Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 Attorney Docket No. OC0003PC
BI-STABLE MICRO SWITCH
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application is being concurrently filed with U.S. Patent
6,388,359 issued on May 14, 2002, entitled "Method of Actuating MEMS switches"
by
Duelli et al.; and U.S. Patent 6,210,540 issued on April 3, 2001, entitled
"Method and
Apparatus for Depositing Thin Films on Vertical Surfaces" by Hichwa.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
OR DEVELOPMENT
Not applicable.
REFERENCE TO MICROFICHE APPENDIX
Not applicable.
FIELD OF THE INVENTION
The present invention is generally related to switches for use in micro
systems, and more particularly to a MEMS switch capable of latching in either
of two
switch positions.
BACKGROUND OF THE INVENTION
Optical switches can be used in a variety of applications, such as optical
fiber transmission networks, to route optical signals along various signal
paths. An optical
switch typically has an optical element, such as a mirror or a filter, that is
switched into
and out of a path of an optical signal beam. Switches are typically
characterized by the
t
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number of input and output port, referred to as NxN. For example, a I x2
switch would
switch one input between two outputs.
Switches can often be described as "latching" or "non-latching". A
latching switch reliably remains in a known position, even if the power is
removed or lost.
A non-latching switch may revert to an unknown position, or even a position
intermediate
between switch states, when the power is lost, for example if current provided
to an
electro-magnetic solenoid or thermal actuator is lost. One type of latching
switch reverts
to a known default position (state), no matter what state the switch was in
when power was
lost. Another type of latching switch preserves the switch state, no matter
what that state
was. The latter case is known as a "bi-stable" switch.
Bi-stable optical switches are desirable for use in optical
telecommunication systems because they preserve the network configuration
associated
with the position of the switch(es) when the power was lost. Various
approaches have
been used to produce bi-stable optical switches. One approach uses a permanent
magnet
in conjunction with a piece of magnetic material to hold the switch element in
the desired
position. Other approaches use a mechanical latch to hold the switch element
in the
desired position.
In a particular application, as illustrated and described in U.S. Patent No.
5,994,816 entitled THERMAL ARCHED BEAM MICROELECTROMECHANICAL
DEVICES AND ASSOCIATED FABRICATION METHODS by Dhuler et al., issued
Nov. 30, 1999, a mechanical latch is used in a micro-electro-mechanical system
("MEMS") (See, e.g. Fig. 11, re nums. 69 and 68c). A thermal arched beam
actuator is
used to move a switch element back and, with a thermally activated latch
holding the
switch element in the desired position(s). However, having contact surfaces
between the
latch and the switch element can result in the mechanism sticking or produces
"sticktion
(i.e. sticking friction), thus altering the force required to change switch
states. This
sticking or sticktion can not only affect the reliability of switch operation,
but also affect
the timing of the switch,particularly with fast (i.e. <_1 ms) in light of the
need to time the
operation of the latch with the operation of the electrostatic comb.
U.S. Patent No. 5,994,816 also describes a latching mechanism that uses an
electrostatic field to clamp a movable portion of the switch to the switch
body (substrate).
Clamping allows the relatively high current flow to the thermal beam actuator
to be
removed without losing the clamped switched state, thus conserving power.
However, if
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the voltage to the electrostatic clamping circuit is removed the switch may
revert to a state
other than what was previously held.
Accordingly, it is desirable to provide a bi-stable MEMS switch without
mechanical contacting surfaces between moving and static surfaces of the
switch. It is
further desirable that the optical switch be repeatable and have a high
switching lifetime,
and maintain a present switch state when power to the switch is removed.
SUlViMARY OF THE INVENTION
The present invention provides a bi-stable MEMS switch without contact
surfaces between the moving and static portions of the switch. In a preferred
embodiment,
a switch element or center body is movable in relation to the switch body or
substrate (i.e.
MEMS chip). The switch element is suspended between portions of the switch
body by a
plurality of spring arms attached at walls of hollow body portions of a center
beam and
can serve to operate in a relay, a valve, or an optical switch, for example.
An actuator,
such as an electrostatic comb drive motor, thermal beam actuator, or magnetic
motor,
provides a motive force to the switch element according to an electronic
switch signal.
The spring arms hold the switch element in place in relation to the switch
body and in one
of two switch positions, whether or not the electronic switch signal continues
to be
applied.
The switch is cycled between states by appropriate electronic switch
signals. For example, if an electrostatic comb drive motor is used, a first
electronic switch
signal, such as a pulse or series of pulses, causes the switch to assume a
first switch
position. A second electronic switch signal causes the switch to assume a
second switch
position. The electrostatic comb drive motor uses two electrostatic arrays,
one array
configured to move a movable portion of the switch in a first direction, and
the other array
configured to move the movable portion of the switch in a second direction in
response to
applied electrical signals. In the event of a thermal beam motor, an electric
current might
be temporarily applied to a first set of heaters to set the switch in a first
position, and a
temporary electric current might be applied to a second set of heaters to set
the switch in a
second position. Using either type of actuator, the switch will remain in its
present state if
power is lost, but typically power is removed to conserve power consumption of
the
switch.
The spring arms and hollow beam walls deform in response to the motiye
force of the actuator, and attain an equilibrium position in either of the
switch states. In a
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preferred embodiment, one switch position has a lower potential energy than
the other
switch position; however, the switch is bi-stable.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1A is a simplified diagram of a portion of a bi-stable latching MEMS
switch device according to the present invention in a first position;
Fig. 1 B is a simplified diagram of the portion of the device shown in Fig.
1 A in a second position;
Fig. 1 C is a simplified graph of the potential mechanical energy state of a
bi-stable switch during operation according to an embodiment of the present
invention;
Fig. 2A is a simplified representation of a portion of a switching device
according to the present invention illustrating various aspects used in
modeling the
operation of the device;.
Figs. 2B-2D are simplified representations of a spring model of a bi-stable
switch according to an embodiment of the present invention;
Fig. 3A is a simplified top view of a portion of a switch device and
electrostatic comb drive actuator according to an embodiment of the present
invention;
Fig. 3B is a simplified cross section of a portion of a MEMS device
illustrating separation between the substrate and movable portions of the
device;
Fig. 4A is a simplified perspective view of a portion of a MEMS chip
according to the present invention illustrating 2x2 optical switch in the bar
state;
Fig. 4B is a simplified perspective view of the portion of the MEMS chip in
Fig. 4A in the cross state;
Fig. 4C is a simplified side view of a packaged bi-stable optical MEMS
switch according to an embodiment of the present invention;
Fig. 4D is a simplified side view of a packaged bi-stable optical MEMS
switch according to an embodiment of the present invention;
Fig. 5A is a simplified graphical representation of a switching signal
according to an embodiment of the present invention;
Fig. 5B is a simplified graphical representation of another embodiment of a
switching signal according to an embodiment of the present invention; and
Figs. 6A and 6B are simplified flow charts of processes according to
embodiments of the present invention.
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DETAILED DESCRIPTION OF THE INVENTION
1. Introduction
The present invention provides a bi-stable MEMS switch that retains its
switch state when power to the device is removed or lost. A plurality of
spring arms
5 deform in response to movement of a center body of the switch that is
coupled to an
actuator. The actuator provides motive force to the center body, but is not
required to
maintain a switch state. Generally, the spring arms start at a stable
equilibrium state,
increase in potential mechanical energy trough a maximum as the center body is
moved
from one switch position to another, and then assumes a second stable
equilibrium, i.e.
within a potential energy well below the maximum, in the second switch
position. The
first position is generally the as-fabricated state, and the spring arms store
mechanical
energy in the second state, thus a non-symmetrical switching signal can be
employed,
namely the switch signal applied to switch from the first state to the second
state might
provide less energy than to switch from the second state to the first state.
In a particular
embodiment, an electrostatic comb drive actuator is used. The current drawn by
such an
actuator is so low that the switch can be "strobed", which means that a switch
signal may
be applied to the switch, even if the switch is already in the desired switch
state.
II. A Bi-Stabie MEMS Switch
Fig. IA is a simplified top view of a portion of a bi-stable MEMS switch 20
according to an embodiment of the present invention in a first latched switch
state. The
switch includes a movable center body 22 attached to fixed (static) portions
of the switch
24, 26 by spring arms 28, 30. An actuator 34, such as a thermal beam actuator,
electrostatic comb drive, or electromagnetic actuator, is coupled to the
center body, and
provides a motive force to the center body to move it between a first latched
switch state
and a second latched switch state. The center body includes a hollow beam
portion 36 that
has a hollow center portion 38 and hollow beam walls 40, 42. The hollow beam
walls act
in conjunction with the spring arms 28, 30 to allow deflection of switch
elements and
storing of spring energy. The spring arms 28, 30 are essentially opposite each
other at the
walls of the hollow beam portion. In a preferred embodiment, two pairs of
spring arms,
each in conjunction with a hollow beam portion, are used to stabilize and
guide the center
body between switch positions. In alternative embodiments, other mechanical
elements
might be used to guide and stabilize the center body. The A further
understanding of the
operation of the switch elements is provided in reference to Figs. 2A-2D,
below.
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The MEMS switch may be made in accordance with various known
fabrication processes. In a particular process, the switch is made on a
commercially
available silicon-on-insulator ("SOI") wafer. The SOI wafer includes a single-
crystal base
and approximately 2 microns of thermally grown silicon oxide between the base
and
approximately 73 microns of single-crystal silicon overlying the silicon
oxide. The SOI
wafer was made according to a wafer bonding process, but it is understood that
the wafer
is described for purposes of illustration and discussion only, and might be
made according
to other processes or with other types or thicknesses of layers.
Features of the MEMS switch are formed in the thin layer of silicon
overlying the oxide layer using a highly directional etch technique, such as a
biased
plasma etch technique, as are well known in the art. A hydrofluoric acid
("HF") etch
process is used to remove the oxide underneath portions of the switch movable
in relation
to the base, such as the spring arms and center body. The HF etch is generally
isotropic,
and removes a portion of the oxide underlying static portions of the switch,
but this does
not significantly affect the characteristics, performance, or the reliability
of the switch.
Access by HF to the portions of oxide to be removed is facilitated by keeping
the sections
of movable portions of the switch relatively thin. In general, the
undercutting of static
portions of the switch, which generally have a single-sided exposure to the
HF, will be
half the distance between edges of movable portions having double-sided
exposure to the
HF.
Fig. IB is a simplified top view of the portion of the bi-stable MEMS
switch 20 shown in Fig. lA in a second latched switch state. The actuator 34
has applied a
motive force to the center body 22 to retract the center body from the first
switch position.
The terms "extend" and "ret.ract" are used for purposes of convenient
discussion only.
The spring arms have assumed a different shape from their shape represented in
Fig. 1 A.
This difference in shape will be referred to as "deformation", which may
induce a variety
of shapes in the spring arms, such as an "S" shape or a "C" shape. The
combination of the
spring arms in conjunction with the walls of the hollow beam portion provide a
linear
spring-type of energy storage, as opposed to the non-linear bucking that can
arise with
conventional non-compressible arms.
If arms similar to the spring arms of the present invention are used with a
solid center beam, the strain in the arms can build up in the arm material to
a critical
degree and cause buckling of the arms, i.e. twisting or bending out of the
plane of the
substrate. This buckling is typically non-linear, and can result in
unpredictable switch
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behavior. In the present invention, the deformation of the hollow beam portion
of the
center body provides a restoring (spring) force proportional to the
displacement of the
wall. The displacement of the wall at least partially accommodates the strain
in the arms,
thus avoiding buckling.
Fig. 1C is a simplified graph 50 of potential energy 52 versus displacement
of the center body in reference to the static switch body for a switch
according to an
embodiment of the present invention. The as-fabricated position 56 of the
center body
will be given the reference of zero displacement 54 and zero potential energy
for purposes
of illustration, and will be referred to as the first switch position. Moving
the center body
in either a positive or negative direction increases the potential of the
switch. For
purposes of illustration, movement toward the second switch position will be
termed a
positive displacement.
As the center body is moved toward the second switch position 58, the
potential increases to a potential maximum 60, and then decreases to a local
minimum 62
at the second switch position. Although the local minimum 62 has a higher
potential than
the first switch position (another potential local minimum), it is stable
because energy is
required to move the switch away from the local minima. The potential maximum
represents a compressed state of the spring arms and/or the hollow beam walls.
Some of
this energy is released as the center body moves to the second switch
position.
Generally, switching from the first switch position to the second switch
position requires sufficient energy to move the center body from the reference
(zero
potential) position to the potential maximum. However, switching from the
second switch
position to the first switch position may be different, requiring the energy
necessary to
move the center body from the second switch position over the potential
maximum, which
is generally less but could be essentially the same or even more.
III. Analytical Model
Fig. 2A is a simplified representation of a portion of a switching device
according to the present invention illustrating various aspects used in
modeling the
operation of the device. For purposes of illustration, the actuator described
is an
electrostatic comb drive. The force generated by a voltage V in an
electrostatic comb
drive actuator can be expressed by the formula:
FeS =reshV2 (Eq. 1)
9
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n is the number of comb fingers. A typical figure in the existing designs is
around 100.
The height of the structure h is about 70-75 m. The gap g between the comb
fingers can
be reduced down to 4 m. The dielectric constant of the medium (vacuum, gas or
fluid)
between the comb drive fingers is E.
Using these values one can calculate the typical force generated by a comb
drive actuator for a given voltage in different media:
Air Oil
Force at 70 V 80 pN 160 N
Thus, application of 70 V to the comb actuator should switch the suspension
structure
between its two bi-stable states if the force required to pass the unstable
barrier is <160 N
for oil filled devices and <80 N for air filled devices.
The bi-stable behavior of the structure in Figs. 1 A and 1 B arises from a
lateral force=which holds the structure in its deformed equilibrium state. The
spring model
shown in Figs. 2B-2D more clearly illustrates the forces acting in the
structure of Fig. 2A
as it is switched between states. Figs. 2B-2D are not representative of the
actual physical
deformation in the MEMS device, but rather are illustrative of the analytical
model. In the
rest position, i.e. such as the structure as it is fabricated, all springs are
in a released state
(Fig. 2B). When the structure is moved in the longitudinal direction both the
lateral
springs 70, 71 as well as the longitudinal spring 72 are compressed. After
half the
deformation (Fig. 2C), the lateral springs 70, 71 start decompressing again,
which results
in a force acting into the longitudinal direction. If this force is higher
than the restoring
spring force from the longitudinal springs, the structure will have two static
equilibrium
states. The compression of the hollow beam walls (see e.g. Fig. 2a, ref. num.
42A) are
represented by the lateral springs 70, 71 and the restoring force of the
spring arms is
represented by the longitudinal spring 72.
The structure modeled in Figs. 2B-2D can be realized by fabrication of the
structure shown in Fig. 2A. The longitudinal spring 72 of Fig. 2B is formed by
the 4
spring arms 80, 84, 87, 90 which have a total linear spring constant of:
Ehth
kh - 2 41' - 61;, L,, + 31hLZh (Eq. 2)
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Where E is the Young's modulus of silicon, typically about 170 GPa, h is the
height of the
structure (nominally 75 m), th is the thickness of the flexible hinges 79,
81, 82, 85, 86,
88, 89, 91, lh is the length of the hinges (assumed to be the same for all
hinges in this
model), and Lh is the total length of one suspension arm (also assumed to be
the same for
all arms in this model). The factor 2 takes into account the total number of 4
spring arms.
The 4 lateral springs have a total spring constant of:
16Et; h
k, = 4 3 (Eq. 3)
1r
where tl and 11 are the thickness and respectively the length of this double
clamped
suspension. The suspension is double clamped because both the first opposite
pair of arms
80, 84 and the second pair of arms 87, 90 (in conjunction with the respective
hollow beam
walls 42A, 40A and 42B, 42B) clamp the center body 22A in one of the two
switch states.
The distance between the two equilibrium points can be adjusted by the initial
offset.
To find an analytical expression describing structure of Fig. 2A, we will
first consider the flexible hinges infinitely compliant, i.e. kh = 0. Applying
a force Fy in the
y direction 92 at a clamping point creates a reaction force Fx in x direction
98 on the
mobile structure (i.e. center body). These forces are related to one another
through the
expression below:
Fs = x d Fy (Eq. 4)
Lh
where x is the longitudinal displacement of the mobile structure. In a
particular
embodiment an optical element, such as a mirror, is mounted or formed on a
mounting
portion 95 of the center body. When the mobile structure is displaced in the x
direction
98, the lateral springs will be deformed by the amount Ay:
~y= d2+Lh dx)2+Lh (Eq. 5)
The force Fy in x direction resulting from this Dy can be obtained by
multiplying this
deformation by the spring constant ki and replacing Fy in equation (Eq. 4):
F~y = d x k, [ d Z+ Lh - (d - x)Z + Lh I (Eq. 6)
Lh
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The expression in the brackets [..] is always greater than or equal to zero.
The sign of the
force F. is thus only determined by the expression (x-d)/L, which changes its
sign when x
becomes larger than d, i.e. when the structure passes its center point ( as in
Fig. 2C).
In order to obtain the desired bi-stable behavior this force F. should be
higher than the
5 restoring spring force once the structure has passed into the position shown
in Fig. 2D, as
described above in conjunction with Fig. 1C.
The restoring spring force from the flexible hinges can be expressed by:
F. = xk,, (Eq. 7)
Finally the static equilibrium can be written as:
10 F. - F. - F,,y = 0 (Eq. 8)
which states, that the extemal force, i.e. the electrostatic drive F., has to
counterbalance
the restoring spring force F,,, from the flexible hinges and as well as the
force F,y from the
lateral springs. Those skilled in the art will appreciate that the model
provided above is
simplified and exemplary only, and that other switch structures might be more
accurately
represented by other models.
IV. Further Details of an Exemplary Device
Fig. 3A is a simplified top view of a portion of a MEMS switch device and
electrostatic comb drive according to an embodiment of the present invention.
The spring
arms 80, 84, 87, 90; hinges (not shown in this figure); center body 22A
including a
mounting portion 95; and actuator 34 are fabricated in a nominally 75 micron-
thick layer
of single-crystal silicon overlying a thin (e.g. 2-5 micron) silicon oxide
layer on a silicon
wafer substrate. The actuator includes a first section 35 and a second section
33. Each
section includes an array of opposing "fingers". When a voltage is applied to
the opposite
halves (not shown) of the first section, the center body is retracted from its
as-fabricated
position. When a voltage is applied to the opposite halves of the second
section, the center
body is extended toward its as-fabricated position.
The mounting portion 95 is about 1 micron thick, but can be thicker if
desired. In an exemplary embodiment a thin film of reflective material is
deposited on a
vertical (to the substrate) surface of the mounting portion to serve as a
mirror that is
switched in and out of an optical signal path. In a further embodiment, a
reflective coating
is formed on both surfaces 94, 96 of the mounting portion and the mirrors are
switched
into and out of two optical paths to form a 2x2 optical switch. The mirrors
could be, for
example, sputtered thin film layers of gold about 1000 Angstroms thick, or
other metals or
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reflective coatings. It is generally desirable that the mirror surface be
smooth, highly
reflective (>97%) in the wavelengths of interest, and not transmit light from
one side of
the mounting portion to the other. The etched surface of the mounting portion
can be
smooth enough to serve as a suitable mirror substrate.
Additional features include channels, or grooves, 100, 102, 104, 106, for
mounting four optical fibers (not shown) to the MEMS chip 108. The grooves
were
formed in the structure concurrently with the electrostatic comb drive
actuator and other
features of the device using a deep reactive ion etch (DRIE") process. The
MEMS chip is
sawn or otherwise separated from the substrate and is about 3.4 mm by 4 mm.
Fig. 3B is a simplified cross section of a portion of a MEMS switch
according to an embodiment of the present invention showing the silicon
substrate 110,
the oxide layer 112, and the overlying silicon layer 114. The oxide layer has
been
removed from beneath the center body 22 (the hollow portion) and the spring
arms 28, 30
with an HF etch, which has formed small undercuts 116 underneath the static
portions of
the overlying silicon layer. Devices made in accordance with the present
invention have
been cycled between two bi-stable states for between 200 million and 1 billion
complete
cycles depending on the particular testing protocol. No device failures have
been observed
in these tests. Other material systems may be used with suitable modifications
to the
fabrication techniques, if necessary.
V. A Bi-Stable MEMS Optical 2x2 Switch
Fig. 4A is a simplified perspective view of a portion of a MEMS chip
according to the present invention used in an optical switch 120 in a "bar"
state. Mirrors
have been formed on the surfaces 94, 96 of the mounting portion 95 of the
center body. A
first input fiber 122 provides an optical signal, represented by the arrow 124
to a mirror
coating 126 on the first surface 96 of the mounting portion when the center
body is
extended, and reflects the optical signal to a first output fiber 128. A
second input fiber
130provides a second optical signal, represented by the arrow 132, to a mirror
coating
(hidden in this view) on the second surface 94 of the mounting portion, which
reflects the
second optical signal to a second output fiber 134.
Fig. 4B shows the optical switch of Fig. 4A in a "cross" state with the
center body retracted so that the mirrors are not in the optical paths of the
first or second
optical signals from the first and second input fibers. In this switch state,
the first optical
signal couples to the second output fiber 134 and the second optical signal
couples to the
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fust output fiber 128. The angle between the first input fiber and first
output fiber (and
similariy second input fiber and second output fiber) is 90 , but in an
alternative
embodiment the angle is 70 to reduce polarization-dependant losses. Other
angles may
be used.
Although the embodiment described in conjunction with Figs. 4A and 4B
relate to optical fiber inputs and outputs, it is understood that other
optical transmission
lines may be used, such as thin-film waveguides, and that the optical fibers
may be
mechanically coupled to the MEMS chip in a variety of fashions to result in an
optical
switch. In a particular embodiment, lensed waveguides are employed to improve
the
insertion loss of the switch. For example, mounting cleaved fibers at a
separation suitable
for inserting the mounting portion of the MEMS switch in a fiber-to-fiber
insertion loss of
about 1.3-1.5 dB. Further providing a lens (i.e. a light-gathering structure)
on each fiber
end reduces the insertion loss. If an anti-reflective ("AR") coating is also
provided to the
lensed end the fiber-to-fiber insertion loss can be reduced to about 0.1-0.3
dB. Lensed
fibers are available from suppliers such as OPTOSPEED SA of Mezzovico,
Switzerland and
HIGHWAVE OPTICAL TECHNOLOGIES, of Lannion, France. Alternatively, a discrete
lens
could be attached to the fiber end. The lensed fibers available from OPTOSPEED
SA are
tapered to provide a light focusing/gathering function, not for merely
mechanical
purposes, although the tapered aspect of the fiber end allows the fiber end to
be brought
into closer proximity with the mirror or cross fiber, thus improving insertion
loss.
Although as little as 0.1 dB of insertion loss between fibers is achievable in
the cross mode, this generally requires optical alignment of the cross fibers.
With this
alignment, about 0.4 dB of insertion loss occurs between fibers in the bar
mode of
operation. This results in about 0.3 dB difference in insertion loss depending
on which
state the switch is in (i.e. the optical signal is reflected or directly
transmitted to the
selected output). To reduce this difference, the fibers can be intentionally
mis-aligned or
"de-focused" in the cross mode to result in increased insertion loss. The
terms "focus"
and "de-focus" are used for convenience and illustration, in light of the
dimensions of the
structures and wavelengths of the light signals involved. In some
applications, aligning
the fibers off-axis may improve insertion loss in the bar mode because the
thickness of the
mounting portion creates a difference in signal path length. In a particular
embodiment,
the cross path fibers with aligned to produce an insertion loss in the cross
mode of about
0.3 dB, thus the difference in insertion loss between the cross path and the
bar path was
less than about 25%, which is desirable to avoid signal level difference
between the
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selected switch states. Otherwise, a gain stage might (e.g. a light amplifier
such as a
doped fiber) in series with the signal might have to be adjusted for gain
depending on the
switch state.
Fig. 4C is a simplified side view of a packaged bi-stable optical MEMS
switch 140 according to an embodiment of the present invention. The actual
switching
device is packaged in a TO-5 package 142 that provides a hermetic seal and
protects the
device from external forces as well as a handling and mounting convenience. In
a
preferred embodiment, a low-profile TO-5 package having a can 144 height of
about 4-5
mm is used. The TO-5 package has a small footprint, with a can diameter of
about 8-
9mm. Alternatively, a TO-8 package having a can diameter of about 13-15 mm may
be
used. Packaging the switch in such a small package is enabled by the
combination of the
lensed fibers with the bi-stable micro switch. The latching spring arms avoid
the need for
separate latching.
Input and output optical signals are provided by four (4) optical fibers 122,
130, 134, one of which is not seen in this view but essentially is disposed
opposite to fiber
134 and normal to fibers 122 and 130 to form a "+" configuration when viewed
from the
top. The fibers are attached to the MEMS device by cementing them in grooves
that are
etched into the silicon MEMS chip, and brought through the cars 142 of the
package. The
can 42 is mounted to the header 146 of the package. The fibers are actively
aligned during
assembly of the hybrid MEMS chip/fiber switch. Two isolated (from the
header/package)
electrical leads 148, 150 are brought out to provide isolated electrical
energy to the
actuator when using an isolation chip mounting technique. Alternatively, a
single isolated
electrical lead may be used in conjunction with some types of actuators. In
yet another
configuration two isolated leads are provided, one for a "push" signal and one
for a "pull"
signal, with a common package ground being provided by a ground pin 152. In
any case,
no more than two isolated electrical leads are needed because the latching
technique of the
present invention does not require a separate latching/de-latching signal,
rather, only the
actuator signal. A two-lead (isolated) configuration allows the packaged MEMS
switch to
be electrically isolated from other components, which may be desirable in the
case of
high-voltage (greater than 40 V) switching signals. Alternatively, a voltage
converter chip
can be included in the package to allow a relatively low voltage (e.g. 5 V) to
be delivered
to the package, which is then boosted to a voltage appropriate for driving the
actuator (e.g.
40-80 V) inside the package. Electrical contacts, which provide the driving
signal for the
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switch, are ultrasonic wire bonded from the isolated electrical leads in the
header or
otherwise electrically coupled to appropriate pads on the MEMS chip.
Fig. 4D is an alternative embodiment of a packaged optical MEMS switch
140A according to an alternative embodiment of the present invention. The four
optical
fibers 122, 130, 134, and 128 are brought out through the header 146 of the
package,
rather than the can 142. A seal is made between the fibers and the header with
epoxy or
other sealant.
VI. MEMS Actuator Signal Waveforms
Novel waveforms are applied to the device to optimize switching speed
and stable switch operation, and hence reliable optical output. In prior
devices it has been
common to provide a simple "square wave" electrical pulse to electrostatic
comb
actuators. Referring to Fig. 1C, particular attention is drawn to the higher
stable well 62
and the potential maximum 60. Depending on the design of the switch, the
movable
portion of the switch can oscillate or "ring" after a switching pulse is
applied. Movable
portions with certain mass in combination with spring or actuator elements and
dampening
characteristics can overshoot the well position and spring back toward the
potential
maximum. The waveforms provided below are exemplary only to illustrate the
concept
that after the first, switching pulse, a second pulse is applied. The second
pulse is timed to
retard the acceleration of the actuator toward the target local equilibrium
position or
potential well, such acceleration being caused by the restoring force of the
spring arms
and/or hollow beam walls.
Simple minor ringing might cause variations in the optical signal
amplitude. More severe ringing might move the mirror in and out of the optical
input
signal path(s), causing drop-out of the desired output and cross-talk in the
undesired
output. In some cases, overshoot might cause the movable body to spring back
over the
potential maximum, and settle in another (undesired) potential well, thus
placing the
switch in a non-selected state. In prior switches, oil has been applied to the
switch to
increase dampening. However, embodiments of the present invention avoid such
measures by applying switching (actuator) waveforms having multiple segments.
Fig. 5A is a simplified graph of voltage versus time for a voltage waveform
applied to a MEMS switch according to an embodiment of the present invention.
The
voltage may be applied directly from a signal generator or similar source, or
a lower
voltage may be applied to a voltage converter or amplifier. In a preferred
embodiment, a
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comb drive with two drive units is employed. The fust drive unit causes the
center body
to retract when a "push" signal is applied. The second drive unit causes the
center body to
extend when a "pull" signal is applied to the second drive unit. In either
case, the fingers
of the respective drive units are moving toward each other in response to an
electric signal.
5 A first pulse 200 is provided to a first section of the electrostatic comb
drive to push the center body from its rest (as-fabricated) position over the
potential
maximum. This pulse is about 83 V with a pulse width of about 160 micro-
seconds.
Those skilled in the art will appreciate that the pulse shown is idealized,
and that the pulse
form typically has some rounding of the corners and sloping of the walls.
Furthermore,
10 the voltage and duration are exemplary only, generally chosen according to
a specific
embodiment of switch to reliably and quickly drive the switch from a first
state (Fig. 1 C,
ref. num. 59) to a second state (Fig. 1C, ref. num. 62) over the potential
maximum. Other
switches might optimally switch states with different voltages and pulse
durations
according to the mass and spring constant of the movable portions of the
switch, among
15 other factors, such as the difference between the potential well(s) and the
potential
maximum.
A second pulse 202 is provided to retard the acceleration of the center body
after it has passed the potential maximum, but before the center body has
reached the
second potential well center. The second pulse slows down, or decelerates, the
center
body, which is typically accelerating from the potential maximum due to the
spring energy
stored in the spring arms and hollow beam walls. The second pulse 202 is
provided to a
second section of the electrostatic comb drive to pull the movable portion of
the switch in
the opposite direction from the motion caused by the first pulse. It is
understood that each
of the two sections of the comb drive operates by attracting one half of the
section to an
opposing half of the section, and that "push" and "pull" are defined in terms
of the
movement of the center body. Furthermore, in a particular embodiment the MEMS
device
is fabricated with the center body in an extended (bar) position, thus
"pushing" from this
position up the potential energy curve involves retracting the center body.
The second
pulse has a voltage of about 48 V and a duration of about 140 micro-seconds.
The second
pulse in Fig. 5A is shown as being inverted merely to illustrate that it has a
different effect
on the center body, and does not imply polarity of either pulse. A dwell
period 204 of
about 40 micro-seconds is provided between the first and second pulses to
account for
variations in switch fabrication, actuator performance, and the electrical
pulse supply, for
example. Ideally, an electrostatic switching signal waveform would provide
enough force
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when applied to the actuator to rapidly accelerate the center body past the
position of the
potential maximum, and then start decelerating the center body in a fashion
that would
rapidly allow the center body to assume the desired switch position with
minimal ringing
or overshoot.
Fig. 5B is a simplified graph of voltage versus time illustrating another
waveform according to an embodiment of the present invention. A first push
pulse 200 is
applied, as before in Fig. 5A; however, the dwell period 206 is increased to
allow the
center body to travel through the desired potential well and spring back
through the well
center toward the potential maximum. After the dwell period, a second push
pulse 208 is
applied to keep the center body from traveling too far toward or over the
potential
maximum, thus reducing ringing and overshoot.
The switching waveforms illustrated in Figs. 5A and 5B are for switching
from a first state to a second state. A different waveform might be desired
for switching
from the second state to the first state. For example, referring again to Fig.
1C, the energy
required to overcome the potential maximum from the lower well to the higher
well is
greater than the energy to overcome the potential maximum from the higher well
to the
lower well. Thus, in a further embodiment, two different waveforms are used to
switch
from opposite states, in other words the switching waveforms (signals) are non-
symmetrical in that they deliver different energies to the actuator depending
on the starting
and ending switch states. The pulse width, as well as the voltage, and
temporal shape and
timing may be modified. For example, when switching to the lower well
position, more
spring energy will be released from the spring arms and hollow beam walls,
thus a longer
and/or higher voltage pull pulse (ref. Fig. 5A, ref. num. 202) is appropriate.
Using the
appropriate amount of switching energy reduces overshoot when switching from a
higher
potential state and reduces the power required for a switch cycle, which is
desirable in
power limited situations, such as solar powered or battery powered
applications, or
environments where heat dissipation from the power supply is critical.
The above illustrations are merely examples of methods to apply a second
electronic signal to a MEMS switch to improve operation of a MEMS switch. In
the
particular spring embodiments described above, a second pulse may be applied
to retard
acceleration as the center body travels past the potential maximum position
toward the
second well position. The spring energy stored in the latching spring arm-beam
side wall
structure contributes to this acceleration. Other MEMS switches may be
improved and
switch mechanisms may similarly benefit from an electronic switching signal
applied as a
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series of electronic signals separated by selected periods of time. It is
specifically
understood that it may be desirable to apply more than two segments (e.g.
pulses) of a
switching signal to perform the desired switching function.
Fig. 6A is a simplified flow chart of a process of operating 600 a MEMS
switch according to an embodiment of the present invention. A MEMS switch is
provided
in a first position (step 602). A first pulse of a switching signal is
provided to the MEMS
switch to move the switch toward a second position (step 604). After waiting
for a
selected dwell period (step 606), a second pulse (step 608) is provided to the
MEMS
switch to place the switch in a second position.
In a particular embodiment the MEMS switch includes an electro-static
comb drive actuator, the first pulse is a push pulse and the second pulse is a
pull pulse. In
another embodiment, both pulses are push pulses. In another embodiment, the
switching
from the first position (switch state) to the second is accomplished with the
application of
the switching signal, and switching back from the second position to the first
position is
accomplished with stored mechanical energy. In a non-latching switch, the
switching
signal (i.e. the signal applied to change states) may be followed with a
switch holding
signal (i.e. the signal applied to hold a state). In some embodiments, these
two types of
signals may be continuous in time.
Fig. 6A is a simplified flow chart of a process of operating 600 a MEMS
switch according to an embodiment of the present invention. A MEMS switch is
provided
in a first latched position (step 602). A switching signal is provided to the
MEMS switch
to latch the switch in a second latched position (step 604). No latching or de-
latching
signal apart from the switching signal is required to change switch states.
Fig. 6B is a simplified flow chart of a process of operating 610 a MEMS
switch according to a further embodiment of the present invention. A MEMS
switch is
provided in a first latched position (602). A first switching signal including
a first pulse, a
first dwell period, and a second pulse is provided to the MEMS switch to latch
the switch
in a second latch position (step 614). In further operation, a second
switching signal
including at least a third pulse is provided to the MEMS switch to latch the
switch in the
first latched position (616).
While the description above provides a full and complete disclosure of the
preferred embodiments of the present invention, various modifications,
alternatives, and
equivalents will be obvious to those of skill in the art. For example, while
embodiments
of the invention have been described primarily with reference to monolithic
MEMS
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electrostatic comb drive actuators embodiments of the present invention might
employ
other types of actuators, such as various other electrostatic actuators,
thermal actuators,
and magnetic actuators. Accordingly, the scope of the invention is limited
solely by the
following claims.
