Title: Microelectromechanical device with continuously variable displacement
Abstract: An electrostatic micromechanical device with continuously variable displacement, that includes: a movable member having a first electrode; an opposing surface having a second electrode; a channel separating the movable member from the opposing surface; a liquid situated in the channel, wherein the liquid has a sufficiently high dielectric constant so as to enable continuously variable and stable control of a displacement of the movable member over a travel range spanning at least half of the channel; the displacement being a result of a voltage applied between the first electrode and the second electrode; and at least one solid dielectric layer physically situated between the first electrode and the second electrode.
Patent Number: 6,844,960 Issued on 01/18/2005 to Kowarz
| Inventors:
|
Kowarz; Marek W. (Henrietta, NY)
|
| Assignee:
|
Eastman Kodak Company (Rochester, NY)
|
| Appl. No.:
|
253747 |
| Filed:
|
September 24, 2002 |
| Current U.S. Class: |
359/298; 359/290; 359/291 |
| Intern'l Class: |
G02B 026/08; G02F001/29 |
| Field of Search: |
359/298,290,291,197,295,231,572,573,292,223,224
|
References Cited [Referenced By]
U.S. Patent Documents
| 6144481 | Nov., 2000 | Kowarz et al. | 359/291.
|
| 6210326 | Apr., 2001 | Ehwald | 600/365.
|
| 6215579 | Apr., 2001 | Bloom et al. | 359/298.
|
| 6284149 | Sep., 2001 | Li et al. | 216/64.
|
| 6286943 | Sep., 2001 | Ashe et al. | 347/71.
|
| 6307663 | Oct., 2001 | Kowarz | 359/231.
|
| 6329738 | Dec., 2001 | Hung et al. | 310/309.
|
| 6356248 | Mar., 2002 | Martin et al. | 345/60.
|
| 6362018 | Mar., 2002 | Xu et al. | 438/50.
|
| 6379929 | Apr., 2002 | Burns et al. | 435/91.
|
| 6419335 | Jul., 2002 | Gooray et al. | 347/9.
|
| 6575020 | Jun., 2003 | de Charmoy Grey et al. | 73/54.
|
| 6599781 | Jul., 2003 | Li | 438/142.
|
| 2001/0051408 | Dec., 2001 | Chiu | 438/243.
|
| 2002/0159701 | Oct., 2002 | Katayama et al. | 385/39.
|
Other References
Elmer S. Hung and Stephen D. Senturia, "Extending the Travel Range of
Analog-Tuned Electrostatic Actuators" Journal of Microelectromechanical
Systems, vol. 8, No. 4, Dec. 1999.
Michael S.-C. Lu, et al., "Closed-Loop Control of a Parallel-Plate
Microactuator Beyond the Pull-in Limit" Solid-State Sensor, Actuator and
Microsystems Workshop, Jun. 2-6, 2002, pp. 255-258.
|
Primary Examiner: Mack; Ricky
Assistant Examiner: Thomas; Brandi
Attorney, Agent or Firm: Shaw; Stephen H.
Claims
What is claimed is:
1. An electrostatic micromechanical device with continuously variable
displacement, comprising:
a) a movable member having a first electrode;
b) an opposing surface having a second electrode;
c) a channel separating the movable member from the opposing surface;
d) a liquid situated in the channel, wherein the liquid has a sufficiently
high dielectric constant so as to enable continuously variable and stable
control of a displacement of the movable member over a travel range
spanning at least half of the channel; the displacement being a result of
a voltage applied between the first electrode and the second electrode;
and
e) at least one solid dielectric layer physically situated between the
first electrode and the second electrode.
2. The electrostatic micromechanical device claimed in claim 1, wherein the
travel range spans the entire channel.
3. The electrostatic micromechanical device claimed in claim 2, wherein the
dielectric constant of the liquid is larger than approximately 1.5 d.sub.c
/t.sub..epsilon., d.sub.c being a separation distance between the movable
member and the opposing surface and t.sub..epsilon. being a total
dielectric thickness of the at least one solid dielectric layer.
