U.S. patent application number 15/917936 was filed with the patent office on 2018-07-19 for magnetoresistive memory device.
This patent application is currently assigned to TOSHIBA MEMORY CORPORATION. The applicant listed for this patent is TOSHIBA MEMORY CORPORATION. Invention is credited to Youngmin EEH, Makoto NAGAMINE, Toshihiko NAGASE, Hiroyuki OHTORI, Tadaaki OIKAWA, Kazuya SAWADA, Daisuke WATANABE, Kenichi YOSHINO.
Application Number | 20180205006 15/917936 |
Document ID | / |
Family ID | 59787095 |
Filed Date | 2018-07-19 |
United States Patent
Application |
20180205006 |
Kind Code |
A1 |
WATANABE; Daisuke ; et
al. |
July 19, 2018 |
MAGNETORESISTIVE MEMORY DEVICE
Abstract
A magnetoresistive memory device includes a first magnetic layer
having a variable magnetization direction; a second magnetic layer,
a magnetization direction of the second magnetic layer being
invariable; a first nonmagnetic layer provided between the first
magnetic layer and the second magnetic layer; and a second
nonmagnetic layer provided on the first magnetic layer, which is
opposite the first nonmagnetic layer. The first magnetic layer
having a stacked layer structure in which amorphous magnetic
material layer is sandwiched between crystalline magnetic material
layers.
Inventors: |
WATANABE; Daisuke; (Seoul,
KR) ; NAGASE; Toshihiko; (Seoul, KR) ; EEH;
Youngmin; (Seongnam-si Gyeonggi-do, KR) ; SAWADA;
Kazuya; (Seoul, KR) ; NAGAMINE; Makoto;
(Seoul, KR) ; OIKAWA; Tadaaki; (Seoul, KR)
; YOSHINO; Kenichi; (Seoul, KR) ; OHTORI;
Hiroyuki; (Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOSHIBA MEMORY CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
TOSHIBA MEMORY CORPORATION
Tokyo
JP
|
Family ID: |
59787095 |
Appl. No.: |
15/917936 |
Filed: |
March 12, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15268507 |
Sep 16, 2016 |
9947862 |
|
|
15917936 |
|
|
|
|
62308156 |
Mar 14, 2016 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11C 11/161 20130101;
H01L 43/10 20130101; H01L 27/228 20130101; H01L 43/08 20130101;
H01L 27/222 20130101; H01L 43/02 20130101 |
International
Class: |
H01L 43/08 20060101
H01L043/08; H01L 43/10 20060101 H01L043/10; G11C 11/16 20060101
G11C011/16; H01L 27/22 20060101 H01L027/22; H01L 43/02 20060101
H01L043/02 |
Claims
1. A magnetoresistive memory device comprising: a first magnetic
layer having a variable magnetization direction; a second magnetic
layer, a magnetization direction of the second magnetic layer being
invariable; a first nonmagnetic layer provided between the first
magnetic layer and the second magnetic layer; and a second
nonmagnetic layer provided on the first magnetic layer, which is
opposite the first nonmagnetic layer, the first magnetic layer
having a stacked layer structure in which amorphous magnetic
material layer is sandwiched between crystalline magnetic material
layers.
2. The device of claim 1, further comprising nonmagnetic material
layers provided between one of the crystalline magnetic material
layers and the amorphous magnetic material layer, and between the
other crystalline magnetic layer and the amorphous magnetic
material layer, respectively.
3. A magnetoresistive memory device comprising: a first magnetic
layer having a variable magnetization direction; a second magnetic
layer, a magnetization direction of the second magnetic layer being
invariable; a first nonmagnetic layer provided between the first
magnetic layer and the second magnetic layer; a second nonmagnetic
layer provided on the first magnetic layer, which is opposite the
first nonmagnetic layer; and an amorphous Mo layer provided on the
second nonmagnetic layer, which is opposite the first magnetic
layer.
4. The device of claim 1, wherein a first electrode is provided on
a semiconductor substrate, and the first magnetic layer is disposed
closer to the first electrode than the second magnetic layer.
5. The device of claim 1, wherein a first electrode is provided on
a semiconductor substrate, and the second magnetic layer is
disposed closer to the first electrode than the first magnetic
layer.
6. The device of claim 3, wherein a first electrode is provided on
a semiconductor substrate, and the first magnetic layer is disposed
closer to the first electrode than the second magnetic layer.
7. The device of claim 3, wherein a first electrode is provided on
a semiconductor substrate, and the second magnetic layer is
disposed closer to the first electrode than the first magnetic
layer.
8. A magnetoresistive memory device comprising: a bottom electrode;
a first magnetic layer provided on the bottom electrode, a
magnetization direction of the first magnetic layer being
invariable; a first nonmagnetic layer provided on the first
magnetic layer; a second magnetic layer provided on the first
nonmagnetic layer, a magnetization direction of the second magnetic
layer being variable, the second magnetic layer including Mo; a
second nonmagnetic layer provided on the second magnetic layer; and
an upper electrode provided on the second nonmagnetic layer.
9. The device of claim 8, wherein: the first magnetic layer is a
reference layer having magnetic anisotropy perpendicular to a film
surface; the second magnetic layer is a storage layer having
magnetic anisotropy perpendicular to a film surface; and the first
nonmagnetic layer is a tunnel barrier layer through which a
tunneling current flows.
10. The device of claim 8, wherein the second nonmagnetic layer
comprises MgO.
11. The device of claim 8, wherein the first and the second
nonmagnetic layers are different materials.
12. The device of claim 8, wherein the second magnetic layer
includes Fe or Co.
13. The device of claim 8, wherein the second magnetic layer has a
stacked layer structure in which a middle layer including Mo is
provided between magnetic material layers.
