Patent Publication Number: US-11037945-B2

Title: Bonded three-dimensional memory devices and methods for forming the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is continuation of International Application No. PCT/CN2019/115767, filed on Nov. 5, 2019, entitled “BONDED THREE-DIMENSIONAL MEMORY DEVICES AND METHODS FOR FORMING THE SAME,” which is hereby incorporated by reference in its entirety. This application is also related to U.S. application Ser. No. 16/739,656, filed on Jan. 10, 2020, entitled “BONDED THREE-DIMENSIONAL MEMORY DEVICES AND METHODS FOR FORMING THE SAME,” and U.S. application Ser. No. 16/739,673, filed on Jan. 10, 2020, entitled “BONDED THREE-DIMENSIONAL MEMORY DEVICES AND METHODS FOR FORMING THE SAME,” all of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     Embodiments of the present disclosure relate to three-dimensional (3D) memory devices and fabrication methods thereof. 
     Planar memory cells are scaled to smaller sizes by improving process technology, circuit design, programming algorithm, and fabrication process. However, as feature sizes of the memory cells approach a lower limit, planar process and fabrication techniques become challenging and costly. As a result, memory density for planar memory cells approaches an upper limit. 
     A 3D memory architecture can address the density limitation in planar memory cells. The 3D memory architecture includes a memory array and peripheral devices for controlling signals to and from the memory array. 
     SUMMARY 
     Embodiments of 3D memory devices and fabrication methods thereof are disclosed herein. 
     In one example, a 3D memory device includes a first semiconductor structure and a second semiconductor structure. The first semiconductor structure includes a first memory stack having a plurality of first conductor layers and a first bonding layer having a plurality of first word line bonding contacts conductively connected to the plurality of first conductor layers, respectively. A second semiconductor structure includes a second memory stack having a plurality of second conductor layers and a second bonding layer having a plurality of second word line bonding contacts conductively connected to the plurality of second conductor layers, respectively. The 3D memory device also includes a bonding interface between the first bonding layer and the second bonding layer, at which the first word line bonding contacts are in contact with the second word line bonding contacts, such that at least one of the first conductor layers is conductively connected to at least one of the second conductor layers, respectively. 
     In another example, a 3D memory device includes a stack structure having a plurality of pairs of bonded semiconductor structures. Each of the pairs includes a first semiconductor structure having a first memory stack having a plurality of first conductor layers, and a first bonding layer having a plurality of first word line bonding contacts conductively connected to the plurality of first conductor layers, respectively. Each of the pair also includes a second semiconductor structure having a second memory stack having a plurality of second conductor layers, and a second bonding layer having a plurality of second word line bonding contacts conductively connected to the plurality of second conductor layers, respectively. Each of the pairs further includes a bonding interface between the first bonding layer and the second bonding layer. The first word line bonding contacts may be in contact with the second word line bonding contacts at the bonding interface such that at least one of the first conductor layers is conductively connected to at least one of the second conductor layers, respectively. The 3D memory device further includes a third semiconductor structure bonded to and conductively connected to the stack structure. The third semiconductor structure may include a peripheral circuit of at least one pair of semiconductor structures. 
     In still another example, a method for forming a 3D memory device includes the following operations. First, on a first substrate, a first memory stack having a plurality of first conductor layers and a first bonding layer having a plurality of first word line bonding contacts are formed conductively connected to the first conductor layers to form a first semiconductor structure. On a second substrate, a second memory stack having a plurality of second conductor layers and a second bonding layer having a plurality of second word line bonding contacts are formed conductively connected to the second conductor layers to form a second semiconductor structure. The first semiconductor structure and the second semiconductor structure are bonded in a face-to-face manner, such that (i) the first semiconductor structure is bonded to the second semiconductor structure, and (ii) the first conductor layers are conductively connected to the second conductor layers through the bonded first and the second word line bonding contacts at a bonding interface. 
     In yet another example, a method for forming a 3D memory device, includes alternatingly bonding a plurality of first semiconductor structures and a plurality of second semiconductor structures to form a stack structure having a plurality of pairs of bonded semiconductor structures, conductor layers of at least one pair of bonded semiconductor structures being conductively connected through bonding. Forming the plurality of bonded semiconductor structures includes bonding a second semiconductor structure with a first semiconductor in a face-to-face manner to form a pair of bonded semiconductor structures, the second semiconductor structure being above the first semiconductor structure, bonding another first semiconductor structure with the pair of bonded semiconductor structures, the another first semiconductor structure facing up, and bonding another second semiconductor structure with the another first semiconductor structure in a face-to-face manner to form another pair of bonded semiconductor structures, the pair and the another pair being bonded in a back-to-back manner. 
     In yet another example, a 3D memory device includes a first semiconductor structure having a first memory stack having a plurality of first conductor layers, and a first bonding layer having a plurality of first word line bonding contacts conductively connected to the plurality of first conductor layers, respectively. The 3D memory device also includes a second semiconductor structure having a second memory stack having a plurality of second conductor layers, and a second bonding layer having a plurality of second word line bonding contacts conductively connected to the plurality of second conductor layers, respectively. The 3D memory device further includes a third semiconductor structure between the first and second bonding layers and having a peripheral circuit of at least one of the first and second memory stacks, at least one of the first conductor layers being conductively connected to at least one of the second conductor layers through and to the third semiconductor structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate embodiments of the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable a person skilled in the pertinent art to make and use the present disclosure. 
         FIG. 1A  illustrates a schematic view of a cross-section of an exemplary bonded 3D memory device having bit lines (BLs) routed and conductively connected through bonding, according to some embodiments. 
         FIG. 1B  illustrates a schematic view of a cross-section of another exemplary bonded 3D memory device having BLs routed and conductively connected through bonding, according to some embodiments. 
         FIG. 2A  illustrates a schematic view of a cross-section of an exemplary bonded 3D memory device having word lines (WLs) routed and conductively connected through bonding, according to some embodiments. 
         FIG. 2B  illustrates a schematic view of a cross-section of another exemplary bonded 3D memory device having WLs routed and conductively connected through bonding, according to some embodiments. 
         FIG. 3A  illustrates a schematic view of a cross-section of an exemplary bonded 3D memory device having BLs and WLs respectively routed and conductively connected through bonding, according to some embodiments. 
         FIG. 3B  illustrates a schematic view of a cross-section of another exemplary bonded 3D memory device having BLs and WLs respectively routed and conductively connected through bonding, according to some embodiments. 
         FIG. 4  illustrates a plan view of an exemplary bonded 3D memory device, according to some embodiments. 
         FIGS. 5A-5F  illustrate an exemplary fabrication process to form a bonded 3D memory device having a pair of bonded semiconductor structures with BLs routed and conductively connected through bonding, according to some embodiments. 
         FIGS. 6A and 6B  illustrate a part of an exemplary fabrication process to form a bonded 3D memory device having a pair of bonded semiconductor structures with WLs routed and conductively connected through bonding, according to some embodiments. 
         FIGS. 7A and 7B  illustrate a part of an exemplary fabrication process to form a bonded 3D memory device having a pair of bonded semiconductor structures with BLs and WLs respectively routed and conductively connected through bonding, according to some embodiments. 
         FIGS. 8A and 8B  illustrate a part of an exemplary fabrication process to form another bonded 3D memory device having a pair of bonded semiconductor structures with BLs and WLs respectively routed and conductively connected through bonding, according to some embodiments. 
         FIGS. 9-11  each illustrates an exemplary bonded 3D memory device having a plurality of pairs of bonded semiconductor structures, according to some embodiments. 
         FIGS. 12A and 12B  each illustrate a schematic view of an exemplary bonded 3D memory device having a plurality of pairs of bonded semiconductor structures, according to some embodiments. 
         FIG. 13  illustrates a flowchart of an exemplary fabrication process to form a bonded 3D memory device having a pair of bonded semiconductor structures, according to some embodiments. 
         FIG. 14  illustrates a flowchart of an exemplary fabrication process to form a bonded 3D memory device having a plurality of pairs of bonded semiconductor structures, according to some embodiments. 
         FIG. 15  illustrates a schematic view of an exemplary pair of bonded semiconductor structures having BLs routed and conductively connected, according to some embodiments. 
     
    
    
     Embodiments of the present disclosure will be described with reference to the accompanying drawings. 
     DETAILED DESCRIPTION 
     Although specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present disclosure. It will be apparent to a person skilled in the pertinent art that the present disclosure can also be employed in a variety of other applications. 
     It is noted that references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” “some embodiments,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiments. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it would be within the knowledge of a person skilled in the pertinent art to effect such feature, structure or characteristic in connection with other embodiments whether or not explicitly described. 
     In general, terminology may be understood at least in part from usage in context. For example, the term “one or more” as used herein, depending at least in part upon context, may be used to describe any feature, structure, or characteristic in a singular sense or may be used to describe combinations of features, structures or characteristics in a plural sense. Similarly, terms, such as “a,” “an,” or “the,” again, may be understood to convey a singular usage or to convey a plural usage, depending at least in part upon context. In addition, the term “based on” may be understood as not necessarily intended to convey an exclusive set of factors and may, instead, allow for existence of additional factors not necessarily expressly described, again, depending at least in part on context. 
     It should be readily understood that the meaning of “on,” “above,” and “over” in the present disclosure should be interpreted in the broadest manner such that “on” not only means “directly on” something but also includes the meaning of “on” something with an intermediate feature or a layer therebetween, and that “above” or “over” not only means the meaning of “above” or “over” something but can also include the meaning it is “above” or “over” something with no intermediate feature or layer therebetween (i.e., directly on something). 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “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. 
     As used herein, the term “substrate” refers to a material onto which subsequent material layers are added. The substrate itself can be patterned. Materials added on top of the substrate can be patterned or can remain unpatterned. Furthermore, the substrate can include a wide array of semiconductor materials, such as silicon, germanium, gallium arsenide, indium phosphide, etc. Alternatively, the substrate can be made from an electrically non-conductive material, such as a glass, a plastic, or a sapphire wafer. 
     As used herein, the term “layer” refers to a material portion including a region with a thickness. A layer can extend over the entirety of an underlying or overlying structure or may have an extent less than the extent of an underlying or overlying structure. Further, a layer can be a region of a homogeneous or inhomogeneous continuous structure that has a thickness less than the thickness of the continuous structure. For example, a layer can be located between any pair of horizontal planes between, or at, a top surface and a bottom surface of the continuous structure. A layer can extend horizontally, vertically, and/or along a tapered surface. A substrate can be a layer, can include one or more layers therein, and/or can have one or more layer thereupon, thereabove, and/or therebelow. A layer can include multiple layers. For example, an interconnect layer can include one or more conductor and contact layers (in which interconnect lines and/or via contacts are formed) and one or more dielectric layers. 
     As used herein, the term “nominal/nominally” refers to a desired, or target, value of a characteristic or parameter for a component or a process operation, set during the design phase of a product or a process, together with a range of values above and/or below the desired value. The range of values can be due to slight variations in manufacturing processes or tolerances. As used herein, the term “about” indicates the value of a given quantity that can vary based on a particular technology node associated with the subject semiconductor device. Based on the particular technology node, the term “about” can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value). 
     As used herein, the term “three-dimensional (3D) NAND memory string” refers to a vertically-oriented string of memory cell transistors connected in series on a laterally-oriented substrate so that the string of memory cell transistors extends in the vertical direction with respect to the substrate. As used herein, the term “vertical/vertically” means nominally perpendicular to the lateral surface of a substrate. 
     As used herein, a “wafer” is a piece of a semiconductor material for semiconductor devices to build in and/or on it and that can undergo various fabrication processes before being separated into dies. 
     As 3D NAND memory devices continue to scale up vertically (e.g., having 96-layers or more), it may not be feasible to enable one-step etching of high aspect ratio structures, such as channel holes and gate line slits (GLSs), due to dry etching challenges. Especially, for small-size patterns like channel holes, critical dimension (CD) control can be challenging. On the other hand, higher voltages are desired to program and erase memory cells due to limited carrier transport rate in semiconductor channels with increases lengths. Currently, reducing the thicknesses of conductor/dielectric pairs and stacking multiple decks in a memory stack have been used to increase the number of WLs (conductor layers). However, reducing the thicknesses of conductor/dielectric pairs can result in undesirable coupling between adjacent memory cells. 
     Stacking multiple decks often includes corresponding parts of the decks to be aligned with one another vertically to form direct bonding at the bonding interface. For example, in related art, each memory string in one deck is aligned and bonded with the corresponding memory string in another deck at the bonding interface so that the memory strings in the bonded decks can be conductively connected. The bit line bonding contacts conductively connected to the BLs (e.g., also memory strings) are vertically aligned with the respective memory strings. In another example, the conductive connection of WLs in the two decks require direct aligning and bonding of WLs in the two decks, and word line bonding contacts conductively connected to and vertically aligned with the WLs can be bonded at the bonding interface. The direct alignment and bonding of the memory strings and WLs require high alignment precision to control the overlay between the memory strings and between the WLs in adjacent decks. The distribution and/or layout of the bonding contacts are limited by the dimensions of the memory strings/WLs and/or spacing between adjacent memory strings/WLs, making it challenging to achieve high yield in the fabrication. The existing alignment and bonding methods for forming multi-deck bonded semiconductor device can impair product yield and thus need to be improved. 
     Various embodiments in accordance with the present disclosure provide bonded 3D memory devices having one or more pairs of bonded semiconductor structures. In each pair, corresponding parts in the two semiconductor structures (e.g., a first semiconductor structure and a second semiconductor structure) are routed to a desired bonding region to be bonded, instead of being directly aligned and bonded at the bonding interface in related art. In each pair, the two semiconductor structures are bonded in a face-to-face manner. The corresponding parts in the two semiconductor structures can be conductively connected to respective conductive routings, which route/extend the parts to the bonding region. Bonding contacts of the two semiconductor structures, conductively connect the respective conductive routings, can be bonded at a bonding interface. Accordingly, the corresponding parts in the two semiconductor structures can be conductively connected through conductive routings and bonding contacts at the bonding region. The conductive connection of the corresponding parts in the two semiconductor structures thus does not require direct aligning and bonding the corresponding parts at the bonding interface. The peripheral circuits and any logic process-compatible devices for controlling the operations of each pair of bonded semiconductor structures, can be integrated into another semiconductor structure, e.g., a third semiconductor structure, that is bonded to the one or more pairs. 
     The layouts of the conductive routings and bonding contacts can be flexibly determined to accommodate the layouts of other structures/devices in the semiconductor structures and facilitate easier bonding at the bonding interface. Compared to existing bonding and stacking processes that require direct aligning and bonding of corresponding parts in the semiconductor structures, in the present disclosure, the bonding of the semiconductor structures only requires the alignment of bonding contacts at the bonding interface. The alignment precision required for bonding can be lower. The location, distribution, and/or dimensions of bonding contacts are less limited by the space and location of the corresponding structures in the semiconductor structures and can be optimized to facilitate easy alignment and bonding. The two semiconductor structures can be formed using any suitable fabrication processes, e.g., existing fabrication processes, to maintain the yield and desirable material/electrical properties. In some embodiments, bonding the semiconductor structures in a face-to-face manner can minimize the number and complexity of conductive routings. The fabrication of the pair of bonded semiconductor structures can be easier while maintaining a desirable yield. 
     In the examples of the present disclosure, the two semiconductor structures that are bonded as a pair each include a memory stack. BLs and/or WLs, in the two semiconductor structures, are respectively routed and bonded at the bonding interface in a desired bonding region. The bonding of BLs can conductively connect memory strings in the two semiconductor structures, and the bonding of WLs can conductively connect the WLs (e.g., conductor layers) in the two semiconductor structures. The memory strings in the two semiconductor structures can be formed using any suitable etching process such as one-step etching to form the channel holes, followed by filling up the channel holes with suitable channel-forming materials. The fabrication of the memory strings in the pair can maintain yield, and carrier transport rate in the semiconductor channels can maintain the desirable transport rate. 
     In some examples, more than one pairs of bonded semiconductor structures are bonded in a back-to-back manner in a bonded 3D memory device. In each pair, BLs and/or WLs of the two semiconductor structures are bonded. The peripheral circuits (or other logic process-compatible devices) of all the semiconductor structures in the bonded 3D memory device can be integrated into one semiconductor structure that is formed on one side of the bonded 3D memory device. The chip size of the bonded 3D memory device can be reduced. 
       FIGS. 1-3  illustrate various ways to bond and conductively connect parts in semiconductor structures.  FIG. 4  illustrates a plan view of the bonded 3D memory devices in  FIGS. 1-3 .  FIGS. 1A and 1B  illustrate bonded 3D memory devices  100  an  101  in which BLs of the bonded semiconductor structures are routed and bonded, according to some embodiments.  FIGS. 2A and 2B  illustrate bonded 3D memory devices  200  and  201  in which WLs of the bonded semiconductor structures are routed and bonded, according to some embodiments.  FIGS. 3A and 3B  illustrate bonded 3D memory devices  300  and  301  in which BLs and WLs of the bonded semiconductor structures are both routed and bonded, according to some embodiments. In bonded 3D memory devices  100 ,  200 , and  300 , the semiconductor structure embedded with the peripheral circuits of the memory arrays is located at one end (e.g., an upper end) of the respective bonded 3D memory device. In bonded 3D memory devices  101 ,  201 , and  301 , the semiconductor structure embedded with the peripheral circuits is located between the pair of semiconductor structures. 
