Patent Publication Number: US-9425387-B1

Title: Magnetic element with perpendicular magnetic anisotropy for high coercivity after high temperature annealing

Description:
RELATED PATENT APPLICATIONS 
     This application is related to U.S. Pat. No. 8,946,834; U.S. Pat. No. 9,006,704; and U.S. Patent Application 2015/0008547; assigned to a common assignee and herein incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     This disclosure relates to magnetic devices that utilize magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA), and in particular, to inserting a diffusion barrier between a capping layer and top electrode to maintain coercivity (Hc) in the MTJ stack following annealing or during high temperature semiconductor processes of about 400° C. 
     BACKGROUND 
     Magnetoresistive Random Access Memory (MRAM) has a read function based on a tunneling magnetoresistive (TMR) effect in a MTJ stack wherein a tunnel barrier is formed between a free layer and a reference layer. The free layer serves as a sensing layer by switching the direction of its magnetic moment in response to external fields (media field) while the reference layer has a fixed magnetic moment. The electrical resistance through the tunnel barrier (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby provides an electrical signal that is representative of the magnetic state in the free layer. In MRAM, the MTJ is formed between a top conductor (electrode) and bottom conductor. When a current is passed through the MTJ, a lower resistance is detected when the magnetization directions of the free and reference layers are parallel (“0” memory state), and a higher resistance is noted when they are anti-parallel (“1” memory state). The TMR ratio is dR/R where R is the minimum resistance of the MTJ, and dR is the difference between the lower and higher resistance values. The tunnel barrier is typically about 10 Angstroms thick so that a current through the tunnel barrier can be established by a quantum mechanical tunneling of conduction electrons. 
     Another version of MRAM that relies on a TMR effect, and is referred to as a spintronic device that involves spin polarized current, is called spin-transfer torque (STT) MRAM and is described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996). J-G. Zhu et al. has described another spintronic device called a spin transfer oscillator (STO) in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. 
     MTJ elements wherein one or both of the free layer and reference layer have perpendicular magnetic anisotropy (PMA) are preferred over their counterparts that employ in-plane anisotropy because the former has an advantage in a lower writing current for the same thermal stability, and better scalability. In MTJs with PMA, the free layer has two preferred magnetization orientations that are perpendicular to the physical plane of the layer. Without external influence, the magnetization or magnetic moment of the free layer will align to one of the preferred two directions, representing information “1” or “0” in the binary system. For memory applications, the free layer magnetization direction is expected to be maintained during a read operation and idle, but change to the opposite direction during a write operation if the new information to store differs from its current memory state. The ability to maintain free layer magnetization direction during an idle period is called data retention or thermal stability. The level of stability required is typically related to the memory application. A typical non-volatile memory device may require thermal stability at 125° C. for about 10 years. 
     CoFeB or the like is commonly used as the free layer (FL) and MgO is selected as the tunnel barrier to generate PMA along the FL/MgO interface. However, the physical shape of the free layer having a lateral dimension more than ten times the thickness tends to induce in-plane anisotropy. If the in-plane anisotropy is greater than the PMA component, the FL magnetization direction will be in-plane. As FL thickness (volume) increases, a greater portion of the free layer is away from the FL/tunnel barrier interface. A thicker free layer means a higher in-plane magnetic moment and a reduction in PMA and coercivity. Usually, free layer thickness must be maintained below 20 to 25 Angstroms to realize a condition where the PMA component is greater than the in-plane anisotropy in the magnetic layer. 
     In a memory element such as STT-MRAM, the current needed to change the magnetic orientation of a magnetic region (free layer) is proportional to the net polarization of the current, the volume, magnetization, Gilbert damping constant, and anisotropy field of the magnetic region to be affected. The critical current (i c ) required to perform such a change in magnetization is given in equation (1): 
                     i   c     =         α   ⁢           ⁢   e   ⁢           ⁢   V   ⁢           ⁢   M   ⁢           ⁢   s       g   ⁢           ⁢     h   _         ⁡     [       H     k     eff   ,   ∥         +       1   2     ⁢     H       k   eff     ,   ⊥           ]               (   1   )               
where e is the electron charge, α is a Gilbert damping constant, Ms is the saturation magnetization of the free layer, h is the reduced Plank&#39;s constant, g is the gyromagnetic ratio, H k     eff     ,∥ , is the in-plane anisotropy field, and H k     eff     ,⊥  is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer. For most applications, the spin polarized current must be as small as possible. When the free layer has a magnetization direction perpendicular to the film plane, the critical current needed to switch the magnetic element is directly proportional to the perpendicular anisotropy field as indicated in equation (2):
 
     
       
         
           
             
               
                 
                   
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     Thermal stability is a function of the perpendicular anisotropy field as shown in equation (3) where k B  is the Boltzmann constant and T is the temperature: 
     
       
         
           
             
               
                 
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     and the perpendicular anisotropy field of a magnetic layer is expressed in equation (4) as: 
                     H       k   eff     ,   ⊥       =         -   4     ⁢           ⁢   π   ⁢           ⁢     M   s       +       2   ⁢           ⁢     K   U     ⊥     ,   s               M   s     ⁢   d       +     H     k   ,   χ   ,   ⊥                 (   4   )               
where M s  is the saturation magnetization, d is the thickness of the magnetic layer, H k,χ,⊥  is the crystalline anisotropy field in the perpendicular direction, and K U   ⊥,s  is the surface perpendicular anisotropy of the top and bottom surfaces of the magnetic layer. The shape anisotropy field is represented by the term (−4πM s ).
 
