# Patent

## Title
Magnetic stack having reference layers with orthogonal magnetization orientation directions

## Abstract

A magnetic cell includes a ferromagnetic free layer having a free magnetization orientation direction and a first ferromagnetic pinned reference layer having a first reference magnetization orientation direction that is parallel or anti-parallel to the free magnetization orientation direction. A first oxide barrier layer is between the ferromagnetic free layer and the first ferromagnetic pinned reference layer. The magnetic cell further includes a second ferromagnetic pinned reference layer having a second reference magnetization orientation direction that is orthogonal to the first reference magnetization orientation direction. The ferromagnetic free layer is between the first ferromagnetic pinned reference layer and the second ferromagnetic pinned reference layer.

## Background
CROSS-REFERENCE
This application is a continuation of application Ser. No. 13/176,029 filed Jul. 5, 2011 which is a continuation of application Ser. No. 12/502,209, filed Jul. 13, 2009, now U.S. Pat. No. 7,999,338, the contents of each are hereby incorporated by reference in their entirety.


BACKGROUND
Spin torque transfer technology, also referred to as spin electronics, combines semiconductor technology and magnetics, and is a more recent development. In spin electronics, the spin of an electron, rather than the charge, is used to indicate the presence of digital information. The digital information or data, represented as a “0” or “1”, is storable in the alignment of magnetic moments within a magnetic element. The resistance of the magnetic element depends on the moment's alignment or orientation. The stored state is read from the element by detecting the component's resistive state.
The magnetic element, in general, includes a ferromagnetic pinned layer and a ferromagnetic free layer, each having a magnetization orientation that defines the resistance of the overall magnetic element. Such an element is generally referred to as a “spin tunneling junction,” “magnetic tunnel junction”, “magnetic tunnel junction cell”, and the like. When the magnetization orientations of the free layer and pinned layer are parallel, the resistance of the element is low. When the magnetization orientations of the free layer and the pinned layer are antiparallel, the resistance of the element is high.
Application of spin torque transfer memory has a switching current density requirement generally at 106 to 107 A/cm2, which leads to difficulty in integrating with a regular CMOS process. It is desirable to reduce the switching current density significantly in order to make a feasible product. Various attempts have been made.
However, there is a dilemma between switching current and data stability in spin torque transfer cells. A low switching current can reduce data retention due to thermal instability of the spin torque transfer cells. Spin torque transfer cell design that can achieve both low switching current with sufficient data retention is desired.
BRIEF SUMMARY
The present disclosure relates to magnetic cells, such as a spin torque memory cell, that have magnetic two reference layers or elements that have orthogonal magnetization orientation directions. These spin torque memory cells quickly switch between a high resistance data state and a low resistance data state and include a free magnetic layer between two oxide barrier layers. The two reference layers are aligned perpendicularly.
In an embodiment of this disclosure is a magnetic cell that includes a ferromagnetic free layer having a free magnetization orientation direction and a first ferromagnetic pinned reference layer having a first reference magnetization orientation direction that is parallel or anti-parallel to the free magnetization orientation direction. A first oxide barrier layer is between the ferromagnetic free layer and the first ferromagnetic pinned reference layer. The magnetic cell further includes a second ferromagnetic pinned reference layer having a second reference magnetization orientation direction that is orthogonal to the first reference magnetization orientation direction. The ferromagnetic free layer is between the first ferromagnetic pinned reference layer and the second ferromagnetic pinned reference layer.
These and various other features and advantages will be apparent from a reading of the following detailed description.



BRIEF DESCRIPTION OF THE DRAWINGS
The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying drawings, in which:
FIG. 1A is a schematic side view diagram of a magnetic cell in a low resistance data state and with orthogonal reference layer magnetization orientations;
FIG. 1B is a schematic side view diagram of a magnetic cell in a high resistance data state and with orthogonal reference layer magnetization orientations;
FIG. 2 is a schematic diagram of an illustrative memory unit including a memory cell and a semiconductor transistor;
FIG. 3 is a schematic diagram of an illustrative memory array;
FIG. 4 is a schematic side view diagram of another magnetic cell with orthogonal reference layer magnetization orientations; and
FIG. 5 is a schematic side view diagram of another magnetic cell with orthogonal reference layer magnetization orientations.



