U.S. patent application number 14/518651 was filed with the patent office on 2015-02-05 for mems device.
This patent application is currently assigned to INSIGHTECH, LLC. The applicant listed for this patent is InSighTech, LLC. Invention is credited to Tao JU, Biao ZHANG.
Application Number | 20150033855 14/518651 |
Document ID | / |
Family ID | 49993560 |
Filed Date | 2015-02-05 |
United States Patent
Application |
20150033855 |
Kind Code |
A1 |
ZHANG; Biao ; et
al. |
February 5, 2015 |
MEMS DEVICE
Abstract
A MEMS gyroscope is disclosed herein, wherein the MEMS gyroscope
comprised a magnetic sensing mechanism and a magnetic source that
is associated with the proof-mass. The magnetic sensing mechanism
comprises multiple magnetic field sensors that are designated for
sensing the magnetic field from a magnetic source so as to mitigate
the problems caused by fabrication.
Inventors: |
ZHANG; Biao; (Hinsdale,
IL) ; JU; Tao; (Beijing, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
InSighTech, LLC |
Los Angeles |
CA |
US |
|
|
Assignee: |
INSIGHTECH, LLC
Los Angeles
CA
|
Family ID: |
49993560 |
Appl. No.: |
14/518651 |
Filed: |
October 20, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13559625 |
Jul 27, 2012 |
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14518651 |
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13854972 |
Apr 2, 2013 |
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13559625 |
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Current U.S.
Class: |
73/504.12 |
Current CPC
Class: |
B81C 2201/0197 20130101;
B81C 2203/038 20130101; G01C 19/5776 20130101; B81C 3/001 20130101;
G01C 19/56 20130101; G01C 19/5712 20130101; G01P 15/125 20130101;
G01R 33/093 20130101; B81C 1/00357 20130101; G01R 33/098 20130101;
B81B 5/00 20130101; G01P 15/105 20130101; G01R 33/091 20130101 |
Class at
Publication: |
73/504.12 |
International
Class: |
G01C 19/5712 20060101
G01C019/5712; G01P 15/105 20060101 G01P015/105 |
Claims
1. A MEMS gyroscope, comprising: a first substrate, comprising: a
proof-mass; and a magnetic source attached to the proof-mass for
generating magnetic field; and a second substrate, comprising: a
magnetic sensor for measuring the magnetic field, wherein the
magnetic sensor has an aspect ratio less than 2.
2. The MEMS gyroscope of claim 1, wherein the magnetic source
comprises a conductive wire to which current can be applied so as
to generate a magnetic field.
3. The MEMS gyroscope of claim 1, wherein the magnetic source
comprises a magnetic nanoparticle.
4. The MEMS gyroscope of claim 2, wherein conductive wire has a
length along a length of at least one of the plurality of magnetic
sensing mechanisms.
5. The MEMS gyroscope of claim 2, wherein the magnetic sensor has
an aspect ratio of 1.
6. The MEMS gyroscope of claim 2, wherein the magnetic sensor
comprises a spin-valve structure.
7. The MEMS gyroscope of claim 2, wherein the magnetic sensor
comprises a tunnel-magnetic-resistor.
8. The MEMS gyroscope of claim 2, wherein the magnetic sensors each
has a geometric length and a width, wherein the geometric lengths
of the magnetic sensors are substantially parallel, and are
substantially parallel to the length of the conductive wire.
9. The MEMS gyroscope of claim 2, wherein at least one of the
magnetic sensors comprises a reference sensor and a signal sensor
pair.
Description
CROSS-REFERENCE
[0001] This U.S. utility patent application claims priority from
co-pending U.S. utility patent application "A HYBRID MEMS DEVICE,"
Ser. No. 13/559,625 filed Jul. 27, 2012, which claims priority from
U.S. provisional patent application "A HYBRID MEMS DEVICE," filed
May 31, 2012, Ser. No. 61/653,408 to Biao Zhang and Tao Ju. This
U.S. utility patent application also claims priority from
co-pending U.S. utility patent application "A MEMS DEVICE," Ser.
No. 13/854,972 tiled Apr. 2, 2013 to the same inventor of this U.S.
utility patent application, the subject matter of each of which is
incorporated herein by reference in its entirety.
TECHNICAL FIELD OF THE DISCLOSURE
[0002] The technical field of the examples to be disclosed in the
following sections is related generally to the art of operation of
microstructures, and, more particularly, to operation of MEMS
devices comprising MEMS magnetic sensing structures.
BACKGROUND OF THE DISCLOSURE
[0003] Microstructures, such as microelectromechanical (hereafter
MEMS) devices (e.g. accelerometers, DC relay and RF switches,
optical cross connects and optical switches, microlenses,
reflectors and beam splitters, filters, oscillators and antenna
system components, variable capacitors and inductors, switched
banks of filters, resonant comb-drives and resonant beams, and
micromirror arrays for direct view and projection displays) have
many applications in basic signal transduction. For example, a MEMS
gyroscope measures angular rate.
[0004] A gyroscope (hereafter "gyro" or "gyroscope") is based on
the Coriolis effect as diagrammatically illustrated in FIG. 1.
Proof-mass 100 is moving with velocity V.sub.d. Under external
angular velocity .OMEGA., the Coriolis effect causes movement of
the poof-mass (100) with velocity V.sub.s. With fixed V.sub.d, the
external angular velocity can be measured from V.sub.d. A typical
example based on the theory shown in FIG. 1 is capacitive MEMS
gyroscope, as diagrammatically illustrated in FIG. 2.
[0005] The MEMS gyro is a typical capacitive MEMS gyro, which has
been widely studied. Regardless of various structural variations,
the capacitive MEMS gyro in FIG. 2 includes the very basic theory
based on which all other variations are built. In this typical
structure, capacitive MEMS gyro 102 is comprised of proof-mass 100,
driving mode 104, and sensing mode 102. The driving mode (104)
causes the proof-mass (100) to move in a predefined direction, and
such movement is often in a form of resonance vibration. Under
external angular rotation, the proof-mass (100) also moves along
the V.sub.s direction with velocity V.sub.s. Such movement of
V.sub.s is detected by the capacitor structure of the sensing mode
(102). Both of the driving and sensing modes use capacitive
structures, whereas the capacitive structure of the driving mode
changes the overlaps of the capacitors, and the capacitive
structure of the sensing mode changes the gaps of the
capacitors.
[0006] Current capacitive MEMS gyros, however, are hard to achieve
submicro-g/rtHz because the capacitance between sensing electrodes
decreases with the miniaturization of the movable structure of the
sensing element and the impact of the stray and parasitic
capacitance increase at the same time, even with large and high
aspect ratio proof-masses.
[0007] Therefore, what is desired is a MEMS device capable of
sensing angular velocities and methods of operating the same.
SUMMARY OF THE DISCLOSURE
[0008] In view of the foregoing, a MEMS gyroscope is disclosed, the
MEMS gyrioscope comprising: a first substrate, comprising: a
proof-mass; and a magnetic source attached to the proof-mass for
generating magnetic field; and a second substrate, comprising: a
magnetic sensor for measuring the magnetic field, wherein the
magnetic sensor has an aspect ratio less than 2.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1 diagrammatically illustrates the Coriolis effect in a
MEMS structure;
[0010] FIG. 2 is a top view of a typical existing capacitive MEMS
gyroscope having a driving mode and a sensing mode, wherein both of
the driving and sensing mode utilize capacitance structures;
[0011] FIG. 3 illustrates an exemplary MEMS gyroscope having a
magnetic sensing mechanism;
[0012] FIG. 4 illustrates a top view of a portion of an exemplary
implementation of the MEMS gyroscope illustrated in FIG. 3, wherein
the MEMS gyroscope illustrated in FIG. 4 having a capacitive
driving mode and a magnetic sensing mechanism;
[0013] FIG. 5 illustrates a perspective view of a portion of
another exemplary implementation of the MEMS gyroscope illustrated
in FIG. 3, wherein the MEMS gyroscope illustrated in FIG. 5 having
a magnetic driving mechanism for the driving mode and a magnetic
sensing mechanism for the sensing mode
[0014] FIG. 6 illustrates an exemplary magnetic driving mechanism
of the MEMS gyroscope in FIG. 5;
[0015] FIG. 7 illustrates an exemplary magnetic source of the MEMS
gyroscope illustrated in FIG. 3;
[0016] FIG. 8 illustrates an exemplary magnetic sensing mechanism
that can be used in the MEMS gyroscope illustrated in FIG. 3;
[0017] FIG. 9 shows an exemplary thin-film stack that can be
configured into a CIP or CPP structure for use in the magnetic
sensing mechanism illustrated in FIG. 8;
[0018] FIG. 10 illustrates an exemplary MEMS gyroscope that
comprises multiple magnetic sensing structures;
[0019] FIG. 11 illustrates an exemplary operation for detecting and
measuring an angular velocity using a MEMS gyroscope illustrated in
FIG. 3;
[0020] FIG. 12 illustrates temperature dependence of the coercivity
of a ferromagnetic thin film, wherein the ferromagnetic thin film
can be used in the signal sensor illustrated in FIG. 3;
[0021] FIG. 13 illustrates temperature dependence of the coercivity
of a ferromagnetic thin film, wherein the ferromagnetic thin film
can be used in the signal sensor illustrated in FIG. 3;
[0022] FIG. 14 illustrates the temperature dependence of the
magnetic exchange field between a pining layer and a free layer,
wherein the pinning layer and the free layer can be used in the
signal sensor illustrated in FIG. 3;
[0023] FIG. 15 illustrates the top view of an exemplary sensor
wafer that comprises multiple magnetic sensors, wherein the
multiple magnetic sensors are associated with one magnetic source
of the proof-mass and are designated for measuring the magnetic
field from said magnetic source;
[0024] FIG. 16 illustrates a mass-wafer and a magnetic sensor
wafer, each having multiple dies;
[0025] FIG. 17 illustrates a wafer assembly comprising the
mass-wafer and magnetic sensor wafer of FIG. 16; and
[0026] FIG. 18 illustrates a side view of the wafer assembly of
FIG. 17.
DETAILED DESCRIPTION OF SELECTED EXAMPLES
[0027] Disclosed herein is a MEMS gyroscope and method of using the
same for sensing an angular velocity, wherein the MEMS gyroscope
utilizes a magnetic sensing mechanism. It will be appreciated by
those Skilled in the art that the following discussion is for
demonstration purposes, and should not be interpreted as a
limitation. Many other variations within the scope of the following
disclosure are also applicable. For example, the MEMS gyroscope and
the method disclosed in the following are applicable for use in
accelerometers.
[0028] Referring to FIG. 3, an exemplary MEMS gyroscope is
illustrated herein. In this example, MEMS gyroscope 106 comprises
magnetic sensing mechanism 114 for sensing the target angular
velocity through the measurement of proof-mass 112. Specifically,
MEMS gyroscope 106 comprises mass-substrate 108 and sensor
substrate 110. Mass-substrate 108 comprises proof-mass 112 that is
capable of responding to an angular velocity. The two substrates
(108 and 110) are spaced apart, for example, by a pillar (not shown
herein for simplicity) such that at least the proof-mass (112) is
movable in response to an angular velocity under the Coriolis
effect. The movement of the proof-mass (112) and thus the target
angular velocity can be measured by magnetic sensing mechanism
114.
[0029] The magnetic sensing mechanism (114) in this example
comprises a magnetic source 116 and magnetic sensor 118. The
magnetic source (116) generates a magnetic field, and the magnetic
sensor (118) detects the magnetic field and/or the magnetic field
variations that is generated by the magnetic source (116). In the
example illustrated herein in FIG. 3, the magnetic source is placed
on/in the proof-mass (112) and moves with the proof-mass (112). The
magnetic sensor (118) is placed on/in the sensor substrate (120)
and non-movable relative to the moving proof-mass (112) and the
magnetic source (116), With this configuration, the movement of the
proof-mass (112) can be measured from the measurement of the
magnetic field from the magnetic source (116).
[0030] Other than placing the magnetic source on/in the movable
proof-mass (1112), the magnetic source (116) can be placed on/in
the sensor substrate (120); and the magnetic sensor (118) can be
placed on in the proof-mass (112).
[0031] It is also noted that the MEMS gyroscope illustrated in FIG.
3 can also be used as an accelerometer.
[0032] The MEMS gyroscope as discussed above with reference to FIG.
3 can be implemented in many ways, one of which is illustrated in
FIG. 4. Referring to FIG. 4, the proof-mass (120) is driven by
capacitive, such as capacitive comb. The sensing mode, however, is
performed using the magnetic sensing mechanism illustrated in FIG.
3. For this reason, capacitive combs can be absent from the
proof-mass (120).
[0033] Alternatively, the proof-mass can be driven by magnetic
force, an example of which is illustrated in FIG. 5. Referring to
FIG. 5, the mass substrate (108) comprises a movable proof-mass
(126) that is supported by flexible structures such as flexures
128, 129, and 130. The layout of the flexures enables the
proof-mass to move in a plane substantially parallel to the major
planes of mass substrate 108. In particular, the flexures enables
the proof-mass to move along the length and the width directions,
wherein the length direction can be the driving mode direction and
the width direction can be the sensing mode direction of the MEMS
gyro device. The proof-mass (126) is connected to frame 132 through
flexures (128, 129, and 130). The frame (132) is anchored by
non-movable structures such as pillar 134. The mass-substrate (108)
and sensing substrate 110 are spaced apart by the pillar (134). The
proof-mass (112) in this example is driving by a magnetic driving
mechanism (136). Specifically, the proof-mass (126) can move (e.g.
vibrate) in the driving mode under magnetic force applied by
magnetic driving mechanism 136, which is better illustrated in FIG.
6.
[0034] Referring to FIG. 6, the magnetic driving mechanism 136
comprise a magnet core 138 surrounded by coil 140. By applying an
alternating current through coil 140, an alternating magnetic field
can be generated from the coil 140. The alternating magnetic field
applies magnetic force to the magnet core 140 so as to move the
magnet core. The magnet core thus moves the proof-mass.
[0035] The magnetic source (114) of the MEMS gyroscope (106)
illustrated in FIG. 3 can be implemented in many ways, one of which
is illustrated in FIG. 7. Referring to FIG. 7, conductive wire 142
is displaced on/in proof-mass 112. In one example, conductive wire
142 can be placed on the lower surface of the proof-mass (112),
wherein the lower surface is facing the magnetic sensors (118 in
FIG. 3) on the sensor substrate (110, in FIG. 3). Alternatively,
the conductive wire (142) can be placed on the top surface of the
proof-mass (112), i.e. on the opposite side of the proof-mass (112)
in view of the magnetic sensor (118). In another example, the
conductive wire (142) can be placed inside the proof-mass, e.g.
laminated or embedded inside the proof-mass (112), which will not
be detailed herein as those examples are obvious to those skilled
in the art of the related technical field.
[0036] The conductive wire (142) can be implemented in many
suitable ways, one of which is illustrated in FIG. 7. In this
example, the conductive wire (142) comprises a center conductive
segment 146 and tapered contacts 144 and 148 that extend the
central conductive segment to terminals, through the terminals of
which current can be driven through the central segment. The
conductive wire (142) may have other configurations. For example,
the contact tapered contacts (144 and 148) and the central segment
(146) maybe U-shaped such that the tapered contacts may be
substantially parallel but are substantially perpendicular to the
central segment, which is not shown for its obviousness.
[0037] The magnetic sensor (118) illustrated in FIG. 3 can be
implemented to comprise a reference sensor (150) and a signal
sensor (152) as illustrated in FIG. 8. Referring to FIG. 8,
magnetic senor 118 on/in sensor substrate 120 comprises reference
sensor 150 and signal sensor 152. The reference sensor (150) can be
designated for dynamically measuring the magnetic signal background
in which the target magnetic signal (e.g. the magnetic field from
the conductive wire 146 as illustrated in FIG. 7) co-exists. The
signal sensor (152) can be designated for dynamically measuring the
target magnetic signal (e.g. the magnetic field from the conductive
wire 146 as illustrated in FIG. 7). In other examples, the signal
sensor (152) can be designated for dynamically measuring the
magnetic signal background in which the target magnetic signal
(e.g. the magnetic field from the conductive wire 146 as
illustrated in FIG. 7) co-exists, while the signal sensor (150) can
be designated for dynamically measuring the target magnetic signal
(e.g. the magnetic field from the conductive wire 146 as
illustrated in FIG. 7).
[0038] The reference sensor (150) and the signal sensor (152)
preferably comprise magneto-resistors, such as AMRs,
giant-magneto-resistors (such as spin-valves, hereafter SV), or
tunneling-magneto-resistors (TMR). For demonstration purpose, FIG.
9 illustrates a magneto-resistor structure, which can be configured
into CIP (current-in-plane, such as a spin-valve) or a CPP
(current-perpendicular-to-plane, such as TMR structure). As
illustrated in FIG. 9, the magneto-resistor stack comprises top
pin-layer 154, free-layer 156, spacer 158, reference layer 160,
bottom pin layer 162, and substrate 120. Top pin layer 154 is
provided for magnetically pinning free layer 156. The top pin layer
can be comprised of IrMn, PtMn or other suitable magnetic
materials. The free layer (156) can be comprised of a ferromagnetic
material, such as NiFe, CoFe, CoFeB, or other suitbale materials or
the combinations thereof. The spacer (158) can be comprised of a
non-magnetic conductive material, such as Cu, or an oxide material,
such as Al.sub.2O.sub.3 or MgO or other suitable materials. The
reference layer (160) can be comprised of a ferromagnetic magnetic
material, such as NiFe, CoFe, CoFeB, or other materials or the
combinations thereof. The bottom pin layer (162) is provided for
magnetic pinning the reference layer (160), which can be comprised
of a IrMn, PtMn or other suitable materials or the combinations
thereof. The substrate (120) can be comprised of any suitable
materials, such as glass, silicon, or other materials or the
combinations thereof.
[0039] In examples wherein the spacer (158) is comprised of a
non-magnetic conductive layer, such as Cu, the magneto-resistor
(118) stack can be configured into a CIP structure (i.e.
spin-valve, SV), wherein the current is driven in the plane of the
stack layers. When the spacer (158) is comprised of an oxide such
as Al.sub.2O.sub.3, MgO or the like, the magneto-resistor stack
(118) can be configured into a CPP structure (i.e. TMR), wherein
the current is driven perpendicularly to the stack layers.
[0040] In the example as illustrated in FIG. 9, the free layer
(156) is magnetically pinned by the top pin layer (154), and the
reference layer (160) is also magnetically pinned by bottom pin
layer 162. The top pin layer (154) and the bottom pin layer (162)
preferably having different blocking temperatures. In this
specification, a blocking temperature is referred to as the
temperature, above which the magnetic pin layer is magnetically
decoupled with the associated pinned magnetic layer. For example,
the top pin layer (154) is magnetically decoupled with the free
layer (156) above the blocking temperature T.sub.B of the top pin
layer (154) such that the free layer (156) is "freed" from the
magnetic pinning of top pin layer (154). Equal to or below the
blocking temperature T.sub.B of the top pin layer (154), the free
layer (156) is magnetically pinned by the top pin layer (154) such
that the magnetic orientation of the free layer (156) is
substantially not affected by the external magnetic field.
Similarly, the bottom pin layer (162) is magnetically decoupled
with the reference layer (160) above the blocking temperature
T.sub.B of the bottom pin layer (162) such that the reference layer
(160) is "freed" from the magnetic pinning of bottom pin layer
(162). Equal to or below the blocking temperature T.sub.B of the
bottom pin layer (162), the reference layer (160) is magnetically
pinned by the bottom pin layer (162) such that the magnetic
orientation of the reference layer (162) is substantially not
affected by the external magnetic field.
[0041] The top and bottom pin layers (154 and 162, respectively)
preferably have different blocking temperatures. When the free
layer (156) is "freed" from being pinned by the top pin layer
(154), the reference layer (160) (preferably remains being pinned
by the bottom pin layer (162). Alternatively, when the free layer
(156) is still pinned by the top pin layer (154), the reference
layer (160) can be "freed" from being pinned by the bottom pin
layer (162). In the later example, the reference layer (160) can be
used as a "sensing layer" for responding to the external magnetic
field such as the target magnetic field, while the free layer (156)
is used as a reference layer to provide a reference magnetic
orientation.
[0042] The different blocking temperatures can be accomplished by
using different magnetic materials for the top pin layer (154) and
bottom pin layer (162). In one example, the top pin layer (154) can
be comprised of IrMn, while the bottom pin layer (162) can be
comprised of PtMn, vice versa. in another example, both of the top
and bottom pin layers (154 and 162) may be comprised of the same
material, such as IrMn or PtMn, but with different thicknesses such
that they have different blocking temperatures.
[0043] It is noted by those skilled in the art that the
magneto-resistor stack (118) is configured into sensors for sensing
magnetic signals. As such, the magnetic orientations of the free
layer (156) and the reference layer (160) are substantially
perpendicular at the initial state. Other layers, such as
protective layer Ta, seed layers for growing the stack layers on
substrate 120 can be provided. It is further noted that the
magnetic stack layers (118) illustrated in FIG. 9 are what is often
referred to as "bottom pin" configuration in the field of art. In
other examples, the stack can be configured into what is often
referred as "top pinned" configuration in the field of art, which
will not be detailed herein.
[0044] In some applications, multiple magnetic sensing mechanisms
can be provided, an example of which is illustrated in FIG. 10.
Referring to FIG. 10, magnetic sensing mechanisms 116 and 164 are
provided tier detecting the movements of proof-mass 112. The
multiple magnetic sensing mechanisms can be used for detecting the
movements of proof-mass 112 in driving mode and sensing mode
respectively. Alternatively, the multiple magnetic sensing
mechanisms 116 and 164 can be provided for detecting the same modes
(e.g. the driving mode and/or the sensing mode).
[0045] By using the different blocking temperatures of the sensors
as discussed above with reference to FIG. 9, the reference sensor
(150) and signal sensor (152) can be dynamically activated or
deactivated for sensing the target magnetic field. For
demonstration purpose, FIG. 11 shows an exemplary operation method
of measuring a target magnetic field (e.g. from the wire 146 as
illustrated in FIG. 7) by using the magnetic sensor (118 as
illustrated in FIG. 8).
[0046] With reference to FIG. 7, FIG. 8, and FIG. 11, the wire can
be set to the OFF state by not driving a current through the wire
at the initial time T.sub.0. The reference sensor can be set to the
ON state. The ON state of the reference sensor can be achieved
through "freeing" the free layer of the reference sensor by raising
the temperature of the pin layer that pins the free layer of the
reference sensor above its blocking temperature (e.g. by applying a
series of heating pulses or current pulses) and driving a current
through the reference sensor so as to measure its
magneto-resistance. The signal sensor at this time can be set to
the ON or any other suitable state, even though it is preferred
that the signal sensor can be set to the OFF state to avoid the
magnetic field generated by the current driven through the signal
sensor for setting the signal sensor to the ON state, Because the
wire is set to the OFF state and no current is driven through the
wire, the reference sensor measures the instant magnetic signal
background at time T.sub.0. After the reference sensor finished the
measurement, it locks its instant state at time T.sub.1 by for
example, lowering the temperature of its top pin layer (used for
pinning the free layer) below its blocking temperature such that
the free layer is magnetically coupled to (thus pinned by) the top
pin layer. The reference sensor at this state is referred to as the
"Lock" state.
[0047] At time T.sub.1, the wire remains OFF and the signal sensor
can be at any state, When the reference sensor is stabled at the
"Lock" state (e.g. finishes "locking" the state of its free layer),
the wire is set to the ON state at time T.sub.2, by driving current
with pre-defined amplitude through the wire so as to generate
magnetic field. The current can be DC or AC. After the magnetic
field generated by the wire is stabilized, the signal sensor can be
set to the ON state. Setting the signal sensor to the ON state can
be accomplished by raising the temperature of the pin layer used
for pinning the free layer of the signal sensor above its blocking
temperature so as to free the free layer. A current is driven
through the signal sensor so as to measure its
magneto-resistance.
[0048] After the signal sensor finished the measurement, it locks
its instant state at time T.sub.3 by for example, lowering the
temperature of its top pin layer (used for pinning the free layer)
below its blocking temperature such that the free layer is
magnetically coupled to (thus pinned by) the top pin layer. The
signal sensor at this state is referred to as the "Lock" state.
[0049] When the signal sensor finishes its locking at time T.sub.3,
the reference sensor and the signal sensor can output their
measurements to so to obtain the magnetic field from the magnetic
source attached to the proof-mass, thus extract the information of
the movement of the proof-mass.
[0050] The reference sensor and the signal sensor can be connected
by a Wheatstone bridge, or can be connected directly to an
amplifier or other electrical circuits to obtain the target
magnetic field, which not be detailed herein.
[0051] In the example discussed above, the reference sensor and/or
the signal sensor can be configured to "lock" the status (e.g. the
detected magnetic signal). This locking capability can be
accomplished in many ways. In the following, such locking
capability will be discussed with reference to signal sensor, and
the reference sensor can be implemented in substantially the same
ways.
[0052] In one example, the signal sensor can be configured to be
comprised of a storage layer that comprises a ferromagnetic layer.
The storage layer is connected to electrical leads such that
electrical current can be applied through the storage layer. When
current is applied, the storage layer is heated, and its
temperature can be elevated.
[0053] The material, as well as the geometry (e.g. the thickness)
of the storage layer can be configured such that at the elevated
temperature above a threshold temperature, such as the Currie
temperature, the storage layer is capable of being magnetized by
the target magnetic signal so as to accomplish the detection of the
target magnetic signal. When the temperature of the storage layer
is dropped to a temperature below the threshold temperature, the
storage layer "freezes" its magnetization states so as to
accomplish its "locking" operation. FIG. 12 illustrates such
operation.
[0054] Referring to FIG. 12, the vertical axis plots the coercivity
of the storage layer (e.g. the ferromagnetic layer of the storage
layer); and the horizontal axis plots the temperature. The
coercivity of the storage layer (ferromagnetic layer) decreases
with increased temperature. At room temperature RT, the storage
layer has a coercivity that is higher than the target magnetic
signal H.sub.target, therefore, the storage layer is unable to
detect the target magnetic signal. As the temperature of the
storage layer increases, the coercivity of the storage layer
decreases. At the storing temperature (or the blocking temperature
wherein the signal storage layer transits from ferromagnetic to
paramagnetic or super-paramagnetic), the coercivity of the storage
layer is equal to or less than the target magnetic signal
H.sub.signal such that the storage layer is capable of being
magnetized and thus detecting the target magnetic signal. After the
detection, the temperature of the storage layer can be decreased,
by for example, removing the current applied through the storage
layer (e.g. the ferromagnetic layer of the storage layer). When the
temperature of the storage layer is decreased to a temperature
below the storing temperature, the magnetization state of the
ferromagnetic layer (storage layer) is "locked" because the
coercivity of the storage layer is higher than the target magnetic
field. With this mechanism, the storage layer accomplishes the
"locking process."
[0055] The coercivity of a magnetic thin-film (layer) also varies
with its thickness, as diagrammatically illustrated in FIG. 13. The
storage layer can have a thickness such that the coercivity of the
signal-storage layer is in the vicinity of the target magnetic
field H.sub.target, such as within a range of .+-.0.5%, .+-.1%,
.+-.1.5%, .+-.2%, .+-.2.5%, .+-.3%, .+-.4%, .+-.5%, .+-.8%, .+-.10%
of H.sub.target. Especially when the storage layer has a thickness
such that its coercivity is higher than H.sub.target, a thermal
layer can be provided to adjust the coercivity of the storage
layer.
[0056] In addition to utilizing the temperature dependence of
coercivity of a ferromagnetic layer, a magnetic coupling structure
can be utilized to accomplish the "locking" process, as illustrated
in FIG. 14. Referring to FIG. 4, the signal sensor comprises free
layer 172 and pinning layer 170. The free layer (172) is a
ferromagnetic layer, and is provide for responding to the target
magnetic field to be detected. The pinning layer (170) is an
antiferromagnetic layer, and is provided for magnetically pining
the free layer (172) through the exchange magnetic field
H.sub.exch. It is known in the art that the magnetic exchange field
H.sub.exch changes with temperature. When the temperature is higher
than the blocking temperature T.sub.B that characterizes the
magnetic exchange field between the free layer (172) and pining
layer (170), the magnetic exchange field between the free layer
(172) and pining layer (172) is broken, e.g. reduced to a level
such that the free layer (172.) and pinning layer (172) are
magnetically decoupled. The free layer (172) is not pinned by the
pinning layer (172) at this temperature. By utilizing such
magnetic-couple (pinning) and magnetic-decouple (unpinning), the
signal sensor comprising the free layer (172) and pining layer
(170) can accomplish the state "locking" process.
[0057] For example, when it is desired to detect the target
magnetic field signal, the signal sensor may elevate its
temperature above the blocking temperature T.sub.B by for example,
applying current through the free layer (172) and/or the pining
layer (170). The free layer (172) is thus "freed" and can be used
for picking up the target magnetic field signal. When it is desired
for the signal sensor to lock its detection, for example, after the
detecting the target magnetic signal, the signal sensor can
decrease its temperature below the blocking temperature T.sub.B. At
a temperature below T.sub.B, the free layer (172) is magnetically
pinned by the pinning layer (172). The magnetic states of the free
layer (172), which corresponds to the target magnetic field is thus
"frozen" in the free layer (172).
[0058] With the exchange biased free layer, the magnetic sensor can
have an aspect ratio, which is defined as the ratio between the
width and length of the magnetic sensor, can be less than 3, such
as 2 or less, and more preferably 1. Specifically, the magnetic
sensor can be substantially squared in shape when viewed from the
top of the magnetic sensor.
[0059] In the above discussion, a magnetic sensing mechanism is
provided for detecting the magnetic field from the magnetic source
of the proof-mass. However, the magnetic source and the magnetic
sensors are in different substrates, such as the mass-substrate 108
and sensor substrate 110 as shown in FIG. 3. During fabrication,
the mass-substrate (108) and sensor substrate (110) are aligned and
assembled into a wafer assembly. One of the requirements of the
wafer alignment is that the magnetic source (116) of the
mass-substrate (108) is expected to be aligned to the magnetic
sensor (118) according to a pre-determined relative geometric
positions. In one example as illustrated in FIG. 3, the geometric
center of active area of the magnetic source (116) (e.g. the
geometric center of the conducting wire segment as illustrated in
FIG. 7) is aligned to the geometric center of the active area of
the magnetic sensor (118) when view from the top, In other
examples, the two geometric centers can be offset by a
pre-determined distance when viewed from the top, especially when
the offset position offers better magnetic field gradient, which
will not be detailed herein. In some other examples, the relative
position of the magnetic source and magnetic sensor can be arranged
so as to obtaining better magnetic field signal strength.
[0060] Regardless of various relative position arrangements, the
mass-substrate (108) and sensor substrate 110 (as illustrated in
FIG. 3) are expected to be aligned according to a predetermined
scheme. In fabrication, such expectation however is not always
guaranteed. Alignment error occurs oftentimes during the process of
assembling the mass-substrate (108) and sensor substrate (110).
Such misalignment can be of detrimental because the magnetic field
from the magnetic source decreases with the squared distance. The
magnetic sensor (118) may not be able to detects the magnetic field
from the associated magnetic sensor (116) if the misalignment too
large such that the strength of the magnetic field from the
magnetic source (116) at the location of the associated magnetic
sensor (118) is smaller than the sensitivity of the magnetic sensor
(118), or the magnetic field gradient of the magnetic field from
the magnetic source (116) is too small to be detected by the
magnetic sensor (118).
[0061] The above problem caused by wafer misalignment can be solved
by increasing the accuracy of the alignment during the wafer
assembly, which however, is extremely hard with present fabrication
technologies. This disclosure provides an alternative approach to
remedy the misalignment. Referring to FIG. 15, multiple magnetic
sensors are provided at different locations such as locations A, B,
C. These locations are determined by at least the alignment
accuracy during the wafer assembly. For example, when the alignment
accuracy of the wafer assembly is .DELTA.D (the accuracy along one
direction), the distance between adjacent locations such as between
A, B, and C can be .alpha..DELTA.D, wherein a is a constant,
preferably between 0.1 to 5, and more preferably, between 0.5 to 2,
and more preferably between 1 and 2, such as 1.5. In some examples,
the distance between the adjacent locations A, B, and C can also
include the factor of the magnetic field gradient of the magnetic
field from the magnetic source.
[0062] With the multiple magnetic sensors provided at the
pre-determined different locations, such as A, B, and C, the
problem caused by the misalignment during the wafer assembly can be
mitigated, if not eliminated. For example, the magnetic source
(146) is expected to be at location A and associated with magnetic
sensor 118. After the wafer assembling, the magnetic source may be
in the vicinity of position B or C due to assembling misalignment.
Magnetic sensor 118 in this situation may not be optimal for
detecting the magnetic field from the magnetic source 146. However,
magnetic sensor 182 (for position B) or 184 (for position C) can be
used for effectively detecting the magnetic field from the magnetic
source (146). In this way, the problem caused by misalignment
during wafer assembling can be mitigated or eliminated.
[0063] The magnetic sensors (118, 182, and 184) each have a
reference sensor and a signal sensor as discussed in the above
sections. Alternatively, the magnetic sensors each can be comprised
of a signal sensor, such as those discussed above with reference to
FIG. 8. The magnetic sensor can be a GMR (giant magneto-resistor),
a spin-valve, a MTI (magnetic tunnel junction), a Hall sensor, or
other types of magnetic sensors capable of sensing magnetic field,
or a combination thereof if necessary.
[0064] In another example, a reference sensor can be provided for
multiple signal sensors, as illustrated in FIG. 15. As shown in
FIG. 15, magnetic sensors 118, 150 and 184 are signal sensors
designated for detecting the magnetic field from the magnetic
source of the proof-mass. Reference sensor 152 is designated for
detecting the background magnetic signals. Detections from the
signal sensors 118, 150, and 184 all may use the detection from
reference sensor 152 as a reference signal for comparison. In this
example, multiple signals sensors can share a single reference
sensor for detection.
[0065] The reference sensor (152) can be placed at any suitable
locations within an area in which the associated signal sensors are
disposed. For example, reference sensor 152 can be placed in the
vicinity of signal sensor 118, 150, or 184. However, it is
preferred that the reference sensor (152) is at a location that is
as close to the geometric center of the associated signal sensors
as possible. Alternatively, the reference sensor (152) can be at a
location that is as close to a symmetric center to which the
associated signal sensors are substantially symmetric. By such
configuration, the reference sensor (152) is capable of providing a
substantially "uniform reference signal" to all associated signal
sensors. If the reference sensor (152) is disposed too far away
such that the reference signal provided by such reference sensor
(152) may not be suitable for all signal sensors.
[0066] The MEMS gyroscope as discussed above can be fabricated on a
wafer level. For example, as illustrated in FIG. 16. Wafer 186 can
be of any size, such as 2 inch, 4 inch, 6 inch, 8 inch, 16 inch,
and it can be of any shape, such as circular with a notch,
rectangular, square. Wafer 186 is comprised of a material suitable
for fabricating the MEMS features, such as the proof-mass,
flextures, frames in the mass-substrate (e.g. mass-substrate 108
illustrated in FIG. 3). A plurality of dies, such as die 188, is
formed in wafer 186. The MEMS features in the mass-substrate are
formed in each die.
[0067] Wafer 190 is a magnetic sensor wafer with a plurality of
magnetic sensing mechanisms formed thereon. Wafer 190 can be of any
size and shape as desired as wafer 186. Wafer 190 is comprised of a
material that is suitable for forming the desired magnetic sensors.
Multiple dies are formed in wafer 190, such as die 192. And each
die comprises a desired magnetic sensing mechanism as discussed in
above sections.
[0068] The mass-wafer (186) and the magnetic sensor wafer 190 are
assembled into a wafer assembly as illustrated in FIG. 17 and FIG.
18. The mass wafer (186) and the magnetic sensor wafer 190 are
assembled into wafer assembly 194. The dies in the mass wafer (186)
and magnetic sensor wafer 190 are assembled into die assemblies,
such as die assembly 196. The wafer assembly may comprise other
features, such as saw lines for dicing. The wafer assembly (194)
can further comprise packages that are disposed through a wafer
level packaging, which will not be detailed herein. The wafer
assembly 194 is better illustrated in a side view, as illustrated
in FIG. 18.
[0069] Referring to FIG. 18, wafer assembly 194 comprises mass
wafer 186 and magnetic sensor wafer 190. The mass wafer (186)
comprises multiple dies, such as die 188 having MEMS features
formed thereon. The magnetic sensor wafer (190) comprises multiple
dies such as die 192 having a magnetic sensing mechanism formed
thereon. The dies 188 and 192 are aligned such that, at least the
magnetic source of the proof-mass is aligned to the associated
magnetic sensor. The dies 188 and 192 are assembled into a die
assembly 196. The die assembly can be separated from the wafer
assembly 194 so as to form a desired MEMS gyroscope device.
[0070] It will be appreciated by those of skilled in the art that a
new and useful MEMS gyroscope has been described herein. In view of
the many possible embodiments, however, it should be recognized
that the embodiments described herein with respect to the drawing
figures are meant to be illustrative only and should not be taken
as limiting the scope of what is claimed. Those of skill in the art
will recognize that the illustrated embodiments can be modified in
arrangement and detail. Therefore, the devices and methods as
described herein contemplate all such embodiments as may come
within the scope of the following claims and equivalents thereof.
In the claims, only elements denoted by the words "means for" are
intended to be interpreted as means plus function claims under 35
U.S.C. .sctn.112, the sixth paragraph.
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