Patent Publication Number: US-2015069554-A1

Title: Magnetic memory and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/874,645, filed Sep. 6, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a magnetic memory and a method of manufacturing the same. 
     BACKGROUND 
     MRAM (Magnetic Random Access Memory) is a memory device using a storage element having a magnetoresistive effect for a memory cell that stores information. MRAM attracts attention as a next-generation memory device featuring the high-speed operation, large capacity, and non-volatility. 
     The magnetoresistive effect is a phenomenon in which electric resistance changes in accordance with the magnetization direction of a ferromagnetic substance. In MRAM, the magnetization direction of such a ferromagnetic substance is used to record information and information is read based on the magnitude of electric resistance corresponding thereto. Accordingly, MRAM can be caused to operate as a memory device. 
     In recent years, a ferromagnetic tunnel junction including two CoFeB ferromagnetic layers and an MgO tunnel barrier layer formed therebetween is used in a magnetoresistive effect element. In the ferromagnetic tunnel junction, a huge MR (Magnetic Resistance) ratio of 100% or more can be obtained due to the TMR (Tunnel Magnetic Resistance) effect. Thus, large-capacity MRAM using an MTJ (Magnetic Tunnel Junction) element making use of the TMR effect attracts expectations and attention as a magnetoresistive effect element. 
     When an MTJ element is used to MRAM, one of two ferromagnetic layers sandwiching the tunnel barrier layer therebetween is set as a reference layer in which the magnetization direction is invariable and the other is set as a storage layer in which the magnetization direction is variable. Information can be stored by associating a state in which the magnetization direction of the reference layer and the magnetization direction of the storage layer are parallel and a state in which both magnetization directions are antiparallel with “0” and “1”. When compared with a case in which both magnetization directions are antiparallel, the resistance (barrier resistance) of the tunnel barrier layer is smaller and the tunnel current is larger when both magnetization directions are parallel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a memory cell array of MRAM according to a first embodiment; 
         FIG. 2  is a plan view showing the memory cell array of MRAM according to the first embodiment; 
         FIG. 3  is a sectional view along an A-A′ line in  FIG. 2 ; 
         FIG. 4A  is a sectional view showing an outline configuration of a magnetoresistive effect element; 
         FIG. 4B  is a diagram illustrating a write operation of the magnetoresistive effect element and is a diagram showing a sectional view of the magnetoresistive effect element in a parallel state; 
         FIG. 4C  is a diagram illustrating the write operation of the magnetoresistive effect element and is a diagram showing a sectional view of the magnetoresistive effect element in an antiparallel state; 
         FIG. 5  is a sectional view showing the configuration of the magnetoresistive effect element according to the first embodiment; 
         FIG. 6  is a graph showing standard electrode potentials of various metallic materials; 
         FIG. 7  is a graph showing shunt bit rates of various metallic materials; 
         FIGS. 8 ,  9 ,  10 ,  11 , and  12  are sectional views showing manufacturing processes of the magnetoresistive effect element according to the first embodiment; 
         FIG. 13  is a sectional view showing the configuration of a modification of the magnetoresistive effect element according to the first embodiment; 
         FIG. 14  is a sectional view showing the configuration of the magnetoresistive effect element according to a second embodiment; 
         FIGS. 15 ,  16 ,  17 ,  18 , and  19  are sectional views showing manufacturing processes of the magnetoresistive effect element according to the second embodiment; 
         FIG. 20  is a graph showing a relationship between a dielectric constant and a breakdown field of various oxides; 
         FIGS. 21A and 21B  are sectional views illustrating the thickness of a tunnel barrier layer  43 ; and 
         FIG. 22  is a diagram showing the dielectric constants and breakdown fields of various oxides. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a magnetic memory is disclosed. The memory includes a conductive layer containing a first metallic material; a stacked body formed above the conductive layer and comprising a first magnetic layer containing a second metallic material, a second magnetic layer, and a tunnel barrier layer formed between the first magnetic layer and the second magnetic layer; and an insulating layer formed on a side face of the stacked body and containing an oxide of the first metallic material, A standard electrode potential of the first metallic material is lower than the standard electrode potential of the second metallic material. 
     In an MRAM manufacturing process, an MTJ film that shows the TMR effect is stacked on an electrode (for example, a contact plug) and the MTJ film is selectively etched to form an MTJ element. At this point, re-deposition of processing residue is occurred on the side face of the MTJ element. The processing residue is mainly material of the layer processed last and re-deposited. If a conductive processing residue is re-deposited near the tunnel barrier layer on the side face of the MTJ element, a short fault occurs between upper and lower ferromagnetic layers of the tunnel barrier layer. If the short fault occurs, the amount of current passing between the upper and lower ferromagnetic layers without passing through the tunnel barrier layer increases. As a result, the ratio of resistance changes reflecting a difference of barrier resistance between the parallel state and the antiparallel state of the magnetization direction decreases, leading to a smaller MR ratio. 
     As a countermeasure to prevent the short fault due to the processing residue, re-depositing an insulating processing residue or selecting processing conditions under which no processing residue is re-deposited can be considered. 
     However, to re-deposit an insulating processing residue on the side face of an MTJ element, after processing the conductive layer directly below the tunnel barrier layer, it is necessary to continue the processing down to an interlayer dielectric layer positioned further below. Thus, etching needs to be performed for a long time, which is hard to implement from the viewpoint of controlling dimensions of the MTJ element and shortening the processing time. The processing conditions for re-depositing no processing residue requires processing a sidewall of the MTJ element by active species which is capable of etching the sidewall, thus an edge (side face portion) of the tunnel barrier layer is damaged, and resulting in the short fault. 
     On the other hand, as the other countermeasure to prevent the short fault due to the processing residue, oxidizing the re-deposits to give insulating properties can be considered. However, if a strong oxidation process is needed to convert the re-deposits into insulating materials, up to the edge (side face portion) of the ferromagnetic layer will be oxidized. Damage due to oxidation of the ferromagnetic layer causes problems such as degradation of the MR ratio. 
     In the present embodiment, by contrast, the above problem is solved by constituting the re-deposit with metallic material whose standard electrode potential is small. 
     The present embodiment will be described below with reference to the drawings. The same reference numerals are attached to the same portions in the drawings. In addition, a duplicate description is provided when necessary. 
     First Embodiment 
     MRAM according to the first embodiment will be described below using  FIGS. 1 to 12 . In MRAM according to the first embodiment, a lower electrode  32  positioned below a stacked body includes a storage layer  42 , the tunnel barrier layer  43 , and a reference layer  44  contains a first metallic material having a standard electrode potential smaller than that of any metallic material contained in the storage layer  42  and the reference layer  44 . Thereby, even if a processing residue of the lower electrode  32  is re-deposited on the side face of a stacked body, the re-deposit can easily be oxidized. The first embodiment will be described in detail below. 
