Abstract:
An annular ring assembly is provided in which mechanical elements of the retaining ring assembly maintain strict planar flatness, rigidity, high tolerances and surface stability control. Additionally, glues, adhesives, and epoxies are eliminated from the construction of the plastic retaining and backing ring assembly. Further, adverse chemical reaction and contamination from adhesives that are typically in direct contact with chemical slurry and substrate layers undergoing polishing are eliminated. As a result, the present invention provides a low cost alternative to suppliers and manufacturers of retaining rings and facilitates a method to exchange, recondition and recycle the retaining ring for an infinite period, thus reducing consumable waste materials. Further, the ring assembly maintains uniform mechanical properties and strict tolerances after post reconditioning, thus reducing the variability and maintaining process consistency.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a recyclable retaining ring used on a carrier head in a chemical mechanical polishing system. 
     Many different manufacturing operations use chemical mechanical polishing; one such operation is the manufacturing of integrated circuits. Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly more non-planar. This occurs because the distance between the outer surface and the underlying substrate is greatest in regions of the substrate where the least etching has occurred, and the distance is least in regions where the greatest etching has occurred. With a single patterned underlying layer, this non-planar surface comprises a series of peaks and valleys where the distance between the highest peak and the lowest valley may be on the order of 5,000 to 12,000 Angstroms. With multiple patterned underlying layers, the height difference between the peaks and the valleys becomes even more severe and can reach several microns. 
     This non-planar outer surface presents a problem for integrated circuit manufacturers. If the outer surface is non-planar, then photolithographic techniques to pattern photoresist layers might not be suitable, as a non-planar surface can prevent proper focusing of the photolithography apparatus. Therefore, there is a need to periodically planarize this substrate surface. Planarization, in effect, polishes away peaks and valleys of non-planar outer surface layers of the integrated circuit, whether conductive, semiconductive, or insulative layers, to form a relatively flat smooth surface. Following planarization, additional layers may be deposited on the outer layer to form interconnect lines between features, or the outer layer may be etched to form vias to lower features. 
     Chemical mechanical polishing, commonly referred to as CMP, is a method of planarizing or polishing substrates. In a typical CMP process, a rotating polishing pad, which receives a chemically reactive slurry is used to polish the outermost surface and layers of the substrate. The substrate is positioned over the polishing pad, which is typically mounted in a carrier and retaining ring assembly. The carrier and retaining ring assembly maintains a bias force between the surface of the substrate and the rotating polishing pad. The movement of the slurry-whetted polishing pad across the surface face of the substrate causes material to be chemically and mechanically polished (removed) from that face of the substrate. 
     Different types of pads and slurry mixtures may be used. Each polishing pad provides a polishing surface which, in combination with the particular slurry mixture, can provide specific polishing characteristics. Thus, for any material being polished, the pad and slurry combination is selected to provide a desired finish and flatness on the polished surface. The pad and slurry combination can provide this finish and flatness in a predetermined polishing time. Additional factors, such as the relative speed between the substrate and the pad, and the force pressing the substrate against the pad affect the polishing rate finish and flatness. 
     One problem with conventional CMP processing is the high volume of wearable parts which are consumed as the substrates are polished. Generally, a retaining ring assembly is mounted under a substrate carrier that continually wears down as the polishing pad makes direct contact against featured substrate layer surfaces. Consequently, the retaining ring assembly burdens a significant cost as a consumable item for general CMP systems because the entire assembly needs to be discarded and replaced. Moreover, the retaining ring assembly should be able to stay substantially parallel to the polishing pad after repeated recycling and replacement. The parallel relationship between the polishing pad and the retaining ring assembly is desirable to eliminate any angular deformities that could result in substandard CMP polishing. 
     There is a need to for an apparatus and method, which overcomes the foregoing and other problems and which substantially reduces the cost of the retaining ring assembly in a CMP apparatus. It is to these ends that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     The invention advantageously provides an apparatus and method to reduce consumable operating expenses in CMP processes by utilizing low cost recyclable components without risking or compromising process performance, material stability, and loss of yield. 
     The invention provides an annular ring assembly is provided in which mechanical elements of the retaining ring assembly maintain flatness, rigidity, high tolerances and surface stability control. Additionally, glues, adhesives, and epoxies are eliminated from the construction of the plastic retaining and backing ring assembly. Further, adverse chemical reaction and contamination from adhesives that are typically in direct contact with chemical slurry and substrate layers undergoing polishing are eliminated. 
     As a result, the present invention provides a low cost alternative to suppliers and manufacturers of retaining rings and facilitates a method to exchange, recondition and recycle the retaining ring for an infinite period, thus reducing consumable waste materials. Further, the ring assembly maintains uniform mechanical properties and strict tolerances after post reconditioning, thus reducing the variability and maintaining process consistency. 
     In one aspect the invention provides an annular ring assembly for a chemical mechanical polishing apparatus comprising an annular backing ring having a recessed channel reference guide groove portion arranged circumferentially within the inner surface of the backing ring and an annular retaining ring having an associated raised neck portion extending circumferentially from the inner surface of the retaining ring, wherein the respective groove and neck portions communicate to secure the backing ring and the retaining ring. Thus, the annular retaining ring may be secured with, and is removable from, the channel reference guiding groove. When the annular retaining ring becomes worn from repetitive use of the assembly, the retaining ring may be removed from the backing ring and replaced with a new annular retaining ring. As a result, the discarding of the entire assembly and the waste of high precision material can be avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is an isometric view of a conventional CMP retaining ring assembly using an adhesive mechanical bonding to unite a backing ring and a retaining ring of the assembly; 
     FIG. 1 b  is a cross-sectional view of the conventional CMP retaining ring assembly shown in FIG. 1 a;    
     FIG. 2 is a block representation of a chemical mechanical polishing system, including the carrier head, retaining ring, substrate and polishing pad, with which the invention may be used; 
     FIG. 3 is an isometric view of a first embodiment of a CMP retaining ring assembly apparatus according to the invention; 
     FIG. 4 a  is a bottom view of an annular backing ring portion of the ring assembly apparatus shown in FIG. 3; 
     FIG. 4 b  is a top view of the annular backing ring portion of the ring assembly apparatus shown in FIG. 4 a;    
     FIG. 5 a  is a top view of an annular retaining ring portion assembly apparatus shown in FIG. 3; 
     FIG. 5 b  is a bottom view of the annular retaining ring portion of the ring assembly apparatus shown in FIG. 5 a;    
     FIG. 6 is a cross-sectional view of the CMP retaining ring assembly of FIG. 3, taken along the line  6 — 6 , showing a dovetail locking feature according to the invention; 
     FIG. 7 is an enlarged view of a portion of the cross-sectional view of FIG. 6, taken of the area encompassed by the dotted circle  7 ; 
     FIG. 8 is an exploded view of the CMP retaining ring assembly apparatus shown in FIG. 3; 
     FIG. 9 is an isometric view of a second embodiment of a CMP retaining ring assembly apparatus according to the invention; 
     FIG. 10 a  is a top view of an annular backing ring portion of the ring assembly apparatus shown in FIG. 9; 
     FIG. 10 b  is a bottom view of the annular backing ring portion of the ring assembly apparatus shown in FIG. 10 a;    
     FIG. 11 a  is a top view of an annular retaining ring portion of the ring assembly apparatus shown in FIG. 9; 
     FIG. 11 b  is a bottom view of the annular retaining ring portion of the ring assembly apparatus shown in FIG. 11 a;    
     FIG. 12 is a cross-sectional view of the CMP retaining ring assembly of FIG. 9, taken along the line  12 — 12 , showing a locking feature according to a second embodiment of the invention; 
     FIG. 13 is an exploded view of the CMP retaining ring assembly apparatus shown in FIG. 9; 
     FIG. 14 a  is a top view of a third embodiment of an annular backing ring portion of the ring assembly apparatus of the invention; 
     FIG. 14 b  is a bottom view of the annular backing ring portion of the ring assembly apparatus shown in FIG. 14 a;    
     FIG. 15 a  is a top view of a third embodiment of an annular retaining ring portion of the ring assembly apparatus of the invention; 
     FIG. 15 b  is a bottom view of the annular retaining ring portion of the ring assembly apparatus shown in FIG. 15 a;    
     FIG. 16 is an isometric view of another embodiment of the annular ring assembly according to the invention; 
     FIG. 17 is a cross-sectional view of the CMP annular retaining ring assembly, taken along the line  17 — 17 , showing a screw thread locking feature of the invention; 
     FIG. 18 is an enlarged view of a portion of the cross-sectional view of FIG. 17, taken of the area encompassed by the dotted circle  18 ; and 
     FIG. 19 is an exploded view of the CMP retaining ring assembly shown in FIG.  16 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A conventional retaining ring assembly  1  is shown in FIG. 1 a.  FIG. 1 b  is a cross-sectional view of the retaining ring assembly  1 . The two-piece ring assembly  1  typically comprises a plastic retaining ring  3  attached to an annular stainless steel backing ring  5 . After several hundreds of repeated CMP cycles, the plastic interface of the retaining ring assembly wears down proportional to the number of processed substrates under the carrier head. The entire retaining ring assembly  1  (plastic and annular stainless steel rings) must be discarded and replaced on a frequent interval. The disadvantage of the current retaining ring method is the associated cost of the stainless steel backing ring  5  component in relation to discarding the entire assembly  1 . 