4. The electrostatic micromechanical device claimed in claim 1, wherein the
dielectric constant of the liquid is larger than 2.
5. The electrostatic micromechanical device claimed in claim 2, wherein the
dielectric constant of the liquid is larger than 5.
6. The electrostatic micromechanical device claimed in claim 2, wherein the
dielectric constant of the liquid is between 5 and 100.
7. The electrostatic micromechanical device claimed in claim 1, wherein the
liquid is an alcohol.
8. The electrostatic micromechanical device claimed in claim 1, wherein the
liquid is ethylene glycol.
9. The electrostatic micromechanical device claimed in claim 1, wherein the
movable member is a tensile ribbon element.
10. The electrostatic micromechanical device claimed in claim 1, wherein
the movable member is a doubly-supported beam.
11. The electrostatic micromechanical device claimed in claim 1, wherein
the movable member is a singly-supported beam.
12. The electrostatic micromechanical device claimed in claim 1, wherein
the movable member is a membrane.
13. The electrostatic micromechanical device claimed in claim 1, wherein
the movable member is a plate.
14. The electrostatic micromechanical device claimed in claim 1, wherein
the opposing surface is a silicon substrate.
15. The electrostatic micromechanical device claimed in claim 1, wherein
one solid dielectric layer is silicon nitride.
16. The electrostatic micromechanical device claimed in claim 1, wherein
one solid dielectric layer is silicon oxide.
17. The electrostatic micromechanical device claimed in claim 1, wherein
one solid dielectric layer is on the movable member.
18. The electrostatic micromechanical device claimed in claim 1, wherein
one solid dielectric layer is on the opposing surface.
19. The electrostatic micromechanical device claimed in claim 1, wherein
the liquid surrounds the electrostatic micromechanical device, in addition
to being situated in the channel.
20. An electrostatic micro-optomechanical device with continuously variable
displacement, comprising:
a) at least one movable member having a first electrode;
b) an opposing surface having a second electrode;
c) a channel separating the at least one movable member from the opposing
surface;
d) a liquid situated in the channel, wherein the liquid has a sufficiently
high dielectric constant so as to enable continuously variable and stable
control of a displacement of the at least one movable member over a travel
range spanning at least half of the channel; the displacement being a
result of a voltage applied between the first electrode and the second
electrode, and the travel range being proportional to .lambda., a
wavelength of incident light; and
e) at least one solid dielectric layer physically situated between the
first electrode and the second electrode.
21. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the liquid is transparent.
22. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the at least one movable member is reflecting.
23. The electrostatic micro-optomechanical device claimed in claim 22,
wherein the at least one reflecting movable member is a micromirror.
24. The electrostatic micro-optomechanical device claimed in claim 22,
wherein a plurality of reflecting movable members comprise an
electro-mechanical grating.
25. The electrostatic micro-optomechanical device claimed in claim 24,
wherein the electromechanical grating is a conformal GEMS device.
26. The electrostatic micro-optomechanical device claimed in claim 24,
wherein the electro-mechanical grating is a grating light valve.
27. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the travel range spans the entire channel.
28. The electrostatic micro-optomechanical device claimed in claim 27,
wherein the dielectric constant of the liquid is larger than approximately
1.5 d.sub.c /t.sub..epsilon., d.sub.c being a separation distance between
the at least one movable member and the opposing surface and
t.sub..epsilon. being a total dielectric thickness of the at least one
solid dielectric layer.
29. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the dielectric constant of the liquid is larger than 2.
30. The electrostatic micro-optomechanical device claimed in claim 27,
wherein the dielectric constant of the liquid is larger than 5.
31. The electrostatic micro-optomechanical device claimed in claim 27,
wherein the dielectric constant of the liquid is between 5 and 100.
32. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the liquid is an alcohol.
33. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the liquid is ethylene glycol.
34. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the at least one movable member is a tensile ribbon element.
35. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the at least one movable member is a doubly-supported beam.
36. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the at least one movable member is a singly-supported beam.
37. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the at least one movable member is a membrane.
38. The electrostatic micro-optomechanical device claimed in claim 20,
wherein the at least one movable member is a plate.
39. A method for constructing an electrostatic micromechanical device with
continuously variable displacement, comprising the steps of:
a) providing a movable member having a first electrode and separated by a
channel from an opposing surface having a second electrode, including at
least one solid dielectric layer between the first electrode and the
second electrode; and
b) filling the channel with a liquid, wherein the liquid has a sufficiently
high dielectric constant so as to enable continuously variable and stable
control of a displacement of the movable member over a travel range
spanning at least half of the channel; the displacement being a result of
a voltage applied between the first electrode and the second electrode.
40. The method claimed in claim 39, wherein the step of filling the channel
with a liquid further includes selecting the dielectric constant of the
liquid as larger than approximately 1.5 d.sub.c /t.sub..epsilon., d.sub.c
being a separation distance between the movable member and the opposing
surface and t.sub..epsilon. being a total dielectric thickness of the at
least one solid dielectric layer.
41. The electrostatic micro-optomechanical device claimed in claim 24,
wherein the travel range is equal to approximately .lambda./4n, n being a
refractive index of the liquid.
Description
FIELD OF THE INVENTION
The present invention relates to electrostatic microelectromechanical
devices, and more particularly to microelectromechanical devices with
movable members requiring continuously variable and stable displacement
over a large travel range.
BACKGROUND OF THE INVENTION
Many different types of microelectromechanical (MEMS) devices, such as
variable capacitors, electromechanical gratings and mirrors, inkjet
printheads, and a variety of sensors, rely on electrostatic forces between
two electrodes to produce controlled actuation of a movable member.
However, as is well known, continuous control of the displacement of the
movable member is only possible over a fraction (approximately 1/3) of the
distance between the two electrodes because of the nonlinear nature of the
electrostatic forces. Once the displacement exceeds this fraction,
"pull-in" or "pull-down" occurs, whereby the nonlinear electrostatic force
completely overwhelms the mechanical restoring force of the member.
Different approaches have been used to produce continuously variable
displacement in electrostatic MEMS devices while avoiding the pull-down
instability. The most straightforward is to design the device with a large
enough separation between the two electrodes, thereby enabling sufficient
displacement before reaching the instability point. This approach has been
used by Silicon Light Machines in their analog Grating Light Valve (GLV),
as described by Bloom et al. in U.S. Pat. No. 6,215,579, entitled Method
and Apparatus for Modulating an Incident Light Beam for Forming a
Two-Dimensional Image, issued Apr. 10, 2001. To avoid high operating
voltages caused by increased electrode separation, these analog GLVs are
specifically designed to have low mechanical restoring forces.
Alternatively, a more complex structural design can be used in an
electromechanical grating to obtain continuous actuation over a larger
travel range, as described by Hung et al. in U.S. Pat. No. 6,329,738,
entitled Precision Electrostatic Actuation And Positioning, issued Dec.
11, 2001 and in E. S. Hung and S. D. Senturia, "Extending the Travel Range
of Analog-Tuned Electrostatic Actuators," Journal of
Microelectromechanical Systems, vol. 8, No. 4, pgs. 497-505 (1999).
Another alternative is described in U.S. Pat. No. 6,362,018, entitled
Method for Fabricating MEMS Variable Capacitor with Stabilized
Electrostatic Drive, by Xu et al., issued Mar. 26, 2002, whereby a fixed
series capacitor is added to a variable MEMS capacitor in order to extend
the electromechanical tunability of the variable capacitor. A disadvantage
of this last approach is that the required actuation voltage is raised
significantly.
Recently, an electromechanical conformal grating device, an optical MEMS
device consisting of ribbon elements suspended above a substrate by a
periodic sequence of intermediate supports was disclosed by Kowarz in U.S.
