Fast exchange MRAM with aluminum-manganese-germanium-free layer bonded to chromium diffusion barrier

Abstract

Magnetic tunnel junction (MTJ) pillars (or arrays thereof) are disclosed, each MTJ pillar having a magnetically free layer (160) (including a fast exchange material such as aluminum or a metal such as gallium), a magnetic reference layer (124), and a tunnel barrier layer (150) separating the two magnetic layers. A chromium-containing diffusion barrier layer disposed between the magnetic free layer (160) and the tunnel barrier layer (150) prevents diffusion of aluminum (or gallium) from the magnetic free layer into the tunnel barrier layer of the MTJ pillar. Apparatuses using the fast exchange MTJ (s) and methods of making the fast exchange MTJ (s) are also disclosed. It enables a device with a reduced resistance area (RA). The use of an AlMnGe alloy to fabricate a magnetic free layer having a tetragonal crystal structure and a lower magnetic moment that supports a higher magnetic directional exchange speed is provided.

Description

Background Art

[0001] The present invention relates to magnetic tunnel junction (MTJ) devices, methods of manufacturing such MTJ devices, and the use of such MTJ devices in magnetic random access memories (MRAMs). More specifically, the present invention relates to MTJ devices that include materials and crystal structures in the MTJ free layer and materials in the tunnel barrier layer in combination with a diffusion barrier layer that reduces the MTJ device resistance and increases the MTJ device switching speed.

[0002] MTJ device pillars are used in MTJ devices similar to MRAMs and other MTJ devices. MRAM is a non-volatile random access memory technology in which data is stored using magnetic storage elements in MTJ pillars. MRAM is a viable memory option for both standalone and embedded applications such as caches, eDRAM replacement, Internet of Things (IoT), automotive, or artificial intelligence (AI).

[0003] MTJs are typically formed by two ferromagnetic layers, each of which can maintain magnetization. These magnetized layers are separated by a thin dielectric layer, i.e., a tunnel barrier layer. Embodiments of MTJs used as memory devices or sensors operate during operation using one of the magnetic layers (reference layer) with an unchanged magnetization orientation, while the magnetization orientation of the other layer (free layer) can be modified or changed during operation.

[0004] Typically, during the startup process of a magnetic tunnel junction device, the magnetic orientation of the reference layer is set, for example, by exposure to a strong external magnetic field. In some cases, the strong magnetic field is combined with heat under ultra-high vacuum, e.g., exposure to a strong magnetic field greater than 0.1 Tesla, but in most MRAM cases likely greater than 1.5 Tesla; heating at temperatures above 200C but below 550C; and in an ultra-high vacuum below 10 -5 torr.

[0005] In a more specific embodiment, a magnetic tunnel junction (MTJ) device is a small-sized MTJ pillar (having a typical diameter between 15 and 150 nanometers (nm), but more typically a diameter between 20 and 100 nm) that includes a magnetic reference layer, a magnetic free layer, and a thin tunnel barrier layer separating the magnetic reference layer and the free layer.

[0006] A specific use case for MTJ devices is their implementation as physical bits in magnetic random access memories. In this use, "0" and "1" are encoded as parallel and anti-parallel magnetization states of the free layer and the reference layer. A smaller sense voltage applied to the ends of the columnar MTJ device enables measurement of the resistance of the MTJ pillar.

[0007] The tunneling magnetoresistance effect (TMR) provides a resistance difference between the parallel and antiparallel orientations of the free layer and the reference layer of the MTJ pillar. This resistance difference is used to indicate the bit value of the memory.

[0008] For different memory designs, there are different ways to write to a bit. One method is to use a write current flowing through the MTJ pillar, thereby generating a so-called spin transfer torque (STT). The direction of this write current relative to the pillar geometry determines whether a "0" or a "1" is written by reorienting the magnetization of the free layer. The "direction of the write current relative to the pillar geometry" is determined by whether the voltage across the tunnel barrier layer of the MTJ pillar is positive or negative.

[0009] In some typical MTJ stacks, the reference layer is in direct contact with the tunnel barrier layer. The tunnel barrier layer is typically made of magnesium oxide (MgO). In some embodiments, the free layer is typically composed of a combination of a cobalt-iron-boron alloy (CoFeB) and a cobalt-iron (CoFe) alloy combined with a refractive metal layer (such as tantalum (Ta), tungsten (W), niobium (Nb), zirconium (Zr), or others).

[0010] The MTJ pillar structure is typically formed by patterning and etching a homogeneous film of the MTJ layer, where the homogeneous film of the MTJ layer is in the form of an MTJ stack structure. In some embodiments, the MTJ stack structure is etched, for example, by ion beam etching (IBE) to form a single MTJ pillar or an array of one or more MTJ pillars. Devices (such as MRAM) and other devices can be fabricated by connecting device components (such as MRAM structures) in the back-end-of-line (BEOL) hierarchy to the MTJ pillars. These structures and the methods of manufacturing these structures are known.

[0011] Advanced applications require a random access memory (RAM) with a very fast switching time. A class of MRAM called spin transfer torque MRAM (STT MRAM) devices requires a switching time of less than 10 nanoseconds (ns). In some applications (such as last-level cache or eDRAM replacement), a switching time of approximately 2 ns is required.

[0012] However, traditional MTJ or MRAM layer materials (such as CoFeB or CoFe alloys) do not support a high switching speed because these materials have too high a magnetic moment.

[0013] The tetragonal aluminum-manganese-germanium (AlMnGe) alloy (with an element ratio of 1:1) is a promising material that provides the required magnetism for the MTJ free layer and enables a high switching speed.

