Abstract:
A wafer polishing head utilizes a wafer backing member having a wafer facing pocket which is sealed against the wafer and is pressurized with air or other fluid to provide a uniform force distribution pattern across the width of the wafer inside an edge seal feature at the perimeter of the wafer to urge (or press) the wafer uniformly toward a polishing pad. Wafer polishing is carried out uniformly without variations in the amount of wafer material across the usable area of the wafer. A frictional force between the seal feature of the backing member and the surface of the wafer transfers rotational movement of the head to the wafer during polishing. A pressure controlled bellows supports and presses the wafer backing member toward the polishing pad and accommodates any dimensional variation between the polishing head and the polishing pad as the polishing head is moved relative to the polishing pad. An integral, but independently retractable and extendable retaining ring assembly is provided around the wafer backing member and wafer to uniformly and independently control the pressure of a wafer perimeter retaining ring on the polishing ad of a wafer polishing bed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/201,428, now U.S. Pat. No. 6,652,368 filed Jul. 22, 2002 which is a continuation of U.S. application Ser. No. 09/892,143, filed Jun. 25, 2001 now U.S. Pat. No. 6,443,824, which is a continuation of U.S. application Ser. No. 09/406,027, filed Sep. 27, 1999, now U.S. Pat. No. 6,290,577, which is a continuation of U.S. application Ser. No. 08/488,921, filed Jun. 9, 1995, now U.S. Pat. No. 6,024,630, each of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to mechanical polishing, and in particular to polishing heads used to polish generally circular semiconductor wafers in the semiconductor industry. 
     BACKGROUND OF THE INVENTION 
     This invention provides improved construction and easier operability of polishing heads useful for positioning a substrate, in particular, a semiconductor substrate, on the surface of a polishing pad. Such heads also provide a controllable biasing, or loading, between the surface of the substrate and the polishing surface. 
     A typical substrate polishing apparatus positions a surface of a substrate against a polishing surface. Such a polishing configuration is useful for polishing the substrate after it has been sliced from a boule (single crystal), to provide smoothly planar, parallel, front and back sides thereon. It is also useful for polishing a surface of the substrate on which one or more film layers have been deposited, where polishing is used to planarize the surface of the substrate on which one or more film layers have been deposited. A slurry having both chemically reactive and abrasive components is used in conjunction with the positioning of the film layer surface against a moving polishing surface to provide the desired polishing. This is known as chemical mechanical polishing. 
     A typical wafer polishing apparatus employs a carrier, or polishing head, to hold the substrate and position the film layer surface of the substrate against a polishing surface. The polishing surface is typically provided by placing a large polishing pad, typically as large as one meter in diameter, on a massive rotatable platen. The platen is driven by a motor to rotate the polishing pad and thus provide relative motion between the pad and the film layer surface of the substrate. As the pad rotates, it tends to pull the substrate out of the carrier. Therefore, the carrier also typically includes a recess within which the substrate is received. This recess is commonly provided by extending a retainer downwardly from the substrate receiving surface of the carrier positioned adjacent to, and extending circumferentially around, the edge of the substrate. The apparatus also provides a means for positioning the carrier over the polishing pad and biasing the carrier towards the pad to load the substrate against the pad, and a drive means for providing rotational, vibratory or oscillatory motion to the carrier. 
     An example of a polishing head having a retaining ring is shown in U.S. Pat. No. 5,205,082, by Shendon et al. which discloses pressurized diaphragm arrangement which urges a wafer carrier and wafer retainer toward a polishing pad. 
     In some carrier head configurations, the force urging the retaining ring toward the polishing pad is dependent on the predetermined spring constant of a circular leaf spring and its compression. The spring-loaded retaining rings are subject to bending and torsional deflection due to the spring configuration which does not provide a continuous contact force but provides a series of point loads, clamping the ring to the polishing pad. The retaining ring bends and deflects because it is allowed to flex between these point loads. This flexing can cause variation in the clearance between the ring and pad which affects the depth of slurry that passes under the ring, and it also affects the pad compression adjacent to the edge of the wafer. Variations in the depth of polishing slurry and in pad compression adjacent to the edge of the wafer can cause differential polishing of the wafer to the detriment of polishing uniformity. 
     The object in each head configuration is to provide a fixture which will uniformly polish the wafer across its full width without unacceptable variations in the thickness of the wafer. These prior art configurations as described can introduce polishing variations due to bladder edge effects, non-uniformly distributed force pressing the wafer to the polishing pad, and retaining ring deflections which require close and frequent monitoring to assure satisfactory polishing results. 
