Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROSTATIC MICROACTUATOR
WITH OFFSET AND/OR INCLINED COMB DRIVE FINGERS
SCOPE OF THE INVENTION
The present invention relates generally to electrostatic actuators and more
particularly to
electrostatic microactuators with comb drive assemblies.
BACKGROUND
Electrostatic comb drive microactuators have heretofore been provided. See,
for
example, U.S. Patent Nos. 5,025,346 and 5,998,906. Flexural suspensions for
such
microactuators have generally fallen into there categories: fixed-fixed beams,
crab-leg flexures
and folded flexures. For a discussion of these suspensions, see G. Legtenberg,
et al., "Comb-
Drive Actuators for Large Displacements", J. Micromech. Microeng. 6 (1996), pp
320-329.
1o Folded flexures are further described in U.S. Patent No. 5,025,346 and
Michael Judy's U.C.
Berkeley dissertation, "Mechanisms Using Sidewall Beams", 1994.
The maximum motion of electrostatic comb drive microactuators is often limited
by
electromechanical side instability forces which occur during interdigitation.
In this regard,
undesirable sidewise movement and possible snapover of the comb drive fingers
can result from
such side instability forces. Flexural suspensions can be utilized to
discourage such sidewise
movement. A discussion of this behavior, particularly with respect to fixed-
fixed beams, crab-
leg flexures and folded flexures, is set forth in the G. Legtenberg, et al.
article cited above.
Several notable solutions for minimizing sidewise movement or snapover of comb
drive
fingers in linear electrostatic microactuators are set forth in U.S. Patent
No. 5,998,906. The
linear comb drive assemblies described in the '906 Patent are disposed between
first and second
folded-beam suspensions, which enhance alignment ofthe comb drive fingers
during deflection
and thus minimize nonlinear travel of the comb drive fingers during
deflection. Each of the
folded-beam suspensions therein consists of a pair of beams connected in
series. The pair of
beams of each folded-beam suspension are connected at one end to a common bar.
The opposite
ends of such beams are connected either to a movable shuttle or to the fixed
substrate. The
compliance of a folded-beam suspension in the sideways direction results from
two effects
caused by the side load, namely individual beam extension or contraction in
the sideways
direction and beam distortion in the forward direction. The first term is
mechanical and the
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second term is geometric. The stiffness of the suspension is the inverse of
the compliance and
thus the combination of the mechanical and geometric terms.
A nonfolded flexure suspension for an electrostatic actuator using parallel
plate electrodes
is described in R. Brennen, "Large Displacement Linear Actuator", 1990
technical digest of
IEEE conference on Micro Electro-Mechanical Systems, pp 135-139. The
suspension beams,
connected by a shuttle, are initially inclined relative to the parallel plate
electrodes by an angle
of approximately five degrees. The load to the suspensions is applied normal
to the shuttle. The
motive force is produced by the increase in the proj ected length of the
suspension beams, which
reduces the electrostatic gap between the plates.
to BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are somewhat schematic in many instances and
are
incorporated in and form a part of this specification, illustrate several
embodiments of the
invention and, together with the description, serve to explain the principles
of the invention.
FIG. 1 is a plan view of an optical microswitch incorporating two linear
electrostatic
microactuators having offset and inclined comb drive fingers of the present
invention.
FIG. 2 is a cross-sectional view of one of the linear electrostatic
microactuator of FIG.
1 taken along the line 2-2 of FIG. 1.
FIG. 3 is a plan view of one of the linear electrostatic microactuators of
FIG. 1 in a
second position.
2o FIG. 4 is an enlarged plan view, taken in the section 4-4 of FIG. 1 and
exaggerated in
certain respects, of a portion of one of the linear electrostatic
microactuators of FIG. 1 in which
the offset and inclined comb drive fingers are in a disengaged position.
FIG. 5 is an enlarged plan view, similar to FIG. 4 and exaggerated in certain
respects, of
a portion of one of the linear electrostatic microactuators of FIG. 1 in which
the offset and
inclined comb drive fingers are in an engaged position.
FIG. 6 is a graph depicting the side deflection of a shuttle with nonfolded
suspensions as
a fimction of the shuttle path.
FIG. 7 is a graph depicting the comb tooth misalignment in a microactuator
having a
comb drive assembly with offset and inclined comb teeth as a function of the
path of the shuttle.
3o FIG. 8 is a graph depicting the required side stiffness in a microactuator
having a comb
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drive assembly with offset and inclined comb teeth as a function of the
required side stiffness for
stability of such comb drive assembly.
