Patent Publication Number: US-2013249025-A1

Title: Magnetoresistive element and magnetoresistive random access memory with the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-064376, filed Mar. 21, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a magnetoresistive element and a magnetoresistive random access memory provided with the same. 
     BACKGROUND 
     In recent years, magnetoresistive random access memory (MRAM) utilizing a tunneling magnetoresistive (TMR) material has been proposed as a nonvolatile semiconductor memory. MRAM is a nonvolatile semiconductor memory possessing distinguishing features suitable for high-speed writing and reading, low power consumption, large capacity, and applications to working memory. The MRAM has a magnetic tunnel junction (MTJ) which is magnetoresistive whose resistance changes depending on the magnetizing direction of the magnetizing film in the MTJ element. 
     MRAM systems have traditionally used the magnetic field induced by an electric current flowing through wires close to the MTJ element (magnetic field writing method) to invert the magnetizing direction of the free magnetizing layer in the MTJ element. This method, however, makes MRAM integration difficult because the wires generating the magnetic field have to be directly adjacent to the MTJ element. This has prompted the study of a different technique, the spin injection writing method, in which a spin polarizing current is used to reverse the magnetization of the element. This method inverts the magnetizing direction of the magnetization free layer in the MTJ element by passing a spin-polarized current (inversion current) through it. In the spin injection method, integration of MRAM is easy since each memory cell is essentially a cell selection transistor paired with an MTJ element, similar to DRAM (Dynamic Random Access Memory). 
     An MTJ element that uses the spin polarized current includes a free magnetization layer including a magnetizing film whose magnetizing direction is flipped by the spin-polarized current, a fixed magnetization layer including a magnetized directionally fixed film, and a tunnel barrier layer sandwiched between these two layers. In addition, there are interface layers to maintain a high MR ratio (magnetoresistance ratio) between the free and fixed magnetization layers and the tunnel barrier layer. 
     Broadly speaking, there are two kinds of MTJ elements. The first type include MTJ elements with an in-plane magnetizing mode, where the in-plane magnetizing film has an magnetizing axis substantially parallel to a film plane thereof. The second type of MTJ element employs a vertical magnetizing mode with a magnetizing film having its magnetizing axis almost perpendicular to a film plane thereof. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross section of an MTJ element of a first embodiment. 
         FIG. 2  shows a cross section showing how an MRAM of the first embodiment is fabricated. 
         FIG. 3  shows a cross section showing how the MRAM of the first embodiment is fabricated. 
         FIG. 4  shows a cross section showing how the MRAM of the first embodiment is fabricated. 
         FIG. 5  shows a cross section showing how the MRAM of the first embodiment is fabricated. 
         FIG. 6  is a diagram illustrating first and second embodiments. 
         FIG. 7  is a diagram for illustrating first and second embodiments. 
         FIG. 8  is a cross section of an MTJ element of a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, the device is explained by referring to the drawings attached. However, parts common to the different figures are indicated using the same symbols so as to avoid duplicate explanation. The figures are schematic diagrams used for explaining the embodiments and the precise shape, size, ratio, etc. may differ from those in the actual device. However, they can be modified based on the explanation and technology described below. 
     According to embodiments, a switchable magnetoresistive element useful in magnetic tunnel junction (MTJ) is provided. As will be explained in more detail herein, the magnetoresistive element commonly comprises, in order, a first, fixed magnetic layer, a diffusion barrier layer disposed over the first magnetic layer, an interlayer (e.g., highly orientated magnetic layer) which provides at least one switchable magnetic domain therein, a second diffusion barrier layer and a second magnetic layer. 
     The first and second magnetic layers may contain precious metals, such as platinum or palladium. It has been found that platinum and palladium deteriorate the stability of the switchable magnetic domains in the interface layer. According to present embodiments, the inventors have discovered that using the element hafnium for the diffusion barrier layer ameliorates the affect of platinum or palladium on the stability of the magnetic domains. Accordingly, the reliability of the MTJ may be improved using hafnium in the diffusion barriers. 
