Modified dual magnetic tunnel junction structure suitable for BEOL integration

Abstract

A modified dual magnetic tunnel junction (mDMTJ) structure (100) is provided, comprising a non-magnetic spin-conducting metal layer (106) sandwiched between a magnetic free layer (108) and a first tunnel barrier layer; the first tunnel barrier layer (104) contacts a first magnetic reference layer (102). A second tunnel barrier layer (110) is located on the magnetic free layer (108), and a second magnetic reference layer (112) is located on the second tunnel barrier layer (110). The mDMTJ structure exhibits efficient switching and fast readout.

Description

Background Art

[0001] The present invention relates to magnetoresistive random access memory (MRAM). More specifically, the present invention relates to a modified dual magnetic tunnel junction (mDMTJ) structure that can modify the performance of spin-transfer torque (STT) MRAM and can be integrated into back-end-of-line (BEOL) processing of semiconductor technologies, such as CMOS technologies.

[0002] MRAM is a non-volatile random access memory technology in which data is stored by magnetic storage elements. These elements are typically formed by two ferromagnetic plates, each of which can maintain magnetization, separated by a thin dielectric layer (i.e., a tunnel barrier). One of the two plates (i.e., the magnetic reference layer or pinned layer) is a magnet whose magnetic moment direction is set to a specific direction; the magnetization of the other plate (i.e., the magnetic free layer) can change in at least two different directions, thereby representing different digital states, such as 0 and 1 for memory. In MRAM, such elements may be referred to as magnetic tunnel junction (MTJ) structures. Figure 1 shows a prior art MTJ structure 10, which includes a magnetic reference layer 12, a tunnel barrier layer 14, and a magnetic free layer 16. The only arrow shown in the magnetic reference layer 12 shows the possible orientation of the layer, and the two arrows in the magnetic free layer 16 show that the orientation in the layer can be switched.

[0003] In the MTJ structure 10 of Figure 1, the magnetization of the magnetic reference layer 12 is fixed in one direction (for example, pointing upward), while the orientation of the magnetic free layer 16 can be "switched" by some external force (such as a magnetic field or spin transfer torque that generates a charging current). A small current (of either polarity) can be used to read the device's resistance, which depends on the relative orientation of the magnetizations of the magnetic free layer 16 and the magnetic reference layer 12. When the magnetizations are antiparallel, the resistance is generally high, and when the magnetizations are parallel, the resistance is low (although this can be reversed depending on the material).

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

[0005] In STT MRAM, it is desirable to reduce the switching current to match the small transistor size in order to increase the memory area density. One approach to reducing the switching current by about 2x is the concept of a dual magnetic tunnel junction (DMTJ) structure 20 as shown in FIG2 . The DMTJ structure 20 of FIG2 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 unique arrow shown in each of the first magnetic reference layer 22 and the second magnetic reference layer 30 illustrates the possible orientation of the layer, and the two arrows in the magnetic free layer 26 illustrate that the orientation in the layer can be switched. One disadvantage of the DMTJ structure shown in FIG2 is that while it reduces the switching current, it also reduces the tunnel magnetoresistance (TMR), hindering efficient readout of the device.

[0006] It is therefore desirable to provide a DMTJ structure with reduced switching current while mitigating the reduction in TMR within the structure, such that the DMTJ structure exhibits efficient switching (at low current) and fast readout (high TMR). Summary of the Invention

[0007] A modified dual magnetic tunnel junction (mDMTJ) structure is provided, which includes a non-magnetic spin-conducting metal layer sandwiched between a magnetic free layer and a first tunnel barrier layer; the first tunnel barrier layer contacts a first magnetic reference layer. The second tunnel barrier layer is located on the magnetic free layer, and the second magnetic reference layer is located on the second tunnel barrier layer. The mDMTJ structure of the present invention exhibits efficient switching (at low current) and fast readout (high TMR). "Low current" refers to a current that is less than the current required by a typical single MTJ device to achieve the same error-free write. In some embodiments, the low current can reach or be lower than 20 to 50 μA. "High TMR" refers to a tunnel magnetoresistance that is greater than the tunnel magnetoresistance value 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 a surface of the first magnetic reference layer, a non-magnetic spin-conductive metal layer having a first surface contacting a second surface of the first tunnel barrier layer, the second surface being opposite to 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-conductive metal layer, the second surface being opposite to the first surface of the non-magnetic spin-conductive metal layer, a second tunnel barrier layer having a first surface contacting the second surface of the magnetic free layer, the second surface being opposite to 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, the second surface of the second tunnel barrier layer being opposite to the first surface of the second tunnel barrier layer.

[0009] In another aspect of the present invention, an STM TJ memory element is provided. In one embodiment, the STM TJ memory element includes an 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 a surface of the first magnetic reference layer, a non-magnetic body, a spin-conductive metal layer having a first surface contacting a second surface of the first tunnel barrier layer, the second surface being opposite to the first surface of the first tunnel barrier layer, a magnetic free layer having a first surface contacting a second surface of the non-magnetic spin-conductive metal layer, the second surface being opposite to the first surface of the non-magnetic spin-conductive metal layer, a second tunnel barrier layer having a first surface contacting a second surface of the magnetic free layer, the second surface being opposite to the first surface of the magnetic free layer, and a second magnetic reference layer having a first surface contacting a second surface of the second tunnel barrier layer, the second surface of the second tunnel barrier layer being opposite to the first surface of the second tunnel barrier layer.

[0010] In another aspect of the present invention, a method for forming an mDMTJ structure is provided. In one embodiment, the method includes forming a lower material stack of the mDMTJ structure, wherein the lower material stack includes, from bottom to top, a first magnetic reference layer, a first tunnel barrier layer, and a non-magnetic spin-conducting metal layer. Next, an upper material stack of the mDMTJ structure is formed on the physically exposed topmost surface of the lower material stack of the mDMTJ structure, wherein the upper material stack includes, from bottom to top, a magnetic free layer, a second tunnel barrier layer, a magnetic free layer, and a second magnetic reference layer. In this embodiment, the magnetic free layer forms an interface with the non-magnetic spin-conducting metal layer.

[0011] In some embodiments, and in order to avoid surface contamination, the upper material stack includes another non-magnetic spin-conductive metal layer of the same non-magnetic spin-conductive metal material as the non-magnetic spin-conductive metal layer of the lower material stack, wherein the other non-magnetic spin-conductive metal layer of the upper material stack forms an interface with the non-magnetic spin-conductive metal layer of the lower material stack.

[0012] In another embodiment, the method includes forming a lower material stack of an mDMTJ structure, wherein the lower material stack includes, from bottom to top, a second magnetic reference layer, a second tunnel barrier layer, and a magnetic free layer. Next, an upper material stack of the mDMTJ structure is formed on the physically exposed topmost surface of the lower material stack of the mDMTJ structure, wherein the upper material stack includes, from bottom to top, a non-magnetic spin-conductive metal layer, a first tunnel barrier layer, and a first magnetic reference layer. In this embodiment, the non-magnetic, spin-conductive metal layer of the upper material stack is in direct contact with the surface of the magnetic free layer of the lower material stack.

[0013] In some embodiments, and in order to avoid surface contamination, the lower material stack includes another non-magnetic spin-conductive metal layer of the same non-magnetic spin-conductive metal material as the non-magnetic spin-conductive metal layer of the upper material stack, wherein the another non-magnetic spin-conductive metal layer of the lower material stack forms an interface with the non-magnetic spin-conductive metal layer of the upper material stack.

[0014] The present method of decoupling the formation of non-magnetic, spin-conducting metallic materials into two separate layers allows for greater flexibility in materials and growth engineering and manufacturing optimization. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is a cross-sectional view of a prior art MTJ structure.

[0016] FIG2 is a cross-sectional view of a prior art DMTJ structure.

[0017] Figure 3 is a cross-sectional view of an mDMTJ structure according to an embodiment of the present invention.

[0018] Figure 4 It can be used for Figure 3 A cross-sectional view of a magnetic reference layer of one or both of the first magnetic reference layer and the second magnetic reference layer, the magnetic reference layer comprising a lower magnetic reference layer, a synthetic antiferromagnetic coupling layer, and an upper magnetic reference layer; in such an application, Figure 3 The reference layer moment direction shown in corresponds to the moment direction of one layer of the synthetic antiferromagnetic coupling layer in contact with the tunnel barrier.

[0019] Figure 5Ais a cross-sectional view of an mDMTJ structure according to another embodiment of the present invention. The dashed line indicates a possible "disconnect" point between the lower portion of the non-magnetic, spin-conducting metal layer and the upper portion of the non-magnetic, spin-conducting metal layer.

[0020] Figure 5B is a cross-sectional view of an mDMTJ structure according to another embodiment of the present invention. The dashed line indicates a possible "disconnect" point between the lower portion of the non-magnetic, spin-conducting metal layer and the upper portion of the non-magnetic, spin-conducting metal layer.

[0021] Figure 6 is a cross-sectional view of an mDMTJ structure according to an embodiment of the present invention.

[0022] Figure 7A is a cross-sectional view of an mDMTJ structure according to another embodiment of the present invention. The dashed line indicates a possible "disconnect" point between the lower portion of the non-magnetic, spin-conducting metal layer and the upper portion of the non-magnetic, spin-conducting metal layer.

[0023] Figure 7B is a cross-sectional view of an mDMTJ structure according to another embodiment of the present invention. The dashed line indicates a possible "disconnect" point between the lower portion of the non-magnetic, spin-conducting metal layer and the upper portion of the non-magnetic, spin-conducting metal layer. DETAILED DESCRIPTION

[0024] Embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. It should be noted that the accompanying drawings are provided for illustrative purposes only and, therefore, are not drawn to scale. It should also be noted that identical and corresponding elements are designated by identical reference numerals.

[0025] In the following description, numerous specific details are set forth, such as specific structures, components, materials, dimensions, processing steps and techniques, in order to provide an understanding of the various embodiments of the present invention. However, one of ordinary skill in the art will appreciate that the various embodiments of the present invention can be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the present invention.

[0026] It will be understood that when an element as a layer, region, or substrate is referred to as being "on" or "over" another element, it 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 on" another element, there are no intervening elements. It will also be understood that when an element is referred to as being "beneath" or "under" another element, it can be directly beneath or under the other element, or there may be intervening elements. Conversely, when an element is referred to as being "directly beneath" or "directly under" another element, there are no intervening elements.

[0027] The present invention provides a mDMTJ structure 100, such as in Figure 3 、 5A , 5B, 6, 7A and 7B, the mDMTJ structure 100 can be integrated in the BEOL and used as a component of the STT MRAM. Figure 3 、 5A , 5B, 6, 7A and 7B include a first magnetic reference layer 102, a first tunnel barrier layer 104 having a first surface contacting a surface of the first magnetic reference layer 102, a non-magnetic spin-conductive metal layer 106 having a first surface contacting the first tunnel barrier layer 104 and a second surface opposite to the first surface of the first tunnel barrier layer 104, a magnetic free layer 108 having a first surface contacting the second surface of the non-magnetic spin-conductive metal layer 106, the second surface opposite to the first non-magnetic surface, the spin-conductive metal layer 106, a second tunnel barrier layer 110 having a first surface contacting the second surface of the magnetic free layer 108, the second surface opposite to the first surface of the magnetic free layer 108, and a second magnetic reference layer 112 having a first surface contacting the second surface of the second tunnel barrier layer 110, the second surface opposite to the first surface of the second tunnel barrier layer 110.

[0028] The non-magnetic, spin-conductive metal layer 106 is a spin-preserving metal material with a long spin-flip scattering lifetime. When tunneling through the first tunnel barrier layer 104, the non-magnetic, spin-conductive metal layer 106 receives the spin current polarized by the first magnetic reference layer 102 and effectively transmits the spin current to the magnetic free layer 108 to assist in STT induced switching. At the same time, the non-magnetic spin-conductive metal layer 106 reduces the spin polarization of the density of states (DOS) to near zero at the interface between the non-magnetic spin-conductive metal layer 106 and the first tunnel barrier layer 104. Therefore, the magnetoresistance across the first tunnel barrier layer 104 is reduced to zero, thereby avoiding the magnetoresistance cancellation effect that plagues the prior art DMTJ structure, as shown in Figure 2.

[0029] This reduction in DOS spin polarization at the interface between the non-magnetic, spin-conductive metal layer 106 and the first tunnel barrier layer 104 also significantly reduces the spin current having a polarization defined by the magnetic free layer 108 that can enter the first magnetic reference layer 102, thereby reducing the spin torque and STT-related disturbances on the magnetic reference layer 102 from the magnetic free layer 108. As a result, the mDMTJ structure 100 of the present invention exhibits efficient switching (at low currents as defined above), fast readout (high TMR as defined above), and reduction of potential disturbances on the magnetic state of the reference layer (102) (disturbances that can also cause write errors).

[0030] exist Figures 5A-5B In the embodiments shown in Figures 7A-7B and 7A-7B, the first tunnel barrier layer 104 has a lateral dimension (i.e., width or pillar diameter) that is greater than the lateral dimension of the second tunnel barrier layer 110. This difference in lateral dimensions between the first tunnel barrier layer 104 and the second tunnel barrier layer 110 reduces the need for the first tunnel barrier layer 104 to have an extremely low value of area-specific resistance (i.e., resistance-area product or RA). This also helps maintain a high TMR signal by reducing the overall added resistance to the mDMTJ structure 100.

[0031] The mDMTJ structure 100 of the present invention allows the first tunnel barrier 104 to operate at a lower current density, alleviating concerns about device endurance over repeated write operations. Since this endurance tends to deteriorate rapidly as the operating current density (and voltage) increases, reducing the total tunnel resistance and tunnel current density (through increased area) is a favorable direction for endurance modification.

[0032] First reference Figure 3 , which illustrates an mDMTJ structure 100 according to an embodiment of the present invention. Figure 3The mDMTJ structure 100 includes a first magnetic reference layer 102, a first tunnel barrier layer 104 contacting a surface of the first magnetic reference layer 102, a non-magnetic spin-conducting metal layer 106 contacting a surface of the first tunnel barrier layer 104, a magnetic free layer 108 contacting a surface of the non-magnetic spin-conducting metal layer 106, a second tunnel barrier layer 110 contacting a surface of the magnetic free layer 108, and a second magnetic reference layer 112 contacting a surface of the second tunnel barrier layer 110. As shown, Figure 3 The mDMTJ structure 100 is located between the first electrode 90 and the second electrode 120. The mDMTJ structure 100, the first electrode 90, and the second electrode 120 collectively provide an STTMTJ memory element.

[0033] Despite Figure 3 Alternatively, and not depicted in any of the remaining figures, the mDMTJ structure 100 may be embedded in various dielectric material layers present in the BEOL, including interconnect dielectric materials. Figure 3 A device region that is not depicted in any of the remaining figures but that includes non-MRAM may be positioned laterally adjacent to a device region that includes MRAM that is depicted in the figures.

[0034] The first electrode 90 may be present on the surface (recessed or non-recessed) of a conductive structure (e.g., a copper-containing structure) embedded in a layer of interconnect dielectric material at one of the interconnect levels present in the 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 any other well-known electrode material. The first electrode 90 may have a thickness of from 2 nm to 25 nm; other thicknesses are possible and may be used as the thickness of 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). After depositing the conductive material providing the first electrode 90, a deep etching process, a planarization process (e.g., chemical mechanical polishing or CMP), or a patterning process (e.g., photolithography and etching) may be performed.

[0035] Next and as Figure 3 As shown in FIG, the mDMTJ structure 100 is formed on the first electrode 90. Figure 3 In FIG, the first magnetic reference layer 102 is located at the bottom portion of the mDMTJ structure 100, and the second magnetic reference layer 112 is located at the top portion of the mDMTJ structure 100. Figure 3, each of the first magnetic reference layer 102, the first tunnel barrier layer 104, and the non-magnetic spin-conducting metal layer 106 has a first lateral dimension, and each of the magnetic free layer 108, the second tunnel barrier layer 110, and the second magnetic reference layer 112 has a second lateral dimension that is the same as the first lateral dimension. Figure 3 Each of the different layers present in the mDMTJ structure 100 shown in FIG. 1 has outermost walls that are vertically aligned or aligned with a controlled sidewall slope with respect to each other.

[0036] The different material layers of the mDMTJ structure 100 can be formed by utilizing one or more deposition processes, such as sputtering, plasma enhanced atomic layer deposition (PEALD), plasma enhanced chemical vapor deposition (PECVD), or physical vapor deposition (PVD) including magnetron sputtering. In some embodiments, the different material layers of the mDMTJ structure 100 can be formed without breaking the vacuum between the deposition of the different material layers. In other embodiments, the different material layers of the mDMTJ structure 100 can be formed by breaking the vacuum between the deposition of one or more of the different material layers. In some embodiments of the present invention, the non-magnetic spin-conductive metal layer 106 is formed in a separate deposition step, so that the lower portion of the non-magnetic spin-conductive metal layer 106 is formed, and then the upper portion of the non-magnetic spin-conductive metal layer 106 is formed. The upper and lower portions of the non-magnetic, spin-conductive metal layer 106 are composed of the same non-magnetic, spin-conductive metal material. The presence of the non-magnetic, spin-conducting metal layer 106 makes it feasible to break the vacuum in the middle of forming this material layer because it is easier to "repair" the metal interface of the same material via annealing at high temperature (such as 400° C.) and minimize interface-related defect effects.

[0037] The first magnetic reference layer 102 has a fixed magnetization. The first magnetic reference layer 102 can be composed of a metal or metal alloy (or a stack thereof) including one or more metals that exhibit high spin polarization at the tunnel barrier interface. In alternative 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) high spin polarization regions formed from metals and / or metal alloys using the above metals, and (2) regions composed of one or more materials that exhibit strong perpendicular magnetic anisotropy (strong PMA). Exemplary materials with strong PMA that can be used include metals such as cobalt, nickel, platinum, palladium, iridium, or ruthenium, and can be arranged as alternating layers. The strong PMA region may also include alloys that exhibit strong intrinsic or bulk (as opposed to interface) PMA, where exemplary alloys include cobalt-iron-terbium, cobalt-iron-gadolinium, cobalt-chromium-platinum, cobalt-platinum, cobalt-palladium, iron-platinum, and / or iron-palladium. The alloys may be arranged as alternating layers. In one embodiment, a combination of these materials and regions may also be used as the first magnetic reference layer 102. In some embodiments, the first magnetic reference layer 102 may include a lower magnetic reference layer, a synthetic antiferromagnetic coupling layer, and an upper magnetic reference layer. This embodiment of the first magnetic reference layer 102 will be discussed below with respect to Figure 4 Describe in more detail.

[0038] The first tunnel barrier layer 104 is configured to provide a spin current when coupled to the non-magnetic spin-conductive metal layer 106, but does not contribute to TMR relative to the TMR of the second tunnel barrier. The first tunnel barrier layer 104 is composed of an insulating material such as, for example, magnesium oxide, aluminum oxide, and titanium oxide, or a material having a higher tunnel conductance while maintaining spin polarization (such as a semiconductor or a low-bandgap insulator). Due to the presence of the non-magnetic, spin-conductive metal layer 106, the thickness of the first tunnel barrier layer 104 is not limited to being much lower than the resistance-area product of the second tunnel barrier 110, so as to provide an appropriate tunnel resistance for the entire mDMTJ. In addition, and in some embodiments, the first tunnel barrier layer 104 has a lower magnetoresistance than the second tunnel barrier layer 110 so as not to reduce the total amount of TMR of the mDMTJ stack.

[0039] The non-magnetic, spin-conductive metal layer 106 is a spin-preserving metal material with a long spin-flip scattering lifetime. In one embodiment of the present invention, the non-magnetic, spin-conductive metal layer 106 is composed of a spin conductor such as silver (Ag), copper (Cu), gold (Au), magnesium (Mg), aluminum (Al), chromium (Cr), vanadium (V), or an alloy thereof such as a silver-tin (Ag-Sn) alloy. In some embodiments, the non-magnetic, spin-conductive metal layer 106 has a body-centered cubic (bcc) crystal structure. In other embodiments, the non-magnetic, spin-conductive metal layer 106 has a face-centered cubic (fcc) crystal structure. Other crystal structures sufficient to maintain the spin polarization of the tunnel current through the first tunnel barrier layer 104 are also contemplated. The non-magnetic, spin-conductive metal layer 106 has a thickness sufficient to transmit the received spin current to the magnetically free layer 108. In one embodiment, the thickness of the non-magnetic, spin-conductive metal layer 106 is 20 nm to 55 nm; although other thicknesses are possible as long as the thickness of the non-magnetic, spin-conductive metal layer 106 is sufficient to transfer the received spin current into the magnetically free layer 108 .

[0040] The magnetically free layer 108 may be composed of a magnetic material (or a stack of magnetic materials) having a magnetization that can change direction relative to the magnetization direction of the first magnetic reference layer 102 and the second magnetic reference layer 112. Exemplary magnetic materials for the magnetically free layer 108 include alloys and / or multilayers of cobalt, iron, a cobalt-iron alloy, nickel, a nickel-iron alloy, and a cobalt-iron-boron alloy.

[0041] The second tunnel barrier layer 110 is composed of an insulator material and is formed to a thickness that provides suitable 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 magnesium oxide, aluminum oxide, and titanium oxide, or materials with higher tunnel conductivity, such as semiconductors or low-bandgap insulators. In some embodiments, the second tunnel barrier layer 110 is composed of an insulator 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 insulator material that is compositionally different from the first tunnel barrier layer 104.

[0042] The second magnetic reference layer 112 also has a fixed magnetization. The second magnetic reference layer 112 can be composed of a metal or metal alloy (or a stack thereof) including one or more metals that exhibit high spin polarization. In alternative embodiments, exemplary metals for 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) high spin polarization regions formed from metals and / or metal alloys using the above metals and (2) regions composed of materials that exhibit strong perpendicular magnetic anisotropy (strong PMA). Exemplary materials with strong PMA that can be used include metals such as cobalt, nickel, platinum, palladium, iridium, or ruthenium, and can be arranged as alternating layers. The strong PMA region may also include alloys that exhibit strong intrinsic or bulk (as opposed to interface) PMA, where exemplary alloys include cobalt-iron-terbium, cobalt-iron-gadolinium, cobalt-chromium-platinum, cobalt-platinum, cobalt-palladium, iron-platinum, and / or iron-palladium. The alloys may be arranged as alternating layers. In one embodiment, a combination of these materials and regions may also be used as the second magnetic reference layer 112. In some embodiments, the second magnetic reference layer 112 may include a lower magnetic reference layer, a synthetic antiferromagnetic coupling layer, and an upper magnetic reference layer. Figure 4 This embodiment of the second magnetic reference layer 112 is described in more detail.

[0043] The second electrode 120 may be formed of one of the conductive metal materials as the first electrode 90. In some embodiments, the second electrode 120 may be formed of a conductive metal material that is the same compositionally as the first electrode 90. In another embodiment, the second electrode 120 may be formed of a conductive metal material that is different compositionally from the first electrode 90. The second electrode 120 may have a thickness of from 2 nm to 25 nm; other thicknesses are possible and may be used as the thickness of the second electrode 120. The second electrode 120 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).

[0044] Now refer to Figure 4 , showing a magnetic reference layer 113, which can be used Figure 3 Medium or Figure 5A 、 5B , 6, 7A and 7B show one or both of the first magnetic reference layer 102 and the second magnetic reference layer 112. Figure 4The magnetic reference layer 113 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 one of the magnetic materials described above for the first or second magnetic reference layers 102, 110. The synthetic antiferromagnetic coupling layer 116 is composed of a non-magnetic material that can couple the lower and upper magnetic layers 114, 118 of the magnetic reference layer 113 in an antiparallel manner. Exemplary non-magnetic materials that can be used as 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 can have a thickness of from 0.2 nm to 1.2 nm; however, other thicknesses are possible and can be used as the thickness of the synthetic antiferromagnetic coupling layer 116. The upper magnetic reference layer 118 can be composed of one of the magnetic materials mentioned above for the first or second magnetic reference layers 102, 110. Typically, the upper magnetic reference layer 118 is compositionally different from the lower magnetic reference layer 114 .

[0045] In one embodiment, magnetic reference layer 113 is used as second magnetic reference layer 112 but not as first magnetic reference layer 102. In another embodiment, magnetic reference layer 113 is used as both second magnetic reference layer 112 and first magnetic reference layer 102. In yet another embodiment, magnetic reference layer 113 is used as first magnetic reference layer 102 but not as second magnetic reference layer 112.

[0046] Now see Figures 5A-5B , which illustrates an mDMTJ structure 100 according to another embodiment of the present invention. Figures 5A-5B The mDMTJ structure 100 includes a first magnetic reference layer 102 as defined above, a first tunnel barrier layer 104 as defined above in contact with a surface of the first magnetic reference layer 102, a non-magnetic spin conductive metal layer 106 as defined above in contact with a surface of the first tunnel barrier layer 104, a magnetic free layer 108 as defined above in contact with the non-magnetic surface, the spin conductive metal layer 106, a second tunnel barrier layer 110 as defined above in contact with a surface of the magnetic free layer 108, and a second magnetic reference layer 112 as defined above in contact with a surface of the second tunnel barrier layer 110. Figures 5A-5B The mDMTJ structure 100 shown in illustrates an embodiment in which the first magnetic reference layer 102 is located at a bottom portion of the mDMTJ structure and the second magnetic reference layer 112 is located at a top portion of the mDMTJ structure.

[0047] Although not shown, Figures 5A-5B The mDMTJ structure 100 is located between the first electrode defined above and the second electrode defined above. The mDMTJ structure 100, the first electrode, and the second electrode together provide an STTMTJ memory element.

[0048] exist Figures 5A-5B 7A-7B, the horizontal dashed lines represent possible "break" points between the lower portion of the non-magnetic, spin-conducting metal layer 106 and the upper portion of the non-magnetic, spin-conducting metal layer 106. Figures 5A-5B In the embodiments shown in Figures 7A-7B and 7A-7B, the first tunnel barrier layer 104 has a lateral dimension that is larger than the lateral dimension of the second tunnel barrier layer 110. The difference in lateral dimensions between the first tunnel barrier layer 104 and the second tunnel barrier layer 110 reduces the need for an ultra-low value of the area specific resistance (i.e., resistance-area product or RA) of the tunnel barrier. This also helps maintain a high TMR signal by reducing the overall added resistance to the mDMTJ structure 100. Figures 5A-5B 7A-7B, the nonmagnetic, spin-conducting metal layer 106 has larger lateral dimensions than the magnetic free layer 108, which helps to eliminate resistance in the device.

[0049] Figure 5A An embodiment is shown in which each of the first magnetic reference layer 102, the first tunnel barrier layer 104, and the non-magnetic spin-conductive metal layer 106 has a first lateral dimension, and each of the magnetic free layer 108, the second tunnel barrier layer 110, and the second magnetic reference layer 112 has a second lateral dimension that is smaller than the first lateral dimension.

[0050] Figure 5B Shown with Figure 5A The embodiment shown is similar to the embodiment shown, except that the magnetically free layer 108 is located on a base portion of the non-magnetic, spin-conductive metal layer 106, the base portion of the non-magnetic, spin-conductive metal layer 106 having a second lateral dimension and the remainder of the non-magnetic, spin-conductive metal layer 106 having a first lateral dimension. The base portion is defined as a portion of material that extends upward from the remainder of the same material. In some embodiments, the base portion is created by recessing a portion of the material that is not protected by some type of etch mask. Figure 5B In the embodiment shown, the base portion of the non-magnetic, spin-on conductive metal layer 106 extends outside of the via opening formed in the interconnect dielectric material layer.

[0051] Now see Figure 6 、 7A and 7B , which illustrate an mDMTJ structure 100 according to yet other embodiments of the present invention. Figure 6 、 7AEach of the mDMTJ structures 100 of 7B includes a first magnetic reference layer 102 as defined above, a first tunnel barrier layer 104 as defined above, contacting a surface of the first magnetic reference layer 102, a non-magnetic spin-conductive metal layer 106 (as defined above) contacting a surface of the first tunnel barrier layer 104, a magnetic free layer 108 as defined above contacting a surface of the non-magnetic spin-conductive metal layer 106, a second tunnel barrier layer 110 as defined above contacting a surface of the magnetic free layer 108, and a second magnetic reference layer 112 as defined above contacting a surface of the second tunnel barrier layer 110. Figure 6 、 7A The mDMTJ structure 100 shown in 7B illustrates an embodiment in which the first magnetic reference layer 102 is located at a top portion of the mDMTJ structure and the second magnetic reference layer 112 is located at a bottom portion of the mDMTJ structure.

[0052] Although not shown, Figure 6 、 7A The DMTJ structure 100 of 7B is located 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 together provide an STT M TJ memory element.

[0053] Figure 6 An embodiment is shown in which each of the first magnetic reference layer 102, the first tunnel barrier layer 104, and the non-magnetic spin-conductive metal layer 106 has a first lateral dimension, and each of the magnetic free layer 108, the second tunnel barrier layer 110, and the second magnetic reference layer 112 has a second lateral dimension that is the same as the first lateral dimension. Figure 6 Each of the various layers present in the mDMTJ structure 100 shown in FIG. 1 has outermost sidewalls vertically aligned with each other.

[0054] Figure 7A An embodiment is shown in which each of the first magnetic reference layer 102, the first tunnel barrier layer 104, and the non-magnetic spin-conductive metal layer 106 has a first lateral dimension, and each of the magnetic free layer 108, the second tunnel barrier layer 110, and the second magnetic reference layer 112 has a second lateral dimension that is smaller than the first lateral dimension.

[0055] Figure 7B Shows something like Figure 7AIn the embodiment depicted in FIG, except that the magnetic free layer 108 contacts the surface of the lower portion of the non-magnetic spin-conducting metal layer 104 extending into the via opening present in the interconnect dielectric material layer, the upper portion of the non-magnetic spin-conducting metal layer 104 exists outside the via opening. The lower portion of the non-magnetic spin-conducting metal layer 106 extending into the via opening has a second lateral dimension, and the upper portion of the non-magnetic spin-conducting metal layer 106 has a first lateral dimension.

[0056] Figures 5A-5B The mDMTJ structures shown in Figures 7A-7B are formed using a method that decouples the formation of a lower material stack of the mDMTJ structure from the formation of an upper material stack of the mDMTJ structure. In some embodiments, the decoupling includes forming separate non-magnetic spin-conducting material layers of the same or similar non-magnetic spin-conducting metal material in the lower material stack and the upper material stack. This decoupling allows for greater flexibility in material and growth engineering and manufacturing optimization.

[0057] In one embodiment, it can be used to provide Figure 5A and 5B The method of the mDMTJ structure 100 shown in FIG includes forming a lower material stack of a first magnetic reference layer 102, a first tunnel barrier layer 104, and a lower portion of a non-magnetic, spin-conductive metal layer 106, and thereafter the lower material stack is photolithographically patterned into a pillar shape. Next, an interconnect dielectric material layer is conformally formed laterally adjacent to and above the pillar, and then the formation of a via opening is stopped on the patterned lower portion of the non-magnetic, spin-conductive metal layer 106. The structure is then flattened by, for example, chemical mechanical polishing to remove excess non-magnetic, spin-conductive metal material, exposing the top of the via with a smooth non-magnetic, spin-conductive metal surface. An upper material stack of an additional layer of non-magnetic spin-conductive material (forming the upper portion of the non-magnetic, spin-conductive layer 106), a magnetic free layer 108, a second tunnel barrier layer 110, and a second magnetic reference layer 112 is then formed on the processed lower material stack comprising elements 102 / 104 and the lower portion of the non-magnetic, spin-conductive metal layer 106 and above the via opening. The upper material stack comprising the nonmagnetic, spin-conductive metal layer 106 and the upper portion of the elements 106 / 108 / 110 / 112 can then be photolithographically patterned to the desired diameter and depth for the MTJ memory device, with the etching stopping at the dielectric surface. The diameter of the patterned upper material stack can be different from, and preferably smaller than, the pillar diameter of the first material stack below the via.

[0058] In another embodiment, it can be used to provide Figure 7A and 7BThe method for forming the mDMTJ structure 100 shown in FIG. 1 includes forming the lower material stack of the mDMTJ structure, namely, the second magnetic reference layer 112, the second tunnel barrier layer 110, the magnetic free layer 108, and the lower portion of the nonmagnetic spin-conductive layer 106. Thereafter, the lower portions of the lower material stacks 112 / 110 / 108 and 106 are photolithographically patterned into pillars of the desired diameter for the MTJ memory. Interconnect dielectric material is then conformally deposited laterally adjacent to and above the pillars. Next, a via opening is created on top of the pillars comprising the lower material stack of elements 112 / 110 / 108 and the lower portion of 106, exposing the surface of the lower portion of the nonmagnetic spin-conductive layer 106. An upper material stack comprising the nonmagnetic, spin-conductive metal layer 106, the first tunnel barrier 104, and the upper portion of the first magnetic reference layer is formed above the via opening. This upper material stack is then photolithographically patterned to a diameter that is preferably larger than the diameter of the lower material stack below the via-shaped opening.

[0059] While the present invention has been particularly shown and described with respect to its preferred embodiments, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made 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 dual magnetic tunnel junction (mDMTJ) structure, comprising: a first magnetic reference layer; a first tunnel barrier layer having a first surface in direct physical contact with a surface of the first magnetic reference layer; a non-magnetic spin-conductive metal layer having a first surface in direct physical contact with the second surface of the first tunnel barrier layer, the second surface of the first tunnel barrier layer being opposite to the first surface of the first tunnel barrier layer; a magnetic free layer having a first surface in direct physical contact with a second surface of the nonmagnetic spin-conductive metal layer, the second surface of the nonmagnetic spin-conductive metal layer being opposite to the first surface of the nonmagnetic spin-conductive metal layer; a second tunnel barrier layer having a first surface contacting a second surface of the magnetic free layer, the second surface of the magnetic free layer being opposite to the first surface of the magnetic free layer, and A second magnetic reference layer has a first surface contacting a second surface of the second tunnel barrier layer, the second surface of the second tunnel barrier layer being opposite to the first surface of the second tunnel barrier layer. 2 . The mDMTJ structure of claim 1 , wherein the first magnetic reference layer is located at a bottom portion of the mDMTJ structure, and the second magnetic reference layer is located at a top portion of the mDMTJ structure.

3. The mDMTJ structure according to claim 2, wherein: The second magnetic reference layer includes a lower magnetic reference layer, an antiferromagnetic coupling layer, and an upper magnetic reference layer.

4. The mDMTJ structure according to claim 2, wherein: The first tunnel barrier layer and the second tunnel barrier layer have the same lateral dimensions.

5. The mDMTJ structure according to claim 2, wherein: The first tunnel barrier layer has a first lateral dimension, and the second tunnel barrier layer has a second lateral dimension that is smaller than the first lateral dimension.

6. The mDMTJ structure according to claim 5, wherein: The magnetic free layer is located on a base portion of the nonmagnetic spin-conductive metal layer, the base portion of the nonmagnetic spin-conductive metal layer has the second lateral dimension, and a remaining portion of the nonmagnetic spin-conductive metal layer has the first lateral dimension.

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

8. The mDMTJ structure according to claim 7, wherein: The second magnetic reference layer includes a lower magnetic reference layer, an antiferromagnetic coupling layer, and an upper magnetic reference layer.

9. The mDMTJ structure according to claim 7, wherein: The first tunnel barrier layer and the second tunnel barrier layer have the same lateral dimensions.

10. The mDMTJ structure according to claim 7, wherein: The first tunnel barrier layer has a first lateral dimension, and the second tunnel barrier layer has a second lateral dimension that is smaller than the first lateral dimension.

11. The mDMTJ structure according to claim 10, wherein: The magnetic free layer contacts a lower portion of the nonmagnetic spin-conductive metal layer, the lower portion of the nonmagnetic spin-conductive metal layer has the second lateral dimension, and an upper portion of the nonmagnetic spin-conductive metal layer has the first lateral dimension.

12. A spin transfer torque (STT) magnetic tunnel junction (MTJ) memory element, comprising: A modified double magnetic tunnel junction (mDMTJ) structure as claimed in any preceding claim, sandwiched between a first electrode and a second electrode.

13. A method of forming a modified double magnetic tunnel junction (mDMTJ) structure, the method comprising: forming a lower material stack of the mDMTJ structure, wherein the lower material stack comprises, from bottom to top, a first magnetic reference layer, a first tunnel barrier layer, and a non-magnetic spin-conductive metal layer; as well as An upper material stack of the mDMTJ structure is formed on the physically exposed topmost surface of the lower material stack of the mDMTJ structure, wherein the upper material stack includes, from bottom to top, a first magnetic free layer, a second tunnel barrier layer, a second magnetic free layer, and a second magnetic reference layer, wherein the non-magnetic spin-conductive metal layer has a first surface and a second surface opposite to the first surface, the first surface is in direct physical contact with the surface of the first tunnel barrier layer, and the second surface is in direct physical contact with the surface of the first magnetic free layer.

14. The method according to claim 13, wherein: The upper material stack includes another non-magnetic spin-conductive metal layer of the same non-magnetic spin-conductive metal material as the non-magnetic spin-conductive metal layer of the lower material stack, wherein the another non-magnetic spin-conductive metal layer of the upper material stack forms an interface with the non-magnetic spin-conductive metal layer of the lower material stack.

15. A method of forming a magnetoresistive random access memory (MRAM), the method comprising the method of claim 13 or claim 14.

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