DESCRIPTION OF THE INVENTION
The electrostatic microactuators of the present invention can be used in a
variety of
devices such as an optical microswitch. Exemplary optical microswitch 21 shown
in FIG. 1 is
a substantially planar device formed from a microchip 22 of any suitable size
and shape.
Miicrochip 22, shown in plan in FIG.1, is rectangular in shape and has first
and second opposite
ends 22a and 22b and first and second opposite sides 22c and 22d. The
microchip 22 has a
length ranging from 1000 to 5000 microns and preferably approximately 2500
microns and a
1o width ranging from 1000 to 5000 microns and preferably approximately 2000
microns. The
microchip is formed from a base or substrate 27 made from any suitable
material such as a
silicon wafer.
At least one and as shown a plurality of two electrostatic microactuators in
the form of
first and second linear micromotors 23 and 24 are included in the optical
microswitch 21 (see
FIG. 1). At least one input optical fiber 28 is optionally provided for
carrying input laser light
31 from a laser source (not shown) to the optical microswitch 21.
Alternatively, input laser light
or beam 31 can be supplied from any other suitable source, such as directly
from a laser in close
proximity to or mounted on substrate 27. The optional input fiber 28 can be
mounted to substrate
27 at first end 22a of microchip 22 by an adhesive or any other suitable
means. A conventional
2o collimating lens such as GRIN lens 32 is disposed adjacent the end of input
fiber 28 and is
secured to substrate 27 by an adhesive or other suitable means. The GRIN lens
32 directs input
light 31 along a linear path extending along the longitudinal axis of optical
microswitch 21.
At least one and as shown a plurality of three output optical fibers are
provided in optical
microswitch 21, as shown in FIG. 1. First and second output fibers 36 and 37
are disposed along
first side 22c of microswitch 21 for receiving laser light 31 reflected 90
degrees from the
longitudinal axis of microswitch 21. A third output fiber 38 is disposed on
second end 22b of
the microchip. Output fibers 36-38 can optionally be mounted to substrate 27
by an adhesive or
any other suitable means Conventional collimating lens such as GRIN lenses 41-
43 are disposed
near the respective ends of optical fibers 36-38 and mounted to substrate 27
in the same manner
3o as the output fibers. GRIN lens 43 is linearly aligned with input GRIN lens
32, while GRIN
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lenses 41 and 42 are aligned side by side and parallel to each other but
perpendicular to GRIN
lenses 32 and 43. The end surfaces of optical fibers 28 and 36-38 and GRIN
lenses 32 and 41-43
are coated in a conventional manner with an anti-reflective material.
Substrate 27 is made from any suitable material such as silicon and is
preferably formed
from a silicon wafer. The substrate has a thickness ranging from 200 to 600
microns and
preferably approximately 400 microns. First and second linear micromotors 23
and 24 are
formed atop the substrate 27 by a second or top layer 46 made from a wafer of
any suitable
material such as silicon (see FIG. 2). Top wafer 46 has a thickness ranging
from 10 to 200
microns and preferably approximately 85 microns and is secured at certain
points to the substrate
l0 27 by a suitable means. The top wafer 46 is preferably fusion bonded to the
substrate 27 by a
silicon dioxide layer 47 having a thickness ranging from 0.1 to two microns
and preferably
approximately one micron. Top layer 46 may be lapped and polished to the
desired thickness.
First and second micromotors 23 and 24 are substantially identical in
construction (see
FIG. 1). Each of the micromotors includes a micromirror 56 and at least one
comb drive
assembly. Preferably, each of the micromotors 23 and 24 includes at least one
first comb drive
assembly 57 for extending the respective micromirror 56 towards one of output
lenses 42 and 43
and thus further into the path of input laser light 31 launched from input
lens 32 and at least one
second comb drive assembly 58 for retracting the respective micromirror 56 in
an opposite
direction out of the path of input laser light 31. As shown in FIG. 3, first
comb drive assembly
57 has opposite first and second extremities 57a and 57b and second comb drive
assembly 58 has
opposite first and second extremities 58a and 58b. Each of the comb drive
assemblies 57 and
58 has a length between its extremities ranging from 100 to 5000 microns and
preferably
approximately 1000 microns.
First comb drive assembly 57 includes a first drive member or comb drive 61
formed
from the top wafer 46 and secured to substrate 27 by silicon dioxide layer 47
(see FIGS 1 and
2). The first comb drive 61 has first and second opposite end portions 61a and
61b
corresponding to first and second extremities 57a and 57b of the comb drive
assembly. The first
comb drive assembly 57 further includes a second comb drive member or comb
drive 62 formed
from top wafer 46 and overlying substrate 27. The second comb drive 62 has
opposite first and
second end portions 62a and 62b corresponding to first and second extremities
57a and 57b of
the comb drive assembly.
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First comb drive 61 includes an elongate member or bar 63 and a plurality of
comb drive
teeth or fingers 64 secured to one side of the spine or bar 63 and extending
towards the second
comb drive 62 (see FIGS. 1 and 3). The first comb drive fingers 64, shown as
being linear, have
a length ranging from five to 200 microns and preferably approximately 80
microns and
preferably have a constant width along their length, which width can range
from two to 1 S
microns and is preferably approximately five microns. The first comb drive
fingers 64 are
longitudinally spaced apart along the length of bar 63 at a separation
distance ranging from six
to 50 microns and preferably approximately 25 microns. Each adjacent pair for
first comb drive
fingers 64 has a space 66 therebetween, as shown most clearly in FIGS. 4 and
5, and a midpoint
1o between the comb drive fingers 64 shown in such figures by midpoint line
67. Comb drive
fingers 64 are joined to first bar 63 at an oblique angle, which inclination
angle can range from
zero to five degrees and is preferably approximately three degrees. The first
comb drive fingers
64 are inclined at such angle towards second end portion 61b of the first comb
drive assembly
57.
Second comb drive 62 is similar in construction to the first comb drive 61, as
shown in
FIG. 1. Specifically, second comb drive 62 includes a second elongate member
or bar 71
extending substantially parallel to first bar 63. A plurality of second comb
drive teeth or fingers
72 are secured to one side of the spine or bar 71 in longitudinally spaced-
apart positions along
the length of the bar and extend towards the first comb drive 61: Each of the
second comb drive
2o fingers, shown as being linear, has a length and a constant width along its
length. The second
comb drive fingers 72 can be of any suitable length and width and preferably
have a length and
width corresponding to the length and width of first comb drive fingers 64.
Each of the second
comb drive fingers 72 is joined to second bar 71 at an oblique angle
corresponding to the angle
at which first comb drive fingers 64 are joined to first bar 63. The second
comb drive fingers 72
are each inclined at such oblique angle towards first extremity 57a of the
first comb drive
assembly 57.
When in its rest position, as shown in FIG. l, each of second comb drive
fingers 72 is
offset relative to second bar 71 from the midpoint line 67 between the adj
acent pair of first comb
drive fingers between which the second comb drive finger interdigitates when
second comb drive
62 is electrostatically attracted to first comb drive 61. The offset of second
comb drive fingers
72 can range from zero to two microns and is preferably approximately 0.75
microns in the
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illustrated embodiment. Second comb drive forgers 72 are shown in FIG. 4 as
being spaced apart
by an exaggerated gap of approximately nine microns from one of the adjacent
first comb drive
fingers 64 and six microns from the other of the adj scent first comb drive
fingers 64. Thus, each
second comb drive finger 72 is offset a distance of approximately 1. S microns
from midpoint line
67 when in its rest position in the exaggerated drawings in FIGS. 4 and 5. The
offset of the comb
drive fingers in FIGS. 4 and 5 has been exaggerated to facilitate the
visualization and
understanding thereof.
Second comb drive assembly 58 is substantially identical to first comb drive
assembly
57 and includes a first comb drive 61 and a second comb drive 62 (see FIGS. l
and 3). The first
to bar 63 of second comb drive assembly 58 is spaced apart and parallel to the
first bar 63 of first
comb drive assembly 57. The second bar 71 of first comb drive assembly 57 is
shared with the
second comb drive assembly 58. Thus, second comb drive fingers 72 of first
comb drive
assembly 57 extend from one side of the second bar 71 and second comb drive
fingers 72 of
second comb drive assembly 58 extend from the other side of the second bar 71.
The double-
sided second comb drive 62 shared by first and second comb drive assemblies 57
and 58 is
disposed between the first comb drives 61 of first and second comb drive
assemblies 57 and 58.
Comb drive second end portion 62b of first and second comb drive assemblies 57
and 58
is perpendicularly joined to an elongate, linear shuttle member or shuttle 76
formed from top
wafer 46 and overlying substrate 27. Shuttle 76 has a first or front end
portion 76a and an
opposite second or rear end portion 76b and a width ranging from 10 to 60
microns and
preferably approximately 35 microns. A micromachined mirror holder or bracket
77 is joined
to front end portion 76a of the shuttle 76 and is preferably formed integral
with the shuttle 76.
Micromirror 56 is secured to brackets 77 in any suitable manner such as an
adhesive (not shown)
and can be of any suitable type such as disclosed in U.S. Patent No.
