Background of the Invention
1. Field of the Invention
The invention relates to the filed of micromachining electromagnetic
devices, and in particular. a micromachined magnetic actuator and a method for releasing
the magnetic actuator from its parent wafer.
2. Description of the Prior Art
It is generally known that magnetic actuation provides stronger forces
over a longer distance as compared to electrostatic driving mechanisms. See
W. Gu et al., AIAA Journal. Vol. 31, No. 7, pp 1177-86 (1993); and
K. Rinoie, Aeronautical Journal, Vol. 97 (961), pp 33-38 (1993). Electromagnetic
driving may be used as the motive force in many different configurations, such as
shown by I.J. Busch-Vishniac, Sensors and Actuators, A33 at 207-20 (1992);
and C.H. Ahn et al., IEEE J. Microelectromechanical Systems, Vol. 2 (1) at
15-22 (1993). even in combination with the electrostatic forces. H. Guckel et
al., 1993 IEEE Workshop on Microelectromechanical Systems at 7-11 (1993). The
introduction of electrochemical deposition of Permalloy (e.g. 50/50 FeNi) has dramatically
increased the power of electromagnetic driving mechanisms and efficiency of magnetic
actuators as described in B. Wagner et al., Sensors and Actuators. A(32)
at 598-03 (1982); C. Liu et al., 1994 IEEE Workshop on Microelectromechanical
Systems at 57-62 (1994); and S.W. Yuan, Foundations of Fluid Mechanics, Prentice
What is needed is a design for a micromachined micromagnetic actuator
which can be made by surface micromachining and which are adapted to be reproduced
in large scale arrays.
As will be described below, the illustrated embodiment of the invention
is discussed generally, then specifically, in any array applied to a delta wing.
The delta wing is one of the fundamental configurations for generating lift forces
and its aerodynamic control is a design feature of great importance. When airflow
hits the two leading edges of the wing at a certain angle of attack, two counter-rotating
leading edge vortices are separated from the laminar flow and propagate over the
wing's top surface. The two high momentum, low pressure vortices contribute identical
vortex lifting forces on the two sides of the wing, the sum of these being about
40 percent of the total lifting forces. The strength and position of these two vortices
are determined by the boundary layer conditions near their separation points. The
boundary layer is roughly 1 to 2 millimeters thick at wind tunnel flow speeds of
less than 20 meters per second. The thickness will decrease when the flow speed
What is needed then is some means of controlling these vortices in
order to provide a control function for delta wing.
Brief Summary of the Invention
The invention is a microelectromechanical magnetic actuator according
to claim 1. The actuator comprises a substrate having a surface and a micromachined
flap defined from the surface of the substrate and separated therefrom. At least
one micromachined beam defined from the surface of substrate couples the flap to
the substrate. A magnetic layer is disposed on the flap. A selectively actuatable
magnetic field source is disposed proximate to the actuator to create a magnetic
field in the vicinity of the flap to bend the flap on the beam outwardly from the
plane of the surface of the substrate. As a result, an out of plane magnetically
actuated flap is provided.
In one embodiment the magnetic source is an electromagnet. The magnetic
layer is comprised of a magnetic coil disposed on the flap.
The actuator further comprises a current source. The magnetic coil
is coupled to the source of current by conductive lines extending from the magnetic
coil to the current source. The lines are disposed along the beam. Preferably at
least two beams are provided to couple the flap to the substrate.
In another embodiment the magnetic layer is comprised of a permanent
magnet and the magnetic source is a permanent magnet. More specifically, the permanent
magnet is a layer of Permalloy.
The flap has a plurality of holes defined therein to facilitate separation
of the flap from the underlying substrate.
The invention is also directed to a method of fabricating a microelectromagnetic
magnetic actuator according to claim 21. The method comprises providing a substantially
completed microelectromechanical magnetic actuator on a sacrificial layer disposed
on an underlying substrate. The sacrificial layer upon which the microelectromechanical
magnetic actuator has been fabricated is removed by etching away the sacrificial
layer through at least one opening defined through the microelectromechanical magnetic
actuator to expose the underlying sacrificial layer. The etched device is dried
while simultaneously actuating the microelectromechanical magnetic actuator to maintain
the released portions of the actuator out of contact with the underlying substrate
until the drying is complete.
