Abstract:
A memory device capable of improving the degree of integration and effectively preventing false data reading is obtained. This memory device comprises a pair of bit lines extending in a prescribed direction, a word line arranged to intersect with the pair of bit lines and a memory cell, arranged between the pair of bit lines and the word line, consisting of two capacitance means. Thus, the area of the memory cell is reduced and no reference voltage is required.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a memory device, and more specifically, it relates to a memory device storing data.  
           [0003]    2. Description of the Background Art  
           [0004]    A ferroelectric memory storing data through polarization of a ferroelectric substance is known in general. This ferroelectric memory is watched with interest as a high-speed nonvolatile memory requiring low power consumption. Therefore, the ferroelectric memory is actively researched and developed. A storage capacitance ferroelectric memory writing/reading data in a system similar to that of a DRAM (dynamic random access memory) employs one of two types of representative memory cells, i.e., a one-transistor two-capacitor (hereinafter referred to as  1 T 1 C) memory cell and a two-transistor two-capacitor (hereinafter referred to as  2 T 2 C) memory cell. The  2 T 2 C memory cell is disclosed in “Low-power High-speed LSI Circuits &amp; Technology”, Jan. 31, 1998, pp. 235-245, for example.  
           [0005]    [0005]FIG. 36 is a circuit diagram showing a memory cell part of a conventional  1 T 1 C ferroelectric memory. FIG. 37 is a circuit diagram for illustrating a method of reading data in the conventional  1 T 1 C ferroelectric memory including memory cells and reference cells.  
           [0006]    As shown in FIG. 36, each memory cell  103  of the conventional  1 T 1 C ferroelectric memory is formed by a selection transistor  101  and a ferroelectric capacitor  102 , similarly to that of a DRAM. When the selection transistor  101  is turned on in a reading operation of the ferroelectric memory, the ferroelectric capacitor  102  is connected with a bit line capacitor Cbl. Then, a plate line PL is pulse-driven to transmit charges varying with the direction of polarization of the ferroelectric capacitor  102  to a bit line BLT. Thus, the ferroelectric memory reads data as the voltage of the bit line BLT, similarly to the DRAM. Whether the data is “1” or “0” depends on the direction of polarization of the ferroelectric capacitor  102 . In this case, a reference cell is required for discharging charges in an intermediate quantity between those of charges discharged by the data “1” and “0” in data reading.  
           [0007]    More specifically, reference cells  103   a  are connected to a pair of bit lines BLT and BLB respectively, as shown in FIG. 37. The data read operation is now described in detail with reference to FIG. 37. First, the pair of bit lines BLT and BLB are precharged to 0 V. When a word line WL 1  selects a memory cell  103  connected with the bit line BLT, a word line RefWLB selects the reference cell  103   a  connected with the bit line BLB. When a word line WL 2  selects a memory cell  103  connected with the bit line BLB, a word line RefWLT selects the reference cell  103   a  connected with the bit line BLT. Thereafter the plate line PL is pulse-driven so that charges corresponding to the memory cells  103  and the reference cells  103   a  are discharged to the pair of bit lines BLT and BLB. Thus, the pair of bit lines BLT and BLB obtain data signals of “1” or “0”. A sense amplifier  105  amplifies the difference between the potentials of the signals. Thus, the ferroelectric memory reads data.  
           [0008]    [0008]FIG. 38 is a circuit diagram showing a memory cell part of a conventional  2 T 2 C ferroelectric memory. As shown in FIG. 38, two transistors and two capacitors are connected to a pair of bit lines BLT and BLB in the memory cell part of the  2 T 2 C ferroelectric memory. The two transistors and the two capacitors store complementary data as 1-bit data. In this case, no reference cells are required for preparing reference voltages for reading the complementary data, dissimilarly to the aforementioned  1 T 1 C ferroelectric memory.  
           [0009]    In general, a matrix storage capacitance ferroelectric memory is also proposed. FIG. 39 is a circuit diagram showing memory cells  121  of a conventional matrix ferroelectric memory. As shown in FIG. 39, ferroelectric capacitors  122  are arranged on intersections between word lines WL 1  to WL 4  and bit lines BL 1  to BL 4  in the memory cells  121  of the conventional matrix ferroelectric memory. The matrix ferroelectric memory, reading voltages through capacitive coupling between the bit lines BL 1  to BL 4  and the ferroelectric capacitors  122 , must ensure the capacitance similarly to the  1 T 1 C ferroelectric memory. In the matrix ferroelectric memory, each memory cell  121  is formed by only a single ferroelectric capacitor  122 , whereby the degree of integration can be more improved as compared with the  1 T 1 C ferroelectric memory.  
           [0010]    [0010]FIG. 40 is a schematic diagram for illustrating the operation principle of the matrix ferroelectric memory shown in FIG. 39. Operations of the conventional matrix ferroelectric memory are now described with reference to FIGS. 39 and 40.  
           [0011]    First, each ferroelectric capacitor  122  has first and second ends connected to each word line WL and each bit line BL respectively. Both ends of the ferroelectric capacitor  122  are at the same potential in a standby state. In order to write data “1”, voltages of Vcc and 0 V are applied to the word line WL and the bit line BL respectively. At this time, the voltage Vcc is applied to the ferroelectric capacitor  122 . Thus, the ferroelectric capacitor  122  shifts to a point A in FIG. 40 despite an initial state. When both ends of the ferroelectric capacitor  122  are thereafter set to the same potential, the ferroelectric capacitor  122  makes transition to “1” in FIG. 40. In order to write data “0”, voltages of 0 V and Vcc are applied to the word line WL and the bit line BL respectively. At this time, a voltage −Vcc is applied to the ferroelectric capacitor  122 . Thus, the ferroelectric capacitor  122  shifts to a point B in FIG. 40. When both ends of the ferroelectric capacitor  122  are thereafter set to the same potential, the ferroelectric capacitor  122  makes transition to “0” in FIG. 40.  
           [0012]    In a read operation, the bit line BL is precharged to 0 V. Then, the word line WL is set to the voltage Vcc. Assuming that Ccell represents the capacitance of the ferroelectric capacitor  122  of each memory cell, Cref represents the capacitance of a ferroelectric capacitor  122   a  of each reference cell  121   a  (see FIG. 39), Cbl represents the parasitic capacitance of a bit line BLn and Cblref represents the parasitic capacitance of a reference bit line Blref, the voltage Vcc of the word line WL is capacitively divided by the capacitances Ccell and Cbl as to the bit line BLn, and capacitively divided by the parasitic capacitances Cref and Cblref as to the reference bit line Blref. The capacitance Ccell can be approximated as a capacitance C 0  or C 1  depending on held data. Therefore, a potential V 0  of the bit line BLn holding data “0”, a potential V 1  of the bit line BLn holding the data “1” and a potential Vref of the reference bit line Blref are expressed as follows respectively: 
             V 0={ C 0/( C 0+ Cbl )}× Vcc   (1) 
             V 1={ C 1/( C 1+ Cbl ))× Vcc   (2) 
             Vref= {( Cref /( Cref+Cblref ))× Vcc   (3) 
           [0013]    The potential Vref of the reference bit line Blref is set to an intermediate level between the potential V 0  of the bit line BLn holding the data “0” and the potential V 1  of the bit line BLn holding the data “1”.  
           [0014]    A sense amplifier determines the difference between the potential V 0  or V 1  and the potential Vref thereby performing reading. At this time, data of the memory cell is destroyed and hence a write operation (restoration) responsive to the read data is performed after the reading.  
           [0015]    The conventional  1 T 1 C ferroelectric memory shown in FIG. 36 having the memory cells each formed by only a single transistor and a single capacitor advantageously has a high degree of integration. However, a reference voltage disadvantageously deviates from a design value due to fabrication dispersion of the ferroelectric capacitor  102  of change of the quantity of polarization charges in the write and read operations or resulting from time change. This disadvantageously leads to false data reading.  
           [0016]    In the conventional  2 T 2 C ferroelectric memory shown in FIG. 38, each memory cell is formed by two ferroelectric capacitors and two selection transistors, and hence the degree of integration is inferior to that of the  1 T 1 C ferroelectric memory.  
