Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2000-05-10
2001-08-21
Zarabian, A. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S173000
Reexamination Certificate
active
06278631
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic random access memory, and more particularly, to a magnetic random access memory with a plurality of memory banks, each memory bank having bit lines allocated on both sides of a reference line.
BACKGROUND OF THE INVENTION
A magnetoresistive random access memory (MRAM), which is one of non-volatile memory devices, includes a plurality of magnetic memory cells. It is known that the magnetoresistive effect appears in multi-layer films that are alternately stacked by magnetic layers and non-magnetic layers. Magnetic resistance over a magnetic memory cell indicates minimum and maximum values when magnetization vectors in magnetic layers point in the same or opposite directions, respectively. The same and opposite directions of magnetization vectors in two magnetic layers are called “Parallel” and “Anti-parallel” states, respectively. When magnetic material is employed for a memory device, Parallel and Anti-parallel directions, for example, are logically defined as “0” and “1” states, respectively. The magnetization vectors in magnetic layers are very quickly switched to another direction by an application of a magnetic field over a switching point, and maintain the magnetization direction even without a magnetic field.
The MRAM device normally has magnetic memory cells arranged on intersections of metal lines, which are placed in rows and columns. The MRAM circuit, for instance, is described in a co-pending U.S. patent application entitled “MAGNETORESISTIVE RANDOM ACCESS MEMORY WITH SHARED WORD AND DIGIT LINES,” Motorola docket No. CR98-038, filed July , 1998 assigned to the same assignee, and incorporated herein by reference.
The MRAM device generally is formed on a substrate such as a semiconductor or glass. Magnetic and non-magnetic layers are sequentially deposited on the substrate and etched down to form magnetic memory cells, which are 0.2 &mgr;m by 0.3 &mgr;m in size, for instance. The magnetic layer usually has a 10-100Å thickness while the non-magnetic layer is 10-30Å thick. The thickness of the magnetic and non-magnetic layers are deposited on the substrate as invariably and evenly as possible, which allows each magnetic memory cell on the substrate to provide steady and constant hysteresis characteristics. It, in reality, is very hard to control and keep a thickness of each layer constant to every magnetic memory cell on the entire substrate.
The MRAM device includes magnetic memory cells for storing specific information, and reference magnetic memory cells where a reference state is maintained. In order to read states stored in a magnetic memory cell (active or target cell), a sense current and a reference current are applied to the target cell and a reference magnetic memory cell (reference cell), respectively. The target and reference cells generate voltage drops across the cells that correspond to magnetic resistance values of the target and reference cells, respectively. These voltages are compared to each other to determine states in the target cell. Therefore, it is desirable that each magnetic cell has as same or similar hysteresis characteristics as possible, in order to execute a steady reading process and to get states correctly.
Accordingly, it is a purpose of the present invention to provide an improved MRAM device that has a high-speed, high-density, and low power memory.
It is another purpose of the present invention to provide an improved MRAM device that has a high reliability reading process.
It is still another purpose of the present invention to provide an improved MRAM device that has a high tolerance for temperature variation.
SUMMARY OF THE INVENTION
These needs and others are substantially met through provision of a magnetoresistive random access memory (MRAM) device that includes memory banks, each memory bank having a reference line, bit lines and digit lines. Bit lines are allocated parallel to the reference line and on both sides of the reference line. Word lines and digit lines, which are perpendicular to bit lines, form intersections on which magnetic memory cells are arrayed for storing information. Reference magnetic memory cells are arranged on intersections of the reference line and digit lines. A memory cell has magnetic layers separated by a non-magnetic layer. Magnetic and non-magnetic layers, for example, are deposited on a substrate with thicknesses of 10-100Å and 10-30Å, respectively. The difficulty for uniformly forming the thickness and the size of the memory element over the entire substrate allows memory cells to indicate different hysteresis characteristics that result in a reading error. In order to prevent mistakenly reading states in a magnetic memory cell, the present invention divides a memory array into a plurality of memory banks. Each memory bank includes a reference line, and bit lines that are formed on both sides of the reference line. Since bit lines are formed adjacent to the reference line on the substrate, magnetic memory cells have the same or similar hysteresis characteristics as reference memory cells.
REFERENCES:
patent: 5173873 (1992-12-01), Wu et al.
patent: 5831920 (1998-11-01), Chen et al.
patent: 5894447 (1999-04-01), Takashima
patent: 5946227 (1999-08-01), Naji
patent: 6055178 (2000-04-01), Naji
patent: 0613148 (1994-08-01), None
Koch William E.
Motorola Inc.
Zarabian A.
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