an enlarged, fragmentary portion of the polishing head section of FIG. 2 shown polishing the wafer; 
     FIG. 6 is an enlarged, fragmentary section of the polishing head of the present invention having a sealing ring having a smaller central opening; 
     FIG. 7 is the enlarged, fragmentary section of FIG. 6 shown polishing a wafer; 
     FIG. 8 is an enlarged, fragmentary section of a polishing head of a second embodiment; 
     FIG. 9 is an enlarged, fragmentary section of a third embodiment; and 
     FIG. 10 is a schematic side elevation of the wafer in a bath. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures, specifically to FIGS. 1 and 2, a wafer polishing apparatus, generally indicated at  21 , constructed according to the present invention is shown having a base, generally indicated at  23 , for housing and supporting other elements of the polishing apparatus. The base  23  may be of various configurations, but preferably is formed to provide a stable support for the polishing apparatus  21 . In the preferred embodiment, the base  23  comprises a booth  25  enclosing the wafer polishing apparatus  21  and inhibiting airborne contaminants from entering the booth and contaminating the apparatus and articles to be polished. Except as pointed out hereinafter with regard to the way a semiconductor wafer  35  is held and polished by the polishing apparatus  21  during polishing, the construction of the polishing apparatus is conventional. An example of such a conventional single-sided polishing apparatus  21  of the type discussed herein is the Strasbaugh Model 6DZ, available from Strasbaugh Inc. of San Luis Obispo, Calif. 
     A turntable  27  is mounted on the base  23  for rotation with respect to the base, as shown in FIG.  1 . The turntable  27  is circular and is adapted to support a polishing pad  29  thereon for polishing a front surface  39  of the semiconductor wafer  35  (FIG.  2 ). The polishing pad  29  is preferably adhesive-backed for securing the pad to the turntable  27 . The turntable and polishing pad  29  rotate conjointly relative to the base  23  about an axis A perpendicular to the turntable and polishing pad. The opposite side of the polishing pad  29  comprises a work surface  37  engageable with the front surface  39  of the semiconductor wafer  35  for use in polishing the front surface. Polishing pads are preferably formed from a urethane foam material, for example, RodelÂ® URI100 and SPM3100 pads (available from Rodel, Inc. of Phoenix, Ariz.) or FujimiÂ® SCCB (available from Fujimi Corporation of Elmhurst, Ill.). Other suitable materials are also contemplated as within the scope of the present invention. During polishing, the polishing pad  29  is configured to receive a continuous supply of polishing slurry. The polishing slurry is delivered to the pad  29  via a slurry delivery system (not shown). Polishing pads  29 , polishing slurry, and slurry delivery systems are well known in the relevant art. 
     Continuing with FIG. 1, the base  23 , booth  25 , turntable  27 , and a turntable drive mechanism  41  are each well known in the art and comprise the basic elements of the single-side wafer polishing apparatus  21  noted above. The turntable drive mechanism  41  operatively connects to the turntable  27  for selectively driving rotation of the turntable about axis A. The subject of the present invention is a new and useful addition to such a polishing apparatus  21 , as discussed in greater detail below. 
     The wafer polishing apparatus  21  further comprises a polishing head, generally indicated at  45  (FIGS.  1  and  2 ), pivotably and rotatably connectable to a head drive mechanism  46 . The head drive mechanism is operatively connected to the polishing head  45  for driving rotation of the polishing head about an axis B (FIGS.  1  and  2 ). The primary purpose of the polishing head  45  is holding the wafer  35  securely during polishing so that the wafer may be polished evenly. The polishing head  45  mounts on the lower end of an output shaft  47  so that they rotate conjointly. Polishing heads  45  are conventionally used to perform single-side polishing, but suffer various drawbacks relating to the quality of the polished wafer  35 . The polishing head  45  of the present embodiment avoids those drawbacks by further comprising a sealing ring  49 , as discussed in greater detail below. 
