HK1064795A - Ferromagnetic layer for magnetoresistive element - Google Patents

Description

Ferromagnetic layer for magnetoresistive element

Technical field and background

A conventional magnetic tunnel junction includes pinned ferromagnetic layers, a sensing ferromagnetic layer, and an insulating tunnel barrier layer sandwiched between the ferromagnetic layers. The relative orientation and magnitude of the spin polarization of the ferromagnetic layers determines the resistance of the magnetic tunnel junction. In general, if the magnetization of the sense layer and the magnetization of the pinned layer point in the same direction (referred to as the "parallel" magnetization direction), the resistance of the magnetic tunnel junction has a first value (R)_N) If the sense and pinned layers are in opposite directions (referred to as "antiparallel" magnetization directions), the resistance increases to a second value (R)_N+ΔR_N). When the magnetization direction of the detection layer is rotated from one direction to another, the magnetic tunnel junction has an intermediate resistance value (R)_N＜R＜R_N+ΔR_N)。

The magnetic tunnel junction may be used as a magnetic detector in a read head. The magnetic tunnel junction of the read head can detect data stored on a magnetic storage medium, such as a hard disk drive.

Magnetic tunnel junctions may be used as storage elements in Magnetic Random Access Memory (MRAM) devices. The two magnetization directions, parallel and anti-parallel, represent the logical values "0" and "1". By setting the magnetization direction in either parallel or anti-parallel direction, a logical value can be written on the magnetic tunnel junction. By applying a suitable magnetic field to the magnetic tunnel junction, the magnetization direction can be changed from a parallel direction to an anti-parallel direction and vice versa. By detecting the resistance of the magnetic tunnel junction, a logic value can be read.

FM coupling between the pinned layers and the detection layer may cause problems with the magnetic tunnel junction. In a read head, FM coupling can cause readback signal distortion. Signal distortion may be corrected using an offset technique. However, the biasing technique is complicated and increases the manufacturing cost.

In a magnetic memory element, FM coupling may render the magnetic tunnel junction unusable. An unusable magnetic tunnel junction may reduce the performance of the MRAM, increase the manufacturing cost, and increase the complexity of the read circuit.

Disclosure of Invention

According to an aspect of the present invention, the ferromagnetic layer of the magnetoresistive element includes a crystalline ferromagnetic sublayer and an amorphous ferromagnetic sublayer. Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

Drawings

Fig. 1 shows a magnetoresistive element according to a first embodiment of the present invention;

fig. 2 shows a peak-to-valley height difference on the surface of the pinned layer of the magnetoresistive element;

fig. 3 shows a magnetoresistive element according to a second embodiment of the invention;

FIG. 4 illustrates a hard disk drive including a magnetoresistive read head according to one embodiment of the present invention;

FIG. 5 shows magnetized regions on a hard disk of a disk drive;

FIG. 6 shows a transition curve of a magnetoresistive read head;

FIG. 7 illustrates a method for fabricating a magnetoresistive read head according to an embodiment of the present invention;

FIG. 8 illustrates a data storage device including a magnetoresistive memory element according to an embodiment of the present invention;

FIG. 9 illustrates a method for manufacturing a data storage device according to one embodiment of the present invention; and

fig. 10 shows a response curve of a magnetoresistive memory element.

Detailed Description

Referring to fig. 1, a magnetoresistive element 110 comprising a stack of layers of material is shown. The stack includes a seed layer 112, an Antiferromagnetic (AF) pinning layer 114, a pinned Ferromagnetic (FM) layer 116, a spacer layer 118, and a sensing ferromagnetic layer 120. The seed layer 112 provides the correct crystal orientation for the AF-pinning layer 114. The AF-pinned layer 114 provides a large exchange field that keeps the pinned layer 116 magnetized in one direction. Thus, pinned layer 116 has a magnetization oriented in one plane but fixed so as not to rotate upon application of a magnetic field within the relevant range.

