Abstract:
A program method of a nonvolatile memory device includes programming data of a first bit into a target page of a plurality of pages in a memory cell array, sensing the programmed data and storing the sensed data in a page buffer coupled to the memory cell array, erasing data of the target page, inputting data of a second bit to the page buffer and generating program data by combining the data of the second bit and the data of the first bit stored in the page buffer, and programming the program data into the target page.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Priority to Korean patent application number 10-2010-0066487 filed on Jul. 9, 2010, the entire disclosure of which is incorporated by reference herein, is claimed. 
     BACKGROUND 
     Exemplary embodiments relate to a nonvolatile memory device and a program method thereof and, more particularly, to a nonvolatile memory device and a program method thereof, which are capable of prohibiting a shift in the threshold voltage of an erase cell. 
     In nonvolatile memory devices which enable electrical program and erasure of data without the refresh function of rewriting data at specific intervals, the interval between neighboring memory cells narrows with an increase in the degree of integration. 
       FIGS. 1A and 1B  are diagrams illustrating the influence on program cells and erase cells due to the interference between neighboring memory cells. 
     As shown in  FIGS. 1A and 1B , a program cell is subjected to a coupling effect from neighboring memory cells that are simultaneously being programmed, whereas an erase cell is subjected to a coupling effect due to all neighboring memory cells regardless of whether they are simultaneously erased. Accordingly, a threshold voltage of the erase cell may be more widely distributed than that of the program cell. 
       FIG. 2  shows a shift in the threshold voltage distribution of an erase cell. 
     As shown in  FIG. 2 , the threshold voltage distribution of an erase cell is shifted due to the program operation for neighboring memory cells. For example, in the case where all memory cells adjacent to an erase cell are programmed with a third threshold voltage distribution C, the erase cell may experience the greatest shift in the threshold voltage distribution. Accordingly, at the time of a read operation, the erase cell may be erroneously read as if it has the threshold voltage distribution of a program cell, for example, a first threshold voltage distribution A. 
     BRIEF SUMMARY 
     Exemplary embodiments relate to a program method of a nonvolatile memory device. According to the embodiments, after a least significant bit (LSB) program operation is performed on a selected page, the least significant bit (LSB) data of the selected page is sensed and stored in a page buffer, and an erase operation is then performed on the selected page. Next, most significant bit (MSB) data is inputted to the page buffer, and a program operation is then performed on the selected page using the previously stored least significant bit (LSB) data and the newly inputted most significant bit (MSB) data. Accordingly, a margin for a read operation can be increased due to a lack of a shift in the threshold voltage distribution of erase cells included in the selected page. 
     A program method of a nonvolatile memory device according to an aspect of this disclosure includes programming data of a first bit into a target page of a plurality of pages in a memory cell array, sensing the programmed data and storing the sensed data in a page buffer coupled to the memory cell array, erasing data of the target page, inputting data of a second bit to the page buffer and generating program data by combining the data of the second bit and the data of the first bit stored in the page buffer, and programming the program data into the target page. 
     A program method of a nonvolatile memory device according to another exemplary embodiment of this disclosure includes inputting least significant bit (LSB) data to be programmed into a target page of plurality of pages, included in a memory cell array, to a page buffer coupled to the memory cell array, erasing data of the target page, inputting most significant bit (MSB) data to the page buffer, and combining the least significant bit (LSB) data and the most significant bit (MSB) data and programming the combination data into the target page as program data. 
     A program method of a nonvolatile memory device according to further exemplary embodiment of this disclosure includes inputting least significant bit (LSB) data to a first latch of a page buffer, erasing data of a target page of a plurality of pages in a memory cell array, inputting most significant bit (MSB) data to a second latch of the page buffer, and combining the MSB data and the LSB data stored in the page buffer and programming the combination data into the target page. 