     A polisher arm  53  applies downward pressure to the polishing head  45  during wafer polishing (FIG.  1 ). A hydraulic or pneumatic actuation system is commonly used to articulate the arm  53 , although other articulation systems are contemplated as within the scope of the present invention. These systems are well known in the relevant art and will not be described in detail here. Downward force from the actuation system is transferred to the wafer  35  through the output shaft  47  and polishing head  45 . 
     The axis of rotation of the polishing head (axis B) is spaced apart from an axis of rotation (axis A) of the turntable (FIG.  1 ). This spacing helps ensure that the wafer  35  is subject to even polishing over a substantial portion of the polishing pad  29 . The polishing pad is preferably much wider than the wafer  35  and polishing head  45 , so that no portion of the wafer passes over the central portion of the polishing pad during polishing. This helps increase the longevity of the polishing pad  29  and the evenness of the wafer polish, because the wafer  35  interacts with a majority of the polishing pad. 
     Additionally, the polishing head  45  and the turntable  27  rotate at different relative rotational speeds for more uniform and efficient polishing of the wafer  35 . Regulating the rotational speed of the polishing head  45  impacts the wear pattern of the polishing pad  29 , which in turn impacts wafer  35  flatness and polishing pad life. The rotation of the wafer  35  and the polishing pad  29  can be modeled mathematically to compare the relative velocities of each for determining what relative velocities will likely provide the most even polishing and longest pad life. The polishing head  45  is preferably driven at a rotational speed less that the turntable  27 . Were the wafer  35  and polishing head  45  allowed to freely rotate, they would rotate at approximately the same speed as the polishing pad  29 , leading to uneven wear of the pad. Thus, the head drive mechanism  46  actually throttles the rotational speed of the polishing head  45  so that the polishing head rotates at a rotational speed of between about fifty percent (50%) and about one hundred percent (100%) of the rotational speed of the turntable  27 . More particularly, the best polishing is achieved where the head drive mechanism  46  rotates at a rotational speed of between about ninety percent (90%) and about one hundred percent (100%) of the rotational speed of the turntable  27 . Operating the head drive mechanism  46  and turntable  27  at similar rotational speeds reduces torque on the polishing head  45  and wafer  35 . 
     Turning to the construction of the apparatus  21 , the polishing head  45  mounts on the head drive mechanism  46  for driven rotation of the polishing head (FIGS.  1  and  2 ). The polishing head  45  is adapted to hold the wafer  35  in generally opposed relation with the turntable  27 , for engaging the front surface  39  of the wafer with the work surface  37  of the polishing pad  29 . The polishing head  45  is further attachable to the head drive mechanism  46  via a spherical bearing assembly, generally indicated at  59 , for pivoting of the polishing head about a gimbal point lying near the work surface  37 . The polishing head  45  holds the front surface  39  of the wafer  35  in engagement with the polishing pad  29 , for polishing the wafer and allowing the plane of the front surface of the wafer to continuously align itself to equalize polishing pressure over the front surface of the wafer for more uniform polishing of the wafer. The gimbal point preferably lies no higher than the interface of the front surface  39  of the wafer  35  and the work surface  37  when the polishing head  45  holds the wafer in engagement with the polishing pad  29 . The head drive mechanism  46  drives rotation of the polishing head  45  for maintaining the front surface  39  and work surface  37  in flatwise engagement for more uniform polishing of the wafer  35 . 
     The spherical bearing assembly  59  further comprises an upper conical seat  61  attachable to and rotating with the head drive mechanism  46  (FIG.  2 ). A lower spherical pivot  63  rigidly mounts on the polishing head  45  and extends upward toward the head drive mechanism  46 . The lower spherical pivot  63  is engageable with the upper conical seat  61  for pivotable movement of the polishing head  45  with respect to the head drive mechanism  46 . The lower spherical pivot  63  has an upwardly directed spherical face  65 . Any line normal to the spherical face  65  passes through the gimbal point. The pivoting motion aids in creating uniform pressure over a retaining ring  107  of the polishing head  45  (discussed in greater detail below), enhancing the ability of the retaining ring to retain the wafer  35 . The gimbal point lies at or slightly below an interface of the wafer  35  and the work surface  37  on a side of the interface containing the turntable  27 . This geometry maintains the work surface  37  and the polishing head  45  in flatwise engagement. This configuration further inhibits low pressure points from forming near the trailing edge of the polishing head  45  due to pivoting of the polishing head relative to the turntable  27  and helps retain the wafer. Preferably, the lower spherical pivot  63  is formed from a high strength metal, such as stainless steel, and the upper conical seat  61  is formed from a plastic material, such as PEEK, a polyaryletherketone resin, available from Victrex USA Inc. of Westcheter, Pa., U.S.A. Both surfaces are highly polished to inhibit wear debris generation and to minimize friction within the spherical bearing assembly  59  and create a highly smooth pivoting movement of the bearing assembly. 
