Patent Publication Number: US-7710759-B2

Title: Nonvolatile ferroelectric memory device

Description:
RELATED APPLICATION 
   This application is based upon and claims the priority of benefit to Korean Patent Application No. 10-2005-0096567, filed on Oct. 13, 2005, the entire contents of which are incorporated by reference herein. 
   BACKGROUND 
   1. Technical Field 
   The present invention generally relates to a nonvolatile ferroelectric memory device, and more specifically, to a technology of controlling a read operation of a nonvolatile memory cell using a channel resistance of the memory cell which changes with a polarization state of a nano-scaled ferroelectric material. 
   2. Description of the Related Art 
   Generally, a ferroelectric random access memory (hereinafter referred to as ‘FeRAM’) has attracted considerable attention as next generation memory device because it has a data processing speed as fast as a Dynamic Random Access Memory (hereinafter, referred to as ‘DRAM’) and preserves data even after the power is turned off. 
   A FeRAM having a structure similar to a DRAM includes capacitors made of a ferroelectric substance, so that it utilizes the high residual polarization characteristic of the ferroelectric substance in which data is not deleted even after an electric field is eliminated. 
   A unit cell of a conventional nonvolatile FeRAM device includes a switching element and a nonvolatile ferroelectric capacitor. The switching element performs a switching operation depending on a state of a word line to connect a nonvolatile ferroelectric capacitor to a bit line or disconnect the nonvolatile ferroelectric capacitor from the bit line. The nonvolatile ferroelectric capacitor is connected between a plate line and a terminal of the switching element. Here, the switching element of the conventional FeRAM is an NMOS transistor whose switching operation is controlled by a gate control signal. 
     FIG. 1  is a cross-sectional view of a unit cell of a conventional nonvolatile ferroelectric memory device. 
   A conventional 1-T (One-Transistor) FET (Field Effect Transistor) cell includes an n-type drain  2  and an n-type source  3  which are formed in a p-type substrate  1 . Also, the cell includes an insulation oxide  4 , a ferroelectric layer  5 , and a word line  6  which are sequentially formed on a channel region between the drain  2  and the source  3 . 
   The above-described conventional nonvolatile FeRAM device reads and writes data by using a channel resistance of the memory cell which changes with a polarization state of the ferroelectric layer  5 . 
   Specifically, the channel region has a high resistance when the polarity of the ferroelectric layer  5  induces positive charges to the channel, and a low resistance when the polarity of the ferroelectric layer  5  induces negative charges to the channel. 
   However, in the conventional nonvolatile FeRAM device, when the cell is scaled down, a data retention characteristic is degraded, especially if a nonvolatile ferroelectric memory cell has been fabricated on a nanometer scale. For example, in a read mode, a read voltage may appear at adjacent cells, which can generate crosstalk noise, thereby destroying data stored in these cells. In a write mode, a write voltage may appear at an unselected cell so that data stored in unselected cells are destroyed. As a result, it is difficult to perform a random access operation. 
   SUMMARY 
   Various embodiments of the present invention are directed at providing a nonvolatile ferroelectric memory device including a floating channel layer between a top word line and a bottom word line to improve reliability of cells. 
   Various embodiments of the present invention are directed at providing a nonvolatile ferroelectric memory device including a nano-scaled floating channel layer between a top word line and a bottom word line to reduce the whole size of the cell. 
   Various embodiments of the present invention are directed at providing a nonvolatile ferroelectric memory device including a switching element for controlling a switching operation of a memory cell that has the same structure as that of a cell transistor to simplify the process. 
   Various embodiments of the present invention are directed at providing a nonvolatile ferroelectric memory device including a switching element for controlling a switching operation of a memory cell that has the same structure as that of a cell transistor to easily regulate a voltage applied to a memory cell. 
   Consistent with the present invention, there is provided a nonvolatile ferroelectric memory device that includes a plurality of memory cells connected serially between a bit line and a sensing line; a first switching unit configured to selectively connect the memory cells to the bit line in response to a first selecting signal; and a second switching unit configured to selectively connect the memory cells to the sensing line in response to a second selecting signal. Each of the plurality of memory cells, the first switching unit, and the second switching unit includes a bottom word line; an insulating layer formed on the bottom word line; a floating channel layer formed on the insulating layer; a ferroelectric layer formed on the floating channel layer; and a top word line formed on the ferroelectric layer in parallel with the bottom word line. 
   Consistent with the present invention, there is also provided a nonvolatile ferroelectric memory device that includes a plurality of bit lines; a plurality of sensing lines; a plurality of unit cell arrays connected in common to the bit lines in a column direction and to the sensing lines in a row direction; and a plurality of sense amplifiers connected to the plurality of bit line. Each of the plurality of unit cell arrays includes a plurality of memory cells connected serially between the corresponding bit line and the corresponding sensing line; a first switching unit configured to selectively connect the memory cells to the corresponding bit line in response to a first selecting signal; and a second switching unit configured to selectively connect the memory cells to the corresponding sensing line in response to a second selecting signal. Each of the plurality of memory cells, the first switching unit, and the second switching unit includes a bottom word line; an insulating layer formed on the bottom word line; a floating channel layer formed on the insulating layer; a ferroelectric layer formed on the floating channel layer; and a top word line formed on the ferroelectric layer in parallel with the bottom word line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
       FIG. 1  is a cross-sectional diagram illustrating a unit cell of a conventional nonvolatile ferroelectric memory device; 
       FIGS. 2   a  and  2   b  are cross-sectional diagrams illustrating a unit cell of a nonvolatile ferroelectric memory device according to an embodiment of the present invention; 
       FIG. 2   c  is a circuit diagram illustrating a unit cell of a nonvolatile ferroelectric memory device according to an embodiment of the present invention; 
       FIGS. 3   a  through  3   c  are diagrams illustrating read and write operations of high level data “1” in a unit cell of a nonvolatile ferroelectric memory device according to an embodiment of the present invention; 
       FIGS. 4   a  through  4   c  are diagrams illustrating read and write operations of low level data “0” in a unit cell of a nonvolatile ferroelectric memory device according to an embodiment of the present invention; 
       FIG. 5  is a plane diagram illustrating a unit cell array of a nonvolatile ferroelectric memory device according to an embodiment of the present invention; 
       FIG. 6  is a cross-sectional diagram illustrating the read operation of the low data “0” of the unit cell array of  FIG. 5 ; 
       FIG. 7  is a cross-sectional diagram illustrating the read operation of the high data “1” of the unit cell array of  FIG. 5 ; 
       FIG. 8  is a timing diagram illustrating a write cycle of a nonvolatile ferroelectric memory device according to an embodiment of the present invention; 
       FIG. 9  is a timing diagram illustrating a write operation in a first operation period; 
       FIG. 10  is a timing diagram illustrating a write operation of a second operation period of  FIG. 8 ; 
       FIG. 11  is a timing diagram illustrating a read operation of the unit cell array of  FIG. 5 ; 
       FIG. 12  is a cross-sectional diagram illustrating connection relationships of a memory cell Q 1  and a switching device Q 0  and of a memory cell Q m  and a switching device Q m+1 ; and 
       FIG. 13  is a circuit diagram illustrating a memory array including a plurality of unit cell arrays of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
   The present invention will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like part. 
     FIGS. 2   a  and  2   b  are cross-sectional views of a unit cell  9  of a nonvolatile ferroelectric memory device consistent with the present invention. 
     FIG. 2   a  is a cross-sectional view cut in parallel with a top word line  14 . 
   Referring to  FIG. 2   a , the unit cell  9  includes a bottom word line  10 , an oxide film  11 , a bit line including a p-type floating channel region  12 , a ferroelectric layer  13 , and the top word line  14 . The bottom word line  10  is arranged in parallel with the top word line  14 . Both the bottom word line  10  and the top word line  14  are selectively driven by a row address decoder (not shown). 
     FIG. 2   b  is a cross-sectional view cut perpendicular to the top word line  14 . 
