Fast-switching MRAM combining an aluminum-manganese-germanium free layer with a chromium diffusion barrier

Abstract

Embodiments of the present invention provide magnetic tunnel junction (MTJ) pillars (or arrays thereof), each having a magnetic free layer (comprising a fast-switching material such as aluminum or a metal such as gallium), a magnetic reference layer, and a tunnel barrier layer separating the two magnetic layers. A chromium-containing diffusion barrier layer disposed between the magnetic free layer and the tunnel barrier layer prevents aluminum (or gallium) from diffusing from the magnetic free layer of the MTJ pillar into the tunnel barrier layer. Devices using fast-switching MTJs and methods for fabricating such MTJs are also disclosed. The present invention enables devices with reduced resistance area (RA). In embodiments, an AlMnGe alloy is used to create a magnetic free layer with a tetragonal crystal structure and a lower magnetic moment to accommodate faster magnetic orientation switching speeds.

Description

[Background technology]

[0001] The present invention relates to magnetic tunnel junction (MTJ) devices, methods for making these MTJ devices, and the use of these MTJ devices in magnetic random-access memories (MRAMs). More particularly, the present invention relates to MTJ devices including materials and crystal structures for the MTJ free layer and materials for the tunnel barrier layer combined with a diffusion barrier layer that reduce the resistance of the MTJ device and increase the switching speed of the MTJ device.

[0002] MTJ device pillars are used in MTJ devices such as MRAM and other MTJ devices. MRAM is a non-volatile random access memory technology that stores data using magnetic storage elements within the MTJ pillars. MRAM is a viable memory option for standalone and embedded applications, such as cache, eDRAM replacement, internet of things (IoT), automotive, or artificial intelligence (AI).

[0003] An MTJ is typically formed of two ferromagnetic layers, each capable of holding a magnetization. These magnetic layers are separated by a thin dielectric layer, or tunnel barrier layer. MTJ embodiments used as memory devices or sensors operate with one of these two magnetic layers, the layer whose magnetic orientation does not change during operation (the reference layer). The magnetic orientation of the other layer, the free layer, can be altered or changed during operation.

[0004] Typically, the magnetic orientation of the reference layer is set during the initialization process of the magnetic tunnel junction device, for example, by exposure to a strong external magnetic field. In some cases, a strong magnetic field is combined with heat under ultra-high vacuum. For example, exposure to a strong magnetic field greater than 0.1 Tesla (perhaps greater than 1.5 Tesla in most MRAM cases); application of heat greater than 200°C but less than 550°C; and 10 -5 Torr (approx. 10 -3 Place it under ultra-high vacuum at a pressure lower than 100 Pa.

[0005] In a more specific embodiment, the magnetic tunnel junction (MTJ) device is a small-dimension MTJ pillar (typically between 15 nanometers (nm) and 150 nm in diameter, but more typically between 20 nm and 100 nm in diameter) that comprises 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 particular use case for MTJ devices is their implementation as physical bits in magnetic random access memories, where "0" and "1" are encoded as parallel and antiparallel magnetization states in the free and reference layers. When a small sensing voltage is applied across the pillars of an MTJ device, the resistance of the MTJ pillars can be measured.

[0007] The tunneling-magneto-resistance (TMR) effect creates a resistance difference between the parallel and antiparallel orientations of the free and reference layers of the MTJ pillar, which is used to indicate the bit value of the memory.

[0008] Different memory designs have different methods for writing bits. One method uses a write current through the MTJ pillar, which generates 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 due to the reorientation of the free layer magnetization. The "direction of the write current relative to the pillar geometry" is determined by whether the voltage applied 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, which is often made of magnesium oxide (MgO). In some embodiments, the free layer is typically composed of a combination of cobalt-iron-boron (CoFeB) and cobalt-iron (CoFe) alloys in combination with refractory metal layers such as tantalum (Ta), tungsten (W), niobium (Nb), zirconium (Zr), etc.

[0010] The MTJ pillar structure is typically formed by patterning and etching a blanket film of MTJ layers. The blanket film of MTJ layers 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 MRAMs and other devices, can be fabricated by connecting multiple device elements (such as MRAM structures) to the MTJ pillars at the back-end-of-the-line (BEOL) stage. These structures and methods for fabricating these structures are known.

[0011] Advanced applications require random-access memory (RAM) with extremely fast switching times. A type of MRAM called spin transfer torque MRAM (STT-MRAM) devices requires switching times of less than 10 nanoseconds (ns). Some applications, such as last-level cache or eDRAM replacement, require switching times on the order of 2 ns.

[0012] However, conventional MTJ or MRAM layer materials, such as CoFeB or CoFe alloys, cannot support high switching speeds because the magnetic moments of these materials are too high.

[0013] Tetragonal aluminum-manganese-germanium (AlMnGe) alloy (1:1:1 elemental ratio) is one promising material that can provide the magnetic properties necessary for use in the MTJ free layer and enable high switching speeds.

[0014] However, the aluminum (Al) in the AlMnGe alloy poses a problem. During thermal cycles during MTJ stack processing, if the AlMnGe ordered alloy free layer is in direct contact with the MgO tunnel barrier layer, some of the aluminum from the AlMnGe diffuses into the MgO, forming MgAl oxide. The problems resulting from this oxide formation include: 1. aluminum depletion in the AlMnGe free layer; 2. changes in the magnetic properties of the free layer, i.e., a decrease in magnetoresistance (tunneling magnetoresistance: TMR); 3. an increase in the electrical breakdown of the barrier oxide (TBBD); and 4. an increase in the resistance area product (RA) of the stack.

[0015] To address these issues, efforts have been made to increase the aluminum content of the free layer and / or to make the MgO barrier layer thinner in an attempt to compensate for aluminum diffusion, but experiments have shown that these efforts have not resolved the problem.

[0016] There is a need to suppress the interaction between the aluminum (Al)-containing MTJ (free) layer and the MgO in the MTJ barrier layer without adversely affecting the magnetic properties of the MTJ stack. Summary of the Invention

[0017] Embodiments of the present invention include at least one magnetic tunnel junction (MTJ) pillar with a magnetic free layer made of an aluminum-containing or gallium-containing fast-switching material, and a thin diffusion barrier layer made of elemental chromium that prevents the aluminum (or gallium) from diffusing from the magnetic free layer to the tunnel barrier layer of the MTJ pillar. Devices using the fast-switching MTJ pillar and methods of making the pillar 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 interconnect (BEOL) structures / layers, may be located between the magnetic reference layer and the substrate.) The magnetic reference layers have a first magnetic orientation. The first magnetic orientation generally defines a fixed magnetic orientation.

[0019] The magnetic free layer has a second magnetic orientation that is switchable to be aligned either parallel or antiparallel to the first magnetic orientation. In a preferred embodiment, the magnetic free layer comprises an AlMnGe alloy with a 1:1:1 atomic ratio and a tetragonal crystal structure, resulting in a lower magnetic moment for faster switching speeds. The magnetic free layer in some embodiments is made of a gallium (Ga) alloy, such as, but not limited to, GaMnGe.

[0020] Separating the magnetic reference layer and the magnetic free layer is a tunnel barrier layer, which has a thickness. When a current tunnels through the tunnel barrier layer, a current flows between the magnetic reference layer and the magnetic free layer. When the magnetic reference layer and the magnetic free layer are aligned parallel, the device is in a low resistance state. When the magnetic reference layer and the magnetic free layer are aligned antiparallel, the device is in a high resistance state. In a preferred embodiment, the tunnel barrier layer is made of magnesium oxide (MgO).

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

[0022] In one embodiment, the diffusion barrier layer is made of a thin chromium diffusion barrier layer (approximately 1 to 5 monolayers thick) made of elemental chromium. Other thicknesses are contemplated. In one embodiment, the thin chromium barrier layer interfaces with and is in direct contact with the tunnel barrier layer, preventing diffusion of aluminum (or materials such as gallium) from the magnetic free layer into the tunnel barrier layer of the MTJ pillar.

[0023] In some embodiments, the diffusion barrier layer is a monolayer of chromium that is one side (the connecting side) of the tetragonal unit cell of the free layer, i.e., the interface unit cell (with the tunnel barrier layer). In other embodiments, the diffusion barrier layer can be between 0.2 nanometers (nm) and 1 nm thick, or it can be from 1 to 5 monolayers of chromium thick.

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

[0025] In some embodiments, the diffusion barrier layer is formed by incorporating chromium atoms at the interface surface of the interfacial unit cell to form a tetragonal CrAlMnGe alloy, where Cr substitutes for Mn at the interface surface. In preferred embodiments, the chromium atoms are present only at or include the interface surface, with no chromium atoms present elsewhere in the free layer.