     The major cost associated with the conventional two-piece retaining ring assembly  1  is the backing ring  5 . The stainless steel backing ring  5  is typically highly ground-precision stainless steel with tight tolerances and precisely patterned mounting holes. The backing ring  5  (including the attached plastic ring  3 ) is the direct interface between the carrier head (not shown) of the CMP platform and polishing pad (not shown). Besides mounting the carrier, the backing ring  5  provides stiffness and backing to the plastic-retaining ring  3 . The current design of the plastic retaining ring  3  is fastened to the stainless steel backing ring  5  by a permanent adhesive compound  7 . Consequently, the entire assembly  1  must be discarded after subsequent usage. This limitation is costly and results in waste of high precision material. 
     FIG. 2 shows a chemical mechanical polishing system  20  having a carrier head assembly  22  rotatably mounted to the system  20  via a rotatable axle  24   a.  An annular retaining ring assembly  30  is typically secured to the carrier head assembly  22 , as by means of an opposing threaded engagement interface, which will be described in detail later with respect to a preferred embodiment of the retaining ring assembly  30 . A substrate  26  sits within the retaining ring assembly  30  and is held within the retaining ring assembly  30  and is brought into physical contact with a polishing pad  28  via a U-shaped bladder (not shown) that is disposed within the retaining ring assembly and abuts against an opposing surface of the substrate  26 . The pad  28  operates to polish the substrate  26  which may be supported by a rotatable polishing platen  29 . The rotatable polishing platen  29  may be fixed to the system  20  via a second rotatable axle  24   b.  Axles  24   a  and  24   b  are preferably independent of each other, and preferably rotate in opposing directions (i.e., clockwise and counter-clockwise). In this manner, polishing can be effected in a relatively efficient manner, as the opposing rotational forces operate to maintain frictional contact between the polishing pad  28  and the substrate  26  which is needed to effectively polish the substrate  26 . 
     FIGS. 3,  4   a  and  4   b  show an isometric view of a first embodiment of a chemical mechanical polishing retaining ring assembly  30  according to the invention. The ring assembly  30  may comprise an annular backing ring  32  and an annular retaining ring  34 . The annular backing ring  32  may have a recessed channel or groove  36  arranged circumferentially along the inner surface  32   a  of the backing ring  32 . The respective edges  36   a,    36   b  of the channel or groove  36  may be outwardly tapered at a slight angle so as to form a beveled edge. This beveled edge is similar to a “dovetail” configuration that is a typical forming and aligning technique in which the edges of a groove are tapered so as to be capable of securing and aligning an opposing member inserted within the groove. Preferably, the angle of taper is slight, such as 1°, but the angle of taper could be as large as 5° or more. This relationship can be appreciated with reference to FIGS. 6-7, which show the dovetail locking feature of the invention. 
     The annular retaining ring  34  may have an associated raised neck  38  protruding circumferentially from along the inner surface  34   a  of the retaining ring  34 , as shown in FIGS. 5 a  and  5   a.  This raised neck  38  may be slightly wider than the opposing channel or groove  36  of the backing ring  32  so as to fit snuggly within the narrower channel or groove  36  to prevent sliding motion of the opposing ring portions  32 ,  34  when the ring assembly  30  is mated. Preferably, the width differential between the raised neck  38  and the channel or groove  36  is about 0.002 meters, however, it is not limited to this dimension and could be wider or narrower. This feature is shown in more detail in FIG. 6, and will be described in detail below. 
