Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2002-12-11
2004-07-27
Nguyen, Tan T. (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S158000
Reexamination Certificate
active
06768670
ABSTRACT:
BACKGROUND
1. Technical Field
The present invention relates to a writing method for a magnetic random access memory (abbreviated as ‘MRAM’) including a bipolar junction transistor and, in particular, to an improved writing method for an MRAM having a higher speed than the static random access memory (SRAM), integration as high as the dynamic random access memory (DRAM), and a property of a nonvolatile memory such as a flash memory.
2. Description of the Related Art
Most of the semiconductor memory manufacturing companies have developed an MRAM using a ferromagnetic material and consider the MRAM as one of the next generation memory devices.
The MRAM is a memory device for reading and writing information by forming multi-layer ferromagnetic thin films, and sensing current variations according to magnetization directions of the respective thin films. The MRAM has a high operating speed, low power consumption and high integration density, all of which are made possible by the special properties of the magnetic thin film. MRAM devices may perform nonvolatile memory operations that are presently carried out using a flash memory.
The MRAM embodies a memory device using either a giant magneto resistive (abbreviated as ‘GMR’) phenomenon or a spin-polarized magneto-transmission (SPMT) phenomenon generated when the spin influences electron transmission.
MRAM devices based on the GMR phenomenon utilize the phenomenon that resistance vanes remarkably when spin directions are different in two magnetic layers having a non-magnetic layer therebetween, which is how a GMR magnetic memory device is implemented.
MRAM devices based on the SPMT approach utilize the phenomenon that larger current transmission is generated when spin directions are identical in two magnetic layers having an insulating layer therebetween, which is how a magneto-transmission junction memory device is implemented.
However, MRAM research is still in its early stage. Today, MRAM research is mostly concentrated on the formation of multi-layer magnetic thin films and is less focused on research pertaining to unit cell structure and to peripheral sensing circuits.
FIG. 1
is a cross-sectional diagram illustrating a conventional MRAM using a bipolar junction transistor. As shown in
FIG. 1
, the MRAM includes a semiconductor substrate
111
used as a base of the bipolar junction transistor, an emitter
113
a
and a collector
113
b
formed in an active region of the semiconductor substrate
111
according to an implant process and a stacked structure of an MTJ cell
121
and a word line
123
positioned in the active region between the emitter
113
a
and the collector
113
b
, spaced apart from the emitter
113
a
and the collector
113
b
by a predetermined distance. The emitter
113
a
and the collector
113
b
are formed according to an implant process using a mask. The MRAM also includes a bit line
135
connected to the collector
113
b
and a reference voltage line
127
connected to the emitter
113
a
. Here, a gate oxide film (not shown) is not formed below the MTJ cell
121
or word line
123
.
The MTJ cell
121
comprises a stacked structure including a free ferromagnetic layer
115
, a tunnel barrier layer
117
and a pinned ferromagnetic layer
119
. At this time, at least three multiple data recording states including ‘0’ or ‘1’ can be obtained in one cell of the memory device. These states are created by setting up a magnetization direction of the free ferromagnetic layer
115
to be identical to or opposite to a magnetization direction of the pinned ferromagnetic layer
119
or to have a predetermined angle.
The bit line
135
is connected to the collector
113
b
through a connection line
129
and a contact plug
133
.
With reference to
FIG. 1
, a conventional method for fabricating the MRAM will now be described. A mask layer (not shown) for exposing a predetermined region of the emitter and collector is formed in the active region of the semiconductor substrate
111
. The emitter
113
a
and the collector
113
b
are formed by implanting an impurity into the semiconductor substrate
111
, and the mask layer is then removed.
The stacked structure of the pinned ferromagnetic layer
115
, the tunnel barrier layer
117
and the free ferromagnetic layer
119
is formed over the resultant structure to form the MTJ cell.
The MTJ cell
121
of an island type is formed by patterning the stacked structure of the pinned ferromagnetic layer
115
, the tunnel barrier layer
117
and the free ferromagnetic layer
119
according to a photolithography process using an MTJ cell mask (not shown).
A conductive layer for a word line is formed over the resultant structure, and patterned to form the word line
123
according to a photolithography process using a word line mask (not shown). This procedure forms the stacked structure of the MTJ cell
121
and the word line
123
. Here, the word line
123
may comprise a mask insulating film at its upper portion, which results in an improved insulating property.
The stacked structure of the MTJ cell
121
and the word line
123
is formed in the active region between the emitter
113
a
and the collector
113
b
. The MTJ cell
121
and the word line
123
are spaced apart from the emitter
113
a
and the collector
113
b
by a predetermined distance.
Thereafter, a first interlayer insulating film
125
is formed to planarize the top surface of the resultant structure. Here, the first interlayer insulating film
125
is planarized to expose the upper portion of the word line
123
.
The connection line
129
and the reference voltage line
127
are formed to respectively contact the emitter
113
a
and the collector
113
b
through the first interlayer insulating film
125
.
A second interlayer insulating film
131
is formed over the resultant structure, and evenly etched to planarize the top surface thereof.
A bit line contact plug
133
is formed to contact the connection line
129
through the second interlayer insulating film
131
.
Here, the connection line
129
is exposed by etching the second interlayer insulating film
131
according to a photolithography process using a bit line contact mask (not shown). After etching, a conductive layer for a bit line contact plug is deposited to contact the connection line
129
, and evenly etched to expose the second interlayer insulating film
131
, thereby forming the bit line contact plug
133
.
The bit line
135
is formed to contact the bit line contact plug
133
.
Here, the bit line
135
is formed by depositing and patterning a conductive layer for a bit line contacting the bit line contact plug
133
.
The operation of the MRAM will now be described with reference to FIG.
1
.
A data write operation is performed by applying current to the word line
123
and the bit line
135
regardless of the transistor.
When the current is applied to the word line
123
, the current does not flow toward the transistor due to resistance elements of the tunnel barrier layer
117
formed between the pinned ferromagnetic layer
115
and the free ferromagnetic layer
119
in the MTJ cell
121
. Rather, the current flows toward the word line
123
.
The current applied to the bit line
135
does not flow from the collector of the bipolar junction transistor to the base or emitter thereof, but flows through the bit line itself.
An amount and direction of the current in the word line
123
and the bit line
135
crossing each other in a vertical direction or at a predetermined angle, are controlled. The amount and direction of the current sets up the magnetization direction of the free ferromagnetic layer
119
of the MTJ cell
121
in a desired direction, thereby performing the data write operation.
After the data write operation, the magnetization direction of the free ferromagnetic layer
119
of the MTJ cell
121
is set to be identical to or opposite to the magnetization direction of the pinned ferromagnetic layer
115
. Alternatively, the MTJ cell
121
may be set up at a predetermined angle with respect to the magnetization direction.
The MTJ
Jang In Woo
Kang Hee Bok
Kim Chang Shuk
Kyung Christine H.
Hynix / Semiconductor Inc.
Marshall & Gerstein & Borun LLP
Nguyen Tan T.
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