Title: Fiber optic micro accelerometer
Abstract: An accelerometer includes a wafer, a proof mass integrated into the wafer, at least one spring member connected to the proof mass, and an optical fiber. A Fabry-Perot cavity is formed by a partially reflective surface on the proof mass and a partially reflective surface on the end of the optical fiber. The two partially reflective surfaces are used to detect movement of the proof mass through the optical fiber, using an optical detection system.
Patent Number: 6,921,894 Issued on 07/26/2005 to Swierkowski
| Inventors:
|
Swierkowski; Steve P. (Livermore, CA)
|
| Assignee:
|
The Regents of the University of California (Oakland, CA)
|
| Appl. No.:
|
238660 |
| Filed:
|
September 10, 2002 |
| Current U.S. Class: |
250/227.21; 250/231.1 |
| Intern'l Class: |
G01J 001/34 |
| Field of Search: |
250/22719,227.21,231.1
356/477
|
References Cited [Referenced By]
U.S. Patent Documents
| 4595830 | Jun., 1986 | McMahon.
| |
| 5202939 | Apr., 1993 | Belleville et al.
| |
| 5218420 | Jun., 1993 | Asmar.
| |
| 5276322 | Jan., 1994 | Carome.
| |
| 5392117 | Feb., 1995 | Belleville et al.
| |
| 5628917 | May., 1997 | MacDonald et al.
| |
| 5891747 | Apr., 1999 | Farah.
| |
| 5990473 | Nov., 1999 | Dickey et al.
| |
| 6175108 | Jan., 2001 | Jones et al.
| |
| 6581465 | Jun., 2003 | Waters et al.
| |
| 6671055 | Dec., 2003 | Wavering et al.
| |
| Foreign Patent Documents |
| 1 083 429 | Mar., 2001 | EP.
| |
| 1 083 429 | Jan., 2002 | EP.
| |
Other References
Uttamchandani et al., "A micromachined silicon accelerometer with fibre optic
interrogation," May 29, 1992, IEEE Coloquium on Fibre Optics Sensor Technology,
pp. 4/1-4/4.
Plaza et al., "Stress free quad beam optical silicon accelerometer," Sensors
IEEE, Jun. 12-14 2002, pp. 1064-1068.
Guldimann et al., "Fiber-optic accelerometer with micro-optical shutter modulation
and integrated damping," Optical MEMS IEEE, Aug. 21-24, 2000, pp. 141-142.
|
Primary Examiner: Luu; Thanh X.
Attorney, Agent or Firm: Scott; Eddie E., Thompson; Alan H.
Goverment Interests
The United States Government has rights in this invention pursuant to Contract
No. W-7405-ENG-48 between the United States Department of Energy and the University
of California for the operation of Lawrence Livermore National Laboratory.
Claims
1. An accelerometer, comprising:
a wafer,
a recess in said wafer,
a proof mass integrated into said wafer and located in said recess,
at least one spring member operatively connected to said proof mass,
said proof mass having a proof mass surface and said proof mass surface being
partially reflective,
an optical fiber optically coupled to said partially reflective proof mass surface
and positioned opposite said partially reflective proof mass surface,
a partially reflective surface on said optical fiber that is positioned opposite
said partially reflective proof mass surface,
a gap between said partially reflective proof mass surface and said partially
reflective surface on said optical fiber, and
sensor means operatively connected to said optical fiber for detecting movement
of said proof mass.
2. The accelerometer of claim 1, wherein said wafer is a silicon wafer.
3. The accelerometer of claim 1, wherein said proof mass is a microfabricated
proof mass.
4. The accelerometer of claim 1, wherein said proof mass is a micromachined proof mass.
5. The accelerometer of claim 1, wherein said wafer contains a groove for receiving
said optical fiber.
6. The accelerometer of claim 1, wherein said wafer includes a groove with an
insertion funnel for receiving said optical fiber.
7. The accelerometer of claim 1, wherein said gap between said partially reflective
proof mass surface and said partially reflective surface on said optical fiber
provides a Fabry-Perot cavity between said partially reflective surface on said
optical fiber and said partially reflective proof mass surface.
8. The accelerometer of claim 1, including means for encapsulating said wafer
and said proof mass.
9. The accelerometer of claim 8, wherein said means for encapsulating said wafer
and said proof mass includes a first side plate and a second side plate with said
proof mass positioned between said first side plate and said second side plate
in an aligned manner.
10. The accelerometer of claim 8, wherein said means for encapsulating said wafer
and said proof mass includes a first glass side plate and a second glass side plate
with said proof mass positioned between said first glass side plate and said second
glass side plate.
11. The accelerometer of claim 9 wherein said proof mass is approximately rectangular
with four corners and wherein said at least one spring member comprises four spring
members with one of said four spring members located proximate each of the four
corners of said proof mass.
12. The accelerometer of claim 9 wherein said proof mass is approximately rectangular
with four corners and wherein said at least one spring member comprises eight spring
members with two of said eight spring members located proximate each of the four
corners of said proof mass.
13. The accelerometer of claim 1 wherein said wafer contains a groove for receiving
said optical fiber and including adhesive wicking dump channels operatively connected
to said groove.
