Abstract:
A method of assembling a multi-chip module does not necessitate more the one pass through a die attach machine. The method involves attaching a smaller die to a larger die without using a die attach machine. The larger die may be attached to a support structure using a die attach machine.

Description:
This is a continuation of prior application Ser. No. 09/767,160, now U.S. Pat. No. 6,553,658 filed Jan. 22, 2001, which is a divisional of prior application Ser. No. 09/386,623, filed Aug. 31, 1999, which issued on Apr. 10, 2001 as U.S. Pat. No. 6,212,767. 
    
    
     BACKGROUND 
     This invention relates generally to assembling integrated circuit die into packages. 
     For a number of reasons, it may be desirable to package two die together in a single package. For example, the components provided in a separate die may not be capable of being integrated in a single integrated circuit. For example, different process technologies may be used in each die. In addition, basic incompatibilities between components on each die may necessitate separate die fabrication. Once the die are fabricated independently, it may be desirable to package them in a single package so that a single set of contacts may be used to connect with each of the die. 
     The handling of die, which may be stacked, to form a composite creates a number of difficulties. Predominantly, semiconductor assembly equipment is adapted to produce packages containing a single die. Thus, techniques are needed to assemble the die. 
     Conventional die assembly techniques may involve a pair of passes through a die attach machine. For example, a larger of two die to be stacked may be attached to a support structure in a die attach machine. Thereafter, the support structure may be run through the die attach machine again to attach the smaller die to a larger die. 
     The repeated passes through the die attach machine may be disadvantageous for a number of different reasons. For example, the assembly process is complicated by the multiple passes through the die attach machine. In some cases the die attach process may be relatively expensive. In some cases the thermal budget may be impacted by the die attach process. 
     Die attach machines usually use an organic-based adhesive to attach the support structure to the die or to attach one die to another die. The adhesive may be applied in an uncured or soft phase between the surfaces to be joined. The adhesive is then cured through exposure to elevated temperatures for a specified time period. The carrier material is typically an epoxy resin or polyimide. The carrier provides the adhesion and mechanical strength along the bond line. The carrier may be filled with metal particles in the event electrical and thermal conductivity is required or with non-metallic particles such as alumina, if the ultimate bond should be an insulator. 
     Three types of polymer adhesives that are often used in die attach processes are epoxies, cyanate esters and polyimides. Epoxies use relatively low cure temperatures (125-175° C.) and have moderate glass transition temperatures (100-155° C.). However, epoxies vary in their ability to absorb and outgas moisture and other contaminants. Cyanate ester adhesives use higher cure temperatures (300° C.) and have relatively higher glass transition temperatures (240° C.). However, unlike epoxy adhesives, cyanate esters have low outgassing characteristics as well as low ionic contents. Finally, polyimide adhesives use high cure temperatures (greater than 400° C.) and like cyanate esters have a high glass transition temperature. A variety of conductive fillers including silver, gold or copper and non-conductive fillers, such as alumina, diamond, glass fabric, silica or ceramic may be used with the adhesive. 
     Thus, in addition the complexity inherent in the die attach process, in some cases the thermal exposure may be disadvantageous both to shallow junctions in advanced devices and to the wire bonds or other elements whose electrical performance may be affected. 
     Circuits that are susceptible to outgassing and moisture generally use a metallurgical attachment technique. Solder or metal eutectic alloys may provide good thermal conductivity but because of their electrical conductivity they may not be used if the backside of the die must be insulated from the support structure. Solders and eutectic alloys generally use higher processing temperatures and thereby impose constraints on work flow. For example, if a gold-tin die attach operation is scheduled after other chips in a multi-chip module have been wire bonded, the high temperatures used may degrade the wire bonds or degrade the electrical performance of the actual device. 
     Thus, for a variety of reasons, it may be desirable in making multi-chip modules, to reduce the number of times that the module or any part thereof must be exposed to the die attach process. 