   Referring to  FIG. 2   b , the bit line further includes a p-type drain region  15  and a p-type source region  16 . The bit line is formed of at least one of a carbon nano tube, a silicon, a germanium, and an organic semiconductor. 
   The unit cell  9  reads and writes data by a channel resistance which changes with a polarization state of the ferroelectric layer  13 . 
     FIG. 2   c  is a circuit diagram representing the unit cell  9  of a nonvolatile ferroelectric memory device consistent with the present invention. Labels “TWL” and “BWL” designate the top word line  14  and the bottom word line  10 , respectively. 
     FIGS. 3   a - 3   c  and  4   a - 4   c  are diagrams illustrating read and write operations of the unit cell  9 . 
     FIG. 3   a  is a diagram illustrating the operation of writing a bit of “1” in the unit cell  9 . Referring to  FIG. 3   a , a ground voltage GND (or a positive voltage +V) is applied to the bottom word line  10 , and a negative voltage −Vc is applied to the top word line  14 . The drain region  15  and the source region  16  are grounded. As a result of the voltages applied to the bottom word line  10 , the top word line  14 , the drain region  15 , and the source region  16 , a voltage drop is created across the ferroelectric layer  13  between the top word line  14  and the p-type channel region  12 , resulting in a polarity of the ferroelectric layer  13  that induces positive charges to the top portion of the p-type channel region  12 . Consequently, a bit of “1” is written in the unit cell  9 . 
     FIGS. 3   b  and  3   c  are diagrams illustrating the read operation of the unit cell  9  when a bit of “1” is stored in the unit cell  9 , wherein  FIG. 3   c  is an exaggerated view of  FIG. 3   b.    
   Referring to  FIGS. 3   b  and  3   c , when a positive read voltage +Vrd is applied to the bottom word line  10 , a depletion layer  12   a  is formed in the bottom portion of the p-type channel region  12 . However, since the top word line  14  is at the ground voltage GND and the polarity of the ferroelectric layer  14  induces positive charges to the p-type channel region  12 , a channel exists in the p-type channel region  12 . As a result, a voltage difference between the drain region  15  and the source region  16  will induce a significant amount of current through the p-type channel region, from which it may be determined that a bit of “1” is stored in the unit cell  9 . 
     FIG. 4   a  is a diagram illustrating the operation of writing a bit of “0” in the unit cell  9 . 
   Referring to  FIG. 4   a , the ground voltage GND (or a negative voltage −V) is applied to the bottom word line  10 , and a positive polarization transition threshold voltage +Vc, corresponding to a threshold voltage that is, when applied across the ferroelectric layer  13 , sufficient to set a polarity of the ferroelectric layer  13  that induces negative charges into the portion of the channel region  12  next to the ferroelectric layer  13 , is applied to the top word line  14 . The drain region  15  and the source region  16  are grounded. 
   As a result of the voltages applied to he bottom word line  10 , the top word line  14 , the drain region  15 , and the source region  16 , a voltage drop is created between the top word line  14  and the p-type channel region  12 , resulting in a polarity of the ferroelectric layer  13  that induces negative charges to the top portion of the p-type channel region  12 . Consequently, a bit of “0” is stored in the unit cell  9 . 
     FIGS. 4   b  and  4   c  are diagrams illustrating the read operation of the unit cell  9  when the unit cell  9  has a bit of “0” stored therein. 
   Referring to  FIGS. 4   b  and  4   c , when the positive read voltage +Vrd is applied to the bottom word line  10 , a depletion layer  12   a  is formed in the bottom portion of the p-type channel region  12 . In the mean time, the top word line  14  is grounded. Therefore, a depletion layer  12   b  is formed in the top portion of the p-type channel region  12  because of the polarity of the ferroelectric layer  13 , which induces negative charges to the p-type channel region  12 . The depletion regions  12   a  and  12   b  effectively block a current path between the drain region  15  and the source region  16 . As a result, a voltage difference between the drain region  15  and the source region  16  does not generate a significant current through the p-type channel region  12 , from which it may be determined that a bit of “0” is stored in the unit cell  9 . 
