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Microactuators and their technologies
Mechatronics 10 (2000) 431±455
Microactuators and their technologies Ernst Thielicke, Ernst Obermeier* Technical University of Berlin, Microsensor and Microactuator Technology Center (MAT), TIB 3.1, Gustav-Meyer-Allee 25, 13355, Berlin, Germany
Abstract This paper gives a brief overview of microactuators, focussing on devices made by microfabrication technologies which are based on silicon processes like photolithography, etching, thin ®lm deposition etc. These technologies enable the miniaturization of electrical devices as well as micromechanisms and microactuators. They can be batch fabricated on large area silicon substrates and represent the smallest available in a vast ®eld of actuators. Mentioning the activation principles and the three main fabrication technologies: bulk micromachining, surface macromachining and moulding, the paper focusses on devices, which made their way into industrial applications or prototypes. The far most developed micro electro-mechanical systems (MEMS) are found in micro-¯uidic systems (printheads, microvalves and -pumps) and micro-optical systems (micromirrors, -scanners, -shutters and switches). They can be combined with microelectronics and microsensors to form an integrated on-chip or hybrid-assembled system. Other MEMS-actuators like microgrippers, microrelays, AFM heads or data storage devices, are promising devices for future medical, biological and technical applications like minimal invasive surgery or the vast ®eld of information storage and distribution. 7 2000 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction For more than a decade, microactuators have been developed using structures and technologies based on those of microsensors and semiconductor fabrication technology. Silicon still plays a leading role, not only as the electrical, but also as the mechanical material [1]. * Corresponding author. Tel.: +49-30-314-72-769; fax: +49-30-314-72-603. E-mail address: [email protected] (E. Obermeier). 0957-4158/00/$ - see front matter 7 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 7 - 4 1 5 8 ( 9 9 ) 0 0 0 6 3 - X
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The ®rst devices have been simple mechanical resonant structures agitated by electrostatical forces to detect humidity by analysing the resonant frequency [2] and bulk micromachined valves and membrane pump prototypes using piezoelectric and thermopneumatic forces [3,4]. Later in the 1980s more sophisticated mechanical elements like springs, cranks, gears and other novel micromechanical structures were presented to facilitate, for the ®rst time, rotational microdynamics and complex micromachines [5±7]. The same physical laws and material constants govern the micro as well as the macro world. Scaling macro-actuators down to micro-actuators shifts the in¯uence of individual parameters on the total system dramatically. Compared to macro- or mini-actuators, microactuators allow completely new mechanical designs. Volumes and masses for example decrease superproportionally (cubically) compared to lengths. Scaling down mechanical systems leads to sti and comparatively lightweight structures with high shock resistance. But in addition, one has to keep in mind that thin ®lms, which are used for the batch fabrication of microactuators, often have dierent material properties compared to bulk materials including intrinsic stress induced by high temperature processing and deposition [8,9]. In 1988 the ®rst IC-processed rotational electrostatic micromotor was shown [10]. But these early, small (é 1 100 mm) and weak machines found no way into the application ®eld. Since then, much work has been done to improve the actuators and some applications have been found. Singular problems like friction [11,12] and sticking [13] are still not yet solved to everyone's satisfaction. Because of the small sizes (bearing clearance in the sub-mm range) and limitations in technological feasibility, no ball bearings exist like those in the macroscopic world, although some eorts are made to use electromagnetic, electrostatic, gas ¯ow or ¯uidic bearings [14,15]. If zero-friction-motion is required, e.g. positioning tables without hysteresis, the movable parts have to be suspended by elastic beams or membranes. Until today frictionless motion was implemented in the predominant proportion of all MEMS which were conceived for a technical application in order to avoid mechanical wear and to thus enable long life and high economical bene®t. Micro¯uid devices like micropumps and microvalves are today's furthest developed microactuators and sold on the market in high volumes. They are used for example in micro total analysis systems (m-TAS) or nano-litre dosing systems [16]. Inkjet print heads are the most common and most well-known micro¯uidic devices, although only a few people expect such MEMS in a non-returnable ink cartridge in their desktop bubble jet printer [17]. Apart from these micro¯uidics there are further areas of application for MEMS in microoptics and electronic systems, like (optical) switches and relays, scanners, (mirror based) displays, variable capacitances and inductances with an enormous market in (optical) computer networks and telecommunications [18,19]. Data storage systems, as well as AFM and STM tools, use microactuators in their head carriers to achieve ultra high density recording/scanning [20]. Mostly piezoelectric microactuators are used for the piggy back actuators of hard disk drives, because
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they have a suciently rapid response and sucient force to allow improved control of the servo system which maintains the recording sensor over the center of the track [21]. 2. Actuation principles The scaling does not only have consequences for the mechanical design but also for the actuation of microactuators. The right choice of the actuation principle is dependent on the structural dimensions, the technology, the response time, the force and torque as a function of displacement and the maximum power consumption as well [22]. Force can be generated following two main principles: 1. External forces which are generated in the space between stationary and moving parts using thermopneumatic [23] or electrochemical [24] eects and electrostatic [25] or magnetic ®elds [26]. 2. Inner forces which use special materials having intrinsic actuation capabilities including piezoelectric [27], thermomechanical [28], shape memory [29], electroand magnetostrictive eects [30]. Although nearly every permutation of activation principle and device has been tried, only a few have been leaving the research laboratories. See in addition Table 1 for typical MEMS devices together with their activation principle, which have been successfully realized and implemented in industrial applications or prototypes.