     SUMMARY OF THE INVENTION 
     This invention relates to a polishing head substrate (wafer) backing member facing the back of, and being sealed to, a substrate (wafer) being polished. The wafer is sealed to a cavity located in the member around the perimeter of the cavity and a fluid (preferably gas although it may be a liquid) pressurizes the cavity and the back of the waft against a slurry containing polishing pad. 
     The wafer backing member preferably includes a seal feature, e.g. an O-ring, lip seal, or other seal member which extends from the backing member adjacent to the perimeter of the backing member to form a recess between the wafer and the member to hold a fluid or gas in the recess behind the wafer to provide a uniform pressure across the surface of the wafer being pressed against the polishing pad. A gas tight bellows chamber supports the wafer backing member and urges it toward the polishing pad to provide primary loading of the substrate against the pad. When the bellows is pressurized to urge the substrate against the polishing pad, it compresses the seal. Simultaneously, the pressure in the cavity formed by the seal may be changed, to selectively vary the polishing of the substrate. The cavity may be evacuated, to urge the center of the substrate away from the pad to increase polishing at the substrate edge as compared to its center, and it may be pressurized to enable uniform loading of the substrate against the pad. The pressure in the cavity urges the substrate away from the holding member, and thereby decompresses the seal. The pressure in the cavity may be sufficiently large to separate the substrate from the seal, at which point the cavity pressure will release, or “blow-by,” through the resulting gap between the substrate and the seal. 
     In a further aspect of the invention, a retractable and pressure extendable retaining ring assembly extends around the backing member and prevents the wafer from sliding out from below the surface of the substrate backing member. An annular ring extending bladder extends along the backside of the ring, the bladder when pressurized urges the ring against the pad. The force with which the retaining ring is clamped to the polishing pad is dependant on the gas pressure maintained in this bladder. 
     These inventive configurations, alone or in combination, provide several advantages. One advantage is direct control of a uniform force on the back surface of the wafer being polished within the perimeter of the seal extending between the holding member and the wafer. A pressure is uniformly maintained without the complication or edge effects of an intermediate bladder in direct contact with the substrate. Another advantage is that the total force pressing the wafer backing member toward the wafer is controlled separately by the force created by controlling the pressure within the bellows completely independent of the influence of the pressure cavity formed between the wafer and the backing member. If the force on the wafer due to the pressure behind the wafer in the wafer facing cavity exceeds the force on the seal to the wafer exerted by the pressure in the bellows then the wafer will lift away from its seal and seal blow-by will occur until equilibrium restores the seal. 
     The pressure within the wafer facing cavity controls the distribution pattern by which this total force is transmitted from the wafer backing member to the wafer. Providing a vacuum to the cavity can cause the center of a supported wafer to bow inward, so that only a perimeter polishing contact is achieved. In contrast, positive pressure in excess of the seal contact pressure will cause the wafer to lift off (move away from) the seal and for gas to blow-by (it cannot cause outward bowing of the substrate as the pressure at the center of the substrate can never exceed the pressure at the perimeter of the substrate), and will also cause a uniform pressure on the back of the wafer. The bowing or deflection of the wafer, if any, is controlled and limited by the pressure on the perimeter seal, so long as the internal pressure of the recess or cavity facing the wafer does not exceed the seal pressure and cause seal blow-by. 
     This configuration according to the invention nearly guarantees that, as long as the force provided by the backing pressure urging the wafer from the seal is maintained at or slightly below the pressure on the seal provided by the bellows, the force clamping the wafer to the polishing pad for polishing will be uniform across the area of the wafer. In reality, because it is desired to maintain a gas tight perimeter seal, in operation the pressure in the wafer facing cavity will be slightly less than the pressure at which seal blow-by occurs. Under these conditions, a slightly greater pressure will be present between the substrate and the pad at the seal location which will slightly increase the polishing (material removed) in the perimeter ring (seal) area. However, the outer three millimeters of the substrate are considered to be a non-usable handling margin and therefore slight additional polishing (material removed) in this narrow band at the edge of the substrate is not considered deleterious. 
     The extension and retraction of the wafer retaining ring assembly is independently controlled by the use of the continuous annular bladder positioned around the perimeter of the wafer backing member. Such a configuration can eliminate the pressure variations associated with the point contacts of springs provided to urge the ring into contact with the pad. In one configuration, one or more restoring springs are supported on a rigid portion of the retaining ring backing ring to cause the retaining ring to retract from its lowered position when the extension bladder is depressurized. 
