Abstract:
A chemical mechanical polishing (CMP) device carrier head ( 54 ) utilizing a pressure pack ( 58 ) for transferring a polishing force (D) to a wafer ( 15 ) being polished. The pressure pack may include a silicon gel material ( 60 ) encapsulated within a sealed urethane casing ( 62 ). The pressure pack provides a desired fluid coupling in the polishing force load path, allowing the carrier head to function as a front side floating reference polishing apparatus. The pressure pack may include a plurality of casings ( 70, 72 ) defining a plurality of chambers ( 66, 68 ) in order to affect the distribution of pressure across an abutting pressure plate ( 56, 65 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to the field of substrate polishing, and more specifically the field of polishing semiconductor wafers, and in particular to an improved carrier for a chemical mechanical polishing (CMP) apparatus.  
           [0002]    It is well known in the field of integrated circuit fabrication to polish a semiconductor wafer to achieve a planar surface on one or both sides of the wafer. One method used to polish such wafers is mechanical polishing, where a surface of the wafer is scoured with an abrasive polishing pad. It is also known to utilize a ID chemical slurry in conjunction with the abrasive polishing pad to increase the material removal rate, and such a process is commonly referred to as chemical mechanical polishing (CMP). Many styles of CMP machines are commercially available from manufactures including Applied Materials, Inc., Ebara, Strausbaugh, LAM Research, and SpeedFam-IPEC International.  
           [0003]    [0003]FIG. 1 illustrates a prior art chemical mechanical polishing machine  10  including a polishing pad  12  disposed on a platen  14  being used to polish a semiconductor wafer  15  secured in a wafer carrier  16 . The carrier  16  includes a driving mechanism  18  for rotating the wafer  15  about a central axis and for oscillating the wafer  15  in a horizontal direction and lifting it in a vertical direction. The CMP machine  10  also includes a driving mechanism  20  for rotating the platen about a central vertical axis. Various CMP machine designs may include one or both such driving mechanisms  18 ,  20  for moving the wafer carrier  16  and wafer  15  with respect to the platen  14  and polishing pad  12 . A slurry delivery system  22  provides the chemical agent  24  used in the CMP process. As is known in the art, CMP machine  10  may include a plurality of carriers  16  and an automatic control system for moving a plurality of semiconductor wafers or other such substrates into and away from the carriers for batch processing.  
           [0004]    [0004]FIG. 2 is a partial cross-sectional view of one type of wafer carrier head that may be used on a CMP machine. Wafer carrier head  26  includes a carrier head frame  28  adapted for connection to a drive apparatus  30 . The drive apparatus  30  is operable to impart rotation and horizontal movement while applying downward force D during a wafer polishing operation. A wafer  15  is secured to the carrier head  26  by a retaining ring  32 , as is known in the art. A pressure plate  34  is disposed between the frame  28  and the wafer  15  and is functional to transfer the downward force D to the wafer  15  to urge the surface to be polished against the polishing pad  12 . It is known that pressure plate  34  may be formed of a metal or metal alloy and may be covered by a felt or polyurethane backing film (not illustrated) to protect the wafer  15 .  
           [0005]    The prior art device of FIG. 2 firmly positions the wafer  15  against the pressure plate  34  during a polishing operation. This type of design is sometimes referred to as back-side reference polishing. The wafer surface that is being polished is urged toward the polishing pad  12  in direct response to the movement of the bottom side face  36  of the pressure plate  34 . Such a system is known to produce uneven material removal across the polished face of wafer  15 , since there is no mechanical flexibility provided to accommodate problems such as a lack of parallelism between the pressure plate bottom side face  36  and the top surface  38  of the polishing pad  12 .  
           [0006]    In order to address the inflexibility of the carrier head  26  of FIG. 2, it is known to provide a floating or front-side reference carrier head  40 , as is illustrated in FIG. 3. In this type of design, downward force D is transmitted to wafer  15  by a flexible membrane  42  that is forced against the wafer  15  by a pressurized fluid such as pressurized air  44  contained within a volume  46  within wafer carrier head frame  28 . A felt or urethane film  43  may be disposed between the wafer  15  and the flexible membrane  42 . The pressurized air is provided and vented via a delivery system  48  designed to be compatible with the driving mechanism  30 . A plate  50  having one or more holes  52  formed there through is provided behind the flexible membrane  42 . However, when pressurized by pressurized air  44 , flexible membrane  42  “floats” away from the plate  50 . This allows an even pressure to be applied across the wafer in spite of minor mechanical misalignments and results in a more consistent material removal rate across the polished face of the wafer  15 .  
           [0007]    Although providing superior polishing performance, a front side reference CMP machine may be more costly to manufacture due to the necessity for incorporating a pressurized air delivery system  48  including associated electrical and pneumatic controls. Such systems may also be more expensive to operate, since even a small hole in the flexible membrane  42  will cause unacceptable polishing performance and will necessitate a maintenance outage to replace the membrane  42 . Furthermore, while it would be desirable to retrofit older back side reference  15  machines with floating head capability, it is generally not practical to do so because of the difficulty of routing an air supply system  48  through an existing driving mechanism  30 .  
         SUMMARY OF THE INVENTION  
         [0008]    Thus there is a particular need for a chemical mechanical polishing machine that offers the performance advantages of a front side reference machine without the expense and maintenance disadvantages of prior art machines. It is also desired to be able to upgrade an existing back side reference CMP machine to provide performance equivalent to a floating reference CMP machine.  
           [0009]    Accordingly, an improved carrier head for a chemical mechanical polishing apparatus is described herein as including: a frame adapted for connection to a drive apparatus; a pressure plate adapted for contact with a wafer; and a pressure pack disposed between the frame and the pressure plate for transferring a force there between. The pressure pack provides a desired degree of fluid coupling between the frame and the wafer to ensure that an even pressure is applied across the face of the wafer being polished. The pressure pack may be formed as a gelatinous material encapsulated within a sealed casing. The pressure pack may further be formed of a plurality of chambers to provide a varied force profile across the wafer surface, such as to provide a different down force at the edge of the wafer than at the center of the wafer in response to a single down force applied by the carrier driving mechanism. In order to reduce the risk of damage resulting from a leaking pressure pack, the pack may be formed to have a double casing surrounding an interior fluid.  
           [0010]    An improved wafer polishing apparatus is also described herein as incorporating such an improved carrier head.  
           [0011]    A method of modifying a back side reference polishing apparatus to function as a floating reference polishing device is also described herein. Such a back side polishing apparatus may have a polishing pad affixed to a platen, a wafer carrier for holding a wafer to be polished against the polishing pad, and a driving mechanism for polishing the wafer by moving the wafer carrier with respect to the platen, with the wafer carrier further including a carrier head frame disposed against a pressure plate for urging the wafer against the polishing pad. The method of modifying such a device is described as including the steps of removing the pressure plate; and replacing the pressure plate with a replacement pressure plate and a pressure pack, the pressure pack being disposed between the replacement pressure plate and the carrier head frame for transferring a force there between for urging the wafer against the polishing pad. The pressure pack provides a desired degree of fluid coupling to ensure that an even pressure is applied across the face of the wafer being polished without the necessity for adding a pressurized air supply system to the back side reference device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which:  
         [0013]    [0013]FIG. 1 illustrates a prior art polishing device.  
         [0014]    [0014]FIG. 2 is a partial cross-sectional view of a prior art back side reference wafer carrier head.  
         [0015]    [0015]FIG. 3 is a partial cross-sectional view of a prior art front side reference wafer carrier head.  
         [0016]    [0016]FIG. 4 is a partial cross-sectional view of an improved wafer carrier head incorporating a pressure pack for transmitting force to a wafer being polished against a polishing pad.  
