JP2007317734A - Storage device and memory - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a storage device that can improve thermal stability even when a material in lower saturated magnetization is used for a storage layer. <P>SOLUTION: The storage device is configured such that: a storage layer 17 for holding information depending on the magnetization state of a magnetic material is provided; a pinned magnetic layer 31 is provided to this storage layer 17 through an intermediate layer 16 formed of an insulator; information is recorded to the storage layer 17 when direction of magnetization M1 of the storage layer 17 is varied by injecting the electrons spin-polarized in the laminating direction; and texture structures 11A, 17A having the uniaxial anisotropy are formed on the upper surface of the ferromagnetic layer forming the storage layer 17 or at the lower layer of this ferromagnetic layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention comprises a storage layer for storing the magnetization state of a ferromagnetic layer as information and a magnetization fixed layer whose magnetization direction is fixed, and a current is passed in a direction perpendicular to the film surface to spin-polarized electrons. The present invention relates to a memory element that changes the magnetization direction of the memory layer by injecting, and a memory including the memory element, and is suitable for application to a nonvolatile memory.

In information devices such as computers, DRAMs with high speed and high density are widely used as random access memories.
However, since DRAM is a volatile memory in which information disappears when the power is turned off, a nonvolatile memory in which information does not disappear is desired.

  As a candidate for a non-volatile memory, a magnetic random access memory (MRAM) that records information by magnetization of a magnetic material has attracted attention and is under development (for example, see Non-Patent Document 1).

  In the MRAM, current is supplied to two types of address lines (word lines and bit lines) that are substantially orthogonal to each other, and the magnetization of the magnetic layer of the magnetic memory element at the intersection of the address lines is caused by a current magnetic field generated from each address line. Inverted information is recorded.

A schematic diagram (perspective view) of a general MRAM is shown in FIG.
A drain region 108, a source region 107, and a gate electrode 101 constituting a selection transistor for selecting each memory cell are formed in a portion separated by the element isolation layer 102 of the semiconductor substrate 110 such as a silicon substrate. Has been.
A word line 105 extending in the front-rear direction in the figure is provided above the gate electrode 101.
The drain region 108 is formed in common to the left and right selection transistors in the drawing, and a wiring 109 is connected to the drain region 108.
A magnetic storage element 103 having a storage layer whose magnetization direction is reversed is disposed between the word line 105 and the bit line 106 disposed above and extending in the horizontal direction in the drawing. The magnetic memory element 103 is composed of, for example, a magnetic tunnel junction element (MTJ element).
Further, the magnetic memory element 103 is electrically connected to the source region 107 via the horizontal bypass line 111 and the vertical contact layer 104.
By applying current to each of the word line 105 and the bit line 106, a current magnetic field is applied to the magnetic memory element 103, thereby reversing the magnetization direction of the memory layer of the magnetic memory element 103 and recording information. be able to.

In order to stably hold recorded information in a magnetic memory such as MRAM, it is necessary that a magnetic layer (storage layer) for recording information has a certain coercive force.
On the other hand, in order to rewrite the recorded information, a certain amount of current must be passed through the address wiring.
However, as the elements constituting the MRAM become finer, the address wiring becomes thinner, so that a sufficient current cannot flow.

Accordingly, attention has been paid to a memory having a configuration using magnetization reversal by spin injection as a configuration capable of performing magnetization reversal with a smaller current (for example, Patent Document 1, Patent Document 2, Non-Patent Document 2, and Non-Patent Document 3). reference).
Magnetization reversal by spin injection is to cause magnetization reversal in another magnetic material by injecting spin-polarized electrons that have passed through the magnetic material into another magnetic material.

  For example, when a current is passed through a giant magnetoresistive element (GMR element) or a magnetic tunnel junction element (MTJ element) in a direction perpendicular to the film surface, magnetization of at least a part of the magnetic layer of these elements is performed. Can be reversed.

  Magnetization reversal by spin injection has an advantage that magnetization reversal can be realized without increasing current even if the element is miniaturized.

9 and 10 are schematic diagrams of a memory having a configuration using the above-described magnetization reversal by spin injection. 9 is a perspective view, and FIG. 10 is a cross-sectional view.
A drain region 58, a source region 57, and a gate electrode 51 constituting a selection transistor for selecting each memory cell are formed in a portion separated by the element isolation layer 52 of the semiconductor substrate 60 such as a silicon substrate. Has been. Among these, the gate electrode 51 also serves as a word line extending in the front-rear direction in FIG.
The drain region 58 is formed in common to the left and right selection transistors in FIG. 9, and a wiring 59 is connected to the drain region 58.
A storage element 53 having a storage layer in which the direction of magnetization is reversed by spin injection is disposed between the source region 57 and the bit line 56 disposed above and extending in the left-right direction in FIG.
The storage element 53 is configured by, for example, a magnetic tunnel junction element (MTJ element). In the figure, reference numerals 61 and 62 denote magnetic layers. Of the two magnetic layers 61 and 62, one magnetic layer is a magnetization fixed layer whose magnetization direction is fixed, and the other magnetic layer is a magnetization direction. A changing magnetization free layer, that is, a storage layer is used.
The storage element 53 is connected to the bit line 56 and the source region 57 via the upper and lower contact layers 54, respectively. As a result, a current can be passed through the memory element 53 to reverse the magnetization direction of the memory layer by spin injection.

In the case of a memory that uses such magnetization reversal by spin injection, the device structure can be simplified as compared with the general MRAM shown in FIG.
Further, by utilizing magnetization reversal by spin injection, there is an advantage that the write current does not increase even when the element is miniaturized as compared with a general MRAM in which magnetization reversal is performed by an external magnetic field.

  In the case of an MRAM, a write wiring (word line or bit line) is provided separately from a memory element, and information is written (recorded) by a current magnetic field generated by passing a current through the write wiring. Therefore, a sufficient amount of current required for writing can be passed through the write wiring.

On the other hand, in a memory configured to use magnetization reversal by spin injection, it is necessary to reverse the magnetization direction of the storage layer by performing spin injection with a current flowing through the storage element.
Since the current is directly supplied to the memory element and information is written (recorded) as described above, the memory cell is configured by connecting the memory element to a selection transistor in order to select a memory cell to be written. In this case, the current flowing through the memory element is limited to the magnitude of the current that can flow through the selection transistor (the saturation current of the selection transistor).
Therefore, it is necessary to perform writing with a current lower than the saturation current of the selection transistor, and it is necessary to improve the efficiency of spin injection and reduce the current flowing through the memory element.

Also, in order to increase the read signal, it is necessary to secure a large rate of change in magnetoresistance. To that end, a memory element having a tunnel insulating layer (tunnel barrier layer) as an intermediate layer in contact with both sides of the memory layer is required. The configuration is effective.
When the tunnel insulating layer is used as the intermediate layer as described above, the amount of current flowing through the memory element is limited in order to prevent the tunnel insulating layer from being broken down. From this viewpoint, it is necessary to suppress the current during spin injection.

In order to reduce the current value, the current value is proportional to the film thickness of the storage layer or proportional to the square of the saturation magnetization, and it is understood that these may be adjusted (see, for example, Non-Patent Document 4). .
And the method of reducing an electric current value by reducing the saturation magnetization amount (Ms) of the material of a memory layer is proposed (refer patent document 3).

Nikkei Electronics 2001.1.22 (pages 164-171) Phys. Rev. B 54.9353 (1996) J. et al. Magn. Mat. 159. L1 (1996) F. J. et al. Albert, et al. , Appl. Phy. Lett. 77, 3809 (2000) JP 2003-17782 A US Pat. No. 6,256,223 US Patent Publication No. 2005-0184839

  In order for a memory element configured to reverse the magnetization direction of the memory layer by spin injection to exist as a memory, the amount of current (threshold current) required to reverse the magnetization direction of the memory layer is set to It is necessary to reduce the saturation current to less than the saturation current and to ensure the stability to hold the written information against heat and the like.

In order to reduce the threshold current, it is effective to reduce the saturation magnetization Ms of the storage layer by using a material having a low saturation magnetization Ms for the storage layer, for example, as in Patent Document 3 described above.
However, when a material having a low saturation magnetization Ms is simply used for the memory layer, sufficient stability against heat (thermal stability) cannot be ensured.

  In order to solve the above-described problems, the present invention provides a memory element that can improve thermal stability even when a material having low saturation magnetization is used for a memory layer, and a memory including the memory element. Is.

The storage element of the present invention has a storage layer that holds information according to the magnetization state of the magnetic material, and a magnetization fixed layer is provided via the intermediate layer for the storage layer, and the intermediate layer is made of an insulator. By injecting spin-polarized electrons in the stacking direction, the magnetization direction of the storage layer is changed, information is recorded on the storage layer, and the top surface of the ferromagnetic layer constituting the storage layer, or A texture structure having uniaxial anisotropy is formed below the ferromagnetic layer.
The memory of the present invention includes a memory element having a memory layer that holds information according to the magnetization state of the magnetic material, and two types of wirings that intersect each other, and the memory element has the configuration of the memory element of the present invention. A storage element is arranged near the intersection of two types of wiring and between the two types of wiring, a current in the stacking direction flows through the two types of wiring, and spin-polarized electrons are injected. .

According to the configuration of the above-described storage element of the present invention, the storage layer that holds information by the magnetization state of the magnetic material is provided, and the magnetization fixed layer is provided to the storage layer via the intermediate layer. By injecting electrons that are made of an insulator and spin-polarized in the stacking direction, the magnetization direction of the storage layer changes and information is recorded on the storage layer. Information can be recorded by injecting spin-polarized electrons.
In addition, a texture structure having uniaxial anisotropy is formed on the upper surface of the ferromagnetic layer constituting the memory layer or on the lower layer than the ferromagnetic layer, so that an effective anisotropic magnetic field of the memory layer is formed. Can be increased. Thereby, it is possible to improve the thermal stability of the storage layer. Then, it is possible to improve the thermal stability of the storage layer without increasing the saturation magnetization of the storage layer.
That is, even when a material having a low saturation magnetization is used for the storage layer, it is possible to sufficiently ensure the thermal stability of the storage layer. This makes it possible to reduce the amount of write current required for reversing the magnetization direction of the storage layer by using a material having a low saturation magnetization for the storage layer.

According to the configuration of the memory of the present invention described above, the memory element includes a memory element having a memory layer that holds information according to the magnetization state of the magnetic material, and two types of wiring intersecting each other. The memory element is arranged near the intersection of two types of wiring and between the two types of wiring, and a current in the stacking direction flows through the memory element through these two types of wiring, and spin-polarized electrons are injected. Accordingly, information can be recorded by spin injection by flowing current in the stacking direction of the memory element through two types of wiring.
In addition, it is possible to sufficiently ensure the thermal stability of the storage layer, and to reduce the amount of write current necessary for reversing the magnetization direction of the storage layer of the storage element. Therefore, it is possible to stably hold information recorded in the memory cell without increasing the power consumption of the memory.

According to the present invention described above, even when a material having a low saturation magnetization Ms is used for the storage layer in order to reduce the amount of current (threshold current) required for reversing the direction of magnetization of the storage layer, the information Since the thermal stability that is the holding ability can be ensured, a memory element with excellent characteristic balance can be configured.
Thereby, an operation error can be eliminated and a sufficient operation margin of the memory element can be obtained.

Further, even if the thermal stability necessary for the memory is ensured, the write current does not increase, so that it is not necessary to apply a large voltage, so that the insulator as the intermediate layer is not destroyed.
Therefore, a highly reliable memory that operates stably can be realized.

Furthermore, even if the write current is reduced, it is possible to sufficiently secure the thermal stability required for the memory, so the write current is reduced and the power consumption when writing to the memory element is reduced. It becomes possible to do.
Therefore, the power consumption of the entire memory can be reduced.

First, an outline of the present invention will be described prior to description of specific embodiments of the present invention.
In the present invention, information is recorded by reversing the magnetization direction of the storage layer of the storage element by the spin injection described above. The memory layer is made of a magnetic material such as a ferromagnetic layer, and holds information by the magnetization state (magnetization direction) of the magnetic material.

