Patent Application
20050001018
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.
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.
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.
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.
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.
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.
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.
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.
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.
An example of an apparatus and method in accordance with the invention will now be described with reference to the accompanying drawings, in which:
protect surrounding dice from oxidation during bumping by the bumping assembly;
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.
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
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.
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.
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.
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.
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.
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.