           [0017]    In the conventional matrix ferroelectric memory shown in FIG. 39, false data reading disadvantageously results from fabrication dispersion or fluctuation of a reference voltage caused by change of the quantity of polarization charges. Further, the matrix ferroelectric memory disadvantageously causes a disturbance phenomenon of a non-selected cell in the write and read operations. In the matrix ferroelectric memory, a voltage of ½Vcc is regularly applied to a non-selected bit line BL and a non-selected word line WL and hence the voltage of ½Vcc is applied to the non-selected cell at the maximum. As shown in FIG. 41, therefore, disturbance is repeated due to hysteretic characteristics of a ferroelectric substance to reduce the quantity of polarization charges. When the quantity of polarization charges is reduced, that of each reference cell  122   a  is also reduced, to result in remarkable fluctuation of the aforementioned reference voltage. This disadvantageously further prompts false data reading.  
         SUMMARY OF THE INVENTION  
         [0018]    An object of the present invention is to provide a memory device capable of improving the degree of integration and effectively preventing false reading.  
           [0019]    Another object of the present invention is to effectively prevent a non-selected memory cell from a disturbance phenomenon in the aforementioned memory device.  
           [0020]    A memory device according to an aspect of the present invention comprises a pair of bit lines extending in a prescribed direction, a word line arranged to intersect with the pair of bit lines and a memory cell, arranged between the pair of bit lines and the word line, consisting of two capacitance means.  
           [0021]    The memory device according to this aspect is provided with the memory cell consisting of two capacitance means as described above, whereby the area of the memory cell can be reduced as compared with a conventional memory cell consisting of two capacitance means and two transistors and hence the degree of integration can be improved. When complementary data are written in the two capacitance means respectively, no reference voltage is required and initial potential difference in reading can be increased as compared with a case of employing the reference voltage. Thus, false data reading can be effectively prevented also when the characteristics of the capacitance means are deteriorated due to fabrication dispersion or increase of the number of write/read times.  
           [0022]    In the memory device according to the aforementioned aspect, the capacitance means preferably include a ferroelectric layer. According to this structure, the ferroelectric memory can be formed in a high degree of integration to be capable of effectively preventing false data reading.  
           [0023]    In the memory device according to the aforementioned aspect, the two capacitance means preferably store complementary data respectively, thereby storing one-bit data in the memory cell. According to this structure, no reference voltage is required and initial potential difference in reading can be increased.  
           [0024]    In this case, the memory device preferably applies a pulsing voltage to a selected word line while applying complementary voltages to a selected pair of bit lines when writing the data. According to this structure, the memory device can write first data in a first bit line in a high-level voltage period of a pulse while writing second data in a second bit line in a low-level voltage period of the pulse. Consequently, the memory device can write complementary data in the pair of bit lines in a cycle of one pulse.  
           [0025]    The memory device applying the aforementioned pulsing voltage preferably further comprises a pulse voltage application circuit for applying the pulsing voltage to the selected word line at least when writing the data. According to this structure, the memory device can easily apply the pulsing voltage to the selected word line.  
           [0026]    The memory device applying the aforementioned pulsing voltage preferably further comprises a write voltage application circuit for applying the complementary voltages to the selected pair of bit lines when writing the data. According to this structure, the memory device can easily apply the complementary voltages to the selected pair of bit lines.  
           [0027]    The memory device applying the aforementioned pulsing voltage may apply a prescribed voltage to a selected memory cell while applying a voltage substantially half the prescribed voltage to a non-selected memory cell when writing and reading data.  
           [0028]    The memory device applying the aforementioned pulsing voltage reads the data by detecting difference between the potentials of the pair of bit lines corresponding to the complementary data stored in the two capacitance means respectively. According to this structure, the memory device can easily read the data.  
           [0029]    In this case, the memory device preferably detects the difference between the potentials of the pair of bit lines corresponding to the complementary data stored in the two capacitance means respectively by precharging the pair of bit lines connected with the selected memory cell to a prescribed voltage and thereafter applying the pulsing voltage to the word line connected with the selected memory cell when reading the data. According to this structure, the memory device can easily detect the difference between the potentials of the pair of bit lines in a high-voltage period of the pulsing voltage.  
           [0030]    In this case, the memory device preferably further comprises a read amplifier for amplifying the difference between the potentials of the pair of bit lines corresponding to the complementary data stored in the two capacitance means respectively. According to this structure, the memory device can easily read the data.  
           [0031]    The memory device according to the aforementioned aspect preferably applies a prescribed voltage to a selected memory cell while applying a voltage of substantially one third of the prescribed voltage to a non-selected memory cell when writing and reading data. According to this structure, the non-selected memory cell can be effectively prevented from a disturbance phenomenon.  
           [0032]    In this case, the memory device may apply the prescribed voltage to a selected memory cell connected with the first one of the pair of bit lines while applying the voltage of substantially one third of the prescribed voltage to the non-selected memory cell and a selected memory cell connected with the second one of the pair of bit lines thereby writing prescribed data in the selected memory cell connected with the first one of the pair of bit lines and thereafter applying the prescribed voltage to the selected memory cell connected with the second one of the pair of bit lines while applying the voltage of substantially one third of the prescribed voltage to the non-selected memory cell and the selected memory cell connected with the first one of the pair of bit lines thereby writing data inverse to the prescribed data in the selected memory cell connected with the second one of the pair of bit lines when writing the data. According to this structure, the memory device can write the data while reducing the voltage applied to the non-selected cell to substantially one third of the prescribed voltage.  
           [0033]    In this case, the memory device detects the difference between the potentials of the pair of bit lines corresponding to the complementary data stored in the two capacitance means respectively by precharging the pair of bit lines connected with the selected memory cell to a prescribed first voltage and thereafter applying a prescribed second voltage to the word line connected with the selected memory cell when reading the data. According to this structure, the memory device can read the data while reducing the voltage applied to the non-selected cell to substantially one third of the prescribed voltage.  
           [0034]    The memory device according to the aforementioned aspect is preferably capable of applying a pulse having a prescribed pulse width causing polarization inversion when a high voltage is applied to the capacitance means while causing substantially no polarization inversion when a low voltage is applied to the capacitance means to the memory cell, for applying a pulse of a high voltage having the prescribed pulse width to a selected memory cell while applying a pulse of a low voltage having the prescribed pulse width to a non-selected memory cell at least either in data writing or in data reading. According to this structure, the memory device can write or read data in or from the selected memory cell while causing substantially no polarization inversion in the non-selected memory cell by applying a pulse of a high voltage having the aforementioned prescribed pulse width to the selected memory cell and applying a pulse of a low voltage having the aforementioned prescribed pulse width to the non-selected memory cell at least either in data writing or in data reading. Consequently, the non-selected memory cell can be prevented from disturbance. In this case, the prescribed pulse width is preferably not more than 70 ns.  
           [0035]    In the memory device according to the aforementioned aspect, at least either the bit lines forming the pair of bit lines or the word line has a multilevel structure, and the capacitance means preferably have multilevel structures. When the capacitance means formed between the bit lines and the word line have multilevel structures, the degree of integration can be improved as compared with a case of employing capacitance means having single-level structures.  
           [0036]    In the memory device including the aforementioned capacitance means having multilevel structures, the two capacitance means forming each memory cell may include a first data storage part and a second data storage part storing complementary data respectively, and the first data storage part and the second data storage part may be transversely arranged at a prescribed interval. According to this structure, the memory cell including the first and second data storage parts can be vertically stacked, whereby the degree of integration can be improved.  
           [0037]    In the memory device including the aforementioned capacitance means having multilevel structures, the two capacitance means forming each memory cell may include a first data storage part and a second data storage part storing complementary data respectively, and the first data storage part and the second data storage part may be vertically arranged at a prescribed interval. According to this structure, the first and second data storage parts forming the memory cell can be vertically stacked, whereby the degree of integration can be improved.  
           [0038]    In the memory device including the aforementioned capacitance means having multilevel structures, the two capacitance means forming each memory cell may include a first data storage part and a second data storage part storing complementary data respectively, and the first data storage part and the second data storage part may be obliquely arranged at a prescribed interval. According to this structure, the first and second data storage parts forming the memory cell can be obliquely stacked, whereby the degree of integration can be improved.  
           [0039]    In the memory device including the aforementioned capacitance means having multilevel structures, the bit lines forming the pair of bit lines are preferably arranged above and under the word line respectively, and the capacitance means preferably include a first data storage layer arranged between the bit line located above the word line and the word line and a second data storage layer arranged between the bit line located under the word line and the word line. According to this structure, the capacitance means (data storage layers) can have two-level structures, whereby the degree of integration can be improved as compared with a case of employing capacitance means of single-level structures.  