5,998,906, the entire content
of which is incorporated herein by this referenced. Micromirror 56 has a
reflective face 78
inclined at an oblique angle to the longitudinal axis of shuttle 76 and
preferably disposed at a 45
degree angle to the shuttle. Mirror face 78 is thus disposed at a 45 degree
angle relative to input
laser light 31 launched from GRIN lens 32.
At least one spring member or suspension is included in each of the first and
second
linear micromotors 23 and 24 for supporting the respective first and second
comb drive
assemblies 57 and 58 and shuttle 76 above substrate 27 so as to permit
movement of the second
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comb drives 62 relative to the first comb drives 61. More specifically, first
and second spaced-
apart suspensions 81 and 82 are included in each of the micromotors, as shown
in FIGS. 1 and
3. Each of the suspensions has a length approximating the length of first and
second comb drive
assemblies 57 and 58 and preferably has a length ranging from 100 to 5000
microns and more
preferably approximately 1000 microns. Although the suspensions 81 and 82 can
be of any
suitable construction, each of the suspensions has an elongate beam-like
member or flexural
beam 83 provided with opposite first and second end portions 83a and 83b. The
flexural beam
83 has a rectangular cross section, as shown in FIG. 2, and a width ranging
from one to ten
microns and preferably approximately four microns. First and second thin,
elongate sacrificial
bars 86 and 87, each of a type described in U.S. Patent No. 5,998,906, are
provided for each
flexural beam 83 to enhance even etching and thus the formation of the desired
cross section of
the flexural beam 83. Sacrificial bars 86 and 87 extend parallel to the
respective flexural beam
83 and are spaced apart on opposite sides of the beam.
The axial stiffness of each suspension 81 and 82, that is each flexural beam
83 thereof,
is represented only by a mechanical term and is represented by the following
equation:
kY = Ewh/L,
where E is Young's modulus and w, h and L are the width, height and length of
flexural beam
83.
First and second comb drive assemblies 57 and S8 are disposed between first
and second
2o suspensions 81 and 82. As shown in FIG.1, first suspension 81 is spaced
apart from the outside
from first comb drive 61 of first comb drive assembly 57. Second suspension 82
is spaced apart
from the outside of the first comb drive 61 of second comb drive assembly 58.
Suspensions 81
and 82 extend parallel to first and second comb drive assemblies 57 and 58
when in their
respective at rest positions shown in FIG. 1. The first end portion 83a of
each beam member 83
is secured to substrate 27 in the vicinity of the first extremity of the
respective first comb drive
assembly 57 or second comb drive assembly 58. The second end portion 83b of
each beam
member 83 is secured to the second end portion 62b of the respective second
comb drive 62, by
means of shuttle 76, in the vicinity of the second extremity of the respective
first or second comb
drive assembly. A first attachment block or anchor 91, formed from top wafer
46 and secured
3o to substrate 27 by silicon dioxide layer 47, is provided for so securing
first suspension 81 to the
substrate and a second attachment block or anchor 92, similar in construction
to the first anchor,
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is provided for so securing the second suspension 82 to the substrate.
The second comb drive 62 of first and second comb drive assemblies 57 and 58,
shuttle
76 and first and second suspensions 81 and 82 are spaced above substrate 27 by
an air gap 93,
shown in FIG. 2 with respect to first suspension 81, so as to be electrically
isolated and moveable
relative to the substrate. These structures can have any suitable thickness or
height and
preferably have a thickness ranging from 10 to 200 microns and more preferably
approximately
85 microns. Each of such structures are formed from top wafer 46 and are
preferably etched
from the wafer 46 using high aspect ratio processes such as deep reactive ion
etching (DRIE)
techniques so as to provide the structures with relatively great out-of plane
stiffness for
1o substantially constraining motion to that in the plane of wafer 46. Plated
metal processes such
as the LIGA process can also be utilized in forming such structures.
Second comb drive 62 of each of first and second comb drive assemblies 57 and
58 is
movable between a first position, in which first and second comb drive fingers
64 and 72 are not
substantially fully interdigitated, and a second position in which the first
and second comb drive
fingers 64 and 72 are substantially fully interdigitated. As used herein, not
fully substantially
interdigitated includes positions when first and second comb drive fingers 64
and 72 are spaced
apart or only slightly interdigitated, as shown in FIG. 1 with respect to both
comb drive
assemblies 57 and 58 and in FIG. 3 with respect to first comb drive assembly
57, or when the
comb drive fingers 64 and 72 are only partially interdigitated. As used
herein, substantially fully
2o interdigitated includes positions when first and second comb drive fingers
64 and 72 are more
interdigitated than when not substantially interdigitated, and particularly
includes positions when
the comb drive fingers 64 and 72 are fully interdigitated, as shown in FIG. 3
with respect to
second comb drive assembly 58.
