Patent Publication Number: US-6909130-B2

Title: Magnetic random access memory device having high-heat disturbance resistance and high write efficiency

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-98407, filed Apr. 1, 2003, 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) device, and more particularly to a structure of a memory cell in an MRAM device using magnetic memory cells which store data by the tunneling magnetoresistive effect. 
     2. Description of the Related Art 
     In recent years, there have been proposed many memory devices which store information based on a new principle. As one of such memory device, an MRAM device having both the non-volatility and the rapidity in which a plurality of memory cells including magnetic tunnel junction elements (which will be referred to as MTJ elements hereinafter) having a tunneling magnetoresistive effect are arranged in a matrix form is disclosed in, e.g., 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 pp. 128 to pp. 129. 
     The MTJ element has two magnetic layers which are generally referred as a recording layer and a fixed layer. When programming data in the MTJ element, a current is caused to flow through a write wiring, and a magnetic field in a predetermined direction is applied to the MTJ element, thereby changing the direction of magnetization of the recording layer. 
     Meanwhile, the most serious problem in the MRAM device is a reduction in a write current. The present inventors found that assuring a thermal stability of recorded information is an important problem as a result of an experiment of holding the reliability of the MTJ element. This prehistory will now be described hereinafter. 
     Under the present situation, a write current value of the MTJ element is as large as 8 to 10 mA. For a practical application, the write current value must be lowered to an allowable level. In the case of a test chip of the MRAM device on a 1K bit level manufactured by the present inventors by way of trial, the current write value is 8 to 10 mA as was expected. 
     Further, bit information retention characteristics of the MTJ element were examined. As a result, irrespective of a fact that criteria Ku×V/kB×T of the thermal stability of the recorded information which are usually considered in a magnetic medium of a hard disk storage apparatus are set to be not less than 80, some bit information were reversed. Here, V is a cubic volume of a recording layer of the MTJ element, kB is the Boltzmann constant, and T is an absolute temperature. In case of the MRAM device, Ku is given mainly based on a shape magnetic anisotropy as a general rule, and it is actually a sum of an anisotropic energy and an induced magnetic anisotropy. 
     For improving the thermal stability of the recorded bit information in order to prevent the bit information from being switched, Ku×U is usually set large. By doing so, however, the write current is increased. 
     In the MRAM device, it is desirable to achieve both a reduction in the write current and an assurance of the thermal stability of the recorded information as described above. In the prior art, however, a concrete design plan for this purpose is not proposed. The prehistory that this problem was found will now be described in detail hereinafter. 
     At present, a reported write current value of the MTJ element is at least approximately 8 mA if a cell width is approximately 0.6 μm and a cell length is approximately 1.2 μm. 
     Usually, a shape of a flat surface of the MTJ element is determined as a rectangular or an ellipse, the shape magnetic anisotropy is given to the MTJ element, a direction of magnetization of the MTJ element is stipulated, and the thermal stability of the recorded information is also given. 
     Ku×V is a product of a sum of the shape magnetic anisotropy and the induced magnetic anisotropy of the MTJ element, and a volume of the recording layer of the MTJ element. Here, the induced magnetic anisotropy of the recording layer is given in the same direction as that of the anisotropy based on a shape so as not to generate the dispersion of the anisotropy or the like. However, usually, NiFe used as a material of the recording layer has the induced magnetic anisotropy (several Oe) smaller than the anisotropic magnetic field based on a shape by a single digit, and it is considered that the thermal stability of the recorded information and the switching magnetic field are also substantially determined by the shape magnetic anisotropy. 
     The switching magnetic field Hsw required to rewrite magnetization information of the recording layer is substantially given by the following expression (1).
 
 Hsw= 4 π×Ms×t/F ( Oe )  (1)
 
     Here, Ms is a saturation magnetization of the recording layer, t is a thickness of the recording layer, and F is a width of the recording layer. Further, a sum Ku of the anisotropic energy based on a shape and the induced magnetic anisotropy is substantially given by the following expression (2).
 
 Ku=Hsw×Ms/ 2  (2)
 
     As a method for reducing the write current, coating a conventional write wiring made of, e.g., Cu with a soft magnetic material such as NiFe and using it as a write wiring with a yoke is proposed in, e.g., Saied Tehrani, “Magneto resistive RAM”, 2001 IEDM short course. According to this method, the approximately twofold high-efficiency effect, i.e., the write current value can be reduced to approximately ½. 
       FIG. 1  shows an example of a structure of the write wiring with a yoke described in the above cited reference (“Magneto resistive RAM”), and  FIG. 2  shows a result of examining write characteristics obtained by using the write wiring illustrated in FIG.  1 . As shown in  FIG. 1 , the write wiring with a yoke  10  has a structure that a part of the periphery of a write wiring  11  made of Cu is coated with a yoke  12  made of a soft magnetic material such as NiFe. 
     In  FIG. 2 , characteristics A indicated by a solid line show a state that a width F of a recording layer is reduced and a switching magnetic field Hsw is increased as minuteness of an MTJ element is realized when a CoFeNi thin film having a film thickness of 2 nm is used as the recording layer. It is to be noted that characteristics B show a generated magnetic field when a conventional write wiring without a yoke is used, and characteristics C show a generated magnetic field when a write wiring with a yoke is used. 
