Semiconductor memory device reading data based on memory...

Details Semiconductor memory device reading data based on memory... Semiconductor memory device reading data based on memory...

2002-06-04

2004-09-14

Nguyen, Tan T. (Department: 2818)

Static information storage and retrieval

Read/write circuit

Testing

C365S189070, C365S189090, C365S210130

Reexamination Certificate

active

06791890

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor memory device, and particularly to a semiconductor memory device having memory cells, of which passing current changes depending on storage data during access.
2. Description of the Background Art
In semiconductor memory devices for executing data storage, various forms have been employed for storing data in memory cells. For example, a semiconductor memory device is configured such that a current passing through each memory cell changes depending on storage data during access. In this semiconductor memory device, storage data is read out from a selected memory cell (i.e., access target) in accordance with results of a comparison between the passing current of the selected memory cell and a preset reference current during the access. As a kind of semiconductor memory device having such memory cells, attention is being given to an MRAM (Magnetic Random Access Memory) device, which can nonvolatilely store data with low power consumption.
Particularly, in recent years, it has been announced that a performance of the MRAM device can be dramatically improved by using the thin film magnetic members, which utilize the MTJs (magnetic tunneling junctions), as memory cells. The MRAM device with memory cells having the magnetic tunneling junctions has been disclosed in technical references 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, “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000.
FIG. 16
conceptually shows a structure of a memory cell, which has a magnetic tunneling junction, and may be merely referred to as an “MTJ memory cell” hereinafter.
Referring to
FIG. 16
, a MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance, which is variable in accordance with a data level of magnetically written storage data, and an access transistor ATR. Access transistor ATR is located between a write bit line WBL and a read bit line RBL, and is connected in series to tunneling magneto-resistance element TMR. Typically, access transistor ATR is formed of a field-effect transistor arranged on a semiconductor substrate.
For the MTJ memory cell, the device includes write bit line WBL and a write digit line WDL for carrying a data write current in different directions during a data write operation, respectively, a word line WL for instructing data reading, and read bit line RBL for receiving a data read current. In the data read operation, tunneling magneto-resistance element TMR is electrically coupled between write bit line WBL carrying a ground voltage GND and read bit line RBL in response to turn-on of access transistor ATR.
FIG. 17
conceptually shows an operation of writing data in the MTJ memory cell.
Referring to
FIG. 17
, tunneling magneto-resistance element TMR has a ferromagnetic material layer, which has a fixed and uniform magnetization direction, and may be merely referred to as a “fixed magnetic layer” hereinafter, and a ferromagnetic material layer VL, which is magnetized in a direction depending on an externally applied magnetic field, and may be merely referred to as a “free magnetic layer” hereinafter. A tunneling barrier (tunneling film) TB formed of an insulator film is disposed between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized in the same direction as fixed magnetic layer FL or in the opposite direction in accordance with the level of the storage data to be written. Fixed magnetic layer FL, tunneling barrier TB and free magnetic layer VL form a magnetic tunneling junction.
Tunneling magneto-resistance element TMR has an electric resistance, which is variable depending on a correlation in magnetization direction between fixed magnetic layer FL and free magnetic layer VL. More specifically, the electric resistance value of tunneling magneto-resistance element TMR takes a minimum value Rmin when the magnetization directions of fixed magnetic layer FL and free magnetic layer VL are same (parallel) to each other. When the magnetization directions of them are opposite (untiparallel) to each other, the above electric resistance value takes a maximum value Rmax.
In the data write operation, word line WL is inactive, and access transistor ATR is off. In this state, the data write currents for magnetizing free magnetic layer VL are supplied to bit line BL and write digit line WDL in directions depending on the level of write data, respectively.
FIG. 18
conceptually shows a relationship between the data write current and the magnetization direction of the tunneling magneto-resistance element in the data write operation.
Referring to
FIG. 18
, an abscissa gives a magnetic field, which is applied along an easy axis (EA) to free magnetic layer VL of tunneling magneto-resistance element TMR. An ordinate H(HA) indicates a magnetic field acting along a hard axis (HA) on free magnetic layer VL. Magnetic fields H(EA) and H(HA) correspond to two magnetic fields produced by currents flowing through bit line BL and write digit line WDL, respectively.
In the MTJ memory cell, the fixed magnetization direction of fixed magnetic layer FL is parallel to the easy axis of free magnetic layer VL, and free magnetic layer VL is magnetized in the magnetization easy direction, and particularly in the same parallel direction, which is the same direction as fixed magnetic layer FL, or in the opposite parallel direction, which is opposite to the above direction, depending on the level (“1” or “0”) of the storage data. The MTJ memory cell can selectively store data (“1” and “0”) of one bit corresponding to the two magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be rewritten only when a sum of applied magnetic fields H(EA) and H(HA) falls within a region outside an asteroid characteristic line shown in FIG.
18
. Therefore, the magnetization direction of free magnetic layer VL does not change when the data write magnetic fields applied thereto have intensities corresponding to a region inside the asteroid characteristic line.
As can be seen from the asteroid characteristic line, the magnetization threshold required for changing the magnetization direction along the easy axis can be lowered by applying the magnetic field in the direction of the hard axis to free magnetic layer VL.
When the operation point in the data write operation is designed, for example, as shown in
FIG. 18
, the data write magnetic field in the MTJ cell selected as a data write target is designed such that the data write magnetic field in the direction of the easy axis has an intensity of H
WR
. Thus, the data write current flowing through bit line BL or write digit line WDL is designed to take a value, which can provide the data write magnetic field of H
WR
. In general, data write magnetic field HWR is represented by a sum of a switching magnetic field HSW required for switching the magnetization direction and a margin &Dgr;H. Thus, it is represented by an expression of H
WR
=H
SW
+&Dgr;H.
For rewriting the storage data of the MTJ memory cell, i.e., the magnetization direction of tunneling magneto-resistance element TMR, it is necessary to pass the data write currents at a predetermined level or higher through write digit line WDL and bit line BL. Thereby, free magnetic layer VL in tunneling magneto-resistance element TMR is magnetized in the same parallel direction as fixed magnetic layer FL or untiparallel direction in accordance with the direction of the data write magnetic field along the easy axis (EA). The magnetization direction, which was once written into tunneling magneto-resistance element TMR, and thus the storage data of MTJ memory cell is held nonvolatilely until next data writing is executed.
FIG. 19
conceptually shows an operation of reading data from the MTJ memory c

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