14. An accelerometer, comprising:
a wafer,
a recess in said wafer,
a proof mass located in said recess in said wafer,
at least one spring member operatively connected to said proof mass,
said proof mass having a proof mass surface and said proof mass surface being
partially reflective,
an optical fiber optically coupled to said partially reflective proof mass surface
and positioned opposite said partially reflective proof mass surface,
a partially reflective surface on said optical fiber that is positioned opposite
said partially reflective proof mass surface,
a gap between said partially reflective proof mass surface and said partially
reflective surface on said optical fiber, and
sensor means operatively connected to said optical fiber for detecting movement
of said proof mass.
15. The accelerometer of claim 14, wherein said wafer is a silicon wafer.
16. The accelerometer of claim 14, wherein said proof mass is a microfabricated
proof mass.
17. The accelerometer of claim 14, wherein said gap between said partially reflective
proof mass surface and said partially reflective surface on said optical fiber
provides a Fabry-Perot cavity between said partially reflective proof mass surface
and said partially reflective surface on said optical fiber.
18. A method of producing an accelerometer, comprising the steps of:
microprocessing a wafer to have a recess,
microprocessing said wafer to produce a proof mass located in said recess,
microprocessing said wafer to produce at least one spring member,
microprocessing said wafer to produce a channel for receiving an optical fiber,
providing a partially reflective proof mass surface on said proof mass,
positioning a first side plate and a second side plate adjacent said, wafer,
said proof mass, said at least one spring member, and said channel and connecting
said first side plate and said second side plate to said wafer,
connecting an optical fiber to said wafer in said channel
providing a partially reflective surface on said optical fiber and
positioning said proof mass and said optical fiber in a position wherein said
partially reflective surface on said optical fiber is opposite said partially reflective
proof mass surface and there is a gap between said partially reflective surface
on said optical fiber and said partially reflective proof mass surface.
19. The method of claim 18, wherein said step of positioning and connecting said
a first side plate and a second side plate to said wafer comprises bonding said
first side plate and said second side plate to said wafer.
20. The method of claim 19, wherein said step of bonding said first side plate
and said second side plate to said wafer includes using a temperature-time profile
during said bonding.
21. The method of claim 19, wherein said step of bonding said first side plate
and said second side plate to said wafer comprises fusion at elevated temperatures.
22. The method of claim 19, wherein said step of bonding said first side plate
and said second side plate to said wafer comprises anodic bonding.
23. The method of claim 18, wherein the steps of positioning said proof mass
and said optical fiber in a position wherein said partially reflective surface
on said optical fiber is opposite said partially reflective proof mass surface
and there is a gap between said partially reflective surface on said optical fiber
and said partially reflective proof mass surface provides a Fabry-Perot cavity
between said partially reflective surface on said optical fiber and said partially
reflective proof mass surface.
24. The method of claim 18, including the steps of providing sensor means for
detecting movement of said proof mass optically coupled to said optical fiber.
25. The method of claim 18, including the steps of maintaining said wafer and
said proof mass encapsulated during said step of microprocessing.
26. The method of claim 18, including the steps of maintaining said wafer and
said proof mass encapsulated during processing and subsequently opening said encapsulated
wafer and said proof mass.
27. The method of claim 26, wherein said step of opening said encapsulated wafer
and said proof mass is accomplished by dry or clean etching a fiber opening in wafer.
Description
BACKGROUND
1. Field of Endeavor
The present invention relates to accelerometers and more particularly to a fiber
optic micro accelerometer.
2. State of Technology
U.S. Pat. No. 4,595,830 for a multimode optical fiber accelerometer by Donald
McMahon issued Jun. 17, 1986 provides the following information, "Accelerometers
of the prior art include devices comprising a proof mass affixed to the end of
an elastic beam. The elastic beam, owing to the inertia of the affixed proof mass,
bends upon acceleration, providing a measurement thereof. Alternatively, upon acceleration,
a feedback loop generates a countervailing force which maintains the original position
of the proof mass. The acceleration is derived from a measurement of the necessary
compensating force. Sensors in these prior art devices for detecting relevant changes
from which the acceleration may be calculated, comprise mechanical or electromagnetic
means, or combinations thereof. Thus, there is a need for an efficacious accelerometer
entailing optical, in particular, optical fiber sensors."
U.S. Pat. No. 5,276,322 for a fiber optic accelerometer by Edward Carome issued
Apr. 1, 1994 provides the following information, "Heretofore, the acceleration
and vibration sensors have been primarily of electromechanical nature. One prior
art acceleration sensor uses a mercury switch which is configured such that the
selected acceleration moves the mercury to a position in which it closes an electrical
connection between leads. Prior art vibration sensors utilize electrical conductors
moving with respect to magnetic fields or piezoelectric elements that produce electric
signals proportional to acceleration. Fiber optic accelerometers of numerous designs
are available. The fiber optic systems have numerous advantages over mechanical
and electromechanical accelerometers, such as their increased sensitivity and immunity
to electrical interference. However, the fiber optic accelerometers tend to be
relatively expensive. Moreover, the fiber optic accelerometers are not as amenable
to automated manufacture as the prior art electromechanical and electrical acceleration sensors."
