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MEMS
Class 6
Microactuators
Mohammad Kilani
Actuation principles
•
A complete shift in paradigm becomes necessary to think of
actuation on a miniature scale—a four-stroke engine is not
scalable. The actuation options available in MEMS are:
• electrostatic,
• Piezoelectric
• Thermal
• Magnetic
•
The choice of actuation depends on the nature of the
application, ease of integration with the fabrication process,
the specifics of the system around it, and economic
justification.
Electrostatic Actuation
•
Electrostatic actuators are based on the fundamental principle that
two plates of opposite charge will attract each other. They are quite
extensive as they are relatively straightforward to fabricate. They
do, however, have a nonlinear force-to-voltage relationship.
•
Consider a simple, parallel plate capacitor arrangement having a
gap separation, g, and area of overlap, A, the energy stored at a
given voltage, V, and the corresponding force are
  AV
1
W  CV 2  o r
2
2g
dW o  r AV
F

dg
2g 2
•
2
2
The force is a nonlinear function of both the applied voltage
and the gap separation. Use of closed loop control
techniques may be used to linearize the response.
Electrostatic Actuation
•
An alternative type of electrostatic actuator is the
comb-drive, which is comprised of many interdigitated
electrodes (fingers) that are actuated by applying a
voltage between them. The geometry is such that the
thickness of the fingers is small in comparison to their
lengths and widths. The attractive forces are therefore
mainly due to the fringing fields rather than the parallel
plate fields.
•
The movement generated is in the lateral direction
and because the capacitance is varied by changing
the area of overlap and the gap remains fixed, the
displacement varies as the square of the voltage.
•
The fixed electrode is rigidly supported to the
substrate, and the movable electrode must be held in
place by anchoring at a suitable point away from the
active fingers. Additional parasitic capacitances such
as those between the fingers and the substrate and
the asymmetry of the fringing fields can lead to out-ofplane forces, which can be minimized with more
sophisticated designs.
•
Electrostatic actuation techniques have also been
used to developed rotary motor structures. With these
devices, a central rotor having surrounding capacitive
plates is made to rotate by the application of voltages
of the correct phase to induce rotation.
Example Electrostatic Actuator: Comb drive
Stationary bank
Folding Beam
Shuttle
Anchor
Direction of
shuttle motion
Example Electrostatic Actuator: Folded Beam Resonator
(Comb Drive)
Stationary bank
Folding Beam
Shuttle
Anchor
Example Comb drive application. Ratchet mechanism
Example Comb drive application. Electrostatic Microengine
100
Up
75
Vo
lta
ge
(V)
50
25
0
0
250
500
750
1000
500
750
1000
500
750
1000
750
1000
100
Down
75
Vol
tag
e
50
(V)
25
0
0
250
100
Right
Vol
tag
e
(V)
75
50
25
0
0
250
100
Left
Vol
tag
e
(V)
75
50
25
0
0
250
500
time(ms)
Example Microengine application. a binary encoder
Example Microengine application. Gear chain
Example Microengine application. Linear rack
Example linear rack application. micromirror
Example microengine application. Spiral micropump
Example electrostatic actuator: Torsional Ratcheting Actuator (TRA)
Bond pad
Rotating Ring gear
Oscillating Bank
Stationary Bank
Torsional Spring
Clip
Example TRA application, crescent micropump
Example electrostatic actuation application, diaphragm micropump
•
The basic structure of the pump
consists of a stack of four
wafers. The bottom two wafers
define two check valves at the
inlet and outlet. The top two
wafers form the electrostatic
actuation unit. The overall
dimensions are 7 × 7 × 2 mm3.
•
The application of a voltage
between the top two wafers
actuates the pump diaphragm,
thus expanding the volume of
the pump inner chamber. This
draws liquid through the inlet
check valve to fill the additional
chamber volume. When the
applied ac voltage goes through
its null point, the diaphragm
relaxes and pushes the drawn
liquid out through the outlet
check valve.
•
Each of the check valves
comprises a flap that can move
only in a single direction: The
flap of the inlet check valve
moves only as liquid enters to fill
the pump inner chamber; the
opposite is true for the outlet
check valve.
Example electrostatic actuation application, Digital Micromirror Devices
• The Digital Micromirror Device™ (DMD) is a trademark of Texas
Instruments of Dallas, Texas, which developed and commercialized this
new concept in projection display technology referred to as Digital Light
Processing™ (DLP). Texas Instruments first introduced its new product
family of DLP-based projection systems in 1996
Example electrostatic actuation application, Digital micromirror arrays
•
The basic structure consists of a bottom aluminum layer containing electrodes, a
middle aluminum layer containing a yoke suspended by two torsional hinges, and
a top reflective aluminum mirror. An applied electrostatic voltage on a bias
electrode deflects the yoke and the mirror towards that electrode.