The detection layer 120 has a magnetization direction that is not pinned. But its magnetization direction can be rotated between each of two directions: the direction of magnetization of the nailed layer, or the direction opposite to the direction of magnetization of the nailed layer.

If magnetoresistive element 110 is a Magnetic Tunnel Junction (MTJ), spacer layer 118 is an insulating tunnel barrier layer that enables quantum mechanical tunneling to occur between pinned layer 116 and detection layer 120. This tunneling phenomenon is electron spin dependent, such that the resistance across the pinned layer 116 and the detection layer 120 of the MTJ is a function of the magnetization direction of the detection layer.

If the magneto-resistive element 110 is a giant magneto-resistive (GMR) device, the spacer layer 118 is made of a conductive metal, such as copper. The in-plane resistance across the detection layer 120 of a GMR device is a function of the magnetization direction of the detection layer.

If the magnetoresistive element is an Anisotropic Magnetoresistive (AMR) device, the spacer layer 120 is made of a conductive material and a soft adjacent layer is used in place of the pinned layer. In dual strip AMR devices, the spacer layer is made of an insulating material.

The facing surfaces of the pinned layer 116 and the spacer layer 118 define an interface 122. This interface 122 is referred to as a "spacing interface". During deposition of the pinned layer 116, the pinned layer 116 grows in a columnar shape, which causes grains to bend upward at a large angular slope on the interfacial surface of the pinned layer 116. This bending creates magnetic poles on the edges of the grains of the pinned layer 116. These magnetic poles generate a magnetic field within the detection layer 120.

Referring to fig. 2, the change in peak-to-valley height at the interface surface of the pinned layer 116 is illustrated. To prevent the bowing from causing strong FM coupling between the pinned layer 116 and the detection layer 120, the spacer layer interface 122 is flattened. The spacer layer interface 122 may be smoothed during manufacture by flattening the peaks and filling the valleys on the exposed surface of the pinned layer 116. Flattening the peaks and filling the valleys can reduce the variation in peak-to-valley height. The surface of the nailed layer after flattening is shown in solid lines at 352 and the surface of the nailed layer before flattening is shown in dashed lines at 354. The height variation between flattened peaks and valleys is indicated by the letter X. FIG. 2 is only used to illustrate the difference in peak-to-valley heights before and after the surface of the nailed layer is flattened; it does not provide an accurate representation of the surface 352 or 354 of the nailed layer 116.

It has been found that flattening surface 352 to a critical flatness reduces or eliminates FM coupling. It has been found that the critical flatness is achieved when the peak-to-valley height difference X is only about 1 nanometer.

The flattening process reduces the edge grain angles on the surface of the nailed layer 116. It is believed that shallower angles of the grains can produce fewer poles at the edges. Theoretically, the angle θ from the top of a grain to the intersection with an adjacent grain is about 3 to 6 degrees.

Instead of eliminating FM coupling, the value of FM coupling may be adjusted to reduce or eliminate AF coupling. As the magnetoresistive element 110 becomes smaller, AF coupling increases. Once the pinned layer 116 is patterned, a demagnetizing field is emitted from its edges. Because this magnetic field attempts to complete a loop, it ends up on the detection layer 120 and thereby generates a magnetic field in a direction opposite to the direction of magnetization of the pinned layer. This induced magnetic field, which is greatest at the edges of the detection layer 120, causes AF coupling. Tuning the FM coupling is particularly valuable when the magnetoresistive elements are smaller and thus the AF coupling becomes more severe.

Flattening the spacer interface 122 is not limited to flattening the surface of the nailed layer 116. Magnetoresistive element 110 may include an FM interface layer between pinned layer 116 and spacer layer 118. The interface layer should be part of the spacer layer interface 122. A planarization treatment may be performed on the interface layer surface facing the spacer layer 118. Such flattening of the spacer layer interface 122 may be done instead of flattening the nailed layer surface 352, or in addition to flattening the nailed layer surface, flattening of the spacer layer interface 122 may be done. Flattening the pinned layer 116 may result in a flatter interface layer and a flatter spacer layer 118.