     In order to improve thermal stability by increasing the value K U   ⊥,s , a second free layer/metal oxide interface is commonly introduced on a side of the free layer opposite to the tunnel barrier. The metal oxide may be another MgO layer and is often called a cap layer or a Hk enhancing layer. Thus, a MgO/FL/MgO stack will substantially increase total PMA in the free layer thereby allowing a thicker free layer and higher thermal stability. The cap layer typically contacts an uppermost MTJ layer called a hard mask, which in turn connects to a top electrode and through a top electrode array to complementary-metal-oxide-semiconductor (CMOS) units in a memory chip. The hard mask generally has a thickness in the hundreds of Angstroms, and is comprised of a metal such as Ta, Ru, Mo, MnPt, and their oxides and nitrides as required for conventional techniques in manufacturing integrated circuits. The remainder of the MTJ typically has a thickness of about 100 Angstroms, which means the volume of hard mask material is much greater than that of the other MTJ layers. 
     Since MTJ elements are implemented in CMOS devices, the MTJ must be able to withstand annealing temperatures up to about 400° C. for 30 minutes that are commonly applied to improve the quality of the CMOS units for semiconductor purposes. In most cases, this temperature exceeds the optimum temperature for best magnetic performance in the MTJ or MRAM. MTJs are usually annealed in the 300-330° C. degree range to obtain the desired magnetic properties. 
     One problem resulting from high temperature anneal around 400° C. is the diffusion of hard mask materials into the MTJ. Although a fully oxidized MgO cap layer is a good diffusion blocking material, the resistance×area (RA) product of fully oxidized MgO is quite high and adds a large series resistance to the MTJ that in turn undesirably reduces the signal difference between reading out “0” and “1” information stored in the device. Therefore, it is practical to employ only partially oxidized MgO cap layers less than 10 Angstroms thick. Unfortunately, hard mask materials are heavier than Mg and do not bind well with Mg in the partially oxidized MgO cap thereby enabling easier diffusion of heavier metals to the free layer which then degrades the capacity of the FL/cap layer interface to promote PMA in the free layer. As a result, free layer PMA is reduced and thermal stability is less compared with a condition where the MTJ is annealed only to 330° C., for example. Free layer coercivity is also less after high temperature annealing to around 400° C. than after 300-330° C. annealing. However, it is an important requirement to maintain coercivity after high temperature processing. 
     Thus, there is a significant challenge to increase PMA and enhance thermal stability of free layers to improve the performance of MTJs at elevated temperatures typical of back end of line (BEOL) semiconductor processes. Current technology fails to provide high He and thermal stability in a free layer with PMA character that will withstand high temperature processing up to at least 400° C., which is required in semiconductor fabrication methods. Therefore, an improved MTJ stack is needed to enable a free layer with thermal stability to at least 400° C., and that exhibits PMA for optimum magnetic memory performance. 
     SUMMARY 
     One objective of the present disclosure is to provide a MTJ stack in a magnetic device wherein the free layer has PMA and coercivity (Hc) that is maintained after high temperature processing including annealing at temperatures up to 400° C. 
     A second objective of the present disclosure is to provide a MTJ stack of layers according to the first objective such that other properties including the TMR ratio, and resistance×area (RA) value are maintained at an acceptable level. 
     According to one embodiment of the present disclosure, these objectives are achieved by configuring a magnetic tunnel junction (MTJ) with a tunnel barrier/free layer (FL)/metal oxide stack to include a diffusion barrier between the metal oxide (cap layer) and overlying hard mask to prevent non-magnetic metals in the hard mask from diffusing through the metal oxide (cap) layer and degrading PMA established along the FL/metal oxide interface. In an alternative embodiment wherein the MTJ has a metal oxide/free layer/tunnel barrier stack and the metal oxide serves as a Hk enhancing layer, a diffusion barrier is inserted between the metal oxide and bottom electrode. Thus, the FL interfaces with a metal oxide along both of its top and bottom surfaces to induce or enhance PMA therein. Additional layers may be included such as a seed layer formed at the bottom of the MTJ stack. Furthermore, a second diffusion barrier may be inserted between the reference layer and bottom (or top) electrode. In a MRAM or STT-MRAM device, the hard mask contacts a bottom surface of the top electrode while the seed layer contacts a top surface of the bottom electrode in a bottom spin valve design. 
     The free layer may be a single layer made of Co, Fe, or an alloy thereof with one or more elements including Ni and B. In an alternative embodiment, the free layer has two magnetic layers FL 1  and FL 2  separated by an antiferromagnetic coupling layer such as Ru, or separated by a non-magnetic insertion layer that is Ta or the like which serves as a moment diluting layer. Furthermore, one of the aforementioned free layer configurations may further include a pair of dusting layers formed as bottom and uppermost layers in the free layer stack. The dusting layer is for example, one of Co, Fe, or an alloy thereof, which promotes PMA in the free layer and thereby increases He and Hk. A dusting layer also improves the tunneling magnetoresistance (TMR ratio) in the MTJ. A reference layer adjoins a surface of the tunnel barrier that is opposite the free layer. In a preferred embodiment, the reference layer has a synthetic antiparallel (SyAP) configuration wherein two magnetic layers called AP 1  and AP 2  are antiferromagnetically coupled through a Ru layer, for example. 
     A key feature is the diffusion barrier composition. In one aspect, the diffusion barrier is a single layer consisting of SiN, TiN, TaN, Mo, or CoFeX or CoFeNiX wherein X is one of Zr, Hf, Nb, Cu, Mo, B, P, and Ta. In particular, the diffusion barrier has a lower affinity to form a metal oxide than the metal in the adjoining cap layer or Hk enhancing layer. As a result, the diffusion barrier remains amorphous at temperatures proximate to 400° C., thereby avoiding a crystalline structure that enables non-magnetic metals from the top electrode (or bottom electrode) to pass through and diffuse along crystal boundaries into the cap layer or Hk enhancing layer. In other embodiments, the diffusion barrier is a stack of two or more layers including one or more non-magnetic metal or alloy layers of Mo, Zr, Nb, Hf, SiN, TIN, or TaN, and one or more of CoFeX or CoFeNiX wherein the CoFeX or CoFeNiX layer contacts the cap layer or Hk enhancing layer. The non-magnetic metal or alloy layer contacts the hard mask in a bottom spin valve, or contacts the bottom electrode or optional seed layer in a top spin valve configuration. 
     After all of the layers in the MTJ are laid down, an anneal process comprising a temperature of about 400° C. is performed to enhance PMA in the free layer and reference layer, increase coercivity (Hc) and Hk, and rearrange the lattice structure to obtain a higher TMR ratio. Thereafter, a conventional patterning and etching sequence is employed to convert a MTJ stack of layers formed on a bottom electrode into a plurality of MTJ elements. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a MTJ with a bottom spin valve design with a diffusion barrier formed between a cap layer and a top electrode according to a first embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view of a MTJ with a bottom spin valve design with a diffusion barrier formed between a cap layer and a top electrode according to a second embodiment of the present disclosure. 
         FIG. 3  is a cross-sectional view of the MTJ stack with a top spin valve design with a diffusion barrier formed between a Hk enhancing layer and bottom electrode according to a third embodiment of the present disclosure. 
         FIG. 4  is a cross-sectional view of the MTJ stack with a top spin valve design with a diffusion barrier formed between a Hk enhancing layer and bottom electrode according to a fourth embodiment of the present disclosure. 
         FIG. 5  is a cross-sectional view of a MTJ stack having two diffusion barriers therein according to a fifth embodiment of the present disclosure. 
         FIG. 6  is a cross-sectional view of a MTJ with two diffusion barriers each having a bilayer structure according to a sixth embodiment of the present disclosure. 
         FIG. 7  is a cross-sectional view of a MTJ with two diffusion barriers each having a bilayer structure according to a seventh embodiment of the present disclosure. 
         FIGS. 8-10  are cross-sectional views depicting a sequence of steps during the fabrication of a MTJ with a bottom spin valve design according to an embodiment of the present disclosure. 
         FIG. 11  is a table showing the free energy of oxide formation for various elements. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is a MTJ element wherein a free layer has thermal stability to at least 400° C. because of perpendicular magnetic anisotropy (PMA) therein that is established by interfaces with two metal oxide layers, and wherein a diffusion barrier prevents non-magnetic metals from a hard mask or bottom electrode from diffusing into an adjoining metal oxide layer thereby maintaining PMA and He in the free layer. The MTJ element may be employed in magnetic memory devices such as MRAM and STT-MRAM, in spintronic devices such as MAMR and STO, and as a TMR sensor between top and bottom shields in a read head if high temperature processing is employed. The MTJ may have a bottom spin valve, top spin valve, or dual spin valve design as appreciated by those skilled in the art. Although a plurality of MTJ elements is usually formed on a substrate, only one MTJ is depicted in the exemplary embodiments in order to simplify the drawings. The terms “Hk enhancing” and “cap” may be used interchangeably when referring to a metal oxide layer that induces PMA in an adjoining free layer. A cap layer as defined herein has a position above the free layer in a MTJ stack, and often adjoins a bottom surface of an overlying hard mask layer. On the other hand, a Hk enhancing layer may contact either a bottom surface of the free layer in a top spin valve, or the top surface of the free layer in a bottom spin valve design. 
     In related U.S. Pat. No. 8,946,834, we disclosed a MTJ having improved thermal stability by way of a tunnel barrier/free layer/cap layer configuration in which both of the tunnel barrier and cap layer are a metal oxide, and wherein dusting layers such as Co or CoFe are included as bottom and uppermost layers in the free layer stack comprising CoFeNiB to further improve thermal stability. According to related U.S. Pat. No. 9,006,704, the cap layer may further comprise an uppermost Ru layer in addition to a metal oxide (Hk enhancing) layer that adjoins the free layer. Ru is advantageously used since it has a higher energy of oxide formation than the metal in the adjoining Hk enhancing layer and reduces the likelihood of oxygen from the metal oxide migrating into the uppermost cap layer during high temperature processing. As a result, interfacial perpendicular anisotropy is maintained at the free layer/Hk enhancing layer interface which means PMA is not diminished by a 400° C. anneal process. An alternative structure for retaining a high degree of oxidation in the Hk enhancing layer at the free layer interface is disclosed in related U.S. Patent Application 2015/0008547 wherein the Hk enhancing layer may be a laminated cap layer including a lower interface oxide layer, and one or two overlying transition metal oxide layers that have a lower affinity for oxygen and oxide formation than a metal (Mg) in the interface oxide.  FIG. 11  lists free energy of oxide formation values for various metals. As one proceeds down the list of metals, there is a greater tendency for metal oxide formation when a metal is exposed to oxygen. 
     Now we have discovered that performance in a MTJ with a tunnel barrier/free layer/metal oxide stack of layers is further improved by including a diffusion barrier otherwise known as a protection layer between the metal oxide and an overlying hard mask in a bottom spin valve scheme to maintain FL coercivity after high temperature processing associated with CMOS technology where memory elements are integrated with semiconductor functionality. Alternatively, in a top spin valve with a metal oxide/free layer/tunnel barrier stack, the diffusion barrier is inserted between the metal oxide and bottom electrode (or optional seed layer) to prevent non-magnetic metals in the bottom electrode or optional seed layer from migrating into the metal oxide and disrupting PMA established by the metal oxide/free layer interface. In other words, metal migration through the metal oxide layer could cause a lower oxidation state in the Hk enhancing layer proximate to the free layer but is blocked by the presence of the diffusion barrier. Preferably, the metal selected for the diffusion barrier is heavier (has a higher atomic number) than the metal in the metal oxide layer. 
     Referring to  FIG. 1 , a MTJ  1  formed according to a first embodiment of the present disclosure is depicted wherein an optional seed layer  11 , reference layer  12 , tunnel barrier  14 , free layer  16 , cap layer  17 , diffusion barrier  18 , and hard mask  19  are sequentially formed on a bottom electrode  10  in a MRAM, STT-MRAM, or STO device. The bottom electrode may be formed on a substrate (not shown) that in some embodiments is a semiconductor structure that may include transistors and other associated control devices. In another embodiment, the bottom electrode may be a bottom shield in a TMR sensor or a main pole layer in a STO device. All layers are typically sputter deposited in a sputter deposition mainframe that has at least one oxidation chamber and an etching chamber. In a preferred embodiment, the tunnel barrier is formed by depositing a first metal layer, performing a natural oxidation or radical oxidation to transform the first metal to a first metal oxide, and then depositing a second metal on the first metal oxide. During a subsequent anneal process, the second metal layer absorbs oxygen from the free layer and first metal oxide to generate a substantially uniform metal oxide layer. It should be understood that in a STO device, the reference layer serves as a spin injection layer (SIL) and free layer functions as a field generation layer (FGL). 
     Seed layer  11  is generally employed to maintain or enhance PMA in reference layer  12  and facilitates smooth and uniform crystalline growth in the overlying MTJ layers. The seed layer may be one or more of NiCr, NiFeCr, Pd, Pt, Ta, Ru, or other metals or alloys used in the art. 
     Reference layer  12  preferably is a SyAP stack of layers having an AP 2 /coupling layer/AP 1  configuration wherein the AP 1  layer has an interface with the tunnel barrier, and each of AP 1  and AP 2  are one or more of Co, Fe, CoFe, or alloys thereof with an additional element including one or both of Ni and B. AP 1  and AP 2  layers have magnetic moments in opposite directions because of antiferromagnetic coupling provided by a Ru, Rh, or Ir coupling layer. PMA is induced or enhanced in the reference layer  12  through interface  30  with tunnel barrier  14 . In another embodiment, one or both of the AP 1  and AP 2  layers are a laminated structure that is one of (Co/Ni) n , (Co/Pd) n , (Co/Pt) n , (CoFe/Ni) n , (Co/NiFe) n , (Co/NiCo) n , or other laminated stacks exhibiting intrinsic PMA where n is between 2 and 30. Preferably, each Co or Co alloy layer in the laminated structure has a thickness between 0.5 and 5 Angstroms while each Pd, Pt, Ni, or Ni alloy layer has a thickness from 2 to 10 Angstroms. The Co or Co alloy layer is generally thinner than the Ni, Ni alloy, Pt, or Pd layer. 
     When the AP 1  layer comprises one of the aforementioned laminated structures, a transition layer that is Co, CoFe, or CoFeB may be inserted as the uppermost layer in the AP 1  stack. In some embodiments, a CoFeB/Co transition layer may be formed in which the CoFeB portion is from 4 to 8 Angstroms thick while the Co layer portion has a thickness from 3 to 5 Angstroms. Co is often used as the uppermost layer in the reference layer stack to form an interface  30  with the tunnel barrier layer since Co is more resistant to oxidation than a CoFeB (or CoFe) layer. In other words, oxygen is able to migrate out of a MgO tunnel barrier layer into a reference layer during annealing thereby causing partial oxidation of CoFeB, for example, which leads to a degradation in the MR ratio. 
     The tunnel barrier layer  14  may be any metal oxide layer that induces a spin dependent tunneling effect between the reference layer and free layer. Although MgO is often preferred since it provides an optimum TMR ratio, other materials including MgN, MgON, and oxides, nitrides, or oxynitrides of Al, Ti, Zn, Hf, AlTi, MgZn, or Ta may be employed as the tunnel barrier. Furthermore, laminations of one or more of the aforementioned materials may be selected as the tunnel barrier. 
     The free layer  16  adjoins a top surface of the tunnel barrier  14  in a bottom spin valve design, and according to one embodiment is a single layer that is one of Co, Fe, CoFe, or alloys thereof with an optional additional element including one or both of Ni and B. However, the free layer may have a bilayer or trilayer configuration represented by FL 1 /FL 2  or FL 1 /FL 2 /FL 3  where each of FL 1 , FL 2 , and FL 3  are magnetic layers made of Co, Fe, CoFe or alloys thereof wherein magnetic moments of all the two or three layers are aligned in the same direction through ferromagnetic coupling. The alloys may include Ni, B, or transition metal elements including Ta, Zr, Hf, Mg, Mo, and Nb. In other embodiments, the free layer has a FL 1 /Ru/FL 2  configuration wherein FL 1  and FL 2  layers are antiferromagnetically coupled. In yet another embodiment, the free layer is one of the laminated structures described previously with respect to reference layer composition. A laminated structure that is (Co/Ni) n  or the like is considered to have intrinsic PMA. 
     In a preferred embodiment, the free layer  16  has a FL 1 /M/FL 2  stack in which M is layer with a thickness from 5 to 10 Angstroms and is made of a non-magnetic element such as Ta, Ti, W, Zr, Hf, Nb, Mo, V, Mg, or Cr that provides a moment dilution effect. The FL 1  and FL 2  layers are ferromagnetically coupled through the M layer. In other words, for a fixed free layer thickness d, a FL 1 /M/FL 2  configuration will have a smaller in-plane magnetization component than a FL 1 /FL 2  stack. It follows that interfacial perpendicular anisotropy at the interface  40  between the tunnel barrier and free layer, and at the interface  41  between the free layer and cap layer more easily overcomes the shape anisotropy field and generates substantial PMA in the free layer with a FL 1 /M/FL 2  configuration. 
     PMA in the free layer is induced or enhanced by interfacial perpendicular anisotropy generated in a portion thereof adjacent to a first interface  40  with the tunnel barrier layer  14 , and in a portion adjacent to a second interface  41  with an Hk enhancing layer also known in bottom spin valve designs as cap layer  17 . Preferably, interfacial (surface) perpendicular anisotropy exceeds the shape anisotropy field for the free layer to provide PMA and a net magnetization direction that is perpendicular to the two interfaces. As a result, the free layer will have greater thermal stability, and higher He after high temperature processing up to about 400° C. than a free layer with in-plane magnetization. Here, the term “about” indicates that the temperature may reach 410° C. or 420° C. for a certain period of time due to thermal fluctuations above a target temperature in a process chamber. Note that the thickness of free layer  16  is represented by “d” in equation (4) presented previously. Thus, as the value for “d” is reduced, the perpendicular anisotropy field increases. We have found that PMA is established in the free layer by maintaining thickness d in  FIG. 1  in the 5 to 25 Angstrom range. In many free layer configurations, if thickness d is greater than about 25 Angstroms, then the shape anisotropy field is too large to be overcome by an out-of plane component, and the net magnetization of the free layer remains in-plane. 
     A cap layer  17  is formed on the free layer  16  and is preferably comprised of an oxide that is one of MgTaOx, MgO, SiOx, SrTiOx, BaTiOx, CaTiOx, LaAlOx, MnOx, VOx, Al 2 O 3 , TiOx, BOx, and HfOx. In some embodiments, the cap layer is a lamination of one or more of the aforementioned oxides. Preferably, the cap layer is MgO with a RA value of less than about 1 ohm-mm 2 , which is substantially less than the RA value of the tunnel barrier. In some cases, the total RA for the MTJ is &lt;10 ohm-mm 2  while in other embodiments, the total RA value is below 20 ohm-mm 2 . The cap layer may be formed by a process similar to the one used in tunnel barrier formation in which a metal or alloy layer is deposited and is then oxidized by a radical oxidation (ROX) or natural oxidation (NOX) process. Since the metal oxide cap does contribute to resistance×area (RA) value for the MTJ but does not factor into the TMR ratio, the thickness of a MgO cap layer or the like must be minimized to avoid a higher than desired RA. 
     A key feature of the present disclosure is a diffusion barrier  18  formed between the cap layer and an overlying hard mask  19 . The diffusion barrier is designed to block the migration of non-magnetic metals and their conductive oxides and nitrides from the hard mask into the cap layer  17  during high temperature processing up to about 400° C. thereby preventing a condition where hard mask metal(s) displace a portion of metal oxide along interface  41  and cause a decrease in PMA in free layer  16 . In addition, the diffusion barrier must not migrate into the cap layer. To satisfy this requirement, the material selected for the diffusion barrier should be heavier than Mg or the metal in the metal oxide cap layer. In this context, “heavier” is defined as having a higher atomic number in the Periodic Table. Secondly, the diffusion barrier should bind better to the cap layer than the hard mask materials. The term “bind” is understood to mean dipole-dipole attractions between two atoms or molecules that are also known as van del Waal forces. Furthermore, the diffusion barrier should remain amorphous or have the same crystal structure as the Hk enhancing (cap) layer after a 400° C. anneal. The diffusion barrier preferably has a thickness of 3 to 10 Angstroms and in some bilayer embodiments, may reach a total thickness of 20 Angstroms. A thickness of at least 3 Angstroms is considered necessary to form a continuous film without gaps therein in order to have sufficient blocking capability to prevent easy migration of metals from the hard mask into the cap layer. 
     In the first embodiment, the diffusion layer  18  is non-magnetic and is a single metal or alloy that is one of SiN, TiN, TaN, Mo, Cr, W, V, or Co a Fe b X c  wherein X is one of Zr, Nb, Hf, Mo, Cu, B, P, and Ta, (a+b+c)=100 atomic %, and the X content which is responsible for the non-magnetic character has a maximum content of 50 atomic % in the CoFeX alloy. Moreover, one of “a” or “b” may be zero to yield a CoX or FeX diffusion barrier. In an alternative embodiment, Ni may be included in the diffusion barrier to give a CoFeNiX alloy. 
     Above the diffusion barrier  18  is a hard mask  19  with a thickness in the range of about 50 to 300 Angstroms. One of the hard mask functions is to act as a protective layer with substantial resistance to a subsequent chemical mechanical polish (CMP) process where a top surface of the hard mask is planarized such that it is coplanar with an adjoining insulation layer as described in a later section. The hard mask also serves as an electrical connection from the underlying MTJ layers to a top electrode  20  and offers substantial resistance to oxidation. The hard mask is preferably made of one or more of Ta and Ru, or is Mo, or MnPt. In one example, the hard mask is a Ru/Ta stack where the lower Ru layer contacts a top surface of the diffusion barrier. 
     Above MTJ  1  is a top electrode  20  that contacts a top surface of hard mask  19 . The top electrode is usually one in a plurality of parallel lines that are formed in a MRAM array and typically comprises Cu or another efficient electrical conductor. In some embodiments, a cladding layer (not shown) is formed around an inner conductive layer. Only one top electrode is shown in order to simplify the drawing and focus on the MTJ structure that is the key feature of the present disclosure. For STT-MRAM, a current is directed through the MTJ between the top electrode and bottom electrode  10  for reading or writing purposes. In a MRAM write process, a current in the top electrode and bottom electrode generates a magnetic field on the MTJ that is capable of switching the magnetization direction in the free layer. In STO devices, the bottom electrode may be a main pole while the top electrode is a trailing shield as appreciated by those skilled in the art. 
     According to a second embodiment shown in  FIG. 2 , MTJ  2  retains all the layers previously described in the first embodiment. However, the diffusion barrier  18  is modified to have a multilayer configuration in which an upper layer  18   b  is a so-called “heavy” material to block non-magnetic metal diffusion from the hard mask  19 , and a lower layer  18   a  is employed to bind well with the cap layer  17  but is made of a less “heavy” material than in layer  18   b  to prevent diffusion of the upper layer into the cap layer. In a preferred embodiment, a CoFeB/Mo bilayer is selected for the diffusion barrier. However, another CoFeX or CoFeNiX alloy may replace CoFeB, and one of SiN, TaN, TIN, W, Cr, or V may replace Mo. The lower CoFeB layer contacts the Hk enhancing layer  17  and assures that Mo in the upper layer  18   b  does not diffuse into the Hk enhancing layer. Meanwhile, upper layer  18   b  preferably remains amorphous and binds well to the hard mask to prevent non-magnetic metal migration from the hard mask into the Hk enhancing layer. Lower layer  18   a  and upper layer  18   b  each have a minimum thickness of about 2 to 3 Angstroms. A maximum thickness of diffusion barrier layer  18  is preferably about 20 Angstroms. 
     The present disclosure also anticipates an embodiment with a top spin valve configuration as illustrated for MTJ  3  in  FIG. 3 . All layers are retained from  FIG. 1  except the order of formation is different. In particular, optional seed layer  11 , diffusion barrier  18 , Hk enhancing layer  17 , free layer  16 , tunnel barrier  14 , reference layer  12 , and hard mask  19  are sequentially formed on bottom electrode  10 . Moreover, there is a first interface  41  between a bottom surface of the free layer and a top surface of the Hk enhancing layer, and a second interface  40  where a top surface of the free layer adjoins a bottom surface of the tunnel barrier. There is also an interface  30  where the tunnel barrier contacts a bottom surface of the reference layer. In this case, the diffusion barrier blocks the migration of non-magnetic metals from the bottom conductor and optional seed layer through the Hk enhancing layer to interface  41  and the free layer. A similar advantage to that associated with MTJ  1  and MTJ  2  is realized. Namely, interfacial perpendicular anisotropy is maintained at interface  41 , and thus PMA as well as thermal stability and coercivity in the free layer is not degraded after high temperature processing to about 400° C. 
     In an alternative embodiment depicted in  FIG. 4  as MTJ  4 , all layers are retained from  FIG. 3  except the diffusion barrier  18  is modified to a bilayer configuration in which previously described layer  18   b  contacts a top surface of the seed layer or bottom electrode, and layer  18   a  is formed on layer  18   b  and adjoins a bottom surface of the Hk enhancing layer  17 . The diffusion barrier provides the same benefits that were explained with respect to the third embodiment in  FIG. 3 . 
     Referring to  FIG. 5 , the present disclosure also encompasses an embodiment with a bottom spin valve design that comprises two diffusion barrier layers wherein a first diffusion barrier is formed between the free layer and a first electrode, and a second diffusion barrier is inserted between the reference layer and a second electrode. In the exemplary embodiment, all of the layers in the first embodiment are retained, and a second diffusion barrier  13  is inserted between the optional seed layer  11  (or bottom electrode  10 ) and reference layer  12 . The second diffusion layer is non-magnetic and is preferably selected from one or more of the materials previously described with regard to diffusion barrier  18 . In an alternative embodiment (not shown), a second non-magnetic diffusion barrier is inserted between the reference layer and hard mask in  FIG. 3 . 
     In another embodiment shown in  FIG. 6 , MTJ  6  may have two diffusion barriers that both have a bilayer stack. The MTJ layers in  FIG. 2  are retained, and a second diffusion barrier  13  with upper layer  13   a  and lower layer  13   b  is inserted between the optional seed layer  11  (or bottom electrode  10 ) and reference layer  12 . Thus, the second diffusion layer may be a bilayer with a Mo/CoFeB configuration, for example, where the bottom layer  13   b  is a CoFeX or CoFeNiX alloy described previously while the upper layer  13   a  is one of SiN, TaN, TIN, Mo, V, Cr, or W. In  FIGS. 5-6 , the second diffusion barrier is advantageously used to prevent heavy metals from the bottom electrode  10  and seed layer  11  from migrating to interface  30  and thereby degrading PMA in the reference layer. 