     In case of using the conventional write wiring (characteristics B), since the generated magnetic field is larger than the switching magnetic field until 1/F is approximately 7, writing is possible. On the other hand, in case of using the conventional write wiring with a yoke (characteristics C), since the generated magnetic field is larger than the switching magnetic field even if 1/F exceeds approximately 7, writing is possible, but the generated magnetic field is smaller than the switching magnetic field when 1/F exceeds approximately 10. 
     As a result of examining the case that write wiring with a yoke formed by a prior art is used based on an experiment and a computer simulation, the approximately twofold high-efficiency effect was confirmed, and the write current can be reduced to 5 mA. However, this is the limit, and it is far from 1 to 2 mA which is a target value required for a practical application. 
     Furthermore, as a result of performing writing at a high speed by using a write current with a short pulse width of approximately 50 nsec, irregularities are generated in the required write current value, there is acquired only the reproducibility which is far below the reproducibility 90% obtained when writing is performed with a fixed write current. 
     On the other hand, even if Ku×V/kB×T of the recording layer is set to be not less than 80, some bit information is reversed. Although a cause is uncertain, the thermal stability of the recorded information is not determined by Ku×V, particles constituting the recording layer in a given defective cell undergo the thermal disturbance, i.e., Kcrysta×Vgrain (Kcrysta is a crystal magnetic anisotropy of a recording layer material, and Vgrain is a cubic volume of particles constituting the recording layer), and this becomes a factor of switching of the magnetization information of the cell. That is, it can be considered that there is a cell that Kcry×v (Kcry is a crystal magnetic anisotropic energy, and v is a cubic volume of one particle) determines the thermal stability of the recorded information. 
       FIG. 3  shows an example of an arrangement relationship between the conventional write wiring with a yoke and the MTJ element. 
     The write wiring with a yoke  10  has a structure that three surfaces of the write wiring  11 , i.e., a bottom surface and both side surfaces are covered with a yoke  12  made of a magnetic material such as NiFe. Such a write wiring with a yoke  10  can generate a large magnetic field by using the same write current as compared with the regular write wiring without a yoke. 
     An MTJ element  20  has a structure that a non-magnetic layer  23  is sandwiched between a recording layer  21  and a fixed layer  22  each made of a magnetic layer. The fixed layer  22  is connected to a bit line (BL)  24 . 
       FIG. 4  shows an example of a relationship between a distance from a yoke edge and a generated magnetic field when the write current is caused to flow through the write wiring  10  with the yoke depicted in FIG.  3 . According to this magnetic field distribution, a certain degree of a large magnetic field is generated in the vicinity of the yoke edge. However, as distanced from the yoke edge, the magnetic field suddenly becomes small. 
     In order to solve such a problem, although using a material having a high crystal magnetic anisotropy in the recording layer was tried, it was revealed that the good outcome cannot be obtained by only using this material as a result of the experiment. 
     First, as the recording layer  21  in which information is written by the write wiring with the yoke  10  depicted in  FIG. 3 , a use of a soft magnetic material such as NiFe having the crystal magnetic anisotropy of approximately 10 3  erg/cc can be considered. In this case, a magnetic flux in the vicinity of the yoke edge is led to a central portion of the recording layer  21  by NiFe, and a large magnetic flux is led even to the center of the recording layer  21 . In an experimental result, a magnetization direction in the recording layer  21  was able to be switched with a small write current value of approximately 2 mA. This can be considered because NiFe has a high magnetic permeability. It is to be noted that the shape magnetic anisotropy is approximately 10 4  erg/cc in this case. 
     However, as a result of examining data retention characteristics of the recording layer  21 , a cell whose magnetization direction is switched was found, and it was revealed that there is a cell that the data retention characteristics of approximately 10 years cannot be expected. That can be considered because the magnetization direction of the recording layer  21  is switched due to reversal of the magnetization direction of magnetic material particles (NiFe particles)  31  at a less yoked cell central portion (central portion of the recording layer  21 ) from the shape anisotropy by the thermal disturbance, as shown in FIG.  5 . 
     Thus, as the recording layer  21  in which information is written by the write wiring with the yoke  10  depicted in  FIG. 3 , a use of, e.g., a Co-based magnetic material having a larger crystal magnetic anisotropy can be considered. As a result of performing an experiment about a use of, e.g., CoNiFe having both the shape magnetic anisotropy and the crystal magnetic anisotropy being approximately 10 4  erg/cc as a material of the recording layer  21 , the write current value used to switch the magnetization direction became as large as 15 mA, irregularities occurred in the write current for each cell, and the largest write current value exceeded 40 mA. 
     When CoNiFe was used, it was predicted that the write current value becomes approximately twofold of that when a NiFe-based material was used. However, the 7.5-fold write current is actually required as an average value, and a result was far from a reduction in the write current. 
     Moreover, since the magnetic flux in the vicinity of the yoke edge cannot be led to the center of the recording layer and the magnetization direction of the recording layer cannot be switched even if the magnetic field of the yoke edge is the same, a very large write current is required. That can be considered because the magnetic permeability of the Co-based material is not high. Additionally, as shown in  FIG. 6 , since irregularities are generated in directions of a crystal  32  of CoNiFe, i.e., since the crystal magnetic anisotropy due to a difference in crystal orientation in a film plane generates irregularities in intensity of the switching magnetic field for each cell in case of the Co-based material, it is estimated that great irregularities occur in the write current value for each cell. 
     Specifically, although the magnetic material is crystal-orientated in the vertical direction relative to the film plane, it cannot help becoming irregular in the film plane, and the crystal magnetic anisotropy competes with the shape magnetic anisotropy or the induced magnetic anisotropy given in a fixed direction of the recording layer. It is to be noted that an arrow in  FIG. 6  indicates a direction of the shape magnetic anisotropy. 
     There are a cell that a direction of the shape magnetic anisotropy or the induced magnetic anisotropy and a direction of a crystal magnetic anisotropy are relatively aligned and a cell that such directions are not aligned, which becomes a factor of irregularities in the switching magnetic field for each cell. In particular, when a size of a cell is approximately 0.1 μm, the number of particles of the recording layer constituting one cell is approximately 10, which results in a further serious problem. It is to be noted that such a phenomenon is not observed in a NiFe-based material with the small crystal magnetic anisotropy, and hence no problem occurs. 
     To sum up, in order to assure the thermal stability of the recorded information, if the idea of using a material with the high crystal magnetic anisotropy to the recording layer is actually applied to the MRAM device, the write current value becomes large beyond expectation, and large irregularities are generated in the switching magnetic field. Therefore, there is demanded an MRAM device which can achieve both a reduction in the write current value and an assurance of the thermal stability of bit information, and has the high thermal stability and the high write efficiency. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a magnetic random access memory device comprises: a magnetoresistive element which stores data in accordance with a size of a resistance value between first and second magnetic layers which varies in response to magnetization arrangement states of the first and second magnetic layers, a non-magnetic layer being sandwiched between the first and second magnetic layers; a write wiring which is arranged in the vicinity of the first magnetic layer, generates an induced magnetic flux when a write current flows therethrough, and writes data in the magnetoresistive element by changing a magnetization direction in the first magnetic layer when the induced magnetic flux is applied to the magnetoresistive element; and a third magnetic layer provided so as to cover at least a peripheral surface of the write wiring including a first part of the write wiring opposed to the first magnetic layer in the peripheral surfaces of the write wiring, wherein the first magnetic layer has a crystal magnetic anisotropy exceeding 10 4  erg/cc, the first magnetic layer is exchange-coupled with the third magnetic layer, and a sum of a magnetic volume of a second part of the third magnetic layer opposed to the first part of the write wiring and that of the first magnetic layer is smaller than a magnetic volume of the third magnetic layer at parts other than the second part. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a perspective view showing an example of a structure of a conventional write wiring with a yoke; 
         FIG. 2  is a characteristic view showing write characteristics of a cell using the write wiring with a yoke depicted in  FIG. 1 ; 
         FIG. 3  is a perspective view showing a conventional MTJ element having the write wiring with a yoke; 
         FIG. 4  is a view showing an example of a magnetic field distribution generated by the write wiring with a yoke depicted in  FIG. 3 ; 
         FIG. 5  is a view showing a state of magnetic material particles when NiFe is used as a recording layer of the MTJ element depicted in  FIG. 3 ; 
         FIG. 6  is a view showing a state of a crystal when CoNiFe is used as the recording layer of the MTJ element depicted in  FIG. 3 ; 
         FIG. 7  is a cross-sectional view schematically showing a general structure of the MTJ element used in an MRAM device; 
         FIGS. 8A and 8B  are views showing directions of magnetization of two magnetic layers in the MTJ element depicted in  FIG. 7 ; 
         FIG. 9  is a view typically showing an example of a plane layout of a cell array of the MRAM device; 
         FIG. 10  is a cross-sectional view taken along the line IX—IX in  FIG. 9 ; 
         FIG. 11  is a cross-sectional view taken along the line X—X depicted in  FIG. 9 ; 
         FIG. 12  is a TMR characteristic curve of the MTJ element depicted in  FIG. 7 ; 
         FIG. 13  is a characteristic view showing an asteroid curve of the MTJ element depicted in  FIG. 7 ; 
         FIG. 14  is a cross-sectional view typically showing a memory cell used in an MRAM device according to a first embodiment of the present invention; 
         FIG. 15  is a characteristic view showing a relationship between a magnetic volume ratio of the MRAM device according to the first embodiment, an intensity of a write current and a write pass ratio; 
         FIG. 16  is a cross-sectional view typically showing a memory cell of an MRAM device according to a second embodiment of the present invention; 
         FIG. 17  is a cross-sectional view typically showing a memory cell of an MRAM device according to a third embodiment of the present invention; 
         FIG. 18  is a cross-sectional view typically showing a memory cell of an MRAM device according to a fourth embodiment of the present invention; 
         FIG. 19  is a block diagram showing a DLS data path part of a digital subscriber line modem as an application example 1 of the MRAM device; 
         FIG. 20  is a block diagram showing a part which realizes a communication function in a cellphone terminal as an application example 2 of the MRAM device; 
         FIG. 21  is a top view showing an example that the MRAM device is applied to an MRAM card; 
         FIG. 22  is a top view showing an insertion type data transfer device as an example of an electronic device which uses the MRAM card; 
         FIG. 23  is a cross-sectional view corresponding to  FIG. 22 ; 
         FIG. 24  is a cross-sectional view showing a fitting type data transfer device as another example of the electronic device; and 
         FIG. 25  is a cross-sectional view showing a slide type data transfer device as still another example of the electronic device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before explaining embodiments according to the present invention, an MTJ element used in an MRAM device according to the embodiments of the present invention will be first described.  FIG. 7  schematically shows a cross-sectional structure of the MTJ element. 
