Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2002-03-14
2003-04-22
Phan, Trong (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S171000, C365S173000
Reexamination Certificate
active
06552926
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a magnetic random-access memory (MRAM) having a magnetic substance structure.
An MRAM is a memory device which utilizes a phenomenon that the resistance to an electric current flowing through a magnetic substance structure varies depending on the direction of electron spin (magnetization direction) in the magnetic substance. A tunneling magnetoresistive (TMR) element is an example of an element conventionally used for performing information storage operation. Each cell of a magnetic memory, capable of storing one bit of information, is made of one each TMR element and metal-oxide-semiconductor (MOS) transistor.
FIG. 16
is a cross-sectional diagram showing the construction of a conventional MRAM device. In this Figure, designated by the numeral
100
is a TMR element having a sandwich structure with a thin insulator layer
102
sandwiched between a first magnetic substance layer
101
and a second magnetic substance layer
103
. Designated by the numeral
150
is a semiconductor substrate (hereinafter referred to simply as the substrate) on which an access transistor, which is a MOS transistor, is formed, and designated by the numeral
155
are source/drain regions of the access transistor. Designated by the numeral
160
is a readout word line which serves as a gate electrode of the access transistor and designated by the numeral
165
is its write word line. Designated by the numeral
170
is an electrode section for connecting one of the source/drain regions
155
to the first magnetic substance layer
101
, designated by the numeral
171
is another electrode section of the other source/drain region
155
, designated by the numeral
175
is an interlayer insulator stacked between the individual layers, and designated by the numeral
180
is a bit line. In this structure, the first magnetic substance layer
101
forms a free-spin layer in which the direction of electron spin is unfixed and variable while the second magnetic substance layer
103
forms a fixed-spin layer in which the direction of electron spin is fixed to a specific direction. Since the sandwich structure of the TMR element
100
has a rectangular shape in top view that is elongate in the direction of the bit line
180
, the spin direction in the first magnetic substance layer
101
could easily become parallel to the direction of the bit line
180
(bit line direction). The spin direction in the second magnetic substance layer
103
is fixed to this bit line direction.
Storage (writing) of data into the TMR element
100
of the aforementioned conventional MRAM device is performed by producing flows of electric currents through the bit line
180
and the write word line
165
and determining the spin direction in the first magnetic substance layer
101
which forms a free-spin layer with the aid of magnetic fields generated by the electric currents, as shown in FIG.
17
. Specifically, a binary “1” or a binary “0” is written in the TMR element
100
depending on whether the spin direction in the first magnetic substance layer
101
is the same (parallel) as or opposite (antiparallel) to that in the second magnetic substance layer
103
. This data write operation requires a specific magnetic field strength to perform. In addition, the data write operation is characterized in that it is carried out in one memory cell where the corresponding bit line
180
and the corresponding write word line
165
intersect each other.
On the other hand, the data stored in the TMR element
100
is read by applying a voltage across the first magnetic substance layer
101
and the second magnetic substance layer
103
and a voltage to the readout word line
160
to turn on the access transistor and then measuring an electric current flowing into the access transistor. The amount of this electric current is large when the spin direction in the first magnetic substance layer
101
is the same as that in the second magnetic substance layer
103
, whereas the amount of the electric current is small when the spin direction in the first magnetic substance layer
101
is opposite to that in the second magnetic substance layer
103
. This property is used in the execution of data read operation. Specifically, the data in the TMR element
100
is read by varying the electrical resistance between the first magnetic substance layer
101
and the second magnetic substance layer
103
, turning on the access transistor, and judging the amount of the electric current flowing from the bit line
180
into the access transistor.
In the aforementioned conventional MRAM device, multiple electrically conductive layers including TMR elements are arranged, forming a multilayer structure with the interlayer insulator
175
or other elements placed between them. In addition, the conventional MRAM device requires separately arranged write word lines and readout word lines. The structure of memory cells which is perpendicular to the plane of the substrate
150
is so complex that its manufacturing processes are complicated and it has been difficult to reduce the area of each cell.
SUMMARY OF THE INVENTION
This invention is intended to provide a solution to the aforementioned problems of the prior art. Accordingly, it is an object of the invention to simplify the structure of memory cells and thereby provide a magnetic random-access memory which enables further miniaturization, a higher level of integration and easier production compared to the prior art.
In a first principal form of the invention, a magnetic random-access memory comprises a semiconductor substrate on which a first word line and a bit line intersecting each other are arranged, a tunneling magnetoresistive element formed in an area of intersection of the first word line and the bit line by stacking a first magnetic substance layer whose magnetization direction is variable, a second magnetic substance layer whose magnetization direction is fixed and an insulator layer placed between the first and second magnetic substance layers, and an access transistor which uses as a gate a second word line extending in a direction intersecting the bit line, wherein the bit line is made of ferromagnetic metal whose magnetization direction is fixed to the longitudinal direction of the bit line, and the bit line acts also as the second magnetic substance layer.
This arrangement of the invention is advantageous in that it serves to simplify the structure of the magnetic random-access memory and facilitate its manufacture.
In one aspect of the invention, the magnetization direction of the bit line is fixed to its longitudinal direction only in its portion where the bit line intersects the first word line.
This makes it possible to easily create unidirectional magnetic domains in which the spin direction is aligned in a single direction as well as to improve the reliability of the magnetic random-access memory.
In a second principal form of the invention, a magnetic random-access memory comprises a semiconductor substrate on which a first word line and a bit line intersecting each other are arranged, a tunneling magnetoresistive element formed in an area of intersection of the first word line and the bit line by stacking a first magnetic substance layer whose magnetization direction is variable, a second magnetic substance layer whose magnetization direction is fixed and an insulator layer placed between the first and second magnetic substance layers, and an access transistor which uses as a gate a second word line extending in a direction intersecting the bit line, wherein the tunneling magnetoresistive element is located in an area of intersection of the second word line and the bit line, and the second word line acts also as the first word line.
This arrangement of the invention is advantageous in that it significantly reduces the complexity involved in creating a multilayer interconnection structure, simplifies the structure and manufacturing processes of the magnetic random-access memory, and enables its miniaturizati
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