U.S. patent application number 14/190721 was filed with the patent office on 2014-09-18 for force feedback electrodes in mems accelerometer.
This patent application is currently assigned to Agency for Science Technology and Research (A*STAR). The applicant listed for this patent is Agency for Science Technology and Research (A*STAR), PGS Geophysical AS. Invention is credited to Sanchitha Nirodha Fernando, Ilker Ender Ocak, Chengliang Sun, Julius Ming-Lin Tsai.
Application Number | 20140260618 14/190721 |
Document ID | / |
Family ID | 50241205 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140260618 |
Kind Code |
A1 |
Ocak; Ilker Ender ; et
al. |
September 18, 2014 |
FORCE FEEDBACK ELECTRODES IN MEMS ACCELEROMETER
Abstract
A microelectromechanical system (MEMS) accelerometer having
separate sense and force-feedback electrodes is disclosed. The use
of separate electrodes may in some embodiments increase the dynamic
range of such devices. Other possible advantages include, for
example, better sensitivity, better noise suppression, and better
signal-to-noise ratio. In one embodiment, the accelerometer
includes three silicon wafers, fabricated with sensing electrodes
forming capacitors in a fully differential capacitive architecture,
and with separate force feedback electrodes forming capacitors for
force feedback. These electrodes may be isolated on a layer of
silicon dioxide. In some embodiments, the accelerometer also
includes silicon dioxide layers, piezoelectric structures, getter
layers, bonding pads, bonding spacers, and force feedback
electrodes, which may apply a restoring force to the proof mass
region. MEMS accelerometers with force-feedback electrodes may be
used in geophysical surveys, e.g., for seismic sensing or acoustic
positioning.
Inventors: |
Ocak; Ilker Ender;
(Singapore, SG) ; Sun; Chengliang; (Singapore,
SG) ; Tsai; Julius Ming-Lin; (San Jose, CA) ;
Fernando; Sanchitha Nirodha; (Singapore, SG) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Agency for Science Technology and Research (A*STAR)
PGS Geophysical AS |
Connexis
Oslo |
|
SG
NO |
|
|
Assignee: |
Agency for Science Technology and
Research (A*STAR)
Connexis
SG
PGS Geophysical AS
Olso
NO
|
Family ID: |
50241205 |
Appl. No.: |
14/190721 |
Filed: |
February 26, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61785851 |
Mar 14, 2013 |
|
|
|
61786259 |
Mar 14, 2013 |
|
|
|
Current U.S.
Class: |
73/514.39 |
Current CPC
Class: |
G01P 2015/0862 20130101;
G01P 2015/0882 20130101; G01V 1/18 20130101; G01P 15/125 20130101;
G01P 15/131 20130101; G01V 1/09 20130101; G01V 1/38 20130101; G01P
2015/0837 20130101 |
Class at
Publication: |
73/514.39 |
International
Class: |
G01P 15/125 20060101
G01P015/125 |
Claims
1. A method, comprising: detecting, by at least one sense capacitor
within an apparatus, an acceleration of a proof mass within the
apparatus; applying a feedback force to the proof mass via at least
one feedback capacitor, wherein the feedback force is based on the
acceleration of the proof mass; and determining an acceleration of
the apparatus based on the feedback force.
2. The method of claim 1, wherein the detecting the acceleration
includes detecting a change in capacitance of the at least one
sense capacitor.
3. The method of claim 2, wherein the detecting a change in the
capacitance includes: detecting a change in the capacitance of a
fully differential set of at least four sense capacitors.
4. The method of claim 1, wherein determining the acceleration
includes determining the Z-axis acceleration.
5. The method of claim 1, wherein determining the acceleration
includes using front-end readout circuitry connected to the at
least one feedback capacitor.
6. An apparatus, comprising: a MEMS accelerometer configured to
measure Z-axis acceleration, wherein the MEMS accelerometer
includes: a sense electrode configured to detect changes in
position of a proof mass in the MEMS accelerometer; and a force
feedback electrode configured to provide a restoring force to the
proof mass, wherein the restoring force is based on the detected
changes in the position of the proof mass.
7. The apparatus of claim 6, wherein the proof mass is adjacent to
at least two vacuum-sealed cavities.
8. The apparatus of claim 6, further comprising a plurality of
piezoelectric elements configured to dampen vibrations of the proof
mass.
9. The apparatus of claim 6, wherein the MEMS accelerometer
includes two sense electrodes arranged in a differential
orientation.
10. The apparatus of claim 6, wherein the MEMS accelerometer
includes four sense electrodes arranged in a fully differential
orientation.
11. The apparatus of claim 6, further comprising closed-loop
circuitry configured to measure the detected changes in the
position of the proof mass and configured to apply a voltage to the
force feedback electrode.
12. The apparatus of claim 11, wherein the apparatus is further
configured to output an acceleration value based on the voltage
applied to the force feedback electrode.
13. The apparatus of claim 12, wherein the acceleration value
output by the apparatus is the voltage applied to the force
feedback electrode.
14. The apparatus of claim 6, wherein the force feedback electrode
forms a capacitor with an electrode on the proof mass.
15. An apparatus, comprising: a central substrate region; a first
bonded substrate opposing a first surface of the central substrate
region; a second bonded substrate opposing a second surface of the
central substrate region; a first sense capacitor and a first force
feedback capacitor formed between the first bonded substrate and
the central substrate region; and a second sense capacitor and a
second force feedback capacitor formed between the second bonded
substrate and the central substrate region.
16. The apparatus of claim 15, wherein the central substrate region
further includes: a proof mass region bounded by a first spring
structure, a second spring structure, a first protection structure,
and a second protection structure.
17. The apparatus of claim 16, further comprising: a vacuum-sealed
cavity bounded in part by the first and second bonded substrates,
the first and second protection structures, a third protection
structure, and a fourth protection structure.
18. The apparatus of claim 17, wherein the first and second
vacuum-sealed cavities are disposed laterally on either side of the
proof mass region, and wherein the first and second bonded
substrates are disposed vertically on either side of the central
substrate region.
19. The apparatus of claim 17, wherein the first, second, third,
and fourth protection structures include silicon dioxide.
20. The apparatus of claim 16, further comprising feedback
circuitry configured to apply a continuous restoring force to the
proof mass region based on measured values of the first and second
sense capacitors.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application No. 61/785,851, filed Mar. 14, 2013, and U.S.
Provisional Application No. 61/786,259, filed Mar. 14, 2013, which
are incorporated by reference herein in their entireties.
BACKGROUND
[0002] Microelectromechanical system (MEMS) accelerometers are
widely used in many different application areas such as geophysical
surveying, underwater imaging, navigation, medical, automotive,
aerospace, military, tremor sensing, consumer electronics, etc.
These sensors typically detect acceleration by measuring the change
in position of a proof mass, for example, by a change in the
associated capacitance. Traditional capacitive MEMS accelerometers
may have poor performance due to low noise suppression and
sensitivity, however.
[0003] Measurement noise and range may vary for different
applications of sensors. For example, for a navigation application,
a measurement range of .+-.20 g may be desired and 1 .mu.g/ Hz
measurement noise for this range could be tolerated. As another
example, a tremor sensing application may desire a .+-.1 g
measurement range and a lower noise floor of .about.10-100 ng/ Hz.
One type of noise affecting this noise floor is Brownian noise.
Brownian noise refers to noise produced by Brownian motion.
Brownian motion refers the random movement of particles suspended
in a liquid or gas resulting from their bombardment by the
fast-moving atoms or molecules in the liquid or gas.
[0004] Accelerometers may have many uses in the field of
geophysical surveying, particularly marine seismic. For example, in
some marine seismic embodiments, a survey vessel may tow one or
more streamers in a body of water. Seismic sources may be actuated
to cause seismic energy to travel through the water and into the
seafloor. The seismic energy may reflect off of the various
undersea strata and be detected via sensors on the streamers, and
the locations of geophysical formations (e.g., hydrocarbons) may be
inferred from these reflections.
[0005] These streamer sensors that are configured to receive the
seismic energy may include accelerometers such as those described
in this disclosure. (Various other sensors may also be included in
some embodiments, such as pressure sensors, electromagnetic
sensors, etc.)
[0006] Additionally, accelerometers may be used to detect the
relative positions of the streamers (or portions thereof) via
acoustic ranging. Acoustic ranging devices typically may include an
ultrasonic transmitter and electronic circuitry configured to cause
the transceiver to emit pulses of acoustic energy. The travel time
of the acoustic energy between a transmitter and receivers (e.g.,
accelerometers) disposed at a selected positions on the streamers
is related to the distance between the transmitter and the
receivers (as well as the acoustic velocity of the water), and so
the distances may be inferred.
[0007] In other marine seismic embodiments, accelerometers
according to this disclosure may also be used in permanent
reservoir monitoring (PRM) applications, for example at a seafloor.
Generally, the term "geophysical survey apparatus" may refer to
streamers, PRM equipment, and/or sensors that form portions of
streamers or PRM equipment.
[0008] Accordingly, improvements in accelerometer technology (e.g.,
allowing better performance and/or lower cost) may provide
substantial benefits in the geophysical surveying field, among
other fields.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIGS. 1A-C are block diagrams illustrating embodiments of a
device;
[0010] FIG. 2 is a block diagram illustrating one embodiment of a
MEMS accelerometer;
[0011] FIGS. 3A-C illustrate an exemplary process flow for the
fabrication of a cap substrate;
[0012] FIGS. 4A-G illustrate an exemplary process flow for the
fabrication of a fully differential MEMS accelerometer;
[0013] FIGS. 5A-E illustrate an exemplary process flow for the
etching of cavities within a substrate; and
[0014] FIGS. 6-7 illustrate methods for the use of an accelerometer
in a geophysical survey according to this disclosure.
DETAILED DESCRIPTION
[0015] This specification includes references to "one embodiment"
or "an embodiment." The appearances of the phrases "in one
embodiment" or "in an embodiment" do not necessarily refer to the
same embodiment. Particular features, structures, or
characteristics may be combined in any suitable manner consistent
with this disclosure.
[0016] The following paragraphs provide definitions and/or context
for terms found in this disclosure (including the appended
claims):
[0017] "Comprising." This term is open-ended. As used herein, this
term does not foreclose additional structure or steps. Consider a
claim that recites: "a system comprising a processor and a memory .
. . ." Such a claim does not foreclose the system from including
additional components such as interface circuitry, a graphics
processing unit (GPU), etc.
[0018] "Configured To" or "Operable To." Various units, circuits,
or other components may be described or claimed as "configured to"
perform a task or tasks. In such contexts, "configured to" is used
to connote structure by indicating that the
units/circuits/components include structure (e.g., circuitry) that
performs those task or tasks during operation. As such, the
unit/circuit/component can be said to be configured to perform the
task even when the specified unit/circuit/component is not
currently operational (e.g., is not on). The
units/circuits/components used with the "configured to" language
include hardware--for example, circuits, memory storing program
instructions executable to implement the operation(s), etc.
Reciting that a unit/circuit/component is "configured to" perform
one or more tasks is expressly intended not to invoke 35 U.S.C.
.sctn.112, sixth paragraph, for that unit/circuit/component.
Additionally, "configured to" can include generic structure (e.g.,
generic circuitry) that is manipulated by software and/or firmware
(e.g., an FPGA or a general-purpose processor executing software)
to operate in manner that is capable of performing the task(s) at
issue.
[0019] "First," "Second," etc. As used herein, these terms are used
as labels for nouns that they precede unless otherwise noted, and
do not imply any type of ordering (e.g., spatial, temporal,
logical, etc.). For example, a "first" computing system and a
"second" computing system can be used to refer to any two computing
systems. In other words, "first" and "second" are descriptors.
[0020] "Based On" or "Based Upon." As used herein, these terms are
used to describe one or more factors that affect a determination.
These terms do not foreclose additional factors that may affect a
determination. That is, a determination may be solely based on the
factor(s) stated or may be based on one or more factors in addition
to the factor(s) stated. Consider the phrase "determining A based
on B." While B may be a factor that affects the determination of A,
such a phrase does not foreclose the determination of A from also
being based on C. In other instances, however, A may be determined
based solely on B.
[0021] FIGS. 1A-1C show block diagrams illustrating some exemplary
embodiments of a device according to this disclosure (devices 100,
102, and 104, respectively), which differ in their capacitive
architecture. These devices include upper substrate 110, interior
substrate 130, and lower substrate 150. In various embodiments,
substrates 110, 130, and 150 contain wafers 110a, 130a and 130f
(e.g., regions of the wafer on substrate 130), and 150a
respectively. In various embodiments, these wafers may be silicon
wafers. As used herein, "wafer" is used broadly to refer to any
substrate used for fabricating microelectromechanical system (MEMS)
devices. As will be recognized by one skilled in the art with the
benefit of this disclosure, "depositing" material on a substrate
may occur according to various methods common in the MEMS device
field. In some embodiments, this deposition method is performed as
described below with reference to FIGS. 3 and 4. As depicted,
interior substrate 130 is split into three portions, proof mass
130a and anchor regions 130f. In the illustrated embodiment, these
portions are separated by cavities 130g. Proof mass 130a may also
be referred to as a proof mass region. Cavities 130g may be etched
by various methods recognized by one skilled in the art with the
benefit of this disclosure, including one embodiment described
below with reference to FIG. 5. As is typically the case in
accelerometer embodiments, proof mass 130a undergoes a change in
position when the device experiences an acceleration. By measuring
the position of proof mass 130a, the magnitude and direction of the
acceleration may be determined.
