U.S. patent application number 11/054735 was filed with the patent office on 2006-08-10 for double-decker mram cells with scissor-state angled reference layer magnetic anisotropy and method for fabricating.
This patent application is currently assigned to Infineon Technologies AG. Invention is credited to Daniel Braun.
Application Number | 20060176734 11/054735 |
Document ID | / |
Family ID | 36779760 |
Filed Date | 2006-08-10 |
United States Patent
Application |
20060176734 |
Kind Code |
A1 |
Braun; Daniel |
August 10, 2006 |
DOUBLE-DECKER MRAM CELLS WITH SCISSOR-STATE ANGLED REFERENCE LAYER
MAGNETIC ANISOTROPY AND METHOD FOR FABRICATING
Abstract
A double-decker MRAM cell is provided, including a stacked
structure of first and second magnetic tunnel junctions. Each
magnetic tunnel junction includes free and fixed magnetic regions
made of magnetic material separated by a tunneling barrier layer
made of non-magnetic material. The fixed magnetic regions are
pinned by at least one pinning layer made of the same
antiferromagnetic material such that in applying an external
magnetic field fixed magnetizations are brought into a scissored
configuration.
Inventors: |
Braun; Daniel; (Paris,
FR) |
Correspondence
Address: |
DICKE, BILLIG & CZAJA, P.L.L.C.
FIFTH STREET TOWERS
100 SOUTH FIFTH STREET, SUITE 2250
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Infineon Technologies AG
Altis Semiconductor
|
Family ID: |
36779760 |
Appl. No.: |
11/054735 |
Filed: |
February 10, 2005 |
Current U.S.
Class: |
365/171 ;
257/E43.006 |
Current CPC
Class: |
H01L 43/12 20130101;
G11C 11/16 20130101 |
Class at
Publication: |
365/171 |
International
Class: |
G11C 11/14 20060101
G11C011/14 |
Claims
1. A method of producing a double-decker magnetoresistive random
access memory cell comprising: providing a stacked structure having
a first magnetic tunnel junction including first and second
magnetic regions made of magnetic material being stacked in a
parallel, overlying relationship separated by a first tunneling
barrier layer made of non-magnetic material; said first magnetic
region of said first magnetic tunnel junction comprising a first
pinned layer having a first fixed magnetization adjacent said first
tunneling barrier layer, said second magnetic region of said first
magnetic tunnel junction exhibiting a first free magnetization
adjacent said first tunneling barrier layer which is free to be
switched between the same and opposite directions with respect to
the first fixed magnetization of said first magnetic region of said
first magnetic tunnel junction; said stacked structure further
having a second magnetic tunnel junction including third and fourth
magnetic regions made of magnetic material being stacked in a
parallel, overlying relationship separated by a second tunneling
barrier layer made of non-magnetic material; said third magnetic
region of said second magnetic tunnel junction comprising a second
pinned layer having a second fixed magnetization adjacent said
second tunneling barrier layer, said fourth magnetic region of said
second magnetic tunnel junction exhibiting a second free
magnetization adjacent said second tunneling barrier layer which is
free to be switched between the same and opposite directions with
respect to the second fixed magnetization of said third magnetic
region of said second magnetic tunnel junction; said free
magnetizations of said first and second magnetic tunnel junctions
being magnetically coupled to magnetic fields generated by first
and second currents made to flow through first and second current
lines, respectively; and pinning said first fixed magnetization
with a first pinning layer made of a first antiferromagnetic
material and pinning said second fixed magnetization with a second
pinning layer made of a second antiferromagnetic material;
antiferromagnetically coupling said first and second pinned layers,
the pinned layers having a small intrinsic anisotropy such that a
further step is enabled.
2. The method of claim 1, further including applying an external
magnetic field such that said first and second fixed magnetizations
are brought into a scissored configuration where the first fixed
magnetization is inclined under a first angle relative to the
external magnetic field and the second fixed magnetization is
inclined under a second angle relative to the external magnetic
field.
3. The method of claim 2, wherein an angle between the first and
second fixed magnetizations is in a range of from 60.degree. to
120.degree..