Table 1 MEMS devices and their activation principle Actuation principle
Typical MEMS devices
piezoelectric electrostatic
micropump [54], microvalve [31], HDD servo system [21] micromotor (shutter) [56], microshutter [32], micromirror [68], microscanner [61], microrelay [33,34] microrelay [35], micropump, -valve [36] microvalve [37], microgripper [65] micropump [23,38], microvalve [39], inkjet printhead [51] microvalve [40], ®ber-optic switch [41]
electromagnetic thermomechanic thermopneumatic shape memory
Electrostatical actuation is the most frequently applied principle combining versatility and simple technology. It needs neither additional elements like coils or cores, nor special materials like shape-memory-alloys or piezoelectric ceramics. Above that, the electrostatical actuation draws its force from the relation of surface to spacing and not from the relation of volume to spacing, i.e. it is less aected by scaling and more favourable for VLSI actuators [42].
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Fig. 1. (a) Schematical view of a comb drive linear actuator. The anchor is suspended by beams. Electrostatical forces pull it towards the right. (b) SEM photograph of the comb drive.
Rotational and linear micromotors are often found to be a key part of micromechanical systems allowing them to perform physical functions. They can be used in x-y-stages, for aperture controlling in microphotonics, driving forces for micro-relays, micro-mirrors and micro-grippers. They also initialise mechanical systems, carry out on-chip assembling and rise pop-up structures. The most commonly used activation principle for micromotors is the electrostatic ®eld between the plates of capacitors including comb drives [43], curved electrodes [44],
Fig. 2. Curved electrode actuator in resting position (upper half of the ®gure). The feather beam is clamped on one side to the ground electrode and forms itself the mobile electrode. If voltage is applied between the stator and the feather beam electrode, the beam is ``rolled up'' by the stator electrode (lower half of the ®gure) [44].
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Fig. 3. Scratch drive actuator (a) and its operating principle (b). It moves forward by parallel plate capacitor and frictional forces with the substrate surface [45].
scratch [45], wobble [46], linear stepping [47] and side drives with synchronous and asynchronous operation [48]. See in addition Figs. 1±6 for the operating principles and some SEM photographs of the devices. Asynchronous operation does not only use attracting, but also repulsive forces. The sliding anchor is made of high resistive and the stator poles are made of low resistive material (e.g. undoped and doped polysilicon). After the anchor is completely charged and the charges stand opposite to each other, the potentials of the stator poles are quickly inverted. The charges in the low resistive anchor-
Fig. 4. Schematical cross section of a wobble micromotor. The rotor is rolling on the isolation layer like a wobbling gyroscope. It is less aected by wear, because the moving part of the bearing is rolling as well (and less sliding) in the bearing shell [46].
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Fig. 5. (a) The four ®nger linear stepping motor uses grip arms that can be moved in two dimensions: perpendicular to the anchor to hold it by frictional forces and parallel to shift it for- or backwards. (b) SEM photograph of the central beam and the grip arms [47].
Fig. 6. Synchronous side drive motor. The stator poles and the sliding anchor are charged antipodally and move the anchor by tangential electrostatical forces as long as the electrode faces remain shifted. When the faces stand opposite to each other, the next stator electrodes are charged and so on [48].
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Fig. 7. Anisotropically etched structures in (100)- and (110)-oriented silicon wafers. Planes with (111)orientation are almost not etched by the KOH solution.
Fig. 8. Partial view of a cross-sectional SEM image of bulk micromachined nozzles and deep narrow grooves for the ink supply.
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Fig. 9. Smart ink jet print head microsystem with 50 nozzles and CMOS circuit on a single chip.
material remain (they relax very slowly) and the anchor is pulled due to attractive forces of the neighbouring stator poles and additionally pushed because of the repulsive forces, which are due to the opposite stator poles. The asynchronous motor leads to a better eciency compared to the synchronous operation mode. 3. Technology and applications for microactuators Starting with the three main technologies for microactuators, some examples and applications will be presented. These technologies are: bulk-micromachining, surface-micromachining and moulding technologies including classical electroplating, the HEXSIL and the LIGA processes. Additional processes are used which are known from standard IC-fabrication. 3.1. Bulk micromachining Bulk micromachining is still commonly performed by aqueous solutions like KOH, TMAH and EDP. Due to the fact that (100) and (110) planes of single crystal silicon are etched up to 100 times faster than (111) planes, anisotropically
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Fig. 10. Simple micropump design with piezoelectrically actuated membrane and bulk micromachined silicon diusion valves. The arrows explain the operating principle in which a big arrow pointing in or out the outlet means high ¯ow and a small arrow means weak ¯ow. The sum of the ¯ows leads to a transport of liquid/gas from outlet B to A.
etched grooves and membranes like the ones shown in Fig. 7 can be realized in a quite simple apparatus [49]. Nowadays it is also possible to etch silicon anisotropically not dependent on the crystal orientation in DRIE-reactors. These processes allow small structures (>2 mm) to be etched with high aspect-ratios (>15) and a very good anisotropy (>99%) [50]. Fig. 8 shows silicon structures anisotropically etched by KOH that belong to a micromachined inkjet printhead, often called bubble jet or drop-on-demand print head which can be seen in Fig. 9. The inkjet print head combines micromechanics, heating actuators, temperature sensors, channels and nozzles with a smart CMOS
Fig. 11. Bulk micromachined, piezoelectric micropump [54].