Example electrostatic actuation application, Binary reflective switches
•
In a 2 × 2 binary
reflective optical
switch, an
electrostatic comb
actuator controls
the position of a
micromirror.
•
In the cross state,
light from an input
fiber is deflected
by 90º. In the bar
state, the light from
that fiber travels
unobstructed
through the switch.
•
Side schematics
illustrate the signal
path for each
state.
Piezoelectric Actuation
•
An applied voltage across the electrodes
of a piezoelectric material will result in a
deformation that is proportional to the
magnitude of the voltage (electric field).
•
Commercially available piezoceramic
cylinders can provide up to a few
newtons of force with applied potentials
on the order of a few hundred volts.
However, thin-film (<5 µm) piezoelectric
actuators can only provide a few
millinewtons. Both piezoelectric and
electrostatic methods offer the advantage
of low power consumption as the electric
current is very small.
•
A piezoelectric unimorph is fabricated by
depositing a piezoelectric film onto a
substrate in the form of a cantilever
beam. The deflection at the free end of
the beam is greater than that produced in
the film itself, thus providing a form of
mechanical amplification to the small
displacement of the piezoelectric film.
Piezoelectric actuator example: membrane pump
•
Piezoelectric actuators are often used in
micropumps as a way of deflecting a thin
membrane, which in turn alters the
volume within a chamber below.
•
The device comprises two silicon wafers
bonded together. The lower wafer
comprises an inlet and outlet port, which
have been fabricated using bulk
micromachining. The upper wafer has
been etched to form the pump chamber.
The shape of the ports gives rise to a
preferential direction for the fluid flow,
although there is a degree of flow in the
reverse direction during pumping. So the
ports behave in a similar manner to
valves.
•
Typical flow rates are in the range of
nanoliters to microliters per minute,
depending on the dimension of the
micropump.
Thermal Actuation
•
A number of distinct approaches have emerged within the
MEMS community. These include bimetallic, thermopneumatic,
differential elongation and shape memory alloy actuation.
•
Thermal actuation techniques tend to consume more power
than electrostatic or piezoelectric methods, but the forces
generated are also greater.
Bimetallic Thermal Actuation
•
Bimetallic actuatoin capitalizes on the difference
in the coefficients of thermal expansion between
two joined layers of dissimilar materials to cause
bending with temperature—One layer expands
more than the other as temperature increases.
This results in stresses at the interface and
consequently bending of the stack. The amount
of bending depends on the difference in
coefficients of thermal expansion and absolute
temperature.
•
Such structures are often referred to as thermal
bimorphs and are analogous to the familiar
bimetallic strips often used in thermostats.
•
In a thermal bimorph, an electric current is
passed through an aluminum layer, it heats up
(Joule heating), thereby causing the free end of
the beam to move. These devices are relatively
straightforward to fabricate and in addition to
consuming relatively large amounts of power,
they also have a low bandwidth because of the
thermal time constant of the overall structure
(i.e., beam and support).
Thermopneumatic Thermal Actuation
•
In thermopneumatic actuation, a liquid is heated inside a sealed
cavity. Pressure from expansion or evaporation exerts a force
on the cavity walls, which can bend if made sufficiently
compliant. This method also depends on the absolute
temperature of the actuator.
Thermopneumatic Actuation Example:
Normally Open Diaphragm valve
In a normally open diaphragm valve, a diaphragm occludes a fluid
port by its flexing action, hence blocking flow. Upon removal of
electrical power, the control liquid entrapped in the sealed cavity
cools down, and the diaphragm returns to its flat position,
consequently allowing flow through the port. The flexing membrane is
in intimate contact with the fluid flow, which increases heat loss by
conduction and severely restricts the operation of the valve.
Thermopneumatic Actuation Example:
Normally Closed Diaphragm valve
•
In a normally closed diaphragm valve, the
diaphragm of the valve normally blocks
fluid flow through the outlet orifice.
•
Heating of the Fluorinert liquid sealed
inside a cavity flexes a thin silicon
diaphragm which in turn causes a
mechanical lever to lift the valve plug.
•
The switching time is typically 1s, and the
corresponding average power
consumption is 1.5W.
•
Because it relies on the absolute
temperature—rather than a differential
temperature—of the control liquid for
actuation, the valve cannot operate at
elevated ambient temperatures.
Consequently, the valve is rated for
operation from 0° to 55ºC.
•
The normally closed valve measures
approximately 6 mm × 6 mm × 2 mm and
is packaged with two attached tubes
Thermopneumatic Actuation Example:
Inkjet print heads
•
Early generations of inkjet heads used electroformed nickel nozzles. More
recent models use nozzle plates drilled by laser ablation. Silicon micromachining
is not likely to compete with these traditional technologies on a cost basis.