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

[0027] In summary, the diffusion barrier layer is a chromium metal layer that directly interfaces with and is located between the tetragonal unit cells of the tunnel barrier layer and the free layer. In one embodiment, the diffusion barrier layer is a monolayer thick layer that contains only chromium atoms. The tunnel interface is the face / surface of the diffusion barrier layer that interfaces with the tunnel barrier layer, and the free layer interface is the face / surface of the diffusion barrier layer that interfaces with the free layer. The free layer does not contain chromium atoms. [Brief explanation of the drawings]

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

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

[0030] [Figure 2]1 is a cross-sectional view of an example MTJ pillar including a diffusion barrier layer that connects and is between an MgO tunnel barrier layer and an aluminum-containing AlMnGe or gallium-containing MnGaGe magnetic free layer.

[0031] [Figure 3] FIG. 1 illustrates a tunnel barrier layer and a diffusion barrier layer in contact with a free layer, where the exemplary magnetic free layer is a stacked tetragonal unit cell structure of an AlMnGe alloy.

[0032] [Figure 4] 1 is a cross-sectional view of an embodiment of an array of MTJ pillars on a substrate.

[0033] [Figure 5] 1 is a graph showing the resistance area product (RA) of three MTJ structures with tunnel barrier layers made of MgO of the same thickness and free layers as follows: a) an AlMnGe free layer with no chromium; b) an AlMnGeCr alloy free layer with chromium throughout the free layer; and c) an AlMnGe free layer with a diffusion barrier layer (preferably chromium monolayer thick) in contact with and between the free layer and tunnel barrier layer.

[0034] [Figure 6] 1 is a flowchart of a process for making a fast-switching MTJ, including an embodiment in which a diffusion barrier layer is deposited between an MgO tunnel barrier layer and an AlMnGe magnetic free layer. DETAILED DESCRIPTION OF THE INVENTION

[0035] It should be understood that embodiments of the present invention are not limited to the exemplary methods, apparatus, structures, systems, and devices disclosed herein, but rather are more broadly applicable to other alternative and broader methods, apparatus, structures, systems, and devices that will become apparent to those skilled in the art in light of this disclosure.

[0036] Furthermore, it should be understood that the various layers, structures, and / or regions illustrated in the accompanying figures are not drawn to scale, and that one or more layers, structures, and / or regions of a commonly used type may not be explicitly shown in a given figure, without implying that the not explicitly shown layers, structures, and / or regions are excluded from the actual device.

[0037] Additionally, certain elements may be omitted from the figures for clarity and / or brevity, without necessarily placing emphasis on the description of such omitted elements. Furthermore, the same or similar reference numbers used throughout the figures may be used to indicate the same or similar features, elements, or structures, and thus, detailed descriptions of the same or similar features, elements, or structures may not be repeated for each figure.

[0038] The semiconductor devices, structures, and methods disclosed according to embodiments of the present invention can be utilized in applications, hardware, and / or electronic systems. Suitable hardware and systems for implementing embodiments of the present invention may include, but are not limited to, personal computers, communication networks, electronic commerce systems, portable communication devices (e.g., mobile phones and smartphones), solid-state media storage devices, expert and artificial intelligence systems, functional circuits, 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 extent of an element (e.g., layer, trench, hole, opening, etc.) in a cross-section or elevation, i.e., as measured from the bottom to the top of the element and / or as measured relative to the surface on which the element is located.

[0040] Conversely, "depth" refers to the vertical extent of an element (e.g., a layer, trench, hole, opening, etc.) in a section or elevation, i.e., the extent measured from the top to the bottom of the element. Terms such as "thick," "thickness," "thin," or derivatives thereof may be used in place of "height" where stated.

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

[0042] As used herein, "width" or "length" refers to the size of an element (e.g., a layer, trench, hole, opening, etc.) in the drawings, i.e., the size of the element as measured from one side to the opposite side. Terms such as "thick," "thickness," "thin," or derivatives thereof, may be used in place of "width" or "length" where applicable.

[0043] As used herein, terms such as "upper," "lower," "right," "left," "vertical," "horizontal," "top," "bottom," and derivatives thereof, 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 a substrate in an elevational view, and "horizontal" refers to a direction parallel to the top surface of a substrate in an elevational view.

[0044] As used herein, unless otherwise specified, terms such as "on," "overlying," "atop," "on top," "positioned on," or "positioned atop" mean that a first element is present on a second element, and there may be intervening elements between the first and second elements. As used herein, unless otherwise specified, the term "directly," or "in contact" or "direct contact," when used in conjunction with the terms "on," "overlying," "atop," "on top," "positioned on," or "positioned atop," or "disposed on," means that a first element and a second element are connected without any intervening elements, such as, for example, intermediate conductive, insulating, or semiconducting layers, between the first and second elements.

[0045] It is understood that these terms may be affected by the orientation of the device being described: for example, the meaning of these descriptions may change if the device is turned upside down, but the descriptions remain valid because they describe relative relationships between features of the invention.

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

[0047] In an MTJ, the magnitude of the tunneling current depends on the relative magnetization orientation of the two magnetic layers. If these magnetic layers are magnetized in the same direction, i.e., parallel alignment or parallel orientation, a larger tunneling current flows. Therefore, the resistance of the device decreases and the device is in a low resistance state. On the other hand, if these magnetic layers are magnetized in opposite directions / orientations, i.e., antiparallel magnetization or antiparallel orientation, a smaller tunneling current flows. Therefore, the resistance of the device increases and the device is in a high resistance state.

[0048] In some embodiments, one of the two magnetic layers acts as a "pinned" or "reference" layer, with its magnetization direction fixed in a predetermined direction, while the other magnetic layer acts as a "free" layer, with its magnetization direction being relatively easy to change relative to the "pinned" or "reference" layer.

[0049] Depending on the relative magnetic orientation of the free layer with respect to the reference layer, the device switches from parallel to antiparallel alignment and vice versa, resulting in the resistance of the device switching from a high to a low resistance state and vice versa.

[0050] In some embodiments, during device operation, a switching current having a magnitude above a switching threshold is used to switch the magnetization orientation of the magnetic free layer. For example, when a switching current above the switching threshold flows from the reference layer to the free layer (across the tunnel barrier), the magnetization direction of the free layer switches from parallel to antiparallel to the reference layer. Alternatively, when a switching current above the switching threshold flows from the free layer to the reference layer (across the tunnel barrier), the magnetization direction of the free layer switches from antiparallel to parallel to the reference layer. In this way, the magnetization direction of the magnetic free layer can be switched from parallel to antiparallel to the magnetization direction of the magnetic pinned layer, and vice versa. Passing a small sensing current through the device in either direction detects whether the device is in a low resistance state or a high resistance state.

[0051] It should be noted that in some embodiments, the magnetic pinned layer and / or the magnetic reference layer may be formed of one or more magnetic layers.

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

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

[0054] Exemplary substrates 105 may be made from a single element (e.g., silicon or germanium) or a compound semiconductor (e.g., gallium arsenide (GaAs)) or a semiconductor alloy (e.g., silicon germanium (SiGe)). Substrates 105 for these devices are well known and diverse. Furthermore, substrate 105 can be as simple as a single dielectric layer or as complex as known substrate front end of the line (FEOL) circuitry. Alternative substrates 105 are known in the art and are contemplated.

[0055] Disposed above substrate 105 are back-end interconnect (BEOL) layers or structures 110, which include a number of well-known layers formed by well-known back-end processes practiced in semiconductor technology. As is well known, it should be noted that BEOL "layers" typically include contacts, components, connections, vias, insulating layers, and metallization interconnect layers, which may have different configurations than the layers referenced in the other "layers" of stacked structure 100. Those skilled in the art will understand how to distinguish between BEOL "layers" and other layers of stacked structure 100, for example, based on the context of the description. For convenience in making this distinction between layers, layers within BEOL 110 will be referred to as BEOL layers (or collectively BEOL layers or structures 110), although it is not believed necessary to formally make this distinction.

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

[0057] In some embodiments, a single layer of ferromagnetic material 125 is disposed on the BEOL layers 110. This layer of ferromagnetic material 125 is a 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 may 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 metals and / or alloys include iron (Fe), nickel (Ni), cobalt (Co), chromium (Cr), boron (B), or manganese (Mn).

