Patent Publication Number: US-2022215879-A1

Title: 3d memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a Continuation of U.S. patent application Ser. No. 17/021,409 filed on Sep. 15, 2020, now Allowed, which claims priority under 35 USC § 119 to Korean Patent Application No. 10-2020-0010031, filed on Jan. 28, 2020, in the Korean Intellectual Property Office, the disclosure of each of which is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND 
     The inventive concept relates to a memory device, and more particularly, to a three-dimensional (3D) memory device and a method of operating the 3D memory device. 
     As non-volatile memory devices, flash memories and resistive memory devices, such as phase change random access memory (PRAM), nano floating gate memory (NFGM), polymer RAM (PoRAM), magnetic RAM (MRAM), ferroelectric RAM (FeRAM), and resistive RAM (RRAM) are known. A resistive memory device may have the high speed of DRAM and non-volatile characteristics of flash memory. Memory cells of the resistive memory device may have a resistance distribution according to programmed data. In the operation of reading data stored in the memory cells, data may be sensed by applying a constant current or voltage to a memory cell and reading a voltage that varies according to the resistance of the memory cell. As the demand for highly integrated memory devices has increased, it is necessary to reduce a power consumption of the memory device. 
     SUMMARY 
     According to an aspect of the inventive concept, there is provided a three-dimensional (3D) memory device including: a memory cell array including a plurality of lower memory cells respectively arranged in regions where a plurality of lower word lines intersect with a plurality of bit lines and a plurality of upper memory cells respectively arranged in regions where a plurality of upper word lines intersect with the plurality of bit lines, a first sense amplifier connected to a first lower word line of the plurality of lower word lines, the first sense amplifier comparing a voltage of the first lower word line with a first reference voltage to perform a data sensing operation on a first lower memory cell connected between a first bit line of the plurality of bit lines and the first lower word line, and a second sense amplifier connected to a first upper word line of the plurality of upper word lines, the second sense amplifier comparing a voltage of the first upper word line with a second reference voltage to perform a data sensing operation on a first upper memory cell connected between the first bit line and the first upper word line, wherein the data sensing operation of the first sense amplifier and the data sensing operation of the second sense amplifier are performed in parallel. 
     According to another aspect of the inventive concept, there is provided a three-dimensional (3D) memory device including: a first semiconductor layer and a second semiconductor layer stacked in a vertical direction, wherein the first semiconductor layer includes a memory cell array including a plurality of lower memory cells respectively arranged in regions where a plurality of lower word lines intersect with a plurality of bit lines and a plurality of upper memory cells respectively arranged in regions where a plurality of upper word lines intersect with the plurality of bit lines, and wherein the second semiconductor layer includes: a first sense amplifier connected to a first lower word line of the plurality of lower word lines, the first sense amplifier performing a data sensing operation on a first lower memory cell connected between a first bit line of the plurality of bit lines and the first lower word line; and a second sense amplifier connected to a first upper word line of the plurality of upper word lines, the second sense amplifier performing a data sensing operation on a first upper memory cell connected between the first bit line and the first upper word line. 
     According to another aspect of the inventive concept, there is provided a three-dimensional (3D) memory device including: a memory cell array including a plurality of lower memory cells respectively arranged in regions where a plurality of lower word lines intersect with a plurality of bit lines and a plurality of upper memory cells respectively arranged in regions where a plurality of upper word lines intersect with the plurality of bit lines, a first row decoder including a plurality of lower row switches respectively connected to a corresponding one of the plurality of lower word lines, the first row decoder performing a selection operation on the plurality of lower word lines, a second row decoder including a plurality of upper row switches respectively connected to a corresponding one of the plurality of upper word lines, the second row decoder performing a selection operation on the plurality of upper word lines, a first sense amplifier connected to a first lower word line of the plurality of lower word lines, the first sense amplifier performing a data sensing operation on a first lower memory cell connected between a first bit line among the plurality of bit lines and the first lower word line, and a second sense amplifier connected to a first upper word line of the plurality of upper word lines, the second sense amplifier performing a data sensing operation on a first upper memory cell connected between the first bit line and the first upper word line. 
    
    
     
       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 of a memory system according to an embodiment of the inventive concept; 
         FIG. 2  is a block diagram of a memory device of  FIG. 1  according to an embodiment of the inventive concept; 
         FIG. 3  illustrates a portion of the memory device of  FIG. 2  in more detail, according to an embodiment of the inventive concept; 
         FIG. 4  illustrates a memory cell according to an embodiment of the inventive concept; 
         FIG. 5A  is a graph showing set write and reset write for a variable resistor element of the memory cell of  FIG. 4 , and  FIG. 5B  is a graph showing the distribution of memory cells according to resistance when the memory cell of  FIG. 4  is a single level cell; 
         FIG. 6A  is a circuit diagram of a memory cell array according to an embodiment of the inventive concept, and  FIG. 6B  is a perspective view of the memory cell array of  FIG. 6A ; 
         FIG. 7  is a flowchart of a method of reading data from a memory device, according to an embodiment of the inventive concept; 
         FIG. 8  is a circuit diagram illustrating components for performing a read operation of a memory device according to an embodiment of the inventive concept; 
         FIG. 9  is a graph showing a read operation of a memory device according to an embodiment of the inventive concept; 
         FIG. 10  is a timing diagram illustrating a read operation for first and second memory cells, according to an embodiment of the inventive concept; 
         FIG. 11  is a timing diagram illustrating a read operation for first and second memory cells, according to an embodiment of the inventive concept; 
         FIG. 12  illustrates a memory device having a cell over peripheral (COP) structure according to an embodiment of the inventive concept; 
         FIG. 13  illustrates a top surface of a second semiconductor layer, according to an embodiment of the inventive concept; 
         FIG. 14  is a cross-sectional view of a memory device taken along line XIV-XIV′ in  FIG. 13 , and  FIG. 15  is a cross-sectional view of the memory device taken along line XV-XV′ in  FIG. 13 ; 
         FIG. 16  illustrates a top surface of a second semiconductor layer, according to an embodiment of the inventive concept; 
         FIG. 17  is a cross-sectional view of a memory device taken along line XVII-XIVII′ in  FIG. 16 ,  FIG. 18  is a cross-sectional view of the memory device taken along line XVIII-XVIII′ in  FIG. 16 ,  FIG. 19  is a cross-sectional view of the memory device taken along line XIX-XIX′ in  FIG. 16 , and  FIG. 20  is a cross-sectional view of the memory device taken along line XX-XX′ in  FIG. 16 ; 
         FIGS. 21 to 24  illustrate memory devices according to some embodiments of the inventive concept; 
         FIG. 25  is a block diagram illustrating an example in which a memory device according to some embodiments of the inventive concept is applied to a solid state drive (SSD) system; and 
         FIG. 26  is a cross-sectional view illustrating a memory device having a chip-to-chip structure, according to some embodiments of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, some example embodiments of the inventive concept will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a block diagram of a memory system  10  according to an embodiment of the inventive concept. 
     Referring to  FIG. 1 , the memory system  10  may include a memory device  100  and a memory controller  200 , and the memory device  100  may include a memory cell array  110 , a first sense amplifier SA 1 , and a second sense amplifier SA 2 . In an embodiment, the memory cell array  110  may include a plurality of resistive memory cells, and the memory device  100  may be referred to as a “resistive memory device”. However, the inventive concept is not limited thereto, and the memory cell array  110  may include various types of other memory cells. 
     The memory device  100  may be implemented in various forms. As an example, the memory device  100  may be a device implemented with one memory chip or one memory die. Alternatively, the memory device  100  may be defined as a device including a plurality of memory chips, and as an example, the memory device  100  may be a memory module in which a plurality of memory chips are mounted on a board. However, embodiments of the inventive concept are not limited thereto, and the memory device  100  may be implemented in various forms such as a semiconductor package including memory dies. 
     The memory controller  200  may control the memory device  100  to read data stored in the memory device  100  or write data to the memory device  100  in response to a write/read request from a host HOST. Specifically, the memory controller  200  may provide an address ADDR, a command CMD, and a control signal CTRL to the memory device  100  to control program (or write), read, and erase operations, etc. on the memory device  100 . Also, write data DATA and read data DATA may be transmitted and received between the memory controller  200  and the memory device  100 . 
     The memory cell array  110  may include a plurality of memory cells respectively arranged in regions where a plurality of first signal lines intersect with a plurality of second signal lines. In an embodiment, the first signal line may be one of a bit line and a word line, and the second signal line may be the other of the bit line and the word line. Accordingly, the memory device  100  may be referred to as a “cross-point memory device”. 
     Each of the plurality of memory cells may be a single level cell storing one bit, or a multi-level cell capable of storing at least 2 bits or more of data. Also, the memory cells may have a plurality of resistance distributions according to the number of bits stored in each memory cell. For example, when one bit of data is stored in each memory cell, the memory cells may have two resistance distributions, and when two bits of data are stored in each memory cell, the memory cells may have four resistance distributions. 
     The memory cell array  110  may include resistive memory cells, each of which includes a variable resistor element (or referred to as “variable resistance element”). For example, when the variable resistor element includes a phase change material and the resistance of the variable resistor element changes with temperature, the resistive memory device may be PRAM. As another example, when the variable resistor element includes an upper electrode, a lower electrode, and a complex metal oxide therebetween, the resistive memory device may be RRAM. As another example, when the variable resistor element includes an upper electrode of a magnetic material, a lower electrode of a magnetic material, and a dielectric material therebetween, the resistive memory device may be MRAM. Hereinafter, the term “memory cell” will be used to refer to a resistive memory cell. 
     The memory cell array  110  may include a plurality of layers, each including a plurality of resistive memory cells, and layers adjacent to each other may be configured to share signal lines (e.g., bit lines). Accordingly, the memory device  100  may be referred to as a “three-dimensional (3D) resistive memory device”. Hereinafter, an embodiment in which the memory device  100  is a 3D resistive memory device will be mainly described. 
     In an embodiment, the memory cell array  110  may include a first layer (e.g., a first layer  310   a  in  FIG. 12 ) and a second layer (e.g., a second layer  310   b  in  FIG. 12 ), the first sense amplifier SA 1  may be connected to the first layer, and the second sense amplifier SA 2  may be connected to the second layer. A connection relationship between the memory cell array  110  and the first and second sense amplifiers SA 1  and SA 2  will be described in more detail with reference to  FIG. 3 . 
       FIG. 2  is a block diagram of the memory device  100  of  FIG. 1  according to an embodiment of the inventive concept. 
     Referring to  FIG. 2 , the memory device  100  may include a memory cell array  110 , a write/read circuit  120 , a control logic  130 , a row decoder  140 , a column decoder  150 , and a voltage generator  160 . The write/read circuit  120  may include a sense amplification block  121  and a write driver  122 . The sense amplification block  121  may include at least a first sense amplifier SA 1  and a second sense amplifier SA 2 . 
     The memory cell array  110  may be connected to a plurality of first signal lines and a plurality of second signal lines. Also, the memory cell array  110  may include a plurality of memory cells respectively arranged in regions where the plurality of first signal lines intersect with the plurality of second signal lines. Hereinafter, the case where the plurality of first signal lines are word lines WL and the plurality of second signal lines are bit lines BL will be described as an example. 
     The write/read circuit  120  may provide a constant voltage or current to a selected memory cell through a selected word line or a selected bit line, which is connected to the selected memory cell, during data write and read operations on the selected memory cell among the plurality of memory cells. For example, when a read operation is performed, the write/read circuit  120  may provide pre-charge voltages to the selected word line and/or the selected bit line, and then may sense a voltage level of the selected word line or the selected bit line. 
     The sense amplification block  121  may be selectively connected to the bit line BL and/or the word line WL and may read data written to the selected memory cell. For example, the sense amplification block  121  may detect a voltage from a word line WL connected to the selected memory cell, amplify the detected voltage, and output read data DATA. The write driver  122  may be selectively connected to the bit line BL and/or the word line WL and may provide a write current to the selected memory cell. As a result, the write driver  122  may program data DATA to be stored in the memory cell array  110 . 
     The control logic  130  may output various control signals required for writing data to the memory cell array  110  or reading data from the memory cell array  110 , based on the command CMD, the address ADDR, and the control signal CTRL received from the memory controller  200  in  FIG. 1 . Specifically, the control logic  130  may provide an operation select signal CTRL_op to the write/read circuit  120 , provide a row address X_ADDR to the row decoder  140 , provide a column address Y_ADDR to the column decoder  150 , and provide a voltage control signal CTRL_vol to the voltage generator  160 . 
     In an embodiment, the control logic  130  may control the first and second sense amplifiers SA 1  and SA 2  such that a data read operation performed on a first lower memory cell arranged in a first layer and a data read operation performed on a first upper memory cell arranged in a second layer are performed in parallel. Specifically, the control logic  130  may control a pre-charge operation to be performed on a first lower word line and a first upper word line in a word line pre-charge period, and may control a pre-charge operation for a first bit line in a bit line pre-charge period after the word line pre-charge period. For example, the memory device may further include a first word line pre-charge circuit configured to apply a first word line pre-charge voltage to the first lower word line in the word line pre-charge period, a second word line pre-charge circuit configured to apply a second word line pre-charge voltage to the first upper word line in the word line pre-charge period, and a bit line pre-charge circuit configured to apply a bit line pre-charge voltage to the first bit line in the bit line pre-charge period, and a voltage level of each of the first and second word line pre-charge voltages is a negative voltage. A read control operation of the control logic  130  will be described later with reference to  FIGS. 9 to 11 . 
     The voltage generator  160  may generate various types of voltages required for performing write, read, and erase operations on the memory cell array  110  based on the voltage control signal CTRL_vol. The row decoder  140  may be connected to the memory cell array  110  through a plurality of word lines WL and may activate a selected word line among the plurality of word lines WL in response to the row address X_ADDR. The column decoder  150  may be connected to the memory cell array  110  through a plurality of bit lines BL and may activate a selected bit line among the plurality of bit lines BL in response to the column address Y_ADDR. 
       FIG. 3  illustrates a portion of the memory device  100  of  FIG. 2  in more detail, according to an embodiment of the inventive concept. 
     Referring to  FIG. 3 , the memory cell array  110  may include lower memory cells or first memory cells MC 1 , which are respectively arranged in regions where lower word lines WL 11  to WL 1   n  intersect with bit lines BL 1  to BLm, and upper memory cells or second memory cells MC 2 , which are respectively arranged in regions where upper word lines WL 21  to WL 2   n  intersect with the bit lines BL 1  to BLm. Here, m and n may be integers of 2 or more. In this case, the first memory cells MC 1  may correspond to a first layer or a lower layer, and the second memory cells MC 2  may correspond to a second layer or an upper layer. However, the inventive concept is not limited thereto, and the memory cell array  110  may have a structure in which three or more layers are vertically stacked. 
     The row decoder  140  may be arranged between the memory cell array  110  and the sense amplification block  121  and may include row switches  141   a ,  141   b ,  142   a , and  142   b . In an embodiment, first row switches  141   a  and  141   b  and second row switches  142   a  and  142   b  may be turned on or off according to a word line select signal LX 0 . For example, the word line select signal LX 0  may be generated from the row decoder  140  based on the row address X_ADDR. However, the inventive concept is not limited thereto. The row decoder  140  may include first row switches  141   a  and  141   b  respectively connected to the lower word lines WL 11  to WL 1   n  and performing a selection operation on corresponding lower word lines and second row switches  142   a  and  142   b  respectively connected to the upper word lines WL 21  to WL 2   n  and performing a selection operation on the corresponding upper word line. According to an embodiment, the row decoder  140  may include a first row decoder including the first row switches  141   a  and  141   b  and a second row decoder including the second row switches  142   a  and  142   b.    
     The column decoder  150  may include column switches  151  and  152  connected to bit lines BL 1  to BLm, respectively, and performing a selection operation on the corresponding bit lines. The column switches  151  and  152  may be respectively turned on or off according to bit line select signals LY 1  and LYm corresponding thereto, and accordingly, the column decoder  150  may select one of the bit lines BL 1  to BLm. For example, the bit line select signals LY 1  and LYm may be generated from the column decoder  150  based on the column address Y_ADDR. 
     The sense amplification block  121  may include at least two sense amplifiers (i.e., the first sense amplifier  121   a  and the second sense amplifier  121   b ) respectively corresponding to the lower layer and the upper layer. The first sense amplifier  121   a  may be connected to the lower word line WL 11  selected from among the lower word lines WL 11  to WL 1   n , and the second sense amplifier  121   b  may be connected to the upper word line WL 21  selected from among the upper word lines WL 21  to WL 2   n . The first sense amplifier  121   a  may include a first sense amplifier SA 1  and a pre-charge transistor TR 1 , and the second sense amplifier  121   b  may include a second sense amplifier SA 2  and a pre-charge transistor TR 2 . The first sense amplifier  121   a  and the second sense amplifier  121   b  may have the same structure. The first sense amplifier  121   a  may be optimized for sensing the data of memory cells connected to the lower word lines WL 11  to WL 1   n , and the second sense amplifier  121   b  may be optimized for sensing the data of memory cells connected to the upper word lines WL 21  to WL 2   n . However, the inventive concept is not limited thereto, and the sense amplification block  121  may further include a third sense amplifier  121   c  and a fourth sense amplifier  121   d . The third sense amplifier  121   c  may be connected to the lower word line WL 1   n  selected from among the lower word lines WL 11  to WL 1   n , and the fourth sense amplifier  121   d  may be connected to the upper word line WL 2   n  selected from the upper word lines WL 21  to WL 2   n.    
     A group including memory cells connected to the first and second sense amplifiers  121   a  and  121   b  may be referred to as a first sense amplifier group, and a group including memory cells connected to the third and fourth sense amplifiers  121   c  and  121   d  may be referred to as a second sense amplifier group. In this case, the first sense amplifier group may be connected to first and second global word lines or first and second data lines DL 1  and DL 2 , and the second sense amplifier group may be connected to third and fourth global word lines or third and fourth data lines DL 3  and DL 4 . Accordingly, lower word lines included in the first sense amplifier group from among the lower word lines WL 11  to WL 1   n  may be selected by the first data line DL 1 , and lower word lines included in the second sense amplifier group from among the lower word lines WL 11  to WL 1   n  may be selected by the third data line DL 3 . Similarly, upper word lines included in the first sense amplifier group from among the upper word lines WL 21  to WL 2   n  may be selected by the second data line DL 2 , and upper word lines included in the second sense amplifier group from among the upper word lines WL 21  to WL 2   n  may be selected by the fourth data line DL 4 . 
       FIG. 4  illustrates a memory cell MC according to an embodiment of the inventive concept. 
     Referring to  FIG. 4 , the memory cell MC may include a variable resistor element R and a switching element (or referred to as “selection element”) SW connected in series. The memory cell MC may correspond to one of the plurality of first memory cells MC 1  and the plurality of second memory cells MC 2  in  FIG. 3 . The variable resistor element R may include a phase change layer  11  (or a variable resistance layer), an upper electrode  12  formed on the phase change layer  11 , and a lower electrode  13  formed below the bottom of the phase change layer  11 . For example, the variable resistor element R may include a phase change material (e.g., Ge—Sb—Te (GST)), a transition metal oxide, or a magnetic material. The switching element SW may be implemented using various elements such as an Ovonic threshold switching (OTS) material, a transistor, and a diode. 
     The upper and lower electrodes  12  and  13  may include various metals, metal oxides, or metal nitrides. The phase change layer  11  may include a bipolar resistance memory material or a unipolar resistance memory material. The bipolar resistance memory material may be programmed to a set or reset state by the polarity of a current, and perovskite-based materials may be used for the bipolar resistance memory material. The unipolar resistance memory material may be programmed to a set or reset state even with a current of the same polarity, and a transition metal oxide such as NiOx or TiOx may be used for the unipolar resistance memory material. 
       FIG. 5A  is a graph showing set write and reset write for the variable resistor element R of the memory cell MC of  FIG. 4 , and  FIG. 5B  is a graph showing the distribution of memory cells according to resistance when the memory cell MC of  FIG. 4  is a single level cell. 
     Referring to  FIGS. 4 and 5A  together, when a phase change material constituting the variable resistor element R is heated to a temperature between a crystallization temperature Tx and a melting point Tm for a certain period of time and then gradually cooled, the phase change material is in a crystalline state. This crystalline state is referred to as a ‘set state’ in which data ‘0’ is stored. On the other hand, when the phase change material is quenched after being heated to a temperature above the melting point Tm, the phase change material is in an amorphous state. This amorphous state is referred to as a ‘reset state’ in which data ‘1’ is stored. Therefore, a current may be supplied to the variable resistor element R to store data, and the resistance value of the variable resistor element R may be measured to read data. 
     Referring to  FIGS. 4 and 5B  together, the horizontal axis of the graph of  FIG. 5B  represents resistance and the vertical axis of the graph of  FIG. 5B  represents the number of memory cells MC. When the memory cell MC is a single level cell, the memory cell MC may be in one of a low resistance state LRS, that is, a set state SET, and a high resistance state HRS, that is, a reset state RESET. Accordingly, the operation of switching the memory cell MC from the low resistance state LRS to the high resistance state HRS may be referred to as a reset operation or a reset write operation. In addition, the operation of switching the memory cell MC from the high resistance state HRS to the low resistance state LRS may be referred to as a set operation or a set write operation. 
       FIG. 6A  is a circuit diagram of a memory cell array  110   a  according to an embodiment of the inventive concept. 
     Referring to  FIG. 6A , the memory cell array  110   a  may include vertically stacked first and second layers, and the first and second layers may share signal lines, for example, bit lines BL 1  to BL 4 . The memory cell array  110   a  may include lower word lines WL 11  and WL 12  extending in a first direction (X direction) and spaced apart from each other in a second direction (Y direction) perpendicular to the first direction, and upper word lines WL 21  and WL 22  extending in the first direction and spaced apart from the lower word lines WL 11  and WL 12  in a third direction (Z direction) perpendicular to the first direction. In addition, the memory cell array  110   a  may include the bit lines BL 1  to BL 4  separated from each of the upper word lines WL 21  and WL 22  and the lower word lines WL 11  and WL 12  in the third direction and extending in the second direction. 
     First memory cells MC 1  may be respectively arranged in regions where the bit lines BL 1  to BL 4  intersect with the lower word lines WL 11  and WL 12 , and second memory cells MC 2  may be respectively arranged in regions where the bit lines BL 1  to BL 4  intersect with the upper word lines WL 21  and WL 22 . The lower word lines WL 11  and WL 12 , the first memory cells MC 1 , and the bit lines BL 1  to BL 4  may constitute the first layer, and the upper word lines WL 21  and WL 22 , the second memory cells MC 2 , and the bit lines BL 1  to BL 4  may constitute the second layer. Arbitrary first and second memory cells MC 1  and MC 2  may be addressed by selectively activating the lower word lines WL 11  and WL 12 , the upper word lines WL 21  and WL 22 , and the bit lines BL 1  to BL 4 . Each of the first and second memory cells MC 1  and MC 2  may include a variable resistor element R and a switching element SW. 
     In the case of the first layer, the variable resistor element R may be connected between one of the bit lines BL 1  to BL 4  and the switching element SW, and the switching element SW may be connected between the variable resistor element R and one of the lower word lines WL 11  and WL 12 . In the case of the second layer, the variable resistor element R may be connected between one of the upper word lines WL 21  and WL 22  and the switching element SW, and the switching element SW may be connected between the variable resistor element R and one of the bit lines BL 1  to BL 4 . However, the inventive concept is not limited thereto, and the arrangement order of the switching element SW and the variable resistor element R may be changed. 
     The switching element SW may control current supply to the variable resistor element R according to voltages applied to a word line and a bit line, which are connected to the switching element SW. For example, the switching element SW may be implemented with an Ovonic Threshold Switching (OTS) material. However, the inventive concept is not limited thereto, and in another embodiment, the switching element SW may be changed to other switchable elements such as a unidirectional diode, a bidirectional diode, and a transistor. 
     A voltage may be applied to the variable resistor element R of each of the first and second memory cells MC 1  and MC 2  through the lower word lines WL 11  and WL 12 , the upper word lines WL 21  and WL 22 , and the bit lines BL 1  to BL 4 , and thus, a current may flow through the variable resistor element R. For example, the variable resistor element R may include a layer of phase change material that may reversibly transition between a first state and a second state. However, the variable resistor element R is not limited thereto and may include any variable resistor having a resistance value that varies depending on an applied voltage. For example, in each of the first and second memory cells MC 1  and MC 2 , the resistance of the variable resistor element R may reversibly transition between the first state and the second state depending on a voltage applied to the variable resistor element R. 
       FIG. 6B  is a perspective view of the memory cell array  110   a  of  FIG. 6A  according to an embodiment. 
     Referring to  FIG. 6B , the memory cell array  110   a  may include lower word lines WL 1 , upper word lines WL 2 , bit lines BL, first memory cells MC 1 , and second memory cells MC 2 . The lower word lines WL 1  may extend in a first direction X and may be arranged parallel to one another in a second direction Y. In this case, the first direction X and the second direction Y may be substantially orthogonal to each other. The bit lines BL may extend in the second direction Y and may be arranged parallel to one another in the first direction X. The upper word lines WL 2  may extend in the first direction X and may be arranged parallel to one another in the second direction Y. The first memory cells MC 1  may be respectively arranged in regions where the lower word lines WL 1  intersect with the bit lines BL, and the second memory cells MC 2  may be respectively arranged in regions where the upper word lines WL 2  intersect with the bit lines BL. 
       FIG. 7  is a flowchart of a method of reading data by a memory device, according to an embodiment of the inventive concept. 
     Referring to  FIG. 7 , the method according to the present embodiment corresponds to an operation of reading data by a memory device according to a request from a host. For example, the method may include operations performed in a time series in the memory device  100  of  FIG. 1 . For example, the memory controller  200  may provide a read command to the memory device  100  according to a request from the host. In Operation S 10 , the memory device  100  receives a read command, and decodes an address provided with the read command to determine selected memory cells. For example, the selected memory cells may be a first memory cell arranged in a first layer and a second memory cell arranged in a second layer. 
     In Operation S 20 , the memory device  100  pre-charges a lower word line and an upper word line, which are respectively connected to one end of the first memory cell and one end of the second memory cell. In Operation S 30 , the memory device  100  pre-charges a bit line commonly connected to the other end of the first memory cell and the other end of the second memory cell. In an embodiment, in Operation S 30 , the lower word line and the upper word line may be floated. For example, a certain word line may be floated when the word line select transistor TRx connected to the certain word line is turned off. However, the inventive concept is not limited thereto. In some embodiments, in Operation S 30 , at least one of row switches connected to the lower word line and the upper word line may be slightly turned on. Operations S 20  and S 30  will be described later with reference to  FIG. 8 . 
     In Operation S 40 , the memory device  100  may connect the lower word line and the upper word line to a first data line and a second data line, respectively. For example, when the lower word line and the first data line are electrically connected to each other, charge sharing may be performed between the lower word line and the first data line and voltage levels of the lower word line and the first data line may be equal to each other. Accordingly, when the first memory cell is in a set state, the voltage level of the first data line may be relatively high, and when the first memory cell is in a reset state, the voltage level of the first data line may be relatively low. 
     In Operation S 50 , the memory device  100  may sense data, that is, data stored in the first and second memory cells, respectively, based on the voltage levels of the first and second data lines. The memory device  100  may compare the voltage level of the first data line with a reference voltage and output a comparison result as first data. In this case, the reference voltage may be set to an intermediate level between the voltage level of the first data line when the first memory cell is in the set state and the voltage level of the first data line when the first memory cell is in the reset state. Therefore, by comparing the voltage level of the first data line with the reference voltage, a comparison result may be output as 1-bit data, for example, 0 or 1. However, the inventive concept is not limited thereto. For example, when the first and second memory cells are multi-level cells, each of the first and second memory cells may have a plurality of states, and in Operation S 50 , voltage levels of the first and second data lines according to the plurality of states of the first and second memory cells may be determined. The memory device may read multi-bit data by comparing the voltage levels of the first and second data lines with a plurality of reference voltages, respectively. 
       FIG. 8  is a circuit diagram illustrating components for performing a read operation of a memory device according to an embodiment of the inventive concept. 
     Referring to  FIG. 8 , a word line WL may be connected to one end of a memory cell MC, and a bit line BL may be connected to the other end of the memory cell MC. A row decoder  140  may be connected to the word line WL. For example, the row decoder  140  may include a plurality of row switches, such as a word line select transistor TRx and a discharge transistor TRd. In  FIG. 8 , the row decoder  140  is illustrated as including one word line select transistor TRx and one discharge transistor TRd for convenience of description. However, the row decoder  140  may include a plurality of word line select transistors TRx and a plurality of discharge transistors TRd, which are connected to a plurality of word lines. In addition, the row decoder  140  may further include other switches and/or control elements. 
     The word line select transistor TRx may be turned on or off in response to a word line select signal LX. The word line select signal LX may be generated from the row decoder  140  based on the row address X_ADDR. When the word line select transistor TRx is turned on, the word line WL may be connected to a sense amplification block  121  through a data line DL. The sense amplification block  121  may correspond to, for example, one of the first to fourth sense amplifiers  121   a  to  121   d  of  FIG. 3 . When the word line select transistor TRx is implemented as an NMOS transistor, the word line select transistor TRx may be turned on when the word line select signal LX is at a high level and may be turned off when the word line select signal LX is at a low level. 
     The discharge transistor TRd may be turned on or off in response to a discharge enable signal WDE. When the discharge transistor TRd is turned on, a discharge voltage Vd may be applied to the word line WL. For example, the discharge voltage Vd may be a ground voltage VSS. In an embodiment, the discharge transistor TRd may be implemented as an NMOS transistor. The discharge transistor TRd of NMOS transistor may be turned on when the discharge enable signal WDE is at a high level and may be turned off when the discharge enable signal WDE is at a low level. In an embodiment, the discharge transistor TRd may be implemented as a PMOS transistor. The discharge transistor TRd of PMOS transistor may be turned on when the discharge enable signal WDE is at a low level and may be turned off when the discharge enable signal WDE is at a high level. For example, in a read operation, when a word line select transistor TRx connected to a selected word line is turned on, a discharge transistor TRd connected to the selected word line may be turned off. Discharge transistors TRd connected to unselected word lines may be turned on, and word line select transistors TRx connected to the unselected word lines may be turned off. 
     A column decoder  150  may be connected to the bit line BL. For example, the column decoder  150  may include a plurality of column switches, such as a bit line select transistor TRy. In  FIG. 8 , the column decoder  150  is illustrated as including one bit line select transistor TRy for convenience of description. However, the column decoder  150  may include a plurality of bit line select transistors TRy respectively connected to a plurality of bit lines BL. Also, the column decoder  150  may further include a plurality of discharge transistors respectively connected to the plurality of bit lines BL. 
     The bit line select transistor TRy may be connected to control switches, for example, a clamping transistor TR CMP  and a bit line pre-charge transistor TRb. The bit line pre-charge transistor TRb and the clamping transistor TR CMP  may be understood as components of the sense amplification block  121 . The bit line select transistor TRy is turned on or off in response to a bit line select signal LY. The bit line select signal LY may be generated from the column decoder  150  based on the column address Y_ADDR. For example, when the bit line select transistor TRy is implemented as a PMOS transistor, the bit line select transistor TRy may be turned on when the bit line select signal LY is at a low level, and turned off when the bit line select signal LY is at a high level. The bit line pre-charge transistor TRb may be turned on or off in response to a bit line pre-charge enable signal BPE. For example, when the bit line pre-charge transistor TRb is implemented as a PMOS transistor, the bit line pre-charge transistor TRb may be turned on when the bit line pre-charge enable signal BPE is at a low level, and turned off when the bit line pre-charge enable signal BPE is at a high level. When the bit line pre-charge transistor TRb is turned on, a second pre-charge voltage Vp 2  may be applied to the bit line BL. In this case, the clamping transistor TR CMP  may be controlled to apply a certain voltage to the bit line BL based on a clamping voltage V CMP . In an embodiment, the clamping transistor TR CMP  may be a PMOS transistor or an NMOS transistor. For example, the clamping transistor TR CMP  of NMOS transistor may be turned on when the clamping voltage V CMP  is at a high level, and turned off when the clamping voltage V CMP  is at a low level. In this case, a voltage level of the clamping voltage V CMP  may be greater than a voltage level of the bit line BL. 
     The sense amplification block  121  may include a word line pre-charge transistor TRa and a sense amplifier SA. The word line pre-charge transistor TRa may be turned on or off in response to a word line pre-charge enable signal WPE. In example embodiments, the word line pre-charge transistor TRa may be implemented as a PMOS transistor. The PMOS transistor of the word line pre-charge transistor TRa may be turned on when the word line pre-charge enable signal WPE is at a low level, and turned off when the word line pre-charge enable signal WPE is at a high level. In example embodiments, the word line pre-charge transistor TRa may be implemented as an NMOS transistor. The NMOS transistor of the word line pre-charge transistor TRa may be turned on when the word line pre-charge enable signal WPE is at a high level, and turned off when the word line pre-charge enable signal WPE is at a low level. When the word line select transistor TRx and the word line pre-charge transistor TRa are turned, a first pre-charge voltage Vp 1  may be applied to the word line WL. In an embodiment, a voltage level of a pre-charge voltage applied to the lower word line may be different from a voltage level of a pre-charge voltage applied to the upper word line. However, the inventive concept is not limited thereto, and the voltage level of the pre-charge voltage applied to the lower word line may be the same as the voltage level of the pre-charge voltage applied to the upper word line 
     The word line WL and the bit line BL may each include a parasitic capacitor, and the capacitance of the parasitic capacitor of the word line WL, for example, a word line capacitor CA, may be less than that of the parasitic capacitor (not shown) of the bit line BL. Accordingly, the sense amplifier SA may be connected to the word line WL having relatively small influence by the parasitic capacitor and sense the voltage level of the word line WL, thereby reading data of a selected memory cell MC. 
     The sense amplifier SA may compare a sensing voltage Vsen of a sensing node SN, for example, a voltage level of the data line DL (in this case, the voltage level of the data line DL is the same as the voltage level of the word line WL), with a reference voltage Vref and output a comparison result as data DATA. For example, the sense amplifier SA may operate as a comparator. When the memory cell MC is in a set state, the sensing voltage Vsen may be higher than the reference voltage Vref, and the sense amplifier SA may output ‘1’ as the data DATA. When the memory cell MC is in a reset state, the sensing voltage Vsen may be lower than the reference voltage Vref, and the sense amplifier SA may output ‘0’ as the data DATA. 
     Hereinafter, the read operation of the memory device will be described with reference to  FIGS. 7 and 8 . In Operation S 20 , the word line select transistor TRx may be turned on to connect the word line WL to the data line DL, and the word line WL and the data line DL may be pre-charged through a pre-charge path connected to the data line DL. In Operation S 30 , the bit line select transistor TRy may be turned on and thus the bit line BL may be pre-charged. 
     The second pre-charge voltage Vp 2  may be higher than the first pre-charge voltage Vp 1 , and a difference between the first pre-charge voltage Vp 1  and the second pre-charge voltage Vp 2  may be greater than a threshold voltage of the memory cell MC, for example, a threshold voltage of the switching element SW. Accordingly, a current (hereinafter referred to as “cell current”) may flow through the memory cell MC and the cell current may charge the word line WL, and thus, the voltage level of the word line WL may increase. In this case, because the amount of cell current varies according to the state of the memory cell MC, for example, according to the set state or the reset state, the voltage level of the word line WL may be changed according to the state of the memory cell MC. For example, when the memory cell MC is in the set state, the voltage level of the word line WL may increase relatively large because the resistance value of the memory cell MC is relatively small and the amount of cell current is relatively large. On the other hand, when the memory cell MC is in the reset state, the voltage level of the word line WL may increase relatively small or not increase because the resistance value of the memory cell MC is relatively large and the amount of cell current is relatively small, 
       FIG. 9  is a graph showing a read operation of a memory device according to an embodiment of the inventive concept. 
     Referring to  FIGS. 8 and 9  together, the horizontal axis of the graph of  FIG. 9  represents time and the vertical axis of the graph of  FIG. 9  represents voltage levels of the bit line BL and the word lines WL. The memory device may pre-charge the word line WL to the first pre-charge voltage Vp 1  in a first pre-charge period T_P 1 , for example, a word line pre-charge period WL_PRC. When the word line select transistor TRx and the word line pre-charge transistor TRa are turned on, the word line WL and the data line DL may be pre-charged to the first pre-charge voltage Vp 1 . In an embodiment, the first pre-charge voltage Vp 1  may be a negative voltage, and the voltage level of the word line WL may drop to the first pre-charge voltage Vp 1 . In this case, the bit line select transistor TRy may be turned off, and thus, the bit line BL may be in a floating state. When the memory cell MC is a selected memory cell, the discharge transistor TRd may maintain a turn-off state during a read operation. 
     The word line WL may be floated in a second pre-charge period T_P 2 , for example, a bit line pre-charge period BL_PRC, and the bit line BL may be pre-charged to the second pre-charge voltage Vp 2 . For example, the word line WL may be floated when the word line select transistor TRx and the discharge transistor TRd both connected to the word line WL are turned off. The bit line select transistor TRy and the bit line pre-charge transistor TRb may be turned on in the second pre-charge period T_P 2 , and thus, the second pre-charge voltage Vp 2  may be applied to the bit line BL. In an embodiment, a power supply voltage may be applied through the bit line pre-charge transistor TRb, and the clamping transistor TR CMP  may maintain the voltage level of the bit line BL as the second pre-charge voltage Vp 2  according to the clamping voltage V CMP . 
     In the second pre-charge period T_P 2 , the voltage level of the bit line BL may increase to the second pre-charge voltage Vp 2 . In this case, when a difference between the voltage level of the bit line BL and the voltage level of the word line WL is equal to or greater than a threshold voltage Vth of the memory cell MC, a cell current may flow in the memory cell MC. When the memory cell MC is in the set state, the voltage level of the word line WL may increase, and the difference between the voltage level of the word line WL and the voltage level of the bit line BL may be maintained above a blocking voltage Vs (i.e., a voltage level at which the cell current of the memory cell MC is blocked). Accordingly, when the memory cell MC is in the set state, the voltage level of the word line WL may increase up to a voltage level obtained by reducing the voltage level of the bit line BL by the blocking voltage Vs. On the other hand, when the memory cell MC is in the reset state, the voltage level of the word line WL may hardly increase or may increase very small. 
     In some embodiments, in the second pre-charge period T_P 2 , the bit line BL may be pre-charged to the second pre-charge voltage Vp 2  while the word line select transistor TRx is slightly turned on. In this case, as the word line select transistor TRx is slightly turned on, the word line WL may be pseudo-floated. For example, when the word line select signal LX has a certain voltage level between a voltage level of the high level and a voltage level of the low level, the word line select transistor TRx may be slightly turned on. Thus, when the selected memory cell is in a set state, the data line DL is slightly charged in the second pre-charge period T_P 2 . As described above, the word line select transistor TRx may be turned on when the word line select signal LX is at a high level, and may be turned off when the word line select signal LX is at a low level. 
     The word line select transistor TRx may be turned on in a sensing period T_S (for example, a data sensing period DL_Sensing), and thus, the word line WL and the data line DL may be electrically connected to each other and charge sharing may be performed. In an embodiment, the bit line BL may be discharged to a certain voltage, for example, a ground voltage VSS in a bit line discharge period (not shown) between the second pre-charge period T_P 2  and the sensing period T_S. For example, a bit line discharge circuit connected to the bit line BL may discharge a voltage of the bit line BL during the bit line discharge period. The charge sharing may be performed after the bit line discharge period in the sensing period T_S. The voltage level of the word line WL may be the same as the voltage level of the data line DL by the charge sharing, and the voltage level of the word line WL may be changed as shown in  FIG. 9 . When the charge sharing is completed, data may be sensed based on the voltage level of the data line DL, for example, the sensing voltage Vsen. The sense amplifier SA may sense data by comparing the reference voltage Vref with the sensing voltage Vsen. 
     In the process of charge sharing, especially when the memory cell MC is in the set state, the voltage level of the word line WL may be reduced by the charge sharing. In this case, when the amount of reduction is large, the sensing margin of the sense amplifier SA may be reduced. However, because the word line select transistor TRx is slightly turned on in the second pre-charge period T_P 2  and thus the data line DL is charged by a leakage current of the word line select transistor TRx, an effect such as an increase in the capacitance of the word line capacitor CA may occur. Accordingly, when the memory cell MC is in the set state, the amount of change in the voltage level of the word line WL may decrease, thereby sufficiently securing the sensing margin SM. 
       FIG. 10  is a timing diagram illustrating a read operation for first and second memory cells MC 1  and MC 2 , according to an embodiment of the inventive concept. 
     Referring to  FIGS. 3, 6A and 8 to 10  together, the first and second memory cells MC 1  and MC 2  may share a bit line BL 1 , and the bit line BL 1  may be driven by the bit line select signal LY. In addition, the first lower word line WL 11  and the first upper word line WL 21  connected to the first memory cell MC 1  and the second memory cell MC 2 , respectively, may be driven by the same word line select signal LX. Accordingly, a read operation for the first memory cell MC 1  and a read operation for the second memory cell MC 2  may be performed in parallel and may be performed substantially simultaneously. Hereinafter, a word line select transistor connected to the first lower word line WL 11  is referred to as a lower word line select transistor (e.g., a first row switch  711  in  FIG. 22 ), and a word line select transistor connected to the first upper word line WL 21  is referred to as an upper word line select transistor (e.g., a second row switch  721  in  FIG. 22 ). 
     In a standby period STB from a time t 0  to a time t 1 , the word line select signal LX may be at a low level, and the bit line select signal LY may be at a high level. Accordingly, the lower word line select transistor (e.g., a first row switch  711  in  FIG. 22 ), the upper word line select transistor (e.g., a second row switch  721  in  FIG. 22 ), and the bit line select transistor TRy may be turned off. The first lower word line WL 11  and the first upper word line WL 21  may be floated or have a voltage level of a ground voltage VSS. For example, the first lower word line WL 11  and the first upper word line WL 21  may have a ground voltage VSS by turning on the discharge transistor TRd of  FIG. 8 . 
     Thereafter, at the time t 1 , the word line select signal LX may transition to a high level, and the first lower word line WL 11  and the first upper word line WL 21  may be pre-charged based on the first pre-charge voltage Vp 1 . In this case, a period from the time t 1  to a time t 2  may be defined as a word line pre-charge period WL_PRC. The first pre-charge voltage Vp 1  may be a negative voltage. Accordingly, a voltage level VWL 11  of the first lower word line WL 11  and a voltage level VWL 21  of the first upper word line WL 21  may drop. In an embodiment, the voltage level of a pre-charge voltage applied to the first lower word line WL 11  and the voltage level of a pre-charge voltage applied to the first upper word line WL 21  may be different from each other. However, the inventive concept is not limited thereto, and the voltage level of the pre-charge voltage applied to the lower word line WL 11  may be the same as the voltage level of the pre-charge voltage applied to the upper word line WL 21 . 
     At the time t 2 , the bit line select signal LY may transition to a low level, and the bit line BL 1  may be pre-charged based on the second pre-charge voltage Vp 2 . In this case, a period from the time t 2  to a time t 4  may be defined as a bit line pre-charge period BL_PRC. The second pre-charge voltage Vp 2  may be higher than the first pre-charge voltage Vp 1  and may be a positive voltage. In an embodiment, the word line select signal LX may transition to a low level, for example, an off level Voff, and accordingly, the first lower word line WL 11  and the first upper word line WL 21  may be floated. In an embodiment, the word line select signal LX may transition to a weak-on level Vwo, where the weak-on level Vwo may be higher than the low level (i.e., the off level Voff) of the word line select signal LX, and may be lower than the high level (i.e., an on level Von) of the word line select signal LX. The lower word line select transistor and the upper word line select transistor may be slightly turned on based on the word line select signal LX having the weak-on level Vwo, and accordingly, the first lower word line WL 11  and the first upper word line WL 21  may be pseudo-floated. 
     As described above, when a difference between the voltage level of the bit line BL 1  and the voltage level of the first lower word line WL 11  is equal to or greater than the threshold voltage of the first memory cell MC 1  (e.g., at the time t 3 ), a cell current may flow through the first memory cell MC 1 . Similarly, when a difference between the voltage level of the bit line BL 1  and the voltage level of the first upper word line WL 21  is equal to or higher than the threshold voltage of the second memory cell MC 2  (e.g., at the time t 3 ), a cell current may flow through the second memory cell MC 2 . For example, the first memory cell MC 1  in the set state may have a greater amount of cell current than the second memory cell MC 2  in the reset state, and as the cell current charges the parasitic capacitor CA of the first lower word line WL 11 , the voltage level VWL 11  of the first lower word line WL 11  may increase. In this case, when the lower word line select transistor is slightly turned on, the voltage level of the data line DL may rise as a leakage current of the lower word line select transistor charges a capacitor CDL of the data line DL. In the case of the second memory cell MC 2  in the reset state, the voltage level VWL 21  of the first upper word line WL 21  may increase very small or may hardly increase. 
     At the time t 4 , as the bit line select signal LY and the word line select signal LX transitions to a high level and the bit line select transistor TRy is turned off and the lower word line select transistor is turned on, the first lower word line WL 11  may be electrically connected to a first data line corresponding thereto, and thus, charge sharing may be performed between the first lower word line WL 11  and the first data line DL 1 . Because the voltage level of the first data line DL 1  is lower than the voltage level of the first lower word line WL 11 , the voltage level of the first lower word line WL 11  decreases and the voltage level of the first data line DL 1  increases, and thus, the voltage levels of the first lower word line WL 11  and the first data line DL 1  may be the same. Thereafter, data sensing for the first memory cell MC 1  may be performed from a time t 5  to a time t 6 , that is, in a data sensing period. 
     Similarly, at the time t 4 , as the word line select signal LX transitions to a high level and the upper word line select transistor is turned on, the first upper word line WL 21  may be electrically connected to a second data line corresponding thereto, and thus, charge sharing may be performed between the first upper word line WL 21  and the second data line DL 2 . Because the voltage level of the second data line DL 2  is the same as the voltage level of the first upper word line WL 21 , the voltage level of the first upper word line WL 21  may be maintained. Thereafter, data sensing for the second memory cell MC 2  may be performed from the time t 5  to the time t 6 , that is, in the data sensing period. 
     As described above, according to the present embodiment, a data read operation for the first memory cell MC 1  and a data read operation for the second memory cell MC 2  may be performed in parallel based on one word line select signal LX and one bit line select signal LY. Accordingly, a read speed for the first and second memory cells MC 1  and MC 2  may be improved, and the read power consumption of the memory device including the first and second memory cells MC 1  and MC 2  may be reduced. 
       FIG. 11  is a timing diagram illustrating a read operation for first and second memory cells MC 1  and MC 2 , according to an embodiment of the inventive concept. Referring to  FIG. 11 , the read operation for the first and second memory cells MC 1  and MC 2  according to the present embodiment corresponds to a modification of the read operation illustrated in  FIG. 10 . When comparing the read operation according to the present embodiment with the read operation of  FIG. 10 , the voltage level of the word line select signal LX in the bit line pre-charge period BL_PRC according to the present embodiment may be different than that in the bit line pre-charge period BL_PRC according to the read operation of  FIG. 10 . 
     At a time t 2 , the bit line select signal LY may transition to a low level, and the bit line BL 1  may be pre-charged based on the second pre-charge voltage Vp 2 . In addition, at the time t 2 , the word line select signal LX may transition to a weak-on level Vwo, and at a time t 4 , the word line select signal LX may transition to a low level Voff. Accordingly, the lower word line select transistor and the upper word line select transistor may be slightly turned on in a portion of the bit line pre-charge period BL_PRC and turned off in the remaining period. In this case, the period from the time t 2  to the time t 4  may be adjusted based on the capacitance of the word line capacitor CA. For example, as the capacitance of the word line capacitor CA increases, the period in which the word line select transistor TRx is slightly turned on in the bit line pre-charge period BL_PRC may be reduced. 
       FIG. 12  illustrates a memory device  300  having a cell over peripheral (COP) structure according to an embodiment of the inventive concept. 
     Referring to  FIG. 12 , the memory device  300  may include first and second semiconductor layers  310  and  320  stacked in a vertical direction Z. The first semiconductor layer  310  may include first and second layers  310   a  and  310   b . In some embodiments, the first semiconductor layer  310  may further include one or more layers on the second layer  310   b . The first layer  310   a  may include lower word lines WL 1 , the second layer  310   b  may include upper word lines WL 2 , and the first layer  310   a  and the second layer  310   b  may share bit lines BL. 
     The first layer  310   a  may further include first memory cells respectively arranged in regions where the lower word lines WL 1  intersect with the bit lines BL, and the second layer  310   b  may further include second memory cells respectively arranged in regions where the upper word lines WL 2  intersect with the bit lines BL. A peripheral region including peripheral circuits may be arranged on and in the second semiconductor layer  320 . For example, a write/read circuit (for example, a write driver/sensing amplifier WD/SA)  321  and a control logic  322  may be arranged on and in the second semiconductor layer  320 . However, the inventive concept is not limited thereto, and various types of peripheral circuits related to memory operations may be arranged on and in the second semiconductor layer  320 . 
       FIG. 13  illustrates a top surface of a second semiconductor layer  420  in a memory device  400  (see  FIG. 14 ) having a COP structure, according to an embodiment of the inventive concept.  FIG. 14  is a cross-sectional view of the memory device  400  taken along line XIV-XIV′ in  FIG. 13 , and  FIG. 15  is a cross-sectional view of the memory device  400  taken along line XV-XV′ in  FIG. 13 . 
     Referring to  FIGS. 13 to 15  together, the memory device  400  having the COP structure may include first and second semiconductor layers  410  and  420 , which are vertically stacked, and the top surface of the second semiconductor layer  420  may be divided into first to fifth regions  421  to  425 . The first and second regions  421  and  422  may be adjacent to each other in the first direction X, and the third and fourth regions  423  and  424  may be adjacent to each other in the first direction X. The fifth region  425  may be arranged at a central portion of the upper surface of the second semiconductor layer  420 , and a column decoder CD for driving a bit line BL may be arranged in the fifth region  425 . 
     First row decoders RD 1  for driving lower word lines WL 1 , first global decoders GD 1  for driving global lower word lines connected to the lower word lines WL 1 , and lower sense amplifiers SA 1  for sensing first memory cells MC 1  may be arranged in the first and fourth regions  421  and  424 . Second row decoders RD 2  for driving upper word lines WL 2 , second global decoders GD 2  for driving global upper word lines connected to the upper word lines WL 2 , and upper sense amplifiers SA 2  for sensing second memory cells MC 2  may be arranged in the second and third regions  422  and  423 . 
     A first lower word line WL 11  and a first upper word line WL 21  may extend in the first direction X, and the bit line BL may extend in the second direction Y. Each of the first row decoders RD 1  may include a first row switch TRx 1 , each of the first global decoders GD 1  may include a first global switch TRgx 1 , each of the second row decoders RD 2  may include a second row switch TRx 2 , and each of the second global decoders GD 2  may include a second global switch TRgx 2 . In this case, the first and second row switches TRx 1  and TRx 2  may be driven by a word line select signal LX 0 , and the first and second global switches TRgx 1  and TRgx 2  may be driven by a global word line select signal GXO. Accordingly, read operations may be simultaneously performed on the first and second memory cells MC 1  and MC 2  connected to the bit line BL. As described above, memory cells, which are driven by the same word line select signal and the same global word line select signal and simultaneously read, may be defined as a sense amplifier group. For example, the memory device  400  may include first to fourth sense amplifier groups SAG 1  to SAG 4 . 
     The first row decoder RD 1  may be connected to the lower word line WL 1  through a first contact plug CP 1 , and may be electrically connected to the first global decoder GD 1  and the first sense amplifier SA 1  through a contact plug CP and a metal pattern MP. The second row decoder RD 2  may be connected to the upper word line WL 2  through a second contact plug CP 2 , and may be electrically connected to the second global decoder GD 2  and the second sense amplifier SA 2  through a contact plug CP and a metal pattern MP. The column decoder CD may be connected to the bit line BL through a third contact plug CP 3 . 
       FIG. 16  illustrates a top surface of a second semiconductor layer  520  in a memory device  500  (see  FIG. 17 ) having a COP structure, according to an embodiment of the inventive concept.  FIG. 17  is a cross-sectional view of the memory device  500  taken along line XVII-XVII′ in  FIG. 16 ,  FIG. 18  is a cross-sectional view of the memory device  500  taken along line XVIII-XVIII′ in  FIG. 16 ,  FIG. 19  is a cross-sectional view of the memory device  500  taken along line XIX-XIX′ in  FIG. 16 , and  FIG. 20  is a cross-sectional view of the memory device  500  taken along line XX-XX′ in  FIG. 16 . 
     Referring to  FIGS. 16 to 20  together, the memory device  500  having the COP structure may include first and second semiconductor layers  510  and  520 , which are vertically stacked, and the top surface of the second semiconductor layer  520  may be divided into a first tile region  521  and a second tile region  522 . A first memory array, that is, a first tile, may be arranged on the top in a vertical direction Z with respect to the first tile region  521 , and a second memory array, that is, a second tile, may be arranged on the top in the vertical direction Z with respect to the second tile region  522 . Accordingly, the structure of the first tile region  521  may be substantially the same as the structure of the second tile region  522 . Hereinafter, the first tile region  521  will be mainly described. 
     The first tile region  521  may be divided into first to fourth regions  521   a  to  521   d . For example, the first tile region  521  may be divided into the first to fourth regions  521   a  to  521   d  according to a windmill structure. The first region  521   a  and the second region  521   b  may be adjacent to each other in the first direction X, and the third region  521   c  and the fourth region  521   d  may be adjacent to each other in the first direction X. The first region  521   a  and the third region  521   c  may be adjacent to each other in the second direction Y, and the second region  521   b  and the fourth region  521   d  may be adjacent to each other in the second direction Y. Hereinafter, components arranged in the first and second regions  521   a  and  521   b  will be mainly described. 
     In the first region  521   a , a first row decoder RD 1  for driving lower word lines WL 1 , a first global decoder GD 1  for driving global lower word lines connected to the lower word lines WL 1 , a first sense amplifier SA 1  for sensing first memory cells MC 1 , a second sense amplifier SA 2  for sensing second memory cells MC 2 , a second global decoder GD 2  for driving global upper word lines connected to upper word lines WL 2 , and a second row decoder RD 2  for driving the upper word lines WL 2  may be arranged in a line in the first direction X. 
     In the second region  521   b , a first peripheral circuit PERIL a global decoder GY, and a column decoder CD may be arranged in a line in the second direction Y. The column decoder CD is a circuit for driving the bit lines BL, and the global decoder GY is a circuit for driving global bit lines connected to the bit lines BL. The first peripheral circuit PERI 1  may include, for example, a write driver (e.g., the write driver  122  in  FIG. 2 ). However, the inventive concept is not limited thereto. 
     The first row decoder RD 1  may be connected to the lower word line WL 1  through a first contact plug CP 1 , and may be electrically connected to the first global decoder GD 1  and the first sense amplifier SA 1  through a contact plug CP and a metal pattern MP. The second row decoder RD 2  may be connected to the upper word line WL 2  through a second contact plug CP 2 , and may be electrically connected to the second global decoder GD 2  and the second sense amplifier SA 2  through a contact plug CP and a metal pattern MP. The column decoder CD may be connected to the bit line BL through a third contact plug CP 3 . 
       FIG. 21  illustrates a memory device  600  according to an embodiment of the inventive concept. Referring to  FIG. 21 , the memory device  600  may include first to fourth memory arrays, for example, first to fourth tiles TL 1  to TL 4 , and the first to fourth tiles TL 1  to TL 4  may be arranged in a line in the first direction X. The first to fourth tiles TL 1  to TL 4  may be implemented based on the embodiments described above with reference to  FIGS. 1 to 4, 5A, 5B, 6A, 6B, and 7 to 20 . For example, each of the first to fourth tiles TL 1  to TL 4  may correspond to the first semiconductor layer  310  of  FIG. 12 . 
     Each of the first to fourth tiles TL 1  to TL 4  may include vertically stacked first and second layers L 1  and L 2 , and the first and second layers L 1  and L 2  may share bit lines. In addition, word line select transistors in each of the first and second layers L 1  and L 2  may be driven by the same word line select signal LX 0 . Accordingly, read operations may be performed in parallel on eight memory cells in each of the first to fourth tiles TL 1  to TL 4  through voltage control of the word line select signal LX 0 . Accordingly, a read speed for the eight memory cells may be improved, and the read power consumption of the memory device  600  may be reduced. 
       FIG. 22  illustrates a memory device  700  according to an embodiment of the inventive concept. 
     Referring to  FIG. 22 , the memory device  700  may include a second row decoder (RD 2 )  720 , a second sense amplifier (SA 2 )  740 , a first sense amplifier (SA 1 )  730 , and a first row decoder (RD 1 )  710 , which are arranged in a line in the first direction X. In an embodiment, the memory device  700  may have a COP structure, and the second row decoder  720 , the second sense amplifier  740 , the first sense amplifier  730 , and the first row decoder  710  may be arranged below first and second memory cells MC 1  and MC 2  in the vertical direction Z. 
     A lower word line WL 1  and an upper word line WL 2  may extend in the first direction X, and the upper word line WL 2  may be arranged above the lower word line WL 1  in the vertical direction Z. A bit line BL may extend in the second direction Y. The first memory cell MC 1  may be arranged in a region where the lower word line WL 1  intersects the bit line BL, and the second memory cell MC 2  may be arranged in a region where the upper word line WL 2  intersects the bit line BL. 
     The first row decoder  710  may include a first row switch  711 , and the first row switch  711  may be implemented as a transistor having a gate to which a word line select signal LX 0  is applied. For example, the first row switch  711  may have a source connected to the lower word line WL 1  and a drain connected to the first sense amplifier  730 . According to an embodiment, a first global decoder (e.g., the first global decoder GD 1  in  FIG. 13 or 15 ) may be arranged between the first row decoder  710  and the first sense amplifier  730 . 
     The second row decoder  720  may include a second row switch  721 , and the second row switch  721  may be implemented as a transistor having a gate to which the word line select signal LX 0  is applied. For example, the second row switch  721  may have a source connected to the upper word line WL 2  and a drain connected to the second sense amplifier  740 . According to an embodiment, a second global decoder (e.g., the second global decoder GD 2  in  FIG. 13 or 15 ) may be arranged between the second row decoder  720  and the second sense amplifier  740 . 
       FIG. 23  illustrates a memory device  800  according to an embodiment of the inventive concept. 
     Referring to  FIG. 23 , the memory device  800  may include first row decoders  810   a  and  810   b , second row decoders  820   a  and  820   b , first sense amplifiers  830   a  and  830   b , and second sense amplifiers  840   a  and  840   b . The second row decoder  820   a , the second sense amplifier  840   a , the first sense amplifier  830   a , and the first row decoder  810   a  may be arranged in a line in the first direction X. The second row decoder  820   b , the second sense amplifier  840   b , the first sense amplifier  830   b , and the first row decoder  810   b  may be arranged in a line in the first direction X. In an embodiment, the memory device  800  may have a COP structure, and the first row decoders  810   a  and  810   b , the second row decoders  820   a  and  820   b , the first sense amplifiers  830   a  and  830   b , and the second sense amplifiers  840   a  and  840   b  may be arranged below first memory cells MC 1   a  and MC 1   b  and second memory cells MC 2   a  and MC 2   b  in the vertical direction Z. 
     Lower word lines WL 11  and WL 12  and upper word lines WL 21  and WL 22  may extend in the first direction X. The upper word lines WL 21  and WL 22  may be respectively arranged above the lower word lines WL 11  and WL 12  in the vertical direction Z. A bit line BL may extend in the second direction Y. The first memory cell MC 1   a  may be arranged in a region where the lower word line WL 11  intersects the bit line BL, and the second memory cell MC 2   a  may be arranged in a region where the upper word line WL 21  intersects the bit line BL. 
     The first row decoder  810   a  may include a first row switch  811 , and the first row switch  811  may be implemented as a transistor having a gate to which a first word line select signal LX 1  is applied. For example, the first row switch  811  may have a source connected to the lower word line WL 11  and a drain connected to the first sense amplifier  830   a . The first row decoder  810   b  may include a first row switch  812 , and the first row switch  812  may be implemented as a transistor having a gate to which the first word line select signal LX 1  is applied. For example, the first row switch  812  may have a source connected to the lower word line WL 12  and a drain connected to the first sense amplifier  830   b . According to the driving of the first word line select signal LX 1 , the first row switches  811  and  812  may be simultaneously driven, and read operations on the first memory cells MC 1   a  and MC 1   b  may be performed in parallel by using the first sense amplifiers  830   a  and  830   b.    
     The second row decoder  820   a  may include a second row switch  821 , and the second row switch  821  may be implemented as a transistor having a gate to which a second word line select signal LX 2  is applied. For example, the second row switch  821  may have a source connected to the upper word line WL 21  and a drain connected to the second sense amplifier  840   a . The second row decoder  820   b  may include a second row switch  822 , and the second row switch  822  may be implemented as a transistor having a gate to which the second word line select signal LX 2  is applied. For example, the second row switch  822  may have a source connected to the upper word line WL 22  and a drain connected to the second sense amplifier  840   b . According to the driving of the second word line select signal LX 2 , the second row switches  821  and  822  may be simultaneously driven, and read operations for the second memory cells MC 2   a  and MC 2   b  may be performed in parallel by using the second sense amplifiers  840   a  and  840   b.    
     According to an embodiment, a first global decoder (e.g., the first global decoder GD 1  in  FIG. 13 or 15 ) may be arranged between the first row decoder  810   a  and the first sense amplifier  830   a  and between the first row decoder  810   b  and the first sense amplifier  830   b . In addition, according to an embodiment, a second global decoder (e.g., the second global decoder GD 2  in  FIG. 13 or 15 ) may be arranged between the second row decoder  820   a  and the second sense amplifier  840   a  and between the second row decoder  820   b  and the second sense amplifier  840   b.    
     As such, the memory device  800  may include four sense amplifiers, for example, the first sense amplifiers  830   a  and  830   b  and the second sense amplifiers  840   a  and  840   b , and by controlling the first and second word line select signals LX 1  and LX 2 , read operations for the four memory cells, for example, the first memory cells MC 1   a  and MC 1   b  and the second memory cells MC 2   a  and MC 2   b  may be performed in parallel. Accordingly, a read speed for the first memory cells MC 1   a  and MC 1   b  and the second memory cells MC 2   a  and MC 2   b  may be improved, and the read power consumption of the memory device  800  may be reduced. In some examples, the first and second word line select signals LX 1  and LX 2  may be generated based on t an address associated with the same page of the memory cell array. In some examples, in a certain mode of the memory device, each of the first and second word line select signals LX 1  and LX 2  may be generated based on a different address. In this case, a read operation of the first memory cells MC 1   a  and MC 1   b  and a read operation of the second memory cells MC 2   a  and MC 2   b  may be independently performed. 
       FIG. 24  illustrates a memory device  900  according to an embodiment of the inventive concept. 
     Referring to  FIG. 24 , the memory device  900  may include first row decoders  910   a  and  910   b , second row decoders  920   a  and  920   b , first sense amplifiers  930   a  and  930   b , and second sense amplifiers  940   a  and  940   b . The second row decoder  920   a , the second sense amplifier  940   a , the first sense amplifier  930   a , and the first row decoder  910   a  may be arranged in a line in the first direction X. The second row decoder  920   b , the second sense amplifier  940   b , the first sense amplifier  930   b , and the first row decoder  910   b  may be arranged in a line in the first direction X. In an embodiment, the memory device  900  may have a COP structure, and the first row decoders  910   a  and  910   b , the second row decoders  920   a  and  920   b , the first sense amplifiers  930   a  and  930   b , and the second sense amplifiers  940   a  and  940   b  may be arranged below the first memory cells MC 1   a  and MC 1   b  and the second memory cells MC 2   a  and MC 2   b  in the vertical direction Z. 
     The first row decoder  910   a  may include a first row switch  911 , and the first row switch  911  may be implemented as a transistor having a gate to which a first word line select signal LX 1  is applied. For example, the first row switch  911  may have a source connected to a lower word line WL 11  and a drain connected to the first sense amplifier  930   a . The first row decoder  910   b  may include a first row switch  912 , and the first row switch  912  may be implemented as a transistor having a gate to which a first word line select signal LX 1 ′ is applied. For example, the first row switch  912  may have a source connected to a lower word line WL 12  and a drain connected to the first sense amplifier  930   b . According to the driving of the first word line select signals LX 1  and LX 1 ′, the first row switches  911  and  912  may be simultaneously driven, and read operations for the first memory cells MC 1   a  and MC 1   b  may be performed in parallel by using the first sense amplifiers  930   a  and  930   b.    
     The second row decoder  920   a  may include a second row switch  921 , and the second row switch  921  may be implemented as a transistor having a gate to which a second word line select signal LX 2  is applied. For example, the second row switch  921  may have a source connected to an upper word line WL 21  and a drain connected to the second sense amplifier  940   a . The second row decoder  920   b  may include a second row switch  922 , and the second row switch  922  may be implemented as a transistor having a gate to which a second word line select signal LX 2 ′ is applied. For example, the second row switch  922  may have a source connected to an upper word line WL 22  and a drain connected to the second sense amplifier  940   b . According to the driving of the second word line select signals LX 2  and LX 2 ′, the second row switches  921  and  922  may be simultaneously driven, and read operations for the second memory cells MC 2   a  and MC 2   b  may be performed in parallel by using the second sense amplifiers  940   a  and  940   b.    
     According to an embodiment, a first global decoder (e.g., the first global decoder GD 1  in  FIG. 13 or 15 ) may be arranged between the first row decoder  910   a  and the first sense amplifier  930   a  and between the first row decoder  910   b  and the first sense amplifier  930   b . In addition, according to an embodiment, a second global decoder (e.g., the second global decoder GD 2  in  FIG. 13 or 15 ) may be arranged between the second row decoder  920   a  and the second sense amplifier  940   a  and between the second row decoder  920   b  and the second sense amplifier  940   b.    
     As such, the memory device  900  may include four sense amplifiers, that is, the first sense amplifiers  930   a  and  930   b  and the second sense amplifiers  940   a  and  940   b , and by controlling the first word line select signals LX 1  and LX 1 ′ and the second word line select signals LX 2  and LX 2 ′, read operations for the four memory cells, for example, the first memory cells MC 1   a  and MC 1   b  and the second memory cells MC 2   a  and MC 2   b  may be performed in parallel. Accordingly, a read speed for the first memory cells MC 1   a  and MC 1   b  and the second memory cells MC 2   a  and MC 2   b  may be improved. In some examples, the first word line select signals LX 1  and LX 1 ′ and the second word line select signals LX 2  and LX 2 ′ may be generated based on an address associated with the same page of the memory cell array. In some examples, in a certain mode of the memory device, each of the first word line select signals LX 1  and LX 1 ′ and the second word line select signals LX 2  and LX 2 ′ may be generated based on a different address. In this case, a read operation of each memory cell of the first memory cells MC 1   a  and MC 1   b  and the second memory cells MC 2   a  and MC 2   b  may be independently performed. 
       FIG. 25  is a block diagram illustrating an example in which a memory device according to some embodiments of the inventive concept is applied to a solid state drive (SSD) system  1000 . Referring to  FIG. 25 , the SSD system  1000  may include a host  1100  and an SSD  1200 . The SSD  1200  may exchange signals SGL with the host  1100  through a signal connector and receive power PWR through a power connector. The SSD  1200  may include an SSD controller  1210 , an auxiliary power supply  1220 , and memory devices (MEM)  1230 ,  1240 , . . . , and  1250 . The SSD controller  1210  may communicate with the memory devices  1230 ,  1240 , . . . , and  1250  respectively through channels Ch 1 , Ch 2 , . . . , and Chn. The memory devices  1230 ,  1240 , . . . , and  1250  may be implemented using the embodiments described above with reference to  FIGS. 1 to 4, 5A, 5B, 6A, 6B, and 7 to 24 . 
       FIG. 26  is a cross-sectional view illustrating a memory device having a chip-to-chip structure, according to some embodiments of the inventive concept. 
