Abstract:
A substrate carrier having a deformable surface for receiving a substrate. Non annular pressure application zones apply pressure to the deformable surface, and addressable transducers within the pressure application zones receive a signal and applying a selectable amount of pressure in response to the signal. In this manner, the amount of pressure provided by the substrate carrier differs from one portion of the substrate to another in a selectable manner. Thus, the pressure applied to the substrate can be tailored to the non uniform thickness of the layer that is being thinned. In other words, portions of the substrate where the layer is thicker can be pressed upon with a greater force by the substrate carrier, thus urging the substrate more forcefully into the polishing pad in those portions, and thereby removing material from the layer at a greater rate of speed in those portions. Similarly, portions of the substrate where the layer is thinner can be pressed upon with a lesser force by the substrate carrier, thus urging the substrate less forcefully into the polishing pad in those portions, and thereby removing material from the layer at a reduced rate of speed in those portions. In this manner, substrates having layers of non uniform thickness can be processed such that the resulting thickness of the thinned layer is extremely uniform.

Description:
FIELD  
         [0001]    This invention relates to the field of integrated circuit processing. More particularly, this invention relates to tooling used for handling integrated circuits in wafer form.  
         BACKGROUND  
         [0002]    Integrated circuits such as those formed in silicon, germanium, or III-IV compounds are typically fabricated as a batch of devices on a common substrate, commonly referred to as a wafer. Wafers typically vary in size from about six inches to about twelve inches in diameter. Although wafers can be smaller or larger than this, most wafers currently used for the production of integrated circuits fall within this range of sizes.  
           [0003]    As the wafers are processed, the various layers which form the elements of the integrated circuits are created. Such processes included, for example, deposition processes where a layer of material is deposited across the surface of the wafer, etch processes where portions of a deposited layer or other material are removed, and doping processes where an atomic dopant is impregnated through the surface of the wafer and buried within the substrate. It is appreciated that there are other processes to which the wafer is also subjected, and many variations on these general process types.  
           [0004]    Most processes tend to have some level of spatial variation, in that they produce somewhat non uniform results as measured across the surface of the wafer. For example, a non uniform etch process may remove more material from an upper layer in first portions of the substrate, and less material from the upper layer in second portions of the substrate. Thus, during such an etching process, the etching of the upper layer in the first portions will achieve the desired end point before the etching of the upper layer in the second portions does. Similarly, a non uniform deposition process deposits material faster in first portions of the substrate, and deposits material slower in second portions of the substrate. Thus, the material is deposited to the desired end point in the first portions of the substrate sooner than it is in the second portions of the substrate.  
           [0005]    Many different methods are used to reduce the degree of non uniformity of the various processes used to fabricate integrated circuits. Unfortunately, it is an elusive goal to completely remove non uniformity from a given process. Adding to the problem is the fact that non uniformity can multiply through the process sequence by which the integrated circuits are fabricated. For example, a non uniformity of a first layer can be further compounded by a non uniformity of a second layer. Similarly, the non uniformity of a first process can be further compounded by a non uniformity of a second process.  
           [0006]    As a more specific example, one method by which a layer of material is thinned on a substrate is called chemical mechanical polishing. During chemical mechanical polishing, the back of the substrate is held with a substrate carrier and the face of the substrate is brought into contact with a rotating polishing pad. A slurry, typically containing both chemical and physical etchants, is applied and the surface of the substrate is eroded as the surface of the substrate and the polishing pad are moved relative to one another. Most preferably the layer of material on the surface of the substrate is removed at an even rate across the surface of the substrate so that a uniform thickness in the layer of material is simultaneously achieved across the entire surface of the substrate.  
           [0007]    Unfortunately, non uniformities in the chemical mechanical polishing process tend to work against the goal of achieving a thinned layer having a perfectly uniform thickness. Further compounding the problem is the fact that the layer of material as formed or deposited is typically non uniform to begin with, in that the thickness of the layer to be thinned was not uniform at the beginning of the process. In such a case, even when the chemical mechanical polishing process removes material at a uniform rate across the entire surface of the substrate, the resultant layer will not have a uniform thickness at the end of the process because it did not start the chemical mechanical polishing process with a uniform thickness.  
           [0008]    Thus, there is a continual need for methods and equipment that increase the uniformity of processing across the surface of a substrate as integrated circuits are fabricated.  
         SUMMARY  
         [0009]    The above and other needs are met by a substrate carrier according to a preferred embodiment of the present invention. The substrate carrier has a deformable surface for receiving a substrate. Non annular pressure application zones apply pressure to the deformable surface, and addressable transducers within the pressure application zones receive a signal and applying a selectable amount of pressure in response to the signal.  
           [0010]    In this manner, the amount of pressure provided by the substrate carrier differs from one portion of the substrate to another in a selectable manner. Thus, the pressure applied to the substrate can be tailored to the non uniform thickness of the layer that is being thinned. In other words, portions of the substrate where the layer is thicker can be pressed upon with a greater force by the substrate carrier, thus urging the substrate more forcefully into the polishing pad in those portions, and thereby removing material from the layer at a greater rate of speed in those portions. Similarly, portions of the substrate where the layer is thinner can be pressed upon with a lesser force by the substrate carrier, thus urging the substrate less forcefully into the polishing pad in those portions, and thereby removing material from the layer at a reduced rate of speed in those portions. In this manner, substrates having layers of non uniform thickness can be processed such that the resulting thickness of the thinned layer is extremely uniform.  
           [0011]    In various preferred embodiments of the invention, the deformable surface is either a metal fabric or an elastomer, or a combination of the two. The non annular pressure application zones are preferably disposed in a grid pattern. Most preferably, each of the non annular pressure application zones is between about one square millimeter and about six hundred square millimeters in size. In alternate embodiments the addressable transducers are either solenoids or piezoelectric transducers. Most preferably, the addressable transducers comprise digitally addressable devices.  
           [0012]    In an especially preferred embodiment, pressure transducers are disposed within each of the pressure application zones. The pressure transducers sense pressures applied from the substrate to the substrate carrier during processing, and send pressure signals to a controller. The controller receives the pressure signals and individually and selectively controls the addressable transducers based at least in part on the pressure signals. Thus, in this embodiment there is real time feedback to the system for dynamically adjusting the polishing rate in different portions of the substrate, thereby continuously compensating for process variations. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein:  
         [0014]    [0014]FIG. 1 is a functional block diagram representing a chemical mechanical polisher holding a substrate with a substrate carrier according to a preferred embodiment of the present invention,  
         [0015]    [0015]FIG. 2 is a top plan view of the substrate carrier according to a preferred embodiment of the present invention, and  
         [0016]    [0016]FIG. 3 is a topographical depiction of a substrate with a layer that has a non uniform thickness profile. 
     
    
     DETAILED DESCRIPTION  
       [0017]    Referring now to FIG. 1, there is depicted a functional block diagram of a chemical mechanical polisher  10  holding a substrate  16  with a substrate carrier  12  according to a preferred embodiment of the present invention. The chemical mechanical polisher  10  thins a layer on the surface of the substrate  16  as described above, by holding the surface of the substrate  16  against a rotating polishing pad  26 . Both the mechanical action of the pad and the chemical and mechanical action of the slurry that is introduced cause the material at the surface of the substrate  16  to be eroded from the substrate  16 , thereby thinning the layer. As mentioned above, it is desirable to remove as much non uniformity from the process as possible, so as to produce a layer that has as uniform a thickness as possible.  
         [0018]    To this end, the substrate carrier  12  includes a deformable surface  14  that receives the substrate  16 . The deformable surface  14  may also include means whereby the substrate  16  is retained to the substrate carrier  12 , such as electrostatic means, or such retaining means may be otherwise provided, such as clamps that engage a portion of the substrate  16 . The deformable surface  14  is preferably formed of a durable material that is both somewhat resistant to the chemical and physical environment of the chemical mechanical polisher  10 .  
         [0019]    For example, the deformable surface  14  is preferably resistant to the chemicals in the slurry, and is also reasonably resilient to the abrasive particles in the slurry and the pressures that are exerted between it and the substrate  16 , as described in more detail hereafter. Furthermore, the deformable surface  14  is preferably formed of a material that is not a source of contamination to the slurry or the substrate  16 , or to the devices that are formed on the substrate  16 . In a most preferred embodiment, the deformable surface  14  is formed of a metal fabric or an elastomer.  
         [0020]    The deformable surface  14  of the substrate carrier  12  is preferably logically divided into pressure application zones  28 , as depicted in FIG. 2, which depicts the substrate carrier  12  of FIG. 1, such as from a view looking up at the deformable surface  14 . The lines on the deformable surface  14  are representational of the logical divisions between different pressure application zones  28 , and would preferably not be visible in an actual implementation of the substrate carrier  12 . In other words, the orthogonal lines are present so as to better convey the concept of the pressure application zones  28 , and do not represent a physical aspect of the substrate carrier  12 .  
         [0021]    The pressure application zones  28  represent a position on the deformable surface  14  of the substrate carrier  12  where pressure can be individually and selectively applied by the substrate carrier  12  through the deformable surface  14  and against the substrate  16 . This pressure is most preferably applied such as by moving the deformable surface  14  toward the substrate  16  within a given one of the pressure application zones  28  relative to other neighboring pressure application zones  28 . Thus, if one pressure application zone  28  sticks out further toward the substrate  16  than its eight nearest neighbor pressure application zones  28 , for example, it is said that that one pressure application zone  28  is exerting a greater pressure on the substrate  16  than are the eight nearest neighbor pressure application zones  28 .  
         [0022]    The pressure application zones  28  are not disposed in an annular pattern, such as two, three, four, or more concentric rings in a bulls-eye configuration, but are rather preferably disposed in a grid pattern as depicted in FIG. 2. In this manner, the patterns of pressure that can be applied by the pressure application zones  28  are far more configurable than could be had with an annular pattern of concentric rings. Most preferably the pressure application zones  28  each have a surface area of between about one square millimeter and about six hundred square millimeters in size.  
         [0023]    The size that is selected for each pressure application zone  28  represents a balance between different competing parameters. For example, for very large substrates  16 , such as those that are about three hundred millimeters in diameter and larger, pressure application zones  28  on the larger end of the range given above may be most appropriate, as a finer degree of control may not be needed. On the other hand, even with such large substrates  16 , if there is a large variation in the thickness of the layer to be thinned during the chemical mechanical polishing process, then smaller pressure application zones  28  may be desired so as to provide a finer degree of control over the thinning of the layer.  
         [0024]    Similarly, for very small substrates  16 , such as those that are about seventy-five millimeters in diameter and smaller, pressure application zones  28  on the smaller end of the range given above may be most appropriate, as they would tend to provide more control over the thinning process within the smaller substrate  16 . However, even with such small substrates  16 , if there is only a very small variation in the thickness of the layer to be thinned during the chemical mechanical polishing process, then larger pressure application zones  28  may be desired, because a finer degree of control over the thinning of the layer may not be required.  
         [0025]    The pressure that may be so applied in each of the pressure application zones  28  is preferably provided by addressable transducers  18  that are disposed within each of the pressure application zones  28 . In one embodiment the addressable transducers  18  are solenoids, and in an alternate embodiment the addressable transducers  18  are piezoelectric transducers. Preferably, the transducers  18  are digitally and individually selectable, as described in more detail hereafter.  
         [0026]    To continue briefly a topic that was introduced above, the size of the pressure application zones  28  may also be determined at least in part upon the type of transducer  18  that is employed within each pressure application zone  28 , in that some transducers  18  may be smaller than others and may fit more readily within smaller pressure application zones  28 , and other transducers  18  may be larger than others and may fit more readily within larger pressure application zones  28 . The size of the pressure application zones  28  may also depend at least in part upon the size and presence of other elements that are to be disposed within each of the pressure application zones  28 , as described in more detail below. It is appreciated that not all of the pressure application zones  28  need be of the same size or shape.  
         [0027]    The pressure applied by the addressable transducers  18  is preferably controlled by a controller  24 , as depicted in FIG. 1, which may reside in either the main part of the chemical mechanical polisher  10  or within the substrate carrier  12 , and which communicates with the addressable transducers  18  via line  22 . One reason why it is preferred that the addressable transducers  18  be digitally selectable is so that fewer lines  22  are required to individually select the addressable transducers  18 , and thus the lines  22  do not require much room in the arm  36 . This is beneficial because it is desirable to not increase the size of the arm  36  to accommodate a large bundle of lines  22 , and also because the substrate carrier  12  preferably rotates on the end of the arm  26 , and connections for many lines  22  through the rotating connections would be expensive and complicated.  
         [0028]    Within each pressure application zone  28  there is also preferably disposed a pressure transducer  20 , which senses the pressure applied from the substrate  16  to the substrate carrier  12  during processing. To explain more fully, as the substrate  16  is pressed against the polishing pad  26  by the substrate carrier  12  during processing, the substrate  16  in effect presses back against the substrate carrier  12 . The pressure transducers  20  sense the force of the pressure of the substrate  16  against the deformable surface  14 , and generate signals representing the force of that pressure, which are preferably sent back to the controller  24  along the lines  22 . The benefit of this preferred embodiment is discussed in more detail hereafter.  
         [0029]    With reference now to FIG. 3, there is depicted a topographical representation of the surface of a substrate  16 , showing first portions  30 , second portions  32 , and third portions  34 . The pattern of the various portions of the substrate  16  is representative only, and is not intended to limit the invention in any way. The pattern represents a varying thickness of a layer that is deposited or formed on the surface of the substrate  16 . For example, portion  30  could be those sections of the layer that have a thickness within a first thickness range, portion  32  could be those sections of the layer that have a thickness within a second thickness range, and portion  34  could be those sections of the layer that have a thickness within a third thickness range.  
         [0030]    For the purposes of example herein, it is specified that portion  30  is the section of the substrate  16  where the layer is the thinnest, portion  34  is the section of the substrate  16  where the layer is the thickest, and portion  32  is the section of the substrate  16  where the layer has an intermediate thickness. Such deposition patterns as represented in FIG. 3 may be caused, for example, by a non uniform gas flow within a deposition chamber, a non uniform heating of the substrate  16  during deposition, a non uniform energy field during deposition, or a combination of these or other variables.  
         [0031]    Regardless of the cause or the exact pattern of the non uniform layer thickness, it is desirable to thin the layer to a uniform thickness across the surface of the substrate  16  during the chemical mechanical polishing process. The various embodiments of the invention as described and claimed herein are particularly adapted to achieving that goal. For example, if the pattern of non uniformity, as exemplified in FIG. 3, is consistent from substrate  16  to substrate  16 , or in other words the pattern of non uniformity does not change much from substrate  16  to substrate  16 , then the substrate carrier  12  can be programmed by the controller  24  so that the addressable transducers  18  press harder against the deformable surface  14  in those pressure application zones  28  that underlie the portions  34  where the layer is the thickest. In this manner, because the substrate  16  is preferably somewhat flexible, the portions  34  are pressed with a greater force into the polishing pad  26  during the chemical mechanical polishing process, and thus tend to be eroded at a greater rate than other portions of the substrate  16 .  
         [0032]    Similarly, the substrate carrier  12  can be programmed by the controller  24  so that the addressable transducers  18  press with slightly less force against the deformable surface  14  in those pressure application zones  28  that underlie the portions  32  where the layer is of intermediate thickness. In this manner, the layer in the portions  32  is eroded somewhat less vigorously during the chemical mechanical polishing process than it is in the portions  34 . Thus, over the length of time that the chemical mechanical polishing process is performed, the layer can be thinned to the same thickness in both the portions  32  and the portions  34 .  
         [0033]    In a similar vein, the substrate carrier  12  can be programmed by the controller  24  so that the addressable transducers  18  press with an even lesser degree of force against the deformable surface  14  in those pressure application zones  28  that underlie the portions  30  where the layer is the thinnest. Thus, at the end of the process, the thickness of the layer can be the same thickness in all of the different portions of the substrate  16 .  
         [0034]    Although the above configuration is useful, it is appreciated that there are some difficulties in its use. For example, although the non uniformity of the thickness of the layer across the surface of the substrates  16  may be quite similar from substrate  16  to substrate  16 , it is not often exactly the same, and thus small variations in the programming of the substrate carrier  12  would be preferred. These small programming variations may be time consuming and thus expensive to accomplish for each substrate  16 . In addition, learning exactly how hard a given addressable transducer  18  should press against the substrate  16  in order to compensate for a given layer thickness is also a time consuming proposition. However, even those these issue may consume a certain amount of time and effort, they can be accomplished and the substrates  16  can be processed with a greater degree of uniformity.  
         [0035]    However, with the addition of the pressure transducers  20 , many of these issues can be reduced in scope. When the substrate  16  is held against the polishing pad  26  by the substrate carrier  12 , those portions of the substrate  16  where the layer is relatively thicker tend to contact the polishing pad  26  before those portions of the substrate  16  where the layer is relatively thinner. Thus, as the substrate  16  is pressed against the polishing pad  26 , the substrate  16  tends to exert a greater pressure against the substrate carrier  12  in those portions of the substrate  16  where the layer is relatively thicker.  
         [0036]    As a specific example, and with reference to FIG. 3, portions  34  of the substrate  16  tend to press against the deformable surface  14  of the substrate carrier  12  with a greater force than do portions  32 , which tend to press against the deformable surface  14  of the substrate carrier  12  with a greater force than do portions  30 . Thus, the pressure transducers  20  can detect from moment to moment those portions of the substrate  16  that have a relatively thicker layer remaining. The pressure transducers  20  preferably generate signals in response to such sensed pressure, which are sent back to the controller  24  as described above. The controller  24  then uses those signals as a feed back to determine which of the addressable transducers  18  should be instructed to press harder against the substrate  16 , so as to cause those relatively thicker portions of the substrate  16  to erode faster. In this manner, a dynamic feed back system is instituted which can automatically adapt to the non uniformities of a starting substrate  16 , and also to the non uniformities that may be introduced during the chemical mechanical polishing process itself.  
         [0037]    Most preferably there is a single pressure transducer  20  associated with each single addressable transducer  18  within each pressure application zone  28 . However, it is appreciated that many other configurations are possible. For example, there may be several addressable transducers  18  for each pressure transducer  20 . In this manner, the additional addressable transducers  18  can be used to interpolate the pressure between adjacent pressure transducers  20 . Likewise, there may be several pressure transducers  20  for each addressable transducer  18 , which may be used to apply a pressure based upon an average of the pressure readings from the pressure transducers  20 , for example.  
         [0038]    The foregoing embodiments of this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as is suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.