Abstract:
The invention provides an apparatus and a method for forming conductive bumps on a plurality of semiconductor devices with an oxidizable material. The apparatus comprises a bump forming device, a chamber system adapted to house the semiconductor devices and a gas supply for supplying an inert gas into the chamber system. A support table is provided for supporting the semiconductor devices during bumping, and the said support table is operative to move the semiconductor devices from a bumping site into the chamber system after bumping.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to an apparatus usable in the stud bumping of semiconductor wafers to form conductive terminals on the wafer, and in particular, to stud bumping of wafers mechanically with material that is prone to oxidation, such as copper.  
       BACKGROUND AND PRIOR ART  
       [0002]     Flip chip microelectronic assembly is already established in the semiconductor industry. The assembly process involves the direct electrical connection of face-down electronic components onto substrates, circuit boards or carriers by means of conductive bumps on the chip bond pads. In contrast, older technology involves wire bonding of each bond pad of face-up chips to substrates, circuit boards or carriers. The advantages of flip chip packaging over older technology include smaller size, better performance, increased flexibility, reliability and lower cost. For example, eliminating packages and bond wires reduces the required board area by up to 95%, and requires far less height. Weight can be less than 5% of packaged device weight. In fact, the flip chip can be even smaller than Chip Scale Packages (CSP) because its size is the size of a chip. Flip chip materials are also becoming more widely available, further lowering production costs.  
         [0003]     A conductive bump serves several functions in the flip chip assembly. Electrically, the bump provides a conductive path from chip to substrate. The bump also provides a thermally conductive path to carry heat from the chip to the substrate. In addition, the bump provides part of the mechanical mounting of the chip to the substrate. Finally, the bump provides a spacer, preventing electrical contact between the chip and substrate conductors, and acting as a short lead to relieve mechanical strain between board and substrate.  
         [0004]     There are several ways of forming bumps on semiconductor wafers. These include solder bumping, plating and stud bumping. The present invention relates to stud bumping, which bumps wafer dice by a wire bonding technique that is modified from the older wire bonding technology. This technique makes a ball for wire bonding by melting the end of a wire to form a sphere. The ball is attached to the chip bond pad as the first part of a wire bond. To form bumps instead of wire bonds, wire bonders are modified to break off the wire after attaching the ball to the chip bond pad. The ball, or “stud bump” remaining on the bond pad provides a permanent connection to the underlying metal on the chip.  
         [0005]     Traditionally, gold wire is used in stud bumping. However, there has recently been increased interest in using copper wire instead. Copper bumps have been found to offer increased reliability, extended temperature range, greater mechanical strength, higher connection density, improved manufacturability, and better electrical and heat-dissipatng performance. Nevertheless, a challenge faced in copper stud bumping is that copper reacts with oxygen and oxidizes at the high temperatures that stud bumping processes are carried out. The formation of copper oxide, which is non-conductive, result in defects in the final product, and this needs to be avoided as far as possible. It would thus be desirable to have access to an apparatus that allows copper stud bumping to be performed at a high temperature while reducing the risk of oxidation of the copper bumps.  
       SUMMARY OF THE INVENTION  
       [0006]     It is therefore an object of the invention to seek to provide an improved apparatus which reduces exposure of stud bumps to oxygen in the atmosphere during a stud bumping process with material that is prone to oxidation so as to minimize oxidation of the bumps.  
         [0007]     According to a first aspect of the invention, there is provided an apparatus for forming conductive bumps on a plurality of semiconductor devices with an oxidizable material, comprising: a bump forming device; a chamber system adapted to house the semiconductor devices; a gas supply for supplying an inert gas into the chamber system-; and a support table for supporting the semiconductor devices during bumping, said support table being operative to move the semiconductor devices from a bumping site into the chamber system after bumping.  
         [0008]     According to a second aspect of the invention, there is provided a method for forming conductive bumps on a plurality of semiconductor devices with an oxidizable material, comprising the steps of providing a support table for supporting the semiconductor devices; forming bumps on the semiconductor devices on the support table at a bumping site; and moving the semiconductor devices from the bumping site into a chamber system after bumping while supplying an inert gas into the chamber system.  
