Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2003-10-01
2004-09-21
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S421000, C257S422000, C257S428000, C257S775000, C257S390000, C365S171000, C365S173000
Reexamination Certificate
active
06794697
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetic memory devices and in particular to ultra-high density asymmetrical magnetic random access memory (commonly referred to as “MRAM”).
BACKGROUND OF THE INVENTION
Today's computer systems are becoming increasingly sophisticated, permitting users to perform an ever greater variety of computing tasks at faster and faster rates. The size of the memory and the speed at which it can be accessed bear heavily upon the overall speed of the computer system.
Generally, the principle underlying the storage of data in a magnetic media (main or mass storage) is the ability to change, and/or reverse, the relative orientation of the magnetization of a storage data bit (i.e. the logic state of a “0” or a “1”). The coercivity of a material is the level of demagnetizing force that must be applied to a magnetic particle to reduce and/or reverse the magnetization of the particle. Generally speaking, the smaller the magnetic particle the higher it's coercivity.
A prior art magnetic memory cell may be a tunneling magneto-resistance memory cell (TMR), a giant magneto-resistance memory cell (GMR), or a colossal magneto-resistance memory cell (CMR). These types of magnetic memory are commonly referred to as magnetic tunnel junction memory (MTJ). As shown in prior art
FIGS. 1A and 1B
a magnetic tunnel junction memory
100
generally includes a data layer
101
(also called a storage layer or bit layer), a reference layer
103
, and an intermediate layer
105
between the data layer
101
and the reference layer
103
. The data layer
101
, the reference layer
103
, and the intermediate layer
105
can be made from one or more layers of material.
The data layer
101
is usually a layer of magnetic material that stores a bit of data as an orientation of magnetization M
2
that may be altered in response to the application of an external magnetic field or fields. More specifically, the orientation of magnetization M
2
of the data layer
101
representing the logic state can be rotated (switched) from a first orientation representing a logic state of “0” to a second orientation, representing a logic state of “1”, and/or vice versa.
The reference layer
103
is usually a layer of magnetic material in which an orientation of magnetization M
1
is “pinned”, as in fixed, in a predetermined direction. Often several layers of magnetic material are required and function as one to effectuate a stable pinned reference layer
103
. The direction is predetermined and established by microelectronic processing steps employed in the fabrication of the magnetic memory cell.
The data layer
101
and reference layer
103
may be thought of as stacked bar magnets, each long on the X axis
107
and short on the Y axis
109
. The magnetization of each layer has a strong preference to align along the easy axis, generally the long X axis
107
. The short Y axis
109
is the hard axis. As with traditional bar magnets, the data layer and reference layer each have magnetic poles, one at either end of the easy axis.
The lines of magnetic force that surround a bar magnet are three-dimensional and flow from the North to the South pole.
FIG. 2A
is a simplified side view illustration of a typical bar magnet
200
, it's magnetic orientation M and the surrounding magnetic field also referred to as lines of force (represented by arrows
201
). As is shown in
FIGS. 2B and 2C
, generally, like poles repel and unlike poles attract. When opposite poles of two bar magnets (
203
and
203
′) are brought together, the lines of force
201
join up and pull the magnets together as in FIG.
2
B. When like poles of two bar magnets (
205
and
205
′) are brought together, the lines of force
201
push away from each other and the magnets repel each other as in FIG.
2
C.
These forces are most pronounced at either pole. As a result when two bar magnets of substantially equal length are evenly stacked lengthwise, both poles either simultaneously attract or simultaneously repel as they are directly proximity to one another.
Typically, the logic state (a “0” or a “1”) of a magnetic memory cell depends on the relative orientations of magnetization in the data layer
101
and the reference layer
103
. For example, when an electrical potential bias is applied across the data layer
101
and the reference layer
103
in a MTJ
100
, electrons migrate between the data layer
101
and the reference layer
103
through the intermediate layer
105
. The intermediate layer
105
is typically a thin dielectric layer commonly referred to as a tunnel barrier layer. The phenomena that cause the migration of electrons through the barrier layer may be referred to as quantum mechanical tunneling or spin tunneling.
Continuing with the model of an elemental bar magnets, the magnetization of the data layer
101
is free to rotate, but with a strong preference to align in either direction along the easy axis
107
of the data layer
101
. The reference layer
103
likewise is aligned along the easy axis
107
of the reference layer
103
, but is pinned in a fixed alignment. The logic state may be determined by measuring the resistance of the memory cell. For example, if the overall orientation of the magnetization in the data layer
101
is parallel to the pinned orientation of magnetization in the reference layer
103
the magnetic memory cell will be in a state of low resistance. If the overall orientation of the magnetization in the data layer
101
is anti-parallel (opposite) to the pinned orientation of magnetization in the reference layer
103
the magnetic memory cell will be in a state of high resistance.
As the data layer
101
and reference layer
103
are substantially equal in length, and as the physical ends of the data layer
101
and reference layer
103
are symmetrically aligned, the poles of each layer are also proximate to one another. When the magnetic fields M
1
and M
2
are anti-parallel, as in
FIG. 1A
, there exists a strong magnetic attraction between both ends as illustrated by joined field lines
111
and
113
. When the magnetic fields M
1
and M
2
are parallel, as in
FIG. 1B
, the magnetic fields emanating from the poles repel one another, as illustrated by field lines
115
and
117
. As the poles are pre-disposed to attract, there is a strong desire for both poles of the data layer
101
to rotate away from their matching pole in the reference layer
103
, as represented by arrows
119
. This symmetric set of forces operating upon both ends of the data layer
101
and reference layer
103
at substantially the same time may be described simplistically as “two-end involvement.”
Storing a binary one or zero in the data layer
101
may require the orientation of the data layer
101
to be rotated, an event that may force the like poles to align, a condition they will fight, or permit opposite poles to align, a condition they desire. In either case, both poles of the data layer
101
and the reference layer
103
are involved and must be coerced to accept the new orientation. While the attraction between the poles reduces the required field to shift the orientation into anti-parallel, the repulsion at both ends requires a greater field to shift the orientation into parallel.
In an ideal setting the orientation of the alterable magnetic field in the data layer
101
would be either parallel or anti-parallel with respect to the field of the reference layer
103
. As the data layer
101
and the reference layer
103
are generally both made from ferromagnetic materials and are positioned in close permanent proximity to each other, the generally stronger reference layer
103
may affect the orientation of the data layer
101
. More specifically, the magnetization of the reference layer
103
may generate a demagnetization field that extends from the reference layer
103
into the data layer
101
.
The result of this demagnetization field from the reference layer
103
is an offset in the coercive switching field. This offset can result in asymmetry
Hewlett--Packard Development Company, L.P.
Nelms David
Tran Mai-Huong
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