JP6643609B2 - Storage device and manufacturing method thereof - Google Patents

Description

  The present invention relates to magnetic memory devices in electronics technology, mainly data storage technology.

In order to suppress the power consumption for holding the memory, the bit information of “0” and “1” is held not by charge storage but by a magnetic moment.
Specifically, a structure in which an insulator is sandwiched between ferromagnetic materials is defined as a 1-bit element.
When a resistance is high when a current flows through the ferromagnetic material through a tunnel through the insulator, "1" is read, and "0" is read when the resistance is low.

In the magnetic memory element, in order to change the resistance, the magnetization of one of the ferromagnetic materials is fixed, and the other is changed by an external field.
In the old days, magnetization was changed using an induced magnetic field by wiring, but this limits the integration of magnetic memory elements and unintentionally rewrites nearby memory cells due to leaked magnetic fields since there are drawbacks such as, in recent years to invert the by Ri magnetization spin as shown in FIG. 1 (a) have been made (non-patent document 1).

Further, as a method of inverting magnetization more easily than spin injection, a method of inverting magnetization by changing a voltage has been recently adopted.
This is based on the principle that, by applying a voltage, the Fermi level of the magnetic material is changed to change the occupancy of the electron orbits that carry the magnetic moment.
FIG. 1B shows the structure of the element (Patent Document 1, Non-Patent Document 2).

  The following explains why changing the occupancy of the electron orbits that carry the magnetic moment leads to the reversal of the magnetization. When iron is used as a magnetic material, it is the electron spin of the 3d orbit of the iron atom that bears the magnetization. However, when the electron spin is aligned as shown schematically in FIG. The direction of the externally applied magnetic field is determined.

  However, when the spin-orbit interaction of electrons is large, the spin direction is determined with respect to the direction of the iron 3d electron orbit, and the electron orbit is determined by the crystal structure. Is determined in a direction determined with respect to the crystal orientation of the magnetic crystal (Non-Patent Document 3). This is shown schematically in FIG.

By applying an electric field to the occupancy of the electron orbitals, that is, by changing the Fermi level, the number of electrons occupying the 3d orbitals of the iron atom changes.
It utilizes in principle the fact that the direction in the crystal, which facilitates magnetization, changes.
FIG. 1B shows an element that performs magnetization reversal by applying a potential from an insulated gate that surrounds a variable magnetization region.
Even when an element is formed of a magnetic material using an element other than iron (such as cobalt), a technique based on the same principle can be used.

JP 2014-53546 A

Yiming.Huai, “Spin-Transfer Torque MRAM (STT-MRAM): Challenges and Prospects”, AAPPS Bulletin December 2008, Vol.18, No.6, page.33) T.Maruyama, Y.Shiota, T.Nozaki, K.Ohta, N.Toda, M.Mizuguchi, AATulapurkar, T.Shinjo, M.Shiraishi, S.Mizukami, Y.Ando & Y.Suzuki, “Large voltage -induced magnetic anisotropy change in a few atomic layers of iron ”, Nature Nanotechnology 4, 158, (2009) T. Oda, A. Pasquarello, and R. Car, “Fully Unconstrained Approach to Noncollinear Magnetism: Application to Small Fe Clusters” Phys. Rev. Lett. 80, 3622, (1998) P. Hohenberg and W. Kohn, “Inhomogeneous Electron Gas” Phys. Rev. 136, 864, (1964) W. Kohn and L. Sham, “Self-Consistent Equations Including Exchange and Correlation Effects” Phys. Rev. 140, 1133, (1965) Y.Lee, S.Bae, H.Jang, S.Jang, S.-E.Zhu, S.-H.Sim, Y.-I.Song, B.-H.Hong, and J.-H. Ahn, “Wafer-Scale Synthesis and Transfer of Graphene Films” Nano Lett. 10, 490 (2010)

  In order to change the direction of magnetization in the magnetization variable section constituting the magnetic memory element, it is necessary to reduce the magnetic anisotropy energy of the magnetic material.

