Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2002-09-12
2004-11-23
Le, Thong Q. (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S161000
Reexamination Certificate
active
06822897
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film magnetic memory device, and particularly to a thin film magnetic memory device provided with memory cells having MTJs (Magnetic Tunnel Junctions).
2. Description of the Background Art
In recent years, attention has been given to a MRAM (Magnetic Random Access Memory) as a storage device, which can nonvolatilely store data with low power consumption. The MRAM device uses a plurality of thin film magnetic elements formed in a semiconductor integrated circuit for nonvolatilely storing data, and can perform random access to the thin film magnetic elements and others.
FIG. 15
conceptually shows a data storing principle of a memory cell having a magnetic tunnel junction, which will be merely referred to as an “MTJ memory cell” hereinafter.
Referring to
FIG. 15
, the MTJ memory cell includes a tunneling magneto-resistance element TMR having a MR (Magneto-Resistive) effect, in which an electric resistance of a material changes depending on a direction of magnetization of a magnetic material. As a distinctive feature, tunneling magneto-resistance element TMR can achieve a remarkable MR effect and a high MR ratio (electric resistance ratio depending on the magnetization direction) even at a room temperature.
Tunneling magneto-resistance element TMR includes ferromagnetic material films
201
and
202
as well as an insulating film (tunneling film)
203
. In tunneling magneto-resistance element TMR, a magnitude of a tunneling current flowing through insulating film
203
, which is located between ferromagnetic material films
201
and
202
, changes in accordance with a direction of spin of electrons, which depends on the magnetization directions of ferromagnetic material films
201
and
202
. The number of states, which spin electrons in ferromagnetic material films
201
and
202
can enter, depends on the magnetization direction. Therefore, the tunneling current increases when ferromagnetic material films
201
and
202
are magnetized in the same direction. When these are magnetized in the opposite directions, respectively, the tunneling current decreases.
This phenomenon is utilized as follows. The magnetization direction of ferromagnetic material film
202
is switched in accordance with storage data while fixing the magnetization direction of ferromagnetic material film
201
, and the magnitude of the tunneling current flowing through tunneling film
203
and thus the electric resistance of tunneling magneto-resistance element TMR are detected. Thereby, the tunneling magneto-resistance element TMR can be used as a memory cell for storing data of one bit. The magnetization direction of ferromagnetic material film
201
is fixed by an antiferromagnetic material or the like, and is generally referred to as a “spin valve”.
In the following description, ferromagnetic material film
201
having a fixed magnetization direction may also referred to as a “fixed magnetic film
201
”, and ferromagnetic material film
202
having a magnetization direction corresponding to the storage data may also be referred to as a “free magnetic film
202
”.
For achieving a memory device having a high density, it is desired that MTJ memory cells formed of such tunneling magneto-resistance element TMRs are arranged in a two-dimensional array form. In general, the ferromagnetic material has a direction, which allows easy magnetization owing to crystal structures, forms and others (and thus requires a low energy for magnetization), and this direction is generally referred to as an easy axis. The magnetization direction of free magnetic film
202
corresponding to the storage data is set along this easy axis. Conversely, the direction, in which the ferromagnetic material cannot be magnetized easily (and a high energy is required for magnetization), is referred to as a hard axis.
FIG. 16
conceptually shows data write magnetic fields applied to the MTJ memory cell in a data write operation.
In
FIG. 16
, an abscissa gives a data write magnetic field H(EA) in the direction of the easy axis, and an ordinate gives a data write magnetic field H(HA) in the direction of the hard axis. When a vector sum of data write magnetic fields H(EA) and H(HA) reaches a region exceeding an asteroid curve
205
, the magnetization direction of tunneling magneto-resistance element TMR (magnetization direction of free magnetic film
202
) is rewritten into the direction of the easy axis.
Conversely, when the data write magnetic field within the region surrounded by asteroid curve
205
is applied, the magnetization direction of tunneling magneto-resistance element TMR is not renewed, and the stored contents are nonvolatilely held.
As shown in
FIG. 16
, data write magnetic field H(EA) required for the data writing is reduced by simultaneously applying data write magnetic field H(HA). Thus, operation points
206
and
207
during the data writing are represented by vector sums of data write magnetic field H(HA) in the uniform direction not affected by the write data and data write magnetic field H(EA) in the direction corresponding to the write data. Further, each of the data write magnetic fields H(HA) and H(EA) at operation points
206
and
207
is designed not to reach the region exceeding asteroid curve
205
.
FIG. 17
conceptually shows an arrangement of data write interconnections in the memory cell array formed of the MTJ memory cells.
Referring to
FIG. 17
, the memory cell array, in which tunneling magneto-resistance elements TMR each forming the MTJ memory cell are arranged in rows and columns, is provided with data write lines
210
and
215
arranged in a grid-like form. Each of data write lines
210
and
215
is supplied with a data write current for generating or the other of data write magnetic fields H(EA) and H(HA).
For example, data write line
210
generates data write magnetic field H(HA), and data write line
215
generates data write magnetic field H(EA). For this, data write line
210
is selectively supplied with the data write current in the uniform direction, and data write line
215
is selectively supplied with the data write current in the direction corresponding to the write data. For the MTJ memory cell, which is designated as a target of data writing, corresponding data write interconnections
210
and
215
are both supplied with the data write currents.
Therefore, the data can be selectively written into the plurality of tunneling magneto-resistance elements TMR arranged in a two-dimensional fashion by controlling supply of the data write currents to data write interconnections
210
and
215
in accordance with the address selection.
FIG. 18
conceptually shows a structure for reading data from the MTJ memory cell.
Structures similar to that in
FIG. 18
are disclosed in technical references such as “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, TA7.2, February 2000, “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000, and “A 256 kb 3.0V 1T1MTJ Nonvolatile Magneto-resistive RAM”, ISSCC Digest of Technical Papers, TA7.6, February 2001.
Referring to
FIG. 18
, the writing of data into the MTJ memory cell, i.e., into tunneling magneto-resistance element TMR is executed, as already described, by the magnetic fields, which are generated by the data write currents flowing through a digit line DL and a bit line BL, respectively. For example, digit line DL corresponds to data write line
210
shown in
FIG. 17
, and bit line BL corresponds to data write line
215
.
As an access element for reading data from tunneling magneto-resistance element TMR, the structure is provided with an access transistor ATR, which is turned on and off in accordance with a voltage on a word line WL. Access transistor ATR is typically formed of a MOS (Metal Oxide Silicon) transistor. One of source/drain regions of access transistor ATR is electrically coupled to tun
Le Thong Q.
McDermott Will & Emery LLP
Renesas Technology Corp.
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