Narrow-base modified double magnetic tunnel junction structures with small RA

Abstract

A modified double magnetic tunnel junction (mDMTJ) structure is provided that includes a narrow base and the use of a spin diffusion layer (i.e., a non-magnetic spin conducting metal layer) that reduces the resistance area (RA) of the tunnel barrier layer that forms the interface with the spin diffusion layer.

Description

[Technical Field]

[0001] The present invention relates to magnetoresistive random access memory (MRAM), and more particularly to a modified double magnetic tunnel junction (mDMTJ) structure that improves the performance of spin-transfer torque (STT) MRAM and can be integrated into back-end-of-the-line (BEOL) processing of semiconductor technologies, such as complementary metal oxide semiconductor (CMOS) technologies. [Background technology]

[0002] MRAM is a nonvolatile random-access memory technology that stores data using magnetic storage elements. These elements typically consist of two ferromagnetic plates, each capable of retaining its magnetization, separated by a thin dielectric layer (i.e., a tunnel barrier layer). One of the two plates (i.e., the magnetic reference layer or magnetic pinned layer) is a magnet with a fixed magnetic moment orientation, while the magnetization of the other plate (i.e., the magnetic free layer) can be swept to at least two different orientations, representing different digital states such as 0 and 1 for memory applications. In MRAM, such elements are sometimes called magnetic tunnel junction (MTJ) structures. Figure 1 shows a prior-art MTJ structure 10, including a magnetic reference layer 12, a tunnel barrier layer 14, and a magnetic free layer 16. The single arrow shown on the magnetic reference layer 12 indicates the possible orientations of that layer, while the two arrows shown on the magnetic free layer 16 indicate that the orientation of that layer can be switched.

[0003] 1, the magnetization of the magnetic reference layer 12 is fixed in one direction (e.g., pointing up), but the orientation of the magnetic free layer 16 can be "switched" by some external force, such as a magnetic field or a spin transfer torque that generates a charge current. A small current (of either polarity) can be used to read the resistance of the device, which depends on the relative orientations of the magnetizations of the magnetic free layer 16 and magnetic reference layer 12. The resistance is typically high when the magnetizations are antiparallel and low when they are parallel (although this can be reversed for some materials).

[0004] One type of MRAM that can use the MTJ structure 10 shown in FIG. 1 is STT MRAM. STT MRAM has the advantages of lower power consumption and greater scalability than traditional MRAM, which uses magnetic fields to flip valued elements. STT MRAM uses spin transfer torque to flip (switch) the orientation of the magnetic free layer. In STT MRAM devices, a current passing through the MTJ structure is used to switch, or "write," the bit state of the MTJ memory element. A current passing downward through the MTJ structure aligns the magnetic free layer 16 parallel to the magnetic reference layer 12, while a current passing upward through the MTJ structure aligns the magnetic free layer 16 antiparallel to the magnetic reference layer 12.

[0005] STT MRAM requires reduced switching current to accommodate smaller transistor sizes to improve memory areal density. One approach to reducing switching current by approximately a factor of two is the concept of a double magnetic tunnel junction (DMTJ) structure 20, as shown in Figure 2. The DMTJ structure 20 in Figure 2 includes a first magnetic reference layer 22, a first tunnel barrier layer 24, a magnetic free layer 26, a second tunnel barrier layer 28, and a second magnetic reference layer 30. The single arrow shown for each of the first and second magnetic reference layers 22 and 30 indicates the possible orientations of that layer, while the two arrows for the magnetic free layer 26 indicate that the orientation of that layer can be switched. One drawback of the DMTJ structure shown in Figure 2 is that while it reduces switching current, it also reduces tunnel magnetoresistance (TMR), inhibiting efficient readout of the device.

[0006] Therefore, there is a need to provide a DMTJ structure that reduces switching current while minimizing degradation of TMR in the structure so that the DMTJ structure exhibits efficient switching (at small currents) and fast readout (high TMR). Summary of the Invention

[0007] A modified double magnetic tunnel junction (mDMTJ) structure is provided that includes a narrow base and the use of a spin diffusion layer (i.e., a non-magnetic spin-conducting metal layer) that provides a small resistance-area product (RA) for the tunnel barrier layer that interfaces with the spin diffusion layer. A "narrow base" means that the diameter of the junction is approximately the same at the bottom of the junction as it is at the top of the junction. A "small RA" is less than 1 ohm-micron. 2The following RA is meant: The mDMTJ structures of the present invention exhibit efficient switching (small current) and rapid readout (high TMR). "Small current" refers to a current smaller than that required by a typical single MTJ device to achieve the same degree of error-free writing. In some embodiments, the small current can reach or be less than 20-50 μA. "High TMR" refers to a higher value of tunneling magnetoresistance than that achieved by a typical DMTJ device. In some embodiments, the high TMR can reach or exceed 100-200%.

[0008] In one aspect of the present invention, an mDMTJ structure exhibiting efficient switching and fast readout is provided. In one embodiment, the mDMTJ structure includes a first magnetic reference layer, a first tunnel barrier layer having a first surface contacting the surface of the first magnetic reference layer, a non-magnetic spin conducting metal layer (i.e., a spin diffusion layer) having a first surface contacting a second surface of the first tunnel barrier layer opposite the first surface of the first tunnel barrier layer, a magnetic free layer having a first surface contacting the second surface of the non-magnetic spin conducting metal layer opposite the first surface of the non-magnetic spin conducting metal layer, a second tunnel barrier layer having a first surface contacting the second surface of the magnetic free layer opposite the first surface of the magnetic free layer, and a second magnetic reference layer having a first surface contacting the second surface of the second tunnel barrier layer opposite the first surface of the second tunnel barrier layer. According to the present invention, the first tunnel barrier layer has a first resistance-area product RA, and the second tunnel barrier layer has a second RA, the first RA being at most less than one-fifth of the second RA.

[0009] In another aspect of the present invention, an STT MTJ memory element is provided. In one embodiment, the STT MTJ memory element includes a mDMTJ structure sandwiched between a first electrode and a second electrode. The mDMTJ structure includes a first magnetic reference layer, a first tunnel barrier layer having a first surface contacting the surface of the first magnetic reference layer, a non-magnetic spin conducting metal layer having a first surface contacting the second surface of the first tunnel barrier layer opposite the first surface of the first tunnel barrier layer, a magnetic free layer having a first surface contacting the second surface of the non-magnetic spin conducting metal layer opposite the first surface of the non-magnetic spin conducting metal layer, a second tunnel barrier layer having a first surface contacting the second surface of the magnetic free layer opposite the first surface of the magnetic free layer, and a second magnetic reference layer having a first surface contacting the second surface of the second tunnel barrier layer opposite the first surface of the second tunnel barrier layer. According to the present invention, the first tunnel barrier layer has a first resistance-area product RA, and the second tunnel barrier layer has a second RA, the first RA being at most less than one-fifth of the second RA. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a prior art MTJ structure. [Figure 2] FIG. 1 is a cross-sectional view of a prior art DMTJ structure. [Figure 3] FIG. 1 is a cross-sectional view of an mDMTJ structure according to one embodiment of the present invention. [Figure 4] FIG. 4 is a cross-sectional view of a magnetic reference layer that may be employed for one or both of the first and second magnetic reference layers shown in FIG. 3, the magnetic reference layer including a lower magnetic reference layer, a synthetic antiferromagnetic coupling layer, and an upper magnetic reference layer, and in such applications, the orientation of the moment of the reference layer shown in FIG. 3 corresponds to the orientation of the moment of a synthetic antiferromagnetic coupling layer in contact with the tunnel barrier. [Figure 5] FIG. 1 is a cross-sectional view of an mDMTJ structure according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Embodiments of the present invention will now be described in more detail with reference to the following discussion and the accompanying drawings, in which it is noted that the drawings are provided for illustrative purposes only and, as such, are not drawn to scale, and it should also be noted that like corresponding elements are referred to by like reference numerals.

[0012] In the following description, numerous specific details are set forth regarding particular structures, components, materials, dimensions, processing steps, and techniques to facilitate an understanding of various embodiments of the present invention. However, it will be understood by those skilled in the art that various embodiments of the present invention may be practiced without these specific details. In other instances, well-known structures or processing steps are not described in detail to avoid obscuring the present invention.

[0013] When an element, such as a layer, region, or substrate, is referred to as being "on" or "over" another element, it is understood that the element can be directly on the other element, or intervening elements may be present. Conversely, when an element is referred to as being "directly on" or "directly over" another element, there are no intervening elements present. Also, when an element is referred to as being "beneath" or "under" another element, it is understood that the element can be directly beneath or directly under the other element, or that intervening elements may be present. Conversely, when an element is referred to as being "directly beneath" or "directly under" another element, there are no intervening elements present.

[0014] U.S. Patent Application No. 16 / 671,995 provides an mDMTJ structure including two tunnel barrier layers and two magnetic reference layers. This mDMTJ structure exhibits smaller switching currents than single-tunnel junction devices because spin torque is applied to the magnetic free layer from both the upper and lower interfaces. To reduce the parasitic resistance of the first tunnel barrier layer (reducing magnetoresistance), the first tunnel barrier layer is made wider than the second tunnel barrier layer. See Figures 5A, 5B, 7A, and 7B of U.S. Patent Application No. 16 / 671,995. "Wider than" means that the diameter of the first tunnel barrier is two to four times larger than the diameter of the second tunnel barrier. The spin diffusion layer is a nonmagnetic spin-conducting metal and is used to collect spins that have tunneled through the first tunnel barrier layer and transport them to the magnetic free layer. The spin diffusion layer allows the first tunnel barrier layer to have a large area (and therefore low resistance) while keeping the magnetic free layer small in area (and therefore low switching current).

[0015] The present invention provides an mDMTJ structure including a narrow base as defined above, and a spin diffusion layer (hereinafter referred to as a non-magnetic spin conducting metal layer) that exhibits a small RA as defined above relative to a tunnel barrier layer (i.e., a first tunnel barrier layer) that forms an interface with the non-magnetic spin conducting metal layer. The mDMTJ structure of the present invention exhibits efficient switching (small current as defined above) and rapid readout (high TMR as defined above).

[0016] The non-magnetic spin conducting metal layer material and the first tunnel barrier material are preferably 1 ohm-micron thick as defined above, with the tunnel barrier thickness not being thick enough to reliably avoid pinholes. 2For example, for an MgO tunnel barrier layer, a tunnel barrier layer thickness of about 1 nm is typically required to reliably avoid pinholes. For most electrode materials (including most spin diffusion layer materials and most magnetic materials such as CoFeB), this thickness is on the order of 5-20 ohm-microns. 2 However, some non-magnetic spin conducting metal layer materials (defined below) have an RA of 1 ohm-micron. 2 Some exhibit a small RA of less than 1000 rad / s, yet still provide spin torque. This small RA barrier allows the use of a narrow base, resulting in the entire structure having approximately the same diameter. This avoids the processing complexities of wider-base modified double magnetic tunnel junctions and significantly reduces cost.

[0017] In particular, the present invention provides an mDMTJ structure 100, for example as shown in Figures 3 and 5. The mDMTJ structure 100 can be integrated into BEOL and used as a component of STT MRAM. For example, an mDMTJ structure 100 as shown in FIGS. 3 and 5 includes a first magnetic reference layer 102, a first tunnel barrier layer 104 having a first surface in contact with the surface of the first magnetic reference layer 102, a non-magnetic spin conducting metal layer 106 having a first surface in contact with a second surface of the first tunnel barrier layer 104 opposite the first surface of the first tunnel barrier layer 104, a magnetic free layer 108 having a first surface in contact with the second surface of the non-magnetic spin conducting metal layer 106 opposite the first surface of the non-magnetic spin conducting metal layer 106, a second tunnel barrier layer 110 having a first surface in contact with the second surface of the magnetic free layer 108 opposite the first surface of the magnetic free layer 108, and a second magnetic reference layer 112 having a first surface in contact with the second surface of the second tunnel barrier layer 110 opposite the first surface of the second tunnel barrier layer 110.

[0018] In accordance with the present invention, the first tunnel barrier layer 104 has a first resistance-area product RA and the second tunnel barrier layer 110 has a second RA, the first RA being at most 5 times less than the second RA, more typically at most 7 times less than the second RA, and even more typically at most 10 times less than the second RA. In some embodiments of the present invention, the first RA of the first tunnel barrier layer 110 is less than 1 ohm-micron. 2 In another embodiment of the present invention, the first RA of the first tunnel barrier layer 110 is 0.1 ohm-microns. 2 ~2 ohms-microns 2 It should be noted that despite the small RA, the first tunnel barrier layer 104 still provides a spin torque to the magnetic free layer 108.

[0019] In the mDMTJ structure of the present invention, the non-magnetic spin conducting metal layer 106 has a long spin flip scattering lifetime and an RA of 1 ohm-micron or less. 2 The nonmagnetic spin-conserving metallic material has a small spin-flip scattering lifetime of less than 10 nm. The term "long spin-flip scattering lifetime" means that electrons can travel a distance greater than 10 nanometers before their spin flips. The nonmagnetic spin-conducting metallic layer 106 receives the spin current polarized by the first magnetic reference layer 102 as it tunnels through the first tunnel barrier layer 104 and effectively transfers the spin current to the magnetic free layer 108, assisting in STT-induced switching. At the same time, the nonmagnetic spin-conducting metallic layer 106 eliminates the spin polarization of the density of states (DOS) at the interface between the nonmagnetic spin-conducting metallic layer 106 and the first tunnel barrier layer 104. This results in zero magnetoresistance across the first tunnel barrier layer 104, thus avoiding the magnetoresistance cancellation effect that plagues prior art DMTJ structures, such as those shown in FIG. 2.

[0020] This reduction in the DOS spin polarization at the interface between the nonmagnetic spin conducting metal layer 106 and the first tunnel barrier layer 104 also significantly reduces the spin current with the polarization defined by the magnetic free layer 108 that can enter the first magnetic reference layer 102, thus reducing the spin torque and STT-related disturbances of the magnetic reference layer 102 from the magnetic free layer 108. Thus, the mDMTJ structure 100 exhibits efficient switching (small current as defined above), fast readout (high TMR as defined above), and reduced potential disturbances to the magnetic state of the reference layer (102) that could also cause write errors.

[0021] Referring initially to FIG. 3 , an mDMTJ structure 100 according to an embodiment of the present invention is illustrated. The mDMTJ structure 100 of FIG. 3 includes a first magnetic reference layer 102, a first tunnel barrier layer 104 in contact with the surface of the first magnetic reference layer 102, a non-magnetic spin conducting metal layer 106 in contact with the surface of the first tunnel barrier layer 104, a magnetic free layer 108 in contact with the surface of the non-magnetic spin conducting metal layer 106, a second tunnel barrier layer 110 in contact with the surface of the magnetic free layer 108, and a second magnetic reference layer 112 in contact with the surface of the second tunnel barrier layer 110. As illustrated, the mDMTJ structure 100 of FIG. 3 is disposed between a first electrode 90 and a second electrode 120. The mDMTJ structure 100, the first electrode 90, and the second electrode 120 collectively provide a STT MTJ memory element.

[0022] Although not depicted in Figure 3 or any of the remaining figures, the mDMTJ structure 100 can be embedded in various dielectric material layers, including wiring dielectric materials present in the BEOL. Although not depicted in Figure 3 or any of the remaining figures, device regions that do not include MRAM can be located adjacent to device regions that include MRAM that are depicted in the figures.

[0023] The first electrode 90 may reside on the surface (recessed or non-recessed) of an electrically conductive structure, such as a copper-containing structure, embedded in an interconnect dielectric material layer in one of the interconnect levels present in the back-end of the electronics (BEOL). The first electrode 90 may be composed of Ta, TaN, Ti, TiN, Ru, RuN, RuTa, RuTaN, Co, CoWP, CoN, W, WN, or any combination thereof. The first electrode 90 may also be composed of other well-known electrode materials. The first electrode 90 may have a thickness of 2 nm to 25 nm, although other thicknesses are possible and may be used for the first electrode 90. The first electrode 90 may be formed by a deposition process such as sputtering, atomic layer deposition (ALD), chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), or physical vapor deposition (PVD). An etch-back process, a planarization process (such as chemical mechanical polishing (CMP)), or a patterning process (such as lithography and etching) may follow the deposition of the conductive material that forms the first electrode 90.

[0024] Next, as shown in FIG. 3, an mDMTJ structure 100 is formed on the first electrode 90. In FIG. 3, the first magnetic reference layer 102 is located at the bottom of the mDMTJ structure 100, and the second magnetic reference layer 112 is located at the top of the mDMTJ structure 100. In FIG. 3, the first magnetic reference layer 102, the first tunnel barrier layer 104, and the non-magnetic spin conducting metal layer 106 each have a first lateral dimension, and the magnetic free layer 108, the second tunnel barrier layer 110, and the second magnetic reference layer 112 each have a second lateral dimension, which is the same as the first lateral dimension. Thus, each of the various layers present in the mDMTJ structure 100 shown in FIG. 3 has outermost sidewalls relative to one another that are aligned vertically or with a controlled sidewall slope.

[0025] The various material layers of the mDMTJ structure 100 can be formed using one or more deposition processes, such as, for example, sputtering, plasma-enhanced atomic layer deposition (PEALD), physical vapor deposition (PVD), including plasma-enhanced chemical vapor deposition (PECVD), or magnetron sputtering. In some embodiments, the various material layers of the mDMTJ structure 100 can be formed without breaking vacuum between the deposition of the various material layers. In other embodiments, the various material layers of the mDMTJ structure 100 can be formed by breaking vacuum between the deposition of one or more of the various material layers. In some embodiments of the present invention, the non-magnetic spin conducting metal layer 106 is formed in separate deposition steps, such that the lower portion of the non-magnetic spin conducting metal layer 106 is formed followed by the upper portion of the non-magnetic spin conducting metal layer 106. The upper and lower portions of the non-magnetic spin conducting metal layer 106 are composed of the same non-magnetic spin conducting metal material. The presence of the non-magnetic spin conducting metal layer 106 makes it feasible to break the vacuum partway through the formation of that material layer, since the same material metal interface is more easily "healed" by high temperature (e.g., 400°C) annealing, minimizing the effects of interface-related defects.

[0026] The first magnetic reference layer 102 has a fixed magnetization. The first magnetic reference layer 102 can be composed of a metal or a metal alloy (or stack) containing one or more metals that exhibit a large spin polarization at the tunnel-barrier interface. In other embodiments, exemplary metals for forming the first magnetic reference layer 102 include iron, nickel, cobalt, chromium, boron, or manganese. Exemplary metal alloys can include the metals exemplified above. In another embodiment, the first magnetic reference layer 102 can be a multilayer arrangement having (1) a region of large spin polarization formed from a metal or metal alloy, or a combination thereof, using the metals described above, and (2) a region composed of one or more materials exhibiting a strong perpendicular magnetic anisotropy (strong PMA). Exemplary materials with a strong PMA that can be used include metals such as cobalt, nickel, platinum, palladium, iridium, and ruthenium, arranged as alternating layers. The strong PMA region can also include alloys that exhibit strong intrinsic or bulk (as opposed to interfacial) PMA; exemplary alloys include cobalt-iron-terbium, cobalt-iron-gadolinium, cobalt-chromium-platinum, cobalt-platinum, cobalt-palladium, iron-platinum, or iron-palladium, or combinations thereof. The alloys can be arranged as alternating layers. In one embodiment, combinations of these materials and regions can also be employed as the first magnetic reference layer 102. In some embodiments, the first magnetic reference layer 102 can include a lower magnetic reference layer, a synthetic antiferromagnetic coupling layer, and an upper magnetic reference layer. This embodiment for the first magnetic reference layer 102 is described in more detail herein below with respect to FIG. 4.

[0027] The first tunnel barrier layer 104 is configured to generate spin current when it interfaces with the non-magnetic spin conducting metal layer 106 but not contribute to TMR that counteracts the TMR of the second tunnel barrier. The first tunnel barrier layer 104 is composed of an insulating material, such as magnesium oxide, aluminum oxide, and titanium oxide, or a material that maintains spin polarization and has high electrical tunneling conductance, such as a semiconductor or low bandgap insulator. In a preferred embodiment, the first tunnel barrier layer 104 is composed of magnesium oxide.

[0028] The non-magnetic spin conducting metal layer 106 is a spin storing metal material with a long spin flip scattering lifetime as defined above and is 1 ohm-micron thick on the first tunnel barrier layer 104. 2 The non-magnetic spin conducting metal layer 106 is typically made of a material that provides an RA of less than 100 rad / s. Examples of materials for the non-magnetic spin conducting metal layer 106 include, but are not limited to, pure copper (Cu), copper (Cu) containing less than 1% of other elements, such as nitrogen (N), boron (B), or beryllium (Be), copper nitride (CuN) alloys, where the nitride content of the CuN alloy is less than 1 atomic %, or copper boron (CuB) alloys, where the boron content of the CuB alloy is less than 20 atomic %. The materials exemplified for the non-magnetic spin conducting metal layer 106 are typically used in combination with the first tunnel barrier layer 104 made of magnesium oxide, providing the first tunnel barrier layer 104 with a first RA.

[0029] In some embodiments of the present invention, the material of the non-magnetic spin conducting metal layer 106 has a selected crystallographic orientation that can promote a high DOS at the interface between the non-magnetic spin conducting metal layer 106 and the first tunnel barrier layer 104. For example, a copper layer with an FCC111 texture (i.e., pure Cu or Cu containing any of the elements mentioned above) can be employed.

[0030] The non-magnetic spin conducting metal layer 106 has a thickness appropriate for transferring the received spin current to the magnetic free layer 108. In one embodiment, the thickness of the non-magnetic spin conducting metal layer 106 is between 2 nm and 20 nm. However, other thicknesses are possible as long as the thickness of the non-magnetic spin conducting metal layer 106 is sufficient to transfer the received spin current to the magnetic free layer 108.

[0031] The magnetic free layer 108 can be composed of a magnetic material (or magnetic material stack) whose magnetization orientation can be changed relative to the magnetization orientations of the first magnetic reference layer 102 and the second magnetic reference layer 112. Exemplary magnetic materials for the magnetic free layer 108 include alloys or multilayers of cobalt, iron, cobalt-iron alloys, nickel, nickel-iron alloys, cobalt-iron-boron alloys, or combinations thereof.

[0032] The second tunnel barrier layer 110 is composed of an insulating material and is formed with a thickness that provides an adequate tunneling resistance between the magnetic free layer 108 and the second magnetic reference layer 112. Exemplary materials for the second tunnel barrier layer 110 include high electrical tunneling conductance materials, such as magnesium oxide, aluminum oxide, and titanium oxide, or semiconductors or low bandgap insulators. In some embodiments, the second tunnel barrier layer 110 is composed of an insulating material that is compositionally the same as the first tunnel barrier layer 104. In other embodiments, the second tunnel barrier layer 110 is composed of an insulating material that is compositionally different from the first tunnel barrier layer 104. The second tunnel barrier layer 110 has a thickness sufficient to avoid pinhole formation.

[0033] The second magnetic reference layer 112 also has a fixed magnetization. The second magnetic reference layer 112 can be composed of a metal or a metal alloy (or stack) containing one or more metals that exhibit a large spin polarization. In other embodiments, exemplary metals forming the second magnetic reference layer 112 include iron, nickel, cobalt, chromium, boron, or manganese. Exemplary metal alloys can include the metals exemplified above. In another embodiment, the second magnetic reference layer 112 can be a multilayer arrangement having (1) a region with large spin polarization formed from a metal or metal alloy, or a combination thereof, using the metals described above, and (2) a region composed of one or more materials that exhibit a strong perpendicular magnetic anisotropy (strong PMA). Exemplary materials with a strong PMA that can be used include metals such as cobalt, nickel, platinum, palladium, iridium, and ruthenium, and can be arranged as alternating layers. The strong PMA region can also include alloys that exhibit strong intrinsic or bulk (as opposed to interfacial) PMA; exemplary alloys include cobalt-iron-terbium, cobalt-iron-gadolinium, cobalt-chromium-platinum, cobalt-platinum, cobalt-palladium, iron-platinum, or iron-palladium, or combinations thereof. The alloys can be arranged as alternating layers. In one embodiment, combinations of these materials and regions can also be employed as the second magnetic reference layer 112. In some embodiments, the second magnetic reference layer 112 can include a lower magnetic reference layer, a synthetic antiferromagnetic coupling layer, and an upper magnetic reference layer. This embodiment for the second magnetic reference layer 112 is described in more detail herein below with respect to FIG. 4.

[0034] The second electrode 120 can be composed of one of the conductive metal materials, similar to the first electrode 90. In some embodiments, the second electrode 120 can be composed of the same conductive metal material as the first electrode 90. In other embodiments, the second electrode 120 can be composed of a different conductive metal material than the first electrode 90. The second electrode 120 can have a thickness of 2 nm to 25 nm, although other thicknesses are also possible and can be used for the second electrode 120. The second electrode 120 can be formed by a deposition process such as, for example, sputtering, atomic layer deposition (ALD), chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), or physical vapor deposition (PVD).

[0035] Referring now to FIG. 4, a magnetic reference layer 113 is illustrated that can be employed for one or both of the first magnetic reference layer 102 and the second magnetic reference layer 112 shown in FIG. 3 or FIG. 5. The magnetic reference layer 113 of FIG. 4 includes a lower magnetic reference layer 114, a synthetic antiferromagnetic coupling layer 116, and an upper magnetic reference layer 118. The lower magnetic reference layer 114 can be composed of any of the magnetic materials described above for the first magnetic reference layer 102 or the second magnetic reference layer 112. The synthetic antiferromagnetic coupling layer 116 is composed of a nonmagnetic material that can couple the lower and upper magnetic layers 114, 118 of the magnetic reference layer 113 in an antiparallel manner. Exemplary nonmagnetic materials that can be used for the synthetic antiferromagnetic coupling layer 116 include, but are not limited to, ruthenium (Ru), iridium (Ir), or rhodium (Rh). In one embodiment, the synthetic antiferromagnetic coupling layer 116 may have a thickness of 0.2 nm to 1.2 nm, although other thicknesses are possible and may be used as the thickness of the synthetic antiferromagnetic coupling layer 116. The upper magnetic reference layer 118 may be composed of one of the magnetic materials described above as the first magnetic reference layer 102 or the second magnetic reference layer 112. Typically, the upper magnetic reference layer 118 is compositionally different from the lower magnetic reference layer 114.

[0036] In one embodiment, the magnetic reference layer 113 is employed as the second magnetic reference layer 112, but not as the first magnetic reference layer 102. In another embodiment, the magnetic reference layer 113 is employed as both the second magnetic reference layer 112 and the first magnetic reference layer 102. In yet another embodiment, the magnetic reference layer 113 is employed as the first magnetic reference layer 102, but not as the second magnetic reference layer 112.

[0037] 5, an mDMTJ structure 100 according to yet another embodiment of the present invention is illustrated, which is a 180° inversion of the mDMTJ structure shown in FIG. 3, such that the first magnetic reference layer 102 is located at the top of the structure. The mDMTJ structure 100 of FIG. 5 includes the first magnetic reference layer 102, as defined above, the first tunnel barrier layer 104, as defined above, in contact with the surface of the first magnetic reference layer 102, the non-magnetic spin conducting metal layer 106, as defined above, in contact with the surface of the first tunnel barrier layer 104, the magnetic free layer 108, as defined above, in contact with the surface of the non-magnetic spin conducting metal layer 106, the second tunnel barrier layer 110, as defined above, in contact with the surface of the magnetic free layer 108, and the second magnetic reference layer 112, as defined above, in contact with the surface of the second tunnel barrier layer 110. The mDMTJ structure 100 shown in FIG. 5 illustrates an embodiment in which the first magnetic reference layer 102 is located at the top of the mDMTJ structure and the second magnetic reference layer 112 is located at the bottom of the mDMTJ structure.

[0038] Although not shown, the DMTJ structure 100 of Figure 5 is disposed between a first electrode, as defined above, and a second electrode, as defined below. The mDMTJ structure 100, the first electrode, and the second electrode collectively provide an STT MTJ memory element.

[0039] In FIG. 5 (a 180° inversion of the structure shown in FIG. 3 ), which illustrates an mDMTJ 100 having a first magnetic reference layer 102 located at the top of the mDMTJ structure and a second magnetic reference layer 112 located at the bottom of the mDMTJ structure, the first magnetic reference layer 102, the first tunnel barrier layer 104, and the nonmagnetic spin conducting metal layer 106 each have a first lateral dimension, and the magnetic free layer 108, the second tunnel barrier layer 110, and the second magnetic reference layer 112 each have a second lateral dimension that is the same as the first lateral dimension. Thus, each of the various layers present in the mDMTJ structure 100 shown in FIG. 5 has its outermost sidewalls aligned perpendicular to one another.

[0040] While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the scope of the invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.

Claims

1. A modified double magnetic tunnel junction (mDMTJ) structure, comprising: a first magnetic reference layer; a first tunnel barrier layer having a first surface in contact with a surface of the first magnetic reference layer; a non-magnetic spin conducting metal layer having a first surface in contact with a second surface of the first tunnel barrier layer opposite the first surface of the first tunnel barrier layer, the non-magnetic spin conducting metal layer reducing the spin polarization of the density of states at its interface with the first tunnel barrier layer to zero and providing zero magnetoresistance across the first tunnel barrier layer; a magnetic free layer having a first surface in contact with a second surface of the non-magnetic spin conducting metal layer opposite the first surface of the non-magnetic spin conducting metal layer; a second tunnel barrier layer having a first surface in contact with a second surface of the magnetic free layer opposite the first surface of the magnetic free layer; a second magnetic reference layer having a first surface in contact with a second surface of the second tunnel barrier layer opposite the first surface of the second tunnel barrier layer; wherein the first tunnel barrier layer has a first resistance-area product RA, and the second tunnel barrier layer has a second RA, and the first RA is at most five times smaller than the second RA.

2. The first RA is 1 ohm-micron ２ 2. The mDMTJ structure of claim 1, wherein:

3. The non-magnetic spin conducting metal layer has a resistivity of 1 ohm micron to the first tunnel barrier layer. ２ 10. The mDMTJ structure of claim 1, wherein the mDMTJ structure provides a small RA of less than 1000 .ANG. and further provides spin torque.

4. 2. The mDMTJ structure of claim 1, wherein the first tunnel barrier layer is composed of magnesium oxide (MgO), and the non-magnetic spin conducting metal layer is composed of copper (Cu) or primarily copper (Cu) containing less than 1% of other elements.

5. 5. The mDMTJ structure of claim 4, wherein the copper has an FCC111 texture.

6. 2. The mDMTJ structure of claim 1, wherein the first tunnel barrier layer is composed of MgO, and the non-magnetic spin conducting metal layer is composed of a copper nitride (CuN) alloy having a nitride content of less than 1 atomic % or a copper boron (CuB) alloy having a boron content of less than 20 atomic %.

7. The mDMTJ structure of claim 1 , wherein the first magnetic reference layer is located at the bottom of the mDMTJ structure and the second magnetic reference layer is located at the top of the mDMTJ structure.

8. 8. The mDMTJ structure of claim 7, wherein at least one of the first magnetic reference layer and the second magnetic reference layer comprises a lower magnetic reference layer, an antiferromagnetic coupling layer, and an upper magnetic reference layer.

9. 8. The mDMTJ structure of claim 7, wherein the first magnetic reference layer, the first tunnel barrier layer, the non-magnetic spin conducting metal layer, the magnetic free layer, the second tunnel barrier layer, and the second magnetic reference layer have the same lateral dimensions.

10. The mDMTJ structure of claim 1 , wherein the second magnetic reference layer is located at the bottom of the mDMTJ structure and the first magnetic reference layer is located at the top of the mDMTJ structure.

11. 11. The mDMTJ structure of claim 10, wherein at least one of the first magnetic reference layer and the second magnetic reference layer comprises a lower magnetic reference layer, an antiferromagnetic coupling layer, and an upper magnetic reference layer.

12. 11. The mDMTJ structure of claim 10, wherein the first magnetic reference layer, the first tunnel barrier layer, the non-magnetic spin conducting metal layer, the magnetic free layer, the second tunnel barrier layer, and the second magnetic reference layer have the same lateral dimensions.

13. 1. A spin transfer torque (STT) magnetic tunnel junction (MTJ) memory element, comprising:

13. An STT MTJ memory element comprising the modified double magnetic tunnel junction (mDMTJ) structure of claim 1 sandwiched between a first electrode and a second electrode.

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