           [0040]    In the memory device including the capacitance means having the aforementioned multilevel structures, the bit lines forming the pair of bit lines preferably include at least first- and second-level bit lines, the word line preferably includes at least first- and second-level word lines, and the capacitance means preferably include a first data storage layer arranged between the first-level bit line and the first-level word line and a second data storage layer arranged between the second-level bit line and the second-level word line, while the memory device preferably further comprises an insulator layer formed between a first region formed with the first data storage layer, the first-level word line and the first-level bit line and a second region formed with the second data storage layer, the second-level word line and the second-level bit line for isolating the first region and the second region from each other. According to this structure, ferroelectric capacitors vertically adjacent to each other can be isolated from each other.  
           [0041]    In the memory device including the capacitance means having the aforementioned multilevel structures, the bit lines forming the pair of bit lines preferably include at least first-, second- and third-level bit lines, the word line preferably includes at least first- and second-level word lines, and the capacitance means preferably include a first data storage layer arranged between the first-level bit line and the first-level word line, a second data storage layer arranged between the first-level word line and the second-level bit line, a third data storage layer arranged between the second-level bit line and the second-level word line and a fourth data storage layer arranged between the second-level word line and the third-level bit line. According to this structure, memory cells vertically adjacent to each other can share the second-level bit line, whereby the number of interconnection layers (bit lines and word lines) can be reduced.  
           [0042]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]    [0043]FIG. 1 is a block diagram showing the overall structure of a ferroelectric memory according to a first embodiment of the present invention;  
         [0044]    [0044]FIG. 2 is an equivalent circuit diagram showing the structure of a portion around a memory cell array of the ferroelectric memory according to the first embodiment shown in FIG. 1;  
         [0045]    [0045]FIG. 3 shows voltage waveforms of respective parts in the ferroelectric memory according to the first embodiment shown in FIG. 1;  
         [0046]    [0046]FIG. 4 illustrates voltages of the respective parts in respective operations of the ferroelectric memory according to the first embodiment shown in FIG. 1;  
         [0047]    [0047]FIG. 5 is a schematic diagram for illustrating voltages applied to word lines WL and pairs of bit lines BLT and BLB when writing data “0” in a ferroelectric memory according to a second embodiment of the present invention;  
         [0048]    [0048]FIG. 6 is a schematic diagram for illustrating voltages applied to the word lines WL and the pairs of bit lines BLT and BLB when writing data “1” in the ferroelectric memory according to the second embodiment of the present invention;  
         [0049]    [0049]FIGS. 7 and 8 are schematic diagrams for illustrating a read operation of the ferroelectric memory according to the second embodiment of the present invention;  
         [0050]    [0050]FIG. 9 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a third embodiment of the present invention;  
         [0051]    [0051]FIG. 10 is a sectional view of the memory cell array part of the ferroelectric memory according to the third embodiment shown in FIG. 9 as viewed from a direction A;  
         [0052]    [0052]FIG. 11 is a sectional view of the memory cell array part of the ferroelectric memory according to the third embodiment shown in FIG. 9 as viewed from a direction B;  
         [0053]    [0053]FIG. 12 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a fourth embodiment of the present invention;  
         [0054]    [0054]FIG. 13 is a sectional view of the memory cell array part of the ferroelectric memory according to the fourth embodiment shown in FIG. 12 as viewed from a direction A;  
         [0055]    [0055]FIG. 14 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a first modification of the fourth embodiment of the present invention;  
         [0056]    [0056]FIG. 15 is a sectional view of the memory cell array part of the ferroelectric memory according to the first modification of the fourth embodiment shown in FIG. 14 as viewed from a direction B;  
         [0057]    [0057]FIG. 16 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a second modification of the fourth embodiment of the present invention;  
         [0058]    [0058]FIG. 17 is a sectional view of the memory cell array part of the ferroelectric memory according to the second modification of the fourth embodiment shown in FIG. 16 as viewed from a direction B;  
         [0059]    [0059]FIG. 18 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a fifth embodiment of the present invention;  
         [0060]    [0060]FIG. 19 is a sectional view of the memory cell array part of the ferroelectric memory according to the fifth embodiment shown in FIG. 18 as viewed from a direction A;  
         [0061]    [0061]FIG. 20 is a sectional view of the memory cell array part of the ferroelectric memory according to the fifth embodiment shown in FIG. 18 as viewed from a direction B;  
         [0062]    [0062]FIG. 21 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a sixth embodiment of the present invention;  
         [0063]    [0063]FIG. 22 is a sectional view of the memory cell array part of the ferroelectric memory according to the sixth embodiment shown in FIG. 21 as viewed from a direction A;  
         [0064]    [0064]FIG. 23 is a sectional view of the memory cell array part of the ferroelectric memory according to the sixth embodiment shown in FIG. 21 as viewed from a direction B;  
         [0065]    [0065]FIG. 24 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a seventh embodiment of the present invention;  
         [0066]    [0066]FIG. 25 is a sectional view of the memory cell array part of the ferroelectric memory according to the seventh embodiment shown in FIG. 24 as viewed from a direction A;  
         [0067]    [0067]FIG. 26 is a sectional view of the memory cell array part of the ferroelectric memory according to the seventh embodiment shown in FIG. 24 as viewed from a direction B;  
         [0068]    [0068]FIG. 27 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to an eighth embodiment of the present invention;  
         [0069]    [0069]FIG. 28 is a sectional view of the memory cell array part of the ferroelectric memory according to the eighth embodiment shown in FIG. 27 as viewed from a direction A;  
         [0070]    [0070]FIG. 29 is a sectional view of the memory cell array part of the ferroelectric memory according to the eighth embodiment shown in FIG. 27 as viewed from a direction B;  
         [0071]    [0071]FIG. 30 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a ninth embodiment of the present invention;  
         [0072]    [0072]FIG. 31 is a sectional view of the memory cell array part of the ferroelectric memory according to the ninth embodiment shown in FIG. 30 as viewed from a direction A;  
         [0073]    [0073]FIG. 32 is a sectional view of the memory cell array part of the ferroelectric memory according to the ninth embodiment shown in FIG. 30 as viewed from a direction B;  
         [0074]    [0074]FIG. 33 is a perspective view showing the structure of a memory cell array part of a ferroelectric memory according to a tenth embodiment of the present invention;  
         [0075]    [0075]FIG. 34 is a sectional view of the memory cell array part of the ferroelectric memory according to the tenth embodiment shown in FIG. 33 as viewed from a direction A;  
         [0076]    [0076]FIG. 35 is a correlation diagram for illustrating the principle of a method of reducing disturbance according to a modification of the second embodiment of the present invention;  
         [0077]    [0077]FIG. 36 is a circuit diagram showing a memory cell part of a conventional one-transistor one-capacitor ( 1 T 1 C) ferroelectric memory;  
         [0078]    [0078]FIG. 37 is a circuit diagram for illustrating a read operation in the conventional  1 T 1 C ferroelectric memory shown in FIG. 36;  
         [0079]    [0079]FIG. 38 is a circuit diagram showing a memory cell part of a conventional two-transistor two-capacitor ( 2 T 2 C) ferroelectric memory;  
         [0080]    [0080]FIG. 39 is a circuit diagram showing a conventional matrix ferroelectric memory;  
         [0081]    [0081]FIG. 40 is a hysteretic diagram for illustrating the operation principle of the conventional matrix ferroelectric memory shown in FIG. 39; and  
         [0082]    [0082]FIG. 41 is a hysteretic diagram for illustrating a disturbance phenomenon in the conventional matrix ferroelectric memory. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0083]    Embodiments of the present invention are now described with reference to the drawings.  
         [0084]    (First Embodiment)  
         [0085]    The overall structure of a ferroelectric memory according to a first embodiment of the present invention is described with reference to FIGS. 1 and 2. The ferroelectric memory according to the first embodiment includes a memory cell array  1 , a row decoder  2 , a column decoder  3 , a row address buffer  4 , a column address buffer  5 , a write amplifier  6 , an input buffer  7 , a read amplifier  8 , an output buffer  9 , a voltage generation circuit  10  and a clock generation part  11 .  
         [0086]    As shown in FIG. 2, the memory cell array  1  includes a plurality of memory cells  41  each consisting of only two ferroelectric capacitors  42   a  and  42   b . The ferroelectric capacitors  42   a  and  42   b  are examples of the “capacitance means” in the present invention. The row decoder  2  is connected to word lines WL 1  to WL 4 , and the column decoder  3  is connected to bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B. The read amplifier  8  is connected to the write amplifier  6  through the column decoder  3 . The input buffer  7  is connected to the write amplifier  6 , and the output buffer  9  is connected to the read amplifier  8 .  
