Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2002-07-08
2004-09-14
Hoang, Huan (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S173000, C365S209000
Reexamination Certificate
active
06791875
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film magnetic memory device and, more particularly, to a random access memory including a memory cell having a magnetic tunnel junction (MTJ).
2. Description of the Background Art
As a memory device capable of storing data in a nonvolatile manner with lower power consumption, attention is 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 elements formed in a semiconductor integrated circuit and each of the thin film magnetic elements can be accessed at random.
Particularly, in recent years, a technique of using a thin film magnetic element using a magnetic tunnel junction (MTJ) as a memory cell to realize dramatically advanced performance of an MRAM device has been announced. An MRAM device including a memory cell having a magnetic tunnel junction is disclosed in technical literature 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. 14
is a schematic diagram showing the configuration of a memory cell having the magnetic tunnel junction (hereinbelow, also simply called “MTJ memory cell”).
Referring to
FIG. 14
, an MTJ memory cell includes a tunneling magneto-resistance element TMR whose electric resistance changes according to the level of stored date and an access transistor ATR for generating a path of a sense current Is passing through tunneling magneto-resistance element TMR in a data reading operation. Access transistor ATR takes the form of, for example, a field effect transistor and is coupled 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 data writing operation, a read word line RWL for executing data reading operation, and a bit line BL as a data line for transmitting an electric signal corresponding to the data level of stored data in the data reading operation and the data write operation are disposed.
FIG. 15
is a conceptual diagram for explaining operation of reading data from the MTJ memory cell.
Referring to
FIG. 15
, tunneling magneto-resistance element TMR has a ferromagnetic layer FL having a predetermined magnetization direction (hereinbelow, also simply called a “fixed magnetic layer”) and a ferromagnetic layer VL magnetized in a direction according to a magnetic field applied from the outside (hereinbelow, also simply called a “free magnetic layer”). Between fixed magnetic layer FL and free magnetic layer VL, a tunneling barrier TB formed 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 different from fixed magnetic layer FL in accordance with the data level of stored data.
In the data reading operation, when read word line RWL is activated, access transistor ATR is turned on. It makes it possible to pass sense current Is to a current path of bit line BL, tunneling magneto-resistance element TMR, access transistor ATR, and ground voltage Vss.
The electric resistance of tunneling magneto-resistance element TMR changes according to the relation between the magnetization direction of fixed magnetic layer FL and that of free magnetic layer VL. Concretely, when the magnetization direction of fixed magnetic layer FL and that written in free magnetic layer VL are parallel to each other (the same), the electric resistance of tunneling magneto-resistance element TMR is lower than that in the case where the magnetization directions of the layers are opposite (anti-parallel) to each other. In the specification, the electric resistances of tunneling magneto-resistance element TMR corresponding to stored data “1” and “0” will be described as R
1
and R
0
, respectively. R
1
is higher than R
0
(R
1
>R
0
).
As described above, the electric resistance of tunneling magneto-resistance element TMR changes according to the magnetization directions. By associating the two ways of the magnetization direction of free magnetic layer VL in tunneling magneto-resistance element TMR with levels (“1” and “0”) of storage data, respectively, data storing operation can be executed.
A voltage change which is caused in tunneling magneto-resistance element TMR by sense current Is varies according to the magnetization direction of free magnetic layer VL, that is, the level of stored data. For example, when bit line BL is precharged to a predetermined voltage and, after that, sense current Is is passed to tunneling magneto-resistance element TMR, by detecting a change in voltage level of bit line BL, the data stored in the MTJ memory cell can be read.
FIG. 16
is a conceptual diagram for explaining operation of writing data to the MTJ memory cell.
Referring to
FIG. 16
, in the data write operation, read word line RWL is made inactive and access transistor ATR is turned off. In this case, a data write current for magnetizing free magnetic layer VL in a direction according to write data is passed to each of write word line WWL and bit line BL. The magnetization direction of free magnetic layer VL is determined by a combination of the directions of the data write currents flowing through write word line WWL and bit line BL.
FIG. 17
is a conceptual diagram for explaining the relation between the direction of the data write current and the magnetization direction in the data write operation.
Referring to
FIG. 17
, the lateral axis Hx denotes the direction of a data write magnetic field H (BL) generated by the data write current flowing in bit line BL. On the other hand, the vertical axis Hy denotes the direction of a data write magnetic field H (WWL) generated by the data write current flowing in write word line WWL.
The magnetization direction of free magnetic layer VL can be newly rewritten only in the case where the sum of data write magnetic fields H(BL) and H(WWL) reaches the area on the outside of the asteroid characteristic line shown in the diagram.
Specifically, when the data write magnetic field applied has the strength corresponding to the area on the inside of the asteroid, the magnetization direction of free magnetic layer VL does not switch. Therefore, in order to update the data stored in the MTJ memory cell, a current at a predetermined level or higher has to be passed to both of write word line WWL and bit line BL. The magnetization direction once written in the tunneling magneto-resistance element, that is, data stored in the MTJ memory cell is held in a nonvolatile manner until new data writing operation is executed.
In the data reading operation as well, sense current Is is passed to bit line BL. However, since sense current Is is generally set to be lower than the data write current by about 1 or 2 digits, the possibility that data stored in the MTJ memory cell is erroneously rewritten in the data reading operation due to an influence of sense current Is is low.
In the above-described technical literature, a technique of constructing a random access memory MRAM device by integrating such MTJ memory cells on a semiconductor substrate is disclosed.
FIG. 18
is a conceptual diagram showing MTJ memory cells packed and arranged in a matrix.
By arranging the MTJ memory cells in a matrix on the semiconductor substrate, a high-density MRAM device can be realized.
FIG. 18
shows the configuration in which MTJ memory cells are disposed in rows of n in number and columns of m in number (n and m: natural numbers). As already described, bit line BL, write word line WWL, and read word line RWL have to be disposed for each of the MTJ memory cells. Therefore, for (n×m) MTJ memory cells arranged in the matrix, n write word lines WWL
1
to WWLn, n read word lines RWL
1
to RWLn, and m bit lines BL
1
to BLm are arrang
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