Static information storage and retrieval – Addressing – Particular decoder or driver circuit
Reexamination Certificate
2002-12-31
2003-09-09
Tran, Andrew Q. (Department: 2824)
Static information storage and retrieval
Addressing
Particular decoder or driver circuit
C365S230060, C365S230030, C365S173000, C365S171000
Reexamination Certificate
active
06618317
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 memory cells having magnetic tunnel junctions (MTJs).
2. Description of the Background Art
As a memory capable of storing nonvolatile data with low power consumption, attention is being paid to an MRAM Magnetic Random Access Memory) device. The MRAM device is a memory which stores nonvolatile data using a plurality of thin film magnetic materials formed on a semiconductor integrated circuit and which can randomly access the respective thin film magnetic materials.
Recently, in particular, it has been made public that the performance of an MRAM device dramatically advances by employing tunnel magneto-resistive elements which are thin film magnetic materials using magnetic tunnel junctions in memory cells. The MRAM device which includes memory cells each having a magnetic tunnel junction is disclosed by 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, Feb. 2000., “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical papers, TA7.3, Feb. 2000., and “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC Digest of Technical Papers, TA7.6, Feb. 2001.
FIG. 30
is a schematic diagram showing the configuration of a memory cell having a magnetic tunnel junction (which will be also referred to simply as “MTJ memory cell” hereinafter).
Referring to
FIG. 31
, each MTJ memory cell includes a tunnel magneto-resistive element TMR the electric resistance of which changes according to stored data level, and an access element ATR for forming the path of a sense current Is which passes through tunnel magneto-resistive element TMR during data read. In the following description, access element ATR will be also referred to as “access transistor ATR”. Access transistor ATR is connected in series to tunnel magneto-resistive element TMR.
For each MTJ memory cell, a digit line DL for instructing data write, a word line WL for executing data read and a bit line BL which is a data line for transmitting an electrical signal corresponding to the data level of stored data during the data read and data write are arranged.
FIG. 31
is a conceptual view for describing a data read operation for reading data from a MTJ memory cell.
Referring to
FIG. 30
, tunnel magneto-resistive element TMR includes a ferromagnetic material layer FL which has a fixed, constant magnetization direction (which will be also referred to simply as “fixed magnetic layer” hereinafter) and a ferromagnetic material VL magnetized in a direction according to a magnetic field applied from externally (which will be also referred to simply as “free magnetization layer” hereinafter). A tunnel barrier (tunnel film) TB formed out of an insulating film is provided between fixed magnetization layer FL and free magnetization layer VL. Free magnetization layer VL is magnetized in the same direction or the opposite direction to that of fixed magnetization layer FL in accordance with the level of stored data to be written. Fixed magnetization layer FL, tunnel barrier TB and free magnetization layer VL form a magnetic tunnel junction.
During data read, access transistor ATR is turned on in response to the activation of word line WL and tunnel magneto-resistive element TMR is connected between bit line BL and a ground voltage GND. As a result, a bias voltage in accordance with a bit line voltage is applied to the both ends of tunnel magneto-resistive element TMR and a tunnel current flows in tunnel film (tunnel barrier) TB. By using such a tunnel current, it is possible to carry a sense current to the current path of bit line BL—tunnel magneto-resistive element TMR—access transistor ATR—ground voltage GND.
The electrical resistance of tunnel magneto-resistive element TMR changes according to the relative relationship between the magnetization direction of fixed magnetization layer FL and that of free magnetization layer VL. Specifically, if the magnetization direction of fixed magnetization layer FL is parallel to that of free magnetization layer VL, the electrical resistance value of tunnel magneto-resistive element TMR is a minimum value Rmin. If these magnetization directions are opposite (non-parallel) to each other, the electrical resistance value of tunnel magneto-resistive element TMR is a maximum value Rmax.
Accordingly, if free magnetization layer VL is magnetized in a direction according to stored data, a voltage change which occurs to tunnel magneto-resistive element TMR due to sense current differs according to the level of the stored data. Therefore, if sense current Is is carried to tunnel magneto-resistive element TMR after precharging bit line BL with, for example, a constant voltage, the stored data of the MTJ memory cell can be read by sensing the voltage of bit line BL.
FIG. 32
is a conceptual view for describing a data write operation for writing data to the MTJ memory cell.
Referring to
FIG. 32
, during data write, word line WL is deactivated and access transistor ATR is turned off. In this state, a data write current for magnetizing free magnetization layer VL in a direction according to the write data, is carried to each of digit line DL and bit line BL.
FIG. 33
is a conceptual view for describing the relationship between the data write current and the magnetization direction of tunnel magneto-resistive element TMR during data write.
Referring to
FIG. 33
, the horizontal axis H(EA) indicates a magnetic field applied in an easy axis (EA: Easy Axis) direction in free magnetization layer VL in tunnel magneto-resistive element TMR. The vertical axis H(HA) indicates a magnetic field applied in a hard axis (HA: Hard Axis) direction in free magnetization layer VL. Magnetic fields H(EA) and H(HA) correspond to two magnetic fields generated by currents carried to bit line BL and digit line DL, respectively.
In the MTJ memory cell, the fixed magnetization direction of fixed magnetization layer FL is along the easy axis of free magnetization layer VL. Free magnetization layer VL is magnetized in a direction parallel or non-parallel (opposite) to fixed magnetization layer FL along the easy axis direction in accordance with the level of stored data (“1” or “0”). The MTJ memory cell can store 1-bit data (“1” and “0”) corresponding to the two magnetization directions of free magnetization layer VL, respectively.
The magnetization direction of free magnetization layer VL can be rewritten only if the sum of magnetic fields H(EA) and H(HA) applied to free magnetization layer VL reaches a region outside of an asteroid characteristic line shown in FIG.
33
. In other words, if the data write magnetic field applied to free magnetization layer VL has an intensity corresponding to the region inside of the asteroid characteristic line, the magnetization direction of free magnetization layer VL has no change.
As shown in the asteroid characteristic line, if a magnetic field in the hard axis direction is applied to free magnetization layer VL, it is possible to decrease a magnetization threshold necessary to change the magnetization direction of free magnetization layer VL along the easy axis.
If operation points during data write are designed as shown in the example of
FIG. 33
, the data write magnetic field in the easy axis direction is designed so as to have an intensity of H
WR
in the MTJ memory cell to which the data is to be written. That is, the value of the data write current carried to each of bit line BL and digit line DL is designed so as to obtain this data write magnetic field H
WR
. Generally, data write magnetic field H
WR
is expressed by the sum of a switching magnetic field H
SW
necessary to change over a magnetization direction and a margin &Dgr;H, i.e., H
WR
=H
SR
+&Dgr;H.
To rewrite the stored data of the MTJ memory cell, i.e., to rewrite the magnetization direction o
Ooishi Tsukasa
Tsuji Takaharu
McDermott & Will & Emery
Tran Andrew Q.
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