Not Applicable
Not Applicable
The present invention relates to a magnetic random access memory (MRAM) and, more specifically, to a perpendicular MRAM with high-speed writing that can be arranged in a three-dimensional architecture.
Magnetic random access memory (MRAM) is a new memory technology that will likely provide a superior performance over existing semiconductor memories including flash memory and may even replace hard disk drives in certain applications requiring a compact non-volatile memory device. In MRAM bit of data is represented by a magnetic configuration of a small volume of ferromagnetic material and its magnetic state that can be measured during a read-back operation. The MRAM typically includes a two-dimensional array of memory cells wherein each cell comprises one magnetic tunnel junction (MTJ) element that can store at least one bit of data, one selection transistor (T) and intersecting conductor lines (so-called 1T-1MTJ design).
Conventional MTJ element represents a patterned thin film multilayer that includes at least a pinned magnetic layer and a free magnetic layer separated from each other by a thin tunnel barrier layer. The free layer has two stable orientations of magnetization that are parallel or anti-parallel to the fixed orientation of magnetization in the pinned layer. Resistance of the MTJ depends on the mutual orientation of the magnetizations in the free and pinned layers and can be effectively measured. A resistance difference between the parallel and anti-parallel states of the MTJ can exceed 600% at room temperature.
The orientation of the magnetization in the free layer may be changed from parallel to anti-parallel or vice-versa by applying two orthogonal magnetic fields to the selected MTJ, by passing a spin-polarized current through the selected junction in a direction perpendicular to the junction plane, or by using a hybrid switching mechanism that assumes a simultaneous application of the external magnetic field and spin-polarized current to the selected MTJ. The hybrid switching mechanism looks the most attractive among all others since it can provide good cell selectivity in the array, relatively low switching current and high write speed.
To write a data to the MTJ element 21, a bias electric current IB is applied to the bit line 19. The current IB induces a magnetic bias field HB that affects the free layer 23 along its hard magnetic axis. The field HB forces the magnetization in the free layer 23 from its equilibrium state that is parallel to the major axis of the MTJ element 21. By applying a voltage to the gate 15 through the word line 16 the selection transistor 12 can be turned on. The transistor 12 delivers a spin-polarized current IS to the MTJ element 21. The current IS running through the element 21 produces a spin momentum transfer that together with the bias field HB provides a reversal of magnetization in the free layer 23. The orientation of magnetization in the free layer 23 is controlled by a direction of the spin-polarized current IS. Magnitude of the spin-polarized current IS required to reverse the magnetization in the free layer 23 depends on the strength of the bias field HB that tilts the orientation of magnetization in the free layers relatively its equilibrium state. The switching current IS can be reduced more than twice by a relatively small bias magnetic field HB.
The MTJ with in-plane magnetization requires a high magnitude of the switching current IS even with applied magnetic bias field HB. Magnitude of the spin-polarized current IS defines a write speed of the memory cell; the speed increases with the current. The spin-polarized current IS of the cell 10 is limited by a saturation current of the transistor 12 that is proportional to a gate width W. The selection transistor 12 has the gate width W=2F, where F is a width of the elliptical MTJ element 21. This gate width is incapable to deliver the required magnitude of the current IS. To overcome the above obstacles the gate width W of the transistor 12 needs to be substantially increased. However that will result in considerable increase of memory cell size and in MRAM density reduction.
Majority of the current MRAM designs uses the free and pinned layers made of magnetic materials with in-plane anisotropy. The in-plane MRAM (i-MRAM) suffers from a large cell size, low thermal stability, poor scalability, necessity to use MTJ with a special elliptical shape, and from other issues, which substantially limit a possibility of i-MRAM application at technology nodes below 90 nm.
MRAM with a perpendicular orientation of magnetization in the free and pinned layers (p-MRAM) does not suffer from the above problems since perpendicular magnetic materials have a high intrinsic crystalline anisotropy. The high anisotropy provides p-MRAM with the excellent thermal stability and scalability, and with a possibility to use junctions of any shape. Nevertheless the existing p-MRAM designs have a large cell size and require a high switching current.
What is needed is a simple design of p-MRAM having high switching speed at low current, small cell size, high capacity and excellent scalability.
The present invention provides a three-dimensional magnetic random access memory (3D-MRAM) with a perpendicular magnetization for high-speed writing.
A magnetic random access memory according to an aspect of the present invention comprises a selection transistor comprising a gate width, the selection transistor is formed on a substrate and is electrically connected to a word line; a plurality of memory layers sequentially disposed above the substrate, wherein each of the plurality of the memory layers includes a plurality of magnetoresistive elements and wherein each of the plurality of the magnetoresistive elements comprises an element width and includes at least a pinned layer comprising a fixed magnetization oriented substantially perpendicular to a layer plane, a free layer comprising a changeable magnetization oriented substantially perpendicular to a layer plane in its equilibrium state, and a tunnel barrier layer residing between the pinned layer and the free layer; a plurality of conductor layers disposed alternately with the memory layers beginning with the memory layer positioned adjacent to the substrate, wherein each of the plurality of the conductor layers comprises a plurality of parallel bit lines intersecting the word line, and wherein the bit line is disposed adjacent to the free layer and is electrically connected with the magnetoresistive element; wherein the gate width is substantially larger than the element width, and wherein the magnetoresistive elements of the memory layer are electrically connected in parallel to the selection transistor.
A method of writing to a magnetic random access memory according to another aspect of the present invention comprises: providing a selection transistor disposed on a substrate and comprising a gate width; a word line connected to the selection transistor; a plurality of memory layers disposed above the substrate and comprising a plurality of magnetoresistive elements, wherein each of the plurality of the magnetoresistive elements comprises an element width and includes at least: a pinned layer with a fixed magnetization oriented perpendicular to a layer plane, a free layer with a changeable magnetization oriented perpendicular to a layer plane in its equilibrium state, and tunnel barrier layer residing between the pinned and free layers; a plurality of conductor layers comprising pluralities of parallel bit lines intersecting the word line, wherein the adjacent conductor layers are spaced from each other by the memory layer; and wherein the magnetoresistive elements are electrically connected in parallel to the selection transistor and the gate width is substantially larger than the element width; driving a bias current pulse through the bit line in a proximity to but not through the magnetoresistive element and producing a bias magnetic field along a hard magnetic axis of both the pinned layer and the free layer; driving a spin-polarized current pulse through the magnetoresistive element along an easy axis of both the pinned layer and the free layer and producing a spin momentum transfer; whereby the magnetization in the free layer will be switched by a collective effect of the substantially superimposed pulses of the bias and spin-polarized currents.
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown be way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
The leading digits of reference numbers appearing in the Figures generally corresponds to the Figure number in which that component is first introduced, such that the same reference number is used throughout to refer to an identical component which appears in multiple Figures.
The memory cell 20 has a 1T-2MTJ (one transistor-two MTJs) design. Number of the MTJ elements in the cell 20 can be any. Each MTJ element of the memory cell 20 has a unique combination of the bit and word lines that provides its selection in the MRAM array. For instance, to write data to the MTJ element 21-1 the bias current IB needs to be run in the bit line 19-1 and the spin-polarized current IS should run through the element in direction perpendicular to its plane. The spin-polarized current IS is controlled by the word line 16 that intersects the bit line 19-1 in vicinity of the MTJ element 21-1. Combined effect of the bias IB and spin-polarized IS currents will reverse the magnetization in the free layer 23 of the element 21-1.
In some embodiments of the present invention the MTJ element 21 has multilayer structure of the pinned 22 and free 23 layers. The
Pulse of the bias current IB running in the bit line 19 induces a bias magnetic field HB that is applied to the free layer 23 along its hard axis lying in the layer plane. The field HB tilts the magnetization M32 in the soft magnetic underlayer 32 on the angle Θ32 but does not change the orientation of magnetizations M22 and M34 in the pinned 22 and storage 34 layers, respectively. The angle Θ32 depends on the bias current magnitude, on thickness and magnetic properties of the soft magnetic underlayer 32 and the storage layer 34, and on the strength of the magnetic coupling between them. Tilting of the magnetization M32 in the soft magnetic underlayer 32 provides a significant reduction of the magnitude of spin-polarized current pulse IS that is required to the reverse the magnetization in the storage layer 34. The spin-polarizing layer 37 offers a high spin polarization of the switching current that is also important for reduction of IS magnitude. The material of the spin-polarizing layer can have perpendicular or in-plane anisotropy. The orientation of magnetization in the spin-polarized layer 37 does not change under the bias field HB due to its strong magnetic coupling with the reference layer 38. The magnetizations in the soft-magnetic underlayer 32 and in the spin-polarizing layer 37 are substantially collinear (parallel or anti-parallel) in the equilibrium state. That is important for providing a high output signal during read operation. The saturation current of the transistor 12 does not limit the magnitude of the spin-polarized current IS since the transistor has a large gate width W. At the same time, the bias field HB offers a significant reduction of the spin-polarized current IS and an additional opportunity of the write speed increase.
There is wide latitude for the choice of materials and their thicknesses within the embodiments of the present invention.
The pinned layer 22 has a thickness of about 10-100 nm and more specifically of about 25-50 nm and coercivity measured along its easy axis above 1000 Oe and more specifically of about 2000-5000 Oe. The layer 22 is made of magnetic material with perpendicular anisotropy such as Co, Fe or Ni-based alloys or their multilayers such as Co/Pt, Co/Pd, Co/Au, CoFe/Pt, Fe/Pt, Fe/Pd, Ni/Cu or similar.
The free layer 23 has a thickness of about 1-30 nm and more specifically of about 5-15 nm and coercivity less than 1000 Oe and more specifically of about 100-300 Oe. The layer 23 is made of soft magnetic material with perpendicular anisotropy such as Co, Fe or Ni-based alloys or multilayers such as Co/Pt, Co/Pd, Co/Au, CoFe/Pt, Fe/Pt, Fe/Pd, Ni/Cu or similar.
The tunnel barrier layer 24 has a thickness of about 0.5-25 nm and more specifically of about 0.5-1.5 nm. The tunnel barrier layer is made of MgO, Al2O3, Ta2O5, TiO2, Mg—MgO and similar materials or their based laminates.
The seed 26 and cap 27 layers have a thickness of 1-100 nm and more specifically of about 5-25 nm. The layers are made of Ta, W, Ti, Cr, Ru, NiFe, NiFeCr, PtMn, IrMn or similar conductive materials or their based laminates.
The conductor lines 18 and 19 are made of Cu, Al, Au, Ag, AlCu, Ta/Au/Ta, Cr/Cu/Cr and similar materials or their based laminates.
The soft magnetic underlayer 32 is 0.5-5 nm thick and is made of a soft magnetic material with a substantial spin polarization and coercivity of about 1-200 Oe such as CoFe, CoFeB, NiFe, Co, Fe, CoPt, FePt, CoPtCu, FeCoPt and similar or their based laminates such as CoFe/Pt, CoFeB/P and similar. The material of the soft magnetic underlayer 74 can have either in-plane or perpendicular anisotropy.
The storage layer 34 has a thickness of 5-25 nm and more specifically of about 8-15 nm; and coercivity less than 1000 Oe and more specifically of about 200-500 Oe. The storage layer 76 is made of magnetic material with a substantial perpendicular anisotropy such as Co, Fe or Ni-based alloys or multilayers such as Co/Pt, Co/Pd, Co/Au, CoFe/Pt, Fe/Pt, Fe/Pd, Ni/Cu or similar.
The spin-polarizing layer 37 has a thickness of 0.5-5 nm and is made of a soft magnetic material with a coercivity of about 1-200 Oe and a substantial spin polarization such as CoFe, CoFeB, NiFe, Co, Fe, CoPt, FePt, CoPtCu, FeCoPt and similar or their based laminates such as CoFe/Pt, CoFeB/P and similar. The material of the spin-polarizing layer 37 can have either in-plane or perpendicular anisotropy.
The reference layer 38 has a thickness of 10-100 nm and more specifically of about 20-50 nm; and coercivity above 1000 Oe and more specifically of about 2000-5000 Oe. The reference layer 38 is made of magnetic material with a substantial perpendicular anisotropy such as Co, Fe or Ni-based alloys or multilayers such as Co/Pt, Co/Pd, Co/Au, CoFe/Pt, Fe/Pt, Fe/Pd, Ni/Cu or similar.
It is understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should be, therefore, determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.
This invention claims benefit of U.S. Provisional Patent Application No. 61/227,364 entitled “3D Magnetic Random Access Memory with High Speed Writing” filed Jul. 21, 2009, which is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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61227364 | Jul 2009 | US |