     [Basic Configuration Example of MRAM According to the First Embodiment] 
     A basic configuration example of MRAM according to the first embodiment will be described using  FIG. 1  to  FIG. 4 . 
       FIG. 1  is a circuit diagram showing a memory cell array of MRAM according to the first embodiment. 
     As shown in  FIG. 1 , a memory cell in a memory cell array MA comprises a serially connected body of a magnetoresistive effect element  33  and a switch element (for example, FET) T. One end of the serially connected body (one end of the magnetoresistive effect element  33 ) is electrically connected to a bit line BL and the other end (one end of the switch element T) of the serially connected body is electrically connected to a source line SL. A control terminal of the switch terminal T, for example, a gate electrode of FET is electrically connected to a word line WL. 
     The potential of the word line WL is controlled by a first control circuit  11 . The potentials of the bit line BL and the source line SL are controlled by a second control circuit  12 . 
       FIG. 2  is a plan view showing the memory cell array of MRAM according to the first embodiment.  FIG. 3  is a sectional view along an A-A′ line in  FIG. 2 .  FIG. 3  also shows a cross section of a source line contact  35  together with the cross section of the magnetoresistive effect element  33 . 
     As shown in  FIGS. 2 and 3 , as an example, a plurality of word lines WL and a plurality of dummy word lines DWL extending in a Y direction and a plurality of bit lines BL and a plurality of source lines SL extending in an X direction perpendicular to the Y direction are arranged in the memory cell array MA. Two word lines WL and one dummy word line DWL are alternately arranged along the X direction. The bit line BL and the source line SL are alternately arranged along the Y direction. 
     In addition, a device isolation insulating layer extending in the X direction is provided in a surface region of a p-type semiconductor substrate (for example, a silicon substrate)  21  in the memory cell array MA and this region becomes an element isolation region  26 . The surface region of the semiconductor substrate  21  in which the element isolation insulating layer is not provided becomes an active area AA. That is, the element isolation region  26  and the active area AA are alternately formed along the Y direction. The element isolation insulating layer is formed of, for example, STI (Shallow Trench Isolation). As the element isolation insulating layer, an insulating material having a high filling characteristic such as silicon nitride (SiN) is used. 
     As shown in  FIG. 3 , a select transistor using, for example, an n-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is provided on a semiconductor substrate  21  as the switch element T. The select transistor has a structure in which a recess is formed in the semiconductor substrate  21  and the gate electrode  23  containing, for example, polycrystalline silicon is embedded in this recess. 
     More specifically, a select transistor T includes a gate insulating layer  22 , the gate electrode  23 , and two diffusion layers  25  (a drain-side diffusion layer and a source-side diffusion layer). 
     The gate insulating layer  22  is formed on an inner surface on the lower side of a recess extending in the Y direction formed on the surface of the semiconductor substrate  21 . The gate electrode  23  is formed on the inner surface of the gate insulating layer  22  like filling in the lower side of the recess. This gate electrode  23  corresponds to the word line WL. An insulating layer  24  made of, for example, SiN is formed on top surfaces of the gate insulating layer  22  and the gate electrode  23  like filling in an upper side of the recess. The top surface of the insulating layer  24  is approximately as high as the top surface (top surface of the diffusion layers  25  described later) of the semiconductor substrate  21 . 
     The two diffusion layers  25  are formed in the surface of the semiconductor substrate  21  like sandwiching the gate insulating layer  22 , the gate electrode  23 , and the insulating layer  24  therebetween. The diffusion layers  25  positioned between two neighboring memory cells along the X direction are shared by the two neighboring memory cells. On the other hand, the diffusion layers  25  are isolated by the element isolation region  26  along the Y direction. In other words, the two diffusion layers  25  adjacent along the Y direction are adjacent via the element isolation region  26 . That is, the diffusion layers  25  are positioned outside the formation region of the gate insulating layer  22 , the gate electrode  23 , and the insulating layer  24  in the active area AA. An interlayer dielectric layer  31  is formed on the semiconductor substrate  21  (on the insulating layer  24  and the diffusion layer  25 ). 
     A lower electrode  32 , a magnetoresistive effect element  33 , and an upper electrode  34  are formed in this order on one of the diffusion layers  25  (drain-side diffusion layer) inside the interlayer dielectric layer  31 . 
     More specifically, the lower electrode  32  is formed so as to be in contact with a portion of the top surface of one of the diffusion layers  25  (drain-side diffusion layer) and a portion of the top surface of the insulating layer  24 . In other words, the lower electrode  32  and the diffusion layer  25  partially overlap in a plane. This is because the processing method of the lower electrode  32  and that of the diffusion layer  25  (recess) are different. The plane shape of the interlayer dielectric layer  31  is, for example, square. Details of the lower electrode  32  will be described later using  FIGS. 5 and 6 . 
     The magnetoresistive effect element  33  is formed so as to be in contact with the top surface of the lower electrode  32 . The magnetoresistive effect element  33  has, for example, a circular plane shape and is formed in a cylindrical shape. In other words, the magnetoresistive effect element  33  and a lower electrode  49   a  overlap in a plane. It is desirable that the plane area of the magnetoresistive effect element  33  be smaller than the plane area of the lower electrode  32 . Thereby, the whole bottom surface of the magnetoresistive effect element  33  can be in contact with the top surface of the lower electrode  32 , and reducing the contact resistance thereof. 
       FIG. 4A  is a sectional view showing an outline configuration of a magnetoresistive effect element. Here, the storage layer  42 , the tunnel barrier layer  43 , and the reference layer  44  are mainly shown as the magnetoresistive effect element  33 . 
     As shown in  FIG. 4A , the magnetoresistive effect element  33  contains a stacked body comprising the storage layer  42  that is a ferromagnetic (may simply be called magnetic) layer, the reference layer  44  that is a ferromagnetic layer, and the tunnel barrier layer  43  that is a nonmagnetic layer formed therebetween. 
     The storage layer  42  is a ferromagnetic layer in which the magnetization direction is variable and has a perpendicular magnetic anisotropy that is perpendicular or approximately perpendicular to the film surface (top surface/bottom surface). Here, the magnetization direction is variable indicates that the magnetization direction changes for a predetermined write current. In addition, Being approximately perpendicular means that the direction of residual magnetization is in the range of 45°&lt;θ≦90° with respect to the film surface. 
     The tunnel barrier layer  43  is formed on the storage layer  42 . The tunnel barrier layer  43  is a nonmagnetic layer and is formed of, for example, MgO. 
     The reference layer  44  is formed on the tunnel barrier layer  43 . The reference layer  44  is a ferromagnetic layer in which the magnetization direction is invariable, and has a perpendicular magnetic anisotropy that is perpendicular or approximately perpendicular to the film surface. Here, the magnetization direction is invariable indicates that the magnetization direction does not change for a predetermined write current. That is, the reference layer  44  has a larger reversal energy barrier than the storage layer  42 . 
       FIG. 4B  is a diagram illustrating a write operation of the magnetoresistive effect element and is a diagram showing a sectional view of the magnetoresistive effect element in a parallel state.  FIG. 4C  is a diagram illustrating the write operation of the magnetoresistive effect element and is a diagram showing a sectional view of the magnetoresistive effect element in an antiparallel state. 
     The magnetoresistive effect element  33  is, for example, a spin injection type magnetoresistive effect element. Thus, when data is written into the magnetoresistive effect element  33  or data is read from the magnetoresistive effect element  33 , a current is passed to the magnetoresistive effect element  33  bidirectionally in a direction perpendicular to the film surface. 
     More specifically, data is written into the magnetoresistive effect element  33  as described below. 
     As shown In  FIG. 4B , When a current flows from the lower electrode  32  to the upper electrode  34 , that is, electrons (electrons from the reference layer  44  to the storage layer  42 ) are supplied from the side of the upper electrode  34 , electrons spin-polarized in the same direction as the magnetization direction of the reference layer  44  are injected into the storage layer  42 . In this case, the magnetization direction of the storage layer  42  is aligned with the same direction as the magnetization direction of the reference layer  44 . Thereby, the magnetization direction of the reference layer  44  and the magnetization direction of the storage layer  42  are parallel arrays. In this parallel state, the value of resistance of the magnetoresistive effect element  33  is the smallest. This case is defined as, for example, data “0”. 
     On the other hand, when, as shown in  FIG. 4C , a current flows from the upper electrode  34  to the lower electrode  32 , that is, when electrons (electrons from the storage layer  42  to the reference layer  44 ) are supplied from the side of the lower electrode  32 , electrons spin-polarized in a direction opposite to the magnetization direction of the reference layer  44  injected into the storage layer  42  due to being reflected by the reference layer  44 . Thereby, the magnetization direction of the reference layer  44  and the magnetization direction of the storage layer  42  are antiparallel arrays. In this antiparallel state, the value of resistance of the magnetoresistive effect element  33  is the largest. This case is defined as, for example, data “1”. 
     In addition, data is read from the magnetoresistive effect element  33  as described below. 
     A read current is supplied to the magnetoresistive effect element  33 . This read current is set to a value (value smaller than the write current) at which the magnetization direction of the storage layer  42  is not reversed. The data “0” or “1” can be read by detecting changes of the value of resistance of the magnetoresistive effect element  33  at this point. 
     As shown in  FIG. 3 , the upper electrode  34  is formed so as to be in contact with the top surface of the magnetoresistive effect element  33 . The bit line BL is formed on the upper electrode  34  so as to be in contact therewith. That is, the upper electrode  34  is a bit line contact. 
     In addition, a source line contact  35  is formed on the other diffusion layer  25  (source-side diffusion layer) inside the interlayer dielectric layer  31 . The source line contact  35  is formed so as to be in contact with the top surface of the other diffusion layer  25 . The source line SL is formed on this source line contact  35  so as to be in contact therewith. The other diffusion layer  25  and the source line contact  35  are shared by two neighboring memory cells. 
     Among the three gate electrodes  23  adjacent in the X direction, the two gate electrodes  23  are electrically connected to the magnetoresistive effect element  33  and correspond to the word like WL, and the one gate electrode  23  is not electrically connected to the magnetoresistive effect element  33  and corresponds to the dummy word line DWL. 
     [Configuration of the Magnetoresistive Effect Element According to the First Embodiment] 
     The configuration of the magnetoresistive effect element  33  according to the first embodiment will be described using  FIGS. 5 to 7 . 
       FIG. 5  is a sectional view showing the configuration of the magnetoresistive effect element according to the first embodiment. Here, in addition to the magnetoresistive effect element  33 , the lower electrode  32  positioned below the magnetoresistive effect element  33  and the upper electrode  34  positioned above the magnetoresistive effect element  33  are also shown. 
     As shown in  FIG. 5 , MRAM comprises the lower electrode  32 , the upper electrode  34 , and the magnetoresistive effect element  33 . The lower electrode  32  is formed in an interlayer dielectric layer  31 A and the upper electrode  34  is formed in an interlayer dielectric layer  31 C. The magnetoresistive effect element  33  is formed between the lower electrode  32  and the upper electrode  34 , and an interlayer dielectric layer  32 B is formed between the neighboring magnetoresistive effect elements  33 . 
     The magnetoresistive effect element  33  comprises the underlying layer  41 , the storage layer  42 , the tunnel barrier layer  43 , the reference layer  44 , an intermediate layer  45 , and a shift cancelling layer  46 . 
     The underlying layer  41  is formed on the lower electrode  32 . The underlying layer  41  contains a nonmagnetic material having electric conductivity. 
     The storage layer  42  is formed on the underlying layer  41 . The storage layer  42  contains a ferromagnetic material like, for example, Co and Fe (second metallic material). In addition, B is added to the ferromagnetic material for the purpose of adjusting saturation magnetization or crystal magnetic anisotropy. That is, the storage layer  42  comprises a compound, for example, CoFeB or the like. 
     The tunnel barrier layer  43  is formed on the storage layer  42 . The tunnel barrier layer  43  contains a nonmagnetic material, for example, MgO or the like. However, the present embodiment is not limited to such an example and the tunnel barrier layer  43  may contain metallic oxide such as Al 2 O 3 , MgAlO, ZnO, or TiO. 
     The reference layer  44  is formed on the tunnel barrier layer  43 . The reference layer  44  comprises, for example, a laminated structure of a first magnetic layer, a nonmagnetic layer, and a second magnetic layer formed from the side of the tunnel barrier layer. 
     The first magnetic layer contains a ferromagnetic material like, for example, Co and Fe (second metallic material). B is added to the ferromagnetic material for the purpose of adjusting saturation magnetization or crystal magnetic anisotropy. That is, the first magnetic layer is formed of, for example, a compound such as CoFeB same as the storage layer  42 . The first magnetic layer is a layer contributing to the MR ratio. The nonmagnetic layer is formed between the first magnetic layer and the second magnetic layer. The nonmagnetic layer contains a nonmagnetic material like Ta, W, or Hf. The second magnetic layer contains a ferromagnetic material and a nonmagnetic material. For example, Pt can be cited as the nonmagnetic material. As the ferromagnetic material, for example, Co is contained as a ferromagnetic material. That is, the second magnetic layer comprises a stacked film, for example, a Pt layer and a Co layer. This stacked film comprises a plurality of Pt layers and a plurality of Co layers being alternately stacked. The second magnetic layer contributes to perpendicular magnetic anisotropy. 
     The shift cancelling layer  46  is formed on the reference layer  44  via the intermediate layer  45 . The intermediate layer  45  contains, for example, a nonmagnetic material having electric conductivity such as Ru. The shift cancelling layer  46  is a magnetic layer in which the magnetization direction is invariable and has a perpendicular magnetic anisotropy that is perpendicular or approximately perpendicular to the film surface. In addition, the magnetization direction thereof is a direction opposite to the magnetization direction of the reference layer  44 . Thereby, the shift cancelling layer  46  can cancel out, a leakage magnetic field from the reference layer  44 , which is applied to the storage layer  42 . In other words, the shift cancelling layer  46  has an effect of adjusting, an offset of reversal characteristics for the storage layer  42  due to the leakage magnetic field from the reference layer  44 , to the opposite direction. This shift cancelling layer  46  comprises, for example, an artificial lattice having a stacked structure of a ferromagnetic material like such as Ni, Fe, or Co and a nonmagnetic material such as Cu, Pd, or Pt. The upper electrode  34  is formed on the shift cancelling layer  46 . 
     In addition, the plane shape of the underlying layer  41 , the storage layer  42 , the tunnel barrier layer  43 , the reference layer  44 , the intermediate layer  45 , and the shift cancelling layer  46  is, for example, circular. Thus, the magnetoresistive effect element  33  is formed in a pillar shape. However, the present embodiment is not limited to such an example and the plane shape of the magnetoresistive effect element  33  may be square, rectangular, or elliptic. 
     In addition, the storage layer  42  and the reference layer  44  may have dimensional differences in a plane. For example, the diameter of the reference layer  44  in a plane may be smaller than the diameter of the storage layer  42 . Moreover, an insulating layer having dimensional differences from the storage layer  42  may be formed as a sidewall of the reference layer  44 . Thereby, an electric short between the storage layer  42  and the reference layer  44  can be prevented. 
     In addition, the order of arrangement may be reversed in the configuration of the magnetoresistive effect element  33 . That is, the shift cancelling layer  46 , the intermediate layer  45 , the second magnetic layer  44 C, the nonmagnetic layer  44 B, the first magnetic layer  44 A, the tunnel barrier layer  43 , the storage layer  42 , and the underlying layer  41  may be formed in this order on the lower electrode  32 . 
     The lower electrode  32  in the first embodiment comprises a conductive layer containing the first metallic material having a standard electrode potential smaller than that of the second metallic material (for example, Co and Fe) contained in the storage layer  42  and the reference layer  44 . The lower electrode  32  will be described in detail below. 
       FIG. 6  is a graph showing standard electrode potentials of various metallic materials. 
     As shown in  FIG. 6 , when the storage layer  42  and the reference layer  44  comprise CoFeB, the lower electrode  32  contains the first metallic material having a standard electrode potential smaller than that of Fe having the smallest standard electrode potential among CoFeB. That is, the standard electrode potential of the first metallic material is smaller than the standard electrode potential (−0.447 V) of Fe. Thus, the first metallic material is a material that is more easily oxidized than Fe. As illustrated, such a first metallic material contains one of Be, Al, Zn, Mg, Yb, Y, Ga, Ca, Eu, Er, Ho, Lu, Zr, Mn, Nd, Sc, Cr, Sr, Tb, Sm, Ce, Dy, Tm, Gd, V, Hf, Ta, Nb, Pa, Ti, and Th or an alloy including at least two of the elements. 
     In addition, the lower electrode  32  is processed a portion of the surface at an end of thereof at the time of processing of the magnetoresistive effect element  33 . Thus, the lower electrode  32  has a step along the plane shape of the magnetoresistive effect element  33 . In other words, the top surface at the end of the lower electrode  32  is lower than the top surface in the center portion thereof. 
     The insulating layer  32 B is formed on the side face (perimeter) of the magnetoresistive effect element  33 . This insulating layer  32 B is obtained by oxidizing processing residue of the lower electrode  32 . More specifically, the insulating layer  32 B is obtained by oxidizing re-deposits formed by processing the surface of the lower electrode  32  at the time of processing the magnetoresistive effect element  33 . That is, the insulating layer  32 B is an oxide of the first metallic material contained in the lower electrode  32 . In addition, the insulating layer  32 B is a re-deposit of the lower electrode  32  and so has a taper shape in which the thickness becomes thinner from the lower electrode  32  toward the upper electrode  34 . 
     A material having a still lower standard electrode potential is desirable as the first metallic material contained in the lower electrode  32  and the insulating layer  32 B. For example, as shown in  FIG. 7 , Hf having a still lower standard electrode potential has, when compared with Ta, a lower shunt bit rate. From the viewpoint of the shunt bit rate, when compared with Ta, it is desirable to use Hf having a still lower standard electrode potential as the first metallic material. 
     An insulating layer  47  comprising, for example, SiN, SiO X , MgO, or AlO X  is formed on the side face of the insulating layer  32 B. The insulating layer  47  functions as a protective layer of the magnetoresistive effect element  33 . 
     [Method of Manufacturing the Magnetoresistive Effect Element According to the First Embodiment] 
     The method of manufacturing the magnetoresistive effect element  33  according to the first embodiment will be described using  FIGS. 8 to 12 . 
       FIGS. 8 to 12  are sectional views showing manufacturing processes of the magnetoresistive effect element according to the first embodiment. 
     First, as shown in  FIG. 8 , the interlayer dielectric layer  31 A containing, for example, SiO X  is formed on the semiconductor substrate  21  by, for example, CVD (Chemical Vapor Deposition) method. Next, a hole not shown reaching the semiconductor substrate  21  is formed in the interlayer dielectric layer  31 A by, for example, lithography technology. 
     Next, the lower electrode  32  is formed in the hole of the interlayer dielectric layer  31 A by, for example, CVD method. When the storage layer  42  and the reference layer  44  comprises CoFeB, the lower electrode  32  contains the first metallic material having a standard electrode potential smaller than that of Fe having the smallest standard electrode potential. The first metallic material contains one of Be, Al, Zn, Mg, Yb, Y, Ga, Ca, Eu, Er, Ho, Lu, Zr, Mn, Nd, Sc, Cr, Sr, Tb, Sm, Ce, Dy, Tm, Gd, V, Hf, Ta, Nb, Pa, Ti, and Th or an alloy including at least two of the elements. 
     Next, as shown in  FIG. 9 , the underlying layer  41  is formed on the lower electrode  32  and the interlayer dielectric layer  31 A by, for example, sputtering method. The underlying layer  41  contains a nonmagnetic material having electric conductivity. 
     Next, the storage layer  42  is formed on the underlying layer  41  by, for example, sputtering method. The storage layer  42  contains, for example, a ferromagnetic material such as Co and Fe (second metallic material). In addition, B is added to the ferromagnetic material for the purpose of adjusting saturation magnetization or crystal magnetic anisotropy. That is, the storage layer  42  comprises, for example, a compound such as CoFeB. 
     Next, the tunnel barrier layer  43  is formed on the storage layer  42 . The tunnel barrier layer  43  contains, for example, a nonmagnetic material such as MgO. The MgO layer constituting the tunnel barrier layer  43  may be formed by a direct film formation of MgO layer by sputtering method targeting MgO. Moreover, the MgO layer may be formed by forming an Mg layer by sputtering method targeting Mg and then oxidizing the Mg layer. As an oxidation method of the Mg layer, oxidation by oxygen gas, oxygen plasma, oxygen radical, or ozone can be cited. To improve the MR ratio, it is desirable to directly form the MgO layer by the sputtering method targeting MgO. In addition, the MgO layer may be formed by MBE (Molecular Beam Epitaxy) method, the ALD (Atomic Layer Deposition) method, or the CVD method. 
     Next, the reference layer  44  is formed on the tunnel barrier layer  43  by, for example, sputtering method. The reference layer  44  comprises, for example, a stacked structure of a first magnetic layer, a nonmagnetic layer, and a second magnetic layer formed from the side of the tunnel barrier layer. 
     The first magnetic layer contains, for example, a ferromagnetic material such as Co and Fe (second metallic material). In addition, B is added to the ferromagnetic material for the purpose of adjusting saturation magnetization or crystal magnetic anisotropy. That is, the first magnetic layer comprises, for example, a compound such as CoFeB same as the storage layer  42 . The first magnetic layer is a layer contributing to the MR ratio. The nonmagnetic layer is formed between the first magnetic layer and the second magnetic layer. The nonmagnetic layer contains a nonmagnetic material such as Ta, W, or Hf. The second magnetic layer contains a ferromagnetic material and a nonmagnetic material. For example, Pt can be cited as the nonmagnetic material. As the ferromagnetic material, for example, Co is contained as a ferromagnetic material. That is, the second magnetic layer comprises, for example, a stacked film of a Pt layer and a Co layer. This stacked film comprises a plurality of Pt layers and a plurality of Co layers being alternately stacked. The second magnetic layer contributes to perpendicular magnetic anisotropy. The second magnetic layer as described above is formed by changing the target in the sputtering method. 
     Next, the intermediate layer  45  made of Ru is formed on the reference layer  44  by, for example, sputtering method, and the shift cancelling layer  46  is formed on this intermediate layer  45  by, for example, sputtering method. The shift cancelling layer  46  comprises an artificial lattice having a stacked structure of a ferromagnetic material like such as Ni, Fe, or Co and a nonmagnetic material such as Cu, Pd, or Pt. 
     Thereafter, each layer of the magnetoresistive effect element  33  is crystallized by performing annealing. 
     Next, as shown in  FIG. 10 , a hard mask not shown is formed on the shift cancelling layer  46  and is patterned so that the plane shape thereof is, for example, circular. The hard mask comprises a metallic material having electric conductivity, for example, comprises TiN. The metallic material is not limited to the above example and the hard mask may comprise a film containing one of Ti, Ta, and W or a stacked film thereof. Thereby, the hard mask does not need to be removed later and can be used as a contact portion for the upper electrode  34 . 
     Next, the shift cancelling layer  46 , the intermediate layer  45 , the reference layer  44 , the tunnel barrier layer  43 , the storage layer  42 , and the underlying layer  41  are processed by physical etching such as ion milling method using the hard mask as a mask. Thereby, the shift cancelling layer  46 , the intermediate layer  45 , the reference layer  44 , the tunnel barrier layer  43 , the storage layer  42 , and the underlying layer  41  are patterned in the same manner as the hard mask and the plane shape thereof becomes circular. 
     At this point, down to the surface of the lower electrode  32  is processed. Thereby, the first metallic material contained in the lower electrode  32  is re-deposited on the side face of the magnetoresistive effect element  33 , so that the conductive layer  32 A is formed. Therefore, the conductive layer  32 A contains the first metallic material having a standard electrode potential smaller than that of Fe having the smallest standard electrode potential among CoFeB contained in the storage layer  42  and the reference layer  44 . Then, electrons move from the conductive layer  32 A to the storage layer  42  and the reference layer  44 . As a result, the storage layer  42  and the reference layer  44  are charged at δ− and the conductive layer  32 A is charged at δ+. 
     Next, as shown in  FIG. 11 , the conductive layer  32 A is oxidized by one of various oxidation methods to form an insulating layer  32 B. Thus, the insulating layer  32 B is an oxide of the first metallic material contained in the lower electrode  32 . Oxidation by an oxygen gas, oxygen plasma, oxygen radical, or ozone can be cited as the oxidation method. At this point, the storage layer  42  and the reference layer  44  are charged at δ− and the conductive layer  32 A is charged at δ+ and thus, the conductive layer  32 A is more likely to be oxidized than the storage layer  42  and the reference layer  44 . Therefore, only the conductive layer  32 A can be oxidized even by weak oxidation. That is, oxidation of the storage layer  42  and the reference layer  44  by the oxidation process can be suppressed. 
     Next, as shown in  FIG. 12 , the insulating layer  47  comprising, for example, SiN, SiO X , MgO, or AlO X  is formed by, for example, sputtering method, CVD method, or ALD method. The insulating layer  47  functions as a protective layer of the magnetoresistive effect element  33  in the next step. 
     Next, an interlayer dielectric layer  31 B containing, for example, SiO X  is formed on the entire surface by, for example, CVD method. Thereby, the interlayer dielectric layer  31 B is embedded between the neighboring magnetoresistive effect elements  33 . Thereafter, the interlayer dielectric layer  31 B formed on the magnetoresistive effect elements  33  is etched back after being planarized. Thereby, the top surface of the magnetoresistive effect elements  33  is exposed. 
     Next, as shown in  FIG. 5 , the interlayer dielectric layer  31 C containing, for example, SiO X  is formed on the magnetoresistive effect elements  33  and the interlayer dielectric layer  31 B. Next, a hole not shown reaching the magnetoresistive effect elements  33  is formed in the interlayer dielectric layer  31 A by, for example, lithography technology. Thereafter, the upper electrode  34  is formed in the hole by, for example, CVD method and electrically connected to the magnetoresistive effect elements  33 . 
     In this manner, the magnetoresistive effect elements  33  according to the first embodiment is formed. 
     [Effect According to the First Embodiment] 
     According to the first embodiment described above, the lower electrode  32  positioned below a stacked body includes the storage layer  42 , the tunnel barrier layer  43 , and the reference layer  44  contains the first metallic material having the standard electrode potential smaller than that of the second metallic material contained in the storage layer  42  and the reference layer  44 . Then, the conductive layer  32 A containing the first metallic material is re-deposited on the side face of the stacked body by the processing of the lower electrode  32 . Because the first metallic material can easily be oxidized, the insulating layer  32 B can be formed by oxidizing the conductive layer  32 A while oxidation damage to the storage layer  42  and the reference layer  44  being suppressed. That is, degradation of the MR ratio can be suppressed while the short fault in the stacked body is prevented. 
     [Modification of the First Embodiment] 
       FIG. 13  is a sectional view showing the configuration of a modification of the magnetoresistive effect element according to the first embodiment. 
     As shown in  FIG. 13 , in the modification, the lower electrode  32  comprises a first electrode  32 C on the lower side and a second electrode  32 D on the upper side. In other words, the second electrode  32 D is formed near the surface of the lower electrode  32 . 
     The first electrode  32 C contains various conductive materials with high embedding properties. The first electrode  32 C contains, for example, TiN, but is not limited to such an example and may contain W. 
     The second electrode  32 D is formed on the first electrode  32 C and is formed in contact with the magnetoresistive effect element  33 . The second electrode  32 D contains the first metallic material, that is, one of Be, Al, Zn, Mg, Yb, Y, Ga, Ca, Eu, Er, Ho, Lu, Zr, Mn, Nd, Sc, Cr, Sr, Tb, Sm, Ce, Dy, Tm, Gd, V, Hf, Ta, Nb, Pa, Ti, and Th or an alloy including at least two of the elements. 
     When the magnetoresistive effect element  33  is processed, a portion of the surface at an end of the second electrode  32 D is processed. Thus, the second electrode  32 D has a step along the plane shape of the magnetoresistive effect element  33 . In other words, the top surface at the end of the second electrode  32 D is lower than the top surface in the center portion thereof. 
     In this way, according to the modification, the first electrode  32 C containing various metallic materials with high embedding properties is formed on the lower side of the lower electrode  32  and the second electrode  32 D containing the first metallic material that is easily oxidized is formed only in a portion of the surface on the upper side. Thereby, restrictions of materials in manufacturing processes or the like can be limited to a necessary minimum. 
     Second Embodiment 
     MRAM according to the second embodiment will be described below using  FIGS. 14 to 19 . In MRAM according to the first embodiment, re-deposition of materials of the lower electrode  32  by the surface of the lower electrode  32  being processed poses a problem. Thus, in the first embodiment, the lower electrode  32  contains the first metallic material having the standard electrode potential smaller than that of the second metallic material contained in the storage layer  42  and the reference layer  44 . 
     However, even if the surface of the lower electrode  32  is not substantially processed (when the minimum is processed), re-deposition of materials of the underlying layer  41  positioned thereon poses a problem. 
     In the second embodiment, by contrast, the underlying layer  41  contains the first metallic material having a standard electrode potential smaller than that of the second metallic material contained in the storage layer  42  and the reference layer  44 . Thereby, even if the processing residue of the underlying layer  41  is re-deposited on the side face of a stacked body, the re-deposit can easily be oxidized. The second embodiment will be described in detail below. 
     In the second embodiment, the description similar to that in the first embodiment is omitted and differences will mainly be described. 
     [Configuration of the Magnetoresistive Effect Element According to the Second Embodiment] 
     The configuration of the magnetoresistive effect element  33  according to the second embodiment will be described using  FIG. 14 . 
       FIG. 14  is a sectional view showing the configuration of the magnetoresistive effect element according to the second embodiment. Here, in addition to the magnetoresistive effect element  33 , the lower electrode  32  positioned below the magnetoresistive effect element  33  and the upper electrode  34  positioned above the magnetoresistive effect element  33  are also shown. 
     As shown in  FIG. 14 , the difference of the second embodiment from the first embodiment is that the underlying layer  41  contains the first metallic material having the standard electrode potential smaller than that of the second metallic material contained in the storage layer  42  and the reference layer  44 . 
     More specifically, when the storage layer  42  and the reference layer  44  comprise CoFeB, the underlying layer  41  in the second embodiment contains the first metallic material having a standard electrode potential smaller than that of Fe having the smallest standard electrode potential among CoFeB. That is, the standard electrode potential of the first metallic material is smaller than the standard electrode potential (−0.447 V) of Fe. Thus, the first metallic material is a material that is more easily oxidized than Fe. The first metallic material as described above contains one of Be, Al, Zn, Mg, Yb, Y, Ga, Ca, Eu, Er, Ho, Lu, Zr, Mn, Nd, Sc, Cr, Sr, Tb, Sm, Ce, Dy, Tm, Gd, V, Hf, Ta, Nb, Pa, Ti, and Th or an alloy including at least two of the elements. 
     An insulating layer  41 B is formed on the side face (perimeter) of the magnetoresistive effect element  33 . The insulating layer  41 B is obtained by oxidizing processing residue of the underlying layer  41 . More specifically, the insulating layer  41 B is obtained by oxidizing re-deposits formed by processing the underlying layer  41  when the magnetoresistive effect element  33  is processed. That is, the insulating layer  41 B is an oxide of the first metallic material contained in the underlying layer  41 . In addition, the insulating layer  41 B is a re-deposit of the underlying layer  41  and so has a taper shape in which the thickness becomes thinner from the lower side toward the upper side. 
     A material having a still lower standard electrode potential is desirable as the first metallic material contained in the underlying layer  41  and the insulating layer  41 B. For example, as shown in  FIG. 7 , Hf having a still lower standard electrode potential has, when compared with Ta, a lower shunt bit rate. From the viewpoint of the shunt bit rate, when compared with Ta, it is desirable to use Hf having a still lower standard electrode potential as the first metallic material. 
     The lower electrode  32  contains various conductive materials with high embedding properties. The lower electrode  32  contains, for example, TiN, but is not limited to such an example and may contain W. In addition, when the magnetoresistive effect element  33  is processed, the lower electrode  32  is substantially not processed. Thus, the top surface of the lower electrode  32  is substantially flat. 
     [Method of Manufacturing the Magnetoresistive Effect Element According to the Second Embodiment] 
     The method of manufacturing the magnetoresistive effect element  33  according to the second embodiment will be described using  FIGS. 15 to 19 . 
       FIGS. 15 to 19  are sectional views showing manufacturing processes of the magnetoresistive effect element according to the second embodiment. 
     First, as shown in  FIG. 15 , the interlayer dielectric layer  31 A containing, for example, SiO X  is formed on the semiconductor substrate  21  by, for example, CVD method. Next, a hole not shown reaching the semiconductor substrate  21  is formed in the interlayer dielectric layer  31 A by, for example, lithography technology. 
     Next, the lower electrode  32  is formed in the hole of the interlayer dielectric layer  31 A by, for example, CVD method. The lower electrode  32  contains various conductive materials with high embedding properties. The lower electrode  32  contains, for example, TiN, but is not limited to such an example and may contain W. 
     Next, as shown in  FIG. 16 , the underlying layer  41  is formed on the lower electrode  32  and the interlayer dielectric layer  31 A by, for example, sputtering method. When the storage layer  42  and the reference layer  44  comprise CoFeB, the underlying layer  41  contains the first metallic material having a standard electrode potential smaller than that of Fe having the smallest standard electrode potential. The first metallic material contains one of Be, Al, Zn, Mg, Yb, Y, Ga, Ca, Eu, Er, Ho, Lu, Zr, Mn, Nd, Sc, Cr, Sr, Tb, Sm, Ce, Dy, Tm, Gd, V, Hf, Ta, Nb, Pa, Ti, and Th or an alloy including at least two of the elements. 
     Thereafter, like in the first embodiment, the storage layer  42 , the tunnel barrier layer  43 , the reference layer  44 , the intermediate layer  45 , and the shift cancelling layer  46  are formed in this order on the underlying layer  41 . Next, each layer of the magnetoresistive effect element  33  is crystallized by performing annealing. 
     Next, as shown in  FIG. 17 , a hard mask not shown is formed on the shift cancelling layer  46  and patterned so that the plane shape thereof is, for example, circular. Thereafter, the shift cancelling layer  46 , the intermediate layer  45 , the reference layer  44 , the tunnel barrier layer  43 , the storage layer  42 , and the underlying layer  41  are processed by physical etching such as ion milling method using the hard mask as a mask. Thereby, the shift cancelling layer  46 , the intermediate layer  45 , the reference layer  44 , the tunnel barrier layer  43 , the storage layer  42 , and the underlying layer  41  are patterned in the same manner as the hard mask and the plane shape thereof becomes circular. 
     At this point, even if the surface of the lower electrode  32  is substantially not processed, a conductive layer  41 A is formed on the side face of the magnetoresistive effect element  33  by the first metallic material contained in the underlying layer  41  being re-deposited thereon. Therefore, the conductive layer  41 A contains the first metallic material having the standard electrode potential smaller than that of Fe having the smallest standard electrode potential among CoFeB contained in the storage layer  42  and the reference layer  44 . Then, electrons move from the conductive layer  41 A to the storage layer  42  and the reference layer  44 . As a result, the storage layer  42  and the reference layer  44  are charged at δ− and the conductive layer  41 A is charged at δ+. 
     Next, as shown in  FIG. 18 , the conductive layer  41 A is oxidized by one of various oxidation methods to form the insulating layer  41 B. Thus, the insulating layer  41 B is an oxide of the first metallic material contained in the lower electrode  32 . Oxidation by an oxygen gas, oxygen plasma, oxygen radical, or ozone can be cited as the oxidation method. At this time, the storage layer  42  and the reference layer  44  are charged at  6 - and the conductive layer  41 A is charged at 6+ and thus, the conductive layer  41 A is more likely to be oxidized than the storage layer  42  and the reference layer  44 . Therefore, only the conductive layer  41 A can be oxidized even by weak oxidation. That is, oxidation of the storage layer  42  and the reference layer  44  by the oxidation process can be suppressed. 
     Next, like in the first embodiment, as shown in  FIG. 19 , the insulating layer  47  and the interlayer dielectric layer  31 B are formed. Thereby, the interlayer dielectric layer  31 B is embedded between the neighboring magnetoresistive effect elements  33 . Thereafter, the top surface of the magnetoresistive effect element  33  is exposed, then, as shown in  FIG. 5 , the interlayer dielectric layer  31 C is formed. Next, a hole not shown reaching the magnetoresistive effect element  33  is formed in the interlayer dielectric layer  31 A and the upper electrode  34  is formed in the hole and electrically connected to the magnetoresistive effect element  33 . 
     In this manner, the magnetoresistive effect elements  33  according to the second embodiment is formed. 
     [Effect According to the Second Embodiment] 
     According to the second embodiment described above, the underlying layer  41  positioned below a stacked body comprising the storage layer  42 , the tunnel barrier layer  43 , and the reference layer  44  contains the first metallic material having the standard electrode potential smaller than that of the second metallic material contained in the storage layer  42  and the reference layer  44 . Then, even if the lower electrode  32  is substantially not processed, the conductive layer  41 A containing the first metallic material is re-deposited on the side face of the stacked body by the processing of the underlying layer  41 . Because the first metallic material can easily be oxidized, the insulating layer  41 B can be formed by oxidizing the conductive layer  41 A while oxidation damage to the storage layer  42  and the reference layer  44  being suppressed. That is, degradation of the MR ratio can be suppressed while the short fault in the stacked body is prevented. 
     Third Embodiment 
     MRAM according to a third embodiment will be described below using  FIGS. 20 to 22 . 
     When, for example, Ta is used as the underlying layer  41 , the insulating layer  41 B made of an oxide (Ta 2 O 5 ) of Ta (re-deposit) is formed on the side face of the magnetoresistive effect element  33 . The dielectric constant of Ta 2 O 5  is 24 which is larger than the dielectric constant 9.65 of MgO used in the tunnel barrier layer  43 . As will be described later, the breakdown field decreases with an increasing of dielectric constant. That is, the breakdown field of Ta 2 O 5  is smaller than the breakdown field of MgO. Thus, even if a short fault is prevented by oxidizing a re-deposit, the withstand voltage is degraded if the breakdown field of oxide of the re-deposit is smaller than that of MgO. In addition, electronic polarization increases and thus, a leak current via a re-deposit is more likely to flow and the breakdown field decreases. 