     The MTJ element  20  has a structure in which one non-magnetic layer (tunnel barrier film)  23  is sandwiched between two magnetic layers  21  and  22  each made of a ferromagnetic layer or a ferromagnetic material film. The MTJ element stores a logic “1” level data or level “0” data depending on whether directions of magnetization of the two magnetic layers  21  and  22  are parallel or anti-parallel. 
     An anti-ferromagnetic layer  25  is arranged on the magnetic layer  22 . The anti-ferromagnetic layer  25  is a member used to change only a direction of magnetization of the magnetic layer  21  by fixing a direction of magnetization of the magnetic layer  22 , and thereby readily rewrite data. Here, the magnetic layer  22  whose direction of magnetization is fixed is referred to as a fixed layer or a pinned layer, and the magnetic layer  21  whose direction of magnetization is variable is referred to as a free layer or a recording layer. 
       FIGS. 8A and 8B  show two states of directions of magnetization of the two magnetic layers  21  and  22  of the MTJ element  20  depicted in FIG.  7 . 
     As shown in  FIG. 8A , when the directions (directions of arrows in the drawing) of magnetization of the two magnetic layers  21  and  22  are parallel (same), a tunnel resistance of the tunnel barrier film  23  sandwiched between the two magnetic layers  21  and  22  is lowest. In this case, a tunnel current is largest. 
     As shown in  FIG. 8B , when the directions of magnetization of the two magnetic layers  21  and  22  are anti-parallel, the tunnel resistance of the tunnel barrier film  23  sandwiched between the two magnetic layers  21  and  22  is highest. In this case, the tunnel current becomes minimum. 
     In the MRAM device, the two states that the MTJ elements have different resistance values are associated with a data storage state on the logic “1” level (state “1”) and a data storage state on the logic “0” level (state “0”). 
       FIG. 9  typically shows an example of a plane layout of a cell array of the MRAM device constituted by two-dimensionally arranging a plurality of MTJ elements in a row direction and a column direction. 
     A plurality of write/read bit lines BL and a plurality of write word lines WWL are arranged in directions orthogonal to each other. Further, each MTJ element  20  is arranged in accordance with an intersection between each bit line BL and each write word line WWL. To each MTJ element  20  is given a magnetization direction along a direction of a long side of a rectangular in such a manner that the long side of the rectangular is parallel with the write word line WWL and a short side of the same is parallel with the bit line BL. Each bit line BL is electrically connected to one magnetic layer (reference numeral  21  or  22  in  FIG. 7 ) of each of a plurality of the MTJ elements  20  in the same row (or column). Each write word line WWL is arranged so as to be opposed to the other magnetic layer (reference numeral  22  or  21  in  FIG. 1 ) of each of a plurality of the MTJ elements in the same column (or row) in close proximity. 
       FIGS. 10 and 11  show cross-sectional structures in different directions when attention is paid to one memory cell in cases where a memory cell is constituted by connecting an NMOSFET as a read cell selection switch element to each MTJ element  20  depicted in FIG.  9 . 
     In  FIGS. 10 and 11 , reference numeral  40  denotes a semiconductor substrate (e.g., a P-type Si substrate);  41 , a shallow trench type isolation region (STI);  42 , a gate oxide film;  43 , an impurity diffusion region (N + ) as a drain or a source region of a read cell selection transistor (NMOSFET);  44 , a gate electrode (GC);  45 , a first metal wiring layer (M 1 );  46 , a second metal wiring layer (M 2 );  47 , an MTJ connection wiring made of a third metal wiring layer (M 3 );  48 , an electro-conductive contact used to electrically connect the first metal wiring layer  45  to the diffusion layer  43 ;  49 , a contact used to electrically connect the second metal wiring layer  46  to the first metal wiring layer  45 ;  50 , a contact used to electrically connect the third metal wiring layer  47  to the second metal wiring layer  46 ;  20 , an MTJ element;  24 , a fourth wiring layer (M 4 );  51 , a contact used to electrically connect the fourth metal wiring layer  24  to the MTJ element  20 ; and  52 , an interlayer insulating film. 
     It is to be noted that BL denotes a write/read bit line; WWL, a write word line; SL, a source line; and RWL, a read word line as use application of wirings, and the source line SL is connected to a ground potential. 
     An operation principle of the MTJ element will now be described with reference to  FIGS. 9  to  11 . 
     Writing with respect to the MTJ element  20  is carried out by causing a current to flow through the write word line WWL and the bit line BL and setting a direction of magnetization of the MTJ element  20  to be parallel or anti-parallel by using a combined magnetic field formed by currents flowing through the both wirings. 
     That is, when writing data to the MTJ element  20 , a magnetic field Hx is generated by causing a current set to a first direction or a second direction opposite to the first direction to flow through the bit line BL in accordance with data, and a magnetic field Hy is generated by causing only a current set to a fixed direction to flow through the write word line WWL, thereby writing information by using the combined magnetic field. At this time, when the current set to the first direction is caused to flow through the bit line BL, the direction of magnetization of the MTJ element  20  becomes parallel. When the current set to the second direction is caused to flow through the bit line BL, the direction of magnetization of the MTJ element  20  becomes anti-parallel. 
