Abstract:
An improved wafer polishing machine is disclosed. In one embodiment, the wafer polishing machine has a movable polishing surface and a holder that holds an object, such as a semiconductor wafer, against the movable polishing surface. The holder includes a support structure that supports the object in contact with the polishing surface and an annular retaining ring that retains the object in alignment with the support structure. The retaining ring has a plurality of projections projecting inwardly from its inner circumference. The projections are evenly spaced around the inner circumference of the retaining ring. In one embodiment, the projections on the retaining ring define a circle with a diameter no less than the diameter of the object being polished. In an alternative embodiment, the retaining ring has a smooth, circular inner circumference formed from a flexible material which distends to from a continuous arc of contact with the wafer during polishing. Each retaining ring disclosed herein forms multiple points of contact or a continuous arc of contact between the retaining ring and the wafer, thereby reducing wafer buckling during polishing and improving surface uniformity.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to semiconductor wafer processing, and in particular to a retaining ring of a wafer holder for wafer polishing. 
     BACKGROUND OF THE INVENTION 
     In polishing applications such as chemical mechanical polishing, measures are taken to ensure that the surface being polished is subjected to uniform, isotropic polishing forces. The uniformity of the polishing force applied to the surface is a significant factor in determining the degree of surface uniformity that can be attained through polishing. 
     Thus, for example, in a chemical mechanical polishing machine with continuous belt polishing, the longitudinal motion of the belt is often supplemented by lateral and rotational motion of the wafer to ensure that every area of the wafer is subjected to uniform, isotropic polishing forces. 
     The force generated by friction between the wafer and the belt will, at any given instant, be exerted primarily in the direction of the belt movement across the surface of the wafer. Likewise, in other polishing configurations, a frictional force will be exerted by the polishing surface in the direction of movement of the polishing surface relative to the wafer. A retaining ring is generally used to counter this force and hold the wafer in position. The frictional force of the polishing surface impels the wafer against the retaining ring, which exerts a counterbalancing force to maintain the wafer in position. 
     The frictional force of the polishing surface and the reactive force exerted by the retaining ring on the wafer may be sufficient to cause the wafer to buckle. This buckling of the wafer may resemble a so-called Euler column familiar to those skilled in the art of material strain analysis. This buckling may result in uneven polishing of the wafer surface, particularly near the edge of the wafer. This problem has been observed in high-speed polishing, particularly for large-diameter, thin wafers. 
     SUMMARY OF THE INVENTION 
     Thus, a need has arisen for a wafer polishing machine that addresses the disadvantages and deficiencies of the prior art. In particular, a need has arisen for a wafer polishing machine with a retaining ring that prevents wafer buckling. 
     Accordingly, an improved wafer polishing machine is disclosed. In one embodiment, the wafer polishing machine has a movable polishing surface and a holder that holds an object, such as a semiconductor wafer, against the movable polishing surface. The holder includes a support structure that supports the object in contact with the polishing surface and an annular retaining ring that retains the object in alignment with the support structure. The retaining ring has a plurality of projections projecting inwardly from its inner circumference. The projections are evenly spaced around the inner circumference of the retaining ring. In one embodiment, the projections on the retaining ring define a circle with a diameter no less than the diameter of the object being polished. 
     In an alternative embodiment, the retaining ring has a circular inner circumference formed from a flexible material. The inner circumference distends to from a continuous arc of contact with the object during polishing. 
     A technical advantage of one embodiment of the present invention is that the projections on the retaining ring create multiple points of contact between retaining ring and the wafer, thereby distributing the pressure of the retaining ring on the wafer. Another technical advantage of the various embodiments of the present invention is that the multiple points of contact or continuous arc of contact between the retaining ring and the wafer reduce wafer buckling during polishing, thereby improving surface uniformity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
     FIGS. 1A and 1B are simplified front and perspective views of a chemical mechanical polishing machine constructed in accordance with the present invention; 
     FIG. 2 is a simplified cross section of a polishing head for use in the chemical mechanical polishing machine; 
     FIG. 3 is a front view of a retaining ring constructed in accordance with one aspect of the present invention; 
     FIG. 4 is a front view of an alternative retaining ring constructed in accordance with one aspect of the present invention; 
     FIGS. 5A and 5B are front and perspective views of another alternative retaining ring constructed in accordance with one aspect of the present invention; and 
     FIG. 6 is a front view of yet another alternative retaining ring constructed in accordance with one aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention and their advantages are best understood by referring to FIGS. 1 through 6 of the drawings. Like numerals are used for like and corresponding parts of the various drawings. 
