Patent Publication Number: US-2022223530-A1

Title: Architecture for Computing System Package

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application claims the benefit of the following provisionally filed U.S. Patent application: Application No. 63/137,375, filed on Jan. 14, 2021, and entitled “SoIC Technology for New Immersion in Memory Compute System Architecture;” which application is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     The packages of integrated circuits are becoming increasing complex, with more device dies integrated in the same package to achieve more functions. For example, system packages have been developed to include a plurality of device dies such as processors and memory cubes in the same package. In the system packages, device dies formed using different technologies and having different functions can be bonded in both 2D side-by-side and 3D stacking manner to form a system, which has high computing efficiency, high bandwidth, high functionality packing density, low communication latency, and low energy consumption per bit data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS. 1A, 1B, 1C, 1D, 1E, 1F, and 1G  illustrate the cross-sectional views, perspective views, and top and bottom views of a computing system package in accordance with some embodiments. 
         FIGS. 2 through 14  illustrate the cross-sectional views of intermediate stages in the formation of computing system packages in accordance with some embodiments. 
         FIGS. 15A, 15B, 15C, 16A, 16B, 16C, 17A, 17B, and 17C  illustrate the cross-sectional views of computing system packages in accordance with some embodiments. 
         FIGS. 18A and 18B  illustrate the top and bottom views of computing system packages in accordance with some embodiments. 
         FIGS. 19A, 19B, 20A, 20B, 21A, 21B, 22A, and 22B  illustrate the cross-sectional views of computing system packages in accordance with some embodiments. 
         FIG. 23  illustrates a perspective view of a die-to-wafer bonding process in accordance with some embodiments. 
         FIG. 24  illustrates a perspective view of a wafer-to-wafer bonding process in accordance with some embodiments. 
         FIG. 25  illustrates a part of an interconnect structure in a bridge die in accordance with some embodiments. 
         FIG. 26  illustrates the capacitors in a bridge die in accordance with some embodiments. 
         FIG. 27  illustrates an amplified view of the encapsulants in a computing system package in accordance with some embodiments. 
         FIG. 28  illustrates a process flow for forming a computing system package in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “underlying,” “below,” “lower,” “overlying,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated  90  degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     A computing system package and the method of forming the same are provided in accordance with various embodiments. The computing system package may include three tiers, with the middle tier including logic dies, and the bottom tier and top tier including memory dies. Accordingly, logic dies have shortest paths to the memory dies they access. Bridge dies are in the bottom tier, and are used to interconnect logic dies. Each of the logic dies thus has maximized access to other logic dies and memory dies, without increasing the complexity of the system. Also, the scalability of the system is improved due to the adoption of arrays of logic dies, memory dies, and bridge dies. With this setting, the computing efficiency may be improved, the bandwidth of the system may be increased, and the latency may be reduced due to the close proximity of the memory dies and logic dies, and the efficient layout. The intermediate stages in the formation of the package are illustrated in accordance with some embodiments. Some variations of some embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. 
       FIGS. 1A, 1B, 1C, 1D, 1E, 1F, and 1G  illustrate the cross-sectional views, perspective views and top and bottom views of a computing system package  100  in accordance with some embodiments. The computing system package  100  includes logic dies, memory dies, and bridge dies distributed in a plurality of tiers including tier-1, tier-2 and tier-3, which are encapsulated in encapsulant  130 ,  132 , and  134 , respectively. Tier-1 may include memory dies MD 1  and bride dies BD. Tier-2 may include logic dies LD. Tier-3 may include memory dies MD 3 . Logic dies LD perform computing functions, and the logic dies LD access memory dies MD 1  and MD 3 . In the drawings of the present disclosure, the reference numbers of device dies may start with symbol “LD,” symbol “MD,” or symbol “BD.” Symbol “LD” is used to represent that the corresponding die is a logic die. Symbol “MDI” is used to represent that the corresponding die is a memory die in tier-1, and symbol “MD 3 ” is used to represent that the corresponding die is a memory die in tier-3. Memory dies MDI and MD 3  are collectively referred to as memory dies MD. Letter “BD” is used to represent that the corresponding die is a bridge die. In each of the tiers tier-1, tier-2 and tier-3, the number of logic dies and the number of memory dies may be more than illustrated. It is appreciated that although three-tier packages are shown as examples, the computing system packages may include more than three tiers such as four tiers, five tiers, or more, and the addition tiers may be under the illustrated tier-1, and/or over the illustrated tier-3. 
     In accordance with some embodiments of the present disclosure, the logic dies LD may be Application Processor (AP) dies, Graphics Processing Unit (GPU) dies, Field Programmable Gate Array (FPGA) dies, Application Specific Integrated Circuit (ASIC) dies, Input-Output (IO) dies, Network Processing Unit (NPU) dies, Tensor Processing Unit (TPU) dies, Artificial Intelligence (AI) engine dies, or the like. 
     In accordance with some embodiments of the present disclosure, the memory dies MD 1  and MD 3  may include Static Random Access Memory (SRAM) dies, Dynamic Random Access Memory (DRAM) dies, wide I/O memory dies, NAND memory dies, Resistive Random Access Memory (RRAM) dies, Magneto-resistive Random Access Memory (MRAM) dies, Phase Change Random Access Memory (PCRAM) dies, or the like, or other types of volatile or non-volatile memory dies. The memory dies may include, or may be free from, controllers therein. In the embodiments in which memory dies do not include controller, the controllers may be built in logic dies. The memory dies may also be in the form of single memory die or pre-stacked memory cubes. 
     In computing system package  100 , and possibly in each of the tiers, different types of memory dies may be mixed. For example, tier-1 may adopt one type of memory dies as aforementioned, and tier-3 may adopt another type of memory dies. All of the memory dies in tier-1 may, however, be of the same type and are identical to each other, and the memory dies in tier-3 may be of the same type and are identical to each other in order to improve the scalability of the system, and to reduce the thickness of the package (since if different types are mixed, the thickness of the corresponding tier is determined by the thickest type). The logic dies LD in tier-2 may include different types of logic dies, which may include the aforementioned. Alternatively, all of the logic dies in tier-2 may be of the same type and identical to each other. 
     Tier-1 may include a plurality of memory dies MD 1  and a plurality of bridge dies BD, and may be, or may not be, free from other types of dies such as logic dies, independent passive device dies, and the like. Tier-2 may include a plurality of logic die LD, and may be, or may not be, free from other types of dies such as memory dies, bridge dies, passive device dies, and the like. Tier-3 may include a plurality of memory dies MD 3 , and may be, or may not be, free from other types of dies such as logic dies, bridge dies, passive device dies, and the like. 
     Each of Logic dies LD, memory dies MD 1 /MD 3 , and bridge dies BD may include semiconductor substrates  20 A,  20 B, or  20 C, which may be silicon substrates. Interconnect structures  22  are formed on the corresponding semiconductor substrates  20 A/ 20 B/ 20 C, and are used to interconnect the devices in the corresponding dies. Through-substrate vias  26 A and  26 B may be formed to penetrate through the corresponding semiconductor substrates  20  of the tier-1 dies and tier-2 dies, and are used for the interconnection of the overlying components to the underlying components. Furthermore, electrical connectors  28  may be formed for bonding to other device dies. Electrical connectors  28  are used for the bonding between the dies in different tiers, and may be metal pads, metal pillars, solder regions, or the like. In accordance with some embodiments, electrical connectors  28  are metal pillars (such as copper pillars), and are in the corresponding surface dielectric layers  30 . In accordance with some embodiments, surface dielectric layers  30  are formed of or comprise silicon oxide. In accordance with other embodiments, surface dielectric layers  30  include a polymer such as polybenzoxazole (PBO), polyimide, benzocyclobutene (BCB), or the like. Throughout the description, the side of semiconductor substrate  20  having interconnect structure  22  (and active device such as transistors, not shown) are referred to as a front side (or “face”), and the opposite side is referred to as a backside (or “back”). Accordingly, depending on which sides of the dies are bonded to each other, the bonding may be face-to-back bonding, face-to-face bonding, back-to-back bonding. For example, in  FIG. 1A , face-to-back bonding is adopted. 
     Further referring again to  FIG. 1A, 1F , or  1 G, neighboring tiers are bonded to each other, either through direct metal-to-metal bonding, solder bonding, or hybrid bonding. Hybrid bonding includes both dielectric-to-dielectric (also known as fusion bonding, in which Si—O—Si bonds may be formed between two bonded dielectric layers). 
     Underlying tier-1 dies may have an interconnect structure (redistribution structure)  50 . Redistribution structure  50  may include dielectric layers  54 ,  108  and  116 , RDLs  106  and  112 , and Under-Bump Metallurgies (UBMs)  114 . Electrical connectors  142 , which may include solder regions, metal pillars, micro-bumps, or the like, are formed at the bottom surface of redistribution structure  50 . 
       FIGS. 1B and 1C  illustrate a top view and a bottom view, respectively, of the computing system package  100  as shown in  FIG. 1A . The cross-sections of the computing system package  100  may be found referring to  FIGS. 1A, 1F, and 1G , with  FIG. 1A  illustrating reference cross-section  1 A- 1 A in  FIGS. 1B and 1C ,  FIG. 1F  illustrating reference cross-section  1 F- 1 F in  FIGS. 1B and 1C , and  FIG. 1G  illustrating reference cross-section  1 G- 1 G in  FIGS. 1B and 1C . As shown in  FIG. 1B , memory dies MD 1  and bridge dies BD are illustrated as being dashed since they are under logic dies DL. In accordance with some embodiments, memory dies MD 3  may be arranged as an array. Logic dies LD may be arranged as an array. Bridge dies BD may be arranged as an array, and memory dies MD 1  may also be arranged as an array. Although 3×3 arrays are illustrated as an example, larger arrays may be formed. Forming device dies as arrays has the advantageous feature of high scalability. This is particularly useful for increasing the computing power through adding more logic dies, memory dies, and bridge dies. As will be discussed in subsequent paragraphs, the co-operation of the logic dies may be achieved through sharing memory dies, and interacting through bridge dies, so that scaling up the system and increasing the computing power may be easily achieved by enlarging the device arrays. 
     In accordance with some embodiments, all of logic dies LD are identical to each other. In accordance with alternative embodiments, some logic dies LD are identical to each other, and are different from other logic dies LD, which are also identical to each other. For example, a first plurality of logic dies LD may be identical to each other, and a second plurality of logic dies LD may be identical to each other, and are different from the first plurality of logic dies LD. The first and the second plurality of logic dies LD may be laid out in an alternating layout, for example, alternating in each of the rows and columns of the array. 
     In accordance with some embodiments, each of memory dies MD 3  is bonded to, and signally accessed by, one of logic dies LD. Each of memory dies MD 1  is bonded to, and signally accessed by, two neighboring logic dies LD, which are also illustrated in  FIG. 1G . With this layout, each logic die may access three memory dies directly without having routing lines in between. This significantly increases the amount of memory accessed by logic dies without increasing the power consumption and latency. 
     Further referring to  FIGS. 1B and 1C , each of bridge dies BD is bonded to, and interconnects, four logic dies LD. Bridge dies BD are used for the intercommunication of the four connecting logic dies with each other. For example, bridge dies BD may include conductive lines therein, which directly interconnect each pair of the four logic dies LD. Bridge dies BD may also include networking circuits (and hence may be a networking-on-chip die), which include switches, router circuits, or the like, for switching signals between each pair of the four logic dies. Accordingly, through the bridge dies BD, all four logic dies LD may function as an integrated system. Furthermore, each of the logic dies LD is connected to four bridge dies, and hence can route signals from any of the four bridge dies to the other. Accordingly, all of the logic dies LD may work with each other (through bridge dies BD) to form an integrated computing system, and to achieve parallel computing. As can be conceived from  FIGS. 1B and 1C , the computing system package  100  can be scaled up to increase computing power by replicating and enlarging the arrays of logic dies LD, memory dies MD 1  and MD 3 , and bridge dies BD. 
       FIGS. 1D and 1E  illustrate the perspective views of computing system package  100  in accordance with some embodiments,  FIG. 1D  illustrates a perspective view from top side, and Figure lE illustrates a perspective view from bottom side. 
       FIG. 1F  illustrates the reference cross-section  1 F- 1 F as shown in  FIGS. 1B and 1C . Memory dies MD 1  are not illustrated in  FIG. 1F  since memory dies MD 1  are not in the illustrated cross-section. The two logic dies LD connected to the same bridge die are illustrated, while the other two logic dies connecting to the same bridge die BD are not illustrated since they are not in the illustrated cross-section. 
       FIG. 25  schematically illustrates a bridge die BD, which includes bridging structure  34  for interconnecting logic dies LD. In accordance with some embodiments, bridging structure  34  is formed in interconnect structure  22  of bridge die BD. For example, interconnect structure  22  may include dielectric layers (sometimes referred to as Inter-Metal Dielectrics (IMDs)), which may include low-k dielectric materials. Bridging structure  34  may include metal lines and vias, which may extend into a plurality of metallization layers in interconnect structure  22 . The metal lines and vias are interconnected to form a plurality of electrical paths  36 , with opposing ends of each of electrical paths  36  connecting to an electrical connector  28 A, which may include a metal pad, a metal pillar, a solder region, or the like. Logic dies LD have electrical connectors  28 B, which are bonded to electrical connectors  28 A. Bridging structure  34  may also include digital switches, routers, or the like, which may include the electrical paths, and switches (which include active device such as transistors and control circuits). 
     Referring back to  FIG. 1F , bridge dies BD may also include passive devices  42 / 43  such as capacitors, resistors, inductors, or the like.  FIG. 26  illustrates an example bridge die BD. In accordance with some embodiments, bridge die BD includes deep trench capacitor  42  and/or Metal-Insulator-Metal (MIM) capacitor  43 . Deep trench capacitor  42  may include capacitor electrodes  42 B and insulator  42 A between capacitor electrodes  42 B, with deep trench capacitor  42  extending into a trench formed in semiconductor substrate  20 , so that the capacitance may be increased. MIM capacitor  43  may include capacitor electrodes  43 B and insulator  43 A between capacitor electrodes  43 B, and may be formed in the interconnect structure  22  in bridge die BD. In accordance with some embodiments, MIM capacitor  43  and the electrical paths  36  ( FIG. 25 ) extend into the same interconnect structure  22  in bridge die BD. 
       FIG. 1G  illustrates the reference cross-section  1 G- 1 G as shown in  FIGS. 1B and 1C . Bridge dies BD are not illustrated in  FIG. 1G  since bridge dies BD are not in the illustrated cross-section. 
     As shown in  FIGS. 1A, 1F, and 1G , through-vias  26 A are formed in memory dies MD 1  and bridge dies BD, and penetrate through the semiconductor substrates  20 A of memory dies MD 1  and bridge dies BD. Through-vias  26 A are used to electrically and signally couple redistribution structure  50  to logic dies LD. As shown in  FIGS. 1F , and  1 G, through-vias  120  (through-molding vias) are formed to penetrate through encapsulant  130 , and are used to electrically and signally couple redistribution structure  50  to logic dies LD. 
       FIGS. 2 through 14  illustrate the cross-sectional views of intermediate stages in the formation of computing system packages  100  as shown in  FIGS. 1A, 1B, 1C, 1D, 1E, 1F, and 1G  in accordance with some embodiments of the present disclosure. In accordance with some embodiments, as shown in  FIGS. 2 through 14 , an RDL-first (with “RDL” representing “redistribution line”) approach is adopted, with redistribution structure  50  ( FIG. 1A ) being formed before placing and bonding dies. The corresponding processes are also reflected schematically in the process flow shown in  FIG. 28 . In accordance with alternative embodiments, an RDL-last approach may be adopted, with the dies placed and bonded first, and then redistribution structure  50  is formed. 
       FIG. 2  illustrates carrier  102  and release film  104  formed on carrier  102 . Carrier  102  may be a glass carrier, a silicon wafer, an organic carrier, or the like. Carrier  102  may have a round top-view shape in accordance with some embodiments. Release film  104  may be formed of a polymer-based material (such as a Light-To-Heat-Conversion (LTHC) material), which is capable of being decomposed under heat-carrying radiation such as a laser beam, so that carrier  102  may be de-bonded from the overlying structures that will be formed in subsequent processes. In accordance with some embodiments of the present disclosure, release film  104  is formed of an epoxy-based thermal-release material, which is coated onto carrier  102 . 
     A plurality of dielectric layers and a plurality of RDLs are formed over the release film  104 , as shown in  FIGS. 2 through 5 . Referring to  FIG. 2 , dielectric layer  54  is formed on release film  104 . In accordance with some embodiments of the present disclosure, dielectric layer  54  is formed of a polymer, which may also be a photo-sensitive material such as polybenzoxazole (PBO), polyimide, benzocyclobutene (BCB), or the like, that may be patterned using a photo lithography process including a light-exposure process and a development process. 
     Redistribution Lines (RDLs)  106  are formed over dielectric layer  54  in accordance with some embodiments. The respective process is illustrated as process  202  in the process flow  200  as shown in  FIG. 28 . The formation of RDLs  106  may include forming a metal seed layer (not shown) over dielectric layer  54 , forming a patterned mask (not shown) such as a photo resist over the metal seed layer, and then performing a metal plating process on the exposed seed layer. The patterned mask and the portions of the seed layer covered by the patterned mask are then removed, leaving RDLs  106  as shown in  FIG. 2 . In accordance with some embodiments of the present disclosure, the seed layer includes a titanium layer and a copper layer over the titanium layer. The seed layer may be formed using, for example, Physical Vapor Deposition (PVD) or a like process. The plating may be performed using, for example, an electrochemical plating process or an electro-less plating process. 
     Referring to  FIG. 3 , dielectric layer  108  is formed on RDLs  106 . The respective process is illustrated as process  204  in the process flow  200  as shown in  FIG. 28 . The bottom surface of dielectric layer  108  is in contact with the top surfaces of RDLs  106  and dielectric layer  54 . In accordance with some embodiments of the present disclosure, dielectric layer  108  is formed of a polymer, which may be a photo-sensitive material such as PBO, polyimide, BCB, or the like. Alternatively, dielectric layer  108  may include a non-organic dielectric material such as silicon oxide, silicon nitride, silicon carbide, silicon oxynitride, or the like. Dielectric layer  108  is then patterned to form openings  110  therein. Some portions of RDLs  106  are exposed through the openings  110  in dielectric layer  108 . 
     Next, referring to  FIG. 4 , RDLs  112  are formed to connect to RDLs  106 . The respective process is illustrated as process  206  in the process flow  200  as shown in  FIG. 28 . RDLs  112  include metal traces (metal lines) over dielectric layer  108 . RDLs  112  also include vias extending into the openings  110  in dielectric layer  108 . RDLs  112  may also be formed through a plating process, wherein each of RDLs  112  includes a seed layer (not shown) and a plated metallic material over the seed layer. In accordance with some embodiments, the formation of RDLs  112  may include depositing a blanket metal seed layer extending into the via openings, and forming and patterning a first plating mask (such as photo resist), with openings formed over and joining the via openings. A plating process is then performed to plate a metallic material, which fully fills the via openings  110  ( FIG. 3 ), and has some portions higher than the top surface of dielectric layer  108 . The first plating mask is then removed. 
     The metal seed layer and the plated material may be formed of the same material or different materials. The metallic material in RDLs  112  may include a metal or a metal alloy including copper, aluminum, tungsten, or alloys thereof. RDLs  112  include RDL lines (also referred to as traces or trace portions)  112 L and via portions (also referred to as vias)  112 V, wherein trace portions  112 L are over dielectric layer  108 , and via portions  112 V are in dielectric layer  108 . Since trace portions  112 L and via portions (also referred to as vias)  112 V are formed in a same plating process, there is no distinguishable interface between vias  112 V and the corresponding overlying trace portions  112 L. Also, each of vias  112 V may have a tapered profile, with the upper portions wider than the corresponding lower portions. 
     Further referring to  FIG. 4 , conductive bumps  114  are formed on RDLs  112 . The respective process is illustrated as process  208  in the process flow  200  as shown in  FIG. 28 . It is appreciated that although one RDL layer  112  is illustrated in the example embodiments as shown, there may be more RDL layers formed over and electrically connecting to RDL layer  112 . The subsequent formation process is related to the bonding scheme for bonding device dies MD 1  ( FIG. 7 ) and BD ( FIG. 1A ), and related to whether device dies are placed facing down or facing up, and whether solder bonding, direct metal-to-metal bonding, or hybrid bonding is used. Accordingly, although one formation process is discussed as an example, other formation processes and structures are in the scope of the present disclosure. 
     In accordance with some embodiments, conductive bumps  114  are plated using a second plating mask, and may be plated using a same metal seed layer as for plating RDLs  112 . Conductive bumps  114  may comprise copper, nickel, gold, or the like. After conductive bumps  114  are plated, the second plating mask is removed, followed by an etching process to remove the exposed portions of the metal seed layer, which was previously covered by the second plating mask and the plated RDLs  112 . The remaining portions of the metal seed layer are also considered as being parts of RDLs  112 . Redistribution structure  50  is thus formed. 
     Next, as shown in  FIG. 5 , dielectric layer  116  is formed. The respective process is illustrated as process  210  in the process flow  200  as shown in  FIG. 28 . A planarization process may be performed to level the top surfaces of conductive bumps  114  and dielectric layer  116 . When hybrid bonding is to be performed, dielectric layer  116  may comprise a silicon-containing dielectric material such as silicon oxide. In accordance with alternative embodiments, conductive bumps  114  may be formed after the formation of dielectric layer  116 , and the formation process may include forming openings in dielectric layer  116  to reveal the underlying RDLs  112 , and then forming conductive bumps  114 . The corresponding dielectric layer  116  may comprise an organic material such as PBO, polyimide, BCB, or the like, or an inorganic dielectric material such as silicon oxide, silicon nitride, silicon oxynitride, or the like. The formation process also include forming a metal seed layer, forming a plating mask, plating a metallic material, removing the plating mask, and then etching undesirable portions of the metal seed layer. 
     Next, as shown in  FIG. 6 , metal posts  120  are formed. The respective process is illustrated as process  212  in the process flow  200  as shown in  FIG. 28 . The formation process may include forming a metal seed layer, forming a plating mask (not shown, may be a photo resist) over the metal seed layer, patterning the plating mask to reveal the underlying metal seed layer, and then plating a metallic material in the openings in the plating mask. Metal posts  120  are alternatively referred to as through-vias or through-molding vias since they will penetrate through the subsequently formed encapsulating material (which may be a molding compound). The plated metallic material may be copper or a copper alloy. Metal posts  120  may have substantially vertical and straight edges. In accordance with alternative embodiments, conductive bumps  114 A are not formed in the preceding processes. Rather, they are formed in the same process for forming metal posts  120 . 
       FIG. 7  illustrates the placement/attachment of tier-1 dies, which includes memory dies MD 1  and bridge dies BD (also refer to  FIG. 1A ). The respective process is illustrated as process  214  in the process flow  200  as shown in  FIG. 28 . Bridge dies BD are in an un-illustrated cross-section, and hence are not shown in  FIG. 7 . In accordance with some embodiments, tier-1 dies MD 1  and BD faces down, and the electrical connectors  122  in the tier-1 dies MDI and BD are bonded to conductive bumps  114 . In accordance with alternative embodiments, for example, as shown in  FIGS. 16A, 16B, and 16C , dies MD 1  and BD may face up, and the electrical connectors on the backside of device dies MD 1  and BD are bonded to conductive bumps  114 . 
     Memory dies MD 1  and bridge dies BD may have electrical connectors  124 A (such as metal pads, metal bumps, or the like) pre-formed as portions of the respective device die. Electrical connectors  124 A are on the backside of the respective dies. Dielectric layer  126 A may also be formed on the back surface of memory dies MD 1  and bridge die BD. In accordance with alternative embodiments, electrical connectors  124 A are not pre-formed in memory dies MD 1  and bridge dies BD. Rather, through-vias  26 A extend to an intermediate level between a top surface and back surface of semiconductor substrate  20 , and the electrical connectors are formed after memory dies MD 1  and bridge die BD are encapsulated, and formed between the process shown in  FIG. 8  and the process shown in  FIG. 9 . 
     Next, tier-1 dies MD 1  and BD and metal posts  120  are encapsulated in encapsulant  130 , as shown in  FIG. 8 . The respective process is illustrated as process  216  in the process flow  200  as shown in  FIG. 28 . Encapsulant  130  fills the gaps between neighboring through-vias  120  and tier-1 dies MD 1  and BD. Encapsulant  130  may include a molding compound, a molding underfill, an epoxy, and/or a resin. When formed of molding compound, encapsulant  130  may include a base material, which may be a polymer, a resin, an epoxy, or the like, and filler particles in the base material. The filler particles may be dielectric particles of SiO 2 , Al 2 O 3 , silica, or the like, and may have spherical shapes. Also, the spherical filler particles may have a plurality of different diameters. 
     A planarization process such as a Chemical Mechanical Polish (CMP) step or a mechanical grinding step is then performed to thin encapsulant  130 , until through-vias  120  and tier-2 dies MD 1  and BD are exposed. Due to the planarization process, the top ends of through-vias  120  are substantially level (coplanar) with the top surfaces of electrical connectors  124 A (if pre-formed), and are substantially coplanar with the top surface of encapsulant  130 . Throughout the description, tier-1 dies and encapsulant  130  are collectively referred to reconstructed wafer  131 . 
     Referring to  FIG. 9 , tier-2 dies LD are bonded to tier-1 dies MD 1  and BD and through-vias  120  (and to reconstructed wafer  131 ). The respective process is illustrated as process  218  in the process flow  200  as shown in  FIG. 28 . In the illustrated example embodiments, tier-2 dies LD are bonded to tier-1 dies MDI and BD and through-vias  120  directly, with no RDLs therebetween. In accordance with alternative embodiments, an additional fan-out redistribution structure (not shown) including dielectric layers and RDLs may be formed on and connecting to tier-1 dies MD 1  and BD and through-vias  120 , and tier-2 dies LD are bonded to the additional fan-out redistribution structure. Similar to tier-1 dies, electrical connectors  124 B and dielectric layers  126 B may be pre-formed in tier-2 dies LD, or may be formed in another additional fan-out redistribution structure over tier-2 dies LD. 
       FIG. 10  illustrates the encapsulation of tier-2 dies LD in encapsulant  132 , which may be similar to or the same as encapsulant  130 . A planarization process is then performed to level the top surfaces of tier-2 dies LD and encapsulant  132 . The respective process is illustrated as process  220  in the process flow  200  as shown in  FIG. 28 . Throughout the description, tier-2 dies LD and encapsulant  132  are collectively referred to reconstructed wafer  133 . 
     In the example embodiments as shown in  FIGS. 9 and 10 , tier-1 dies are first placed and encapsulated to form a reconstructed wafer  131 , and discrete tier-2 dies are placed on the reconstructed wafer  131  through die-to-wafer bonding. A perspective view of the die-to-wafer bonding process is shown in  FIG. 23 , wherein the reconstructed wafer  131  includes tier-1 dies MDI and BD and encapsulant  130 . Tier-2 dies LD are placed onto reconstructed wafer  131 .  FIG. 24  illustrates an alternative embodiment, in which both of reconstructed wafers  131  and  133  are pre-formed, and reconstructed wafer  133  is bonded to the reconstructed wafer  131  through wafer-to-wafer bonding. Similar to what are shown in  FIGS. 23 and 24 , tier-3 dies MD 3  may also be bonded to tier-2 dies through die-to-wafer bonding or wafer-to-wafer bonding. 
     Referring to  FIG. 11 , tier-3 dies MD 3  are bonded to tier-2 dies LD (and to reconstructed wafer  133 ). The respective process is illustrated as process  222  in the process flow  200  as shown in  FIG. 28 . In the illustrated example embodiments, tier-3 dies MD 3  are bonded to tier-2 dies LD directly, with no RDLs in between. In accordance with alternative embodiments, an additional fan-out redistribution structure (not shown) including dielectric layers and RDLs may be formed on and connecting to tier-2 dies LD, and tier-3 dies MD 3  are bonded to the additional fan-out redistribution structure. 
       FIG. 12  illustrates the encapsulation of tier-3 dies MD 3  in encapsulant  134 , which may be similar to or the same as encapsulants  130  and/or  132 . The respective process is illustrated as process  224  in the process flow  200  as shown in  FIG. 28 . A planarization process is then performed to level the top surfaces of tier-3 dies MD 3  and encapsulant  134 . Tier-2 dies MD 3  and encapsulant  134  are collectively referred to reconstructed wafer  135 . Throughout the description, the structure including dielectric layer  54  and the overlying structure is referred to as a reconstructed wafer  100 , which is also referred to as computing system package  100 . Next, reconstructed wafer  100  is de-bonded from carrier  102  ( FIG. 11 ), for example, by projecting a laser beam on release film  104 , so that release film  104  is decomposed. The respective process is illustrated as process  226  in the process flow  200  as shown in  FIG. 28 . 
       FIG. 13  illustrates the formation of electrical connectors  142 , which may include solder regions, metal pads, metal pillars, or combinations thereof. The respective process is illustrated as process  228  in the process flow  200  as shown in  FIG. 28 . The formation process may include forming openings in dielectric layer  54 , and forming electrical connectors  142  extending into the openings to contact RDLs  106 . 
       FIG. 14  illustrates the bonding of reconstructed wafer  100  to package component  144 , which may be or may include a printed circuit board, a package substrate, a silicon interposer, an organic interposer, a power module, a socket, or the like. The respective process is illustrated as process  230  in the process flow  200  as shown in  FIG. 28 . Underfill  146  is dispensed into the gap between reconstructed wafer  100  and package component  144 . Package  148  is thus formed. In accordance with some embodiments, a connector  152 , which may be an adapter, a socket (including pin holes to insert pins), or the like, may be formed in package  148 , for example, attached to package component  144 , so that the circuits in package  148  may be electrically connected to external components. 
     In accordance with some embodiments, the entire reconstructed wafer  100  that is not sawed is bonded to package component  144 , and is included in the resulting package  148 . Accordingly, reconstructed wafer  100  in package  148  may have a round top view, similar to what are shown in  FIGS. 23 and 24 . In accordance with alternative embodiments, reconstructed wafer  100  is trimmed to remove the portions that have no device dies and conductive lines, while the parts including devices and conductive line containing parts are not trimmed. In accordance with yet alternative embodiments, reconstructed wafer  100  is sawed along scribe lines  141  ( FIG. 13 ) into a plurality of identical packages, each including all of the illustrated multiple device dies as shown in  FIG. 13 , and one of the identical package is used to form the package as in  FIG. 14 . 
       FIG. 27  illustrates an amplified view of region  150  in  FIG. 14 . As shown in  FIG. 27 , encapsulant  130  includes base material  130 A and filler particles  130 B in base material  130 A. Encapsulant  132  includes base material  132 A and filler particles  132 B in base material  130 A. Encapsulant  134  includes base material  134 A and filler particles  134 B in base material  134 A. Since no planarization is performed on the bottom surface of encapsulant  130 , the spherical particles  130 B that are in contact with redistribution structure  50  are rounded, with the rounded surfaces in contact with redistribution structure  50 . The portions of encapsulant  130  in contact with encapsulant  132  (or the additional redistribution structure (if any) have been planarized in the step shown in  FIG. 8 . Accordingly. The spherical particles  130 B at the top surface of encapsulant  130  are partially polished during the planarization, and hence will have substantially planar top surfaces. Similar, in each of encapsulant  132  and  134 , the spherical particles  132 B/ 134 B at the bottom surfaces are not polished, and are spherical, while the spherical particles  132 B/ 134 B at the top surfaces are polished, and are partial spherical particles with round bottom surface and planar top surfaces. 
       FIGS. 15A, 15B, 15C, 16A, 16B, 16C, 17A, 17B, 17C, 19A, 19B, 20A, 20B, 21A, 21B, 22A, and 22B  illustrate the cross-sectional views of computing system packages  100  in accordance with alternative embodiments. These embodiments are similar to the embodiments shown in  FIGS. 1A, 1B, 1C, 1D, 1E, 1F, and 1G  (and  FIGS. 2 through 13 ), with some parts modified. Accordingly, the discussion provided in preceding embodiments may also be applied to these embodiments whenever applicable. 
     The embodiments shown in  FIGS. 15A, 15B, 15C, 16A, 16B, 16C, 17A, 17B , and  17 C may also be obtained from the reference cross-sections  1 A- 1 A,  1 F- 1 F, and  1 G- 1 G in  FIGS. 1B and 1C . 
       FIGS. 15A, 15B, and 15C  illustrate computing system package  100  in accordance with some embodiments. These embodiments are similar to the embodiments in  FIG. 1A, 1B, 1C, 1D, 1E, 1F, and 1G  (which is also the structure in  FIG. 13 ), except that in  FIG. 13 , each of the memory dies MD 1  and MD 3  is a single memory die, while in the embodiments in  FIGS. 15A, 15B, and 15C , memory stacks MD 1 ′ and MD 3 ′ may be used, which includes a plurality of stacked memory dies MD 1  and MD 3 , respectively. The plurality of memory dies MD 1  may be interconnected through the through-substrate vias formed therein. Tie-1 dies, tier-2 dies, and tier-3 dies face down in accordance with these embodiments. Tier-3 dies MD 3  may also be parts of die stacks MD 3 ′. 
       FIGS. 16A, 16B, and 16C  illustrate computing system package  100  in accordance with some embodiments. These embodiments are similar to the embodiments in  FIG. 1A, 1B, 1C, 1D, 1E, 1F, and 1G  (which is also the structure in  FIG. 13 ), except that in  FIG. 13 , tier-1 dies MD 1  and BD face down, while in the embodiments in  FIGS. 16A, 16B, and 16C , tier-1 dies MD 1  and BD face up. Tier-2 dies LD and tier-3 dies MD 3  still face down. 
       FIGS. 17A, 17B, and 17C  illustrate the computing system package  100  in accordance with some embodiments. These embodiments are similar to the embodiments in  FIGS. 15A, 15B, and 15C , except that in  FIGS. 15A, 15B, and 15C , the memory dies MD 1  and bridge dies BD face down, while in the embodiments in  FIGS. 17A, 17B, and 17C , the memory dies MD 1  and bridge dies BD face up. Tier-2 dies LD and tier-3 dies MD 3  still face down. 
       FIGS. 18A and 18B  illustrate a top view and a bottom view, respectively, of a computing system package  100  in accordance with some embodiments, these embodiments are similar to the embodiments as shown in  FIGS. 1B and 1C , except that besides bridge dies BD (marked as BD 1 ) that interconnect four logic dies, bridge dies BD (marked as BD 2 ) may be added to interconnect two neighboring logic dies LD. In accordance with alternative embodiments, the bridge dies BD 1  are not formed, while bridge dies BD 2  are formed. In subsequent  FIGS. 19A, 19B, 20A, 20B, 21A, 21B, 22A, and 22B , the figures whose figure numbers including letter “A” are obtained from the reference cross-section A-A in  FIGS. 18A and 18B , and the figures whose figure numbers including letter “B” are obtained from the reference cross-section B-B in  FIGS. 18A and 18B . 
       FIGS. 19A and 19B  illustrate computing system package  100  in accordance with some embodiments. These embodiments are similar to the embodiments in  FIGS. 1A, 1B, 1C, 1D, 1E, 1F, and 1G  (which is also the structure in  FIG. 13 ), except bridge dies BD 2  are added, and each of the illustrated bridge dies BD 2  interconnects two tier-2 dies LD, rather than interconnects four tier-2 dies LD as bridge dies BD 1 . The memory dies in tier-1 and tier-3 are single memory dies. 
       FIGS. 20A and 20B  illustrate computing system package  100  in accordance with some embodiments. These embodiments are similar to the embodiments in  FIGS. 19A and 19B , except in  FIGS. 20A and 20B , memory die stacks MD 1 ′ and MD 3 ′ are used. 
       FIGS. 21A and 21B  illustrate computing system package  100  in accordance with some embodiments. These embodiments are similar to the embodiments in  FIGS. 19A and 19B , except that in  FIGS. 19A and 19B , tier-1 dies MD 1  and BD 1  face down, while in  FIGS. 21A and 21B , tier-1 dies MD 1  and BD 1  face up. 