     When reading data from the MTJ element  20 , the read word line RWL is activated, the NMOSFET in  FIG. 10  connected to a selected MTJ element  20  is turned on in order to make a current path, and the current is caused to flow through the source line SL from the selected bit line BL. As a result, since the current according to its resistance value of the selected MTJ element  20  flows through only the selected MTJ element  20 , data can be read by detecting its current value. 
     A mechanism that the direction of magnetization of the MTJ element is changed will now be briefly described with reference to  FIGS. 12 and 13 . 
       FIG. 12  shows TMR characteristics curve of the magnetic field applied to the MTJ element and the resistance value of the MTJ element.  FIG. 13  shows an asteroid curve of the MTJ element. 
     Like the TMR curve depicted in  FIG. 12 , when the magnetic field Hx is applied in an easy-axis direction of the MTJ element, the resistance value of the MTJ element is changed, e.g., approximately 17%. This change ratio, i.e., a ratio of the resistances difference between two states and parallel state resistance is referred to as an MR ratio. It is to be noted that the MR ratio varies depending on properties of the magnetic layers of the MTJ element. At present, an MTJ element having the MR ratio of approximately 50% is available. The MTJ element is subjected to a magnetic field obtained by combining the magnetic field Hx in the easy-axis direction and a magnetic field Hy in a hard-axis direction. 
     As indicated by solid lines and broken lines in  FIG. 12 , the intensity of the magnetic field Hx in the easy-axis direction required to change the resistance value of the MTJ element also varies depending on the intensity of the magnetic field Hy in the hard-axis direction. Utilizing this phenomenon can write data only in the MTJ element arranged in accordance with an intersection of a selected write word line WWL and a selected bit line BL in a plurality of memory cells arranged in an array form. 
     That is, as shown in  FIG. 13 , if the intensity of the combined magnetic field obtained by combining the magnetic field Hx in the easy-axis direction and the magnetic field Hy in the hard-axis direction corresponds to the outside of the asteroid curve, e.g., positions of black circles in the drawing, the directions of magnetization of the magnetic layers of the MTJ element can be switched. That is, data can be written. 
     On the contrary, if the intensity of the combined magnetic field obtained by combining the magnetic field Hx in the easy-axis direction and the magnetic field Hy in the hard-axis direction corresponds to the inside of the asteroid curve, e.g., positions of write circles in the drawing, the directions of magnetization of the magnetic layers of the MTJ element cannot be switched. That is, data cannot be written. 
     Therefore, changing the intensity of the combined magnetic field obtained by combining the magnetic field Hx in the easy-axis direction and the magnetic field Hy in the hard-axis direction and changing a position of the combined magnetic field in an Hx-Hy plane can control writing of data with respect to the MTJ element. 
     Furthermore, when to the MTJ element is applied the magnetic field Hx only in the easy-axis direction, i.e., the magnetic field Hx obtained by only the current flowing through the write word line or the magnetic field Hy only in the hard-axis direction, i.e., the magnetic field Hx obtained by only the current flowing through the write bit line, since the combined magnetic field does not protrude to the outside of the asteroid curve, data cannot be written. 
     As to the write operation in the MRAM device, always correctly writing the write data to the MTJ element, i.e., stabilization of the write characteristics is required. Stabilization of the write characteristics is particularly important when stored data in the MTJ element is different from the write data. In such a case, the magnetization direction of the recording layer of the MTJ element must be stably reversed. 
     (First Embodiment) 
       FIG. 14  is a cross-sectional view typically showing an arrangement relationship between the MTJ element, the write wiring and the bit line in connection with an example of the memory cell used in an MRAM device according to a first embodiment of the present invention. 
     In  FIG. 14 , the MTJ element  20  is the same as the MTJ element described in conjunction with  FIG. 3  in that the tunnel magnetoresistive effect is provided by the structure that the tunnel barrier layer  23  is sandwiched between the recording layer  21  and the fixed layer  22  each made of a magnetic film, but different from that shown in  FIG. 3  in the following point. 
     As the recording layer  21 , there is used a material with a high magnetic anisotropy that a crystal magnetic anisotropy exceeds 10 4  erg/cc, e.g., a Co-based material such as CoFeNi, CoFe, CoCr, CoPt and others (characteristics 1). 
     A laminated film made of, e.g., PtMn and CoFe is used as the fixed layer  22  of the MTJ element  20  and, e.g., AlOx is used as the tunnel barrier layer  23 . The MTJ element  20  according to this example has a rectangular or elliptic shape and is arranged in such a manner its long side direction is parallel to the write wiring  10 , and a direction of magnetization is given along the long side direction. 
     As the write wiring  10 , there is used a write wiring with a yoke that at least one surface opposed to the MTJ element  20 , or all four surfaces in this example, of peripheral surfaces of a regular write wiring  11  made of, e.g., Cu is covered with a magnetic layer  12  made of a magnetic material such as NiFe or CoZrNb. 
     A recording layer  21  of the MTJ element  20  is superimposed on a part of the magnetic layer  12  of the write wiring with a yoke  10  through, e.g., a ruthenium (Ru) film  26 . Magnetic particles constituting the recording layer  21  of the MTJ element  20  are exchange-coupled with the magnetic layer  12  of the write wiring with a yoke  10 . This exchange coupling may be either ferro-coupling or anti-ferro-coupling. Alternatively, it may be 90-degree coupling. 
