Magnetic memory

Details Magnetic memory Magnetic memory

2002-04-11

2003-08-26

Lam, David (Department: 2818)

Static information storage and retrieval

Systems using particular element

Magnetic thin film

C365S173000, C365S158000

Reexamination Certificate

active

06611455

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic memories, such as magnetic random access memories (MRAMs), using magnetoresistive elements (hereinafter also referred to as “magnetic memory elements”) in which data is written by a magnetization direction and data is read by a magnetoresistance effect.
2. Description of the Related Art
Although a MRAM is a solid-state memory with no active parts as in the case of a semiconductor memory, in the MRAM, data is not lost even if a power supply is cut off, rewriting can be performed for an unlimited number of times, and there is no danger that memory contents may disappear due to exposure to radiation, all of which are advantageous in comparison with the semiconductor memory.
As the memory element used for the MRAM, a magnetoresistive element is preferably used, in which an external magnetic field is applied to the magnetic layers while a predetermined current is applied between the magnetic layers, the resistance changes in response to the relative angle between the magnetization directions of both magnetic layers. When the magnetization directions of the magnetic layers are parallel to each other (i.e., the magnetization directions of the magnetic layers are the same), the minimum resistance occurs, and when the magnetization directions are antiparallel to each other (i.e., the magnetization directions of the magnetic layers are opposite to each other), the maximum resistance occurs.
Accordingly, by using the magnetic layers having different coercive forces, the parallel state and the antiparallel state can be brought about in response to the strength of the magnetic field, and thus the magnetization state can be sensed by the change in resistance.
Recently, tunneling magnetoresistive (TMR) elements have been developed, in which a surface-oxidized Al film is used as the tunneling barrier layer sandwiched between two magnetic layers, resulting in a magnetic memory element exhibiting a rate of change in magnetoresistance of approximately 20%. Therefore, it is highly possible to apply the tunneling magnetoresistive element in magnetic heads and magnetic memory elements. Magnetoresistive elements exhibiting such a large rate of change in magnetoresistance have been reported, for example, in the Journal of Applied Physics, vol. 79, 4724 4729, 1996. With respect to memory elements used for magnetic memories, U.S. Pat. No. 6,219,275 discloses a magnetic memory using a magnetoresistive element in which perpendicular magnetization films are used as magnetic films.
In order to write data in a MRAM, currents are passed through lines placed in the vicinity of the individual memory cells to generate magnetic fields, and since the magnetization directions of magnetic layers (memory layers) are determined by the magnetic fields, data is written. Therefore, in order to perform writing, currents which can generate magnetic fields sufficient for reversing the magnetization directions of the memory layers must be passed through the lines. For that purpose, considerably large currents of approximately several to 10 mA are required.
In order to reduce a current to be passed through the lines during writing, for example, U.S. Pat. No. 5,894,447 discloses a configuration in which in-plane magnetization films are used as the magnetic layers, upper and lower write word lines are placed so as to sandwich the magnetic layers, and the ends of the upper and lower write word lines are connected to each other so that the current flows from the upper write word line to the lower write word line in a turn-back manner.
FIGS. 12A
to
12
C are schematic diagrams showing a configuration of a conventional magnetic memory.
FIG. 12A
is a sectional view of a memory cell,
FIG. 12B
is a plan view showing a plurality of memory cells adjacent to each other, and
FIG. 12C
is a sectional view of a cell array along word lines.
FIG. 12C
also shows a drive circuit for driving the word lines.
As shown in
FIGS. 12A and 12B
, in each memory cell
36
, bit lines
31
are placed orthogonally to an upper word line
32
and a lower word line
33
formed above and below the bit lines
31
, respectively. A giant magneto-resistance (GMR) film
34
is formed at the intersection, both ends of the GMR film
34
being connected to the bit lines
31
. That is, the upper word line
32
is formed directly above the GMR film
34
, and the lower word line
33
is formed beneath the GMR film
34
. The upper word line
32
and the lower word line
33
overlap in the vertical direction with the GMR film
34
and interlayers therebetween. The interlayers electrically insulate the upper word line
32
from the lower word line
33
and electrically insulate the upper and lower word lines
32
and
33
from the GMR film
34
and the bit lines
31
.
When a current is passed through the upper word line
32
of the memory cell
36
, for example, toward the front as shown in
FIG. 12A
by a circular mark having a dot therein, and a current is passed through the lower word line
33
toward the back as shown in
FIG. 12A
by a circular mark having a cross therein, both magnetic fields generated by the currents flowing through the upper word line
32
and the lower word line
33
are directed rightward in the drawing in accordance with the Ampere's corkscrew rule. As a result, a combined magnetic field is produced by combining the magnetic fields generated by the currents flowing through the upper word line
32
and the lower word line
33
, and the combined magnetic field is applied to the GMR film
34
. The combined magnetic field applied to the GMR film
34
has a magnetic intensity approximately twice the intensity of the magnetic field generated by one word line on the assumption that the magnitude of the current supplied is the same.
For example, as shown in
FIG. 12C
, in a memory cell array including memory cells
36
a
to
36
l,
the upper word line
32
and the lower word line
33
are extended from the memory cell
36
a
on the extreme left to the memory cell
36
l
on the extreme right, and the upper word line
32
and the lower word line
33
are connected in series by a contact
37
at the left edge of the cell array. Furthermore, the right ends of the upper and lower word lines
32
and
33
are connected to a drive circuit
35
, and a current is supplied in the directions indicated by the arrows in the drawing by applying voltages V
1
and V
2
thereto.
As a result, at the same current consumption as in the case of a traditional magnetic memory, the combined magnetic field having a magnetic intensity approximately twice the magnetic intensity in the traditional magnetic memory can be produced by the current flowing through both word lines.
However, in the configuration which uses the in-plane magnetization films as the magnetic layers and in which word lines are provided so as to sandwich the memory cells as described above, the fabrication process is difficult, for example, because the upper lines and the lower lines must be connected to each other at the ends by through holes or the like. Additionally, parasitic capacitance occurs between the upper word lines and the lower word lines, which may give rise to a problem, in particular, when high-speed driving is performed. Moreover, because of the multilayered structure, the aspect ratio (x/y shown in the drawing) of the lines tends to increase, and therefore, it is difficult to decrease the opposing areas of the upper and lower word lines which are directly related to parasitic capacitance.
Additionally, when the in-plane magnetization films are used for memory cells, it is difficult to miniaturize the memory elements under the influence of curling of magnetization, etc. In order to solve this problem, the configuration in which perpendicular magnetization films are used as the magnetic layers as disclosed in U.S. Pat. No. 6,219,275 may be employed. However, since the intensity of magnetic fields for reversing the magnetization is increased as memory elements are further miniaturized in

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