U.S. patent application number 11/804177 was filed with the patent office on 2008-05-01 for self-stabilizing, floating microelectromechanical device.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Jason Vaughn Clark.
Application Number | 20080100175 11/804177 |
Document ID | / |
Family ID | 35185699 |
Filed Date | 2008-05-01 |
United States Patent
Application |
20080100175 |
Kind Code |
A1 |
Clark; Jason Vaughn |
May 1, 2008 |
Self-stabilizing, floating microelectromechanical device
Abstract
The present invention relates to MicroElectroMechanical Systems
(MEMS), devices and applications thereof in which a proof mass is
caused to levitate by electrostatic repulsion. Configurations of
electrodes are described that result in self-stabilized floating of
the proof mass. The electrical properties of the electrodes causing
floating, such as currents and/or voltages, typically change in
response to environmental perturbations affecting the proof mass.
Measuring such currents and/or voltages allow immediate and
accurate measurements to be performed related to those
perturbations affecting the location and/or the orientation of the
proof mass. Additional sensing electrodes can be included to
further enhance sensing capabilities. Drive electrodes can also be
included that allow forces to be applied to the charged proof mass
resulting in a floating, electrically controllable MEMS device.
Several applications are described including accelerometers,
inertial sensors, resonators and filters for communication devices,
gyros, one and two axis mirrors and scanners, among other devices.
Several fabrication methods are also described.
Inventors: |
Clark; Jason Vaughn; (San
Pablo, CA) |
Correspondence
Address: |
MICHAELSON & ASSOCIATES
P.O. BOX 8489
RED BANK
NJ
07701
US
|
Assignee: |
The Regents of the University of
California
Oakland
CA
|
Family ID: |
35185699 |
Appl. No.: |
11/804177 |
Filed: |
May 16, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10836562 |
Apr 30, 2004 |
7225674 |
|
|
11804177 |
|
|
|
|
Current U.S.
Class: |
310/309 ;
73/514.18 |
Current CPC
Class: |
B81B 5/00 20130101; B81B
2201/0235 20130101; G02B 26/001 20130101; G01P 15/18 20130101; G01P
15/125 20130101; G02B 26/0841 20130101 |
Class at
Publication: |
310/309 ;
73/514.18 |
International
Class: |
H02N 1/06 20060101
H02N001/06; G01P 15/125 20060101 G01P015/125 |
Claims
1-20. (canceled)
21. A floating MEMS device comprising: a) an electrostatically
charged proof mass; and, b) at least two stabilizing electrode
structures carrying charges that repel said charges on said proof
mass, wherein i) said proof mass is located with respect to said
stabilizing electrode structures so as to levitate due to
like-charge repulsion between said proof mass and said stabilizing
electrode structures; and wherein ii) said proof mass and said
stabilizing electrode structures are located so as to produce
electrostatic repulsion leading to stable levitation such that
displacements of said proof mass from its equilibrium floating
position result in electrostatic forces on said proof mass tending
to return said proof mass to said equilibrium position; and further
comprising, c) a plurality of side electrodes disposed about the
periphery of said proof mass, having first ends of said side
electrodes integrally attached to said proof mass and having the
opposite, second ends of said side electrodes projecting outward
from said proof mass, and wherein said side electrodes carry at
least a portion of said electrostatic charge of said proof mass,
and wherein each of said second ends of said side electrodes
project into one of said stabilizing electrode structures; and
further comprising, d) at least one drive electrode located so as
to apply force to said proof mass under external control.
22. A floating MEMS device comprising: a) an electrostatically
charged proof mass; and, b) at least two stabilizing electrode
structures carrying charges that repel said charges on said proof
mass, wherein i) said proof mass is located with respect to said
stabilizing electrode structures so as to levitate due to
like-charge repulsion between said proof mass and said stabilizing
electrode structures; and wherein ii) said proof mass and said
stabilizing electrode structures are located so as to produce
electrostatic repulsion leading to stable levitation such that
displacements of said proof mass from its equilibrium floating
position result in electrostatic forces on said proof mass tending
to return said proof mass to said equilibrium position; and further
comprising, c) a plurality of side electrodes disposed about the
periphery of said proof mass, having first ends of said side
electrodes integrally attached to said proof mass and having the
opposite, second ends of said side electrodes projecting outward
from said proof mass, and wherein said side electrodes carry at
least a portion of said electrostatic charge of said proof mass,
and wherein each of said second ends of said side electrodes
project into one of said stabilizing electrode structures; and
wherein, d) two side electrodes provide an axis of rotation for
said proof mass.
23. A floating MEMS device as in claim 22 further comprising at
least one drive electrode capable of providing force to said proof
mass for rotation of said proof mass about said axis.
24. A floating MEMS device as in claim 22 further comprising a
third stabilizing electrode as a substantially circular track so as
to allow said two side electrodes substantially free traverse
around said track.
25. A floating MEMS device as in claim 24 further comprising at
least one drive electrode located so as to apply force to said
proof mass under external control.
