U.S. patent application number 11/899486 was filed with the patent office on 2008-05-15 for tunneling magnetoresistance (tmr) device, its manufacture method, magnetic head and magnetic memory using tmr device.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Takahiro Ibusuki, Masashige Sato, Shinjiro Umehara.
Application Number | 20080112093 11/899486 |
Document ID | / |
Family ID | 39368955 |
Filed Date | 2008-05-15 |
United States Patent
Application |
20080112093 |
Kind Code |
A1 |
Sato; Masashige ; et
al. |
May 15, 2008 |
Tunneling magnetoresistance (TMR) device, its manufacture method,
magnetic head and magnetic memory using TMR device
Abstract
A barrier layer is disposed over a pinned layer made of
ferromagnetic material having a fixed magnetization direction, the
barrier layer having a thickness allowing electrons to transmit
therethrough by a tunneling phenomenon. A first free layer is
disposed over the barrier layer, the first free layer being made of
amorphous or fine crystalline soft magnetic material which changes
a magnetization direction under an external magnetic field. A
second free layer is disposed over the first free layer, the second
free layer being made of crystalline soft magnetic material which
changes a magnetization direction under an external magnetic field
and being exchange-coupled to the first free layer. A tunneling
magnetoresistance device is provided which has good magnetic
characteristics and can suppress a tunnel resistance change rate
from being lowered.
Inventors: |
Sato; Masashige; (Kawasaki,
JP) ; Umehara; Shinjiro; (Kawasaki, JP) ;
Ibusuki; Takahiro; (Kawasaki, JP) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR, 25TH FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
Fujitsu Limited
Kawasaki-shi
JP
|
Family ID: |
39368955 |
Appl. No.: |
11/899486 |
Filed: |
September 6, 2007 |
Current U.S.
Class: |
360/324.2 ;
G9B/5.117 |
Current CPC
Class: |
B82Y 40/00 20130101;
H01F 10/3204 20130101; B82Y 10/00 20130101; H01F 10/3272 20130101;
H01L 43/08 20130101; Y10T 428/1114 20150115; G11B 5/3909 20130101;
G01R 33/093 20130101; H01F 41/303 20130101; H01F 10/3295 20130101;
G01R 33/098 20130101; B82Y 25/00 20130101; G11B 5/3906 20130101;
H01L 43/12 20130101; H01F 10/3254 20130101 |
Class at
Publication: |
360/324.2 |
International
Class: |
G11B 5/33 20060101
G11B005/33 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2006 |
JP |
2006-307987 |
Claims
1. A tunneling magnetoresistance device comprising: a pinned layer
made of ferromagnetic material having a fixed magnetization
direction; a barrier layer disposed over the pinned layer and
having a thickness allowing electrons to transmit therethrough by a
tunneling phenomenon; a first free layer disposed over the barrier
layer and made of amorphous or fine crystalline soft magnetic
material which changes a magnetization direction under an external
magnetic field; and a second free layer disposed over the first
free layer and made of crystalline soft magnetic material which
changes a magnetization direction under an external magnetic field
and being exchange-coupled to the first free layer.
2. The tunneling magnetoresistance device according to claim 1,
wherein the first free layer is made of the soft magnetic material
of CoFe added with at least one element selected from a group
consisting of B, C, Al, Si and Zr.
3. The tunneling magnetoresistance device according to claim 1,
wherein the first free layer is made of CoFeB and a B concentration
is 10 atom % or higher.
4. The tunneling magnetoresistance device according to claim 1,
wherein the second free layer is polycrystalline having a face
centered cubic structure, and has non-orientation or has a (111)
plane oriented preferentially in parallel to a substrate
surface.
5. The tunneling magnetoresistance device according to claim 1,
wherein a coercive force of the second free layer is smaller than a
coercive force of the first free layer.
6. The tunneling magnetoresistance device according to claim 1,
further comprising a crystallization suppressing layer disposed
between the first and second free layers, the crystallization
suppressing layer preventing the first free layer from being
crystallized by inheriting a crystal structure of the second free
layer.
7. The tunneling magnetoresistance device according to claim 6,
wherein the crystallization suppressing layer is made of Ta.
8. A method for manufacturing a tunneling magnetoresistance device,
comprising steps of: (a) forming a pinning layer made of
antiferromagnetic material on a support substrate; (b) forming a
pinned layer over the pinning layer, the pinned layer being made of
ferromagnetic material whose magnetization direction is fixed by an
exchange interaction with the pinning layer; (c) forming a barrier
layer over the pinned layer, the barrier layer having a thickness
allowing electrons to transmit therethrough by a tunneling
phenomenon; (d) forming a first free layer made of amorphous or
fine crystalline soft magnetic material over the barrier layer; (e)
exposing a surface of the first free layer to nitrogen plasma; (f)
forming a second free layer made of crystalline soft magnetic
material over the first free layer exposed to the nitrogen plasma;
and (g) conducting a regularizing heat treatment process for the
pinning layer by disposing a lamination structural body between the
support substrate and the second free layer in a magnetic
field.
9. The method for manufacturing the tunneling magnetoresistance
device according to claim 8, wherein the step (g) is performed
under a condition that crystallization will not progress from an
interface between the first and second free layers toward an inside
of the first free layer.
10. The method for manufacturing the tunneling magnetoresistance
device according to claim 8, wherein the first free layer is made
of the soft magnetic material of CoFe added with at least one
element selected from a group consisting of B, C, Al, Si and
Zr.
