Abstract:
Methods and systems for reinforcing the periphery of a semiconductor wafer bonded to a carrier are disclosed. In one embodiment, additional adhesive is applied to the semiconductor wafer prior to bonding. The additional adhesive seeps into a crevice between the carrier and wafer and provides reinforcement. In another embodiment, adhesive is applied to the crevice by a dispenser after the wafer is bonded to the glass carrier.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to semiconductor fabrication, and more particularly, to the handling of semiconductor wafers. 
       BACKGROUND OF THE INVENTION 
       [0002]    Modern semiconductor fabrication processes often require very thin semiconductor wafers, typically silicon wafers. When the silicon wafers are very thin (e.g. less than 200 micrometers), they become fragile, and hence, are often bonded to a glass plate, typically with an adhesive in between the glass plate and the silicon wafer, forming a glass-wafer structure. The glass plate provides support to facilitate handling of thin silicon wafers. The process of forming a thin silicon wafer starts with bonding a thick (e.g. 750 micrometers) silicon wafer to a glass plate. The silicon is ground from the thick wafer (e.g. around 750 micrometers) to a thin wafer (less than 200 micrometers). As a result of this process, a crevice is formed around the periphery of the glass-wafer structure, and the outermost edge of the silicon wafer protrudes out from the edge of the glass carrier. In the process of grinding and polishing this glass-wafer structure, the very thin silicon is often chipped. Sometimes the chips are quite large (more than several millimeters). These edge chips can cause problems during downstream processing. For instance when a sensor looks for an alignment notch in a wafer, it may also detect one or more large chips. Some tools will simply error-out while others will randomly pick one of the notch-like chips as the notch. This results in erroneous processing of the wafer. Therefore, it is desirable to have a system and method to reduce edge chipping during wafer handling. 
       SUMMARY OF THE INVENTION 
       [0003]    In one embodiment of the present invention, a method is provided for reducing edge chipping of a semiconductor wafer during processing. The method comprises: dispensing adhesive in a center zone on a top surface of the semiconductor wafer; dispensing adhesive in an outer zone on a top surface of the semiconductor wafer; and disposing a glass carrier onto the top surface of the semiconductor wafer, thereby forming a glass-wafer structure comprising a crevice between the glass carrier and the semiconductor wafer, and forming an adhesive accumulation in an area on the periphery of the glass-wafer structure in the crevice. 
         [0004]    In another embodiment of the present invention, a method is provided for reducing edge chipping of a semiconductor wafer during processing. The method comprises: dispensing adhesive in a center zone on a top surface of the semiconductor wafer; disposing a glass carrier onto the top surface of the semiconductor wafer, thereby forming a glass-wafer structure comprising a crevice between the glass carrier and the semiconductor wafer; and applying a fill material in the crevice. 
         [0005]    In another embodiment of the present invention, a system is provided for reducing edge chipping of a semiconductor wafer during processing. The system comprises: a motor; a rotatable shaft mechanically linked to said motor; a chuck affixed to said shaft, said chuck configured to hold a glass-wafer structure, the glass-wafer structure comprising a crevice at the periphery of the glass-wafer structure; and a dispenser disposed to dispense fill material into the crevice. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The structure, operation, and advantages of the present invention will become further apparent upon consideration of the following description taken in conjunction with the accompanying figures (FIGS.). The figures are intended to be illustrative, not limiting. 
           [0007]    Certain elements in some of the figures may be omitted, or illustrated not-to-scale, for illustrative clarity. The cross-sectional views may be in the form of “slices”, or “near-sighted” cross-sectional views, omitting certain background lines which would otherwise be visible in a “true” cross-sectional view, for illustrative clarity. 
           [0008]    Often, similar elements may be referred to by similar numbers in various figures (FIGS.) of the drawing, in which case typically the last two significant digits may be the same, the most significant digit being the number of the drawing figure (FIG.). 
           [0009]      FIG. 1  shows a prior art glass-wafer structure. 
           [0010]      FIG. 2  shows a glass-wafer structure with a reinforced crevice. 
           [0011]      FIG. 3  shows a prior art adhesive application process. 
           [0012]      FIG. 4  shows an adhesive application process for an embodiment of the present invention. 
           [0013]      FIG. 5  shows a bonding process for an embodiment of the present invention. 
           [0014]      FIG. 6  shows a top-down view of a wafer, indicating adhesive application zones. 
           [0015]      FIG. 7  shows an alternative embodiment, using an extruding applicator device. 
           [0016]      FIG. 8  shows an alternative embodiment, using a brush applicator device. 
           [0017]      FIG. 9  is a flowchart showing process steps for an embodiment of the present invention. 
           [0018]      FIG. 10  is a flowchart showing process steps for an alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a prior art glass-wafer structure  100 , which comprises glass carrier  102  affixed to a thinned silicon wafer  104  with adhesive. The outermost edge  105  of the silicon wafer  104  is very thin, and is also not supported by the glass carrier  102 , due to inherent rounding of corners of the glass carrier  102  and wafer  104 , which forms a crevice  103 . This makes edge  105  very prone to chipping during wafer handling, subsequent polishing, and other processing steps. 
         [0020]      FIG. 2  shows a glass-wafer structure  200  with a reinforced crevice. Embodiments of the present invention place a fill material into the crevice in order to reinforce the crevice, which serves to reduce the risk of chipping. In this case, adhesive  206  serves as the fill material, and is disposed between wafer  204  and glass carrier  202  at the outermost edge of the glass-wafer structure. The adhesive provides support for the thin outermost edge of the wafer  204 , thereby reducing the risk of wafer chipping. In one embodiment, the adhesive is a polyimide adhesive. In a particular embodiment, the adhesive used is HD-3007, distributed by Hitachi DuPont MicroSystems, LLC, of Parlin, N.J. 