4. The method of claim 2, further including selecting a magnitude
of said applied external magnetic field such that an absolute value
of a difference of said first and second angles amounts to
approximately 90.degree..
5. The method of claim 2, further including applying said external
magnetic field in the direction of one of said first and second
fixed magnetizations.
6. The method of claim 2, wherein said second and fourth magnetic
regions respectively being comprised of a plurality of N
ferromagnetic free layers which are antiferromagnetically coupled,
where N is an integer greater than or equal to two.
7. The method of claim 6, wherein said second and fourth magnetic
regions respectively being comprised of a tri-layered structure
including two ferromagnetic layers being antiferromagnetically
coupled by an intermediate layer made of antiferromagnetic coupling
material.
8. The method of claim 2, further including applying said external
magnetic field in direction of one of said first and second current
lines.
9. The method of claim 2, further including selecting the same
antiferromagnetic material for the first and second pinning layers
in the first and second magnetic tunnel junctions.
10. The method of claim 2, further including selecting a different
antiferromagnetic material for the first and second pinning layers
in the first and second magnetic tunnel junctions.
11. A method of producing a double-decker magnetoresistive random
access memory cell comprising: providing a stacked structure having
a first magnetic tunnel junction including first and second
magnetic regions made of magnetic material being stacked in a
parallel, overlying relationship separated by a first tunneling
barrier layer made of non-magnetic material; said first magnetic
region of said first magnetic tunnel junction comprising a first
pinned layer having a first fixed magnetization adjacent said first
tunneling barrier layer, said second magnetic region of said first
magnetic tunnel junction exhibiting a first free magnetization
adjacent said first tunneling barrier layer which is free to be
switched between the same and opposite directions with respect to
the first fixed magnetization of said first magnetic region of said
first magnetic tunnel junction; said stacked structure further
having a second magnetic tunnel junction including third and fourth
magnetic regions made of magnetic material being stacked in a
parallel, overlying relationship separated by a second tunneling
barrier layer made of non-magnetic material; said third magnetic
region of said second magnetic tunnel junction comprising a second
pinned layer having a second fixed magnetization adjacent said
second tunneling barrier layer, said fourth magnetic region of said
second magnetic tunnel junction exhibiting a second free
magnetization adjacent said second tunneling barrier layer which is
free to be switched between the same and opposite directions with
respect to the second fixed magnetization of said third magnetic
region of said second magnetic tunnel junction; said free
magnetizations of said first and second magnetic tunnel junctions
being magnetically coupled to magnetic fields generated by first
and second currents made to flow through first and second current
lines, respectively; pinning said first and second fixed
magnetizations by a pinning layer made of antiferromagnetic
material; and said first and second pinned layers being
antiferromagnetically coupled and having a small intrinsic
anisotropy such that a further step is enabled.
12. The method of claim 11, further including applying an external
magnetic field such that said first and second fixed magnetizations
are brought into a scissored configuration where the first fixed
magnetization is inclined under a first angle relative to the
external magnetic field and the second fixed magnetization is
inclined under a second angle relative to the external magnetic
field.
13. The method of claim 8, wherein an angle between the first and
second fixed magnetizations is in a range of from 60.degree. to
120.degree..
14. The method of claim 12, further including selecting a magnitude
of said applied external magnetic field such that an absolute value
of a difference of said first and second angles amounts to
approximately 90.degree..
15. The method of claim 12, further including applying said
external magnetic field in the direction of one of said first and
second fixed magnetizations.
16. The method of claim 12, wherein said second and fourth magnetic
regions respectively being comprised of a plurality of N
ferromagnetic free layers which are antiferromagnetically coupled,
where N is an integer greater than or equal to two.
17. The method of claim 16, wherein said second and fourth magnetic
regions respectively being comprised of a tri-layered structure
including two ferromagnetic layers being antiferromagnetically
coupled by an intermediate layer made of antiferromagnetic coupling
material.
18. The method of claim 12, further including applying said
external magnetic field in direction of one of said first and
second current lines.
19. The method of claim 12, further including selecting the same
antiferromagnetic material for the first and second pinning layers
in the first and second magnetic tunnel junctions.
