Patent Publication Number: US-5898220-A

Title: Multi-chip device and method of fabrication employing leads over and under processes

Description:
This is a continuation of application Ser. No. 08/574,994, filed Dec. 19, 1995, U.S. Pat. No. 5,689,135. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved semiconductor device and method for increasing semiconductor device density. In particular, the present invention relates to a device and method utilizing leads over and under processes such that two superimposed semiconductor dice can be attached to a single lead frame. 
     2. Background Art 
     High performance, low cost, increased miniaturization of components, and greater packaging density of integrated circuits have long been the goals of the computer industry. Greater integrated circuit package density, for a given level of component and internal conductor density, is primarily limited by the space available for die mounting and packaging. For lead frame mounted dies, this limitation is a result of conventional lead frame design. Conventional lead frame design inherently limits potential single-die package density because the die-attach paddle of the lead frame must be as large 25 or larger than the die securing it. The larger the die, the less space (relative to size of the die) that remains around the periphery of the die-attach paddle for bond pads for wire bonding. Furthermore, the inner lead ends on a lead frame provide anchor points for the leads when the leads and the die are encapsulated in plastic. The anchor points may be emphasized as lateral flanges or bends or kinks in the lead. Therefore, as the die size is increased in relation to the package size, there is a corresponding reduction in the space along the sides of the package for the encapsulating plastic which joins the top and bottom portions of the molded plastic body at the mold part line and anchors to the leads. As the leads are subjected to the normal stresses of forming and assembly operations, the encapsulating plastic may crack, which may destroy the package seal and substantially increase the probability of premature device failure. 
     One method of increasing integrated circuit density is to stack dice vertically. U.S. Pat. No. 5,012,323 (&#34;the &#39;323 patent&#34;) issued Apr. 30, 1991 to Farnworth teaches combining a pair of dice mounted on opposing sides of a lead frame. An upper die is back-bonded to the upper surface of the leads of the lead frame via a first adhesively coated, insulated film layer. The lower die is face-bonded to the lower lead frame die-bonding region via a second, adhesively coated, insulative, film layer. The wirebonding pads on both the upper die and lower die are interconnected with the ends of their associated lead extensions with gold or aluminum wires. The lower die needs to be slightly larger than the upper die in order that the lower die bonding pads are accessible from above through an aperture in the lead frame such that gold wire connections can be made to the lead extensions. However, this arrangement has a major disadvantage from a production standpoint, since the different size dice require that different equipment produce the different dice or that the same equipment be switched over in different production runs to produce the different dice. Moreover, the lead frame design employed by Farnworth employs long conductor runs between the die and the exterior of the package, and the lead frame configuration is specialized and rather complex. 
     U.S. Pat. No. 5,291,061 issued Mar. 1, 1994 to Ball teaches a multiple stacked die device that contains up to four dies which does not exceed the height of current single die packages. The low profile of the device is achieved by close-tolerance stacking which is made possible by a low-loop-profile wirebonding operation and thin-adhesive layers between the stack dies. However, Ball secures all of the dice to the same (upper) side of the lead frame, necessarily increasing bond wire length, even if some of the leads are bent upwardly. Moreover, Ball employs a die paddle to support the die stack, a technique which requires an extra die-attach step, and which increases the distance between the inner lead ends and even the lowermost die in the stack, resulting in longer bond wires. 
     U.S. Pat. No. 4,862,245 issued Aug. 29, 1989 to Pashby discloses a &#34;leads over chip&#34; (LOC) configuration, wherein the inner lead ends of a standard dual-in-line package (DIP) lead frame configuration extend over and are secured to the upper (active) surface of the die through a dielectric layer. The bond wire length is thus shortened by placing the inner lead ends in closer proximity to a central row of die bond pads, and the lead dimensions purportedly enhance heat transfer from the die. However, the Pashby LOC configuration as disclosed relates to mounting and bonding only a single die. 
     Therefore, it would be advantageous to develop a technique and device for increasing integrated circuit density using substantially similar or identically sized dies with non-complex lead frame configurations, provide short bond wire lengths and achieve a lower profile than state-of-the-art designs, which configuration is readily susceptible to plastic packaging techniques, such as transfer molding. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a device and method for increasing integrated circuit density. The device comprises a pair of superimposed dies with a plurality of leads disposed between the dies. The device is produced by providing a lower die which has a plurality of bond pads on a face side or active surface of the lower die. A layer of dielectric shielding such as a polyimide is applied over the lower die face side without covering the lower die bond pads. Leads are secured to an upper surface of the shielding layer. The lower die-to-leads connection is thus the previously referenced LOC configuration. 
     A plurality of lower die bond wires are then attached between the lower die bond pads and an upper surface of their associated, respective leads in an LOC chip wirebonding process. In wirebonding, gold or aluminum bond wires are attached, one at a time, from each bond pad on the die and to a corresponding lead. The bond wires are generally attached through one of three industry standard wirebonding techniques: ultrasonic bonding--using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bonding--using a combination of pressure and elevated temperature to form a weld; and thermosonic bonding--using a combination of pressure, elevated temperature, and ultrasonic vibration bursts. 
