Abstract:
The invention provides an ultrasonic transducer assembly for a bonding apparatus, comprising a bonding tool mounted to an amplifying horn secured between first and second ultrasonic-generating means. A method of forming a transducer for a bonding apparatus is also provided, comprising the steps of providing an amplifying horn, securing first and second ultrasonic-generating means to the amplifying horn such that the amplifying horn is located between said first and second ultrasonic-generating means and mounting a bonding tool to the amplifying horn.

Description:
FIELD OF THE INVENTION 
     The invention relates to ultrasonic transducers, and in particular, to ultrasonic transducers for bond formation (for example, welding of fine wires and metallic joints) in semiconductor packaging equipment. 
     BACKGROUND AND PRIOR ART 
     During a semiconductor packaging process, electrical wire connections are typically made between different electronic components, for example, between a semiconductor chip and a leadframe substrate. One method is to use ultrasonic welding equipment, such as an ultrasonic transducer. An ultrasonic welding operation is carried out using a transducer that is caused to vibrate at ultrasonic frequencies. The ultrasonic energy generated by the transducer is transmitted to materials to be bonded (for example, gold or aluminum wire for wire bonding and chips with gold bumps for thermosonic flip chip bonding) through a bonding tool, which is normally in the form of a wedge, capillary or collet. The ultrasonic transducer may therefore use ultrasonic energy to attach a length of bonding wire to contact surfaces or pads of the electronic components. 
       FIG. 1  is a side view of a conventional ultrasonic transducer  100  of the prior art. It comprises a plurality of piezoelectric elements, such as piezoelectric ceramic rings  102  disposed in a stack, for generating ultrasonic vibrations when electricity is passed through the ceramic rings  102 . The ceramic rings  102  are formed with central holes and held together in compression by back masses  104 . The ceramic rings  102  are connected to control circuitry and receive input signals that cause the ceramic elements to generate ultrasonic vibrations. The ultrasonic vibrations are amplified by an elongate amplifying horn  108  at the free end of which is a bonding tool, illustrated herein in the form of a capillary  110 . The capillary  110  is used to apply compressive bonding force to bonding components being welded together. A barrel  106  is provided between the ceramic rings  102  and the amplifying horn  108  to allow the transducer  100  to be mounted to a wire bonding apparatus. The transducer  100  vibrates along its axis, and its amplitude of vibration is represented by a line  112  in  FIG. 1 . The maximum displacement amplitude is at the free end of the amplifying horn  108  where the capillary is located. 
     A problem with such a conventional transducer is that the bonding force bends or deforms the transducer during bonding due to a reaction force, when the bonding tool is exerting pressure on a bonding component to be welded. This bending or deformation may result in poor coplanarity, and in turn lead to uneven distribution of bonding force exerted over different locations to be bonded. 
     In a particular instance, during thermosonic flip chip bonding, welding is performed on the number of gold bumps on a semiconductor chip. The bonding force could be in the order of 10 kg. The huge bonding force could deform the transducer and result in poor coplanarity on the bonding tool tip. The poor coplanarity leads to an uneven distribution of bonding force among gold bumps. A significant bonding strength difference may thus result among each bonding bump. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an improved transducer that avoids the aforesaid disadvantage of prior art ultrasonic transducers. 
     According to a first aspect of the invention, there is provided a transducer for a bonding apparatus, comprising a bonding tool mounted to an amplifying horn secured between first and second ultrasonic-generating means. 
     According to a second aspect of the invention, there is provided a method of forming a transducer for a bonding apparatus, comprising the steps of: providing an amplifying horn; securing first and second ultrasonic-generating means to the amplifying horn such that the amplifying horn is located between said first and second ultrasonic-generating means; and mounting a bonding tool to the amplifying horn. 
     It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of a preferred embodiment of an ultrasonic transducer in accordance with the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a side view of a conventional ultrasonic transducer of the prior art; 
         FIG. 2  is an exploded view of an ultrasonic transducer according to the preferred embodiment of the invention; 
         FIGS. 3(   a ) and  3 ( b ) are comparison charts showing axial vibration profiles of a conventional ultrasonic transducer as compared to the said ultrasonic transducer according to the preferred embodiment of the invention; 
         FIG. 4  is a schematic side view of the said transducer showing poling directions of its piezoelectric elements in use; 
         FIGS. 5(   a ) and  5 ( b ) are side view illustrations of deformations of a conventional transducer and the said transducer respectively experienced when applying a bond force; 
         FIG. 6  shows impedance and phase characteristics of the said transducer over a selected frequency range; and 
         FIG. 7  is a graphical illustration of vibration displacement against power output characteristics of the said transducer at its bond tip as compared to a conventional transducer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  is an exploded view of an ultrasonic transducer  10  according to the preferred embodiment of the invention. Ultrasonic energy is provided from first and second ultrasonic-generating means, which may be in the form of two sets of piezoelectric elements, typically made of ceramic rings  18  stacked and distributed evenly on either side of an amplifying horn  22 . The amplifying horn is preferably bi-conical in shape. In this embodiment, each piezoelectric stack comprises four ceramic rings  18 . The bi-conical amplifying horn  22  has a tapered geometry towards its centre which allows a maximum vibration amplitude at its centre. The transducer  10  is mountable to a bonding apparatus, such as a wire bonder machine, by way of a mounting device  12  with double flanges  14  and a mounting support  16  associated with each flange  14 . 
     The ceramic rings  18  and mounting supports  16  are affixed to the bi-conical amplifying horn  22  by back masses  20 , which are secured by fastening devices, such as tightening bolts  28 , to the bi-conical amplifying horn  22 . The mounting supports  16  are located at the vibration nodal points of the transducer  10 . A bonding tool, for example in the form of a capillary  24 , is preferably mounted substantially at the centre of the bi-conical amplify horn  22 , where vibration amplitude of the transducer  10  is the highest. It should be appreciated that the transducer  10  vibrates along its axis  30 . 