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Fig. 12. Single layer surface micromachining process for ®xed and movable structures.
circuit including signal processing [51]. The outside dimensions amount to 4.9 4.0 mm2 with nozzle dimensions of 20 40 mm2. Droplet velocities between 10 and 15 m/s and droplet masses between 60 and 110 ng meet conventional inkjet printer requirements. Other MEMS fabricated using bulk-micromachining technology, like valves and micropumps have found their way into industrial applications since many years. Fig. 10 shows the schematical structure of a micropump with piezoelectric actuation and bulk micromachined silicon diusion valves. But also designs with watertight and/or airtight valves and integrated or hybrid-assembled ¯ow sensor and control unit are available [52]. The ¯ow rates range from single drops in the sub nanolitre range up to several ml per min. The substrates (glass+silicon or silicon+silicon with intermediate glass layer) are anodically bonded, leading to a stable covalent bond between both materials [53]. Fig. 11 shows the Debiotech (Switzerland) micropump developed to be the heart of an implantable drug infusion system [54]. The device is based on silicon bulk micromachining, silicon pyrex (glass) anodic bonding and piezoelectric
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Fig. 13. Surface micromachining of rotational micromotors (cross section).
actuation. The pumping mechanism has been designed for maximum safety and reliability together with high open-loop accuracy (210%) in a low ¯ow rate range (0±100 ml/h). The overall size is 16 12 mm2.
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Fig. 14. SEM photograph of the central bearing of a surface micromachined rotational actuator.
3.2. Surface micromachining Surface micromachining mainly makes use of polysilicon as the ``active'' material (mechanically and electrically) and silicon-oxide as the sacri®cial layer. Several active and intermediate sacri®cial layers can be grown one on top of each other on a silicon wafer. Another possibility is to use aluminium as the active layer and organic compounds (polyimid, photoresist) as the sacri®cial layer. The layers are patterned by classical photolithographical processes and wet or plasma etch. Finally the sacri®cial layer(s) are etched away to release the structures [55].
Fig. 15. SEM photograph of the driving poles of a surface micromachined electrostatical micromotor. The poles are connected by polysilicon microbridges (see arrow).
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Fig. 16. SEM-picture of the micro-shutter fabricated using surface (motor) and bulk micromachining technology (aperture).
Surface micromachined structures are always built upwards and remain on the surface of the substrate during the whole fabrication process and in the application. Fig. 12 shows the main fabrication steps of a double clamped beam (bridge) using surface micromachining technology. Even complicated mechanical components, like beams, guide ways, bearings, hinges and locking mechanisms can be realized by choosing a suitable combination of layers and their appropriate processing. Many of the micromotor
Fig. 17. (a) Conventional apparatus for a temperature radiation measurement unit with pyroelectric detector, electromagnetic motor and chopper disk; (b) its micro-electro-mechanical counterpart which integrates the motor, the chopper disk and the aperture hole in a single device contained together with the pyroelectric detector in a single housing.
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Fig. 18. Output signal of the pyroelectric detector when excited by heat radiation. The radiation is modulated by a surface micromachined chopper turning 180 rpm.
designs presented in Section 2 have been realized using surface micromachining, which necessarily leads to a multi-layer surface micromachining process. Fig. 13 schematically shows the fabrication steps for an electrostatically driven rotational micromotor with central bearing. The bearing clearance is only 400 nm. The distance between stator and anchor poles amounts to 2 mm with a polysilicon layer thickness of 2 mm. Figs. 14 and 15 show close-ups of the central bearing and the poles.
Fig. 19. SEM photograph of a pop-up micromirror [60,61].
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Fig. 20. Partial view of a patterned photoresist mould for a synchronous micro linear actuator in gold electroplating technology.
An SEM-photograph of the whole device can be seen in Fig. 16. Fig. 17 illustrates its application as a micro-shutter for the modulation of radiation in optical or thermal detectors. The rotor diameter of the microshutter amounts to between 800 mm and 1200 mm. An aperture of 100 200 mm2 to 200 300 mm2 is etched from the backside of the substrate with bulk micromachining technology. Parts of the rotor and the whole backside are coated with a thin gold layer to reduce IR transmittance. The shutter is applied in a pyroelectric detector with numbers of revolution ranging from 0 rpm up to 7000 rpm. The minimum driving voltage is 35 V [56]. Pyroelectric detectors only generate output signals when they have excitation and refreshing cycles, i.e. when the radiation is chopped. Only 180 revolutions (rpm) are necessary to achieve an output signal like the one shown in Fig. 18.
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Fig. 21. Partial view of the electroplated gold structures that were grown into the mould shown in Fig. 20.
The linear stepping microactuator presented in Section 2 (Fig. 5) has an overall dimension of 1 2 mm2, yielding ``high'' forces (some 10 mN) and ``large'' displacements (>100 mm) with an accuracy in the nm range which are limited only by the length of the central beam. Surface micromachined devices can be made quite large in the lateral dimension (up to 4 mm2), but the maximum height amounts to between 2 and 10 mm, because the thicknesses of the layers are limited [57,58]. A way out is oered by pop-up-structures realized by hinges and connecting rods [59]. Fig. 19 shows a two axis micromirror for an optical scanner which is tilted by comb drives. 3.3. Moulding Moulding processes can be divided into three main groups: classical
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Fig. 22. Steps of the HEXSIL process.
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Fig. 23. Partial view of a micro-tweezer fabricated using the HEXSIL process [65].
electroplating, LIGA and HEXSIL processes. All moulding technologies allow high aspect ratios and a structure height of more than 100 mm, but the mechanical structures are more simple and less diverse compared with surface micromachining [62]. 3.3.1. Electroplating Classical electroplating technology uses photoresist or other photo-structurable, organic materials for the female form of the structures. They are deposited on various substrates with low resistivity or metal coating (starting layer) [63]. After the moulds are patterned, metals like Au, Ni, Cu or alloys are grown into the moulds by electrodeposition starting from the metal layer. Like surface
Fig. 24. Process steps of a suspended structure using LIGA technology.
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Fig. 25. SEM photograph of an electromagnetically actuated micro-relay fabricated using the LIGA process [67].
micromachining technology, sacri®cial layers can be used to realize bridges, suspending beams or sliding anchors. The structures are freed by stripping the photoresist. Each mould can only be used once. Figs. 20 and 21 show a partial view of a patterned photoresist mould and the gold structures after the plating process and resist stripping took place.
Fig. 26. Three arrays of electrostatically actuated micromirrors for applications in colour projection displays [68].
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Fig. 27. Schematical view of a micromirror with electrostatical actuation.
3.3.2. HEXSIL The HEXSIL process was developed at UC Berkeley. It uses a low-pressure chemical vapour deposition (LPCVD) process and silicon moulds fabricated by deep reactive ion etching (DRIE) of silicon wafers. The devices do not remain on the substrate where they were fabricated, so that the moulds can be used several times. Fig. 22 shows the steps of HEXSIL processing. An SEM photograph of a HEXSIL micro-tweezer is shown in Fig. 23. It is actuated by thermomechanical forces and has an overall dimension of 2 mm 1.4 mm 80 mm. An application for micro-tweezers will be the handling and assembling of microparts to build up microsystems with growing complexity [64]. Another future operational area could be in the ®eld of minimal invasive surgery. 3.3.3. LIGA The LIGA process [66] (RoÈntgentiefenLithographie, Galvanoformung and Abformung=deep x-ray lithography, electroforming and moulding/embossing) was developed at Forschungszentrum Karlsruhe (FZK), Germany. It starts with a thick layer (up to several 100 mm) of PMMA (polymethylmethacrylate, plexiglass) which is deposited on a metal substrate. The structures are patterned by parallel and high energy x-ray lithography. The gaps are ®lled up with metal (Au, Cu, Ni, NiFe etc.) by electrodeposition. The electroforming process starts on the metal substrate. Either the PMMA or the released metal structures can be used as the female mould. Mass production of polymer, metal and even ceramic microcomponents makes use of hot embossing and injection moulding. Cantilever LIGA-structures can be realized by sacri®cial layer etching (e.g. titanium). Fig. 24 shows the process steps of a suspended LIGA-microstructure. Fig. 25 shows an SEM photograph of an electromagnetic micro-relay fabricated using FeNi (permalloy) in the LIGA process. Much eort is made to develop mSMD compatible, fast switching low-power micro-relays for applications in e.g. telecommunication and automated test equipment (ATM). A growing number of devices are fabricated using a mix of several technologies. Often CMOS circuits are integrated to create a smart microsystem. Microsensors,
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IC signal processing and microactuators are combined on a single chip allowing the system to step into interaction with the environment. Compatibility of processes plays an important role. Hot processes like surface micromachining, diusion or annealing have to be performed ®rst. Bulk micromachining processes are implemented last, because they are cold (60±908C) and produce deep grooves with high aspect ratios that mostly do not allow any further lithographical steps. Very complex microsystems have been fabricated and brought to market like the Texas Instruments digital micromirror device (DMD) pixel array shown in Fig. 26. A schematical view of an electrostatically-driven mirror which represents one single pixel is shown in Fig. 27. A light source illuminates the micromirror array in such a way that the light beams are de¯ected either to the screen or not, depending on the tilting state of the mirror (in plane or out of plane). Each pixel on the screen can thus be switched on or o by actuating the appropriate mirror, leading to a digital image. RGB light beams and three DMD arrays (or one DMD array together with serial colour modulation) allow the screening of a coloured image. The devices are used in desktop digital light projectors, featuring up to 1280 1024 pixels. The advantage compared to conventional technology is that a bright light source can be used thus achieving a brightly shining image. 4. Conclusion Micromachined actuators are no longer just designed in research laboratories to show state of the art technology. Some applications have been found in the last ten years and put to a rapidly growing market. However microsystem technology is still in the initial phase of its development. The future of microactuators will be marked by new applications that have sophisticated requirements concerning small size, low weight, low power consumption, high shock resistance and high mechanical cut-o frequency. Already today, ®rst applications have been implemented that would have been inconceivable with conventional techniques. Microactuators cannot be designed just by scaling macro- or miniactuators, and the intention of researchers is not to replace conventional technology. They oer new solutions for applications and enlarge the ®eld of actuators, which will lead to a coexistance of micro- and conventional actuators. It is clearly essential, that only micromachines will ®t into small environments and that, for example, a small manipulator can handle microobjects much more gently and dexterously than its macro counterparts. With microactuators, ®ner positioning in shorter response time is possible than with macroscopic machines. Only a few statements can be made in order to decide about an actuation principle or microactuator design, because their usefulness is strongly coupled to the application. It has been shown that various types of actuation principles and dierent fabrication technologies are available. They can be combined to achieve the required functionality, yield and costs. Still a set of problems is unsolved. Friction, wear, sticking and fatigue reduce the life time and reliability. Much more eort will be required to introduce new materials, designs and technological concepts to solve these and future problems. The fusion of knowledge from many dierent disciplines is essential for
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a well-balanced and accelerated growth of microsystem technology and will allow new applications in further technical, medical and biological areas.
