Patent Publication Number: US-2023145467-A1

Title: Nonvolatile memory device having multi-stack memory block and method of operating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 to Korean Patent Application Nos. 10-2021-0154261, filed on Nov. 10, 2021, and 10-2022-0002349, filed on Jan. 06, 2022 in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference in their entireties herein. 
     1. Technical Field 
     The inventive concept relates to semiconductor memory devices, and more particularly, to a nonvolatile memory device having a multi-stack memory block, and a method of operating the nonvolatile memory device. 
     2. Discussion of Related Art 
     A system using semiconductor chips typically uses dynamic random access memory (DRAM) as a main memory and a nonvolatile memory to store data or instructions. An increase in a capacity of a storage device causes an increase in the number of memory cells stacked on a substrate of a nonvolatile memory, an increase in the number of word lines, and an increase in the number of bits of data stored in a memory cell. A three-dimensional (3D) NAND flash memory device, having memory cells stacked in a 3D structure may be used to implement the storage device to have a higher storage capacity and degree of integration. 
     The 3D NAND flash memory device includes a memory cell array formed in a multi-stack memory block structure. The memory cell array includes a plurality of cell strings respectively disposed between a plurality of bit lines and a source line in a vertical direction with respect to a substrate. The multi-stack memory block structure may have stacked memory stacks having gate lines corresponding to word lines and include an inter-stack portion between the memory stacks. In this case, the inter-stack portion may be formed to be relatively longer than a length between gate lines of a memory stack in a manufacturing process. The inter-stack portion is included in a channel region of each cell string, and thus, it is required even in the inter-stack portion that a channel potential or a channel voltage be equalized. If the channel potential is not equalized in the inter-stack portion, a hot carrier injection (HCI) may occur when channel boosting occurs by word lines in a program operation or a read operation. 
     SUMMARY 
     At least one embodiment of the inventive concept provides a nonvolatile memory device having a multi-stack memory block, of which a channel potential is equalized by differently controlling operating time points of word lines adjacent to an inter-stack portion between memory stacks, and a method of operating the nonvolatile memory device. 
     According to an embodiment of the inventive concept, there is provided a nonvolatile memory device including: a memory cell array and a control circuit. The memory cell array includes a plurality of cell strings in which a plurality of memory cells are disposed in a vertical direction, respectively. The memory cell array is divided into a plurality of memory stacks disposed in the vertical direction. Inter-stack portions are disposed between the plurality of memory stacks. Word lines of the plurality of memory cells are stacked in the vertical direction in each of the plurality of memory stacks. A channel hole passes through the word lines of each of the plurality of memory stacks. The control circuit is configured to determine, as inter-stack word lines, some word lines adjacent to the inter-stack portions among the word lines of each of the plurality of memory stacks and perform a channel voltage equalization operation of the plurality of memory stacks while differently controlling setup time points for applying a pass voltage to the inter-stack word lines, according to sizes of the channel hole of the inter-stack word lines. The pass voltage is set to a voltage by which the plurality of memory cells are turned on. 
     According to an embodiment of the inventive concept, there is provided a method of operating a nonvolatile memory device. The method includes: dividing, into a plurality of memory stacks, a memory cell array including a plurality of cell strings in which a plurality of memory cells are disposed in a vertical direction, respectively, wherein inter-stack portions are disposed between the plurality of memory stacks, word lines of the plurality of memory cells are stacked in the vertical direction in each of the plurality of memory stacks, and a channel hole passes through the word lines of each of the plurality of memory stacks; determining, as inter-stack word lines, some word lines adjacent to the inter-stack portions among the word lines of each of the plurality of memory stacks; and performing a channel voltage equalization operation of the plurality of memory stacks while differently controlling setup time points for applying a pass voltage to the inter-stack word lines, according to sizes of the channel hole of the inter-stack word lines. The pass voltage is set to a voltage by which the plurality of memory cells are turned on. 
     According to an embodiment of the inventive concept, there is provided a method of operating a nonvolatile memory device. The method includes: dividing, into a plurality of memory stacks, a memory cell array including a plurality of cell strings in which a plurality of memory cells are disposed in a vertical direction, respectively, wherein inter-stack portions are disposed between the plurality of memory stacks, word lines of the plurality of memory cells are stacked in the vertical direction in each of the plurality of memory stacks, and a channel hole passes through the word lines of each of the plurality of memory stacks; determining, as inter-stack word lines, some word lines adjacent to the inter-stack portions among the word lines of each of the plurality of memory stacks; and performing a channel voltage equalization operation of the plurality of memory stacks while differently controlling recovery time points for applying a recovery voltage to the inter-stack word lines, according to sizes of the channel hole of the inter-stack word lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the inventive concept will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG.  1    is a block diagram illustrating a memory system according to an embodiment of the inventive concept; 
         FIG.  2    is a block diagram illustrating a memory device according to an embodiment of the inventive concept; 
         FIG.  3    is a cross-sectional view for describing a structure of a memory device according to an embodiment of the inventive concept; 
         FIG.  4    is a perspective view illustrating a memory block according to an embodiment of the inventive concept; 
         FIG.  5    is a cross-sectional view for describing an example of an inter-stack portion included in the memory block of  FIG.  4   ; 
         FIG.  6    is an equivalent circuit diagram of the memory block of  FIG.  4   ; 
         FIG.  7    illustrates a threshold voltage distribution when write data is written to memory cells shown in  FIG.  6   ; 
         FIG.  8    is a circuit diagram illustrating a program bias condition according to an embodiment of the inventive concept; 
         FIGS.  9 A and  9 B  are timing diagrams for describing a program operation according to an embodiment of the inventive concept; 
         FIG.  10    is a cross-sectional view for describing another example of the inter-stack portion included in the memory block of  FIG.  4   ; 
         FIG.  11    is a timing diagram for describing a program operation according to an embodiment of the inventive concept; 
         FIG.  12    is a flowchart illustrating a method of operating a nonvolatile memory device, according to an embodiment of the inventive concept; 
         FIG.  13    illustrates a read operation associated with the threshold voltage distribution of the memory cells shown in  FIG.  7   ; 
         FIG.  14    is a timing diagram for describing a read operation according to an embodiment of the inventive concept; 
         FIG.  15    is a cross-sectional view illustrating a memory block according to an embodiment of the inventive concept; and 
         FIG.  16    is a block diagram illustrating a system including a nonvolatile memory device, according to an embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG.  1    is a block diagram illustrating a memory system  100  according to an embodiment of the inventive concept. 
     Referring to  FIG.  1   , the memory system  100  include a memory controller  110  (e.g., a control circuit) and at least one memory device, e.g., a nonvolatile memory device  120 . Although a plurality of conceptual hardware components included in the memory system  100  are shown in the present embodiment, the present embodiment is not limited thereto, and other components may be included. The memory controller  110  control writing of data to the memory device  120 , in response to a write request from a host, or control reading of data from the memory device  120 , in response to a read request from the host. 
     In some embodiments, the memory system  100  may be an internal memory embedded in an electronic device. For example, the memory system  100  may be an embedded universal flash storage (UFS) memory device, an embedded multi-media card (eMMC), or a solid state drive (SSD). In some embodiments, the memory system  100  may be an external memory detachably attachable to an electronic device. For example, the memory system  100  may include at least one of a UFS memory card, a compact flash (CF) card, a secure digital (SD) card, a micro secure digital (Micro-SD) card, a mini secure digital (Mini-SD) card, an extreme digital (xD) card, and a memory stick. 
     The memory device  120  may perform an erase operation, a program operation, a read operation, or the like under control by the memory controller  110 . The memory device  120  may receive a command CMD and an address ADDR from the memory controller  110  through input/output lines and transmit and receive data DATA for a program operation or a read operation to and from the memory controller  110 . In addition, the memory device  120  may receive a control signal CTRL through a control line. The memory device  120  may include a memory cell array  122  and a control circuit  124 . 
     The memory cell array  122  may include a plurality of memory blocks, and each of the plurality of memory blocks may include a plurality of memory cells, e.g., flash memory cells. Hereinafter, embodiments of the inventive concept are described in detail based on an example in which the plurality of memory cells are NAND flash memory cells. The memory cell array  122  may include a three-dimensional (3D) memory cell array including a plurality of cell strings, and this is described in detail with reference to  FIGS.  3  to  6   . 
     The 3D memory cell array is monolithically formed in at least one physical level of memory cell arrays having an active area disposed on a silicon substrate and a circuit formed on or in the substrate as a circuit associated with an operation of memory cells. The term “monolithic” indicates that layers of each level forming the array are stacked immediately on layers of each lower level in the array. In an embodiment of the inventive concept, a 3D memory cell array includes cell strings disposed in a vertical direction so that at least one memory cell is disposed on another memory cell. The at least one memory cell may include a charge trap layer. U.S. Pat. Publication Nos. 7,679,133, 8, 553, 466, 8, 654, 587, and 8, 559, 235 and U.S. Pat. Application No. 2011/0233648, which disclose in detail appropriate configurations of a 3D memory array in which a 3D memory array is formed in a plurality of levels, and word lines and/or bit lines are shared among the levels, are herein incorporated by reference in their entireties. 
     A memory block in the memory cell array  122  may include a first memory stack ST 1  and a second memory stack ST 2  stacked in the vertical direction, as shown in  FIG.  4   . An inter-stack portion INT-ST may be included between the first memory stack ST 1  and the second memory stack ST 2 . According to an embodiment, the memory block may include three or more memory stacks ST 1 , ST 2 , and ST 3 , as shown in  FIG.  15   . 
