U.S. patent application number 10/720962 was filed with the patent office on 2005-05-26 for magnetic tunnel junction with improved tunneling magneto-resistance.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Parkin, Stuart Stephen Papworth, Samant, Mahesh Govind.
Application Number | 20050110004 10/720962 |
Document ID | / |
Family ID | 34591696 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050110004 |
Kind Code |
A1 |
Parkin, Stuart Stephen Papworth ;
et al. |
May 26, 2005 |
Magnetic tunnel junction with improved tunneling
magneto-resistance
Abstract
A magnetic tunnel element that can be used, for example, as part
of a read head or a magnetic memory cell, includes a first layer
formed from an amorphous material, an amorphous tunnel barrier
layer, and an interface layer between the first layer and the
tunnel barrier layer. The interface layer is formed from a material
that is crystalline when the material is in isolation from both the
first layer and the tunnel barrier layer. Alternatively, the
thickness of the interface layer is selected so that the interface
layer is not crystalline. The first layer is formed from at least
one material selected from the group consisting of amorphous
ferromagnetic material, amorphous ferrimagnetic materials, and
amorphous non-magnetic materials. The interface layer is formed
from a material selected from the group consisting of a
ferromagnetic material and a ferrimagnetic material.
Inventors: |
Parkin, Stuart Stephen
Papworth; (San Jose, CA) ; Samant, Mahesh Govind;
(San Jose, CA) |
Correspondence
Address: |
JOSEPH P. CURTIN
1469 N.W. MORGAN LANE
PORTLAND
OR
97229
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
34591696 |
Appl. No.: |
10/720962 |
Filed: |
November 24, 2003 |
Current U.S.
Class: |
257/30 ;
257/E43.004 |
Current CPC
Class: |
B82Y 40/00 20130101;
G11C 11/16 20130101; H01F 10/3254 20130101; H01L 43/08 20130101;
H01F 41/307 20130101; B82Y 25/00 20130101; H01F 10/3204
20130101 |
Class at
Publication: |
257/030 |
International
Class: |
H01L 029/06 |
Claims
What is claimed is:
1. A magnetic tunnel element, comprising: a first layer formed from
an amorphous material; an amorphous tunnel barrier layer; and an
interface layer between and in proximity with the first layer and
the tunnel barrier layer, the interface layer being formed from at
least one material selected from the group consisting of
ferromagnetic materials and ferrimagnetic materials, wherein the
interface layer material is crystalline when it is in isolation
from both the first layer and the tunnel barrier layer.
2. The magnetic tunnel element according to claim 1, wherein the
first layer is formed from at least one material selected from the
group consisting of amorphous ferromagnetic materials and amorphous
ferrimagnetic materials.
3. The magnetic tunnel element according to claim 1, further
comprising a second layer in contact with the tunnel barrier layer
and including at least one material selected from the group
consisting of ferromagnetic materials and ferrimagnetic materials,
and wherein the first layer, the interface layer, the tunnel
barrier layer and the second layer form a magnetic tunnel
junction.
4. The magnetic tunnel junction according to claim 3, wherein the
magnetic tunnel junction has a tunneling magnetoresistance that is
greater than 50%.
5. The magnetic tunnel junction according to claim 3, wherein the
magnetic tunnel junction has a tunneling magnetoresistance that is
greater than 60%.
6. The magnetic tunnel junction according to claim 3, wherein the
magnetic tunnel junction has a tunneling magnetoresistance that is
greater than 65%.
7. The magnetic tunnel element according to claim 3, wherein the
interface layer is configured to increase the tunneling
magnetoresistance of the magnetic tunnel junction.
8. The magnetic tunnel element according to claim 1, further
comprising: a metal-containing layer in contact with the tunnel
barrier layer; and a semiconductor layer that is in contact with
the first layer, wherein the metal-containing layer, the tunnel
barrier, the interface layer, the first layer and the semiconductor
layer form a magnetic tunneling transistor.
9. The magnetic tunnel element according to claim 1, further
comprising a semiconductor material layer in proximity with the
tunnel barrier layer, wherein the semiconductor layer, tunnel
barrier layer, the interface layer and the first layer form a
spin-injector or detector device.
10. The magnetic tunnel element according to claim 1, wherein the
interface layer includes at least one of Fe and an Fe-containing
alloy.
11. The magnetic tunnel element according to claim 10, wherein the
Fe-containing alloy includes Co.
12. The magnetic tunnel element according to claim 11, wherein the
CoFe alloy contains between about 10 atomic percent and 95 atomic
percent Fe.
13. The magnetic tunnel element according to claim 10, wherein the
Fe-containing alloy includes Ni.
14. The magnetic tunnel element according to claim 10, wherein the
Fe-containing alloy is formed from Fe and at least one of Co and
Ni.
15. The magnetic tunnel element according to claim 1, wherein the
tunnel barrier layer includes an oxide of at least one of Al, Ga
and In.
16. The magnetic tunnel element according to claim 1, wherein the
first layer includes an alloy of Co, Fe and B.
17. The magnetic tunnel element according to claim 16, wherein the
CoFeB alloy is an alloy of the form
(CO.sub.70Fe.sub.30).sub.100-xB.sub.x.
18. The magnetic tunnel element according to claim 17, wherein x is
between about 15 and 20.
19. The magnetic tunnel element according to claim 1, wherein the
first layer includes an alloy of Co, Fe, X and Y, wherein X and Y
are independent and chosen from the group consisting of B, Hf, Zr,
C, Be, Si, Ge, P and Al.
20. The magnetic tunnel element according to claim 19, wherein at
least one of X and Y causes the alloy to be amorphous.
21. The magnetic tunnel element according to claim 1, wherein the
first layer includes an alloy of Co, Fe and Zr.
22. The magnetic tunnel element according to claim 1, wherein the
thickness of the interface layer is less than 30 .ANG..
23. The magnetic tunnel element according to claim 1, wherein the
thickness of the interface layer is less than 20 .ANG..
24. The magnetic tunnel element according to claim 1, wherein the
thickness of the interface layer is selected so that the interface
layer is amorphous.
25. A magnetic tunnel element, comprising: a first layer formed
from an amorphous material; an amorphous tunnel barrier layer; and
an interface layer between and in proximity with the first layer
and the tunnel barrier layer, the interface layer being formed from
at least one material selected from the group consisting of
ferromagnetic materials and ferrimagnetic materials, wherein the
interface layer material is crystalline when it is in isolation
from both the first layer and the tunnel barrier layer, the
thickness of the interface layer being selected so that the
interface layer is not crystalline.
26. The magnetic tunnel element according to claim 25, wherein the
first layer is formed from at least one material selected from the
group consisting of amorphous ferromagnetic materials and amorphous
ferrimagnetic materials.
27. A memory device, comprising: a first plurality of conductive
lines; a second plurality of conductive lines overlapping the first
plurality of conductive lines at a plurality of intersecting
regions; and a plurality of nonvolatile memory cells formed at
respective intersecting regions, at least one nonvolatile memory
cell including a magnetic tunnel element comprising a first layer
formed from an amorphous material, an amorphous tunnel barrier
layer, and an interface layer between the first layer and the
tunnel barrier layer, wherein the interface layer is formed from at
least one material that is crystalline when the material is in
isolation from both the first layer and the tunnel barrier layer,
and wherein the interface layer is formed from a material selected
from the group consisting of ferromagnetic materials and
ferrimagnetic materials.
28. The memory device according to claim 27, wherein the first
layer is formed from at least one material selected from the group
consisting of amorphous ferromagnetic materials and amorphous
ferrimagnetic materials.
29. The memory device according to claim 28, wherein the magnetic
tunnel element further includes a second layer in contact with the
tunnel barrier layer and formed from at least one material selected
from the group consisting of ferromagnetic materials and
ferrimagnetic materials, and wherein the first layer, the interface
layer, the tunnel barrier layer and the second layer form a
magnetic tunnel junction.