         [0017]    [0017]FIG. 5 is a cross-sectional view of a multi-chamber pressure pack. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    [0018]FIG. 4 illustrates an improved wafer carrier head  54  that may be used in a chemical mechanical polishing machine such as the machine  10  of FIG. 1. The carrier head  54  includes a frame  28  adapted to be moved relative to a platen  14  and polishing pad  12  by a driving mechanism  30 . The carrier head  54  includes a retaining ring  32  and is adapted to position a semiconductor wafer  15  for polishing against polishing pad  12 . A force D is applied through frame  28  to urge the wafer  15  against the polishing pad  12  during the polishing operation. A pressure plate  56  is disposed between the frame  28  and the wafer  15  for transmitting polishing force D there between. As is known in the art, pressure plate  56  may be fabricated from a hard substance, such as metal or ceramic, but may include a covering of a softer substance, such as a felt pad or polyurethane in order to protect wafer  15  from scratching. In one embodiment, the hard material may be omitted and only a softer substance may be used to transfer the force to the wafer. The downward force D is transmitted to pressure plate  56  from the carrier frame  28  through a pressure pack  58 . Pressure pack  58  provides a desired degree of fluid coupling between the frame  28  and the wafer  15  to ensure that an even force is applied across pressure plate  56  in spite of any slight misalignment between the Gus abutting surfaces of the frame  28 , pressure plate  56  and wafer  15 . Adequate space is made available between frame  28  and pressure plate  56  to permit the pressure plate  28  to more about freely to accommodate the interactions between wafer  15  and pressure pack  58 . Pressure pack  58  includes a volume of fluid  60  encapsulated within a sealed outer casing  62 . The outer casing  62  may be a high quality urethane and the fluid  60  may be water, silicone or other viscous material. The flexible, fluid properties of pressure pack  58  allow the pressure plate  56  to “float” relative to the frame  28 , thus ensuring that carrier  54  functions as a front side reference polishing device.  
         [0019]    A back side reference polishing apparatus such as wafer carrier head  26  of FIG. 2 may be modified to function as a floating reference polishing device by incorporating a pressure pack  58  into an existing carrier frame  28 . In order to provide space for pressure pack  58 , the prior art pressure plate  34  of carrier  26  must be removed. Pressure pack  58  and a replacement pressure plate  56  are then installed into the carrier head frame  28  in order to provide the desired fluid coupling in the load path between driving mechanism  30  and wafer  15 . Depending upon the particular carrier head being modified, other parts may have to be replaced or modified in order to accommodate the pressure pack  58 . Importantly, however, there is no need to provide a supply of pressurized air behind a membrane, as is described in the prior art carrier head  40  of FIG. 3. Fluid  60  is sealed within casing  62  and therefore is available for repeated polishing operations without the need to be replenished.  
         [0020]    Contamination is a constant concern during semiconductor fabrication processes. It is important that pressure pack  58  have a low incidence of failure, and that in the event of a failure, the spread of contaminants is minimized. Toward that end, it is preferred that fluid  60  be a highly viscous material, such as a gelatinous material, so that in the event of a failure of casing  62 , the spread of the fluid  60  will be minimized for easier clean up. It is also possible to manufacture pressure pack  58  from a highly elastic solid material that does not have liquid properties, such as a material having a consistency such as soft clay. Furthermore, materials of construction for pressure pack  58  should be selected to avoid any highly harmful or hard to clean materials.  
         [0021]    [0021]FIG. 5 illustrates another embodiment of a pressure pack  64  for use in a wafer polishing apparatus. Pressure pack  64  contains two chambers, a first chamber  66  in a central portion of the pressure pack  64 , and a second chamber  68  forming a donut-shaped volume around the perimeter of the pressure pack  64 . One may appreciate that any number of individual chambers may be provided. The chambers  66 ,  68  of pressure pack  64  are defined by two separate casings  70 ,  72  respectively. Advantageously, the fluid contained in first chamber  68  is encapsulated by both casing  72  and casing  70 , so that a leak in either one of the casings will not result in contamination of the associated machinery. Such a double casing design may also be incorporated on a single chamber pressure pack. An inner casing may be selected for its compatibility with an encapsulated fluid and an outer casing may be selected for its toughness or elasticity properties.  
         [0022]    Advantageously, the physical properties of the two chambers may be selected to be different so that the distribution of a force across an abutting pressure plate may be made non-equal while still maintaining a fluid coupling. Such a dual chamber pressure pack  64  may advantageously be used with a two-part pressure plate  65  having corresponding center  67  and edge  69  portions. In order to achieve a desired pressure loading on an abutting pressure plate, the fluid pressure in the respective chambers  66 ,  68  may be different in a relaxed state. For example, by filling fluid in chamber  66  to a higher pressure than fluid in chamber  68 , the pressure transferred by the pressure pack  64  between abutting frame and pressure plate members will be higher at the center portion of the pressure pack  64  than at the periphery portion of the pressure pack  64 . Similarly, the fluid material contained within the two chambers  66 ,  68  may be selected to be different to achieve a desired mechanical effect. For example, the two fluids may have a different viscosity. Furthermore, the material of casing  70  may be different than the material of casing  72  in order to provide a desired mechanical effect and/or to improve the compatibility with the encapsulated fluids.  
         [0023]    While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.