Spin-transfer torque magnetic random access memory having magnetic tunnel junction with perpendicular magnetic anisotropy

A spin-torque transfer memory random access memory (STTMRAM) element includes a fixed layer formed on top of a substrate and a tunnel layer formed upon the fixed layer and a composite free layer formed upon the tunnel barrier layer and made of an iron platinum alloy with at least one of X or Y material, X being from a group consisting of: boron (B), phosphorous (P), carbon (C), and nitride (N) and Y being from a group consisting of: tantalum (Ta), titanium (Ti), niobium (Nb), zirconium (Zr), tungsten (W), silicon (Si), copper (Cu), silver (Ag), aluminum (Al), chromium (Cr), tin (Sn), lead (Pb), antimony (Sb), hafnium (Hf) and bismuth (Bi), molybdenum (Mo) or rhodium (Ru), the magnetization direction of each of the composite free layer and fixed layer being substantially perpendicular to the plane of the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a spin-transfer torque (STT) magnetic random access memory (MRAM), and, more particularly, to an STTMRAM element having magnetic moments oriented perpendicular to the plane of the substrate, and having lower programming current density while maintaining higher thermal stability.

2. Description of the Prior Art

Magnetic random access memory (MRAM) and resistive RAM (RRAM), which are a type of non-volatile memory, have gained great notoriety within recent years; however, efforts to improve their practicality of manufacturing and operation are well under way. These types of memory, which switch between a parallel and an anti-parallel states through the application of spin polarized current during programming, is also being investigated.

One type of MRAM is spin-transfer torque magnetic random access memory (STTMRAM). While STTMRAM are expected to be a universal memory solution, various hurdles stand in the way. The programming current of STTMRAM is still very high as main memory, therefore, the cell size is too large thereby preventing its adoption in consumer devices. Scaling STTMRAM below 65 nm is ideal; however reducing the programming current and shrinking the cell size introduces a variety of issues, such as thermal instability.

STTMRAM has significant advantages over magnetic-field-switched (toggle) MRAM, which has been recently commercialized. The main hurdles associated with field switched MRAM are its more complex cell architecture, which utilizes bypass line and remote write lines. Additional hurdles include its high write current (currently in the order of milliamps (mA)) and poor scalability, which cannot scale beyond 65 nano meters (nm). The fields and the currents required to write the bits increase rapidly as the size of the MTJ elements shrink. STT writing technology, by directly passing a current through the MTJ, overcomes these hurdles with much lower switching current (in the order of microamps (uA)), simpler cell architecture which results in a smaller cell size (for single-bit cells) and reduced manufacturing cost, and more importantly, improved scalability.

One of the challenges for implementing STT, is that during writing mode, such memory used in high-density and high-speed memory applications, require substantial reduction of the intrinsic current density to switch the magnetization of the free layer while maintaining high thermal stability, which is required for long-term data retention. Minimal switching (write) current is required mainly for reducing the size of the select transistor of the memory cell, which is typically coupled, in series, with MTJ to achieve the highest possible memory density. The channel width (in unit of F) of the transistor is proportional to the write current for a given transistor current drivability (uA/um). Minimal channel width or the width of MTJ element is required for achieving a reduced STTMRAM cell size. Second, smaller voltage across MTJ decreases the probability of tunneling barrier degradation and breakdown, ensuring write endurance of the device. This is particularly important for STTMRAM, because both sense and write currents are driven through MTJ cells.

One of the efficient way to reduce the programming current in STTMRAM is to use a MTJ with perpendicular anisotropy.

Incorporation of MgO-based MTJ with conventional perpendicular anisotropy material(s), such as FePt, into STTMRAM with such MTJ designs cause high damping constant, high magnetic anisotropy leading to undesirably high switching current density. Furthermore, during manufacturing, conventional higher ordering transformation temperature required for forming L10 order structure could undesirably degrades or even obliterates tunneling magneto-resistance (TMR) performance.

