Abstract:
In a stacked die integrated circuit structure, the structure can subsequently be tested by removing any packaging material and separating the die from a die paddle and from each other. The separation can involve the use of chemicals or heat, with or without the use of mechanical force. One aspect of the invention includes making use of specifically chosen adhesives to secure the die to the die paddle and to each other, so that any subsequent removal can readily be achieved.

Description:
FIELD OF THE INVENTION 
   The invention relates to stacked die and a process for testing stacked die that are subsequently found to display faulty behavior. The invention also provides a new process for forming stacked die that makes it easier to subsequently test the die for identifying faults on the die. 
   BACKGROUND OF THE INVENTION 
   An ongoing trend in integrated circuits (ICs) is the attempt to reduce the footprint of the IC. This is addressed, for example, by introducing new processes that allow the gate length to be reduced and thereby allow more transistors to be formed on an IC. 
   Another recent solution is to provide more than one die on a packaged chip, wherein the die are stacked on top of each other and separated by an insulating material. A typical stacked die is shown in  FIG. 1 , which shows a bottom dice  100  secured by means of silver epoxy  102  (such as Ablestik 8340) to a die paddle  104 . A second, or top, dice  106  is secured to the bottom dice  100  by means of an adhesive layer  108  which can be a Teflon based epoxy such as Loctite Qmi 500. The entire structure, comprising the paddle  104 , bottom die  100  and top die  106  are encased in a packaging material, typically referred to as a package (not shown in  FIG. 1 ) and commonly made of a plastics material. In order to provide electrical contact to the two die  100 ,  106 , each die is provided with contacts along its periphery, and bonding wires (not shown) are connected to the contacts by means of gold ball bonds  110 . As shown in  FIG. 1 , in order to avoid the top die  106  from interfering with the gold ball bonds  110  of the bottom die  100 , the top die  106 , in this embodiment, is smaller than the bottom die  100 . However, it should be noted that this configuration is shown for illustration purposes only. More than two die can be stacked on top of each other and at the time of this application, stacking four die on top of each other is known in the art. Also, it is not necessary that the die be smaller toward the top of the stack. The epoxy layers and spacers between the die can sufficiently space the die to accommodate the gold ball bonds. 
   A problem facing the industry is in the testing of such stacked die devices after they have been stacked and packaged. It is common for faulty die to be returned by customers (e.g. original equipment manufacturers (OEMs)) to the manufacturer for analysis to determine the cause of the fault. In the case of non-stacked die this involves the decapping of the IC, i.e. the top of the package is removed by mechanical grinding or chemical etching. With the IC exposed, electrical testing contacts (other than the I/O contacts on the periphery of the IC become accessible for attaching needles of testing equipment or otherwise testing the IC. However, in the case of a stacked device, there is more than one die. Thus, at least some of the electrical contacts and circuitry of the bottom die are typically covered by the top die, and cannot be accessed. Furthermore, it may not always be possible to isolate the characteristics of individual devices since the functioning of devices on one die is impacted by the devices on the other die, since the die are interconnected to allow all of the die to work as one large IC. This interconnection may take place externally (on the printed circuit board on which the stacked device is mounted), or by die-to-die gold wire bonds, or internally by having vias through the epoxy between the die, to thereby allow electrical connection between contacts on one die and contacts of another die. 
   The present invention seeks to provide a way of testing stacked ICs and also seeks to provide a process for forming stacked ICs that makes the ICs more amenable to subsequent testing. 
   SUMMARY OF THE INVENTION 
   The invention relates to a method of testing a stacked, packaged IC that includes an IC package, at least two die packaged in the package, a die paddle on which the die are mounted, and bonding wires connected to at least one of the die, the method comprising removing at least part of the IC package, and separating the die from each other and from the die paddle. 
