Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2003-03-14
2004-08-17
Lam, David (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S158000, C365S230070
Reexamination Certificate
active
06778432
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film magnetic memory device and a method of fabricating the same, and more particularly to a thin film magnetic memory device having a redundant configuration for repairing a defective memory cell and a method of fabricating the same.
2. Description of the Background Art
As a memory device capable of storing data in a nonvolatile manner with low power consumption, attention is being paid to an MRAM (Magnetic Random Access Memory) device. The MRAM device is a memory device for storing data in a nonvolatile manner by using a plurality of thin film magnetic materials formed on a semiconductor integrated circuit. Each of the thin film magnetic materials can be accessed at random.
Particularly, in recent years, it has been announced that the performance of an MRAM device is dramatically improved by using a thin film magnetic material utilizing a magnetic tunnel junction (MTJ) as a memory cell. An MRAM device including memory cells each having a magnetic tunnel junction is disclosed in the following Literature 1 to 3.
(Literature 1)
Roy Scheuerlein and six others, “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and, FET Switch in each Cell”, (U.S.A.), 2000 IEEE ISSCC Digest of Technical Papers, TA7.2
(Literature 2)
M. Durlam and five others, “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, (U.S.A.), 2000 IEEE ISSCC Digest of Technical Papers, TA7. 3
(Literature 3)
Peter K. Naji and four others, “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, (U.S.A.), 2001 IEEE ISSCC Digest of Technical Papers, TA7.6
FIG. 16
is a schematic diagram showing the configuration of a memory cell having a magnetic tunnel junction (hereinafter, also simply referred to as “MTJ memory cell”).
Referring to
FIG. 16
, an MTJ memory cell has a tunneling magneto-resistance element TMR in which electric resistance changes according to the level of stored data and an access element ATR for forming a path of a sense current Is passing through tunneling magneto-resistance element TMR when data is read. Since access element ATR is formed representatively by a field effect transistor, access element ATR will be also referred to as an access transistor ATR hereinafter. Access transistor ATR is connected between tunneling magneto-resistance element TMR and a fixed voltage (ground voltage Vss).
For the MTJ memory cell, a write word line WWL for instructing writing of data, a read word line RWL for reading data, and a bit line BL as a data line for transmitting an electric signal corresponding to the level of stored data in a data reading/writing operation are disposed.
FIG. 17
is a conceptual diagram for describing reading of data from the MTJ memory cell.
Referring to
FIG. 17
, tunneling magneto-resistance element TMR has a ferromagnetic layer (hereinafter, also simply referred to as “fixed magnetic layer”) FL having a fixed predetermined magnetization direction and a ferromagnetic layer (hereinafter, also simply referred to as “free magnetic layer”) VL magnetized in the direction according to a magnetic field applied from the outside. Between fixed magnetic layer FL and free magnetic layer VL, a tunneling barrier layer (tunnel film) TB made by an insulating film is provided. Free magnetic layer VL is magnetized in the same direction as fixed magnetic layer FL or in the direction opposite to fixed magnetic layer FL in accordance with the level of storage data to be written. By fixed magnetic layer FL, tunneling barrier TB, and free magnetic layer VL, a magnetic tunnel junction is formed.
At the time of reading data, in response to activation of read word line RWL, access transistor ATR is turned on. It enables sense current Is to be passed through a current path constructed by bit line BL, tunneling magneto-resistance element TMR, access transistor ATR, and ground voltage Vss.
Electric resistance of tunneling magneto-resistance element TMR changes according to the relative relation between the magnetization direction of fixed magnetic layer FL and the magnetization direction of free magnetic layer VL. Concretely, when the magnetization direction of fixed magnetic layer FL and that of free magnetic layer VL are the same (parallel), as compared with the case where the magnetization directions are opposite (anti-parallel) to each other, the electric resistance of tunneling magneto-resistance element TMR is lower.
Therefore, by magnetizing free magnetic layer VL in one of the two kinds of directions in accordance with storage data, a voltage change occurring in tunneling magneto-resistance element TMR by sense current Is varies according to the level of the storage data. For example, after precharging bit line BL to a predetermined voltage, sense current Is is passed to tunneling magneto-resistance element TMR, and by detecting the voltage of bit line BL, data stored in the MTJ memory cell can be read.
FIG. 18
is a conceptual diagram for describing an operation of writing data to the MTJ memory cell.
Referring to
FIG. 18
, at the time of writing data, read word line RWL is made inactive and access transistor ATR is turned off. In this state, a data write current for magnetizing free magnetic layer VL in the direction according to write data is passed to write word line WWL and bit line BL. The magnetization direction of free magnetic layer VL is determined by the data write current flowing in write word line WWL and the data write current flowing in bit line BL.
FIG. 19
is a conceptual diagram showing the relation between the data write current at the time of writing data to the MTJ memory cell and the magnetization direction of the tunneling magneto-resistance element.
Referring to
FIG. 19
, a horizontal axis H (EA) indicates a magnetic field applied in the direction of a magnetization easy axis (EA) in free magnetic layer VL in tunneling magneto-resistance element TMR. On the other hand, a vertical axis H (HA) indicates a magnetic field acting in the direction of a magnetization hard axis (HA) in free magnetic layer VL. The magnetic fields H (EA) and H (HA) correspond to two magnetic fields generated by the current passing through bit line BL and the current passing through write word line WWL.
In the MTJ memory cell, the fixed magnetization direction of fixed magnetic layer FL is along the magnetization easy axis of free magnetic layer VL, and free magnetic layer VL is magnetized in parallel with (in the same direction as) or in anti-parallel (opposite) with fixed magnetic layer FL along the magnetization easy axis direction in accordance with the level of the stored data (“1” and “0”). In the specification, the electric resistance of tunneling magneto-resistance element TMR corresponding to the two kinds of magnetization directions of free magnetic layer VL is expressed by Rmax and Rmin (where Rmax>Rmin). The MTJ memory cell can store one-bit data (“1” or “0”) in correspondence with the two kinds of magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be newly rewritten only in the case where the sum of magnetic fields H (EA) and H (HA) applied reaches the outside of the asteroid characteristic curve shown in the diagram. In other words, when the data write magnetic field applied has an intensity corresponding to the inside area of the asteroid, characteristic curve the magnetization direction of free magnetic layer VL does not change.
As shown by the asteroid characteristic curve, by applying a magnetic field in a magnetization hard axis direction to free magnetic layer VL, a magnetization threshold value necessary to change the magnetization direction along the magnetization easy axis can be decreased.
In the case where the operating point at the time of writing data is designed as shown in the example of
FIG. 19
, in an MTJ memory cell to which data is to be written, a data write magnetic field in the magnetization easy axis direction is designed so that its intensity becomes H
WR
. Specifically, the v
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