Static information storage and retrieval – Systems using particular element – Ferroelectric
Reexamination Certificate
2002-06-20
2004-01-27
Le, Thong (Department: 2818)
Static information storage and retrieval
Systems using particular element
Ferroelectric
C365S097000, C365S171000
Reexamination Certificate
active
06683802
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic memory device and a method of designing the same. More particularly, the present invention relates to a magnetic memory device which carries out a reading/writing operation of data using magnetic reversal in a magnetic substance element, and a method of designing the same.
2. Description of the Related Art
In recent years, the development of a magnetic random access memory (MRAM) using a magnetic resistance element as a memory element has carried forward. In the magnetic resistance element, a reading/writing operation is carried out using magnetic reversal which is brought about by a local magnetic field. The MRAM is expected to be a memory device of the next generation, because with MRAM, it is possible to operate at high speed.
The structure in which a tunnel insulating film is put between two magnetic substance films is called tunneling magnetroregistance (TMR). The TMR structure will be described below as an example of the magnetic resistance element.
FIG. 1
shows an example of the TMR reported in 2000 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS (p. 128). Referring to
FIG. 1
, an anti-ferromagnetic substance layer
30
formed of FeMn (10 nm), a ferromagnetic substance pin layer
31
formed of CoFe (2.4 nm), the tunnel insulating layer
32
formed of Al
2
O
3
, and a ferromagnetic substance free layer
33
formed of NiFe (5 nm) are stacked.
Conductive wiring lines are connected with the anti-ferromagnetic substance layer
30
and the free layer
33
to make it possible to apply a voltage. A magnetization direction of the pin layer
31
is fixed on a specific direction by the anti-ferromagnetic substance layer
30
. The free layer
33
is formed to be easily magnetized and the magnetization direction of the free layer
33
can be changed by an externally applied magnetic field. It should be noted that a direction to which it is easy for the free layer to be magnetized is referred to as an easy axis and a direction to which it is difficult for the free layer to be magnetized and which is orthogonal to the easy direction is referred to as a hard axis.
In the TMR of the above structure, current flows through the tunnel insulating film
32
when a voltage is applied between the free layer
33
and the pin layer
31
. In this case, a resistance value changes based on the relation of the magnetization of the free layer
33
and the magnetization of the pin layer
31
. That is, when the magnetization directions are the same, the resistance is low. When the magnetization directions are different from each other, the resistance becomes high. Therefore, data can be stored by changing the magnetization direction of the free layer
33
.
Next, an example using the above TMR as a memory element of a nonvolatile memory will be described with reference to FIG.
2
. This example is reported in 2000 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS (p. 130). Referring to
FIG. 2
, in this example, the TMRs
34
are arranged at the intersections of upper wiring lines B (B
1
, B
2
)
35
and lower wiring lines D (D
1
, D
2
, D
3
)
38
in an array manner. The upper wiring line
35
is connected with a free layer of the TMR
34
. An anti-ferromagnetic substance layer of the TMR
34
is connected with the drain of a transistor
37
through a third wiring line
36
. The gate of transistor
37
is controlled by read word line W (W
1
, W
2
, W
3
)
39
. By flowing current through the two wiring lines B and D, a synthetic magnetic field is generated in the neighborhood of the intersection. The magnetization direction of the free layer is set based on the direction of the current. Thus, the resistance value of the TMR
34
can be changed.
FIG. 3
shows an example of the synthetic magnetic field condition when the magnetic reversal in this example is caused. Referring to
FIG. 3
, the following is described. That is, the magnetic reversal does not occur by only one magnetic field. There are condition areas (Write “0”, Write “1”) for the magnetic reversal using the synthetic magnetic field. Even if a writing operation is carried out to the TMR arranged at the intersection of the two wiring lines using this condition, the magnetic reversal does not occur because the magnetic field of one axis is applied only to the TMRs other than the TMR at the intersection. It should be noted that when a reading operation of the data from the TMR
34
is carried out, the transistor
37
connected with the TMR
34
is set to the ON state using a wiring line W to apply voltage to the TMR
34
through the wiring line B. The resistance value of the TMR is evaluated based on the flowing current.
However, in the memory device in which the conventional magnetic resistance elements are arranged in an array manner, the magnetization states of adjacent magnetic substance elements change by the magnetic field of the wiring line when the writing operation is carried out to the magnetic substance element. As a result, there is a possibility that the data cannot be read out, and the data is changed. This phenomenon will be described with reference to the figures.
Supposing that a weak magnetic field of a degree that the magnetization does not reverse is repeatedly applied to the magnetized magnetic substance in an opposite direction to the magnetization direction, the magnetization state is destroyed. Such a phenomenon is mentioned in “Magnetic Engineering Lecture 5 Magnetic Thin Film Engineering” (Maruzen Co., Ltd., P.174).
FIG. 4
shows a magnetic field condition when the magnetization state changes by applying a magnetic field to the easy axis and the hard axis of the magnetized magnetic substance. Also,
FIG. 4
shows the dependence on the number of times when the magnetic field weaker than the magnetic field necessary for the magnetic reversal is repeatedly applied to the hard axis.
In this reference, a case is described that a sine wave magnetic field with the frequency of 1 kHz and the amplitude of 1.0 Oe is applied 10
5
cycles to the magnetic substance in the hard axis. In this case, the applied magnetic field is about 30 percent of the magnetic coercive force of 3.2 Oe of the magnetic substance, and the magnetization of the magnetic substance has reversed almost fully when the magnetic field 2.0 Oe is applied to the hard axis of the magnetic substance after the above repetitive application of the weak magnetic field. Also, in case that the magnetic fields are applied to both of the easy axis and the hard axis, the application of the magnetic field of 1.5 Oe to the hard axis causes the magnetization reversal when the magnetic field of 0.8 Oe is applied to the easy axis. Specifically, when the magnetic field of 2.0 Oe is applied to the easy axis using a bit line and the magnetic field of 2.0 Oe is applied to the hard axis using a word line, a synthetic magnetic field at the intersection of the bit line and the word line exceeds a critical magnetic field to cause the magnetic reversal. Thus, the magnetic reversal is not caused in an adjacent magnetic substance apart from the intersection and a writing operation can be carried out correctly. In this case, however, when the magnetic field of 1.0 Oe, which is a half of the writing magnetic field, is applied to the adjacent magnetic substance 10
5
times, the magnetic field of 2.0 Oe by the word line changes the magnetization state of the adjacent magnetic substance as mentioned above. In this way, when the number of cycles of the sine wave magnetic field exceeds a predetermined value, the magnetization state is changed with the magnetic field smaller than a static critical magnetic field. Consequently, the reliability of the magnetic memory device reduces remarkably.
In the magnetic memory device at present, the above phenomenon is not a problem because the interval between the word lines or the bit lines is wide so that the magnetic field which reaches an adjacent cell is small. However, when the magnetic memory dev
Le Thong
NEC Corporation
Sughrue & Mion, PLLC
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