Abstract:
A magnetic tunnel junction (MTJ) for a magnetic random access memory (MRAM) includes a magnetic free layer having a variable magnetization direction; an iron (Fe) dusting layer formed on the free layer; an insulating tunnel barrier formed on the dusting layer; and a magnetic fixed layer having an invariable magnetization direction, disposed adjacent the tunnel barrier such that the tunnel barrier is located between the free layer and the fixed layer; wherein the free layer and the fixed layer have perpendicular magnetic anisotropy and are magnetically coupled through the tunnel barrier.

Description:
BACKGROUND 
       [0001]    This disclosure relates generally to the field of magnetoresistive random access memory (MRAM), and more specifically to materials for use in fabrication of magnetic tunnel junctions for spin torque transfer (STT) MRAM. 
         [0002]    MRAM is a type of solid state, non-volatile memory that uses tunneling magnetoresistance (TMR) to store information. MRAM is made up of an electrically connected array of magnetoresistive memory elements, referred to as magnetic tunnel junctions (MTJs). Each MTJ includes a free layer and fixed layer that each include a layer of a magnetic material, and that are separated by a non-magnetic insulating tunnel barrier. The free layer has a variable magnetization direction, and the fixed layer has an invariable magnetization direction. An MTJ stores information by switching the magnetization state of the free layer. When the magnetization direction of the free layer is parallel to the magnetization direction of the fixed layer, the MTJ is in a low resistance state. Conversely, when the magnetization direction of the free layer is antiparallel to the magnetization direction of the fixed layer, the MTJ is in a high resistance state. The difference in resistance of the MTJ may be used to indicate a logical ‘1’ or ‘0’, thereby storing a bit of information. The TMR of an MTJ determines the difference in resistance between the high and low resistance states. A relatively high difference between the high and low resistance states facilitates read operations in the MRAM. 
         [0003]    The magnetization direction of the free layer may be changed by a spin torque switched (STT) write method, in which a write current is applied in a direction perpendicular to the film plane of the magnetic films forming the MTJ. The write current has a tunneling magnetoresistive effect, so as to change (or reverse) the magnetization direction, or state, of the free layer of the MTJ. In STT magnetization reversal, the write current required for the magnetization reversal is determined by the current density. As the area of the surface in an MTJ on which the write current flows becomes smaller, the write current required for reversing the magnetization of the free layer of the MTJ becomes smaller. Therefore, if writing is performed with fixed current density, the necessary write current becomes smaller as the MTJ size becomes smaller. Inclusion of material layers that exhibit perpendicular anisotropy (PMA) in a MTJ also lowers the necessary write current density relative to MTJs having in-plane magnetic anisotropy, lowering the total necessary write current. However, MTJs that include PMA materials may not exhibit sufficient coercivity (H c ) to meet reliability and retention requirements for an MRAM made up of the PMA MTJs. 
       BRIEF SUMMARY 
       [0004]    In one aspect, a magnetic tunnel junction (MTJ) for a magnetic random access memory (MRAM) includes a magnetic free layer having a variable magnetization direction; an iron (Fe) dusting layer formed on the free layer; an insulating tunnel barrier formed on the dusting layer; and a magnetic fixed layer having an invariable magnetization direction, disposed adjacent the tunnel barrier such that the tunnel barrier is located between the free layer and the fixed layer; wherein the free layer and the fixed layer have perpendicular magnetic anisotropy. 
         [0005]    In another aspect, a method of forming a magnetic tunnel junction (MTJ) for a magnetic random access memory (MRAM) includes forming a magnetic free layer having a variable magnetization direction; forming an iron (Fe) dusting layer over the free layer; forming a tunnel barrier comprising an insulating material over the Fe dusting layer; and forming a magnetic fixed layer having an invariable magnetization direction over the tunnel barrier, wherein the free layer and the fixed layer have perpendicular magnetic anisotropy. 
