Abstract:
In a wire bonding apparatus including a horn driver for generating ultrasonic waves, a capillary, an ultrasonic horn formed by a symmetrical section fixed to the horn driver and an asymmetrical section having an end for mounting the capillary, the asymmetrical section is constructed by a spurious vibration suppressing structure for suppressing a vibration component of the ultrasonic horn perpendicular to a propagation direction of the ultrasonic waves with the ultrasonic horn.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wire bonding apparatus. 
     2. Description of the Related Art 
     As wire bonding apparatuses for connecting bonding wires (Au) between bonding pads of semiconductor chips and lead-out terminals of packages, there are a thermopressing type, an ultrasonic thermopressing type and an ultrasonic type. The ultrasonic thermopressing type wire bonding apparatus are widely used in the manufacture of quad flat packages (QFPs) having less than 200 pins. 
     In an ultrasonic thermopressing type wire bonding apparatus, a bonding pad of a semiconductor chip is heated to about 150 to 300° C., and a gold ball formed by gold wire is pushed down by a capillary onto the bonding pad. Then, ultrasonic waves are performed upon the capillary, so that the gold ball is deformed to break oxide on the bonding pad, thus surely connecting the gold ball to the bonding pad. 
     The above-mentioned wire bonding apparatus uses an ultrasonic horn for transmitting ultrasonic waves to the capillary. That is, a prior art wire bonding apparatus is constructed by a horn driver for generating ultrasonic waves, a capillary, an ultrasonic horn including a symmetrical section fixed to the horn driver and an asymmetrical section having an end for mounting the capillary (see JP-A-2000-340598). This will be explained later in detail. 
     In the above-described prior art wire bonding apparatus, however, since the asymmetrical section is tapered, spurious vibration of the capillary occurs so as to reduce the accuracy of the bonding operation. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a wire bonding apparatus capable of improving the accuracy of the bonding operation. 
     According to the present invention, in a wire bonding apparatus including a horn driver for generating ultrasonic waves, a capillary, an ultrasonic horn formed by a symmetrical section fixed to the horn driver and an asymmetrical section having an end for mounting the capillary, the asymmetrical section is constructed by a spurious vibration suppressing structure for suppressing a vibration component of the ultrasonic horn perpendicular to a propagation direction of the ultrasonic waves within the ultrasonic horn. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description set forth below, as compared with the prior art, with reference to the accompanying drawings, wherein: 
     FIG. 1A is a plan view illustrating a prior art wire boding apparatus; 
     FIG. 1B is a side view of the wire bonding apparatus of FIG. 1A; 
     FIG. 1C is a front view of the wire bonding apparatus of FIG. 1A; 
     FIG. 2 is a diagram for explaining the problem in the wire bonding apparatus of FIGS. 1A,  1 B and  1 C; 
     FIG. 3A is a perspective view illustrating a first embodiment of the wire bonding apparatus according to the present invention; 
     FIG. 3B is a side view of the wire bonding apparatus of FIG. 3A; 
     FIG. 4 is a diagram for explaining the effect of the wire bonding apparatus of FIGS. 3A and 3B; 
     FIG. 5A is a side view illustrating a second embodiment of the wire bonding apparatus according to the present invention; 
     FIG. 5B is a cross-sectional view of the wire bonding apparatus of FIG. 5A; and 
     FIG. 6 is a diagram for explaining the effect of the wire bonding apparatus of FIGS.  5 A and  5 B. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the description of the preferred embodiments, a prior art wire bonding apparatus will be explained with reference to FIGS. 1A,  1 B,  1 C and  2 . 
     FIG. 1A is a plan view illustrating a prior art wire bonding apparatus (see JP-A-2000-340598), FIG. 1B is a side view of the wire bonding apparatus of FIG. 1A, and FIG. 1C a front view of the wire bonding apparatus of FIG.  1 A. 
     As illustrated in FIGS. 1A,  1 B and  1 C, a wire bonding apparatus is constructed by an ultrasonic horn  1  made of elastic metal fixed to a horn driver  2  for generating ultrasonic waves. The ultrasonic horn  1  is divided into a straight (symmetrical) section  11  and a sloped (asymmetrical) section  12  at a bonder mounting section  3 . 
     The symmetrical section  11  has a cross section which is symmetrical with respect to the Y-direction and the Z-direction, while the asymmetrical section  12  has a cross section which is symmetrical with respect to the Y-direction but asymmetrical with respect to the Z-direction. 
     A capillary  4  is mounted on a capillary mounting end  12   a  of the asymmetrical section  12 . Note that the capillary  4  is shown in FIG. 1B but not in FIGS. 1A and 1C. 
     Each of the symmetrical section  11  and the asymmetrical section  12  serves as a λ/4 resonator where λ is a wavelength of ultrasonic waves in the ultrasonic horn  1  (λ≈8 cm). 