U.S. Pat. No. 6,175,108 for an accelerometer featuring fiber optic bragg grating
sensor for providing multiplexed multi-axis acceleration sensing by Richard Jones,
et al. issued Jan. 16, 2001 provides the following information, "Accelerometers
are known in the prior art that use an optical fiber. Such accelerometers measure
acceleration by sensing optical fiber surface strain, by sending optical fiber
displacement or microbending, by sensing optical signal intensity, and by sensing
optical signal phase shifts. One disadvantage of the prior art accelerometers is
that they are all complicated point sensors that do not allow multiplexing. Instead,
a separate prior art accelerometer is needed to sense each respective axis."
SUMMARY
Features and advantages of the present invention will become apparent from
the following description. Applicants are providing this description, which includes
drawings and examples of specific embodiments, to give a broad representation of
the invention. Various changes and modifications within the spirit and scope of
the invention will become apparent to those skilled in the art from this description
and by practice of the invention. The scope of the invention is not intended to
be limited to the particular forms disclosed and the invention covers all modifications,
equivalents, and alternatives falling within the spirit and scope of the invention
as defined by the claims.
The present invention provides an accelerometer. The accelerometer includes a
wafer, a proof mass operatively connected to the wafer, at least one spring member
operatively connected to the proof mass, a partially reflective surface integrated
into the proof mass, an optical fiber operatively connected to the wafer, a partially
reflective surface on the optical fiber, and sensor means operatively connected
to the optical fiber for detecting movement of the proof mass. The wafer contains
a groove for receiving the optical fiber. In one embodiment, a Fabry-Perot cavity
is located between the partially reflective surface on the optical fiber and the
partially reflective surface that is operatively connected to the proof mass. In
one embodiment, a means for encapsulating the wafer and the proof mass includes
a first side plate and a second side plate with the proof mass positioned between
the first side plate and the second side plate. In one embodiment, the wafer contains
a groove for receiving the optical fiber and including bonding material wicking
dump channels operatively connected to the groove.
In one embodiment, a method of producing an accelerometer is provided that includes
microprocessing a wafer to produce a proof mass, at least one spring member, and
a channel for receiving an optical fiber. A first side plate and a second side
plate are positioned adjacent the wafer, the proof mass, the at least one spring
member, and the channel. The first side plate and the second side plate are bonded
to the wafer and an optical fiber is connected to the wafer. In one embodiment,
the step of positioning and connecting a first side plate and a second side plate
to the wafer comprises bonding the first side plate and the second side plate to
the wafer. In one embodiment, a temperature-time profile is used during the bonding.
A partially reflective surface is integrated into the proof mass and a partially
reflective surface is provided on the optical fiber. The partially reflective surface
that is operatively connected to the proof mass and the partially reflective surface
on the optical fiber are opposite from each other. In one embodiment, a Fabry-Perot
cavity is located between the partially reflective surface on the optical fiber
and the partially reflective surface that is operatively connected to the proof
mass. Sensor means are provided for detecting movement of the proof mass. The wafer
and the proof mass remain encapsulated during processing.
The invention is susceptible to modifications and alternative forms. Specific
embodiments are shown by way of example. It is to be understood that the invention
is not limited to the particular forms disclosed. The invention covers all modifications,
equivalents, and alternatives falling within the spirit and scope of the invention
as defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated into and constitute a part
of the specification, illustrate specific embodiments of the invention and, together
with the general description of the invention given above, and the detailed description
of the specific embodiments, serve to explain the principles of the invention.
FIG. 1 illustrates one embodiment of a fiber optic micro accelerometer constructed
in accordance with the present invention.
FIG. 2 shows a top view of one embodiment of an accelerometer constructed in
accordance with the present invention.
FIG. 3 shows a side view of one embodiment of an accelerometer constructed in
accordance with the present invention.
FIG. 4 illustrates another embodiment of a fiber optic micro accelerometer constructed
in accordance with the present invention.
FIG. 5 illustrates another embodiment of a fiber optic micro accelerometer constructed
in accordance with the present invention.
FIG. 6 illustrates another embodiment of a fiber optic micro accelerometer constructed
in accordance with the present invention and an embodiment of a method of producing
a fiber optic micro accelerometer.
FIG. 7 illustrates another embodiment of a method of producing a fiber optic
micro accelerometer in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, to the following detailed information, and
to incorporated materials; a detailed description of the invention, including specific
embodiments, is presented. The detailed description serves to explain the principles
of the invention. The invention is susceptible to modifications and alternative
forms. The invention is not limited to the particular forms disclosed. The invention
covers all modifications, equivalents, and alternatives falling within the spirit
and scope of the invention as defined by the claims.
The vast majority of accelerometers use a sensing element on a chip that requires
electrical power input, and the output signal is an electrical output signal. The
present invention is a departure from this. The present invention provides a completely
passive sensing element that is very small, and is optically interrogated remotely-possibly
very far away (e.g., 1 km). This is ideal for explosive environments [no wires
going in/out] and places where readout electronics [often not very small] and power
are not desired, i.e., ends of ship propellers, composite rudder fin structures
of airplanes, automotive sensing, RFI intensive environments-arcing and lightning
proof, in electrical machines, etc.
