# Patent

## Title
Low power magnetoresistive random access memory elements

## Abstract

Low power magnetoresistive random access memory elements and methods for fabricating the same are provided. In one embodiment, a magnetoresistive random access device has an array of memory elements. Each element comprises a fixed magnetic portion, a tunnel barrier portion, and a free SAF structure. The array has a finite magnetic field programming window Hwin represented by the equation Hwin≈(Hsat−σsat)−(Hsw+σsw), where Hsw is a mean switching field for the array, Hsat is a mean saturation field for the array, and Hsw for each memory element is represented by the equation HSW≅√{square root over (HkHSAT)}, where Hk represents a total anisotropy and HSAT represents an anti-ferromagnetic coupling saturation field for the free SAF structure of each memory element. N is an integer greater than or equal to 1. Hk, HSAT, and N for each memory element are selected such that the array requires current to operate that is below a predetermined current value.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a divisional of Ser. No. 10/997,118 filed on Nov. 24, 2004, now U.S. Pat. No. 7,129,098.


FIELD OF THE INVENTION
The present invention generally relates to magnetoelectronic devices, and more particularly relates to magnetoresistive random access memory elements that require low power for operation.
BACKGROUND OF THE INVENTION
Magnetoelectronic devices, spin electronic devices, and spintronic devices are synonymous terms for devices that make use of effects predominantly caused by electron spin. Magnetoelectronics is used in numerous information devices, and provides non-volatile, reliable, radiation resistant, and high-density data storage and retrieval. The numerous magnetoelectronics information devices include, but are not limited to, Magnetoresistive Random Access Memory (MRAM), magnetic sensors, and read/write heads for disk drives.
Typically, a magnetoelectronic information device, such as an MRAM, includes an array of memory elements. Each memory element typically has a structure that includes multiple magnetic layers separated by various non-magnetic layers. Information is stored as directions of magnetization vectors in the magnetic layers. Magnetic vectors in one magnetic layer are magnetically fixed or pinned, while the magnetization direction of another magnetic layer may be free to switch between the same and opposite directions that are called “parallel” and “antiparallel” states, respectively. Corresponding to the parallel and antiparallel magnetic states, the magnetic memory element has low and high electrical resistance states, respectively. Accordingly, a detection of change in the measured resistance allows a magnetoelectronics information device, such as an MRAM device, to provide information stored in the magnetic memory element.
FIG. 1 illustrates a conventional memory element array 10 having one or more memory elements 12. An example of one type of magnetic memory element, a magnetic tunnel junction (MTJ) element, comprises a fixed ferromagnetic layer 14 that has a magnetization direction fixed with respect to an external magnetic field and a free ferromagnetic layer 16 that has a magnetization direction that is free to rotate with the external magnetic field. The fixed layer and free layer are separated by an insulating tunnel barrier layer 18. The resistance of memory element 12 relies upon the phenomenon of spin-polarized electron tunneling through the tunnel barrier layer between the free and fixed ferromagnetic layers. The tunneling phenomenon is electron spin dependent, making the electrical response of the MTJ element a function of the relative orientations and spin polarization of the conduction electrons between the free and fixed ferromagnetic layer.
The memory element array 10 includes conductors 20, also referred to as digit lines 20, extending along rows of memory elements 12 and conductors 22, also referred to as word or bit lines 22, extending along columns of the memory elements 12. A memory element 12 is located at a cross point of a digit line 20 and a bit line 22. The magnetization direction of the free layer 16 of a memory element 12 is switched by supplying currents to digit line 20 and bit line 22. The currents create magnetic fields that switch the magnetization orientation of the selected memory element from parallel to anti-parallel, or vice versa.
FIG. 2 illustrates the fields generated by a conventional linear digit line 20 and bit line 22. To simplify the description of MRAM device 10, all directions will be referenced to an x- and y-coordinate system 50 as shown. A bit current IB 30 is defined as being positive if flowing in a positive x-direction and a digit current ID 34 is defined as being positive if flowing in a positive y-direction. A positive bit current IB 30 passing through bit line 22 results in a circumferential bit magnetic field, HB 32, and a positive digit current ID 34 will induce a circumferential digit magnetic field HD 36. The magnetic fields HB 32 and HD 36 combine to switch the magnetic orientation of the memory element 12.
Large bit and digit line currents are undesirable because memory array power consumption is a serious limiting factor in MRAM applications. High bit and digit currents require larger bit and digit lines and write circuits to handle the high currents. This may result in larger, more expensive MRAM devices. However, there is an ever-increasing demand for smaller memory devices. While smaller device size may be achieved through techniques such as patterning smaller memory elements, a smaller memory element increases the shape component of the anisotropy associated with the memory element. As the anisotropy increases, the amount of current necessary to alter the magnetization direction also increases.
Accordingly, it is desirable to provide a low power MRAM memory element that requires reduced or minimized current to alter the magnetic direction of the element. In addition, it is desirable to provide an MRAM device that requires low power for programming. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
FIG. 1 illustrates a conventional memory element array;
FIG. 2 illustrates magnetic fields generated at a memory element of a conventional memory element array;
FIG. 3 is a cross-sectional view of a memory element in accordance with an exemplary embodiment of the present invention;
FIG. 4 is a plan view of the memory element of FIG. 3 illustrating magnetic fields generated at the memory element;
FIG. 5 is a graphical illustration of a programming window of the memory element of FIG. 3;
FIG. 6 is a cross-sectional view of a memory element in accordance with another exemplary embodiment of the present invention;
FIG. 7 is a graphical illustration of the relationship between an anti-ferromagnetic coupling saturation field of an anti-ferromagnetic coupling material and the thickness of the anti-ferromagnetic coupling material;
FIG. 8 is a cross-sectional view of a memory element in accordance with a further exemplary embodiment of the present invention;
FIG. 9 is a schematic illustration of a memory element array having memory elements, shown in phantom, in accordance with an exemplary embodiment of the present invention;
FIG. 10 is a schematic illustration of a memory element having an elliptical shape; and
FIG. 11 is a schematic illustration of a memory element having a rectangular shape.