     Referring to FIGS. 1A and 1B, simplified front and perspective views of a chemical mechanical polishing (CMP) machine  10  constructed in accordance with the present invention are shown. CMP machine  10  includes a continuous polishing belt  12  which rotates on a pair of rollers  14  and  16 . A motor (not shown) drives the bottom roller  16  in a counterclockwise direction, while top roller  14  is free to rotate as polishing belt  12  rotates. Polishing belt  12  may move at a linear rate of up to 1000 feet per minute, or at even greater speeds depending on the object being polished. 
     A polishing head  20  on each side of CMP machine  10  swivels from a loading and unloading position  20   a  to a polishing position  20   b . In polishing position  20   b , polishing head presses a semiconductor wafer (not shown in FIG. 1) against polishing belt  12  as polishing belt  12  rotates. A support head  18  supports polishing belt  12  from the back side, allowing polishing head  20  to press the wafer against polishing belt  12  with a selected pressure, such as from one to five psi. 
     Polishing head  20  rotates the wafer in a plane parallel to and adjacent to polishing belt  12 , preferably at a rate of 10 to 50 revolutions per minute This rotation, in conjunction with the linear motion of polishing belt  12  against the surface of the wafer, results in polishing forces being applied in all directions along the surface of the wafer, and prevents striations from forming on the surface of the wafer. 
     Polishing head  20  also undergoes lateral oscillation to distribute the wear on polishing belt  12 . This oscillation may have a range of, for example, one inch on either side of the center line of polishing belt  12 . Polishing head  20  may oscillate at a rate of, for example, up to five cycles per minute. Although lateral oscillation of polishing head  20  is not required to polish wafer  15 , lateral oscillation prevents uneven wearing of polishing belt  12 , increases the useful life of polishing belt  12  and enhances the uniformity of wafer polishing. 
     A slurry dispenser (not shown) on each side of CMP machine  10  dispenses a slurry onto polishing belt  12  as polishing belt  12  rotates. The slurry contains abrasive particles which mechanically polish the surface of the wafer when brushed across the surface of the wafer by polishing belt  12 . 
     In loading and unloading position  20   a , polishing head  20  holds the wafer in a horizontal position. In this position, a wafer gripper  24  may descend and grip the wafer to remove the wafer from polishing head  20 . Wafer gripper  24  has a set of pins  26  disposed in a circle that corresponds to the circumference of the wafer. Wafer gripper  24  can move each pin  26  radially inward and outward so as to contract and expand the circle formed by pins  26 . Thus, the circle of pins  26  may be expanded before wafer gripper  24  descends to grip the wafer, thus allowing pins  26  to descend past the edge of the wafer. The circle of pins  26  may then be contracted to grip the wafer, allowing the wafer to be lifted from polishing head  20  and moved over a receptacle (not shown). The circle of pins  26  may then be expanded to drop the wafer into the receptacle. In a similar manner, a new wafer may be taken from another receptacle and loaded on polishing head  20  for polishing. Polishing head  20  then swivels into polishing position  20   b  to polish the new wafer. 
     Further description of an exemplary structure of CMP machine  10  may be found in the U.S. Pat. Application entitled “Modular Wafer Polishing Apparatus And Method,” Ser. No. 08/964,930, filed Nov. 5, 1997, now U.S. Pat. No. 5,757,764 isued on Sep. 28, 1999, which is incorporated herein by reference. 
     Although CMP machine  10  is shown with a vertically oriented polishing surface, it will be understood that the present invention may be advantageously implemented in a horizontal CMP machine, such as those produced by Lain Research in Fremont, Calif. 
     Referring to FIG. 2, a simplified cross section of polishing head  20  is shown. Polishing head  20  includes a support structure  24  and an optional backing film  26  in contact with the back surface of a wafer  28 . A retaining ring  30  extends around the outer circumference of wafer  28 , holding wafer  28  stationary against the frictional force of polishing belt  12 . 