Referring now to FIG. 1, one embodiment of a fiber optic micro accelerometer
constructed in accordance with the present invention is illustrated. The fiber
optic micro accelerometer embodiment is designated generally by the reference numeral
100. The structural elements of the accelerometer
100 will now be
summarized. The accelerometer
100 includes a wafer
101. The wafer
101 is a thin piece of material used as a structural component and/or optical
component. The wafer
101 can be made of silicon or other material. A proof
mass
102 is incorporated in the wafer
101. At least one spring member
103A is operatively connected to the proof mass
102. The accelerometer
100 measures acceleration. Because it is difficult to measure acceleration
directly, the accelerometer
100 measures movement of the proof mass
102
suspended by the at least one spring member
103A. A partially reflecting
surface
104 is integrated into the proof mass
102. An optical fiber
and sensor means
105 for detecting movement of the proof mass
102
is optically coupled to the partially reflecting surface
104. The optical
fiber and sensor means
105 can be any of the state of the art optical fiber
and sensor means. For example, the optical fiber and sensor means
105 can
be a system that uses the white light interferometer effect of a Fabry-Perot cavity.
The accelerometer
100 is useful for detecting and measuring motions in
mechanical structures such as physics experiments, explosive environments, industrial
machinery, bridges, automobiles, planes, missiles and other equipment. The accelerometer
100 provides a passive sensing element that is very small and is optically
interrogated remotely. The accelerometer
100 can be optically interrogated
remotely from long distances. This is ideal for explosive environments. The accelerometer
100 is useful in explosive environments and in environments where electrical
sparks and noise create problems because there are no wires going in or out of
the accelerometer
100 and the readout electronics and power source are at
a remote location. There is no stored energy in the accelerometer. The accelerometer
100 provides results that are useful for determining forces, failures and
failure prediction, and navigation, to name just a few of many applications. It
is particularly well suited to: a.) minimally invasive probes, b.) explosive or
hazardous or extreme environments since no wires are employed and the package is
extremely robust, c.) sensor applications with high EMI, and d.) remote readout
instruments without loss of fidelity.
The accelerometer
100 will now be described in greater detail. The accelerometer
100 is constructed using IC (integrated circuit) microfabrication technologies.
Microfabricated devices are formed using crystalline substrates, such as silicon
and gallium arsenide, but may be formed on non-crystalline materials, such as glass
or certain polymers. The shapes of crystalline devices can be precisely controlled.
Micromachined and etched surfaces and shapes are readily produced. The materials
may be bonded by processes such as adhesive bonding, fusion at elevated temperatures,
anodic bonding, or field-assisted methods. Microfabrication technology enables
the production of electrical, mechanical, electromechanical, optical, chemical
and thermal devices. The integration of these microfabricated devices into a single
system allows for the batch production of microscale instruments.
The proof mass
102 is incorporated in the wafer
101 by microprocessing.
The proof mass
102 can be created by micromachining the wafer
101
to incorporate the proof mass
102 or by other processing methods such as
etching. The proof mass
102 has at least one spring member
103A.
A partially reflecting surface
104 is operatively connected to the proof
mass
102. An optical fiber
105 is optically coupled to the partially
reflecting surface
104. The wafer
101 contains a groove
106
for receiving the optical fiber
105. The groove
106 can include an
insertion funnel for receiving the optical fiber
105. The optical fiber
105 has a partially reflecting end surface
107. A Fabry-Perot cavity
108 is located between the end surface
107 of the optical fiber
105
and the partially reflecting surface
104. A Fabry-Perot cavity is an optical
resonator in which feedback is accomplished by two parallel planes. The partially
reflecting surface
104 forms one mirror of the Fabry-Perot optical cavity
108. The other mirror of the Fabry-Perot cavity
108 is formed by
the partially reflecting end surface
107 of the optical fiber
105.
An optical readout of the position of the proof mass
102, relative to the
wafer
101, is achieved by a remote external optical system that illuminates
the mirrors and also measures the reflected light as is well know in the art.
The basic structural details of one embodiment of a fiber optic micro accelerometer
constructed in accordance with the present invention have been described. Some
additional details of other embodiments of the accelerometer
100 will now
be described. The accelerometer
100 includes a proof mass
102 that
is approximately rectangular with four individual corners. The spring member system
includes at least one spring member
103A operatively connected to the proof
mass
102. Additional spring members may be included. For example, the spring
members
103B,
103C and
103D may be included. The spring members
103A,
103B,
103C and
103D are located proximate each
of the four corners of the proof mass
102. The accelerometer
100
may include means for encapsulating the wafer
101 and the proof mass
102.
The means for encapsulating the wafer
101 and the proof mass
102
can include a first side plate and a second side plate with the proof mass positioned
between the first side plate and the second side plate. A second groove
109
may be provided in the wafer
101. The second groove
109 may contain
a plug
110 or it may contain another optical fiber.
Referring now to FIGS. 2 and 3, another embodiment of a fiber optic micro
accelerometer constructed in accordance with the present invention is illustrated.
This fiber optic micro accelerometer embodiment is designated generally by the
reference numeral
200. A top view of the accelerometer
200 is shown
in FIG. 2 and a side view of accelerometer
200 is shown in FIG.
3.
The accelerometer
200 is constructed using microfabrication technologies.