     A semi-rigid connection, generally indicated at  71 , is attachable to the output shaft  47  and the polishing head  45  for transferring a rotational force from the head drive mechanism  46  to the polishing head, while permitting universal pivoting motion of the polishing head with respect to the head drive mechanism about the spherical bearing assembly  59 . The semi-rigid connection  71  comprises a plurality of shoulder bolts  73  attachable to the polishing head  45  (FIG.  2 ). These shoulder bolts  73  extend upward from the polishing head  45  and pass through a series of radial slots  75  in an annular flange  79  extending laterally from the upper conical seat  61 . The radial slots  75  are sized slightly larger than the shoulder bolts  73  so that as the output shaft  47  rotates, the radial slots engage the bolts for inducing rotation of the polishing head  45 . The additional clearance between the radial slots  75  and the bolts  73  allows the upper conical seat  61  and the lower spherical pivot  63  to pivot slightly with respect to one another. The pivoting allows for more uniform retaining ring pressure and continuous transmission of rotation from the head drive mechanism  46  to the polishing head  45 . The flange  79  and upper conical seat  61  are of unitary, plastic construction. When the head drive mechanism  46  is lifted upward after polishing, a bolt head  83  of each shoulder bolt  73  engages the plastic flange  79 , such that the polishing head  45  is lifted from the work surface  37 . 
     Turning to the novel features of the present invention, the polishing head  45  includes a back plate  89  having at least a central region  91  in opposed relation with a rear surface  93  of the wafer  35  when the wafer engages the polishing head. The back plate  89  is preferably a one-piece, rigid part. The annular sealing ring  49  is mounted on the underside of the polishing head  45  (FIGS.  2  and  3 ). The sealing ring  49  is preferably formed from flexible material having a thickness. The flexible material of the sealing ring  49  is preferably thin and adapted to flex upon receiving the wafer  35  on the polishing head. The sealing ring  49  may comprise an elastomeric material selected from a group including rubber, silicone and urethane. In the preferred embodiment, the sealing ring  49  is formed from 40 durometer EDPM (Ethylene Propylene Diene Monomer). The sealing ring is preferably about 0.79 millimeter (0.031 inch) thick. Other materials are contemplated as within the scope of the present invention. For example, non-contamination materials exhibiting a flexibility adequate to conform to the wafer  35  and a resiliency sufficient to transfer the rotational motion of the polishing head  45  to the wafer may be substituted for the preferred material. 
     The sealing ring  49  is disposed around the central region  91  of the back plate  89  and has a central opening  97  extending through the complete thickness of the sealing ring. The sealing ring is disposed for engaging a peripheral edge margin of the wafer  35 . The sealing ring  49  has a first major surface opposite a second major surface, hereinafter referred to as an outer surface  101  and an inner surface  103 , respectively. At least a portion of the outer surface  101  is engageable with the wafer  35  for mounting and sealing the wafer on the polishing head  45 , whereas the inner surface  103 , opposite the outer surface, faces the polishing head. 
     Referring now to FIGS. 2 and 3, the polishing head  45  further comprises the retaining ring  107  that encircles the sealing ring  49  and is mounted on the polishing head by a series of angularly spaced bolts  108  (only two are shown in FIG.  2 ). A primary function of the retaining ring  107  is to retain the wafer  35  in the polishing head  45  during polishing by forming a barrier against lateral motion of the wafer out from under the polishing head. Thus, the retaining ring  107  extends below the back plate  89  to be in radially opposed relation with a peripheral edge of the wafer (FIG.  4 ). 
     The sealing ring  49  includes an annular bead  109  received within a groove  111  of the back plate  89  for mounting the sealing ring on the polishing head  45 . The retaining ring  107  closes the groove  111  and clamps the sealing ring  49  against the back plate  89 . The portion of the sealing ring  49  not clamped between the retaining ring  107  and the back plate  89  is free to flex inward and outward from the back plate  89  a short distance. As the retaining ring  107  wears in normal use, it becomes thinner. The ability of the free portion of the sealing ring  49  to freely flex relative to the retaining ring  107  assures that the sealing ring will not force the wafer  35  below the bottom edge of the retaining ring. 
     A substantially fluid-tight cavity  115  is defined by the rear surface  93  of the wafer  35 , the sealing ring  49  and the back plate  89  for controlling fluid pressure in the cavity. A source of vacuum, as discussed below, communicates with the polishing head  45  via a series of channels  117  in the output shaft  47  and head (FIG.  2 ). The sealing ring  49  extends outwardly from the retaining ring  107  when the wafer  35  is not received in the polishing head  45  (FIG.  3 ). The sealing ring  49  also extends radially inwardly toward axis B of the polishing head  45  when the wafer  35  is not received in the polishing head, presenting the outer surface  101  for engagement with the rear surface  93  of the wafer. 
     Because the sealing ring  49  extends downwardly and inwardly, the central opening  97  of the sealing ring presents a circular edge for initial engagement with the rear surface  93  of the wafer  35  when the wafer is brought into close proximity with the polishing head  45  (FIG.  3 ). The central opening  97  forms a circular seal with the wafer  35 , so that when a vacuum is drawn in the cavity  115 , the wafer is drawn up into the polishing head  45 . In other words, the greater air pressure outside the cavity  115 , as compared with inside the cavity, lifts the wafer  35  upward toward the polishing head  45  as a vacuum is drawn within the cavity. The free edge portion of the sealing ring  49  is clamped between the wafer  35  and the back plate  89  (FIG.  4 ). The wafer  35  is drawn toward engagement with the back plate  89  so that the polishing head  45  may pick up the wafer. A support pad  119  may also mount on the underside of the back plate  89  for supporting the wafer  35  when held by the polishing head  45 . The support pad  119  is preferably formed from a resilient material less rigid than the back plate  89  for resiliently engaging the wafer  35  when mounting the wafer on the polishing head  45 . For instance, the support pad  119  may be readily formed from used polishing pad material, as described above. Such material is soft enough to resiliently engage the wafer  35  when engaging the polishing head  45  (FIG.  4 ). Moreover, the support pad  119  is preferably non-smooth to reduce the contact area of the support pad engageable with the sealing ring  49 , thereby reducing the adhesive forces and allowing the support pad to release the sealing ring. 
     Alternately, where a portion  125  of the back plate  89  is exposed for engagement with the inner surface  103  of the sealing ring  49  (e.g., FIG.  3 ), such portion may be cross-hatched, textured or otherwise non-smooth. This reduces the contact area of the portion  125  engageable with the sealing ring  49  to reduce the adhesive forces between the sealing ring and back plate  89 , thereby allowing the back plate to release the sealing ring. The support pad  119  also serves this purpose by preventing the sealing ring  49  from adhering to the back plate  89 . 
     A fluid pressure control  127 , such as a source of vacuum (FIG.  1 ), is adapted to affect fluid pressure within the cavity  115 . The pressure control  127  selectively applies vacuum pressure to the cavity  115  for capturing the wafer  35  on the polishing head  45 . At least one orifice  131  in the back plate  89  affects fluid communication of the cavity  115  with the pressure control  127  via the channels  117 . 
