Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2002-11-22
2004-09-07
Le, Thong Q. (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S157000, C365S161000
Reexamination Certificate
active
06788571
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 random access memory provided with memory cells having MTJs (Magnetic Tunnel Junctions).
2. Description of the Background Art
Attention is being given to an MRAM (Magnetic Random Memory) device as a memory device, which can nonvolatilely store data with low power consumption. The MRAM device is a memory device, in which a plurality of thin film magnetic members are formed in a semiconductor integrated circuit for nonvolatilely storing data, and random access to each thin film magnetic member is allowed.
Particularly, in recent years, it has been announced that a performance of the MRAM device can be dramatically improved by using the thin film magnetic members, which utilize the magnetic tunnel junctions, as memory cells. The MRAM device with memory cells having the magnetic tunnel junctions has been 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 and “Nonvolatile RAM Based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000.
FIG. 22
conceptually shows a structure of a memory cell, which has a tunnel junction, and may be merely referred to as a “MTJ memory cell” hereinafter.
Referring to
FIG. 22
, an MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance, which is variable in accordance with a data level of magnetically written storage data, and an access transistor ATR. Access transistor ATR is located between a bit line BL and a ground voltage line GL, and is connected in series to tunneling magneto-resistance element TMR. Typically, access transistor ATR is formed of a field-effect transistor.
For the MTJ memory cell, the device includes bit line BL for carrying a data write current and a data read current in a data write operation and a data read operation, respectively, a write digit line WDL for carrying the data write current in the data write operation, a word line WL for instructing data reading, and ground voltage line GL for puling down tunneling magneto-resistance element TMR to a ground voltage GND in the data read operation.
In the data read operation, tunneling magneto-resistance element TMR is electrically coupled between ground voltage line GL carrying ground voltage GND and bit line BL in response to turn-on of access transistor ATR.
FIG. 23
conceptually shows an operation of writing data in the MTJ memory cell.
Referring to
FIG. 23
, tunneling magneto-resistance element TMR has a magnetic material layer FL, which has a fixed magnetization direction, and may be merely referred to as a “fixed magnetic layer” hereinafter, and a magnetic material layer VL, which is magnetized in a direction depending on a data write magnetic field caused by the data write current, and may be merely referred to as a “free magnetic layer” hereinafter. A tunneling barrier TB formed of an insulator film is disposed between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized in the same direction as fixed magnetic layer FL or in the opposite direction in accordance with the level of the storage data to be written.
Tunneling magneto-resistance element TMR has an electric resistance, which is variable depending on a correlation in magnetization direction between fixed magnetic layer FL and free magnetic layer VL. More specifically, when fixed and free magnetic layers FL and VL are magnetized in the same direction, the electric resistance is smaller than that in the case where these are magnetized in the opposite directions.
In the data write operation, word line WL is inactive, and access transistor ATR is off. In this state, the data write currents for magnetizing free magnetic layer VL are supplied to bit line BL and write digit line WDL in directions depending on the level of write data, respectively. Thus, the magnetization direction of free magnetic layer VL depends on the directions of data write currents flowing through bit line BL and write digit line WDL, respectively.
FIG. 24
conceptually illustrates a relationship between the data write current and the magnetization of the free magnetic layer VL.
Referring to
FIG. 24
, an abscissa H(EA) gives a magnetic field, which is applied in a direction of an easy axis (EA) to free magnetic layer VL of tunneling magneto-resistance element TMR. An ordinate H(HA) indicates a magnetic field acting in a direction of a hard axis (HA) on free magnetic layer VL. Magnetic fields H(EA) and H(HA) correspond to two magnetic fields produced by currents flowing through bit line BL and write digit line WDL, respectively.
In the MTJ memory cell, the fixed magnetization direction of fixed magnetic layer FL is parallel to the easy axis of free magnetic layer VL, and free magnetic layer VL is magnetized in the direction along the easy axis, and particularly in the same parallel direction, which is the same direction as fixed magnetic layer FL, or in the opposite-parallel direction, which is opposite to the above direction, depending on the level (“1” or “0”) of the storage data. The MTJ memory cell can selectively store data (“1” and “0”) of one bit depending on the two magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be rewritten only when a sum of applied magnetic fields H(EA) and H(HA) falls within a region outside an asteroid characteristic line illustrated in FIG.
24
. Therefore, the magnetization direction of free magnetic layer VL does not switch when the data write magnetic fields applied thereto have intensities corresponding to a region inside the asteroid characteristic line.
As can be seen from the asteroid characteristic line, the magnetization threshold required for switching the magnetization direction along the easy axis can be lowered by applying the magnetic field in the direction of the hard axis to free magnetic layer VL.
For rewriting the storage data of the MTJ memory cell, i.e., the magnetization direction of tunneling magneto-resistance element TMR, it is necessary to pass the data write currents at a predetermined level or higher through write digit line WDL and bit line BL. Thereby, free magnetic layer VL in tunneling magneto-resistance element TMR is magnetized in the same direction as fixed magnetic layer FL or the opposite (opposite-parallel) direction depending on the direction of the data write magnetic field along the easy axis (EA). The magnetization direction, which was once written into tunneling magneto-resistance element TMR, and thus the storage data of MTJ memory cell is held nonvolatilely until next data writing is executed.
FIG. 25
conceptually shows an operation of reading data from the MTJ memory cell.
Referring to
FIG. 25
, access transistor ATR is turned on in response to activation of word line WL in the data read operation. Thereby, tunneling magneto-resistance element TMR is electrically coupled to bit line BL while being pulled down with ground voltage GND.
In this state, bit line BL is pulled up with a predetermined voltage, whereby a current path including bit line BL and tunneling magneto-resistance element TMR carries a memory cell current Icell corresponding to the electric resistance of tunneling magneto-resistance element TMR, and thus to the storage data of the MTJ memory cell. For example, this memory cell current Icell is compared with a predetermined reference current, whereby storage data can be read out from the MTJ memory cell.
As described above, the electric resistance of tunneling magneto-resistance element TMR changes in accordance with the magnetization direction, which is rewritable by the data write magnetic field applied thereto. Therefore, nonvolatile data storage can be executed by establishing a correlation of electric resistances Rmax and Rmin of tunneling magneto-resistance element TMR wit
Ishikawa Masatoshi
Ooishi Tsukasa
Le Thong Q.
McDermott Will & Emery LLP
Renesas Technology Corp.
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