U.S. patent application number 10/354251 was filed with the patent office on 2004-07-29 for memory cell structures for latch memory application.
Invention is credited to Arrott, Anthony, Drewes, Joel, Lu, Yong, Zhu, Theodore.
Application Number | 20040145943 10/354251 |
Document ID | / |
Family ID | 32681642 |
Filed Date | 2004-07-29 |
United States Patent
Application |
20040145943 |
Kind Code |
A1 |
Zhu, Theodore ; et
al. |
July 29, 2004 |
MEMORY CELL STRUCTURES FOR LATCH MEMORY APPLICATION
Abstract
A magneto-resistive memory comprising magneto-resistive memory
cells is disclosed, comprising a pinned magnetic layer and a free
magnetic layer. The two magnetic layers are formed having widened
regions at the ends of the layers. As such, the shape made out by
the magneto-resisitve memory, from a top-view perspective, is wide
at the ends and narrower at the mid-, forming an I shape in one
preferred embodiment. The end portions of the free magnetic layer
are allowed to magnetically couple to the end portions of the
pinned magnetic layer such that magnetic coupling is shifted to
these widened regions and coupling in the mid-portion between the
widened regions is minimized. Thus, the influence of the pinned
magnetic layer on the magnetization orientation of the mid-portion
of the free magnetic layer is substantially eliminated, allowing
for increased predictability in switching behavior and increased
write selectivity of memory cells.
Inventors: |
Zhu, Theodore; (Mission
Viejo, CA) ; Lu, Yong; (Rosemount, MN) ;
Arrott, Anthony; (Washington, DC) ; Drewes, Joel;
(Boise, ID) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
32681642 |
Appl. No.: |
10/354251 |
Filed: |
January 29, 2003 |
Current U.S.
Class: |
365/158 ;
257/E27.005; 257/E43.004; 365/171; 365/173 |
Current CPC
Class: |
G11C 11/16 20130101;
H01L 27/222 20130101; G11C 11/15 20130101; H01L 43/08 20130101 |
Class at
Publication: |
365/158 ;
365/171; 365/173 |
International
Class: |
G11C 011/14; G11C
011/15 |
Claims
We claim:
1. A magneto-resistive memory cell comprising: a free magnetic
layer, comprising: a first widened end-portion at a first end of
the free magnetic layer; and a second widened end-portion at a
second end of the free magnetic layer opposite to the first end of
the free magnetic layer; a pinned magnetic layer, comprising: a
third widened end-portion at a first end of the pinned magnetic
layer; and a fourth widened end-portion at a second end of the
pinned magnetic layer opposite to the first end of the free
magnetic layer; and a non-magnetic interlayer, wherein the
non-magnetic interlayer is between the pinned magnetic layer and
the free magnetic layer and wherein a first minimum magnitude of an
applied magnetic field for switching the magneto-resistive memory
cell from a low resistance state to a high resistance state is
about 80-120 percent of a second minimum magnitude of an applied
magnetic field for switching the magneto-resistive memory cell from
a high resistance state to a low resistance state and wherein a
switching magnitude difference of the magneto-resistive memory cell
is less than another switching magnitude difference of an identical
magneto-resistive memory cell without widened end-portions.
2. The magneto-resistive memory cell of claim 1, wherein the first
magnitude is substantially equal to the second magnitude.
3. The magneto-resistive memory cell of claim 1, wherein the first
widened end-portion is directly above with the third widened
end-portion and wherein the second widened end-portion is directly
above the fourth widened end-portion.
4. The magneto-resistive memory cell of claim 1, wherein a first
length and a first width of the first widened end-portion is
substantially equal to a second length and a second width of the
second widened end-portion.
5. The magneto-resistive memory cell of claim 4, wherein a third
length and a third width of the third widened end-portion is
substantially equal to a fourth length and a fourth width of the
fourth widened end-portion.
6. The magneto-resistive memory cell of claim 5, wherein the first
length and the first width of the first widened end-portion is
substantially equal to the third length and the third width of the
third widened end-portion.
7. The magneto-resistive memory cell of claim 1, wherein the
non-magnetic interlayer comprises a conductor.
8. The magneto-resistive memory cell of claim 7, wherein the
conductor comprises copper.
9. The magneto-resistive memory cell of claim 7, wherein the
non-magnetic interlayer has a thickness of about 19 to 40
.ANG..
10. The magneto-resistive memory cell of claim 9, wherein the
non-magnetic interlayer has a thickness of about 20 to 28
.ANG..
11. The magneto-resistive memory cell of claim 10, wherein the
non-magnetic interlayer has a thickness of about 22 to 24
.ANG..
12. The magneto-resistive memory cell of claim 1, wherein the
non-magnetic interlayer comprises an insulator.
13. The magneto-resistive memory cell of claim 12, wherein the
non-magnetic interlayer has a thickness of about 5 to 30 .ANG..
14. The magneto-resistive memory cell of claim 13, wherein the
non-magnetic interlayer has a thickness of about 8 to 15 .ANG..
15. The magneto-resistive memory cell of claim 14, wherein the
non-magnetic interlayer has a thickness of about 10 to 12
.ANG..
16. The magneto-resistive memory cell of claim 1, wherein a
magnetization orientation of the pinned magnetic layer is pinned by
an adjacent layer.
17. The magneto-resistive memory cell of claim 16, wherein the
adjacent layer comprises an antiferromagnetic material.
18. The magneto-resistive memory cell of claim 16, wherein the
adjacent layer comprises a permanent magnet material.
19. The magneto-resistive memory cell of claim 1, wherein the
pinned magnetic layer comprises a permanent magnet.
20. The magneto-resistive memory cell of claim 1, wherein the
pinned magnetic layer comprises a ferromagnetic material with
coercivity sufficiently high such that its magnetization
orientation remains fixed in the presence of an applied magnetic
field of a magnitude sufficient to switch the magnetization
orientation of the free magnetic layer.
21. A magneto-resistive memory cell comprising: a mid-segment; a
first end-segment abutting the mid-segment; and a second
end-segment abutting the mid-segment on a side opposite to a side
abutted by the first end-portion; wherein a first width and a first
length of the first end-segment and a second width and a second
length of the second end-segment are chosen so that a first minimum
magnitude of an applied magnetic field for switching the
magneto-resistive memory cell to a high resistance state is about
80-120 percent of a second minimum magnitude of an applied magnetic
field for switching the magneto-resistive memory cell to a low
resistance state.
22. The magneto-resistive memory cell of claim 21, wherein a first
switching magnitude difference for the magneto-resistive memory
cell is less than a second switching magnitude difference for an
identical magneto-resistive memory cell having a substantially
rectangular shape.
23. The magneto-resistive memory cell of claim 21, wherein the
first magnitude of an applied magnetic field is about 90-110
percent of the second magnitude of an applied magnetic field.
24. The magneto-resistive memory cell of claim 23, wherein the
first magnitude of an applied magnetic field is substantially equal
to the second magnitude of an applied magnetic field.
25. The magneto-resistive memory cell of claim 21, wherein the
first width is substantially equal to the second width.
26. The magneto-resistive memory cell of claim 24, wherein the
first length is substantially equal to the second length.
27. The magneto-resistive memory cell of claim 21, wherein the
first width is at least about 1.5 times greater than the first
length.
28. The magneto-resistive memory cell of claim 27, wherein the
second width is at least about 1.5 times greater than the second
length.
29. The magneto-resistive memory cell of claim 21, wherein a first
shape of the first end-segment and a second shape of a second
end-segment are substantially rectangular.
30. The shape of claim 29, wherein all sides of the first and
second shapes are substantially straight.
31. The magneto-resistive memory cell of claim 30, wherein the
first end-segment and the second end-segment are substantially
parallel.
32. The magneto-resistive memory cell of claim 31, wherein the
first end-segment and the second end-segment are substantially
perpendicular to the mid-segment.
33. The magneto-resistive memory cell of claim 32, wherein the
mid-segment is substantially centered along the length of both the
first end-segment and the second end-segment.
34. A magneto-resistive memory cell, comprising: a mid-portion
having a first length and a first width, wherein the first width is
measured at the mid-point of the first length and wherein a first
ratio of the first length to the first width is at least about
1.5:1; a first end-portion, wherein the first end-portion abuts the
mid-portion and extends along the first width of the mid-portion
and wherein the first end-portion has a second length and a second
width, wherein a second ratio of the second length to the second
width is at least about 1.5 and wherein a third ratio of the first
width to the second width is at least about 1.5; and a second
end-portion, wherein the second end-portion abuts a side of the
mid-portion opposite to a side abutted by the first end-portion,
wherein the second end-portion has a third width and a third
length, wherein a fourth ratio of the third length to the third
width is at least about 1.5 and wherein a fifth ratio of the first
width to the third width is at least about 1.5.
35. The magneto-resistive memory cell of claim 34, wherein a first
magnitude of an applied magnetic field for switching the memory
cell from a low resistance state to a high resistance state is
about 80-120 percent of a second magnitude of an applied magnetic
field for switching the magneto-resistive memory cell from a high
resistance state to a low resistance state.
36. The magneto-resistive memory cell of claim 35, wherein the
first magnitude is about 90-110 percent of the second
magnitude.
37. The magneto-resistive memory cell of claim 36, wherein the
first magnitude is substantially equal to the second magnitude.
38. The magneto-resistive memory cell of claim 34, wherein the
first end-portion is perpendicular to the mid-portion.
39. The magneto-resistive memory cell of claim 38, wherein the
second end-portion is perpendicular to the mid-portion.
40. The magneto-resistive memory cell of claim 34, wherein the
mid-portion is centered along the second width of the first
end-portion.
41. The magneto-resistive memory cell of claim 40, wherein the
mid-portion is centered along the third width of the second
end-portion.
42. The magneto-resistive memory cell of claim 34, wherein a first
shape of the first end-portion a second shape the second
end-portion are substantially rectangular.
43. The magneto-resistive memory cell of claim 34, wherein the
first ratio at least about 3:1.
44. The magneto-resistive memory cell of claim 43, wherein the
second ratio is at least about 2:1.
45. The magneto-resistive memory cell of claim 44, wherein the
third ratio is at least about 2:1.
46. The magneto-resistive memory cell of claim 45, wherein the
fourth ratio is at least about 2:1.
47. The magneto-resistive memory cell of claim 46, wherein the
fifth ratio is at least about 2:1.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to magneto-resistive memories,
and more particularly, to magneto-resistive memory cell structures
that offer superior selectivity of memory cells.
BACKGROUND OF THE INVENTION
[0002] As computer memory technology advances, magneto-resistive
memory has emerged as a possible replacement for conventional
memory devices. Magneto-resistive memories are non-volatile and
employ the magneto-resistive effect to store memory states. These
memories typically use the magnetization orientation of a layer of
magneto-resistive material to represent and to store a binary
state. For example, magnetization orientation in one direction may
be defined as a logic "0", while magnetization orientation in
another direction may be defined as a logic "1".
[0003] The ability to read stored binary states is a consequence of
the magneto-resistive effect. This effect is characterized by a
change in resistance of multiple layers of magneto-resistive
material, depending on the relative magnetization orientations of
the layers. Thus, a magneto-resistive memory cell typically has two
magnetic layers that may change orientation with respect to one
another. Where the directions of the magnetization vectors point in
the same direction, the layers are said to be in a parallel
orientation and where the magnetization vectors point in opposite
directions, the layers are said to be oriented anti-parallel. In
practice, typically one layer, the "free" or "soft" magnetic layer,
is allowed to change orientation, while the other layer, commonly
called the "fixed," "pinned" or "hard" magnetic layer, has a fixed
magnetization orientation to provide a reference for the
orientation of the free magnetic layer. The magnetization
orientation of the two layers may then be detected by determining
the relative electrical resistance of the memory cell. If the
magnetization orientation of its magnetic layers are substantially
parallel, a memory cell is typically in a low resistance state. In
contrast, if the magnetization orientation of its magnetic layers
is substantially anti-parallel, the memory cell is typically in a
high resistance state. Thus, ideally, in typical magneto-resistive
memories, binary logic states are stored as binary magnetization
orientations in magneto-resistive materials and are read as the
binary resistance states of the magneto-resistive memory cells
containing the magneto-resistive materials.
[0004] Giant magneto-resistive (GMR) and tunneling
magneto-resistive (TMR) memory cells are two common types of memory
cells that take advantage of this resistance behavior. In GMR
cells, the flow of electrons through a conductor situated between a
free magnetic layer and a pinned magnetic layer is made to vary,
depending on the relative magnetization orientations of the
magnetic layers on either side of the conductor. By switching the
magnetization orientation of the free magnetic layer, the electron
flow through the conductor is altered and the effective resistance
of the conductor is changed.
[0005] In TMR cells, an electrical barrier layer, rather than a
conductor, is situated between a free magnetic layer and a pinned
magnetic layer. Electrical charges quantum mechanically tunnel
through the barrier layer. Due to the spin dependent nature of the
tunneling, the extent of electrical charges passing through the
barrier vary with the relative magnetization orientations of the
two magnetic layers on either side of the barrier. Thus, the
measured resistance of the TMR cell may be switched by switching
the magnetization orientation of the free magnetic layer.
[0006] Switching the relative magnetization orientations of the
magneto-resistive materials in the memory cell by applying an
external magnetic field is the method commonly used to write a
logic state to a magneto-resistive memory cell. The magnitude of
the applied magnetic field is typically sufficient to switch the
magnetization orientation of the free magnetic layer, but not large
enough to switch the pinned magnetic layer. Thus, depending on the
desired logic state, an appropriately aligned magnetic field is
applied to switch the magnetization orientation of the free
magnetic layer so that the magneto-resistive memory cell is in
either a high or a low resistance state.
[0007] Magneto-resistive memory cells are typically part of an
array of similar cells and the selection of a particular cell for
writing is usually facilitated by use of a grid of conductors.
Thus, magneto-resistive memory cells are usually located at the
intersections of at least two conductors. A magneto-resistive
memory cell is typically selected for writing by applying
electrical currents to two conductors that intersect at the
selected magneto-resistive memory cell. With current flowing
through it, each conductor generates a magnetic field and,
typically, only the selected magneto-resistive memory cell receives
two magnetic fields, one from each conductor. The current flowing
through both conductors is such that the net magnetic field from
the combination of both these magnetic fields is sufficient to
switch the magnetization orientation of the selected cells. Other
magneto-resistive memory cells in contact with a particular
conductor usually receive only the magnetic field generated by that
particular conductor. Thus, the magnitudes of the magnetic fields
generated by each line are usually chosen to be high enough so that
the combination of both fields switches the logic state of a
selected magneto-resistive memory cell but low enough so that the
other magneto-resistive memory cells subject to only one magnetic
field do not switch.
[0008] In addition to the two conductors for writing, memory arrays
with three conductors connecting magneto-resistive memory cells
have also been developed. The additional conductor may be used
exclusively for sensing the resistance of a particular memory cell,
allowing another conductor to be used exclusively for writing. In
this way, writing and reading operations may be performed
simultaneously, increasing the speed of data access. Furthermore,
the third conductor can also be employed to supply an additional
magnetic field during switching operations.
[0009] Magneto-resistive memory technology continues to mature and
work continues in refining implementation of magneto-resistive
memory cells.
SUMMARY OF THE INVENTION
[0010] The preferred embodiments of the present invention provide
magneto-resistive memory cell structures which minimize disruptions
to the magnetization orientation of the free magnetic layer caused
by interactions with pinned magnetic layer magnetic fields. In a
preferred embodiment, looking at a top-view of the memory cell, a
magneto-resistive memory cell is formed with two widened
end-portions on either side of a thinner mid-portion. The two
widened end-portions may each be of different dimensions and may
abut the mid-portion at different angles, so long as the widened
end-portions maintain a width greater than their length. In another
embodiment, the mid-portion is centered along the width of the two
end portions, forming an I shape with the end portions. In other
embodiments, the mid-portion is not centered along the width of the
two end-portions.
[0011] As a consequence of the presence of the widened
end-portions, the mid-portion is substantially free of magnetic
coupling. Thus, the magnitude of an applied magnetic field used to
switch a magnetization orientation of the free magnetic layer in
one direction is substantially equal to the magnitude of an applied
magnetic field used to switch the magnetization orientation of the
free magnetic layer in a substantially opposite direction.
[0012] Other features and advantages of the present invention will
become apparent from the following detailed description, taken in
conjunction with the accompanying drawings, illustrating by way of
example the teachings of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1a is a schematic side-view of a cross-section of a
magnetic material, with arrows indicating, in simplied form, the
magnetization orientation of that material the magnetic lines of
the demagnetization field created by that material;
[0014] FIG. 1b is a schematic side-view of a cross-section of two
layers of a magnetic material separated by a non-magnetic material,
with arrows indicating, in simplied form, the magnetization
orientation of that material and the magnetic lines of the
demagnetization fields created by those layers of materials;
[0015] FIG. 2 is a schematic top-view of possible magnetization
orientations of three portions of a magneto-resistive memory cell
construed in accordance with preferred embodiments of the present
invention;
[0016] FIG. 3 is a generalized representation of the
magneto-resistive behavior of a conventional magneto-resistive
memory cell;
[0017] FIG. 4 is an isometric schematic view of a magneto-resistive
memory cell construed in accordance with preferred embodiments of
the present invention, with arrows indicating, in simplied form,
possible magnetization orientations of that material and the
magnetic lines of the demagnetization fields created by those
layers of materials;
[0018] FIG. 5 is a representation of the magneto-resistive behavior
of a magneto-resistive memory cell construed in accordance with
preferred embodiments of the present invention;
[0019] FIG. 6 is a schematic top-view of a memory cell in
accordance with one illustrative embodiment of the present
invention;
[0020] FIG. 7 is a schematic top-view of a memory cell in
accordance with another illustrative embodiment of the present
invention;
[0021] FIG. 8 is a schematic side view of a memory cell in
accordance with one illustrative embodiment of the present
invention;
[0022] FIG. 9 is a schematic top view of a magneto-resistive memory
array which incorporates memory cells constructed in accordance
with the present teachings;
[0023] FIG. 10 is a schematic top-view of a memory cell in
accordance with one illustrative embodiment of the present
invention; and
[0024] FIG. 11 is a schematic top-view of a memory cell in
accordance with another illustrative embodiment of the present
invention;.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] To reliably store a state using magneto-resistive memory, it
is desirable to have predictability in switching behavior. As such,
it is desirable to have a magneto-resistive memory cell that will
switch to a high resistance state by application of a magnetic
field of a predictable magnitude and that will also switch to a low
resistance state by application of a magnetic field of a
predictable magnitude in the opposite direction. Moreover,
predictability in switching behavior associated with a particular
applied magnetic field will improve the ability to select a
particular magneto-resistive memory cell in a magneto-resistive
memory cell array. Unfortunately, interactions between the pinned
and free magnetic layers of prior magneto-resistive memory cells
may contribute to unpredictability in switching and, thus,
undermine the ability to selectively switch cells.
[0026] While this invention is not limited by theory, the
interactions between pinned and free magnetic layers may be
conceptualized in terms of thermodynamic principles. It is
generally understood that a given system will tend to assume a
configuration which minimizes the free energy of the system. Thus,
in the case of magneto-resistive memory cells, the lowest energy
arrangement for a given system of magnetic layers is the most
stable the most likely to occur.
[0027] Magneto-resistive memory cells typically have well-defined
edges. Consequently, the magnetic layers comprising these
magneto-resistive memory cells typically also terminate in an
abrupt edge. In one theoretical model, uncompensated magnetic poles
may be thought of as occuring along these abrupt edges.
[0028] In cases of a magnetic material without other magnetic
materials nearby, the uncompensated poles create a so-called B
field, or demagnetization field, which opposes another magnetic
field created by the alignment of the uncompensated poles within
the magnetic material. As such, the orientation of the
demagnetization field is essentially anti-parallel to the alignment
of the uncompensated poles. Oriented this way, the demagnetization
field thus creates the closed magnetic flux loop that necessarily
exists with all magnetic materials. This model of the
demagnetization field is represented in FIG. 1a.
[0029] In other cases where one magnetic layer is in close
proximity with another magnetic layer, the two magnetic layers can
interact to create the closed loop of magnetic flux discussed
above. Each magnetic layer generates a demagnetization field that
tends to force the magnetization orientation of the other layer
into an anti-parallel orientation. The resulting anti-parallel
magnetization orientations allow the two magnetic layers to create
a closed magnetic flux loop, as shown in FIG. 1b. This arrangement
of anti-parallel magnetic layers is thermodynamically favored
because it minimizes the free energy of the system. In cases where
the two magnetic layers are the free and fixed layers of
magneto-resistive cells, it is thought that the tendency of closely
arranged magnetic layers to align in an anti-parallel orientation
causes assymetries in switching behavior, as discussed below.
[0030] As noted above, the orientation of a demagnetization field
is governed by the alignment of the uncompensated poles of the
magnetic material. Also, as noted above, these uncompensated poles
occur at the edges of the magnetic layers. It has been found, in
addition, that the alignment of these uncompensated poles is
related to the shape of the magnetic material. A magnetic layer
with an elongated shape has shape anisotropy; that is, where a
magnetic layer has one axis longer than another axis, magnetic
moments will preferentially align along the long axis. As shown in
FIG. 2, where a material is shaped having multiple long axis, the
magnetic moments of the magnetic material along each long axis will
align along that axis. This is because it is energetically
favorable for these uncompensated poles to be minimized to be as
far as possible from one another. In essence, shape anistropy
caused the uncompensated poles to occur along the short axis of an
elongated shape, resulting in the closed magnetic flux loop
illustrated in FIGS. 1a and 1b.
[0031] Thus, where the free and pinned layers of a
magneto-resistive memory cell are rectangularly shaped (from a
top-and side-view perspective), the uncompensated poles align as
described above and the demagnetization fields of the pinned and
the free magnetic layers align along the long axis of a
magneto-resistive memory cell, as illustrated in FIG. 1b. Moreover,
to minimize the free energy of the system comprising the two
magnetic layers, the free and pinned magnetic layers orient with
anti-parrallel magnetization orientations to create a closed
magnetic flux loop. In such an arrangement, the magnetization
fields may be said to be coupled due to the influence of their
respective demagnetization fields.
[0032] Thus, where such coupling occurs, an applied magnetic field
must overcome the influence of this coupling to switch a free
magnetic layer to the parallel orientation, while the
demagnetization fields will augment a magnetic field applied to
switch the same free magnetic layer to the anti-parallel
orientation. Thus, as represented in FIG. 3, the magnitude of the
applied magnetic field needed to switch the free magnetic layer to
the low resistance state is increased while the magnitude of the
applied magnetic field needed to switch the free magnetic layer to
the high resistance state is decreased. As a consequence, coupling
due to demagnetization fields typically increase the current needed
to write the magneto-resistive memory cell to the low resistance
state and may cause accidental writing to the high resistance
state. In extreme cases, the applied magnetic fields may be
insufficient to overcome the demagnetization fields and the free
magnetic layer may remain in the anti-parallel orientation.
[0033] In less extreme cases, where the coupling is strong, but
where an applied magnetic field is able to switch the magnetization
orientation of the free magnetic layer, the coupling between the
free and fixed magnetic layers may still be sufficient to cause the
magnetization orientation of the free layer to switch back to its
previous orientation. In such cases, depending on the strength of
the coupling, the orientation of the free layer may switch
spontaneously, undermining the ability of the cell to be used as a
non-volatile memory device.
[0034] Similarly, interactions between the magnetic layer of one
magneto-resistive memory cell and the magnetic layers of
neighboring magneto-resistive memory cells may cause
demagnetization of those neighboring memory cells. For example,
demagnetization fields originating from the pinned magnetic layer
of one magneto-resistive memory cell may interact with and alter
the magnetization orientation of the free layer of another memory
cell. Moreover, these demagnetization fields may augment a magnetic
field applied to switch the free magnetic layer to one
magnetization orientation, while resisting switching to the
opposite magnetization orientation. Such interactions are of
particular concern as work continues on decreasing the distance
between memory cells to increase the memory cell density of
magneto-resistive memory cell arrays.
[0035] U.S. Pat. No. 6,172,904 discloses one possible structure for
addressing the problem of unpredictable switching behavior caused
by coupling between the pinned magnetic layer and the free magnetic
layer within a particular memory cell. That patent describes a
structure, illustrated in FIG. 4 of that patent, with a free
magnetic layer 4 between two pinned magnetic layers, 6 and 34, with
anti-parallel magnetization orientations M1 and M4. The pinned
layers 6 and 34 couple to the free magnetic layer 4. Thus, coupling
to one pinned layer 6 forces the magnetization orientation M3 of
the free magnetic layer 4 in one direction. However, because the
structure contains another pinned layer 34 with an opposite
magnetization orientation M4 and a magnetic field of the same
magnitude, the second pinned layer 34 also couples to the free
magnetic layer 4. Each pinned magnetic layer 6 and 34 affects the
free magnetic layer 4 in an equal and opposite way, resulting in no
net tendency to push the magnetization orientation M3 of the free
magnetic layer 4 in any particular direction.
[0036] As discussed earlier, pinned and free magnetic layers tend
to couple along their long axis such that their magnetization
orientations are anti-parallel. The preferred embodiments of the
present invention takes advantage of this effect to increase
predictability in switching behavior. In particular, the
illustrated embodiments utilize two elongated, or widened, sections
that are perpendicular and at opposite ends of a magneto-resistive
memory cell.
[0037] Without being limited by theoretical models, such a
structure may be thought of as accomplishing several purposes.
First, because the magnetization orientation of a magnetic material
will align along the long axis of the shape made out by that
material, three alignments will result: each widened end-portion 44
and 48 will align along its respective long axis and the
mid-portion 46 will align along its own long axis (FIG. 2). Second,
widening the end-portions 44 and 48 shifts the location of
uncompensated poles. Because uncompensated poles tend to occur
along the short axis of an elongated shape, the location of the
uncompensated poles will be shifted from the now widened ends 50 of
the magneto-resistive memory cell to the shorter ends 52 of the
elongated end-portions. Ideally, the demagnetization field
generated by the uncompensated poles will align along the long axis
of the widened end-portionss 44 and 48, rather than the long axis
of the memory cell as a whole. Thus, coupling will occur
principally between a widened end-portion 44 or 48 of a pinned
magnetic layer that is immediately above or below another
corresponding widened end-portion 44 or 48 of a free magnetic layer
(FIG. 4). Consequently, in one embodiment where the
magneto-resistive memory cell is in the general shape of an I, the
coupled widened end-portions 44 and 48 will have magnetic
alignments substantially perpendicular to the magnetization
orientation of the mid-portion 46. As such, the demagnetization
fields do not augment or resist a magnetic field applied for
switching the magnetization orientation of the free layer. Thus,
magnetic coupling and the influence of demagnetization fields is
shifted to the widened end-portions 44 and 48, leaving the
mid-portion 46 of the free layer relatively free to switch
magnetization orientations. It will be appreciated that the
magnetization orientations shown in FIG. 4 are illustrative only;
the magnetization orientations of any of portions 44, 46 or 48,
together or separately, may be reversed from that shown.
[0038] Thus, the preferred embodiments of the present invention
take advantage of the interactions between the elongated ends of
the free and pinned magnetic layers to increase predictability in
switching behavior. As represented in FIG. 5, the interactions
between the two magnetic layers allow for switching behavior in
which the magnitudes of the applied magnetic fields used to switch
a magneto-resistive memory cell from a low resistance state to a
high resistance state, and vice versa, may be made substantially
equal. Because of this magneto-resistive behavior, the magnitudes
of the current used to write a logic "0" or "1" are also
substantially equal, and switching predictability is increased.
[0039] In that case, it may be said that the switching magnitude
difference is close to zero. The switching magnitude difference is
the difference in the magnitudes of the applied magnetic fields
used to switch a memory cell opposite states, i.e., the switching
magnitude difference is equal to the magnitude of the applied
magnetic field used to switch a memory cell from a low resistance
state to the high resistance state minus the magnitude of the
applied magnetic field used to switch a memory cell from a high
resistance state to a low resistance state. Thus, in comparison to
an identical magneto-resistive memory cell without widened
end-portions (i.e., a memory cell that is identical in all aspects
to a memory cell formed according to the present invention, except
that the identical magneto-resistive memory cell does not have
widened end-portions with the dimensions taught by the present
invention), the switching magnitude difference for the
magneto-resistive memory cell of the present invention will be
closer to zero than the switching magnitude difference for the
identical magneto-resistive memory cell.
[0040] In one preferred embodiment, a first magnitude of an applied
magnetic field used to switch a magneto-resistive memory cell from
a low resistance state to a high resistance state is about 80-120
percent of a second magnitude of a second applied magnetic field
used to switch a magneto-resistive memory cell from a high
resistance state to a low resistance state. More preferably the
first magnitude is about 85-115 percent of the second magnitude.
Most preferrably, the first magnitude is better than about 90-110
percent of the second magnitude. The dimensions are most preferably
selected to result in the first and second magnitudes being
substantially equal.
[0041] Reference will now be made to FIGS. 6-11 wherein like
numerals refer to like parts throughout. FIG. 6 is a top view of
one illustrative embodiment of the invention. A magneto-resistive
memory cell 2 forms a shape with three portions: one mid-portion
and two widened end-portions at opposite ends of the mid-portion. A
first widened end-portion 34 has a width 8 and a length 10. A
second widened end-portion 38 has a width 4 and a length 6. A
mid-portion 36 has a width 12 and a length 14. Note that while
magneto-resistive memory cell 2 is described as comprising three
portions, 34, 36, and 38, this division into parts is for ease of
description only, as the memory cell 2 is preferably formed as a
single unit.
[0042] The dimensions of the memory cell 2 are not limited by
theory. Theory does not limit the maximum size of mid-portion 36,
nor the maximum size of widened end-portions 34 and 38. The goal of
increasing memory cell density, however, provides a practical
limitation on the sizes of these memory cells; it will be
appreciated that smaller absolute dimensions allow for higher
memory cell densities and are, thus, generally preferred. As to
theoretical minimums, the end-portions 34 and 38 generally will
couple when the widths 8 and 4 are longer than the lengths 10 and
6, repectively. To account for manufacturing imprecision and other
limitations, however, the widths 8 and 4 are preferably longer than
the lengths 10 and 6, respectively, by some nominal amounts, as
discussed below.
[0043] Thus, in one preferred embodiment of the invention,
mid-portion 36 is centered along the widths 8 and 4 of widened
end-portions 34 and 38. Preferably, mid-portion 36 and end-portions
34 and 38 are rectangular in shape, so that the three portions
generally form the shape of an I. To facilitate magnetic coupling
at the end-portions, the width 8 of widened end-portion 34 is
preferably at least about 1.5 times longer than the length 10 of
the end-portion 34. More preferably, the width 8 of end-portion 34
is at least about 2 times longer than the length 10. Most
preferably, the width 8 of end-portion 34 is at least about 3 times
longer than the length 10 of the end-portion 34. Similarly, the
width 4 of end-portion 38 is preferably at least about 1.5 times
longer than the length 6. More preferably width 4 is at least about
2 times longer than length 6, and most preferably width 4 is at
least about 3 times longer than length 6.
[0044] Preferably, the width 8 is at least about 1.5 times longer
than the width 12 of the mid-portion 36. More preferably the width
8 is at least about 2 times longer than the width 12. Similarly,
the width 4 is at least about 1.5 times longer than the width 12
and more preferably the width 4 is at least about 2 times longer
than the width 12.
[0045] Preferably, the dimensions of the end-portions 34 and 38 are
substantially equal; that is, the width 8 and the length 10 of the
end-portion 34 are substantially equal to the width 4 and the
length 6 of the end-portion 38, respectively.
[0046] To facilitate proper alignment of the magnetizaton oriention
of mid-portion 36 perpendicular to the magnetic alignment of the
widened end-portions 34 and 36, the length 14 is preferably at
least about about 1.5 times longer than the width 12. More
preferably, the length 14 is at least about 2 times longer than the
width 12 and most preferably at least about 3 times longer than the
width 12.
[0047] It will be appreciated that the above-mentioned
relationships between various dimensions of a magneto-resistive
memory cell according to the present invention may be represented
as ratios between two relevant dimensions. For example, in an
embodiment where the width 8 is about 1.5 times longer than the
width 10, it may be said that the ratio of the width 8 to the
length 10 is 1.5:1.
[0048] The skilled artisan will appreciate that while the present
teachings indicate particular relationships between the various
dimensions of the widened end-portions 34 and 36 and the
mid-portion 36, these teachings do not demand that the edges of the
cells be straight. Thus, ellipsoids or other elongated shapes are
also contemplated for widened end-portions 34 and 36.
[0049] In actuality, limitations in the manufacture of
magneto-resistive memory cells may prevent the formation of
perfectly straight edges, as illustrated in FIG. 7. Rather than
forming an I shape, magneto-resistive memory cell 2 may have a
so-called dog bone shape and still conform to the present
teachings. In particular, the end-portion 34 still has a width 8,
measured at its widest expanse, and a length 10, measured from the
end of the shape to where the shape begins to narrow. End-portion
38 similarly has a length 6 and a width 4. Also, mid-portion 36
still has a length 14, measured as the distance between the
termination of length 10 and the termination of length 6, and a
width 12, measured at the midpoint of length 14. It will be
appreciated that the preferred dimensions remain as discussed above
with respect to FIG. 6.
[0050] FIG. 8 is a side view of an illustrative embodiment of the
present invetion. The magneto-resistive memory cell 2 comprises a
free magnetic layer 64 and a pinned magnetic layer 66. The free
magnetic layer 64 has switchable magnetization orientation M3 and
the pinned magnetic layer 66 has a fixed magnetization orientation
M1. The pinned magnetic layer 66 is preferably comprised of a
ferromagnetic material, including cobalt, iron-cobalt, nickel iron,
nickel-iron-cobalt, or similar-material. The magnetization
orientation of the pinned magnetic layer 66 is preferably fixed by
an adjacent layer 60 of antiferromagnetic material, in contact with
and located directly below the pinned magnetic layer 66. The
antiferromagnetic material may be iron-manganese, nickel-manganese,
iridium-manganese, platinum-manganese, or similar material.
[0051] In another embodiment, the magnetization orientation of the
pinned magnetic layer 66 is fixed by an adjacent layer 60
comprising a permanent magnet material.
[0052] In yet another embodiment, the pinned magnetic layer 66 is
comprised of a permanent magnet, the orientation of which may be
fixed by exposure to a large external magnetic field. As such, in
this embodiment, there is no layer 60 since the magneto-resistive
memory cell 2 does not require a layer 60 to fix the orientation of
the pinned magnetic layer 66. In another embodiment, the pinned
magnetic layer 66 comprises a ferromagnetic material with high
coercivity such that, in the presence of applied magnetic fields of
magnitudes in a range sufficient to switch the free magnetic layer,
the magnetic moment of this layer is essentially fixed by its
intrinsic magnetic anisotropy.
[0053] As with the pinned magnetic layer 66, the free magnetic
layer 641 is preferably also comprised of a soft ferromagnetic
material, including, but not limited to, ferromagnetic materials
such as cobalt, iron-cobalt, nickel iron, or
nickel-iron-cobalt.
[0054] The free magnetic layer 64 is separated from the pinned
magnetic layer 66 by a non-magnetic interlayer 62. When the
material of the non-magnetic layer 62 is an insulator, the
magneto-resistive memory cell 2 is a spin dependent tunneling
device in which an electrical charge quantum mechanically tunnels
through the tunnel barrier 62 when a read voltage is applied to the
magneto-resistive memory cell 62. In this embodiment, the
interlayer 62 is preferably comprised of an insulating material
such as aluminum oxide. Preferably the insulating material has a
thickness in the range of about 5 .ANG. to 30 .ANG., more
preferably about 8 .ANG. to 15 .ANG., and most preferably about 10
.ANG. to 12 .ANG..
[0055] In another embodiment, the non-magnetic interlayer 62 may be
comprised of a conductor such as copper to take advantage of the
giant magneto-resistive effect, where a read, or sense, current
flows through the conductor of non-magnetic interlayer 62.
Preferably the conductor has a thickness in the range of about 19
.ANG. to 40 .ANG., more preferably about 20 .ANG. to 28 .ANG., and
most preferably about 22 .ANG. to 24 .ANG..
[0056] The skilled artisan will recognize that because the
preferred embodiments employ coupling at the end-portions of the
free magnetic layer 64 and the pinned magnetic layer 66, the
present invention does not depend upon a particular structure or
arrangement of layers for memory cell 2. Thus, the present
teachings do not layers in addition to the layers 64, 62, 66 and 60
shown in FIG. 8, nor whether above, below or to the sides of the
structure shown in FIG. 8, so long as the above described coupling
of the end-portions 34 and 38 is facilitated. In addition, while
the proximity of each layer relative to other layers should be
maintained, the present teachings do not limit the orientation of
the stack of layers as a whole. For example, the stacks of FIG. 8
may be constructed on a substrate such that they appear upside down
relative to the illustrated figure.
[0057] FIG. 9 is a top view of an illustrative magneto-resistive
memory array 30 that incorporates the present teachings. The
magneto-resistive memory array 30 includes an array of
magneto-resistive memory cells, including the magneto-resistive
memory cell 2 and additional magneto-resistive memory cells 24-28,
each similarly made. Each magneto-resistive memory cell is located
at the intersection of at least two conductors, one each from the
sets of conductors, 16, 18 and 20, 22.
[0058] The sets of conductors allow reading from and writing to the
magneto-resistive memory cells. In the illustrated embodiment,
conductors 16 and 18 are perpendicular to conductors 20 and 22. In
other embodiments, the angle 32 formed by conductors 16 and 18 with
conductors 20 and 22 may vary so long as a magnetic-resistive
memory cell 2 at the intersection of two conductors may be selected
and the resistance state switched by a current flowing through the
two conductors.
[0059] In addition, the present teachings do not depend on and, so,
do not limit the relative sizes of the conductor lines 16-22 or the
relative sizes of the magneto-resistive memory cells 2 and 24-28.
Also, while magneto-resistive memory cells are typically connected
to two conductors, and are illustrated as such, it will be
appreciated that this figure only illustrates that, at a minimum,
two conductors are necessary for writing to the memory cell.
Additional conductors, e.g., an additional, separate conductor
(sense line) to sense the resistance of the memory cell, are not
illustrated but can be added.
[0060] With respect to the magnetic-resistive memory cell 2, the
skilled artisan will appreciate that, while preferred embodiments
of the present invention have certain relationships between width 8
and length 10, width 12 and length 14, width 4 and lengths 6, and
width 12 and width 4, these relationships require no specific
orientation of the widened end-portions 34 and 38 with mid-portion
36. For example, with reference to FIG. 6, in the illustrated
embodiment, the mid-portion 36 is perpendicular to and centered
along the widths of the widened end-portions 34 and 38, which are
equal in their dimensions. As such, the three portions form the
preferred general shape of an I. While an I shape is preferred, in
other embodiments, the widened end-portions 34 and 38 need not be
perpendicular to the mid-portion 38. For example, each end-portion
34 or 38 may form angle 40 and 42 (FIG. 6) with the mid-portion 36
that is obtuse or acute. The mid-portion 36 also may be offset and
not centered along the widths 8 or 4 of widened end-portions 34 or
38, as shown in FIGS. 10 and 11. Thus, in one embodiment, the
magneto-resistive memory cell 2 may be in the general shape of a C,
as judged from a top view and as shown in FIG. 10. Less preferably,
the top view of magneto-resistive memory cell 2 shows a general Z
shape, as shown in FIG. 11. Generally, however, the mid-portion 36
preferably does not extend fully to meet the shorter edges of the
widened end-portions 3438, as shown in FIGS. 10 and 11. For
example, with respect to FIG. 10, the long edge of mid-portion 36
is preferebly not flush with the short edges of end-portions 34 or
38.
[0061] Consequently, although this invention has been described in
terms of a certain preferred embodiment and suggested possible
modifications thereto, other embodiments and modifications that may
suggest themselves or be apparent to those of ordinary skill in the
art are also within the spirit and scope of this invention.
Accordingly, the scope of this invention is intended to be defined
by the claims that follow.
* * * * *