Static information storage and retrieval – Read/write circuit – Bad bit
Reexamination Certificate
2002-07-15
2003-12-30
Nguyen, Tan T. (Department: 2818)
Static information storage and retrieval
Read/write circuit
Bad bit
C365S230060
Reexamination Certificate
active
06671213
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film magnetic memory device, and more specifically relates to a thin film magnetic memory device having a redundancy configuration for repairing a defective memory cell.
2. Description of the Background Art
Attention has been paid to an MRAM (Magnetic Random Access Memory) device as a memory device capable of storing nonvolatile data with lower consumption power. The MRAM device is a memory device which stores nonvolatile data using a plurality of thin film magnetic elements formed on a semiconductor integrated circuit and which can randomly access the respective thin film magnetic elements.
In recent years, it has been made public that the performance of the MRAM device surprisingly advances particularly by using a thin film magnetic body using a magnetic tunnel junction (MTJ) as a memory cell.
FIG. 11
is a schematic diagram showing the configuration of a memory cell having a magnetic tunnel junction (which memory cell will be also referred to simply as “MTJ” memory cell hereinafter).
Referring to
FIG. 11
, MTJ memory cell includes a tunneling magneto-resistance element TMR having electric resistance changing according to storage data level, and an access element ATR for forming the path of a sense current Is which passes through tunneling magneto-resistance element TMR during data read. Since access element ATR is typically formed out of a field effect transistor, access element ATR will be also referred to as “access transistor ATR” hereinafter. Access transistor ATR is connected between tunneling magneto-resistance element TMR and a fixed voltage (ground voltage Vss).
FIG. 12
is a conceptual view for explaining data read from an MTJ memory cell.
Referring to
FIG. 12
, tunneling magneto-resistance element TMR includes a ferromagnetic layer FL which has a fixed, uniform magnetization direction (which layer will be also referred to simply as “fixed magnetic layer” hereinafter) and a ferromagnetic layer VL which is magnetized in a direction according to an externally applied magnetic field (which layer will be also referred to simply as “free magnetic layer” hereinafter). A tunneling barrier (tunneling film) TB formed out of an insulating film is provided between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized in the same direction as or the opposite direction to the magnetization direction of fixed magnetic layer FL in accordance with the level of written, stored data. Fixed magnetic layer FL, tunneling barrier TB and free magnetic layer VL form a magnetic tunnel junction.
During data read, access transistor ATR is turned on in accordance with the activation of a word line WL. As a result, sense current Is can be fed to a current path from a bit line BL to tunneling magneto-resistance element TMR, access transistor ATR and a ground voltage Vss.
The electric resistance of tunneling magneto-resistance element TMR changes according to the relative relationship in magnetization direction between fixed magnetic layer FL and free magnetic layer VL. More specifically, if the magnetization direction of fixed magnetic layer FL is the same as (parallel to) that of free magnetic layer VL, the electric resistance of tunneling magneto-resistance element TMR becomes lower than that of tunneling magneto-resistance element TMR if the magnetization direction of fixed magnetic layer FL is opposite (anti-parallel) to that of free magnetic layer FL.
Accordingly, if free magnetic layer VL is magnetized in one of the two directions in accordance with stored data, the voltage change of tunneling magneto-resistance element TMR caused by sense current Is differs according to the level of the stored data. Therefore, if bit line BL is precharged with a constant voltage and then sense current Is is fed to tunneling magneto-resistance element TMR, it is possible to read the data stored in the MTJ memory cell by detecting the voltage of bit line BL.
FIG. 13
is a conceptual view for explaining a data write operation for writing data to the MTJ memory cell.
Referring to
FIG. 13
, during data write, word line WL is inactivated and access transistor ATR is turned off. In this state, a data write current for magnetizing free magnetic layer VL in a direction according to written data is fed to a write digit line WDL and a bit line BL, respectively. The magnetization direction of free magnetic layer VL is determined according to data write currents fed to write digit line WDL and that fed to bit line BL, respectively.
FIG. 14
is a conceptual view for explaining the relationship between the data write current and the magnetization direction of tunneling magneto-resistance element TMR during data write to the MTJ memory cell.
Referring to
FIG. 14
, a horizontal axis H (EA) indicates a magnetic field applied in an easy axis (EA) direction in free magnetic layer VL in tunneling magneto-resistance element TMR. A vertical axis H (HA) indicates a magnetic field applied in a hard axis (HA) direction in free magnetic layer VL. Magnetic fields H(EA) and H(HA) correspond to two magnetic fields generated by currents fed to bit line BL and write digit line WDL, respectively.
In the MTJ memory cell, the fixed magnetization direction of fixed magnetic layer FL is along the easy axis of free magnetic layer VL, and free magnetic layer VL is magnetized in the parallel direction or anti-parallel (opposite) direction to the magnetization direction of fixed magnetic layer FL along the easy axis direction in accordance with the level (“1” or “0”) of the stored data. In the specification, the electric resistances of tunneling magneto-resistance element TMR corresponding to the two magnetization directions of free magnetic layer VL will be denoted by Rmax and Rmin (where Rmax>Rmin) hereinafter, respectively. The MTJ memory cell can store 1-bit data (“1” or “0”) in accordance with one of these two magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be rewritten only if the sum of applied magnetic fields H(EA) and H(HA) reaches a region outside of an asteroid characteristic line shown in FIG.
14
. Namely, if the applied data write magnetic fields have intensity corresponding to a region inside of the asteroid characteristic line, the magnetization direction of free magnetic layer VL has no change.
As shown in the asteroid characteristic line, if a magnetic field in the hard axis (HA) direction is applied to free magnetic layer VL, it is possible to decrease a magnetic threshold value necessary to switch the magnetization direction along the easy axis.
If operation points during data write are designed as shown in the example of
FIG. 14
, the data write magnetic field in the easy axis direction is designed to have an intensity of H
WR
in the data write target MTM memory cell. That is, the value of the data write current fed to bit line BL or write digit line WDL is designed so as to obtain this data write magnetic field H
WR
. Generally, data write magnetic field H
WR
is expressed by the sum of a switching magnetic field H
SW
necessary to change over a magnetization direction and a margin &Dgr;H. That is, data write magnetic field H
WR
is expressed by H
WR
=H
SW
+&Dgr;H.
To rewrite the data stored in the MTJ memory cell, i.e., to rewrite the magnetization direction of tunneling magneto-resistance element TMR, it is necessary to feed a data write current at not lower than predetermined level to each of write digit line WDL and bit line BL. By doing so, free magnetic layer VL in tunneling magneto-resistance element TMR is magnetized in the parallel or opposite (anti-parallel) direction to the magnetization direction of fixed magnetic layer FL in accordance with the direction of the data write magnetic field along the easy axis (EA). The magnetization direction, i.e., the stored data of the MTJ memory cell which has been written to tunneling magneto-resistance element TMR is held in a nonvolatile manner until new data is w
McDermott & Will & Emery
Nguyen Tan T.
Renesas Technology Corp.
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