     SUMMARY 
     In accordance with one aspect, a method of assembling multi-chip modules includes using a die attach machine to attach a first, relatively larger die to a support structure. A second, relatively smaller die is affixed to the larger die without using a die attach machine. 
     Other aspects are set forth in the accompanying detailed description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a process flow in accordance with one embodiment of the present invention; 
     FIG. 2 is a more detailed embodiment of a process flow shown in FIG. 1; 
     FIG. 3 is a greatly enlarged top plan view of a die positioned on a wafer; 
     FIG. 4 is a greatly enlarged cross-sectional view of a smaller die positioned on a larger die; 
     FIG. 5 is a greatly enlarged cross-sectional view of a multi-chip module; 
     FIG. 6 is a more detailed process flow for a process in accordance with the more general flow show in FIG. 1; 
     FIG. 7 is a top plan view of a die positioned on a wafer tape; 
     FIG. 8 is a top plan view of a smaller die positioned on a larger die positioned on a wafer tape; 
     FIG. 9 is a more detailed flow for another embodiment corresponding to the more general flow shown in FIG. 1; 
     FIG. 10 is a top plan view of a larger and a smaller die in position on a wafer tape; 
     FIG. 11 is an enlarged, cross-sectional view showing a larger die that has been die attached to a support structure; and 
     FIG. 12 is a greatly enlarged cross-sectional view of a multi-chip module. 
    
    
     DETAILED DESCRIPTION 
     A process for forming a multi-chip module, shown in FIG. 1, avoids the need to expose the elements of the module to a die attach machine more than one time. As used herein, a multi-chip module is intended to refer to a combination of two die stacked one on top of the other, and coupled to a support structure. A support structure may be any one of a variety of devices used in making multi-chip modules including leadframes, substrates, and laminate cores, as examples. 
     The process flow may include three basic steps. One step is to die attach the larger die to the support structure (block  10 ). This step is conventionally done in a die attach machine. Another step is to affix a smaller die to the larger die without using a die attach machine, as illustrated by block  12 . Finally, the composite of the smaller die over the larger die over a support structure may be packaged as indicated in block  14 . 
     Two different sequences may be utilized to assemble the multi-chip module. In accordance with one sequence, the smaller die may be affixed to the larger die and then that subassembly may be die attached to a support structure as indicated by the arrows A 1  in FIG.  1 . Thereafter, the assembly may be packaged as indicated by arrows B 1 . 
     In accordance with another sequence, initially, the larger die may be die attached to the support structure. Thereafter, as indicated by arrow A 2 , the smaller die may be affixed to the larger die which has already been affixed to the support structure. Finally, as indicated by arrow B 2 , the assembly may be packaged. 
     In general, it may be advantageous, in some embodiments, if the smaller die is sufficiently smaller than the larger die to enable both die to be wire bonded after the smaller die is attached to the larger die, with both die facing upwardly. 
     A more detailed process, shown in FIG. 2, for implementing one of the sequences illustrated in FIG. 1, may begin by singulating a smaller die from a wafer as indicated in block  16 . The smaller die may then be attached to the larger die as indicated in block  18 . In accordance with one aspect of the invention, this attachment is done without using a die attach machine. Referring to FIG. 3, a semiconductor wafer  25  is shown with a smaller die  30  attached to an unsingulated, larger die  28  still in wafer form. 
     The smaller die  30  may be attached to the larger die  28  without using a die attach machine, but using conventional securement techniques. For example, adhesive tape such as lead on chip (LOC) tape, adhesives that do not need to be cured at temperatures above room temperature, solder or other non-die attach machine based techniques may be used. 
     For example, double sided die attach tape may be situated between the smaller and larger die. A suitable tape is Sumioxy 5120T adhesive, 75 to 125 micrometers thick. Bond pads for coupling the external devices may be exposed peripherally about the larger die  28  so as to be accessible around the smaller die  30 . A pick and place machine may be used to locate the smaller die over the larger die. 