     Additionally, the raised neck  38  may be provided with at least one gap  40  disposed at symmetrical distances from the centerline of the raised neck  38 . This gap  40  allows for the raised neck  38  to be compressed during insertion of the raised neck  38  within the channel or groove  36  of the backing ring  32 . Upon assembly, the raised neck  38  relaxes to fit snugly within the channel or groove  36 . As a result, sliding of the ring assembly components is prevented. Preferably, two gap portions  40  are provided at symmetrical distances relative to the centerline of the raised neck  38 , to allow for further compression of the raised neck  38 , however, any number of gap portions  40  will suffice. The amount of force required to compress the raised neck  38  to a minimal width necessary to fit the raised neck  38  within the channel or groove  36  is proportional to both the width of the gap  40  and the number of gaps  40  disposed within the raised neck  38 . 
     FIG. 6 shows a cross-sectional view of the annular ring assembly  30  of FIG. 3 taken along the line  6 — 6 . The above-described dovetail feature will now be explained. As can be seen from the enlarged view of the cross-sectional area of the annular ring assembly  30 , shown in FIG. 7, adjacent mating edges  36   a,    36   b  (represented in FIG. 7 as  36   b ) of the channel or groove  36  of the retaining ring  32  are preferably tapered at a slight angle θ so as to form a beveled edge. The raised neck  38  of the annular retaining ring  34  is designed to be slightly wider that the opposing channel or groove  36  of the backing ring  32 . The gaps  40  of the raised neck portion  38  allow for compression of the neck portion  38  upon mating of the retaining ring  32  and backing ring  34 , so as to secure the retaining ring  32  to the backing ring  34  and prevent slidable motion of the ring assembly  30  when mated. 
     Referring again to FIGS. 3-6, additionally, the backing ring  32  may have a pin  42  that operates to separate the backing ring  32  from the retaining ring  34 , when it is desired to replace the plastic retaining ring  34  which may have become worn due to the chemicals and friction involved in the CMP process. By causing a translational force to be applied to the backing ring  34 , via the removal pin  42 , the separation of the ring assembly  30  can be forced. Typically, the removal pin  42  is a screw, but could be any such pressure exerting means. 
     Additionally, dowel pin cavities  44  may be provided at respective intervals along the inner diameter surface  32   a  of the backing ring  32 . These dowel pin cavities  44  are associated with respective dowel pin insertion holes  44   a  along the inner surface  32   a  of the backing ring  32 . When the annular ring assembly  30  is assembled, the dowel pin insertion holes  44   a  are aligned with dowel pin insertion holes  44   b  located along the top surface  34   a  of the retaining ring  34 . When the ring assembly  30  is mated, dowel pins  46  can be inserted along dowel pin cavities  44  and through dowel pin insertion holes  44   a,    44   b,  thereby preventing sliding motion of the ring assembly  30  during operation. For additional security, the annular retaining ring  34  and the backing ring  32  can be fixed together by a mechanical fastener (not shown). The assembly as described above is represented in the exploded view of FIG.  8 . 
     The foregoing structure of the retaining ring assembly  30  eliminates the need for an adhesive bond, as was required by conventional retaining ring assemblies, and, as such, the annular retaining ring  34  can be easily removed from the backing ring  32  without the need for extensive reconditioning, adhesive stripping or surface interface finishing. Therefore, the annular backing ring  32  can be recycled and refurbished and a new retaining ring  34  can be fastened with the recycled annular backing ring  32 . As such, the entire ring assembly  30  does not have to be discarded when the annular retaining ring  34  becomes worn and ineffective. 
     In the carrier ring assembly  30  of the system  20 , the plastic retaining ring  34  prevents the shear forces created by the motion of polishing pad  26  from pushing the substrate  26  out from underneath carrier head  22 . The retaining ring  34  projects down to the substrate  26  from the outer edge of carrier head  22  to contact the polishing pad  28  and polishing platen  29 . 
     Preferably, the retaining ring  34  is constructed of a plastic composite material, but may also be constructed of other materials. In fact, any rigid, sturdy composition may suffice. The backing ring  32  is preferably constructed of stainless steel, but it may also be constructed of other high tolerance materials, such as titanium or aluminum. Additionally, the bottom surface  34   b  of the annular retaining ring  34  is flat and has a number of slurry channels  48  circumferentially arranged along its surface  34   b,  as shown in FIG. 5 a.  These slurry channels  48  extend from the inner diameter of the ring  34  to the outer diameter of the ring  34  and are disposed at an angle relative to the inner diameter of the ring  34 . The slurry channels  48  operate to provide slurry to the polishing pad (not shown) that contacts the substrate (not shown). 
