U.S. patent application number 13/093287 was filed with the patent office on 2012-10-25 for magnetic stacks with perpendicular magnetic anisotropy for spin momentum transfer magnetoresistive random access memory.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Guohan Hu, Janusz J. Nowak, Philip L. Troilloud, Daniel C. Worledge.
Application Number | 20120267733 13/093287 |
Document ID | / |
Family ID | 47020637 |
Filed Date | 2012-10-25 |
United States Patent
Application |
20120267733 |
Kind Code |
A1 |
Hu; Guohan ; et al. |
October 25, 2012 |
MAGNETIC STACKS WITH PERPENDICULAR MAGNETIC ANISOTROPY FOR SPIN
MOMENTUM TRANSFER MAGNETORESISTIVE RANDOM ACCESS MEMORY
Abstract
A magnetic tunnel junction (MTJ) includes a magnetic free layer,
having a variable magnetization direction; an insulating tunnel
barrier located adjacent to the free layer; a magnetic fixed layer
having an invariable magnetization direction, the fixed layer
disposed adjacent the tunnel barrier such that the tunnel barrier
is located between the free layer and the fixed layer, wherein the
free layer and the fixed layer have perpendicular magnetic
anisotropy; and one or more of: a composite fixed layer, the
composite fixed layer comprising a dusting layer, a spacer layer,
and a reference layer; a synthetic antiferromagnetic (SAF) fixed
layer structure, the SAF fixed layer structure comprising a SAF
spacer located between the fixed layer and a second fixed magnetic
layer; and a dipole layer, wherein the free layer is located
between the dipole layer and the tunnel barrier.
Inventors: |
Hu; Guohan; (Yorktown
Heights, NY) ; Nowak; Janusz J.; (Highland Mills,
NY) ; Troilloud; Philip L.; (Norwood, NJ) ;
Worledge; Daniel C.; (Cortlandt Manor, NY) |
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
Armonk
NY
|
Family ID: |
47020637 |
Appl. No.: |
13/093287 |
Filed: |
April 25, 2011 |
Current U.S.
Class: |
257/421 ;
257/E21.665; 257/E29.323; 438/3 |
Current CPC
Class: |
H01L 43/10 20130101;
H01L 43/12 20130101; G11C 11/161 20130101; H01L 43/08 20130101 |
Class at
Publication: |
257/421 ; 438/3;
257/E29.323; 257/E21.665 |
International
Class: |
H01L 29/82 20060101
H01L029/82; H01L 21/8239 20060101 H01L021/8239 |
Claims
1. A magnetic tunnel junction (MTJ) for a magnetic random access
memory (MRAM), comprising: a magnetic free layer, having a variable
magnetization direction; an insulating tunnel barrier located
adjacent to the free layer; a magnetic fixed layer having an
invariable magnetization direction, the fixed layer disposed
adjacent the tunnel barrier such that the tunnel barrier is located
between the free layer and the fixed layer, wherein the free layer
and the fixed layer have perpendicular magnetic anisotropy; and one
or more of: a composite fixed layer, the composite fixed layer
comprising a dusting layer, a spacer layer, and a reference layer,
wherein the spacer layer is located between the reference layer and
the tunnel barrier, and wherein the dusting layer is located
between the spacer layer and the tunnel barrier; a synthetic
antiferromagnetic (SAF) fixed layer structure, the SAF fixed layer
structure comprising a SAF spacer located between the fixed layer
and a second fixed magnetic layer, wherein the fixed layer and the
second fixed magnetic layer are anti-parallely coupled through the
SAF spacer; and a dipole layer, wherein the free layer is located
between the dipole layer and the tunnel barrier.
2. The MTJ of claim 1, wherein the free layer comprises one of
cobalt-iron-boron (CoFeB), pure iron (Fe), CoFeB|Fe and
Fe|CoFeB.
3. The MTJ of claim 1, further comprising a seed layer underneath
the free layer, the seed layer comprising one of tantalum (Ta) or
tantalum magnesium (TaMg), and wherein the seed layer has a
thickness from about 0.5 nanometers (nm) to about 3 nm.
4. The MTJ of claim 1, wherein the tunnel barrier comprises
magnesium oxide (MgO).
5. The MTJ of claim 1, wherein the dusting layer of the composite
fixed layer comprises one of CoFeB, CoFe, Fe, bilayers of Fe|CoFeB,
bilayers of CoFe|CoFeB, bilayers of CoFeB|Fe, and bilayers of
CoFeB|CoFe.
6. The MTJ of claim 1, wherein the dusting layer of the composite
fixed layer has a thickness from about 0.5 nm to about 2 nm.
7. The MTJ of claim 1, wherein the spacer layer of the composite
fixed layer comprises a non-magnetic material.