20. The method of claim 12, further including selecting a different
antiferromagnetic material for the first and second pinning layers
in the first and second magnetic tunnel junctions.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is also related to Utility patent
application Ser. No. ______, filed on even date herewith, entitled
"DOUBLE-DECKER MRAM CELLS WITH ROTATED REFERENCE LAYER
MAGNETIZATIONS," having Docket No. I433.148.101 and is commonly
assigned to the same assignee as the present invention, and which
is herein incorporated by reference.
BACKGROUND
[0002] The present invention pertains to non-volatile semiconductor
magnetoresistive random access memory (MRAM) chips. More
particularly, the invention relates to so-called double-decker MRAM
cells where each one of the cells comprises two stacks of magnetic
tunnel junctions, and a method for making the same.
[0003] MRAM technology is a non-volatile random access memory
technology that may probably replace present random access memories
as the standard memory technology for computing devices. An MRAM
cell (also referred to as a tunneling magnetoresistive or
TMR-device) includes a structure having ferromagnetic layers
respectively exhibiting a resultant magnetic moment vector
separated by a non-magnetic layer (or tunneling barrier) and
arranged into a magnetic tunnel junction (MTJ). In contrast to
present day's non-volatile DRAM memory technology, digital
information is not stored by power but rather is represented in the
MRAM cell as directions of magnetic moment vectors (magnetization)
in the ferromagnetic layers. More specifically, the magnetic moment
vector of one ferromagnetic layer is magnetically fixed (or
pinned), while the magnetic moment vector of the other
ferromagnetic layer is free to be switched between the two
preferred directions in the magnetization easy axis, which
typically is arranged to be aligned with the fixed magnetization of
the reference layer. Hence, a memory state of an MRAM cell is
maintained by the direction of the magnetization of the free layer
with respect to the direction of the magnetization of the reference
layer.
[0004] Depending upon the two different magnetic states of the free
layer, the MRAM cell exhibits two different resistance values in
response to a voltage applied across the magnetic tunneling
junction barrier. Accordingly, the particular resistance of the
TMR-device reflects the magnetization state of the free layer. In
this way, the resistance is low when the magnetization of the free
layer is parallel to the magnetization of the reference layer, and
high when magnetizations are antiparallel. Hence, a detection of
changes in resistance allows to provide information stored in the
MRAM cell.
[0005] In order to switch MRAM cells, magnetic fields that are
coupled to the freely switchable magnetization of the magnetic free
layer are applied, which typically are generated by supplying
currents to current lines, for example, write bit and write word
lines, usually crossing at right angles with an MRAM cell being
positioned in an intermediate position therebetween and at an
intersection thereof.
[0006] Recently, a new concept of MRAM cells ("toggle cells") has
been proposed, wherein the free layer is designed to be a free
magnetic region including a number of ferromagnetic layers that are
antiferromagnetically coupled, where the number of
antiferromagnetically coupled ferromagnetic layers may be
appropriately chosen to increase the effective magnetic switching
volume of the MRAM device. See, for instance, U.S. Pat. No.
6,531,723 B1 to Engel et al., the disclosure of which is
incorporated herein by reference.
[0007] For switching such magnetoresistive memory cells having a
free magnetic region including antiferromagnetically coupled
ferromagnetic layers, another switching mechanism, the so-called
"adiabatic rotational switching", which is well-known to the
skilled persons, is envisaged. The adiabatic rotational switching
mechanism is, for example, disclosed in U.S. Pat. No. 6,545,906 B1
to Savtchenko et al., the disclosure of which is incorporated
herein by reference. More specifically, adiabatic rotational
switching relies on the "spin-flop" phenomenon, which lowers the
total magnetic energy in an applied magnetic field by rotating the
magnetic moment vectors of the magnetic free region ferromagnetic
layers.
[0008] Now reference is made to FIG. 1A, where a typical stability
diagram for an adiabatic rotation switchable MRAM cell is
illustrated, the abscisse of which represents the bit line magnetic
field H.sub.BL, while the ordinate represents the word line
magnetic field H.sub.WL, which respectively arrive at the MRAM cell
for its switching. Using the spin-flop phenomenon in an MRAM cell
having antiferromagnetically coupled magnetic moment vectors
M.sub.1 and M.sub.2 of the free magnetic region ferromagnetic
layers inclined at a 45.degree. angle to the word and bit lines,
respectively, a timed switching pulse sequence of applied magnetic
fields in a typical "toggling write" mode is described as
follows.