14. The device of claim 8, wherein the second nonmagnetic layer is
an oxide.
15. The device of claim 8, further comprising a first layer
provided between the bottom electrode and the first magnetic
layer.
16. The device of claim 15, wherein the first layer includes at
least one element selected from the group consisting of Al, Be, Mg,
Ca, Sr, Ba, Sc, Y, La, Si, Zr, Hf, W, Cr, Mo, Nb, Ti, Ta, and V, or
a boride of these elements.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation application of U.S. Ser.
No. 15/268,507, filed on Sep. 16, 2016, which claims the benefit of
U.S. Provisional Application No. 62/308,156, filed on Mar. 14,
2016, the entire contents of both of which are incorporated herein
by reference.
FIELD
[0002] Embodiments described herein relate generally to a
magnetoresistive memory device.
BACKGROUND
[0003] Recently, there are expectations and attention on a
large-capacity magnetoresistive random access memory (MRAM) in
which a magnetic tunnel junction (MTJ) element is used. In the MTJ
element, one of the two magnetic layers which sandwich a tunnel
barrier layer is formed as a magnetization fixed layer (a reference
layer) in which the direction of magnetization is fixed to be
invariable, and the other magnetic layer is formed as a
magnetization free layer (a storage layer) in which the direction
of magnetization is made to be easily reversed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is a plan view which schematically illustrates a
magnetoresistive memory device according to a first embodiment.
[0005] FIG. 2 is a cross-sectional view taken along line I-I' of
FIG. 1.
[0006] FIG. 3 is a cross-sectional view taken along line II-II' of
FIG. 1.
[0007] FIG. 4 is a cross-sectional view showing a structure of a
memory cell portion of the magnetoresistive memory device of the
first embodiment.
[0008] FIG. 5 is a cross-sectional view showing a structure of a
memory cell portion according to a modification of the first
embodiment.
[0009] FIG. 6 is a cross-sectional view showing a structure of a
memory cell portion according to another modification of the first
embodiment. FIG. 7 is a characteristic diagram showing the
relationship between annealing temperature T and anisotropic
magnetic field Hk.
[0010] FIG. 8 is a characteristic diagram showing the relationship
between saturation magnetization Mst and anisotropic magnetic field
Hk.
[0011] FIG. 9 is a characteristic diagram showing the relationship
between the annealing temperature and anisotropic magnetic field
Hk.
[0012] FIG. 10 is a cross-sectional view showing a structure of a
memory cell portion of a magnetoresistive memory device of a second
embodiment.
[0013] FIG. 11 is a cross-sectional view showing a structure of a
memory cell portion of a magnetoresistive memory device of a third
embodiment.
[0014] FIG. 12 is a cross-sectional view showing a modification of
the third embodiment.
[0015] FIG. 13 is a cross-sectional view showing a structure of a
memory cell portion of a magnetoresistive memory device of a fourth
embodiment.
[0016] FIG. 14 is a cross-sectional view showing a structure of a
memory cell portion according to a modification of the fourth
embodiment.
[0017] FIG. 15 is a characteristic diagram showing a change in
saturation magnetization Mst when a buffer layer is varied.
[0018] FIG. 16 is a characteristic diagram showing a change in an
anisotropic magnetic field Hk when the buffer layer is varied.
[0019] FIG. 17 is a characteristic diagram showing a change in the
saturation magnetization Mst with respect to a ratio of W or Mo
added to CFB.
[0020] FIG. 18 is a cross-sectional view showing a structure of a
memory cell portion used in a magnetoresistive memory device
according to a fifth embodiment.
[0021] FIG. 19 is a schematic diagram showing a structure of an MTJ
element portion of FIG. 18 in comparison with a comparative
example.
[0022] FIG. 20 is a cross-sectional view showing a modification of
the fifth embodiment.
[0023] FIG. 21 is a characteristic diagram showing the relationship
between a thickness of a magnetic material layer and saturation
magnetization Mst of a storage layer.
[0024] FIG. 22 is a characteristic diagram showing the relationship
between saturation magnetization Mst and anisotropic magnetic field
Hk.
[0025] FIG. 23 is a cross-sectional view showing a structure of an
MTJ element portion used in a magnetoresistive memory device
according to a sixth embodiment.
[0026] FIGS. 24A and 24B are diagrams schematically showing a
structure of the MTJ element portion of FIG. 23.
[0027] FIG. 25 is a cross-sectional view showing a structure of a
memory cell portion according to a modification.
DETAILED DESCRIPTION
[0028] In general, according to one embodiment, a magnetoresistive
memory device comprises: a first magnetic layer in which a
magnetization direction is variable; a first nonmagnetic layer
provided on the first magnetic layer, the first magnetic layer
including Mo; a second magnetic layer provided on the first
nonmagnetic layer, a magnetization direction of the second magnetic
layer being invariable; and a second nonmagnetic layer provided on
the first magnetic layer, which is opposite the first nonmagnetic
layer.
[0029] (First Embodiment)
[0030] FIG. 1 is a plan view which schematically illustrates a
magnetoresistive memory device according to a first embodiment.
FIG. 2 is a cross-sectional view taken along line I-I' of FIG. 1,
and FIG. 3 is a cross-sectional view taken along line II-II' of
FIG. 1.
[0031] A member indicated by a broken line in FIGS. 2 and 3
represents a plug SC at the back side which cannot be seen in the
I-I' and II-II' cross-sections.
[0032] The magnetoresistive memory device of the present embodiment
is an MRAM comprising an MTJ element (a magnetoresistive element)
of a spin-transfer-torque writing method as a storage element. A
perpendicular magnetization film is used in the above MTJ element.
