Patent Publication Number: US-6656024-B1

Title: Method and apparatus for reducing compressed dry air usage during chemical mechanical planarization

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to semiconductor fabrication and, more particularly, to a method and apparatus for reducing consumption of compressed dry air (CDA) during chemical mechanical planarization (CMP) operations. 
     CMP systems are designed to planarize a wafer surface by applying the wafer against a polishing surface in the presence of an abrasive slurry. In some CMP systems, the polishing surface is a belt. For example, the TERESA™ CMP system, which is commercially available from Lam Research Corporation, the assignee of this application, is one such belt-type CMP system. FIG. 1 is a simplified schematic diagram of a conventional belt-type CMP system. In this system, polishing surface  100  is in the form of a belt that is driven by rotors  102 . Wafer carrier  104  supporting a wafer is disposed over polishing surface  100  and forces the wafer against the polishing surface during the CMP process. Air-bearing platen  106  provides friction-free support to the underside of polishing surface  100  through a layer of compressed dry air (CDA) supplied from a CDA source connected to platen  106 . 
     During CMP operations, the air-bearing platen  106  consumes a significant amount of CDA. The amount of CDA is a function of the size of the wafers being processed. Consequently, as chip fabricators shift from 200 millimeter (mm) wafers to 300 mm wafers the annual cost of CDA significantly increases. Because of the high consumption rate of CDA by air-bearing platens, chip fabricators must also incur capital expenditures to add CDA capacity when purchasing additional CMP systems with air-bearing platens. 
     Another shortcoming of the belt-type CUP system of FIG. 1 is the transient losses of the CDA at the edge of platen  106 . Due to inherent transient losses, the support provided for polishing surface  100  degrades at the edges of the platen. Consequently, the removal rate at the edge of the wafer is the most challenging region on the wafer to control during CMP operations. If the removal rate at the edge of the wafer differs from that for the remainder of the wafer, then the wafer is not planarized evenly. Hence, yields and device quality may be negatively impacted. 
     In view of the foregoing, there is a need for a method and apparatus for reducing the consumption of CDA during CMP operations and limiting transient losses around the edge of the wafer to provide more uniform support for the entire surface of the wafer. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the present invention fills this need by providing a retaining ring which reduces the consumption of compressed dry air (CDA) during chemical mechanical planarization (CMP) operations. The present invention also provides a method for reducing a consumption of CDA during a CMP operation In accordance with one aspect of the present invention, a retaining ring is provided. The retaining ring includes a lower annular sleeve having a base. An inner sidewall and an outer sidewall extend from the base. The lower annular sleeve has at least one hole defined therein. An upper annular sleeve is moveably disposed over the lower annular sleeve. The upper annular sleeve has a top that may have one or more holes defined therein. An inner sidewall and an outer sidewall extend from the top. 
     In accordance with another aspect of the invention, a chemical mechanical planarization (CMP) system is provided. The system includes a polishing surface and a platen disposed along an underside of the polishing surface. The platen is configured to be coupled to a first fluid source. A retaining ring surrounds the platen. The retaining ring includes a lower annular sleeve and an upper annular sleeve moveably disposed over the lower annular sleeve. The lower annular sleeve is fixed and has at least one hole configured to be coupled to a second fluid source. 
     In accordance with yet another aspect of the invention, a method for reducing a consumption of CDA during a CMP operation. In this method an air-bearing platen is surrounded by a retaining ring having a moveable sleeve. The moveable sleeve of the retaining ring is moved into close proximity with an underside of a polishing surface. A CMP operation is then conducted during which the retaining ring reduces the consumption of CDA and limits transient losses around the edge of a wafer undergoing the CMP operation. 
    
    
     It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the principles of the invention. 
     FIG. 1 is a simplified schematic diagram of a conventional belt-type CMP system. 
     FIG. 2 is a simplified schematic diagram of a chemical mechanical planarization system (CMP) configured to reduce the consumption of compressed dry air (CDA) in accordance with one embodiment of the invention. 
     FIG. 3 is a simplified cross-sectional view of a platen and a retaining ring in accordance with one embodiment of the invention. 
