U.S. patent application number 10/174866 was filed with the patent office on 2003-12-18 for current-perpendicular-to-plane magnetoresistive device with oxidized free layer side regions and method for its fabrication.
Invention is credited to Childress, Jeffrey R., Dobisz, Elizabeth A., Fontana, Robert E. JR., Ho, Kuok San, Nguyen, Son Van, Tsang, Ching Hwa.
Application Number | 20030231437 10/174866 |
Document ID | / |
Family ID | 29733708 |
Filed Date | 2003-12-18 |
United States Patent
Application |
20030231437 |
Kind Code |
A1 |
Childress, Jeffrey R. ; et
al. |
December 18, 2003 |
Current-perpendicular-to-plane magnetoresistive device with
oxidized free layer side regions and method for its fabrication
Abstract
A current-perpendicular-to the-plane (CPP) magnetoresistive
device has two ferromagnetic layers separated by a nonmagnetic
spacer layer with the free ferromagnetic layer having a central
region of ferromagnetic material and nonmagnetic side regions
formed of one or more oxides of the ferromagnetic material. One
type of CPP device is a magnetic tunnel junction (MTJ)
magnetoresistive read head in which the lower pinned layer has a
width and height greater than the width and height, respectively,
of the overlying central region of the upper free layer, with the
side regions of the free layer being oxidized and therefore
nonmagnetic. The MTJ read head is formed by patterning resist in
the shape of the free layer central region over the stack of layers
in the MTJ, ion milling or etching the stack down into the free
layer, and then exposing the stack to oxygen to oxidize the
ferromagnetic material in the side regions not covered by the
resist.
Inventors: |
Childress, Jeffrey R.; (San
Jose, CA) ; Dobisz, Elizabeth A.; (San Jose, CA)
; Fontana, Robert E. JR.; (San Jose, CA) ; Ho,
Kuok San; (Cupertino, CA) ; Tsang, Ching Hwa;
(Sunnyvale, CA) ; Nguyen, Son Van; (Los Gatos,
CA) |
Correspondence
Address: |
IBM CORPORATION ALMADEN RESEARCH CENTER
INTELLECTUAL PROPERTY LAW
650 HARRY ROAD
C4TA/J2
SAN JOSE
CA
95120
US
|
Family ID: |
29733708 |
Appl. No.: |
10/174866 |
Filed: |
June 17, 2002 |
Current U.S.
Class: |
360/324.12 ;
257/E43.004; 257/E43.006; 29/603.18; 360/324.2; G9B/5.116 |
Current CPC
Class: |
Y10T 29/49032 20150115;
H01L 43/08 20130101; Y10T 29/49052 20150115; B82Y 25/00 20130101;
Y10T 29/49043 20150115; G01R 33/093 20130101; G11B 5/313 20130101;
G11B 2005/3996 20130101; Y10T 29/49044 20150115; Y10T 29/49046
20150115; G11B 5/3116 20130101; G11B 5/3903 20130101; Y10T 29/49041
20150115; B82Y 10/00 20130101; G11B 5/3909 20130101; G11B 5/3163
20130101; H01L 43/12 20130101; Y10T 29/49048 20150115 |
Class at
Publication: |
360/324.12 ;
360/324.2; 29/603.18 |
International
Class: |
G11B 005/39 |
Claims
What is claimed is:
1. A magnetic tunnel junction device comprising: a substrate; a
pinned ferromagnetic layer on the substrate and having a width in a
first dimension in the plane of the pinned layer and a
magnetization direction oriented in a preferred direction and
substantially prevented from rotation in the presence of an applied
magnetic field in the range of interest; an insulating tunnel
barrier layer on the pinned layer; a free ferromagnetic layer on
the tunnel barrier layer and having a width defined by side edges
and less than the width of the pinned layer, the free layer having
a magnetization direction substantially free to rotate in the
presence of an applied magnetic field in the range of interest; and
a nonmagnetic side region located on the tunnel barrier layer on
each side of and adjacent to the side edges of the free layer, each
side region being formed of one or more oxides of the same
ferromagnetic material present in the free layer.
2. The magnetic tunnel junction device according to claim 1 wherein
the magnetization directions of the pinned and free ferromagnetic
layers are substantially parallel or antiparallel to one another in
the absence of an applied magnetic field.
