# Patent

## Title
Low resistance high-TMR magnetic tunnel junction and process for fabrication thereof

## Abstract

A non-volatile magnetic memory element includes a fixed layer, a barrier layer formed on top of the fixed layer, and a free layer formed on top of the barrier layer, wherein the electrical resistivity of the barrier layer is reduced by placing said barrier layer under compressive stress. Compressive stress is induced by either using a compressive stress inducing layer, or by using inert gases at low pressure during the sputtering process as the barrier layer is deposited, or by introducing compressive stress inducing molecules into the molecular lattice of the barrier layer.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of U.S. application Ser. No. 11/674,124 filed on Feb. 12, 2007, entitled “Non-Uniform Switching Based Non-Volatile Magnetic Based Memory,” which claims priority to U.S. Provisional Application No. 60/853,115 filed on Oct. 20, 2006 entitled “Non-Uniform Switching Based Non-Volatile Magnetic Based Memory”; and is a further continuation-in-part of U.S. application Ser. No. 11/678,515 filed Feb. 23, 2007, entitled “A High Capacity Low Cost Multi-State Magnetic Memory,” which claims priority to U.S. Provisional Application No. 60/777,012 filed Feb. 25, 2006 entitled “A High Capacity Low Cost Multi-State Magnetic Memory”; and is a further continuation-in-part of U.S. application Ser. No. 11/739,648, filed Apr. 24, 2007 entitled “Non-Volatile Magnetic Memory with Low Switching Current and High Thermal Stability”; and is a further continuation-in-part of U.S. application Ser. No. 11/740,861, filed Apr. 26, 2007, titled “High Capacity Low Cost Multi-Stacked Cross-Line Magnetic Memory”; and is a further continuation-in-part of U.S. application Ser. No. 11/776,692, filed Jul. 12, 2007, titled “Non-Volatile Magnetic Memory Element with Graded Layer”; and is a further continuation-in-part of U.S. application Ser. No. 11/860,467 filed Sep. 24, 2007, titled “Low cost multi-state magnetic memory”; and is a further continuation-in-part of U.S. application Ser. No. 11/866,830 filed Oct. 3, 2007 entitled “Improved High Capacity Low Cost Multi-State Magnetic Memory”; and is a further continuation-in-part of U.S. application Ser. No. 11/932,940 filed Oct. 31, 2007 entitled “Current-Confined Effect of Magnetic Nano-Current-Channel (NCC) for Magnetic Random Access Memory (MRAM),” which claims priority to U.S. Provisional Application No. 60/863,812 filed Nov. 1, 2006 entitled “Novel Spintronic Device.”


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic based sensors and particularly to low switching current non-volatile magnetic based memory.
2. Description of the Prior Art
Computers conventionally use various media for data storage. Prevalent media include rotating magnetic media, such as hard disk drives (HDDs) for non-volatile storage, or dynamic random access memory (DRAM) and static RAM (SRAM), which are volatile and very costly but have fast random read/write access time. Solid state storage, such as solid-state-nonvolatile-memory (SSNVM) devices having memory structures made of NOR/NAND-based Flash memory, providing fast access time, increased input/output (IOP) speed, decreased power dissipation and physical size and increased reliability but at a higher cost.
There has been an extensive effort in development of Magnetic RAM (MRAM) as an alternative to existing data storage technologies. One of the problems with prior art memory structures is that the current and power requirements are too high to make a functional memory device or cell. This also poses a key concern regarding the reliability of such devices due to likely dielectric breakdown of the tunneling barrier layer and thereby making it non-functional.
The challenge with other prior art techniques has been that the switching current is too high to allow the making of a functional device for memory applications due to the memory's high power consumption. The high switching current is due, at least partly, to the memory element's electrical resistance. Several current solutions suggest that the switching current can be reduced by having the memory element pinned by two anti-ferromagnetic (AF)-couple layers resulting in spin oscillations or “pumping” and thereby reducing the switching current. Although these methods are helpful in reducing the memory element's resistance, further improvements in reducing the resistance of magnetic memory devices is desirable.
One prior art technique disclosed a structure and process for reducing switching current by including a fixed layer, a barrier layer formed on top of the fixed layer, a first free layer formed on top of the barrier layer, a non-uniform switching layer (NSL) formed on top of the first free layer, and a second free layer formed on top of the non-uniform switching layer. In the foregoing, switching current was applied, in a direction substantially perpendicular to the fixed, barrier, first free, non-uniform and the second free layers causing switching between states of the first, second free and non-uniform layers with substantially reduced switching current. Although switching current is reduced, there is still a need for further reduction in switching current.
What is needed is magnetic memory with lower resistance, thus further reducing power consumption associated with reading and switching data bits.
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 structure and a corresponding method for fabrication of 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 current-switching magnetic memory element having a magnetic tunneling junction (MTJ), for storing one or more bits of information. The MTJ includes at least a fixed layer, a barrier layer, and a free layer. When one of several different levels of current is applied to the memory element, the applied current causes the free layer of the MTJ to switch to one of several different magnetic moments, or states.
Of the several layers in a memory element, the barrier layer typically has the highest electrical resistance. By reducing the resistivity of the barrier layer, the memory element's electrical resistivity, and the associated switching is accelerated especially in a memory array where voltage across the memory element depends on the size of the transistor, which advantageously increases scalability of such memory device having multiple memory elements. The reduction in resistance is achieved by placing the barrier layer under compressive stress. As is discussed below, this is accomplished by using a compressive stress inducing layer (CSIL) in the memory element, in one embodiment of the present invention. In another embodiment of the present invention this is achieved by using vacuum or inert gas sputtering and in yet another embodiment of the present invention, this is achieved by introduction of compressive stress-inducing molecules into the barrier layer's molecular lattice.
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.



IN THE DRAWINGS
FIG. 1(a) shows relevant layers of a magnetic tunneling junction (MTJ) to include a fixed layer, a barrier (or tunneling) layer, and a free layer.
FIG. 1(b) shows relevant layers of an exemplary embodiment of a non-uniform switching based non-volatile magnetic memory element.
FIGS. 2(a) and 2(b) show examples of magnetic-tunnel-junctions with barrier layer in conjunction with the CSIL layer.
FIG. 3(a) shows an MTJ with a reduced resistance barrier layer under constant compressive stress caused by a layer of high-thermal expansion material deposited on top of the MTJ, and below the top electrode of the memory element, in accordance with an embodiment of the present invention.
FIG. 3(b) shows an MTJ with a reduced resistance barrier layer under constant compressive stress caused by a layer of high-thermal expansion material deposited at the lower end of the MTJ, above the bottom electrode of the memory element, in accordance with another embodiment of the present invention.
FIG. 4 shows an MTJ with a reduced resistance barrier layer under constant compressive stress caused by a layer of high-thermal expansion material deposited at the higher end of the MTJ, below the top electrode of the memory element as well as in between two MTJs for a multi-state memory design, in accordance with yet another embodiment of the present invention.
FIG. 5 shows the stress forces induced on the various layers in the MTJ after the deposition process is completed, and the layers have cooled to room temperature, according to an embodiment of the present invention.
FIG. 6(a) shows the resistivity of the barrier layer, made of MgO, measured in Ω/cm, as a function of compressive stress measured in MPascals.
FIG. 6(b) shows the resistivity of an MgO barrier (tunneling) layer of the memory element, as a function of the argon pressure during sputtering.
FIG. 7 shows a typical MgO barrier layer in a 3-dimensional lattice to comprise of MgO unit cells.
FIG. 8 shows the 3-dimensional lattice of a barrier layer with compressive stress inducing molecules introduced into the unit cells of the lattice structure according to an embodiment of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
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.
The following are herein incorporated by reference as though set forth in full:

    U.S. application Ser. No. 11/674,124, filed Feb. 12, 2007, titled “Non-Uniform Switching Based Non-Volatile Magnetic Based Memory” by Ranjan et alia,
    U.S. application Ser. No. 11/678,515, Filed Feb. 23, 2007, titled “A high capacity low cost multi-state magnetic memory” by Ranjan et alia,
    U.S. application Ser. No. 11/739,648 Filed Apr. 24, 2007, titled “Non-volatile magnetic memory with low switching current and high thermal stability” by Ranjan et alia,
    U.S. application Ser. No. 11/776,692, filed Jul. 12, 2007, titled “Non-Volatile Magnetic Memory Element with Graded Layer” by Ranjan et alia,
    U.S. application Ser. No. 11/740,861, filed Apr. 26, 2007, titled “High capacity low cost multi-stacked cross-line magnetic memory” by Ranjan et alia,
    U.S. Application No. 60/863,812, filed Nov. 1, 2006, titled “Novel spintronic device” by Wang,
    U.S. application Ser. No. 11/932,940 filed Oct. 31, 2007 titled “current-confined effect of magnetic nano-current-channel (NCC) for magnetic random access memory (MRAM)” by Wang,
    U.S. application Ser. No. 11/866,830 filed Oct. 3, 2007, titled “Improved high capacity low cost multi-state magnetic memory” by Ranjan et alia, and
    U.S. application Ser. No. 11/860,467 filed Sep. 24, 2007, titled “Low cost multi-state magnetic memory” by Ranjan et alia.


Typically, a non-volatile magnetic memory element includes a free layer, a barrier layer, and a fixed layer among a plurality of layers. Together, these layers comprise a magnetic tunneling junction (MTJ). Each memory element is comprised of a single MTJ, or multiple MTJs “stacked” upon each other in order to increase memory storage density. The fixed layer's magnetic polarity is static, or “fixed,” for example by an adjacent “pinning layer.” The free layer's magnetic polarity can be switched between at least two states by passing an electrical current through the MTJ.
As electrons flow through the free layer, the magnetic moment of the free layer polarizes the “spin-states” of the electrons. The quantum tunneling effect, which is well-known to those familiar with the art, causes a portion of the spins to tunnel through the barrier layer. The probability that any given spins will tunnel through the barrier layer is determined by the spin states of that electron and the magnetic moment of the fixed layer.
In non-relativistic quantum mechanics, the transmission coefficient is used to describe the behavior of waves incident on a barrier. The transmission coefficient represents the probability flux of the transmitted wave relative to that of the incident wave. It is often used to describe the probability of a particle tunneling through a barrier. The transmission coefficient is defined in terms of the incident and transmitted probability current density j according to:




  
    
      
        
          T
          =
          
            
              
              
                j
                transmitted
              
              
            
            
              
              
                j
                incident
              
              
            
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          1
          )
        
      
    
  




where jincident is the probability current in the wave incident upon the barrier and jtransmitted is the probability current in the wave moving away from the barrier on the other side.

As used herein, the electrical resistivity of the barrier layer is the inverse of its tunneling coefficient minus one:




  
    
      
        R
        =
        
          
            1
            T
          
          -
          1
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          2
          )
        
      
    
  




Therefore, for example, a barrier layer with a tunneling coefficient of zero is deemed to have infinite resistivity, whereas a barrier layer with a tunneling coefficient of 1 is deemed to have zero resistivity. It should be noted that Equation (2) is shown here to merely illustrate the relationship between a barrier layer's electrical resistivity with its tunneling coefficient. Equation (2) is not necessarily widely used in the art, nor is the value of resistivity necessarily measured in Ohms.
Thus, the barrier layer's resistivity is determined by the probability of spins tunneling through the barrier layer. Conversely, the probability of spins tunneling through the barrier layer is determined by the spin of the electrons. Since the spin of the electrons is determined by the magnetic moment of the free layer, changing the magnetic moment of the free layer affects the electrical resistivity of the barrier layer.
Abstractly, in the magnetic memory element, the state of the MTJ represents a binary value. At any given moment, the state of the MTJ is determined by its electrical resistance, which is a function of the current last applied to the memory element. Each MTJ is capable of having at least two unique resistance values. The resistance value is read to determine the state, in binary format, of the MTJ.
Effectively, the resistance of the MTJ is determined by the direction of the magnetic moments of the MTJs' free and fixed layers in respect to each other. The MTJ takes on one of several states at any given time, each having a unique electrical resistivity. In its simplest form, the MTJ has only two states, representing a binary 1 or 0. Other exemplary embodiments of the MTJ are disclosed in the applications incorporated by reference hereinabove, For example, U.S. Application No. 60/853,115, filed Oct. 20, 2006, titled “Non-Uniform Switching Based Non-Volatile Magnetic Base Memory” by Ranjan, discloses a non-uniform switching layer to reduce the current necessary to switch the magnetic moment of the free layer. Similarly, U.S. application Ser. No. 11/860,467 filed Sep. 24, 2007, titled “Low cost multi-state magnetic memory” by Ranjan et alia discloses MTJs that have several states. The several states represent more than one binary digit. For example, a 4-state MTJ can depict binary values 00, 01, 10, and 11.
Referring now to FIG. 1(a), relevant layers of an MTJ 99 is shown to include a fixed layer 101, a barrier (or tunneling) layer 103, and a free layer 107. The barrier layer 103 is shown formed on top of the fixed layer 101 and the free layer 107 is shown formed on top of the barrier layer 103.
The fixed layer 101 is generally made of magnetic material and in an exemplary embodiment is typically made of CoFe alloys. In one embodiment, the fixed layer 101 comprises of three layers namely, Cobalt-iron-boron alloy, CoFeB, Ruthenium, Ru, and Cobalt-iron alloy, CoFe, with CoFeB alloy layer being largely amorphous and placed adjacent to the barrier layer. The barrier layer 103 is generally made of non-magnetic material, such as magnesium oxide (MgO), titanium oxide (TiO2), aluminum oxide (Al2O3), ruthenium oxide (RuO), strontium oxide (SrO), europium oxide (EuO) and any combination of these as well as other minority oxides. The free layer 107 is generally made of magnetic material and in an exemplary embodiment is made of Cobalt-iron-boron alloy, CoFeB or multi-layers of CoFe or NiFe or other relatively lower anisotropy alloys of Cobalt, Co, iron, Fe, and nickel, Ni.
FIG. 1(b) shows relevant layers of one embodiment of a non-uniform switching based non-volatile magnetic memory element 100, in accordance with an embodiment of U.S. application Ser. No. 11/674,124, filed Feb. 12, 2007, titled “Non-Uniform Switching Based Non-Volatile Magnetic Based Memory” by Ranjan et alia, the contents of which are incorporated by reference hereinabove.
Referring now to FIG. 1(b), relevant layers of an exemplary embodiment of a non-uniform switching based non-volatile magnetic memory element 100 are shown. The memory element 100 is shown to generally include the layers included in the MTJ 99 in addition to other layers. More specifically, the memory element 100 is shown to include a bottom electrode 39, a seed layer 37 shown formed on top of the bottom electrode 39, an anti-ferromagnetic (AF) pinning layer 36, shown formed on top of the seed layer 37, a fixed layer 101 shown formed on top of the AF pinning layer 36, a barrier layer 103 shown formed on top of the fixed layer 101. Shown formed on the barrier layer 103 is a free layer 105, itself shown to be comprised of three layers, a free layer 1 102 shown formed on top of the barrier layer 103, a non-uniform switching-initiator layer (NSL) 104 shown formed on top of the free layer 1 102, and a free layer 2 106 shown formed on top of the NSL 104. Shown formed on top of the free layer 2 106 is a top electrode 38. In this embodiment of the non-uniform switching based non-volatile magnetic memory element 100, the fixed layer 101 is shown to be made of multiple layers.
The memory element 100, among other characteristics, exhibits low switching current when the magnetization switches from one polarity (or state) to another. For example, in one embodiment of the present invention, the memory element 100 has been known to switch with a program current, applied substantially perpendicular to the memory element, within the range of for example, 100 to 1200 μA (micro amps) for memory elements having dimensions of approximately 0.15 μm*0.2 μm.
In one embodiment of the memory element 100, the fixed layer 101 is multi-layered, the multiple layers of which, in an exemplary embodiment are the following layers: cobolt iron chromium, on top of which is formed Ruthenium X (RuX), where X is one or more of the following: Chromium Cr, Molybdenum (Mo) and Tantalum (Ta), on top of which is formed cobolt iron boron chromium x (CoFeBCrx), wherein, x is typically 0-15 atomic percent.
In one embodiment of the memory element 100, the barrier (or tunneling) layer 103, as may be referred thereto, is made of magnesium oxide (MgO) and may contain a thin layer of magnesium (Mg) to ensure minimal damage to the underlying CoFeB layer during the time MgO is deposited, and also to ensure more perfect crystalline growth of the MgO layer.
It is worthy to note that the tunneling layer 103 is crystalline but the underlying CoFeB layer may be mostly amorphous at the time of deposition, but may become crystalline after the annealing process. The amorphous structure of the free layer 101 results in a smoother interface for enhanced magnetic tunneling. The AF pinning layer 36 can be made of either platinum manganese (PtMn) or iron iridium manganese (IrMn), and requires magnetic annealing process to create a preferred anisotropy. BE refers to the bottom electrode 39 which is deposited prior to the fixed-layer 101. The free layer, made of layers 102-106, is deposited directly on top of the barrier layer 103.
When forming the memory element 100, the layers 102, 104 and 106 are deposited sequentially without breaking the vacuum such that there is direct magnetic coupling between the adjacent layers. The non-uniform switching layer 104 is introduced to initiate a non-uniform switching of the free layer during current-induced switching processes leading to a substantially lower switching current being 2 to 5 fold less than that which would have been needed in the absence of the layer 104.
In one embodiment of the present invention, a typical thickness of each of the layers 102 and 106 is less than 10 nanometers (nm). The ratio of thickness of the layer 102 to the layer 106 is typically less than 5. It should be understood that wherever numbers or values or ranges thereof are provided herein, they are to serve as examples only and other dimensions and sizes are contemplated.
The layers 102 and 106 are each typically chosen from alloys having one or more of ferromagnetic primary elements from, for example, the materials Co, Fe and Ni and may include additional non-magnetic elements such as boron (B), chromium (Cr), rhodium (Rh), molybdenum (Mo), silicon (Si), zirconium (Zr), or any other type of glass forming elements or alloy material. In one embodiment of the present invention, the layer 102 is made of cobolt iron boron chromium (CoFeBCrx), where x is in the range of 0-15 molecular % The non-uniform switching layer 104 is typically made of an alloy of Co, Fe, Ni having one or more of these elements and including typically less than 50 mol % of oxides, nitrides, sulfides or phosphides such as titanium dioxide (TiO2), aluminum dioxide (Al2O3), magnesium oxide (MgO), tantalum oxide (Ta2O5), hafnium oxide (HfO2), zirconium dioxide (ZrO2), or tantalum nitride (TaN), and other types of non-conductive material. The choice of these compounds is based on at least two criteria namely, high heat of formation so that it does not decompose during the plasma deposition process using rf- or dc-magnetron sputtering, with or without the presence of reactive gases such as oxygen or nitrogen and oxygen or nitrogen containing gases, and low-level of solubility to the base elements such as Co, Fe, Ni and alloys of these. While specific material and sizes are provided herein, they are to serve merely as examples with other suitable material and sizes being contemplated.
In one embodiment of the present invention, the bottom electrode 39 promotes electrical conductivity. The seed layer 37 acts as a template for growth of the AF pinning layer 36. The fixed layer 101(c) is crystalline, and locks into the AF pinning layer 36. The fixed layer 101(a) is largely amorphous. The fixed layers 101(c) and 101(a) generally have opposing magnetic moments, resulting in substantially zero net magnetic fields on the free-layers 102, 104 and 106.
It should be noted that FIGS. 1(a) and 1(b) depict only two embodiments of an MTJ. In other embodiments, not shown, other variations can be introduced. For example, the order of the layers in the MTJ can be reversed, or multiple MTJs can be stacked on top of each other.
FIG. 7 shows the molecular structure of the barrier (tunneling) layer 103, which is comprised of molecules 500 in a 3-dimensional lattice configuration. The natural tendency of MgO is to form into a lattice structure, absent a crystalline underlying structure. The lattice is often compromised of a plurality of generally cube-shaped “unit cells.” In this context a “unit cell” is often defined as the minimum number of atoms that replicate in the structure.
The molecular lattice of the barrier layer 103 shown in FIG. 7 is not under compressive stress, and has higher electrical resistivity. The higher electrical resistivity is undesirable because it results in a higher reading and switching current. It should be noted that electrical resistivity can be reduced by using a thinner barrier layer 103. However, reducing electrical resistivity by using a thinner barrier layer 103 comes at the expense of performance, reliability and scalability. The electrical resistivity of the barrier layer 103 is reduced when it is placed under compressive stress. The reduction of the electrical resistivity of the barrier layer 103, when placed under compressive stress, is due to modification of the band structure of the MgO through changing of its atom spacing, thus affecting the tunneling coefficient. This phenomenon is well-known to those skilled in the art. Reducing the electrical resistivity of the barrier layer 103 advantageously results in reduced reading and switching current, and also enables getting higher performance and higher reliability products.
Referring now to FIG. 2(a) a current-switching non-volatile magnetic memory element 110 is shown to include a bottom electrode (BE) 39 on top of which is shown formed a plurality of intermediate layers (shown by two discontinued lines), on top of which is shown formed a barrier layer 103, on top of which is shown a plurality of intermediate layers (shown by two discontinued lines), on top of which is shown formed a top electrode (TE) 38, on top of which is shown formed a compressive stress inducing layer (CSIL) 200, in accordance with an embodiment of the present invention. It is noted that wherever discontinued (swigly) lines appear in the figures of this patent document, they indicate layers that are typically formed between two other layers. For example, in one embodiment of the present invention, the intermediate layers between the BE 39 and the barrier layer 103 are analogous to the layers shown formed between the BE 39 and the barrier layer 103 of the memory element 100 (of FIG. 1(b). In another embodiment of the present invention, the intermediate layers are analogous to the layers shown formed in memory element 210 below the barrier layer 103(b). It is understood that other embodiments are similarly analogous.
In one embodiment of the present invention, the intermediate layers between the barrier layer 103 and the TE 38 are analogous to the layers shown formed between the barrier layer 103 and the TE 38 of the memory element 100 (of FIG. 1(b), while in another embodiment of the present invention, the intermediate layer are analogous to the layers shown formed in memory element 210 on top of the barrier layer 103(a).
In an exemplary embodiment of the present invention, the CSIL 200 is made of aluminum (Al) or an alloy of aluminum (Al) having typically a substantial portion of Al. However, in other embodiments of the present invention, the CSIL 200 can be formed from any conducting material whose thermal expansion coefficient exceeds that of the barrier layer 103. As will be discussed shortly, in yet other embodiments of the present invention, the CSIL 200 can be formed from a conducting material whose thermal expansion coefficient is substantially similar to, or even less than the barrier layer 103.
During the deposition process conducted when manufacturing the memory element 110, the CSIL 200 is deposited at high temperatures. Such high temperatures should exceed 250 degrees Celsius, and in an exemplary embodiment, should range between 325 and 400 degrees Celsius. As the CSIL cools, it shrinks. The rate at which a layer shrinks in response to temperature variance is sometimes depicted by a, which is calculated as follows:




  
    
      
        α
        =
        
          
            Δ
            ⁢
            
              
            
            ⁢
            
              l
              /
              l
            
          
          
            Δ
            ⁢
            
              
            
            ⁢
            T
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          3
          )
        
      
    
  




Where Δl/l represents the ratio of change of length, and ΔT represents change in temperature, as measured in degrees Kelvin. The unit for a is commonly K−1.
In one embodiment of the present invention, the α of the CSIL 200 should be at least twice that of the barrier layer 103. In an exemplary embodiment, the CSIL 200 has an α greater than 15×10−6. Table 1 below shows a list of some of the materials that can be used to form the CSIL 200, and their corresponding α:










TABLE 1







Material
α









Al
25 × 10−6



Cu
17 × 10−6



Zn
35 × 10−6



CuZn
19 × 10−6



Ag
18 × 10−6










The α of the barrier layer 103 depends upon the material used. In an exemplary embodiment, where the barrier layer 103 is formed from MgO, it has an α that is typically less than 1e-06.
In one embodiment of the present invention, the CSIL 200 has an α at least twice that of the barrier (tunneling) layer 103. Therefore, upon cooling, the CSIL shrinks more than the other layers. This shrinking differential induces compressive stress upon the other layers.
In other embodiments of the present invention, the CSIL 200 has an α that is substantially similar to, or even less than the α of the barrier layer 103, however, the CSIL 200 induces compressive stress upon the barrier layer 103 because the CSIL 200 is deposited at a substantially higher temperature than the barrier layer 103. Examples of materials with an α that is substantially similar to, or even less than the α of the barrier layer 103 are tungsten, W, tantalum, Ta, titanium-tungsten alloy, TiW, ruthenium, Ru, nickel-aluminum, NiAl, ruthenium-aluminum, RuAl, nickel-niobium, NiNb.
Several applications of the memory element 110 include non-volatile magnetic memory, thin-film head for hard-disc-drive and other sensors, such as pressure-sensor, magnetic field-sensor, stress-sensor, biological-sensors, biometric-sensors. Other applications are contemplated.
Referring now to FIG. 2(b) a current switching non-volatile magnetic memory element 120 is shown to include generally the same layers as that of the embodiment of FIG. 2(a) except that in this embodiment, the CSIL 200 is shown deposited towards the bottom of the MTJ, and more specifically on top of the bottom electrode 39 and below the barrier layer 103. As with the embodiment of FIG. 2(a), the CSIL 200 is deposited at high temperatures. As it cools, the CSIL 200 shrinks, inducing compressive stress upon the other layers, and advantageously reducing electrical resistivity of the barrier layer 103, resulting in reduced reading and switching current, which is desirable.
Referring now to FIG. 3(a) a current switching non-volatile magnetic memory element 130 is shown according to another embodiment of the present invention. The memory element 130 is shown to include generally the same layers as that of the embodiments of FIGS. 3(a) and 3(b) except that in this embodiment, the CSIL 200 is shown formed directly below the top electrode 38 and generally on top of the barrier layer 103.
Referring now to FIG. 3(b) a current switching non-volatile magnetic memory element 140 is shown according to another embodiment of the present invention. The memory element 140 is shown to comprise generally the same layers as that of the embodiments of FIGS. 3(a) and 3(b) except that in this embodiment, the CSIL 200 replaces the top electrode 38. In the embodiment shown in FIG. 3(b), the CSIL 200 is formed directly on top of the free layer, which may be the free layer 106 or 107 of the previous embodiments.
Referring now to FIG. 4, a current switching non-volatile magnetic memory element 210 is shown to include a top MTJ 211 and a bottom MTJ 212 in accordance with another embodiment of the present invention. The top MTJ 211 and bottom MTJ 212 are shown to comprise generally the same layers as that of the embodiments of FIG. 1(a). Also shown is a CSIL 200(b) formed in the bottom MTJ 212, above the free layer 107(b) of the bottom MTJ 212 and below the fixed layer 101(a). Also shown is a CSIL 200(a) formed in the top MTJ 211 above the free layer 107(a) and below the top electrode 38.
The embodiment of FIG. 4 shows how, when multiple MTJs are stacked on top of each other, compressive stress can be induced upon the barrier layer 103 of all of the MTJs by using multiple CSILs 200, thereby advantageously reducing the electrical resistivity of the barrier layers 103, resulting in reduced reading and switching current, which is desirable.
All of these embodiments result in inducing compressive stress upon the barrier layer 103, through the use of an CSIL 200. In other embodiments, the entire MTJ stack can be deposited up-side-down, with the free layer 106 deposited on top of the bottom electrode 39, the non-uniform switching layer 104 deposited on top of the second free layer 106, and so forth. As with the other embodiments, where the MTJ stack is deposited up-side-down, compressive stress is induced on the barrier layer 103 by depositing a CSIL 200, at high temperature, either below the bottom electrode 39, or above it, or in place of it.
Referring now to FIG. 5, the stress forces induced on the various layers in the MTJ 110 is shown after the deposition process is completed, and the layers have cooled to room temperature, according to an embodiment of the present invention. The stress forces are shown to include decompressive stress forces 111 induced upon the CSIL 200, and compressive stress forces 112 induced upon the remaining layers of the MTJ 110.
At room temperature, the CSIL 200 seeks to shrink or compress to a smaller size. The remaining layers in the MTJR 110, however, stop this, and in the process induce tensile stress upon the CSIL 200, as shown by arrows 112. The tensile stress force 111 induced upon the CSIL 200 by the remaining layers, is equal and opposite to the compressive stress force 112 induced upon the remaining layers by the CSIL 200. These two forces cancel each other out, resulting in an MTJ that neither expands nor shrinks.
Referring now to FIG. 6(a) the resistivity of the barrier layer 103, made of MgO, measured in Ω-cm, is shown as a function of compressive stress measured in MPascals.
Since the barrier layer 103 has lower resistivity when it is under compressive stress, and since the barrier layer 103 has often the highest resistance of any of the layers in the MTJ, the induced compressive stress reduces the electrical resistance of the MTJ. This results in reduced switching and reading current.
In other embodiments, compressive stress is induced upon the barrier layer 103 in the deposition process itself, wherein an inert gas, typically Argon (Ar), is present during the sputtering process.
Referring now to FIG. 6(b) the film stress of a layer of MgO, measured in MPascals, is shown as a function of the Ar sputtering pressure, measured in mTorrs. As can be seen from this graph, in order to induce compressive stress on a deposited MgO film, in one embodiment of the present invention, the Ar sputtering pressure should be less than 15 mTorrs. It should be noted that while the exact number of the compressive stress of the film and the argon pressure depends on the details of the sputtering system, cathode design, target-to-substrate spacing, gas flow rates and pumping rates, etc., a preferred argon pressure is generally below 5 mtorr at a power density of below 0.5 w/cm2.
Referring now to FIG. 7, a typical MgO barrier layer 103 in a 3-dimensional lattice is shown to comprise of MgO unit cells 500. As shown in FIG. 7, the unit cell lattice structure is not under compressive stress, and therefore has high electrical resistivity.
Referring now to FIG. 8, the 3-dimensional lattice of a barrier layer 103(2) is shown with CSIMs 501 introduced into the unit cells of the lattice structure according to an embodiment of the present invention.
The CSIMs 501, typically being smaller than the other unit cells 500 in the barrier layer 103(b), induce compressive stress upon the entire barrier layer 103(b), thus reducing its electrical resistivity, Examples of materials that may be used for the CSIM include, but are not limited to Ruthenium oxide, RuO2, strontitum oxide, SrO, strontium titanate, SrTiO3, calcium oxide, CaO, titanium oxide, TiO2, europium oxide, EuO. Of these, RuO2, SrO and EuO are the more preferred as they pertain similar cubic structure as MgO. The typical percentages of these are below 50 mol % with a preferred range being below 10 mol %. The final film can be formed preferably by rf-sputtering form one composite target although these can be premixed in the plasma through multiple target sources or by using reactive sputtering such as in presence of a Ar—O2 or such oxidizing gases.
Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modification as fall within the true spirit and scope of the invention.

## Claims

1. A current-switching non-volatile magnetic memory element including;
a magnetic tunneling junction (MTJ) including a barrier layer, said barrier layer having associated therewith an electrical resistivity, and
a compressive stress inducing layer (CSIL) formed on top of said MTJ and characterized to induce compressive stress on said MTJ thereby reducing electrical resistively of the barrier layer thereof.

2. The current-switching non-volatile magnetic memory element, as recited in claim 1, wherein said MTJ comprises:
a fixed layer;
said barrier layer formed on top of said fixed layer; and
a free layer formed on top of said barrier layer.

3. The current-switching non-volatile magnetic memory element, as recited in claim 1, wherein said MTJ comprises:
a free layer;
said barrier layer formed on top of said free layer; and
a fixed layer formed on top of said barrier layer.

4. The current-switching non-volatile magnetic memory element, as recited in claim 1, wherein said CSIL comprises one or more of the following materials: Al, Cu, Zn, CuZn, Ag., Ru, Ta, W, TiW, NiAL, RuAl, or NiNb.

5. The current-switching non-volatile magnetic memory element, as recited in claim 1, wherein said CSIL is deposited at a temperature in excess of 250° C.

6. A current-switching non-volatile magnetic memory element including;
a compressive stress inducing layer (CSIL); and
a magnetic tunneling junction (MTJ) formed on top of said CSIL,
said MTJ including a barrier layer, said barrier layer having associated therewith an electrical resistivity,
wherein said electrical resistivity of said barrier layer in said MTJ is reduced by compressive stress induced by said CSIL.

