Abstract:
Semiconductor devices and stacked die assemblies, and methods of fabricating the devices and assemblies for increasing semiconductor device density are provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is a division of U.S. patent application Ser. No. 10/389,433, filed on Mar. 14, 2003, which is a division of U.S. patent application Ser. No. 10/068,159, filed on Feb. 5, 2002, presently pending. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention generally relates to assembling and packaging multiple semiconductor dies, and more particularly to a stacked multiple die device and methods for fabricating the device.  
       BACKGROUND OF THE INVENTION  
       [0003]     Miniaturization of wireless products such as cellular phones and handheld computers such as personal digital assistants (PDA), has driven the increased demand for smaller component footprints, which in turn increases the popularity of multi-chip stack BGA packaging. Most multi-chip packages involve stacking dies on top of each other by means of adhesive elements. However, to achieve a low package height for multi-chip stacked die packages, a significantly reduced die thickness is needed together with the use of special wire bond techniques to reduce the height of the wire bond loop height.  
         [0004]     Thin die handling and the required special bonding techniques poses many challenges to the assembly process.  FIGS. 1-3  depict conventional ways of packaging a multi-chip stacked die package. As shown in  FIG. 1 , one prior art package  10  includes two conventional stacked dies, the first (bottom) die  12  being surface mounted by means of an adhesive element  14  to a substrate  16 , and a smaller second (top) die  18  being mounted by a second adhesive element  20  onto the active surface  22  of the bottom die  12 , each of the dies being wire bonded  24  to the substrate  16 .  FIG. 2  illustrates a prior art stack die package  10   a  in which the first (bottom) die  12   a  is mounted to a substrate  16   a  in a flip chip attachment, and the second (top) die  18   a  is surface mounted to the inactive surface  26   a  of the first die  12   a  by means of an adhesive element  20   a  and wire bonded  24   a  to the substrate  16   a .  FIG. 3  shows a prior art three-die stack BGA package  10   b  in which the first bottom die  12   b  is mounted to a substrate  16   b  by an adhesive element  14   b , a second (middle) die  18   b  is mounted on the active surface  22   b  of the bottom die  12   b  by a second adhesive element  20   b , and a third (top) die  28   b  is mounted on a spacer  30   b  mounted on the active surface  32   b  of the second (middle) die  18   b , with each of the dies being wire bonded  24   b  to the substrate  16   b.    
         [0005]     In stacked die assemblies in which the bottom die is a flip chip, there is a limit on the minimum overall thickness of the package that can be achieved. If a solder-bumped wafer having a 150 μm bump height were to be ground to a total thickness of 150 μm to 200 μm, there would be a high occurrence of broken wafers due to the stress induced on the wafers from the bumps. Furthermore, even if the wafer does not crack, the die strength will drop significantly due to the presence of “dimples” on the backside of the wafer. Such dimples are typical defects observed on bump wafers that are ground too thin or an inappropriate backgrinding tape is used in the process.  
         [0006]     In addition, as depicted in  FIG. 3 , with multiple stacked dies, a spacer  30   b  is required to create the minimal clearance for the wire loop height between the second (middle) die  18   b  and the third (top) die  28   b . This results in a higher package height, or requires ultrathin dies in order to meet the package height requirement. Thinner dies translate into a higher possibility of cracked dies during the assembly process.  
         [0007]     In view of these and other deficiencies, improvements in stacked die modules are desirable.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides semiconductor devices and stacked die assemblies, methods of fabricating the devices and assemblies for increasing semiconductor device density, and method of fabricating die packages of the assemblies.  
         [0009]     In one aspect, the invention provides a stacked die assembly. In one embodiment, the stacked die assembly, comprises a first (bottom) die disposed on a substrate, a bonding element connecting bond pads on an active surface of the bottom die to terminal pads on the substrate, and a second die mounted on the bottom die. The second die has a bottom surface with a recessed edge along the perimeter of the die that provides an opening for the bonding element extending from the bond pads of the bottom die, thus eliminating the need for a spacer between the two dies to achieve sufficient clearance for the bonding element. A second bonding element connects the bond pads on the active surface of the second die to terminal pads on the substrate. Adhesive elements are typically disposed between the two dies and the bottom die and the substrate.  
         [0010]     In another embodiment, the stacked die assembly, comprises a first (bottom) die disposed on a substrate, typically through a flip chip attachment, and having a recess formed in the upper (inactive) surface. A second die is at least partially disposed within the recess of the first die. A bonding element connects bond pads on the active surface of the second die to terminal pads on the substrate. An adhesive element can be disposed within the recess to attach the two dies. In a further embodiment of this assembly, a third die is mounted on the second die. The third die has a bottom surface with a recessed edge along the perimeter of the die that provides an opening for the bonding element extending from the bond pads of the second die, thus eliminating the need for a spacer between the two dies for clearance of the bonding element. A second bonding element connects the bond pads on the active surface of the third die to terminal pads on the substrate. An adhesive element can be used to attach the second and third dies.  
         [0011]     In a further embodiment, the stacked die assembly, comprises a first (bottom) die disposed on a substrate, a bonding element connecting bond pads on the active surface of the first die to terminal pads on the substrate, and a second die mounted on the bottom die. A recess is formed on the bottom surface of the first die, and an adhesive element is disposed within the recess to attach to the first die to the substrate. The containment of the adhesive element in the recess rather than being disposed between the die and the substrate as a separate layer decreases the overall height of the die assembly. In an embodiment of this assembly, the second die has a recessed edge along the perimeter of the bottom surface for clearance of the bonding element extending from the bond pads of the second die, thus eliminating the need for a spacer between the two dies. Bond pads on the second die are connected to terminal pads on the substrate by a second bonding element, and an adhesive element can be used to attach the second and third dies.  