26. A resonator comprising: a) an electrostatically charged proof
mass; and, b) at least two stabilizing electrode structures
carrying charges that repel said charges on said proof mass,
wherein i) said proof mass is located with respect to said
stabilizing electrode structures so as to levitate due to
like-charge repulsion between said proof mass and said stabilizing
electrode structures; and wherein ii) said proof mass and said
stabilizing electrode structures are located so as to produce
electrostatic repulsion leading to stable levitation such that
displacements of said proof mass from its equilibrium floating
position result in electrostatic forces on said proof mass tending
to return said proof mass to said equilibrium position; and further
comprising, c) a plurality of side electrodes disposed about the
periphery of said proof mass, having first ends of said side
electrodes integrally attached to said proof mass and having the
opposite, second ends of said side electrodes projecting outward
from said proof mass, and wherein said side electrodes carry at
least a portion of said electrostatic charge of said proof mass,
and wherein each of said second ends of said side electrodes
project into one of said stabilizing electrode structures; and
further comprising, d) at least one sensor for detecting the
position of said proof mass, wherein said at least one sensor
comprises at least one sensing electrode in proximity to said proof
mass; and further comprising, e) at least one drive electrode
capable of driving said proof mass into a resonance mode.
Description
CLAIM TO PRIORITY
[0001] This application is a divisional of, and claims the benefit
of, our co-pending United States application entitled
"Self-Stabilizing, Floating Microelectromechanical Device" filed
Apr. 30, 2004 and assigned Ser. No. 10/836,562, the entire contents
of which, including all attachments thereto, are hereby
incorporated herein by reference for all purposes.
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] This invention relates generally to the field of
microelectromechanical (MEMS) devices and, in particular, to
floating MEMS devices in which a proof mass is electrostatically
levitated.
[0004] 2. Description of the Prior Art
[0005] Microelectromechanical (MEMS) devices are finding
ever-increasing areas of application in the modern economy as
trends towards miniaturization, portability, lightweight, low power
consumption and low cost drive many technologies. Sensors,
actuators and many other devices are all affected by the wider
application of microtechnology and MEMS fabrication techniques.
[0006] The present invention relates to a floating MEMS device
(FLEMS) in which a proof mass is levitated electrostatically as
part of a MEMS device, without the need for feedback control means
and, once levitated, without mechanical contact between the proof
mass and any other portion of the device.
[0007] This non-contact floating MEMS device of the present
invention should be contrasted to prior MEMS devices, particularly
accelerometers, in which acceleration is typically sensed by
deflection of a proof mass mechanically coupled to the device.
Examples include U.S. Pat. Nos. 5,969,848; 5,992,233; 6,067,858;
6,296,779 and 6,250,779.
[0008] Electrostatic levitation has been used in a variety of
non-MEMS fields of application including non-contact materials
processing, acceleration or inertial sensors, and other
applications. However, these devices typically require complex
means to control and stabilize the position of the proof mass. For
example, U.S. Pat. Nos. 5,015,906 and 5,187,399 employ high
frequency sinusoidal excitation circuitry for levitation and
control. U.S. Pat. No. 4,521,854 is an example of electrostatic
levitation requiring a complex sensing and feedback system to
maintain the proof mass in its floating position.
[0009] In view of the foregoing, a need exists in the art for a
floating MEMS device that includes self-stabilizing levitation
without requiring sensing and/or feedback systems for controlling
the levitation of the proof mass.
SUMMARY OF THE INVENTION
[0010] Accordingly and advantageously the present invention
includes MEMS devices and applications thereof in which a proof
mass is caused to levitate by electrostatic repulsion.
[0011] The present invention relates to a floating MEMS device
(FLEMS) in which a proof mass is levitated electrostatically
without the need for feedback control means and, once levitated,
without mechanical contact between the proof mass and any other
portion of the device. This is in contrast with other MEMS devices,
including accelerometers and inertial sensors, in which deflection
of a mechanically-coupled proof mass, cantilevered beam or other
structure is the basis for measuring acceleration. In various
embodiments of the present non-contact floating MEMS device,
mechanical wear is substantially eliminated and mechanical
parasitic effects are sharply reduced or eliminated. Thus, an
important objective of the present invention is to produce a
floating MEMS device in which the proof mass has no mechanical
contact with other portions of the device.
[0012] Configurations of electrodes for a FLEMS device are
described that, when charged with like charges as the proof mass,
result in self-stabilized floating of the proof mass based upon
electrostatic repulsion. Self-stabilization results from a FLEMS
structure such that, when the proof mass is displaced from its
equilibrium position, electrostatic forces arise tending to restore
the proof to its equilibrium position. The magnitude of the
restoring electrostatic forces tend to increase in magnitude as the
proof mass undergoes a larger displacement from its equilibrium
position. Thus, another objective of the present invention is to
produce a self-stabilizing FLEMS device that does not require
feedback control systems. In addition, since only like electrical
charges are in close proximity, the dangers of short circuits are
markedly reduced.
[0013] Perturbations causing displacement of the proof mass can
conveniently be measured by measuring the changes in electrical
properties of the stabilization electrodes or electrical properties
of the electrodes causing floating, and/or the electrical
properties of additional sensing electrodes that can be included to
further enhance sensing capabilities. Drive electrodes can also be
included that allow forces to be applied to the charged proof mass
resulting in a floating, electrically controllable MEMS device.
Numerous applications are possible including: 1) Charge storage
device. 2) GHz resonator. 3) Accelerometer. 4) Wide-angle scanner.
5) Magnetometer. 6) Nanogravity sensor. 7) Inertial navigation. 8)
Magnetic confinement. 9) Quantum gyros. 10) Harsh-environment
applications. 11) Thermal isolation applications. 12) Micro
transport.
[0014] FLEMS devices can conveniently be fabricated by lithographic
technologies developed, in part, for the fabrication of integrated
circuits. In addition, FLEMS devices can be fabricated by means of
various three-dimensional microfabrication technologies that have
been developed by several research groups and publicly available
including EFAB.TM., PolyMUMPs, among others.
[0015] Other embodiments of FLEMS devices include a proof mass
attached to a plurality of side electrodes that carry a substantial
portion of the electrostatic charge of the proof mass. The side
electrodes can be terminated by one or more termination blocks or
other structures that lead to stable levitation of the proof mass
and attached side electrodes when this combined structure is
embedded in a suitable configuration of stabilizing electrodes.
[0016] The proof mass and the stabilizing electrodes can be charged
by a variety of techniques including contact with an external
voltage source, charging by means of electrification by induction
and/or conduction, capacitance, tribocharging, electric discharge,
among other methods.
[0017] Once charged FLEMS, devices typically require almost no
additional electric power, depending on the magnitude of leakage
currents for the structures and materials used in the FLEMS device
and for the environmental conditions in which it is operated. That
is, it is not required to maintain continuous contact between the
charging source and the stabilizing electrodes. The charging source
can be disconnected and re-engaged only when necessary to replenish
the charge that is lost over time due to leakage.
[0018] Further, the operating range of FLEMS devices is, or can be
made, very large. The response of a FLEMS device to various
external perturbations can be changed by changing the amount of
electric charge residing on various portions of the FLEMS device,
including dynamic changes in real-time or near real-time during
operation of the device. In addition, FLEMS devices are typically
largely insensitive to operating temperatures.
[0019] These and other advantages are achieved in accordance with
the present invention as described in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures. The drawings are not to scale and
the relative dimensions of various elements in the drawings are
depicted schematically and not to scale.
[0021] The techniques of the present invention can readily be
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0022] FIG. 1. FIGS. 1a and 1b depict in schematic cross-sectional
view structures arising in one approach to FLEMS fabrication. FIGS.
1c, 1d, 1e depict in schematic cross-sectional view other
structures arising in FLEMS fabrication. All numerical values are
approximate.
[0023] FIGS. 2a and 2b depict in perspective view (2a) and side
view (2b) a FLEMS device. All numerical values are approximate.
[0024] FIGS. 3a-3f are schematic depictions of various charging
mechanisms.
[0025] FIG. 4 is a schematic perspective view of a typical FLEMS
device including sensing and drive electrodes.
[0026] FIGS. 5a-5c are schematic perspective views of a typical
FLEMS mirror/scanner in various angular positions.
[0027] FIGS. 6a-6c are schematic perspective views (6a, 6b) and a
cut-away view (6c) of a typical FLEMS two-axis mirror/scanner.
[0028] FIG. 7 depicts in schematic perspective view the rotor and
stators of a FLEMS flux capacitor.
[0029] FIGS. 8a and 8b depict in top view (8a) and side view (8b)
typical magnetic fields generated by a FLEMS flux capacitor.
[0030] FIG. 9 depicts in schematic perspective view a FLEMS flux
capacitor as in FIG. 7 including typical drive voltages applied to
the stators.
[0031] FIG. 10 depicts an exploded view of the rotor and stators of
a FLEMS flux capacitor as in FIG. 7.
[0032] FIG. 11 depicts in top view, a schematic diagram of a FLEMS
flux capacitor in combination with a SQUID to form a sensitive
gyro.
[0033] FIG. 12 depicts in schematic perspective view (12a) and
schematic cut-away views (12b and 12c) another configuration for a
FLEMS/SQUID device.
DETAILED DESCRIPTION OF THE INVENTION
[0034] After considering the following description, those skilled
in the art will clearly realize that the teachings of the invention
can be readily utilized as Floating MicroElectroMechanical Systems
and devices (FLEMS) as well as for the fabrication of FLEMS devices
and the use thereof in various applications.
[0035] FIG. 1 depicts in schematic cross-sectional view structures
arising in one approach to the fabrication of FLEMS devices
pursuant to some embodiments of the present invention. The FLEMS
fabrication procedures described herein are exemplary and not
intended to be limiting as variations apparent to those with
ordinary skills in the art can also be employed within the intent
and scope of the present invention.
[0036] On a semiconducting substrate 1, typically silicon, a layer
of dielectric 2 is deposited. Dielectric 2 is typically silicon
nitride on a silicon substrate 1 although other combinations of
materials are not excluded. Typically, dielectric layer 2 is about
0.5 .mu.m in thickness (.mu.m=micron=10.sup.-6 meter).
[0037] A layer of conductive material, typically polysilicon
("poly"), is deposited on the dielectric layer 2. The poly layer is
typically about 0.5 .mu.m in thickness. A continuous poly layer is
deposited and then patterned by known conventional lithographic
means to produce an opening in poly layer 3 as depicted in FIG.
1a.
[0038] Another dielectric layer, 4, is then deposited on patterned
layer 3. Dielectric layer 4 is typically silicon dioxide ("oxide")
and has a typical thickness of approximately 2 .mu.m. In practice,
it is expected that the various layers will deposit substantially
conformally in the fabrication of the device depicted in FIG. 1.
However, conformal layering is not depicted in FIG. 1 for
simplicity.
[0039] A second conductive layer 5 is deposited on dielectric layer
4 and patterned so as to have a shape substantially as depicted in
FIG. 1. Typically, dielectric layer 5 is also poly and about 2
.mu.m in thickness. Conductive layer 5 becomes the floating element
or "proof mass."
[0040] A second layer of dielectric 6, typically another layer of
oxide, is deposited on the patterned conductor 4 and comes into
contact with dielectric layer 4 around the periphery of conductor 5
as depicted in FIG. 1. Typically, oxide layer 6 is approximately
0.75 .mu.m thick.
[0041] A third conductive layer 7 is deposited on dielectric layer
6 and patterned so as to have a shape substantially as depicted in
FIG. 1. Conductive layer 7 is typically another layer of poly
having a thickness of approximately 1.5 .mu.m.
[0042] The structure depicted in FIG. 1 is shown in cross-section.
It is convenient for the structure to be fabricated so as to have
approximate rotational symmetry about a central vertical axis 8.
Thus, FIG. 1 depicts a radial cross-sectional view of a
substantially circular structure. However, approximate rotational
symmetry is not required of structures fabricated as depicted in
FIG. 1 and square, rectangular, polygonal, elliptical or any other
shape can be used as convenient for the particular application
under consideration and/or for particular fabrication procedures
and/or materials. Indeed, a linear structure can be fabricated by
means of the procedures described herein with a long z dimension
perpendicular to the plane of FIG. 1 and a substantially constant
cross-section as depicted in FIG. 1 perpendicular to the z axis.
Such a device, when coupled with suitable drive electrodes as
described elsewhere herein, is capable of transporting the proof
mass 5 linearly along the z axis. When coupled with appropriate
driving voltages, the proof mass can be caused to accelerate to
high velocity along the z direction.
[0043] The lateral extent of the structure, for example the length
of conductor 5, is not critical in the fabrication or operation of
the device and can be chosen for ease of fabrication and/or for a
particular field of application. Typical FLEMS applications will
employ structures with conductors 5 having typical lengths around a
hundred to a few hundred microns, although larger or smaller
structures are not excluded.
[0044] Following the fabrication of a structure as depicted
schematically in FIG. 1, insulating layers 4 and 6 are removed.
Hydrofluoric acid, HF, is a commonly-used etchant for silicon
dioxide insulating layers. Thus, conductor 5 becomes detached from
other structures of the device but, due to the overhanging geometry
of conductor 7, remains substantially entrapped in the space
between conductors 7 and 3 as depicted in FIG. 1a.
[0045] The removal of insulating layers 4 and 6 causes conductor 5
to come to rest under the force of its own weight in contact with
conductors 3 for the orientation depicted in FIG. 1a, or conductors
7 for an inverted configuration. To be concrete in our description,
we presume the force of gravity acts downward in FIG. 1a,
understanding thereby that no essential differences occur if the
configuration were inverted.
[0046] FIGS. 1c, 1d and 1e depict alternative structures for the
FLEMS device of FIGS. 1a and 1b. Although the dimensions and
structures depicted in FIGS. 1c, 1d and 1e are expected to be not
too far from realistic for functioning FLEMS devices, the depiction
is schematic and the dimensions are approximate. However, FIG. 1d
depicts negative charges in contrast to the positive charges
depicted in FIG. 1b. This illustrates that the like-charge
repulsion leading to levitation in a FLEMS device can. result from
either (+, +) or (-,-) repulsion. Furthermore, it is quite feasible
that (+,+) repulsion occurs in one portion of a FLEMS device while
(-,-) repulsion occurs in another portion of the device
[0047] One convenient method for charging the conductors involves
the application of a voltage to conductors 3 and 7, thereby
delivering charge to conductor 5 in contact therewith. However,
when charged, conductor 5 will repel from both 3 and 7 to float as
depicted in FIG. 1b. Thus, conductor 3 remains in electrical
contact with conductor 3 for so long as is necessary for conductor
5 to accumulate sufficient charge to repel or "float" and break
electrical contact with conductors 3. In some circumstances it is
advantageous for the floating conductor 5 to receive substantially
more charge than the minimum necessary to cause floating and
severance of electrical contact. Thus, some embodiments of the
present invention include electrical contact(s) for delivering
charge directly to conductor 5. Other embodiments include
mechanical and/or chemical means for maintaining conductor 5 in
electrical contact with conductor 3 despite the generation of
electrical repulsion greatly exceeding the weight of conductor 5.
Such embodiments typically allow for the mechanical, chemical
and/or thermal release (for example, a meltable solder), when the
desired charge has been delivered to floating conductor 5. It is
advantageous that the charging mechanism be such that it can be
conveniently employed numerous times as the device typically
requires numerous re-chargings over its service lifetime.
[0048] FIG. 1b depicts floating conductor 5 suspended by electrical
repulsion between conductors 3 and 7. Positive charges are depicted
in FIG. 1b for convenience but negative charges can also be
employed to provide the mutual repulsions. The charges are depicted
in FIG. 1b schematically to depict mutual repulsion, not to
indicate a realistic charge distribution on conductors 3, 5 or 7.
Charges will distribute themselves over the conductors in response
so the mutual repulsions between like charges within a conductor
and between conductors lead to a distribution having no net forces
on the charges. A somewhat more realistic (but still approximate)
charge distribution is depicted in FIG. 1d.
[0049] It is clear from FIG. 1b that the configuration of the
floating conductor 5 possesses an inherent stability without the
necessity for position detectors, feedback means, or the like. A
displacement of floating conductor 5 in any direction brings like
charges closer together hereby increasing the restoring electrical
repulsive force tending to return the floating conductor to the
position from which it came. Thus, once charged, floating conductor
5 is captured between stabilizing conductors 3 and 7 in FIG. 1b and
can remain so captured indefinitely, so long as the electrical
charges leading to restoring forces remain sufficiently large to
overcome perturbations or disturbances to which the device is
subjected.
[0050] In addition, it is clear from FIG. 1b that the position
and/or motion of conductor 5 can be simply controlled by
time-varying application of different charges to conductors 3
and/or 7. Complex control of motion and/or position may call for
more involved configurations of stabilizing conductors, several
examples of which are described herein while additional examples
would be apparent to those with ordinary skills in the art.
[0051] Conductors 3 and 7 can be in electrical contact with each
other by means of one or more contacts not depicted in FIG. 1b, or
electrically isolated from each other. It is advantageous in some
embodiments of the present invention to provide electrical
isolation for conductors 3 and 7 such that a displacement of
floating conductor 5 towards one stabilizing conductor (say
conductor 3)does not permit charge to flow from conductor 3 to
conductor 7, thereby reducing the stabilization forces on the
floating conductor. Some embodiments of the present invention
involve voltage and/or current sensing devices connected between
stabilizing electrodes in order to detect and perhaps measure
disturbances or perturbations to which floating conductor 5 may be
subject as, for example, in the construction of an accelerometer.
However, in some embodiments, it is advantageous to utilize
additional electrodes in place of, or in addition to, the
stabilizing electrodes for purposes of sensing. Optical sensing
means can also be employed within the scope of the present
invention.
[0052] The devices depicted and described in connection with FIGS.
1a and 1b are conveniently fabricated with conductive components 3,
5 and 7. However, the devices are not inherently limited to
conductive components and can employ semiconductive and/or
dielectric components as 3, 5, and 7 so long as sufficient etching
selectivity is present to permit removal of sacrificial layers 4
and 6. However, in such cases charging of floating component 5 by
gravity-induced contact and electrical conduction with another
element is typically not feasible. However, other charging
mechanisms as described elsewhere herein can be employed.
[0053] The devices depicted and described in connection with FIGS.
1a and 1b use the same materials for layers 3, 5 and 7 (for
example, conductive polysilicon) and for sacrificial layers 2 and 4
(for example, silicon dioxide) as a matter of convenience and not
as an inherent limitation in the devices or fabrication processes.
Different conductive layers 3, 5, and 7 can be employed and 3, 5, 7
need not be wholly made from the same material so long as these
layers have adequate charge-retaining electrical properties and
adequate etching selectivity with respect to the sacrificial layers
2 and 4. Likewise, different sacrificial layers may be employed in
layers 2 and 4 so long as adequate etching selectivity is
retained.
[0054] In addition to fabrication procedures described herein,
FLEMS devices pursuant to some embodiments of the present invention
can also be fabricated by means of the EFAB.TM. fabrication
techniques of Microfabrica, Inc. of Burbank, Calif. (formerly the
MEMGen Corporation). EFAB.TM. technology is essentially an additive
microfabrication process based on electrodeposition of multiple
layers of metals and capable of producing three dimensional
devices. Further information concerning EFAB.TM. is described in
materials printed from the website having URL www.microfabrica.com
which are incorporated hereby by reference.
[0055] Other techniques for fabrication of three dimensional
microscale structures include fabrication techniques developed by
the MCNC Research and Development Institute of Research Triangle
Park, N.C. (founded as the Microelectronics Center of North
Carolina) and applied to the fabrication of vertical interconnects
for integrated circuits. Further description of the MCNC
fabrication techniques is described in materials printed from the
website having the following URL: www.mcnc.org which are
incorporated hereby by reference. A related fabrication technique
"PolyMUMPs" by MEMSCAP, Inc. is described in materials printed from
the website having the following URL: www.mcnc.org which are
incorporated hereby by reference.
[0056] Another embodiment of a FLEMS device pursuant to some
embodiments of the present invention is depicted in perspective
view in FIG. 2a and in cross-sectional view in FIG. 2b. A proof
mass 20 is attached to a plurality of side electrodes 25. The side
electrodes can be terminated by a termination block 26 or other
structure such that, when imbedded in levitating and stabilizing
electrodes 21, an advantageous configuration for floating and
stabilizing the proof mass results.
[0057] As described in detail elsewhere, the forces causing the
proof mass to float and the stabilization forces for the floating
proof mass result chiefly from electrostatic repulsion between the
components of the stabilizing electrodes 21 and the side electrodes
(and termination blocks, if any). In some applications, it is
advantageous for charges to be isolated on the side electrodes
and/or termination blocks and be hindered from flowing away from
the stabilizing electrodes, 21. Thus, it is advantageous in such
cases for the proof mass to be insulating preventing charge from
leaving the side electrodes. It is also advantageous in some
embodiments for all or part of the side electrodes to be
insulating, retaining charge on the termination blocks, 26, and/or
on the portion of side electrodes 25 in proximity to the
stabilizing electrode structure 21. However, aside from these
guidelines, the choice of materials for the FLEMS device depicted
in FIGS. 2a and 2b is largely within the design discretion of the
FLEMS designers.
[0058] However, in other instances, the repulsion of like charges
inherently causes like charges deposited on the proof mass to
separate and to congregate chiefly in the side electrodes when both
proof mass and side electrodes (and termination blocks, if any) are
conductive. In typical instances, this internal repulsion keeps
sufficient charge on or near the stabilizing electrodes to provide
for levitation in spite of repulsion of like charges in the
stabilizing electrodes and the side electrodes tending to drive
charges out of the side electrodes. Thus, both conductive and
partially conductive structures can be employed for all or part of
the assembly consisting of the proof mass and the side
electrodes.
[0059] The side view of the FLEMS device of FIG. 2a is depicted in
FIG. 2b, including exemplary dimensions for various structures and
gaps. These dimensions are both approximate and illustrative as
different dimensions, or different combinations of dimensions, can
be employed within the design discretion of the FLEMS designers and
within the scope of the present invention.
[0060] With both stabilizing electrodes and side electrodes
uncharged, the proof mass typically lies on or near the substrate
27 with the side electrodes or the termination blocks resting on
the lower electrodes 22b of the stabilizing electrodes. Numerous
charging methods can be employed including bringing the side
electrodes and stabilizing electrodes into contact with an external
voltage, or charging by means of electrification by induction
and/or conduction by contact with a charged body as depicted
schematically in FIGS. 3a-3f. Charging can be accomplished by
several methods including, but not limited to, capacitance,
tribocharging, electric discharge, contact with a battery or other
voltage source, among other methods. Upon charging the stabilizing
electrodes and the side electrodes (and possibly the proof mass),
the resulting like-charge repulsion causes the proof mass to repel
from the stabilizing electrodes.
[0061] Once charged, it is advantageous in many applications for
the proof mass to avoid or reduce charge leakage, thereby
increasing the time between re-charging. Thus, it is advantageous
in such instances for the proof mass to be electrically isolated
from various means of charge leakage including sources of
ionization of the ambient gas and the like, for example, by
immersion in a vacuum, reduction of sharp points or edges in
charge-containing regions, among other techniques. Applications
such as accelerometers in space are particularly in need of long
times between charging, and possibly subject to harsh conditions of
ambient ionizing radiation.
[0062] Several advantages derive from the FLEMS structure depicted
in FIGS. 2a and 2b and variations thereof. For example, as a
non-contact floating device, mechanical wear is substantially
eliminated. In addition, if both the side electrodes/proof mass and
the stabilizing electrodes are charged to the same potential, there
is no danger of short-circuiting, in contrast to floating devices
based upon the attraction of opposite charges. Also, since the
proof mass is mechanically decoupled from its surroundings,
mechanical parasitic effects are sharply reduced or eliminated
including, for example, thermal isolation, isolation from
electrical noise and independence from process variations such as
Young's modulus, Poisson's ratio, among others. Additionally, as
noted elsewhere, once charged the FLEMS devices typically require
almost no additional electric power, depending on the magnitude of
leakage currents for the structures and materials used in the FLEMS
device and for the environmental conditions in which it is
operated. To be clear, it is not required to maintain the contact
between the charging source and the stabilizing electrodes. The
charging source may be re-engaged periodically to replenish the
charge that is lost over time due to leakage.
[0063] Further, the operating range of the FLEMS devices pursuant
to some embodiments of the present invention is, or can be made,
very large. Adjustment of the charge carried by the stabilizing
electrodes and/or the side electrodes and proof mass typically
changes the mechanical characteristics of levitation, including
changing the restoring forces experienced in response to
displacements, changing induced voltages and/or currents, among
other effects. Thus, device response to various external
perturbations can be changed merely by changing the charges on
various portions of the FLEMS device, including dynamic changes in
real-time or near real-time during operation of the device.
[0064] In addition, FLEMS devices are typically largely insensitive
to operating temperatures, performing throughout substantially any
temperature range at which the mechanical and/or electrical
properties of the materials are sufficient for stable levitation.
Some applications of FLEMS devices make use of optical, magnetic
and/or other material properties which also should not
substantially degrade over the desired operating range.
[0065] The devices depicted schematically in FIGS. 2a and 2b are
illustrative of typical FLEMS devices and do not encompass the full
range of embodiments within the scope of the present invention. For
example, the number of stabilizing electrodes and side electrodes
need not be four as depicted, but can be either fewer or greater.
Examples of devices having two electrodes are given elsewhere
herein. FIGS. 1a and 1b depict what is essentially a continuum of
side and stabilizing electrodes. Various numbers between these
limits are also possible and included within the scope of the
present invention.
[0066] The present invention is not limited to two-dimensional
embodiments as depicted in FIGS. 1 and 2. Examples follow of
embodiments of FLEMS devices having substantially linear structures
with two stabilizing and side electrodes. Additional stabilizing
and side electrodes can be added analogous to those depicted in
FIG. 2 but in directions perpendicular (above and below) the proof
mass, leading to a family of FLEMS devices in three-dimensions.
EXAMPLES
1. Accelerometer--Inertial Sensor
[0067] Accelerometers and inertial sensors have numerous uses
including guidance and navigation, gravitational field detection,
collision sensors (as for vehicle air bag deployment), among other
applications. The FLEMS embodiments depicted in FIGS. 2a and 2b are
useful for this application. As proof mass 20 experiences a force
such as that caused by an acceleration, impact, gravitational
field, among others, the charges residing on or near termination
blocks 26 and side conductors 25 will undergo a displacement with
respect to charges residing on the stabilizing structures 21. This
charge displacement will induce voltages and/or currents that can
be measured and thereby determine the displacement of proof mass
20. The displacement of proof mass 20 provides information
concerning the acceleration or other environmental perturbation
causing the displacement.
[0068] As depicted in FIGS. 2a and 2b, there are four termination
blocks 26, each of which is in proximity to five charged regions of
a stabilizing structure depicted as blocks in FIGS. 2; two
left-right horizontal blocks, 23a and 23b; two up-down vertical
blocks 22a and 22b, and one in-out block, 28 for each stabilizing
structure, or twenty (20) in all. Recognizing that the proof mass
need not displace in a horizontal plane only but may elevate,
twist, tilt and/or rotate, sensing displacements with respect to
all 20 blocks of the stabilizing structures can provide useful
information and redundant checks on the motion of proof mass 20. In
some applications, it may be economical to sense fewer than all 20
possible displacements or to combine some sensing measurements into
unitary groups, all of which are envisioned within the scope of the
present invention. In addition, displacements can be determined
electronically as a function of time, permitting the determination
of more detailed components and higher derivatives of velocity vs
time behavior than acceleration.
2. RF communications--GHz Filter
[0069] MicroelectroElectroMechanical ("MEMS") resonators and
filters are common devices finding use in many types of cell phone
and other forms of wireless communication. However, conventional
devices are typically mechanically mounted to a substrate and,
consequently, suffer significant energy losses through the
mountings. This is an important problem in low energy wireless
communication in which such MEMS devices are frequently employed.
In addition, such traditional MEMS devices are hindered in
achieving higher quality factor, "higher-Q" components.
[0070] The present FLEMS devices float without mechanical supports
and thus cannot lose energy through mechanical contact. A typical
family of FLEMS devices configured for use as resonators or filters
is depicted in perspective view in FIG. 4. Building on the basic
FLEMS configurations of FIGS. 2a and 2b, a pair of additional side
electrodes 40 are included to drive the bulk material of the proof
mass (having a substantially cylindrical shape in some embodiments)
into a resonance mode. Another pair of electrodes 41 may be added
in these embodiments to act as sensors and to measure the deforming
cylindrical modes.
3. Mirror--Scanner
[0071] Mounting a FLEMS device by means of two stabilizing
structures leads to a device of the type depicted in FIG. 5. In
this embodiment, the stabilizing structures function as virtually
frictionless pin joints or pivots allowing the proof mass free
rotation. Applying a time-varying charge to the electrode beneath
the mirror allows the position of the mirror to be adjusted to a
desired angle. Three typical angular positions are depicted in FIG.
5. Thus, a reflective proof mass functions as a mirror/scanner
under control of the charge delivered to electrode 50. For rapid
positioning of the mirror, it may be advantageous to include a
damping mechanism, typically an electromagnetic and/or
electromechanical damper, not pictured in FIG. 5.
[0072] It is advantageous in many applications for the proof mass
(mirror) in FIG. 5 to have a size and/or shape that permits full
360 deg. rotation without contacting the substrate supporting the
FLEMS device. However, this is not an inherent limitation of the
structure as mirrors having other shapes can be employed along with
software and/or hardware safeguards insuring against damaging
contact between mirror and substrate.
4. Gyro--Two-Axis Mirror-Scanner
[0073] By physically permitting one pair of stabilizing electrodes
to undergo free rotation in a guide track as depicted in FIG. 6,
the proof mass can rotate independently about two perpendicular
axes. FIG. 6a depicts rotation about the local major axis, radially
through the proof mass and substantially parallel to the plane of
the substrate. FIG. 6b depicts rotation about the global z axis
perpendicular to the substrate. The cut-away view in FIG. 6c
depicts the control electrodes 61 to which a sinusoid or other
time-varying charge is applied to control the motion of the proof
mass. Electrodes 60 in FIG. 6c typically remain constantly charged
during the operation of the device. A reflective proof mass results
in a mirror-scanner capable of independent control about two
perpendicular axes. A gyro results from the use of a proof mass
having suitable mass and shape such that, once rotating, the proof
tends to retain its orientation in space regardless of the
rotational motion of the FLEMS device itself.
5. FLEMS Flux Capacitor
[0074] Neither the structure nor the mode of operation are clear
for Dr. Emmett Brown's flux capacitor in the Back to the Future
movie trilogy. However, by "flux capacitor" herein we intend a
capacitive device capable of accepting and retaining a substantial
amount of charge relative to its size and mass; that is, a
substantial charge density. Furthermore, by rotating that stored
charge at a high rate of speed by means of the non-contact FLEMS
technology described herein, a significant amount of magnetic flux
can be generated.
[0075] FIG. 7 depicts a typical FLEMS flux capacitor pursuant to
some embodiments of the present invention in which a floating rotor
70 is charged by any, or any combination, of means disclosed herein
or otherwise known, and located between two stationary stators, 71.
At least one and typically both stators are charged. By rotating
the rotor 70 with respect to the stators 71, magnetic flux is
generated as a magnetic or B-field as depicted in FIG. 8a (top
view) and FIG. 8b (side view). The geometry of the stators shown in
FIGS. 7, 8 and 9, in combination with the time-varying voltages
applied to the stators as depicted in FIG. 9 causes rotor 70 to
attain angular velocity .omega.. FIG. 10 shows the flux capacitor
of FIG. 7 in exploded view as it might be fabricated by a
three-mask fabrication process.
6. FLEMS SQUID-Gyro
[0076] A two-junction Superconducting Quantum Interference Device
(SQUID), or DC SQUID, consists essentially of two superconductors
separated by two thin, insulating layers forming a pair of
Josephson junctions, as depicted in FIG. 11. Among other uses,
SQUIDs can be employed as very sensitive detectors of magnetic flux
and changes in magnetic flux passing through the ring enclosed by
the superconductors and Josephson junctions ("the SQUID ring"). By
using a FLEMS flux capacitor surrounded by a SQUID ring, quantum
changes in the magnetic field generated by the spinning flux
capacitor are sensed by the SQUID. Since the magnetic flux depends
on the angular momentum of the rotating mass, changes in the
magnetic flux sensed by the SQUID yields a very sensitive gyro.
[0077] An alternative embodiment of a FLEMS/SQUID device is given
in perspective view FIG. 12a, cut-away view in FIG. 12b and
cut-away view including typical driving voltages in FIG. 12c. Thus,
FIG. 12 depicts an alternative embodiment for a flux capacitor that
can be coupled with a SQUID to produce a sensitive gyro.
[0078] Various approaches can be used for the functional
combination of a FLEMS device and a SQUID. For example, the device
can initially be placed in an external magnetic field. When the
external field is removed, a magnetic field inside the SQUID ring
is "trapped." During operation, the rotating FLEMS disc adds to the
magnetic flux. Quantum changes in the magnetic flux due to changes
in the rotation of the FLEMS device can be measured with the pickup
coil.
[0079] Although various embodiments which incorporate the teachings
of the present invention have been shown and described in detail
herein, those skilled in the art can readily devise many other
varied embodiments that still incorporate these teachings.
* * * * *
References