Abstract:
A magnetic tunnel junction (MTJ) device in a magnetoresistive random access memory (MRAM) and method of making the same are provided to achieve a high tunneling magnetoresistance (TMR), a high perpendicular magnetic anisotropy (PMA), good data retention, and a high level of thermal stability. The MTJ device includes a first free ferromagnetic layer, a synthetic antiferromagnetic (SAF) coupling layer, and a second free ferromagnetic layer, where the first and second free ferromagnetic layers have opposite magnetic moments.

Description:
FIELD OF DISCLOSURE 
     Various embodiments described herein relate to magnetoresistive random access memory (MRAM), and more particularly, to magnetic tunnel junction (MTJ) in MRAM. 
     BACKGROUND 
     MRAM (Magnetoresistive Random Access Memory) is a non-volatile memory that may utilize MTJ (Magnetic Tunnel Junction) devices, where the state of an MTJ device depends on the magnetic (electron-spin) orientation of its ferromagnetic layers. A STT-MTJ (Spin Torque Transfer MTJ) changes the spin orientation by using a switching current. To achieve high-density MRAM with good thermal stability and low switching current, attempts have been made to develop MTJ devices with a high perpendicular magnetic anisotropy (PMA). In a perpendicular magnetic tunnel junction (p-MTJ) having a free ferromagnetic layer, the orientation of the magnetic field in the free ferromagnetic layer is perpendicular to the interface between the barrier and ferromagnetic layers. It is desirable for a p-MTJ device to have a high tunneling magnetoresistance (TMR), a high PMA, and good data retention. 
     SUMMARY 
     Exemplary embodiments of the invention are directed to a magnetic tunnel junction (MTJ) device and method for making the same, with improved tunneling magnetoresistance (TMR), perpendicular magnetic anisotropy (PMA), data retention, and thermal stability. Moreover, the magnetic and electrical properties of the MTJ device according to embodiments of the invention can be maintained at high process temperatures. 
     In an embodiment, a magnetoresistive random access memory (MRAM) device comprises: a first free ferromagnetic layer having a first magnetic moment; a synthetic antiferromagnetic (SAF) coupling layer disposed on the first free ferromagnetic layer; and a second free ferromagnetic layer disposed on the SAF coupling layer, the second free ferromagnetic layer having a second magnetic moment opposite to the first magnetic moment of the first free ferromagnetic layer. 
     In another embodiment, a magnetic tunnel junction (MTJ) device comprises: a first free ferromagnetic layer having a first magnetic moment; a synthetic antiferromagnetic (SAF) coupling layer disposed on the first free ferromagnetic layer; and a second free ferromagnetic layer disposed on the SAF coupling layer, the second free ferromagnetic layer having a second magnetic moment opposite to the first magnetic moment of the first free ferromagnetic layer. 
     In another embodiment, a method for making a magnetic tunnel junction (MTJ) comprises the steps for: forming a first free ferromagnetic layer having a first magnetic moment; forming a synthetic antiferromagnetic (SAF) coupling layer on the first free ferromagnetic layer; and forming a second free ferromagnetic layer on the SAF coupling layer, the second free ferromagnetic layer having a second magnetic moment opposite to the first magnetic moment of the first free ferromagnetic layer. 
     In yet another embodiment, a method of making a magnetic tunnel junction (MTJ) comprises the steps of: forming a first free ferromagnetic layer having a first magnetic moment; forming a synthetic antiferromagnetic (SAF) coupling layer on the first free ferromagnetic layer, the SAF coupling layer comprising a material selected from the group consisting of ruthenium (Ru) and chromium (Cr); and forming a second free ferromagnetic layer on the SAF coupling layer, the second free ferromagnetic layer having a second magnetic moment opposite to the first magnetic moment of the first free ferromagnetic layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are presented to aid in the description of embodiments of the invention and are provided solely for illustration of the embodiments and not limitations thereof. 
         FIG. 1  is a sectional view of a magnetic tunnel junction (MTJ) in a magnetoresistive random access memory (MRAM) according to embodiments of the present invention. 
         FIG. 2  is a more detailed sectional view of the barrier layer, the synthetic antiferromagnetic (SAF) coupled free ferromagnetic layer structure and the capping layer according to embodiments of the present invention. 
         FIGS. 3A and 3B  are diagrams illustrating opposite magnetic moments of the first and second free ferromagnetic layers of  FIG. 2 . 
         FIG. 4  is a flowchart illustrating a method of making an MTJ device according to embodiments of the present invention. 
         FIG. 5  is a more detailed flowchart illustrating a method of making an MTJ device according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the invention are disclosed in the following description and related drawings directed to specific embodiments of the invention. Alternate embodiments may be devised without departing from the scope of the invention. Additionally, well known elements of the invention will not be described in detail or will be omitted so as not to obscure the relevant details of the invention. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiments of the invention” does not require that all embodiments of the invention include the discussed feature, advantage or mode of operation. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of embodiments of the invention. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or groups thereof. Moreover, it is understood that the word “or” has the same meaning as the Boolean operator “OR,” that is, it encompasses the possibilities of “either” and “both” and is not limited to “exclusive or” (“XOR”), unless expressly stated otherwise. 
       FIG. 1  is a sectional view of a magnetic tunnel junction (MTJ) device  100  in a magnetoresistive random access memory (MRAM) which includes a synthetic antiferromagnetic (SAF) coupled free ferromagnetic layer structure according to embodiments of the present invention. In  FIG. 1 , a bottom electrode  102  is provided, and a seed layer  104  is disposed on the bottom electrode  102  in a conventional manner. In an embodiment, a bottom SAF layer  106  is formed on the seed layer  104 , and an SAF layer  108 , which may comprise ruthenium (Ru) or chromium (Cr), is formed on the bottom SAF layer  106 . In a further embodiment, a top SAF layer  110  is formed on the SAF layer  108 . In yet a further embodiment, a reference layer  112  is formed on the top SAF layer  110 . 
     In an embodiment according to the present invention, a barrier layer  114  is formed on the reference layer  112 . In a further embodiment, the barrier layer  114  comprises magnesium oxide (MgO). In yet a further embodiment, the barrier layer  114  comprises an MgO layer having a surface orientation of (100), which will be discussed in further detail below with respect to  FIG. 2 . Other materials may be implemented in the barrier layer  114  without departing from the scope of the present invention. 
     Moreover, the MTJ device  100  comprises an SAF coupled free ferromagnetic layer structure  116 , an embodiment of which includes a multi-layer structure as shown in  FIG. 2 , which will be discussed in further detail below. Referring to  FIG. 1 , the SAF coupled free ferromagnetic layer structure  116  is disposed on the barrier layer  114 . In an embodiment, a capping layer  118  is formed on the SAF coupled free ferromagnetic layer structure  116 . In a further embodiment, the capping layer  118  comprises an MgO layer having a surface orientation of (100). Alternatively, the capping layer  118  may comprise aluminum oxide (AlO x ). Other materials may be also be implemented in the capping layer  118  without departing from the scope of the present invention. In an embodiment, a top electrode  120  is formed on the capping layer  118 . 
       FIG. 2  is a more detailed sectional view of the barrier layer  114 , the SAF coupled free ferromagnetic layer structure  116  and the capping layer  118  according to embodiments of the present invention. In the embodiment shown in  FIG. 2 , the barrier layer  114  comprises an MgO layer having a surface orientation of (100). In an embodiment, the SAF coupled free ferromagnetic layer structure  116  comprises a first free ferromagnetic layer  202 , which itself comprises a three-layer structure, an SAF coupling layer  204  formed on the first free ferromagnetic layer  202 , and a second free ferromagnetic layer  206 , which itself comprises a two-layer structure, on the SAF coupling layer  204 . In an embodiment, the capping layer  118 , which may comprise an MgO layer, or alternatively, an AlO x  layer, is formed on the second free ferromagnetic layer  206 . 
     In an embodiment, the first free ferromagnetic layer  202  comprises an iron-rich cobalt-iron-boron (Fe-rich CoFeB) layer  202   a  formed on the barrier layer  114 . In a further embodiment, the Fe-rich CoFeB layer  202   a  has an epitaxial relationship with the barrier layer  114  to provide high tunneling magnetoresistance (TMR) and high perpendicular magnetic anisotropy (PMA). In a further embodiment, the Fe-rich CoFeB layer  202   a  is subjected to a high-temperature annealing process to transform the Fe-rich CoFeB material from an amorphous structure to a crystalline structure. 
     In an embodiment, an intermediate layer  202   b  is formed on the Fe-rich CoFeB layer  202   a . In a further embodiment, the intermediate layer  202   b  comprises a cobalt-iron-boron-tantalum (CoFeBTa) layer. In another embodiment, the intermediate layer  202   b  comprises a cobalt-iron-boron-hafnium (CoFeBHf) layer. Alternatively, another element may be used as an alternative to tantalum (Ta) or hafnium (Hf) in a cobalt-iron-boron (CoFeB) structure in the intermediate layer  202   b . In an embodiment, a thin layer of cobalt (Co)  202   c  is formed on the intermediate layer  202   b . In a further embodiment, the Co layer  202   c  is not more than 5 Angstroms in thickness. 
     In an embodiment, the SAF coupling layer  204 , which is formed above the thin Co layer  202   c , comprises ruthenium (Ru). Alternatively, the SAF coupling layer  204  comprises chromium (Cr). Another element may be implemented in the SAF coupling layer  204  instead of Ru or Cr above the thin Co layer  202   c  within the scope of the present invention. The thin Co layer  202   c  helps increase SAF coupling to improve the PMA and prevent diffusion of the SAF coupling layer  204  during post annealing. As shown in  FIG. 2 , the Fe-rich CoFeB layer  202   a , the intermediate layer  202   b , which may comprise either CoFeBTa or CoFeBHf, and the thin Co layer  202   c  together form the first free ferromagnetic layer  202 . 
     In an embodiment, the second free ferromagnetic layer  206  is formed on the SAF coupling layer  204 , which may comprise Ru, Cr or another material. The second free ferromagnetic layer  206 , which is positioned above the SAF coupling layer  204  opposite the first free ferromagnetic layer  202 , enhances the PMA of the MTJ device. In an embodiment, the second free ferromagnetic layer  206  comprises a thin Co layer  206   a  formed above the SAF coupling layer  204 . In a further embodiment, the thin Co layer  206   a  has a thickness of not more than 5 Angstroms. In an embodiment, a thin Fe-rich CoFeB layer  206   b  is formed on the thin Co layer  206   a . The thin Co layer  206   a  and the thin Fe-rich CoFeB layer  206   b  together form the second free ferromagnetic layer  206 . The thin Co layer  206   a  increases SAF coupling, improves PMA, and helps prevent Ru or Cr diffusion from the SAF coupling layer  204  during post annealing. Moreover, the thin Fe-rich CoFeB layer  206   b  further enhances the PMA of the MTJ device. 
     In an embodiment, the capping layer  118  is formed on the Fe-rich CoFeB layer  206   b  of the second free ferromagnetic layer  206 . In an embodiment, the capping layer  118  may be regarded as an integral part of the second free ferromagnetic layer  206 . As discussed above, the capping layer  118  may comprise MgO having a surface orientation of (100), or alternatively, AlO x . In an embodiment, both the barrier layer  114  below the first free ferromagnetic layer  202  and the capping layer  118  above the second free ferromagnetic layer  206  comprise MgO having a surface orientation of (100), which is a surface orientation in reference to a planar interfacing surface  203  between the first free ferromagnetic layer  202  and the SAF coupling layer  204 , or a planar interfacing surface  205  between the SAF coupling layer  204  and the second free ferromagnetic layer  206 . 
       FIGS. 3A and 3B  provide exemplary illustrations of opposite magnetic moments of the first and second free ferromagnetic layers of  FIG. 2 .  FIG. 3A  illustrates the SAF coupled free ferromagnetic layer structure  116  as part of an MTJ device, acting as a memory cell in an MRAM, with the first free ferromagnetic layer  202  having a magnetic moment in a direction indicated by an upward-pointing arrow  302 , whereas the second free ferromagnetic layer  206  having a magnetic moment in a direction indicated by a downward-pointing arrow  304 . The upward-pointing arrow  302  and the downward-pointing arrow  304  are perpendicular to the planar interfacing surfaces  203  and  205 , and thus the MTJ device illustrated in  FIGS. 1 and 2  and described above is called a perpendicular magnetic tunnel junction (p-MTJ) device. In  FIG. 3B , the first free ferromagnetic layer  202  has a magnetic moment in a direction indicated by a downward-pointing arrow  306 , whereas the second free ferromagnetic layer  206  has a magnetic moment in a direction indicated by an upward-pointing arrow  308 . 
     In either  FIG. 3A  or  FIG. 3B , the magnetic moment of the first free ferromagnetic layer  202  is opposite to the magnetic moment of the second free ferromagnetic layer  206 . The SAF coupling layer  204  couples the magnetic orientations of the first and second free ferromagnetic layers  202  and  206  such that their magnetic or electron-spin orientations are opposite to each other, thereby resulting in reduced magnetic offset and reduced interference from stray magnetic fields. In an embodiment, the state of the MRAM memory cell of  FIG. 3A , in which the directions  302  and  304  of magnetic moments of the first and second free ferromagnetic layers  202  and  206  point toward each other, may be regarded as storing a number “0,” whereas the state of the MRAM memory cell of  FIG. 3B , in which the directions  306  and  308  of magnetic moments of the first and second free ferromagnetic layers  202  and  206  point away from each other, may be regarded as storing a number “1.” Alternatively, the state of the MRAM memory cell of  FIG. 3A  may be regarded as storing “1” whereas the state of the MRAM memory cell of  FIG. 3B  may be regarded as storing “0.” 
       FIG. 4  is a flowchart illustrating a method of making an MTJ device according to embodiments of the present invention. In  FIG. 4 , a first free ferromagnetic layer having a first magnetic moment is formed in step  402 . In an embodiment, the first free ferromagnetic layer  202  is formed on a barrier layer  114 , for example, an MgO layer having a surface orientation of (100), as described above with reference to  FIG. 2 . In an embodiment, the first free ferromagnetic layer  202  comprises an Fe-rich CoFeB layer  202   a , an intermediate layer  202   b , which may comprise CoFeBTa or CoFeBHf, and a Co layer  202   c  as described above with reference to  FIG. 2 . 
     Referring to  FIG. 4 , a synthetic antiferromagnetic (SAF) coupling layer is formed on the first free ferromagnetic layer in step  404 . In an embodiment, the SAF coupling layer comprises Ru, or alternatively, Cr, as described above with reference to  FIG. 2 . Referring back to  FIG. 4 , a second free ferromagnetic layer is formed on the SAF coupling layer, the second free ferromagnetic layer having a second magnetic moment opposite to the first magnetic moment of the first free ferromagnetic layer, in step  406 . In an embodiment, the second free ferromagnetic layer  206  comprises a Co layer  206   a  and an Fe-rich CoFeB layer  206   b , as described above with reference to  FIG. 2 . In a further embodiment, a capping layer  118 , for example, an MgO layer having a surface orientation of (100), or alternatively, an AlO x  layer, is formed on the second free ferromagnetic layer  206 , as described above with reference to  FIG. 2 . 
       FIG. 5  is a more detailed flowchart illustrating a method of making an MTJ device according to embodiments of the present invention. In  FIG. 5 , a barrier layer comprising MgO is formed in step  502 . An Fe-rich CoFeB layer is then epitaxially grown on the barrier layer in step  504 , and the Fe-rich CoFeB layer is annealed to form a crystalline Fe-rich CoFeB structure in step  506 . In an embodiment, the Fe-rich CoFeB layer is subjected to a high-temperature annealing process to transform the Fe-rich CoFeB material from an amorphous structure to a crystalline structure. 
     In an embodiment, an intermediate layer comprising a material selected from the group consisting of CoFeBTa and CoFeBHf is formed on the Fe-rich CoFeB layer in step  508 . A Co layer is then formed on the intermediate layer in step  510 . In an embodiment, a thin layer of cobalt with a thickness of not more than 5 Angstroms is formed on the intermediate layer, which may be either CoFeBTa or CoFeBHf. The Fe-rich CoFeB layer, the intermediate layer and the Co layer made according to steps  504 ,  506 ,  508  and  510  together form a first free ferromagnetic layer, such as the first free ferromagnetic layer  202  described above with reference to  FIG. 2 . 
     Referring to  FIG. 5 , a synthetic antiferromagnetic (SAF) coupling layer is formed on the first free ferromagnetic layer in step  512 . As described above, the SAF coupling layer may comprise ruthenium, or alternatively, chromium. A Co layer is then formed on the SAF coupling layer in step  514 . In an embodiment, the Co layer formed on top of the SAF coupling layer may be a thin layer of cobalt with a thickness of not more than 5 Angstroms. Subsequently, an Fe-rich CoFeB layer is formed on the Co layer in step  516 . The Co layer and the Fe-rich CoFeB layer made according to steps  514  and  516  together form a second free ferromagnetic layer, such as the second free ferromagnetic layer  206  described above with reference to  FIG. 2 . In a further embodiment, a capping layer  118 , such as an MgO layer with a surface orientation of (100), or alternatively, an AlO x  layer, may be formed on top of the Fe-rich CoFeB layer made according to step  516 . 
     While the foregoing disclosure describes illustrative embodiments of the invention, it should be noted that various changes and modifications could be made herein without departing from the scope of the invention as defined by the appended claims. The functions, steps or actions in the method and apparatus claims in accordance with the embodiments of the invention described herein need not be performed in any particular order unless explicitly stated otherwise. Furthermore, although elements of the invention may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated.