Abstract:
An improved wire bonding capillary used in the bonding of wires to the bond pads of a semiconductor device and the leads of a lead frame, the wire bonding capillary having a working tip having a working surface including a flat annular portion surrounding the wire feed aperture in the capillary and a concave surface extending therefrom to the intersection with the radius extending from the external tip diameter of the working tip.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 10/624,796, filed Jul. 22, 2003, pending, which is a continuation of application Ser. No. 10/155,317, filed May 23, 2002, now U.S. Pat. No. 6,595,406, issued Jul. 22, 2003, which is a continuation of application Ser. No. 09/940,203, filed Aug. 27, 2001, now U.S. Pat. No. 6,439,450, issued Aug. 27, 2002, which is a continuation of application Ser. No. 09/649,209, filed Aug. 28, 2000, now U.S. Pat. No. 6,311,890, issued Nov. 6, 2001, which is a continuation of application Ser. No. 09/162,649, filed Sep. 29, 1998, now U.S. Pat. No. 6,158,647, issued Dec. 12, 2000. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Field of the Invention  
         [0003]     The invention relates to semiconductor manufacturing. More particularly, the invention relates to an improved wire bonding capillary used in the bonding of wires to the bond pads of a semiconductor device and the leads of a lead frame.  
         [0004]     In semiconductor device manufacturing, a wire bonding process is used to electrically connect the metal bond pads on the active surface of a semiconductor die to the leads or lead fingers of a lead frame. Wire bonding devices are well known in the art. U.S. Pat. Nos. 3,894,671, 4,877,173, and 5,082,154 illustrate wire bonding devices.  
         [0005]     The wire bonding process occurs during the final stages of manufacture of the semiconductor device before the semiconductor device is enclosed within a sealed or encapsulated package. Although a variety of different packaging systems are used, most packaging systems include a lead frame on which the semiconductor die is mounted. The lead frame has lead fingers which are connected to the bond pads on the active surface of the semiconductor die through the use of wires bonded to the lead fingers and bond pads. Subsequently, the semiconductor die, wires, and portions of the lead fingers and lead frame are encapsulated in plastic during a molding process. After the molding process, the portions of the lead fingers extending beyond the molded plastic material are trimmed and formed into the desired configuration for the use and mounting of the semiconductor device on a substrate.  
         [0006]     The wires used to connect the bond pads of the semiconductor die to the lead fingers of the lead frame are small in diameter, have as short a length as possible, and are typically of gold. During the wire bonding process, a heat block heats the semiconductor die and the lead frame to a temperature in the range of 150° C. to 350° C. A bonding capillary tool mechanically presses the wire against a bond pad on the active surface of the semiconductor die and then to a bonding site on the desired lead finger of the lead frame. The bond wire is threaded through the bonding capillary for use in the bonding process. The end of the wire threaded through the bonding capillary is then heated by an electrical discharge or a hydrogen torch to a molten state, thereby forming a ball of molten material on the end of the bond wire extending from the bonding capillary. The molten ball is pressed by the bonding capillary against the heated bond pad on the active surface of the semiconductor die to alloy the metallic elements of the wire and the bond pad, thereby bonding the wire to the bond pad in a ball type wire bond. In some instances, ultrasonic vibration of the bonding capillary may be used during the bonding process. After the bonding of the wire to the bond pad on the active surface of a semiconductor die, the bonding capillary tool is moved to a bonding site on the desired lead finger of the lead frame. The wire is pressed against the heated lead finger of the lead frame to bond the wire to the lead finger. The bond wire is then tensioned by the bonding capillary until the wire is sheared, making a stitch or wedge type wire bond on the lead finger. The bonding process is repeated for the desired bond pads on the active surface of the semiconductor die for connections to the desired lead fingers of the lead frame.  
         [0007]     One type of wire bonding capillary design is illustrated in U.S. Pat. No. 4,415,115. The bonding capillary tip is formed having a flat bonding surface and an annular cutting ridge raised from the flat bond surface surrounding the aperture through which the wire is fed in the bonding capillary. Alternately, the cutting ridge may comprise a semicircle shape rather than an annular shape.  
         [0008]     Another type of wire bonding capillary is illustrated in U.S. Pat. No. 5,421,503. The bonding capillary is for use in automatic gold ball bonders for bonding fine gold wires onto closely spaced bond pads on semiconductor devices. The bonding capillary comprises a cylindrical body portion which fits into an ultrasonic transducer having a bottle-neck working tip on the other end of the body portion. The working tip includes a working face comprising an annular ring between the chamfer diameter of the working tip and the working tip diameter and a face angle of approximately 22 degrees plus or minus 3 degrees.  
         [0009]     Yet another wire bonding capillary is illustrated in U.S. Pat. No. 5,662,261. The wire bonding capillary includes a working tip having an elongated hole therethrough and a face extending away from the hole at multiple angles of increasing magnitude.  