[0009] At a time t.sub.0 neither a word line current nor a bit line
current are applied resulting in a zero magnetic field H.sub.0 of
both H.sub.BL and H.sub.WL. At a time t1, the word line current is
increased to result in magnetic field H.sub.1 and magnetic moment
vectors M.sub.1 and M.sub.2 begin to rotate either clockwise or
counter-clockwise, depending on the direction of the word line
current, to align themselves nominally orthogonal to the field
direction. At a time t.sub.2, the bit line current is switched on.
The bit line current is chosen to flow in a certain direction so
that both magnetic moment vectors M.sub.1 and M.sub.2 are further
rotated in the same clockwise or counter-clockwise direction as the
rotation caused by the word line magnetic field. At this time
t.sub.2, both the word and bit line currents are on, resulting in
magnetic field H.sub.2 with magnetic moment vectors M.sub.1 and
M.sub.2 being nominally orthogonal to the net magnetic field
direction, which is 45.degree. with respect to the current
lines.
[0010] At a time t.sub.3, the word line current is switched off,
resulting in magnetic field H.sub.3, so that magnetic moment
vectors M.sub.1 and M.sub.2 are being rotated only by the bit line
magnetic field. At this point of time, magnetic moment vectors
M.sub.1 and M.sub.2 have generally been rotated past their hard
axis instability points. Finally, at a time t.sub.4, the bit line
current is switched off, again resulting in zero magnetic field
H.sub.0, and magnetic moment vectors M.sub.1 and M.sub.2 will align
along the preferred anisotropy axis (easy axis) in a 180.degree.
angle rotated state as compared to the initial state. Accordingly,
with regard to the magnetic moment vector of the reference layer,
the MRAM cell has been switched from its parallel state into its
anti-parallel state, or vice versa, depending on the state
switching ("toggling") starts off with.
[0011] In order to successfully switch the MRAM cell, it is
essential that magnetic field sequence applied thereon results in a
magnetic field path crossing a diagonal line being a straight
connection between a minimum switching field H.sub.SF ("toggling
point" ) for reversal of the free magnetization and another
critical magnetic field value H.sub.SAT ("saturation point"), at
which both magnetic moment vectors M.sub.1 and M.sub.2 of
antiferromagnetically coupled ferromagnetic layers of the free
magnetic region are forced to align with the applied external
magnetic field in a parallel configuration.
[0012] Usually, the first and third quadrant of the
H.sub.BL-H.sub.WL-plane are used for switching the cell.
Apparently, as can be seen from FIG. 1A, no magnetic fields are
applied in the second and fourth quadrant leaving room to operate
another (second) magnetic tunnel junction in the same memory cell,
the reference layer magnetization is rotated by 90 degrees relative
to the first one.
[0013] Reference is now made to FIG. 1B. Assuming a second magnetic
tunnel junction similar to above (first) magnetic tunnel junction
except for a 90.degree. angled reference layer magnetization, a
timed switching pulse sequence of applied magnetic fields in the
second quadrant is typically as follows: a time t.sub.0 neither a
word line current nor a bit line current are applied resulting in a
zero magnetic field H.sub.0 of both H.sub.BL and H.sub.WL. At a
time t1, the word line current being reversed to the previous case
is increased to result in magnetic field H.sub.1 and magnetic
moment vectors M.sub.1 and M.sub.2 of the second MTJ begin to
rotate either clockwise or counter-clockwise, depending on the
direction of the word line current, to align themselves nominally
orthogonal to the field direction. At a time t.sub.2, the bit line
current is switched on. The bit line current is chosen to flow in a
certain direction so that both magnetic moment vectors M.sub.1 and
M.sub.2 are further rotated in the same clockwise or
counter-clockwise direction as the rotation caused by the word line
magnetic field. At this time t.sub.2, both the word and bit line
currents are on, resulting in magnetic field H.sub.2 with magnetic
moment vectors M.sub.1 and M.sub.2 being nominally orthogonal to
the net magnetic field direction, which is 45.degree. with respect
to the current lines. At a time t.sub.3, the word line current is
switched off, resulting in magnetic field H.sub.3, so that magnetic
moment vectors M.sub.1 and M.sub.2 are being rotated only by the
bit line magnetic field. At this point of time, magnetic moment
vectors M.sub.1 and M.sub.2 have generally been rotated past their
hard axis instability points. Finally, at a time t.sub.4, the bit
line current is switched off, again resulting in zero magnetic
field H.sub.0, and magnetic moment vectors M.sub.1 and M.sub.2 will
align along the preferred anisotropy axis (easy axis) in a
180.degree. angle rotated state as compared to the initial state.
Accordingly, with regard to the magnetic moment vector of the
reference layer, the second MTJ of the MRAM cell has been switched
from its parallel state into its anti-parallel state, or vice
versa, depending on the state switching starts off with. As with
the first cell, magnetic field sequence applied on the second MTJ
crosses a diagonal line being a straight connection between a
minimum switching field H.sub.SF for reversal of the free
magnetization and another critical magnetic field value H.sub.SAT,
at which both magnetic moment vectors M.sub.1 and M.sub.2 of
antiferromagnetically coupled ferromagnetic layers of the free
magnetic region are forced to align with the applied external
magnetic field in a parallel configuration.
[0014] As above described, in order to successfully switch two
different MTJs in a single memory cell, it is necessary that
reference layer magnetizations are inclined at an angle of
90.degree.. Such situation is illustrated in FIG. 2, where a
stacked structure 1 of two magnetic tunnel junctions (MTJs) of a
memory cell is positioned in between bit and word lines at an
intersection thereof and having reference layer magnetizations 2, 3
exhibiting a 90.degree. angle in between. (FIG. 2 illustrates
different cases of orientations of the two reference layer
magnetizations, each one having a 90.degree. angle in between.)
[0015] A possible realization of two different MTJs in a single
cell is the so-called "double-decker MRAM cell"-concept having a
stacked structure of two MTJs. Using such a double-decker MRAM cell
allows for storing two bits of information (1.sup.st and 2.sup.nd
bit) in a single memory cell. Thus, half the effective cell size
per MTJ as compared to the convenient case having only one MTJ per
memory cell can be achieved. However, in such double-decker MRAM
cell, reference layer magnetizations have to be inclined in an
angle of 90.degree. in order to selectively switch the MTJs for
which reason the pinning layers for pinning of the reference layers
in the state of the art necessarily have to be made of different
antiferromagnetic materials having sufficiently different setting
(Neel) temperatures. Accordingly, optimizing the antiferromagnetic
materials such that they have as high a difference in setting
temperatures as possible while meeting other requirements like
pinning strength, thermal stability etc. is a big challenge and
often results in a rather dissatisfying trade-off of desired
characteristics. Accordingly, there is a need for the present
invention.
SUMMARY
[0016] One embodiment of the present invention includes a
double-decker MRAM cell with a stacked structure and a method of
fabricating the same. The stacked structure has first and second
magnetic tunnel junctions. Each of the magnetic tunnel junctions
include free and fixed magnetic regions made of magnetic material
separated by a tunneling barrier layer made of non-magnetic
material. The fixed magnetic regions are pinned by at least one
pinning layer made of the same antiferromagnetic material such that
in applying an external magnetic field fixed magnetizations are
brought into a scissored configuration.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The accompanying drawings are included to provide a further
understanding of the present invention and are incorporated in and
constitute a part of this specification. The drawings illustrate
the embodiments of the present invention and together with the
description serve to explain the principles of the invention. Other
embodiments of the present invention and many of the intended
advantages of the present invention will be readily appreciated as
they become better understood by reference to the following
detailed description. The elements of the drawings are not
necessarily to scale relative to each other. Like reference
numerals designate corresponding similar parts.
[0018] FIGS. 1A and 1B illustrate stability diagrams of prior art
toggle cells in which toggle switching scenarios relating to a
single-MTJ toggle cell (FIG. 1A) and to a two-MTJ (double-decker)
toggle cell (FIG. 1B) are illustrated.
[0019] FIG. 2 illustrates a typical configuration of a
double-decker toggle cell.
[0020] FIG. 3 illustrates a cross-sectional view of a double-decker
toggle cell according to one embodiment of the invention.
[0021] FIGS. 4A and 4B illustrate a configuration of fixed
magnetizations without having applied an external magnetic field
(FIG. 4A) and in their scissored configuration (FIG. 4B) after
having applied a magnetic field.
[0022] FIGS. 5A to 5D illustrate respective cross-sectional views
of double-decker toggle cells according to alternative embodiments
of the invention.
DETAILED DESCRIPTION
[0023] In the following Detailed Description, reference is made to
the accompanying drawings, which form a part hereof, and in which
is shown by way of illustration specific embodiments in which the
invention may be practiced. In this regard, directional
terminology, such as "top," "bottom," "front," "back," "leading,"
"trailing," etc., is used with reference to the orientation of the
Figure(s) being described. Because components of embodiments of the
present invention can be positioned in a number of different
orientations, the directional terminology is used for purposes of
illustration and is in no way limiting. It is to be understood that
other embodiments may be utilized and structural or logical changes
may be made without departing from the scope of the present
invention. The following detailed description, therefore, is not to
be taken in a limiting sense, and the scope of the present
invention is defined by the appended claims.
[0024] According to one embodiment of the invention, a
double-decker MRAM cell is provided having two MTJs in a single
cell without the need of necessarily choosing different
antiferromagnetic materials for the pinning layers for pinning of
reference layers in both MTJs.
[0025] In one embodiment of the invention, a method of producing a
double-decker magnetoresistive random access memory (MRAM) cell is
given, which includes the step of providing a stacked structure
having first and second magnetic tunnel junctions (MTJs). In such
stacked structure, the first MTJ includes first and second magnetic
regions made of magnetic material being stacked in a parallel,
overlying relationship separated by a first tunneling barrier layer
made of non-magnetic material. The first magnetic region includes a
first pinned layer (or reference layer) having a first fixed
magnetization adjacent the first tunneling barrier layer, while the
second magnetic region exhibits a first free magnetization adjacent
the first tunneling barrier layer which is free to be switched
between the same and opposite directions with respect to the first
fixed magnetization of the first magnetic region. Further, in such
stacked structure, the second MTJ includes third and fourth
magnetic regions made of magnetic material being stacked in a
parallel, overlying relationship separated by a second tunneling
barrier layer made of non-magnetic material. The third magnetic
region includes a second pinned layer (or reference layer) having a
second fixed magnetization adjacent the second tunneling barrier
layer, while the fourth magnetic region exhibits a second free
magnetization adjacent the second tunneling barrier layer which is
free to be switched between the same and opposite directions with
respect to the second fixed magnetization of the third magnetic
region of the second MTJ.
[0026] In such stacked structure, the free magnetizations of the
first and second MTJs are magnetically coupled to magnetic fields
generated by first and second currents made to flow through first
and second current lines, respectively. In order to practically
realize the first and second fixed magnetizations these are pinned
by first and second pinning layers, respectively, made of first and
second antiferromagnetic materials. The first and second
antiferromagnetic materials may be chosen to be the same material.
Also, the first and second pinned layers (first and second fixed
magnetizations) are coupled antiferromagnetically, for example, via
magnetostatic interaction and have a sufficiently small intrinsic
magnetic anisotropy such that in applying an external magnetic
field the first and second fixed magnetizations are brought into a
so-called "scissored configuration." In a scissored configuration
the first fixed magnetization is inclined under a first angle
relative to the external magnetic field and the second fixed
magnetization is inclined under a second angle relative to the
external magnetic field, such that an angle between the first and
second fixed magnetizations lies in a range of from 60.degree. to
120.degree. to allow for a proper discrimination between the four
resistance values.
[0027] In one case, the above angle between the first and second
fixed magnetizations amounts to about 90.degree. resulting in an
arrangement similar to the conventional case where both fixed
magnetizations are perpendicularly aligned. In the latter case, the
first and second angles have about the same absolute values and
opposite signs.