7. The current-switching non-volatile magnetic memory element, as recited in claim 6, wherein said MTJ comprises:
a fixed layer, said barrier layer formed on said fixed layer; and
a free layer formed on said barrier layer.

8. The current-switching non-volatile magnetic memory element, as recited in claim 6, wherein said MTJ comprises:
a free layer, said barrier layer formed on top of said free layer; and
a fixed layer formed on top of said barrier layer.

9. The current-switching non-volatile magnetic memory element, as recited in claim 6, wherein said CSIL comprises one or more of the materials: Al, Cu, Zn, CuZn, Ag., Ru, Ta, W, TiW, NiAL, RuAl, or NiNb.

10. The current-switching non-volatile magnetic memory element, as recited in claim 6, wherein said CSIL is deposited at a temperature in excess of 250° C.

11. A method of manufacturing current-switching non-volatile magnetic memory element comprising:
forming a magnetic tunneling junction (MTJ) including a barrier layer, said barrier layer having associated therewith an electrical resistivity; and
depositing at high temperature, a compressive stress inducing layer (CSIL) on top of said MTJ,
wherein said CSIL, upon cooling, shrinks, thus inducing compressive stress on said barrier layer and resulting in reduction of electrical resistivity of said barrier layer.

12. The method of manufacturing current-switching non-volatile magnetic memory, as recited in claim 11, wherein said forming the MTJ step comprises:
forming a fixed layer;
depositing said barrier layer on top of said fixed layer; and
depositing a free layer on top of said barrier layer.

13. The method of manufacturing current-switching non-volatile magnetic memory, as recited in claim 11, wherein said forming the MTJ step comprises:
forming a free layer;
depositing said barrier layer on top of said free layer; and
depositing a fixed layer on top of said barrier layer.

14. The method of manufacturing current-switching non-volatile memory element, as recited in claim 11, wherein the CSIL is deposited at a temperature in excess of 250° C.

15. The method of manufacturing current-switching non-volatile memory element, as recited in claim 11, wherein said CSIL comprises one or more of the materials: Al, Cu, Zn, CuZn, Ag., Ru, Ta, W, TiW, NiAL, RuAl, or NiNb.

16. A method of manufacturing current-switching non-volatile magnetic memory element comprising:
forming a magnetic tunneling junction (MTJ) including a barrier layer, said barrier layer having associated therewith an electrical resistivity,
wherein electrical resistivity of said barrier layer is reduced by depositing said barrier layer under compressive film stress in a sputtering process with an inert gas.

17. The method of manufacturing current-switching-non-volatile memory element, as recited in claim 16, wherein said inert gas used in said sputtering process is comprised substantially of Ar, Kr, or Xe.

18. The method of manufacturing current-switching non-volatile memory element, as recited in claim 16, wherein said inert gas used during said sputtering process is at pressures less than 15 m Torrs.

19. The method of manufacturing current-switching-non-volatile memory element, as recited in claim 16, wherein said MTJ is formed by;
forming a free layer;
depositing said barrier layer on top of said free layer, and
depositing a fixed layer on top of said barrier layer.

20. The method of manufacturing current-switching-non-volatile memory element, as recited in claim 16, wherein said MTJ is formed by the steps of:
forming a fixed layer;
depositing said barrier layer on top of said fixed layer; and
depositing a free layer on top of said barrier layer.

21. The method of manufacturing current-switching non-volatile magnetic memory element comprising:
forming a magnetic tunneling junction (MTJ) including a barrier layer, said barrier layer having associated therewith an electrical resistivity,
wherein electrical resistivity of said barrier layer is reduced by dissolving into a molecular lattice of said barrier layer, compressive-stress-inducing molecules (CSIM).

22. The method of manufacturing current-switching-non-volatile memory element, as recited in claim 21, wherein said MTJ is formed by the steps of:
forming a free layer;
depositing said barrier layer on top of said free layer; and
depositing a fixed layer on top of said barrier layer.

23. The method of manufacturing current-switching-non-volatile memory element, as recited in claim 21, wherein said MTJ is formed by the steps of:
forming a fixed layer;
depositing said barrier layer on said fixed layer; and
depositing a free layer on said barrier layer.

24. The method of manufacturing current-switching non-volatile magnetic memory element, as recited in claim 21, wherein the barrier layer is formed substantially from MgO, and the CSIM that is dissolved into the barrier layer molecular lattice is comprised substantially of: RuO2, SrO, TiO2, SrTiO2, CaO, or EuO.

25. The method of manufacturing current-switching non-volatile magnetic memory element, as recited in claim 21, wherein the percent of CSIM dissolved into the molecular lattice of the barrier layer is below 10 mol %.