Abstract:
A system comprises a collet is configured for holding a die surface against the bearing surface and for simultaneously pushing outward on the center region of the die so held.

Description:
[0001]    This application is a divisional of application Ser. No. 12/844,937 filed Jul. 28, 2010, which is a divisional of application Ser. No. 11/690,808 filed Mar. 24, 2007, the contents of both are herein incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to electronic semiconductor devices and manufacturing. More particularly, the invention relates to semiconductor device manufacturing and packaging and to die handling apparatus and related methods. 
       BACKGROUND OF THE INVENTION 
       [0003]    In conventional semiconductor device assembly, it is known to use adhesive to permanently mount a semiconductor die to a mounting pad or substrate. Typical adhesive die attach processes use curable adhesive, such as epoxy or polyimide, as die attach material to affix the die to a die pad, leadframe, substrate, or socket, for convenience referred to herein generally as the die pad. It is common in the art to dispense die attach material in a controlled amount on a die pad. Die handling equipment used for die attach processes typically employs a pick-and-place tool to lift a die from a wafer tape or other holding mechanism and place it on a die pad. The portion of the die handling equipment that actually makes contact with the die is referred to as a collet. The die is placed on the collet, either by surface contact alone or with assistance from a mechanical ejector pin guiding the die onto the collet. A vacuum force exerted within the collet holds the die in the collet while the tool moves it into the appropriate position for placement on the pre-applied adhesive on the die pad. 
         [0004]    Die handling presents technical challenges. Particularly for thin dice, which are becoming increasingly common in the arts, handling during die attach requires great care to avoid cracks or other damage. Some examples of die attach-related failure mechanisms known in the art include backside tool marks, scratches, or microcracks, which can eventually lead to die cracking. Thinner dice are in particular danger from microcracks, which can result from excessive flexing of the die during handling. The contact surfaces of die attach collets are sometimes made from relatively soft plastic or elastomeric materials instead of metal in an effort to avoid causing mechanical damage on the die surface. The practice of using a vacuum to hold the die in the collet is another example of efforts to avoid inflicting damage to fragile dice. The use of a prior art vacuum collet, however, tends to cause a thin die to flex forming a concavity during die placement, which can cause further problems. Damage to the surface of the die can also occur, particularly in the central region, due to contact with the collet. 
         [0005]    The amount and distribution of die attach material between the die and the die pad can be crucial to the secure attachment of the die to the die pad and to the long term reliability of the completed assembly. Achieving the appropriate depth and uniform distribution of the die attach adhesive layer, also called the bond line, is a significant challenge. If the bond line is too thin, the bond may be insufficient to hold the die to the die pad. If the bond line is too thick, curing may be inhibited or prolonged, the bond may tend to weaken over time, thermal performance may suffer, or other problems may result. An uneven bond line resulting from non-uniform adhesive distribution can result in similar problems or in a combination of such problems. One of the most threatening problems a non-uniform distribution of adhesive can create is the formation of voids in the adhesive between the die and the die pad. Voids can lead to failures due to insufficient adhesive coverage or thermally induced stresses, for example. The formation of a concavity in the die surface, caused by the flexing of a thin die placed in a vacuum collet common in the arts, can induce these and other problems. 
         [0006]    Generally, in addition to the formation of the bond line, a quantity of adhesive is pressed from between the die and the bond pad during die placement. The formation of adhesive that builds from the bond pad to the edges of the die is known as the die attach fillet. The formation of the fillet can be adversely affected by the excess, lack, or non-uniform distribution of adhesive in the formation of the bond line. Excessive die attach fillet may lead to die attach contamination of the die surface. Too little fillet may reduce the strength of the attachment and lead to eventual problems such as die lifting or die cracking. The formation of a proper fillet may be impeded by excessive, inconsistent, or unpredictable die flexion during placement of the die on the adhesive by a die handling collet. 