     Support structure  24  includes a drive plate, bellows, sub-carrier, lift plate and bladder as described in the co-pending U.S. Patent Application entitled “A Polishing Head for a Chemical Mechanical Polishing Apparatus,” Ser. No. 09/116,160 now U.S. Pat. No. 6,159,083, filed herewith and incorporated herein by reference. Backing film  26  is unnecessary in this support configuration. Alternatively, support structure  24  may simply comprise a sub-carrier made from a rigid, non-porous material such as stainless steel, in which case backing film  26  is preferably used to cushion and support wafer  28 . Support structure  24  may alternatively comprise any other conventional support structure. 
     Backing film  26  may comprise a porous, soft material such as IC 1000 or SUBA IV manufactured by Rodel, Incorporated in Newark, Del. Backing film  26  may be attached to support structure  24  by double-sided adhesive tape (not shown). 
     Retaining ring  30 , according to one aspect of the present invention, has projections  36  extending inwardly from its inner circumference. Projections  36 , which will be described more fully below, contact wafer  28  to hold wafer  28  stationary against the frictional force of polishing belt  12 . Projections  36  are illustrated as having a thickness less than the thickness of the body of retaining ring  30 . However, projections  36  may be as thick as the body of retaining ring  30 . 
     Referring to FIG. 3, a front view of retaining ring  30  is shown. Retaining ring  30  may be made of a rigid polymer such as Techtron PPS (polyphenylene sulfide), available from E. Jordan Brookes Company in Fremont, Calif., or polyethylene terephthalate (PET). Retaining ring  30  has an outer circumference  32  and an inner circumference  34 . For polishing a 200±0.2 mm diameter wafer, retaining ring  30  may have an outer circumference  32  with a diameter of, for example, 10.125 inches (257.18 mm). Inner circumference  34  may have a diameter of, for example, 8.10 inches (205.74 mm). 
     Along inner circumference  34  is a series of projections  36  projecting radially inward from inner circumference  34 . Projections  36  are evenly spaced around inner circumference  34 , separated by intervals of 60°, for a total of six projections  36 . The tips of projections  36  form a circle with a diameter of, for example, approximately 7.89 inches (200.41 mm), which is slightly larger than the largest diameter wafer to be held by retaining ring  30 . 
     The tips of projections  36  are the only points of contact between retaining ring  30  and wafer  28 . During polishing, retaining ring  30  and wafer  28  rotate as previously described, while friction with polishing belt  12  forces wafer  28  to one side of retaining ring  30 . Thus, the edge of wafer  28  is in contact with two adjacent projections  36  at most times during polishing. Wafer  28  is in contact with only one projection  36  for brief periods when a projection is approximately aligned with the center of wafer  28  in the direction of the polishing force exerted by polishing belt  12 . 
     Wafer  28  is therefore held in place at most times by retaining ring  30  as a result of force applied at two contact points separated by 60° along the edge of wafer  28 . With the frictional force of polishing distributed between two contact points, the buckling of wafer  28  due to the polishing force is significantly reduced. The degree of wafer surface uniformity attainable through polishing is correspondingly increased. In particular, since wafer buckling primarily occurs near the edge of the wafer in typical CMP machines, the surface uniformity near the edge of wafer  28  is increased by the present invention. 
     Referring to FIG. 4, a front view of an alternative retaining ring  40  is shown. Like retaining ring  30 , retaining ring  40  may be made of a rigid polymer such as PPS or PET. Retaining ring  40  has an outer circumference  42  and an inner circumference  44 . For polishing a 200±0.2 mm diameter wafer, retaining ring  40  may have the same inner and outer circumference measurements as retaining ring  30 . 
     Along inner circumference  44  is a series of projections  46  projecting radially inward from inner circumference  44 . Projections  46  are evenly spaced around inner circumference  44 , separated by intervals of 30°, for a total of twelve projections  46 . The tips of projections  46  form a circle with a diameter of, for example, approximately 7.89 inches (200.41 mm). 
     As with retaining ring  30 , the tips of projections  46  are the only points of contact between retaining ring  40  and wafer  28 . During polishing, retaining ring  40  and wafer  28  rotate while friction with polishing belt  12  forces wafer  28  to one side of retaining ring  40 . Thus, the edge of wafer  28  is at most times in contact with two adjacent projections  46  at most times during polishing. Wafer  28  is therefore held in place by retaining ring  40  at most times as a result of force applied at two contact points separated by 30° along the edge of wafer  28 . 
     The 30° contact point separation offered by retaining ring  40  has been determined to be less beneficial with regard to surface uniformity than the 60° separation offered by retaining ring  30 . However, both retaining rings  30  and  40  offer significant improvements in wafer surface uniformity over that attainable by polishing with a smooth, rigid, circular retaining ring. 
     Referring to FIGS. 5A and 5B, front and perspective views of another alternative retaining ring  50  are shown. Like retaining rings  30  and  40 , retaining ring  50  may be made of a rigid polymer such as PPS or PET. Retaining ring  50  has an outer circumference  52  and an inner circumference  54 . For polishing a 200±0.2 mm diameter wafer, retaining ring  50  may have an outer circumference  52  with a diameter of, for example, 10.2 inches (259.08 mm). Inner circumference  54  may have a diameter of, for example, 8.37 inches (212.60 mm). 
     Along inner circumference  54  is a series of projections  56  projecting radially inward and diagonally in a clockwise direction. In this example, projections  56  are evenly spaced around inner circumference  54  and separated by intervals of 6°, for a total of 60 projections  56 . The tips of projections  56  form a circle with a diameter of, for example, approximately 7.89 inches (200.41 mm). 
     Unlike retaining rings  30  and  40 , retaining ring  50  has flexible projections  56  that provide multiple points of contact for wafer  28 . In one embodiment, each projection  56  has a length of 0.35 inches, a width (measured in a radial direction with respect to retaining ring  50 ) of 0.1 inches, and a thickness (measured in an axial direction with respect to retaining ring  50 ) of 0.175 inches. 
     Because projections  56  are relatively long and thin, each projection is capable of bending outward toward inner circumference  54  when a load such as wafer  28  is applied. The amount of deflection (Δ) is approximated by the following equation: 
     
       
         Δ=PL 2 /3EI 
       
     
     in which P is the load applied to the projection  56 , L is the length of the projection  56 , E is a material property of the projection  56 , and I is the moment of inertia of the projection  56 . 
     As illustrated in FIGS. 5A and 5B, each projection  56  overlaps the base of an adjacent projection  56 . Thus, the deflection of one projection  56  may cause the deflection of adjacent projections  56  in a domino-like effect. This effect, along with the close proximity of projections  56  to each other, creates a flexible cushion for wafer  28 , with many points of contact along a broad arc of the perimeter of wafer  28 . The dimensions and material properties of projections  56  are preferably selected to provide support for wafer  28  along a 60° arc, so as to minimize the buckling of wafer  28  caused by friction with polishing belt  12 . 
     Referring to FIG. 6, a front view of yet another alternative retaining ring  60  is shown. Retaining ring  60  has an outer circumference  62  with a diameter of, for example, 10.2 inches, and an inner circumference  64  with a diameter of, for example, 7.89 inches. Retaining ring  60 , unlike the retaining rings previously described, has a smooth inner circumference  64  with no projections thereon. The body of retaining ring  60  is made of a rigid polymer such as PPS or PET. Inner circumference  64  is constructed of a flexible material such as Viton available from DuPont Dow Elastomers in Wilmington, Del., or the terpolymer elastomer of ethylene-propylene diene monomer (commonly termed EPDM). The thickness of inner circumference  64  is typically less than the thickness of the body of retaining ring  60 . However, inner circumference  64  may be as thick as the body of retaining ring  60 . During polishing, when wafer  28  is pressed against a portion of retaining ring  60 , inner circumference  64  distends to provide a continuous arc of contact between retaining ring  60  and wafer  28 . As with retaining ring  50 , the dimensions and material properties of retaining ring  60  are preferably selected to provide support for wafer  28  along an arc of at least 30°, preferably approximately 60°, so as to minimize the buckling of wafer  28  caused by friction with polishing belt  12 . 
     Although CMP machine  10  and retaining rings  30 ,  40 ,  50  and  60  have been described with reference to semiconductor wafer polishing, it will be understood that retaining rings  30 ,  40 ,  50  and  60  may be advantageously implemented in other polishing or lapping applications, such as the polishing or lapping of disks and thin film heads for hard disk drives. Furthermore, although a vertical continuous belt CMP machine  10  has been used to illustrate the present invention, it will be understood that the invention may be advantageously implemented in other conventional CMP machine designs, such as those with horizontal belt, disk, or planetary polishing surfaces. 
     Thus, although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.