The basic operation of reversing the magnetization direction of the magnetic layer by spin injection is perpendicular to the film surface of a storage element composed of a giant magnetoresistive element (GMR element) or a magnetic tunnel junction element (MTJ element). A current of a certain threshold value (Ic) or more flows in the direction. At this time, the polarity (direction) of the current depends on the direction of magnetization to be reversed.
When a current having an absolute value smaller than this threshold is passed, magnetization reversal does not occur.

  When the magnetization direction of the magnetic layer is reversed by spin injection, the required current threshold Ic is phenomenologically expressed by the following (formula 1) (for example, FJAlbert et al., Appl. Phys Lett., 77, p. 3809, 2000, etc.).

なお、（式１）において、Ａは定数、αはスピン制動定数、ηはスピン注入効率、Ｍｓは飽和磁化量、Ｖは磁性層（記憶層）の体積である。

In (Expression 1), A is a constant, α is a spin braking constant, η is a spin injection efficiency, Ms is a saturation magnetization, and V is a volume of a magnetic layer (storage layer).

In the present invention, as represented by (Equation 1), the threshold value of the current is arbitrarily set by controlling the volume V of the magnetic layer, the saturation magnetization Ms of the magnetic layer, the spin injection efficiency, and the braking constant. Take advantage of what is possible.
Then, a storage element having a magnetic layer (storage layer) capable of holding information depending on the magnetization state and a magnetization fixed layer in which the magnetization direction is fixed is configured.
In order to be able to exist as a memory, it must be able to hold the written information. As an index of the ability to hold information, it is determined by the value of the thermal stability index Δ described above. The thermal stability index Δ of the magnetic layer (memory layer) is expressed by the following (formula 2).

なお、（式２）において、Ｈｋは実効的な異方性磁界、ｋ_Ｂはボルツマン定数、Ｔは温度、Ｍｓは飽和磁化量、Ｖは記憶層の体積である。

Note that in Equation (2), Hk is effective anisotropy field, k _B is the Boltzmann constant, T is the temperature, Ms is the saturation magnetization, V is the volume of the storage layer.

  The effective anisotropy magnetic field Hk incorporates effects such as shape magnetic anisotropy, induced magnetic anisotropy, and magnetocrystalline anisotropy. When a single domain coherent rotation model is assumed, It is equivalent to magnetic force.

  Generally, in order to retain the stored information at 85 ° C. for 10 years, a value of 60 or more is required as the value of the thermal stability index Δ. The thermal stability index Δ and the current threshold value Ic often have a trade-off relationship, and in order to maintain memory characteristics, it is often a problem to achieve both of them.

The threshold value of the current for changing the magnetization state of the storage layer is actually a tunnel magnetoresistive effect element (TMR element) having a substantially elliptical shape in which the thickness of the storage layer is 2 nm and the planar pattern is 100 nm × 150 nm, for example. The threshold value on the + side + Ic = + 0.5 mA, the threshold value on the − side, −Ic = −0.3 mA, and the current density at that time is about 3.5 × 10 ⁶ A / cm ² . These substantially coincide with the above (Equation 1).

On the other hand, a normal MRAM that performs magnetization reversal by a current magnetic field requires a write current of several mA or more.
On the other hand, when the magnetization reversal is performed by spin injection, the threshold value of the write current becomes sufficiently small as described above, which is effective for reducing the power consumption of the integrated circuit.
In addition, the current magnetic field generating wiring (105 in FIG. 11) required in a normal MRAM is not necessary, and this is advantageous in terms of integration as compared with a normal MRAM.

When magnetization reversal is performed by spin injection, information is written (recorded) by directly passing a current through the memory element. Therefore, the memory element is connected to a selection transistor in order to select a memory cell to be written. Thus, a memory cell is configured.
In this case, the current flowing through the memory element is limited to the magnitude of the current that can flow through the selection transistor (the saturation current of the selection transistor), and thus the allowable range of the write current is also limited.

On the other hand, if the amount of magnetization of the storage layer is reduced, the threshold value of the write current can be reduced and the allowable range can be expanded. However, as described above, the thermal stability index of the storage layer is impaired. . In order to configure the memory, the thermal stability index needs to be a certain size or more.
In order to make the critical current Ic of magnetization reversal by spin injection smaller than the saturation current of the selection transistor, the saturation magnetization amount Ms of the storage layer may be reduced from (Equation 1). (See Patent Document 3), the thermal stability of information storage in the storage layer is significantly impaired, and the memory function cannot be achieved.

As a result of various studies by the inventors of the present application, a layer having a texture structure with uniaxial anisotropy is introduced below the ferromagnetic layer constituting the memory layer or on the upper surface of the ferromagnetic layer. As a result, the effective anisotropic magnetic field Hk can be increased, and the thermal stability index Δ (= KV / k _B T) is improved even when a material having a low saturation magnetization Ms is used for the storage layer. It has been found that a stable memory can be formed.

When a texture structure is introduced below the ferromagnetic layer constituting the memory layer, a texture structure is formed on the upper surface of any one or more of the layers below the ferromagnetic layer constituting the memory layer. To do.
For example, even when a texture structure is formed in a layer that is relatively below the memory element, such as an underlayer, the irregularities due to the texture structure are also reflected in the upper layer, so the irregularities are also reflected in the lower surface of the memory layer. Therefore, the anisotropic magnetic field of the memory layer can be increased.

  Note that the texture structure can be formed in any layer below the memory layer, but the tunnel insulating layer of the TMR element is thinner than the other layers. It is desirable to form a texture structure in a layer other than the tunnel insulating layer because the characteristics may be impaired.

  For the formation of the texture structure, physical forming methods and chemical forming methods such as polishing and etching can be used.

The surface roughness of the texture structure is desirably 1.1 nm or more in terms of average roughness (arithmetic average roughness) Ra. And by making average roughness Ra 1.1 nm or more, the effect which forms a texture structure and raises the effective anisotropic magnetic field of a memory | storage layer can fully be exhibited.
The average roughness (arithmetic mean roughness) Ra is extracted from the roughness curve, the reference length L in the direction of the average line, the direction of the extracted portion is the x axis, and the height direction with respect to the surface is the y axis. When the curve is expressed by y = f (x), it can be obtained by the following (Equation 3).

Here, the texture structure formed in the memory element will be described with reference to FIGS. 4A to 4D.
First, although not shown, a film or a layer forming a texture structure is formed on the wafer surface.
Next, a texture structure is formed on the formed film or layer. An enlarged plan view of the main part of the wafer at this time is shown in FIG. 4A. As shown in FIG. 4A, texture structures 41 are formed in parallel in a specific direction X over the entire wafer.
The specific cross-sectional shape of the texture structure 41 is not particularly limited, and can be, for example, a triangular cross-section shown in FIG. 4B or a rectangular cross-section shown in FIG. 4C. It is good. Further, at least a part of the side surface of the texture structure 41 may be a curved surface.
Then, each layer above the film or layer in which the texture structure 41 is formed is formed, and a laminated film constituting the memory element is manufactured.
After that, for each storage element corresponding to each memory cell, a storage element is formed by patterning in a predetermined pattern (for example, an ellipse). At this time, the pattern of the memory element is formed so as to have a predetermined relationship with the direction of the texture structure 41. Here, FIG. 4D shows an exploded plan view of a film or layer in which a texture structure is formed in the case of patterning into an elliptical shape. As shown in FIG. 4D, a memory structure 42 that is patterned in an elliptical shape is formed with a texture structure 41 that is parallel to the constant direction X and is parallel to the major axis of the ellipse. Note that the direction X in FIG. 4D is the same as the direction X in FIG. 4A in the wafer state.

  And as shown to FIG. 4D, by setting it as the structure where the direction X of the texture structure 41 is parallel to the major axis of an ellipse, the uniaxial anisotropy by the texture structure 41, the shape anisotropy by an ellipse pattern, and Can be formed in a consistent direction. Thereby, the anisotropic magnetic field of a memory layer can be strengthened effectively.

Furthermore, in the present invention, in consideration of the saturation current value of the selection transistor, a magnetic tunnel junction (MTJ) using a tunnel insulating layer made of an insulator as a nonmagnetic intermediate layer between the storage layer and the magnetization fixed layer. Configure the element.
By constructing a magnetic tunnel junction (MTJ) element using a tunnel insulating layer, a magnetoresistance change rate (MR ratio) is compared with a case where a giant magnetoresistive effect (GMR) element is constructed using a nonmagnetic conductive layer. This is because the read signal intensity can be increased.

In addition, by using magnesium oxide (MgO) as a material of the tunnel insulating layer, the magnetoresistance change rate (MR ratio) is made larger than when aluminum oxide that has been generally used so far is used. be able to.
In general, the spin injection efficiency depends on the MR ratio, and the higher the MR ratio, the higher the spin injection efficiency and the lower the magnetization reversal current density.
Therefore, by using magnesium oxide as the material of the tunnel insulating layer, which is an intermediate layer, and simultaneously using the memory layer having the above-described structure, the write threshold current due to spin injection can be reduced, and information can be written (recorded) with a small current. )It can be performed. In addition, the read signal intensity can be increased.
As a result, the MR ratio (TMR ratio) can be ensured, the write threshold current by spin injection can be reduced, and information can be written (recorded) with a small current. In addition, the read signal intensity can be increased.

  When the tunnel insulating layer is formed of a magnesium oxide (MgO) film, it is more desirable that the MgO film is crystallized and the crystal orientation is maintained in the 001 direction.

In the present invention, the intermediate layer between the storage layer and the magnetization fixed layer is made of magnesium oxide (tunnel insulating layer), for example, aluminum oxide, aluminum nitride, SiO ₂ , Bi ₂ O ₃ , MgF ₂ , CaF, SrTiO ₂ , AlLaO ₃ , Al—N—O, and other various insulators, dielectrics, and semiconductors can also be used.

Furthermore, in order to obtain excellent magnetoresistance effect characteristics (MR characteristics) when magnesium oxide is used for the intermediate layer, the annealing temperature should be set to a high temperature of 300 ° C. or higher, desirably 340 ° C. to 360 ° C. Required. This is higher than the annealing temperature range (250 ° C. to 280 ° C.) in the case of aluminum oxide conventionally used for the intermediate layer.
This is considered to be necessary for forming an appropriate internal structure or crystal structure of the tunnel insulating layer such as magnesium oxide.

  For this reason, excellent MR characteristics can be obtained by using a heat-resistant ferromagnetic material so that the ferromagnetic layer of the memory element is resistant to this high temperature annealing.

The sheet resistance value of the tunnel insulating layer needs to be controlled to about several tens of Ωμm ² or less from the viewpoint of obtaining a current density necessary for reversing the magnetization direction of the storage layer by spin injection.
In the tunnel insulating layer made of the MgO film, the film thickness of the MgO film needs to be set to 1.5 nm or less in order to make the sheet resistance value in the above range.

In addition, it is desirable to make the memory element small so that the magnetization direction of the memory layer can be easily reversed with a small current.
Accordingly, the area of the memory element is preferably 0.04 μm ² or less.

Note that it is also possible to directly stack a storage layer having the above-described configuration conditions and another ferromagnetic layer having a different material or composition range. It is also possible to stack a ferromagnetic layer and a soft magnetic layer, or to stack a plurality of ferromagnetic layers via a soft magnetic layer or a nonmagnetic layer. The effect of the present invention can be obtained even when stacked in this way.
In particular, when a plurality of ferromagnetic layers are laminated via a nonmagnetic layer, the strength of interaction between the ferromagnetic layers can be adjusted. Even if it becomes below, the effect that it becomes possible to suppress so that a magnetization reversal current may not become large is acquired. The material of the nonmagnetic layer in this case is Ru, Os, Re, Ir, Au, Ag, Cu, Al, Bi, Si, B, C, Cr, Ta, Pd, Pt, Zr, Hf, W, Mo. , Nb or their alloys can be used.

The magnetization fixed layer preferably has unidirectional anisotropy, and the storage layer preferably has uniaxial anisotropy.
Moreover, it is preferable that each film thickness of a magnetization fixed layer and a memory layer is 1 nm-30 nm.

  The other configuration of the storage element can be the same as a conventionally known configuration of the storage element that records information by spin injection.

The magnetization fixed layer has a configuration in which the magnetization direction is fixed only by the ferromagnetic layer or by using the antiferromagnetic coupling between the antiferromagnetic layer and the ferromagnetic layer.
In addition, the magnetization fixed layer has a single-layered ferromagnetic layer structure or a laminated ferrimagnetic structure in which a plurality of ferromagnetic layers are stacked via a nonmagnetic layer.
When the magnetization pinned layer has a laminated ferrimagnetic structure, the sensitivity of the magnetization pinned layer to the external magnetic field can be reduced. Therefore, unnecessary magnetization fluctuations in the magnetization pinned layer due to the external magnetic field are suppressed, and the memory element operates stably. Can be made. Furthermore, the film thickness of each ferromagnetic layer can be adjusted, and the leakage magnetic field from the magnetization fixed layer can be suppressed.
Co, CoFe, CoFeB, or the like can be used as the material of the ferromagnetic layer constituting the magnetization fixed layer having the laminated ferrimagnetic structure. Moreover, Ru, Re, Ir, Os etc. can be used as a material of a nonmagnetic layer.

Examples of the material of the antiferromagnetic layer include magnetic materials such as FeMn alloy, PtMn alloy, PtCrMn alloy, NiMn alloy, IrMn alloy, NiO, and Fe ₂ O ₃ .
In addition, nonmagnetic elements such as Ag, Cu, Au, Al, Si, Bi, Ta, B, C, O, N, Pd, Pt, Zr, Hf, Ir, W, Mo, and Nb are included in these magnetic materials. Can be added to adjust the magnetic properties and other physical properties such as crystal structure, crystallinity and material stability.

  In addition, the film configuration of the storage element has no problem whether the storage layer is disposed above the magnetization fixed layer or the lower layer.

  As a method for reading information recorded in the memory layer of the memory element, a ferromagnetic layer that flows through the insulating layer is provided by providing a magnetic layer serving as a reference of information via a thin insulating film in the memory layer of the memory element. Reading may be performed by a tunnel current or may be performed by a magnetoresistive effect.

Next, embodiments of the present invention will be described.
As an embodiment of the present invention, a schematic configuration diagram (perspective view) of a memory is shown in FIG.
In this memory, a storage element capable of holding information in a magnetized state is arranged near the intersection of two types of address lines (for example, a word line and a bit line) orthogonal to each other.
That is, the drain region 8, the source region 7, and the gate electrode 1 that constitute a selection transistor for selecting each memory cell in a portion separated by the element isolation layer 2 of the semiconductor substrate 10 such as a silicon substrate, Each is formed. Of these, the gate electrode 1 also serves as one address wiring (for example, a word line) extending in the front-rear direction in the figure.
The drain region 8 is formed in common to the left and right selection transistors in the figure, and a wiring 9 is connected to the drain region 8.

The storage element 3 is disposed between the source region 7 and the other address wiring (for example, bit line) 6 disposed above and extending in the left-right direction in the drawing. The storage element 3 has a storage layer composed of a ferromagnetic layer whose magnetization direction is reversed by spin injection.
The storage element 3 is arranged near the intersection of the two types of address lines 1 and 6.
The storage element 3 is connected to the bit line 6 and the source region 7 through upper and lower contact layers 4, respectively.
As a result, a current in the vertical direction can be passed through the storage element 3 through the two types of address lines 1 and 6, and the magnetization direction of the storage layer can be reversed by spin injection.

A cross-sectional view of the memory element 3 of the memory according to the present embodiment is shown in FIG.
As shown in FIG. 2, the storage element 3 is provided with a fixed magnetization layer 31 in the lower layer with respect to the storage layer 17 in which the direction of the magnetization M1 is reversed by spin injection. The antiferromagnetic layer 12 is provided under the magnetization fixed layer 31, and the magnetization direction of the magnetization fixed layer 31 is fixed by the antiferromagnetic layer 12.
An insulating layer 16 serving as a tunnel barrier layer (tunnel insulating layer) is provided between the storage layer 17 and the magnetization fixed layer 31, and the storage layer 17 and the magnetization fixed layer 31 constitute an MTJ element.
A base layer 11 is formed below the antiferromagnetic layer 12, and a cap layer 18 is formed on the storage layer 17.

The magnetization fixed layer 31 has a laminated ferrimagnetic structure.
Specifically, the magnetization fixed layer 31 has a configuration in which two ferromagnetic layers 13 and 15 are stacked via a nonmagnetic layer 14 and antiferromagnetically coupled.
Since each of the ferromagnetic layers 13 and 15 of the magnetization fixed layer 31 has a laminated ferrimagnetic structure, the magnetization M13 of the ferromagnetic layer 13 is directed to the right, and the magnetization M15 of the ferromagnetic layer 15 is directed to the left. It has become. Thereby, the magnetic fluxes leaking from the ferromagnetic layers 13 and 15 of the magnetization fixed layer 31 cancel each other.

The material of the ferromagnetic layers 13 and 15 of the magnetization fixed layer 31 is not particularly limited, and an alloy material composed of one or more of iron, nickel, and cobalt can be used. Furthermore, transition metal elements such as Nb, Zr, Gd, Ta, Ti, Mo, Mn, and Cu, and light elements such as Si, B, and C can also be included. Alternatively, the ferromagnetic layers 13 and 15 may be configured by directly stacking a plurality of films of different materials (not via a nonmagnetic layer) such as a CoFe / NiFe / CoFe stacked film.
As the material of the nonmagnetic layer 14 constituting the laminated ferrimagnetic pinned layer 31, ruthenium, copper, chromium, gold, silver or the like can be used.

In the present embodiment, in particular, the memory element 3 has the above-described uniaxial anisotropic texture structure on the upper surface of the memory layer 17 or on the lower layer than the memory layer 17.
Note that FIG. 2 is a cross-sectional view of the storage layer 17 and the magnetization fixed layer 31 (13, 15) of the storage element 3 in the direction of the easy axis of magnetization, so that the texture structure does not appear.
3A and 3B are sectional views of the storage layer 17 and the magnetization fixed layer 31 (13, 15) of the storage element 3 in the hard axis direction so that the texture structure appears. FIG. 3A shows a case where a texture structure is formed on the upper surface of the storage layer 17. FIG. 3B shows a case where a texture structure is formed on the upper surface of the underlying layer 11 below the storage layer 17.

  As shown in FIG. 3A, when the texture structure 17A is formed on the upper surface of the memory layer 17, the uniaxial anisotropy due to the texture structure 17A is formed on the upper surface of the memory layer 17, and the cap is formed thereon. Layer 18 is formed.

As shown in FIG. 3B, when the texture structure 11 </ b> A is formed on the upper surface of the foundation layer 17, the antiferromagnetic layer 12 is formed on the texture structure 11 </ b> A on the upper surface of the foundation layer 11.
The unevenness of the texture structure 11 </ b> A on the upper surface of the base layer 11 is reflected in the upper layers 12, 13, 14, 15, and 16, although not shown, and the insulating layer 16 below the storage layer 17 is also uniaxially different. Isotropic irregularities are formed.
Thereby, uniaxially anisotropic irregularities are also formed on the lower surface of the memory layer 17.

When a texture structure is formed below the storage layer 17, the texture structure may be formed in a layer other than the base layer 11 as long as the characteristics of the storage element 3 are not impaired.
Even if the texture structure is formed in any layer from the base layer 11 to the insulating layer 16, it is reflected in the upper storage layer 17.
Note that the insulating layer 16 is thinner than the other layers as described above, and if the texture structure is formed on the insulating layer 16, the state and characteristics (breakdown voltage and MR ratio) of the layer may be impaired. For this reason, it is desirable to form the texture structure in a layer other than the texture insulating layer 16.

  For the formation of the texture structure, as described above, methods such as polishing and etching can be used.

Furthermore, in this embodiment, when the insulating layer 16 that is an intermediate layer is a magnesium oxide layer, the magnetoresistance change rate (MR ratio) can be increased.
By increasing the MR ratio in this way, the efficiency of spin injection can be improved, and the current density required for reversing the direction of the magnetization M1 of the storage layer 17 can be reduced.

  The memory element 3 of the present embodiment is manufactured by continuously forming the base layer 11 to the cap layer 18 in a vacuum apparatus, and then forming a pattern of the memory element 3 by processing such as etching. Can do.

According to this embodiment described above, the upper surface of the memory layer 17 or the lower layer of the memory layer 17 has the above-described uniaxial anisotropic texture structure, so that the upper surface or the lower surface of the memory layer 17 is uniaxially formed. An anisotropic unevenness is formed. Thereby, since the effective anisotropic magnetic field of the memory layer 17 can be increased, the thermal stability of the memory layer 17 can be improved.
Since the thermal stability of the storage layer 17 can be improved without increasing the saturation magnetization amount Ms of the storage layer 17, even if a material having a low saturation magnetization amount Ms is used for the storage layer 17, sufficient In addition, the thermal stability of the storage layer 17 can be ensured. This makes it possible to reduce the amount of write current required to reverse the direction of the magnetization M1 of the storage layer 17 by using a material having a low saturation magnetization amount Ms for the storage layer 17.

  By improving the thermal stability of the memory layer 17, it is possible to expand an operation area in which information is recorded by passing a current to the memory element 3, and a wide operation margin is ensured. Can be operated stably.

Further, even if sufficient thermal stability is ensured for the memory layer 17 of the memory element 3, the write current does not increase, so that it is not necessary to apply a large voltage. It will not be done.
Therefore, a highly reliable memory that operates stably can be realized.

Furthermore, even if the write current is reduced, sufficient thermal stability can be ensured, so the write current can be reduced and the power consumption when writing to the memory element 3 can be reduced. become.
Thereby, it becomes possible to reduce the power consumption of the entire memory in which the memory cell is configured by the storage element 3 of the present embodiment.

  Accordingly, a highly reliable memory that has excellent information retention characteristics and operates stably can be realized, and power consumption can be reduced in the memory including the memory element 3.

Further, the memory having the memory element 3 shown in FIG. 2 and having the configuration shown in FIG. 1 has an advantage that a general semiconductor MOS formation process can be applied when the memory is manufactured. For example, it is possible to withstand annealing at 340 ° C. to 360 ° C. without deteriorating the magnetic characteristics of the memory layer 17.
Therefore, a memory including the memory element 3 of the present embodiment can be applied as a general-purpose memory.

Here, in the configuration of the memory element of the present invention, the characteristics and the properties were examined by selecting the material and film thickness of each layer, such as the ferromagnetic material that specifically configures the memory layer.
Specifically, an element in which a layer having a texture structure with uniaxial anisotropy is introduced on the lower layer of the ferromagnetic layer constituting the memory layer or on the upper surface of the ferromagnetic layer,
As shown in FIGS. 1 and 9, in an actual memory, there are switching semiconductor circuits and the like in addition to the memory element, but the description of the manufacturing process of the selection transistor and the lower layer wiring is omitted.

A thermal oxide film having a thickness of 300 nm was formed on a silicon substrate having a thickness of 0.725 mm, and the memory element 3 having the configuration shown in FIG. 2 was formed thereon.
Specifically, in the memory element 3 having the configuration shown in FIG. 2, the material and thickness of each layer are as follows: the underlayer 11 is a Ta film with a thickness of 10 nm, the antiferromagnetic layer 12 is a PtMn film with a thickness of 20 nm, and the magnetization The ferromagnetic layer 13 constituting the fixed layer 31 is a CoFe film having a thickness of 2 nm, the ferromagnetic layer 15 is a CoFeB film having a thickness of 2.5 nm, and the nonmagnetic layer 14 constituting the magnetization fixed layer 31 having a laminated ferrimagnetic structure is formed. A 0.8 nm Ru film, a tunnel insulating layer (barrier layer) 16 as a 0.9 nm thick magnesium oxide film, a memory layer 17 as a ferromagnetic layer as a 2.5 nm thick CoFeB film, and a cap layer 18. Was selected as a Ta film having a thickness of 5 nm, and a Cu film (not shown) having a thickness of 100 nm (to be described later) was provided between the base film 11 and the antiferromagnetic layer 12 to form each layer. .
In the above film configuration, the composition of the CoFeB film of the ferromagnetic layer of the memory layer 17 was Co45Fe30B25 (atomic%), the composition of the PtMn film was Pt50Mn50 (atomic%), and the composition of the CoFe film was Co90Fe10 (atomic%).

The texture structure was introduced after a Ta film having a thickness of 10 nm for the underlayer 11 was formed or after a CoFeB film for the memory layer 17 was formed. When the texture structure 17A was introduced on the upper surface of the memory layer 17, the effective film thickness of the memory layer 17 was adjusted to 2.5 nm.
The texture structure was formed by etching.
Each layer other than the insulating layer 16 made of a magnesium oxide film was formed using a DC magnetron sputtering method.
The insulating layer 16 made of a magnesium oxide (MgO) film was formed using an RF magnetron sputtering method.
Further, after each layer of the memory element 3 was formed, a heat treatment was performed at 10 kOe · 360 ° C. for 2 hours in a magnetic field heat treatment furnace, and an orderly heat treatment was performed on the PtMn film of the antiferromagnetic layer 12.
However, the heat treatment in the sample for evaluating the anisotropic magnetic field due to the texture structure was performed under the conditions of 0 kOe (no magnetic field applied), 360 ° C., and 2 hours.

Next, after masking the word line portion by photolithography, the word line (lower electrode) was formed by performing selective etching with Ar plasma on the laminated film other than the word line. At this time, except for the word line portion, the substrate was etched to a depth of 5 nm.
Thereafter, a mask of the pattern of the memory element 3 was formed by an electron beam drawing apparatus, and selective etching was performed on the laminated film to form the memory element 3. Except for the memory element 3 portion, the etching was performed up to the Cu layer of the word line.
In addition, in order to generate the spin torque necessary for the magnetization reversal, it is necessary to flow a sufficient current through the storage element for the characteristic evaluation storage element, and thus it is necessary to suppress the resistance value of the tunnel insulating layer. Therefore, the pattern of the memory element 3 is an ellipse having a minor axis of 0.09 μm and a major axis of 0.18 μm, and the area resistance value (Ωμm ² ) of the memory element 3 is 20 Ωμm ² .
Next, the portions other than the memory element 3 portion were insulated by sputtering of Al ₂ O ₃ having a thickness of about 100 nm.
Thereafter, a bit line to be an upper electrode and a measurement pad were formed using photolithography.
In this way, a sample of the memory element 3 was produced.
For the evaluation of the anisotropic magnetic field, a solid film sample of 8 mm × 8 mm square was further prepared.

Each sample having the memory element 3 in which the surface roughness (average roughness Ra) of the texture structures 11A and 17A introduced into the underlayer 11 or the memory layer 17 was changed by the above-described manufacturing method was produced. Moreover, the sample which has not formed the texture structure was produced as a comparative example. The formation conditions for each sample are shown in Table 1. Sample No. 1 is a sample of the comparative example, sample No. 2-No. 5 is a sample in which the texture structure 17A is formed on the upper surface of the memory layer 17, sample No. 5; 6-No. Reference numeral 10 denotes a sample in which the texture structure 11 </ b> A is formed on the upper surface of the base layer 11.
As the surface roughness, the above-described average roughness Ra was adopted. Then, the average roughness Ra of the texture structures 11A and 17A was measured using a Nano Scope III contact AFM manufactured by Digital Instruments. The average roughness Ra of the comparative example is a value obtained by measuring the upper surface of the memory layer 17.

The characteristics of each sample of the produced memory element 3 were evaluated as follows.
Prior to the measurement, the storage element 3 was configured to be able to apply a magnetic field from the outside in order to control the positive and negative values of the reversal current to be symmetrical. Further, the voltage applied to the memory element 3 was set to 1 V within a range where the insulating layer 16 was not broken.

(Measurement of anisotropic magnetic field and saturation magnetization)
By applying a VSM (Vibrating Sample Magnetometer) measurement when a magnetic field is applied in a direction parallel to the texture structure and in a direction perpendicular to the texture structure, and obtaining a magnetic field at which the magnetization of the storage layer is saturated when the magnetic field is applied in the vertical direction. The magnitude of the anisotropic magnetic field was determined. The sample for anisotropic magnetic field measurement is heat-treated under the conditions of an applied magnetic field of 0 kOe (no magnetic field applied), 360 ° C., and 2 hours.
The saturation magnetization was also determined by VSM measurement.

(Measurement of coercive force)
The coercivity of the memory element was measured.
First, the resistance value of the memory element was measured while applying a continuously changing external magnetic field to the memory element. At this time, the temperature was set to room temperature of 25 ° C., and the bias voltage applied to the word line terminal and the bit line terminal was adjusted to 10 mV.
Then, an external magnetic field is applied in a direction opposite to the magnetization direction of the storage layer, and when the external magnetic field exceeds the coercive force of the storage layer, the magnetization direction of the storage layer is reversed. Since the resistance value of the memory element changes due to the reversal of the magnetization direction, the magnitude of the external magnetic field when this resistance value changes is considered to be equal to the coercivity of the memory element, and the coercivity of the memory element is Obtained.

(Measurement of reversal current value and thermal stability)
In order to evaluate the writing characteristics of the memory element according to the present invention, the inversion current value was measured.
A current having a pulse width of 10 μs to 100 ms was passed through the memory element, and then the resistance value of the memory element was measured. Further, the amount of current flowing through the storage element was changed to obtain a current value at which the magnetization of the storage layer was reversed. A value obtained by extrapolating the pulse width dependence of the current value to a pulse width of 1 ns was defined as an inversion current value.