     In particular, the invention is useful to enable a thin diffusion barrier layer, which unlike thicker diffusion barrier layers, does not attenuate the magnetic coupling between the first magnetic layer and the switchable interface layer. Whereas a thick diffusion barrier layer ameliorates the detrimental effect the presence of platinum or palladium has in the magnetic layer, it also has the detrimental effect of attenuating the magnetic coupling between the first magnetic layer and the switchable interface layer. Thus, a thin barrier layer having a thickness on the order of 0.6-0.8 nm is enabled by incorporating hafnium therein, when the magnetic layer includes platinum or palladium, and the attenuation of the magnetic coupling caused by the thicker barrier layer is substantially reduced. 
     According to an embodiment, there is provided a magnetoresistive element capable of preventing diffusion of precious metals from the fixed and free magnetization layers into the interface layer during heat treatment, without hindering magnetization bonding between the free and fixed magnetization layers and the interface layer. 
     In general, according to one embodiment, a magnetoresistive element possesses a bottom electrode, a first magnetic layer with an easy axis of magnetization nearly perpendicular to a film plane thereof, a first interface layer formed on top of the first magnetic layer, an MgO insulating layer on the first interface, a second interface layer on the insulating layer, a second magnetic layer formed on top the second interface layer with an easy axis of magnetization nearly perpendicular to a film plane thereof, and a top electrode on the second magnetic layer. The MTJ cell has a diffusion barrier layer between the first magnetic layer and the first interface layer when the first magnetic layer contains Pt, and a diffusion barrier layer between the second magnetic layer and the second interface layer when the second magnetic layer contains Pt. The diffusion barrier layer contains Hf and has a film thickness of 0.6 nm to 0.8 nm. 
     Embodiment 1 
     An embodiment is explained in  FIG. 1  which shows a cross section of an MRAM  1 . In what follows we will describe an MTJ element (Magnetic Tunnel Junction element)  30  which employs a vertical magnetizing film. That is, the vertical magnetizing film is a magnetizing film having a magnetizing direction (easy axis direction of magnetization) substantially perpendicular to a film plane of the magnetizing film in this disclosure. 
     As shown in  FIG. 1 , the MTJ element  30  in the present embodiment has a bottom electrode  116  on which a crystal orientation controlling film  117 , a fixed magnetization layer (first magnetic layer)  118 , a diffusion barrier layer  100 , a highly oriented magnetic layer (first interface layer)  119 , a tunnel barrier layer (insulating layer)  120 , a highly oriented magnetic layer (second interface layer)  121 , a diffusion barrier layer  200 , a free magnetization layer (second magnetic layer)  122 , and a top electrode  123  are sequentially laminated. 
     As explained in detail below, the MTJ element  30  in this embodiment has diffusion barrier layers  100 ,  200  that block diffusion of precious metals from the fixed and free magnetization layers  118  and  122  into the highly oriented magnetic layers  119 ,  121  when the MRAM  1  (e.g., see  FIG. 5 ) is heat treated during fabrication. The diffusion barrier layers  100 ,  200  also inhibit crystal orientation in the highly oriented magnetic layer  119  from being influenced by the crystal orientation of the fixed magnetization layer  118  when the MTJ element  30  is fabricated. As a result, the highly oriented magnetic layer  119  can be formed with a good crystal structure. In addition, the free magnetization layer  122  can be formed with a good crystal structure because the highly oriented magnetic layer  121  cannot influence the crystal orientation of the free magnetization layer  122  due to the existence of the diffusion barrier layer  200 . Specifically, the highly oriented magnetic layer  119  and the fixed magnetization layer  118  differ in their crystal structure or direction, as do the free magnetization layer  122  and the highly oriented magnetic layer  121 . Because controlling the crystal orientation as well as blocking the diffusion of precious metal is important, a high MR ratio can thus be achieved in the MTJ element  30  in this embodiment. The details of the diffusion barrier layers  100 ,  200  will be explained later. 
     In order to fix the magnetizing direction of the fixed magnetization layer  118  in one direction, an anti-ferromagnetic layer (not shown in the figure) may be provided adjacent to the fixed magnetization layer  118 . The anti-ferromagnetic layer can be sandwiched between the fixed magnetization layer  118  and the diffusion barrier layer  100  or between the diffusion barrier layer  100  and the highly oriented magnetic layer  119 . The anti-ferromagnetic layer may be formed of FeMn, NiMn, PtMn, PtPdMn, RuMn, OsMn, IrMn, CrPtMn, etc., which are manganese alloys (Mn) with iron (Fe), nickel (Ni), platinum (Pt), palladium (Pd), ruthenium (Ru), osmium (Os), iridium (Ir), etc. 
     The bottom electrode  116  can, for instance, be a tantalum (Ta) film of thickness 5 nm. 
     The orientation controlling film  117  can, for instance, be a 5 nm thick Pt film with (001) crystal orientation. The film  117  need not be Pt′. Ir, Ru and laminated films thereof, for example, can also be used for the orientation controlling film  117 . 
     The fixed magnetization layer  118  is a vertical magnetizing film containing precious metals such as Pt, cobalt (Co), etc. The fixed magnetizing layer  118  can be a 10 nm thick Fe 50 Pt 50 -containing magnetizing film. In addition, the fixed magnetization layer  118  need not be a Fe 50 Pt 50  vertical magnetizing film and one could also use Co 50 Pt 50 , Co 30 Fe 20 Pt 50 , or (Fe 50 Pt 50 ) 88 — (SiO 2 ) 12 , which has a partitioned structure obtained by interspersing the film with silicon oxide (SiO 2 ) or magnesium oxide (MgO) film. For ease of control and fabrication, according to an embodiment, the fixed magnetization layer  118  is a vertical magnetizing film with a high magnetization and to contain precious metals such as Pt, Co, etc. 
     Diffusion barrier layers  100 ,  200  are hafnium (Hf) films of thickness 0.6 nm to 0.8 nm. They block the diffusion of the precious metals from the fixed and free magnetization layers  118  and  122  into the highly oriented magnetizing layers  119 ,  121  during the heat treatment used to fabricate the MRAM  1 . Further details are explained later. 
     The highly oriented magnetic layers  119 ,  121  should ideally be vertically magnetizing film a having a high polarization rate such as Co 50 Fe 50  film, etc. In order to obtain a high MR ratio and low inversion current, and the film thickness should range from 1 nm to 1.5 nm to make the magnetizing direction substantially perpendicular to the film plane. 
     The tunnel barrier layer  120  can be a MgO film of thickness 1.0 nm. The tunnel barrier layer  120  need not be a MgO film, other films can be used. the MR ratio of MTJ element  30  may degrade if the precious metals in fixed and free magnetization layers  118  and  122  diffuse into the tunnel barrier layer  120 . However, due to the highly oriented magnetic layers  119 ,  121  between the tunnel barrier layer  120  and the fixed or free magnetization layers  118  or  122 , the distance between the tunnel barrier layer  120  and the fixed or free magnetization layers  118  or  122  is increased. Accordingly, the diffusion barrier layers  100 ,  200  of the MTJ element  30  in this embodiment inhibit the diffusion of the precious metals from the fixed and free magnetization layers  118  and  122  into the tunnel barrier layer  120 , thereby preventing degradation of the MR ratio. 
     The free magnetization layer  122  is a vertical magnetizing film containing a precious metal such as Pt, Co, etc. and, for instance, a laminated film [Co/Pt]  5  obtained by layering  5  pairs of 0.4 nm-thick Co film and 0.8 nm-thick Pt film. The free magnetization layer  122  need not be a laminated film, an artificial Co/Pd lattice can be used instead. In addition, the number of pairs of the laminated film can be changed between 1 and 10 depending on the desired characteristics of the MTJ element  30 . Alloys of Co and Pt can also be used for the free magnetization layer  122 . For ease of control and fabrication, the free magnetization layer  122  should ideally be a vertical magnetizing film with a high magnetization containing precious metals such as Pt, Co, etc. 
     The top electrode  123  can, for instance, be a Ta film of thickness 10 nm. 
     Furthermore, the layered structure of the MTJ element  30  in this embodiment is not limited to that shown in  FIG. 1 , various shapes can be used. Thus, additional layers can be added, or existing layers can be removed. The MTJ element  30  in this embodiment need not have both diffusion barriers layers  100 ,  200 , it is possible to have just one diffusion barrier layer and still block diffusion of precious metals from the fixed and free magnetization layers  118  and  122  during heat treatment. 
     In some cases the layered structure of the MTJ element  30  in this embodiment may be such that the interface between the layers is not clear. For instance, the fixed magnetization layer  118 , diffusion barrier layer  100  and highly oriented magnetic layer  119  may be a monolithic layer. Similarly, the highly oriented magnetic layer  121 , diffusion barrier layer  200  and free magnetization free  122  may sometimes have the form of a monolithic layer. In these cases, when a 1 nm-thick MgO film is used as the tunnel barrier layer  120  in the MTJ element  30 , Hf atoms in 100 or 200 are 1.886 to 2.500 times to the Mg atoms. A method for fabricating the MRAM  1  having the MTJ element  30  shown in  FIG. 1  is explained with reference to  FIG. 2  to  FIG. 5 , which show cross-sections of the MRAM  1 . However, the present disclosure is not limited to the method of MRAM  1  fabrication described below. 
     First, referring to  FIG. 2 , an isolation groove is formed next to a transistor active region by the usual method in the surface of the p-type semiconductor substrate  10 , such as reactive ion etch of silicon. An insulating SiO 2  film, etc. is deposited in the groove to form a shallow trench (STI (Shallow Trench Isolation))  101 . 
     A transistor for a switching operation is fabricated. First, an oxide film  102  of thickness of about 6 nm is formed on the semiconductor substrate  10  by thermal oxidation, and an arsenic-doped n +  type polycrystalline silicon film  103  is deposited on the oxide film  102 , followed by deposition of tungsten silicide (WSi x ) film  104  and a nitride film  105 . Using photolithographic and RIE (Reactive Ion Etching) techniques, the polycrystalline silicon film  103 , tungsten silicide film  104  and nitride film  105  are then patterned to form a gate electrode  20  in the multilayer structure. A nitride film  106  is deposited for the side wall of the gate electrode  20 . A spacer including the nitride film  106  is formed on the side of the gate electrode  20  by RIE to form side walls. A source-drain region  107  is formed, next to the gate electrode  20 , in the semiconductor substrate  10  by ion injection and heat treatment. The result is shown in  FIG. 2 . 
     Then, referring to  FIG. 3 , a silicon oxide film  108  is then deposited by a CVD (Chemical Vapor Deposition) on the transistor in the semiconductor substrate  10 , and the top of the silicon oxide film  108  is polished flat by CMP (Chemical Mechanical Polishing). In addition, a contact hole  109  connected to one side of the source-drain region  107  is formed using traditional lithographic and RIE techniques. 
     Then, a thin titanium film is deposited on the inside of the contact hole  109  by sputtering or CVD and heat treated in a forming gas containing N, such as NH 3 , to form a titanium nitride film (TiN)  110  coating the inside of the contact hole  109 . A tungsten film  111  is deposited on the inside of the contact hole  109 , already coated with the TiN film  110 , by CVD using a tungsten hexa-fluoride gas (WF 6 ), and a portion of the tungsten film  111  sticking out from the contact hole  109  is removed by CMP to form a contact plug  40 . 
     A silicon nitride film  112  is deposited over the oxide film  108  by CVD and a contact hole  113  connecting with the other source-drain region  107  is formed by using lithographic and RIE techniques as explained previously. A titanium nitride film  114  coating the inside of the contact hole  113  is formed as described before, and a tungsten film  115  is deposited on the inside of the contact hole  113  coated with the titanium nitride film  114 , and a portion of the tungsten film  115  sticking out from the contact hole  113  is then removed to form a contact plug  50  connected to MTJ element  30 . The resulting structure is shown in  FIG. 3 . 
     Referring again to  FIG. 1 , a film stack for forming an MTJ element as shown in  FIG. 4  begins with sputtering a Ta film of thickness 5 nm, for example, to form a bottom electrode  116  of the MTJ element  30 . 
     A Pt film of thickness 5 nm, for example, is sputtered onto the bottom electrode  116  to form a crystal orientation controlling film  117  of the MTJ element  30 . As explained previously, the crystals in the orientation controlling film  117  have the (001) orientation. 