What is needed is a STTMRAM element having a MTJ with perpendicular magnetic anisotropy material(s) with an optimal combination of saturation magnetization (Ms) and anisotropy constant (Ku) to lower the damping constant and the magnetic anisotropy of the MTJ yielding a lower switching current density associated with the MTJ while maintaining high thermal stability and high TMR performance.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and a corresponding structure for a magnetic storage memory device that is based on current-induced-magnetization-switching having reduced switching current in the magnetic memory.

Briefly, an embodiment of the present invention includes a spin-torque transfer memory random access memory (STTMRAM) element that has a fixed layer having a first magnetization that is substantially fixed in one direction and formed on top of a substrate and a tunnel layer formed upon the fixed layer and a composite free layer having a second magnetization that is switchable in two directions and formed upon the tunnel barrier layer. The composite free layer is made of an iron platinum alloy with at least one of X or Y material, X being from a group consisting of: boron (B), phosphorous (P), carbon (C), and nitride (N) and Y being from a group consisting of: tantalum (Ta), titanium (Ti), niobium (Nb), zirconium (Zr), tungsten (W), silicon (Si), copper (Cu), silver (Ag), aluminum (Al), chromium (Cr), tin (Sn), lead (Pb), antimony (Sb), hafnium (Hf) and bismuth (Bi), molybdenum (Mo) or rhodium (Ru), the magnetization direction of each of the composite free layer and fixed layer being substantially perpendicular to the plane of the substrate. During a write operation, a bidirectional electric current is applied across the STTMRAM element switching the second magnetization between parallel and anti-parallel states relative to the first magnetization.These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the preferred embodiments illustrated in the several figures of the drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration of the specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized because structural changes may be made without departing from the scope of the present invention. It should be noted that the figures discussed herein are not drawn to scale and thicknesses of lines are not indicative of actual sizes.

In accordance with an embodiment of the present invention, a spin-transfer torque magnetic random access memory (STTMRAM) element includes a magnetic tunnel junction (MTJ) having a fixed layer and a free layer wherein the magnetic orientation of fixed layer and free layer is substantially perpendicular to the plane of the layers. The selection of the magnetic alloy for use in each of the fixed layer and the free layer, as disclosed herein, advantageously reduces the switching current density, and increases reliability in single domain switching. In an exemplary embodiment, the switching current density is less than 1 mega amp per centimeter squared (MA/cm2) while maintaining high thermal stability. Thermal stability is defined by KuV/kT. Ku represents magnetic anisotropy constant, V represents the volume of the free layer, k is Boltzmann's constant and T represents temperature. In an exemplary embodiment, high thermal stability is greater than 40.

In one embodiment, the fixed layer and free layer each are made of iron platinum doped alloy (FePtXY). X and Y each represent a material. For example, X represents any one of the materials: boron (B), phosphorous (P), carbon (C), nitride (N) and Y represents any of the materials: tantalum (Ta), titanium (Ti), niobium (Nb), zirconium (Zr), tungsten (W), silicon (Si), copper (Cu), silver (Ag), aluminum (Al), chromium (Cr), tin (Sn), lead (Pb), antimony (Sb), hafnium (Hf) and bismuth (Bi), molybdenum (Mo) or rhodium (Ru). In one embodiment, either X or Y is employed. In another embodiment, both X and Y are employed. X and Y are each doping materials (dope the essential material, such as FePt of FePtNi) and dilute the FePt or FePtNi alloys to obtain the desired Ku (anisotropy constant) and Ms (saturation magnetization) combination for reducing the damping constant, magnetization (Ms), and for reducing the distribution of perpendicular magnetic anisotropy. X is an interstitial element and Y is a trapping element. That is, Y serves to trap X during annealing, otherwise without the presence of Y, X may be undesirably diffused during annealing. In one embodiment, each of the materials X and Y have a concentration of 3-15 atomic percentage. X and/or Y also aid in lattice matching of subsequent (adjacent) layers above the layer which contains the X and/or Y.

While either the free layer or fixed layer may be formed on material that is doped with X or Y, it is understood that the damping constant and the magnetic perpendicular anisotropy of the fixed layer are much larger than that of the free layer.

Annealing, as known to those skilled in the art, is performed during manufacturing of the MTJ of the STTMRAM element to promote desired crystalline texture for higher TMR performance. In addition, the doped alloys, such as FePtXY, ensure reliable single domain switching. Single domain refers to uniform magnetization orientation throughout the same layer. In accordance with an embodiment of the present invention, all materials and alloys are selected with various goals in mind. First, an optimized combination of magnetic anisotropy constant (Ku), saturation magnetization (Ms) for low damping, and single domain reliable switching are achieved. Doping elements such as, B, P, C, N, Ta, Ti, Nb, Zr, W, Si, Cu, and Ag are used to dope an essential element, such as FePt. These elements lower the Msand Ku, ensuring that the anisotropy field of the MTJ is substantially perpendicular, and greater than 4πMs, where π represents ‘pi’ and commonly used in mathematical equations. Second, the various layers of the embodiments of the present invention may be grown in an L10 crystal structure, ideal for perpendicular anisotropy. Third, each layer may be grown to match the lattice constant of the subsequent (or adjacent) layer, in cubic (001) texture, to increase the tunnel magnetoresistance (TMR) effect. Fourth, a lower damping constant reduces the programming current density (Jco) of the MTJ. Fifth, an enhanced spin-polarization interface layer may be formed between the magnesium oxide (MgO) barrier layer and the adjacent pinned (or fixed) layer and the barrier layer and the free layer. This enhanced spin-polarization interface layer (SPEL) may be made of a cobolt iron boron (CoFeB)-based alloy. In alternative embodiments, CoFe-based alloys are employed, such as but not limited to CoFeTa or CoFeZr.

It is noted that by doping XY into FePt, the L10 ordering transformation temperature is lowered to preferably below 400 degrees C. The L10 ordering transformation temperature as used herein refers to the L10 FePt structure required for effectuating perpendicular anisotropy field, Hk.

The dependence of magnetic properties on a preferred direction of magnetization is referred to as “Magnetic anisotropy”. Magnetic anisotropy is also of considerable practical importance because it is exploited in the design of most magnetic materials of commercial importance.

Referring now toFIG. 1, the relevant layers of a STTMRAM element100are shown, in accordance with an embodiment of the present invention. STTMRAM element100is shown to comprise: bottom electrode (BE)101, one or more underlayer(s)102, fixed layer103, spin polarization enhanced (interface) layer (SPEL) layer105, tunnel (or barrier) layer107, SPEL layer108, free layer110, cap layer112and top electrode (TE)111.

The BE101is, as known to those skilled in the art, typically formed on top of the substrate (not shown inFIG. 1). The underlayer102, which in one embodiment is multi-layered and in another embodiment is a single layer, is formed on top of the BE101. The fixed layer103is formed on top of the underlayer102. The SPEL105is formed on top of the fixed layer103. The tunnel layer107is formed on top of the SPEL105. The SPEL108is formed on top of the tunnel layer107. The free layer110is formed on top of the SPEL108. The cap layer112is formed on top of the free layer110and the TE111is formed on top of the cap layer112.

The cap layer helps the free layer. That is, in one embodiment of the present invention, the cap layer112reduces damping during switching of the magnetization direction of the element100, when such switching occurs, for example, during a write operation. In one embodiment, the cap layer112is made of X and/or Y, as discussed above. In another embodiment, the cap layer112is made of a thin insulating oxide layer and on top thereof is formed the X and/or Y, accordingly, making the cap layer112a multi-layered structure. In one embodiment, the thin insulating layer comprising the cap layer112is made of MgO, AlO, HfO or any other oxide layer.

The SPEL108and the free layer110, in combination, comprise a composite free layer114. The fixed layer103and the SPEL105, in combination, comprise a composite fixed layer115. An MTJ120is formed from the combination of the composite fixed layer115, the tunnel layer107and the composite free layer114. In one embodiment of the present invention, the SPEL105is optional and in its absence, the tunnel layer107is formed directly on top of the fixed layer103with no SPEL105between the fixed layer103and the tunnel layer107.

The magnetization of the fixed layer103, as with respect to all of the fixed layers of the various embodiments of the present invention, is substantially fixed in one direction but the magnetization of composite free layer114, as with respect to all of the fixed layers of the various embodiments of the present invention, is switchable. InFIG. 1, the magnetization direction of both the fixed layer103and the free layer110is substantially perpendicular relative to the plane of the substrate onto which the element100is formed.