  To change the magnetization direction, it is necessary to apply a voltage to the magnetic material.However, at the interface between the insulator and the ferromagnetic material, a potential is applied to the interface when an electric field is applied due to the mixing of the electron orbits of each material. There is a problem that a drop occurs and a sufficient electric field is not applied to the magnetic body.

  That is, when the electron orbits are mixed at the interface, an electron orbit that spreads in a direction perpendicular to the interface is generated, and the electrons easily block the applied electric field.

  Also in the method based on voltage application described in the background art, similar orbital mixing occurs at the interface between the insulating gate to which voltage is applied and the ferromagnetic material, and the electric field does not reach the ferromagnetic material due to the potential drop at the interface.

In view of the above, it is necessary to apply an applied potential higher than the potential difference applied to the magnetic material, and it is necessary to set the applied potential at the time of the writing operation to be high.
This causes an increase in power consumption (Joule heat) due to a leak current which cannot be avoided by applying a voltage.

In order to suppress the chemical bonding at the interface between the ferromagnetic material and the insulator constituting the magnetic memory element, a thin material having no chemical bonding with both materials forming the interface is inserted.
It is preferable that the inserted thin film itself does not shield the applied electric field, for example, a low-dimensional substance made of an atomic layer such as graphene or hexagonal boron nitride.

By inserting such a thin material, hybridization of electron orbits between the ferromagnetic material and the insulating material is suppressed, and a potential drop at the interface when an electric field is applied to the element is suppressed.
Hereinafter, specific features will be described.
1. A magnetic memory element in which a first electrode, a magnetization direction variable layer, an insulating layer, a magnetization direction stationary phase, and a second electrode are sequentially stacked, wherein atoms are further provided between the magnetization direction variable layer and the insulating layer. A layer made of a layered material arranged in a layered manner is inserted to suppress hybridization of electron orbits at an interface between the magnetization direction variable layer and the insulating layer, and when an electric field is applied between the first electrode and the second electrode, A magnetic memory element wherein a potential drop at the interface is suppressed.
2. Further, a side wall insulator surrounding the magnetization direction variable layer and a third electrode in contact with the side wall insulator are provided, and a layer made of a layered material in which the atoms are arranged in a layered manner is referred to as Instead of the insertion between the insulating layers, it is inserted between the magnetization direction variable layer and the sidewall insulator to suppress the mixing of electron orbits at the interface between the magnetization direction variable layer and the sidewall insulator. 2. The magnetic memory element according to claim 1, wherein a potential drop at the interface is suppressed when an electric field is applied between the second electrode and the third electrode.
3. The layer made of a layered material in which the atoms are arranged in a layered manner has a two-dimensional structure having a thickness of several layers of atoms. The magnetic memory element as described in the above.
4. 4. The magnetic memory device according to claim 3, wherein the layered material is one of graphene, hexagonal boron nitride, and a metal disulfide compound.
5. 5. A method for reading a memory of a magnetic memory element according to any one of the items 3 to 4, wherein the memory reading method has a two-dimensional structure having a thickness corresponding to the insulating portion and the atomic number layer. Reading a resistance value of a current that tunnels through the memory.
6. 5. A memory writing method for a magnetic memory element according to any one of items 1 to 4, wherein the memory writing method comprises: injecting a spin current into the magnetization direction variable layer; Characterized in that a pulse-like voltage is applied between the first electrode and the second electrode or between the second electrode and the third electrode, and the spin direction is controlled by adjusting the pulse application voltage. Method.
7. When the magnetization direction variable layer is iron, the insulating layer is magnesium oxide, and by providing one layer of graphene as the layered material, the magnetic anisotropy energy of the magnetization direction variable layer can be reduced. 7. The memory writing method for a magnetic memory element according to claim 5, wherein the pulse application voltage width can be reduced by about one digit by reducing the pulse application voltage width by about one tenth.
8. A memory cell comprising the magnetic memory element according to claim 4.