     Referring to  FIG. 26 , a memory device  2000  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, different from the first wafer, and then connecting the upper chip and the lower chip in a bonding manner. For example, the bonding manner may include a method of electrically connecting a bonding metal formed on an uppermost metal layer of the upper chip and a bonding metal formed on an uppermost metal layer of the lower chip. For example, when the bonding metals may be formed of copper (Cu), the bonding manner may be a Cu—Cu bonding, and the bonding metals may also be formed of aluminum or tungsten. 
     Each of the peripheral circuit region PERI and the cell region CELL of the memory device  2000  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  2110 , an interlayer insulating layer  2115 , a plurality of circuit elements  2120   a ,  2120   b , and  2120   c  formed on the first substrate  2110 , first metal layers  2130   a ,  2130   b , and  2130   c  respectively connected to the plurality of circuit elements  2120   a ,  2120   b , and  2120   c , and second metal layers  2140   a ,  2140   b , and  2140   c  formed on the first metal layers  2130   a ,  2130   b , and  2130   c . Each of the circuit elements  2120   a ,  2120   b , and  2120   c  may include one or more transistors. In an example embodiment, the first metal layers  2130   a ,  2130   b , and  2130   c  may be formed of tungsten having relatively high resistance, and the second metal layers  2140   a ,  2140   b , and  2140   c  may be formed of copper having relatively low resistance. 
     In an example embodiment illustrate in  FIG. 26 , although the first metal layers  2130   a ,  2130   b , and  2130   c  and the second metal layers  2140   a ,  2140   b , and  2140   c  are shown and described, they are not limited thereto, and one or more metal layers may be further formed on the second metal layers  2140   a ,  2140   b , and  2140   c . At least a portion of the one or more metal layers formed on the second metal layers  2140   a ,  2140   b , and  2140   c  may be formed of aluminum or the like having a lower resistance than those of copper forming the second metal layers  2140   a ,  2140   b , and  2140   c.    
     The interlayer insulating layer  2115  may be disposed on the first substrate  2110  and cover the plurality of circuit elements  2120   a ,  2120   b , and  2120   c , the first metal layers  2130   a ,  2130   b , and  2130   c , and the second metal layers  2140   a ,  2140   b , and  2140   c . The interlayer insulating layer  2115  may include an insulating material such as silicon oxide, silicon nitride, or the like. 
     Lower bonding metals  2171   b  and  2172   b  may be formed on the second metal layer  2140   b  in the word line bonding area WLBA. In the word line bonding area WLBA, the lower bonding metals  2171   b  and  2172   b  in the peripheral circuit region PERI may be electrically connected to upper bonding metals  2271   b  and  2272   b  in the cell region CELL in a bonding manner, and the lower bonding metals  2171   b  and  2172   b  and the upper bonding metals  2271   b  and  2272   b  may be formed of aluminum, copper, tungsten, or the like. Further, the upper bonding metals  2271   b  and  2272   b  in the cell region CELL may be referred as first metal pads and the lower bonding metals  2171   b  and  2172   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  2210  and a common source line  2220 . On the second substrate  2210 , a plurality of word lines  2231  to  2238  (i.e.,  2230 ) may be stacked in a direction (a Z-axis direction), perpendicular to an upper surface of the second substrate  2210 . At least one string select line and at least one ground select line may be arranged on and below the plurality of word lines  2230 , respectively, and the plurality of word lines  2230  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, perpendicular to the upper surface of the second substrate  2210 , and pass through the plurality of word lines  2230 , 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 third metal layer  2250   c  and a fourth metal layer  2260   c . For example, the third metal layer  2250   c  may be a bit line contact, and the fourth metal layer  2260   c  may be a bit line. In an example embodiment, the bit line  2260   c  may extend in a first direction (a Y-axis direction), parallel to the upper surface of the second substrate  2210 . 
     In an example embodiment illustrated in  FIG. 26 , an area in which the channel structure CH, the bit line  2260   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  2260   c  may be electrically connected to the circuit elements  2120   c  providing a page buffer  2293  in the peripheral circuit region PERI. For example, the bit line  2260   c  may be connected to upper bonding metals  2271   c  and  2272   c  in the cell region CELL, and the upper bonding metals  2271   c  and  2272   c  may be connected to lower bonding metals  2171   c  and  2172   c  connected to the circuit elements  2120   c  of the page buffer  2293 . 
     In the word line bonding area WLBA, the plurality of word lines  2230  may extend in a second direction (an X-axis direction), parallel to the upper surface of the second substrate  2210 , and may be connected to a plurality of cell contact plugs  2241  to  2247  (i.e.,  2240 ). The plurality of word lines  2230  and the plurality of cell contact plugs  2240  may be connected to each other in pads provided by at least a portion of the plurality of word lines  2230  extending in different lengths in the second direction. A third metal layer  2250   b  and a fourth metal layer  2260   b  may be connected to an upper portion of the plurality of cell contact plugs  2240  connected to the plurality of word lines  2230 , sequentially. The plurality of cell contact plugs  2240  may be connected to the circuit region PERI by the upper bonding metals  2271   b  and  2272   b  of the cell region CELL and the lower bonding metals  2171   b  and  2172   b  of the peripheral circuit region PERI in the word line bonding area WLBA. 
     The plurality of cell contact plugs  2240  may be electrically connected to the circuit elements  2120   b  providing a row decoder  2294  in the peripheral circuit region PERI. In an example embodiment, operating voltages of the circuit elements  2120   b  providing the row decoder  2294  may be different than operating voltages of the circuit elements  2120   c  providing the page buffer  2293 . For example, operating voltages of the circuit elements  2120   c  providing the page buffer  2293  may be greater than operating voltages of the circuit elements  2120   b  providing the row decoder  2294 . 
     A common source line contact plug  2280  may be disposed in the external pad bonding area PA. The common source line contact plug  2280  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  2220 . A third metal layer  2250   a  and a fourth metal layer  2260   a  may be stacked on an upper portion of the common source line contact plug  2280 , sequentially. For example, an area in which the common source line contact plug  2280 , the third metal layer  2250   a , and the fourth metal layer  2260   a  are disposed may be defined as the external pad bonding area PA. 
     Input-output pads  2105  and  2205  may be disposed in the external pad bonding area PA. Referring to  FIG. 26 , a lower insulating film  2101  covering a lower surface of the first substrate  2110  may be formed below the first substrate  2110 , and a first input-output pad  2105  may be formed on the lower insulating film  2101 . The first input-output pad  2105  may be connected to at least one of the plurality of circuit elements  2120   a ,  2120   b , and  2120   c  disposed in the peripheral circuit region PERI through a first input-output contact plug  2103 , and the first input-output pad  2105  may be separated from the first substrate  2110  by the lower insulating film  2101 . In addition, a side insulating film may be disposed between the first input-output contact plug  2103  and the first substrate  2110  to electrically separate the first input-output contact plug  2103  and the first substrate  2110 . 
     Referring to  FIG. 26 , an upper insulating film  2201  covering the upper surface of the second substrate  2210  may be formed on the second substrate  2210 , and a second input-output pad  2205  may be disposed on the upper insulating layer  2201 . The second input-output pad  2205  may be connected to at least one of the plurality of circuit elements  2120   a ,  2120   b , and  2120   c  disposed in the peripheral circuit region PERI through a second input-output contact plug  2203 . 
     According to embodiments, the second substrate  2210  and the common source line  2220  may not be disposed in an area in which the second input-output contact plug  2203  is disposed. Also, the second input-output pad  2205  may not overlap the word lines  2230  in the third direction (the Z-axis direction). Referring to  FIG. 26 , the second input-output contact plug  2203  may be separated from the second substrate  2210  in a direction, parallel to the upper surface of the second substrate  2210 , and may pass through an interlayer insulating layer  2215  of the cell region CELL to be connected to the second input-output pad  2205  and an upper metal pattern  2272   a  of the cell region CELL. 
     According to embodiments, the first input-output pad  2105  and the second input-output pad  2205  may be selectively formed. For example, the memory device  2000  may include only the first input-output pad  2105  disposed on the lower insulating film  2101  in contact with the first substrate  2110  or the second input-output pad  2205  disposed on the upper insulating film  2201  in contact with the second substrate  2210 . Alternatively, the memory device  2000  may include both the first input-output pad  2105  and the second input-output pad  2205 . 
     A metal pattern in 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  2000  may include a lower metal pattern  2173   a , corresponding to the upper metal pattern  2272   a  formed in an uppermost metal layer of the cell region CELL, and having the same shape as the upper metal pattern  2272   a  of the cell region CELL, in an uppermost metal layer of the peripheral circuit region PERI. In the peripheral circuit region PERI, the lower metal pattern  2173   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, an upper metal pattern, corresponding to the lower metal pattern formed in an uppermost metal layer of the peripheral circuit region PERI, and having the same shape as a lower metal pattern of the peripheral circuit region PERI, may be formed in an uppermost metal layer of the cell region CELL. 
     The lower bonding metals  2171   b  and  2172   b  may be formed on the second metal layer  2140   b  in the word line bonding area WLBA. In the word line bonding area WLBA, the lower bonding metals  2171   b  and  2172   b  of the peripheral circuit region PERI may be electrically connected to the upper bonding metals  2271   b  and  2272   b  of the cell region CELL by a Cu—Cu bonding. 
     Further, the bit line bonding area BLBA, an upper metal pattern  2292 , corresponding to a lower metal pattern  2152  formed in the uppermost metal layer of the peripheral circuit region PERI, and having the same shape as the lower metal pattern  2152  of the peripheral circuit region PERI, may be formed in an uppermost metal layer of the cell region CELL. A contact may not be formed on the upper metal pattern  2292  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 shape as the metal pattern may be formed in an uppermost metal layer in another one of the cell region CELL and the peripheral circuit region PERI, and a contact may not be formed on the reinforcement metal pattern. 
     In an example embodiment, the memory device  2000 , such as described in  FIG. 26 , can operate and can include device components according to one or more of the example embodiments described previously. 
     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.