U.S. patent application number 11/255703 was filed with the patent office on 2006-02-23 for system and method for fixing a direction of magnetization of pinned layers in a magnetic field sensor.
This patent application is currently assigned to Honeywell International Inc.. Invention is credited to Hong Wan, Lakshman S. Withanawasam.
Application Number | 20060039091 11/255703 |
Document ID | / |
Family ID | 32868198 |
Filed Date | 2006-02-23 |
United States Patent
Application |
20060039091 |
Kind Code |
A1 |
Wan; Hong ; et al. |
February 23, 2006 |
System and method for fixing a direction of magnetization of pinned
layers in a magnetic field sensor
Abstract
A spin valve GMR sensor configured in a bridge configuration is
provided. The bridge includes two spin valve element pairs. The
spin valve elements include a free layer, a space layer, a pinned
layer, and a bias layer. The bias layer includes a first bias layer
and a second bias layer. The first and second spin valve element
pairs are formed on separate metal layers and a current pulse is
applied to the metal layers, which sets the direction of
magnetization in the pinned layer of the first pair of spin valve
elements to be antiparallel to the direction of magnetization in
the pinned layer of the second pair of spin valve elements. The
same effect can be accomplished by making the pinned layer
substantially thicker than the second bias layer in the first spin
valve element pair and the pinned layer is substantially thinner
than the second bias layer in the second spin valve element pair
and applying a magnetic field to the first and the second spin
valve element pairs.
Inventors: |
Wan; Hong; (Plymouth,
MN) ; Withanawasam; Lakshman S.; (Maple Grove,
MN) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD
P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Assignee: |
Honeywell International
Inc.
Morristown
NJ
|
Family ID: |
32868198 |
Appl. No.: |
11/255703 |
Filed: |
October 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10370652 |
Feb 20, 2003 |
|
|
|
11255703 |
Oct 20, 2005 |
|
|
|
Current U.S.
Class: |
360/324.1 |
Current CPC
Class: |
B82Y 25/00 20130101;
G01R 33/093 20130101; G11B 2005/0008 20130101 |
Class at
Publication: |
360/324.1 |
International
Class: |
G11B 5/127 20060101
G11B005/127 |
Claims
1-9. (canceled)
10. A method of fabricating a spin valve giant magnetoresistive
sensor in a Wheatstone bridge configuration, comprising in
combination: depositing at least one metal layer; depositing spin
valve element layers to form spin valve elements; depositing
dielectric layers substantially between the at least one metal
layer and the spin valve elements; and applying a current pulse to
the at least one metal layer.
11. The method of claim 10, wherein a separate metal layer is
deposited for each spin valve element.
12. The method of claim 10, wherein a separate metal layer is
deposited for each spin valve element pair.
13. The method of claim 10, wherein the spin valve element layers
include a free layer, a space layer, a pinned layer, and a bias
layer.
14. The method of claim 10, wherein the current pulse is applied
after forming the spin valve elements.
15. The method of claim 10, wherein the current pulse sets a
direction of magnetization in the spin valve elements.
16. The method of claim 10, wherein the current pulse is applied
for approximately 1 microsecond and has a peak greater than 100
milliamperes.
17. The method of claim 10, wherein the current pulse generates a
first magnetic field in a first pair of spin valve elements and a
second magnetic field in a second pair of spin valve elements.
18. The method of claim 17, wherein the first magnetic field is in
a direction substantially opposite to that of the second magnetic
field.
19. The method of claim 17, wherein the first magnetic field and
the second magnetic field set a direction of magnetization in a
pinned layer of the first spin valve element pair to be
substantially antiparallel to a direction of magnetization in a
pinning layer of the second spin valve element pair.
20-38. (canceled)
Description
FIELD
[0001] The present invention relates generally to giant
magnetoresistive ("GMR") sensors. More specifically, the present
invention relates to a GMR sensor in which a direction of
magnetization of pinned layers in the GMR sensor may be easily
fixed.
BACKGROUND
[0002] The ability to sense and measure a magnetic field is
important in many areas. For example, magnetic sensors may be used
for compassing, navigation, magnetic anomaly detection, and
identifying position. As a result, magnetic sensors may be found in
medical, laboratory, and electronic instruments; weather buoys;
virtual reality systems; and a variety of other systems.
[0003] Such applications frequently employ magnetoresistive ("MR")
sensors capable of sensing small magnetic fields. MR sensors are
often formed using integrated circuit fabrication techniques and
are typically composed of a nickel-iron (permalloy) thin film
deposited on a silicon wafer, or other type of substrate, and
patterned as resistive strips. The resistance of the strips varies
with respect to an angle formed between a sensed magnetic field and
current direction within the sensor. The strip resistance is
maximized when the magnetic field and the current direction are
parallel to each other.
[0004] During the manufacture of an MR sensor, the easy axis (the
preferred direction of magnetization) is set to a direction along
the length of the film to allow the maximum change in resistance of
the strip. However, the influence of a strong magnetic could rotate
the magnetization of the film, changing the sensor's
characteristics. Following such changes, a strong restoring
magnetic field can be applied to the sensor to restore, or set, the
sensor's characteristics.
[0005] In certain designs, large external magnets can be placed
adjacent to the sensor to set the sensor's characteristics.
However, such an implementation may not be feasible when the MR
sensor has already been packaged into a system. Particularly, some
applications require several sensors within a single package to be
magnetized in different directions. In such applications, instead
of using large external magnets, individual coils may be wrapped
around each sensor to restore the sensor's characteristics.
Alternatively, current straps, also known as set-reset straps, may
be used to restore the sensor's characteristics. The use of current
straps in a magnetic field sensing device is discussed in U.S. Pat.
No. 5,247,278 to Bharat B. Pant, assigned to the same assignee as
the current application. U.S. Pat. No. 5,247,278 is fully
incorporated herein by reference.
[0006] Another type of magnetic sensor is a giant magnetoresistive
("GMR") sensor. GMR sensors are typically employed in applications
that require measurements of relatively small magnetic fields. GMR
sensors may be manufactured using thin film technology and may
include multiple layers of alternating ferromagnetic and
non-magnetic materials. Generally, a GMR sensor includes two
magnetic layers separated by a non-magnetic layer. The resistance
of the magnetic layers is related to the direction of magnetization
between the two magnetic layers.
[0007] Some of the structures currently being used to fabricate GMR
elements include unpinned sandwich, antiferromagnetic multilayer,
spin valve structures, and spin dependent tunnel structures.
[0008] The unpinned sandwich structure may include two magnetic
layers separated by a conducting non-magnetic layer. For example,
an unpinned sandwich structure may consist of two permalloy layers
separated by a layer of copper.
[0009] An antiferromagnetic multilayer structure may consist of
multiple repetitions of alternating conducting magnetic layers and
non-magnetic layers. In this structure, each magnetic layer may
have a direction of magnetization antiparallel to the direction of
magnetization of the magnetic layers on either side.
[0010] Spin valve structures may include a pinned magnetic layer
and a free magnetic layer, with a nonmagnetic layer, such as
copper, located between the two magnetic layers. The pinned layer
may have a fixed magnetization direction, while the free layer may
rotate in the presence of an external magnetic field.
[0011] Spin dependent tunnel structures are similar to spin valve
structures; however, the non-magnetic layer is a non-conductive
material, such as an oxide, and current flows from one magnetic
layer to another magnetic layer through a tunnel current in the
non-conductive layer.
[0012] Magnetic field sensors using GMR elements are often
fabricated in a Wheatstone bridge configuration. A Wheatstone
bridge can be fabricated using four GMR elements, such as spin
valves. One of the biggest challenges of fabricating a spin valve
GMR sensor in a Wheatstone bridge configuration is producing two
GMR element pairs that respond differently to the same external
magnetic field. For a spin valve GMR sensor, the direction of
magnetization of the pinned layers in adjacent legs of the bridge
should be antiparallel in order to utilize the GMR ratio fully.
[0013] U.S. Pat. No. 5,617,071 entitled "Magnetoresistive structure
comprising ferromagnetic thin films and intermediate alloy layer
having magnetic concentrator and shielding permeable masses"
discloses one approach to fix the direction of magnetization of the
pinned layers in adjacent legs to be antiparallel by shielding one
pair of Wheatstone bridge elements. By shielding opposing GMR
elements with a highly permeable material, the shielded pair may
not experience the effects of an applied magnetic field that
rotates the direction of magnetization of the non-shielded pair.
However, this approach limits the range of the output signal,
reduces sensitivity of the sensor in half, and requires extra
processing steps to fabricate the shielded layer.
[0014] U.S. Pat. No. 5,561,368 (hereinafter referred to as the '368
patent) entitled "Bridge Circuit Magnetic Field Sensor Having Spin
Valve Magnetoresistive Elements Formed on Common Substrate"
discloses another approach for producing two GMR element pairs that
respond differently to the same external magnetic field. According
to the '368 patent, four GMR spin valve elements are formed on the
same substrate. The free layers of all four of the spin valve
elements have their magnetization axes parallel to one another. The
pinned layers of two spin valve elements have their magnetization
axes antiparallel to the direction of magnetization of the pinned
layers of the other two spin valve elements.
[0015] The magnetic field sensor in the '368 patent further
includes an electrically conductive fixing layer (a current strap)
formed on the substrate. The application of current through the
fixing conductor during fabrication of the sensor fixes the
direction of magnetization of two of the pinned layers to be
antiparallel to the direction of magnetization of the other two
pinned layers. While the current is applied to the fixing
conductor, the sensor is first heated and then cooled.
[0016] The application of the current during the sensor fabrication
may be difficult and not feasible, especially when many sensors are
fabricated on a single wafer. Multiple power supplies may be
required to supply the current to the fixing conductors or
individual sensors may have to be linked. Thus, the methods
described in the '368 patent may require a complicated
manufacturing process. Furthermore, applying heat during the
process may reduce the GMR ratio for the material.
[0017] Therefore, a need exists for a simple method of setting the
magnetization directions in the pinned layers of spin valve GMR
sensors configured in a Wheatstone bridge configuration.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] An exemplary embodiment of the present invention is
described below with reference to the drawings, in which:
[0019] FIG. 1 is a schematic diagram of a GMR sensor, according to
an exemplary embodiment;
[0020] FIG. 2 is a cross sectional view of a spin valve element,
according to an exemplary embodiment;
[0021] FIG. 3 is a cross sectional view of a GMR sensor, according
to an exemplary embodiment;
[0022] FIG. 4 is a cross sectional view of one half of a spin valve
GMR sensor in a Wheatstone bridge configuration, according to an
exemplary embodiment;
[0023] FIG. 5 is a flow chart diagram of a method of fabricating a
GMR sensor, according to an exemplary embodiment;
[0024] FIG. 6 is a cross sectional view of a GMR sensor, according
to an exemplary embodiment; and
[0025] FIG. 7 is a flow chart diagram of a method of fabricating a
GMR sensor, according to another exemplary embodiment.
DETAILED DESCRIPTION
[0026] FIG. 1 is a schematic diagram illustrating a giant
magnetoresistive ("GMR") sensor 100, according to an exemplary
embodiment. The GMR sensor 100 includes four spin valve elements
102, 104, 106, and 108 arranged in a bridge configuration, such as
a Wheatstone bridge. Other bridge configurations may be used. The
GMR sensor 100 may be packaged as an integrated circuit.
[0027] Spin valve elements 102-108 may be composed of GMR thin film
layers as described below with reference to FIG. 2. Spin valve
element 102 may respond to an external magnetic field substantially
in the same manner as spin valve element 106. Spin valve element
104 may respond to an external magnetic field substantially in the
same manner as spin valve 108.
[0028] Each of the spin valve elements 102-108 has a length and a
width. Each spin valve element 102-108 may be arranged so that each
length is parallel to lengths of the other spin valve elements.
Further, each spin valve element 102-108 may be several hundred
microns long and a few microns wide. The width of each spin valve
element 102-108 may vary based on the GMR sensor's sensitivity
requirements, while the length of each element may vary based on
the GMR sensor's resistance and size requirements.
[0029] The GMR sensor 100 may include four terminals 110, 112, 114,
and 116. As depicted in FIG. 1, terminal 110 is located between
spin valve elements 102 and 108, terminal 112 is located between
spin valve elements 102 and 104, terminal 114 is located between
spin valve elements 104 and 106, and terminal 116 is located
between spin valve elements 106 and 108. A power supply may be
applied across terminals 110 and 114, which may result in an output
of the bridge across terminals 112 and 116. Alternatively, the
power supply may be applied across terminals 112 and 116, resulting
in an output of the bridge across the terminals 110 and 114.
[0030] FIG. 2 is a cross sectional view of a spin valve element
200. Spin valve element 200 may be substantially the same as spin
valve elements 102-108 depicted in FIG. 1. The spin valve element
200 may include a free layer 202, a space layer 204, a pinned layer
206, and a bias layer 208. As depicted in FIG. 2, the pinned layer
206 is located substantially above the bias layer 208, the space
layer 204 is located substantially above the pinned layer 206, and
the free layer 202 is located substantially above the space layer
204. The spin valve element 200 may include additional layers not
shown in FIG. 2, such as a capping layer or a buffer layer.
[0031] The layers 202-208 of the spin valve element 200 may be
deposited on a substrate using standard semiconductor deposition
processes. For example, the layers 202-208 may be deposited using
sputtering. The substrate may be composed of a semiconductor
material such as silicon or gallium arsenide.
[0032] The layers 202-208 of the spin valve element 200 may be
deposited in an external magnetic field. As a result, the free
layer 202 and the pinned layer 206, which are both magnetic layers,
may each possess a magnetic easy axis (a preferred direction of
magnetization) due to their grain structure. The easy axis of the
free layer 202 may be substantially parallel to the length of the
spin valve element 200. The easy axis of the pinned layer 206 may
be substantially perpendicular to the length of the spin valve
element 200.
[0033] The free layer 202 may be composed of a ferromagnetic
material, such as cobalt, iron, nickel, and their related alloys.
In a preferred embodiment, the free layer may be permalloy.
However, other ferromagnetic materials may be used to form the free
layer. The free layer 202 may be free to rotate its direction of
magnetization in response to an externally applied magnetic
field.
[0034] The space layer 204 may be composed of a nonmagnetic
material, such as copper (Cu). However, other nonmagnetic materials
may be used to form the space layer. The space layer 204 may
separate the two magnetic layers (e.g., the free layer 202 and the
pinned layer 206). The space layer 204 may be thin enough so that
the two magnetic layers 202, 206 may be coupled. For example, the
space layer 204 may be approximately 1-4 nm. Accordingly, when one
of the layers changes magnetic orientation, the other magnetic
layer may also change its orientation.
[0035] The pinned layer 206 may also be composed of a ferromagnetic
material. While a layer composed of cobalt (Co) or a CoFe alloy is
preferred for the pinned layer, other ferromagnetic materials may
be used. The bias layer 208 may fix the magnetization direction of
the pinned layer 206. Accordingly, when the pinned layer 206 is
exposed to an externally applied magnetic field, the direction of
magnetization of the pinned layer 206 may remain in its preferred
or fixed orientation.
[0036] The bias layer 208 may be composed of an antiferromagnetic
material. The bias layer 208 may be composed of two layers, a first
bias layer and a second bias layer. In a preferred embodiment, the
first bias layer may be composed of ruthenium (Ru), while the
second bias layer may be composed of cobalt (Co). The first bias
layer may be located substantially above the second bias layer. If
the pinned layer 206 is composed of cobalt, the pinned layer 206
and the bias layer 208 may form a "sandwich" with the ruthenium
layer located between the two cobalt layers. The bias layer 208 may
fix the magnetization direction of the pinned layer 206.
[0037] According to an exemplary embodiment, the thickness of the
pinned layer 206 and the thickness of the cobalt layer in the bias
layer 208 may be different. The thickness of the ruthenium layer in
the bias layer 208 may be selected so that it provides
antiferromagnetic coupling between the two adjacent cobalt layers.
As a result the magnetizations of these two cobalt layers have
opposite directions. For example, the thickness of the ruthenium
layer may be approximately 4-7 Angstroms. In such an embodiment,
the total magnetization from the two cobalt layers is very small,
and thus, may not be easily influenced by an external magnetic
field with low or moderate strength.
[0038] FIG. 3 is a cross-sectional view of a GMR sensor 300. FIG. 3
depicts spin valve elements 302, 304, 306, and 308 in a bridge
arrangement, similar to the configuration depicted in FIG. 1. Spin
valve elements 302-308 are substantially the same as spin valve
element 200 depicted in FIG. 2. In FIG. 3, the spin valve elements
are shown with a free layer consisting of permalloy, a space layer
consisting of copper (Cu), a pinned layer consisting of cobalt
(Co), and a bias layer consisting of a ruthenium/cobalt (Ru/Co)
layer. However, the spin valve element layers may be composed of
different materials as discussed with reference to FIG. 2. FIG. 3
also depicts the direction of magnetization in each of the layers
of the spin valve elements 302-308.
[0039] According to an exemplary embodiment, the direction of the
magnetization of the pinned layer in one of the spin valve elements
is antiparallel to the direction of the magnetization of the pinned
layers in adjacent spin valve elements (e.g., compare element 302
with elements 304 and 308). The arrows in the pinned layers
represent the direction of the magnetization in these layers. The
direction of magnetization in each of the pinned layers may be
critical to the operation of the GMR sensor 300. The layer of
cobalt in the bias layer may have a magnetization in a direction
opposite to that of the magnetization of the pinned layer in each
of the spin valve elements 302-308.
[0040] In the absence of a magnetic field, the direction of
magnetization in the free layers (permalloy layers) of each of the
spin valve elements 302-308 may be the same. As depicted in FIG. 3,
the free layers have a direction of magnetization into the page,
which is perpendicular to the direction of the magnetizations in
the two cobalt layers. This arrangement may provide the most linear
response for the spin valve elements 302-308.
[0041] The bridge arrangement of the GMR sensor 300 may be balanced
so that the spin valve elements 302-308 have equal resistance when
not exposed to a magnetic field. If a voltage is applied across two
opposite terminals of a balanced GMR sensor 300 that is not exposed
to an external magnetic field, the differential output across the
two other terminals will equal zero.
[0042] However, when the balanced GMR sensor 300 is exposed to a
magnetic field, the free layers may rotate while the pinned layers
remain fixed, resulting in a change of resistance of each of the
spin valve elements 302-308. The resistance change may be
proportional to the angle between the magnetization direction of
the fixed pinned layer and the magnetization direction of the
rotating free layer. This change in resistance may be detected and
the magnitude and polarity of the applied magnetic field may be
determined from the resistance change.
[0043] One of the biggest challenges of manufacturing a spin valve
GMR sensor in a Wheatstone bridge or similar configuration is
fabricating the pinned layers to have antiparallel magnetic
directions in adjacent legs of the bridge. In one embodiment,
applying a current pulse to an isolated metal layer may be used to
set the direction of magnetization in the pinned layers.
[0044] FIG. 4 is a cross sectional view of one half of a Wheatstone
bridge configuration spin valve GMR sensor 400. It is understood
that the GMR sensor 400 may have a second half substantially
similar to the half depicted in FIG. 4. Spin valve elements 402 and
404 are substantially the same as spin valve elements 302 and 304
depicted in FIG. 3. The connections between spin valve elements 402
and 404 are not shown in FIG. 4 for the sake of simplicity, but it
is understood that spin valve element 402 is connected to spin
valve element 404 in such a manner that element 402 is located
adjacent to element 404 in a bridge configuration. Accordingly, it
is desirable for the direction of the magnetization in the pinned
layer of spin valve element 402 to be antiparallel to the direction
of the magnetization in the pinned layer of spin valve element
404.
[0045] Spin valve elements 402, 404 may be formed on dielectric
layers 406, 408. The dielectric layers 406, 408 may be deposited on
metal layers 410, 412. The metal layers 410, 412 may be deposited
on a substrate. Standard semiconductor deposition processes may be
used to deposit the metal layers 410, 412; the dielectric layers
406, 408; and the layers of the spin valve elements 402, 404.
[0046] The metal layers 410, 412 may be composed of copper,
aluminum, or other conducting material. The dielectric layers 406,
408 may be composed of an insulating material, such as silicon
dioxide or silicon nitride. The substrate may be composed of a
semiconductor material such as silicon or gallium arsenide.
[0047] The metal layers 410, 412 may be connected to electrodes or
other device terminals. As such, a source may be applied to the
metal layers 410, 412 after device fabrication. For example, a
current source may be applied to the metal layers 410, 412. The
metal layers 410, 412 may be connected to the same current source,
or each metal layer 410, 412 may be connected to a different
current source.
[0048] Metal layer 410 may be located on the same substrate as
metal layer 412. However, metal layer 410 may be electrically
isolated from metal layer 412. Standard semiconductor isolation
techniques may be used to provide the isolation. Accordingly,
current flowing through metal layer 410 may not impact metal layer
412, and vice versa.
[0049] After the GMR sensor has been fabricated, a current pulse
may be applied to each of the metal layers 410, 412. The current
pulse may be characterized as having a large peak and a short
width. The peak of the current pulse should be large enough to
provide sufficient magnetic field to set the direction of
magnetization in the spin valve elements 402, 404, while the width
should be short enough to avoid generating too much heat. For
example, the current peak may range from 100 milliamperes to
several amperes and the pulse may be approximately 1 microsecond.
However, other current pulses with different peaks and widths may
be used.
[0050] Applying a current pulse to each of the metal layers 410,
412 may generate a localized magnetic field. The magnetic field
generated by current flowing through metal layer 410 may affect
spin valve element 402, but not spin valve element 404. Likewise,
the magnetic field generated by current flowing through metal layer
412 may affect spin valve element 404, but not spin valve element
402.
[0051] The current flowing through metal layer 410 may be designed
to flow in a direction opposite to that of the current flowing
through metal layer 412. The opposite directions of current flow
through the metal layers 410, 412 may generate the localized
magnetic fields that cause spin valve elements 402, 404 to orient
in opposite directions. In response to the pulse current, the layer
of cobalt in the bias layer of spin valve element 402 may be fixed
in a direction antiparallel to the layer of cobalt in the bias
layer of spin valve element 404.
[0052] The current pulse may affect the total magnetization from
the pinned and biasing layers. The total magnetization may be the
difference between the magnetizations of the pinned and biasing
layers because the moments are in opposite directions. The total
magnetization may align to the magnetic field direction generated
by the current pulse.
[0053] FIG. 5 is a flow chart diagram of a method 500. The method
500 provides a method of fabricating a spin valve GMR sensor in a
bridge configuration, such as a Wheatstone bridge. Block 502
specifies depositing metal layers. The metal layers may be
deposited onto a substrate using standard semiconductor fabrication
techniques. The metal layers may be composed of copper, aluminum,
or other conducting material. A separate metal layer may be
deposited for each spin valve element. Alternatively, a separate
metal layer may be deposited for each pair of spin valve elements
having the same direction of magnetization in their respective
pinned layers. In yet another embodiment, a single metal layer may
be deposited for all four spin valve elements in the GMR
sensor.
[0054] Block 504 specifies depositing dielectric layers. The
dielectric layers may be composed of an insulating material, such
as silicon dioxide or silicon nitride. The dielectric layers may be
deposited onto the metal layers using standard semiconductor
fabrication techniques. Accordingly, there may be a separate
dielectric layer for each spin valve element or for each pair of
spin valve elements having the same direction of magnetization in
their respective pinned layers.
[0055] Block 506 specifies depositing spin valve element layers.
The spin valve element layers may include a free layer, a space
layer, a pinned layer, and a bias layer. The spin valve element
layers may be deposited onto the dielectric layers using standard
semiconductor fabrication techniques. Four spin valve elements may
be formed for each GMR sensor fabricated in a bridge configuration.
The four spin valve elements may be formed on separate dielectric
layers. Alternatively, one pair of spin valve elements may be
formed on one dielectric layer and a second pair of spin valve
elements may be formed on a second dielectric layer.
[0056] Block 508 specifies applying a current pulse to the metal
layers. The current pulse may fix the direction of the
magnetizations in the pinned layers to be in the same direction
within each pair. Additionally, the current pulse may fix the
direction of the magnetizations of the pinned layers in one pair to
be antiparallel to the magnetizations of the pinned layers in the
other pair. The current pulse may be applied to electrodes
connected to the metal layers when the fabrication of the GMR
sensor is substantially complete.
[0057] In an alternate embodiment, the spin valve elements may be
deposited prior to depositing the dielectric layers and the metal
layers. The dielectric layers may be located substantially between
the spin valve element layers and the metal layers.
[0058] This method of making the pinned layers have antiparallel
magnetic directions in the adjacent legs of a bridge may be
advantageous because the current pulse is applied after the GMR
sensor is fabricated. Applying a current to a current strap during
fabrication may be very difficult, especially when there are many
sensors on a single wafer. In addition, by limiting the width of
the current pulse, degradation of the GMR sensor due to heat may be
avoided. The resulting bipolar GMR sensor may be highly sensitive
to wide range of magnetic fields.
[0059] FIG. 6 is a cross sectional view of a GMR sensor 600. The
GMR sensor 600 includes two spin valve element pairs. Spin valve
elements 602 and 606 form the first pair, while spin valve elements
604 and 608 form the second pair. The first pair of spin valve
elements may have a pinned layer (cobalt layer) that is
substantially thicker than the layer of cobalt in the bias layer.
In contrast, the second pair of spin valve elements may have a
pinned layer (cobalt layer) that is substantially thinner than the
layer of cobalt in the bias layer. For example, the cobalt layer
thickness may be in the range of 3 to 20 nanometers. Additionally,
the difference in thickness between the two cobalt layers may be in
the range of 0.4 to 10 nanometers. Other thicknesses may also be
used.
[0060] The first spin valve element pair (spin valve elements 602
and 606) may be formed on a substrate 610. Spin valve elements 602
and 606 may be located diagonally in the bridge, such as spin valve
elements 102 and 106 in FIG. 1. Each spin valve element in the
first spin valve element pair may include a free layer, a space
layer, a pinned layer, and a bias layer. When the layers of the
first spin valve element pair are deposited, the pinned layer
(cobalt layer) may be substantially thicker than the layer of
cobalt in the bias layer.
[0061] A first dielectric layer 612 may be deposited substantially
around the top and two sides of the first spin valve element pair,
using standard semiconductor deposition processes. The first
dielectric layer 612 may be composed of an insulating material,
such as silicon dioxide or silicon nitride.
[0062] A second dielectric layer 614 may be deposited on the
substrate 610 substantially adjacent to the first spin valve
element pair. The dielectric layer 614 may be composed of an
insulating material, such as silicon dioxide or silicon nitride.
Alternatively, the first dielectric layer 612 may be deposited on
the substrate 610 substantially adjacent to the first spin valve
element pair, eliminating the need for the second dielectric layer
614.
[0063] The second spin valve element pair (spin valve elements 604
and 608) may be formed on the dielectric layer 614. Each spin valve
element in the second spin valve element pair may include a free
layer, a space layer, a pinned layer, and a bias layer. When the
layers of the second spin valve element pair are deposited, the
pinned layer (cobalt layer) may be substantially thinner than the
layer of cobalt in the bias layer.
[0064] After fabricating GMR sensor 600, the GMR sensor 600 may be
magnetized by a large external magnetic field. The magnetic field
may be greater than 10 Gauss, depending on the film thickness. The
magnetizations of the thicker layers of cobalt (e.g., the pinned
layer of the first pair of spin valve elements and the layer of
cobalt in the bias layer of the second pair of spin valve elements)
may align in the same direction. The magnetizations of the thicker
layers of cobalt may be substantially aligned in the direction of
the applied magnetic field.
[0065] Through the coupling of the layer of ruthenium (Ru) in the
bias layer, the magnetizations of the thinner cobalt layers may
align in a direction opposite to that of the thicker cobalt layers.
As a result, the orientation of the magnetization of the pinned
layers in the first spin valve element pair may be antiparallel to
the magnetization of the pinned layers in the second spin valve
element pair.
[0066] FIG. 7 is a flow chart diagram of a method 700. The method
700 provides a method of fabricating a spin valve GMR sensor in a
bridge configuration. For example, the bridge configuration may be
a Wheatstone bridge. Block 702 specifies forming a first spin valve
element pair. Standard semiconductor deposition processes may be
used to deposit the various layers of the two spin valve elements
onto a substrate. The layers of the first spin valve element pair
may be deposited such that the layer of cobalt in the bias layers
are thinner than the pinned layers, which may also be composed of
cobalt. A dielectric layer may be deposited substantially around
the top and two sides of the spin valve element pair.
[0067] Block 704 specifies forming a second spin valve element
pair. A dielectric layer may be deposited on the substrate
substantially adjacent to the first spin valve element pair.
Standard semiconductor deposition processes may be used to deposit
the various layers of the two spin valve elements onto the
dielectric layer located substantially adjacent to the first spin
valve element pair. The layers of the second spin valve element
pair may be deposited such that the layer of cobalt in the bias
layers are thicker than the pinned layers, which may also be
composed of cobalt.
[0068] Block 706 specifies applying a magnetic field. The magnetic
field may be applied to the first and second spin valve element
pairs after they have been fabricated. The magnetic field may cause
the magnetizations of the thicker layers of cobalt to align in the
direction of the applied magnetic field. Through the coupling of
the layer of ruthenium in the bias layer, the magnetizations of the
thinner cobalt layers may align in a direction opposite to that of
the thicker cobalt layers. As a result, the orientation of
magnetization of the pinned layers in the first spin valve element
pair may be antiparallel to the magnetization of the pinned layers
in the second spin valve element pair.
[0069] This method of making the pinned layers have antiparallel
magnetic directions in adjacent legs of a bridge may be
advantageous because simple fabrication methods are used. Because
the applied magnetic field is applied to both spin valve element
pairs, fabricating a shield layer for half of the bridge may be
unnecessary, which reduces the complexity of manufacturing the GMR
sensor. The resulting bipolar GMR sensor may be highly sensitive to
wide range of magnetic fields.
[0070] Methods 500 and 700 may also used for fabricating GMR
sensors using spin dependent tunnel elements. A difference between
the spin valve elements and a spin dependent tunnel element is the
type of material used in fabricating the space layer. While a space
layer in a spin valve may be composed of copper, the space layer in
a spin dependent tunnel structure may be composed of an oxide. A
change in the type of spacer material may not impact the
effectiveness of the systems and methods described above with
respect to a GMR sensor using spin valve elements.
[0071] It should be understood that the illustrated embodiments are
exemplary only and should not be taken as limiting the scope of the
present invention. The claims should not be read as limited to the
described order or elements unless stated to that effect.
Therefore, all embodiments that come within the scope and spirit of
the following claims and equivalents thereto are claimed as the
invention.
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