     A vertical magnetizing film containing Fe 50 Pt 50  of thickness 10 nm, for example, is then sputtered on the orientation controlling film  117  to form the fixed magnetization layer  118 . 
     An Hf film of thickness 0.6 nm to 0.8 nm serving as the diffusion barrier layer  100  is then deposited on the fixed magnetization layer  118 . More specifically, an Hf barrier film  100  of thickness 0.8 nm, for example, can be formed in 14 seconds by sputtering in Ar flowing at 60 sccm at a sputtering power of 200 W. 
     Next, a first Co 40 Fe 40 B 20  film of thickness 1 nm to 1.5 nm, for example, is sputtered on the diffusion barrier layer  100  to form a first highly oriented magnetizing layer  119 . 
     Thereafter, a MgO film of thickness 1.0 nm, for example, serving as the tunnel barrier  120  is sputtered on the highly oriented magnetizing layer  119 . 
     A second CO 40 Fe 40 B 20  film of thickness 1 nm to 1.5 nm, for instance, is sputtered on the tunnel barrier layer  120  to form a second highly oriented magnetizing layer  121 . 
     An Hf film of 0.6 to 0.8 nm thick is then deposited on the highly oriented magnetizing layer  121  to form the diffusion barrier layer  200 . Since the Hf diffusion barrier  200  is formed in the same way as the diffusion barrier layer  100 , further details are omitted. 
     Next, the free magnetization layer  122  including a vertical magnetizing layer sputtered on the diffusion barrier layer  200 . As explained previously, the free magnetization layer  122  is a laminated film [Co/Pt]  5  obtained by 5 cycles of Co film having thickness of 0.4 nm and Pt film of thickness 0.8 as one cycle, for example. 
     A Ta film of thickness 10 nm, for example, is then sputtered to form the top electrode  123 . 
     Crystallization annealing of the MgO film tunnel barrier  120  is then performed at 360° C. in vacuum for 1 hour. Although the annealing temperature does not have to reach 360° C., to get MgO films with a good crystal structure it should be at least 350° C. After annealing, both the MgO film tunnel barrier layer  120  and the Co 40 Fe 40 B 20  film in the highly oriented magnetic layers  119 ,  121  are crystallized. At that time, the boron (B) in the highly oriented magnetic layers  119 ,  121  then diffused out so that the highly orientated magnetic layers  119 ,  121  become Co 50 Fe 50  films. 
     An silicon oxide film  124  useful as a mask and photoresist (not shown) is deposited on the electrode  123 . The oxide film  124  is patterned by photolithographic and RIE techniques. The photoresist is removed, and the film stack is etched by RIE to form the top electrode  123 , the free magnetization layer  122 , the diffusion barrier layer  200 , the highly oriented magnetic layer  121 , the tunnel barrier layer  120 , the highly oriented magnetic layer  119 , the diffusion barrier layer  100 , the fixed magnetization layer  118 , the orientation control film  117 , and the bottom electrode  16 , in a single region confined over the contact plug  50  and adjacent to the nitride layer  112 . The resulting MTJ element  30  is formed on a contact plug  50  to give the structure shown in  FIG. 4 . 
     Now, referring to  FIG. 5 , a protective silicon nitride film  125  of thickness 5 nm, for example, is then formed by CVP on the top and sides of the MTJ element  30 . 
     In addition, an interlayer dielectric  126  including an SiO 2  film covering the MTJ element  30  and silicon nitride film  112  is formed by CVD. In more detail, the interlayer dielectric  126  including the SiO 2  film is formed using TEOS (tetraethoxysilane) and oxygen by RF plasma processing at a substrate temperature of 350° C. 
     Two contact holes are formed simultaneously in the interlayer dielectric  126  to form a contact plug  70  connected to the top electrode  123  of the MTJ element  30  and a contact plug  60  connected to the contact plug  40 . 
     The TiN barrier layer to cover the inside of the contact holes (not shown) is then formed by CVD from titanium tetrachloride (TiCl 4 ) and ammonia (NH 3 ) at 350° C. The tungsten film (not shown) is deposited by CVD from tungsten hexafluoride (WF 6 ) gas to fill the inside of the contact holes already coated with the barrier layer, and a portion of tungsten film projecting from the holes is removed by CMP to form the contact plugs  60 ,  70 . 
     An upper wiring  135  is formed on the contact plugs  60 ,  70  by the usual method. 
     An interlayer dielectric  132  is further deposited on the interlayer dielectric  126  and a contact hole to contact the upper wiring  135  is formed by lithographic and RIE techniques. An aluminum (Al) film is applied to the contact hole and polished flat by CMP to form a contact plug  80 . An interlayer dielectric  138  is then formed on the interlayer dielectric  132  and a wiring groove to hold the wiring is made by lithography and RIE in the interlayer dielectric  138  on the contact plug. The Al film is then filled in the wiring groove and polished flat by CMP to form a second upper wiring  137 . The resulting MRAM  1  is shown in  FIG. 5 . 
     In the present embodiment, diffusion of precious metals in the fixed and free magnetization layers  118  and  122  into the highly oriented magnetic layers  119 ,  121  during heat treatment when the MRAM  1  is being fabricated can be prevented, due to the 0.6 to 0.8 nm thickness Hf diffusion barrier layers  100 ,  200  in the MTJ element  30 . This will be explained in detail below. 
     Because the MTJ elements previously used lack the hafnium based diffusion barrier layers  100 ,  200  included in the present embodiment, during heat treatment at or above 350° C., the precious metals in the fixed and free magnetization layers  118  and  122  diffuse into the highly oriented magnetic layers  119 ,  121  and disrupt the crystal structure, thereby degrading the MR ratio of a MTJ device. 
     However, the MTJ element  30  in the present embodiment can avoid degradation of MR ratio of MTJ element  30  since it has Hf in the diffusion barrier layers  100 ,  200  of thickness 0.6 nm to 0.8 nm. In detail, since the Hf film can retain a high residual magnetization, magnetic coupling between the fixed magnetization layer  118  and the highly oriented magnetic layer  119  and between the free magnetization layer  122  and the highly oriented magnetic layer  121  is not hindered. Even during heat treatment, where temperatures of 350° C. and above are applied to the MTJ element  30 , the diffusion barrier layers  100 ,  200  keep the precious metals in the fixed and free magnetization layers  118  and  122  from diffusing into the highly oriented magnetic layers  119 ,  121 , so that the MR ratio of the MTJ element  30  remains good. 
     However, according to embodiments, the Hf diffusion barrier layers  100 ,  200  are 0.6 nm to 0.8 nm thick, because then the magnetic coupling between the fixed magnetization layer  118  and the highly oriented magnetic layer  119  and between the free magnetization layer  122  and the highly oriented magnetic layer  121  is not hindered and diffusion of precious metals in the fixed and free magnetization layers  118  and  122  into the highly oriented magnetic layers  119 ,  121  during heat treatment can be prevented. This film thickness is found experimentally by the present inventors as explained below. 
     First, sample MTJ elements used in the experiment will be explained. They are obtained by sandwiching a highly oriented magnetic layers laminate of Co 50 Fe 50  film obtained by sandwiching a tunnel barrier layer including a MgO film of thickness 1 nm by a vertical magnetizing layer containing Pt and Co through the diffusion barrier layer, which contained an Hf film of various thicknesses. Each layer in the sample MTJ element is formed in the same manner as in the first embodiment. 
     The present inventors measured the residual magnetization of the MTJ samples and the results are shown in  FIG. 6 . In  FIG. 6 , the x-axis plots the thickness of the Hf film diffusion barrier layer, and the y-axis shows the magnitude of the residual magnetization relative to the saturated magnetization per unit area. It is clear from  FIG. 6  that the MTJ element retains a high residual magnetization when the Hf film diffusion barrier layer thickness is 5 Å (0.5 nm) to 8 Å (0.8 nm). 
     The samples are annealed at various temperatures under vacuum (1×10 −4  Pa) for 1 hour to obtain results shown in  FIG. 7 . In  FIG. 7 , the x-axis is the film thickness of the Hf film diffusion barrier layer, and the y-axis shows the MR ratio relative to the MR of an MTJ element having an Hf film of thickness 5 Å which is not annealed. Further, the annealing temperature is shown for four categories: no annealing, 350° C., 375° C., and 400° C. It is clear from  FIG. 7  that when annealed at 350° C. or higher, the MR ratio is degraded for MTJ elements having a diffusion barrier layer either without any Hf film or with an Hf film less than 6 Å (0.6 nm) thickness, whereas the MR ratio for an MTJ element with a diffusion barrier layer with an Hf film 6 Å (0.5 nm) to 8 Å (0.8 nm) thickness is hardly degraded. 
     The above results clearly show that that the Hf film in the diffusion barrier layers  100 ,  200  should have a thickness between 0.6 nm and 0.8 nm in order to maintain a high residual magnetization without hindering the magnetizing coupling between the fixed magnetization layer  118  and the highly oriented magnetic layer  119  and between the free magnetization layer  122  and the highly oriented magnetic layer  121 , and to prevent diffusion of precious metals in the fixed and free magnetization layers  118 ,  122  into the highly oriented magnetic layers  119 ,  121 . 
     Thus, since the MTJ element  30  in this embodiment has diffusion barrier layers  100 ,  200  with an Hf film of thickness between 0.6 nm and 0.8 nm, magnetic coupling between the fixed magnetization layer  118  and the highly oriented magnetic layer  119  and between the free magnetization free layer  122  and the highly oriented magnetic layer  121  is not hindered, and diffusion of precious metals in the fixed and free magnetization layers  118 ,  122  into the highly oriented magnetic layers  119 ,  121  can be prevented. In this embodiment, the MR ratio of the MTJ element  30  can be kept high. Furthermore, because diffusion of the precious metals is blocked by the diffusion barrier layers  100 ,  200 , the tunnel barrier layer  120  can be crystallized at high temperature to obtain a good crystal structure so that an MTJ element  30  obtains high MR ratio. According to the experiments of the present inventors, a high MR ratio of 140 is obtained even when an MTJ element  30  whose diffusion barrier layers  100 ,  200  contain Hf film of thickness of 0.6 nm or higher is annealed in vacuum for 1 hour at 350° C. 
     Owing to the diffusion barrier layers  100 ,  200  of the MTJ element  30  in this embodiment, crystal growth in the highly oriented magnetic layer  119  influenced by the crystal structure of the fixed magnetization layer  118  in the MTJ element  30  can be inhibited. Thus, the highly oriented magnetic layer  119  with a good crystal structure can be formed and the same goes for the free magnetization layer  122 , because crystal growth in the free magnetization layer  122  influenced by the crystal structure in the highly oriented magnetic layer is inhibited. The MTJ element  30  in this embodiment therefore has a high MR ratio. 
     Embodiment 2 
     This embodiment differs from the first embodiment in that the lamination order of the layers making up the MTJ element is reversed. The MTJ elements of  FIG. 8  have diffusion barrier layers containing Hf film of thickness 0.6 nm to 0.8 nm as in the first embodiment, so that magnetic coupling between the fixed magnetization layer and highly oriented magnetic layer and between the free magnetization layer and highly oriented magnetic layer is not hindered, and precious metals are prevented from diffusing from the fixed and free magnetization layers into the highly oriented magnetic layer. 
     This embodiment will now be explained for the case of an MTJ element  30  with a vertical magnetizing film.  FIG. 8  shows a cross section of the MTJ element. In explaining this embodiment, parts analogous to the corresponding parts in the first embodiment are denoted by the same symbols and their explanation is omitted. 
     The MTJ element  30  in this embodiment shown in  FIG. 8  has a bottom electrode  116  containing a Ta film of thickness 5 nm, for instance, on which the following layers are laminated in order: an orientation controlling film  117 , for example, containing a Pt film with crystal orientation (001) and thickness 5 nm, a free magnetization layer (first magnetic layer)  322 , a diffusion barrier layer  300  containing Hf film, a highly oriented magnetic layer  319 , for example, containing a Co 50 Fe 50  film of thickness 1 nm to 1.5 nm, a tunnel barrier layer  320 , for example, containing an MgO film of thickness 1.0 nm, a highly oriented magnetic layer  321  containing a Co 50 Fe 50  film, for example, of thickness 1 nm to 1.5 nm, diffusion barrier layer  400  containing Hf film, a fixed magnetization layer (second magnetic layer)  318  containing an Fe 50 Pt 50  film, for example, of thickness 10 nm, and a top electrode  323 , for example, containing a Ta film of thickness 10 nm. 