The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number.
DETAILED DESCRIPTION
This disclosure is directed to magnetic stacks or cells (e.g., spin torque memory (STRAM) cells) having magnetic two reference layers or elements that have orthogonal magnetization orientation directions. These spin torque memory cells quickly switch between a high resistance data state and a low resistance data state and include a free magnetic layer between two oxide barrier layers. The two reference layers are aligned perpendicularly. This data cell construction increases the write speed and improves the tunneling magneto-resistance ratio of the data cell over conventional data cells that do not have perpendicularly aligned reference layers.
In the following description, reference is made to the accompanying set of drawings that form a part hereof and in which are shown by way of illustration several specific embodiments. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense. Any definitions provided herein are to facilitate understanding of certain terms used frequently herein and are not meant to limit the scope of the present disclosure.
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein.
As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” encompass embodiments having plural referents, unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
It is noted that terms such as “top”, “bottom”, “above, “below”, etc. may be used in this disclosure. These terms should not be construed as limiting the position or orientation of a structure, but should be used as providing spatial relationship between the structures.
While the present disclosure is not so limited, an appreciation of various aspects of the disclosure will be gained through a discussion of the examples provided below.
FIG. 1A is a schematic side view diagram of a magnetic cell 10 in a low resistance data state and with orthogonal reference layer magnetization orientations. FIG. 1B is a schematic side view diagram of a magnetic cell 10 in a high resistance data state and with orthogonal reference layer magnetization orientations. The magnetic tunnel junction cell 10 includes a first ferromagnetic pinned reference layer or element 14 having a first reference magnetization orientation direction MR1, a ferromagnetic free element or layer 18 having a free magnetization orientation direction MF and a first tunneling barrier 16 separating the first ferromagnetic pinned reference magnetic element 14 from the ferromagnetic free element 18. A second ferromagnetic pinned reference layer or element 13 has a second reference magnetization orientation direction MR2 that is orthogonal to the first reference magnetization orientation direction MR1. The ferromagnetic free layer is between the first ferromagnetic pinned reference layer 14 and the second ferromagnetic pinned reference layer 13. In many embodiments, a second tunneling barrier 15 separates the second ferromagnetic pinned reference magnetic element 13 from the ferromagnetic free element 18.
These elements or layers are disposed electrically between a first electrode 13 and a second electrode 19. While a single magnetic tunnel junction cell 10 is shown, it is understood that a plurality of magnetic tunnel junction cell 10 can be arranged in an array to form a memory array. Other layers, such as seed or capping layers, are not depicted for clarity.
The ferromagnetic free element 18 has a free magnetization orientation direction MF that is switchable between a high resistance data state (i.e., anti-parallel direction relative to the first ferromagnetic pinned reference magnetic element 14 magnetization orientation direction MR1 and illustrated in FIG. 1B) and a low resistance data state (i.e., parallel direction relative to the first ferromagnetic pinned reference magnetic element 14 magnetization orientation direction MR1 and illustrated in FIG. 1A). The ferromagnetic free element or layer 18, first ferromagnetic pinned reference magnetic element 14, and second ferromagnetic pinned reference magnetic element 13 have in-plane magnetic anisotropy.
While the first ferromagnetic pinned reference element 14 is illustrated as a single layer, it is understood that this element 14 can include two or more layer such as, a ferromagnetic reference (pinned) layer and a antiferromagnetic reference (pinning) layer, where the antiferromagnetic reference layer serves to fix the magnetization of the ferromagnetic reference layer. In other embodiments, the first ferromagnetic pinned reference element 14 includes more than one ferromagnetic layer that are coupled anti-ferromagnetically to each other (e.g., synthetic antiferromagnet). The ferromagnetic reference layer can be formed of any useful material such as, for example, alloys and materials including Co, Fe, and/or Ni. Ternary alloys, such as CoFeB, may be particularly useful because of their lower moment and high polarization ratio, which are desirable for the spin-current switching. The antiferromagnetic reference layer can be formed of any useful material such as, for example, IrMn, FeMn, and/or PtMn.
