U.S. patent application number 11/983329 was filed with the patent office on 2009-05-14 for tmr device with low magnetostriction free layer.
This patent application is currently assigned to Headway Technologies, Inc.. Invention is credited to Min Li, Hui-Chuan Wang, Kunliang Zhang, Tong Zhao.
Application Number | 20090122450 11/983329 |
Document ID | / |
Family ID | 40623475 |
Filed Date | 2009-05-14 |
United States Patent
Application |
20090122450 |
Kind Code |
A1 |
Wang; Hui-Chuan ; et
al. |
May 14, 2009 |
TMR device with low magnetostriction free layer
Abstract
A high performance TMR sensor is fabricated by employing a free
layer comprised of CoB.sub.X with a .lamda. between
-5.times.10.sup.-6 and 0 on a MgO.sub.X tunnel barrier. Optionally,
a FeCo/CoB.sub.X free layer configuration may be used where x is
about 1 to 30 atomic %. Trilayer configurations represented by
FeCo/CoFeB/CoB.sub.X, FeCo/CoB.sub.X/CoFeB,
FeCo.sub.Y/CoFe.sub.W/CoB.sub.X, or FeCo.sub.Y/FeB/CoB.sub.X may
also be employed. Alternatively, CoNiFeB or CoNiFeBM formed by
co-sputtering CoB with CoNiFe or CoNiFeM, respectively, where M is
V, Ti, Zr, Nb, Hf, Ta, or Mo may be substituted for CoBx in the
aforementioned embodiments. A 15 to 30% in improvement in TMR ratio
over a conventional CoFe/NiFe free layer is achieved while
maintaining a low Hc and RA<3 ohm-um.sup.2. In bilayer or
trilayer embodiments, .lamda. between -5.times.10.sup.-6 and
5.times.10.sup.-6 is achieved by combining CoBx (-.lamda.) and one
or more layers having a positive .lamda..
Inventors: |
Wang; Hui-Chuan;
(Pleasanton, CA) ; Zhao; Tong; (Fremont, CA)
; Li; Min; (Dublin, CA) ; Zhang; Kunliang;
(Fremont, CA) |
Correspondence
Address: |
SAILE ACKERMAN LLC
28 DAVIS AVENUE
POUGHKEEPSIE
NY
12603
US
|
Assignee: |
Headway Technologies, Inc.
|
Family ID: |
40623475 |
Appl. No.: |
11/983329 |
Filed: |
November 8, 2007 |
Current U.S.
Class: |
360/324.2 |
Current CPC
Class: |
B82Y 25/00 20130101;
H01F 10/3272 20130101; H01L 43/12 20130101; H01L 43/10 20130101;
G11B 5/3912 20130101; H01F 10/3254 20130101; H01L 43/08 20130101;
G01R 33/093 20130101; Y10T 428/1114 20150115; G11B 5/3909 20130101;
G11C 11/161 20130101; G11B 5/3906 20130101; G11B 2005/3996
20130101; H01F 10/3295 20130101; G01R 33/18 20130101; B82Y 10/00
20130101 |
Class at
Publication: |
360/324.2 |
International
Class: |
G11B 5/33 20060101
G11B005/33 |
Claims
1. A magnetoresistive element in a magnetic device, comprising: (a)
a stack of layers comprised of a seed layer, anti-ferromagnetic
(AFM) layer, and a pinned layer sequentially formed on a substrate;
(b) a tunnel barrier layer made of MgOx on the pinned layer; (c) a
free layer comprised of CoB.sub.X or FeB.sub.V formed on the tunnel
barrier layer where x and v are from about 1 to 30 atomic %; and
(d) a capping layer on the free layer.
2. The magnetoresistive element of claim 1 wherein said free layer
is a single layer comprised of CoB.sub.X or FeB.sub.V with a
thickness from about 20 to 60 Angstroms.
3. The magnetoresistive element of claim 1 wherein the free layer
has a bilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer and an
upper CoB.sub.X layer having a .lamda. between about
-5.times.10.sup.-6 and 0, said FeCo.sub.Y layer has a thickness
between about 2 and 10 Angstroms and said CoB.sub.X layer has a
thickness between about 20 and 50 Angstroms.
4. The magnetoresistive element of claim 1 wherein the free layer
has a trilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer, a
middle Co.sub.UFe.sub.WB.sub.Z layer having a thickness of about 5
to 20 Angstroms where u is from about 1 to 95 atomic %, w is from 0
to about 70 atomic %, and z is from about 1 to 30 atomic %, and an
upper CoB.sub.X layer having a .lamda. between about
-5.times.10.sup.-6 and 0, said lower FeCo layer has a thickness
between about 2 and 10 Angstroms and said upper CoB.sub.X layer has
a thickness between about 20 and 40 Angstroms.
5. The magnetoresistive element of claim 1 wherein the free layer
has a trilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer, a
middle CoB.sub.X layer having a .lamda. from about
-5.times.10.sup.-6 to 0, and an upper Co.sub.UFe.sub.WB.sub.Z layer
having a thickness of about 5 to 20 Angstroms where u is from about
1 to 95 atomic %, w is from 0 to about 70 atomic %, and z is from
about 1 to 30 atomic %, said lower FeCo.sub.Y layer has a thickness
between about 2 and 10 Angstroms, and said middle CoB.sub.X layer
is between about 20 to 40 Angstroms thick.
6. The magnetoresistive element of claim 1 wherein the free layer
has a trilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer, a
middle CoFe.sub.W layer where w is from 0 to 100 atomic % and is
unequal to y, and an upper CoB.sub.X layer, said lower FeCo.sub.Y
and middle CoFe.sub.W layers each have a thickness between about 2
and 10 Angstroms and said upper CoB.sub.X layer has a thickness
from about 20 to 40 Angstroms.
7. The magnetoresistive element of claim 1 wherein the free layer
has a trilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % and having a thickness of about 2 to 10
Angstroms formed on the tunnel barrier layer, a middle FeB.sub.v
layer having a thickness of about 2 to 10 Angstroms, and an upper
CoB.sub.X layer with a thickness from about 20 to 40 Angstroms.
8. A magnetoresistive element in a magnetic device, comprising: (a)
a stack of layers comprised of a seed layer, anti-ferromagnetic
(AFM) layer, and a pinned layer sequentially formed on a substrate;
(b) a tunnel barrier layer made of MgOx on the pinned layer; (c) a
free layer comprised of Co.sub.PNi.sub.RFe.sub.SB.sub.T or CoNiFeBM
formed on the tunnel barrier layer wherein p is from about 5 to 90
atomic %, r is from about 5 to 20 atomic %, s is between about 5
and 90 atomic %, t is from about 1 to 30 atomic %, and p+r+s+t=100
atomic %, and M is one of V, Ti, Zr, Nb, Hf, Ta, or Mo; and (d) a
capping layer on the free layer.
9. The magnetoresistive element of claim 8 wherein said free layer
is a single layer comprised of Co.sub.PNi.sub.RFe.sub.SB.sub.T or
CoNiFeBM with a thickness from about 20 to 60 Angstroms, and M has
a content of <10 atomic % in the CoNiFeBM alloy.
10. The magnetoresistive element of claim 8 wherein the free layer
has a bilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer and an
upper Co.sub.PNi.sub.RFe.sub.SB.sub.T or CoNiFeBM layer, said
FeCo.sub.Y layer has a thickness between about 2 and 10 Angstroms
and said Co.sub.PNi.sub.RFe.sub.SB.sub.T or CoNiFeBM layer has a
thickness between about 20 and 50 Angstroms.
11. The magnetoresistive element of claim 8 wherein the free layer
has a trilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer, a
middle Co.sub.UFe.sub.WB.sub.Z layer having a thickness of about 5
to 20 Angstroms where u is from about 1 to 95 atomic %, w is from 0
to about 70 atomic %, and z is from about 1 to 30 atomic %, and an
upper Co.sub.PNi.sub.RFe.sub.SB.sub.T or CoNiFeBM layer, said lower
FeCo layer has a thickness between about 2 and 10 Angstroms and
said upper Co.sub.PNi.sub.RFe.sub.SB.sub.T or CoNiFeBM layer has a
thickness between about 20 and 40 Angstroms.
12. The magnetoresistive element of claim 8 wherein the free layer
has a trilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer, a
middle Co.sub.PNi.sub.RFe.sub.SB.sub.T or CoNiFeBM layer, and an
upper Co.sub.UFe.sub.WB.sub.Z layer where u is from about 1 to 95
atomic %, w is from 0 to about 70 atomic %, and z is from about 1
to 30 atomic %.
13. The magnetoresistive element of claim 8 wherein the free layer
has a trilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer, a
middle CoFe.sub.W layer where w is from 0 to 100 atomic % and is
unequal to y, and an upper Co.sub.PNi.sub.RFe.sub.SB.sub.T or
CoNiFeBM layer.
14. The magnetoresistive element of claim 8 wherein the free layer
has a trilayer configuration with a lower FeCo.sub.Y layer where y
is from 0 to 100 atomic % formed on the tunnel barrier layer, a
middle FeB.sub.V layer, and an upper
Co.sub.PNi.sub.RFe.sub.SB.sub.T or CoNiFeBM layer.
15. A method of forming a magnetoresistive element in a TMR sensor,
comprising: (a) sequentially forming a seed layer, AFM layer, and a
pinned layer on a substrate; (b) forming a tunnel barrier layer on
said pinned layer by depositing a first Mg layer on the pinned
layer, performing a natural oxidation process to form a MgOx layer,
and then depositing a second Mg layer on the MgOx layer; (c)
forming a free layer on the MgOx tunnel barrier layer, said free
layer is comprised of CoB.sub.X or FeB.sub.V where x and v are from
about 1 to 30 atomic %; and (d) forming a capping layer on the free
layer.
16. The method of claim 15 wherein the free layer has a bilayer
configuration with a lower FeCo.sub.Y layer where y is from 0 to
100 atomic % formed on the tunnel barrier layer and an upper
CoB.sub.X layer having a k between about -5.times.10.sup.-6 and 0,
said FeCo.sub.Y layer has a thickness between about 2 and 10
Angstroms and said CoB.sub.X layer has a thickness between about 20
and 50 Angstroms.
17. The method of claim 15 wherein the free layer has a trilayer
configuration with a lower FeCo.sub.Y layer where y is from 0 to
100 atomic % formed on the tunnel barrier layer, a middle
Co.sub.UFe.sub.WB.sub.Z layer having a thickness of about 5 to 20
Angstroms where u is from about 1 to 95 atomic %, w is from 0 to
about 70 atomic %, and z is from about 1 to 30 atomic %, and an
upper CoB.sub.X layer having a .lamda. between about
-5.times.10.sup.-6 and 0, said lower FeCo layer has a thickness
between about 2 and 10 Angstroms and said upper CoB.sub.X layer has
a thickness between about 20 and 40 Angstroms
18. The method of claim 15 wherein the free layer has a trilayer
configuration with a lower FeCo.sub.Y layer where y is from 0 to
100 atomic % formed on the tunnel barrier layer, a middle
CoFe.sub.W layer where w is from 0 to 100 atomic % and w is unequal
to y, and an upper CoB.sub.X layer having a .lamda. between about
-5.times.10.sup.-6 and 0, said lower FeCoy and middle CoFe.sub.W
layers each have a thickness between about 2 and 10 Angstroms and
said upper CoB.sub.X layer has a thickness from about 20 to 40
Angstroms.
19. The method of claim 15 wherein the free layer has a trilayer
configuration with a lower FeCo.sub.Y layer where y is from 0 to
100 atomic % and having a thickness of about 2 to 10 Angstroms
formed on the tunnel barrier layer, a middle FeB.sub.V layer having
a thickness of about 2 to 10 Angstroms, and an upper CoB.sub.X
layer with a .lamda. between about -5.times.10.sup.-6 and 0 and a
thickness from about 20 to 40 Angstroms.
20. A method of forming a free layer in a TMR sensor in a magnetic
device, comprising: (a) sequentially forming a seed layer, AFM
layer, and a pinned layer on a substrate; (b) forming a tunnel
barrier layer on said pinned layer by depositing a first Mg layer
on the pinned layer, performing a natural oxidation process to form
a MgOx layer, and then depositing a second Mg layer on the MgOx
layer; (c) forming a free layer comprised of
Co.sub.PNi.sub.RFe.sub.SB.sub.T or CoNiFeBM on the tunnel barrier
layer wherein p is from about 5 to 90 atomic %, r is from about 5
to 20 atomic %, s is between about 5 and 90 atomic %, t is from
about 1 to 30 atomic %, and p+r+s+t=100 atomic %, and M is one of
V, Ti, Zr, Nb, Hf, Ta, or Mo; and (d) forming a capping layer on
the free layer.
Description
RELATED PATENT APPLICATION
[0001] This application is related to the following: Docket #
HT05-015, Ser. No. 11/180,808, filing date Jul. 13, 2005; assigned
to a common assignee, and which is herein incorporated by reference
in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates to a high performance tunneling
magnetoresistive (TMR) sensor in a read head and a method for
making the same, and in particular, to a composite free layer
comprised of CoB that reduces magnetostriction while achieving
acceptable RA (resistance.times.area) and dR/R values.
BACKGROUND OF THE INVENTION
[0003] A TMR sensor otherwise known as a magnetic tunneling
junction (MTJ) is a key component (memory element) in magnetic
devices such as Magnetic Random Access Memory (MRAM) and a magnetic
read head. A TMR sensor typically has a stack of layers with a
configuration in which two ferromagnetic layers are separated by a
thin non-magnetic insulator layer. The sensor stack in a so-called
bottom spin valve configuration is generally comprised of a seed
(buffer) layer, anti-ferromagnetic (AFM) layer, pinned layer,
tunnel barrier layer, free layer, and capping layer that are
sequentially formed on a substrate. The free layer serves as a
sensing layer that responds to external fields (media field) while
the pinned layer is relatively fixed and functions as a reference
layer. The electrical resistance through the tunnel barrier layer
(insulator layer) varies with the relative orientation of the free
layer moment compared with the reference layer moment and thereby
converts magnetic signals into electrical signals. In a magnetic
read head, the TMR sensor is formed between a bottom shield and a
top shield. When a sense current is passed from the top shield to
the bottom shield (or top conductor to bottom conductor in a MRAM
device) in a direction perpendicular to the planes of the TMR
layers (CPP designation), a lower resistance is detected when the
magnetization directions of the free and reference layers are in a
parallel state ("1" memory state) and a higher resistance is noted
when they are in an anti-parallel state or "0" memory state.
Alternatively, a TMR sensor may be configured as a current in plane
(CIP) structure which indicates the direction of the sense
current.
[0004] A giant magnetoresistive (GMR) head is another type of
memory device. In this design, the insulator layer between the
pinned layer and free layer in the TMR stack is replaced by a
non-magnetic conductive layer such as copper.
[0005] In the TMR stack, the pinned layer may have a synthetic
anti-ferromagnetic (SyAF) configuration in which an outer pinned
layer is magnetically coupled through a coupling layer to an inner
pinned layer that contacts the tunnel barrier. The outer pinned
layer has a magnetic moment that is fixed in a certain direction by
exchange coupling with the adjacent AFM layer which is magnetized
in the same direction. The tunnel barrier layer is so thin that a
current through it can be established by quantum mechanical
tunneling of conduction electrons.
[0006] A TMR sensor is currently the most promising candidate for
replacing a GMR sensor in upcoming generations of magnetic
recording heads. An advanced TMR sensor may have a cross-sectional
area of about 0.1 microns.times.0.1 microns at the air bearing
surface (ABS) plane of the read head. The advantage of a TMR sensor
is that a substantially higher MR ratio can be realized than for a
GMR sensor. In addition to a high MR ratio, a high performance TMR
sensor requires a low areal resistance RA (area.times.resistance)
value, a free layer with low magnetostriction (.lamda.) and low
coercivity (Hc), a strong pinned layer, and low interlayer coupling
(Hin) through the barrier layer. The MR ratio (also referred to as
TMR ratio) is dR/R where R is the minimum resistance of the TMR
sensor and dR is the change in resistance observed by changing the
magnetic state of the free layer. A higher dR/R improves the
readout speed. For high recording density or high frequency
applications, RA must be reduced to about 1 to 3 ohm-um.sup.2.
[0007] A MgOx based MTJ is a very promising candidate for high
frequency recording applications because its tunneling
magnetoresistive (TMR) ratio is significantly higher than for AlOx
or TiOx based MTJs. In order to achieve a smaller Hc but still
maintain a high TMR ratio, the industry tends to use CoFeB as the
free layer in a TMR sensor. Unfortunately, the magnetostriction
(.lamda.) of a CoFeB free layer is considerably greater than the
maximum acceptable value of about 5.times.10.sup.-6 for high
density memory applications. A free layer made of a CoFe/NiFe
composite has been employed instead of CoFeB because of its low
.lamda. and soft magnetic properties. However, when using a
CoFe/NiFe free layer, the TMR ratio will degrade. Thus, an improved
free layer in a TMR sensor is needed that provides low
magnetostriction in combination with a high TMR ratio, low RA
value, and low coercivity.
[0008] U.S. Pat. No. 7,035,058 discloses a GMR-CPP device with a
free layer made of Co, CoFe alloys, Ni, and NiFe alloys. A high
resistance layer formed on or within the free layer may be
comprised of CoNiFeB or CoB with an unspecified composition.
However, this patent teaches a high resistance property that is not
acceptable for TMR where resistance.times.area must be low.
[0009] In U.S. Patent Application No. 2007/0139827, a free layer is
described that includes a sense enhancing layer (Ta) sandwiched
between a first ferromagnetic layer and a second ferromagnetic
layer. The first ferromagnetic layer has a positive
magnetostriction and is made of CoFeB or CoFe based alloys while
the second ferromagnetic layer has a negative magnetostriction and
is comprised of CoFe, Ni, or NiFe based alloys.
[0010] U.S. Patent Application No. 2007/0047159 teaches a free
layer comprised of a CoFe/CoFeB/NiFe composite to achieve low
coercivity and low magnetostriction for either GMR-CPP or TMR
sensors.
[0011] U.S. Patent Application No. 2007/0070553 describes a
magnetoresistance effect device that has a CoNiFeB free layer
formed by co-sputtering CoFeB and Ni and with a Ni content between
5 and 17 atomic % to maintain a .lamda. value between
-1.times.10.sup.-6 and 1.times.10.sup.-6.
SUMMARY OF THE INVENTION
[0012] One objective of the present invention is to provide a TMR
sensor with a free layer composition that improves the TMR ratio by
at least 15 to 30% compared with a conventional FeCo/NiFe free
layer.
[0013] A second objective of the present invention is to provide a
TMR sensor with a free layer according to the first objective that
also has a low magnetostriction between -5.times.10.sup.-6 and
5.times.10.sup.-6, a low RA value below 3 ohm-.mu.m.sup.2, and a
low coercivity in the range of 4 to 6 Oe.
[0014] A further objective of the present invention is to provide a
method of forming a TMR sensor that satisfies the first and second
objectives and is cost effective.
[0015] According to one embodiment of the present invention, these
objectives are achieved by forming a TMR sensor on a suitable
substrate such as a bottom shield in a read head. The TMR sensor
may have a bottom spin valve configuration comprised of a seed
layer, AFM layer, pinned layer, tunnel barrier layer, free layer,
and capping layer which are formed sequentially on the bottom
shield. The tunnel barrier layer is made of MgOx and the free layer
is comprised of a low magnetostriction CoB.sub.X layer where x is
from 0 to 30 atomic %, or a FeB.sub.V layer where v is from 0 to 30
atomic %. In an alternative embodiment, the free layer is a
composite represented by a FeCo.sub.Y/CoB.sub.X configuration where
y is from 0 to 100 atomic % and the FeCo.sub.Y layer contacts the
tunnel barrier layer. Optionally, the free layer may have a
trilayer configuration represented by
FeCo.sub.Y/CoFe.sub.WB.sub.Z/CoB.sub.X,
FeCo.sub.Y/CoB.sub.X/CoFe.sub.WB.sub.Z,
FeCo.sub.Y/CoFe.sub.W/CoB.sub.X, or FeCo.sub.Y/FeB.sub.V/CoB.sub.X
where w is from 0 to 70 atomic %, and v and z are from 0 to 30
atomic %.
[0016] In a second embodiment, the tunnel barrier layer is made of
MgOx and the free layer may have a configuration represented by
FeCo.sub.Y/Co.sub.PNi.sub.RFe.sub.SB.sub.T in which the CoNiFeB
layer is formed by co-sputtering targets made of CoB and CoNiFe,
and p is from 5 to 90 atomic %, r is from 5 to 20 atomic %, s is
between 5 and 90 atomic %, t is from 1 to 30 atomic %, and
p+r+s+t=100 atomic %. Alternatively, the free layer may have a
trilayer configuration represented by
FeCo.sub.Y/CoFe.sub.WB.sub.Z/CoNiFeB,
FeCo.sub.Y/CoNiFeB/CoFe.sub.WB.sub.Z,
FeCo.sub.Y/CoFe.sub.W/CoNiFeB, or FeCo.sub.Y/FeB.sub.V/CoNiFeB
where the CoNiFeB layer is made by co-sputtering CoB and CoNiFe. In
yet another embodiment, the CoNiFeB layer in one of the previous
embodiments may be replaced by a CoNiFeBX layer that is formed by
co-sputtering CoB with CoNiFeX where X is an element such as V, Ti,
Zr, Nb, Hf, Ta, or Mo and the content of the X element in the
CoNiFeBX layer is <10 atomic %.
[0017] Typically, a TMR stack of layers is laid down in a
sputtering system. All of the layers may be deposited in the same
sputter chamber. Preferably, the MgOx tunnel barrier is formed by
depositing a first Mg layer on the pinned layer followed by a
natural oxidation process on the first Mg layer to form a MgOx
layer and then depositing a second Mg layer on the MgOx layer. The
oxidation step is performed in an oxidation chamber within the
sputtering system. The TMR stack is patterned by a conventional
method prior to forming a top shield on the cap layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a cross-sectional view showing a TMR stack of
layers according to one embodiment of the present invention.
[0019] FIG. 2 is a cross-sectional view showing a TMR stack of
layers that has been patterned to form a MTJ element during an
intermediate step of fabricating the TMR sensor according to one
embodiment of the present invention.
[0020] FIG. 3 is a cross-sectional view of a TMR read head having a
MTJ element interposed between a top shield and bottom shield and
formed according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention is a high performance TMR sensor
having a free layer comprised of CoB or CoNiFeBX and a method for
making the same. While the exemplary embodiment depicts a TMR
sensor in a read head, the present invention may be employed in
other devices based on a tunneling magnetoresistive element such as
MRAM structures, or in a GMR-CPP sensor. The TMR sensor may have a
bottom spin valve, top spin valve, or multilayer spin value
configuration as appreciated by those skilled in the art. Drawings
are provided by way of example and are not intended to limit the
scope of the invention. For example, various elements are not
necessarily drawn to scale and their relative sizes may differ
compared with those in an actual device.
[0022] Referring to FIG. 1, a portion of a partially formed TMR
sensor 1 of the present invention is shown from the plane of an air
bearing surface (ABS). There is a substrate 10 that in one
embodiment is a bottom lead otherwise known as a bottom shield (S1)
which may be a NiFe layer about 2 microns thick that is formed by a
conventional method on a substructure (not shown). It should be
understood that the substructure may be comprised of a wafer made
of AlTiC, for example.
[0023] A TMR stack is formed on the substrate 10 and in the
exemplary embodiment has a bottom spin valve configuration wherein
a seed layer 14, AFM layer 15, pinned layer 16, tunnel barrier
layer 17, free layer 18, and capping layer 19 are sequentially
formed on the substrate. The seed layer 14 may have a thickness of
10 to 100 Angstroms and is preferably a Ta/Ru composite but Ta,
Ta/NiCr, Ta/Cu, Ta/Cr or other seed layer configurations may be
employed, instead. The seed layer 14 serves to promote a smooth and
uniform grain structure in overlying layers. Above the seed layer
14 is an AFM layer 15 used to pin the magnetization direction of
the overlying pinned layer 16, and in particular, the outer portion
or AP2 layer (not shown). The AFM layer 15 has a thickness from 40
to 300 Angstroms and is preferably comprised of IrMn. Optionally,
one of PtMn, NiMn, OsMn, RuMn, RhMn, PdMn, RuRhMn, or MnPtPd may be
employed as the AFM layer.
[0024] The pinned layer 16 preferably has a synthetic anti-parallel
(SyAP) configuration represented by AP2/Ru/AP1 where a coupling
layer made of Ru, Rh, or Ir, for example, is sandwiched between an
AP2 layer and an AP1 layer (not shown). The AP2 layer which is also
referred to as the outer pinned layer is formed on the AFM layer 15
and may be made of CoFe with a composition of about 10 atomic % Fe
and with a thickness of about 10 to 50 Angstroms. The magnetic
moment of the AP2 layer is pinned in a direction anti-parallel to
the magnetic moment of the AP1 layer. For example, the AP2 layer
may have a magnetic moment oriented along the "+x" direction while
the AP1 layer has a magnetic moment in the "-x" direction. A slight
difference in thickness between the AP2 and AP1 layers produces a
small net magnetic moment for the pinned layer 16 along the easy
axis direction of the TMR sensor to be patterned in a later step.
Exchange coupling between the AP2 layer and the AP1 layer is
facilitated by a coupling layer that is preferably comprised of Ru
with a thickness from 3 to 9 Angstroms. The AP1 layer is also
referred to as the inner pinned layer and may be a single layer or
a composite layer. In one aspect, the AP1 layer is amorphous in
order to provide a more uniform surface on which to form the tunnel
barrier layer 17.
[0025] In the exemplary embodiment that features a bottom spin
valve configuration, the tunnel barrier layer 17 is comprised of
MgOx because a MgOx tunnel barrier is known to provide a higher TMR
ratio than a TMR stack made with an AlOx or TiOx tunnel barrier.
The MgOx tunnel barrier layer is preferably formed by depositing a
first Mg layer having a thickness between 4 and 14 Angstroms on the
pinned layer 16, oxidizing the Mg layer with a natural oxidation
(NOX) process, and then depositing a second Mg layer with a
thickness of 2 to 8 Angstroms on the oxidized first Mg layer. Thus,
the tunnel barrier is considered as having a MgOx/Mg configuration.
The second Mg layer serves to protect the subsequently deposited
free layer from oxidation. It is believed that excessive oxygen
accumulates at the top surface of the MgOx layer as a result of the
NOX process and this oxygen can oxidize a free layer that is formed
directly on the MgOx portion of the tunnel barrier layer. Note that
the RA and MR ratio for the TMR sensor may be adjusted by varying
the thickness of the two Mg layers in tunnel barrier layer 17 and
by varying the natural oxidation time and pressure. A thicker MgOx
layer resulting from longer oxidation time and/or higher pressure
would increase the RA value.
[0026] All layers in the TMR stack may be deposited in a DC
sputtering chamber of a sputtering system such as an Anelva C-7100
sputter deposition system which includes ultra high vacuum DC
magnetron sputter chambers with multiple targets and at least one
oxidation chamber. Typically, the sputter deposition process
involves an argon sputter gas and a base pressure between
5.times.10.sup.-8 and 5.times.10.sup.-9 torr. A lower pressure
enables more uniform films to be deposited.
[0027] The NOX process may be performed in an oxidation chamber
within the sputter deposition system by applying an oxygen pressure
of 0.1 mTorr to 1 Torr for about 15 to 300 seconds. In the
exemplary embodiment, no heating or cooling is applied to the
oxidation chamber during the NOX process. Oxygen pressure between
10.sup.-6 and 1 Torr is preferred for an oxidation time mentioned
above in order to achieve a RA in the range of 0.5 to 5
ohm-um.sup.2. A mixture of O.sub.2 with other inert gases such as
Ar, Kr, or Xe may also be used for better control of the oxidation
process.
[0028] The present invention anticipates that a MgOx barrier layer
17 could be formed by depositing a MgOx layer on pinned layer with
a rf-sputtering or reactive sputtering method. It should be
understood that the performance of a TMR sensor fabricated with a
barrier layer comprised of sputtered MgO will not be as desirable
as one made according to the preferred embodiment of this
invention. For example, the inventors have observed that the final
RA uniformity (1.sigma.) of 0.6 um circular devices is more than
10% when the MgOx tunnel barrier layer is rf-sputtered and less
than 3% when the MgOx tunnel barrier is formed by DC sputtering a
Mg layer followed by a NOX process.
[0029] Optionally, other materials such as TiOx, TiAlOx, MgZnOx,
AlOx, ZnOx, or any combination of the aforementioned materials
including MgOx may be used as the tunnel barrier layer 21.
[0030] Returning to FIG. 1, an important feature of the present
invention is the free layer 18 formed on the tunnel barrier layer
17. In one embodiment, the free layer 18 has a thickness from 20 to
60 Angstroms and is comprised of a low magnetostriction CoB.sub.X
layer where x is from about 1 to 30 atomic % and .lamda. is
preferably between -5.times.10.sup.-6 and 0, or a FeB.sub.V layer
where v is from about 1 to 30 atomic % and .lamda. is preferably
between 0 and 5.times.10.sup.-6. It should be understood that in a
binary composition, the total atomic % for the two elements is 100
atomic %. In an alternative bilayer embodiment, the free layer 18
is a composite represented by a FeCo.sub.Y/CoB.sub.X configuration
where y is from 0 to 100 atomic % and the FeCo.sub.Y layer contacts
the tunnel barrier layer 17. The FeCo.sub.Y layer may have a
thickness between 2 and 10 Angstroms and the CoB.sub.X layer may be
from 20 to 50 Angstroms thick. Optionally, the free layer 18 may
have a trilayer configuration represented by
FeCo.sub.Y/Co.sub.UFe.sub.WB.sub.Z/CoB.sub.X,
FeCo.sub.Y/CoB.sub.X/Co.sub.UFe.sub.WB.sub.Z,
FeCo.sub.Y/CoFe.sub.W/CoB.sub.X, or FeCo.sub.Y/FeB.sub.V/CoB.sub.X
where u is from 1 to 95 atomic %, w is from 0 to 70 atomic %, v and
z are from about 1 to 30 atomic %, and u+w+z=100 atomic % In the
trilayer embodiments, the FeCo.sub.Y and CoFe.sub.W layers have a
thickness between 2 and 10 Angstroms and y is unequal to w, the
CoB.sub.X layer has a thickness from 20 to 40 Angstroms, the
Co.sub.UFe.sub.WB.sub.Z layer is from 5 to 20 Angstroms thick, and
the CoFe.sub.W and FeB.sub.V layers have a thickness between 2 and
10 Angstroms.
[0031] The inventors have discovered that a CoB.sub.X layer with a
small negative magnetostriction between -5.times.10.sup.-6 and 0,
or a FeB.sub.V layer with a small positive magnetostriction between
0 and 5.times.10.sup.-6 may be used as a free layer to afford a 15
to 30% improvement in TMR ratio compared with a conventional
FeCo/NiFe free layer and still maintain low Hc (4-6 Oe) and low RA
values necessary for high performance TMR sensors. Alternatively, a
CoB.sub.X layer with a small negative .lamda. may be combined with
one or more materials having a positive .lamda. value to form a
free layer composite with a bilayer or trilayer configuration
having a .lamda. value between -5.times.10.sup.-6 and
5.times.10.sup.-6 with TMR ratio, RA, and Hc values similar to
those described for a CoB.sub.X free layer.
[0032] In an alternative embodiment, the free layer 18 may have a
configuration represented by Co.sub.PNi.sub.RFe.sub.SB.sub.T or
FeCo.sub.Y/Co.sub.PNi.sub.RFe.sub.SB.sub.T in which the CoNiFeB
layer may be formed by co-sputtering targets made of CoB and CoNiFe
or CoNiFeB, and p is from 5 to 90 atomic %, r is from 5 to 20
atomic %, s is between 5 and 90 atomic %, t is from about 1 to 30
atomic %, and p+r+s+t=100 atomic %. Alternatively, the free layer
18 may have a trilayer configuration represented by
FeCo.sub.Y/CoFe.sub.WB.sub.Z/Co.sub.PNi.sub.RFe.sub.SB.sub.T,
FeCo.sub.Y/Co.sub.PNi.sub.RFe.sub.SB.sub.T/CoFe.sub.WB.sub.Z,
FeCo.sub.Y/CoFe.sub.W/Co.sub.PNi.sub.RFe.sub.SB.sub.T, or
FeCo.sub.Y/FeB.sub.V/Co.sub.PNi.sub.RFe.sub.SB.sub.T where the
CoNiFeB layer is made by co-sputtering CoB and CoNiFe, or CoB and
CoNiFeB. CoB is preferably used in the co-sputtering process as a
means of adjusting k.
[0033] In another embodiment, CoB may be co-sputtered with CoNiFeM
where M is V, Ti, Zr, Nb, Hf, Ta, or Mo to produce a free layer 18
represented by CoNiFeBM in which the M content is <10 atomic %.
Moreover, the present invention also anticipates that the free
layer 18 may have a FeCo.sub.Y/CoNiFeBM configuration. Optionally,
a CoNiFeBM layer may be substituted for the CoNiFeB layer in one of
the aforementioned trilayer configurations. Preferably, the CoNiFeB
layer and CoNiFeBM layer in the embodiments described herein have a
.lamda. between -5.times.10.sup.-6 and 5.times.10.sup.-6. A
magnetic layer comprised of CoNiFeB or CoNiFeBM in combination with
one or more other magnetic materials such as FeCo is expected to
provide a similar improvement in TMR ratio compared to a FeCo/NiFe
layer while maintaining low RA and low Hc values.
[0034] Once the TMR stack is complete, the partially formed read
head 1 may be annealed in a vacuum oven within the range of
240.degree. C. to 340.degree. C. with an applied magnetic field of
at least 2000 Oe, and preferably 8000 Oe for about 2 to 10 hours to
set the pinned layer and free layer magnetization directions. It
should be understood that under certain conditions, depending upon
the time and temperature involved in the anneal process, the tunnel
barrier layer 17 may become a uniform MgOx tunnel barrier layer as
unreacted oxygen diffuses into the adjacent Mg layer.
[0035] Referring to FIG. 2, the TMR stack is patterned by following
a conventional process sequence. For example, a photoresist layer
20 may be coated on the capping layer 19. After the photoresist
layer 20 is patterned, a reactive ion etch (RIE), ion beam etch
(IBE), or the like is used to remove underlying layers in the TMR
stack that are exposed by openings in the photoresist layer. The
etch process stops on the bottom shield 10 or between the bottom
shield and a barrier layer (not shown) to give a TMR sensor with a
top surface 19a and sidewalls 21.
[0036] Referring to FIG. 3, an insulating layer 22 may be deposited
along the sidewalls 21 of the TMR sensor. The photoresist layer 20
is then removed by a lift off process. A top lead otherwise known
as a top shield 25 is then deposited on the insulating layer 22 and
top surface 19a of the TMR sensor. Similar to the bottom shield 10,
the top shield 25 may also be a NiFe layer about 2 microns thick.
The TMR read head 1 may be further comprised of a second gap layer
(not shown) disposed on the top shield 25.
COMPARATIVE EXAMPLE 1
[0037] A first experiment was conducted to demonstrate the improved
performance achieved by implementing a free layer in a TMR sensor
according to the present invention. A TMR stack of layers,
hereafter referred to as MTJ Sample 1 and shown in Table 1, was
fabricated as a reference and is comprised of a conventional
CoFe/NiFe free layer wherein the lower CoFe layer is 10 Angstroms
thick and the upper NiFe layer is 40 Angstroms thick. MTJ Sample 1
has a seed/AFM/AP2/Ru/AP1/MgO/free layer/capping layer
configuration. The pinned layer has an AP2/Ru/AP1 structure in
which the AP2 layer is a 25 Angstrom thick CO.sub.70Fe.sub.30
layer, the Ru coupling layer has a 7.5 Angstrom thickness, and the
AP1 layer is a 25 Angstrom thick CO.sub.70Fe.sub.30 layer. The MgOx
tunnel barrier was formed by depositing a 7 Angstrom thick lower Mg
layer that was subjected to a NOX process before a 3 Angstrom thick
upper Mg layer was deposited. The thicknesses in Angstroms of the
other layers are given in parentheses: Ta(20)/Ru(20) seed layer;
IrMn (70) AFM layer; and Ru(10)/Ta(60) capping layer. The TMR stack
was formed on a NiFe shield and was annealed under vacuum at
250.degree. C. for 5 hours with an applied field of 8000 Oe.
TABLE-US-00001 TABLE 1 Hc, .lamda. results for TMR sensors with
Seed/AFM/AP2/Ru/AP1/MgOx/ free/cap configurations MTJ Hc Sample
Free Layer Composition (Oe) Lambda 1
Fe.sub.70Co.sub.3010/Ni.sub.90Fe.sub.1040 4.13 1.80 .times.
10.sup.-6 2 Fe.sub.70Co.sub.303/Co.sub.72Fe.sub.8B.sub.2030 5.15
9.45 .times. 10.sup.-6 3 Fe.sub.70Co.sub.303/Co.sub.80B.sub.2030
4.38 1.20 .times. 10.sup.-6 4
Fe.sub.70Co.sub.303/Co.sub.72Fe.sub.8B.sub.2010/Co.sub.80B.sub.2030
5.49 4.00 .times. 10.sup.-6 5
Fe.sub.70Co.sub.303/Co.sub.72Fe.sub.8B.sub.2010/ 4.26 3.50 .times.
10.sup.-6 CoB--CoNiFeB co-sputter 40
[0038] MTJ Sample 2 was fabricated with the same stack of layers as
in MTJ Sample 1 except the free layer was changed to a conventional
FeCo/CoFeB configuration in which the lower FeCo layer is 3
Angstroms thick and the upper CoFeB layer has a 30 Angstrom
thickness. Samples 3 to 5 are MTJs in which a free layer formed
according to an embodiment of the present invention has been
inserted. For example, MTJ Sample 3 has a free layer comprised of a
3 Angstrom thick lower Fe.sub.70CO.sub.30 layer and a 30 Angstrom
thick upper CO.sub.80B.sub.20 layer. MTJ Sample 4 has a 3 Angstrom
thick lower Fe.sub.70CO.sub.30 layer, a 10 Angstrom thick middle
CO.sub.72Fe.sub.8B.sub.20 layer, and a 30 thick upper
CO.sub.80B.sub.20 layer. MTJ Sample 5 is the same as MTJ Sample 4
except the upper CoB layer is replaced by a 40 Angstrom thick
CO.sub.56Ni.sub.8Fe.sub.16B.sub.20 layer formed by co-sputtering
CoB and CoNiFeB.
[0039] In Table 1, MTJ Samples 3-5 with a free layer comprised of
CoB or CoNiFeB have a magnetostriction in the range of
1.times.10.sup.-6 to 4.times.10.sup.-6 that is substantially less
than shown for MTJ Sample 2 which has a conventional CoFeB free
layer. Furthermore, low Hc values are achieved with Samples 3-5
that are similar to the Hc for a conventional CoFe/NiFe free
layer.
COMPARATIVE EXAMPLE 2
[0040] A second experiment was performed to compare MTJ Samples 1
and 2 with MTJ Samples 3-5 with regard to TMR ratio (dR/R) and RA
results. The data shown in Table 2 was generated on 6 inch device
wafers with the same TMR stacks shown in FIG. 1. Note that a
combination of high TMR ratio and low RA values are achieved with a
free layer formed according to an embodiment of the present
invention. In particular, MTJ Samples 3-5 exhibit a 14-25% higher
TMR ratio than the reference MTJ Sample 1 while maintaining the
same RA in the range of 2.3 to 2.5 ohm-.mu.m.sup.2.
TABLE-US-00002 TABLE 2 TMR, RA results for TMR sensors with
Seed/AFM/AP2/Ru/AP1/ MgO/free/cap configurations MTJ RA Sample Free
Layer Composition ohm-.mu.m.sup.2 dR/R 1
Fe.sub.70Co.sub.3010/Ni.sub.90Fe.sub.1040 2.40 51.0% 2
Fe.sub.70Co.sub.303/Co.sub.72Fe.sub.8B.sub.2030 2.27 61.5% 3
Fe.sub.70Co.sub.303/Co.sub.80B.sub.2030 2.34 64.2% 4
Fe.sub.70Co.sub.303/Co.sub.72Fe.sub.8B.sub.2010/Co.sub.80B.sub.2030
2.49 64.0% 5 Fe.sub.70Co.sub.303/Co.sub.72Fe.sub.8B.sub.2010/ 2.39
58.0% CoB--CoNiFeB co-sputter 40
[0041] The advantages of the present invention are that a high TMR
ratio of greater than 60% can be achieved simultaneously with a low
RA value (<3 ohm-um.sup.2) and low magnetostriction which is a
significant improvement over conventional TMR sensors (MTJs) based
on a FeCo/NiFe free layer (low TMR ratio), or based on a FeCo/CoFeB
free layer (high magnetostriction). TMR sensors formed with a free
layer according to the present invention are able to achieve a
14-25% increase in TMR ratio compared with a FeCo/NiFe free layer
and a 50% reduction in .lamda. compared with a FeCo/CoFeB free
layer while maintaining acceptable RA and Hc results.
[0042] The free layers disclosed in the embodiments found herein
may be fabricated without additional cost since no new sputtering
targets or sputter chambers are required. Furthermore, a low
temperature anneal process may be employed which is compatible with
the processes for making GMR sensors. Therefore, there is no change
in process flow and related processes compared with current
manufacturing schemes.
[0043] While this invention has been particularly shown and
described with reference to, the preferred embodiment thereof, it
will be understood by those skilled in the art that various changes
in form and details may be made without departing from the spirit
and scope of this invention.
* * * * *