Abstract:
A method for operating a page buffer of a nonvolatile memory device includes activating a first latch circuit of the page buffer in a programming operation and inactivating the first latch circuit in a copy-back programming operation. A second latch circuit is activated in both the copy-back programming operation and the programming operation.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to methods for operating page buffers in nonvolatile memory devices and more particularly, to methods for operating a page buffer of a NAND flash memory device.  
         [0003]     2. Discussion of Related Art  
         [0004]     Semiconductor memory devices are known to use electrically programmable and erasable components with refresh functions for restoring data in predetermined periods. Herein, “programming” means an operation to write data in memory cells.  
         [0005]     NAND flash memory devices have strings containing a number of memory cells serially connected to enable a high integration of a memory device (i.e., adjacent memory cells share a drain or source). NAND flash memory devices are types of memory devices that are configured to read out information in a sequence, which is different from NOR flash memory devices.  
         [0006]     A NAND flash memory device employs page buffers to store a large amount of data into memory cells or to read out information from the memory cells. The page buffers receive a large amount of data through input/output pads and then provide the data to the memory cells, or output the data after storing the data of the memory cells. The page buffer can consist of a single register to temporarily store data, or a dual register to raise a programming speed in programming a large amount of data.  
         [0007]     A copy-back function is required when memory cells are defective. Data in defective memory cells is transferred to other normal memory cells by way of the page buffers.  
         [0008]      FIG. 1  is a block diagram showing a copy-back programming operation in a conventional NAND flash memory device.  
         [0009]     Referring to  FIG. 1 , a conventional copy-back programming operation includes the steps of: reading out a data bit of a defective memory cell of a memory cell array  10  through a bitline (e.g., BLe) selected by a bitline selection/bias circuit  21  and a sensing node SO and then storing the read data bit into a main latch circuit  23  of a page buffer  20  (step  41 ); transferring the data bit from the main latch circuit  23  to a cache latch circuit  24  (step  42 ); returning the data bit from the cache latch circuit  24  to the main latch circuit  23  (step  43 ); and then reprogramming the data bit of the main latch circuit  23  in another memory cell (a normal memory cell) by way of the selected bitline and the sensing line SO (step  44 ).  
         [0010]     However, such a copy-back programming scheme can result in a high probability of errors while transferring data between the main latch circuit  23  and the cache latch circuit  24 .  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention provides a method for operation of a page buffer in a nonvolatile memory device, capable of shortening a copy-back programming time with transmission errors between latch circuits.  
         [0012]     In one embodiment of the present invention, a nonvolatile memory device comprises: a memory cell array including a number of memory cells that are arranged on intersections of wordlines and bitlines. A number of page buffers are connected to the memory cell array through a sensing line. Each page buffer comprises: a first and second latch circuit configured to store a program data bit; a data transmission module configured to transfer the program data bit to a selected bitline through the sensing line during a programming operation, the program data bit transferred from the first latch circuit and stored in the second latch circuit; and a copy-back programming circuit connected between the second latch circuit and the sensing line and configured to conduct a copy-back programming operation. In one embodiment, the first latch circuit is activated only in the programming operation and inactivated in the copy-back programming operation, and the second latch circuit is activated in both the copy-back programming operation and the programming operation.  
         [0013]     In another embodiment, during the copy-back programming operation, the second latch circuit reads and stores a data bit through a selected bitline and the sensing line. The data bit has been programmed in a defective memory cell among the memory cells. The copy-back programming circuit inverses the read data bit stored in a first node of the second latch circuit and transfers the inverse data bit to the selected bitline through the sensing line so as to reprogram the data bit into a normal memory cell among the memory cells.  
         [0014]     In one embodiment, the copy-back programming circuit comprises: an inverter configured to invert a data bit of the first node of the second latch circuit during a copy-back programming operation; and a transfer module configured to transfer the inverse data bit from the inverter to the selected bitline through the sensing line.  
         [0015]     In another embodiment, the first latch circuit comprises: a latch configured to store the program data bit, which is supplied from an external source in the programming operation; a first transfer module configured to transfer the program data bit to the first latch circuit, the first transfer module connected to first and second nodes of the first latch circuit; and a second transfer module configured to transfer a data bit from the first node of the first latch circuit to the second latch circuit through the sensing line.  
         [0016]     In still another embodiment, the second latch circuit comprises: a latch configured to store a data bit in response to a voltage level of the sensing line during the copy-back programming operation and the programming operation; and a discharge module configured to discharge a first node of the latch in response to the voltage level of the sensing line in the copy-back programming operation, the programming operation, or a reading operation.  
         [0017]     In one embodiment, the data transmission module comprises: a programming switch module configured to transfer a data bit from a second node of the second latch circuit to the selected bitline through the sensing line so as to program the memory cell, the programming switch module active in the programming operation; and reading a transfer module configured to transfer a data bit from the second node of the second latch circuit to an external device through a data line, the transfer module active in a reading operation.  
         [0018]     In another embodiment, the nonvolatile memory device further comprises a verifying switch module configured to detect a programmed state of pass or failure from reading a data bit of a second node of the second latch circuit.  
         [0019]     In still another embodiment, the page buffer comprises: a precharging circuit configured to preliminarily charge the sensing line while reading data programmed in the memory cells; and a bitline selection/bias circuit configured to select one of the bitlines and connect the selected bitline to the sensing line.  
         [0020]     In another embodiment of the present invention, a method is provided for operating a page buffer of a nonvolatile memory device. The nonvolatile memory device includes a memory cell array composed of memory cells arranged on intersections of wordlines and bitlines, and a number of page buffers connected to the memory cell array through a sensing line. Each page buffer has a first and second latch circuit. The method comprises: activating the first and second latch circuits during a programming operation; and inactivating the first latch circuit during a copy-back programming operation.  
         [0021]     In one embodiment, the copy-back programming operation is carried out with the steps of: reading a data bit which has been programmed in a defective memory cell through a selected bitline and sensing line; storing the read data bit into a second latch circuit; inversing the read data bit stored in the second latch circuit; transferring the inverse data bit to the selected bitline; and reprogramming the data bit into a normal memory cell among the memory cells.  
         [0022]     In another embodiment, the copy-back programming operation comprises: precharging the sensing line; detecting a precharged or discharged state on the sensing line; and storing the data bit of the defective memory cell into the second latch circuit.  
         [0023]     In still another embodiment, the step of inversing comprises: inversing a data bit of a first node of the second latch circuit. The programming operation may be carried out with steps comprising: storing a program data bit in the first latch circuit; transferring the program data bit from the first latch circuit to the second latch circuit through the sensing line; and transferring the program data bit to a selected bitline through the sensing line and programming the transferred data bit into the memory cell. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated herein and constitute a part of this specification. The drawings illustrate example embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the drawings:  
         [0025]      FIG. 1  is a block diagram showing a copy-back programming operation in a conventional NAND flash memory device;  
         [0026]      FIG. 2  is a block diagram showing a copy-back programming operation in a NAND flash memory device in accordance with one embodiment of the present invention;  
         [0027]      FIG. 3  is a circuit diagram illustrating the NAND flash memory device shown in  FIG. 2 ;  
         [0028]      FIG. 4  is a circuit diagram showing a copy-back programming operation in the NAND flash memory device shown  FIG. 3 ; and  
         [0029]      FIG. 5  is a timing diagram showing a copy-back programming operation in the NAND flash memory device shown  FIG. 3 .  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0030]     Embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numerals refer to like elements throughout the specification.  
         [0031]     Hereinafter, it will be described about an exemplary embodiment of the present invention in conjunction with the accompanying drawings.  
         [0032]      FIG. 2  is a block diagram showing a copy-back programming operation in a NAND flash memory device in accordance with one embodiment of the present invention.  
         [0033]     Referring to  FIG. 2 , the NAND flash memory device is programmed by first reading out a data bit into a main latch circuit  240  through a bitline (e.g., BLE) selected by a bitline selection/bias circuit  210  (step  401 ). The data bit comes from a defective memory cell of a memory cell array  100 . The data bit is then transfered from main latch circuit  240  to a selected bitline through the copy-back programming circuit  230 . The data bit in main latch circuit  240  is then reprogrammed in a normal memory cell of the memory cell array  100  (step  402 ).  
         [0034]     The data bit fetched to the main latch circuit  240  is transferred to a cache latch circuit  250  and the data bit returns to the main latch circuit to be reprogrammed in the normal memory cell. The procedure shown in  FIG. 2  reprograms the data bit, which is fetched to the main latch circuit  240 , directly in a normal memory cell without returning it to a cache latch circuit  250 .  
         [0035]      FIG. 3  is a circuit diagram illustrating the NAND flash memory device shown in  FIG. 2 .  
         [0036]     Referring to  FIG. 3 , the NAND flash memory device includes a memory cell array  100 , a page buffer  200 , and a column selection circuit  300 .  
         [0037]     In memory cell array  100 , the reference numeral BLe denotes even-ordered bitlines while the reference numeral BLo denotes odd-ordered bitlines. A number of memory cells MC 1 ˜MCn are connected to the even-ordered bitlines BLe, while other memory cells are connected to the odd-ordered bitlines BLo. A memory cell (e.g., MC 1 ) is controlled by a single wordline (e.g., WL 1 ), belonging to a single page.  
         [0038]     The page buffer  200  is coupled between the memory cell array  100  and the column selection circuit  300 . Bitlines BLe and BLo are coupled to the page buffer  200  through a sensing line SO.  FIG. 3  illustrates a simplified page buffer in convenience of description. It should be appreciated that any number of page buffers may be used in page buffer  200 . Page buffer  200  includes a bitline selection/bias circuit  210 , a precharging circuit  220 , a copy-back programming circuit  230 , a main latch circuit  240 , and a cache latch circuit  250 .  
         [0039]     The bitline selection/bias circuit  210  includes NMOS transistors N 11 ˜N 14 . NMOS transistor N 11  is connected between the bitline BLe and a line supplying a voltage supply signal VIRPWR. NMOS transistor N 11  is turned on or off in response to a gate control signal DISe that is applied to a gate of transistor N 11 . NMOS transistor N 11  can be turned on in response to the gate control signal DISe, applying a voltage supply signal VIRPWR as a power source voltage to bitline BLe when a data bit is to be programmed in bitline BLo. NMOS transistor N 12  is connected between the bitline BLo and the line supplying the voltage supply signal VIRPWR. NMOS transistor N 12  is turned on or off in response to a gate control signal DISo that is applied to a gate of transistor N 12 . NMOS transistor N 12  can be turned on in response to the gate control signal DISo, applying a voltage supply signal VIRPWR as a power source voltage to bitline BLo when a data bit is to be programmed in bitline BLe. In one embodiment, voltage supply signal VIRPWR is set to the same level as power source voltage VCC during the programming operation. NMOS transistor N 13  connects bitline BLe to the sensing line SO in response to a bitline selection signal BSLe. NMOS transistor N 14  connects bitline BLo to sensing line SO in response to a bitline selection signal BSLo.  
         [0040]     Precharging circuit  220  is connected between a power source voltage VCC and a sensing line SO. The precharging circuit  220  includes a PMOS transistor P 11  that may be turned on or off in response to a precharge signal PRECHb applied to a gate of transistor P 11 . PMOS transistor P 11  precharges sensing line SO up to the power source voltage VCC during the reading operation, supplying a current to bitline BLe or BLo through sensing line SO.  
         [0041]     Main latch circuit  240  includes NMOS transistors N 21 ˜N 23  and a first latch LT 1 . The first latch LT 1  includes inverters IV 1  and IV 2 , configured to temporarily store a data bit read out from a memory cell. The NMOS transistor N 21  is turned on or off in response to a signal on the sensing line SO. NMOS transistor N 22  is turned on or off in response to a main latch signal MLCH. In one embodiment, NMOS transistor N 22  is turned on when NMOS transistor N 21  is turned on, changing a node QAb of the first latch LT 1  to a logic low (e.g., ‘0’) while a node QA of the first latch LT 1  is changed to a logic high (e.g., ‘1’). NMOS transistor N 23  is coupled between the node QA of the first latch LT 1  and a ground voltage VSS, initializing the node QA of the first latch LT 1  to ‘0’ and node QAb to ‘1’ in response to a reset signal MRST applied to a gate of transistor N 23 .  
         [0042]     Copy-back programming circuit  230  includes an inverter IV 3  and an NMOS transistor N 24 . The inverter IV 3  outputs an inverse signal from a signal of the node QAb of the first latch LT 1 . The NMOS transistor N 24  is coupled between the sensing line SO and the main latch circuit  240 . NMOS transistor N 24  is turned on in response to a copy-back signal CPBK applied to a gate of transistor N 24 . The NMOS transistor N 24  connects the main latch circuit  240  to the sensing line SO, in order to reprogram a data bit of a defective memory cell, which is stored in the main latch circuit  240 , into a normal cell in the copy-back programming operation.  
         [0043]     Cache latch circuit  250  includes NMOS transistors N 40 ˜N 43  and N 47 , and a second latch LT 2 . The second latch LT 2  is includes inverters IV 4  and IV 5  configured to temporarily store a data bit transferred from the main latch circuit  240 . NMOS transistor N 40  is coupled between the node QB of the second latch LT 2  and a ground voltage VSS, initializing the node QB of the second latch LT 2  to logic low (e.g., ‘0’) and the node QBb to logic high (e.g., ‘1’) in response to a reset signal CSET applied to a gate of transistor N 40 . NMOS transistor N 41  stores a program data bit which is transferred from an external source into the second latch LT 2  through a data line in response to a data input signal nDI. NMOS transistors N 42  and N 43  store data to be programmed, which is transferred from an external source into the second latch LT 2  through a data line in response to a data input signal DI. NMOS transistor N 47  is turned on in response to a program dump signal PDUMP in the program operation, transferring a data bit from node QBb of the second latch LT 2  to the main latch circuit  250  through the sensing line SO.  
         [0044]     In one embodiment, page buffer  200  includes NMOS transistors N 44 ˜N 46 , a PMOS transistor P 12 , a bitline selection/bias circuit  210 , a precharging circuit  220 , a copy-back programming circuit  230 , a main latch circuit  240 , and a cache latch circuit  250 . NMOS transistor N 44  is turned on in response to a program signal PGM in the programming operation, transferring a program data bit to a selected bitline (e.g., BLe) through sensing line SO. An example of a program data bit is a data bit of node QA of first latch LT 1 . NMOS transistor N 45  is turned on in response to a read signal PBDO in the read operation, transferring a data bit from the selected bitline to the data line DL through column selection circuit  300 . PMOS transistor P 12  is connected between the power source voltage VCC and a node nWDO. Transistor P 12  is turned on or off in response to the data bit of the node QA of the first latch LT 1  so as to verify pass or fail of the memory cell by the programming or erasing operation.  
         [0045]     Column selection circuit  300  includes two NMOS transistors N 51  and N 52  controlled by column selection signals YA and YB. The NMOS transistors, N 51  and N 52 , function to connect the page buffer  200  to the data line DL in the reading and programming operations. The column selection signals, YA and YB, are generated from a column address.  
         [0046]      FIGS. 4 and 5  are circuit and timing diagrams showing the copy-back programming operation in the NAND flash memory device shown  FIG. 3 .  
         [0047]     In accordance with one embodiment of the present invention, to illustrate the copy-back programming operation, assume that a defective memory cell is MC 1 . A data bit of the defective memory cell MC 1  is read into the first latch circuit  240  and then reprogrammed into a normal memory cell (e.g., MC 2 ).  
         [0048]     In one embodiment, NMOS transistor N 13  is turned on to select a wordline WL 1  and a bitline BLe in response to the bitline selection signal BSLe. This is done to read out a data bit from the memory cell MC 1  and reprogram it into another normal memory cell.  
         [0049]     As illustrated in  FIGS. 4 and 5 , the reset signal MRST initializes (e.g., produces a pulse) so that node QA of the first latch LT 1  is set to logic low (e.g., ‘0’) and the node QAb of the first latch LT 1  is set to logic high (e.g., ‘1’). Sensing line SO is precharged to equal the level of power source voltage VCC. As memory cell MC 1  remains a program cell, sensing line SO retains the precharged voltage equal to power source voltage VCC. NMOS transistors N 21  and N 22  are turned on as illustrated in  FIG. 5 , so that the node QAb of the first latch LT 1  is set to ‘0’ and the node QA of the first latch LT 1  is changed to ‘1’ (reading step  401 ).  
         [0050]     During this process, inverter IV 3  turns the node QA of the first latch LT 1  from ‘1’ to ‘0’. As NMOS transistor N 24  is turned on in response to the copy-back signal CPBK, the data bit ‘0’ output from the inverter UV 3  is transferred to the bitline BLe and the data bit of the memory cell MC 1  is reprogrammed into the normal memory cell MC 2  (programming step  402 ).  
         [0051]     In accordance with one embodiment of the present invention, the copy-back programming operation may be executed with just a main latch circuit without a cache latch circuit.  
         [0052]     In accordance with one embodiment of the present invention, it is possible to reprogram a data bit from a defective memory cell into a normal memory cell directly through a bitline without using a cache latch. This can raise a speed of the copy-back programming operation.  
         [0053]     Although the present invention has been described in connection with the embodiment of the present invention illustrated in the accompanying drawings, it is not limited by the embodiments described. It will be apparent to those skilled in the art that various substitutions, modifications and changes may be made without departing from the scope and spirit of the present invention.