While the second ferromagnetic pinned reference element 13 is illustrated as a single layer, it is understood that this element 13 can include two or more layer such as, a ferromagnetic reference (pinned) layer and an antiferromagnetic reference (pinning) layer, where the antiferromagnetic reference layer serves to fix the magnetization of the ferromagnetic reference layer. In other embodiments, the second ferromagnetic pinned reference element 13 includes more than one ferromagnetic layer that are coupled anti-ferromagnetically to each other (e.g., synthetic antiferromagnet). The ferromagnetic reference layer can be formed of any useful material such as, for example, alloys and materials including Co, Fe, and/or Ni. Ternary alloys, such as CoFeB, may be particularly useful because of their lower moment and high polarization ratio, which are desirable for the spin-current switching. The antiferromagnetic reference layer can be formed of any useful material such as, for example, IrMn, FeMn, and/or PtMn.
The ferromagnetic free element 18 can be formed of any useful soft magnetic material that allows a magnetization orientation of the ferromagnetic free element 18 to switch between a first magnetization orientation and an opposing second magnetization orientation. In many embodiments the ferromagnetic free element 18 is formed of a CoFeB material such as, Co65Fe30B15 and having a magnetic saturation in a range from 1200 to 500 emu/cc, for example. The first magnetization orientation can be parallel with a magnetization orientation of the first ferromagnetic pinned reference element 14, forming a low resistance data state or a “0” data state. The second magnetization orientation can be anti-parallel with a magnetization orientation of the first ferromagnetic pinned reference element 14, forming a high resistance data state or a “1” data state. The ferromagnetic free layer can be formed of any useful material such as, for example, alloys and materials including Co, Fe, and/or Ni. Ternary alloys, such as CoFeB, may be particularly useful because of their lower moment and high polarization ratio, which are desirable for the spin-current switching. Thus the ferromagnetic free element 18 can be switched due to spin torque transfer induced by a current passing through the magnetic cell 10.
The first and second tunneling or oxide barrier 15, 16 is an electrically insulating and non-magnetic material. The tunneling or oxide barrier 15, 16 can be formed of any useful electrically insulating and non-magnetic material such as, AlO, MgO, and/or TiO, for example. In some embodiments, the oxide barrier layers 15, 16 have a thickness of about 0.5-2 nm.
Electrodes 13, 19 electrically connect the magnetic tunnel junction cell 10 to a control circuit providing read and write currents through the magnetic tunnel junction cell 10. Resistance across the magnetic tunnel junction cell 10 is determined by the relative orientation of the magnetization vectors or magnetization orientations of ferromagnetic layers 14, 18. The magnetization directions of the ferromagnetic pinned reference layers 14, 13 are pinned in a predetermined direction while the magnetization direction of ferromagnetic free layer 18 is free to rotate under the influence of spin torque when a current flows through the magnetic tunnel junction cell 10.
Switching the resistance state and hence the data state of magnetic tunnel junction cell 10 via spin-torque transfer occurs when a current, passing through a magnetic layer of magnetic tunnel junction cell 10, becomes spin polarized and imparts a spin torque on the ferromagnetic free layer 18 of magnetic tunnel junction cell 10. When a sufficient spin torque is applied (sufficient to overcome the energy barrier E) to ferromagnetic free layer 18, the magnetization orientation of the ferromagnetic free layer 18 can be switched between two opposite directions and accordingly, magnetic tunnel junction cell 10 can be switched between the parallel state (i.e., low resistance state or “0” data state) and anti-parallel state (i.e., high resistance state or “1” data state).
FIG. 2 is a schematic diagram of an illustrative memory unit including a memory unit 20 and a semiconductor transistor 22. Memory unit 20 includes a magnetic tunnel junction cell 10, as described herein, electrically coupled to semiconductor transistor 22 via an electrically conducting element 24. Transistor 22 includes a semiconductor substrate 21 having doped regions (e.g., illustrated as n-doped regions) and a channel region (e.g., illustrated as a p-doped channel region) between the doped regions. Transistor 22 includes a gate 26 that is electrically coupled to a word line WL to allow selection and current to flow from a bit line BL to memory cell 10. An array of memory units 20 can be formed on a semiconductor substrate utilizing semiconductor fabrication techniques.
FIG. 3 is a schematic diagram of an illustrative memory array 30. Memory array 30 includes a plurality of word lines WL and a plurality of bit lines BL forming a cross-point array. At each cross-point a memory cell 10, as described herein, is electrically coupled to word line WL and bit line BL. A select device (not shown) can be at each cross-point or at each word line WL and bit line BL.
FIG. 4 is a schematic side view diagram of another magnetic cell 40 with orthogonal reference layer magnetization orientations. The magnetic tunnel junction cell 40 includes a first ferromagnetic pinned reference layer or element 14 having a first reference magnetization orientation direction, a ferromagnetic free element or layer 18 having a free magnetization orientation direction and a first tunneling barrier 16 separating the first ferromagnetic pinned reference magnetic element 14 from the ferromagnetic free element 18. A second ferromagnetic pinned reference layer or element 13 has a second reference magnetization orientation direction that is orthogonal to the first reference magnetization orientation direction. The ferromagnetic free layer 18 is between the first ferromagnetic pinned reference layer 14 and the second ferromagnetic pinned reference layer 13. In many embodiments, a second tunneling barrier 15 separates the second ferromagnetic pinned reference magnetic element 13 from the ferromagnetic free element 18.