     FIG. 4 is a top view of an upper annular sleeve of a retaining ring in accordance with one embodiment of the invention. 
     FIG. 5 is a top view a lower annular sleeve of a retaining ring in accordance with one embodiment of the invention. 
     FIG. 6 is a top view of an upper annular sleeve of a retaining ring in accordance with one embodiment of the invention. 
     FIG. 7 is a side view that shows channels formed in the top surface of the two curved members of an annular sleeve in accordance with one embodiment of the invention. 
     FIG. 8 is a cross-sectional view of a retaining ring with an upper annular sleeve in a relaxed state in accordance with one embodiment of the invention. 
     FIG. 9 is a cross-sectional view of the retaining ring shown in FIG. 8 with the upper annular sleeve in a raised state. 
     FIG. 10 is a partial cross-sectional view of a retaining ring. 
     FIG. 11 is a cross-sectional view of the upper annular sleeve and the lower annular sleeve of the retaining ring. 
     FIG. 12 is a flowchart diagram of the method operations performed in reducing consumption of compressed dry air (CDA) during a chemical mechanical planarization (CMP) operation in accordance with one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Several exemplary embodiments of the invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is discussed above in the “Background of the Invention” section. 
     FIG. 2 is a simplified schematic diagram of a chemical mechanical planarization system (CMP) configured to reduce the consumption of compressed dry air (CDA) in accordance with one embodiment of the invention. A polishing surface  116  is mounted on rotors  114 . Air-bearing platen  112  is disposed under polishing surface  116  and between rotors  114 . As is well known to those skilled in the art, air-bearing platen  112  provides low friction support for the underside of polishing surface  116 . Retaining ring  118  surrounding platen  112 . Wafer carrier  108  is disposed over polishing surface  116  and supports wafer  110 . During operation, rotors  114  rotate around their axis and drive polishing surface  116  in a linear direction over air-bearing platen  112 . As wafer carrier  108  forces wafer  110  against the top surface of polishing surface  116 , a layer of compressed dry air (CDA) from air bearing platen  112  supports polishing surface  116 . Retaining ring  118  constrains the CDA layer between polishing surface  116  and platen  112 . As will be explained in more detail below, retaining ring  118  is configured to minimize CDA losses without perturbing the interaction angle between polishing surface  116  and wafer  110 . 
     FIG. 3 is a simplified cross-sectional view of a platen and a retaining ring in accordance with one embodiment of the invention. As shown therein, retaining ring  118  includes upper annular sleeve  118   a  and lower annular sleeve  118   b . Upper annular sleeve  118   a  is moveably disposed over lower annular sleeve  118   b  and is capable of automatically aligning to the underside of polishing surface  116 , as will be described in more detail below with reference to FIGS. 8-11. Lower annular sleeve  118   b  is fixed, i.e., rigidly attached, to a suitable part of the CMP system. It will be apparent to one skilled in the art that lower annular sleeve  118   b  can be attached to any parts of the CMP system that are capable of providing rigid support for the lower annular sleeve. In one embodiment, lower annular sleeve  118   b  is attached to platen  112 . When upper annular sleeve  118   a  is in a raised position as shown in FIG. 3, the CDA from air-bearing platen  112  is constrained in a region defined by the upper annular sleeve, platen  112  and polishing surface  116 . Additionally, transient losses at edge  124  of platen  112  are reduced, which in turn provides for tighter control of the removal rate at the edge of the wafer being planarized. It should be appreciated that the retaining ring allows for the controlled release of the constrained air, e.g., through the gap between the top of the upper annular sleeve and the underside of the polishing surface, to preclude chattering of the polishing surface. However, the amount of air lost via this controlled release is significantly reduced relative to the amount of air lost in conventional CMP systems. 
     FIG. 4 is a top view of an upper annular sleeve of a retaining ring in accordance with one embodiment of the invention. Upper annular sleeve  118   a  of the retaining ring has a top surface  119  with outer sidewall  120  extending from top surface  119 . An inner sidewall  117  also extends from top surface  119 . A plurality of holes  126  extend through top surface  119  of upper annular sleeve  118   a . Holes  126  allow for lubrication of the interface between the retaining ring and polishing surface as will be explained in more detail in reference to FIGS. 6 and 7. One skilled in the art will appreciate that holes  126  can be configured in any pattern that allows for upper annular sleeve  118   a  to move in close proximity to the underside of the polishing surface. 
