Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N50/00—Galvanomagnetic devices
  • H10N50/10—Magnetoresistive devices
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11C—STATIC STORES
  • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
  • G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
  • G11C11/16—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect
  • G11C11/161—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect details concerning the memory cell structure, e.g. the layers of the ferromagnetic memory cell
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11C—STATIC STORES
  • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
  • G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
  • G11C11/16—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using elements in which the storage effect is based on magnetic spin effect
  • G11C11/165—Auxiliary circuits
  • G11C11/1675—Writing or programming circuits or methods
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11C—STATIC STORES
  • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
  • G11C11/56—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency
  • G11C11/5607—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency using magnetic storage elements
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10B—ELECTRONIC MEMORY DEVICES
  • H10B61/00—Magnetic memory devices, e.g. magnetoresistive RAM [MRAM] devices
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11C—STATIC STORES
  • G11C2211/00—Indexing scheme relating to digital stores characterized by the use of particular electric or magnetic storage elements; Storage elements therefor
  • G11C2211/56—Indexing scheme relating to G11C11/56 and sub-groups for features not covered by these groups
  • G11C2211/561—Multilevel memory cell aspects
  • G11C2211/5615—Multilevel magnetic memory cell using non-magnetic non-conducting interlayer, e.g. MTJ

Definitions

• the present invention pertains to magnetic tunneling junctions for memory cells and more specifically to very small magnetic junctions for very high density arrays of memory cells.
• a magnetic random access memory is a nonvolatile memory which basically includes a giant
• GMR magnetoresistive
• MTJ magnetic tunneling junction
• the MRAM employs the magnetic vectors to store memory states. Magnetic vectors in one or all of the layers of GMR material or MTJ are switched very quickly from one
• one direction can be defined as a
• the GMR material or MTJ maintains these states even without a magnetic field being applied.
• the states stored in the GMR material or MTJ can be read by passing a sense stored, for example, one direction can be defined as a logic "0", and another direction can be defined as a logic "1".
• the GMR material or MTJ maintains these states even without a magnetic field being applied.
• the states stored in the GMR material or MTJ can be read by passing a sense current through the cell in a sense line because of the difference between the resistances of the two states.
• a dummy magnetic layer is added to a two magnetic layer stack and coupled to one of the two magnetic layers so that the other magnetic layer is a free layer.
• a drawback of the dummy magnetic layer approach is that it relies on cancellation of magnetostatic interaction between the two magnetic layers and this magnetostatic interaction strength depends on the geometry of the cell and the interlayer spacing. These parameters change as the critical dimension shrinks.
• two layer magnetic memory cells e.g. standard tunneling cells
• the aspect ratio goes below 5
• the amount of magnetic field required for switching states of the cell increases dramatically.
• the layers of the cells are made thinner to reduce the amount of magnetic field required for switching states of the cell, since the magnetic moment (determined by the material) times the thickness of the layer determines the required switching field.
• Softer magnetic material can also be used to reduce the magnetic moment but the reduction in switching field is limited for ultra small memory cells.
• the cells are made smaller they become unstable because, for example, when the size of the memory cell is 10 nm or less, the energy barrier of the magnetization, which is proportional to the cell volume, is decreased due to the decreased volume and is close to the thermal fluctuation energy, which is KT. Accordingly, it is highly desirable to provide magnetic random access memories and memory cells which are capable of being written (stored states switched) with less magnetic field and which have a volume sufficient to not be effected by the thermal fluctuation energy.
• a low switching field magnetoresistive tunneling junction memory cell including a antiferromagnetically coupled multi-layer structure having first and second magnetoresistive layers with a non-magnetic conducting layer situated in parallel juxtaposition between the pair of magnetoresistive layers.
• the pair of magnetoresistive layers in the antiferromagnetically coupled multi-layer structure are constructed to switch at different magnetic fields, by having different thicknesses or different magnetic material.
• the pair of magnetoresistive layers in the antiferromagnetically coupled multi-layer structure each have a magnetic vector which are anti- parallel with no applied magnetic field due to the antiferromagnetic coupling of the pair of layers and the aspect ratio.