3. The magnetic tunnel junction device according to claim 1 wherein
the magnetization directions of the pinned and free ferromagnetic
layers are substantially perpendicular to one another in the
absence of an applied magnetic field.
4. The magnetic tunnel junction device according to claim 1 wherein
the pinned layer has a height in a second dimension in the plane of
the pinned layer perpendicular to said first dimension and wherein
the free layer has a height less than the height of the pinned
layer.
5. The magnetic tunnel junction device according to claim 1 further
comprising a capping layer on the free layer.
6. The magnetic tunnel junction device according to claim 1 further
comprising a layer of antiferromagnetic material on the substrate
below the pinned layer for pinning the magnetization of the pinned
layer by antiferromagnetic exchange coupling.
7. The magnetic tunnel junction device according to claim 1 wherein
the tunnel barrier layer is formed substantially of alumina.
8. The magnetic tunnel junction device according to claim 1 further
comprising an insulating cover on each insulating side region and
formed of material having a composition different from the
composition of the side region.
9. The magnetic tunnel junction device according to claim 1 wherein
the substrate is a magnetic shield layer formed on the trailing
surface of a head carrier.
10. The magnetic tunnel junction device according to 9 further
comprising a nonmagnetic electrically conductive lead layer on the
shield layer.
11. The magnetic tunnel junction device according to claim 1
wherein the free layer is formed of an alloy comprising Co and Fe,
and wherein the nonmagnetic side regions are formed of one or more
oxides of Co and Fe.
12. The magnetic tunnel junction device according to claim 1
wherein the free layer is formed of an alloy comprising Ni and Fe,
and wherein the nonmagnetic side regions are formed of one or more
oxides of Ni and Fe.
13. The magnetic tunnel junction device according to claim 1
wherein the free layer is formed of an alloy comprising Co, Ni and
Fe, and wherein the nonmagnetic side regions are formed of one or
more oxides of Co, Ni and Fe.
14. A magnetic tunnel junction read head for sensing data recorded
on a magnetic recording disk, the head comprising: a first magnetic
shield layer; a fixed ferromagnetic layer over the shield layer and
having a width W along a dimension corresponding to the trackwidth
TW dimension of the disk and a height H along a dimension
substantially perpendicular to the TW dimension, the magnetization
of the fixed layer being fixed in a direction along its height; an
insulating tunnel barrier layer on the fixed ferromagnetic layer; a
free ferromagnetic layer formed of an alloy comprising the elements
of Fe and one or more of Co and Ni on the tunnel barrier layer and
having a width TW defined by side edges and less than W and a
stripe height SH less than H, the free layer having a magnetization
direction in the TW dimension in the absence of an applied field,
the magnetization direction of the free layer being substantially
free to rotate in the presence of magnetic fields from the disk; a
nonmagnetic side region located on the tunnel barrier layer on each
side of and adjacent to the side edges of the free layer, each side
region being formed of one or more oxides of the elements in the
alloy of said ferromagnetic free layer; and a second magnetic
shield layer over the free layer and nonmagnetic side regions.
15. The magnetic tunnel junction read head according to claim 14
further comprising a first nonmagnetic electrically conductive
bottom lead layer between the first shield layer and the fixed
layer, and a second nonmagnetic electrically conductive top lead
layer between the free layer and the second shield layer.
16. The magnetic tunnel junction read head according to claim 15
further comprising an antiferromagnetic layer on the bottom lead
layer, the fixed layer being located on and in contact with the
antiferromagnetic layer and exchange coupled with the fixed layer
for pinning the magnetization of the fixed layer in said direction
along its height.
17. The magnetic tunnel junction read head according to claim 14
wherein H/W is greater than one.
18. A current-perpendicular to the plane magnetoresistive sensor
comprising: a substrate; a pinned ferromagnetic layer on the
substrate and having a width in a first dimension in the plane of
the pinned layer and a magnetization direction oriented in a
preferred direction and substantially prevented from rotation in
the presence of an applied magnetic field in the range of interest;
a nonmagnetic spacer layer on the pinned layer; a free
ferromagnetic layer on the spacer layer and having a width defined
by side edges and less than the width of the pinned layer, the free
layer having a magnetization direction substantially free to rotate
in the presence of an applied magnetic field in the range of
interest; and a nonmagnetic side region located on the spacer layer
on each side of and adjacent to the side edges of the free layer,
each side region being formed of one or more oxides of the same
ferromagnetic material present in the free layer.