DETAILED DESCRIPTION OF THE INVENTION
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
Turning now to FIG. 3, in accordance with an exemplary embodiment of the present invention, a simplified sectional view of an MRAM array 100 comprises a scalable magnetoresistive memory element 102. In this illustration, only a single magnetoresistive memory element 102 is shown for simplicity in describing the embodiments of the present invention, but it will be understood that MRAM array 100 may consist of a number of magnetoresistive memory elements 102.
Magnetoresistive memory element 102 is sandwiched between a bit line 122 and a digit line 120. Bit line 122 and digit line 120 include conductive material such that a current can be passed therethrough. In this illustration, bit line 122 is positioned on top of magnetoresistive memory element 102 and digit line 120 is positioned on the bottom of magnetoresistive memory element 102, and is directed at a 90-degree angle to bit line 122. While bit line 122 and digit line 120 are illustrated with physical contact to memory element 102, it will be understood that the various embodiments of the present invention are not so limited and bit line 122 and/or digit line 120 may be physically separated from memory element 102. In addition, while bit line 122 is illustrated positioned above digit line 120, it will be understood that the reverse positioning of digit line 120 and bit line 122 may be utilized.
Magnetoresistive memory element 102 comprises a first magnetic region 104, a second magnetic region 106, and a tunnel barrier 108 disposed between first magnetic region 104 and second magnetic region 106. In one embodiment of the invention, magnetic region 104 includes a synthetic anti-ferromagnetic (SAF) structure 110, a structure having an anti-ferromagnetic coupling spacer layer 134 sandwiched between two ferromagnetic portions 130 and 132. Further, second magnetic region 106 may have an SAF structure 112, which has an anti-ferromagnetic coupling spacer layer 144 disposed between two ferromagnetic portions 140 and 142. However, it will be appreciated that second magnetic region 106 may have any structure suitable for forming an operable memory element 102.
Ferromagnetic portions 130 and 132 each have a magnetic moment vector 150 and 152, respectively, that are usually held anti-parallel by the anti-ferromagnetic coupling spacer layer 134. Magnetic region 104 has a resultant magnetic moment vector 154 and magnetic region 106 has a resultant magnetic moment vector 156. Resultant magnetic moment vectors 154 and 156 are oriented along an anisotropy easy-axis in a direction that is at an angle from bit line 122 and digit line 120. In one embodiment of the invention, the resultant magnetic moment vectors 154 and 156 are oriented at angle in the range of about 30 degrees to about 60 degrees from bit line 122 or digit line 120. In a preferred embodiment of the invention, the resultant magnetic moment vectors 154 and 156 are oriented at an angle of about 45 degrees from bit line 122 and digit line 120. Further, magnetic region 104 is a free ferromagnetic region, meaning that resultant magnetic moment vector 154 is free to rotate in the presence of an applied magnetic field. Magnetic region 106 is a pinned ferromagnetic region, meaning that resultant magnetic moment vector 156 is not free to rotate in the presence of a moderate applied magnetic field and is used as the reference layer.
The magnetic moment vectors 150 and 152 of the two ferromagnetic portions 130 and 132 can have different thicknesses or material to provide resultant magnetic moment 154 given by ΔM=M2−M1. In a preferred embodiment of the invention, the SAF structure 110 will be substantially balanced; that is, ΔM is less than 15 percent of the average of M2−M1 (otherwise simply stated as “the imbalance is less than 15 percent) and is more preferably as near to zero as can be economically fabricated in production lots.
During fabrication of MRAM array 100, each succeeding layer, discussed in more detail below, is deposited or otherwise formed in sequence and each memory element 102 may be defined by selective deposition, photolithography processing, etching, etc. using any of the techniques known in the semiconductor industry. During deposition of at least the ferromagnetic portions 130 and 132, a magnetic field is provided to set a preferred anisotropy easy-axis (induced intrinsic anisotropy). The provided magnetic field creates a preferred anisotropy easy-axis for magnetic moment vectors 150 and 152. As described in more detail below, in addition to intrinsic anisotropy, memory elements having aspect ratios greater than one may have a shape anisotropy that defines an easy axis that is parallel to a long axis of the memory element. This easy axis may also be selected to be at about a 30 to 60 degree angle, preferably at about a 45-degree angle, between the bit line 122 and the digit line 120.
