WO2020173026A1 - 一种磁性随机存储器存储单元及磁性随机存储器 - Google Patents

一种磁性随机存储器存储单元及磁性随机存储器 Download PDF

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WO2020173026A1
WO2020173026A1 PCT/CN2019/093736 CN2019093736W WO2020173026A1 WO 2020173026 A1 WO2020173026 A1 WO 2020173026A1 CN 2019093736 W CN2019093736 W CN 2019093736W WO 2020173026 A1 WO2020173026 A1 WO 2020173026A1
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layer
cofeb
free layer
magnetic
free
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PCT/CN2019/093736
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French (fr)
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张云森
郭一民
陈峻
肖荣福
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上海磁宇信息科技有限公司
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Priority to US16/490,548 priority Critical patent/US20210193735A1/en
Publication of WO2020173026A1 publication Critical patent/WO2020173026A1/zh

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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10B—ELECTRONIC MEMORY DEVICES
    • H10B61/00—Magnetic memory devices, e.g. magnetoresistive RAM [MRAM] devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/10—Magnetoresistive devices
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
    • G11C11/16—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect
    • G11C11/161—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect details concerning the memory cell structure, e.g. the layers of the ferromagnetic memory cell
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
    • G11C11/14—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using thin-film elements
    • G11C11/15—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using thin-film elements using multiple magnetic layers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/80—Constructional details
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/80—Constructional details
    • H10N50/85—Materials of the active region

Definitions

  • the invention relates to the field of magnetic random access memory, in particular to a magnetic random access memory storage unit and a magnetic random access memory with a double-layer free layer.
  • MRAM using Magnetic Tunnel Junction is considered to be the future solid-state non-volatile memory, which has the characteristics of high-speed reading and writing, large capacity and low energy consumption.
  • Ferromagnetic MTJ is usually a sandwich structure, which has a magnetic memory layer (free layer), which can change the magnetization direction to record different data; an insulating tunnel barrier layer located in the middle; a magnetic reference layer, located on the other side of the tunnel barrier layer Side, its magnetization direction remains unchanged.
  • STT-MRAM spin momentum transfer or Spin Transfer Torque (STT, Spin Transfer Torque) conversion technology.
  • STT-MRAM spin momentum transfer or Spin Transfer Torque (STT, Spin Transfer Torque) conversion technology.
  • STT-MRAM Spin Transfer Torque
  • pSTT-MRAM vertical STT-MRAM
  • PMA perpendicular anisotropy
  • the free layer for storing information it has two magnetization directions in the vertical direction, namely: upward and downward, corresponding to the "0" and "0" in the binary system. "1".
  • the magnetization direction of the free layer remains the same when reading information or when it is left empty; during the writing process, if there is a signal input in a different state from the existing signal, the magnetization direction of the free layer will change A 180-degree flip occurs in the vertical direction.
  • the industry refers to the ability of the free layer of the magnetic memory to maintain the magnetization direction as data retention or thermal stability.
  • the requirements are different in different application scenarios.
  • the thermal stability requirement is that the data can be stored for 10 years at 125°C.
  • the MTJ which is the core memory cell of the magnetic memory (MRAM)
  • MRAM magnetic memory
  • FIG. 1 is a schematic diagram of the structure of a conventional magnetic random access memory storage unit.
  • the free layer is generally composed of CoFeB, CoFe/CoFeB, Fe/CoFeB or CoFeB/(Ta, W, Mo, Hf)/CoFeB, etc., which is equivalent to the first free layer in the patent of the present invention.
  • the critical dimension of the magnetic tunnel junction has become smaller and smaller.
  • the thermal stability (Thermal Stability Factor) of the magnetic tunnel junction will be found to deteriorate sharply.
  • the thickness of the free layer can be generally reduced, the saturation susceptibility of the free layer can be reduced, or the interface anisotropy can be increased. If the thickness of the free layer is reduced, the tunneling magnetoresistance (Tunnel Magnetoresistance Rational, TMR) will decrease, which will increase the error rate of the read operation. Under the condition of the same thickness, add or remove the free layer in the free layer. Changing the layer to a material with a low saturation magnetic susceptibility will also reduce the TMR, which is not conducive to the read operation of the device.
  • TMR tunneling magnetoresistance Rational
  • the present invention provides a magnetic random access memory storage unit and a magnetic random access memory with a double-layer free layer.
  • a second free layer is interspersed between the first free layer and the cover layer of Magnetic Radom Access Memory.
  • the present invention provides a magnetic random access memory cell with a double-layer free layer, including a reference layer, a barrier layer, a first free layer, and a second free layer, and a vertical coupling thereunder. Layer and the magnetic damping barrier layer thereon, the magnetization vector in the second free layer is always perpendicular to the interface of the first free layer and parallel to the magnetization vector in the first free layer;
  • the first free layer includes a first free layer (I), a first free layer (II) and a first free layer (III) that are stacked, and the vertical coupling layer is provided on the first free layer and the second free layer. Between the free layers, the perpendicular magnetic coupling layer is used to achieve magnetic coupling between the first free layer and the second free layer.
  • the second free layer material is selected from Fe, Co, Ni, CoFe, FeB, CoB, W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt or CoFeB.
  • the second free layer has CoFeB, CoFe/CoFeB, Fe/CoFeB, CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os , Ru, Rh, Ir, Pd, Pt)/CoFeB, Fe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh , Ir, Pd, Pt)/CoFeB, CoFe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd , Pt)/CoFeB or interspersing non-magnetic metals W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru,
  • the second free layer has CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt )/CoFeB structure
  • the thickness of the first layer of CoFeB is 0.2-1.4nm
  • the atomic ratio of Co:Fe is 1:3 to 3:1
  • the atomic percentage of B is 15%-40%
  • the second layer is non-magnetic
  • the metal is W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd and/or Pt
  • the thickness is 0.1-0.6nm
  • the thickness of the third layer of CoFeB is 0.2-1.0 nm, the atomic ratio of Co:Fe is 1:3 to 3:1, and the atomic percentage of B is 15%-40%;
  • the total thickness of the second free layer is 0.5-2 nm.
  • the barrier layer is made of a non-magnetic metal oxide, and the non-magnetic metal oxide includes MgO, MgZn x O y , MgB x O y or MgAl x O y .
  • the first free layer has a variable magnetic polarization
  • the first free layer (260) has CoFeB, CoFe/CoFeB, Fe/CoFeB, CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt)/CoFeB, Fe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt)/CoFeB or CoFe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt)/CoFeB or CoFe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr
  • the present invention provides a magnetic random access memory, including the memory cell as described above, and further including a bottom electrode, a seed layer, an antiparallel ferromagnetic superlattice layer, a lattice barrier layer, a cover layer, and a top electrode,
  • the top electrodes are arranged sequentially in layers.
  • the antiparallel ferromagnetic superlattice layer includes a lower ferromagnetic superlattice layer, an antiparallel ferromagnetic coupling layer and an upper ferromagnetic layer, and the antiparallel ferromagnetic superlattice layer has [Co/Pt] n Co/(Ru, Ir, Rh), [Co/Pt] n Co/(Ru, Ir, Rh)/(Co, Co[Pt/Co] m ), [Co/Pd] n Co/(Ru, Ir, Rh), [Co/Pt] n Co/(Ru, Ir, Rh)/(Co, Co[Pd/Co] m ), [Co/Ni] n Co/(Ru, Ir, Rh) or [ Co/Ni] n Co/(Ru, Ir, Rh)/(Co, Co[Ni/Co] m ) superlattice structure.
  • the bottom electrode is made of Ti, TiN, Ta, TaN, W, WN or a combination thereof;
  • the top electrode is made of Ta, TaN, Ti, TiN, W, WN or a combination thereof.
  • the seed layer is made of Ta, Ti, TiN, TaN, W, WN, Ru, Pt, Ni, Cr, NiCr, CrCo, CoFeB or a combination of materials, and the seed layer has Ta/Ru, Ta /Pt or Ta/Pt/Ru multilayer structure;
  • the lattice barrier layer is made of Ta, W, Mo, Hf, Fe, Co (Ta, W, Mo or Hf), Fe (Ta, W, Mo or Hf), FeCo (Ta, W, Mo or Hf) or Made of FeCoB (Ta, W, Mo or Hf);
  • the covering layer is made of W, Mo, Mg, Nb, Ru, Hf, V, Cr or Pt material, and the covering layer has a (W, Mo, Hf)/Ru double-layer structure or Pt/(W, Mo ,Hf)/Ru three-layer structure.
  • the seed layer, the antiparallel ferromagnetic superlattice layer, the lattice barrier layer, the reference layer, the barrier layer, the first free layer, the vertical coupling layer, the magnetic damping barrier layer, and the covering layer After the top electrode is deposited, an annealing operation is performed at a temperature of 400° C. for at least 90 minutes.
  • the magnetic random access memory memory cell with a thermal stability enhancement layer provided by the present invention can produce the following beneficial effects: the addition of an additional second free layer in the present invention does not affect TMR, increases the thickness of the free layer, and reduces the damping coefficient , Increase the thermal stability factor, and the critical write current will not increase.
  • the second free layer and the first free layer exhibit ferromagnetic coupling. If the magnetization vector of the free layer is to be reversed under thermal disturbance or an external magnetic field, it must provide greater energy barrier and thermal stability enhancement layer energy than the free layer The energy of the sum of the barriers greatly improves the thermal stability;
  • a non-magnetic metal layer is deposited before and after the second free layer is deposited, the material of which is: MgO, MgZn x O y, MgB x O y , MgAl x O y , the thickness of which is 0.3nm ⁇ 1.5nm respectively And 0.5nm ⁇ 3.0nm.
  • This can provide an additional source of interface anisotropy, thereby further increasing thermal stability; in addition, due to the addition of the magnetic damping barrier layer on the second free layer, the damping coefficient of the entire film structure is effectively reduced, which is beneficial to Reduction of write current;
  • the addition of the second free layer increases the thickness of the free layer, which is conducive to the reduction of the damping coefficient, so that the critical writing current will not increase.
  • FIG. 1 is a schematic structural diagram of a magnetic random access memory storage unit in the prior art
  • FIG. 2 is a schematic diagram of the structure of a magnetic random access memory storage unit provided by an embodiment of the present invention
  • FIG. 3 is a schematic structural diagram of a magnetic random access memory storage unit provided by a preferred embodiment of the present invention.
  • FIG. 4 is a schematic diagram of the comparison of the flipping behavior of the free layer under an external magnetic field before and after the addition of the second free layer according to an embodiment of the present invention.
  • the reference numerals include: 110-bottom electrode, 210-seed layer, 220-antiparallel ferromagnetic superlattice layer, 221-lower ferromagnetic layer, 222-antiparallel ferromagnetic coupling layer, 223-upper ferromagnetic layer , 230-lattice barrier layer, 240-reference layer, 250-barrier layer, 260-first free layer, 261-first free layer (I), 262-first free layer (II), 263-first Free layer (III), 271-vertical coupling layer, 272-second free layer, 272a-second free layer (I), 272b-second free layer (II), 272c-second free layer (III), 273 -Magnetic damping barrier layer, 280-cover layer, 310-top electrode.
  • a magnetic random access memory memory cell with a double free layer is provided.
  • the memory cell includes a stacked reference layer 240, a barrier layer 250, a first free layer 260, and a second free layer 272, and a vertical coupling layer 271 underneath and a magnetic damping barrier layer 273 thereon.
  • the magnetization vector in the second free layer 273 is always perpendicular to the first free layer 260 and parallel to the magnetization vector in the first free layer 260;
  • the first free layer 260 includes a first free layer (I) 261, a first free layer (II) 262, and a second free layer (III) 263 that are stacked, and the vertical coupling layer 271 is disposed on the first Between the free layer 260 and the second free layer 272, the vertical coupling layer 271 is used to achieve magnetic coupling between the first free layer 260 and the second free layer 272.
  • a magnetic random access memory which includes the memory cell as described above, and further includes a bottom electrode 110, a seed layer 210, an antiparallel ferromagnetic superlattice layer 220, and a lattice partition
  • the layer 230, the cover layer 280 and the top electrode 310, the bottom electrode 110, the seed layer 210, the antiparallel ferromagnetic superlattice layer 220, the lattice barrier layer 230, the reference layer 240, the barrier layer 250, the first free layer 260, the vertical coupling layer 271, the second free layer 272, the magnetic damping barrier layer 273, the cover layer 280, and the top electrode 310 are sequentially stacked.
  • the bottom electrode 110 is composed of Ti, TiN, Ta, TaN, W, WN, or a combination thereof, which is generally realized by physical vapor deposition (Physical Vapor Deposition, PVD), which is usually flattened after deposition. Treatment to achieve the surface flatness of the magnetic tunnel junction.
  • PVD Physical Vapor Deposition
  • the seed layer 210 is generally composed of Ta, Ti, TiN, TaN, W, WN, Ru, Pt, Ni, Cr, CrCo, CoFeB or a combination thereof, and further, it may be Ta/Ru, Ta/Pt or Ta/ Multi-layer structure such as Pt/Ru. It is used to optimize the crystal structure of the subsequent antiferromagnetic layer 220.
  • Anti-Parallel Magnetic Supper-lattice 220 is also called Synthetic Anti-Ferrimagnet (SyAF) generally composed of [Co/Pt] n Co/(Ru, Ir, Rh) , [Co/Pt] n Co/(Ru,Ir,Rh)/(Co,Co[Pt/Co] m ), [Co/Pd] n Co/(Ru,Ir,Rh), [Co/Pt] n Co/(Ru, Ir, Rh)/(Co, Co[Pd/Co] m ), [Co/Ni] n Co/(Ru, Ir, Rh) or [Co/Ni] n Co/(Ru, Ir, Rh)/(Co, Co[Ni/Co] m ) superlattice composition, the antiparallel ferromagnetic superlattice layer 220 has strong perpendicular anisotropy (PMA).
  • PMA perpendicular anisotropy
  • the reference layer 240 has magnetic polarization invariance under the ferromagnetic coupling of the antiparallel ferromagnetic superlattice layer 220, and its constituent materials are generally Co, Fe, Ni, CoFe, CoFeB, or a combination thereof. Since the antiparallel ferromagnetic superlattice layer 120 has a face-centered cubic (FCC) crystal structure, and the reference layer 140 has a body-centered cubic (BCC) crystal structure, the crystal lattices are not matched.
  • FCC face-centered cubic
  • BCC body-centered cubic
  • a lattice barrier layer 230 is generally added between the two layers of materials, and its constituent materials are generally Ta, W, Mo, Hf, Fe, Co (Ta, W, Mo or Hf), Fe (Ta, W, Mo or Hf), FeCo (Ta, W, Mo or Hf) or FeCoB (Ta, W, Mo or Hf), etc.
  • the barrier layer 250 is a non-magnetic metal oxide, preferably MgO, MgZn x O y , MgB x O y or MgAl x O y . Furthermore, MgO can be selected.
  • the first free layer 260 has a variable magnetic polarization, generally composed of CoFeB, CoFe/CoFeB, Fe/CoFeB, CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt)/CoFeB, Fe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt)/CoFeB or CoFe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt)/CoFeB and CoFe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc
  • CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh , Ir, Pd, Pt)/CoFeB structure represents a three-layer structure, the first and third layers are made of CoFeB material, the middle layer is made of W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta , Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd or Pt materials, the following structural expressions are similar, and the explanation is not repeated.
  • the second free layer 273 has the same magnetization direction as the first free layer 260, and its composition material is similar to that of the free layer, and is generally composed of Fe, Co, Ni, CoFe, FeB, CoB, W, Mo, V, Nb, Cr, One or more of Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd, Pt or CoFeB, etc.
  • the specific structure can be CoFeB, CoFe/CoFeB, Fe/CoFeB, CoFeB/(W, Mo, V, Nb, Cr, Hf)/CoFeB, Fe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn , Ru, Os, Ru, Rh, Ir, Pd, Pt)/CoFeB, CoFe/CoFeB/(W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os , Ru, Rh, Ir, Pd, Pt)/CoFeB or interspersing non-magnetic metals W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta between CoFeB, CoFe/CoFeB, Fe/CoFeB many times, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, P
  • the second free layer 272 includes two free layers (I) 272a, a second free layer (II) 272b and a second free layer Layer (III) 272c, wherein the thickness of the first layer of CoFeB is 0.2nm ⁇ 1.4nm, the atomic ratio of Co:Fe can be adjusted from 3:1 to 1:3, and the atomic percentage of B is 15% ⁇ 40 %, the second layer of non-magnetic metals is W, Mo, V, Nb, Cr, Hf, Ti, Zr, Ta, Sc, Y, Zn, Ru, Os, Ru, Rh, Ir, Pd and/or Pt, etc.
  • the thickness of the third layer of CoFeB is 0.2nm ⁇ 1.0nm
  • the atomic ratio of Co:Fe can be adjusted from 3:1 to 1:3
  • the atomic percentage of B is 15% ⁇ 40%
  • the effect of the thermal stability enhancement layer can be optimized by changing PVD parameters such as the deposition power or air pressure, and can be selectively modified by plasma etching after the second layer of CoFeB.
  • a non-magnetic metal layer is deposited before and after the second free layer 272 is deposited, and its material is: MgZn x O y, MgB x O y , MgAl x O y or a combination of them, preferably, you can choose
  • the thickness of MgO is 0.3nm ⁇ 1.5nm and 0.5nm ⁇ 3.0nm respectively. This can also provide a source of interface anisotropy, thereby increasing thermal stability.
  • the damping coefficient of the entire film structure is effectively reduced, which is beneficial to the reduction of the writing current.
  • Fig. 4 is a better implementation case of the present invention.
  • Ms*t increases It is equivalent to the precondition that Hk and Ms remain unchanged.
  • the thickness of the free layer is increased, thereby increasing the thermodynamic barrier of free flip.
  • the covering layer 280 is composed of W, Mo, Mg, Nb, Ru, Hf, V, Cr or Pt, etc., preferably (W, Mo, Hf)/Ru or /Pt/(W, Mo, Hf)/ Ru structure.
  • the top electrode 290 can be selected from Ta, TaN, TaN/Ta, Ti, TiN, TiN/Ti, W, WN, WN/W, or a combination thereof.
  • an annealing at a temperature of 400° C. for 90 minutes is selected to make the reference layer, the first free layer and the second free layer change from an amorphous phase to a body-centered cubic (BCC) crystal structure.
  • BCC body-centered cubic
  • the present invention provides a magnetic random access memory thermal stability enhancement layer.
  • PVD Physical Vapor Deposition
  • the magnetization vector is always perpendicular to the free layer and parallel to the magnetization vector of the first free layer. Because the second free layer and the first free layer exhibit ferromagnetic coupling, under thermal disturbance or external magnetic field conditions , In order to reverse the magnetization vector of the first free layer, it is necessary to provide energy greater than the sum of the energy barrier of the first free layer and the energy barrier of the second free layer.
  • a non-magnetic metal layer is deposited before and after the second free layer is deposited.
  • Its material is: MgO, MgZn x O y , MgAl x O y , Mg or their combination, etc., so that an additional one can be provided
  • the source of interface anisotropy which increases thermal stability.
  • the damping coefficient of the entire film structure is effectively reduced, which is beneficial to the reduction of the writing current.
  • the thickness of the free layer is increased, which is conducive to the reduction of the damping constant ( ⁇ ), and at the same time, the choice of the first free layer/second free layer coupling
  • materials with a low damping coefficient can be preferred, which can further reduce the damping coefficient.
  • Data Retention the data retention capacity
  • ⁇ is the time for the magnetization vector to remain unchanged under thermal disturbance conditions
  • ⁇ 0 is the trial time (typically 1 ns)
  • E is the energy barrier of the free layer
  • k B is the Boltzmann constant
  • T is the operating temperature.
  • the thermal stability factor (Thermal Stability factor) can be expressed as the following formula:
  • K eff is the effective isotropic energy density of the free layer
  • V is the volume of the free layer
  • K V is the volume anisotropy constant
  • M s is the saturation susceptibility of the free layer
  • N z is the demagnetization constant in the vertical direction
  • t is the free
  • K i is the interface anisotropy constant
  • CD is the critical dimension of the magnetic random access memory (ie: the diameter of the free layer)
  • a s is the stiffness integral exchange constant
  • k is the free layer flip mode from the magnetic domain flip (ie :Magnetization switching processed by “macro-spin” switching) to reverse domain nucleation/growth (ie: Magnetization switching processed by nucleation of a reversed domain and propagation of a domain wall)
  • the critical size for the mode transition is the critical size for the mode transition.
  • the critical current I c0 of the writing operation is strongly related to thermal stability, and the relationship can be expressed as the following formula:
  • ⁇ is the damping constant
  • Is the reduced Planck constant
  • ⁇ is the spin polarizability
  • the addition of the additional second free layer of the present invention does not affect the TMR, increases the thickness of the free layer, reduces the damping coefficient, and increases the thermal stability factor, while the critical writing current does not increase.

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Abstract

本发明公开了一种磁性随机存储器存储单元及磁性随机存储器，存储单元包括层叠设置的参考层、势垒层、第一自由层，还包括第二自由层，以及其下的垂直磁耦合层和其上的磁阻尼阻挡层，第二自由层中的磁化矢量始终垂直于第一自由层，并与第一自由层中的磁化矢量平行；垂直磁耦合层用于实现第一自由层与第二自由层的强磁性耦合，并提供额外的垂直界面各项异性来源；磁阻尼阻挡层提供额外的各向异性来源，并同时减少膜层的磁阻尼系数。本发明中额外的第二自由层的加入，并不会影响TMR，增加了自由层的厚度，降低磁阻尼系数，增加热稳定性因子，而临界写电流并不会增加。

Description

一种磁性随机存储器存储单元及磁性随机存储器 技术领域
本发明涉及磁性随机存储器领域，特别涉及一种具有双层自由层的磁性随机存储器存储单元及磁性随机存储器。
背景技术
近年来，采用磁性隧道结(Magnetic Tunnel Junction,MTJ)的MRAM被人们认为是未来的固态非易失性记忆体，它具有高速读写、大容量以及低能耗的特点。铁磁性MTJ通常为三明治结构，其中有磁性记忆层(自由层)，它可以改变磁化方向以记录不同的数据；位于中间的绝缘隧道势垒层；磁性参考层，位于隧道势垒层的另一侧，它的磁化方向不变。
为能在这种磁电阻元件中记录信息，建议使用基于自旋动量转移或称自旋转移矩(STT，Spin Transfer Torque)转换技术的写方法，这样的MRAM称为STT-MRAM。根据磁极化方向的不同，STT-MRAM又分为面内STT-MRAM和垂直STT-MRAM(即pSTT-MRAM)，后者有更好的性能。在具有垂直各向异性(PMA)的磁性隧道结(MTJ)中，作为存储信息的自由层，在垂直方向拥有两个磁化方向，即：向上和向下，分别对应二进制中的“0”和“1”。在实际应用中，在读取信息或者空置的时候，自由层的磁化方向保持不变；在写的过程中，如果有与现有不同状态的信号输入的时候，那么自由层的磁化方向将会在垂直方向上发生180度的翻转。业界把这种空置状态之下，磁性存储器的自由层保持磁化方向不变得能力叫做数据保存能力(Data Retention)或者热稳定性(Thermal Stability)。在不同的应用场景中要求不一样。对于一个典型的非易失存储器(Non-volatile Memory，NVM)的热稳定性要求是在125℃的条件可以保存数 据10年。
另外，作为磁性存储器(MRAM)的核心存储单元的MTJ，还必须和CMOS工艺相兼容，必须能够承受在400℃条件下的长时间退火。
图1为现有的磁性随机存储器存储单元的结构示意图。自由层一般由CoFeB、CoFe/CoFeB、Fe/CoFeB或CoFeB/(Ta，W，Mo，Hf)/CoFeB等组成，相当于本发明专利中的第一自由层，为了提高磁性存储器的密度，近年来，磁性隧道结的关键尺寸(Critical Dimension)做得越来越小。当尺寸进一步缩小时，会发现磁性隧道结的热稳定性(Thermal Stability Factor)急剧变差。对于超小尺寸的MRAM磁性存储单元而言，为了提高热稳定，通常可以降低自由层的厚度，降低自由层的饱和磁化率或者增加界面各向异性。如果降低自由层的厚度，则隧穿磁阻率(Tunnel Magnetoresistance Ration，TMR)将会降低，这将会增加读操作时候错误率；在厚度不变的条件下，在自由层里添加或把自由层改为低饱和磁化率的材料，同样会使TMR降低，不利于器件的读操作。
发明内容
为了解决现有技术的问题，本发明提供了一种具有双层自由层的磁性随机存储器存储单元及磁性随机存储器，在具有垂直各向异性(Perpendicular Magnetic Anisotropy，PMA)的磁性随机存储器(MRAM，Magnetic Radom Access Memory)的第一自由层和覆盖层之间穿插一层第二自由层，技术方案如下：
一方面，本发明提供了一种具有双层自由层的磁性随机存储器存储单元，包括层叠设置的参考层、势垒层、第一自由层，还包括第二自由层，以及其下的垂直耦合层和其上的磁阻尼阻挡层，所述第二自由层中的磁化矢量始终垂直于第一自由层界面，并与第一自由层中的磁化矢量平行；
所述第一自由层包括层叠设置的第一自由层(I)、第一自由层(II)和第一自由层(III)，所述垂直耦合层设置在所述第一自由层与第二自由层之间，所述垂直磁耦合层用于实现所述第一自由层与第二自由层的磁性耦合。
进一步地，所述第二自由层材料选自Fe,Co，Ni，CoFe，FeB，CoB，W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt或CoFeB。
进一步地，所述第二自由层具有CoFeB、CoFe/CoFeB、Fe/CoFeB、CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB，Fe/CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB、CoFe/CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB或在CoFeB、CoFe/CoFeB、Fe/CoFeB之间多次穿插非磁性金属W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd或Pt的结构。
进一步地，所述第二自由层具有CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB结构，第一层的CoFeB的厚度为0.2-1.4nm，Co：Fe的原子比例为1:3至3:1，B的原子百分比为15％-40％；第二层的非磁性金属为W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd和/或Pt，其厚度为0.1-0.6nm；第三层的CoFeB的厚度为0.2-1.0nm，Co：Fe的原子比例为1:3至3:1，B的原子百分比为15％-40％；
所述第二自由层的总厚度为0.5-2nm。
进一步地，所述势垒层由非磁性金属氧化物制成，所述非磁性金属氧化物包括MgO，MgZn xO y，MgB xO y或MgAl xO y。
进一步地，所述第一自由层为可变磁极化，所述第一自由层(260)具有CoFeB、CoFe/CoFeB、Fe/CoFeB、CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB、Fe/CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB或CoFe/CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB结构。
另一方面，本发明提供了一种磁性随机存储器，包括如上所述的存储单元，还包括底电极、种子层、反平行铁磁超晶格层、晶格隔断层、覆盖层及顶电极， 所述底电极、种子层、反平行铁磁超晶格层、晶格隔断层、参考层、势垒层、第一自由层、自由层铁磁耦合层、磁阻尼阻挡层、覆盖层及顶电极顺序层叠设置。
进一步地，所述反平行铁磁超晶格层包括下铁磁超晶格层、反平行铁磁耦合层和上铁磁层，所述反平行铁磁超晶格层具有[Co/Pt] nCo/(Ru，Ir，Rh)、[Co/Pt] nCo/(Ru，Ir，Rh)/(Co，Co[Pt/Co] m)、[Co/Pd] nCo/(Ru，Ir，Rh)、[Co/Pt] nCo/(Ru，Ir，Rh)/(Co，Co[Pd/Co] m)、[Co/Ni] nCo/(Ru，Ir，Rh)或[Co/Ni] nCo/(Ru，Ir，Rh)/(Co，Co[Ni/Co] m)超晶格结构。
进一步地，所述底电极由Ti、TiN、Ta、TaN、W、WN或其组合材料制成；
所述顶电极由Ta、TaN、Ti、TiN、W、WN或其组合材料制成。
进一步地，所述种子层由Ta、Ti、TiN、TaN、W、WN、Ru、Pt、Ni、Cr、NiCr、CrCo、CoFeB或其组合材料制成，所述种子层具有Ta/Ru、Ta/Pt或Ta/Pt/Ru的多层结构；
所述晶格隔断层由Ta，W，Mo，Hf，Fe，Co(Ta，W，Mo或Hf)，Fe(Ta，W，Mo或Hf)，FeCo(Ta，W，Mo或Hf)或FeCoB(Ta，W，Mo或Hf)制成；
所述覆盖层由W、Mo、Mg、Nb、Ru、Hf、V、Cr或Pt材料制成，所述覆盖层具有(W,Mo,Hf)/Ru双层结构或Pt/(W,Mo,Hf)/Ru三层结构。
进一步地，在所述底电极、种子层、反平行铁磁超晶格层、晶格隔断层、参考层、势垒层、第一自由层、垂直耦合层、磁阻尼阻挡层、覆盖层和顶电极沉积之后，在400℃的温度下进行至少90分钟的退火操作。
本发明提供的具有热稳定性增强层的磁性随机存储器存储单元能够产生以下有益效果：本发明中额外的第二自由层的加入，并不会影响TMR，增加了自由层的厚度，降低阻尼系数，增加热稳定性因子，而临界写电流并不会增加。
a.第二自由层和第一自由层呈现铁磁耦合，在热扰动或者外加磁场条件下，要想使自由层磁化矢量发生翻转，就必须提供大于自由层能量壁垒和热稳定性增强层能量壁垒之和的能量，大大提高了热稳定性；
b.第二自由层的加入，对TMR没有影响；
c.在沉积第二自由层之前和之后都会沉积一层非磁性金属层，其材料为：MgO,MgZn xO y,MgB xO y，MgAl xO y，其厚度分别为0.3nm～1.5nm和0.5nm～3.0nm。这样可以额外提供一个界面各向异性的来源，从而进一步增加了热稳定；另外，由于第二自由层之上的磁阻尼阻挡层的加入，有效降低了整个膜层结构的阻尼系数，有利于写电流的降低；
d.能够经受得住400℃下的长时间退火；
e.第二自由层的加入，增加了自由层的厚度，有利于在阻尼系数的降低，从而临界写电流并不会增加。
附图说明
为了更清楚地说明本发明实施例中的技术方案，下面将对实施例描述中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本发明的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他的附图。
图1为现有技术中的磁性随机存储器存储单元的结构示意图；
图2为本发明实施例提供的磁性随机存储器存储单元结构示意图；
图3为本发明优选实施例提供的磁性随机存储器存储单元的结构示意图；
图4是本发明实施例提供的第二自由层添加前后，在外磁性场下自由层的翻转行为对比示意图。
其中，附图标记包括：110-底电极，210-种子层，220-反平行铁磁超晶格层，221-下铁磁层，222-反平行铁磁耦合层，223-上铁磁层，230-晶格隔断层，240-参考层，250-势垒层，260-第一自由层，261-第一自由层(I)，262-第一自由层(II)，263-第一自由层(III)，271-垂直耦合层，272-第二自由层，272a-第二自由层(I)，272b-第二自由层(II)，272c-第二自由层(III)， 273-磁阻尼阻挡层，280-覆盖层，310-顶电极。
具体实施方式
为了使本技术领域的人员更好地理解本发明方案，下面将结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例仅仅是本发明一部分的实施例，而不是全部的实施例。基于本发明中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都应当属于本发明保护的范围。
需要说明的是，本发明的说明书和权利要求书及上述附图中的术语“第一”、“第二”等是用于区别类似的对象，而不必用于描述特定的顺序或先后次序。应该理解这样使用的数据在适当情况下可以互换，以便这里描述的本发明的实施例能够以除了在这里图示或描述的那些以外的顺序实施。此外，术语“包括”和“具有”以及他们的任何变形，意图在于覆盖不排他的包含，例如，包含了一系列步骤或单元的过程、方法、装置、产品或设备不必限于清楚地列出的那些步骤或单元，而是可包括没有清楚地列出的或对于这些过程、方法、产品或设备固有的其它步骤或单元。
在本发明的一个实施例中，提供了一种具有双层自由层的磁性随机存储器存储单元，在MRAM磁性隧道结多层膜的物理气相沉积(Physical Vapor Deposition,PVD)的过程中，在不隔断真空的条件下，在第一自由层顶部(free layer)和覆盖层(capping layer)之间穿插一层第二自由层，如图2所示，所述具有双层自由层的磁性随机存储器存储单元包括层叠设置的参考层240、势垒层250、自第一自由层260，还包括第二自由层272，以及其下的垂直耦合层271和其上的磁阻尼阻挡层273，所述第二自由层273中的磁化矢量始终垂直于第一自由层260，并与第一自由层260中的磁化矢量平行；
所述第一自由层260包括层叠设置的第一自由层(I)261、第一自由层(II)262和第二自由层(III)263，所述垂直耦合层271设置在所述第一自由层260与第二自由层272之间，所述垂直耦合层271用于实现所述第一自由层260与第 二自由层272的磁性耦合。
在本发明的一个较佳实施例中，提供了一种磁性随机存储器，包括如上所述的存储单元，还包括底电极110、种子层210、反平行铁磁超晶格层220、晶格隔断层230、覆盖层280及顶电极310，所述底电极110、种子层210、反平行铁磁超晶格层220、晶格隔断层230、参考层240、势垒层250、第一自由层260、垂直耦合层271、第二自由层272、磁阻尼阻挡层273、覆盖层280及顶电极310顺序层叠设置。
其中，底电极110组成材料为Ti，TiN，Ta，TaN，W,WN或者它们的组合，一般采用物理气相沉积(Physical Vapor Deposition,PVD)的方式实现，通常在沉积之后，都会对其平坦化处理，以达到制作磁性隧道结的表面平整度。
种子层210一般由Ta,Ti，TiN，TaN，W，WN，Ru，Pt，Ni，Cr,CrCo，CoFeB或它们的组合构成，更进一步地，可以是Ta/Ru，Ta/Pt或Ta/Pt/Ru等多层结构。用以优化后续的反铁磁层220的晶体结构。
反平行铁磁超晶格层(Anti-Parallel Magnetic Supper-lattice)220也叫合成反铁磁层(Synthetic Anti-Ferrimagnet，SyAF)一般由[Co/Pt] nCo/(Ru，Ir，Rh)、[Co/Pt] nCo/(Ru，Ir，Rh)/(Co，Co[Pt/Co] m)、[Co/Pd] nCo/(Ru，Ir，Rh)、[Co/Pt] nCo/(Ru，Ir，Rh)/(Co，Co[Pd/Co] m)、[Co/Ni] nCo/(Ru，Ir，Rh)或[Co/Ni] nCo/(Ru，Ir，Rh)/(Co，Co[Ni/Co] m)超晶格组成，反平行铁磁超晶格层220具有很强的垂直各向异性(PMA)。
参考层240在反平行铁磁超晶格层220的铁磁耦合下，具有磁极化不变性，其组成材料一般为Co，Fe,Ni，CoFe，CoFeB或它们的组合等。由于反平行铁磁超晶格层120具有面心立方(FCC)晶体结构，而参考层140的晶体结构为体心立方(BCC)，晶格并不匹配，为了实现从反平行铁磁超晶格层220到参考层240的过渡和铁磁耦合，一般会在两层材料之间添加一层晶格隔断层230，其组成材料一般为Ta，W，Mo，Hf，Fe，Co(Ta，W，Mo或Hf)，Fe(Ta，W，Mo或Hf)，FeCo(Ta，W，Mo或Hf)或FeCoB(Ta，W，Mo或Hf)等。
势垒层250为非磁性金属氧化物，优选MgO，MgZn xO y，MgB xO y或MgAl xO y。 更进一步可以选择MgO。
第一自由层260具有可变磁极化，一般由CoFeB、CoFe/CoFeB、Fe/CoFeB，CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB，Fe/CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB或CoFe/CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB等组成，更进一步地可以选择CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB，Fe/CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB或CoFe/CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB结构。以第一自由层结构为例进行说明，在本领域中，CoFeB/(W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB结构表示三层结构，第一层和第三层结构均由CoFeB材料制成，中间层由W，Mo，V,Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd或Pt材料制成，下述结构表述方式类同，不再赘述解释说明。
第二自由层273具有和第一自由层260相同的磁化方向，其组成材料也和自由层类似，一般由Fe,Co，Ni，CoFe，FeB，CoB，W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt或CoFeB等中其中一种或几种组成，其具体结构可以是CoFeB，CoFe/CoFeB、Fe/CoFeB，CoFeB/(W，Mo，V,Nb，Cr，Hf)/CoFeB，Fe/CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB，CoFe/CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB或在CoFeB，CoFe/CoFeB、Fe/CoFeB之间多次穿插非磁性金属W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，和/或Pt等，其总厚度为0.5nm到2nm，在具体工艺过程中，通过调整PVD沉积条件改变材料组成成分，并可添加等离子刻蚀工艺来对材料进行改性以获得最优的性能。
更进一步地，如图3所示，在本发明的一个较优实施案例中，选用CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd， Pt)/CoFeB做为第二自由层，所述第二自由层272包括二自由层(I)272a、第二自由层(II)272b和第二自由层(III)272c，其中，第一层CoFeB的厚度为0.2nm～1.4nm，Co：Fe的原子比例可以从3:1到1:3之间进行调整，B的原子百分比为15％～40％，第二层非磁性金属为W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd和/或Pt等，其厚度为0.1nm～0.6nm，第三层CoFeB的厚度为0.2nm～1.0nm，Co：Fe的原子比例可以从3:1到1:3之间进行调整，B的原子百分比为15％～40％，通过改变沉积的功率或气压等PVD参数使热稳定性增强层的效果达到最佳，并可以选择性地在第二层CoFeB之后进行等离子刻蚀处理来进行改性。
通常，在沉积第二自由层272之前和之后都会沉积一层非磁性金属层，其材料为：由MgZn xO y,MgB xO y，MgAl xO y或它们组合，更优地，可以选择MgO，其厚度分别是为0.3nm～1.5nm和0.5nm～3.0nm。这样同时也可以为提供一个界面各向异性的来源，从而增加了热稳定。另外，由于第二自由层272之上的磁阻尼阻挡层273的加入，有效降低了整个膜层结构的阻尼系数，有利于写电流的降低。
图4为本发明的一个较优的实施案例，额外的第二自由层添加前后，在外磁性场下，自由层的翻转行为，可以明显的看出在第二自由层添加之后，Ms*t增加了很多，相当于在Hk，Ms不变的前提条件了，增加了自由层的厚度，从而增加了自由翻转的热力学势垒。
覆盖层280为W，Mo,Mg，Nb，Ru，Hf，V，Cr或Pt等组成，较优地可以选择(W,Mo,Hf)/Ru或/Pt/(W,Mo,Hf)/Ru等结构。
顶电极290可以选择Ta，TaN，TaN/Ta，Ti，TiN，TiN/Ti，W，WN，WN/W或它们的组合。
在所有膜层沉积之后，选择400℃的温度下，90分钟的退火，以使得参考层，第一自由层和第二自由层从非晶相变为体心立方(BCC)的晶体结构。
本发明提供的一种磁性随机存储器热稳定性增强层，在MRAM磁性隧道结多层膜的物理气相沉积(Physical Vapor Deposition,PVD)的过程中，在不隔断真空的条件下，在自由层顶部(free layer)和覆盖层(capping layer)之间穿 插一层二外的第二自由层。
在第二自由层之中，磁化矢量始终垂直于自由层，并和第一自由层的磁化矢量平行，由于第二自由层和第一自由层呈现铁磁耦合，在热扰动或者外加磁场条件下，要想使第一自由层磁化矢量发生翻转，就必须提供大于第一自由层能量壁垒和第二自由层能量壁垒之和的能量。
实验表明，额外的第二自由层的加入，并不会影响TMR。
同时，通常，在沉积第二自由层之前和之后都会沉积一层非磁性金属层，其材料为：MgO，MgZn xO y，MgAl xO y，Mg或它们的组合等，这样可以额外提供一个界面各向异性的来源，从而增加了热稳定。另外，由于第二自由层之上的磁阻尼阻挡层的加入，有效降低了整个膜层结构的阻尼系数，有利于写电流的降低。
由于在选择第一自由层材料和覆盖层材料的时候，成功避免了Ta及其氮化物，使其能够经受得住400℃下的长时间退火。
更进一步地，由于额外的第二自由层的加入，增加了自由层的厚度，有利于在阻尼系数(damping constant，α)的降低，同时，在选择第一自由层/第二自由层的耦合层和覆盖层的材料时候，可以优选阻尼系数低的材料，这样可以进一步降低了阻尼系数。在对器件进行写操作的时候，尽管热稳定性因子增加了，但是由于阻尼系数的降低，临界写电流并不会增加。
更进一步地，数据保存能力(Data Retention)可以用下面的公式进行计算：
Figure PCTCN2019093736-appb-000001
其中，τ为在热扰动条件下磁化矢量不变的时间，τ 0为尝试时间(一般为1ns)，E为自由层的能量壁垒，k B为玻尔兹曼常数，T为工作温度。
热稳定性因子(Thermal Stability factor)则可以表示为如下的公式：
Figure PCTCN2019093736-appb-000002
其中，K eff为自由层的有效各向能量密度，V为自由层的体积，K V为体各向异性常数M s为自由层饱和磁化率，N z垂直方向的退磁化常数，t为自由层的厚度，K i为界面各向异性常数，CD为磁性随机存储器的关键尺寸(即：自由层的直径)，A s为刚度积分交换常数，k为自由层翻转模式从磁畴翻转(即：Magnetization switching processed by“macro-spin”switching)到反向畴成核/长大(即：Magnetization switching processed by nucleation of a reversed domain and propagation of a domain wall)模式转变的临界尺寸。实验表明当自由层的厚度较厚时表现为面内各向异性，较薄时，表现为垂直各向异性，K V一般可以忽略不计，而退磁能对垂直各向异性的贡献为负值，因此垂直各向异性完全来自界面效应(K i)。
此外，随着磁性自由层的体积的缩减，写或转换操作需注入的自旋极化电流也越小，写操作的临界电流I c0和热稳定性强相关，其关系可以表达如下的公式：
Figure PCTCN2019093736-appb-000003
其中，α为阻尼系数(damping constant)，
Figure PCTCN2019093736-appb-000004
为约化普朗克常数，η为自旋极化率。
本发明额外的第二自由层的加入，并不会影响TMR，增加了自由层的厚度，降低阻尼系数，增加热稳定性因子，而临界写电流并不会增加。
以上所述仅为本发明的较佳实施例，并不用以限制本发明，凡在本发明的精神和原则之内，所作的任何修改、等同替换、改进等，均应包含在本发明的保护范围之内。

Claims (10)

  1. 一种具有双层自由层的磁性随机存储器存储单元，包括层叠设置的参考层(240)、势垒层(250)、第一自由层(260)、垂直耦合层(271)、第二自由层(272)、磁阻尼阻挡层(273)、覆盖层(280)，其特征在于，所述垂直耦合层(271)提供垂直界面各向异性及强铁磁耦合予第二自由层(272)的磁化矢量和第一自由层(260)中的磁化矢量，使其始终垂直于第一自由层(260)和第二自由层(272)的平面；
    所述磁阻尼阻挡层(273)设置在所述第二自由层(272)之上，提供一个垂直界面各向异性予第二自由层(272)的磁化矢量，并减少整个膜层的磁阻尼系数。
  2. 根据权利要求1所述的存储单元，其特征在于，所述垂直耦合层(271)为MgO,MgZn xO y,MgB xO y或MgAl xO y，其厚度为0.3nm～1.5nm。
  3. 根据权利要求1所述的存储单元，其特征在于，所述磁阻尼阻挡层(273)为MgO,MgZn xO y,MgB xO y或MgAl xO y，其厚度为0.5nm～3.0nm。
  4. 根据权利要求1所述的存储单元，其特征在于，所述势垒层(250)由非磁性金属氧化物制成，所述非磁性金属氧化物包括MgO,MgZn xO y,MgB xO y或MgAl xO y。
  5. 根据权利要求1所述的存储单元，其特征在于，所述第一自由层(260)具有CoFeB、CoFe/CoFeB、Fe/CoFeB、CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB、Fe/CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB 或CoFe/CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB结构。
  6. 根据权利要求1所述的存储单元，其特征在于，所述第二自由层(272)具有CoFeB、CoFe/CoFeB、Fe/CoFeB、CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB，Fe/CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB、CoFe/CoFeB/(W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd，Pt)/CoFeB或在CoFeB、CoFe/CoFeB、Fe/CoFeB之间多次穿插非磁性金属W，Mo，V，Nb，Cr，Hf，Ti，Zr，Ta，Sc，Y，Zn，Ru，Os，Ru，Rh，Ir，Pd或Pt的结构；
    所述第二自由层(272)的总厚度为0.5-2nm。
  7. 一种磁性随机存储器，其特征在于，包括如权利要求1-6中任意一项所述的存储单元，还包括底电极(110)、种子层(210)、反平行铁磁超晶格层(220)、晶格隔断层(230)、覆盖层(280)及顶电极(310)，所述底电极(110)、种子层(210)、反平行铁磁超晶格层(220)、晶格隔断层(230)、参考层(240)、势垒层(250)、第一自由层(260)、自由层铁磁耦合层(271)、第二自由层(272)、磁阻尼阻挡层(273)、覆盖层(280)及顶电极(310)顺序层叠设置。
  8. 根据权利要求6所述的磁性随机存储器，其特征在于，所述底电极(110)由Ti、TiN、Ta、TaN、W、WN或其组合材料制成；
    所述顶电极(290)由Ta、TaN、Ti、TiN、W、WN或其组合材料制成。
  9. 根据权利要求6所述的磁性随机存储器，其特征在于，所述种子层(210)由Ta、Ti、TiN、TaN、W、WN、Ru、Pt、Cr、Ni、NiCr、CrCo、CoFeB或其组合材料制成，所述种子层(210)具有Ta/Ru、Ta/Pt或Ta/Pt/Ru的多层结构；
    所述晶格隔断层(230)由Ta，W，Mo，Hf，Fe，Co(Ta，W，Mo或Hf)，Fe(Ta，W，Mo或Hf)，FeCo(Ta，W，Mo或Hf)或FeCoB(Ta，W，Mo或Hf)制成；
    所述覆盖层(280)由W、Mo、Mg、Nb、Ru、Hf、V、Cr或Pt材料制成，所述覆盖层具有(W,Mo,Hf)/Ru双层结构或Pt/(W,Mo,Hf)/Ru三层结构。
  10. 根据权利要求6所述的磁性随机存储器，其特征在于，在所述种子层(210)、反平行铁磁超晶格层(220)、晶格隔断层(230)、参考层(240)、势垒层(250)、第一自由层(260)、自由层铁磁耦合层(271)、第二自由层(272)、磁阻尼阻挡层(273)、覆盖层(280)沉积之后，在400℃的温度下进行至少90分钟的退火操作。
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