   In the unit cell  9  consistent with the present invention, the top word line  14  is at the ground voltage GND in a read mode. As a result, a voltage stress is avoided and the data retention characteristic of the unit cell  9  is improved. 
     FIG. 5  is a diagram illustrating a memory cell array of a nonvolatile ferroelectric memory device consistent with the present invention. As shown in  FIG. 5 , the cell array includes switching elements Q 0 , Q m+1  and a plurality of memory cells Q 1 ˜Q m . 
   The switching element Q 0  is connected between a bit line BL and a memory cell Q 1  to connect the memory cell Q 1  to the bit line BL in response to a voltage applied to a selecting line SEL 1 . The switching element Q 0  may have the same structure as the memory cells Q 1 ˜Q m . The switching element Q m+1  is connected between a sensing line SL and the memory cell Q m  to connect the memory cell Q m  to the sensing line SL in response to a voltage applied to a selecting line SEL 2 . The switching element Q m+1  may also have the same structure as the memory cells Q 1 ˜Q m . 
   The plurality of memory cells Q 1 ˜Q m  are connected serially between the switching elements Q 0  and Q m+1 , and are selected by voltages applied to the corresponding top word lines TWL 1 ˜TWLm and bottom word lines BWL 1 ˜BWLm. 
     FIG. 6  is a cross-sectional diagram illustrating the read operation of the cell array of  FIG. 5 . Each of the memory cells Q 1 ˜Q m  has the same structure as the unit cell  9  shown in  FIG. 2 . It is assumed that the memory cell Q 1  is selected and that a bit of “0” is stored in the memory cell Q 1 . 
     FIG. 6  shows that each of the switching elements Q 0  and Q m+1  includes a bottom electrode  20 , an oxide film  21 , a floating channel layer, a ferroelectric layer  23  and a top electrode  24 . Here, the floating channel layer includes a p-type channel region  22 , a p-type drain region  25  and a p-type source region  26 . Both the bottom electrode  20  and the top electrode  24  of the switching element Q 0  are connected to the selecting line SEL 1 . Both the bottom electrode  20  and the top electrode  24  of the switching element Q m+1  are connected to the selecting line SEL 2 . 
   A ground voltage is applied to the top electrode  24  and the bottom electrode  20  of the switching element Q 0  through the selecting line SEL 1 . As a result, a current path exists between the bit line BL and the memory cell Q 1 . 
   The top word lines TWL 1 ˜TWLm are grounded. A positive read voltage +Vrd is applied to the bottom word line BWL 1  of the selected cell Q 1 , and the bottom word lines BWL 2 ˜BWLm are grounded. 
   Because the top word lines TWL 2 ˜TWLm and the bottom word lines BWL 2 ˜BWLm are grounded, a current path exists through the memory cells Q 2 ˜Q m , and the amount of current through the bit line depends on a current path through the memory cell Q 1 . Because of the read voltage +Vrd applied to the bottom word line BWL 1  of the memory cell Q 1 , a depletion region such as depletion region  12   a  shown in  FIGS. 3   c  and  4   c  is formed in a portion of the channel region  12  next to the bottom word line BWL 1 . Also, because a bit of “0” is stored in the memory cell Q 1 , a depletion region such as the depletion region  12   b  as shown in  FIG. 4   c  is formed in a portion of the channel region  12  next to the top word line TWL 1 . The depletion regions  12   a  and  12   b  effectively block a current path between the drain region  15  and the source region  16  of the memory cell Q 1 . As a result, a voltage difference between the drain region  15  and the source region  16  of the memory cell Q 1  does not generate a significant current through the p-type channel region  12 , from which it may be determined that a bit of “0” is stored in the memory cell Q 1 . 
     FIG. 7  is a cross-sectional diagram illustrating the read operation of the cell array of  FIG. 5 . It is assumed that the memory cell Q 1  is selected and that a bit of “0” is stored in the memory cell Q 1 . 