FIG. 4 illustrates a simplified plan view of MRAM array 100 in accordance with an embodiment of the present invention. To simplify the description of magnetoresistive memory element 102, all directions will be referenced to an x- and y-coordinate system 160 as shown. To further simplify the description, only the magnetic moment vectors of region 104 are illustrated since they will be switched. As shown, a bit current IB 170 is defined as being positive if flowing in a positive x-direction and a digit current ID 172 is defined as being positive if flowing in a positive y-direction. A positive bit current IB 170 passing through bit line 122 results in a circumferential bit magnetic field, HB 174, and a positive digit current ID 172 will induce a circumferential digit magnetic field HD 176. The magnetic fields HB 174 and HD 176 combine to switch the magnetic orientation of first magnetic region 104 of memory element 102.
FIG. 5 is a graphical representation 200 of a programming region or window, in terms of magnetic field HB 174 and magnetic field HD 176, within which first magnetic region 104 may be switched reliably. In MRAM array 100, an individual memory element is programmed by flowing current through the bit line and the digit line proximate to the individual memory element. Information is stored by selectively switching the magnetic moment direction of first magnetic region 104 of the individual memory element 102. The memory element state is programmed to a “1” or “0” depending on the previous state of the bit; that is, a “1” is switched to a “0” or a “0” to a “1”. All other memory elements 102 are exposed only to fields from a single line (1⁄2-selected memory elements), or no lines. A memory element is switched reliably when the magnetic region 104 of the memory element switches deterministically between a “0” state and a “1” state upon application or withdrawal of a magnetic field. A memory element that switches somewhat randomly between a “0” state and a “1” state upon application or withdrawal of a magnetic field does not provide reliable or desirable switching.
Due to process and material variations, an array of memory elements 102 has a distribution of switching fields with a mean value Hsw and a standard deviation σsw. Typically, the array of memory elements 102 is required to meet a predetermined switching or programming error rate. Accordingly, to program the memory elements 102 in MRAM array 100 with approximately the same currents, the applied field produced from the currents preferably is larger than the mean switching field Hsw by no less than approximately Nσsw, where N is a positive number large enough to ensure the actual switching error rate does not exceed the predetermined programming error rate, and is typically greater than or equal to 6 for memories whose size are about 1 Mbit or larger.
In addition, there is a maximum saturation field HSAT that can be applied to a selected memory element to ensure reliable switching. The field HSAT corresponds to that field which, when applied to magnetic region 104, causes magnetic moment vector 150 and 152 to be aligned approximately parallel. Therefore, HSAT is known as the saturation field of the SAF structure in region 104 and is a measure of the anti-ferromagnetic coupling between layers 130 and 132. Also due to process and material variations, an array of memory elements 102 has a distribution of saturation fields with a mean value HSAT and a standard deviation □sat. Therefore, the applied field preferably is kept less than approximately HSAT−□sat or the selected memory element will not be programmed reliably.
Thus, for reliable programming that meets a predetermined switching error rate or has an error rate below the predetermined switching error rate, there is an operating window 202 for an applied magnetic field H that results from programming fields HB 174 and HD 176. The magnitude of the operating window, Hwin, along the dotted line shown in FIG. 5 is represented approximately by the equation Hwin≈(Hsat−□sat)−(Hsw+□sw). Inside this window 202, substantially all the memory elements can be programmed without error. Outside this window, the memory elements cannot be programmed or cannot be programmed without possible errors. For example, the region 204 of graphical representation 200 is that region where a magnetic field H applied to memory element 102 by bit current IB 170 and digit current ID 172 is greater than HSAT and first magnetic region 104 of magnetoresistive memory element 102 does not switch reliably between both the “1” and “0” states. The region 206 of graphical representation 200 is that region where the applied field H is less than the switching field HSW and first magnetic region 104 of magnetoresistive memory element 102 does not switch.
The magnetic switching field for writing to memory element 102 is represented by the equation:

HSW≅√{square root over (HkHSAT)},

where Hk is the total anisotropy of first magnetic region 104 and HSAT, as described above, is the anti-ferromagnetic coupling saturation field, that is, HSAT is the maximum magnetic field at which first magnetic region 104 of magnetoresistive memory element 102 will switch reliably. Hk may be represented by the equation:

Hk(total)=Hk(intrinsic)+Hk(shape),

where Hk(intrinsic) is the intrinsic anisotropy of the material comprising magnetic region 104 and Hk(shape) is the anisotropy due to the shape of magnetic region 104. Similarly, HSAT may be represented by the equation:

HSAT(total)=HSAT(intrinsic)+HSAT(shape).

In this equation, HSAT(intrinsic) is the magnetic field at which the magnetic layers of first magnetic region 104 are substantially parallel to each other when formed as continuous films and HSAT(shape) represents the magnetostatic coupling of the magnetic layers of magnetic region 104 as a result of the shape of the magnetic region 104.
Accordingly, to reduce the power required by magnetoresistive memory element 102, that is, to reduce or minimize the current required to switch first magnetic region 104 of magnetoresistive memory element 102, HSW of magnetic region 104 may be reduced or minimized. To minimize HSW, Hk(total) or HSAT(total) or both may be reduced or minimized. Thus, in accordance with an embodiment of the invention, Hk(intrinsic), Hk(shape), HSAT(intrinsic), or HSAT(shape), or any combination thereof, may be reduced or minimized.
Referring again to FIG. 3, in accordance with an exemplary embodiment of the present invention, to reduce or minimize the current required to switch first magnetic region 104, and thus reduce the power required by memory element 102, ferromagnetic portions 130 and 132 may be fabricated such that magnetic region 104 has a low Hk(total) value. However, in a preferred embodiment of the invention, magnetic region 104 may not have an Hk(total) value that is so low that magnetic region 104 and, hence, magnetoresistive memory element 102, are thermally unstable and volatile. Thermal instability refers to the switching of the memory state due to thermal fluctuations in the magnetic layers 130 and 132. The energy barrier Eb to thermal fluctuations for first magnetic region 104 is represented approximately by the equation Eb=MS×V×Hk, where MS is the saturation magnetization of the magnetic material in layers 130 and 132, V is the total volume (area×thickness) of layers 130 and 132, and Hk is as defined above. In one embodiment of the invention, Hk(total) has a value of less than about 15 Oe-microns divided by region width, where the “region width” is the dimension (in microns) of the first magnetic region 104 that is orthogonal to the longitudinal axis of the first magnetic region 104 and the thickness of the first magnetic region 104. In a preferred embodiment of the present invention, Hk(total) has a value in the range of from about 10 Oe-microns÷region width (in microns) to about 15 Oe-microns÷region width (in microns).
In one embodiment of the invention, to reduce Hk(total) and, hence, to reduce the power requirements of memory element 102, ferromagnetic portions 130 and 132 may be formed of one or more layers of material or materials having a low Hk(intrinsic) value. As used herein, the term low Hk(intrinsic) value means an Hk(intrinsic) value of less than or equal to about 10 Oe. Examples of materials that have a low Hk(intrinsic) value that is suitable for forming ferromagnetic portions 130 and 132 of magnetic region 104 but that does not render magnetic region 104 thermally unstable include nickel (Ni), iron (Fe), cobalt (Co), or alloys of Ni, alloys of Fe, or alloys of Co, such as NiFeB, NiFeMb, NiFeTa, NiFeCo, and the like. Ferromagnetic portions 130 and 132 may be formed of the same material or may be formed of different materials having a low Hk(intrinsic) value.
In accordance with another embodiment of the present invention, to reduce the power requirements of memory element 102, magnetic region 104 may be fabricated utilizing a material or materials that produce a low Hk(shape) value to form ferromagnetic portions 130 and 132. Again, however, it is preferred that the material that forms magnetic region 104 may not produce an Hk(total) value that is so low that magnetic region 104 and, hence, magnetoresistive memory element 102, are thermally unstable and volatile. As discussed above, materials producing a low Hk(shape) value for a given memory element shape include materials having a low saturation magnetization MS. As used herein, the term “low saturation magnetization”, or “low magnetization”, refers to those materials having a magnetization that is less than or equal to the magnetization of Ni80Fe20. Ni80Fe20 has a magnetization approximately equal to 800 kA/m