     The frictional force between the seal at the perimeter of the wafer backing member is sufficient such that when the polishing head is rotated during polishing while the wafer is in contact with the polishing slurry on the polishing pad, there is sufficient frictional force that the wafer rotates with the polishing head and overcomes the resistance to rotation with the head due to the motion of the pad and the polishing media on the polishing pad. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross section of an embodiment according to the invention; 
         FIG. 2  is a close up view of the right side of  FIG. 1  showing the periphery of the wafer backing member with an O-ring seal; and 
         FIG. 3  is a close up view of the right side of  FIG. 1  showing the periphery of the wafer backing member with a lip seal. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a polishing head assembly  100  in a configuration according to the invention. The polishing head  100  includes a polishing head housing support plate  102  which is integral with its rod or stem support member. This support plate  102  is generally circular so as to match the circular configuration of the substrate or wafer  142  to be polished. A polishing head housing descending wall  104  is attached to the bottom of the support plate  102  by a descending wall top flange  106 . The descending wall  104  includes a lower lip  110  which curves inward toward the wafer  142 . The descending wall  104  encloses a wafer perimeter retaining ring assembly  146  enclosing a wafer backing member  124 . The wafer backing member  124  is attached to the support plate  102  by a bellows  118  which allows a vertically variable vacuum seal. The bellows  118  encloses a bellows chamber  120 . The bellows chamber  120  can be pressurized positively or negatively through a gas passage  112  to which is connected the inside of the bellows. 
     An Overview Of The Apparatus 
     One typical substrate polishing apparatus generally includes a large rotating polishing pad, typically larger than, and more typically several times larger than, the surface area of the substrate being polished. Also included is a polishing head within which the substrate is mounted for positioning a surface of the substrate against the polishing surface. The head is typically supported over the pad, and fixed relative to the surface of the pad, by a support member. This support member provides a fixed bearing location from which head may extend, to provide a desired unit loading of the substrate against the pad. Loading means to enable this loading of the substrate against the polishing pad include hydraulic and pneumatic pistons which extend between the polishing head  100  and the support member (not shown). Additionally, the polishing head  100  will also typically be rotatable, which enables rotation of the substrate on the pad. Likewise, the pad is typically rotated, to provide a constantly changing surface of the pad against the substrate. This rotation is typically provided by separate electric motors (not shown) coupled to the head and a polishing platen on which the pad is received. 
     The polishing head  100  of the present invention provides a mechanism to position and to uniformly load the surface of the wafer  142  against a polishing pad  182  located in a stationary or rotating polishing bed  180 . Generally, the polishing head  100  can be considered to comprise three systems: a loading member which supplies the downward loading of the wafer against the polishing surface; a mounting portion which allows a uniform pattern loading of the wafer against the polishing surface; and a retaining assembly which ensures that the wafer will not slip out from beneath the mounting portion during polishing operations. Each of these three members or systems provide improvements in polishing head designs, and may be used independently or in combination. 
     The loading member generally comprises the bellows  118  and the bellows chamber  120  provided by the attachment of the bellows to the upper surface of the backing member  124  and the interior surface of the support plate  102 . By pressurizing the bellows chamber  120 , force is exerted on the backing member  124 , and thus on the wafer  142 , to load the wafer  142  against the polishing surface of the polishing pad  182 . The mounting portion includes a separate sealed pocket  123 , one wall of which is firmed by the wafer, to provide an even, hydrostatic, loading across the backside of the wafer. The retaining ring assembly  146  includes an extendable retainer  162  which circumscribes the wafer  142 . 
     The Structure Of The Loading Member And The Mounting Portion 
     To provide the mounting portion, the backing member  124  includes a wafer facing recess  126 . The perimeter of the backing member  124  is configured to receive an edge seal feature  130 , e.g., an O-ring (not shown in the empty O-ring groove of  FIG. 2 ) or other type of seal. The edge seal  130  is located and configured to engage the perimeter portion of the backside of the wafer  142  and thereby form, in combination with the recess  126 , a pressurizable pocket  123 . The pocket includes the recess  126  and the area within the seal  130  over the backside of the wafer. When the backing member  124  is rotated, this feature provides a frictional force between the wafer  142  and the backing member  124  so that the substrate  142  generally turns with the backing member  124 . 