     A nonvolatile memory device according to an aspect of this disclosure includes a memory cell array including a plurality of pages and a page buffer coupled to the memory cell array and including first, second, and third latches. During a program operation, the page buffer is configured to input least significant bit (LSB) data to the first latch, erase data of a target page, input most significant bit (MSB) data to the second latch, generate program data by combining the most significant bit (MSB) data and the least significant bit (LSB) data, store the program data in the third latch, and program the program data into the target page. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are diagrams illustrating the influence on program cells and erase cells due to the interference between neighboring memory cells; 
         FIG. 2  shows a shift in the threshold voltage distribution of an erase cell; 
         FIG. 3  is a configuration showing a memory cell array and a page buffer of a nonvolatile memory device; 
         FIG. 4  is a flowchart illustrating a program method according to a first exemplary embodiment of this disclosure; and 
         FIG. 5  is a flowchart illustrating a program method according to a second exemplary embodiment of this disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, some exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The figures are provided to enable those of ordinary skill in the art to make and use the exemplary embodiments of the disclosure. 
       FIG. 3  is a configuration showing a memory cell array and a page buffer of a nonvolatile memory device and. 
     Referring to  FIG. 3 , the memory cell array  100  includes a plurality of strings. Each of the strings includes a drain select transistor DST, a plurality of memory cells MCn to MC 0 , and a source select transistor SST coupled in series between a bit line (for example, BLe) and a common source line CSL. 
     Memory cells coupled to the same word line, among the plurality of memory cells of the memory cell array  100 , are classified as the same page. For example, the memory cells MCn−1 included in respective strings and coupled to a word line WLn−1 are defined as a page. 
     The page buffer  200  is coupled to the bit lines BLe and BLo of the memory cell array  100 . The page buffer  200  includes a bit line selector  210 , a precharge unit  220 , a first latch  230 , a second latch  240 , and a third latch  250 . The bit line selector  210  selectively couples the even bit line BLe or the odd bit line BLo to the sense node SO of the page buffer  200 . The precharge unit  220  precharges the sense node SO by supplying a power supply voltage to the sense node SO. The first latch  230  temporarily stores least significant bit (LSB) of program data received from an I/O terminal YA when an operation of inputting the program data is performed. The first latch  230  may send the stored program data to the memory cell of a target page through the bit line selectively coupled to the sense node SO through the bit line selector  210 . The first latch  230  may sense a program state of the memory cell of the target page through the bit line selectively coupled to the sense node SO when a sense operation is performed and store the sensed data. The second latch  240  temporarily stores most significant bit (MSB) of the program data received from the I/O terminal YA when the operation of inputting the program data is performed. The third latch  250  combines the least significant bit (LSB) data and the most significant bit (MSB) data, respectively stored in the first and second latches  230  and  240 . When an operation of programming both the least significant bit (LSB) data and the most significant bit (MSB) data is performed, the third latch  250  sends a result of the combination to the memory cell as the program data. 
       FIG. 4  is a flowchart illustrating a program method according to a first exemplary embodiment of this disclosure. 
     The program method of a semiconductor memory device according to the exemplary embodiment of this disclosure is described below with reference to  FIGS. 3 and 4 . 
     1) Input of Least Significant Bit (LSB) Data ( 410 ) 
     Least significant bit (LSB) data to be programmed into a target page of the memory cell array  100  is stored in the first latch  230  of the page buffer  200 . More particularly, the least significant bit (LSB) data is transmitted to the first latch  230  through the I/O terminal YA coupled to the page buffer  200  and then temporarily stored therein. 
     2) Program of Least Significant Bit (LSB) Data ( 420 ) 
     The least significant bit (LSB) data is programmed into the target page of the memory cell array  100  by performing a least significant bit (LSB) data program operation. More particularly, the bit line selector  210  of the page buffer  200  selects one (for example, the even bit line BLe) of the even and odd bit lines BLe and BLo and couples the selected bit line BLe to the sense node SO. The voltage level of the sense node SO is controlled to be a high level or a low level in response to the least significant bit (LSB) data stored in the first latch  230 , and the voltage level of the bit line BLe coupled to the sense node SO is also controlled in response to the controlled voltage level of the sense node SO. Next, a program voltage is supplied to the word line WL&lt;n−1&gt; coupled to the target page, so that the least significant bit (LSB) data is programmed into the selected memory cell MCn−1 of the target page. 