   The method may also comprise separating the bonding wires from the die. The bonding wires may be connected to the die by means of gold ball bonds and the separating may include severing the bond wires and then breaking or cutting the bonding wires or mechanically grinding the bond wires off at the neck of the bond. The breaking may include repeated bending to break the wires by wire fatigue. The cutting may include microtome cutting. The removal of at least part of the package may include, for example, chemical etching or mechanical grinding. The stacked IC may include adhesive such as epoxy for securing the die to each other and to the die paddle, and the removal of the die from the die paddle and from each other may include placing the IC in an acid bath, or applying heat to at least part of the IC, or thermally shocking the IC by first heating and then suddenly cooling it, and may include applying a mechanical force between the two parts being separated. The mechanical force may be applied by securing the one part and applying force, e.g. by means of a shear tool, to the other part. The heat may, for example, be applied by passing heated inert gas over at least part of the IC or by placing at least part of the IC on a heated hot plate surface. The sudden cooling in order to shock the IC, may include placing the IC in a water bath, e.g. water at room temperature (e.g. approximately 15 to 35 degrees Celsius). The one part may be secured by placing it against a barrier, e.g. a metal barrier that is held in place relative to a surface on which the IC is placed, or is temporarily or permanently secured to the surface on which the IC is placed. The surface on which the IC is placed may be a heated hot plate surface. The microtome cutting of the bond wires may include mounting the IC on a microtome tool, which may include securing the IC by means of wax to the microtome tool. The cutting typically comprises slicing the neck of the bond, where the bond wire connects to the gold ball bonds. The method may further include cleaning epoxy or other adhesive material from the die once they are separated from the die paddle and when they are separated from each other, e.g., by oxygen plasma cleaning. 
   Further, according to the invention, there is provided a method of forming a stacked IC that includes at least two die stacked on top of each other, comprising connecting the die to each other by means of a light sensitive adhesive, such as a ultraviolet sensitive resin, which breaks down when exposed to certain frequencies of light. The stacked IC may include a die paddle on which the die are mounted. The die may be secured to the die paddle by means of a light sensitive adhesive, such as a ultraviolet sensitive resin, which breaks down when exposed to certain frequencies of light. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows one embodiment of a simple prior art stacked IC; 
       FIG. 2  shows one embodiment of the method of the invention for testing a stacked IC such as the IC of  FIG. 1 ; 
       FIG. 3  shows another step in the method of  FIG. 2 , and 
       FIG. 4  shows yet another step in the method of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In one embodiment of the invention, stacked IC such as the one illustrated in  FIG. 1  is tested by separating the individual die  100 ,  106  making up the IC, into separate parts, and then connecting each of the dice to a separate lead frame for facilitating applying power and input/output (I/O) signals to the dice. For ease of discussion, the reference numerals used in  FIG. 1  will be used in this embodiment of the invention to refer to the parts of the stacked IC. 
   To simplify this process, it will be appreciated that the structural integrity of each of the dice cannot be compromised, the die have to be clean and the existing contacts (in this case, the gold ball bonds) should remain intact. 
   In one embodiment of the invention, the packaging material is removed using mechanical grinding or acid etching in a manner known in the art. Thereafter, the bond wires are cut, e.g., using wire cutters, and then the die are separated from the die paddle  104 , and the die  100 ,  106 , are separated from each other. The die are then cleaned by removing any adhesive material adhering to the die surfaces and by breaking, cutting or grinding the bond wire pieces sticking out beyond the neck of the bond. 
   The removal of the bond wires will be discussed in further detail below with respect to  FIG. 4 . 
   The separation of the stacked die from the die paddle  104  is done either by immersing the device (with the packaging and bond wires removed) into an acid bath, e.g. fuming nitric (having a concentration of about 86% or more), or by applying heat to the device, e.g., by placing the die paddle onto a hot plate surface at 330 degrees Celsius, as shown in the embodiment of  FIG. 2 , or, as proposed in one embodiment, by placing the die paddle onto a hot plate surface at 330° C. for about one minute followed by immersion in water at 20° C. In another embodiment, heat may be applied to the die paddle  104  and die  102 ,  106  by exposing the structure to heated inert gas, e.g., in a gas chamber, or the heated inert gas may be applied to the structure by passing heated inert gas over the structure. It will be appreciated that the term structure refers here to the die paddle  104  and stacked set of die, in this case, die  102 ,  106 . In this embodiment the separation of the two die  103 ,  106  from the die paddle  104  is assisted by exerting a mechanical force on the die  102 ,  106  using a shear tool  206  (in one embodiment comprising a flexible piece of heat resistant Teflon), while holding the die paddle  104  in place, e.g., using a metal barrier  202  that is physically held in place by a person or secured relative to the hot plate surface  200 . 
