Abstract:
A method for fabricating a wire bonding structure is disclosed, which includes: providing a substrate having a plurality of bonding pads; and foaming a ball end of a bonding wire on at least one of the bonding pads by performing a scrubbing process along a path around a periphery of the bonding pad, thereby preventing delamination of the ball end of small size from the bonding pad.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to semiconductor processes, and more particularly, to a method for fabricating a wire bonding structure. 
         [0003]    2. Description of Related Art 
         [0004]    Current semiconductor packaging technologies generally involve performing wire bonding processes to form gold or copper bonding wires for electrically connecting semiconductor chips and carriers such as lead frames or packaging substrates. Further, the carriers are electrically connected to external devices such as circuit boards through conductive elements, for example, solder balls. 
         [0005]    To perform such a wire bonding process, a ball end of a bonding wire is formed by the tip of a capillary through ultrasonic vibration on a bonding pad of a semiconductor device or a carrier and then bonded to the bonding pad by pressure. Thereafter, a wire body of the bonding wire is extended from the ball end and bonded by pressure to a bonding pad of another semiconductor device or carrier. Thereafter, the bonding wire is cut by the tip of the capillary. 
         [0006]      FIG. 1A  is a schematic cross-sectional view of a conventional wire bonding structure  1 . Referring to  FIG. 1A , a ball end  12   a  of a bonding wire  12  is formed by an ultrasonic vibration device through a scrubbing process on a bonding pad  11  of a substrate  10 . Then, the ball end  12   a  is bonded to the bonding pad  11  by pressure. Thereafter, the wire body of the bonding wire  12  is extended from the ball end  12   a.    
         [0007]    During the scrubbing process for forming the ball end  12   a,  particles and organic thin films generated during fabrication of the substrate  10  can be removed. Further, the scrubbing process is performed by the ultrasonic vibration device along a path in a vertical direction (Y direction) or a horizontal direction (X direction) to form a solder material on the bonding pad  11 , thereby forming the ball end  12   a,  as shown in  FIG. 1B . 
         [0008]    The above-described scrubbing process is suitable for forming a ball end on the bonding pad  11  of large size, for example, on the bonding pad  11  having a width d greater than 40 um. 
         [0009]    However, as circuits are developed toward fine pitches, the width d of the bonding pad  11  is continuously reduced. Accordingly, the size of the ball end  12   a  needs to be reduced. When the width d of the bonding pad  11  is reduced below 40 um, if the ball end  12   a  is formed through the above-described scrubbing process, delamination easily occurs between the ball end  12   a  and the bonding pad  11 . 
         [0010]    Further, the bonding wire  12  is generally made of a hard material such as copper or gold. When such a bonding wire is bonded to a very thin substrate such as a semiconductor chip, the bonding pad is easily damaged by the above-described scrubbing process for forming the ball end  12   a.    
         [0011]    Therefore, there is a need to provide a method for fabricating a wire bonding structure so as to overcome the above-described drawbacks. 
       SUMMARY OF THE INVENTION 
       [0012]    In view of the above-described drawbacks, the present invention provides a method for fabricating a wire bonding structure, which comprises the steps of providing a substrate having a plurality of bonding pads; and forming a ball end of a bonding wire on at least one of the bonding pads by performing a scrubbing process along a path around a periphery of the bonding pad. 
         [0013]    In the above-described method, the bonding pad can have a width less than 40 um. 
         [0014]    In the above-described method, the path of the scrubbing process around the periphery of the bonding pad can be clockwise or counterclockwise. 
         [0015]    In the above-described method, the path of the scrubbing process can be from a center of the bonding pad to the periphery of the bonding pad and then around the periphery of the bonding pad. 
         [0016]    In the above-described method, the bonding wire can be made of made of copper, silver or gold. 
         [0017]    After the ball end is formed, the above-described method can further comprise pressing the ball end on the bonding pad. 
         [0018]    After the ball end is formed, the above-described method can further comprise forming a wire body of the bonding wire that extends from the ball end. 
         [0019]    Therefore, the present invention performs a scrubbing process along a path around a periphery of a bonding pad so as to form a ball end of a bonding wire on the bonding pad, which not only effectively removes particles and organic thin films generated during the fabrication process, but also strengthens the bonding between the ball end and the bonding pad to thereby prevent delamination of the ball end from the bonding pad. 
