Abstract:
A method of bonding a wire to a bond pad on an electronic or photonic device is provided. A section of the wire is held within a bond head of the wirebonding apparatus. A laser beam is directed onto the bond pad. Energy of the laser beam heats the bond pad to the temperature that is higher than the temperature of the device. The bond head is subsequently moved toward the device to bring a portion of the wire into contact with the bond pad. Ultrasonic energy is provided to an interface between the portion of the wire and the bond pad.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1). Field of the Invention  
           [0002]    This invention relates to a method and apparatus for bonding a wire to a bond pad on a device, such as an electronic or a photonic device.  
           [0003]    2). Discussion of Related Art  
           [0004]    Electronic and photonic devices usually have a number of bond pads on upper surfaces thereof that are connected to bond pads on package substrates or other devices for purposes of providing electronic signals to and from the electronic or photonic devices.  
           [0005]    A common technique for interconnecting such bond pads is wirebonding. A portion of a wire is held against a bond pad, and ultrasonic energy is provided through an ultrasonic bond head to an interface between the portion of the wire and the bond pad. Such energy by itself is usually insufficient to fuse the wire to the bond pad. The electronic device is usually heated to approximately 180° C. by a substrate workholder during the bonding process. Heat energy from the workholder supplements the ultrasonic energy, and the combination of the heat energy and ultrasonic energy is sufficient to fuse the wire to the bond pad.  
           [0006]    Several electronic and photonic devices will be damaged at high temperatures, and often require processing conditions much below the 180° C. to which a device has to be heated for purposes of ultrasonic bonding. The maximum temperatures for some integrated circuit packages having certain thin films may, for example, be below 100° C. Ultrasonic bonding at such low temperatures is usually not possible. Ultrasonic bonding of gold wires to aluminum-capped wirebond pads at temperatures below 125° C., for example, is generally not possible.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The invention is described by way of example with reference to the accompanying drawings, wherein:  
         [0008]    [0008]FIG. 1 is a cross-sectional side view illustrating a wirebonding apparatus for carrying out a wirebonding method, according to an embodiment of the invention;  
         [0009]    [0009]FIG. 2 is a cross-sectional plan view on  2 - 2  in FIG. 1, illustrating the location of a laser beam that is used to heat a bond pad;  
         [0010]    [0010]FIG. 3 is a view similar to FIG. 1, illustrating the use of an ultrasonic bonding apparatus to bond a wire to the bond pad which has been heated with the laser beam;  
         [0011]    [0011]FIG. 4 is a cross-sectional side view, illustrating bonding of the wire at a secondary bond pad; and  
         [0012]    [0012]FIG. 5 is a view similar to FIG. 3, illustrating the apparatus after the wire has been severed and a free end of the wire is moved into a position to attach a free end of the wire to a further bond pad.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    [0013]FIG. 1 of the accompanying drawings illustrates a wirebonding apparatus  10  used for bonding wires  12  to bond pads  14 A and  14 B on an electronic or photonic device  16 , according to an embodiment of the invention. The wirebonding apparatus  10  includes a holder  18  for the device  16 , ultrasonic bonding apparatus  20 , and laser heating apparatus  22 .  
         [0014]    One skilled in the art will appreciate that some of the components illustrated in FIG. 1 are conventional to a wirebonding apparatus. A typical wirebonding apparatus also includes a frame and many other components that move relative to one another to, for example, feed wire, cut wire, etc. Such components are not discussed in detail herein, so as not to obscure the invention. For example, FIG. 1 illustrates a holder  18  and a component  24 . The holder  18  and the component  24  are both mounted to a frame, and the frame is configured to allow for vertical movement of the component  24  toward and away from the holder  18 . The exact configuration of the frame, its actuators, etc., are beyond the scope of this invention.  
         [0015]    The ultrasonic bonding apparatus  20  includes an ultrasonic bond head  26  and an ultrasonic source  28  connected to the ultrasonic bond head  26 . The ultrasonic bond head  26  is mounted to the component  24  so as to be movable together with the component  24  when the component  24  moves vertically toward and away from the holder  18 . The ultrasonic bond head  26  has a passage  30  extending vertically therethrough.  
         [0016]    The ultrasonic source  28 , when energized, can provide an alternating voltage to the ultrasonic bond head  26  at ultrasonic frequencies. Ultrasonic energy is thereby provided from the ultrasonic source  28  to the ultrasonic bond head  26 .  
         [0017]    The laser heating apparatus  22  includes a soft beam laser  34 , an optic fiber  36 , and an alignment piece  38 .  
         [0018]    The alignment piece  38  is secured to the component  24 . The alignment piece  38  can move together with the component  24  and the ultrasonic bond head  26  in a vertical direction toward and away from the holder  18 .  
         [0019]    The optic fiber  36  has an end that is inserted and held in alignment within a passage that extends through the alignment piece  38 . The end of the optic fiber  36  held by the alignment piece  38  points vertically downward. An opposing end of the optic fiber  36  is connected to the laser  34 . The laser  34  is typically mounted in a stationary position relative to the holder  18 , and movement of the alignment piece  38  relative to the laser  34  is allowed for due to flexibility of the optic fiber  36 .  
         [0020]    In use, the electronic or photonic device  16  is located on an upper horizontal surface of the holder  18 . An end  40  of one of the wires  12  is fed through the passage  30  in the ultrasonic bond head  26 , and a section  42  of the wire  12  is held and aligned by the passage  30 . The ultrasonic source  28  and the laser  34  are at this stage still switched off. The electronic or photonic device  16  and the bond pads  14 A and  14 B are at the same temperature, for example, 20° C. The component  24  is moved in horizontal x- and y-directions, so that the free end of the optic fiber  36  and the end  40  of the wire  12  are moved so that they are simultaneously in positions over the bond pad  14 A.  
         [0021]    The laser  34  is then switched on, or energized, and a laser beam  44  is emitted thereby, which propagates through the optic fiber  36 , is emitted by the free end of the optic fiber  36 , held by the alignment piece  38 , and then propagates through air onto the bond pad  14 A. FIG. 2 illustrates the location at which the laser beam  44  strikes the bond pad  14 A on the right-hand side of the bond pad  14 A, and the position of the end  40  of the wire  12  above the left-hand side of the bond pad  14 A.  
         [0022]    The laser beam may also be focused closer to the bond location or directly at the wire to pad interface. The laser pad heating and ultrasonic wirebonding could be done in parallel (the laser is on during the wirebond fusion). Substrate pad heating may also be required (not just the die pad), since the entire package will be at a lower temperature.  
         [0023]    Energy from the laser beam  44  causes localized heating of the bond pad  14 A, so that a temperature of the bond pad  14 A increases from 20° C. to approximately 125° C., while a majority (e.g., at least 99%) of the electronic or photonic device  16  and the other bond pad  14 B remain at 20° C. The temperature of 125° C. to which the bond pad  14 A is heated is sufficiently high to allow for subsequent ultrasonic bonding of the wire  12  to the bond pad  14 A. Such a high temperature could, however, cause damage to circuitry or photonics of the electronic or photonic device  16 . Localized heating of the bond pad  14 A, however, avoids damage to electronics or photonics of the device  16 . In other embodiments, the temperature of the bond pad  14 A may increase to between 100° C. and 150° C. In other embodiments, the temperature of the bond pad  14 A may be at least 50° C. more than the temperature of the electronic or photonic device  16 . In other embodiments, the electronic or photonic device  16  is preferably always below 50° C.  
         [0024]    The laser beam  44  preferably has an energy density of between 10 and 100 mJ/cm 2 . Such an energy density is sufficiently low to cause gentle heating of the bond pad  14 A. The laser beam may have a wavelength of between 1.06 μm (CO 2  laser) to 10 μm (Seed ND:YAG laser). In order to heat a 50 μm by 60 μm aluminum-capped wirebond pad, the beam  44  would be a continuous wave expanded and sent through a mask to produce a final spot size of approximately 50 μm in diameter. The process time would be less than 200 ms, and the energy density would be approximately 50 mJ/cm 2 .  
         [0025]    The laser beam  44  is subsequently switched off. As illustrated in FIG. 3, the component  24 , together with the ultrasonic bond head  26 , and the alignment piece  38 , are subsequently moved downward toward the electronic or photonic device  16 . The end ( 40  in FIG. 1) of the wire  12  is deformed between opposing surfaces of the ultrasonic bond head  26  and the bond pad  14 A to form a metal mass  50 . In another embodiment, the wire  12  may be deformed into the different shape or may not be deformed at all. An interface  52  is created between the metal mass  50  of the wire  12  and the bond pad  14 A.  
         [0026]    The ultrasonic source  28  is then switched on, or energized, so that ultrasonic energy is provided thereby to the ultrasonic bond head  26 . The ultrasonic energy is transmitted from the ultrasonic bond head  26  to the interface  52 . The ultrasonic energy provided to the interface  52  by itself is generally insufficient to fuse the metal mass  50  to the bond pad  14 A. However, heat energy provided by the heated bond pad  14 A supplements the ultrasonic energy to fuse the metal mass  50  to the bond pad  14 A at the interface  52 . The ultrasonic source  28  is then switched off.  
         [0027]    As illustrated in FIG. 4, the ultrasonic bond head  26  is then moved sequentially upward in a z-direction, sideways in an x-direction, and downward in the z-direction to another bond pad  14 C. The wire  12  is then attached to the bond pad  14 C, so as to interconnect the bond pads  14 A and  14 C. A blade  54  is then used to sever the wire  12  in a direction  56 . Thereafter, a new free end of the wire  12  is held by the ultrasonic bond head  26 .  
         [0028]    As illustrated in FIG. 5, the newly created free end of the wire  12  may then be moved into a position over the bond pad  14 B. The process, illustrated with reference to FIGS. 1, 2, and  3  for attaching the wire  12  to the bond pad  14 A may then be repeated to attach the wire  12  to the bond pad  14 B.  
         [0029]    It can thus be seen that wires  12  can be bonded to the bond pads  14 A,  14 B, and  14 C using an ultrasonic bonding apparatus  20  without heating the electronic or photonic device  16  to temperatures that may cause damage to electronics and photonics thereof. The laser heating apparatus  22  provides fast (less than 200 ms), localized (50 μm beam) heating of the bond pads  14 A,  14 B, and  14 C to facilitate ultrasonic bonding.  
         [0030]    Although ultrasonic bonding has been described, by way of example, using a ball bonding process, one skilled in the art will appreciate that the invention may be applied with minor modification to other wirebonding processes such as wedge bonding, reverse bonding, stud bump bonding, etc.  
         [0031]    While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described since modifications may occur to those ordinarily skilled in the art.