Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2001-02-12
2003-01-21
Lebentritt, Michael S. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S173000
Reexamination Certificate
active
06510078
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a memory cell array having memory elements with magnetoresistive effect and a method for manufacturing it.
Technologie Analyse XMR-Technologien, Technologie-früh-erkennung [Technology Analysis XMR Technologies, Early Recognition of Technology], by Stefan Mengel, Publisher VDI-Technologiezentrum Physikalische Technologie, discloses layered structures with magnetoresistive effect. Depending on its design, the layered structure is classified as a GMR (giant magnetoresistance) element, TMR (tunneling magnetoresitive) element, AMR (anisotropic resistance) element or CMR (colossal magnetoresistance) element. The term GMR element is used in the art to designate layered structures which have at least two ferromagnetic layers and a nonmagnetic, conductive layer arranged between them and exhibit the GMR (giant magnetoresistance) effect, that is to say exhibit a large magnetoresistive effect in comparison with the AMR (anisotropic magnetoresistance) effect. The GMR effect is understood as referring to the fact that the electrical resistance of the GMR element is dependent on whether the magnetizations in the two ferromagnetic layers are oriented in parallel or antiparallel both for currents which are parallel (CIP current in plane) and perpendicular (CPP current perpendicular to plane) to the layer planes. The resistance changes here as a function of the orientation of the magnetizations by &Dgr;R/R=5 percent to 20 percent at room temperature.
The term TMR element is used in the specialist field for “Tunneling Magnetoresistance” layered structures which have at least two ferromagnetic layers and an insulating, nonmagnetic layer arranged between them. The insulating layer has such a small thickness that a tunnel current occurs between the two ferromagnetic layers. These lead structures also exhibit a magnetoresistive effect which is brought about by spin-polarized tunnel current through the insulating, nonmagnetic layer arranged between the two ferromagnetic layers. In this case also, the electrical resistance of the TMR element (CPP arrangement) is dependent on whether the magnetizations in the two ferromagnetic layers are oriented in parallel or antiparallel. The resistance varies by &Dgr;R/R=10 percent to approximately 30 percent at room temperature.
The AMR effect is due to the fact that the resistance in magnetized conductors parallel to the magnetization direction varies from that of magnetized conductors which are perpendicular to the magnetization direction. It is a volume effect and thus occurs in ferromagnetic single layers.
A further magnetoresistive effect which is referred to as colossal magnetoresistance effect due to its magnitude (&Dgr;R/R=100 percent to 400 percent at room temperature) requires a high magnetic field for switching between the magnetization states owing to its high coercitive forces.
It has been proposed (see for example D. D. Tang, P. K. Wang, V. S. Speriosu, S. Le, K. K. Kung, “Spin Valve RAM Cell”, IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1996, page 3206) to use GMR elements as memory elements in a memory cell array. The magnetization direction of the one ferromagnetic layer of the GMR element is held here, for example, by an adjacent antiferromagnetic layer. Intersecting x and y lines are provided. In each case a memory element is arranged at the points of intersection of the x/y lines. In order to write information, the x/y lines are supplied with signals which bring about at the point of intersection a magnetic field which is sufficient for the change of polarity. In order to read out the information, the x/y lines can be supplied with a signal which switches the respective memory cell to and fro between the two magnetization states. The current through the memory element from which the resistance value, and thus the information, is determined is measured.
In order to write and read, local magnetic fields of 10 Oe to approximately 100 Oe corresponding to 8 A/cm to 80 A/cm are necessary. It is desirable here for the magnetic fields to be generated by the smallest possible current in the lines.
However, as miniaturization progresses, the current densities necessary to generate the local magnetic fields become greater. In addition, an effect has been observed (see M. H. Kryder, Kie Y. Ahn, N. J. Mazzeo, S. Schwarzl, and S. M. Kane, “Magnetic Properties and Domain Structures in Narrow NiFe Stripes”, IEEE Transactions on Magnetics, Vol. Mag.-16, No. 1, January 1980, page 99), in which the magnetic switching field thresholds increase as the dimensions become smaller, that is to say higher currents become necessary for switching.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a memory cell array and a method for manufacturing the memory cell array which overcomes the above-mentioned disadvantageous of the prior art memory cell arrays and methods for producing arrays of this general type. In particular, it is an object of the invention to provide a memory cell array having a memory element with magnetoresistive effect which can be programmed with lower currents and current densities than in the prior art.
With the foregoing and other objects in view there is provided, in accordance with the invention a memory cell array, that includes: a substrate having a main face; a first insulating layer configured on the main face of the substrate, the first insulating layer formed with a trench having a bottom and edges; a first line configured in the trench of the first insulation layer; a second line; a memory element configured at a point of intersection between the first line and the second line, the memory element being switched between the first line and the second line; a first yoke disposed adjacent the bottom and the edges of the trench of the first insulation layer, the first yoke configured such that a magnetic flux through the first yoke is essentially closed in the memory element, the first yoke including a magnetizable material with a relative permeability of at least 10; and a line selected from the group consisting of the first line and the second line being supplied with current during a write access and being partially surrounded by the first yoke.
In other words, at least a first line, a second line and a memory element with magnetoresistive effect which is arranged at a point of intersection between the first line and the second line are provided in the memory cell array. Preferably, the memory element is switched between the first line and the second line. In addition, a yoke is provided which partially surrounds at least one of the lines and which contains magnetizable material with a relative permeability of at least 10. The yoke is arranged in such a way that the magnetic flux path through the yoke is closed essentially by means of the memory element. In order to write to the memory cell, the first line and the second line are supplied with current in such a way that superposition of the magnetic fields of the first line and of the second line at the location of the memory element generates a magnetic field which exceeds the switching threshold of the memory element.
The yoke is magnetized here by the magnetic field of the line through which current flows, the line being partially surrounded by the yoke. As a result, the induction flux density B is increased by a factor &mgr;
r
, the relative permeability. As a result, magnetic poles are produced at the end faces of the yoke and a magnetic field is generated between the poles. This magnetic field assumes very high values depending on the selection of the material of the yoke and is used to switch the memory element. Given identical current density in the line, considerably higher magnetic fields are thus achieved for switching the memory element.
Part of the yoke can be formed from all ferromagnetic and ferromagnetic materials.
The yoke is preferably formed from soft-magnetic, ferromagnetic layers, in particular composed of Fe, Ni, Co, Mn, M
Greenberg Laurence A.
Lebentritt Michael S.
Mayback Gregory L.
Phung Anh
Siemens Aktiengesellschaft
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