         [0010]     While the size of semiconductor devices continues to decrease and the number of bond pads on the active surface continues to increase, having smaller pitch between adjacent bond pads and the width of lead fingers of lead frames decreases, it is necessary to have a wire bonding capillary which will form effective ball type wire bonds on the bond pads of the semiconductor device and stitch or wedge type wire bonds on the lead fingers of the lead frame having acceptable pull-off strength characteristics without damaging the circuitry of the semiconductor device. Such requirements illustrate the need for a wire bonding capillary which will effectively make a ball type wire bond on the bond pad of the semiconductor device without contacting the surface of the semiconductor device adjacent thereto and will effectively make stitch or wedge type wire bonds on the lead fingers of lead frames.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention relates to an improved wire bonding capillary used in the bonding of wires to the bond pads of a semiconductor device and the leads of a lead frame. The wire bonding capillary has a working tip having a working surface including a flat annular portion surrounding the wire feed aperture in the capillary and a concave surface extending therefrom to the intersection with the radius extending from the external tip diameter of the working tip.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0012]      FIGS. 1 through 4  are cross-sectional views showing the steps of a typical wire bonding operation;  
         [0013]      FIG. 5  is a cross-sectional view of a prior art working tip of a wire bonding capillary having a flat surface thereon;  
         [0014]      FIG. 6  is a cross-sectional view of a prior art working tip of a wire bonding capillary having an angled surface thereon;  
         [0015]      FIG. 7  is a cross-sectional view of a working tip of a wire bonding capillary of the present invention;  
         [0016]      FIG. 8  is a cross-sectional view of a working tip of a wire bonding capillary of the present invention making a ball type wire bond on a bond pad of a semiconductor device; and  
         [0017]      FIG. 9  is a cross-sectional view of a working tip of a wire bonding capillary of the present invention making a stitch or wedge type bond on a lead finger of a lead frame. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring to drawing  FIGS. 1 through 4 , a typical wire bonding operation using a capillary is illustrated. A wire  40 , typically of gold or copper, is threaded through a hole  20  in the capillary tip  22 . The end of wire  40  is heated by an electrical discharge or a hydrogen torch to a molten state, thereby forming a ball  42  of molten metal on the end of wire  40 . Capillary tip  22  is lowered toward bond pad  44  on the active surface of a semiconductor device  46 . The bond pad  44  is typically of aluminum or has an aluminum layer thereon. However, the bond pad  44  may be of any suitable conductive metal. The semiconductor device  46  is heated to a temperature of 150° C. to 350° C. by a heated block  48 . Molten metal ball  42  is pressed by the capillary tip  22  against the heated bond pad  44  to alloy the metallic elements of the wire  40  and bond pad  44 , thereby bonding the wire  40  to the bond pad  44 . In some instances, ultrasonic vibrations are applied to the capillary tip  22  as the molten metal ball  42  on the end of wire  40  is pressed against the bond pad  44 .  
         [0019]     Referring specifically to drawing  FIG. 4 , capillary tip  22  is then raised, causing the wire  40  to feed through the aperture in the capillary tip  22  and moved to a bonding site on a lead finger  50  of a lead frame. The lead finger  50  is heated to a temperature of 150° C. to 350° C. by heated block  48 . The wire  40  is pressed against the heated lead finger  50  to alloy the metallic elements of wire  40  and lead finger  50 , thereby bonding the wire  40  to the lead finger  50 . If desired, ultrasonic vibrations may be applied to the capillary tip  22  as wire  40  is pressed against lead finger  50 . The face of the capillary tip  22  severs the wire  40  to form a stitch type wire bond of the wire  40  to the lead finger  50 , thereby allowing the capillary tip  22  to be used to repeat the process with respect to other bond pads  44  on the active surface of the semiconductor device  46  and lead fingers  50  of the lead frame, depending upon the uniformity of the thickness of the coatings  49  and/or  47  ( FIG. 5 ) on the semiconductor device  46 .  
         [0020]     Referring to drawing  FIG. 5 , the tip portion of a prior art wire bonding capillary  60  is illustrated forming a molten metal ball  42  on the bond pad  44  of a semiconductor device  46  having a first coating  47  thereon and a second coating  49  over the first coating  47 . The tip portion of the bonding capillary  60  includes a central aperture  62  having a frusto-conical surface  63  at the lower end of the aperture, a flat annular surface  64  extending from the frusto-conical surface  63  to an annular curved surface  66  which terminates in the outer surface  68  of the tip. The frusto-conical surface  63  and flat annular surface  64  help to form the required wire bonds to the bond pad  44  and lead finger (not shown) of a lead frame. However, the flat annular surface  64  may contact the second coating  49  on the semiconductor device  46  during the bonding of the molten metal ball  42  of the wire  40  to the bond pad  44 , causing damage to the semiconductor device  46 .  
         [0021]     Referring to drawing  FIG. 6 , the tip portion of a prior art wire bonding capillary  70  is illustrated forming a molten metal ball  42  on the bond pad  44  of a semiconductor device  46  having a first coating  47  thereon and a second coating  49  over the first coating  47 . The tip portion of the bonding capillary  70  includes a central aperture  72  having a first frusto-conical surface  73  at the lower end of the central aperture  72 , a second frusto-conical surface  74  extending from the first frusto-conical surface  73  to an annular curved surface  76  which terminates in the outer surface  78  of the tip. The first frusto-conical surface  73  and second frusto-conical surface  74  help to form the required wire bonds to the bond pad  44  and lead finger (not shown) of a lead frame. The second frusto-conical surface  74  is formed generally at an angle of four degrees (4°) with respect to a horizontal plane extending through the upper surface of the second coating  49  on the semiconductor device  46 . However, the second frusto-conical surface  74  may contact the second coating  49  on the semiconductor device  46  during the bonding of the molten metal ball  42  of the wire  40  to the bond pad  44 , causing damage to the semiconductor device  46 , depending upon the uniformity of thickness of the coating  49  and/or  47  on the semiconductor device  46 .  
         [0022]     Referring to drawing  FIG. 7 , the tip of a wire bonding capillary  80  of the present invention is illustrated. The tip portion of the bonding capillary  80  includes a central aperture  82  having a first frusto-conical surface  84  and second frusto-conical surface  86  extending from the lower end of the first frusto-conical surface  84 , both located at or near the lower end of the central aperture  82 , and, on the lower surface of the tip, a flat annular surface  88  extending for a distance between diameter “a” to diameter “b” of the tip from the second frusto-conical surface  86  of the central aperture  82  and a face having an annular concave surface  90  extending from the diameter “b” of the flat annular surface  88  into the tip and outwardly a distance to a point of inflection (tangency)  91  to an annular curved surface  92  defined by a radius “r” of curvature of the tip, which, in turn, intersects at point  94 , the lower annular radial diameter “R” of the wire bonding capillary  80 , with the annular conical outer surface  96  of the tip. A radius of curvature “RR” is used for the formation of the annular concave surface  90 , the size of the radius “RR” determining the degree of curvature of the annular concave surface  90  for the relative clearance between the tip of the wire bonding capillary  80  and a second coating  49  on the surface of the semiconductor device  46 . The first frusto-conical surface  84  and second frusto-conical surface  86  of the central aperture  82  and the flat annular surface  88  help to form the required wire bonds to the bond pad  44  and lead finger (not shown) of a lead frame. The flat annular surface  88  of the lower surface of the tip is used to sever the wire  40  during the formation of a stitch or wedge type wire bond on the lead finger (not shown) of a lead frame during the wire bonding process. The width of the flat annular surface  88  is determined by the size of the bond pad  44  of the semiconductor device upon which the wire bonding capillary  80  is to be used.  
         [0023]     Referring to drawing  FIG. 8 , the tip of a wire bonding capillary  80  of the present invention is illustrated in relation to a semiconductor device  46  having a second coating  49  and a bond pad  44  on the active surface thereof. The first frusto-conical surface  84 , second frusto-conical surface  86 , and flat annular surface  88  form the molten metal ball  42  with the bond pad  44 , the annular concave surface  90  providing clearance with the second coating  49  on the active surface of the semiconductor device  46  to prevent contact of the tip of the wire bonding capillary  80 .  
         [0024]     Referring to drawing  FIG. 9 , the wire bonding capillary  80  of the present invention is illustrated in conjunction with a lead finger  50  of a lead frame. The wire  40  is bonded in the bond area  52  of the lead finger  50  by the wire bonding capillary  80  with the flat annular surface  88 , annular concave surface  90 , and annular curved surface  92  of the tip deforming the wire  40  into engagement with a portion of the lead finger  50 . The flat annular surface  88  of the wire bonding capillary  80  is used to sever the wire  40  after the bonding thereof to the lead finger  50 . The annular concave surface  90  and annular curved surface  92  of the wire bonding capillary  80  are used in conjunction with the flat annular surface  88  thereof to form the bond area  52  of the connection of the wire  40  to the lead finger  50 , the bond area  52  having a convex portion  90 ′ which is formed by annular concave surface  90  of the tip of wire bonding capillary  80  and a curved portion  92 ′ which is formed by the annular curved surface  92  of the tip of wire bonding capillary  80 . The flat annular surface  88 , annular concave surface  90 , and annular curved surface  92  provide a gradual transition between the wedge deformity of the wire  40  and the existing circular shape of the wire  40  extending beyond the bond area  52 , the wire  40  being heated by suitable means before the formation of the stitch or wedge type bond on the lead finger  50  while the lead finger  50  is heated before the formation of the wire bond thereto.  
         [0025]     From the foregoing, it is apparent that changes, modifications, and deletions may be made to the capillary bonding tool of the present invention which fall within the scope of the invention, for instance, varying the length or size of the radii “r”, “R”, or “RR” and the radial dimensions of the flat annular surface  88  between the dimensions “a” and “b.”