The spacer layer interface 122 may also include a sublayer of the spacer layer 118. If the spacer layer 118 is formed within a sub-layer (e.g., an insulating tunnel barrier layer formed by multiple deposition steps), at least one of the sub-layers may be planarized. Such flattening of the spacer layer interface 122 may be done instead of or in addition to flattening of the nailed layer surface 352, the spacer layer interface 122 may be flattened. Planarizing the underlying sublayer may result in a more planar spacer layer 118.

The spacer interface 122 may be planarized using an ion etching process or other process that does not destroy the performance of the magnetoresistive element. With ion etch times in a range (which range depends on the FM material), the FM coupling can be monotonically reduced. A typical ion etch rate is on the order of 1 nm/min. The ion etch time can be adjusted so that the FM coupling can accurately compensate for the AF coupling regardless of the device size used in the application design center.

The spacer interface 122 may be further planarized by adding a layer of amorphous FM material to the magnetoresistive element. A magnetoresistive element having an amorphous FM layer is shown in fig. 3.

Referring to fig. 3, the magnetoresistive element 310 includes a seed layer 312, an AF pinning layer 314, a pinned layer 316, a spacer layer 318, and a detection layer 320. Element 310 also includes first and second interface FM layers 324a, 324b on opposite sides of spacer layer 318. The pinned layer 316 includes a sublayer 316a of amorphous ferromagnetic material that is located between sublayers 316b and 316c of crystalline ferromagnetic material (the interface layer 324a and the sublayer 316b may also be made as one layer rather than as separate layers). For this application, crystalline FM material refers to both monocrystalline and polycrystalline FM material. Amorphous sublayer 316a may be an FM material (e.g., NiFe, NiFeCo alloy, CoFe) to which an amorphizing agent (e.g., B, Nb, Hf, Zr) is added. Crystalline sublayers 316b and 316c may be made of a material such as NiFe. The final thickness of these sub-layers 316a, 316b, 316c may be between 1 and 3 nanometers.

The amorphous sublayer 316a may break the boundary structure of the grains in the AF pinning layer 316. This deliberate disruption of the grain structure reduces the severity of the peaks and valleys formed at the spacer layer interface 122. If the AF-pinned layer comprises a material that can diffuse into the crystalline ferromagnetic material (e.g. manganese), the amorphous sub-layer 316a provides the additional advantage of preventing said diffusion.

The crystalline sublayers 316b, 316c of the pinned layer preferably have a high magnetic moment and polarization. Increasing the magnetic moment/polarization of these crystalline sublayers 316b, 316c can increase the TMR of the magnetoresistive element 310.

The pinned layer 316 may instead include only one layer of amorphous ferromagnetic material 316a, or may include an amorphous sublayer 316c and one layer of crystalline sublayer 316 b. However, eliminating one or both of the crystal sublayers 316b, 316c reduces the pinning field (reducing the crystals reduces the pinning field) in addition to reducing the TMR of the magnetoresistive element 310.

The upper surface of amorphous sub-layer 316a and the lower surface of insulating tunnel barrier layer 318 define a spacer layer interface 322. The interface may be planarized by planarizing the upper surface of at least one of the following layers: an amorphous sublayer 316a, a crystalline sublayer 316b, and an interface layer 324 a. The spacer interface 322 may also be planarized by planarizing the lower surface of the insulating tunnel 318.

Alternatively, none of these surfaces are flattened. However, flattening the surface of the amorphous sublayer 316a can improve the grain structure of the overlying crystalline sublayer 316 b. In addition, flattening either of these surfaces can further reduce or enable tuning of FM coupling.

The magnetoresistive element described above is not limited to any particular application. Two exemplary applications are described below: hard disk drives and MRAM devices.