         [0006]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0008]      FIG. 1  is a cross sectional view illustrating an embodiment of an MTJ with an iron (Fe) dusting layer located between the free layer and the tunnel barrier. 
           [0009]      FIG. 2  is a pair of graphs that illustrate the relationship between TMR and perpendicular field for a plurality of MTJs in an MRAM array with and without an Fe dusting layer between the free layer and the tunnel barrier. 
           [0010]      FIG. 3  is a pair of graphs that illustrate the relationship between TMR and perpendicular field for a plurality of MTJs in an MRAM array with different compositions of the tunnel barrier. 
           [0011]      FIG. 4  is a cross sectional view illustrating an embodiment of an MTJ with an Fe dusting layer located between the free layer and the tunnel barrier, and an interfacial layer located between the fixed layer and the tunnel barrier. 
           [0012]      FIG. 5  is a cross sectional view illustrating an embodiment of an MTJ with an Fe dusting layer located between the free layer and the tunnel barrier, and a fixed layer comprising a synthetic anti-ferromagnetic (SAF) structure. 
           [0013]      FIG. 6  is a cross sectional view illustrating an embodiment of an MTJ with an Fe dusting layer located between the free layer and the tunnel barrier, and a dipole layer. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Embodiments of an MTJ with an iron (Fe) dusting layer located between the free layer and the tunnel barrier are provided, with exemplary embodiments being discussed below in detail. The addition of the Fe dusting layer increases the H c  in MTJs that include PMA materials. The Fe dusting layer may be relatively thin, for example, from about 0.2 angstroms ({acute over (Å)}) to about 2 {acute over (Å)} thick in some embodiments. A PMA MJT stack that includes an Fe dusting layer may be grown at room temperature, reducing manufacturing complexity for an MRAM comprising the PMA MTJs. 
         [0015]    Referring initially to  FIG. 1 , there is shown a cross sectional view of an MTJ with an Fe dusting layer in accordance with an exemplary embodiment. As is shown, the MTJ  100  includes a seed layer  101  having free layer  102  grown thereon. The seed layer  101  may include, for example, tantalum (Ta) or tantalum magnesium (TaMg) in some embodiments. The free layer  102  may include cobalt-iron-boron (CoFeB), for example. An Fe dusting layer  103  is then formed on the free layer  102 . Next, a tunnel barrier  104  is formed on the Fe dusting layer  103 , wherein the tunnel barrier  104  may include a non-magnetic insulating material such as magnesium oxide (MgO), for example. Following the formation of the tunnel barrier  104 , a fixed layer  105  is formed on top of the tunnel barrier  104 . The fixed layer  105  may include, for example one or more interfacial layers, or spacers, and cobalt-platinum (COO or cobalt-palladium (Co|Pd), in multilayers or a mixture, in various embodiments. The Fe dusting layer  103  may be formed by sputtering, as may various other layers that make up MTJ  100 . The free layer  102  and the fixed layer  105  have perpendicular magnetic anisotropy. 
         [0016]    The presence of the Fe dusting layer  103  on top of a CoFeB free layer  102  significantly increases the H c  of the MTJ devices. For example, in an MTJ  100  with a free layer  102  made of 7CoFe 20 B 20  and a dusting layer  103  that is about 0.4{acute over (Å)} thick, the H c  of a MTJ having a diameter of about 120 nanometer (nm) is about 600 to 700 Oersteds (Oe), compared to about 200 Oe for an MTJ with a 7CoFe 20 B 20  free layer and no dusting layer, as illustrated by graphs  200   a  and  200   b  of  FIG. 2 . More specifically, graph  200   a  shows the relationship between TMR and the perpendicular field for 128 MTJs in a 4 kb MRAM array with a CoFeB free layer with an Fe dusting layer between the free layer and the tunnel barrier, while graph  200   b  shows the relationship between TMR and perpendicular field for 128 MTJs in a 4 kb MRAM array with a pure CoFeB free layer and no dusting layer. 
         [0017]    For a given thickness of CoFeB in the free layer  102 , as the Fe dusting layer is made thicker (e.g., greater than about 2 {acute over (Å)}), the H c  of the MTJ eventually decreases because of the increase of total moment and weaker PMA. A relatively thick Fe dusting layer  103  may also increase the switching voltage (i.e., the voltage required to change the magnetization direction of the free layer, V c ) of the MTJ. Depending on the specific requirements for H c  (for retention) and V c  (for switching) for the MRAM comprising the MTJs, optimal CoFeB and Fe relative thicknesses may be selected. The thickness of the Fe dusting layer  103  may be from about 0.2 {acute over (Å)} to about 2 {acute over (Å)} thick in some embodiments. 
         [0018]    The MgO tunnel barrier  104  may be formed by radiofrequency (RF) sputtering in some embodiments. Alternatively, the MgO tunnel barrier  104  may be formed by oxidation (either natural or radical) of a layer of Mg in other embodiments. After oxidation, the MgO layer may then be capped with a second layer of Mg. The second layer of Mg may have a thickness of about 5 {acute over (Å)} or less in some embodiments. The H c  of the free layer  102  may vary based on the method chosen to form the MgO tunnel barrier  104 . For example, in the case of an MgO tunnel barrier  104  made by radical oxidation and capped with a second layer of Mg, the thickness of the second Mg layer may significantly impact the H c  of the free layer. For a first exemplary MTJ, when the barrier is made of 9 {acute over (Å)} Mg|Radical Oxidation|3 {acute over (Å)} Mg, an H c  of about 120 Oe is observed. For a second exemplary MTJ having the same free layer and fixed layer materials as the first exemplary MTJ, when the barrier is made of 9 {acute over (Å)} Mg|Radical Oxidation|2 {acute over (Å)} Mg, a H c  of about 270 Oe is observed. This is illustrated in graphs  300   a  and  300   b  of  FIG. 3 . More specifically, graph  300   a  shows the relationship between TMR and perpendicular field for a set of 128 MTJs in a 4 kb MRAM array, with each MTJ having a CoFeB free layer, a 9 {acute over (Å)} Mg|Radical Oxidation|2 {acute over (Å)} Mg tunnel barrier. Graph  300   b  shows the relationship between TMR and perpendicular field for a set of 128 MTJs in a 4 kb MRAM array, with each MTJ having a CoFeB free layer, a 9 {acute over (Å)} Mg|Radical Oxidation|3 {acute over (Å)} Mg tunnel barrier. 
         [0019]    Referring now to  FIG. 4 , there is shown a cross sectional view of an MTJ  400 , in accordance with another embodiment. Similar to the embodiment of  FIG. 1 , the MTJ includes a free layer  402  formed on a seed layer  401 , a dusting layer  403  formed on the free layer  402 , and a tunnel barrier  404  formed on the dusting layer  403 . The various materials, thicknesses, and manner of forming the layers  401 - 404  may be similar to those shown in  FIG. 1 . Here, however, the MTJ  400  further includes an interfacial layer  405  formed on the tunnel barrier  404 . In the embodiment depicted, the interfacial layer  405  includes a first layer of, for example, Fe, and a second layer of, for example, CoFeB. In an exemplary embodiment, the combined Fe/CoFeB interfacial layer  405  may have a total thickness of about 5 {acute over (Å)} to about 15 {acute over (Å)}. 
         [0020]    As further depicted in  FIG. 4 , a spacer layer  406  is formed on the opposite side of the interfacial layer  405 , with respect to the tunnel barrier  404 . The spacer layer  406  may be formed a material such as Ta, for example, at an exemplary thickness of about 5 {acute over (Å)} or less. Finally, a fixed layer  407  is formed on the spacer layer  406 , at an opposite side of the spacer layer  406  with respect to the interfacial layer  405 . The fixed layer  407  may include, for example, Co|Pd or Co|Pt multilayers. As is the case with the embodiment of  FIG. 1 , the free layer  402  and the fixed layer  407  have perpendicular magnetic anisotropy. 
         [0021]      FIG. 5  is a cross sectional view illustrating another embodiment of a MTJ  500 . Once again, the MTJ  500  includes, similar to the  FIG. 1  and  FIG. 4  embodiments, a free layer  502  formed on a seed layer  501 , a dusting layer  503  formed on the free layer  502 , and a tunnel barrier  504  formed on the dusting layer  503 . In this particular embodiment, the MTJ  500  further includes a fixed layer shown collectively as layers  505 - 507  in  FIG. 5 , and which comprise a synthetic anti-ferromagnetic (SAF) structure. The SAF structure includes Co|Pd multilayers  505  and  507  that are coupled anti-ferromagnetically through a ruthenium (Ru) spacer  506  disposed therebetween. The SAF fixed layer structure  505 - 507  may reduce the offset field in the MTJ  500 . Similar to the embodiments described above, the seed layer  501  may include Ta or TaMg while the free layer  502  may include CoFeB. The Fe dusting layer  503  may be from about 0.2 {acute over (Å)} to about 2 {acute over (Å)} thick in some embodiments. The tunnel barrier  504  may include a non-magnetic insulating material such as MgO, and may be formed by oxidation of Mg metal layers or RF sputtering. The Fe dusting layer  503  may be formed by sputtering, as may various other layers that make up MTJ  500 . The free layer  502  and fixed layer  505 - 507  have perpendicular magnetic anisotropy. 
         [0022]      FIG. 6  is a cross sectional view illustrating still another embodiment of a MTJ  600 . In this embodiment, however, a dipole layer below the free layer is used to reduce the offset field of the MTJ, in contrast to the SAF fixed layer structure in the embodiment of  FIG. 5 . As shown in  FIG. 6 , the MTJ  600  includes, similar to the above described embodiments, a free layer  602  formed on a seed layer  601 , a dusting layer  603  formed on the free layer  602 , and a tunnel barrier  604  formed on the dusting layer  503 . In addition, an interfacial layer  605  is formed on the tunnel barrier  604 , and a fixed layer  606  is formed on the interfacial layer  605 . As is the case with previous embodiments, the free layer  602  may include CoFeB, and have a thickness from about 5 {acute over (Å)} to about 15 {acute over (Å)}, while the Fe dusting layer  603  may be from about 0.2 {acute over (Å)} to about 2 {acute over (Å)} thick. The tunnel barrier  604  may include a non-magnetic insulating material such as MgO, and may be formed by oxidation of Mg metal layers or RF sputtering. Further, the interfacial layer  405  may include, for example, both Fe and CoFeB. As further shown in  FIG. 6 , a dipole layer  607  is formed beneath the free layer  602  to reduce the offset field of the MTJ  600 . The dipole layer  607  may include, for example, cobalt-nickel (Co|Ni), Co|Pt or Co|Pd multilayers, which exhibit PMA. As is the case with other embodiments, the free layer  602  and the fixed layer  606  have perpendicular magnetic anisotropy. 
         [0023]    It should be appreciated that the exemplary MTJ embodiments  100 ,  400 ,  500 , and  600  discussed above with respect to FIGS.  1  and  4 - 6  are shown for illustrative purposes only, and it is contemplated that other suitable MTJ structures may be formed in which an Fe dusting layer is disposed between a free layer and a tunnel barrier so as to provide sufficient coercivity (H c ) to meet reliability and retention requirements for an MRAM made up of the PMA MTJs. 
         [0024]    The technical effects and benefits of exemplary embodiments include increased coercivity and magnetoresistance in a MTJ through addition of the Fe dusting layer. 
         [0025]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting 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” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0026]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.