Patent Publication Number: US-9892795-B2

Title: Nonvolatile memory device and method of operating the nonvolatile memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Korean Patent Application No. 10-2015-0176054, filed on Dec. 10, 2015, and entitled, “Nonvolatile Memory Device and Method of Operating the Nonvolatile Memory Device,” is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     One or more embodiments described herein relate to a nonvolatile memory device and a method of operating a nonvolatile memory device. 
     2. Description of the Related Art 
     Storage devices store data under the control of a host device such as a computer, a smart phone, a smart pad, etc. Examples of storage devices include a device for storing data in a magnetic disk (e.g., a hard disk drive) and a device for storing data in a semiconductor memory, e.g., a nonvolatile memory like a solid state drive, a memory card, etc. Examples of a nonvolatile memory include a read only memory (ROM), a programmable ROM, an electrically programmable ROM, an electrically erasable and programmable ROM, a flash memory, a phase-change random access memory (RAM), a magnetic RAM, a resistive RAM, and ferroelectric RAM. In order to keep pace with advancements in semiconductor manufacturing technology, attempts are being made to increase the operation speed and capacity of the host device of the memory. 
     SUMMARY 
     In accordance with one or more embodiments, a nonvolatile memory device includes a memory cell array including a plurality of cell strings connected to a plurality of select lines, each of the select lines connected to two or more cell strings, each of the cell strings including memory cells connected to a plurality of word lines, and a select transistor connected to a corresponding one of the select lines among the plurality of select lines; a row decoder to sequentially select the plurality of select lines in a read operation; a page buffer to obtain a read result of the two or more cell strings when a corresponding select line is selected and to accumulate read results of the plurality of cell strings when the plurality of select lines is sequentially selected; and control logic to control a subsequent operation based on the accumulated read results. 
     In accordance with one or more other embodiments, a method is provided for operating a nonvolatile memory device including a plurality of cell strings, each of the cell strings including a plurality of memory cells. The method includes obtaining a first read result by performing a read operation with respect to first cell strings among the plurality of cell strings; obtaining a second read result by performing a read operation with respect to second cell strings among the plurality of cell strings; generating a third read result by accumulating the first read result and the second read result; and performing a subsequent operation according to the third read result. 
     In accordance with one or more other embodiments, an apparatus includes first logic to perform a read operation for first cell strings of a memory to generate a first read result; second logic to perform a read operation for second cell strings of the memory to generate a second read result; and third logic to perform a subsequent operation based on a third read result, wherein the third read result is based on the first read result and the second read result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features will become apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which: 
         FIG. 1  illustrates an embodiment of a nonvolatile memory device; 
         FIG. 2  illustrates an embodiment of a memory block; 
         FIG. 3  illustrates an embodiment of a page buffer circuit; 
         FIG. 4  illustrates an embodiment of a method to operate a nonvolatile memory device; 
         FIG. 5  illustrates an extension example of a memory block; 
         FIG. 6  illustrates an application example of a read operation; 
         FIG. 7  illustrates an example of a timing diagram for voltages applied to memory cells and operations performed for a nonvolatile memory device during a single erase loop; 
         FIG. 8  illustrates an example of an operation for a page buffer during an erase loop; 
         FIG. 9  illustrates an example of accumulated for verify read operations; 
         FIG. 10  illustrates an example of a pass/fail check read operation; 
         FIG. 11  illustrates an application example of a read operation; 
         FIG. 12  illustrates another example of a pass/fail check read operation; 
         FIG. 13  illustrates another example of a pass/fail check read operation; 
         FIG. 14  illustrates an example of a reliability test read operation; 
         FIG. 15  illustrates another application example of a read operation; 
         FIG. 16  illustrates another application example of a read operation; 
         FIG. 17  illustrates an embodiment of a storage device; 
         FIG. 18  illustrates an embodiment of a controller; and 
         FIG. 19  illustrates an embodiment of a computing device. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an embodiment of a nonvolatile memory device  110  including a memory cell array  111 , a row decoder circuit  113 , a page buffer circuit  115 , a pass/fail check circuit (PFC), a data input/output circuit  117  and a control logic circuit  119 . 
     The memory cell array  111  includes a plurality of memory blocks BLK 1 ˜BLKz. Each memory block may be connected to the row decoder circuit  113  through at least one ground select line GSL, a plurality of word lines WL, and at least one string select line SSL. Each memory block may be connected to the page buffer circuit  115  through a plurality of bit lines BL. The memory blocks BLK 1 ˜BLKz may be connected to the bit lines BL in common. In one embodiment, memory cells of memory blocks BLK 1 ˜BLKz may all have the same structure. 
     Each of the memory blocks BLK 1 ˜BLKz may include an erase operation unit. Memory cells of the memory cell array  111  may be erased at the same time by a memory block unit. Each of the memory blocks BLK 1 ˜BLKz may be divided into a plurality of sub blocks. Each sub block may include an erase operation unit. 
     Each of the memory blocks BLK 1 ˜BLKz may include a physical storage space discriminated by a block address. Each of the word lines WL may correspond to a physical storage space discriminated by a row address. Each of the bit lines BL may correspond to a physical storage space discriminated by a column address. 
     Each memory block includes a plurality of physical pages and each physical page may include a plurality of memory cells. Each physical page may be a program operation unit. Memory cells of each physical page may be programmed at the same time. Each physical page may include a plurality of logical pages. Bits programmed in each of memory cells of each physical page may form logical pages. First bits programmed in memory cells of each physical page may form a first logical page. Kth bits programmed in memory cells of each physical page may form a Kth logical page. 
     The row decoder circuit  113  is connected to the memory cell array  111  through a plurality of ground select lines GSL, a plurality of word lines WL, and a plurality of string select lines SSL. The row decoder circuit  113  operates under the control of the control logic circuit  119 . The row decoder circuit  113  may decode an address received from a controller  120  through an input/output channel and control voltages applied to the string select lines SSL, the word lines WL and the ground select lines GSL according to the decoded address. 
     For example, in a program operation, the row decoder circuit  113  may apply a program voltage to a selected word line of a selected memory block by an address and may apply a pass voltage to unselected word lines of the selected memory block. In a read operation, the row decoder circuit  113  may apply a select read voltage to a selected word line of a selected memory block by an address and may apply an unselect read voltage to unselected word lines of the selected memory block. In an erase operation, the row decoder circuit  113  may apply erase voltages (e.g., a ground voltage or low voltages having levels substantially equal to ground voltage) to words line of a selected memory block by an address. 
     The page buffer circuit  115  is connected to the memory cell array  111  through the bit lines BL. The page buffer circuit  115  is connected to the data input/output circuit  117  through a plurality of data lines DL. The page buffer circuit  115  operates according to a control of the control logic circuit  119 . 
     In a program operation, the page buffer circuit  115  may store data to be programmed in memory cells. The page buffer circuit  115  may apply voltages to the bit lines BL based on the stored data. For example, the page buffer circuit  115  may function as a write driver. In a read operation or a verify-read operation, the page buffer circuit  115  may sense voltages of the bit lines BL and store a sensing result. For example, the page buffer circuit  115  may function as a sense amplifier. 
     After the verify-read operation, the pass/fail check circuit (PFC) may receive a sensing result from the page buffer circuit  115 . Based on the received sensing result, the pass/fail check circuit (PFC) may determine a fail or a pass. For example, in a program verify read operation, if the number of on-cells is greater than a threshold value, the pass/fail check circuit (PFC) may determine a fail. If the number of on-cells is less than the threshold value, the pass/fail check circuit (PFC) may determine a pass. For example, in an erase verify read operation, the page buffer circuit  115  may count the number of off-cells being turned off. If the number of off-cells is greater than a threshold value, the pass/fail check circuit (PFC) may determine a fail. If the number of off-cells is less than the threshold value, the pass/fail check circuit (PFC) may determine a pass. A determination result of pass or fail is transmitted to control logic circuit  119 . 
     The data input/output circuit  117  is connected to the page buffer circuit  115  through the data lines DL. The data input/output circuit  117  may output data read by the page buffer circuit  115  and transmit data received from the controller  120  through an input/output channel to the page buffer circuit  115 . 
     The control logic circuit  119  may receive a command from the controller  120  through the input/output channel and may receive a control signal through a control channel. In response to the control signal. The control logic circuit  119  may receive a command through the input/output channel, route an address received through the input/output channel to the row decoder circuit  113 , and route data received through the input/output channel to the data input/output circuit  117 . The control logic circuit  119  may decode the received command and control the nonvolatile memory device  110  according to the decoded command. 
     In a read operation, the control logic circuit  119  may generate a data strobe signal DQS from a read enable signal (/RE) received through the control channel. The generated data strobe signal DQS may be output to the controller  120  through the control channel. In a program operation, the control logic circuit  119  may receive the data strobe signal DQS from the controller  120  through the control channel. 
     According to control of the control logic circuit  119 , a program operation, an erase operation, and a read operation may be performed in memory cells of each memory block. The program operation may include a plurality of program loops. For example, a program loop may be repeated until a pass is determined. 
     Each program loop may include a program operation and a verify operation. In the program operation, the page buffer circuit  115  may apply voltages to bit lines BL according to data to be programmed. For example, a ground voltage and a low voltage having a level equal to, or substantially equal to, a ground voltage may be applied to a bit line corresponding to a memory cell of which a threshold voltage increases. A power supply voltage and a positive voltage having a level equal to, or substantially equal to, the power supply voltage may be applied to a memory cell (e.g., a memory cell in which a program will be prohibited) of which a threshold voltage does not increase. The row decoder circuit  113  may apply a program voltage to a word line connected to selected memory cells and may apply a pass voltage to the remaining word lines. In a verify operation, a program result may be verified. 
     The verify operation may include a verify-read operation and a pass/fail determination. In the verify read operation, the page buffer circuit  115  may apply a power supply voltage and a positive voltage having a level equal to, or substantially equal to, the power supply voltage to the bit lines BL or bit lines corresponding to memory cells which are targets of the verify read. The row decoder circuit  113  may apply a verify voltage to a word line connected to selected memory cells which are verify targets and may apply a read pass voltage to the remaining word lines. A verify read result may be sensed in the page buffer circuit  115  and may be transmitted to the pass/fail check circuit (PFC). In a pass/fail determination operation, the pass/fail check circuit (PFC) may determine a pass or a fail according to the verify-read result. 
     In a program operation of the nonvolatile memory device  110 , the nonvolatile memory device  110  may sequentially receive bits to be programmed in each memory cell of a selected physical page of the memory cell array  111  and may complete a program of the selected physical page through a single program operation based on the sequentially received bits. Completion of a program may correspond, for example, to the case where bits to be programmed in a corresponding physical page are all programmed in a readable state and an additional program with respect to the corresponding physical page is prohibited. 
     A read operation is performed similar to a verify-read operation. In the read operation, the page buffer circuit  115  may apply a power supply voltage or a positive voltage having a similar level to the power supply voltage to the bit lines BL or bit lines corresponding to memory cells which are targets of verify read. The row decoder circuit  113  may apply a read voltage to a word line connected to selected memory cells which are targets of read operation and apply a read pass voltage to the remaining word lines. A result of read operation may be sensed in the page buffer circuit  115  and may be output through data input/output circuit  117 . 
     An erase operation may include a plurality of erase loops. For example, an erase loop may be repeated until a pass is determined. Each erase loop may include erase and verification operations. In an erase operation, the row decoder circuit  113  may apply a ground voltage or low voltages having a level equal to, or substantially equal to, the ground voltage to word lines connected to selected memory cells. An erase voltage may be applied to channels of the selected memory cells through a substrate. In a verify operation, an erase result may be verified. The verify operation may include a verify read and a pass/fail determination. In a verify-read operation, the page buffer circuit  115  may apply a power supply voltage and a positive voltage having a level equal to, or substantially equal to, the power supply voltage to the bit lines BL or bit lines corresponding to memory cells which are targets of the verify read. The row decoder circuit  113  may apply an erase verify voltage to word lines connected to selected memory cells which are verify targets. A result of verify read may be sensed in the page buffer circuit  115  and transmitted to the pass/fail check circuit (PFC). In a pass/fail determination, the pass/fail check circuit (PFC) may determine a pass or a fail according to the result of verify read. 
       FIG. 2  illustrates an embodiment of a memory block BLKa which includes a plurality of cell strings (CS 11 ˜CS 21 , CS 12 ˜CS 22 ). The cell strings (CS 11 ˜CS 21 , CS 12 ˜CS 22 ) may be arranged along a row direction and a column direction to form rows and columns. For example, the cell strings CS 11  and CS 12  along the row direction may form a first row and the cell strings CS 21  and CS 22  along the row direction may form a second row. The cell strings CS 11  and CS 21  along the column direction may form a first column and the cell strings CS 12  and CS 22  along the column direction may form a second column. 
     Each cell string may include a plurality of transistors. The cell transistors include ground select transistors GST, memory cells MC 1 ˜MC 6 , and string select transistors SSTa and SSTb. The ground select transistor GST, the memory cells MC 1 ˜MC 6  and string select transistors SSTa and SSTb of each cell string may be stacked in a height direction perpendicular to or otherwise crossing a plane (e.g., a plane on a substrate of the memory block BLKa) on which the cell strings (CS 11 ˜CS 21 , CS 12 ˜CS 22 ) are arranged along rows and columns. 
     The cell transistors may be charge trap type transistors with threshold voltages that vary based on the amount of charges trapped in an insulating layer. 
     Sources of the lowermost ground select transistors GST may be connected to a common source line CSL in common. 
     Control gates of the ground select transistors GST of the cell strings CS 11  and CS 12  of the first row are connected to a ground select line GSL 1  in common and control gates of the ground select transistors GST of the cell strings CS 21  and CS 22  of the second row are connected to a ground select line GSL 2  in common. For example, cell strings of different rows are connected to different ground select lines. 
     The memory block BLKa may be changed so that ground select transistors of different heights of the same row are connected to different ground select lines. For example, the memory block BLKa may be changed so that ground select lines connected to ground select transistors of the same height of different rows are connected to one another to be controlled in common. The memory block BLKa may be changed so that ground select lines connected to ground select transistors are connected to one another to be controlled in common. 
     Control gates of memory cells at the same height (or order) from a substrate (or ground select transistors GST) may be connected to one word line in common. Control gates of memory cells at different heights (or orders) from the substrate (or ground select transistors GST) may be connected to different word lines WL 1 ˜WL 6  respectively. For example, the memory cells MC 1  are connected to the word line WL 1  in common. The memory cells MC 2  are connected to the word line WL 2  in common. The memory cells MC 3  are connected to the word line WL 3  in common. The memory cells MC 4  are connected to the word line WL 4  in common. The memory cells MC 5  are connected to the word line WL 5  in common. The memory cells MC 6  are connected to the word line WL 6  in common. 
     At first string select transistors SSTa of the same height (or order) of the cell strings (CS 11 ˜CS 21 , CS 12 ˜CS 22 ), control gates of the first string select transistors SSTa of different rows are connected to different string select lines SSL 1   a ˜SSL 2   a  respectively. For example, the first string select transistors SSTa of the cell strings CS 11  and CS 12  are connected to the string select line SSL 1   a  in common. The first string select transistors SSTa of the cell strings CS 21  and CS 22  are connected to the string select line SSL 2   a  in common. 
     At second string select transistors SSTb of the same height (or order) of the cell strings (CS 11 ˜CS 21 , CS 12 ˜CS 22 ), control gates of the second string select transistors SSTb of different rows are connected to different string select lines SSL 1   b ˜SSL 2   b  respectively. For example, the second string select transistors SSTb of the cell strings CS 11  and CS 12  are connected to the string select line SSL 1   b  in common. The second string select transistors SSTb of the cell strings CS 21  and CS 22  are connected to the string select line SSL 2   b  in common. 
     For example, cell strings of different rows are connected to different string select lines. String select transistors of the same height (or order) of cell strings of the same row are connected to the same string select line. String select transistors of different heights (or orders) of cell strings of the same row are connected to different string select lines. 
     String select transistors of cell strings of the same row may be connected to one string select line in common. For example, the string select transistors SSTa and SSTb of the cell strings CS 11  and CS 12  of the first row may be connected to one string select line in common. The string select transistors SSTa and SSTb of the cell strings CS 21  and CS 22  of the second row may be connected to one string select line in common. 
     Columns of the cell strings (CS 11 ˜CS 21 , CS 12 ˜CS 22 ) are connected to different bit lines BL 1  and BL 2  respectively. For example, the string select transistors SSTb of the cell strings CS 11 ˜CS 21  of the first column are connected to the bit line BL 1  in common. The string select transistors SSTb of the cell strings CS 12 ˜CS 22  of the second column are connected to the bit line BL 2  in common. 
     In the memory block BLKa, memory cells at the same height from the substrate may be characterized by sharing a word line. In other memory blocks, word lines may be characterized not to be shared. For example, a memory cell of a first height of a first memory block may share a word line with other memory cells of the first height of the first memory block. The memory cell of the first height of the first memory block may not share a word line with a memory cell of the first height of a second memory block. A sub block may be characterized as a part of the memory block BLKa. 
     The cell strings CS 11  and CS 12  may form a first plane. The cell strings CS 21  and CS 22  may form a second plane. 
     In a memory block BLKa, memory cells of each height of each plane may form a physical page. The physical page may be a write unit and a read unit. For example, one plane of the memory block BLKa may be selected by the string select lines SSL 1   a , SSL 1   b , SSL 2   a  and SSL 2   b . When a turn-on voltage is supplied to the string select lines SSL 1   a  and SSL 1   b  and a turn-off voltage is supplied to the string select lines SSL 2   a  and SSL 2   b , the cell strings CS 11  and CS 12  of the first plane are connected to the bit lines BL 1  and BL 2 . That is, the first plane is selected. When a turn-on voltage is supplied to the string select lines SSL 2   a  and SSL 2   b  and a turn-off voltage is supplied to the string select lines SSL 1   a  and SSL 1   b , the cell strings CS 21  and CS 22  of the second plane are connected to the bit lines BL 1  and BL 2 . That is, the second plane is selected. 
     In the selected plane, one row of the memory cells MC may be selected by the word lines WL 1 ˜WL 6 . In the selected row, a select voltage may be applied to the second word line WL 2  and an unselect voltage may be applied to the remaining word lines WL 1  and WL 3 ˜WL 6 . That is, a physical page corresponding to the second word line WL 2  of the second plane may be selected by controlling voltages of the string select lines SSL 1   a , SSL 1   b , SSL 2   a  and SSL 2   b  and the word lines WL 1 ˜WL 6 . In the memory cells MC 2  of the selected physical page, a write or read operation may be performed. 
     Two or more bits may be written in each of the memory cells MC. Bits written in each of the memory cells MC that belong to one physical page form logical pages. A first bit written in each of the memory cells MC that belong to one physical page forms a first logical page. An Nth bit written in each of the memory cells MC that belong to one physical page forms an Nth logical page. The logical page may be a unit of data access. When a read operation is performed in one physical page, data may be accessed by a logical page unit. 
     In the memory block BLKa, an erase of the memory cells MC 1 ˜MC 6  may be performed by a memory block unit or a sub block unit. When an erase operation is performed by a memory block unit, all the memory cells MC of the memory block BLKa may be erased at the same time according to an erase request (e.g., an erase request from an external memory controller). When an erase operation is performed by a sub block unit, part of the memory cells MC 1 ˜MC 6  of the memory block BLKa may be erased at the same time according to an erase request and the remaining memory cells may be erase-prohibited. A low voltage (for example, a ground voltage or a voltage having a level substantially equal to ground voltage) may be supplied to a word line connected to memory cells MC being erased. A word line connected to erase-prohibited memory cells MC may be floated. 
     The memory block BLKa illustrated in  FIG. 2  is merely illustrative. In another embodiment, the number of rows of cell strings may more or less than in  FIG. 2 . As the number of rows of cell strings changes, the number of string select lines or ground select lines connected to rows of the cell strings and the number of cell strings connected to one bit line may change. 
     The number of columns of cell strings may more or less than in  FIG. 2 . As the number of columns of cell strings changes, the number of bit lines connected to columns of the cell strings and the number of cell strings connected to one string select line may change. 
     The height of the cell strings may be more or less than in  FIG. 2 . For example, the number of ground select transistors, memory cells, or string select transistors that are stacked on each cell string may increase or decrease. 
     Memory cells MC that belong to one physical page may correspond to at least three logical pages. For example, k (k is a positive integer greater than 2) number of bits may be programmed in one memory cell MC. In the memory cells MC that belong to one physical page, k number of bits being programmed in each memory cell MC may form k number of logical pages respectively. 
     In one embodiment, a three dimensional (3D) memory array is provided. The 3D memory array is monolithically formed in one or more physical levels of arrays of memory cells having an active area disposed above a silicon substrate and circuitry associated with the operation of those memory cells, whether such associated circuitry is above or within such substrate. The term “monolithic” may correspond to the case where layers of each level of the array are directly deposited on the layers of each underlying level of the array. 
     In one embodiment, the 3D memory array includes vertical NAND strings that are vertically oriented such that at least one memory cell is located over another memory cell. The at least one memory cell may include a charge trap layer. Each vertical NAND string may include at least one select transistor over memory cells, the at least one select transistor having the same structure with the memory cells and formed monolithically with the memory cells. 
     The following documents are incorporated by reference herein and describe example configurations for three-dimensional memory arrays, in which the three-dimensional memory array includes a plurality of levels and in which word lines and/or bit lines shared between levels: U.S. Pat. Nos. 7,679,133; 8,553,466; 8,654,587; 8,559,235; and US Pat. Pub. No. 2011/0233648. 
       FIG. 3  illustrates an embodiment of the page buffer circuit  115  of  FIG. 1 . A constitution of the page buffer circuit  115  corresponding to one bit line BL is illustrated in  FIG. 3 . 
     Referring to  FIGS. 1 and 3 , the page buffer circuit  115  includes a sense latch SL and first through third data latches DL 1 ˜DL 3  corresponding to one bit line BL. In a program operation, the first through third data latches DL 1 ˜DL 3  may correspond to respective logical pages that will be programmed in a selected physical page. For example, a bit of a first logical page that will be programmed in the selected logical page may be loaded into the first data latch DL 1 . A bit of a second logical page that will be programmed in the selected logical page may be loaded into the second data latch DL 2 . A bit of a third logical page that will be programmed in the selected logical page may be loaded into the third data latch DL 3 . 
     The sense latch SL may control whether to program a selected memory cell that belongs to a selected physical page and corresponds to a bit line BL based on the first through third data latches DL 1 ˜DL 3 . For example, when the selected memory cell is a program target, the sense latch SL may be set as a first value. When the selected memory cell is program-prohibited, the sense latch SL may be set as a second value different from the first value. 
     In a read operation or a verify-read operation, the sense latch SL may latch a voltage of the bit line BL. The voltage of the bit line BL is a result of the read operation or the verify-read operation and may include information about a threshold voltage of the selected memory cell that belongs to the selected physical page and is connected to the bit line BL. A result of the read operation or the verify-read operation obtained by the sense latch SL may be dumped in one of the first through third data latches DL 1 ˜DL 3 . For example, the result of the read operation or the verify-read operation obtained by the sense latch SL may be dumped in the first data latch DL 1 . 
     In the verify-read operation, information dumped in the first data latch DL 1  may be transmitted to the pass/fail check circuit (PFC). The pass/fail check circuit (PFC) may determine a pass or a fail based on the result of the verify-read operation. 
     In  FIG. 3 , the page buffer circuit  115  has three data latches DL 1 ˜DL 3  corresponding to one bit line BL. In another embodiment, the page buffer circuit  115  may have a different number of data latches DL 1 ˜DL 3  with respect to one bit line BL. 
       FIG. 4  illustrates an embodiment of a method for operating a nonvolatile memory device  110 . As an example, a method is described for the case where the nonvolatile memory device  110  performs a read operation with respect to a memory block BLKa. 
     Referring to  FIGS. 1 through 4 , in operation S 110 , the nonvolatile memory device  110  selects a first string select line in the memory block BLKa. For example, the nonvolatile memory device  110  may select a first select line (e.g., SSLa or SSLb) among string select lines (e.g., SSL 1   a  and SSL 2   a , or SSL 1   b  and SSL 2   b ) at the same height. 
     In operation S 120 , the nonvolatile memory device  110  performs a read operation in relation to the selected string select line. For example, the row decoder circuit  113  may apply voltages for performing a read operation with respect to select transistors or memory cells associated with the selected string line to the select lines (SSL 1   a , SSL 1   b , SSL 2   a , SSL 2   b , GSL 1 , GSL 2 ) and the word lines WL. 
     In operation S 130 , the nonvolatile memory device  110  accumulates a result of the read operation. For example, the page buffer circuit  115  may update one (e.g., DL 1 ) among the data latches DL 1 ˜DL 3  based on the result of the read operation obtained by the sense latch SL. 
     In operation S 140 , the nonvolatile memory device  110  determines whether the selected string select line is the last string select line. If the selected string select line is not the last string select line, in operation S 150  the nonvolatile memory device  110  selects a next string select line. Subsequently, the nonvolatile memory device  110  may perform operation S 120  again. 
     If the selected string select line is the last string select line, in operation S 160  the nonvolatile memory device  110  may perform a subsequent operation based on the accumulated result of the read operation. 
     As illustrated in  FIG. 4 , the nonvolatile memory device  110  may sequentially select a plurality of string select lines that belongs to the memory block BLKa to perform a read operation. A result of the read operation may be accumulated in the page buffer circuit  115 . The accumulated result of the read operation may include information relating to all the read operations performed in relation to the string select lines. If the read operations are completed, the nonvolatile memory device  110  may perform a subsequent operation based on the result of the read operation accumulated in the page buffer circuit  115 . 
       FIG. 5  illustrates an extension example of the memory block BLKa of  FIG. 2 . Referring to  FIGS. 1 and 5 , cell strings (CS 11 ˜CS 14 , CS 21 ˜CS 24 , CS 31 ˜CS 34 , CS 41 ˜CS 44 ) are provided to the memory block BLKa. The cell strings CS 11 ˜CS 14  of a first row may be connected to first string select lines (SSL 1   a , SSL 1   b ) and a first ground select line GSL 1 . The cell strings CS 21 ˜CS 24  of a second row may be connected to second string select lines (SSL 2   a , SSL 2   b ) and a second ground select line GSL 2 . The cell strings CS 31 ˜CS 34  of a third row may be connected to third string select lines (SSL 3   a , SSL 3   b ) and a third ground select line GSL 3 . The cell strings CS 41 ˜CS 44  of a fourth row may be connected to fourth string select lines (SSL 4   a , SSL 4   b ) and a fourth ground select line GSL 4 . 
     Cell strings CS 11 ˜CS 41  of a first column may be connected to a first bit line BL 1 . Cell strings CS 12 ˜CS 42  of a second column may be connected to a second bit line BL 2 . Cell strings CS 13 ˜CS 43  of a third column may be connected to a third bit line BL 3 . Cell strings CS 14 ˜CS 44  of a fourth column may be connected to a fourth bit line BL 4 . The cell strings (CS 11 ˜CS 14 , CS 21 ˜CS 24 , CS 31 ˜CS 34 , CS 41 ˜CS 44 ) may be connected to the word lines WL 1 ˜WL 6  respectively. 
       FIG. 6  illustrates an application example of a read operation described with reference to  FIG. 4 . As an example, the read operation of  FIG. 4  corresponds to a verify read of each erase loop of an erase operation in  FIG. 6 . 
     Referring to  FIGS. 1, 2, 3, 5 and 6 , the nonvolatile memory device  110  performs an erase operation in operation S 210 . For example, an erase voltage may be applied to bodies of memory cells MC of the cell strings (CS 11 ˜CS 14 , CS 21 ˜CS 24 , CS 31 ˜CS 34 , CS 41 ˜CS 44 ). The row decoder circuit  113  may float the string select lines (SSL 1   a ˜SSL 4   a , SSL 1   b ˜SSL 4   b ) and the ground select lines GSL 1 ˜GSL 4  and apply word line erase voltages to the word lines WL 1 ˜WL 6 . The word line erase voltages may be a ground voltage or low voltages having a level similar to the ground voltage. 
     In operation S 220 , the nonvolatile memory device  110  selects a first string select line. For example, the nonvolatile memory device  110  may select the first string select line (SSL 1   a  or SSL 1   b ) among the string select lines (SSL 1   a ˜SSL 4   a  or SSL 1   b ˜SSL 4   b ) at the same height. 
     In operation S 230 , the nonvolatile memory device  110  performs a verify-read operation. The verify-read operation may proceed in the order of a precharge, develop, and a latch. During the precharge time, the page buffer circuit  115  may float the bit lines BL 1 ˜BL 4  after charging the bit lines BL 1 ˜BL 4  with a power supply voltage. During the develop time, the row decoder circuit  113  may apply a turn-on voltage to the select lines (SSL 1   a , SSL 1   b , GSL 1 ) associated with the selected string select line (SSL 1   a  or SSL 1   b ) and may apply an erase verify voltage to the word lines WL 1 ˜WL 6 . The turn-on voltages may be voltages that turn on the select transistors (SSTa, SSTb, GST). The erase verify voltage may correspond to an upper limit target value of threshold voltages which memory cells of an erase state have. During the latch time, the page buffer circuit  115  may set values of the sense latches SL according to voltages of the bit lines BL 1 ˜BL 4 . The values set in the sense latches SL may be a result of the verify-read operation. 
     In operation S 240 , the nonvolatile memory device  110  accumulate the result of the verify-read operation. For example, the page buffer circuit  115  may perform a dump operation that updates the data latch (e.g., DL 1 ) among the data latches DL 1 ˜DL 3  according to the value of the sense latch SL in relation to each bit line. 
     In operation S 250 , a determination is made as to whether the selected string select line is the last string select line. For example, it may be determined whether verify read operations are performed with respect to all the memory cells MC that belong to the memory block BLKa. If the selected string select line is not the last string select line, a next string is selected in operation S 260 . After that, the nonvolatile memory device  110  may perform operation S 230  again. If the selected string select line is the last string select line, an operation S 270  is performed. 
     In operation S 270 , the nonvolatile memory device  110  performs a pass/fail check. For example, the page buffer circuit  115  may output the result of the verify-read operation accumulated in the data latch DL 1  to the pass/fail check circuit (PFC). The pass/fail check circuit (PFC) may determine a pass or a fail according to the accumulated result of the verify-read operation. For example, if the number of bits having a first value among the accumulated result of the verify-read operation is greater than a critical value, the pass/fail check circuit (PFC) may determine a fail. If a fail is determined, the nonvolatile memory device  110  may perform a next erase loop from operation S 210 . If the number of bits having a first value among the accumulated result of the verify-read operation is less than a critical value, the pass/fail check circuit (PFC) may determine a pass. If a pass is determined, the nonvolatile memory device  110  may finish an erase operation. 
       FIG. 7  illustrates an example of a timing diagram for voltages applied to memory cells and operations performed in each circuit of a nonvolatile memory device while a single erase loop is performed. In  FIG. 7 , a horizontal axis indicates time (T) and a vertical axis indicates a voltage and operations of circuits of the nonvolatile memory device  110 .  FIG. 8  illustrates an example of an operation performed in a page buffer circuit  115  while an erase loop of  FIG. 7  is performed. 
     Referring to  FIGS. 1 through 3, and 5 through 8 , an erase operation (operation S 210 ) is performed at a first time (T 1 ). An erase voltage VERS may be applied to bodies of memory cells MC and word line erase voltages VWE may be applied to control gates. 
     A verify-read operation is performed with respect to the first string line (SSL 1   a  or SSL 1   b ) during a second time (T 2 ) through a sixth time (T 6 ). The page buffer circuit  115  may perform a precharge PRE during the second time (T 2 ). The page buffer circuit  115  may float the bit lines BL 1 ˜BL 4  after charging the bit lines BL 1 ˜BL 4  with a power supply voltage. The page buffer circuit  115  performs a develop DEV at a third time (T 3 ). For example, the page buffer circuit  115  may standby during a predetermined time. The row decoder circuit  113  may perform a first selection S 1  selecting the first string select line (SSL 1   a  or SSL 1   b ). The row decoder circuit  113  may apply turn-on voltages to select lines (SSL 1   a , SSL 1   b , GSL 1 ) associated with the selected string line (SSL 1   a  or SSL 1   b ). The row decoder circuit  113  may apply erase verify voltages VFY to the memory cells MC through the word lines WL 1 ˜WL 6 . 
     The page buffer circuit  115  performs a latch LAT at a fourth time (T 4 ). As illustrated by an arrow in  FIG. 8 , the page buffer circuit  115  may set values of the sense latches SL according to voltages of the bit lines BL 1 ˜BL 4 . The page buffer circuit  115  performs a dump operation DMP at a fifth time (T 5 ). The page buffer circuit  115  may update the data latches DL 1  according to the values of the sense latches SL. 
     A verify-read operation is performed with respect to the second string select line (SSL 2   a  or SSL 2   b ) through a second selection S 2  during a sixth time (T 6 ) through a tenth time (T 10 ). The page buffer circuit  115  may perform a precharge PRE at the sixth time (T 6 ). The page buffer circuit  115  may perform a develop DEV at the seventh time (T 7 ). The page buffer circuit  115  may perform a latch LAT at the eighth time (T 8 ). As illustrated by an arrow in  FIG. 8 , the page buffer circuit  115  may set values of the sense latches SL according to voltages of the bit lines BL 1 ˜BL 4 . The page buffer circuit  115  performs a dump operation DMP at a ninth time (T 9 ). The page buffer circuit  115  may update the data latches DL 1  according to the values of the sense latches SL. 
     A verify-read operation is performed with respect to the third string select line (SSL 3   a  or SSL 3   b ) through a third selection S 3  during a tenth time (T 10 ) through a fourteenth time (T 14 ). The page buffer circuit  115  may perform a precharge PRE at the tenth time (T 10 ). The page buffer circuit  115  may perform a develop DEV at the eleventh time (T 11 ). The page buffer circuit  115  may perform a latch LAT at the twelfth time (T 12 ). As illustrated by an arrow in  FIG. 8 , the page buffer circuit  115  may set values of the sense latches SL according to voltages of the bit lines BL 1 ˜BL 4 . The page buffer circuit  115  performs a dump operation DMP at a thirteenth time (T 13 ). The page buffer circuit  115  may update the data latches DL 1  according to the values of the sense latches SL. 