     In the present disclosure, for the simplicity of illustration, similar or same objects are labeled with the same numerals in  FIGS. 1-3  and  FIGS. 5-8 , respectively. It should be noted that objects depicted in the figures of the present disclosure are for illustrative purposes only. The positions, dimensions, shapes, and numbers of any object do not reflect the actual position, dimensions, shape, and number of the object. The illustrated locations of the bonding contacts in the figures do not reflect the actual locations where the bonding contacts are located. It is noted that x-, y-, and z-axes are added in  FIGS. 1-3 and 5-12 , to further illustrate the spatial relationship of the components in bonded 3D memory devices. It is noted that x and y axes are included in these figures to illustrate two orthogonal directions in the plane of a semiconductor structure. The x-direction is the word line direction (e.g., the direction along which the WLs/conductor layers extend), and the y-direction is the bit line direction (e.g., the direction along which the BLs extend). The substrate of a semiconductor structure (e.g., the first/second/third semiconductor structure) or a 3D NAND memory device (e.g., 3D memory devices  100 ,  101 ,  200 ,  201 ,  300 , and  301 ) includes two lateral surfaces extending laterally in the x-y plane: a top surface on the front side of the semiconductor structure/3D NAND memory device, and a bottom surface on the backside opposite to the front side. The z axis is perpendicular to both the x and y axes. As used herein, whether one component (e.g., a layer or a device) is “on,” “above,” or “below” another component (e.g., a layer or a device) of a semiconductor structure/3D NAND memory device is determined relative to the substrate of the semiconductor device in the z-direction (the vertical direction perpendicular to the x-y plane) when the substrate is positioned in the lowest plane of the semiconductor device in the z-direction. The same notion for describing the spatial relationship is applied throughout the present disclosure. 
     In the present disclosure, for ease of description, “conductively-connected BLs” refer to the BLs that are routed and conductively connected through bonding, “conductively-connected WLs” refer to the WLs that are routed and conductively connected through bonding, and “conductively-connected 3D memory strings” refer to the 3D memory strings that are routed and conductively connected through bonding. 
     In  FIGS. 1-3 , bonded 3D memory devices  100 ,  101 ,  200 ,  201 ,  300 , and  301  may include a first (1 ST ) semiconductor structure, a second (2 ND ) semiconductor structure, and a third (3 RD ) semiconductor structure. In  FIGS. 1A, 2A, and 3A , the first and second semiconductor structures may be directly bonded in a face-to-face manner, and the third semiconductor structure is bonded to one of the first and second semiconductor structures at one end of the bonded 3D memory device. For illustrative purposes, the second semiconductor structure is above the first semiconductor structure, and the third semiconductor structure, above the second semiconductor structure, is bonded to the second semiconductor structure in a face-down manner. In  FIGS. 1B, 2B, and 3B , the first and second semiconductor structures may be bonded in a face-to-face manner with the third semiconductor structure in between in a face-up manner. 
     In some embodiments, the first and second semiconductor structures each include a memory stack, facing each other. The third semiconductor structure may include peripheral circuits for the memory stacks in both first and second semiconductor structures. In the present disclosure, semiconductor structures may be bonded by a suitable bonding method, such as hybrid bonding (also known as “metal/dielectric hybrid bonding”), which is a direct bonding technology (e.g., forming bonding between surfaces without using intermediate layers, such as solder or adhesives) and can obtain metal-metal bonding and dielectric-dielectric bonding simultaneously. Unless specified otherwise, the bonding between any two of the first, second, and third semiconductor structures include hybrid bonding. 
     As shown in  FIG. 1A , bonded 3D memory device  100  further includes a bonding interface  126  where the first semiconductor structure and the second semiconductor structure are bonded, and a bonding interface  164  where the second semiconductor structure and the third semiconductor structure are bonded. As described below in detail, the first, second, and third semiconductor structures can be fabricated separately (and in parallel in some embodiments) such that the thermal budget of fabricating one of the first, second, and third semiconductor structures does not limit the processes of fabricating another one of the first, second, and third semiconductor structures. Moreover, a large number of bonding contacts can be formed through bonding interfaces  126  and  164  to make electrical connections between the first semiconductor structure and the second semiconductor structure, and between the second semiconductor structure and the third semiconductor structure, respectively. The bonding contacts, conductively connected to corresponding structures (e.g., BLs and/or WLs) in respective semiconductor structures, can be formed in a desired bonding region away from the memory stacks. Data transfer between the NAND memory (e.g., the memory stacks) in the pair of bonded semiconductor structures and the peripheral circuits (and any other logic process-compatible devices) can be performed through the bonding contacts and conductive routings across bonding interfaces  126  and  164 . By vertically integrating the first, second, and third semiconductor structures, the chip size can be reduced, and the memory cell density can be increased. Furthermore, as a “unified” chip, by integrating multiple discrete chips (e.g., various processors, controllers, and memories) into a single bonded chip (e.g., bonded 3D memory device  100 ), faster system speed and smaller PCB size can be achieved as well. The details of each semiconductor structure in bonded 3D memory device  100  are now described below. 
     The first semiconductor structure, as a part of bonded 3D memory device  100 , can include a substrate  102 , which can include silicon (e.g., single crystalline silicon, c-Si), silicon germanium (SiGe), gallium arsenide (GaAs), germanium (Ge), silicon on insulator (SOI), or any other suitable materials. 
     The first semiconductor structure can include a memory stack  108  above substrate  102 . Substrate  102  may include two lateral surfaces (e.g., a top surface and a bottom surface) extending laterally in the x-axis (the lateral direction or width direction). As used herein, whether one component (e.g., a layer or a device) is “on,” “above,” or “below” another component (e.g., a layer or a device) of a bonded 3D memory device (e.g., bonded 3D memory device  100 ) is determined relative to the substrate of the bonded 3D memory device (e.g., substrate  102 ) in the y-axis (the vertical direction or thickness direction) when the substrate is positioned in the lowest plane of the bonded 3D memory device in the y-axis. The same notion for describing the spatial relationship is applied throughout the present disclosure. 
     In some embodiments, the first semiconductor structure of bonded 3D memory device  100  includes a NAND flash bonded 3D memory device in which memory cells are provided in the form of an array of 3D NAND memory strings  132 . Each 3D NAND memory string  132  extends vertically through a plurality of pairs each including a conductor layer  108 - 1  and a dielectric layer  108 - 2 , according to some embodiments. The stacked and interleaved conductor layers  108 - 1  and dielectric layer  108 - 2  are also referred to herein as a memory stack  108 . Interleaved conductor layers  108 - 1  and dielectric layers  108 - 2  in memory stack  108  alternate in the vertical direction, according to some embodiments. In other words, except for the ones at the top or bottom of memory stack  108 , each conductor layer  108 - 1  can be adjoined by two dielectric layers  108 - 2  on both sides, and each dielectric layer  108 - 2  can be adjoined by two conductor layers  108 - 1  on both sides. Conductor layers  108 - 1  can each have the same thickness or different thicknesses. Similarly, dielectric layers  108 - 2  can each have the same thickness or different thicknesses. Conductor layers  108 - 1  can include conductor materials including, but not limited to, W, Co, Cu, Al, doped silicon, silicides, or any combination thereof. Dielectric layers  108 - 2  can include dielectric materials including, but not limited to, silicon oxide, silicon nitride, silicon oxynitride, or any combination thereof. Conductor layers  108 - 1  may be a part of a WL. Unless specified otherwise, in the present disclosure, “WL” and “conductor layer” are used interchangeably. That is, the bonding of WLs may be equivalent to the bonding of conductor layers in bonded semiconductor structures. In some embodiments, each conductor layer  108 - 1  is in contact with and conductively connected to one or more word line contacts  150 , which is in contact with and conductively connected to an interconnect layer of the first semiconductor structure. 
     In some embodiments, each 3D NAND memory string  132  is a “charge trap” type of NAND memory string including a semiconductor channel and a memory film. In some embodiments, the semiconductor channel includes silicon, such as amorphous silicon, polysilicon, or single crystalline silicon. In some embodiments, the memory film is a composite dielectric layer including a tunneling layer, a storage layer (also known as “charge trap/storage layer”), and a blocking layer. Each 3D NAND memory string  132  can have a cylinder shape (e.g., a pillar shape). The semiconductor channel, the tunneling layer, the storage layer, and the blocking layer of the memory film are arranged along a direction from the center toward the outer surface of the pillar in this order, according to some embodiments. The tunneling layer can include silicon oxide, silicon oxynitride, or any combination thereof. The storage layer can include silicon nitride, silicon oxynitride, silicon, or any combination thereof. The blocking layer can include silicon oxide, silicon oxynitride, high dielectric constant (high-k) dielectrics, or any combination thereof. In one example, the blocking layer can include a composite layer of silicon oxide/silicon oxynitride/silicon oxide (ONO). In another example, the blocking layer can include a high-k dielectric layer, such as aluminum oxide (Al 2 O 3 ), hafnium oxide (HfO 2 ) or tantalum oxide (Ta 2 O 5 ) layer, and so on. 
     In some embodiments, 3D NAND memory strings  132  further include a plurality of control gates (each being part of a WL). Each conductor layer  108 - 1  in memory stack  108  can act as a control gate for each memory cell of 3D NAND memory string  132 . In some embodiments, each 3D NAND memory string  132  includes two plugs at a respective end in the vertical direction. As used herein, the “upper end” of a component (e.g., 3D NAND memory string  132 ) is the end farther away from substrate  102  in the y-axis, and the “lower end” of the component (e.g., 3D NAND memory string  132 ) is the end closer to substrate  102  in the y-axis when substrate  102  is positioned in the lowest plane of bonded 3D memory device  100 . The plug at the lower end of 3D NAND memory string  132  and in contact with the semiconductor channel can include a semiconductor material, such as single-crystal silicon, that is epitaxially grown from substrate  102 . This plug can function as the channel controlled by a source select gate of 3D NAND memory string  132 . The other plug at the upper end and in contact with the semiconductor channel can include semiconductor materials (e.g., polysilicon). By covering the upper end of 3D NAND memory string  132  during the fabrication of the first semiconductor structure, the plug at the other end can function as an etch stop layer to prevent etching of dielectrics filled in 3D NAND memory string  132 , such as silicon oxide and silicon nitride. In some embodiments, the plug at the upper end functions as the drain of 3D NAND memory string  132 . 
     It is understood that 3D NAND memory strings  132  are not limited to the “charge trap” type of 3D NAND memory strings and may be “floating gate” type of 3D NAND memory strings in other embodiments. Substrate  102  may include polysilicon as the source plate of the “floating gate” type of 3D NAND memory strings. In some embodiments, memory stack  108  includes a plurality of 2D NAND memory strings. 
     In some embodiments, the first semiconductor structure of bonded 3D memory device  100  further includes an interconnect layer  110  above memory stack  108  to transfer electrical signals. Interconnect layer  110  can include a plurality of interconnects. As used herein, the term “interconnects” can broadly include any suitable types of interconnects, such as middle-end-of-line (MEOL) interconnects and back-end-of-line (BEOL) interconnects. In some embodiments, the interconnects in interconnect layer  110  also include local interconnects, such as bit line contacts and word line contacts. Interconnect layer  110  can further include one or more interlayer dielectric (ILD) layers in which the interconnects and via contacts can form. The interconnects and via contacts in interconnect layer  110  can include conductive materials including, but not limited to W, Co, Cu, Al, silicides, or any combination thereof. ILD layers in interconnect layer  110  can include dielectric materials including, but not limited to, silicon oxide, silicon nitride, silicon oxynitride, low-k dielectrics, or any combination thereof. 
     The first semiconductor structure may include a plurality of BLs  134  in contact with and conductively connected to the other plugs at the other ends of 3D NAND memory strings  132 . BLs  134  may be a part of interconnect layer  110  and conductively connected to 3D NAND memory strings  132  through the bit line contacts. In some embodiments, BLs  134  are at a lower end of interconnect layer  110  and above 3D NAND memory strings  132 . 
     Interconnect layer  110  may also include a plurality of first conductive routings  140 , disposed at any suitable locations in interconnect layer  110 . For example, first conductive routings  140  may be above and/or around BLs  134 . First conductive routings  140  may be conductively connected to BLs  134  through any suitable electrical connection means such as vias. First conductive routings  140  may be designed to route BLs  134  (e.g., route the electrical signals of BLs) flexibly to desired locations (e.g., bonding regions) at bonding interface  126 . In some embodiments, first conductive routings  140  route BLs  134  to be in contact with and/or conductively connected to a plurality of first bit line bonding contacts in a first bonding layer  112  of the first semiconductor structure at bonding interface  126 . 
     First bonding layer  112 , as a part of the first semiconductor structure, may be located above interconnect layer  110  and at bonding interface  126 . In some embodiments, first bonding layer  112  may be a part of interconnect layer  110 . First bonding layer  112  may include the plurality of first bit line bonding contacts  142  in contact with and/or conductively connected to first conductive routings  140  so that electrical signals from/to BLs  134  may be transmitted to first bit line bonding contacts  142  through first conductive routings  140 . First bonding layer  112  may include a dielectric layer in which first bit line bonding contacts  142  are positioned. In some embodiments, first bit line bonding contacts  142  may be located at a suitable bonding region of the first semiconductor structure to be bonded with a plurality of second bit line bonding contacts  144  of the second semiconductor structure. The locations and distribution of first and second bit line bonding contacts  142  and  144  may be determined based on factors such as the layouts of devices/structures of the chip, the available space for placing first and second bit line bonding contacts  142  and  144 , the spacing between adjacent bit line bonding contacts, and/or the fabrication process to form, align, and bond first and second bit line bonding contacts  142  and  144 . In some embodiments, first bonding layer  112  may include a plurality of bonding contacts which are conductively connected to word line contacts  150  through any suitable interconnects and/or routings (e.g., in interconnect layer  110 ) so that WLs (e.g., conductor layers  108 - 1 ) may be conductively connected to peripheral circuits and/or other logic process-compatible devices in the third semiconductor structure through bonding. 
     First conductive routings  140 , first bit line bonding contacts  142 , word line contacts  150 , word line bonding contacts, and the interconnects in interconnect layer  110  can include conductive materials including, but not limited to W, Co, Cu, Al, silicides, or any combination thereof. Any ILD layers in interconnect layer  110  and dielectric layers in first bonding layer  112  can include dielectric materials including, but not limited to, silicon oxide, silicon nitride, silicon oxynitride, low-k dielectrics, or any combination thereof. Unless specified otherwise, the conductive structures (e.g., interconnects and bonding contacts) in the interconnect layers and bonding layers of the present disclosure may include, but not limited to, W, Co, Cu, Al, silicides, or any combination thereof, and the dielectric structures (e.g., ILD and dielectric layers) in the interconnect layers and bonding layers of the present disclosure may include, but not limited to, silicon oxide, silicon nitride, silicon oxynitride, low-k dielectrics, or any combination thereof. 
     The second semiconductor structure may have a semiconductor layer  104  above and in contact with a memory stack  118 , which includes a plurality of interleaved conductor layers  118 - 1  and dielectric layers  118 - 2 . Conductor layers  118 - 1  may each be in contact with and conductively connected to one or more word line contacts  174 , which may be conductively connected to a plurality of bonding contacts that are conductively connected to peripheral circuits and/or other logic process-compatible devices in the third semiconductor structure through bonding. Semiconductor layer  104  may be formed from a thinned substrate, on which memory stack  118  is formed. Semiconductor layer  104  may include similar or the same materials as substrate  102 . The second semiconductor structure may also include a plurality of 3D NAND memory strings  136  extending vertically through memory stack  118  and in contact with semiconductor layer  104 . The second semiconductor structure may also include a plurality of BLs  138  in contact with and/or conductively connected to 3D NAND memory strings  136  and conductive routings  146  in an interconnect layer  116 . In some embodiments, interconnect layer  116  may be under BLs  138  and above a second bonding layer  114 , which includes a plurality of second bit line bonding contacts  144  that are in contact with and conductively connected to first bit line bonding contacts  142 . In some embodiments, memory stack  118 , 3D NAND memory strings  136 , BLs  138 , word line contacts  174 , interconnect layer  116 , second bonding layer  114  are respectively similar to memory stack  108 , 3D NAND memory strings  132 , BLs  134 , word line contacts  150 , interconnect layer  110 , and first bonding layer  112 , and the detailed descriptions of these components are not repeated. However, the specific number of staircases/memory cells in each memory stack, the patterns and distribution of conductive routings in each interconnect layer may be the same or different, e.g., subjected to the design and fabrication of the respective semiconductor structures, and are thus not limited by the embodiments of the present disclosure. 
