US20140037991A1

US20140037991A1 - Magnetic random access memory with synthetic antiferromagnetic storage layers

Info

Publication number: US20140037991A1
Application number: US13/562,873
Authority: US
Inventors: David W. Abraham; Michael C. Gaidis
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2012-07-31
Filing date: 2012-07-31
Publication date: 2014-02-06
Also published as: US9015927B2; US20140033516A1

Abstract

A synthetic antiferromagnetic device includes a first tantalum layer, a reference layer disposed on the first tantalum layer and including a first cobalt iron boron layer, a second cobalt iron boron layer disposed on the first cobalt iron boron layer, a third cobalt iron boron layer and a second tantalum layer disposed between the second and third cobalt iron boron layers, a magnesium oxide spacer layer disposed on the reference layer and a cap layer disposed on the magnesium oxide spacer layer.

Description

BACKGROUND

The present invention relates to magnetic random access memory devices, and more specifically, to magnetic random access memory with a synthetic antiferromagnetic (SAF) freelayer with a non-exchange-coupled spacer layer.
SAF storage layers are implemented in magnetic random access memory devices (MRAM) with certain advantages. For example, it appears that higher activation energy can be obtained for equivalent switching currents when using a SAF structure versus a simple single free layer. Currently, the results in the literature that demonstrate such advantage are obtained with low-temperature annealing and processing. However, for many semiconductor applications (particularly for embedded memory application) it is important that the process be compatible with existing CMOS fabrication requirements. In practice, this means that the MRAM device must be able to withstand high temperature annealing and processing, with temperatures in the range of 400° C. for 1 hour total exposure time. Currently, it is also possible to fabricate SAF structures, which survive 400° C. anneals, demonstrating strong coupling in the SAF. Spin torque switching has been observed for anneal temperatures of up to 350° C. However, the current density can be too large to be practically employed, and even for these demonstrations the activation energy reported was only 62 kT. For applications in MRAM, an activation energy of at least 80 kT is typically required in order to retain information for sufficiently long times.

SUMMARY

Exemplary embodiments include synthetic antiferromagnetic device, including a first tantalum layer, a reference layer disposed on the first tantalum layer and including a first cobalt iron boron layer, a second cobalt iron boron layer disposed on the first cobalt iron boron layer, a third cobalt iron boron layer and a second tantalum layer disposed between the second and third cobalt iron boron layers, a magnesium oxide spacer layer disposed on the reference layer and a cap layer disposed on the magnesium oxide spacer layer.
Additional exemplary embodiments include a synthetic antiferromagnetic device including a reference layer, a spacer layer disposed on the reference layer, a storage layer disposed on the spacer layer and a cap layer disposed on the storage layer, wherein at least one of the reference layer and the storage layer include a refractory metal layer disposed between two layers of cobalt iron boron.
Further exemplary embodiments include a synthetic antiferromagnetic device, including a substrate, a first tantalum layer disposed on the substrate, a first cobalt iron boron layer disposed on the first tantalum layer, a second cobalt iron boron layer disposed on the first cobalt iron boron layer, a second tantalum layer disposed on the second cobalt iron boron layer, a third cobalt iron boron layer disposed on the second tantalum layer, a magnesium oxide spacer layer disposed the third cobalt iron boron layer, a fourth cobalt iron boron layer disposed on the magnesium oxide spacer layer, a third tantalum layer disposed on the fourth cobalt iron boron layer, a fifth cobalt iron boron layer disposed on the third tantalum layer, a cap layer disposed on the fifth cobalt iron boron layer, a hard mask tantalum nitride layer disposed on the cap layer, a bottom contact coupled to the substrate and a top contact coupled to the hard mask tantalum nitride layer, wherein the fourth cobalt iron boron layer, the third tantalum layer and the fifth cobalt iron boron layer; form a storage layer.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates an exemplary a SAF device;

FIG. 2 illustrates exemplary SAF devices in further detail; and

FIG. 3 illustrates a SAF fabrication method in accordance with exemplary embodiments.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary a SAF device 100 that includes high anneal temperature SAF storage (free) layers for MRAM devices. The device 100 illustrates two SAF devices 105, although it will be appreciated that additional or fewer SAF devices are contemplated in other exemplary embodiments. The device 100 includes a top contact 110 and a bottom contact 115 coupled to the SAF devices 105.
In exemplary embodiments, each of the SAF devices 105 includes a reference (pinned or non-pinned) layer 120 coupled to the bottom contact 115. In exemplary embodiments, the reference layer 120 can be any standard reference layer to provide a reference electrode. The SAF devices 105 further include a spacer layer 125 disposed on the reference layer 120 and a storage layer 130 disposed on the spacer 125. The SAF devices 105 further include a cap layer 135 (which can include a hard mask) disposed between the storage layer 130 and the top contact 110.
In exemplary embodiments, the SAF devices 105 described herein incorporate the storage layer 130, which combines both thermal stability at high anneal temperatures implemented in device fabrication for the required period and switching with appropriately low switching currents, device resistance, activation energy and coercivity to allow proper functioning in an MRAM device.
In exemplary embodiments, the systems and methods described herein combine lower switching current with high temperature stability in SAF devices. As such, MRAM devices include an SAF freelayer with a non-exchange-coupling spacer layer. The non-exchange-coupled SAF can implement refractory materials and similar spacers and caps as further described herein. In exemplary embodiments, an SAF device for an MRAM device includes a magnetic storage with a spacer made of Ta, TaN, W, or similar refractory (i.e. low diffusion) material. This structure can be placed either below or above the tunneling barrier (e.g., MgO). The seed or cap layers can be one of the same materials used for the spacer, and these can be mixed and matched (i.e. Ta seed, W spacer and the like). In exemplary embodiments, the SAF devices described herein can include no pinning layer, but instead implement a double SAF device. The reference layer SAF is identical in structure to the freelayer SAF, except that the magnetic films are thicker. In this way, the magnetic layers including the reference layer do not switch.
FIG. 2 illustrates the SAF devices 105 in further detail. FIG. 3 illustrates a SAF fabrication method 300 in accordance with exemplary embodiments. At block 305, the substrate 200 is prepared. The substrate 200 can be silicon (Si). In order to attain magnetic tunnel junction (MTJ) performance in the final MRAM device, preparation of the substrate 200 includes attaining atomic-scale flatness over areas of the order of the size of an MTJ. Atomic scale flatness reduces dipolar magnetic coupling effects and makes for a well-controlled MRAM device. Substrates for the processes are thus generally prepared either with a careful silicon wafer oxidation/cleaning or with the deposition of a dielectric such as silicon nitride (SiN) on the substrate 200, followed by a chemical mechanical planarization (CMP) step to smooth the surface.
At block 310, the reference layer 120 is deposited. As described herein, the reference layer 120 can include any standard reference layer, which can include a first tantalum (Ta) layer 205 to promote polycrystalline growth. In the example of FIG. 2, the first Ta layer 205 could be tantalum nitride (TaN). In exemplary embodiments, depending on the implementation, the reference layer 120 can then further include a variety of other layers including but not limited to an antiferromagnet for strong pinning of the reference layer (e.g., PtMn or IrMn), and an antiferromagnetically exchange-biased pair of ferromagnets (e.g., CoFe/Ru/CoFe). In the example of FIG. 2 a first cobalt iron boron (CoFeB) layer 210 is deposited on the seed layer 205, followed by a second CoFeB layer 211. It will be appreciated that the first and second CoFeB layers can be a single integrated CoFeB layer. A second. Ta layer 215 is then deposited on the second CoFeB layer 211. Finally, a third CoFeB layer 212 is deposited on the second Ta layer 215.
At block 315, the spacer layer 125 is deposited on the reference layer 120. In exemplary embodiments, the spacer layer 125 is a suitable insulating electrical bridge (e.g., aluminum oxide (Al₂O₃) or magnesium oxide (MgO)). In the example in FIG. 2, the spacer layer is an MgO layer 220.
At block 320, the storage layer 130 is deposited on the spacer layer 125. In the example in FIG. 2, a fourth CoFeB layer 230 is deposited on the spacer layer 125, followed by a deposition of third Ta layer 235 and a fifth CoFeB layer 240.
At block 325, a suitable cap layer 135 is deposited on the storage layer 130. In the example shown in FIG. 2, the cap layer 135 includes a fourth Ta layer 245 deposited on the storage layer 130 followed by a deposition of a ruthenium (Ru) layer 250. In exemplary embodiments a hardmask TaN layer 255 can be deposited on the Ru layer 250. The cap layer 135 and the hardmask TaN layer protect the SAF device 105 during the aggressive etch which defines the device 100
As described herein, the exemplary SAF devices 105 include a storage layer 130 with a spacer made of Ta, TaN, W, or similar refractory (i.e. low diffusion) material. This structure can be placed either below or above the spacer layer 125. FIG. 2 illustrates this structure both below and above the spacer layer 125 respectively as the layers including the second CoFeB layer 211, the second Ta layer 215 and the third CoFeB layer 212, and as the layers including the fourth CoFeB layer 230, the third Ta layer 235 and the fifth CoFeB layer 240. In addition, the seed or cap layers can be one of the same materials used for the spacer in the SAF storage layer 130, and these materials can be mixed and matched (i.e., Ta seed, W spacer and the like). As shown the first Ta layer 205 is the seed layer and the cap layer 135 includes the fourth Ta layer 245.
At block 330, further semiconductor processing can be implemented. For example, the SAF device 100 can be encapsulated in a dielectric. The encapsulation of the SAF device 100 protects the SAF device 100 while at the same time forming the environment in which the attachment of the top and bottom contacts 110, 115 can be implemented. The choice of encapsulation is determined from three requirements: a) it must not damage the SAF devices 100; b) it must adhere well to the substrate 200; and c) it should closely emulate the interlayer dielectrics (ILDs) that would be used in a fully integrated wafer processes. For example, damage to the MTJs can arise from chemical interactions and thermal stress. Standard semiconductor-industry dielectrics typically are deposited or cured at temperatures around 400° C., whereas degradation in submicron MTJs can set in at temperatures below 350° C. Thus, a major challenge to the integrator of MRAM devices is the development and utilization of suitable low temperature dielectrics. However, it will be appreciated that the SAF device can withstand the high anneal temperatures discussed above. Adhesion of the dielectric to the substrate can be particularly problematic given the characteristics of the magnetic films being used. Noble-metal-containing antiferromagnets can be particularly difficult to adhere to, and, if exposed by the etching used for MTJ patterning, can require specialized surface-cleaning or surface-preparation techniques to promote adhesion to the encapsulating dielectric. The dielectric thickness is chosen such that it will be thick enough to provide the environment for the wiring level above the MTJs.
Other semiconductor processing includes suitable patterning of the spacer layer 125. Patterning of the MTJ introduces device-to-device isolation in the counter-electrode the conductive portion of the storage layer 130 above the spacer layer 125), but maintains electrical continuity between desired devices in the base electrode (the conductive portion of the reference layer 120 below the spacer layer 125). Often negligible in fully integrated wafers, the resistance of the base electrode after MTJ patterning is germane to the short loop. The use of a continuous planar base electrode incurs additional measurement error at final electrical testing if the base electrode possesses a high sheet resistance. Subject to the constraint of emulating the stack used in fully functional wafers, the magnetic stack of the short loop will therefore include thick or low resistivity films beneath the tunnel barrier. A commonly used, straightforward approach to patterning the MTJs is through the use of a conducting hard mask. The conducting mask is later utilized as a self-aligned stud bridging the conductive MT wiring to the active magnetic films in the device. Such a processing scheme is among the simplest and fastest ways of creating and contacting the MTJs, making it an ideal approach for use in the short loop. Choices for the hard mask are numerous, with necessary characteristics being etchability and a resistance that is negligible when compared with MTJ resistance. Refractory materials commonly used in the semiconductor industry such as Ta, TaN, and TiN are suitable as masks for MTJ patterning. The MTJ shapes are defined in the hard mask by transfer from a first photomask level in a process such as the following: apply resist/expose and develop/reactive ion etching (RIE) through hard mask/strip resist. The pattern is further transferred downward to penetrate to (or through) the tunnel barrier, leaving behind a low-resistance base layer which covers the entire wafer.
Other semiconducting fabrication processes can include planarization. To facilitate industry standard damascene copper wiring, the wafers generally undergo a gentle dielectric CMP process at this stage. The purpose of the CMP is to remove topography from the surface that is caused by the underlying MTJs. This step is also the first check of the adhesion of the dielectrics to the underlying metal films, as well as the cohesion of the metal films to each other. If the encapsulating dielectrics are suitably planarizing in their deposition, this CMP planarization step can be eliminated for faster turnaround time and potentially higher yield.
After completion of the steps for MRAM development (e.g., layer formation, patterning, and encapsulation), the wiring (e.g., applying the bottom and top contacts 110, 115) is instituted in the simplest manner consistent with the available tooling. Relying on well-established semiconductor-industry techniques, a photomask defined trench is etched into the dielectric with RIE, to be filled with a liner and high-conductivity copper. The depth of the trench is sufficient to expose a portion of the conducting hard-mask stud (the counter-electrode), while not so deep as to create a short circuit to the planar base electrode. Endpointing during the trench RIE can facilitate the proper choice of trench depth even for relatively thin hard-mask films. After the trench etching and a suitable cleaning step, the wiring liner film is deposited, along with thin copper seed layer. This deposition is followed by the electroplating of copper to completely fill the trench and provide enough overburden so that the ensuing CMP step will planarize the metal coincident with the surface of the dielectric. This final CMP step can be aggressive enough to cause shear failure of the films on the wafers, and care must be taken to prevent such delamination. A post-polish cleaning of the wafers is the final preparation step before electrical testing.