     The asymmetrical section  12  is conically-tapered, exponentially-tapered or catenoidally-tapered, so that the cross section of the asymmetrical section  12  is gradually decreased from the symmetrical section  11  to the capillary  4 . As a result, the amplitude of ultrasonic waves propagating within the asymmetrical section  12  is mechanically amplified to apply the ultrasonic waves to the capillary  4 . 
     In the wire bonding apparatus of FIGS. 1A,  1 B and  1 C, the bottom level of the symmetrical section  11  is the same as that of the asymmetrical section  12 , so that a clearance between the ultrasonic horn  1  and an object surface to be bonded can be definite, thus easily carrying out a wire bonding operation. 
     In the wire bonding apparatus of FIGS. 1A,  1 B and  1 C, as illustrated in FIG. 2, ultrasonic waves W 1  propagate within the symmetrical section  11  and the asymmetrical section  12  and are incident perpendicularly to the capillary  4 . As a result, the capillary  4  vibrates along the X-direction. However, ultrasonic waves W 2  propagate within the symmetrical section  11  and the asymmetrical section  12  and are incident to a sloped boundary of the asymmetrical section  12 . As a result, the asymmetrical section  12  vibrates along the Z-direction as well as along the X-direction, that is, the capillary  4  vibrates along the Z-direction as well as along the X-direction. The vibration of the capillary  4  along the Z-direction is spurious so as to reduce the accuracy of the wire bonding operation. 
     FIG. 3A is a perspective view illustrating a first embodiment of the wire bonding apparatus according to the present invention, and FIG. 3B is a side view of the wire bonding apparatus of FIG.  3 A. In FIGS. 3A and 3B, the asymmetrical section  12  of FIGS. 1A,  1 B and  1 C is changed into an asymmetrical section  12 ′ which includes an asymmetrical section  121  and a symmetrical section  122 . 
     The symmetrical section  122  has a cross section which is symmetrical with respect to the Y-direction and the Z-direction. In this case, the cross section of the symmetrical section  122  is smaller than that of the symmetrical section  11 . On the other hand, the asymmetrical section  121  has a cross section which is symmetrical with respect to the Y-direction but asymmetrical with respect to the Z-direction. 
     The cross section of the asymmetrical section  121  is gradually decreased from the symmetrical section  11  to the symmetrical section  122 . As a result, the amplitude of ultrasonic waves propagating within the asymmetrical section  121  is mechanically amplified to apply the ultrasonic waves to the capillary  4 . 
     Additionally, stepped grooves  121   a  are formed on an upper portion of the asymmetrical section  121 . Each of the stepped grooves  121   a  has a face in parallel with the X-direction and a face in parallel with the Z-direction. 
     In the wire bonding apparatus of FIGS. 3A and 3B, as illustrated in FIG. 4, ultrasonic waves W 1  propagate within the symmetrical section  11  and the asymmetrical section  12 ′ and are incident perpendicularly to the capillary  4 . As result, the capillary  4  vibrates along the X-direction. On the other hand, ultrasonic waves W 2  propagate within the symmetrical section  11  and the asymmetrical section  121  and are incident to the faces of the asymmetrical section  121  in parallel with the Z-direction. As a result, the asymmetrical section  12  vibrates along the X-direction, that is, the capillary  4  also along the X-direction. 
     In the above-described first embodiment, both the ultrasonic waves W 1  and W 2  vibrate the capillary  4  only along the X-direction, in other words, the spurious vibration of the capillary  4  along the Z-direction can be suppressed, thus improving the accuracy of the wire bonding operation. 
     FIG. 5A is a side view illustrating a second embodiment of the wire bonding apparatus according to the present invention, and FIG. 5B is a side view of the wire bonding apparatus of FIG.  5 A. In FIGS. 5A and 5B, the asymmetrical section  121  of FIG. 3A and 3B is changed into an asymmetrical section  121 ′ in which a hollow  5  is formed. Note that the hollow  5  is also formed within the symmetrical section  11 . 
     Even when the hollow  5  is formed within the symmetrical section  11  and the asymmetrical section  121 ′, the ultrasonic horn  1  has a sufficient rigidity, and accordingly, the wire bonding apparatus can be small in weight. 
     In the wire bonding apparatus of FIGS. 5A and 5B, as illustrated in FIG. 6, ultrasonic waves W 1  propagate within the symmetrical section It and the asymmetrical section  12 ′ and are incident perpendicularly to the capillary  4 . As result, the capillary  4  vibrates along the X-direction. On the other hand, there are no ultrasonic waves W 2  as shown in FIG.  4 . In other words, the spurious vibration of the capillary  4  along the Z-direction can also be suppressed, thus improving the accuracy of the wire bonding operation. 
     The present invention can be applied to an ultrasonic type wire bonding apparatus as well as an ultrasonic thermopressing type wire bonding apparatus. 
     As explained hereinabove, according to the present invention, since spurious vibration of the capillary can be suppressed, the accuracy of wire bonding operation can be improved.