These elements or layers are disposed electrically between a first electrode 13 and a second electrode 19. While a single magnetic tunnel junction cell 10 is shown, it is understood that a plurality of magnetic tunnel junction cell 10 can be arranged in an array to form a memory array. Other layers, such as seed or capping layers, are not depicted for clarity.
The first ferromagnetic pinned reference layer or element 14 includes a first synthetic anti-ferromagnetic element SAF1 and a first antiferromagnetic reference (pinning) layer AFM1. The first synthetic anti-ferromagnetic element SAF1 includes two ferromagnetic layers FM1, FM2 anti-ferromagnetically coupled and separated by a non-magnetic and electrically conducting spacer layer SP1. The second ferromagnetic pinned reference layer or element 13 includes a second synthetic anti-ferromagnetic element SAF2 and a second antiferromagnetic reference (pinning) layer AFM2. The second synthetic anti-ferromagnetic element SAF2 includes two ferromagnetic layers FM3, FM4 anti-ferromagnetically coupled and separated by a non-magnetic and electrically conducting spacer layer SP2.
In many embodiments the first antiferromagnetic reference (pinning) layer AFM1 has a different material composition than the second antiferromagnetic reference (pinning) layer AFM2. The first antiferromagnetic reference (pinning) layer AFM1 can have a greater blocking temperature than the second antiferromagnetic reference (pinning) layer AFM2. Thus the first ferromagnetic pinned reference layer or element 14 can have its magnetization orientation set at a higher temperature than the later formed second ferromagnetic pinned reference layer or element 13. Then the second ferromagnetic pinned reference layer or element 13 can have its magnetization orientation set at a lower temperature than the prior formed first ferromagnetic pinned reference layer or element 14.
FIG. 5 is a schematic side view diagram of another magnetic cell 50 with orthogonal reference layer magnetization orientations. The magnetic tunnel junction cell 50 includes a first ferromagnetic pinned reference layer or element 14 having a first reference magnetization orientation direction, a ferromagnetic free element or layer 18 having a free magnetization orientation direction and a first tunneling barrier 16 separating the first ferromagnetic pinned reference magnetic element 14 from the ferromagnetic free element 18. A second ferromagnetic pinned reference layer or element 13 has a second reference magnetization orientation direction that is orthogonal to the first reference magnetization orientation direction. The ferromagnetic free layer 18 is between the first ferromagnetic pinned reference layer 14 and the second ferromagnetic pinned reference layer 13. In many embodiments, a second tunneling barrier 15 separates the second ferromagnetic pinned reference magnetic element 13 from the ferromagnetic free element 18.
These elements or layers are disposed electrically between a first electrode 13 and a second electrode 19. While a single magnetic tunnel junction cell 10 is shown, it is understood that a plurality of magnetic tunnel junction cell 10 can be arranged in an array to form a memory array. Other layers, such as seed or capping layers, are not depicted for clarity.
The first ferromagnetic pinned reference layer or element 14 includes a first synthetic anti-ferromagnetic element SAF1 and a antiferromagnetic reference (pinning) layer AFM. The first synthetic anti-ferromagnetic element SAF1 includes two ferromagnetic layers FM1, FM2 anti-ferromagnetically coupled and separated by a non-magnetic and electrically conducting spacer layer SP1. The second ferromagnetic pinned reference layer or element 13 includes a second synthetic anti-ferromagnetic element SAF2 and a permanent magnet PM. The second synthetic anti-ferromagnetic element SAF2 includes two ferromagnetic layers FM3, FM4 anti-ferromagnetically coupled and separated by a non-magnetic and electrically conducting spacer layer SP2. The magnetization orientation of the first ferromagnetic pinned reference layer or element 14 can be set with a magnetic set anneal and the magnetization orientation of the second ferromagnetic pinned reference layer or element 13 can be set with the permanent magnet PM.
The various structures of this disclosure may be made by thin film techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), sputter deposition, and atomic layer deposition (ALD).
Thus, embodiments of the MAGNETIC STACK HAVING REFERENCE LAYERS WITH ORTHOGONAL MAGNETIZATION ORIENTATION DIRECTIONS are disclosed. The implementations described above and other implementations are within the scope of the following claims. One skilled in the art will appreciate that the present disclosure can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.
The use of numerical identifiers, such as “first”, “second”, etc. in the claims that follow is for purposes of identification and providing antecedent basis. Unless content clearly dictates otherwise, it should not be implied that a numerical identifier refers to the number of such elements required to be present in a device, system or apparatus. For example, if a device includes a first layer, it should not be implied that a second layer is required in that device.

## Claims

1. A magnetic cell comprising:
a ferromagnetic free layer having a free magnetization orientation direction;
a first ferromagnetic pinned reference layer having a first reference magnetization orientation direction that is parallel or anti-parallel to the free magnetization orientation direction, and the first ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element having a first blocking temperature;
a first oxide barrier layer between the ferromagnetic free layer and the first ferromagnetic pinned reference layer; and
a second ferromagnetic pinned reference layer having a second reference magnetization orientation direction that is orthogonal to the first reference magnetization orientation direction, the ferromagnetic free layer between the first ferromagnetic pinned reference layer and the second ferromagnetic pinned reference layer, and the second ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element having a second blocking temperature;
wherein the second blocking temperature is less than the first blocking temperature.

2. The magnetic cell of claim 1 wherein the ferromagnetic free layer, first ferromagnetic pinned reference layer, and second ferromagnetic pinned reference layer, have in-plane magnetic anisotropy.

3. The magnetic cell of claim 1 further comprising a second oxide barrier layer between the ferromagnetic free layer and the second ferromagnetic pinned reference layer.

4. The magnetic cell of claim 1 wherein the first ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element.

5. The magnetic cell of claim 1 wherein the second ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element.

6. The magnetic cell of claim 1 wherein the ferromagnetic free layer switches between a high resistance data state and a low resistance data state due to spin torque transfer induced by a current passing through the magnetic cell.

7. The magnetic cell of claim 1 wherein the free magnetization orientation direction is orthogonal to the second reference magnetization orientation direction.

8. A spin torque transfer magnetic cell comprising:
a ferromagnetic free layer having an in-plane free magnetization orientation direction that switches between a high resistance data state and a low resistance data state due to spin torque transfer induced by a current passing through the magnetic cell, and the first ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element having a first blocking temperature;
a first ferromagnetic pinned reference layer having a first reference magnetization orientation direction that is parallel or anti-parallel to the free magnetization orientation direction;
a first oxide barrier layer between the ferromagnetic free layer and the first ferromagnetic pinned reference layer;
a second ferromagnetic pinned reference layer comprising a permanent magnet and having an in-plane second reference magnetization orientation direction that is orthogonal to the free magnetization orientation direction, and the second ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element having a second blocking temperature; and
a second oxide barrier layer between the ferromagnetic free layer and the second ferromagnetic pinned reference layer;
wherein the second blocking temperature is less than the first blocking temperature.

9. The spin torque transfer magnetic cell of claim 8 wherein the first ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element.

10. The spin torque transfer magnetic cell of claim 8 wherein the second ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element.

11. The spin torque transfer magnetic cell of claim 8 wherein the free magnetization orientation direction is orthogonal to the second reference magnetization orientation direction.

12. A spin torque transfer magnetic cell comprising:
a ferromagnetic free layer having an in-plane free magnetization orientation direction that switches between a high resistance data state and a low resistance data state due to spin torque transfer induced by a current passing through the magnetic cell;
a first ferromagnetic pinned reference layer having a first reference magnetization orientation direction that is parallel or anti-parallel to the free magnetization orientation direction, and the first ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element having a first blocking temperature;
a first oxide barrier layer between the ferromagnetic free layer and the first ferromagnetic pinned reference layer;
a second ferromagnetic pinned reference layer comprising a permanent magnet and having an in-plane second reference magnetization orientation direction that is orthogonal to the first reference magnetization orientation direction, and the second ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element having a second blocking temperature; and
a second oxide barrier layer between the ferromagnetic free layer and the second ferromagnetic pinned reference layer;
wherein the second blocking temperature is less than the first blocking temperature.

13. The spin torque transfer magnetic cell of claim 12 wherein the first ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element.

14. The spin torque transfer magnetic cell of claim 12 wherein the second ferromagnetic pinned reference layer comprises a synthetic anti-ferromagnetic element.