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Microactuators and their technologies Ernst Thielicke, Ernst Obermeier* Technical University of Berlin, Microsensor and Microactuator Technology Center (MAT), TIB 3.1, Gustav-Meyer-Allee 25, 13355, Berlin, Germany
Abstract This paper gives a brief overview of microactuators, focussing on devices made by microfabrication technologies which are based on silicon processes like photolithography, etching, thin ®lm deposition etc. These technologies enable the miniaturization of electrical devices as well as micromechanisms and microactuators. They can be batch fabricated on large area silicon substrates and represent the smallest available in a vast ®eld of actuators. Mentioning the activation principles and the three main fabrication technologies: bulk micromachining, surface macromachining and moulding, the paper focusses on devices, which made their way into industrial applications or prototypes. The far most developed micro electro-mechanical systems (MEMS) are found in micro-¯uidic systems (printheads, microvalves and -pumps) and micro-optical systems (micromirrors, -scanners, -shutters and switches). They can be combined with microelectronics and microsensors to form an integrated on-chip or hybrid-assembled system. Other MEMS-actuators like microgrippers, microrelays, AFM heads or data storage devices, are promising devices for future medical, biological and technical applications like minimal invasive surgery or the vast ®eld of information storage and distribution. 7 2000 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction For more than a decade, microactuators have been developed using structures and technologies based on those of microsensors and semiconductor fabrication technology. Silicon still plays a leading role, not only as the electrical, but also as the mechanical material [1]. * Corresponding author. Tel.: +49-30-314-72-769; fax: +49-30-314-72-603. E-mail address: [email protected] (E. Obermeier). 0957-4158/00/$ - see front matter 7 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 7 - 4 1 5 8 ( 9 9 ) 0 0 0 6 3 - X
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The ®rst devices have been simple mechanical resonant structures agitated by electrostatical forces to detect humidity by analysing the resonant frequency [2] and bulk micromachined valves and membrane pump prototypes using piezoelectric and thermopneumatic forces [3,4]. Later in the 1980s more sophisticated mechanical elements like springs, cranks, gears and other novel micromechanical structures were presented to facilitate, for the ®rst time, rotational microdynamics and complex micromachines [5±7]. The same physical laws and material constants govern the micro as well as the macro world. Scaling macro-actuators down to micro-actuators shifts the in¯uence of individual parameters on the total system dramatically. Compared to macro- or mini-actuators, microactuators allow completely new mechanical designs. Volumes and masses for example decrease superproportionally (cubically) compared to lengths. Scaling down mechanical systems leads to sti and comparatively lightweight structures with high shock resistance. But in addition, one has to keep in mind that thin ®lms, which are used for the batch fabrication of microactuators, often have dierent material properties compared to bulk materials including intrinsic stress induced by high temperature processing and deposition [8,9]. In 1988 the ®rst IC-processed rotational electrostatic micromotor was shown [10]. But these early, small (é 1 100 mm) and weak machines found no way into the application ®eld. Since then, much work has been done to improve the actuators and some applications have been found. Singular problems like friction [11,12] and sticking [13] are still not yet solved to everyone's satisfaction. Because of the small sizes (bearing clearance in the sub-mm range) and limitations in technological feasibility, no ball bearings exist like those in the macroscopic world, although some eorts are made to use electromagnetic, electrostatic, gas ¯ow or ¯uidic bearings [14,15]. If zero-friction-motion is required, e.g. positioning tables without hysteresis, the movable parts have to be suspended by elastic beams or membranes. Until today frictionless motion was implemented in the predominant proportion of all MEMS which were conceived for a technical application in order to avoid mechanical wear and to thus enable long life and high economical bene®t. Micro¯uid devices like micropumps and microvalves are today's furthest developed microactuators and sold on the market in high volumes. They are used for example in micro total analysis systems (m-TAS) or nano-litre dosing systems [16]. Inkjet print heads are the most common and most well-known micro¯uidic devices, although only a few people expect such MEMS in a non-returnable ink cartridge in their desktop bubble jet printer [17]. Apart from these micro¯uidics there are further areas of application for MEMS in microoptics and electronic systems, like (optical) switches and relays, scanners, (mirror based) displays, variable capacitances and inductances with an enormous market in (optical) computer networks and telecommunications [18,19]. Data storage systems, as well as AFM and STM tools, use microactuators in their head carriers to achieve ultra high density recording/scanning [20]. Mostly piezoelectric microactuators are used for the piggy back actuators of hard disk drives, because
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they have a suciently rapid response and sucient force to allow improved control of the servo system which maintains the recording sensor over the center of the track [21]. 2. Actuation principles The scaling does not only have consequences for the mechanical design but also for the actuation of microactuators. The right choice of the actuation principle is dependent on the structural dimensions, the technology, the response time, the force and torque as a function of displacement and the maximum power consumption as well [22]. Force can be generated following two main principles: 1. External forces which are generated in the space between stationary and moving parts using thermopneumatic [23] or electrochemical [24] eects and electrostatic [25] or magnetic ®elds [26]. 2. Inner forces which use special materials having intrinsic actuation capabilities including piezoelectric [27], thermomechanical [28], shape memory [29], electroand magnetostrictive eects [30]. Although nearly every permutation of activation principle and device has been tried, only a few have been leaving the research laboratories. See in addition Table 1 for typical MEMS devices together with their activation principle, which have been successfully realized and implemented in industrial applications or prototypes.
Table 1 MEMS devices and their activation principle Actuation principle
Typical MEMS devices
piezoelectric electrostatic
micropump [54], microvalve [31], HDD servo system [21] micromotor (shutter) [56], microshutter [32], micromirror [68], microscanner [61], microrelay [33,34] microrelay [35], micropump, -valve [36] microvalve [37], microgripper [65] micropump [23,38], microvalve [39], inkjet printhead [51] microvalve [40], ®ber-optic switch [41]
electromagnetic thermomechanic thermopneumatic shape memory
Electrostatical actuation is the most frequently applied principle combining versatility and simple technology. It needs neither additional elements like coils or cores, nor special materials like shape-memory-alloys or piezoelectric ceramics. Above that, the electrostatical actuation draws its force from the relation of surface to spacing and not from the relation of volume to spacing, i.e. it is less aected by scaling and more favourable for VLSI actuators [42].
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Fig. 1. (a) Schematical view of a comb drive linear actuator. The anchor is suspended by beams. Electrostatical forces pull it towards the right. (b) SEM photograph of the comb drive.
Rotational and linear micromotors are often found to be a key part of micromechanical systems allowing them to perform physical functions. They can be used in x-y-stages, for aperture controlling in microphotonics, driving forces for micro-relays, micro-mirrors and micro-grippers. They also initialise mechanical systems, carry out on-chip assembling and rise pop-up structures. The most commonly used activation principle for micromotors is the electrostatic ®eld between the plates of capacitors including comb drives [43], curved electrodes [44],
Fig. 2. Curved electrode actuator in resting position (upper half of the ®gure). The feather beam is clamped on one side to the ground electrode and forms itself the mobile electrode. If voltage is applied between the stator and the feather beam electrode, the beam is ``rolled up'' by the stator electrode (lower half of the ®gure) [44].
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Fig. 3. Scratch drive actuator (a) and its operating principle (b). It moves forward by parallel plate capacitor and frictional forces with the substrate surface [45].
scratch [45], wobble [46], linear stepping [47] and side drives with synchronous and asynchronous operation [48]. See in addition Figs. 1±6 for the operating principles and some SEM photographs of the devices. Asynchronous operation does not only use attracting, but also repulsive forces. The sliding anchor is made of high resistive and the stator poles are made of low resistive material (e.g. undoped and doped polysilicon). After the anchor is completely charged and the charges stand opposite to each other, the potentials of the stator poles are quickly inverted. The charges in the low resistive anchor-
Fig. 4. Schematical cross section of a wobble micromotor. The rotor is rolling on the isolation layer like a wobbling gyroscope. It is less aected by wear, because the moving part of the bearing is rolling as well (and less sliding) in the bearing shell [46].
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Fig. 5. (a) The four ®nger linear stepping motor uses grip arms that can be moved in two dimensions: perpendicular to the anchor to hold it by frictional forces and parallel to shift it for- or backwards. (b) SEM photograph of the central beam and the grip arms [47].
Fig. 6. Synchronous side drive motor. The stator poles and the sliding anchor are charged antipodally and move the anchor by tangential electrostatical forces as long as the electrode faces remain shifted. When the faces stand opposite to each other, the next stator electrodes are charged and so on [48].
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Fig. 7. Anisotropically etched structures in (100)- and (110)-oriented silicon wafers. Planes with (111)orientation are almost not etched by the KOH solution.
Fig. 8. Partial view of a cross-sectional SEM image of bulk micromachined nozzles and deep narrow grooves for the ink supply.
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Fig. 9. Smart ink jet print head microsystem with 50 nozzles and CMOS circuit on a single chip.
material remain (they relax very slowly) and the anchor is pulled due to attractive forces of the neighbouring stator poles and additionally pushed because of the repulsive forces, which are due to the opposite stator poles. The asynchronous motor leads to a better eciency compared to the synchronous operation mode. 3. Technology and applications for microactuators Starting with the three main technologies for microactuators, some examples and applications will be presented. These technologies are: bulk-micromachining, surface-micromachining and moulding technologies including classical electroplating, the HEXSIL and the LIGA processes. Additional processes are used which are known from standard IC-fabrication. 3.1. Bulk micromachining Bulk micromachining is still commonly performed by aqueous solutions like KOH, TMAH and EDP. Due to the fact that (100) and (110) planes of single crystal silicon are etched up to 100 times faster than (111) planes, anisotropically
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Fig. 10. Simple micropump design with piezoelectrically actuated membrane and bulk micromachined silicon diusion valves. The arrows explain the operating principle in which a big arrow pointing in or out the outlet means high ¯ow and a small arrow means weak ¯ow. The sum of the ¯ows leads to a transport of liquid/gas from outlet B to A.
etched grooves and membranes like the ones shown in Fig. 7 can be realized in a quite simple apparatus [49]. Nowadays it is also possible to etch silicon anisotropically not dependent on the crystal orientation in DRIE-reactors. These processes allow small structures (>2 mm) to be etched with high aspect-ratios (>15) and a very good anisotropy (>99%) [50]. Fig. 8 shows silicon structures anisotropically etched by KOH that belong to a micromachined inkjet printhead, often called bubble jet or drop-on-demand print head which can be seen in Fig. 9. The inkjet print head combines micromechanics, heating actuators, temperature sensors, channels and nozzles with a smart CMOS
Fig. 11. Bulk micromachined, piezoelectric micropump [54].
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Fig. 12. Single layer surface micromachining process for ®xed and movable structures.
circuit including signal processing [51]. The outside dimensions amount to 4.9 4.0 mm2 with nozzle dimensions of 20 40 mm2. Droplet velocities between 10 and 15 m/s and droplet masses between 60 and 110 ng meet conventional inkjet printer requirements. Other MEMS fabricated using bulk-micromachining technology, like valves and micropumps have found their way into industrial applications since many years. Fig. 10 shows the schematical structure of a micropump with piezoelectric actuation and bulk micromachined silicon diusion valves. But also designs with watertight and/or airtight valves and integrated or hybrid-assembled ¯ow sensor and control unit are available [52]. The ¯ow rates range from single drops in the sub nanolitre range up to several ml per min. The substrates (glass+silicon or silicon+silicon with intermediate glass layer) are anodically bonded, leading to a stable covalent bond between both materials [53]. Fig. 11 shows the Debiotech (Switzerland) micropump developed to be the heart of an implantable drug infusion system [54]. The device is based on silicon bulk micromachining, silicon pyrex (glass) anodic bonding and piezoelectric
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Fig. 13. Surface micromachining of rotational micromotors (cross section).
actuation. The pumping mechanism has been designed for maximum safety and reliability together with high open-loop accuracy (210%) in a low ¯ow rate range (0±100 ml/h). The overall size is 16 12 mm2.
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Fig. 14. SEM photograph of the central bearing of a surface micromachined rotational actuator.
3.2. Surface micromachining Surface micromachining mainly makes use of polysilicon as the ``active'' material (mechanically and electrically) and silicon-oxide as the sacri®cial layer. Several active and intermediate sacri®cial layers can be grown one on top of each other on a silicon wafer. Another possibility is to use aluminium as the active layer and organic compounds (polyimid, photoresist) as the sacri®cial layer. The layers are patterned by classical photolithographical processes and wet or plasma etch. Finally the sacri®cial layer(s) are etched away to release the structures [55].
Fig. 15. SEM photograph of the driving poles of a surface micromachined electrostatical micromotor. The poles are connected by polysilicon microbridges (see arrow).
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Fig. 16. SEM-picture of the micro-shutter fabricated using surface (motor) and bulk micromachining technology (aperture).
Surface micromachined structures are always built upwards and remain on the surface of the substrate during the whole fabrication process and in the application. Fig. 12 shows the main fabrication steps of a double clamped beam (bridge) using surface micromachining technology. Even complicated mechanical components, like beams, guide ways, bearings, hinges and locking mechanisms can be realized by choosing a suitable combination of layers and their appropriate processing. Many of the micromotor
Fig. 17. (a) Conventional apparatus for a temperature radiation measurement unit with pyroelectric detector, electromagnetic motor and chopper disk; (b) its micro-electro-mechanical counterpart which integrates the motor, the chopper disk and the aperture hole in a single device contained together with the pyroelectric detector in a single housing.
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Fig. 18. Output signal of the pyroelectric detector when excited by heat radiation. The radiation is modulated by a surface micromachined chopper turning 180 rpm.
designs presented in Section 2 have been realized using surface micromachining, which necessarily leads to a multi-layer surface micromachining process. Fig. 13 schematically shows the fabrication steps for an electrostatically driven rotational micromotor with central bearing. The bearing clearance is only 400 nm. The distance between stator and anchor poles amounts to 2 mm with a polysilicon layer thickness of 2 mm. Figs. 14 and 15 show close-ups of the central bearing and the poles.
Fig. 19. SEM photograph of a pop-up micromirror [60,61].
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Fig. 20. Partial view of a patterned photoresist mould for a synchronous micro linear actuator in gold electroplating technology.
An SEM-photograph of the whole device can be seen in Fig. 16. Fig. 17 illustrates its application as a micro-shutter for the modulation of radiation in optical or thermal detectors. The rotor diameter of the microshutter amounts to between 800 mm and 1200 mm. An aperture of 100 200 mm2 to 200 300 mm2 is etched from the backside of the substrate with bulk micromachining technology. Parts of the rotor and the whole backside are coated with a thin gold layer to reduce IR transmittance. The shutter is applied in a pyroelectric detector with numbers of revolution ranging from 0 rpm up to 7000 rpm. The minimum driving voltage is 35 V [56]. Pyroelectric detectors only generate output signals when they have excitation and refreshing cycles, i.e. when the radiation is chopped. Only 180 revolutions (rpm) are necessary to achieve an output signal like the one shown in Fig. 18.
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Fig. 21. Partial view of the electroplated gold structures that were grown into the mould shown in Fig. 20.
The linear stepping microactuator presented in Section 2 (Fig. 5) has an overall dimension of 1 2 mm2, yielding ``high'' forces (some 10 mN) and ``large'' displacements (>100 mm) with an accuracy in the nm range which are limited only by the length of the central beam. Surface micromachined devices can be made quite large in the lateral dimension (up to 4 mm2), but the maximum height amounts to between 2 and 10 mm, because the thicknesses of the layers are limited [57,58]. A way out is oered by pop-up-structures realized by hinges and connecting rods [59]. Fig. 19 shows a two axis micromirror for an optical scanner which is tilted by comb drives. 3.3. Moulding Moulding processes can be divided into three main groups: classical
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Fig. 22. Steps of the HEXSIL process.
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Fig. 23. Partial view of a micro-tweezer fabricated using the HEXSIL process [65].
electroplating, LIGA and HEXSIL processes. All moulding technologies allow high aspect ratios and a structure height of more than 100 mm, but the mechanical structures are more simple and less diverse compared with surface micromachining [62]. 3.3.1. Electroplating Classical electroplating technology uses photoresist or other photo-structurable, organic materials for the female form of the structures. They are deposited on various substrates with low resistivity or metal coating (starting layer) [63]. After the moulds are patterned, metals like Au, Ni, Cu or alloys are grown into the moulds by electrodeposition starting from the metal layer. Like surface
Fig. 24. Process steps of a suspended structure using LIGA technology.
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Fig. 25. SEM photograph of an electromagnetically actuated micro-relay fabricated using the LIGA process [67].
micromachining technology, sacri®cial layers can be used to realize bridges, suspending beams or sliding anchors. The structures are freed by stripping the photoresist. Each mould can only be used once. Figs. 20 and 21 show a partial view of a patterned photoresist mould and the gold structures after the plating process and resist stripping took place.
Fig. 26. Three arrays of electrostatically actuated micromirrors for applications in colour projection displays [68].
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Fig. 27. Schematical view of a micromirror with electrostatical actuation.
3.3.2. HEXSIL The HEXSIL process was developed at UC Berkeley. It uses a low-pressure chemical vapour deposition (LPCVD) process and silicon moulds fabricated by deep reactive ion etching (DRIE) of silicon wafers. The devices do not remain on the substrate where they were fabricated, so that the moulds can be used several times. Fig. 22 shows the steps of HEXSIL processing. An SEM photograph of a HEXSIL micro-tweezer is shown in Fig. 23. It is actuated by thermomechanical forces and has an overall dimension of 2 mm 1.4 mm 80 mm. An application for micro-tweezers will be the handling and assembling of microparts to build up microsystems with growing complexity [64]. Another future operational area could be in the ®eld of minimal invasive surgery. 3.3.3. LIGA The LIGA process [66] (RoÈntgentiefenLithographie, Galvanoformung and Abformung=deep x-ray lithography, electroforming and moulding/embossing) was developed at Forschungszentrum Karlsruhe (FZK), Germany. It starts with a thick layer (up to several 100 mm) of PMMA (polymethylmethacrylate, plexiglass) which is deposited on a metal substrate. The structures are patterned by parallel and high energy x-ray lithography. The gaps are ®lled up with metal (Au, Cu, Ni, NiFe etc.) by electrodeposition. The electroforming process starts on the metal substrate. Either the PMMA or the released metal structures can be used as the female mould. Mass production of polymer, metal and even ceramic microcomponents makes use of hot embossing and injection moulding. Cantilever LIGA-structures can be realized by sacri®cial layer etching (e.g. titanium). Fig. 24 shows the process steps of a suspended LIGA-microstructure. Fig. 25 shows an SEM photograph of an electromagnetic micro-relay fabricated using FeNi (permalloy) in the LIGA process. Much eort is made to develop mSMD compatible, fast switching low-power micro-relays for applications in e.g. telecommunication and automated test equipment (ATM). A growing number of devices are fabricated using a mix of several technologies. Often CMOS circuits are integrated to create a smart microsystem. Microsensors,
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IC signal processing and microactuators are combined on a single chip allowing the system to step into interaction with the environment. Compatibility of processes plays an important role. Hot processes like surface micromachining, diusion or annealing have to be performed ®rst. Bulk micromachining processes are implemented last, because they are cold (60±908C) and produce deep grooves with high aspect ratios that mostly do not allow any further lithographical steps. Very complex microsystems have been fabricated and brought to market like the Texas Instruments digital micromirror device (DMD) pixel array shown in Fig. 26. A schematical view of an electrostatically-driven mirror which represents one single pixel is shown in Fig. 27. A light source illuminates the micromirror array in such a way that the light beams are de¯ected either to the screen or not, depending on the tilting state of the mirror (in plane or out of plane). Each pixel on the screen can thus be switched on or o by actuating the appropriate mirror, leading to a digital image. RGB light beams and three DMD arrays (or one DMD array together with serial colour modulation) allow the screening of a coloured image. The devices are used in desktop digital light projectors, featuring up to 1280 1024 pixels. The advantage compared to conventional technology is that a bright light source can be used thus achieving a brightly shining image. 4. Conclusion Micromachined actuators are no longer just designed in research laboratories to show state of the art technology. Some applications have been found in the last ten years and put to a rapidly growing market. However microsystem technology is still in the initial phase of its development. The future of microactuators will be marked by new applications that have sophisticated requirements concerning small size, low weight, low power consumption, high shock resistance and high mechanical cut-o frequency. Already today, ®rst applications have been implemented that would have been inconceivable with conventional techniques. Microactuators cannot be designed just by scaling macro- or miniactuators, and the intention of researchers is not to replace conventional technology. They oer new solutions for applications and enlarge the ®eld of actuators, which will lead to a coexistance of micro- and conventional actuators. It is clearly essential, that only micromachines will ®t into small environments and that, for example, a small manipulator can handle microobjects much more gently and dexterously than its macro counterparts. With microactuators, ®ner positioning in shorter response time is possible than with macroscopic machines. Only a few statements can be made in order to decide about an actuation principle or microactuator design, because their usefulness is strongly coupled to the application. It has been shown that various types of actuation principles and dierent fabrication technologies are available. They can be combined to achieve the required functionality, yield and costs. Still a set of problems is unsolved. Friction, wear, sticking and fatigue reduce the life time and reliability. Much more eort will be required to introduce new materials, designs and technological concepts to solve these and future problems. The fusion of knowledge from many dierent disciplines is essential for
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a well-balanced and accelerated growth of microsystem technology and will allow new applications in further technical, medical and biological areas.
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