     In this example, the magnetic layer  12  covering the peripheral surface of the write wiring with a yoke  10  opposed to the MTJ element  20  has a concave cross section, and a lower portion of the MTJ element  20  or at least the recording layer  21  is provided in the concave portion  13 . In other words, a part  12   a  of the magnetic layer  12  which is opposed to the recording layer  21  is formed thinner than a thickness of any other part, and a part of the magnetic layer  12  exists so as to be opposed to both ends of the recording layer  21  of the MTJ element  20  in a widthwise direction of the write wiring  10 , i.e., a direction crossing the extending direction of the write wiring  10  (characteristics 3). 
     Incidentally, it is good enough for the magnetic layer  12  to have a shape that the magnetic flux converges toward the MTJ element  20 , and this shape can be modified in many ways. Furthermore, the magnetic layer  12  has the larger magnetic permeability than that of the recording layer  21 . Moreover, the magnetic layer  12  has a higher saturation magnetic flux density than that of the recording layer  21 . 
     Moreover, a sum of magnetic volumes (Ms×t) of the part  12   a  of the magnetic layer  12  at a portion where the MTJ element  20  is superimposed on the write wiring with a yoke  10  and of the recording layer  21  is set smaller than a magnetic volume (Mx′×t′) of the magnetic layer  12  at any other portion. In this case, it is desirable to set the volume ratio (Ms×t/Ms′×t′) smaller than 0.9, or preferably smaller than 0.3 (characteristics 4). 
     According to the structure shown in  FIG. 14 , the Ru film  26  is interposed and superimposed between the magnetic layer  12  on the top face of the write wiring with a yoke  10  and the recording layer  21  of the MTJ element  20 . Therefore, the flatness of the superimposed surface is improved, and a magnitude of exchange coupling of the recording layer  21  and the magnetic layer  12  can be controlled by controlling a thickness of the Ru film  26 . 
     In case of constituting the cell array by arranging the memory cells having the above-described structure on a semiconductor layer, e.g., an Si substrate in a matrix form, the cell array is configured as described above in conjunction with  FIGS. 9  to  11 , the write wirings WWL are extended in the row direction so as to get closer to respective free layers  22  of the MTJ elements  20  in the same row on the lower layer side of the MTJ elements  20 , and the bit lines BL are extended in the column direction so as to be continuous with the recording layers  21  of the MTJ elements  20  in the same column on the upper layer side of the MTJ elements  20 . 
     A principle of the write operation with respect to the memory cell having the above structure will now be described. 
     Writing data to the MTJ element  20  can be achieved by causing a current proceeding in a first direction or a second direction opposite to the first direction according to write data to flow through the write wiring  10 , and making a direction of magnetization of the recording layer  21  of the MTJ element  20  parallel or anti-parallel by using a magnetic field formed by this current. 
     It is to be noted that the magnetization direction of the recording layer  21  can be slightly inclined in writing by causing the current in a fixed direction to flow through the bit line BL as an auxiliary write current in writing. Therefore, as described in conjunction with the prior art, the write current is further reduced by making the direction of magnetization of the recording layer  21  parallel or anti-parallel by using a combined magnetic filed of magnetic fields respectively formed by the currents flowing through the write wiring  10  and the bit line BL. 
     In case of reading data from the MTJ element  20 , a path through which the current is passed to the selected MTJ element  20  from the selected bit line BL is formed, a current according to a resistance value of the MTJ element  20  is caused to flow, and a current value is detected by using a sense amplifier connected to the bit line BL, thereby reading data. 
     In the writing operation, the magnetic flux of the yoke is converged to the part  12   a  of the magnetic layer  12  on which the MTJ element  20  is superimposed and the recording layer  21  based on the structural characteristics (1) to (3). Since a magnetic leakage flux from the yoke edge also flows to the recording layer  21 , the write current is greatly reduced. Furthermore, since the recording layer  21  is exchange-coupled with the magnetic layer  12  of the write wiring  10  having a yoke, differences in crystal orientations can be averaged, and irregularities in write current value can be greatly reduced. Moreover, it was confirmed that the write current value can be decreased to approximately 0.2 mA based on the characteristics (4). 
     Therefore, according to the first embodiment, it is possible to realize the MRAM device which can reduce the write current value to a practical level, eliminate irregularities in switching current value, assure the sufficient thermal stability of the recorded information, and decrease the write current value. 
     The advantage of the first embodiment will now be concretely described hereinafter. 
     First, the structural characteristics (1) were adopted, and the MTJ element  20  having the recording layer  21  made of a high magnetic anisotropic material of not less than 10 4  erg/cc was arranged in close proximity to the write wiring with a yoke  10  covered with the magnetic layer  12 . As a result of conducting an experiment with respect to such a memory cell, no switched cell was generated even though a shelf test was carried out. 
     Additionally, the structural characteristics (2) were adopted, the MTJ element  20  was superimposed on the part  12   a  of the magnetic layer  12  of the write wiring with a yoke  10 , and the recording layer  21  was exchange-coupled with the magnetic layer  12  of the write wiring with a yoke  10 . As a resulting of conducting an experiment with respect to such a memory cell, the write current value was approximately 1 mA, irregularities in the write current values were almost eliminated, and the maximum write current was reduced to 1.2 mA. 
     Further, the structural characteristics (3) were adopted, and a sum (Ms×t) of a magnetic volume of the part  12   a  of the magnetic layer  12  on which the MTJ element  20  is superimposed and that of the recording layer  21  was set smaller than a magnetic volume (Ms′×t′) of any other part of the magnetic layer  12 . Concretely, a magnetic volume ratio (Ms×t/Ms′×t′) was set smaller than 0.3 (characteristics 4). As a result of conducting an experiment with respect to such a memory cell, the write current value was reduced to be not more than 0.5 mA as shown in FIG.  15 . 
       FIG. 15  is a characteristic view showing a relationship between the magnetic volume ratio (Ms×t/Ms′×t′) in the memory cell having the structure depicted in  FIG. 14 , the write current (characteristics A) and a write pass ratio (characteristics B). 
     It can be understood from this characteristic view that the write current becomes approximately 0.5 mA when the magnetic volume ratio (Ms×t/Ms′×t′) is approximately 0.3, and the write pass ratio becomes maximum when the magnetic volume ratio is not more than approximately 0.3. Furthermore, even if the magnetic volume ratio substantially exceeds 0.3, the write current reduction effect can be provided, but irregularities in the write current were as large as approximately 0.5 mA. Even if the magnetic volume ratio substantially exceeds 0.3, the write pass ratio is not less than 80% in an area that the magnetic volume ratio is not more than approximately 0.9. 
     (Second Embodiment) 
       FIG. 16  is a cross-sectional view typically showing a structure of an example of a memory cell used in an MRAM device according to a second embodiment of the present invention. 
     This memory cell is different from the memory cell according to the first embodiment in that peripheral surfaces of the write wiring with a yoke  10  which are opposite to a peripheral surface of the same opposed to the MTJ element  20  among four peripheral surfaces of the write wiring with a yoke  10 , i.e., three peripheral surfaces other than a bottom surface in this example are covered with the magnetic layer  12 , namely, a yoke of the bottom surface portion is eliminated, and any other part is the same, thereby eliminating the redundant explanation. 
     According to such a structure, since the yoke (magnetic layer  12 ) at the bottom surface portion of the write wiring  10  is eliminated, the write current becomes slightly bigger than that shown in FIG.  15 . However, the MRAM device having such a structure is preferable when there is an enough cell array area. Furthermore, the MRAM device according to the second embodiment can reduce the number of steps in manufacture. 
     (Third Embodiment) 
       FIG. 17  is a cross-sectional view typically showing a structure of an example of a memory cell used in an MRAM device according to a third embodiment of the present invention. 
     This memory cell is different from the memory cell according to the first embodiment in that peripheral surfaces other than both side surfaces continuous with a peripheral surface opposed to the MTJ element  20  in four peripheral surfaces of the write wiring  10  are covered with a yoke, i.e., yokes at the both side parts of the write wiring  10  are eliminated, and any other part is the same, thereby eliminating the redundant explanation. 
     According to such a structure, as compared with a case that the four peripheral surfaces of the write wiring  10  are covered with a yoke, the write current becomes slightly large. However, the MRAM device having such a structure is preferable when there is an enough cell array area. Moreover, the MRAM device according to the third embodiment can reduce the number of steps in manufacture. 
     (Fourth Embodiment) 
       FIG. 18  is a cross-sectional view typically showing a structure of an example of a memory cell used in an MRAM device according to a fourth embodiment of the present invention. 
     This memory cell is different from the memory cell according to the first embodiment in that a write cell selection transistor  60  is connected to the write wiring  10 , and any other part is the same, thereby eliminating the redundant explanation. 
     Selection of the write cell and separation from any other cell can be carried out by connecting the write cell selection transistor  60  to the write wiring  10  as described above and controlling the transistor  60  to be switched in accordance with a control signal. In this case, as described above, since the write current can be set to a sub mA, a size of the write cell selection transistor  60  can be reduced as small as that of the MTJ element  20 , and the transistor  60  can be also integrated in the memory cell array. 
     According to such a structure, erroneous writing can be completely eliminated. Moreover, it is possible to adopt a heat assist recording mode which performs writing while heating the recording layer  21  of a selected cell by using a write current or the like in writing. As a result, a size of the MTJ element  20  can be refined to 50 nm order, and the MRAM device according to the fourth embodiment can be substituted for a DRAM device in a system in which the DRAM device is incorporated as a memory. 
     On the contrary, since a concept of the conventional heat assist recording mode does not have the selectivity of cells in writing, a certain degree of heat is applied to cells other than the selected one, which is a factor provoking magnetization reversion of the recording layer due to the thermal disturbance. 
     It is to be noted that a description of the MRAM device according to the fourth embodiment has been given as to the case that the write cell selection transistor  60  is provided to the memory cell in the MRAM device according to the first embodiment depicted in  FIG. 16 , but the write cell selection transistor  60  can be likewise provided to the MRAM device according to the second or third embodiment. 
     Moreover, although a description has been given as to the case that the recording layer  21  of the MTJ element  20  is a single magnetic layer in each of the foregoing embodiments, the recording layer  21  may be a layer obtained by laminating multiple magnetic layers. 
     The MRAM device according to the first to fourth embodiments can be applied in many ways. Some of such application examples will now be described hereinafter. 