     The first semiconductor structure and the second semiconductor structure may be bonded at bonding interface  126 . In some embodiments, each first bit line bonding contact  142  is aligned and bonded with a respective second bit line bonding contact  144  at bonding interface  126 , and dielectric layers of first and second bonding layers  112  and  114  are bonded together. 
     The bonding of the first and second semiconductor structures may include the bonding of first and second bit line bonding contacts  142  and  144 , which conductively connect each 3D NAND memory string  132  with a respective 3D NAND memory string  136 . Memory cells in memory stacks  108  and  188  may be conductively connected through the bonding of BLs  134  and  138 . In some embodiments, the bonding of the first and second semiconductor structures requires only the alignment of first and second bit line bonding contacts  142  and  144  along the vertical direction. In some embodiments, to facilitate easier bonding, first and second bit line bonding contacts  142  and  144  may be formed at a desired bonding region, such that the placement/layout of memory stacks  108  and  118 , as well as other structures in the chip, may be optimized. In some embodiments, the spacing between adjacent first bit line bonding contacts  142  and adjacent second bit line bonding contacts  144 , and the contact area between each first bit line bonding contact  142  and the respective second bit line bonding contact  144  may be sufficiently large to tolerate higher overlay error during alignment. The bonding may then be less limited by the dimensions of and/or the spacing between 3D NAND memory strings  136  and  134 . Compared to conventional direct bonding of first and second semiconductor structures, in which 3D NAND memory strings in different memory stacks are directly aligned and bonded together, the alignment and bonding of bit line bonding contacts, which facilitates the conductive connection of 3D NAND memory strings in different memory stacks, may require lower overlay alignment precision and contribute higher yield. 
     In some embodiments, first and second bit line bonding contacts  142  and  144  may be formed at a BL bonding region. The location of the BL bonding region may be determined based on the factors aforementioned and may or may not be located between BLs  134  and  138 . In some embodiments, the BL bonding region may be away from BLs  134  and  138  in the x-y plane. The bonded first and second bit line bonding contacts  142  and  144  may be conductively connected to the peripheral circuits and any other logic process-compatible devices in the third semiconductor structure through the same interconnect or conductive routing in interconnect layer  110  or  116 . For illustrative purposes, in  FIG. 1A , second conductive routings  146  and a through-silicon via (TSV)  148  may be depicted as transmitting the electrical signals between the conductively-connected BLs  134  and  138  (i.e., conductively-connected 3D NAND memory strings  136  and  132 ) and the third semiconductor structure. In other words, BLs  134  and  138  (i.e., 3D NAND memory strings  136  and  132 ) share the same interconnect for transmission of electrical signals. In some embodiments, as shown in  FIG. 1A , electrical signals between WLs (or conductor layers  108 - 1  and  118 - 1 ) of memory stacks  108  and  118  may be transmitted separately to the peripheral circuits and any other logic process-compatible devices through respective interconnects, e.g., TSVs  154 , and bonding contacts  158 . 
     In some embodiments, the second semiconductor structure includes a bonding layer  180  that has the plurality of bonding contacts  158  distributed at bonding interface  164  and above semiconductor layer  104 , insulated by dielectric materials. Bonding contacts  158  may include various bonding contacts that are respectively and conductively connected to different devices/structures in the first and second semiconductor structures. For example, bonding contacts  158  may include bonding contacts that are respectively and conductively connected to TSVs  148  and  154 , to transmit electrical signals for the conductively-connected BLs (i.e.,  134  and  138 ) and separated WLs (i.e.,  108 - 1  and  118 - 1 ). 
     The third semiconductor structure may include the peripheral circuits for memory stacks  108  and  118  (and any other logic process-compatible devices) and be bonded with the pair of bonded semiconductor structures at bonding interface  164 . The third semiconductor structure may include a semiconductor layer  106  above a device layer  122  and under a pad-out interconnect layer  124 . Device layer  122  may include a plurality of transistors  168  formed “on” semiconductor layer  106 , in which the entirety or part of transistors  168  are formed under semiconductor layer  106  (e.g., above the bottom surface of semiconductor layer  106 ) and/or directly under semiconductor layer  106 . Semiconductor layer  106  may be formed from a thinned substrate, similar to semiconductor layer  104 . Isolation regions (e.g., shallow trench isolations (STIs))  170  and doped regions (e.g., source regions and drain regions of transistors  168 ) can be formed in semiconductor layer  106  as well. Isolation regions  170  may include any suitable dielectric materials such as silicon oxide, silicon nitride, and/or silicon oxynitride. Transistors  168  are high-speed with advanced logic processes (e.g., technology nodes of 90 nm, 65 nm, 45 nm, 32 nm, 28 nm, 20 nm, 16 nm, 14 nm, 10 nm, 7 nm, 5 nm, 3 nm, etc.), according to some embodiments. 
     Transistors  168  may form or be part of various devices in device layer  122 . In some embodiments, device layer  122  includes peripheral circuits formed entirely or partially by transistors  168 . The peripheral circuits may be part or the entirety of the peripheral circuits for controlling and sensing the NAND memory of bonded 3D memory device  100 . In some embodiments, transistors  168  form peripheral circuit, i.e., any suitable digital, analog, and/or mixed-signal control and sensing circuits used for facilitating the operation of the NAND memory including, but not limited to, a page buffer, a decoder (e.g., a row decoder and a column decoder), a sense amplifier, a driver (e.g., a word line driver), a charge pump, a current or voltage reference, or any active or passive components of the circuit (e.g., transistors, diodes, resistors, or capacitors). 
     In some embodiments, the third semiconductor structure further includes an interconnect layer  120  under device layer  122  and above bonding interface  164  to transfer electrical signals from/to the peripheral circuits. Interconnect layer  120  can include a plurality of interconnects  166 , including lateral interconnect lines and vertical interconnect access (via) contacts. Interconnect layer  120  can further include one or more interlayer dielectric ILD layers in which the interconnect lines and via contacts can form. That is, interconnect layer  120  can include interconnects and via contacts in multiple ILD layers. In some embodiments, the devices in device layer  122  are electrically connected to one another through the interconnects in interconnect layer  120 . 
     Pad-out interconnect layer  124  can include interconnects, e.g., contact pads  172 , in one or more ILD layers. Pad-out interconnect layer  124  and interconnect layer  120  can be formed at opposite sides of semiconductor layer  106 . In some embodiments, interconnects  130  in pad-out interconnect layer  124  can extend into the third semiconductor structure and transfer electrical signals between bonded 3D memory device  100  and outside circuits, e.g., for pad-out purposes. Unless specified otherwise, the contact pads and interconnects in the pad-out interconnect layers of the present disclosure may include, but not limited to, W, Co, Cu, Al, silicides, or any combination thereof, and the dielectric structures (e.g., ILD and dielectric layers) in the pad-out interconnect layers of the present disclosure may include, but not limited to, silicon oxide, silicon nitride, silicon oxynitride, low-k dielectrics, or any combination thereof. 
     The third semiconductor structure can further include a bonding layer  162  at bonding interface  164  and under interconnect layer  120 . Bonding layer  162  can include a plurality of bonding contacts  160  and dielectrics electrically isolating bonding contacts  160 . Bonding contacts  160  may respectively be aligned, in contact with, and conductively connected to bonding contacts  158  at bonding interface  164  to transmit electrical signals between peripheral circuits (and other logic process-compatible devices) and devices/structures in the first and second semiconductor structures, e.g., conductively-connected BLs and separate WLs. Bonding contacts  160  and  158  and surrounding dielectrics in bonding layers  162  and  180  can be used for hybrid bonding. 
     It is understood that the relative positions of the first, second, and third semiconductor structures are not limited.  FIG. 1B  illustrates a schematic view of another exemplary bonded 3D memory device  101 , according to some embodiments. Different from bonded 3D memory device  100  in  FIG. 1A  in which the third semiconductor structure is bonded with the first and second semiconductor structures from above, forming bonding with only the second semiconductor structure, in bonded 3D memory device  101 , the third semiconductor structure is between the first and second semiconductor structures, forming bonding with both the first and second semiconductor structures. The pad-out interconnect layer may be formed as a part of the second semiconductor structure. In some embodiments, the third semiconductor structure is bonded to the first and second semiconductor structures in a face-up manner. 
     As shown in  FIG. 1B , the third and first semiconductor structures may be bonded at a bonding interface  127 , and the second and third semiconductor structures may be bonded at a bonding interface  179 . Because the stacking order of the first, second, and third semiconductor structures in bonded 3D memory device  101  is different from that of bonded 3D memory device  100 , in some embodiments, the interconnect layers and bonding layers of the first, second, and third semiconductor structures in bonded 3D memory device  101  may be different from those in bonded 3D memory device  100 . 
     In some embodiments, the first semiconductor structure may include an interconnect layer  111  above memory stack  108  and a first bonding layer  113  above interconnect layer  111 . First bonding layer  113  may be bonded with a bonding layer  181  of the third semiconductor structure at bonding interface  127 . In some embodiments, the third semiconductor structure may include a semiconductor layer  107  above bonding layer  181  and under device layer  122 . The third semiconductor structure may also include an interconnect layer  121  above device layer  122  and a bonding layer  163  above device layer  122 . Bonding layer  163  may be bonded with a second bonding layer  165  of the second semiconductor structure at bonding interface  179 . In some embodiments, the second semiconductor structure may include an interconnect layer  119  above second bonding layer  165  and under memory stack  118 . The second semiconductor structure may also include a semiconductor layer  105  above and in contact with memory stack  118 . The pad-out interconnect layer  124  may be formed above semiconductor layer  105 . Pad-out interconnect layer  124  may include contact pads  172  and interconnect  130  extending into the second semiconductor structure for transmitting electrical signals between bonded 3D memory device  101  and the outside circuits. In some embodiments, semiconductor layers  105  and  107  may be similar to semiconductor layers  106  and  104 , and the detailed descriptions are not repeated herein. 
     First bonding layer  113  may include a plurality of first bit line bonding contacts  143  distributed at bonding interface  127 , aligned and bonded with bonding contacts  145  of the third semiconductor structure. Second bonding layer  165  may include a plurality of second bit line bonding contacts  115  distributed at bonding interface  179 , aligned and bonded with bonding contacts  117  of the third semiconductor structure. First bit line bonding contacts  143  may be conductively connected to BLs  134  through first conductive routings  141 , and second bit line bonding contacts  115  may be conductively connected to BLs  138  through second conductive routings  147 . First and second bit line bonding contacts  143  and  115  may be conductively connected through bonding contacts  117  (in bonding layer  163 ) and  145  (in bonding layer  181 ), and one or more interconnects such as a TSV  149  extending in the third semiconductor structure. BLs  134  and  138 , and 3D NAND memory strings  132  and  136  may thus be conductively connected. Suitable interconnects in interconnect layer  121  of the third semiconductor structure may conductively connect BLs  134  and  138  and conductively connected 3D NAND memory strings  132  and  136  (or TSV  149 ) to the peripheral circuits (or other logic process-compatible devices). 
     The distribution and/or location of first and second bit line bonding contacts  143  and  115  in the respective bonding layer may be the same as or different from the distribution and/or location of first and second bit line bonding contacts  142  and  144 . Depending on the distribution of first and second bit line bonding contacts  143  and  115 , the distribution of first and second conductive routings  141  and  147  may be determined/adjusted accordingly to conductively connect first and second bit line bonding contacts  143  and  115  respectively to BLs  134  and  138 . In some embodiments, conductor layers  108 - 1  and  118 - 1  are conductively connected to the third semiconductor structure through respective word line contacts  150  and  174 , and separate interconnects in interconnect layer  121 . Thus, conductively-connected 3D NAND memory strings  132  and  136  may be conductively connected to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure through the same interconnect, and WLs  108 - 1  and  118 - 2  may be conductively connected to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure through separate interconnects. 
     Although not shown in the figures, in some embodiments, the third semiconductor may be located at the lower end of the respective bonded 3D memory device and conductively connected to the bonded pair of semiconductor structures from below. In this case, the substrate of the third semiconductor structure may function as the substrate of the bonded 3D memory device. If the semiconductor layer of the second semiconductor structure is facing up, a pad-out interconnect layer may be formed above the semiconductor layer. The placement of the first, second, and third semiconductor structures along the vertical direction may be flexibly determined based on the overall layout/placement of devices/structures in the bonded 3D memory device/chip, and should not be limited by the embodiments of the present disclosure. 
       FIG. 2A  illustrates a bonded 3D memory device  200  in which WLs (or conductor layers) of the pair of bonded semiconductor structures are bonded together to conductively connect the memory stacks (and memory cells) in the pair of bonded semiconductor structures, according to some embodiments. In some embodiments, the conductively-connected WLs share the same interconnect that transmits electrical signals from/to the peripheral circuits (or other logic process-compatible devices). In bonded 3D memory device  200 , electrical signals from/to the peripheral circuits (or other logic process-compatible devices) are transmitted separately for the BLs. As shown in  FIG. 2A , the first and second semiconductor structures are bonded at a bonding interface  226 , and the third and second semiconductor structures are bonded at a bonding interface  264 . The first semiconductor structure may include an interconnect layer  210  above memory stack  108  and under bonding interface  226 , and a first bonding layer  212  at bonding interface  226 . The second semiconductor structure may include an interconnect layer  216  under memory stack  118  and above bonding interface  226 , and a second bonding layer  214  at bonding interface  226 . The second semiconductor structure may also include a bonding layer  280  above semiconductor layer  104  and at bonding interface  264 . The third semiconductor structure may include a bonding layer  262  at bonding interface  264  and an interconnect layer  220  between bonding layer  262  and device layer  122 . 
     In some embodiments, first bonding layer  212  includes a plurality of first word line bonding contacts  242  distributed on bonding interface  226 , and second bonding layer  214  includes a plurality of second word line bonding contacts  244  distributed on bonding interface  226 . Each of first word line bonding contacts  242  may be conductively connected to a respective WL  108 - 1  (or conductor layer  108 - 1 ) of the first semiconductor structure, and each of second word line bonding contacts  244  may be conductively connected to a respective WL  118 - 1  (or conductor layer  118 - 1 ) of the second semiconductor structure. In some embodiments, each WL is conductively connected to the respective bonding contact through respective conductive routings and/or interconnects in respective interconnect layer (e.g.,  210  or  216 ). For example, first conductive routings  240  may be conductively connected to WLs  108 - 1  (or conductor layers  108 - 1 ) through word line contacts  150  and other necessary vias/interconnects, and second conductive routings  246  may be conductively connected to WLs  118 - 1  (or conductor layers  118 - 1 ) through word line contacts  174  and other necessary vias/interconnects. Similar to first and second conductive routings  140  and  146 , first and second conductive routings  246  and  240  may be flexibly designed to route the electrical signals from/to the respective WLs to the respective word line bonding contacts, which are positioned at a desired bonding region at interface  126 . First word line bonding contacts  242  and second word line bonding contacts  244  may then be aligned with each other and bonded at interface  126 , and WLs  108 - 1  and  118 - 1  may be conductively connected. 
     Interconnects in interconnect layer  210  or  216  may conductively connect the conductively-connected WLs to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure. For ease of illustration, second conductive routings  246  and a TSV  248  are depicted to conductively connected the conductively-connected WLs to a bonding contact  258  in bonding layer  280 . Bonding contact  258  may be bonded with a bonding contact  260  in bonding layer  262  of the third semiconductor structure at bonding interface  264 . Suitable interconnects in interconnect layer  220  may conductively connect the bonded bonding contacts  258  and  260  to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure. The conductively-connected WLs may then be conductively connected to the peripheral circuits (and/or other logic process-compatible devices). In some embodiments, bonding contacts  258  also includes other bonding contacts to conductively connect other structures/devices to the third semiconductor structure. In some embodiments, BLs  134  and  138  are conductively connected to the third semiconductor structure separately, e.g., through separate interconnects and bonding contacts  258 . 
       FIG. 2B  illustrates another bonded 3D memory device  201  in which WLs (or conductor layers) of the pair of bonded semiconductor structures are bonded together to conductively connect the memory stacks (and memory cells) in the pair of bonded semiconductor structures, according to some embodiments. Different from bonded 3D memory device  200 , the third semiconductor structure, positioned between the first and second semiconductor structures, may be bonded to both the first and second semiconductor structures. As shown in  FIG. 2B , the first and third semiconductor structures may be bonded at a bonding interface  227 , and the third and second semiconductor structures may be bonded at a bonding interface  279 . The pad-out interconnect layer may be formed above the second semiconductor structure. In some embodiments, the third semiconductor structure is bonded to the first and second semiconductor structures in a face-up manner Because the stacking order of the first, second, and third semiconductor structures in bonded 3D memory device  201  is different from that of bonded 3D memory device  200 , in some embodiments, the interconnect layers and bonding layers of the first, second, and third semiconductor structures in bonded 3D memory device  201  may be different from those of bonded 3D memory device  200 . 
     In some embodiments, the first semiconductor structure may include an interconnect layer  211  above memory stack  108  and a first bonding layer  213  above interconnect layer  211 . First bonding layer  213  may be bonded with a bonding layer  281  of the third semiconductor structure at bonding interface  227 . The third semiconductor structure may also include an interconnect layer  221  above device layer  122  and a bonding layer  263  bonded with a second bonding layer  265  of the second semiconductor structure at bonding interface  279 . In some embodiments, the second semiconductor structure may include an interconnect layer  219  above second bonding layer  265  and under memory stack  118 . The pad-out interconnect layer  124  may be formed above semiconductor layer  105 . 
     First bonding layer  213  may include a plurality of first word line bonding contacts  243  distributed at bonding interface  227 , aligned and bonded with bonding contacts  245  of the third semiconductor structure. Second bonding layer  265  may include a plurality of second word line bonding contacts  215  distributed at bonding interface  279 , aligned and bonded with bonding contacts  217  of the third semiconductor structure. First word line bonding contacts  243  may be conductively connected to WLs  108 - 1  (or conductor layers  108 - 1 ) through first conductive routings  241 , and second word line bonding contacts  215  may be conductively connected to WLs  118 - 1  (or conductor layers  118 - 1 ) through second conductive routings  247 . First and second word line bonding contacts  243  and  215  may be conductively connected through bonding contacts  217  (in bonding layer  263 ) and  245  (in bonding layer  281 ), and an interconnect  249 , e.g., a TSV, that extends in the third semiconductor structure. WLs  108 - 1  and  118 - 1  (or conductor layers  108 - 1  and  118 - 1  may thus be conductively connected. Suitable interconnects in interconnect layer  221  of the third semiconductor structure may conductively connect conductively-connected WLs  108 - 1  and  118 - 1  (or interconnect  249 ) to the peripheral circuits (or other logic process-compatible devices). 
     The distribution and/or location of first and second word line bonding contacts  243  and  215  in the respective bonding layer may be the same as or different from the distribution and/or location of first and second word line contacts  242  and  244 . Depending on the distribution of first and second word line bonding contacts  243  and  215 , the distribution of first and second conductive routings  241  and  247  may be determined/adjusted accordingly to conductively connect first and second word line bonding contacts  243  and  215  respectively with WLs  108 - 1  and  118 - 1 . In some embodiments, BLs  134  and  138  are conductively connected to the third semiconductor structure through separate interconnects in interconnect layer  221 . Thus, the conductively-connected WLs  108 - 1  and  118 - 1  may be conductively connected to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure through the same interconnect, and BLs  134  and  138  may be conductively connected to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure through separate interconnects. 
       FIG. 3A  illustrates a bonded 3D memory device  300  in which both BLs and WLs (or conductor layers) of the pair of bonded semiconductor structures are bonded together to conductively connect the memory stacks (and memory cells) in the pair of bonded semiconductor structures, according to some embodiments. In some embodiments, the conductively-connected WLs and BLs respectively share the same interconnect that transmits electrical signals from/to the peripheral circuits (or other logic process-compatible devices). As shown in  FIG. 3A , the first and second semiconductor structures are bonded at a bonding interface  326 , and the third and second semiconductor structures are bonded at a bonding interface  364 . The first semiconductor structure may include an interconnect layer  310  above memory stack  108  and under bonding interface  326 , and a first bonding layer  312  at bonding interface  326 . The second semiconductor structure may include an interconnect layer  316  under memory stack  118  and above bonding interface  326 , and a second bonding layer  314  at bonding interface  326 . The second semiconductor structure may also include a bonding layer  380  above semiconductor layer  104  and at bonding interface  364 . The third semiconductor structure may include a bonding layer  362  at bonding interface  364  and an interconnect layer  320  between bonding layer  362  and device layer  122 . 
     In some embodiments, first bonding layer  312  includes a plurality of first bit line bonding contacts  342   a  and a plurality of first word line bonding contacts  342   b  distributed on bonding interface  326 , and second bonding layer  314  includes a plurality of second bit line bonding contacts  344   a  and a plurality of second word line bonding contacts  344   b  distributed on bonding interface  326 . Each of first bit line bonding contacts  342   a  may be conductively connected to respective BL  134 , and each first word line bonding contacts  342   b  may be conductively connected to a respective WL  108 - 1  (i.e., conductor layer  108 - 1 ), of the first semiconductor structure. Each of second bit line bonding contacts  344   a  may be conductively connected to a respective bit line  138  and each of second word line bonding contacts  344   b  may be conductively connected to a respective WL  118 - 1  (i.e., conductor layer  118 - 1 ), of the second semiconductor structure. In some embodiments, BLs and WLs are each conductively connected to the respective bonding contact through suitable vias and/or interconnects in the respective interconnect layer (e.g.,  310  or  316 ). For example, first conductive routings  340   a  and  340   b  (and other necessary interconnects/vias) may conductively connect BLs  134  and WLs  108 - 1  (or conductor layers  108 - 1 ) to respective first bit line bonding contacts  342   a  and first word line bonding contacts  342   b , and second conductive routings  346   a  and  346   b  may conductively connect BLs  138  and WLs  118 - 1  (or conductor layers  118 - 1 ) to respective second bit line bonding contacts  344   a  and second word line bonding contacts  344   b . First and second conductive routings  340  (i.e.,  340   a  and  340   b ) and  346  (i.e.,  346   a  and  346   b ) may be flexibly designed to route the electrical signals from/to the respective BLs and WLs to the respective bit line bonding contacts and word line bonding contacts, which are positioned at desired locations (e.g., bonding regions) at interface  326 . At interface  126 , first and second bit line bonding contacts  342   a  and  344   a  may then be aligned bonded with each other, and first and second word line bonding contacts  342   b  and  344   b  may be aligned bonded with each other. Accordingly, 3D NAND memory strings  132  and  136 , and WLs  108 - 1  and  118 - 1 , may be conductively connected. 
     Interconnects in interconnect layer  310  or  316  may conductively connect the conductively-connected BLs (i.e.,  134  and  138 ) and conductively-connected WLs (e.g.,  108 - 1  and  118 - 1 ) to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure. For ease of illustration, second conductive routings  346   a  and  346   b , and TSVs  348   a  and  348   b  are depicted to conductively connected the conductively-connected BLs and conductively-connected WLs to respective bonding contacts  358  in bonding layer  380 . Bonding contacts  358  may be bonded with bonding contacts  360  in bonding layer  362  of the third semiconductor structure. Suitable interconnects in interconnect layer  320  may conductively connect the bonded bonding contacts  358  and  360  to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure. Conductively-connected BLs  134  and  138  and conductively-connected WLs  108 - 1  and  118 - 1 , as well as conductively-connected 3D NAND memory strings  132  and  136 , may then be conductively connected to the peripheral circuits (and/or other logic process-compatible devices). 
       FIG. 3B  illustrates another bonded 3D memory device  301  in which BLs and WLs (or conductor layers) of the pair of bonded semiconductor structures are bonded together to conductively connect the memory stacks (and memory cells) in the pair of bonded semiconductor structures, according to some embodiments. Different from bonded 3D memory device  300 , the third semiconductor structure, positioned between the first and second semiconductor structures, may be bonded to both the first and second semiconductor structures. As shown in  FIG. 3B , the first and third semiconductor structures may be bonded at a bonding interface  327 , and the third and second semiconductor structures may be bonded at a bonding interface  379 . The pad-out interconnect layer may be formed above the second semiconductor structure. In some embodiments, the third semiconductor structure is bonded to the first and second semiconductor structures in a face-up manner Because the stacking order of the first, second, and third semiconductor structures in bonded 3D memory device  301  is different from that of bonded 3D memory device  300 , in some embodiments, the interconnect layers and bonding layers of the first, second, and third semiconductor structures in bonded 3D memory device  301  may be different from those of bonded 3D memory device  300 . 
     In some embodiments, the first semiconductor structure may include an interconnect layer  311  above memory stack  108  and a first bonding layer  313  above interconnect layer  311 . First bonding layer  313  may be bonded with a bonding layer  381  of the third semiconductor structure at bonding interface  327 . The third semiconductor structure may also include an interconnect layer  321  above device layer  122  and a bonding layer  363  bonded with a second bonding layer  365  of the second semiconductor structure at bonding interface  379 . Bonding layers  381  and  363  may each include a plurality of bonding contacts, e.g.,  345  and  317 , for conductively connect bit line bonding contacts and word line bonding contacts in the first and second semiconductor structures. For example, bonding contacts  345  may include a plurality of bonding contacts for conductively connect first bit line bonding contacts  343   a  and another plurality of bonding contacts for conductively connect first word line bonding contacts  343   b , and bonding contacts  317  may include a plurality of bonding contacts for conductively connect second bit line bonding contacts  315   a  and another plurality of bonding contacts for conductively connect first word line bonding contacts  315   b . In some embodiments, the second semiconductor structure may include an interconnect layer  319  above second bonding layer  365  and under memory stack  118 . The pad-out interconnect layer  124  may be formed above semiconductor layer  105 . 
     First bonding layer  313  may include a plurality of first bit line bonding contacts  343   a  and a plurality of first word line bonding contacts  343   b  distributed at bonding interface  327  and bonded with bonding contacts  345  of the third semiconductor structure. Second bonding layer  365  may include a plurality of second bit line bonding contacts  315   a  and a plurality of second word line bonding contacts  315   b  distributed at bonding interface  379  and bonded with bonding contacts  317  of the third semiconductor structure. First bit line bonding contacts  343   a  may be conductively connected to BLs  134  through first conductive routings  357 , and second bit line bonding contacts  315   a  may be conductively connected to BLs  138  through second conductive routings  347 . First word line bonding contacts  343   b  may be conductively connected to WLs  108 - 1  (or conductor layers  108 - 1 ) through first conductive routings  357 , and second word line bonding contacts  315   b  may be conductively connected to WLs  118 - 1  (or conductor layers  118 - 1 ) through second conductive routings  347 . First and second bit line bonding contacts  343   a  and  315   a  may be conductively connected through bonding contacts  317  (in bonding layer  363 ) and  345  (in bonding layer  381 ) in the third semiconductor structure, and an interconnect  349   a , e.g., a TSV, that extends in the third semiconductor structure. First and second word line bonding contacts  343   b  and  315   b  may be conductively connected through bonding contacts  317  (in bonding layer  363 ) and  345  (in bonding layer  381 ), and an interconnect  349   b , e.g., a TSV, that extends in the third semiconductor structure. BLs  134  and  138 , WLs  108 - 1  and  118 - 1  (or conductor layers  108 - 1  and  118 - 1 ), and memory stacks  108  and  118  may thus be conductively connected. 
     The distribution and/or location of first and second bit line bonding contacts  343   a  and  315   a  may be the same as or different from the distribution and/or location of first and second bit line bonding contacts  342   a  and  344   a . The distribution and/or location of first and second word line bonding contacts  343   b  and  315   b  may be the same as or different from the distribution and/or location of first and second word line bonding contacts  342   b  and  344   b . Depending on the distribution of these bonding contacts, the distribution of first and second conductive routings  357  and  347  may be determined/adjusted accordingly to conductively connect first and second bit line bonding contacts  343   a  and  315   a  respectively to BLs  134  and  138 , and conductively connect first and second word line bonding contacts  343   b  and  315   b  respectively to WLs  108 - 1  and  118 - 1 . Thus, the conductively-connected WLs  108 - 1  and  118 - 1  may be conductively connected to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure through the same interconnects, and BLs  134  and  138  may be conductively connected to the peripheral circuits (and/or other logic process-compatible devices) in the third semiconductor structure through the same interconnects. 3D NAND memory strings  132  and  136  in the bonded memory stacks  108  and  118  and WLs (or conductor layers  108 - 1  and  118 - 1 ) may then be conductively connected. 
       FIG. 4  illustrates a schematic plan view  400  of a bonded 3D memory device having conductively-connected BLs and/or conductively-connected WLs, according to some embodiments. The plan view may illustrate the layout/location of conductively-connected BLs and/or conductively-connected WLs in bonded 3D memory devices  100 ,  101 ,  200 ,  201 ,  300 , and  301 . As shown in  FIG. 4 , plan view  400  may include a core array region, one or more (e.g., a pair) of array common source (ACS) driver pad-out regions, one or more (e.g., a pair) of BL bonding and pad-out regions, one or more (e.g., a pair) of WL bonding and pad-out regions, and a bonding pad region. The core array region may represent a region that memory arrays are placed. The ACS driver pad-out regions may represent regions where contact pads conductively connecting the ACS of the memory arrays and the peripheral circuits are located. The bonding pad region may represent a region that contact pads for the overall chip (e.g., peripheral circuits) are located. The BL bonding and pad-out regions may represent regions in which the BLs of the bonded memory stacks are conductively connected through bonding and interconnects conductively connected to the conductively-connected BLs are located. The WL bonding and pad-out regions may represent regions in which the WLs of the bonded memory stacks are conductively connected through bonding and interconnects conductively connected to the conductively-connected WLs are located. In some embodiments, the number, locations, and/or area of the BL and WL bonding and pad-out regions are determined (e.g., optimized), e.g., to accommodate the layout and/or arrangement of the overall chip. 
     In various embodiments, each of the memory stacks (e.g.,  108  and  118 ) of the pair of bonded semiconductor structures may be flexibly placed in the core array region at a respective desired location, e.g., at a center of the chip, to optimize the overall arrangement of structures/devices in the chip. The memory stacks (e.g.,  108  and  118 ) may or may not be aligned with each other vertically (e.g., along the z-axis). In some embodiments, the memory stacks may be arranged in a staggered manner vertically. That is, the memory stacks are not aligned with one another along the z-axis, e.g., to optimize the overall arrangement/routing for the chip. The conductive routings (e.g.,  140 ,  146 ,  141 ,  147 ,  240 ,  246 ,  241 ,  247 ,  340   a ,  340   b ,  346   a , and/or  346   b ) in the respective semiconductor structure may be designed to conductively connect and route the respective WLs and/or BLs to the respective bonding and pad-out regions to be conductively connected to respective bonding contacts. That is, one end of each conductive routing may be in contact with and/or conductively connected to the respective BL/WL, and the other end of the conductively routing may be in contact with and/or conductively connected to the respective bonding contact. The arrangement of the conductive routing between the two ends may be subjected to the layout of structures/devices in the respective semiconductor structure. In various embodiments, the arrangement of conductive routings in each semiconductor structure may be the same or different. 
     The bonding contacts, in contact with or conductively connected to the respective conductive routing, may be distributed in the respective bonding layer at the respective bonding and pad-out region, bonded with respective bonding contacts in the other bonding layer. Depending on factors such as the total number of bonding contacts and/or the locations of the memory stacks, in some embodiments, bonding contacts of the pair of bonded semiconductor structures are divided into more than one portions, which are respectively located in the respective bonding and pad-out region. The division of bonding contacts may allow the arrangement of the conductive routings and/or interconnects to be optimized. For example, the lengths and complexity of the conductive routings may be minimized by routing the BLs and/WLs to a respective bonding and pad-out region that is closer to the BLs and/or WLs. In some embodiments, memory stacks  108  and  118  may be placed in the core array region. 
     In an example, for bonded 3D memory devices  100  and  101 , first and second bit line bonding contacts (e.g.,  142  and  144  in bonded 3D memory device  100 , and  143  and  145  in bonded 3D memory device  101 ) may be bonded in the BL bonding and pad-out regions. In some embodiments, depending on the distances from the BLs of each semiconductor structure to each BL bonding and pad-out region, the total bonded first and second bit line bonding contacts are divided into a plurality of portions (e.g., two portions) so each BL bonding and pad-out region includes a portion of the bonded first and second bit line bonding contacts. For example, the bonded first and second bit line bonding contacts may be evenly divided to distribute in the two BL bonding and pad-out regions. Meanwhile, a portion of WLs  108 - 1  (or conductor layers  108 - 1 ) and a portion of WLs  118 - 1  (or conductor layers  118 - 1 ) may be separately routed to one of the WL bonding and pad-out regions by respective word line contacts  150  and  174  and any suitable interconnects and/or bonding contacts. The other portion of WLs  108 - 1  (or conductor layers  108 - 1 ) and the other portion of WLs  118 - 1  (or conductor layers  118 - 1 ) may be separately routed to the other one of the WL bonding and pad-out regions by respective word line contacts  150  and  174  and any suitable interconnects and/or bonding contacts. In some embodiments, the WLs are evenly divided into two portions, each being routed to the closer WL bonding and pad-out region. 
     In this example, in the BL bonding and pad-out regions, the bonded first and second bit line bonding contacts may further be conductively connected to peripheral circuits and/or contact pads (e.g.,  172 ) through suitable interconnects (e.g.,  148  or  149 ) and bonding contacts (e.g.,  158  and  160 ). In the WL bonding and pad-out regions, word line contacts  150  and  174  may, separately, be conductively connected to peripheral circuits and/or contact pads (e.g.,  172 ) through suitable interconnects (e.g.,  154 ) and bonding contacts (e.g.,  158  and  160 ). 
     In another example, for bonded 3D memory devices  200  and  201 , first and second word line bonding contacts (e.g.,  242  and  244  in bonded 3D memory device  200 , and  243  and  245  in bonded 3D memory device  201 ) may be bonded respectively in the WL bonding and pad-out regions. In some embodiments, depending on the distances from the WLs of each semiconductor structure each WL bonding and pad-out region, the bonded first and second word line bonding contacts are divided, such that each WL bonding and pad-out region includes a portion of the total bonded first and second word line bonding contacts. For example, the bonded first and second word line bonding contacts may be evenly divided to distribute in the two WL bonding and pad-out regions. Meanwhile, a portion of BLs  134  and a portion of BLs  138  may be separately routed to one of the BL bonding and pad-out regions by respective interconnects and/or bonding contacts. The other portion of BLs  134  and the other portion of BLs  138  may be separately routed to the other one of the BL bonding and pad-out regions by respective interconnects and/or bonding contacts. In some embodiments, BLs  134  and  138  are evenly divided into two portions, each being routed to the closer BL bonding and pad-out region. 
     In this example, in the WL bonding and pad-out regions, the bonded first and second word line bonding contacts may further be conductively connected to peripheral circuits and/or contact pads (e.g.,  172 ) through suitable interconnects (e.g.,  248  or  249 ) and bonding contacts (e.g.,  258  and  260 ). In the BL bonding and pad-out regions, bonding contacts (e.g.,  258 ) separately and conductively connected to BLs  134  and  138  may be conductively connected to peripheral circuits and/or contact pads (e.g.,  172 ) through suitable interconnects and bonding contacts (e.g.,  258  and  260 ). 
     In a third example, for bonded 3D memory devices  300  and  301 , first and second bit line bonding contacts (e.g.,  342   a  and  344   a  in bonded 3D memory device  300 , and  343   a  and  315   a  in bonded 3D memory device  301 ) may be aligned and bonded respectively in the BL bonding and pad-out regions, and first and second word line bonding contacts (e.g.,  342   b  and  344   b  in bonded 3D memory device  300 , and  343   b  and  315   b  in bonded 3D memory device  301 ) may be aligned and bonded respectively in the WL bonding and pad-out regions. In some embodiments, depending on the distances from the BLs of each semiconductor structure to each BL bonding and pad-out region, and/or from the WLs of each semiconductor structure to each WL bonding and pad-out region, the bonded first and second bit line bonding contacts and the bonded first and second word line bonding contacts are each divided, such that each BL bonding and pad-out region has a portion of the total bonded first and second bit line bonding contacts, and each WL bonding and pad-out region has a portion of the total bonded first and second word line bonding contacts. For example, the bonded first and second word line bonding contacts may be evenly divided to distribute in the two WL bonding and pad-out regions. Meanwhile, the bonded first and second bit line bonding contacts may be evenly divided to distribute in the two BL bonding and pad-out regions. In this example, in the WL bonding and pad-out regions, the bonded first and second word line bonding contacts may further be conductively connected to peripheral circuits and/or contact pads (e.g.,  172 ) through suitable interconnects (e.g.,  348   b  or  349   b ) and bonding contacts (e.g.,  358  and  360  in bonded 3D memory device  300 , and  315   b ,  317 ,  345 , and  343   b  in bonded 3D memory device  301 ). In the BL bonding and pad-out regions, the bonded first and second bit line bonding contacts may further be conductively connected to peripheral circuits and/or contact pads (e.g.,  172 ) through suitable interconnects (e.g.,  348   a  or  349   a ) and bonding contacts (e.g.,  358  and  360  in bonded 3D memory device  300 , and  315   a ,  317 ,  345 , and  343   a  in bonded 3D memory device  301 ). 
       FIGS. 5A-5F  illustrate an exemplary fabrication process to form a bonded 3D memory device having a pair of bonded semiconductor structures with BLs conductively connected, according to some embodiments.  FIGS. 6A and 6B  illustrate a part of an exemplary fabrication process to form a bonded 3D memory device having a pair of bonded semiconductor structures with WLs bonded together, according to some embodiments.  FIGS. 7A and 7B  illustrate a part of an exemplary fabrication process to form a bonded 3D memory device having a pair of bonded semiconductor structures with both BLs and WLs conductively connected, respectively, according to some embodiments.  FIG. 13  is a flowchart of an exemplary method  1300  for forming the bonded 3D memory device, according to some embodiments. It is understood that the operations shown in method  1300  are not exhaustive and that other operations can be performed as well before, after, or between any of the illustrated operations. Further, some of the operations may be performed simultaneously, or in a different order than shown in  FIG. 13 . Method  1300  may be employed to form bonded 3D memory devices shown in  FIGS. 1A, 2A, and 3B . 
     Referring to  FIG. 13 , method  1300  starts at operation  1302 , in which a first semiconductor structure and a second semiconductor structure are formed. The first semiconductor structure includes a plurality of first BLs, a plurality of first conductor layers, and a first bonding layer with a plurality of first bit line bonding contacts and/or a plurality of first word line bonding contacts. The second semiconductor structure includes a plurality of second BLs, a plurality of second conductor layers, and a second bonding layer with a plurality of second bit line bonding contacts and/or a plurality of second word line bonding contacts.  FIGS. 5A, 6A, and 7A  illustrate corresponding structures. 
       FIG. 5A  illustrates the first and second semiconductor structures each having a bonding layer with a plurality of bit line bonding contacts. As shown in  FIG. 5A , the first and second semiconductor structures may be formed. The first semiconductor structure may include a substrate  502 , a memory stack  506  above substrate  502 , a plurality of 3D NAND memory strings  516  extending vertically through memory stack  506 , a plurality of first BLs  522 - 1  conductively connected to 3D NAND memory strings  516 . Memory stack  506  may include a plurality of interleaved conductor layers  506 - 1  and dielectric layers  506 - 2 . Conductor layers  506 - 1  may each be a part of a WL, conductively connected to one or more word line contacts  524 - 1 . The first semiconductor structure may also include an interconnect layer  508  above memory stack  506  and a first bonding layer  538  above or as a part of interconnect layer  508 . First bonding layer  538  may include a plurality of first bit line bonding contacts  518 - 1 , which are conductively connected to first BLs  522 - 1  through a plurality of first conductive routings  520 - 1  in interconnect layer  508 . 
     The second semiconductor structure may be similar to the first semiconductor structure. As shown in  FIG. 5A , the second semiconductor structure may include a substrate  504 , a memory stack  510  above substrate  504 , a plurality of 3D NAND memory strings  514  extending vertically through memory stack  510 , a plurality of second BLs  522 - 2  conductively connected to 3D NAND memory strings  514 . Memory stack  510  may include a plurality of interleaved conductor layers  510 - 1  and dielectric layers  510 - 2 . Conductor layers  510 - 1  may each be a part of a WL, conductively connected to one or more word line contacts  524 - 2 . The first semiconductor structure may also include an interconnect layer  512  above memory stack  510  and a second bonding layer  536  above or as a part of interconnect layer  512 . Second bonding layer  536  may include a plurality of second bit line bonding contacts  518 - 2 , which are conductively connected to second BLs  522 - 2  through a plurality of second conductive routings  520 - 2  in interconnect layer  512 . Memory stacks  506  and  510  may have the same or different levels/number of staircases. In some embodiments, each first bit line bonding contact  518 - 1  corresponds to respective second bit line bonding contact  518 - 2 . 
       FIG. 6A  illustrates the first and semiconductor structures each having a bonding layer with a plurality of word line bonding contacts. Different from the first and second semiconductor structures shown in  FIG. 5A , in  FIG. 6A , first bonding layer  538  of the first semiconductor structure includes a plurality of first word line bonding contacts  620 - 1 , which are conductively connected to WLs  506 - 1  (or conductor layers  506 - 1 ) through a plurality of first conductive routings  622 - 1  that are conductively connected to word line contacts  524 - 1 . Second bonding layer  536  of the second semiconductor structure includes a plurality of second word line bonding contacts  620 - 2 , which are conductively connected to WLs  510 - 1  (or conductor layers  510 - 1 ) through a plurality of second conductive routings  622 - 2  that are conductively connected to word line contacts  524 - 1 . In some embodiments, first and second conductive routings  622 - 1  and  622 - 2  are respectively in interconnect layers  508  and  512 . In some embodiments, each first word line bonding contact  620 - 1  corresponds to a respective second word line bonding contact  620 - 2 . 
       FIG. 7A  illustrates the first and semiconductor structures each having a bonding layer with a plurality of bit line bonding contacts and a plurality of word line bonding contacts. Different from the first and second semiconductor structures shown in  FIGS. 5A and 6A , in  FIG. 7A , first bonding layer  538  of the first semiconductor structure includes a plurality of first bit line bonding contacts  718 - 1  and a plurality of first word line bonding contacts  728 - 1 . First bit line bonding contacts  718 - 1  may be conductively connected to first BLs  522 - 1  through a plurality of first conductive routings  720 - 1 , and first word line bonding contacts  728 - 1  may be conductively connected to WLs  506 - 1  (or conductor layers  506 - 1 ) through a plurality of first conductive routings  730 - 1  that are conductively connected to word line contacts  524 - 1 . Second bonding layer  536  of the second semiconductor structure includes a plurality of second bit line bonding contacts  718 - 2  and a plurality of second word line bonding contacts  728 - 2 . Second bit line bonding contacts  718 - 2  may be conductively connected to second BLs  522 - 2  through a plurality of second conductive routings  720 - 2 , and second word line bonding contacts  728 - 2  may be conductively connected to WLs  510 - 1  (or conductor layers  510 - 1 ) through a plurality of second conductive routings  730 - 2  that are conductively connected to word line contacts  524 - 2 . In some embodiments, each first bit line bonding contact  718 - 1  corresponds to a respective second bit line bonding contact  718 - 2 , and each first word line bonding contact  728 - 1  corresponds to a respective second word line bonding contact  728 - 2 . 
     The first and second semiconductor structures may be formed by similar methods/processes. In some embodiments, a memory stack is formed above a substrate, and an array of 3D NAND memory strings extending vertically through the memory stack are formed. As illustrated in  FIGS. 5A, 6A, and 7A , interleaved sacrificial layers (not shown) and dielectric layers (e.g.,  506 - 2  or  510 - 2 ) are formed above the substrate (e.g.,  502  or  504 ). The interleaved sacrificial layers and dielectric layers can form a dielectric stack (not shown). In some embodiments, each sacrificial layer includes a layer of silicon nitride, and each dielectric layer includes a layer of silicon oxide. The interleaved sacrificial layers and dielectric layers can be formed by one or more thin film deposition processes including, but not limited to, chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layered deposition (ALD), or any combination thereof. In some embodiments, a memory stack (e.g.,  506  or  510 ) can be formed by a gate replacement process, e.g., replacing the sacrificial layers with a plurality of conductor layers (e.g.,  506 - 1  or  510 - 1 ) using wet/dry etch of the sacrificial layers selective to the dielectric layers and filling the resulting recesses with the conductor layers. As a result, the memory stack (e.g.,  506  or  510 ) can include interleaved conductor layers (e.g.,  506 - 1  or  510 - 1 ) and dielectric layers (e.g.,  506 - 2  or  510 - 2 ). In some embodiments, each conductor layer includes a metal layer, such as a layer of tungsten. It is understood that the memory stack may be formed by alternatingly depositing conductor layers (e.g., doped polysilicon layers) and dielectric layers (e.g., silicon oxide layers) without the gate replacement process in other embodiments. In some embodiments, a pad oxide layer including silicon oxide is formed between the memory stack and the substrate. 
     A plurality of 3D NAND memory strings (e.g.,  516  or  514 ) may be formed above the substrate, each extending vertically through interleaved conductor layers and dielectric layers of the memory stack. In some embodiments, fabrication processes to form 3D NAND memory strings include forming a channel hole through the memory stack and into the substrate using dry etching/and or wet etching, such as deep reactive-ion etching (DRIE), followed by epitaxially growing a plug in the lower portion of the channel hole from the substrate. In some embodiments, the fabrication processes to form the 3D NAND memory strings also include subsequently filling the channel holes each with a plurality of layers, such as a memory film (e.g., a tunneling layer, a storage layer, and a blocking layer) and a semiconductor layer, using thin film deposition processes such as ALD, CVD, PVD, or any combination thereof. In some embodiments, the fabrication processes to form the 3D NAND memory strings further include forming another plug in the upper portion of each channel hole by etching a recess at the upper ends of the 3D NAND memory strings, followed by filling the recess with a semiconductor material using thin film deposition processes such as ALD, CVD, PVD, or any combination thereof. 
     An interconnect layer (e.g.,  508  or  512 ) may be formed above the memory stack (e.g.,  506  or  510 ) and the array of 3D NAND memory strings. The interconnect layer can include interconnects of MEOL and/or BEOL in a plurality of ILD layers to make electrical connections with the array of 3D NAND memory strings. In some embodiments, the interconnect layer includes multiple ILD layers and interconnects therein formed in multiple processes. For example, the interconnects in the interconnect layer can include conductive materials deposited by one or more thin film deposition processes including, but not limited to, CVD, PVD, ALD, electroplating, electroless plating, or any combination thereof. Fabrication processes to form the interconnects can also include photolithography, CMP, wet/dry etch, or any other suitable processes. The ILD layers can include dielectric materials deposited by one or more thin film deposition processes including, but not limited to, CVD, PVD, ALD, or any combination thereof. 
     In some embodiments, the formation of the interconnect layer (e.g.,  508  or  512 ) also includes forming a plurality of BLs (e.g.,  522 - 1  or  522 - 2 ) conductively connected to the 3D NAND memory strings (e.g.,  516  or  514 ) and a plurality of conductive routings (e.g.,  520 - 1 ,  520 - 1 ,  720 - 1 , and  720 - 2 ) conductively connected to the BLs. In some embodiments, the formation of the interconnect layer also includes forming a plurality of conductive routings (e.g.,  622 - 1 ,  622 - 2 ,  730 - 1 , and  730 - 2 ) conductively connected to the WLs (e.g.,  506 - 1  and  510 - 1 ). Suitable vias and/or interconnects may be formed between the BLs and the conductive routings to make electrical connections. The formation of the BLs and the conductive routings may be a part of the formation of the interconnects, and the detailed description is thus not repeated. 
     A bonding layer (e.g.,  538  or  536 ) may be formed above the interconnect layer. The bonding layer can include a plurality of bit line bonding contacts (e.g.,  518 - 1 ,  518 - 2 ,  718 - 1 , and  718 - 2 ) and/or a plurality of word line bonding contacts (e.g.,  620 - 1 ,  620 - 2 ,  728 - 1 , and  728 - 2 ), surrounded by dielectrics. In some embodiments, a dielectric layer is deposited on the top surface of the interconnect layer (e.g.,  508  or  512 ) by one or more thin film deposition processes including, but not limited to, CVD, PVD, ALD, or any combination thereof. The bonding contacts then can be formed through the dielectric layer and in contact with the interconnects in the interconnect layer by first patterning contact holes through the dielectric layer using a patterning process (e.g., photolithography and dry/wet etch of dielectric materials in the dielectric layer). The contact holes can be filled with a conductor (e.g., copper). In some embodiments, filling the contact holes includes depositing an adhesion (glue) layer, a barrier layer, and/or a seed layer before depositing the conductor. In some embodiments, the bit line bonding contacts (e.g.,  518 - 1 ,  518 - 2 ,  718 - 1 , and  718 - 2 ) are conductively connected to and/or in contact with respective conductive routings (e.g.,  520 - 1 ,  520 - 2 ,  720 - 1 , and  720 - 2 ) to be conductively connected to the BLs (e.g.,  522 - 1  and  522 - 2 ). In some embodiments, the word line bonding contacts (e.g.,  620 - 1 ,  620 - 2 ,  728 - 1 , and  728 - 2 ) are conductively connected to and/or in contact with respective conductive routings (e.g.,  622 - 1 ,  622 - 2 ,  730 - 1 , and  730 - 2 ). 
     Referring back to  FIG. 13 , after the formation of the first and second semiconductor structures, method  1300  proceeds to operation  1304 , in which the first and second semiconductor structures are bonded in a face-to-face manner to conductively connect the first bit line bonding contacts with the second bit line bonding contacts and/or the first word line bonding contacts with the second word line bonding contacts.  FIGS. 5B, 6B, and 7B  illustrate corresponding structures. 
     As shown in  FIGS. 5B, 6B, and 7B , the first and second semiconductor structures may be bonded in a face-to-face manner, thereby forming a bonding interface. In some embodiments, the second semiconductor structure is flipped to be up-side-down and the first semiconductor structure is facing up. The second semiconductor structure may then be above the first semiconductor structure with substrate  504  facing up. In some embodiments, bonding interfaces  570 ,  670 , and  770  are respectively formed between the first and second semiconductor structures in  FIGS. 5B, 6B, and 7B . In some embodiments, a treatment process, e.g., a plasma treatment, a wet treatment, and/or a thermal treatment, is applied to the bonding surfaces prior to the bonding. The bonding can include hybrid bonding. In some embodiments, each first bit line bonding contact (e.g.,  518 - 1  or  718 - 1 ) is aligned with and bonded to a respective second bit line bonding contact (e.g.,  518 - 2  or  718 - 2 ), as shown in  FIGS. 5B and 7B . In some embodiments, each first word line bonding contact (e.g.,  620 - 1  or  728 - 1 ) is aligned with and bonded to a respective second word line bonding contact (e.g.,  620 - 2  or  728 - 2 ), as shown in  FIGS. 6B and 7B . The bonding may include hybrid bonding. 
     A schematic view of a pair of bonded semiconductor structures is shown in  FIG. 15 , according to some embodiments.  FIG. 15  illustrates the spatial relationship of different components, such as BLs, WLs, bit line bonding contacts, conductive routings, and memory strings, in a pair of bonded semiconductor structures. 
     As shown in  FIG. 15 , a second semiconductor structure is bonded with a first semiconductor structure at a bonding interface. The first semiconductor structure may include a plurality of first memory strings (e.g.,  516 ), a plurality of first BLs BLa- 1 , BLa- 1 , . . . , BLa-n (e.g.,  522 - 1 ) conductively connected to the first memory strings, a plurality of first bit line bonding contacts distributed on the bonding interface, and a plurality of first conductive routings (e.g.,  520 - 1 ) conductively connecting the bit line bonding contacts and the BLs. Similarly, the second semiconductor structure may include a plurality of second memory strings (e.g.,  514 ), a plurality of second BLs BLb- 1 , BLb- 2 , . . . , BLb-n (e.g.,  522 - 2 ) conductively connected to the second memory strings, a plurality of second bit line bonding contacts distributed on the bonding interface, and a plurality of second conductive routings (e.g.,  520 - 2 ) conductively connecting the second bit line bonding contacts and the second BLs. 
     In some embodiments, each BL may be conductively connected to the respective conductive routings through suitable vias, which can be a part of the respective interconnect layer (e.g.,  508  or  512 ) of the respective semiconductor structure. As shown in  FIG. 15 , the first BLs may be conductively connected to the first conductive routings through first vias, and the second BLs may be conductively connected to the second conductive routings through second vias. In some embodiments, one end of a first/second conductive routing is in contact with a respective via, and the other end of the first/second conductive routings is in contact with the respective bonding layer (e.g., the respective bit line bonding contacts in the bonding layer that is laterally away from the respective BLs, not shown). The first and second vias may extend vertically in the respective interconnect layer. The other end of the first and second bit line conductive routings may extend to a desired bonding location (e.g., bit line bonding regions) in the respective bonding layer to route/extend the electrical signals of the first and second BLs to the bonding location. The first and second bit line bonding contacts may be distributed on the bonding layers and bonded together at the bonding interface. Thus, the first and second BLs can be routed and conductively connected at the desired bonding location on the bonding interface. 
     As shown in  FIG. 15 , the first and second semiconductor structures may also include first and second conductor layers (e.g.,  506 - 1  and  510 - 1 ), functioning as the first and second WLs of the first and second semiconductor structures. The first and second conductor layers may each be a part of a respective memory stack (e.g.,  506  and  510 ). The first and second 3D NAND memory strings may extend vertically through the respective memory stack. Although not shown in the figures, the first and second WLs may also be routed to a desired bonding location (e.g., word line bonding regions) in the respective bonding layer, by suitable interconnects such as word line contacts (e.g.,  524 - 1  and  524 - 1 , and the respective conductive routings). The first and second word line bonding contacts may be distributed on the bonding layers and bonded together at the bonding interface. Thus, the first and second WLs can be routed and conductively connected at the desired bonding location on the bonding interface. In some embodiments, the first and second BLs and the first and second WLs are both routed and conductively connected at respective bonding locations on the bonding interface. 
     Referring back to  FIG. 13 , after the bonding of the first and second semiconductor structures, method  1300  proceeds to operation  1306 , in which a substrate of one of the first and second semiconductor structures is thinned to form a semiconductor layer.  FIGS. 5C, 6B, and 7B  illustrate corresponding structures. 
     As shown in  FIGS. 5C, 6B, and 7B , substrate  504  at the top of the pair of bonded semiconductor structures is thinned, so that thinned top substrate  504  can serve as a semiconductor layer  540 , for example, a single-crystal silicon layer or a polysilicon layer. The thickness of semiconductor layer  540  can be between about 200 nm and about 5 μm, such as between 200 nm and 5 μm, or between about 150 nm and about 50 μm, such as between 150 nm and 50 μm. Substrate  504  can be thinned by processes including, but not limited to, wafer grinding, dry etch, wet etch, CMP, any other suitable processes, or any combination thereof. 
     In some embodiments, after semiconductor layer  540  is formed, a bonding layer  546  is formed above semiconductor layer  540 . Bonding layer  546  may include a plurality of bonding contacts  544  surrounded by dielectrics. Bonding contacts  544  may be conductively connected to any interconnects in the second semiconductor structure that are to be conductively connected to peripheral circuits (and other logic process-compatible devices). For example, bonding contacts  544  may be conductively connected to interconnects in interconnect layer  512  to be further conductively connected to second conductive routings (e.g.,  520 - 2 ,  620 - 2 ,  720 - 2 , and  730 - 2 ). Accordingly, bonding contacts  544  may be conductively connected to any conductively-connected BLs (e.g.,  522 - 1  and  522 - 2 ) and/or any conductively-connected WLs (e.g.,  506 - 1  and  510 - 1 ). 
     Bonding layer  546  may be formed by depositing a dielectric layer on semiconductor layer  540 .  FIG. 5C  illustrates a corresponding structure as an example. The dielectric layer can be formed by one or more thin film deposition processes including, but not limited to, CVD, PVD, ALD, or any combination thereof. Bonding contacts  544  can be formed through the dielectric layer and in contact with the interconnects and/or conductive routings in interconnect layer  512  by first patterning contact holes through the dielectric layer using patterning process (e.g., photolithography and dry/wet etch of dielectric materials in the dielectric layer). The contact holes can be filled with a conductor (e.g., copper). In some embodiments, filling the contact holes includes depositing an adhesion (glue) layer, a barrier layer, and/or a seed layer before depositing the conductor. Interconnects, such as TSVs  542 , can be formed in the second semiconductor structure and/or first semiconductor structure to conductively connect BLs  522 - 1  and  522 - 2  to respective bonding contacts  544 . Although not shown in the figures, bonding layer same as or similar to bonding layer  546  may also be formed above semiconductor layer  540  in  FIGS. 6C and 7C . 
     Referring back to  FIG. 13 , after the substrate is thinned and the bonding layer above the substrate is formed, method  1300  proceeds to operation  1308 , in which a third semiconductor structure is bonded to the semiconductor layer.  FIGS. 5D and 5E  illustrate corresponding structures. 
     Before bonding the third semiconductor structure, as shown in  FIG. 5D , the third semiconductor structure can be formed in a separate fabrication process. The third semiconductor structure may include a substrate  526 , a device layer  548  above substrate  526 , an interconnect layer  550  above device layer  548 , and a bonding layer  552  above or as a part of interconnect layer  550 . Substrate  526  may be the same as or similar to substrates  502  and  504 . Device layer  548  may include peripheral circuits for memory stacks  506  and  510 , and any other logic process-compatible devices. A plurality of transistors may form the entire or a part of the peripheral circuits and any other logic process-compatible devices. Interconnect layer  550  may include any suitable conductive interconnects to conductively connect the peripheral circuits and any other logic process-compatible devices with structures in the first and second semiconductor structures. Bonding layer  552  may include a plurality of bonding contacts  554  for bonding with bonding contacts  544  in bonding layer  546 . Bonding contacts  554  may be conductively connected to the interconnects in interconnect layer  550 . 
     In some embodiments, to form the third semiconductor structure, device layer  548  having the peripheral circuits (and any other logic process-compatible devices) is formed on substrate  526 , interconnect layer  550  is formed above device layer  548 , and bonding layer  552  is formed above or as a part of interconnect layer  550 . In some embodiments, to form the peripheral circuits (and any other logic process-compatible devices), a plurality of transistors are formed on substrate  526 . 
     In some embodiments, a plurality of transistors are formed on substrate  526 . The transistors can be formed by a plurality of processes including, but not limited to, photolithography, dry/wet etch, thin film deposition, thermal growth, implantation, CMP, and any other suitable processes. In some embodiments, doped regions are formed in substrate  526  by ion implantation and/or thermal diffusion, which function, for example, as source regions and/or drain regions of the transistors. In some embodiments, isolation regions (e.g., STIs) are also formed in substrate  526  by wet/dry etch and thin film deposition. The transistors can form device layer  548  on substrate  526 . In some embodiments, other logic process-compatible devices in device layer  548  are formed by similar fabrication processes. 
     Further, interconnect layer  550  can be formed above device layer  548 . Interconnect layer  550  can include interconnects of MEOL and/or BEOL in a plurality of the formation of interconnect layer  550  may be the same as or similar to the formation of interconnect layers  508  and  512 , and the detailed description is not repeated herein. The ILD layers and interconnects can be collectively referred to as interconnect layer  550 . Bonding layer  552  may be formed above interconnect layer  550 . Bonding layer  552  can include a plurality of bonding contacts  554  surrounded by dielectrics. The formation of bonding layer  552  may be the same as or similar to the formation of bonding layers  546 ,  538 , and  536 , and the detailed description is not repeated herein. 
     As shown in  FIG. 5E , the third semiconductor layer is flipped upside down and bonded to bonding layer  546 . A bonding interface  580  may be formed between the second and third semiconductor structures, e.g., between bonding layers  546  and  550 . The third semiconductor layer may be bonded to the first and second semiconductor structures in a face-down manner. In some embodiments, the bonding includes hybrid bonding, similar to the bonding of the first and second semiconductor structures. At bonding interface  580 , bonding contacts  554  may be bonded with bonding contacts  554 , and the peripheral circuits (and any other logic process-compatible devices) can be conductively connected to corresponding structures (e.g., conductively-connected BLs  522 - 1  and  522 - 2  and/or conductively-connected WLs  506 - 1  and  510 - 1 ) in the bonded first and second semiconductor structures. 
     Referring back to  FIG. 13 , after the bonding of the third semiconductor structure, method  1300  proceeds to operation  1310 , in which a pad-out interconnect layer is formed.  FIG. 5F  illustrates a corresponding structure. 
     As shown in  FIG. 5F , a pad-out interconnect layer  576 , including a contact pad  572  and a plurality of interconnects  574  conductively connected to contact pad  572 , may be formed. In some embodiments, before the formation of pad-out interconnect layer  576 , substrate  526  is thinned to form a semiconductor layer  560 . The formation of semiconductor layer  560  can be similar to the formation of semiconductor layer  540 , and the detailed description is not repeated herein. 
     Pad-out interconnect layer  576  can include interconnects, such as contact pads  572 , formed in one or more ILD layers. Interconnects  574  can be in contact with the interconnects in interconnect layer  550  and contact pads  572 . Contact pads  572  can be formed by depositing a conductive material such as W over semiconductor layer  560  to conductively connect interconnects  574 . 
     Operations  1306 - 1310  illustrated in  FIGS. 5C-5F  may also be performed on the pair of bonded semiconductor structures in  FIGS. 6B and 7B  to bond the third semiconductor structure to the pair of bonded semiconductor structures. The peripheral circuits and any other logic process-compatible devices in the third semiconductor structure can be conductively connected to the corresponding structures (e.g., the conductively-connected WLs  506 - 1  and  510 - 1  and/or conductively-connected BLs  522 - 1  and  522 - 2 ) in the pair of bonded semiconductor structures. 
       FIGS. 8A and 8B  illustrate part of a bonding method to form a bonded 3D memory device with the third semiconductor bonded between the first and second semiconductor structures, according to some embodiments. For ease of illustration, operations similar to or the same as the operations in method  1300  are not depicted or described in detail. In some embodiments, the third semiconductor is bonded in a face-up manner. The bonding method may be employed to form semiconductor structures  1 B,  2 B, and  3 B. 
     As shown in  FIG. 8A , the first, second, and third semiconductor structures may be formed by separate fabrication processes. Interconnect layer  508  of the first semiconductor structure may include a plurality of first conductive routings  806 - 1  conductively connected to BLs  522 - 1  and/or WLs  506 - 1  (or conductor layers  506 - 1 ) through any suitable interconnects. First bonding layer  838  above or as a part of interconnect layer  508  may include a plurality of first bit line bonding contacts  804 - 1  and/or a plurality of first word line bonding contacts  808 - 1 . In some embodiments, first bit line bonding contacts  804 - 1  and/or first word line bonding contacts  808 - 1  are conductively connected to respective first conductive routings  806 - 1 , to be conductively connected to BLs  522 - 1  and/or WLs  506 - 1 . Similarly, interconnect layer  512  of the second semiconductor structure may include a plurality of second conductive routings  806 - 2  conductively connected to BLs  522 - 2  and/or WLs  510 - 1  (or conductor layers  510 - 1 ) through any suitable interconnects. Second bonding layer  836  above or as a part of interconnect layer  512  may include a plurality of second bit line bonding contacts  816 - 2  and/or a plurality of second word line bonding contacts  814 - 2 . In some embodiments, second bit line bonding contacts  816 - 2  and/or second word line bonding contacts  814 - 2  are conductively connected to respective second conductive routings  806 - 2 , to be conductively connected to BLs  522 - 2  and/or WLs  510 - 1 . 
     Substrate  526  of the third semiconductor structure may be thinned to form a semiconductor layer  860 , similar to semiconductor layer  540 / 560 . A bonding layer  854  may be formed on semiconductor layer  860  and a plurality of bonding contacts  804 - 2  and/or  808 - 2 , conductively connected to interconnect layer  550 , may be formed in bonding layer  854 . In some embodiments, to form bonding layer  854 , the substrate of the third semiconductor structure is thinned to form semiconductor layer  860 , and the third semiconductor flipped upside down for dielectrics to be deposited on semiconductor layer  860 . Bonding contacts  804 - 2  can be formed in the dielectrics, forming bonding layer  854 . The process to form bonding layer  854  is similar to the process to form bonding layer  546 . Interconnect layer  550  of the third semiconductor structure may include a plurality of interconnects, e.g., TSVs  818  and/or  810 , conductively connected to bonding contacts  804 - 2  and/or  808 - 2 . In some embodiments, interconnect layer  550  includes any suitable interconnects (e.g., interconnect  822 ,  818 , and/or  810 ) that conductively connect bonding contacts  804 - 2 ,  816 - 1 ,  808 - 2 , and/or  814 - 1  to the peripheral circuits (and/or other logic process-compatible devices) in device layer  548 . Bonding layer  852  of the third semiconductor layer may include a plurality of bonding contacts  816 - 1  and/or  814 - 1 , conductively connected to bonding contacts  804 - 2  and  808 - 2 , respectively. 
     As shown in  FIG. 8A , the first and third semiconductor structures may be bonded together. The bonding may include hybrid bonding. In some embodiments, both the first and third semiconductor structures are bonded in a face-up manner A bonding interface  870  is formed between the first and third semiconductor structures (i.e., between first bonding layer  838  and bonding layer  854 ). In some embodiments, at the bonding interface, first bit line bonding contacts  804 - 1  are aligned with and bonded to bonding contacts  804 - 2 , and first word line bonding contacts  808 - 1  are aligned with and bonded to bonding contacts  808 - 2 . 
     The second semiconductor structure may then be flipped upside down and bonded to the third semiconductor structure. The bonding of the first and third semiconductor structures, and the bonding of the third and second semiconductor structure may each include hybrid bonding. As shown in  FIG. 8B , a bonding interface  880  may be formed between the second and third semiconductor layers (i.e., between second bonding layer  836  and bonding layer  852 ). In some embodiments, at bonding interface  880 , second bit line bonding contacts  816 - 2  are aligned with and bonded to bonding contacts  816 - 1  and second word line bonding contacts  814 - 2  are aligned with and bonded to bonding contacts  814 - 1 . First bit line bonding contacts  804 - 1  may then be conductively connected to second bit line bonding contacts  816 - 2  through bonding contacts  804 - 2  and  816 - 1 , and any suitable interconnects such as TSV  818 . Also, first word line bonding contacts  808 - 1  may then be conductively connected to second word line bonding contacts  814 - 2  through bonding contacts  808 - 2  and  814 - 1 , and any suitable interconnects such as TSV  810 . That is, BLs (e.g.,  522 - 1  and  522 - 2 ) and WLs (e.g.,  506 - 1  and  510 - 1 ) may respectively be conductively connected in the third semiconductor structure. Conductively-connected BLs  522 - 1  and  522 - 2  and the conductively-connected WLs  506 - 1  and  510 - 1  may be respectively connected to the peripheral circuits and any other logic process-compatible devices through any suitable interconnects, such as TSVs  808  and  810 , and interconnects  822 . 
     In some embodiments, after the second semiconductor structure is bonded with the third semiconductor structure, substrate  504  of the second semiconductor structure is thinned to form a semiconductor layer  840 . A pad-out interconnect layer may then be formed above semiconductor layer  840 . The bonding, the formation of bonding layers (e.g.,  838 ,  854 ,  852 , and  836 ), the formation of semiconductor layers  860  and  840 , and the formation of pad-out interconnect layer may be referred to the description of method  1300 . The detailed description is not repeated herein. 
       FIGS. 12A and 12B  illustrates an exemplary block diagram of stack structures  1200  and  1201  each having N pairs of bonded semiconductor structures, each pair being bonded back-to-back to another pair along the vertical direction, with N being a positive integer. In stack structure  1200 , the third semiconductor structure containing the peripheral circuits (and/or any other logic process-compatible devices) of one or more pairs (e.g., N pairs) may be located above the N pair. In stack structure  1201 , the third semiconductor structure may be located under the N pair. Stack structures  1200  and  1201  may each include a substrate, located at the bottom of stack structure  1200 / 1201 . Each pair may include a first and a second semiconductor structures bonded in a face-to-face manner. In some embodiments, in each pair, the second semiconductor structure is above the first semiconductor structure. Each of the first and second semiconductor structures also includes a plurality of 3D NAND memory strings, a plurality of BLs, and a plurality of WLs (e.g., conductor layers). In some embodiments, in stack structure  1200 , the substrate of the first semiconductor structure of the Nth pair is retained as the substrate of stack structure  1200 , and memory stack in the first semiconductor structure in the N th  pair is formed above the substrate, and the memory stack in all other first and second semiconductor structure in the N pairs and the third semiconductor structure is formed on a respective semiconductor layer. In some embodiments, a pad-out interconnect layer, including one or more contact pads, may be formed above the semiconductor layer of the third semiconductor structure. In some embodiments, in stack structure  1201 , the substrate of the third semiconductor structure is retained as the substrate of stack structure  1201 , and the memory stack in each of the first and second semiconductor structures in the N pairs is formed on a respective semiconductor layer. In some embodiments, the pad-out interconnect layer may be formed above the semiconductor layer of the second semiconductor structure in the first pair. 
     In various embodiments, the corresponding structures bonded in one pair may be the same as or different from another pair. For example, in the first pair, the BLs of the first and second semiconductor structures may be bonded and conductively connected to the third semiconductor structure through the same interconnects, and the WLs of the first and second semiconductor structures may not be bonded and may be conductively connected to the third semiconductor structure through two different interconnects (or two different sets of interconnects). In the second pair, the WLs of the first and second semiconductor structures may be bonded and conductively connected to the third semiconductor structure through the same interconnects, and the BLs of the first and second semiconductor structures may not be bonded and may be conductively connected to the third semiconductor structure through two different interconnects (or two different sets of interconnects). In the third pair, the WLs of the first and second semiconductor structures may be bonded and conductively connected to the third semiconductor structure through the same interconnects, and the BLs of the first and second semiconductor structures may be conductively connected to the third semiconductor structure through the same interconnects. In some embodiments, the bonded structures in all pairs may be the same. For example, BLs in all pairs may be bonded and WLs in all pairs may be separate. The specific structures bonded in each pair, the number of pairs with the same/different bonded structures, and/or the order in which the pairs of particular bonded structures are arranged should not be limited by the embodiments of the present disclosure. 
       FIGS. 9-11  illustrate stack structures  900 ,  1000 , and  1100 , each having a plurality of pairs of bonded semiconductor structures, sharing the same semiconductor structure that has the peripheral circuits for all pairs of bonded semiconductor structures, according to some embodiments. Suitable interconnects, such as TSVs, may be employed to extend through and conductively connect the plurality of pairs. Bonding contacts may be disposed at each bonding interface to conductively connect any suitable interconnects in the adjacent semiconductor structures. Each pair may include a pair of memory stacks, and a plurality of conductively-connected BLs and/or a plurality of conductively-connected WLs. For ease of illustration, each stack structure includes two pairs of bonded semiconductor structures. 
     Each of stack structures  900 ,  1000 , and  1100  may include a substrate (e.g.,  902 ,  1002 , and  1102 ) at the bottom of the respective stack structure, above which a plurality of pairs of memory stacks are bonded in a face-to-face manner Except for the memory stack at the bottom of the stack structure, each memory stack may be on a semiconductor layer, which can be formed by the thinning of a substrate where the memory stack is formed above. Each semiconductor structure in a pair may include a memory stack, a plurality of 3D NAND memory strings, a plurality of BLs, and a plurality of WLs (e.g., conductive layers). Each semiconductor structure may be bonded with another semiconductor structure through hybrid bonding. Each pair of bonded semiconductor structures may be bonded to another pair through hybrid bonding in a back-to-back manner. 
     As shown in  FIG. 9 , stack structure  900  includes two pairs of semiconductor structures, each includes a first semiconductor structure and a second semiconductor structure. In some embodiments, a first pair is bonded with a second pair in a back-to-back manner, at a bonding interface  934 - 2 . The first pair may include a first semiconductor structure  912  bonded with a second semiconductor structure  914  in a face-to-face manner at a bonding interface  934 - 1 . The second pair may include a first semiconductor structure  916  bonded with a second semiconductor structure  918  in a face-to-face manner at a bonding interface  934 - 3 . A third semiconductor structure  920 , with a pad-out interconnect layer  922  at the top surface, may be bonded with the second pair in the face-down manner at a bonding interface  934 - 4 . In some embodiments, BLs of the first and second semiconductor structures in each pair are routed and bonded at the respective bonding interface through respective bit line bonding contacts (e.g.,  924 - 1  and  924 - 2 , and  926 - 1  and  926 - 2 ) and are conductively connected to third semiconductor structure  920  through the same interconnects. In some embodiments, WLs (e.g., conductor layers) of the first and second semiconductor structures in each pair are conductively connected to third semiconductor structure  920  separately. 
     As shown in  FIG. 10 , stack structure  1000  includes two pairs of semiconductor structures, each includes a first semiconductor structure and a second semiconductor structure. In some embodiments, a first pair is bonded with a second pair in a back-to-back manner, at a bonding interface  1034 - 2 . The first pair may include a first semiconductor structure  1012  bonded with a second semiconductor structure  1014  in a face-to-face manner at a bonding interface  1034 - 1 . The second pair may include a first semiconductor structure  1016  bonded with a second semiconductor structure  1018  in a face-to-face manner at a bonding interface  1034 - 3 . A third semiconductor structure  1020 , with a pad-out interconnect layer  1022  at the top surface, may be bonded with the second pair in the face-down manner at a bonding interface  1034 - 4 . In some embodiments, WLs (e.g., conductor layers) of the first and second semiconductor structures in each pair are routed and bonded at the respective bonding interface through respective WL line bonding contacts (e.g.,  1032 - 1  and  1032 - 2 , and  1036 - 1  and  1036 - 2 ) and are conductively connected to third semiconductor structure  1020  through the same interconnects. In some embodiments, BLs of the first and second semiconductor structures in each pair are conductively connected to third semiconductor structure  1020  separately. 
     As shown in  FIG. 11 , stack structure  1100  includes two pairs of semiconductor structures, each includes a first semiconductor structure and a second semiconductor structure. In some embodiments, a first pair is bonded with a second pair in a back-to-back manner, at a bonding interface  1134 - 2 . The first pair may include a first semiconductor structure  1112  bonded with a second semiconductor structure  1114  in a face-to-face manner at a bonding interface  1134 - 1 . The second pair may include a first semiconductor structure  1116  bonded with a second semiconductor structure  1118  in a face-to-face manner at a bonding interface  1134 - 3 . A third semiconductor structure  1120 , with a pad-out interconnect layer  1122  at the top surface, may be above may be bonded with the second pair in the face-down manner at a bonding interface  1134 - 4 . In some embodiments, at the respective bonding interface, BLs of the first and second semiconductor structures in each pair are routed and bonded through respective bit line bonding contacts (e.g.,  1124 - 1  and  1124 - 2 , and  1126 - 1  and  1126 - 2 ), and WLs (e.g., conductor layers) of the first and second semiconductor structures in each pair are routed and bonded through respective word line bonding contacts (e.g.,  1132 - 1  and  1132 - 2 , and  1136 - 1  and  1136 - 2 ). The conductively-connected BLs in each pair may be conductively connected to third semiconductor structure  1120  through respective same interconnects, and the conductively-connected WLs in each pair may be conductively connected to third semiconductor structure  1120  through respective same interconnects. 
       FIG. 14  is a flowchart of an exemplary method  1400  for forming a stack structure  1200 , according to some embodiments. It is understood that the operations shown in method  1400  are not exhaustive and that other operations can be performed as well before, after, or between any of the illustrated operations. Further, some of the operations may be performed simultaneously, or in a different order than shown in  FIG. 14 . For ease of illustration, method  1400  may be described in view of  FIG. 5  and  FIGS. 9-12 . 
     Method  1400  starts at operation  1402 , in which a plurality of pairs of semiconductor structures are formed. Each pair includes a first semiconductor structure and a second semiconductor structure, non-bonded. For example, N pairs of semiconductor structures are formed (e.g., referring to stack structure  1200 ). Each pair may include a first semiconductor and a second semiconductor structure, formed in separate fabrication processes (e.g., referring to the description of first and second semiconductor structures in  FIG. 5A ). The first and second semiconductor structures may each include a memory stack above a substrate, a plurality of 3D NAND memory strings extending through the memory stack, a plurality of BLs conductively connected to the 3D NAND memory strings, and a plurality of WLs (or conductor layers) as a part of the memory stack. In each pair, the BLs and/or WLs of the first and second semiconductor structures are routed to the respective bonding region (e.g., referring to the description of  FIG. 4 ) to be subsequently bonded once the first and second semiconductor structures are bonded together as a pair. 
     In some embodiments, each pair includes a pair of bonding layers subsequently located on opposite sides of the bonding interface. Except for the N t h pair, each pair also includes a bonding layer on each of the top and the bottom surfaces, for bonding with other pairs/the third semiconductor structure (e.g., referring to bonded 3D memory devices  900 - 1100 ). Each bonding layer may include a plurality of bonding contacts, e.g., bonding contacts conductively connected to the conductively-connected BLs, the conductively-connected WLs of the respective pair, and/or the separate BLs and WLs, for forming conductive contacts with the another pair (e.g., referring to the structure shown in  FIGS. 9-12 ). 
     After the plurality of pairs of semiconductor structures are formed, method  1400  proceeds to operations  1404  and  1406 , in which the pairs of bonded semiconductor structures are bonded, and after N pairs of bonded semiconductor structures are formed, the substrate of the second semiconductor structure at the top of the stack is thinned to form a top semiconductor layer. In some embodiments, the first and second semiconductor structures in one pair are bonded in a face-to-face manner, and the pairs are bonded with one another in a back-to-back manner. For ease of description, the second semiconductor structure is above the first semiconductor structure in each pair. 
     The first semiconductor structure of the N th  pair (e.g., the bottom pair) may be at the bottom of the stack structure. The substrate of the first semiconductor structure in the N th  pair may be maintained to function as the substrate for the stack structure. The second semiconductor structure of the N th  pair may be flipped upside down to be aligned and bonded with the first semiconductor structure of the N th  pair (e.g., referring to the description of  FIG. 5B ). The substrate of the second semiconductor structure in the N th  pair may be thinned to form a semiconductor layer. Optionally, a bonding layer may be formed above the semiconductor layer (e.g., referring to the description of  FIG. 5C ). 
     The first semiconductor structure of the (N−1) th  pair, with the substrates thinned, may then be bonded the second semiconductor structure of the N th  pair in a back-to-back manner so the first semiconductor structure of the (N−1) th  pair is above the N th  pair, facing up. In some embodiments, the substrate of the first semiconductor structure of the (N- 1 )′ h  pair is thinned before the (N−1) th  pair is bonded to the N th  pair. The second semiconductor structure of the (N−1) th  pair may then be aligned with and bonded to the first semiconductor structure of the (N−1) th  pair. In some embodiments, the substrate of the second semiconductor structure of the (N−1) th  pair is thinned after it is bonded to the first semiconductor structure of the (N−1) th  pair. In some embodiments, one or more pairs may be stacked above the N th  pair, by repeating the bonding and thinning process until a desired number of pairs is formed. In some embodiments, the first semiconductor structures and the second semiconductor structures are alternatingly bonded along the z-axis to form the N pairs of bonded semiconductor structures. In some embodiments, the substrates of each of the one or more pairs may be thinned to form a respective semiconductor layer. The conductive connection between adjacent pairs may be formed by the bonding contacts at each bonding interface and interconnects (e.g., TSVs) between bonding interfaces. In some embodiments, the substrate of the second semiconductor structure in the first pair (e.g., the top pair of the stack structure) is thinned to form a top semiconductor structure (referring to the description of  FIGS. 5C and 5D ). 
     After the N pairs are bonded together, method  1400  proceeds to operation  1408 , in which a third semiconductor structure is bonded to the top semiconductor layer. The third semiconductor structure may be bonded to the N pairs in a face-down manner (Referring to the description of  FIG. 5E , and  FIGS. 9-12 ), by hybrid bonding. A conductive connection between the third semiconductor structure and the N pairs of bonded semiconductor structures may be formed at the bonding interface. In some embodiments, the substrate of the third semiconductor structure is thinned to form another semiconductor layer (referring to the description of  FIG. 5E ). 
     After the another semiconductor layer is formed, method  1400  proceeds to operation  1410 , in which a pad out interconnect layer is formed above the another semiconductor layer (referring to the description of  FIG. 5F  and  FIGS. 9-12 ). The bonding and thinning operations in method  1400  may be referred to the description of method  1300  and are not repeated herein. 
     Although not shown, in some embodiments, the N pairs of bonded semiconductor structures are formed above the third semiconductor structure (e.g., referring to stack structure  1201 ), and a pad-out interconnect layer is formed above the top semiconductor layer which is formed by thinning of the substrate of the second semiconductor structure of the first pair. In this case, the third semiconductor structure may be at the bottom of the bonded 3D memory device, and the substrate of the third semiconductor structure may be retained as the substrate of the bonded 3D memory device. The substrate of the first bonded 3D memory device of the N th  pair may be thinned to form a semiconductor layer before the first bonded 3D memory device is bonded to the third bonded 3D memory device, using hybrid bonding. The second semiconductor structure of the N th  pair may then be bonded to the first semiconductor structure in a face-to-face manner, with the second semiconductor structure above the first semiconductor structure. The substrate of the first semiconductor structure of the N th  pair may then be the thinned to form another semiconductor layer (e.g., referring to the description of  FIGS. 5B and 5C ). Repeatedly, the (N−1) th  pair may be bonded to the N th  pair in a back-to-back manner, and the substrates of the first and second semiconductor structures may both be thinned to form a respective semiconductor layer. In some embodiments, one or more pairs may be stacked above the N th  pair, by repeating the bonding and thinning process until a desired number of pairs is formed. In some embodiments, the substrates of each of the one or more pairs may be thinned to form a respective semiconductor layer. The conductive connection between adjacent pairs and between the third semiconductor and the N pairs of bonded semiconductor structures may be formed by the bonding contacts at each bonding interface and interconnects (e.g., TSVs) between bonding interfaces. In some embodiments, the substrate of the second semiconductor structure in the first pair (e.g., the top pair of the stack structure) is thinned to form a top semiconductor structure (referring to the description of  FIGS. 5C and 5D ), and the pad-out interconnect layer above the top semiconductor layer. 
     According to the embodiments of the present disclosure, a 3D memory device includes a first semiconductor structure and a second semiconductor structure. The first semiconductor structure includes a first memory stack having a plurality of first conductor layers and a first bonding layer having a plurality of first word line bonding contacts conductively connected to the plurality of first conductor layers, respectively. A second semiconductor structure includes a second memory stack having a plurality of second conductor layers and a second bonding layer having a plurality of second word line bonding contacts conductively connected to the plurality of second conductor layers, respectively. The 3D memory device also includes a bonding interface between the first bonding layer and the second bonding layer, at which the first word line bonding contacts are in contact with the second word line bonding contacts, such that at least one of the first conductor layers is conductively connected to at least one of the second conductor layers, respectively. 