         [0009]     It would be convenient hereinafter to 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  
       [0010]     An example of an apparatus and method in accordance with the invention will now be described with reference to the accompanying drawings, in which:  
         [0011]      FIG. 1  is a plan view of a bumping assembly located over a wafer array including an oxidation reduction device positioned adjacent the bumping assembly for reducing oxidation while bumping;  FIG. 2  is an isometric view of a shroud with a detachable window cover to  
         [0012]     protect surrounding dice from oxidation during bumping by the bumping assembly;  
         [0013]      FIG. 3  is an isometric view of an apparatus according to the preferred embodiment of the invention including a chamber system looking from one end of the chamber system;  
         [0014]      FIG. 4  is an isometric view of the apparatus looking from an opposite end of the chamber system of  FIG. 3 ;  
         [0015]      FIG. 5  is an isometric view of the apparatus of  FIGS. 3 and 4  with an open outer chamber cover revealing a removable inner chamber;  
         [0016]      FIG. 6  is an isometric view of the removable inner chamber that has been removed from the outer chamber; and  
         [0017]      FIG. 7  is an isometric view of the removable inner chamber from an opposite end to that of  FIG. 6  illustrating its detachable tubing connectors. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]      FIG. 1  is a plan view of a bumping assembly  10  that is usable for stud bumping located over a plurality of semiconductor devices, such as that comprised in a wafer array  18 . A bump forming device in the form of a bonding horn  12  of an ultrasonic transducer wire bonder that includes a capillary at its end creates ball bumps on the surface of dice that form the wafer array  18  at a bumping site  23 . An electrical discharge electrode  14  is positioned next to the bonding horn  12  and capillary to provide electrical sparks to form the ball bumps. Since wire which is prone to oxidation at typical bumping temperatures—in this case copper wire—is used for bumping, an oxidation reduction device  16  with an inert gas nozzle  17  is positioned adjacent the bonding horn  12  to blow an inert gas such as nitrogen or argon gas (or a combination of nitrogen and hydrogen gas) to the bumping site  23  for reducing oxidation of the copper ball bumps that are formed.  
         [0019]     A fixed shroud  20  with a detachable window cover  22  covers a portion of the wafer  18  and is positioned around the bumping site  23  for covering one or more bumped semiconductor devices. The detachable window cover  22  can be changed to offer a different opening size based on the area of the bumping site  23  that is required. The detachable window cover  22  is usually made of regular metal, but may be made of a porous metal material if it is desired to generate a supply of the inert gas through the body of the cover  22 . It is preferable that the shroud  20  also has nozzles or outlets blowing inert gas onto the wafer  18  and particularly the bumping site  23 , to further reduce the risk of oxidation.  
         [0020]      FIG. 2  is an isometric view of a shroud  20  with a detachable window cover  22  to protect surrounding dice on the wafer  18  from oxidation during bumping by the bumping assembly  10 . As mentioned above, it is preferable that the shroud  20  include a number of gas nozzles  24  on its bottom surface to continuously introduce the inert gas onto the surface of the wafer  18  during bumping to further reduce the risk of oxidation of bumped dice located away from the bumping site  23 .  
         [0021]      FIG. 3  is an isometric view of an apparatus according to the preferred embodiment of the invention including a chamber system looking from one end of the chamber system. The chamber system comprises an outer chamber  30  and an inner chamber  40  that is houseable within the outer chamber  30  (see  FIG. 5 ). The outer chamber  30  is enclosed on three sides, with one open side that enables a support table, which may be in the form of a wafer table  34 , to slide into and out of the outer chamber  30 . The outer chamber  30  has a top cover  32  which can be opened to enable access to its contents.  
         [0022]     The wafer table  34  is designed to support and hold a wafer array  18  in a relatively fixed position to allow bumping by the bonding horn  12  to be carried out. In this embodiment, bumping is carried out at a bumping site  23  adjacent to an opening of and just outside the confines of the outer chamber  30 . The wafer table  34  is operative to move the semiconductor devices of the wafer array  18  from the bumping site  23  into the chamber system after bumping. The shroud  20  is fixed together with the bumping assembly  10  just outside the opening of the outer chamber  30  such that its detachable window  22  is located at the bumping site  23  where dice are to be bumped. Thus, the wafer  18  generally moves relative to the bumping assembly  10  and shroud  20 , which maintain relatively fixed positions. The wafer table  18  includes a heater block (not shown) and a detachable top plate  35  (see  FIG. 6 ) attached on top of the heater block.  
         [0023]      FIG. 4  is an isometric view of the apparatus looking from an opposite end of the chamber system of  FIG. 3 . The wafer table  34  carrying the wafer  18  is slidable into and out of the outer chamber  30 . To control the positions of the wafer table  34  and thus the wafer  18  relative to the bumping assembly  10 , the wafer table and outer chamber  30  of the chamber system may be coupled to a positioning device, preferably an XY table. The wafer table  34  may be coupled to a Y stage  36  of the XY table whereas the outer chamber  30  may be coupled to an X stage  38  of the XY table. Since the wafer table  34  is slidable with respect to the outer chamber  30  in the Y direction along a first axis, the Y stage  36  may determine the extent to which the wafer table  34  is retracted into the outer chamber  30 . This, together with movement of the X stage  38  along a second axis perpendicular to the first axis, allows access by the relatively fixed bumping assembly  10  to every semiconductor device on the wafer  18 .  
         [0024]     A typical stud bumping process may now be described. The process starts with the wafer table  34  in an open position so that the whole wafer is exposed. The wafer table  34  is heated to a temperature of between  200 ° C and  300 ° C to prepare the wafer  18  for bumping. The Y and X stages  36 ,  38  of the XY table position the wafer table  34  and outer chamber  30  respectively such that the bumping assembly  10  and window cover  22  defining the bumping site  23  are located over an innermost semiconductor device on the wafer array  18 , that is, the portion of the wafer  18  nearest the outer chamber  30 . The Y position of the wafer  18  is then fixed, and the wafer  18  is moved in the X direction to bump a row of dice in sequence. The bumping assembly  10  conducts copper stud bumping or bonding on the wafer surface. During bumping, the oxidation reduction device  16  blows an inert gas to the bumping site  23  to protect the copper balls formed from oxidation. Gas nozzles  24  under the fixed shroud  20  also blow the inert gas to and beyond the bumping site  23  to protect the bumped studs both under the shroud  20  and inside the bumping site  23 .  
         [0025]     After a row of dice at a Y position has been bumped, that row of dice on the wafer table  34  is moved into the outer chamber  30  (and inner chamber  40 ) to protect them from the atmosphere. The next row of dice in the next Y position is then bumped in sequence by movement in the X direction. This process will be continued until all rows of dice in the wafer array  18  are bumped. The whole wafer  18  is then slid into the outer chamber  30  (and inner chamber  40 ). The gas supply for continuously supplying an inert gas into the inner chamber  40  of the chamber system is required as the wafer  18  is still hot and the finished copper studs should not be exposed to oxygen.  
         [0026]      FIG. 5  is an isometric view of the apparatus of  FIGS. 3 and 4  with an open outer chamber cover  32  revealing a removable inner chamber  40 . The inner chamber  40  is removable from the outer chamber  30  and another inner chamber may be put in its place for conducting stud bumping on another wafer. The usefulness of the removable inner chamber  40  lies in the fact that the wafers  18  are heated to high temperatures during bumping, and therefore, the copper stud bumps on the wafer  18  are still susceptible to oxidation until the wafer  18  cools sufficiently. Containing the wafer  18  inside an enclosed inner chamber  40  prevents exposure of the wafer  18  to the atmosphere until it has cooled sufficiently. The removal of the inner chamber  40  allows another wafer to be bumped while a completed wafer  18  is allowed time to cool.  
         [0027]     The inner chamber  40  may be inserted into or removed from the outer chamber  30  either manually or automatically. Whilst it may be removable by opening the outer chamber cover  32  first, it may also be removable by sliding it out-of the open end of the outer chamber  30  without opening the outer chamber cover  32 .  
         [0028]      FIG. 6  is an isometric view of the removable inner chamber  40  that has been removed from the outer chamber  30 . The inner chamber  40  includes an inner chamber cover  42  with a bracket that allows the wafer table  34  to slide relative to it. The wafer table  34 , together with the inner chamber cover  42 , respectively form an enclosure for the wafer  18  when the wafer table  34  is fully retracted. The wafer table  34  may be fully extended to expose the whole of the wafer  18  for bumping, or fully retracted to protect the heated wafer  18  from oxygen in the atmosphere. The removable part of the inner chamber  40  comprises the inner chamber cover  42  and top plate  35  (not including the heater block) of the wafer table  34  for forming an enclosure around bumped semiconductor devices of the wafer array  18 .  
         [0029]     Furthermore, in a preferred embodiment, the inner chamber cover  42  has a number of gas nozzles on its inside surface to blow an inert gas into the inner chamber  40  onto the wafer  18  when the wafer table  34  is retracted. Preferably, the inner chamber cover  42  also has a recessed portion on its inside surface to enable the inner chamber cover  42  to be dropped on top of the top plate  35  before the inner chamber  40  is removed from the outer chamber, such that the wafer  18  can be set inside the recessed portion for closer protection.  
         [0030]      FIG. 7  is an isometric view of the removable inner chamber  40  from an opposite end to that of  FIG. 6  illustrating its detachable gas tubing connectors  46 . These connectors  46  are designed to receive detachable tubing (not shown) that are attached to outside gas sources and detachably connectable to the inner chamber  40  to introduce an inert gas into the inner chamber  40  during bumping. As mentioned, the reason is so that stud bumps that have been formed do not oxidize while inside the inner chamber  40 . Therefore, there are preferably internal tunnels (not shown) in the inner chamber cover  42  to direct the inert gas to gas nozzles on the inside surface of the inner chamber cover  42 . The detachable tubing may be disconnected from an inner chamber  40  once bumping of a wafer  18  inside it has been completed, allowing the inner chamber  40  to be removed from the outer chamber  30 . Another inner chamber with another wafer may be put into the outer chamber  30 , and the tubing connected to the new inner chamber.  
         [0031]     It should be appreciated that the described embodiment of the invention provides oxidation prevention for copper stud bumping or bonding during the whole process from stud bumping to finished wafer transferring. The apparatus can cater for wafers  18  of different diameter and is relatively easy to operate, thereby promoting flexibility and savings in cost. By moving the chamber system together with an XY table, a relatively compact apparatus can be obtained. Therefore, a relatively smaller amount of inert gas is required for the more compact apparatus. Further, machine idling time is minimized because no time is needed to cool down the finished wafer or the heater bock before removing it from the machine. The finished wafer can be removed from the outer chamber  30  immediately after stud bumping is completed, and another wafer can be processed.  
         [0032]     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.