U.S. patent application number 14/278243 was filed with the patent office on 2015-11-19 for reduction of barrier resistance x area (ra) product and protection of perpendicular magnetic anisotropy (pma) for magnetic device applications.
This patent application is currently assigned to Headway Technologies, Inc.. The applicant listed for this patent is Headway Technologies, Inc.. Invention is credited to Huanlong Liu, Keyu Pi, Ru-Ying Tong, Jian Zhu.
Application Number | 20150333254 14/278243 |
Document ID | / |
Family ID | 52463216 |
Filed Date | 2015-11-19 |
United States Patent
Application |
20150333254 |
Kind Code |
A1 |
Liu; Huanlong ; et
al. |
November 19, 2015 |
Reduction of Barrier Resistance X Area (RA) Product and Protection
of Perpendicular Magnetic Anisotropy (PMA) for Magnetic Device
Applications
Abstract
A method of forming a MTJ with a tunnel barrier having a high
tunneling magnetoresistance ratio, and low resistance x area value
is disclosed. The method preserves perpendicular magnetic
anisotropy in bottom and top magnetic layers that adjoin bottom and
top surfaces of the tunnel barrier. A key feature is a passive
oxidation step of a first Mg layer that is deposited on the bottom
magnetic layer wherein a maximum oxygen pressure is 10.sup.-5 torr.
A bottom portion of the first Mg layer remains unoxidized thereby
protecting the bottom magnetic layer from substantial oxidation
during subsequent oxidation and anneal processes that are employed
to complete the fabrication of the tunnel barrier and MTJ. An
uppermost Mg layer may be formed as the top layer in the tunnel
barrier stack before a top magnetic layer is deposited.
Inventors: |
Liu; Huanlong; (San Jose,
CA) ; Zhu; Jian; (San Jose, CA) ; Pi;
Keyu; (San Jose, CA) ; Tong; Ru-Ying; (Los
Gatos, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Headway Technologies, Inc. |
Milpitas |
CA |
US |
|
|
Assignee: |
Headway Technologies, Inc.
Milpitas
CA
|
Family ID: |
52463216 |
Appl. No.: |
14/278243 |
Filed: |
May 15, 2014 |
Current U.S.
Class: |
438/3 |
Current CPC
Class: |
H01L 43/12 20130101;
H01F 41/307 20130101; G01R 33/098 20130101; G01R 33/096 20130101;
G11B 2005/3996 20130101; G11B 5/3909 20130101; H01L 43/08 20130101;
G11C 11/161 20130101 |
International
Class: |
H01L 43/12 20060101
H01L043/12 |
Claims
1. A method of forming a magnetic tunnel junction (MTJ) stack of
layers including a tunnel barrier layer between two magnetic
layers, comprising: (a) providing a bottom magnetic layer with
perpendicular magnetic anisotropy (PMA); (b) depositing a first
metal layer that forms a bottom magnetic layer/first metal layer
interface; (c) performing a first passive oxidation process with a
maximum oxygen pressure of about 10.sup.-5 torr, the first passive
oxidation process oxidizes an upper portion of the first metal
layer while a bottom portion of the first metal layer along the
bottom magnetic layer/first metal layer interface remains
unoxidized; (d) forming one or more metal or metal oxide layers on
the oxidized portion of the first metal layer wherein steps (b)-(d)
form a tunnel barrier layer; and (e) depositing a top magnetic
layer on a top surface of the tunnel barrier layer.
2. The method of claim 1 wherein the first passive oxidation
process has a maximum duration of about 1000 seconds.
3. The method of claim 1 wherein the first metal layer has a
thickness from about 1 to 6 Angstroms.
4. The method of claim 1 where the one or more metal oxide layers
formed on the oxidized portion of the first metal layer are formed
by one or more conventional methods comprising: (a) direct
deposition of a metal oxide layer; (b) depositing a metal layer and
then oxidizing all or part of the metal layer with an oxygen
pressure that is at least 10.sup.-3 torr; and (c) any combination
or repetition of steps (a) and (b) above.
5. The method of claim 1 wherein the first metal layer, and the one
or more metal oxide layers are comprised of a metal or alloy
selected from Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and
MgTa.
6. The method of claim 1 wherein the bottom magnetic layer is part
of a synthetic antiferromagnetic (SAF) layer and is
antiferromagnetically coupled to a second magnetic layer through a
coupling layer in a Ruderman-Kittel-Kasuya-Yosida (RKKY)
interaction.
7. The method of claim 1 wherein the top magnetic layer is part of
a SAF layer and is antiferromagnetically coupled to a second
magnetic layer through a coupling layer in a
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction.
8. The method of claim 1 wherein the passive oxidation process is
further comprised of nitrogen.
9. The method of claim 1 wherein at least one of the one or more
metal oxide layers formed on the top surface of the oxidized
portion of the first metal layer is further comprised of nitrogen
and has a metal oxynitride composition.
10. The method of claim 1 further comprising an anneal process
during or following the deposition of the MTJ stack, the anneal
process comprises a temperature up to about 450.degree. C. for a
duration up to about 90 minutes.
11. The method of claim 1 wherein the first metal layer is
comprised of a different metal than a metal in the one or more
metal or metal oxide layers.
12. The method of claim 1 further comprised of forming a capping
layer on the top magnetic layer.
13. The method of claim 12 wherein the capping layer is a metal
oxide layer that is formed by a process sequence comprising one or
more of a direct deposition method, a passive oxidation process, or
an oxidation process comprising an oxygen pressure of at least
10.sup.-3 torr.
14. The method of claim 1 further comprising the formation of a
dual spin valve MTJ stack by a process comprising: (a) forming a
second stack of layers on a top surface of the top magnetic layer,
the second stack is formed by a process comprising; (1) depositing
a second metal layer that contacts a top surface of the top
magnetic layer; (2) performing a second passive oxidation process
with a maximum oxygen pressure of about 10.sup.-5 torr, the second
passive oxidation process oxidizes an upper portion of the second
metal layer while a bottom portion of the second metal layer at an
interface with the top magnetic layer remains unoxidized; and (3)
forming one or more metal or metal oxide layers on the oxidized
portion of the second metal layer, steps (1)-(3) form a second
tunnel barrier; and (b) depositing a third magnetic layer on a top
surface of the second tunnel barrier.
15. A method of forming a magnetic tunnel junction (MTJ) stack of
layers including a tunnel barrier layer between two magnetic
layers, comprising: (a) providing a bottom magnetic layer with
perpendicular magnetic anisotropy (PMA); (b) depositing a first
metal layer that forms a bottom magnetic layer/first metal layer
interface; (c) performing a first passive metal nitride deposition
process with a maximum nitrogen pressure of about 10.sup.-5 torr,
the first passive metal nitride deposition process deposits a first
metal nitride layer on the first metal layer while keeping the
bottom magnetic layer/first metal layer interface from reacting
with nitrogen; (d) forming one or more metal, metal oxide, metal
oxynitride, or metal nitride layers on the first metal nitride
layer, steps (b)-(d) form a tunnel barrier layer; and (e)
depositing a top magnetic layer on a top surface of the tunnel
barrier layer.
16. The method of claim 15 wherein the first metal layer has a
thickness from about 1 to 6 Angstroms.
17. The method of claim 15 wherein the first metal layer, the first
metal nitride layer, and the one or more metal, metal oxide, metal
oxynitride, or metal nitride layers comprise a metal selected from
Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and MgTa.
18. The method of claim 13 wherein the first metal layer is
comprised of a different metal than in the first metal nitride
layer, and in the one or more metal, metal oxide, metal oxynitride,
or metal nitride layers.
19. A method of forming a spin torque oscillator (STO) device,
comprising: (a) forming a first metal oxide layer on a substrate
that includes the steps of: (1) forming a first metal layer on the
substrate and oxidizing an upper portion thereof with a first
passive oxidation having a maximum oxygen pressure of about
10.sup.-5 torr; and (2) forming one or more metal or metal oxide
layers on the oxidized upper portion of the first metal layer; (b)
forming a spin polarization (SP) layer on a top surface of the
first metal oxide layer; (c) forming a non-magnetic layer on the SP
layer; (d) forming an oscillation layer (OL) on the non-magnetic
layer; and (e) forming a second metal oxide layer on the OL with a
process comprising: (1) forming a second metal layer on the OL and
oxidizing an upper portion thereof with a second passive oxidation
process having a maximum oxygen pressure of 10.sup.-5 torr; and (2)
forming one or more metal or metal oxide layers on the oxidized
upper portion of the second metal layer.
20. The method of claim 19 wherein the first and second passive
oxidation processes have a maximum duration of about 1000 seconds,
the maximum oxygen pressure is determined by directly controlling
the oxygen pressure in a closed chamber, or by controlling an
oxygen flow rate in a vented chamber.
21. The method of claim 19 wherein the first metal layer and the
second metal layer each have a thickness from about 1 to 6
Angstroms.
22. The method of claim 19 where the one or more metal oxide layers
formed on the oxidized upper portion of the first metal layer and
the second metal layer are formed by one or more conventional
methods comprising: (a) direct deposition of a metal oxide layer;
(b) depositing a metal layer and then oxidizing all or part of the
metal layer with an oxygen pressure that is at least 10.sup.-3
torr; and (c) any combination or repetition of steps (a) and (b)
above.
23. The method of claim 19 wherein the first and second metal
layers, and the one or more metal or metal oxide layers formed on
the oxidized upper portion of the first metal layer and the second
metal layer are comprised of a metal selected from Mg, Al, Ta, Ti,
Zn, Sn, MgZn, AITi, CoMg, and MgTa.
24. The method of claim 19 wherein the first and second passive
oxidation processes are further comprised of nitrogen.
25. The method of claim 19 wherein at least one of the one or more
metal oxide layers formed on the top surface of the oxidized
portion of the first metal layer is further comprised of nitrogen
and has a metal oxynitride composition.
26. The method of claim 19 further comprising an anneal process
following the formation of the second metal oxide layer, the anneal
process comprises a temperature up to about 450.degree. C. for a
duration up to about 90 minutes.
27. The method of claim 19 wherein the first metal layer is
comprised of a different metal than in the second metal layer or in
the one or more metal or metal oxide layers formed on an oxidized
upper portion of the first metal layer.
28. The method of claim 19 wherein the second metal layer is
comprised of a different metal than in the first metal layer or in
the one or more metal or metal oxide layers formed on an oxidized
upper portion of the second metal layer.
29. A method of forming an RF signal generation device, comprising:
(a) forming a spin torque oscillator (STO) with a top surface and
having at least one magnetic reference layer (MRL) that contacts a
first terminal, a magnetic oscillation layer (MOL), and a first
junction layer formed between the MRL and MOL; (b) forming a
non-magnetic spacer layer on the MOL, the non-magnetic spacer layer
is connected to a second terminal; (c) forming a magnetoresistive
(MR) sensor on the non-magnetic spacer, the MR sensor has at least
one magnetic sensing layer that is magnetostatically coupled with
said MOL, a second magnetic reference layer, and a second junction
layer that is a metal oxide formed between the magnetic sensing
layer and the second magnetic reference layer, the metal oxide is
formed by a process comprising: (1) depositing a first metal layer
on the magnetic sensing layer; (2) oxidizing an upper portion of
the first metal layer with a passive oxidation process having a
maximum oxygen pressure of 10.sup.-5 torr; and (3) forming one or
more metal or metal oxide layers on an oxidized upper portion of
the first metal layer; and (d) forming a third terminal on the MR
sensor, the magnetic sensing layer has an oscillation state with an
oscillation frequency that is induced in said MOL when a magnetic
field is applied to said STO and MR sensor in a direction
perpendicular to the STO top surface concurrently with a first
electric current flowing between the first and second terminals,
and the magnetostatic coupling generates magnetic oscillation with
an RF frequency in the magnetic sensing layer that produces a
varying voltage across the MR sensor when a second electric current
flows between the second and third terminals.
30. The method of claim 29 wherein the passive oxidation process
has a maximum duration of about 1000 seconds, and the maximum
oxygen pressure is controlled by directly controlling the oxygen
pressure in a closed chamber, or by controlling an oxygen flow rate
in a vented chamber.
31. The method of claim 29 wherein the first metal layer has a
thickness from about 1 to 6 Angstroms.
32. The method of claim 29 where the one or more metal oxide layers
formed on the oxidized upper portion of the first metal layer is
formed by one or more conventional methods comprising: (a) direct
deposition of a metal oxide layer; (b) depositing a metal layer and
then oxidizing all or part of the metal layer with an oxygen
pressure that is at least 10.sup.-3 torr; and (c) any combination
or repetition of steps (a) and (b) above.
33. The method of claim 29 wherein the first metal layer, and the
one or more metal or metal oxide layers formed on the oxidized
upper portion of the first metal layer are comprised of a metal
selected from Mg, Al, Ta, Ti, Zn, Sn, MgZn, AITi, CoMg, and
MgTa.
34. The method of claim 29 wherein the passive oxidation process is
further comprised of nitrogen.
35. The method of claim 29 wherein at least one of the one or more
metal oxide layers formed on the top surface of the oxidized
portion of the first metal layer is further comprised of nitrogen
and has a metal oxynitride composition.
36. The method of claim 29 further comprising an anneal process
following formation of the MR sensor, the anneal process comprises
a temperature up to about 450.degree. C. for a duration up to about
90 minutes.
37. The method of claim 29 wherein the first metal layer is
comprised of a different metal than in the one or more metal or
metal oxide layers formed on an oxidized upper portion of the first
metal layer.
38. A method of forming a three terminal device, comprising: (a)
forming a bottom magnetic layer as a polarizing layer that contacts
a first terminal; (b) forming a non-magnetic metal layer or a low
RA tunnel barrier on the polarizing layer, where the low RA tunnel
barrier is formed by a process comprising: (1) depositing a first
metal layer on the bottom magnetic layer; (2) oxidizing an upper
portion of the first metal layer with a passive oxidation process
having a maximum oxygen pressure of 10.sup.-5 torr; and (3) forming
one or more metal or metal oxide layers on the oxidized upper
portion of the first metal layer; (c) depositing a middle magnetic
layer as a free layer on the non-magnetic metal layer or the low RA
tunnel barrier, the free layer contacts a second terminal, a
current flowing between the first terminal and second terminal is
used during a write operation; (d) forming a tunnel barrier on the
free layer where the tunnel barrier formation process comprises:
(1) depositing a second metal layer on the free layer; (2)
oxidizing an upper portion of the second metal layer with a passive
oxidation process having a maximum oxygen pressure of 10.sup.-5
torr; and (3) forming one or more metal or metal oxide layers on an
oxidized upper portion of the second metal layer; and (e)
depositing a top magnetic layer as a reference layer on the tunnel
barrier, the reference layer contacts a third terminal, a current
flowing between the second terminal and third terminal is employed
during a read operation.
39. A method of forming a three terminal device, comprising: (a)
forming a bottom magnetic layer as a reference layer that contacts
a first terminal; (b) forming a tunnel barrier on the reference
layer where the tunnel barrier formation process comprises: (1)
depositing a first metal layer on the reference layer; (2)
oxidizing an upper portion of the first metal layer with a passive
oxidation process having a maximum oxygen pressure of 10.sup.-5
torr; and (3) forming one or more metal or metal oxide layers on an
oxidized upper portion of the first metal layer; (c) depositing a
middle magnetic layer as a free layer on the tunnel barrier, the
free layer contacts a second terminal, a current flowing between
the first terminal and second terminal is used during a read
operation; (d) forming a non-magnetic metal layer or a low RA
tunnel barrier on the free layer, where the low RA tunnel barrier
is formed by a process comprising: (1) depositing a second metal
layer on the free magnetic layer; (2) oxidizing an upper portion of
the second metal layer with a passive oxidation process having a
maximum oxygen pressure of 10.sup.-5 torr; and (3) forming one or
more metal or metal oxide layers on the oxidized upper portion of
the second metal layer; and (e) depositing a top magnetic layer as
a polarizing layer on the non-magnetic metal layer or low RA
barrier, the polarizing layer contacts a third terminal, a current
flowing between the second terminal and third terminal is employed
during a write operation.
Description
RELATED PATENT APPLICATIONS
[0001] This application is related to the following: U.S. Pat. No.
8,557,407; U.S. Pat. No. 8,592,927; U.S. Pat. No. 8,609,262; US
2012/0205758; and US 2013/0175644; assigned to a common assignee
and herein incorporated by reference in their entirety.
TECHNICAL FIELD
[0002] The present disclosure relates to high performance Magnetic
Tunneling Junction (MTJ) elements that include an oxide based
tunnel barrier and/or oxide cap layer such as MgO, and in
particular, to a method of forming the oxide layer to provide a low
resistance x area (RA) product for good writability and
reliability, and to protect (maintain) interfacial perpendicular
anisotropy at interfaces with adjoining magnetic layers.
BACKGROUND
[0003] Magnetoresistive Random Access Memory (MRAM) is based on the
integration of silicon CMOS with MTJ technology, and is a major
emerging technology that is highly competitive with existing
semiconductor memories such as SRAM, DRAM, and Flash. Spin-transfer
(spin torque) magnetization switching described by C. Slonczewski
in "Current driven excitation of magnetic multilayers", J. Magn.
Magn. Mater. V 159, L1-L7 (1996), is important due to its potential
application for spintronic devices such as Spin-Torque MRAM on a
gigabit scale.
[0004] Both field-MRAM and Spin-Torque MRAM have a MTJ element
based on a tunneling magneto-resistance (TMR) effect wherein a
stack of layers has a configuration in which two magnetic layers
called the reference layer and free layer are separated by a thin
non-magnetic dielectric layer that is called a tunnel barrier
layer, or more simply, the tunnel barrier. The MTJ element is
typically formed between a bottom electrode such as a first
conductive line and a top electrode which is a second conductive
line at locations where the top electrode crosses over the bottom
electrode.
[0005] In various designs of a Spin-Torque MRAM wherein one or both
of the reference layer and free layer has perpendicular magnetic
anisotropy (PMA), the tunnel barrier contributes to the write
function by generating spin polarized current. A low RA product is
needed for the tunnel barrier for good writability and good
reliability. Theoretically, the current density needed to write one
MRAM device depends only on the free layer properties. Therefore,
lowering the RA value in the tunnel barrier means less voltage is
required for writing. In addition, the write voltage is
proportional to the stress applied on the tunnel barrier during a
write operation. Too much stress will lead to an endurance problem,
for example, that will affect the reliability of writing the same
device multiple times without damaging the tunnel barrier. It is
commonly believed that although the strength of the tunnel barrier
against applied voltages is reduced as RA decreases, the writing
voltage is reduced even faster. As a result, one will improve
writing reliability by reducing the tunnel barrier RA value. A well
oxidized interface between the free layer and tunnel barrier is
preferred to enhance PMA in the free layer. An oxidized cap layer
on the free layer may further enhance PMA along a second interface
with the free layer.
[0006] When a spin-polarized current transverses a magnetic
multilayer in a current perpendicular-to-plane (CPP) configuration,
the spin angular moment of electrons incident on a magnetic layer
interacts with magnetic moments of the magnetic layer near the
interface between the magnetic layer and non-magnetic spacer.
Through this interaction, the electrons transfer a portion of their
angular momentum to the magnetic layer. As a result, spin-polarized
current can switch the magnetization direction of the free layer if
the current density is sufficiently high, and if the dimensions of
the multilayer are small. The difference between a Spin-Torque MRAM
and a conventional MRAM is only in the write operation mechanism.
The read mechanism is the same.
[0007] An important consideration when fabricating a tunnel barrier
is the oxidation process employed to convert a metal layer into an
oxide without creating cracks through which metal ions can easily
migrate. The tunnel barrier is typically formed by deposition and
oxidation of a thin Mg layer, and is required to have a low RA for
good reliability. PMA in one or both of the free layer and
reference layer must be maintained for optimum Spin-Torque MRAM
performance. The origin of PMA is from the interface between the
magnetic layer and the tunnel barrier where electron orbits at the
interface have less symmetry than their counterparts in the bulk of
the magnetic layer. Orbits that mostly maintain in the plane of the
interface are energetically favorable, resulting in PMA in the
magnetic layer. When oxidation conditions during the formation of
the tunnel barrier are too strong, a magnetic layer that interfaces
with the tunnel barrier may become partially oxidized, causing a
loss of PMA. Thus, the oxidation process must be cleverly designed
and carefully controlled to preserve PMA in adjoining magnetic
layers.
[0008] For Spin-Torque MRAM applications, an ultra small MTJ
element also referred to as a nanomagnet must exhibit a high TMR
ratio or dR/R of about 100% or higher at low resistance x area (RA)
values of less than 20 ohm-.mu.m.sup.2. Note that dR is the maximum
change in resistance in a MTJ and R is the minimum resistance of
the MTJ. In many cases, MgO is preferred as the tunnel barrier
layer since it provides a higher MR value than other oxides.
Improvements in tunnel barrier layer quality are still needed in
order to preserve PMA while optimizing RA and TMR ratio in the
device for Spin-Torque MRAM to be viable in the 90 nm technology
node and beyond.
SUMMARY
[0009] One objective of the present disclosure is to provide a MTJ
element that is able to satisfy design requirements for advanced
MRAM and Spin-Torque MRAM devices wherein substantial PMA in one or
both magnetic layers are required along with low RA of .ltoreq.20
ohm-.mu.m.sup.2, and a dR/R greater than 100%.
[0010] A second objective of the present disclosure is to provide a
method for forming the MTJ in the first objective wherein the
tunnel barrier is fabricated by an oxidation method that minimizes
or prevents oxidation in the adjoining magnetic layers thereby
preserving PMA therein and enabling a high TMR ratio.
[0011] According to one embodiment, these objectives are achieved
by formation of a MTJ element wherein a stack includes a first or
bottom magnetic layer and a second or upper magnetic layer that are
separated by a tunnel barrier comprised substantially of a metal
oxide. The metal oxide may be MgO, or other metal oxides used in
the art, or may be a lamination of one or more different metal
oxides. Moreover, the bottom magnetic layer may be a reference
layer in a bottom spin valve configuration, a free layer in a top
spin valve configuration, or another functional layer such as a
polarizing layer in a three terminal spin-transfer switching
device. Furthermore, one or both of the bottom and top magnetic
layers may be part of a synthetic antiferromagnetic (SAF)
multilayer which contains two magnetic layers antiferromagnetically
coupled across a non-magnetic layer (typically Ru) through
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The metal oxide
fabrication may comprise one or more oxidation steps wherein a
first step is a passive oxidation with an oxygen pressure of
10.sup.-5 torr or less that is substantially weaker than subsequent
oxidation steps where oxygen pressure is typically 10.sup.-3 torr
or greater. In one aspect where the metal oxide is MgO, a composite
tunnel barrier with a Mg/MgO configuration is formed after the one
or more oxidation steps. The lower Mg layer contacts the bottom
magnetic layer and has a substantially smaller thickness than the
overlying MgO layer. Oxidation steps following the initial passive
oxidation may involve conventional oxidation processes including
but not limited to sequential deposition of one or more metal
layers wherein each metal deposition is followed by an oxidation in
which the oxygen pressure is typically about 10.sup.-3 torr or
greater, or at least 10 to 100 times higher than in the initial
passive oxidation step. Thus, a lower portion of the MgO layer is
formed by passive oxidation while an upper portion is formed by one
or more conventional methods such as natural oxidation (NOX), or by
direct deposition such as radio frequency (RF) sputtering of MgO
which may require no further oxidation steps.
[0012] The metal oxide may have a laminated oxide structure wherein
two different metals or alloys (M.sub.1, M.sub.2) are employed such
that the tunnel barrier has a M.sub.1/M.sub.1Ox/M.sub.2Ox
configuration where a first metal oxide (M.sub.1Ox) is formed by a
passive oxidation method and M.sub.2Ox is a second metal oxide that
is formed by direct deposition or by one or more conventional
oxidation steps having an oxygen pressure of 10.sup.-3 torr or
greater. M.sub.1 and M.sub.2 are selected from Mg, MgZn, Zn, Al,
Ti, AITi, CoMg, Ta, MgTa, Hf, and Zr. After annealing, the tunnel
barrier has a M.sub.1Ox/M.sub.2Ox configuration.
[0013] The present disclosure also anticipates a dual spin valve
design having a reference layer 1/tunnel barrier 1/free
layer/tunnel barrier 2/reference layer 2 stack or a free layer
1/tunnel barrier 1/reference layer/tunnel barrier 2/free layer 2
stack where both tunnel barriers are made by an oxidation process
as described herein. One or both tunnel barriers may be MgO, MgZnO,
ZnO, AIOx, TiOx, AlTiOx, CoMgO, TaOx, MgTaOx, HfOx, or ZrOx, or one
or both tunnel barriers have a laminated oxide structure as
explained above.
[0014] After the MTJ stack is completed by depositing the top
magnetic layer on the tunnel barrier and then depositing one or
more overlying layers such as a capping layer, an anneal process is
used to promote a high TMR ratio. Under certain annealing
conditions, oxygen in the one or more metal oxide layers of the
tunnel barrier may diffuse into the lower Mg layer to form a
substantially uniform tunnel barrier where a metal oxide that is
preferably MgO interfaces with the bottom magnetic layer and the
top magnetic layer to promote PMA in the adjoining magnetic layers
for greater thermal stability. The lower Mg layer serves as a
buffer to limit the amount of oxygen reaching the bottom magnetic
layer so that undesired oxidation is avoided. In other words, the
lower Mg layer, or M1 layer in an alternative embodiment, prevents
the bottom magnetic layer from being oxidized to an extent that PMA
is degraded. In addition, higher TMR ratio is realized.
[0015] In a preferred embodiment that relates to fabricating a
tunnel barrier in a bottom spin valve MTJ configuration, a first Mg
layer about 1 to 6 Angstroms thick is deposited on a top surface of
the reference layer. The reference layer may have intrinsic PMA
that is enhanced by contact with an appropriate seed layer along a
bottom surface of the reference layer. Then a passive oxidation
comprised of an oxygen pressure of 10.sup.-5 torr or less is
applied to oxidize an upper portion of the first Mg layer while a
bottom portion of the Mg layer along an interface with the
reference layer remains unoxidized. Thereafter, the tunnel barrier
formation process continues with one or more conventional oxidation
steps. In one embodiment, a second Mg layer is deposited on the
partially oxidized first Mg layer. The second Mg layer is
essentially completely oxidized by a second oxidation process such
as a natural oxidation (NOX) involving an oxygen pressure of
10.sup.-3 torr or higher. Thus, the second oxidation involves
substantially stronger oxidation conditions than the first passive
oxidation. Conditions for the NOX step are selected so that oxygen
does not penetrate into the weakly oxidized Mg layer and cause
further oxidation therein. A third Mg layer may be deposited on the
second oxidized Mg layer. Next, a free layer is deposited on the
third Mg layer or on the oxidized second metal oxide layer followed
by one or more layers such as a capping layer to complete the MTJ
stack. Finally, an anneal process is performed by heating the MTJ
stack at a temperature up to 450.degree. C. for a duration up to 90
minutes. As a result, the first and third Mg layers absorb oxygen
from the adjoining second oxidized Mg layer to form a MgO tunnel
barrier.
[0016] In a second embodiment, a third Mg layer is deposited on the
second oxidized Mg layer and a third oxidation process is performed
to form a third oxidized Mg layer before sequentially depositing a
fourth Mg layer, depositing the free layer and one or more
overlying layers, and applying an annealing process to form a
reference layer/MgO tunnel barrier/free layer configuration and
complete the formation of the MTJ stack. In yet another embodiment,
the fourth Mg layer in the second embodiment is treated with a
fourth oxidation process to form a fourth oxidized Mg layer. Then a
fifth Mg layer, free layer and one or more overlying layers are
deposited, and an annealing step is performed to yield a MgO tunnel
barrier. In all oxidations after the initial passive oxidation
step, oxygen pressure is at least 100 times higher than in the
first oxidation step. Optionally, a MgO or metal oxide layer may be
deposited by RF sputtering as the uppermost layer in the tunnel
barrier stack.
[0017] The MTJ stack is an improvement over the prior art since
oxidation of the bottom magnetic layer that is a reference layer in
a bottom spin valve is minimized or avoided such that PMA is
preserved in the bottom magnetic layer. In other words, PMA is
enhanced compared with prior art MTJ structures where overoxidation
of the bottom magnetic layer causes a loss of PMA. The benefits of
enhanced PMA in the bottom magnetic layer are higher TMR ratio and
a reduction in RA which leads to better writing performance and
reliability.
[0018] The present disclosure also encompasses a spin torque
oscillator (STO) structure wherein PMA in a spin polarization layer
is preserved and enhanced by an adjoining metal oxide layer that is
formed by an oxidation process comprising a passive oxidation as
defined herein.
[0019] In another embodiment relating to a three terminal device
where read and write circuits are separated by placing a conductive
layer between a STO stack and a RF generator, a tunnel barrier made
by an embodiment of the present disclosure may be used in the RF
generator stack of layers.
[0020] In yet another embodiment relating to a three terminal
spin-transfer switching device where the read and write circuits
are separated through the electrical terminals on a polarizing
layer, free layer, and reference layer, an oxidation process
according to an embodiment of the present disclosure may be used to
fabricate the tunnel barrier between the free layer and reference
layer in the read circuit, and the low RA tunnel barrier between
the free layer and polarizing layer in the write circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a cross-sectional view showing a conventional
oxidation method to form a metal oxide tunnel barrier layer where
oxygen pressure and process time are large enough to cause cracks
that extend to an underlying magnetic layer.
[0022] FIG. 2 is a cross-sectional view depicting the result of a
passive oxidation process where a thin metal layer is preserved at
an interface with a bottom magnetic layer in a bottom spin valve
configuration after an initial tunnel barrier oxidation step
according to a first embodiment of the present disclosure.
[0023] FIG. 3 is a cross-sectional view showing the deposition of a
second metal layer on the metal/metal oxide tunnel barrier stack
formed in FIG. 2.
[0024] FIG. 4 is a cross-sectional view of the tunnel barrier in
FIG. 3 after a second oxidation process is used to oxidize the
second metal layer according to an embodiment of the present
disclosure.
[0025] FIG. 5 is a cross-sectional view of the tunnel barrier in
FIG. 4 after a third metal layer is deposited on the oxidized
second metal layer.
[0026] FIG. 6 is a cross-sectional view of the composite tunnel
barrier in FIG. 4 after one or more metal layers are deposited and
oxidized by one or more oxidation methods on the partially oxidized
first metal layer according to an embodiment of the present
disclosure.
[0027] FIG. 7 is a cross-sectional view of a tunnel barrier
according to the present disclosure after the deposition of an
uppermost metal layer on the tunnel barrier structure shown in FIG.
6.
[0028] FIG. 8 is a cross-sectional view depicting a free layer
formed on the tunnel barrier in FIG. 7 according to an embodiment
of the present disclosure.
[0029] FIG. 9 is a cross-sectional view of the bottom magnetic
layer/tunnel barrier/top magnetic layer stack in a bottom spin
valve structure following an anneal process.
[0030] FIG. 10 is a cross-sectional view depicting a MTJ nanopillar
having a bottom spin valve configuration according to an embodiment
of the present disclosure.
[0031] FIG. 11 is a cross-sectional view depicting the passive
oxidation process on a bottom magnetic layer in a top spin valve
configuration according to another embodiment of the present
disclosure.
[0032] FIG. 12 is a cross-sectional view of a partially formed MTJ
wherein a metal/metal oxide stack and an upper magnetic layer are
formed on a bottom magnetic layer in a top spin valve
configuration.
[0033] FIG. 13 is a cross-sectional view of the bottom magnetic
layer/tunnel barrier/top magnetic layer stack in a top spin valve
configuration following an anneal process.
[0034] FIG. 14 is a cross-sectional view showing a MTJ nanopillar
having a top spin valve configuration according to an embodiment of
the present disclosure.
[0035] FIG. 15 shows a cross-sectional view of a dual spin valve
structure wherein one or both tunnel barriers are formed by a
process of the present disclosure.
[0036] FIG. 16 is a cross-sectional view of a bottom magnetic
layer/tunnel barrier/top magnetic layer stack where the tunnel
barrier has a M.sub.1/M.sub.1Ox/M.sub.2Ox configuration according
to an embodiment of the present disclosure.
[0037] FIG. 17 is a cross-sectional view of a bottom magnetic
layer/tunnel barrier/top magnetic layer stack where the tunnel
barrier has a M.sub.1Ox/M.sub.2Ox configuration following an anneal
step according to an embodiment of the present disclosure.
[0038] FIG. 18 is a cross-sectional view of a STO device wherein a
metal oxide layer is formed by an oxidation process according to an
embodiment described herein.
[0039] FIG. 19 is a cross-sectional view of a three terminal device
wherein a tunnel barrier of a MR sensor component is formed
according to an oxidation process of the present disclosure.
[0040] FIGS. 20-21 are embodiments of a three terminal
spin-transfer switching device where one or both of the tunnel
barrier and low RA tunnel barrier are formed according to an
oxidation process of the present disclosure.
DETAILED DESCRIPTION
[0041] The present disclosure is a method of forming a high
performance MTJ element for an ultra high density MRAM, Spin-Torque
MRAM, or Spin Torque Oscillator (STO) device wherein RA is reduced,
and PMA is better preserved in one or both of a free layer and
reference layer by employing a tunnel barrier formation process
that prevents substantial oxidation of the free layer and reference
layer. Meanwhile, a first (oxide/reference layer) interface and a
second (oxide/free layer) interface are used to generate
interfacial perpendicular anisotropy and enhance PMA in the
adjoining magnetic layers.
[0042] As magnetic devices require higher areal density, MTJ
elements tend to become smaller with shrinking in-plane dimensions
and thicknesses for layers including the reference/pinned layer,
tunnel barrier, and free layer. Control of the tunnel barrier
oxidation process is especially critical in order to generate a
uniform tunnel barrier with low RA in perpendicularly magnetized
MTJ devices. In the prior art, formation of MgO and other tunnel
barrier oxides has been accomplished with a single oxidation or
with multiple oxidation steps applied in a sequential manner to a
plurality of Mg layers, or by direct deposition of a metal oxide
(MgO) layer. Typically, the oxidation steps involve oxidation
conditions with oxygen pressure greater than 10.sup.-3 torr in
order to achieve the desired RA and TMR ratio. However, prior art
MgO fabrication is not compatible with next generation MTJ devices
where the magnetic layers at top and bottom surfaces of the tunnel
barrier preferably have PMA in order to promote higher thermal
stability while maintaining a high TMR ratio. In particular, CoFeB
layers in a CoFeB/MgO/CoFeB reference layer/tunnel barrier/free
layer design that were previously in the 20-30 Angstrom thickness
range are now approaching 10 Angstroms thick or less in order to
improve the PMA properties therein. Accordingly, it becomes
necessary to develop an improved MgO fabrication process that is
compatible with the new reference layer and free layer design
requirements. A higher degree of control must be incorporated in
the MgO (or metal oxide) fabrication to avoid or minimize oxygen
incursion into the adjoining magnetic layers while reducing the
number of cracks in the metal oxide layer that might degrade
properties such as the RA value.
[0043] As shown in FIG. 1, conventional oxidation 12 of a Mg layer
11 that is deposited on a reference layer 10 generates cracks 13 in
the resulting MgO barrier 11a such that a certain number of cracks
extend to an interface 14 with the reference layer. The cracks are
formed due to the fact that MgO has a smaller lattice than Mg.
Cracks allow oxygen diffusion during a subsequent oxidation process
into a CoFeB magnetic layer, for example, that lead to undesirable
RA and TMR ratio.
[0044] We have discovered an improved tunnel barrier process that
may be applied to the formation of MgO or related oxides such as
AIOx, MgTaOx, TiO, ZnO, and native CoFeB oxide. The key aspect is
to insert two extra steps comprising a thin metal (Mg) layer
deposition followed by a passive oxidation involving an oxygen
pressure of 10.sup.-5 torr or less. Thereafter, one or more metal
layers are deposited and each deposition is followed by a
conventional oxidation having an oxygen pressure of at least
10.sup.-3 torr. Passive oxidation as defined herein means that the
kinetic energy of the oxygen atoms in the flow is essentially at
the minimum level that is reproducible in a manufacturing
environment. Typically, the pressure of the oxygen flow is less
than 10.sup.-5 torr, and preferably about 10.sup.-6 torr during
passive oxidation. In conventional oxidation methods, the pressure
of oxygen flow is usually above 10.sup.-3 torr, and at least 10 to
100 times greater than in passive oxidation. The unusually weak
oxidation condition applied in the passive oxidation step is
employed to minimize the extent of oxidation of the thin metal
layer to prevent oxygen diffusion into an underlying (bottom)
magnetic layer and to avoid cracks that extend through the thin
metal layer to a top surface of the bottom magnetic layer.
[0045] According to a preferred embodiment wherein the thin metal
layer is Mg, only a top surface of the Mg layer is gently oxidized
by passive oxidation to form a first MgO layer whereas the bottom
portion of the Mg layer and the bottom magnetic layer remain
unoxidized. Moreover, the gently oxidized first MgO layer will
prevent oxygen during subsequent oxidation steps from causing a
substantial amount of oxidation in the bottom magnetic layer, even
when standard oxidation conditions comprising .gtoreq.10.sup.-3
torr oxygen pressures are employed to oxidize subsequently
deposited metal layers to complete the tunnel barrier formation.
One or more additional metal oxide layers may be formed on the
oxidized upper portion of the first metal (Mg) layer. The one or
more additional metal oxide layers may be formed by (a) direct
deposition of a metal oxide by a conventional method such as
sputtering a metal oxide target, or by (b) depositing a metal layer
and then oxidizing with a process comprising an oxygen pressure of
at least 10.sup.-3 torr. It should be understood that when two or
more metal oxide layers are formed on a top surface of the oxidized
portion of the first metal layer, any combination or repetition of
steps (a) and (b) above may be used to form a plurality of metal
oxide layers. However, passive oxidation is a key discovery that
enables PMA of the bottom magnetic layer to be better preserved
than in prior art tunnel barrier fabrications where only
conventional methods are used to form one or more metal oxide
layers.
[0046] In a first embodiment depicted in FIGS. 2-6, a series of
process steps are employed to fabricate a tunnel barrier and begin
with deposition of a first metal layer on a bottom magnetic layer
followed by a passive oxidation. Then, a second metal layer is
deposited and is oxidized by a conventional oxidation method. A
third (upper) metal layer may be deposited on the oxidized second
metal layer. In the exemplary embodiments, Mg is used as the metal
for tunnel barrier fabrication. However, one or more other metals
including Al, Ta, Zn, Ti, and Sn, may be selected instead of Mg.
For example, the first (lower) metal layer and upper metal layer
may be comprised of a first metal or alloy while one or more
intermediate metal layers may be selected from a second metal or
alloy unequal to the first metal or alloy. However, for the purpose
of improving throughput, all metal layers are preferably selected
from the same metal or alloy.
[0047] Referring to FIG. 2 that relates to a MTJ having a bottom
spin valve configuration, a bottom magnetic layer that is reference
layer 10 is provided and may be Co, CoFeB, or another alloy
comprising two or more of Co, Fe, Ni, and B and deposited on a seed
layer 8 (FIG. 10). Furthermore, the bottom magnetic layer may be a
composite with a lower laminated stack of layers such as
(Co/Ni).sub.n in which n is a lamination number. The laminated
stack is preferably formed on a seed layer and there may be an
upper CoFeB layer (not shown) to give a (Co/Ni).sub.n/CoFeB
configuration, for example. However, an (A1/A2).sub.n laminate may
be selected rather than (Co/Ni).sub.n. Al may be Co, CoFe, or a
CoFeR alloy where R is one of Ru, Rh, Pd, Ti, Zr, Hf, Ni, Cr, Mg,
Mn, or Cu. A2 may be Ni, NiCo, NiFe, Pt or Pd. Preferably, Co,
CoFeB, or the alloy that is the uppermost layer in the bottom
magnetic layer is less than about 20 Angstroms thick to enable
intrinsic perpendicular magnetic anisotropy (PMA) that is enhanced
when forming an interface with MgO or a metal oxide tunnel barrier
in a later step.
[0048] In an alternative embodiment relating to a top spin valve
configuration in FIG. 12, the reference layer 10 becomes the top
magnetic layer while the free layer 30 is the bottom magnetic
layer. In this case, the top magnetic layer may have a
CoFeB/(Co/Ni).sub.n or CoFeB/(A1/A2).sub.n configuration where the
CoFeB layer contacts a top surface of tunnel barrier 20.
[0049] Returning to FIG. 2, a first metal layer 21 such as Mg with
a thickness between 1 and 6 Angstroms is deposited by a sputter
deposition method on the bottom magnetic layer. A Mg film with a
thickness less than 1 Angstrom is likely to be discontinuous and
comprise gaps between adjacent grains that extend vertically
through the entire film. On the other hand, a Mg layer that is
thicker than 6 Angstroms may not become fully oxidized during a
subsequent oxidation and/or anneal step which means degraded
interfacial perpendicular anisotropy along the interface 14 with
the bottom magnetic layer due to the absence of a metal
oxide/magnetic layer interface.
[0050] A critical feature of the tunnel barrier fabrication
sequence as disclosed herein is a passive oxidation step 16 that is
performed to transform an upper portion of Mg layer 21 into MgO
while a lower portion of the Mg layer remains unoxidized with no
cracks. The upper MgO layer 21a is advantageously used to prevent
oxygen during later conventional oxidation steps with oxygen
pressure .gtoreq.10.sup.-3 torr from penetrating Mg layer 21. As
indicated previously, pressure of the oxygen flow in a conventional
oxidation process is generally a factor of at least 10, and
preferably, about 100 greater in magnitude than employed during our
passive oxidation of the first Mg layer. The extremely weak
oxidation condition with a maximum oxygen pressure of 10.sup.-5
torr and preferably 10.sup.-6 torr for a maximum duration of 1000
seconds guarantees that only an upper portion of the first Mg layer
is oxidized and no cracks are formed through the first Mg layer.
Maximum oxygen pressure is determined by controlling oxygen
pressure in a closed chamber, or by controlling the oxygen flow
rate in a vented chamber. Note that there are oxidized indentations
15 in oxidized layer 21a but they do not touch interface 14.
[0051] Referring to FIG. 3, the following step involves the
deposition of a second Mg layer 22 on oxidized layer 21a. The
second Mg layer and all subsequent Mg layers have a minimum
thickness of 1 Angstrom to yield a continuous film. The maximum
thickness for the second Mg layer depends on the desired RA value
for the tunnel barrier, the number of Mg layers deposited during
tunnel barrier fabrication, and the oxidation condition employed
during the second oxidation step also referred to as the first
conventional oxidation process shown in FIG. 4. In general, RA
tends to become larger with an increasing number of Mg layers that
are oxidized with a conventional oxidation process.
[0052] In FIG. 4, a first conventional oxidation process 17 that
may be a natural oxidation (NOX), for example, is performed using
conditions that completely convert the second Mg layer into an
oxide layer. The oxidized second Mg layer and the oxidized portion
of the first Mg layer form MgO layer 21b. Preferably, the oxygen
flow rate during the first conventional oxidation process is at
least 1 sccm for a period of 10 seconds, an oxidation condition
that is considered moderate in comparison to the weak passive
oxidation 16. Optionally, a relatively strong oxidation condition
may be used where a flow rate of >1 sccm is applied for a period
up to 600 seconds or with a pressure in the range of 0.1 mtorr to 1
torr. However, the first conventional oxidation process should not
generate cracks that extend to first Mg layer 21 or allow oxygen to
diffuse through MgO layer 21a to further oxidize the first Mg
layer. The second Mg layer reduces the energy of oxygen that passes
through layer 22 during the first conventional oxidation process,
and MgO layer 21a further prevents the NOX oxygen from reaching the
first Mg layer 21. The integrity of an oxidized metal layer may be
determined by transmission electron microscopy (TEM) analysis to
confirm whether or not cracks are created by a particular oxidation
condition that may be too extreme for a certain Mg thickness. In
other words, cracking is observed by TEM analysis if oxidation
conditions are too strong.
[0053] According to one embodiment depicted in FIG. 5 that
corresponds to a tunnel barrier fabrication sequence involving the
fewest number of oxidation steps according to the present
disclosure, an uppermost Mg layer 23 is deposited on MgO layer 21b
to form a composite tunnel barrier layer 20 having a Mg/MgO/Mg
configuration. As described in a later section, an anneal step (not
shown) may be used following completion of the MTJ stack shown in
FIG. 8 to cause diffusion of oxygen from the middle MgO layer 21b
(or MgO layer 21c in an alternative embodiment where a plurality of
conventional oxidation processes is performed) into Mg layers 21,
23 thereby forming a MgO layer 21d which contacts both of bottom
magnetic layer 10 and top magnetic layer 30 (FIG. 9). Optionally, a
passive oxidation is followed by a direct deposition of a MgO layer
to give a tunnel barrier formation process with the fewest number
of oxidation steps.
[0054] In a second embodiment shown in FIG. 6, the sequence
illustrated in FIGS. 3-4 may be repeated one or more times to give
a MgO layer 21c having a thickness t2 on the unoxidized portion of
Mg layer 21 that has a thickness t1. Thus, the MgO layer in FIG. 6
may result from a second Mg layer deposition followed by a first
conventional oxidation process, and then a third Mg layer
deposition (not shown) followed by a second conventional oxidation
process. The second conventional oxidation may also comprise NOX
conditions described previously. In the exemplary embodiment,
t2>t1, but the present disclosure also anticipates a structure
where t1>t2 since it is well known that MgO has a smaller
lattice size than that of Mg. For instance, in an embodiment where
the first Mg layer has a 3 to 4 Angstrom thickness, and each of the
second and third Mg layers are about 2 Angstroms thick, oxidation
of the second and third Mg layers may result in a MgO layer 21c
thickness less than 4 Angstroms. It should be understood that
oxidation pressure and duration used to oxidize the second Mg layer
may be unequal to the oxidation pressure and duration for oxidation
of the third Mg layer. Furthermore, one or both of the first and
second conventional oxidation processes may include more than one
step. For example, a conventional oxidation sequence may have a
first step with a first oxygen flow rate and pressure, and a second
step with a second oxygen flow rate and pressure unequal to the
conditions in the first step. It is important that all conventional
processes employed during the formation of MgO layer 21c be at
least 10 to 100 times stronger in terms of oxygen pressure compared
with the passive oxidation of the first Mg layer in order to ensure
that the second and third Mg layers are completely oxidized.
[0055] The intermediate tunnel barrier structure depicted in FIG. 6
also encompasses a third embodiment wherein a fourth Mg layer (not
shown) is deposited on the oxidized third Mg layer followed by a
third conventional oxidation process to oxidize the fourth Mg
layer. Thereafter, an uppermost (fifth) Mg layer (not shown) may be
deposited and remains unoxidized until an optional anneal process
after the MTJ stack of layers is complete. As indicated earlier,
the number of Mg layers that are deposited and oxidized by a
conventional oxidation process, and the thickness of each Mg layer
may be adjusted to influence the RA value for the tunnel barrier.
An anneal process with a temperature up to 450.degree. C. for a
duration up to 90 minutes may be employed during or after the
deposition of the MTJ stack in any of the aforementioned
embodiments.
[0056] The present disclosure also encompasses a tunnel barrier
fabrication wherein the oxidation sequence in the second embodiment
is modified such that a second passive oxidation (PO) process
replaces one of the conventional oxidation processes. Thus, there
may be a plurality of PO steps employed during fabrication of the
tunnel barrier. In one aspect, a first Mg layer is partially
oxidized by a first passive oxidation, a second Mg layer is
oxidized by a first NOX step, and a third Mg layer is partially
oxidized by a second passive oxidation before an uppermost Mg layer
is deposited. This oxidation sequence may be represented by
PO/NOX/PO. However, the second Mg layer may be partially oxidized
by a second PO process and the third Mg layer may be oxidized by a
NOX method in a PO/PO/NOX scheme before an uppermost Mg layer is
deposited and remains unoxidized until a subsequent anneal process.
Preferably, at least one NOX step is retained to ensure that a
sufficient amount of oxygen is contained within the oxidized Mg
layers to enable diffusion into unoxidized portions of Mg layers
during the anneal process and thereby forming an essentially
uniform MgO tunnel barrier, or metal oxide tunnel barrier in
embodiments where the metal is not Mg.
[0057] It should be understood that the third embodiment may be
modified wherein one or more of the NOX steps are replaced by a
passive oxidation. According to one fabrication sequence, a first
Mg layer is deposited and partially oxidized by a first passive
oxidation process, a second Mg layer is deposited and oxidized by a
first NOX process, a third Mg layer is deposited and oxidized by a
second NOX process, and then a fourth Mg layer is deposited and
partially oxidized by a second passive oxidation before the
uppermost Mg layer is deposited. This oxidation scheme is
represented by PO/NOX/NOX/PO. Instead of a PO/NOX/NOX/PO sequence,
a series of oxidation steps represented by PO/PO/NOX/PO,
PO/PO/PO/NOX, or PO/NOX/PO/PO may be used wherein at least one
oxidation involves a NOX step to ensure a sufficient quantity of
oxygen within the tunnel barrier layer stack to completely oxidize
all metal layers therein following free layer formation and a
subsequent anneal process.
[0058] The present disclosure also anticipates that the passive
oxidation process may comprise nitrogen gas so that an upper
portion of the first metal layer deposited in a tunnel barrier
stack becomes a metal oxynitride. The first metal layer preferably
has a thickness between 1 and 6 Angstroms. In an alternative
embodiment, nitrogen in the absence of oxygen is used to deposit a
first metal nitride layer on the first metal layer. Typically, a
metal nitride is deposited by using ionized nitrogen atoms and Ar
to hit a metal target. As a result, the metal nitride is sputter
deposited onto a substrate. One can control the flow rate of
nitrogen gas to change the ratio between Ar and ionized nitrogen
atoms and thereby change the nitrogen content in the metal nitride
such as MgN.sub.x. This process may be defined as a passive
nitridation process if oxygen is excluded and there is a maximum
nitrogen pressure of 10.sup.-5 torr. As a result, the bottom
magnetic layer/first metal layer interface does not react with
nitrogen and a first metal/first metal nitride stack is formed.
Subsequent layers formed on the metal oxynitride or first metal
nitride layer may be metal oxide layers fabricated with a
conventional oxidation method of a metal layer, or by direct
deposition, or one or more of the subsequent layers may have a
metal oxynitride or metal nitride composition. Thereafter, an
uppermost metal layer may be deposited on a top surface of an
underlying metal oxide, metal oxynitride, or metal nitride layer.
An anneal process at a temperature up to 450.degree. C. and with a
duration up to 90 minutes may be performed during the uppermost
metal deposition or after a top magnetic layer and capping layer
are sequentially formed on the uppermost metal layer.
[0059] Referring to FIG. 7, an uppermost Mg layer 23 is deposited
on MgO layer 21c once the final conventional oxidation process or
second passive oxidation is completed in the aforementioned
embodiments related to FIG. 6. The thickness t3 of the final Mg
layer is preferably at least 1 Angstrom. A maximum thickness for t3
is determined in part by the temperature and time involved in a
subsequent annealing step. In particular, t3 should not be so large
that oxygen from MgO layer 21c does not diffuse into all portions
of Mg layer 23 and fail to oxidize a portion thereof along top
surface 23s. It is important that a metal oxide/top magnetic layer
interface be formed in order to maximize PMA in the top magnetic
layer. As mentioned previously, an anneal process may be employed
during deposition of the uppermost metal layer 23. In addition, a
second anneal process may occur after the MTJ stack is completed.
As temperature is increased up to 450.degree. C. and/or process
time is lengthened in any of the anneal processes, then oxygen
diffuses a greater distance into layer 23. Note that a relatively
thick Mg layer 23 (t3.gtoreq.3 Angstroms) will provide a
substantial contribution to the final RA value. Therefore, t3 is
preferably kept between 1 to 3 Angstroms. It is important that
unoxidized Mg layers 21, 23 are maintained on opposite surfaces of
MgO layer 21c during subsequent steps related to formation of a top
magnetic layer and overlying layers such as a capping layer so that
oxygen does not penetrate and oxidize a portion of the bottom
magnetic layer 10 and top magnetic layer 30. Thus, metal layers 21,
23 prevent oxidation of adjoining magnetic layers but serve as a
pathway for the tunnel barrier to become completely oxidized at a
later time after the MTJ stack is completed.
[0060] In FIG. 8 that relates to a bottom spin valve configuration,
MTJ stack 40 is shown after a free layer 30 is formed as the top
magnetic layer on the uppermost Mg layer 23. In this intermediate
structure, a first interface 14 has a CoFeB/Mg composition, for
example, while a second interface 28 may also have an Mg/CoFeB
composition. In an alternative embodiment, the top magnetic layer
may be comprised of Fe, or an alloy of two or more of Co, Fe, Ni,
and B. The present disclosure also encompasses embodiments where
the free layer has a synthetic antiferromagnetic (SAF)
configuration wherein two ferromagnetic layers are separated and
antiferromagnetically coupled that a layer such as Ru. Moreover,
the top magnetic layer may have a moment diluting layer such as Ta
or Mg formed between two magnetic layers that are ferromagnetically
coupled. In yet another embodiment, the free layer may be a
composite with a CoFeB, Co, or CoFe layer that has a bottom surface
along interface 28, and a laminated stack such as (Co/Ni).sub.n, or
(A1/A2).sub.n described previously formed on a top surface of the
CoFeB, Co, or CoFe layer.
[0061] MTJ stack 40 may further comprise a capping layer (not
shown) formed on a top surface 30s of free layer 30. For example,
the capping layer may include one or more of Ru and Ta to protect
the free layer during subsequent process steps such as a chemical
mechanical polish process that produces a smooth top surface on the
MTJ stack. In another embodiment, the capping layer may be a metal
oxide to generate interfacial perpendicular anisotropy along the
top surface 30s and enhance PMA within the top magnetic layer.
According to one aspect of the present disclosure, a metal oxide
capping layer may be formed by employing the tunnel barrier
formation process disclosed herein. Thus, both of the tunnel
barrier layer and capping layer may be MgO, for example, that has
been fabricated by depositing a first Mg layer followed by a
passive oxidation step. A bottom portion of the first metal layer
in the capping layer remains unoxidized to prevent oxidation of the
free layer. Thereafter, at least a second Mg layer is deposited on
the partially oxidized first Mg layer followed by a conventional
oxidation process. The formation of an uppermost Mg layer that is
not subjected to an oxidation process may be omitted during capping
layer formation since there is no subsequently deposited magnetic
layer that requires protection from oxidation. During the
subsequent anneal process described previously, oxygen from the
oxidized second Mg layer diffuses into the bottom portion of the
capping layer to form a metal oxide interface with a top surface of
the top magnetic layer. In an alternative embodiment, the tunnel
barrier may be an oxide made of a first metal or alloy such as
MgTaO while the capping layer is made of a second metal or alloy
that is MgO, for example.
[0062] Referring to FIG. 9, an anneal process is employed to cause
diffusion of oxygen within MgO layer 21c (FIG. 8) into adjoining Mg
layers. As a result, MgO layer 21d is formed that has a bottom
surface along first interface 14 and a top surface along second
interface 28 to enhance PMA within bottom magnetic layer 10 and top
magnetic layer 30, respectively. The anneal process comprises
applying a temperature up to 450.degree. C. for a period of up to
90 minutes. Anneal temperatures around 400.degree. C. are preferred
when the resulting MTJ structure is incorporated in a CMOS
device.
[0063] According to an embodiment depicted in FIG. 10, a MTJ
nanopillar 40n is fabricated after the MTJ stack is completed and
annealed by following a conventional patterning and etching
sequence. In the exemplary embodiment, the MTJ nanopillar is formed
on a substrate 6 such as a bottom electrode. The MTJ nanopillar
comprises a seed layer 8 on the substrate and an uppermost capping
layer 35 having a planar top surface 35s. A sidewall 38 extends
from the top surface to substrate 6. In this drawing, the x-axis
and y-axis directions are in the planes of the layers while a
thickness of each MTJ layer is determined along the z-axis
direction. The MTJ nanopillar top surface 35s may have a circular
or elliptical shape from a top-down view along the z-axis. A
plurality of MTJ nanopillars is typically arrayed in a design with
columns and rows on the substrate.
[0064] Referring to FIG. 11, the present disclosure also
encompasses a method of forming a tunnel barrier in a MTJ with a
top spin valve structure. In this process flow, a first metal (Mg)
layer 21 is deposited on a bottom magnetic layer that is a free
layer 30. Thereafter, a passive oxidation as described earlier is
performed to gently oxidize a top portion of the first metal layer
to form a first metal oxide layer 21a while a bottom portion of the
first metal layer and bottom magnetic layer remain unoxidized.
[0065] FIG. 12 depicts a MTJ structure after one or more metal
layers are deposited on metal oxide layer 21a. Each of the one or
more metal layers is oxidized by a conventional oxidation process
as described previously to form metal oxide layer 21c that includes
the first metal oxide layer. Optionally, one of the conventional
oxidation processes may be replaced by a second passive oxidation.
Then, an uppermost metal layer 23 is deposited. Once the uppermost
metal layer in the tunnel barrier stack is laid down, a top
magnetic layer that is a reference layer 10 is deposited. The top
magnetic layer forms an interface 27 with metal layer 23 while the
bottom magnetic layer forms an interface 16 with first metal layer
21.
[0066] FIG. 13 illustrates the MTJ structure in FIG. 12 after an
anneal process is performed. As a result, metal oxide layer 21d is
the tunnel barrier that has a first interface 16 with the bottom
magnetic layer, and a second interface 27 with the top magnetic
layer. PMA in preserved an enhanced in both magnetic layers because
of the controlled oxidation processes involved in preparing the
tunnel barrier, especially the passive oxidation applied to the
first metal layer.
[0067] In FIG. 14, one example of a MTJ with a top spin valve
structure having a tunnel barrier 21d formed according to an
embodiment of the present disclosure is depicted. All layers are
retained from the bottom spin valve stack in FIG. 10 except that
magnetic layers 10 and 30 are switched.
[0068] Another embodiment of the present disclosure is illustrated
in FIG. 15 where a MTJ nanopillar 70n has a dual spin valve
structure having a first tunnel barrier 21d1 between a first
reference layer 10a and free layer 30, and a second tunnel barrier
21d2 between the free layer and a second reference layer 10b. Both
tunnel barrier layers may be fabricated according to a process flow
that includes a passive oxidation of a first metal layer as
described in one of the previously described embodiments.
Alternatively, the dual spin valve may have a configuration (not
shown) represented by FL1/tunnel barrier 1/reference layer/tunnel
barrier 2/FL2 where FL1 is a first free layer and FL2 is a second
free layer. Formation of both tunnel barriers (layers 21d1, 21d2)
may proceed according to an embodiment described previously related
to tunnel barrier 21d. The dual spin valve structure may be
fabricated by a sequence wherein a second tunnel barrier 21d2 is
formed on stack that has a first magnetic layer 10a/first tunnel
barrier 21d1/magnetic layer 30 configuration. Then a third magnetic
layer 10b is deposited on the second tunnel barrier. With regard to
tunnel barrier 21d2, a first metal layer (not shown) is deposited
on magnetic layer 30 followed by a passive oxidation process with a
maximum oxygen pressure of 10.sup.-5 torr for up to 1000 seconds.
The passive oxidation process oxidizes an upper portion of the
first metal layer while a bottom portion of the first metal layer
at an interface with a top surface of magnetic layer 30 remains
unoxidized. One or more metal oxide layers may then be formed on
the oxidized upper portion of the first metal layer according to
methods described in previous embodiments. Thereafter, an uppermost
metal layer may be deposited on a top surface of the one or more
metal oxide layers before a third magnetic layer 10b is formed.
There may be a capping layer 35 formed on a top surface of magnetic
layer 10b to complete the MTJ stack. An anneal process with a
temperature up to 450.degree. C. for up to 90 minutes may be
performed during deposition of a first metal layer in both tunnel
barriers 21d1 and 21d2, or an anneal process may be performed after
all layers in the dual spin valve MTJ are formed.
[0069] According to another embodiment shown in FIG. 16, a
composite tunnel barrier 50 is formed according to an embodiment
where a first metal layer (M.sub.1) 21 is deposited and an upper
portion thereof is partially oxidized by a passive oxidation to
form a first metal oxide layer 21a represented by a
M.sub.1/M.sub.1Ox configuration. Then, one or more metal layers
made of a second metal (M.sub.2) where M.sub.2 with a different
from that of M.sub.1 may be deposited and oxidized by one or more
conventional oxidation processes to form a second metal oxide layer
22a. Optionally, one of the conventional oxidation processes may be
replaced by a second passive oxidation. An uppermost metal layer 23
that may have either a M.sub.1 or M.sub.2 composition is deposited
on metal oxide layer 22a.
[0070] In FIG. 17, the MTJ structure from FIG. 16 is shown after an
anneal is performed wherein a first metal oxide layer 21e is formed
as a result of oxygen diffusion into first metal layer 21 and
oxidation thereof to yield an essentially uniform M.sub.1Ox layer
from the intermediate M.sub.1/M.sub.1Ox stack. There is also a
second metal oxide layer 22b resulting from oxygen diffusion into
uppermost metal layer 23 and oxidation thereof to form an
essentially uniform M.sub.2Ox layer from the intermediate
M.sub.2Ox/uppermost metal oxide stack. Therefore, a composite
tunnel barrier 50 is formed wherein at least the first metal oxide
layer is fabricated by a process including a passive oxidation of a
metal layer.
[0071] The present disclosure also anticipates an embodiment
relating to a STO device wherein a metal oxide layer made according
to a process sequence disclosed herein adjoins a spin polarization
(SP) layer in order to preserve and even enhance PMA in the SP
layer. Previously, we disclosed a spin torque oscillator (STO)
device in U.S. Pat. No. 8,582,240 wherein non-magnetic layers
formed adjacent to a spin polarization layer and oscillation layer
may be metal oxides.
[0072] Referring to FIG. 18, a MAMR writer based on perpendicular
magnetic recording (PMR) is depicted. There is a main pole 81 with
a sufficiently large local magnetic field to write the media bit 85
in medium bit layer 84. Magnetic flux 88 in the main pole proceeds
through the air bearing surface (ABS) 86-86 and into medium bit
layer 84 and soft underlayer (SUL) 87. A portion of the flux (not
shown) returns to the write head where it is collected by write
shield 82. For a typical MAMR writer, the magnetic field generated
by the main pole itself is not strong enough to flip the
magnetization 89 of the medium bit in order to accomplish the write
process. However, writing becomes possible when assisted by a spin
torque oscillator (STO) 83 positioned between the main pole and
write shield.
[0073] The STO is comprised of a high moment magnetic layer 90, and
a second magnetic layer 91 that preferably has perpendicular
magnetic anisotropy (PMA). Between layers 82 and 90, 90 and 91, and
91 and 81, there are nonmagnetic layers 92, 93, 94, respectively,
to prevent strong magnetic coupling between adjacent magnetic
layers. Non-magnetic layer 94 may be a metal oxide layer in order
to form a metal oxide/magnetic layer interface with magnetic layer
91 and thereby preserving or enhancing PMA therein. Likewise,
non-magnetic layer 92 may be a metal oxide layer to preserve or
enhance PMA in non-magnetic layer 90.
[0074] An external current source 98 creates a bias current across
the main pole and write shield. The applied dc results in a current
flow in a direction from lead 101 into oscillation layer (OL) 90
and then through non-magnetic layer 93 and into SP layer 91 before
exiting through lead 100. Direct current generated by source 98 is
spin polarized by magnetic layer 91, interacts with magnetic layer
90, and produces a spin transfer torque that causes oscillation
with a precession angle 95 in magnetic layer 90 hereafter called
the oscillation layer (OL). The large angle oscillatory
magnetization of OL 90 generates a radio frequency (1) usually with
a magnitude of several to tens of GHz. This rf field (not shown)
interacts with magnetization 89 of medium bit 85 and makes the
magnetization oscillate into a precessional state 97 thereby
reducing the coercive field of medium bit 85 to allow switching by
the main pole field 88.
[0075] A key feature of the present disclosure is to provide a
metal oxide composition in one or both of non-magnetic layers 92,
94 made by a passive oxidation process as disclosed in one of the
previous embodiments. As a result, PMA is preserved in an adjoining
magnetic layer that is SP 91 or OL 90, respectively. According to
one embodiment, layer 94 is a metal oxide formed by depositing a
first metal layer on the main pole layer and then performing a
passive oxidation. One or more metal oxide layers are formed on the
upper oxidized portion of the first metal layer before an uppermost
metal layer is laid down. Then, layers 91, 93, 90, and 92 are
sequentially formed before the write shield is fabricated. In one
aspect, layer 92 is formed by depositing a first metal layer on OL
90 and then performing a passive oxidation. Next, one or more oxide
layers are formed on an oxidized upper portion of layer 92 before
an uppermost metal layer is deposited. An anneal process may be
performed at this point or when each of the uppermost metal layers
are deposited. As a result, oxidation processes to form metal oxide
layers 92, 94 are well controlled and prevent substantial oxygen
incursion into SP layer 91 and OL layer 90. In an alternative
embodiment, the STO layers may be formed in reverse order on the
main pole layer. Other aspects of previous embodiments are retained
including the composition of metal oxide layers, methods to form
one or more oxide layers on the oxidized first metal layer, and an
anneal process during deposition of the uppermost metal layer or
after all STO layers are laid down.
[0076] Another embodiment of the present disclosure is related to a
perpendicular spin torque oscillator (PSTO) device wherein a high
density STO current is isolated from a low density RF generation
current that we previously disclosed in U.S. Pat. No. 8,203,389. In
particular, a tunnel barrier in a RF generator portion of the three
terminal device may be a metal oxide layer such as MgO that is
formed by a process described in a previous embodiment.
[0077] Referring to FIG. 19, a PSTO device is shown with a STO
component 103 and a RF generation component 104 hereafter referred
to as "RF generator" or "MR sensor" that are separated by a
non-magnetic conductive layer 114. In one aspect, STO 103 is a
giant magnetoresistive junction comprised of a PMA magnetic layer
111 that serves as a magnetic reference layer (MRL), and a stack
including a first junction layer also known as non-magnetic spacer
112, second PMA magnetic layer 113a, and a soft magnetic layer 113b
that are sequentially formed on the MRL. PMA layer 111 and second
PMA layer 113a may be comprised of a (A1/A2).sub.n laminate as
described earlier.
[0078] Spacer 112 may be made of a conductive material such as Cu,
or may have a confining current pathway (CCP) configuration in
which Cu pathways are formed in an oxide matrix such as AlO.sub.x.
Layers 113a, 113b are exchange coupled to each other and form a
composite magnetic oscillation layer (MOL) wherein the
magnetization in each layer is free to oscillate when subjected to
an applied magnetic field perpendicular to the planes of the
layers, and when an electric current of sufficiently high density
flows in a direction perpendicular to the planes of the layers from
a first electrical terminal 122 to a second electrical terminal
121. The high current density is preferably in the range of
1.times.10.sup.7 to 1.times.10.sup.9 Amps/cm.sup.2 in order to
exceed the critical current density for causing a spin torque
effect on the MOL. It is believed that reflected electrons from the
MRL/spacer interface excite the MOL layer and thereby induce an
oscillation state in layers 113a, 113b with significant in-plane
amplitude. Note that PMA layer 113a has the same oscillation
frequency as soft magnetic layer 113b but a smaller in-plane
magnetization component. Soft magnetic layer 113b may be made of
CoFe, a CoFe alloy, or a composite thereof.
[0079] Non-magnetic conductive layer 114 is preferably a metal made
of Cu or the like, or a metal alloy having a bottom surface that
contacts an uppermost layer of STO 103, and with a top surface that
adjoins a bottom layer in RF generator 104. Preferably, conductive
layer 114 has a width in an in-plane direction along the x-axis
that is greater than the width w of the layers in the STO and RF
generator in order to allow an electrical connection to a first
electrical terminal hereafter referred to as first terminal
122.
[0080] According to one embodiment, RF generator 104 is a
magnetoresistive (MR) sensor with a TMR configuration in which a
MTJ has a magnetic sensing layer 125, a second junction layer
hereafter referred to as tunnel barrier 126, reference layer 127,
exchange coupling layer 128, pinned layer 129, and AFM layer 130
are sequentially formed on a top surface of conductive layer 114.
Optionally, when reference layer 127 has PMA, layers 128-130 may be
omitted. An important feature is that magnetic sensing layer 125
should have a Mst value within about .+-.50% of the Mst value for
MOL layer (113a, 113b). Moreover, magnetic sensing layer 125 may be
a single layer or a composite and is magnetostatically coupled to
soft magnetic layer 113b such that when an oscillating state is
established in the MOL, an oscillation state is induced in the
sensing layer with substantially the same frequency as in layers
113a, 113b. Preferably, in an embodiment wherein MR sensor 104 and
STO 103 have essentially the same width w, the MR sensor is aligned
vertically above the STO such that sidewalls 103s, 104s are
substantially coplanar in order to provide an efficient
magnetostatic coupling between soft magnetic layer 113b and sensing
layer 125. A key aspect is that tunnel barrier 126 is a metal oxide
made by a process including partial oxidation of a first metal
layer by a passive oxidation process as described previously to
preserve PMA in magnetic sensing layer 125, and in reference layer
127 in an embodiment where layers 128-130 are omitted. As a result,
RF generator 104 has lower RA and higher TMR ratio compared with
prior art PSTO devices where the tunnel barrier is fabricated by
conventional oxidation processes.
[0081] During an operating mode, an external magnetic field 105 is
applied to the entire PSTO structure including STO 103 and RF
generator 104 in either a (+) or (-) y-axis direction to align the
perpendicular magnetization components of MRL 111, MOL 113a/113b,
and magnetic sensing layer 125 in the same direction as the field
direction. Preferably, MRL 111 has an entirely perpendicular to
plane magnetization orientation while the MOL and magnetic sensing
layer magnetizations are tilted partially out of the film plane.
When a high density current flows from first terminal 122 to second
terminal 121, electrons pass through the MOL layer to MRL 111. A
portion of the electrons are reflected from the MRL/spacer 112
interface back into the MOL to excite the MOL magnetization from a
quiescent state into a significant in-plane oscillation.
Subsequently, the oscillating in-plane magnetization component in
MOL 113a/113b produces an oscillating magnetic field in magnetic
sensing layer 125. The in-plane magnetization oscillation of the
magnetic sensing layer has a 180 degree phase difference compared
with that of MOL which means the MOL and magnetic sensing layer are
in a pseudo anti-ferromagnetic coupled FMR mode. Therefore, with
magnetic sensing layer 25 being part of MR sensor 104 and a DC
current flowing between first terminal 122 and third terminal 120
in either direction, an AC voltage signal can be generated between
the first and third terminals from a resistance change in the MR
sensor due to magnetostatic coupling between the magnetic sensing
layer and the oscillating MOL.
[0082] According to another embodiment, a three terminal
spin-transfer switching device shown in FIG. 20 may comprise a
tunnel barrier layer and a low RA barrier formed by using the
passive oxidation process and one or more methods such as NOX and
RF sputtering described previously. Similar to the three terminal
structure we have disclosed in U.S. Pat. No. 7,978,505, three
magnetic layers including a polarizing layer, free layer, and a
reference layer are separated by two non-magnetic layers. In FIG.
20, the non-magnetic layer 214 between the free layer 213 and the
reference layer 215 is a tunnel barrier with normal RA to produce
the read signal in the read circuit 221, which contains the free
layer and its terminal (electrode 202), the normal RA tunnel
barrier, the reference layer and its terminal (electrode 203) and
the source of read signal and a detecting setup, a sense amplifier
(not shown), for example. The non-magnetic layer 212 between the
free layer and the polarizing layer 211 may be a metal layer or a
low RA tunnel barrier to produce the spin-transfer torque for
switching the free layer in the write circuit 220, which contains
the free layer and its terminal (electrode 202), the metal spacer
or low RA tunnel barrier, the polarizing layer and its terminal
(electrode 201) and the source of writing voltage (not shown). The
separation of the write and read circuits ensures that the write
circuit, where the writing current can be much larger than that in
a normal two terminal device, has a low RA while the read circuit
has a normal RA to generate decent read signals. The previously
disclosed oxidation process may be used to fabricate the normal RA
barrier 214 to enhance MR ratio and preserve PMA, and may also be
employed in formation of the low RA barrier 212 to further reduce
the RA value.
[0083] In an alternative embodiment depicted in FIG. 21, the order
of forming the layers 211-215 in the three terminal device shown in
FIG. 20 may be reversed such that the reference layer 215 is formed
as the bottommost layer followed by the tunnel barrier 214, free
layer 213, metal or low RA barrier 212, and a polarizing layer as
the uppermost layer. In this case, the read circuit 221 comprises a
first terminal (electrode 201) attached to the reference layer, a
second terminal (electrode 202) connected to the free layer, and
layers 213-215 between the first and second terminals. Meanwhile,
the write circuit 220 comprises the second terminal, a third
terminal (electrode 203) attached to the polarizing layer, and
layers 211-213 between the second and third terminals.
[0084] To demonstrate the effectiveness of the tunnel barrier
fabrication method of the present invention, an experiment was
performed to build MTJ nanopillars in 10 Mb memory device arrays
with a reference layer/MgO tunnel barrier/free layer/cap layer
configuration. A current-in-plane tunneling (CIPT) technique was
used to measure RA of the stack and TMR ratio of each MTJ
nanopillar. In all examples in Table 1, the bottom and top magnetic
layers that adjoin the tunnel barrier are both CoFeB and the
capping layer is MgO.
[0085] A conventional tunnel barrier formation process currently
practiced by the inventors is used for the MTJ shown in row 1 of
Table 1 and includes deposition of three Mg layers with thicknesses
of 6.6 Angstroms (first Mg layer), 3 Angstroms (second Mg layer),
and 2.5 Angstroms for the uppermost Mg layer. The first Mg layer is
oxidized with a NOX process comprised of a 5 sccm O.sub.2 flow rate
for 80 seconds, and the second Mg layer is oxidized with a two part
NOX process where the first step has a 5 sccm O.sub.2 flow rate for
20 seconds and the second step has a O.sub.2 pressure control of 1
torr for 600 seconds. After the third Mg layer is deposited on the
oxidized second Mg layer and a capping layer is formed as the
uppermost layer, the complete MTJ is annealed with a process
comprising 400.degree. C. for 30 minutes that is common to all
three MTJ nanopillar structures.
[0086] Row 2 represents MTJ with an MgO tunnel barrier built
according to an embodiment of the present disclosure. A key feature
is that the first Mg layer with a 3.3 Angstrom thickness is treated
with a passive oxidation (PO) method with an oxygen flow pressure
<10.sup.-6 torr for 20 seconds. Then a second Mg layer with a
3.3 Angstrom thickness is deposited and a NOX process is performed
with a 5 sccm O.sub.2 flow rate for 80 seconds. Next, a third Mg
layer is deposited and a two part NOX process is applied wherein a
first step comprises a 5 sccm O.sub.2 flow rate for 20 seconds, and
a second step has a pressure control at 1 torr for 600 seconds.
Finally, a fourth Mg layer with a 2.5 Angstrom thickness is
deposited and the structure is annealed at 400.degree. C. after the
MTJ stack is completed.
[0087] Row 3 represents a MTJ with an MgO tunnel barrier built
according to another embodiment of the present disclosure. A key
feature is that the first Mg layer with a 2.75 Angstrom thickness
is treated with a passive oxidation (PO) method with an oxygen flow
pressure of <10.sup.-6 torr for 20 seconds. Then a second Mg
layer with a 2.25 Angstrom thickness is deposited and a NOX process
is performed with a 5 sccm O.sub.2 flow rate for 80 seconds. Next,
a third Mg layer with a 3 Angstrom thickness is deposited and a two
part NOX process is applied with a 5 sccm, 20 second O.sub.2 flow
for the first step and a 18 sccm, 500 second O.sub.2 flow for the
second step. Then a fourth Mg layer having a 4.5 Angstrom thickness
is deposited and a third NOX process is performed wherein O.sub.2
flow rate is 5 sccm for 20 seconds. Finally, a fifth Mg layer with
a 2.5 Angstrom thickness is deposited and the structure is annealed
at 400.degree. C. after the MTJ stack is completed.
[0088] In rows 1 and 2 in Table 1, the total thickness of all
deposited Mg layers is around 12 Angstroms. There are five separate
Mg layers in the third example (row 3 process) with a combined
thickness of 15 Angstroms that leads to a slightly higher RA value
than in row 2 but RA is still lower than that shown for the MTJ in
row 1.
TABLE-US-00001 TABLE 1 Magnetic Properties of patterned MTJ
nanopillars with CoFeB/MgO/CoFeB/MgO configuration after anneal at
400.degree. C. for 30 min. # MgO tunnel barrier formation process
RA TMR % 1 Mg6.6/NOX(5 sccm, 80 s)/3Mg/NOX(5 sccm, 20 20 130
s)/NOX(1 torr, 600 s)/Mg2.5 2 Mg3.3/PO(<10.sup.-6 torr, 20
s)/3.3Mg/NOX(5 12 140 sccm, 80 s)/3Mg/NOX(5 sccm, 20 s)/NOX(1 torr,
600 s)/Mg2.5 3 Mg2.75/PO(<10.sup.-6 torr, 20 s)/2.25Mg/NOX(5 17
140 sccm, 80 s)/3Mg/NOX(5 sccm, 20 s + 18 sccm, 500 s)/Mg4.5/NOX(5
sccm, 20 s)/Mg2.5
[0089] A comparison of MTJ nanopillar in row 2 to the conventional
MTJ in row 1 clearly indicates several benefits associated with
having a passive oxidation as the initial oxidation step in
fabricating a tunnel barrier. In particular, there is an increase
in TMR ratio from 130% to 140%, and a decrease in RA from 20 to 12.
It is important to note that the increase in TMR ratio is due to
better preservation of PMA in the CoFeB magnetic layers. The row 3
MTJ nanopillar has a tunnel barrier made with a process that has
one additional Mg layer deposition and an extra NOX oxidation
compared with the MTJ in row 2 to intentionally produce a thicker
MgO layer with slightly higher RA. The example in row 3 also
exhibits a better TMR ratio and lower RA compared with the MTJ in
row 1 with a conventional MgO tunnel barrier. Thus, we have
demonstrated that the improved tunnel barrier formation process
described herein has flexibility in fabricating a variety of tunnel
barriers.
[0090] Table 1 results suggest that the tunnel barrier fabrication
of the present disclosure enables PMA in the reference layer and
free layer to be maintained or even enhanced as demonstrated by the
larger TMR ratio. Moreover, thermal stability of at least
400.degree. C. in MTJ nanopillars which is required for
compatibility with CMOS processes is achieved since all of the
desired properties in Table 1 were measured after an anneal process
for 30 minutes at 400.degree. C. An elevated anneal temperature
near 400.degree. C. is also beneficial in crystallizing amorphous
magnetic layers such as CoFeB and the MgO tunnel barrier to ensure
a higher TMR ratio.
[0091] While this disclosure 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 disclosure.
* * * * *