Magnetic random access memory

Details Magnetic random access memory Magnetic random access memory

2001-06-19

2002-10-08

Elms, Richard (Department: 2824)

Static information storage and retrieval

Systems using particular element

Magnetoresistive

C365S171000, C365S173000

Reexamination Certificate

active

06462981

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to nonvolatile random access memories and more specifically to a magnetic random access memory using a magnetoresistance element.
2. Description of the Related Art
U.S. Pat. No. 5,748,519 discloses a magnetic RAM in which magnetic memory cells are each comprised of a giant magnetoresistance element and organized into first and second array portions. In each row of the first array portion, the GMRs are connected in series between a row transistor and a first common output line. Likewise, in each row of the second array portions, the GMRs are connected in series between a row transistor and a second common output line. During a read mode, a current is produced in the GMRs of a selected row of the first cell array portion as well as in the GMRs of the corresponding row of the second cell array portion. One of the cell array portions is used to produce a reference voltage. The voltage developed on each common output line is proportional to the total value of the resistances of the series-connected memory cells. A differential amplifier is connected to the first and second common lines to produce an output voltage representing the difference between the voltages developed on the first and second output lines. However, parasitic elements arise from the physical separation of the cell array into the two array portions. Therefore, if variability exists in the operating characteristics of the magnetoresistance elements of the memory it is difficult to implement a magnetic RAM having a sufficient amount of operating margin. In addition, since the total resistance of a selected row contributes to a significant portion of the voltage developed at the common output line, the amount of the voltage contributed by a memory cell of the selected row is small. As a result, the prior art magnetic RAM is less tolerant of cell variability and noise. The problem could be overcome only at the cost of an increase in the resistance of each GMR element, which would require an increase both in power consumption and memory chip size. Furthermore, the total value of the resistances of each selected row inherently includes the turn-on resistance of each row transistor. Since the GMR is a type of device whose resistance is of the same order of magnitude as the resistance of the associated line, the voltage drop contributed by the row transistor and the associated line resistance to the output voltage cannot be ignored, requiring precision sense amplifiers.
U.S. Pat. No. 5,640,343 discloses a magnetic memory array. Each memory cell consists of a magnetic tunnel junction (TMR) element and a diode electrically connected in series. However, a paper titled “Bias Voltage Dependence of Tunneling Magnetoresistance and Annealing Effect in Spin Dependent Tunnel Junctions”, J. J. Sun et al, Journal of Magnetics Society of Japan, Vol. 23, No. 1-2, pages 55-57, describes that as the voltage across the tunnel junction increases the magnetoresistance (MR) ratio decreases due to the known bias effect, and hence precision sense amplifiers would be required to detect voltage variations. Further, the tunneling magnetoresistance element cannot tolerate high voltages as described in a paper under the title of “Observation and analysis of breakdown of magnetic tunnel junctions”, W. Oepts, et al, Journal of Magnetism and Magnetic Materials Vol. 198-199, pages 164-166. The application of a high voltage across a tunneling magnetoresistance element would break down the tunnel barrier and shorten its lifetime.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a magnetic random access memory tolerant of variability of the operating characteristics of magnetoresistance elements and capable of operating with a sufficient margin of allowance.
A second object of the present invention is to provide a magnetic random access memory capable of precision readout operation by compensating for the voltage drop caused by the line resistance and transistor's turn-on resistance connected in series.
A third object of the present invention is to provide a magnetic random access memory capable of high speed readout operation by simultaneously reading stored information from a plurality of memory cells connected to the same word line.
A fourth object of the present invention is to provide a magnetic random access memory using tunneling magnetoresistance elements in the memory cells capable of preventing the breakdown of their tunnel barrier and their bias effect of the tunneling magnetoresistance elements.
A fifth object of the present invention is to provide a magnetic random access memory of small chip size by utilizing parasitic capacitances of the sense lines.
A sixth object of the present invention is to provide a low power consumption magnetic random access memory by utilizing energy stored on capacitors.
According to a first aspect, the present invention provides a magnetic random access memory which comprises a plurality of word lines, a plurality of sense lines, a matrix array of memory cells, each memory cell being located on an intersection of a corresponding one of the word lines and a corresponding one of the sense lines, each memory cell including a magnetoresistance element and a switching element which establishes a resistive connection between the corresponding sense line and the magnetoresistance element when the corresponding word line is addressed, and a plurality of sense circuits respectively corresponding to the sense lines, each sense circuit including a capacitive element connected to the corresponding sense line and a switching element for applying a voltage to the capacitive element and removing the voltage when the corresponding sense line is addressed, thereby discharging energy from the capacitive element through the resistive connection to the magnetoresistance element.
According to a second aspect, the present invention provides a magnetic random access memory comprising a plurality of word lines, a plurality of pairs of sense lines, a matrix array of memory cells, each memory cell being located on an intersection of a corresponding one of the word lines and a corresponding one of the pairs of the sense lines, each memory cell including a first magnetoresistance element and a first switching element which establishes a first resistive connection between a first one of the corresponding pair of sense lines and the first magnetoresistance element when the corresponding word line is addressed, each memory cell further including a second magnetoresistance element and a second switching element which establishes a second resistive connection between a second one of the corresponding pair of sense lines and the second magnetoresistance element when the corresponding word lines is addressed, and a plurality of sense circuits respectively corresponding to the pairs of sense lines, each sense circuit including a first capacitive element connected to a first one of the corresponding pair of sense lines and a first switching element for applying a voltage to the first capacitive element and removing the voltage when the first one of the corresponding pair of sense lines is addressed, thereby discharging energy from the first capacitive element through the first resistive connection to the first magnetoresistance element, each sense circuit further including a second capacitive element connected to a second one of the corresponding pairs of sense lines and a second switching element for applying a voltage to the second capacitive element and removing the voltage when the second one of the corresponding pair of sense lines is addressed simultaneously with the first one of the corresponding pair of sense lines, thereby discharging energy from the second capacitive element through the second resistive connection to the second magnetoresistance element. A plurality of differential amplifiers may be provided respectively corresponding to the sense circuits, each of the differential amplifiers producing a

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