     Thereafter, the larger die  28  is singulated as indicated in block  20  of FIG.  2 . FIG. 4 shows a cross-sectional view of the smaller die  30  attached by a securement medium  33  to the larger die  28  after the larger die has been singulated in accordance with the step illustrated by block  20 . 
     The stack of the smaller die  30  and the larger die  28  may then be die attached to a support structure, as indicated in block  22  in FIG.  2 . Similarly, FIG. 5 shows the subassembly of the smaller die  30  and larger die  28  attached to a support structure  34  through a die attach  32 . The die attach  32  may be a heat curable adhesive deposited by syringe-dispensing, screen printing or stamping paste adhesive, as examples. Alternatively, perforated film adhesives may be used as the die attach. Thereafter, conventional packaging techniques may be utilized, as indicated in block  24  in FIG. 2, including encapsulation. 
     The die attach process may be otherwise conventional in all ways. Thus, depending on the adhesive that is utilized, the necessary time and temperature may be determined, as will be apparent to those skilled in the art. For example, a conductive adhesive may be formed as an epoxy paste with a silver filler. Its glass transition temperature is 127° C. Similarly, a nonconductive adhesive may be formed of epoxy paste with an alumina filler. Its glass transition temperature is 85° C. 
     In accordance with still another embodiment, a flow for assembling a multi-chip module using only one pass through a die attach machine may begin by locating a larger die on a wafer tape as illustrated at block  40  in FIG.  6 . The wafer tape, sometimes called a wafer dicing tape, is a tape with high toughness used to hold wafers during the dicing process. An example of wafer tape is a polyvinyl chloride membrane with adhesive applied to one side and mounted in a frame. 
     Referring to FIG. 7, the larger die  28  is shown in position on the adhesive side of a wafer tape  48 . The tape  48  includes an adhesive covered polymer membrane  52  secured in a frame  50 . The frame  50  is illustrated by a rectangular frame but other shapes including ring-shaped frames may be utilized as well. 
     The die  28  is held in position by the adhesive on the membrane  52 . While only a single die is shown in position on the wafer tape  48  in FIG. 7, one or more die may be positioned on the tape  48  at one time, in some embodiments of the present invention. 
     Referring back to FIG. 6, the next step is to attach the smaller die to the larger die using a pick and place machine. A pick and place machine transfers the die  28  from the wafer tape  48  onto the larger die  28 , as indicated in block  42 . A die attach material may be positioned between the stacked die. The result, shown in FIG. 8, has a smaller die  30  on top of the larger die  28  still secured to the wafer tape  48 . 
     The stacked die are then die attached to a support structure as indicated in block  44 . In the die attach machine, the die attach layers between the die and between the layer die and the support structure may be simultaneously activated and cured. The resulting composite, shown in FIG. 12, may use die attach material as the layers  32 . Thereafter, the composite is packaged as indicated in block  46  in FIG.  6 . 
     In accordance with still another embodiment, a larger die  28  may be located onto the wafer tape  48  as indicated in block  54  in FIG.  9 . Thereafter, a smaller die  30  may be attached next to the larger die  28  on the wafer tape  48  as indicated in block  56 . Thus, referring to FIG. 10, the larger die  28  may be positioned next to the smaller die  30  on the adhesive bearing side of the wafer tape  48 . 
     The larger die  28  then may be stacked on a support structure, as indicated in block  58  in FIG. 9, using a die attach machine. The resulting composite is shown in FIG.  11 . The larger die  28  is mounted on a die attach  32  over the support structure  34 . 
     Next the smaller die  30  is mounted onto the larger die  28  (block  60  in FIG.  9 ). This may be done using the die attach machine. The composite of the die attach materials, the larger and smaller die  28  and  30  and the support structure  34  is then secured together in a die attach machine. The composite, shown in FIG. 12 includes die attach layers  32  between the die  28  and  30  and the support structure  34 . The composite is then packaged as indicated in block  62  in FIG.  9 . 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.