and a saturation flux density of approximately 1 Tesla. As the magnetization of the material(s) that form ferromagnetic portions 130 and 132 also directly affect the magnetostatic coupling of the layers, the use of a low magnetization material(s) for ferromagnetic portions 130 and 132 also serves to reduce or minimize HSAT(shape). Accordingly, the lower the magnetization of the material(s) of portions 130 and 132 is, the lower the Hk(shape) and the HSAT(shape) values are. Low magnetization materials suitable for forming ferromagnetic portions 130 and 132 comprise Ni80Fe20 and alloys of Ni, alloys of Fe, or alloys of Co, such as, for example, NiFeB, NiFeMb, NiFeTa, and NiFeCo. Again, ferromagnetic portions 130 and 132 may be formed of the same or different low magnetization materials.
The doping of Ni80Fe20 with materials such as molybdenum, tantalum, boron, and the like also may result in a material with a low Hk(intrinsic) value and a magnetization less than those of Ni80Fe20, thus facilitating fabrication of a low power memory element 102. However, doping with such materials also may decrease the magnetoresistance through tunnel barrier 108, and thus decrease the performance of memory element 102. Although the spin polarization of the tunneling electrons determines the magnetoresistance, low magnetization materials typically also have low spin polarization. Accordingly, in one alternative embodiment of the invention, as illustrated in FIG. 6, a magnetoresistive memory element 250 may have a ferromagnetic portion 132 that comprises two materials, a first material 252 with a low magnetization that reduces the value of Hk(shape) of magnetic region 104 and a second material 254, disposed close to the tunnel barrier 108, with a high polarization that compensates for the decrease in the magnetoresistance due to the first material 252. As used herein, the term “high polarization material” is any material having a spin polarization that is greater than or equal to Ni80Fe20. Second material 254 may comprise material such as, for example, Co, Fe, and CoFe and may also comprise Ni80Fe20 when the first material 252 has a magnetization lower than Ni80Fe20. In a preferred embodiment of the invention, first material 252 and/or second material 254 comprise materials that also have a low Hk(intrinsic), as described above. As first magnetic region 104 is preferably a moment-balanced SAF structure, in one embodiment of the invention, ferromagnetic portion 130 has a thickness such that the magnetic moments of ferromagnetic portions 132 and 130 have the same magnitude. In another embodiment of the invention, ferromagnetic portion 130 also comprises first material 252 and second material 254.
The Hk(shape) of a single magnetic layer is approximately proportional to Nd×Ms×t/w where Nd is a demagnetizing factor that increases with aspect ratio, t is the thickness of the layer, and w is the region width. This formula also applies for the layers in the SAF structure of first magnetic region 104. Although the SAF structure of first magnetic region 104 does reduce Hk(shape) compared to a single film of comparable thickness 2×t, the Hk(shape) is still finite due to asymmetry in the switching process. The magnetic layers are not perfectly antiparallel during switching, so that each layer's magnetostatic fields (that produce Hk(shape)) do not perfectly cancel one another.
In another embodiment of the present invention, magnetic region 104 may be fabricated with the minimum possible thickness t for ferromagnetic layers 130 and 132. As discussed above, a thinner thickness t will result in a smaller Hk(shape) and Hsat(shape) since the magnetostatic fields that produce Hk(shape) and Hsat(shape) are proportional to thickness. The minimum thickness possible is limited by the requirement of thermal stability. Note that by reducing t, both Hk(shape) and total volume V of layers 130 and 132 are reduced for the bit, so that the energy barrier is reduced by approximately t2. In addition to the thermal stability requirement, the minimum thickness is also limited by the ability to grow a high quality continuous magnetic film on top of the tunnel barrier. In one embodiment of the invention, the optimum minimum thickness t of layers 130 and 132 is within a range of from about 3.5 nm to about 5 nm.
Referring again to FIG. 3, in accordance with a further embodiment of the present invention, to reduce the power requirements of memory element 102, first magnetic region 104 also may be fabricated to have a low Hk(shape) value by forming it in a shape having a low aspect ratio. In one embodiment of the invention, first magnetic region 104 has a length preferably measured along a long axis of region 104, and a width measured orthogonal to the length, and a length/width ratio in a range of about 1 to about 3 for a non-circular plan. For example, as illustrated in FIG. 10, in one embodiment of the invention, a memory element 400, which may be the same as memory element 102, may have a first magnetic region 104 of an elliptical shape with a length 402 and width 404 and with a length/width ratio of about 1 to about 3. In another embodiment of the invention, as illustrated in FIG. 11, a memory element 410, which may be the same as memory element 102, may have a first magnetic region 104 of a rectangular shape with a length 412 and width 414 and having a length/width ratio of about 1 to about 3. Alternatively, the first magnetic region 104 of a memory element may be circular in shape (length/width ratio of 1) to minimize the contribution to the switching field from shape anisotropy Hk(shape) and also because it is easier to use photolithographic processing to scale the device to smaller dimensions laterally. However, it will be understood that first magnetic region 104 can have any other suitable shape, such as square or diamond. In a preferred embodiment of the invention, first magnetic region 104 has a length/width ratio in a range of about 2 to about 2.5.
In accordance with yet another embodiment of the present invention, magnetic region 104 may be fabricated to reduce or minimize HSAT(total) to reduce the power requirements of memory element 102. Again, however, as discussed above with reference to FIG. 5, magnetic region 104 may not have an HSAT(total) value that is so low that there is no operable programming window. In other words, while HSAT(total) may be reduced or minimized, its value preferably is such that the programming window operable for switching magnetic region 104 can be defined as above by the equation Hwin≈(HSAT−□sat)−(Hsw+□sw), where Hwin is a magnetic field applied to magnetoresistive memory element 102 by currents ID and IB to switch magnetic region 104. In one embodiment of the invention, HSAT(total) has a value in the range of from about 150 Oe to about 350 Oe. In a preferred embodiment, HSAT(total) has a value less than or equal to approximately 180/w0.5 (Oe), where w is the region width of magnetic region 104, as previously described.
At present memory element dimensions in the range of 0.5 to 1 micron, the dominant contribution to HSAT(total) is from HSAT(intrinsic). HSAT(intrinsic) is determined by the anti-ferromagnetic coupling material that comprise anti-ferromagnetic coupling spacer layer 134 and its thickness. Generally, anti-ferromagnetic coupling spacer layer 134 comprises one of the elements ruthenium, osmium, rhenium, chromium, rhodium, copper, or combinations thereof. Preferably, anti-ferromagnetic coupling spacer layer 134 comprises ruthenium. In one embodiment of the present invention, HSAT(intrinsic), and hence HSAT(total), may be reduced or minimized by fabricating anti-ferromagnetic coupling spacer layer 134 with a thickness such that magnetic region 104 comprises a second order SAF. FIG. 7 is a graph that illustrates a typical relationship between the value of HSAT(intrinsic) and the thickness of an anti-ferromagnetic coupling material, such as ruthenium, that may be used to form anti-ferromagnetic coupling spacer layer 134. As shown in FIG. 7, the anti-ferromagnetic coupling material operates as an anti-ferromagnetic coupling spacer layer 134 at a first peak or first range of thicknesses 280. At first peak 280, the anti-ferromagnetic coupling spacer layer 134 forms a first order SAF with ferromagnetic layers 130 and 132 of FIG. 3. The anti-ferromagnetic coupling material also may operate as an anti-ferromagnetic coupling spacer layer 134 at a second peak or range of thicknesses 282, thus forming a second order SAF with ferromagnetic layers 130 and 132. As illustrated in FIG. 7, the values of HSAT(intrinsic) are relatively higher at the first peak 280 than at the second peak 282. Thus, by forming magnetic region 104 as a second order SAF, that is, with an anti-ferromagnetic coupling spacer layer 134 having a thickness within the range of thicknesses of the second peak 282, HSAT(total) may be reduced or minimized, thus reducing or minimizing HSW. In addition, the second peak is much flatter as a function of spacer layer thickness compared to the first order peak, so that the spacer layer thickness can vary over a wider range and still supply an HSAT(intrinsic) of nominally the same magnitude. HSAT insensitivity to spacer layer thickness may be desirable for robust and reproducible manufacturing.
