Abstract:
A polishing apparatus includes a rotatable head assembly including a substrate retainer that incorporates a cavity in the face surface of the substrate retainer for temporarily holding polishing slurry during operation of the polishing apparatus. The cavity resides in the face surface of the substrate retainer at a location adjacent the perimeter surface of the retainer. During operation of the polishing apparatus, slurry flowing along the surface of the polishing pad flows into the cavity where a portion of the slurry is temporarily held. As the head assembly of the polishing apparatus rotates against the polishing pad, slurry continuously flows from the cavity across the polishing pad and is uniformly distributed across the exposed surface of the substrate being polished. An offset in the cavity wall permits used slurry to flow away from the substrate retainer during rotation of the head assembly.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to polishing equipment and, more particularly, to polishing heads and head sub-assemblies for use with semiconductor polishing equipment. 
     BACKGROUND OF THE INVENTION 
     Semiconductor wafer polishing apparatus are well-known in the art and are conventionally used to planarize a semiconductor wafer in a process known as chemical-mechanical-polishing (CMP). Such polishing apparatus typically include one or more polishing heads, each of which supports a respective semiconductor wafer and positions the wafer adjacent a polishing surface, such as a polishing pad. The polishing head is moved relative to the polishing pad and a suitable polishing slurry is introduced between the wafer and the pad. 
     Typically, a polishing head includes a central substrate carrier which is surrounded by a substrate retainer. The substrate carrier and the substrate retainer cooperate to form a substrate-receiving pocket that prevents the substrate from moving laterally with respect to the polishing head during polishing operations. To polish the surface of a substrate, the wafer carrier is brought into contact with the polishing pad. Exposed surface layers of the substrate are removed by a combination of chemical reaction and frictional forces brought to bear upon the substrate surface. The frictional forces are created by relative movement of the polishing head and the polishing pad. For example, in one common arrangement, the polishing head is rotated about a rotational axis while the polishing pad undergoes lateral translation relative to the rotating polishing head. Both the polishing head and the polishing pad are placed in rotational motion. 
     During polishing operations, it is important that an adequate supply of slurry be maintained between the substrate and the polishing pad. It is of considerable importance that the wafer polishing machine be able to planarize substantially the entire exposed surface of the substrate. Difficulty often arises with respect to the marginal edge of the substrate, which can often be polished at a rate that is different than the center of the substrate. If the polishing rate at the periphery of the wafer differs excessively from the polishing rate at the center of the wafer, the periphery of the substrate may not be suitable for use in subsequent semiconductor processing stages. The edge-to-center polishing uniformity can be affected by a variance in the amount of slurry at the center of the substrate versus the periphery of the substrate. In order to provide a uniform amount of slurry between the substrate and the polishing pad, polishing equipment manufacturers have developed various techniques for delivering slurry to the polishing pad. For example, one or more nozzles can be provided within the polishing head to deliver slurry to the polishing pad. 
     In one method, one or more slurry nozzles are mounted near the perimeter of the polishing head and slurry is dispensed onto the polishing pad during rotation of the polishing head. Despite the application of slurry nozzles to the polishing head, non-uniform polishing of semiconductor substrates continues to be a problem in polishing operations. Accordingly, further development of the polishing equipment is necessary to provide more uniform substrate polishing. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a polishing apparatus includes a polishing pad and a polishing head assembly coupled to a rotatable shaft. A retainer is engaged with the head assembly such that a substrate is held against a substrate carrier mounted to the polishing head. The retainer ring has a face surface opposite the surface of the polishing table. When a polishing slurry is applied to the surface of the polishing pad, a portion of the slurry flows into the cavity such that, during operation of the polishing apparatus, slurry continuously flows from the cavity onto the polishing table. The continual release of polishing slurry from the cavity provides a uniform amount of slurry between the substrate and the polishing table during operation of the polishing apparatus. 
     In another aspect of the invention, a polishing head assembly is provided that includes a substrate carrier having an annular indentation at the perimeter of the substrate carrier. A substrate retainer is positioned within the annular indentation such that a face surface of the retainer is positioned opposite a polishing pad. The face surface has a cavity that is configured to cooperate with a polishing surface in contact with the face surface to provide a liquid reservoir. The substrate carrier is configured to move in relation to a polishing surface that supports a polishing slurry. The cavity retains a portion of the polishing slurry during the movement of the substrate carrier. An offset in the cavity wall permits used slurry to flow away from the substrate retainer during rotation of the head assembly. 
     In yet another aspect of the invention, a substrate retainer for use in a polishing apparatus includes a continuous annular member having a face surface and a perimeter surface. A cavity resides in a portion of the face surface in proximity to the perimeter surface of the annular member. 
     The cavity in the annular member can have several different geometric configurations, such as a hollowed-out region in the face surface, an elongated channel, and the like. Further, a plurality of cavities can reside in the face surface of the annular member in which each cavity is separated by a non-cavity containing portion of the face surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates, in cross-section, a portion of a substrate polishing head assembly arranged in accordance with one embodiment of the invention; 
     FIG. 2 a  illustrates, in cross-section, a substrate retainer in accordance with one embodiment of the invention; 
     FIG. 2 b  is a bottom view of the substrate retainer illustrated in FIG. 2 a;    
     FIG. 3 is an exploded view of a portion of the substrate retainer illustrated in FIG. 2 a;    
     FIGS. 4 a-c  are bottom views of a substrate retainer arranged in accordance with another embodiment of the invention in successive stages of rotation; and 
     FIG. 5 is a cross-sectional view illustrating a portion of a substrate retainer and substrate carrier configured in accordance with an embodiment of the invention. 
    
    
     It will be appreciated that, for simplicity and clarity of illustration, elements shown in the FIGURES have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other for clarity. Further, where considered appropriate, reference numerals have been repeated among the FIGURES to indicate corresponding elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a cross-sectional view of a portion of a polishing head assembly  10 . Polishing head assembly  10  is configured to rotate about a rotational axis  12 . A substrate  14  is positioned against a substrate carrier  16  in the lower portion of polishing head assembly  10 . Substrate  14  can be any material requiring polishing, including a semiconductor substrate, a refractory metal substrate, a metal alloy substrate, and the like. 
     In a preferred embodiment of the invention, polishing head assembly  10  is configured for polishing a semiconductor substrate having any of several different thin film materials thereon. For example, substrate  14  can be a semiconductor substrate having a thick dielectric material, such as silicon oxide thereon. Further, semiconductor substrate  14  can be a semiconductor substrate having a semiconductive material, such as polycrystalline silicon, a refractory metal silicide, amorphous silicon, and the like thereon. Also, substrate  14  can be a semiconductive substrate having electrically conductive metals, such as aluminum, aluminum-silicon alloys, copper, copper alloys, and the like thereon. 
     Also shown in FIG. 1 is a portion of a polishing pad  18 . Polishing pad  18  can be one of a number of different types of polishing pads commonly used in a (CMP) apparatus. For example, polishing pad  18  can be a polyurethane material and the like. Generally, polishing pad  18  includes a support layer  20  and a polishing surface layer  22 . 
     During operation, polishing pad  20  is engaged in relative motion to polishing head assembly  10 . For example, in one common CMP apparatus, polishing pad  18  moves in a lateral direction, as indicated by arrow  24 , while head assembly  10  rotates about rotational axis  12 . A slight downward pressure is exerted upon head assembly  10  to cause substrate  14  to come into contact with polishing surface layer  22  of polishing pad  18 . 
     Substrate  14  is positioned against a substrate support surface  26  of substrate carrier  16 . A substrate retainer  28  surrounds substrate support surface  26  and protrudes below substrate support surface  26  by an amount sufficient to form a continuous surface with substrate  14 . By extending below substrate support surface  26  of substrate carrier  16 , substrate retainer  28  cooperates with substrate carrier  16  to form a substrate receiving area in which substrate  14  is contained during polishing operations. 
     Substrate retainer  28  is positioned within an annular indentation  30  located at the perimeter of substrate carrier  16 . In one embodiment of the invention, substrate retainer  28  is attached to substrate carrier  16  by a torque pin  32 . Although only one torque pin is shown, those skilled in the art will appreciate that two or more pins are typically used to couple the substrate retainer to the head assembly. Torque pin  32  allows for relative movement of substrate carrier  16  relative to the remaining structure of polishing head assembly  10 . Alternatively, substrate retainer  28  can be flexibly attached to substrate carrier  16  by a clip or other fastening mechanisms. 
     Shown in FIG. 2 a  is a cross-sectional view of substrate retainer  28 . Substrate retainer  28  includes an annular member  33  having a recessed surface portion  34 . In accordance with the invention, annular member  33  also includes a cavity  36  that is formed in a portion of annular member  33 . As shown in the bottom view of FIG. 2 b,  cavity  36  resides in a recessed surface portion  34  and is formed by a lip  38  that extends about a portion of the perimeter of annular member  33 . Annular member  33  defines a central opening  39  for receiving substrate  14 . 
     An exploded view of a portion of annular member  33  is illustrated in cross-section in FIG.  3 . Lip  38  forms a distal wall surface  40  of cavity  36  on a first side and a portion of a perimeter surface  41  on a second side. Annular member  33  also includes a face surface  42  that contacts polishing layer  22  during operation of the CMP apparatus. More importantly, lip  38  does not form a continuous surface with face surface  42 . As shown in FIG. 3 by the line laterally extended from face surface  42 , the lower portion of lip  38  is offset from face surface  42 . The offset forms a gap  43  between the bottom of lip  38  and any flat surface coming into contact with face surface  42 . As will subsequently be described, gap  43  plays an important roll in the distribution of slurry during operation of the CMP apparatus. 
     A bottom view of substrate retainer  28  in successive stages of rotation is illustrated in FIGS. 4 a - 4   c.  In the illustrated embodiment, two cavities are formed in the substrate retainer. In accordance with the illustrative embodiment, substrate retainer  28  has cavity  36  and a cavity  48 . Cavity  36  is separated from cavity  48  by non-cavity portions  50  and  52  of substrate retainer  28 . 
     In accordance with the invention, during operation of a CMP apparatus, a polishing slurry is introduced onto the polishing pad. A typical polishing slurry is an aqueous composition including an abrasive material, surfactants and can include chemicals that react with the thin film materials formed on the surface of substrate  14 . 
     As illustrated in FIGS. 4 a - 4   c,  the slurry is depicted as undergoing a translational motion relative to the rotational motion of substrate retainer  28 . As substrate retainer  28  rotates, a portion of the slurry is captured within first and second cavities  36  and  48 . By capturing a portion of the slurry, slurry reservoirs are created at the perimeter of substrate retainer  28 . First and second cavities  36  and  48  are configured such that slurry continuously flows from first and second cavities  36  and  48  onto the polishing pad. By providing the continuous flow of slurry, first and second cavities  36  and  48  assist in the distribution of the slurry during polishing operations. In particular, the continuous flow of slurry provided by first and second cavities  36  and  48  enables a more uniform polishing operation by uniformly distributing the slurry across the face of substrate  14 . 
     A portion of substrate carrier  16 , substrate retainer  28  and polishing surface layer  22  are illustrated in cross-section in FIG.  5 . As shown by the arrows, slurry generally flows along polishing surface layer  22  and into cavity  36 . Cavity  36  cooperates with polishing surface layer  22  to provide a liquid reservoir in which a portion of the slurry is temporarily held. Under the continual translational motion of polishing surface layer  22  and the rotational motion of substrate retainer  28  and substrate carrier  16 , the slurry flows from cavity  36  across face surface  42  of substrate retainer  28  and across an exposed surface  54  of substrate  14 . As the polishing process proceeds, the polishing action of polishing surface layer  22  and the slurry removes portions of substrate  14  at exposed surface  54 . The removed portions of exposed surface  54  are partially dissolved and entrained within the slurry. It is important to note that the thickness of the slurry layer is greatly exaggerated in FIG. 5 for purposes of illustration. In practice, surfaces  42  and  54  are in substantially direct contact with polishing surface layer  22 . 
     As described above, when face surface  42  comes into contact with polishing surface layer  22 , a gap  43  is formed between lip  38  and polishing surface layer  22 . Gap  43  allows the used slurry that contains dissolved and entrained portions of exposed surface  54  to be removed from cavity  36  at the trailing edge of rotational cycle. Cavity  36  is positioned at the trailing edge when substrate carrier  16  rotates cavity  36  into a downstream position with respect to the general direction of slurry flow (shown by the directional arrows in FIG.  5 ). In the absence of gap  43 , used slurry would become trapped in cavities  36  and  48 . By providing for the release of used slurry, a fresh reservoir of slurry can be maintained within the cavities for enhanced polishing uniformity. 
     The cavity structure in the substrate retainer provides several advantages in a polishing operation. For example, the ability to hold a slurry reservoir at the perimeter of the substrate improves both the polishing removal rate and the substrate polishing uniformity. Also, the continuous flow of slurry from the cavity can reduce the total slurry consumption during substrate polishing. Further, the polishing variation associated with the placement of slurry delivery systems is reduced. 
     Those skilled in the art will appreciate that the functional aspects of the invention can be carried out with a variety of geometric configurations. For example, a plurality of cavities can be formed in the substrate retainer. The number of cavities used will depend upon several processing parameters, such as the diameter of the substrates being polished, the flow characteristics of the slurry, the slurry retention capability of the polishing pad, and the like. Further, although the cavity has been illustrated having a particular cross-sectional geometry, those skilled in the art will appreciate that various cross-sectional geometries can be implemented. For example, a rectangular shaped cross-sectional geometry, a square shaped cross-sectional geometry, a circular cross-sectional geometry, and the like can be used. 
     Additionally, the invention provides a uniform disbursement of slurry across the polishing pad downstream from polishing head assembly  10 . This feature is important in a CMP apparatus that employs a pad conditioner in tandem with a polishing head assembly. By uniformly distributing slurry across the polishing pad downstream from the polishing head, the pad conditioner can more effectively condition the pad for subsequent polishing operations. 
     Thus, it is apparent that there has been described, in accordance with the invention, a polishing apparatus and substrate retainer ring providing continuous slurry distribution that fully provides the advantages set forth above. Although the polishing apparatus has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. For example, the polishing pad can be placed in rotational motion as the rotating head assembly is brought into contact with the polishing pad. Further, the cavity can be a plurality of circular depressions within the face surface of the substrate retainer. Those skilled in the art will recognize that other variations and modifications can be made without departing from the spirit of the invention. It is, therefore, intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.