Abstract:
An improved PMA STT MTJ storage element, and a method for forming it, are described. By inserting a suitable oxide layer between the storage and cap layers, improved PMA properties are obtained, increasing the potential for a larger Eb/kT thermal factor as well as a larger MR. Another important advantage is better compatibility with high processing temperatures, potentially facilitating integration with CMOS.

Description:
TECHNICAL FIELD 
     The disclosed structure relates to the Spin Transfer Torque Magnetic Random Access Memory (STT MRAM) including its implementation via Magnetic Tunnel Junction (MTJ) elements with Perpendicular Magnetic Anisotropy (PMA), i.e. the magnetization of the free layer and of the reference layer being perpendicular to the plane of the films. 
     BACKGROUND 
     Among the critical requirements for PMA STT MRAM MTJ storage is to provide (1) a strong PMA, implying a high magnetocrystalline anisotropy field (Hk) to ensure a high thermal stability factor Eb/kT, (2) a high magnetoresistance ratio (MR), and (3) preferably, compatibility with high-temperature processing at or above 300° C. (which helps to increase the MR for the most commonly used MgO-based MTJs). 
     PMA STT storage elements proposed to date typically utilize Co- and/or Fe-based magnetic layers or multilayers, most commonly a CoFeB-based layer, grown on top of the MgO MTJ tunnel barrier. An example is shown in  FIG. 1 . Represented there as layer  1  are the MTJ bottom layers (seed layer, antiferromagnetic layer etc.). Layer  2  is the reference layer, in contact with magnesia tunnel barrier layer  3  whose upper side is contacted by (free) storage layer  4 . 
     The PMA of storage layer  4  is induced by the interfacial anisotropy at the MgO/storage layer interface where the lattice mismatch between layers  3  and  4  generates strain at their interface. Completing this prior art design is protective cap layer  5  most commonly made of Ta and selected to not deteriorate the storage layer&#39;s magnetic and magnetoresistive properties. 
     Such storage layer designs, however, suffer from several drawbacks, including (1) a weak PMA as a consequence of being induced at only one interface and thus not allowing magnetic elements, such as CoFeB, to be thick enough for Eb to maximize the MR, and (2) making possible diffusion of cap layer material into the storage layer and/or the MgO barrier during processing at or above 300° C., resulting in loss of PMA and/or MR. 
     While there have been attempts at mitigating drawback (1), by applying a cap layer that provides additional interfacial anisotropy, there remains a need to further improve the strength of the storage layer&#39;s PMA. Drawback (2) remains largely unaddressed in the designs that are currently being described in the prior art. 
     SUMMARY 
     It has been an object of at least one embodiment of the present disclosure to provide an MTJ having enhanced PMA. 
     Another object of at least one embodiment of the present disclosure has been to provide an MTJ having increased high temperature stability together with an increased MR. 
     Still another object of at least one embodiment of the present disclosure has been to provide a method to manufacture said MTJ. 
     A further object of at least one embodiment of the present disclosure has been that said method be compatible with existing methods for manufacturing MTJs. 
     These objects have been achieved by inserting a suitable oxide layer between the storage and cap layers. This oxide layer, which is native to the materials making up the storage layer, can be formed through natural oxidation of the top surface of the storage layer or it may be deposited thereon using a standard deposition technique. 
     The inserted oxide layer introduces anisotropic vertical strain at its interface with the storage layer which induces additional PMA in the storage layer, over and above the PMA originating at the storage layer-to-barrier layer interface that is already present in the storage layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a magnetic tunnel junction storage device, of the prior art, that includes PMA. 
         FIG. 2  shows how PMA in the device of  FIG. 1  can be enhanced by inserting an oxide layer that contacts the storage layer. 
         FIG. 3  illustrates formation of the oxide layer 
         FIG. 4  shows the relative positions of the oxide and storage layers 
         FIG. 5  shows a preferred embodiment of the disclosed structure 
     
    
    
     DETAILED DESCRIPTION 
     The subject of this disclosed structure is an improved and simplified storage element for the PMA STT MRAM that provides enhanced interfacial PMA, thus allowing for thicker storage elements and higher Eb, which are important for practical realization of high-density high-performance STT MRAM products. 
     As shown schematically in  FIG. 2 , the structure that is disclosed here includes magnetic layer (or multilayer)  20  that contacts MgO tunnel barrier  3  and has a thickness in a range of between about 1 and 5 nanometers, with a thickness between about 1.5 and 3 nanometers being preferred. This is then contacted by layer  21  of an oxide that is native to (i.e. derived from) the magnetic material(s) that make up the top-most portions of layer  20  (typically material that was within about 6 Angstroms from the top surface of layer  20  before layer  21  was formed either through deposition onto layer  20  or through the oxidation of above-mentioned part of layer  20 . 
     The preferred process for forming layer  21  was natural oxidation of the top portion of layer  20 . Details of this oxidation process (which is schematically illustrated in  FIG. 3 ) were as follows: Layer  20  was exposed to an 18 SCCM oxygen flow for about 30 seconds in an evacuated (pumped) vacuum chamber. Natural oxidation was generally preferred because this process generates a unique stress and strain pattern in the magnetic element which further enhances the interfacial PMA properties. 
     However, the added top oxide layer could have been made by any of several other methods, including, but not limited to, radical oxidation, reactive physical deposition in the presence of oxygen or direct deposition of the oxide through physical or chemical deposition. 
     As illustrated in  FIG. 4 , to preserve its properties, the structure was then covered with protective layer  41  of a material such as Ru or _Cu, Pt, Pd, Co, Rh, Ag, and Au that have an affinity to oxygen lower than that of layer  21 . 
     In  FIG. 5  we show a preferred embodiment of the disclosed structure. It was made up of the following elements:
     Layer  51 : TaN  — 20 A/Mg 7 A   Layer  52 : NiCr 50 A   Layer  53 : [Ni6/Co2.5]×4/Ru 4 A/Co20Fe60B20 8 A   Layer  3 : MgO 10 A   Layer  54 : Co20Fe60B 16 A   Layer  55 : Co20Fe60B Oxide ˜6 A   Layer  41 : Ru 50 A   

     It should be noted that although CoFeB has been cited as the material used for layer  20 , it could be replaced with other magnetic materials including Co, Fe, Ni, their alloys, as well as their alloys with Boron, Si or other Ms-diluting element without removing the advantages offered by the disclosed structure. 
     High Temperature Stability: 
     The structure shown in  FIG. 5 , was annealed at 300° C. for 10 minutes, following which its primary characteristics were measured. It was found that the PMA properties were preserved up to a large magnetic thickness of the CoFeB layer (at least 22 Å in this case), thus indicating that a very strong interfacial PMA had been induced. This PMA is much stronger than that found in prior-art structures, similar to  FIG. 1  but with a Ta cap, for which the maximum CoFeB thickness that can be used is 14 Å. The advantage of a thicker CoFeB in the storage element is that it helps to increase Eb and to maximize the MR. 
     It should also be noted that the presence of top oxide layer  21  can introduce a series parasitic resistance in the storage element, thus lowering the effective MTJ MR. The thickness of the oxide layer should therefore be kept small, preferably &lt;10 Å, or other measures intended to lower its resistance should be adopted. Examples include doping the oxide or the magnetic layer before oxidation with additional conducting elements like Cu, or etching back the formed oxide using plasma treatment. 
     It should additionally be noted that although the benefits of the disclosed structure are most apparent for the PMA STT MRAM, the basic approach taught in this disclosure can also be advantageously applied for the In-Plane STT MRAM where it could be used to lower the Out-of-Plane magnetic anisotropy of the In-Plane storage element, thus lowering the its critical current for switching.