As described above, while it is preferable that HSAT(total) be minimized to lower the power requirements of magnetoresistive memory element 102, HSAT(total) preferably is large enough that there exists an operable programming window for programming memory element 102. Thus, while it may be desirable to fabricate magnetic region 104 as a second order SAF, HSAT(total) may be too low to provide a satisfactory programming window for memory element 102. As illustrated by the third peak 284 in FIG. 7, the presence of a material that produces higher anti-ferromagnetic exchange coupling, such as a material comprising Co, Fe, or CoFe, disposed proximate to a surface of anti-ferromagnetic coupling spacer layer 134 may increase HSAT(intrinsic) to acceptable values. Accordingly, referring to FIG. 8, in another embodiment of the present invention, a magnetoresistive memory element 300 may comprise a first interface layer 302 disposed at a first surface of anti-ferromagnetic coupling spacer layer 134 and/or a second interface layer 304 disposed at a second surface of anti-ferromagnetic coupling spacer layer 134. Materials suitable for forming interface layers 302 and 304 comprise materials such as Co, Fe, CoFe, and alloys of Co or alloys of Fe, such as, for example, CoFeTa or CoFeB.
Referring again to FIG. 7, in another embodiment of the invention, magnetic region 104 may be fabricated as a first order SAF, that is, with an anti-ferromagnetic coupling spacer layer 134 having a thickness within the range of thicknesses of the first peak 280. Preferably, however, anti-ferromagnetic coupling spacer layer 134 has a thickness that is larger than a thickness tmax that results in a maximum HSAT(intrinsic). In this regard, HSAT(intrinsic) may be optimized along first peak 280 to reduce the power requirements of memory element 102 but also to provide a suitable programming window within which memory element 102 may be switched.
In yet another embodiment of the invention, when magnetic region 104 is fabricated as a first order SAF, HSAT(intrinsic) may be further optimized by utilizing interface layers 302 and/or 304, as illustrated in FIG. 8. For practical reasons, it may be desirable to fabricate magnetic region 104 with an anti-ferromagnetic coupling spacer layer thickness that exhibits an HSAT(intrinsic) that is approximately equal to or below a predetermined HSAT(intrinsic). For example, it may be desirable to form the anti-ferromagnetic coupling spacer layer with a thickness such that any deviations of thickness during processing do not significantly affect the value of HSAT(intrinsic). In other words, it may be desirable to form the anti-ferromagnetic coupling spacer layer with a thickness that is at a flatter or more stable region of the first peak 280. However, at this thickness, HSAT(intrinsic) may be below a desired HSAT(intrinsic). Thus, interface layers 302 and/or 304, as illustrated in FIG. 8, may be utilized to increase the HSAT(intrinsic) to the desired value.
HSW also may be reduced or minimized, thus reducing the power requirements of memory element 102, by reducing or minimizing HSAT(shape). As described above, in one embodiment of the present invention, HSAT(shape) may be reduced or minimized by fabricating magnetic layers 130 and 132 from a low magnetization material. Also as described above, in another embodiment of the present invention, HSAT(shape) may be reduced or minimized by fabricating magnetic layers 130 and 132 with a minimum thickness t. In another exemplary embodiment of the present invention, HSAT(shape) also may be reduced by fabricating memory element 102 with a shape having one or more substantially sharp or pointed ends along the anisotropy axis that exhibit magnetostatic coupling of ferromagnetic layers 130 and 132 that is lower than the magnetostatic coupling of layers 130 and 132 of a memory element 102 having a shape with substantially rounded ends, such as a circular-shaped memory element 102. For example, as illustrated in FIG. 9, memory element 102 may be formed in the shape of an ellipse that comprises substantially sharp or pointed ends 320 along a longitudinal axis 322 of the memory element. A memory element 102 having this shape will exhibit less magnetostatic coupling, and hence a lower HSAT(shape) value, than a comparable memory element 102 having a circular shape or an elliptical shape with substantially rounded ends. It will be appreciated, however, that memory element 102 may be fabricated with a variety of other shapes, such as a diamond shape, that will exhibit reduced magnetostatic coupling and hence a reduced or minimized HSAT(shape).
Accordingly, magnetoresistive random access memory elements that require lower power for programming in accordance with the present invention have been described. The power requirements for programming the memory elements are related to the magnetic switching field HSW represented by the equation HSW≅√{square root over (HkHSAT)}. The embodiments of the present invention provide methods and structures for reducing and/or minimizing Hk and HSAT. While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.

## Claims

1. A magnetoresistive random access device having an array of memory elements, each memory element comprising:
a fixed magnetic portion,
a tunnel barrier portion disposed proximate to said fixed magnetic portion; and
a free SAF structure disposed proximate to said tunnel barrier portion, wherein:
the array of memory elements has a finite magnetic field programming window Hwin represented by the equation Hwin≈(Hsat−σsat)−(Hsw+σsw);
Hsw is a mean switching field for said array;
Hsat is a mean saturation field for said array;
Hsw for said each memory element is represented by the equation HSW≅√{square root over (HkHSAT)}, wherein Hk represents a total anisotropy field of said free SAF structure of said each memory element and HSAT represents a total anti-ferromagnetic coupling saturation field for said free SAF structure of said each memory element;
N is an integer greater than or equal to 1;
σsw is a standard deviation for Hsw; and
σsat is a standard deviation for HSAT,

and wherein said free SAF structure is configured to have Hk, HSAT, and N values such that the array of memory elements is thermally stable and requires current to operate that is below a predetermined current value.

2. The magnetoresistive random access device of claim 1, wherein Hk of said free SAF structure of said each memory element of said array has a value that is no greater than 15 Oe-microns divided by a width of said free SAF structure, wherein said width is a dimension (in microns) of said free SAF structure that is orthogonal to a longitudinal axis of said free SAF structure.

3. The magnetoresistive random access device of claim 1, wherein said free SAF structure comprises two magnetic portions, each of said two magnetic portions comprising a layer of low magnetization material.

4. The magnetoresistive random access device of claim 3, each of said at least two magnetic portions comprising a material selected from the group consisting of Ni, Fe, Co, alloys of Ni, alloys of Fe, and alloys of Co.

5. The magnetoresistive random access device of claim 3, wherein said low magnetization material is doped with at least one material selected from the group consisting of molybdenum, tantalum, and boron.

6. The magnetoresistive random access device of claim 3, at least one of said two magnetic portions further comprising a layer of high spin polarization material.

7. The magnetoresistive random access device of claim 1, wherein said free SAF structure comprises two magnetic portions, each of said two magnetic portions having a thickness no greater than about 5 nm.

8. The magnetoresistive random access device of claim 1, wherein said free SAF structure has a length and a width and a length/width ratio in the range of from about 1 to about 3.

9. The magnetoresistive random access device of claim 8, said free SAF structure having a length/width ratio in the range of from about 2 to about 2.5.

10. The magnetoresistive random access device of claim 1, said free SAF structure having a value of HSAT in the range of from about 150 Oe to about 350 Oe.

11. The magnetoresistive random access device of claim 1, said free SAF structure is configured to have a value of HSAT less than approximately 180/w0.5 (Oe), where w is the width (in microns) of said free SAF structure.

12. The magnetoresistive random access device of claim 1, wherein said free SAF structure is a second order SAF structure.

13. The magnetoresistive random access device of claim 12, wherein said free SAF structure comprises two magnetic portions separated by an anti-ferromagnetic coupling spacer layer and a layer of material that produces a higher anti-ferromagnetic exchange coupling than said anti-ferromagnetic coupling spacer layer alone.

14. The magnetoresistive random access device of claim 1, wherein said free SAF structure comprises two magnetic portions separated by an anti-ferromagnetic coupling spacer layer, wherein a thickness of said anti-ferromagnetic coupling spacer layer is such that said free SAF structure is a first order SAF structure and wherein said thickness of said anti-ferromagnetic coupling spacer layer is greater than a thickness at which an anti-ferromagnetic coupling saturation field HSAT of said anti-ferromagnetic coupling material is a maximum.

15. The magnetoresistive random access device of claim 1, wherein said free SAF structure has an anisotropy axis and has a shape with at least one substantially pointed end disposed substantially along said anisotropy axis.