8. The MTJ of claim 1, wherein the spacer layer of the composite
fixed layer comprises one of chromium (Cr), ruthenium (Ru),
titanium nitride (TiN), titanium (Ti), vanadium (V), tantalum (Ta),
tantalum nitride (TaN), aluminum (Al), magnesium (Mg) and MgO.
9. The MTJ of claim 1, wherein the spacer layer of the composite
fixed layer comprises a tri-layer structure comprising a center
magnetic spacer layer disposed between two non-magnetic spacer
layers.
10. The MTJ of claim 9, wherein the center magnetic spacer layer
comprises one of CoFeB, Fe, and CoFe.
11. The MTJ of claim 9, wherein the center magnetic layer has a
thickness from about 0.1 nm to about 0.5 nm.
12. The MTJ of claim 1, wherein the fixed layer comprises one of
cobalt-platinum (Co|Pt) and cobalt-palladium (Co|Pd).
13. The MTJ of claim 1, wherein the SAF spacer comprises ruthenium,
and wherein the second fixed magnetic layer comprises one of
cobalt-nickel (Co|Ni), Co|Pd and Co|Pt.
14. The MTJ of claim 1, wherein the dipole layer comprises one of
cobalt chromium platinum (CoCrPt), Co|Ni, Co|Pd, and Co|Pt
multilayers.
15. The MTJ of claim 14, wherein the dipole layer further comprises
a CoFeB layer.
16. The MTJ of claim 1, wherein the fixed layer is magnetized in a
direction opposite to a magnetization direction of the second fixed
magnetic layer in the SAF structure.
17. The MTJ of claim 1, wherein the MTJ comprises both the dipole
layer and the SAF structure, and the dipole layer and the second
fixed magnetic layer of the SAF structure are magnetized in the
same direction, and the fixed layer is magnetized in a direction
that is opposite to the magnetization direction of the dipole layer
and the second fixed magnetic layer.
18. The MTJ of claim 1, wherein the dipole layer is magnetized in a
direction opposite to a magnetization direction of the fixed
layer.
19. A method of forming a magnetic tunnel junction (MTJ) for a
magnetic random access memory (MRAM), the method comprising:
forming a magnetic free layer having a variable magnetization
direction; forming a tunnel barrier over the free layer, the tunnel
barrier comprising an insulating material; forming a magnetic fixed
layer having an invariable magnetization direction over the tunnel
barrier, wherein the free layer and the fixed layer have
perpendicular magnetic anisotropy; and forming one or more of: a
composite fixed layer, the composite fixed layer comprising a
dusting layer, a spacer layer, and a reference layer, wherein the
spacer layer is located between the reference layer and the tunnel
barrier, and wherein the dusting layer is located between the
spacer layer and the tunnel barrier; a synthetic antiferromagnetic
(SAF) fixed layer structure, the SAF fixed layer structure
comprising a SAF spacer located between the fixed layer and a
second fixed magnetic layer, wherein the fixed layer and the second
fixed magnetic layer are anti-parallelly coupled through the SAF
spacer; and a dipole layer, wherein the free layer is located
between the dipole layer and the tunnel barrier.
20. The method of claim 19, wherein forming the free layer
comprises growing the free layer on a seed layer comprising one of
tantalum (Ta) or tantalum magnesium (TaMg), and wherein the seed
layer has a thickness from about 0.5 nanometers (nm) to about 3
nm.
21. The method of claim 19, wherein the tunnel barrier comprises
magnesium oxide (MgO) and is formed by one of natural oxidation,
radical oxidation, and radio frequency (RF) sputtering.
22. The method of claim 19, wherein the fixed layer is magnetized
in a direction opposite to a magnetization direction of the second
fixed magnetic layer in the SAF structure.
23. The method of claim 19, wherein the MTJ comprises both the
dipole layer and the SAF structure, and the dipole layer and the
second fixed magnetic layer of the SAF structure are magnetized in
the same direction, and the fixed layer is magnetized in a
direction that is opposite to the magnetization direction of the
dipole layer and the second fixed magnetic layer.
24. The method of claim 19, wherein the dipole layer is magnetized
in a direction opposite to a magnetization direction of the fixed
layer.
25. The method of claim 19, wherein the free layer comprises one of
cobalt-iron-boron (CoFeB), pure iron (Fe), CoFeB|Fe and Fe|CoFeB,
and the fixed layer comprises one of cobalt-platinum (Co|Pt) and
cobalt-palladium (Co|Pd).
Description
BACKGROUND
[0001] This disclosure relates generally to the field of
magnetoresistive random access memory (MRAM), and more specifically
to spin momentum transfer (SMT) MRAM.
[0002] MRAM is a type of solid state memory that uses tunneling
magnetoresistance (MR) to store information. MRAM is made up of an
electrically connected array of magnetoresistive memory elements,
referred to as magnetic tunnel junctions (MTJs). Each MTJ includes
a magnetic free layer having a magnetization direction that is
variable, and a magnetic fixed layer having a magnetization
direction that is invariable. The free layer and fixed layer are
separated by an insulating non-magnetic tunnel barrier. An MTJ
stores information by switching the magnetization state of the free
layer. When the magnetization direction of the free layer is
parallel to the magnetization direction of the fixed layer, the MTJ
is in a low resistance state. When the magnetization direction of
the free layer is anti-parallel to the magnetization direction of
the fixed layer, the MTJ is in a high resistance state. The
difference in resistance of the MTJ may be used to indicate a
logical `1` or `0`, thereby storing a bit of information. The MR of
an MTJ determines the difference in resistance between the high and
low resistance states. A relatively high difference between the
high and low resistance states facilitates read operations in the
MRAM.
[0003] The magnetization state of the free layer may be changed by
a spin torque switched (STT) write method, in which a write current
is applied in a direction perpendicular to the film plane of the
magnetic films forming the MTJ. The write current has a tunneling
magnetoresistive effect, so as to change (or reverse) the
magnetization state of the free layer of the MTJ. In STT
magnetization reversal, the write current required for the
magnetization reversal is determined by the current density. As the
area of the surface in an MTJ on which the write current flows
becomes smaller, the write current required for reversing the
magnetization of the free layer of the MTJ becomes smaller.
Therefore, if writing is performed with fixed current density, the
necessary write current becomes smaller as the MTJ size becomes
smaller.
[0004] MTJs that include material layers that exhibit perpendicular
anisotropy (PMA) may be switched with a relatively low current
density as compared to MTJs having in-plane magnetic anisotropy,
also lowering the total necessary write current. However, MTJs made
using PMA materials may have a relatively low MR because of
structural and chemical incompatibility between the various
material layers that comprise a PMA MTJ. A relatively low MR may
result in difficulty with read operations in the STT MRAM, as the
difference in resistance between the high and low resistance states
of the MTJs will also be relatively low. In a PMA MTJ, the fixed
layer and the free layer may be magnetized in directions that are
parallel or anti-parallel to one another, and the fixed layer may
apply a relatively strong dipolar field to the free layer. The
fixed layer dipolar field may offset the free layer loop by about
1000 oersteds (Oe) or more. The free layer H.sub.c needs to be
higher than the offset field from the fixed layer; otherwise, there
is only one stable resistance state instead of the two stable
resistance states (referred to as bistability) needed to store
information in the MTJ.
[0005] MRAM is formed from a layered sheet comprising a magnetic
stack of the various MTJ layers that is patterned to form
individual MTJs. The MTJs may take the form of relatively small
cylinders, each comprising the layered magnetic stack. In a sheet
film, there is a Neel coupling between the various layers, and the
fixed layer dipolar field is not present. The fixed layer dipolar
field becomes apparent after the MTJs are patterned, as the dipolar
field originates at the edge of the MTJ device. The fixed layer
dipolar field becomes stronger as the MTJs are made smaller and is
non-uniform across an MTJ. The fixed layer dipolar field creates a
number of issues in MTJ devices, including increasing the free
layer loop offset and the minimum required free layer H.sub.c
needed to ensure bistability of the MTJ. The minimum free layer
H.sub.c must be maintained across the full temperature operating
range of the device. The fixed layer dipolar field may also change
the switching mode of an MTJ, and the impact on device
functionality increases as MTJ size is scaled down.
BRIEF SUMMARY
[0006] In one aspect, a magnetic tunnel junction (MTJ) for a
magnetic random access memory (MRAM) includes a magnetic free
layer, having a variable magnetization direction; an insulating
tunnel barrier located adjacent to the free layer; a magnetic fixed
layer having an invariable magnetization direction, the fixed layer
disposed adjacent the tunnel barrier such that the tunnel barrier
is located between the free layer and the fixed layer, wherein the
free layer and the fixed layer have perpendicular magnetic
anisotropy; and one or more of: a composite fixed layer, the
composite fixed layer comprising a dusting layer, a spacer layer,
and a reference layer, wherein the spacer layer is located between
the reference layer and the tunnel barrier, and wherein the dusting
layer is located between the spacer layer and the tunnel barrier; a
synthetic antiferromagnetic (SAF) fixed layer structure, the SAF
fixed layer structure comprising a SAF spacer located between the
fixed layer and a second fixed magnetic layer, wherein the fixed
layer and the second fixed magnetic layer are anti-parallely
coupled through the SAF spacer; and a dipole layer, wherein the
free layer is located between the dipole layer and the tunnel
barrier.
[0007] In another aspect, a method of forming a magnetic tunnel
junction (MTJ) for a magnetic random access memory (MRAM) includes
forming a magnetic free layer having a variable magnetization
direction; forming a tunnel barrier over the free layer, the tunnel
barrier comprising an insulating material; forming a magnetic fixed
layer having an invariable magnetization direction over the tunnel
barrier, wherein the free layer and the fixed layer have
perpendicular magnetic anisotropy; and forming one or more of: a
composite fixed layer, the composite fixed layer comprising a
dusting layer, a spacer layer, and a reference layer, wherein the
spacer layer is located between the reference layer and the tunnel
barrier, and wherein the dusting layer is located between the
spacer layer and the tunnel barrier; a synthetic antiferromagnetic
(SAF) fixed layer structure, the SAF fixed layer structure
comprising a SAF spacer located between the fixed layer and a
second fixed magnetic layer, wherein the fixed layer and the second
fixed magnetic layer are anti-parallelly coupled through the SAF
spacer; and a dipole layer, wherein the free layer is located
between the dipole layer and the tunnel barrier.
[0008] Additional features are realized through the techniques of
the present exemplary embodiment. Other embodiments are described
in detail herein and are considered a part of what is claimed. For
a better understanding of the features of the exemplary embodiment,
refer to the description and to the drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0009] Referring now to the drawings wherein like elements are
numbered alike in the several FIGURES:
[0010] FIG. 1 is a cross sectional view illustrating an embodiment
of a magnetic stack with a composite fixed layer on top of the
magnetic stack.
[0011] FIG. 2 is a cross sectional view illustrating an embodiment
of a magnetic stack with a composite fixed layer on the bottom of
the magnetic stack.
[0012] FIG. 3 is a cross sectional view illustrating an embodiment
of a magnetic stack with a composite fixed layer and an SAF
structure on top of the magnetic stack.
[0013] FIG. 4 is a cross sectional view illustrating an embodiment
of a magnetic stack with a composite fixed layer and an SAF
structure on the bottom of the magnetic stack.
[0014] FIG. 5 is a cross sectional view illustrating an embodiment
of a magnetic stack with a composite fixed layer and an SAF
structure on top of the magnetic stack, and a dipole layer on the
bottom of the magnetic stack.
[0015] FIG. 6 is a cross sectional view illustrating an embodiment
of a magnetic stack with a composite fixed layer and an SAF
structure on the bottom of the magnetic stack, and a dipole layer
on top of the magnetic stack.
[0016] FIG. 7 is a cross sectional view illustrating an embodiment
of a magnetic stack with a composite fixed layer on top of the
magnetic stack, and a dipole layer on the bottom of the magnetic
stack.
[0017] FIG. 8 is a cross sectional view illustrating an embodiment
of a magnetic stack with a composite fixed layer on the bottom of
the magnetic stack, and a dipole layer on top of the magnetic
stack.
DETAILED DESCRIPTION
[0018] Embodiments of PMA magnetic stacks for STT MRAM are
provided, with exemplary embodiments being discussed below in
detail. The PMA magnetic stacks form MTJs that exhibit a high MR
and a reduced fixed layer dipolar field thus commensurately reduced
free layer loop offset through inclusion of one or more of a
composite fixed layer, a synthetic antiferromagnetic (SAF)
structure in the fixed layer, and a dipole layer. A composite fixed
layer includes three layers: a dusting layer, a spacer layer, and a
reference layer. A fixed layer SAF structure includes a SAF spacer
located between two layers of magnetic material that are
anti-parallelly coupled through the SAF spacer. The magnetization
of the two layers of magnetic material in the SAF structure may be
adjusted such that they are aligned opposite to one another,
reducing the overall fixed layer dipole field. A dipole layer is
located on the opposite side of the free layer from the tunnel
barrier, and may be magnetized in a direction opposite to the fixed
layer to cancel out the fixed layer dipole field. A PMA MTJ may be
formed with one or more of the composite fixed layer, the SAF
structure, and the dipole layer, in any combination, to reduce the
fixed layer dipole field and free layer loop offset of the PMA
MTJ.
[0019] Referring initially to FIG. 1, there is shown a cross
sectional view of a PMA magnetic stack 100 with a composite fixed
layer 107. The composite fixed layer 107 includes dusting layer
104, spacer 105, and reference layer 106. The MTJ 100 also includes
a free layer 102 that is grown on a seed layer 101. Seed layer 101
may include tantalum (Ta), or tantalum magnesium (TaMg) with a
percentage of Mg that is less than 20%, in some embodiments. The
thickness of seed layer 102 may be about 0.5 nanometers (nm) or
more, and from about 1 nm to about 3 nm in some exemplary
embodiments. Free layer 102 may include cobalt-iron-boron (CoFeB)
with various compositions; the Co composition may be less than 90%,
and the Fe may be from about 10% to about 100% (pure Fe) in various
embodiments. Free layer 102 may also comprise CoFeB|Fe or Fe|CoFeB.