Pat. No. 6,307,663, entitled Spatial Light Modulator With Conformal
Grating Device, issued Oct. 23, 2001. The electromechanical conformal
grating device is operated by electrostatic actuation, which causes the
ribbon elements to conform around the support substructure, thereby
producing a grating. The device of '663 has more recently become known as
the conformal GEMS device, with GEMS standing for grating
electromechanical system. The conformal GEMS device provides high-speed
light modulation with high contrast, good efficiency and digital
operation. However, for applications that require amplitude modulation of
light intensity, analog operation with continuous control of the
displacement of the ribbon elements is needed. In addition, the approaches
mentioned earlier for producing continuously variable displacement while
avoiding the pull-down instability are ill-suited for the conformal GEMS
device.
There is a need, therefore, for an electrostatic microelectromechanical
device that has a continuously variable displacement and avoids the
problems noted above.
SUMMARY OF THE INVENTION
The above need is met according to the present invention by providing an
electrostatic micromechanical device with continuously variable
displacement, that includes: a movable member having a first electrode; an
opposing surface having a second electrode; a channel separating the
movable member from the opposing surface; a liquid situated in the
channel, wherein the liquid has a sufficiently high dielectric constant so
as to enable continuously variable and stable control of a displacement of
the movable member over a travel range spanning at least half of the
channel; the displacement being a result of a voltage applied between the
first electrode and the second electrode; and at least one solid
dielectric layer physically situated between the first electrode and the
second electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a cross-sectional view of a prior art tensile ribbon element in
an unactuated state;
FIG. 1b is a rotated cross-sectional view of (prior art) three parallel
tensile ribbon elements in an unactuated state;
FIG. 2a is a cross-sectional view of a prior art tensile ribbon element in
a partially actuated state;
FIG. 2b is a cross-sectional view of a prior art tensile ribbon element in
a fully actuated state;
FIG. 3a shows the profile of a prior art ribbon element in a gas atmosphere
for various actuation voltages;
FIG. 3b shows the profile of a ribbon element immersed in a liquid with a
high dielectric constant for various actuation voltages;
FIG. 4a shows a plot of the midpoint deflection of a ribbon element in a
gas atmosphere as a function of applied voltage, illustrating the presence
of pull-down and release instabilities;
FIG. 4b shows a plot of the midpoint deflection of a ribbon element in a
liquid with a high dielectric constant as a function of applied voltage,
illustrating the elimination of pull-down and release instabilities;
FIG. 5 shows a plot of the critical voltages of a ribbon element in liquid
as a function of the liquid dielectric constant;
FIG. 6 is a perspective, partially cut-away view of two conformal GEMS
devices in a linear array;
FIG. 7 is a top view of four conformal GEMS devices in a linear array;
FIGS. 8a and 8b are cross-sectional views through line 8--8 in FIG. 7,
showing the operation of a conformal GEMS device in an unactuated state
and a fully actuated state, respectively;
FIGS. 9a and 9b are cross-sectional views through line 9--9 in FIG. 7,
showing the conformal GEMS device in an unactuated state and a fully
actuated state, respectively;
FIG. 10 shows theoretical plots of the reflected light intensity as a
function of applied voltage for a conformal GEMS device, comparing various
liquids to a gas atmosphere; and
FIG. 11 shows experimental plots of the reflected light intensity as a
function of applied voltage for a conformal GEMS device, comparing various
liquids to a gas atmosphere.
DETAILED DESCRIPTION OF THE INVENTION
In its broadest embodiment, the present invention increases the usable
travel range of a variety of movable members used in electrostatic
microelectromechanical (MEMS) devices. Within the usable travel range, the
displacement of the movable members is continuously variable and stable.
Although the invention is described primarily for the specific case of an
electrostatic MEMS device with one or more tensile ribbon elements, it
will be apparent to those skilled in the art that the invention also
pertains to devices containing other microelectromechanical structures.
These movable members can include, for example, singly-clamped beams,
doubly-clamped beams, membranes or rigid plates on torsional hinges.
Furthermore, multiple movable members can be used to create more complex
moving structures, such as electrostatic comb drives or electromechanical
gratings.
FIG. 1a illustrates the cross-section of a typical tensile ribbon element
2a that is a movable member in a MEMS device. Tensile stress in the
dielectric ribbon material 7 keeps the tensile ribbon element 2a separated
from the opposing surface, a conducting substrate 9. The channel 4 is
usually evacuated or filled with an inert gas atmosphere, such as nitrogen
or a nobel gas. To deform the ribbon into the channel 4, a voltage is
applied between the first electrode 6 and the conducting substrate 9,
which serves as the second electrode. In FIGS. 1a and 1b, the applied
voltage is zero. FIG. 1b shows a rotated cross-section of the same tensile
ribbon element 2a with two neighboring elements 2b and 2c.
FIGS. 2a and 2b show cross-sectional views of the tensile ribbon element
2a, with a voltage applied between the first electrode 6 and the
conducting substrate 9. When the applied voltage is slightly below the
pull-down voltage V.sub.PD, the tensile ribbon element 2a is suspended as
illustrated in FIG. 2a. Once the applied voltage exceeds V.sub.PD, the
nonlinear electrostatic force completely overwhelms the tensile restoring
force. The tensile ribbon element 2a then snaps into contact with the
conducting substrate 9, producing the ribbon profile shown in FIG. 2b. As
already mentioned, it is not possible to produce continuously variable
profiles for applied voltages close to V.sub.PD.
In the present invention, the channel 4 is filled with a liquid that has a
high dielectric constant and can withstand high electrostatic fields. By
careful selection of this liquid, the pull-down instability can be
eliminated, thereby enabling continuously variable and stable displacement
of the tensile ribbon element 2a over a travel range spanning the entire
channel 4.
The dielectric constant requirements for the liquid can be established by
considering the effective electrostatic thickness t of the device.
Including the effect of the liquid-filled channel 4, the effective
electrostatic thickness t of the layers between the first electrode 6 and
the conducting substrate 9 is given by the expression
t=d.sub.c +.epsilon.t.sub..epsilon.,
where
##EQU1##
is the total dielectric thickness of the solid dielectrics; d.sub.c is the
depth of the channel 4; and .epsilon. is the dielectric constant of the
liquid. In the total dielectric thickness t.sub..epsilon., the summation
is over all solid dielectrics between the two electrodes with the
thickness t.sub.m of each solid dielectric reduced by its dielectric
constant .epsilon..sub.m. It can be shown that the tensile ribbon element
2a can be displaced smoothly throughout a travel range spanning the entire
channel 4 if the depth of the channel d.sub.c is less than approximately
0.388 t. To satisfy this requirement, the dielectric constant of the
liquid should satisfy the inequality
.epsilon.>1.58 d.sub.c /t.sub..epsilon. (Equation 1)
A liquid that satisfies Equation 1 eliminates the pull-down instability,
thereby allowing for a continuously variable and stable displacement over
the entire depth of the channel 4. Liquids with lower dielectric constants
can be used to increase the travel range beyond what is usually possible
with a gas-filled or evacuated channel 4. Such lower dielectric constant
liquids are still considered to be within the scope of this invention.
FIGS. 3a through 5 illustrate the use of a liquid to eliminate the
pull-down instability of a tensile ribbon element. In this example, the
channel depth d.sub.c is 150 nanometers and the total dielectric thickness
t.sub..epsilon. is 24 nanometers. Based on Equation 1, a liquid with a
dielectric constant of greater than 9.8 enables travel range spanning the
entire channel 4. FIG. 3a shows a plot of the ribbon profile as a function
of increasing voltage for a gas-filled channel (.epsilon.=1). The change
in ribbon profile is discontinuous and unstable near the pull-down voltage
V.sub.PD =22.6 V. FIG. 3b shows the same device with the channel 4 filled
with a liquid with .epsilon.=18. A continously variable deflection and
stable deflection can now be obtained over a travel range spanning the
entire channel 4, with only a slight increase in voltage required.
FIGS. 4a and 4b compare the deflection at the midpoint of a ribbon as a
function of voltage for the gas-filled and liquid-filled devices of FIGS.
3a and 3b, respectively. For the gas-filled device (FIG. 4a), as the
voltage is increased from zero, the midpoint displacement increases
continously until the voltage reaches the pull-down voltage of 22.6 V. At
this point, the ribbon slams into the substrate, and further increases in
voltage changes the ribbon's profile as depicted in FIG. 3a. With the
ribbon now in contact with the substrate, the voltage can be decreased
below the pull-down voltage while maintaining contact. At the release
voltage V.sub.RL of 11.7 V, the tensile stress overcomes the electrostatic
attraction and the ribbon is released from the substrate. This type of
hysteresis curve is well-known in electrostatic MEMS devices and is often
used to describe the instability associated with electrostatic actuation.
As shown in FIG. 4a, the response is identical for negative applied
voltages.
For the liquid-filled device (.epsilon.=18), the midpoint displacement is a
smooth function of applied voltage as shown in FIG. 4b. The instabilities
associated with the critical pull-down and release voltages are absent and
the hysteresis disappears.
FIG. 5 is a plot of the two critical voltages, V.sub.PD and V.sub.RL, as a
function of the dielectric constant of the liquid. When these two voltages
become equal, i.e., when .epsilon.=9.8 for this example, the travel range
spans the entire depth of the channel 4. Further increasing the dielectric
constant reduces the sensitivity of displacement to voltage, thereby
improving the ability to control displacement.
As is well known, tensile ribbon elements, such as those discussed above,
are basic building blocks that can be used to form more complex MEMS
devices. For example, large numbers of parallel ribbon elements can be
used to produce electromechanical gratings, such as the conformal GEMS
device or the GLV. Individual ribbon elements can be used to make variable
capacitors or capacitive sensors.
The conformal Grating Electromechanical System (GEMS) devices disclosed in
'663 are illustrated in FIGS. 6 through 9. FIG. 6 shows two side-by-side
conformal GEMS devices 5a and 5b in an unactuated state. The conformal
GEMS devices 5a and 5b are formed on top of a substrate 10 covered by a
bottom conductive layer 12, which acts as an electrode to actuate the
devices 5a, 5b. The bottom conductive layer 12 is covered by a dielectric
protective layer 14 followed by a standoff layer 16 and a spacer layer 18.
On top of the spacer layer 18, a ribbon layer 20 is formed which is
covered by a reflective and conductive layer 22. The reflective and
conductive layer 22 provides electrodes for the actuation of the conformal
GEMS devices 5a and 5b. Accordingly, the reflective and conductive layer
22 is patterned to provide electrodes for the two conformal GEMS devices
5a and 5b. The ribbon layer 20, preferably, comprises a material with a
sufficient tensile stress to provide a large restoring force. Each of the
two conformal GEMS devices 5a and 5b has an associated elongated ribbon
element 23a and 23b, respectively, patterned from the reflective and
conductive layer 22 and the ribbon layer 20. The elongated ribbon elements
23a and 23b are supported by end supports 24a and 24b, formed from the
spacer layer 18, and by one or more intermediate supports 27 that are
uniformly separated in order to form equal-width channels 25. The
elongated ribbon elements 23a and 23b are secured to the end supports 24a
and 24b, and to the intermediate supports 27. A plurality of standoffs 29
is patterned at the bottom of the channels 25 from the standoff layer 16.
These standoffs 29 reduce the possibility of the elongated ribbon elements
23a and 23b sticking when actuated.
A top view of a four-device linear array of conformal GEMS devices 5a, 5b,
5c and 5d is shown in FIG. 7. The elongated ribbon elements 23a, 23b, 23c,
and 23d are depicted partially removed over the portion of the diagram
below the line A--A in order to show the underlying structure. For best
optical performance and maximum contrast, the intermediate supports 27
should preferably be completely hidden below the elongated ribbon elements
23a, 23b, 23c, and 23d. Therefore, when viewed from the top, the
intermediate supports 27 should not be visible in the gaps 28 between the
conformal GEMS devices 5a-5d. Here, each of the conformal GEMS devices
5a-5d has three intermediate supports 27 with four equal-width channels
25. The center-to-center separation A of the intermediate supports 27
defines the period of the conformal GEMS devices in the actuated state.
The elongated ribbon elements 23a-23d are mechanically and electrically
isolated from one another, allowing independent operation of the four
conformal GEMS devices 5a-5d. The bottom conductive layer 12 of FIG. 6 can
be common to all of the conformal GEMS devices 5a-5d.
FIG. 8a is a side view, through line 8--8 of FIG. 7, of two channels 25 of
the conformal GEMS device 5b in an unactuated state. FIG. 8b shows the
same view for an actuated state. For operation of the device, an
attractive electrostatic force is produced by applying a voltage
difference between the bottom conductive layer 12 and the reflective and
conductive layer 22 of the elongated ribbon element 23b. In the unactuated
state (see FIG. 8a), with no voltage difference, the ribbon element 23b is
suspended flat between the supports. In this state, an incident light beam
30 is primarily reflected into a 0th order light beam 32, as in a simple
planar mirror. To obtain the actuated state, a voltage is applied to the
conformal GEMS device 5b, which deforms the elongated ribbon element 23b
and produces a partially conformal GEMS with period A. FIG. 8b shows the
device 5b (as shown and described in FIGS. 6 and 7) in the fully actuated
state with the elongated ribbon element 23b in contact with standoffs 29.
The height difference between the bottom of element 23b and the top of the
standoffs 29 is chosen to be approximately 1/4 of the wavelength .lambda.
of the incident light. The optimum height depends on the specific
conformal shape of the actuated device. In the actuated state, the
incident light beam 30 is primarily diffracted into the +1st order light
beam 35a and -1st order light beam 35b, with additional light diffracted
into the +2nd order 36a and -2nd order 36b. A small amount of light is
diffracted into even higher orders and some light remains in the 0th
order. In general, one or more of the various beams can be collected and
used by an optical system, depending on the application. When the applied
voltage is removed, the forces due to tensile stress and bending restores
the ribbon element 23b to its original unactuated state, as shown in FIG.
8a.
FIGS. 9a and 9b show a side view through line 9--9 of FIG. 7 of the
conformal GEMS device 5b in the unactuated and actuated states,
respectively. The conductive reflective ribbon element 23b is suspended by
the end support 24b and the adjacent intermediate support 27 (not shown in
this perspective). The application of a voltage actuates the device as
illustrated in FIG. 9b.
FIGS. 10 and 11 illustrate the application of the present invention to
remove the pull-down and release instabilities in a conformal GEMS device.
With these instabilities eliminated, it is possible to control the
diffracted or reflected light intensity in a continuous manner by varying
the voltage applied to the device. The channels 25 are filled with a
transparent liquid that can withstand high electric fields. In addition to
providing continuously variable control of light intensity, the liquid
increases the effective optical depth of the actuated conformal GEMS
device and, therefore, reduces the required travel range of the ribbon
elements. For example, the travel range needed to minimize the 0th order
light beam 32 or to maximize diffraction into the non-zero diffracted
orders (+1.sup.st order 35a, -1st order 35b, +2nd order 36a, -2nd order
36b, or even higher orders) is reduced by the refractive index n of the
liquid. Specifically, for maximum diffraction into the +1.sup.st order 35a
and -1st order 35b, the required travel range is reduced to approximately
.lambda./4n in liquid from approximately .lambda./4 in a gas atmosphere.
In practice, since the refractive index of many liquids is near 1.4, the
required travel range is reduced by approximately 30%.
Referring to FIG. 10, the theoretical response of a conformal GEMS device
to applied voltage is shown, comparing a gas atmosphere to immersion in
liquids. The curves in FIG. 10, which represent the normalized intensity
of the 0.sup.th order reflected light beam, illustrate the elimination of
the pull-down instability. With a gas atmosphere, .epsilon.=1.0, pull-down
occurs near 24V. At the pull-down voltage, the normalized reflected
intensity drops significantly and dramatically from about 0.85 to about
0.22. Further increases in voltage only produce small differences in
intensity because the ribbon elements are in contact with the underlying
standoffs with voltages above 24V. For liquids with relatively large
dielectric constants, a smooth, more controlled reduction in reflected
intensity as a function of applied voltage is expected. Indeed, the curves
for liquids with .epsilon.=18.0 and .epsilon.=37.0 show that it is
theoretically possible to obtain continously variably control of the
reflected light intensity. The larger dielectric constant, .epsilon.=37.0,
reduces the sensitivity of intensity to voltage and, therefore, improves
controllability. However, the larger dielectric constant also increases
the required voltage.
Referring to FIG. 11, the actual response of a conformal GEMS device
immersed in known substances with different dielectric constants is shown
to closely follow the theoretical expections disclosed in FIG. 10 above.
As expected, a device in air, which has a dielectric constant of 1.0,
exhibits a sharp pull-down instability of the ribbon elements at
approximately 24V. In contrast, Isopropyl Alcohol, having a dielectric
constant of 18, provides a significantly increased control of the ribbon
elements as voltage is applied. Ethylene Glycol, having a dielectric
constant of 37, further reduces the sensitivity to applied voltage,
thereby providing more precise control. The response of a conformal GEMS
device immersed in several other liquids (not shown in FIG. 11) was also
characterized, for example, in Isopar.RTM. manufactured by Exxon Mobil and
in methanol. Isopar.RTM., with a dielectric contant of 2, increased the
travel range, but did not completely eliminate the pull-down instability.
Methanol, however, caused problems because of the occurrence of
electrochemical reactions.
A robust implementation of the invention requires careful selection of both
the applied voltage waveform and the liquid. As disclosed by Kowarz et al.
in U.S. Pat. No. 6,144,481, entitled Method and System for Actuating
Electro-Mechanical Ribbon Elements in Accordance to a Data Stream, issued
Nov. 7, 2000, a bipolar voltage waveform reduces charging in
electromechanical ribbons. Furthermore, as described by Gooray et al. in
U.S. Pat. No 6,419,335, Electronic Drive Systems and Methods, issued Jul.
16, 2002, a high-frequency bipolar waveform used in conjunction with a
liquid-filled electrostatic MEMS device reduces electrochemical reactions
and dielectric breakdown of the liquid. To obtain the results shown in
FIG. 11, a 4 MHz bipolar voltage waveform with a constant RMS
(root-mean-squared) value was used to actuate the conformal GEMS device.
Because of the viscous damping introduced by the liquid, the ribbon
elements do not respond to the high frequency and only respond to the RMS
value. Therefore, the high frequency component does not show up in the
optical response.
The invention has been described in detail with particular reference to
certain preferred embodiments thereof, but it will be understood that
variations and modifications can be effected within the spirit and scope
of the invention.
Parts List
2a tensile ribbon element
2b tensile ribbon element
2c tensile ribbon element
4 channel
5a conformal GEMS device
5b conformal GEMS device
5c conformal GEMS device
5d conformal GEMS device
6 first electrode
7 dielectric ribbon material
9 conducting substrate
10 substrate
12 bottom conductive layer
14 dielectric protective layer
16 standoff layer
18 spacer layer
20 ribbon layer
22 reflective and conductive layer
23a elongated ribbon element
23b elongated ribbon element
23c elongated ribbon element
23d elongated ribbon element
24a end support
24b end support
25 channel
27 intermediate support
28 gap
29 standoff
30 incident light beam
32 0.sup.th order light beam
Parts List--Continued
35a +1.sup.st order light beam
35b -1.sup.st order light beam
36a +2.sup.nd order light beam
36b -2.sup.nd order light beam
*
