JP2012174708A - Magnetic tunnel junction element and magnetic random access memory - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem of difficulty that a sufficient large MR ratio cannot be obtained with a GMR element and to reduce inversion current in an MTJ element by which the large MR ratio can be achieved.SOLUTION: A perpendicular magnetic anisotropic film is formed on a lower electrode with an easy magnetization direction being in a thickness direction. A spacer layer formed of nonmagnetic material is arranged on the perpendicular magnetic anisotropic film. A base layer including amorphous conductive material is arranged on the spacer layer. A magnetization free layer is arranged on the base layer with the easy magnetization direction being in an in-plane direction. A tunnel barrier layer is arranged on the magnetization free layer. A magnetization fixed layer is arranged on the tunnel barrier layer with a magnetization direction being fixed in the in-plane direction. The spacer layer has a thickness not allowing exchange interaction to occur between the perpendicular magnetic anisotropic film and the magnetization free layer and is thinner than a spin relaxation length.

Description

  The present invention relates to a magnetic tunnel junction element and a magnetic random access memory in which a tunnel barrier layer is sandwiched between a magnetization free layer and a magnetization fixed layer.

  A spin transfer torque type magnetic random access memory (STT-MRAM) using a magnetic tunnel junction (MTJ) element in which a tunnel barrier layer is sandwiched between a magnetization free layer and a magnetization fixed layer as a memory cell has attracted attention. In an MTJ element using an in-plane magnetization film, the demagnetizing field Hd in the perpendicular direction in the magnetization free layer is one factor in increasing the reversal current that reverses the magnetization direction.

  It has been reported that the switching speed of a giant magnetoresistive effect (GMR) element can be increased by combining a perpendicular magnetization film with a magnetization free layer. In this element, spin-transfer torque acts on the magnetization of the magnetization free layer by injecting vertically polarized electrons from the perpendicular magnetization film into the magnetization free layer. Thereby, the magnetization reversal of the magnetization free layer is likely to occur. For this reason, it is possible to increase the switching speed and reduce the inversion current.

  A technique that enables high-efficiency oscillation of a spin torque oscillator having a spin valve structure by using a perpendicular magnetization film and an in-plane magnetization free layer has been reported.

JP 2007-81280 A

J. C. Slonczewski, Currents, torques, and polarization factors in magnetic tunnel junctions, Physical review B 71, 024411 (2005) O. J. Lee et al., “Ultrafast switching of a nanomagnet by a combined out-of-plane and in-plane polarized spin current pulse”, Applied Physics Letters 95, 012506 (2009) D. Houssameddine et al., “Spin-torque oscillator using a perpendicular polariser and a plamar free layer”, Nature Materials Vol.6, pp.447-453 (2007) T. Seki et al., “Magnetization reversal by spin-transfer torque in 90 ° configuration with a perpendicular spin polarizer”, Applied Physics Letters 89, 172504 (2006) C. Papusoi et al., “100 ps precessional spin-transfer switching of a planar magnetic random access memory cell with perpendicular spin polarizer”, Applied Physics Letters 95, 072506 (2009) J. Z. Sun, “Spin-current interaction with a monodomain magnetic body: A model study”, Physical Review B 62, 570 (2000)

  With a GMR element, it is difficult to obtain a sufficiently large MR ratio. In an MTJ element capable of realizing a large MR ratio, it is desired to reduce the inversion current.

According to one aspect of the invention,
A perpendicular magnetic anisotropy film formed on the lower electrode and having an easy magnetization direction in the thickness direction;
A spacer layer disposed on the perpendicular magnetic anisotropic film and made of a nonmagnetic material;
An underlayer made of an amorphous conductive material disposed on the spacer layer;
A magnetization free layer disposed on the underlayer and having an easy magnetization direction facing an in-plane direction;
A tunnel barrier layer disposed on the magnetization free layer;
A magnetization fixed layer disposed on the tunnel barrier layer and having a magnetization direction fixed in an in-plane direction;
The spacer layer has a thickness that prevents exchange interaction between the perpendicular magnetic anisotropic film and the magnetization free layer, and provides a magnetic tunnel junction element that is thinner than the spin relaxation length.

  According to another aspect of the present invention, a magnetic random access memory using the above-described magnetic tunnel junction element is provided.

  By arranging the perpendicular magnetic anisotropic film, the write current can be reduced. Further, by disposing the underlayer, a higher MR ratio can be obtained as compared with the case where the magnetization free layer is formed directly on the spacer layer.