11. The method for manufacturing the tunneling magnetoresistance
device according to claim 8, wherein the first free layer is made
of CoFeB and a B concentration is 10 atom % or higher.
12. The method for manufacturing the tunneling magnetoresistance
device according to claim 8, wherein the second free layer is
polycrystalline having a face centered cubic structure, and has
non-orientation or has a (111) plane oriented preferentially in
parallel to a surface of the support substrate.
13. The method for manufacturing the tunneling magnetoresistance
device according to claim 8, wherein a coercive force of the second
free layer is smaller than a coercive force of the first free
layer.
14. A method for manufacturing a tunneling magnetoresistance
device, comprising steps of: (a) forming a pinning layer made of
antiferromagnetic material on a support substrate; (b) forming a
pinned layer over the pinning layer, the pinned layer being made of
ferromagnetic material whose magnetization direction is fixed by an
exchange interaction with the pinning layer; (c) forming a barrier
layer over the pinned layer, the barrier layer having a thickness
allowing electrons to transmit therethrough by a tunneling
phenomenon; (d) forming a first free layer made of amorphous or
fine crystalline soft magnetic material over the barrier layer; (e)
forming a crystallization suppressing layer over the first free
layer; (f) forming a second free layer made of crystalline soft
magnetic material over the crystallization suppressing layer; and
(g) conducting a regularizing heat treatment process for the
pinning layer by disposing a lamination structural body between the
support substrate and the second free layer in a magnetic field,
wherein the crystallization suppressing layer suppressing the first
free layer from being crystallized by inheriting a crystal
structure of the second free layer during the step (g).
15. The method for manufacturing the tunneling magnetoresistance
device according to claim 14, wherein the first free layer is made
of the soft magnetic material of CoFe added with at least one
element selected from a group consisting of B, C, Al, Si and
Zr.
16. The method for manufacturing the tunneling magnetoresistance
device according to claim 14, wherein the first free layer is made
of CoFeB and a B concentration is 10 atom % or higher.
17. The method for manufacturing the tunneling magnetoresistance
device according to claim 14, wherein the second free layer is
polycrystalline having a face centered cubic structure, and has
non-orientation or has a (111) plane oriented preferentially in
parallel to a substrate surface.
18. The method for manufacturing the tunneling magnetoresistance
device according to claim 14, wherein a coercive force of the
second free layer is smaller than a coercive force of the first
free layer.
19. A magnetic head comprising: a pinned layer made of
ferromagnetic material having a fixed magnetization direction; a
barrier layer disposed over the pinned layer and having a thickness
allowing electrons to transmit therethrough by a tunneling
phenomenon; a first free layer disposed over the barrier layer and
made of amorphous or fine crystalline soft magnetic material which
changes a magnetization direction under an external magnetic field;
and a second free layer disposed over the first free layer and made
of crystalline soft magnetic material which changes a magnetization
direction under an external magnetic field and being
exchange-coupled to the first free layer.
20. A magnetic memory comprising: a tunneling magnetoresistance
device; recording means for applying a magnetic field to the
tunneling magnetoresistance device to change magnetization
directions of first and second free layers of the tunneling
magnetoresistance device; and reproducing means for applying a
sense current through the tunneling magnetoresistance device to
detect a resistance of the tunneling magnetoresistance device,
wherein the tunneling magnetoresistance device comprises: a pinned
layer made of ferromagnetic material having a fixed magnetization
direction; a barrier layer disposed over the pinned layer and
having a thickness allowing electrons to transmit therethrough by a
tunneling phenomenon; the first free layer disposed over the
barrier layer and made of amorphous or fine crystalline soft
magnetic material which changes a magnetization direction under an
external magnetic field; and the second free layer disposed over
the first free layer and made of crystalline soft magnetic material
which changes a magnetization direction under an external magnetic
field and being exchange-coupled to the first free layer.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is based on and claims priority of Japanese
Patent Application No. 2006-307987 filed on Nov. 14, 2006, the
entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] A) Field of the Invention
[0003] The present invention relates to a tunneling
magnetoresistance device and its manufacture method, and more
particularly to a tunneling magnetoresistance device which changes
its electric resistance depending on an external magnetic field and
is applied to a reproducing head of a magnetic recording apparatus
and a magnetic memory, and its manufacture method.
[0004] B) Description of the Related Art
[0005] In a junction having a "metal/insulating film/metal"
structure consisting of the insulating film and metal films
sandwiching the insulating film therebetween, as a voltage is
applied across opposing metal layers, a small current flows if the
insulating film is sufficiently thin. Generally, current does not
flow through an insulating film. However, if the insulating film is
sufficiently thin, e.g., several nm or thinner, electrons transmit
through the insulating film at some probability because of the
quantum mechanics effects. Current of electrons transmitting
through an insulating film is called a "tunnel current" and its
structure is called a "tunnel junction".
[0006] Generally, a metal oxide film is used as the insulating film
of the tunnel junction. For example, a thin insulating film of
aluminum oxide is formed by natural oxidation, plasma oxidation or
thermal oxidation of a surface layer of aluminum. By controlling
oxidation conditions, an insulating film can be formed which is
applicable to the tunnel junction and has a thickness of several
nm.
[0007] A device having a tunnel junction exhibits nonlinear
current-voltage characteristics and has been used as a nonlinear
device.
[0008] The structure of the tunnel junction whose opposing metal
layers are made of ferromagnetic material is called a
"ferromagnetic tunnel junction". A tunnel probability (tunnel
resistance) of a ferromagnetic tunnel junction is dependent upon a
magnetization state of opposing ferromagnetic materials. Therefore,
the tunnel resistance can be changed by controlling the
magnetization state by applying an external magnetic field. A
tunnel resistance R can be expressed by the following equation:
R=Rs+0.5.DELTA.R(1-cos.theta.)
where .theta. is a relative angle between magnetization directions
of opposing ferromagnetic materials. Rs represents a tunnel
resistance at the magnetization direction relative angle .theta. of
0, i.e., at parallel magnetization directions, and .DELTA.R
represents a difference between tunnel resistances at the
magnetization direction relative angle .theta. of 180.degree.,
i.e., at counter-parallel magnetization directions and the tunnel
resistance at the parallel magnetization directions.
[0009] A phenomenon that a tunnel resistance changes depending on a
magnetization direction of ferromagnetic material results from
polarization of electrons in ferromagnetic material. Generally,
there exist in metal, spin-up electrons in an upward spin state and
spin-down electrons in a downward spin state. There exist in
nonmagnetic metal, the same number of spin-up electrons and
spin-down electrons. Therefore, no magnetism is exhibited as a
whole. In ferromagnetic material, the number of spin-up electrons
(Nup) is different from the number of spin-down electrons (Ndown)
so that the ferromagnetic material exhibits spin-up or spin-down
magnetism as a whole.
[0010] It is known that when an electron transmits through a
barrier layer by the tunnel phenomenon, the spin state of the
electron is retained. Therefore, if there is a vacant electron
quantum level in a tunnel destination ferroelectric material, an
electron can transmit through the barrier layer. If there is no
vacant electron quantum level, an electron cannot transmit through
the barrier layer.
[0011] A change rate .DELTA.R/Rs of a tunnel resistance is
expressed by the following equation:
.DELTA.R/Rs=2P.sub.1P.sub.2/(1-P.sub.1P.sub.2)
wherein P.sub.1 and P.sub.2 are spin polarizabilities of
ferroelectric material on both sides of a barrier layer. The spin
polarizability is given by the following equation:
P=2(Nup-Ndown)/(Nup+Ndown)
[0012] Tunneling magnetoresistance devices are reported in
"Japanese Patent Publication No. 2871670", "Yuasa et al., Nature
Materials vol. 3 (2004) p. 868-p. 871", "Parkin et al., Nature
Materials vol. 3 (2004) p. 862-p. 867", and "Tsunekawa et al.,
Effect of Capping Layer Material on Tunnel Magnetoresistance in
CoFeB/MgO/CoFeB magnetic Tunnel Junctions, International Magnetic
Conference 2005, HP-08, p. 992".
SUMMARY OF THE INVENTION
[0013] Magnetic characteristics of a tunneling magnetoresistance
device are improved. A tunnel resistance change rate of a tunneling
magnetoresistance device can be suppressed from being lowered.
Further, a manufacture method for such a tunneling
magnetoresistance device is provided.
[0014] According to one aspect of the present invention, there is
provided a tunneling magnetoresistance device including:
[0015] a pinned layer made of ferromagnetic material having a fixed
magnetization direction;
[0016] a barrier layer disposed over the pinned layer and having a
thickness allowing electrons to transmit therethrough by a
tunneling phenomenon;
[0017] a first free layer disposed over the barrier layer and made
of amorphous or fine crystalline soft magnetic material which
changes a magnetization direction under an external magnetic field;
and
[0018] a second free layer disposed over the first free layer and
made of crystalline soft magnetic material which changes a
magnetization direction under an external magnetic field and being
exchange-coupled to the first free layer.
[0019] According to another aspect of the present invention, there
is provided a method for manufacturing a tunneling
magnetoresistance device, including steps of:
[0020] (a) forming a pinning layer made of antiferromagnetic
material on a support substrate;
[0021] (b) forming a pinned layer over the pinning layer, the
pinned layer being made of ferromagnetic material whose
magnetization direction is fixed by an exchange interaction with
the pinning layer;
[0022] (c) forming a barrier layer over the pinned layer, the
barrier layer having a thickness allowing electrons to transmit
therethrough by a tunneling phenomenon;
[0023] (d) forming a first free layer made of amorphous or fine
crystalline soft magnetic material over the barrier layer;
[0024] (e) exposing a surface of the first free layer to nitrogen
plasma;
[0025] (f) forming a second free layer made of crystalline soft
magnetic material over the first free layer exposed to the nitrogen
plasma; and
[0026] (g) conducting a regularizing heat treatment process for the
pinning layer by disposing a lamination structural body between the
support substrate and the second free layer in a magnetic
field.
[0027] According to still another aspect of the present invention,
there is provided a method for manufacturing a tunneling
magnetoresistance device, including steps of:
[0028] (a) forming a pinning layer made of antiferromagnetic
material on a support substrate;
[0029] (b) forming a pinned layer over the pinning layer, the
pinned layer being made of ferromagnetic material whose
magnetization direction is fixed by an exchange interaction with
the pinning layer;
[0030] (c) forming a barrier layer over the pinned layer, the
barrier layer having a thickness allowing electrons to transmit
therethrough by a tunneling phenomenon;
[0031] (d) forming a first free layer made of amorphous or fine
crystalline soft magnetic material over the barrier layer;
[0032] (e) forming a crystallization suppressing layer over the
first free layer;
[0033] (f) forming a second free layer made of crystalline soft
magnetic material over the crystallization suppressing layer;
and
[0034] (g) conducting a regularizing heat treatment process for the
pinning layer by disposing a lamination structural body between the
support substrate and the second free layer in a magnetic
field,
[0035] wherein the crystallization suppressing layer suppressing
the first free layer from being crystallized by inheriting a
crystal structure of the second free layer during the step (g).
[0036] According to still another aspect of the present invention,
there is provided a magnetic head provided with the tunneling
magnetoresistance device.
[0037] According to still another aspect of the present invention,
there is provided a magnetic memory including:
[0038] the tunneling magnetoresistance device;
[0039] recording means for applying a magnetic field to the
tunneling magnetoresistance device to change magnetization
directions of first and second free layers of the tunneling
magnetoresistance device; and
[0040] reproducing means for applying a sense current through the
tunneling magnetoresistance device to detect a resistance of the
tunneling magnetoresistance device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIGS. 1A and 1B are a cross sectional view and a plan view
of a tunneling magnetoresistance device according to a first
embodiment.
[0042] FIGS. 2A to 2D are cross sectional views of the tunneling
magnetoresistance device during manufacture of the first
embodiment.
[0043] FIGS. 3A and 3B are graphs showing the relation between a
resistance change rate and an applied magnetic field of tunneling
magnetoresistance devices of the first embodiment and a comparative
example, respectively.
[0044] FIGS. 4A and 4B are TEM photographs in section of the
tunneling magnetoresistance devices of the first embodiment and the
comparative example, respectively.
[0045] FIG. 5 is a cross sectional view of a tunneling
magnetoresistance device according to a second embodiment.
[0046] FIGS. 6A and 6B are graphs showing the relation between a
resistance change rate and an applied magnetic field of tunneling
magnetoresistance devices of the second embodiment and a
comparative example, respectively.
[0047] FIG. 7 is a front view of a magnetic head using the
tunneling magnetoresistance device of each of the first and second
embodiments.
[0048] FIG. 8A is a cross sectional view of an MRAM using the
tunneling magnetoresistance device of each of the first and second
embodiments, and FIG. 8B is an equivalent circuit of MRAM.
[0049] FIG. 9A is a cross sectional view of a tunneling
magnetoresistance device consisting of a CoFeB/MgO/CoFeB lamination
structure, and FIG. 9B is a graph showing the relation between a
resistance change rate and an applied magnetic field.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] A tunneling magnetoresistance device consisting of a
CoFeB/MgO/CoFeB lamination structure is explained below prior to
describing embodiments.
[0051] FIG. 9A shows an example of a tunneling magnetoresistance
device consisting of a CoFeB/MgO/CoFeB lamination structure. An
underlying layer 101 of Ta having a thickness of 50 nm, a pinning
layer 102 of PtMn having a thickness of 15 nm, a first pinned layer
103 of CoFe having a thickness of 3 nm, a non-magnetic coupling
layer 104 of Ru having a thickness of 0.8 nm, a second pinned layer
105 of CoFeB having a thickness of 3 nm, a barrier layer 106 of MgO
having a thickness of 2 nm, a free layer 107 of CoFeB having a
thickness of 3 nm, a first cap layer 108 of Ta having a thickness
of 10 nm and a second cap layer 109 of Ru having a thickness of 10
nm are formed in this order on a support substrate 100 made of Si
or SiO.sub.2.
[0052] FIG. 9B shows a relation between an external magnetic field
and a resistance change rate. A resistance change rate is defined
by (R-Rs)/Rs where Rs is a device resistance when a magnetization
direction of the pinned layers 103 and 105 is parallel to a
magnetization direction of the free layer 107, and R is a device
resistance when an external magnetic field is applied. It can be
seen that the maximum resistance change rate of about 200% is
obtained.
[0053] When a tunneling magnetoresistance device is applied to a
magnetic head, it is required to have desired magnetic
characteristics, that is magnetization characteristics,
magnetostriction characteristics, coercive force, magnetic
anisototropy and the like. For example, it can be known from the
measurement results shown in FIG. 9B that a magnetic field
(coercive force) of about 50 Os is required to reverse
magnetization of the free layer 107 of the tunneling
magnetoresistance device. In order to apply the device to a
magnetic head, it is necessary to lower the coercive force. An
effective coercive force can be lowered by stacking soft magnetic
material having a smaller coercive force than CoFeB on the free
layer 107 made of CoFeB.
[0054] If a layer of soft magnetic material having a small coercive
force such as NiFe is stacked on the free layer of CoFeB, a
resistance change rate lowers.
[0055] A first embodiment is described below.
[0056] FIGS. 1A and 1B are a cross sectional view and a plan view
of a tunneling magnetoresistance device according to the first
embodiment. FIG. 1A corresponds to a cross sectional view taken
along one-dot chain line 1A-1A of FIG. 1B.
[0057] As shown in FIG. 1A, a conductive layer 12 of NiFe is formed
on a support substrate 10 having an SiO.sub.2 film formed on Si.
Other materials such as ceramic material such as AlTiC, and quartz
glass may be used as the material of the support substrate 10. The
surface of the NiFe conductive layer 12 is planarized by chemical
mechanical polishing (CMP). A tunneling magnetoresistance device 40
of a cylindrical shape is formed on a partial area of the
conductive layer 12.
[0058] The tunneling magnetoresistance device 40 is formed by
laminating a first underlying layer 13, a second underlying layer
14, a pinning layer 18, a first pinned layer 20, a non-magnetic
coupling layer 21, a second pinned layer 22, a barrier layer 25, a
first free layer 30, a second free layer 32, a first cap layer 35
and a second cap layer 36 in this order.
[0059] The first underlying layer 13 is made of Ta and has a
thickness of about 5 nm. The first underlying layer 13 may be made
of Cu or Au, or may be a lamination layer of these materials. The
second underlying layer 14 is made of Ru and has a thickness of
about 2 nm.
[0060] The pinning layer 18 is made of IrMn and has a thickness of
about 7 nm. The pinning layer 18 may be made of antiferromagnetic
material other than IrMn, such as alloy of Mn and at least one
element selected from a group consisting of Pt, Pd, Ni, Ir and Rh.
A thickness of the pinning layer 18 is preferably in a range
between 5 nm and 30 nm, and more preferably in a range between 10
nm and 20 nm. The pinning layer 18 is regularized by heat treatment
in a magnetic field after it is deposited, and exhibits
antiferromagnetism.
[0061] The first pinned layer 20 is made of Co.sub.74Fe.sub.26 and
has a thickness of, e.g., 2 nm. The non-magnetic coupling layer 21
is made of Ru and has a thickness of, e.g., 0.8 nm. The second
pinned layer 22 is made of CO.sub.60Fe.sub.20B.sub.20 and has a
thickness of, e.g., 2 nm. A magnetization direction of the first
pinned layer 20 is fixed to a certain direction by an exchange
interaction with the pinning layer 18. Namely, the magnetization
direction of the first pinned layer 20 does not change even if an
external magnetic field is applied if the magnetic field intensity
is weaker than the exchange interaction. The first pinned layer 20
and second pinned layer 22 exchange-couple antiferromagnetically
with each other via the non-magnetic coupling layer 21.
[0062] A thickness of the non-magnetic coupling layer 21 is set in
a range allowing that the first pinned layer 20 and second pinned
layer 22 exchange-couple antiferromagnetically with each other. The
thickness is in a range between 0.4 nm and 1.5 nm, and preferably
between 0.4 nm and 0.9 nm. The first pinned layer 20 and second
pinned layer 22 may be made of ferromagnetic material which
contains one of Co, Ni and Fe. The non-magnetic coupling layer 21
may be made of non-magnetic material such as Rh, Ir, Ru-based
alloy, Rh-based alloy and Ir-based alloy, in addition to Ru. Alloy
containing Ru and at least one element selected from a group
consisting of Co, Cr, Fe, Ni and Mn may be cited as an example of
the Ru-based alloy.
[0063] The magnetization direction of the first pinned layer 20 and
the magnetization direction of the second pinned layer 22 are
counter-parallel so that an intensity of a net leakage magnetic
field from the first and second pinned layers 20 and 22 lowers.
This mitigates the adverse effect that the leakage magnetic field
changes the magnetization directions of the first and second free
layers 30 and 32. Accordingly, magnetization of the first and
second free layers 30 and 32 can respond correctly to a leakage
magnetic field from a magnetic recording medium, and detection
accuracy for magnetization recorded in the magnetic recording
medium is improved.
[0064] The barrier layer 25 is made of MgO and has a thickness of,
e.g., 1.0 nm. It is preferable that MgO of the barrier layer 25 is
crystalline, and it is particularly preferable that the (001) plane
of MgO is oriented generally in parallel to the substrate surface.
A thickness of the barrier layer 25 is preferably in a range
between 0.7 nm and 2.0 nm from the viewpoint of good film quality.
The barrier layer 25 may be made of AlO.sub.x, TiO.sub.x,
ZrO.sub.x, AlN, TiN, ZrN or the like, in place of MgO. If the
barrier layer 25 is made of material other than MgO, its thickness
is preferably in a range between 0.5 nm and 2.0 nm, and more
preferably in a range between 0.7 nm and 1.2 nm.
[0065] The first free layer 30 is made of amorphous
CO.sub.60Fe.sub.20B.sub.20 and has a thickness of about 2 nm. From
the viewpoint that the first free layer 30 is easy to be amorphous,
a B concentration is preferably in a range between 10 atom % and 25
atom %. The first free layer 30 may be made of soft magnetic
material added with at least one element selected from a group
consisting of B, C, Al, Si and Zr, in place of CoFeB.
[0066] The second free layer 32 is made of Ni.sub.80Fe.sub.20 and
has a thickness of, e.g., 4 nm. The second free layer 32 is made of
soft magnetic material having a smaller coercive force than that of
the first free layer 30. CoNiFe having a composition allowing a
face centered cubic structure may be cited as an example of the
material of the second free layer 32, in place of NiFe. At least
one element selected from a group consisting of B, C, Al, Si and Zr
may be added to NiFe and CoNiFe. A concentration of the added
element is set lower than that of the element added to the first
free layer 30.
[0067] By ferromagnetically coupling the second free layer 32
having a smaller coercive force to the first free layer 30,
sensitivity to a change in the external magnetic field can be
improved. Generally, a ferromagnetic film is more sensitive to a
change in the direction of an external magnetic field, the smaller
the coercive force is. Since the coercive force of the second free
layer 32 is lower than that of the first free layer 30, as the
direction of an external magnetic field changes, the magnetization
direction of the second free layer 32 changes before the
magnetization direction of the first free layer 30 changes. Since
the first free layer 30 is ferromagnetically exchange-coupled to
the second free layer 32, the magnetization direction of the first
free layer 30 changes following a change in the magnetization
direction of the second free layer 32. Therefore, the magnetization
direction of the first free layer 30 is more susceptible to a
change in the direction of an external magnetic field. Since the
magnetization direction of the first free layer 30 contributes to
the resistance change rate, a sensitivity of the tunneling
magnetoresistance device can be improved by disposing the second
free layer 32.
[0068] The first cap layer 35 is made of Ta and has a thickness of,
e.g., 5 nm. The second cap layer 36 is made of Ru and has a
thickness of, e.g., 10 nm. The first cap layer 35 and second cap
layer 36 prevent the underlying ferromagnetic layer and the like
from being oxidized during heat treatment. The first cap layer 35
may be made of Ru, and the second cap layer 36 may be made of Ta.
More generally, the cap layer may be made of non-magnetic metal
such as Au, Ta, Al, W and Ru, or may be made of a lamination
structure of layers made of these metals. A total thickness of the
cap layers is preferably in a range between 5 nm and 30 nm.
[0069] Of the surface of the conductive layer 12, the region where
the tunneling magnetoresistance device 40 is not disposed is
covered with an insulating film 48 of insulating material such as
SiO.sub.2. A first electrode 45 is formed on the tunneling
magnetoresistance device 40 and insulating film 48. The first
electrode 45 is electrically connected to the second cap layer 36.
A via hole is formed through the insulating film 48, reaching the
conductive layer 12. The via hole is filled with a second electrode
46. The second electrode 46 is electrically connected to the
conductive layer 12. The first electrode 45 and second electrode 46
are made of, e.g., Cu.
[0070] Next, with reference to FIGS. 2A to 2D, description will be
made on a manufacture method for the tunneling magnetoresistance
device of the first embodiment.
[0071] As shown in FIG. 2A, the layers from the conductive layer 12
to first free layer 30 are formed on the support substrate 10 by
using a magnetron sputtering system.
[0072] As shown in FIG. 2B, the first free layer 30 is exposed to
nitrogen plasma 38. For example, this plasma process is executed
under the following conditions: [0073] Nitrogen gas flow rate: 100
sccm [0074] RF power: 50 W [0075] Process time: 30 sec
[0076] As shown in FIG. 2C, the second free layer 32, first cap
layer 35 and second cap layer 36 are formed on the first free layer
30 subjected to the surface treatment by nitrogen plasma, by using
the magnetron sputtering system. Thereafter, the substrate is
disposed in vacuum and a regularizing heat treatment process is
performed for the pinning layer 18, in the state that a magnetic
field is applied. A heat treatment temperature is, e.g.,
270.degree. C. and a heat treatment time is, e.g., four hours. The
heat treatment temperature may be in a range between 250.degree. C.
and 400.degree. C.
[0077] As shown in FIG. 2D, the layers between the first underlying
layer 13 and the second cap layer 36 are patterned to form the
tunneling magnetoresistance device 40 of a cylindrical shape.
Patterning these layers may be performed by Ar ion milling.
Thereafter, as shown in FIG. 1A, the insulating film 48, first
electrode 45, via hole through the insulating film 48 and second
electrode 46 are formed.
[0078] FIG. 3A shows a resistance change rate of a tunneling
magnetoresistance device manufactured by the method of the first
embodiment. For the reference sake, a resistance change rate is
also shown for a comparative example manufactured without the
nitrogen plasma process shown in FIG. 2B. The tunneling
magnetoresistance device of the comparative example without the
plasma nitrogen process has the maximum resistance change rate of
about 20%, whereas the tunneling magnetoresistance device
manufactured by the first embodiment method has the maximum
resistance change rate of about 60%. Since the second free layer 32
having a smaller coercive force is disposed on the first free layer
30, an effective coercive force of the free layer was 50 Os or
weaker. The advantage of disposing the second free layer 32 is
apparent as compared to the coercive force of about 500 Os of the
free layer 107 of the tunneling magnetoresistance device shown in
FIG. 9A.
[0079] The coercive force can be made small by disposing the second
free layer 32. However, if the second free layer 32 is disposed
simply on the first free layer 30, the resistance change rate
lowers as shown in FIG. 3B. As in the first embodiment, the
resistance change rate can be maintained high by exposing the first
free layer 30 to the nitrogen plasma 38 after the first free layer
30 is formed and before the second free layer 32 is formed. In the
following, studies will be made on the reason why the resistance
change rate can be maintained high.
[0080] FIGS. 4A and 4B show transmission electron microscope (TEM)
photographs of the cross sections of tunneling magnetoresistance
devices of the first embodiment and a comparative example,
respectively. In the comparative example, it can be seen from FIG.
4B that the first free layer 30 of CoFeB is polycrystallized. It
can be considered that crystallization progresses from the
interface between the first free layer 30 and second free layer 32
toward the inside of the first free layer 30 during the
regularizing heat treatment for the pinning layer 18 and other heat
treatment. Therefore, although the first free layer 30 is amorphous
immediately after film formation, the first free layer 30 is
crystallized by the succeeding heat treatment. It can be understood
from a distance between crystallographic planes in the TEM
photograph that CoFeB of the first free layer 30 has the (111)
plane preferentially oriented in parallel to the substrate
surface.
[0081] As shown in FIG. 4A, in the first embodiment, a crystalline
structure is not observed in the first free layer 30, and the first
free layer 30 is amorphous. It is known that if the ferromagnetic
layer being in contact with the barrier layer 25 has the (111)
orientation, the resistance change rate lowers. In the first
embodiment, the resistance change rate is suppressed from lowering,
by making the first free layer 30 amorphous.
[0082] It can be seen from the TEM photograph that NiFe of the
second free layer 32 of the tunneling magnetoresistance device of
the first embodiment has the (111) orientation. Since the second
free layer 32 is not in contact with the barrier layer 25, the
(111) orientation of the second free layer 32 does not cause a
lowered resistance change rate.
[0083] In the first embodiment, the second free layer 32 is made of
crystalline ferromagnetic material having the face centered cubic
structure and the (111) orientation. After the first free layer 30
is formed, the surface thereof is subjected to the plasma process.
It is therefore possible to prevent the first free layer 30 from
being crystallized by inheriting the crystal structure of the
second free layer 32 on the first free layer 30. Even if the
orientation of crystalline grains of the second free layer 32 is
random (namely the second free layer 32 has non-orientation.), it
is possible to suppress the resistance change rate from being
lowered, by making the first free layer 30 amorphous.
[0084] In the first embodiment, a composition ratio of Co, Fe and B
constituting the first free layer 30 is set to 60 atom %, 20 atom %
and 20 atom %, respectively. B is added in order to make CoFe alloy
amorphous. In order to make the first free layer 30 amorphous, it
is preferable to set a B concentration to 10 atom % or higher.
[0085] Generally, it is difficult to definitely distinguish
amorphous state from fine crystalline state. As shown in FIG. 4B,
if clear crystal lattice images can be observed in the first free
layer 30, it can be defined that the first free layer 30 is
crystalline. If clear crystal lattice images cannot be observed, it
can be defined that the first free layer 30 is either amorphous or
fine crystalline. Even if the first free layer 30 is fine
crystalline, it is possible to suppress the resistance change rate
from being lowered more than if the first free layer 30 is
crystalline. If a sharp peak does not appear in an X-ray
diffraction pattern of CoFeB constituting the first free layer 30,
it can be defined that the first free layer 30 is either amorphous
or fine crystalline.
[0086] A very thin region near the interface between the barrier
layer 25 and first free layer 30 is crystallized in some cases.
However, if most of the region of the first free layer 30 are
amorphous or fine crystalline, it is possible to obtain sufficient
advantages of suppressing the resistance change rate from being
lowered. If a very thin crystallized region has a thickness of at
most 0.5 nm, it can be defined that the first free layer 30 is
amorphous or fine crystalline as a whole.
[0087] FIG. 5 is a cross sectional view of a tunneling
magnetoresistance device according to the second embodiment. In the
second embodiment, a crystallization suppressing layer 50 is
inserted between the first fee layer 30 and second free layer 32.
The crystallization suppressing layer 50 is a Ta layer having a
thickness of, e.g., 0.2 nm, and is formed by magnetron sputtering.
In the second embodiment, the surface of the first free layer 30 is
not processed by nitrogen plasma shown in FIG. 2B of the first
embodiment. Other structures are the same as those of the first
embodiment.
[0088] In the second embodiment, the crystallization suppressing
layer 50 suppresses crystallization of the first free layer 30
during the regularizing heat treatment process for the pinning
layer 18. Therefore, as in the case of the first embodiment, the
first free layer 30 can be maintained in an amorphous state. In
order to exchange-couple the first free layer 30 to the second free
layer 32, it is preferable to set a thickness of the
crystallization suppressing layer 50 to 0.5 nm or thinner. The
crystallization suppressing layer 50 may be thinned to one atomic
layer if the crystallization suppressing effect is ensured.
[0089] FIG. 6A shows the relation between a resistance change rate
of the tunneling magnetoresistance device of the second embodiment
and an applied magnetic field. For the comparison sake, FIG. 6B
shows the relation between a resistance change rate of a tunneling
magnetoresistance device not provided with the crystallization
suppressing layer 50 and an applied magnetic field. The maximum
resistance change rate of the tunneling magnetoresistance device of
the second embodiment was about 62%, whereas the maximum resistance
change rate of the comparative example was about 17%. Coercive
forces of the tunneling magnetoresistance devices of the second
embodiment and comparative example were 4.9 Os and 4.3 Os,
respectively. It can be seen that a large resistance change rate
can be obtained by disposing the crystallization suppressing layer
50.
[0090] It is possible to use as the material of the crystallization
suppressing film 50, other conductive materials capable of
suppressing crystallization of the first free layer 30. Hf, Zr, Pd
and the like may be cited as usable material of the crystallization
suppressing film 50.
[0091] FIG. 7 shows the main portion of the surface facing a
magnetic recording medium, of a magnetic head including the
tunneling magnetoresistance device of each of the first and second
embodiments. An alumina film 76 is formed on a base body 75 made of
Al.sub.2O.sub.3--TiC or the like. A reproducing unit 80 is disposed
on the alumina film 76, and an induction type recording unit 90 is
disposed on the reproducing unit 80.
[0092] The induction type recording unit 90 includes a lower
magnetic pole 91, an upper magnetic pole 92 and a recording gap
layer 93 disposed between the poles. The upper magnetic pole 92 has
a width corresponding to a track width of the magnetic recording
medium. The induction type recording unit 90 further includes a
yoke (not shown) for magnetically coupling the lower magnetic pole
91 to the upper magnetic pole 92, and a coil (not shown) wound
around the yoke. As a recording current flows through the coil, a
recording magnetic field is induced.
[0093] The lower magnetic pole 91 and upper magnetic pole 92 are
made of soft magnetic material. Material having a large saturation
magnetic flux density, such as Ni.sub.80Fe.sub.20, CoZrNb, FeN,
FeSiN, FeCo alloys may be preferably used as the material of the
lower magnetic pole 91 and upper magnetic pole 92. The induction
type recording unit 90 may be replaced by a recording unit having
another structure.
[0094] Next, the structure of the reproducing unit 80 will be
described. A lower electrode 81 is formed on the alumina film 76. A
tunneling magnetoresistance device 85 is formed on a partial
surface area of the lower electrode 81. The tunneling
magnetoresistance device 85 has the same structure as that of the
tunneling magnetoresistance device of the first or second
embodiment.
[0095] An insulating film 82 covers the sidewall of the tunneling
magnetoresistance device 85 and the surface of the lower electrode
81 continuous with the sidewall. Magnetic domain control films 83
are disposed on both sides of the tunneling magnetoresistance
device 85. Each of the magnetic domain control films 83 has a
lamination structure of, e.g., a Cr film and a ferromagnetic CoCrPt
film stacked in this order from the lower electrode 81 side. The
magnetic domain control films 83 make each of the pinned layers and
free layers constituting the tunneling magnetoresistance device 85
have a single magnetic domain to thereby prevent generation of
Barkhausen noises.
[0096] An alumina film 86 is formed on the tunneling
magnetoresistance device 85 and magnetic domain control films 83,
and an upper electrode 87 is formed on the alumina film 86. A
portion of the upper electrode 87 penetrates the alumina film 86
and is electrically connected to the upper surface of the tunneling
magnetoresistance device 85.
[0097] The lower electrode 81 and upper electrode 87 are made of
soft magnetic alloy such as NiFe and CoFe, and has a function as a
magnetic shielding function as well as a sense current flow path. A
conductive film of Cu, Ta, Ti or the like may be disposed at the
interface between the lower electrode 81 and tunneling
magnetoresistance device 85.
[0098] The reproducing unit 80 and induction type recording unit 90
are covered with an alumina film, a carbon hydride film or the like
in order to prevent corrosion and the like.
[0099] A sense current flows through the tunneling
magnetoresistance device 85 in a thickness direction thereof. A
change in the tunnel resistance of the tunneling magnetoresistance
device 85 is detected as a voltage change.
[0100] FIG. 8A is a cross sectional view of a magnetic random
access memory (MRAM) using the tunneling magnetoresistance device
of the first or second embodiments, and FIG. 8B is an equivalent
circuit of MRAM. Disposed on the surface of a silicon substrate 60
are a reproducing word line 62, a MOS transistor 63, a recording
word line 68, a bit line 69 and a tunneling magnetoresistance
device 70. The reproducing word line 62 and recording word line 68
are in one-to-one correspondence and extend in a first direction (a
direction perpendicular to the drawing sheet surface of FIG. 8A, a
vertical direction in FIG. 8B). The bit line 69 extends in a second
direction (horizontal directions in FIGS. 8A and 8B) crossing the
first direction.
[0101] The MOS transistor 63 is disposed at a cross point between
the reproducing word line 62 and bit line 69. The reproducing word
line 62 serves also as the gate electrode of the MOS transistor 63.
Namely, the conduction state of the MOS transistor 63 is controlled
by a voltage applied to the reproducing word line 62.
[0102] The tunneling magnetoresistance device 70 is disposed at a
cross point between the recording word line 68 and bit line 69, and
has the same structure as that of the tunneling magnetoresistance
device of the first or second embodiment.
[0103] As a recording current flows through the recording word line
68 and bit line 69, a magnetization direction changes in the free
layer of the tunneling magnetoresistance device 70 positioned at
the cross point of the recording word line 68 and bit line 69. Data
is written by changing the magnetization direction. In tunneling
magnetoresistance devices disposed at positions different from the
cross point between the recording word line 68 and bit line 69
through which the recording current flowed, data is not written
because a magnetic field is not generated having an intensity
sufficient for changing the magnetization direction of the free
layer.
[0104] The lowermost conductive layer of the tunneling
magnetoresistance device 70 is connected to one impurity diffusion
region 61 of the MOS transistor 63 via a wiring 67 and a plurality
of plugs 64 penetrating a multilayer wiring layer and isolated
wirings 65. The uppermost conductive layer of the tunneling
magnetoresistance device 70 is connected to the bit line 69.
Namely, the wiring 67 and bit line 69 are used as the electrodes
for applying a sense current through the tunneling
magnetoresistance device 70 in the thickness direction thereof.
[0105] The other impurity diffusion region 61 of the MOS transistor
63 is connected to a plate line 66 via a plug 64. As the MOS
transistor 63 is made on-state, current depending on the resistance
of the tunneling magnetoresistance device 70 flows between the bit
line 69 and plate line 66. By judging a magnitude of this current,
data can be read.
[0106] By utilizing the same structure as that of the first or
second embodiment for the tunneling magnetoresistance device 70, it
is possible to lower the coercive force of the free layer and
increase a current change amount. It is therefore possible to lower
the recording current and retain a large margin when recorded data
is reproduced.
[0107] The present invention has been described in connection with
the preferred embodiments. The invention is not limited only to the
above embodiments. It will be apparent to those skilled in the art
that other various modifications, improvements, combinations, and
the like can be made.
* * * * *