The invention can alternatively be regarded as an improvement in a
method of fabricating a surface micromachine cantilevered layer disposed over an
underlying substrate. The improvement comprises providing the cantilevered layer
on a sacrificial layer which in turn is disposed upon the substrate. A magnetic
layer is disposed on the cantilevered layer. The sacrificial layer beneath at least
part of the cantilevered layer is removed to release the cantilevered layer from
the substrate. The cantilevered layer is simultaneously maintained apart from the
substrate layer by exposing the magnetic layer to a magnetic field, which tends
to lift the cantilevered layer away from the substrate. The cantilevered is maintained
separate from the substrate until removal of the sacrificial layer is completed
and possibility of adhesion of the cantilevered layer to the substrate substantially
Removing the sacrificial layer from underneath at least part of the
cantilevered layer comprises wet etching the sacrificial layer away and where simultaneously
maintaining the cantilevered layer out of contact with the substrate is performed
until the microelectromagnetic device is dried.
In another embodiment the improvement further comprising disposing
an organic polymer on at least the cantilevered layer prior to disposition of the
magnetic layer thereon and selectively removing the organic polymer layer and the
magnetic layer disposed thereon after removal of the sacrificial layer is complete.
The improvement further comprises disposing an organic polymer on
at least the cantilevered layer prior to disposition of the magnetic layer thereon
and selectively removing the organic polymer layer and the magnetic layer disposed
thereon after removal of the sacrificial layer is complete.
The invention is again regarded as an improvement in a method of controlling
turbulent flow across the surface of an object. The improvement comprises disposing
a plurality of microelectromechanical actuators each having selectively operable
flap disposable out of the plane of the microactuator into a boundary layer above
the surface over which the turbulent flow is established. At least some of the plurality
of microelectromechanical actuators are selectively actuated to dispose their corresponding
flaps into the boundary layer to thereby effect turbulent flow.
While the illustrated embodiment describes movement of flaps 14 in
a direction out of the plane of the wafer or substrate, it must be expressly understood
that deflection in all directions is included within the scope of the invention.
The beam structure could be modified consistent with the teachings of the invention
to include lateral deflection of a single or multiple beams in the plane of the
wafer or substrate or any combination thereof.
Selectively disposing the corresponding flaps into the boundary layer
comprises electromagnetically actuating the flaps to bend the flaps into the boundary
layer out to the plane of the actuator.
In one embodiment the surface is an airplane control surface and selectively
disposing the corresponding flaps serves to provide a control force to the airplane
Alternatively, selectively disposing the corresponding flaps into
the boundary layer serves to reduce fluid drag of the surface for any purpose.
The invention may be better visualized by now turning to the following
drawings wherein like elements are referenced by like numerals.
Brief Description of the Drawings
- Figure 1 is a perspective view of a first embodiment of an out-of-plane magnetic
actuator devised according to the invention.
- Figure 2 is a cross-sectional view of the embodiment of Figure 1 taken through
section lines 2-2.
- Figures 3a-f are simplified cross-sectional views in enlarged scale showing
the method of fabrication of the actuator of Figures 1 and 2.
- Figure 4 is a top plan view of a second embodiment of an out-of-plane magnetic
actuator devised according to the invention.
- Figure 5 is a cross-sectional view of the embodiment of Figure 4 taken through
section lines 5-5.
- Figure 6a and b is a simplified side elevational diagram illustrating the operation
of the actuator of the invention.
- Figures 7a-d are simplified cross-sectional views in enlarged scale showing
the method of fabrication of the actuator of Figures 4 and 5.
- Figures 8a-h are cross-sectional views of a wafer illustrating the fabrication
of a multiple coil actuator.
- Figure 8i is a top plan view of the device fabricated according to the method
illustrated in Figures 8a-h.
- Figure 9 is a simplified top plan view of a delta wing illustrating application
of the actuators of the invention therein.
- Figure 10 is a simplified cross-sectional view in enlarged scale of a portion
of the delta wing of Figure 9.
- Figure 11a-e are cross-sectional view of a wafer illustrating a bulk machining
method of fabricating an actuatable shutter.
- Figure 12 is top plan view of the shutter fabricated according to the method
of Figures 11a-e.
The invention as set forth in its illustrated embodiments and other
embodiments disclosed in the specification can now be better understood by turning
to the following detailed description.
Detailed Description of the Preferred Embodiments
A surface micromachined micromagnetic actuator is provided with a
flap capable of achieving large deflections above 100 microns using magnetic force
as the actuating force. The flap is coupled by one or more beams to a substrate
and is cantilevered over the substrate. A Permalloy layer or a magnetic coil is
disposed on the flap such that when the flap is placed in a magnetic field, it can
be caused to selectively interact and rotate out of the plane of the magnetic actuator.
The cantilevered flap is released from the underlying substrate by
etching out an underlying sacrificial layer disposed between the flap and the substrate.
The etched out and now cantilevered flap is magnetically actuated to maintain it
out of contact with the substrate while the just etched device is dried in order
to obtain high release yields.
Figure 1 is a perspective view of a micromachined magnetic actuator
10, and Figure 2 is a cross section view taken through section lines of 2-2 of Figure
1. Magnetic actuator 10 is comprised of a spiral magnetic coil 12 disposed on the
top of a flap 14 hinged by two cantilever beams 16 extending from side 18 of flap
14 to an opposing side 20 of a surrounding substrate 22.
Actuator 10 of Figure 1 is a surface-micromachined magnetic actuator
and, as will be described below, is used as an integral part of a microelectromechanical
system to control turbulence for drag reduction. In this application in order to
be effective, flaps 14 are required to achieve a vertical deflection of at least
+/- 100 microns at the end-point of their travel with a bandwidth of over 10 kHz.
Forces of the order of 1-10 micro-Newtons are needed for flaps 14 to effectively
The size of flap 14 may vary typically from 250 microns to 900 microns
on a side with beam 16 having a length varying from 100 to 360 microns and a width
from 14 to 50 microns. Etch holes 24. which typically have a size of 15 x 15 micron2
are strategically disposed through flap 14 to allow faster etching of a polysilicon
glass sacrificial layer 28 as shown in Figure 2. to ensure that all structures in
a wafer are released in roughly the same amount of time as will become more apparent
when the method of fabrication discussed in connection with Figures 3a-f is considered.
In addition to the suspension of laminated thin film flap 14 by a
pair of linear parallel beams 16. it is within the scope of the invention that flap
14 could also be supported by a plurality of serpentine beams defined at each of
It is to be understood that according to the invention either the
polarity of the magnetic field to which the actuator is exposed or the polarity
of the coils on flap 14 itself may be switched or inverted. Thus in an array of
actuators the polarity and phase of current flowing in the coils can be changed
or varied to obtain a distributed motion in the array of the flaps.
Furthermore, the effective internal magnetization of Permalloy is
about 2.2 Oe (1 Oe = 7,96.101 Am-1) so that it is a soft magnet.
Its magnetic polarity can thus be readily changed by an impressed field. If the
polarization of magnetization of the permalloy layer is such that the flap rotates
down into the silicn substrate and is stopped by it instead of being rotated out
of the plane. then an unstable high energy configuration is assumed by the system.
The polarity of the Permalloy layer spontaneously changes or inverts so that the
flap will now rotate out of the plane of the substrate. The inclusion of mechanical
stops to limit the downward motion of the flap insures that this spontaneous magnetization
There are four factors to consider in design of a magnetic actuator
as described: the factors are magnetic, mechanical, thermal, electrical and fluid
dynamic. The magnetic force that the flap experiences in a nonuniform magnetic field
is given by the following equation (1) where B is the magnetic flux density vector,
N the number of turns of the coil, I the current that passes through the coil, Rav
the average radius of the coil, and N the normal vector to the current loop.
F = NI π (Rav)2 (n·▿)Z
Mechanically, the choice of spring constant of flap 14 must be a compromise.
In order to achieve a large displacement, flap 14 should have a small spring constant.
However, having a large bandwidth requires a large spring constant. The force constant
of flap 14 is obtained by using an approximate composite layer model as described
by W.C. Young, Roark's Formulas for Stress and Strain, 6th Edition, McGraw
Hill (1989), or by using finite element simulation. In the presently preferred embodiment,
all flaps 14 are designed to have force constants in the range of 0.001-0.010 Newtons
The intrinsic stresses of the different layers contribute to a bending
moment acting on flap 14, which results in flap 14 having a curved rather than a
flat configuration at rest. The magnitude of curvature of a thin bi-material plate
is calculated by the following equation (2)
Fh2 = E1I1 + E2I2ρ
where F is the lateral force due to the intrinsic stress, H is
the total height of the bi-material layer, E1 and I1 are Young's
modulus and moment of inertia for the top layer with E2 and I2
being Young's modulus and moment of inertia for the bottom layer, with ρ being
the curvature of the plate.
Thermally, as the temperature of flap 14 rises, the thermal mismatch
of different materials in the composite layers will cause the flap to bend down,
which is generally undesirable in the case of use on an aerodynamic surface. Given
the geometry and material composition. this bending can be calculated as described
S. Timoshenko, "Analysis of Bi-Metal Thermostats," Journal O.S.A.
and R.S.I. 11 at 233-55 (Sept. 1925). Geometry and material composition are thus
chosen to minimize thermal bending.
Low electrical resistance is desirable to minimize heat generation
and, thus, thermal bending of flap 14. The total resistance is comprised of the
metal coil resistance, the contact resistance between the metal and doped polysilicon.
and the resistance of the polysilicon flap 14. The resistance of polysilicon flap
14 contributes 60 to 70 percent of the total resistance which ranges from 30 to
70 ohms and hence the majority of the heating.