         [0007]    Due to these and other problems, it would be useful and advantageous to provide semiconductor die handling apparatus and manufacturing methods with improved die handling capabilities, particularly for use with relatively thin and delicate dice. 
       SUMMARY OF THE INVENTION 
       [0008]    In carrying out the principles of the present invention, using methods and equipment compatible with established manufacturing processes, improved semiconductor die handling collets contribute useful advantages to the art. The invention provides apparatus for the handling and placement of semiconductor dice and superior die attach techniques, resulting in improved semiconductor device assemblies. 
         [0009]    According to one aspect of the invention, a method for attaching a semiconductor die to a die pad includes steps for positioning a die on a bearing surface of a collet and retaining the die on the bearing surface of the collet using a vacuum force. A pushing force is exerted on the die adjacent to the applied vacuum force. The pushing force is used to oppose flexion of the die in the direction of the vacuum force. In further steps, the die is placed on the die pad, with die attach adhesive interposed between the die and the die pad, the die is then released from the collet. 
         [0010]    According to another aspect of the invention, in a method for attaching a semiconductor die to a die pad using a preferred embodiment of a collet, a pushing force is applied to the die in order to bow the central region of the die toward the die pad. 
         [0011]    According to another aspect of the invention, a collet for handling a semiconductor die includes a body having a bearing surface for receiving the die. A chamber in the body has an open side bounded by the bearing surface, with a vacuum groove included in the bearing surface for holding a die against the bearing surface. A port is provided for transmitting an expelled gas to the chamber. The parts are arranged so that the vacuum force is adapted for holding the die surface against the bearing surface and the expelled gas is adapted for pushing the center of the die out from the interior of the chamber. 
         [0012]    According to yet another aspect of the invention, in a preferred embodiment, a collet is provided wherein an expelled gas may be applied for pushing outward on the center of a die held therein such that the die extends outward beyond the plane of the bearing surface. 
         [0013]    According to another aspect of the invention, a preferred embodiment of a collet includes a flexible skin attached to the body of the collet and situated for supporting a die held by the collet during handling. 
         [0014]    According to still another aspect of the invention, a preferred embodiment of a semiconductor die attach system includes a collet having a die bearing surface. The collet is configured for retaining the die on the bearing surface using a vacuum force and also for applying a pushing force to an adjacent portion of the die in opposition to inward flexion of the die in the direction of the vacuum force. The system also includes a handling tool for moving the collet to a die pad, and placing the die on the die pad. 
         [0015]    According to another aspect of the invention, a collet system features an arrangement whereby a pushing force may be applied to cause outward flexion of a die held by the collet such that a handling tool may bring the center region of the die into contact with a die pad in advance of the periphery. 
         [0016]    According to another aspect of the invention, a collet system includes provisions for using pressurized gas for applying a pushing force to the center region of a die held to the collet by a vacuum force. 
         [0017]    According to still another aspect of the invention, a semiconductor device assembly of the invention includes a flexed die with a cured bond line formed with the central region of the die thinner than the periphery. 
         [0018]    The invention has advantages including but not limited to providing methods, apparatus, and systems offering improvements in die handling capabilities useful in the manufacture of semiconductor device packages. These and other features, advantages, and benefits of the present invention can be understood by one of ordinary skill in the arts upon careful consideration of the detailed description of representative embodiments of the invention in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention will be more clearly understood from consideration of the following detailed description and drawings in which: 
           [0020]      FIG. 1  is a bottom view of a die handling collet according to an example of a preferred embodiment of the invention; 
           [0021]      FIG. 2A  is a cutaway side view of the die handling collet according to the example of a preferred embodiment of the invention of  FIG. 1  shown in the context of a die attach system and method; 
           [0022]      FIG. 2B  is a cutaway side view of the die handling collet in a continuation of the example of a preferred embodiment of the invention of  FIG. 2A  shown in the context of a die attach system and method; 
           [0023]      FIG. 2C  is a cutaway side view of the die handling collet in a continuation of the example of a preferred embodiment of the invention of  FIGS. 2A and 2B  shown in the context of a die attach system and method; 
           [0024]      FIG. 3  is a bottom view of a die handling collet according to an example of an alternative preferred embodiment of the invention; 
           [0025]      FIG. 4A  is a cutaway side view of the die handling collet according to the example of a preferred embodiment of the invention of  FIG. 3  shown in the context of a die attach system and method; 
           [0026]      FIG. 4B  is a cutaway side view of the die handling collet in a continuation of the example of a preferred embodiment of the invention of  FIG. 4A  shown in the context of a die attach system and method; and 
           [0027]      FIG. 4C  is a cutaway side view of the die handling collet in a continuation of the example of a preferred embodiment of the invention of  FIGS. 4A and 4B  shown in the context of a die attach system and method for implementing a preferred embodiment of a semiconductor device assembly of the invention. 
