Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2003-05-12
2004-06-01
Hoang, Huan (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S225500, C365S171000
Reexamination Certificate
active
06744662
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a magnetoresistive memory (MRAM) having a multiplicity of magnetic memory cells, which are disposed at the crossover points of a cell array constructed from a matrix of column and row leads and are connected to the leads, which are provided for conducting read and write currents. In which case, in the event of a write operation, the magnetic fields generated by the write currents in the respective leads are added at a random crossover point and thereby enable a magnetization reverse of the memory cell there.
Such a nonvolatile magnetic random access memory is disclosed for example in Published, Non-Prosecuted German Patent Application DE 198 07 361 A1, corresponding to U.S. Pat. No. 5,902,690.
At the present time, a multiplicity of digital memories are being developed for read and write operations, which memories, as memory chips having a high packing density on a magnetic basis (MRAM), could, at least in some instances, replace the conventional silicon chips (DRAM) as early as in a few years. The MRAM concept provides for a respective bit of information, i.e. the logic zero (“0”) or one (“1”) state, to be stored in a memory cell which, in principle, contains two magnetized layers, which may be magnetized in a parallel or antiparallel manner with respect to one another. The cell array of the multiplicity of memory cells is constructed from a matrix of column and row leads. The leads contain conductive materials and the actual memory cell is situated at the crossover points of the leads. In order to achieve a change in the magnetization of an individual memory cell, it is necessary to generate a magnetic field, whose strength exceeds a specific threshold value, selectively, that is to say as far as possible only in direct proximity to a freely addressable crossover point. The required magnetic field is achieved in accordance with the customary selection mode only by vector addition of the two magnetic fields that are associated with a specific crossover point and are generated by the column and row leads.
While the outlined principles of the write operations in MRAMs have already been known for a long time, more recently the main emphasis of development has been on progressive read operations. Present-day individual memory cells usually contain at least two magnetic layers separated by an intermediate layer. The individual memory cells are furthermore connected by a matrix of leads. The leads not only serve, as described above, for generating magnetic fields for write operations, rather they also conduct the read currents for reading out the binary information present in the individual memory cells. The magnetic storage state of a memory cell is no longer determined by external magnetic sensors, but rather by measurement of a property, namely of the resistance, at the and through the memory cell itself.
Nowadays consideration is already being given to a plurality of different magnetoresistance effects that are based in each case on different physical principles of action. In practice, what is involved is, in the context of changing the magnetization orientation from parallel to antiparallel and vice versa, the realization of large changes in resistance in the region of a few percent, for example by the giant magnetoresistance effect (GMR) or the tunneling magnetoresistance effect (TMR, other abbreviations are also common).
The general advantage of MRAMs over conventional semiconductor memories consists in the persistent storage of the information. Consequently, after the device in which the memory cells are used has been switched off and switched on again, the stored information is immediately available. Moreover, the energy-expending refresh cycles in silicon chips could also be obviated. In notebooks, for example, the “refresh” necessitates the use of large and heavy rechargeable batteries.
What is problematic in the case of such MRAMs is that magnetic leakage fields from outside the memory or from adjacent cells can cause errors in the memory content if they have a sufficient magnitude. Since magnetic fields can be localized only with difficulty, there is the risk, particularly in the case of high packing densities and, consequently, leads or memory cells lying close together, that the magnetic state and thus the memory content of adjacent cells will be altered.
Therefore, Published, Non-Prosecuted German Patent Application DE 198 07 361 A1 cited in the introduction proposed a shielding layer made of a material having a high permeability. The shielding layer shields external magnetic leakage fields from the coated memory cell and, moreover, concentrates the magnetic fields generated by the write currents at the respective memory cell onto the memory cell, so that a lower current intensity is required to generate a total magnetic field sufficient for writing.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a magnetoresistive memory that overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a magnetoresistive memory. The magnetoresistive memory contains a cell array having a matrix formed of leads including column leads and row leads and a multiplicity of magnetic memory cells disposed at crossover points of the column leads and the row leads. The magnetic memory cells are connected to the column leads and the row leads for conducting read currents and write currents. In an event of a write operation, magnetic fields generated by the write currents in the leads are added at a random crossover point and thereby enable a magnetization reverse of the magnetic memory cell. The leads have a form optimized by deviating from a square cross section thereof in such a way that a magnetic field component lying in a cell array plane decreases sufficiently rapidly with increasing distance from a respective crossover point.
It is an aim of the present invention to configure a magnetoresistive memory (MRAM) of the type mentioned in the introduction in such a way as to produce, without complicated interventions in the fabrication sequence, a controlled, in particular greater, localization of the magnetic field at the respective memory cell, which can thus be addressed with higher selectivity. Moreover, the intention is to be able to generate the magnetic fields as far as possible with a relatively low current intensity.
In the case of a magnetoresistive memory (MRAM) of the type mentioned in the introduction, the aim is achieved according to the invention by virtue of the fact that the form of the leads is optimized by deviation from a square cross section thereof in such a way that the magnetic field component lying in the cell array plane decreases sufficiently rapidly with increasing distance from the crossover point.
Since the use of leads with an approximately square cross section for the construction of an MRAM has been taken as a basis heretofore, the invention is now based on the idea of generating magnetic fields that are localized to a greater extent than heretofore from the outset by optimizing the form of the leads, also resulting at the same time in possibilities for optimizing the required current intensity relative to the demanded threshold value strength and selectivity of the magnetic field.
According to the invention, the new memory structures can be integrated onto the customary wafers e.g. by thin-film technology and optical lithography, a CMOS circuit known per se, for example, on the MRAM chip controlling the read and write operations. Accordingly, the leads of the memory cells are realized by special interconnects made of Cu which are disposed for example between the penultimate and last wiring planes. In terms of the compatibility with the rest of the interconnect system and with regard to properties such as simple producibility or high current-carrying capacity, (approximately) square interconnect cross sections are actually ap
Freitag Martin
Gogl Dietmar
Hoenigschmid Heinz
Lammers Stefan
Hoang Huan
Mayback Gregory L.
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