30. The memory device according to claim 29, wherein the magnetic
tunnel junction has a tunneling magnetoresistance that is greater
than 50%.
31. The memory device according to claim 29, wherein the magnetic
tunnel junction has a tunneling magnetoresistance that is greater
than 60%.
32. The memory device according to claim 29, wherein the magnetic
tunnel junction has a tunneling magnetoresistance that is about
65%.
33. The memory device according to claim 27, wherein the thickness
of the interface layer is less than 30 .ANG..
34. The memory device according to claim 27, wherein the thickness
of the interface layer is less than 20 .ANG..
35. The memory device according to claim 27, wherein the thickness
of the interface layer is selected so that the interface layer is
amorphous.
36. A method for forming a magnetic tunnel element, comprising:
forming an amorphous tunnel barrier layer; and forming an interface
layer on the tunnel barrier layer, the interface layer being formed
from a material that is crystalline when the material is in
isolation from the tunnel barrier layer.
37. The method according to claim 36, wherein the interface layer
has a thickness selected so that it is amorphous.
38. The method according to claim 36, wherein forming the interface
layer includes rapidly quenching the interface layer to make the
interface layer amorphous.
39. The method according to claim 36, wherein forming the interface
layer includes depositing the interface layer on the tunnel barrier
layer at a cryogenic temperature, the interface layer including at
least one of a ferromagnetic film and a ferrimagnetic film.
40. The method according to claim 36, further comprising bombarding
the interface layer with energetic ions after the interface layer
has been formed on the tunnel barrier layer, the interface layer
including at least one of a ferromagnetic film and a ferrimagnetic
film.
41. The method according to claim 36, wherein the interface layer
is formed from at least one material selected from the group
consisting of ferromagnetic materials and ferrimagnetic
materials.
42. The method according to claim 36, further comprising forming a
first layer on the interface layer, the first layer being formed
from an amorphous material.
43. The method according to claim 41, wherein the interface layer
is formed from a material that is crystalline when it is in
isolation from both the tunnel barrier layer and the first
layer.
44. The method according to claim 41, wherein the first layer is
formed from at least one material selected from the group
consisting of amorphous ferromagnetic materials and amorphous
ferrimagnetic materials.
45. The method according to claim 41, further comprising forming a
second layer in contact with the tunnel barrier layer, the second
layer including at least one material selected from the group
consisting of ferromagnetic materials and ferrimagnetic materials,
and wherein the first layer, the interface layer, the tunnel
barrier layer, and the second layer form a magnetic tunnel
junction.
46. The method according to claim 45, wherein the magnetic tunnel
junction has a tunneling magnetoresistance that is greater than
50%.
47. The method according to claim 45, wherein the magnetic tunnel
junction has a tunneling magnetoresistance that is greater than
60%.
48. The method according to claim 45, wherein the magnetic tunnel
junction has a tunneling magnetoresistance that is greater than
65%.
49. The method according to claim 42, further comprising: forming a
metal-containing layer in contact with the tunnel barrier layer;
and forming a semiconductor layer that is in contact with the first
layer, wherein the metal-containing layer, the tunnel barrier, the
interface layer, the first layer and the semiconductor layer form a
magnetic tunneling transistor.
50. The method according to claim 42, further comprising forming a
semiconductor material layer in proximity with the tunnel barrier
layer, wherein the semiconductor layer, tunnel barrier layer, the
interface layer and the first layer form a device that can be used
for at least one of spin injection and spin detection.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to Magnetic Tunnel Junction
(MTJ) Magnetoresistive (TMR) devices for use as magnetic field
sensors, such as read heads for reading magnetically recorded data,
as memory cells in nonvolatile magnetic random access memory (MRAM)
cells, and for magnetic logic and spintronic applications. More
particularly, the present invention relates to magnetic tunneling
elements and MTJ devices having significantly improved
magnetoresistance.
[0003] 2. Description of the Related Art
[0004] Magnetic Tunnel Junctions (MTJ) are promising candidates for
memory storage cells by enabling a nonvolatile, high performance
Magnetic Random Access Memory (MRAM). An MTJ-based MRAM has the
potential to rival conventional Dynamic Random Access Memory (DRAM)
in density and cost, and conventional Static Random Access Memory
(SRAM) in speed. In addition, MRAM is truly nonvolatile, that is,
the state of the memory is maintained even after the power has been
removed from the memory. Furthermore, each MTJ bit can be read
non-destructively without changing its magnetic state. For MTJ-MRAM
to directly replace conventional semiconductor memory technologies,
however, it is preferred that the materials making up an MTJ memory
are built on complementary metal oxide semiconductor (CMOS)
circuits that are necessary to read and write the state of the MTJ
cells. For example, MTJ-MRAM arrays having a cross-point
architecture are described in U.S. Pat. Nos. 6,097,625 to
Scheuerlein and 5,640,343 to Gallagher et al.
[0005] An MTJ includes two ferromagnetic layers separated by a thin
insulating layer, wherein the resistance (or conductance) through
the layers depends on the relative orientation of the magnetic
moments of the ferromagnetic layers. The change in resistance as
the orientation of the moments is changed from parallel to
anti-parallel divided by the resistance for the parallel state is
the tunneling magnetoresistance (TMR). The most useful MTJ for
memory cells has the magnetic moment of one of the ferromagnetic
(FM) layers (termed a storage layer) free to rotate and the
magnetic moment of the other ferromagnetic layer fixed or pinned
(termed a reference layer) by being exchange-biased with a thin
antiferromagnetic (AF) layer.
[0006] In a cross-point MRAM, the MTJ devices are switched by the
application of two magnetic fields that are applied along two
orthogonal directions, the "easy" and "hard" axes of the MTJ
device. Typically, the magnetic easy and hard anisotropy axes are
defined by the shape of the MTJ element through the self
demagnetizing fields. The MRAM array contains a series of MTJ
elements arranged along a series of word and bit lines, typically
arranged orthogonal to one another. For example, FIG. 1 depicts an
exemplary cross-point array 100 of an MTJ-MRAM. Cross-point array
100 includes a plurality of MTJ memory cells 01, a plurality of row
lines 102 (also referred to as word lines), and a plurality of
column lines 103 (also referred to as bit lines). An MTJ memory
cell 101 is located at an intersection of a row line 102 and a
column line 103. The magnetic switching fields are realized by
passing currents along the word and bit lines. All of the cells
along a particular word or bit line are subjected to the same word
or bit line field. Thus, the width of the distribution of switching
fields for the selected MTJs (those subject to the vector sum of
the bit and word line fields) must be sufficiently narrow that it
does not overlap the distribution of switching fields for the
half-selected devices. The magnetic moment within the MTJ devices
lies along a particular direction, which is referred to as the
"easy" axis. The orthogonal direction is the magnetic "hard"
axis.
[0007] One of the most challenging problems for the successful
development of MTJ memory storage cells is to obtain sufficiently
uniform switching fields for a large array of MTJ cells. This
uniformity can be characterized in various ways. One method is to
use a parameter termed the "array quality factor" (AQF). The AQF
represents the mean switching field at zero hard-axis field divided
by the standard deviation of the same distribution for an array of
MTJ devices. The AQF parameter is useful when the observed
distribution of switching fields follows a Gaussian distribution.
In some cases, though, the observed distribution of switching
fields may not follow a simple Gaussian form, for example, when the
magnetic elements may have two different ground states having
similar energies.
[0008] Based on a statistical model that takes into account the MTJ
memory cell size, shape, and pitch along the word and bit lines
together with the pattern of written states of the MTJ elements for
an 1 Mbit array, it is estimated that an AQF above .about.10 to 20
is needed for reliable write operation (write yield close to 100%)
of such a MRAM chip having elliptical cells formed at 0.18 micron
ground rules. The AQF required is sensitive to details of the
writing procedure. The observed AQF is influenced by various
factors including the lithographic patterning of the individual MTJ
storage elements, as well as details of the process integration,
especially through the dielectric material surrounding the
patterned edges of the MTJ elements, and by the MTJ materials and
structure itself. One clear limitation on the AQF is the
polycrystalline nature of the MTJ materials, which becomes
especially important for high density MRAM in which the size of the
MTJ element is so small that the MJT element contains only a small
number of crystalline grains. It has been hypothesized that for
very small MTJ cells, the relatively small number of crystalline
grains having random anisotropy axes can lead to significant
variations in magnetic switching field from device to device.
Improved AQFs may be obtained by either reducing the crystalline
grain size or by using amorphous ferromagnetic (a-FM) materials
having no macroscopic granular structure.
[0009] MTJs with a-FM storage layers, in addition to having a
narrower distribution of magnetic switching fields, are also
expected to have other improved magnetic properties, such as lower
values of the coercive fields H.sub.c. The latter property is
advantageous as this allows for reduced write currents. Typically,
an amorphous FM will have a lower magnetic moment than its
crystalline counterpart because the alloy is made amorphous by the
addition of non-magnetic elements, thereby diluting the
magnetization. This aspect can be advantageous for MRAM
applications because the self-demagnetization fields, which are
directly proportional to the magnetization of the ferromagnet for
otherwise identical structures, would thereby be reduced. The Curie
temperature (T.sub.C) of amorphous alloys can be varied by varying
the alloy composition, but can be sufficiently high for MRAM and
other applications. For example, this may be useful for
thermally-assisted writing techniques. See, for example, U.S. Pat.
No. 6,538,919 to Abraham et al. Similarly, other magnetic
properties, for example, the magnetostriction of the amorphous
alloy can often be tuned to the required value by choice of the
alloy composition. In this regard, it is useful to be able to add a
considerable number of different constituents to the amorphous
alloy to be able to engineer the magnetic properties of the alloy
as is desired.
[0010] Amorphous FM films are also expected to have improved
elastic properties because such films will resist plastic
deformation. The lack of an ordered atomic lattice implies the
absence of dislocations. More importantly, a-FM layers should
exhibit improved corrosion resistance due to the absence of grain
boundaries along which contaminants can diffuse. Similarly, the
thermal stability of MTJs having a-FM layers will be more thermally
stable because diffusion of material is enhanced along grain
boundaries in thin film materials.
[0011] Previous studies have disclosed that magnetic storage layers
consisting of CoFeB or CoFeNbB, or CoNbB a-FM alloys yield MTJs or
giant magnetoresistive (GMR) devices having softer magnetic
characteristics (i.e., lower coercivity) than equivalent MTJs or
GMR devices having crystalline magnetic layers. See, for example,
U.S. Pat. No. 6,436,526 to Odagawa et al. Also, see U.S. Pat. No.
6,028,786 to Nishimura, which describes the use of CoFeB alloys in
an MTJ stack.
[0012] What is needed is an MTJ element having a high TMR and a
thermal stability that can withstand typical MRAM processing
thermal budgets.
BRIEF SUMMARY OF THE INVENTION
[0013] The present invention provides a magnetic tunneling element
which provides a source of highly spin polarized electrons for a
variety of applications. In particular, the magnetic tunneling
element can be incorporated as part of a magnetic tunneling
junction (MTJ) device, which thereby has higher TMR than prior art
devices, while at the same time having high thermal stability that
can withstand a typical MRAM processing thermal budget. The MTJ
device with improved TMR can be used, for example, as a storage
element in an MRAM, or as a sensing element, such as in a magnetic
recording disk drive read head.
[0014] The advantages of the present invention are provided by a
magnetic tunnel element having a first layer formed from an
amorphous material, an amorphous tunnel barrier layer, such as
Al.sub.2O.sub.3, and an interface layer between the first layer and
the tunnel barrier layer. According to the present invention, the
interface layer is formed from a material that is typically
crystalline and typically not amorphous when the material is in
isolation from both the first layer and the tunnel barrier layer.
For a limited range of thickness of the interface layer, the
magneto-tunneling characteristics of the magnetic tunneling element
are improved. For example, the thickness of the interface layer is
preferably less than .about.25 .ANG. when comprised of an alloy of
Co and Fe. Alternatively, the thickness of the interface layer is
selected so that the interface layer is not crystalline. The first
layer is preferably formed from at least one material selected from
the group consisting of an amorphous ferromagnetic material and an
amorphous ferrimagnetic material. For example, the first layer can
be a CoFeB alloy of the form (CO.sub.70Fe.sub.30).sub.100-xB.sub.x
in which x is between about 15 and 20. Alternatively, the first
layer is an alloy of the form CoFeX or CoFeXY, in which X and Y are
chosen from the group consisting of B, Hf, Zr, C, Be, Si, Ge, P and
Al, so that at least one of X and Y of the alloy causes the alloy
to be amorphous. The interface layer is advantageously formed from
at least one material selected from the group consisting of
ferromagnetic materials and ferrimagnetic materials. For example,
the interface layer can be an Fe-containing alloy, such as CoFe
containing between about 10 atomic percent and 60 atomic percent
Fe. As another alternative, the Fe-containing alloy is NiFe. Or as
another alternative, the Fe-containing alloy can be formed from a
CoNiFe alloy.
[0015] The magnetic tunnel element can further include a second
layer that is in contact with the tunnel barrier layer and formed
from at least one material selected from the group consisting of
ferromagnetic materials and ferrimagnetic materials. The first
layer, the interface layer, the tunnel barrier layer and the second
layer form a magnetic tunnel junction that has an improved
tunneling magnetoresistance that is preferably greater than
50%.
[0016] Alternatively, the magnetic tunnel element can include a
second interface layer that is formed from a material selected from
the group consisting of ferromagnetic materials and ferrimagnetic
materials and that is in contact with the tunnel barrier, as well
as a second layer that is formed from at least one material
selected from the group consisting of amorphous ferromagnetic
materials and amorphous ferrimagnetic materials. The first layer,
the interface layer, the tunnel barrier layer, the second interface
layer, and the second layer form a magnetic tunnel junction that
has an improved tunneling magnetoresistance that is preferably
greater than 50%.
[0017] Alternatively, the magnetic tunnel element can further
include a metal-containing layer that is in contact with the tunnel
barrier layer and a semiconductor layer that is in contact with the
first layer. The metal-containing layer includes a metallic layer
that is formed from at least one of a non-ferromagnetic material
and a non-ferrimagnetic material. The metal-containing layer, the
tunnel barrier, the interface layer, the first layer and the
semiconductor layer together form a magnetic tunneling
transistor.
[0018] As yet another alternative, the magnetic tunnel element can
further include a semiconductor material layer that is in contact
with the tunnel barrier layer. The semiconductor layer, tunnel
barrier layer, the interface layer and the first layer together
form a spin-injector device. Additionally, there may be a metal
containing layer between the tunnel barrier and the
semiconductor.
[0019] The present invention also provides a method for forming a
magnetic tunnel element in which an amorphous tunnel barrier layer
is formed and an interface layer is formed on the tunnel barrier
layer. According to the invention, the interface layer is formed
from a material that is typically crystalline when the material is
in isolation from the tunnel barrier layer. Alternatively, the
thickness of the interface layer is selected so that the interface
layer is not crystalline. The interface layer is formed from a
material selected from the group consisting of ferromagnetic
materials and ferrimagnetic materials. One technique that can be
used so that the interface layer is not crystalline includes
rapidly quenching the interface layer. Another technique that can
be used so that the interface layer is not crystalline includes
depositing the interface layer on the tunnel barrier layer at a
cryogenic temperature when the interface layer is formed from one
at least one of a ferromagnetic film and a ferrimagnetic film.
Still another technique that can be used so that the interface
layer is not crystalline includes bombarding the interface layer
with energetic ions after the interface layer has been formed on
the tunnel barrier layer when the interface layer is formed from at
least one of a ferromagnetic film and a ferrimagnetic film.
[0020] A first layer can be formed on the interface layer, such
that the first layer is formed from an amorphous material (example,
at least one material selected from the group consisting of
amorphous ferromagnetic materials, amorphous ferrimagnetic
materials and amorphous non-ferromagnetic or non-ferrimagnetic
materials). The interface layer is formed from a material that is
crystalline when the material is in isolation from both the tunnel
barrier layer and the first layer.
[0021] A second layer, formed from at least one material selected
from the group consisting of ferromagnetic materials and
ferrimagnetic materials, can be formed in contact with the tunnel
barrier layer. Accordingly, the first layer, the interface layer,
the tunnel barrier layer, and the second layer together form a
magnetic tunnel junction having a tunneling magnetoresistance that
is greater than 50%.
[0022] Alternatively, a metal-containing layer can be formed in
contact with the tunnel barrier, and a semiconductor layer can be
formed that is in contact with the first layer. The
metal-containing layer includes a metallic layer formed from at
least one of a non-ferromagnetic material and a non-ferrimagnetic
material. The metal-containing layer, the tunnel barrier, the
interface layer, the first layer and the semiconductor layer
together form a magnetic tunneling transistor.
[0023] As yet another alternative, a semiconductor material layer
can be formed in contact with the tunnel barrier layer. The
semiconductor layer, tunnel barrier layer, the interface layer and
the first layer together form a spin-injector device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The present invention is illustrated by way of example and
not by limitation in the accompanying figures in which like
reference numerals indicate similar elements and in which:
[0025] FIG. 1 depicts an exemplary cross-point array 100 of an
MTJ-MRAM;
[0026] FIG. 2 depicts a cross-sectional diagram of an exemplary
structural arrangement of a conventional magnetic tunnel
junction;
[0027] FIG. 3 depicts a cross-sectional diagram of an exemplary
structural arrangement of an MTJ element having a thin interface
layer of CoFe inserted between a tunnel barrier and a layer of
amorphous material;
[0028] FIGS. 4A and 4B are graphs respectively showing the TMR and
resistance for an exemplary MTJ element according to the present
invention as a function of the thickness of an interface CoFe layer
that is between an Al.sub.2O.sub.3 insulating barrier and a CoFeHfB
layer;
[0029] FIG. 5 is a graph showing TMR as a function of thickness of
an interface layer formed from CO.sub.70Fe.sub.30 and an amorphous
layer formed from a B-doped CO.sub.70Fe.sub.30 layer according to
the present invention;
[0030] FIGS. 6A and 6B are graphs respectively showing resistance
versus field loops for the same MTJ sample subjected to fields
varying between .+-.2000 Oe in the plane of the sample and to
fields between .+-.200 Oe in the plane of the sample, where the
sample has an interface layer formed from CoFe and an amorphous
ferromagnetic layer formed from CoFeZr;
[0031] FIGS. 7A and 7B are graphs respectively showing
representative thermal stability for TMR and resistance for MTJ
devices with CoFeX and CoFe/CoFeX layers in which CoFeX is
(CO.sub.70Fe.sub.30).sub.85Zr.sub.15,
(CO.sub.70Fe.sub.30).sub.85Hf.sub.15, and
(CO.sub.70Fe.sub.30).sub.85C.su- b.15;
[0032] FIG. 8 is a graph showing values of spin polarization as a
function of the thickness of a CoFe interface layer for a magnetic
tunneling element with CoFe interface layers and an amorphous
ferromagnetic layer formed from a
(CO.sub.70Fe.sub.30).sub.85Zr.sub.15 alloy and a
(CO.sub.70Fe.sub.30).sub.85Hf.sub.15 alloy;
[0033] FIGS. 9A-9C depict cross-sectional diagrams of exemplary MTJ
structures according to the present invention having high-spin
polarization of tunneling current; and
[0034] FIG. 10 depicts a cross-sectional diagram of an exemplary
MTJ device according to the present invention having a thin spacer
layer between a tunnel barrier and a ferromagnetic layer.
DETAILED DESCRIPTION OF THE INVENTION
[0035] The present invention provides MTJ structures having
Al.sub.2O.sub.3 tunnel barriers exhibiting TMR of nearly 70%.
According to the present invention, one layer of an MTJ element is
formed from the class of ferromagnetic (FM) materials of the form
CoFeX or CoFeXY in which the dopants X and Y are chosen from B, Hf,
Zr, C, Be, Si, Ge, P and Al, or combinations of two or more of
these elements. Use of a thin interface layer of CoFe along with
these CoFeX and CoFeXY alloys significantly enhances the TMR of an
MTJ element according to the present invention. The enhanced TMR
values together with excellent magnetic-switching characteristics
(high AQF values) make CoFeX and CoFeXY alloys with CoFe interface
layers extremely attractive for MRAM applications as well as other
applications, for example, magnetic recording read heads, in which
high TMR values are needed in comparatively low magnetic fields.
The layers of an MTJ device according to the present invention show
very good thermal stability, sufficient to survive typical MRAM
processing thermal budgets. Using an appropriate choice of CoFe and
CoFeX or CoFeXY, the layers are stable to temperatures that are
typically greater than 400 C.
[0036] FIG. 2 depicts a cross-sectional diagram of an exemplary
structural arrangement of a conventional MTJ device 200. A typical
conventional MTJ structure includes a seed layer, an
anti-ferromagnetic exchange-bias layer, a reference ferromagnetic
electrode, a tunnel barrier, a storage, or "free," ferromagnetic
layer and a cap layer. In particular, MTJ element 200 includes a
bottom "fixed" ferromagnetic (F) layer 218, an insulating tunnel
barrier layer 224, and a top "free" ferromagnetic layer 234. MTJ
200 has bottom and top electrical leads 212 and 236, respectively,
with bottom lead 212 being formed on a suitable substrate 211, such
as a silicon oxide layer. Ferromagnetic layer 218 is called the
fixed layer because its magnetic moment is prevented from rotating
in the presence of an applied magnetic field in the desired range
of interest for a MTJ device, e.g., the magnetic field caused by a
write current applied to the memory cell from the read/write
circuitry of the MRAM or the magnetic field from the recorded
magnetic layer in a magnetic recording disk. The magnetic moment of
ferromagnetic layer 218, the direction of which is indicated by
arrow 290 in FIG. 2, can be fixed by forming ferromagnetic layer
218 from a high coercivity magnetic material or by
exchange-coupling ferromagnetic layer 218 to an antiferromagnetic
layer 216. The magnetic moment of free ferromagnetic layer 234 is
not fixed, and is thus free to rotate in the presence of an applied
magnetic field in the range of interest. In the absence of an
applied magnetic field, the respective moments of ferromagnetic
layers 218 and 234 are aligned generally parallel (or
anti-parallel) in an MTJ memory cell (as indicated by double-headed
arrow 280 in FIG. 2) and generally perpendicular in a MTJ
magnetoresistive read head. The relative orientation of the
respective magnetic moments of ferromagnetic layers 218 and 234
affects the tunneling current and, thus, the electrical resistance
of the MTJ device. Bottom lead 212, antiferromagnetic layer 216,
and fixed ferromagnetic layer 218 together may be regarded as
constituting a lower electrode 210.
[0037] Conventional MTJ devices have been prepared on silicon
substrates with 250 .ANG. thick thermally oxidized SiO.sub.2 in a
high-vacuum chamber having a base pressure of
.about.2.times.10.sup.-9 Torr. The layers were sequentially
deposited by magnetron sputtering in .about.3 mTorr of Argon
(although a range of sputtering pressures was also used). In order
to explore the properties of a wide range of MTJ structures and
materials, MTJ devices have been patterned using metal shadow
masks. A sequence of three metal shadow masks has been used,
without breaking vacuum, to define the bottom electrode, the tunnel
barrier and the top electrode. The junction area for these MTJ
devices was .about.80.times.80 .mu.m.sup.2.
[0038] Such a conventional MTJ structure, shown in FIG. 2, in which
the reference ferromagnetic layer, i.e., layer 218, is fixed by
exchange bias with an antiferromagnetic layer, has been referred to
as a "simple-pinned" MTJ. The reference layer is formed from a FM
layer, which can be a single ferromagnetic layer or a stack of
multiple ferromagnetic layers. The reference layer could
alternately be an "artificial antiferromagnet (AAF)" in which the
FM layer is replaced by a sandwich of two thin FM layers that are
separated by an antiferromagnetic (AF) coupling spacer layer formed
from a non-ferromagnetic conducting material, most commonly Ru or
Os. See, for example, U.S. Pat. No. 5,841,692 to Gallagher et al.
The thickness of the spacer layer is chosen so that the two FM
layers are strongly AF exchanged coupled, thereby ensuring that the
magnetic moments of the two ferromagnetic are aligned anti-parallel
to one another. See, for example, U.S. Pat. No. 5,341,118 to Parkin
et al. Such an AAF structure provides greater magnetic stability
for the reference layer and also can be used to control the
coupling via magnetostatic fields at the edges of this layer to the
storage layer, as described in U.S. Pat. No. 5,465,185 to Parkin et
al.
[0039] For conventional MTJ 200, underlayer 212 and cap layer 236
are each typically formed from 50 .ANG. TaN/50 .ANG. Ta, although
these thicknesses have been varied over a wide range. The TaN layer
has been deposited in a manner consistent with previously
established procedures and, in particular, by reactive sputtering
of Ta in an Ar--N.sub.2 gas mixture, such as described in U.S. Pat.
No. 6,518,588 to Parkin et al. The AF layer is formed from
.about.150-250 .ANG. Ir--Mn in which the Ir content of the alloy is
in the range 20 to 30 atomic percent. Other AF materials can also
be used including Pt--Mn, Os--Mn, Ni--Mn, Pd--Mn, Os--Ir--Mn,
CrPtMn, etc. For simple pinned MTJs, fixed FM layer 218 is
.about.35 .ANG. CO.sub.70Fe.sub.30 and for MTJs having AAF
reference layers, the AAF layer is .about.20 .ANG. FM/x Ru/17 .ANG.
FM, with x in the range from 6 to 11 .ANG., which gives rise to AF
coupling of the FM layers. The tunnel barrier is formed by
depositing 22 .ANG. Al, which is subsequently plasma oxidized in
.about.100 mTorr oxygen to form an Al.sub.2O.sub.3 barrier. Other
methods could be used to form tunnel barrier 224, including
reactive sputtering of Al in an Ar+O.sub.2 mixture or by natural
oxidation of metallic Al. Storage layer 234 is 75 .ANG. thick.
[0040] The magnetic and magneto-transport properties of simple
pinned MTJs having FM storage layers that are formed from an
extensive range of B-doped CoFe alloys have been explored.
Crystalline Co--Fe alloys are well known to become amorphous when B
is added to these alloys in the range of .about.10-25 atomic
percent. See, for example, A. R. Ferchmin et al., Amorphous
Magnetism and Metallic Magnetic Materials Digest, North Holland
Publishing Company, New York (1983). The TMR of the MTJs depends on
both the B and the Co--Fe composition of the alloy. For CoFeB
alloys, with B contents of between 15 and 20 atomic percent, the
TMR increases as the Fe content of the CoFe alloy increases from
.about.10 to .about.30 atomic percent. For an CoFeB alloy with a
Co:Fe ratio of 70:30 the maximum TMR was found for a B content of
.about.15%.
[0041] The highest TMR for MTJs having CoFeB alloy storage layers
was found to be the alloy with composition
(CO.sub.70Fe.sub.30).sub.85B.sub.1- 5 for which the TMR is 62% at
room temperature. This value is significantly higher than found for
MTJs containing storage layers formed from crystalline Co--Fe
alloys, for which the maximum TMR observed was .about.55%.
[0042] The magnetic switching properties of CoFeB alloys have been
examined by carrying out AQF studies on lithographically patterned
arrays of magnetic nanoelements with structures of the type: 50
.ANG. TaN.vertline.50 .ANG. Ta.vertline.10 .ANG. Al plasma oxidized
.vertline.t CoFeB alloy .vertline.100 .ANG. TaN in which the
thickness of the storage layer t was varied in the range 20-100
.ANG.. These structures include only the storage layer and so allow
for the determination of AQF factors without regard to the
reference layer. The dependence of the AQF on storage layer
thickness allows surface contributions to be distinguished from
volume effects. The MJT storage elements in these studies were of
an elliptical shape and were of .about.0.3.times.0.6 .mu.m in size.
Arrays of (CO.sub.70Fe.sub.30).sub.1-x B.sub.x elements displayed
AQF values of 20 at a switching field of .about.100 Oe, which meets
the write yield requirements for MRAM arrays. Typically, much
higher AQF values were found for structures having amorphous CoFeB
storage layers than for otherwise identical arrays of magnetic
nanoelements with storage layers that were formed from crystalline
CoFe alloys. Thus, amorphous free layers have considerable
advantages for MRAM storage cells.
[0043] CoFe and other 3d transition metal ferromagnets can be made
amorphous by adding one or more of a range of non-magnetic
elements, including B, Hf, Zr, C, Be, Si, Ge, P and Al. In
particular, MTJs have been studied in which the storage layer was
formed from various alloys of the form CoFeXY, in which X and Y are
chosen from B, Hf, Zr, C, Be, Si, Ge, P and Al. The CoFeXY alloys
become amorphous when the concentration of X and/or Y exceed a
critical value, which depends on various factors, including the
nature of the underlayer (in this case, tunnel barrier
Al.sub.2O.sub.3), and the overlayer (in this case, cap layer TaN),
as well as the thickness of the CoFeXY layer and the temperature at
which the layer is deposited or subsequently is subjected to. The
CoFeXY layer, if amorphous, will typically undergo an amorphous to
crystalline transition at some critical temperature that is
sensitive to the concentration of X and Y, as well as to the
underlayer and cap layer, and the thickness of the CoFeXY layer.
Thus the amorphous state is typically not the thermodynamically
lowest energy state but is rather a metastable state.
[0044] MTJs having CoFeXY storage layers in which X and Y are not
boron display lower TMR values those found than for CoFeB or for
crystalline CoFe layers. For example, MTJs with
(CO.sub.70Fe.sub.30).sub.85Zr.sub.15 storage layers show TMR values
of up to 45-50% which are nearly the same as crystalline CoFe
layers. On the other hand MTJs with
(CO.sub.70Fe.sub.30).sub.80Si.sub.20 show TMR values of only
.about.25-30%, which are much lower than those found for
crystalline CoFe storage layers. Because, however, amorphous
storage layers have significant benefits with respect to magnetic
switching uniformity and control, studies have been carried out in
which a thin interface layer of CoFe was inserted between the
tunnel barrier and the CoFeXY storage layer. In particular, FIG. 3
depicts a cross-sectional diagram of an exemplary structural
arrangement of an MTJ element 300 according to the present
invention having a thin interface layer of CoFe inserted between a
tunnel barrier and a layer of amorphous material. MTJ element 300
includes an antiferromagnetic layer 316, a bottom "fixed"
ferromagnetic (or ferrimagnetic) (F) layer 318, an insulating
tunnel barrier layer 324, a thin interface layer of CoFe 328, and a
top "free" ferromagnetic layer 334. MTJ element 300 has bottom and
top electrical leads 312 and 336, respectively, with bottom lead
312 being formed on a suitable substrate 311, such as a silicon
oxide layer. The exemplary structure for MTJ 300 provides TMR
values that are much higher than those obtained for storage layers
formed only from CoFe or from a CoFeXY alloy, as illustrated in
FIGS. 4A and 4B for the case of X=Hf and Y=B. Thus, according to
the present invention, the introduction of thin, nominally
crystalline, CoFe layers at the interface between CoFeXY alloy 334
and Al.sub.2O.sub.3 tunnel barrier 324 significantly increases the
tunneling magnetoresistance and gives rise to higher TMR values
than are possible with either crystalline CoFe alloys or amorphous
CoFeXY alloys, alone.