     Once the lower die bond wires are attached, a second layer of shielding is applied over the leads and the portion of the lower die bond wires which extends over the lead upper surface. A back side of the upper die is attached to an upper surface of the second shielding layer in a &#34;leads under chip&#34; (LUC) die-attach. A plurality of upper die bond wires are attached between a plurality of bond pads on a face side or active surface of the upper die and the upper surface of their associated, respective leads in a LUC wirebonding process. 
     Alternately, the lower die bond wires can be attached with a more vertical arrangement such that only a small portion of each of the lower die bond wires extends over the lead upper surfaces. Alternately, the lower bond wires can be attached to the lead tipper surface in a direction extending away from the assembly. With either arrangement, the lower die bond wires will not cause electrical interference with the operation of the upper die, thus no second shielding layer is needed to cover the portion of the lower die bond wires which extends over the lead upper surfaces. 
     Generally, the device has leads with inner ends of substantially the same length to which the dies are attached as discussed above. However, the device can be constructed with lead-ends of differing lengths. The differing lead-end length arrangement can consist of a plurality of long leads with an alternating plurality of short leads. The long leads extend between the upper die and lower die and may be connected to the lower bond pads by the lower bond wires. The short leads, which preferably do not extend between the upper die and lower die, are connected to the upper bond pads by the upper die bond wires. This arrangement assists in reducing the potential for bond wire to lead shorting. 
     Preferably, the tipper and lower dies are identical, such as a pair of 2 Meg VRAMs. Thus, the above discussed arrangement would achieve a 4 Meg VRAM, yielding more memory in a low-profile, small package with a smaller lead pitch. Alternately, 8 MEG memory may be achieved by using two 4 MEG DRAMS while 32 MEG memory may be achieved by using two 16 MEG DRAMS. However, the upper and lower dies do not have to be identical in size. Furthermore, the dies can have differing bond pad arrangements. For example, the lower die may have a plurality of lower bond pads in a row in approximately the middle of the face side of the lower die. Such device is constructed by applying the parallel layers of shielding adhesive over the lower die face side on either side of the row of lower bond pads. The leads are adhered to the upper surfaces of the shielding layer. The plurality of lower die bond wires are attached between the lower die bond pads and the upper surface of their respective leads. 
     Once the lower bond wires are attached, a second set of parallel layers of shielding is applied over the leads. The lower die bond wires may or may not be covered with the second shielding layer. The back of the upper die is adhered to the upper surfaces of the second shielding layer. The plurality of upper die bond wires is attached between the plurality of bond pads on a face side of the upper die and the upper surface of their associated, respective leads. 
     If the lower bond wires are not covered with the second shielding layer, a conformal coating or potting compound may be applied over and around the lower bond wires. Alternately, the encapsulation material used to encapsulate the bare die assembly may be permitted to flow around the lower die bond wires. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a side cross-sectional view of one preferred assembly of the present invention, 
     FIG. 2 is a side cross-sectional view of a second preferred assembly of the present invention; 
     FIG. 3 is a side cross-sectional view of a third preferred assembly of the present invention; 
     FIG. 4 is a top view of one lead arrangement suitable for use in the present invention; and 
     FIG. 5 is a plan view of an alternate lead arrangement suitable for use in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a preferred bare multiple-die assembly 10 of the present invention. The assembly 10 comprises a lower semiconductor die 12 and an opposing upper semiconductor die 14 with a plurality of leads 16 (usually of a lead frame as known in the art) disposed therebetween. Fabrication of the assembly 10 begins with providing the lower die 12 which has a plurality of bond pads 18 on a face side 22 of the lower die 12. A layer of dielectric shielding 20 is applied over the lower die face side 22. The leads 16 are adhered to an upper surface 24 of the shielding layer 20. Shielding layer 20 may preferably comprise a polyimide tape such as Kapton™ film or tape, having a suitable adhesive on each side. Alternatively, it may comprise a liquid, gel or paste layer having adhesive characteristics for attachment of the dies and the leads of the lead frame. All of the foregoing options are known in the art. A plurality of lower die bond wires 26 are attached between the lower die bond pads 18 and an upper surface 28 of their respective leads 16. 
     Once the lower die bond wires 26 are attached, a second layer of dielectric shielding 30 is applied over the leads 16 and the portion of the lower bond wires 26 extending over the lead upper surfaces 28. A back side 32 of the upper die 14 is adhered to an upper surface 34 of the second shielding layer 30. Layer 30 may also comprise a polyimide, other suitable dielectric tape or film or other suitable material. A plurality of upper die bond wires 36 is attached between a plurality of bond pads 38 on a face side 40 of the upper die 14 and the upper surface 28 of their respective leads 16. A die coat may be applied to the face side of upper die 14 after wire bonding, if desired. 
