Multilayer thin film and method for manufacturing the same

Abstract

To provide a new multilayer thin film exhibiting a tunnel magnetoresistance effect.SOLUTION: The present invention is a multilayer thin film having an insulating layer and a magnetic layer sandwiching the insulating layer. At least one of the magnetic layers is made of a Co-based alloy containing Al and / or Ga. For example, Co is contained in an amount of 58.5 to 63 at% with respect to the entire Co-based alloy. The Co-based alloy constituting a magnetic layer may be a β phase. The insulating layer is, for example, MgO or MgAl2O4. The magnetic layer on the other side of the insulating layer may be CoFeB (alloy) in addition to CoAl. Each layer is formed by sputtering, and planarization, crystallization, and the like are performed by heat treatment. A magnetic tunnel junction (MTJ) having the multilayer thin film of the present invention exhibits a high magnetoresistance change ratio (MR ratio), and can be applied to a next-generation magnetoresistance memory (MRAM) or the like.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to multilayer thin films and the like.

Background Art

[0002] In the field of spintronics that applies both the charge and spin characteristics of electrons, research and development of magnetic materials that exhibit the magnetoresistance effect in which electrical resistance changes due to an external magnetic field have been mainly carried out.

[0003] As a representative example, there is a magnetic tunnel junction (MTJ: Magnetic Tunnel Junction) having a stacked structure (magnetic layer / insulating layer / ferromagnetic layer) in which one insulating layer is sandwiched between two (ferromagnetic) magnetic layers. In the MTJ, the probability (tunnel probability) that electrons flowing in the plane perpendicular direction of the stacked structure pass through the insulating layer changes due to an external magnetic field, and the electrical resistance (tunnel resistance) when the magnetization directions of the two magnetic layers are not parallel is larger than the electrical resistance when they are parallel, resulting in the tunnel magnetoresistance (TMR: Tunnel Magneto Resistance) effect.

[0004] The MTJ is expected to be used in energy-saving next-generation magnetoresistive memories (MRAM: Magnetic Random Access Memory), high-sensitivity magnetic sensors, etc. For example, there are descriptions (research reports) related to the following Non-Patent Documents 1 to 6.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

[0006] Non-patent documents 1-6 only report on MTJs (devices) in which the insulating layer is MgO or MgAl2O4 and the magnetic layer is CoFeB or Co2MnSi.

[0007] Non-patent documents 7-18 report on CoAl. However, these non-patent documents contain no descriptions or suggestions related to the use of CoAl in MTJs or its electron spin conduction characteristics.

[0008] This invention has been made in view of these circumstances, and aims to provide a new multilayer thin film suitable for magnetic tunnel junctions and the like. [Means for solving the problem]

[0009] As a result of the inventors' intensive research to solve this problem, it has been newly found that a magnetic tunnel junction (MTJ) using CoAl in the magnetic layer exhibits a high magnetoresistance change rate (MR ratio). By developing this result, the present invention as described below has been completed.

[0010] 《Multilayer thin film》 (1) The present invention is a multilayer thin film having an insulating layer and a magnetic layer sandwiching the insulating layer, wherein at least one of the magnetic layers is a multilayer thin film made of a Co-based alloy containing Al and / or Ga.

[0011] (2) By using the multilayer thin film of the present invention, deterioration of the laminated structure can be suppressed, and an MTJ element or the like that can stably exhibit a high MR ratio can be realized.

[0012] 《Method for manufacturing a multilayer thin film》 The present invention can also be grasped as a method for manufacturing a multilayer thin film. For example, the present invention may be a method for manufacturing a multilayer thin film including a layer forming step of forming a magnetic layer made of a Co-based alloy and a heating step of heating the magnetic layer at 500 to 675°C.

[0013] 《Others》 (1) In this specification, unless otherwise specified, the composition is shown in atomic ratio (at%). Alloys and compounds denoted as AB or ABC only indicate the constituent elements (A, B, C...), and do not indicate the atomic ratio of each element (for example, 1:1) unless otherwise specified.

[0014] In this specification, unless otherwise specified, up (side, direction) and down (side, direction) mean the formation order (precedence relationship) of the layers. For example, the lower layer is formed earlier than the upper layer. The lower side means that it was formed prior to the upper side. Also, "side" and "direction" do not matter whether there is adjacency (contact) or not. When referring to "the magnetic layer above the insulating layer", it means that the magnetic layer is formed after the insulating layer, and does not necessarily mean that the two layers are in contact. For example, another layer may be interposed, such as an insulating layer / intermediate layer / magnetic layer. The same applies when referring to "the magnetic layer below the insulating layer".

[0015] (2) Unless otherwise specified, "x~y" as used herein includes the lower limit x and the upper limit y. Any numerical value included in the various numerical values ​​or ranges described herein may be used to create new lower or upper limits, such as a range "a~b". Also, "x~ynm" as used herein means xnm~ynm. The same applies to other unit systems. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic diagram showing the layered structure of the fabricated sample. [Figure 2] This is an electron microscope image of the layered cross-section of sample 3. [Figure 3] This graph shows the relationship between the amount of Co contained in the magnetic layer and the MR ratio of the multilayer thin film. [Modes for carrying out the invention]

[0017] The contents described herein may apply not only to multilayer thin films but also to their manufacturing methods. One or more components arbitrarily selected from this specification may be added as components of the present invention. Components related to the manufacturing method may also be components of a physical object. Which embodiment is best depends on the subject, required performance, etc.

[0018] "background" The process leading to the completion of this invention is as follows. The tunnel magnetoresistance (TMR) effect is evaluated by the rate of change of the electrical resistance of the element due to an external magnetic field (magnetoresistance change rate: MR ratio). The MR ratio improves as the spin polarization (ratio of up-spin electrons to down-spin electrons) in the magnetic tunnel junction (MTJ) increases. In order to improve the MR ratio, the following research and studies have been conducted on the insulating layer and (ferro)magnetic layer, which are the main components of the MTJ.

[0019] First, Al2O3 was considered as the insulating layer, but currently MgO and MgAl2O4 are considered optimal. When current is passed through these crystals in the

[0001] direction, only up-spin electrons flow in the Δ1 band, which is mainly responsible for coherent tunnel conduction. Therefore, an insulating layer that contributes to improving the MR ratio can be obtained by controlling the orientation of MgO or MgAl2O4 crystals in the

[0001] direction.

[0020] Next, regarding the magnetic layer, we began by considering CoFe, and currently, CoFeB and half-metals (especially Heusler alloys such as Co2MnSi) are being considered as promising candidates. Immediately after deposition, CoFeB is in an amorphous state. When this CoFeB is heated to over 300°C, B atoms diffuse into other layers, and CoFe grows epitaxially in a BCC structure while lattice matching with the MgO of the insulating layer.

[0021] This CoFe has a Δ1 band and can exchange up-spin electrons tunneled through the Δ1 band of the insulating layer without scattering. Therefore, in an MTJ (CoFe / MgO / CoFe or CoFe / MgAl2O4 / CoFe) where an insulating layer made of MgO or MgAl2O4 is sandwiched between magnetic layers made of CoFe, a current flows due to almost completely spin-polarized up-spin electrons, and a high tunnel magnetoresistance effect can be exhibited. However, atomic diffusion that occurs during the heating process above 300°C as described above can affect each layer, which is several nanometers thick, potentially leading to degradation of the TMR (decrease in the MR ratio).

[0022] Co-based Heusler alloys such as Co2MnSi are half-metallic, and at their Fermi level, they contain only up-spin electrons, resulting in complete spin polarization. Therefore, Co2MnSi can conduct perfectly spin-polarized currents without any special treatment (such as heat treatment). Furthermore, since Co2MnSi also possesses a Δ1 band, a high MR ratio can be expected in MTJs with a magnetic layer made of Co2MnSi. However, Co2MnSi can generate paramagnetic Mn ions (MnOx) at the interface with MgO (insulating layer), which can degrade the TMR properties (especially its temperature dependence).

[0023] The Co-based alloy according to the present invention does not contain B, Mn, or other elements that can degrade the multilayer structure through thermal diffusion. Therefore, by using the multilayer thin film of the present invention, it becomes possible to realize, for example, an MTJ element that stably exhibits a high MR ratio.

[0024] 《Magnetic layer》 (1) Composition The Co-based alloy constituting the magnetic layer is, for example, a Co-Al alloy or a Co-Ga alloy. Taking the total as 100 at%, the Co content is, for example, 55-65 at%, 58.5-63 at%, 59-62.5 at%, 59.5-62 at%, or 60-61.5 at%. The remainder is, for example, Al and / or Ga.

[0025] Co-based alloys can be any binary alloy as described above, excluding impurities. Al and Ga, both group 13 elements, may be partially substituted for each other. In other words, Co-Al alloys may contain Ga, and Co-Ga alloys may contain Al. Regardless of the amount of substitution, a Co-based alloy can be Co(Al,Ga).

[0026] (2) Structure Co-based alloys are preferably composed of a β phase with a body-centered cubic (BCC) crystal structure and are ferromagnetic. Incidentally, the crystal structure of Co is either face-centered cubic (FCC) or hexagonal close-packed (HCP). The crystal structure of Al is face-centered cubic (FCC). Ga has a low melting point (approximately 30°C), and its stable crystal structure differs depending on the pressure and temperature.

[0027] (3) Thickness The thickness of the magnetic layer made of Co-based alloy is, for example, 2-100 nm, 3-75 nm, 4-50 nm, or 5-25 nm.

[0028] (4) Combinations Each magnetic layer sandwiching the insulating layer may have the same or different component composition. For example, both layers may be the Co-based alloys described above. In that case, the Co content may differ from layer to layer, or one layer may be a CoAl alloy and the other a CoGa alloy.

[0029] Furthermore, one magnetic layer (for example, the layer below the insulating layer) may be made of the aforementioned Co-based alloy, while the other magnetic layer (for example, the layer above the insulating layer) may be made of a different Co-based alloy (for example, a CoFeB alloy). The CoFeB alloy can be any alloy in which Co, Fe, and B are essential elements; its specific alloy composition is not limited. For example, a ternary alloy with Fe: 20-60 at%, B: 20-30 at%, and Co: the remainder may be used. The CoFeB alloy may also contain small amounts of impurities or modifying elements in addition to these three elements. A magnetic layer made of such an alloy is also preferably a BCC crystal structure (β phase, etc.).

[0030] Insulating layer The insulating layer is, for example, an MgO layer or an MgAl2O4 layer. The MgO layer is often used as the insulating layer.

[0031] The magnetic layer below the insulating layer is preferably made of the aforementioned Co-based alloy (CoAl or CoGa). This allows for epitaxial growth between the magnetic layer and the insulating layer, resulting in crystal orientation (matching). A combination of a CoAl layer and an MgO layer is preferred.

[0032] 《Other layers》 The substrate may have single-crystal surfaces (film-forming surfaces) made of MgO, Si, sapphire, SrTiO3, SiC, etc. The single-crystal surfaces may be the surface of the substrate itself, or they may be layers (films) formed separately on the substrate.

[0033] Intermediate layers such as buffer layers with lattice constants that mitigate interlayer lattice mismatches, and barrier layers that suppress interlayer elemental diffusion, may be appropriately interposed. Furthermore, a diamagnetic layer or protective layer may be formed above the magnetic layer formed on the insulating layer. The diamagnetic layer can make the magnetization directions of opposing magnetic layers antiparallel. Examples of diamagnetic layers include Mn-Ir layers and Pt-Mn layers. Protective layers are provided to prevent oxidation of the multilayer thin film. Examples of protective layers include Ru layers, Ta layers, and Cr layers. The thickness of each layer is, for example, approximately 1-20 nm, 2-10 nm, and 3-5 nm.

[0034] 《Manufacturing method》 (1) Stratification Multilayer thin films are typically obtained by sequentially forming layers of different compositions on or beneath a substrate. Each layer is formed by known thin-film methods such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). With PVD methods such as vacuum deposition (sputtering, vacuum heating deposition, pulsed laser deposition, etc.), it is easy to form layers of desired composition by changing or combining the target (raw material).

[0035] Vacuum deposition is, for example, 10 -6 ~10 -10 Pa and furthermore 10 -7 ~10 -9 This process is best carried out under (ultra) high vacuum conditions of around Pa. The temperature during stratification (substrate temperature, base temperature) is, for example, around room temperature (below 50°C, or even below 30°C) to 600°C.

[0036] (2) Heat treatment Each layer is heat-treated as appropriate to achieve planarization, crystallization, etc. The heating temperature is, for example, 500-675°C or 550-650°C. The heating time is, for example, 0.1-2 hours or 0.2-1 hour. The heat source is electric heating, radiant heat, laser, etc. The heating atmosphere is under the high vacuum or inert gas atmosphere described above. The heat treatment may be performed after each layer is formed, or it may be performed all at once after the formation of multiple layers.

[0037] 《Application》 Multilayer thin films are used in various spintronic devices (including elements) such as magnetic random access memory (MRAM), magnetic sensors, and magnetic logic circuits. [Examples]

[0038] Related to magnetic tunnel junctions (MTJs) Magnetic layer MTJ elements (samples) having a magnetic layer made of a Co-based alloy were fabricated, and their properties (MR ratio) were evaluated. The present invention will be described in more detail based on these specific examples.

[0039] Sample preparation (1) Overview As shown in Figure 1, a sample (multilayer thin film) was fabricated by the thin-film method. Specifically, the process is as follows. For the sake of explanation, the directions indicated by the arrows in Figure 1 are referred to as the vertical direction (layering direction) and the horizontal direction.

[0040] Using an ultra-high vacuum multi-component sputtering apparatus (ULVAC, Inc. MPS-2000-C8 / simply referred to as "the apparatus"), a film was deposited on an MgO single crystal surface (100) that had been heated and cleaned under vacuum (600°C) and then cooled to near room temperature. The resulting single crystal surface (1 cm square) was formed by polishing the surface of the MgO substrate.

[0041] Vacuum level achieved before film deposition: 1 × 10⁻⁶ -7 The film thickness was set to Pa or less and the film shape to φ8 mm. Film thickness was calculated from the product of the film deposition rate (0.1 nm / sec or less) and the film deposition time.

[0042] For the target (raw material), a pure metal appropriate to each layer was used. For the alloy layer, an alloy prepared in advance to the desired composition may be used as the target.

[0043] (2) Stratification First, a Cr layer (50 nm thick) was deposited on an MgO single crystal plane at room temperature. This Cr layer was then heated to 600°C in the apparatus to flatten it.

[0044] Next, after cooling the Cr layer (underlayer) to below 50°C, a CoAl layer (40 nm thick) was deposited by a binary sputtering method using Co and Al. The CoAl layer was heated to 600°C in the apparatus to form a magnetic layer consisting of the β-CoAl phase. The composition (Co content) of the CoAl (Co-based alloy) was changed for each sample, as shown in Table 1. Unless otherwise specified, the composition is expressed as the atomic ratio to the total.

[0045] After cooling the magnetic layer to below 50°C, the following layers were stacked in order: Mg layer (thickness 0.4 nm), MgO (thickness 2 nm), CoFeB layer (thickness 3 nm), Ru layer (thickness 0.95 nm), Co layer (thickness 2 nm), Mn-Ir layer (thickness 10 nm), Cr layer (thickness 5 nm), and Ru layer (thickness 7 nm).

[0046] The entire laminate was heated (325°C) in a vacuum inside the apparatus in an oriented magnetic field (5kOe). The direction of the magnetic field was MgO <100> This was done to induce crystallization of the amorphous CoFeB layer and to bring about exchange magnetic anisotropy in the Co layer / Mn-Ir layer.

[0047] The MgO layer acts as an insulating layer, while the CoFeB layer above it acts as the other (ferro)magnetic layer. The extremely thin Mg layer is provided to recover any remaining oxygen in the apparatus and to stabilize the MgO layer (preventing the diffusion of oxygen into the magnetic layer). The Mn-Ir layer acts as an antiferromagnetic layer, and the Ru layer acts as a protective layer. The combination of each layer on the insulating layer (MgO layer) fabricated in this embodiment is well known as a multilayer structure of an MTJ.

[0048] The resulting laminate was cooled to room temperature and then removed into the atmosphere. In this way, the samples (multilayer thin films) shown in Table 1 were obtained. The compositions shown in Table 1 represent the atomic ratio of Co to the total amount of Al, and are in close agreement with the results of analysis using an inductively coupled plasma (ICP) emission spectrometer.

[0049] "observation" Sample 3 (Co 60 Al 40 A cross-section of the sample was observed using a scanning transmission electron microscope (STEM). The observed image is shown in Figure 2.

[0050] As can be seen in Figure 2, it was confirmed that the magnetic layer (CoAl layer) and the insulating layer (MgO layer) were grown epitaxially with controlled crystal orientation.

[0051] "measurement" The MR ratio of each sample was measured. The MR ratio was calculated using the formula: MR ratio = 100 × (rap - rp) / rp (%), where the electrical resistance (rp) is when the magnetizations of the magnetic layer (CoAl layer) and the magnetic layer (CoFeB layer) are parallel, and the electrical resistance (rap) is when their magnetizations are antiparallel.

[0052] Electrical resistance was measured using the four-terminal method with a CIPT apparatus. When no magnetic field is applied to the sample, the magnetization directions of each magnetic layer are antiparallel along their in-plane direction. When a magnetic field (150 Oe) is applied to the sample, the magnetization directions of each magnetic layer become parallel. The measurements were performed at room temperature.

[0053] The MR ratios of each sample obtained in this way are shown in Table 1. Furthermore, Figure 3 shows the relationship between the Co content (at%) of the magnetic layer of each sample and the MR ratio.

[0054] As can be seen from Table 1 and Figure 3, all samples exhibited the tunnel magnetoresistance effect (TMR). In particular, when the amount of Co contained in the magnetic layer beneath the insulating layer was 58.5–63.5 at% and even 59–62.5 at%, the MR ratio exceeded 100%.

[0055] Incidentally, when the amount of Co increased (Co ≥ 64 at%), a mixed structure of CoAl phase and Co phase appeared in the magnetic layer, and the MR ratio decreased. Conversely, when the amount of Co in the magnetic layer decreased (Co ≤ 58 at%), although the CoAl phase was formed well, antiferromagnetization or paramagnetization caused the saturation magnetization to decrease (and even become nearly zero), resulting in deterioration of the TMR (decrease in the MR ratio).

[0056] Thus, the present invention provides a new multilayer thin film that stably exhibits the tunnel magnetoresistance effect.

[0057] Table 1

Claims

1. A multilayer thin film having an insulating layer and a magnetic layer sandwiching the insulating layer, At least one of the magnetic layers is made of a Co-based alloy consisting of Al and the remainder Co. A multilayer thin film containing 58.5 to 63.5 at% of Co relative to the entire Co-based alloy.

2. The multilayer thin film according to claim 1, wherein Co is contained in an amount of 59 to 62.5 at% of the entire Co-based alloy.

3. The multilayer thin film according to claim 1, wherein the Co-based alloy is the β phase.

4. The multilayer thin film according to claim 1, wherein the magnetic layer has a thickness of 2 to 100 nm.

5. The multilayer thin film according to claim 1, wherein the magnetic layer below the insulating layer is formed on an MgO single crystal, or on a Cr layer located on the MgO single crystal.

6. The magnetic layer below the insulating layer is made of the Co-based alloy. The multilayer thin film according to claim 1, wherein the magnetic layer above the insulating layer is made of the Co-based alloy or CoFeB-based alloy.

7. The insulating layer is MgO or MgAl ２ O ４ A multilayer thin film according to claim 1, comprising the above.

8. A method for manufacturing a multilayer thin film according to any one of claims 1 to 7, A layering step for forming a magnetic layer made of the aforementioned Co-based alloy, A heating step of heating the magnetic layer to 500 to 675°C, A method for manufacturing a multilayer thin film, comprising the features described above.

9. The method for manufacturing a multilayer thin film according to claim 8, wherein the layering step is performed by sputtering.

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