In an exemplary embodiment, the tunnel layer107is made of magnesium oxide (MgO). In an exemplary embodiment, the BE101is made of metal, such as but not limited to copper (Cu) or tungsten (W). In an exemplary embodiment, the layers105and108are each made of CoFeB. In an exemplary embodiment, the cap layer112is made of tantalum (Ta) or titanium (Ti) and the TE111is made of metal, such as but not limited to copper (Cu) or tungsten (W).

Proper formation of underlayer(s)102is critical to ensure proper crystalline growth of the subsequently deposited layers, i.e., layers103-112. In one embodiment, the crystalline growth of the fixed layer103and the free layer110is L10, which is a well known type of crystal structure. Arrows131and133indicate the magnetization direction of composite fixed layer115and composite free layer114, respectively. Arrows131and133indicate that the magnetization direction of each of layers115and114is perpendicular relative to the plane of layers101-108,110,112and111.

In embodiments where the SPEL105is not employed, the arrow131indicates the magnetic orientation of the fixed layer103.

In prior art memory modules with MTJs and STTMRAMs, the magnetic orientation of the free layer (or composite free layer) and fixed layer (or composite fixed layer) is parallel to the surface of the various layers (or substrate). Achieving perpendicular magnetic orientation of the free and fixed layers requires proper growth methods and alloy selection.

The underlayer102serves to promote a desired crystalline growth, such as L10, for the layers subsequently above the underlayer102during film deposition and/or during the post annealing process.

In some embodiments, fixed layer103is a magnetic alloy, particularly an FePt with at least one of X or Y alloy, to ensure proper cubic (001) crystal growth. In one embodiment and method of the present invention, a thin SPEL (SPEL105), typically less than 3 nano meters (nm) thick, is deposited upon fixed layer103, prior to the deposition of tunnel layer107. The SPEL105may be largely amorphous (greater than 50% by volume), to ensure cubic (001) growth of the crystal planes of tunnel layer107. Cubic-on-cubic growth of an amorphous SPEL105and tunnel layer107ensures a high tunnel magnetoresistance (TMR) within MTJ120.

Composite free layer114, in an exemplary embodiment, comprises at least two layers, in accordance with an embodiment of the present invention. Specifically, as shown inFIG. 1, composite free layer114comprises SPEL108, for enhancing the spin-polarization of electrons passing through MTJ120, and free layer110. In an exemplary embodiment, free layer110is made of FePt with at least X or Y alloy, having, as described earlier, a perpendicular magnetic orientation.

During a write operation, a bidirectional electric current is applied across the STTMRAM element100including the MTJ120. The electric current becomes spin polarized by transmission through or by reflection from the fixed layer103. The spin polarized current exerts a spin torque on the magnetic moment of the free layer110when it passes through it. The spin torque causes the magnetic moment of the free layer110to switch from a parallel state to an anti-parallel state or vice versa when the electric current density is sufficiently high. The magnetic moment of the free layer110can be switched to be parallel or anti-parallel relative to the magnetization direction of the fixed layer103, depending on the direction of electric current flow.

For example, in the embodiment ofFIG. 1, when electric current flows from the cap layer112down to the bottom electrode, in the manner shown by the arrow1000, the magnetization direction of the free layer110is switched to a parallel direction relative to that of the fixed layer103and if electric current flows from the BE101to the cap layer112, in a manner shown by the arrow1001, the magnetization direction of the free layer110is anti-parallel relative to the fixed layer103. The parallel magnetization configuration corresponds to a low resistance state (the logical “0”) of the MTJ120. The anti-parallel magnetization configuration corresponds to a high resistance state (the logical “1”) of the MTJ120. When the free layer110is in a parallel magnetization state, the arrow133is in the same direction as that of the arrow131and when the free layer110is in an anti-parallel magnetization state, the arrow133is in an opposite direction as that of the arrow131.

Stated differently, switching current, which is the spin polarized current, when applied and flowing in a direction shown by the arrow1001, flows through the STTMRAM element100in a direction from the layer101through subsequent layers and finally through the cap layer112or in a direction opposite thereto that is in a direction from the top electrode111through to the bottom electrode layer101, as shown by the arrow1000. While, inFIG. 1, the STTMRAM element100is shown to include bottom stacking where the free layer110is formed on top of the fixed layer103, top stacking may be employed where the fixed layer103is formed on top of the free layer110.

Hk, which represents the perpendicular anisotropy field of the element100, is preferably of the following relationship with respect to Ms: Hk>4πMs, in order to ensure a perpendicular anisotropy orientation.

The composite fixed layer115or the fixed layer103, each has a magnetization that is substantially fixed in one direction. The composite free layer114has a magnetization that is switchable between a parallel state, where the direction of its magnetization is parallel relative to that of the composite fixed layer115, and anti-parallel state where the direction of its magnetization is anti-parallel relative to that of the composite fixed layer113. During a write operation, a bidirectional electric current (1000and1001) is applied across the STTMRAM element100to switch the magnetization of the free layer110between parallel and anti-parallel states relative to the magnetization of the composite fixed layer115.

It should be pointed out that while the SPEL layers105and108, when deposited by themselves, tend to prefer to have an in-plane (i.e., perpendicular to the arrows131and133) magnetization orientation, that in the presence of the fixed and free layers, respectively, the magnetization orientation of the SPEL layers105and108become perpendicular, as shown by the arrows131and133, respectively.

Referring now toFIG. 2, relevant layers of STTMRAM200are shown, in accordance with another embodiment of the present invention. STTMRAM200is analogous to STTMRAM100, except for the composition of the free layer210of the composite free layer214. The composite free layer214includes one more layer than that of the composite free layer114. The composite free layer214is shown formed on top of the tunnel layer107and above the composite layer214is shown formed the cap layer112. The free layer210is shown formed of the SPEL108.

In one embodiment of the present invention, the additional layer of the composite free layer214is a non-magnetic spacing (or coupling) layer216. That is, the composite free layer214is shown comprising the SPEL108, the non-magnetic spacing layer216and a sub-free layer218. The free layer is itself composite and formed on the layers216and218. The layer216is shown formed on top of the SPEL108and on top of the layer216is shown formed the sub-free layer218and on top of the layer218is shown formed the cap layer112. The layers of the composite free layer214are shown on the right side ofFIG. 2. In one embodiment, the layer216is made of a ruthenium alloy (RuZ where Z is one of a number of materials, such as chromium, Cr, molybdenum, Mo, tungsten, W, iridium, Ir, tantalum, Ta, or rhodium, Rh). In one embodiment, Z is less than (<) 40 atomic percent of the RuZ material.

The layer218and the SPEL108are interlayer exchange coupled through the non-magnetic spacing layer216. In one embodiment, the layer218and the SPEL108are ferromagnetically coupled and in another embodiment, they are anti-ferromagnetically coupled.

In an exemplary embodiment, the sub-free layer218is made of FePt with either X, Y or both alloys (XY). Layer216is deposited between SPEL108and the sub-free layer218to increase magnetic coupling of the same.

In an exemplary embodiment, the layer216is made of the alloy RuZ where Z is less than 40 atomic percent of one or more of the following elements: Chromium (Cr), Molybdenum (Mo), W, Iridium (Ir), Tanatalum (Ta), and/or Rhodium (Rh).

As discussed relative toFIG. 1, the element200is written thereto by applying a electric current thereto, which results in spin current or switching current flowing through layers to switch the magnetic state of the composite free layer214.

Referring now toFIG. 3, relevant layers of STTMRAM element300are shown, in accordance with another embodiment of the present invention. STTMRAM element300is shown to comprise: bottom electrode101, underlayer(s)102, fixed layer305, non-magnetic spacing layer307, fixed layer309, SPEL311, tunnel layer313, SPEL315, free layer317, cap layer319and TE111. BE101is formed on a substrate or film (not shown inFIG. 3). Underlayer102is shown formed on top of the BE101, fixed layer305is shown formed on top of underlayer102, non-magnetic spacing layer307is shown formed on top of fixed layer305, fixed layer309is shown formed on top of the non-magnetic spacing layer307, SPEL311is shown formed on top of the fixed layer309, the tunnel layer313is shown formed on top of the SPEL311, the SPEL315is shown formed on top of the tunnel layer313, the free layer317is shown formed on top of the SPEL315, the cap layer319is shown formed on top of the free layer317and the TE is shown formed on top of the cap layer319.

The layers305-317together comprise the MTJ320. The fixed layer305, the non-magnetic spacing layer307, the fixed layer309, and the SPEL311, together comprise composite fixed layer321. The free layer317and the SPEL315together form the composite free layer323. In one embodiment of the present invention, the layer307is made of RuZ, as earlier discussed. As earlier indicated, in an exemplary embodiment, the SPEL311is made of CoFeB or other material indicated herein. The fixed layers305and309each may be made of an FePtXY alloy. The pinned magnetic orientations of the fixed layers305and309are anti-parallel relative to each other. Arrows331,332, and333indicate the magnetic orientation of fixed layer305, fixed layer309, and free layer317, respectively. As indicated by arrows331-333, the magnetic orientation of these layers is perpendicular to the surface of the layers (or the plane of the film or substrate) through which tunneling electrons pass. Having two fixed layers (305and309) with perpendicular, anti-parallel magnetic orientation advantageously ensures low demagnetizing field of the free layer323by cancelling or adjusting the net magnetization as indicated by arrows331and332. Such structures tend to provide a more symmetric switching current whereby the switching current when the free layer is switching from a parallel to an anti-parallel state is nearly close to that of as the switching current when the free layer is switching from an anti-parallel to a parallel state. To achieve substantially identical switching current density, Jco, associated with the MTJ320, from a parallel to an anti-parallel state and from an anti-parallel to a parallel state, the thicknesses of the fixed layers305and309may need to be optimized.

In an exemplary embodiment, the fixed layer305and the fixed layer309may be each made of an alloy of FePt with X and/or Y (or FePtX, FePtY or FePtXY), where X and Y are made of material indicated hereinabove. For example, alloy FePtXY may be an alloy of FePtNiTaB. Where the FePtXY alloy includes Ni, it may include less than 40 atomic percent of Ni, to enable a lower Msand lower transformation temperature, ensuring an L10 crystalline structure for the magnetic fixed layers305and309. Utilizing Cu and B also help to reduce the L10 transformation temperature to below 400° C. The Msor magnetization of an alloy containing only FePt is about 1140 electron mass unit (emu)/cubic centimeter (cc). When the alloy is diluted with Ni, Ta, Ti, and/or B, the Msis quickly reduced. For example, 10 atomic percent of Ni drops the Msof the alloy to 1070 emu/cc. Addition of other materials reduces the alloy's Msfurther still. Including 10 atomic percent of Ta as well as Ti, for an FePtNiTa(Y) alloy, drops the alloy's Msto about 700-800 emu/c.

Various other suitable FePtXY alloys are contemplated in accordance with an embodiment of the present invention. These include, for example, FePtTaB, FePtTiB, FePtCuB, FePtNiTaB. Further examples of appropriate FePtXY alloys include, for example, an FePtTaB alloy of less than 50 atomic percent of Fe, less than 50 (and substantially equal to that of the Fe) atomic percent of Pt, 3-20 atomic percent of Ta, and 3-20 atomic percent of B (with the understanding that the total atomic percent should clearly not exceed 100%); an FeNiPtTaB alloy of 45 atomic percent of Fe, 10 atomic percent of Ni, 45 atomic percent of Pt, 2-12 atomic percent of Ta, and 12-26 atomic percent of B; an FePtTaCuB alloy of 40 atomic percent of Fe, 50 atomic percent of Pt, 2-8 atomic percent of Ta, 6-12 atomic percent of Cu, and 10-22 atomic percent of B; and an FeNiPtTaCuB alloy of 45 atomic percent of Fe, 10 atomic percent of Ni, 45 atomic percent of Pt, 2-8 atomic percent of Ta, 6-12 atomic percent of Cu, and 10-22 atomic percent of B.

BE101may be a layer that efficiently conducts current, and may comprise one or more layers of Ta, NiNb, TaN, TiN, CrN, or AlN. BE101enables crystalline growth of the CrX alloy(s) of underlayer102. Ensuring crystalline growth of the Cr (200) planes of underlayer102ensures (001) cubic crystalline growth of the magnetic FePtXY layers, through lattice matching epitaxial growth. Crystalline plane growth increases the TMR, and reduces the resistance-area product (RA) of MTJ320, thus improving the performance of the MTJ.

In the various embodiments of the present invention, the underlayer102may be or may include as one of its layers, a seed layer. In an exemplary embodiment, this seed layer is made of Ta, NiNb, TaN, TiN, CrN or AlN.

In accordance with an alternative embodiment of the present invention, a thin layer of Pt may be deposited upon the underlayer102, and before the deposition of fixed layer305.

The composition of underlayer102may be selected from a variety of CrX alloys, where X may be one of numerous elements, such as, for example Cr—Mo, Cr—W, Cr—Ru, Cr—Ta, Cr—Ti, Cr—B, or Cr—Zr. Further, the composition of the underlayer102may be selected from a combination of these, such as, for example, CrMoB, CrTiB, CrMoTa, or CrMoW. For these alloys, X should be less than 30 atomic percent of the selected element(s).

The layer307may be an Ru—Z alloy where Z is less than 40 atomic percent of one or more of the following elements: Cr, Mo, W, Ir, Ta, and/or Rh.

In an exemplary embodiment, the cap layer319is made of Ta, the layer317is made of FePtTaB, the layers315and311are each made of material that is analogous to that of the SPEL108or105, and the tunnel layer313is made of material that is analogous to that of the layer107.

The layer305and the layer309are interlayer exchange coupled through the non-magnetic spacing layer307. In one embodiment, the layer305and the layer309are ferromagnetically coupled and in another embodiment, they are preferably anti-ferromagnetically coupled.

FIG. 4shows a STTMRAM element400in accordance with another embodiment of the present invention. The element400is analogous to the element300ofFIG. 3except that a substrate1009is shown upon which the element400is formed and on top of a metal area1111of the substrate1009is shown formed a bottom electrode1007and on top of the BE1007is shown formed a seed layer1005and on top of the seed layer1005is shown formed the underlayer102with the remainder of the layers of the element400being the same as and in the same location as the element300.

In one embodiment of the present invention, the underlayer102is made of CrX with X being less than thirty atomic percept of one or more of the materials: Mo, W, Ru, Ta, Ti, B or Zr.

In an exemplary embodiment, the seed layer1005is made of one or more conducting layers, such as but not limited of Ta, NiB, NiNb NiAl, NiRu, TaN, TiN, CrN or AlN. The role of the seedlayer is to allow better crystalline growth of the underlayer, which leads to improved crystalline growth of subsequent magnetic layers.

FIG. 5shows a STTMRAM element500in accordance with another embodiment of the present invention. The element500ofFIG. 5is analogous to the element400ofFIG. 4except that the free layer of the element500is made of multiple layers, namely, the first free layer317formed on top of the SPEL315and the second free layer318formed on top of the first free layer317. The cap layer319is formed on top of the second free layer318. In the foregoing embodiments, the SPELs311and315need not have a perpendicular magnetic moment.

In an exemplary embodiment, the first free layer has a higher anisotropy than the second free layer. For example, the second free layer may have an anisotropy that is less than 50% of that of the first free layer. In yet another exemplary embodiment, the first free layer317has a higher dispersion of the easy axis of magnetization than that of the second free layer318. The dispersion of the easy axis of magnetization is typically represented by delta-theta 50 (ΔΘ50) which is measured by using an x-ray diffractometer.

A higher ΔΘ50 refers to having higher dispersion and vice versa. A lower value of ΔΘ50 represents a more oriented magnetic structure. In one embodiment, the ΔΘ50 of the free layer317is greater than 10 degrees and the ΔΘ50 of the free layer318is less than 10 degrees and more typically below 7 degrees. A typical thickness of free layer317and free layer318is 0.2 nm to 3 nm. In one embodiment the free layer317is 0.5-1 nm and the free layer318is 0.8-2 nm.