     Beyond applying vacuum pressure to pick up the wafer  35  (FIGS.  3  and  4 ), the pressure control  127  is also adapted to selectively apply positive air pressure within the cavity  115  for urging the wafer  35  toward the polishing pad  29  to polish the front surface  39  of the wafer, as shown in FIG.  5 . The pressure control  127  increases the air pressure within the cavity  115  until the wafer  35  engages the polishing pad  29  with sufficient force to polish the wafer. The sealing ring  49  flexes outward to engage the retaining ring  107  and wafer  35 , to maintain a fluid tight seal of the cavity  115 . The use of fluid pressure in combination with the flexible sealing ring  49  allows the pressure to equalize over the back surface  93  of the wafer  35  throughout polishing. The operation of the polishing head  45  will be discussed in greater detail below. 
     The size of the central opening  97  is also important for adjusting the polishing attributes of the apparatus  21 . Preferably, the inner diameter of the central opening  97  as measured when not engaging the wafer  35  (or when just engaging the wafer, as shown in FIG. 3) is between about 50% and about 95% of the wafer diameter. For a wafer  35  with a diameter of 200 millimeters (7.9 inches), the central opening  97  is preferably between about 100 millimeters (3.9 inches) and about 190 millimeters (7.5 inches). More specifically, the inner diameter is between about 80% and about 90% of the wafer  35  diameter. For a wafer  35  with a diameter of 200 millimeters (7.9 inches), the central opening  97  is preferably between about 160 millimeters (6.3 inches) and 180 millimeters (7.1 inches) in diameter. When the central opening  97  is about 85% of the wafer  35  diameter, the polisher polishes optimally. For a wafer 200 millimeters in diameter, this corresponds to a central opening  97  of 170 millimeters (6.7 inches). For a wafer 300 millimeters in diameter, the optimal diameter central opening  97  increases to 255 millimeters (10 inches). These preferred central opening  97  sizes are based upon the preferred sealing ring  49  material disclosed above, and those preferred sizes may change with a different sealing ring material. 
     During polishing, the sealing ring  49  may stretch slightly due to the application of pressure, slightly increasing the size of the central opening  97  from its nominal size. Changes in the durometer of the material selected for the sealing ring  49  may also drive alteration of the appropriate size of the central opening  97 . Where the sealing ring  49  is formed from a more flexible material, it will flex more during use and the central opening  97  need not be as large to ensure an adequate stretch of the sealing ring for proper contact with the wafer  35  (FIGS.  6  and  7 ). An opening  97  smaller than the examples noted above is not desirable, however, because it creates additional, unnecessary engagement area between the wafer and the sealing ring  49 . Less engagement of the wafer  35  and sealing ring  49  (i.e., a larger opening  97 ) is more desirable because more wafer area is subject to the direct engagement of uniform air pressure within the cavity  115  and wafer contamination is lessened due to any contaminants present on the sealing ring. 
     Conversely, a sealing ring  49  formed from a more inelastic material may require a larger opening  97  because the material is less flexible and is less likely to stretch to conform with the wafer  35  without a larger opening. An example of such an inelastic material is a fluorocarbon rubber, such as VitonÂ®, available from E. I. Dupont de Nemours Company of Wilmington, Del. A larger opening  97 , such as those in the preferred ranges noted above, provides more area over the rear surface  93  of the wafer for uniform pressure application. Moreover, a larger opening  97  may allow the sealing ring  49  to further conform to the retaining ring  107  and wafer  35 , encouraging more uniform application of pressure on the peripheral edge of the wafer  35 . Too large of an opening  97 , however, may implicate another problem, sealing ring  49  blowout. As the pressure within the cavity  115  increases, such as during polishing, the sealing ring  49  must have the strength to remain inwardly directed, so that the cavity  115  remains intact. Where the opening  97  is too large, the pressure may cause the sealing ring  49  to slide off the wafer  35 , causing it to blowout and release the wafer  35 . Furthermore, too large an opening  97  reduces the contact area with the wafer  35 , thus reducing the frictional force holding the wafer. Because torque must be applied to the wafer  35 , such a reduction in friction may lead to wafer slippage and backside polishing. 