In the magnetic memory element according to the configuration of the present invention, the mixing of electron orbitals between the ferromagnetic material and the insulating material is suppressed, and the potential drop at the interface when an electric field is applied is suppressed, so that power consumption due to leakage current (Joule heat) The increase is prevented, and the stability of the element is improved.
Therefore, when the magnetic memory element of the present invention is configured as a memory cell, thermal design is facilitated and operation stability is brought to the cell.

FIG. 1A is a structural diagram of an MRAM element. "1" and "0" are determined by the writing by spin injection and the magnitude of the resistance between the ferromagnetic materials sandwiching the insulating layer. In this element, the read and write circuits are common. FIG. 1B is a structural diagram of an MRAM three-terminal element. It is the magnetization direction variable layer surrounded by the insulating layer that changes the magnetization direction in the magnetic body by applying a voltage. As in FIG. 1A, “1” and “0” are determined by the magnitude of the tunnel resistance sandwiching the insulating layer separating the variable magnetization direction layer and the stationary phase. FIG. 2A is an MRAM diagram showing the present invention, in which a graphene layer is inserted at the interface between the insulating layer and the magnetization direction variable layer of the MRAM element of FIG. 1A. FIG. 2B is a diagram showing the present invention, in which the graphene layer is inserted at the interface between the insulating layer and the magnetization direction variable layer of the MRAM three-terminal element of FIG. 1B. The change in the magnetization direction in the magnetic material due to the application of a voltage is caused by the magnetization direction variable layer surrounded by the sidewall insulating layer, and the graphene is inserted at the interface between the sidewall insulating layer and the magnetization direction variable layer. The ellipse indicates the electron orbit, and the white circle at the center of the ellipse indicates the atomic position in the crystal. In the left panel, the black and gray arrows indicate that the electron spin can be directed in any of the alternative directions, and in the right panel, the spin can easily be turned to the right by applying an external magnetic field. Indicates that it can be done. When the spin-orbit interaction is large, in the left panel, the elliptical electron orbit is oriented to reflect the crystal structure, and the direction of the electron spin is also determined to reflect the crystal structure via the spin-orbit interaction . Even if an external magnetic field is applied as in the right panel, the spin direction cannot be easily changed. Atomic arrangement at the Fe / MgO interface considered in this first-principles simulation. Atomic arrangement with one layer of graphene inserted at the Fe / MgO interface considered in this first-principles simulation. It is a figure showing change of magnetic anisotropy energy when an electric field (1 V / () is applied to each of Fe / MgO and a structure in which one graphene layer is inserted into Fe / MgO. FIG. 4 is an explanatory diagram of a memory cell growth step of the MRAM element.

In the present invention, as shown in FIG. 2A, the graphene layer is inserted as a thin material at the interface between the insulating layer and the magnetization direction variable layer of the MRAM element of FIG. 1A to improve the memory characteristics.
Similarly, in the present invention, as shown in FIG. 2B, a graphene layer is inserted as an above-described thin material at the interface between the sidewall insulating layer 7 and the magnetization direction variable layer 4 of the MRAM three-terminal element of FIG. To improve memory characteristics.
Hereinafter, the effect of the present invention in the case where one graphene layer is inserted at the interface between magnesium oxide (MgO) as an insulating layer and iron (Fe) crystal as a magnetization direction variable layer will be described in detail.

Using a first-principles calculation method that strictly describes the electron orbit, the magnetic anisotropy of Fe at the interface consisting of iron (Fe) crystal as an example of a ferromagnetic material and magnesium oxide (MgO) as an example of an insulator The calculation results are shown with and without the application of an electric field for the application of neutral energy.
Further, as an example of an atomic layer low-dimensional material, a single graphene layer was inserted into Fe / MgO, and the same calculation was performed.

The first-principles calculation method is based on the Korn-Sham equation (Non-patent Document 4 and Non-Patent Document 5) that approximates the Schrodinger equation, which deals with the many-body problem of electrons based on quantum mechanics, based on the behavior of electrons in matter. ) Is a method of calculating the numerical solution of the above by a computer.
By performing this calculation, it is possible to accurately compare the total energies of different magnetized structures after determining the type and arrangement of the atoms constituting the material.
It is now widely known that simulations using real-world principles can be used to simulate actual materials in a computer, which is like conducting a virtual experiment on a material in a computer. .

In this example, the calculated value of the magnetic anisotropy energy at the interface between Fe and MgO (hereinafter referred to as Fe / MgO) is shown.
In the Fe / MgO structure shown in FIG. 5, the magnetic anisotropy energy was 0.66 meV per Fe atom at the interface.
On the other hand, when one layer of graphene is inserted into Fe / MgO as shown in FIG. 6, the magnetic anisotropy energy is slightly increased to 1.02 meV. Therefore, the coercive force of the magnetization variable section is improved by inserting one graphene layer.

  As a result of the first-principles calculation in FIGS. 5 and 6, the axes of easy magnetization in the structure in which one layer of graphene is inserted at the interface between Fe / MgO and Fe / MgO are parallel to the direction perpendicular to the interface.

Further, FIG. 7 shows a change in magnetic anisotropy energy when an electric field (1 V / Å) is applied to Fe / MgO and a structure in which one graphene layer is inserted into Fe / MgO.
It can be seen from the figure that the magnetic anisotropy energy at the Fe / MgO interface has little dependence on the applied electric field strength.

On the other hand, when an electric field of the same intensity is applied to a structure in which one layer of graphene is inserted into Fe / MgO, the sign is inverted to −0.12 meV when the electric field intensity is changed in a negative direction (ie, a downward direction in FIG. 6). (The amount of drop is -1.14 meV per unit cell of 6 Mg atoms, 6 oxygen atoms, and 3 Fe atoms.)
The direction of magnetization easy changes from a direction horizontal to the Fe / MgO interface to a direction perpendicular to the Fe / MgO interface, and becomes the same direction as when no graphene layer is inserted, but can be changed to an arbitrary direction due to a decrease in magnetic anisotropy energy. It is easy to be.
That is, the application of an electric field means that the magnetization direction changes spontaneously.
This mechanism is as described in the background art.
From the above, it was shown that the insertion of a single layer of graphene was superior in terms of the coercive force of the magnetization direction when there was no electric field and the ease of changing the magnetization direction when the electric field was applied.

The simulations shown here show the effect of inserting a single layer of graphene, but it is possible to insert graphene for several atomic layers in practice.
However, the number of layers of the graphene layer is limited to a level that does not induce electric field shielding in the vertical direction.
The condition that electric field shielding is not induced is similarly required for a two-dimensional material composed of a layered substance in which atoms other than graphene are arranged in a layered manner.

When compared under the same applied electric field, the decrease in magnetic anisotropy energy was -0.07 meV per atom in Fe / MgO, and -1.14 meV when one layer of graphene was inserted into the Fe / MgO interface.
Approximately, considering that the amount of decrease in magnetic anisotropy depends linearly on the applied voltage, a structure in which a single layer of graphene is inserted at the Fe / MgO interface realizes a decrease in magnetic anisotropy energy of -0.07 meV. The applied electric field required for this is roughly estimated to require an applied voltage one order of magnitude lower than when no graphene layer is inserted.
When the electric field applied at the time of writing does not have a graphene layer, the electric field is effectively reduced due to the effect of the shielding described above, but by reducing the shielding effect by inserting the graphene layer, the applied electric field strength is reduced. It can be set very low.

  As described above, by inserting the graphene layer, the applied voltage at the time of writing can be reduced, thereby enabling normal writing. Specifically, as described in Non-Patent Document 2, there is a method of adjusting the direction in which the spin is to be directed by adjusting the pulse application voltage between the swing widths of -200 V to 200 V. This enables arbitrary rewriting of "0" or "1". According to the present invention, as described in the preceding paragraph, the swing width of the voltage is reduced to one digit or less.

  The method for manufacturing the memory cell having the interface structure shown in the first embodiment is performed in the following order. (See Fig. 8)

In circuit patterning of a memory cell, the number of bits in a certain area is determined in the same manner as in a conventional MRAM, and then a ferromagnetic material is grown on a substrate according to a wiring pattern of resistance measurement for voltage application and reading. For growth, a CVD growth method is preferable to a sputtering method in order to obtain a ferromagnetic material having good crystallinity.
This magnetic material becomes a stationary phase 2 (see FIGS. 1 and 2) for fixing the magnetization direction.

