# Patent

## Title
Method for production of MRAM elements

## Abstract

Magneto-resistive random access memory elements include a ferromagnetic layer having uniaxial anisotropy provided by elongate structures formed in the ferromagnetic film. The magnetic dipole aligns with the long axis of each structure. The structures can be formed in a variety of ways. For example, the ferromagnetic film can be applied to a seed layer having a textured surface. Alternatively, the ferromagnetic film can be stressed to generate the textured structure. Chemical mechanical polishing also can be used to generated the structures.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is a continuation of U.S. application Ser. No. 11/700,958, filed on Feb. 1, 2007 now U.S. Pat. No. 7,470,552, which is a continuation of U.S. application Ser. No. 10/610,823 filed on Jul. 2, 2003 now U.S. Pat. No. 7,189,583, the disclosures of which are herewith incorporated by reference in their entirety.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to methods of producing magnetoresistive elements, and more particularly, to methods of producing magnetoresistive elements having an induced secondary anisotropy.
2. Description of the Related Art
Storage memories of various types are used extensively in digital systems such as microprocessor-based systems, digital processing systems, and the like. Recently, magnetic random access memory (MRAM) devices have been investigated for possible use in non-volatile random access memory. Information is stored in MRAM devices based on a magnetoresistive effect, in which memory cells formed of ferromagnetic layers in the device have resistances that change based on the magnetized state of a free ferromagnetic layer compared to that of a pinned (fixed) ferromagnetic layer. The magnetic moment of the pinned layer remains fixed while the magnetic moment of the free layer can change depending on an externally-applied magnetic field or potential. The relative magnetic directions of the free layer to the pinned layer typically are referred to as “parallel” and “antiparallel.”
A magnetic memory element, such as a magnetic tunnel junction (MTJ), is formed on a wafer substrate. The structure includes free and pinned ferromagnetic layers separated by a non-magnetic tunnel junction barrier. The magnetic memory elements are formed using thin-film materials and can be manufactured on the sub-micron level.
In response to parallel and antiparallel magnetic states, the magnetic memory element represents two different resistances to a current provided across the memory element in a direction perpendicular to the plane of the ferromagnetic layers. The tunnel barrier is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs across the barrier junction between the two separated sets of ferromagnetic layers. The resistance across the element has minimum and maximum values corresponding to whether the magnetization vectors of the free and pinned layers are parallel or antiparallel.
Consequently, it is necessary when producing magnetic memory elements to provide layers having magnetic moments that are selectively aligned. Magnetic memory element structures include very thin layers, also known as ultrathin films, some of which are tens of angstroms or less in thickness. The structure of ultrathin films has a strong influence on their magnetic properties. Small variations in thickness and surface morphology can impact the magnetic characteristics of an ultrathin magnetic film layer.
Magnetic anisotropy, the tendency of the magnetic moments in the layer to align in a given direction, can be influenced by the shape of the layer. Magnetic moments tend to align head to tail, rather than head to head, so by forming a layer as a rectangle, for example, the magnetic moments will tend to align parallel to the longer dimension of the layer. This phenomenon is known as shape-induced anisotropy. It would be advantageous to be able to induce anisotropy without regard to the overall shape of the film layer.
It is known in the prior art to form layer shapes using photolithography. Photolithography is susceptible, however, to image distortion and instabilities, problems that are exacerbated at the sub-micron level at which magnetic memory elements are being manufactured. It would be advantageous to be able to induce magnetic anisotropy in layers of a magnetic memory element by methods other than photolithography, which methods are accurate and reproducible at the sub-micron level of production.
BRIEF SUMMARY OF THE INVENTION
The present invention overcomes the problems in the prior art by providing induced uniaxial anisotropy rather than shape anisotropy to generate preferential alignment in submicron MRAM elements.
According to a preferred embodiment, the invention relies on preferential patterning or roughening of the starting substrate surface, which patterning translates to magnetic alignment within the MRAM structure itself. The magnetic moments of ferromagnetic films produced according to the present invention will align preferentially along grooves or patterns generated in the starting surface. By generating very fine surface patterns, anisotropy is induced independent of the overall shape of the element, even if the element is circular or symmetric. This method allows control of the anisotropy or switching field to be generated by processes which are more reliable than photoimaging.
Various methods can be used to create the anisotropy-inducing features. Grooves or patterns can be generated in a seed layer or a magnetic layer by masking the surface and etching. According to another embodiment, the patterns also can be generated by stressing the films on the surface and allowing them to relax and form ripples in a deposited film. Additionally, chemical mechanical polishing (CMP) can be used to generate surface texture. Other techniques include nano-fabrication methods such as self-assembly, self-organization, and nano-patterning.
Texturing an underlayer (seed layer) currently is preferred as the easiest and most repeatable method for inducing a secondary anisotropy in a magnetic film. This can be accomplished by utilizing off axis angular deposition which creates ripples by generating shadow effects as the film grows. The geometry and roughness that induce the anisotropy are controlled by deposition angle and thickness, which are very easy to repeat in a deposition system.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a magnetic tunnel junction structure according to an exemplary embodiment of the present invention.
FIG. 2 is a cross-sectional illustration taken along the line II-II in FIG. 1 of a portion of the film stack shown in which secondary anisotropy is imparted to a ferromagnetic layer according to an exemplary embodiment of the present invention.
FIG. 3 illustrates an exemplary embodiment of the present invention in which aligning structures are formed directly in a ferromagnetic film supported on a substrate;
FIG. 4 is a plan view of the planes IV-IV in FIGS. 2 and 3 schematically illustrating secondary anisotropy imparted to a ferromagnetic layer in the film stack.
FIG. 5 illustrates an MRAM array according to an exemplary embodiment of the present invention.
FIG. 6 illustrates a microprocessor-based system including an MRAM according to an exemplary embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
A generalized, exemplary magneto-resistive element structure 2 is shown in FIG. 1. The structure includes a free stack 4 and a pinned stack 6 formed above a substrate 20. Free stack 4 includes a seed layer 8 on which is disposed a free ferromagnetic layer 10. Above free stack 4 is pinned stack 6, separated by a tunnel barrier layer 12. Pinned stack 6 is made up of a pinned ferromagnetic layer 14, an antiferromagnetic pinning layer 16, and a capping layer 18. Other arrangements can be used. For example, the free and pinned ferromagnetic layers 10 and 14 may each be formed as a plurality of stacked individual layers. The stacks also can include offset control and coupling layers.
FIG. 2 shows a partial cross-section of film stack 2 taken along the line II-II in FIG. 1. As illustrated in FIG. 2, seed layer 8 provided above substrate 20 is textured. The texture of the seed layer 8 imparts uniaxial anisotropy in ferromagnetic film 10 grown above seed layer 8. Alignment of the magnetic moments in ferromagnetic film 10 is illustrated in FIG. 4, described further below.
The seed layer 8 can be textured by various methods known in the art, including patterning and etching, off-axis deposition, in which thin films are deposited at an angle not perpendicular to the substrate, dimple etching, or by chemical mechanical polishing (CMP), for example. In a representative CMP process, an oxide slurry having a neutral pH is applied with pressure to a substrate. The presence of oxide in the slurry and the physical pressure lead to micro-scratches in the surface. Polishing is carried out so that the micro-scratches are aligned to impart the uniaxial anisotropy to the deposited ferromagnetic film layer 10.
In addition, various nanofabrication techniques can be utilized to impart texture to seed layer 8. In general, nanofabrication techniques include lithographic techniques using ultraviolet light, X-rays, ion beams, and electron beams; scanning probe techniques for nano-patterning, (scanning tunneling microscopy (STM) and atomic force microscopy (AFM); and self-assembly or self-organization of nanostructures. Examples of the latter include quantum dots and quantum wires. See T. Ogino et al., “Bottom-up approach in Si technology based on surface structure design,” Electrochemical Society Proceedings, Vol. 2002-2, p. 992-1001, the entire disclosure of which is incorporated herein by reference.
The textured seed layer may be a dielectric, metal, or other thin film that would impart controlled morphology to the ferromagnetic film. Typically, the surface texture features are tens of microns in length, and a few nanometers to a few tenths of nanometers in width and depth.
According to an alternative method of the invention, aligning structures 28 are formed more directly in a ferromagnetic film 30, shown supported on a substrate 40 in FIG. 3. The aligning structures are formed by stressing film 30, applying tensile or compressive force, for example. Alternatively, aligning structures can be formed in the ferromagnetic film by applying techniques discussed above in connection with forming aligning structures in the seed layer directly to the ferromagnetic film layer.
FIG. 4 depicts the magnetic dipole alignment in ferromagnetic film 10 along the trenches or long axis of any structures or textures formed in the ferromagnetic films 10 or 30.
FIG. 5 illustrates an MRAM including an array made up of magneto-resistive memory elements 2. MRAM array 50 is formed over a substrate 52 and includes column lines 54, 56, 58, and 60, and row lines 62, 64, 66, and 68. Columns and rows are selected by column and row line circuits 70 and 72, respectively. At the intersection of each column and row line is an MTJ memory element 2 fabricated in accordance with the invention.
FIG. 6 illustrates a microprocessor-based system that includes an MRAM containing an MRAM array made up of magneto-resistive memory elements 2. FIG. 6 illustrates a processor system 80 in which an MRAM 82 according to the present invention may be utilized. System 80 includes a CPU 84 and a user input/output (I/O) device 86 connected to a system bus 88. System 80 also includes MRAM 82 which communicates with the CPU 84 over system bus 88. Other peripheral devices include a disk drive 90 and a CD ROM drive 92.
While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.

## Claims

1. An assembly for use in a magneto-resistive cell structure, said assembly consisting of:
a substrate, the substrate being made of a silicon material;
a seed layer formed in direct physical contact with the substrate and having a textured surface; and
a ferromagnetic layer formed in direct physical contact with the textured surface and having uniaxial magnetic anisotropy.

2. The assembly of claim 1, wherein the textured surface is imparted to the ferromagnetic layer by patterning and etching.

3. The assembly of claim 1, wherein the textured surface is imparted to the ferromagnetic layer by chemical mechanical polishing.

4. The assembly of claim 1, wherein the textured surface is imparted by depositing a thin film on a wafer surface at an oblique angle to the wafer surface.

5. The assembly of claim 1, wherein the textured surface is a series of elongate surface inequalities.

6. The assembly of claim 5, wherein the series of elongate surface inequalities includes a pattern of alternating parallel ridges and channels.

7. The assembly of claim 1, wherein the textured surface is imparted by applying the ferromagnetic layer to the substrate, and stressing the film.

8. The assembly of claim 1, wherein the textured surface is imparted to the ferromagnetic layer as a plurality of substantially parallel, elongate trenches.

9. The assembly of claim 1, wherein the textured surface is imparted to the ferromagnetic layer by off-axis angular deposition of the seed layer.

10. The assembly of claim 1, wherein the textured surface is imparted to the ferromagnetic layer by dimple etching the seed layer.

11. The assembly of claim 1, wherein the textured surface is imparted to the ferromagnetic layer using a lithographic technique.

12. A memory device comprising:
a three-layered magneto-resistive film stack comprising:
a ferromagnetic material formed in direct physical contact with a seed material, the seed material being in direct physical contact with a substrate and having at least one surface exhibiting a physical texture, wherein the ferromagnetic material exhibits uniaxial magnetic anisotropy and the substrate is made of a silicon material.

13. The memory device of claim 12, wherein the physical texture is imparted by generating a physically textured seed layer, and applying the ferromagnetic film to the textured seed layer.

14. The memory device of claim 12, wherein the seed layer comprises either a dielectric material or a metal material.

15. The memory device of claim 12, wherein the physical texture is a series of elongate surface inequalities including a pattern of alternating parallel ridges and channels.

16. The memory device of claim 12, wherein the physical texture is imparted to the ferromagnetic film as a plurality of substantially parallel, elongate trenches.

17. A magneto-resistive random access memory comprising:
a plurality of magneto-resistive memory cells, each magneto-resistive memory cell comprising a three-layered magneto-resistive film stack consisting of:
a substrate made of a silicon material, located in direct physical contact to a seed layer; and
a ferromagnetic film, located in direct physical contact to the seed layer including at least two elongate structures, having a magnetic dipole due to a physical surface texture on at least one surface of the seed layer.

18. The magneto-resistive random access memory of claim 17, wherein the at least two elongate structures are formed in the film in a regular, repeating pattern.

19. The magneto-resistive random access memory of claim 17, wherein the at least two elongate structures are formed by forming the ferromagnetic layer on a textured underlayer.

20. The magneto-resistive random access memory of claim 17, wherein the at least two elongate structures are formed by stressing the ferromagnetic film.