     The present invention is ideally suited for polishing a wafer  35  previously polished on a double-side polished wafer polisher. Such a wafer  35  is already polished substantially flat, so that any additional polishing is aimed at removing a uniform layer of silicon material over the entirety of the wafer, without generally impacting wafer flatness. The sealing ring  49  configuration of the present invention is particularly well suited for such a purpose. As the retaining ring  107  is pressed firmly against the polishing pad  29  for retaining the wafer  35 , the sealing ring  49  and uniform air pressure across the rear surface  93  of the wafer allows the wafer to conform to the polishing pad for removal of a uniform layer of silicon. Moreover, the flexibility of the sealing ring  49  allows it to conform to the rear surface  93  of the wafer  35 , particularly the peripheral edge of the rear surface. By conforming more closely to the peripheral edge of the wafer  35 , the pressure within the polishing head  45  is exerted more uniformly upon the entire rear surface  93  of the wafer, including the lateral edges. Such uniform polishing pressure has advantages over a polisher using a rigid surface to support a wafer  35  during polishing. First, the polishing head  45  retains the wafer  35  without an adhesive, thereby reducing complexity and eliminating a possible contaminant. The polishing head  45  initially secures the wafer  35  with a vacuum, eliminating one source of potential contamination. Second, because the polishing pressure is applied to the wafer  35  directly by a fluid and only at the wafer periphery by the sealing ring  49 , there is less concern of contamination. Any particulate matter on the rear surface  93  of the wafer  35  coincident with the central opening  97  is not likely to impact polishing, as it may with rigid wafer support structures, because the air in the cavity  115  applies pressure directly to the rear surface, irrespective of the contaminants. Moreover, any particulate matter inadvertently caught between the wafer  35  and the sealing ring  169  is less likely to affect the polished surface. With conventional rigid support systems, particulate matter can become lodged between the wafer  35  and the rigid support structure, creating dimples in the polished surface. The foregoing benefits are also realized by the current configuration over conventional thin backing film configurations, which apply mechanical pressure to the wafer by a soft pad. Any method that applies mechanical pressure to the wafer is prone to generate uneven polishing and material removal. Primary reasons include uneven mechanical pressure because of local stiffness variations in the soft backing pad and uneven flatness of the surface to which the pad is mounted. In contrast, air pressure applied directly to the wafer inherently results in uniform polishing pressure. 
     During polishing, particulate matter puts pressure on the rear surface of the wafer, thereby pushing a small portion of the wafer outward toward the polishing pad. The polishing operation seeks to flatten the wafer, and typically flattens this small portion of the wafer pushed outward by the foreign matter. Once the wafer is removed from the rigid support, the portion of the wafer pushed out by the particulate matter returns to its original position, leaving a dimple defect in the polished surface. With a sealing ring  49 , any particulate matter lodged between the sealing ring and the wafer  35  will temporarily deform the sealing ring, not the wafer  35 , allowing the wafer to be polished without dimpling. Moreover, any particulate matter on the rear surface  93  of the wafer is less likely to affect the polish because the air imparts polishing pressure directly upon the wafer  35 . 
     Additionally, the sealing ring  49  betters conventional polisher configurations, specifically membrane configurations, because it eliminates superfluous membrane material that adds no additional polishing benefits. The sealing ring  49  is large enough to transmit torque and create a seal for the cavity  115  without any material engaging the center of the wafer  35 . Moreover, the sealing ring  49  provides the advantage of quickly and efficiently picking up and releasing the wafer  35 . The central opening  97  of the sealing ring  49  readily engages the back surface  93  of the wafer  35  to create a seal, while the majority of the back surface is free from engagement with the sealing ring. This allows the vacuum created within the cavity  115  to quickly pull the wafer  35  into engagement with the polishing head. During release, the wafer  35  more quickly disengages from the polishing head  45  because a large portion of the back surface  93  of the wafer receives the full force of the air pressure returning to the cavity  115 . Membrane configurations require a much greater contact area between the wafer and the polishing head, thereby increasing the adhesive forces between the two. These adhesive forces impede the ability of the polishing head to release the wafer after polishing. Moreover, membrane configurations are generally complicated mechanically, as compared with the present configuration. 