     In some embodiments, the first semiconductor structure includes the first bonding layer at the bonding interface, the first memory stack under the first bonding layer, the first memory stack having interleaved the plurality of first conductor layers and a plurality of first insulating layers, and a plurality of first NAND memory strings extending vertically through the first memory stack. In some embodiments, the second semiconductor structure includes the second bonding layer at the bonding interface, the second memory stack above the second bonding layer, the second memory stack having interleaved the plurality of second conductor layers and a plurality of second insulating layers, and a plurality of second NAND memory strings extending vertically through the second memory stack. 
     In some embodiments, the plurality of first and second conductor layers are routed and conductively connected at a bonding region that is laterally away from the first and second conductor layers. 
     In some embodiments, the first semiconductor structure includes a first interconnect layer having a plurality of first conductive routings between the plurality of first NAND memory strings and the first bonding layer and the second semiconductor structure includes a second interconnect layer having a plurality of second conductive routings between the plurality of second NAND memory strings and the second bonding layer. The first conductor layers may be conductively connected to and routed by the first conductive routings to the bonding region and the second conductor layers may be conductively connected to and routed by the second conductive routings to the bonding region. 
     In some embodiments, the first memory stack and the second memory stack are vertically arranged in a staggered manner; each of the first conductor layers is conductively connected to the respective first word line bonding contact through a first via structure and the first conductive routings; and each of the second conductor layers is conductively connected to the respective second word line bonding contact through a second via structure and the second conductive routings. 
     In some embodiments, the 3D memory device further includes a third semiconductor structure above the first semiconductor structure, the third semiconductor structure having a peripheral circuit of the first and second memory stacks and a second bonding interface between the second semiconductor structure and a third bonding layer of the third semiconductor structure. 
     In some embodiments, the second semiconductor structure includes a semiconductor layer above and in contact with the second NAND memory strings. In some embodiments, the first semiconductor structure includes a substrate under and in contact with the first NAND memory strings. In some embodiments, the third semiconductor structure includes a third interconnect layer between the third bonding layer and the peripheral circuit, the peripheral circuit above and in contact with the third interconnect layer, a second semiconductor layer above and in contact with the peripheral circuit, and a pad-out interconnect layer above the second semiconductor layer. 
     In some embodiments, the 3D memory device further includes a third semiconductor structure under the first semiconductor structure, the third semiconductor structure having a peripheral circuit of the first and second memory stacks. In some embodiments, the 3D memory device further includes a second bonding interface between the first semiconductor structure and a third bonding layer of the third semiconductor structure. 
     In some embodiments, the second semiconductor structure includes a semiconductor layer above and in contact with the second NAND memory strings, and a pad-out interconnect layer above the semiconductor layer and conductively connected to the peripheral circuit. In some embodiments, the first semiconductor structure includes a second semiconductor layer under and in contact with the first NAND memory strings. In some embodiments, the third semiconductor structure includes a third interconnect layer under and in contact with the third bonding layer, the peripheral circuit under and in contact with the third interconnect layer, and a substrate under and in contact with the peripheral circuit. 
     In some embodiments, the 3D memory device further includes a plurality of via structures extending vertically from the pad-out interconnect layer into the third interconnect layer to be conductively connected to the peripheral circuit. 
     In some embodiments, the peripheral circuit is conductively connected to the third bonding layer, which is conductively connected to the first and second conductor layers. 
     In some embodiments, the first semiconductor structure further includes a plurality of first BLs conductively connected to the plurality of first NAND memory strings. In some embodiments, the second semiconductor structure further includes a plurality of second BLs conductively connected to the plurality of second NAND memory strings, the first BLs and second BLs being non-bonded and conductively connected to the peripheral circuit through the third bonding layer separately. 
     According to the embodiments of the present disclosure, a 3D memory device includes a stack structure having a plurality of pairs of bonded semiconductor structures. Each of the pairs includes a first semiconductor structure having a first memory stack having a plurality of first conductor layers, and a first bonding layer having a plurality of first word line bonding contacts conductively connected to the plurality of first conductor layers, respectively. Each of the pair also includes a second semiconductor structure having a second memory stack having a plurality of second conductor layers, and a second bonding layer having a plurality of second word line bonding contacts conductively connected to the plurality of second conductor layers, respectively. Each of the pairs further includes a bonding interface between the first bonding layer and the second bonding layer. The first word line bonding contacts may be in contact with the second word line bonding contacts at the bonding interface such that at least one of the first conductor layers is conductively connected to at least one of the second conductor layers, respectively. The 3D memory device further includes a third semiconductor structure bonded to and conductively connected to the stack structure. The third semiconductor structure may include a peripheral circuit of at least one pair of semiconductor structures. 
     In some embodiments, the first semiconductor structure includes the first bonding layer at the bonding interface, the first memory stack under the first bonding layer, and a plurality of first insulating layers, and a plurality of first NAND memory strings extending vertically through the first memory stack. The first memory stack may include interleaved the plurality of first conductor layers. In some embodiments, the second semiconductor structure includes the second bonding layer at the bonding interface, the second memory stack above the second bonding layer a plurality of second NAND memory strings extending vertically through the second memory stack. The second memory stack may have interleaved the plurality of second conductor layers and a plurality of second insulating layers. 
     In some embodiments, the plurality of first and second conductor layers are routed and conductively connected at a bonding region that is laterally away from the first and second conductor layer. 
     In some embodiments, the first semiconductor structure includes a first interconnect layer having a plurality of first conductive routings between the plurality of first NAND memory strings and the first bonding layer. In some embodiments, the second semiconductor structure includes a second interconnect layer having a plurality of second conductive routings between the plurality of second NAND memory strings and the second bonding layer. In some embodiments, the first conductor layers are conductively connected to and routed by the first conductive routings to the bonding region and the second conductor layers are conductively connected to and routed by the second conductive routings to the bonding region. 
     In some embodiments, the first memory stack and the second memory stack are vertically arranged in a staggered manner; each of the first conductor layers is conductively connected to the respective first word line bonding contact through a first via structure and the first conductive routings; and each of the second conductor layers is conductively connected to the respective second word line bonding contact through a second via structure and the second conductive routings. 
     In some embodiments, the stack structure includes a semiconductor layer above and in contact with the second NAND memory strings in a top second semiconductor structure, and a substrate under and in contact with the first NAND memory strings in a bottom first semiconductor structure. In some embodiments, the third semiconductor structure includes a third bonding layer above and in contact with the semiconductor layer, the peripheral circuit above the third bonding layer, a second semiconductor layer above the peripheral circuit, and a pad-out interconnect layer above the second semiconductor layer. 
     In some embodiments, the stack structure includes a semiconductor layer above and in contact with the second NAND memory strings in a top second semiconductor structure, a pad-out interconnect layer above the semiconductor layer and in contact with the second NAND memory strings, and a second semiconductor layer under and in contact with the first NAND memory strings in a bottom first semiconductor structure. In some embodiments, the third semiconductor structure includes a third bonding layer under and in contact with the second semiconductor layer, the peripheral circuit under and in contact with the third bonding layer, and a substrate under the peripheral circuit. 
     In some embodiments, the 3D memory device further includes a plurality of via structures extending vertically from the pad-out interconnect layer to be conductively connected to the peripheral circuit. 
     In some embodiments, each pair of semiconductor structures further includes a plurality of first BLs conductively connected to the plurality of first NAND memory strings; and a plurality of second bit lines (BLs) conductively connected to the plurality of second NAND memory strings. The plurality of first BLs and second BLs may be non-bonded and conductively connected to the peripheral circuit through the third bonding layer separately. 
     In some embodiments, each pair of semiconductor structures is bonded in a back-to-back manner to one another through hybrid bonding. 
     According to the embodiments of the present disclosure, a method for forming a 3D memory device includes the following operations. First, on a first substrate, a first memory stack having a plurality of first conductor layers and a first bonding layer having a plurality of first word line bonding contacts are formed conductively connected to the first conductor layers to form a first semiconductor structure. On a second substrate, a second memory stack having a plurality of second conductor layers and a second bonding layer having a plurality of second word line bonding contacts are formed conductively connected to the second conductor layers to form a second semiconductor structure. The first semiconductor structure and the second semiconductor structure are bonded in a face-to-face manner, such that (i) the first semiconductor structure is bonded to the second semiconductor structure, and (ii) the first conductor layers are conductively connected to the second conductor layers through the bonded first and the second word line bonding contacts at a bonding interface. 
     In some embodiments, forming the first semiconductor structure includes forming a first interconnect layer having a plurality of first conductive routings above and conductively connected to the first conductor layers and the first word line bonding contacts, and forming the first bonding layer above and conductively connected to the first interconnect layer. In some embodiments, forming the second semiconductor structure includes forming a second interconnect layer having a plurality of second conductive routings above and conductively connected to the second conductor layers and the second word lien bonding contacts, and forming the second bonding layer above and conductively connected to the second interconnect layer. 
     In some embodiments, forming the first semiconductor structure further includes forming the first memory stack above the first substrate and forming a plurality of first NAND memory strings extending vertically through the first memory stack. The first memory stack may include interleaved the plurality of first conductor layers and a plurality of first insulating layers. In some embodiments, forming the second semiconductor structure further includes forming the second memory stack above the second substrate and forming a plurality of second NAND memory strings extending vertically through the second memory stack. The second memory stack may include interleaved the plurality of second conductor layers and a plurality of second insulating layers. 
     In some embodiments, the method further includes thinning the second substrate to form a semiconductor layer, bonding a third semiconductor structure to the semiconductor layer, thinning a third substrate of the third semiconductor structure to form a second semiconductor layer, and forming a pad-out interconnect layer above the second semiconductor layer. 
     In some embodiments, the method further includes thinning the first substrate to form a semiconductor layer, bonding a third semiconductor structure to the semiconductor layer, thinning a second substrate of the second semiconductor structure to form a second semiconductor layer, and forming a pad-out interconnect layer above the second semiconductor layer. 
     In some embodiments, forming the third semiconductor structure includes forming a peripheral circuit above the third substrate, forming a third interconnect layer above the peripheral circuit, and forming a third bonding layer above the third interconnect layer. In some embodiments, bonding the third semiconductor structure to the semiconductor layer includes bonding the third bonding layer to the semiconductor layer. 
     In some embodiments, the bonding of the first semiconductor structure and the second semiconductor structure includes hybrid bonding. 
     According to embodiments of the present disclosure, a method for forming a 3D memory device, includes alternatingly bonding a plurality of first semiconductor structures and a plurality of second semiconductor structures to form a stack structure having a plurality of pairs of bonded semiconductor structures, conductor layers of at least one pair of bonded semiconductor structures being conductively connected through bonding. Forming the plurality of bonded semiconductor structures includes bonding a second semiconductor structure with a first semiconductor in a face-to-face manner to form a pair of bonded semiconductor structures, the second semiconductor structure being above the first semiconductor structure, bonding another first semiconductor structure with the pair of bonded semiconductor structures, the another first semiconductor structure facing up, and bonding another second semiconductor structure with the another first semiconductor structure in a face-to-face manner to form another pair of bonded semiconductor structures, the pair and the another pair being bonded in a back-to-back manner. 
     In some embodiments, forming the first semiconductor structure includes forming a first memory stack having a plurality of first conductor layers, and a first bonding layer having a plurality of first word line bonding contacts conductively connected to the first conductor layers, respectively. In some embodiments, forming the second semiconductor structure includes forming a second memory stack having a plurality of second conductor layers and a second bonding layer having a plurality of second word line bonding contacts conductively connected to the second BLs, respectively. 
     In some embodiments, forming the another first semiconductor structure includes forming another first memory stack having a plurality of another first conductor layers and another first bonding layer having a plurality of another first word line bonding contacts conductively connected to the another first conductor layers, respectively. In some embodiments, forming the another second semiconductor structure includes forming another second memory stack having a plurality of another second conductor layers and another second bonding layer having a plurality of another second word line bonding contacts conductively connected to the another second conductor layers, respectively. 
     In some embodiments, bonding the first and second semiconductor structures in a face-to-face manner includes aligning and bonding the first and second word line bonding contacts at a bonding interface such that (i) the first semiconductor structure is bonded to the second semiconductor structure, and (ii) the first conductor layers are conductively connected to the second conductor layers through the bonded first and second word line bonding contacts. In some embodiments, bonding the another first and the second semiconductor structures in a face-to-face manner includes aligning and bonding the another first and second word line bonding contacts at another bonding interface such that (i) the another first semiconductor structure is bonded to the another second semiconductor structure, and (ii) the another first word lines are conductively connected to the another second word lines through the bonded another first and second word line bonding contacts. 
     In some embodiments, forming the first, the second, the another first, and the another second semiconductor structures each includes forming a respective interconnect layer having a plurality of first conductive routings above the respective NAND memory strings and conductively connected to the respective conductor layers and the respective word line bonding contacts. 
     In some embodiments, the method further includes thinning a substrate of the second semiconductor structure, the another first semiconductor structure, and the another second semiconductor structure to form a respective semiconductor layer. 
     In some embodiments, the method further includes bonding a third semiconductor structure in a face-down manner to the semiconductor layer of the second semiconductor structure, thinning a third substrate of the third semiconductor structure to form a second semiconductor layer, and forming a pad-out interconnect layer above the second semiconductor layer. 
     In some embodiments, the method further includes thinning the first substrate to form a respective semiconductor layer, bonding a third semiconductor structure in a face-up manner to the semiconductor layer of the first semiconductor structure, and forming a pad-out interconnect layer above the semiconductor layer of the another second semiconductor structure. 
     In some embodiments, forming the third semiconductor structure includes forming a peripheral circuit above the third substrate, forming a third interconnect layer above the peripheral circuit, and forming a third bonding layer above the third interconnect layer. In some embodiments, bonding the third semiconductor structure to the semiconductor layer includes bonding the third bonding layer to the semiconductor layer. 
     In some embodiments, the bonding between the semiconductor structures in a pair and between the pairs include hybrid bonding. 
     According to the embodiments of the present disclosure, a 3D memory device includes a first semiconductor structure having a first memory stack having a plurality of first conductor layers, and a first bonding layer having a plurality of first word line bonding contacts conductively connected to the plurality of first conductor layers, respectively. The 3D memory device also includes a second semiconductor structure having a second memory stack having a plurality of second conductor layers, and a second bonding layer having a plurality of second word line bonding contacts conductively connected to the plurality of second conductor layers, respectively. The 3D memory device further includes a third semiconductor structure between the first and second bonding layers and having a peripheral circuit of at least one of the first and second memory stacks, at least one of the first conductor layers being conductively connected to at least one of the second conductor layers through and to the third semiconductor structure. 
     In some embodiments, the first semiconductor structure includes the first bonding layer under the third semiconductor structure, and the first memory stack under the first bonding layer, the first memory stack having interleaved the plurality of first conductor layers and a plurality of first insulating layers. In some embodiments, the second semiconductor structure includes the second bonding layer above the third semiconductor structure, and a second memory stack above the second bonding layer, the second memory stack having interleaved the plurality of second conductor layers and a plurality of second insulating layers. 
     In some embodiments, the first semiconductor structure further includes a plurality of first NAND memory strings extending vertically through the first memory stack, and the second semiconductor structure further includes a plurality of second NAND memory strings extending vertically through the second memory stack. 
     In some embodiments, the first semiconductor structure includes a first interconnect layer having a plurality of first conductive routings between the plurality of first NAND memory strings and the first bonding layer. In some embodiments, the second semiconductor structure includes a second interconnect layer having a plurality of second conductive routings between the plurality of second NAND memory strings and the second bonding layer. The first conductor layers may be conductively connected to and routed by the first conductive routings to a bonding region and the second conductor layers are conductively connected to and routed by the second conductive routings to the bonding region, the bonding region being laterally away from the first and second conductor layers. 
     In some embodiments, the first memory stack and the second memory stack are vertically arranged in a staggered manner, each of the first conductor layers is conductively connected to the respective first word line bonding contact through a first via structure and the first conductive routings, and each of the second conductor layers is conductively connected to the respective second word line bonding contact through a second via structure and the second conductive routings. 
     In some embodiments, the third semiconductor structure includes an upper bonding layer, a lower bonding layer, and the peripheral circuit between the upper bonding layer and the lower bonding layer. The upper bonding layer may include a plurality of upper bonding contacts conductively connected to the second word line bonding contacts. The lower bonding layer may include a plurality of lower bonding contacts conductively connected to the first word line bonding contacts. 
     In some embodiments, the second semiconductor structure includes a semiconductor layer above and in contact with the second NAND memory strings and a pad-out interconnect layer above the semiconductor layer. In some embodiments, the first semiconductor structure includes a substrate under and in contact with the first NAND memory strings. In some embodiments, the third semiconductor structure includes a third interconnect layer between the upper bonding layer and the peripheral circuit, the peripheral circuit under and in contact with the third interconnect layer, and a second semiconductor layer between and in contact with the peripheral circuit and the lower bonding layer. 
     The foregoing description of the specific embodiments will so reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
     Embodiments of the present disclosure have been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
     The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way. 
     The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.