         [0021]      FIG. 3  shows a prior art adhesive application process. Wafer  304  is secured to a chuck  312  which is secured to a shaft  314 . The shaft  314  is mechanically linked to a motive source (not shown) such as an electric motor, allowing it to rotate, and thus “spin” the wafer  304 . Adhesive dispenser  308  dispenses adhesive  310  in a center zone of the wafer. The wafer  304  is then spun at a high speed in order to evenly distribute the adhesive  310  over the top surface  307  of wafer  304 . 
         [0022]      FIG. 4  shows an adhesive application process for an embodiment of the present invention. This step occurs in addition to the center zone application described in  FIG. 3 . Adhesive dispenser  408  is positioned over an outer zone of wafer  404 , and deposits a small amount of adhesive  410 . Adhesive  410  may be the same adhesive as adhesive  310  of  FIG. 3 . Alternatively, adhesive  410  may be a different compound than adhesive  310  of  FIG. 3 . 
         [0023]    The adhesive  410  is preferably deposited while wafer  404  is spinning at a slow speed (10-50 rpm). After application, the wafer may then be spun at a high speed (800-2000 rpm) to evenly distribute the adhesive  410 . The effect of applying extra adhesive  410  at the outer periphery of wafer  404  is that the extra adhesive will seep out when the glass carrier is applied, forming an accumulation of adhesive in the crevice. The adhesive in the crevice hardens during the curing process, serving to reinforce the outermost edge of the wafer  404 . 
         [0024]      FIG. 5  shows a bonding process for an embodiment of the present invention. Wafer  504  is pressed against glass carrier  502  by a bonder tool. The bonder tool may comprise a chuck  512  and shaft  514  which apply upward force F 1 , and chuck  516  and shaft  518  which apply downward force F 2 . The total compressive force may be in the range of 5 to 100 kilo-Newtons, and be applied in an environment having a temperature ranging from 100 to 400 degrees Celsius for a time period of 10 to 600 seconds. 
         [0025]      FIG. 6  shows a top-down view of a wafer  630 , indicating adhesive application zones. Center zone  633  is the region enclosed by dotted circle  634  having radius R 1 , and the outer zone  637  is the region between outer edge  632  of wafer  630 , and dotted circle  636  having radius R 2 . Wafer  630  has radius R 3 , and R 3 &gt;R 2 &gt;R 1 . In one embodiment, radius R 1  is between 0.1(R 3 ) and 0.4(R 3 ) and radius R 2  is between 0.8(R 3 ) and 0.99(R 3 ). The wafer  630  is spun along direction D during the adhesive application process. Wafer  630  may be spun clockwise or counterclockwise. 
         [0026]      FIG. 7  shows an alternative embodiment, using an extruding applicator  740 . In this embodiment, the wafer  704  is first bonded to the glass carrier  702  using conventional methods. Then, an extruding applicator  740  applies a bead of adhesive to the crevice  703 . In one embodiment, the glass-wafer structure  700  is secured to a chuck  712  that is rotated by shaft  714 , which is mechanically connected to a motive force, such as an electric motor  719 . Once the adhesive applied via applicator  740  has cured, the wafer  704  can then be ground to the desired thickness, preferably in the range of 30 to 500 micrometers. 
         [0027]      FIG. 8  shows an alternative embodiment similar to that shown in  FIG. 7 , except that a brush applicator  840  is used to apply the adhesive to the crevice  803 . Once the adhesive applied via applicator  840  has cured, the wafer  804  can then be ground to the desired thickness. 
         [0028]      FIG. 9  is a flowchart showing process steps for an embodiment of the present invention. In process step  950 , a wafer is placed in an apply tool and spun at a slow speed (10-50 rpm). The speed used depends on the viscosity of the adhesive. In process step  952 , adhesive is applied to the center zone (see adhesive  310  of  FIG. 3 , and center zone  633  of  FIG. 6 ). In process step  954 , a fast spin (800-2000 rpm) is applied to evenly distribute the adhesive on the top surface of the wafer. In process step  956 , the spin rate is again changed to the slow spin rate in preparation for step  958 , application of adhesive to the outer zone (see adhesive  410  of  FIG. 4 , and outer zone  637  of  FIG. 6 ). In process step  960 , the wafer is again spun at the fast speed (800-2000 rpm) to evenly distribute the adhesive. In process step  962 , the silicon wafer is bonded to the glass carrier (see  FIG. 5 ). In process step  964 , the wafer is ground (preferably to a thickness in the range of 30 to 500 micrometers), resulting in the glass-wafer structure shown in  FIG. 2 . 
         [0029]      FIG. 10  is a flowchart showing process steps for an alternative embodiment of the present invention. In process step  1050 , a wafer is placed in an apply tool and spun at a slow speed (10-50 rpm). In process step  1052 , adhesive is applied to the center zone (see adhesive  310  of  FIG. 3 , and center zone  633  of  FIG. 6 ). In process step  1054 , a fast spin (800-2000 rpm) is applied to evenly distribute the adhesive on the wafer. In process step  1056 , the wafer is bonded to the glass carrier. In process step  1058 , adhesive is applied to the crevice between the glass carrier and wafer (see  740  of  FIG. 7 and 840  of  FIG. 8 ). In process step  1060 , the wafer is ground, (preferably to a thickness in the range of 30 to 500 micrometers), resulting in the glass-wafer structure shown in  FIG. 2 . 
         [0030]    Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, certain equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.) the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiments of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several embodiments, such feature may be combined with one or more features of the other embodiments as may be desired and advantageous for any given or particular application.