[0022] In the embodiment shown in FIGS. 1A-C, upper substrate 110
is bonded to interior substrate 130, and lower substrate 150 is
also bonded to interior substrate 130. Bonding may occur using any
suitable method known in the art. In various embodiments, bonding
between substrates 110 and 130 and between 150 and 130 occurs using
any suitable bonding method. In one embodiment, cavity 120 between
upper substrate 110 and interior substrate 130 and cavity 140
between lower substrate 150 and interior substrate 130 are
vacuum-sealed. Cavities 130g may also be vacuum-sealed. In some
embodiments, cavities 120 and 140 may be vacuum-sealed in part by
bonding substrates 110, 130, and 150 together in a vacuum
environment. Substrate 130 may have portions etched away such that
vacuum-sealed cavities 130g, cavity 120, and cavity 140 may be in
fluid communication with each other (e.g., they may possess a
common vacuum).
[0023] Turning now to FIG. 1A, a six-capacitor embodiment is shown
as device 100. Substrates 110, 130, and 150 are divided into two
parts: the wafers of each substrate (110a; 130a and 130f together;
and 150a, respectively), and sets of electrodes (these are shown as
elements 110b, 110p, and 110c; 130b, 130p, and 130c; 130d, 130q,
and 130e; 150b, 150q, and 150c). These electrodes are typically
deposited on an insulator layer (not shown) such as silicon
dioxide, silicon nitride, etc. in order to electrically isolate
them from one another, and they are configured to form the plates
of respective capacitors. Two sets of electrodes are
deposited/situated/disposed on interior substrate 130: the first
set being on the upper surface, forming electrodes 130b, 130p, and
130c; and the second set being on the lower surface, forming
electrodes 130d, 130q, and 130e. Said differently, the sets of
electrodes on the interior substrate are deposited on the top and
bottom of the interior substrate, or on opposite sides of the
interior substrate. (Note that the phrase "opposite sides" of a
structure such as a substrate is not limited to the top and bottom
of a structure; instead, the phrase may be used to variously refer
to the left and right sides of a structure, or the front and back
sides of a structure. Of course, the characterization of different
portions of a structure as top, bottom, left, right, front, and
back depends on a particular vantage point.)
[0024] In one embodiment, a set of electrodes is deposited on the
lower surface of upper substrate 110, forming electrodes 110b,
110p, and 110c. A set of electrodes is also deposited on the upper
surface of lower substrate 150, forming electrodes 150b, 150q, and
150c. Both of these sets of electrodes on upper substrate 110 and
lower substrate 150 may be referred to as a set of electrodes
deposited, situated, or disposed on a surface opposing a surface of
interior substrate 130 (i.e., the respective upper and lower
surfaces of interior substrate 130). In some embodiments, the sets
of electrodes may be deposited as metallic layers on each
substrate.
[0025] In the embodiment shown in FIG. 1A, the set of electrodes on
the upper substrate 110 and the opposing set of electrodes on the
upper surface of interior substrate 130 are configured to form
three capacitors. (E.g., electrodes 110b and 130b are configured to
form one capacitor; electrodes 110p and 130p a second capacitor;
and electrodes 110c and 130c, the third capacitor.) Similarly, the
set of electrodes on lower substrate 150 and the opposing set of
electrodes on the lower surface of interior substrate 130 are
configured to form three capacitors.
[0026] As used herein, "opposing" surfaces are those that face each
other. "Opposing" surfaces may be on the same substrate or on
different substrates. For example, electrodes 130b and 130d are on
opposing surfaces of substrate 130; electrodes 110b and 130b are on
opposing surfaces of different substrates. As shown, in this
embodiment the electrodes on opposing surfaces of different
substrates may be formed such that they are in corresponding
positions. This arrangement allows each pair of electrodes (e.g.,
110b and 130b) to act as plates of a capacitor. As used herein, the
term "deposited" refers to any fabrication technique in which a
type of material is placed on at least a portion of an underlying
material or layer.
[0027] The term "layer" is to be construed according to its
ordinary usage in the art, and may refer to a material that covers
an entire portion of one or more underlying materials, as well as
discrete regions situated on top of the underlying material(s).
Accordingly, a "layer" may be used to refer, for example, to the
set of electrodes 130b, 130p, and 130c depicted in FIG. 1A, which
may result from a continuous deposition of material that is
deposited and then partially etched away. In some embodiments--for
example as described below with reference to FIG. 4--a certain
layer may fall "below" another layer that was deposited first
because the first deposited layer is not continuous. For example, a
deposition of a piezoelectric material may be processed such that
the layer contains discrete portions. Accordingly, when another
spring layer is deposited, some portions of the spring layer may
fall "below" the piezoelectric layer since it is not continuous.
Thus, portions of the spring layer may appear to be at the same
vertical level as the piezoelectric layer. Accordingly, in some
instances, the term "layer" refers to the order of deposition, and
not necessarily the vertical position (e.g., height) of materials
in reference to one another.
[0028] Continuing with the discussion of FIG. 1A, the capacitors
formed by electrodes 110b and 130b, 110c and 130c, 130d and 150b,
and 130e and 150c may all be used as sense capacitors. What is
meant by this is that they are operable to detect the movement of
proof mass 130a by using a system configured to detect changes in
the capacitances. Because sets of electrodes deposited on the
substrates are used for sensing the acceleration in device 100,
these electrodes may be referred to as sensing electrodes or sense
electrodes. The system detecting the changes in the capacitances
may be any system that is configured to use the capacitances--for
example, a closed-loop readout circuit. In other embodiments, along
with vacuum packaging and piezoelectric damping, this capacitive
architecture may be used in closed-loop accelerometer systems, as
well as any other resonating MEMS structure. Together, the four
capacitors may form a fully differential architecture. In one
instance, as proof mass 130a is displaced along the Z-axis 155 by
an applied acceleration, two of the capacitors are increasing in
capacitance, while the other two are decreasing equally. The
differences in capacitances in each capacitor, as measured by any
system configured to use capacitances, indicate the position of
proof mass 130a. In certain embodiments, with proper full bridge
connection of these four capacitors, the architecture of device 100
may avoid some of the disadvantages in traditional capacitive
sensing architectures--for example, less sensitivity, low noise
suppression, and low SNR. These disadvantages may arise in part
from Brownian noise.
[0029] In other embodiments, however, a simpler capacitive
architecture may be used. For example, two sense capacitors may be
used instead of four. In this embodiment, the capacitors may be
arranged such as proof mass 130a is displaced along the Z-axis by
an applied acceleration, one capacitor is increasing in
capacitance, while the other is decreasing. This is known as a
"differential" architecture, in that the difference between
capacitances is the figure of merit.
[0030] In yet other embodiments, a single sense capacitor may be
used. In that embodiment, as proof mass 130a is displaced along the
Z-axis in one direction by an applied acceleration, the capacitance
increases; as proof mass 130a is displaced along the Z-axis in the
other direction, the capacitance decreases. This embodiment may be
referred to as a "single-ended" capacitive architecture.
[0031] A differential architecture typically provides a higher S/N
ratio than a single-ended architecture, and a fully differential
architecture typically provides even further S/N improvement. The
differential and single-ended designs may be used in accordance
with the present disclosure, however.