         [0012]     In yet another embodiment, the stacked die assembly, comprises a first (bottom) die disposed on a substrate, typically through a flip chip attachment, and a second die having a recess formed in the bottom (inactive) surface. The first die is at least partially disposed in the recess of the second die, and a bonding element connects bonding pads on the second die. An adhesive element can be disposed within the recess to attach the two dies.  
         [0013]     In another aspect, the invention provides a semiconductor package. In various embodiments, the package comprises a stacked die assembly according to the invention, at least partially encapsulated. The package can further include external contacts disposed on the second surface of the substrate for attaching the package as a component to an external electrical apparatus or device.  
         [0014]     In another aspect, the invention provides methods of fabricating the foregoing stacked die assemblies and semiconductor packages. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Preferred embodiments of the invention are described below with reference to the following accompanying drawings, which are for illustrative purposes only. Throughout the following views, the reference numerals will be used in the drawings, and the same reference numerals will be used throughout the several views and in the description to indicate same or like parts.  
         [0016]      FIGS. 1-3  depict cross-sectional, side elevational views of prior art embodiments of stacked die packages.  
         [0017]      FIG. 4  is a cross-sectional, side elevational view of an embodiment of a stacked die package according to the invention.  
         [0018]      FIG. 5  is an enlarged partial view of the package of  FIG. 4 , showing the recessed edge and opening between the stacked dies.  
         [0019]      FIGS. 6-11  illustrate sequential processing steps in the fabrication of the stacked die package of  FIG. 4 , according to an embodiment of a method of invention.  FIG. 6  is a bottom, perspective view of the second die of the package of  FIG. 4 , showing the removed (etched) portion of the die forming the recessed edge along the perimeter of the die.  FIGS. 7 and 9 - 11  are cross-sectional, side elevational views of sequential steps in the mounting of the dies.  FIG. 8  is a top plan view of a panel with multiple die packages disposed thereon.  
         [0020]      FIG. 12  is a cross-sectional, side elevational view of another embodiment of a stacked die package according to the invention.  
         [0021]      FIGS. 13-16  illustrate sequential processing steps in the fabrication of the stacked die package of  FIG. 12 , according to an embodiment of a method of invention.  FIG. 14  is a top, perspective view of the first (bottom) die of the package of  FIG. 12 , showing the recess formed in the die.  FIGS. 13 and 15 - 16  are cross-sectional, side elevational views of sequential steps in the mounting of the dies.  
         [0022]      FIG. 17  is a cross-sectional, side elevational view of another embodiment of a stacked die package according to the invention.  
         [0023]      FIGS. 18-19  illustrate sequential processing steps in the fabrication of a portion of the stacked die package of  FIG. 17 , according to an embodiment of a method of invention, showing the mounting of the third (top) die.  
         [0024]      FIG. 20  is a cross-sectional, side elevational view of another embodiment of a stacked die package according to the invention.  
         [0025]      FIGS. 21-24  illustrate sequential processing steps in the fabrication of the stacked die package of  FIG. 20 , according to an embodiment of a method of invention.  FIG. 22  is a bottom, perspective view of the first (bottom) die of the package of  FIG. 20 , showing the recess formed in the bottom surface of the die.  FIGS. 21 and 23 - 24  are cross-sectional, side elevational views of sequential steps in the mounting of the dies.  
         [0026]      FIG. 25  is a cross-sectional, side elevational view of another embodiment of a stacked die package according to the invention.  
         [0027]      FIGS. 26-29  illustrate sequential processing steps in the fabrication of the stacked die package of  FIG. 25 , according to an embodiment of a method of invention.  FIG. 26  is a bottom, perspective view of the second (top) die of the package of  FIG. 25 , showing the recess formed in the bottom surface of the die.  FIGS. 27-29  are cross-sectional, side elevational views of sequential steps in the mounting of the dies. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The invention will be described generally with reference to the drawings for the purpose of illustrating embodiments only and not for purposes of limiting the same. The figures illustrate processing steps for use in fabricating semiconductor devices in accordance with the present invention. It should be readily apparent that the processing steps are only a portion of the entire fabrication process.  
         [0029]     The terms “top” and “bottom”, and “upper” and “lower” are used herein for convenience and illustrative purposes only, and are not meant to limit the description of the invention inasmuch as the referenced item can be exchanged in position.  
         [0030]     The invention advantageously reduces the overall height of stacked die packages, achieves a desirably low package profile, allows the use of thicker dies in the stack assembly to reduce the number of cracked dies, eliminate the need for a spacer between dies to provide clearance for bond wires extending from an underlying die, and reduces the number of passes required for manufacturing multiple stacked dies by eliminating the need for mounting a spacer. The invention further offers more reliable adhesion bleed out control, and the benefits increase as more dies are stacked. The method of the invention can be utilized to fabricate an assembly comprising additional stacked die layers to those of the illustrated embodiments using the described concepts herein.  
         [0031]     In each of the described embodiments, prior to mounting the individual dies of a stacked assembly, the backside (inactive surface) of a die (wafer) can be backgrinded or otherwise processed to a desired thickness, flatness value and texture using conventional methods in the art.  