The structure and design parameters of magnetic actuator 10 now having
been generally described, consider the method of its manufacture as set forth in
Figures 3a-3f. A 2.5 micron thick phosphosilicate glass layer 28 is provided as
a sacrificial layer with a measured 6 percent phosphorous content. Layer 28 is first
deposited on the wafer surface using low pressure chemical vapor deposition and
is followed by a 6,000 angstrom (1 Angstrom = 1.10-10 m) thick low pressure
chemical vapor deposition of polysilicon layer 30 as shown in Figure 3b disposed
at 620 degrees Centigrade. In order to dope polysilicon layer 30, the wafer is coated
with a 5,000 angstrom layer 32 of phosphosilicate glass as shown in Figure 3b and
then annealed at 950 degrees C. for 1 hour to release its intrinsic stress. During
the annealing, polysilicon layer 30 is doped by phosphorous diffusion and the resulting
sheet resistivity is of the order of 50.5 ohms per square centimeter. Top glass
layer 32 is subsequently removed and polysilicon layer 30 is patterned by photolithography
as shown at Figure 3c.
Thereafter, a 3,000 angstrom low pressure chemical vapor deposition,
low stress, silicon nitride layer 34 is deposited at 820 degrees C. to cover and
insulate polysilicon flap 14. The nitride is then patterned to define the contact
holes as shown in Figure 3d.
A 4,000 angstrom aluminum layer 36 is then disposed by vapor deposition
and patterned to define coils 12 as shown in Figure 3e. Buffered hydrofluoric acid
and oxide pad etchant. Type 777, from Olin Hunt Specialty Products Inc., are then
employed to etch away the underlying sacrificial layer 28 to obtain flap release
as depicted in Figure 3f.
To completely underetch at 200 x 200 square micro flap 14, pad etchant
required approximately 3 hours, and buffered hydrofluoric acid approximately 30
minutes. The slow etch rate, together with the low etching selectivity over aluminum
metalization, may compromise yield. Yields may be improved by using a chromium/gold
metalization in place of aluminum layer 36. An adhesion layer of 100 angstrom chromium
beneath a 4,000 angstrom gold layer can be used in combination with a 49 percent
hydrofluoric acid etchant to completely undercut the plate structure without damaging
metalization and to increase yield. The etching process in this case takes about
2 minutes to complete the amount of etching on flap 14 of the polysilicon nitride
layers is minimal.
A subsequent drying process is also essential for obtaining a high
yield. Different drying techniques are known, such as described by G.T. Mulhern
et al., "Supercritical Carbon Dioxide Drying of Microstructures," Technical
Digest of Transducers '93 at 296-98 (1993); C. Mastrangelo et al.,
"A Dry Release Method Based on Polymer Columns for Microstructure Fabrication."
IEEE Microelectromechanical Systems Workshop, Fort Lauderdale, Florida, at 77-81
(1992); R.L. Alley et al., "The Effect of Release-Etch Processing of Surface
Microstructure Stiction," IEEE Solid State Sensor and Actuator Workshop, Hiltonhead
Island, South Carolina, at 202-07 (1992); and T. Hirano et al.,
"Dry Releasing of Electroplated Rotational and Overhanging Structures,"
IEEE Microelectromechanical Systems Workshop, Fort Lauderdale, Florida, at 278-83
In the preferred embodiment, the process includes rinsing the etched
wafer in deionized water for 20 minutes and in acetone followed by an alcohol rinse
of 1 minute each. The alcohol is removed by 10 minutes of deionized water rinse
and the wafer is baked dry by an infrared lamp. The flap stiction to the substrate
is almost negligible and a yield near 100 percent is obtained. Drying with the use
of an infrared lamp can be used in conjunction with a convection oven at 120 degrees
In order to prevent flap 14 from sagging down to the substrate 22
and thus forming a permanent bond during fabrication, silicon nitride tethers may
be used to hold flap 14 in place during the sacrificial etching process of Figure
3f. In the preferred process, the tethers are typically 100 microns long, 6 microns
wide and 30 angstroms thick, being of the same dimensions as low stress nitride
layer 34. The tethers are broken manually by manipulator probes once the plates
Since manual tether breaking in large scale arrays is inefficient,
photoresist tethers of the same or similar dimensions may be substituted and then
removed by oxygen plasma ashing after the plates are freed and dried. Photoresist
tethers, however, are not able to withstand the 49 percent hydrofluoric etching
process and most of them may peel off within 2 minutes of exposure to the etch.
It has been observed in the fabrication process that intrinsic flap
bending caused by intrinsic stresses are substantially independent of the metalization.
Therefore, intrinsic bending can be modeled assuming a nitride polysilicon laminate
for flap 14. Further, the amount of plate bending is generally much larger than
beam bending and in the illustrated embodiment. was found to be of the order about
700 microns. When aluminum metalization is used, subsequent annealing to reduce
aluminum to polysilicon contact resistance can significantly increase aluminum stress
thereby increasing the bending by approximately 25 percent.
Thermal motion of flap 14 can be approximately modeled by considering
actuator 10 as being a bi-layer thermostat composed of a gold layer and a composite
nitride/polysilicon layer. Horizontal and vertical deflections of more than 100
microns are typically observed indicating a temperature of 300 degrees C. The frequency
response of thermal actuation of the device is for a flap 300 microns2
with cantilevered beams 200 microns long and 18 microns wide. has a bandwidth of
about 1 kHz with a first mode resonant frequency at about 1 kHz. Smaller resonant
peaks are observed at 180 Hz and 360 Hz. The motion of flap 14 will be the result
of both thermal effects as well as magnetic effects. The two can be separated by
first passing a DC current through coil 12 and observing the thermal motion of the
flap until it comes to rest. Thereafter, current is applied to electromagnetic coil
12 and the motion followed.
The external electromagnetic field has been created both by permanent
and electromagnets typically disposed underneath flap 14 which is then biased with
DC currents ranging from 0 to 50 milliamps. Field strengths for the electromagnet
generated field are variable with a peak value of 1.76 kGauss (1 Gauss = 1·10-4
T) at 2.5 amps current input with a permanent magnet providing a constant magnetic
flux density of approximately 2.8 kGauss as measured at the permanent magnet's surface.
The gradient of the magnetic field, B, near flap 14 is about 280 Gauss per centimeter.
Under a 1.4 kGAuss magnetic flux density, and a 40 ma coil current (70 milliwatts)
flowing through three turns of coil 12, a flap having a size of 420 microns2
and suspended on size by two beams 16 280 microns long and 20 microns wide make
a +/- 100 micron vertical deflection.
A typical flap 14 described above will survive a 50 meter per second
air flow or greater when the coil side of flap 14 faces the wind. Flap 14 will fold
over by 180 degrees and break in about 20 meters per second air flow in the opposite
direction. Improvement in intrinsic bending is expected to be realized by designing
laminate layers that have zero combined stress.
The micro actuator described utilized an electromagnetic coil 12,
however, coil 12 may be replaced by electroplated permanent magnets in order to
avoid thermal induced bending. Figure 4 shows in top plan view with Figure 5 being
a cross sectional longitudinal view seen through section lines 5-5 of Figure 4.
As before, magnetic actuator 10 of Figure 4 is comprised of a suspended polysilicon
plate 40 with an electroplated Permalloy layer 42 disposed thereover. An external
magnetic field produced by conventional means is provided perpendicular to the surface
of substrate 22 and deflects the flap 14 out of the plane of substrate 22.
The physics of the magnetic actuation are illustrated in connection
with Figures 6a-b. Figure 6a shows actuator 10 in a rest position in ambient magnetic
fields with electromagnet 44 off. Figure 6b is a simplified side elevational view
of the actuator of Figure 6a with electromagnet 44 energized to produce magnetic
forces F1 and F2 on upper edge 46 and lower edge 48 of flap 14. Assuming that two
magnetic poles of opposite polarities are fixed at the two ends of Permalloy plate
42, forces will be developed in the direction shown by the arrows for F1 and F2
in Figure 6b which will deflect flap 14 out of the plane of substrate 22. Flap 14
can be regarded as essentially rigid so that the entire bending is taken up by beams
16. The result is a counterclockwise torque arising from F1 and a downward force
arising from F2 - F1. The counterclockwise torque is dominate with the result that
beams 16 are deflected out of plane. The downward deflection on beam 16 caused by
the net downward force will be approximately 8 to 10 times smaller than that caused
by the out-of-plane torque.
The maximum strain of beam 16 can be computed and a maximum angle
of bend determined at which fracture will take place. It can be computationally
predicted that flap 14 can bend by as much as 118 degrees before fracture occurs
in the case of silicon beams. This implies that flaps 14 will never reach their
fracture point in a uniform magnetic field with field lines perpendicular to the
surface of substrate 22. However, it should be noted that once flaps 14 are in a
flow field, flow induced bending and vibration can theoretically be larger than
a fracture angle.
Figure 7a-7d illustrates the major fabrication steps in the actuator
of Figures 4 and 5. Conventional surface micromachining procedures are followed
to fabricate the polysilicon plates/beam structures on top of a 3 micron thick phosphosilicate
glass sacrificial layer 46 disposed on substrate 22 as shown in Figure 7a. Polysilicon
layers 48 are selectively disposed through conventional photolithographic techniques.
Polysilicon layer 48 is then covered with a thin 0.5 micron thick phosphosilicate
glass layer 50 as shown in Figure 7a which serves as a complementary phosphorous
doping source. During the one hour, 1,050 degrees Centigrade stress relief annealing,
polysilicon layer 48 is doped from both sides to avoid intrinsic bending due to
unbalanced doping concentrations. Top phosphosilicate glass layer 50 is later removed
by a buffered hydrofluoric acid etch.
A 200 angstrom chromium 1800 angstrom cooper thin film is then vapor
deposited over polysilicon layer 48 as a conductive seed layer 52 as shown in Figure
7b. A 5 micron thick photoresist 54 is selectively applied and patterned to form
molding frames inside which Permalloy (Ni80Fe20) are electroplated.
A frame plating technique which is used is known in the thin film magnetic industry
and which creates high quality Permalloy films.
During the plating process, the wafer is affixed to the cathode and
is oriented in such a way that the external magnetic field is parallel to the supporting
beam 16. Electroplating rate is approximately 5 microns per hour under a bias current
density of 8 milliamps per square centimeter. The resulting Permalloy has a saturated
magnetization of 1.35 Tesla, relative permeability of 4500. a small remnant magnetization
between I and 10 Gauss and a coercive force of 4 Oe.
After electroplating, the wafer is flooded with ultraviolet light
and frame photoresist 54 is removed leaving the patterned plated Permalloy 56 as
shown in Figure 7c. Seed layer 52 is etched away using a cooper etchant and standard
chromium mask etchant. Flaps 14 are then released by etch in 50 percent hydrofluoric
acid for 20 minutes as shown in Figure 7d. To facilitate a sacrificial release process,
etch holes 24 approximately 30 microns by 30 microns in size and 250 microns apart
are open through flap 14.
Since microflaps 14 have large surface areas and supporting beams
16 are soft with a spring constant of about 100 microNewtons per millimeter, they
can be easily pulled down by surface tension to the substrate and form permanent
bonds if conventional drying techniques are used after the last etch. Under room
temperature and pressure the drying technique of the invention can provide yields
of 100 percent.
Prior attempts to solve the release problem have focused on eliminating
the liquid-vapor phase transformation which induces deflection in flap 14. For example,
liquid freezing/sublimation techniques have been applied at different temperatures
by Gucket et al., "The Application of Fine-Grain, Tensile Polysilicon
to Mechanically Resonant Transducers," Sensors and Actuators, Vol. 821, at 346-51
(1990), and Takeshimo et al., "Electrostatic Parallelogram Actuators.
" Transducers '91 at 63-6 (1991). Another solution based on a transformation from
a supercritical liquid to air has been used as shown by G. T. Mulhern et al,
"Supercritical Carbon Dioxide Drying Microstructures," proceedings, Transducer
'93 at 296 (1993). Mulhern's method involves counteracting surface tension
induced deflection during the drying process by using polymeric anchors, which can
be subsequently removed by plasma etching. A third solution is based on a surface
treatment which stops the formation of permanent bonds between flap 14 and its underlying
substrate in layers after they have been brought together. See R.L. Alley et
al., "The Effect of Release-Etch Processing on Surface Microstructure Stiction,"
IEEE Solid State Sensor and Actuator Workshop, Hiltonhead Island. South Carolina,
at 202-07 (1992).
According to the drying process of the invention, microstructures
of flap 14 are prevented from being pulled down to the substrate by levitating them
out of the plane during drying. This levitation is provided by attaching a thin
film permanent magnet at the end of the microstructures, which is inherent in a
magnetic actuator in any case, then applying an external magnetic field to mildly
deflect the structure upward.
In the cases where the Permalloy layer 50 is not desired as a part
of the final product, it can be deposited over a polymeric layer and later removed
by undercutting and stripping the underlying polymer by dry etching. In the case
where a polymer layer is used, release can be obtained using a dry plasma etch instead
of a wet etch. The dry plasma etch may be combined with the magnetic release described
above if needed.
There are other magnetic compounds which can be used in place of Permalloy.
These compounds can be deposited selectively in the structures and later dry etched
with fluorine- or chlorine-based gasses. The exact etching parameters, the selection
of gas, the pressure and power can be calibrated so that the organic compound is
etched at a much faster rate than other exposed materials in the device.
Figures 8a-i illustrate the method of fabricating another embodiment
of the invention in which multiple coils are employed on flap 14 and on adjacent
substrate 22. As before a front side polished silicon substrate 22 is provided as
shown in Figure 8a. A polysilicon glass layer 28 is deposited at Figure. 8b. A nitride
layer 34 is deposited at the step of Figure 8c and selectively patterned using conventional
photolithography techniques at as shown at Figure 8d. A first metalization 36a,
which is preferably a laminate of 10 nm of Cr and 400 nm of Au, for defining at
least a first coil is deposited on nitride layer 34 and selectively patterned as
shown at Figure 8e. A photoresist layer 70 is spun onto the surface and patterned
to form contact holes 72 over metalization 36a. A second metalization 36b is then
deposited of the same type as first metalization 36a, but at a slower rate to avoid
burning photoresist 70. Second metalization 36b is patterned as shown in Figure
8g and photoresist 70 removed, and sacrificial layer 28 selectively removed to create
flap 14 as described above and shown in Figure 8h to produce a double layer coil
actuator 10 as shown in plan top view in Figure 8i.
It is entirely within the scope of the invention that more than two
coils or metalizations 36 could be similarly devised on flap 14. Further, one or
more coils 74 may be similarly fabricated and placed on the adjacent regions of
the wafer as shown in Figure 8i, although not described in connection with the fabrication
of the flap coils in Figures 8ah. Coils 74 would then be used to generate the magnetic
field in which flap 14 would be operable. Second metalization 36b may be formed
as an air bridge over metalization 36a. Air bridges for the metalizations may be
used both for the coils on flap 14 and coils 74. For example, some of the air bridges
formed by contact the coil of the first metalization only at its center and one
outside coil as suggested in the cross-sectional view of Figure 8h, or may use anchors
at midway points to prevent sagging of the bridge, which would short out one or
more turns of the coil. The air bridges are substantially more flexible than beams
16 and hence would not materially affect the deflection of flap 14.
The invention now having been described in terms of the structure
of the actuator and the method of its manufacture, consider some of the applications
to which it may be put. In the delta wing application of the illustrated embodiment,
an array of microflaps as described above, are mounted on the wing surface to deflect
1 to 2 millimeters out of the plane of the wing or at least through a substantial
thickness of the boundary layer over the wing. The system may include an array of
shear stress sensors disposed on the wing for sensing the turbulence of the vortices
propagating across the wing. An on-chip neural network processes the sensor signals
according to a built-in feedback algorithm. The output of the signals drive the
micromagnetic flap array to reduce the vortices.
In the illustrated embodiment, the delta wing is assumed to have a
top angle of 67 degrees as shown in Figure 9. Two grooves 58 parallel to leading
edges 60 of the wing are opened on the bottom side with a delta wing as better shown
in Figure 10. Each groove may for example be 250 millimeters by 4 millimeters in
size and 5 millimeters away from the leading edge of the wing. A plurality of electromagnets
64 are disposed in groove 58 in linear arrays of magnetic flap actuators of the
type described above in connection with Figures 4-6. Actuators 10 are mounted on
top of electromagnets 60 flush with the delta wing surface. A current is supplied
through the electromagnets to generate a perpendicular magnetic field through the
linear array of actuators 10 with a field strength of approximately 2.1 times 104
amperes per meter on the actuator plane. Actuation of the electromagnets create
a rolling moment on the wing. A highly repeatable rolling moment to vortex lift
moment is created at various air flow speeds across the wing. The maximum ratio
of roll moment to vortex lift moment was about 1.2 percent at a flow speed of 16
meters per second.
As a microactuator flaps 10 in groove 58 are positioned closer to
leading edge 60. the local flow speed is higher and the boundary layer thinner resulting
in a more significant rolling moment. Roll moments to vortex lift moment ratio as
high as 10 percent can be achieved at 16 meter per second air flow speeds when the
passive flap is right on leading edge 60. In a delta wing F15 fighter, this would
be sufficient to turn the fighter 360 degrees in about 1 second. It is expected
that the fluid loading of flaps 14 will be increased by increasing both the flexibility
of flaps 14 and their robustness and further increasing the Permalloy/magnetic field
Therefore, it can now be appreciated that by using arrays of micromachines
the size of pinpoints which may be computer controlled, the fluid dynamic control
of airplanes, ships and vehicles may be more intelligent controlled to reduce turbulent
drag in such vehicles and other devices. The application is not limited to the control
of fluid dynamic surface flows on planes, ships, and vehicles, but can be used anywhere
where fluid flows over a surface, including biomedical applications such as in vascular
system, pipes, hoses, conduits and the like.
The advantages to be realized by the reduction of turbulent drag are
believed to be significant. For example, reduction of turbulent drag in an airplane
by only 1 percent, may reduce operating costs by 20 percent or more. Furthermore,
control of a large aircraft using out-of-plane microactuated flaps is expected to
eliminate or reduce the need for rudders, aerolons, elevators, flaps, spoilers,
and similar aerodynamic devices used by conventional aircraft for control, which
devices contribute substantially to the radar cross section of the aircraft.
Further, the microactuators of the invention could be installed within
the blades of jet turbines to improve flow of air and fuel to allow higher engine
operating temperatures that increase efficiency. Other applications of the these
devices could be used to suppress jet engine exhaust noise.
More particularly, arrays of actuators 10 can be disposed on a surface
at a position just upstream in a fluid flow over the surface from where flow separation
would normally occur. By oscillating actuators 10 at a selected frequency in response
to the flow dynamics, flow separation can be avoided in circumstances where it may
otherwise occur leading to many consequent advantages including dramatically increased
heat exchange between the fluid and surface.
The applications of the invention are not limited to aerodynamics
or fluid dynamics, but can be used generally in optical and microwave fields. Figures
11a-e illustrate a bulk micromachining application wherein a shutter, mirror or
antenna is fabricated. A <100> silicon substrate 22 with a 35 micron epitaxial
layer 78 of silicon and a 4 micron boron etch stop layer 80 is coated on its front
and back sides with photoresist 82 and patterned for the definition of double alignment
marks as shown in Figure 11a. The wafer is then bulk machined by plasma etching
to create membrane 85 and the alignment marks transferred to the silicon as shown
in Figure 11b.
Photoresist 82 is removed and the wafer is oxidized with a 5000 Angstrom
thick silicon dioxide layer 84, a 200 Angstrom Ti and a 1000 Angstrom Cu seedlayer
86 is disposed on oxide layer 84 by vapor deposition. A layer 88 of photoresist
is spun on and patterned to form a mold for the Permalloy layer 90 to be later deposited
as shown in Figure 11b. The wafer is then electroplated with 5-7 microns of Permalloy
and photoresist 88 removed to create the patterned Permalloy layer 90 as shown in
Figure 11c on the top surface of the wafer.
The exposed portions of seedlayer 86 is then etched away and a new
20 micron photoresist layer 92 is spun on and patterned to form an reactive ion
etchant mask as shown in Figure 11d. The wafer is then reactive ion etched until
the structure is free as shown in Figure 11e and layer 92 removed.
Figure 12 is a plan top view of the fabricated device which results
from the method of Figures 11a-e. Wafer 94 is separated from shutter 96 which carries
Permalloy plate 98 and is connected to wafer 94 by means of two cantilevered serpentine
beams 100. In the bulk machined actuator plate 98 is 2.9mm by 1.6mm and 5-7 microns
thick. Shutter 96 is 3mm by 1.8mm and is 39 microns thick. Beams 100 are 120 microns
wide and 39 microns thick.
In one embodiment mechanical stops for shutter 96 were fabricated
below membrane 85 from Permalloy beams 8 microns wide and 200 microns long formed
in the shape of an X to prevent shutter 96 from descending below the plane of the
wafer. An external 500 Gauss magnetic field is sufficient to deflect shutter 96
of the order of 1mm at low frequencies out of the plane of wafer 94 at an angle
of 30 degrees. Increasing the field to 1000 Gauss causes deflections of up to 60
degrees. Shutters without stops vibrate above and below the plane of the wafer depending
on the initial orientation of the wafer relative to the field. A downward inclination
of the wafer resulted in downward deflection of the shutter and vice versa. Shutters
with stops always move out of the plane of the wafer.
A surface micromachine shutter can also be fabricated using techniques
similar to the surface micromachine actuators as described in connection with Figures
1 and 4. A rear wafer opening can be formed by wet etching the back side of the
wafer beneath flap 14. Performance of surface micromachined shutters were comparable
to bulk machined shutters as described above.
While a shutter has been described in the context of Figures 11a-e
and 12, shutter 94 can be processed to act as a mirror by providing an appropriate
reflective surface on the smooth or polished silicon surface. Applications for controllable
reflectors is expected to have utility in laser disk equipment and optical communications.
Alternatively, further processing steps could add coils or antennas which would
be used as orientable arrays of high frequency antennas to create variable focused
high frequency beams or receivers. In the instances where the antenna elements are
made from doped silicon their radar cross-section could then be varied both by altering
their conductivity using antifuse technologies and altering their orientation.
Many alterations and modifications may be made by those having ordinary
skill in the art without departing from the scope of the invention. Therefore, it
must be understood that the illustrated embodiment has been set forth only for the
purposes of example and that it should not be taken as limiting the invention as
defined by the following claims.