     FIG. 5 is a top view a lower annular sleeve of a retaining ring in accordance with one embodiment of the invention. Lower annular sleeve  118   b  includes base  122  that has inner sidewall  123  and outer sidewall  127  extending from base  122 . Holes  136  extend through base  122  of lower annular sleeve  118   b . As will be explained in more detail with respect to FIG. 10, holes  136  are configured to be connected to a fluid source. The fluid source provides a fluid flow to lower annular sleeve  118   b  which in turn causes the upper annular sleeve to move as will be described in more detail with reference to FIGS. 9 and 10. It should be appreciated that upper annular sleeve  118   a  of FIG. 4 nests with lower annular sleeve  118   b  to form the retaining ring. 
     FIG. 6 is a top view of an upper annular sleeve of a retaining ring in accordance with one embodiment of the invention. Upper annular sleeve  118   a ′ the same as upper annular sleeve  118  of FIG. 4, however, upper annular sleeve  118   a ′ is quartered as depicted by upper curved members  118   a - 1 ,  118   a - 2 ,  118   a - 3  and  118   a - 4 . Of course, each of upper curved members  118   a - 1 ,  118   a - 2 ,  118   a - 3  and  118   a - 4  is moveably disposed over corresponding lower curved members. That is, lower annular sleeve  118   b  of FIG. 5 would be simnilarly quartered into lower curved members and nested with upper annular sleeve  118 ′. Gaps  128  between each of the upper curved members  118   a - 1 ,  118   a - 2 ,  118   a - 3  and  118   a - 4  provide controlled release points to avoid chattering of the polishing surface. Alternatively, upper annular sleeve  118   a  may include relief channels to systematically release the CDA from air-bearing platen  112  as shown in FIG.  7 . The systematic release of the CDA avoids the build-up of pressure between platen  112  and the polishing surface when the upper annular sleeve is in close proximity to the underside of the polishing surface. It will be apparent to one skilled in the art that the configuration of annular ring  118   a ′ allows for the individual control of each curved member. Thus, variations or localized deflections of the polishing surface are more easily accommodated. FIG. 6 illustrates retaining ring  118   a ′ as four (4) curved members for exemplary purposes only and is not meant to be limiting, as retaining ring  118   a ′ can be configured in any number of curved members. 
     FIG. 7 is a side view that shows channels formed in the top surface of the two curved members of an annular sleeve in accordance with one embodiment of the invention. Relief channels  129  allow for the controlled release of compressed dry air to preclude chattering of the polishing surface. One skilled in the art will appreciate that relief channels  129  can be implemented in numerous ways such as providing a v-shaped channel across the top surface of curved members  118   a - 1  and  118   a - 2  of the upper annular sleeve between holes  126 . As shown in FIG. 7, relief notches  129  provide a mechanism for the systematic release of CDA in addition to gap  128 . While relief channels  129  are depicted as a V-shaped channel across the top surface of the upper annular sleeve, it will be apparent to one skilled in the art that a number of other geometric configurations also can be used, e.g., rectangular-shaped channels or U-shaped channels. 
     FIG. 8 is a cross-sectional view of a retaining ring with an upper annular sleeve in a relaxed state in accordance with one embodiment of the invention. As shown here, it can be seen that upper annular sleeve  118   a  is a sleeve disposed over lower annular sleeve  118   b . As shown in FIG. 8, the inner and outer sidewalls of lower annular sleeve  118   b  are contained between the inner and outer sidewalls of upper annular sleeve  118   a . Thus, a gap  130  exists between the inner and outer sidewalls of upper annular sleeve  118   a  and the corresponding inner and outer sidewalls of lower annular sleeve  118   b  in one embodiment. As discussed in more detail with respect to FIG. 9, gap  130  can act as a release for excess fluid to flow out of the region between lower annular sleeve  118   b  and upper annular sleeve  118   a . In a relaxed state, i.e., where no fluid flow is being supplied through lower annular sleeve  118   b , upper annular sleeve  118   a  is not in close proximity to the underside of polishing surface  116 . Thus, CDA supplied from air-bearing platen  112  is not constrained in a region defined between platen  112  retaining ring  118  and polishing surface  116 . 
