© 2002 by CRC Press LLC amount of backlash built into the system. A third problem relates to the ability of the hinge to pivot. Often there is not a large rotational moment to rotate the mirror out of plane when planar hinged structures are connected to planar-surface-micromachined actuators. This means that the designer must take great care to ensure that the hinge always rotates in the correct direction. 27.5 Failure Mechanisms in MEMS One of the most effective ways we can learn is to learn from our own mistakes. This can be a memorable experience but in the field of MEMS it can be a very expensive and inefficient one. One reason is that the time between design completion and testing is usually measured in months and the price per fabri- cation run is many thousands of dollars. As of the year 2000, there were only rudimentary modeling and simulation tools available for surface-micromachined mechanisms. This section seeks to share learning obtained at the expense of others by describing some mechanical failures in surface-micromachined mechanisms. The hope is that the reader will gain a deeper appreciation for the complexities of surface- micromachined mechanism design and learn about some of the pitfalls. 27.5.1 Vertical Play and Mechanical Interference in Out-of-Plane Structures Surface-micromachined parts typically have a thickness that is very small in relationship to their width or breadth. In the out-of-plane direction, the thickness is limited to a few micrometers due to the limited deposition rates of low-pressure chemical vapor deposition (LPCVD) systems and the stresses in the deposited films. In the plane of the substrate, structures can be millimeters across. These factors typically lead to surface-micromachined structures that have a very small aspect ratio as well as stiffness issues in the out-of-plane direction due to the limited thickness of the parts. The result is that designers of surface- micromachines need to design structures in three dimensions and account for potential movements out of the plane of the substrate. One instance of a potential problem is when gears fabricated in the same structural layer of polysilicon fail to mesh because one or both of the gears are tilted. Another is when structures moving above or below another structure mechanically interfere with each other when it was the intended for them to clear each other without touching. Both of these instances will be examined separately. An example of the out-of-plane movement of gears is illustrated in Figure 27.26. In this instance, the driven gear in the top of the figure has been wedged underneath the large load gear at the bottom of the photograph. The way to prevent this situation is to understand the forces that create the out-of-plane motion FIGURE 27.26 The gear teeth of the small gear are wedged underneath the teeth of the large diameter gear. In this case, gear misalignment is about 2.5 µm in the vertical direction. © 2002 by CRC Press LLC amount of backlash built into the system. A third problem relates to the ability of the hinge to pivot. Often there is not a large rotational moment to rotate the mirror out of plane when planar hinged structures are connected to planar-surface-micromachined actuators. This means that the designer must take great care to ensure that the hinge always rotates in the correct direction. 27.5 Failure Mechanisms in MEMS One of the most effective ways we can learn is to learn from our own mistakes. This can be a memorable experience but in the field of MEMS it can be a very expensive and inefficient one. One reason is that the time between design completion and testing is usually measured in months and the price per fabri- cation run is many thousands of dollars. As of the year 2000, there were only rudimentary modeling and simulation tools available for surface-micromachined mechanisms. This section seeks to share learning obtained at the expense of others by describing some mechanical failures in surface-micromachined mechanisms. The hope is that the reader will gain a deeper appreciation for the complexities of surface- micromachined mechanism design and learn about some of the pitfalls. 27.5.1 Vertical Play and Mechanical Interference in Out-of-Plane Structures Surface-micromachined parts typically have a thickness that is very small in relationship to their width or breadth. In the out-of-plane direction, the thickness is limited to a few micrometers due to the limited deposition rates of low-pressure chemical vapor deposition (LPCVD) systems and the stresses in the deposited films. In the plane of the substrate, structures can be millimeters across. These factors typically lead to surface-micromachined structures that have a very small aspect ratio as well as stiffness issues in the out-of-plane direction due to the limited thickness of the parts. The result is that designers of surface- micromachines need to design structures in three dimensions and account for potential movements out of the plane of the substrate. One instance of a potential problem is when gears fabricated in the same structural layer of polysilicon fail to mesh because one or both of the gears are tilted. Another is when structures moving above or below another structure mechanically interfere with each other when it was the intended for them to clear each other without touching. Both of these instances will be examined separately. An example of the out-of-plane movement of gears is illustrated in Figure 27.26. In this instance, the driven gear in the top of the figure has been wedged underneath the large load gear at the bottom of the photograph. The way to prevent this situation is to understand the forces that create the out-of-plane motion FIGURE 