     A verify-read operation is performed with respect to the fourth string select line (SSL 4   a  or SSL 4   b ) through a fourth selection S 4  during a fourteenth time (T 14 ) through an eighteenth time (T 18 ). The page buffer circuit  115  may perform a precharge PRE at the fourteenth time (T 14 ). The page buffer circuit  115  may perform a develop DEV at the fifteenth time (T 15 ). The page buffer circuit  115  may perform a latch LAT at the sixteenth time (T 16 ). As illustrated by an arrow in  FIG. 8 , the page buffer circuit  115  may set values of the sense latches SL according to voltages of the bit lines BL 1 ˜BL 4 . The page buffer circuit  115  performs a dump operation DMP at a seventeenth time (T 17 ). The page buffer circuit  115  may update the data latches DL 1  according to the values of the sense latches SL. 
     Verify-read operations with respect to the memory cells MC of the memory block BLKa are completed, the page buffer circuit  115  may output the accumulated result of the verify-read operations. As illustrated by an arrow in  FIG. 8 , the page buffer circuit  115  may output values stored in the data latches DL 1  to the pass/fail check circuit (PFC) as the accumulated result of the verify-read operations. 
     The pass/fail check circuit (PFC) may perform a pass/fail check CHK at a nineteenth time (T 19 ). 
       FIG. 9  illustrates an example of accumulated results of verify read operations. Referring to  FIGS. 5 and 9 , in the first bit line BL 1 , results of verify read operations associated with the first through fourth string select lines SSL 1   a ˜SSL 4   a  may all indicate a pass (P). At this time, the accumulated result may indicate a pass (P). In the second bit line BL 2 , results of verify read operations associated with the first through third string select lines SSL 1   a ˜SSL 3   a  may indicate a pass (P) and a result of verify read operation associated with the fourth string select line SSL 4   a  may indicate a fail (F). At this time, the accumulated result may indicate a fail (F). 
     In the third bit line BL 3 , results of verify read operations associated with the first and second string select lines (SSL 1   a , SSL 2   a ) may indicate a pass (P) and results of verify read operations associated with the third and fourth string select lines (SSL 3   a , SSL 4   a ) may indicate a fail (F). At this time, the accumulated result may indicate a fail (F). In the fourth bit line BL 4 , a result of verify read operation associated with the first string select line SSL 1   a  may indicate a pass (P) and results of verify read operations associated with the second through fourth string select lines SSL 2   a ˜SSL 4   a  may indicate a fail (F). At this time, the accumulated result may indicate a fail (F). 
     For example, while the verify-read operations are performed, if at least one result of the verify-read operation indicates a fail (F), an accumulated result associated with a corresponding bit line may indicate a fail (F). While the verify-read operations are performed, if results of the verify-read operations all indicate a pass (P), an accumulated result associated with a corresponding bit line may indicate a pass (P). 
     For example, the nonvolatile memory device  110  may have a first operation mode in which the nonvolatile memory device  110  performs a verification read and a pass-fail check for each plane. The nonvolatile memory device  110  may have a second operation mode in which the nonvolatile memory device  110  performs verification reads of entire planes of the memory block BLKa and then performs a pass-fail check once. 
     For example, if reliability of the nonvolatile memory device  110  is high, the nonvolatile memory device  110  may operate in the second operation mode. If the nonvolatile memory device  110  wears out and is unreliable, the nonvolatile memory device  110  may operate in the first operation mode. 
     For example, pass or fail may be determined by comparing the verification read result with the first critical value in the first operation mode. Pass or fail may be determined by comparing the verification read result with the second critical value. The second critical value may greater than the first critical value. The first critical value and the second critical value may be the numbers of on cells (e.g., failed cells) or off cells (e.g., passed cells). For example, the second critical value may be greater than the first critical value m-times, where m is the number of planes or a similar value. 
       FIG. 10  illustrates an embodiment corresponding to a pass-fail check read performed by selection transistors. In  FIG. 10 , the horizontal axis indicates threshold voltages Vth of the selection transistors and the vertical axis indicates the number of the selection transistors. Referring  FIGS. 1, 2, 3, 5, and 10 , the string selection transistors SSTa or SSTb or the ground selection transistors GST may have the same structure with memory cells. For example, the string selection transistors SSTa or SSTb or the ground selection transistors GST may have the threshold voltage Vth. The threshold voltages of the string selection transistors SSTa or SSTb or the ground selection transistors GST may be controlled within a certain range for normal read, write, or erase operations of the memory cells MC. For example, the threshold voltage Vth of the string selection transistors SSTa or SSTb or the ground selection transistors GST may be controlled below a upper limit voltage VU and above a lower limit voltage VL. 
     During programming, read and erase operations are performed in the nonvolatile memory device  110  and threshold voltages of the string select transistors (SSTa or SSTb) or the ground select transistors GST may be increased by disturbance. For example, the threshold voltages of the string select transistors (SSTa or SSTb) or the ground select transistors GST may become greater than an upper limit voltage VU by a disturbance. 
     As time goes by after the threshold voltages of the string select transistors (SSTa or SSTb) or the ground select transistors GST are controlled, the threshold voltages of the string select transistors (SSTa or SSTb) or the ground select transistors GST may become low. For example, the threshold voltages of the string select transistors (SSTa or SSTb) or ground select transistors GST may become less than a lower limit voltage VL. 
     If the threshold voltages of the string select transistors (SSTa or SSTb) or the ground select transistors GST become less than the lower limit voltage VL or greater than the upper limit voltage VU, an incorrect operation may occur in the nonvolatile memory device  110 . To prevent an incorrect operation from occurring in the nonvolatile memory device  110 , a pass/fail check read operation may be performed. 
     For example, in a pass/fail check read operation, the nonvolatile memory device  110  may perform a read operation of the string select transistors (SSTa or SSTb) or the ground select transistors GST using the higher limit voltage VU and may perform an off-cell count that counts off-cells being turned off in a read operation. For example, in a pass/fail check read operation, the nonvolatile memory device  110  may perform a read operation of the string select transistors (SSTa or SSTb) or the ground select transistors GST using the lower limit voltage VL and may perform an on-cell count that counts on-cells being turned on in a read operation. 
       FIG. 11  illustrates an application example of the read operation of  FIG. 4  applied to a pass/fail check read of select transistors. Referring to  FIGS. 1, 2, 3, 5, 10, and 11 , in operation S 310  the nonvolatile memory device  110  may select selection transistors corresponding to a first string select line. For example, the nonvolatile memory device  110  may select the string select transistors (SSTa or SSTb) or the ground select transistors GST corresponding to the first string select line (SSL 1   a  or SSL 1   b ) among the string select lines (SSL 1   a ˜SSL 4   a  or SSL 1   b ˜SSL 4   b ) at the same height. 
     In operation S 320 , the nonvolatile memory device  110  performs a read operation with respect to selected select transistors using the lower limit voltage VL or the upper limit voltage VU. The read operation may proceed in the order of precharge, develop, and latch. During the precharge time, the page buffer circuit  115  may float the bit lines BL 1 ˜BL 4  after charging the bit lines BL 1 ˜BL 4  with a power supply voltage. During the develop time, the row decoder circuit  113  may apply the lower limit voltage VL or the upper limit voltage VU to a selected select line connected to the selected string select transistors and may apply turn-on voltages to unselected select lines and the word lines connected to the unselected select transistors. The turn-on voltages may be voltages that turn on the select transistors and the memory cells MC. During the latch time, the page buffer circuit  115  may set values of the sense latches SL according to voltages of the bit lines BL 1 ˜BL 4 . 
     In operation S 330 , the nonvolatile memory device  110  may accumulate a result of the read operation. For example, the page buffer circuit  115  may update the data latches DL 1  according to the values of the sense latches SL. 
     In operation S 340 , the nonvolatile memory device  110  determines whether selected select transistors correspond to the last string select line. For example, the nonvolatile memory device  110  may determine whether pass/fail check read operations are completed with respect to the whole string select transistors (SSTa or SSTb) or the ground select transistors GST. For example, the nonvolatile memory device  110  may determine whether pass/fail check read operations are completed with respect to some select transistors which are check targets among the string select transistors (SSTa or SSTb) or the ground select transistors GST. 
     If the pass/fail check read operation is not completed, in operation S 350 , the nonvolatile memory device  110  may select selection transistors corresponding to a next string select line. If the pass/fail check read operation is completed, operation S 360  is performed. 
     In operation S 360 , the nonvolatile memory device  110  may perform a pass/fail check operation. For example, when the pass/fail check read operation is performed using the lower limit voltage VL, the pass/fail check circuit (PFC) may perform an on-cell count and compare a count result with a critical value. When the pass/fail check read operation is performed using the higher limit voltage VU, the pass/fail check circuit (PFC) may perform an off-cell count and compare a count result with a critical value. 
     In operation S 370 , the nonvolatile memory device  110  determines a pass or a fail. For example, if the count result is less than the critical value, the pass/fail check circuit (PFC) may determine a pass. If a pass is determined, the pass/fail check read operation may be finished. If the count result is greater than the critical value, the pass/fail check circuit (PFC) may determine a fail. If a fail is determined, in operation S 380 , the nonvolatile memory device  110  may perform a select transistor fail process. For example, the select transistor fail process may include an operation of adjusting threshold voltages of the select transistors and an operation of setting the memory block BLKa as a bad block. 
       FIG. 12  illustrates an application example of a pass/fail check read operation with reference to  FIG. 11 . Referring to  FIGS. 1, 2, 3, 5, 10, and 12 , in operation S 410 , the nonvolatile memory device  110  may perform a pass/fail check read operation using the lower limit voltage VL. The pass/fail check read operation may be performed according to the method of  FIG. 11 . After performing the pass/fail check read operation using the lower limit voltage VL, in operation S 420 , the nonvolatile memory device  110  may perform a pass/fail check read operation using the upper limit voltage VU. The pass/fail check read operation may be performed according to the method described with reference to  FIG. 11 . For example, pass/fail check read operations may be sequentially performed with respect to the string select transistors (SSTa or SSTb) or the ground select transistors GST using the lower limit voltage VL and the upper limit voltage VU. 
       FIG. 13  illustrates another application example of a pass/fail check read operation relating to  FIG. 11 . Referring to  FIGS. 1, 2, 3, 5, 10, and 13 , in operation S 510 , the nonvolatile memory device  110  may perform a pass/fail check read operation with respect to the string select transistors SSTa. The pass/fail check read operation may be performed using the lower limit voltage VL and the upper limit voltage VU according to the method of  FIG. 11 . In operation S 520 , the nonvolatile memory device  110  may perform a pass/fail check read operation with respect to the string select transistors SSTb. The pass/fail check read operation may be performed using the lower limit voltage VL and the upper limit voltage VU according to the method of  FIG. 11 . In operation S 530 , the nonvolatile memory device  110  may perform a pass/fail check read operation with respect to the ground select transistors GST. The pass/fail check read operation may be performed using the lower limit voltage VL and the upper limit voltage VU according to the method of  FIG. 11 . For example, pass/fail check read operations may be sequentially performed with respect to select transistors of different heights that belong to the memory block BLKa. 
       FIG. 14  illustrates an example of a reliability test read operation performed for memory cells. In  FIG. 14 , the horizontal axis indicates threshold voltages Vth of memory cells MC and the vertical axis indicates the number of the memory cells MC. Referring to  FIGS. 1, 2, 3, 5, and 14 , the memory cells MC may be programmed to have an erase state E and first through seventh program states P 1 ˜P 7 . 
     While program, read, and erase operations are performed in the nonvolatile memory device  110 , threshold voltages of the memory cells MC may be increased by disturbance. For example, threshold voltages of memory cells having the erase state E may increase the most. 
     As time goes by after the memory cells MC are programmed, threshold voltages of the memory cells MC may become low. For example, a reduction in threshold voltages of memory cells having the seventh program state P 7  may be to a greatest extent. 
     If threshold voltages of memory cells of the erase state E increase or threshold voltages of memory cells of the seventh program state P 7  decrease, a read error may occur in a read operation. To prevent a read error from occurring, a reliability test read operation may be performed. 
     For example, in the reliability test read operation, the nonvolatile memory device  110  may perform a read operation with respect to memory cells MC using a first read voltage VR 1  and may perform an off-cell count that counts the number of off-cells having a threshold voltage higher than the first read voltage VR 1  among memory cells programmed to the erase state E. The nonvolatile memory device  110  may perform an on-cell count that counts the number of on-cells having a threshold voltage lower than the first read voltage VR 1  among memory cells programmed to the erase state E and estimate a result of the off-cell count according to a result of the on-cell count. 
     For example, in the reliability test read operation, the nonvolatile memory device  110  may perform a read operation with respect to memory cells MC using a second read voltage VR 2  and may perform an on-cell count that counts the number of on-cells having a threshold voltage lower than the second read voltage VR 2  among memory cells programmed to the seventh program state P 7 . The nonvolatile memory device  110  may perform an off-cell count that counts the number of off-cells having a threshold voltage higher than the second read voltage VR 2  among memory cells programmed to the seventh program state P 7  and may estimate a result of the on-cell count according to a result of the off-cell count. 
       FIG. 15  illustrates another application example of a read operation applied to a reliability test read operation of the memory cells MC. Referring to  FIGS. 1, 2, 3, 5, 14, and 15 , in operation S 610 , the nonvolatile memory device  110  may select a word line that will perform a reliability test read operation. 
     In operation S 620 , the nonvolatile memory device  110  may select a first string select line. For example, the nonvolatile memory device  110  may select the first select line (SSL 1   a  or SSL 1   b ) among the string select lines (SSL 1   a ˜SSL 4   a  or SSL 1   b ˜SSL 4   b ) at the same height. 
     In operation S 630 , the nonvolatile memory device  110  may perform a read operation using the first read voltage VR 1  or the second read voltage VR 2 . The read operation may proceed in the order of precharge, develop, and latch. During the precharge time, the page buffer circuit  115  may float the bit lines BL 1 ˜BL 4  after charging the bit lines BL 1 ˜BL 4  with a power supply voltage. During the develop time, the row decoder circuit  113  may apply a turn-on voltage to the string select lines (SSL 1   a , SSL 1   b ) corresponding to a selected string select line and a ground select line GSL 1 , may apply a turn-on voltage to unselected word lines, and may apply the first read voltage VR 1  or the second read voltage VR 2  to a selected word line. During the latch time, the page buffer circuit  115  may set values of the sense latches SL according to voltages of the bit lines BL 1 ˜BL 4 . 
     In operation S 640 , the nonvolatile memory device  110  may accumulate a result of the read operation. For example, the page buffer circuit  115  may update the data latches DL 1  according to the values of the sense latches SL. 
     In operation S 650 , the nonvolatile memory device  110  determines whether the selected string select line is the last string select line. For example, the nonvolatile memory device  110  may determine whether reliability test read operations are completed with respect to the whole string select lines (SSL 1   a ˜SSL 4   a  or SSL 1   b ˜SSL 4   b ). For example, the nonvolatile memory device  110  may determine whether reliability test read operations are completed with respect to some select lines which are check targets among the string select lines (SSL 1   a ˜SSL 4   a  or SSL 1   b ˜SSL 4   b ). 
     If the reliability test read operations are not completed, in operation S 660 , the nonvolatile memory device  110  may select a next string select line. If the reliability test read operations are completed, operation S 670  is performed. 
     In operation S 670 , the nonvolatile memory device  110  may perform a pass/fail check operation. For example, when the reliability test read operation is performed using the first read voltage VR 1 , the pass/fail check circuit (PFC) may perform an off-cell count and compare a count result with a critical value. When the reliability test read operation is performed using the second read voltage VR 2 , the pass/fail check circuit (PFC) may perform an on-cell count and compare a count result with the critical value. 
     In operation S 680 , the nonvolatile memory device  110  determines a pass or a fail. For example, if the count result is less than the critical value, the pass/fail check circuit (PFC) may determine a pass. If a pass is determined, the reliability test read operation may be finished. If the count result is greater than the critical value, the pass/fail check circuit (PFC) may determine a fail. If a fail is determined, in operation S 690 , the nonvolatile memory device  110  may perform a reliability fail process. For example, the reliability fail process may include a refresh operation in which data is read from the memory cells MC of the memory block BLKa and the read data is programmed in other memory block. 
       FIG. 16  illustrates another application example of a read operation applied to a reliability test read operation of the memory cells MC. Referring to  FIGS. 1, 2, 3, 5, 14, and 16 , in operation S 710 , the nonvolatile memory device  110  may select a first string select line. For example, the nonvolatile memory device  110  may select the first string select line (SSL 1   a  or SSL 1   b ) among the string select lines (SSL 1   a ˜SSL 4   a , SSL 1   b ˜SSL 4   b ) at the same height. 
     In operation S 720 , the nonvolatile memory device  110  may select a physical page for performing a reliability test read operation. 
     In operation S 730 , the nonvolatile memory device  110  may perform a read operation using the first read voltage VR 1  or the second read voltage VR 2 . The read operation may proceed in the order of precharge, develop, and latch. 
     In operation S 740 , the nonvolatile memory device  110  may accumulate a result of the read operation. For example, the page buffer circuit  115  may update the data latches DL 1  according to values of the sense latches SL. 
     In operation S 750 , the nonvolatile memory device  110  may determine whether the selected string select line is the last string select line. For example, the nonvolatile memory device  110  may determine whether reliability test read operations are completed with respect to the whole string select lines (SSL 1   a ˜SSL 4   a  or SSL 1   b ˜SSL 4   b ). For example, the nonvolatile memory device  110  may determine whether reliability test read operations are completed with respect to some select lines which are check targets among the string select lines (SSL 1   a ˜SSL 4   a  or SSL 1   b ˜SSL 4   b ). 
     If the reliability test read operations are not completed, in operation S 760 , the nonvolatile memory device  110  may select a next string select line. If the reliability test read operations are completed, operation S 770  is performed. 
     In the operation S 770 , the nonvolatile memory device  110  may perform a pass/fail check operation. In operation S 780 , the nonvolatile memory device  110  determines a pass or a fail. For example, if a count result is less than a critical value, the pass/fail check circuit (PFC) may determine a pass. If a pass is determined, a reliability test read operation may be finished. If the count result is greater than the critical value, the pass/fail check circuit (PFC) can determine a fail. If a fail is determined, in operation S 790 , the nonvolatile memory device  110  may perform a reliability fail process. 
     In comparison with the operation method of  FIG. 15 , in  FIG. 16  the nonvolatile memory device  110  may perform a reliability test read operation with respect to different physical pages from one another when sequentially selecting the string select lines (SSL 1   a ˜SSL 4   a  or SSL 1   b ˜SSL 4   b ). 
       FIG. 17  illustrates an embodiment of a storage device  100  which includes a nonvolatile memory device  110 , a controller  120 , and a RAM  130 . The nonvolatile memory device  110  may operate according to the methods with reference to  FIGS. 1 through 16 . For example, the nonvolatile memory device  110  may perform an erase operation according to the method of  FIG. 6 , may perform a pass/fail check read operation according to the method of  FIG. 11 , or may perform a reliability test read operation according to the method with reference to  FIG. 15 or 16 . Thus, operation speed of the nonvolatile memory device  110  may be improved and operation speed of the storage device  100  may be improved. 
     The nonvolatile memory device  110  may perform write, read, and erase operations according to a control of the controller  120 . The nonvolatile memory device  110  may receive a command and an address from the controller through an input/output channel. The nonvolatile memory device  110  may exchange data with the controller  120  through the input/output channel. 
     The nonvolatile memory device  110  may exchange a control signal with the controller through a control channel. For example, the nonvolatile memory device  110  may receive a chip enable signal /CE selecting at least one nonvolatile memory chip among a plurality of nonvolatile memory chips of the nonvolatile memory device  110 , a command latch enable signal CLE indicating that a signal being received from the controller  120  through an input/output channel is a command, an address latch enable signal ALE indicating that a signal being received from the controller  120  through the input/output channel is an address, a read enable signal /RE that in a read operation, is generated by the controller  120  and is periodically toggled to be used to adjust timing, a write enable signal /WE being activated when a command or an address is transmitted, a write protection signal /WP being activated by the controller  120  to prevent an unwanted write or erase when a power supply is changed, and a data strobe signal DQS that in a write operation, is generated by the controller  120  and is periodically toggled to be used to adjust a sync of data being transmitted through the input/output channel from the controller  120 . For example, the nonvolatile memory device  110  may output a ready &amp; busy signal R/nB indicating that the nonvolatile memory device  110  is performing a program, erase, or read operation and a data strobe signal DQS generated from the read enable signal /RE by the nonvolatile memory device  110  and periodically toggled for adjusting an output sync of data to the controller  120 . 
     The nonvolatile memory device  110  may include, for example, one or more of a flash memory or a nonvolatile memory such as a PRAM (phase-change RAM), a MRAM (magnetic RAM), a RRAM (resistive RAM), or a FeRAM (ferroelectric RAM). 
     The controller  120  may control the nonvolatile memory device  110 . For example, the controller  120  may control the nonvolatile memory device  110  through the input/output channel and the control channel so that the nonvolatile memory device  110  performs a write, read, or erase operation. 
     The controller  120  may control the nonvolatile memory device  110  under the control of an external host device. For example, the controller  120  may communicate with the external host device according to a format different from a format being used when communicating with the nonvolatile memory device  110 . A unit of data by which the controller  120  communicates with the nonvolatile memory device  110  may be different from a unit of data by which the controller  120  communicates with the external host device. 
     The controller  120  may use the RAM  130  as a buffer memory, a cache memory, or an operation memory. The controller  120  may store data and/or code for managing the nonvolatile memory device  110  in the RAM  130 . For example, the controller  120  may read data and/or code for managing the nonvolatile memory device  110  from the nonvolatile memory device  110  and load them into the RAM  130 . The RAM  130  may include at least one among various random access memory devices such as a DRAM (dynamic RAM), a SRAM (static RAM), a SDRAM (synchronous DRAM), a PRAM (phase-change RAM), a MRAM (magnetic RAM), a RRAM (resistive RAM), or a FeRAM (ferroelectric RAM). 
     The nonvolatile memory device  110  may include a plurality of nonvolatile memory chips. For example, the controller  120  and the nonvolatile memory chips may be connected to one another based on a channel and a way. One channel may include one data channel and one control channel. One channel may include one data channel and one control channel. One data channel may include, for example, eight data lines. One control channel may include control lines that transmit the chip enable signal /CE, the command latch enable signal CLE, the address latch enable signal ALE, the read enable signal /RE, the write enable signal /WE, the write protection signal /WP and the ready &amp; busy signal R/nB described above. 
     Nonvolatile memory chips connected to one channel may form a way. If n number of nonvolatile memory chips is connected to one channel, an n-way may be formed. Nonvolatile memory chips that belong to one way may share data lines and control lines that transmit the command latch enable signal CLE, the address latch enable signal ALE, the read enable signal /RE, the write enable signal /WE, and the write protection signal /WP. Each of nonvolatile memory chips that belong to one way may communicate with the controller  120  through control lines only for transmitting the chip enable signal /CE and the ready &amp; busy signal R/nB. 
     The controller  120  may alternately access nonvolatile memory chips of n-way connected to one channel. The controller  120  may independently access nonvolatile memory chips connected to different channels from one another. The controller  120  may alternately or concurrently access nonvolatile memory chips connected to different channels from one another. 
     The nonvolatile memory chips may be connected to the controller  120  in the form of wide I 0 . For example, nonvolatile memory chips connected to different channels from one another may share a control line of one chip enable signal /CE. Nonvolatile memory chips that share a control line of one chip enable signal /CE may be accessed at the same time. Since data lines of different channels are used at the same time, a wide input/output bandwidth may be accomplished. 
     The storage device  100  may include a SSD (solid state drive) or a HDD (hard disk drive). The storage device  100  may include memory cards such as a PC card (PCMCIA, personal computer memory card international association), a compact flash card (CF), a smart media card (SM, SMC), a memory stick, a multimedia card (MMC, RS-MMC, MMCmicro), a SD card (SD, miniSD, microSD, SDHC), a USB (universal serial bus) memory card, a universal flash storage (UFS), etc. The storage device  100  may include a mounting type memory such as an eMMC (embedded multimedia card), a UFS, a PPN (perfect page NAND), etc. 
     In  FIG. 17 , the storage device  100  may include the RAM  130  outside the controller  120 . However, the storage device  100  may not include the RAM  130  outside the controller  120 . The controller  120  may use an internal RAM (e.g., refer to  FIG. 18 ) as a buffer memory, an operation memory, or a cache memory. 
       FIG. 18  illustrates an embodiment of a controller  120 . Referring to  FIGS. 17 and 18 , the controller  120  includes a bus  121 , a processor  122 , a RAM  123 , an error correction block  124 , a host interface  125 , a buffer control circuit  126 , and a memory interface  127 . The bus  121  may provide a channel between constituent elements of the controller  120 . 
     The processor  122  may control the overall operation of the controller  120  and perform a logical operation. The processor  122  may communicate with the external host device through the host interface  125 , may communicate with the nonvolatile memory device  110  through the memory interface  127 , and may communicate with the RAM  130  through the buffer control circuit  126 . The processor  122  may use the RAM  123  as an operation memory, a cache memory, or a buffer memory in order to control the storage device  100 . 
     The RAM  123  may be used as an operation memory, a cache memory, or a buffer memory of the processor  122 . The RMA  123  may store codes and commands executed by the processor  122 . The RAM  123  may store data processed by the processor  122 . The RAM  123  may include a SRAM (static RAM). 
     The error correction block  124  performs an error correction operation. For example, the error correction block  124  may perform error correction encoding based on data to be written in the nonvolatile memory device  110  through memory interface  127 . The error correction encoded data may be transmitted to the nonvolatile memory device  110  through the memory interface  127 . The error correction block  124  may perform error correction decoding with respect to data received from the nonvolatile memory device  110  through the memory interface  127 . The error correction block  124  may be in the memory interface  127 , for example, as a constituent part of memory interface  127 . 
     The host interface  125  communicates with an external host under control of the processor  122 . The host interface  125  may perform communications through at least one of a plurality of communication protocols or devices, e.g., a USB (universal serial bus), a SAS (serial attachment SCSI), a SATA (serial AT attachment), a SAS (serial attachment SCSI), a HSIC (high speed interchip), a SCSI (small computer system interface), a PCI (peripheral component interconnection), a PCIe (PCI express), a NVMe (nonvolatile memory express), a UFS (universal flash storage), a SD (secure digital), a MMC (multimedia card), an eMMC (embedded MMC), a DIMM (dual in-line memory module), a RDIMM (registered DIMM), or a LRDIMM (load reduced DIMM). 
     The buffer control circuit  126  controls the RAM  130  according to a control of the processor  122 . 
     The memory interface  127  communicates with the nonvolatile memory device  110  under control of the processor  122 . As described with reference to  FIG. 1 , the memory interface  127  may communicate a command, an address, and data with the nonvolatile memory device  110  through the input/output channel. The memory interface  127  may communicate a control signal with the nonvolatile memory device  110  through the control channel. 
     When the RAM  130  is not provided to the storage device  100 , the buffer control circuit  126  may not be provided to the controller  120 . 
     The processor  122  may control the controller  120  using codes. The processor  122  may load code from a nonvolatile memory device (e.g., read only memory) inside the controller  120 . The processor  122  may load code from the nonvolatile memory device  110  through the memory interface  127 . 
     The bus  121  of the controller  120  may be divided into a control bus and a data bus. The data bus may transmit data in the controller  120  and the control bus may transmit control information (e.g., a command and an address) in the controller  120 . In one embodiment, the data bus and the control bus may be separate from each other and may not interfere with each other or affect each other. The data bus may be connected to the host interface  125 , the buffer control circuit  126 , the error correction block  124 , and the memory interface  127 . The control bus may be connected to the host interface  125 , processor  122 , buffer control circuit  126 , the RAM  123 , and memory interface  127 . 
       FIG. 19  illustrates an embodiment of a computing device  1000  which includes a processor  1100 , a memory  1200 , a storage device  1300 , a modem  1400 , and a user interface  1500 . The processor  1100  may control overall operation of the computing device  1000  and may perform one or more logical operations. The processor  1100  may be, for example, a hardware-based data processing device including a circuit physically constituted to execute operations expressed by code or a command in a program. For example, the processor  1100  may be constituted by a SoC (system-on-chip). The processor  1100  may be, for example, a general purpose processor, a special purpose processor, or an application processor. 
     The RAM  1200  communicates with the processor  1100  and may be, for example, a main memory of the processor  1100  or the computing device  1000 . The processor  1100  may temporarily store code or data in the RAM  1200 . The processor  1100  may execute code and process data using the RAM  1200 . The processor  1100  may execute various software, e.g., an OS (operating system) and an application, stored in the RAM  1200 . The processor  1100  may control overall operation of the computing device  1000  using the RAM  1200 . The RAM  1200  may be, for example, a volatile memory device such as a SRAM (static RAM), a DRAM (dynamic RAM), a SDRAM (synchronous DRAM), etc., or a nonvolatile memory device such as a PRAM (phase-change RAM), a MRAM (magnetic RAM), a RRAM (resistive RAM), a FRAM (ferroelectric RAM), etc. 
     The storage device  1300  may communicate with the processor  1100 . The storage device  1300  may store data to be preserved on a long term basis. In one embodiment, the processor  1100  may store data to be preserved for a long time in the storage device  1300 . The storage device  1300  may store a boot image for driving the computing device  1000 . The storage device  1300  may store source code of various software, e.g., an OS and an application. 
     The processor  1100  may drive various software, such as an OS and an application, by loading source code stored in the storage device  1300  into the RAM  1200  and executing the loaded source code into the RAM  1200 . The processor  1100  may load data stored in the storage device  1300  into the RAM  1200  and may process data loaded into the RAM  1200 . The processor  1100  may store data to be preserved in the long term among data stored in the RAM  1200  in the storage device  1300 . 
     The storage device  1300  may include a nonvolatile memory device such as a flash memory, a PRAM (phase-change RAM), a MRAM (magnetic RAM), a RRAM (resistive RAM), a FRAM (ferroelectric RAM), etc. 
     The modem  1400  may perform communications with an external device under control of the processor  1100 . For example, the modem  1400  may perform a wired or wireless communication with an external device. The modem  1400  may perform communications based on at least one of various wireless communication methods such as a long term evolution (LTE), a WiMax, a global system for mobile communication (GSM), a code division multiple access (CDMA), a Bluetooth, a near field communication (NFC), a WiFi, a radio frequency Identification (RFID), or at least one of various wired communication methods such as a universal serial bus (USB), a serial at attachment (SATA), a HSIC (high speed interchip), a small computer small interface (SCSI), a Firewire, a peripheral component interconnection (PCI), a PCI express (PCIe), a nonvolatile memory express (NVMe), a universal flash storage (UFS), a secure digital (SD), a SDIO, a universal asynchronous receiver transmitter (UART), a SPI (serial peripheral interface), a high speed SPI (HS-SPI), a RS232, an inter-intergrated circuit (I2C), a HS-I2C, an integrated-interchip sound (I2S), a sony/philips digital interface (S/PDIF), a multimedia card (MMC), an embedded MMC (eMMC), etc. 
     The user interface  1500  may communicate with a user under control of the processor  1100 . For example, the user interface  1500  may include user input interfaces such as a keyboard, a keypad, a button, a touch panel, a touch screen, a touch pad, a touch ball, a camera, a mike, a gyroscope sensor, a vibration sensor, etc. The user interface  1500  may include user output interfaces such as a liquid crystal display (LCD), an organic light emitting diode (OLED) display, an active matrix OLED (AMOLED) display, a LED, a speaker, a motor, etc. 
     The storage device  1300  may include a storage device  100  in accordance with one or more of the aforementioned example embodiments. The processor  1100 , the RAM  1200 , the modem  1400 , and the user interface  1500  may form a host device that communicates with the storage device  1300 . 
     The storage device  1300 , as described with reference to  FIG. 17 , may include the nonvolatile memory device  110  that operate according to any of the method embodiments described with reference to  FIGS. 1 to 16 . Thus, if the operation speed of the storage device  1300  is improved, the operation speed of the computing device  1000  that accesses the storage device  1300  may also be improved. 
     In accordance with one embodiment, an apparatus includes first logic to perform a read operation for first cell strings of a memory to generate a first read result; second logic to perform a read operation for second cell strings of the memory to generate a second read result; and third logic to perform a subsequent operation based on a third read result, wherein the third read result is based on the first read result and the second read result. The first, second, and third logic may correspond to the constituent elements corresponding to any of the aforementioned embodiments. The read operations for the first and second cell strings may be erase verify read operations, pass/fail check read operations with respect to select transistors of the first and second cell strings, or reliability test read operations with respect to some of a plurality of memory cells of the first and second cell strings. The memory may be, for example, a three-dimensional memory. 
     The logic, logic circuits, decoders, encoders, selectors, and selection circuits described herein may include hardware, software, or both. When implemented at least partially in hardware, the logic, logic circuits, decoders, encoders, selectors, and selection circuits may be, for example, any one of a variety of integrated circuits including but not limited to an application-specific integrated circuit, a field-programmable gate array, a combination of logic gates, a system-on-chip, a microprocessor, or another type of processing or control circuit. 
     When implemented in at least partially in software, the logic, logic circuits, decoders, encoders, selectors, and selection circuits may include, for example, a memory or other storage device for storing code or instructions to be executed, for example, by a computer, processor, microprocessor, controller, or other signal processing device. The computer, processor, microprocessor, controller, or other signal processing device may be those described herein or one in addition to the elements described herein. Because the algorithms that form the basis of the methods (or operations of the computer, processor, microprocessor, controller, or other signal processing device) are described in detail, the code or instructions for implementing the operations of the method embodiments may transform the computer, processor, controller, or other signal processing device into a special-purpose processor for performing the methods herein. 
     In accordance with one or more of the aforementioned embodiments, when verify read operations are performed with respect to a plurality of string select lines, a pass/fail check operation is performed corresponding to a number of times less than the number of the verify read operations. Thus, a nonvolatile memory device having improved operation speed and a method for operating the nonvolatile memory device may be provided. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. The embodiments may be combined to form additional embodiments. In some instances, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise Accordingly, various changes in form and details may be made without departing from the spirit and scope of the embodiments set forth in the claims.