     As in the first embodiment, and for the same reasons, it is best for the diffusion barrier layers  300 ,  400  to contain an Hf film of thickness 0.6 nm to 0.8 nm. 
     Also, as in the first embodiment, the free magnetization layer  322  contains a vertical magnetizing film which, in more detail, is a lamination [Co/Pt]  5  structure obtained by laminating 5 cycles of Co film of thickness 0.4 nm and a Pt film of thickness 0.8 nm as one cycle, for example. 
     As in the first embodiment, an anti-ferromagnetic layer (not shown) may be inserted next to the fixed magnetization layer  318  to fix the magnetizing direction of the fixed magnetization layer  318  in one direction. More concretely, the anti-ferromagnetic layer may be sandwiched between the fixed magnetization layer  318  and the diffusion barrier layer  400 , or between the diffusion barrier layer  400  and the highly oriented magnetic layer  321 . The same film as in the first embodiment can be used as the anti-ferromagnetic layer. 
     As in the first embodiment the lamination structure of the MTJ element  30  in this embodiment, need not be as shown in  FIG. 8 , and various shapes can be used. Thus, as in the first embodiment, additional layers may be added or existing layers may be omitted. Again, as in the first embodiment, the MTJ element  30  need not have both the diffusion barrier layers  300 ,  400 , but may have only one. 
     As in the first embodiment, the interfaces between the layers may sometimes be unclearly defined in the laminated structure of the MTJ element  30 . For instance, sometimes the fixed magnetization layer  318 , diffusion barrier layer  400 , and highly oriented magnetic layer  321  appear as a monolithic layer. The highly oriented magnetic layer  319 , diffusion barrier layer  300 , and free magnetization layer  322  may also appear as a monolithic layer. In those cases, when a MgO film having thickness 1 nm is used as the tunnel barrier layer  320  of the MTJ element  30 , the number of Hf atoms in the monolithic layer (fixed magnetization layer  318  plus diffusion barrier layer  400  plus highly oriented magnetic layer  321 ) or the monolithic layer (highly oriented magnetic layer  319  plus diffusion barrier layer  300  plus free magnetization free layer  322 ) ranges from 1.886 to 2.500 times the number of Mg atoms in one MTJ element  30 . 
     Each layer in the MTJ element  30  in this embodiment can be formed in the same manner as in the first embodiment. The fabrication of the MRAM  1  having the MTJ element  30  shown in  FIG. 8  is the same as in the first embodiment and its explanation is omitted. 
     In this embodiment, the MTJ element  30  has diffusion barrier layers  300 ,  400  containing Hf film of thickness 0.6 nm to 0.8 nm as in the first embodiment, and magnetic coupling between the fixed magnetization layer  318  and the highly oriented magnetic layer  321  and between the free magnetization layer  322  and the highly oriented magnetic layer  319  is not hindered, so precious metals are prevented from diffusing from the fixed and free magnetization layers  318  and  322  into the highly oriented magnetic layers  319 ,  321 . According to this embodiment, the MR ratio of MTJ element  30  can be kept high. Furthermore, since diffusion of precious metals is blocked by the diffusion barrier layers  300 ,  400 , the tunnel barrier layer  320  can be crystallized at high temperature to obtain a tunnel barrier layer  320  with a good crystal structure so that an MTJ element  30  with a good MR ratio is obtained. 
     As in the first embodiment, the diffusion barrier layers  300 ,  400  in the MTJ element  30  in this embodiment inhibit crystal growth in the highly oriented magnetic layer  319  influenced by the crystal structure of the free magnetization layer  322  when the MTJ element  30  is being fabricated, and crystal growth in the fixed magnetization layer  318  influenced by the crystal structure in the highly oriented magnetic layer  321  is likewise inhibited. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions, and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.