FIG. 6 is a cross-sectional view of the magnetic tunnel junction device according to Example 1 and a diagram for explaining an operation when reversing the magnetization from a parallel state to an antiparallel state. FIG. 6 is a cross-sectional view of the magnetic tunnel junction device according to Example 1 and a diagram for explaining the operation when the magnetization is reversed from the antiparallel state to the parallel state. (3A) is a graph showing the relationship between the thickness of the Al spacer layer and the element resistance RA, and (3B) is a graph showing the relationship between the thickness of the Al spacer layer and the MR ratio. (4A) is a graph showing the relationship between the thickness of the CoFeB magnetization free layer and the element resistance RA, and (4B) is a graph showing the relationship between the thickness of the CoFeB magnetization free layer and the MR ratio. (5A) is a graph showing the relationship between the thickness of the Ta underlayer and the element resistance RA, and (5B) is a graph showing the relationship between the thickness of the Ta underlayer and the MR ratio. (6A) is a graph showing the relationship between the thickness of the CoFeB magnetization free layer and the element resistance RA, and (6B) is a graph showing the relationship between the thickness of the CoFeB magnetization free layer and the MR ratio. (7A) is a graph showing the relationship between the thickness of the Ru spacer layer and the element resistance RA, and (7B) is a graph showing the relationship between the thickness of the Ru spacer layer and the MR ratio. It is a graph which shows the relationship between the thickness of a magnetization free layer, and MR ratio. FIG. 3 is a cross-sectional view of a magnetic tunnel junction device according to Example 1 in the middle of manufacturing. FIG. 3 is a cross-sectional view of a magnetic tunnel junction device according to Example 1 in the middle of manufacturing. FIG. 3 is a cross-sectional view of a magnetic tunnel junction device according to Example 1 in the middle of manufacturing. 6 is an equivalent circuit diagram of an STT-MRAM according to Embodiment 2. FIG. 12 is a cross-sectional view of an STT-MRAM according to Example 2 in the middle of manufacture. FIG.

  FIG. 1 is a schematic diagram of an MTJ element according to Example 1. A lower electrode 11 is formed on the substrate 10. For example, Ta is used for the lower electrode 11. On a partial region of the lower electrode 11, the buffer layer 12, the perpendicular magnetic anisotropic film 13, the spin polarization enhancement film 14, the spacer layer 15, the underlayer 16, the magnetization free layer (magnetization free layer) 17, and the tunnel barrier layer 18, a magnetization fixed layer (magnetization pinned layer) 19, an antiferromagnetic layer (magnetization pinning layer) 20, an upper electrode 21, and a connection layer 22 are laminated in this order.

  The buffer layer 12 has a function of improving the crystal quality of the perpendicular magnetic anisotropy film 13 disposed thereon and developing perpendicular magnetic anisotropy. For example, Ru, Pt, Rh, Pd, Cu or the like is used for the buffer layer 12, and the thickness thereof is, for example, 2 nm to 10 nm.

  The perpendicular magnetic anisotropic film 13 is made of a ferromagnetic material whose easy magnetization direction is perpendicular to the film surface. The perpendicular magnetic anisotropic film 13 directs the spin direction of conduction electrons in a direction perpendicular to the film surface. In this specification, the direction perpendicular to the film surface is simply referred to as “vertical direction”. For the perpendicular magnetic anisotropic film 13, for example, CoPt, FePt, CoPd, FePd, or the like is used. The perpendicular magnetic anisotropic film 13 may have a multilayer structure. Examples of the multilayer film structure include an alternately laminated structure of Fe layer and Pt layer, an alternately laminated structure of Fe layer and Pd layer, an alternately laminated structure of Co layer and Pt layer, and an alternately laminated structure of Co layer and Pd layer. , And an alternately laminated structure of Co layers and Ni layers. The thickness of the perpendicular magnetic anisotropy film 13 is preferably 4 nm or more in order to develop sufficient perpendicular magnetic anisotropy. Moreover, it is preferable to set it as 10 nm or less from a viewpoint of workability.

  The spin polarization enhancement film 14 increases the spin polarizability of conduction electrons. For the spin polarization enhancement film 14, a magnetic material having a higher spin polarizability than that of the perpendicular magnetic anisotropic film 13, such as CoFe or CoFeB, is used. The direction of easy magnetization of the spin polarization enhancement film 14 using these ferromagnetic materials is in the in-plane direction. However, when the spin polarization enhancing film 14 and the perpendicular magnetic anisotropic film 13 are exchange-coupled, the magnetization direction of the spin polarization enhancing film 14 becomes the perpendicular direction.

  If the spin polarization enhancement film 14 is made too thick, the magnetic anisotropy of the spin polarization enhancement film 14 itself becomes more dominant than the exchange interaction with the perpendicular magnetic anisotropy film 13 and has in-plane magnetization. It becomes like this. In order to direct the magnetization of the spin polarization enhancing film 14 in the vertical direction, the thickness is preferably 1 nm or less.

  Conversely, if the spin polarization enhancement film 14 is too thin, the effect of increasing the spin polarizability of conduction electrons will be reduced. In order to ensure a high spin polarizability, the thickness of the spin polarization enhancing film 14 is preferably set to 0.4 nm or more.

  The spacer layer 15 is used to prevent exchange interaction between the magnetic film including the perpendicular magnetic anisotropic film 13 and the spin polarization enhancement film 14 and the magnetization free layer 17 or between the two. Arranged to weaken the working exchange interaction. In order to allow the electrons polarized by the spin polarization enhancement film 14 to reach the magnetization free layer 17 while maintaining the spin state, it is preferable to use a nonmagnetic conductive material having a long spin relaxation length for the spacer layer 15. . The thickness of the spacer layer 15 is preferably shorter than the spin relaxation length of the material. Furthermore, it is preferable to select a material that does not disturb the crystallinity of the magnetization free layer 17 for the spacer layer 15.

  Examples of materials that satisfy the above conditions include Al and Cu. The spin relaxation length of Al and Cu is about several μm. When Cu is used for the spacer layer 15, the spin polarization enhancing film 14 and the magnetization free layer 17 may be exchange-coupled depending on the film thickness within a range of 3 nm or less. Therefore, when Cu is used for the spacer layer 15 and the film thickness is 3 nm or less, the film thickness of the spacer layer 15 is set so that the spin polarization enhancement film 14 and the magnetization free layer 17 are not exchange-coupled. There is a need. When Al is used for the spacer layer 15, the thickness is preferably in the range of 0.3 nm to 1.5 nm. In the case where Cu is used for the spacer layer 15, the thickness is preferably in the range of 0.3 nm to 2.0 nm.

  The spin relaxation length of Ru is several nm, which is shorter than the spin relaxation length of Al or Cu, but Ru can also be used for the spacer layer 15. When Ru is used for the spacer layer 15, it is preferable that the spacer layer 15 be thinner than the spin relaxation length and have a thickness that weakens the exchange interaction between the spin polarization enhancement film 14 and the magnetization free layer 17. Specifically, the thickness of the spacer layer 15 is preferably in the range of 0.8 nm to 1.1 nm.

  When the magnetization free layer 17 such as CoFeB is formed directly on the spacer layer 15, the magnetization free layer 17 has a (110) -oriented body-centered cubic lattice structure. If the magnetization free layer 17 is (110) oriented, a large MR ratio cannot be secured.

  The underlayer 16 has a role of forming a (001) -oriented body-centered cubic lattice structure in the magnetization free layer 17 such as CoFeB. The MR ratio can be increased by orienting the magnetization free layer 17 in (001) orientation. An amorphous conductive material is used for the underlayer 16. For example, Ta, CoFeBTa, or the like can be used. In order to improve the orientation of the magnetization free layer 17, it is preferable that the thickness of the underlayer 17 is 0.1 nm or more. If the underlayer 16 is made too thick, spin scattering by the underlayer 16 becomes obvious, so the underlayer 16 is preferably 0.4 nm or less.

  For the magnetization free layer 17, a ferromagnetic material whose easy magnetization direction is in the in-plane direction is used. Examples of such a magnetic material include CoFe, NiFe, CoNiFe, CoFeB, NiFeB, CoNiFeB, and FeB. The reversal current (write current) and the information retention capability (retention) when the MTJ element is applied to the STT-MRAM depend on the thickness of the magnetization free layer 17. When Co42Fe42B16 is used for the magnetization free layer 17, the thickness is preferably in the range of 0.9 nm to 1.8 nm.

  The tunnel barrier layer 18 is set to a thickness at which electrons move between the magnetization free layer 17 and the magnetization fixed layer 19 by the quantum tunnel effect. The material and thickness of the tunnel barrier layer 18 contribute to the element resistance RA. As an example, when MgO is used for the tunnel barrier layer 18, the thickness is set to 1 nm. In addition to MgO, oxides such as AlO, ZnO, and HfO may be used.

  The magnetization fixed layer 19 is, for example, in order from the tunnel barrier layer 18 side, a CoFeB layer having a thickness of 2.5 nm, a CoFe layer having a thickness of 0.5 nm, a Ru layer having a thickness of 0.7 nm, and a CoFe layer having a thickness of 3 nm. It has a laminated ferri structure in which layers are laminated.

  The antiferromagnetic layer 20 is made of an antiferromagnetic material such as PtMn or IrMn. As an example, a PtMn layer having a thickness of 15 nm is used. The antiferromagnetic layer 20 fixes the magnetization direction of the magnetization fixed layer 19 in one direction in the plane by exchange coupling with the magnetization fixed layer 19.

  For the upper electrode 21, for example, a Ru layer having a thickness of 5 nm is used. For the connection layer 22, for example, a Ta layer having a thickness of 100 nm is used. The connection layer 22 is used as a hard mask when the layers from the upper electrode 21 to the buffer layer 12 are patterned.

  The state where the magnetization direction of the magnetization free layer 17 is parallel to the magnetization direction of the magnetization fixed layer 19 (parallel state) corresponds to the low resistance state of the MTJ element and is antiparallel (antiparallel state). ) Corresponds to the high resistance state.

  The operation of reversing the magnetization direction of the magnetization free layer 17 from the parallel state to the antiparallel state will be described.

When shifting from the parallel state to the anti-parallel state, the write current I ₁ flows from the upper electrode 21 toward the lower electrode 11. The conduction electrons e move from the lower electrode 11 toward the upper electrode 21. When the conduction electrons e are injected into the perpendicular magnetic anisotropic film 13, the spins of the conduction electrons e are oriented in the direction of localized spins in the perpendicular magnetic anisotropic film 13, that is, in the perpendicular direction. The spin polarization enhancement film 14 increases the spin polarizability of the conduction electrons e. When conduction electrons e pass through the magnetization free layer 17, the magnetization M ₁ of the magnetization free layer 17, acts spin transfer torque from the conduction electrons e, the direction of magnetization M ₁ is inclined to the film surface. Further, the spin of the conduction electron e is directed in the in-plane direction.

Conduction electrons e having spins antiparallel to the magnetization M _{2 of the} magnetization fixed layer 19 are reflected at the interface between the tunnel barrier layer 18 and the magnetization fixed layer 19 and reinjected into the magnetization free layer 17. By the action of the spin transfer torque from the re-injected conduction electrons e, the orientation of magnetization M ₁ of the magnetization free layer 17 is reversed.

When the by conduction electrons e reflected on magnetization fixed layer 19 is re-injected into the magnetization free layer 17, the magnetization M ₁ of the magnetization free layer 17 is inclined to the film surface. For this reason, they are easy to reversing the magnetization M ₁ of the magnetization free layer 17. Thus, with less current, and high speed, it is possible to reverse the magnetization M ₁ of the magnetization free layer 17.

With reference to FIG. 2, an operation when the antiparallel state is reversed to the parallel state will be described. In this case, a write current I ₂ from the lower electrode 11 toward the upper electrode 21 is passed. Conduction electrons e are injected from the magnetization fixed layer 19 into the magnetization free layer 17. The spin of the conduction electron e is oriented in the direction of the magnetization M ₂ of the magnetization fixed layer 19. Due to the spin transfer torque acting on the magnetization M ₁ of the magnetization free layer 17, the magnetization M ₁ of the magnetization free layer 17 is inverted from the antiparallel state to the parallel state.

The relationship between the thickness of the Al spacer layer 15 and the element characteristics will be described with reference to FIGS. 3A and 3B. The produced sample for evaluation has the same stacked structure as the MTJ element of Example 1 shown in FIG. A plurality of evaluation samples having different thicknesses of the Al spacer layer 15 were prepared. The material and thickness of each layer are as follows.
Buffer layer 12: Ru film with a thickness of 2 nm. Perpendicular magnetic anisotropic film 13: CoPt film with a thickness of 6 nm. Spin polarization enhancement film 14: CoFe film with a thickness of 0.6 nm. 4 nm to 1.2 nm Al film, underlayer 16: Ta film with a thickness of 0.2 nm, magnetization free layer 17: CoFeB film with a thickness of 1.4 nm, tunnel barrier layer 18: MgO film with a thickness of 1.0 nm Magnetization fixed layer 19: CoFeB layer having a thickness of 2.0 nm, CoFe layer having a thickness of 0.5 nm, Ru layer having a thickness of 0.75 nm, and CoFe layer / antiferromagnetic layer 20 having a thickness of 3.0 nm: Thickness 15 nm PtMn film • Upper electrode 21: Ru layer 5 nm thick • Connection layer 22: Ta layer 100 nm thick FIG. 3A shows the relationship between the element resistance RA and the thickness of the spacer layer 15. The relationship between the MR ratio and the thickness of the spacer layer 15 Indicates the person in charge. 3A and 3B, the horizontal axis represents the thickness of the spacer layer 15 in the unit “nm”, the vertical axis in FIG. 3A represents the element resistance RA in the unit “Ωμm ² ”, and the vertical axis in FIG. 3B represents the MR. The ratio is expressed in the unit “%”.

  As the spacer layer 15 increases in thickness, the element resistance RA increases and the MR ratio decreases. This is considered because Al of the spacer layer 15 diffuses into the tunnel barrier layer 18 when the spacer layer 15 becomes thick. From the viewpoint of suppressing Al diffusion, the thickness of the Al spacer layer 15 is preferably 1.2 nm or less.

With reference to FIGS. 4A and 4B, the relationship between the thickness of the CoFeB magnetization free layer 17 and the element characteristics will be described. The produced sample for evaluation has the same stacked structure as the MTJ element of Example 1 shown in FIG. The materials and thicknesses of the spacer layer 15, the underlayer 16, and the magnetization free layer 17 are as follows. The material and thickness of the other layers are the same as those of the evaluation sample in which the measurement of FIGS. 3A and 3B was performed.
Spacer layer 15: Al film with a thickness of 1.0 nm Base layer 16: Ta film with a thickness of 0.1 nm to 0.2 nm Magnetization free layer 17: CoFeB film with a thickness of 1.2 nm to 1.5 nm FIG. 4B shows the relationship between the element resistance RA and the thickness of the magnetization free layer 17, and FIG. 4B shows the relationship between the MR ratio and the thickness of the magnetization free layer 17. 4A and 4B, the horizontal axis represents the thickness of the magnetization free layer 17 in the unit “nm”, the vertical axis in FIG. 4A represents the element resistance RA in the unit “Ωμm ² ”, and the vertical axis in FIG. The MR ratio is expressed in the unit “%”. 4A and 4B indicate the measurement results of the samples in which the thickness of the base layer 16 is 0.1 nm and 0.2 nm, respectively.

  As the thickness of the magnetization free layer 17 increases within the range of 1.2 nm to 1.5 nm, the element resistance RA decreases and the MR ratio increases.

With reference to FIGS. 5A and 5B, the relationship between the thickness of the Ta underlayer 16 and the element characteristics will be described. The produced sample for evaluation has the same stacked structure as the MTJ element of Example 1 shown in FIG. The material and thickness of the spacer layer 15 and the underlayer 16 are as follows. The material and thickness of the other layers are the same as those of the evaluation sample in which the measurement of FIGS. 3A and 3B was performed.
Spacer layer 15: Al film having a thickness of 0.4 nm and 1.0 nm. Base layer 16: Ta film having a thickness of 0 to 0.4 nm. FIG. 5A shows the relationship between the element resistance RA and the thickness of the base layer 16. FIG. 5B shows the relationship between the MR ratio and the thickness of the underlayer 16. 5A and 5B, the horizontal axis represents the thickness of the underlying layer 16 in the unit “nm”, the vertical axis in FIG. 5A represents the element resistance RA in the unit “Ωμm ² ”, and the vertical axis in FIG. 5B represents the MR. The ratio is expressed in the unit “%”. Circle symbols and square symbols in FIGS. 5A and 5B indicate the measurement results of the samples in which the thickness of the spacer 15 is 0.4 nm and 1.0 nm, respectively.

  It can be seen that the MR ratio is significantly lower in the sample in which the thickness of the underlayer 16 is 0, that is, in the sample in which the underlayer 16 is not disposed, compared to the sample in which the underlayer 16 is disposed. By arranging the underlayer 16, the MR ratio can be increased. This is because the underlayer 16 orients the magnetization free layer 17 in (001) orientation.

  Even when Cu or Ru is used for the spacer layer 15, the same effect can be obtained by disposing the base layer 16.

The relationship between the thickness of the magnetization free layer 17 and element characteristics will be described with reference to FIGS. 6A and 6B. The produced sample for evaluation has the same stacked structure as the MTJ element of Example 1 shown in FIG. The materials and thicknesses of the spacer layer 15, the underlayer 16, and the magnetization free layer 17 are as follows. The material and thickness of the other layers are the same as those of the evaluation sample in which the measurement of FIGS. 3A and 3B was performed.
Spacer layer 15: Cu film with a thickness of 1.5 nm Base layer 16: Ta film with a thickness of 0.1 nm to 0.2 nm Magnetization free layer 17: CoFeB film with a thickness of 1.1 nm to 1.4 nm FIG. 6B shows the relationship between the element resistance RA and the thickness of the magnetization free layer 17, and FIG. 6B shows the relationship between the MR ratio and the thickness of the magnetization free layer 17. 6A and 6B, the horizontal axis represents the thickness of the magnetization free layer 17 in the unit “nm”, the vertical axis in FIG. 6A represents the element resistance RA in the unit “Ωμm ² ”, and the vertical axis in FIG. The MR ratio is expressed in the unit “%”. The circle symbols and square symbols in FIGS. 6A and 6B indicate the measurement results of the samples in which the thickness of the base layer 16 is 0.1 nm and 0.2 nm, respectively.

  4A and 4B show the evaluation results of the sample using Al for the spacer layer 15, but even if Cu is used for the spacer layer 15, the same evaluation results as in FIGS. 4A and 4B are obtained.

