Abstract:
A method of implementing three-dimensional (3D) integration of multiple integrated circuit (IC) devices includes forming a first insulating layer over a first IC device; forming a second insulating layer over a second IC device; forming a 3D, bonded IC device by aligning and bonding the first insulating layer to the second insulating layer so as to define a bonding interface therebetween, defining a first set of vias within the 3D bonded IC device, the first set of vias landing on conductive pads located within the first IC device, and defining a second set of vias within the 3D bonded IC device, the second set of vias landing on conductive pads located within the second device, such that the second set of vias passes through the bonding interface; and filling the first and second sets of vias with a conductive material.

Description:
BACKGROUND 
     The present invention relates generally to semiconductor device manufacturing techniques and, more particularly, to implementing three-dimensional integration of multiple integrated circuit (IC) devices. 
     The packaging density in electronic industry continuously increases in order to accommodate more electronic devices into a package. In this regard, three-dimensional (3D) wafer-to-wafer stacking technology substantially contributes to the device integration process. Typically, a semiconductor wafer includes several layers of integrated circuitry (e.g., processors, programmable devices, memory devices, etc.) built on a silicon substrate. A top layer of the wafer may be connected to a bottom layer of the wafer through silicon interconnects or vias. In order to form a 3D wafer stack, two or more wafers are placed on top of one other and bonded. 
     3D wafer stacking technology offers a number of potential benefits, including, for example, improved form factors, lower costs, enhanced performance, and greater integration through system-on-chip (SOC) solutions. In addition, the 3D wafer stacking technology may provide other functionality to the chip. For instance, after being formed, the 3D wafer stack may be diced into stacked dies or chips, with each stacked chip having multiple tiers (i.e., layers) of integrated circuitry. SOC architectures formed by 3D wafer stacking can enable high bandwidth connectivity of products such as, for example, logic circuitry and dynamic random access memory (DRAM), that otherwise have incompatible process flows. At present, there are many applications for 3D wafer stacking technology, including high performance processing devices, video and graphics processors, high density and high bandwidth memory chips, and other SOC solutions. 
     SUMMARY 
     In an exemplary embodiment, a method of implementing three-dimensional (3D) integration of multiple integrated circuit (IC) devices includes forming a first insulating layer over a first IC device; forming a second insulating layer over a second IC device; forming a 3D, bonded IC device by aligning and bonding the first insulating layer of the first IC device to the second insulating layer of the second IC device so as to define a bonding interface therebetween, wherein the bonding interface is absent of electrically conductive materials; subsequent to the bonding, defining a first set of vias within the 3D bonded IC device, the first set of vias landing on conductive pads located within the first IC device, and defining a second set of vias within the 3D bonded IC device, the second set of vias landing on conductive pads located within the second device, such that the second set of vias passes through the bonding interface; and filling the first and second sets of vias with a conductive material, and electrically connecting at least one via of the first set of vias to at least one via of the second set of vias, thereby establishing electrical communication between the first and second ICs of the 3D bonded IC device. 
     In another embodiment, a method of implementing three-dimensional (3D) integration of multiple integrated circuit (IC) devices includes forming a first IC device having a semiconductor substrate, front-end-of-line (FEOL) structures, middle-of-line (MOL) structures, and back-end-of-line (BEOL) structures, with a first insulating layer over the BEOL structures of the first IC device; forming a second IC device having a semiconductor substrate, FEOL structures, MOL structures, and BEOL structures, with a second insulating layer over the BEOL structures of the second IC device; forming a 3D, bonded IC device by aligning and bonding the first insulating layer of the first IC device to the second insulating layer of the second IC device so as to define a first bonding interface therebetween, wherein the first bonding interface is absent of electrically conductive materials; subsequent to the bonding, defining a first set of vias within the 3D bonded IC device, the first set of vias landing on conductive pads located within the first IC device, and defining a second set of vias within the 3D bonded IC device, the second set of vias landing on conductive pads located within the second device, such that the second set of vias passes through the first bonding interface; and filling the first and second sets of vias with a conductive material, and electrically connecting at least one via of the first set of vias to at least one via of the second set of vias, thereby establishing electrical communication between the first and second ICs of the 3D bonded IC device. 
     In another embodiment, a three-dimensional (3D) integrated circuit (IC) device includes a first IC device bonded to a second IC device at a first bonding interface therebetween, thereby defining a 3D, bonded IC device, the first bonding interface defined between a first insulating layer of the first IC device and second insulating layer of the second IC device, wherein the first bonding interface is absent of electrically conductive materials; a first set of vias defined within the 3D bonded IC device, the first set of vias landing on conductive pads located within the first IC device, and a second set of vias defined within the 3D bonded IC device, the second set of vias landing on conductive pads located within the second device, such that the second set of vias passes through the first bonding interface; and the first and second sets of vias filled with a conductive material, and electrically connecting at least one via of the first set of vias to at least one via of the second set of vias, thereby establishing electrical communication between the first and second ICs of the 3D bonded IC device. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIGS. 1 through 16  are a series of cross-sectional views illustrating a method of implementing three-dimensional integration of multiple integrated circuit (IC) devices, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     One disadvantage associated with the current 3D wafer stacking technology concerns the use of adhesives to bond the wafers. Such adhesives limit the amount of practical, in-fab processing and raised reliability concerns due to undesirable inherent polymeric adhesive properties, such as thermal stability. In addition, achieving acceptable alignment of pre-existing through silicon vias (TSV) across the entire diameter of a pair of wafers is also difficult, which also creates reliability concerns. 
     Accordingly, disclosed herein is a method and resulting structure for 3D wafer integration bonding in which the TSVs are formed post bonding. In this manner, the actual bonding involves only oxide-to-oxide bonding (or more generally insulator-to-insulator bonding) of the wafers, in that because the TSVs are not formed on the individual wafers prior to bonding, there are no alignment issues therebetween with respect to the vias. 
     It should be appreciated that although specific wafer substrate bonding process flows are depicted herein, such descriptions are exemplary only, and that the principles disclosed herein are also applicable to various types of TSV conductive materials, dielectric and adhesive interface materials, and multiple types of semiconductor wafers and substrates. 
     Referring initially to  FIG. 1( a ) , there is shown a cross-sectional view of a first wafer  100  to be integrated and bonded with one or more additional wafers. In the exemplary embodiment depicted, the wafer  100  represents a memory wafer having front-end-of-line (FEOL), middle-of-line (MOL), and back-end-of-line (BEOL) structures formed thereon, as known in the art. In addition, the exemplary memory wafer  100  is shown having a thick sacrificial base layer  102  (e.g., a heavily doped P+ layer), and a lightly doped epitaxial layer  104  formed on the sacrificial base layer. The sacrificial base layer  102  may have a dopant concentration of, for example, 1000 times that of the epi-layer  104 . The FEOL structures are formed in the epi-layer  104 , which serves as the memory layer substrate. As indicated above, this layer  104  could be embodied by a bulk substrate of semiconducting material or a semiconductor-on-insulator (SOI) substrate. 
     As will be appreciated, the wiring layers (e.g.,  106  and  108 ) shown in the MOL and BEOL regions of the wafer are illustrative only. In an actual device, there may be several layers of insulator materials and associated wiring formed therein. As also shown in  FIG. 1( a ) , one or more strap/landing pads  110  are formed in the wiring layers. For purposes of illustration, the pads are shown at the M 1  (first) level of wiring, although such pads can be formed at various levels within the device. 
     Referring now to  FIG. 1( b ) , there is shown a cross-sectional view of a second wafer  200  to be integrated and bonded with the first wafer  100  shown in  FIG. 1( a ) . In the exemplary embodiment depicted, the wafer  200  represents a processor wafer having FEOL, MOL and BEOL structures formed thereon, as known in the art. In addition, the exemplary memory wafer  200  is shown having a base substrate layer  202  (e.g., a P-type layer), which may be embodied by a bulk substrate of semiconducting material such as silicon or an SOI substrate. Again, the wiring layers (e.g.,  206  and  208 ) shown in the MOL and BEOL regions of the wafer  200  are illustrative only. As is the case with the memory wafer  100 , the processor wafer  200  also includes metal strap/landing pads  210  formed in one or more of the wiring layers. 
       FIGS. 2( a ) and 2( b )  depict passivation of the wafers  100 ,  200 , respectively, with an oxide layer  120 ,  220 , or other suitable type of insulator material (including any adhesive material) in preparation of wafer bonding. Both passivated wafers  100 ,  200  are then shown together in  FIG. 3  where, in particular, memory wafer  100  is flipped and aligned with processor wafer  200 . Such alignment may be implemented through any known techniques such as, for example, infrared (IR) alignment or other suitable method. It will be noted that since no TSVs have been formed in either of the wafers  100 ,  200  to this point, there is no need to align any conductor materials between the wafers. 
     As then shown in  FIG. 4 , the wafers  100 ,  200  are bonded together to form an integrated wafer, now depicted generally at  300 . Where oxide is used as the passivation material for the individual wafers, the bonding may be, for example, oxide-to-oxide bonding (e.g., by annealing), oxide/adhesive bonding, or any other suitable technique known in the art that results in a strong bond between electrically insulating layers. Thus bonded, integrated wafer  300  has a bonding interface  302  between layers  120  and  220 , wherein the interface is comprised entirely of insulating materials, and no conducting materials such as vias. 
     It should be appreciated at this point that the exemplary wafers  100 ,  200  that are bonded to form integrated wafer  300  need not be the specific types of wafers presented in the above example. For instance, a processor wafer could also be “flipped” and bonded to a memory wafer. In addition, one memory wafer could be flipped and bonded in the above described manner to another memory wafer. Even more generally, the wafers  100 ,  200  may represent any type of integrated circuit device formed on a substrate where it is desired to integrate the same or other types of integrated circuit devices in a 3D fashion. 
     In any case,  FIG. 5  illustrates the removal of the thick sacrificial base layer used in forming the memory portion of the wafer  300 . The removal may be implemented by any suitable means in the semiconductor arts including, for example, techniques such as grinding, chemical mechanical polishing (CMP), etching, etc., as well as combinations thereof. By doping the sacrificial base layer at such a heavy concentration with respect to the epitaxial layer  104 , a strong etch selectivity between the two layers results. Thus, etching becomes one suitable technique for removal of the sacrificial base layer. Upon removal of the sacrificial base layer, the epitaxial layer  104  of the memory portion of the integrated wafer  300  is now exposed for further processing. 
     Referring now to  FIG. 6 , the processing steps for the “via last” TSV formation are commenced. This may include, for example, forming a passivation layer  304  (e.g., an oxide) on the epitaxial layer  104 . Then, first and second sets of TSVs are defined by via patterning and etching. More specifically, a “shallow” set of TSVs  306  is formed through the passivation layer  304 , the epitaxial layer  104 , and one or more of the wiring layers  106 ,  108  corresponding to the particular locations of the landing pads  110  of the memory portion of the wafer  300 . In addition, a “deep” set of TSVs  308  is formed through the entirety of the memory portion of the wafer  300 , as well as the passivation layers  120  and  220 , down to the landing pads  210  in the processor portion of the wafer  300 . As is the case with the landing pads  110  in the memory portion, the landing pads  210  in the processor portion may also be located in various wiring levels, and thus the deep TSV etch may continue through one or more of the wiring layer  206 ,  208  in the event the landing pads  210  are present at these levels. 
     In an exemplary embodiment, the shallow TSVs  306  may have a diameter of about 2-3 microns (μm), a total depth of about 8-15 μm, and a pitch (spacing) of about 10 μm or more. The deep TSVs  308  may have a diameter of about 5-10 μm, a total depth of about 25-40 μm, and a pitch of about 10 μm or more. After the initial etch of both sets of the TSVs, an oxide liner is formed on sidewalls thereof in order to prevent subsequent diffusion of the conductive via fill materials. The deposition of a thin oxide liner is then followed up by an anisotropic etch to remove the liner from the horizontal surfaces, such as the metallic landing pads  110 ,  210 . Then, standard processing may be carried out to form a via liner layer (e.g., tantalum, tantalum nitride, etc.), metal seed layer, and metal fill  310  (e.g., copper), after which the excess material is planarized such as by CMP, as shown in  FIG. 7 . 
     At this point in the process, any connection straps desired between shallow and deep TSVs (thereby defining electrical communication between the processor and memory portions of the integrated wafer  300 ) may be formed such as shown in  FIG. 8 . As is shown, another passivation (e.g., oxide) layer  312  is formed, followed by patterning etching, liner layer, seed layer, metal plating and CMP to form a connection strap  314  between a shallow/deep TSV pair. Although the embodiment depicted is an example of single damascene processing, it will be understood that dual damascene processing can also be used to simultaneously define and fill the via and strap structures. 
     In the event the resulting 3D integrated structure were substantially complete at this point, final processing steps would then be implemented, such as forming a top passivation layer (not shown), patterning the passivation layer and forming a conductive capture pad (not shown) or other metallurgy for an external connection such as a C 4  solder ball. However, for purposes of illustration, it will be assumed that further 3D wafer integration is desired, such as (for example) the addition of more memory chips. Accordingly, as shown in  FIG. 9 , another layer  320  of passivation is formed on the wafer. Layer  320  may be, for example, an oxide layer such as used for layers  120 ,  220 , or other suitable type of insulator material including any adhesive material in preparation of wafer bonding. 
       FIG. 10  illustrates the alignment of a second memory wafer  400  with the integrated wafer  300 . The second memory wafer  400  is similar in construction with respect to the first memory wafer  100  shown in  FIG. 3 , in that the wafer  400  includes a thick sacrificial base layer  402  (e.g., a heavily doped P+ layer), a lightly doped epitaxial layer  404  formed on the sacrificial base layer  402 , one or more wiring layers  406 ,  408  in the MOL and BEOL regions, and one or more strap/landing pads  410  formed in the wiring layers. As then shown in  FIG. 11 , the wafers  300  and  400  are bonded together to form a single integrated wafer, now depicted generally at  500 . Again, where oxide is used as the passivation material for the individual wafers, the bonding may be, for example, oxide-to-oxide bonding (e.g., by annealing), oxide/adhesive bonding, or any other suitable technique known in the art that results in a strong bond between electrically insulating layers. Thus bonded, integrated wafer  500  has a second bonding interface  502  between layers  320  and  520 , wherein (like the first bonding interface  302 ) the second bonding interface  502  is comprised entirely of insulating materials, and no conducting materials such as vias. 
     The next steps in the processing sequence are similar to that shown in  FIGS. 5-8 . For example, in  FIG. 12 , the thick sacrificial base layer  402  used in forming the second memory wafer  400  is removed by any suitable means in the semiconductor arts such as grinding, CMP, etching, etc. Upon removal of the sacrificial base layer, the epitaxial layer  404  of the second memory portion of the integrated wafer  500  is now exposed for further processing.  FIG. 13  then illustrates the formation of another set of TSVs, including forming a passivation layer  504  (e.g., an oxide) on the epitaxial layer  404 . Once again, a shallow set of TSVs  506  is formed through the passivation layer  504 , the epitaxial layer  404 , and one or more of the wiring layers  406 ,  408  corresponding to the particular locations of the landing pads  410  of the second memory portion of the wafer  500 . In addition, a deep set of TSVs  508  is formed through the entirety of the second memory portion of the wafer  500 , as well as the passivation layers  420  and  320 . In the exemplary embodiment depicted, one of the deep TSVs  508  lands on the strap  314  that connects circuitry between the processor and first memory portions of the wafer  500 . Another of the deep TSVs  508  is shown to connect to an earlier formed TSV, thereby extending the total depth of the TSV  508  from the top of the device down to the landing strap  210  in the processor portion. 
     After the etch of both sets of the TSVs  506 ,  508 , an oxide liner is formed on sidewalls thereof in order to prevent subsequent diffusion of the conductive via fill materials. The deposition of the thin oxide liner is then followed up by an anisotropic etch to remove the liner from the horizontal surfaces. Then, standard processing may be carried out to form a via liner layer (e.g., tantalum, tantalum nitride, etc.), metal seed layer, and metal fill  510  (e.g., copper), after which the excess material is planarized such as by CMP, as shown in  FIG. 14 . In  FIG. 15 , connection straps between shallow and deep TSVs are formed. In particular, another passivation (e.g., oxide) layer  512  is formed, followed by patterning etching, liner layer, seed layer, metal plating and CMP to form connection straps  514  between the shallow/deep TSV pair. Again, although the illustrated embodiment is an example of single damascene processing, it will be understood that dual damascene processing can also be used to simultaneously define and fill the via and strap structures. 
     Finally,  FIG. 16  depicts a capture pad  516  (e.g., C 4 ) formed atop the wafer  500  for providing external electrical contact for the 3D integrated wafer  500 . The capture pad  516  may be formed with or without another passivation layer (not shown). Again, prior to formation of any external capture pads, additional wafer layer may also be stacked and bonded, in the manner described above, prior to a “via last” definition that electrically interconnects the bonded wafers. In so doing, all bonding processes are, in essence, insulator-to-insulator in that no alignment/bonding of conductive structures need take place for the integration. It is noted that although the deep TSVs ultimately pass through a wafer bonding interface, the vias themselves do not comprise a part of that interface since the metal fill takes place post-bonding. 
     It should be understood that the exemplary process flow described herein may have many variations including, but not limited to, the use of straight, “via only” wafers, wafers with only wiring redistribution and wafer connection TSVs, and wafers having specific features such as capacitors, voltage regulator modules (VRMs), etc. In addition, the various wafers (e.g., processors, wafers) may also include buried oxide (BOX) layers therein for SOI applications. 
     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.