Note: Descriptions are shown in the official language in which they were submitted.
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TITLE:
Fiber optic MEMS seismic sensor with mass supported by hinged beams
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority from U.S. Provisional Patent Application No.
60/794,948, filed on April 26, 2006, and titled FIBER OPTIC MEMS SEISMIC
SENSOR WITH MASS SUPPORTED BY HINGED BEAMS, and from U.S. Patent
Application No. 11/705,224, filed on February 12, 2007 and titled FIBER OPTIC
MEMS
SEISMIC SENSOR WITH MASS SUPPORTED BY HINGED BEAMS.
FIELD OF THE INVENTION
The present invention generally relates to mechanical optical devices, and,
more
particularly, to micro-electro-mechanical optical devices having a mass
supported by
hinged beams.
BACKGROUND
The traditional method for detecting land seismic signals has been the coil-
type
geophone. Geophone sensors consist of a mass-spring assembly contained in a
cartridge
about 3cm long and weighing about 75 grams. In a typical geophone sensor, the
spring is
soft and as the cartridge case moves the mass (coil) is held in place by its
own inertia.
Thus, the coil acts as a reference for measurement of the cartridge
displacement. This
sensor arrangement is ideal for measurement of large, oscillatory
displacements on the
order of millimeters with sub-micrometer resolution. However, the frequency
range of
these sensors is limited. For best sensitivity to small displacements, a given
sensor has a
mechanical bandwidth of about 10Hz. Sensors can be designed with center
frequencies
from 20Hz to 100Hz.
Micro-Electro-Mechanical Systems (MEMS) are miniature mechanical
components fabricated in silicon wafers. The fabrication methods are based on
the same
photolithographic and etching processes used to manufacture electronic
circuits in
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silicon. In fact, most MEMS devices include not only miniature mechanical
components
such as nozzles, gears, etc. but also integrated electronic components to
provide local
signal conditioning. Unfortunately, the integrated circuits limit the maximum
operating
temperature of electronic MEMS to 75 C. The maximum temperature limit can be
extended to 400 C or more if optical fiber sensors are integrated with
mechanical MEMS
components so that no electronics are needed in the high temperature
environment.
Recently, MEMS accelerometers have been developed for 3-component (3C) land
seismic measurements. In the MEMS accelerometer, a mass-spring assembly is
also used,
but, unlike the geophone, the spring is stiff and the mass moves with the case
that houses
the MEMS. The inertia of the mass causes strain and deflection of the spring
and the
deflection or strain that can be measured with a sensor to determine the
acceleration of an
object. Capacitance sensors may also be incorporated into high performance 3C
MEMS
accelerometers to determine the acceleration of an object.
The measurement range of accelerometers is specified in units of 'G' where 1 G
9.8m/s2. Commercial specifications include 12OdBV dynamic range (1 G to 10"6G)
and
500Hz mechanical bandwidth with 24-bit digital resolution equivalent to a
noise limited
performance of 10"7 G/(Hz)1/2. The accelerometer is fabricated on a silicon
chip on the
order of 100mm2. Three single-axis accelerometers (each with an application
specific
integrated circuit (ASIC) for signal conditioning) are packaged to measure in
three
orthogonal directions. The limitation of these accelerometers is an upper
limit on the
operating temperature of 75 C, which is imposed by the electronic integrated
circuits and
is not a fundamental limitation of silicon itself.
SUMMARY OF INVENTION
The present invention relates to a proof mass supported by a frame having
supporting beams. The proof mass is positioned within the frame and has a
hinged
attachment to the beams. The proof mass has a sensor gap having a first
reflector and a
second reflector positioned at opposing ends of the sensor gap. An optical
fiber injects
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light into the sensor gap and light is reflected to determine seismic movement
of the
proof mass with respect to the frame. Stops are provided for limiting the
movement of the
proof mass to minimize strain on the attachment of the beams and the proof
mass.
DESCRIPTION OF THE DRAWINGS
Operation of the invention may be better understood by reference to the
following
detailed description taken in connection with the following illustrations,
wherein:
FIGURE 1 is a perspective view of a proof mass supported by hinged support
beams in an embodiment of the present invention;
FIGURE 2A is a side perspective view of an assembled MEMS device in an
embodiment of the present invention;
FIGURE 2B is a cross-sectional view taken generally along line A-A of the
assembled MEMS device of FIGURE 2A;
FIGURE 3 is a cross-sectional view of the top portion of a sensor assembly in
an
embodiment of the present invention;
FIGURE 4 is a detailed view of a fiber optic sensor with a proof mass in an
embodiment of the present invention;
FIGURE 5 is a perspective view of a variation of the fiber optic sensor;
FIGURE 6 is a cross-sectional view of the variation of the fiber optic sensor;
FIGURE 7 is a perspective view of the first embodiment of the triaxial
assembly,
where three sensors are mounted on the three mutually perpendicular surfaces
at the apex
of a corner of a cube in an embodiment of the present invention; and
FIGURE 8 is a perspective view of an alternative embodiment of the triaxial
assembly, where two sensors are shown, the third sensor would exit the page
and be
perpendicular to the two sensors shown; and
DETAILED DESCRIPTION
While the present invention is described with reference to the embodiments
described herein, it should be clear that the present invention should not be
limited to
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such embodiments. Therefore, the description of the embodiments herein is
illustrative of
the present invention and should not limit the scope of the invention as
claimed.
The present invention relates to a micro-electro mechanical system (MEMS)
sensor. More specifically, the present invention relates to an interferometric
MEMS
optical sensor that may be used for seismic sensing. In an embodiment of the
invention,
the components of the optical seismic sensor are positioned to form an
interferometric
gap.
Figures 1 and 2B illustrate a frame 12 supporting beams 16a-16d and a proof
mass 14 that may be incorporated into a sensor assembly 10. The beams 16a-16d
support
the proof mass 14 that may be positioned within the frame 12. In an
embodiment, the
proof mass 14 and the beams 16a-16d are made of silicon. One of ordinary skill
in the art
will appreciate that other materials may be used and that the present
invention is not
deemed as limited to any specific type of material. The beams 16a-16d are
secured to the
frame 12 to provide a stable and robust arrangement of the sensor assembly 10.
At an end of the beams 16a-16d opposite the frame 12, the beams 16a-16d are
secured to the proof mass 14, as illustrated in Figure 1. In an embodiment,
the beams
16a-16d are secured to the proof mass 14 with hinges and/or secondary beams.
In a
preferred embodiment, the beams 16a-16d are integrally formed with proof mass
14 and
frame 12, and the beams 16a-16d have as stiffness that allows them to act as
"spring-
loaded" hinges. To this end, the hinged attachment of the beams 16a-16d to the
proof
mass 14 allows the proof mass 14 to move with respect to the beams 16a-16d,
but
resiliently act against that movement. The beams 16a-16d are shown to be
secured to the
proof mass 14 at corners of the proof mass 14. In addition, the beams 16a-16d
may have
a uniform thickness between the frame 12 and the proof mass 14, or may be
composed of
parallel strips of material. In an embodiment, the proof mass 14 moves in the
y-
direction as shown in Figure 1. The performance of sensor assembly 10 will
depend
upon the material and design chosen. For example, in a design where proof mass
moves
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in the y-direction, the movement of each beam 16a-16d may be characterized by
the
formula K= (Ehjw)/L3, where K is the stiffness, E is Young's Modulus of the
material
used, L is length of the beam, h is the height if the beam and w is the width
of the beam.
Figure 2A illustrates a side view of the MEMS sensor device 10 in an
embodiment of the present invention. In the embodiment of the invention
illustrated in
Figure 2A and B, the frame 12 is made of silicon and positioned between glass
wafers 22.
In a preferred embodiment, the glass wafers 22 are borosilicate glass and are
bonded to
the top and the bottom surface of the frame 12. The glass wafers 22 as
referred herein
should not be deemed as limited to glass or borosilicate glass and may be any
other
materials as will be appreciated by one of ordinary skill in the art.
A cross-sectional view taken generally along line A-A of the MEMS sensor
device 10 is illustrated in Figure 2B. An optical fiber assembly 31 can be
inserted into
hole 24 in glass wafer 22 to direct light to the proof mass 14. In this
embodiment, the
beams 16a and 16b have a uniform thickness. The glass wafers 22 are
illustrated on
opposing sides of the frame 12.
Figure 3 illustrates a perspective view of the optical fiber assembly 31 in an
embodiment of the present invention. The optical fiber assembly 31 has an
optical fiber
30 held within the optical fiber assembly 31. The optical fiber 30 extends
toward proof
mass 14 as shown in Figure 4 to transmit light to and/or receive reflected
light from
reflector R2 on proof mass 14. There are many ways of mounting optical fiber
assembly
31. Referring to Figure 3, a tube 32 houses the optical fiber assembly 31,
with the optical
fiber 30 contained within optical fiber assembly 31. Tube 32 is supported by a
top layer
34, and as shown in Figure 2B, may extend through top layer 34. Optical fiber
assembly
31 may be soldered into place as is known in the art. Furthermore, those
skilled in the art
will recognize that other means may be used to mount an optical fiber to the
sensor.
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An optical fiber 30 injects light into the sensor assembly 10. For example,
the
optical fiber 30 may inject light in the C band (at approximately 1550nm)
remotely by a
signal conditioner/interrogator. Light exits the end of the optical fiber 30
as illustrated in
Figure 4. In one embodiment, the end of optical fiber 30 may be angle polished
and
coated with an antireflection film, such as a metal or dielectric material, to
reduce back
reflection into the optical fiber 30. In another embodiment, where the end of
the optical
fiber 30 is used as the first reflector, a reflective coating may be applied
instead.
Light is transmitted to the proof mass 14 and a portion of the light is
reflected by
reflector R1, as illustrated in Figure 4. The portion of the light reflected
from the reflector
Rl is indicated as Beam A. The Rl surface, which may be a glass surface, such
as a
borosilicate glass surface, may be inserted after the end of the optical fiber
30. The R1
surface may also be the end of the optical fiber 30 as shown in Figure 4. In
either case, a
coating may be applied onto the reflector R1 surface to increase the
reflectance. The
amount of reflectance of the reflector RI may be set to a predetermined level
by, for
example, selecting a substance and/or a coating for the glass surface or the
end of the
optical fiber 30 that provides the predetermined level of reflectance.
Light not reflected at the reflector R1 travels to reflector R2 and is
reflected as
illustrated in Figure 4. Reflectors R1 and R2 need to reflect light back in
the same
direction, such that the reflective surfaces of R1 and R2 need to be optically
parallel, such
that light returns through optical fiber 30 to the analyzer (not shown). The
light reflected
from the reflector R2 is indicated as Beam B. Beam B is reflected by the
surface carried
by proof mass 14. The sensor gap is defined by the separation between the
reflector R1,
which is either the end of optical fiber 30 or a fixed partially reflective
surface, and the
reflector R2, which acts as a moving reflective surface. Proof mass 14 may be
designed
to act as reflector R2, or reflector R2 may be integrally formed with, carried
by, or
otherwise movable with proof mass 14. In a preferred embodiment, the
reflectance of the
reflector R2 is similar to the reflectance of the reflector R1, but may be up
to two or three
times different. In such an embodiment, the reflector R2 is a coating that
increases the
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reflectance. For example, in an exemplary embodiment, the reflector R2 is a
gold coating.
The reflector R2 is positioned and/or deposited and thus is bonded to the
proof mass 14..
Other materials may be chosen for the reflectors Rl and R2 as shown in Figure
4.
In an embodiment, bare borosilicate glass (having a 3.7 % reflectance) is used
for the
reflector Rl, but other materials may be used to increase the reflectance. For
example, a
high index-low index dielectric stack may be used for the reflector R1 to
increase the
reflectance to 40% or more, if desired. In addition, materials other than the
gold coating
for the reflector R2 may be used. For example, aluminum, silver and/or a
dielectric stack
may be deposited onto proof mass 14 in order to obtain a high reflectance for
the reflector
R2. The present invention should not be deemed as limited to any specific
material for
the reflectors R1 and R2.
Movement of the proof mass 14 with respect to the frame 12 changes the sensor
gap defined as the separation between reflectors R1 and R2, with the amount of
movement being related to the acceleration of the sensor 10. The Beams A and B
reflect
back into the optical fiber 30 and may, for example, interfere on the surface
of a
photodiode detector in the signal conditioner (not shown). The interference
signal of the
Beams A and B is analyzed to precisely determine the sensor gap, and thus the
acceleration of the sensor 10. The sensor 10 is, therefore, capable of sensing
seismic
movement.
The sensor 10 as described above and depicted in the drawings may be
fabricated
using wafer processing technology, such as, for example, masking, etching and
bonding
methods, which are well known in the art. For example, the MicragemTM process
employed by Micralyne Inc. based in Edmonton, Alberta may be used to obtain
satisfactory results. The Micragem process uses glass etching, anodic bonding
of Pyrex
glass and an SOI (Silicon On Insulator) wafer, KOH etching of the handle wafer
of SOI
wafer, and DRIE (Deep Reactive Ion Etching) of the device layer of the SOI
wafer. In
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employing these steps, favorable results have been obtained by leaving a small
portion of
the handle wafer beneath what becomes the proof mass 14.
The supporting glass wafer 22 is bonded to the frame 12 as shown in Figure 2b.
The supporting glass wafer 22 has stops 53a, 53b that are etched into the
wafer 22 to limit
the movement of the proof mass 14. The supporting wafer 22 is preferably
borosilicate
glass and anodically bonded to the frame 12. For example, the stops 53a, 53b
will limit
the movement of the proof mass 14 to a predetermined distance to prevent
excessive
stress and possible breakage and failure of the beams 16a-16d, such as a shock
of
approximately 1500g. In a preferred embodiment, the proof mass 14 is limited
to a
displacement of approximately five micrometers in order to maintained the
stress on the
beams 16a-16d at or below approximately 24 x 106 Pa or about 0.3% of the
tensile
strength of silicon.
Variations
The above description is directed mainly toward a proof mass 14 positioned in
a
plane perpendicular to the optical fiber 30 with four support beams 16a-16d
attaching it
to the frame 12. However, other designs are also possible. For example,
referring to
Figure 6, proof mass 14 may be positioned in a plane parallel to the optical
fiber 30 rather
than perpendicular. The plane of proof mass 14 that is used to define the
orientation is
the plane formed by beams 16a-16d as compared to the optical axis of the
optical fiber
30. As shown in Figure 5, frame 12 and proof mass 14 are integrally formed
with beams
16a-16d and the portion that secures an optical fiber. Frame 12 also includes
stops 53a-
53d to limit the movement of proof mass 14. Frame 12 is then mounted between
wafers
22. Reflector R2 is positioned on the edge of proof mass 14, and may be coated
or
otherwise have a reflective coating mounted accordingly, if proof mass 14
itself is
insufficiently reflective. In this embodiment, reflector surface R1 is shown
to be a
reflective surface that is inserted into frame 12 instead using the end of
optical fiber 30 as
the reflector R1 as shown in Figure 4. Figure 5 also shows an alternative
method of
securing optical fiber 30, where frame 12 has inwardly protruding guides 50 to
help
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center an optical fiber or optical fiber assembly as it is inserted, and a
fiber stop 51 to
indicate when the optical fiber 31 is fully inserted. In this embodiment,
reflector Ri is
mounted on the other side of fiber stop 51. Referring to Figure 6, optical
fiber 30 may
them be soldered into place using known techniques.
In use, the sensor 10 may be attached and/or secured in various orientations
to
accurately determine seismic movement. Figure 7 illustrates a triaxial
assembly of the
sensors 100 attached to the corner of a cube 101. In this embodiment, the
sensors 100 are
positioned on three mutually perpendicular surfaces at the apex of corners of
the cube
101. Figure 8 illustrates another embodiment of the orientation of the sensors
100
where two sensors 100a, 100b are positioned at adjacent edges perpendicular to
each
other. The third sensor (not shown) is positioned out of the page and mutually
perpendicular to the sensors 100a, 100b.
The invention has been described above and, obviously, modifications and
alternations will occur to others upon a reading and understanding of this
specification.
The claims as follows are intended to include all modifications and
alterations insofar as
they come within the scope of the claims or the equivalent thereof.