[0028] For a more detailed description of the scissored
configuration, see B. D. Cullity, Introduction to Magnetic
Materials, pages 239-240, the disclosure of which is incorporated
herein by reference.
[0029] In accordance with one method of the invention, it is
possible to use the same material for both first and second pinning
layers thus avoiding to be forced to select two antiferromagnetic
materials having substantially different setting temperatures.
Thus, a stacked double-decker MRAM cell can be realized in which
the two ferromagnetic fixed (pinned) layers are pinned by the same
antiferromagnetic material. Upon doing so, it is possible to
arrange two separate antiferromagnetic pinning layers made of the
same material, and, alternatively, even a single antiferromagnetic
pinning layer of that material may be provided, if it is thick
enough.
[0030] According to one embodiment of the invention, a method of
producing a double-decker MRAM cell is given, in that the first and
second fixed magnetizations respectively are pinned by a (single)
pinning layer made of antiferromagnetic material which has a small
intrinsic anisotropy such that a further step is enabled, wherein,
in applying an external magnetic field, the first and second fixed
magnetizations are brought into above-described scissored
configuration.
[0031] In one embodiment of the invention, selecting different
magnitudes of the applied external magnetic field will result in
different angles of first and second fixed magnetizations relative
to the direction of the external magnetic field.
[0032] Provided that fixed magnetizations are aligned with one of
the first and second current lines crossing at right angles before
they are brought into their scissored configuration, the external
magnetic field is applied in one case in direction of that one of
the first and second current lines to then obtain a scissored
configuration of first and second fixed magnetizations which are
respectively inclined in an angle of 45.degree. to each one of the
first and second current lines.
[0033] According to one embodiment of the invention, the second and
fourth magnetic regions respectively are comprised of a plurality
of N ferromagnetic free layers which are antiferromagnetically
coupled, where N is an integer greater than or equal to two. In
another case, the second and fourth magnetic regions respectively
are comprised of a tri-layered structure including two
ferromagnetic layers being antiferromagnetically coupled by an
intermediate layer made of antiferromagnetic coupling material.
[0034] FIG. 3 illustrates a cross-sectional view of a double-decker
toggle cell according to one embodiment of the present invention.
In one case, a double-decker toggle cell is fabricated which
includes a first MTJ with first and second magnetic regions 13, 4.
First and second magnetic regions 13, 4 are made of magnetic
material stacked in a parallel, overlying relationship and are
separated by a first tunneling barrier layer 5, which is made of
non-magnetic material. The cell also includes a second MTJ that
similarly includes third and fourth magnetic regions 14, 10. Third
and fourth magnetic regions 14, 10 are made of magnetic material
stacked in a parallel, overlying relationship and are separated by
a second tunneling barrier layer 9, which is made of non-magnetic
material. First and second MTJs are separated by conductive layer
7. Each one of the first and third magnetic regions 13, 14 of the
first and second MTJs, respectively, includes a pinned
magnetization reference layer 6, 8, which respectively are pinned
by antiferromagnetic pinning layers 11, 12. The second and fourth
magnetic regions 4, 10 of both MTJs exhibit a free magnetization
adjacent their respective tunneling barrier layers 5, 9, which is
free to be switched between the same and opposite directions with
respect to the fixed magnetization of the respective reference
layer 6, 8 fixed magnetizations. Further, the second and fourth
magnetic regions 4, 10 of both MTJs, respectively, include a
tri-layered structure including two ferromagnetic layers 15, 16 and
17, 18 being antiferromagnetically coupled by an intermediate layer
(not shown in the drawings) made of antiferromagnetic coupling
material.
[0035] According to one embodiment of the invention, first and
second antiferromagnetic materials of antiferromagnetic pinning
layers 11, 12 are chosen to be made of the same material.
Additionally, the first and second pinned layers 6, 8 are
antiferromagnetically coupled and have a sufficiently small
intrinsic magnetic anisotropy such that in applying an external
magnetic field a scissored configuration of the first and second
fixed magnetizations is enabled.