• the cell further includes a magnetoresistive structure having a magnetic vector with a fixed relationship to the vector of the second magnetoresistive layer. Electrically insulating material is situated in parallel juxtaposition between the antiferromagnetic coupled multi-layer structure and the magnetoresistive structure to form a magnetic tunneling junction.
• the magnetic field required to switch states in the cell is dictated by the difference between the two magnetoresistive layers in the antiferromagnetically coupled multi-layer structure.
• each of the first and second structures have, or can be constructed to have, little net magnetic moment and, hence, the memory cell has little net magnetic moment so that it can be positioned closer to adjacent cells without affecting adjacent cells.
• FIG. 1 is a simplified side view of a low switching field magnetic tunneling junction memory cell in accordance with the present invention
• FIG. 2 is a view similar to FIG. 1 illustrating a different mode and a modification
• FIG. 3 through FIG. 9 illustrate steps in switching the low switching field magnetic tunneling junction memory cell from the mode of FIG. 1 to the mode of FIG. 2;
• FIG. 10 is a simplified view in top plan of an array of low switching field magnetic tunneling junction memory cell in accordance with the present invention.
• FIG. 1 illustrates an enlarged, simplified side view of a low switching field magnetic tunneling junction memory cell 10 in accordance with the present invention.
• Magnetic tunneling junction 10 is generally formed on a supporting substrate 11 and includes a magnetoresistive structure 12 supported on substrate 11, an electrically insulating material layer 13 positioned on structure 12, and a magnetoresistive structure 15 positioned on electrically insulating material layer 13 so as to sandwich layer 13 between magnetoresistive structures 12 and 15 and form a tunneling junction.
• Magnetoresistive structure 15 is comprised of an antiferromagnetically coupled multi-layer structure including magnetoresistive layers 17 and 18 having a nonmagnetic conducting layer 19 situated in parallel juxtaposition between magnetoresistive layers 17 and 18.
• Magnetoresistive layer 17 has a magnetic vector 20 lying along a preferred magnetic axis parallel to the planar surface of substrate 11 and magnetoresistive layer 18 has a magnetic vector 21.
• Magnetic vectors 20 and 21 are anti-parallel with no magnetic field applied to magnetic tunneling junction 10, due to the antiferromagnetic coupling between magnetoresistive layers 17 and 18 and/or an aspect ratio less than approximately 5.
• the layers of magnetic material (Layers 17 and 18 in FIG.
• antiferromagnetically coupled means that either of the antiparallel states (illustrated in FIGS. 1 and 2) is stable and that the parallel state is unstable and does require a constant magnetic field, since the magnetic vectors always tend to move to an antiparallel state (pointing in opposite directions) .
• shape and size of magnetoresistive structure 15 vectors 20 and 21 can be constrained to lie along the preferred magnetic axis by either shape or magnetic crystalline anisotropy.
• the preferred magnetization direction can be determined by uniaxial crystal field anisotropy (or magnetic crystalline anisotropy) .
• This preferred magnetization direction is set during film deposition by a bias field or by annealing the film after deposition in a high magnetic field (e.g. several kOe) at elevated temperatures (e.g. 200°C to 300°C) .
• a high magnetic field e.g. several kOe
• elevated temperatures e.g. 200°C to 300°C
• the uniaxial crystal anisotropy can be set along a diagonal direction of the square.
• the uniaxial crystal anisotropy can be set along the long axis of the cell.
• a preferred feature here is to minimize the shape effect, which contributes to the rise in required switching fields at narrow cell widths, and to utilize magneto-crystalline anisotropy to set the preferred magnetization direction needed by a memory cell.
• magnetoresistive layer 17 is constructed to switch direction of magnetic vector 20 at a different magnetic field intensity than the switching of magnetic vector 21 of magnetoresistive layer 18. This feature can be accomplished in several different ways, including by forming layer 17 thinner (with less material) than layer
• layer 17 by forming layer 17 with different magnetization (e.g. forming layer 17 of softer magnetic material than layer 18) , or some combination of the size and magnetization.
• different magnetization e.g. forming layer 17 of softer magnetic material than layer 18
• Magnetoresistive layers 17 and 18 each can be single layers of ferromagnetic materials such as a layer of nickel, iron, cobalt, or alloys thereof including alloys having palladium or platinum therein.
• layers 17 and 18 can be a composite ferromagnetic layer, such as a layer of nickel-iron-cobalt covering a layer of cobalt-iron or three layer structures including layers of cobalt-iron and nickel-iron-cobalt and cobalt-iron with cobalt-iron at the interface with adjacent layers.
• Materials that are suitable for nonmagnetic conducting layer 19 include most electrically conductive materials such as copper and the like.
• Magnetoresistive structure 12 including at least one magnetoresistive layer having a magnetic vector parallel to the preferred magnetic axis, is positioned on substrate 11 with electrically insulating material layer 13 situated in parallel juxtaposition between structures 12 and 15 to form magnetic tunneling junction 10.
• magnetoresistive structure 12 is illustrated as similar to structure 15 and includes magnetoresistive layers 25 and 26 separated by a non-magnetic conducting layer 27.
• magnetoresistive layer 26 has a magnetic vector 28 which, generally, in the operation of magnetic tunneling junction 10, is fixed in one direction along the preferred magnetic axis.
• magnetoresistive structure 12 can have substantially any configuration that includes a magnetoresistive layer with a fixed magnetic vector adjacent electrically insulating material layer 13 to produce a magnetic tunneling junction and which, preferably has a substantially zero magnetic moment to produce a minimum effect on adjacent cells.
• layer 13 is a barrier or tunneling layer, the provision of which between antiferromagnetic layers 17 and 26 produces a tunneling junction that allows a flow of current perpendicularly through layer 13, from layer 17 to layer 26 (or vice versa) .
• magnetic tunneling junction 10 appears as a relatively high impedance (referred to herein as a resistance R) , which has dependence on the square area of the cell and the dielectric structure, generally several thousand ohms, e.g. 10 to 1000 kohms .
• the resistance R of magnetic tunneling junction 10 remains very high.
• the resistance R of magnetoresistive tunneling junction 10 drops perceptibly.
• layer 17 is formed of cobalt (Co) approximately 50 A thick
• layer 13 is formed of aluminum oxide (AI 2 O 3 ) approximately 15 A thick
• layer 26 is formed of nickel iron (NiFe) approximately 50 A thick.
• Layer 18, which in this example is thicker than layer 17, is approximately 60 A thick and layer 25, if present, has a similar thickness.
• the state of magnetic tunneling junction 10 is relatively easily sensed by passing a sense current therethrough from layer 18 to layer 25 (or vice versa) .
• the change of resistance in magnetic tunneling junction 10 is easily read as a change in voltage drop across magnetic tunneling junction 10 which can conveniently be used in conjunction with memory arrays and the like.
• FIG. 2 a structure similar to that of FIG. 1 is illustrated in a different mode, and a slightly different embodiment is illustrated with similar components being designated with similar numbers having a prime added to indicate the different embodiment.
• the antiparallel coupling between layers 17' and 18' of structure 15' is reinforced by the addition of flux closure material 30' positioned to enclose exposed edges of layers 17', 18' and 19'.
• Flux closure material 30' is any soft magnetic material which encloses flux lines, or completes a magnetic circuit, between the various layers. Soft magnetic material 30' simply guides magnetic field lines from layers 17' and 18', respectively, into a closed loop to further reduce the end magnetic poles and greatly reduce stray magnetic fields. Similar flux closure material can be used in conjunction with layers 25', 26', and 27' if the layers are present.
• FIG. 3 illustrates vector 20 of magnetoresistive layer 17 and vector 21 of magnetoresistive layer 18 in the antiparallel position of FIG. 1.
• vector 20 is also antiparallel to vector 28 of magnetoresistive layer 26 so that magnetic tunneling junction memory cell 10 is in the high resistance mode.
• an initial small magnetic field is applied to magnetic tunneling junction memory cell 10 sufficient to switch vector 20 into a parallel position with vector 21, as illustrated in FIG. 4.
• a larger positive magnetic field is then applied to magnetoresistive tunneling junction memory cell 10, which causes vectors 20 and 21 to rotate in opposite directions 180°, as denoted by the transitional states illustrated in FIGS. 5, 6, and 7, respectively.
• vectors 20 and 21 are switched into the opposite direction illustrated in FIG. 8 and, when the magnetic field is removed, vector 20 again assumes an antiparallel state, illustrated in FIG. 9.
• the antiparallel state illustrated in FIG. 9 is a stable state and, represents the low resistance mode of magnetic tunneling junction memory cell 10. Because magnetic vectors 20 and 21 of magnetoresistive layers 17 and 18 rotate in opposite directions 180° , the magnetic moments tend to offset each other and a minimum amount of magnetic field is required to perform the switching operation.
• the amount of switching magnetic field required to perform the switching operation is primarily dependent upon the difference in thickness or material between magnetoresistive layers 17 and 18. That is, because the stable modes for vectors 20 and 21 are antiparallel, by forming one of layers 17 and 18 so that it switches at a different magnetic field intensity than the other of layers 17 and 18, the mode switching results in rotation of vectors 20 and 21 in opposite directions with the resulting advantageous offsetting magnetic moments.
• the antiparallel vectors 20 and 21 result in very little total magnetic moment for magnetoresistive tunneling junction memory cell 10. This low magnetic moment is further enhanced by the flux closure material 30' illustrated in FIG. 2.
• the energy barrier of the magnetization (which is proportional to the volume) is close to the thermal fluctuation energy (KT) . This causes the cell to be unstable.
• the switching field can be reduced substantially without reducing the total size to an unstable level.
• an advantage of having two layers switching in opposite directions at the same time is that the switching field is determined by the difference in thickness between the two switching layers or, more accurately, the difference between the product of thickness and magnetization.
• both layers can be sufficiently thick so that the energy barrier for the two different magnetic states is higher than the thermal fluctuation energy.
• FIG. 10 a simplified view in top plan is illustrated of an high density array 45 of low switching field magnetoresistive tunneling junction memory cells 46 in accordance with the present invention.
• Array 45 is formed on a substrate structure 47 which may include control electronics and other peripheral equipment, if practical.
• the layer may be formed as a blanket layer so as to cooperate with each cell 46.
• Cells 46 lying in a common row for example, have the top magnetic layer connected to the bottom magnetic layer of the adjacent cell to form a common sense line 48.
• word lines 49 are coupled to cells 46 lying in a common column for purposes of writing information into the cells, as described above. Because of the zero, or essentially zero, magnetic moment of cells 10, cells of this type can be positioned very close and the density of an array of these cells can be greatly increased. Thus, new and improved magnetic random access memories and memory cells which are capable of being written (stored states switched) with less magnetic field have been disclosed. Also, the new and improved multi- state, multi-layer magnetic memory cell with antiferromagnetically coupled magnetic layers is capable of being written (stored states switched) with less magnetic field and has a volume sufficient to not be effected by the thermal fluctuation energy.
• the new and improved multi-state, multi-layer magnetic memory cell with antiferromagnetically coupled magnetic layers which is disclosed produces less magnetic interaction with adjacent cells in an array and can be fabricated very small and with an aspect ratio less than 5. Further, the new and improved multi-state, multi-layer magnetic memory cell is simpler to manufacture and to use and, because of its size, results in a high density array of cells.

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• Engineering & Computer Science (AREA)
• Computer Hardware Design (AREA)
• Hall/Mr Elements (AREA)
• Semiconductor Memories (AREA)
• Mram Or Spin Memory Techniques (AREA)

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• 1998
  • 1998-07-20 US US09/118,979 patent/US5953248A/en not_active Expired - Lifetime
• 1999
  • 1999-07-19 WO PCT/US1999/016314 patent/WO2000004552A1/en not_active Ceased
  • 1999-07-19 JP JP2000560586A patent/JP4815051B2/ja not_active Expired - Lifetime
  • 1999-07-19 EP EP99935700A patent/EP1038299B1/en not_active Expired - Lifetime
  • 1999-07-19 DE DE69932589T patent/DE69932589T2/de not_active Expired - Fee Related
  • 1999-11-16 TW TW088112229A patent/TW451192B/zh active

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