       
    
    
       [0028]    References in the detailed description correspond to like references in the various drawings unless otherwise noted. Descriptive and directional terms used in the written description such as first, second, top, bottom, upper, lower, side, and so forth, refer to the drawings themselves as laid out on the paper and not to physical limitations of the invention unless specifically noted. The drawings are not to scale, and some features of embodiments shown and discussed are simplified or amplified for illustrating the principles, features, and advantages of the invention. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    In general, the invention provides a die handling collet and related systems and methods for improved die handling in semiconductor device manufacturing processes, particularly die attach processes. Referring primarily to  FIG. 1  and  FIG. 2A , a bottom view ( FIG. 1 ) and cutaway side view ( FIG. 2A ) of a collet  10  according to a preferred embodiment of the invention is described. A body  12 , preferably made from plastic, metal, or other suitably rigid material, is capable of receiving a die  14  ( FIG. 2A ). A vacuum groove  16  is provided at the edge of the body  12 . The vacuum groove  16  is incorporated into a die-bearing surface  18  of the body  12 , preferably entirely around the periphery. The vacuum groove  16  is in communication with vacuum ports  20  for transmitting a vacuum force, indicated by arrow  22 , generated by a suitable mechanism such as a pump (not shown). The vacuum groove  16  preferably distributes the vacuum force around the periphery of the die-bearing surface  18  for holding a die  14  during handling and placement. A chamber  24  is incorporated within the body  12  and is preferably encompassed by the die-bearing surface  18 . One side of the chamber  24  is open such that a die  14  placed on the bearing surface  18  completes the enclosure. Within the chamber  24 , a port  26  is provided for expelling pressurized gas, preferably air, indicated by arrow  28 . The expelled gas  28  pressurizes the chamber  24 , exerting a pushing force on the adjacent surface of the die  14 . The pushing force preferably opposes the die flexion which tends to occur due to the application of the vacuum force  22 , preventing or reducing the temporary formation of a concavity on the outer surface of the die  14  due to flexing. Preferably, during die attach, the chamber  24  is sufficiently pressurized to cause the die  14  to bow outward slightly in a position convex to the adjacent die pad  32  or intervening die attach adhesive  30  ( FIG. 2A ). 
         [0030]    Now referring primarily to  FIG. 2B , the collet  10  is shown in the context of further steps in a die attach method according to preferred embodiments of the invention. As illustrated, using the preferred embodiment of the collet  10  shown and described above, the expelled air  28  within the chamber  24  is preferably used to flex the die  14  in order to present a convex surface to the die attach material  30  pre-applied to the die pad  32 . The convex surface of the die  14 , as indicated by arrows  34 , tends to expel air from between the die  14  and die attach adhesive  30  during the ultimate placement of the die, reducing the frequency and magnitude of void formation. Although the use of a convex surface is preferred, in an alternative embodiment, the die may be flexed by the expelled air  28  by an amount adapted to counter any inward flexion caused by the vacuum  22 , and calculated to prevent the outward flexion of the die  14 . This implementation may be preferred for example with particularly delicate dice, preventing or attenuating flexion and presenting a substantially flat die surface to the die pad  32  and the intervening die attach adhesive  30 , promoting a uniform thickness of die attach material  30 . It should be appreciated by those skilled in the arts that other alternative embodiments are possible without departure from the invention, for example, die attach processes using die adhesive film may also advantageously use the invention. As depicted in  FIG. 2C , as the collet  10  brings the die  14  into position on the die attach adhesive  30 , the pushing and vacuum forces may be reduced or eliminated, ultimately enabling the collet  10  to be removed after the die  14  is placed. 