     FIG. 9 is a cross-sectional view of the retaining ring shown in FIG. 8 with the upper annular sleeve in a raised state. A flow of fluid is supplied through lower annular sleeve  118   b . The pressure created by the fluid flow forces upper annular sleeve  118   a  to rise. One skilled in the art will appreciate that the fluid flow rate, the area of hole  126 , and the size of gap  130  between the lower annular sleeve  118   b  and the upper annular sleeve  118   a  impact the distance traveled by upper annular sleeve  118   a . As mentioned above, these parameters are configured so that upper annular sleeve  11   8   a  can move into close proximity with the underside of polishing surface  116  without perturbing polishing surface  116 . Accordingly, a wafer interaction angle is controlled by the distance of platen  112  from polishing surface  116  and not by the movement of retaining ring  118 . It should be further appreciated that the configuration illustrated in FIG. 9 allows for a gimbal effect between upper annular sleeve  118   a  and lower annular sleeve  118   b , so that the upper annular sleeve can self-align to the underside of polishing surface  116 . 
     The interaction angle between polishing surface  116  and a wafer being planarized impacts the removal rate at the edge of the wafer particularly in a region within  10  millimeters of the wafer edge. This angle is controlled in part by regulating the distance between platen  112  and polishing surface  116 . By providing a floating retaining ring  118 , i.e., a retaining ring  118  with a moveable upper annular sleeve  118   a , the interaction angle remains controllable by the distance between platen  112  and polishing surface  116 . Additionally, when upper annular sleeve  118   a  of retaining ring  118  is raised, transient losses of CDA at the edge of platen  112  are reduced. Therefore, the steady-state performance of the layer of CDA for supporting polishing surface  116  is improved at the edge of platen  112 . In turn, the removal rate at the edge of a wafer subjected to the CMP process is able to be more tightly controlled because of the increased support for the polishing surface at the edge of platen  112 . 
     Still referring to FIG. 9, the fluid is supplied to lower annular sleeve  118   b  which manifolds the DIW to upper annular sleeve  118   a . Upper annular sleeve  118   a  travels along a vertical axis of the retaining ring in response to the fluid flow to lower annular sleeve  118   b . In one embodiment, the fluid provided to activate upper annular sleeve  118   a  is de-ionized water (DIW). A portion of the fluid supplied to lower annular sleeve  118   b  flows through hole  126  to lubricate the interface between upper annular sleeve  118   a  and polishing surface  116 . As mentioned previously, gap  130 , between lower annular sleeve  118   b  and upper annular sleeve  118   a , allows excess fluid to escape. The fluid portions that flow through gap  130  or holes  126  can be collected and recycled in one embodiment of the present invention. A travel limiter, as discussed with respect to FIGS. 10 and 11, can limit the distance upper annular sleeve  118   a  traverses from a relaxed position to a fully raised position. 
     FIG. 10 is a partial cross-sectional view of a retaining ring. Upper annular sleeve  118   a  is raised by a pressure created by a flow of fluid through lower annular sleeve  118   b . One skilled in the art will appreciate that upper annular sleeve  118   a  can be made from any suitable material compatible with the fluid and the CMP process. Exemplary materials include general purpose plastic materials. In one embodiment, upper annular sleeve  118   a  is comprised of a friction resistant polymeric material such as DELRIN™ acetal resins. Holes  126  allow the fluid to lubricate the interface between the polishing surface and the top surface of upper annular sleeve  118   a  during CMP operations. While FIG. 10 displays two holes  126  along the cross-sectional view of the top of upper annular sleeve  118   a , those skilled in the art will recognize that any number or pattern of holes  126  can be used which allow the interface to be lubricated without perturbing the polishing surface. Of course, the pattern of holes are configured to allow a pressure from the fluid flow through lower annular sleeve  118   b  to lift upper annular sleeve  118   a  into close proximity to the underside of the polishing surface. 