[0045] FIGS. 4A and 4B are graphs respectively showing the TMR and
resistance (R) for an exemplary MTJ element according to the
present invention as a function of the thickness of an interface
CoFe layer that is placed in contact with an Al.sub.2O.sub.3
insulating barrier under a CoFeHfB storage layer. Specifically,
FIGS. 4A and 4B show the average TMR and resistance, respectively,
of ten MTJs formed by shadow masks, each having an area of
.about.80.times.80 .mu.m.sup.2 and the corresponding error bars
show the distribution of TMR and R (.+-.1.sigma.). Sample MTJ
devices having the exemplary structure shown in FIG. 3 were sputter
deposited onto silicon oxide layers 311 formed on a silicon
substrate. Underlayer 312 is formed from a bilayer of 50 .ANG. TaN
and 50 .ANG. Ta in which these layers are deposited by magnetron
sputtering in .about.3 mTorr of Argon at nominally room temperature
by first reactively sputtering a layer of Ta in an Ar--N.sub.2 gas
mixture with .about.10% N.sub.2 followed by sputtering of Ta in
pure Argon. An antiferromagnetic exchange bias layer 316 of 150
.ANG. IrMn, in which the Ir content is .about.28 atomic percent Ir,
is subsequently formed on top of Ta layer 312. A fixed
ferromagnetic layer 318 is formed from 35 .ANG. CO.sub.70Fe.sub.30.
Tunnel barrier 324 is formed by first depositing a layer of Al
metal 22 .ANG. thick, which is then plasma oxidized in 100 mTorr
oxygen for 240 seconds. Storage layer 330 is formed from a bilayer
of CoFe and CoFeHfB. CoFe interface layer 328 is formed from an
alloy of composition CO.sub.70Fe.sub.30 having a thickness t in the
range from zero up to 20 .ANG.. Amorphous ferromagnetic layer 334
is formed from an alloy of
(CO.sub.70Fe.sub.30).sub.80Hf.sub.10B.sub.10 in which the layer
contains about 10 atomic percent of both Hf and B. The CoFe content
of layer 334 has about 30 atomic percent Fe. The total thickness of
storage layer 330 is maintained at .about.75 .ANG. so that layer
334 has a thickness of (75-t) A. Finally, cap layer 336 is formed
from a bilayer of 50 .ANG. TaN and 50 .ANG. Ta. The MTJs are then
annealed at 280 C for 90 minutes in a field of IT to set the
exchange bias and so fix the moment direction of the fixed layer
318.
[0046] FIG. 4A shows that the TMR for MTJ elements having no
interface layer 328 (i.e., thickness equals 0) is low (.about.38%),
but that the TMR increases substantially with the inclusion of a
CoFe interface layer 328 that is only 5 .ANG. thick. As the
interface layer thickness is further increased, the TMR also
increases and reaches a maximum value for a CoFe interface
thickness of .about.10-15 .ANG.. FIG. 4B shows that, even though
the TMR varies substantially with the thickness of interface layer
328, the resistance of the MTJ devices is not very sensitive to the
presence and thickness of the interface layer. The dependence of
TMR and R shown in FIGS. 4A and 4B are typical for the family of
amorphous CoFeXY ferromagnetic materials, although the magnitude of
the TMR varies depending on the detailed composition of CoFeXY.
FIG. 5 is a graph showing TMR as a function of thickness of an
interface layer 328 formed from CO.sub.70Fe.sub.30 and the
amorphous layer 334 is formed from a B-doped CO.sub.70Fe.sub.30
layer such that the B concentration is 10 atomic percent. In
contrast to FIG. 4A, layer 334 has a constant thickness of 100
.ANG. for all the MTJ element samples shown in FIG. 5. There are
other minor differences as well, such as the underlayer for the
MTJs of FIG. 5 was formed from 100 .ANG. Ta without a TaN layer and
the IrMn layer had a thickness of .about.250 .ANG.. Cap layer 336
was formed from a bilayer of 50 .ANG. TaN and 75 .ANG. Ta.
Notwithstanding these small structural differences, the basic
results are similar by exhibiting strongly enhanced TMR values in
comparison to the TMR values of conventional MTJs having storage
layers formed from either the CoFeB alloy only (i.e., t=0) or only
CoFe (i.e., no amorphous layer 334). In particular, the TMR is
increased for a range of thickness of CoFe interface layer 328 from
.about.5 to 25 .ANG.. This range of thickness depends on the
composition of layers 328 and 334.
[0047] MTJs having storage layers formed from bilayers of CoFe and
CoFeZr have the highest TMR values of nearly 70% amongst the
various combinations of interface layers 328 and storage layers 334
that were considered. Typical data for such devices are shown in
FIGS. 6A and 6B in which resistance versus field loops are shown
for the same MTJ sample, but subjected to fields varying between
.+-.2000 Oe in the plane of the sample in FIG. 6A and to fields
between .+-.200 Oe in the plane of the sample in FIG. 6B. The
exemplary structural configuration of the MTJ device for FIGS. 6A
and 6B is 50 .ANG. TaN/50 .ANG. Ta/150 .ANG. IrMn/35 .ANG.
CO.sub.70Fe.sub.30/22 .ANG. Al plasma oxidized for 240 seconds/10
.ANG. CO.sub.70Fe.sub.30/65 .ANG.
(CO.sub.70Fe.sub.30).sub.85Zr.sub.15/10- 0 .ANG. TaN/100 .ANG. Ta.
Interface layer 328 (FIG. 3) is formed from 10 .ANG.
CO.sub.70Fe.sub.30 and amorphous layer 334 is formed from 65 .ANG.
(CO.sub.70Fe.sub.30).sub.85Zr.sub.15. The TMR obtained for this
exemplary structural configuration was .about.67%, which is the
highest TMR yet reported for any MTJ sample having an
Al.sub.2O.sub.3 barrier. The minor loop shown in FIG. 6B has a
square shape indicating excellent switching characteristics for
this storage layer. Note that the coercivity of the storage layer
for this structural configuration is significantly lower than that
for a storage layer of CO.sub.70Fe.sub.30 alone and is similar to
that for (CO.sub.70Fe.sub.30).sub.85Zr.sub.15 without any CoFe
interface layer.
[0048] The successful integration of MTJs into MRAM requires that
the MTJ devices survive whatever is the integrated thermal anneal
budget as determined by the various processing steps to define the
MRAM chip. Thus, an important property of MTJ devices having CoFeX
and CoFe/CoFeX storage layers is their thermal stability.
Representative results are shown in FIGS. 7A and 7B for CoFeX and
CoFe/CoFeX in which CoFeX is (CO.sub.70Fe.sub.30).sub.85Zr.sub.15,
(CO.sub.70Fe.sub.30).sub.85Hf.sub.1- 5, and
(CO.sub.70Fe.sub.30).sub.85C.sub.15. FIGS. 7A and 7B are graphs
respectively showing representative thermal stability for TMR and
resistance for MTJ devices according to the present invention
having CoFeX and CoFe/CoFeX layers in which CoFeX is
(CO.sub.70Fe.sub.30).sub.85- Zr.sub.15,
(CO.sub.70Fe.sub.30).sub.85Hf.sub.15, and
(CO.sub.70Fe.sub.30).sub.85C.sub.15. The thermal stability
measurements were carried out in a specially designed annealing
furnace having a high-vacuum chamber with a base pressure of
5.times.10.sup.-8 torr and capable of heating an MTJ sample to 400
C in presence of an applied field of up to 4000 Oe. The annealing
measurement displayed in FIGS. 7A and 7B consisted of a sequential
series of ramp, soak, cool, and transport measurement steps.
Typically, the sample temperature was ramped to the chosen anneal
temperature at a ramp rate of .about.10 C/minute and soaked at this
temperature for {fraction (1/2)} hour, then cooled to 25 C. The
transport properties were then measured at 25 C. This was followed
by the next temperature step. FIGS. 7A and 7B also include data on
the thermal stability of MTJs having
CoFe/(CO.sub.70Fe.sub.30).sub.85Zr.sub.15 storage layers for
various CoFe interface layer thicknesses. These results show that
the variation of TMR and R with anneal temperature are not very
sensitive to the presence and thickness of the CoFe interface
layer. For all of the examples shown in FIGS. 7A and 7B having CoFe
interface layers, excellent thermal stability is found with
significant TMR of >45% even after annealing for {fraction
(1/2)} hour at 400 C.
[0049] CoFeXY layers are amorphous when the amount of X and/or Y
exceeds some critical value. The thermal stability of the MTJs
containing these amorphous layers is limited by the crystallization
temperature of the amorphous CoFeXY alloy. Typically, the
crystallization of the a-FM layers results in a dramatic increase
in coercivity of the storage layer and a corresponding drop in TMR.
The crystallization temperature is strongly influenced by the
concentration of the dopants X and Y and typically was found to
increase significantly with increasing amount of X and Y. The TMR,
however, often drops when the dopant concentration X and Y is
increased beyond some value. Thus, for MTJs having amorphous
ferromagnetic storage layers and no interface layer 328 (FIG. 3), a
compromise must be made between higher crystallization temperatures
and, thereby, higher thermal stability or higher TMR. With the
presence of interface layer 328, no such compromise must be made
because the magnitude of TMR is largely determined by the
properties of interface layer 328. Thus, layer 334 can be formed
from amorphous alloys that have very high crystallization
temperatures--much higher than those that the MTJ stack will be
subjected to during the fabrication of the MTJ devices. For
example, when the MTJ devices are incorporated into the
back-end-of-line (BEOL) of a CMOS technology, the devices will
likely be subjected to thermal treatments in the range of 400 to
450 C. Thus, the crystallization temperature of the CoFeXY alloys
must be above these temperatures. If the MTJs are the memory
storage cells in an embedded memory, then these devices may have to
withstand even higher temperatures.
[0050] Note the crystallization temperature of a-FM layer 334 may
be strongly influenced by the material and composition and
thickness of cap layer 336. The highest crystallization
temperatures were obtained with TaN cap layers. Similar MTJ
structures having Ta or TaCr cap layers (the latter are amorphous)
exhibit much lower crystallization temperatures. For TaCr, this was
due to the comparatively low crystallization temperature of
TaCr.
[0051] The TMR of MTJs is determined by, in large part, by the spin
polarization of the conduction electrons of the FM layers on either
side of the tunnel barrier. See, for example, M. Julliere, Phys.
Lett. 54A, 225 (1975). The spin polarization is defined as the
difference in density of states (DOS) of spin up and spin down
electrons at the Fermi energy divided by the total DOS at the Fermi
energy. Thus, the increased TMR found for thin CoFe interface
layers could be a result of an increase in the spin polarized
density of states in this layer. One possibility is that electrons
are confined in this thin interface layer because of a mis-match in
the band structures of ferromagnetic interface layer 328 and
ferromagnetic amorphous layer 334. The band structures of
insulating tunnel barrier 324 and metallic interface layer 328 are
clearly mis-matched because of the large band gap in the
Al.sub.2O.sub.3 insulating layer. Confinement, however, in metallic
non-ferromagnetic interface layers gives rise to a diminished TMR
and not an enhanced TMR. See, for example, S. Yuasa et al., "Spin
polarized resonant tunneling in magnetic tunnel junctions", Science
297, 234 (2002). Moreover, a more likely explanation is revealed by
cross-sectional transmission electron microscopy (XTEM) studies
that show that for the range of thickness of the CoFe interface
layer for which the TMR is enhanced, the CoFe layer is unexpectedly
amorphous in structure. As used herein amorphous means the absence
of crystalline order on larger length scales (for example, the
absence of any obvious granularity, such as crystalline grains) but
does not necessarily exclude the possibility of nano-crystallinity
on very short length scales (such as atomic length scales). Usually
CoFe is crystalline for the whole composition range of Co--Fe
alloys from pure Co to pure Fe. Moreover, for CoFe interface layers
thicker than about .about.20 .ANG., XTEM studies reveal that the
CoFe structure reverts to being crystalline. Thus, CoFe interface
layer 328 becomes amorphous in structure when layer 328 is very
thin, presumably due to its contact with the amorphous
Al.sub.2O.sub.3 tunnel barrier on one of its surfaces and the
amorphous ferromagnetic layer 334 on its other surface. Thus, there
is a clear correlation between enhanced TMR and the amorphous
structure of thin CoFe interface layers. This hypothesis is
confirmed by soft resonant emission x-ray spectroscopy studies on
bilayers of CoFe/CoFeB grown on Al.sub.2O.sub.3 that reveal
additional structure in the energy dependence of the density of
states at the Fermi Energy of this metallic bilayer for thin CoFe
interface layers. In particular, a sharp feature in the Fe 3d local
density of states is found for thin CoFe layers over the same range
of thickness for which the TMR is enhanced and the structure is
found to be amorphous. Interestingly, no such feature is found in
the Co 3d density of states. Thus, the enhanced TMR can be
attributed to an enhancement of the local Fe 3d density of states,
which itself is a result of the amorphization of thin CoFe layers.
Thus, a technique for enhancing the TMR of MTJs is revealed by
these observations. By taking nominally crystalline ferromagnetic
metals and making such metals amorphous, their density of states
can be enhanced leading to enhanced TMR values. According to the
present invention, the amorphization is carried out by sandwiching
thin ferromagnetic films between materials, which are amorphous. It
may also be possible to induce an amorphous structure to thin
ferromagnetic layers by other techniques that might include rapid
quenching of the structure of the thin film, by, for example,
depositing a ferromagnetic film at very low temperatures onto
tunnel barrier 324. Amorphization might also be possible by
bombarding the ferromagnetic layer, after deposition, with
energetic ions.
[0052] The high TMR of the MTJ device shown in FIG. 3 results from
the high-spin polarization of ferromagnetic layer 328 at the
interface with amorphous tunnel barrier 324. The high-spin
polarization results from the amorphization of normally crystalline
ferromagnetic alloys. In a simple description, the randomization of
the crystal structure of the alloy can be considered to result in a
reduction in the effective number of bonds between individual atoms
in the alloy thereby leading to a narrower conduction band, an
increase in the density of states at the Fermi energy and to a
higher spin polarization. What is needed is the amorphization of
interface layer 328, which is achieved by sandwiching a normally
crystalline material between two amorphous layers, that is, tunnel
barrier layer 324 and amorphous ferromagnetic layer 334. Note that
layer 334 does not need to be ferromagnetic and, indeed, for some
applications it may be advantageous to form layer 334 from a
material that is non-ferromagnetic to reduce the total magnetic
moment of storage layer 330. Because the magnetization of the
storage layer couples to the magnetization of the reference layer
through magnetostatic coupling at the edges of patterned elements,
it is often useful to reduce this effect by reducing the respective
magnetic moments of these layers. For example, layer 334 can be
formed from alloys of Co and Fe, which are made non-magnetic by the
addition of sufficiently high quantities of B or Zr or Hf or other
elements that make the layer amorphous. It may be advantageous to
form layer 334 from non-magnetic alloys containing Co and Fe
because these layers in conjunction with CoFe interface layers 324
are likely to be more thermally stable than alloys that are more
dissimilar from the alloy forming interface layer 324.
[0053] Layer 334 can be formed from other amorphous ferromagnetic
alloys including alloys of Co and Fe with rare-earth elements such
as Gd or Tb.
[0054] Although the preferred embodiment of the current invention
has been described with regard to a tunnel barrier layer 324 formed
from an amorphous layer of Al.sub.2O.sub.3, the tunnel barrier can
also be formed from other amorphous materials including, for
example, Ga.sub.2O.sub.3 and In.sub.2O.sub.3 and related materials.
See, for example, U.S. Pat. No. 6,359,289 to Parkin.
[0055] High TMR values are obtained in MTJs by using thin interface
layers of CoFe alloys 328 that are made amorphous by sandwiching
these layers between an amorphous tunnel barrier 324 and an
amorphous metallic layer 334. The high TMR follows from the high
spin polarization of the tunneling current that can be directly
measured using the technique of superconducting tunneling
spectroscopy in which lower electrode 310 is replaced by a thin
superconducting layer of Al. See, for example, R. Meservey et al.,
Phys. Rep. 238, 173 (1994). FIG. 8 is a graph showing values of
spin polarization P as a function of the thickness of CoFe
interface layer 324 for a structure comprised of 45 .ANG. Al/14
.ANG. Al plasma oxidized for 180 or 240 seconds/t
CO.sub.70Fe.sub.30/300 .ANG. (CO.sub.7Fe.sub.30).sub.85Zr.sub.15 or
300 .ANG. (CO.sub.70Fe.sub.30).sub- .85Hf.sub.15 fort ranging from
zero to 50 .ANG. for the (CO.sub.7Fe.sub.30).sub.85Zr.sub.15 alloy
and from zero to 15 .ANG. for the
(CO.sub.70Fe.sub.30).sub.85Hf.sub.15 alloy. As shown in FIG. 8, the
polarization P attains values of about 56% for thicknesses of
CO.sub.70Fe.sub.30 in the range from 15 to 20 .ANG., which is
consistent with the high values of TMR observed in MTJs for
CO.sub.70Fe.sub.30 interface layers 328 in this thickness
range.
[0056] Although the preferred embodiment has been described with
respect to an MTJ with a storage or free ferromagnetic layer formed
from an interface layer of a nominally crystalline ferromagnetic or
ferromagnetic material and an amorphous layer formed from a
non-magnetic, ferromagnetic or ferromagnetic material, the improved
magnetic tunneling element of the current invention can also be
used in an inverted MTJ structure in which the storage layer is
formed below the tunnel barrier and the fixed layer is formed above
the tunnel barrier. Similarly, the improved magnetic tunneling
element of the current invention may be used both for the storage
(or free) layer as well as the fixed layer so that the MTJ would be
comprised of a first layer formed from one or more of the group of
amorphous materials comprised of non-magnetic materials,
ferromagnetic materials and ferromagnetic materials; a first
interface layer formed from one or more of a ferromagnetic or
ferromagnetic material which is typically crystalline in isolation
of the first layer; an amorphous tunnel barrier which is formed on
top of this first interface layer; and a second interface layer,
formed from one or more of a ferromagnetic or ferromagnetic
material which is typically crystalline in isolation; and a second
layer, formed from one or more of the group of amorphous materials
comprised of non-magnetic materials, ferromagnetic materials and
ferromagnetic materials.
[0057] The high-spin polarization of the tunneling current using
the MTJ structure of the present invention can be applied to a wide
variety of tunnel junction devices in which one or the other or
both of the metal layers on either side of the amorphous tunnel
barrier do not need to be ferromagnetic. Illustrations of such
exemplary structures are shown in FIGS. 9A-9C. FIG. 9A shows a
tunnel junction device 900 in which the upper electrode is formed
from a non-magnetic metal layer 334' that is adjacent to an
amorphous tunnel barrier 324'. A lower electrode 312 is formed from
an exchange-biased ferromagnetic layer in which layer 316 is an
antiferromagnetic layer, such as IrMn, that is used to exchange
bias, or fix, the magnetic moment of a ferromagnetic reference
layer formed from a combination of an amorphous ferromagnetic layer
318, such as CoFeZr or CoFeB, and a thin ferromagnetic normally
crystalline interface layer 319, such as a CoFe alloy. A tunneling
current is passed through device 900, as shown by arrow 902 and is
spin polarized by ferromagnetic layer 319. FIG. 9B illustrates an
MTJ device 910 that is similar to MTJ device 900 in which the lower
ferromagnetic electrode 312 is formed without an antiferromagnetic
exchange-bias layer. Such a structure may be useful as part of a
magnetic tunnel transistor (MTT). The MTT is a three-terminal
device that is typically formed from a magnetic tunnel junction in
combination with a semiconductor collector in which the
semiconductor can be formed from, for example, GaAs or Si. One
ferromagnetic electrode in the MTJ device forms the emitter, and
the other ferromagnetic layer forms the base of the MTT
three-terminal device between the tunnel barrier of the MTJ and the
collector of the semiconductor. The MTJ component of the MTT could
be formed as described herein with an amorphous tunnel barrier. It
may, however, also be advantageous for certain applications that
the emitter of the magnetic tunnel transistor be formed from a
non-magnetic metal according to the structure shown in FIG. 9B.
[0058] FIG. 9C illustrates a tunnel junction device 920 in which
the lower electrode is formed from a non-magnetic metal layer 312'.
Layer 312' may be formed from more than one metal layers, including
the possibility that the layer 312' may contain ferromagnetic
layers that are not immediately adjacent to tunnel barrier 324'.
The structure of tunnel junction device 920 may be useful as part
of a MTT in which the emitter of the MTT is ferromagnetic, but the
base layer is non-magnetic. In the structure illustrated in FIG.
9C, the interface layer is shown as layer 334 and the amorphous
layer is shown as a non-magnetic amorphous layer 336. The direction
of the magnetic moment of interface layer 334 is shown by arrow
380.
[0059] While a ferromagnetic interface layer is preferably in
direct contact with amorphous tunnel barrier 324', it is also
possible to separate the ferromagnetic electrode and the tunnel
barrier by a thin spacer layer, providing that the spacer layer
does not significantly diminish the spin polarization of the
electrons tunneling through the tunnel barrier 324'. FIG. 10
illustrates a tunnel junction device 1000 having a thin spacer
layer 325 between tunnel barrier 324' and free ferromagnetic layer
334 in the magnetic tunnel junction device of FIG. 3. As described
in U.S. Pat. No. 5,764,567 to Parkin with reference to magnetic
tunnel junction devices formed with alumina tunnel barriers, the
ferromagnetic electrodes in such MTJs can be separated from the
tunnel barrier by thin spacer layers formed from Cu and other
non-magnetic metallic materials while maintaining significant
tunneling magnetoresistance. The types of non-magnetic metallic
materials that are preferred are those that display large values of
giant magnetoresistance in metallic spin-valve structures or in
metallic magnetic multilayers. These include Ag and Au, as well as
Cu. The non-magnetic spacer layer could also be formed from a
metallic-oxide layer, such as RuO.sub.2 or a Sr--Ru oxide. As
illustrated in FIG. 10, free ferromagnetic layer 334 and spacer
layer 325 form an overlayer. Generally, the magnetic tunnel
junction will include an overlayer formed on top of amorphous
tunnel barrier 324', which may be formed of one or more
ferromagnetic layers with or without a non-magnetic spacer layer,
or, more generally, from a multiplicity of ferromagnetic and
non-ferromagnetic, non-ferrimagnetic layers. Similarly, the
magnetic tunnel junction will include an underlayer beneath tunnel
barrier 324', which may be formed of one or more ferromagnetic
layers with or without a non-magnetic spacer layer (e.g., spacer
layer 326), or, more generally, from a multiplicity of
ferromagnetic and non-ferromagnetic, non-ferrimagnetic layers.
[0060] Although the foregoing invention has been described in some
detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
that are within the scope of the appended claims. Accordingly, the
present embodiments are to be considered as illustrative and not
restrictive, and the invention is not to be limited to the details
given herein, but may be modified within the scope and equivalents
of the appended claims.
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