Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2002-02-06
2002-12-31
Tran, Andrew Q. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S171000, C365S158000, C365S189090, C365S201000, C365S225700
Reexamination Certificate
active
06501679
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention 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). 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 that stores data in a non-volatile manner using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and is capable of random access to each thin film magnetic element.
In particular, recent announcement shows that significant progress in performance of the MRAM device is achieved by using thin film magnetic elements having a magnetic tunnel junction (MTJ) as memory cells. 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, and “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000.
FIG. 83
is a schematic diagram showing the structure of a memory cell having a magnetic tunnel junction (hereinafter, also simply refereed to as “MTJ memory cell”).
Referring to
FIG. 83
, the MTJ memory cell includes a magnetic tunnel junction MTJ whose resistance value varies according to the storage data level, and an access transistor ATR. The access transistor ATR is formed from a field effect transistor, and is coupled between the magnetic tunnel junction MTJ and the ground voltage Vss.
For the MTJ memory cell are provided a write word line WWL for instructing a data write operation, a read word line RWL for instructing a data read operation, and a bit line BL serving as a data line for transmitting an electric signal corresponding to the storage data level in the data read and write operations.
FIG. 84
is a conceptual diagram illustrating the data read operation from the MTJ memory cell.
Referring to
FIG. 84
, the magnetic tunnel junction MTJ has a magnetic layer FL having a fixed magnetic field of a fixed direction (hereinafter, also simply referred to as “fixed magnetic layer FL”), and a magnetic layer VL having a free magnetic field (hereinafter, also simply referred to as “free magnetic layer VL”). A tunnel barrier TB formed from an insulator film is provided between the fixed magnetic layer FL and the free magnetic layer VL. According to the storage data level, either a magnetic field of the same direction as that of the fixed magnetic layer FL or a magnetic field of the direction different from that of the fixed magnetic layer FL has been written to the free magnetic layer VL in a non-volatile manner.
In reading the data, the access-transistor ATR is turned ON in response to activation of the read word line RWL. As a result, a sense current Is flows through a current path formed by the bit line BL, magnetic tunnel junction MTJ, access transistor ATR and ground voltage Vss. The sense current Is is supplied as a constant current from a not-shown control circuit.
The resistance value of the magnetic tunnel junction MTJ varies according to the relative relation of the magnetic field direction between the fixed magnetic layer FL and the free magnetic layer VL. More specifically, in the case where the fixed magnetic layer FL and the free magnetic layer VL have the same magnetic field direction, the magnetic tunnel junction MTJ has a smaller resistance value as compared to the case where both magnetic layers have different magnetic field directions.
Accordingly, in the data read operation, a voltage change produced at the magnetic tunnel junction MTJ due to the sense current Is varies according to the magnetic field direction stored in the free magnetic layer VL. Thus, by starting supply of the sense current Is with the bit line BL precharged to a high voltage, the storage data level in the MTJ memory cell can be read by monitoring a voltage level change on the bit line BL.
FIG. 85
is a conceptual diagram illustrating the data write operation to the MTJ memory cell.
Referring to
FIG. 85
, in the data write operation, the read word line RWL is inactivated, and the access transistor ATR is turned OFF. In this state, a data write current for writing a magnetic field to the free magnetic layer VL is applied to the write word line WWL and the bit line BL. The magnetic field direction of the free magnetic layer VL is determined by combination of the respective directions of the data write current flowing through the write word line WWL and the bit line BL.
FIG. 86
is a conceptual diagram illustrating the relation between the respective directions of the data write current and the magnetic field in the data write operation.
Referring to
FIG. 86
, a magnetic field Hx of the abscissa indicates the direction of a magnetic field H(WWL) produced by the data write current flowing through the write word line WWL. A magnetic field Hy of the ordinate indicates the direction of a magnetic field H(BL) produced by the data write current flowing through the bit line BL.
The magnetic field direction stored in the free magnetic layer VL is updated only when the sum of the magnetic fields H(WWL) and H(BL) reaches the region outside the asteroid characteristic line shown in the figure. In other words, the magnetic field direction stored in the free magnetic layer VL is not updated when a magnetic field corresponding to the region inside the asteroid characteristic line is applied.
Accordingly, in order to update the storage data of the magnetic tunnel junction MTJ by the data write operation, a current must be applied to both the write word line WWL and the bit line BL. Once the magnetic field direction, i.e., the storage data, is stored in the magnetic tunnel junction MTJ, it is held therein in a non-volatile manner until a new data write operation is conducted.
The sense current Is flows through the bit line BL in the data read operation. However, the sense current Is is generally set to a value that is smaller than the above-mentioned data write current by about one or two orders of magnitude. Therefore, it is less likely that the storage data in the MTJ memory cell is erroneously rewritten during the data read operation due to the sense current Is.
The above-mentioned technical documents disclose a technology of forming an MRAM device, a random access memory, having such MTJ memory cells integrated on a semiconductor substrate.
FIG. 87
is a conceptual diagram showing the MTJ memory cells arranged in rows and columns in an integrated manner.
Referring to
FIG. 87
, with the MTJ memory cells arranged in rows and columns on the semiconductor substrate, a highly integrated MRAM device can be realized.
FIG. 87
shows the MTJ memory cells arranged in n rows by m columns (where n, m is a natural number).
As described before, the bit line BL, write word line WWL and read word line RWL must be provided for each MTJ memory cell. Accordingly, n write word lines WWL
1
to WWLn, n read word lines RWL
1
to RWLn, and n bit lines BL
1
to BLm are required for the n×m MTJ memory cells.
Thus, the MTJ memory cells are generally provided with the independent word lines for the read and write operations.
FIG. 88
is a structural diagram of the MTJ memory cell provided on the semiconductor substrate.
Referring to
FIG. 88
, the access transistor ATR is formed in a p-type region PAR of the semiconductor main substrate SUB. The access transistor ATR has source/drain regions (n-type regions)
110
,
120
and a gate
130
. The source/drain region
110
is coupled to the ground voltage Vss through a metal wiring formed in a first metal wiring layer Ml. A metal wiring formed in a second metal wiring layer M
2
is used as the write word line WWL. The bit line BL is provided in a third metal wiring layer M
3
.
The magnet
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
Tran Andrew Q.
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