       FIGS. 22A and 22B  illustrate computing system package  100  in accordance with some embodiments. These embodiments are similar to the embodiments in  FIGS. 20A and 20B , except that in  FIGS. 20A and 20B , bridge dies BD and tier-1 dies MD 1  in memory die stacks MD 1 ′ face down, while in in  FIGS. 22A and 22B , bridge dies BD and tier-1 dies MD 1  in memory die stacks MD 1 ′ face up. 
     The embodiments of the present disclosure have some advantageous features. By forming computing system packages including three tiers, with the middle tier having logic dies, and memory dies in the upper and lower tiers, logic dies have shortest paths to the memory dies. Bridge dies are formed to interconnect neighboring logic dies. The scalability of the system is improved due to the adoption of arrays of logic dies, memory dies, and bridge dies. The computing efficiency may be improved, the bandwidth of the system may be increased, and the latency may be reduced. 
     In accordance with some embodiments of the present disclosure, a method comprises forming a reconstructed wafer, which comprises forming a redistribution structure over a carrier; bonding a first plurality of memory dies over the redistribution structure; bonding a plurality of bridge dies over the redistribution structure; bonding a plurality of logic dies over the first plurality of memory dies and the plurality of bridge dies, wherein each of the plurality of bridge dies interconnects, and is overlapped by corner regions of, four of the plurality of logic dies; and bonding a second plurality of memory dies over the plurality of logic dies, wherein the plurality of logic dies form a first array, and the second plurality of memory dies form a second array. In accordance with an embodiment, the method further comprises bonding a package component to the reconstructed wafer to form an additional package. In accordance with an embodiment, at a time of the package component is bonded to the reconstructed wafer, the reconstructed wafer comprises the first array and the second array. In accordance with an embodiment, the method further comprises attaching a socket to the additional package, wherein at a time after the socket is attached, the reconstructed wafer comprises both of the first array and the second array. In accordance with an embodiment, the method further comprises encapsulating the first plurality of memory dies and the plurality of bridge dies in a first encapsulant; encapsulating the plurality of logic dies in a second encapsulant; and encapsulating the second plurality of memory dies in a third encapsulant. In accordance with an embodiment, the reconstructed wafer is free from logic dies in the first encapsulant and the third encapsulant, and free from memory dies in the second encapsulant. In accordance with an embodiment, the plurality of logic dies have bottom surfaces in physical contact with a top surface of the first encapsulant. In accordance with an embodiment, the second encapsulant is encapsulated over, and is in physical contact with, the first encapsulant. In accordance with an embodiment, the first plurality of memory dies are identical to each other, the plurality of logic dies are identical to each other, and the second plurality of memory dies are identical to each other. In accordance with an embodiment, the plurality of logic dies are bonded to the first plurality of memory dies through hybrid bonding. In accordance with an embodiment, the bonding the plurality of logic dies over the first plurality of memory dies and the plurality of bridge dies is performed through die-to-wafer bonding. In accordance with an embodiment, the bonding the plurality of logic dies over the first plurality of memory dies and the plurality of bridge dies is performed through a wafer-to-wafer bonding process comprising encapsulating the first plurality of memory dies and the plurality of bridge dies to form a first reconstructed wafer; encapsulating the plurality of logic dies to form a second reconstructed wafer; and bonding the second reconstructed wafer to the first reconstructed wafer through wafer-to-wafer bonding. 
     In accordance with some embodiments of the present disclosure, a package comprises a redistribution structure; a first plurality of memory dies over the redistribution structure; a plurality of bridge dies over the redistribution structure; a plurality of logic dies over the first plurality of memory dies and the plurality of bridge dies, wherein each of the plurality of bridge dies interconnects, and is overlapped by corner regions of, at least two of the plurality of logic dies, wherein the plurality of logic dies are identical to each other; and a second plurality of memory dies over and bonded to the plurality of logic dies, wherein the plurality of logic dies form a first array, and the second plurality of memory dies form a second array. In accordance with an embodiment, the first plurality of memory dies are identical to each other, the second plurality of memory dies are identical to each other, the plurality of logic dies are identical to each other, and the plurality of bridge dies are identical to each other. In accordance with an embodiment, the plurality of bridge dies further comprise capacitors therein. In accordance with an embodiment, the package further comprises a first encapsulant encapsulating the first plurality of memory dies therein; a second encapsulant encapsulating the plurality of logic dies therein, wherein the plurality of logic dies have bottom surfaces contacting a top surface of the first encapsulant; and a third encapsulant encapsulating the second plurality of memory dies therein. In accordance with an embodiment, the third encapsulant is in physical contact with the second encapsulant. 
     In accordance with some embodiments of the present disclosure, a package comprises a reconstructed wafer, which comprises a redistribution structure comprising a plurality of redistribution lines; a plurality of bridge dies over and bonded to the redistribution structure; a plurality of logic dies over and bonded to the plurality of bridge dies, wherein at least one of the plurality of bridge dies is bonded to corner regions of four of the plurality of logic dies; and a second plurality of memory dies over and bonded to the plurality of logic dies, wherein the second plurality of memory dies are bonded to the plurality of logic dies. In accordance with an embodiment, the package further comprises a package component bonding to the reconstructed wafer. In accordance with an embodiment, the package further comprises a socket bonding to the package component. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.