However, applications that require high resolution printing will probably benefit
from micromachined nozzles. At a resolution of 1,200 dots per inch (dpi), the
spacing between adjacent nozzles in a linear array is about 21 µm. A greater
number of laser-drilled nozzles on a head raises the cost, while the cost remains
constant as holes are added using batch-fabrication methods.
•
Nonetheless, the nozzles continue to be made in nickel plates, but
micromachining technology is now necessary to integrate a large number of
microheaters on a silicon chip. High-performance inkjet technology represents an
excellent illustration of how micromachining has become a critical and enabling
element in a more complex system.
Thermopneumatic Actuation Example: Inkjet print heads
•
The device from Hewlett-Packard illustrates the basic principle of thermal inkjet
printing. A well under an orifice contains a small volume of ink held in place by
surface tension. To fire a droplet, a thin-film resistor made of tantalum-aluminum alloy
locally superheats the water-based ink beneath an exit nozzle to over 250ºC. Within 5
µs, a bubble forms with peak pressures reaching 1.4 MPa (200 psi) and begins to
expel ink out of the orifice.
•
After 15 µs, the ink droplet, with a volume on the order of 10−10 liter, is ejected from
the nozzle. Within 24 µs of the firing pulse, the tail of the ink droplet separates, and
the bubble collapses inside the nozzle, resulting in high cavitation pressure. Within
less than 50 µs, the chamber refills, and the ink meniscus at the orifice settles.
Diifferential Elongation Thermal Actuation
•
Diifferential elongation actuation utilizes a suspended beam of a same
homogeneous material with one end anchored to a supporting frame
of the same material. Heating the beam to a temperature above that
of the frame causes a differential elongation of the beam’s free end
with respect to the frame. Holding this free end stationary gives rise to
a force proportional to the beam length and temperature differential.
Such an actuator delivers a maximal force with zero displacement,
and conversely, no force when the displacement is maximal. Designs
operating between these two extremes can provide both force and
displacement. A system of mechanical linkages can optimize the
output of the actuator by trading off force for displacement, or vice
versa. Actuation in this case is independent of fluctuations in ambient
temperature because it relies on the difference in temperature
between the beam and the supporting frame.
Shape memory alloy thermal actuation
•
The shape memory effect is a property of a special class of metal alloys
know as shape-memory alloys. When these materials are heated beyond a
critical transition temperature, they return to a predetermined shape.
•
The SMA material has a temperature-dependent crystal structure such that,
at temperatures below the transition point, it possesses a low yield strength
crystallography referred to as a Martensite. In this state, the alloy is
relatively soft and easy to deform into different shapes. It will retain this
shape until the temperature exceeds the phase transition temperature, at
which point the material reverts to its parent structure known as Austenite.
•
One of the most widely used SMA materials is an alloy of nickel and titanium
called Nitinol. This has excellent electrical and mechanical properties and a
long fatigue life. In its bulk form, it is capable of producing up to 5% strain.
The transition temperature of Nitinol can be tailored between –100°C and
+100°C by controlling the impurity concentration. The material has been
used in MEMS by sputter depositing TiNi thin-film layers
•
Shape-memory alloys offer the highest energy density available for
actuation. The effect can provide very large forces when the temperature of
the material rises above the critical temperature, typically around 100ºC.
•
The challenge with shape-memory alloys lies in the difficulty of integrating
their fabrication with conventional silicon manufacturing processes.
Magnetic Actuation
•
Lorentz forces form the dominant mechanism in magnetic actuation
on a miniaturized scale. This is largely due to the difficulty in
depositing permanently magnetized thin films.
•
Electrical current in a conductive element that is located within a
magnetic field gives rise to an electromagnetic force—the Lorentz
force—in a direction perpendicular to the current and magnetic field.
This force is proportional to the current, magnetic flux density, and
length of the element.
•
A conductor 1mm in length carrying 10 mA in a 1-T magnetic field is
subject to a force of 10 µN. Lorentz forces are useful for closed-loop
feedback in systems employing electromagnetic sensing.
Example Magnetic Actuator: Yaw Rate Sensor
•
The CRS family of yaw-rate sensors
uses a vibratory ring shell. Electric
current loops in a magnetic field excite
the primary mode of resonance. These
same loops provide the sense signal to
detect the angular position of the
vibration pattern.
•
The ring is suspended by eight flexural
beams anchored to a square frame.
Eight equivalent current loops span
every two adjacent support beams. A
current loop starts at a bond pad on the
frame, traces a support beam to the
ring, continues on the ring for one
eighth of the circumference, then moves
onto the next adjacent support beam,
before ending on a second bond pad.
Under this scheme, each support beam
carries two conductors.