   As shown in  FIG. 3 , the top die  106  is then separated from the bottom die  100  in a similar fashion, by placing the lower die  104  on the hot plate surface  200  and securing it using the metal barrier or shear stop  202 . ( FIG. 3  does not show the adhesive layer  102  but, in practice, at least some of the epoxy or other adhesive will still be adhering to the lower surface of the lower die  100 . The upper die  106  is then pushed mechanically, again by making use of the shear tool  206 . It will be appreciated that the advantage of using a hot plate as the heating element rather than an inert gas is that it allows the mechanical shearing force to easily be applied at the same time rather than having the heating and shearing force steps be conducted sequentially, or having to make use of a mechanical or remotely manipulated shear tool while the device is located in an inert gas chamber. 
   Once the die in the stacked device have been separated by the method described above or another method such as an acid bath or heat alone, any resin adhering to the surfaces of the die has to be removed. Once common resin used between the lower dice and the die paddle is silver epoxy to a thickness of about 50 μm, while a Teflon based epoxy to a thickness of about 10 μm is commonly used between the die. In the embodiment shown in  FIG. 2 , the adhesive layer  102  is silver epoxy 50 μm in thickness, and the adhesive layer  108  is a Teflon based epoxy 10 μm in thickness. 
   In one embodiment, the cleaning of the die is done by placing the die in an oxygen plasma, however, other embodiments have made use of sulphuric acid (e.g at 95%) or other solvents. 
     FIG. 4  shows the process of removing the bond wires  400 , sticking out beyond the neck  402  of the gold ball bond  110  of the die  100 . As shown in  FIG. 4 , each gold ball bond  110  is connected to a bond pad  410 . In this embodiment, the bond wire  400  sticking out from the neck  402  is removed by making use of a microtome tool having a sample mount  420  on which the die  100  is mounted, and a microtome blade  422 . The blade  422  is slidably connected to the mount  420  and adjustable in distance relative to the mount  420  to allow the position of the blade to be adjusted to coincide with the necks of the bonds. The blade is then moved parallel to the surface of the die  100  to cut the bond wires  400  at the necks  402  of the bonds. In this embodiment the die were first cleaned of adhesive residue before the pieces of bond wire  400  were cut. It will be appreciated that the cleaning step could also be done after the bond wires are removed. 
   The rest of the testing of the die would be done in a manner known in the art for separate die. For instance, each of the die would be connected to a separate lead frame by means of new bond wires, bonded to existing cut balls or exposed bonding pads, and then the power and input/output leads would be applied to the dice, using the lead frame. CONFIRM. A test apparatus would supply the necessary power and input signals and would monitor the output signals to determine whether there is any faulty behavior and to narrow down the area of concern before the die is further broken down, e.g., by parallel lapping or by cross-sectioning. 
   Currently die are typically attached to the die paddle using an epoxy such as Ablestik 8340, supplied by Ablestik, which is about 25 to 125 μm thick and has a silver filler, or using a die attach film (DAF) such as Nitto EM 100 DAF, supplied by Nitto Denko, which is only about 10–40 μm thick and has a silica filler. Dice are, in turn, typically attached to each other by means of an epoxy such as Loctite Qmi 550 with spacers, supplied by Loctite, which is about 25 to 125 μm thick and has a Teflon filler, or using a die attach film such as Hitachi FH-800 DAF (supplied by Hitachi) which is only about 10–40 μm thick and has a silica filler. 
   As a further feature of the invention and as yet another embodiment of a method of separating the die for testing, the present application provides for a method of connecting the die to each other and to the die paddle by making use of an adhesive that degenerates rapidly or breaks down under certain conditions, e.g. under certain radiation conditions such as ultraviolet (UV) light. Applicant is aware of Hitachi FH-800, a resin which is cured by exposure to UV light. Applicant is currently working on obtaining details on resins or other adhesives that rapidly break down when exposed to UV light. Thus in one embodiment, the die would be attached to each other and to the die paddle by a UV sensitive adhesive such as a UV sensitive epoxy. In the absence of exposure to UV radiation, the die and die paddle would remain firmly secured to each other. However, when the need arises to test the die individually, they would readily be removable from the die paddle and each other by exposing the die and die paddle to UV radiation. It will be appreciated that other types of adhesives could be used that rapidly break down when exposed to certain radiation or chemical. This will allow not only for the easy separation of the die from the die paddle and each other but also simplifies or automatically takes care of the removal of any remaining adhesive adhering to the die surface. Thus this part of the cleaning step can be eliminated altogether. 
   More generally, while the invention was described with respect to a specific embodiment and by referring to a few specific examples, the invention is not so limited but includes any embodiments within the scope of the claims.