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

221

2 2o rAVW CVg

2

22o rAVdW

Fdg g

• 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-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.

Example Electrostatic Actuator: Comb drive

Direction of

shuttle motion

Anchor

Shuttle

Folding Beam

Stationary bank

Example Electrostatic Actuator: Folded Beam Resonator (Comb Drive)

Anchor

Shuttle

Folding Beam

Stationary bank

Example Comb drive application. Ratchet mechanism

Example Comb drive application. Electrostatic Microengine

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Down

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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.

• 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: Normally Closed Diaphragm valve

• 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.

Thermopneumatic Actuation Example: Inkjet print heads

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.