27.26 The gear teeth of the small gear are wedged underneath the teeth of the large diameter gear. In this case, gear misalignment is about 2.5 µm in the vertical direction. © 2002 by CRC Press LLC 28 Microrobotics 28.1 Introduction MEMS as the Motivation for Robot Miniaturization 28.2 What is Microrobotics? Task-Specific Definition of Microrobots • Size- and Fabrication-Technology-Based Definitions of Microrobots • Mobility- and Functional-Based Definition of Microrobots 28.3 Where To Use Microrobots? Applications for MEMS-Based Microrobots • Microassembly 28.4 How To Make Microrobots? Arrayed Actuator Principles for Microrobotic Applications • Microrobotic Actuators and Scaling Phenomena • Design of Locomotive Microrobot Devices Based on Arrayed Actuators 28.5 Microrobotic Devices Microgrippers and Other Microtools • Microconveyers • Walking MEMS Microrobots 28.6 Multirobot System (Microfactories and Desktop Factories) Microrobot powering • Microrobot communication 28.7 Conclusion and Discussion 28.1 Introduction 28.1.1 MEMS as the Motivation for Robot Miniaturization The microelectromechanical systems (MEMS) field has traditionally been dominated by silicon micro- machining. In the early days, efforts were concentrated on fabricating various silicon structures and relatively simple components and devices were then developed. For describing this kind of microelec- tromechanical structures the acronym MEMs is used. A growing interest in manufacturing technologies other than the integrated circuit (IC)-inspired silicon wafer and batch MEMs fabrication is evident in the microsystem field today. This interest in alternative technologies has surfaced with the desire to use new MEMs materials, that enable a greater degree of geometrical freedom than materials that rely on planar photolithography as a means to define the structure. One such new MEMs material is plastic, which can be used to produce low-cost, disposable microdevices through microreplication. The micro- machining field has also matured and grown from a technology used to produce simple devices to a technology used for manufacturing complex miniaturized systems which has shifted the acronym from representing structures to microelectromechanical systems . Microsystems encompass microoptical systems (microoptoelectromechanical systems, MOEMS), microfluidics (micro-total analysis systems, µ -TAS) etc. These systems contain micromechanical components including moveable mirrors and lenses, sensors, light sources, pumps and valves and passive components such as optical and fluidic waveguides, as well as electrical components and power sources of various types. Thorbjörn Ebefors Royal Institute of Technology Göran Stemme Royal Institute of Technology © 2002 by CRC Press LLC 29 Microscale Vacuum Pumps 29.1 Introduction 29.2 Fundamentals Basic Principles • Conventional Types of Vacuum Pumps • Pumping Speed and Pressure Ratio • Definitions for Vacuum and Scale 29.3 Pump Scaling Positive-Displacement Pumps • Kinetic Pumps • Capture Pumps • Pump-Down and Ultimate Pressures for MEMS Vacuum Systems • Operating Pressures and Requirements in MEMS Instruments • Summary of Scaling Results 29.4 Alternative Pump Technologies Outline of Thermal Transpiration Pumping • Accommodation Pumping 29.5 Conclusions Acknowledgments 29.1 Introduction Numerous potential applications for meso- and microscale sampling instruments are based on mass spectrometry [Nathanson et al., 1995; Ferran and Boumsellek, 1996; Orient et al., 1997; Piltingsrud, 1997; Wiberg et al., 2000; White et al., 1998; Freidhoff et al., 1999; Short et al., 1999] and gas chroma- tography [Terry et al., 1979]. Other miniaturized instruments utilizing electron optics [Chang et al., 1990; Park et al., 1997; Callas, 1999] will require both high-vacuum and repeated solid-sample transfers from higher pressure environments. The mushrooming interest in chemical laboratories on chips will likely evolve toward some manifestations requiring vacuum capabilities. At present, there are no microscale or mesoscale vacuum pumps to pair with the embryonic instruments and laboratories that are being devel- oped. Certainly, small vacuum pumps will not always be necessary. Some of the new devices are attractive because of low quantities of waste and rapidity of analysis, not directly because they are small, energy efficient, or portable. However, for other applications involving portability and/or autonomous opera- tions, appropriately small vacuum pumps with suitably low power requirements will be necessary. This chapter addresses the question of how to approach providing microscale and mesoscale vacuum pumping capabilities consistent with the volume and energy requirements of meso- and microscale instruments and processes in need of similarly sized vacuum pumps. It does not review existing microscale pumping devices because none are available with attractive performance characteristics (a review of the attempts has recently been presented by Vargo, 2000; see also NASA/JPL, 1999). In the macroscale world, vacuum pumps are not very efficient machines, ranging in thermal efficiencies from very small fractions of one percent to a few percent. They generally do not scale advantageously to N ˙ E. Phillip Muntz University of Southern California Stephen E. Vargo SiWave, Inc. © 2002 by CRC Press LLC 30 Microdroplet Generators 30.1 Introduction 30.2 Operation Principles of Microdroplet Generators Pneumatic Actuation • Piezoelectric Actuation • Thermal-Bubble Actuation • Thermal-Buckling Actuation • Acoustic-Wave Actuation • Electrostatic Actuation • Inertial Actuation 30.3 Physical and Design Issues Frequency Response • Thermal/Hydraulic Cross-Talk and Overfill • Satellite Droplets • