Keynote Papers

Keynote Papers Presented at the Opening Session

Assembly of Microsystems

H. Van Brussel l)l, J. Peirs, D. Reynaertsl,A. Delchambre2, G. Reinhart M3, N. Roth

Katholieke Universiteit Leuven, Leuven, Belgium

M. Weck Division Production engineering. Machine design and Automation, Dept. of Mechanical

lI5, E. Zussman (216 ngineering,

2Universite Libre de Bruxelles, Faculte des Sciences Appliquees, Campus du Solbosch, Brussels, Belgium 3Technische Universitat Munchen, iwb-Lehrstuhl fur Montagesystemtechnik und Betriebswissenschaften,

Garching, Germany 4Siemens AG, CorporateTechnology/Production Processes, Section of Mechatronics Production, Munchen, Germany

5University ofTechnology (RWTH) Aachen, Chair of MachineTools, Lab. of MachineTools and Prod. Eng. (WZL), Aachen, Germany

6Dept. of Mechanical Engineering,Technion - Israel Institute ofTechnology, Haifa, Israel

Abstract In the microworld, as well as in the macroworld, assembly is a crucial operation in the genesis of a product. This keynote paper focusses on the assembly problems occurring in the manufacturing cycle of microsystems. Scaling effects make that the assembly problems are different in the microworld. The different assembly operations and techniques, like manipulation by physical contact, non-contact manipulation, smart assembly techniques, and joining methods are thoroughly discussed. Finally, some relevant examples of micro-assembly systems and of assembled microproducts are given.

Keywords: Assembly systems, Miniaturisation, Micro-electromechanical systems

1 TERMS OF REFERENCE Where natural creatures grow from a single cell, man- made artifacts, like industrial products, are mostly assembled from different components. Assembly is therefore a very important process in the genesis of a product. There are many reasons why products may consist of different parts. Different functions require different materials in one and the same device. In mechanisms, pivots require flexible materials while the links themselves must be stiff. Electrical actuators are made of conducting as well as isolating materials. The rotor and the stator of a motor are different parts by their very nature. Products may be too complex to be produced as a single part. Some parts are wearing out and must be regularly replaced. The abovementioned arguments are true for macro- as well as for microproducts. This keynote paper focusses on assembly problems occurring in the manufacturing cycle of microsystems. For our considerations, microproducts have dimensions in the millimetre range and below. We do not include structures in the nanometre range. Miniaturisation of electromechanical systems is a hot issue in modern technology. The resulting products, called micro-electromechanical systems (MEMS), are claimed to have a vast - yet virtually untapped - potential. Although the field is still to mature, a multibillion dollar market is predicted [l]. It therefore belongs to the mission of a scientific organisation like CIRP to closely monitor the evolution of this emerging technology. A quick survey of past publications, in the ClRP Annals, on manufacturing issues of MEMS yields only a small harvest [65,66,76,95,97,98,100]. The many contributions, from ClRP members, to this keynote paper however indicate a vivid interest in microsystems technology (MST) within the ClRP community. To miniaturise a product it is not sufficient to simply reduce its dimensions. A lot of new problems emerge, related to scaling effects, manufacturing problems and, of course, assembly problems. One of those problems has

to do with the fact that, up to now, nearly all production techniques for microsystems have their origin in the microelectronics technology, and are essentially 2D- processes. Assembly is required to realise 3D-parts out of parts made by those 2D manufacturing methods. Assembly further allows the manufacture of objects consisting of parts requiring incompatible technologies. For instance, processing of GaAs is not compatible with Si processing, especially with the high-temperature oxidation and dopant diffusion steps that destroy the GaAs material and contaminate the Si circuitry and furnaces. Through an assembly step, GaAs optoelectronic devices can be integrated on Si VLSl circuitry. Assembly also allows the choice of the most optimal production method for each part. For instance, it is difficult to produce high-quality-factor coils on-chip, mainly due to the difficulty to create the third dimension on-chip. By producing these coils by conventional winding and by assembling coil and actuator afterwards, both quality and production cost can be improved. The paper is organised as follows. The typical features of assembly in the microworld are outlined in section 3. Section 4 classifies the assembly systems relevant for microsystems. Manipulation issues are crucial to assembly ; manipulation with physical contact is explained in section 5, while section 6 deals with non- contact manipulation. Some so-called ‘smart’ assembly techniques, particular for microsystems, are dealt with in section 7. Joining of microparts is considered in section 8. Some industrial examples of micro-assembly systems and of assembled microproducts are given in sections 9 and 10 respectively. Some closing remarks conclude the paper.

2 MARKET PROSPECTS FOR MEMS AND MST [2] In 1998, NEXUS (The European Network of Excellence in Multifunctional Microsystems) established a task force entitled ‘Market Analysis MST’ to prepare an applications- oriented in-depth analysis of MST markets from 1996 through the year 2002. The total world market for

Annals of the ClRP Vol. 49/2/2000 45 1

microsystems is expected to grow fiom 14.4 billion Euro in 1996, to 38 billion Euro by the year 2002. This reflects growth rates of 18% per year. The top seven markets in 2002 are predicted to be: 0

0

0

0

0

0

Six main application domains have been identified: 0 IT peripherals 0 Medicalhiomedical applications 0

0 Automotive applications 0 Environmental monitoring Products that have a high probability of being on the market by 2002 are found to be: 0 Drug-delivery systems 0 Optical switches 0

0 Magneto-optical heads 0 Projection light valves 0 Coil-on-chip 0 Micro-relays 0 Micromotors Integrating silicon micromachining with CMOS technology remains difficult, while hybrid (multichip) microsystems have gained in importance. Microstructuring, not only in silicon, but also in polymers, metals, and ceramics has been enabled by advances in microfabrication techniques. These latter include laser machining, high- aspect ratio microreplication based on lithographic patterning, electrodischarge machining (EDM), diamond milling and other precision mechanical removal processes. There is a need for fully-3D batch production micromachining processes. Micro-EDM, combined with micro die casting may provide a solution.

Hard disk drive heads (12 billion Euro) lnkjet printer heads (10 billion Euro) Heart pace makers (3.7 billion Euro) In-vitro diagnostic devices (2.8 billion Euro) Hearing aids (2 billion Euro) Pressure sensors (1.3 billion Euro) Chemical sensors (0.8 billion Euro)

Industry and automation (including aerospace)

Lab-on-chip systems (DNA, HPLC, ..)

3 ASSEMBLY PROBLEMS IN THE MICROWORLD The main difference between macro- and micro-assembly is the required positional accuracy of automatic assembly machines. In the macroworld, a precision of a few hundred microns is typical for serial link robotic manipulators with four to six axes. In the microworld, submicron precision is often required, comparable to wafer stepper precision. This degree of precision is beyond the calibration range of conventional open-loop precision assembly devices used in industry. Closed-loop strategies are required to compensate for poor kinematic models and thermal effects. Real-time vision feedback is perfectly suited for this application. Moreover, a manipulator should have a 3D workspace instead of the wafer stepper's 2D workspace, making the manipulation problem considerably harder. Obtaining accurate sensor information to close the loop is also difficult as sensors can be too bulky to be placed on tiny precision instruments and they have to be extremely sensitive as forces and displacements are very small. The alternative of image processing has its problems too: it is slow, costly, difficult to program, and susceptible to reflection, light condition, colour changes, ... Moreover, the view may be obstructed by tools that are orders of magnitude larger than the parts being handled, or the lenses, cameras and other optical instruments may obstruct proper manipulation of the object, especially as multiple optical axes are required for real 3D manipulation. Thus

to enable micromanipulation, miniature sensitive sensors and extremely accurate 3D robotic manipulators have to be built. A second major difference between assembly in micro- and macrodomains is the mechanics of object interactions. In the macroworld, the mechanics of manipulation are predictable, e.g. when a gripper opens, gravity causes the part to drop. In the microworld, forces other than gravity dominate due to scaling effects. Surface-related forces, such as electrostatic, van der Waals and surface tension forces become dominant over gravitational forces. Mass decreases with L3 while stiffness for bending and tensile strength are proportional to L and L2 respectively. Due to this unevenly scaling behaviour, manipulation in the microworld is completely different from manipulation in the macroworld. Manipulation in this 'strange' world, therefore, requires training of the human operator. A difficult problem in manual handling in the microworld is the loss of direct hand-eye co-ordination. The microscopes and tools limit the ability to directly see and sense the objects to be handled. The tools used to manipulate the objects have less degrees of freedom than the human hand and there is no force feedback. The operator's view is restricted for a number of reasons. First, the high magnification restricts the view to a very small area, such that the operator lacks global information about the object. Therefore, variable magnification in a wide range is indispensable. Second, in an optical microscope with very high magnification, the depth of focus is often very short in comparison with the width of the field of view. The limited depth of view impedes clear images of non-planar objects or moving or vibrating structures because the structure moves in and out the focussed plane. Third, the working distance (the distance between objective lens and object) also tends to be short. This hinders manipulation of objects and tools. Weck et al. [3] built an assembly system in the vacuum chamber of a large-chamber scanning electron microscope, with a volume of nearly 2 m3. The main disadvantages of this solution are its high price and the long evacuation time. A big issue is also the trade off between field of view and resolution. An elegant solution of this problem is the so called 'eye on the hand', i.e. a camera is mounted on the arm of the robot [IOO] or in the gripper [3]. The main advantage of this solution is that the tool centre point is always in focus. The main challenge is then to fabricate a small-sized camera with a lean structure. Weck et al. [3] solved this problem by integrating a miniature endoscope into the gripper. Finally also the cost of the manipulation has to be considered. Most microparts are produced in batch processes, with hundreds or thousands of them on a single wafer. This massively parallel production mode is the main factor for cost reduction. This cost reduction is one of the major reasons for microsystems to replace their macroscale counterparts. When assembly of these microsystems is performed one by one, be it manually or automated, it will considerably increase the production cost. Therefore, massively parallel micro-assembly systems may be required.

4 MICRO-ASSEMBLY SYSTEMS The dimensional and physical mismatch between the 'micro- and macroworld and the required precision, operator stress and eye strain associated with assembling minute parts under a microscope ask for advanced micro-assembly tools.

452

Keynote Papers

4.1 Master-slave systems As the assembly and grasping forces are too small to be sensed by a human operator, manual assembly with tweezers is performed purely based on visual feedback. In the macroscopic world a human operator uses vision for coarse and non-contact servoing and combines it with force feedback for accurate positioning under contact. A good micro-assembly system should measure these extremely small forces with microsensors and feed it back to the human operator. A master-slave macro-micro tele- operation system has been built and tested by Kaneko et al. [4]. The position of the master arm, manipulated by the operator, is scaled down and used to control the position of the slave. The forces measured by the slave are amplified and applied to the master arm through some transfer function. How to set up these transfer function gains to provide the operator with a natural sensation remains to be investigated. From an intuitive point of view however, force scaling should not be dynamic as this introduces lead or lag between input and output signals. This would make it difficult for the operator to tele- manipulate the objects with real-time sensation, and could cause the slave to break or damage the objects. Figure 1 illustrates the layout of a tele-micro-surgical system used in an experiment of suturing an artificial blood vessel of 1 mm diameter [ 5).

Internet 700km Okayama Tokyo acccSS point access point

Surgcry site Operation room

Figure 1: Tele-micro-surgical system [5].

4.2 Automatic assembly machines To reduce the assembly cost, automatic micro-assembly machines might be a good solution. Zhou et al. [6] use a combination of visual and force feedback to control the grasping force. Visual servoing allows for controlled motion at mm/s speeds and submicron repeatability. However, nanometre repeatability cannot be achieved with visual feedback alone. Force sensing yields much higher resolution, but works only when in contact with the object. A combination of both force and visual feedback allows fast movement in free space without the danger of large impact forces when the contact is made. Contact forces of 2 nN with impact forces of 9 nN were achieved for micropart approach velocities of 80 pm/s. Feddema et al. [7] use a CAD-driven technique, where the objects, their position and orientation are recognised by comparison with a synthetic image from CAD. Diffraction and out-of-focus effects are added to the synthetic image to make it more resemble the expected real image. A modular micro-assembly system with 4 degrees of freedom is shown in figure 2 [8]. It consists of an x-y positioning table, above which an overhead manipulator and a stereomicroscope with a CCD camera are located. The overhead manipulator can move along the z-axis and rotate around this axis; it can be equipped with different grippers or applicators by an automated turret tool changer. The positioning accuracy of the micro-assembly

system is 2 pm. It is used to assemble micro-optical duplexers, which consist of two 0.9 mm spherical lenses, a 3x3~1 mm3 wavelength filter, and a glass fibre cable.

Figure 2: Modular micro-assembly system [ 81.

4.3 Assembly by microrobots As tolerances in micromechanics lie in the nanometre range, assembly robots should be very precise. The manipulation accuracy of conventional robots is mechanically limited, since disturbing influences which are often negligible in the macroworld, such as fabrication defects, friction, thermal expansion or computational errors, play an important role in the microworld. Furthermore, these robots are subject to mechanical wear, and must undergo regular maintenance and calibration, which makes them expensive. As an alternative, Fatikow [9][10] proposes micro- assembly with microrobots. Figure 3 illustrates the approach. The microrobots, 50 to 80 mm in size, stand on piezoelectric legs and move based on the stick-slip principle. This allows very fine resolution down to 10 nm, while speeds up to 30 mmls can be obtained. The robots move on a glass plate with three degrees of freedom (two translational and one rotational degree of freedom). They are equipped with a gripper which has three rotational degrees of freedom, such that the robots can reach any point in the workspace. The tools can be easily exchanged.

I _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Figure 3 Micro-assembly with microrobots [ 101.

The glass plate is mounted on an x-y positioning table such that each assembly cell can be brought in the field of view of a microscope. A camera mounted on the microscope forms a sensor system for fine positioning. Coarse positioning of the robots is supervised by a second camera and a laser measurement system. The power of this system lies in the flexibility of the microrobots. Different robot types can operate on the same platform at the same time, each with its own

453

specialisation. One robot can transport parts while another performs assembly, or they can cooperate during an assembly. Furthermore, the robots and their tools can be easily exchanged.

5 MANIPULATION BY PHYSICAL CONTACT Typical problems in assembly are related to the way the part can be picked up, how it can be positioned and how it can be released.

5.1 Sticking effects in microparts handling When parts to be handled are less than one millimetre in size, adhesive forces between gripper and object can be significant compared to gravitational forces. These surface forces can be used in grippers as an adhesive force to pick up the object, but as these forces are almost not controllable, they are more likely to disturb the process rather than to help it. As the gripper approaches, the object can jump off the surface into the gripper, with an orientation depending on the initial charge distribution. When the part is placed at the desired location, it may adhere better to the gripper than to the substrate, preventing accurate placement. These adhesive forces arise primarily from electrostatic attraction, van der Waals forces and surface tension. The balance between these forces depends on the environmental conditions, such as humidity, temperature, surrounding medium, surface condition, material, and relative motion. Tsuchitani et al. [11,12] have studied the surface forces in microstructures. They concluded that the dominant surface force in usual microstructures is (1) the liquid bridge force due to the capillary condensation of the water when the humidity of the atmosphere around the two contacting surfaces is high (over 60% RH); (2) the hydrogen bonding force between water molecules adsorbed on the two surfaces when the humidity is relatively low; and (3) the van der Waals force when adsorbed water molecules on the surface have almost disappeared. Sticking effects are not only problematic for assembly, they also create problems during production of microstructures. For instance, surface machined cantilever beams can stick to the substrate after removal of the sacrificial layer. The surface tension of the rinse liquid is sufficiently strong to pull the suspended cantilever in contact with the substrate, leading to permanent adhesion by van der Waals bonding.

Electrostatic force The electrostatic forces arise from charge generation (triboelectrification) or charge transfer during contact. The force per unit area (pressure) for parallel plates is:

with E is the permittivity of the dielectric, E the electric field strength and os the surface charge density. At atmospheric pressure and centimetre-size gaps, the breakdown strength of air (about 30 kV/cm) limits the maximum charge density to about 3 x C/m2, or peak pressures of about 50 Pa. However, at very small gaps of the order of 1 pm (less than the mean free path of an electron in air), fields of two orders of magnitude higher have been observed. With good insulators such as smooth silica and mica, the charge density can rise up to 10 mC/m2 with pressures in the order of 1 MPa at 1 pm distance [ 131. When two materials with different contact potentials are brought in contact, charge flows between them to

equalise this potential. Consider two metal spheres (insulated from their surroundings) brought into contact, then slowly separated. With a contact potential of 0.5 V, the initial charge density will be about 4 mC/m2, with field strengths of 5 MV/cm [13]. For small gaps (order 1 nm), electron tunneling and field emission will transfer charge, and for larger gaps (order 1 pm) air breakdown can occur. In principle, using conductive grippers can reduce static charging effects. However, the objects to be handled, such as silicon parts, may be covered with insulators, such as native oxides. On silicon, up to 1 nm of native oxide can build up after several days in air at room temperature. This native oxide is a very good insulator and can withstand a maximum field strength of up to 30 MV/cm [13]. This implies that significant amounts of charge can be stored in the oxide. With the permittivity of silicon (E = 3.9 EO), peak pressures are of the order of 100 MPa. When a grounded gripper grasps an initially charged object, charges will be induced in the regions of the dielectric that are not in contact. Surface roughness can prevent charge neutralisation through intimate contact of oppositely charged regions. The residual charges can cause adhesion.

Van der Waals force The van der Waals force (sometimes called London's or dispersion force) is the force that holds together the inert gas crystals and many plastics. It is a much weaker bond than ion and covalent bonds. Many molecules that are too stable to become an integral part, interact with each other through the van der Waals force. Van der Waals forces are due to instantaneous polarisation of atoms and molecules when they are set close. The force between a sphere and a flat gripper can be approximated by [ 141:

Hr 62 * F =-

wlw

with H the Hamaker constant, z the distance between the surfaces, and r the radius of the sphere. When the atomic distance goes to zero, the attractive forces change into strongly repulsive forces. Therefore, the minimum distance between the surfaces is commonly assumed to be approximately 0.2 nm. For example, the force between a SiO2 sphere 100 pm in diameter and a silicon gripper equals 2.3 pN [15]. Of course, this formula assumes atomically smooth surfaces as the van der Waals forces fall off very rapidly with distance. For that reason, F. Arai et al. [16] cover the gripper surface with small pyramids. The pyramids are made by anisotropic etching of Si and are placed at intervals of 10 pm and are a few micrometre wide and high. A second advantage of the pyramids is the self-discharge possibility due to the high electric field strength at the tips. Sharp tips enhance discharge and reduce contact area but may damage the surface. Small particles scraped off by the micropyramids can contaminate the environment. The micropyramids are coated with a thin metal layer to enhance the effect.

Surface tension When an object is exposed to the environment, a thin film of water is formed on its surface by adsorption of moisture. When two objects are brought together very closely, the films touch first and melt together. Due to the surface tension, the objects are pulled together. High humidity, large radii of curvature, long contact times and hydrophilic surfaces increase the adhesion force. The Kelvin equation (171 gives a measure for the minimum distance that should be kept between surfaces as a function of the relative humidity in order to prevent the formation of a stable layer of water between the two objects.

454

Keynote Papers

This moisture layer causes several problems: the surfaces are pulled together prohibiting proper functioning of mechanisms or the release of objects from grippers, electric short circuits occur, and friction increases. The solution consists of removing the water by drying techniques such as critical point drying and freeze-drying. Also dimples and sharp corners on the contact surfaces can be used to reduce sticking. But this can only remove moisture due to fabrication methods, for instance after rinsing in sacrificial layer removal techniques. When the microstructure is not enclosed in a sealed container, water vapour can also condense on it during use as the native oxide layers of silicon and other non-precious metals are hydrophilic. An effective way to avoid in-use sticking is the application of hydrophobic coatings. The droplets will then exert a repulsive force instead of an attractive force. To be effective, the contact angle between water and coating should be greater than 90". Several hydrophobic coatings have been studied such as the self-assembled monolayer coating (SAM), silicon- hydrogen bonds using ammonium fluoride treatment and plasma polymerisation of fluorocarbon films. A more detailed overview of several surface forces can be found in [ 181.

Comparison Figure 4 shows the different sticking forces and the gravitational force as a function of the object radius. It is assumed that the object is a silicon sphere picked up by a gripper with flat jaw surfaces. For accurate placement, adhesion forces should be an order of magnitude less than gravitational forces. Surface tension forces dominate and must be avoided. Van der Waals forces can become significant below about 100 pm radius and generated electric charges from contacts could prevent reliable manipulation for parts less than 10 pm in size. While figure 4 shows electrostatic forces to be the least significant adhesion force, it can be argued that it is actually the most significant force for grasping and manipulation of 10 pm to 1 mm parts. First, the van der Waals force is only significant for gaps less than about 100 nm. Unless the objects are very smooth, the effective distance between the object and the gripper will be large except at a few points of contact. Second, actual contact with a fluid layer needs to be made for surface tension to be significant, and a dry or vacuum environment could be used to eliminate surface tension effects. As the electrostatic forces can be active over ranges of the order of the object radius, surface roughness is much less influential for electrostatic forces than for van der Waals forces.

Forces acting on microstructures - lE-03 ~

1E-04 -

1E-10 - -x-Ftens (N)

0.001 0.01 0.1 1

Radius (mm)

Figure 4 : Adhesion forces acting between spherical object and gripper compared to gravitational force [ 19).

Conclusions By making the proper choice for the gripper materials and geometry, adhesion can be reduced. The following rules can be derived from the above discussion [ 131:

Use materiais with a small contact potential difference between gripper and object. Use conductive materials which do not easily form highly insulating native oxides. Keep the contact area small. Therefore, spherical fingertips are preferred above planar ones. The contact area can also be reduced by increasing the roughness of the gripper. This will considerably reduce the van der Waals forces. High contact pressures, caused by the adhesion forces, can cause local deformations at the contact site (Hertzian deformations). This deformation will increase the contact area and hence the net adhesive force. Therefore, hard materials are preferable. The surface tension effect can be reduced with a dry atmosphere and hydrophobic coatings. An attractive alternative is assembly while immersed in a fluid, which eliminates electrostatic and surface tension effects [20]. The surface tension effect can also be used to help parts adhere better to the target location than the gripper. Free charges such as in ionised air can combine with and neutralise exposed surface charges.

5.2 Releasing To release an object from the gripper, if the gravitational forces are dominant, the object will drop by itself on the right place. Otherwise, there are several methods that can be used [21]:

Glueing the part to the substrate on the right place [21 I. Releasing the component by positive mechanical engagement [21][22]. Injection of gas: a small puff of gas pushes the object while removing the grippers[21][23]. This technique is especially suited for vacuum grippers. Mechanical release mechanism with needle: the needles push the object and since the contact surface between the object and the needles is very small, the gravity becomes dominant again and the object will stay in place when removing the needles.

Destruction of the gripping mechanism. For instance, with a gripper using the surface tension force to pick an object, the object can be released by heating the gripper and evaporating the adhesive liquid. Another example is the ice microgripper described in [ 241. Vibration of the gripper [25]. Mechanical release by stripping off against a sharp edge [23]. Using the adhesion effects described above: the adhesion between the substrate and the microcomponent must be greater than that between the microcomponent and the gripper [ 231. To remove the object from the gripper, the gripper can make a rolling motion as shown in figure 5. To pick up the object, the tool is brought into contact with the object with its flat side. If the adhesion forces between tool and object are larger than those between object and surface, the object will adhere to the tool. To release the object from the tool after

[211[231.

455

positioning it on the surface, the' tool makes a rolling motion such that the object makes contact with the tool's edge. The reduced contact area causes less adhesion force such that the object remains on the surface.

1 I ........ ............. I l... -. - ........ J . ..... .............. i i !

Pick up POsitMn Release

Figure 5 : Releasing an object by rolling motion.

5.3 Mechanical grippers Mechanical grippers are traditionally used for manipulating large, macroscopic objects. In the microworld, mechanical grasping induces a number of problems: too high forces damage the object or the object may jump away and get lost: too low forces may also lead to lost parts. Therefore, the force applied by the gripper should be precisely controlled. The best way to grasp micro-objects is to use a microgripper where grasping forces and jaw size match the requirements. Many kinds of microgrippers have been built. Figure 6 shows an electrostatically driven gripper which has a total length of 400 pm and a thickness of 2.5 pm [26]. The gripper closes completely by applying a voltage of 45 V. Theoretically, a maximum force of 0.1 pN is obtained at 50 V. The gripper has been used to seize various microscopic objects including 2.7 pm diameter polystyrene spheres, dried red blood cells, and various protozoa. Sticking problems have been observed. Microgrippers are very fragile and should be used, stored and transported with care. The gripper in figure 6 for instance can already break by air movements stronger than those of normal handling.

Figure 6: Electrostatic microgripper [ 261.

Hydraulically or pneumatically driven grippers are found in Suzumori et al. [27] and Peirs et al. [28]. In the first one, one of the jaws is a flexible pressure chamber that bends when pressurised, pressing against the other, rigid jaw. The prototype is 8 mm wide, 18 mm long and has a gripping force of 2N. The gripper of Peirs et al. [28] consists of a piston driving both jaws simultaneously via linkages. The prototype has a diameter of 5 mm, is 18 mm long and generates a gripping force of 5 N at 10 bar.

It has a Hall sensor incorporated to measure the gripper opening. Other designs use a kind of chop sticks attached to a piezoelectric Stewart platform. Tanikawa et al. [29] use two six-degrees-of-freedom Stewart platforms. Each platform drives one stick, which is in fact a glass needle. Glass balls having a size of 2 pm can be picked up, placed down and rotated with a positioning accuracy of 0.1 pm. Kallio et al. [30] use a three-degrees-of-freedom Stewart platform. In [31], Weck proposes two types of microgrippers. They consist of two 0.2 mm-thick arms made out of copper- beryllium alloy and holding the two jaws achieving the gripping. Those arms are fixed to a plate made of non- magnetic high-quality steel. The jaws can be designed in different shape and material. For instance, in [31] they used 50 pm wide and 10 pm thick tweezers fixed to the arms by use of a resin adhesive. The material for those tweezers can be either titanium or high-quality steel. A U- shaped spring is placed between the two arms and causes the opening of the jaws. The deformation of the spring was made through a force applied exactly in the centre of the "U" stripe in order to obtain a symmetrical opening of the jaws. The force is obtained either byTg piezoelectric disk translator or a piezoelectric Picomotor (New Focus) (see figure 7). A - non-magnetic - CuBez alloy was chosen for the jaws.

Figure 7 : Top, gripper with disk translator as drive: Bottom, gripper with PicomotorTM as drive [31].

Carrozza et al. [32] and Ando et al. [33] built parallel jaw grippers driven by a piezoelectric element of which the motion is amplified through lever mechanisms with flexible hinges. Breguet et al. [34] cut the whole gripper out of a piezoceramic plate as shown in figure 8. The electrodes are designed to render active only the parts of the piezo which are underneath them. A piezoelectric force sensor can be easily integrated in one of the jaws. The gripper is about 3 cm long and opens f 18 pm at the tip for a voltage of 150 V. A similar approach is followed by Bellouard et al. [35] for the design of a SMA gripper. The whole gripper structure is cut out of a single SMA plate. One prototype uses two- way shape memory effect, the other, which is shown in figure 9, uses the elasticity of the structure itself as bias spring. The latter prototype uses local heating with a laser beam to actuate one part of the arm. The rest of the arm remains unaffected and acts as a bias spring. Hesselbach et al. [36] use two antagonistic SMA wires to deform an elastic structure with flexible notch hinges. The elastic structure is made of superelastic material. The

456

Keynote Papers

Figure 8 : Piezoelectric microgripper [34].

resistance of the SMA wire is used as a feedback sensor. By using an antagonistic design, a short closelopen time of 0.5 s is obtained. lkuta [37] uses a cantilevered finger driven by antagonistic SMA springs. By measuring the electric resistance and the position of the finger, the applied force can be determined without the use of a force sensor. The gripper has a size of 30 x 40 x 14 mm and has a minimum response time of 0.7 sec. An alternative design uses a SMA wire embedded in rubber [38]. As the wire is located eccentrically in the rubber, the rubber bends when the wire is heated.

Figure 9: Monolithic SMA gripper [35].

Other thermal grippers, based on differential thermal expansion, are made by Greitmann et al. [40] and Keller et al. [41]. Greitmann et al. use a thermal bimorph actuator for one jaw, while the other jaw is not actuated but contains piezoresistive sensors to measure forces. The gripper is about 1.5 mm long (bimorph only), has a maximum tip displacement of 0.8 mm, generates gripping forces up to 1 mN and measures forces with a resolution of 0.2 pN. The microtweezers of Keller et al. use a

Figure 10: Thermal microtweezers [39].

longitudinally expanding beam element that drives and elastic structure to enlarge the motion as shown in figure 1 Ob. Figure 1 Oa shows a variant that can be provided with different types of tweezer tips, depending on the application. The structure is about 2 mm wide and 9 mm long.

5.4 Adhesive gripper The disturbing sticking forces discussed in section 5.1 can also be used to build grippers. The van der Waals force is difficult to control and its influence should be kept as small as possible. The electrostatic force on the other hand, is relatively easy to control, but is not good for the manipulation of charge-sensitive lC devices. Surface tension forces due to air humidity can be controlled by incorporating a microheater in the gripper [15]. In the cold condition, an object can be picked up simply by touching it. To release the object, the heater evaporates the water in the contact. Although the above principle may work, most adhesive grippers have a dispenser in the gripper that forms a small drop at the gripper's surface, instead of using natural moisture layers. When brought in contact with the object, the capillary forces act perpendicularly to the gripping surface while the surface tension offers automatic centring of the component to the surface of the gripper. Besides the self-centring effect, the liquid bridge offers also compliance in the horizontal plane during assembly while stiffness in the vertical direction is large, quite similar to the SCARA-type robots. The adhesive gripper of Westkamper et al [42] uses alcohol and achieves position tolerances better than k 20 pm for 4.2 x 4.2 mm2 chips, without any active tolerance compensation. Figure 11 shows the different phases in the operation of an adhesive gripper.

1 Approach 2 Dispense

4 Centre 3 Establish contact

5 Hold 6 Release I -

t I l _ _ - i - &,ha -

- . . , .-

Figure 11: The different phases in the operation of an adhesive gripper [42].

Gripping is a dynamic process. Especially the spreading of the drop takes some time. This causes a difference in pulling force between approach and departure of the gripper as the wetting angle differs during the dynamic spreading and retracting of the liquid [43]. Quickly spreading fluids have greater, retracting liquids have lower wetting angles as in the static case. Even static

457

wetting angles can be influenced in that way. In most cases wetting angles are larger after spreading than after retracting. When the two surfaces come so close together that the fluid brims over, the meniscus changes from concave to convex and a pressure is generated instead of a pulling force.

5.5 Vacuum gripper A vacuum gripper is very simple as it consists mainly of a thin tube or pipette connected to a vacuum pump. This makes this kind of gripper cheap and easy to replace. This is important as microtools are fragile and have to be replaced frequently [23]. A particular problem when manipulating microparts is that the tube has to be very thin and, therefore, is easily obstructed by small particles. An interesting alternative is to generate the vacuum in the gripper itself as presented in [44]. The surface of the tip is covered by microholes of 10 or 15 pm diameter. The holes are made by isotropic etching of Si. Before contacting the object, the tip is heated. After contacting the object, the temperature is decreased and pressure drops inside the microholes with 0.1 8 bar according to calculations. The measured adhesion force was higher (= double), probably due to the contribution of other surface forces. To release the object, the temperature of the holes is raised again. This system realises quick response due to its small dimension. The gripper and the parts require smooth surfaces to prevent air leakage.

5.6 Ultrasonic travelling waves Objects can be transported on a travelling wave generated on a flat surface. Two types of waves are used: Lamb and Rayleigh waves. Lamb waves cause bending of the plate over its whole thickness (the substrate thickness is inferior to the wavelength), while Rayleigh waves only propagate in the surface layer of the plate. Objects placed on the surface move in the opposite direction of the travelling wave. The plate consists of a piezoelectric material on which interdigital transducer (IDT) electrodes are patterned which generate the travelling wave. Care has to be taken to avoid reflections at the plate border. The construction and working principle is identical to the linear ultrasonic travelling wave motor. Helin et al. [45] tested prototypes of Lamb and Rayleigh wave microconveyors working at respectively 10 and 20 MHz. A speed of 18 cm/s is reported. Motion control is obtained by adjusting IDT driving time. A driving time of 100 ps moves the object 20 nm, while a driving time of 50 ms moves the object 1 mm.

through the use of near-field levitation. Figure 12 shows non-tactile single wafer transport using ultrasonic travelling waves.

5.7 Distributed micromotion Distributed micromotion systems use arrays of tiny simple actuators that co-operate to move objects over relatively large distances and possibly in different directions and orientations. The actuators are often thermally activated cantilevers a few 100 pm in length. These actuators are called cilia in analogy to their biological counterparts which can be found for instance in the human respiratory tract. Cilia arrays can move small objects placed on them by synchronous vibrations of the tiny hairs. Figure 13a shows an array of thermal cilia capable of generating motion in one direction (both senses). Half of the cilia are oriented in one sense, the other half in the other sense. Both groups of cilia are driven with a phase difference of 90" as shown in figure 13b. The last two steps both generate a small displacement. Speeds of 27 pmls and operating frequencies of 1 Hz are obtained. To move objects in X- as well as in Y-direction, four cilia, one for each direction and sense, are grouped in motion pixels. The motion system consists then of an array of these motion pixels. Appropriate control allows motion in any direction.

fa1 Ib) Figure 13: Ciliary micro-actuator system for locomotion

(University of Tokyo, Institute of Industrial Science).

Besides thermal cilia, also other actuation principles are used. Coutinho et al. [47] use cilia driven by Lorentz forces. Suh et al. [48] use a combination of thermal and electrostatic actuation of the cilia. Thermal actuation provides the initial large-angle movement of each actuator and transition to electrostatic actuation (70 V) allows a low power hold-down mode. The capacitor plates also allow sensing and feedback control capability. Lifting forces of 70 pN/mm2, maximum step sizes of 9 pm per cycle, and speeds of 200 pm/s [49] are demonstrated. Bohringer et al. [50] use electrostatic torsional resonators generating a force of 28 pN/mm2. Each resonator consists of a plate suspended on two torsional beams and resonated in the high kHz range. In the system developed by Konishi et al. [51], each motion pixel contains four micronoules, one for each direction. The air flow is not only used to drive the objects but also for levitation. This avoids friction which is normally an important problem in micromechanics. Motion is fast but hard to control because of lack of damping. The nozzles can be selectively closed by electrostatic force. With a multi-layer method, it is possible to integrate micro- actuators, sensors and circuits. More control on the object is obtained when the motion array is divided into zones which are driven independently. By driving one side of the object in one direction and another side in another direction, the object can also rotate. Most control on the object is obtained when each motion pixel is controlled indet>endentlv.

Figure 12: Non-tactile transport using ultrasonic waves

According to Reinhart et al. [46], microcomponents can be positioned and manipulated within a standing wave

[461.

field, while a non-tactile transfer system can be conceived

458

Keynote Papers

Several modes which correspond to specific tasks of the conveyance system can be generated as shown in figure 14: (1) transporting mode, (2) aligning mode, (3) positioning mode and (4) rotating mode. If the system is able to detect the position of the objects, the system could move several objects at the same time into different directions. When all actuators have to be controlled independently, wiring becomes a limitation to the scale of the device. Therefore, Mita et al. [52] propose an autonomous distributed system concept: the behaviour of the total system is decided mainly by interactions of local controllers. Since micro-actuators are very fragile, the micro conveyance system must be robust against local defects. For that reason, fault recovery or fault avoidance logic is added. B6hringer et al. [49][50] derived and tested actuator control strategies to orientate and position parts from an arbitrary initial position and orientation. With their theory of programmable vector fields, vector fields can be cascaded into multi-step strategies, allowing unique sorting and positioning of objects without sensor feedback.

Figure 14: Different operation modes of a motion array 151 I.

5.8 STM The scanning tunneling microscope (STM), originally developed to image atomically sized objects, becomes more and more regarded as the ultimate in manipulation technology. In principle, it can manipulate single atoms. However, it can neither handle microparts as large as 100 nm nor can it correct microdefects of a VLSl mask [53]. Manipulating single atoms appears promising for building small structures or store massive amounts of data, but we must realise that diffusion processes may destroy the structure quite rapidly. It is not expected that STM technology is a valuable candidate for assembly of microparts in the near future.

6 NON-CONTACT MANIPULATION

6.1 Aerostatic gripper Many components used in microsystem technology are highly sensitive to mechanical handling forces. Examples are thin, fracture-sensitive GaAs wafers or contact- sensitive, surface-structured components. Therefore, Hohn et al. [54] developed an aerostatic gripper for contactless manipulation. The part is retained by suction nozzles in the gripper on the one hand and - similar to an air bearing - separated from the gripper surface by an air cushion. The equilibrium of the forces holds the part in a suspended state without any mechanical contact.

Laterally mounted stops on the gripper head prevent lateral slipping of the part from the gripper.

6.2 Optical trap or laser tweezers The optical pressure that occurs when light is refracted, absorbed or reflected by an object, can be used to manipulate objects with sizes ranging from a few micrometre to a few hundred micrometres. Figure 15a shows the forces that occur due to refraction in a microparticle. To trap the object, a focussed laser beam with a spot diameter of a few micron is aimed at the object. When the object moves away from the beam, the optical forces result in a restoring force. It works as well in liquid as in gas or vacuum. A liquid has the advantages that viscous damping increases the trap’s stability and that the buoyancy force helps to levitate the object. For metallic objects, which have a high density, an additional levitating electrostatic force is best used, otherwise the required laser power will cause heating effects [55]. Disadvantages of a liquid medium are the disturbing forces due to fluid motion. Besides Brownian motions, fluid motions can also be generated by thermal gradients caused by absorbed laser light. Manipulation of the objects can also be disturbed by van der Waals forces which are orders of magnitude larger than the optical forces. This causes the objects to stick to other objects or to the recipient wall.

Figure 15: Optical pressure on a microparticle caused by refraction (a) levitation and trapping, (b) rotation [ 561.

Morishima et al. [57] successfully used laser tweezers to trap single Escherichia coli bacteria, while the rest of the bacteria are carried off by dielectrophoretic forces. Rotation of the object can be obtained by asymmetry of that object geometry. In the example given in figure 15b, the laser beam enters the object in the centre and perpendicular to the paper. The optical pressure on surface 1 generates a torque as indicated, while the resultant force on surface 2 goes through the object’s centre and generates no resultant torque. No rays reach surface 3. Torques of lo-’’ Nm at rotational speeds of 500 rpm are reported for 3 pm diameter glass rods, 10 pm long [56]. Rotation can also be obtained by circularly polarised [58] or rotating higher-order mode laser beams [59], but reported rotational speeds are low: 0.7 rpm and 6 rpm respectively. Multiple objects can be trapped or manipulated by scanning the different objects. At high enough scanning frequencies, disturbances have not enough time to change the object’s position.

6.3 Magnetic fields The objects to be manipulated must be intrinsically magnetic. If not, small magnetic particles can be attached to the objects to be manipulated. For this purpose,

459

superparamagnetic particles can be used with high magnetic susceptibilities and saturation magnetisation but a very weak magnetic hysteresis [60]. Such particles become magnetic dipoles when placed in a magnetic field but lose their magnetism when the field is turned off. Hence, individual particles can be readily removed and resuspended after exposure since no permanent magnetic dipoles are sustained in these particles. For separating particles from solutions, magnetic separation techniques provide one of the most rapid and convenient methods. A micromachined magnetic particle separator is built by C. H. Ahn et al. [60].

6.4 Electric fields A particle in an electric field is subject to two forces: the Coulomb force and the dielectrophoretic force. The Coulomb force is caused by the attraction of a charged object to the oppositely charged electrode. This effect, called electrophoresis, only occurs with charged objects. The dielectrophoretic force on the other hand, works also with uncharged objects and is based on the polarisation of the object by the electric field. The object does not have the same electric properties as the surrounding medium, its electrical conductivity and dielectric constant are different. Therefore, charges are induced on the boundary between the particle and the medium. The object has now the characteristics of an electric dipole. In a uniform field, the forces working on these poles cancel out, but in a non-uniform field they result in a net force acting on the object. The dielectrophoretic force is proportional to the gradient of the square of the electric field and acts in the direction of this gradient. As illustrated in figure 16, the particles move to or away from places with high field line densities. This allows the transportation of particles and trapping them at spots with high or low field line densities. For the electrophoretic effect, the direction of the force depends on the sense of the field. Therefore, in AC electric fields above 1 kHz the particle's inertia causes the electrophoretic effect to become vanishingly small. The dielectrophoretic force on the contrary, does not depend on the sense of the field. Therefore, it can exert its influence up to frequencies of 50 MHz and beyond. Thus also charged particles can be manipulated by dielectrophoresis without disturbance of the electrophoretic effect. The high-frequency electric fields have also the advantage that no electrolysis of the medium, generally water, occurs.

n

Figure 16: Two different particles in a non-uniform electric field. The particle on the left is more polarisable than the surrounding medium and is attracted towards the strong

field at the pin electrode, whilst the particle of low polarisability on the right is directed away from the strong

field region [61].

Electrophoresis and dielectrophoresis are used in biotechnology to separate, trap and classify cells, bacteria, viruses and DNA. Electrophoresis is for instance used to produce DNA fingerprints. The DNA is broken up in several particles by enzymes and put in a gel. Different

Figure 17: Polarisation of DNA molecule and dielectrophoretic force [ 621.

DNA will be cut in different ways. The distance each of the DNA particles travels through the gel by the electrophoretic effect, depends on their length, and is thus characteristic for the original DNA molecule. Morishima et al. use an electrode configuration as shown in figure 17 to transport Escherichia Coli bacteria [57] and DNA molecules [62]. Voltages of 10 to 100 V peak-to- peak are applied at frequencies of 1 MHz. Between the angled electrodes the electric field is not uniform but has a gradient in the vertical direction of the picture. The selected bacteria are trapped by a laser, and the other bacteria are driven away by the dielectrophoretic effect to the small exit. The DNA molecules on the other hand are too small to be trapped by laser. They are separated by a third electrode located at the middle of the exit of the two angled electrodes. Particles can be transported by travelling waves. Figure 18 shows a junction of two travelling wave tracks. Each track contains four electrode groups, energised by sinusoidal voltages that are separated 90" in phase. The electrodes are typically 5 to 10 pm in width. The frequency of the travelling fields on the two tracks is chosen differently, each according to the dielectric properties of the particles to be separated or mixed together.

Figure 18: Travelling wave dielectrophoresis junction [ 63).

Non-uniform fields can sometimes induce a torque on a particle, causing it to rotate. However, the controlled way to induce spin is to subject the particle to a rotating electrical field [63]. Depending upon the phase angle of the induced dipole moment, the rotational torque acting on the particle will be in the same or opposite direction of the field rotation.

7 SMART ASSEMBLY TECHNIQUES Despite the large amount of research in microtechnology, assembly of microsystems is still immature. Furthermore, assembly is a costly process. Therefore, most designs avoid assembly by producing monolithic systems and

460

Keynote Papers

using sacrificial layer techniques. On the other hand, techniques are developed that simplify or automate the assembly procedure: on-the-machine assembly, folding techniques inspired by origami, flip-up mechanisms, batch assembly, preadjusted assembly magazines, and even self-assembly.

7.1 Sacrificial layer technique Techniques have been developed that enable the production of complete microsystems with actuator, sensor and mechanisms consisting of different materials and parts without any assembly. They are mainly implemented on silicon and include deposition techniques, doping and sacrificial layers. A large number of materials can be deposited ranging from simple metal and oxide layers to piezoelectric, magnetic, shape memory and biosensing materials. Several techniques are used: sputtering, chemical vapour deposition, electroplating, surface oxidation.

(a) (b)

Figure 19: (a) Part of a mechanism produced by the sacrificial layer technique, (b) Cross-section of the central joint prior to removal of the sacrificial layers (black) [ 641.

By using sacrificial layers the different parts can be made together in the assembled state and only at the end of the process they are separated by etching away the sacrificial layers. Figure 19a shows part of a mechanism produced by surface micromachining with sacrificial layers. A cross- section of the central joint is shown in figure 19b prior to removal of the sacrificial oxide layers (coloured black). After removal of these black layers, the grey part belonging to the upper link can rotate freely around the white part belonging to the horizontal link.

7.2 On-the-machine assembly When a microproduct is dismounted from the machine on which it is produced, information about its position is lost. This makes subsequent assembly difficult. On-the- machine assembly, as presented by Langen et al. [65], is one of the solutions for this problem. The idea is to assemble the product directly on the production machine, and even to produce the tools on the same machine [66]. Figure 20 shows how this can be realised on an electro- discharge machine (EDM). First, a part is produced with W EDG (wire-electro-discharge grinding). The pin at the bottom of the part is then used to drill a hole by micro- EDM into the underlying plate (a). The pin is inserted into the hole while applying ultrasonic vibration to the worktable until the force reaches a threshold value (b). Then the pin is twisted to break the neck (c). In the final step, the remains of the neck are removed by micro-EDM (d). The resulting part can be used as a workpiece, tool or a (sub)assembly setup. Since the space required for micromachining itself is usually small, the machine table can accommodate several modules on which tool-making, machining, product storage and assembly can take place.

w t lil t . . .

a) Micro-EDM b) Insert c) Twist d) Micro-EDM

Figure 20: On-the-machine assembly [65].

7.3 Folding, origami, flip-up mechanisms Out of plate-like material, a flat structure consisting of plates and hinges is manufactured, e.g. by etching, and subsequently folded into a 3D structure, similar to a flat sheet of paper that is folded into a 3D object. When the hinges have little stiffness, the folded parts can be locked together by snap fits [70]. This folding technique allows the creation of large motions out of the plane and rotational joints under any angle. Folding is performed manually or by integrated micro- actuators, or by a physical effect, like e.g. surface tension. A layer of solid solder is deposited on the hinges during manufacturing. During the assembly step, the structure is heated above the melting temperature of the solder such that the surface tension in the solder pulls the parts together into a programmable angle [68][69]. The flexible hinges can even be omitted when the molten solder itself acts as a hinge. An example is the construction shown in figure 21, developed by Yeh et al. [70] as a single link for a multiple- degree-of-freedom manipulator. A hollow beam is created by three plates that click together by snap fits. Also the lever arm is fixed to the beam by snap fits.

Figure 22: Flip-up microscanner [ 671.

46 1

Another example is the out-of-plane mirror shown in figure 22. The assembly can be performed manually [71][72], but automatic assembly by integrated micro- actuators is preferred when batch fabrication is envisaged. Reid et al. [73] use on-chip actuators controlled by a CMOS controller. One of the problems is to generate the initial out-of-plane motion as all mechanisms and actuators lie in the same plane. This is solved by adding additional vertical actuators which lift parts of the mechanism out of the plane after which a horizontal actuator drives this mechanism to assemble the mirror. The comb actuator visible in figure 22 is not used for assembly, but drives the mirror after it has been assembled. One of the problems with this flip-up technique is that the pre-assembled mechanism and the actuators performing the assembly, take up a large wafer area.

7.4 Flip-chip wafer-to-wafer transfer One of the technical problems in MEMS is to integrate sensors and actuator processes with existing electronics and packaging technology. A common approach is to fabricate the MEMS and electronics on the same substrate in an integrated monolithic process. Even though this approach has been shown to improve device performance, it usually results in a large number of process steps, thus increasing process complexity and reducing yield. Hybrid packages, on the other hand, are expensive and may not be compatible with MEMS due to stray parasitics. Flip-chip bonding is a hybrid technology that combines the advantages of monolithic and hybrid MEMS and can integrate MEMS into electronics to meet new specifications. Flip-chip is defined as mounting the chip on a substrate using various interconnected materials and methods (e.g. tape-automated bonding, fluxless solder bumps) as long as the chip surface (circuit) is facing (oriented in the direction of) the substrate. The interconnect materials and methods and their related geometrical and physical properties dictate the planar alignment and vertical gap between the chip and the substrate. When all bonding process and manufacturing parameters are within variation limits, this feature makes it possible, to adhere to passive alignment, in contrast to active alignment where each chip has to be manipulated individually.

/' 1 '~. Top MOEMS '~ substrate

Figure 23: Schematic diagram of a prototype MEMS structure for optical I/O couplers which is composed of a

flip-chip photonic device and a MEMS substrate [74].

A schematic diagram of a MEMS device where hybridisation consists of flip-chip bonding is shown in figure 23. The optical I/O coupler is composed of three components: photo-detectors, integrated electrical multichip modules, and optical waveguide/fibre patterns and micromachined mirrors. The three components are independently fabricated, and then assembled to construct the optical I/O coupler using flip-chip bonding techniques. The interconnect materials used are micromachined conductive polymer bumps. The top MEMS substrate includes contact metal pads, electrical metal lines, side alignment pedestal bumps, and through

holes. The side alignment pedestal bumps improve planar alignment of the chip and the substrate. Another example is a rate gyroscope shown in figure 24 [75]. The dynamic element which includes a proof mass attached to a rotating frame by an elastic beam is bonded to a CMOS chip. A metal plate is placed on the chip, and another plate is placed on the side of the proof mass facing the chip, forming an air capacitor (as shown in figure 24b). Bonding is carried out by using indium bumps technology. This method assures the appropriate electrical and mechanical connectivity between the chip and the dynamical element. Aluminum stoppers are deposited to guarantee an even gap between the chip and the substrate.

Figure 24: Schematic view of the rate gyroscope (a) top- view (b) side view [75].

Based on the model developed by Salalha et al. [76], it was found that the yield of solder flip-chip assemblies is related to the following: dimensional variation of the interconnect solder bumps, the non-flatness of the chip/substrate, dimensional variations of the connecting pads, and the initial misalignment of the chip relative to the substrate. In addition, physical parameters related to the bonding process; non-simultaneous solidification of the joints, and variable surface tension can affect the final location of the chip. A modified flip-chip method for the transfer and assembly of MEMS using sacrificial-layer micromachining has been proposed by Singh et al. [78] and Irwin et al. [77]. The approach to low-cost and high-performance packaging has been to shift from chip (die) to wafer scale methods. Wafers enable a high degree of parallelism (100 or more devices per wafer). The wafer-to-wafer transfer of a surface micromachined structure is depicted in figure 25. Lid structures are fabricated in 4 pm polysilicon on top of a PSG (phosphosilicate glass) sacrificial layer. Gold break-away tethers hold the lids in place. Gold bumps patterned on the polysilicon enable bonding and transfer to a target substrate. Then, donor and target substrates are aligned and compressed at room temperature and

462

Keynote Papers

process can be carried out with simple tube-like tools.

7.6 Self-assembly The concept of self-assembly is inspired by biological processes in which antigens on viruses and bacteria bind to antibodies in a serum through the matching of biochemicals. In the non-biological world, objects can be assembled in a similar way, not necessarily based on chemical receptors but also simply based on geometry. The advantage of self-assembly is that it is a batch process, in which hundreds, thousands or even more objects are assembled at a time. When the interface is well designed, the objects are also self-aligned. Also different types of objects can be assembled simultaneously by using different interfaces. According to Yeh et al.[80], self-assembly maximises also the amount of useful devices that can be integrated on a wafer. It allows also optimal use of material in comparison with flip-chip methods. When for instance small GaAs LEDs have to be placed on top of large Si circuits, then, with flip-chip assembly, the LEDs have to be positioned at the mirror position of the circuits. As the LEDs are much smaller, most of the expensive GaAs material is unused. With an assembly method, the LEDs cover the whole GaAs wafer, are cut and distributed over the target Si wafer. The self-assembly method can position the parts directly on the target substrate, or the objects can be positioned on an intermediate, reusable template which is then used to transfer the objects in parallel to the target, like during a flip-chip operation.

Based on geometry An experiment is performed by Yeh et al. [80], in which trapezoidal GaAs LEDs were placed in an array of trapezoidal pockets on a Si wafer by a self-assembly process. The LEDs are suspended in a fluid that is dispensed over the Si wafer. The selection mechanism is based purely on the geometry of object and pocket. Before evaporation of the fluid, 90 Yo of the holes are correctly filled with LEDs. However, during evaporation, surface tension pulls some objects out of the holes, reducing the yield to 30% to 70% locally. This problem can be addressed by using liquids with lower surface tension or by using supercritical drying methods.

Electrostatic attraction The attractive force to the target and the selection mechanism can also be provided by electrostatic fields. Cohn [81] and Bohringer et al. [82] use a template consisting of a pair of oppositely charged planar electrodes. The upper electrode contains a multiplicity of apertures. In the vicinity of the apertures, the electric field emerges at the template surface. Dielectric material is attracted to these regions with high electric field strength. At first, the template is vibrated strongly such that the objects move randomly. With time, the vibration amplitude is gradually reduced and the objects are attracted to the apertures and trapped thereat. The

preadjusted mounting of gear assembly wheels in a batch magazine process

Figure 25: Transfer and assembly of a microstructure using sacrificial-layer micromachining and flip-chip

bonding [78].

separated. Released structures separate at the donor's substrate-gold interface or at the polysilicon tethers. The transfer method seems to be relatively insensitive to imperfections up to 500 nm in size. Defects in one feature will not severely impact overall transfer yield as each bump compresses and bonds independently.

7.5 Preadjusted assembly magazines This method is especially suited for moulded microparts. A mould generally contains a multitude of cavities in which the products are formed. In this method (791, a common base plate is added to the microparts such that they form one single structure with relatively large size. The form and size of the base plate facilitates demoulding, handling, storage, and assembly. The position of the parts on the base plate corresponds to their assembled position. Figure 26 shows the different steps in the assembly process. After the plate has been removed from the mould, the microstructures are embedded by an appropriate resin. Then the superfluous filling material and the base plate are removed by milling, grinding, and polishing. The result is a polymer magazine in which the microstructures are fixed laterally. The resin features a rather weak adhesion to the component material such that the parts remain movable in the axial direction. In a final step, the magazine is positioned above the microsystem and the microparts are pushed out, right in place. This technology additionally allows the simultaneous assembly of groups of microstructures at predefined locations. Thus no complex tools like microgrippers are required. With the use of specific macroscopic grabbing tools the magazine can be handled and positioned. The assembly

embedding of removing of structuresole microstmctures and supeffluous filling

compound

Figure 26: Assembly using a common base plate [ 791.

463

particle’s bottom surface is coated with a conductive film which has the same pattern as the aperture. When the particles pattern fits on the aperture, the conductive film stops the field lines at the surface, so only one particle is trapped at the aperture. Discrimination between different types of particles and orientation is obtained by the shape of the pattern. Trap of the wrong particle or bad orientation result in a smaller energy decrease than when the correct particle is trapped in the correct orientation. These local minima are avoided by slowly decreasing the vibration amplitude. An excess of particles helps to cover all sites.

Magnetic attraction In analogy to the electrostatic attraction and selection mechanism, also magnetic templates can be used [81]. One method is to use a magnetic medium such as a floppy disk on which a magnetic pattern is created. The particles are then coated with a corresponding permalloy pattern. Patterns on the template may be rapidly, economically and microscopically created in arbitrary planar configurations by means of a magnetic recording head.

Hydrophobic or hydrophilic attraction [ S l ] When particles with mating surfaces, suspended in water, are coated with a hydrophobic film (e.g. oil), the mutual attraction of the oily surfaces causes them to assemble together, minimising surface area. The oil provides both attractive force and lubrication, which reduces friction, accelerating the self-assembly process and making it more accurate. Analogously, the particles may be coated or permeated with water, and suspended in a hydrophobic medium. These self-assembly methods look tempting because of the simplicity and automatism of the process. However, none of them is 100 % reliable which limits their practical use.

8 JOINING OF MICROPARTS Once the parts have been put into place, the next step is to securely lock them into position by a process called joining. At the microlevel, joining parts is as difficult a job as the assembly step itself. Joints occupy space, are often less strong than the bulk material, require additional production steps and parts, and are often difficult to realise on these small scales. Although avoiding assembly and joining is the best solution, it is often impossible due to technological and economical reasons. Special techniques have been developed to join parts made by silicon microtechnology, like e.g. anodic and fusion bonding, but also more traditional methods are

being miniaturised like bolting, riveting, welding, glueing, etc. Of course, each joining method has a size limit of applicability, and often changes are needed to the macrotechnology to make it applicable on a microscale.

8.1 Mechanical fasteners

Miniature screws Screws and bolts belong to the most frequently used joining methods in today’s macroscale production. Down to a size of 1 mm diameter, they are still easy to find. Smaller ones, down to a diameter of 0.3 mm, can be found in the watch industry. To tap holes that small, great care has to be taken not to break the taps.

Micronvets Shivkumar et al. [83] developed a micro-riveting technique for packaging of MEMS. Tapered holes 300x300 pm or 1300x1300 pm in size are etched through the covering wafer by anisotropic etching. After evaporating a seed layer on the base wafer, both wafers are clamped together and nickel is plated through the holes of the covering wafer as shown in figure 27. No plating occurs in the gap between the wafers as no potential gradient is present between the seed layer and the electrolyte trapped between the two wafers. The tapered form of holes and rivets prevents the wafers to separate. An advantage over anodic and fusion bonding is that this technique allows packaging at low temperatures and low electric fields which could damage ICs. Also surface preparation and mating are more relaxed. It is a simple process, allowing batch processing of the rivets and the bonded wafers can be diced with normal wafer dicing saws without rivet failure. Highest strength was obtained under shear: 350 MPa. Under normal load, the rivets failed by rivet deformation between 1 and 3 MPa for rivets respectively 30 and 70 pm thick. Thicker rivets would further increase normal strength.

Snap fit Yeh et al. [70] use silicon micromachined snap fits to assemble 3D structures from planar plate-and-hinge structures, as the one shown in figure 21. Plates and hinges are produced by a planar silicon process, after which the 3D structure is assembled by folding the hinges and clicking the structure together with the snap fits shown in figure 28a. Although the technique looks simple, the folding and locking of the parts is a manual job, performed on each structure separately. The backlash and reduced stiffness in the joints and snap fits, cause lower stiffness, strength and accuracy of the final structure than when the connections would be rigid. Figure 28b shows snap fits with less backlash.

Figure 28: Snapfits [70].

A variant to the snap fit is a kind of micro-Velcro [84]. It consists of a two-dimensional array of mushroom-like

Figure 27: Bonding of two wafers by electroplating Structures placed on both mating surfaces. The ‘mushrooms’ have a pitch of 22 pm. When pressed microrivets [ 831.

464

Keynote Papers

together, the hoods of the 'mushrooms' hook behind each other. Unlike real VelcroTM, the bond cannot be broken without damaging the structures. A tensile strength of 240 kPa is obtained and could increase to 1.1 MPa when 100 % of the microstructures would latch and bear the load evenly.

Mating parts Gonzalez et al. (851 developed micromechanical connections similar to the ones used in wood constructions: dovetail joints, slot joints, dado or rabbet joints, diamond joints, finger joints, and mortise and tenon joints. The mating parts of the joints are produced by anisotropic etching and with a dicing saw. However, LlGA and micromilling are also possible machining techniques. Depending on the tolerance set on the mating parts, these joints can be used to rigidly fix parts or to constrain the movements in one or more degrees of freedom. The technique also allows quick and reversible connection and disconnection of parts. This makes it possible to have modular parts that can be built together in different configurations to fulfil different functions. An example of a finger joint connection is shown in figure 29. A fluidic microconnector based on mortise and tenon joints is depicted in figure 30.

Figure 29: Connection using Figure 30: Mortise and

interconnection [85]. finger joints [85]. tenon microfluidic

8.2 Press fit The parts to join are machined on their joining faces with high accuracy and a small overlap. To join them, the parts are pressed together and the elastic joint stresses keep them together. Press fits are often used for joining cylindrical parts. In macro-applications, one of the parts is cooled or heated to let it shrink or expand, such that the part can easily be assembled without force. When the part has returned to environmental temperature, thermal expansion or shrinkage presses both parts together (shrink fit). For the assembly of microparts, shrink fits are not realistic because of the small thermal time constants of the parts. The main advantage of a press fit is that no additional parts are needed, what is favourable for miniaturisation. It is often also a very simple operation that can be easily automated. However, tolerances on product size are close and may increase cost. As relative accuracy of micromachining techniques is generally lower than that of macromachining, producing press-fits becomes more difficult with miniaturisation.

8.3 Sillcon-related bonding techniques

Anodic bonding Anodic bonding of silicon to glass is an important method for encapsulating sensors. It allows the silicon chip itself to be used as a package because it is hermetically sealed from the surrounding environment. A polished glass wafer is placed on the silicon wafer and a cathode electrode is held against the outer surface of the glass wafer. The whole assembly is placed on a hot plate heating it up to 300 - 400 "C (861. A potential of 200 to 1000 V is applied

between the electrode and the hot plate, which acts as anode. At the elevated temperatures, the glass becomes slightly conductive and opposite charges accumulate at the Si-glass interface. These charges pull both wafers together into close contact forming a strong bond. Electrostatic pressures of 34 GPa are reported [87]. A disadvantage of this method are the thermal stresses, although the thermal expansion coefficients of silicon and the glasses that are used are fairly close to each other. Figure 31 shows the principle of the anodic bonding process [88].

Figure 31 : The anodic bonding process [ 881.

Fusion bonding Fusion bonding involves bonding of two or more silicon wafers. As each of these wafers can be patterned, complex microstructures can be produced. The basic principles of wafer fusion bonding include pretreatment, room temperature mating, and thermal annealing, typically at 1000 "C. Fusion bonding techniques require well-polished and particle-free surfaces (i.e. a clean room environment). In addition, due to its high-temperature nature and high sensitivity to wafer strain, fusion bonding must be performed as early as possible in the process sequence. Since the wafers are made from the same material, no thermal stress is expected.

8.4 Glueing Glueing has a number of advantages: no additional parts are needed, enhancing miniaturisation and production cost, materials with very different chemical composition can be bonded, tolerances can be larger as the gap is filled with glue, the connecting forces are spread over a relatively large surface, and a hermetic sealing of the connection is possible. The many parameters influencing quality and strength of the bond make glueing a difficult operation. Important factors are surface cleanliness, surface roughness, glue viscosity, wettability and material type. To get a surface free of any contamination, soap or solvents are not sufficient. The remaining contamination, a few atom layers thick, can be removed with a UV/ozone or a plasma treatment [89]. Important families of glues are acrylates and epoxies. Among the acrylates, there exist cyanoactylates, well known as 'superglue', and anaerobic glues. Cyanoacrylates are one-component glues that cure due to moisture absorbed on the substrate. Due to the low viscosity, the glue penetrates well into the pores and a strong bond is created on nearly every material. Due to their sensitivity to humidity, temperature, shocks and peeling, they should not be used where high durability is required. Their fast curing, from seconds to minutes, may be interesting in many cases, but it impedes accurate positioning of the parts. Another kind of acrylates are the anaerobic ones. These have also a low viscosity and harden when excluded from air. They withstand well humidity and some withstand temperatures up to 250 "C. Drawback for the assembly of

465

miniature parts is that the depth of the gaps is not sufficient to exclude the air, such that the glue does not cure. Placing the parts in a vacuum chamber solves this problem but is quite expensive and cumbersome. Epoxies are two-component glues available in different forms: paste, film, powder. The properties can be adapted to the application: viscosity, high shear strength, high peeling strength, shock resistance, heat resistance and durability. Compared to other glues, epoxies fill gaps very well and their shrinkage is low. Epoxies are often used for their high strength, filling capacity and the large positioning time. Glueing small objects is a problem for a number of reasons. Application of liquid glue with a dropper is impossible due to the relatively large size of a drop compared to the miniature objects. To apply smaller quantities, needles or capillaries should be used. An interesting way to apply small amounts of glue, is to use a microdrop dispenser, similar to the systems used in some ink jet printers. This way, Eberhardt et al. [go] could dispense 50 pm diameter adhesive drops with a reproducibility of 1 % on the volume. Also the inclusion of air bubbles is a large problem [91]. These bubbles are introduced during mixing or during application, for instance when glue is inserted into a narrow blind hole. Despite all the knowledge and the use of finite element programs, the design of an optimal bond still requires tests, experience and accurate control of conditions. Figure 32 shows an automatic glueing station for the assembly of microgenerators for kinetic watches at Kinetron Systems B.V., the Netherlands.

Figure 32: Automatic glueing station for the assembly of microgenerators (Kinetron Systems B.V.).

8.5 Welding

Laser welding With laser welding, as well metals as plastics can be joined. Which type of laser is most suited for the process at hand depends on the materials that have to be welded together, because the absorption ratio depends on the frequency of the laser light and the material itself. In this respect, there is a large correspondence with laser cutting. The main difference with macroscopic-size welding is that a higher positioning accuracy and a smaller spot size are required. Another difference is that masks can be used, a technique often applied in microtechnology .

Friction welding (wire bonding, ultrasonic welding) Ultrasonic bonding is often used for wire bonding of chips. The wire and bonding surface (pad or frame) are forced together. The tool and ultrasonic vibration are used to compress the surfaces together to achieve the desired bond. Ultrasonic bonding can be used when the device cannot or should not be heated.

8.6 Soldering Flip-chip solder bonding is a well established method for performing simultaneous interconnection and mechanical mounting of microelectronic components. The process is explained in figure 33. First, an array of solder spheres is created on one of the components to be joined. The solder is constrained to defined areas by first depositing ”wettable metal” pads over otherwise non-wettable surfaces. Solder is deposited over the wettable metal and then melted, whereupon surface tension forces shape the solder into perfect spherical caps. The mating component is similarly prepared with a spatially matching array of wettable pads. These are roughly aligned with the solder spheres and the assembly is heated to re-melt the solder. As the solder wets, surface tension provides a restorative force that precisely aligns the components. The energy balance of the surface energy of the solder joint, the potential energy of the solder joint, and the wetting energy controls the relative horizontal and vertical locations. The normal force is an order of magnitude higher than the lateral force, making the joint more susceptible to lateral misalignment compared to the axial displacement [92]. Under normal circumstances, the alignment tolerance achieved between components is f 2 pm in the horizontal plane and f 0.5 pm in the vertical direction [93]. The reason for the relatively poor alignment in the horizontal plane is because the restoring force goes very fast to zero when the parts become aligned. Alignment better than 0.25 pm can be achieved by deliberately misaligning the pairs of wettable metal pads, as illustrated in figure 34 [93]. Then, the equilibrium position is a balance of opposing forces. The magnitude of the alignment force in lateral and axial direction reduces with increase in solder volume [92]. Precision alignment in the vertical direction is thus obtained by maximising the number of interconnects of the smallest possible diameter. This combination provides the highest surface tension force and internal pressure in the solder spheres, and allows vertical alignment to f 0.2

Watabb meld &a As deposilea SddOr md1.d

Rwph l p n m M l Solder n.1l.d AYg”M W M

Figure 33: Flip-chip solder bonding [93].

Figure 34: Enhanced component alignment by deliberate misalignment of the wettable pads [ 93).

466

Keynote Papers

pm [93]. According to Patra et at. [92], the axial misalignment is primarily dictated by the accuracy of the solder deposition height. The components can thus be joined with sub-micron precision and repeatability in three orthogonal directions. State-of-the-art processes have been developed capable of making over 100,000 interconnects simultaneously with a 30 pm pitch [93].

9 EXAMPLES OF MICRO-ASSEMBLY SYSTEMS SPI (Scientific Precision Instruments) commercialises a modular assembly robot. The basis is an orthogonal three-axial system driven by hybrid stepper motors and precision micrometre spindles, having a resolution of 10 nm. The gripper consists of two elastomer fingers, a passive one fixed to the basic three-axial robot, and an active finger driven by a compact three-axial system fixed to the basic three-axial robot. This allows not only gripping of the part, but also rotating it between the fingers. Assembly is monitored through two camera microscopes giving a top and side view. The camera system is mounted on a separate three-axial positioning system. By moving the cameras synchronously with the assembly robot, the gripper and parts can be observed over large distances from the storage area to the assembly place. A general overview is obtained from a stationary camera with lower magnification. At the Technical University of Vienna [94] a prototype system with manipulation, assembly and testing capabilities of microparts has been developed. All operations can be performed either under normal air (optical microscope) or under vacuum conditions (in a SEM chamber under remote control). The system includes one gripper and a pair of tweezers driven by a single control unit. They are mounted on a three-axial table to perform an XYZ-movement. The ranges are 20 mm in X and Y and 5 mm in Z-direction. The gripper is controlled by one piezo-actuator. The arms of the tweezers are independently controlled by two piezo- elements. A similar system was built at IPT, Aachen [31] for use in a large-chamber SEM. Two independent Z-slides are

Figure 35: Micro-assembly system operating in ultrahigh vacuum with tools for surface activated bonding [ 961.

provided with microgrippers driven by piezoactuators (figure 7). An XY-plus-rotation table carries the products to be assembled. This arrangement allows to keep the focus of the LC-SEM system on the gripper jaws all the time. A small gear pump has been assembled as a case study. Hatamura et al. [95] developed a three-dimensional fabrication and assembly system, consisting of three chambers: a shaping, a handling and a buffer chamber. The shaping chamber contains a fast neutral atom beam and optical microscopes. The handling chamber contains several manipulators and a multi-view SEM. The part remains fixed on a pallet which travels between the three chambers. This avoids loss of coordinate information between the different production and assembly steps. The manipulators are arranged spherically around a single point and make spherical movements, such that the endeffector stays at a single point in the view. The process table and manipulators are based on coarse-fine motion, with the coarse motion generated by pulse motors and ultrasonic motors, and the fine motion by piezo devices. Each manipulator is equipped with a multi- axis force sensor. Image data is used to compute the relative positions of the parts. A micro-assembly system for MEMS, using the surface activated bonding (SAB) technique has been developed at RCAST, University of Tokyo [96]. The basic concept of SAB is that two atomically clean solid surfaces under contact can develop a very strong adhesive force. Atomically clean surfaces can be obtained by energetic particle bombardment in ultrahigh vacuum (UHV). The micro-assembly system developed for this purpose is shown in figure 35. It consists of an UHV chamber, a multi-axis manipulator, a SEM, and an Ar-FAB source. The two parts to be bonded together are brought in each- other's vicinity by the multi-axial stage (14 degrees of freedom (dofs)) and the manipulator (3 dofs). Their positions are monitored by the SEM. The surface of the specimen is then sputter-cleaned by Ar-FAB irradiation of about 1-2 keV for several minutes. After the native oxide or contamination layer is removed, they are brought into contact under slight pressure. A powerful graphical user interface (virtual manipulation system) has been developed to operate the system. Feldmann [97][98] has developed several robotised assembly systems for 3D-assembly of SMDs (surface mounted devices) in products made according to the MID (Moulded Interconnect Device) technology. Figure 36 shows an assembly robot and a few examples of assembled products.

Figure 36: Robotised 3D-assembly of SMDs in MID products (FABS, Univ. of Erlangen, Germany).

467

planning, and feeding/handling/assembling. A pilot plant for the assembly of the miniature gear trains used in modern radio-controlled wrist-watches has been developed by Reinhart [loo]. The required assembly accuracy is in the region of 20 pm. The assembly is performed with a SCARA robot with passive tolerance compensation using a compliance in both the gripper and the workpiece carrier. The gripper tool has integrated solid-state joints free of backlash and friction, which permit one rotary and three translatory compensating movements. The workpiece carrier is mounted on an air bearing which is also free of friction and allows two translations and one rotation. The resetting force which is needed to centre the workpiece carrier is realised in a contactless way with four pairs of permanent magnets. The sensitive gears are protected against damage and contamination in gel packs, where they are retained in position by adhesive forces. A stationary camera roughly locates the gears at low resolution. In a second stage, a camera mounted on the robot, operating with a reduced field of view but at higher resolution, determines the precise position and orientation of the components. Weule et al. [ l o l l use a coarse-fine manipulator for the handling of microparts produced by powder injection moulding. The manipulator consists of an industrial 6- axes robot (needed for large distances) combined with an ultraprecise hexapod robot (needed for very small distances). A batch-suction-gripper is used, able to grip 9 microparts at the same time. A spin-off company of the University of Twente, 3T, has developed a modular assembly technology (MATAS) for hybrid micro total analysis systems (pTAS) [102]. These latter systems are used for DNA analysers, online blood or water analysis, medicine dosage systems, etc. The essence of MATAS technology is a series of modules designed to fit a class of sensor types that fit into holes in a printed circuit board PCB). Electrical and mechanical connections are made by soldering, fluidic interconnections are realised by O-rings. Figure 38 a,b and c illustrate the technology. The assembly technology for surface mounted devices (SMD) can be used for assembling the modules on the PCB. In [103], an assembly system for beam transformation systems of high-power laser diode bars for material processing is described. Laser diode bars of 1 cm width are formed from 20 to 50 high-power diode lasers (HPDL). A beam transformation system circularises the

Figure 37: Microfabrication process centre with its main challenges [99j.

In (991, a microfab for the micro-assembly of optical lenses for endoscopes is described. The microfab concept was developed to provide innovative medium- size companies with a customised concept for the commercially viable introduction of microfabrication technology. The microproduction process centre consists of a number of tools, one or more handling units, and standardised input/output interfaces for wafers, substrates and semi-finished components. The control is decentralised and the modular tools are capable of working separately as well as working in linear or cluster- like arrangements (figure 37). The assembly of microcomponents onto a preprocessed substrate can be decomposed into a series of operations like picking components, dispensing adhesives, positioning, assembly and inspection steps at chip level. The main challenges, apart from developing suitable processing steps, for a successful development of such processing centres are contamination control, standard mechanical and software interfaces, process control, production

(c) Assembly of the modules and the wet circuit board.

Figure 38: Assembly of the pTAS [ 1021. Figure 39: Beam transformation system for high-power

laser diode bars [ 1031.

468

Keynote Papers

beam by means of an actively aligned fast-axis collimation (FAC) lens, as shown in figure 39. The central part of the assembly system is a 6-axis positioning system. The most critical step in the assembly process is the adjustment and fixing of the FAC lens. To give an idea, for a 200 pm fibre coupled system a 1 pm FAC height decentring results in about 50% excess loss. Assembly is carried out in closed loop with feedback from an intensity profile measurement. In its final position, the lens is glued to the heat sink while the laser operates at maximum output power.

10 EXAMPLES OF ASSEMBLED MICROPRODUCTS Siemens developed a small-signal relay fabricated by silicon micromachining and using an electrostatic actuator [I 051. The actuator consists of a variable capacitor with two electrodes, a first flat and fixed electrode on an insulating carrier chip and a second in the form of a bent silicon leaf fixed at one side to a silicon chip forming a 'moving wedge' in between. The actuator size is about 1.5 mm2 and low pull-in voltages below 24V are achieved. Figure 40 shows a double make relay with a three-chip structure consisting of a glass carrier chip, a silicon chip with etched spring blades and a glass cover chip fitted together by anodic bonding without any organic adhesive. A hermetically sealed contact chamber can be formed, evacuated or filled with protective gas to provide long electrical life times. In [104], the fabrication of a fully integrated electromagnetically actuated micromachined relay, enclosed in ceramic and plastic packages, is described. A multi-layer Cu coil is used to actuate a movable NiFe armature, thereby closing the Au contacts. With the current design, magnetic forces of around 1 mN can be achieved. The fabrication process starts with two chips, which, after processing, are flip-chip bonded together using a low-temperature ( ~ 3 5 0 "C) soldering process. The electrical contacts are housed in a hermetically sealed cavity with a predetermined atmosphere. The relay size is about 5x4~1 mm3, excluding the package. Figure 41 shows the heart of the microrelay, illustrating the flip- chip assembly. The assembly process is based on the eutectic bonding between electroplated SnPb and gold. Figure 42 shows the electronics of a pacemaker, developed at IPK, Berlin [106]. The multi-chip module is completely automatically assembled on a flexible substrate.

Figure 40: Double make relay with three-chip structure [105].

11 CLOSING REMARKS One of the main conclusions to be drawn from this keynote paper is that, compared to the macroworld, the solution of assembly problems in the microworld requires a fundamentally new approach to the different technological problem areas of the assembly sequence: gripping and grippers, manipulation, joining.

mg F&iS&r(- *)

lFeNiWBO1-w -1 t m z B i I m m

Figure 41 : Close-up view of the heart of the microrelay, illustrating the flip-chip assembly [ 1041.

Figure 42: Pacemaker electronics [ 1061.

Consequently, a completely different assembly system technology is needed. Furthermore, a whole array of alternative assembly techniques emerge, like the sacrificial layer technique, folding, flip-chip wafer-to-wafer transfer, self-assembly. The MEMS technology, and its associated manufacturing technologies are still in their infancy. The industrial potential is enormous however. Some sources predict growth figures that are higher than those of the microelectronics market. It is therefore a challenge for the CIRP community to adopt the MEMS technology as one of its key research areas. The three keynote presentations on MEMS-related subjects at this year's ClRP General Assembly indicate that the STCs start to realise that something important is emerging here. It should be seriously considered whether there is a need to establish a new STC on MEMS technology within CIRP. As is clear from this keynote paper the technologies involved are so different from the mainstream technologies considered in the present STCs that grouping them in a separate STC might be justified.

12 ACKNOWLEDGEMENTS Special thanks are due to following colleagues who contributed valuable material : Prof. H. Makino, Prof. H. Weule, Prof. T. Arai, Prof. G. Seliger, Prof. K. Feldmann, Prof. H.K. Tonshoff, Dr. K. Baert, Prof. D. Milutinovic.

13 REFERENCES Wechsung, R., Unal, N., Eloy, J.C., Wicht, H., October 1998, Market Analysis for Microsystems 1996 - 2002, NEXUS Task Force Report, Berlin. N., 1998, Market analysis for microsystems 1996- 2002, mst news 3:38-41. Weck, M., Petersen, B., 1999, High Precision Positioning Stage for Micro Assembly Purposes, Symposium on Design, Test, and Microfabrication of MEMS and MOEMS, SPIE 3680:734-739.

469

[4] Kaneko, K., Tokashiki, H., Tanie K., Komoriya, K., 1997, Impedance Shaping based on Force Feedback Bilateral Control in macro-Micro Teleopeation System, Proc. ICRA, 71 0-71 7. Mitsuishi, M., lizuka, Y., Watanabe, H., Hashizume, H., Fujiwara, K., 1998, Remote operation of a micro- surgical system, Proc. ICRA, Int. Conf. on Robotics and Automation, 1013-1019.

[6] Zhou, Y., Nelson, B.J., Vikramaditya, B., 1998, Fusing Force and Vision Feedback for Micromanipulation, Proc. ICRA, Int. Conf. on Robotics an Automation, 1220-1 225.

[7] Feddema, J.T., Simon, R.W., 1998, CAD-Driven Microassembly and Visual Servoing, Proc. ICRA,

[8] Gengenbach, U., et al., 1995, Ein System zur automatischen Montage von Mikrosystemen, 2. Statuskolloquium des Projektes Mikrosystemtechnik, Tagungsband, Forschungszentrum Karlsruhe, pp.

[9] Fatikow, S., 1997, Microrobots Take On Microassembly Tasks, mst news, 22:20-26.

[lo] Fatikow, S., 1998, Microrobot-Based Assembly of Microsystems, NEXUS Research News, 1 :12-14.

[ll] Tsuchitani, S., et al., 1994, Measurement of the Surface Force in Micro Structures and Its Reduction, Trans. of the Society of Instrument and Control Engineers, 30/2:136-142 (in Japanese).

[12] Tsuchitani, S., Suzuki, S., et al, 1996, Theoretical Study on the Surface Force in Microstructures, Trans. of the Society of Instrument and Control Engineers, 32/5:637-645 (in Japanese).

[13] Fearing, R.S., 1995, Survey of Sticking Effects for Micro Parts Handling, Proc. IROS '95, IEEE/RSJ Int. Conf. on Intelligent Robots and Systems, 2:212-217.

[14] Drexler, K.E., 1992, Nanosystems, Molecular Machinery, Manufacturing, and Computation, pp. 65, John Wiley & Sons.

[I51 Arai, F., Ando, D., Fukuda, T., Nonoda, Y., Oota, T., 1995, Micro Manipulation Based on Micro Physics - Strategy Based on Attractive Force Reduction and Stress Measurement, Proc. IROS '95, 2:236-241.

[16] Arai, F., Andou, D., Nonoda, Y., Fukuda, T., Iwata, H., Itoigawa, K., 1996, Micro Endeffector with Micro Pyramids and Integrated Piezoresistive force Sensor, Proc. IROS '96, 2:842-849.

[17] Adamson, A.W., 1990, Physical Chemistry of Surfaces, Fifth edition, pp. 58-59, Wiley&Sons.

[18] Maboudian, R., Howe, R.T., 1997, Critical Review: Adhesion in surface micromechanical structures, J. Vac. Sci. Technol. B, 15/1 :I-20.

[19] Fearing, R.S., 1995, Grasping for Macro-scale and Micro-scale, IROS '95, Workshop on "Working in the micro-world: Systems to enable the manipulation and machining of micro-objects", 13-40.

[20] Yeh, H., Smith, J.S., 1994, Fluidic self-assembly for the integration of GaAs light-emitting diodes on Si substrates, IEEE Photonics Technology Letters,

[21] Bark, C., Binnenboese, T., 1998, Gripping with low viscosity fluids, IEEE Int. Workshop on MEMS, 301- 305.

[22] Bbhringer, K.F., Prasad, R., McDonald, N.C., 1995, Design, fabrication and characterization of single crystal silicon latching snap fasteners for micro assembly, Proc. of ASME Int. Mech. Eng. Congress and Exposition (IMECE95).

[5]

121 2-1 21 9.

62-66.

6/6:706-708.

[23] Zesch, W., Brunner, M. , Weber, A., 1997, Vacuum tool for handling micro-objects with a nano-robot, Proc. ICRA, 1761-1766.

[24] El-Khoury, M., 1998, Ice gripper handles micro-sized components, Design News, September, pp. 8.

[25] Bbhringer, K.F., Fearing, R.S., Golgberg K.Y., 1998, Chapter Micro-assembly, to appear in The Handbook of Industrial Robotics, Second Edition, Wiley & Sons.

[26] Kim, C.-J., Pisano, A.P., Muller, R.S., 1992, Silicon- Processed Overhanging Microgripper, Journal of Microelectromechanical Systems, 1/1:31-36.

[27] Suzumori, K., likura, S., Tanaka, H., 1991, Flexible Microactuator for Miniature Robots, Proc. IEEE MEMS workshop, 204-209.

[28] Peirs, J., Reynaerts, D., Van Brussel, H., A Micro Robotic Arm for a Self Propelling Colonoscope, 1998, Proc. Actuator '98, 576-579.

[29] Tanikawa, T., Arai, T., Masuda, T., 1996, Development of Micro Manipulation System with Two-Finger Micro Hand, Proc. IROS, 2:850-855.

[30] Kallio, P., Lind, M., Zhou, Q., Koivo, H.N., 1998, A 3 DOF Piezohydraulic Parallel Micromanipulator, Proc.

[31] Weck, M., Hummler, J., Petersen, B., 1997, Assembly of hybrid micro systems in a large- chamber electron microscope by use of mechanical grippers, Proc. of SPIE, Micromachining and Microfabrication Process Technology 111, 3223:223- 229.

[32] Carroua, M.C., Dario, P., Menciassi, A., Fenu, A., 1998, Manipulating Biological and Mechanical Micro- Objects Using LIGA-Microfabricated End-Effectors, Proc. ICRA, 181 1-1816.

[33] Ando, Y., 1992, Micro Grippers, J. Robotics Mechatronics, 2:214-217.

[34] Breguet, J.-M., Henein, S., Mericio, R., Clavel, R., 1997, Monolithic Piezoceramic Flexible structures for Micromanipulation, Proc. of the gth Int. Precision Engineering Seminar, 397-400.

[35] Bellouard, Y., Clavel, R., Gotthardt, R., Bidaux J.-E., Sidler, T., 1998, A New Concept of Monolithic Shape Memory Alloy Micro-devices used in Micro-robotics, Proc. Actuator 98, 499-502.

[36] Hesselbach, J., Pittschellis, R., Hornbogen, E., Mertmann, M., 1997, Shape memory alloys for use in miniature grippers, Proc. SMST, Int. Conf. On Shape Memory and Superelastic Technologies, 251 - 256.

[37] Ikuta, K., 1990, Micro/Miniature Shape Memory Alloy Actuator, Proc. ICRA, 2156-21 61.

[38] Furuya, Y., Shimada, H., 1990, Shape Memory Actuators for Robotic Applications, in Engineering Aspects of Shape Memory Alloys, Buttennrorth- Heinemann, 338-355.

[39] http://bsac.eecs.berkeley.edu/, Berkeley Sensor & Actuator Center web page.

[40] Greitmann, G., Buser, R.A., 1996, Tactile microgripper for automated handling of microparts, Sensors and Actuators, A53:410-415.

[41] Keller, C.G., Howe, R.T., 1997, Hexsil Tweezers for Teleoperated Micro-Assembly, IEEE Workshop on Micro Electro Mechanical Systems, 72-73.

[42] Westkiimper, E., Schraft, R.D., Bark, C., V6gele, G., Weisener, T., 1996, Adhesive Gripper - a new

ICRA, 1823-1 828.

470

Keynote Papers

approach to handling MEMS, Proc. Actuator 96,

[43] Grutzeck, H., Kiesewetter, L., 1998, Downscaling of Adhesive and Capillary Grippers for Micro Assembly, Micro System Technologies 98, 6‘h Int. Conf. On Micro Electro, Opto, Mechanical Systems and Components, 237-242.

[44] Arai, F., Fukuda, T., 1997, Adhesion-type Micro Endeffector for Micromanipulation, Proc. ICRA,

[45] Helin, P., Druon, C., Sadaune, V., 1996, A Microconveyor Using Surface Acoustic Waves in the HF Band, Proc. Mecatronics ’96, 580-582.