   Similarly, a ground voltage is applied to the top electrode  24  and the bottom electrode  20  of the switching element Q 0  through the selecting line SEL 1 . The top word lines TWL 1 ˜TWLm are grounded. A positive read voltage +Vrd is applied to the bottom word line BWL 1  of the selected cell Q 1 , and the bottom word lines BWL 2 ˜BWLm are grounded. As a result, a current path exists through the switching elements Q 0  and Q m+1 , and the memory cells Q 2 ˜Q m . Thus, the amount of current through the bit line BL is determined by a current path through the memory cell Q 1 . 
   Because of the read voltage +Vrd applied to the bottom word line BWL 1  of the memory cell Q 1 , a depletion region such as depletion region  12   a  shown in  FIGS. 3   c  and  4   c  is formed in a portion of the channel region  12  next to the bottom word line BWL 1 . However, because a bit of “1” is stored in the memory cell Q 1 , positive charges are induced to the portion of the channel region  12  next to the top word line TWL 1 . As a result, a current path exists through the memory cell Q 1 . As a result, a voltage difference between the drain region  15  and the source region  16  of the memory cell Q 1  generates a significant current through the p-type channel region  12 , from which it may be determined that a bit of “1” is stored in the memory cell Q 1 . 
   It is to be understood that, although  FIGS. 6 and 7  indicate that the memory cells Q 2 ˜Q m  each have a bit of “0” stored therein, the memory cells Q 2 ˜Q m  do not have to have bits of “0” stored therein. As long as the portion of the floating layer  12  of each of the memory cells Q 2 ˜Q m  next to the corresponding bottom word line is not depleted, a current path exists through the corresponding memory cell so that the selected memory cell can be properly read. 
     FIG. 8  is a timing diagram illustrating a write cycle of a nonvolatile ferroelectric memory device consistent with the present invention. 
   Referring to  FIG. 8 , a write cycle includes two steps. In a first step, bits of “1” are written into some of the memory cells of the memory device. In a second step, bits of “0” are written into the remaining memory cells of the memory device. 
   After the bits of “1” are written during the first step, the bits of “1” are maintained during the second period, by a voltage having a potential half that of the polarization transition threshold voltage Vc applied to the corresponding bit line BL. 
     FIG. 9  is a timing diagram of the first step, during which bits of “1” are written into the memory device. It is assumed in  FIG. 9  that the memory device includes such a memory array as shown in  FIG. 5  and that the first memory cell Q 1  of the cell array of  FIG. 5  is selected. 
     FIG. 9  shows that the first step includes five time periods, t 00 ˜t 04 . The first period t 00  and the fifth period t 04  are precharge periods, during which a high level voltage +Vh is applied to selecting lines SEL 1  and SEL 2 , and other lines are at the ground state GND. 
   In the second period t 01 , the selecting lines SEL 1  and SEL 2  are grounded. 
   In a third period t 02 , a negative polarization transition threshold voltage −Vc is applied to the top word line TWL 1  of the selected memory cell Q 1 . The negative polarization transition threshold voltage −Vc is a voltage that, when applied across the ferroelectric layer  13 , is sufficient to set the polarity of the ferroelectric layer  13  to induce positive charges into the portion of the channel region  12  next to the ferroelectric layer  13 . As a result, a bit of “1” is written into the selected memory cell Q 1 . 
   In the fourth period t 03 , the top word line TWL 1  of the selected memory cell Q 1  is grounded. 
     FIG. 10  is a timing diagram illustrating the second step of the write operation, during which bits of “0” are written into the memory device. It is assumed in  FIG. 9  that the memory device includes such a memory array as shown in  FIG. 5  and that the first memory cell Q 1  of the cell array of  FIG. 5  is selected. 
     FIG. 10  shows that the second step includes seven time periods, t 10 ˜t 16 . In the first period t 10  and the seventh period t 16 , a high level voltage +Vh is applied to the selecting line SEL 1 , and other lines are at the ground state GND. 
   In the second period t 11 , the selecting line SEL 1  connected to the switching element Q 0  is grounded, and a high level voltage +Vh is applied to the selecting line SEL 2  connected to the switching element Q m+1 . 
