Abstract:
A microelectronic device and method for manufacture. In one embodiment, two microelectronic substrates are directly bonded to each other without an intermediate adhesive material. For example, each microelectronic substrate can include a first surface, a second surface opposite the first surface, and a functional microelectronic feature coupled to a connection terminal of the microelectronic substrate. The connection terminals can be coupled to a support member, such as a leadframe or a printed circuit board, with the bond plane between the microelectronic substrates either aligned with or transverse to the support member. The microelectronic substrates can be enclosed in a protective packaging material that can include a transparent window to allow selected radiation to strike one or the other of the microelectronic substrates.

Description:
This application is a Division of U.S. application Ser. No. 09/634,056, filed on Aug. 9, 2000. 
    
    
     TECHNICAL FIELD 
     This invention relates to microelectronic devices having multiple substrates and s methods for manufacturing such microelectronic devices. 
     BACKGROUND OF THE INVENTION 
     Microelectronic devices, such as memory chips and microprocessor chips, typically include a microelectronic substrate die encased in a plastic, ceramic or metal protective covering. The die includes functional features, such as memory cells, processor circuits, and interconnecting circuitry. The die also typically includes bond pads electrically coupled to the functional features. The bond pads are coupled to terminals, such as pins, that extend outside the protective covering for connecting to buses, circuits and/or other microelectronic devices. 
     Conventional microelectronic devices are typically arranged side-by-side on a is circuit board or other support device that is incorporated into a computer, mobile phone or other larger electronic product. One drawback with this arrangement is that the circuit board may have a large surface area to accommodate a large number of microelectronic devices. Accordingly, it may be difficult to fit the circuit board into a housing of a compact electronic product. 
     One approach to address this problem is to stack one microelectronic die on top of another to reduce the surface area occupied by the dies. Typically, the stacked microelectronic dies are connected to each other with an intermediate adhesive layer that is heat cured to securely bond the dies to each other. However, the adhesive can have several drawbacks. For example, at high temperatures, the adhesive can emit gases that leave deposits on the bond pads of dies. The deposits can inhibit secure electrical connections between the bond pads and the terminals of the die. Another drawback is that the adhesive layer can have a different coefficient of thermal expansion than the dies to which it is attached. Accordingly, the adhesive layer can put stresses on the dies as the ambient temperature changes. In some cases, these stresses can crack or fracture the dies. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward compound microelectronic devices and methods for manufacturing compound microelectronic devices. A method in accordance with one aspect of the invention includes forming a first microelectronic substrate having a first surface, a second surface opposite the first surface, a first operable microelectronic device, and a first connection terminal coupled to the first microelectronic device. The method further includes forming a second microelectronic substrate having a first surface, a second surface opposite the first surface, a second operable microelectronic device, and a second connection terminal coupled to the second microelectronic device. The first and second microelectronic substrates are bonded together by placing the second surface of the second microelectronic substrate directly against the first or second surface of the first microelectronic substrate. 
     In a further aspect of the invention, the method can further include selecting the first and second microelectronic substrates to include microelectronic dies separated from one or more microelectronic wafers. Alternatively, when the first and second substrates are microelectronic wafers, the method can further include separating the wafers into bonded pairs of dies after bonding the wafers together. 
     The invention is also directed toward a compound microelectronic device. In one aspect of the invention, the compound microelectronic device can include a first microelectronic substrate having a first surface, a second surface opposite the first surface, a first functional or operable microelectronic feature or device, and a first connection terminal coupled to the first microelectronic feature or device. The compound device can further include a second microelectronic substrate having a first surface, a second surface opposite the first surface, a second functional or operable microelectronic feature or device, and a second connection terminal coupled to the second feature or device. The second surface of the second microelectronic substrate is bonded directly to the first or second surface of the first microelectronic substrate. 
     In a further aspect of the invention, the compound device can include a conductive support member engaged with at least one of the microelectronic substrates and conductive couplings extending between the conductive support member and the connection terminal. In still a further aspect of the invention, the first and second microelectronic substrates can each include an intermediate surface between the first and second surfaces, and the connection terminals of the substrates can be positioned on the intermediate surfaces. The compound device can further include a support member and the microelectronic substrates can be coupled to the support member as a unit with the first and second connection terminals electrically coupled to the support member and with the intermediate surfaces of the microelectronic substrates facing toward the support member. 
     In yet another aspect of the invention, the compound device can include a packaging material encapsulating the microelectronic substrates in a single enclosure. When one of the microelectronic substrates includes an imaging device, the enclosure can include a lens at least partially transparent to a selected radiation and positioned proximate to the imaging device to allow the radiation to pass to the imaging device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top isometric view of two microelectronic substrate wafers bonded to each other in accordance with an embodiment of the invention. 
         FIG. 2  is a top isometric view of two dies separated from the wafers shown in  FIG. 1  and mounted to a support member in accordance with an embodiment of the invention. 
         FIG. 3  is a side isometric view of a packaged device formed from the dies shown in  FIG. 2  in accordance with an embodiment of the invention. 
         FIG. 4  is a top isometric view of two bonded dies edge-mounted to a support member in accordance with another embodiment of the invention. 
         FIG. 5  is a side isometric view of two dies edge-mounted to a support member in accordance with still another embodiment of the invention. 
         FIG. 6  is a top isometric view of two dies bonded to each other and mounted to a support member in accordance with still another embodiment of the invention. 
         FIG. 7  is a top isometric view of two dies bonded and electrically coupled to each other in accordance with yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes microelectronic devices and methods for forming such devices. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 1–7  to provide a thorough understanding of these embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the invention may be practiced without several of the details described below. 
       FIG. 1  is a top isometric view of a first microelectronic substrate  20   a  bonded to a second microelectronic substrate  20   b  (referred to collectively as “microelectronic substrates  20 ”) in accordance with one embodiment of the invention. In one aspect of this embodiment, the first microelectronic substrate  20   a  includes a first wafer  21   a  and the second microelectronic substrate  20   b  includes a second wafer  21   b  (referred to collectively as “wafers  21 ”). The wafers  21  can include silicon or another suitable substrate material. Each wafer  21  has a first surface  22  and a second surface  23  opposite the first surface. The wafers  21  each have a plurality of operable microelectronic devices, such as memory devices, processors, and/or other microelectronic devices arranged to form separable, stand-alone dies  30 . For example, when the microelectronic devices are memory devices, the dies  30  can have memory capacity of 128 megabits or more. Each die  30  has one or more connection terminals  31  at the first surface  22  for coupling to other dies or devices. The dies  30  shown in  FIG. 1  have six connection terminals  31 , but can have more or fewer connection terminals in other embodiments. 
     The wafers  21  are bonded directly to each other by a technique commonly referred to as “wafer direct bonding.” This technique is described by Andreas Ploessl and Gertrude Krauter in an article entitled “Wafer Direct Bonding: Tailoring Adhesion Between Brittle Materials,” Materials Science and Engineering, R25 (1999) at 1–88 (Elsevier Science S.A.), which is incorporated by reference herein in its entirety. Accordingly, the second surfaces  23  of each wafer  21  are planarized, polished or otherwise processed to be extremely flat. The second surfaces  23  are then brought into contact with each other and directly bonded by attractive forces between the wafers  21  without requiring an intermediate adhesive. In one aspect of this embodiment, the temperature and/or pressure of the wafers  21  can be elevated while the second surfaces  23  are in contact with each other to strengthen the bond between the wafers  21 . Alternatively, the wafers  21  can be attached without elevated pressure or temperature. In either embodiment, the bond between the wafers  21  can be extremely strong and can equal the bulk strength of the material comprising the wafers  21 . 
     In one embodiment, the wafers  21  can be cut along the lines delineating the dies  30  after the wafers  21  are bonded to each other. Alternatively, the dies  30  can be separated from each other before bonding the wafers  21  together, and pairs of the individual dies  30  can be bonded together using the direct bonding technique described above. In either embodiment, the result (as shown in  FIG. 2 ) is a first die  30   a  bonded directly to a second die  30   b  without an intermediate adhesive. Each die  30  has an outwardly facing first surface  32  (having the connection terminals  31 ), and a second surface  33  bonded directly to the corresponding second surface  33  of the adjacent die  30 . 
     In one aspect of the embodiment shown in  FIG. 2 , the dies  30  are mounted to a conductive support  40  to provide connections between the dies  30  and other devices. The conductive support  40  can include a leadframe  41  having an outer ring  43  with leadfingers  42  that extend inwardly toward the dies  30 . The second die  30   b  can be mounted to the leadfingers  42  with adhesive strips  44  that extend along opposing edges of the second die  30   b . Alternatively, each leadfinger  42  can be bonded to the second die  30   b  with an individual portion of adhesive. In either of these embodiments, the dies  30  are then electrically connected to the leadframe  41  by connecting wire bonds  35  between the connection terminals  31  of the dies  30  and the leadfingers  42  of the leadframe  41 . In an alternate arrangement, the dies  30  can be inverted as a unit so the first die  30   a  is mounted to the leadfingers  42  with the adhesive strips  44 . The connection terminals  31  of both dies  30  can be connected to the same leadfingers  42  as shown in  FIG. 2  or alternatively, the connection terminals  31  of the first die  30   a  and the connection terminals of the second die  30   b  can be connected to different leadfingers  42 . 
     Next, the outer ring  43  can be separated from the leadfingers  42  by cutting the leadframe  42  along line “A.” A protective coating is disposed around the dies  30  and the leadfingers  42  up to line “B.” The protective coating can be applied in accordance with one or more known techniques, such as pouring a liquid epoxy directly over the dies  30 , placing the dies  30  in a cavity and injecting epoxy into the cavity, or placing the dies within a preformed package. In any of these embodiments, the protective coating forms a package enclosure  60 , shown in  FIG. 3 , that protects the dies  30  from contaminants and contact with other devices. The leadfingers  42  can be bent downwardly (as shown in  FIG. 3 ) or upwardly to forms pins  45  for connecting the resulting packaged compound microelectronic device  10  to other devices and/or other circuits (not shown). 
     One feature of the processes and devices described above with reference to  FIGS. 1–3  is that the dies  30  are bonded directly to each other without an intermediate adhesive layer. By eliminating the adhesive layer, this embodiment is expected to eliminate contamination caused by outgassing of an intermediate layer. Furthermore, some conventional adhesives used to bond the dies  30  have a different coefficient of thermal expansion than the dies  30  and may crack the dies  30  at elevated temperatures. Accordingly, eliminating the adhesive is expected to eliminate this potential source of damage to the dies  30 . Yet another advantage is that the packaged microelectronic device  10  will be shorter (in the vertical direction, as shown in  FIGS. 1–3 ) than a conventional packaged device because the thickness of the adhesive is eliminated. Accordingly, the microelectronic device  10  can more easily be placed in locations having limited vertical clearance. Still another advantage is that the first surfaces  32  of the dies  30  are more likely to be parallel to each other and to the conductive support  40  when the dies are directly bonded to each other. This advantage can be particularly important when one or more of the dies  30  has optical features, as will be discussed in greater detail below with reference to  FIG. 7 . 
       FIG. 4  is a top isometric view of a compound microelectronic device  110  in accordance with another embodiment of the invention. The microelectronic device  110  includes a first die  130   a  bonded to a second die  130   b  (referred to collectively as dies  130 ), either before or after the dies  130  are separated from corresponding wafers  21  ( FIG. 1 ). Each die  130  has an outwardly facing first surface  132 , an inwardly facing second surface  133  and an intermediate surface  134  between the first surface  132  and the second surface  133 . The second surface  133  of the first die  130   a  is bonded to the corresponding second surface  133  of the second die  130   b  without an intermediate adhesive in a manner generally similar to that discussed above with reference to  FIG. 1 . 
     The first surface  132  of each die  130  includes a plurality first connection terminals  131   a  and the intermediate surface  134  of each die  130  includes one or more second connection terminals  131   b  electrically coupled to the first connection terminals  131   a  with couplings  136 . In one aspect of this embodiment, the first connection terminals  131   a  are connected to the second connection terminals  131   b  when the die or the wafer is fabricated. Alternatively, the first connection terminals  131   a  can be connected to the second connection terminals after fabrication. In still another embodiment, the first connection terminals  131   a  can be eliminated and the functional microelectronic features within the dies  130  can be connected directly to the second connection terminals  131   b  positioned on the intermediate surface  134  of the dies  130 . 
     In any of the above embodiments discussed with reference to  FIG. 4 , the dies  130  are positioned on a support member  150  with the intermediate surfaces  134  facing the support member  150  and the second surfaces  133  extending transversely away from the support member  150 . The support member  150  can include connection sites for connecting to the second connection terminals  131   b  of the dies  130 . For example, when the second connection terminals  131   b  include solder balls  139 , the support member  150  can include solder pads  151  aligned with the solder balls  139 . The solder pads  151  are connected to leads  153  for coupling the dies  130  to other devices. Alternatively, the device  110  can include other arrangements for coupling the second connection terminals  131   b  to the support member  150 . In either embodiment, the device  110  can include an enclosure surrounding the dies  130  in a manner similar to that discussed above with reference to  FIG. 3 , or the device  110  can be coupled to the support member  150  without an enclosure. 
     One feature of the compound microelectronic device  110  discussed above with reference to  FIG. 4  is that the dies  130  are rotated 90 degrees relative to the dies  30  discussed above with reference to  FIGS. 1–3  before they are attached to the support member  150 . Accordingly, the intermediate surfaces  134  of the dies  130  face the support member  150 . An advantage of this feature is that the dies  130  may take up less surface area of the support member than the dies  30  discussed above with reference to  FIGS. 1–3 . 
       FIG. 5  is a top isometric view of a compound microelectronic device  210  that includes a first die  230   a  coupled to a second die  230   b , both of which are coupled to a support member  250  in accordance with another embodiment of the invention. Each die  230  includes an outwardly facing first surface  232  having connection terminals  231 , an inwardly facing second surface  233 , and an intermediate surface  234  between the first and second surfaces. In one aspect of this embodiment, the second surfaces  233  of each of the dies  230  are bonded to each other with a die adhesive layer  238   a  that extends between the two dies  230 . Alternatively, the dies  230  can be directly bonded in a manner similar to that discussed above with reference to  FIGS. 1–4 . In either embodiment, the device  210  can include a support adhesive layer  238   b  that extends between the support member  250  and the intermediate surfaces  234  of the dies  230  to bond the dies  230  to the support member  250 . Alternatively, the intermediate surfaces  234  of the dies  230  can also include solder balls generally similar to those discussed above with reference to  FIG. 4  connected to the support member  250 . The support member  250  includes support member pads  252  connected to the connection terminals  231  of the dies  230  by wire bonds  235 . The support member pads  252  can be coupled to other devices with leads  253 . 
     One feature of the compound device  210  discussed above with reference to  FIG. 5  is that the dies  230  are connected to each other with an adhesive layer  238   a . Accordingly, an advantage of the device  210  is that the second surfaces  233  of the dies  230  need not be made as flat as the corresponding directly bonded surfaces discussed above with reference to  FIGS. 1–4 . Conversely, an advantage of the directly bonded substrates discussed above with reference to  FIGS. 1–4  is that the adhesive layer  238   a  is eliminated to reduce the likelihood of contaminating and/or cracking the dies. 
     Another feature of the compound device  210  shown in  FIG. 5  is that the connection terminals  231  are bonded directly to the support member pads  252  without the need for intermediate connection terminals on the intermediate surface  234  of the dies  230 . An advantage of this feature is that it can eliminate the fabrication process required to provide the additional connection terminals. Conversely, an advantage of the arrangement discussed above with reference to  FIG. 4  is that it may be easier to connect terminals that face directly toward each other than to connect terminals that do not directly face each other. 
       FIG. 6  is a top isometric view of a compound microelectronic device  310  having a first die  330   a  and a second die  330   b  coupled to a support member  350  in accordance with another embodiment of the invention. In one aspect of this embodiment, the first die  330   a  has a first surface  332  facing downwardly toward the support member  350  and a second surface  333  facing upwardly toward the second die  330   b . The second die  330   b  has an upwardly facing first surface  332  and a downwardly facing second surface  333  bonded to the second surface  333  of the first die  330   a  in accordance with any of the direct bonding processes discussed above with reference to  FIGS. 1–3 . 
     In a further aspect of this embodiment, the first die  330   a  has first connection terminals  331   a  in the form of solder balls  339  that are bonded to corresponding solder pads  351  of the support member  350 . The second die  330   b  has second connection terminals  331   b  in the form of bond pads that are coupled to corresponding support member pads  352  with wire bonds  335 . The second die  330   b  can have a planform shape that is smaller than the planform shape of the first die  330   a  (as shown in  FIG. 6 ). For example, the first die  330   a  can include an SRAM device and the second die  330   b  can include a flash memory device. Alternatively, the second die  330   b  can have a planform shape that is the same as or larger than the planform shape of the first die  330   a . In either of these embodiments, an advantage of the device  310  compared to an embodiment of the device  10  discussed above with reference to  FIGS. 1–3  is that it may be easier in some cases to bond the solder balls  339  to the solder pads  351  than to wire bond the connection terminals  31  to the leadframe  41 . 
       FIG. 7  is a top isometric view of a compound microelectronic device  410  having a first die  430   a  electrically coupled to a second die  430   b  and a support member  450  in accordance with yet another embodiment of the invention. In one aspect of this embodiment, each die  430  has an upwardly facing first surface  432  and a downwardly facing second surface  433  opposite the first surface  432 . The first die  430   a  has first connection terminals  431   a  and the second die  430   b  has second connection terminals  431   b . In a further aspect of this embodiment, the first and second connection terminals (collectively referred to as “connection terminals  431 ”) can include wire bond pads on the upwardly facing first surfaces  432 . 
     In one aspect of the embodiment shown in  FIG. 7 , the second surface  433  of the second die  430   b  is directly bonded to the first surface  432  of the first die  430   a  using a method generally similar to that discussed above with reference to  FIGS. 1–3 . For example, when the first surface  432  of the first die  430   a  includes a passivation layer, the passivation layer and the second surface  433  of the second die  430   b  are planarized, polished or otherwise processed to be extremely flat. In another aspect of this embodiment, at least some of the second connection terminals  431   b  of the second die  430   b  are connected directly to corresponding first connection terminals  431   a  of the first die  430   a . At least some of the first connection terminals  431   a  of the first die  430   a  are connected to support member pads  452  on the support member  450  using wire bonds  435 . Such an arrangement may be suitable where it is advantageous to minimize the lengths of the electrical paths between the first die  430   a  and the second die  430   b . For example, when the first die  430   a  is a processor and the second die  430   b  includes an imaging device, the direct connection between the two dies  430  can increase the speed and timing accuracy with which signals are transmitted between the processor and the imaging device. 
     In still another aspect of the embodiment shown in  FIG. 7 , the device  410  can include a package enclosure  460 , a portion of which is shown in  FIG. 7 . In one aspect of this embodiment (for example, when the second die  430   b  includes an imaging device), the package enclosure  460  can include a lens  461  that transmits radiation to the second die  430   b . Alternatively, the second die  430   b  can emit light and the lens  461  can focus light leaving the package enclosure  460 . In either embodiment, the package enclosure  460  can be pre-formed so that the lens  461  is parallel to the support member  450 . Accordingly, a feature of directly bonding the second die  430   b  to the first die  430   a  is that it is more likely that the second die  430   b  will remain parallel to the support member  450  and, therefore, parallel to the lens  461 . This feature is advantageous because if the second die  430   b  is not parallel to the lens  461 , the device  410  may not operate properly because the lens  461  may not accurately focus radiation received by and/or emitted by the second die  430   b.    
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, the operable microelectronic devices discussed above can include operable microelectronic components or features that combine to form the operable microelectronic devices, or that define standalone operable elements. Accordingly, the invention is not limited except as by the appended claims.