Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2001-09-04
2003-07-01
Mai, Son (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S158000, C365S171000
Reexamination Certificate
active
06587371
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a memory device and, more particularly, to a memory device typified by a thin film magnetic memory device, including a memory cell having an electric resistance value which varies according to the level of storage data.
2. Description of the Background Art
As a non-volatile memory device capable of storing data with low power consumption, attention is being paid to an MRAM (Magnetic Random Access Memory) device. The MRAM device is a non-volatile memory device for storing data by using a plurality of thin film magnetic elements formed on a semiconductor integrated circuit. Each of the thin film magnetic elements can be accessed at random.
Particularly, in recent years, it was announced that the performance of an MRAM device is dramatically improved by using a thin film magnetic element using a magnetic tunnel junction (MTJ) as a memory cell. An MRAM device including memory cells each having the magnetic tunnel junction is disclosed in technical document 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. 20
is a schematic diagram showing the configuration of a memory cell having a magnetic tunnel junction (hereinbelow, also simply called an MTJ memory cell).
Referring to
FIG. 20
, the MTJ memory cell has a magnetic tunnel junction MTJ in which a resistance value changes according to the level of storage data and an access transistor ATR. The access transistor ATR takes the form of a field effect transistor and is coupled between the magnetic tunnel junction MTJ and a ground voltage Vss.
For the MTJ memory cell, a write word line WWL for instructing data writing, a read word line RWL for instructing data reading, and a bit line BL as a data line for transmitting an electric signal corresponding to the level of the storage data at the time of data reading and data writing are disposed.
FIG. 21
is a conceptual diagram for explaining an operation of reading data from the MTJ memory cell.
Referring to
FIG. 21
, the magnetic tunnel junction MTJ has a magnetic layer having a fixed magnetic field in a fixed direction (hereinbelow, also simply called a fixed magnetic layer) FL and a magnetic layer having a free magnetic field (hereinbelow, also simply called a free magnetic layer) VL. Between the fixed magnetic layer FL and the free magnetic layer VL, a tunnel barrier TB formed by an insulating film is disposed. In the free magnetic layer VL, according to the level of storage data, either the magnetic field in the same direction as the fixed magnetic layer or the magnetic field in the direction different from the fixed magnetic layer FL is written in a non-volatile manner.
In a data reading operation, the access transistor ATR is turned on in response to activation of the read word line RWL. By the turn-on, a sense current Is supplied as a data read current at a fixed level is passed from a not-illustrated data read circuit to a current path constructed by the bit line BL, magnetic tunnel junction MTJ, access transistor ATR and ground voltage Vss.
The electric resistance value of the magnetic tunnel junction MTJ changes according to the relation between the magnetic field direction of the fixed magnetic layer FL and that of the free magnetic layer VL. To be specific, when the magnetic field direction of the fixed magnetic layer FL and that written in the free magnetic layer VL are the same, the electric resistance value of the magnetic tunnel junction MTJ is smaller as compared with the case where the magnetic field directions are different from each other.
In the data reading operation, therefore, a voltage drop occurring in the magnetic tunnel junction MTJ by the sense current Is varies according to the magnetic field direction stored in the free magnetic layer VL. Consequently, when the supply of the sense current Is is started after the bit line BL is once precharged to a high voltage, by detecting a change in voltage level of the bit line BL, data stored in the MTJ memory cell can be read.
FIG. 22
is a conceptual diagram for explaining a data writing operation to the MTJ memory cell.
Referring to
FIG. 22
, in the data writing operation, the read word line RWL is made inactive, and the access transistor ATR is turned off. In such a state, a data write current for writing the magnetic field to the free magnetic layer VL is passed to the write word line WWL and the bit line BL. The magnetic field direction of the free magnetic layer VL is determined by a combination of the direction of the data write current flowing in the write w word line WWL and the direction of the data write current flowing in the bit line BL.
FIG. 23
is a conceptual diagram for explaining the relation between the direction of the data write current and the magnetic field direction in the data writing operation.
Referring to
FIG. 23
, a magnetic field Hx on the lateral axis indicates the direction of a magnetic field H(WWL) generated by the data write current flowing in the write word line WWL. On the other hand, a magnetic field Hy on the vertical axis denotes a direction of the magnetic field H(BL) generated by the data write current flowing in the bit line BL.
The magnetic field direction stored in the free magnetic layer VL is newly written only when the sum of the magnetic fields H(WWL) and H(BL) reaches the area outside of the asteroid characteristic line shown in the diagram. That is, in the case where the magnetic field corresponding to the area inside the asteroid characteristic line is applied, the magnetic field direction stored in the free magnetic layer VL is not updated.
Therefore, in order to update the stored data in the magnetic tunnel junction MTJ by a writing operation, a current has to be passed to both the write word line WWL and the bit line BL. The magnetic field direction once stored in the magnetic tunnel junction MTJ, that is, storage data is held in a non-volatile manner until a new data writing operation is executed.
In the data reading operation as well, the sense current Is is passed through the bit line BL. The sense current Is is, however, generally set so as to be lower than the data write current by about one or two digits, the possibility that the stored data in the MTJ memory cell is erroneously rewritten by an influence of the sense current Is in the data reading operation is low.
The above-described technical documents discloses a technique of constructing an MRAM device as a random access memory (RAM) by integrating such MTJ memory cells on the semiconductor substrate.
FIG. 24
is a diagram showing the structure of the MTJ memory cell disposed on the semiconductor substrate.
Referring to
FIG. 24
, the access transistor ATR is formed in the p-type area PAR on a semiconductor main substrate SUB. The access transistor ATR has source/drain areas
110
and
120
as n-type areas and a gate
130
. The source/drain area
110
is coupled to the ground voltage Vss via a metal line formed in a first metal wiring layer M
1
. As the write word line WWL, a metal wiring formed in a second metal wiring layer M
2
is used. The bit line BL is formed in a third metal wiring layer M
3
.
The magnetic tunnel junction MTJ is disposed between the second metal wiring layer M
2
in which the write word line WWL is provided and the third metal wiring layer M
3
in which the bit line BL is provided. The source/drain area
120
of the access transistor ATR is electrically coupled to the magnetic tunnel junction MTJ via a metal film
150
formed in a contact hole, the first and second metal wiring layers M
1
and M
2
, and a barrier metal
140
. The barrier metal
140
is a buffer material provided to electrically couple the magnetic tunnel junction MTJ and a metal line.
As already described, in the MTJ memory cell, the read word line RWL is provided as a line in
Mai Son
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
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