         [0087]    The row decider  2  includes a NAND circuit  21 , an inverter circuit  22  and transfer gates  23  and  24 . A pulse line WL PULSE is connected to the transfer gate  24 . The transfer gate  24  and the pulse line WL PULSE form a “pulse application circuit”. The column decoder  3  includes a NAND circuit  31 , an inverter circuit  32  and transfer gates  33 ,  34 ,  35  and  36 .  
         [0088]    Operations of the aforementioned ferroelectric memory according to the first embodiment are now described with reference to FIGS.  1  to  4 . It is assumed that a memory cell  41  arranged on the intersections between the word line WL 2  and the bit lines BL 2 T and BL 2 B is selected. In a standby time, the row decoder  2  and the column decoder  3  apply a voltage of ½Vcc to all word lines WL 1  to WL 4  and all bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B. In other words, both of row address lines (Row Add.) and column address lines (Col. Add) are low (0 V), while outputs of the NAND circuits  21  and  31  are high (Vcc). Thus, the transfer gates  23 ,  33  and  35  enter open states for supplying the voltage of ½Vcc to the word line WL 2  and the bit lines BL 2 T and BL 2 B.  
         [0089]    In order to write data “0”, the ferroelectric memory applies a pulse WL PULSE to the word line WL 2  selected by the row decoder  2  through the transfer gate  24 . The write amplifier  6  applies voltages Vcc and 0 V to the pair of bit lines BL 2 T and BL 2 B selected by the column decoder  3  through the transfer gates  34  and  36  respectively. The write amplifier  6  is an example of the “write voltage application circuit” in the present invention. Thus, the ferroelectric memory writes data “1” in the ferroelectric capacitor  42   b  connected with the bit line BL 2 B while the word line WL 2  is at the voltage Vcc, and writes the data “0” in the ferroelectric capacitor  42   a  connected with the bit line BL 2 T while the word line WL 2  is at the voltage 0 V. Thus, the ferroelectric memory writes complementary data in the two ferroelectric capacitors  42   a  and  42   b . In this case, it follows that the data “0” is written as the data of the memory cell  41 . Thereafter the ferroelectric memory resets the row address lines (Row Add.) and the column address lines (Col. Add.) to 0 V, thereby returning the voltages of the word line WL 2  and the bit lines BL 2 T and BL 2 B to the standby level of ½Vcc and terminating the write operation.  
         [0090]    In order to write the data “1”, the ferroelectric memory inverts data input in the write amplifier  6  from the input buffer  7  from that in the aforementioned write operation for the data “0”. More specifically, the ferroelectric memory applies the voltages 0 V and Vcc to the bit lines BL 2 T and BL 2 B respectively. Therefore, the data “1” is written as cell data.  
         [0091]    In the read operation, the read amplifier  8  precharges the pair of bit lines BL 2 T and BL 2 B connected with the memory cell  41 , selected by the column decoder  3 , to 0 V. Thereafter the ferroelectric memory applies the pulse WL PULSE to the word line WL 2  selected by the row decoder  2  through the transfer gate  24 , similarly to the write operation. In this case, however, the ferroelectric memory delays selection of the word line WL 2  by the row decoder  2  by delaying settlement of a row address or the like, in order drive the word line WL after precharging the pair of bit lines BL 2 T and BL 2 B to 0 V. The pulse WL PULSE is initially at the potential Vcc, and hence potential difference substantially corresponding to the voltage Vcc is developed between both poles of the two ferroelectric capacitors  42   a  and  42   b  of the selected cell  41 . Complementary data are written in the two ferroelectric capacitors  42   a  and  42   b , whereby different voltages appear on the pair of bit lines BL 2 T and BL 2 B. The difference between the voltages is amplified by the read amplifier  8  and output through the output buffer  9 . The pair of bit lines BL 2 T and BL 2 B may alternatively be precharged to a voltage level other than 0 V.  
         [0092]    The aforementioned read operation is destructively performed, and hence the data settled by the read amplifier  8  must be rewritten (restored) in the selected cell  41 . This rewrite operation is carried out similarly to the aforementioned data write operation. In other words, the ferroelectric memory rewrites the data “1” in the ferroelectric capacitor  42   a  or  42   b  connected with the bit line BL 2 T or BL 2 B at the voltage 0 V in the first half period when the word line WL 2  is at the voltage Vcc and rewrites the data “0” in the ferroelectric capacitor  42   a  or  42   b  connected with the bit line BL 2 T or BL 2 B at the voltage Vcc in the second half period when the word line WL 2  is at the voltage 0 V.  
         [0093]    Thereafter the ferroelectric memory resets the row address lines (Row Add.) and the column address lines (Col. Add.) to 0 V, thereby returning the voltages of the word line WL 2  and the bit lines BL 2 T and BL 2 B to the standby level of ½Vcc. Thus, the ferroelectric memory terminates the read operation.  
         [0094]    The voltage of ½Vcc is regularly applied to the non-selected bit lines BL 1 T and BL 1 B, BL 3 T and BL 3 B and BL 4 T and BL 4 B and the non-selected word lines WL 1 , WL 3  and WL 4  in the write operation and the read operation, and hence it follows that a voltage of ½Vcc at the maximum is applied to the ferroelectric capacitors  42   a  and  42   b  of the non-selected cells  41 . Similarly to the conventional ferroelectric memory shown in FIG. 41, therefore, disturbance is repeated due to the hysteresis of a ferroelectric substance to reduce the quantity of polarization charges. Basically, however, no polarization inversion is caused with this potential difference and hence no problem arises if the reduction of the quantity of polarization charges is small. In other words, the ferroelectric memory according to the first embodiment detects the difference between the potentials of the complementary data without employing a reference voltage, whereby the ferroelectric memory is hardly influenced by reduction of the quantity of polarization charges as compared with the case of employing the reference voltage.  
         [0095]    According to the first embodiment, each memory cell  41  is formed only by the two ferroelectric capacitors  42   a  and  42   b  as hereinabove described, whereby the memory cell area can be reduced as compared with that in a  2 T 2 C ferroelectric memory having memory cells each formed by two selection transistors and two ferroelectric capacitors and hence the degree of integration can be improved. The two ferroelectric capacitors  42   a  and  42   b  forming each memory cell  41  store complementary data respectively, whereby no reference voltage is required and initial potential difference in reading can be increased. Thus, even if the characteristics of the ferroelectric capacitors  42   a  and  42   b  are deteriorated due to fabrication dispersion or increase of the number of write/read times, the ferroelectric memory can be effectively prevented from false data reading.  
         [0096]    (Second Embodiment)  
         [0097]    FIGS.  5  to  8  show a ferroelectric memory according to a second embodiment of the present invention. Referring to FIGS.  5  to  8 , the ferroelectric memory according to the second embodiment applies a voltage of ⅓Vcc at the maximum to ferroelectric capacitors  42   a  and  42   b  of non-selected cells  41 , dissimilarly to the aforementioned first embodiment. When a problem of disturbance arises due to application of a voltage of ½Vcc, the maximum voltage applied to the ferroelectric capacitors  42   a  and  42   b  of the non-selected cells  41  can be reduced to ⅓Vcc by employing voltages of ⅓Vcc and ⅔Vcc. The remaining structure of the ferroelectric memory according to the second embodiment is similar to that of the ferroelectric memory according to the first embodiment.  
         [0098]    Operations of the ferroelectric memory according to the second embodiment are now described. In a standby state, the ferroelectric memory sets all word lines WL 1  to WL 4  and all bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B to the voltage of ½Vcc. It is assumed that the memory cell  41  arranged on the intersections between the word line WL 2  and the pair of bit lines BL 2 T and BL 2 B is selected.  
         [0099]    [0099]FIG. 5 shows voltages applied to the word lines WL 1  to WL 4  and the pairs of bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B for writing data “0”. According to the second embodiment, the disturbance voltage is set to ⅓Vcc at the maximum and hence data cannot be simultaneously written in the ferroelectric capacitors  42   a  and  42   b  connected with the bit lines BL 2 T and BL 2 B respectively. Therefore, the write operation must be performed twice while varying the voltages applied to the word lines WL 1  to WL 4  and the bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B, as shown in FIG. 5.  
         [0100]    The data may be first written in either the ferroelectric capacitor  42   a  or  42   b  connected with the bit line BL 2 T or BL 2 B.  
         [0101]    As shown in FIG. 5, the ferroelectric memory writes the data “0” in the ferroelectric capacitor  42   a  connected with the bit line BL 2 T while setting the word line WL 2  and the bit line BL 2 T to the voltages 0 V and Vcc respectively. Further, the ferroelectric memory writes inverse data “1” in the ferroelectric capacitor  42   b  connected with the bit line BL 2 B while setting the word line WL and the bit line BL 2 B to the voltages Vcc and 0 V respectively. At this time, the voltages applied to the word lines WL 1  to WL 4  and the bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B are set as shown in FIG. 5 so that voltages applied to the non-written ferroelectric capacitor  42   a  or  42   b  of the selected cell  41  and all ferroelectric capacitors  42   a  and  42   b  of the non-selected cells  41  can be reduced to ⅓Vcc. Thereafter the ferroelectric memory returns all word lines WL 1  to WL 4  and all pairs of bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B to standby states of ½Vcc, thereby terminating the write operation.  
         [0102]    [0102]FIG. 6 shows voltages applied to the word lines WL 1  to WL 4  and the bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B for writing the data “1”. In this case, data inverse to those in the aforementioned case of writing the data “0” are written in the ferroelectric capacitors  42   a  and  42   b  connected with the bit lines BL 2 T and BL 2 B respectively. In correspondence thereto, the voltages applied to the word lines WL 1  to WL 4  and the bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B are set as shown in FIG. 6 so that voltages applied to the non-written ferroelectric capacitor  42   a  or  42   b  of the selected cell  41  and all ferroelectric capacitors  42   a  and  42   b  of the non-selected cells  41  can be reduced to ⅓Vcc.  
         [0103]    In reading, voltages applied to the word lines WL 1  to WL 4  and the bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B are set as shown in FIG. 7 thereby precharging the selected pair of bit lines BL 2 T and BL 2 B to 0 V. After this precharging, the pair of bit lines BL 2 T and BL 2 B are brought into Hi-Z (high impedance) states at 0 V while the selected word line WL 2  is set to the voltage Vcc as shown in FIG. 8, thereby obtaining complementary signal voltages corresponding to the data of the selected cell  41  on the selected pair of bit lines BL 2 T and BL 2 B. The complementary signal voltages are amplified by a read amplifier and output from an output buffer. The pair of bit lines BL 2 T and BL 2 B may alternatively be precharged to a voltage level other than 0 V.  
         [0104]    Also in the second embodiment, the read operation is destructively performed similarly to the first embodiment, and hence the data settled by the read amplifier must be rewritten (restored) in the selected cell  41 . This rewrite operation is carried out similarly to the aforementioned data write operation. In other words, the ferroelectric memory rewrites the data “0” in the ferroelectric capacitors  42   a  and  42   b  connected with the bit lines BL 2 T and BL 2 B respectively at the voltages shown in FIG. 5 while rewriting the data “1” therein at the voltages shown in FIG. 6. Thereafter the ferroelectric memory returns all word lines WL 1  to WL 4  and all pairs of bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B to standby states of ½Vcc, thereby terminating the write operation  
         [0105]    According to the second embodiment, the ferroelectric memory sets the voltages applied to the word lines WL 1  to WL 4  and the pairs of bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B so that the voltage applied to the ferroelectric capacitors  42   a  and  42   b  of the non-selected cells  41  is ⅓Vcc at the maximum as hereinabove described, whereby the non-selected memory cells  41  can be effectively prevented from a disturbance phenomenon.  
         [0106]    According to the second embodiment, further, each memory cell  41  is formed by only the two ferroelectric capacitors  42   a  and  42   b  similarly to the aforementioned first embodiment, whereby the areas of the memory cells  41  can be reduced as compared with the conventional  2 T 2 C memory cells each consisting of two transistors and two ferroelectric capacitors, for improving the degree of integration. Further, the ferroelectric memory writing complementary data in the two ferroelectric capacitors  42   a  and  42   b  of each memory cell  41  requires no reference voltage and can increase initial potential difference in reading as compared with a case of employing a reference voltage. Also when the characteristics of the ferroelectric capacitors  42   a  and  42   b  are deteriorated due to fabrication dispersion or increase of the number of writing/reading times, therefore, the ferroelectric memory can be effectively prevented from false data reading.  
         [0107]    (Third Embodiment)  
         [0108]    FIGS.  9  to  11  show the structure of a memory cell array part of a ferroelectric memory according to a third embodiment of the present invention. According to the third embodiment, a ferroelectric layer  52  forming ferroelectric capacitors  42   a  and  42   b  has a single-level structure. Referring to FIG. 9, the ferroelectric layer  52  is omitted for facilitating understanding of interconnection structures of bit lines BL 1 T, BL 1 B, BL 2 T and BL 2 B and word lines WL 1 , WL 2 , WL 3  and WL 4 .  
         [0109]    According to the third embodiment, the bit lines BL 1 T, BL 1 B, BL 2 T and BL 2 B and the word lines WL 1  to WL 4  are arranged to intersect with each other in the form of a matrix. The single ferroelectric layer  52  is arranged between the word lines WL 1  to WL 4  and the bit lines BL 1 T, BL 1 B, BL 2 T and BL 2 B. The ferroelectric layer  52  is an example of the “capacitance means” and the “data storage layer” in the present invention.  
         [0110]    As shown in FIG. 11, a data storage part  52   a  of the ferroelectric layer  52 , the word line WL 4  located on the data storage part  52   a  and the bit line BL 1 T located under the data storage part  52   a  form a first ferroelectric capacitor  42   a . Further, a data storage part  52   b  of the ferroelectric layer  52 , the word line WL 4  located on the data storage part  52   b  and the bit line BL 1 B located under the data storage part  52   b  form a second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form a single memory cell  41 .  
         [0111]    The data storage parts  52   a  and  52   b  of the ferroelectric capacitors  42   a  and  42   b  store complementary data.  
         [0112]    In the ferroelectric memory according to the third embodiment, each memory cell  41  consisting of only two ferroelectric capacitors  42   a  and  42   b  can be easily formed due to the aforementioned structure. Thus, the areas of the memory cells  41  can be reduced as compared with the conventional  2 T 2 C memory cells each consisting of two transistors and two ferroelectric capacitors, whereby the degree of integration can be improved.  
         [0113]    The data storage parts  52   a  and  52   b  storing complementary data, transversely adjacently arranged in the third embodiment, may alternatively be transversely unadjacently arranged. In the aforementioned structure of the ferroelectric memory according to the third embodiment, further, the word lines WL 1  to WL 4  and the bit lines BL 1 T, BL 1 B, BL 2 T and BL 2 B may alternatively be arranged vertically oppositely to each other.  
         [0114]    (Fourth Embodiment)  
         [0115]    [0115]FIGS. 12 and 13 show the structure of a memory cell array part of a ferroelectric memory according to a fourth embodiment of the present invention. According to the fourth embodiment, two ferroelectric layers  52  are employed for forming ferroelectric capacitors  42   a  and  42   b , dissimilarly to the aforementioned third embodiment. Referring to FIG. 12, the ferroelectric layers  52  are omitted for facilitating understanding of interconnection structures of bit lines BL 1 T, BL 2 T, BL 1 B and BL 2 B and word lines WL 1 , WL 2 , WL 3  and WL 4 .  
         [0116]    In the ferroelectric memory according to the fourth embodiment, the bit lines BL 1 T and BL 2 T and the bit lines BL 1 B and BL 2 B are formed above and under the word lines WL 1  to WL 4  respectively, as shown in FIGS. 12 and 13. The ferroelectric layers  52  are formed between the upper bit lines BL 1 T and BL 2 T and the word lines WL 1  to WL 4  and between the lower bit lines BL 1 B and BL 2 B and the word lines WL 1  to WL 4  respectively. In other words, the ferroelectric memory has two ferroelectric layers  52 .  
         [0117]    A data storage part  52   a  of the upper ferroelectric layer  52 , the word line WL 1  and the upper bit line BL 1 T form an upper ferroelectric capacitor  42   a . A data storage part  52   b  of the lower ferroelectric layer  52 , the word line WL 1  and the lower bit line BL 1 B form a lower ferroelectric capacitor  42   b . The upper and lower ferroelectric capacitors  42   a  and  42   b  form a single memory cell  41 . According to this structure, the word line WL 1  serves both as a lower electrode of the upper ferroelectric capacitor  42   a  and as an upper electrode of the lower ferroelectric capacitor  42   b . The data storage parts  52   a  and  52   b  of the upper and lower ferroelectric capacitors  42   a  and  42   b  store complementary data.  