         [0032]     Referring to  FIG. 4 , a first embodiment of a multiple chip die assembly package  40  according to the invention is depicted in a cross-sectional, side elevational view. The package  40  comprises a first (bottom) die  42  mounted to a support substrate  44 , and a second (top) die  46  mounted on the bottom die  42 . Bond pads  48   a ,  48   b  on the first and second dies  42 ,  44  are wire bonded  50   a ,  50   b , respectively, to terminal pads  52   a ,  52   b  on the support substrate  44 . Substrate  44  further includes external contacts  54 , for example, in the form of conductive solder balls, to connect the die package  40  to an external electrical apparatus (not shown). As best seen in  FIG. 5 , a portion or thickness  60  (shown in phantom) along the perimeter  56  of the second (inactive) surface  58  of the second (top) die  46  is removed (e.g, etched) to provide a recess (recessed edge)  62 . The recessed edge  62  has a height (h) and provides an opening  63  for sufficient clearance of the bond wires  50   a  (or other connecting member such as TAB tape) extending from the bond pads  48   a  on the bottom die  42  to the substrate  44 . This eliminates the need for a spacer (e.g.,  FIG. 3, 30   b ) between the two overlying dies to provide the necessary clearance for bond wires extending from the lower die  42 , and thus achieves a lower overall package height  67 . Adhesive elements  64 ,  66  can be utilized, respectively, to secure the bottom die  42  onto the support substrate  44 , and the second (top) die  46  onto the bottom die  42 .  
         [0033]      FIGS. 6-11  illustrate an embodiment of a process flow and method for forming the stacked die package  40  of  FIG. 4 .  
         [0034]     Prior to mounting, a portion or thickness of the second (inactive) surface  58  of the second (top) die  46  can be removed to form the recessed edge  62 . As shown in  FIGS. 5-6 , a portion  60  (shown in phantom in  FIG. 5 ) of the surface  58  has been removed along the perimeter  56  of the die. Known methods in the art can be used to selectively remove a portion  60  along the perimeter of the die  46  such that when the die  46  is subsequently mounted onto the first (bottom) die  42 , the recessed edge  62  provides an opening with sufficient clearance for the bond wires  50   a  extending from the bottom die  42  to the support substrate  44 . The recessed edge  62  can be formed using known techniques in the art, for example, a chemical wet etch or dry etch, laser ablation, or other mechanical means of reducing the bottom surface  58  of the top die  46  to a predetermined depth.  
         [0035]     Referring now to  FIGS. 7-9 , the first (bottom) die  42  is mounted on a first surface  68  of the support substrate  44 . The bottom die  42  comprises a first (active) surface  70  with a plurality of bond pads  48   a  along the periphery thereof, and a second (bottom) surface  72 . As shown in  FIG. 7 , the bottom surface  72  of the bottom die  42  is aligned with and facing the first surface  68  of the support substrate  44  prior to assembly.  
         [0036]     The support substrate  44  can comprise an electrically insulating polymer material such as a resin reinforced with glass fibers, for example, bismaleimide triazine (BT) resin, epoxy resins such as FR-4 or FR-5 laminates, ceramics, and polyimide resins; a metal leadframe (e.g., Alloy42 or copper); a flexible polyimide film (e.g., KAPTON from DuPont, Wilmington, Del., or UPILEX from Ube Industries, Ltd., Japan); among other substrates. A representative thickness of the substrate is about 50 μm to about 500 μm. As shown in  FIG. 8 , the support substrate  44  can be in the form of a strip or panel  74  on which multiple die packages  40  are formed, whereby the panel  74  can be singulated, for example, by cutting or shearing along an expansion slot  76 , into individual packages.  
         [0037]     Referring to  FIG. 7 , the bottom die  42  can be attached to the support substrate  44  by use of an adhesive element  64 . The adhesive element  64  can be applied onto the bottom surface  72  of the bottom die  42  (as shown), and/or to the first surface  68  of the support substrate  44 . The adhesive element  64  can comprise any suitable adhesive material known in the art, including contact adhesives, thermoplastic adhesives and thermosetting adhesives, for example, a die-attach epoxy or equivalent, or a double-sided, multi-layered adhesive tape such as polyimide film coated on both sides with adhesive. The bottom die  42  and/or the support substrate  44  can be provided in a pre-taped form with an adhesive tape attached thereto, or an adhesive element  64  can be applied to either or both of the bottom die  42  and the support substrate  44  during fabrication of a stacked die package. Many suitable adhesive application methods for liquid or gel adhesive application are known in the art, such as screen printing, roller applicator, spray, and transfer. Similarly, an adhesive tape may be applied from a dispenser and severed from a roll of tape, or applied from a transfer (carrier) film.  
         [0038]     Referring to  FIG. 9 , with the first (bottom) die  42  mounted on the substrate  44 , the bond pads  48   a  of the first (bottom) die  42  are then electrically connected to the terminal pads  52   a  on the support substrate  44 , for example, by wire bonding (as shown) or by tape automated bonding (“TAB”). For example, ball bonds (not shown) can be thermosonically bonded to the bond pads  48   a , and the bond wires  50   a  extended and bonded to the terminal pads  52   a  on the support substrate  44 . In other embodiments, TAB bonding and ultrasonic bonding, as known in the art, can be used to connect the bond pads  48   a  and the terminal pads  52   a.    
         [0039]     Referring to  FIGS. 10-11 , the second (top) die  46  is then mounted onto the first (active) surface  70  of the first (bottom) die  42  to form the stacked die assembly  78 . The second die  46  comprises a first (active) surface  80  with a plurality of bond pads  48   b  along the periphery thereof, and a second (inactive) surface  58 . As shown in  FIG. 10 , the second surface  58  of the second (top) die  46  is aligned with and facing the first surface  70  of the first (bottom) die  42  prior to assembly. The recessed edge  62  between the first (bottom) die  42  and the overlying second die  46  is sized with a height (h) to provide an opening  63  for sufficient clearance of the bond wires  50   a  extending from the bottom die  42  to the support substrate  44 .  
         [0040]     The second (top) die  46  can be attached to the bottom die  42  by means of an adhesive element  66 , for example, a tape or die-attach adhesive as described with reference to adhesive element  64 . The first (bottom) die  42  and/or the second (top) die  46  can be provided in a pre-taped form with an adhesive tape attached thereto, or an adhesive element  66  can be applied to either or both dies during mounting of the second (top) die onto the first (bottom) die.  
         [0041]     As shown in  FIG. 11 , the bond pads  48   b  of the second (top) die  46  are then electrically connected to the terminal pads  52   b  on the support substrate  44 , for example, by wire bonding, as shown in the illustrated example, or by TAB bonding, resulting in the wire-bonded stacked die assembly  78 .  
         [0042]     The die assembly  78  can be partially or fully encapsulated with a dielectric encapsulation material  82 , typically a thermoset resin, the assembly  78  can be encapsulated using known techniques in the art, for example, screen printing, glob-top, pot molding, and transfer molding, resulting in the encapsulated stacked die package  40  depicted in  FIG. 4 . In one embodiment, a number of die assemblies  78  can be placed in a lower mold plate or half of an open multi-cavity mold, one assembly within each cavity, and following encapsulation, the mold plates are separated and the individual packages  40  can be singulated.  
         [0043]     In the embodiment illustrated in  FIG. 4 , external contacts  54 , typically in the form of conductive solder balls (or other suitable conductive material such as conductive epoxies or conductor-filled epoxies), columns, pins, and the like, are mounted on the second (bottom) surface  84  of the support substrate  44  for electrical connection of the encased die package  40  as a component to an external electrical apparatus (not shown). Examples of such electric apparatus include a PCB or other external circuitry (not shown) such as a motherboard of a computer, program logic controller (PLC), a testing apparatus, among others. The support substrate  44  typically includes a variety of conductive through-holes or vias  86  that extend through the cross-section of the substrate and establish routing of the conductive elements through the substrate  44 , and further include electrically conductive metal lines or traces and pads formed on the second (bottom) surface  84  on which the external contacts  54  are mounted.  
         [0044]     Where multiple die packages are fabricated on a panel substrate (e.g. panel  74 ,  FIG. 8 ), the panel can be singulated into individual die packages  40 , for example, by cutting or shearing.  
         [0045]     Another embodiment of a multiple chip die assembly package according to the invention is depicted in a cross-sectional, side elevational view in  FIG. 12 . The package  40 ′ comprises a first (bottom) die  42 ′ mounted to a support substrate  44 ′ in a flip chip attachment, and a second (top) die  46 ′ mounted in a recess  88 ′ formed in the first (upper) surface  72 ′ of the bottom die  42 ′. Bond pads  48   b ′ on the second (top) die  46 ′ are wire bonded  50   b ′ to terminal pads  52   b ′ on the support substrate  44 ′. The substrate further includes external contacts  54 ′ (e.g. solder balls) for connection of the die package  40 ′ as a component to an external electrical apparatus (not shown). The recess  88 ′ in the bottom die  42 ′ allows the second (top) die  46 ′ to be inset into the bottom die  42 ′, thus achieving a lower overall package height  67 ′. An adhesive element  66 ′ can be utilized to attach the second (top) die  46 ′ onto the bottom die  42 ′.  
         [0046]      FIGS. 13-16  illustrate an embodiment of a process flow and method for forming the stacked die package  40 ′ of  FIG. 12 .  
         [0047]      FIGS. 13 and 15  depict simplified cross-sectional views of the mounting and bonding of the first (bottom) die  42 ′ in a flip chip attachment to the substrate  44 ′. As shown, the first die  42 ′ comprises a first (active) surface  70 ′ and a second (inactive) surface  72 ′. The active surface  70 ′ of the first die  42 ′ includes a plurality of bond pads with conductive bumps  90 ′ mounted thereon, which are arranged in a predetermined configuration. The conductive bumps  90 ′ typically comprise a metal or alloy such as copper, silver or gold, or a conductive polymer material, and can be formed by known methods in the art, for example, electroplating, metal stud bumping by wire bonders, and stenciling. The support substrate  44 ′ can be in a form as described, for example, with respect to the support substrate  44  (die package  40 ) ( FIGS. 6-11 ).  
         [0048]     Prior to mounting, a recess  88 ′ can be formed in the second (inactive) surface  72 ′ of the first (bottom) die  42 ′, as shown in cross-section in  FIG. 13 , and in a top perspective view in  FIG. 14 . The recess  88 ′ is sized and configured to receive the second die  46 ′ therein in a subsequent step. The recess  88 ′ can be formed in any suitable shape, such as square, rectangular, oval, and circular. The recess  88 ′ can be formed to a predetermined depth and width to accommodate the placement of the second die therein using known methods in the art, for example, patterning and utilizing a chemical wet etch or dry etch, laser ablation, or other mechanical means of removing the second (inactive) surface  72 ′ of the die. Dry etchers are commercially available, for example, from SECON, having an etch rate of 25 μm/min. for an 8-inch wafer. The recess can be formed at the wafer level, the die level (i.e., singulated die), or on a strip level after the die  42 ′ is mounted on the substrate (e.g., strip).  
         [0049]     The bottom die  42 ′ can be mounted on the support substrate  44 ′ by conventional flip chip methodology. As shown in  FIG. 13 , the active surface  70 ′ of the bottom die  42 ′ is aligned with and facing the first (upper) surface  68 ′ of the support substrate  44 ′ prior to assembly. Traces and electrical connections (not shown) on the first surface  68 ′ of the support substrate  44 ′ are configured to correspond to the configuration of bond pads and the conductive bumps  90 ′ of the bottom die  42 ′. The conductive bumps  90 ′ in the form of solder bumps can be reflowed to physically and electrically bond with the traces or other conductive elements on the first (upper) surface  68 ′ of the support substrate  44 ′, or cured in the case of conductive polymer bumps, although other methods such as thermal compression can also be used. Terminal pads  52   b ′ on the first surface  68 ′ of the support substrate  44 ′ are exposed along the periphery.  
         [0050]     Referring to  FIGS. 15-16 , the second (top) die  46 ′ is then mounted in the recess  88 ′ of the bottom die  42 ′. The second (top) die  46 ′ comprises a first (active) surface  80 ′ with a plurality of bond pads  48   b ′ along the periphery thereof, and a second (bottom) surface  58 ′. As shown in  FIG. 15 , the second (bottom) surface  58 ′ of the second (top) die  46 ′ is aligned with and facing the recess  88 ′ in the second surface  72 ′ of the bottom die  42 ′ prior to assembly.  
         [0051]     The second (top) die  46 ′ can be attached to the bottom die  42 ′ by means of an adhesive element  66 ′. The adhesive element  66 ′ can be applied within the recess  88 ′ to the recess surface  92 ′ of the bottom die  42 ′, and/or to the second surface  58 ′ of the top die  46 ′ (as shown). The adhesive element  66 ′ can comprise any suitable adhesive material known in the art, for example, a tape adhesive or die attach adhesive, as described with respect to adhesive element  64 ′. The adhesive element  66 ′ can have a thickness such that it functions as a spacer to control the degree of insertion of the second die  46 ′ into the recess  88 ′. The first and/or second dies  42 ′,  46 ′ can be provided in a pre-taped form with an adhesive tape attached thereto, or an adhesive element  66 ′ can be applied to either or both dies during fabrication of the stacked die package  40 ′. The adhesive element  66 ′ can be applied by conventional methods known in the art.  
         [0052]     As depicted in  FIG. 16 , the bond pads  48   b ′ of the second (top) die  46 ′ are then electrically connected by wire bonds  50   b ′ to the terminal pads  52   b ′ on the support substrate  44 ′, for example, by wire bonding (as shown) or by TAB bonding.  
         [0053]     The wire bonded stacked die assembly  78 ′ can then be partially or fully encapsulated with a dielectric encapsulation material  82 ′ using known methods in the art to form the encapsulated stacked die package  40 ′ shown in  FIG. 12 .  
         [0054]     External contacts  54 ′ (e.g., conductive solder balls can then be mounted on the second (bottom) surface  84 ′ of the support substrate  44 ′ for connecting the die package  40 ′ to a motherboard or other electrical apparatus (not shown).  
         [0055]     Where applicable, a panel substrate comprising a plurality of dies (e.g.,  FIG. 8 , panel  74 ) can then be singulated into individual die packages  40 ′.  
         [0056]     A further embodiment of a multiple chip die assembly package according to the invention is depicted in a cross-sectional, side elevational view in  FIG. 17 . The die package  40 ″ incorporates features of the die packages  40 ,  40 ′ depicted in  FIGS. 4 and 12 .  
         [0057]     As illustrated in  FIG. 17 , the package  40 ″ comprises a first (bottom) die  42 ″ mounted onto a support substrate  44 ″ in a flip chip attachment, and a second (middle) die  46 ″ at least partially received within a recess  88 ″ in the bottom die  42 ″, similar to the die package  40 ′ ( FIG. 12 ). The package  40 ″ further comprises a third (top) die  94 ″ mounted on the first (active) surface  80 ″ of the second (middle) die  46 ″, similar to the die package  40  ( FIG. 4 ). Bond pads  48   b ″,  48   c ″ on the second (middle) die  46 ″ and the third (top) die  94 ″ are wire bonded ( 50 B″,  50   c ″) to terminal pads  52   b ″,  52   c ″, respectively, on the support substrate  44 ″. Substrate  44 ″ further includes external contacts  54 ″ (e.g., solder balls) for connecting the die package  40 ″ as a component to an electrical apparatus (not shown). A portion along the perimeter of the second (inactive) surface  96 ″ of the third (top) die  94 ″ is partially removed to provide a recessed edge  62 ″ to provide an opening  63 ″ for sufficient clearance of the bond wires  50   b ″ connecting the bond pads  48   b ″ on the second (middle) die  46 ″ to the substrate  44 ″, thus eliminating the need for a spacer between the two dies  46 ″,  94 ″. The recess  88 ″ in the bottom die  42 ″ allows the second (middle) die  46 ″ to be inserted (nested) therein. The recess features  62 ″,  88 ″ advantageously combine to achieve a lower overall package height  67 ″. Adhesive members  66 ″,  98 ″ can be utilized, respectively, to attach the second (middle) die  46 ″ to the bottom die  42 ″, and the third (top) die  94 ″ to the second (middle) die  46 ″.  