The thickness of the free layer 102 may be from about 0.6 nm to
about 2 nm, depending on the composition of free layer 102. Tunnel
barrier 103 is located on free layer 102, and comprises an
insulating material such as magnesium oxide (MgO). An MgO tunnel
barrier 103 can be formed by natural oxidation, radical oxidation
or radiofrequency (RF) sputtering.
[0020] In the embodiment shown in FIG. 1, composite fixed layer 107
is located on top of tunnel barrier 103. The dusting layer 104 and
the reference layer 106 are magnetically coupled through the spacer
105 and have PMA. The dusting layer 104 may be pure CoFeB, CoFe,
Fe, or bilayers of Fe|CoFeB, CoFe|CoFeB, CoFeB|Fe or CoFeB|CoFe in
various embodiments. The thickness of the dusting layer 104 may be
from about 0.5 nm to about 2 nm. The spacer 105 comprises a
non-magnetic material, such as chromium (Cr), ruthenium (Ru),
titanium nitride (TiN), titanium (Ti), vanadium (V), tantalum (Ta),
tantalum nitride (TaN), aluminum (Al), magnesium (Mg) or oxides
such as MgO in various embodiments. The thickness of the spacer 105
may be from about 0.1 nm to about 1 nm in some embodiments, or
thicker than 1 nm in other embodiments; the thickness of spacer 105
must allow dusting layer 104 and reference layer 106 to be
magnetically coupled together through spacer 105. The spacer layer
105 may alternately have a tri-layer structure, with a relatively
thin center magnetic layer (which may comprise CoFeB, Fe, or CoFe)
sandwiched by two non-magnetic layers (which may comprise Cr, Ru,
TiN, Ti, V, Ta, TaN, Al, Mg or oxides such as MgO) in some
embodiments. For a tri-layer spacer 105, the thickness of the
center magnetic layer may be from about 0.1 nm to about 0.5 nm.
Reference layer 106 may comprise cobalt-platinum (Co|Pt) or
cobalt-palladium (Co|Pd), in multilayers or a mixture, in various
embodiments. Additional embodiments of MTJs that may comprise a
composite fixed layer including a dusting layer, spacer, and
reference layer are shown in FIGS. 2-8; composite fixed layers 204,
307, 406, 507, 606, 707, and 804 may comprise any of the materials,
structures, and thicknesses listed above with respect to composite
fixed layer 107 of FIG. 1.
[0021] FIG. 2 shows an embodiment of a PMA magnetic stack 200 with
the composite fixed layer 204 located on the bottom. The composite
fixed layer 204 includes dusting layer 203, spacer 202, and
reference layer 201. Magnetic stack 200 further includes tunnel
barrier 205 and free layer 206. Tunnel barrier 205 comprises an
insulating material such as MgO. An MgO tunnel barrier 205 can be
formed by natural oxidation, radical oxidation or RF sputtering.
Free layer 206 may include CoFeB with various compositions; the Co
composition may be less than 90% and the Fe may be from about 10%
to about 100% (pure Fe) in various embodiments. Free layer 206 may
also comprise CoFeB|Fe or Fe|CoFeB.
[0022] A composite fixed layer may be incorporated into a SAF fixed
layer structure, as shown in PMA magnetic stacks 300 and 400 of
FIGS. 3 and 4. By adjusting the magnetic moments and the magnetic
anisotropy of the magnetic material located on either side of the
SAF spacer in the SAF structure such that the magnetic moments are
aligned opposite to one another, a reduced fixed layer dipole field
and a centered (or zero offset) free layer loop can be obtained. In
FIG. 3, composite fixed layer 307, including dusting layer 304,
spacer 305, and reference layer 306, is anti-parallelly coupled to
a top reference layer 309 through an SAF spacer 308. Top reference
layer 309 may comprise cobalt-nickel (Co|Ni), Co|Pd or Co|Pt, in
multilayers or a mixture. SAF spacer 308 may comprise Ru, and may
be from about 8 angstroms to about 10 angstroms thick in some
embodiments. MTJ 300 further includes seed layer 301, free layer
302, and tunnel barrier 303. Seed layer 301 may include Ta, or TaMg
with a percentage of Mg that is less than 20%, in some embodiments.
The seed layer 301 thickness may be about 0.5 nm or more, and from
about 1 nm to about 3 nm in some exemplary embodiments. Free layer
302 may include CoFeB with various compositions; the Co composition
may be less than 90% and the Fe may be from about 10% to about 100%
(pure Fe) in various embodiments. Free layer 302 may also comprise
CoFeB|Fe or Fe|CoFeB. The thickness of the free layer 302 may be
from about 0.6 nm to about 2 nm depending on the free layer
composition. Tunnel barrier 303 is located on free layer 302, and
comprises an insulating material such as MgO. An MgO tunnel barrier
303 can be formed by natural oxidation, radical oxidation or
radiofrequency (RF) sputtering. Composite fixed layer 307 may
comprise any of the materials, structures, and thicknesses listed
above with respect to composite fixed layer 107 of FIG. 1 in some
embodiments. In other embodiments of an MTJ with a fixed layer SAF
structure, composite fixed layer 307 may be replaced by a simple
fixed layer comprising an appropriate magnetic material, omitting
dusting layer 304 and spacer 305.
[0023] In FIG. 4, PMA magnetic stack 400 includes a composite fixed
layer 406, with dusting layer 405, spacer 404, and reference layer
403, anti-parallelly coupled through SAF spacer 402 to bottom
reference layer 401. Bottom reference layer 401 may comprise Co|Ni,
Co|Pd or Co|Pt, in multilayers or a mixture. SAF spacer 402 may
comprise Ru, and may be from about 8 angstroms to about 10
angstroms thick in some embodiments. PMA magnetic stack 400 further
includes tunnel barrier 407 and free layer 408. Tunnel barrier 407
comprises an insulating material such as MgO. An MgO tunnel barrier
407 can be formed by natural oxidation, radical oxidation or RF
sputtering. Free layer 408 may include CoFeB with various
compositions; the Co composition may be less than 90% and the Fe
may be from about 10% to about 100% (pure Fe) in various
embodiments. Free layer 408 may also comprise CoFeB|Fe or Fe|CoFeB.
Composite fixed layer 406 may comprise any of the materials,
structures, and thicknesses listed above with respect to composite
fixed layer 107 of FIG. 1 in some embodiments. In other embodiments
of a PMA magnetic stack with a fixed layer SAF structure, composite
fixed layer 406 may be replaced by a simple fixed layer comprising
an appropriate magnetic material, omitting dusting layer 405 and
spacer 404.
[0024] In the embodiments of PMA magnetic stacks 300 and 400 with
fixed layer SAF structures shown in FIGS. 3 and 4, because top
reference layer 309/bottom reference layer 401 is further away from
the free layer 302/408 than the composite fixed layer 307/406, the
magnetic moment of top reference layer 309/bottom reference layer
401 has to be larger than the magnetic moment of composite fixed
layer 307/406 in order to cancel the dipole field from composite
fixed layer 307/406. In the case that both reference layer 306/403
and top reference layer 309/bottom reference layer 401 comprise
multilayers, top reference layer 309/bottom reference layer 401
must include more layer repeats, and therefore be thicker, than
reference layer 306/403. As top reference layer 309/bottom
reference layer 401 is made thicker, the compensation effect of
additional layer repeats becomes weaker, as the additional layer
repeats are further away from the free layer 302/408. Thicker
multilayer stacks may also lead to longer deposition times during
magnetic stack fabrication and more materials expense. Therefore, a
dipole layer that is magnetized in a direction that is the same as
that of top reference layer 309/bottom reference layer 401 and
opposite to that of the reference layer 306/403 may be incorporated
into the PMA magnetic stack in addition to the fixed layer SAF
structure so as to reduce the thickness of the top reference layer
309/bottom reference layer 401 that is necessary to cancel out the
dipole field from the composite fixed layer 307/406. Magnetic
stacks including a composite fixed layer, a fixed layer SAF
structure, and a dipole layer are shown in FIGS. 5 and 6.
Alternately, the dipole layer may be incorporated into a PMA
magnetic stack that omits the fixed layer SAF structure, as shown
below with respect to FIGS. 7 and 8, and the dipole layer may be
magnetized in a direction that is opposite to the fixed layer.
[0025] The PMA magnetic stack 500 of FIG. 5 includes a fixed layer
SAF structure with composite fixed layer 507 anti-parallelly
coupled to a top reference layer 509 through an SAF spacer 508. Top
reference layer 509 may comprise Co|Ni, Co|Pd or Co|Pt, in
multilayers or a mixture. SAF spacer 508 may comprise Ru, and may
be from about 8 angstroms to about 10 angstroms thick in some
embodiments. The dipole layer 510 may comprise cobalt chromium
platinum (CoCrPt), Co|Ni, Co|Pd, or Co|Pt multilayers in some
embodiments. To increase the magnetic moment and reduce the
thickness of the dipole layer 510, a CoFeB dusting layer (not
shown), which has relatively large saturation moment, may be grown
directly on top of the CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers
as part of the dipole layer 510 in some embodiments. Dipole layer
510 is magnetized in a direction that is the same as that of top
reference layer 509 and opposite to that of the reference layer 506
to cancel out the fixed layer dipole field and reduce the loop
offset of the free layer 502. A seed layer 501 located between the
free layer 502 and dipole layer 510 provided magnetic separation
between the dipole layer 510 and free layer 502. Seed layer 501 may
include Ta, or TaMg with a percentage of Mg that is less than 20%,
in some embodiments. The seed layer 501 thickness may be 0.5 nm or
more, and from about 1 nm to about 3 nm in some exemplary
embodiments. Free layer 502 may include CoFeB with various
compositions; the Co composition may be less than 90% and the Fe
may be from about 10% to about 100% (pure Fe) in various
embodiments. Free layer 502 may also comprise CoFeB|Fe or Fe|CoFeB.
The thickness of the free layer 502 may be from about 0.6 nm to
about 2 nm depending on the free layer composition. Tunnel barrier
503 is located on free layer 502, and comprises an insulating
material such as MgO. An MgO tunnel barrier 503 can be formed by
natural oxidation, radical oxidation or RF sputtering. Composite
fixed layer 507, including dusting layer 504, spacer 505, and
reference layer 506, may comprise any of the materials, structures,
and thicknesses listed above with respect to composite fixed layer
107 of FIG. 1 in some embodiments. In other embodiments of a PMA
magnetic stack with a fixed layer SAF structure and a dipole layer,
composite fixed layer 507 may be a simple fixed layer comprising an
appropriate magnetic material, omitting dusting layer 504 and
spacer 505.
[0026] In FIG. 6, PMA magnetic stack 600 includes a fixed layer SAF
structure including composite fixed layer 606 anti-parallelly
coupled through SAF spacer 602 to bottom reference layer 601.
Bottom reference layer 601 may comprise Co|Ni, Co|Pd or Co|Pt, in
multilayers or a mixture. SAF spacer 602 may comprise Ru, and may
be from about 8 angstroms to about 10 angstroms thick in some
embodiments. The dipole layer 609 may comprise CoCrPt, Co|Ni,
Co|Pd, or Co|Pt multilayers in some embodiments. To increase the
magnetic moment and reduce the thickness of the dipole layer 609, a
CoFeB dusting layer (not shown), which has relatively large
saturation moment, may be grown directly on the bottom of the
CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers as part of the dipole
layer 609 in some embodiments. Dipole layer 609 is magnetized in a
direction that is the same as that of bottom reference layer 601
and opposite to that of the reference layer 603 to cancel out the
fixed layer dipole field and reduce the loop offset of the free
layer 608. A cap layer 610 located between the free layer 608 and
dipole layer 609 provides magnetic separation between the dipole
layer 609 and free layer 608. Tunnel barrier 607 comprises an
insulating material such as MgO. An MgO tunnel barrier 607 can be
formed by natural oxidation, radical oxidation or RF sputtering.
Free layer 608 may include CoFeB with various compositions; the Co
composition may be less than 90% and the Fe may be from about 10%
to about 100% (pure Fe) in various embodiments. Free layer 608 may
also comprise CoFeB|Fe or Fe|CoFeB. Composite fixed layer 606, with
dusting layer 605, spacer 604, and reference layer 603, may
comprise any of the materials, structures, and thicknesses listed
above with respect to composite fixed layer 107 of FIG. 1 in some
embodiments. In other embodiments of a PMA magnetic stack with a
fixed layer SAF structure and a dipole layer, composite fixed layer
606 may be a simple fixed layer comprising an appropriate magnetic
material, omitting dusting layer 605 and spacer 604.
[0027] The dipole fields generated by top reference layer
509/bottom reference layer 601 and dipole layers 510 and 609 in
FIGS. 5 and 6 together compensate the dipole field generated by the
reference layer 506/603. As long as the H.sub.c of composite fixed
layer 507/606 is either greater or less than the H.sub.c of both
the top reference layer 509/bottom reference layer 601 and the
dipole layer 510/609, the magnetization of the three layers can be
adjusted to reduce the offset field of free layer 502/608.
[0028] A dipole layer that is magnetized in a direction opposite to
the fixed layer may also be used to cancel out the fixed layer
dipole field and center the magnetization loop of the free layer in
the absence of a fixed layer SAF structure, as shown in FIGS. 7 and
8. In FIG. 7, PMA magnetic stack 700 includes a composite fixed
layer 707 and a dipole layer 708. The dipole layer 708 may comprise
CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers in some embodiments. To
increase the magnetic moment and reduce the thickness of the dipole
layer 708, a CoFeB dusting layer (not shown), which has relatively
large saturation moment, may be grown directly on top of the
CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers as part of the dipole
layer 708 in some embodiments. A seed layer 701 located between the
free layer 702 and dipole layer 708 provided magnetic separation
between the dipole layer 708 and free layer 702. Seed layer 701 may
include Ta, or TaMg with a percentage of Mg that is less than 20%,
in some embodiments. The seed layer 701 thickness may be 0.5 nm or
more, and from about 1 nm to about 3 nm in some exemplary
embodiments. Free layer 702 may include CoFeB with various
compositions; the Co composition may be less than 90% and the Fe
may be from about 10% to about 100% (pure Fe) in various
embodiments. Free layer 702 may also comprise CoFeB|Fe or Fe|CoFeB.
The thickness of the free layer 702 may be from about 0.6 nm to
about 2 nm depending on the free layer composition. Tunnel barrier
703 is located on free layer 702, and comprises an insulating
material such as MgO. An MgO tunnel barrier 703 can be formed by
natural oxidation, radical oxidation or RF sputtering. Composite
fixed layer 707, with dusting layer 704, spacer 705, and reference
layer 706, may comprise any of the materials, structures, and
thicknesses listed above with respect to composite fixed layer 107
of FIG. 1 in some embodiments. In other embodiments of a PMA
magnetic stack with a dipole layer, composite fixed layer 707 may
be a simple fixed layer comprising an appropriate magnetic
material, omitting dusting layer 704 and spacer 705.
[0029] In FIG. 8, PMA magnetic stack 800 includes composite fixed
layer 804 and a dipole layer 807. The dipole layer 807 may comprise
CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers in some embodiments. To
increase the magnetic moment and reduce the thickness of the dipole
layers 807, a CoFeB dusting layer (not shown), which has relatively
large saturation moment, may be grown directly on the bottom of the
CoCrPt, Co|Ni, Co|Pd, or Co|Pt multilayers as part of the dipole
layer 807 in some embodiments. A cap layer 808 located between the
free layer 806 and dipole layer 807 provides magnetic separation
between the dipole layer 807 and free layer 806. Tunnel barrier 805
comprises a insulating material such as MgO. An MgO tunnel barrier
805 can be formed by natural oxidation, radical oxidation or
radiofrequency (RF) sputtering. Free layer 806 may include CoFeB
with various compositions; the Co composition may be less than 90%
and the Fe may be from 10% to about 100% (pure Fe) in various
embodiments. Free layer 806 may also comprise CoFeB|Fe or Fe|CoFeB.
Composite fixed layer 804, with dusting layer 803, spacer 802, and
reference layer 801, may comprise any of the materials, structures,
and thicknesses listed above with respect to composite fixed layer
107 of FIG. 1 in some embodiments. In other embodiments of an PMA
magnetic stack with a dipole layer, composite fixed layer 804 may
be a simple fixed layer comprising an appropriate magnetic
material, omitting dusting layer 803 and spacer 802.
[0030] In the PMA magnetic stacks 700 and 800 of FIGS. 7 and 8, the
dipole layer 708/807 and the reference layer 706/801 are magnetized
to opposite directions, such that their dipole fields cancel each
other. The H.sub.c of the dipole layer 708/807 and the H.sub.c of
the reference layer 706/801 may not be equal. A wide resetting
field window can be achieved compared to the SAF/dipole PMA
magnetic stacks 500 and 600 of FIGS. 5 and 6. However, a relatively
thick dipole layer 708/807 is required in PMA magnetic stacks 700
and 800 of FIGS. 7 and 8, as compared to the SAF/dipole PMA
magnetic stacks 500 and 600 of FIGS. 5 and 6.
[0031] The technical effects and benefits of exemplary embodiments
include PMA magnetic stacks for MTJs having a relatively high
magnetoresistance and a relatively low fixed layer dipolar field
and free layer offset loop.
[0032] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an", and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
[0033] The corresponding structures, materials, acts, and
equivalents of all means or step plus function elements in the
claims below are intended to include any structure, material, or
act for performing the function in combination with other claimed
elements as specifically claimed. The description of the present
invention has been presented for purposes of illustration and
description, but is not intended to be exhaustive or limited to the
invention in the form disclosed. Many modifications and variations
will be apparent to those of ordinary skill in the art without
departing from the scope and spirit of the invention. The
embodiment was chosen and described in order to best explain the
principles of the invention and the practical application, and to
enable others of ordinary skill in the art to understand the
invention for various embodiments with various modifications as are
suited to the particular use contemplated.
* * * * *