  Subsequently, an insulating layer is grown on the ferromagnetic material. In order to lower the resistance of the tunnel current, a film forming method with good crystallinity of this insulating layer is also desired, and a CVD method is more preferable than a sputtering method.

Subsequently, a layered material is grown on the insulating layer. The growth method is preferably CVD, but may be a method of transferring a material separated from a bulk parent material in which a plurality of layered substances are stacked.
Regarding the transfer method, there is a report example in the case of graphene, for example (Non-Patent Document 6).

  Finally, a ferromagnetic material is grown again on the layered material, and wiring for reading is performed.

As more of Engineering is to be performed for each bit cell, using a mask pattern.

DESCRIPTION OF SYMBOLS 1 1st electrode 2 Magnetization direction fixed phase 3 Insulating layer 4 Magnetization direction variable layer 5 2nd electrode 6 Control (read: resistance measurement, write: voltage application, or spin current injection)
7 Insulating layer 8 Third electrode 9 Control (writing: voltage application)
10 Insert a graphene layer 11 Insert graphene at the interface between the insulating layer 7 (side or side wall) and the magnetization direction variable layer 4

Claims (8)

A first electrode;
A magnetization direction variable layer,
An insulating layer,
A magnetization direction stationary phase,
A second electrode;
Are provided sequentially,
A sidewall insulator surrounding the magnetization direction variable layer,
A third electrode in contact with the sidewall insulator;
A layer that is provided between the sidewall insulator and the magnetization direction variable layer and is made of a layered material in which atoms are arranged in a layered manner;
A magnetic memory element further comprising:   The magnetic memory device of claim 1, wherein the layered material is one of graphene, hexagonal boron nitride, and a metal disulfide compound. It is a memory read method of the magnetic memory element of Claim 1 or 2 , Comprising:
A memory reading method for a magnetic memory device, comprising reading a resistance value with respect to a current that tunnels through the insulating layer. A first electrode;
A magnetization direction variable layer,
A layer made of graphene or a metal disulfide compound, which is a layered substance in which atoms are arranged in a layered manner,
An insulating layer,
A magnetization direction stationary phase,
A second electrode;
Is a memory reading method of a magnetic memory element provided sequentially,
A memory reading method for a magnetic memory device, comprising reading a resistance value against a current that tunnels through the insulating layer and the layer made of graphene or a metal disulfide compound. A memory writing method for a magnetic memory element according to claim 1 or 2 ,
Injecting a spin current into the magnetization direction variable layer, or applying a pulsed voltage between the second electrode and the third electrode and controlling the spin direction by adjusting the pulse applied voltage. Characteristic memory writing method for a magnetic memory element. A first electrode;
A magnetization direction variable layer,
A layer made of graphene or a metal disulfide compound, which is a layered substance in which atoms are arranged in a layered manner,
An insulating layer,
A magnetization direction stationary phase,
A second electrode;
Is a memory writing method of a magnetic memory element sequentially provided,
Injecting a spin current into the magnetization direction variable layer, or applying a pulsed voltage between the first electrode and the second electrode and controlling the spin direction by adjusting the pulse applied voltage. Characteristic memory writing method for a magnetic memory element. Memory cells, comprising the magnetic memory element according to claim 1 or 2. A first electrode;
A magnetization direction variable layer,
A layer made of graphene or a metal disulfide compound, which is a layered substance in which atoms are arranged in a layered manner,
An insulating layer,
A magnetization direction stationary phase,
A second electrode;
Are provided sequentially,
0 or 1 is set in the direction of spin in the magnetization direction variable layer, and the set 0 or 1 is read based on the magnitude of resistance between the first electrode and the second electrode.
A magnetic memory element,
The layer made of the graphene or the metal disulfide compound is in contact with the magnetization direction variable layer and the insulating layer
Magnetic memory element.

Images