Micromechanical Devices and Systems

424 Micromechanical Devices and Systems

Micromechanical Devices and Systems H Fujita, University of Tokyo, Tokyo, Japan & 2005, Elsevier Ltd. All Rights Reserved.

Introduction The successful extension of semiconductor technology to fabricate mechanical parts of the sizes from 10–100 mm opened wide ranges of possibilities for micromechanical devices and systems. The fabrication technique is called micromachining. Micromachining processes are based on silicon integrated circuits (IC) technology and used to build threedimensional (3D) structures and movable parts by the combination of lithography, etching, film deposition, and wafer bonding. The same technology base that enabled miniaturization and large-scale integration of electronics offers three distinctive features defining micromachined devices and systems: miniaturization, multiplicity, and microsystem integration. Miniaturization is clearly essential. Small parts respond fast, constitute miniature machines that work in extremely shallow spaces, add functionality to portable or wearable devices, and realize tools to investigate the nanometric world. Millions of such parts can work cooperatively to do things impossible for a single device alone. Thus, multiplicity is one key to successful micromechanical systems. The coordination of these parts is accomplished by integrating them with electrical circuits. Furthermore, optical, sensing, fluidic, and biological elements are to be integrated in multi-functional microsystems in a cost-effective manner. The research field concerning micromechanical devices is referred to as MEMS (microelectromechanical

systems) in USA, Micromachine in Japan, and MST (micro system technology) in Europe. The root of MEMS research can be found in the research of silicon sensors. A noticeable turning point from sensor research toward MEMS research was the demonstration of micromachined gears and turbines made on a silicon chip in 1987. Since then, development has continued in micromachining processes, material varieties, microactuators, and the application of MEMS.

Micromachining MEMS generally requires complex microstructures that are thick, 3D, and movable. Therefore, many technologies have been developed on the basis of semiconductor microfabrication processes including lithography, etching, and deposition. Pre-designed patterns are transferred by exposure to resists that have sensitivity to light, X-ray, or electron beam and those that are coated on substrates. Photo lithograph through a glass-chromium mask is most commonly used. Parts of the resist remain on the substrate according to the exposed patterns by development. Using the resist pattern as a masking layer, one can selectively etch the substrate or the film on it. In another process, the resist mold pattern can be replicated by deposition. After resist removal, the next material is deposited on the patterned substrate. The material is, then, patterned by lithography and etching. These sequences are very similar to the IC fabrication. There are special processes for micromachining to obtain 3D structures and movable parts (Table 1). To construct 3D microstructures, researchers have used

Table 1 Micromachining processes Process (material)

Feature

Application

Wet anisotropic etching (single-crystal silicon)

Precise structures defined by crystal facets

Dry anisotropic etching (silicon)

3D structure defined by mask patterns

Sacrificial layer etching (various thin films)

Integrated circuits compatible fabrication of micromovable structures Closed cavity formation, 3D assembly of devices on substrates

Pressure sensor membrane, V-groove (channel, fiber alignment), Optical flat mirror surface High aspect ratio microstructure, through hole Integrated sensor, Arrayed MEMS (e.g., digital micromirror device)

Wafer bonding (Si or glass substrate)

Electroforming (metal) Injection molding and thermal embossing (polymer)

Replication of resist patterns by electroplating through the resist mask Low cost replication from precise master

Hybrid integration of multifunctional system, Packaging and encapsulation Metallic 3D structures, Mater for injection molding 3D microstrutures for fiber alignment or microchannels

Micromechanical Devices and Systems 425 Table 2 Selectivity of etching Etching liquid/ gas

Etched material

Resistant material

HF ( þ NH4F) KOH Hot H3PO4 XeF2 (gas) SF6 (plasma) O2 (plasma)

SiO2 Si Si3N4 Si Si Polymer

Si SiO2, Si3N4 SiO2, Si SiO2, Si3N4 Ni, Al Al

anisotropic wet etching, deep reactive ion etching, and replication of deep lithography patterns. Another 3D fabrication technique is capable of folding micromachined plates out from the substrate by removing either a thin film under it or the substrate itself. The removal process is called sacrificial layer etching. Selective etching of the ‘‘sacrificial’’ material should be performed without damaging microstructures. Table 2 shows the choice of etching solution or gas, the structural material and the sacrificial material. Even after release etching, microstructures tend to stick to the substrate. Sublimation drying, CO2 supercritical drying, release etching by plasma or gas, and surface coating are typical methods to prevent sticking. Wafer bonding is also essential to micromachining. 3D stacking of structures, sealing of cavities and channels, hermetic or vacuum encapsulation of sensing/actuation elements and combination of devices made of different materials and processes are accomplished by bonding wafers together. Glass and silicon substrates are bonded by applying 400–500 V at 600–800 K; this is called anodic bonding. Two silicon substrates can be fusion bonded at B1370 K after careful cleaning of the surfaces to be bonded. Replication of micro molds is the inexpensive way of fabricating many microstructures. Molds can be made by deep lithography or deep etching. Electroplating of metals, chemical vapor deposition (CVD) of polysilicon films, and injection molding of polymers are commonly used for replication. Finally, direct fabrication of microstructures may be possible by beam assisted deposition or solidification by laser beam, electron beam assisted deposition, and focused ion beam (FIB) etching.

