Magnetic random access memory capable of writing information...

Details Magnetic random access memory capable of writing information... Magnetic random access memory capable of writing information...

2000-12-28

2003-01-21

Wilson, Allan R. (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Magnetic field

C365S171000

Reexamination Certificate

active

06509621

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to magnetic memory devices and more particularly to a magnetic random access memory capable of writing information with a reduced write-current.
A magnetic random access memory (MRAM) is a magnetic memory device that stores information in the form of magnetization in a ferromagnetic layer constituting a part of a magneto-resistive sensor. The information thus written into the ferromagnetic layer is read out by detecting a magneto-resistance of the magneto-resistive sensor.
A MRAM is characterized by high operational speed comparable to that of a static random access memory (SRAM) and a simple construction suitable of forming a high-density integrated circuit. In addition, a MRAM has an advantageous feature of immunity to soft-errors caused by penetration of charged particles such as cosmic ray.
With the progress in the art of GMR (giant magneto-resistive) sensors, particularly with the progress of a TMR (tunneling magneto-resistive) sensor, application of MRAM is expanding rapidly in various fields of electronics including computers and telecommunication.
FIGS. 1A-1C
are diagrams showing the principle of a MRAM according to a related art (S. S. P. Parkin, et al., J. Appl. Phys. vol.85, pp.5828, 1999).
Referring to
FIG. 1A
showing a single MRAM cell, the MRAM cell includes a word line
56
extending in a first direction and a bit line
57
extending in a second, different direction, and a TMR sensor is interposed between the word line
56
and the bit line
57
in correspondence to an intersection part where the word line
56
and the bit line
57
cross with each other.
More specifically, the TMR sensor includes a ferromagnetic free layer
52
provided on the word line
56
via an intervening foundation layer
51
, and a tunneling barrier layer
53
of AlO
x
and a ferromagnetic pinned layer
54
are formed consecutively on the foregoing ferromagnetic free layer
52
. Further, there is provided an anti-ferromagnetic pinning layer
55
between the ferromagnetic pinned layer
54
and the bit line
57
, wherein the anti-ferromagnetic pinning layer
55
causes a pinning of magnetization in the pinned layer
54
in a predetermined direction represented in an arrow.
When writing information, a word line current is caused to flow through the word line
56
in the direction represented by an arrow in
FIG. 1B
together with a bit line current, which is caused to flow through the bit line
57
in the direction represented by another arrow in FIG.
1
B. Thereby, the bit line current and the word line current induce respective magnetic fields
57
and
58
as represented in
FIG. 1B
, and a synthetic magnetic is field formed as a result of a sum of the magnetic fields
57
and
58
. The synthetic magnetic field thus formed induces a magnetization in the ferromagnetic free layer
25
as represented by an arrow in FIG.
1
B. In the state of
FIG. 1B
, it can be seen that the magnetization in the free layer
52
and the magnetization in the pinned layer
53
are in an anti-parallel relationship. The state of
FIG. 1B
represents a data bit “0.”
Alternatively, the ferromagnetic free layer
52
may be magnetized in a parallel relationship as represented in FIG.
1
C. The state of
FIG. 1C
may be designated as a data bit “1.” The information thus written into the MRAM cell is held stably even when the electric power to the MRAM cell is turned off. In other words, MRAMs function as a non-volatile memory.
In the state of
FIG. 1B
, there is formed a large magneto-resistance between the ferromagnetic free layer
52
and the ferromagnetic pinned layer
54
in correspondence to the anti-parallel relationship of the magnetization in the layers
52
and
54
, while in the state of
FIG. 1C
, there is formed a small magneto-resistance in correspondence to the parallel relationship of the magnetization. Thus, reading of information can be achieved by detecting a magneto-resistance of the TMR sensor formed between the word line
56
and the bit line
57
. Such reading of information is non-destructive reading.
In a TMR sensor, it should be noted that a ratio of tunneling resistance change or TMR ratio &Dgr;R is defined as
&Dgr;
R=
2
P
1
×P
2
/(1
−P
1
×P
2
)   Eq.(1)
wherein P
1
represents a spin polarization of the pinned layer
54
while P
2
represents a spin polarization of the free layer
52
.
Thus, a TMR ratio &Dgr;R of about 50% is achieved when a NiFe alloy, having a spin polarization of about 45%, is used for the free layer
52
and also for the pinned layer
54
:
Δ
⁢
 
⁢
R
max
=
2
×
0.45
×
0.45
⁢
(
1
-
0.45
×
0.45
)
=
0.405
/
0.7975
≈
0.508
.
When writing information into the magnetic random access memory of
FIG. 1A
, it is necessary to apply an external magnetic field (H
x
, H
y
) such that the external magnetic field exceeds a critical value given by a relationship of
(
H
x
/H
x0
)
⅔
+(
H
y
/H
y0
)
⅔
=1   Eq.(2)
wherein H
x0
represents a coercive force in the direction of easy axis of magnetization while H
y0
represents a coercive force in the direction of hard axis of magnetization.
FIG. 2
shows an asteroid curve corresponding to the relationship of Eq.(2) above, wherein there occurs an inversion or reversal of magnetization in the free layer
52
when the external magnetic field (H
x
, H
y
), caused by the magnetic fields
57
and
58
, has exceeded the closed region defined by the asteroid curve.
From
FIG. 2
, it will be understood that the word line current and the bit line current for creating the inverting magnetic field (H
x
, H
y
) should have the same magnitude in order to minimize the magnitude of the word line current and further the bit line current.
On the other hand, such a MRAM has a problem in that the magnitude of the external magnetic field needed for inverting the magnetization of the magnetic free layer
52
increases with increasing degree of device miniaturization. It should be noted that the relative ratio of the thickness of the ferromagnetic free layer
52
to the lateral size thereof, and hence the structural anisotropy of the ferromagnetic free layer
52
in which the information is stored, increases with increasing degree of device miniaturization.
In the case a ferromagnetic free layer
52
is formed of a NiFe alloy, the external magnetic field needed for causing an inversion of magnetization in the free layer
52
reaches as much as 50-100 Oe when the MRAM is formed according to the 0.1 &mgr;m design rule, in which the bit lines
57
are formed with a 0.1 &mgr;m line-and-space pitch. In order to create the foregoing magnetic field by the electric current flowing through the bit line
57
and the word line
52
, a current density of as much as 3-5×10
7
A/cm
2
is needed, provided that the word line
52
and the bit line
57
are offset from a thickness center of the MRAM cell by a distance of 0.1 &mgr;m. With such a large electric current density, even a Cu conductor pattern, which is thought immune to electro-migration up to the current density of 10
6
A/cm
2
, would cause electro-migration. This means that miniaturization beyond the 0.1 &mgr;m design rule is not possible in the MRAM of FIG.
1
A.
Of course, the word line current and the bit line current needed in a MRAM for writing information can be reduced by adding impurity elements to the ferromagnetic free layer
52
such that the coercive force thereof is reduced or the saturation magnetization is reduced. However, such an approach is thought undesirable as it would degrade the performance of the magnetic material and hence the TMR ratio &Dgr;R of the TMR sensor and the S/N ratio of the MRAM.
Generally, the TMR ratio &Dgr;R of a TMR sensor can be increased when the value of the spin polarization P
1
or P
2
is increased. For example, the TMR ratio &Dgr;R becomes theoretically infinite when magnetic materials having P
1
=1 and P
2
=1 are used for the ferromagnetic free layer
52
and the

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