     The control circuit  124  may perform a program operation in response to a program command from the memory controller  110  so that threshold voltages of memory cells of a certain page, which are adjacent to each other at the same location from a substrate of the memory cell array  122 , have a plurality of target states. A program operation may be performed by program loops based on a voltage increase portion of a program voltage, and each of the program loops may include a program period and a verify period. The control circuit  124  may perform a read operation on a memory cell selected from among memory cells included in the memory cell array  122 , in response to a read command from the memory controller  110 . 
     In an embodiment, the control circuit  124  includes an inter-stack word line manager  129 . The inter-stack word line manager  129  stores channel hole profile information of some word lines adjacent to inter-stack portions INT-ST. The inter-stack portions INT-ST may be defined in a manufacturing process step of the nonvolatile memory device  120 . The inter-stack word line manager  129  may determine, to be inter-stack word lines, some word lines adjacent to inter-stack portions INT-ST among word lines of each of a plurality of memory stacks in the memory cell array  122 , based on the channel hole profile information. The channel hole profile information may indicate addresses or locations of word lines adjacent or near a given inter-stack portion and whether they are considered to have high or low resistance. For example, the channel hole profile information may include for a given inter-stack portion, locations or addresses of one or more first word lines that are near or adjacent the given inter-stack portion that have a first resistance and locations or addresses of one or more second word lines that are near or adjacent the given inter-stack portion that have a second other different resistance. The inter-stack word lines may include the first word lines and the second word lines. 
     According to an embodiment, the inter-stack word line manager  129  performs a channel voltage equalization operation of the plurality of memory stacks while differently controlling setup time points for applying a pass voltage to the inter-stack word lines, according to sizes of a channel hole of the inter-stack word lines. The inter-stack word line manager  129  may first set a first inter-stack word line having a larger channel hole and later set a second other inter-stack word line having a smaller channel hole when the inter-stack word lines are set to the pass voltage. For example, the pass voltage may be first applied to the first inter-stack word line (e.g., WL 4 ) of a given inter-stack portion and the pass voltage may then be applied to the second inter-stack word line (e.g., WL 5 ) of the given inter-stack portion. 
     According to an embodiment, the inter-stack word line manager  129  performs a channel voltage equalization operation of the plurality of memory stacks while differently controlling recovery time points for applying a recovery voltage to the inter-stack word lines, according to sizes of a channel hole of the inter-stack word lines. In an embodiment, the inter-stack word line manager  129  first recovers an inter-stack word line having a smaller channel hole and later recovers an inter-stack word line having a larger channel hole when the inter-stack word lines are recovered to the recovery voltage. For example, the recovery voltage may be first applied to the first inter-stack word line (e.g., WL 4 ) and the recovery voltage may then be applied to the second inter-stack word line (e.g., WL 5 ) of the given inter-stack portion. 
     Although  FIG.  1    shows that the inter-stack word line manager  129  is included in the control circuit  124 , the inter-stack word line manager  129  according to an embodiment of the inventive concept may be implemented as a separate component outside the control circuit  124 . 
       FIG.  2    is a block diagram illustrating the memory device  120  according to embodiments of the inventive concept.  FIG.  2    illustrates a schematic configuration of a flash memory device. The configuration of the flash memory device shown in  FIG.  2    is provided as an example, and  FIG.  2    is not necessarily an actual flash memory device configuration. In addition, the configuration of the flash memory device shown in  FIG.  2    does not indicate or imply limitation to the inventive concept. For convenience of description, the memory device  120  may be referred to as a flash or nonvolatile memory device  120 . 
     Referring to  FIGS.  1  and  2   , the memory device  120  may include the memory cell array  122 , a row decoder  394  (e.g., a decoder circuit), the control circuit  124 , a page buffer  393 , an input/output (I/O) circuit  126 , and a voltage generator  127 . Although not shown, the memory device  120  may further include an I/O interface. 
     The memory cell array  122  may be connected to a plurality of word lines WL, a plurality of string select lines SSL, a plurality of ground select lines GSL, and a plurality of bit lines BL. The memory cell array  122  may be connected to the row decoder  394  via the plurality of word lines WL, the plurality of string select lines SSL, and the plurality of ground select lines GSL and connected to the page buffer  393  via the plurality of bit lines BL. The memory cell array  122  may include a plurality of memory blocks BLK 1  to BLKn. 
     Each of the plurality of memory blocks BLK 1  to BLKn may include a plurality of memory cells and a plurality of select transistors. The plurality of memory cells may be respectively connected to the plurality of word lines WL, and the plurality of select transistors may be respectively connected to the plurality of string select lines SSL or the plurality of ground select lines GSL. Each memory cell may store one or more bits, and for example, each memory cell may correspond to a multi-level cell (MLC), a triple-level cell (TLC), or a quadruple-level cell (QLC). 
     The row decoder  394  may be connected to the memory cell array  122  via the plurality of string select lines SSL, the plurality of word lines WL, and the plurality of ground select lines GSL. In a program operation or a read operation, the row decoder  394  may determine one of the plurality of word lines WL as a selected word line and the other word lines WL as unselected word lines, based on a row address R_ADDR provided from the control circuit  124 . In addition, in a program operation or a read operation, the row decoder  394  may determine one of the plurality of string select lines SSL as a selected string select line and the other string select lines SSL as unselected string select lines, based on the row address R_ADDR provided from the control circuit  124 . 
     The control circuit  124  may output various kinds of internal control signals for performing program, read, and erase operations on the memory cell array  122 , based on the command CMD, the address ADDR, and the control signal CTRL transmitted from the memory controller  110 . The control circuit  124  may provide the row address R_ADDR to the row decoder  394 , a column address to the I/O circuit  126 , and a voltage control signal CTRL_VOL to the voltage generator  127 . 
     The page buffer  393  may operate as a write driver or a sense amplifier according to an operating mode. In a read operation, the page buffer  393  may sense a bit line BL of a selected memory cell under control by the control circuit  124 . Sensed data may be stored in latches included in the page buffer  393 . The page buffer  393  may transfer data stored in the latches to the I/O circuit  126  via data lines DL under control by the control circuit  124 . 
     The I/O circuit  126  may be connected to the page buffer  393  via the data lines DL. In a program operation, the I/O circuit  126  may receive program data from the memory controller  110  and provide the program data to the page buffer  393 , based on a column address provided from the control circuit  124 . In a read operation, the I/O circuit  126  may provide read data stored in the page buffer  393  to the memory controller  110 , based on a column address provided from the control circuit  124 . 
     The voltage generator  127  may generate various types of voltages for performing program, read, and erase operations on the memory cell array  122 , based on the voltage control signal CTRL_VOL. Particularly, the voltage generator  127  may generate a word line voltage VWL, e.g., a program voltage, a verify voltage, a read voltage, a pass voltage, an erase voltage, an erase verify voltage, and the like. 
     In an embodiment, the control circuit  124  includes the inter-stack word line manager  129 . The inter-stack word line manager  129  may be configured to differently control operating time points of word lines adjacent to an inter-stack portion between memory stacks. The inter-stack word line manager  129  may be implemented by hardware, firmware, software, or a combination thereof to control or manage inter-stack word lines. Although it is described in embodiments below that the inter-stack word line manager  129  determines, as inter-stack word lines, some word lines adjacent to an inter-stack portion among word lines of memory stacks, based on channel hole profile information, and controls the inter-stack word lines, embodiments of the inventive concept are not limited thereto. For example, the inter-stack word line manager  129  corresponds to a component included in the control circuit  124 , and it may be described that the control circuit  124  controls the inter-stack word lines. 
       FIG.  3    is a cross-sectional view for describing a structure of the memory device  120  according to an embodiment of the inventive concept. 
     Referring to  FIG.  3   , the memory device  120  may have a chip-to-chip (C2C) structure. The C2C structure may refer to a structure formed by manufacturing an upper chip including a cell region CELL on a first wafer, manufacturing a lower chip including a peripheral circuit region PERI on a second wafer, separate from the first wafer, and then bonding the upper chip and the lower chip to each other. Here, the bonding process may include a method of electrically connecting a bonding metal formed on an uppermost metal layer of the upper chip to a bonding metal formed on an uppermost metal layer of the lower chip. For example, when the bonding metals may include copper (Cu) using Cu-to-Cu bonding. The example embodiment, however, may not be limited thereto. For example, the bonding metals may also be formed of aluminum (Al) or tungsten (W). 
     Each of the peripheral circuit region PERI and the cell region CELL of the memory device  120  may include an external pad bonding area PA, a word line bonding area WLBA, and a bit line bonding area BLBA. 
     The peripheral circuit region PERI may include a first substrate  210 , an interlayer insulating layer  215 , a plurality of circuit elements  220   a ,  220   b , and  220   c  formed on the first substrate  210 , first metal layers  230   a ,  230   b , and  230   c  respectively connected to the plurality of circuit elements  220   a ,  220   b , and  220   c , and second metal layers  240   a ,  240   b , and  240   c  formed on the first metal layers  230   a ,  230   b , and  230   c . In an example embodiment, the first metal layers  230   a ,  230   b , and  230   c  may be formed of tungsten having relatively high electrical resistivity, and the second metal layers  240   a ,  240   b , and  240   c  may be formed of copper having relatively low electrical resistivity. 
     Although only the first metal layers  230   a ,  230   b , and  230   c  and the second metal layers  240   a ,  240   b , and  240   c  are shown and described in  FIG.  3   , embodiments of the inventive concept are not limited thereto, and one or more additional metal layers may be further formed on the second metal layers  240   a ,  240   b , and  240   c . At least a portion of the one or more additional metal layers formed on the second metal layers  240   a ,  240   b , and  240   c  may be formed of aluminum or the like having a lower electrical resistivity than those of copper forming the second metal layers  240   a ,  240   b , and  240   c . 
     The interlayer insulating layer  215  may be disposed on the first substrate  210  and cover the plurality of circuit elements  220   a ,  220   b , and  220   c , the first metal layers  230   a ,  230   b , and  230   c , and the second metal layers  240   a ,  240   b , and  240   c . The interlayer insulating layer  215  may include an insulating material, such as silicon oxide, silicon nitride, or the like. 
     Lower bonding metals  271   b  and  272   b  may be formed on the second metal layer  240   b  in the word line bonding area WLBA. In the word line bonding area WLBA, the lower bonding metals  271   b  and  272   b  in the peripheral circuit region PERI may be electrically bonded to upper bonding metals  371   b  and  372   b  of the cell region CELL. The lower bonding metals  271   b  and  272   b   and the upper bonding metals  371   b  and  372   b  may be formed of aluminum, copper, tungsten, or the like. Furthermore, the upper bonding metals  371   b  and  372   b  in the cell region CELL may be referred as first metal pads and the lower bonding metals  271   b  and  272   b  in the peripheral circuit region PERI may be referred as second metal pads. 
     The cell region CELL may include at least one memory block. The cell region CELL may include a second substrate  310  and a common source line  320 . On the second substrate  310 , a plurality of word lines  331  to  338  (i.e.,  330 ) may be stacked in a direction (a Z-axis direction), perpendicular to an upper surface of the second substrate  310 . At least one string select line and at least one ground select line may be arranged on and below the plurality of word lines  330 , respectively, and the plurality of word lines  330  may be disposed between the at least one string select line and the at least one ground select line. 
     In the bit line bonding area BLBA, a channel structure CH may extend in a direction (the Z-axis direction), perpendicular to the upper surface of the second substrate  310 , and pass through the plurality of word lines  330 , the at least one string select line, and the at least one ground select line. The channel structure CH may include a data storage layer, a channel layer, a buried insulating layer, and the like, and the channel layer may be electrically connected to a first metal layer  350   c  and a second metal layer  360   c . For example, the first metal layer  350   c  may be a bit line contact, and the second metal layer  360   c  may be a bit line. In an example embodiment, the bit line  360   c  may extend in a first direction (a Y-axis direction), parallel to the upper surface of the second substrate  310 . 
     In the example embodiment illustrated in  FIG.  3   , an area in which the channel structure CH, the bit line  360   c , and the like are disposed may be defined as the bit line bonding area BLBA. In the bit line bonding area BLBA, the bit line  360   c  may be electrically connected to the circuit elements  220   c  providing a page buffer  393  in the peripheral circuit region PERI. The bit line  360   c  may be connected to upper bonding metals  371   c  and  372   c  in the cell region CELL, and the upper bonding metals  371   c  and  372   c  may be connected to lower bonding metals  271   c  and  272   c  connected to the circuit elements  220   c  of the page buffer  393 . 
     In the word line bonding area WLBA, the plurality of word lines  330  may extend in a second direction (an X-axis direction), parallel to the upper surface of the second substrate  310  and perpendicular to the first direction, and may be connected to a plurality of cell contact plugs  341  to  347  (i.e.,  340 ). The plurality of word lines  330  and the plurality of cell contact plugs  340   may be connected to each other in pads provided by at least a portion of the plurality of word lines  330  extending in different lengths in the second direction. A first metal layer  350   b  and a second metal layer  360   b  may be connected to an upper portion of the plurality of cell contact plugs  340  connected to the plurality of word lines  330 , sequentially. The plurality of cell contact plugs  340  may be connected to the peripheral circuit region PERI by the upper bonding metals  371   b  and  372   b  of the cell region CELL and the lower bonding metals  271   b  and  272   b  of the peripheral circuit region PERI in the word line bonding area WLBA. 
     The plurality of cell contact plugs  340  may be electrically connected to the circuit elements  220   b  forming a row decoder  394  in the peripheral circuit region PERI. In an example embodiment, operating voltages of the circuit elements  220   b  of the row decoder  394  may be different than operating voltages of the circuit elements  220   c  forming the page buffer  393 . For example, operating voltages of the circuit elements  220   c  forming the page buffer  393  may be greater than operating voltages of the circuit elements  220   b  forming the row decoder  394 . 
     A common source line contact plug  380  may be disposed in the external pad bonding area PA. The common source line contact plug  380  may be formed of a conductive material, such as a metal, a metal compound, polysilicon, or the like, and may be electrically connected to the common source line  320 . A first metal layer  350   a  and a second metal layer  360   a  may be stacked on an upper portion of the common source line contact plug  380 , sequentially. For example, an area in which the common source line contact plug  380 , the first metal layer  350   a , and the second metal layer  360   a  are disposed may be defined as the external pad bonding area PA. 
     Input-output pads  205  and  305  may be disposed in the external pad bonding area PA. Referring to  FIG.  3   , a lower insulating film  201  covering a lower surface of the first substrate  210  may be formed below the first substrate  210 , and the first input-output pad  205  may be formed on the lower insulating film  201 . The first input-output pad  205  may be connected to at least one of the plurality of circuit elements  220   a ,  220   b , and  220   c  disposed in the peripheral circuit region PERI through a first input-output contact plug  203 , and may be separated from the first substrate  210  by the lower insulating film  201 . In addition, a side insulating film may be disposed between the first input-output contact plug  203  and the first substrate  210  to electrically separate the first input-output contact plug  203  and the first substrate  210 . 
     Referring to  FIG.  3   , an upper insulating film  301  covering the upper surface of the second substrate  310  may be formed on the second substrate  310 , and the second input-output pad  305   may be disposed on the upper insulating layer  301 . The second input-output pad  305  may be connected to at least one of the plurality of circuit elements  220   a ,  220   b , and  220   c  disposed in the peripheral circuit region PERI through a second input-output contact plug  303 . In the example embodiment, the second input-output pad  305  is electrically connected to a circuit element  220   a . 
     According to an embodiment, the second substrate  310  and the common source line  320  are not disposed in an area in which the second input-output contact plug  303  is disposed. Also, in an embodiment, the second input-output pad  305  does not overlap the word lines  330  in the third direction (the Z-axis direction). Referring to  FIG.  3   , the second input-output contact plug  303  may be separated from the second substrate  310  in a direction, parallel to the upper surface of the second substrate  310 , and may pass through the interlayer insulating layer  315  of the cell region CELL to be connected to the second input-output pad  305 . 
     According to embodiments, the first input-output pad  205  and the second input-output pad  305  may be selectively formed. For example, the memory device  120  may include only the first input-output pad  205  disposed on the first substrate  210  or the second input-output pad  305  disposed on the second substrate  310 . Alternatively, the memory device  120  may include both the first input-output pad  205  and the second input-output pad  305 . 
     A metal pattern provided on an uppermost metal layer may be provided as a dummy pattern or the uppermost metal layer may be absent, in each of the external pad bonding area PA and the bit line bonding area BLBA respectively included in the cell region CELL and the peripheral circuit region PERI. 
     In the external pad bonding area PA, the memory device  120  may include a lower metal pattern  273   a , corresponding to an upper metal pattern  372   a  formed in an uppermost metal layer of the cell region CELL, and having the same cross-sectional shape as the upper metal pattern  372   a  of the cell region CELL so as to be connected to each other, in an uppermost metal layer of the peripheral circuit region PERI. In the peripheral circuit region PERI, the lower metal pattern  273   a  formed in the uppermost metal layer of the peripheral circuit region PERI may not be connected to a contact. Similarly, in the external pad bonding area PA, the upper metal pattern  372   a , corresponding to the lower metal pattern  273   a  formed in an uppermost metal layer of the peripheral circuit region PERI, and having the same shape as a lower metal pattern  273   a  of the peripheral circuit region PERI, may be formed in an uppermost metal layer of the cell region CELL. 
     The lower bonding metals  271   b  and  272   b  may be formed on the second metal layer  240   b  in the word line bonding area WLBA. In the word line bonding area WLBA, the lower bonding metals  271   b  and  272   b  of the peripheral circuit region PERI may be electrically connected to the upper bonding metals  371   b  and  372   b  of the cell region CELL by Cu-to-Cu bonding. 
     Furthermore, in the bit line bonding area BLBA, an upper metal pattern  392 , corresponding to a lower metal pattern  252  formed in the uppermost metal layer of the peripheral circuit region PERI, and having the same cross-sectional shape as the lower metal pattern  252  of the peripheral circuit region PERI, may be formed in an uppermost metal layer of the cell region CELL. In an embodiment, a contact is not formed on the upper metal pattern  392  formed in the uppermost metal layer of the cell region CELL. 
     In an example embodiment, corresponding to a metal pattern formed in an uppermost metal layer in one of the cell region CELL and the peripheral circuit region PERI, a reinforcement metal pattern having the same cross-sectional shape as the metal pattern may be formed in an uppermost metal layer in the other one of the cell region CELL and the peripheral circuit region PERI. A contact may not be formed on the reinforcement metal pattern. 
     The memory device  120  may have a memory cell array formed in a multi-stack memory block structure, the memory cell array including a plurality of cell strings respectively disposed between a plurality of bit lines and a source line in the vertical direction with respect to a substrate. In the bit line bonding area BLBA of  FIG.  3   , the plurality of word lines  331  to  338  (i.e.,  330 ) between string select lines and a ground select line may be grouped into two memory stacks, as shown in  FIG.  4   . An inter-stack portion INT-ST (see  FIG.  4   ) may be between the two memory stacks. A channel structure CH (see  FIG.  4   ) is formed even in the inter-stack portion INT-ST, and this is particularly described with reference to  FIGS.  4  and  5   . 
       FIG.  4    is a perspective view illustrating the memory block BLK 1  according to an embodiment of the inventive concept.  FIG.  5    is a cross-sectional view for describing an example of an inter-stack portion INT-ST included in the memory block BLK 1  of  FIG.  4   .  FIG.  4    representatively shows the memory block BLK 1  among the plurality of memory blocks BLK 1  to BLKn of  FIG.  2   . The memory block BLK 1  may include NAND strings or cell strings formed in a 3D structure or a vertical structure. The memory block BLK 1  may include structures extending in a plurality of directions (X-, Y-, and Z-axes directions). 
     Referring to  FIG.  4   , the memory block BLK 1  is formed in the vertical direction (the Z-axis direction) with respect to a substrate SUB. The substrate SUB may have a first conductive type (e.g., p type), and a common source line CSL doped with impurities of a second conductive type (e.g., n type) may be formed in the substrate SUB. 
     Above a region of the substrate SUB between common source lines CSL, a plurality of insulating materials IL extending in a second horizontal direction (the Y-axis direction) are sequentially disposed in the vertical direction (the Z-axis direction). For example, the plurality of insulating materials IL may be formed by being separated by a certain distance in a first horizontal direction (the X-axis direction). For example, the plurality of insulating materials IL may include an insulating material, such as silicon oxide. 
     On the substrate SUB between the common source lines CSL, a plurality of channel structures CH sequentially arranged in the second horizontal direction (the Y-axis direction) and passing through the plurality of insulating materials IL in the vertical direction (the Z-axis direction) are formed. For example, the plurality of channel structures CH may be connected to the substrate SUB by passing through the plurality of insulating materials IL. For example, each channel structure CH may include a plurality of materials. A surface layer S of each channel structure CH may include a silicon material having the first conductive type and function as a channel region. In some embodiments, the channel structure CH may be referred to as a vertical channel structure or a pillar. An inner layer I of each channel structure CH may include an insulating material, such as silicon oxide, or an air gap. 
     A charge storage layer CS is provided along the plurality of insulating materials IL, the plurality of channel structures CH, and an exposed surface of the substrate SUB. The charge storage layer CS may include a gate insulating layer (or referred to as “tunneling insulating layer”), a charge trap layer, and a blocking insulating layer. For example, the charge storage layer CS may have an oxide-nitride-oxide (ONO) structure. In addition, a gate electrode GE including a ground select line GSL, a string select line SSL, and first to eighth word lines WL 1  to WL 8  is provided on an exposed surface of the charge storage layer CS. 
     Drain contacts or drains DR are provided on the plurality of channel structures CH, respectively. For example, the drains DR may include a silicon material doped with impurities having the second conductive type. On the drains DR, bit lines, e.g., first to third bit lines BL 1  to BL 3  extending in the first horizontal direction (the X-axis direction) and separated by a certain distance in the second horizontal direction (the Y-axis direction), are disposed. 
     The memory block BLK 1  may include a first memory stack ST 1  and a second memory stack ST 2  stacked in the vertical direction (the Z-axis direction). For example, the first memory stack ST 1  may include the first to fourth word lines WL 1  to WL 4 , and the second memory stack ST 2  may include the fifth to eighth word lines WL 5  to WL 8 . The inter-stack portion INT-ST may be formed between the first memory stack ST 1  and the second memory stack ST 2  to ensure the structural stability of the memory device  120  during a process of manufacturing the memory device  120 . While  FIG.  4    illustrates each stack including four word lines, embodiments of the inventive concept are not limited thereto as each stack may include more than four word lines and a different number of word lines from one another. 
     Referring to  FIG.  5   , a channel hole constituting a cell string in each channel structure CH may include a first sub-channel hole  510  and a second sub-channel hole  520 . The first sub-channel hole  510  formed in the first memory stack ST 1  may include a channel layer  511 , an internal material  512 , and an insulating layer  513 . The second sub-channel hole  520  formed in the second memory stack ST 2  may include a channel layer  521 , an internal material  522 , and an insulating layer  523 . The channel layer  511  of the first sub-channel hole  510  may be connected to the channel layer  521  of the second sub-channel hole  520 . 
     At a connection part between the channel layer  511  of the first sub-channel hole  510  and the channel layer  521  of the second sub-channel hole  520  in the inter-stack portion INT-ST, a size  502  of the second sub-channel hole  520  may be smaller than a size  501  of the first sub-channel hole  510 . Accordingly, an overlap margin may be ensured at the connection part between the channel layer  511  of the first sub-channel hole  510  and the channel layer  521  of the second sub-channel hole  520 . To increase the overlap margin in a manufacturing process, the inter-stack portion INT-ST may be formed to be long in the vertical direction. Accordingly, a length L2 of the inter-stack portion INT-ST may be relatively greater than a gap L1 between every two of the first to fourth word lines WL 1  to WL 4  and the fifth to eighth word lines WL 5  to WL 8  in the first and second memory stacks ST 1  and ST 2 . 
     In each of the first to eighth word lines WL 1  to WL 4  and WL 5  to WL 8 , a remaining region excluding a region through which the first or second sub-channel hole  510  or  520  passes from a corresponding region of the gate electrode GE is represented by a resistance value of a corresponding one of the first to eighth word lines WL 1  to WL 4  and WL 5  to WL 8 . The greater a size of the first or second sub-channel hole  510  or  520 , which each of the first to eighth word lines WL 1  to WL 4  and WL 5  to WL 8  has, the greater a word line resistance value, and the smaller the size of the first or second sub-channel hole  510  or  520 , which each of the first to eighth word lines WL 1  to WL 4  and WL 5  to WL 8  has, the smaller the word line resistance value. 
     For example, the fourth word line WL 4  at a top of the first memory stack ST 1  may have a relatively larger size of the first sub-channel hole  510  than the other word lines, i.e., the first to third word lines WL 1  to WL 3  of the first memory stack ST 1 . Therefore, a resistance value of the fourth word line WL 4  at the top of the first memory stack ST 1  may be relatively large. The fifth word line WL 5  at a bottom of the second memory stack ST 2  may have a relatively smaller size of the second sub-channel hole  520  than the other word lines, i.e., the sixth to eighth word lines WL 6  to WL 8  of the second memory stack ST 2 . Therefore, a resistance value of the fifth word line WL 5  at the bottom of the second memory stack ST 2  may be relatively small. 
     According to a resistance value difference between the fourth word line WL 4  and the fifth word line WL 5 , which are adjacent to the inter-stack portion INT-ST, a first channel voltage VCH 1  of a cell string of the first memory stack ST 1  and a second channel voltage VCH 2  of a cell string of the second memory stack ST 2  may be differently boosted. As shown in  FIG.  5   , the first channel voltage VCH 1  associated with the fourth word line WL 4  having a large resistance value may be less boosted, and the second channel voltage VCH 2  associated with the fifth word line WL 5  having a small resistance value may be more boosted. Because the inter-stack portion INT-ST is included in a channel region of each cell string, if a difference between the first channel voltage VCH 1  and the second channel voltage VCH 2  is greater than a certain range, hot carrier injection (HCI) due to the channel voltage difference may occur. Accordingly, a threshold voltage state of a memory cell adjacent to the inter-stack portion INT-ST may be distorted. 
     In a process step of forming the first sub-channel hole  510  and the second sub-channel hole  520 , which constitute a channel hole of each cell string, channel hole profile information may be obtained. A channel hole profile may be defined in a manufacturing process step of the nonvolatile memory device  120 , and the inter-stack word line manager  129  may store channel hole profile information of word lines adjacent to the inter-stack portion INT-ST, which is defined in the manufacturing process step. In an embodiment, the inter-stack word line manager  129  determines, to be inter-stack word lines, some word lines having a large resistance value difference among word lines adjacent to the inter-stack portion INT-ST, based on the channel hole profile information. 
     The inter-stack word line manager  129  may determine, to be inter-stack word lines, word line(s) adjacent to the inter-stack portion INT-ST in each of the first and second memory stacks ST 1  and ST 2 . For example, the inter-stack word line manager  129  may determine the fourth word line WL 4  of the first memory stack ST 1  and the fifth word line WL 5  of the second memory stack ST 2  to be inter-stack word lines of the inter-stack portion INT-ST. To prevent HCI due to a difference between the first channel voltage VCH 1  and the second channel voltage VCH 2  in the inter-stack portion INT-ST, the inter-stack word line manager  129  may control operating time points of the inter-stack word lines (e.g., the fourth and fifth word lines WL 4  and WL 5 ). By controlling the operating time points of the inter-stack word lines (e.g., the fourth and fifth word lines WL 4  and WL 5 ), channel potential equalization of the inter-stack portion INT-ST may be implemented. 
       FIG.  6    is an equivalent circuit diagram of the memory block BLK 1  of  FIG.  4   . 
     Referring to  FIG.  6   , the memory block BLK 1  may include NAND strings, e.g., cell strings NS 11  to NS 33 , the first to eighth word lines WL 1  to WL 8 , the first to third bit lines BL 1  to BL 3 , ground select lines, e.g., first to third ground select lines GSL 1  to GSL 3 , string select lines, e.g., first to third string select lines SSL 1  to SSL 3 , and a common source line CSL. Although  FIG.  6    shows that each of the cell strings NS 11  to NS 33  includes eight memory cells MC respectively connected to eight word lines, e.g., the first to eighth word lines WL 1  to WL 8 , the inventive concept is not limited thereto. 
     Each cell string (e.g., NS 11) may include a string select transistor SST, a plurality of memory cells MC, and a ground select transistor GST, which are connected in series. The string select transistor SST is connected to a corresponding string select line, e.g., the first string select line SSL 1 . The plurality of memory cells MC are connected to corresponding word lines, e.g., the first to eighth word lines WL 1  to WL 8 , respectively. The ground select transistor GST is connected to a corresponding ground select line, e.g., the first ground select line GSL 1 . The string select transistor SST is connected to a corresponding bit line BL 1 , BL 2 , or BL 3 , and the ground select transistor GST is connected to the common source line CSL. 
     According to an embodiment, in each cell string, one or more dummy memory cells may be provided between a string select transistor SST and memory cells MC. In each cell string, one or more dummy memory cells may be provided between a ground select transistor GST and memory cells MC. In each cell string, one or more dummy memory cells may be provided between memory cells MC. The dummy memory cells have the same structure as the memory cells MC and may not be programmed (e.g., program-prohibited) or may be programmed to be different from the memory cells MC. For example, when the memory cells MC are programmed to have two or more threshold voltage distributions, the dummy memory cells may be programmed to have one threshold voltage distribution range or a less number of threshold voltage distributions than the memory cells MC. 
       FIG.  7    illustrates a threshold voltage distribution when write data is written to the memory cells MC shown in  FIG.  6   . In  FIG.  7   , the horizontal axis indicates threshold voltages of the memory cells MC, and the vertical axis indicates cell counts, i.e., the number of memory cells MC. 
     Referring to  FIGS.  6  and  7   , one or more bits may be programmed in a memory cell MC. According to the number of bits stored in the memory cell MC, the memory cell MC may be classified into a single-level cell (SLC), an MLC, a TLC, or a QLC. The memory cell MC may have a plurality of states according to the numbers of bits stored in the memory cell MC. The plurality of states may be defined by ranges of a threshold voltage. In  FIG.  7   , the memory cell MC may be a QLC, and a threshold voltage of the memory cell MC may be programmed to one of 16 states S 1  to S 16 . Each of the states S 1  to S 16  may correspond to a distribution range of a threshold voltage Vth of memory cells MC, respectively. 
       FIG.  8    is a circuit diagram illustrating a program bias condition according to ab embodiment of the inventive concept.  FIG.  8    shows, for convenience, the cell strings NS 11  and NS 21  connected to the first bit line BL 1  and the cell strings NS 12  and NS 22  connected to the second bit line BL 2  among the cell strings NS 11  to NS 33  (see  FIG.  6   ). 
     Referring to  FIG.  8   , the first bit line BL 1  is a program permission bit line to which a relatively low program permission voltage VPER (see  FIGS.  9 A and  9 B ), e.g., a ground voltage VSS, is applied, and the second bit line BL 2  is a program inhibition bit line to which a relatively high program inhibition voltage VINH (see  FIGS.  9 A and  9 B ), e.g., a power source voltage VDD, is applied. 
     If it is assumed that the cell string NS 21  between the cell strings NS 11  and NS 21  connected to the first bit line BL 1  is selected, in a program period PROGRAM (see  FIGS.  9 A and  9 B ), a turn-off voltage of a level of the ground voltage VSS may be applied to the first string select line SSL 1  connected to the cell string NS 11 , and a first turn-on voltage VSSL1, e.g., the power source voltage VDD, greater than or equal to the threshold voltage Vth of each string select transistor SST may be applied to the second string select line SSL 2  connected to the cell string NS 21 . 
     The turn-off voltage of the level of the ground voltage VSS may be applied to the first and second ground select lines GSL 1  and GSL 2 , and the ground voltage VSS may be applied to the common source line CSL. A program voltage VPGM (see  FIGS.  9 A and  9 B ) (e.g., 18 V) may be applied to a selected word line (e.g., WL 2 ), a program pass voltage VPASS 1 (see  FIGS.  9 A and  9 B ) (e.g., 8 V) may be applied to unselected word lines (e.g., WL 1  and WL 3 ), and the program pass voltage VPASS1 (see  FIGS.  9 A and  9 B ) may also be applied to inter-stack word lines (e.g., WL 4  and WL 5 ). Although not shown, the unselected word lines may also include the sixth, seventh, and eighth word lines WL 6 , WL 7 , and WL 8 . The program pass voltage VPASS1 may be set to a voltage by which a plurality of memory cells are turned on or always turned on. 
     Under this program bias condition, 18 V is applied to a gate of a memory cell A, and a channel voltage is 0 V. Because a strong electric field is formed between the gate of the memory cell A and a channel, the memory cell A is programmed. Meanwhile, because a channel voltage of a memory cell B is the power source voltage VDD, and because a weak electric field is formed between a gate of the memory cell B and a channel, the memory cell B is not programmed. Because channels of memory cells C and D are in a floating state, a channel voltage is boosted to a boosting level due to the program pass voltage VPASS 1, and the memory cells C and D are not programmed. 
       FIGS.  9 A and  9 B  are timing diagrams for describing a program operation according to embodiments of the inventive concept.  FIGS.  9 A and  9 B  assume that, with respect to the memory cell A programmed in the cell string NS 21 , a selected word line WLs indicates the second word line WL 2 , unselected word lines WLu indicate the first, third, sixth, seventh, and eighth word lines WL 1 , WL 3 , WL 6 , WL 7 , and WL 8 , inter-stack word lines indicate the fourth and fifth word lines WL 4  and WL 5 , a selected string select line SSL indicates the second string select line SSL 2 , an unselected string select line SSL indicates the first string select line SSL 1 , a selected ground select line GSL indicates the second ground select line GSL 2 , an unselected ground select line GSL indicates the first ground select line GSL 1 , a program permission bit line BL indicates the first bit line BL 1 , and a program inhibition bit line BL indicates the second bit line BL 2 . 
     Referring to  FIG.  9 A , for memory cells connected to the selected word line WLs, a plurality of program loops LOOP 1 , LOOP 2 , LOOP 3 , ... may be sequentially performed until a program has completed according to an ISPP. As the plurality of program loops LOOP 1 , LOOP 2 , LOOP 3 , ... are progressive, program voltages VPGM1, VPGM2, VPGM3, ... may increase step-by-step. Each program loop LOOPi (i is a natural number) may include a program period PROGRAM, in which the program voltage VPGM is applied to the selected word line WLs to program a selected memory cell, and a verify period VERIFY, in which a verify voltage VVFY is applied to the selected word line WLs to verify whether the programming is successful. 
     The program period PROGRAM may include an inter-stack word line setup period PIWLS, a channel precharge and bit line setup period PBLS, a program execution period PEXE, and a program recovery period PRCV. For example, the program period PROGRAM may occur when a program operation is performed on memory cells connected to the selected word line WLs such as WL 2 . 
     In the inter-stack word line setup period PIWLS, a channel potential equalization operation on the inter-stack portion INT-ST may be performed by controlling the inter-stack word lines WL 4  and WL 5  adjacent to the inter-stack portion INT-ST. In the inter-stack word line setup period PIWLS, the program pass voltage VPASS1 may be applied to the inter-stack word lines WL 4  and WL 5 . According to information indicating that a resistance value of the inter-stack word line WL 4  among the inter-stack word lines WL 4  and WL 5  is greater than a resistance value of the inter-stack word line WL 5 , the program pass voltage VPASS1 is applied to the inter-stack word line WL 4  at a time point T a   1  and applied to the inter-stack word line WL 5  at a time point T a   2 . That is, for channel potential equalization, the program pass voltage VPASS1 is first applied to the inter-stack word line WL 4  having a large resistance value and later applied to the inter-stack word line WL 5  having a small resistance value. 
     In the channel precharge and bit line setup period PBLS, an operation of initializing or precharging channels of a plurality of unselected cell strings (hereinafter, referred to as unselect string initial precharge (USIP)) may be performed before a program loop by using a gate induced drain leakage (GIDL) phenomenon. In the channel precharge and bit line setup period PBLS, the power source voltage VDD, which is the program inhibition voltage VINH, may be applied to the program inhibition bit line BL, and the ground voltage VSS of a program permission voltage level may be applied to the program permission bit line BL. The first turn-on voltage VSSL1 may be applied to the selected string select line SSL, and the turn-off voltage (e.g., the ground voltage VSS) may be applied to the unselected string select line SSL. A turn-on voltage VGSL may be applied to the ground select lines GSL, and the power source voltage VDD, which is a precharge voltage VPC, may be applied to the common source line CSL. 
     In the channel precharge and bit line setup period PBLS, the channels of the plurality of unselected cell strings may be initialized by a gate induced drain leakage (GIDL) phenomenon induced by a selected string select transistor SST connected to the selected string select line SSL and initialized by the GIDL phenomenon induced by ground select transistors GST connected to the ground select lines GSL. Thereafter, the first turn-on voltage VSSL1 may be maintained for the selected string select line SSL, the turn-off voltage may be applied to the unselected string select line SSL, the ground voltage VSS may be applied to the common source line CSL and the ground select lines GSL. 
     In the program execution period PEXE, the program pass voltage VPASS1 may be applied to the selected word line WLs and the unselected word lines WLu, and after a certain time elapses, the program voltage VPGM may be applied to the selected word line WLs. 
     In the program recovery period PRCV, the bit lines BL, the string select lines SSL, the selected word line WLs, the unselected word lines WLu, and the inter-stack word lines WL 4  and WL 5  may be recovered or set to the ground voltage VSS. In this case, the inter-stack word line WL 5  having a small resistance value among the inter-stack word lines WL 4  and WL 5  may be recovered or set to the ground voltage VSS at a time point T b   1 , and the inter-stack word line WL 4  having a large resistance value may be recovered or set to the ground voltage VSS at a time point T b   2 . That is, in the program recovery period PRCV, the inter-stack word line WL 5  having a small resistance value may be program-recovered first, and the inter-stack word line WL 4  having a large resistance value may be program-recovered later. 
     According to an embodiment, in the program recovery period PRCV, the selected word line WLs, the unselected word lines WLu, and the inter-stack word lines WL 4  and WL 5  may be recovered to a recovery voltage VRCV, as shown in  FIG.  9 B . The control circuit  124  of the nonvolatile memory device  120  may determine the recovery voltage VRCV by considering characteristics of memory cells MC having an initial state after a manufacturing process and/or characteristics of the memory cell A having a programmed state in a cell string. The recovery voltage VRCV may be set the same as an external power source voltage applied to the nonvolatile memory device  120  or an internal power source voltage generated from the external power source voltage in the nonvolatile memory device  120 . In this case, the inter-stack word line WL 5  having a small resistance value among the inter-stack word lines WL 4  and WL 5  may be recovered to the recovery voltage VRCV at the time point T b   1 , and the inter-stack word line WL 4  having a large resistance value may be recovered to the recovery voltage VRCV at the time point T b   2 . That is, in the program recovery period PRCV, the inter-stack word line WL 5  having a small resistance value may be program-recovered first, and the inter-stack word line WL 4  having a large resistance value may be program-recovered later. 
     In  FIGS.  9 A and  9 B , channel potential equalization of the inter-stack portion INT-ST in the program period PROGRAM may be implemented by applying the program pass voltage VPASS1 first to the inter-stack word line WL 4  having a large resistance value and later to the inter-stack word line WL 5  having a small resistance value in the inter-stack word line setup period PIWLS, and recovering first the inter-stack word line WL 5  having a small resistance value to the ground voltage VSS or the recovery voltage VRCV and later recovering the inter-stack word line WL 4  having a large resistance value to the ground voltage VSS or the recovery voltage VRCV in the program recovery period PRCV. 
     In  FIG.  9 A , the verify period VERIFY may include the inter-stack word line setup period RIWLS, a verify read period RVFY, and a read recovery period RRCV. Similar to the inter-stack word line setup period PIWLS of the program period PROGRAM, in the inter-stack word line setup period PIWLS, a read pass voltage VPASS2 may be applied first to the inter-stack word line WL 4  having a large resistance value and later to the inter-stack word line WL 5  having a small resistance value. The read pass voltage VPASS2 may be applied to the inter-stack word line WL 4  at a time point T c   1  and applied to the inter-stack word line WL 5  at a time point T c   2 . In an embodiment, the read pass voltage VPASS2 is higher than the program pass voltage VPASS1. 
     In the verify read period RVFY, the read pass voltage VPASS2, by which a memory cell is turned on or always turned on regardless of a program state of the memory cell, may be applied to the unselected word lines WLu, the verify voltage VVFY may be applied to the selected word line WLs, the turn-on voltage VGSL may be applied to the selected ground select line GSL, and the turn-off voltage (i.e., the ground voltage VSS) may be applied to the unselected ground select line GSL. 
     In the read recovery period RRCV, the bit lines BL, the string select lines SSL, the selected word line WLs, the unselected word lines WLu, and the inter-stack word lines WL 4  and WL 5  may be recovered or set to the ground voltage VSS. Similar to the program recovery period PRCV of the program period PROGRAM, in the read recovery period RRCV, the inter-stack word line WL 5  having a small resistance value may be first read-recovered, and the inter-stack word line WL 4  having a large resistance value may be read-recovered later. The inter-stack word line WL 5  may be first recovered to the ground voltage VSS at a time point T d   1 , and the inter-stack word line WL 4  may be recovered to the ground voltage VSS at a time point T d   2 . 
     According to an embodiment, in the read recovery period RRCV, the selected word line WLs, the unselected word lines WLu, and the inter-stack word lines WL 4  and WL 5  may be recovered to the recovery voltage VRCV as shown in  FIG.  9 B . The inter-stack word line WL 5  having a small resistance value among the inter-stack word lines WL 4  and WL 5  may be recovered or set to the recovery voltage VRCV at the time point T d   1 , and the inter-stack word line WL 4  having a large resistance value may be recovered or set to the recovery voltage VRCV at the time point T d   2 . That is, in the read recovery period RRCV, the inter-stack word line WL 5  having a small resistance value may be first recovered, and the inter-stack word line WL 4  having a large resistance value may be recovered later. 
     In  FIGS.  9 A and  9 B , channel potential equalization of the inter-stack portion INT-ST in the verify period VERIFY may be implemented by applying the program pass voltage VPASS1 first to the inter-stack word line WL 4  having a large resistance value and later to the inter-stack word line WL 5  having a small resistance value in the inter-stack word line setup period PIWLS, and recovering first the inter-stack word line WL 5  having a small resistance value to the ground voltage VSS and later recovering the inter-stack word line WL 4  having a large resistance value to the ground voltage VSS in the read recovery period RRCV. 
       FIG.  10    is a cross-sectional view for describing an example of the inter-stack portion INT-ST included in the memory block BLK 1  of  FIG.  4   .  FIG.  11    is a timing diagram for describing a program operation according to an embodiment of the inventive concept. In the program operation of  FIG.  11   , a timing diagram of inter-stack word lines WL 3 , WL 4 , WL 5 , and WL 6  of  FIG.  10    is shown, and a timing diagram of the selected word line WLs, the unselected word lines WLu, the selected/unselected string select lines SSL, the selected/unselected ground select lines GSL, and the program permission/inhibition bit lines BL described with reference to  FIGS.  9 A and  9 B  is omitted herein for simplification of drawing. 
     Referring to  FIGS.  10  and  11   , in the inter-stack word line setup period PIWLS of the program period PROGRAM, the program pass voltage VPASS1 is applied to the inter-stack word line WL 4  of the first memory stack ST 1  at a time point T a   1 , applied to the inter-stack word line WL 3  of the first memory stack ST 1  at a time point T a   2 , applied to the inter-stack word line WL 5  of the second memory stack ST 2  at a time point T a   3 , and applied to the inter-stack word line WL 6  of the second memory stack ST 2  at a time point T a   4 . That is, in the inter-stack word line setup period PIWLS of the program period PROGRAM, the program pass voltage VPASS1 is applied first to the inter-stack word line WL 4  closer to the inter-stack portion INT-ST among the inter-stack word lines WL 3  and WL 4  having a large resistance value and applied later to the inter-stack word line WL 6  farther from the inter-stack portion INT-ST among the inter-stack word lines WL 5  and WL 6  having a small resistance value. 
     In the program recovery period PRCV, the inter-stack word line WL 3  of the first memory stack ST 1  is recovered or set to the ground voltage VSS or the recovery voltage VRCV at a time point T b   1 , the inter-stack word line WL 4  of the first memory stack ST 1  is recovered or set to the ground voltage VSS or the recovery voltage VRCV at a time point T b   2 , the inter-stack word line WL 6  of the second memory stack ST 2  is recovered or set to the ground voltage VSS or the recovery voltage VRCV at a time point T b   3 , and the inter-stack word line WL 5  of the second memory stack ST 2  is recovered or set to the ground voltage VSS or the recovery voltage VRCV at a time point T b   4 . That is, in the program recovery period PRCV, the inter-stack word line WL 3  farther from the inter-stack portion INT-ST among the inter-stack word lines WL 3  and WL 4  having a large resistance value is first program-recovered, and the inter-stack word line WL 5  closer to the inter-stack portion INT-ST among the inter-stack word lines WL 5  and WL 6  having a small resistance value is program-recovered later. 
     In the inter-stack word line setup period RIWLS of the verify period VERIFY, the read pass voltage VPASS2 is applied to the inter-stack word line WL 4  of the first memory stack ST 1  at a time point T c   1 , applied to the inter-stack word line WL 3  of the first memory stack ST 1  at a time point T c   2 , applied to the inter-stack word line WL 5  of the second memory stack ST 2  at a time point T c   3 , and applied to the inter-stack word line WL 6  of the second memory stack ST 2  at a time point T c   4 . That is, in the inter-stack word line setup period RIWLS of the verify period VERIFY, the read pass voltage VPASS2 is applied first to the inter-stack word line WL 4  closer to the inter-stack portion INT-ST among the inter-stack word lines WL 3  and WL 4  having a large resistance value and applied later to the inter-stack word line WL 6  farther from the inter-stack portion INT-ST among the inter-stack word lines WL 5  and WL 6  having a small resistance value. In an embodiment, the read pass voltage VPASS2 is higher than the program pass voltage VPASS 1. 
     In the read recovery period RRCV of the verify period VERIFY, the inter-stack word line WL 3  of the first memory stack ST 1  is recovered to the ground voltage VSS or the recovery voltage VRCV at a time point T d   1 , the inter-stack word line WL 4  of the first memory stack ST 1  is recovered to the ground voltage VSS or the recovery voltage VRCV at a time point T d   2 , the inter-stack word line WL 6  of the second memory stack ST 2  is recovered to the ground voltage VSS or the recovery voltage VRCV at a time point T d   3 , and the inter-stack word line WL 5  of the second memory stack ST 2  is recovered to the ground voltage VSS or the recovery voltage VRCV at a time point T d   4 . That is, in the read recovery period RRCV of the verify period VERIFY, the inter-stack word line WL 3  farther from the inter-stack portion INT-ST among the inter-stack word lines WL 3  and WL 4  having a large resistance value is first read-recovered, and the inter-stack word line WL 5  closer to the inter-stack portion INT-ST among the inter-stack word lines WL 5  and WL 6  having a small resistance value is read-recovered later. 
       FIG.  12    is a flowchart illustrating a method of operating a nonvolatile memory device, according to an embodiment of the inventive concept. 
     Referring to  FIG.  12   , in operation S 1210 , a memory cell array including a plurality of cell strings, in which a plurality of memory cells are respectively disposed in the vertical direction between a plurality of bit lines and a source line, is divided into a plurality of stacks. The memory cell or a memory block may be divided into two memory stacks as described with reference to  FIG.  5    or divided into three memory stacks as to be described with reference to  FIG.  15   . Although not shown, it will be understood by those of ordinary skill in the art that the memory block may be divided into four or more memory stacks in a similar manner. 
     In operation S 1220 , the inter-stack word line manager  129  determines, as inter-stack word lines, some word lines adjacent to the inter-stack portions INT-ST among word lines of the plurality of memory stacks based on channel hole profile information. The channel hole profile information of the inter-stack word lines may be stored in the memory device  120  during a manufacturing process step of the nonvolatile memory device  120 . 
     In operation S 1230 , the inter-stack word lines determined by the inter-stack word line manager  129  are sequentially set according to resistance values of the inter-stack word lines. The inter-stack word line manager  129  may first set an inter-stack word line having a large resistance value and later set an inter-stack word line having a small resistance value when the inter-stack word lines are set to the pass voltage. In the inter-stack word line setup period PIWLS, as shown in  FIGS.  9 A and  9 B , the inter-stack word line manager  129  first sets the inter-stack word line WL 4  having a large resistance value, which is adjacent to the inter-stack portion INT-ST, and later sets the inter-stack word line WL 5  having a small resistance value. As shown in  FIG.  11   , the inter-stack word line manager  129  first sets the inter-stack word line WL 4  closer to the inter-stack portion INT-ST among the inter-stack word lines WL 3  and WL 4  having a large resistance value and later sets the inter-stack word line WL 6  farther from the inter-stack portion INT-ST among the inter-stack word lines WL 5  and WL 6  having a small resistance value. 
     In operation S 1240 , a channel voltage of the plurality of cell strings is initialized or equalized by sequentially setting up the inter-stack word lines of operation S 1230  according to resistance values. 
     In operation S 1250 , a program or read operation on a selected cell string may be performed. The program or read operation on the selected cell string may be performed based on bias conditions of the program execution period PEXE or the verify read period RVFY described with reference to  FIGS.  9 A and  9 B  or a read period RD to be described with reference to  FIG.  14   . 
     In operation S 1260 , the inter-stack word lines are recovered in a recovery operation performed after the program or read operation. The inter-stack word line manager  129  may first recover an inter-stack word line having a small resistance value and later recover an inter-stack word line having a large resistance value when the inter-stack word lines are recovered to the ground voltage VSS or the recovery voltage VRCV. In the program recovery period PRCV and the read recovery period RRCV, as shown in  FIGS.  9 A and  9 B , the inter-stack word line manager  129  first recovers the inter-stack word line WL 3  having a smaller resistance value among the inter-stack word lines WL 3  and WL 4  and later recovers the inter-stack word line WL 3  having a greater resistance value. As shown in  FIG.  11   , the inter-stack word line manager  129  first recovers the inter-stack word line WL 3  farther from the inter-stack portion INT-ST among the inter-stack word lines WL 3  and WL 4  having a large resistance value and later recovers the inter-stack word line WL 5  closer to the inter-stack portion INT-ST among the inter-stack word lines WL 5  and WL 6  having a small resistance value. 
       FIG.  13    illustrates a read operation associated with the threshold voltage distribution of the memory cells shown in  FIG.  7   . 
     Referring to  FIGS.  2 ,  7 , and  13   , when each of the memory cells is a QLC, a state of each of the memory cells may correspond to one of 16 states, e.g., first to sixteenth states S1 to S16. Memory cells connected to one word line WL may include a least significant bit (LSB) page, a first center significant bit (CSB 1) page, a second center significant bit (CSB 2 ) page, and a most significant bit (MSB) page. 
     The control circuit  124  may perform an operation of searching for valley positions VR1 to VR15 of a threshold voltage of a memory cell, an operation of inferring optimal read voltages (e.g., first to fifteenth read voltages VRD1 to VRD15) based on the valley positions VR 1  to VR15, and a page read operation on each of the LSB page, the CSB 1  page, the CSB 2  page, and the MSB page by using the first to fifteenth read voltages VRD1 to VRD15. 
     In the read operation on the LSB page, the eleventh and twelfth states S11 and S12 may be determined by applying the eleventh read voltage VRD11 to the selected word line WL, and then the sixth and seventh states S6 and S7, the fourth and fifth states S4 and S5, and the first and second states S1 and S2 may be determined by sequentially applying the sixth read voltage VRD6, the fourth read voltage VRD4, and the first read voltage VRD1 to the selected word line WL, respectively. 
     In the read operation on the CSB 1  page, the memory device  120  may determine the thirteenth and fourteenth states S 13  and S 14 , the ninth and tenth states S9 and S 10 , the seventh and eighth states S7 and S8, and the third and fourth states S3 and S4 may be determined by sequentially applying the thirteenth read voltage VRD13, the ninth read voltage VRD9, the seventh read voltage VRD7, and the third read voltage VRD3 to the selected word line WL, respectively. 
     In the read operation on the CSB 2  page, the memory device  120  may determine the fourteenth and fifteenth states S 14  and S 15 , the eighth and ninth states S8 and S9, and the second and third states S2 and S3 by sequentially applying the fourteenth read voltage VRD14, the eighth read voltage VRD8, and the second read voltage VRD2 to the selected word line WL, respectively. 
     In the read operation on the MSB page, the memory device  120  may determine the fifteenth and sixteenth states S 15  and S 16 , the twelfth and thirteenth states S 12  and S 13 , the tenth and eleventh states S 10  and S 11 , and the fifth and sixth states S5 and S6 by sequentially applying the fifteenth read voltage VRD15, the twelfth read voltage VRD12, the tenth read voltage VRD10, and the fifth read voltage VRD5 to the selected word line WL, respectively. 
       FIG.  14    is a timing diagram for describing a read operation according to an embodiment of the inventive concept. The read operation of  FIG.  14    may be similar to the verify period VERIFY of the program operation of  FIG.  9   . 
     Referring to  FIGS.  8 ,  10 , and  14   , a read period READ includes the inter-stack word line setup period RIWLS, a read period RD, and a read recovery period RRCV. Similar to the inter-stack word line setup period PIWLS of the verify period VERIFY, in the inter-stack word line setup period PIWLS, the read pass voltage VPASS2 is applied first to the inter-stack word line WL 4  closer to the inter-stack portion INT-ST among the inter-stack word lines WL 3  and WL 4  having a large resistance value and applied later to the inter-stack word line WL 6  farther from the inter-stack portion INT-ST among the inter-stack word lines WL 5  and WL 6  having a small resistance value. The read pass voltage VPASS2 is applied to the inter-stack word line WL 4  of the first memory stack ST 1  at a time point T e   1 , applied to the inter-stack word line WL 3  of the first memory stack ST 1  at a time point T e   2 , applied to the inter-stack word line WL 5  of the second memory stack ST 2  at a time point T e   3 , and applied to the inter-stack word line WL 6  of the second memory stack ST 2  at a time point T e   4 . 
     In the read period RD, the read pass voltage VPASS2, by which a memory cell is always turned on regardless of a program state of the memory cell, may be applied to the unselected word lines WLu, the read voltage VRD of  FIG.  12    may be applied to the selected word line WLs, the turn-on voltage VGSL may be applied to the selected ground select line GSL, and the turn-off voltage (i.e., the ground voltage VSS) may be applied to the unselected ground select line GSL. 
     In the read recovery period RRCV, the selected word line WLs, the unselected word lines WLu, and the inter-stack word lines WL 4  and WL 5  may be recovered or set to the ground voltage VSS or the recovery voltage VRCV. Similar to the read recovery period RRCV of the verify period VERIFY, in the read recovery period RRCV, the inter-stack word line WL 3  farther from the inter-stack portion INT-ST among the inter-stack word lines WL 3  and WL 4  having a large resistance value is first read-recovered, and the inter-stack word line WL 5  closer to the inter-stack portion INT-ST among the inter-stack word lines WL 5  and WL 6  having a small resistance value is read-recovered later. The inter-stack word line WL 3  of the first memory stack ST 1  is recovered or set to the ground voltage VSS or the recovery voltage VRCV at a time point T f   1 , the inter-stack word line WL 4  of the first memory stack ST 1  is recovered or set to the ground voltage VSS or the recovery voltage VRCV at a time point T f   2 , the inter-stack word line WL 6  of the second memory stack ST 2  is recovered or set to the ground voltage VSS or the recovery voltage VRCV at a time point T f   3 , and the inter-stack word line WL 5  of the second memory stack ST 2  is recovered or set to the ground voltage VSS or the recovery voltage VRCV at a time point T f   4 . 
       FIG.  15    is a cross-sectional view illustrating a memory block BLKla according to an embodiment of the inventive concept. Hereinafter, a subscript (e.g., a in BLK1a) attached to the same reference sign in different drawings is used to identify a plurality of circuits configured to perform similar functions or the same function. For example, memory block of the memory cell array  122  of  FIG.  2    may be replaced with the memory block BLK1a. 
     Referring to  FIGS.  2  and  15   , the memory block BLKla include three memory stacks ST 1 , ST 2 , and ST 3 . Inter-stack portions  1510  and  1520  are included between the memory stacks ST 1 , ST 2 , and ST 3 . For example, a first inter-stack portion  1510  is located between stacks ST 1  and ST 2  and a second inter-stack portion  1520  is located between stacks ST 2  and ST 3 . 
     The inter-stack word line manager  129  may store channel hole profile information of word lines adjacent to the inter-stack portions  1510  and  1520 . The channel hole profile information may be defined in a manufacturing process step. The channel hole profile information may also be store outside of the inter-stack word line manager  129  and be accessible to the inter-stack word line manager  129 . The inter-stack word line manager  129  may determine, as inter-stack word lines  1512  and  1522 , one word line adjacent to each of the inter-stack portions  1510  and  1520  among word lines of each of the memory stacks ST 1 , ST 2 , and ST 3  based on the channel hole profile information. According to an embodiment, the inter-stack word line manager  129  determines, as inter-stack word lines  1514  and  1524 , two word lines adjacent to each of the inter-stack portions  1510  and  1520  among the word lines of each of the memory stacks ST 1 , ST 2 , and ST 3  based on the channel hole profile information. 
     The inter-stack word line manager  129  may perform a channel voltage equalization operation of a plurality of memory stacks while differently controlling setup time points for applying the pass voltage to the inter-stack word lines  1512 ,  1522 ,  1514 , and  1524   according to resistance values of the inter-stack word lines  1512 ,  1522 ,  1514 , and  1524  The inter-stack word line manager  129  may first set an inter-stack word line having a large resistance value and later set an inter-stack word line having a small resistance value when the inter-stack word lines  1512 ,  1522 ,  1514 , and  1524  are set to the pass voltage. 
     The inter-stack word line manager  129  may perform a channel voltage equalization operation of the plurality of memory stacks while differently controlling recovery time points for applying the ground voltage VSS or the recovery voltage VRCV to the inter-stack word lines  1512 ,  1522 ,  1514 , and  1524  according to resistance values of the inter-stack word lines  1412 ,  1422 ,  1414 , and  1424 . The inter-stack word line manager  129  may first recover an inter-stack word line having a small resistance value and later recover an inter-stack word line having a large resistance value when the inter-stack word lines  1512 ,  1522 ,  1514 , and  1524  are recovered to the ground voltage VSS or the recovery voltage VRCV. 
       FIG.  16    is a block diagram illustrating a system  3000  including a nonvolatile memory device, according to an embodiment of the inventive concept. 
     Referring to  FIG.  16   , the system  3000  may include a camera  3100 , a display  3200 , an audio processor  3300 , a modem  3400 , dynamic random access memories (DRAMs)  3500   a  and  3500   b , flash memory devices  3600   a  and  3600   b , I/O devices  3700   a  and  3700   b , and an application processor (hereinafter, referred to as “AP”)  3800 . The system  3000  may be implemented by a laptop computer, a mobile phone, a smartphone, a tablet personal computer (PC), a wearable device, a healthcare device, or an Internet of Things (IoT) device. Alternatively, the system  3000  may be implemented by a server or a PC. 
     The camera  3100  may capture a still image or a video under control by a user and store the captured image/video data or transmit the same to the display  3200 . The audio processor  3300  may process audio data included in content of the flash memory devices  3600   a  and  3600   b  or a network. The modem  3400  may modulate and transmit a signal and demodulate a modulated signal to an original signal at a reception side, for wired/wireless data transmission and reception. The I/O devices  3700   a  and  3700   b  may include devices configured to provide a digital unput function and/or a digital output function, such as a universal serial bus (USB) or a storage, a digital camera, an SD card, a digital versatile disc (DVD), a network adapter, and a touchscreen. 
     The AP  3800  may control a general operation of the system  3000 . The AP  3800  may control the display  3200  so that a portion of content stored in the flash memory devices  3600   a   and  3600   b  is displayed on the display  3200 . When a user input is received via the I/O devices  3700   a  and  3700   b , the AP  3800  may perform a control operation corresponding to the user input. The AP  3800  may include a controller  3810  and an interface  3830  and include an accelerator block, which is a circuit for artificial intelligence (AI) data computation, or have an accelerator chip  3820 , which is separated from the AP  3800 . The DRAM  3500   b  may be additionally mounted on the accelerator block or the accelerator chip  3820 . An accelerator is a functional block configured to expertly perform a particular function and may include a graphics processing unit (GPU), which is a functional block configured to expertly perform graphics data processing, a neural processing unit (NPU), which is a block configured to expertly perform AI computation and inference, and a data processing unit (DPU), which is a block configured to expertly perform data transmission. 
     The system  3000  may include a plurality of DRAMs  3500   a  and  3500   b . The AP  3800  may control the DRAMs  3500   a  and  3500   b  by a command according to a Joint Electron Device Engineering Council (JEDEC) standard and a mode register set (MRS) or perform communication by setting a DRAM interface protocol to use company-specific functions, such as low voltage/high speed/reliability, and cyclic redundancy check (CRC)/error correction code (ECC) functions. For example, the AP  3800  may communicate with the DRAM  3500   a  by using an interface according to a JEDEC standard, such as fourth generation low power double data rate (LPDDR4) or fifth generation LPDDR (LPDDR5), and the accelerator block or the accelerator chip  3820  may perform communication by setting a new DRAM interface protocol to control the DRAM  3500   b  for an accelerator, which has a wider bandwidth than the DRAM  3500   a . 
     Although  FIG.  16    shows only the DRAMS  3500   a  and  3500   b , the inventive concept is not limited thereto, and any memory, such as phase change random access memory (PRAM), static random access memory (SRAM), magnetic random access memory (MRAM), resistive random access memory (RRAM), ferroelectric random access memory (FRAM), or hybrid RAM, may be used only if a bandwidth, a reaction speed, and a voltage condition of the AP  3800  or the accelerator chip  3820  are satisfied. The DRAMs  3500   a  and  3500   b  have a relatively lower latency and narrower bandwidth than the I/O devices  3700   a  and  3700   b  or the flash memory devices  3600   a  and  3600   b . The DRAMs  3500   a  and  3500   b  may be initialized when the system  3000  is turned on, and used as a temporary storage of an operating system and application data after the operating system and the application data are loaded therein, or used as an execution space of various kinds of software codes. 
     In the DRAMs  3500   a  and  3500   b , the four fundamental arithmetic operations of addition/subtraction/multiplication/division, a vector operation, an address operation, or a fast Fourier transform (FFT) operation may be performed. In addition, in the DRAMs  3500   a  and  3500   b , a functional function used for inference may be performed. Herein, inference may be performed in a deep learning algorithm using an artificial neural network. The deep learning algorithm may include a training step of training a model by using various pieces of data and an inference step of recognizing data by using the trained model. As an embodiment, an image captured by a user using the camera  3100  may be signal-processed and stored in the DRAM  3500   b , and the accelerator block or the accelerator chip  3820  may perform AI data computation for recognizing data by using data stored in the DRAM  3500   b  and a function used for inference. 
     The system  3000  may include a plurality of storages or a plurality of flash memory devices  3600   a  and  3600   b , which have a greater capacity than the DRAMs  3500   a  and  3500   b . The accelerator block or the accelerator chip  3820  may perform the training step and the AI data computation by using the flash memory devices  3600   a  and  3600   b . As an embodiment, the flash memory devices  3600   a  and  3600   b  may more efficiently perform the training step and the inference step (the AI data computation), which the AP  3800  and/or the accelerator chip  3820  performs, by using a computation device included in a memory controller  3610 . The flash memory devices  3600   a  and  3600   b  may store a picture taken using the camera  3100  or store data received via a data network. For example, augmented reality, virtual reality, high definition (HD), or ultra high definition (UHD) content may be stored. 
     The flash memory devices  3600   a  and  3600   b  may include the inter-stack word line manager  129  described with reference to  FIGS.  1  to  15   . The inter-stack word line manager  129  may store channel hole profile information of word lines adjacent to an inter-stack portion defined in a manufacturing process step. The channel hole profile information may be stored in the flash memory devices  3600   a  and  3600   b  outside of the inter-stack word line manager  129 , but be accessible to the inter-stack word line manager  129 . The inter-stack word line manager  129  may determine, as inter-stack word lines, some word lines adjacent to inter-stack portions among word lines of memory stacks based on the channel hole profile information. The inter-stack word line manager  129  may perform a channel voltage equalization operation of a plurality of memory stacks while differently controlling setup time points for applying a pass voltage to the inter-stack word lines, according to sizes of a channel hole of the inter-stack word lines. The inter-stack word line manager  129  may first set an inter-stack word line having a larger channel hole and later set an inter-stack word line having a smaller channel hole when the inter-stack word lines are set to the pass voltage. The inter-stack word line manager  129  may perform a channel voltage equalization operation of the plurality of memory stacks while differently controlling recovery time points for applying a ground voltage to the inter-stack word lines, according to sizes of the channel hole of the inter-stack word lines. The inter-stack word line manager  129  may first recover an inter-stack word line having a smaller channel hole and later recover an inter-stack word line having a larger channel hole when the inter-stack word lines are recovered to the ground voltage. 
     While the inventive concept has been particularly shown and described with reference to embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims.