Abstract:
A perpendicular magnetoresistive element comprises a novel buffer layer having rocksalt crystal structure interfacing to a CoFeB-based recording layer has (100) plane parallel to the substrate plane and with {110} lattice parameter being slightly larger than the bcc CoFe lattice parameter along {100} direction, and crystallization process of amorphous CoFeB material in the recording layer during thermal annealing leads to form bcc CoFe grains having epitaxial growth with in-plane expansion and out-of-plane contraction. Accordingly, a perpendicular anisotropy, as well as a perpendicular magnetization, is induced in the recording layer. The invention preferably includes materials, configurations and processes of perpendicular magnetoresistive elements suitable for perpendicular spin-transfer torque MRAM applications.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of U.S. Provisional Application No. 61,740,764, filed Dec. 21, 2012, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to the field of perpendicular magnetoresistive elements. More specifically, the invention comprises perpendicular spin-transfer-torque magnetic-random-access memory (MRAM) using perpendicular magnetoresistive elements as basic memory cells which potentially replace the conventional semiconductor memory used in electronic chips, especially mobile chips for power saving and non-volatility. 
         [0004]    2. Description of the Related Art 
         [0005]    In recent years, magnetic random access memories (hereinafter referred to as MRAMs) using the magnetoresistive effect of ferromagnetic tunnel junctions (also called MTJs) have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can cope with high-speed reading and writing, large capacities, and low-power-consumption operations. A ferromagnetic tunnel junction has a three layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulating spacing layer, and a fixed layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction. 
         [0006]    As a write method to be used in such magnetoresistive elements, there has been suggested a write method (spin torque transfer switching technique) using spin momentum transfers. According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to the magnetoresistive element. Furthermore, as the volume of the magnetic layer forming the recording layer is smaller, the injected spin-polarized current to write or switch can be also smaller. Accordingly, this method is expected to be a write method that can achieve both device miniaturization and lower currents. 
         [0007]    Further, as in a so-called perpendicular MTJ element, both of the two magnetization films have easy axis of magnetization in a direction perpendicular to the film plane due to their strong magnetic crystalline anisotropy (shape anisotropies are not used), and accordingly, the device shape can be made smaller than that of an in-plane magnetization type. Also, variance in the easy axis of magnetization can be made smaller. Accordingly, by using a material having a large magnetic crystalline anisotropy, both miniaturization and lower currents can be expected to be achieved while a thermal disturbance resistance is maintained. 
         [0008]    There has been a known technique for achieving a high MR ratio by forming a crystallization acceleration film that accelerates crystallization and is in contact with an interfacial magnetic film having an amorphous structure. As the crystallization acceleration film is formed, crystallization is accelerated from the tunnel barrier layer side, and the interfaces with the tunnel barrier layer and the interfacial magnetic film are matched to each other. By using this technique, a high MR ratio can be achieved. However, where a MTJ is formed as a device of a perpendicular magnetization type, the materials of the recording layer typically used in an in-plane MTJ for both high MR and low damping constant as required by low write current application normally don&#39;t have enough magnetic crystalline anisotropy to achieve thermally stable perpendicular magnetization against its demagnetization field. In order to obtain perpendicular magnetization with enough thermal stability, the recording layer has to be ferromagnetically coupled to additional perpendicular magnetization layer, such as TbCoFe, or CoPt, or multilayer such as (Co/Pt)n, to obtain enough perpendicular anisotropy. Doing so, reduction in write current becomes difficult due to the fact that damping constant increases from the additional perpendicular magnetization layer and its associated seed layer for crystalline matching and material diffusion during the heat treatment in the device manufacturing process. 
         [0009]    In a spin-injection MRAM using a perpendicular magnetization film, a write current is proportional to the damping constant and inversely proportional to a spin polarization, and increases in proportional to a square of an area size. Therefore, reduction of the damping constant, increase of the spin polarization, maintain of the perpendicular anisotropy and reduction of an area size are mandatory technologies to reduce the write current. 
       BRIEF SUMMARY OF THE PRESENT INVENTION 
       [0010]    The present invention comprises perpendicular magnetoresistive element for perpendicular spin-transfer-torque MRAM. The perpendicular magnetoresistive element in the invention are sandwiched between an upper electrode and a lower electrode of each MRAM memory cell, which also comprises a write circuit which bi-directionally supplies a spin polarized current to the magnetoresistive element and a select transistor electrically connected between the magnetoresistive element and the write circuit. 
         [0011]    The invention includes a magnetoresistive element comprising: a recording layer having magnetic anisotropy in a direction perpendicular to a film surface and having a variable magnetization direction; a reference layer having magnetic anisotropy in a direction perpendicular to a film surface and having an invariable magnetization direction; a spacing layer provided between the recording layer and the reference layer; and a buffer layer provided on a surface of the recording layer, which is opposite to a surface of the recording layer where the spacing layer is provided, wherein at least the portion of the buffer layer interfacing to the recording layer contains a rocksalt crystal structure having the (100) plane parallel to the substrate plane and with lattice parameter along its {110} direction being slightly larger than the bcc (body-centered cubic)-phase Co lattice parameter along {100} direction; and a base layer provided on a surface of the buffer layer, which is opposite to a surface of the buffer layer where the recording layer is provided. 
         [0012]    As an amorphous ferromagnetic material, like CoFeB, in the recording layer is thermally annealed, a crystallization process occurs to form bcc CoFe grains having epitaxial growth with (100) plane parallel to surface of the rocksalt crystal buffer layers with in-plane expansion and out-of-plane contraction. Accordingly, a perpendicular anisotropy, as well as a perpendicular magnetization, is induced in the recording layer. The invention preferably includes materials, configurations and processes of perpendicular magnetoresistive elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view showing a configuration of an MTJ element  10  according to the first embodiment; 
           [0014]      FIG. 2  is a cross-sectional view showing a configuration of an MTJ element  10  according to the second embodiment; 
           [0015]      FIG. 3  is a cross-sectional view showing a configuration of an MTJ element  10  according to the third embodiment; 
           [0016]      FIG. 4  is a cross-sectional view showing a configuration of an MTJ element  10  according to the fourth embodiment; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    In general, according to one embodiment, there is provided a magnetoresistive element comprising:
       a recording layer having magnetic anisotropy in a direction perpendicular to a film surface and having a variable magnetization direction;   a reference layer having magnetic anisotropy in a direction perpendicular to a film surface and having an invariable magnetization direction;   a spacing layer provided between the recording layer and the reference layer;   a buffer layer provided on a surface of the recording layer, which is opposite to a surface of the recording layer where the spacing layer is provided, wherein at least the portion of the buffer layer interfacing to the recording layer contains a rocksalt crystal structure having the (100) plane parallel to the substrate plane and with lattice parameter along its {110} direction being larger than the Co lattice parameter along {100} direction in its bcc (body-centered cubic) phase.   and a base layer provided on a surface of the buffer layer, which is opposite to a surface of the buffer layer where the recording layer is provided.       
 
       First Embodiment 
       [0023]      FIG. 1  is a cross-sectional view showing a configuration of an MTJ element  10  as a MTJ element according to the first embodiment. The MTJ element  10  is configured by stacking an upper electrode  11 , a reference layer  12 , a spacing layer (tunnel barrier layer)  13 , a recording layer  14 , a buffer layer  15 , and a base layer  16  in this order from the top. 
         [0024]    The recording layer  14  and reference layer  12  each are made of a ferromagnetic material, and have uni-axial magnetic anisotropy in a direction perpendicular to a film surfaces. Further, directions of easy magnetization of the recording layer  14  and reference layer  12  are also perpendicular to the film surfaces. In another word, the MTJ element  10  is a perpendicular MTJ element in which magnetization directions of the recording layer  14  and reference layer  12  face in directions perpendicular to the film surfaces. A direction of easy magnetization is a direction in which the internal magnetic energy is at its minimum where no external magnetic field exists. Meanwhile, a direction of hard magnetization is a direction which the internal energy is at its maximum where no external magnetic field exists. 
         [0025]    The recording layer  14  has a variable (reversible) magnetization direction. The reference layer  12  has an invariable (fixing) magnetization direction. The reference layer  12  is made of a ferromagnetic material having a perpendicular magnetic anisotropic energy which is sufficiently greater than the recording layer  14 . This strong perpendicular magnetic anisotropy can be achieved by selecting a material, configuration and a film thickness. In this manner, a spin polarized current may only reverse the magnetization direction of the recording layer  14  while the magnetization direction of the reference layer  12  remains unchanged. An MTJ element  10  which comprises a recording layer  14  having a variable magnetization direction and a reference layer  12  having an invariable magnetization direction for a predetermined write current can be achieved. 
         [0026]    The spacing layer  13  is made of a non-magnetic material for which a non-magnetic insulating metal oxide or nitride can be used. 
         [0027]    The buffer layer  15  may serve to introduce or improve perpendicular magnetic anisotropy of the recording layer  14 . A damping constant of the recording layer  14  sometimes increases (deteriorates) depending on a material in contact with the recording layer  14 , which is known as a spin pumping effect. The buffer layer  15  may also have a function to prevent increase of the damping constant of the recording layer  14  by reducing the spin pumping. The buffer layer  15  is made of an oxide (or nitride, chloride) layer which has a rocksalt crystalline as its naturally stable structure thereof will be described later. 
         [0028]    An example configuration of the MTJ element  10  will be described below. The reference layer  12  is made of TbCoFe (around 10 nm)/CoFeB (around 2 nm). The spacing layer  13  is made of MgO (around 1 nm). The recording layer  14  is made of CoFeB (around 1.2 nm). The buffer layer  15  is made of MgZnO (around 0.8 nm). The base layer  16  is made of Ta (around 20 nm)/Cu (around 20 nm)/Ta (around 20 nm). Each element written in the left side of “/” is stacked above an element written in the right side thereof. 
         [0029]    Since a high resistance layer can be formed when the MgZnO buffer layer is used, a read output is caused to decrease when a read current flows across MgZnO buffer layer. A resistance of the MgZnO buffer layer can be reduced and decrease of the read output can accordingly be reduced by adopting a surface oxidization process, i.e. by using of a mixed gas containing natural oxygen (O.sub.2), or radical or ionized oxygen and Argon (Ar) after co-sputtering Mg and Zn metal layer. Such MgZnO composition contains less oxygen in the portion of the buffer layer facing to the base layer than the composition by sputtering of MgZnO or co-sputtering of Mg and Zn in a mixed gas containing oxygen (O.sub.2) and Argon (Ar). 
         [0030]    The CoFeB (with B content no less than 10% and no more than 30%) layer comprised in the recording layer  14  is formed into an amorphous state as deposited. The MgZnO layer comprised in the buffer layer  15  is formed into rocksalt crystal grains with the (100) plane parallel to the substrate plane. In the rocksalt crystal structure, two fcc sublattices for metal atom (Mg or Zn) and O, each displaced with respect to the other by half lattice parameter along the [100] direction. Its lattice parameter along the {110} direction is ranged from 2.98 to 3.02 angstrom, which has slightly larger than bcc CoFe lattice parameter along {100} direction and has a lattice mismatch between 4% and 7%. After thermal annealing with a temperature higher than 250-degree, the amorphous CoFeB is crystallized to form bcc CoFe grains having epitaxial growth with (100) plane parallel to surface of the rocksalt crystal buffer layers with in-plane expansion and out-of-plane contraction. Accordingly, a perpendicular magnetization is induced in the recording layer. 
       Second Embodiment 
       [0031]      FIG. 2  is a cross-sectional view showing an example configuration of the MTJ element  10  according to the second embodiment. As shown in  FIG. 2 , the buffer layer  15  has a bi-layer structure in which a first buffer layer  15 A, and a second buffer layer  15 B are stacked in this order from the bottom. The first buffer layer  15 A is made of MgO, and the second buffer layer  15 B is made of ZnO. Naturally, MgO can easily form stable rocksalt crystal grains, while ZnO typically forms hexagonal crystal structure, instead of rocksalt crystal. Once a MgO seed layer having a rocksalt crystal with a cubic lattice plane (100) as a substrate, ZnO rocksalt crystal grains can readily epitaxially grow on top of MgO layer. The lattice mismatch between ZnO along {110} direction and bcc CoFe along {100} direction is slightly higher than that between MgO and bcc CoFe, a stronger perpendicular anisotropy is expected in the recording layer. Accordingly, the thermal energy stability of the perpendicular MRAM improves. 
       Third Embodiment 
       [0032]      FIG. 3  is a cross-sectional view showing an example configuration of the MTJ element  10  according to the third embodiment. As shown in  FIG. 3 , the recording layer  14  has a multi-layer structure in which a first ferromagnetic layer  14 A, a nonmagnetic insertion layer  14 B, and a second ferromagnetic layer  14 C are stacked in this order from the bottom, and rest layers are the same as shown in  FIG. 1 . An example configuration will be described below. The first ferromagnetic layer  14 A is CoFeB (around 0.8 nm), the insertion layer  14 B is Ta (around 0.3 nm), and the second magnetic layer  14 C is CoFeB (around 0.6 nm). From layer  14 A to layer  14 C, the Fe composition relative to Co is increased to improve MR ratio. Further, the perpendicular magnetic anisotropy can be improved by a thermal annealing process in which B atoms move toward the insertion Ta layer. 
       Fourth Embodiment 
       [0033]      FIG. 4  is a cross-sectional view showing an example configuration of the MTJ element  10  according to the fourth embodiment. The base layer  16  has a bi-layer structure in which a first base layer  16 A is TbCoFe (around 20 nm), and a second base layer  16 B is CoFeB (around 2 nm). The reference layer  12  is a multi-layer (Pd/Co)n/CoFeB (around 1 nm). (Pd/Co)n is a superlattice structure which enables a strong perpendicular anisotropy. Both the base layer and reference layer have perpendicular magnetizations, however a careful selection of these layer structures can be made to make them have different perpendicular anisotropy or coercive forces so that they can be set towards opposite directions by applying external perpendicular magnetic fields. A careful selection of magnetic base layer and reference layer with opposite perpendicular magnetizations would lead near-zero or zero perpendicular stray field acting on the recording layer, accordingly, the thermal stability is improved. 
         [0034]    While certain embodiments have been described above, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. For an example, the perpendicular MTJ element in each embodiment may have reversed layer-by-layer sequence. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.