     FIG. 9 is an isometric view of a second embodiment of the CMP retaining ring assembly  80  according to the invention. In this embodiment, like parts are denoted by like numerals. The ring assembly  80  shown in FIGS. 9-12 is similar to the first embodiment described above. Referring to FIG. 13, the second embodiment differs from the first embodiment in that dowel pin holes  86   a  may be provided at respective intervals along the outer diameter  82   a  of the backing ring  82 . These dowel pin holes  86   a  are associated with respective dowel pin insertion holes  86   b  along the outer diameter surface  38   a  of the raised neck  38  of the retaining ring  84 . When the annular ring assembly  80  is mated, respective dowel pin insertion holes  86   a  are aligned with respective dowel pin insertion holes  86   b  and locking pins  86  can be inserted through like hole pairs  86   a,    86   b  and operate to lock the assembly  80  to prevent slidable motion of the components during operation. For additional security, the annular retaining ring  84  and the backing ring  82  can be fixed by a mechanical fastener (not shown). 
     Subsequently, the retaining ring assembly  80  eliminates the need for an adhesive bond, as was required by conventional retaining ring assemblies, and, as such, the annular retaining ring  84  can be easily removed from the backing ring  82  without the need to consider extensive reconditioning, adhesive stripping or surface interface finishing. Thus, the annular backing ring  82  can be recycled and refurbished and fastened with a new annular retaining ring  84 . Therefore, the entire ring assembly  80  does not need be discarded when the annular retaining ring  84  becomes worn and ineffective. 
     Yet another alternative embodiment will now be explained with reference to FIGS. 14A-16. In this embodiment, like features are represented by like numerals. FIGS. 14 a  and  14   b  show respective top and bottom views of a third embodiment of an annular backing ring  132  that makes up a portion of a ring assembly  130 . The annular backing ring  132  may have a channel or groove  36  arranged circumferentially along the inner surface  132   a  of the backing ring  132 . The groove  36  may extend from the inner diameter  132   a ′ of the annular backing ring  132  to a threaded flange edge  136   a  arranged along the outer diameter  132   a ″ of the annular backing ring  132 . When the annular retaining ring  132  and an annular backing ring  134  are mated, the threaded edge  136   b  of the annular retaining ring  134  and the threaded edge  136   a  of the annular backing ring  132  may be secured via the respective threaded interface relationship. This feature will be explained in detail herein with reference to FIGS. 17 and 18. 
     As just explained, an annular retaining ring  134  has an associated raised neck  38  protruding circumferentially from along the inner surface  134   a  of the retaining ring  134 , as shown in FIGS. 15 a  and  15   b,  respectively. The raised neck  38  may extend from the inner diameter  134   a ′ of the annular retaining ring  134  to a distance slightly narrower in width than the length of the inner surface  134   a  of the retaining ring  134 . The outer edge  136   b  of the raised neck portion  38  may be a threaded edge  136   b.  When the assembly  130  is mated, the opposing threaded edges interface to secure the retaining ring  134  to the backing ring  132 . Therefore, no additional locking pins (not shown) are required. 
     Referring now to FIGS. 16-18, a cross-sectional view of the annular ring assembly  130  is shown taken along the line  17 — 17 . When mated, the raised neck  38  of the annular retaining ring  134  together with the threaded edge  136   b  is designed to be slidably connected with the channel or groove  36  of the backing ring  132  and its respective threaded flange edge  136   a.  The ring assembly  130  is locked and held in place via this threaded relationship, as can be seen in the enlarged view of the connection interface in FIG.  18 . Thus, slidable motion between the components of the assembly  130  is prevented. An exploded view of the assembly  130  is shown in FIG.  19 . 
     The design of the invention maintains the necessary characteristics of structural rigidity, flatness and parallelism equivalent to conventional CMP two-piece retaining ring assembly products available in the market. However, unlike conventional assemblies, the backing ring and retaining ring can be recycled with minimum reconditioning costs and expenses. 
     While the foregoing has been described with reference to particular embodiments of the invention, it will be appreciated by those skilled in the art that changes in these embodiments may be made without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims.