Patent Publication Number: US-6912152-B2

Title: Magnetic random access memory

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-46964, filed Feb. 22, 2002, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a magnetic random access memory (MRAM) which utilizes a magnetoresistive effect. 
   2. Description of the Related Art 
   In recent years, many memories which store data by new principles have been proposed. One of them is a magnetic random access memory which utilizies the tunneling magnetoresistive (to be referred to as TMR hereinafter) effect. 
   As a proposal for a magnetic random access memory, for example, Roy Scheuerlein et al, “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest, p. 128 is known. 
   A magnetic random access memory stores “1”- and “0”-data using MTJ (Magnetic Tunnel Junction) elements. As the basic structure of a MTJ element, an insulating layer (tunneling barrier) is sandwiched between two magnetic layers (ferromagnetic layers). However, various kinds of MTJ element structures have been proposed to, e.g., optimize the MR (MagnetoResistive) ratio. 
   Data stored in the MTJ element is determined on the basis of whether the magnetizing states of the two magnetic layers are parallel or antiparallel. “Parallel” means that the two magnetic layers have the same magnetizing direction. “Antiparallel” means that the two magnetic layers have opposite magnetizing directions. 
   Normally, one (fixed layer) of the two magnetic layers has an antiferromagnetic layer. The antiferromagnetic layer serves as a member for fixing the magnetizing direction of the fixed layer. In fact, data (“1” or “0”) stored in the MTJ element is determined by the magnetizing direction of the other (free layer) of the two magnetic layers. 
   When the magnetizing states in the MTJ element are parallel, the tunneling resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers of the MTJ element is minimized. For example, this state is defined as a “1”-state. When the magnetizing states in the MTJ element are antiparallel, the tunneling resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers of the MTJ element is maximized. For example, this state is defined as a “0”-state. 
   Currently, various kinds of cell array structures have been examined for a magnetic random access memory from the viewpoint of increasing the memory capacity or stabilizing write/read operation. 
   For example, currently, a cell array structure in which one memory cell is formed from one MOS transistor and one MTJ element is known. Additionally, a magnetic random access memory which has such a cell array structure and stores 1-bit data using two memory cell arrays so as to realize stable read operation is also known. 
   However, in these magnetic random access memories, it is difficult to increase the memory capacity. This is because one MOS transistor corresponds to one MTJ element in these cell array structures. 
   As a magnetic random access memory which needs no MOS transistors in the memory cell array, a cross-point cell array structure is conventionally known. A cross-point cell array structure has a simple structure with an MTJ element being arranged at the inter-connection of a word line and a bit line. As a characteristic feature, no select transistor is arranged in the memory cell array. 
   According to the cross-point cell array structure, the memory cell size can be reduced because no select MOS transistors are used. As a consequence, the memory capacity can be increased. 
   For example, when the minimum size of design rule is defined as “F”, the size of a memory cell formed from a select MOS transistor and MTJ element is 8F 2 . However, a memory cell including only an MTJ element is 4F 2 . That is, the memory cell including only an MTJ element can realize a cell size about ½ that of the memory cell formed from a select MOS transistor and MTJ element. 
   However, when a magnetic random access memory is formed by employing a cross-point cell array structure, there is posed a problem of breakdown of the insulting layer (tunneling barrier layer) of a TMR (MTJ) element in write operation. 
   More specifically, in the cross-point cell array structure, an MTJ element is arranged at the intersection of a word line and a bit line while being in contact with them. Write currents having the same value are supplied to the word line and bit line (the directions of the write currents supplied to the word line and bit line change in accordance with the data value) to generate a magnetic field. The direction of magnetization of the MTJ element arranged between the word line and the bit line is thus determined. 
   The word line and bit line have interconnection resistances. The value of the interconnection resistance across the word line and bit line increases as they become long. That is, when the write current is flowing, the potential at a position close to the driver of the word line or bit line is higher than that at a position close to the sinker of the word line or bit line. 
   Hence, in write operation, a potential difference may be generated across the MTJ element in accordance with its position. This potential difference may cause voltage stress on the tunneling barrier layer of the MTJ element and then dielectric breakdown of the tunneling barrier layer. 
   This problem will be described in detail. 
   An MTJ element (worst case) which is arranged at a position closest to a word line driver WD (farthest from a word line sinker WS) and closest to a bit line sinker BS (farthest from a bit line driver BD), as shown in  FIG. 107 , will be examined. 
   The potential at the word-line-side end portion of the MTJ element is, e.g., Vp because the end portion is in contact with the word line at a position closest to the word line driver WD. On the other hand, the potential at the bit-line-side end portion of the MTJ element is, e.g., Vp-α because the end portion is in contact with the bit line at a position farthest from the bit line driver BD, and a voltage drop occurs due to an interconnection resistance r of the bit line. 
   That is, the potential of the bit-line-side end portion of the MTJ element is lower than that of the word-line-side end portion by α. As a result, the potential difference α is generated across the MTJ element arranged at the closest to the word line driver WD and bit line sinker BS. 
   Assume that dielectric breakdown of the tunneling barrier layer is caused by an electric field more than 10 [MV/cm] at a very high probability. 
   When the sheet resistance of the word line and bit line is 100 [mΩ], and the size of the memory cell array is 1750 (1.75 kilo) cells×1750 (1.75 kilo) cells, the interconnection resistance r from one end to the other end of the word line or bit line is as follows. 
   In the cross-point cell array structure, memory cells are arranged along the word lines and bit lines from one end to the other end of each of them. When a memory cell has a minimum process size (design rule) in the direction in which the word line or bit line runs, the pitch between the memory cells in that direction is also set to the minimum process size (pitch). 
   That is, the length of a word line or bit line corresponds to an array of 1750×2 memory cells. Hence, the interconnection resistance r from one end to the other end of the word line or bit line is 350 [Ω] (when the memory cell array becomes large, the word lines and bit lines become long, and the interconnection resistance r increases). 
   When the interconnection resistance r is 350 [Ω], and a write current Ip is 2 [mA], a potential difference of 0.7 (=0.002×350) [V] is generated across each of the word lines and bit lines. 
   When the thickness of the tunneling barrier layer of the MTJ element (when the MTJ element has a plurality of tunneling barrier layers, the total thickness of the tunneling barrier layers) is 0.7 [nm], and the potential difference across the MTJ element is 0.7 [V], an electric field of 10 [MV/cm] is generated in the MTJ element. 
   To avoid dielectric breakdown of the tunneling barrier layer under the above conditions, the size of one memory cell array surrounded by the word line driver/sinker and bit line driver/sinker must be set to 1.75 kilo×1.75 kilo or less. 
   As described above, in the cross-point cell array structure, when dielectric breakdown of the tunneling barrier layer of the MTJ element in write operation is taken into consideration, the upper limit of the memory cell array size is determined. Hence, the degree of integration of MTJ elements cannot be sufficiently increased. 
   In addition, the write current Ip does not always flow to the word line or bit line. The write current Ip is supplied to the word line or bit line only in the write operation. That is, the potential at a position closest to the word line or bit line sometimes exceeds Vp due to overshoot phenomenon. 
   In consideration of this overshoot phenomenon, an electric field more than 10 [MV/cm] may be generated in the MTJ element under the above conditions. 
   Assume that the sheet resistance of the word line and bit line, the write current Ip, and the thickness of the tunneling barrier layer are constant. In this case, to avoid probable generation of an electric field more than 10 [MV/cm] in the MTJ element at a high possibility, the memory cell array size must be further reduced to decrease the voltage drop amount due to the interconnection resistance r of the word line or bit line. 
   For example, overshoot of the potential on the word line or bit line will be examined under the above conditions. The upper limit size of one memory cell array must be decreased from 3 mega (=1.75 kilo×1.75 kilo) to 1.5 mega. 
   A clamp circuit which clamps the potential of the word line or bit line may be newly arranged as a peripheral circuit of the memory cell array to prevent the overshoot/undershoot phenomenon. 
   In this case, however, the size of the peripheral circuits increases as the clamp circuit is added. In addition, the clamp circuit has a function of suppressing abrupt increase/decrease in potential of the word line or bit line. For this reason, changing the potential of the word line or bit line to Vp takes a long time, resulting in a decrease in write speed. 
   BRIEF SUMMARY OF THE INVENTION 
   A magnetic random access memory according to a first example of the present invention comprises a memory cell array having memory cells which utilizes a magnetoresistive effect, a first functional line which runs in a first direction in the memory cell array and is commonly connected to one terminal of each of the memory cells, second functional lines which are arranged in correspondence with the memory cells and run in a second direction perpendicular to the first direction in the memory cell array, and a third functional line which is separated from the memory cells and shared by the memory cells. The other terminal of each of the memory cells is independently connected to one of the second functional lines, and one terminal of each of the memory cells is directly connected to the first functional line. 
   A magnetic random access memory according to a second example of the present invention comprises a memory cell array having a memory cell which utilizes a magnetoresistive effect, a first functional line which runs in a first direction in the memory cell array and is connected to one terminal of the memory cell, a second functional line which runs in a second direction perpendicular to the first direction in the memory cell array and is connected to the other terminal of the memory cell, and a third functional line which is separated from the memory cell and generates a magnetic field to write data in the memory cell. One terminal of the memory cell is directly connected to the first functional line, and the other terminal of the memory cell is directly connected to the second functional line. 
   A read method of a magnetic random access memory according to a third example of the present invention comprises fixing all the second functional lines to a first potential, setting the first functional line to a second potential different from the second potential, individually supplying a read current to the memory cells, and reading out data from the memory cells on the basis of a value of the read current. 
   A write method of a magnetic random access memory according to a fourth example of the present invention comprises supplying a first write current flowing in one direction to one of the second functional lines, supplying a second write current having a direction depending on write data to the third functional line, and writing the write data in one of the memory cells using a magnetic field generated by the first and second write currents. 
   A write method of a magnetic random access memory according to a fifth example of the present invention comprises supplying a first write current having a direction depending on write data to one of the second functional lines, supplying a second write current flowing in one direction to the third functional line, and writing the write data in one of the memory cells using a magnetic field generated by the first and second write currents. 
   A manufacturing method of a magnetic random access memory according to a sixth example of the present invention comprises the first step of forming a gate electrode of a MOS transistor in a peripheral circuit region and simultaneously forming, in a memory cell array region, dummy interconnections equidistantly, periodically, or in a layout uniform as a whole, the second step of forming a first interlayer dielectric film which covers the MOS transistor and dummy interconnections, the third step of forming a memory cell having a magnetoresistive effect in a surface region of the first interlayer dielectric film in the memory cell array region, and the fourth step of forming a second interlayer dielectric film which covers the memory cell. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a circuit diagram showing a magnetic random access memory according to Structural Example 1 of the present invention; 
       FIG. 2  is a sectional view showing Device Structure  1  according to Structural Example 1; 
       FIG. 3  is a plan view showing Device Structure  1  according to Structural Example 1; 
       FIG. 4  is a sectional view showing Device Structure  2  according to Structural Example 1; 
       FIG. 5  is a plan view showing Device Structure  2  according to Structural Example 1; 
       FIG. 6  is a plan view showing Device Structure  2  according to Structural Example 1; 
       FIG. 7  is a plan view showing Device Structure  2  according to Structural Example 1; 
       FIG. 8  is a sectional view showing Device Structure  3  according to Structural Example 1; 
       FIG. 9  is a plan view showing Device Structure  3  according to Structural Example 1; 
       FIG. 10  is a plan view showing Device Structure  3  according to Structural Example 1; 
       FIG. 11  is a plan view showing Device Structure  3  according to Structural Example 1; 
       FIG. 12  is a plan view showing Device Structure  3  according to Structural Example 1; 
       FIG. 13  is a view showing the outline of a magnetic random access memory according to Structural Example 2 of the present invention; 
       FIG. 14  is a sectional view showing Device Structure  1  according to Structural Example 2; 
       FIG. 15  is a sectional view showing Device Structure  2  according to Structural Example 2; 
       FIG. 16  is a circuit diagram showing a magnetic random access memory according to Structural Example 3 of the present invention; 
       FIG. 17  is a circuit diagram showing the magnetic random access memory according to Structural Example 3 of the present invention; 
       FIG. 18  is a sectional view showing a device structure according to Structural Example 3; 
       FIG. 19  is a plan view showing a device structure according to Structural Example 3; 
       FIG. 20  is a plan view showing a device structure according to Structural Example 3; 
       FIG. 21  is a plan view showing a device structure according to Structural Example 3; 
       FIG. 22  is a plan view showing a device structure according to Structural Example 3; 
       FIG. 23  is a plan view showing a device structure according to Structural Example 3; 
       FIG. 24  is a circuit diagram showing a magnetic random access memory according to Structural Example 4 of the present invention; 
       FIG. 25  is a circuit diagram showing the magnetic random access memory according to Structural Example 4 of the present invention; 
       FIG. 26  is a sectional view showing a device structure according to Structural Example 4; 
       FIG. 27  is a plan view showing a device structure according to Structural Example 4; 
       FIG. 28  is a plan view showing a device structure according to Structural Example 4; 
       FIG. 29  is a plan view showing a device structure according to Structural Example 4; 
       FIG. 30  is a plan view showing a device structure according to Structural Example 4; 
       FIG. 31  is a plan view showing a device structure according to Structural Example 4; 
       FIG. 32  is a plan view showing a device structure according to Structural Example 4; 
       FIG. 33  is a plan view showing a device structure according to Structural Example 4; 
       FIG. 34  is a circuit diagram showing a magnetic random access memory according to Structural Example 5 of the present invention; 
       FIG. 35  is a circuit diagram showing the magnetic random access memory according to Structural Example 5 of the present invention; 
       FIG. 36  is a sectional view showing a device structure according to Structural Example 5; 
       FIG. 37  is a plan view showing a device structure according to Structural Example 5; 
       FIG. 38  is a plan view showing a device structure according to Structural Example 5; 
       FIG. 39  is a plan view showing a device structure according to Structural Example 5; 
       FIG. 40  is a plan view showing a device structure according to Structural Example 5; 
       FIG. 41  is a plan view showing a device structure according to Structural Example 5; 
       FIG. 42  is a plan view showing a device structure according to Structural Example 5; 
       FIG. 43  is a plan view showing a device structure according to Structural Example 5; 
       FIG. 44  is a circuit diagram showing a magnetic random access memory according to Structural Example 6 of the present invention; 
       FIG. 45  is a circuit diagram showing the magnetic random access memory according to Structural Example 6 of the present invention; 
       FIG. 46  is a sectional view showing a device structure according to Structural Example 6; 
       FIG. 47  is a plan view showing a device structure according to Structural Example 6; 
       FIG. 48  is a plan view showing a device structure according to Structural Example 6; 
       FIG. 49  is a plan view showing a device structure according to Structural Example 6; 
       FIG. 50  is a plan view showing a device structure according to Structural Example 6; 
       FIG. 51  is a plan view showing a device structure according to Structural Example 6; 
       FIG. 52  is a plan view showing a device structure according to Structural Example 6; 
       FIG. 53  is a circuit diagram showing a magnetic random access memory according to Structural Example 7 of the present invention; 
       FIG. 54  is a sectional view showing a device structure according to Structural Example 7; 
       FIG. 55  is a plan view showing a device structure according to Structural Example 7; 
       FIG. 56  is a plan view showing a device structure according to Structural Example 7; 
       FIG. 57  is a plan view showing a device structure according to Structural Example 7; 
       FIG. 58  is a circuit diagram showing a magnetic random access memory according to Structural Example 8 of the present invention; 
       FIG. 59  is a circuit diagram showing a magnetic random access memory according to Structural Example 9 of the present invention; 
       FIG. 60  is a sectional view showing a device structure according to Structural Example 10; 
       FIG. 61  is a view showing a structural example of an MTJ element; 
       FIG. 62  is a view showing a structural example of the MTJ element; 
       FIG. 63  is a view showing a structural example of the MTJ element; 
       FIG. 64  is a view showing a circuit example of a write word line driver/sinker; 
       FIG. 65  is a view showing a circuit example of the write word line driver/sinker; 
       FIG. 66  is a view showing a circuit example of a row decoder; 
       FIG. 67  is a view showing a circuit example of a column decoder &amp; read column select line driver; 
       FIG. 68  is a view showing a circuit example of a write bit line driver/sinker; 
       FIG. 69  is a view showing a circuit example of a write bit line driver/sinker; 
       FIG. 70  is a view showing a circuit example of a column decoder &amp; write word line driver/sinker; 
       FIG. 71  is a view showing a circuit example of a row decoder; 
       FIG. 72  is a view showing a circuit example of a write word line driver; 
       FIG. 73  is a view showing a circuit example of a row decoder &amp; read line driver; 
       FIG. 74  is a circuit diagram showing a magnetic random access memory according to Structural Example 11 of the present invention; 
       FIG. 75  is a view showing a circuit example of a write bit line driver/sinker of  FIG. 74 ; 
       FIG. 76  is a view showing a circuit example of a write bit line driver/sinker of  FIG. 74 ; 
       FIG. 77  is a view showing a circuit example of a read circuit; 
       FIG. 78  is a view showing a circuit example of a read circuit; 
       FIG. 79  is a view showing a circuit example of a sense amplifier &amp; bit line bias circuit; 
       FIG. 80  is a view showing a circuit example of a sense amplifier; 
       FIG. 81  is a view showing a circuit example of a reference potential generation circuit; 
       FIG. 82  is a view showing a circuit example of an operational amplifier; 
       FIG. 83  is a view showing a circuit example of a sense amplifier &amp; bit line bias circuit; 
       FIG. 84  is a view showing MTJ elements arranged symmetrically with respect to a write line; 
       FIG. 85  is a view showing MTJ elements arranged symmetrically with respect to a write line; 
       FIG. 86  is a view showing MTJ elements arranged symmetrically with respect to a write line; 
       FIG. 87  is a view showing MTJ elements arranged symmetrically with respect to a write line; 
       FIG. 88  is a view showing MTJ elements arranged symmetrically with respect to a write line; 
       FIG. 89  is a view showing MTJ elements arranged symmetrically with respect to a write line; 
       FIG. 90  is a view showing a circuit example of a write bit line driver/sinker; 
       FIG. 91  is a sectional view showing a device structure to which a manufacturing method according to the example of the present invention is applied; 
       FIG. 92  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 93  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 94  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 95  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 96  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 97  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 98  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 99  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 100  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 101  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 102  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 103  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 104  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 105  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 106  is a sectional view showing a step in manufacturing according to the example of the present invention; 
       FIG. 107  is a view showing a problem of a cross-point cell array structure; 
       FIG. 108  is a circuit diagram showing a magnetic random access memory according to Modification example of Structural Example 8; 
       FIG. 109  is a circuit diagram showing a magnetic random access memory according to Modification example of Structural Example 8; 
       FIG. 110  is a circuit diagram showing a magnetic random access memory according to Modification example of Structural Example 8; 
       FIG. 111  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 112  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 113  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 114  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 115  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 116  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 117  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 118  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 119  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 120  is a circuit diagram showing a magnetic random access memory according to Structural Example 12 of the present invention; 
       FIG. 121  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 122  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 123  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 124  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 125  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 126  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 127  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 128  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 129  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 130  is a circuit diagram showing a magnetic random access memory according to Structural Example 13 of the present invention; 
       FIG. 131  is a circuit diagram showing a magnetic random access memory according to Structural Example 14 of the present invention; and 
       FIG. 132  is a circuit diagram showing a magnetic random access memory according to Structural Example 15 of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A magnetic random access memory according to an example of the present invention will be described below in detail with reference to the accompanying drawing. 
   1. Cell Array Structure 
   The cell array structure of the magnetic random access memory according to the example of the present invention will be described first. 
   As a characteristic feature of the cell array structure according to the example of the present invention, in a cell array structure in which one terminal of each of a plurality of MTJ elements which form a read block is commonly connected, and the other terminal is independently connected to a read bit line, one terminal of each of the plurality of MTJ elements is directly connected to a read word line without intervening a read select switch. 
   That is, no read select switch (e.g., a MOS transistor) is arranged in the read block. Consequently, a memory cell array can be formed from only MTJ elements. 
   According to this cell array structure, no switch element is arranged in the memory cell array. Hence, the density of MTJ elements can be increased, and the underlying layer of the MTJ elements can be planarized (the magnetoresistive value and MR ratio can be uniformed). In addition, since one of two write lines is separated from MTJ elements, no potential difference is generated across an MTJ element in write operation, unlike a cross-point cell array structure. Hence, the tunneling barrier layer of the MTJ element is not broken. 
   (1) STRUCTURAL EXAMPLE 1 
   In Structural Example 1, one read block is formed from four MTJ elements. 
   {circle around (1)} Circuit Structure 
   The circuit structure will be described first. 
     FIG. 1  shows the main part of a magnetic random access memory according to Structural Example 1 of the present invention. 
   A memory cell array  11  has a plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi (i=1, . . . , j). The read word line RWLi runs in the Y-direction. One read word line RWLi is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi is connected to a ground point VSS through a column select switch CSW formed from, e.g., a MOS transistor. 
   The column select switches CSW are arranged outside the memory cell array  11 . Hence, no switch elements (MOS transistors) are arranged in the memory cell array  11 . 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4. That is, the four read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 run in the X-direction. One end of each read bit line is connected to a common data line  30  through a row select switch (MOS transistor) RSW 2 . The common data line  30  is connected to a read circuit  29 B (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 111 and 121 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 - 1 , . . . ,  25 - n  output the row select line signals RLi. 
   As shown in  FIG. 111 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 121 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 - 1 , . . . ,  25 - n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 run in the X-direction and also function as write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4, respectively. 
   One end of each of the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4 is connected to a write word line driver  23 A through the row select switches RSW 2  and common data line  30 . The other end of each write word line is connected to a corresponding one of write word line sinkers  24 - 1 , . . . ,  24 - n.    
   One write bit line WBLi (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi is arranged in one column. 
   One end of each write bit line WBLi is connected to a circuit block  29 A including a column decoder and write bit line driver/sinker. The other end is connected to a circuit block  31  including a column decoder and write bit line driver/sinker. 
   In write operation, the circuit blocks  29 A and  31  are set in an operative state. A write current flows to the write bit lines WBLi in accordance with write data in a direction toward the circuit block  29 A or  31 . 
   In the write operation, the row decoder  25 - n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A supplies a write current to the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4 in the selected row. The write current is absorbed by the write word line sinker  24 - n.    
   In read operation, the row decoder  25 - n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32  selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   In the magnetic random access memory according to Structural Example 1, one terminal of each of the plurality of MTJ elements in a read block is commonly connected. The other terminal is connected to a corresponding one of different read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4. 
   Hence, data of the plurality of MTJ elements in one read block can be read at once by one read step. 
   The read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 also function as the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4, respectively. Since no interconnections which exclusively serve as write word lines need be arranged in the cell array, the cell array structure can be simplified. 
   As described above, as the characteristic feature of Structural Example 1, a read block has no read select switch for selecting it. In this case, the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 in an unselected row are biased to same potential to those in a selected row and the write word line WBLj in an unselected column is set in a floating state. 
   For this reason, the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 in all rows are set to the same potentials. 
   In Structural Example 1, in the read operation, for example, the potentials of the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 in the selected row are fixed to identical values. That is, the potentials of the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 in the selected row are fixed, and a change in read current flowing to the MTJ elements is detected. 
   The circuit (clamp circuit) for fixing the potentials of the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 in the selected row will be described later in detail in association with a read circuit. 
   If the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 in all rows always have the same potential in the read operation, no sneak current flows between the read bit lines through the plurality of unselected MTJ elements and poses no problem in determining the data value of the selected MTJ element. 
   In Structural Example 1, since no read select transistor is arranged in the read block, a current path is formed through the MTJ elements in an unselected block in the read operation. However, the resistance value of the MTJ element is sufficiently large. The read current is much smaller than the write current. Hence, an increase in current consumption poses no serious problem. 
   In the write operation, when the write current flows to the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4 in the selected row, the read word line RWLi is charged through the MTJ elements in the selected row. The read word line RWLi is in the floating state. Hence, it is only charged. No potential difference is generated across the MTJ element. 
   {circle around (2)} Device Structure  1   
   Device Structure  1  will be described next. 
   [1] Sectional Structure 
     FIG. 2  shows Device Structure  1  corresponding to one block of the magnetic random access memory according to Structural Example 1 of the present invention. 
   The same reference numerals as in  FIG. 1  denote the same elements in  FIG. 2  to show the correspondence between the elements. 
   A read word line RWL 1  running in the Y-direction is formed on a semiconductor substrate  41 . No switch element is arranged immediately under the read word line RWL 1 . Four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  arrayed in the Y-direction are formed above the read word line RWL 1 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is commonly connected to an upper electrode  44 . A contact plug  42  electrically connects the upper electrode  44  to the read word line RWL 1 . 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is electrically connected to a corresponding one of read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ). The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are independently connected to the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4 , respectively. That is, the four read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  are arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   A write bit line WBL 1  is formed above and near the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . The write bit line WBL 1  runs in the Y-direction (column direction). 
   In Structural Example 1, one write bit line WBL 1  is arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  which construct a read block. Instead, for example, the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  may be stacked, and four write bit lines may be arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   In Structural Example 1, the write bit line WBL 1  running in the Y-direction is arranged above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  running in the X-direction are arranged under the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   However, the positional relationship of the write bit line WBL 1  and read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  with respect to the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is not limited to this. 
   For example, the write bit line WBL 1  running in the Y-direction may be arranged under the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  running in the X-direction are arranged above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   According to this device structure, the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block are electrically connected to the different read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ), respectively. For this reason, data of the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block can be read at once by one read step. 
   One terminal of each of the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block is commonly connected. The connection point is directly connected to the read word line RWL 1  without intervening a read select switch. In addition, the write bit line WBL 1  running in the Y-direction is shared by the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block. For this reason, the degree of integration of MTJ elements can be increased, and their characteristic can be improved. 
   [2] Plane Structure 
     FIG. 3  shows the positional relationship between the MTJ elements, the read bit lines (write word lines), and the write bit line in the device structure shown in FIG.  2 . 
   The upper electrode  44  of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  has, e.g., a rectangular shape and has, as a portion, a contact region for the contact plug. 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are arranged in the Y-direction. Their axis of easy magnetization (a direction parallel to the long sides of the MTJ elements) is the X-direction. That is, each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  has a rectangular shape long in the X-direction. 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are arranged in a region where the write bit line WBL 1  and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ) cross each other. 
   {circle around (3)} Device Structure  2   
   Device Structure  2  will be described next. 
   [1] Sectional Structure 
     FIG. 4  shows Device Structure  2  corresponding to one block of the magnetic random access memory according to Structural Example 1 of the present invention. 
   The same reference numerals as in  FIG. 1  denote the same elements in  FIG. 4  to show the correspondence between the elements. 
   The read word line RWL 1  running in the Y-direction is formed on the semiconductor substrate  41 . No switch element is arranged immediately under the read word line RWL 1 . The four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  arrayed in the Y-direction are formed above the read word line RWL 1 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is commonly connected to the upper electrode  44 . The contact plug  42  and a conductive layer  43  electrically connect the upper electrode  44  to the read word line RWL 1 . 
   Device Structure  2  is different from Device Structure  1  in the position where the contact plug  42  is formed. More specifically, in Device Structure  1 , the contact plug  42  is formed at an end portion in the Y-direction. In Device Structure  2 , the contact plug  42  is arranged at the central portion of the upper electrode  44 . 
   When the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are uniformly arranged to be symmetrical with respect to the contact plug  42 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43  may be integrated with the upper electrode  44 . That is, the conductive layer  43  and upper electrode  44  may be formed simultaneously using the same material. 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is electrically connected to a corresponding one of the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ). The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are independently connected to the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4 , respectively. That is, the four read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  are arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   The write bit line WBL 1  is formed above and near the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . The write bit line WBL 1  runs in the Y-direction (column direction). 
   In Structural Example 1, one write bit line WBL 1  is arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  which construct a read block. Instead, for example, the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  may be stacked, and four write bit lines may be arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   In Structural Example 1, the write bit line WBL 1  running in the Y-direction is arranged above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  running in the X-direction are arranged under the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   However, the positional relationship of the write bit line WBL 1  and read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  with respect to the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is not limited to this. 
   For example, the write bit line WBL 1  running in the Y-direction may be arranged under the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  running in the X-direction are arranged above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   According to this device structure, the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block are electrically connected to the different read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ), respectively. For this reason, data of the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block can be read at once by one read step. 
   One terminal of each of the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block is commonly connected. The connection point is directly connected to the read word line RWL 1  without intervening a read select switch. In addition, the write bit line WBL 1  running in the Y-direction is shared by the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block. For this reason, the degree of integration of MTJ elements can be increased, and their characteristic can be improved. 
   [2] Plane Structure 
     FIGS. 5  to  7  show the layouts of the respective interconnection layers in Device Structure  2  shown in FIG.  4 . The section shown in  FIG. 4  corresponds to the section taken along a line IV—IV in  FIGS. 5  to  7 . 
     FIG. 5  shows the layout of read word lines. 
   The read word lines RWL 1  run in the Y-direction. The contact plug  42  is arranged on each read word line RWL 1 . 
     FIG. 6  shows the layout of the read bit lines and MTJ elements. 
   The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ) run in the X-direction. The interval between the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are arranged on the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4 , respectively. The axis of easy magnetization of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is the X-direction. 
   The read bit line RBL 1  is commonly connected to the MTJ elements MTJ 1  arranged in the X-direction. The read bit line RBL 2  is commonly connected to the MTJ elements MTJ 2  arranged in the X-direction. The read bit line RBL 3  is commonly connected to the MTJ elements MTJ 3  arranged in the X-direction. The read bit line RBL 4  is commonly connected to the MTJ elements MTJ 4  arranged in the X-direction. 
   The conductive layer  43  is arranged on the contact plug  42 . 
     FIG. 7  shows the layout of write bit lines. 
   The upper electrode  44  having a rectangular pattern is arranged on the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  and conductive layer  43 . The upper electrode  44  are in contact with the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  and conductive layer  43 . 
   The write bit lines WBL 1  are arranged immediately on the upper electrodes  44 . The write bit lines WBL 1  run in the Y-direction. 
   {circle around (4)} Device Structure  3   
   Device Structure  3  will be described next. 
   [1] Sectional Structure 
     FIG. 8  shows Device Structure  3  corresponding to one block of the magnetic random access memory according to Structural Example 1 of the present invention. 
   The same reference numerals as in  FIG. 1  denote the same elements in  FIG. 8  to show the correspondence between the elements. 
   The write bit line WBL 1  running in the Y-direction is formed on the semiconductor substrate  41 . No switch element is arranged immediately under the write bit line WBL 1 . A lower electrode  44  having, e.g., a rectangular pattern is formed above the write bit line WBL 1 . 
   The four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  arrayed in the Y-direction are formed on the lower electrode  44 . 
   The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ) are formed on the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , respectively. The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  are in contact with the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , respectively. The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are independently connected to the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4 , respectively. That is, the four read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  are arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   The contact plug  42  and conductive layer  43  are formed on the lower electrode  44 . The contact plug  42  and conductive layer  43  electrically connect the lower electrode  44  to the read word line RWL 1 . 
   The contact plug  42  is arranged at the central portion of the lower electrode  44 . When the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are uniformly arranged to be symmetrical with respect to the contact plug  42 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The read word line RWL 1  is formed above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . The read word line RWL 1  runs in the Y-direction (column direction). 
   In Structural Example 1, one write bit line WBL 1  is arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  which construct a read block. Instead, for example, the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  may be stacked, and four write bit lines may be arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   In Structural Example 1, the write bit line WBL 1  running in the Y-direction is arranged under the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  running in the X-direction are arranged above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   However, the positional relationship of the write bit line WBL 1  and read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  with respect to the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is not limited to this. 
   For example, the write bit line WBL 1  running in the Y-direction may be arranged above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  running in the X-direction are arranged under the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   According to this device structure, the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block are electrically connected to the different read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ), respectively. For this reason, data of the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block can be read at once by one read step. 
   One terminal of each of the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block is commonly connected. The connection point is directly connected to the read word line RWL 1  without intervening a read select switch. In addition, the write bit line WBL 1  running in the Y-direction is shared by the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block. For this reason, the degree of integration of MTJ elements can be increased, and their characteristic can be improved. 
   Furthermore, the contact portion between the lower electrode  44  and the read word line RWL 1  is formed in the region between the MTJ elements MTJ 1  and MTJ 2  and the MTJ elements MTJ 3  and MTJ 4 . When the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are uniformly arranged to be symmetrical with respect to the contact portion of the lower electrode  44 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   [2] Plane Structure 
     FIGS. 9  to  12  show the layouts of the respective interconnection layers in Device Structure  3  shown in FIG.  8 . The section shown in  FIG. 8  corresponds to the section taken along a line VIII—VIII in  FIGS. 9  to  12 . 
     FIG. 9  shows the layout of write bit lines. 
   The write bit lines WBL 1  run in the Y-direction. The lower electrode  44  having a rectangular shape is arranged on each write bit line WBL 1 . 
     FIG. 10  shows the layout of MTJ elements. 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  and conductive layer  43  are arranged on the lower electrode  44  having a rectangular pattern. 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  on the lower electrode  44  are arranged in the Y-direction. The axis of easy magnetization of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is the X-direction. 
     FIG. 11  shows the layout of read bit lines. 
   The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ) are arranged on the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , respectively. 
   The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  run in the X-direction. The interval between the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The read bit line RBL 1  is commonly connected to the MTJ elements MTJ 1  arranged in the X-direction. The read bit line RBL 2  is commonly connected to the MTJ elements MTJ 2  arranged in the X-direction. The read bit line RBL 3  is commonly connected to the MTJ elements MTJ 3  arranged in the X-direction. The read bit line RBL 4  is commonly connected to the MTJ elements MTJ 4  arranged in the X-direction. 
   The contact plug  42  is arranged on the conductive layer  43 . 
     FIG. 12  shows the layout of read word lines. 
   The read word lines RWL 1  run in the Y-direction. The read word line RWL 1  is in contact with the contact plug  42 . 
   (2) STRUCTURAL EXAMPLE 2 
   {circle around (1)} Outline 
     FIG. 13  shows the outline of a magnetic random access memory according to Structural Example 2 of the present invention. 
   The same reference numerals as in  FIG. 1  denote the same elements in  FIG. 13  to show the correspondence between the elements. 
   As a characteristic feature of Structural Example 2, a plurality of stages of memory cell arrays  11 - 1 ,  11 - 2 , . . . ,  11 - m  according to Structural Example 1 are stacked on a semiconductor substrate (chip)  10 . Each of the memory cell arrays  11 - 1 ,  11 - 2 , . . . ,  11 - m  corresponds to the memory cell array  11  shown in FIG.  1 . 
   {circle around (2)} Device Structure  1   
   In Device Structure  1  of Structural Example 2, a plurality of stages of memory cell arrays in Device Structure 2 ( FIG. 4 ) of Structural Example 1 are stacked. 
     FIG. 14  shows Device Structure  1  corresponding to one block of the magnetic random access memory according to Structural Example 2 of the present invention. 
   [1] First Stage (Memory Cell Array  11 - 1 ) 
   A read word line RWL 1 - 1  running in the Y-direction is formed on a semiconductor substrate  41 . No switch element is arranged immediately under the read word line RWL 1 - 1 . Four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  arrayed in the Y-direction are formed above the read word line RWL 1 - 1 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is commonly connected to an upper electrode  44 - 1 . A contact plug  42 - 1  and conductive layer  43 - 1  electrically connect the upper electrode  44 - 1  to the read word line RWL 1 - 1 . 
   The contact plug  42 - 1  is arranged at the central portion of the upper electrode  44 - 1 . When the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 1 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43 - 1  may be integrated with the upper electrode  44 - 1 . That is, the conductive layer  43 - 1  and upper electrode  44 - 1  may be formed simultaneously using the same material. 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is electrically connected to a corresponding one of read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ). The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are independently connected to the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1 , respectively. That is, the four read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are arranged in correspondence with the four MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . 
   A write bit line WBL 1 - 1  is formed above and near the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . The write bit line WBL 1 - 1  runs in the Y-direction (column direction). 
   [2] Second Stage (Memory Cell Array  11 - 2 ) 
   A read word line RWL 1 - 2  running in the Y-direction is formed on the write bit line WBL 1 - 1  in the memory cell array  11 - 1  of the first stage. Four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  arrayed in the Y-direction are formed above the read word line RWL 1 - 2 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is commonly connected to an upper electrode  44 - 2 . A contact plug  42 - 2  and conductive layer  43 - 2  electrically connect the upper electrode  44 - 2  to the read word line RWL 1 - 2 . 
   The contact plug  42 - 2  is arranged at the central portion of the upper electrode  44 - 2 . When the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 2 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43 - 2  may be integrated with the upper electrode  44 - 2 . That is, the conductive layer  43 - 2  and upper electrode  44 - 2  may be formed simultaneously using the same material. 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is electrically connected to a corresponding one of read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ). The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are independently connected to the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2 , respectively. That is, the four read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are arranged in correspondence with the four MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . 
   A write bit line WBL 1 - 2  is formed above and near the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . The write bit line WBL 1 - 2  runs in the Y-direction (column direction). 
   [3] Others 
   Referring to  FIG. 14 , the memory cell arrays  11 - 1  and  11 - 2  according to Device Structure  2  of the Structural Example 1 are stacked in two stages on the semiconductor substrate  41 . In principle, the memory cell arrays may be stacked in three or more stages (there is no upper limit). 
   According to Device Structure  1  of Structural Example 2, a plurality of stages of memory cell arrays according to Device Structure  2  of Structural Example 1 are stacked on the semiconductor substrate. For this reason, the density of MTJ elements can be increased. 
   {circle around (3)} Device Structure  2   
   In Device Structure  2  of Structural Example 2, a plurality of stages of memory cell arrays in Device Structure  3  ( FIG. 8 ) of Structural Example 1 are stacked. 
     FIG. 15  shows Device Structure  2  corresponding to one block of the magnetic random access memory according to Structural Example 2 of the present invention. 
   [1] First Stage (Memory Cell Array  11 - 1 ) 
   The write bit line WBL 1 - 1  running in the Y-direction is formed on the semiconductor substrate  41 . No switch element is arranged immediately under the write bit line WBL 1 - 1 . A lower electrode  44 - 1  having, e.g., a rectangular pattern is formed above the write bit line WBL 1 - 1 . 
   The four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  arrayed in the Y-direction are formed on the lower electrode  44 - 1 . 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ) are formed on the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , respectively. The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are in contact with the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , respectively. The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are independently connected to the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1 , respectively. That is, the four read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are arranged in correspondence with the four MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . 
   The contact plug  42 - 1  and conductive layer  43 - 1  are formed on the lower electrode  44 - 1 . The contact plug  42 - 1  and conductive layer  43 - 1  electrically connect the lower electrode  44 - 1  to the read word line RWL 1 - 1 . 
   The contact plug  42 - 1  is arranged at the central portion of the lower electrode  44 - 1 . When the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 1 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The read word line RWL 1 - 1  is formed above the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . The read word line RWL 1 - 1  runs in the Y-direction (column direction). 
   [2] Second Stage (Memory Cell Array  11 - 2 ) 
   The write bit line WBL 1 - 2  running in the Y-direction is formed on the semiconductor substrate  41 . No switch element is arranged immediately under the write bit line WBL 1 - 2 . A lower electrode  44 - 2  having, e.g., a rectangular pattern is formed above the write bit line WBL 1 - 2 . 
   The four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  arrayed in the Y-direction are formed on the lower electrode  44 - 2 . 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ) are formed on the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , respectively. The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are in contact with the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , respectively. The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are independently connected to the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2 , respectively. That is, the four read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are arranged in correspondence with the four MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . 
   The contact plug  42 - 2  and conductive layer  43 - 2  are formed on the lower electrode  44 - 2 . The contact plug  42 - 2  and conductive layer  43 - 2  electrically connect the lower electrode  44 - 2  to the read word line RWL 1 - 2 . 
   The contact plug  42 - 2  is arranged at the central portion of the lower electrode  44 - 2 . When the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 2 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The read word line RWL 1 - 2  is formed above the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . The read word line RWL 1 - 2  runs in the Y-direction (column direction). 
   [3] Others 
   Referring to  FIG. 15 , the memory cell arrays  11 - 1  and  11 - 2  according to Device Structure  3  of the Structural Example 1 are stacked in two stages on the semiconductor substrate  41 . In principle, the memory cell arrays may be stacked in three or more stages (there is no upper limit). 
   According to Device Structure  2  of Structural Example 2, a plurality of stages of memory cell arrays according to Device Structure  3  of Structural Example 1 are stacked on the semiconductor substrate. For this reason, the density of MTJ elements can be increased. 
   (3) STRUCTURAL EXAMPLE 3 
   {circle around (1)} Outline 
   Structural Example 3 is an improvement of Structural Example 2. In Structural Example 2, the plurality of stages of the memory cell arrays  11 - 1 ,  11 - 2 , . . . ,  11 - m  according to Structural Example 1 are stacked on the semiconductor substrate (chip). 
   Even in Structural Example 3, a plurality of stages of memory cell arrays according to Structural Example 1 are stacked on a semiconductor substrate (chip). In Structural Example 3, the number of interconnections in the memory cell arrays is decreased to planarize the underlying layer of MTJ elements (improve the characteristic of MTJ elements). For this purpose, one interconnection is shared by memory cell arrays of different stages. 
   {circle around (2)} Circuit Structure 
   In Structural Example 3, in a plurality of stages of memory cell arrays  11 - 1 ,  11 - 2 , . . . ,  11 - m  stacked, as shown in  FIG. 13 , the write bit line of the memory cell array of the lower stage and the read word line of the memory cell array of the upper stage are integrated and shared as one write bit line/read word line. 
     FIGS. 16 and 17  show the main part of a magnetic random access memory according to Structural Example 3 of the present invention. 
   [1] First Stage (Lower Stage) 
     FIG. 16  shows the cell array structure of the first stage of Structural Example 3. 
   The memory cell array  11 - 1  has a plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11 - 1  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi- 1  (i=1, . . . , j). The read word line RWLi- 1  runs in the Y-direction. One read word line RWLi- 1  is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi- 1  (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi- 1  is connected to a ground point VSS through a column select switch CSW formed from, e.g., a MOS transistor. 
   The column select switches CSW are arranged outside the memory cell array  11 - 1 . Hence, no switch elements (MOS transistors) are arranged in the memory cell array  11 - 1 . 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1. That is, the four read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 run in the X-direction. One end of each read bit line is connected to a common data line  30 ( 1 ) through a row select switch (MOS transistor) RSW 2 . The common data line  30 ( 1 ) is connected to a read circuit  29 B( 1 ) (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 112 and 122 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 ( 1 )- 1 , . . . ,  25 ( 1 )- n  output the row select line signals RLi. 
   As shown in  FIG. 112 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 122 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 ( 1 )- 1 , . . . ,  25 ( 1 )- n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 run in the X-direction and also function as write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1, respectively. 
   One end of each of the write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1 is connected to a write word line driver  23 A( 1 ) through the row select switches RSW 2  and common data line  30 ( 1 ). The other end of each write word line is connected to a corresponding one of write word line sinkers  24 ( 1 )- 1 , . . . ,  24 ( 1 )- n.    
   One write bit line WBLi- 1  (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi- 1  is arranged in one column. 
   One end of each write bit line WBLi- 1  is connected to a circuit block  29 A( 1 ) including a column decoder and write bit line driver/sinker through a switching circuit  22 . The other end of the write bit line WBLi- 1  is connected to a circuit block  31 ( 1 ) including a column decoder and write bit line driver/sinker through a disconnecting circuit  21 . 
   The disconnecting circuit  21  and switching circuit  22  are controlled by a memory cell array select signal SEL. In write operation, when the memory cell array  11 - 1  of the first stage (lower stage) is selected, the switching circuit  22  electrically connects one end of the write bit line WBLi- 1  to the circuit block  29 A( 1 ). The disconnecting circuit  21  electrically connects the other end of the write bit line WBLi- 1  to the circuit block  31 ( 1 ). 
   In the write operation, the circuit blocks  29 A( 1 ) and  31 ( 1 ) are set in an operative state. A write current flows to the write bit lines WBLi- 1  in accordance with write data in a direction toward the circuit block  29 A( 1 ) or  31 ( 1 ). 
   In the write operation, the row decoder  25 ( 1 )- n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A( 1 ) supplies a write current to the write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1 in the selected row. The write current is absorbed by the write word line sinker  24 ( 1 )- n.    
   In read operation, the row decoder  25 ( 1 )- n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32 ( 1 ) selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   [2] Second Stage (Upper Stage) 
     FIG. 17  shows the cell array structure of the second stage of Structural Example 3. 
   The memory cell array  11 - 2  has the plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11 - 2  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi- 2  (i=1, . . . , j). The read word line RWLi- 2  runs in the Y-direction. One read word line RWLi- 2  is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi- 2  (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi- 2  is connected to the ground point VSS through the column select switch CSW formed from the switching circuit  22  and a MOS transistor. 
   The other end of the read word line RWLi- 2  is connected to the circuit block  31 ( 1 ) including a column decoder and write bit line driver/sinker through the disconnecting circuit  21 . 
   The disconnecting circuit  21 , switching circuit  22 , and column select switches CSW are arranged outside the memory cell array  11 - 2 . Hence, no switch elements (MOS transistors) are arranged in the memory cell array  11 - 2 . 
   The disconnecting circuit  21  and switching circuit  22  are the disconnecting circuit  21  and switching circuit  22  in the cell array structure of the memory cell array of the first stage shown in FIG.  16 . 
   The disconnecting circuit  21  and switching circuit  22  are controlled by the memory cell array select signal SEL. 
   As described above, in the write operation, when the memory cell array  11 - 1  of the first stage (lower stage) is selected, the switching circuit  22  electrically connects one end of the write bit line WBLi- 1  to the circuit block  29 A( 1 ). The disconnecting circuit  21  electrically connects the other end of the write bit line WBLi- 1  to the circuit block  31 ( 1 ). 
   In the read operation, when the memory cell array  11 - 2  of the second stage (upper stage) is selected, the switching circuit  22  electrically connects one end of the read word line RWLi- 2  to the column select switch CSW. The disconnecting circuit  21  electrically disconnects the other end of the read word line RWLi- 2  from the circuit block  31 ( 1 ). 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2. That is, the four read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 run in the X-direction. One end of each read bit line is connected to a common data line  30 ( 2 ) through a row select switch (MOS transistor) RSW 2 . The common data line  30 ( 2 ) is connected to a read circuit  29 B( 2 ) (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 113 and 123 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 ( 2 )- 1 , . . . ,  25 ( 2 )- n  output the row select line signals RLi. 
   As shown in  FIG. 113 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 123 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 ( 2 )- 1 , . . . ,  25 ( 2 )- n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 run in the X-direction and also function as write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2, respectively. 
   One end of each of the write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2 is connected to a write word line driver  23 A( 2 ) through the row select switches RSW 2  and common data line  30 ( 2 ). The other end of each write word line is connected to a corresponding one of write word line sinkers  24 ( 2 )- 1 , . . . ,  24 ( 2 )- n.    
   One write bit line WBLi- 2  (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi- 2  is arranged in one column. 
   One end of each write bit line WBLi- 2  is connected to a circuit block  29 A( 2 ) including a column decoder and write bit line driver/sinker. The other end of the write bit line WBLi- 2  is connected to a circuit block  31 ( 2 ) including a column decoder and write bit line driver/sinker. 
   In the write operation, the circuit blocks  29 A( 2 ) and  31 ( 2 ) are set in an operative state. A write current flows to the write bit lines WBLi- 2  in accordance with write data in a direction toward the circuit block  29 A( 2 ) or  31 ( 2 ). 
   In the write operation, the row decoder  25 ( 2 )- n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A( 2 ) supplies a write current to the write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2 in the selected row. The write current is absorbed by the write word line sinker  24 ( 2 )- n.    
   In the read operation, the row decoder  25 ( 2 )- n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32 ( 2 ) selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   {circle around (3)} Device Structure (Sectional Structure) 
   As a characteristic feature of the device structure of Structural Example 3, in the memory cell array of Device Structure  1  ( FIG. 14 ) of Structural Example 2, a write bit line WBL 1 - 1  of the lower stage (first stage) and a read word line RWL 1 - 2  of the upper stage (second stage) are integrated and shared as one write bit line/read word line WBL 1 - 1 /RWL 1 - 2 . 
     FIG. 18  shows a device structure corresponding to one block of the magnetic random access memory according to Structural Example 3 of the present invention. 
   [1] First Stage (Memory Cell Array  11 - 1 ) 
   The read word line RWL 1 - 1  running in the Y-direction is formed on a semiconductor substrate  41 . No switch element is arranged immediately under the read word line RWL 1 - 1 . Four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  arrayed in the Y-direction are formed above the read word line RWL 1 - 1 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is commonly connected to an upper electrode  44 - 1 . A contact plug  42 - 1  and conductive layer  43 - 1  electrically connect the upper electrode  44 - 1  to the read word line RWL 1 - 1 . 
   The contact plug  42 - 1  is arranged at the central portion of the upper electrode  44 - 1 . When the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 1 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43 - 1  may be integrated with the upper electrode  44 - 1 . That is, the conductive layer  43 - 1  and upper electrode  44 - 1  may be formed simultaneously using the same material. 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is electrically connected to a corresponding one of read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ). The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are independently connected to the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1 , respectively. That is, the four read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are arranged in correspondence with the four MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . 
   A write bit line WBL 1 - 1  is formed above and near the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . The write bit line WBL 1 - 1  runs in the Y-direction (column direction). 
   [2] Second Stage (Memory Cell Array  11 - 2 ) 
   The write bit line WBL 1 - 1  in the memory cell array  11 - 1  of the first stage also functions as the read word line RWL 1 - 2  in the memory cell array  11 - 2  of the second stage. 
   More specifically, in the write operation, when the memory cell array  11 - 1  of the first stage is selected, the write bit line/read word line WBL 1 - 1 /RWL 1 - 2  are used as the write bit line WBL 1 - 1 . In the read operation, when the memory cell array  11 - 2  of the second stage is selected, the write bit line/read word line WBL 1 - 1 /RWL 1 - 2  is used as the read word line RWL 1 - 2 . 
   Four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  arrayed in the Y-direction are formed above the read word line RWL 1 - 2 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is commonly connected to an upper electrode  44 - 2 . A contact plug  42 - 2  and conductive layer  43 - 2  electrically connect the upper electrode  44 - 2  to the read word line RWL 1 - 2 . 
   The contact plug  42 - 2  is arranged at the central portion of the upper electrode  44 - 2 . When the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 2 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43 - 2  may be integrated with the upper electrode  44 - 2 . That is, the conductive layer  43 - 2  and upper electrode  44 - 2  may be formed simultaneously using the same material. 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is electrically connected to a corresponding one of read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ). The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are independently connected to the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2 , respectively. That is, the four read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are arranged in correspondence with the four MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . 
   A write bit line WBL 1 - 2  is formed above and near the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . The write bit line WBL 1 - 2  runs in the Y-direction (column direction). 
   [3] Others 
   In the example shown in  FIG. 18 , the memory cell arrays  11 - 1  and  11 - 2  are stacked in two stages on the semiconductor substrate  41 . In principle, the memory cell arrays may be stacked in three or more stages (there is no upper limit). 
   According to the device structure of Structural Example 3, the memory cell array  11 - 1  of the lower stage and the memory cell array  11 - 2  of the upper stage according to Device Structure  1  of Structural Example 2 share one interconnection. For this reason, the density of MTJ elements can be increased. In addition, the underlying layer of the MTJ elements can be planarized (the characteristic of the MTJ elements can be improved). 
   {circle around (4)} Device Structure (Plane Structure) 
     FIGS. 19  to  23  show the layouts of the respective interconnection layers in Device Structure  1  shown in FIG.  18 . The section shown in  FIG. 18  corresponds to the section taken along a line XVIII—XVIII in  FIGS. 19  to  23 . 
     FIG. 19  shows the layout of read word lines of the first stage. 
   The read word lines RWL 1 - 1  run in the Y-direction. The contact plug  42 - 1  is arranged on each read word line RWL 1 - 1 . 
     FIG. 20  shows the layout of read bit lines of the first stage and MTJ elements of the first stage. 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ) run in the X-direction. The interval between the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are arranged on the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1 . The axis of easy magnetization of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is the X-direction. 
   The read bit line RBL 1 - 1  is commonly connected to the MTJ elements MTJ 1 - 1  arranged in the X-direction. The read bit line RBL 2 - 1  is commonly connected to the MTJ elements MTJ 2 - 1  arranged in the X-direction. The read bit line RBL 3 - 1  is commonly connected to the MTJ elements MTJ 3 - 1  arranged in the X-direction. The read bit line RBL 4 - 1  is commonly connected to the MTJ elements MTJ 4 - 1  arranged in the X-direction. 
   The conductive layer  43 - 1  is arranged on the contact plug  42 - 1 . 
     FIG. 21  shows the layout of write bit lines of the first stage/read word lines of the second stage. 
   The upper electrodes  44 - 1  each having a rectangular pattern are arranged on the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  and conductive layers  43 . The upper electrodes  44 - 1  are in contact with the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  and conductive layers  43 - 1 . 
   The write bit lines/read word lines WBL 1 - 1 /RWL 1 - 2  are arranged immediately on the upper electrodes  44 - 1 . The write bit lines/read word lines WBL 1 - 1 /RWL 1 - 2  run in the Y-direction. 
   The contact plug  42 - 2  is arranged on each write bit lines/read word lines WBL 1 - 1 /RWL 1 - 2 . 
     FIG. 22  shows the layout of read bit lines of the second stage and MTJ elements of the second stage. 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ) run in the X-direction. The interval between the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are arranged on the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2 . The axis of easy magnetization of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is the X-direction. 
   The read bit line RBL 1 - 2  is commonly connected to the MTJ elements MTJ 1 - 2  arranged in the X-direction. The read bit line RBL 2 - 2  is commonly connected to the MTJ elements MTJ 2 - 2  arranged in the X-direction. The read bit line RBL 3 - 2  is commonly connected to the MTJ elements MTJ 3 - 2  arranged in the X-direction. The read bit line RBL 4 - 2  is commonly connected to the MTJ elements MTJ 4 - 2  arranged in the X-direction. 
   The conductive layer  43 - 2  is arranged on the contact plug  42 - 2 . 
     FIG. 23  shows the layout of write bit lines of the second stage. 
   The upper electrodes  44 - 2  each having a rectangular pattern are arranged on the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  and conductive layer  43 - 2 . The upper electrodes  44 - 2  are in contact with the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  and conductive layers  43 - 2 . 
   The write bit lines WBL 1 - 2  are arranged immediately on the upper electrodes  44 - 2 . The write bit lines WBL 1 - 2  run in the Y-direction. 
   (4) STRUCTURAL EXAMPLE 4 
   {circle around (1)} Outline 
   Structural Example 4 is also an improvement of Structural Example 2. In Structural Example 4, a plurality of stages of memory cell arrays are stacked on a semiconductor substrate (chip), and one interconnection is shared by memory cell arrays of different stages, as in Structural Example 3. With this arrangement, the number of interconnections in the memory cell arrays is decreased to planarize the underlying layer of MTJ elements (improve the characteristic of MTJ elements). 
   Structural Example 4 is different from Structural Example 3 in the positional relationship of an interconnection to be shared. More specifically, in Structural Example 3, one interconnection is shared as a write bit line of the memory cell array of the lower stage and a read word line of the memory cell array of the upper stage. In Structural Example 4, one interconnection is shared as a read word line of the memory cell array of the lower stage and a write bit line of the memory cell array of the upper stage. 
   {circle around (2)} Circuit Structure 
   In Structural Example 4, in a plurality of stages of memory cell arrays  11 - 1 ,  11 - 2 , . . . ,  11 - m  stacked, the read word line of the memory cell array of the lower stage and the write bit line of the memory cell array of the upper stage are integrated and shared as one write bit line/read word line. 
     FIGS. 24 and 25  show the main part of a magnetic random access memory according to Structural Example 4 of the present invention. 
   [1] First Stage (Lower Stage) 
     FIG. 24  shows the cell array structure of the first stage of Structural Example 4. 
   The memory cell array  11 - 1  has a plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11 - 1  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi- 1  (i=1, . . . , j). The read word line RWLi- 1  runs in the Y-direction. One read word line RWLi- 1  is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi- 1  (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi- 1  is connected to a ground point VSS through a switching circuit  22  and a column select switch CSW formed from a MOS transistor. 
   The other end of the read word line RWLi- 1  is connected to a circuit block  31 ( 2 ) including a column decoder and write bit line driver/sinker through a disconnecting circuit  21 . 
   The disconnecting circuit  21 , switching circuit  22 , and column select switches CSW are arranged outside the memory cell array  11 - 1 . Hence, no switch elements (MOS transistors) are arranged in the memory cell array  11 - 1 . 
   The disconnecting circuit  21  and switching circuit  22  are controlled by a memory cell array select signal SEL. 
   For example, in read operation, when the memory cell array  11 - 1  of the first stage (lower stage) is selected, the switching circuit  22  electrically connects one end of the read word line RWLi- 1  to the column select switch CSW. The disconnecting circuit  21  electrically disconnects the other end of the read word line RWLi- 1  from the circuit block  31 ( 2 ). 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1. That is, the four read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 run in the X-direction. One end of each read bit line is connected to a common data line  30 ( 1 ) through a row select switch (MOS transistor) RSW 2 . The common data line  30 ( 1 ) is connected to a read circuit  29 B( 1 ) (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 114 and 124 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 ( 1 )- 1 , . . . ,  25 ( 1 )- n  output the row select line signals RLi. 
   As shown in  FIG. 114 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 124 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 ( 1 )- 1 , . . . ,  25 ( 1 )- n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 run in the X-direction and also function as write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1, respectively. 
   One end of each of the write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1 is connected to a write word line driver  23 A( 1 ) through the row select switches RSW 2  and common data line  30 ( 1 ). The other end of each write word line is connected to a corresponding one of write word line sinkers  24 ( 1 )- 1 , . . . ,  24 ( 1 )- n.    
   One write bit line WBLi- 1  (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi- 1  is arranged in one column. 
   One end of each write bit line WBLi- 1  is connected to a circuit block  29 A( 1 ) including a column decoder and write bit line driver/sinker. The other end of the write bit line WBLi- 1  is connected to a circuit block  31 ( 1 ) including a column decoder and write bit line driver/sinker. 
   In the write operation, the circuit blocks  29 A( 1 ) and  31 ( 1 ) are set in an operative state. A write current flows to the write bit lines WBLi- 2  in accordance with write data in a direction toward the circuit block  29 A( 1 ) or  31 ( 1 ). 
   In the write operation, the row decoder  25 ( 1 )- n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A( 1 ) supplies a write current to the write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1 in the selected row. The write current is absorbed by the write word line sinker  24 ( 1 )- n.    
   In read operation, the row decoder  25 ( 1 )- n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32 ( 1 ) selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   [2] Second Stage (Upper Stage) 
     FIG. 25  shows the cell array structure of the second stage of Structural Example 4. 
   The memory cell array  11 - 2  has the plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11 - 2  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi- 2  (i=1, . . . , j). The read word line RWLi- 2  runs in the Y-direction. One read word line RWLi- 2  is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi- 2  (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi- 2  is connected to the ground point VSS through the column select switch CSW formed from, e.g., a MOS transistor. 
   The column select switches CSW are arranged outside the memory cell array  11 - 2 . Hence, no switch elements (MOS transistors) are arranged in the memory cell array  11 - 2 . 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2. That is, the four read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 run in the X-direction. One end of each read bit line is connected to a common data line  30 ( 2 ) through a row select switch (MOS transistor) RSW 2 . The common data line  30 ( 2 ) is connected to a read circuit  29 B( 2 ) (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 115 and 125 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 ( 2 )- 1 , . . . ,  25 ( 2 )- n  output the row select line signals RLi. 
   As shown in  FIG. 115 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 125 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 ( 2 )- 1 , . . . ,  25 ( 2 )- n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 run in the X-direction and also function as write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2, respectively. 
   One end of each of the write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2 is connected to a write word line driver  23 A( 2 ) through the row select switches RSW 2  and common data line  30 ( 2 ). The other end of each write word line is connected to a corresponding one of write word line sinkers  24 ( 2 )- 1 , . . . ,  24 ( 2 )- n.    
   One write bit line WBLi- 2  (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi- 2  is arranged in one column. 
   One end of each write bit line WBLi- 2  is connected to a circuit block  29 A( 2 ) including a column decoder and write bit line driver/sinker through the switching circuit  22 . The other end of the write bit line WBLi- 2  is connected to a circuit block  31 ( 2 ) including a column decoder and write bit line driver/sinker through the disconnecting circuit  21 . 
   The disconnecting circuit  21  and switching circuit  22  are the disconnecting circuit  21  and switching circuit  22  in the cell array structure of the memory cell array of the first stage shown in FIG.  24 . 
   The disconnecting circuit  21  and switching circuit  22  are controlled by the memory cell array select signal SEL. 
   As described above, in the read operation, when the memory cell array  11 - 1  of the first stage (lower stage) is selected, the switching circuit  22  electrically connects one end of the read word line RWLi- 1  to the column select switch CSW. The disconnecting circuit  21  electrically disconnects the other end of the read word line RWLi- 1  from the circuit block  31 ( 2 ). 
   In the write operation, when the memory cell array  11 - 2  of the second stage (upper stage) is selected, the switching circuit  22  electrically connects one end of the write bit line WBLi- 2  to the circuit block  29 A( 2 ). The disconnecting circuit  21  electrically connects the other end of the write bit line WBLi- 2  to the circuit block  31 ( 2 ). 
   In the write operation, the circuit blocks  29 A( 2 ) and  31 ( 2 ) are set in an operative state. A write current flows to the write bit lines WBLi- 2  in accordance with write data in a direction toward the circuit block  29 A( 2 ) or  31 ( 2 ). 
   In the write operation, the row decoder  25 ( 2 )- n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A( 2 ) supplies a write current to the write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2 in the selected row. The write current is absorbed by the write word line sinker  24 ( 2 )- n.    
   In the read operation, the row decoder  25 ( 2 )- n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32 ( 2 ) selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   {circle around (3)} Device Structure (Sectional Structure) 
   As a characteristic feature of the device structure of Structural Example 4, in the memory cell array of Device Structure  2  ( FIG. 15 ) of Structural Example 2, a read word line RWL 1 - 1  of the lower stage (first stage) and a write bit line WBL 1 - 2  of the upper stage (second stage) are integrated and shared as one read word line/write bit line RWL 1 - 1 /WBL 1 - 2 . 
     FIG. 26  shows a device structure corresponding to one block of the magnetic random access memory according to Structural Example 4 of the present invention. 
   [1] First Stage (Memory Cell Array  11 - 1 ) 
   The write bit line WBL 1 - 1  running in the Y-direction is formed on a semiconductor substrate  41 . No switch element is arranged immediately under the write bit line WBL 1 - 1 . A lower electrode  44 - 1  having, e.g., a rectangular pattern is formed above the write bit line WBL 1 - 1 . 
   The four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  arrayed in the Y-direction are formed on the lower electrode  44 - 1 . 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ) are formed on the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , respectively. The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are in contact with the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , respectively. The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are independently connected to the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1 , respectively. That is, the four read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are arranged in correspondence with the four MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . 
   The contact plug  42 - 1  and conductive layer  43 - 1  are formed on the lower electrode  44 - 1 . The contact plug  42 - 1  and conductive layer  43 - 1  electrically connect the lower electrode  44 - 1  to the read word line RWL 1 - 1 . 
   The contact plug  42 - 1  is arranged at the central portion of the lower electrode  44 - 1 . When the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 1 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43 - 1  may be integrated with the contact plug  42 - 1 . More specifically, the conductive layer  43 - 1  may be omitted, and the contact plug  42 - 1  may be brought into direct contact with the lower electrode  44 - 1 . 
   The read word line RWL 1 - 1  is formed above the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . The read word line RWL 1 - 1  runs in the Y-direction (column direction). 
   [2] Second Stage (Memory Cell Array  11 - 2 ) 
   The read word line RWL 1 - 1  in the memory cell array  11 - 1  of the first stage also functions as the write bit line WBL 1 - 1  in the memory cell array  11 - 2  of the second stage. 
   More specifically, in the read operation, when the memory cell array  11 - 1  of the first stage is selected, the read word line/write bit line RWL 1 - 1 /WBL 1 - 2  is used as the read word line RWL 1 - 1 . In the write operation, when the memory cell array  11 - 2  of the second stage is selected, the read word line/write bit line RWL 1 - 1 /WB 1 - 2  is used as the write bit line WBL 1 - 2 . 
   A lower electrode  44 - 2  having, e.g., a rectangular pattern is formed above the write bit line WBL 1 - 2 . The four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  arrayed in the Y-direction are formed on the lower electrode  44 - 2 . 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ) are formed on the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , respectively. The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are in contact with the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , respectively. The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are independently connected to the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2 , respectively. That is, the four read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are arranged in correspondence with the four MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . 
   The contact plug  42 - 2  and conductive layer  43 - 2  are formed on the lower electrode  44 - 2 . The contact plug  42 - 2  and conductive layer  43 - 2  electrically connect the lower electrode  44 - 2  to the read word line RWL 1 - 2 . 
   The contact plug  42 - 2  is arranged at the central portion of the lower electrode  44 - 2 . When the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 2 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43 - 2  may be integrated with the contact plug  42 - 2 . More specifically, the conductive layer  43 - 2  may be omitted, and the contact plug  42 - 2  may be brought into direct contact with the lower electrode  44 - 2 . 
   The read word line RWL 1 - 2  is formed above the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . The read word line RWL 1 - 2  runs in the Y-direction (column direction). 
   [3] Others 
   In the example shown in  FIG. 26 , the memory cell arrays  11 - 1  and  11 - 2  are stacked in two stages on the semiconductor substrate  41 . In principle, the memory cell arrays may be stacked in three or more stages (there is no upper limit). 
   According to the device structure of Structural Example 4, the memory cell array  11 - 1  of the lower stage and the memory cell array  11 - 2  of the upper stage according to Device Structure  2  of Structural Example 2 share one interconnection. For this reason, the density of MTJ elements can be increased. In addition, the underlying layer of the MTJ elements can be planarized (the characteristic of the MTJ elements can be improved). 
   {circle around (4)} Device Structure (Plan Structure) 
     FIGS. 27  to  33  show the layouts of the respective interconnection layers in device structure shown in FIG.  26 . The section shown in  FIG. 26  corresponds to the section taken along a line XXVI—XXVI in  FIGS. 27  to  33 . 
     FIG. 27  shows the layout of write bit lines of the first stage. 
   The write bit lines WBL 1 - 1  run in the Y-direction. The lower electrode  44 - 1  having a rectangular shape is arranged on each write bit line WBL 1 - 1 . 
     FIG. 28  shows the layout of MTJ elements of the first stage. 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  and conductive layer  43 - 1  are arranged on the lower electrode  44 - 1  having a rectangular pattern. 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  on the lower electrode  44 - 1  are arranged in the Y-direction. The axis of easy magnetization of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is the X-direction. 
     FIG. 29  shows the layout of read bit lines of the first stage. 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ) are arranged on the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , respectively. 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  run in the X-direction. The interval between the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The read bit line RBL 1 - 1  is commonly connected to the MTJ elements MTJ 1 - 1  arranged in the X-direction. The read bit line RBL 2 - 1  is commonly connected to the MTJ elements MTJ 2 - 1  arranged in the X-direction. The read bit line RBL 3 - 1  is commonly connected to the MTJ elements MTJ 3 - 1  arranged in the X-direction. The read bit line RBL 4 - 1  is commonly connected to the MTJ elements MTJ 4 - 1  arranged in the X-direction. 
   The contact plug  42 - 1  is arranged on the conductive layer  43 - 1 . 
     FIG. 30  shows the layout of read word lines of the first stage/write bit lines of the second stage. 
   The read word lines/write bit lines RWL 1 - 1 /WBL 1 - 2  run in the Y-direction. The read word line/write bit line RWL 1 - 1 /WBL 1 - 2  is in contact with the contact plug  42 - 1 . 
     FIG. 31  shows the layout of MTJ elements of the second stage. 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  and conductive layer  43 - 2  are arranged on the lower electrode  44 - 2  having a rectangular pattern. 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  on the lower electrode  44 - 2  are arranged in the Y-direction. The axis of easy magnetization of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is the X-direction. 
     FIG. 32  shows the layout of read bit lines of the second stage. 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ) are arranged on the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , respectively. 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  run in the X-direction. The interval between the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The read bit line RBL 1 - 2  is commonly connected to the MTJ elements MTJ 1 - 2  arranged in the X-direction. The read bit line RBL 2 - 2  is commonly connected to the MTJ elements MTJ 2 - 2  arranged in the X-direction. The read bit line RBL 3 - 2  is commonly connected to the MTJ elements MTJ 3 - 2  arranged in the X-direction. The read bit line RBL 4 - 2  is commonly connected to the MTJ elements MTJ 4 - 2  arranged in the X-direction. 
   The contact plug  42 - 2  is arranged on the conductive layer  43 - 2 . 
     FIG. 33  shows the layout of read word lines of the second stage. 
   The read word lines RWL 1 - 2  run in the Y-direction. The read word line RWL 1 - 2  is in contact with the contact plug  42 - 2 . 
   (5) STRUCTURAL EXAMPLE 5 
   {circle around (1)} Outline 
   In Structural Examples 3 and 4, one interconnection is shared as interconnections having different functions of two memory cell arrays (lower and upper stages). 
   In Structural Example 5, one interconnection is shared as interconnections having identical functions of two memory cell arrays. When one interconnection is shared as interconnections having identical functions, the switching circuit and disconnecting circuit in Structural Examples 3 and 4 can be omitted. Hence, the peripheral circuit arrangement is simplified. 
   {circle around (2)} Circuit Structure 
   In Structural Example 5, in a plurality of stages of memory cell arrays  11 - 1 ,  11 - 2 , . . . ,  11 - m  stacked, the write bit line of the memory cell array of the lower stage and that of the memory cell array of the upper stage are integrated and shared as one write bit line. 
     FIGS. 34 and 35  show the main part of a magnetic random access memory according to Structural Example 5 of the present invention. 
   [1] First Stage (Lower Stage) 
     FIG. 34  shows the cell array structure of the first stage of Structural Example 5. 
   The memory cell array  11 - 1  has a plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11 - 1  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi- 1  (i=1, . . . , j). The read word line RWLi- 1  runs in the Y-direction. One read word line RWLi- 1  is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi- 1  (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi- 1  is connected to a ground point VSS through a column select switch CSW formed from, e.g., a MOS transistor. 
   The column select switches CSW are arranged outside the memory cell array  11 - 1 . Hence, no switch elements (MOS transistors) are arranged in the memory cell array  11 - 1 . 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1. That is, the four read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 run in the X-direction. One end of each read bit line is connected to a common data line  30 ( 1 ) through a row select switch (MOS transistor) RSW 2 . The common data line  30 ( 1 ) is connected to a read circuit  29 B( 1 ) (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 116 and 126 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 ( 1 )- 1 , . . . ,  25 ( 1 )- n  output the row select line signals RLi. 
   As shown in  FIG. 116 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 126 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 ( 1 )- 1 , . . . ,  25 ( 1 )- n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 run in the X-direction and also function as write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1, respectively. 
   One end of each of the write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1 is connected to a write word line driver  23 A( 1 ) through the row select switches RSW 2  and common data line  30 ( 1 ). The other end of each write word line is connected to a corresponding one of write word line sinkers  24 ( 1 )- 1 , . . . ,  24 ( 1 )- n.    
   One write bit line WBLi- 1  (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi- 1  is arranged in one column. 
   The write bit line WBLi- 1  also functions as a write bit line WBLi- 2  (i=1, . . . , j) in the memory cell array of the second stage. 
   Each write bit line WBLi- 1  is connected to a circuit block  29 A including a column decoder and write bit line driver/sinker. The other end of the write bit line WBLi- 1  is connected to a circuit block  31  including a column decoder and write bit line driver/sinker. 
   In the write operation, the circuit blocks  29 A and  31  are set in an operative state. A write current flows to the write bit lines WBLi- 1  in accordance with write data in a direction toward the circuit block  29 A or  31 . 
   In the write operation, the row decoder  25 ( 1 )- n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A( 1 ) supplies a write current to the write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1 in the selected row. The write current is absorbed by the write word line sinker  24 ( 1 )- n.    
   In read operation, the row decoder  25 ( 1 )- n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32 ( 1 ) selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   [2] Second Stage (Upper Stage) 
     FIG. 35  shows the cell array structure of the second stage of Structural Example 5. 
   The memory cell array  11 - 2  has the plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11 - 2  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi- 2  (i=1, . . . , j). The read word line RWLi- 2  runs in the Y-direction. One read word line RWLi- 2  is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi- 2  (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi- 2  is connected to the ground point VSS through the column select switch CSW formed from a MOS transistor. 
   The column select switches CSW are arranged outside the memory cell array  11 - 2 . Hence, no switch elements (MOS transistors) are arranged in the memory cell array  11 - 2 . 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2. That is, the four read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 run in the X-direction. One end of each read bit line is connected to a common data line  30 ( 2 ) through a row select switch (MOS transistor) RSW 2 . The common data line  30 ( 2 ) is connected to a read circuit  29 B( 2 ) (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 117 and 127 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 ( 2 )- 1 , . . . ,  25 ( 2 )- n  output the row select line signals RLi. 
   As shown in  FIG. 117 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 127 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 ( 2 )- 1 , . . . ,  25 ( 2 )- n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 run in the X-direction and also function as write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2, respectively. 
   One end of each of the write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2 is connected to a write word line driver  23 A( 2 ) through the row select switches RSW 2  and common data line  30 ( 2 ). The other end of each write word line is connected to a corresponding one of write word line sinkers  24 ( 2 )- 1 , . . . ,  24 ( 2 )- n.    
   One write bit line WBLi- 2  (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi- 2  is arranged in one column. 
   As described above, the write bit line WBLi- 2  is also used as the write bit line WBL 1 - 1  of the memory cell array of the first stage. 
   One end of each write bit line WBLi- 2  is connected to the circuit block  29 A including a column decoder and write bit line driver/sinker. The other end of the write bit line WBLi- 2  is connected to the circuit block  31  including a column decoder and write bit line driver/sinker. 
   In the write operation, the circuit blocks  29 A and  31  are set in an operative state. A write current flows to the write bit lines WBLi- 2  in accordance with write data in a direction toward the circuit block  29 A or  31 . 
   In the write operation, the row decoder  25 ( 2 )- n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A( 2 ) supplies a write current to the write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2 in the selected row. The write current is absorbed by the write word line sinker  24 ( 2 )- n.    
   In the read operation, the row decoder  25 ( 2 )- n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32 ( 2 ) selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   {circle around (3)} Device Structure (Sectional Structure) 
   As a characteristic feature of the device structure of Structural Example 5, Device Structure  2  ( FIG. 4 ) of Structural Example 1 is employed for the memory cell array of the first stage, Device Structure  3  ( FIG. 8 ) of Structural Example 1 is employed for the memory cell array of the second stage, and the write bit line is shared. 
     FIG. 36  shows a device structure corresponding to one block of the magnetic random access memory according to Structural Example 5 of the present invention. 
   [1] First Stage (Memory Cell Array  11 - 1 ) 
   A read word line RWL 1 - 1  running in the Y-direction is formed on a semiconductor substrate  41 . No switch element is arranged immediately under the read word line RWL 1 - 1 . Four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  arrayed in the Y-direction are formed above the read word line RWL 1 - 1 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is commonly connected to an upper electrode  44 - 1 . A contact plug  42 - 1  and conductive layer  43 - 1  electrically connect the upper electrode  44 - 1  to the read word line RWL 1 - 1 . 
   The contact plug  42 - 1  is arranged at the central portion of the upper electrode  44 - 1 . When the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 1 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43 - 1  may be integrated with the upper electrode  44 - 1 . That is, the conductive layer  43 - 1  and upper electrode  44 - 1  may be formed simultaneously using the same material. 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is electrically connected to a corresponding one of read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ). The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are independently connected to the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1 , respectively. That is, the four read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are arranged in correspondence with the four MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . 
   A write bit line WBL 1 - 1  is formed above and near the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . The write bit line WBL 1 - 1  runs in the Y-direction (column direction). 
   [2] Second Stage (Memory Cell Array  11 - 2 ) 
   A write bit line WBL 1 - 1  in the memory cell array  11 - 1  of the first stage also functions as a write bit line WBL 1 - 2  in the memory cell array  11 - 2  of the second stage. 
   More specifically, in write operation, when the memory cell array  11 - 1  of the first stage is selected, and the memory cell array  11 - 2  of the second stage is selected, a write current flows to the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   Four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  arrayed in the Y-direction are formed above the write bit line WBL 1 - 2 . 
   One terminal (lower end in this example) of each of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is commonly connected to a lower electrode  44 - 2 . A contact plug  42 - 2  and conductive layer  43 - 2  electrically connect the lower electrode  44 - 2  to the read word line RWL 1 - 2 . 
   The contact plug  42 - 2  is arranged at the central portion of the lower electrode  44 - 2 . When the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 2 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43 - 2  may be integrated with contact plug  42 - 2 . More specifically, the conductive layer  43 - 2  may be omitted, and the contact plug  42 - 2  may be brought into direct contact with the lower electrode  44 - 2 . 
   The other terminal (upper end in this example) of each of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is electrically connected to a corresponding one of read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ). The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are independently connected to the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2 , respectively. That is, the four read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are arranged in correspondence with the four MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . 
   A write bit line WBL 1 - 2  is formed above and near the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . The write bit line WBL 1 - 2  runs in the Y-direction (column direction). 
   [3] Others 
   In the example shown in  FIG. 36 , the memory cell arrays  11 - 1  and  11 - 2  are stacked in two stages on the semiconductor substrate  41 . In principle, the memory cell arrays may be stacked in 2×a (a is a natural number) stages. The memory cell arrays may be stacked in three or more stages (there is no upper limit) by combining Structural Example 5 and Structural Example 6 (to be described later). 
   According to the device structure of Structural Example 5, the memory cell array  11 - 1  of the lower stage and the memory cell array  11 - 2  of the upper stage share one interconnection. For this reason, the degree of integration of MTJ elements can be increased, and the underlying layer of the MTJ elements can be planarized (the characteristic of the MTJ elements can be improved). 
   {circle around (4)} Device Structure (Plane Structure) 
     FIGS. 37  to  43  show the layouts of the respective interconnection layers in the device structure shown in FIG.  36 . The section shown in  FIG. 36  corresponds to the section taken along a line XXXVI—XXXVI in  FIGS. 37  to  43 . 
     FIG. 37  shows the layout of read word lines of the first stage. 
   The read word lines RWL 1 - 1  run in the Y-direction. The contact plug  42 - 1  is arranged on each read word line RWL 1 - 1 . 
     FIG. 38  shows the layout of read bit lines of the first stage and MTJ elements of the first stage. 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ) run in the X-direction. The interval between the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are arranged on the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1 . The axis of easy magnetization of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is the X-direction. 
   The read bit line RBL 1 - 1  is commonly connected to the MTJ elements MTJ 1 - 1  arranged in the X-direction. The read bit line RBL 2 - 1  is commonly connected to the MTJ elements MTJ 2 - 1  arranged in the X-direction. The read bit line RBL 3 - 1  is commonly connected to the MTJ elements MTJ 3 - 1  arranged in the X-direction. The read bit line RBL 4 - 1  is commonly connected to the MTJ elements MTJ 4 - 1  arranged in the X-direction. 
   The conductive layer  43 - 1  is arranged on the contact plug  42 - 1 . 
     FIG. 39  shows the layout of write bit lines of the first stage/write bit lines of the second stage. 
   The upper electrodes  44 - 1  each having a rectangular pattern are arranged on the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  and conductive layers  43 . The upper electrodes  44 - 1  are in contact with the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  and conductive layers  43 - 1 . 
   The write bit lines WBL 1 - 1 /WBL 1 - 2  are arranged immediately on the upper electrodes  44 - 1 . The write bit lines WBL 1 - 1 /WBL 1 - 2  run in the Y-direction. 
     FIG. 40  shows the layout of lower electrodes of the second stage. 
   The lower electrodes  44 - 2  each having a rectangular pattern are arranged on the write bit lines WBL 1 - 1 /WBL 1 - 2 . The upper electrodes  44 - 1  and lower electrodes  44 - 2  may be arranged to be symmetrical with respect to the write bit lines WBL 1 - 1 /WBL 1 - 2 , as in this example, or may be arranged asymmetrically. 
     FIG. 41  shows the layout of MTJ elements of the second stage. 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  and conductive layers  43 - 2  are arranged on the lower electrodes  44 - 2  each having a rectangular pattern. 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  on the lower electrodes  44 - 2  are arranged in the Y-direction. The axis of easy magnetization of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is the X-direction. 
     FIG. 42  shows the layout of read word lines of second stage. 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ) are arranged on the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , respectively. 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  run in the X-direction. The interval between the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The read bit line RBL 1 - 2  is commonly connected to the MTJ elements MTJ 1 - 2  arranged in the X-direction. The read bit line RBL 2 - 2  is commonly connected to the MTJ elements MTJ 2 - 2  arranged in the X-direction. The read bit line RBL 3 - 2  is commonly connected to the MTJ elements MTJ 3 - 2  arranged in the X-direction. The read bit line RBL 4 - 2  is commonly connected to the MTJ elements MTJ 4 - 2  arranged in the X-direction. 
   The contact plug  42 - 2  is arranged on the conductive layer  43 - 2 . 
     FIG. 43  shows the layout of read word lines of the second stage. 
   The read word lines RWL 1 - 2  run in the Y-direction. The read word line RWL 1 - 2  is in contact with the contact plug  42 - 2 . 
   (6) STRUCTURAL EXAMPLE 6 
   {circle around (1)} Outline 
   In Structural Example 6, one interconnection is shared as interconnections having identical functions of two memory cell arrays, like Structural Example 5. In Structural Example 5, a write bit line is shared. However, in Structural Example 6, a read word line is shared. 
   When one interconnection is shared as interconnections having identical functions, the switching circuit and disconnecting circuit in Structural Examples 3 and 4 can be omitted. Hence, the peripheral circuit arrangement is simplified. 
   {circle around (2)} Circuit Structure 
   In Structural Example 6, in a plurality of stages of memory cell arrays  11 - 1 ,  11 - 2 , . . . ,  11 - m  stacked, the read word line of the memory cell array of the lower stage and that of the memory cell array of the upper stage are integrated and shared as one read word line. 
     FIGS. 44 and 45  show the main part of a magnetic random access memory according to Structural Example  6  of the present invention. 
   [1] First Stage (Lower Stage) 
     FIG. 44  shows the cell array structure of the first stage of Structural Example 6. 
   The memory cell array  11 - 1  has a plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11 - 1  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi- 1  (i=1, . . . , j). The read word line RWLi- 1  also functions as a read word line RWLi- 2  of the memory cell array of the second stage (to be described later). The read word line RWLi- 1  runs in the Y-direction. One read word line RWLi- 1  is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi- 1  (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi- 1  is connected to a ground point VSS through a column select switch CSW formed from a MOS transistor. 
   The column select switches CSW are arranged outside the memory cell array  11 - 1 . Hence, no switch elements (MOS transistors) are arranged in the memory cell array  11 - 1 . 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1. That is, the four read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 run in the X-direction. One end of each read bit line is connected to a common data line  30 ( 1 ) through a row select switch (MOS transistor) RSW 2 . The common data line  30 ( 1 ) is connected to a read circuit  29 B( 1 ) (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 118 and 128 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 ( 1 )- 1 , . . . ,  25 ( 1 )- n  output the row select line signals RLi. 
   As shown in  FIG. 118 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 128 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 ( 1 )- 1 , . . . ,  25 ( 1 )- n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL{ 4 (n−1)+1}−1, RBL{ 4 (n−1)+2}−1, RBL{ 4 (n−1)+3}−1, and RBL{ 4 (n−1)+4}−1 run in the X-direction and also function as write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1, respectively. 
   One end of each of the write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1 is connected to a write word line driver  23 A( 1 ) through the row select switches RSW 2  and common data line  30 ( 1 ). The other end of each write word line is connected to a corresponding one of write word line sinkers  24 ( 1 )- 1 , . . . ,  24 ( 1 )- n.    
   One write bit line WBLi- 1  (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi- 1  is arranged in one column. 
   One end of each write bit line WBLi- 1  is connected to a circuit block  29 A( 1 ) including a column decoder and write bit line driver/sinker. The other end of the write bit line WBLi- 1  is connected to a circuit block  31 ( 1 ) including a column decoder and write bit line driver/sinker. 
   In the write operation, the circuit blocks  29 A( 1 ) and  31 ( 1 ) are set in an operative state. A write current flows to the write bit lines WBLi- 2  in accordance with write data in a direction toward the circuit block  29 A( 1 ) or  31 ( 1 ). 
   In the write operation, the row decoder  25 ( 1 )- n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A( 1 ) supplies a write current to the write word lines WWL{ 4 (n−1)+1}−1, WWL{ 4 (n−1)+2}−1, WWL{ 4 (n−1)+3}−1, and WWL{ 4 (n−1)+4}−1 in the selected row. The write current is absorbed by the write word line sinker  24 ( 1 )- n.    
   In read operation, the row decoder  25 ( 1 )- n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32  selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   [2] Second Stage (Upper Stage) 
     FIG. 45  shows the cell array structure of the second stage of Structural Example 6. 
   The memory cell array  11 - 2  has the plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11 - 2  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is connected to, e.g., a read word line RWLi- 2  (i=1, . . . , j). The read word line RWLi- 2  also functions as the read word line RWLi- 1  of the memory cell array of the first stage. The read word line RWLi- 2  runs in the Y-direction. One read word line RWLi- 2  is arranged in one column. 
   The MTJ elements  12  in the read blocks BKik arranged in one column are directly connected to the read word lines RWLi- 2  (i=1, . . . , j) without intervening read select switches (MOS transistors). One end of each read word line RWLi- 2  is connected to the ground point VSS through the column select switch CSW formed from, e.g., a MOS transistor. 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2. That is, the four read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 run in the X-direction. One end of each read bit line is connected to a common data line  30 ( 2 ) through a row select switch (MOS transistor) RSW 2 . The common data line  30 ( 2 ) is connected to a read circuit  29 B( 2 ) (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 119 and 129 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 ( 2 )- 1 , . . . ,  25 ( 2 )- n  output the row select line signals RLi. 
   As shown in  FIG. 119 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 129 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 ( 2 )- 1 , . . . ,  25 ( 2 )- n  output the row select line signals RLi and the inverting signal thereof. 
   The read bit lines RBL{ 4 (n−1)+1}−2, RBL{ 4 (n−1)+2}−2, RBL{ 4 (n−1)+3}−2, and RBL{ 4 (n−1)+4}−2 run in the X-direction and also function as write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2, respectively. 
   One end of each of the write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2 is connected to a write word line driver  23 A( 2 ) through the row select switches RSW 2  and common data line  30 ( 2 ). The other end of each write word line is connected to a corresponding one of write word line sinkers  24 ( 2 )-i, . . . ,  24 ( 2 )- n.    
   One write bit line WBLi- 2  (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write bit line WBLi- 2  is arranged in one column. 
   One end of each write bit line WBLi- 2  is connected to a circuit block  29 A( 2 ) including a column decoder and write bit line driver/sinker. The other end of the write bit line WBLi- 2  is connected to a circuit block  31 ( 2 ) including a column decoder and write bit line driver/sinker. 
   In the write operation, the circuit blocks  29 A( 2 ) and  31 ( 2 ) are set in an operative state. A write current flows to the write bit lines WBLi- 2  in accordance with write data in a direction toward the circuit block  29 A( 2 ) or  31 ( 2 ). 
   In the write operation, the row decoder  25 ( 2 )- n  selects one of the plurality of rows on the basis of a row address signal. The write word line driver  23 A( 2 ) supplies a write current to the write word lines WWL{ 4 (n−1)+1}−2, WWL{ 4 (n−1)+2}−2, WWL{ 4 (n−1)+3}−2, and WWL{ 4 (n−1)+4}−2 in the selected row. The write current is absorbed by the write word line sinker  24 ( 2 )- n.    
   In the read operation, the row decoder  25 ( 2 )- n  selects one of the plurality of rows on the basis of a row address signal. In the read operation, a column decoder  32  selects one of the plurality of columns on the basis of column address signals CSL 1 , . . . , CSLj to turn on the column select switch CSW arranged in the selected column. 
   {circle around (3)} Device Structure (Sectional Structure) 
   As a characteristic feature of the device structure of Structural Example 6, Device Structure  3  ( FIG. 8 ) of Structural Example 1 is employed for the memory cell array of the first stage, Device Structure  2  ( FIG. 4 ) of Structural Example 1 is employed for the memory cell array of the second stage, and the read word line is shared. 
     FIG. 46  shows a device structure corresponding to one block of the magnetic random access memory according to Structural Example 6 of the present invention. 
   [1] First Stage (Memory Cell Array  11 - 1 ) 
   The write bit line WBL 1 - 1  running in the Y-direction is formed on a semiconductor substrate  41 . No switch element is arranged immediately under the write bit line WBL 1 - 1 . A lower electrode  44 - 1  having, e.g., a rectangular pattern is formed above the write bit line WBL 1 - 1 . 
   The four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  arrayed in the Y-direction are formed on the lower electrode  44 - 1 . 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ) are formed on the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , respectively. The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are in contact with the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , respectively. The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are independently connected to the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1 , respectively. That is, the four read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  are arranged in correspondence with the four MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . 
   A contact plug  42 - 1  and conductive layer  43 - 1  are formed on the lower electrode  44 - 1 . The contact plug  42 - 1  and conductive layer  43 - 1  electrically connect the lower electrode  44 - 1  to the read word line RWL 1 - 1 . 
   The contact plug  42 - 1  is arranged at the central portion of the lower electrode  44 - 1 . When the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 1 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The read word line RWL 1 - 1  is formed above the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 . The read word line RWL 1 - 1  runs in the Y-direction (column direction). 
   [2] Second Stage (Memory Cell Array  11 - 2 ) 
   The read word line RWL 1 - 1  in the memory cell array  11 - 1  of the first stage also functions as the read word line RWL 1 - 2  in the memory cell array  11 - 2  of the second stage. 
   More specifically, in the read operation, when the memory cell array  11 - 1  of the first stage is selected, and the memory cell array  11 - 2  of the second stage is selected, the read word line RWL 1 - 1 /RWL 1 - 2  is short-circuited to the ground point. 
   An upper electrode  44 - 2  having, e.g., a rectangular pattern is formed above the read word line RWL 1 - 2 . The four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  arrayed in the Y-direction are formed immediately under the upper electrode  44 - 2 . 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ) are formed immediately under the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , respectively. The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are in contact with the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , respectively. The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are independently connected to the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2 , respectively. That is, the four read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  are arranged in correspondence with the four MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . 
   A contact plug  42 - 2  and conductive layer  43 - 2  are formed between the upper electrode  44 - 2  and the read word line RWL 1 - 2 . The contact plug  42 - 2  and conductive layer  43 - 2  electrically connect the upper electrode  44 - 2  to the read word line RWL 1 - 2 . 
   The contact plug  42 - 2  is arranged at the central portion of the upper electrode  44 - 2 . When the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are uniformly arranged to be symmetrical with respect to the contact plug  42 - 2 , signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The write bit line WBL 1 - 2  is formed above the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 . The write bit line WBL 1 - 2  runs in the Y-direction (column direction). 
   [3] Others 
   In the example shown in  FIG. 46 , the memory cell arrays  11 - 1  and  11 - 2  are stacked in two stages on the semiconductor substrate  41 . In principle, the memory cell arrays may be stacked in 2×a (a is a natural number) stages. The memory cell arrays may be stacked in three or more stages (there is no upper limit) by combining Structural Examples 5 and 6. 
   According to the device structure of Structural Example  6 , the memory cell array  11 - 1  of the lower stage and the memory cell array  11 - 2  of the upper stage share one interconnection. For this reason, the degree of integration of MTJ elements can be increased, and the underlying layer of the MTJ elements can be planarized (the characteristic of the MTJ elements can be improved). 
   {circle around (4)} Device Structure (Plan Structure) 
     FIGS. 47  to  52  show the layouts of the respective interconnection layers in device structure shown in FIG.  46 . The section shown in  FIG. 46  corresponds to the section taken along a line XLVI—XLVI in  FIGS. 47  to  52 . 
     FIG. 47  shows the layout of write bit lines of the first stage. 
   The write bit lines WBL 1 - 1  run in the Y-direction. The lower electrode  44 - 1  having a rectangular shape is arranged on each write bit line WBL 1 - 1 . 
     FIG. 48  shows the layout of MTJ elements of the first stage. 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  and conductive layer  43 - 1  are arranged on the lower electrode  44 - 1  having a rectangular pattern. 
   The MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  on the lower electrode  44 - 1  are arranged in the Y-direction. The axis of easy magnetization of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1  is the X-direction. 
     FIG. 49  shows the layout of read bit lines of the first stage. 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  (write word lines WWL 1 - 1 , WWL 2 - 1 , WWL 3 - 1 , and WWL 4 - 1 ) are arranged on the MTJ elements MTJ 1 - 1 , MTJ 2 - 1 , MTJ 3 - 1 , and MTJ 4 - 1 , respectively. 
   The read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  run in the X-direction. The interval between the read bit lines RBL 1 - 1 , RBL 2 - 1 , RBL 3 - 1 , and RBL 4 - 1  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The read bit line RBL 1 - 1  is commonly connected to the MTJ elements MTJ 1 - 1  arranged in the X-direction. The read bit line RBL 2 - 1  is commonly connected to the MTJ elements MTJ 2 - 1  arranged in the X-direction. The read bit line RBL 3 - 1  is commonly connected to the MTJ elements MTJ 3 - 1  arranged in the X-direction. The read bit line RBL 4 - 1  is commonly connected to the MTJ elements MTJ 4 - 1  arranged in the X-direction. 
   The contact plug  42 - 1  is arranged on the conductive layer  43 - 1 . 
     FIG. 50  shows the layout of read word lines of the first stage/read word lines of the second stage. 
   The read word lines RWL 1 - 1 /RWL 1 - 2  run in the Y-direction. The read word line RWL 1 - 1 /RWL 1 - 2  is in contact with the contact plug  42 - 1 . The contact plug  42 - 2  is formed on the read word line RWL 1 - 1 /RWL 1 - 2 . 
     FIG. 51  shows the layout of read bit lines of the second stage and MTJ elements of the second stage. 
   The read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  (write word lines WWL 1 - 2 , WWL 2 - 2 , WWL 3 - 2 , and WWL 4 - 2 ) run in the X-direction. The interval between the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  are arranged on the read bit lines RBL 1 - 2 , RBL 2 - 2 , RBL 3 - 2 , and RBL 4 - 2 . The axis of easy magnetization of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  is the X-direction. 
   The read bit line RBL 1 - 2  is commonly connected to the MTJ elements MTJ 1 - 2  arranged in the X-direction. The read bit line RBL 2 - 2  is commonly connected to the MTJ elements MTJ 2 - 2  arranged in the X-direction. The read bit line RBL 3 - 2  is commonly connected to the MTJ elements MTJ 3 - 2  arranged in the X-direction. The read bit line RBL 4 - 2  is commonly connected to the MTJ elements MTJ 4 - 2  arranged in the X-direction. 
   The conductive layer  43 - 2  is arranged on the contact plug  42 - 2 . 
     FIG. 52  shows the layout of write bit lines of the second stage. 
   The upper electrodes  44 - 2  each having a rectangular pattern are arranged on the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  and conductive layer  43 - 2 . The upper electrodes  44 - 2  are in contact with the MTJ elements MTJ 1 - 2 , MTJ 2 - 2 , MTJ 3 - 2 , and MTJ 4 - 2  and conductive layers  43 - 2 . 
   The write bit lines WBL 1 - 2  are arranged immediately on the upper electrodes  44 - 2 . The write bit lines WBL 1 - 2  run in the Y-direction. 
   (7) STRUCTURAL EXAMPLE 7 
   Structural Example 7 is a modification of Structural Example 1. As its characteristic feature, the axis of easy magnetization of the MTJ element of Structural Example 1 is rotated by 90°. 
   In Structural Example 1, the axis of easy magnetization of the MTJ element is the X-direction (row direction), and the axis of hard magnetization is the Y-direction (column direction). That is, the MTJ element has a rectangular shape long in the X-direction. To the contrary, in Structural Example 7, the axis of easy magnetization of the MTJ element is the Y-direction, and the axis of hard magnetization is the X-direction. That is, the MTJ element has a rectangular shape long in the Y-direction. 
   In a magnetic random access memory, basically, data is written in a memory cell (the direction of magnetization of the pinning layer is determined) by changing the direction of a write current flowing to a write line that runs in a direction parallel to the axis of hard magnetization. 
   Hence, in this example, data to be written in a memory cell is determined by controlling the direction of a write current flowing to a write bit line (read bit line) that runs in the X-direction in write operation. 
   Generally, a write line that runs along the axis of hard magnetization (in a direction parallel to the short axis of an MTJ) is called a write bit line. 
   {circle around (1)} Circuit Structure 
     FIG. 53  shows the main part of a magnetic random access memory according to Structural Example 7 of the present invention. 
   A memory cell array  11  has a plurality of MTJ elements  12  arranged in an array in the X- and Y-directions. For example, j MTJ elements  12  are arranged in the X-direction, and 4×n MTJ elements  12  are arranged in the Y-direction. 
   The four MTJ elements  12  arranged in the Y-direction form one read block BKik (i=1, . . . , j, and k=1, . . . , n). One row is constructed by j read blocks BKik arranged in the X-direction. The memory cell array  11  has n rows. In addition, one column is constructed by n read blocks BKik arranged in the Y-direction. The memory cell array  11  has j columns. 
   One terminal of each of the four MTJ elements  12  in the block BKik is commonly connected. The connection point is directly connected to a read word line RWLi (i=1, . . . , j) without intervening read select switches. The read word line RWLi runs in the Y-direction. One read word line RWLi is arranged in one column. 
   Each read word line RWLi is connected to a ground point VSS through a column select switch CSW formed from, e.g., a MOS transistor. 
   In read operation, in a selected row, a row select switch RSW 2  is turned on. In a selected column, the column select switch CSW is turned on. For this reason, the potential of the read word line RWLi becomes the ground potential VSS. A read current flows to the MTJ elements  12  in the read block BKik located at the intersection between the selected row and the selected column. 
   The other terminal of each of the four MTJ elements  12  in the read block BKik is independently connected to a corresponding one of read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4. That is, the four read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 are arranged in correspondence with the four MTJ elements  12  in one read block BKik. 
   The read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 run in the X-direction. One end of each read bit line is connected to a common data line  30 A through the row select switch (MOS transistor) RSW 2 . The common data line  30 A is connected to a read circuit  29 B (including, e.g., a sense amplifier, selector, and output buffer). 
   For example, as shown in  FIGS. 120 and 130 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 - 1 , . . . ,  25 - n  output the row select line signals RLi. 
   As shown in  FIG. 120 , the bias transistor BT is a PMOS transistor, when the RLi is input to the bias transistor BT. As shown in  FIG. 130 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 - 1 , . . . ,  25 - n  output the row select line signals RLi and the inverting signal thereof. 
   In this example, the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 run in the X-direction and also function as write bit lines WBL 4 (n−1)+1, WBL 4 (n−1)+2, WBL 4 (n−1)+3, and WBL 4 (n−1)+4, respectively. 
   One end of each of the write bit lines WBL 4 (n−1)+1, WBL 4 (n−1)+2, WBL 4 (n−1)+3, and WBL 4 (n−1)+4 is connected to a write bit line driver/sinker  23 AR through the row select switches RSW 2  and common data line  30 A. The other end of each write bit line is connected to a write bit line driver/sinker  23 AS through a common data line  30 B. 
   One write word line WWLi (i=1, . . . , j) which is shared by the four MTJ elements  12  of one read block BKik and run in the Y-direction is arranged near the MTJ elements  12  constituting the read block BKik. One write word line WWLi is arranged in one column. 
   One end of each write word line WWLi is connected to a circuit block  29 AR including a column decoder and write word line driver. The other end is connected to a circuit block  31 R including a column decoder and write word line sinker. 
   In write operation, the circuit blocks  29 AR and  31 R are set in an operative state. A write current flows to the write word lines WWLi in a direction from the circuit block  29 AR to the circuit  31 R. 
   In the write operation, the row decoder  25 - n  selects one of the plurality of rows on the basis of a row address signal. The write bit line drivers/sinkers  23 AR and  23 AS supply a write current having a direction corresponding to write data to one of the write bit lines WBL 4 (n−1)+1, WBL 4 (n−1)+2, WBL 4 (n−1)+3, and WBL 4 (n−1)+4 in the selected row. 
   In the read operation, the row decoder  25 - n  selects one of the plurality of rows on the basis of a row address signal. 
   A column decoder  32  selects one of the plurality of columns on the basis of column address signals and outputs column select signals CSL 1 , . . . , CSLj. The column select switch CS Warranted in the selected column is turned on. 
   {circle around (2)} Device Structure 
   The device structure will be described next. 
   [1] Sectional Structure 
     FIG. 54  shows a device structure corresponding to one block of the magnetic random access memory according to Structural Example 7 of the present invention. 
   The same reference numerals as in  FIG. 53  denote the same elements in  FIG. 54  to show the correspondence between the elements. 
   A read word line RWL 1  running in the Y-direction is formed on a semiconductor substrate  41 . No switch element is arranged immediately under the read word line RWL 1 . Four MTJ elements (Magnetic Tunnel Junction elements) MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  arrayed in the Y-direction are formed above the read word line RWL 1 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is commonly connected to an upper electrode  44 . A contact plug  42  and conductive layer  43  electrically connects the upper electrode  44  to the read word line RWL 1 . 
   The contact portion between the upper electrode  44  and the read word line RWL 1  is formed in the region between the MTJ elements MTJ 1  and MTJ 2  and the MTJ elements MTJ 3  and MTJ 4 . When the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are uniformly arranged to be symmetrical with respect to the contact portion, signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43  may be integrated with the upper electrode  44 . That is, the conductive layer  43  and upper electrode  44  may be formed simultaneously using the same material. 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is electrically connected to a corresponding one of the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write bit lines WBL 1 , WBL 2 , WBL 3 , and WBL 4 ). The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are independently connected to the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4 , respectively. That is, the four read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  are arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   The write word line WWL 1  is formed immediately on and near the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . The write word line WWL 1  runs in the Y-direction (column direction). 
   In this example, one write word line WWL 1  is arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  which construct a read block. Instead, for example, the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  may be stacked, and four write word lines may be arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   In this example, the write word line WWL 1  running in the Y-direction is arranged above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  running in the X-direction are arranged under the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   Instead, for example, the write word line WWL 1  running in the Y-direction may be arranged under the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , and the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  running in the X-direction are arranged above the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   According to this device structure, the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block are electrically connected to the different read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write bit lines WBL 1 , WBL 2 , WBL 3 , and WBL 4 ), respectively. For this reason, data of the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block can be read at once by one read step. 
   One terminal of each of the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block is commonly connected. The connection point is directly connected to the read word line RWL 1  without intervening a read select switch. In addition, the write word line WWL 1  running in the Y-direction is shared by the plurality of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read block. For this reason, the degree of integration of MTJ elements can be increased, and their characteristic can be improved. 
   [Plane Structure] 
     FIGS. 55  to  57  show the layouts of the respective interconnection layers in device structure shown in FIG.  54 . The section shown in  FIG. 54  corresponds to the section taken along a line LIV—LIV in  FIGS. 55  to  57 . 
     FIG. 5  shows the layout of read word lines. 
   The read word lines RWL 1  run in the Y-direction. The contact plug  42  is arranged on each read word line RWL 1 . 
     FIG. 56  shows the layout of the read bit lines and MTJ elements. 
   The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write bit lines WBL 1 , WBL 2 , WBL 3 , and WBL 4 ) run in the X-direction. The interval between the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  can be set to, e.g., the minimum size (or design rule) processible by photolithography. 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are arranged on the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4 , respectively. The axis of easy magnetization of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 , i.e., the direction parallel to the long sides of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is the Y-direction. 
   The read bit line RBL 1  is commonly connected to the MTJ elements MTJ 1  arranged in the X-direction. The read bit line RBL 2  is commonly connected to the MTJ elements MTJ 2  arranged in the X-direction. The read bit line RBL 3  is commonly connected to the MTJ elements MTJ 3  arranged in the X-direction. The read bit line RBL 4  is commonly connected to the MTJ elements MTJ 4  arranged in the X-direction. 
   The conductive layer  43  is arranged on the contact plug  42 . 
     FIG. 57  shows the layout of write bit lines. 
   The upper electrode  44  having a rectangular pattern is arranged on the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  and conductive layer  43 . The upper electrode  44  are in contact with the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  and conductive layer  43 . 
   The write word lines WWL 1  are arranged immediately on the upper electrodes  44 . The write word lines WWL 1  run in the Y-direction. 
   (8) STRUCTURAL EXAMPLES 8, 9, and 10 
   Structural Examples 8, 9, and 10 as improvements of Structural Example 1 will now be described. 
   {circle around (1)} Structural Example 8 
     FIG. 58  shows the main part of a magnetic random access memory according to Structural Example 8 of the present invention. 
   As a characteristic feature of Structural Example 8, in read operation, a bias voltage VC is applied to one terminal of each of four MTJ elements  12  that form a read block BKik. 
   More specifically, in Structural Example 1 (FIG.  1 ), the read word line RWLi is connected to the ground point VSS through the column select switch CSW, and the bias voltage VC is generated by the read circuit  29 B. In Structural Example 8, a read word line RWLi is connected to a bias line  34  through a column select switch CSW, and the bias voltage VC is supplied to the bias line  34 . 
   Hence, in the read operation, the bias voltage VC can be applied to the bias line  34 , and a read current can be supplied from the bias line  34  to the MTJ element  12 . In a mode (e.g., write operation) except the read operation, a ground potential VSS is applied to the bias line  34 . 
   In Structural Example 8, the potential of the read word line RWLi can be changed. Hence, in the read operation, the bias voltage VC can be applied to the read word line RWLi, and the read current can be supplied to the MTJ element  12  in the read block BKik. 
   For example, as shown in  FIG. 131 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RLi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  25 - 1 , . . . ,  25 - n  output the row select line signals RLi. 
   As shown in  FIG. 131 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  25 - 1 , . . . ,  25 - n  output the row select line signals RLi and the inverting signal thereof. 
   {circle around (2)} Structural Example 9 
     FIG. 59  shows the main of a magnetic random access memory according to Structural Example 9 of the present invention. 
   As a characteristic feature of Structural Example 9, a write word line driver is arranged in one row of a memory cell array. 
   In Structural Example 1 (FIG.  1 ), only one write word line driver  23 A is commonly arranged for all rows of the memory cell array  11  and connected to the common data line (common driver line)  30 . In this case, however, elements having resistances, i.e., the common data line and row select switches are connected between the write word line driver and the write word line. Since a voltage drop due to these elements becomes large, the write current becomes small. 
   In Structural Example 9, write word line drivers  33 - 1 , . . . ,  33 - n  are arranged for rows of a memory cell array  11 , respectively. 
   More specifically, in each row of the memory cell array  11 , a corresponding one of the write word line drivers  33 - 1 , . . . ,  33 - n  is connected between row select switches RSW 2  and write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4. 
   In this case, the write word line drivers  33 - 1 , . . . ,  33 - n  need to drive only the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4. 
   Hence, the driving force for the write word line drivers  33 - 1 , . . . ,  33 - n  can be decreased. This contributes to decreasing power consumption and increasing the operation speed. 
   Since the read current is much smaller than the write current, the driving force of the row select switches RSW 2  need not be increased. 
   The write word line drivers  33 - 1 , . . . ,  33 - n  are controlled by output signals (word line enable signals) WLEN 1 , . . . , WLEN 4  from row decoders  25 - 1 , . . . ,  25 - n . More specifically, in the write operation, the row decoders  25 - 1 , . . . ,  25 - n  are activated to select one row. In the selected row, one of the output signals (word line enable signals) WLEN 1 , . . . , WLEN 4  changes to “H”. 
   In Structural Example 1, the row select switches RSW 2  are controlled by the output signals from the row decoders  25 - 1 , . . . ,  25 - n  which are activated only in the write operation. In Structural Example 9, the row select switches RSW 2  are controlled by the output signals from circuit blocks  23 B- 1 , . . . ,  23 B- n  each including a row decoder and read line driver. 
   That is, the gates of the row select switches (MOS transistors) RSW 2  are connected to read lines RW 1 , . . . , RWn. 
   The reason why this structure is employed is as follows. The write word line drivers  33 - 1 , . . . ,  33 - n  are arranged for the respective rows. Hence, in the write operation, all the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4 must be disconnected from the common data line  30 . 
   More specifically, the circuit blocks  23 B- 1 , . . . ,  23 B- n  each including a row decoder and read line driver are activated only in the read operation. In the write operation, the row select switches RSW 2  of all rows are turned off, so all the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL 4 (n−1)+4 are disconnected from the common data line  30 . 
   For example, as shown in  FIG. 132 , the read bit line is connected to a bias transistor BT which sets the bit line potential to VC. 
   A row select line signal RWi (i=1, . . . , n) is input to each row select switch RSW 2 . Row decoders  23 B- 1 , . . . ,  23 B- n  output the row select line signals RWi. 
   As shown in  FIG. 132 , the bias transistor BT is a NMOS transistor, when the inverting signal from RLi is input to the bias transistor BT. Row decoders  23 B- 1 , . . . ,  23 B- n  output the row select line signals RWi and the inverting signal thereof. 
   {circle around (3)} Structural Example 10 
     FIG. 60  shows the main part of a magnetic random access memory according to Structural Example 10 of the present invention. 
   As a characteristic feature of Structural Example 10, MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in a plurality of or all read blocks BK 1 x and BK 1 (x+1) in one column (Y-direction) share one upper electrode  44 . 
   In Structural Example 1, the upper electrode  44  for the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is arranged for each read block. However, the upper electrodes  44  for the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in read blocks in one column are short-circuited through the read word line RWL 1 . 
   Hence, the upper electrodes  44  for the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the read blocks in one column may be short-circuited. However, the upper electrodes  44  for the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in read blocks in one row (X-direction) must be disconnected from each other. 
   In Structural Example 10, the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  in the plurality of or all the read blocks BK 1 x and BK 1 (x+1) in one column share one upper electrode  44 . 
   According to Structural Example 10, since no contact plug  42  must be arranged for each read block, the density of MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  can be increased. That is, theoretically, at least one contact plug  42  suffices between a read word line RWL 1  and the shared upper electrode  44 . Actually, a plurality of contact plugs  42  are preferably arranged in one column equidistantly in consideration of the interconnection resistance and the like. 
   Structural Example 10 has been described as a modification of Structural Example 1. However, Structural Example 10 can be applied to all of Structural Examples 2 to 9. 
   (9) Others 
   As described above, the present invention is applied to a magnetic random access memory which has a cell array structure in which one terminal of each of a plurality of MTJ elements of a read block is commonly connected, and the other terminal is independently connected to a read bit line. When a select switch (e.g., a MOS transistor) is arranged in a read block, the degree of integration of MTJ elements can hardly be increased. 
   Normally, a select switch is formed in the surface region of a semiconductor substrate. An MTJ element is formed above the select switch. In this case, a contact hole is necessary for electrically connecting the select switch and MTJ element. That is, since no MTJ element can be arranged in the region where the contact hole is formed, the area of the memory cell array increases. 
   On the other hand, the resistance value of the MTJ element which forms a memory cell of the magnetic random access memory is sufficiently large. The read current is much smaller than the write current. That is, even when the select transistor in the read block is omitted, an increase in current consumption due to the read current flowing to MTJ elements in an unselected block poses no serious problem. 
   In the present invention, first, in the cell array structure in which one terminal of each of the plurality of MTJ elements of a read block is independently connected to a read bit line, the select switch which selects a read block is omitted. That is, no select switch (MOS transistor) is arranged in the memory cell array (immediately under the MTJ elements). 
   According to the characteristic feature of the present invention, since no select switch is present in the memory cell array, the MTJ elements can be arranged at a high density. In addition, since no select switch (semiconductor element) is present immediately under the MTJ elements, the planarity of the underlying layer of the MTJ elements can be improved, and the characteristics (uniform MR ratio or the like) of the MTJ elements can be improved. 
   To further improve the planarity of the underlying layer of the MTJ elements, a dummy pattern, e.g., a dummy interconnection pattern which does not function as an actual interconnection is arranged immediately under the MTJ elements. 
   Normally, to reduce the manufacturing cost (proportional to the number of times of PEP, the members (MTJ elements and the like) of the memory cell array portion and the members (interconnections) of the peripheral circuit portion are simultaneously processed as much as possible. However, when no select switch is present immediately under the MTJ elements, a step difference is generated between the memory cell array portion and the peripheral circuit portion. This step difference degrades the process accuracy of photolithography. 
   To prevent this, a dummy pattern is arranged immediately under the MTJ elements to increase the planarity of the underlying layer of the MTJ elements. More specifically, the step difference between the memory cell array portion and the peripheral circuit portion is eliminated. As the dummy pattern, a periodical (a repeat of a predetermined pattern) or a pattern which is uniform as a whole is used. 
   When the plurality of MTJ elements in a read block are arrayed in a direction parallel to the surface of the semiconductor substrate, i.e., arrayed in a line in the horizontal direction, a plurality of stages of memory cell arrays are stacked. When no select switch is present in the read block, a plurality of stages of memory cell arrays can be stacked. 
   The MTJ elements are also arranged in a direction perpendicular to the surface of the semiconductor substrate, i.e., in the vertical direction. That is, since the MTJ elements are arranged three-dimensionally, the density of MTJ elements can be increased as compared to a two-dimensional cell array structure. In addition, when a predetermined interconnection is shared by the memory cell array of the upper stage and that of the lower stage, the manufacturing cost can be reduced, and the insulating layer of each stage can be planarized (the characteristic of the MTJ elements can be increased). 
   In the cell array structure having the above characteristic features, an interconnection which functions only as a read bit line is connected to one terminal of each of the plurality of MTJ elements of a read block. That is, one of two write lines for a write is not electrically connected to the plurality of MTJ elements. 
   Hence, in the write operation, even when a potential difference is generated due to the interconnection resistance of the two write lines, the potential difference is not generated across the MTJ elements. According to the device structure of the present invention, dielectric breakdown (breakdown of the tunneling barrier layer of the MTJ element) in the write operation poses no problem, unlike a cross-point cell array structure. 
   As a switch for the magnetic random access memory, a MIS (Metal Insulator Semiconductor) transistor (including a MOS transistor), MES (Metal Semiconductor) transistor, junction transistor, bipolar transistor, or diode can be used. 
   2. Structural Examples of MTJ Element 
     FIGS. 61  to  63  show structural examples of the MTJ element. 
   The MTJ element shown in  FIG. 61  has the most basic structure having two ferromagnetic layers and a tunneling barrier layer sandwiched between these layers. 
   An antiferromagnetic layer for fixing the magnetizing direction is added to a fixed layer (pinning layer) of the two ferromagnetic layers, in which the magnetizing direction is fixed. The magnetizing direction in a free layer (storing layer) of the two ferromagnetic layers, in which the magnetizing direction can be freely changed, is determined by a synthesized magnetic field formed by a write word line and write bit line. 
   The MTJ element shown in  FIG. 62  has two tunneling barrier layers in it to make the bias voltage higher than in the MTJ element shown in FIG.  61 . 
   The MTJ element shown in  FIG. 62  can be regarded to have a structure (double junction structure) in which two MTJ elements shown in  FIG. 61  are connected in series. 
   In this example, the MTJ element has three ferromagnetic layers. Tunneling barrier layers are inserted between the ferromagnetic layers. Antiferromagnetic layers are added to the two ferromagnetic layers (pinning layers) at two ends. The middle layer in the three ferromagnetic layers serves as a free layer (storing layer) in which the magnetizing direction can be freely changed. 
   The MTJ element shown in  FIG. 63  can easily close lines of magnetic force in the ferromagnetic layer serving as a storing layer, as compared to the MTJ element shown in FIG.  61 . 
   For the MTJ element of this example, it can be regarded that the storing layer of the MTJ element shown in  FIG. 61  is replaced with a storing layer formed from two ferromagnetic layers and a nonmagnetic metal layer (e.g., an aluminum layer) sandwiched between those layers. 
   When the storing layer of the MTJ element has a three-layered structure made of two ferromagnetic layers and a nonmagnetic metal layer sandwiched between these layers, lines of magnetic force in the two ferromagnetic layers of the storing layer readily close. That is, since any antimagnetic field component in the two ferromagnetic layers of the storing layer can be prevented, the MR ratio can be improved. 
   The structural examples of the MTJ element have been described above. In the present invention (circuit structure, device structure, read operation principle, read circuit, and manufacturing method), the structure of the MTJ element is not particularly limited. The above-described three structural examples are mere representative examples of the MTJ element structure. 
   3. Examples of Peripheral Circuits 
   Circuit examples of the write word line driver/sinker, circuit examples of the write bit line driver/sinker, circuit examples of the read word line driver, circuit examples of the row decoder, circuit examples of the column decoder, and circuit examples of a read circuit (including a sense amplifier) will be sequentially described below. 
   (1) Write Word Line Driver/Sinker 
     FIG. 64  shows a circuit example of the write word line driver/sinker. 
   Assume that a read block is formed from four MTJ elements, and each of the four MTJ elements is selected by lower two bits CA 0  and CA 1  of a column address signal.  FIG. 64  shows a write word line driver/sinker of only one row. 
   The write word line driver  23 A includes PMOS transistors QP 1 , QP 2 , QP 3 , and QP 4 , and NAND gate circuits ND 1 , ND 2 , ND 3 , and ND 4 . The write word line sinker  24 - n  is formed from NMOS transistors QN 1 , QN 2 , QN 3 , and QN 4 . 
   The source of the PMOS transistor QP 1  is connected to a power supply terminal VDD. The drain is connected to one end of the write word line WWL 4 (n−1)+1 through the common data line (common driver line)  30  and row select switch RSW 2 . The output terminal of the NAND gate circuit ND 1  is connected to the gate of the PMOS transistor QP 1 . The source of the NMOS transistor QN 1  is connected to the ground terminal VSS. The drain is connected to the other end of the write word line WWL 4 (n−1)+1. 
   When the output signal from the NAND gate circuit ND 1  is “0”, a write current flows to the write word line WWL 4 (n−1)+1 in the selected row (the row whose row select switch RSW 2  is turned on). 
   The source of the PMOS transistor QP 2  is connected to the power supply terminal VDD. The drain is connected to one end of the write word line WWL 4 (n−1)+2 through the common data line (common driver line)  30  and row select switch RSW 2 . The output terminal of the NAND gate circuit ND 2  is connected to the gate of the PMOS transistor QP 2 . The source of the NMOS transistor QN 2  is connected to the ground terminal VSS. The drain is connected to the other end of the write word line WWL 4 (n−1)+2. 
   When the output signal from the NAND gate circuit ND 2  is “0”, a write current flows to the write word line WWL 4 (n−1)+2 in the selected row (the row whose row select switch RSW 2  is turned on). 
   The source of the PMOS transistor QP 3  is connected to the power supply terminal VDD. The drain is connected to one end of the write word line WWL 4 (n−1)+3 through the common data line (common driver line)  30  and row select switch RSW 2 . The output terminal of the NAND gate circuit ND 3  is connected to the gate of the PMOS transistor QP 3 . The source of the NMOS transistor QN 3  is connected to the ground terminal VSS. The drain is connected to the other end of the write word line WWL 4  (n−1)+3. 
   When the output signal from the NAND gate circuit ND 3  is “0”, a write current flows to the write word line WWL 4 (n−1)+3 in the selected row (the row whose row select switch RSW 2  is turned on). 
   The source of the PMOS transistor QP 4  is connected to the power supply terminal VDD. The drain is connected to one end of the write word line WWL 4 (n−1)+4 through the common data line (common driver line)  30  and row select switch RSW 2 . The output terminal of the NAND gate circuit ND 4  is connected to the gate of the PMOS transistor QP 4 . The source of the NMOS transistor QN 4  is connected to the ground terminal VSS. The drain is connected to the other end of the write word line WWL 4 (n−1)+4. 
   When the output signal from the NAND gate circuit ND 4  is “0”, a write current flows to the write word line WWL 4 (n−1)+4 in the selected row (the row whose row select switch RSW 2  is turned on). 
   A write signal WRITE is input to the NAND gate circuits ND 1 , ND 2 , ND 3 , and ND 4 . In the write operation, the write signal WRITE changes to “H”. In addition, different lower column address signals CA 0 , /CA 0 , CA 1 , and /CA 1  are input to the NAND gate circuits ND 1 , ND 2 , ND 3 , and ND 4 . 
   That is, in this example, column address signal bits bCA 0  and bCA 1  are used to select one write word line WWL 4 (n−1)+1 of the four write word lines (read bit lines) in the selected row and input to the NAND circuit ND 1 . 
   Column address signal bits CA 0  and bCA 1  are used to select one write word line WWL 4 (n−1)+2 of the four write word lines (read bit lines) in the selected row and input to the NAND circuit ND 2 . 
   Column address signal bits bCA 0  and CA 1  are used to select one write word line WWL 4 (n−1)+3 of the four write word lines (read bit lines) in the selected row and input to the NAND circuit ND 3 . 
   The column address signal bits CA 0  and CA 1  are used to select one write word line WWL 4 (n−1)+4 of the four write word lines (read bit lines) in the selected row and input to the NAND circuit ND 4 . 
   Note that the signal bits bCA 0  and bCA 1  are inverted signal bits with inverted levels of CA 0  and CA 1 . 
   In this write word line driver/sinker, in the write operation, the write signal WRITE changes to “H”. For example, one of the output signals from the four NAND gate circuits ND 1 , ND 2 , ND 3 , and ND 4  changes to “L”. 
   For example, when both CA 0  and CA 1  are “0”, all input signals to the NAND gate circuit ND 1  are “1”. The output signal from the NAND gate circuit ND 1  is “0”. As a result, the PMOS transistor QP 1  is turned on. The write current flows to the write word line WWL 4 (n−1)+1. 
   When CA 0  is “1” and CA 1  is “0”, all input signals to the NAND gate circuit ND 2  are “1”. The output signal from the NAND gate circuit ND 2  is “0”. As a result, the PMOS transistor QP 2  is turned on. The write current flows to the write word line WWL 4 (n−1)+2. 
   When CA 0  is “0” and CA 1  is “1”, all input signals to the NAND gate circuit ND 3  are “1”. The output signal from the NAND gate circuit ND 3  is “0”. As a result, the PMOS transistor QP 3  is turned on. The write current flows to the write word line WWL 4 (n−1)+3. 
   When both CA 0  and CA 1  are “1”, all input signals to the NAND gate circuit ND 4  are “1”. The output signal from the NAND gate circuit ND 4  is “0”. As a result, the PMOS transistor QP 4  is turned on. The write current flows to the write word line WWL 4 (n−1)+4. 
   (2) Write Bit Line Driver/Sinker 
     FIG. 65  shows a circuit example of the write bit line driver/sinker. 
   The write bit line driver/sinker  29 A is formed from PMOS transistors QP 5  and QP 6 , NMOS transistors QN 5  and QN 6 , NAND gate circuits ND 5  and ND 6 , AND gate circuits AD 1  and AD 2 , and inverters INV 1  and INV 2 . 
   The PMOS transistor QP 5  is connected between the power supply terminal VDD and one end of the write bit line WBL 1 . The output signal from the NAND gate circuit ND 5  is supplied to the gate of the PMOS transistor QP 5 . The NMOS transistor QN 5  is connected between one end of the write bit line WBL 1  and the ground terminal VSS. The output signal from the AND gate circuit AD 1  is supplied to the gate of the NMOS transistor QN 5 . 
   The PMOS transistor QP 6  is connected between the power supply terminal VDD and one end of the write bit line WBLj. The output signal from the NAND gate circuit ND 6  is supplied to the gate of the PMOS transistor QP 6 . The NMOS transistor QN 6  is connected between one end of the write bit line WBLj and the ground terminal VSS. The output signal from the AND gate circuit AD 2  is supplied to the gate of the NMOS transistor QN 6 . 
   The write bit line driver/sinker  31  is formed from PMOS transistors QP 7  and QP 8 , NMOS transistors QN 7  and QN 8 , NAND gate circuits ND 7  and ND 8 , AND gate circuits AD 3  and AD 4 , and inverters INV 3  and INV 4 . 
   The PMOS transistor QP 7  is connected between the power supply terminal VDD and the other end of the write bit line WBL 1 . The output signal from the NAND gate circuit ND 7  is supplied to the gate of the PMOS transistor QP 7 . The NMOS transistor QN 7  is connected between the other end of the write bit line WBL 1  and the ground terminal VSS. The output signal from the AND gate circuit AD 3  is supplied to the gate of the NMOS transistor QN 7 . 
   The PMOS transistor QP 8  is connected between the power supply terminal VDD and the other end of the write bit line WBLj. The output signal from the NAND gate circuit ND 8  is supplied to the gate of the PMOS transistor QP 8 . The NMOS transistor QN 8  is connected between the other end of the write bit line WBLj and the ground terminal VSS. The output signal from the AND gate circuit AD 4  is supplied to the gate of the NMOS transistor QN 8 . 
   In the write bit line drivers/sinkers  29 A and  31  with the above structures, when the output signal from the NAND gate circuit ND 5  is “0”, and the output signal from the AND gate circuit AD 3  is “1”, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line WBL 1 . 
   When the output signal from the NAND gate circuit ND 7  is “0”, and the output signal from the AND gate circuit AD 1  is “1”, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line WBL 1 . 
   In the write bit line drivers/sinkers  29 A and  31 , in the write operation, the write signal WRITE is “1”. In the selected column, all bits of the upper column address signal are “1”. Hence, a write current having a direction corresponding to the value of write data DATA flows to the write bit line WBLi (i=1, . . . , j) in the selected column. 
   The direction of write current flowing to the write bit line WBLi in the selected column is determined in accordance with the value of the write data DATA. 
   For example, when the write bit line WBL 1  is selected, and the write data DATA is “1”, the output signal from the NAND gate circuit ND 5  is “0”. The output signal from the AND gate circuit AD 3  is “1”. As a result, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line WBL 1 . 
   Conversely, when the write data DATA is “0”, the output signal from the NAND gate circuit ND 7  is “0”. The output signal from the AND gate circuit AD 1  is “1”. As a result, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line WBL 1 . 
   (3) Row Decoder 
     FIG. 66  shows a circuit example of the row decoder. 
   The row decoder  25 - 1  can have, e.g., the following structure.  FIG. 66  shows the row decoder of only one row. 
   The row decoder  25 - 1  is formed from an AND gate circuit AD 11 . A row address signal is input to the AND gate circuit AD 11 . In the selected row, all the bits of the row address signals are “H”. Hence, an output signal RL 1  from the row decoder  25 - 1  changes to “H”. 
   (4) Column Decoder &amp; Read Column Select Line Driver 
     FIG. 67  shows a circuit example of the column decoder &amp; read column select line driver. 
     FIG. 67  illustrates the column decoder &amp; read column select line driver of only one column of the memory cell array. 
   The column decoder &amp; read column select line driver  32  is formed from an AND gate circuit AD 10 . A read signal READ and upper column address signal are input to the AND gate circuit AD 10 . 
   In the read operation, the read signal changes to “H”. That is, in a mode other than the read operation, the potential of the output signal (column select signal) CSL 1  from the column decoder &amp; read column select line driver  32  does not change to “H”. In the read operation, in the selected column, all bits of the column address signal are “H”. Hence, the potential of the output signal CSL 1  from the column decoder &amp; read column select line driver  32  changes to “H”. 
   (5) Write Bit Line Driver/Sinker 
   A circuit example of the write bit line driver/sinker used in Structural Example 7 ( FIG. 53 ) will be described. 
     FIGS. 68 and 69  show a circuit example of the write bit line driver/sinker. 
   The write bit line driver/sinker  23 AR is formed from PMOS transistors QP 5 , QP 6 , QP 7 , and QP 8 , NMOS transistors QN 5 , QN 6 , QN 7 , and QN 8 , NAND gate circuits ND 5 , ND 6 , ND 7 , and ND 8 , AND gate circuits AD 1 , AD 2 , AD 3 , and AD 4 , and inverters INV 1 , INV 2 , INV 3 , and INV 4 . 
   The PMOS transistor QP 5  is connected between the power supply terminal VDD and the common data line  30 A. The output signal from the NAND gate circuit ND 5  is supplied to the gate of the PMOS transistor QP 5 . The NMOS transistor QN 5  is connected between the common data line  30 A and the ground terminal VSS. The output signal from the AND gate circuit AD 1  is supplied to the gate of the NMOS transistor QN 5 . 
   The PMOS transistor QP 6  is connected between the power supply terminal VDD and the common data line  30 A. The output signal from the NAND gate circuit ND 6  is supplied to the gate of the PMOS transistor QP 6 . The NMOS transistor QN 6  is connected between the common data line  30 A and the ground terminal VSS. The output signal from the AND gate circuit AD 2  is supplied to the gate of the NMOS transistor QN 6 . 
   The PMOS transistor QP 7  is connected between the power supply terminal VDD and the common data line  30 A. The output signal from the NAND gate circuit ND 7  is supplied to the gate of the PMOS transistor QP 7 . The NMOS transistor QN 7  is connected between the common data line  30 A and the ground terminal VSS. The output signal from the AND gate circuit AD 3  is supplied to the gate of the NMOS transistor QN 7 . 
   The PMOS transistor QP 8  is connected between the power supply terminal VDD and the common data line  30 A. The output signal from the NAND gate circuit ND 8  is supplied to the gate of the PMOS transistor QP 8 . The NMOS transistor QN 8  is connected between the common data line  30 A and the ground terminal VSS. The output signal from the AND gate circuit AD 4  is supplied to the gate of the NMOS transistor QN 8 . 
   The write bit line driver/sinker  23 AS is formed from PMOS transistors QP 9 , QP 10 , QP 11 , and QP 12 , NMOS transistors QN 9 , QN 10 , QN 11 , and QN 12 , NAND gate circuits ND 9 , ND 10 , ND 11 , and ND 12 , AND gate circuits AD 5 , AD 6 , AD 7 , and AD 8 , and inverters INV 5 , INV 6 , INV 7 , and INV 8 . 
   The PMOS transistor QP 9  is connected between the power supply terminal VDD and the common data line  30 B. The output signal from the NAND gate circuit ND 9  is supplied to the gate of the PMOS transistor QP 9 . The NMOS transistor QN 9  is connected between the common data line  30 B and the ground terminal VSS. The output signal from the AND gate circuit AD 5  is supplied to the gate of the NMOS transistor QN 9 . 
   The PMOS transistor QP 10  is connected between the power supply terminal VDD and the common data line  30 B. The output signal from the NAND gate circuit ND 10  is supplied to the gate of the PMOS transistor QP 10 . The NMOS transistor QN 10  is connected between the common data line  30 B and the ground terminal VSS. The output signal from the AND gate circuit AD 6  is supplied to the gate of the NMOS transistor QN 10 . 
   The PMOS transistor QP 11  is connected between the power supply terminal VDD and the common data line  30 B. The output signal from the NAND gate circuit ND 11  is supplied to the gate of the PMOS transistor QP 11 . The NMOS transistor QN 11  is connected between the common data line  30 B and the ground terminal VSS. The output signal from the AND gate circuit AD 7  is supplied to the gate of the NMOS transistor QN 11 . 
   The PMOS transistor QP 12  is connected between the power supply terminal VDD and the common data line  30 B. The output signal from the NAND gate circuit ND 12  is supplied to the gate of the PMOS transistor QP 12 . The NMOS transistor QN 12  is connected between the common data line  30 B and the ground terminal VSS. The output signal from the AND gate circuit AD 8  is supplied to the gate of the NMOS transistor QN 12 . 
   In the write bit line drivers/sinkers  23 AR and  23 AS with the above structures, for example, when the output signal from the NAND gate circuit ND 5  is “0”, and the output signal from the AND gate circuit AD 5  is “1”, a write current from the write bit line driver/sinker  23 AR to the write bit line driver/sinker  23 AS flows to the write bit line WBL 4 (n−1)+1 in the row selected by the row select switch RSW 2 . 
   For example, when the output signal from the NAND gate circuit ND 9  is “0”, and the output signal from the AND gate circuit AD 1  is “1”, a write current from the write bit line driver/sinker  2 AS to the write bit line driver/sinker  23 AR flows to the write bit line WBL 4 (n−1)+1 in the row selected by the row select switch RSW 2 . 
   In the write bit line drivers/sinkers  23 AR and  23 AS, in the write operation, the write signal WRITE is “1”. In this example, one read block BKik is selected by the row address signal and upper column address signal (signal bits except lower two bits of the column address signal). 
   The four MTJ elements are present in the selected read block BKik. To select one of the four MTJ elements, the lower two bits CA 0  and CA 1  of the column address signal are used. 
   The direction of write current flowing to the write bit line WBL 4 (n−1)+1, which is to be used to write data in the selected MTJ element in the selected read block BKik, is determined in accordance with the value of the write data DATA. 
   For example, when the write bit line WBL 4 (n−1)+1 is selected, and the write data DATA is “1”, the output signal from the NAND gate circuit ND 5  is “0”. The output signal from the AND gate circuit AD 5  is “1”. As a result, a write current from the write bit line driver/sinker  23 AR to the write bit line driver/sinker  23 AS flows to the write bit line WBL 4 (n−1)+1. 
   Conversely, when the write data DATA is “0”, the output signal from the NAND gate circuit ND 9  is “0”. The output signal from the AND gate circuit AD 1  is “1”. As a result, a write current from the write bit line driver/sinker  23 AS to the write bit line driver/sinker  23 AR flows to the write bit line WBL 4 (n−1)+1. 
   (6) Column Decoder &amp; Write Word Line Driver/Sinker 
   A circuit example of the column decoder &amp; write word line driver/sinker used in Structural Example 7 ( FIG. 53 ) will be described. 
     FIG. 70  shows a circuit example of a column decoder &amp; write word line driver/sinker. 
   The column decoder &amp; write word line driver/sinker  29 AR is formed from NAND gate circuits ND 1 , . . . , NDj and PMOS transistor QP 1 , . . . , QPj. 
   Each of the PMOS transistor QP 1 , . . . , QPj is connected between the power supply terminal VDD and one end of a corresponding one of write word lines WWL 1 , . . . , WWLj. The output signals from the NAND gate circuits ND 1 , . . . , NDj are supplied to the gates of the PMOS transistor QP 1 , . . . , QPj, respectively. 
   In the write operation, the write signal WRITE is “1”. In the selected column, all the upper column address signal bits are “1”. Hence, the output signals from the NAND gate circuits ND 1 , . . . , NDj are “0”, and the PMOS transistor QP 1 , . . . , QPj are turned on. 
   The write word line sinker  31 R is formed from NMOS transistor QN 1 , . . . , QNj. 
   Each of the NMOS transistor QN 1 , . . . , QNj is connected between the ground terminal VSS and the other end of a corresponding one of the write word lines WWL 1 , . . . , WWLj. The NMOS transistor QN 1 , . . . , QNj are always ON because the power supply potential VDD is supplied to their gates. 
   (7) Row Decoder 
   A circuit example of the row decoder used in Structural Example 9 ( FIG. 59 ) will be described. 
     FIG. 71  shows a circuit example of the row decoder. 
     FIG. 71  shows the row decoder  25 - 1  of only one row. 
   The row decoder  25 - 1  is formed from four AND gate circuit AD 13  to AD 16 . The write signal WRITE, row address signal, and lower two bits CA 0  and CA 1  of the column address signal are input to the AND gate circuit AD 13  to AD 16 . 
   In the write operation, the write signal WRITE changes to “H”. In the selected row, all bits of the row address signal change to “H”. In the selected row, one of the four MTJ elements in the selected read block, i.e., one of the four write word lines is selected on the basis of the lower two bits CA 0  and CA 1  of the column address signal. 
   (8) Write Word Line Driver 
   A circuit example of the write word line driver used in Structural Example 9 ( FIG. 59 ) will be described. 
     FIG. 72  shows a circuit example of the write word line driver. 
     FIG. 72  shows the write word line driver of only one row. 
   The write word line driver  33 - 1  is formed from PMOS transistors P 1 , P 2 , P 3 , and P 4  connected to the write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 , respectively. 
   Each of the PMOS transistors P 1 , P 2 , P 3 , and P 4  is connected between the power supply terminal VDD and a corresponding one of the write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4  and controlled by a corresponding one of word line enable signals WLEN 1  to WLEN 4 . The word line enable signals WLEN 1  to WLEN 4  are obtained by decoding lower two bits of the row address signal and column address signal by the row decoder shown in FIG.  71 . 
   (9) Row Decoder &amp; Read Line Driver 
   A circuit example of the row decoder &amp; read line driver used in Structural Example 9 ( FIG. 59 ) will be described. 
     FIG. 73  shows a circuit example of the row decoder &amp; read line driver.  FIG. 73  shows the row decoder &amp; read line driver of only one row. 
   The row decoder &amp; read line driver  23 B- 1  is formed from an AND gate circuit AD 9 . The read signal READ and row address signal are input to the AND gate circuit AD 9 . 
   In the read operation, the read signal READ changes to “H”. That is, in a mode except the read operation, the potential of the read word line RWL 1  does not change to “H”. In the read operation, in the selected row, all bits of the row address signal change to “H”. Hence, the potential of the read line RWL 1  is “H”. 
   (10) Column Decoder &amp; Write Bit Line Driver/Sinker 
     FIG. 74  is a circuit diagram showing a magnetic random access memory according to Structural Example 11 of the present invention. 
   Structural Example 11 has a characteristic that the write word lines WWLj are extended in column direction and the write bit lines WBL 4 (n−1)+1, . . .  4  (n−1)+4 are extended in row direction. 
   A circuit example of the column decoder &amp; write bit line driver/sinker used in Structural Example 11 will be described. 
     FIGS. 75 and 76  show a circuit example of the column decoder &amp; write bit line driver/sinker. 
     FIGS. 75 and 76  show the column decoder &amp; write bit line driver/sinker of only one column. 
   In this example, assume that a read block is formed from four MTJ elements, and the four MTJ elements in the Structural Example are selected by lower two bits CA 0  and CA 1  of the column address signal. In addition, a column of the memory cell array is selected by upper column address signal bits, i.e., a column address signal excluding its lower two bits CA 0  and CA 1 . 
   The write bit line driver/sinker  29 A is formed from the PMOS transistors QP 5 , QP 6 , QP 7 , and QP 8 , NMOS transistors QN 5 , QN 6 , QN 7 , and QN 8 , NAND gate circuits ND 5 , ND 6 , ND 7 , and ND 8 , AND gate circuits AD 1 , AD 2 , AD 3 , and AD 4 , and inverters INV 1 , INV 2 , INV 3 , and INV 4 . 
   The PMOS transistor QP 5  is connected between the power supply terminal VDD and one end of the write bit line BL 1 . The output signal from the NAND gate circuit ND 5  is supplied to the gate of the PMOS transistor QP 5 . The NMOS transistor QN 5  is connected between one end of the write bit line BL 1  and the ground terminal VSS. The output signal from the AND gate circuit AD 1  is supplied to the gate of the NMOS transistor QN 5 . 
   The PMOS transistor QP 6  is connected between the power supply terminal VDD and one end of the write bit line BL 2 . The output signal from the NAND gate circuit ND 6  is supplied to the gate of the PMOS transistor QP 6 . The NMOS transistor QN 6  is connected between one end of the write bit line BL 2  and the ground terminal VSS. The output signal from the AND gate circuit AD 2  is supplied to the gate of the NMOS transistor QN 6 . 
   The PMOS transistor QP 7  is connected between the power supply terminal VDD and one end of the write bit line BL 3 . The output signal from the NAND gate circuit ND 7  is supplied to the gate of the PMOS transistor QP 7 . The NMOS transistor QN 7  is connected between one end of the write bit line BL 3  and the ground terminal VSS. The output signal from the AND gate circuit AD 3  is supplied to the gate of the NMOS transistor QN 7 . 
   The PMOS transistor QP 8  is connected between the power supply terminal VDD and one end of the write bit line BL 4 . The output signal from the NAND gate circuit ND 8  is supplied to the gate of the PMOS transistor QP 8 . The NMOS transistor QN 8  is connected between one end of the write bit line BL 4  and the ground terminal VSS. The output signal from the AND gate circuit AD 4  is supplied to the gate of the NMOS transistor QN 8 . 
   The write bit line driver/sinker  31  is formed from the PMOS transistors QP 9 , QP 10 , QP 11 , and QP 12 , NMOS transistors QN 9 , QN 10 , QN 11 , and QN 12 , NAND gate circuits ND 9 , ND 10 , ND 11 , and ND 12 , AND gate circuits AD 5 , AD 6 , AD 7 , and AD 8 , and inverters INV 5 , INV 6 , INV 7 , and INV 8 . 
   The PMOS transistor QP 9  is connected between the power supply terminal VDD and the other end of the write bit line BL 1 . The output signal from the NAND gate circuit ND 9  is supplied to the gate of the PMOS transistor QP 9 . The NMOS transistor QN 9  is connected between the other end of the write bit line BL 1  and the ground terminal VSS. The output signal from the AND gate circuit AD 5  is supplied to the gate of the NMOS transistor QN 9 . 
   The PMOS transistor QP 10  is connected between the power supply terminal VDD and the other end of the write bit line BL 2 . The output signal from the NAND gate circuit ND 10  is supplied to the gate of the PMOS transistor QP 10 . The NMOS transistor QN 10  is connected between the other end of the write bit line BL 2  and the ground terminal VSS. The output signal from the AND gate circuit AD 6  is supplied to the gate of the NMOS transistor QN 10 . 
   The PMOS transistor QP 11  is connected between the power supply terminal VDD and the other end of the write bit line BL 3 . The output signal from the NAND gate circuit ND 11  is supplied to the gate of the PMOS transistor QP 11 . The NMOS transistor QN 11  is connected between the other end of the write bit line BL 3  and the ground terminal VSS. The output signal from the AND gate circuit AD 7  is supplied to the gate of the NMOS transistor QN 11 . 
   The PMOS transistor QP 12  is connected between the power supply terminal VDD and the other end of the write bit line BL 4 . The output signal from the NAND gate circuit ND 12  is supplied to the gate of the PMOS transistor QP 12 . The NMOS transistor QN 12  is connected between the other end of the write bit line BL 4  and the ground terminal VSS. The output signal from the AND gate circuit AD 8  is supplied to the gate of the NMOS transistor QN 12 . 
   In the write bit line drivers/sinkers  29 A and  31  with the above structures, when the output signal from the NAND gate circuit ND 5  is “0”, and the output signal from the AND gate circuit AD 5  is “1”, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line BL 1 . 
   When the output signal from the NAND gate circuit ND 9  is “0”, and the output signal from the AND gate circuit AD 1  is “1”, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line BL 1 . 
   When the output signal from the NAND gate circuit ND 6  is “0”, and the output signal from the AND gate circuit AD 6  is “1”, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line BL 2 . 
   When the output signal from the NAND gate circuit ND 10  is “0”, and the output signal from the AND gate circuit AD 2  is “1”, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line BL 2 . 
   When the output signal from the NAND gate circuit ND 7  is “0”, and the output signal from the AND gate circuit AD 7  is “1”, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line BL 3 . 
   When the output signal from the NAND gate circuit ND 11  is “0”, and the output signal from the AND gate circuit AD 3  is “1”, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line BL 3 . 
   When the output signal from the NAND gate circuit ND 8  is “0”, and the output signal from the AND gate circuit AD 8  is “1”, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line BL 4 . 
   When the output signal from the NAND gate circuit ND 12  is “0”, and the output signal from the AND gate circuit AD 4  is “1”, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line BL 4 . 
   In the write bit line drivers/sinkers  29 A and  31 , in the write operation, the write signal WRITE is “1”. In the selected column, all bits of the upper column address signal, i.e., all bits of the column address signal excluding the lower two bits CA 0  and CA 1  are “1”. 
   The lower two bits CA 0  and CA 1  of the column address signal are signal bits for selecting one of the four write bit lines BL 1 , BL 2 , BL 3 , and BL 4  in the selected column. A write current having a direction corresponding to the value of write data DATA flows to the selected write bit line. 
   The direction of write current flowing to the selected write bit line in the selected column is determined in accordance with the value of the write data DATA. 
   For example, when the write bit line BL 1  is selected (CA 0 =“0”, and CA 1 =“0”), and the write data DATA is “1”, the output signal from the NAND gate circuit ND 5  is “0”. The output signal from the AND gate circuit AD 5  is “1”. As a result, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line BL 1 . 
   Conversely, when the write data DATA is “0”, the output signal from the NAND gate circuit ND 9  is “0”. The output signal from the AND gate circuit AD 1  is “1”. As a result, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line BL 1 . 
   When the write bit line BL 2  is selected (CA 0 =“1”, and CA 1 =“0”), and the write data DATA is “1”, the output signal from the NAND gate circuit ND 6  is “0”. The output signal from the AND gate circuit AD 6  is “1”. As a result, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line BL 2 . 
   Conversely, when the write data DATA is “0”, the output signal from the NAND gate circuit ND 10  is “0”. The output signal from the AND gate circuit AD 2  is “1”. As a result, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line BL 2 . 
   When the write bit line BL 3  is selected (CA 0 =“0”, and CA 1 =“1”), and the write data DATA is “1”, the output signal from the NAND gate circuit ND 7  is “0”. The output signal from the AND gate circuit AD 7  is “1”. As a result, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line BL 3 . 
   Conversely, when the write data DATA is “0”, the output signal from the NAND gate circuit ND 11  is “0”. The output signal from the AND gate circuit AD 3  is “1”. As a result, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line BL 3 . 
   When the write bit line BL 4  is selected (CA 0 =“1”, and CA 1 =“1”), and the write data DATA is “1”, the output signal from the NAND gate circuit ND 8  is “0”. The output signal from the AND gate circuit AD 8  is “1”. As a result, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line BL 4 . 
   Conversely, when the write data DATA is “0”, the output signal from the NAND gate circuit ND 12  is “0”. The output signal from the AND gate circuit AD 4  is “1”. As a result, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line BL 4 . 
   (11) Read Circuit 
     FIG. 77  shows a circuit example of the read circuit. 
   In this example, assume that four MTJ elements are arranged in a read block of one column, and the MTJ elements are independently connected to read bit lines. That is, four read bit lines are arranged in one column. These read bit lines are connected to the read circuit  29 B through the column select switch. 
   The read circuit  29 B of this example is applied to a 1-bit-type magnetic random access memory which outputs read data bits one by one. 
   Hence, the read circuit  29 B has four sense amplifiers &amp; bias circuits  29 B 11 ,  29 B 12 ,  29 B 13 , and  29 B 14 , a selector  29 B 2 , and an output buffer  29 B 3 . 
   In the read operation, read data are simultaneously read from four MTJ elements in the selected read block. These four read data are input to and sensed by the sense amplifiers &amp; bias circuits  29 B 11 ,  29 B 12 ,  29 B 13 , and  29 B 14 , respectively. 
   On the basis of the lower two bits CA 0  and CA 1  of the column address signal, the selector  29 B 2  selects one of the four read data output from the sense amplifiers &amp; bias circuits  29 B 11 ,  29 B 12 ,  29 B 13 , and  29 B 14 . The selected read data is output from the magnetic random access memory as output data through the output buffer  29 B 3 . 
   This example assume that the read circuit  29 B is applied to a 1-bit-type magnetic random access memory. 
   However, when the read circuit  29 B is applied to, e.g., a 4-bit-type magnetic random access memory which outputs 4-bit read data, the selector  29 B 2  can be omitted. To the contrary, four output buffers  29 B 3  are required in correspondence with the sense amplifiers &amp; bias circuits  29 B 11 ,  29 B 12 ,  29 B 13 , and  29 B 14 . 
     FIG. 78  shows a circuit example of the read circuit applied to a 4-bit-type magnetic random access memory. 
   The read circuit  29 B has four sense amplifiers &amp; bias circuits  29 B 11 ,  29 B 12 ,  29 B 13 , and  29 B 14  and four output buffers  29 B 31 ,  29 B 32 ,  29 B 33 , and  29 B 34 . 
   In the read operation, read data are simultaneously read from four MTJ elements in the selected read block. These four read data are input to and sensed by the sense amplifiers &amp; bias circuits  29 B 11 ,  29 B 12 ,  29 B 13 , and  29 B 14 , respectively. 
   The output data from the sense amplifiers &amp; bias circuits  29 B 11 ,  29 B 12 ,  29 B 13 , and  29 B 14  are output from the magnetic random access memory through the output buffers  29 B 31 ,  29 B 32 ,  29 B 33 , and  29 B 34 . 
     FIG. 79  shows a circuit example of the sense amplifier &amp; bias circuit. 
   This sense amplifier &amp; bias circuit corresponds to one of the four sense amplifiers &amp; bias circuits shown in  FIG. 77  or  78 . 
   A sense amplifier S/A is formed from, e.g., a differential amplifier. 
   A PMOS transistor QP 14  and NMOS transistor QN 13  are connected in series between the power supply terminal VDD and the column select switch  29 C. The negative input terminal of an operational amplifier OP is connected to a node n 2 . The output terminal of the operational amplifier OP is connected to the gate of the NMOS transistor QN 13 . A clamp potential VC is input to the positive input terminal of the operational amplifier OP. 
   The operational amplifier OP equalizes the potential of the node n 2  with the clamp potential VC. The clamp potential VC is set to a predetermined positive value. 
   A constant current source Is 1  generates a read current Iread. The read current Iread flows to a bit line BLi through a current mirror circuit formed from a PMOS transistor QP 13  and the PMOS transistor QP 14 . The sense amplifier formed from, e.g., a differential amplifier senses the data of a memory cell (MTJ element) on the basis of the potential of a node n 1  when the read current Iread is flowing. 
     FIG. 80  shows a circuit example of the sense amplifier.  FIG. 81  shows a circuit example of the reference potential generating circuit of the sense amplifier. 
   The sense amplifier S/A is formed from, e.g., a differential amplifier. The sense amplifier S/A compares a potential Vn 1  of the node n 1  with a reference potential Vref. 
   The reference potential Vref is generated by an MTJ element which stores “1” data and an MTJ element which stores “0” data. 
   A PMOS transistor QP 16  and NMOS transistors QN 14  and QN 15  are connected in series between the power supply terminal VDD and the MTJ element which stores “1” data. A PMOS transistor QP 17  and NMOS transistors QN 16  and QN 17  are connected in series between the power supply terminal VDD and the MTJ element which stores “0” data. 
   The drains of the PMOS transistors QP 16  and QP 17  are connected to each other. The drains of the NMOS transistors QN 15  and QN 17  are also connected to each other. 
   The operational amplifier OP equalizes the potential of a node n 4  with the clamp potential VC. A constant current source Is 2  generates the read current Iread. The read current Iread flows to the MTJ element which stores “1” data and MTJ element which stores “0” data through a current mirror circuit formed from the PMOS transistors QP 15  and QP 16 . 
   The reference potential Vref is output from a node n 3 . 
     FIG. 82  shows a circuit example of the operational amplifier OP shown in  FIGS. 79 and 81 . 
   The operational amplifier OP is formed from PMOS transistors QP 18  and QP 19  and NMOS transistors QN 18 , QN 19 , and QN 20 . When an enable signal Enable changes to “H”, the operational amplifier OP is set in an operative state. 
     FIG. 83  shows a circuit example of the sense amplifier &amp; bias circuit. 
   This sense amplifier &amp; bias circuit corresponds to one of the four sense amplifiers &amp; bias circuits shown in  FIGS. 77 and 78 . 
   The sense amplifier &amp; bias circuit of this example is applied to Structural Example 8 (FIG.  58 ). 
   When the sense amplifier &amp; bias circuit is applied to Structural Example 8 (FIG.  58 ), NMOS transistors QN 24  and QN 25  in  FIG. 83  have the same size as that of the column select switch CSW shown in FIG.  58 . NMOS transistors QN 20  and QN 21  in  FIG. 83  have the same size as that of the row select switch RSW 2  shown in FIG.  58 . 
   NMOS transistors QN 17 , QN 18 , and QN 19  in  FIG. 83  have the same size such that they have the same driving capability. 
   With this structure, the positive input potential of the operational amplifier is an almost intermediate potential between the negative input potential of the operational amplifier when “1” data is read out and that of the operational amplifier when “0” data is read out in FIG.  58 . Hence, the positive input potential of the operational amplifier functions as a reference potential in the data read mode. 
   A signal VtA input to the gates of the NMOS transistors QN 18  and QN 19  equals the data discrimination voltage of the sense amplifier S/A. The read signal READ which changes to “H” in the read operation is input to the gates of the NMOS transistors QN 20 , QN 21 , QN 24 , and QN 25 . 
   Referring to  FIG. 83 , “1” indicates that the MTJ element stores “1” data, and “0” indicates that the MTJ element stores “0” data. VC equals the bias potential VC applied to the bias line  34  of Structural Example 8 (FIG.  58 ). 
   4. Write/Read Operation Principle 
   The write/read operation principle of the magnetic random access memory of the present invention will be described. 
   (1) Write Operation Principle 
   A write in MTJ elements is executed at random. For example, the row decoders  25 - 1 , . . . ,  25 - n  select one row on the basis of a row address signal. In the selected row, an output signal RLk from a row decoder  25 - k  changes to “H”, so the row select switch RSW 2  is turned on. 
   The column decoder &amp; read column select line driver  32  is activated only in the read operation. Hence, all the read word lines RWL 1 , . . . , RWLj are in a floating state. 
   The write word line driver  23 A selects one of the four MTJ elements in the selected read block BKik and, more specifically, one of the four write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL(n−1)+4 on the basis of, e.g., the lower two bits CA 0  and CA 1  of the column address signal. 
   The write word line driver  23 A supplies a write current to the selected write word line through the common data line (common driver line)  30  and row select switch RSW 2 . 
   The column decoders &amp; write bit line drivers/sinkers  29 A and  31  select a column on the basis of, e.g., upper column address signal bits (column address signal excluding the lower two bits CA 0  and CA 1 ) and supplies the write current to the write bit line WBLi in the selected column. 
   The column decoders &amp; write bit line drivers/sinkers  29 A and  31  determine the direction of write current to be supplied to the write bit line WBLi in the selected column in accordance with the value of write data. 
   The magnetizing direction of the free layer (storing layer) of the selected MTJ element is determined by the synthesized magnetic field generated by the write current flowing to the write word line and the write current flowing to the write bit line, and “1”/“0” information is stored in the MTJ element. 
   In this write operation principle, one terminal of the MTJ element is connected to the read word line RWLi in the floating state in the write operation. Hence, charges are injected into the read word line RWLi from the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL(n−1)+4, though no potential difference is generated across the MTJ element. 
   Hence, even when the write word line and write bit line have different potentials at a predetermined portion in the write operation due to the interconnection resistance of the write word line and write bit line, no potential difference is generated across the MTJ element, and the tunneling barrier layer is not broken. 
   (2) Read Operation Principle 
   A read from MTJ elements is executed for each read block. For example, the row decoders  25 - 1 , . . . ,  25 - n  select one row on the basis of a row address signal. In the selected row, the output signal RLk from the row decoder  25 - k  changes to “H”, so the row select switch RSW 2  is turned on. 
   The column decoder &amp; read column select line driver  32  selects a column on the basis of upper column address signal bits. In the selected column, the output signal from the column decoder &amp; read column select line driver  32 , i.e., the column select signal CSLi changes to “H”, so the column select switch CSW is turned on. 
   That is, the potential of the read word line RWLi in the selected column is the ground potential VSS. The read word lines RWLi in the remaining unselected columns are set in the floating state. 
   In the read operation, the write word line driver  23 A and column decoders &amp; write bit line drivers/sinkers  29 A and  31  are in an inoperative state. 
   The read circuit  29 B generate, e.g., a read current. The read current flows to only the plurality of MTJ elements  12  in the read block which is present in the selected row and column. 
   More specifically, the read current is absorbed by the ground point VSS through the row select switches RSW 2  in the selected row, the MTJ elements  12  in the read block, and the column select switch CSW in the selected column. 
   In the read operation, one terminal of each of the MTJ elements in read blocks that are present in the selected row and unselected columns is short-circuited. The read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 in the selected row are short-circuited through the MTJ elements. 
   This problem can be solved by, in the read operation, fixing the potentials of the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 by a clamp circuit and detecting data on the basis of a change in amount of the read current. 
   The direction of read current is not particularly limited. The read current may flow in a direction in which the read current is absorbed by the read circuit  29 B. 
   The change in amount of the read current flowing to the read bit lines RBL 4 (n−1)+1, RBL 4 (n−1)+2, RBL 4 (n−1)+3, and RBL 4 (n−1)+4 is detected by the sense amplifier in the read circuit  29 B. 
   The data in each MTJ element is sensed by the sense amplifier in the read circuit  29 B and then output from the magnetic random access memory. The data bits of the plurality of MTJ elements  12  in the read block may be output one by one or simultaneously. 
   To sequentially output the data bits of the plurality of MTJ elements one by one, one of the data of the plurality of MTJ elements  12  is selected using the lower column address signal bits CA 0  and CA 1 . 
   (3) STRUCTURAL EXAMPLE 9 (FIG.  59 ) 
   {circle around (1)} Write Operation Principle 
   The row decoders  25 - 1 , . . . ,  25 - n  select one row on the basis of a row address signal. In the selected row, the output signals WLEN 1  to WLEN 4  from the row decoder  25 - k  change to “H”. Hence, the write word line driver  33 - k  is activated, and a write current is supplied to the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL(n−1)+4. 
   To write data in the MTJ elements at random, the lower two bits CA 0  and CA 1  of the column address signal, which select one of the four write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL(n−1)+4, are input to the row decoders  25 - 1 , . . . ,  25 - n , as shown in, e.g., FIG.  71 . 
   That is, in Structural Example 9, four row decoders are arranged in one row, and different lower two bits CA 0  and CA 1  of column address signals are input to the row decoders, as shown in FIG.  71 . In addition, the four word line enable lines WLEN 1  to WLEN 4  are arranged in one row such that the four write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL(n−1)+4 can be independently driven. 
   The row decoders &amp; read line drivers  23 B- 1 , . . . ,  23 B- n  and column decoder &amp; read column select line driver  32  are activated only in the read operation. 
   For this reason, all the read word lines RWL 1 , . . . , RWLj are in the floating state, and the write word lines WWL 4 (n−1)+1, WWL 4 (n−1)+2, WWL 4 (n−1)+3, and WWL(n−1)+4 are electrically disconnected from the common data line  30 . 
   The column decoders &amp; write bit line drivers/sinkers  29 A and  31  select a column on the basis of, e.g., upper column address signal bits (column address signal excluding the lower two bits CA 0  and CA 1 ) and supplies a write current to the write bit line WBLi in the selected column. 
   The column decoders &amp; write bit line drivers/sinkers  29 A and  31  determine the direction of write current to be supplied to the write bit line WBLi in the selected column in accordance with the value of write data. 
   The magnetizing direction of the free layer (storing layer) of the selected MTJ element is determined by the synthesized magnetic field generated by the write current flowing to the write word line and the write current flowing to the write bit line, and “1”/“0” information is stored in the MTJ element. 
   {circle around (2)} Read Operation Principle 
   A read from MTJ elements is executed for each read block. In Structural Example 9, the row decoders  25 - 1 , . . . ,  25 - n  are in the inoperative state in the read operation. That is, all the output signals WLEN 1  to WLEN 4  from the row decoders  25 - 1 , . . . ,  25 - n  are “L”. 
   The row decoders &amp; read line drivers  23 B- 1 , . . . ,  23 B- n  select one row on the basis of the row address signal. In the selected row, the output signal from the row decoder &amp; read line driver  23 B- k , i.e., the potential of the read line RWk changes to “H”, so the row select switch RSW 2  is turned on. 
   The column decoder &amp; read column select line driver  32  selects one column on the basis of upper column address signal bits. In the selected column, the output signal from the column decoder &amp; read column select line driver  32 , i.e., the column select signal CSLi changes to “H”, so the column select switch CSW is turned on. 
   In the read operation, the write word line driver  33 - k  and column decoders &amp; write bit line drivers/sinkers  29 A and  31  are in the inoperative state. 
   The read circuit  29 B generates, e.g., the read current. The read current flows to only the plurality of MTJ elements  12  in the read block which is present in the selected row and column. 
   More specifically, the read current is absorbed by the ground point VSS through the row select switches RSW 2  in the selected row, the MTJ elements  12  in the read block, and the column select switch CSW in the selected column. 
   The direction of read current is not particularly limited. The read current may flow in a direction in which the read current is absorbed by the read circuit  29 B. 
   5. Positional Relationship Between Pinning Layer and Storing Layer of Each MTJ Element 
   As in Structural Example 5 (e.g., the sectional view shown in FIG.  36 ), when MTJ elements are arranged on the upper and lower sides of a write line (write word line or write bit line), and data is to be written in the MTJ element on the upper or lower side of the write line using a magnetic field generated by a write current that flows to the write line, the positional relationship between the pinning layer (fixed layer) and the storing layer (free layer) or the magnetizing direction of the pinning layer in each MTJ element must be examined. 
   This is because the write operation principle or the write circuit arrangement changes depending on the direction of the current flowing to the write line. 
   (1) Positional Relationship Between Pinning Layer and Storing Layer of Each MTJ Element 
   As shown in  FIG. 84 , the positional relationship (relative relationship) between the pinning layer and the storing layer of each MTJ element (MTJ element) is preferably symmetrical with respect to a write line to be used. 
   For example, when MTJ elements are arranged on the upper and lower sides of a write line (write word line or write bit line), and data is to be written in the MTJ element on the upper or lower side of the write line using a magnetic field generated by a write current that flows to the write line, the positional relationship between the pinning layer and the storing layer of each MTJ element is set to be symmetrical with respect to the write line. 
   More specifically, assume that the MTJ element on the lower side of the write line has a storing layer on a side close to the write interconnection and a pinning layer on a side far from the write interconnection. In this case, the MTJ element on the upper side of the write line also has a storing layer on a side close to the write interconnection and a pinning layer on a side far from the write interconnection. 
   Similarly, assume that the MTJ element on the lower side of the write line has a pinning layer on a side close to the write interconnection and a storing layer on a side far from the write interconnection. In this case, the MTJ element on the upper side of the write line also has a pinning layer on a side close to the write interconnection and a storing layer on a side far from the write interconnection. 
   Note that this positional relationship is ensured for all MTJ elements in the memory cell array. In addition, for all write lines in the memory cell array, the MTJ element arranged on the upper side and that arranged on the lower side are symmetrically arranged. 
   With this positional relationship, the distance from a write line to a storing layer is substantially the same for all MTJ elements. That is, since the influence of a magnetic field generated by a write current flowing to a write line due to the write current flowing to the write line is the same for all MTJ elements. Hence, all MTJ elements can have the same write characteristic. 
   In this case, the direction of the MTJ element arranged on the lower (or upper) side of the write line is opposite to the direction of the MTJ element arranged on the upper (or lower) side of the write line. 
   However, that the directions of all the MTJ elements in the memory cell array are not the same, and, for example, the directions of the MTJ elements change for each stage is no disadvantage for the present invention (directions here include only two directions: upward and downward, and the semiconductor substrate side is defined as the lower side). 
   This is because in forming MTJ elements, the directions of the MTJ elements can easily be changed only by changing the order of forming the layers of MTJ elements. 
   (2) Magnetizing Direction of Pinning Layer of MTJ Element 
   When MTJ elements are arranged on the upper and lower sides of a write line (write word line or write bit line), and data is to be written in the MTJ element on the upper or lower side of the write line using a magnetic field generated by a write current that flows to the write line, the write operation principle and read operation principle must be changed depending on the magnetizing direction of the pinning layer of the MTJ element. 
   This is because the direction of a magnetic field applied to an MTJ element arranged on the upper side of a write line is opposite to that of a magnetic field applied to an MTJ element arranged on the lower side of the write line even though the direction of a current that flows to the write line is constant. 
   {circle around (1)} When Magnetizing Directions of Pinning Layers are Individually Set 
   When the magnetizing directions of pinning layers can be individually set, the magnetizing direction of the pinning layer of each MTJ element that is present on the lower side of a write line (write word line or write bit line) is made opposite to that of the pinning layer of each MTJ element that is present on the upper side of the write line. With this arrangement, the normal read operation principle and write operation principle can be applied. 
   That is, a state wherein the magnetizing direction of the pinning layer is the same as that of the storing layer can be defined as “1”. A state wherein the magnetizing direction of the pinning layer is different from that of the storing layer can be defined as “0”. 
   A detailed example will be described below. 
   As a presupposition, the axes of easy magnetization of the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  are directed in the X-direction (a direction in which the write word lines run), as shown in  FIGS. 85 and 86 . In addition, the magnetizing direction of the pinning layer of the MTJ element MTJ 1 - 1  arranged on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is leftward. The magnetizing direction of the pinning layer of the MTJ element MTJ 1 - 2  arranged on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is rightward. 
   Furthermore, write data is determined by the direction of a write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2 . Only a write current directed in one direction flows to the write word lines WWL 1 - 1  and WWL 1 - 2 . 
   When Data Is to Be Written in MTJ Element on Lower Side of Write Bit Line 
   [“1”-Write] 
   As shown in  FIG. 85 , a write current directed in one direction is supplied to the write word line WWL 1 - 1 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current is absorbed in the direction perpendicular to the drawing surface. A magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle clockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a leftward magnetic field is applied to the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is leftward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is parallel, and data “1” is written. 
   [“0”-Write] 
   A write current directed in one direction is supplied to the write word line WWL 1 - 1 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current comes out from the direction perpendicular to the drawing surface. A magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle counterclockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a rightward magnetic field is applied to the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is rightward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is antiparallel, and data “0” is written. 
   When Data Is to Be Written in MTJ Element on Upper Side of Write Bit Line 
   If the same data can be written in the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  under the same write condition as for the MTJ element MTJ 1 - 1 , the write and read operations can be executed for the two MTJ elements MTJ 1 - 1  and MTJ 1 - 2  using the same write circuit (write bit line driver/sinker) and same read circuit. 
   [“1”-Write] 
   As shown in  FIG. 86 , a write current directed in one direction is supplied to the write word line WWL 1 - 2 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current is absorbed in the direction perpendicular to the drawing surface. 
   This write condition is the same as the “1”-write condition for the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . At this time, a magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle clockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a rightward magnetic field is applied to the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is rightward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is parallel, and data “1” is written. 
   As described above, when the magnetizing directions of the pinning layers of the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  are opposite to each other, the same data can be written in the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  under the same write condition. 
   [“0”-Write] 
   A write current directed in one direction is supplied to the write word line WWL 1 - 2 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current comes out from the direction perpendicular to the drawing surface. 
   This write condition is the same as the “0”-write condition for the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . At this time, a magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle counterclockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a leftward magnetic field is applied to the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is leftward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is antiparallel, and data “0” is written. 
   As described above, when the magnetizing directions of the pinning layers of the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  are opposite to each other, the same data can be written in the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  under the same write condition. 
   {circle around (2)} When Pinning Layers of All MTJ Elements Have Same Magnetizing Direction 
   When the pinning layers of all the MTJ elements have the same magnetizing direction, for example, after the wafer process is ended, the magnetizing direction of the pinning layers of all the MTJ elements can be instantaneously determined by simultaneously applying magnetic fields in the same direction to the pinning layers of all the MTJ elements. 
   Especially, when the temperature of the wafer is increased in applying the magnetic field, the magnetizing directions of the pinning layers of all the MTJ elements can easily be determined. 
   In this case, however, identical data cannot be written in the MTJ elements arranged on the lower side of a write line and MTJ elements arranged on the upper side of the write line under the same condition. 
   The following two countermeasures can be used: A. the arrangement of the read circuit is changed without changing the arrangement of the write circuit (write bit line driver/sinker), i.e., the write condition, and B. the arrangement of the write circuit (write bit line driver/sinker), i.e., the write condition is changed without changing the arrangement of the read circuit. 
   A detailed example will be described below. 
   As a presupposition, the axes of easy magnetization of the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  are directed in the X-direction (a direction in which the write word lines run), as shown in  FIGS. 87 and 88 . In addition, both the magnetizing direction of the pinning layer of the MTJ element MTJ 1 - 1  arranged on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  and the magnetizing direction of the pinning layer of the MTJ element MTJ 1 - 2  arranged on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  are leftward. 
   Furthermore, write data is determined by the direction of a write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2 . Only a write current directed in one direction flows to the write word lines WWL 1 - 1  and WWL 1 - 2 . 
   A. When Write Condition Is Not Changed 
   When Data Is to Be Written in MTJ Element on Lower Side of Write Bit Line 
   [“1”-Write] 
   As shown in  FIG. 87 , a write current directed in one direction is supplied to the write word line WWL 1 - 1 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current is absorbed in the direction perpendicular to the drawing surface. A magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle clockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a leftward magnetic field is applied to the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is leftward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is parallel, and data “1” is written. 
   [“0”-Write] 
   A write current directed in one direction is supplied to the write word line WWL 1 - 1 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current comes out from the direction perpendicular to the drawing surface. A magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle counterclockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a rightward magnetic field is applied to the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is rightward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is antiparallel, and data “0” is written. 
   When Data Is to Be Written in MTJ Element on Upper Side of Write Bit Line 
   For the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2 , the write operation is executed using the same write condition, i.e., the same write circuit (write bit line driver/sinker) as that for the MTJ element MTJ 1 - 1 . 
   [“1”-Write] 
   As shown in  FIG. 88 , a write current directed in one direction is supplied to the write word line WWL 1 - 2 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current is absorbed in the direction perpendicular to the drawing surface. 
   This write condition is the same as the “1”-write condition for the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . At this time, a magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle clockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a rightward magnetic field is applied to the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is rightward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is antiparallel, i.e., data “0” is stored. 
   The write data for the MTJ element MTJ 1 - 2  is “1”. Hence, in the read mode, the “0”-data stored in the MTJ element MTJ 1 - 2  must be read out not as “0” but as “1”. 
   To do this, the arrangement of the read circuit is slightly changed. 
   Basically, since write data in an inverted state is stored in the MTJ element that is present on the upper side of the write bit line, one inverter is added to the output section (final stage) of the read circuit for reading the data of the MTJ element that is present on the upper side of the write bit line. 
   For example, in Structural Example 5 (FIG.  36 ), the write bit line WBL 1 - 1 /WBL 1 - 2  is arranged between the MTJ element MTJ 1 - 1  of the first stage and the MTJ element MTJ 1 - 2  of the second stage. For example, when the so-called batch read operation principle is applied, one inverter is added to each of the output sections of the logic circuits for discriminating data. 
   When the pinning layers of the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  have the same magnetizing direction, data opposite to write data is stored in one of the MTJ element arranged on the upper side of the write line and that arranged on the lower side of the write line. 
   Hence, when one inverter is added to the output section (final stage) of the read circuit for reading the data of the MTJ element that stores opposite data, the write operation can be executed without changing the arrangement of the write circuit (write bit line driver/sinker) 
   [“0”-Write] 
   A write current directed in one direction is supplied to the write word line WWL 1 - 2 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current comes out from the direction perpendicular to the drawing surface. 
   This write condition is the same as the “0”-write condition for the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . At this time, a magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle counterclockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a leftward magnetic field is applied to the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is leftward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is parallel, i.e., data “1” is stored. 
   The write data for the MTJ element MTJ 1 - 2  is “0”. Hence, in the read mode, the “1”-data stored in the MTJ element MTJ 1 - 2  must be read out not as “1” but as “0”. 
   When one inverter is added to the output section (final stage) of the read circuit for reading the data of the MTJ element that is present on the upper side of the write bit line, as described above, the data can be read without any problem. 
   B. When Write Condition Is Changed 
   When the write condition is changed, both the states of the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  can be set to parallel when the write data is “1”. When the write data is “0”, both the states of the MTJ elements MTJ 1 - 1  and MTJ 1 - 2  can be set to antiparallel. 
   That is, the read circuit need not be changed. 
   When Data Is to Be Written in MTJ Element on Lower Side of Write Bit Line 
   [“1”-Write] 
   As shown in  FIG. 87 , a write current directed in one direction is supplied to the write word line WWL 1 - 1 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current is absorbed in the direction perpendicular to the drawing surface. A magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle clockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a leftward magnetic field is applied to the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is leftward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is parallel, and data “1” is written. 
   [“0”-Write] 
   A write current directed in one direction is supplied to the write word line WWL 1 - 1 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current comes out from the direction perpendicular to the drawing surface. A magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle counterclockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a rightward magnetic field is applied to the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is rightward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2  is antiparallel, and data “0” is written. 
   When Data Is to Be Written in MTJ Element on Upper Side of Write Bit Line 
   [“1”-Write] 
   As shown in  FIG. 89 , a write current directed in one direction is supplied to the write word line WWL 1 - 2 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current comes out from the direction perpendicular to the drawing surface. 
   This write condition is different from the “1”-write condition for the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . That is, if the write data is the same, the direction of the write current to be supplied to the write line changes depending on whether the MTJ element is present on the upper or lower side of the write line. 
   A write circuit (write bit line driver/sinker) which realizes such operation will be described later. 
   At this time, a magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle counterclockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a leftward magnetic field is applied to the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is leftward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is parallel, i.e., data “1” is stored. 
   [“0”-Write] 
   A write current directed in one direction is supplied to the write word line WWL 1 - 2 . A write current is supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  in a direction in which the current is absorbed in the direction perpendicular to the drawing surface. 
   This write condition is different from the “0”-write condition for the MTJ element MTJ 1 - 1  on the lower side of the write bit line WBL 1 - 1 /WBL 1 - 2 . That is, if the write data is the same, the direction of the write current to be supplied to the write line changes depending on whether the MTJ element is present on the upper or lower side of the write line. 
   At this time, a magnetic field generated by the write current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  forms a circle clockwise about the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In this case, a rightward magnetic field is applied to the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2 . For this reason, the magnetizing direction of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is rightward. 
   Hence, the magnetizing state of the MTJ element MTJ 1 - 2  on the upper side of the write bit line WBL 1 - 1 /WBL 1 - 2  is antiparallel, i.e., data “0” is stored. 
   {circle around (3)} Arrangement of Write Circuit (Write Bit Line Driver/Sinker) When Pinning Layers of All MTJ Elements Have Same Magnetizing Direction 
     FIG. 90  shows a circuit example of the write bit line drivers/sinkers. 
   The circuit shown in  FIG. 90  is applied to the magnetic random access memory according to Structural Example 5 (FIGS.  34  and  35 ). As a characteristic feature, this circuit has a function of changing the direction of write current on the basis of the position information (lower or upper side) of MTJ elements. 
     FIG. 90  shows write bit line drivers/sinkers corresponding to only one column. 
   The write bit line driver/sinker  29 A is formed from the PMOS transistor QP 1 , NMOS transistor QN 1 , NAND gate circuit ND 1 , AND gate circuit AD 1 , exclusive OR circuit Ex-OR 1 , and exclusive NOR circuit Ex-NOR 1 . 
   The write bit line driver/sinker  31  is formed from the PMOS transistor QP 2 , NMOS transistor QN 2 , NAND gate circuit ND 2 , AND gate circuit AD 2 , exclusive OR circuit Ex-OR 2 , and exclusive NOR circuit Ex-NOR 2 . 
   The PMOS transistor QP 1  is connected between the power supply terminal VDD and the write bit line WBL 1 - 1 /WBL 1 - 2 . The NMOS transistor QN 1  is connected between the write bit line WBL 1 - 1 /WBL 1 - 2  and the ground terminal VSS. The PMOS transistor QP 2  is connected between the power supply terminal VDD and the write bit line WBL 1 - 1 /WBL 1 - 2 . The NMOS transistor QN 2  is connected between the write bit line WBL 1 - 1 /WBL 1 - 2  and the ground terminal VSS. 
   When the output signal from the NAND gate circuit ND 1  is “0”, and the output signal from the AND gate circuit AD 2  is “1”, a write current from the write bit line driver/sinker  29 A toward the write bit line driver/sinker  31  flows to the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   When the output signal from the NAND gate circuit ND 2  is “0”, and the output signal from the AND gate circuit AD 1  is “1”, a write current from the write bit line driver/sinker  31  toward the write bit line driver/sinker  29 A flows to the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   In such write bit line drivers/sinkers  29 A and  31 , the write signal WRITE is “1” in the write operation. Additionally, in the selected column, all the upper column address signal bits are “1”. 
   In this example, the direction of write current to be supplied to the write bit line WBL 1 - 1 /WBL 1 - 2  is determined using a select signal ZA 0  for selecting a memory cell array (upper or lower stage). 
   When Write Data Is “1” 
   When write data is “1” (DATA=“1”), the direction of current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  is as follows. 
   When the memory cell array (MTJ elements) of the first stage is selected, ZA 0 =“0”. The output signals from the exclusive OR circuits Ex-OR 1  and Ex-OR 2  are “1”, and the output signals from the exclusive NOR circuits Ex-NOR 1  and Ex-NOR 2  are “0”. 
   Hence, the output signal from the NAND gate circuit ND 1  is “0”, and the output signal from the AND gate circuit AD 2  is “1”. As a result, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   When the memory cell array (MTJ elements) of the second stage is selected, ZA 0 =“1”. The output signals from the exclusive OR circuits Ex-OR 1  and Ex-OR 2  are “0”, and the output signals from the exclusive NOR circuits Ex-NOR 1  and Ex-NOR 2  are “1”. 
   Hence, the output signal from the NAND gate circuit ND 2  is “0”, and the output signal from the AND gate circuit AD 1  is “1”. As a result, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   When Write Data Is “0” 
   When write data is “0” (DATA=“0”), the direction of current flowing to the write bit line WBL 1 - 1 /WBL 1 - 2  is as follows. 
   When the memory cell array (MTJ elements) of the first stage is selected, ZA 0 =“0”. The output signals from the exclusive OR circuits Ex-OR 1  and Ex-OR 2  are “0”, and the output signals from the exclusive NOR circuits Ex-NOR 1  and Ex-NOR 2  are “1”. 
   Hence, the output signal from the NAND gate circuit ND 2  is “0”, and the output signal from the AND gate circuit AD 1  is “1”. As a result, a write current from the write bit line driver/sinker  31  to the write bit line driver/sinker  29 A flows to the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   When the memory cell array (MTJ elements) of the second stage is selected, ZA 0 =“1”. The output signals from the exclusive OR circuits Ex-OR 1  and Ex-OR 2  are “1”, and the output signals from the exclusive NOR circuits Ex-NOR 1  and Ex-NOR 2  are “0”. 
   Hence, the output signal from the NAND gate circuit ND 1  is “0”, and the output signal from the AND gate circuit AD 2  is “1”. As a result, a write current from the write bit line driver/sinker  29 A to the write bit line driver/sinker  31  flows to the write bit line WBL 1 - 1 /WBL 1 - 2 . 
   6. Manufacturing Method 
   The cell array structure, MTJ element structure, read circuit, and read operation principle of the magnetic random access memory of the present invention have been described above. Finally, a manufacturing method for implementing the magnetic random access memory of the present invention will be described. 
   The manufacturing method to be described below is related to Device Structure  2  of Structural Example 1. Device Structures  1  and  3  of Structural Example 1 and Structural Examples 2 to 10 can also easily be formed using the following manufacturing method. 
   (1) Cell Array Structure to Be Manufactured 
   The cell array structure completed by the manufacturing method of the present invention will be briefly described first. Then, the manufacturing method of the cell array structure will be described. 
     FIG. 91  shows a cell array structure including the characteristic feature of Device Structure  2  of Structural Example 1. 
   Element isolation insulating layers  45  having an STI (Shallow Trench Isolation) structure are formed in the semiconductor substrate  41 . Dummy interconnections  46  are formed on the element isolation insulating layers  45 . The dummy interconnections  46  are formed in a periodical pattern (a repeat of a predetermined pattern) or a pattern uniform as a whole. In this example, the dummy interconnections  46  are arranged equidistantly. 
   The dummy interconnections  46  are made of the same material as that of interconnections of peripheral circuits arranged around the memory cell array, e.g., the gate interconnections of MOS transistors. 
   The read word line RWL 1  running in the Y-direction is formed on the dummy interconnections  46 . The four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  arrayed in the Y-direction are arranged on the read word line RWL 1 . 
   One terminal (upper end in this example) of each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is commonly connected to the upper electrode  44 . The contact plug  42  and conductive layer  43  electrically connect the upper electrode  44  to the read word line RWL 1 . 
   The contact portion between the upper electrode  44  and the read word line RWL 1  is formed in the region between the MTJ elements MTJ 1  and MTJ 2  and the MTJ elements MTJ 3  and MTJ 4 . When the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are uniformly arranged to be symmetrical with respect to the contact portion, signal margin in the read operation due to the interconnection resistance or the like can be maximized. 
   The conductive layer  43  may be integrated with the upper electrode  44 . That is, the conductive layer  43  and upper electrode  44  may be formed simultaneously using the same material. 
   The other terminal (lower end in this example) of each of the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  is electrically connected to a corresponding one of the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  (write word lines WWL 1 , WWL 2 , WWL 3 , and WWL 4 ). The read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  run in the X-direction (row direction). 
   The MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4  are independently connected to the read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4 , respectively. That is, the four read bit lines RBL 1 , RBL 2 , RBL 3 , and RBL 4  are arranged in correspondence with the four MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . 
   The write bit line WBL 1  is formed immediately on and near the MTJ elements MTJ 1 , MTJ 2 , MTJ 3 , and MTJ 4 . The write bit line WBL 1  runs in the Y-direction. 
   (2) Steps in Manufacturing Method 
   The manufacturing method for implementing the cell array structure shown in  FIG. 91  will be described below. A detailed manufacturing method (e.g., employment of a dual damascene process) will be described here. Hence, note that elements that are not illustrated in the cell array structure of  FIG. 91  will be mentioned. However, the outline of the finally completed cell array structure is almost the same as that shown in FIG.  91 . 
   [1] Element Isolation Step 
   First, as shown in  FIG. 92 , an element isolation insulating layer  52  having an STI (Shallow Trench Isolation) structure is formed in a semiconductor substrate  51 . 
   The element isolation insulating layer  52  can be formed by, e.g., the following process. 
   A mask pattern (e.g., a silicon nitride film) is formed on the semiconductor substrate  51  by PEP (Photo Engraving Process). The semiconductor substrate  51  is etched by RIE (Reactive Ion Etching) using the mask pattern as a mask to form a trench in the semiconductor substrate  51 . This trench is filled with an insulating layer (e.g., a silicon oxide layer) using, e.g., CVD (Chemical Vapor Deposition) and CMP (Chemical Mechanical Polishing). 
   After that, a p-type impurity (e.g., B or BF 2 ) or an n-type impurity (e.g., P or As) is doped into the semiconductor substrate by, e.g., ion implantation, as needed, to form a p-type well region or an n-type well region. 
   [2] MOSFET Forming Step 
   Next, as shown in  FIG. 93 , a MOS transistor functioning as a read select switch is formed on the surface region of the semiconductor substrate  51 . 
   Dummy interconnections are formed in the memory cell array region simultaneously when the MOS transistor is formed (FIG.  95 ). 
   The MOS transistor can be formed by, e.g., the following process. 
   An impurity for controlling the threshold value of the MOS transistor is ion-implanted into the channel portion in the element region surrounded by the element isolation insulating layer  52 . A gate insulating film (e.g., a silicon oxide film)  53  is formed in the element region by thermal oxidation. A gate electrode material (e.g., polysilicon containing an impurity) and cap insulating film (e.g., a silicon nitride film)  55  are formed on the gate insulating film  53  by CVD. 
   The cap insulating film  55  is patterned by PEP. Then, the gate electrode material and gate insulating film  53  are processed (etched) by RIE using the cap insulating film  55  as a mask. As a consequence, gate electrodes  54  running in the X-direction are formed on the semiconductor substrate  51 . 
   A p- or n-type impurity is doped into the semiconductor substrate  51  by ion implantation using the cap insulating film  55  and gate electrodes  54  as a mask. Lightly-doped impurity regions (LDD regions or extension regions) are formed in the semiconductor substrate. 
   An insulating film (e.g., a silicon nitride film) is formed on the entire surface of the semiconductor substrate  51  by CVD. After that, the insulating film is etched by RIE to form sidewall insulating layers  57  on the side surfaces of the gate electrodes  54  and cap insulating films  55 . A p- or n-type impurity is doped into the semiconductor substrate  51  by ion implantation using the cap insulating films  55 , gate electrodes  54 , and sidewall insulating layers  57  as a mask. As a result, source regions  56 A and drain regions  56 B are formed in the semiconductor substrate  51 . 
   After that, an interlayer dielectric film (e.g., a silicon oxide layer)  58  that completely covers the MOS transistor is formed on the entire surface of the semiconductor substrate  51  by CVD. In addition, the surface of the interlayer dielectric film  58  is planarized by CMP. 
   [3] Contact Hole Forming Step 
   Next, as shown in  FIG. 94 , contact holes  59  that reach the source regions  56 A and drain regions  56 B of MOS transistors are formed in the interlayer dielectric film  58  on the semiconductor substrate  51 . 
   The contact holes  59  can easily be formed by, e.g., forming a resist pattern on the interlayer dielectric film  58  by PEP and etching the interlayer dielectric film  58  by RIE using the resist pattern as a mark. After etching, the resist pattern is removed. 
   [4] Interconnection Trench &amp; First Interconnection Layer Forming Step 
   As shown in  FIGS. 95 and 96 , interconnection trenches  60  are formed in the interlayer dielectric film  58  on the semiconductor substrate  51 . In the memory cell array region, the interconnection trenches  60  are trenches in which read word lines should be formed and run in, e.g., the Y-direction. The interconnection trenches  60  are indicated by broken lines in  FIGS. 95 and 96 . 
   The interconnection trenches  60  can easily be formed by, e.g., forming a resist pattern on the interlayer dielectric film  58  by PEP and etching the interlayer dielectric film  58  by RIE using the resist pattern as a mark. After etching, the resist pattern is removed. 
   As shown in  FIGS. 96 and 97 , a barrier metal layer (e.g., a multi-layer of Ti and TiN)  61  is formed on the interlayer dielectric film  58 , the inner surfaces of the contact holes  59 , and the inner surfaces of the interconnection trenches  60  by, e.g., sputtering. Subsequently, a metal layer (e.g., a W layer)  62  that completely fills the contact holes  59  and interconnection trenches  60  is formed on the barrier metal layer  61  by, e.g., sputtering. 
   After that, the metal layer  62  is polished by, e.g., CMP and left only in the contact holes  59  and interconnection trenches  60 . The metal layer  62  remaining in each contact hole  59  forms a contact plug. 
   As shown in  FIG. 98 , the metal layer  62  remaining in each interconnection trench  60  forms a first interconnection layer (read word line). 
   As shown in  FIG. 99 , an interlayer dielectric film (e.g., a silicon oxide layer)  63  is formed on the interlayer dielectric film  58  by CVD. 
   The step comprising the contact hole forming step, the interconnection trench forming step, and the first interconnection layer forming step is called a dual damascene process. 
   [5] Interconnection Trench Forming Step 
   Next, as shown in  FIG. 100 , interconnection trenches  64  are formed in the interlayer dielectric film  63 . In this example, the interconnection trenches  64  serve as trenches used to form write word lines (read bit lines) and run in the X-direction. Sidewall insulating layers (e.g., silicon nitride layers) for increasing the insulating function may be formed on the side surfaces of the interconnection trenches  64 . 
   The interconnection trenches  64  can easily be formed by, e.g., forming a resist pattern on the interlayer dielectric film  63  by PEP and etching the interlayer dielectric film  63  by RIE using the resist pattern as a mask. After etching, the resist pattern is removed. 
   The sidewall insulating layers can easily be formed by forming an insulating film (e.g., a silicon nitride film) on the entire surface of the interlayer dielectric film  63  by CVD and etching the insulating film by RIE. 
   [6] Second Interconnection Layer Forming Step 
   Next, as shown in  FIG. 101 , a contact hole  65  that reaches the metal layer  62  serving as the read word line is formed in the interconnection trench  64 . 
   The contact hole  65  can easily be formed by, e.g., forming a resist pattern on the interlayer dielectric film  63  by PEP and etching the interlayer dielectric film  63  by RIE using the resist pattern as a mask. After etching, the resist pattern is removed. 
   After that, a barrier metal layer (e.g., a multi-layer of Ta and TaN)  66  is formed on the interlayer dielectric film  63  and the inner surfaces of the interconnection trenches  64  and contact hole  65  by, e.g., sputtering. Subsequently, a metal layer (e.g., a Cu layer)  67  that completely fills the interconnection trenches  64  and contact hole  65  is formed on the barrier metal layer  66  by, e.g., sputtering. 
   After that, the metal layer  67  is polished by, e.g., CMP and left only in the interconnection trenches  64  and contact hole  65 . The metal layer  67  remaining in each interconnection trench  64  forms a second interconnection layer that functions as a write word line (read bit line). The metal layer  67  remaining in the contact hole  65  forms a contact plug. 
   [7] MTJ Element &amp; Lower Electrode Forming Step 
   As shown in  FIG. 102 , a lower electrode (e.g., a Ta layer)  68  is formed on the interlayer dielectric film  63  by sputtering. Subsequently, a plurality of layers  69  for MTJ elements are formed on the lower electrode  68 . The plurality of layers  69  include, e.g., a tunneling barrier layer, two ferromagnetic layers that sandwich the tunneling barrier layer, and an antiferromagnetic layer. 
   After that, as shown in  FIG. 103 , the plurality of layers  69  for MTJ elements are patterned to form a plurality of MTJ elements  69 A on the lower electrode  68 . Each of the e plurality of MTJ elements  69 A finally has the structure shown in, e.g.,  FIG. 61 ,  62 , or  63 . 
   The plurality of layers  69  for MTJ elements can easily be patterned by forming a resist pattern on the plurality of layers  69  by PEP and etching the plurality of layers  69  by RIE using the resist pattern as a mask. After etching, the resist pattern is removed. 
   Subsequently, the lower electrode  68  for the MTJ elements is patterned. 
   The lower electrode  68  can easily be patterned by forming a resist pattern on the lower electrode  68  by PEP and etching the lower electrode  68  by RIE using the resist pattern as a mask. After etching, the resist pattern is removed. 
   After that, an interlayer dielectric film  70  that completely covers the MTJ elements  69 A is formed by CVD. In addition, the interlayer dielectric film  70  is polished and planarized by CMP and left only between the MTJ elements  69 A. 
   [8] Step of Forming Upper Electrode for MTJ Elements 
   As shown in  FIG. 104 , a contact hole that reaches the metal layer  67  serving as a contact plug is formed in the interlayer dielectric film  70 . 
   The contact hole can easily be formed by, e.g., forming a resist pattern on the interlayer dielectric film  70  by PEP and etching the interlayer dielectric film  70  by RIE using the resist pattern as a mask. After etching, the resist pattern is removed. 
   After that, a metal layer (e.g., a Ta layer)  71  as the upper electrode for the MTJ elements  69 A is formed on the MTJ elements  69 A and interlayer dielectric film  70  by sputtering such that the contact hole is completely filled. In addition, the metal layer  71  is polished by CMP to planarize the surface of the metal layer  71 . 
   The upper electrode  71  for the MTJ elements  69 A is patterned. 
   The upper electrode  71  for the MTJ elements  69 A can easily be patterned by, e.g., forming a resist pattern on the upper electrode  71  by PEP and etching the upper electrode  71  by RIE using the resist pattern as a mask. After etching, the resist pattern is removed. 
   With this patterning, the upper electrode  71  may be formed for each read block, as in Structural Example 1. Alternatively, the upper electrode  71  may be shared by read blocks in one column, as in Structural Example 10. 
   [9] Third Interconnection Layer Forming Step 
   Next, as shown in  FIG. 105 , an interlayer dielectric film  72  which completely covers the upper electrode  71  for the MTJ elements  69 A is formed on the interlayer dielectric film  70  by CVD. In addition, the interlayer dielectric film  72  is polished by CMP to planarize the surface of the interlayer dielectric film  72 . 
   Interconnection trenches are formed in the interlayer dielectric film  72 . The interconnection trenches are trenches in which write bit lines are to be formed and run in the Y-direction. Sidewall insulating layers (e.g., silicon nitride layers) for increasing the insulating function may be formed on the side surfaces of the interconnection trenches. 
   The interconnection trenches can easily be formed by, e.g., forming a resist pattern on the interlayer dielectric film  72  by PEP and etching the interlayer dielectric film  72  by RIE using the resist pattern as a mask. After etching, the resist pattern is removed. 
   The sidewall insulating layers can easily be formed by forming an insulating film (e.g., a silicon nitride film) on the entire interlayer dielectric film  72  by CVD and etching the insulating layer by RIE. 
   After that, a barrier metal layer (e.g., a multi-layer of Ta and TaN)  73  is formed on the interlayer dielectric film  72  and inner surfaces of the interconnection trenches by, e.g., sputtering. Subsequently, a metal layer (e.g., a Cu layer)  74  that completely fills the interconnection trenches is formed on the barrier metal layer  73  by, e.g., sputtering. 
   The metal layer  74  is polished by, e.g., CMP and left only in the interconnection trenches. The metal layer  74  remaining in each interconnection trench forms a third interconnection layer that functions as a write bit line. 
     FIG. 106  shows the final structure in which the upper electrode  71  is shared by read block in one column. 
   (3) Conclusion 
   According to this manufacturing method, a cell array structure in which no read select switch is connected between a read word line and one terminal of an MR element in a read block, and one of two write lines is not in contact with the MTJ element can be implemented. 
   No select switch (MOS transistor) is formed immediately under the MTJ element. Instead, for example, a plurality of dummy interconnections are equidistantly formed. For this reason, the interlayer dielectric film can be planarized, and the characteristics of the MTJ element can be improved. 
   In this example, to form an interconnection layer, a damascene process and dual damascene process are employed. Instead, for example, a process of forming an interconnection layer by etching may be employed. 
   7. Others 
   Application examples of Structural Example 8 shown in  FIG. 58  will be described briefly. 
   As a characteristic feature of the example shown in  FIG. 58 , the bias voltage VC is applied to the selected read word line RWLi in the read mode. The following modifications can also be made for the MRAM having this characteristic feature. 
   As a characteristic feature of an example shown in  FIG. 108 , the read circuit  29 B is connected to the read word lines RWL 1 , . . . , RWLi. The column select switch CSW is connected between the read circuit  29 B and the read word lines RWLi, . . . , RWLi. One bias circuit BIAS which generates the bias voltage VC is arranged in correspondence with one of the read word lines RWL 1 , . . . , RWLi. In the read operation, the row decoders  25 - 1 , . . . ,  25 - n  select one read bit line RBLi. The selected read bit line RBLi is connected to the ground point through the read bit line sinker  23 A. All the remaining unselected read bit lines are set in the floating state. 
   An MRAM shown in  FIG. 109  includes the characteristic feature of the MRAM shown in FIG.  108 . As a characteristic feature of the example shown in  FIG. 109 , a plurality of read circuits  29 B are present, and a plurality of bit data can simultaneously be read out from a plurality of memory cells (MTJ elements) by read operation of one cycle, unlike the example shown in FIG.  108 . 
   An MRAM shown in  FIG. 110  includes the characteristic feature of the MRAM shown in FIG.  108 . As a characteristic feature of the example shown in  FIG. 110 , a plurality of read circuits  29 B are present, and a plurality of bit data can simultaneously be read out from a plurality of memory cells (MTJ elements) by read operation of one cycle, unlike the example shown in FIG.  108 . 
   In the above description, a MTJ element is used as a memory cell of the magnetic random access memory. However, even when the memory cell is formed from a GMR (Giant MagnetoResistance) element or CMR (Colossal MagnetoResistance) element, the present invention, i.e., various kinds of cell array structures, the read operation principle, and the detailed example of the read circuit can be applied. 
   The structure of a MTJ element, GMR element, or CMR element and the materials thereof are not particularly limited in applying the present invention. In this example, the number of MTJ elements in one read block is four. However, the number of MTJ elements in one read block is not limited to four and can freely be set. 
   As a switch element such as the row/column select switch of the magnetic random access memory, a bipolar transistor, diode, MIS (Metal Insulator Semiconductor) transistor (including a MOSFET), MES (Metal Semiconductor) transistor, or junction transistor can be used. 
   As has been described above, according to the present invention, a magnetic random access memory having a cell array structure which can implement an increase in memory capacity without forming any select switch in a read block and also prevent the MTJ elements from breaking in a write mode can be provided. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.