     APPLICATION EXAMPLE 1 
     As one of application examples of the MRAM device,  FIG. 19  shows a DSL data path part of a digital subscriber line (DSL) modem. This modem includes a programmable digital signal processor (DSP)  151 , an analog to digital converter (ADC) and a digital to analog converter (DAC)  152 , a transmission driver  153 , and a receiver amplifier  154 . In  FIG. 19 , a band pass filter is eliminated, an MRAM  155  according to the present invention and an EEPROM  156  are shown as various kinds of optional memories which can hold a line code program instead of the band pass filter. 
     It is to be noted that this application example uses two types of memories, i.e., the MRAM and the EEPROM as memories used to hold a line code program, but the EEPROM may be substituted by the MRAM. That is, only the MRAM may be used without using the two types of memories. 
     APPLICATION EXAMPLE 2 
     As another application example of the MRAM device,  FIG. 20  shows a part in a cellphone terminal  300  which realizes a communication function. As shown in  FIG. 30 , the part which realizes the communication function includes a transmission/reception antenna  201 , an antenna duplexer  202 , a receiver  203 , a base band processor  204 , a Digital Signal Processor (DSP)  205  uses as an audio codec, a speaker  206 , a microphone  207 , a transmitter  208 , and a frequency synthesizer  209 . 
     Moreover, as shown in  FIG. 20 , to the cellphone terminal  300  is provided a controller  200  which controls each portion in the cellphone terminal. The controller  200  is a microcomputer in which a CPU  221 , a ROM  222 , an MRAM  223  according to the present invention and a flash memory  224  are connected through a CPU bus  225 . 
     Here, the ROM  222  previously stores a program executed by the CPU  221  or required data such as fonts for display. Additionally, the MRAM  223  is mainly used as a working area, and it is used when storing data in the middle of calculation according to needs in execution of a program by the CPU  221  or when temporarily storing data transmitted/received with each portion. Further, the flash memory  224  stores immediately preceding setting conditions or the like even if a power supply of the cellphone terminal  300  is turned off, and stores setting parameters in case of adopting a usage that the same settings are employed at the time of turning on the power supply next time. That is, the flash memory  224  is a non-volatile memory in which data stored therein is not eliminated even if the power supply of the cellphone terminal is turned off. 
     In this application example, although the ROM  222 , the MRAM  223  and the flash memory  224  are used, the flash memory  224  may be substituted by the MRAM according to the present invention. Furthermore, the ROM  222  can be also substituted by the MRAM according to the present invention. 
     It is to be noted that, in  FIG. 20 , reference numeral  211  denotes an audio data reproduction processor;  212 , an external terminal connected to the audio data reproduction processor  211 ;  213 , an LCD controller;  214 , an LCD connected to the LCD controller  213 ;  215 , a ringer;  231 , an interface provided between the CPU bus  225  and an external memory slot  232 ,  233 , an interface provided between the CPU bus  225  and a key operation unit  234 ; and  235 , an interface between the CPU bus  225  and the external terminal  236 . An external memory  240  is inserted into the external memory slot  232 . 
     APPLICATION EXAMPLE 3 
       FIGS. 21  to  25  show examples that the MRAM device according to the present invention is applied to a card (MRAM card) that embodies a removable media such as a Smart Media card. 
     In a top view of  FIG. 21 , reference numeral  400  designates an MRAM card main body;  401 , an MRAM chip;  402 , an opening portion;  403 , a shutter; and  404 , a plurality of external terminals. An MRAM chip  401  is accommodated in the card main body  400  and exposed to the outside from the opening portion  402 . When carrying the MRAM card, the MRAM chip  401  is covered with the shutter  403 . The shutter  403  is constituted by a material having an effect to block off an external magnetic field, e.g., ceramics or the like. In case of transferring data, the shutter  403  is opened, and the MRAM chip  401  is exposed. The external terminal  404  is used to fetch content data stored in the MRAM card to the outside. 
       FIGS. 22 and 23  are a top view and a side view of a card insertion type transfer device used to transfer data to the MRAM card. A second MRAM card  450  used by an end user is inserted from an insertion portion  510  of the transfer device  500  and pushed until the card stops at a stopper  520 . The stopper  520  is also used as a member to position a first MRAM card  550  and the second MRAM card  450 . With the second MRAM card  450  being arranged at a predetermined position, data stored in the first MRAM card  550  is transferred to the second MRAM card  450 . 
       FIG. 24  is a side view of a fitting type transfer device. As indicated by an arrow in the drawing, this device is of a type which mounts the second MRAM card  450  on the first MRAM card  550  so as to be fitted in with the stopper  520  being used as a target. The transfer method is the same as that of the card insertion type, thereby eliminating its explanation. 
       FIG. 25  is a side view of a slide type transfer device. Like a CD-ROM drive, a DVD drive and others, a sliding tray  560  is provided in the transfer device  500 , and the sliding tray  560  slides as indicated by an arrow in the horizontal direction in the drawing. When the sliding tray  560  has moved to a state indicated by a broken line in the drawing, the second MRAM card  450  is mounted on the sliding tray  560 . Thereafter, the sliding tray  560  carries the second MRAM card  450  into the transfer device  500 . Since the point that the second MRAM card  450  is carried in such a manner that an end portion of this card is brought into contact with the stopper  520  and the transfer method are the same as those of the card insertion type, their explanation is eliminated. 
     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 invention concept as defined by the appended claims and their equivalents.