U.S. patent application number 10/094971 was filed with the patent office on 2003-05-15 for asymmetric mram cell and bit design for improving bit yield.
Invention is credited to Deak, James G..
Application Number | 20030090932 10/094971 |
Document ID | / |
Family ID | 26789406 |
Filed Date | 2003-05-15 |
United States Patent
Application |
20030090932 |
Kind Code |
A1 |
Deak, James G. |
May 15, 2003 |
ASYMMETRIC MRAM CELL AND BIT DESIGN FOR IMPROVING BIT YIELD
Abstract
An asymmetric cell and bit design for an MRAM device. The design
is asymmetrical with respect to the easy-axis of the cell and has a
centroid displaced from bit center along the hard-axis of the cell.
This asymmetry is large enough so that manufacturing process
variations do not substantially change the switching fields of the
bits. In addition, the asymmetry causes the ends of the bits to
align in opposite directions in small half-select fields and
parallel to each other at large half-select fields, which increases
the difference in the switching fields between selected and
unselected bits. The combined effect of these two characteristics
results in increased bit yield (relative to similarly sized
symmetric bits) due to a smaller overlap between selected and
unselected bit switching distributions.
Inventors: |
Deak, James G.; (Boise,
ID) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L STREET NW
WASHINGTON
DC
20037-1526
US
|
Family ID: |
26789406 |
Appl. No.: |
10/094971 |
Filed: |
March 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60331421 |
Nov 15, 2001 |
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Current U.S.
Class: |
365/158 |
Current CPC
Class: |
G11C 11/161 20130101;
G11C 11/16 20130101; G11C 11/155 20130101; G11C 11/15 20130101 |
Class at
Publication: |
365/158 |
International
Class: |
G11C 011/14 |
Claims
1. A magnetic memory cell comprising: a first magnetic layer having
a pinned magnetization direction; a non-magnetic layer provided on
said first magnetic layer; and a second magnetic layer provided on
said non-magnetic layer, said second magnetic layer stores a
magnetization that is switchable between a first direction that
aligns with said pinned magnetization direction and a second
direction that opposes said pinned magnetization direction, wherein
said cell comprises a hard-axis and an easy-axis, an intersection
of the hard-axis and easy-axis defining a center of said cell, and
at least said second magnetic layer has a centroid that is
displaced along the hard-axis from the center.
2. The magnetic memory cell of claim 1 wherein said second magnetic
layer is patterned into a lopsided shape with its centroid
displaced along the hard-axis from the center.
3. The magnetic memory cell of claim 1 wherein each of said layers
has an associated centroid that is displaced from the center along
the hard-axis.
4. The magnetic memory cell of claim 1 wherein each of said layers
has an associated centroid and is patterned into a lopsided shape
with its respective centroid displaced along the hard-axis from the
center.
5. The magnetic memory cell of claim 1 wherein said second magnetic
layer comprises first and second end regions and a main region,
said main region being parallel to said easy-axis and stores the
magnetization that is switchable between the first and second
directions, and said first and second ends store magnetic fields
that align in opposite directions with respect to each other when
low half-select fields are applied to the cell.
6. The magnetic memory cell of claim 1 wherein said second magnetic
layer comprises first and second end regions and a main region,
said main region being parallel to said easy-axis and stores the
magnetization that is switchable between the first and second
directions, and said first and second ends store magnetic fields
that align in a same direction with respect to each other when high
half-select fields are applied to the cell.
7. A magnetic memory device comprising: a plurality of magnetic
memory cells organized into an array of rows and columns, each
magnetic memory cell comprising: a first magnetic layer having a
pinned magnetization direction; a non-magnetic layer provided on
said first magnetic layer; and a second magnetic layer provided on
said non-magnetic layer, said second magnetic layer stores a
magnetization that is switchable between a first direction that
aligns with said pinned magnetization direction and a second
direction that opposes said pinned magnetization direction, wherein
said cell comprises a hard-axis and an easy-axis, an intersection
of the hard-axis and easy-axis defining a center of said cell, and
at least said second magnetic layer has a centroid that is
displaced along the hard-axis from the center.