In addition, the slope of the pulse width dependency of the inversion current value corresponds to the above-described thermal stability index (Δ = KV / k _B T) of the memory element. It means that the more the reverse current value does not change with the pulse width (the smaller the slope), the stronger the heat disturbance. As described above, a thermal stability index of 60 or more is necessary for use in a memory.
Then, in order to take into account the variation between the storage elements, about 20 storage elements having the same configuration were manufactured, and the above-described measurement was performed to obtain the average value of the reversal current value and the thermal stability index.
The measurement results of each sample are shown in FIGS. In FIG. 5 to FIG. 6-No. 10 shows the measurement results of 10 samples. 2-No. 5 shows the measurement results of the sample No. 5, and the comparative example No. The measurement result of the sample No. 1 is shown as a value of 0.68 nm for the base layer type and the memory layer type.

FIG. 5 shows the relationship between the average roughness Ra of the texture structure and the anisotropic magnetic field Han.
From FIG. 5, even when a texture structure having uniaxial anisotropy is introduced into any one of the underlayer 11 and the storage layer 17, when the average roughness Ra is 1.1 nm or more, the anisotropic magnetic field Han Can be confirmed to be 50 [Oe] or more. The anisotropic magnetic field Han appears to have a peak when the average roughness Ra is about 1.5 nm. This is because when the average roughness Ra becomes larger than necessary, the magnetic characteristics of the storage layer are deteriorated due to the unevenness.

The relationship between the average roughness Ra of the texture structure and the coercive force Hc is shown in FIG.
From FIG. 6, it was confirmed that the coercive force increased corresponding to the increase of the anisotropic magnetic field Han shown in FIG.

The relationship between the average roughness Ra of the texture structure and the thermal stability index KV / k _B T is shown in FIG.
From FIG. 7, it can be seen that the thermal stability index KV / k _B T increases greatly when the average roughness Ra is 1.1 nm or more, corresponding to the coercive force shown in FIG. From FIG. 7, it can be seen that by introducing the texture structure, the thermal stability index KV / k _B T is improved by about 30% or more compared to the case without the texture structure.

FIG. 8 shows the relationship between the average roughness Ra of the texture structure and the saturation magnetization Ms of the storage layer 17.
FIG. 8 confirms that there is no change in the saturation magnetization amount Ms of the storage layer even when the texture structure is introduced. Accordingly, it can be seen that the increase in the thermal stability index KV / k _B T shown in FIG. 7 is not simply caused by the increase in the saturation magnetization amount Ms of the storage layer.

From the results of FIGS. 5 to 8 described above, by introducing a texture structure and setting the average roughness Ra of the texture structure to 1.1 nm or more, thermal stability is achieved when the anisotropic magnetic field is 50 [Oe] or more. It was confirmed that the sex index KV / k _B T was greatly improved.
Therefore, by using the configuration of the present invention, the thermal stability index KV / k _B T can be intentionally improved, and information recording is possible even when a material having a low saturation magnetization Ms is used for the storage layer. It becomes possible to maintain the thermal stability of.

  In the present invention, not only the film configuration of the memory element 3 shown in each of the embodiments described above but also various film configurations can be adopted.

  In each of the embodiments described above, the magnetization fixed layer 31 has a laminated ferrimagnetic structure composed of two ferromagnetic layers 13 and 15 and a nonmagnetic layer 14. For example, the magnetization fixed layer is a single-layer ferromagnetic layer. You may comprise by.

  The present invention is not limited to the above-described embodiment, and various other configurations can be taken without departing from the gist of the present invention.

1 is a schematic configuration diagram (perspective view) of a memory according to an embodiment of the present invention. FIG. 2 is a cross-sectional view (a cross-sectional view in the easy axis direction) of the memory element in FIG. 1. FIGS. 2A and 2B are cross-sectional views in the hard axis direction of the memory element of FIG. It is a figure explaining the state of AD texture structure. It is a figure which shows the relationship between average roughness Ra and anisotropic magnetic field Han. It is a figure which shows the relationship between average roughness Ra and coercive force Hc. It is a graph showing the relationship between the average roughness Ra and the thermal stability index KV / k B _T. It is a figure which shows the relationship between average roughness Ra and the saturation magnetization amount Ms. It is a schematic block diagram (perspective view) of the memory using the magnetization reversal by spin injection. FIG. 10 is a cross-sectional view of the memory of FIG. 9. It is the perspective view which showed the structure of the conventional MRAM typically.

Explanation of symbols

  3,42 Memory element, 11 Underlayer, 11A, 17A, 41 Texture structure, 12 Antiferromagnetic layer, 13,15 Ferromagnetic layer, 14 Nonmagnetic layer, 16 Insulating layer, 17 Memory layer, 18 Cap layer, 31 Magnetization Fixed layer

Claims (4)

It has a storage layer that holds information according to the magnetization state of the magnetic material,
A magnetization fixed layer is provided via an intermediate layer for the storage layer,
The intermediate layer is made of an insulator;
By injecting spin-polarized electrons in the stacking direction, the magnetization direction of the storage layer changes, and information is recorded on the storage layer.
A memory element, wherein a texture structure having uniaxial anisotropy is formed on an upper surface of a ferromagnetic layer constituting the memory layer or on a lower layer than the ferromagnetic layer.   The memory element according to claim 1, wherein the magnitude of an anisotropic magnetic field of the ferromagnetic layer constituting the memory layer is 50 [Oe] or more.   The memory element according to claim 1, wherein an average roughness Ra of the texture structure is 1.1 nm or more. A storage element having a storage layer for retaining information by the magnetization state of the magnetic material;
Two types of wiring crossing each other,
The storage element is provided with a magnetization fixed layer via an intermediate layer with respect to the storage layer, the intermediate layer is made of an insulator, and the storage layer is injected with spin-polarized electrons. The direction of magnetization of the recording layer is changed, information is recorded on the storage layer, and uniaxial anisotropy is formed on the upper surface of the ferromagnetic layer constituting the storage layer or on the lower layer than the ferromagnetic layer. It is a configuration in which the texture structure is formed,
The storage element is disposed near the intersection of the two types of wiring and between the two types of wiring,
A memory in which a current in the stacking direction flows in the memory element through the two types of wirings.

Images