Non-volatile memory device capable of generating accurate...

Details Non-volatile memory device capable of generating accurate... Non-volatile memory device capable of generating accurate...

2003-02-12

2004-08-24

Tran, Andrew Q. (Department: 2824)

Static information storage and retrieval

Systems using particular element

Magnetoresistive

C365S210130, C365S209000, C365S207000, C365S206000, C365S173000, C365S171000

Reexamination Certificate

active

06781873

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a non-volatile memory device, and more particularly, to an MRAM (Magnetic Random Access Memory).
2. Description of the Background Art
The MRAM is a memory using a magnetic body as a storage element. A phenomenon in which the resistance of a material changes depending on the direction of magnetization of the magnetic body is called an MR (Magneto Resistive) effect. The MR effect is further classified by operating principles. One of such called a TMR (Tunneling Magneto Resistive) phenomenon has been identified as having a high MR ratio (a resistance ratio by magnetization of the magnetic body), and a TMR element has been studied as an element for the MRAM.
The TMR phenomenon is such a phenomenon that the magnitude of tunneling current flowing through an insulation film interposed between magnetic bodies changes in accordance with the direction of electron spin determined by magnetization of the magnetic body.
FIG. 13
schematically shows a conventional thin film having a magnetic tunnel junction (MTJ) at which the TMR phenomenon occurs. The thin film is also referred to as an MTJ element or a tunneling magneto resistive (TMR) element.
Referring to
FIG. 13
, ferromagnetic films
701
,
703
are arranged with an insulation film
702
interposed therebetween. Spin electrons within the magnetic bodies change their state in accordance with the direction of magnetization. Here, if magnetic film
701
has the same direction of magnetization as that of magnetic film
703
, the tunneling current increases. By contrast, if the direction of magnetization of magnetic film
701
is opposite to that of magnetic film
703
, the tunneling current decreases.
Using such a phenomenon, the direction of magnetization of magnetic film
701
is changed and the magnitude (resistance) of the tunneling current is detected, to allow the TMR element to be used as a storage element. Magnetic film
703
may have the direction of magnetization fixed by an antiferromagnetic body, which is called a spin valve.
In order to realize a high-density non-volatile memory device, memory cells are preferably arranged in a two-dimensional array. A ferromagnetic body has an easily-magnetized direction (low energy state) due to a crystal structure, a shape or the like, the direction being referred to as an easy axis. Magnetization of a storage element is held in a direction along the easy axis. By contrast, a hardly-magnetized direction is called a hard axis.
FIG. 14
shows an asteroid curve for illustrating reversal of magnetization.
Referring to
FIG. 14
, in order to reverse the direction of magnetization, a magnetic field is applied in a direction opposite to the present magnetization with respect to the easy axis to change the direction of magnetization of a storage element. It is known here that, if a magnetic field is applied in the direction of the hard axis, the direction of magnetization is reversed with a smaller magnetic filed in the direction of the easy axis compared to the case where no magnetic field applied in the direction of the hard axis. The asteroid curve shown in
FIG. 14
illustrates the relationship between the magnitude of the magnetic field and the threshold for the reversal of magnetization in the directions of the easy axis and the hard axis.
FIG. 15
shows a two-dimensional arrangement of memory cells in the MRAM.
Referring to
FIG. 15
, a plurality of interconnection lines
801
are arranged orthogonal to a plurality of interconnection lines
802
arranged parallel to the X-axis. At each intersection of interconnection lines
801
and
802
, a magnetic body
803
is interposed between interconnection lines
801
and
802
.
If current is provided for a specific line selected from the lines in the X- and Y-directions, magnetization of magnetic body
803
is reversed only in memory cells located at intersections to which a magnetic filed is applied both in the hard and easy axis directions, allowing data rewriting. No magnetic filed exceeding the threshold value is applied to magnetic bodies in the other numerous memory cells, so that no rewriting occurs. Accordingly, writing to the two-dimensional memory array can be realized.
Data reading can be detected by, for example, a method of comparing tunneling current flowing in magnetic body
803
with a reference cell.
FIG. 16
schematically shows a conventional MRAM memory cell (for example, disclosed in the document by Roy, et al., which will be described later).
Referring to
FIG. 16
, a TMR element
852
in a spin valve form is used as a storage element of a memory cell. A transistor
855
is used as a switch element for reading.
As interconnection lines for transmitting signals, a bit line
851
used at the time of reading and writing, a word line
854
rendering transistor
855
conductive at reading, a digit line
853
providing current at writing, and a source line connected to the source of a transistor (not shown) are provided.
At writing, current is applied to digit line
853
and bit line
851
to generate a synthetic magnetic field at a cell position of interest so as to control the direction of magnetization of a magnetic body in the TMR element.
At reading, voltage is applied to word line
854
, rendering transistor
855
conductive. Current is provided from bit line
851
to the source line via TMR element
852
and transistor
855
. Here, the magnitude of the flowing current varies depending on the direction of spinning of the TMR element. This current is compared with the current flowing in the reference cell to determine that the data stored in the TMR element is either a logic high of “H” or a logic low of “L.” Current flowing at the time of reading is considerably lower than the current flowing at the time of writing. Thus, data reading would not apply a magnetic field exceeding the threshold to the TMR element, so that the direction of magnetization of the magnetic body remains unchanged. Hence, the MRAM is capable of nondestructive read out.
It is noted that technical information for the MRAM are disclosed in the documents as follows:
Roy Scheuerlein, et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell,” ISSCC Digest of Technical Papers, February 2000, TA7.2;
M. Durlam, et al., “Nonvolatile RAM based on Magnetic Tunnel Junction Elements,” ISSCC Digest of Technical Papers, February 2000, TA7.3, pp. 130-131;
U.S. Pat. No. 6,269,040 (FIG. 4A); and
U.S. Pat. No. 6,317,376.
In the MRAM, determination of data is performed by comparing a reference signal with a signal from a memory cell to be read out. U.S. Pat. No. 6,269,040 and U.S. Pat. No. 6,317,376 disclose that a plurality of reference memory cells are used to generate a reference signal. The TMR element, however, has such a characteristic that the resistance value changes in accordance with a voltage applied to the both ends thereof.
FIGS. 17 and 18
illustrate a problem occurring when a plurality of TMR elements are used as reference cells to produce a reference signal.
Referring to
FIG. 17
, in order to discriminate a memory cell storing “H” from a memory cell storing “L,” an intermediate value between a current value IH and a current value IL is required as reference current.
Reference cell
871
stores data “H,” whereas reference cell
872
stores data “L.” A voltage V is applied to the both ends of reference cells
871
,
872
connected in parallel, to provide current of IH+IL flowing as reference current Iref. Current with half the magnitude of the reference current is required as the reference value for data discrimination.
Even if the voltage applied to the both ends of the parallely connected reference memory cells
871
,
872
is changed from voltage V to ½·V, reference current Irefa obtained here will not be half the value of IH+IL, as shown in FIG.
18
. This is because the voltage applied to both ends of TMR element reduced to ½ would change the resistance value of the TMR element. Henc

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