     3) Sense of Least Significant Bit (LSB) Data ( 430 ) 
     The data programmed into the selected memory cell MCn−1 of the target page of the memory cell array  100  is sensed and stored in the page buffer  200 . More particularly, a read voltage is supplied to the word line WL&lt;n−1&gt; coupled to the target page, and a pass voltage is supplied to the remaining word lines WL&lt;0&gt; to WL&lt;n−2&gt; and WL&lt;n&gt; except the word line WL&lt;n−1&gt;. Next, the bit line selector  210  of the page buffer  200  selects the even bit line BLe and couples it to the sense node SO. The voltage levels of the even bit line BLe and the sense node SO are maintained in a high level or discharged to a low level according to a program state of the selected memory cell MCn−1. The first latch  230  senses the voltage level of the sense node SO and stores the sensed voltage level as the least significant bit (LSB) data of the selected memory cell MCn−1. 
     4) Erase of Target Page Data ( 440 ) 
     Data stored in the memory cells of the target page of the memory cell array  100  is erased. Here, the erase operation is performed on only the memory cells of the target page. An example of the erase operation performed per page is described below. 
     First, an operation voltage of 0 V is supplied to the selected word line WL&lt;n−1&gt; coupled to the target page, and an erase-inhibition voltage of about 5 V is supplied to the remaining word lines WL&lt;0&gt; to WL&lt;n−2&gt; and WL&lt;n&gt; except the word line WL&lt;n−1&gt;. Next, an erase voltage of about 18 V to 20 V is supplied to the P well of the semiconductor substrate in which the memory cell array  100  is disposed. Electric charges stored in the floating gates of the memory cells coupled to the word line WL&lt;n−1&gt; tunnel toward semiconductor substrate by means of the erase voltage and thus are erased. However, electric charges stored in the floating gates of memory cells coupled to the remaining word lines WL&lt;0&gt; to WL&lt;n−2&gt; and WL&lt;n&gt; may not tunnel through due to the erase-inhibition voltage supplied to the gates of the memory cells although the erase voltage is supplied to the P well of the semiconductor substrate. Accordingly, the electric charges stored in the floating gates of memory cells coupled to the remaining word lines WL&lt;0&gt; to WL&lt;n−2&gt; and WL&lt;n&gt; are not erased. In this manner, only the memory cells of the target page are erased. 
     5) Input of Most Significant Bit (MSB) Data ( 450 ) 
     Most significant bit (MSB) data to be programmed into the target page of the memory cell array  100  is stored in the second latch  240  of the page buffer  200 . More particularly, the most significant bit (MSB) data is transmitted to the second latch  240  through the I/O terminal YA coupled to the page buffer  200  and then temporarily stored therein. 
     6) Program of Least Significant Bit (LSB) Data and Most Significant Bit (MSB) Data ( 460 ) 
     The sensed least significant bit (LSB) data and the most significant bit (MSB) data respectively stored in the first latch  230  and the second latch  240  are combined and stored in the third latch  250 . Next, the combination data stored in the third latch  250  is transmitted to the selected memory cell (for example, MCn−1) of the target page through the sense node SO and programmed into the memory cell MCn−1, by performing a program operation. 
     As described above, according to the exemplary embodiment of this disclosure, after the programmed least significant bit (LSB) data is sensed, a target page data is erased, and the least significant bit (LSB) data and most significant bit (MSB) data are combined and programmed. Accordingly, a shift in the threshold voltage of a memory cell due to interference, occurring when a least significant bit (LSB) program operation is performed, can be prohibited. 
       FIG. 5  is a flowchart illustrating a program method according to a second exemplary embodiment of this disclosure. 
     The program method of a semiconductor memory device according to another exemplary embodiment of this disclosure is described below with reference to  FIGS. 3 and 5 . 
     1) Input of Least Significant Bit (LSB) Data ( 510 ) 
     Least significant bit (LSB) data to be programmed into a target page of the memory cell array  100  is stored in the first latch  230  of the page buffer  200 . More particularly, the least significant bit (LSB) data is transmitted to the first latch  230  through the I/O terminal YA coupled to the page buffer  200  and is temporarily stored in the first latch  230 . 
     2) Erase of Target Page Data ( 520 ) 
     Data stored in the memory cells of the target page of the memory cell array  100  is erased. Although a program operation has not been performed on the target page, the threshold voltages of the memory cells of the target page may rise due to interference occurring when the program operation was performed on a previous page that was programmed. For this reason, the erase operation is performed on the target page. Here, the erase operation is performed per page in order to erase the data stored in, for example, only the memory cells of the target page. An example of the erase operation performed per page is described below. 
     First, an operation voltage of 0 V is supplied to a selected word line (for example, WL&lt;n−1&gt;) coupled to the target page, and an erase-inhibition voltage of about 5 V is supplied to the remaining word lines WL&lt;0&gt; to WL&lt;n−2&gt; and WL&lt;n&gt; except the word line WL&lt;n−1&gt;. Next, an erase voltage of about 18 V to 20 V is supplied to the P well of the semiconductor substrate in which the memory cell array  100  is disposed. Electric charges stored in the floating gate of memory cells coupled to the word line WL&lt;n−1&gt; tunnel toward the semiconductor substrate by means of the erase voltage and thus are erased. However, electric charges stored in the floating gates of memory cells coupled to the remaining word lines WL&lt;0&gt; to WL&lt;n−2&gt; and WL&lt;n&gt; may not tunnel through due to the erase-inhibition voltage supplied to the gates of the memory cells although the erase voltage is supplied to the P well of the semiconductor substrate. Accordingly, the electric charges stored in the floating gates of memory cells coupled to the remaining word lines WL&lt;0&gt; to WL&lt;n−2&gt; and WL&lt;n&gt; are not erased. In this manner, only the memory cells of the target page are erased. 
     3) Input of Most Significant Bit (MSB) Data ( 530 ) 
     Most significant bit (MSB) data to be programmed into the target page of the memory cell array  100  is stored in the second latch  240  of the page buffer  200 . More particularly, the most significant bit (MSB) data is transmitted to the second latch  240  through the I/O terminal YA coupled to the page buffer  200  and is then temporarily stored in the second latch  240 . 
     4) Program of Least Significant Bit (LSB) Data and Most Significant Bit (MSB) Data ( 540 ) 
     The least significant bit (LSB) data and the most significant bit (MSB) data respectively stored in the first latch  230  and the second latch  240  are combined and stored in the third latch  250 . Next, the combination data stored in the third latch  250  is transmitted to the selected memory cell (for example, MCn−1) of the target page through the sense node SO and programmed into the memory cell MCn−1, by performing a program operation. 
     As described above, according to the present embodiment of this disclosure, after the least significant bit (LSB) data is inputted to the page buffer, a target page data is erased, and the least significant bit (LSB) data and most significant bit (MSB) data are combined and programmed. Accordingly, a shift in the threshold voltage of a memory cell due to interference, occurring when a least significant bit (LSB) program operation is performed, can be prohibited. 
     According to the present disclosure, after a least significant bit (LSB) program operation is performed on a selected page, a least significant bit (LSB) data of the selected page is sensed and stored in the page buffer, and a data of the selected page is then erased. Next, after a most significant bit (MSB) data is inputted to the page buffer, a program operation is performed on the selected page using the least significant bit (LSB) data and the most significant bit (MSB) data stored in the page buffer. Accordingly, a shift in the threshold voltage distributions of erase cells of a selected page can be prohibited and so a margin for a read operation can be increased.