The perpendicular magnetization film is a magnetization film in
which the direction of magnetization (direction of axis of easy
magnetization) is substantially perpendicular to the film plane of
the perpendicular magnetization film.
[0033] In the drawings, 101 indicates a silicon substrate (a
semiconductor substrate), and an element isolation region 102 is
formed on a surface of the silicon substrate 101. The element
isolation region 102 defines an active region.
[0034] The MRAM of the present embodiment comprises a first select
transistor in which a gate electrode is word line WL1, a first MTJ
element M which is connected to source/drain region 104 (drain
region D1) on one side of the first select transistor, a second
select transistor in which a gate electrode is word line WL2, and a
second MTJ element M which is connected to source/drain region 104
(drain region D2) on one side of the second select transistor. In
the drawing, 103 indicates a protective insulating film. That is, a
memory cell of the present embodiment is constituted of an MTJ (a
storage element) and a select transistor, and two select
transistors of the adjacent two memory cells share source/drain
region 104 (source regions S1, S2), which is the source/drain
region on the other side.
[0035] A gate (a gate insulating film and a gate electrode) of the
select transistor of the present embodiment is buried in a surface
of the silicon substrate 101. That is, the gate of the present
embodiment has a buried gate (BG) structure. Similarly, a gate
(word line WL) for element isolation has the BG structure.
[0036] Source/drain region 104 (D1) on one side of the first select
transistor is connected to the lower part of the first MTJ element
M via a bottom electrode BEC. The upper part of the first MTJ
element M is connected to a bit line BL2 via a top electrode TEC.
Source/drain region 104 (S1) on the other side of the first select
transistor is connected to bit line BL1 via the plug SC.
[0037] In the present embodiment, while a planar pattern of each of
the bottom electrode BEC, the MTJ element M, the top electrode TEC,
and the plug SC is circular, they may be formed in another
shape.
[0038] Source/drain region 104 (D2) on one side of the second
select transistor is connected to the lower part of the second MTJ
element M via a bottom electrode BEC. The upper part of the second
MTJ element M is connected to bit line BL2 via a top electrode TEC.
Source/drain region 104 (S2) on the other side of the second select
transistor is connected to bit line BL1 via the plug SC.
[0039] The first select transistor, the first MTJ element M, the
second select transistor, and the second MTJ element M (i.e., two
memory cells) are provided in every active region. The two adjacent
active regions are separated from each other by the element
isolation region 102.
[0040] Word lines WL3 and WL4 correspond to word lines WL1 and WL2,
respectively. Accordingly, two memory cells are constituted by a
first select transistor in which word line WL3 is a gate, a first
MTJ element M which is connected to a source/drain region on one
side of the first select transistor, a second select transistor in
which word line WL4 is a gate, and a second MTJ element M which is
connected to a source/drain region on one side of the second select
transistor.
[0041] Note that the layout of the MTJ element, BL, WL, etc., is in
no way limited to the illustration of FIGS. 1 to 3. For example,
BL2 may be arranged at a lower layer than BL1. Further, an active
region may be inclined with respect to a gate electrode.
[0042] FIG. 4 is a cross-sectional view showing a specific
structure of an MTJ element portion employed in the present
embodiment.
[0043] On the bottom electrode (BEC) connected to the drain region
of the select transistor, an underlayer (UL [second nonmagnetic
layer]) 12 is formed via a buffer layer 11 (BuL). As the buffer
layer 11, Al, Be, Mg, Ca, Sr, Ba, Sc, Y, La, Si, Zr, Hf, W, Cr, Mo,
Nb, Ti, Ta, V, etc., may be used. Also, a boride of the above
elements may be included.
[0044] The boride is not limited to a binary compound consisting of
two elements, but may be a ternary compound consisting of three
elements. That is, the boride may be a mixture of a binary
compound. For instance, examples of such a boride are HfB, MgAlB,
HfAlB, ScAlB, ScHfB, and HfMgB. Further, the aforementioned
materials may be stacked.
[0045] The underlayer 12 may be a nitrogen compound or an oxygen
compound such as MgN, ZrN, NbN, SiN, AIN, HfN, TaN, WN, CrN, MoN,
TiN, VN, and MgO, or a mixture of these compounds. That is, the
underlayer 12 is not limited to a binary compound consisting of two
elements, but may be a ternary compound consisting of three
elements such as aluminum titanium nitride (AlTiN).
[0046] The nitrogen compound and the oxygen compound suppress an
increase in a damping constant of a magnetic layer which is in
contact with these compounds, and an advantage of reducing write
current can be obtained. Further, by using a nitrogen compound or
an oxygen compound of high-melting-point metal, it is possible to
suppress diffusion of an underlayer material into a magnetic layer
and prevent deterioration of an MR ratio. Here, a
high-melting-point metal is a material having a higher melting
point than Fe or Co, and is, for example, Zr, Hf, W, Cr, Mo, Nb,
Ti, Ta, and V.
[0047] When a nitrogen compound or an oxygen compound is used as
the underlayer 12, the magnetic anisotropy of a storage layer 20,
described later, can be greatly improved. That is, by using MgO,
for example, for the underlayer 12 and a tunnel barrier layer (TB)
30, the storage layer 20 is sandwiched between layers of MgO.
Accordingly, at the interface between the storage layer 20 and the
underlayer 12, and the interface between the storage layer 20 and
the tunnel barrier layer 30, the interfacial anisotropy can be
developed, and perpendicular magnetic anisotropy of the storage
layer 20 can be doubled in principle.
[0048] On the underlayer 12, the storage layer 20 (SL [first
magnetic layer]) in which a middle layer 22 is sandwiched between
magnetic material layers 21 and 23 is formed. That is, the storage
layer 20 in which three layers 21, 22, and 23 are stacked is
formed. Each of the magnetic material layers 21 and 23 is an alloy
including Fe and Co, and further, B, may be included. Further, the
middle layer 22 includes Mo. Here, since Mo is a material which has
a higher melting point than the magnetic material layers 21 and 22,
and is hard to oxidize, it is suitable as the material used for the
middle layer 22.
[0049] On the storage layer 20, a reference layer 40 (RL [second
magnetic layer]) is formed via the tunnel barrier layer (first
nonmagnetic layer) 30. After these layers 11, 12, 20, 30, and 40
have been stacked on the BEC, by selectively etching the layers by
ion beam etching (IBE) or reactive ion etching (RIE), etc., they
are processed into a cell pattern. Further, the top electrode (TEC)
is formed on the reference layer 40.
[0050] The storage layer 20 has magnetic anisotropy perpendicular
to a film surface, and a magnetization direction is variable. The
magnetic material layers 21 and 23 of the storage layer 20 are not
limited to CoFeB, and various magnetic materials can be used. For
example, CoB or FeB can also be used.
[0051] If the middle layer 22 has a great film thickness, magnetic
coupling between the magnetic material layers 21 and 23 may be cut.
Accordingly, the film thickness of the middle layer 22 must be set
within a range that allows the magnetic material layers 21 and 23
to be magnetically coupled. For example, the film thickness should
preferably be 0.5 nm or less. Since Mo is a material which has a
higher melting point than CoFeB, and is also hard to oxidize, this
material does not cause unnecessary diffusion in annealing, or
decrease the MR ratio.
[0052] Also, the middle layer 22 is not necessarily limited to a
single layer of Mo. It is sufficient if the storage layer 20
includes magnetic materials such as Fe and Co, and Mo. For example,
a structure in which a layer including Mo is inserted into a part
of the storage layer 20 of CoFeB, or a single-layer alloy of
CoFeBMo, for example, as shown in FIG. 5, may be adopted.
[0053] The tunnel barrier layer 30 is a layer for passing a
tunneling current, and various nonmagnetic materials can be used.
In the present embodiment, while the tunnel barrier layer 30 is
formed of MgO, an oxide including Cu, Si, Ba, Ca, La, Mn, Zn, Hf,
Ta, Ti, B, Cr, V or Al can also be used.
[0054] The reference layer 40 is an alloy including Fe and Co, for
example, and may be CoFeB further including B. The reference layer
40 has magnetic anisotropy perpendicular to a film surface, and a
magnetization direction is fixed. The material of the reference
layer 40 is not limited to CoFeB, and various magnetic materials
can be used. For example, CoPt, CoNi, CoPd, etc., can be used.
Further, Fe/Pt (a superlattice structure formed by a stacked layer
structure of Fe and Pt), Fe/Pd, Co/Pt, and Co/Pd can be used.
[0055] Further, as shown in FIG. 6, in order to cancel or reduce a
stray magnetic field, a shift canceling layer 50 having a
magnetization direction opposite to that of the reference layer 40
may be provided on the reference layer 40. As the material of the
shift canceling layer 50, various magnetic materials can be used
likewise the reference layer 40.
[0056] FIG. 7 is a graph showing the relationship between annealing
temperature T and anisotropic magnetic field Hk. As shown in FIG.
7, the higher the annealing temperature is, the smaller the
magnetic anisotropy of the storage layer 20 becomes. However, in
cases where the middle layer 22 is formed of W and Mo,
respectively, the annealing temperature dependence is lower than
that of a case where the middle layer 22 is formed of Ta. Further,
when a high-temperature annealing process is performed, W and Mo
have greater magnetic anisotropy than Ta.
[0057] FIG. 8 is a graph showing the relationship between
saturation magnetization Mst and anisotropic magnetic field Hk of
the storage layer 20 when the MTJ element portion is annealed. As
shown in FIG. 8, the greater the saturation magnetization Mst is,
the smaller the magnetic anisotropy of the storage layer 20
becomes. Also, greater magnetic anisotropy can be obtained with Mo
as compared to Ta and W.
[0058] As in the present embodiment, by using Mo as the material of
the middle layer 22 of the storage layer 20, improvement in the
thermal resistance can be achieved. In addition, if the same
material, i.e., MgO, is used for the underlayer 12 and the tunnel
barrier layer 30, the MgO-CoFeB interfacial anisotropy can be
developed from both of the two interfaces of the storage layer 20,
and perpendicular magnetic anisotropy of the storage layer 20 can
be doubled in principle. As a result, thermal stability (.DELTA.)
and reduction of write current can be achieved. This advantage
becomes more significant when the film thickness of the storage
layer 20 is increased.
[0059] Here, since a single layer of CoFeB sandwiched between MgO
layers is hard to crystallize, Ta has been considered as the middle
layer which absorbs B and promotes crystallization. However, since
the thermal resistance is low in the middle layer of Ta,
perpendicular magnetic anisotropy may be degraded by a
high-temperature heat treatment whereby a high TMR can be
obtained.
[0060] In contrast, in the present embodiment, by using Mo instead
of Ta, thermal resistance is greatly improved, and greater
perpendicular magnetic anisotropy than when a Ta middle layer is
used in a high-temperature heat treatment is realized. Preferably,
the thickness of an Mo film of the middle layer should be 5 .ANG.
at a maximum which allows magnetic coupling between two magnetic
layers.
[0061] Also, when the middle layer 20 is formed of Mo, an advantage
that an anisotropic magnetic field Hk can be more increased with an
oxide underlayer (MgO) than with a nitride underlayer (AlN) is
obtained.
[0062] FIG. 9 is a characteristic diagram showing the relationship
between the annealing temperature and the anisotropic magnetic
field Hk of the storage layer 20.