   In the third period t 12 , the previously written bits of “1” are maintained, by a voltage having a potential of (+½ Vc) applied to the bit line BL. 
   In the fourth period t 13 , the positive polarization transition threshold voltage +Vc is applied to the top word line TWL 1  of the selected memory cell Q 1 . Because of the voltage of +½ Vc is applied to the bit line BL corresponding to the memory cell Q 1 , the bit of “1” stored in the memory cell Q 1  is maintained. In the mean time, bits of “0” may be written into other selected memory cells (not shown) associated with bit lines biased at a ground level. 
   In the fifth period t 14 , the top word line TWL 1  connected to the selected memory cell Q 1  is grounded. 
   In the sixth period t 15 , the bit line BL is grounded. 
     FIG. 11  is a timing diagram illustrating a read operation of a ferroelectric memory device including the unit cell array of  FIG. 5 . It is assumed in  FIG. 11  that the first memory cell Q 1  of the unit cell array of  FIG. 5  is selected. 
   The read operation is divided into seven time periods, t 20 ˜t 26 . The first period t 20  and the seventh period t 26  are precharge periods, during a high level voltage +Vh is applied to selecting lines SEL 1  and SEL 2 , and other lines are grounded. 
   In the second period t 21 , the selecting lines SEL 1  and SEL 2  are grounded. 
   In the third period t 22 , a positive read voltage Vrd is applied to the bottom word line BWL 1  of the selected memory cell Q 1 . 
   In the fourth period t 23 , a positive sensing voltage +Vsen is applied to the bit line BL, and current flowing through the bit line BL is sensed. If the current through the bit line BL is small and the potential on the bit line BL is maintained at the level of the sensing voltage +Vsen, it may be determined that a bit of “0” is stored in the selected memory cell Q 1 . Conversely, if the current through the bit line BL is significant and the potential on the bit line BL decreases to lower than the sensing voltage +Vsen, it may be determined that a bit of “1” is stored in the selected memory cell Q 1 . 
   In the fifth period t 24 , the bit line BL is grounded. 
   In the sixth period t 25 , the bottom word line BWL 1  of the selected memory cell Q 1  is grounded. 
     FIG. 12  is a cross-sectional view illustrating the connection between the memory cell Q 1  and the switching device Q 0  and between the memory cell Q m  and the switching device Q m+1  of  FIG. 5 . 
   As  FIG. 12  shows, the p-type source region  26  of the switching element Q 0  is connected to the bit line BL, and the p-type drain region  25  is connected to the p-type source region  16  of the memory cell Q 1 . 
   The p-type source region  26  of the switching element Q m+1  is connected to the sensing line SL, and the p-type drain region  25  is connected to the p-type source region  16  of the memory cell Q m . 
     FIG. 13  is a circuit diagram illustrating a memory array including a plurality of cell arrays. 
   Referring to  FIG. 13 , in each column, a plurality of cell arrays  30  are connected in common to a corresponding one of bit lines BL 1 ˜BLn and a corresponding one of sense amplifiers  40 . 
   In each row, the plurality of cell arrays  30  are connected in common to selecting lines SEL 1 , SEL 2 -SEL 2n−1 , SEL 2n , and the corresponding one of sensing lines SL 1 ˜SLn. 
   As described above, a nonvolatile ferroelectric memory device consistent with the present invention includes a floating channel layer formed between a top word line and a bottom word line to improve the reliability of cells. Also, the nonvolatile ferroelectric memory device includes a nano-scaled floating channel layer formed between a top word line and a bottom word line to reduce the whole size of cells. Moreover, a switching element for controlling a switching operation of the memory cell is configured to have the same structure as that of a cell transistor, thereby simplifying the process for manufacturing a memory device consistent with the present invention. In addition, voltages applied to the memory cell consistent with the present invention can be more easily regulated. 
   The foregoing description of various embodiments of the invention has been presented for purposes of illustrating and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. Thus, the embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.