8. The magnetic memory device of claim 7 wherein said second
magnetic layer is patterned into a lopsided shape that is
asymmetrical with respect to the easy-axis and its centroid
displaced along the hard-axis from the center.
9. The magnetic memory device of claim 7 wherein each layer has an
associated centroid that is displaced from the center along the
hard-axis.
10. The magnetic memory device of claim 7 wherein each layer has an
associated centroid, is patterned into a lopsided shape that is
asymmetrical with respect to the easy-axis, and has its respective
centroid displaced along the hard-axis from the center.
11. The magnetic memory device of claim 7 wherein said second
magnetic layer comprises first and second end regions and a main
region, said main region being parallel to said easy-axis and
stores the magnetization that is switchable between the first and
second directions, and said first and second ends store magnetic
fields that align in opposite directions with respect to each other
when low half-select fields are applied to the cell.
12. The magnetic memory device of claim 7 wherein said second
magnetic layer comprises first and second end regions and a main
region, said main region being parallel to said easy-axis and
stores the magnetization that is switchable between the first and
second directions, and said first and second ends store magnetic
fields that align in a same direction with respect to each other
when high half-select fields are applied to the cell.
13. The magnetic memory device of claim 7, wherein said device is a
magnetoresistive random access memory (MRAM) device.
14. A processor system comprising: a processor; and a magnetic
memory device coupled to said processor, said magnetic memory
device comprising a plurality of magnetic memory cells organized
into an array of rows and columns, each magnetic memory cell
comprising: a first magnetic layer having a pinned magnetization
direction; a non-magnetic layer provided on said first magnetic
layer; and a second magnetic layer provided on said non-magnetic
layer, said second magnetic layer stores a magnetization that is
switchable between a first direction that aligns with said pinned
magnetization direction and a second direction that opposes said
pinned magnetization direction, wherein said cell comprises a
hard-axis and an easy-axis, an intersection of the hard-axis and
easy-axis defining a center of said cell, and at least said second
magnetic layer has a centroid that is displaced along the hard-axis
from the center.
15. The system of claim 14 wherein said second magnetic layer has a
lopsided shape that is asymmetrical with respect to the easy-axis
and its centroid displaced along the hard-axis from the center.
16. The system of claim 14 wherein each layer has a centroid that
is displaced along the hard-axis from the center.
17. The system of claim 14 wherein each layer has a centroid and is
patterned into a lopsided shape having its respective centroid
displaced from the center along the hard-axis.
18. The system of claim 14 wherein said second magnetic layer
comprises first and second end regions and a main region, said main
region being parallel to said easy-axis and stores the
magnetization that is switchable between the first and second
directions, and said first and second ends store magnetic fields
that align in opposite directions with respect to each other when
low half-select fields are applied to the cell.
19. The system of claim 14 wherein said second magnetic layer
comprises first and second end regions and a main region, said main
region being parallel to said easy-axis and stores the
magnetization that is switchable between the first and second
directions, and said first and second ends store magnetic fields
that align in a same direction with respect to each other when high
half-select fields are applied to the cell.
20. The system of claim 14 wherein said magnetic memory device is a
magnetoresistive random access memory (MRAM) device.
21. The system of claim 14 wherein said magnetic memory device is
embedded on a same chip as said processor.
22. A method of storing information in a magnetic memory cell, said
method comprises the steps of: providing a memory cell having a
hard-axis and an easy-axis of magnetization, an intersection of the
hard-axis and easy-axis defining a center of said cell, the memory
cell comprising a magnetic field that is aligned with the easy-axis
in a first direction, the memory cell having a centroid that is
displaced along the hard-axis from the center; applying a
half-select magnetic field to the cell; and applying a switching
magnetic field to the cell, said switching magnetic field causing
the magnetic to switch from the first direction to the second
direction.
23. The method of claim 22, wherein a domain wall is formed on the
cell before the magnetic field is switched to the second
direction.
24. The method of claim 22, wherein the memory comprises a magnetic
layer having first and second end regions and a main region, said
method further comprises the step of applying a low half-select
field to the cell to cause a first magnetic field to be stored in
the first end region and a second magnetic field to be stored in
the second end region, wherein the first and second magnetic fields
are aligned in opposite directions with respect to each other.
25. The method of claim 22, wherein the memory comprises a magnetic
layer having first and second end regions and a main region, said
method further comprises the step of applying a high half-select
field to the cell to cause a first magnetic field to be stored in
the first end region and a second magnetic field to be stored in
the second end region, wherein the first and second magnetic fields
are aligned in a same direction with respect to each other.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/331,421, filed on Nov. 15, 2001, the
entire contents of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to memory devices
and, more particularly, to an asymmetric cell and bit design for
improving the bit yield of a magnetoresistive random access memory
(MRAM) device.
BACKGROUND OF THE INVENTION
[0003] Integrated circuit designers have always sought the ideal
semiconductor memory: a device that is randomly accessible, can be
written or read very quickly, is non-volatile, but indefinitely
alterable, and consumes little power. MRAM technology has been
increasingly viewed as offering all these advantages.
[0004] An MRAM device typically includes an array of magnetic
memory cells. A typical magnetic memory cell has a structure which
includes magnetic layers separated by a non-magnetic layer.
Magnetic vectors in one magnetic layer, typically referred to as
the pinned layer, are magnetically fixed or pinned in one
direction. The magnetic vectors of the other magnetic layer, often
referred to as the storage or sense layer, are not fixed so that
its magnetization direction is free to switch between "parallel"
and "anti-parallel" states relative to the pinned layer. In
response to the parallel state, the magnetic memory cell will have
a low resistance state. Conversely and in response to the
anti-parallel state, the magnetic memory cell will have a high
resistance state. The MRAM device associates these two resistance
states with either a logical "1" or a "0" bit value.
[0005] A logical "1" or "0" is usually written into the magnetic
memory cell by applying external magnetic fields (via an electrical
current) that rotate the magnetization direction in the storage
layer. Typically, the orientation of magnetization in the storage
layer aligns along an axis known as the easy-axis. The external
magnetic fields are applied to flip the orientation of
magnetization in the storage layer along its easy-axis to either
the parallel or anti-parallel orientation with respect to the
orientation of magnetization in the pinned layer depending on the
desired logic state.
[0006] MRAM devices usually include an array of row lines and
column lines that are used to apply the external magnetic fields to
the magnetic memory cells during writing. The magnetic memory cells
are usually located at intersections of the row lines and column
lines. A selected magnetic memory cell is usually written by
applying electrical currents to the particular row and column lines
that intersect at the selected magnetic memory cell.
[0007] FIG. 1 illustrates a portion of an array 10 of magnetic
memory cells 11 found in the typical MRAM device. The cells 11 are
arranged into rows and columns with each row having an associated
row line 12 and each column having an associated column line 14. In
addition, the cells 11 are arranged with their long axis extending
parallel to the row lines 12 and their transverse axis extending
parallel to the column lines 14. Referring to FIGS. 1 and 2a, each
cell 11 has an easy-axis 19 of magnetization directed parallel with
the long axis (length) of the cell and a hard-axis 20 of
magnetization directed parallel with the short axis (width) of the
cell. Each cell 11 has a column line 14 that generates an easy-axis
magnetic field when current is applied through it and a row line 12
that generates a hard-axis magnetic field when current is applied
through it. The manner in which currents generate magnetic fields
in magnetic memory devices is well known in the art and is not
discussed herein.
[0008] The magnetic field aligned to the easy-axis is referred to
herein as the easy-axis write field while the other field is
referred to as the hard-axis write field. It is desired that only
the selected magnetic memory cell receives both the easy-axis and
hard-axis write fields. Each write field is commonly referred to as
a half-select field because individually they cannot switch the
contents of cell. In practice, however, the hard-axis write field
is usually referred to as the half-select field, while the
easy-axis write field is referred to as the switching field.
[0009] The bit stored in the selected memory cell is referred to
herein as a "selected bit." All of the remaining memory cells
coupled to the column line or row line, which are not the desired
selected cell are referred to herein as "unselected cells" and
their corresponding bits are "unselected bits." The unselected
cells coupled to the particular column line usually receive only
the easy-axis write field. Similarly, the unselected cells coupled
to the particular row line usually receive only the hard-axis write
field. The magnitudes of the easy-axis and hard-axis write fields
are usually chosen to be high enough so that the stored bit in the
selected magnetic memory cell switches its logic state, but are low
enough so that the stored bits in the unselected memory cells, that
are subject to only one of the write fields, do not switch. An
undesirable switching of a stored bit in an unselected magnetic
memory cell (i.e., one that receives only one of the write fields)
is commonly referred to as half-select switching.
[0010] A serious problem that needs to be overcome in order to
build reliable MRAM devices is the distribution of the switching
fields that occur in the selected and unselected bits. A
distribution of selected or unselected write fields strongly
degrades bit yield. This is due to an overlap in the distribution
of the write currents between the selected and unselected bits. It
has been determined that this problem is attributable in part to
the shape of the memory cells.
[0011] Referring again to FIG. 2a and as discussed above, the
typical memory cell 11 has multiple layers of magnetoresistive
material. For example, the illustrated cell 11 includes a first
magnetic layer 16 and a second magnetic layer 17, which are
separated by a first conducting or insulating spacer layer 18. The
stack of magnetic and non-magnetic layers are often patterned into
symmetrical shape such as an ellipse, rectangle or hexagon. FIG. 2a
illustrates a memory cell 11 with a rectangular shape. In the
illustrated rectangular cell 11, the layers 16 and 17 have a
magnetization vector 21 that is positioned substantially along the
length or easy-axis of the cell 11. The vector 21 is depicted with
an arrowhead at each end to represent the two different
magnetization directions within the cell 11. As discussed above,
the magnetization in one of the layers 16/17 is generally pinned
while the magnetization of the other layer 17/16 is free to rotate
into either of the two positions represented by the vector 21.
[0012] The problem with the shape of the current magnetic memory
cell (i.e., ellipse, rectangle, hexagon) is that they are perfectly
symmetrical. Any slight deviation from the perfectly symmetrical
shape due to, for example, manufacturing process variations can
cause a significant change in the magnetic fields and currents
required to write a bit into the cells increasing the distribution
of write currents within the array. This decreases write margin
(i.e., the difference between the write currents of selected and
unselected bits), which reduces bit yield.
[0013] Accordingly, there is a desire and need for a cell and bit
design that increases the write margin and bit yield in an MRAM
device.
SUMMARY OF THE INVENTION
[0014] The present invention provides a design for memory cells of
an MRAM device that increases the write margin and bit yield of the
MRAM device.
[0015] The above and other features and advantages are achieved by
providing an asymmetric cell and bit design, rather than a
symmetric design, for an MRAM device. The design is asymmetric when
reflected about the easy-axis and has a centroid that is displaced
from the bit center along the hard-axis. This asymmetry is large
enough so that manufacturing process variations do not
substantially change the switching fields of the stored bits. In
addition, the asymmetry causes the ends of the bits to align in
opposite directions in small half-select fields and parallel to
each other at large half-select fields, which increases the
difference in the switching fields between selected and unselected
bits. The combined effect of these two characteristics results in
increased bit yield (relative to similarly sized symmetric cells
and bits) due to a smaller overlap between selected and unselected
bit switching distributions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing and other advantages and features of the
invention will become more apparent from the detailed description
of exemplary embodiments provided below with reference to the
accompanying drawings in which:
[0017] FIG. 1 illustrates a portion of an array of magnetic memory
cells in an MRAM device;
[0018] FIG. 2a illustrates a simplified view of a memory cell in
the array illustrated in FIG. 1;
[0019] FIG. 2b illustrates a bit center and coordinate system for
defining bit symmetry/asymmetry;
[0020] FIGS. 3a-3c illustrate exemplary bit shapes that can be used
in an MRAM device;
[0021] FIGS. 4a-4d illustrate different magnetization patterns for
a bit shape that is asymmetrical and has a centroid displaced from
the bit center along the hard-axis;
[0022] FIGS. 5a-5d illustrate different magnetization patterns for
a bit shape that is rotated with respect to the row line;
[0023] FIGS. 6a-6d illustrate different magnetization patterns for
a symmetrical bit shape;
[0024] 7a-7d illustrate exemplary bit shapes used in an MRAM
device;
[0025] FIG. 8 is a simulation of the switching astroids of the bit
shapes shown in FIGS. 7a-7d;
[0026] FIGS. 9a-9c. illustrate an example reversal mode for an
asymmetric bit shape with a centroid that is displaced from bit
center along the hard-axis with a small hard-axis bias;
[0027] FIGS. 10a-10c. illustrate an example reversal mode for an
asymmetric bit shape with a centroid displaced along the hard-axis
from the bit center with a large hard-axis bias; and
[0028] FIG. 11 illustrates a processor system incorporating an MRAM
memory circuit constructed in accordance with an exemplary
embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0029] In the following detailed description, reference is made to
various specific embodiments in which the invention may be
practiced. These embodiments are described with sufficient detail
to enable those skilled in the art to practice the invention, and
it is to be understood that other embodiments may be employed, and
that structural and electrical changes may be made without
departing from the spirit or scope of the present invention.
[0030] As noted above, a fundamental problem that needs to be
overcome in any MRAM device is the distribution of switching fields
that results in poor bit yield due to an overlap between the write
current distributions of selected and unselected bits. It has been
determined that there are various parameters intrinsic to the MRAM
cell/bit that could affect the selected and unselected bit
distribution overlap. These include, but are not limited to random
variations in bit shape from bit to bit, random variations in
material parameters from bit to bit, the sequence of fields by
which the bit is written, the value of the fields used to write a
bit, and magnetic noise.
[0031] A major contributor to the width of the switching field
distributions relates to the magnetization reversal mode. The
magnetization reversal mode can be defined as the sequence of
magnetization patterns during the magnetization reversal process
(i.e., the switching of the magnetization direction of vector 21
illustrated in FIG. 2a). These magnetization patterns can be
selected by bit shape and by different combinations of the write
fields. The term "bit shape" is used throughout the remainder of
this specification, but it should be appreciated that bit shape
results from the shape of the memory cell storing the bit. Thus,
for example, a reference to a symmetrical bit shape should also be
deemed as referring to a symmetrical memory cell.
[0032] It has been determined that if properly selected, the bit
shape and write fields used to write the bit can be used to
increase the distance between the mean of the selected and
unselected write field distributions (i.e., increase the write
margin) and to decrease the width of the write field distributions
by locking in a preferred reversal mode.
[0033] Bit shape and corresponding symmetry/asymmetry can be
quantified using the bit centroid and rotation with respect to the
coordinate system illustrated in FIG. 2b. The x-axis is parallel to
the row line and the y-axis is parallel to the column line. The
length of the bit is measured along the x-axis and the width is
measured along the y-axis. The coordinate system used for defining
bit symmetry/asymmetry is centered at 22 as shown in FIG. 2b.
[0034] The x and y coordinates of a bit's centroid are defined as
follows:
[0035] (1) <x>=.intg..intg.xdxdy/.intg..intg.dxdy; and
[0036] (2) <y>=.intg..intg.ydxdy/.intg..intg.dxdy.
[0037] The bit rotation is defined as:
[0038] (3)
<.theta.>=.intg..intg..theta.(x,y)dxdy/.intg..intg.dxdy.
[0039] Using these definitions, bit symmetry/asymmetry is defined
as follow:
[0040] (4) <x>=0; <y>=0;
<.theta.>=0symmetric;
[0041] (5) <x>=0; <y>=0;
<.theta.>.noteq.0rotated; and
[0042] (6) <x>=0; <y>.noteq.0;
<.theta.>=0lopsided with respect to the hard-axis (i.e., the
centroid is displaced from the bit's center along the
hard-axis).
[0043] FIG. 3a illustrates a symmetrical ellipse-shaped bit
corresponding to equation (4). FIG. 3b illustrates a rotated
parallelogram-shaped bit corresponding to equation (5). FIG. 3c
illustrates a bit shape that is asymmetrical with a centroid that
is displaced from the bit center along the hard-axis (y) that
corresponds to equation (6). In FIGS. 3a-3c, the x-axis is the
easy-axis while the y-axis is the hard-axis.
[0044] Bit symmetry/asymmetry strongly determines magnetization
patterns, since the magnetization prefers to align parallel to the
edges of the bit. FIGS. 4a-4d illustrate different magnetization
patterns for a bit 30 that is asymmetrical and has a centroid
displaced along the hard-axis from the bit's center (i.e.,
<y>.noteq.0). FIGS. 4a, 4b illustrate the patterns for low
hard-axis write fields. With low hard-axis write fields, the
magnetization patterns in the ends 32, 34 of the asymmetrical bit
30 align in opposite directions (i.e., vector 32 is point up while
vector 34 is pointing down). In FIG. 4a, the magnetization vector
21 is pointing to the right and in FIG. 4b, the vector 21 has been
switched or reversed such that it is now pointing to the left.
FIGS. 4c, 4d illustrate the patterns for high hard-axis write
fields. With high hard-axis write fields, the patterns in the ends
32, 34 of the asymmetrical bit 30 align in the same direction. In
FIG. 4c, the magnetization vector 21 is pointing to the right and
in FIG. 4d, the vector 21 has been switched or reversed such that
it is now pointing to the left. The field required to switch the
direction of the magnetization vector 21 is greater when the
patterns in the ends 32, 34 are aligned in opposite directions than
when they are aligned in the same direction.
[0045] FIGS. 5a-5d illustrate different magnetization patterns for
a bit 35 having a shape that is rotated with respect to the x-axis
(<.theta.>.noteq.0). FIGS. 5a, 5b illustrate the patterns for
low hard-axis write fields. With low hard-axis bias fields, the
magnetization patterns in the ends 36, 36 of the bit 35 align in
the up direction when the magnetization vector 21 is pointing to
the right and in the down direction when the vector 21 has been
reversed such that it is pointing to the left. FIGS. 5c, 5d
illustrate the patterns for high hard-axis bias fields. With high
hard-axis bias fields, the patterns in the ends 36, 38 of the bit
35 align in the same direction and always up. As the hard-axis
magnetic write field increases it becomes easier to magnetize the
vector 21 to the right than to the left.
[0046] FIGS. 6a-6d illustrate different magnetization patterns for
a symmetrical bit 40. FIGS. 6a, 6b illustrate the patterns for low
hard-axis bias fields. With low hard-axis bias fields and perfect
symmetry, the magnetization patterns in the ends 42, 44 of the bit
40 split (point up and down) so that they align parallel to the bit
edge. Slight deviation in bit symmetry, which could result from
process variations, will cause the bit to magnetize as illustrated
in FIGS. 4 and 5. This will cause a large scatter in the write
fields required to reverse the direction of the magnetization
vector 21. FIGS. 6c, 6d illustrate the patterns for high hard-axis
bias fields. With high hard-axis bias fields, the patterns in the
ends 42, 44 of the bit 40 align in the same direction and parallel
to the hard-axis bias field.
[0047] As shown above with respect to FIGS. 3a-6d, bit
symmetry/asymmetry strongly determines magnetization patterns. Four
different bit shapes were simulated to determine their
magnetization patterns. The shapes are illustrated in FIGS. 7a-7d
and are defined as follows:
[0048] (7) ellipse=<x>=0 nm; <y>=0 nm;
<.theta.>=0 degrees;
[0049] (8) asym#2=<x>=0 nm; <y>=-8 nm;
<.theta.>=0 degrees;
[0050] (9) asym#3=<x>=0 nm; <y>=-4 nm;
<.theta.>=0 degrees; and
[0051] (10) asym#4=<x>=0 nm; <y>=0 nm;
<.theta.>=-15 degrees.
[0052] All bits in the simulation are nominally 270 nm long, by 180
nm wide and 4 nm thick ellipses. FIG. 7a illustrates the
symmetrical ellipse 40 corresponding to equation (7). FIG. 7b
illustrates a first asymmetrical bit 30a that has a centroid that
is displaced from the center along the hard-axis (i.e., y-axis) and
corresponds to equation (8). FIG. 7c illustrates a second
asymmetrical bit 30b that has a centroid displaced along its
hard-axis (i.e., y-axis) from the bit center and corresponds to
equation (9). FIG. 7d illustrates a third ellipse that is rotated
with respect to its easy-axis (i.e., x-axis) and corresponds to
equation (10).
[0053] FIG. 8 shows the effects of changes in bit symmetry on the
mean magnetic field required to write a bit. There are four curves
with each curve comprised of two lines on the graph. Each curve
represents the bits illustrated in FIGS. 7a-7d. The curve for the
ellipse corresponds to the first and second lines 50a, 50b, the
curve for asym#2 corresponds to the third and fourth lines 60a,
60b, the curve for asym#3 corresponds to the fifth and sixth lines
70a, 70b and the curve for asym#4 corresponds to the seventh and
eighth lines 80a, 80b.
[0054] The axes for the graph are the hard-axis write field (Hy)
and the easy-axis write fields (Hx). That is, Hy is the half-select
field, and Hx is the switching field. The units for the fields in
the graph are oersted (Oe). The curves represented by lines 50a,
50b, 60a, 60b, 70a, 70b, 80a, 80b are usually referred to as
switching astroids. The memory cell operating points are
represented by Hx, Hy pairs. If the operating point is between the
two lines of the curve (i.e., inside the astroid), a bit cannot be
written into the cell. If, however, the operating point is on the
outer left or right of the two lines of the curve (e.g., outside
the astroid), the bit can be written into the cell.
[0055] The subtle differences in bit shape illustrated in FIGS.
7a-7d cause significant variations in the switching field at fixed
values of the half-select field, as indicated by the different
curves shown in FIG. 8. From FIG. 8, it is apparent that when a bit
has an asymmetry characterized by a centroid displaced along the
y-axis (illustrated by the asym#2 and asym#3 curves), it shows a
stronger drop in Hx with increasing Hy and would thus be preferred
in terms of improving bit yield. Thus, the present invention uses
asymmetric bit shapes that have a centroid displaced from the bit
center along the hard-axis to overcome one of the major problems
with the prior art MRAM cells.
[0056] It should be noted that the present invention also
encompasses the effects of material parameter variation as well as
dimensional variation. Had material parameter variation and/or
dimensional variation been introduced into the above example, the
switching field distribution at fixed values of Hy would be larger
than those shown in FIG. 8. As noted above, increasing the
difference in Hx between the unselected and selected states (Hy=0
and Hy.noteq.0) improves bit yield. The bit shape characterized by
equation (6) would have this effect.
[0057] Two exemplary switching modes are illustrated in FIGS. 9a-9c
and 10a-10c. FIGS. 9a-9c illustrate the switching mode that tends
to occur in asymmetric bits characterized by <y>.noteq.0
(i.e., the centroid is displaced from center along the hard-axis)
and at values of Hy below a threshold value. FIG. 9a illustrates a
first magnetization pattern 100 in which it main magnetization 102
is pointing to the left. An end region 104 has a magnetization
direction pointing down while a second end region 106 has a
magnetization direction pointing up. FIG. 9b illustrates an
intermediate magnetization pattern 110 that consists of a main
magnetization portion 112 with its direction pointing to the left.
The first end region 114 has a magnetization direction pointing
down while a second end region 116 has a magnetization direction
pointing up. FIG. 9c illustrates a magnetization pattern 120 after
the reversal. The pattern 120 comprises a portion 122 with a
magnetization direction pointing to the right, an end region 124
has a magnetization direction pointing up while a second end region
126 has a magnetization direction pointing down. This switching
mode occurs because it is energetically favorable for the ends of
these bits to align in opposite directions and thus, they tend to
form a domain wall (e.g., regions 114, 116) as the Hx is increased
(FIG. 9b).
[0058] FIGS. 10a-10c illustrate the mode that occurs in asymmetric
bits characterized by <y>.noteq.0 (i.e., centroid displaced
from bit center along the hard-axis) but at high bias values of Hy
(i.e., above a certain threshold value). FIG. 10a illustrates a
magnetization pattern 140 in which it main magnetization 142 is
pointing to the left while its end regions 144, 146 have a
magnetization that are pointing down and up, respectively. FIG. 10b
illustrates an intermediate magnetization pattern 150 after both Hx
and Hy have been turned on that consists of a main magnetization
portion 152 with its direction pointing up, and left and right end
regions 154, 156 also pointing up. FIG. 10c illustrates a
magnetization pattern 160 after the reversal and after Hy is set to
zero. The pattern 160 comprises a main magnetization region 162
pointing to the right, and end regions 164, 166 having
magnetization directions pointing up and down, respectively. A
shape such as <y>.noteq.0 asymmetry that exhibits this mode
change with increasing Hy is highly desirable since it increases
write margin due to the difference in switching fields between the
reversal modes illustrated in FIGS. 9 and 10.
[0059] Thus, simulations of the lopsided asymmetrical bits (asym#2
and asym#3 illustrated in FIGS. 7b, 7c) tend to show that it is
possible to design a bit that changes its magnetization reversal
mode upon application of Hy and thus shows an improved write margin
over that of a symmetric bit. Secondly, it is possible to design
the <y>.noteq.0 asymmetry so that it is just large enough to
overwhelm any changes in symmetry that may result due to processing
variation. This would lock in a reversal mode and narrow the
distribution of switching fields by eliminating modes with
different switching astroids. FIG. 8 illustrates that although bits
asym#2 and asym#3 have slightly different shapes, their astroids
nearly identical.
[0060] It should be noted that the cells with the asymmetric bit
shapes such that the centroid is displaced from the bit center
along the hard-axis can be manufactured by any processing method or
technique and that the invention is not to be limited to any such
method. Moreover, the layers (e.g., layers 16-18 illustrated in
FIG. 2a) that make up the cells are not limited to any specific
material or layers of materials. All that is required is the
ability to form cells with the desired shape. The layers 16-18
illustrated in FIG. 2a are suitable to practice the invention as
long as they are patterned into the asymmetrical shape with the
centroid displaced from the bit center along the hard-axis. It
should also be appreciated that only the storage layer 17 needs to
have a shape with its centroid displaced from the bit center along
the hard-axis to practice the invention. That is, if it is
desirable to have a symmetrical pinned layer 16 and non-magnetic
layer 18, then the invention could still be practiced solely by
shaping the storage layer 17 with its centroid displaced from the
bit center along the hard-axis.
[0061] FIG. 11 illustrates a processor system 200 incorporating an
MRAM memory circuit 212 constructed in accordance with an
embodiment of the invention. That is, the MRAM memory circuit 212
comprises an array of magnetic memory cells that are asymmetrical
and have a centroid displaced from bit center along the hard-axis
and thus, have improved bit yield as explained above with respect
to FIGS. 3b, 4a-4d, 8 and 9a-9c. The system 200 may be a computer
system, a process control system or any other system employing a
processor and associated memory.
[0062] The system 200 includes a central processing unit (CPU) 202,
e.g., a microprocessor, that communicates with the memory circuit
212 and an I/O device 208 over a bus 220. It must be noted that the
bus 220 may be a series of buses and bridges commonly used in a
processor system, but for convenience purposes only, the bus 220
has been illustrated as a single bus. A second I/O device 210 is
illustrated, but is not necessary to practice the invention. The
system 200 may also include additional memory devices such as a
read-only memory (ROM) device 214, and peripheral devices such as a
floppy disk drive 204 and a compact disk (CD) ROM drive 206 that
also communicates with the CPU 202 over the bus 220 as is well
known in the art. It should be noted that the memory 212 may be
embedded on the same chip as the CPU 202 if so desired.
[0063] While the invention has been described and illustrated with
reference to exemplary embodiments, many variations can be made and
equivalents substituted without departing from the spirit or scope
of the invention. Accordingly, the invention is not to be
understood as being limited by the foregoing description, but is
only limited by the scope of the appended claims.
[0064] What is claimed as new and desired to be protected by
Letters Patent of the United States is:
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