[0063] In FIG. 9, (1) corresponds to a case where the underlayer is
formed of AlN and the middle layer is formed of Mo, (2) corresponds
to a case where the underlayer is formed of AlN and the middle
layer is formed of W, (3) corresponds to a case where the
underlayer is formed of AlN and the middle layer is formed of Ta,
(4) corresponds to a case where the underlayer is formed of MgO and
the middle layer is formed of W, (5) corresponds to a case where
the underlayer is formed of MgO and the middle layer is formed of
Mo, and (6) corresponds to a case where the underlayer is formed of
MgO and the middle layer is formed of Ta. As can be seen from this
diagram, when the middle layer is formed of Mo, regardless of the
annealing temperature, the anisotropic magnetic field Hk is
increased when MgO is used for the underlayer. Therefore, when the
middle layer 20 is formed of Mo, it is preferable that an oxide
such as MgO, instead of a nitride such as AlN, be used as the
underlayer 12. This also applies to a case where the storage layer
20 is a single-layer alloy including Mo, as shown in FIG. 5.
[0064] As can be seen, according to the present embodiment, by
using Mo as the material of the middle layer 22 of the storage
layer 20 having a three-layer structure, it is possible to improve
the magnetic anisotropy of the storage layer 20, and also to
improve the thermal resistance. Moreover, when Mo is used for the
middle layer 22, by using an oxide such as MgO as the underlayer
12, there is also an advantage that the anisotropic magnetic field
Hk can further be increased. Accordingly, a magnetoresistive memory
device having good magnetic properties and thermal resistance can
be realized.
[0065] (Second Embodiment)
[0066] FIG. 10 is a cross-sectional view showing the structure of
an MTJ element portion used in a magnetoresistive memory device
according to a second embodiment. It should be noted that the same
portions as those of FIG. 4 will be given the same reference
numbers, and detailed explanations of them will be omitted.
[0067] The point in which the present embodiment is different from
the first embodiment described above is that a nitrogen compound is
used for an underlayer 12, and Mo or W is used for a middle layer
22 of a storage layer 20. Here, the nitrogen compound is the same
as that described in the first embodiment, and is, for example,
MgN, ZrN, NbN, SiN, AIN, HfN, TaN, WN, CrN, MoN, TiN, or VN, or a
mixture of the aforementioned materials. That is, the underlayer 12
is not limited to a binary compound consisting of two elements, but
may be a ternary compound consisting of three elements such as
aluminum titanium nitride (AlTiN). As described in the first
embodiment, even in a case where the nitrogen compound is used for
the underlayer 12, interfacial anisotropy of CoFeB can be developed
from both of the two interfaces of the storage layer 20, and
perpendicular magnetic anisotropy of the storage layer 20 can be
increased. Further, when a nitrogen compound is used as the
underlayer 12, an advantage of reducing a damping constant and
reducing write current can also be obtained. Also, since each of Mo
and W is a material which has a higher melting point than magnetic
material layers 21 and 22, and is harder to oxidize than Ta, Mo and
W are suitable as the materials used for the middle layer 22.
[0068] Note that the middle layer 22 is not necessarily limited to
a single layer of Mo or W. It is sufficient if the storage layer 20
includes magnetic materials such as Fe and Co, and Mo or W. For
example, a structure in which a layer including W is inserted into
the storage layer 20 of CoFeB, or an alloy of CoFeBW, for example,
may be adopted.
[0069] As can be seen, according to the present embodiment, by
using a nitrogen compound as the underlayer 12 of the storage layer
20, and using Mo or W as the material of the middle layer 22 of the
storage layer 20, it is possible to improve the magnetic anisotropy
of the storage layer 20, and also to improve the thermal
resistance. Accordingly, an advantage similar to that of the first
embodiment can be obtained. Further, an advantage of reducing a
damping constant and reducing write current can also be
obtained.
[0070] (Third Embodiment)
[0071] FIG. 11 is a cross-sectional view showing the structure of
an MTJ element portion used in a magnetoresistive memory device
according to a third embodiment. It should be noted that the same
portions as those of FIG. 4 will be given the same reference
numbers, and detailed explanations of them will be omitted.
[0072] The point in which the present embodiment is different from
the first and second embodiments is that a plurality of middle
layers are provided. In other words, a storage layer 20 is formed
such that it is constituted of more than three layers. More
specifically, the storage layer 20 has a five-layer structure
including a first magnetic material layer 21, a first middle layer
22, a second magnetic material layer 23, a second middle layer 24,
and a third magnetic material layer 25. The middle layers 22 and 24
are formed of Mo or W, and an underlayer 12 is a nitrogen compound
or an oxygen compound, or a mixture of these compounds. Note that
the number of middle layers is not limited to two, and may be three
or more so that the storage layer 20 as a whole is formed of seven
layers or more. FIG. 12. shows an example of a case in which the
number of the middle layers is three, and the number of the storage
layers is seven. In FIG. 12, reference number 26 denotes a third
middle layer, and reference number 27 denotes a fourth magnetic
material layer.
[0073] Even with such a structure, an advantage similar to those of
the first and second embodiments can surely be obtained. Also, in
the present embodiment, by providing a plurality of middle layers,
i.e., the middle layers 22 and 24, in the storage layer 20, the
second magnetic material layer 23 is sandwiched between the middle
layers without contacting an underlayer 12.
[0074] In this case, since the second magnetic material layer 23 is
not crystallized, saturation magnetization Mst is reduced.
Therefore, there is an advantage that an activation volume is
increased, and the thermal stability .DELTA. is increased.
[0075] (Fourth Embodiment)
[0076] FIG. 13 is a cross-sectional view showing the structure of
an MTJ element portion used in a magnetoresistive memory device
according to a fourth embodiment. It should be noted that the same
portions as those of FIG. 4 will be given the same reference
numbers, and detailed explanations of them will be omitted.
[0077] The point in which the present embodiment is different from
the first and second embodiments is that two buffer layers, i.e.,
buffer layers 61 and 62, are provided beneath an underlayer 12.
[0078] On a bottom electrode (BEC) connected to a drain region of a
select transistor, a first buffer layer (BuL1) 61 and a second
buffer layer (BuL2) 62 are stacked. Preferably, the material of the
second buffer layer 62 on the lower side should be amorphous, or a
material which does not inhibit the amorphous property of the
buffer layer 61. It is sufficient if the second buffer layer 62 on
the lower side is one which has a small lattice mismatching with
the underlayer 12 in order to form the underlayer 12 with good
crystallinity. More specifically, the same material as that used
for the buffer layer 11 described in the first embodiment can be
used. It suffices that the first buffer layer 61 on the upper side
is one which includes W or Mo in Co and Fe in order for it to be
amorphous. For example, W or Mo is added to CoFeB alloy.
[0079] On the buffer layer 61, the underlayer 12 is formed. The
underlayer 12 is, for example, an oxygen compound or a mixture
thereof. Further, on the underlayer 12, as in the first embodiment,
a storage layer 20 (SL [first magnetic layer]), a tunnel barrier
layer 30, and a reference layer 40 are formed. As in the first or
second embodiment, the storage layer 20 has a stacked layer
structure in which a middle layer 22 formed of Mo or W is
sandwiched between magnetic material layers 21 and 23 of CoFeB.
Moreover, the storage layer 20 is not necessarily limited to a
stacked layer structure, and may be a single-layer alloy of
CoFeBMo, for example, or a single-layer alloy of CoFeBW, for
example, as shown in FIG. 14, may be adopted. Further, the storage
layer 20 is not necessarily limited to one which includes Mo or W,
and it may be a single magnetic layer of CoFeB or CoFe, etc.,
without a middle layer.
[0080] As in the present embodiment, by providing the amorphous
buffer layer 61, the orientation of the underlayer 12 can be
improved. When the orientation of the underlayer 12 is improved,
the crystallinity of CoFeB formed thereon is improved. In this way,
the element characteristics can be improved.
[0081] Here, by the form of a layer beneath the underlayer 12,
saturation magnetization or an anisotropic magnetic field of the
MTJ element portion show different values. Hence, the saturation
magnetization Mst and the anisotropic magnetic field Hk have been
verified for three cases where (a) when only the second buffer
layer 62 is provided beneath the MgO underlayer 12, (b) when a
stacked layer structure of CFB and the second buffer layer 62 is
provided, and (c) when a stacked layer structure of CFB-W (alloy in
which W is added to CFB: first buffer layer 61) and the second
buffer layer 62 is provided. FIGS. 15 and 16 show the results.
[0082] As shown in FIG. 15, in case (b) corresponding to the
stacked layer structure of CFB and the second buffer layer 62, the
saturation magnetization Mst is great since the buffer layers are
magnetic materials. In contrast, when W is added to the CFB (case
c), since the buffer layers become nonmagnetic, the saturation
magnetization can be made small.
[0083] Also, as shown in FIG. 16, as compared to case (a) where
only the second buffer layer 62 is provided beneath the MgO
underlayer 12, in case (b) corresponding to the stacked layer
structure of CFB and the second buffer layer 62, the anisotropic
magnetic field Hk is increased. Further, when W is added to the CFB
(case c), the anisotropic magnetic field Hk is further increased.
It has been found that also in a case where Mo is added to CFB, the
characteristics tend to be similar to those indicated in FIGS. 15
and 16.
[0084] In view of the foregoing, anisotropy of the storage layer 20
is maintained by providing the first buffer layer 61 and the second
buffer layer 62 beneath the underlayer 12. Further, since the
buffer layers are nonmagnetic, the buffer layers do not adversely
affect a write current to the storage layer 20 or a stability
factor.
[0085] FIG. 17 shows the relationship between the ratio of W or Mo
in the buffer layer 61 and the saturation magnetization Mst. In
order to make Mst of the buffer layer 61 sufficiently small, the
ratio of W or Mo should preferably be 30 atm % or more. However, if
the buffer layer 61 is crystallized as a result of too high a ratio
of W or Mo, the MgO underlayer 12 is crystallized in an undesired
orientation. Accordingly, the ratio of W or Mo may be set at a
level which allows the buffer layer 61 to maintain the amorphous
state.
[0086] Also, the magnetic material in the buffer layer 61 is not
limited to CoFeB, and various magnetic materials can be used. For
example, CoB or FeB can also be used.
[0087] As can be seen, according to the present embodiment, by
providing the second buffer layer 62 and the amorphous first buffer
layer 61 beneath the underlayer 12, the underlayer 12 can be
crystalline-oriented, and the crystallinity of the storage layer 20
can be improved. Thereby, the element characteristics of the
magnetoresistive memory device can be improved.
[0088] Note that even if a nitrogen compound or an oxygen compound
other than MgO, or a mixture of such compounds is used as the
underlayer 12, an advantage of improving the orientation of the
underlayer 12 achieved by forming two buffer layers can be
obtained.
[0089] (Fifth Embodiment)
[0090] FIG. 18 is a cross-sectional view showing the structure of a
memory cell portion used in a magnetoresistive memory device
according to a fifth embodiment. It should be noted that the same
portions as those of FIG. 4 will be given the same reference
numbers, and detailed explanations of them will be omitted. Also,
FIG. 19 schematically shows a difference between an MTJ element
portion shown in FIG. 18 and a comparative example.
[0091] The point in which the present embodiment is different from
the first embodiment described above is that a magnetic material
layer 72 formed of amorphous CoFeB is used instead of the middle
layer 22. That is, an underlayer (UL) 12 is provided on a bottom
electrode (BEC) via a buffer layer (BuL) 11. On the underlayer 12,
a storage layer 20 (SL [first magnetic layer]) in which the
amorphous magnetic material layer 72 is sandwiched between
crystalline magnetic material layers 21 and 23 is formed. That is,
the storage layer 20 in which three layers, i.e., layers 21, 72,
and 23, are stacked is formed. Further, a reference layer 40 is
formed on the magnetic material layer 23 via a tunnel barrier layer
30.
[0092] Here, the magnetic material layers 21 and 23 are formed of
crystalline CoFeB, and the barrier layer 30 is formed of MgO, AlO,
MgAlO, or ZnO. The CoFeB of the magnetic material layers 21 and 23
is a crystalline material of bcc (001) orientation, and the
concentration of B is greater than or equal to 0 at % and less than
or equal to 30 at %. CoFeB of the magnetic material layer 72 is an
amorphous material having less crystallinity than the magnetic
material layers 21 and 23, and the concentration of B is 0 to 30 at
%. Note that as the magnetic material layers 21 and 23 and the
barrier layer 30, various materials described in the first
embodiment can be used. Further, as the underlayer 12, various
materials described in the first embodiment can be used.
[0093] In order to improve retention of the storage layer 20 of the
MTJ element portion and reduce a write current, it is necessary to
reduce the saturation magnetization Mst of the storage layer 20.
However, if the Mst is simply reduced, the TMR and perpendicular
magnetic anisotropy are adversely affected. The CoFeB used for the
storage layer 20 is crystallized from the amorphous state at the
time of film formation when subjected to heat treatment, and a high
TMR and perpendicular magnetic anisotropy are exhibited. That is,
preferably, an interface between the barrier layer 30 and the
underlayer 12 should be structured such that the TMR and the
perpendicular magnetic anisotropy are maintained by the crystalline
CoFeB, and the Mst is reduced by arranging the amorphous CoFeB as
the middle layer of the storage layer 20.
[0094] Further, in order to form the storage layer 20 with bcc-CFB
and amorphous CFB separately, the concentration of B may be
adjusted. Generally, when the concentration of B is greater than or
equal to 0 at % and less than or equal to 30 at %, a region which
is crystallized is greater than a region which is amorphous. When
the concentration of B is 25 at % or more, the amorphous region is
greater. In the range of 25 to 30 at %, according to the condition
of crystallization, a crystallized region and an amorphous region
are mixed.
[0095] As can be seen, in the present embodiment, by using
crystalline CoFeB for the magnetic material layers 21 and 23 of the
storage layer 20, and using amorphous CoFeB for the magnetic
material layer 72 in the middle, it is possible to reduce the
saturation magnetization Mst in the storage layer 20 while
maintaining the TMR and the perpendicular magnetic anisotropy.
[0096] FIG. 20 is a further improvement of the present embodiment,
and the difference is that a nonmagnetic material layer is provided
between amorphous CoFeB and crystalline CoFeB. That is, in addition
to the structure illustrated in FIG. 18, a nonmagnetic material
layer 71 formed of W, etc., is inserted between the magnetic
material layer 21 formed of crystalline CoFeB and the magnetic
material layer 72 formed of amorphous CoFeB, and a nonmagnetic
material layer 73 formed of W, etc., is inserted between the
magnetic material layer 72 formed of amorphous CoFeB and the
magnetic material layer 23 formed of crystalline CoFeB.
[0097] Here, as the nonmagnetic material layers 71 and 73, a
high-melting point material such as W, Mo, or Ta should preferably
be used. In this example, by inserting the nonmagnetic material
layers 71 and 73, a difference between crystallinity of the
crystalline layer and the amorphous layer can be exhibited.
[0098] FIG. 21 is a characteristic diagram showing the relationship
between the thickness of the magnetic material layer 72 and the
saturation magnetization of the storage layer 20 multiplied by the
film thickness ("saturation magnetization.times.film
thickness")[Mst]. Here, [Mst] in FIG. 21 is product of the
saturation magnetization of the storage layer 20 and the film
thickness of the storage layer 20. FIG. 22 is a characteristic
diagram showing the relationship between the saturation
magnetization multiplied by the film thickness and the anisotropic
magnetic field Hk. In the drawing, (1) corresponds to a case where
only the nonmagnetic material layer is provided without the
magnetic material layer 72, (2) corresponds to a case where the
nonmagnetic material layers 71 and 73 are formed of Mo, and (3)
corresponds to a case where the nonmagnetic material layers 71 and
73 are formed of W.
[0099] As shown in FIG. 21, as compared to (1) in which no magnetic
material layer 72 is provided, Mst can be reduced in (2) and (3)
including CoFeB. The nonmagnetic material layers 71 and 73 being
formed of W can more reduce the saturation magnetization multiplied
by the film thickness than the nonmagnetic material layers 71 and
73 being formed of Mo. Also, as shown in FIG. 22, the smaller the
saturation magnetization Mst is, the greater the anisotropic
magnetic field Hk is. However, a structure in which the nonmagnetic
material layers 71 and 73 are arranged respectively between the
middle magnetic material layer 72 and the magnetic material layer
21, and between the middle magnetic material layer 72 and the
magnetic material layer 23 can more increase the anisotropic
magnetic field Hk. Accordingly, by adopting the structures of (2)
and (3), the saturation magnetization can be reduced without
degrading the magnetic anisotropy.
[0100] (Sixth Embodiment)
[0101] FIG. 23 is a cross-sectional view showing the structure of
an MTJ element portion used in a magnetoresistive memory device
according to a sixth embodiment. It should be noted that the same
portions as those of FIG. 4 will be given the same reference
numbers, and detailed explanations of them will be omitted. Also,
the schematic structure of the MTJ element portion is shown in
FIGS. 24A and 24B. FIG. 24A shows an example in which an SL is
formed of three layers indicated as 21 to 23, and FIG. 24B shows an
example in which the SL is formed of a single layer.
[0102] The point in which the present embodiment is different from
the first and the second embodiment is that two buffer layers,
i.e., buffer layers 81 and 82, are provided below an underlayer 12.
Further, the point in which the present embodiment is different
from the fourth embodiment is that amorphous Mo is used for the
buffer layer 81.
[0103] On a bottom electrode (BEC) connected to a drain region of a
select transistor, a first buffer layer (BuL1) 81 and a second
buffer layer (BuL2) 82 are stacked. The buffer layer 81 on the
upper side should preferably be amorphous, and is formed of, for
example, Mo. The amorphous Mo is realized by adding a semimetal
element such as B or Si to Mo, or by a thin layer of Mo before
being subjected to crystal growth of 10 .ANG. or so.
[0104] Preferably, the material of the buffer layer 82 on the lower
side should be amorphous, or a material which does not inhibit the
amorphous property of the buffer layer 81. More specifically, the
same material as that of the buffer layer 11 described in the first
embodiment can be used.
[0105] On the buffer layer 81, the underlayer 12 is formed. The
underlayer 12 is an oxygen compound, which is MgO, for example.
Further, on the underlayer 12, as in the first embodiment, a
storage layer 20 (SL [first magnetic layer]), a tunnel barrier
layer 30, and a reference layer 40 are formed. Likewise the first
or the second embodiment, the storage layer 20 has a stacked layer
structure in which a middle layer 22 formed of Mo or W is
sandwiched between magnetic material layers 21 and 23 of CoFeB.
[0106] Further, the storage layer 20 is not necessarily limited to
a stacked layer structure, and may be a single-layer alloy of
CoFeBMo, for example, or a single-layer alloy of CoFeBW, for
example. Furthermore, the storage layer 20 is not necessarily
limited to one which includes Mo or W, and it may be a single
magnetic layer of CoFeB or CoFe, etc.
[0107] As in the present embodiment, by providing the buffer layer
81 of amorphous Mo, the orientation of the MgO underlayer 12 can be
improved. When the orientation of the MgO underlayer 12 is
improved, the crystallinity of CoFeB formed thereon is improved. In
this way, the element characteristics can be improved. When
amorphous Mo is used for the buffer layer 81, if the thickness is
around 10 .ANG., substantially the same level of anisotropic
magnetic field Hk as CoFeBMo can be obtained.
[0108] When MgO is used for the underlayer 12 of the storage layer
20 of the MTJ element portion, perpendicular magnetic anisotropy
PMA is developed from both of the two interfaces of the storage
layer 20, and retention can be improved. Since the PMA is improved
in accordance with (001) orientation of the NaCl structure of the
MgO underlayer, a material of the buffer layer of the MgO
underlayer 12, in particular, the buffer layer 81 immediately below
the MgO underlayer 12 is important. While using quarternary alloy
(CFB-Mo) obtained by adding Mo to CoFeB as in the fourth embodiment
can improve the orientation of the MgO underlayer 12, a similar
advantage can also be obtained with a single layer as in the
present embodiment. The Mo buffer layer 81 should preferably be
amorphous, and can be realized by adding a semimetal element such
as B or Si to Mo, or by a thin layer of Mo before being subjected
to crystal growth of approximately 10 to 30 .ANG..
[0109] Note that the underlayer 12 is not necessarily limited to
MgO, and it is expected that a similar advantage can be obtained as
long as the underlayer 12 is an oxide.
[0110] As described above, in the present embodiment, likewise the
fourth embodiment which has already been explained, the element
characteristics of the magnetoresistive memory device can be
improved. Further, there is also an advantage that the parasitic
resistance of the MgO underlayer is reduced, and the TMR may be
improved when the buffer layer 81 of Mo is used as in the present
embodiment, as compared to a case where the buffer layer 61 formed
of CoFeBMo is used as in the fourth embodiment.
[0111] (Modification)
[0112] Note that the embodiments are not limited to those described
above.
[0113] In the present embodiments, the storage layer 20 is arranged
on the side of the substrate, and the reference layer 30 is
arranged on the opposite side. However, the positional relationship
between these layers may be made opposite, as shown in FIG. 25. The
underlayer 12 is not necessarily provided on a substrate 10, and it
is sufficient if the underlayer 12 contacts the storage layer
20.
[0114] In the first to third embodiments, when the underlayer 12
can be formed with good crystallinity even if the buffer 11 is not
provided, the buffer layer 11 can be omitted. Also, the number of
middle layers in the storage layer 20 is not limited to one or two.
That is, three or more middle layers may be provided.
[0115] In the fourth embodiment, when the crystallinity of the
underlayer 12 is sufficiently good even if the second buffer layer
62 is not provided, the second buffer layer 62 can be omitted.
Further, in a case where the crystallinity of the underlayer 12 is
improved and sufficient characteristics as the storage layer 20 can
be obtained by simply providing the first buffer layer 61, the
storage layer 20 may be formed as a single layer, instead of
forming it into stacked layer structure. Also, in order to make the
buffer layer amorphous, CFB may be mixed into W or Mo, instead of
adding W or Mo to a magnetic material of CFB, etc. Furthermore, as
long as the buffer layer can be formed amorphous, Mo or a material
including Mo as the main component may be used.
[0116] In addition, the material of each layer is not limited to
that described in the above embodiments, and can be changed as
appropriate according to the specification. Further, the film
thickness of each layer can be changed as appropriate according to
the specification.
[0117] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
* * * * *