     In contrast, the third embodiment is a modification of the second embodiment and an example of suppressing degradation of the breakdown field due to a re-deposit by limiting the dielectric constant of oxide of the first metallic material contained in the underlying layer  41  in accordance with the thickness of the tunnel barrier layer  43 . The third embodiment will be described in detail below. 
     In the third embodiment, the description similar to that in the second embodiment is omitted and differences will mainly be described. 
     [Dielectric Constant of Oxide of the First Metallic Material Contained in the Underlying Layer According to the Third Embodiment] 
     The dielectric constant of oxide of the first metallic material contained in the underlying layer  41  according to the third embodiment will be described using  FIGS. 20 to 22 . 
       FIG. 20  is a graph showing the relationship between the dielectric constant and the breakdown field of various oxides. 
     As shown in  FIG. 20 , the dielectric constant k and the breakdown field Ebd [MV/cm] of various oxides have the relationship of Formula (1). 
         Ebd= 22.511× k   (−0.5424)   (1)
 
       FIGS. 21A and 21B  are sectional views illustrating the thickness of the tunnel barrier layer  43 . More specifically,  FIG. 21A  is a diagram showing the ideal thickness of tunnel barrier layer  43  after the oxidation process of a re-deposit and  FIG. 21B  is a diagram showing the actual thickness of the tunnel barrier layer  43  after the oxidation process of the re-deposit. 
     As shown in  FIG. 21A , the thickness of the tunnel barrier layer  43  is a uniform thickness d1 after the ideal processing of the magnetoresistive effect element  33 . However, as shown in  FIG. 21B , in the actual processing of the magnetoresistive effect element  33 , the oxidation region increases under the influence of the oxidation step of the re-deposit. Thus, the thickness of the tunnel barrier layer  43  increases near the end thereof. That is, while the thickness of the tunnel barrier layer  43  is the thickness d1 in the center portion, the thickness thereof at an end is a thickness d2, which is thicker than the thickness d1. A breakdown field Ebd2 to be satisfied at an end of the tunnel barrier layer  43  is represented by formula (2) using the breakdown field Ebd1 in the center portion. 
         Ebd 2=( d 1/ d 2) Ebd 1  (2)
 
     As shown in Formula (2), the breakdown field Ebd2 at an end of the tunnel barrier layer  43  can be smaller as the thickness thereof increases. In other words, with an increasing thickness at an end of the tunnel barrier layer  43 , the distance between electrodes, that is between the storage layer  42  and the reference layer  44  increases. Thereby, the breakdown field Ebd2 at an end of a large thickness may be smaller than the breakdown field Ebd1 in the center portion which has a small thickness. 
     Here, in order to prevent the decreasing of breakdown field due to the re-deposit, a breakdown field Ebd3 of oxide (insulating layer  41 B) of the re-deposit needs to be equal to or more than the breakdown field Ebd2 at the end in the tunnel barrier layer  43 . Thus, from formulas (1) and (2), formula (3) holds for the breakdown field Ebd3. 
         Ebd 3=22.511× k   (−0.5424) ≧( d 1/ d 2) Ebd 1  (3)
 
       FIG. 22  is a diagram showing the dielectric constants and breakdown fields of various oxides. Here, each breakdown field is calculated from formula (1). “NG” in  FIG. 22  indicates that the standard electrode potential in the second embodiment is not in the allowable range. 
     In order for the insulating layer  41 B to satisfy formula (3), similarly an oxide of the first metallic material contained in the underlying layer  41  needs to satisfy formula (3). 
     For example, when the tunnel barrier layer  43  has the ideal thickness shown in  FIG. 21A , the insulating layer  41 B (oxide of the first metallic material) needs to have a dielectric constant equal to or less a dielectric constant of MgO. That is, as shown in  FIG. 22 , it is desirable to use Be, Zn, or Al as the first metallic material. It is noted that Ga, Y, or Yb may be all right since it has a dielectric constant approximately equal to that of MgO. 
     As described above, actually, the tunnel barrier layer  43  has the shape shown in  FIG. 21B , the allowable range of the dielectric constant of oxide of the first metallic material broadens. More specifically, the dielectric constant of oxide of the first metallic material may be larger than that of MgO. In addition, the allowable range of the dielectric constant of oxide of the first metallic material broadens as the thickness of the tunnel barrier layer  43  increases. Thus, the first metallic material may be Be, Al, Zn, Mg, Yb, Y, Ga, Ca, Eu, Er, Ho, Lu, Zr, Mn, Nd, Sc, Cr, Sr, Tb, Sm, Ce, Dy, Tm, Gd, V, or Hf. 
     In addition, the underlying layer  41  may contain a nitride or boride of the first metallic material. 
     Particularly, the underlying layer  41  desirably contains a boride of the first metallic material. This is because of the following reason. When the underlying layer  41  contains the boride of the first metallic material, boron atoms become boron oxide (B 2 O 3 ) after the processing step of the magnetoresistive effect element  33  and the oxidation step of a re-deposit. Then, boron oxide (B 2 O 3 ) is formed on the side face of the tunnel barrier layer  43  together with oxide of the first metallic material. In other words, the insulating layer  41 B contains the oxide of the first metallic material and B 2 O 3 . As shown in  FIG. 22 , B 2 O 3  has a larger breakdown field than MgO. As a result, the breakdown field can be made larger than a case where only the oxide of the first metallic material is formed. 
     On the other hand, when the underlying layer  41  contains a nitride of the first metallic material, nitrogen atoms are desorbed from the magnetoresistive effect element  33  after the processing step of the magnetoresistive effect element  33  and the oxidation step of a re-deposit. Thus, only the oxide (insulating layer  42 B) of the first metallic material is formed on the side face of the tunnel barrier layer  43 . That is, this is the same as in a case where the underlying layer  41  contains the first metallic material. 
     [Effect According to the Third Embodiment] 
     According to the third embodiment described above, the dielectric constant of oxide of the first metallic material contained in the underlying layer  41  is limited in accordance with the thickness at the end of the tunnel barrier layer  43 . More specifically, control is exercised so that the above formula (3) holds. Thereby, the reduction of breakdown field due to the oxide of a re-deposit (insulating layer  41 B) caused by the underlying layer  41  can be suppressed. 
     In the present example, the limitation of dielectric constant in the third embodiment is applied to the underlying layer  41  in the second embodiment, but it is not limited to such an example. The limitation of dielectric constant in the third embodiment can also be applied to the lower electrode  32  in the first embodiment. 
     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.