     In  FIG. 7 , the present disclosure further anticipates a top spin valve (MTJ  7 ) embodiment wherein  FIG. 4  is modified to include a second non-magnetic diffusion barrier  13  having a bilayer configuration that is formed between the reference layer  12  and hard mask  19 . In this case, bottom layer  13   a  contacts a top surface of the reference layer and top layer  13   b  adjoins a bottom surface of the hard mask. Moreover, a second Hk enhancing layer (not shown) may be included between the reference layer and second diffusion barrier to establish a second metal oxide/reference layer interface and further enhance PMA in the reference layer. 
     The present disclosure also encompasses a method of forming a MTJ element having a configuration as disclosed herein between bottom and top electrodes. Referring to  FIG. 8 , a MTJ stack of layers is formed on a substrate that comprises a bottom electrode layer  10 . In one embodiment, the seed layer  11 , reference layer  12 , tunnel barrier  14 , free layer  16 , Hk enhancing layer  17 , diffusion barrier  18 , and hard mask  19  are sequentially formed on the substrate. All layers may be laid down by a sputter deposition technique. In a preferred embodiment, the tunnel layer is formed by depositing a first metal (or alloy) layer, oxidizing the first metal layer with a NOX or ROX method, and then depositing a second metal or alloy layer on the oxidized first metal layer. In some cases, the second metal layer may be oxidized and a third metal layer is deposited on the oxidized second metal layer. Thereafter, the free layer is depositing on the uppermost metal layer. 
     Once all of the layers in the multilayer stack and MTJ are formed, an annealing process may be performed that is comprised of at least a final treatment at about 400° C. for 1 minute to 5 hours, and preferably 30 to 50 minutes. The annealing process transforms the tunnel barrier stack into a substantially uniform metal oxide layer. 
     Referring to  FIG. 9 , a photoresist layer is deposited on a top surface of the MTJ and is patterned by a conventional method to form a photoresist mask  50  with a top-down view (not shown) in the desired shape of the MRAM device. Thereafter, an ion beam etch or reactive ion etch is used to transfer the photoresist mask pattern through the underlying layers in the MTJ to form sidewalls  1   s . In the exemplary embodiment, sidewalls  1   s  and  50   s  are formed along the same plane that is essentially orthogonal to a top surface of bottom electrode  10 . In other embodiments, sidewall  1   s  may be aligned with a slope such that the seed layer has a greater width in the x-axis direction than hard mask  19 . Here, the thickness of each MTJ layer is related to a distance along the y-axis direction. Preferably, the free layer  16  and reference layer  14  have a magnetization either in the (+) y-axis or (−) y-axis direction. 
     Referring to  FIG. 10 , an insulation layer  55  such as alumina or SiO 2  is deposited that contacts sidewall  1   s  to a thickness at least as great as the MTJ stack of layers. Then, a chemical mechanical polish (CMP) process is performed to remove the photoresist mask and form a top surface  55   t  of the insulation layer that is coplanar with top surface  19   t  of the uppermost layer in the MTJ. The MTJ is now often referred to as a nanopillar because the structure resembles a pillar with lateral dimensions along the x and z axes that is typically smaller than the thickness in the y-axis direction. From a top-down view (not shown), the MTJ nanopillar may have a circular, elliptical, or polygonal shape. Thereafter, a top electrode layer  20  is deposited and patterned to give a plurality of parallel lines by a well known process to form a magnetic memory device as depicted in  FIG. 1  or  FIG. 2 . 
     Formation of a MTJ with a top spin valve design such as one depicted in  FIG. 3  or  FIG. 4  follows the same process flow represented in  FIGS. 9-11  with the exception that the order of the layers deposited in  FIG. 9  is changed. In particular, an optional seed layer  11 , diffusion barrier  18 , Hk enhancing layer  17 , free layer  16 , tunnel barrier  14 , reference layer  12 , and hard mask  19  are sequentially formed on a bottom electrode  10  before sidewall  1   s  is generated in a patterning and etch process previously described with respect to  FIG. 10 . 
     To demonstrate the benefits of a diffusion barrier according to the present disclosure, patterned MTJ elements were fabricated as circular 100 nm MRAM devices and the He field for each was measured. Reference MTJ A has a seed layer/reference layer/MgO tunnel barrier/free layer/MgO cap layer/Ru/Ta configuration wherein the reference layer and free layer are CoFeB based material, the Hk enhancing layer is a MgO cap layer, and the hard mask is Ru/Ta. MTJ B and MTJ C are formed according to an embodiment of the present disclosure and differ from the reference MTJ A only by including a diffusion barrier between the Hk enhancing layer and the hard mask. All structures were deposited at room temperature and annealed at 330° C. for 30 minutes. A first coercivity measurement was taken after the first anneal step and then a second anneal process at 400° C. for 30 minutes was performed. A second coercivity measurement was obtained following the second anneal. As shown in Table 1, all MTJs have He=3200 Oe after the first anneal. However, He for MTJ A is undesirably reduced to 1600 Oe after the second anneal. A significant improvement is achieved in MTJs with a diffusion barrier since He is maintained at 3200 Oe in MTJ B, or even increased to 3300 Oe in MTJ C following the second anneal. In some applications, a minimum He of &gt;3200 Oe is required after high temperature anneal. In addition, it is important that He after a 400° C. anneal should not decrease compared with a value obtained after a 330° C. anneal. Note that 400° C. is an industry standard for semiconductor manufacturing to reduce the defect rate of the product and therefore becomes a requirement for embedded MRAM applications. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Coercivity results for MTJ elements with free layer/Hk enhancing  
               
               
                 layer/diffusion barrier/hard mask configurations 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Hk  
                   
                 Hc (Oe)  
                 Hc (Oe)  
               
               
                   
                 enhancing 
                 Diffusion 
                 after 330° C., 
                 after 400° C., 
               
               
                 MTJ 
                 layer 
                 barrier 
                 30 min. anneal 
                 30 min. anneal 
               
               
                   
               
               
                 A 
                 MgO 
                 None 
                 3200 
                 1600 
               
               
                 B 
                 MgO 
                 5 Angstroms Mo 
                 3200 
                 3200 
               
               
                 C 
                 MgO 
                 5 Angstroms CoFeB/ 
                 3200 
                 3300 
               
               
                   
                   
                 5 Angstroms Mo 
               
               
                   
               
            
           
         
       
     
     The benefits of the present disclosure are an improved PMA (Hc and Hk) for a free layer in a magnetic device that will increase the retention time for a memory element such as a MTJ following high temperature processing up to 400° C. that is required for MRAM or STT-MRAM devices integrated in CMOS structures. Furthermore, the processes and materials described herein are compatible with the design and processing requirements of magnetic devices. The fabrication of a MTJ according to an embodiment of this disclosure is not expected to significantly impact the manufacturing cost. 
     While this disclosure has been particularly shown and described with reference to, the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this disclosure.