Materials for MEMS Silicon is the most common material in micromachining because (1) the fabrication processes are well established, (2) it has excellent mechanical properties, and (3) integration with electronic circuits and sensors is possible. It also has piezoresistive properties used for strain detection. SiO2 and SiN films serve as electrical and thermal insulators. They are also excellent masking layers for anisotropic wet

etching by TMAH and KOH. Optical waveguides may be made of SiO2. SiN and other nitride films, e.g., TiN and A1N, are very hard and used for low friction and antiwear coating. Other materials that may not be common for IC fabrication are also useful in MEMS. Actuator materials produce force and displacement. They include piezoelectric materials (PZT, ZnO, quartz), magnetostrictive material (TbFe), shape memory alloy (TiNi), and some conductive polymers. Compound semiconductors such as GaAs, GaN and InGaAs can be micromachined in a way similar to silicon, and offer optically active and passive components. Metals can be deposited by electroplating, sputtering, and vacuum evaporation. Electroplated copper is widely used for conductors as well as Al thin films. Electroplated nickel is favored for both structural and magnetic elements. Researchers have developed batch fabrication processes for all of those materials achieving the features of MEMS mentioned above.

Microactuators Microactuators are key devices allowing MEMS to perform physical functions. Many types of microactuators have been successfully operated. Some of them are driven by force associated with physical fields. Force can be generated in the space between stationary and moving parts using electric, magnetic, and flow fields. Some others utilize actuator materials introduced before. Thermal expansion and phase transformations, such as the shape-memory effect and bubble formation, cause shape or volume changes. Micromachining technology allows one to make structures in which a well-controlled field is generated or to deposit and pattern actuator materials. Typical sizes of microactuators are from 10 mm–1 mm. Although the physical principle that describes the motion of macroscopic objects is still applicable to microactuators, the relative importance of various forces changes in small dimensions. It is called a scaling effect. Table 3 shows the dependence of some physical parameters and forces on the characteristic dimension of an object. The scaling effect and the compatibility with micromachining process are important issues when a microactuator is designed. One of the most significant scaling effects is that friction dominates over inertia forces in the micro world. Researchers have tried many ways of minimizing friction by suspending movable parts with flexures, applying smooth coatings, using rotational contacts, levitating objects, and using friction drive mechanism.

426 Micromechanical Devices and Systems Table 3 Scaling effect (L: characteristic dimension) Parameter

Formula

Scaling

Comment

Gravity, fg Inertial force, fi Elasticity, fe Surface tension, Fs Resonant frequency, o Moment of inertia, I Reinors number, Re Heat conduction, Qc Thermal expansion, Fr Magnetic force, Fm Electrostatic force, Fe

Mg Ma eSD L =L Lg pffiffiffiffiffiffiffiffiffiffiffi K =m a mr 2 fi =ff l d TA= d eSD LðT Þ=L m SH 2 =2 e SE 2 =2

L3 L4 L2 L L
1 L5 L2 L L2 L4 L2

g: gravity const., m: mass a: acceleration e: Yang’s modulus g: surface wetting constant M: spring constant (pL) a: shape constant, r: radius of rotor Ratio between inertia and viscosity (pL2) d T: temperature difference, l: heat conductivity, A: cross-section area (pL2) Valid for piezoelectric deformation (D L(E)) m: permittivity, H: magnetic field (pL) e: permittivity, E: electric field (constant)

Table 4 Comparison of driving principles

Speed Force Integration with circuits Power consumption Robustness in harsh environments

Electrostatic

Magnetic

Piezoelectric

Shape memory alloy

Thermal

Excellent Poor Excellent Excellent Poor

Good Good Good Fair Good

Good Excellent Fair Good Good

Fair Excellent Good Fair Excellent

Poor Excellent Excellent Poor Excellent

Each actuation principle has its own advantages and disadvantages (Table 4). The choice and optimization of an approach should be made according to the requirements of a particular application. Generally speaking, the electrostatic actuator is more suitable for performing tasks that can be completed within a chip (e.g., positioning of devices/heads/ probes, sensors with servo feedback for self-test or readout, light deflection and modulation) since it is easily integrated on a chip, easily controlled, and consumes little power. On the contrary, the other types of actuators are more robust, more capable of producing larger forces, and more suitable for performing external tasks (propulsion, manipulation of objects, etc.).

Applications Figure 1 maps the prospective applications in optics, transportation and aerospace, robotics, chemical analysis systems, biotechnologies, medical engineering, and microscopy using scanning microprobes. Most of these applications have a common feature in that only very lightweight objects such as mirrors, heads, valves, cells, and microprobes are driven, and very little physical interaction with the external environment is necessary. Sensors

The first successful application of MEMS was pressure sensors for the precise control of an automobile

engine combustion for clean exhaust in 1980s. MEMS accelerometers are currently used for air bag ignition sensors. MEMS gyroscopes, or angular rate sensors, are applied to navigation systems, active suspensions, and spin suppression systems. Precisely micromachined membranes serve as pressure sensing element; its deflection by applied pressure is detected by piezoresistive gauges made on the silicon membrane. In the accelerometer, a proof mass suspended by a flexible suspension is micromachined (Figure 2). When acceleration is applied to the proof mass, the initial force displaces the mass and the suspension; this motion is detected by either the strain gauge, that is, an integrated piezoresistor, or the change in capacitance between the mass and a fixed electrode. Better linearity and stability can be achieved by feedback measurement in which the inertia force is cancelled by electrostatic force; the magnitude of the force is proportional to the acceleration. Printers

The ink jet printer head is another example of successful MEMS products. Micro nozzles and channels of 10–20 mm in size are micromachined in an array. A microheater is attached to each channel. The heater generates a bubble in the channel when a current pulse flows through it. An ink droplet is ejected from the nozzle by the pressure pulse caused by the bubble. The amount of the droplet is in the order of a few pL now. A typical head has hundreds

Micromechanical Devices and Systems 427

Optics

Microprobe-based disk storage Magnetic disk

Display

Optical manipulation

Optical disk Micro-optical bench

Network sensor

Optical switch Variable attenuator

Relay matrix

Microsatellite

Optical modulator Inkjet printer Laser printer

NSOM

Mill-wave switch

AFM Atom/molecular manipulation

Active skin for aerodynamic control

Chopper Scanner

Active suspension Laser radar Fuel infection

STM

Air bag

Multiprobes

Automatic driving Navigation

Transportation & aerospace

Scanning probe microscopes

Communication and information apparatus

Human interface Protein analysis Miniature robot DNA analysis Channel Single molecular Microteleoperator Valve Low-invasive characterization Pump Mixer surgery Microreactor Mobile sensor Heat exchanger Active endscope Gas/liquid chromatography Total analysis system Combinatorial synthesis

Robot

Biotechnology

DNA manipulation

Nerve stimulator Microblood analyzer

Implantable artificial organ

Microrobot

High-throughput screening

Fluidics and chemical system

Medical

Figure 1 A map of MEMS applications.

Piezo resistive gauge

Driving electrode for servo feedback detection

Acceleration

Capacitive detection electrode Figure 2 Micromachined accelerometer.

of nozzles and channels which operate as fast as 10 kHz. This device takes the advantage of the scaling effect, that is, the speed of heating and cooling increases with decrease in size. Thus, fine pictures and documents are printed on paper. Some heads use piezoactuators and electrostatic actuators to eject ink droplets. Optical Devices

MEMS is very suitable to optical applications. Light beams can be controlled easily by moving mirrors, shutters, or gratings. MEMS with small feature sizes, precise motion and arraying capability can realize wide varieties of micro optical MEMS devices and systems based on interference, diffraction and

refraction. MEMS technology also enables precisely aligned integration of active and passive optical devices, micromechanical elements and optical fibers. Therefore, optical MEMS devices and systems are used for optical scanners, micro-optical benches, arrayed micromirror displays, optical switches, modulators and attenuators. The most successful example is a display. An array of movable micromirrors corresponding to pixels reflects light and projects an image on the screen. Each mirror is randomly accessible and tilted either 7101 by electrostatic force. The pixel becomes bright or dark depending on the angle of the mirror. The mirror moves within 0.1 ms and controls the brightness by pulse-width-modulation of reflected light. The array is micromachined on top of the integrated

428 Micromechanical Devices and Systems

inserted at appropriate positions to reflect beams and connect them to the output optical fibers. The alignment between fibers and mirrors is precisely determined by micromachined structures. Arrayed movable mirrors of several hundreds of micrometers in size are driven by microactuators. An on-off motion of the N2 mirrors is necessary in the switch. The port count, N, is from 2–32. For port counts as large as a few hundreds, 3D MEMS switches are used. The schematic configuration is shown in Figure 4. Micromirrors have motion with two degrees of freedom, that is, rotation around two orthogonal axes. Two sets of arrays consisting of N such mirrors are used. Collimated input light beams hit one of the mirrors in the first array that direct the beam to one of the mirrors in the second array. The second mirror reflects the light straight into the output fiber. The analog control of mirror angles allows cross connection of a large number of signals. Those switches have application to optical cross connects, add-drop switches for wavelength divided multiplexing networks, and back-up or inspection apparatuses. Optical scanners based on movable MEMS mirrors have many applications including bar-code readers, laser range finders, scanning laser microscopes, variable optical attenuators, and laser projection display. Some of them are already introduced in the market.

circuits which allow the individual control of many mirrors within a short time. MEMS based optical switches have been intensively studied. Optical switches, by means of mechanical movement, have favorable characteristics such as wave-length independence, polarization independence, large contrast, and small crosstalk. 2D MEMS optical switches change the optical path either straight or to a predetermined angle and connect it to one of the two output paths. The typical configuration of an N port 2D MEMS optical switch is shown in Figure 3. Light beams from input optical fibers are collimated by lenses. Micromirrors are

Micromirrors

Input optical fibers

Collimater lenses

Output optical fibers

Chemistry and Biomedical Applications

Figure 3 Principle of a 2D MEMS optical switch. Each micromirror is individually driven by a microactuator in the on-off fashion.

Handling fluid in micromachined channels offers benefits in chemistry and biotechnology because of

Input optical fiber array

2-DoF Micromirror array

Collimater lens array Collimater lens array

Optical path

2-DoF Micromirror array Output optical fiber array

Figure 4 Principle of 3D MEMS optical switch.

Two-degree-of- freedom (2-DoF) micromirror

Micromechanical Devices and Systems 429 Optical detection of molecules Buffer Sample

Waste

migration speed depending on their length. Separation is completed within seconds and detected by fluorescence.

Future Development of MEMS

Separation of target molecules

Figure 5 Principle of a chemical analysis chip based on capillary electrophoresis.

the scaling effect. The chemical reaction is completed within a few seconds due to short diffusion time in micro channels and reactors. Fast and precise temperature control is possible. Arrayed channels allow simultaneous chemical processing of a large variety of reactions. Only a small amount of samples or chemicals are required for chemical analysis and medical diagnosis. Even individual handling and observation of a cell or a molecule has been accomplished. A typical microchemical chip is composed of one layer having grooves of 10–100 mm in dimension and another layer for sealing and connection to external pipes. Fluids flow in the channel to be mixed, heated, reacted, extracted, separated, and analyzed. Glass and PDMS (poly di-methyl siloxane) are two favored materials for the chip. Applications of microchemical chips include DNA analysis and amplification, blood analysis, environmental monitoring, combinatorial chemical synthesis, bioreactors and sensors, and production of fine chemicals. The most successful product is the DNA electrophoresis in the micro channel. The device has two orthogonal channels (Figure 5). The sample solution containing DNA that is labeled by a fluorescent dye is introduced in the shorter channel by electroosmotic effect. A small amount of the sample is injected into the long separation channel by introducing a buffer solution from the end of the other channel. DNA fragments are driven by an electric field in the channel and separated due to the difference in

The future prospects of MEMS for making contributions to the future society are envisaged in three areas: (1) offering easier access to information for a wider public, (2) making human lifestyles more compatible with the environment, and (3) improving people’s social welfare. Breakthroughs to be accomplished by MEMS would be in five general areas: machine intelligence, downsizing and parallelism, biomimetics, informatics, and environment monitoring/preservation. Downsizing and parallelism will be the direct benefit from miniaturization and multiplicity. Machine intelligence and information networks can be very much improved by introducing MEMS integrated with microelectronics. These breakthroughs are likely to be realized by MEMS technologies within ten years. They may, for example, be used in healthcare for minimally invasive diagnosis and treatment. See also: Integrated Circuits; Microelectromechanical Systems; Quantum Devices of Reduced Dimensionality.

PACS: 07.10.Cm Further Reading Elwenspoek M and Wiegerink R (2001) Mechanical Microsensors. Berlin: Springer. Fujita H (ed.) (2003) Micromachines as Tools for Nanotechnology. Berlin: Springer. Kensall DW (1998) Special Issue on Integrated Sensors, Microactuators, and Microsystems (MEMS). Proceedings of the IEEE, 21 August 86(8): 1531–1787. Nam-Trung N and Steven TW (2002) Fundamentals and Applications of Microfluidics. Boston, MA: Artech House Publishers. Petersen KE (1982) Silicon as a Mechanical Material. Proceedings of the IEEE, 70: 420. Rai-Choudhury P (ed.) (1997) Handbook of microlithography, micromachining and microfabrication. Micromachining and Microfabrication, vol. 2. Bellingham, WA: SPIE Optical Engineering Press. Senturia SD (2001) Microsystem Design. Boston: Kluwer Academic.