     Preferably, the lower die bond wires 26 are attached using a low-loop-profile wirebonding technique such that the lower die bond wires 26 lay substantially flush to the lead upper surface 28. 
     The die assembly 10 may be encapsulated with plastic by transfer molding or other process known in the art, as shown in broken lines at 42, the outer ends of leads 16 extending to the exterior of the package. 
     FIG. 2 illustrates a second preferred bare die assembly 50 of the present invention. Components common to both FIG. 1 and FIG. 2 retain the same numeric designation. The assembly 50 is similar to the assembly 10 of FIG. 1 in that it comprises the lower die 12 and the superimposed upper die 14 with the plurality of leads 16 disposed therebetween. The lower die 12 has the plurality of bond pads 18 on the face side 22 of the lower die 12. The layer of shielding 20 is applied over the lower die face side 22. The leads 16 are adhered to the upper surface 24 of the shielding layer 20. The plurality of lower bond wires 26 is attached between the lower die bond pads 18 and the tipper surface 28 of their respective leads 16. 
     Once the lower bond wires 26 are attached, the second layer of shielding 30 is applied over the leads 16. In this embodiment, the lower die bond wires are attached with a more vertical arrangement such that there is only a small portion of each of the lower die bond wires 26 extending over the lead upper surfaces 28. Alternately, the lower die bond wires 26 can be attached to the lead upper surface 28 in a direction which extends away from the assembly 50 (not shown). With either arrangement, no portion of the second shielding layer 30 covers the portion of the lower die bond wires 26 which extend over the lead upper surfaces 28. 
     The back side 32 of the tipper die 14 is adhered to the tipper surface 34 of the second shielding layer 30. The plurality of upper die bond wires 36 is attached between the plurality of bond pads 38 on the face side 40 of the upper die 14 and the upper surface 28 of their respective leads 16. 
     FIG. 3 illustrates a third preferred bare die assembly 60 of the present invention. Components common to the previous figures and FIG. 3 retain the same numeric designation. The assembly 60 is similar to the assembly 10 of FIG. 1 in that it comprises the lower die 12 and the superimposed upper die 14 with the plurality of leads 16 disposed therebetween. However, the lower die 12 has a plurality of lower bond pads 62 in one or more rows in approximately the middle of the FIG. 3 of the lower die 12. Two parallel layers of dielectric shielding 20A and 20B are applied over the lower die face side 22 on either side of the row of lower bond pads 62. The leads 16 are adhered to the upper surfaces 24A and 24B of the shielding layers 20A and 20B, respectively. The plurality of lower die bond wires 26 is attached between the lower die bond pads 62 and the upper surface 28 of their associated, respective leads 16. 
     Once the lower die bond wires 26 are attached, a second set of parallel layers of dielectric shielding 30A and 30B are applied over the leads 16. In this embodiment, the lower die bond wires 26 may or may not be covered with the second shielding layers 30A or 30B. The back side 32 of the upper die 14 is adhered to the upper surfaces 34A and 34B of the second shielding layers 30A and 30B, respectively. A plurality of upper die bond wires 36 are attached between a plurality of bond pads 38 on a face side 40 of the upper die 14 and the upper surface 28 of their respective leads 16. 
     A conformal coating or potting material (not shown) may be disposed within the assembly 60 around the lower die bond wires 26. Alternately, a plastic encapsulation material (not shown) used to encapsulate the bare die assembly 60 is also injected into the bare die assembly 60 which acts to insulate the lower die bond wires 26. 
     FIG. 4 illustrates a top view of a bare die assembly 70 of the present invention showing one embodiment of a lead arrangement. Components common to the previous figures and FIG. 4 retain the same numeric designation. Assembly 70 may have the same structural assembly of FIGS. 1, 2 or 3 comprising leads 16 (shown partially hidden) of substantially the same length which are attached as discussed above. A bond pad configuration as employed in the assemblies of FIGS. 1 and 2 is depicted. 
     FIG. 5 illustrates a top view of a bare die assembly 80 of the present invention showing an alternate lead arrangement. Components common to the previous figures and FIG. 5 retain the same numeric designation. Assembly 80 may have the same structural assembly of FIGS. 1, 2 or 3 comprising leads of differing length. A plurality of extended leads 82 (shown partially in shadow is dispersed along the length of the assembly 80 with an alternating plurality of truncated leads 84. The extended leads 82 extend between the upper die 14 and lower die 12 and are connected to the lower bond pads 18 (shown in shadow) by the lower die bond wires 26 (also shown in shadow). The truncated leads 84 preferably do not extend between the upper die 14 and lower die 12, but terminate adjacent the edge of the dies 12 and 14, respectively. The truncated leads 84 are connected to the upper bond pads 38 by the upper die bond wires 36. This arrangement assists in reducing the potential for shorting between bond wires and leads. The use of truncated leads 84 does not impair the structural stability of the assembly 80 because both dies are secured to the extended leads 82 and the entire assembly 80 is usually encased in an encapsulating material (not shown). 
     Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope thereof.