[0058] In alternative embodiments, the magnetic reference layer 125 is formed as a multilayer structure 123 having (1) high spin polarization regions 125 / 125-1 formed from metals and / or metal alloys (using the metals described above) and (2) regions 120 / 120-1 composed of one or more materials exhibiting strong perpendicular magnetic anisotropy (strong PMA). In some embodiments, the multilayer structure 123 is a single ferromagnetic material layer or a pair of high spin polarization layer 125 and a single PMA layer 120. In other embodiments, the multilayer structure 124 of the magnetic reference layer 124 has multiple pairs 124 of high spin polarization layer 125-1 and PMA layer 120-1. Exemplary materials 120 / 120-1 exhibiting strong PMA that may be used include metals such as cobalt, nickel, platinum, palladium, iridium, or ruthenium, including those arranged as alternating layers. The strong PMA regions / layers may also include alloys that exhibit strong PMA, with exemplary alloys including cobalt-iron-terbium, cobalt-iron-gadolinium, cobalt-chromium-platinum, cobalt-platinum, cobalt-palladium, iron-platinum, and / or iron-palladium. In some embodiments, some or all of the highly spin-polarized layers are omitted.

[0059] Again, these alloys may be arranged as alternating layers, as shown in region 123 or alternating layers 120 / 125 overlying region 123. In some embodiments, combinations of these materials and regions may also be utilized. In these cases, layer combination 124 functions as magnetic pinned layer 124 or magnetic reference layer 124. All of these embodiments and combinations thereof are contemplated.

[0060] The thicknesses of the magnetic reference layers (125, 120 / 125, 123, 124) vary depending on the materials selected. Exemplary thicknesses 125T-1 of high spin layer 125-1 and 120T1 of PMA layer 120-1 range from 0.3 nanometers (nm) to 3 nm. However, other thicknesses are contemplated, and the thicknesses (120T, 120T1, 125T, 125T1, etc.) vary depending on the materials selected.

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

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

[0063] In some embodiments, the presence of 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 for embodiments of the present invention. For example, in some embodiments, the free layer 160 can be deposited first on the BEOL layers 110, followed by the magnetic reference layer 124. This results in the device being "upside down" from that shown in FIG. 1 and subsequent figures.

[0064] Deposition of the reference layer 123 / 124 is performed by known techniques (e.g., physical vapor deposition (PVD)) using known equipment such as a PVD cluster tool. Typically, multiple layers can be deposited using the same tool by using subchambers to avoid vacuum breakdown, as is well known.

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

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

[0067] The tunnel barrier layer 150 is made of an insulating material and has a thickness that provides an appropriate tunnel resistance. An exemplary tunnel barrier layer 150 material includes magnesium oxide (MgO). The thickness 150T of the tunnel barrier layer 150 varies depending on the selected material. In one example, the thickness 150T of the tunnel barrier layer 150 may be 0.5 nm to 1.5 nm. The tunnel barrier layer 150 is deposited by known techniques such as atomic layer deposition (ALD), physical vapor deposition (PVD), or 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 is aluminum-free.

[0068] In most embodiments, MgO is the preferred material for the tunnel barrier layer 150, although materials such as titanium dioxide (TiO) are also used, although this material is less preferred. While MgO is the preferred material for use in the tunnel barrier layer 150, all embodiments are contemplated for use with the present invention. The preferred method for depositing such an MgO barrier layer is physical vapor deposition.

[0069] The diffusion barrier layer 181 is comprised of a thin barrier layer 181 of chromium atoms (e.g., a monolayer, i.e., approximately the diameter of a chromium atom, thick 181T, or between one and five monolayers thick 181T). The diffusion barrier layer 181 is disposed on and connects to the tunnel barrier layer 150, and is in direct contact with the tunnel barrier layer at a tunnel interface 181I, and is between the tunnel barrier layer 150 and the magnetic free layer 160. The diffusion barrier layer 181 connects to the free layer 160, and is in direct contact with the free layer at a free-layer interface 183I. Other diffusion barrier layer 181 thicknesses 181T are also contemplated as disclosed herein.

[0070] The chromium contained in the diffusion barrier layer 181 prevents aluminum (and, in some embodiments, gallium) from diffusing across the tunnel interface 181I from the magnetic free layer 160 into the tunnel barrier layer 150, even during the manufacturing thermal cycle process when forming the MTJ pillar.

[0071] As described in more detail below, in one embodiment, the 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 layer of chromium as a connecting surface of the interface unit cell of the free layer 160. The deposition of elemental chromium can be performed by PVD in ultra-high vacuum (UHV), as described herein. See, for example, FIG. 6. The use of UHV and an inert gas environment prevents oxidation of the chromium and the formation of electrically insulating chromium oxides. In some embodiments, the thickness 181T of the diffusion barrier layer 181 can be a monolayer of chromium atoms thick, or range between 1 angstrom and 5 angstroms, or preferably between 2 angstroms 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 1 and 5 atomic layers of chromium atoms. See the description of FIG. 3 for more details.

[0072] In one embodiment, the entire magnetic free layer 160 is deposited on top of the diffusion barrier layer 181. Thus, as described above, the magnetic free layer 160 directly connects with the diffusion barrier layer 181 at the free layer interface 183I, with the diffusion barrier layer 181 separating the tunnel barrier layer 150 and the magnetic free layer 160.

[0073] The material of the magnetic free layer 160 is a magnetic material (or a stack of magnetic materials) that is typically capable of changing its magnetization orientation relative to the magnetization orientation of the magnetic material of the magnetic reference layer 124. In particular, the material of the magnetic reference layer 160 can be switched quickly, for example, faster 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 memu / cm and 0.1 memu / cm.

[0074] In some embodiments, the magnetic free layer 160 is made of a material that includes aluminum, such as an aluminum-containing free layer 160. In yet 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 1:1:1 between Al, Mn, and Ge. In some embodiments, the magnetic free layer 160 is made of AlMnGe, which has tetragonal symmetry.

[0075] In alternative embodiments, the magnetic free layer 160 may be made of MnAlCo, MnAlCo, or MnAl. In further alternative embodiments, the free layer 160 may include a metal such as gallium (Ga) and may be made of MnGaGe, MnGaCo, MnGaCo, 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, for example, PVD using a cluster deposition tool, as described above.

[0077] In some embodiments, the techniques and apparatus described above are used to deposit an upper MgO layer 150U on the magnetic free layer 160. The upper layer 150U, e.g., the MgO layer 150U, is made of a material (typically MgO) that is 150UT thick and adds nothing (or negligible series resistance) to the overall stack 100. However, this upper MgO layer 150U is optional and will be omitted in the remainder of this description without loss of generality.

[0078] (However, it should be noted that in view of 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 100. In some embodiments, the capping layer is made of a conductive metal used as an electrical contact / electrode for the device. For example, the metal for the capping layer 175 can be 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, such as PVD (e.g., as described above), CVD, ALD, and sputtering. Various thicknesses are contemplated for the capping layer 175. In some embodiments, the thickness of the capping layer 175 is between 20 nm and 100 nm.

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

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

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

[0083] In some embodiments, the etch 275 is ion beam etching (IBE). The IBE is timed to stop upon reaching the surface of the BEOL layer 110, using known techniques, creating MTJ pillars 250 in the shape and location defined by the mask 225. The width 280 of the MTJ pillars 250 is in the range of 20-100 nm. The mask 225 may be further processed using known methods.

[0084] 3 illustrates a diffusion barrier layer 181 connecting 181I (for clarity, not shown as in direct contact) with the tunnel barrier layer 150 at the tunnel interface 181I and connecting 183I with the (magnetic) free layer 160 at the free layer interface 183I. The exemplary magnetic free layer 160 is a conductive stack 311 structure made of a tetragonal unit cell 305 / 310 of an AlMnGe alloy.

[0085] In one embodiment, the interface unit cell 305 of the magnetic free layer 160 includes a connection surface 183I that functions as a diffusion barrier layer 181 having a thickness 181T of a monolayer. The connection surface 183I of the interface unit cell 305 becomes the diffusion barrier layer 181 after manganese atoms 320Mn at the connection surface 183I are replaced with chromium atoms 320Cr. Thus, a monolayer of elemental chromium 320Cr / 181 is formed as the diffusion barrier layer 181, which connects 181I with the tunnel barrier layer 150 at the tunnel interface 181I and connects 183I with the magnetic free layer 160 at the free layer interface 183I.

[0086] The thickness 181T of the diffusion barrier layer 181 can be increased by depositing more chromium, for example, by the process described in FIG.

[0087] In a preferred embodiment, the unit cells 310 that are not the interfacial unit cells 305, i.e., the non-interfacial unit cells 310, make up the majority of the thickness 160T of the magnetic free layer 160. The non-interfacial unit cells 310 do not contain chromium.

[0088] The term "tetragonal unit cell structure" refers to a crystal structure that has a unit cell with three axes, two of which are equal in length and perpendicular to each other, and a third axis that is perpendicular to the other two. A tetragonal lattice results from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base (e.g., sides x and x) and a height (y, which is different from x).

[0089] 3, the multiple stacked tetragonal unit cells (non-interface unit cells) 310 in the magnetic free layer 160 are made of an AlMnGe alloy 310. Exceptionally, the interface unit cell 305 in the magnetic free layer 160 is made of a CrAlMnGe alloy, with Cr atoms 320Cr substituting for Mn atoms 320Mn to form the diffusion barrier layer 181.

[0090] As mentioned above, this diffusion barrier interface (chromium layer) 181 acts as a barrier to prevent aluminum (Al) (or gallium (Ga)) from diffusing 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] The chromium-containing diffusion barrier 181 is also believed to prevent materials other than aluminum from diffusing from the free layer 160 into the tunnel barrier layer 150. For example, if the free layer 160 were made of GaMnGe or other Ga-containing compounds, the chromium-containing interfacial unit cell 305 would prevent the diffusion of gallium (Ga) from the free layer 160 into the tunnel barrier layer 150.

[0092] As a further non-limiting example, the chromium-containing diffusion barrier 181 prevents Al from diffusing into the tunnel barrier layer 150 when the free layer 160 is made of MnAlCo, MnAlCo, and MnAl. Additionally, the chromium-containing diffusion barrier 181 prevents Ga from diffusing into the tunnel barrier layer 150 when the free layer 160 is made of MnGaCo, MnGaCo, and MnGa.

[0093] 4 is a cross-sectional view of an array 400 of MTJ pillars 250 / 251 on a substrate 105. The array 400 is fabricated from the MTJ stack 100 using techniques described above, for example, by pattern etching 275 through a mask 225, as shown in FIG. 2. Using these known techniques, the array 400 of MTJ pillars 250 / 251 can be fabricated 410 to span both directions in the plane of the substrate 105 / BEOL layers 110. By forming connections to the MTJ pillars 250 / 251 and other components through the BEOL layers 110, the MRAM is fabricated.

[0094] Figure 5 shows the resistance-area product (RA, ohm-micrometer) for three MTJ structures 525 with tunnel barrier layers made of MgO of the same thickness and free layers as follows: 2 (Ω·μm 2 1 is a graph 500 showing the free layer temperature (measured in .mu.m) 505. The free layers are a) a chromium-free AlMnGe free layer 510, b) an AlMnGeCr alloy free layer 520 containing chromium throughout the free layer, and c) an AlMnGe free layer 530 with a diffusion barrier layer 181 between and connecting the free layer and the tunnel barrier layer, as in the present invention.

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

[0096] The diffusion barrier layer 181 allows for a reduced R for a given tunnel barrier layer 150 thickness 150T, so that the nominal thickness of the MgO tunnel barrier can be increased in the presence of the diffusion barrier without increasing the resistance of the device. A thicker barrier is less prone to defects and generates a smaller electric field across such a thick barrier for a given voltage. Fewer defects and a smaller electric field across the MgO barrier reduce the likelihood of the barrier failing due to electrical breakdown, extending the device lifetime and the reliability of memories formed with MTJ pillar devices.

[0097] FIG. 6 is a flowchart of a process 600 for making a fast switching MTJ 200 / 400 that includes a diffusion barrier layer 181 between the tunnel barrier layer 150 and the aluminum-containing (AlMnGe) (or other) magnetic free layer 160.

[0098] The process 600 begins at step 605, where the MTJ stack 100 described in the description of FIG. 1 is constructed after the tunnel barrier layer 150 is deposited.

[0099] Step 610 deposits the diffusion barrier layer 181 and the free layer 160, where a layer of elemental chromium 181 is deposited first, followed by the growth of the chromium-free free layer 160.

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

[0101] After the elemental chromium is deposited, a Mn-Ge-Al alloy is deposited, typically in a 1:1:1 ratio, to form the interfacial unit cell 305 and the remaining non-interfacial unit cells 310. The atomic ratio of Mn:Ge:Al as deposited may deviate from 1:1:1, with the Mn:Al ratio being up to 1:2 and the Mn:Ge ratio being up to 1:1.5. As mentioned above, after the deposition of the chromium layer 181 that interfaces with the tunnel barrier layer 150, the interfacial unit cell 305 and the free layer unit cell 310 are grown using standard PVD techniques and PVD cluster equipment as described.

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

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

[0104] In step 620, the MTJ pillars 250 / 251 are connected to circuitry (eg, in the BEOL layer 110) using known methods to form the circuitry (eg, MRAM).

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

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

[0107] The description of various embodiments of the present invention has been presented for illustrative purposes and is not intended to be exhaustive or to be limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope of the described embodiments. For example, the semiconductor devices, structures, and methods disclosed in accordance with embodiments of the present invention can be utilized in applications, hardware, and / or electronic systems. Suitable hardware and systems for implementing embodiments of the present invention may include, but are not limited to, personal computers, communication networks, e-commerce systems, portable communication devices (e.g., mobile phones and smartphones), solid-state media storage devices, expert and artificial intelligence systems, functional circuits, and the like. Systems and hardware incorporating semiconductor devices are contemplated embodiments of the present invention.

[0108] The terms used herein have been selected to explain the principles and practical applications of the embodiments or technical improvements of the technology found in the market, or to otherwise enable others skilled in the art to understand the embodiments disclosed herein. Devices, components, elements, features, apparatuses, systems, structures, techniques, and methods that perform substantially the same function, operate in substantially the same way, are used in substantially the same way, and / or perform similar steps, that are described using various terms, are contemplated as embodiments of the present invention.

Claims

1. Magnetic reference layer; 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; Equipped with the diffusion barrier layer is disposed between the tunnel barrier layer and the magnetic free layer, the tunnel interface and the free layer interface are on opposite sides of the diffusion barrier layer, the diffusion barrier layer is made of elemental chromium, the tunnel interface is in direct contact with the tunnel barrier layer, and the free layer interface is in direct contact with the magnetic free layer; the diffusion barrier layer prevents one of aluminum and gallium from diffusing from the magnetic free layer into the tunnel barrier layer; Magnetic tunnel junction (MTJ).

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

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

4. The MTJ of claim 1 , wherein the diffusion barrier layer has a thickness between 0.2 nanometers (nm) and 1 nm.

5. The MTJ of claim 1 wherein the diffusion barrier layer is one monolayer of chromium thick.

6. The MTJ of claim 1 , wherein the diffusion layer has a thickness between 1 and 5 monolayers.

7. The MTJ of claim 1 , wherein the diffusion barrier layer is electrically conductive.

8. 2. The MTJ of claim 1, wherein the magnetic free layer is made of a stack of multiple tetragonal unit cells including multiple non-interface unit cells arranged on an interface unit cell, the interface unit cell having a connection surface, the connection surface being the tunnel interface that directly contacts the tunnel barrier layer.

9. The MTJ of claim 8 , wherein the interfacial unit cell is made of CrAlMnGe.

10. The MTJ of claim 8 , wherein the non-interfacial unit cell is made of AlMnGe.

11. The MTJ of claim 8 wherein the non-interfacial unit cell is chromium-free.

12. The MTJ of claim 1 , wherein the tunnel barrier layer has an aluminum content of less than 5%.

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

14. The resistance area product (RA) of the MTJ is 2 ohm-micrometers. ２ and 25 ohm-micrometer ２ 2. The MTJ of claim 1, wherein:

15. The magnetic free layer is made of Mn ２ AlCo, MnAlCo ２ 10. The MTJ of claim 1, made of one of AlMnGe, AlMnGe, and MnAl.

16. The magnetic free layer is made of Mn ２ GaCo, MnGaCo ２ 10. The MTJ of claim 1, made of one of MnGa and MnGaGe.

17. The MTJ of claim 1 , wherein the MTJ is one of a plurality of MTJs in an array of MTJs.

18. 1. A method of fabricating a magnetic tunnel junction (MTJ), comprising: Building the MTJ stack up to the tunnel barrier layer; forming a diffusion barrier layer comprising chromium over the tunnel barrier layer; forming a magnetic free layer on the diffusion barrier layer; and Etching the MTJ stack to form one or more MTJ pillars. A method for providing the above.

19. 20. The method of claim 18, wherein the diffusion barrier layer is a 1 to 5 monolayer thick layer of chromium.

20. The method of claim 18 , wherein a plurality of non-interface unit cells in the magnetic free layer are free of chromium.

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