[0014] However, aluminum (Al) in the AlMnGe alloy causes problems. When the AlMnGe ordered alloy free layer directly contacts the MgO tunnel barrier layer during the thermal cycling of processing the MTJ stack, some of the aluminum in the aluminum from the AlMnGe diffuses into the MgO, thus forming MgAl oxide. The problems caused by this oxide formation include: 1. Depletion of aluminum in the AlMnGe free layer, 2. Alteration of the magnetic properties of the free layer, i.e., reduction of magnetoresistance (tunnel magnetoresistance - TMR), 3. Increase in the breakdown of the barrier oxide (TBBD), and 4. Increase in the resistance area product (RA) of the stack.

[0015] To address these problems, the free layer is given a higher aluminum content to compensate for aluminum diffusion, and / or the MgO barrier layer has been made thinner. However, experiments have shown that these efforts have failed to solve these problems.

[0016] It is necessary to suppress the interaction between the MTJ (free) layer containing aluminum (Al) and the MgO in the MTJ barrier layer without adversely affecting the magnetic MTJ stack properties. SUMMARY OF THE INVENTION

[0017] Embodiments of the present invention include at least one magnetic tunnel junction (MTJ) pillar having a magnetic free layer made of a rapid exchange material containing aluminum or containing gallium and a thin diffusion barrier layer made of elemental chromium that prevents aluminum (or gallium) from diffusing from the magnetic free layer into the tunnel barrier layer of the MTJ pillar. Devices using (multiple) rapid exchange MTJ pillars and methods of manufacturing (multiple) rapid exchange MTJ pillars are also disclosed.

[0018] Embodiments of the pillar include one or more magnetic reference layers disposed on a substrate. (Other layers or structures (such as back-end-of-line (BEOL) structures / layers) may be located between the magnetic reference layer and the substrate.) The magnetic reference layer has a first magnetic orientation. The first magnetic orientation is typically in a fixed magnetic orientation.

[0019] The magnetic free layer has a second magnetic orientation. The second magnetic orientation can be exchanged to align parallel or antiparallel with the first magnetic orientation. In a preferred embodiment, the magnetic free layer includes an AlMnGe alloy having a 1:1 atomic ratio in a tetragonal crystal structure, which has a lower magnetic moment to support a higher exchange speed. Some embodiments of the magnetic free layer are made of a gallium (Ga) alloy, including but not limited to GaMnGe.

[0020] A tunnel barrier having a tunnel barrier thickness separates a magnetic reference layer and a magnetic free layer. Current tunnels through the tunnel barrier when flowing between the magnetic reference layer and the magnetic free layer. When the magnetic reference layer and the magnetic free layer are parallel-aligned, the device is in a low-resistance state. When the magnetic reference layer and the magnetic free layer are anti-parallel-aligned, the device is in a high-resistance state. In a preferred embodiment, the tunnel barrier is made of magnesium oxide (MgO).

[0021] A diffusion barrier is disposed between the magnetic free layer and the tunnel barrier.

[0022] In one embodiment, the diffusion barrier is made of a thin chromium diffusion barrier (on the order of one to five atomic monolayers in thickness) made of the element chromium. Other thicknesses are conceivable. In one embodiment, the thin chromium barrier layer abuts and is in direct contact with the tunnel barrier layer and prevents aluminum (or a material such as gallium) from diffusing from the magnetic free layer into the tunnel barrier layer of the MTJ pillar.

[0023] In some embodiments, the diffusion barrier is a monolayer-thick chromium that is the face (interface) of the tetragonal unit cell of the free layer (i.e., the interfacial unit cell (having the tunnel barrier layer)). In other embodiments, the diffusion barrier thickness can be between 0.2 nanometers (nm) and 1 nm, or alternatively from one atomic monolayer thickness to five atomic monolayer thicknesses of chromium.

[0024] In some embodiments, the diffusion barrier is in direct contact with the tunnel barrier layer. In a preferred embodiment, the diffusion barrier (chromium layer) is in direct contact with both the tunnel barrier layer and the free layer (and between the tunnel barrier layer and the free layer).

[0025] In some embodiments, the diffusion barrier is formed by incorporating chromium atoms into the interface of the interfacial unit cell to form a tetragonal CrAlMnGe alloy, where Cr replaces Mn in the interface. In a preferred embodiment, the chromium atoms are only in the interface or only include the interface, and there are no chromium atoms elsewhere in the free layer.

[0026] The diffusion barrier has two sides or surfaces: 1. The "tunnel interface" is the side / surface in contact with the tunnel barrier layer, and 2. The "free layer interface" is the side / surface in contact with the free layer. Thus, the tunnel interface and the free layer interface are opposite surfaces on the diffusion barrier and are opposite each other.

[0027] In summary, the diffusion barrier layer is a chromium metal layer that directly interfaces with the four-sided cell interfaces of the tunnel barrier layer and the free layer and is between them. In one embodiment, the diffusion barrier layer is a layer on the order of a single atomic layer in thickness that contains only chromium atoms. The tunnel interface is the side / surface of the diffusion barrier layer that interfaces with the tunnel barrier layer interface, and the free layer interface is the side / surface of the diffusion barrier layer that interfaces with the free layer interface. The free layer does not contain chromium atoms. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Various embodiments of the present invention will now be described in more detail with reference to the accompanying drawings, which are briefly described herein. The drawings show various devices, structures, and related method steps of the present invention.

[0029] Figure 1 is a cross-sectional view of a non-limiting example of an intermediate magnetic tunnel junction (MTJ) stack structure including a diffusion barrier layer between a MgO tunnel barrier layer and an AlMnGe (or MnGaGe containing gallium, e.g., MnGaGe) magnetic free layer containing aluminum.

[0030] Figure 2 is a cross-sectional view of an example MTJ pillar including a diffusion barrier layer that interfaces with and is between a MgO tunnel barrier layer and an AlMnGe or MnGaGe magnetic free layer containing aluminum.

[0031] Figure 3 is a schematic diagram showing a diffusion barrier layer that interfaces with a tunnel barrier layer and a free layer, where the exemplary magnetic free layer is a stacked four-sided crystal unit cell structure of an AlMnGe alloy.

[0032] Figure 4 is a cross-sectional view of an array embodiment of MTJ pillars on a substrate.

[0033] Figure 5 is a graph showing the resistance area product (RA) of three MTJ structures having tunnel barrier layers and free layers made of MgO of the same thickness, as follows: a) an AlMnGe free layer without chromium, b) an AlMnGeCr alloy free layer with chromium throughout the free layer, and c) an AlMnGe free layer having a diffusion barrier layer (preferably a single atomic layer of chromium thickness) that interfaces with and is between the free layer and the tunnel barrier layer.

[0034] Figure 6 is a flowchart of a process for fabricating a fast-switching MTJ that includes an embodiment of depositing a diffusion barrier layer between a MgO tunnel barrier layer and an AlMnGe magnetic free layer. DETAILED DESCRIPTION

[0035] It will be understood that embodiments of the present invention are not limited to the illustrative methods, apparatuses, structures, systems, and devices disclosed herein, but rather, more broadly apply to other alternative and broader methods, apparatuses, structures, systems, and devices, which will be apparent to those skilled in the art given this disclosure.

[0036] Furthermore, it will be understood that the various layers, structures, and / or regions shown in the figures are not drawn to scale, and one or more layers, structures, and / or regions of the type commonly used may not be explicitly shown in a given figure. This does not mean that the layers, structures, and / or regions not explicitly shown are omitted from the actual device.

[0037] In addition, for clarity and / or simplicity, certain elements may be omitted from the views when the explanation need not focus on those omitted elements. Also, the same or similar reference numerals are used in all the figures to denote the same or similar features, elements, or structures, and thus, a detailed explanation of the same or similar features, elements, or structures may not be repeated for each figure in the figures.

[0038] The semiconductor devices, structures, and methods disclosed according to embodiments of the present invention can be used in applications, hardware, and / or electronic systems. Suitable hardware and systems for implementing embodiments of the present invention can include, but are not limited to, personal computers, communication networks, e-commerce systems, portable communication devices (e.g., cellular and smart phones), solid-state media storage devices, expert and artificial intelligence systems, functional circuitry, neural networks, etc. Systems and hardware incorporating semiconductor devices and structures are contemplated embodiments of the present invention.

[0039] As used herein, "height" refers to the vertical dimension of an element (e.g., a layer, trench, hole, opening, etc.) measured in a cross-section or front view from the bottom surface to the top surface of the element and / or relative to the surface on which the element is located.

[0040] Conversely, "depth" refers to the vertical dimension of an element (e.g., a layer, trench, hole, opening, etc.) measured in a cross-section or front view from the top surface to the bottom surface of the element. In the cases indicated, terms such as "thick", "thickness", "thin", or their derivatives may be used in place of "height".

[0041] As used herein, "lateral", "lateral side", "side", and "lateral surface" refer to the side surfaces of an element (e.g., a layer, opening, etc.), such as the left or right side surfaces in the figures.

[0042] As used herein, "width" or "length" refers to the dimension of an element in a drawing (e.g., a layer, trench, hole, opening, etc.) measured from a side surface of the element to an opposite surface. In the indicated cases, terms such as "thick", "thickness", "thin", or their derivatives may be used in place of "width" or "length".

[0043] As used herein, terms such as "upper", "lower", "right", "left", "vertical", "horizontal", "top", "bottom", and their derivatives shall refer to the disclosed structures and methods as oriented in the drawings. For example, as used herein, "vertical" refers to a direction perpendicular to the top surface of the substrate in a front view, and "horizontal" refers to a direction parallel to the top surface of the substrate in a front view.

[0044] As used herein, unless otherwise specified, terms such as "on", "overlying", "on top of", "above", "positioned on", or "positioned on top of" mean that a first element is present on a second element, where an intermediate element may be present between the first and second elements. As used herein, unless otherwise specified, the term "directly" used in conjunction with the terms "on", "overlying", "on top of", "above", "located on" or "located on top of", "being disposed on" or the term "contact" or "direct contact" means that the first and second elements are connected without any intermediate element between the first and second elements, such as an intermediate conductive, insulating, or semiconductor layer.

[0045] It will be understood that these terms may be affected by the orientation of the described device. For example, although the meanings of these descriptions may change if the device is rotated upside down, these descriptions remain valid as they describe the relative relationships between the features of the present invention.

[0046] A magnetic tunnel junction (MTJ) has two magnetic conductive metal layers separated by a thin insulator layer, a tunnel barrier layer. The tunnel barrier layer is thin enough so that if a bias voltage is applied between two metal electrodes or connections attached to the respective magnetic metal layers, electrons can tunnel through the tunnel barrier. In some embodiments, the bias voltage is applied through connections in the BEOL layer.