     Still referring to FIG. 10, protrusions  132   a  and  13   b  of lower annular sleeve  118   b  and corresponding protrusions  134   a  and  134   b  of upper annular sleeve  118   a  act as travel limiters. In particular, as the fluid forces the upper annular sleeve  118   a  to rise, protrusion  134   a  and protrusion  134   b  will limit the travel of upper annular sleeve  118   a  as they meet protrusion  132   a  and protrusion  132   b , respectively. It will be apparent to one skilled in the art, that any configuration can be applied in place of the protrusions  132   a  and  132   b  and  134   a  and  134   b , as long as upper annular sleeve  118   a  is limited in the distance that the upper annular sleeve can travel above lower annular sleeve  118   b.    
     FIG. 11 is a cross-sectional view of the upper annular sleeve and the lower annular sleeve of the retaining ring. As shown here, lower annular sleeve  118   b  includes hole  136 . Hole  136  enables fluid from a fluid source to be supplied to lower annular sleeve  118   b . Lower annular sleeve  118   b  manifolds the fluid to upper annular sleeve  118   a  which results in upper annular sleeve  118   a  moving to a close proximity to the underside of the polishing surface. Of course, protrusions  132   a ,  132   b ,  134   a  and  134   b  limit the movement of upper annular sleeve  118   a  to preclude the upper annular sleeve from being forced off of lower annular sleeve  118   b . In one embodiment, the pressure created by the fluid flow is sufficient to raise upper annular sleeve  118   a  into close proximity with the underside of the polishing surface and provide lubrication to an interface between the polishing surface and upper annular sleeve  118   a . While one hole is shown in FIG. 11, it will be apparent to one skilled in the art that any number of holes  136  can be defined in the base of lower annular sleeve  118   b    
     FIG. 12 is a flowchart diagram of the method operations performed in reducing consumption of compressed dry air (CDA) during a chemical mechanical planarization (CMP) operation in accordance with one embodiment of the invention. The method begins in operation  138  where an air-bearing platen is surrounded by a retaining ring. An example of a suitable retaining ring is the retaining ring described with reference to FIGS. 4-11; however, other suitable retaining rings also may be used. The method then advances to operation  140  where the moveable sleeve, e.g., the upper sleeve of the retaining ring, moves into close proximity with the underside of a polishing surface. As discussed above with reference to FIGS. 9 and 10, a fluid flow supplied to the lower sleeve of the retaining ring creates a pressure which forces the moveable sleeve of the retaining ring into close proximity with the underside of a polishing surface. In one embodiment, travel limiters governing the maximum distance the moveable sleeve can travel are provided. By adjusting the moveable sleeve into close proximity with the underside of the polishing surface, the compressed dry air supplied to the air-bearing platen for supporting the underside of the polishing surface is constrained within a region defined between the retaining ring, the platen and the polishing surface. Thus, the moveable sleeve acts as a barrier to the transient losses at the edge of the platen. 
     When the moveable sleeve acts as a barrier to the transient losses, one skilled in the art will appreciate that the controlled release of the constrained air precludes chattering of the polishing surface. As mentioned above, the release of the air can be regulated by channels included in the top surface of the moveable sleeve of the retaining ring. Alternatively, the pressure created by the fluid flow to the lower sleeve of the retaining ring can regulate the distance the moveable sleeve travels in order to moderate the loss of compressed dry air. While there is a controlled release of the constrained air, it should be appreciated that the losses are significantly reduced as compared to when there is no retaining ring surrounding the platen. The method then moves to operation  142  where the CMP operation is conducted. As the moveable sleeve is raised, the compressed dry air is constrained and transient losses near the edge of the platen are reduced. Therefore, during the CMP operation tighter control over the removal rate near the edge of the wafer being subjected to the CMP operation is provided. 
     In summary, the present invention provides a retaining ring that constrains the compressed dry air within a region between the retaining ring, the platen and the polishing surface and a method for reducing consumption of compressed dry air during CMP operations. The invention has been described herein in terms of several exemplary embodiments. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. The embodiments and preferred features described above should be considered exemplary, with the invention being defined by the appended claims.