Static information storage and retrieval – Read/write circuit – Having particular data buffer or latch
Reexamination Certificate
2003-01-30
2004-08-17
Tran, Andrew Q. (Department: 2824)
Static information storage and retrieval
Read/write circuit
Having particular data buffer or latch
C365S220000, C365S173000, C365S158000, C365S230030, C365S230070, C365S210130, C365S209000, C365S230080, C365S233100, C365S163000, C365S225700
Reexamination Certificate
active
06778445
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to nonvolatile memory devices. In particular, the present invention relates to a nonvolatile memory device having memory cells with the electric resistance varied according to the level of storage data written by a data write current.
2. Description of the Background Art
An MRAM (Magnetic Random Access Memory) device is now the focus of attention as a new-generation nonvolatile memory device. The MRAM device uses a plurality of thin-film magnetic elements formed in a semiconductor integrated circuit to store data in nonvolatile manner, with each of the thin-film magnetic elements being randomly accessible.
In recent years, it has been published that memory cells of thin-film magnetic elements with magnetic tunnel junctions are used to achieve dramatic improvements in performance of the MRAM device. The MRAM device including memory cells with magnetic tunnel junctions is disclosed for example in technical papers: “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, “Nonvolatile RAM based on Magnetic Tunnel Junction Elements,” ISSCC Digest of Technical Papers, TA7.3, February 2000 and “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM,” ISSCC Digest of Technical Papers, TA7.6, February 2001.
FIG. 14
schematically shows a structure of a memory cell having a magnetic tunnel junction (this memory cell is hereinafter referred to as “MTJ memory cell”).
Referring to
FIG. 14
, the MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance varying according to the data level of magnetically written storage data and includes an access transistor ATR. The access transistor ATR is connected in series with the tunneling magneto-resistance element TMR between a bit line BL and a source voltage line SL. The access transistor ATR is typically a field-effect transistor formed on a semiconductor substrate.
To the MTJ memory cell, the bit line BL and a write digit line WDL for allowing respective data write currents to flow in different directions respectively in data writing, a word line WL for reading data, and the source voltage line SL for pulling down the tunneling magneto-resistance element TMR to a ground voltage GND in data reading are provided. In data reading, the access transistor ATR is turned on and, in response to this turn-on, the tunneling magneto-resistance element TMR is electrically coupled between the source line SL and the bit line BL.
FIG. 15
conceptually shows an operation of writing data into the MTJ memory cell.
Referring to
FIG. 15
, the tunneling magneto-resistance element TMR includes a ferromagnetic layer FL having a fixed direction of magnetization (hereinafter referred to as “fixed magnetic layer”), and a ferromagnetic layer VL magnetized in a direction according to an externally applied magnetic field (hereinafter referred to as “free magnetic layer”). Between the fixed magnetic layer FL and the free magnetic layer VL, a tunneling barrier (tunneling film) TB formed of an insulating film is provided. According to the level of storage data to be written, the free magnetic layer VL is magnetized in the same direction as or in a different direction from the direction in which the fixed magnetic layer FL is magnetized. The fixed magnetic layer FL, tunneling barrier TB and free magnetic layer VL constitute a magnetic tunnel junction.
The tunneling magneto-resistance element TMR has an electric resistance varying according to a relative relation between respective directions of magnetization of the fixed magnetic layer FL and the free magnetic layer VL. Specifically, the tunneling magneto-resistance element TMR has a minimum electric resistance Rmin when the fixed magnetic layer FL has a magnetization direction which is the same as (in parallel with) that of the free magnetic layer VL and has a maximum electric resistance Rmax when respective magnetization directions of the fixed magnetic layer FL and the free magnetic layer VL are opposite to (in antiparallel with) each other.
In data writing, the word line WL is inactivated to turn off the access transistor ATR. In this state, a data write current for magnetizing the free magnetic layer VL flows through each of the bit line BL and the write digit line WDL in respective directions according to the level of data to be written.
FIG. 16
conceptually shows a relation between the data write current and the magnetization direction of the tunneling magneto-resistance element in data writing.
Referring to
FIG. 16
, the horizontal axis H (EA) represents a magnetic field applied in the direction of an easy axis (EA) in the free magnetic layer VL in the tunneling magneto-resistance element TMR. The vertical axis H (HA) represents a magnetic field acting in the direction of a hard axis (HA) in the free magnetic layer VL. The magnetic field H (EA) and the magnetic field H (HA) correspond respectively to two magnetic fields generated by respective currents flowing through the bit line BL and the write digit line WDL.
In the MTJ memory cell, the fixed magnetization direction of the fixed magnetic layer FL is in parallel with the easy axis of the free magnetic layer VL, and the free magnetic layer VL is magnetized in the direction which is in parallel or antiparallel with (opposite to) the fixed magnetic layer FL in the direction of the easy axis according to the level (“1” or “0”) of the storage data. The MTJ memory cell is capable of storing 1-bit data (“1” or “0”) according to the two magnetization directions of the free magnetic layer VL.
The magnetization direction of the free magnetic layer VL is only rewritable when the sum of the applied magnetic fields H (EA) and H (HA) falls within the region outside the asteroid characteristic line shown in FIG.
16
. In other words, if the intensity of the applied data write magnetic fields corresponds to the region inside the asteroid characteristic line, the magnetization direction of the free magnetic layer VL is not switched.
As indicated by the asteroid characteristic line, a magnetic field in the direction of the hard axis can be applied to the free magnetic layer VL to reduce a magnetization threshold which is necessary for changing the magnetization direction along the easy axis. Suppose that operating points for data writing are designed as shown in FIG.
16
. Then, for the MTJ memory cell into which data is to be written, a data write magnetic field in the direction of the easy axis is designed to have its intensity equal to H
WR
. More specifically, the value of a data write current flowing through the bit line BL or the write digit line WDL is designed to obtain this data write magnetic field H
WR
. In general, the data write magnetic field H
WR
is represented by the sum of a switching magnetic field H
SW
necessary for changing the magnetization direction and a margin &Dgr;H: H
WR
=H
SW
+&Dgr;H.
In order to rewrite storage data of the MTJ memory cell, i.e., switch the magnetization direction of the tunneling magneto-resistance element TMR, a data write current of at least a predetermined level must be flown through both of the write digit line WDL and the bit line BL. Accordingly, the free magnetic layer VL in the tunneling magneto-resistance element TMR is magnetized in the direction in parallel with or opposite to (antiparallel with) the fixed magnetic layer FL according to the direction of a data write magnetic field along the easy axis (EA). The magnetization direction once written into the tunneling magneto-resistance element TMR, i.e., storage data in the MTJ memory cell, is held in nonvolatile manner until execution of writing of new data.
FIG. 17
conceptually shows an operation of reading data from the MTJ memory cell.
Referring to
FIG. 17
, in the data reading operation, the access transistor ATR is turned on in response to activation of the word line WL. Then, the tunneling magneto-resistance element TMR pull
Ooishi Tsukasa
Tanizaki Hiroaki
Renesas Technology Corp.
Tran Andrew Q.
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