     Finally, unlike membrane configurations, as the sealing ring  49  stretches during use, the additional material is less likely to wrinkle and cause uneven polishing pressure on the wafer  35 . Any additional material engaging the wafer merely creates a potential for wrinkling as the membrane stretches, which may ultimately lead to uneven polishing and inadequate frictional force between the wafer and membrane. 
     In a second embodiment of the present invention, the sealing ring  49  mounts on the polishing head  45  in a novel way. As shown in FIG. 8, the outer edge of the sealing ring  49  no longer includes a bead, as with the previous embodiment, but is clamped between the retaining ring  107  and the back plate  89 . In all other respects, the apparatus  21  is identical to the first embodiment. Similarly, FIG. 9 depicts a sealing ring  49  configuration of a third embodiment. Here, the polishing head  45  includes an annular hoop  141  that clamps the sealing ring  49  between itself and the back plate  89 . In all other respects, the apparatus  21  is identical to the first embodiment. 
     The present invention further comprises a method of polishing a semiconductor wafer  35 . The method comprises multiple steps, which may be carried out with the apparatus  21  described above. The rear surface  93  of the wafer  35  is placed in engagement with the sealing ring  49  of the polishing head  45  of the wafer polishing apparatus  21 , forming the fluid pressure cavity  115 , defined by the rear surface  93  of the wafer, the seal and the polishing head. The seal of the polishing head  45  is preferably the sealing ring  49  as set forth above. Relative motion between the wafer and the polishing pad is then obtained, as described in detail above. Selectively applying air pressure within the cavity  115  urges the front surface  39  of the wafer  35  against the work surface  37  for pressing the wafer surface uniformly against the polishing pad  29 . Air within the cavity  115  directly engages a majority of the rear surface  93  of the wafer  35 , creating more uniform pressure application of the wafer. Moreover, because the sealing ring  49  conforms more closely to the lateral edges of the rear surface  93  of the wafer  35 , polishing pressure at the lateral edge of the wafer is increased to levels adequate to more evenly polish the edge of the wafer. As discussed previously, the sealing ring  49  of the present method provides substantial benefits over traditional configurations incorporating rigid backing plates or membranes. Finally, the wafer  35  is held on the polishing head  45  by re-applying a vacuum and then removed from the polishing head  45  by applying positive pressure. 
     Another embodiment of the present invention comprises a polishing method generally as set forth above with an additional processing step of forming an oxide layer on a rear surface  93  of the semiconductor wafer  35 . Because the wafer  35  is free-mounted on the polishing head (i.e., without the use of a wax layer), the rear side  93  of the wafer of the present invention is susceptible to damage and must be protected during processing. During polishing, some polishing slurry may inadvertently squeeze between the sealing ring  49  and the wafer  35 . Such slurry can stain the rear surface  93  of the wafer  35  or increase backpolishing and scratching of the rear surface, both of which are undesirable. Moreover, even small amounts of sliding between the sealing ring  49  and the rear surface  93  of the wafer  35  may create microscopic scratches. Such sliding may occur from torque, as described above, or from very slight movement of the sealing ring  49  as pressure is applied. The additional processing step of forming an oxide layer on the rear surface  93  of the wafer  35  protects the rear surface from staining, backpolishing and scratches due to processing. 
     An oxide layer may be formed on a wafer  35  in a number of different ways. As shown in FIG. 10, the wafer  35  may be placed in a bath  151  to form an oxide layer. Such a bath  151  preferably comprises an aqueous solution of approximately 0.5 molar hydrogen peroxide and 0.03 molar ammonia for soaking a wafer  35  for at least four minutes. Alternately, a weaker solution and a longer time will yield a similar oxide layer and similar beneficial results. During polishing, such an oxide layer will protect the polysilicon underneath the oxide layer from harm. Because the entire wafer  35  is placed within the bath  151 , an oxide layer will also form on the front surface  39  of the wafer. Polishing such a front surface  39  after immersion in the bath  151  will readily remove the oxide layer from the front surface. Other methods of forming an oxide layer on a wafer  35 , such as an aqueous solution of oxide bath, are also contemplated as within the scope of the present invention. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.