Static information storage and retrieval – Addressing – Particular decoder or driver circuit
Reexamination Certificate
2003-01-02
2004-06-08
Elms, Richard (Department: 2824)
Static information storage and retrieval
Addressing
Particular decoder or driver circuit
C365S177000, C365S230030
Reexamination Certificate
active
06747910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a thin film magnetic memory device. More particularly, the present invention relates to a random access memory (RAM) including memory cells having a magnetic tunnel junction (MTJ).
2. Description of the Background Art
An MRAM (Magnetic Random Access Memory) device has attracted attention as a memory device capable of non-volatile data storage with low power consumption. The MRAM device is a memory device capable of non-volatile data storage using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and also capable of random access to each thin film magnetic element.
In particular, recent announcement shows that the use of tunneling magneto-resistance elements (i.e., thin film magnetic elements having a magnetic tunnel junction (MTJ)) in memory cells significantly improves performance of the MRAM device. The MRAM device including memory cells having a magnetic tunnel junction is disclosed in technical documents 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 Magnetoresistive RAM”, ISSCC Digest of Technical Papers, TA7.6, February 2001.
FIG. 44
schematically shows the structure of a memory cell having a magnetic tunnel junction (hereinafter, sometimes simply referred to as “MTJ memory cell”).
Referring to
FIG. 44
, the MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance varying according to the storage data level, and an access element ATR for forming a path of a sense current Is flowing through tunneling magneto-resistance element TMR in data read operation. Since a field effect transistor is typically used as access transistor ATR, access element ATR is hereinafter sometimes referred to as access transistor ATR. Access transistor ATR is connected in series with tunneling magneto-resistance element TMR.
A digit line DL for data write operation, a word line WL for data read operation, and a bit line BL are provided for the MTJ memory cell. Bit line BL is a data line for transmitting an electric signal corresponding to the storage data level in data read and write operations.
FIG. 45
is a conceptual diagram illustrating data read operation from the MTJ memory cell.
Referring to
FIG. 45
, tunneling magneto-resistance element TMR has a ferromagnetic material layer FL having a fixed magnetization direction (hereinafter, sometimes simply referred to as “fixed magnetic layer”), and a ferromagnetic material layer VL that is magnetized in the direction according to an external magnetic field (hereinafter, sometimes simply referred to as “free magnetic layer”). A tunneling barrier (tunneling film) TB of an insulator film is interposed between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized either in the same (parallel) direction as, or in the opposite (antiparallel) direction to, that of fixed magnetic layer FL according to the write data level. Fixed magnetic layer FL, tunneling barrier TB and free magnetic layer VL form a magnetic tunnel junction.
In data read operation, access transistor ATR is turned ON in response to activation of word line WL, and tunneling magneto-resistance element TMR is connected between bit line BL and ground voltage GND. As a result, a bias voltage according to a bit line voltage is applied across tunneling magneto-resistance element TMR, and a tunneling current is supplied to the tunneling film. The use of such a tunneling current enables a sense current to be supplied to a current path formed by bit line BL, tunneling magneto-resistance element TMR, access transistor ATR and ground voltage GND in data read operation.
The electric resistance of tunneling magneto-resistance element TMR varies according to the relation of the magnetization direction between fixed magnetic layer FL and free magnetic layer VL. More specifically, tunneling magneto-resistance element TMR has a minimum electric resistance value Rmin when fixed magnetic layer FL and free magnetic layer VL have parallel magnetization directions, and has a maximum electric resistance value Rmax when they have opposite (antiparallel) magnetization directions.
Accordingly, provided that free magnetic layer VL is magnetized in the direction according to the storage data level, a voltage change caused by sense current Is at tunneling magneto-resistance element TMR varies depending on the storage data level. For example, by supplying sense current Is to tunneling magneto-resistance element TMR after precharging bit line BL to a prescribed voltage, the storage data in the MTJ memory cell can be read by sensing a voltage on bit line BL.
FIG. 46
is a conceptual diagram illustrating data write operation to the MTJ memory cell.
Referring to
FIG. 46
, in data write operation, word line WL is inactivated and access transistor ATR is turned OFF. In this state, a data write current is applied to digit line DL and bit line BL in order to magnetize free magnetic layer VL in the direction according to the write data.
FIG. 47
is a conceptual diagram illustrating the relation between the data write current and the magnetization direction of the tunneling magneto-resistance element in data write operation.
Referring to
FIG. 47
, the abscissa H(EA) indicates a magnetic field that is applied to free magnetic layer VL of tunneling magneto-resistance element TMR in the easy-axis (EA) direction. The ordinate H(HA) indicates a magnetic field that is applied to free magnetic layer VL in the hard-axis (HA) direction. Magnetic field H(EA) corresponds to one of two magnetic fields produced by the currents flowing through bit line BL and digit line DL, and magnetic field H(EA) corresponds to the other magnetic field.
In the MTJ memory cell, fixed magnetic layer FL is magnetized in the fixed direction along the easy axis of free magnetic layer VL. Free magnetic layer VL is magnetized either in the direction parallel or antiparallel (opposite) to that of fixed magnetic layer FL along the easy axis according to the storage data level (“1” and “0”). The MTJ memory cell is thus capable of storing 1-bit data (“1” and “0”) by using the two magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL is rewritable only when the sum of the applied magnetic fields H(EA), H(HA) reaches the region outside the asteroid characteristic line shown in the figure. In other words, the magnetization direction of free magnetic layer VL will not change if an applied data write magnetic field corresponds to the region inside the asteroid characteristic line.
As shown by the asteroid characteristic line, applying a magnetic field of the hard-axis direction to free magnetic layer VL enables reduction in a magnetization threshold value required to change the magnetization direction along the easy axis.
When the operation point of the data write operation is designed as in the example of
FIG. 47
, a data write magnetic field of the easy-axis direction is designed to have strength H
WR
in the MTJ memory cell to be written. In other words, the data write current to be applied to bit line BL or digit line DL is designed to produce such a data write magnetic field H
WR
. In general, data write magnetic field H
WR
is defined by the sum of a switching magnetic field H
SW
required to switch the magnetization direction and a margin &Dgr;H. Data write magnetic field H
WR
is thus defined by H
WR
=H
SW
+&Dgr;H.
In order to rewrite the storage data of the MTJ memory cell, that is, the magnetization direction of tunneling magneto-resistance element TMR, a data write current of at least a prescribed level must be applied to both digit line DL and bit line BL. Free magnetic layer VL in tunneling magneto-resi
Elms Richard
Nguyen Hien
Renesas Technology Corp.
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