Example

In one embodiment, the best thermal stability after the different curing/annealing fabrication steps and combination of switching voltage and magnetoresistance, occurred for the composition as now described. The first Ta layer 205 is 20 angstroms (Å). The first CoFeB layer 210 is 25 Å and the specific composition is Co₂₀Fe₆₀B₂₀. The second CoFeB layer 211 is 10 Å and the specific composition is Co₂₀Fe₆₀B₂₀. The second Ta layer 215 is 10 Å. The third CoFeB layer is 30 Å and the specific composition is Co₂₀Fe₆₀B₂₀. The spacer layer 125 of MgO is formed by depositing 8 Å of Mg following by a 60 second exposure to oxygen gas under a rate of 100 standard centimeter cube per minute (sccm). Another layer of 3 Å of Mg is deposited followed by a 60 second exposure to oxygen under a rate of 50 sccm. Finally, another layer of 2 Å of Mg is deposited. The storage layer 130 is then deposited with 15 Å of the fourth CoFeB layer 230. The third Ta layer 245 is then deposited at a thickness of 10 Å. The fifth layer of CoFeB 240 is then deposited at a thickness of 15 Å and a specific composition of Co₂₀Fe₆₀B₂₀. The cap layer 135 is then deposited including the fourth Ta layer 245 at a thickness of 10 Å, and the Ru layer 250 at a thickness of 200 Å. Finally, the hard mask TaN layer 255 is deposited at a thickness of 1000 Å.
It has been determined that the exemplary SAF devices (e.g., the SAF device 100) described herein combine lower switching current with high temperature stability in SAF devices for implementation in MRAM devices. The non-exchange-coupled SAF can implement refractory materials and similar spacers and caps as further described herein. This structure can be placed either below or above the tunneling barrier. The seed or cap layers can be one of the same materials used for the spacer, and these can be mixed and matched. In exemplary embodiments, the SAF devices described herein can include no pinning layer, but instead implement a double SAF stack. The reference layer SAF is identical in structure to the freelayer SAF, except that the magnetic films are thicker. In this way, the reference layers don't switch. The SAF devices described herein allow for improved thermal stability of the SAFs employed in the stack. Even though no exchange coupling is afforded by this SAF structure, the ultimate performance is good, and improved thermal stability is a significant improvement over conventional Ru-coupled SAF structures, for example. Note that magnetically, the exemplary SAF devices described herein behave much differently than the Ru-exchange-coupled SAFs (either AP or P orientation). However, experimental evidence suggests that the exemplary devices improve switching performance even if they do so in a different mode.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof.
The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated
The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.
While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.

Claims

What is claimed is:

1. A synthetic antiferromagnetic (SAF) device, comprising:

a first tantalum layer;

a reference layer disposed on the first tantalum layer and including:

a first cobalt iron boron (CoFeB) layer;

a second CoFeB layer disposed on the first CoFeB layer;

a third CoFeB layer; and

a second Ta layer disposed between the second and third CoFeB layers;

a magnesium oxide (MgO) spacer layer disposed on the reference layer; and

a cap layer disposed on the MgO spacer layer.

2. The device as claimed in claim 1 further comprising a storage layer disposed between the MgO spacer layer and the cap layer.

3. The device as claimed in claim 2 wherein the storage layer comprises:

a fourth CoFeB layer;

a fifth CoFeB layer; and

a third Ta layer disposed between the fourth and fifth CoFeB layers.

4. The device as claimed in claim 3 wherein the cap layer comprises:

a fourth Ta layer disposed on the fifth CoFeB layer; and

a ruthenium (Ru) layer disposed on the fourth Ta layer.

5. The device as claimed in claim 1 further comprising a substrate, wherein the first Ta layer is disposed on the substrate.

6. The device claimed in claim 2 wherein the MgO spacer layer is magnetically coupled to the storage layer, after the device is subject to anneal temperatures of about 375° C. to 400° C.

7. The device as claimed in claim 3 wherein the first, second, third, fourth and fifth CoFeB layers are Co₂₀Fe₆₀B₂₀.

8. A synthetic antiferromagnetic (SAF) device, comprising:

a reference layer;

a spacer layer disposed on the reference layer;

a storage layer disposed on the spacer layer; and

a cap layer disposed on the storage layer,

wherein at least one of the reference layer and the storage layer include a refractory metal layer disposed between two layers of cobalt iron boron CoFeB.

9. The device as claimed in claim 8 wherein the cap layer comprises a ruthenium (Ru) layer disposed on a tantalum layer.

10. The device as claimed in claim 9 further comprising:

a substrate; and

a seed layer disposed on the substrate,

wherein the reference layer is disposed on the seed layer.

11. The device as claimed in claim 8 wherein the reference layer comprises:

a first tantalum (Ta) layer; and

a first CoFeB layer disposed on the first Ta layer;

12. The device as claimed in claim 11 wherein when the refractory metal layer disposed between the two layers of CoFeB is disposed in the reference layer, the reference layer further comprises:

a second CoFeB layer disposed on the first CoFeB layer;

a third CoFeB layer;

and a second Ta layer disposed between the second and third CoFeB layers.

13. The device as claimed in claim 11 wherein when the refractory metal layer disposed between the two layers of CoFeB is disposed in the storage layer, the storage layer further comprises:

a fourth CoFeB layer disposed on the spacer layer;

a fifth CoFeB layer;

and a third Ta layer disposed between the fourth and fifth CoFeB layers.

14. The device as claimed in claim 8 further comprising a hard mask tantalum nitride (TaN) layer disposed on the cap layer.

15. The device as claimed in claim 8 wherein the refractory metal layer is a tantalum (Ta) layer.

16. The device claimed in claim 8 wherein the spacer layer is magnetically coupled to the storage layer, after the device is subject to anneal temperatures of about 375° C. to 400° C.

17. The device as claimed in claim 16 wherein two layers CoFeB are Co₂₀Fe₆₀B₂₀.

18. A synthetic antiferromagnetic (SAF) device, comprising:

a substrate;

a first tantalum (Ta) layer disposed on the substrate;

a first cobalt iron boron (CoFeB) layer disposed on the first Ta layer;

a second CoFeB layer disposed on the first CoFeB layer;

a second Ta layer disposed on the second CoFeB layer;

a third CoFeB layer disposed on the second Ta layer;

a magnesium oxide (MgO) spacer layer disposed the third CoFeB layer;

a fourth CoFeB layer disposed on the MgO spacer layer;

a third Ta layer disposed on the fourth CoFeB layer;

a fifth CoFeB layer disposed on the third Ta layer;

a cap layer disposed on the fifth CoFeB layer;

a hard mask tantalum nitride (TaN) layer disposed on the cap layer;

a bottom contact coupled to the substrate; and

a top contact coupled to the hard mask TaN layer,

wherein the fourth CoFeB layer, the third Ta layer and the fifth CoFeB layer; form a storage layer.

19. The device claimed in claim 18 wherein the MgO spacer layer is magnetically coupled to the storage layer, after the device is subject to anneal temperatures of about 375° C. to 400° C.

20. The device as claimed in claim 18 wherein the first, second, third, fourth and fifth CoFeB layers are Co₂₀Fe₆₀B₂₀.