         [0020]    Further, by performing the scrubbing process of the present invention, a hard wire material can be bonded to a very thin substrate without causing damage of the bonding pad. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1A  is a schematic cross-sectional view of a conventional wire bonding structure; 
           [0022]      FIG. 1B  is a planar top view of the scrubbing path of a conventional scrubbing process; 
           [0023]      FIGS. 2A and 2B  are schematic cross-sectional views showing a method for fabricating a wire bonding structure according to the present invention; and 
           [0024]    FIGS.  3 A to  3 B′ are planar top views of the scrubbing path of a scrubbing process according to different embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects can be apparent to those in the art after reading this specification. 
         [0026]    It should be noted that all the drawings are not intended to limit the present invention. Various modifications and variations can be made without departing from the spirit of the present invention. Further, terms such as “on”, “a” etc. are merely for illustrative purposes and should not be construed to limit the scope of the present invention. 
         [0027]      FIGS. 2A and 2B  are schematic cross-sectional views showing a method for fabricating a wire bonding structure  2  according to the present invention. 
         [0028]    Referring to  FIGS. 2A and 3A , a substrate  21  having at least a bonding pad  21  is provided. A scrubbing process is performed by an ultrasonic vibration device (not shown) along a path S 1  of  FIG. 3A  around a periphery of the bonding pad  21  so as to form a ball end  22   a  on the bonding pad  21 . 
         [0029]    Referring to  FIG. 2B , the ball end  22   a  is pressed on the bonding pad  21  and a wire body  22   b  is extended from the ball end  22   a  to form a bonding wire  22 . The bonding wire  22  is comprised of the ball end  22   a  and the wire body  22   b.    
         [0030]    In the present embodiment, the substrate  20  can be a packaging substrate or a lead frame. The bonding pad can be an electrical contact pad or a lead. The bonding wire  22  can be made of copper, silver or gold. 
         [0031]    The bonding pad  21  can have a width less than 40 um. 
         [0032]    Referring to  FIG. 3A , the scrubbing process is performed first along an upper camber line path L 1  from the center of the bonding pad  21  to a right side of the periphery of the bonding pad  21  and then along the path S 1  around the periphery of the bonding pad  21  in a clockwise direction or along a path S 1 ′ of FIG.  3 A′ in a counterclockwise direction so as to form the ball end  22   a.  In another embodiment, the scrubbing process can be performed first along a lower camber line path L 2  from the center of the bonding pad  24  to a left side of the periphery of the bonding pad  21 . 
         [0033]    Further, position setting of the ultrasonic vibration device is described as follows. Since the ultrasonic vibration device has an original output direction of Y direction, the output direction of the ultrasonic vibration process in the scrubbing process can be compensated in a horizontal direction, i.e, X direction, such that the bottom surface of the ball end  22   a  (i.e, the surface in contact with the bonding pad  21 ) after the scrubbing process is close to a circular shape. 
         [0034]    In another embodiment, referring to  FIG. 3B , the scrubbing process can be performed first along an upper camber line path L 3  from the center of the bonding pad  21  to the left side of the periphery of the bonding pad  21  and then along the path S 2  around the periphery of the bonding pad  21  in a counterclockwise direction or along the path S 2 ′ of FIG.  3 B′ in a clockwise direction so as to form the ball end  22   a.  In a further embodiment, the scrubbing process can be performed first along a lower camber line path L 4  from the center of the bonding pad  21  to the right side of the periphery of the bonding pad  21 . 
         [0035]    Therefore, there are various combinations of the camber line paths L 1 , L 2 , L 3 , L 4  and the circular paths S 1 , S 2 . But it should be noted that the present invention is not limited thereto. 
         [0036]    Therefore, even if a bonding pad  21  has a width less than 40 um, the present invention can effectively form a ball end  22   a  of a bonding wire  22  on the bonding pad  21  by performing a scrubbing process along a path around a periphery of the bonding pad  21 , thereby preventing delamination of the ball end  22   a  from the bonding pad  21 . 
         [0037]    Further, by performing the scrubbing process of the present invention, stresses applied by the wire material on the bonding pad  21  can be dispersed. As such, a hard wire material can be bonded to a very thin substrate without causing damage of the bonding pad  21 . 
         [0038]    The above-described descriptions of the detailed embodiments are only to illustrate the preferred implementation according to the present invention, and it is not to limit the scope of the present invention. Accordingly, all modifications and variations completed by those with ordinary skill in the art should fall within the scope of present invention defined by the appended claims.