With reference to FIGS. 7A and 7B, the relationship between the thickness of the Ru spacer layer 15 and the element characteristics will be described. The produced sample for evaluation has the same stacked structure as the MTJ element of Example 1 shown in FIG. The materials and thicknesses of the spacer layer 15, the underlayer 16, and the magnetization free layer 17 are as follows. The material and thickness of the other layers are the same as those of the evaluation sample in which the measurement of FIGS. 3A and 3B was performed.
Spacer layer 15: Ru film having a thickness of 0.4 nm to 1.2 nm. Base layer 16: Ta film having a thickness of 0.1 nm. Magnetization free layer 17: CoFeB film having a thickness of 1.1 nm to 1.4 nm. FIG. 7B shows the relationship between the element resistance RA and the thickness of the spacer layer 15, and FIG. 7B shows the relationship between the MR ratio and the thickness of the spacer layer 15. 7A and 7B, the horizontal axis represents the thickness of the spacer layer 15 in the unit “nm”, the vertical axis in FIG. 7A represents the element resistance RA in the unit “Ωμm ² ”, and the vertical axis in FIG. The ratio is expressed in the unit “%”. The circle symbol, triangle symbol, square symbol, and hexagon symbol in FIG. 7A and FIG. 7B are for samples having a magnetization free layer 17 thickness of 1.1 nm, 1.2 nm, 1.3 nm, and 1.4 nm, respectively. The measurement results are shown.

  In the region where the thickness of the spacer layer 15 is less than 0.7 nm, the MR ratio is remarkably lower than that in the region of 0.8 nm to 1.1 nm. This is because an exchange interaction works between the spin polarization enhancement film 14 whose magnetization direction is perpendicular to the magnetization free layer 17. Even if the spacer layer 15 has a thickness of 1.2 nm or more, the MR ratio is lowered because the exchange interaction acts between the spin polarization enhancing film 14 and the magnetization free layer 17. Therefore, when Ru is used for the spacer layer 15, the thickness is preferably in the range of 0.8 nm to 1.1 nm.

  With reference to FIG. 8, the effect of disposing the underlayer 16 will be described. FIG. 8 shows the measurement results of the MR ratio of various MTJ elements. The manufactured MTJ element includes a lower electrode 11, a magnetization free layer 17, a tunnel barrier layer 18, and a magnetization fixed layer 19. CoFeB was used for the magnetization free layer 17 and MgO was used for the tunnel barrier layer 19. The magnetization fixed layer 20 has the same laminated ferrimagnetic structure as that of the magnetization fixed layer 20 of the first embodiment shown in FIG. Five types of samples using Ta, Ru, Cr, Pt, and Ti as the material of the lower electrode 11 were produced.

  The horizontal axis in FIG. 8 represents the film thickness of the magnetization free layer 17 in the unit “nm”, and the vertical axis represents the MR ratio in the unit “%”. The element symbol attached to the curve in FIG. 8 indicates the material of the lower electrode 11. When Ta is used for the lower electrode 11, a larger MR ratio is obtained compared to the case where other materials are used.

  When Ta is used for the lower electrode 11, crystallization proceeds downward from the tunnel barrier layer 18 in the magnetization free layer 17 during the crystallization heat treatment after film formation. Since Ta forming the lower electrode 11 is in an amorphous state, Ta in the lower electrode 11 does not become a crystal growth nucleus. For this reason, crystallization hardly proceeds from the lower electrode 11. Therefore, the quality of the interface between the magnetization free layer 17 and the tunnel barrier layer 18 can be improved.

  On the other hand, when a material other than Ta is used for the lower electrode 11, crystallization proceeding from the lower electrode 11 toward the magnetization free layer 17 becomes dominant. For this reason, the quality of the interface between the magnetization free layer 17 and the tunnel barrier layer 18 is degraded. Due to the deterioration of the interface quality, the MR ratio is reduced.

  In the MTJ element according to the first embodiment, the underlayer 16 is disposed under the magnetization free layer 17. The underlayer 16 has the same function as the lower electrode 11 made of Ta shown in FIG. By disposing the underlayer 16, crystallization from the spacer layer 15 into the magnetization free layer 17 can be suppressed, and a reduction in MR ratio can be avoided.

  With reference to FIGS. 9A to 9F, a method for manufacturing an MTJ element according to Example 1 will be described.

  As shown in FIG. 9A, each layer from the lower electrode 11 to the connection layer 22 is formed on the substrate 10 by sputtering. A conductive plug 10 </ b> A connected to the base electrode 11 is embedded in the substrate 10. After the connection layer 22 is formed, antiferromagnetism appears in the antiferromagnetic layer 27 by performing heat treatment in a magnetic field. During this heat treatment, the magnetization free layer 17 is crystallized.

As shown in FIG. 9B, the connection layer 22 is patterned so that the connection layer 22 remains in the region where the MTJ stacked structure is to be disposed. For patterning the connection layer 22, for example, a silicon oxide film is used as an etching mask, and a Cl ₂ gas is used as an etching gas. After the connection layer 22 is patterned, the silicon oxide film used as an etching mask is removed. The planar shape of the patterned connection layer 22 is, for example, a rectangle or an ellipse. In a plan view, the patterned connection layer 22 does not overlap the conductive plug 10A.

As shown in FIG. 9C, each layer from the upper electrode 21 to the buffer layer 12 is etched using the patterned connection layer 22 as an etching mask. For this etching, reactive ion etching using a mixed gas of CO and NH ₃ is applied. The flow rate ratio between CO and NH ₃ is, for example, 1:10, and the pressure in the etching chamber is, for example, 10 Pa. Note that methanol gas can also be used as an etching gas.

  When the lower electrode 11 made of Ta is exposed, the surface of the lower electrode 11 is oxidized, and a tantalum oxide layer is formed. Since the tantalum oxide layer acts as an etching stopper, the lower electrode 11 is not etched. Through the steps so far, the laminated structure 30 including the layers from the buffer layer 12 to the connection layer 22 is formed.

As shown in FIG. 9D, a photoresist film is formed on the laminated structure 30 and the lower electrode 11, and this photoresist film is patterned to form a resist pattern 33. The resist pattern 33 covers the laminated structure 30 and the lower electrode 11 around it. In plan view, the resist pattern 33 includes the conductive plug 10A. The lower electrode 11 is etched using the resist pattern 33 as an etching mask. For this etching, for example, reactive ion etching using Cl ₂ as an etching gas is applied. After the etching, the resist pattern 33 is removed. Through the steps so far, the MTJ element shown in FIG. 1 is formed. The patterned lower electrode 11 is electrically connected to the conductive plug 10A.

  As shown in FIG. 9E, an interlayer insulating film 35 is formed on the substrate 10, the lower electrode 11, and the laminated structure 30. For the interlayer insulating film 35, for example, an insulating material such as silicon nitride or silicon oxide is used. For example, chemical vapor deposition (CVD) is applied to the formation of the interlayer insulating film 35.

  As shown in FIG. 9F, a via hole exposing the upper surface of the connection layer 22 is formed in the interlayer insulating film 35, and a conductive plug 37 is filled therein. The conductive plug 37 is electrically connected to the upper electrode 21 of the MTJ element through the connection layer 22.

  FIG. 10 shows an equivalent circuit diagram of a spin torque injection type MRAM (STT-MRAM) according to the second embodiment. A plurality of word lines 53 extend in the vertical direction of FIG. 10, and a plurality of bit lines 65 extend in the horizontal direction of FIG. Corresponding to the intersection of the word line 53 and the bit line 65, a memory cell is arranged. The memory cell includes a switching element 52 and an MTJ element 60. For example, a MOS transistor is used as the switching element 52. The control terminal of the switching element 52 (the gate electrode of the MOS transistor) is connected to the corresponding word line 53. On / off control of the switching element 52 is performed by an electrical signal applied to the word line 53. One current terminal of the switching element 52 is grounded, and the other current terminal is connected to the corresponding bit line 65 via the MTJ element 60.

  A manufacturing method of the STT-MRAM according to the second embodiment will be described with reference to FIGS. 11A to 11C. 11A to 11C show cross-sectional views of a portion corresponding to one memory cell.

  As shown in FIG. 11A, an element isolation insulating film 51 is formed on a surface layer portion of a semiconductor substrate 50 such as silicon to define an active region. In this active region, a MOS transistor 52 is formed. The gate electrode of the MOS transistor 52 also serves as the word line 53 (FIG. 10). An interlayer insulating film 55 made of silicon oxide or the like is deposited on the semiconductor substrate 50 and the MOS transistor 52 by, for example, chemical vapor deposition (CVD). After deposition, the surface of the interlayer insulating film 55 is planarized by chemical mechanical polishing (CMP).

  A via hole is formed in the interlayer insulating film 55, and the via hole is filled with a conductive plug 56 such as tungsten. For example, TiN is used as the barrier metal. The conductive plug 56 is connected to one impurity diffusion region of the MOS transistor 52.

  A ground wiring 57 connected to the conductive plug 56 is formed on the interlayer insulating film 55. An interlayer insulating film 58 made of silicon oxide or the like is deposited on the interlayer insulating film 55 and the ground wiring 57 by, for example, CVD. After deposition, the surface of the interlayer insulating film 58 is planarized by CMP.

  As shown in FIG. 11B, via holes are formed in the interlayer insulating films 55 and 58, and the via holes are filled with conductive plugs 59 such as tungsten. For example, TiN is used as the barrier metal. Conductive plug 59 is connected to the other impurity diffusion region of MOS transistor 52.

  An MTJ element 60 is formed on the interlayer insulating film 58. The MTJ element 60 has the same stacked structure as the stacked structure from the lower electrode 11 to the connection layer 22 of the first embodiment shown in FIG. The MTJ element 60 is manufactured by the same method as in the first embodiment. The lower electrode 11 is connected to the conductive plug 59.

  As shown in FIG. 11C, an interlayer insulating film 63 made of silicon oxide or the like is deposited on the MTJ element 60 and the interlayer insulating film 58 by, for example, CVD. Thereafter, the surface of the interlayer insulating film 63 is planarized by CMP. A via hole is formed at a position overlapping with the MTJ element 60, and the via hole is filled with a conductive plug 64. For the conductive plug 64, for example, an Al / TiN laminated film is used.

  A bit line 65 is formed on the interlayer insulating film 63. For example, the bit line 65 has a three-layer structure in which a Ti layer having a thickness of 10 nm, a NiFe layer having a thickness of 30 nm, and an Al layer having a thickness of 600 nm are sequentially deposited. The bit line 65 is connected to the conductive plug 64.

  An upper wiring layer and electrode pads are formed on the bit line 65 and the interlayer insulating film 63 as necessary.

  In the STT-MRAM according to the second embodiment, the MTJ element according to the first embodiment is used. For this reason, it is possible to reduce the write current and increase the speed of magnetization reversal.

  The MTJ element according to the first embodiment can be applied to, for example, a spin torque oscillator in addition to the STT-MRAM.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 10 Substrate 10A Conductive plug 11 Lower electrode 12 Buffer layer 13 Vertical magnetic anisotropic film 14 Spin polarization enhancement film 15 Spacer layer 16 Underlayer 17 Magnetization free layer 18 Tunnel barrier layer 19 Magnetization fixed layer 20 Antiferromagnetic layer 21 Upper electrode 22 Connection layer 30 Laminated structure 33 Resist pattern 35 Interlayer insulating film 37 Conductive plug 50 Semiconductor substrate 51 Element isolation insulating film 52 Switching element (MOS transistor)
53 Word line 55 Interlayer insulation film 56 Conductive plug 57 Ground wiring 58 Interlayer insulation film 59 Conductive plug 60 MTJ element 63 Interlayer insulation film 64 Conductive plug 65 Bit line

Claims (7)

A perpendicular magnetic anisotropy film formed on the lower electrode and having an easy magnetization direction in the thickness direction;
A spacer layer disposed on the perpendicular magnetic anisotropic film and made of a nonmagnetic material;
An underlayer made of an amorphous conductive material disposed on the spacer layer;
A magnetization free layer disposed on the underlayer and having an easy magnetization direction facing an in-plane direction;
A tunnel barrier layer disposed on the magnetization free layer;
A magnetization fixed layer disposed on the tunnel barrier layer and having a magnetization direction fixed in an in-plane direction;
The spacer layer is a magnetic tunnel junction element having a thickness that prevents exchange interaction between the perpendicular magnetic anisotropic film and the magnetization free layer and being thinner than a spin relaxation length.   2. The magnetic tunnel junction device according to claim 1, wherein the magnetization free layer is made of CoFe, NiFe, CoNiFe, CoFeB, NiFeB, CoNiFeB, or FeB, and the underlayer includes Ta.   The perpendicular magnetic anisotropy film is made of CoPt, FePt, CoPd, or FePd, or an alternately laminated structure of an Fe layer and a Pt layer, an alternately laminated structure of an Fe layer and a Pd layer, a Co layer and a Pt layer An alternate lamination structure of Co layers and Pd layers, or an alternate lamination structure of Co layers and Ni layers, and the thickness of the perpendicular magnetic anisotropic film is in the range of 4 nm to 10 nm. The magnetic tunnel junction device according to claim 1 or 2.   The spacer layer may be an Al layer having a thickness of 0.3 nm to 1.5 nm, a Cu layer having a thickness of 0.3 nm to 2 nm, or a Ru having a thickness of 0.8 nm to 1.1 nm. The magnetic tunnel junction element according to claim 1, wherein the magnetic tunnel junction element is a layer.   5. A spin polarization enhancement film disposed between the perpendicular magnetic anisotropic film and the spacer layer and formed of a material having a higher spin polarizability than the perpendicular magnetic anisotropic film. The magnetic tunnel junction device according to any one of the above.   6. The spin polarization enhancement film is formed of a ferromagnetic material having an easy magnetization direction in an in-plane direction, and the magnetization direction is directed to the perpendicular direction by exchange coupling with the perpendicular magnetic anisotropic film. Magnetic tunnel junction element. A plurality of word lines formed on the substrate and extending in one direction;
A plurality of bit lines extending in a direction crossing the word line;
Memory cells arranged corresponding to the intersections of the word lines and the bit lines;
The memory cell is
7. A serial connection of the magnetic tunnel junction device and the switching device according to claim 1, wherein one terminal of the serial connection is connected to the corresponding bit line, and the other terminal is grounded. And the switching element is controlled to be turned on and off by an electric signal applied to the corresponding word line.

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