Thin film magnetic memory device including memory cells...

Details Thin film magnetic memory device including memory cells... Thin film magnetic memory device including memory cells...

2002-06-21

2004-08-24

Mai, Son (Department: 2818)

Static information storage and retrieval

Systems using particular element

Magnetic thin film

C365S158000, C365S171000

Reexamination Certificate

active

06781874

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally 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).
2. 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 capable of non-volatile data storage using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and also capable of random access to each thin film magnetic element.
In particular, recent announcement shows that the use of thin film magnetic elements having a magnetic tunnel junction (MTJ) as memory cells significantly improves performance of the MRAM device. 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. 48
is a conceptual diagram illustrating the structure of a memory cell having a magnetic tunnel junction (hereinafter, sometimes simply referred to as “MTJ memory cell”) and the data read operation.
Referring to
FIG. 48
, the MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance varying according to the storage data level, and an access transistor ATR for forming a path of a sense current flowing through tunneling magneto-resistance element TMR in the data read operation. For example, access transistor ATR is a field effect transistor, and is coupled between tunneling magneto-resistance element TMR and a ground voltage VSS.
Tunneling magneto-resistance element TMR has a ferromagnetic material layer FL having a fixed magnetization direction (hereinafter, sometimes simply referred to as “fixed magnetic layer”), and a ferromagnetic material layer VL that is magnetized in the direction corresponding to an external magnetic field (hereinafter, sometimes simply referred to as “free magnetic layer”). A tunneling barrier TB of an insulator film is interposed between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized either in the same (parallel) direction as, or in the opposite (antiparallel) direction to, that of fixed magnetic layer FL according to the storage data level.
For the MTJ memory cell are provided a write word line WWL for data write operation, a read word line RWL for 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 data read and write operations.
In data read operation, access transistor ATR is turned ON in response to activation of read word line RWL. This allows a sense current Is to flow through a current path formed by bit line BL, tunneling magneto-resistance element TMR, access transistor ATR and ground voltage VSS.
The electric resistance value of tunneling magneto-resistance element TMR varies according to the relation between the respective magnetization directions of fixed magnetic layer FL and free magnetic layer VL. More specifically, when fixed magnetic layer FL and free magnetic layer VL have the same (parallel) magnetization direction, tunneling magneto-resistance element TMR has a smaller electric resistance value than when they have opposite (antiparallel) magnetization directions. Hereinafter, the electric resistance values of the tunneling magneto-resistance element corresponding to the storage data levels “1”, “0” are respectively denoted with R
1
and R
0
, where R
1
>R
0
.
The electric resistance value of the tunneling magneto-resistance element TMR varies according to the magnetization direction. Accordingly, two magnetization directions of free magnetic layer VL in tunneling magneto-resistance element TMR can be stored as two storage data levels (“1”, “0”), respectively. In other words, free magnetic layer VL corresponds to a storage node of the MTJ memory cell.
A voltage change that occurs at tunneling magneto-resistance element TMR in response to sense current Is varies depending on the magnetization direction of free magnetic layer VL, that is, the storage data level. Therefore, sense current Is is supplied to tunneling magneto-resistance element TMR after precharging bit line BL to a prescribed voltage, and the storage data in the MTJ memory cell can be read by sensing a voltage change on bit line BL.
FIG. 49
is a conceptual diagram illustrating data write operation to the MTJ memory cell.
Referring to
FIG. 49
, in data write operation, read word line RWL is inactivated and access transistor ATR is turned OFF. In this state, a data write current is supplied to write word line WWL and bit line BL in order to magnetize free magnetic layer VL in the direction corresponding to the write data. The magnetization direction of free magnetic layer VL is determined by combination of the respective directions of the data write current flowing through write word line WWL and bit line BL.
FIG. 50
is a conceptual diagram illustrating the relation between the direction of the data write current and the magnetization direction in data write operation.
Referring to
FIG. 50
, the abscissa Hx indicates the direction of a data write magnetic field H(BL) generated by the data write current flowing through bit line BL. The ordinate Hy indicates the direction of a data write magnetic field H(WWL) generated by the data write current flowing through write word line WWL.
The magnetization direction of free magnetic layer VL can be rewritten only when the sum of the data write magnetic fields H(BL) and H(WWL) reaches the region outside the asteroid characteristic line shown in the figure. In other words, the magnetization direction of free magnetic layer VL will not change if an applied data write magnetic field corresponds to the region inside the asteroid characteristic line.
In order to rewrite the data stored in tunneling magneto-resistance element TMR by data write operation, a current of at least a prescribed level must be applied to both write word line WWL and bit line BL. Once the magnetization direction, that is, the storage data, is written to tunneling magneto-resistance element TMR, it is held in a non-volatile manner until another data write operation is conducted.
In data read operation as well, sense current Is flows through bit line BL. However, sense current Is is generally about one to two orders smaller than the data write current. Therefore, it is less likely that the storage data in the MTJ memory cell is erroneously rewritten by sense current Is in the data read operation.
With reduction in memory cell size, the MRAM device using such a tunneling magneto-resistance element TMR has the following problems:
The MTJ memory cell stores the data according to the magnetization direction of free magnetic layer VL. Provided that the magnetic layer has a thickness T and a length L in its magnetization direction, the magnetic field strength that must be applied to rewrite the magnetization direction of the free magnetic layer (hereinafter, sometimes referred to as “switching magnetic field strength”) is proportional to T/L. Accordingly, with reduction in memory cell size, the switching magnetic field strength is increased according to the scaling of the size in the in-plane direction.
Moreover, with reduction in memory cell size, magnetic field interference between the fixed magnetic layer and the free magnetic layer is increased inside and outside the MTJ memory cell. As a result, the threshold value of a data write magnetic field required for data write operation (which corresponds to the asteroid characteristic line in
FIG. 50
) varies depending on the write data pattern or becomes a

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