     Gas or other fluid (preferably an inert gas) is supplied to or evacuated from the pocket through a gas passage  125  which is connected through a hose  122  coiled inside the bellows  118  and supplied from a gas line  114 . The selective pressurization of the pocket  123  and the bellows chamber  120  provides the loading of the wafer on the polishing pad  182 . Additionally, the bellows enables the backing member  124 , and thus the wafer  142 , to move rotationally with respect to the support plate  102  and in the x, y, and z directions during polishing. 
     The bellows  118 , in combination with the upper surface of the backing member  124 , the lower surface of the support plate  102  and a pressure source (not shown), provide the loading member. In one mode of operation, the pressure in the bellows chamber  120  is controlled to be constant and the flexibility of the bellows  118  accommodates misalignments or changes in clearance between the backing member  124  and the surface of the polishing pad  182 . The pressure in the bellows chamber  120  is selected to provide the desired loading of the wafer  142  against the polishing pad  182 . In this configuration, the pressure in the bellows chamber  120  provides a regulatable uniform force pressing the backing member  124  toward the surface of the polishing pad  182  regardless of the extension of the bellows  118 . 
     In turn, pressurizing the recess  126  behind the wafer  142  enables a uniform contact pressure to exist between the polishing pad  182  and the wafer  142  across the entire surface of the wafer contacting the polishing pad  182 . 
     The extension or retraction of the bellows  118  is controlled by pressurizing or depressurizing the bellows chamber  120  via the gas passage  112 . The pressurization or depressurization of the recess  126  in the backing member  124  either pressurizes or depressurized the pocket  123 . A negative differential pressure due to vacuum bends the wafer  142  upwardly. A sufficient positive pressure creates a separating force greater than the force from the bellows  118  which forces the seal wafer. 
     The polishing head configuration of  FIG. 1  also overcomes the comparative difficulty encountered in prior art head designs when loading and unloading the wafer from the head, and in ensuring that the wafer does not slip from beneath the backing member  124 . 
     In the present head design, the pressure maintained in the pocket may be changed to provide a super-atmospheric pressure to separate the wafer from the carrier when polishing is completed, and to provide a vacuum pressure (preferably of up to approximately 100 torr less than atmospheric pressure) behind the wafer thereby causing atmospheric pressure to maintain the wafer on the head as the head is loaded onto the polishing pad  182 . 
     When the wafer is attached to the backing member  124  by maintaining a vacuum in the pocket, the wafer may deflect inwardly toward the recess  126 . The recess  126  is sufficiently shallow that the total possible deflection of the wafer into the recess, when considered in combination with the span of the wafer  142  across the recess  126 , will impose stresses in the wafer  142  which are less than the strength or yield limits of the wafer material. 
     The vacuum need be maintained in the pocket only during the period of time that the polishing head is removed from the polishing pad  182 . Once the polishing head and the wafer  142  are repositioned on the polishing pad  182 , the pressure in the pocket is increased, until a pressure above atmospheric pressure is maintained therein. Simultaneously, the pressure in the bellows chamber  120  is increased, to provide a load force to load the wafer  142  against the polishing pad  182 . 
     As the pressure in the bellows chamber  120  is increased, it loads the seal  130  received in the backing member  124  into contact with the backside of the wafer. The seal will compress under this load, which will enhance the sealing characteristics of the seal  130 . Therefore, as the pressure in the bellows chamber  120  increases, the threshold pressure at which gas maintained in the pocket  123  will leak past, or “blow-by”, the seal  130 , also increases. Blow-by occurs when the head and the seal lift off the wafer. This condition occurs when the pressure in the pocket, when multiplied by the surface area of the wafer  142  circumscribed by the seal  130 , exceeds the load force on the seal-wafer interface. In the configuration of the head, as shown in  FIG. 3 , the area of the backing member  124  which is circumscribed by the bellows  118  is smaller than the area of the wafer  142  circumscribed by the seal  130 . Therefore, the pressure in the bellows cavity must exceed the pressure maintained in the pocket to prevent blow-by. 
     Preferably, the pressure maintained in the pocket is approximately 75 torr less than the threshold at which blow-by will occur. At these pressures, the entire backside of the wafer, less a very small annular area outward of the seal  130 , will have a uniform pressure on the back surface thereof which ensures that the front surface of the wafer is uniformly loaded against the polishing pad  182 . However, it is specifically contemplated, although not preferred, that higher pressures, including a pressure at or above blow-by, may be used. Where such higher pressures are used, the seal-wafer interface will serve as a relief valve, and blow-by will occur periodically to maintain a desired pressure within the pocket  123 . 