STTMRAM element500is expected to have a lower switching current density in that the switching typically starts in the free layer317at a lower current and thereby initiates the magnetization switching of the entire free layer in the manner taught in U.S. patent application Ser. No. 11/776,692 filed on Jul. 12, 2007, and entitled “Non-volatile magnetic memory element with graded layer” by Rajiv Yadav Ranjan, which is herein incorporated by reference as though set forth in full.

FIG. 6shows a STTMRAM600in accordance with yet another embodiment of the present invention. The element600is analogous to that of the element500except that a second non-magnetic spacing layer308is shown formed between the first free layer317and the second free layer318. The second non-magnetic spacing layer308is made of the same material as the layer307. The direction of magnetization, shown by the arrows131and133, depends on the thickness of the non-magnetic spacing layer308which is comprised of Ru—Z alloy described earlier. The magnetic coupling is ferromagnetic if the two magnetization vectors (shown by the arrows) are aligned in the same direction and this typically happens when the Ru—Z spacing layer is below 0.85 nanometers (nm). The magnetic coupling is anti-ferromagnetic when the magnetization vectors are anti-parallel (as shown for the composite fixed layer321). STTMRAM600is expected to have a lower switching current due to the lower demagnetizing field.

In the various embodiments of the present invention, an optimal Ku and Ms combination of Ku and Ms are employed for reliable single domain switching and to lower the damping constant.

In some embodiments, some of the free layers of the various embodiments of the present invention are made of at least two layers of FePtXY and in this respect, the free layer is considered to be a graded layer with one embodiment thereof having one free layer have a higher anisotropy than the free layer that is formed on top thereof. In another embodiment, the graded design includes a higher dispersion angle associated with one free layer than that of the free layer that is formed on top thereof.

In some embodiments, the free layer is synthetic and has a perpendicular magnetic moment. In some embodiments, the free and fixed layers are synthetic. In some embodiments, the free layer is comprised of FePtXY like a granular film and likely to provide better uniformity of the magnetic properties over the wafer or from wafer-to-wafer. In some embodiments, the free layer is granular and made of FePtXY and the free layer and the fixed layer are synthetic.

Various alternative embodiments of the present invention are now discussed. In each of the embodiments ofFIGS. 1 through 3, the underlayer can be multiple layers. In one embodiment, the seedlayer can contain one or more conducting layers made of: Ta, NiNb, TaN, TiN, CrN, or AlN.

In some embodiments, some of the free layers of the various embodiments of the present invention are made of at least two layers of FePtXY and in this respect, the free layer is considered to be a graded layer with one embodiment thereof having one free layer have a higher anisotropy than the free layer that is formed on top thereof. In another embodiment, the graded design includes a higher dispersion angle associated with one free layer than that of the free layer that is formed on top thereof.

In an alternative embodiment, FePtNi replaces FePt in the free or fixed layer.

In some embodiments, the cap layer and the underlayer can each include at least one of Y. In another embodiment, Y is <30 atomic percent (%). In other embodiments, the cap layer and the underlayer can each include X.

In one embodiment, FePt has roughly atomic percentage that is equal between Fe and Pt.

In some embodiments, the SPELs do not have perpendicular anisotropy.

The STTMRAM element of the various embodiments of the present invention are submicron-sized or nano-scaled.

In any of the foregoing embodiments where the material FePtXY can be employed, FePtXYZ1 where ‘Z1’ is one or more of SiO2, TiO2, Ta2O5, ZrO2, TaN, TiN may be employed in place of FePtXY. The FePtXYZ1 mentioned above is also referred to as a granular alloy, mentioned above. In such cases the magnetic grains of the FePtXY alloy is generally isolated by segregation of Z1 (one or more of SiO2, TiO2, Ta2O5, ZrO2, TaN, TiN).

The structures including layers of the various drawings included herein are not necessarily drawn to scale. The thicknesses of the lines in the drawings are arbitrary and do not represent actual thicknesses or sizes of structures associated therewith.