[46] Reinhart, G., Hoppner, J., 1999, Beruhrungslose Handhabung mit Leistungsschall, Werkstattstechnik,

[47] Coutinho, M.G., Will, P.M., Viswanathan, P.S., 1997, The Intelligent Motion Surface: A hardware/software tool for the assembly of meso-scale devices, Proc.

(481 Suh, J.W., Glander, S.F., Darling, R.B., Storment, C.W., Kovacs, G.T.A., 1996, Combined Organic Thermal and Electrostatic Omnidirectional Ciliary Microactuator Array for Object Positioning and Inspection, MEMS Workshop, 168-173.

[49] Bohringer, K.-F., Suh, J.W., Donald, B.R. Kovacs, G.T.A., 1997, Vector Fields for Task-level Distributed Manipulation: Experiments with Organic Micro Actuator Arrays, Proc. ICRA, 1779-1786.

[50] Bohringer, K.-F., Donald, B.R., Mihailovich, R., MacDonald, N.C., 1994, A Theory of Manipulation and Control for Microfabricated Actuator Arrays,

[51] Konishi, S., Fujita, H., 1996, Two-dimensional Conveyance System using Cooperative Motions of Many Microactuators, Proc. IROS ‘96, 2:988-992.

[52] Mita, Y., Konishi, S., Fujita, H., 1996, Implementation of a micro conveyance system controller, Proc. of the 3rd France-Japan Congress & 1 st Europe-Asia Congress on Mechatronics,

[53] Sato, T., 1996, Micro/Nano Manipulation World, Proc. IROS ‘96, 2:834-841.

[54] Hohn, M., Jacob, D., 1999, Montagetechnik fur Mikrosysteme, iwb-newsletter 7,1/2:2-3.

[55] Rambin, C.L., Warrington, R.O., 1994, Micro- assembly with a focused laser beam, Proc. IEEE

[56] Higurashi, E., Ukita, H., Tanaka, H., Ohguchi, O., 1994, Rotational control of anisotropic micro-objects by optical pressure, Proc. IEEE MEMS, 291-296.

[57] Morishima, K., Arai, F., Fukuda, T., Matsuura, H., Yoshikawa, K., 1998, Bio-Micromanipulation System for High Throughput Screening of Microbes in Microchannel, Proc. ICRA, 1198-1203.

[58] Sugiura, T., Kawata, S., Minami, S., 1990, Optical rotation of small particles by a circulary-polarized laser beam in an optical microscope, J. Spectrosc. SOC. Jpn., 39:342.

[59] Sato, S., Ishigure, M., Inaba, H., 1991, Optical trapping and rotational manipulation of microscopic particles and biological cells using higher-order mode Nd:YAG laser beams, Electron. Lett., 27: 1831 .

[60] Ahn, C.H., Allen, M.G., 1994, A Fully Integrated Micromachined Magnetic Particle Manipulator and Separator, Proc IEEE MEMS, 91-96.

100-1 03.

1472-1477.

89191429-432.

ICRA, 1755-1760.

IEEE MEMS, 102-1 07.

2:583-588.

MEMS, 285-290.

[61] http://www.sees.bangor.ac.uk/-rslee/biochip/paged science/dep.htm, 2000, Internet pages of the Institute of Molecular & Biomolecular Electronics, University of Wales, United Kingdom.

[62] Morishima, K., Fukuda, T., Arai, F., Yoshikawa, K., 1997, Manipulation of DNA Molecule Utilizing the Conformational Transition in the Higher Order Structure of DNA, Proc. ICRA, 1454-1459.

[63] http://www.sees.bangor.ac.uk/-rslee/biochip/mainpa ges/frontframe.htm, lntemet pages of the Institute of Molecular & Biomolecular Electronics, University of Wales, United Kingdom.

[64] Sniegowski, J.J., 1996, Multi-level polysilicon surface-micromachining technology: applications and issues, ASME Int. Mechanical Engineering Congress and Exposition, Proc. ASME Aerospace Division, AD-Vol. 52:751-759.

[65] Langen, H.H., Masuzawa, T. , Fujino, M., 1995, Modular Method for Microparts Machining and Assembly with Self-Alignment, Annals of the CIRP,

[66] Masuzawa, T., Tonshoff, H.K., 1997, Three- Dimensional Micromachining by Machine Tools, Annals of the CIRP, 46/2:621-628.

[67] Muller, R.S., Lau, K.Y., 1998, Surface- Micromachined Microoptical Elements and Systems, Proc. of the IEEE, Special Issue: Integrated Sensors, Microactuators, & Microsystems (MEMS),

[68] Syms, R.R.A., 1995, Equilibrium of Hinged and Hingeless Structures Rotated Using Surface Tension Forces, Journal of Microelectromechanical Systems, 4/4:177-184.

[69] Green, P.W., Syms, R.R.A., Yeatman, E.M., 1995, Demonstration of Three-Dimensional Microstructure Self-Assembly, Journal of Microelectromechanical Systems, 4/4:170-176.

[70] Yeh, R., Kruglick, E.J.J., Pister, K.S.J., 1996, Surface-Micromachined Components for Articulated Microrobots, Journal of Microelectromechanical Systems, 511 : 10-1 7.

[71] Kiang, M.-H., Solgaard, O., Lau, K.Y., Muller, R.S., 1998, Electrostatic Combdrive-Actuated Micromirrors for Laser-Beam Scanning and Positioning, Journal of Microelectromechanical Systems, 7/1:27-37.

[72] Kiang, M.-H., Solgaard, O., Lau, K.Y., Muller, R.S., 1998, Polysilicon optical microscanners for laser displays, Sensors and Actuators, A70: 195-1 99.

[73] Reid, J.R., Bright, V.M., Butler, J.T., 1998, Automated assembly of flip-up micromirors, Sensors and Actuators, A66:292-298.

[74] Oh, K.W., Ahn, C.H., Roenker, K.P., 1999, Flip-Chip packaging using micromachined conductive polymer bumps and alignment pedestals for MOEMS, IEEE J. of selected topics in quantum electronics,

[75] Seter, D.J., Degani, O.B., Klador, S., Scher, E., Nemirovsky, Y., 1999, Characterization of a novel micromachined optical vibrating rate gyroscope, Review of scientific instruments, 70:2.

[76] Salalha, W., Zussman, E., Kaldor, S., 2000, Prediction of yield for flip-chip packaging, Proc. of the CIRP design seminar.

[77] Irwin, R., Zhang, W., Harsh, K., and Lee, Y.C., 1999, Quick prototyping of flip-chip assembly with MEMS,

44/1:173-176.

86/8: 1705-1 720.

5/11 1 19-1 26.

47 1

Technical Report, Department of Mechanical Eng., University of Colorado at Boulder.

[78] Singh, A., Horsley, D.A., Cohn, M.B., Pisano, A.P., and Howe, R.T., 1999, Batch transfer of microstructures using flip-chip solder bonding, IEEE J. of microelectromechanical systems, 8/1:27-33.

[79] Begemann, M., Ehtfeld, W., Michel, R., Weber, L., Berg, U., 1998, Preadjusted Assembly Magazines, Micro System Technologies 98, 6Ih International Conference on Micro Electro, Opto, Mechanical Systems and Components, 676-678.

[80] Yeh H.-J.J., Smith, J.S., 1994, Fluidic Self-Assembly of Microstructures and its Application to the Integration of GaAs on Si, Proc. IEEE MEMS, 279- 284.

[81] Cohn, M.B., 1994, Method and Apparatus for the Assembly of Microfabricated Devices, US Patent nr. 5 355 577.

[82] Bohringer, K.-F., Goldberg, K., Cohn, M., Howe, R., Pisano, A., 1998, Parallel Microassembly with Electrostatic Force Fields, Proc. ICRA, 1204-121 1.

[83] Shivkumar, B., Kim, C.-J., 1997, Microrivets for MEMS Packaging: Concept, Fabrication, and Strength Testing, Journal of Microelectromechanical Systems, 6/3:217-225.

[84] Han, H., Weiss, L.E., Reed, M.L., 1992, Micromechanical Velcro, Journal of Microelectromechanical Systems, 1/1:37-43.

[85] Gonzalez, C., Smith, R.L., Howitt, D.G., Collins, S.D., 1998, MicroJoinery: concept, definition, and application to microsystem development, Sensors and Actuators, A66:315-332.

[86] Lapadatu, A.C., 1999, Anodic Bonding for the Fabrication of Self-Testable pressure Sensors, PhD thesis, Catholic University Leuven, Belgium.

[87] Schulze, S., 1995, Silicon bonding in microsystem technology, Proc. of the NEXUS-Workshop on Micro-Machining.

[88] Madou, M., 1997, Fundamentals of Microfabrication, p. 389, CRC-Press.

[89] Poulis, J.A., Cool, J.C., Logtenberg, E.H.P., 1994, Lijmen van kleine onderdelen (2), UV/ozon en andere reinigingsmethoden vergeleken, Mikroniek

[go] Eberhardt, R., Scheller, T., Menzel, E.-M., Doring, M., Lenhardt, K., Kopp, H., 1998, Automated assembly of Microcomponents supported by a 3D- inspection system , Micro System Technologies 98, 6'h Int. Conf. On Micro Electro, Opto, Mechanical Systems and Components, 249-255.

[91] Poulis, J.A., Cool, J.C., Logtenberg, E.H.P., 1993, Lijmen van kleine onderdelen (l), Mikroniek 5140- 147.

[92] Patra, S.K., Sritharan, S.S., Lee, Y.C., 1995, Quantitative Characterisation of a Flip-Chip Solder Joint, Transactions of the ASME, Journal of Applied Mechanics, 62:390-397.

[93] Humpston, G., 1997, Flip-Chip Solder Bonding for Microsystems, IEE Colloquium on assembly and connections in microsystems, 3:l-3.

[94] Brenner, W. , Petrovic, D., Chatzitheodoridis, E., Popovic, G., Haddad, G., Kukla, C., Vujanic, A., 1998, A new Manipulation System for use under SEM Observation, Micro System Technologies 98, 6th Int. Conf. on Micro Electro, Opto, Mechanical Systems and Components, 728-730.

1 :24-29.

[95] Hatamura, Y., Nakao, M., Sato, T., 1997, Construction of an Integrated Manufacturing System for 3D Microstructure - Concept, Design and Realization, Annals of the CIRP, 46/1:313-318.

[96] Suga, T., Ishii, Y., Hosoda, N., 1997, Microassembly System for Integration of MEMS Using the Surface Activated Bonding Method, IEICE Trans. on Electronics, E80-C/2:297-302.

[97] Feldmann, K., Krimi, S., 1998, Alternative Placement Systems for Three-Dimensional Circuit Boards, Annals of the CIRP,47/1:23-26.

[98] Feldmann, K., Brand, A., 1994, Analytical and Experimental Research on Assembly Systems for Molded Interconnection Devices (3D-MID), Annals of the CIRP, 43/1:15-18.

[99] Schuenemann, M., Grimme, R., Kaufmann, T., Schwaab, G., Bader, U., Schaefer W., Dorner, J., 1997, Manufacturing Concepts and Development Trends in the Industrial Production of Microelectromechanical Systems, Proc. of SPIE: Microrobots and Microsystem Fabrication, 3202:l- 12.

[IOOIReinhatt, G., Hohn, M., 1997, Growth into Miniaturisation - Flexible Microassembly Automation, Annals of the CIRP, 46/1:7-10.

[101]Weule, H., Spath, D., Blessing, A., Peles, T., 2000, Handling of miniaturized components by sensor-led hybrid kinetics, Proc. IAS6.

[I 021 Wissink, J.M., 2000, MATAS: A modular assembly technology for hybrid pTAS, mst news, 1:20-22.

[103]Gattner, C., Blumel, V., Kraplin, A., PoOner, T., 2000, Micro-assembly processes for beam transformation systems of high-power laser diode bars, mst news, 1:23-24.

104]Tilmans, H.A.C., Fullin, E., Ziad, H., Van de Peer, M.D.J., Kesters, J., Van Geffen, E., Bergqvist, J., Pantus, M., Beyne, E., Baert, K., Naso, F., 1999, A fully-packaged electromagnetic microrelay, MEMS

105]Schlaak, H.F., Arndt, F., Hanke, M., 1997, Silicon- microrelay - a small signal relay with electrostatic actuator, 45Ih Annual Int. Relay Conf.

[106]Schaldach, M., 1999, Synergetic Engineering - Entwicklung eines neuen Integrations- und Fertigungskonzeptes anhand mikroelektronischer Implantate, PhD thesis, Technical University Berlin.

'99,25-30.

472