19. The sensor according to claim 18 wherein the spacer layer is
electrically insulating.
20. The sensor according to claim 18 wherein the spacer layer is
electrically conducting.
21. A method for making a current-perpendicular to the plane
magnetoresistive sensor comprising: depositing on a substrate in
succession a layer of antiferromagnetic material, a first layer of
ferromagnetic material, a spacer layer of nonmagnetic material, a
second layer of ferromagnetic material, and a layer of capping
material; providing a mask over a central region of the capping
layer and underlying central region of the second ferromagnetic
layer; removing the capping layer and a portion of the second
ferromagnetic layer in side regions not covered by the mask;
oxidizing the remaining material in the second ferromagnetic layer
in the side regions not covered by the mask; and removing the
mask.
22. The method according to claim 21 further comprising depositing
over the oxidized side regions electrically insulating cover
material different from the material of the oxidized side
regions.
23. The method according to claim 21 wherein depositing the
nonmagnetic spacer layer comprises depositing electrically
insulating material.
24. The method according to claim 23 wherein depositing
electrically insulating material comprises depositing a layer of
aluminum and then oxidizing the aluminum.
Description
TECHNICAL FIELD
[0001] The invention relates generally to a
current-perpendicular-to-the-p- lane (CPP) magnetoresistive device
that operates with the sense current directed perpendicularly to
the planes of two ferromagnetic layers separated by a nonmagnetic
spacer layer, and more particularly to a magnetic tunnel junction
(MTJ) type of CPP device and method for its fabrication.
BACKGROUND OF THE INVENTION
[0002] A magnetic tunnel junction (MTJ) has two metallic
ferromagnetic layers separated by a very thin nonmagnetic
insulating tunnel barrier layer, wherein the tunneling current
perpendicularly through the layers depends on the relative
orientation of the magnetizations in the two ferromagnetic layers.
The high magnetoresistance at room temperature and generally low
magnetic switching fields of the MTJ makes it a promising candidate
for the use in magnetic sensors, such as a read head in a magnetic
recording disk drive, and nonvolatile memory elements or cells for
magnetic random access memory (MRAM).
[0003] IBM's U.S. Pat. No. 5,650,958 describes an MTJ for use as a
magnetoresistive read head and as a non-volatile memory cell
wherein one of the ferromagnetic layers has its magnetization
fixed, such as by being pinned by exchange coupling with an
adjacent antiferromagnetic layer, and the other ferromagnetic layer
is "free" to rotate in the presence of an applied magnetic field in
the range of interest of the read head or memory cell. When the MTJ
is a disk drive magnetoresistive read head, the magnetization of
the fixed or pinned ferromagnetic layer will be generally
perpendicular to the plane of the disk, and the magnetization of
the free ferromagnetic layer will be generally parallel to the
plane of the disk but will rotate slightly when exposed to magnetic
fields from the recorded data on the disk. When the MTJ is a memory
cell, the magnetization of the free ferromagnetic layer will be
either parallel or antiparallel to the magnetization of the pinned
ferromagnetic layer.
[0004] IBM's U.S. Pat. No. 5,729,410 describes an MTJ
magnetoresistive read head with longitudinal biasing of the free
ferromagnetic layer in which the MTJ device has electrical leads
that connect to the sense circuitry. The leads are in contact with
the insulating material in the read gap and the gap material is in
contact with the magnetic shields so that the leads are
electrically insulated from the shields. IBM's U.S. Pat. No.
5,898,548 describes an MTJ magnetoresistive read head with a narrow
gap in which the leads are in direct contact with the magnetic
shields, so that the shields also carry current from the sense
circuitry.
[0005] In addition to MTJ devices, there are other
current-perpendicular-t- o-the-plane (CPP) sensors that operate
with the sense current directed perpendicularly to the planes of
two ferromagnetic layers separated by a nonmagnetic spacer layer.
One other type of CPP sensor is a spin-valve (SV) sensor in which
the nonmagnetic spacer layer is electrically conductive. Thus in a
MTJ magnetoresistive read head, the spacer layer is typically
alumina (Al.sub.2O.sub.3) while in a CPP SV magnetoresistive read
head the spacer layer is typically copper. CPP SV read heads are
described by A. Tanaka et al., "Spin-valve heads in the
current-perpendicular-to-plane mode for ultrahigh-density
recording", IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1
January 2002.
[0006] In the previously cited '958 patent, the pinned
ferromagnetic layer is the lower ferromagnetic layer and has an
outer perimeter greater than that of the upper free ferromagnetic
layer. This MTJ device is patterned by ion milling down through the
upper free ferromagnetic layer, stopping at the barrier layer.
Alumina is then deposited on the sides of the free ferromagnetic
layer on top of the barrier layer. The ion milling process suffers
from the disadvantages of redeposition of conductive material and
the inability to precisely control the removal process due to
uncertainties in the ion milling rate and film thicknesses, which
makes it difficult to avoid damaging the pinned ferromagnetic
layer.
[0007] What is needed is an MTJ device with a pinned ferromagnetic
layer having an outer perimeter greater than that of the free
ferromagnetic layer and that can be fabricated without the
disadvantages of the prior art ion milling process.
SUMMARY OF THE INVENTION
[0008] The invention is a CPP device wherein the free ferromagnetic
layer has a central region of ferromagnetic material defined by
side edges, and nonmagnetic side regions adjacent the edges of the
central region formed of one or more oxides of the ferromagnetic
material. In one embodiment the device is a MTJ magnetoresistive
read head formed between two magnetic shields, with the pinned
ferromagnetic layer on a first nonmagnetic spacer on the bottom
shield, the insulating tunnel barrier layer on the pinned layer,
the free ferromagnetic layer on the tunnel barrier layer, a second
nonmagnetic spacer on the free ferromagnetic layer and the top
shield on the free ferromagnetic layer. The pinned layer has a
width and height greater than the width and height, respectively,
of the overlying central region of the free layer, with the regions
of the free layer other than the central region being oxidized and
therefore nonmagnetic. The MTJ read head is formed by patterning
resist in the shape of the free layer central region over the stack
of layers in the MTJ, ion milling the stack down into the free
layer, and then exposing the stack to oxygen to oxidize the
ferromagnetic material in the side regions not covered by the
resist. The material of the free layer as deposited is an alloy
comprising Fe and one or more of Co and Ni, which remains in the
central region, with the side regions becoming one or more
nonmagnetic oxides of Fe and Co and/or Ni. Additional insulating
material different from the oxides, such as Al.sub.2O.sub.3 or
SiO.sub.2, can be formed as cover layers over the nonmagnetic side
regions of the free layer.
[0009] For a fuller understanding of the nature and advantages of
the present invention, reference should be made to the following
detailed description taken together with the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWING
[0010] FIG. 1 is a cross-sectional view a conventional prior art
MTJ device.
[0011] FIG. 2 is a cross-sectional view of the MTJ device of the
present invention in the form of an MTJ read head for a magnetic
recording disk drive.
[0012] FIG. 3 is a perspective view of the MTJ read head of FIG. 2
with the top lead, insulating covers and insulating outer regions
removed.
[0013] FIGS. 4A-4I illustrate the process sequence for forming the
MTJ read head of the present invention with the selective oxidation
process to form the oxidized side region of the free layer.
[0014] FIG. 5 is a graph showing the effect of oxidation on the
ferromagnetic free layer material vs. time.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Prior Art
[0016] FIG. 1 illustrates in cross-sectional view a conventional
prior art MTJ device. The device includes a substrate 9, a base
multilayer stack 10, a spacer layer of an insulating tunnel barrier
layer 20, a top stack 30, an insulating layer 40 surrounding top
stack 30 and bottom stack 10, and a top wiring layer or electrical
lead 50. The tunnel barrier layer 20 is sandwiched between the two
stacks 10 and 30.
[0017] The base stack 10 formed on substrate 9 includes a first
seed layer 12 deposited on substrate 9, an optional "template"
ferromagnetic layer 14 on the seed layer 12, a layer of
antiferromagnetic material 16 on the template layer 14, and a
"pinned" ferromagnetic layer 18 formed on and exchange coupled with
the underlying antiferromagnetic layer 16. The ferromagnetic layer
18 is called the pinned layer because its magnetization direction
(shown by arrow 19) is prevented from rotation in the presence of
applied magnetic fields in the desired range of interest for the
MTJ device, i.e., the field from the write current if the device is
a MTJ memory cell, or the field from the data recorded on the disk
if the device is a MTJ read head. The top electrode stack 30
includes a "free" ferromagnetic layer 32 and a protective or
capping layer 34 formed on the free layer 32. The magnetization
direction of the ferromagnetic layer 32 is not pinned by exchange
coupling, and is thus free to rotate in the presence of applied
magnetic fields in the range of interest. When the device is an MTJ
memory cell, the magnetization direction of ferromagnetic layer 32
will be either parallel or antiparallel to the magnetization
direction 19 of pinned ferromagnetic layer 18. When the device is
an MTJ sensor, such as a disk drive read head, the magnetization
direction of pinned ferromagnetic layer 18 will be oriented into
the paper in FIG. 1 and the magnetization direction of free
ferromagnetic layer 32 will be oriented in the plane of the paper
in FIG. 1 (perpendicular to the magnetization direction of pinned
ferromagnetic layer 18) in the absence of an applied magnetic
field, but will rotate slightly when exposed to a magnetic field
from the recorded data on the disk, as described in the previously
cited '410 patent.
[0018] The materials for MTJ devices with the structure illustrated
in FIG. 1 are well known, and representative ones will be
described. The MTJ base stack 10 comprises a stack of 200 .ANG.
Pt/40 .ANG. Ni.sub.81Fe.sub.19/100 .ANG. Mn.sub.50Fe.sub.50/80
.ANG. Ni.sub.81Fe.sub.19 (layers 12, 14, 16, 18, respectively)
grown on substrate 9. In addition to Pt, other conducting
underlayers include Ta, Cu and Au. Other CoFe and NiFe alloys may
be used for the ferromagnetic layers and other antiferromagnetic
materials include NiMn, PtMn and IrMn. Substrate 9 would be a
silicon wafer if the device is a memory cell. Substrate 9 would
typically be the bottom electrically conductive lead located on
either the alumina gap material or the magnetic shield material on
the trailing surface of the head carrier if the device is a read
head, as shown in the previously cited '410 patent. The stack 10 is
grown in the presence of a magnetic field applied parallel to the
surface of the substrate wafer. The magnetic field serves to orient
the Mn.sub.50Fe.sub.50 antiferromagnetic layer 16. Layer 16 pins
the magnetization direction of the NiFe free ferromagnetic layer 18
by exchange coupling. Next, the tunnel barrier layer 20 is formed
by depositing and then oxidizing a 5-15 .ANG. Al layer. This
creates the Al.sub.2O.sub.3 insulating tunnel barrier layer 20.
While Al.sub.2O.sub.3 is the most common tunnel barrier material, a
wide range of other materials may be used, including MgO, AlN,
aluminum oxynitride, oxides and nitrides of gallium and indium, and
bilayers and trilayers of such materials. The MTJ top stack 30 is
an 80 .ANG. Co/200 .ANG. Pt stack (layers 32, 34, respectively)
having a cross-sectional area of a few microns or less. The free
ferromagnetic layer 32 is preferably either a single layer of an
alloy of Fe and one or more of Co and Ni, or a bilayer of a CoFe
alloy and a NiFe alloy. The top stack 30 is surrounded by an
insulation layer 40, which is typically SiO.sub.2 if the device is
a memory cell and alumina if the device is a read head. The
junction is contacted by a 200 .ANG. Ag/3000 .ANG. Au contact layer
50 that serves as the top wiring lead. Other capping or lead
materials include Ta, Ti, Ru and Rh.
[0019] This MTJ structure is fabricated by sputtering all the
layers in the junction stack (layers 12, 14, 16, 18, 20, 32, 34)
onto the substrate 9, followed by ion milling down through the free
ferromagnetic layer 32 to the barrier layer 20. This process of
direct subtractive removal of the free ferromagnetic layer by ion
milling or reactive ion etching (RIE) suffers from the
disadvantages of redeposition of conductive material, inability to
precisely control the removal process, and ion damage that can
extend 20-40 .ANG. below the etched surface. The ion milling or RIE
of the free layer and pinned layer can cause redeposition of the
material in these layers onto the edges of the tunnel barrier layer
20 and electrically "short" the insulating tunnel barrier at its
edges. In addition, uncertainties in the ion milling rate and film
thicknesses make it difficult to avoid damaging the underlying
layers. Typical ion milling rates are 1 .ANG./sec for capping
material (Ta) and free layer material (NiFe or CoFe). Typical
capping layer thickness is 100 .ANG. to 200 .ANG. and typical free
layer thickness is 30 .ANG. to 40 .ANG.. The film thickness
uniformity and ion mill removal rate uniformity are each
approximately 5%. Thus the use of ion milling or RIE to precisely
remove the capping layer 34 and free layer 32 and stop at the
tunnel barrier layer 20 has an inherent uncertainty of 13 .ANG. to
24 .ANG. in the removal process. This uncertainty is greater than
or equivalent to the thickness of the tunnel barrier layer 20.
[0020] The Invention
[0021] The MTJ device of the present invention is shown in FIG. 2
in the form of an MTJ read head for a magnetic recording disk
drive. The cross-sectional view of FIG. 2 is essentially the read
head as would be viewed from the disk with "TW" representing the
trackwidth of the data tracks on the disk. The layers formed on the
substrate 109, which is typically the permalloy (NiFe) bottom
shield or the alumina gap material in the head structure, are the
bottom electrical lead layer 102, seed layer 112, antiferromagnetic
layer 116, fixed or pinned ferromagnetic layer 118 with its
magnetization direction 119 being shown as into the paper,
nonmagnetic insulating tunnel barrier layer 120, free ferromagnetic
layer 132 with its magnetization direction 135 being in the plane
of the paper and perpendicular to direction 119 in the absence of
an applied field from the recorded data on the disk, capping layer
134 and top electrical lead 150. The top magnetic shield (not
shown) or alumina gap material (not shown) would then be formed on
top lead 150, as depicted in the previously cited '410 and '548
patents. The bottom lead 102 and top lead 150 are formed of
nonmagnetic materials and thus serve as first and second spacer
layers to separate the ferromagnetic layers of the device from the
bottom magnetic shield 109 and top magnetic shield,
respectively.
[0022] Typical material compositions and thicknesses for layers 102
through 134 are as follows:
[0023] 20-50 .ANG. Ru or Ta lead layer/20-50 .ANG. NiFe or NiFeCr
seed layer/200 .ANG. PtMn or IrMn antiferromagnetic layer/30 .ANG.
NiFe or CoFe or NiFe--CoFe bilayer pinned layer/10-20 .ANG.
Al.sub.2O.sub.3 tunnel barrier layer/30 .ANG. NiFe or CoFe or
CoFe--NiFe bilayer free layer/50-100 .ANG. Ta, Ru or Ti capping
layer.
[0024] The device is similar to the prior art of FIG. 1 with the
primary difference being that there are nonmagnetic side regions
142 adjacent free layer 132 that are formed of oxides of the
ferromagnetic material making up free layer 132. The side regions
142 are formed by selectively oxidizing the free layer to render it
locally nonmagnetic (substantially incapable of conducting magnetic
flux) and electrically insulating. Insulating alumina covers 140
are formed on top of the oxidized side regions 142. Additional
alumina is located in outer regions 147 surrounding the outer edges
of the tunnel barrier layer 120 and the layers beneath it. Covers
140 and outer regions 147 may be formed of other insulating
material, such as SiO.sub.2.
[0025] FIG. 3 is a perspective view of the MTJ read head with the
top lead 150 and alumina covers 140 and outer regions 147 removed.
Because the pinned layer 118 has a width W wider than the width
(trackwidth TW) of the free layer 132 in the trackwidth direction,
better longitudinal biasing of the free layer in this direction is
achieved since the edge domain effects of the pinned layer are
physically separated from the edges of the free layer by the
nonmagnetic side regions 142. FIG. 3 also illustrates that the
pinned layer 118 can have a height H greater than the height
(stripe height SH) of the free layer 132 in the stripe height
dimension perpendicular to the trackwidth dimension. Because the
pinned layer 118 can be formed with an aspect ratio (H/W) greater
than unity, better stabilization of the pinned layer magnetization
direction 119 along its height H can be achieved.
[0026] The MTJ device of the present invention is fabricated using
controlled oxidation of selected regions 142 of the free layer 132
to render the free layer nonmagnetic and non-conducting in these
selected regions above the pinned layer 118. The oxidation process
does not penetrate the previously oxidized tunnel barrier layer 120
and therefore can not damage the underlying pinned layer.
[0027] FIGS. 4A-4I illustrate the process sequence for forming the
MTJ device with the selective oxidation process. The process begins
(FIG. 4A) by sputter depositing on the substrate (not shown) the
MTJ layers 102 through 118 followed by a layer of Al, typically
5-15 .ANG. thick. The Al is then oxidized by evacuation of the Ar
sputtering gas and then either introduction of oxygen or exposure
to an oxygen plasma. This forms the alumina (Al.sub.2O.sub.3)
tunnel barrier layer 120, typically 10-20 .ANG. thick. The
remaining layers 132 and 134 are then sputter deposited over the
tunnel barrier layer, resulting in the stack shown in FIG. 4A. A
mask of resist 800 is then patterned on a central region of the
capping layer 134 to define the lateral edges (TW and SH) of the
free layer 132, as shown in FIG. 4B. The stack is then moved to the
RIE or ion milling tool where it is etched through the capping
layer 134 and ending at or into the free layer 132 (FIG. 4C).
[0028] Next, the exposed portions of free layer 132 are oxidized in
the RIE or ion milling tool to render these regions 142 of the free
layer nonmagnetic and non-conducting (FIG. 4D). Suitable oxidation
processes include ozone treatment, air oxidation, thermal
oxidation, plasma oxidation, electrolytic oxidation, implantation
of oxygen or molecular oxygen (O.sub.2, O.sub.3) ions or neutrals.
Reactive oxygen plasma induced oxidation can be performed in a RF
coupled plasma, electron cyclotron resonance coupled plasma, or an
inductively coupled plasma (ICP) system. A preferred process for
oxidation of the free layer is with an ICP tool, which generates a
dense plasma of oxygen radicals and allows the substrate bias to be
controlled separately from the plasma source. When etching a test
wafer with photoresist in the ICP system in an oxygen plasma under
our typical plasma oxidation conditions, the etch rate is uniform
across an entire 5 inch wafer to within 3%. The ICP oxidation
process that induced demagnetization of the ferromagnetic layer had
parameters of 30 scc O.sub.2/min, 20.degree. C. substrate
temperature, 10 mT chamber pressure, 50W @ 13.56 MHz applied to the
source coils, and 18W @ 13.56 MHz applied to the substrate.
[0029] The regions 142 become oxides of the material in the free
layer 132, e.g., one or more oxides of Fe and Co and/or Ni. Because
the underlying layer 120 is already oxidized, the oxidation process
is self-limiting, so control of the oxidation time is not critical.
Next, insulation material, typically alumina or SiO.sub.2, is
deposited to form covers 140 above the nonmagnetic side regions
142, followed by lift-of of the resist 800, resulting in the
structure shown in FIG. 4E. While the deposition of the covers 140
is preferred, this additional step may not be necessary if the
oxide regions 142 are sufficiently free of pin holes. New resist
820 is then patterned to define the outside lateral extent of the
pinned layer 118 as well as the other layers in the stack (FIG.
4F). The stack is then ion milled or RIE to remove all the layers
down to the substrate (FIG. 4G), and an insulating layer of alumina
or SiO.sub.2 is then deposited to form the outer regions 147 (FIG.
4H). Finally the top electrical lead 150, typically 200 .ANG. of
Ta, Au or Cu, is deposited and patterned over the capping layer 134
(FIG. 4I). FIG. 5 is a graph showing the effect of oxidation on the
effective ferromagnetic layer thickness (as calculated from
measurements of magnetic moment) vs. time. Shown in the plot are
data points for two compositions: a 31 .ANG. CoFe single layer and
a 10 .ANG. CoFe/32 .ANG. NiFe bilayer. While these two compositions
have different physical thicknesses, they each have substantially
the same effective magnetic thicknesses as a 36 .ANG. film of NiFe.
The oxidation time was approximately 6 minutes to demagnetize the
CoFe single layer, and approximately 14 minutes to demagnetize the
CoFe/NiFe bilayer.
[0030] While the device and method for its fabrication have been
described above with respect to an MTJ device, particularly an MTJ
sensor in the form an MTJ read head, the invention is also
applicable to current-perpendicular-to-the-plane or CPP spin-valve
(SV) sensors. A CPP SV read head has a structure substantially the
same as the above-described MTJ read head, with the exception that
the spacer layer is electrically conductive instead of insulating.
For example, a copper spacer layer can replace the alumina
tunneling barrier layer.
[0031] While the present invention has been particularly shown and
described with reference to the preferred embodiments, it will be
understood by those skilled in the art that various changes in form
and detail may be made without departing from the spirit and scope
of the invention. Accordingly, the disclosed invention is to be
considered merely as illustrative and limited in scope only as
specified in the appended claims.
* * * * *