[0047] In an MTJ, the magnitude of the tunneling current depends on the relative orientation of the magnetization of the two magnetic layers. When the magnetic layers are magnetized in the same direction, i.e., parallel aligned or parallel oriented, more tunneling current flows, so the resistance of the device decreases and the device is in a low resistance state. On the other hand, when the magnetic layers are magnetized in opposite directions / orientations, i.e., anti-parallel magnetized or anti-parallel oriented, less tunneling current flows, so the resistance of the device increases and the device is in a high resistance state.

[0048] In some embodiments, one of the magnetic layers serves as a "fixed layer" or "reference layer" whose magnetization direction is fixed to a given direction. Alternatively, another magnetic layer serves as a "free layer" whose magnetization can relatively easily change its direction with respect to the "fixed" or "reference" layer.

[0049] Based on the relative magnetic orientation of the free layer with respect to the reference layer, the device switches from a parallel alignment to an antiparallel alignment and vice versa. As a result, the resistance of the device switches from a high-resistance state to a low-resistance state and vice versa.

[0050] In some embodiments, during operation of the device, an exchange current having a current amplitude higher than an exchange amplitude threshold is used to switch the magnetization orientation of the magnetic free layer. For example, an exchange current higher than the exchange amplitude threshold in the direction from the reference layer to the free layer (through the tunnel barrier) switches the magnetization direction of the free layer with respect to the reference layer from parallel to antiparallel. Alternatively, an exchange current higher than the exchange magnitude threshold in the direction from the free layer to the reference layer (across the tunnel barrier) switches the magnetization direction of the free layer with respect to the reference layer from antiparallel to parallel. In this way, the magnetization direction of the magnetic free layer can be switched from a parallel orientation to an antiparallel orientation with respect to the magnetization direction of the magnetic fixed layer and vice versa. A sense current having a lower amplitude passing through the device in either direction will sense whether the device is in a low-resistance state or a high-resistance state.

[0051] Note that in some embodiments, the magnetic fixed layer and / or the reference layer may be formed in one or more magnetic layers.

[0052] A diffusion barrier layer, i.e., a diffusion barrier layer made of chromium, separates the tunnel barrier layer and the magnetic free layer, whereby the diffusion barrier layer prevents aluminum (or other substances such as gallium) from diffusing from the magnetic free layer (free layer) into the tunnel barrier layer.

[0053] Figure 1 is a cross-sectional view of a temporary magnetic tunnel junction (MTJ) stack structure 100 built on a substrate 105.

[0054] Exemplary substrates 105 can be made of a single element (such as silicon or germanium) or a compound semiconductor (such as gallium arsenide (GaAs)) or a semiconductor alloy (such as silicon germanium (SiGe)). Substrates 105 for these devices are well known and vary. In addition, the substrate 105 can be as simple as a single dielectric layer or as complex as a known front-end-of-line (FEOL) circuit. Alternative substrates 105 are known in the art and foreseeable.

[0055] The Back-End-of-Line (BEOL) layer or structure 110 is disposed on the substrate 105 and includes a plurality of known layers formed by known BEOL processes performed in semiconductor technology. Note that it is well known that BEOL "layers" typically include contacts, elements, connections, vias, insulating layers, and metallization and interconnect layers, which have different forms from the layers mentioned in other "layers" in the stacked structure 100. Those skilled in the art will know how to distinguish the "layers" in BEOL from the other layers of the stacked structure 100, for example, based on the context of this specification. The layers in BEOL 110 will be referred to as BEOL layers (or generally as BEOL layer or structure 110) to facilitate this distinction between the layers, even if it is not considered necessary to make such a distinction formally.

[0056] As is known, circuits fabricated in the FEOL layer can be connected to the MRAM or MTJ pillars (see element 250 below) using the interconnect structures in the BEOL layer 110 and / or the substrate 105. In some embodiments, some MRAM circuits are formed in the BEOL layer, and this MRAM circuit is connected to the MTJ pillar (see 250 below) connection to complete the formation of the MRAM device.

[0057] In some embodiments, there is a single ferromagnetic material layer 125 disposed on the BEOL layer 110. This ferromagnetic material layer 125 is the magnetic reference layer 125 and is set to one magnetic polarity or orientation. In some embodiments, the reference layer 125 is a permanent magnet 125 or other fixed magnetic material. For example, the reference layer 125 can be composed of one or more metals or metal alloys that exhibit high spin polarization. Non-limiting examples of metals in the reference layer 125 metal and / or alloy include: iron (Fe), nickel (Ni), cobalt (Co), chromium (Cr), boron (B), or manganese (Mn).

[0058] In an alternative embodiment, the magnetic reference layer 125 is formed as a multilayer arrangement 123, which has (1) a high spin polarization region formed by a metal and / or metal alloy using the above-mentioned metals 125 / 125-1, and (2) a region composed of one or more materials that exhibit strong perpendicular magnetic anisotropy (strong PMA) 120 / 120-1. In other embodiments, there are multiple pairs 124 of high spin polarization layers 125-1 and PMA layers 120-1 in the multilayer arrangement 124 of the magnetic reference layer 124. Exemplary materials having strong PMA 120 / 120-1 that can be used include metals such as cobalt, nickel, platinum, palladium, iridium, or ruthenium, and these metals arranged as alternating layers. The strong PMA region / layer can also include alloys that exhibit strong PMA, exemplary alloys including cobalt-iron-terbium, cobalt-iron-gadolinium, cobalt-chromium-platinum, cobalt-palladium, iron-platinum, and / or iron-palladium. In some embodiments, some or all of the layers having high spin polarization are omitted.

[0059] Similarly, the alloy can be arranged as alternating layers as shown in region 123 or alternating layers 120 / 125 above region 123. In some embodiments, combinations of these materials and regions can also be employed. In these instances, the combination of layer 124 serves as the magnetic fixing layer 124 or the magnetic reference layer 124. All of these embodiments and their combinations are foreseeable.

[0060] The thickness of the magnetic reference layer (125, 120 / 125, 123, 124) will depend on the materials selected. The exemplary thickness 125T-1 of the high-spin layer 125-1 and the exemplary thickness 120T1 of the PMA layer 120-1 range between 0.3 nanometers (nm) and 3 nm. However, other thicknesses can also be contemplated, and the thicknesses (120T, 120T1, 125T, 125T1, etc.) depend on the materials selected.

[0061] In an alternative embodiment, only the polarization enhancement layer 140 (described below) has high spin polarization, and high spin polarization is not necessary for the other layers 124.

[0062] In some embodiments, the magnetic reference layer 124 includes a series of alternating platinum (Pt) PMA layers 120 (120-1) and cobalt (Co) layers 125 (125-1) layer 123, which constitute the magnetic fixing layer 124 disposed on the BEOL layer 110.

[0063] In some embodiments, having multiple layers in the reference layer 123 / 124 makes the reference layer 123 / 124 more stable and reliable. Different configurations of the reference layer 123 / 124 are known and are contemplated as embodiments of the present invention. For example, in some embodiments, the free layer 160 can be deposited first on the BEOL layer 110, and then the magnetic reference layer 124 can be deposited, so that the device will be "upside down" compared to the device shown in Figure 1 and the later figures.

[0064] The reference layer 123 / 124 is deposited by known techniques (such as physical vapor deposition (PVD)) using known tools (such as PVD cluster tools). Generally, as is well known, multiple layers can be deposited using the same tool by using a sub-chamber to avoid breaking the vacuum.

[0065] In some embodiments, a non-magnetic metal spacer layer 130 is deposited on the reference layer 123 / 124, for example, using the above PVD method. In some embodiments, the non-magnetic metal spacer layer 130 is made of tantalum, tungsten, or other refractory metals, and its thickness 130T is between 0.1 nanometers (nm) and 0.4 nm. In some embodiments, this layer 130 is omitted. These and other alternative embodiments of the non-magnetic metal layer 130 can be contemplated.

[0066] In some embodiments, the polarization enhancement layer 140 is deposited on the non-magnetic metal spacer layer 130 using known techniques, such as PVD using the above-described PVD cluster tool. In some embodiments, the polarization enhancement layer 140 is made of a bilayer of CoFeB or CoFeB|Fe, and the total thickness 140T is between 0.5 nm and 1.5 nm. In some embodiments, the polarization enhancement layer 140 has a thickness 140T of approximately 1 nm.

[0067] The tunnel barrier layer 150 is made of an insulator material and is formed with a thickness that provides an appropriate tunneling resistance. Exemplary materials for the tunnel barrier layer 150 include magnesium oxide (MgO). The thickness 150T of the tunnel barrier layer 150 will depend on the material selected. In one example, the tunnel barrier layer 150 can have a thickness 150T ranging from 0.5 nm to 1.5 nm. The tunnel barrier layer 150 is deposited by known techniques, including atomic layer deposition (ALD), physical vapor deposition (PVD), and chemical vapor deposition (CVD). In a preferred embodiment, the tunnel barrier layer 150 contains less than 5% aluminum, and more preferably, the tunnel barrier layer 150 contains 0% aluminum.

[0068] For most embodiments, MgO is the preferred material for the tunnel barrier layer 150, while materials such as titanium dioxide (TiO2) are used, but are not preferred. Although MgO is the preferred material for the tunnel barrier layer 150, all embodiments are considered for the present invention. The preferred method for depositing such an MgO barrier layer is physical vapor deposition.

[0069] The diffusion barrier layer 181 includes a thin (e.g., atomic monolayer, approximate thickness 181T of the chromium atom diameter or one to five atomic monolayers of thickness 181T) barrier layer 181 of chromium atoms. The diffusion barrier layer 181 is disposed on the tunnel barrier layer 150, in interfacial contact therewith, in direct contact therewith at the tunnel interface 181I, and is located between the tunnel barrier layer 150 and the magnetic free layer 160. The diffusion barrier layer 181 is in dielectric contact with and in direct contact with the free layer 160 at the free layer interface 183I. Other diffusion barrier layer 181 thicknesses 181T can be considered as disclosed herein.

[0070] The chromium in the diffusion barrier layer 181 is a substance that prevents aluminum (and in some embodiments, gallium) from diffusing from the magnetic free layer 160 across the tunnel interface 181I into the tunnel barrier layer 150, even during the manufacturing heat cycle steps during the formation of the MTJ pillar.

[0071] As described in more detail below, in one embodiment, a diffusion barrier layer 181 is formed by depositing elemental chromium on the surface of the tunnel barrier layer 150 in a vacuum / inert gas environment to form a chromium layer as an interfacial surface of the interfacial unit of the free layer 160. Elemental chromium can be deposited by PVD in ultra-high vacuum (UHV) as described herein. For example, see Figure 6 , the use of UHV and an inert gas environment prevents the oxidation of chromium and the formation of electrically insulating chromium oxides. In some embodiments, the thickness 181T of the diffusion barrier layer 181 is the thickness of a single atomic layer of chromium atoms, or can be in the range between 1 and 5 angstroms, or preferably between 2 and 3 angstroms. In other embodiments, the thickness 181T of the diffusion barrier layer 181 is between 0.2 nanometers (nm) and 1 nm, or between one atomic layer and five atomic layers of chromium atoms. See Figure 3 for more details in the description of

[0072] In one embodiment, the entire magnetic free layer 160 is deposited on the diffusion barrier layer 181. Thus, as described, the magnetic free layer 160 is directly joined to the diffusion barrier layer 181 at the free layer interface 183I, such that the diffusion barrier layer 181 separates the tunnel barrier layer 150 and the magnetic free layer 160.

[0073] The magnetic free layer 160 material is a magnetic material (or a stack of magnetic materials) having magnetization that can be changed in an orientation relative to the magnetization orientation of the magnetic material in the magnetic reference layer 124. In particular, the material in the magnetic reference layer 160 can exchange at high speed, for example, at a speed higher than 10 ns, or more preferably between 2 ns and 5 ns. In a preferred embodiment, the magnetic moment per unit area of the free layer is between 0.02 and 0.1 memu / cm 2 between.

[0074] In some embodiments, the magnetic free layer 160 is made of a material containing aluminum, such as an aluminum-containing free layer 160. In other embodiments, the aluminum-containing magnetic free layer 160 is made of an alloy of aluminum (Al), manganese (Mn), and germanium (Ge), typically with an atomic ratio of Al, Mn, and Ge of 1:1. In some embodiments, the magnetic free layer 160 is made of AlMnGe having a tetragonal crystal symmetry.

[0075] In an alternative embodiment, the magnetic free layer 160 can be made of Mn2AlCo, MnAlCo2, or MnAl. In additional alternative embodiments, the free layer 160 can contain a metal such as gallium (Ga), and can be made of MnGaGe, Mn2GaCo, MnGaCo2, and MnGa.

[0076] In some embodiments, the thickness 160T of the magnetic free layer 160 is between 2 nm and 2.5 nm and is deposited by known techniques, e.g., PVD using a cluster deposition tool as described above.

[0077] In some embodiments, an upper MgO layer 150U is deposited on the magnetic free layer 160 using the techniques and equipment described above. The upper layer 150U, e.g., the MgO layer 150U, is made of a material (typically MgO) of thickness 150UT that does not add (or can be negligible in series resistance) through the stack structure 100. However, this upper MgO layer 150U is optional and will be omitted in the remainder of this specification without loss of generality.

[0078] (However, note that given this disclosure, an optional chromium diffusion layer (not shown) can be added between the upper MgO layer 150U and the magnetic free layer 160)

[0079] A capping layer 175 is deposited as the final layer of the MTJ stack structure 100. In some embodiments, the capping layer is made of a conductive metal that serves as an electrical contact / electrode for the device. For example, metals for the capping layer 175 include tantalum (Ta), titanium (Ti), tantalum nitride (TaN), titanium nitride (TiN), ruthenium (Ru), tungsten (W), or any combination thereof. The capping layer 175 is deposited using known deposition techniques, including PVD (e.g., as described above), CVD, ALD, and sputtering. It is contemplated that the capping layer 175 can have various thicknesses. In some embodiments, the thickness of the capping layer 175 is between 20 nm and 100 nm.

[0080] The capping layer 175 serves as an electrical connection for the device. In some embodiments, other electrical connections are made through the BEOL layer 110.

[0081] Figure 2 is a cross-sectional view of an MTJ pillar 200 / 250 having an MTJ pillar magnetic free layer 160, a tunnel barrier layer 150, and a diffusion barrier layer 181.

[0082] An MTJ pillar 250 or an array of two or more MTJ pillars 250 is generated from one or more MTJ stack structures by performing a masking 225 etching 275 process that is generally known. The hard mask 225 is a protective hard material deposited by standard techniques.

[0083] In some embodiments, the etching 275 is an ion beam etching (IBE). Once the surface of the BEOL layer 110 is reached by known techniques and MTJ pillars 250 are created in the shape and location defined by the mask 225, the IBE is time-controlled to stop. The width 280 of the MTJ pillar 250 is in the range of 20 to 100 nm. The mask 225 can be further processed by known methods.

[0084] Figure 3 This is a diagram showing that the diffusion barrier layer 181 interfaces with the tunnel barrier layer 150 at the tunnel interface 181I (not shown in direct contact for clarity) 181I and interfaces with the (magnetic) free layer 160 at the free layer interface 183I 183I. An exemplary magnetic free layer 160 is a conductive stacked 311 structure of a tetragonal crystal cell 305 / 310 of an AlMnGe alloy.

[0085] In one embodiment, the interfacial unit cell 305 of the magnetic free layer 160 includes an interface 183I, which serves as the diffusion barrier layer 181 and has a monolayer of atoms with a thickness 181T. After the manganese atoms 320Mn in the interface 183I are replaced by chromium atoms 320Cr, the interface 183I of the interfacial unit cell 305 becomes the diffusion barrier layer 181. Thus, a single atomic layer 320Cr / 181 of element chromium is formed as the diffusion barrier layer 181, which interfaces with the tunnel barrier layer 150 at the tunnel interface 181I and interfaces with the magnetic free layer 160 at the free layer interface 183I 183I.

[0086] The thickness 181I of the diffusion barrier layer 181 can be increased, for example, by depositing more chromium during the process described in Figure 6 which is described.

[0087] In a preferred embodiment, the unit 310 that is not the interfacial unit 305, i.e., the non - interfacial unit 310, constitutes most of the thickness 160T of the magnetic free layer 160. The non - interface unit 310 does not contain chromium.

[0088] The term "structure of a tetragonal crystal cell" refers to a crystal structure having a cell that includes three axes, where two axes have the same length and are perpendicular to each other, and where the third axis is perpendicular to these two axes. A tetragonal lattice is produced by stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base (e.g., side x by x) and a height (y, which is different from x).

[0089] In some embodiments, as Figure 3 shown, multiple stacked tetragonal crystal unit cells (non - interfacial unit cells) 310 in the magnetic free layer 160 are made of an AlMnGe alloy 310. As an exception, the interfacial cell 305 in the magnetic free layer 160 is made of a CrAlMnGe alloy, where Cr atoms 320Cr replace Mn atoms 320Mn to form the diffusion barrier layer 181.

[0090] As described, this diffusion barrier interface (chromium layer) 181 serves as a barrier layer for aluminum (Al) (or gallium (Ga)) to diffuse from the free layer 160 / 305 / 310 through the free layer interface 183I of the diffusion barrier layer 181 into the tunnel barrier layer 150.

[0091] It is believed that the chromium-containing diffusion barrier layer 181 will also prevent materials other than aluminum from diffusing from the free layer 160 into the tunnel barrier layer 150. For example, the chromium-containing interface cell 305 will prevent gallium (Ga) from diffusing from the free layer 160 into the tunnel barrier layer 150, where the free layer 160 is made of GaMnGe or other Ga-containing compounds.

[0092] As a continuing non-limiting example, in the case where the free layer 160 is made of Mn2AlCo, MnAlCo2, and MnAl, the chromium-containing diffusion barrier layer 181 will prevent Al from diffusing into the tunnel barrier layer 150. In addition, in the case where the free layer 160 is made of Mn2GaCo, MnGaCo2, and MnGa, the chromium-containing diffusion barrier layer 181 will prevent Ga from diffusing into the tunnel barrier layer 150.

[0093] Figure 4 is a cross-sectional view of an array 400 of MTJ pillars 250 / 251 on a substrate 105. The array 400 is produced from the MTJ stack structure 100 using the mask 225 patterning etch 275 techniques described above (e.g.) Figure 2 as described. Using these known techniques, an array 400 of MTJ pillars 250 / 251 can be produced that extends 410 in two directions in the plane of the substrate / BEOL layer 105 / 110. The MRAM is produced by forming connections through the BEOL layer 110 to the MTJ pillars 250 / 251 and other components.

[0094] Figure 5 is a graph 500 showing the resistance area product (RA, measured in ohm-square microns (Ω-μm 2 )) 505 of three MTJ structures 525 having tunnel barrier layers and free layers of the same thickness made of MgO, as follows: A) an AlMnGe free layer 510 without chromium, B) an AlMnGeCr alloy free layer with chromium throughout the free layer 520, and C) an AlMnGe free layer with a diffusion barrier layer 181 that is between and interfaces with the free layer and the tunnel barrier layer 530, as described in the present invention.

[0095] As shown in graph 500, the resistance area product RA, 505 of case 530 with the chromium diffusion barrier layer 181 is lower than the two cases 510 / 520. The RA is approximately 7 ohm-square microns (Ω-μm 2 ) or less, where 530 is the chromium diffusion layer 181.

[0096] Since the diffusion barrier 181 can reduce the RA of a given tunnel barrier 150 thickness 150T, the nominal thickness of the MgO tunnel barrier can be thicker when a diffusion barrier is present, without increasing the device resistance. The thicker barrier will be less prone to defects, and for a given voltage, the electric field across this thicker barrier will be smaller. Fewer defects and a smaller electric field across the MgO barrier will make the barrier less likely to fail due to electrical breakdown, and will extend the life of the device and the reliability of the memory formed by the MTJ pillar device.

[0097] Figure 6 is a flowchart of a process 600 for manufacturing a spin-transfer torque magnetic random access memory (STT-MRAM) device 200 / 400 including a diffusion barrier 181 between a tunnel barrier 150 and an aluminum (AlMnGe) (or other) magnetic free layer 160.

[0098] Process 600 begins at step 605, which constructs MTJ stack structure 100 as described in Figure 1 until after deposition of tunnel barrier 150.

[0099] In step 610, diffusion barrier 181 and free layer 160 are deposited by first depositing a layer of elemental chromium 181, and then growing free layer 160 without chromium.

[0100] Diffusion barrier 181 is formed by depositing a layer of elemental chromium using PVD techniques and the PVD cluster tool described above (see Figure 3 ). In some embodiments, this process step 610 is performed in a vacuum environment such that no oxides are formed and diffusion barrier 181 remains conductive. In some embodiments, diffusion barrier 181 is sputtered using a PVD cluster tool in an inert gas environment, such as argon (Ar) or krypton (Kr) or other inert gas, or in an environment of an inert gas mixture. Sputtering can occur at room temperature for a duration of 0.5 to 100 seconds, although other temperatures and durations are contemplated. Sputtering continues until the desired diffusion barrier 181 thickness 181T is obtained.

[0101] After the elemental chromium is deposited, the Mn-Ge-Al alloy is deposited, typically in a 1:1 ratio, to form interface unit 305 and the remaining non-interface unit 310. The as-deposited Mn:Ge:Al atomic ratio can deviate from 1:1 and allows an Mn:Al ratio of up to 1:2 and an Mn:Ge ratio of up to 1:1.5. As described above, after deposition of chromium layer 181 adjacent to tunnel barrier 150, the adjacent unit cell 305 and free layer unit cell 310 are grown using the standard PVD techniques and PVD cluster tool described.

[0102] Form enough of these non-interface units 310 to obtain the desired thickness 160T of the magnetic free layer 160.

[0103] In step 615, the remainder of the MTJ stack structure is formed together with the MTJ pillar 250 as described above.

[0104] In step 620, a circuit, such as an MRAM, is formed by connecting the MTJ pillars 250 / 251 to a circuit in, for example, the BEOL layer 110 using known methods.

[0105] As a result of this process 600, and as Figure 3 shown in the description, the chromium atoms 320Cr form a single atomic layer of chromium diffusion barrier layer 181 contained in the interface cell 305. The thickness 181T of the diffusion barrier layer 181 can be increased by depositing more chromium on the single atomic layer of chromium.

[0106] The diffusion barrier layer 181 effectively blocks the migration of aluminum (Al) and other metals (such as germanium (Ge)) into the tunnel barrier layer 150 while keeping the diffusion barrier layer 181 thin 181T. Thus, these metals can be used in the low magnetic moment magnetic free layer 160 to enhance the device switching time without degrading the tunnel barrier layer 150.

[0107] The description of the various embodiments of the present invention has been given for purposes of illustration, but is not intended to be exhaustive or limited to the disclosed embodiments. Many modifications and variations will be apparent to a person of ordinary skill in the art without departing from the scope and spirit of the described embodiments. For example, the semiconductor devices, structures, and methods disclosed in embodiments of the present invention can be used in applications, hardware, and / or electronic systems. Suitable hardware and systems for implementing the embodiments of the present invention can include, but are not limited to, personal computers, communication networks, e-commerce systems, portable communication devices (such as cellular phones and smart phones), solid state media storage devices, expert and artificial intelligence systems, functional circuits, etc. Systems and hardware incorporating semiconductor devices are contemplated embodiments of the present invention.

[0108] The terms used herein are chosen to explain the principles and practical applications of the embodiments or to improve the technology existing in the market, or otherwise to enable other ordinary skilled persons in the art to understand the embodiments disclosed herein. Devices, components, elements, features, devices, systems, structures, technologies, and methods that perform substantially the same functions, work in substantially the same manner, have substantially the same uses, and / or perform similar steps described in different terms are considered to be embodiments of the present invention.

Claims

1. A magnetic tunnel junction (MTJ) comprising: A magnetic reference layer; A magnetic free layer; A tunnel barrier layer disposed between the magnetic reference layer and the magnetic free layer, and A diffusion barrier layer having a diffusion barrier layer thickness, a tunnel interface, and a free layer interface, the diffusion barrier layer being disposed between the tunnel barrier layer and the magnetic free layer, the tunnel interface and the free layer interface being opposite each other across the diffusion barrier layer, the diffusion barrier layer being made of the element chromium, the tunnel interface being in direct contact with the tunnel barrier layer, and the free layer interface being in direct contact with the magnetic free layer, Wherein the diffusion barrier layer prevents one of aluminum and gallium from diffusing from the magnetic free layer into the tunnel barrier layer.

2. The MTJ according to claim 1, wherein the magnetic free layer is made of AlMnGe.

3. The MTJ according to claim 1, wherein the tunnel barrier layer is made of MgO.

4. The MTJ according to claim 1, wherein the diffusion barrier layer thickness is between 0.2 nanometers (nm) and 1 nm.

5. The MTJ according to claim 1, wherein the diffusion barrier layer thickness is the thickness of a single atomic monolayer of chromium.

6. The MTJ according to claim 1, wherein the diffusion layer thickness is a thickness between a single atomic monolayer and five atomic monolayers.

7. The MTJ according to claim 1, wherein the diffusion barrier layer is conductive.

8. The MTJ according to claim 1, wherein the magnetic free layer is made of a stack of a plurality of quadrilateral unit cells, the plurality of quadrilateral unit cells including a plurality of non-interface unit cells disposed on an interface unit cell, the interface unit cell having an interface surface which is the tunnel interface in direct contact with the tunnel barrier layer.

9. The MTJ according to claim 8, wherein the interface unit cell is made of CrAlMnGe.

10. The MTJ according to claim 8, wherein the non-interface unit cells are made of AlMnGe.

11. The MTJ according to claim 8, wherein the non-interface unit cells do not contain chromium.

12. The MTJ according to claim 1, wherein the tunnel barrier layer contains less than 5% aluminum.

13. The MTJ according to claim 1, wherein the thickness of the tunnel barrier layer is less than 1.5 nanometers (nm).

14. The MTJ according to claim 1, wherein the MTJ has a resistance-area (RA) product between 2 and 25 ohm-square microns.

15. The MTJ according to claim 1, wherein the magnetic free layer is made of one of the following: Mn2AlCo, MnAlCo2, AlMnGe, and MnAl.

16. The MTJ according to claim 1, wherein the magnetic free layer is made of one of the following: Mn2GaCo, MnGaCo2, MnGa, and MnGaGe.

17. The MTJ according to claim 1, which is one of the MTJs in an MTJ array.

18. A method of manufacturing a magnetic tunnel junction (MTJ), the method comprising the following steps: Construct an MTJ stack structure up to the tunnel barrier layer; Form a diffusion barrier layer containing chromium on the tunnel barrier layer; Form a magnetic free layer on the diffusion barrier layer; and Etch the MTJ stack structure to form one or more MTJ pillars.

19. The method according to claim 18, wherein the diffusion barrier layer is a chromium layer with a thickness of one atomic monolayer to five atomic monolayers.

20. The method according to claim 18, wherein a plurality of non-interface unit cells in the magnetic free layer do not contain chromium.

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