         [0058]     The stacked die package  40 ″ of  FIG. 17  can be fabricated utilizing the process steps described above in fabricating packages  40 ,  40 ′.  
         [0059]     Prior to mounting, the recesses  88 ″,  62 ″ can be formed in the first (bottom) die  42 ″ and the third (top) die  94 ″, respectively.  
         [0060]     A recess  88 ″ can be formed in the second (inactive) surface  72 ″ of the bottom die  42 ″ ( FIG. 18 ), as described with respect to die  42 ′ (package  40 ) and as depicted in  FIGS. 13-14 . The recess  88 ″ is sized and configured to receive the second (middle) die  46 ″ therein in a subsequent step, and can be suitably shaped to correspond with the shape of the second die.  
         [0061]     A recessed edge  62 ″ along the perimeter  56 ″ of the second (inactive) surface  58 ″ of the third (top) die  94 ″ can be formed as described previously for the second die  46  of package  40  and as depicted in  FIGS. 4-6 . A portion  60 ″ of the third (top) die  94 ″ is removed along the second (bottom) surface  58 ″ to provide a recessed edge  62 ″.  
         [0062]     Similar to the mounting of the first die  42 ′ on the substrate  44 ′ shown in  FIGS. 13 and 15 , the first (bottom) die  42 ″ is mounted on a support substrate  44 ″ using flip chip technology, with the terminal pads  52   a ″,  52   b ″ on the surface of the support substrate  44 ″ exposed along the periphery.  
         [0063]     The second (middle) die  46 ″ is then mounted in the recess  88 ″ of the bottom die  42 ″, as depicted in  FIGS. 15-16 . The second die  46 ″ comprises a plurality of bond pads  48   b ″ on a first (active) surface  80 ″, and a second (bottom) surface  58 ″. The bottom surface  58 ″ of the second die  46 ″ is mounted onto the recess surface  92 ″ of the bottom die  42 ″ by means of an adhesive element  66 ″, such as a tape or die-attach adhesive as described with respective to adhesive element  64 . The dies  42 ″,  46 ″ can be pre-taped or an adhesive element  66 ″ can be applied to the surface of either or both dies during fabrication of the package.  
         [0064]     The bond pads  48   b ″ of the second die  46 ″ are then electrically connected to the terminal pads  52   b ″ on the support substrate  44 ″, for example, by wire bonding or by TAB binding, resulting in a structure similar to that shown in  FIG. 16 .  
         [0065]     Referring now to  FIG. 18 , the third (top) die  94 ″ can then be mounted on the second (middle) die  46 ″ similar to the mounting of the second die  46  on the bottom die  42  shown in  FIGS. 10-11 . The third (top) die  94 ″ is mounted onto the first (active) surface  80 ″ of the second die  46 ″ to form the stacked die assembly  78 ″, as depicted in  FIG. 19 . The third (top) die  94 ″ comprises a first (active) surface  100 ″ with a plurality of bond pads  48   c ″ along the periphery thereof, and a second surface  96 ″ with recessed edge  62 ″. As shown in  FIG. 18 , the second surface  96 ″ of the third (top) die  94 ″ is aligned with and facing the first (active) surface  80 ″ of the second (middle) die  46 ″ prior to assembly.  
         [0066]     The third (top) die  100 ″ can be attached to the second die  46 ″ by means of an adhesive element  98 ″, for example, a tape or die attach adhesive, as described hereinabove with respect to adhesive element  64 . The dies  46 ″,  100 ″ can be provided in a pre-taped form or an adhesive element  98 ″ can be applied to either or both dies during mounting of the third die  94 ″ onto the second die  46 ″. The recessed edge  62 ″ of the third (top) die  94 ″ has a height (h″) to provide an opening  63 ′″ with sufficient clearance for the bond wires  50   b ″ extending from the second die  46 ″ to the support substrate  44 ″.  
         [0067]     Referring to  FIG. 19 , the bond pads  48   c ″ of the third (top) die  94 ″ are then electrically connected to the terminal pads  52   c ″ on the support substrate  44 ″, for example, by wire bonding ( 50   c ″) or TAB bonding.  
         [0068]     The die assembly  78 ″ can be partially or fully encapsulated  82 ″ resulting in the die package  40 ″ depicted in  FIG. 17 . External contacts  54 ″ in the form of conductive solder balls (or other suitable conductive material or form) are mounted on the second (bottom) surface  84 ″ of the support substrate  44 ″ to provide electrical connection of the die package  40 ″ to an electrical apparatus (not shown). Thereafter, a multi-die panel can be singulated into individual die packages.  
         [0069]     Referring to  FIG. 20 , another embodiment of a multiple chip die assembly package according to the invention is depicted in a cross-sectional, side elevational view. The package  40 ′″ comprises a first (bottom) die  42 ′″ mounted to a support substrate  44 ′″, and a second (top) die  46 ′″ mounted on the bottom die  42 ′″. The second die  46 ′″ comprises a first (active) surface  80 ′″ with bond pads  48   b ′″ along the periphery thereof, and a second (inactive) surface  58 ′″. As illustrated, the second die  46 ′″ is larger in size, i.e., a greater width (w) and/or length (l) than the bottom die (see  FIG. 6 ). Bond pads  48   a ′″,  48   b ′″, on the first and second dies  42 ′″,  46 ′″ are wire bonded  50   a ′″,  50   b ′″ to terminal pads  52   a ′″,  52   b ′″ on the support substrate  44 ′″, which further includes external contacts  54 ′″ to connect the die package  40 ′″ to an electrical apparatus. Similar to the die  46  depicted and described with respect to  FIGS. 4-6 , a portion of the second (inactive) surface  58 ′″ of the second (top) die  46 ′″ is removed to provide a recessed edge  62 ′″ for sufficient clearance for the bond wires  50   a ′″ mounted on the underlying bottom die  42 ′″. A cavity or recess  102 ′″ is also etched in the second (bottom) surface  72 ′″ of the bottom die  42 ′″, and is sized for receiving an adhesive element  104 ′″ therein to secure the bottom die  42 ′″ to the support substrate  44 ′″. The recess features  62 ′″,  102 ′″ combine to achieve a lower overall package height  67 ′″ for the package  40 ′″ by eliminating the need for a spacer between the top and bottom dies, and mounting the adhesive element  104 ′″ as an insert into the recess  102 ′″ in the bottom die  42 ′″ rather than as a distinct layer between the bottom die  42 ′″ and the substrate  44 ′″. In addition, the recess  102 ′″ contains a die-attach adhesive therein and limits the amount of adhesive (epoxy) bleed onto bond fingers and/or other components on the substrate adjacent to the die edge.  
         [0070]      FIGS. 21-24  illustrate an embodiment of a method and process flow for forming the stacked die package of  FIG. 20 .  
         [0071]     Prior to mounting, the recesses  102 ′″,  62 ′″ can be formed in the first (bottom) die  42 ′″ and the second (top) die  46 ′″, respectively.  
         [0072]     As shown in  FIG. 21 , and in a bottom perspective view in  FIG. 22 , a recess  102 ′″ is formed in the second (bottom) surface  72 ′″ of the first (bottom) die  42 ′″. The recess  102 ′″ is sized and configured to receive an adhesive member  104 ′″ therein for attachment of the die  42 ′″ to the substrate  44 ′″. The recess  102 ′″ can be formed in any suitable shape, such as square, rectangular, oval, and circular. The recess  102 ′″ can be formed using known methods in the art, for example, patterning and utilizing a chemical wet etch or dry etch, mechanical drilling or punching, and laser ablation of the second surface  72 ′″ of the die  42 ′″. The recess  102 ′″ can be formed at the wafer level or the die level (i.e., singulated die).  
         [0073]     A recessed edge  62 ′″ along the perimeter  56 ′″ of the second (inactive) surface  58 ′″ of the second (top) die  46 ′″ can be formed as described previously for the second die  46  (package  40 ) depicted in  FIGS. 4-6 . A portion of the die  46 ′″ is removed such that, when the second die  46 ′″ is then mounted onto the first die  42 ′″, the recessed edge  62 ′″ provides an opening  63 ′″ for sufficient clearance of the bond wires  50   a ′″ extending from the first die  42 ′″ to the terminal pads  52   a ′″ on the support substrate  44 ′″.  
         [0074]     Referring to  FIG. 21 , the second (bottom) surface  71 ′″ of the first (bottom) die  42 ′″ is aligned with and facing the first (upper) surface  68 ′″ of the support substrate  44 ′″ prior to assembly.  
         [0075]     The first die  42 ′″ is attached to the support substrate  44 ′″ by means of an adhesive element  104 ′″. The adhesive element  104 ′″ can be applied to the recess surface  106 ′″ of the recess  102 ′″ of the first (bottom) die  42 ′″, and/or onto the first (upper) surface  68 ′″ of the substrate  44 ′″ and aligned with the recess  102 ′″ to be received therein. The adhesive element  104 ′″ can comprise an adhesive gel or tape, as described hereinabove with respect to adhesive element  64  (package  40 ). The first die  44 ′″ and/or the substrate  44 ′″ can be provided in a pre-taped form, or an adhesive element  104 ′″ can be applied to the surface of either or both the first die  42 ′″ and the substrate  44 ′″ during the attachment step. The first die  42 ′″ is attached to the substrate  44 ′″ such that the terminal pads  52   a ′″,  52   b ′″ on the surface of the substrate are exposed.  
         [0076]     Referring to  FIGS. 23-24 , the second (top) die  46 ′″ is then mounted onto the first (bottom) die  42 ′″ to form the stacked die assembly  78 ′″. As shown in  FIG. 23 , the second surface  58 ′″ of the second (top) die  46 ′″ is aligned with and facing the first (active) surface  70 ′″ of the first (bottom) die  42 ′″ prior to assembly. The second (top) die  46 ′″ can be attached to the first die by means of an adhesive element  66 ′″, for example, with a tape or die attach adhesive, as described with respect to the adhesive element  64  (die package  40 ). Either or both of the first and second dies  42 ′″,  46 ′″ can be provided in a pre-taped form or the adhesive element  66 ′″ can applied to either or both dies during the mounting step.  
         [0077]     As depicted in  FIG. 24 , the bond pads  48   b ′″ of the second (top) die  46 ′″ can then be electrically connected to the terminal pads  52   b ′″ on the substrate  44 ′″. The recessed edge  62 ′″ of the second (top) die  46 ′″ has a height (h′″) sufficient to provide an opening  63 ′″ for adequate clearance of the bonding wires  50   a ′″ extending from the second die  46 ′″ to the substrate  44 ′″.  
         [0078]     The wire-bonded stacked die assembly  78 ′″ can be partially or fully encapsulated with an encapsulant material  82 ′″ using known techniques in the art to form the encapsulated stacked die package  40 ′″ as depicted in  FIG. 20 . Thereafter, external contacts  54 ′″ can be mounted on the second (bottom) surface  84 ′″ of the support substrate  44 ′″ for electrical connection of the die package  40 ′″ to an external electrical apparatus (not shown).  
         [0079]     Singulation of a multiple die panel or strip can then be performed to provide individual die packages  40 ′″.  
         [0080]     Referring to  FIG. 25 , a further embodiment of a multiple chip die assembly package according to the invention, is depicted in a cross-sectional, side elevational view. The package  40 ″″ comprises a first (bottom) die  42 ″″ mounted in a flip chip attachment to a support substrate  44 ″″, and a larger sized, second (top) die  46 ″″ mounted on the first (bottom) die  42 ″″. Bond pads  48   b ″″ on the second (top) die  46 ″″ are wire bonded  50   b ″″ to terminal pads  52   b ″″ on the support substrate  44 ″″. External contacts  54 ″″ are mounted on the second (bottom) surface of the substrate  44 ″″ for connecting the package  40 ″″ to an external electrical apparatus (not shown). A portion of the second (bottom) surface  58 ″″ of the second (top) die  46 ″″ is removed to provide a recess  108 ″″ for receiving the bottom die  42 ″″ therein. The recess feature  108 ″″ helps achieve a lower overall package height  67 ″″ for the package  40 ″″ by nesting the first die  42 ″″ within the overlying second die  46 ″″.  
         [0081]      FIGS. 26-29  illustrate an embodiment of a method and process flow for forming the stacked die package of  FIG. 25 .  
         [0082]     Prior to mounting, the recess  108 ″″ can be formed in the second (bottom) surface  58 ″″ of the second (top) die  46 ″″, as shown in  FIG. 25  and in a bottom perspective view in  FIG. 26 . The recess  108 ″″ can be formed at the wafer level or the die level. The recess  108 ″″ is sized and configured to receive the bottom die  42 ″″ therein, and can be formed in any suitable shape, such as square, rectangular, oval, and circular using known techniques in the art.  
         [0083]     As depicted in  FIG. 27 , the first (active) surface  70 ″″ of the first (bottom) die  42 ″″ is aligned with and facing the first (upper) surface  68 ″″ of the support substrate  44 ″″ prior to assembly. The active surface  70 ″″ of the first die  42 ″″ includes a plurality of bond pads with conductive bumps  90 ″″ mounted thereon, which are arranged in a predetermined configuration. The bottom die  42 ″″ can be mounted on the support substrate  44 ″″ according to conventional flip chip techniques, resulting in the structure shown in  FIG. 28 .  
         [0084]     The second (top) die  46 ″″ is then mounted onto the first (bottom) die  42 ″″ to form the stacked die assembly  78 ″″. The second die  46 ″″ comprises a first (active) surface  80 ″″ with bond pads  48   b ″″, and a second (inactive) surface  58 ″″. As shown in  FIG. 28 , the second surface  58 ″″ of the second (top) die  46 ″″ is aligned with and facing the second surface  72 ″″ of the first (bottom) die  42 ″″ prior to assembly. The first (bottom) die  42 ″″ is received at least partially in the recess  108 ″″ and can be attached to the recess surface  106 ″″ of the second die  46 ″″ by means of an adhesive element  66 ″″ such as a tape or die attach adhesive as described with respect to the adhesive element  64  (die package  40 ). Either or both of the first and second dies  42 ″″,  46 ″″ can be provided in a pre-taped form, or the adhesive element  66 ″″ can applied to either or both dies during the mounting step.  
         [0085]     Referring to  FIG. 29 , the bond pads  48   b ″″ on the first (active) surface  80 ″″ of the second (top) die  46 ″″ can then be electrically connected to the terminal pads  52   b ″″ on the substrate  44 ″″.  
         [0086]     Partial or full encapsulation of the die assembly  78 ″″ can be performed using known techniques in the art to form the encapsulated package  40 ″″ shown in  FIG. 25 . External contacts  54 ″″ can then be mounted on the second (bottom) surface  84 ″″ of the substrate  44 ″″ to facilitate electrical connection of the component die package  40 ″″ to an external electrical apparatus (not shown).  
         [0087]     Individual die packages of a multiple die panel (e.g., as shown with reference to panel  74  in  FIG. 8 ) can be separated by a singulation technique.  
       COMPARATIVE EXAMPLE 1 AND EXAMPLE 2  
       [0088]     A comparison of the package design shown in  FIG. 2  (prior art) with the package design shown in  FIG. 12 .  
                                                                                                                                                                          Bottom die, thickness   6   mils   6   mils           Second die, thickness   6   mils   6   mils           Bond line, thickness   1   mil   1   mil                Slot (recess) depth   —   4   mils                Overall total thickness of the   13   mils   9   mils           stacked dies                      
 
         [0089]     By utilizing a package design according to the invention, a lower package height can be achieved using thicker dies. In addition, thicker dies can be utilized to help reduce the number of cracked dies that occur during the assembly process.  
         [0090]     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.