         [0031]    An alternative preferred embodiment of a collet  40  of the invention is depicted in a bottom view in  FIG. 3 , and in corresponding cutaway side views in  FIG. 4A through 4C  showing an example of a system and method for its use. As described elsewhere herein, the collet  40  has a body  12  preferably made from plastic, metal, or like material and is capable of receiving a die  14  as shown. A vacuum groove  16  is incorporated into the die-bearing surface  18  of the collet  40 , preferably at the edge of the body  12  and around its periphery. The vacuum groove  16  is provided with a vacuum force  22  through suitable vacuum ports  20 . The vacuum groove  16  preferably evenly distributes the vacuum force  22  around the periphery of the die-bearing surface  18  for holding a die  14  during handling and placement. An interior chamber  24  is incorporated within the die-bearing surface  18  of the body  12 . A port  26  is provided for expelling pressurized air or other gas  28  into the chamber  24 . As in the other preferred embodiment described, the expelled air  28  pressurizes the chamber  24  to prevent the formation of a concavity in the outer surface of an adjacent die  14  due to flexion in response to the application of the vacuum force  22 . Preferably, during die attach the chamber  24  is sufficiently pressurized to cause the die to bow outward slightly in a position convex to the adjacent die attach adhesive  30  as shown in  FIG. 4A  and  FIG. 4B . In this alternative embodiment, the collet  40  also includes a support skin  42 . The support skin  42  is preferably permanently attached to the die bearing surface  18  of the collet body  12 . The support skin  42  is made from a flexible material such as, for example, a thin film of Teflon, Mylar, (both registered trademarks of DuPont Corporation), polymer, or the like. In operation, while the vacuum  22  exerted in the vacuum groove  16  holds the die  14 , the pushing force of air  28  expelled into the cavity  24  pressurizes the support skin  42 . As a result, the center region of the die  14  may be caused to bow outward in a shape convex relative to the die attach adhesive  30 . As with the above-described embodiments, using this preferred method, the bowed center region of the die  14  die contacts the die attach adhesive  30  first and then spreads outward toward the periphery as it is moved toward the die pad and as the pushing force on the die  14  is diminished. This sequence avoids the trapping of air during die attach, helps to form the die attach adhesive into a bond line  30  of uniform thickness, and fosters the formation of suitable fillets  36 . Alternatively, for example in cases where the die  14  may be particularly susceptible to damage from flexing, the outward pressure  28  may be regulated to hold the die  14  substantially flat relative to the die pad  32  and attach adhesive  30 . Thus the invention may be used for regulating the shape of the die surface presented to the die attach locale, balancing against inward flexion exerted by the vacuum force  22 , but refraining from bowing the die  14  outward in order to prevent inducing stress on the die  14  in cases where increased gentleness is required. In another alternative embodiment, illustrated in the final position of the die  14  in  FIG. 4C , the bond line  30  is cured with a “smile” profile as shown, preferably uniformly thinner in the central region of the die  14  and progressively thicker approaching the periphery. 
         [0032]    As shown and described herein, preferred embodiments of the invention contribute one or more useful advantages to the art. The invention provides advantages including but not limited to improved die handling capabilities and reductions in damage during die attach in microelectronic semiconductor device package assembly. While the invention has been described with reference to certain illustrative embodiments, the methods, apparatus, and systems described are not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other advantages and embodiments of the invention will be apparent to persons skilled in the art upon reference to the description, drawings, and claims.