Each of the second comb drive forgers 72 is substantially centered on the
midpoint line
67 between the adjacent first comb drive fingers 62 when the second comb drive
62 is
substantially fully interdigitated with the first comb drive 61, as shown in
FIG. 5. Although the
second comb drive fingers 72 are shown as being centered on line 67 in FIG. 5,
the invention is
broad enough to cover comb drive assemblies where the second comb drive
fingers 72 are
centered on line 67 slightly before full interdigitation and thus move past
line 67 at fixll
interdigitation. Comb drive fingers 72 which approach but do not actually
center on line 67 at
fully interdigitation are also contemplated herein.
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When the micromotor 23 or 24 is in its rest position, as shown in FIG. 1, the
second comb
drive fingers 72 of each of first and second comb drive assemblies 57 and 58
are not substantially
fully interdigitated with the respect to the first comb drive fingers 64.
Movement of the second
comb drive 62 of first comb drive assembly 57 in a first direction
substantially perpendicular to
second bar 71 results in first and second comb drive fingers 64 and 72 of such
comb drive
assembly becoming substantially fully interdigitated (not shown) and
micromirror 56 moving
to a fizlly extended position. Movement of second comb drive 62 of second comb
drive assembly
58 in an opposite direction substantially perpendicular to second bar 71
results in first and second
comb drive fingers 64 and 72 of the second comb drive assembly becoming
substantially fully
to interdigitated, as shown in FIG. 3 with respect to first micromotor 23, and
the micromirror 56
moving to a filly retracted position.
Shuttle 76 and second comb drive fingers 72 do not move along a straight line
as the
shuttle moves from its fully retracted position, shown in FIG. 3, to its fully
extended position
(not shown). The curvature induced shortening of 81 and 82 results in the
shuttle 76 moving
along a shallow curve approximated by the formula
y(x) _ ~2 x2 / 16 L,
where the x axis extends parallel to the linear shuttle 76, the y axis extends
towards the first
extremities of comb drive assemblies 57 and 58 and L refers to the length of
flexural beam 83.
The path of a shuttle 76 where flexural beams 83 have a length of one
millimeter is shown in
2o FIG. 6. As first and second suspensions 81 and 82 bend, the second comb
drives 62 of the first
and second comb drive assemblies 57 and 58 are pulled by the suspensions
towards the
respective first extremities 57a and 58a of the comb drive assemblies. As
such, the second comb
drive forgers 72 of first and second comb drive assemblies 57 and 58 travel
through an arcuate
or parabolic path as second bar 71 moves between a fully retracted position in
which the first and
second comb drive fingers of second comb drive assembly 58 are fully
interdigitated and a fully
extended position in which the first and second comb drive fingers of first
comb drive assembly
57 are fully interdigitated.
The curved path of shuttle 76 would result in a traditional comb drive finger,
oriented
along the direction of travel and not offset from the midpoint between the
stationary comb drive
3o fingers, having a three micron side deflection resulting in a six micron
difference in electrostatic
gap after 70 microns of forward travel. Such a differential in electrostatic
gap would produce
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a significant force imbalance and probably cause the comb drive fingers to
snap over.
Side instability between comb drive fingers occurs when the derivative of the
net side
forces becomes positive, that is when the negative feedback of the forces from
suspensions 81
and 82 becomes smaller than the positive feedback of the electrostatic forces
between comb drive
fingers 64 and 72. Therefore, stability requires that
~vzC ~ 3+ ~ 3~<xy
(g-v) (g+v>
where N is the number of comb drive fingers 72 in the comb drive assemblies 57
and 58, h is the
thickness or depth of the comb drive fingers 72, V is the drive voltage, a is
the free space
dielectric constant, y is the comb misalignment and ky is the side suspension
stiffness. Utilizing
the above equation, the misalignment, y, of comb drive fingers 72 relative to
comb drive fingers
64 is comprised of the arcuate or parabolic path of shuttle 76 and the initial
inclination and offset
of the comb drive fingers and is represented by the equation
y = (~zxz/16L) + mx + b,
where m is the inclination of the comb drive fingers and b is the initial
offset of the comb fingers.
The values of m and b may be chosen to maximize stability by minimizing the
derivative of the
net side force over the forward deflection range. The graphs in FIGS. 7 and 8
show the path of
shuttle 76 and the required side stiffness for stability where first and
second comb drive fingers
64 and 72 are inclined at an angle of three degrees and the second comb drive
fingers 72 are
provided with an initial offset of 0.75 microns.
Means is optionally included within each of the first and second micromotors
23 and 24
2o for limiting the movement of second comb drives 62 in each of the opposite
first and second
directions. In this regard, opposite first and second stubs 97 and 98 extend
from respective first
and second end portions 62a and 62b of second bar 71. A stop block 101 formed
from top wafer
46 is secured to substrate 22 by layer 47 at each end of comb drive assembly
57 for limiting the
interdigitation of the second comb drive fingers 72 with the first comb drive
fingers 64 of the
first comb drive assembly. A similar second stop 102 is secured to substrate
27 at each end of
second comb drive assembly S 8 for limiting the interdigitation of the second
comb drive fingers
72 with the first comb drive fingers 64 of second comb drive assembly 58.
Electrical means is included within optical microswitch 21 for driving the
second comb
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drives 62 of each of the first and second micromotors 23 and 24 in the
opposite first and second
directions. Such electrical means includes a controller and voltage generator
106 that is
electrically connected by means of a plurality of electrical leads 107 to a
plurality of electrodes
provided on substrate 27. Each of the micromotors 23 and 24 has a first or
ground or common
electrode 108 joined to second anchor 92 by a trace 109 for grounding first
and second
suspensions 81 and 82, shuttle 76 and second comb drives 62. A second or drive
electrode 112
is electrically connected by a trace 109 to first end portion 61 of first comb
drive assembly 57
for providing an electrical potential to first comb drive 61 of the comb drive
assembly 57. A
similar third or drive electrode 113 is connected by means of a trace 109 to
first end portion 61 a
of the second comb drive assembly 58 for providing an electrical potential to
the first comb drive
61 of the second comb drive assembly 58. Electrodes 108, 112 and 113 and
traces 109 are each
formed from top wafer 46 and secured to substrate 27 by means of silicon
dioxide layer 47. A
metal layer (not shown) of aluminum or any other suitable material is created
on the portion of
the top surface of wafer 36 forming electrodes 108, 112 and 113 and traces 109
for facilitating
the creation of such electrodes and traces. Electrodes 108, 112 and 113 are
advantageously
placed along one of the ends or sides of microchip 22 so as to facilitate
access to the electrodes
by leads 107. As shown, the electrodes for first micromotor 23 are placed
along first side 22a
of the microswitch 21 and the electrodes for second micromotor 24 are placed
along the second
side 22b of the microswitch. For simplicity, controller 106 is shown in FIG. 1
as being
2o electrically coupled only to electrodes 108, 112 and 113 of second
micromotor 24.
The design of each micromotor is defined by five dimension, namely , the
length and
width of flexural beams 83 as identified respectively by the letters LS"~, in
FIG. 1 and the letters
WS"~ in FIG. 3, the maximum forward travel of each of first and second comb
drive assemblies
as identified by the letters X",~ in FIG. 3 with respect to the second comb
drive assembly 58, the
gap between comb drive fingers 64 and 72 as identified by the letter g in FIG.
4 and the width
of shuttle 76 as identified by the letters Wb~, in FIG. 3. When Xmax is set,
Wb~, is chosen to
achieve sufficient stiffness while minimizing mass. The length of shuttle 76
is equal to
approximately:
(4*X~"~)+(3 * Wbar.)
3o WS,~p is set to the minimum line width of the fabrication process and LS,~p
is chosen to satisfy the
resonant frequency and drive voltage constraints. The electrostatic gap
between comb drive
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fingers 64 and 72 is determined by the choice of suspension length LS"~.
First and second linear micromotors 23 and 24 are disposed end to end adjacent
second
side 22b of microchip 22. The shuttles 76 and micromirrors 56 of the
micromotors 23 and 24
are disposed adj acent to each other so as to desirably limit the path length
of input laser light 31
within optical microswitch 21. Means in the form of a closed looped servo
control can be
included in optical microswitch 21 for monitoring the position of the second
comb drives 62 of
each of first and second micromotors 23 and 24 and thus the position of
micromirrors 56 within
the microswitch 21. For example, controller 106 can determine the position of
a movable comb
drive 62 by means of a conventional algorithm included in the controller for
measuring the
to capacitance between the second comb drive fingers 72 of the movable comb
drive 62 and the first
comb drive fingers 64 of the related stationary comb drive 61. A signal
separate from the drive
signal to the respective comb drive assembly 57 and 58 can be transmitted by
controller 106 to
the micromotor for measuring such capacitance.
In operation and use, optical microswitch 21 is particularly suited for use in
a fiber-optic
network of a telecommunications system for directing laser light to one of the
three output fibers
36-38. Micromirror 36 of first linear micromotor 23 can be moved by the first
and second comb
drive assemblies 57 and 58 of such micromotor from a first or retracted
position out of the path
of input laser light 31 to a second or extended position into such path for
directing light Slat a
90 degree angle into GRIN lens 41 for output via first output fiber 36. When
micromirror 56 of
2o first micromotor 23 is in its retracted position, micromirror 56 of second
micromotor 24 can be
similarly utilized for directing laser light 31 at a 90 degree angle into GRIN
lens 42 for output
via second output fiber 37. When the micromirrors 56 of both first and second
micromotors 23
and 24 are in a retracted position, the laser light 31 continues in its linear
path to GRIN lens 43
and third output fiber 38.
When it is desired to move micromirror 56 of one of first and second
micromotors 23 and
24 from its home position shown in FIG. 1, suitable voltage potentials can be
supplied by
controller 106 to first and second comb drive assemblies 57 and 58 in any
suitable procedure
such as described in U.S. Patent No. 5,998,906. In one exemplary procedure,
micromirror 56
can be retracted by supplying a voltage potential to first comb drive 61 of
second comb drive
3o assembly 58 so as to cause second comb drive fingers 72 of such comb drive
assembly to be
electrostatically attracted to first comb drive fingers 64 of the related
first comb drive 61. Such
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attraction force causes the second comb drive fingers 72 to move towards and
interdigitate with
the first comb drive fingers 64. The amount of such interdigitation, and thus
the amount of
retraction of the micromirror 56, can be controlled by the amount of voltage
supplied to the first
comb drive 61 of second comb drive assembly 58. When it is desired to move the
micromirror
56 from its rest position to a position where the mirror is further extended
into the path of input
light 31, a suitable voltage potential is supplied by the controller 106 to
the first comb drive 61
of first comb drive assembly 57 to cause second comb drive fingers 72 of such
comb drive
assembly to move towards and interdigitate with the first comb drive fingers
64 of the first comb
drive assembly. Suitable voltage potentials to first and second comb drive
assemblies 57 and 58
1o can range from 20 to 300 volts and preferably range from 70 to 140 volts.
Micromirror 56 of each of the first and second micromotors 23 and 24 is
capable of
extension or retraction from its rest position shown in FIG. 1 of
approximately 70 microns, for
an ~ aggregate travel between its fully retracted position to its fully
extended position of
approximately 140 microns. The amount of travel is dependent in part on the
number of comb
drive fingers 64 and 72, the gap between the comb drive fingers and the length
and width of the
first and second suspensions 81 and 82.
In an alternative electrical drive configuration for electrostatic
microactuators 23 or 24,
controller 106 applies a ground potential to electrode 112 coupled to first
comb drive 61 of first
comb drive assembly 57 and a fixed maximum potential to electrode 113 coupled
to first comb
2o drive 61 of second comb drive assembly 58. A variable potential between the
ground potential
and the fixed maximum potential is applied by the controller to common
electrode 108 coupled
to second anchor 92 and hence second comb drives 62. When the potential
applied to common
electrode 108 is equal to half of the maximum potential, an equal potential
difference exists
between electrodes 113 and 108 and between electrodes 112 and 108 resulting in
approximately
equal forces tending to move micromirror 56 in forward and rearward directions
and thus
resulting in no movement of the micromirror 56. As the drive voltage applied
to common
electrode 108 is varied from this half value, an increasing net force is
provided which results in
movement of the micromirror 56 from its rest position of FIG. 1. When the
applied potential
to common electrode 108 is at either ground or the fixed maximum value, a
maximum force
3o substantially equal to the drive force that occurs when a similar maximum
drive voltage is
applied to either electrode 113 or 112 and common electrode 108 is grounded is
then applied so
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as to cause movement of micromirror 56. Specifically, when the common
electrode is grounded,
the micromirror 56 retracts; when the common electrode is provided with the
fixed maximum
value, the micromirror 56 extends. Similar voltages to those discussed above
can be applied and
similar travel distances can be achieved. This alternative drive configuration
requires only a
single variable potential source and smoothly varies the position of
micromirror 56 with optical
microswitch 21 by varying only a single source. The number of electrical
components in
controller 106 and thus the cost of the actuator system can be reduced with
this drive
configuration.
Single, nonfolded beams 83 contribute to the relatively large travel distances
of
micromirrors 56 by providing a relatively large suspension stiffness to first
and second
suspensions 81 and 82. Sideways movement of the suspensions, resulting from
forward or
rearward movement of shuttle 76, produces only a small deflection in the
second comb drive
fingers 72 of the advancing comb drive 62 that is mostly forward and only
slightly sideways
relative to the first comb drive fingers 64 into which the second comb drive
fingers 72 are
interdigitating. Thus, such side loads on the suspensions 81 and 82 cause the
shuttle 76 and
second comb drive fingers 72 to move along essentially the same path as the
shuttle and comb
drive fingers 72 travel when forward loads are exerted on the shuttle. The
increase in side
stability forces provided by beams 83 provides micromotors 23 and 24 with a
significant
improvement in performance, either in switching speed or drive volta.g~:,.
2o The offset and inclined comb drive fingers of first and second comb drive
assemblies
contribute to the stability of the first and second micromotors 23 and 24. The
offset alignment
of second comb drive fingers 72 relative to first comb drive fingers 64
ensures that the second
comb drive fingers 72 will be substantially centered on midpoint line 67, as
shown in FIG. 5,
when the first and second comb drive fingers are fully interdigitated. Since
the comb drive
fingers 72 are substantially centered, the derivative of the net side force is
substantially
minimized and the side stability is maximized at the fully interdigitated
position.
The complimentary inclination of first and second comb drive fingers 64 and 72
further
minimizes side instability of the first and second comb drive assemblies 57
and 58. As discussed
above, first comb drive fingers 64 are inclined toward the second extremity of
the respective first
3o and second comb drive assemblies 57 and 58 and second comb drive fingers 72
are inclined at
an equal angle towards the first extremity of the respective first and second
comb drive assembly.
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The cooperative inclination of the first and second comb drive fingers
contributes to each second
comb drive finger 72 being more centered relative to the respective pair of
adjacent first comb
drive forgers 64 during interdigitation of the first and second comb drive
fingers 64 and 72.
Since the comb drive fingers remain more centered, the side stability is
maximized during
interdigitation. The combination of initial offset and inclination allows the
side stability to be
maximized throughout the full deflection range. It should be appreciated that
the invention is
broad enough to cover microactuators having comb drive assemblies with comb
drive fingers that
are offset but not inclined, inclined but not offset or not offset or
inclined.
Optical microswitch 21 is relatively compact in design. In this regard, first
and second
to micromotors 23 and 24 are piggy-backed together in an end-to-end
configuration to
advantageously place the two micromirrors 56 close together in the optical
microswitch 21. The
electrodes 108,112 and 113 for electrically accessing the micromotors are
disposed along edges
of the microchip 22 to eliminate spacial requirements otherwise required by
the threading of
electrical leads between adjacent micromotors. The nonfolded suspensions 81
and 82 require
less surface area for deflection than folded springs and thus permit more
surface area to be
allocated to comb drive assemblies 57 and 58 in the micromotors of optical
microswitch 21.
Although optical microswitch 21 has been disclosed for use in a fiber-optic
network of
a telecommunications system, it should be appreciated that the microswitch can
be used in other
applications within the scope of the invention. For example, the microswitch
21 can be used in
2o an optical data storage system of the type disclosed in copending U.S.
Patent Application Serial
No. 09/135,236 filed August 17,1998 and in optical scanners, optical
spectrometers and optical
phase compensators. It should also be appreciated that microswitch 21 can be
bidirectional or
unidirectional in another direction from that described above. For example,
any of optical fibers
36-38 can be utilized as an input fiber and optical fiber 28 can be utilized
as an output fiber. In
addition, it is contemplated that micromotors 23 and 24 can have applications
other than in an
optical switch. For example, micromotors 23 and 24 can be used to rotate or
translate
components such as optical waveplates and diffraction gratings. The various
features of first and
second comb drive assemblies 57 and 58, including the offset and inclined comb
drive fingers
64 and 72 thereof, can also be incorporated into other micoactuators such as
rotary electrostatic
3o microactuators, including the type disclosed copending U.S. patent
application Serial No.
09/464,361 filed December 15, 1999, the entire content of which is
incorporated herein by this
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reference.
As can be seen from the foregoing, an electrostatic microactuator having an
improved
suspension has been provided. The microactuator has improved side stability
and includes a
suspension that provides side stiffiiess to the comb drive fingers that is
substantially independent
of the forward deflection of the microactuator. The microactuator has
nonfolded suspension
members and is of a reduced size and complexity. It can be provided with a
comb drive
assembly having comb teeth that are inclined relative to the comb drive bar.
The comb drive
assembly of the microactuator can further include movable comb teeth that are
offset from the
midpoint between the stationary comb teeth of the comb drive assembly.