Referring to FIG. 4, a hard disk drive 410 including a magnetic media disk 412 is shown. User data is stored on concentric circular tracks on the surface of each disc 412. The disc drive 410 further comprises a sensor 414 for reading and writing operations on the disc 412. Each sensor 414 includes a magnetoresistive read head for performing read operations (each sensor 414 may also include a thin film inductive magnetic head for performing write operations).

With reference to fig. 5. During a read operation, magnetized regions 510 of the disk pass under the head of transducer 414. The read head detects changes in the magnetized regions 510. A fringing magnetic field 512 emanates from the grains at the grain boundaries. Has 3 net magnetization states: when a fringing magnetic field is applied out of the plane of the disk 412 (the net magnetic field is represented by arrow 514); when a fringing magnetic field is applied in the plane of the disk 412 (the net magnetic field is represented by arrow 516); and when the fringing fields cancel (no net field, hence no arrow). The net magnetic field applied to the outside of the disk 412 causes the magnetization direction of the sense layer to rotate in one direction, and the net magnetic field 516 applied to the inside of the disk 412 causes the magnetization direction of the sense layer to rotate in the other direction.

During the detection of the net magnetic field, the read head generates a readback signal. The magnitude of the readback signal depends on the magnetic resistance of the read head. The flattening process can be controlled to adjust the position of the transition curve so that the amplitude is linear with the read head resistance. This in turn reduces the distortion of the readback signal during read operations.

With additional reference to FIG. 6, there is shownThe transition curve 610 of the read head. The transition curve 610 has a substantially linear region between point a and point B. It may be desirable to center this linear region on a 0 field (H ═ 0). When the magnetizations of the pinned layer 116 and the detection layer 120 are in the same direction, the magnetic tunnel junction has a normal resistance (R1 ═ R)_N). When the magnetizations of the pinned layer 116 and the detection layer 120 are in opposite directions, the magnetic tunnel junction has a higher resistance (R2 ═ R)_N+ΔR_N). The magnetic tunnel junction has an intermediate resistance (R1 < R2) when the magnetization direction of the sense layer is rotated from one direction to the other.

The FM coupling may also be adjusted to improve the degree to which the transition curve 610 is centered about the 0 field. The AF coupling tends to move the transition curve 610 toward the left in fig. 6, so that the resistance of the magnetoresistive element 110 at zero magnetic field is in a high resistance state.

In addition to adjusting the magnetic field coupling, leveling can have a number of advantages, in the case of magnetic tunnel junctions, the insulating tunnel barrier layer is more evenly distributed over the pinned layer. The more uniform distribution of the barrier material enables the thickness of the insulating tunnel barrier to be reduced without creating pinholes (which can greatly increase magnetic coupling and short circuit the magnetic tunnel junction). Reducing the thickness of the barrier layer in turn results in a reduction in the resistance of the read head, which results in a reduction in the RC constant of the read head. Thus, the response time of the read head is accelerated. Reducing the resistance of the read head may also reduce power consumption.

Flattening the spacer layer interface also improves the uniformity of the resistance of the read head. The uniformity of the read head simplifies the design of the read head and reduces disk drive adjustments to achieve read head performance. The flat interface also reduces the number of shorted read heads, thereby improving yield.

Amorphous FM material may be used only in the pinned layer. However, adding crystalline FM material to the pinned layer can increase signal strength.

The magnetoresistive element is not limited to longitudinal recording as shown in fig. 5. Perpendicular recording can be performed using a magnetoresistive element, in which the magnetic field from the crystal grains is measured.

Referring now to FIG. 7, a method of manufacturing a plurality of read heads is shown. Depositing a stack of the following materials on a wafer: a shielding material, a seed layer material, an AF-nail rolled material, and a nailed FM layer material (710). The thickness of the deposited nailed layer material is increased to compensate for the underlying ion etch.

The exposed upper surface of the pinned layer is ion etched to have a critical flatness 712. Ion etching may be performed by bombarding the pinned layer with argon ions or other non-reactive ions. The ion etching may be performed in situ within the deposition chamber. With an ion etch time in a particular range (which range depends on the material), the FM coupling can be monotonically reduced. This enables adjustment of the magnetic interaction so that the read head operates linearly.

An interface layer material is then deposited on the etched surface, and the upper surface of the interface layer material is ion etched (714). An insulating tunnel barrier material is then deposited 716. Barrier layer materials such as alumina can be formed by r-f sputtering or by depositing aluminum and then oxidizing the aluminum, such as by plasma oxidation.

Interface layer material, detection layer material, and shielding layer material are then deposited (718). For GMR and AMR devices, an insulating dielectric layer is formed between the detection layer material and the shield material. The resulting stack is then shaped into a plurality of read heads (720).

The shielding material may be an electrically and magnetically conductive material such as NiFe. The seed layer material may be any material capable of establishing the desired grain orientation of the AF-pinned layer. Candidate materials for the seed layer include titanium (Ti), tantalum (Ta), and platinum (Pt). Candidate materials for the AF nail layer 36 include platinum-manganese (PtMn), manganese-iron (MnFe), nickel-manganese (NiMn), and iridium-manganese (IrMn). Candidate materials for insulating tunnel barrier layers include aluminum oxide (Al)₂O₃) Silicon dioxide (SiO)₂) Tantalum oxide (Ta)₂O₅) Silicon nitride (SiN)₄)，Other dielectric materials, and some semiconductor materials. Candidate materials for the pinned and sense layers include NiFe, iron oxide (Fe)₃O₄) Chromium oxide (CrO)₂) Any alloy of Ni, Fe and Co (e.g., CoFe, NiCoFe), as well as other ferromagnetic and ferrimagnetic materials.

Referring now to fig. 8, an exemplary MRAM device 810 including an array 812 of memory cells 814 is shown. Each memory cell 814 may include one or several magnetoresistive elements. To simplify the illustration of MRAM device 810, only a relatively small number of magnetoresistive elements 814 are shown. In fact, any size array 812 may be used.

The word lines 816 extend along rows of the memory cells 814 and the bit lines 818 extend along columns of the memory cells 814. Each row of array 812 may have a wordline 816 and each column of array 812 may have a bitline 818. Each memory cell 814 is located at an intersection of a wordline 816 and a bitline 818.

MRAM device 810 also includes read/write circuitry 820 for performing read/write operations on selected memory cells 814. The read and write circuits 820 detect the resistance of the selected memory cell 814 during a read operation and set the magnetization direction of the selected memory cell 814 during a write operation.

Fig. 9 illustrates a method for fabricating an MRAM device. Read and write circuits and other circuits are fabricated on a substrate (910). A conductive material is then deposited over the substrate and formed into bit lines 912. A dielectric material is then deposited between the bitlines. The material for the seed layer and the material for the AF-nail layer are then deposited 914.

The nailed layer material is then deposited. The material for the amorphous FM sublayer is deposited and the upper exposed surface is ion etched to critical flatness 916. A material for the crystalline FM sublayer is deposited and the upper exposed surface is ion etched to a critical flatness 918. Ion etching the upper surface of the amorphous layer may improve the grain structure of the crystalline sub-layer. The thickness of the deposited material is increased to compensate for the ion etch.

An insulating tunnel barrier material (920) is formed over the etched material. Barrier layer materials such as alumina can be deposited by r-f sputtering or by depositing aluminum and then oxidizing the aluminum, such as by plasma oxidation. If the insulating tunnel barrier material is formed in multiple stages, at least one of the stages may be ion etched.

The material for the detected FM layer is then deposited 922. The resulting stack is then shaped into bit lines, and the spaces between the bit lines are filled with a dielectric material (924). A conductive material is then disposed over the dielectric layer and shaped to become a word line (924).

The resulting array is then planarized. A new array can be formed on top of the planarized array.

Fig. 9 illustrates the fabrication of an MRAM device. In practice, however, many MRAM devices may be fabricated on a single wafer at the same time.

Referring to fig. 10, a response curve 1010 of a magnetoresistive element of an MRAM device is shown. Flattening may be performed to adjust the position of the response curve and improve the uniformity of the array switching. The switching points a and B may be centered around a zero magnetic field. Improving switching uniformity can reduce the number of magnetic tunnel junctions that do not switch when there is a sufficient magnetic field, but do not switch unintentionally even when the magnetic field is insufficient. Thus, leveling can reduce the number of magnetic tunnel junctions that cannot be used.

Planarization can also reduce the number of tunnel junctions that cannot be used by reducing the number of magnetic tunnel junctions that are shorted by pinholes.

Reducing the number of tunnel junctions that cannot be used may increase the storage capacity of the MRAM device. The burden of performing error correction can also be reduced. Thus, reducing the number of magnetic tunnel junctions that cannot be used can improve the performance of MRAM and reduce manufacturing costs.

The flattening may improve the uniformity of the tunnel junction resistance of the array. Improving the uniformity may reduce the complexity of reading selected memory cells among multiple columns of memory cells.

Amorphous ferromagnetic sublayers may also reduce the roughness of the interface. If the antiferromagnetic pinning layer includes manganese, the amorphous sublayer may also block the diffusion of manganese into the crystalline ferromagnetic layer.

Although each memory cell is shown with only one magnetoresistive element, the MRAM device is not so limited. Each memory cell may include more than one magnetoresistive element.

The memory cell is not limited to magnetoresistive devices, such as a magnetic tunnel junction. Other magnetoresistive devices may be used.

The present invention is not limited to the magnetoresistive elements described above. The ferromagnetic layers (pinned or unstripped, crystalline or amorphous) of the magnetoresistive elements may be replaced by a combination of crystalline and amorphous ferromagnetic sublayers.

Instead of being placed at the bottom of the stack, the AF-pinned layer may be placed near the top of the stack, whereby the pinned layer is placed after the detection layer. If the detection layer material is disposed before the stapled layer material, the detection layer will have an interface including peaks and valleys. The interface of the detection layer should be planarized.

The present invention is not limited to the specific embodiments described above, but should be defined in accordance with the following claims.

Claims (9)

1. A magnetoresistive device (310), comprising:

a pinned ferromagnetic layer (316);

a sensing ferromagnetic layer (320); and

a spacer layer (318) between the ferromagnetic layers (316 and 320);

the pinned layer (316) comprises an amorphous sublayer (316a) and a crystalline sublayer (316b), the crystalline sublayer (316b) being between the amorphous sublayer (316a) and the spacer layer (318).

2. The device of claim 1, wherein at least one of the amorphous and crystalline layers (316a and 316b) is planarized.

3. The device of claim 2, wherein the surface of the planarized sub-layer is planarized to a peak to valley height difference of only about 1 nanometer.

4. The device of claim 3, wherein an angle from a top of a die to an intersection with an adjacent die is between about 3 degrees and 6 degrees.

5. The device of claim 1, wherein the pinned layer (316) further comprises a second crystalline ferromagnetic layer (316c), the amorphous ferromagnetic layer (316a) being sandwiched between the two crystalline ferromagnetic layers (316a and 316 c).

6. The device of claim 1, further comprising an antiferromagnetic pinning layer (314) comprising manganese, wherein the pinned layer (316) is disposed on the antiferromagnetic layer; and wherein the surface of the pinned layer is planarized to adjust the antiferromagnetic coupling between the antiferromagnetic layer and the pinned layer.

7. A read head (414) for a data storage device (410), the read head (414) comprising the device (310) of claim 1.

8. A data storage device (810) comprising an array (812) of memory cells (814), wherein each memory cell (814) comprises at least one device (310) according to claim 1.

9. A method for manufacturing a device (310) according to claim 1, the method comprising:

depositing an amorphous ferromagnetic layer (916);

ion etching the exposed surface of the amorphous ferromagnetic layer to a critical flatness (916); and

a crystalline ferromagnetic layer is deposited over the ion etched surface of the amorphous ferromagnetic layer (918).