Microfabrication technology enables the production of electrical, mechanical, electromechanical,
optical, chemical and thermal devices. The accelerometer
200 is useful for
determining forces, failures and failure prediction, and navigation, to name just
a few of many applications. The accelerometer
200 can detect and measure
motions in mechanical structures such as physics experiments, explosive environments,
industrial machinery, bridges, automobiles, planes, missiles and other equipment.
The accelerometer
200 is particularly well suited to: a.) minimally invasive
probes, b.) explosive or hazardous or extreme environments since no wires are employed
and the package is extremely robust, c.) sensor applications with high EMI, and
d.) remote readout instruments without loss of fidelity. The accelerometer
200
provides a passive sensing element that is very small and is optically interrogated
remotely. The accelerometer
200 can be optically interrogated remotely from
long distances. This is ideal for explosive environments. The accelerometer
200
is useful in explosive environments and in environments where electrical sparks
and noise create problems because there are no wires going in or out of the accelerometer
200 and the readout electronics and power source are at a remote location.
There is no stored energy in the accelerometer.
Referring now to FIG. 2, a top view of the accelerometer
200 is
shown. The accelerometer
200 comprises a wafer
201. The wafer
201
is a thin piece of material used as a structural component and optical component.
The wafer shown is 7 mm long and 4.4 mm wide; however, it is to be understood that
the wafer and other components can be made much smaller. The wafer
201 can
be made of silicon or other material. A proof mass
202 is incorporated in
the wafer
201. The proof mass
202 is incorporated in the wafer
201
by microprocessing. The proof mass
202 can be created by micromachining
the wafer
201 to incorporate the proof mass
202 or by other processing
methods such as etching. The proof mass
202 is approximately rectangular
with four individual corners
203,
204,
205, and
206.
Spring members are operatively connected to the proof mass
202. The spring
members
207,
208,
209, and
210 are located proximate
each of the corners
203,
204,
205, and
206 of the proof
mass
202. The spring members
207,
208,
209, and
210
enable a high compliance only along the desired sensitive axis, which is collinear
with the microetched fiber channel. Because it is difficult to measure acceleration
directly, the accelerometer
200 measures movement of a proof mass suspended
by the spring members
207,
208,
209, and
210.
An optical fiber
212 is connected to the wafer
201. The optical
fiber
212 has a 125 micrometer diameter fiber; however, it is to be understood
that other diameter fibers can be used. The optical fiber
212 is a typical
multimode optical fiber. While it is possible to use a single mode optical fiber,
there are advantages in using a multimode optical fiber. The multimode wide bandwidth
(i.e., white light) system has many practical advantages. The wafer
201
contains a groove
213 for receiving the optical fiber
212. The groove
213 can include an insertion funnel
214 for receiving the optical
fiber
212. Optionally, another groove
218 may be provided in the
wafer
201. The second groove
218 may contain a plug
219 or
it may contain another optical fiber. The optical fiber
212 is bonded to
the wafer
201 by bond material
217 such as an adhesive or a solder
alloy for hermetic sealing and fixing the position of the partially reflecting
mirror
215.
The optical fiber
212 has a partially reflecting end surface
215.
The partially reflecting surface
211 is integrated into the proof mass
202.
A Fabry-Perot cavity
216 is located between the partially reflecting end
surface
215 of the optical fiber
212 and the partially reflecting
surface
211. The Fabry-Perot (FP) cavity was invented in the late 1800s.
It essentially consists of two parallel plane optical mirrors; the mirrors may
be fully or partially reflective and partially transmissive. For example, a partial
mirror may be obtained on the flat end of an optical fiber. A partially reflective,
non-transmissive mirror may be a micromachined smooth surface on silicon. The FP
cavity operation has been thoroughly studied for many years and it is discussed
in many optics textbooks; for example the classic textbook "Principles of Optics,"
by Born and Wolf, MacMillan 1959 discusses the operating principles. The performance
of the FP cavity is strongly determined by the mirror gap spacing-as well as many
other factors such as mirror smoothness, mirror flatness, geometrical alignment
to an optical axis, degree of parallelism, etc. The cavity gap determines the optical
resonance of the cavity where single wavelength light is used; this effect has
been used for many, many years for optical filtering. For the broadband light that
is used to interrogate the accelerometer
200, the cavity gap (which is about
5-25 micrometers) strongly determines the phases present for the different wavelengths
that are present in the reflected light. This complex reflected spectrum has phase
information in it that can be processed by an optical receiver system that will
determine the gap at the FP cavity with a very high degree of accuracy and stability,
as has been previously demonstrated.
The transmissive and/or reflective properties are a strong function of the optical
and geometrical factors making up the cavity. Any physical effect, such as temperature,
that changes a cavity parameter, can be sensed by the optical characterization
of the cavity. In fact, much extreme effort has been put into making various FP
systems invariant to environmental factors such as temperature, mechanical strain,
pressure, acceleration, etc. The use of an optical interferometric component(s)
to make a physical parameter sensing system has been used for many years. It is
the detail of the system design and its fabrication method, that makes each system
unique, and specialized. One major difficulty has always been to make a FP sensor
that senses the desired physical parameter well, while also discriminating against
other effects.
The Fabry-Perot cavity
216 is an optical resonator in which feedback is
accomplished by two parallel planes. The partially reflecting surface
211
forms one mirror of the Fabry-Perot optical cavity
216. The other mirror
of the Fabry-Perot cavity
216 is formed by the partially reflecting end
surface
215 of the optical fiber
212. An optical readout of the position
of the proof mass
202, relative to the wafer
201, is achieved by
a remote external optical system that illuminates the mirrors and also measures
the reflected light as is well know in the art.
Referring now to FIG. 3, a side view of the accelerometer
200 is
shown. The accelerometer
200 includes means for encapsulating the wafer
201 and the proof mass
202. A first side plate
220 and a second
side plate
221 are used to hermetic seal the accelerometer
200. The
first side plate
220 and the second side plate
221 have recess sections
224 and
225 that provide a space for the proof mass
202. The
recess sections
224 and
225 provide a simple non-critically aligned
glass recess feature makes sure the proof mass is constrained for large out-of-plane
motion, protecting the device from severe overrange handling, which can cause beam
breakage. Metallic screening electrodes
222 and
223 are positioned
in the recess sections
224 and
225. The accelerometer
200
measures acceleration. Because it is difficult to measure acceleration directly,
the accelerometer
200 measures movement of the proof mass suspended by the
spring members
207,
208,
209, and
210.
The first side plate
220 and the second side plate
221 are positioned
over and below the wafer
201 and the proof mass
202 and bonded to
the wafer
201. A glass/silicon/glass wafer bonding scheme is used with integral
alignment to 50 micrometers. All 200+ parts on a single small 75 mm wafer are aligned
and packaged at once; the package/sensor is bonded at +350° C. A temperature-time
profile is used during the bonding process that not only produces a very strong
bond, but also minimizes the residual strain (upon cooling to ambient) between
the glass and the silicon. If uncontrolled, the silicon shrinks a lot more than
the glass when cooling. This, in turn, puts the proof mass suspension beams under
tension, which can have an adverse effect on their deflection characteristics and
change the responsivity [nanometers of deflection/g of acceleration] of the device.
The accelerometer
200 has been successfully plunge tested into liquid nitrogen
(˜-;200° C.) and into boiling water (+100° C). This demonstrates
that the accelerometer package is extremely robust.
Referring now to FIG. 4, another embodiment of a fiber optic micro accelerometer
constructed in accordance with the present invention is illustrated. This fiber
optic micro accelerometer embodiment is designated generally by the reference numeral
400. The accelerometer
400 is constructed using microfabrication
technologies. The accelerometer
400 can detect and measure motions in mechanical
structures such as physics experiments, explosive environments, industrial machinery,
bridges, automobiles, planes, missiles and other equipment.
The accelerometer
400 comprises a wafer
401. The wafer
401
is a thin piece of material used as a structural component and/or optical component.
The wafer
401 can be made of silicon or other material. A proof mass
402
is incorporated in the wafer
401. The proof mass
402 is incorporated
in the wafer
401 by microprocessing. The proof mass
402 is approximately
rectangular with four individual corners
403,
404,
405, and
406. Eight spring member are operatively connected to the proof mass
402.
The spring members
407,
408,
409,
410,
411,
412,
413, and
414 are located proximate the corners
403,
404,
405, and
406 of the proof mass
402.
The spring members provide a complex proof mass suspension system. The proof
mass
402 is suspended by the four primary ribbon shaped beams
407,
408,
409, and
410 that enable a high compliance only along
the desired sensitive axis, which is colinear with the microetched fiber channel.
The primary beams
407,
408,
409, and
410 are terminated
near the frame at the center of small orthogonal secondary stress relieving beams
411,
412,
413, and
414. The secondary beams
411,
412,
413, and
414 are designed to accommodate virtually all
of any residual bonding stress caused by the different thermal expansion coefficients
of silicon and glass. In other words, they relieve the tensional stress in the
primary beams
407,
408,
409, and
410. These secondary
beams
411,
412,
413, and
414 are also designed to be
ribbon shaped, so that the out-of-plane compliance is very low. The suspension
system has one fundamental low frequency resonance, well below other higher order
modes, that is along the desired sensitive axis.
The accelerometer
400 has special design fillets for concave corner rounding
at the ends of all the suspension beams and inside corners. This greatly improves
their strength as it reduces the stress concentration. This special fillet has
a gradual tapered section
415, in addition to the normal corner round. This
feature negates the detrimental artifact of RIE etching where on inside corners,
the proximity effect of the two adjacent sidewalls produces a slightly enhanced
lateral etch rate. Without this special fillet, the suspension beams would experience
a slight "necking" at their attachment points, caused by artifacts of micromachining,
thus promoting premature failure at this location.
Because it is difficult to measure acceleration directly, the accelerometer
400 measures movement of the proof mass
402 suspended by the spring
members
407,
408,
409,
410,
411,
412,
413, and
414. An optical fiber
420 is connected to the wafer
401. The optical fiber
420 has a 125 micrometer diameter fiber. The
wafer
401 contains a groove
419 for receiving the optical fiber
420.
Another groove
421 may be provided in the wafer
401. The second groove
421 may contain a plug
422 or it may contain another optical fiber.
The optical fiber
420 has a partially reflecting end surface
423.
The partially reflecting surface
423 is optically coupled to the proof mass
402. A Fabry-Perot cavity
424 is located between the partially reflecting
end surface
423 of the optical fiber
420 and a partially reflecting
surface
425 on the proof mass
402. The Fabry-Perot cavity
424
essentially consists of two parallel plane optical mirrors; the mirrors may be
fully or partially reflective and partially transmissive. The Fabry-Perot cavity
424 is an optical resonator in which feedback is accomplished by two parallel
planes. The partially reflecting surface
425 forms one mirror of the Fabry-Perot
optical cavity
424. The other mirror of the Fabry-Perot cavity
424
is formed by the partially reflecting end surface
423 of the optical fiber
420. An optical readout of the position of the proof mass
402, relative
to the wafer
401, is achieved by a remote external optical system that illuminates
the mirrors and also measures the reflected light as is well know in the art.
Referring now to FIG. 5, another embodiment of a fiber optic micro accelerometer
constructed in accordance with the present invention is illustrated. This fiber
optic micro accelerometer embodiment is designated generally by the reference numeral
500. The accelerometer
500 is constructed using microfabrication
technologies. Because it is difficult to measure acceleration directly, the accelerometer
500 measures movement of a proof mass suspended by the spring members. The
accelerometer
500 can detect and measure motions in mechanical structures
such as physics experiments, explosive environments, industrial machinery, bridges,
automobiles, planes, missiles and other equipment.
The accelerometer
500 is constructed in a similar fashion to the accelerometer
previously described. The accelerometer
500 comprises a wafer
501
that is a thin piece of material used as a structural component and/or optical
component. A proof mass
502 is incorporated in the wafer
501 by microprocessing.
Spring members as previously described provide a complex proof mass suspension
system. An optical fiber
503 is connected to the wafer
501. The optical
fiber
503 is located in a groove
504.
The optical fiber
503 has a partially reflecting end surface
505.
The partially reflecting surface
505 is optically coupled to a partially
reflecting surface
506 on the proof mass
502. A Fabry-Perot cavity
507 is located between the partially reflecting end surface
505 of
the optical fiber
503 and a partially reflecting surface
506 on the
proof mass
502.
The optical fiber
503 is retained in the groove
504 by adhesives,
soldering, or fusion bonding. Adhesive
508 is deposited at the entrance
to the groove
504 between the optical fiber
503 and the groove
504.
The groove is rectangular in shape and the optical fiber is generally round in
shape. This leaves a space between the groove
504 and the optical fiber
503. The adhesive
508 is drawn into the space by capillary action.
The accelerometer
500 includes adhesive wicking "dump" channels
509
on the sides of the main fiber channel
504. These narrow RIE etched grooves
509 strongly wick and divert the UV curable adhesive
508 as it initially
wicks down the fiber channel
504 from the outside. These side channels
509
divert excess adhesive so it does not penetrate onto the fiber end
505,
the Fabry-Perot cavity
507, or the partially reflecting surface
506.
Adhesive on the internal surfaces of the accelerometer such as the fiber end
505,
the Fabry-Perot cavity
507, or the partially reflecting surface
506
would ruin the device.
The accelerometer
500 is constructed using hermetic sealing to maintain
the internal parts in strictly clean condition. Means are provide for encapsulating
the wafer
501 and the proof mass
502. A first side plate and a second
side plate are used to hermetic seal the accelerometer
500. The adhesive
508 that is deposited in the groove
504 between the optical fiber
503 and the groove
504 maintains the hermetic sealing. The accelerometer
500 has a tapered opening or funnel
510 for the main channel
504
of the optical fiber
503. It is exposed in the final RIE fiber port opening
process. It makes it much easier to physically insert the fiber
503 into
its precision machined channel
504 before the fiber
503 is bonded
into place. Once the fiber
503 is bonded into the wafer
501, the
sensor construction is complete.
Referring now to FIG. 6, another embodiment of a fiber optic micro accelerometer
constructed in accordance with the present invention is illustrated and an embodiment
of a method of producing a fiber optic micro accelerometer is described. This fiber
optic micro accelerometer embodiment is designated generally by the reference numeral
600. The accelerometer
600 comprises a wafer
601. The wafer
601 is a thin piece of material used as a structural component and/or optical
component. The accelerometer is typically 7 mm long and 4.4 mm wide, but could
be much smaller. The wafer
601 can be made of silicon or other material
and is typically 75 mm diameter. A proof mass
602 is incorporated in the
wafer
601. The proof mass
602 is incorporated in the wafer
601
by microprocessing. The proof mass
602 can be created by micromachining
the wafer
601 to incorporate the proof mass
602 or by other processing
methods such as etching.
The accelerometer
600 is constructed using microfabrication technologies.
Microfabricated devices are formed using crystalline substrates, such as silicon
and gallium arsenide, but may be formed on non-crystalline materials, such as glass
or certain polymers. The shapes of crystalline devices can be precisely controlled.
Micromachined and etched surfaces and shapes are readily produced. The materials
may be bonded by processes such as fusion at elevated temperatures, anodic bonding,
or field-assisted methods. Microfabrication technology enables the production of
electrical, mechanical, electromechanical, optical, chemical and thermal devices.
The integration of these microfabricated devices into a single system allows for
the batch production of microscale instruments.
The accelerometer
600 is produced by a "bulk" and "dry" micromachining
process. This is used for many reasons. It is easy to make with common reactive
ion etch (RIE). It makes useable sidewall mirrors. It has a large proof mass [etching
all the way through silicon wafer versus "surface" type devices] and thus lower
resonant frequency. The proof mass motion is in-plane which eliminates air cushion
effect from normal-to-the-plane mass motion. RIE can accommodate arbitrary shaped
designs for the geometrical features of the part (i.e., beams, fiber guides, proof
mass, alignment holes and features, etc.). It has simple mask design, simple lithography
and processing and avoids critical alignment and anisotropic wet etching of wafers
used by many MEMS efforts. Dry processing avoids water spots and other liquid etch
processing problems of microairbubbles. It provides uniformity, temperature control, etc.
The wafer
601 and the proof mass
602 are produced using a glass
photomask. The desired pattern is produced on the starting wafer using an etch
mask. The starting wafer is processed using an RIE etch. This is done from both
sides of the starting wafer. The starting wafer is processed into the accelerometer
frame
601 and the proof mass
602. The wafer
601 and the proof
mass
602 are encapsulated between a first side plate
603 and a second
side plate
604 to hermetic seal the accelerometer
600. A glass/silicon/glass
wafer bonding scheme with integral alignment to 50 micrometers is used. The package/sensor
is bonded at +350° C. A temperature-time profile is used during the bonding
process that not only produces a very strong bond, but also minimizes the residual
strain (upon cooling to ambient) between the glass and the silicon. If uncontrolled,
the silicon shrinks a lot more than the glass when cooling. This, in turn, puts
the proof mass suspension beams under tension, which can ruin their deflection
characteristics and change the responsivity [nanometers of deflection/g of acceleration]
of the device.
Metallic screening electrodes
605 and
606 are deposited on
the plates
603 and
604, and they are simply pin-aligned through holes
609 and
610 with a shadow mask, and it makes an automatic connection
to the patterned silicon just prior to anodic bonding. This eliminates a severe
problem of electrostatic attraction that is always present during anodic bonding
that can, and will usually, snap-down the proof mass
602 to one of the plates
603 or
604 and permanently stick it, thus ruining the part. This
is a particularly severe problem when glass side plate recess regions are very
close to the proof mass, which is desirable for large overrange protection. The
glass recess feature
607 and
608 makes sure the proof mass is constrained
for large out-of-plane motion, protecting the device from severe overrange handling,
which can cause beam breakage.
The accelerometer
600 is produced using hermetic sealing during the bonding
process, which enables wafer-level part dicing, dicing line
612, with a
precision diamond grit saw, without risk of internal part contamination by wafer
saw water and grit. The internal pressure inside the accelerometer
600 is
maintained during the processing operations and external fluid and particles are
prevented from entering the accelerometer
600. Once the channel
611
is opened during the final processing, by dry etching away the temporary plug
613,
the internal pressure vents outward. The final parts are completely dry and clean
when the fiber ports are opened up by one last short, non-critical, non-masked
RIE process. The fiber port opening process insures that the residual moderate
pressure inert gas inside the part flows from inside the part into the RIE etch
chamber when the fiber port is first etched open, thus removing any particulate
debris and preventing it from contaminating the interior of the device. The accelerometer
600 is easily amenable to practical mass production.
Referring now to FIG. 7, another embodiment of a method of producing a
micro fiber optic accelerometer in accordance with the present invention is illustrated
and described. This embodiment is designated generally by the reference numeral
700. A wafer disk
701, typically 75 mm, 100 mm, or larger contains
a large number of individual sensor components
702 and other components
703. The wafer disk
701 allows the micro accelerometer to be produced
by mass production. The method comprises producing an accelerometer including the
steps of: microprocessing a wafer to produce a proof mass, at least one spring
member, and a channel for receiving an optical fiber, positioning a first side
plate and a second side plate adjacent the, wafer, the proof mass, the at least
one spring member, and the channel and connecting the first side plate and the
second side plate to the wafer, connecting an optical fiber to the wafer in the
channel. The integration of these microfabricated devices into a single system
allows for the batch production of microscale instruments.
The accelerometer is produced using hermetic sealing during the packaging process,
which enables wafer-level part dicing with a precision diamond grit saw, without
risk of internal part contamination by wafer saw water and grit. The accelerometer
is produced by microprocessing a wafer to produce for each sensor component or
chip: a proof mass, at least one spring member, and a channel for receiving an
optical fiber, positioning a first side plate and a second side plate adjacent
the, wafer, the proof mass, the at least one spring member, and the channel and
connecting the first side plate and the second side plate to the wafer, connecting
an optical fiber to the wafer in the channel.
While the invention may be susceptible to various modifications and alternative
forms, specific embodiments have been shown by way of example in the drawings and
have been described in detail herein. However, it should be understood that the
invention is not intended to be limited to the particular forms disclosed. Rather,
the invention is to cover all modifications, equivalents, and alternatives falling
within the spirit and scope of the invention as defined by the following appended claims.
*
