Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2001-03-30
2003-06-10
Le, Vu A. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S171000
Reexamination Certificate
active
06577526
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
Magnetoresistive elements are increasingly used as sensor elements or as memory elements for memory cell configurations, so-called MRAMs (see S. Mengel, “Technologieanalyse Magnetismus Band 2, XMR-Technologien”, published by VDI Technologiezentrum Physikalische Technologien, August 1997). In the art, the term “magnetoresistive element” refers to a structure which comprises at least two ferromagnetic layers and a nonmagnetic layer disposed in between these. Depending on the design of the layer structure, a distinction is made between GMR element, TMR element, and CMR element.
The term “GMR element” is used in the art for layer structures which comprise at least two ferromagnetic layers and a nonmagnetic, conductive layer disposed between these and which exhibit the so-called GMR (giant magnetoresistance) effect. The GMR effect refers to the fact that the electrical resistance of the GMR element depends on whether the magnetizations in the two ferromagnetic layers are oriented parallel or antiparallel. The GMR effect is large compared with the so-called AMR (anisotropic magnetoresistance) effect. The AMR effect refers to the fact that the resistance in magnetized conductors parallel and perpendicular to the direction of magnetization differs. The AMR effect is a bulk effect which occurs in ferromagnetic single layers.
The term “TMR element” is used in the art for tunneling magnetoresistance structures which comprise at least two ferromagnetic layers and an insulating, nonmagnetic layer disposed between these. The insulating layer is so thin as to give rise to a tunneling current between the two ferromagnetic layers. These layer structures likewise exhibit a magnetoresistive effect which is caused by a spin-polarized tunneling current through the insulating, nonmagnetic layer disposed between the two ferromagnetic layers. In this case, too, the electrical resistance of the TMR element depends on whether the magnetizations in the two ferromagnetic layers are oriented parallel or antiparallel. The relative change in resistance here is from about 6 percent to about 30 percent.
The term “CMR effect” describes a further magnetoresistive effect. Because of its measurement (relative change in resistance by from 100 to 400 percent at room temperature) it is referred to as colossal magnetoresistance (CMR) effect. It requires a high magnetic field, because of the high coercitivities it involves, to switch between the magnetization states.
U.S. Pat. No. 5,477,482 proposed an annular configuration of the ferromagnetic layers and the nonmagnetic layer of a CMR element, the rings being stacked on top of one another or being nested concentrically.
It has been proposed (see for example S. Tehrani et al., IEDM 96-193 and D. D. Tang et al., IEDM 95-997) to use GMR elements or TMR elements as memory elements in a memory cell configuration. The memory elements are connected in series via read lines. Running transversely to these are word lines which are insulated both with respect to the read lines and with respect to the memory elements. Signals applied to the word lines give rise to a magnetic field which is a consequence of the current flowing within the word line and which, if sufficiently strong, affects the memory elements located underneath. The memory cell configuration exploits the fact that the resistance of the memory elements differs, depending on whether the magnetizations in the two ferromagnetic layers are oriented parallel or antiparallel to one another. To write information, the direction of magnetization of the one ferromagnetic layer is therefore pinned, while that of the other ferromagnetic layer is switched. To this end, crossing lines, which are also referred to as xy lines and which cross at the memory cell to be written to, are fed with signals in such a way that a magnetic field sufficient for remagnetization is produced at the crossing point.
Pinning the direction of magnetization in the one ferromagnetic layer is achieved by an adjacent antiferromagnetic layer which pins the magnetization (see D. D. Tang et al., IEDM 95-997) or by differing layer thicknesses of the ferromagnetic layers (see S. Tehrani et al., IEDM 95-193). Here, the antiferromagnetic layer differs in material composition from the adjacent ferromagnetic layer whose magnetization state is pinned.
As a result of the different layer thicknesses of the two ferromagnetic layers, a higher magnetic field is required in the one ferromagnetic layer to affect the direction of magnetization than in the other one. To write information, the magnetic field is assigned such a level that it is able to affect only the direction of magnetization in the one of the two ferromagnetic layers. The direction of magnetization in the other ferromagnetic layer, which can only be switched over by means of an increased magnetic field, therefore remains unaffected thereby.
As the layer thickness of the ferromagnetic layer cannot, on the one hand, drop below a minimum layer thickness of about 5 nm owing to fabrication constraints, and on the other hand the maximum layer thickness of the ferroelectric layer in a GMR or TMR element is also limited—by the fact that a defined direction of magnetization parallel to the layer plane must be present—it is necessary in this case for the switching magnetic field to be set precisely.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a magnetoresistive element which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which can be fabricated in good yields within the context of semiconductor process technology and which is insensitive in terms of setting the switching magnetic field.
With the above and other objects in view there is provided, in accordance with the invention, which can be used advantageously, inter alia, as a memory element in a memory cell configuration or as a sensor element.
In a first embodiment, the magnetoresistive element comprises the following features:
a plurality of planar layer elements, including a first ferromagnetic layer element, a nonmagnetic layer element on the first ferromagnetic layer element and forming an interface therewith, and a second ferromagnetic layer element on the nonmagnetic layer element and forming an interface therewith, the planar layer elements defining a stack with a layer sequence;
the first ferromagnetic layer element and the second ferromagnetic layer element comprising essentially the same material and having respective measurements in a dimension perpendicular to the layer sequence differing by at least percent relative to one another, and preferably by 30%.
In other words, the magnetoresistive element comprises a first ferromagnetic layer element, a nonmagnetic layer element, and a second ferromagnetic layer element which are arranged in such a way that the nonmagnetic layer element is disposed between the first ferromagnetic layer element and the second ferromagnetic layer element. In this arrangement, the nonmagnetic layer element has one interface each both with the first ferromagnetic layer element and with the second ferromagnetic layer element. The first ferromagnetic layer element and the second ferromagnetic layer element comprise essentially the same material. The first ferromagnetic layer element and the second ferromagnetic layer element have different measurements in at least one dimension parallel to the interface with the nonmagnetic layer element.
As a result of this different shape of the first ferromagnetic layer element and the second ferromagnetic layer element, the magnetic fields required to switch over the directions of magnetization in the ferromagnetic layer elements differ. This effect is referred to as shape anisotropy. Since in each layer element the measurements perpendicular to the layer thickness are distinctly larger than the layer thickness, larger differences in this measurement are possible in the present magnetoresistive element than is possible regarding the lay
Greenberg Laurence A.
Infineon - Technologies AG
Le Vu A.
Mayback Gregory L.
Stemer Werner H.
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