[0036] According to one embodiment of the invention, first and
second fixed magnetizations have been brought into the scissored
configuration in applying a magnetic external field. In the
scissored configuration of FIG. 3, an approximately orthogonal
alignment of fixed magnetizations of the reference layers 6, 8 is
achieved by applying an appropriately sized external magnetic
field. In other words, an absolute value of a difference of the
first and second angles, that is, an angle between the first fixed
magnetization and the second fixed magnetization amounts to about
90.degree. resulting in an arrangement similar to the conventional
case where both fixed magnetizations are perpendicularly aligned.
First fixed magnetization of first reference layer 6 is in parallel
alignment to free magnetizations of the first magnetic region, and,
second fixed magnetization of second reference layer 8 is in
parallel alignment to free magnetizations of the second magnetic
region. In one case, both first and second fixed magnetizations
being orthogonally aligned are inclined in an angle of 45.degree.
to each one of the first and second current lines (for instance bit
and word lines).
[0037] FIGS. 4A and 4B illustrate a configuration of first and
second fixed magnetizations in the double-decker MRAM cell of FIG.
3 without having applied an external magnetic field (FIG. 4A) and
in their scissored configuration (FIG. 4B) after having applied an
external magnetic field. Accordingly, first 19 and second 20 fixed
magnetizations, without having applied an external magnetic field,
are oppositely directed due to antiferromagnetism. Applying an
external magnetic field 21 (H.sub.x) results in a new orientation
of both first and second fixed magnetizations into their scissored
state having a first angle .THETA. between first fixed
magnetization 19 and magnetic field 21 and a second angle -.THETA.
between second fixed magnetization 20 and magnetic field 21, where
first and second angles .THETA., -.THETA. have the same absolute
value and different signs. The absolute values of both first and
second angles .THETA., -.THETA. respectively amount to
approximately 45.degree. resulting in a sum (absolute value) of
approximately 90.degree..
[0038] FIGS. 5A through 5D illustrate respective cross-sectional
views of double-decker toggle cells according to alternative
embodiments of the invention. In order to avoid unnecessary
repetitions, only the differences as to the embodiment of FIG. 3
are explained, otherwise reference is made to the description made
in connection with FIG. 3.
[0039] FIG. 5A illustrates an embodiment having a single
antiferromagnetic pinning layer 22 instead of first and second
antiferromagnetic pinning layers 11, 12 as in FIG. 3. The single
antiferromagnetic pinning layer 22 has to be sufficiently thick to
ensure pinning action for both first and second fixed
magnetizations of first 6 and second 8 reference layers.
[0040] FIGS. 5B to 5 D illustrate different cases similar to FIG.
3, where each system comprised of antiferromagnetic pinning layer
and reference layer is varied to be arranged on top/bottom of
antiferromagnetically coupled ferromagnetic free regions, that is,
second and fourth magnetic regions. FIG. 5B illustrates an
embodiment having the first magnetic region 13 comprised of
antiferromagnetic pinning layer 11 and reference layer 6 arranged
on top of the second magnetic region 4 separated by the first
tunneling barrier layer 5. FIG. 5C illustrates an embodiment having
the third magnetic region 14 comprised of antiferromagnetic pinning
layer 12 and reference layer 8 arranged at the bottom of the fourth
magnetic region 10 separated by the second tunneling barrier layer
9. FIG. 5D illustrates an embodiment having the first magnetic
region 13 comprised of antiferromagnetic pinning layer 11 and
reference layer 6 arranged on top of the second magnetic region 4
separated by the first tunneling barrier layer 5 and also having
the third magnetic region 14 comprised of antiferromagnetic pinning
layer 12 and reference layer 8 arranged at the bottom of the fourth
magnetic region 10 separated by the second tunneling barrier layer
9.
[0041] Obviously many modifications and variations of the present
invention are possible in light of the above description. It is
therefore to be understood, that within the scope of appended
claims, the invention may be practiced otherwise than as
specifically devised.
[0042] Although specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that a variety of alternate and/or equivalent
implementations may be substituted for the specific embodiments
shown and described without departing from the scope of the present
invention. This application is intended to cover any adaptations or
variations of the specific embodiments discussed herein. Therefore,
it is intended that this invention be limited only by the claims
and the equivalents thereof.
* * * * *