[0032] Continuing with the discussion of FIG. 1A, the capacitors
formed by electrodes 110p and 130p, and 130q and 150q, respectively
may be used as force feedback capacitors. In some embodiments of
known devices, a capacitor may be used as both a sense capacitor
and a force feedback capacitor, e.g. with the use of switching
electronics. In such embodiments, only a portion (typically 50%) of
the capacitor's duty cycle is allocated to each task, which may
limit the maximum feedback force that can be applied.
[0033] According to the present disclosure, however, the use of
separate force feedback electrodes may provide for continuous
feedback, which may substantially increase the dynamic range
compared to designs that switch the function of a capacitor
according to a duty cycle. Further, the design of a device
according to this disclosure may further be simplified through the
omission of switching circuitry.
[0034] In the embodiment shown in FIG. 1A, device 100 is configured
to perform in a fully differential capacitive architecture. As
shown, the four outer capacitors (i.e., those formed by electrodes
110b and 130b, 110c and 130c, 130d and 150b, and 130e and 150c,
respectively) act as sense capacitors, and the two inner capacitors
(i.e., those formed by electrodes 110p and 130p, and 130q and 150q,
respectively) act as force feedback capacitors. In other
embodiments, the various capacitors may take on other roles. For
example, the outer capacitors may form four force feedback
capacitors, and the inner capacitors may form sense capacitors. It
may be advantageous in some embodiments for the force feedback
capacitor(s) to be symmetric with respect to the center of mass of
proof mass 130a. This feature may allow the force feedback
capacitors to provide a linear restoring force without providing a
torque.
[0035] The fully differential capacitive architecture embodiment
depicted in FIG. 1A may allow the differences between capacitors
(e.g., voltage, current, or capacitance) to be measured by another
circuit (not shown). In some embodiments, a fully differential
capacitive architecture may allow the capacitors to be connected
using a full bridge connection or a Wheatstone bridge connection.
In another embodiment, the fully differential capacitive
architecture may be connected to differential readout circuitry,
for example, using a differential operational amplifier. In some
embodiments, these configurations may avoid the disadvantages of a
low signal-to-noise ratio found in traditional MEMS
accelerometers.
[0036] In addition, the architecture shown in FIG. 1A allows
measurement of acceleration along an axis that perpendicularly
intersects substrates 110, 130, and 150 (referred to as the
"Z-axis" herein). Because proof mass 130a is separated from anchor
regions 130f by cavities 130g, anchor regions 130f act as an
anchor/stabilizer when proof mass 130a moves upwards and downwards
along Z-axis 155. This movement leads to slight variations in the
position of proof mass 130a, which leads to slight changes in the
capacitance of the capacitors arranged in the fully differential
architecture. This change in capacitance allows the capacitors to
detect a change in the position of proof mass 130a. The fully
differential capacitive architecture shown in FIG. 1 thus allows a
Z-axis acceleration to be measured. In another embodiment, device
100 may contain additional electrodes or capacitors situated
surrounding interior substrate 130. With additional structural
modifications, known to one skilled in the art, these additional
electrodes or capacitors allow measurement of the acceleration of
proof mass 130a as it moves side-to-side (i.e., to the left or
right of interior substrate 130) or front-to-back (i.e., into and
out of sheet 1). In such an embodiment, device also includes
lateral accelerometer capabilities. Accordingly, in one embodiment,
device 100 may measure acceleration along Z-axis 155, as well as in
an X-Y plane perpendicular to Z-axis 155 (i.e., a plane parallel to
substrate 130). This allows an acceleration to be measured or
detected in three dimensions.
[0037] Force feedback electrodes as shown in FIG. 1A may be used to
maintain proof mass 130a relatively close to its equilibrium, or
rest, position. By design, proof mass 130a tends to deviate from
its equilibrium position as device 100 undergoes acceleration;
however by maintaining proof mass 130a relatively close to the
equilibrium position, the system may be maintained in an
approximately linear region of operation. Compared to an embodiment
that does not use force feedback, this may allow for an increased
range of accelerations measurable by device 100. In some
embodiments, the output of device 100 may be based on the amount of
force necessary to maintain proof mass 130a at or near its
equilibrium position, because this amount is directly related to
the acceleration being experienced by device 100.
[0038] In other embodiments, electrodes may be formed such that
only one sense capacitor and only one force feedback capacitor are
used. In yet other embodiments, various numbers of sense capacitors
and various numbers of force feedback capacitors may be used.
[0039] As noted above, Brownian noise is an important consideration
in the design of devices such as device 100. The Brownian noise
associated with a sensor such as a MEMS accelerometer may be
represented by the following equation:
Noise.sub.MEMS= 4k.sub.BTb/M
In this equation, k.sub.B is Boltzmann's constant
(1.381.times.10.sup.-23 J/K), T represents the ambient temperature
in K, b represents the damping coefficient in N s/m, and M
represents the mass of the resonating structure. As can be seen by
this equation, the Brownian noise of the system can be decreased by
increasing the mass and decreasing the air damping of the system.
By designing a huge mass for the accelerometer, thermal noise can
be decreased down to on the order of hundreds of ng/ Hz levels, but
practically, MEMS devices are not typically designed with such
large sensor dimensions.
[0040] A high vacuum level may be used to decrease the Brownian
noise by reducing the quantity of random interactions of air
molecules with the sensor. Accordingly, the use of a vacuum may in
some embodiments reduce the noise floor of the system to ng/ Hz
levels. Thus in some embodiments, the use of a vacuum-sealed
cavity, for example 120, 130g, and 140, may reduce the Brownian
noise inside device 100.
[0041] The use of a vacuum may, however, in some embodiments,
increase the resonant quality factor of the system greatly. In some
embodiments, the quality factor may increase to levels over 10,000.
Such a high quality factor may contribute to undesirable
instabilities in the operation of device 100. In some embodiments,
piezoelectric damping may be used to at least partially counteract
the effect of the high vacuum level. Piezoelectric damping
transforms the kinetic oscillation energy of an accelerometer to
electrical energy that may be dissipated outside the system, for
example, by connecting the piezoelectric structures to a tunable
external load (e.g., a tunable resistive load). Thus the quality
factor may be decreased to manageable levels.
[0042] Besides the effects of Brownian noise on measurement noise
and measurement range, non-linearities may affect the performance
of MEMS devices. As one skilled in the art with the benefit of this
disclosure will recognize, non-linearity of a MEMS device may be
affected by frequency response, sensing architecture, springs,
and/or the readout circuit. In particular, an accelerometer may
have a region of approximate linearity while the proof mass is near
its rest or equilibrium position. However, the farther the proof
mass travels from its rest position, the readout may depart from
the ideal linear response.
[0043] As noted above, the non-linearities in device 100 may in
some embodiments be reduced by using a closed-loop readout circuit,
which may be used to stabilize proof mass 130a within a MEMS
accelerometer to the region of its equilibrium position via the use
of force feedback electrode(s). For example, a closed loop
.SIGMA.-.DELTA. circuit may be used.
[0044] In certain embodiments, a closed-loop readout circuit
includes the sensing capacitors, as well as one or more force
feedback electrodes. With these elements connected in a closed
loop, the accelerometer may adjust the position of the proof mass
to maintain linear operation, using the acceleration detected by
the capacitors and a force applied by the force feedback
electrodes. Thus, using a closed-loop circuit architecture with a
MEMS accelerometer may avoid some of the disadvantages of such
non-linearities.
[0045] Turning now to FIG. 1B, device 102 is shown. Device 102 is
broadly similar to device 100, discussed above (and with
corresponding reference numerals), but it has a different
capacitive architecture. FIG. 1B depicts a "differential," rather
than a "fully differential" capacitive architecture. What is meant
by this is that only two sense capacitors are used, instead of
four.
[0046] In the embodiment depicted as device 102, for example, the
capacitors formed by electrodes 110c and 130c, and 130e and 150c,
respectively, may be used as sense capacitors. The capacitors
formed by electrodes 110p and 130p, and 130q and 150q,
respectively, may be used as force feedback capacitors. In other
embodiments, these roles may be changed; however, it may in some
embodiments be advantageous for the force feedback capacitors to be
symmetric with respect to the center of mass of proof mass 130a, as
discussed above.
[0047] Turning now to FIG. 1C, another related device, device 104,
is shown. Device 104 is broadly similar to device 102, discussed
above (and with corresponding reference numerals), but the
capacitors are arranged differently.
[0048] In this embodiment, four capacitors are disposed
symmetrically on device 104. These may be used, in various
embodiments, as either sense or force feedback capacitors. For
example, the capacitors formed by electrodes 110p and 130p, and
130e and 150c, respectively, may be used as sense capacitors. The
capacitors formed by electrodes 110c and 130c, and 130q and 150q,
respectively, may be used as force feedback capacitors. In other
embodiments, these roles may be changed; however, it may in some
embodiments be advantageous for the force feedback capacitors to be
symmetric with respect to the center of mass of proof mass 130a, as
discussed above.
[0049] Turning now to FIG. 2, a block diagram illustrating one
embodiment of a MEMS accelerometer 200 is shown. As depicted,
accelerometer 200 includes upper substrate 210, interior substrate
230, and lower substrate 250. In various embodiments, substrates
210, 230, and 250 contain wafers 210a, 230a and 230f together, and
250a respectively, all of which are similarly numbered to FIGS.
1A-1C, and may be configured as described above with reference to
those figures. Additionally, in the embodiment shown, the wafer of
interior substrate 230 is split into three portions, proof mass
230a and anchor regions 230f. In the illustrated embodiment, these
portions are separated by cavities 230g and bounded by protection
structures 230h. In one embodiment, protection structures 230h may
be silicon dioxide. In this embodiment, cavity 220 between upper
substrate 210 and interior substrate 230, cavity 240 between lower
substrate 250 and interior substrate 230, and cavities 230g are
vacuum-sealed. By vacuum-sealing, or vacuum-packaging, these
cavities, certain embodiments of accelerometer 200 may avoid some
of the disadvantages of Brownian noise discussed above.
[0050] Interior substrate 230 may include several parts: the
silicon wafer, composed of proof mass 230a and anchor regions 230f;
cavities 230g (which may in some embodiments become vacuum-sealed
cavities during processing), bounded by protection structures 230h;
sets of electrodes 230b and 230c; spring layers 230d and 230e;
piezoelectric structures 230j; and pairs of electrodes 230k
situated on piezoelectric structures 230j. In one embodiment,
substrates 210 and 250 are divided into four parts: the wafers of
each substrate, 210a and 250a respectively; sets of electrodes 210b
and 250b respectively; oxide layers, 210c and 250c respectively;
and getter layers 210d and 250d. In this embodiment, the central
ones of electrodes 210b, 230b, 230c, and 250b may be used as
separate force feedback electrodes. The other ones of those
electrodes may be used as sensing electrodes.
[0051] In the embodiment shown in FIG. 2, upper substrate 210 is
bonded to interior substrate 230, and lower substrate 250 is bonded
to interior substrate 230 as well. In various embodiments of FIG.
2, bonding between substrates 210 and 230 and between 250 and 230
occurs using any suitable bonding technique. As depicted,
substrates 210, 230, and 250 are bonded to each other using bonding
structures 260. Bonding structures 260 may be composed of any
material known to one skilled in the art that may suitably vacuum
seal cavities 220 and 240. In one embodiment, bonding structures
260 may be composed of silicon dioxide; in another, a metallic
material or composition such as copper and tin. In other
embodiments, bonding structures 260 may be composed of metallic
compositions such as gold and tin, or aluminum and germanium.
Alternately, bonding structures 260 may be composed of both silicon
dioxide and metallic contacts. Cavities 220 and 240 may be
vacuum-sealed in part by bonding structures 260. Substrates 210,
230, and 250 may also assist in vacuum-sealing cavities 220 and
240. In some embodiments, cavities 220 and 240 may be vacuum-sealed
in part by bonding substrates 210, 230, and 250 together. Spring
layers 230d and 230e may have portions etched away such that
vacuum-sealed cavities 230g, 220, and 240 may be in fluid
communication with each other (e.g., they may possess a common
vacuum). Thus, this vacuum-sealed cavity may be bounded in part by
upper substrate 210, lower substrate 250, and protection structures
230f. Bonding structures 260 and substrate 230 may also bound in
part the common vacuum throughout cavities 220, 230g, and 240.
[0052] In one embodiment, spring layers 230d and 230e are grown on
opposing surfaces of interior substrate 230. In addition, as used
herein, the term "grown" refers to any fabrication technique in
which a type of material is formed on at least a portion of an
underlying material or layer. This may be accomplished, for
example, by heating that material or layer to high temperatures, by
wet oxidation, etc. For example, heating a silicon substrate to
high temperatures may create bonds with oxygen atoms in the air so
that silicon dioxide is formed. Thus an insulating silicon dioxide
layer may be formed using thermal oxidation of silicon. Spring
layers 230d and 230e may be composed of an oxide such as silicon
dioxide. Spring layers 230d and 230e allow proof mass 230a to vary
in position within interior substrate 230, with anchor regions 230f
assisting by adding stability to interior substrate 230.
Vacuum-sealed cavities 230g may assist in avoiding noise (e.g.,
Brownian noise) caused by the impingement of gas particles on proof
mass 230a. Oxide layers 210c and 250c are grown, or disposed, on
the lower surface of upper substrate 210 and the upper surface of
lower substrate 250, respectively. Oxide layers 210c and 250c may
be composed of silicon dioxide. Getter layers 210d and 250d, which
assist in maintaining the common vacuum of vacuum-sealed cavities
220, 240, and 230g, are deposited on oxide layers 210c and 250c. In
some embodiments, getter layers 210d and 250d may be deposited on
any portion of substrates 210, 230, and 250 exposed to the
vacuum-sealed cavity. In one embodiment, a single getter layer may
exist within accelerometer 200, deposited on some portion of
substrates 210, 230, and/or 250. Getter layers 210d and 250d may be
composed of any suitable material known to those skilled in the
art, and may assist, in some embodiments, in avoiding some of the
disadvantages of Brownian noise within vacuum-sealed cavities 220,
230g, and 240.
[0053] As shown, two sets of electrodes 230b and 230c may be
deposited on spring layers 230d and 230e--the first on the upper
surface of interior substrate 230; and the second, on the lower
surface. Said differently, sets of electrodes 230b and 230c may be
deposited on opposite sides of the interior substrate. A set of
electrodes 210b is deposited on the lower surface of upper
substrate 210. A set of electrodes 250b is also deposited on the
upper surface of lower substrate 250. Both sets of electrodes 210b
and 250b on upper substrate 210 and lower substrate 250
respectively may be referred to as a set of electrodes deposited on
an opposing surface from the upper and lower surface respectively
of interior substrate 230.
[0054] In the embodiment shown, sets of electrodes 210b and 230b
are configured to form three capacitors. Similarly, sets of
electrodes 230c and 250b are configured to form three capacitors.
Overall, by forming these six capacitors, accelerometer 200 is
configured to perform in a fully differential capacitive
architecture with separate force feedback electrodes, for example,
as described above with reference to FIG. 1A. Accordingly, in the
embodiment depicted in FIG. 2, the fully differential capacitive
architecture may allow the capacitors to operate together to detect
changes in an acceleration of proof mass 230a as it moves upwards
and downwards along Z-axis 155. For example, the capacitors formed
by sets of electrodes 210b, 230b, 230c, and 250c may detect an
acceleration of accelerometer 200. Then, a closed-loop circuit or
system may determine an acceleration of accelerometer using the
measured electrical current, change in capacitance, or change in
voltage of these capacitors. In some embodiments, this closed-loop
circuit or system may be referred to as front-end readout
circuitry, which may use a differential operational amplifier
configuration. In some embodiments, accelerometer 200 may contain
additional electrodes or capacitors situated surrounding interior
substrate 230. With additional structural modifications known to
one skilled in the art these additional electrodes or capacitors
allow measurement of acceleration in an X-Y plane perpendicular to
Z-axis 155. Such modifications would allow acceleration to be
measured or detected in three dimensions.
[0055] In one embodiment, accelerometer 200 also includes
piezoelectric structures 230j disposed on spring layers 230d and
230e. Piezoelectric structures 230j may be composed of any
piezoelectric material. Piezoelectric structures 230j translate
mechanical energy from spring layers 230d and 230e into electrical
energy, which may be dissipated externally via pairs of electrodes
230k disposed on each piezoelectric structure 230j. The
piezoelectric material may bend due to the movement of proof mass
130a, which is translated to mechanical energy by spring layers
230d and 230e. The addition of this piezoelectric damping may
reduce the Q-factor of accelerometer 200. The Q-factor may be
adjusted by tuning the load connected to electrodes 230k.
[0056] FIGS. 3A-C illustrate an exemplary process flow for the
fabrication of a cap substrate (e.g., a substrate similar substrate
110 or substrate 210). Turning now to FIG. 3A, substrate 310 may be
a silicon wafer, etched for the later deposition of getter layers.
Layer 320 is deposited or grown on substrate 310. Layer 320 may be
further patterned. In one embodiment, layer 320 may be silicon
dioxide.
[0057] Turning now to FIG. 3B, a set of electrodes 350 and metallic
contacts 360 and 365 are deposited on layer 320. Notably, the set
of electrodes 350 are isolated from one another on layer 320. Set
of electrodes 350 may be any type of metallic contact. Metallic
contacts 360 and 365 may in some embodiments be made of chromium,
copper and tin, gold and tin, aluminum and germanium, etc., which
may be patterned with any suitable method, such as lift-off or
etching. In this embodiment, layer 320 is patterned further for the
deposition of metallic contacts 365. In other embodiments, metallic
contacts 360 and 365 may be deposited on another layer, which may
be silicon dioxide, especially patterned for their deposition. This
additional layer may be deposited partially on layer 320, for
example, deposited only in the regions of metallic contacts 360 and
365.
[0058] Turning now to FIG. 3C, spacers 370 are deposited on layer
320. In some embodiments, spacers 370 may also be deposited on
another layer, which may be silicon dioxide, especially patterned
for their deposition. As depicted, spacers 370 may be silicon
dioxide. In some embodiments, metallic contacts 360 and 365 and
spacers 370 may operate as a bonding region to be bonded to another
substrate as described below with reference to FIGS. 4A-G. In one
embodiment, spacers 370 may be referred to as bonding spacers.
[0059] FIGS. 4A-G illustrate an exemplary process flow for the
fabrication of a fully differential MEMS accelerometer with a
separate force feedback electrode, according to one embodiment of
this disclosure. Turning now to FIG. 4A, substrate 430 may be a
silicon wafer. Trenches 415 may be filled with silicon dioxide. In
one specific embodiment, trenches 415 may be 3 .mu.m wide. To fill
trenches 415, trenches 415 may be etched first by any method known
to one skilled in the art. For example, in one embodiment, using
deep reactive-ion etching (DRIE), 3 .mu.m wide trenches are opened
on the silicon wafer. Then, to fill trenches 415, oxide is grown on
the surface of substrate 430. In another embodiment, this oxide may
be used as a masking layer for etching in later fabrication stages,
for example, XeF.sub.2 (gaseous) etching to remove portions of
substrate 430. The depth of trenches 415 may affect the thickness
of the accelerometer proof mass because substrate 430 is part of
the fully fabricated accelerometer. Referring briefly to FIG. 4D,
because trenches 415 isolate proof mass 430a from anchor regions
430f, trenches 415 may be referred to as isolation trenches.
Trenches 415 may also protect proof mass 410a and anchor regions
410f from possible later etching steps. Thus trenches 415 may also
be referred to as protection trenches. Layer 420 is deposited/grown
on substrate 430, also covering trenches 415. Layer 420 may be
silicon dioxide. In one embodiment, layer 420 may also be patterned
for deposition of subsequent layers or deposited portions. In this
accelerometer embodiment, layer 420 may be referred to as a "spring
layer." One of ordinary skill in the art will understand that layer
420 may be of any suitable thickness according to design
parameters. In one specific embodiment, the thickness of layer 420
may be 4 .mu.m.
[0060] Turning now to FIG. 4B, metallic contacts 425 are deposited
and patterned for deposition of piezoelectric structures 432.
Metallic contacts 425 may be various metals, known to one skilled
in the art. Piezoelectric structures 432 may be various
piezoelectric materials, known to one skilled in the art. To form
piezoelectric structures 432, piezoelectric material is deposited.
In certain embodiments, both metallic contact 425 and piezoelectric
structure 432 may be referred to as the piezoelectric
structure.
[0061] Turning now to FIG. 4C, layer 440 is deposited/grown on
layer 420 and patterned to protect the side walls of piezoelectric
structures 432. Layer 440 may be silicon dioxide. Then top
metallization is deposited on layer 440. This metallic deposition
forms set of electrodes 450. Set of electrodes 450 are deposited so
that the electrodes are isolated from each other on layer 440. In
one embodiment with a further etching step, set of electrodes 450
may also be patterned. Layer 440 may also include another thin
layer of silicon dioxide. That layer may be patterned so that the
bonding regions, the regions extending laterally outwards from
piezoelectric structures 432 (or the region surrounding and
including bonding pads 427) are defined for the later bonding of
spacers 460, which are depicted in FIG. 4D. These bonding regions
may also be referred to as wafer bonding areas. Bonding pads 427
are deposited in the same metallic deposition as sets of electrodes
450. In one embodiment, bonding pads 427 may also be patterned, for
example, especially for later bonding of a cap wafer. Finally, in
the same metallic deposition, pairs of electrodes 455 are deposited
on piezoelectric structures 432 and partially on layer 440. In the
wafer bonding areas, set of electrodes 450, and pairs of electrodes
455, chromium may be patterned with lift off. In another
embodiment, bonding pads 427, sets of electrodes 450, and pairs of
electrodes 455 may be deposited and patterned in separate steps.
For example, these elements may be deposited or electroplated. In
some embodiments, the metals used for bonding pads 427, sets of
electrodes 450, and pairs of electrodes 455 may be gold, aluminum,
or chromium.
[0062] Turning now to FIG. 4D, cap wafer 475 is bonded to substrate
430 by any bonding process known to one skilled in the art. As part
of substrate 430, proof mass 430a is bounded in part by trenches
415, as well as spring layers 420 and 480. Spacers 460 may be
aligned with the bonding region on substrate 430, and bonding pads
427 may be aligned with an opposing contact on cap wafer 475
between bonding spacers 460. In some embodiments, for example in
accelerometer 200 as described above with reference to FIG. 2, the
bonding process may establish the height of cavities 220 and 240
such that the capacitors formed by the electrodes have desired
values. Because of this bonding process, in one embodiment, spacers
460 may be referred to as bonding spacers. In some embodiments, cap
wafer 475 may be a cap wafer fabricated by the process illustrated
in FIG. 3. Thus cap wafer 475 contains layer 470, which may be
silicon dioxide, isolating the set of electrodes on cap wafer 475
opposing set of electrodes 450.
[0063] During this bonding process, set of electrodes 450 are
aligned to oppose the set of electrodes on cap wafer 475 so that at
least a portion of these sets of electrodes may form the capacitors
to be used in a fully differential capacitive architecture.
According to any suitable bonding process, the spacing of set of
electrodes 450 from the set of electrodes on cap wafer 475 may be
determined in order to give the capacitors formed thereby their
desired values. In certain embodiments, the center electrode of set
of electrodes 450 and the opposing electrode on cap wafer 475 may
form electrode contacts to be used for force feedback. That is,
these electrodes are operable to apply a feedback force to proof
mass 430a.
[0064] MEMS accelerometers, in order to operate in a regime of
approximate linearity, may use electrodes to apply a force to the
proof mass. In the embodiment depicted in FIG. 4D, the center
electrode of set of electrodes 475 and the opposing electrode on
cap wafer 475 may form electrode contacts to be used for force
feedback. In some embodiments, this may avoid some of the
disadvantages of MEMS accelerometers that use electrodes for
sensing and force feedback at the same time. Certain MEMS
accelerometers may switch between integration and feedback in a
closed loop circuit, which may increase circuit complexity and may
decrease the maximum feedback force applied. For example, because
only a portion (typically 75-80%) of the duty cycle of such a
switched-function electrode is available for force feedback, only a
limited amount of feedback force may be applied.
[0065] But to operate in a closed loop circuit, accelerometers may
need to apply force to the proof mass or structure. Thus in the
embodiment shown, separate electrodes (in this embodiment, the
center electrode of set of electrodes 450 and the opposing
electrode on cap wafer 475) are used to apply force to the proof
mass. Because these electrodes are used solely to apply force,
these electrodes may be referred to as force feedback electrodes.
These force feedback electrodes may receive feedback from an
external circuit based on measurements taken at the sense
electrodes to apply a force to the proof mass region, which may
allow accelerometer 400 to avoid operating in a non-linear manner.
Such force feedback electrodes may also allow accelerometer 400 to
avoid switching complexity from an external circuit and may
increase the measurement range of accelerometer 400.
[0066] The readout of such an accelerometer with separate force
feedback electrodes may also be simplified. In one embodiment, the
readout may simply be based on the force applied by the feedback
electrodes. This is due to the fact that, in order to keep the
proof mass near its equilibrium position, a force is required that
is proportional to the overall acceleration being experienced by
the accelerometer.
[0067] As shown in FIG. 4D, after cap wafer 475 is bonded to
substrate 430, substrate 430 is ground from bottom up to the tip of
trenches 415. (In some embodiments, substrate 410 may be ground
somewhat beyond the tips of trenches 415.) Then layer 480 is
grown/deposited on the bottom (or may be referred to as backside)
of substrate 430. One of ordinary skill in the art will recognize
that layer 480 may be of any suitable thickness according to design
requirements. In one specific embodiment, the thickness of layer
480 may be 4 .mu.m. In this accelerometer embodiment, layer 480 is
referred to as a spring layer.
[0068] Turning now to FIG. 4E, layer 481, piezoelectric structures
486 (including metallic contacts), set of electrodes 490, pairs of
electrodes 493, and bonding pads 494 are deposited onto layer 480
using the same or a similar process as outlined above with
reference to FIGS. 4A-C, with similar corresponding elements.
[0069] Turning now to FIG. 4F, substrate 430 may be etched to form
cavities between trenches 415 as described below with reference to
FIG. 5. For example, XeF.sub.2 gas may be used to etch the
cavities. After etching, proof mass 430a is separated from anchor
regions 430f by isolation trenches 415 and the cavities defined
thereby, which may become vacuum-sealed in a later processing
step.
[0070] Turning now to FIG. 4G, cap wafer 495 is bonded to substrate
430 using the same or similar process described above with
reference to FIG. 4D. Spacers 460 may be aligned with the bonding
region on substrate 430, and bonding pads may be aligned with an
opposing contact on cap wafer 495 between bonding spacers 460 using
known techniques. In some embodiments, cap wafer 495 may be a cap
wafer fabricated by the same or similar process illustrated in FIG.
3. Thus cap wafer 495 contains layer 470, which may be silicon
dioxide, isolating the set of electrodes on cap wafer 495 opposing
the corresponding set of electrodes on substrate 430. At least a
portion of these sets of electrodes may form the capacitors to be
used in a fully differential capacitive architecture, and a
separate portion may be used to form force feedback electrodes.
Thus substrate 430 and cap wafers 475 and 495 are now fabricated to
form, in this embodiment, a fully differential MEMS accelerometer.
In certain embodiments, the bottom center electrode of substrate
430 and center electrode on cap wafer 495 may be selected to form
electrode contacts to be used for force feedback. The use of
electrodes at the center of proof mass 430a for force feedback may
have the advantageous effect of applying a linear force, but no
torque, to proof mass 430a. In some embodiments, for example in
accelerometer 200, this last bonding step may form a common
vacuum-sealed cavity throughout cavities 220, 230g, and 240.
[0071] FIGS. 5A-E illustrate an exemplary process flow for the
etching of cavities within a substrate. Turning now to FIG. 5A,
substrate 530 may be a silicon wafer. Trenches 515 may be etched by
any method known to one skilled in the art. For example, in one
embodiment, using deep reactive-ion etching (DRIE), 3 .mu.m wide
trenches are opened on the silicon wafer. Trenches 515 may be used
as protection layers, or protection structures, during the later
isotropic release processes. The depth of trenches 515 may affect
the thickness of the accelerometer proof mass. In an accelerometer
implementation, for example in accelerometer 200, trenches 515 may
be referred to as protection trenches.
[0072] Turning now to FIG. 5B, trenches 515 are
grown/filled/deposited, with an oxide, for example silicon dioxide
conformally. This filling process deposits a layer of oxide on the
surface of substrate 530, which is removed with chemical mechanical
polishing (CMP). Then layer 520 is grown/deposited on substrate
530. The thickness of layer 520, which may be silicon dioxide, may
affect the thickness of the spring layers used in an accelerometer
implementation. For example, layer 520 may correspond to spring
layer 230d on substrate 230 in accelerometer 200. Thus, in some
embodiments, precise thickness control of layer 520 may be used
during deposition.
[0073] Turning now to FIG. 5C, a bottom portion of substrate 530 is
removed, for example through grinding and CMP. The removal may be
up to the bottom of trenches 515. Then, turning now to FIG. 5D,
layer 540 is grown/deposited on substrate 530. The thickness of
layer 540, which may be silicon dioxide, may affect the thickness
of a spring layer used in an accelerometer implementation. For
example, layer 520 may correspond to spring layer 230e on substrate
230 in accelerometer 200. Thus, in some embodiments, precise
thickness control of layer 540 may be used during deposition.
Layers 520 and 540 may be surface patterned for use as spring
layers in an accelerometer embodiment.
[0074] Turning now to FIG. 5E, using either or both of photo resist
and silicon dioxide as a mask layer, bulk silicon regions between
trenches 515 are etched through substrate 530. In some embodiments,
this etching process may be performed using dry vertical etching
techniques, known to one skilled in the art. In some embodiments,
the etching may be omitted; it may be advantageous to conduct the
etching, however, in order to decrease the processing time of the
subsequent processing step. After these trenches are etched, bonded
wafers are placed into XeF.sub.2 (gaseous) for isotropic release of
substrate 530. Silicon dioxide covering all surfaces (i.e., through
layers 520 and 540 and filled trenches 515) of substrate 530 act as
a masking layer. Substrate 530 is etched as shown in FIG. 5E,
leaving cavities 550. As discussed above, vacuum-sealing, or
vacuum-packaging, cavities 550, implemented in an accelerometer,
may avoid some of the disadvantages of Brownian noise.
[0075] Turning now to FIG. 6, a method in accordance with one
embodiment of this disclosure is provided. Flow begins at step
600.
[0076] At step 600, a survey vessel tows a streamer including at
least one accelerometer in accordance with this disclosure. In
various embodiments, the streamer may include a plurality of
accelerometers in accordance with this disclosure, and it may also
include other sensors (e.g., pressure sensors and/or
electromagnetic sensors). In some instances, the survey vessel may
tow a plurality of such streamers. Flow proceeds to step 602.
[0077] At step 602, one or more seismic sources are actuated. These
may be located on the survey vessel, towed by the survey vessel,
towed by a different vessel, etc. Seismic energy from the seismic
sources travels through the water and into the seafloor. The
seismic energy then reflects off of the various geophysical
formations. Various portions of the seismic energy may then be
reflected upward toward the streamer, in some instances
incorporating time delays and/or phase shifts that may be
indicative of the geophysical formations. Flow proceeds to step
604.
[0078] At step 604, seismic energy is received at the
accelerometers located on the streamers. Different portions of the
seismic energy may reach the accelerometers either directly from
the seismic sources, or after one or more reflections at the
seafloor and/or water surface. Data based on the received seismic
energy may then be used to infer information about geological
structures that may exist under the seafloor. Flow ends at step
604.
[0079] Turning now to FIG. 7, an additional method in accordance
with one embodiment of this disclosure is provided. Flow begins at
step 700.
[0080] At step 700, a survey vessel tows streamers including
acoustic transmitters, and also including accelerometers in
accordance with this disclosure. In some instances, the acoustic
transmitters and the accelerometers may be combined into an
acoustic transceiver. Flow proceeds to step 702.
[0081] At step 702, one or more of the acoustic transmitters are
actuated. The acoustic energy produced by the transmitters may
travel through the water toward the other streamers. Flow proceeds
to step 704.
[0082] At step 704, the acoustic energy is received by an
accelerometer. The delay between the actuation of the acoustic
transmitters and the reception at the accelerometer may be based in
part on the distance between them. Flow proceeds to step 706.
[0083] At step 706, the positions of the streamers (or portions
thereof) are determined. For example, such positions may be
determined based on the distances between pairs of acoustic
transmitters and accelerometers. Flow ends at step 706.
[0084] One of ordinary skill in the art with the benefit of this
disclosure will understand that various aspects of this disclosure
may in some embodiments be implemented via computer systems. Such
computer systems may in some embodiments include various types of
non-transitory computer-readable media, such as hard disks, CDs,
DVDs, RAM, ROM, tape drives, floppy drives, etc.
[0085] Although specific embodiments have been described above,
these embodiments are not intended to limit the scope of the
present disclosure, even where only a single embodiment is
described with respect to a particular feature. Examples of
features provided in the disclosure are intended to be illustrative
rather than restrictive unless stated otherwise. The above
description is intended to cover such alternatives, modifications,
and equivalents as would be apparent to a person skilled in the art
having the benefit of this disclosure.
[0086] The scope of the present disclosure includes any feature or
combination of features disclosed herein (either explicitly or
implicitly), or any generalization thereof, whether or not it
mitigates any or all of the problems addressed herein. Accordingly,
new claims may be formulated during prosecution of this application
(or an application claiming priority thereto) to any such
combination of features. In particular, with reference to the
appended claims, features from dependent claims may be combined
with those of the independent claims and features from respective
independent claims may be combined in any appropriate manner and
not merely in the specific combinations enumerated in the appended
claims.
[0087] Numerous variations and modifications will become apparent
to those skilled in the art once the above disclosure is fully
appreciated. It is intended that the following claims be
interpreted to embrace all such variations and modifications.
* * * * *