MEMS

1

• MEMS
• Class 6
• Microactuators
• Mohammad Kilani

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Actuation principles

• A complete shift in paradigm becomes necessary to
  think of actuation on a miniature scalea
  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.

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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

• 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.

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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-of-plane 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.

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Example Electrostatic Actuator Comb drive

Stationary bank

Folding Beam


Shuttle

Anchor

Direction of shuttle motion
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Example Electrostatic Actuator Folded Beam
Resonator (Comb Drive)
Stationary bank


Folding Beam


Shuttle

Anchor

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Example Comb drive application. Ratchet mechanism
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Example Comb drive application. Electrostatic
Microengine
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Example Microengine application. a binary encoder
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Example Microengine application. Gear chain
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Example Microengine application. Linear rack
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Example linear rack application. micromirror
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Example microengine application. Spiral micropump
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Example electrostatic actuator Torsional
Ratcheting Actuator (TRA)
Bond pad

Rotating Ring gear
Oscillating Bank

Stationary Bank
Torsional Spring
Clip

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Example TRA application, crescent micropump
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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.

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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

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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.

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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.

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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 (lt5 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.

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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.

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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.

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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 temperatureOne
  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).

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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.

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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.
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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
  temperaturerather than a differential
  temperatureof 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

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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.

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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.

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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 beams 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.

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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 100C and 100C 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.

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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 forcethe Lorentz forcein 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.

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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.