U.S. patent application number 14/104424 was filed with the patent office on 2015-11-05 for baw gyroscope with bottom electrode.
This patent application is currently assigned to Analog Devices, Inc.. The applicant listed for this patent is Analog Devices, Inc.. Invention is credited to Firas N. Sammoura, Kuang L. Yang.
Application Number | 20150318190 14/104424 |
Document ID | / |
Family ID | 44913450 |
Filed Date | 2015-11-05 |
United States Patent
Application |
20150318190 |
Kind Code |
A1 |
Sammoura; Firas N. ; et
al. |
November 5, 2015 |
BAW Gyroscope with Bottom Electrode
Abstract
A bulk acoustic wave gyroscope has a primary member in a member
plane, and an electrode layer in an electrode plane spaced from the
member plane. The electrode layer has a first portion that is
electrically isolated from a second portion. The first portion,
however, is mechanically coupled with the second portion and faces
the primary member (e.g., to actuate or sense movement of the
primary member). For support, the second portion of the electrode
is directly coupled with structure in the member plane.
Inventors: |
Sammoura; Firas N.;
(Melrose, MA) ; Yang; Kuang L.; (Newton,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Analog Devices, Inc. |
Norwood |
MA |
US |
|
|
Assignee: |
Analog Devices, Inc.
Norwood
MA
|
Family ID: |
44913450 |
Appl. No.: |
14/104424 |
Filed: |
December 12, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12983476 |
Jan 3, 2011 |
8616056 |
|
|
14104424 |
|
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|
12940354 |
Nov 5, 2010 |
8631700 |
|
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12983476 |
|
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Current U.S.
Class: |
438/50 |
Current CPC
Class: |
H01L 21/4846 20130101;
H01L 21/84 20130101; Y10T 29/49002 20150115; G01C 19/5698
20130101 |
International
Class: |
H01L 21/48 20060101
H01L021/48; H01L 21/84 20060101 H01L021/84; G01C 19/5698 20060101
G01C019/5698 |
Claims
1. A method of forming a bulk acoustic wave gyroscope, the method
comprising: forming a primary member in a member plane; forming an
electrode layer in an electrode plane that is spaced from the
member plane; removing a portion of the electrode layer to form a
trench; and filling the trench with a non-conductive material to
produce a first portion of the electrode layer and a second portion
of the electrode layer, the non-conductive material mechanically
connecting the first portion of the electrode layer and the second
portion of the electrode layer, the nonconductive material
electrically isolating the first portion of the electrode layer
from the second portion of the electrode layer, the second portion
being separate from the primary member and stabilizing the
electrode layer.
2. The method as defined by claim 1, further comprising forming an
anchor through the second portion of the electrode layer and into
structure in the member plane.
3. The method as defined by claim 1, wherein forming the primary
member and forming the electrode layer comprises processing a
silicon-on-insulator wafer.
4. The method as defined by claim 1, further comprising releasing
the primary member to form a space between the primary member and
the electrode layer.
5. The method as defined by claim 4, wherein releasing comprises
timing the release to maintain insulator material between the
member plane and the electrode plane.
6. The method as defined by claim 1, further comprising: forming a
conductive path extending from the member plane and into the second
portion of the electrode layer.
7. The method as defined by claim 6, wherein forming the conductive
path comprises: forming a via.
8. The method as defined by claim 7, wherein the via includes doped
polysilicon.
9. The method as defined by claim 1, wherein the second portion of
the electrode layer is grounded.
10. The method as defined by claim 1, wherein the first portion of
the electrode layer is configured to actuate movement of the
primary member.
11. The method as defined by claim 10, wherein the first portion of
the electrode layer is configured to actuate a flexural mode of the
primary member.
12. The method as defined by claim 1, wherein the first portion of
the electrode layer is configured to sense movement of the primary
member.
13. The method as defined by claim 12, wherein the first portion of
the electrode layer is configured to sense a flexural mode of the
primary member.
14. The method as defined by claim 1, further comprising: forming a
side electrode in the member plane, the side electrode being
radially spaced from the primary member.
15. The method as defined by claim 14, wherein the side electrode
is configured to actuate movement of the primary member.
16. The method as defined by claim 15, wherein the side electrode
is configured to actuate a bulk mode of the primary member.
17. The method as defined by claim 14, wherein the side electrode
is configured to sense movement of the primary member.
18. The method as defined by claim 17, wherein the side electrode
is configured to sense a bulk mode of the primary member.
Description
PRIORITY
[0001] This patent application is a divisional of U.S. patent
application Ser. No. 12/983,476, filed Jan. 3, 2011, and entitled,
"BAW Gyroscope with Bottom Electrode," which is a
continuation-in-part of U.S. patent application Ser. No.
12/940,354, filed Nov. 5, 2010, and entitled, "RESONATING SENSOR
WITH MECHANICAL CONSTRAINTS;" each of these patent applications is
hereby incorporated herein by reference in its entirety.
TECHNICAL FIELD
[0002] The invention generally relates to bulk acoustic wave
sensors and, more particularly, the invention relates to electrodes
in bulk acoustic wave sensors.
BACKGROUND ART
[0003] Bulk acoustic wave ("BAW") gyroscope use has increased in
recent years. This trend is driven by their many benefits
including, among other things, their high gain factor, which causes
them to use less power than conventional gyroscopes. In addition,
such gyroscopes generally cost less to manufacture.
[0004] To those ends, many bulk acoustic wave gyroscopes known to
the inventors have a disk with a crystal lattice that, during
either or both an actuation or detection phase, vibrates/resonates
at a very high frequency, typically in the megahertz range. This is
in contrast to gyroscopes having a disk mechanically moving back
and forth about a substrate in both phases. When the crystal
lattice of the disk vibrates, the disk is considered to be
operating in a "bulk" mode.
SUMMARY OF THE INVENTION
[0005] In accordance with one embodiment of the invention, a bulk
acoustic wave gyroscope has a primary member in a member plane, and
an electrode layer in an electrode plane spaced from the member
plane. The electrode layer has a first portion that is electrically
isolated from a second portion. The first portion, however, is
mechanically coupled with the second portion and faces the primary
member (e.g., to actuate or sense movement of the primary member).
For support, the second portion of the electrode is directly
coupled with structure in the member plane.
[0006] The gyroscope may have conductive path extending from
structure in the member plane and into the second portion of the
electrode layer. Among other things, the conductive path may
include a via. Alternatively or in addition, an oxide may directly
couple the second portion of the electrode layer with the structure
in the member plane.
[0007] The second portion of the electrode layer may be grounded,
while the first portion may hold a potential. In some embodiments,
to isolate the two portions of the electrode layer, a trench
separates the first portion of the electrode layer from the second
portion of the electrode layer. This trench may be at least partly
filled with a dielectric material to provide the electrical
isolation and yet mechanically connect the portions. Moreover, the
gyroscope may have a side electrode, in the member plane, that is
radially spaced from the primary member.
[0008] A number of materials may form the layers. For example, the
primary member and electrode layer may be formed at least in part
from a silicon-on-insulator wafer. Moreover, in certain
embodiments, the primary member forms a disk that resonates in a
flexure mode response to receipt of an electrostatic signal from
the first portion of the electrode layer.
[0009] In accordance with another embodiment of the invention, a
bulk acoustic wave gyroscope has a resonating member in a member
plane, and an electrode layer in an electrode plane. The member and
electrode planes are spaced apart, and the resonating member is
formed at least in part from a first layer of a silicon on
insulator wafer. In a similar manner, the bottom electrode layer is
formed at least in part from a second layer of the same silicon on
insulator wafer. The bottom electrode layer has a first portion and
a second portion, where the first portion is electrically isolated
from, and mechanically secured to, the second portion. The first
portion faces the resonating member and is configured to
electrostatically actuate the resonating member when subjected to a
resonating voltage. The second portion of the electrode layer is
anchored to structure in the member plane.
[0010] In accordance with other embodiments of the invention, a
method of forming a bulk acoustic wave gyroscope forms a primary
member in a member plane, forms an electrode layer in an electrode
plane that is longitudinally spaced from the member plane, and
removes a portion of the electrode layer to form a trench. The
method also fills the trench with a non-conductive material to
produce a first portion of the electrode layer and a second portion
of the electrode layer. The non-conductive material mechanically
connects the first portion of the electrode layer and the second
portion of the electrode layer, while electrically isolating the
first portion from the second portion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Those skilled in the art should more fully appreciate
advantages of various embodiments of the invention from the
following "Description of Illustrative Embodiments," discussed with
reference to the drawings summarized immediately below.
[0012] FIG. 1 schematically shows a perspective view of a packaged
inertial sensor having a bulk acoustic wave gyroscope configured in
accordance with illustrative embodiments of the invention.
[0013] FIG. 2 schematically shows a perspective view of a bulk
acoustic wave gyroscope configured in accordance with illustrative
embodiments of the invention. This figure has a partial cutaway
view to show the vibrating disk. FIG. 2 is rotated 180 degrees from
FIG. 3, which is oriented appropriately for use of the terms "top"
and "bottom."
[0014] FIG. 3 schematically shows a cross-sectional view of the
bulk acoustic wave gyroscope of FIG. 2 along line 3-3.
[0015] FIG. 4 schematically shows a top perspective view of a
bottom electrode in the gyroscope of FIG. 2.
[0016] FIG. 5 schematically shows a perspective close-up view of a
portion of the gyroscope shown in FIG. 4 with a partial cut-away
portion.
[0017] FIG. 6 schematically shows a top perspective view of the
device layer, including the resonating/vibrating disk, in
accordance with illustrative embodiments of the invention.
[0018] FIG. 7 shows a process of forming a bulk acoustic wave
inertial sensor in accordance with illustrative embodiments of the
invention.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0019] In illustrative embodiments, a bulk acoustic wave gyroscope
has an electrode on a different plane than that of its resonating
primary member. In addition, that electrode has first and second
portions that are electrically isolated from and mechanically
coupled to each other. Details of illustrative embodiments are
discussed below.
[0020] FIG. 1 schematically shows a perspective view of a packaged
inertial sensor 10 having a bulk acoustic wave gyroscope 12 (FIG. 2
and others, discussed below) configured in accordance with
illustrative embodiments of the invention. This package protects
its interior gyroscope 12 from the environment. As shown, the
package has a top portion 14 that connects with a bottom portion 16
to form an interior (not shown) for containing the gyroscope 12.
Although not necessary, some embodiments of the invention
hermetically seal the package interior. Other embodiments of the
package, however, do not provide a hermetic seal.
[0021] The package can be any of a variety of different types, such
as, among other things, a pre-molded leadframe package, a substrate
package, or a ceramic package. The top portion 14 and/or the bottom
portion 16 can be planar or form a cavity. In either case, the top
and bottom portions 14 and 16 should appropriately couple to
protect the gyroscope 12. For example, if the top portion 14 is
flat, then the bottom portion 16 should have a cavity, or there
should be some spacing apparatus to form the interior with an
appropriate volume for containing the gyroscope 12.
[0022] In alternative embodiments, the package is a conventional
post-molded, plastic leadframe package. Specifically, as known by
those skilled in the art, this relatively inexpensive package type
molds plastic, in liquid form, directly around the gyroscope die
12. This packaging process therefore can damage the gyroscope 12 if
it is not properly sealed. In that case, the sensitive
microstructure within the gyroscope 12 preferably is hermetically
sealed or otherwise protected from the molding process.
[0023] It should be noted that although this discussion and
attendant figures describes specific gyroscope, other bulk
gyroscope designs also may incorporate principles of various
embodiments. This discussion of a specific bulk acoustic wave
gyroscope therefore is for illustrative purposes only and not
intended to limit all embodiments of the invention.
[0024] The packaged gyroscope 10 may be used in any number of
different applications. For example, it could be part of a larger
guidance system in an aircraft, or part of a satellite sensor in an
automobile that cooperates with a stabilization system to maintain
a smooth ride. The packaged gyroscope 10 thus has a plurality of
interfaces (not shown) for communicating with exterior
components.
[0025] To those ends, the packaged gyroscope 10 may have a
plurality of pins (not shown) on its bottom, top, and/or side
surfaces for making a mechanical and electrical connection with an
underlying system, such as a printed circuit board. Alternatively,
the package may have a plurality of pads (not shown) for surface
mounting the package to an underlying printed circuit board.
Conventional soldering techniques should suffice to make this
connection. The printed circuit board may have additional
components that interact with the device to both control the
gyroscope die 12, and receive output signals indicating rotational
acceleration of the overall system. For example, the printed
circuit board also may have one or more application-specific
integrated circuits (ASICs) and other circuit devices for
controlling operation.
[0026] FIG. 2 schematically shows a perspective view of a bulk
acoustic wave gyroscope 12 that may be configured in accordance
with illustrative embodiments of the invention. This figure also
has a partial cutaway view to show its vibrating disk 18, and an
outline of a member stabilizing a portion of that disk 18 (shown in
dashed lines). To further illustrate this embodiment, FIG. 3
schematically shows a rotated, cross-sectional view of the bulk
acoustic wave gyroscope of FIG. 2 along line 3-3.
[0027] Specifically, this description uses the terms "top,"
"bottom," and the like for descriptive purposes only. Those terms
are used with respect to the frame of reference of FIG. 3. FIG. 2,
however, is rotated 180 degrees (i.e., the top is down and the
bottom is up) to better show the components. Accordingly, for
elements identified as "top" elements in FIG. 3, the correct
orientation is on the bottom side of FIG. 2. For example, FIG. 2
shows a top substrate 40 near the top of the structure, while FIG.
3 shows that same top substrate 40 near the bottom of the
structure--because FIG. 3 is rotated 180 degrees from the frame of
reference figure.
[0028] This gyroscope 12 is a two dimensional gyroscope that
measures rotational movement about the X-axis and Y-axis shown in
FIG. 2. Accordingly, those skilled in the art refer to this type of
gyroscope as an X/Y gyroscope, or a two dimensional gyroscope. It
nevertheless should be reiterated that illustrative embodiments
apply to gyroscopes that measure rotation about its other axes,
such as the Z-axis alone, about the X-axis and Z-axis, or about all
three axes, among other things. Accordingly, discussion of this
specific two-dimensional bulk acoustic wave gyroscope 12 should not
limit various embodiments of the invention.
[0029] At its core, the bulk acoustic wave gyroscope 12 has a
generally planar disk 18 (noted above and also referred to as a
"primary member") that resonates in a flexure mode upon receipt of
an electrostatic actuation signal. In particular, during the
flexure mode, a bottom electrode 22 (discussed below) produces an
electrostatic force that causes portions of the disk 18 to vibrate
in and out of the plane of the disk 18. As a bulk acoustic wave
gyroscope, however, the crystal lattice of the disk 18 itself
vibrates in response to both a rotation and the continued actuation
by the noted electrostatic signal. This is in contrast to other
types of gyroscopes that have a shuttle/mass vibrating back and
forth above a substrate during both actuation and detection phases.
To that end, the embodiment shown in FIGS. 2 and 3 has the above
noted bottom electrode 22 for actuating/vibrating the disk 18 in a
flexure mode at a preselected frequency. As known by those skilled
in the art, this frequency can be quite high, such as on the order
of about 1-20 Megahertz.
[0030] The disk 18 is configured to vibrate in a predetermined
manner at the known vibration frequency. For example, the vibration
frequency may be the resonant frequency of the disk 18 itself. As
such, the disk 18 vibrates in and out of plane in a non-uniform
manner. Specifically, parts of the disk 18 may vibrate, while other
parts of the disk 18 may remain substantially stable; i.e., the
stable portions will vibrate at approximately zero Hertz. In other
words, the stable portions substantially do not vibrate at all. The
stable portions are known as "nodes 24" and preferably are located
generally symmetrically about the top and bottom faces of the disk
18. For example, when vibrating at the resonant frequency, the
bottom face of a 200 micron radius disk 18 may have a node 24 that
forms a general ellipse about the center of the disk 18. This
elliptical node 24 may take on the shape of a circle with a radius
of between about ten and fifteen microns.
[0031] Rotation about the X-axis or Y-axis causes the shape of the
disk 18 to change. To detect this change in shape, the gyroscope 12
has a plurality of side electrodes 26 generally circumscribing the
disk 18. For example, the cutaway of FIG. 2 shows four side
electrodes 26 that can detect this change. More specifically, the
side electrodes 26 form a variable capacitor with the side wall of
the disk 18. A change in the shape of the disk 18, in the bulk
mode, causes at least a portion of its side wall to change its
position, thus changing the distance between it and the side
electrode 26. This changes the variable capacitance measured by the
side electrode 26. It is this capacitance change that provides the
necessary movement information.
[0032] A plurality of pads 28 formed on the same layer as the
bottom electrode 22 electrically connect the bottom and top
electrodes 22 and 26 to other circuitry. Off-chip circuitry or
on-chip circuitry (not shown) thus detects the noted capacitance
change as a changing signal, which includes the necessary
information for identifying the degree and type of rotation. The
larger system then can take appropriate action, such as controlling
the rotation of tires in an automobile for stabilization control,
or changing the trajectory of a guided missile.
[0033] Naturally, the disk 18 should be supported to function most
effectively. To that end, the gyroscope 12 has a bottom substrate
30 mechanically bonded to the bottom of the disk 18. In
illustrative embodiments, the bottom substrate 30 is formed from a
single crystal silicon wafer and hermetically bonded to the layer
having the bottom electrode 22 and pads 28. For example, a ring of
seal glass 32, or glass frit, can hermetically seal this bottom
substrate 30 to the disk/electrode structure.
[0034] The bottom substrate 30 shown in FIGS. 2 and 3 also has a
bottom support portion 34 that mechanically connects to the bottom
face of the disk 18. In illustrative embodiments, the bottom
support portion 34 is connected directly to the node 24 on the
bottom face of the disk 18. As noted above, this node 24
substantially does not vibrate when the disk 18 as actuated at its
resonant frequency. The bottom support portion 34 can be formed
from any number of materials. For example, this structure can be a
solid piece of polysilicon, or a part of the layer forming the
bottom electrode 22 and seal glass 32. Alternatively, the bottom
support can be formed from the same material as the bottom
substrate 30--e.g., one or more pedestals formed from a timed etch
of the bottom substrate 30. In that case, the bottom support is
integral with the bottom substrate 30, and formed from the same
material as the bottom substrate 30 (e.g., single crystal
silicon).
[0035] Conventional micromachining processes may form the disk 18
and layer immediately beneath the disk 18 in any number of known
ways. For example, that portion of the gyroscope 12 may be formed
from a micromachined silicon-on-insulator wafer (also known as an
"SOT" wafer). In that case, the disk 18 may be formed from the top,
single crystal silicon layer of the SOI wafer (often referred to as
the "device layer" of the SOI wafer). Moreover, the side electrodes
26 may be formed from deposited polysilicon and electrically
connected with the bond pads 28, which may be formed from deposited
metal.
[0036] As known by those skilled in the art, the top SOI layer is
typically much thinner than the bottom, "handle," layer 36 of the
SOI wafer, which also is formed from single crystal silicon. The
layer having the bottom electrode 22 (referred to as the "bottom
layer 36" or "electrode layer 36"), however, is thinner than the
layer having the disk 18 (referred to as the "top layer 38").
Although not necessary, illustrative embodiments thin this bottom
layer 36 to reduce the profile of the overall sensor, and improve
the performance of the bottom electrode 22. For example, the disk
18 may have a thickness of about 50 microns, while the bottom
electrode 22 may have a thickness of about 40 microns.
[0037] The gyroscope 12 also has a top substrate 40 secured to the
top node region 24 of the disk. To that end, the top substrate 40
may be considered to have a top support portion 42 secured directly
to the node region 24 of the top surface of the disk. In a manner
similar to the bottom support portion 34, the top support portion
42 may be formed in any number of manners. For example, the top
support portion(s) 42 may be formed as an anchor having a
silicon-to-silicon bond with the disk 18. Moreover, the top support
portion(s) 42, which, like the bottom support(s) 34, may include a
number of separate members, illustratively symmetrically positioned
and spaced about the top surface of the disk.
[0038] The top substrate 40 also has an annular sealing region 44
that forms a seal with the bottom layer 36 of the disk/lower
electrode apparatus. In a manner similar to the bottom substrate
30, the top substrate 40 may not provide a hermetic seal. When both
substrates provide a hermetic seal, however, those skilled in the
art should expect the disk 18 to be fully protected by the chamber
formed by both of the substrates.
[0039] FIGS. 4-5 schematically show additional details of the
bottom electrode/bottom layer 36 of the gyroscope 12. Specifically,
FIG. 4 schematically shows a top perspective view of the bottom
electrode/bottom layer 36 of the gyroscope 12 shown in FIGS. 2 and
3, while FIG. 5 schematically shows a close up, partial
cross-sectional view of the gyroscope from the perspective of FIG.
4.
[0040] As shown in FIG. 4, the bottom electrode 22 may comprise
twelve separate electrodes (referred to as "actuating portions 48")
that cooperate to actuate the disk 18 in a flexure mode.
Specifically, the bottom electrode 22 shown in FIG. 4 has two sets
of six electrodes that each provide opposite force to the disk
18--one set pushes while the other pulls. Both sets alternate
(i.e., they are about 180 degrees out of phase) according to the
actuation frequency. A first pad controls one set of electrodes,
while a second pad controls the second set of electrodes. A pair of
generally circular, concentric metallic traces 46A connects the
bottom electrodes 22 in the desired manner. More particularly, each
of the two traces 46A electrically connects every other electrode
to form the two sets.
[0041] In accordance with illustrative embodiments of the
invention, the bottom electrode 22 is separated into a plurality of
different portions that together form a single, mechanically
connected (substantially integral) unit. These portions, however,
fall into two sets of portions; namely, actuating portions 48 for
actuating the disk, and stabilizing, grounded portions
("stabilizing portions 50") for stabilizing the entire bottom
electrode layer 22/36.
[0042] To those ends, as shown in greater detail in FIG. 5, the
bottom electrode 22 has a plurality of isolation trenches 52 that
separate the actuating portions 48 from the stabilizing portions
50. Specifically, the as discussed in greater detail below in FIG.
7, the isolation trenches 52 have nitride lined walls and
polysilicon between the nitride lined walls for mechanical
integrity and planarization. Accordingly, these nitride lined,
polysilicon filled trenches 52 provide the requisite electrical
isolation between the actuating portions 48 and the stabilizing
portions 50.
[0043] It should be noted, however, that those skilled in the art
can use other materials to perform the same electrical isolation
and structural function. For example, the trenches 52 can be lined
with another dielectric material and filled/planarized by an oxide
or multi-crystalline silicon or germanium. Accordingly, discussion
of nitride and polysilicon is not intended to limit various
embodiments the invention.
[0044] Each isolation trench 52 effectively forms a lead 54 that
electrically connects its local actuation portion 48 with a bond
pad 28. Specifically, as best shown in FIG. 4 and noted above, one
pad 28 controls a first set of actuation portions 48 while another
pad 28 controls a second set of actuation portions 48.
[0045] In a corresponding manner, the stabilizing portions 50 all
may be at a different potential (e.g., at a ground potential) than
those of the actuation portions 48. To that end, illustrative
embodiments form one or more vias 56 through the device wafer and
into the stabilizing portions. Among other things, the vias 56
include doped polysilicon. Accordingly, the vias 56 both
electrically contact the stabilizing portions 50 and mechanically,
directly connect/anchor the stabilizing portions 50 to the device
layer/wafer 38. Alternatively, or in addition, a layer of oxide
between the stabilizing portions 50 and device wafer 38 may provide
similar mechanical stability. In either case, the stabilizing
portions 50 may be considered to be directly coupled with structure
in the device layer plane (e.g., the interior face of the device
layer 38).
[0046] FIG. 6 shows the electrical connections between the side
electrodes 26 and the pads 28, as well as the top face of the disk
18. Unlike the schematic diagram of FIG. 2, this embodiment shows
twelve side electrodes 26. In a manner similar to the bottom
electrode, three concentric, circular conductive traces 46B
electrically connect various combinations of the side electrodes 26
to the pads 28.
[0047] Any number of different processes may form the gyroscope 12.
For example, much of the process discussed in parent patent
application Ser. No. 12/940,354, noted and incorporated above,
should suffice. As a second example, FIG. 7 shows a process of
forming a gyroscope in accordance with illustrative embodiments of
the invention. This process forms the gyroscope 12 primarily from a
SOI wafer. It nevertheless should be noted that other processes may
use other types of wafers, such as bulk silicon wafers.
Accordingly, discussion of SOI wafers is by example only.
[0048] It should be noted that for simplicity, this described
process is a significantly simplified version of an actual process
used to fabricate the gyroscope 12 discussed above. Accordingly,
those skilled in the art would understand that the process may have
additional steps and details not explicitly shown in FIG. 7.
Moreover, some of the steps may be performed in a different order
than that shown, or at substantially the same time. Those skilled
in the art should be capable of modifying the process to suit their
particular requirements.
[0049] The process begins at step 700, by providing an SOI wafer.
The next three steps, step 702, 704 and 706, which, as noted above,
may be performed either at the same time or in a different order,
form the microstructure making up the gyroscope 12. Specifically,
step 702 forms the disk 18 (also referred to as the "primary
member") and side electrodes 26 primarily on/from the top layer 38
of the SOI wafer, while step 706 forms the bottom electrode 22 from
the handle wafer (a/k/a the top layer 38 of the SOI wafer).
Illustrative embodiments form the bottom electrode 22 to be
self-aligning and thus, use a single mask. Moreover, step 704 thins
the handle wafer. Micromachining processes also form additional
microstructure.
[0050] In addition, step 702 also forms the vias 56 that connect
the stabilizing portions 50 of the electrode layer 22/36 to the
device wafer 38. To that end, among other ways, the process may
etch a square or round "donut" shaped trench through the device
wafer 38 using deep reactive ion etching techniques. Next, the
process may deposit polysilicon in the trench, and planarize the
polysilicon with an etch back step. Accordingly, the trench and
polysilicon forms a mechanically continuous boundary structure with
a central region filled with silicon.
[0051] The process then may etch a square or round channel/trench
through the central region of the boundary structure in the device
layer, again using deep reactive ion etching techniques. Next, the
process punches through the buried oxide layer of the SOI wafer and
etches into the stabilizing portions 50 of the bottom electrode
layer. Finally, the process may fill this new trench with
polysilicon, which, like the boundary structure, also may be
planarized with an etch back step.
[0052] After completing step 702, 704, and 706, or while completing
those steps, the process continues to step 708, which forms
trenches 52 around what will become the bottom electrode leads 54.
Again, deep reactive ion etching techniques may form these trenches
54. Step 710 then may fill the trenches 54; first with an isolation
material, such as nitride liner (i.e., a dielectric), and then with
polysilicon to provide the structural connection. Again,
conventional etch back techniques may planarize the polysilicon.
Accordingly, step 708 and 710 effectively form the two separate
portions/types of the electrode layer.
[0053] Step 712 then releases the primary member/disk 18, thus
removing the oxide between the actuating portions 48 of the
electrode layer 22/36 and the disk 18, and much of the rest of the
(buried) oxide layer of the SOI wafer. For example, the process may
release the disk 18 by immersing the structure in a bath of
hydrofluoric acid. This step also can remove some of the oxide
between the stabilizing portions 50 of the electrode layer 22/36
and the device layer 38. Embodiments using vias 56, however, should
provide structural integrity to anchor the stabilizing portions 50
in a sufficient manner. Alternative embodiments may use a timed
etch to maintain some of the oxide layer between the stabilizing
portions 50 and the device layer 38.
[0054] Next, step 714 deposits the metal to form, among other
things, the concentric metallic traces 46A and top surface of the
pads 28 on the electrode layer 22/36. Another deposited metal
portion formed by this step includes the traces 46B. Rather than
release (step 712) and then apply metal (step 714), other
embodiments apply metal before releasing (i.e., step 714 and then
step 712).
[0055] Finally, the process concludes at step 716 by bonding the
top and bottom substrates 40 and 30 to the microstructure formed by
the previous steps. Specifically, the top substrate 40 may be
formed from a single crystal silicon wafer having an etched cavity
that fits over the processed top layer 38 of the (former) SOI
wafer. This etched cavity forms the above noted annular sealing
region 44 of the top substrate 40, which forms a seal with the
bottom layer 36 of the disk/lower electrode apparatus. To that end,
conventional processes bond the annual seal region 44 to the bottom
layer 36 of the SOI wafer. For example, a glass frit 32 may provide
a hermetic or non-hermetic seal at that point.
[0056] In addition, the interior of the cavity bonds directly with
the support portion 42 extending from the disk 18. A glass frit
(not shown here) also may make this connection. Alternatively, some
of the embodiments that form the support portions directly from the
top substrate 40 (e.g., forming the top support portions 42 as
pedestals with a timed etch) may simply make a direct
silicon-to-silicon bond with the node(s) 24 on the top surface of
the disk 18.
[0057] The process may bond the bottom substrate 30 to the bottom
layer 36 of the processed SOI wafer in a similar manner. For
example, a glass frit 32 may couple the bottom substrate 30 about
the edges of the bottom layer 36 to provide a hermetic or
non-hermetic seal. In addition, a silicon-to-silicon bond, or other
bond as discussed above with the top substrate 40, also may secure
the bottom substrate 30 to the node(s) 24 and bottom support
portions 34 on the bottom surface of the disk 18.
[0058] The BAW gyroscope therefore provides a bottom electrode 22
with two electrically isolated but generally integrally connected
portions--the actuating portions 48 and grounded stabilizing
portions 50. Alternatively, some embodiments do not ground the
stabilizing portions 50. Vias 56 connecting to the stabilizing
portions 50 support the entire electrode layer 22, and electrically
permit control of the potential of the stabilizing portions 50.
[0059] It should be noted that some embodiments may actuate and
detect movement of the disk 18 in other modes than those discussed.
For example, some embodiments may actuate in a bulk mode.
Accordingly, discussion of the specific modes is for illustrative
purposes only.
[0060] Although the above discussion discloses various exemplary
embodiments of the invention, it should be apparent that those
skilled in the art can make various modifications that will achieve
some of the advantages of the invention without departing from the
true scope of the invention.
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