     Locating the bonding tool or capillary  24  at the centre of the two mounting supports  16  allows a good bonding coplanarity even under large compressive bond force as will be illustrated below. Further, as the bonding tool is located at the centre of the whole transducer  10 , it allows a rotary moment of inertia to be reduced to a minimum when an axis passing through the longitudinal axis of the bonding tool is aligned with its axis of rotation, as in the instant design. 
       FIGS. 3(   a ) and  3 ( b ) are comparison charts showing axial vibration profiles of a conventional ultrasonic transducer  100  as compared to the said ultrasonic transducer  10  according to the preferred embodiment of the invention. Both transducers are operating at full wavelength mode. In the drawings, the alphabet “R” represents the ability of the transducers  100 ,  10  to rotate about rotational axes  26 , which rotational axes  26  preferably pass through the centre of the capillaries  110 ,  24  or their longitudinal axes. Rotation may be necessary to align the capillaries  110 ,  24  with respect to a bonding location. U x  represents vibration amplitude. In  FIG. 3(   a ), the capillary  110  of the conventional transducer  100  is vibrating at its maximum amplitude at one vibrational wavelength distance, λ, from an opposite end of the transducer  100 . On the other hand, in  FIG. 3(   b ), the capillary  24  of the transducer  10  according to the preferred embodiment of the invention is vibrating at its maximum amplitude, A max , at λ/2 distance from either end of the transducer  10 . 
       FIG. 4  is a schematic side view of the said transducer  10  showing poling directions  32  of its piezoelectric elements or ceramic rings  18  in use. The drawing shows that an electrical current source  34  delivers a current and electric fields to all the ceramic rings  18 . Piezoelectric material has a poling direction. When we apply an electric field to a piezoelectric element in the poling direction, the piezoelectric element stretches horizontally in the electric field and shrinks vertically in accordance with Poisson&#39;s ratio. When we apply an electric field in the reverse direction, it shrinks horizontally in the electric field and stretches vertically. 
     By arranging the poling configurations or directions of the two sets of ceramic rings  18   a ,  18   b  such that they are in opposite poling directions  32 , the ceramic rings  18  are synchronized such that one set of rings  18   a  contracts (pull) while the other  18   b  expands (push). That is, one set of ceramic rings  18   a  will contract and the other  18   b  will expand under a single electrical source  34 . The whole transducer  10  vibrates at an ultrasonic frequency with the axial vibration profile as shown in  FIG. 3 . Due to the bi-conical shape of the amplifying horn  22 , the tapered gemetory amplifies the vibration amplitude towards its centre. 
       FIGS. 5(   a ) and  5 ( b ) are side view illustrations of deformations of a conventional transducer  100  and the said transducer  10  respectively experienced when applying a bond force. In the case of a conventional transducer  100  ( FIG. 5(   a )), when it is experiencing a bond force, the axis of its capillary  110  will shift away from the vertical so that there is a deviation between the actual bonding axis and the vertical. Such deviation will cause inaccuracy in bond force application. 
     In the case of the said transducer  10  according to the preferred embodiment, since its capillary  24  is equally supported on both sides by the bi-conical amplifying horn  22 , the axis of the capillary  24  maintains substantially aligned with the vertical. This is even though some deformation of the bi-conical amplifying horn  22  is experienced from applying the bond force. Application of a bond force is thus more accurate and repeatable. 
       FIG. 6  shows some electrical characteristics of the said transducer  10 , namely its impedance and phase characteristics over a selected frequency range. A resonance frequency is reached at about 62 kHz. 
       FIG. 7  is a graphical illustration of vibration displacement against power output characteristics of the said transducer  10  at its bond tip as compared to a conventional transducer  100 . The profile of the said transducer  10  is represented by squares, whereas the profile of the conventional transducer  100  is represented by circles. It can be seen from  FIG. 7  that displacement of the said transducer  10  can be significantly increased for any level of power output to the transducer  10 , as compared to a conventional transducer  100 . One reason for this increase is that two sets of piezoelectric element stacks  18   a ,  18   b  have been used as compared to one set for conventional transducers. 
     It would be appreciated that the preferred embodiment of the invention is advantageous in that the bonding coplanarity is independent of bond force. To ensure good alignment of the bonding tool, a hole for fixing the bonding tool may be drilled after the whole assembly as assembled. This assembly method may ensure a good perpendicularity of the bonding tool  24  with the mounting support  16  and simplify the transducer alignment procedure. 
     In a rotating bondhead mechanism, the rotational axis  26  is preferably aligned with the axis of the bonding tool  24 . As the bonding tool  24  of the transducer  10  is aligned with respect to the plane of the rotational axis  26 , the centre of gravity of the transducer  10  will fall in the rotational axis  26 , so that the rotation inertia of the device will be reduced to a minimum. 
     One of the main advantages of the present invention is the automatic maintenance of the coplanarity of the bonding tool with the rotary axis  26 , even at high bond forces. When a conventional transducer  100  is put under a high bond force, the transducer  100  will deform and cause the bonding tool tip to be no longer parallel to the bonding target. This lack of coplanarity becomes worse as the bond force increases. The present transducer  10 , however, can maintain the coplanarity of the bonding tool surface and the bonding target even under deformation as shown in  FIG. 5 . Thus, the volume of piezoelectric elements  18  can be increased without an increased tendency to deform. Moreover, it would be appreciated that the transducer  10  has a higher power handling capability than conventional transducers. 
     The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.