         [0118]    The ferroelectric memory according to the fourth embodiment has the two ferroelectric layers  52  forming the ferroelectric capacitors  42   a  and  42   b  as hereinabove described, whereby the degree of integration can be further improved as compared with the ferroelectric memory according to the third embodiment having the single ferroelectric layer  52 .  
         [0119]    [0119]FIGS. 14 and 15 show a ferroelectric memory according to a first modification of the fourth embodiment. Referring to FIGS. 14 and 15, ferroelectric capacitors  42   a  and  42   b  forming memory cells  41  are transversely adjacently formed in the ferroelectric memory according to the first modification of the fourth embodiment, dissimilarly to the ferroelectric memory according to the fourth embodiment shown in FIGS. 12 and 13. The ferroelectric memory according to the first modification of the fourth embodiment also has two ferroelectric layers  52 , and hence the degree of integration can be improved similarly to the ferroelectric memory according to the fourth embodiment.  
         [0120]    [0120]FIGS. 16 and 17 show a ferroelectric memory according to a second modification of the fourth embodiment. Referring to FIGS. 16 and 17, ferroelectric capacitors  42   a  and  42   b  forming each memory cell  41  are obliquely adjacently arranged in the ferroelectric memory according to the second modification of the fourth embodiment. Also according to this structure, the ferroelectric memory can have two ferroelectric layers  52  forming the ferroelectric capacitors  42   a  and  42   b  similarly to the fourth embodiment, whereby the degree of integration can be improved as compared with the ferroelectric memory according to the third embodiment having the single ferroelectric layer  52 . In the second modification of the fourth embodiment, the ferroelectric capacitors  42   a  and  42   b  may not necessarily be obliquely adjacent to each other but may be formed on obliquely separate positions.  
         [0121]    (Fifth Embodiment)  
         [0122]    FIGS.  18  to  20  show the structure of a memory cell array part of a ferroelectric memory according to a fifth embodiment of the present invention. Referring to FIGS.  18  to  20 , the ferroelectric memory according to the fifth embodiment is provided with two levels of word lines WL 11  to WL 14  and WL 21  to WL 24  and two levels of bit lines BL 11 T, BL 11 B, BL 12 T, BL 12 B, BL 21 T, BL 21 B, BL 22 T and BL 22 B respectively. Referring to FIG. 18, ferroelectric layers  52  and an insulator layer  53  are omitted for facilitating understanding of interconnection structures of the word lines WL 11  to WL 14  and WL 21  to WL 24  and the bit lines BL 11 T, BL 11 B, BL 12 T, BL 12 B, BL 21 T, BL 21 B, BL 22 T and BL 22 B.  
         [0123]    In the ferroelectric memory according to the fifth embodiment, the first-level word lines WL 11 , WL 12 , WL 13  and WL 14  are formed on the first-level bit lines BL 11 T, BL 11 B, BL 12 T and BL 12 B through the first ferroelectric layer  52 , as shown in FIGS.  18  to  20 . The insulator layer  53  is formed to cover the first-level word lines WL 11 , WL 12 , WL 13  and WL 14 . The second-level bit lines BL 21 T, BL 21 B, BL 22 T and BL 22 B are formed on the insulator layer  53 . The second-level word lines WL 21 , WL 22 , WL 23  and WL 24  are formed on the second-level bit lines BL 21 T, BL 21 B, BL 22 T and BL 22 B through the second ferroelectric layer  52 .  
         [0124]    According to the fifth embodiment, further, a data storage part  52   a  of the first ferroelectric layer  52 , the bit line BL 11 T arranged under the data storage part  52   a  and the word line WL 14  arranged on the data storage part  52   a  form a first ferroelectric capacitor  42   a , as shown in FIG. 20. A data storage part  52   b  of the first ferroelectric layer  52 , the first-level bit line BL 11 B located under the data storage part  52   b  and the word line WL 14  located on the data storage part  52   b  form a second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form a single memory cell  41 . The data storage parts  52   a  and  52   b  of the first and second ferroelectric capacitors  42   a  and  42   b  store complementary data.  
         [0125]    According to the fifth embodiment, the ferroelectric capacitors  42   a  and  42   b  forming each memory cell  41  are transversely adjacently arranged. The ferroelectric capacitors  42   a  and  42   b  may not necessarily be adjacent to each other but may be transversely arranged.  
         [0126]    According to the fifth embodiment, the ferroelectric memory has the two ferroelectric layers  52  as hereinabove described, whereby the degree of integration can be improved as compared with the ferroelectric memory according to the third embodiment having the single ferroelectric layer  52 .  
         [0127]    (Sixth Embodiment)  
         [0128]    FIGS.  21  to  23  show the structure of a memory cell array part of a ferroelectric memory according to a sixth embodiment of the present invention. Referring to FIGS.  21  to  23 , the ferroelectric memory according to the sixth embodiment has three ferroelectric layers  52  forming ferroelectric capacitors  42   a  and  42   b . Referring to FIG. 21, the ferroelectric layers  52  and insulator layers  53  are omitted for facilitating understanding of interconnection structures of word lines WL 11  to WL 14 , WL 21  to WL 24  and WL 31  to WL 34  and bit lines BL 11 T, BL 11 B, BL 12 T, BL 12 B, BL 21 T, BL 21 B, BL 22 T, BL 22 B, BL 31 T, BL 31 B, BL 32 T and BL 32 B.  
         [0129]    In the ferroelectric memory according to the sixth embodiment, the first-level word lines WL 11 , WL 12 , WL 13  and WL 14  are formed on the first-level bit lines BL 11 T, BL 11 B, BL 12 T and BL 12 B through the first ferroelectric layer  52 , as shown in FIGS.  21  to  23 . The first insulator layer  53  is formed to cover the first-level word lines WL 11 , WL 12 , WL 13  and WL 14 . The second-level bit lines BL 21 T, BL 21 B, BL 22 T and BL 22 B are formed on the first insulator layer  53 . The second-level word lines WL 21 , WL 22 , WL 23  and WL 24  are formed on the second-level bit lines BL 21 T, BL 21 B, BL 22 T and BL 22 B through the second ferroelectric layer  52 . The second insulator layer  53  is formed to cover the second-level word lines WL 21 , WL 22 , WL 23  and WL 24 . The third-level bit lines BL 31 T, BL 31 B, BL 32 T and BL 32 B are formed on the second insulator layer  53 . The third-level word lines WL 31 , WL 32 , WL 33  and WL 34  are formed on the third-level bit lines BL 31 T, BL 31 B, BL 32 T and BL 32 B through the third ferroelectric layer  52 .  
         [0130]    A data storage part  52   a  of the first ferroelectric layer  52 , the first-level bit line BL 11 T located under the data storage part  52   a  and the word line WL 14  located on the data storage part  52   a  form a first ferroelectric capacitor  42   a . A data storage part  52   b  of the first ferroelectric layer  52 , the first-level bit line BL 11 B located under the data storage part  52   b  and the first-level word line WL 14  located on the data storage part  52   b  form a second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form a single memory cell  41 . The data storage parts  52   a  and  52   b  store complementary data.  
         [0131]    The ferroelectric capacitors  42   a  and  42   b  forming each memory cell  41  are transversely adjacently arranged. The ferroelectric capacitors  42   a  and  42   b  may not necessarily be adjacent to each other but may be transversely unadjacently arranged.  
         [0132]    According to the sixth embodiment, the ferroelectric memory has the three ferroelectric layers  52  forming the ferroelectric capacitors  42   a  and  42   b  as hereinabove described, whereby the degree of integration can be improved as compared with the ferroelectric memory having the two ferroelectric layers  52 .  
         [0133]    (Seventh Embodiment)  
         [0134]    FIGS.  24  to  26  show the structure of a memory cell array part of a ferroelectric memory according to a seventh embodiment of the present invention. Referring to FIGS.  24  to  26 , the ferroelectric memory according to the seventh embodiment has four ferroelectric layers  52  forming ferroelectric capacitors  42   a  and  42   b . Referring to FIG. 24, the ferroelectric layers  52  and insulator layers  53  are omitted for facilitating understanding of interconnection structures of word lines WL 11  to WL 14 , WL 21  to WL 24 , WL 31  to WL 34  and WL 41  to WL 44  and bit lines BL 11 T, BL 11 B, BL 12 T, BL 12 B, BL 21 T, BL 21 B, BL 22 T, BL 22 B, BL 31 T, BL 31 B, BL 32 T, BL 32 B, BL 41 T, BL 41 B, BL 42 T and BL 42 B.  
         [0135]    In the ferroelectric memory according to the seventh embodiment, the first-level word lines WL 11 , WL 12 , WL 13  and WL 14  are formed on the first-level bit lines BL 11 T, BL 11 B, BL 12 T and BL 12 B through the first ferroelectric layer  52 . The first insulator layer  53  is formed to cover the first-level word lines WL 11 , WL 12 , WL 13  and WL 14 . The second-level bit lines BL 21 T, BL 21 B, BL 22 T and BL 22 B are formed on the first insulator layer  53 . The second-level word lines WL 21 , WL 22 , WL 23  and WL 24  are formed on the second-level bit lines BL 21 T, BL 21 B, BL 22 T and BL 22 B through the second ferroelectric layer  52 . The second insulator layer  53  is formed to cover the second-level word lines WL 21 , WL 22 , WL 23  and WL 24 . The third-level bit lines BL 31 T, BL 31 B, BL 32 T and BL 32 B are formed on the second insulator layer  53 .  
         [0136]    The third-level word lines WL 31 , WL 32 , WL 33  and WL 34  are formed on the third-level bit lines BL 31 T, BL 31 B, BL 32 T and BL 32 B through the third ferroelectric layer  52 . The third insulator layer  53  is formed to cover the third-level word lines WL 31 , WL 32 , WL 33  and WL 34 . The fourth-level bit lines BL 41 T, BL 41 B, BL 42 T and BL 42 B are formed on the third insulator layer  53 . The fourth-level word lines WL 41 , WL 42 , WL 43  and WL 44  are formed on the fourth-level bit lines BL 41 T, BL 41 B, BL 42 T and BL 42 B through the fourth ferroelectric layer  52 .  
         [0137]    In the seventh embodiment, a data storage part  52   a  of the first ferroelectric layer  52 , the first-level bit line BL 11 T located under the data storage part  52   a  and the word line WL 14  located on the data storage part  52   a  form a first ferroelectric capacitor  42   a , as shown in FIG. 26. A data storage part  52   b  of the first ferroelectric layer  52 , the first-level bit line BL 11 B located under the data storage part  52   b  and the first-level word line WL 14  located on the data storage part  52   b  form a second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form a single memory cell  41 . The data storage parts  52   a  and  52   b  of the first and second ferroelectric capacitors  42   a  and  42   b  store complementary data.  
         [0138]    According to the seventh embodiment, the ferroelectric capacitors  42   a  and  42   b  forming each memory cell  41  are transversely adjacently arranged. The ferroelectric capacitors  42   a  and  42   b  may not necessarily be adjacent to each other but may be transversely unadjacently arranged.  
         [0139]    According to the seventh embodiment, the ferroelectric memory has the four ferroelectric layers  52  forming the ferroelectric capacitors  42   a  and  42   b  as hereinabove described, whereby the degree of integration can be further improved as compared with the ferroelectric memories according to the aforementioned third to sixth embodiments.  
         [0140]    (Eighth Embodiment)  
         [0141]    FIGS.  27  to  29  show the structure of a memory cell array part of a ferroelectric memory according to an eighth embodiment of the present invention. Referring to FIGS.  27  to  29 , the ferroelectric memory according to the eighth embodiment has four ferroelectric layers  52  forming ferroelectric capacitors  42   a  and  42   b , while the ferroelectric capacitors  42   a  and  42   b  forming each memory cell  41  are vertically arranged. Referring to FIG. 27, the ferroelectric layers  52  and an insulator layer  53  are omitted for facilitating understanding of interconnection structures of word lines WL 11  to WL 14  and WL 21  to WL 24  and bit lines BL 11 B, BL 12 B, BL 13 B, BL 14 B, BL 11 T, BL 12 T, BL 13 T, BL 14 T, BL 21 B, BL 22 B, BL 23 B, BL 24 B, BL 21 T, BL 22 T, BL 23 T and BL 24 T.  
         [0142]    In the ferroelectric memory according to the eighth embodiment, the first-level word lines WL 11 , WL 12 , WL 13  and WL 14  are formed on the first-level bit lines BL 11 B, BL 12 B, BL 13 B and BL 14 B through the first ferroelectric layer  52 , as shown in FIGS.  27  to  29 . The second-level bit lines BL 11 T, BL 12 T, BL 13 T and BL 14 T are formed on the first-level word lines WL 11 , WL 12 , WL 13  and WL 14  through the second ferroelectric layer  52 . The insulator layer  53  is formed on the second-level bit lines BL 11 T, BL 12 T, BL 13 T and BL 14 T. The third-level bit lines BL 21 B, BL 22 B, BL 23 B and BL 24 B are formed on the insulator layer  53 . The second-level word lines WL 21 , WL 22 , WL 23  and WL 24  are formed on the third-level bit lines BL 21 B, BL 22 B, BL 23 B and BL 24 B through the third ferroelectric layer  52 . The fourth-level bit lines BL 21 T, BL 22 T, BL 23 T and BL 24 T are formed on the second-level word lines WL 21 , WL 22 , WL 23  and WL 24  through the fourth ferroelectric layer  52 .  
         [0143]    As shown in FIG. 29, a data storage part  52   a  of the second ferroelectric layer  52 , the first-level word line WL 14  located under the data storage part  52   a  and the second-level bit line BL 11 T located on the data storage part  52   a  form a first ferroelectric capacitor  42   a . A data storage part  52   b  of the first ferroelectric layer  52 , the first-level bit line BL 11 B located under the data storage part  52   b  and the first-level word line WL 14  located on the data storage part  52   b  form a second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form a single memory cell  41 . The data storage parts  52   a  and  52   b  of the first and second ferroelectric capacitors  42   a  and  42   b  store complementary data.  
         [0144]    According to the eighth embodiment, the ferroelectric capacitors  42   a  and  42   b  are vertically adjacently arranged. The ferroelectric capacitors  42   a  and  42   b  may not necessarily be vertically adjacent to each other but may be transversely or obliquely arranged.  
         [0145]    Also in the eighth embodiment, the ferroelectric memory has the four ferroelectric layers  52  forming the ferroelectric capacitors  42   a  and  42   b  similarly to the seventh embodiment, whereby the degree of integration can be further improved as compared with the ferroelectric memories according to the aforementioned third to sixth embodiments.  
         [0146]    (Ninth Embodiment)  
         [0147]    FIGS.  30  to  32  show the structure of a memory cell array part of a ferroelectric memory according to a ninth embodiment of the present invention. Referring to FIGS.  30  to  32 , the ferroelectric memory according to the ninth embodiment has six ferroelectric layers  52  forming ferroelectric capacitors  42   a  and  42   b . Referring to FIG. 30, the ferroelectric layers  52  are omitted for facilitating understanding of interconnection structures of bit lines BL 11  to BL 14 , BL 21  to BL 22 , BL 31  to BL 34  and BL 41  to BL 44  and word lines WL 11  to WL 14 , WL 21  to WL 24  and WL 31  to WL 34 .  
         [0148]    In the ferroelectric memory according to the ninth embodiment, the first-level word lines WL 11 , WL 12 , WL 13  and WL 14  are formed on the first-level bit lines BL 11 , BL 12 , BL 13  and BL 14  through the first ferroelectric layer  52 , as shown in FIGS.  30  to  32 . The second-level bit lines BL 21 , BL 22 , BL 23  and BL 24  are formed on the first-level word lines WL 11 , WL 12 , WL 13  and WL 14  through the second ferroelectric layer  52 . The second-level word lines WL 21 , WL 22 , WL 23  and WL 24  are formed on the second-level bit lines BL 21 , BL 22 , BL 23  and BL 24  through the third ferroelectric layer  52 . -The third-level bit lines BL 31 , BL 32 , BL 33  and BL 34  are formed on the second-level word lines WL 21 , WL 22 , WL 23  and WL 24  through the fourth ferroelectric layer  52 . The third-level word lines WL 31 , WL 32 , WL 33  and WL 34  are formed on the third-level bit lines BL 31 , BL 32 , BL 33  and BL 34  through the fifth ferroelectric layer  52 . The fourth-level bit lines BL 41 , BL 42 , BL 43  and BL 44  are formed on the third-level word lines WL 31 , WL 32 , WL 33  and WL 34  through the sixth ferroelectric layer  52 .  
         [0149]    As shown in FIG. 32, a data storage part  52   a  of the first ferroelectric layer  52 , the first-level bit line BL 11  located under the data storage part  52   a  and the word line WL 14  located on the data storage part  52   a  form a first ferroelectric capacitor  42   a . A data storage part  52   b  of the second ferroelectric layer  52 , the first-level word line WL 14  located under the data storage part  52   b  and the second-level bit line BL 21  located on the data storage part  52   b  form a second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form a single memory cell  41 . The data storage parts  52   a  and  52   b  of the first and second ferroelectric capacitors  42   a  and  42   b  store complementary data 1 and 0 respectively.  
         [0150]    As shown in FIG. 31, a data storage part  52   c  of the first ferroelectric layer  52 , the first-level bit line BL 11  located under the data storage part  52   c  and the word line WL 11  located on the data storage part  52   c  form another first ferroelectric capacitor  42   a . A data storage part  52   d  of the second ferroelectric layer  52 , the first-level word line WL 11  located under the data storage part  52   d  and the second-level bit line BL 21  located on the data storage part  52   d  form another second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form another single memory cell  41 . The data storage parts  52   c  and  52   d  of the first and second ferroelectric capacitors  42   a  and  42   b  store complementary data 1 and 0 respectively.  
         [0151]    As shown in FIG. 31, a data storage part  52   e  of the third ferroelectric layer  52 , the second-level bit line BL 21  located under the data storage part  52   e  and the second-level word line WL 21  located on the data storage part  52   e  form still another first ferroelectric capacitor  42   a . A data storage part  52   f  of the fourth ferroelectric layer  52 , the second-level word line WL 21  located under the data storage part  52   f  and the third-level bit line BL 31  located on the data storage part  52   f  form still another second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form still another single memory cell  41 . The data storage parts  52   e  and  52   f  of the first and second ferroelectric capacitors  42   a  and  42   b  store complementary data 1 and 0.  
         [0152]    More specifically, the bit lines BL 11  and BL 21  are employed for reading/writing the complementary data with respect to the word line WL 11 , as shown in FIG. 31. Further, the bit lines BL 21  and BL 31  are employed for reading/writing the complementary data with respect to the word line WL 21 . In this case, vertically adjacent memory cells  41  can share the bit line BL 21 , whereby the number of interconnection layers can be reduced. In other words, the bit lines BL 11  to BL 14 , BL 21  to BL 24 , BL 31  to BL 34  and BL 41  to BL 44  can be provided in four levels and word lines WL 11  to WL 14 , WL 21  to WL 24  and WL 31  to WL 34  can be provided in three levels according to the ninth embodiment despite the six ferroelectric layers  52 .  
         [0153]    In the ferroelectric memory according to the ninth embodiment having the six ferroelectric layers  52  forming the ferroelectric capacitors  42   a  and  42   b  as hereinabove described, the degree of integration can be further improved as compared with the aforementioned third to eighth embodiments.  
         [0154]    The ferroelectric capacitors  42   a  and  42   b  forming each memory cell  41 , vertically arranged in the ninth embodiment, may alternatively be obliquely arranged.  
         [0155]    (Tenth Embodiment)  
         [0156]    [0156]FIGS. 33 and 34 show the structure of a memory cell array part of a ferroelectric memory according to a tenth embodiment of the present invention. In the ferroelectric memory according to the tenth embodiment, word lines WL 21  and WL 22  and word lines WL 11  and WL 12  are arranged on and under bit lines BL 1 T, BL 1 B, BL 2 T and BL 2 B respectively. Referring to FIG. 33, ferroelectric layers  52  are omitted for facilitating understanding of interconnection structures of the bit lines BL 1 T, BL 1 B, BL 2 T and BL 2 B and the word lines WL 11 , WL 12 , WL 21  and WL 22 .  
         [0157]    In the ferroelectric memory according to the tenth embodiment, the bit lines BL 1 T, BL 1 B, BL 2 T and BL 2 B are formed on the first-level word lines WL 11  and WL 12  through the first ferroelectric layer  52 , as shown in FIGS. 33 and 34. The second-level word lines WL 21  and WL 22  are formed on the bit lines BL 1 T, BL 1 B, BL 2 T and BL 2 B through the second ferroelectric layer  52 . A data storage part  52   a  of the first ferroelectric layer  52 , the first-level word line WL 11  located under the data storage part  52   a  and the bit line BL 1 T located on the data storage part  52   a  form a first ferroelectric capacitor  42   a . A data storage part  52   b  of the second ferroelectric layer  52 , the bit line BL 1 B located under the data storage part  52   b  and the second-level word line WL 21  located on the data storage part  52   b  form a second ferroelectric capacitor  42   b . The first and second ferroelectric capacitors  42   a  and  42   b  form a single memory cell  41 .  
         [0158]    In the ferroelectric memory according to the tenth embodiment, the ferroelectric capacitors  42   a  and  42   b  forming each memory cell  41  are obliquely adjacently arranged. The data storage parts  52   a  and  52   b  store complementary data. The ferroelectric capacitors  42   a  and  42   b  may not necessarily be obliquely adjacently arranged but may be arranged on obliquely separate positions.  
         [0159]    According to the tenth embodiment, the word lines WL 11  and WL 21  are driven with a time lag in read and write operations. Thus, the ferroelectric memory according to the tenth embodiment can read/write data.  
         [0160]    Also in the ferroelectric memory according to the tenth embodiment having the two ferroelectric layers  52 , the degree of integration can be further improved as compared with the ferroelectric memory having the single ferroelectric layer  52 .  
         [0161]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.  
         [0162]    For example, while each of the above embodiments has been described with reference to a ferroelectric memory having ferroelectric capacitors, the present invention is not restricted to this but is also applicable to a memory including capacitance means other than the ferroelectric capacitors.  
         [0163]    While the ferroelectric memory according to the aforementioned second embodiment sets the voltages applied to the word lines WL 1  to WL 4  and the bit lines BL 1 T and BL 1 B to BL 4 T and BL 4 B so that the voltage applied to the ferroelectric capacitors  42   a  and  42   b  of the non-selected cells  41  is ⅓Vcc at the maximum thereby reducing disturbance of the non-selected memory cells  41 , the present invention is not restricted to this but the ferroelectric memory may alternatively reduce disturbance of the non-selected memory cells  41  by another method in place of the ⅓Vcc method. For example, the ferroelectric memory may reduce the disturbance by applying voltage pulses to the selected cell  41  and the non-selected cells  41  while controlling the times for applying the pulses.  
         [0164]    [0164]FIG. 35 is a correlation diagram for illustrating the aforementioned operation principle. This correlation diagram shows the relation between pulse widths and quantities of polarization inversion charges in a case of applying pulses to a ferroelectric capacitor employing an SBT film as a ferroelectric layer with parameters of applied voltages. As clearly understood from FIG. 35, the quantity of polarization inversion charges is substantially saturated at about 13 RC/cm 2  when the pulse width is not more than 70 nm and a high voltage (e.g., 3 V) is applied. On the other hand, it is also understood that substantially no polarization inversion is caused when a low voltage (e.g., 1.0 V) is applied.  
         [0165]    Thus, dipoles of a ferroelectric substance, which are inverted under a high voltage, substantially remain unmoved under a low voltage when the pulse width is relatively small. When a high voltage pulse is applied to a selected cell with a small pulse width while applying a low voltage pulse to non-selected cells with a small pulse width, therefore, voltages necessary for writing and reading can be applied to ferroelectric layers of the selected cell while causing no change in molecular structure in ferroelectric layers of the non-selected cells. When employing this operation principle, a simple matrix ferroelectric memory can perform a memory operation with no disturbance. When applying the aforementioned pulses, the width of the pulses applied to the word line WL 2  in writing and reading may be set to not more than  70  ns in the timing chart shown in FIG. 3, for example.  
         [0166]    While the ferroelectric layer(s) is entirely formed between the word lines WL and the bit lines BL in each of the aforementioned third to tenth embodiments, the present invention is not restricted to this but the ferroelectric layer(s) (data storage parts) necessary for operations of the ferroelectric memory may simply be formed at least on the intersections between the word lines WL and the bit lines BL.  
         [0167]    In each of the aforementioned third to tenth embodiments, further, ferroelectric layers or insulator layers may be arranged on regions between the adjacent word lines WL and between the adjacent bit lines BL.