       FIG. 2  shows a close up of the right side of the polishing head of FIG.  1 . The seal  130  in this configuration is an O-ring  134  located in an O-ring groove  132  (i.e., collectively: an annular extending portion). This seal is located at the perimeter of the wafer  142  surrounding the recess  126  (and the associated pocket). The perimeter of the backing member  124  is surrounded by the retaining ring assembly  146 . The retaining ring includes a the retaining ring  162  which is attached to the backing ring  148 . A series of compression springs  172  (i.e., first set of elastic members) support the backing ring  148  on the lip  110  of the descending wall  104 . An expandable retaining ring extending bladder  170  can be pressurized through gas supply passage  171  (i.e., a second set of elastic members). When bladder  170  is pressurized, the retaining ring assembly  146  is extended to a location adjacent the wafer  142  as shown by the dashed lines  146   a  in FIG.  2 . 
     A second configuration of the polishing head of the present invention is shown in  FIG. 3 , wherein the seal  130  is a downwardly extending lip seal  136  received on the outer perimeter of the backing member  124 , and secured thereon by a backing ring  138  extending about the outer circumference of the lip seal  36 . The lip seal  136  is preferably a thin, elastic, member having a rectangular cross section. A portion of the lip seal  136  extends from the underside, or wafer engaging side, of the backing member  124 , to engage the upper surface of the wafer  142  immediately inwardly of the perimeter of the wafer  142 . As with the O-ring  134 , the engagement of the lip seal  136  with the wafer forms a pocket (including wafer recess  126  and a shoulder area inside lip seal) which may be evacuated or pressurized. The lip seal  136  and the O-ring  134  provide sufficient contact between the surface of the substrate and the surface of the seal to create a rotational force due to friction between the two to keep them in contact so that the substrate turns with the polishing head. 
     The Retaining Ring 
     Referring again to  FIG. 1 , the polishing head  100  also includes a retaining ring assembly  146  to ensure that the wafer  142  does not slip out from beneath the head during polishing operations. The retaining ring  162  has through holes  164  and counterbores  166  therein (FIG.  3 ). Retaining ring screws  168  are placed therethrough and threaded into a series of backing-ring bottom-surface threaded holes  160  to hold the retaining ring  162  to a backing ring  148 . The retaining ring  162  is preferable made of Delrin or similar plastic material. The backing ring  148  is preferably made of aluminum as are all of the other metal pieces except for the bellows which is stainless steel. The backing ring  148  has a bottom surface  158  facing the retaining ring  162 . The backing ring  148  includes an outside flange  152  having a top face  154  facing the bladder  170  and a bottom face  156  facing the series of compression springs  172 . The backing ring  148  has an inside flange  150  having a lower face  151  which extends inwardly over the diameter of the retaining member  124   a  such that when the backing member  124   a  is raised beyond a certain point the backing ring assembly  146  also rises. 
       FIGS. 2 and 3  show details of the retaining ring assembly  146 . The backing ring  148  is urged upwardly away from the lip  110  of the descending wall  104  by a plurality of (for example 6-12) compression springs  172 . When the bladder  170  is pressurized to extend the retaining ring assembly  146  to its operating position as shown by the dashed lines  146   a  in  FIG. 2 , the retaining ring  162  surrounds the edge of the wafer being polished. This prevents the wafer from sliding out under the wafer backing member  124 , or  124   a . Inflation of the bladder  170  through the gas passage  171  provides a downward force to oppose the compression springs  172  and forces the retaining ring  162  toward and possibly against the polishing pad  182 . A continuous continuously pressurized bladder could be employed to replace the series of springs  172  to provide uniformly distributed retracting forces. 
     The lower surface  151  of the backing ring inside flange  150  is configured so that as the plastic Delrin material of the wafer perimeter retaining ring  162  wears away, the travel of retaining ring is limited by the interference between the lower surface  151  of the upper flange  150  and the top of the wafer backing member  124   a  so that the head of the retaining ring retaining screws  168  cannot touch the polishing pad. This prevents the heads of retaining screws  168  from coming in contact with the polishing pad and introducing undesirable contaminants. The perimeter retaining ring can also be mounted without screws, such as by use of key slots requiring insertion and partial rotation to retain the key and opposing grooves having O-rings sized to engage and span the space between grooves. 
     While the invention has been described with regard to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention.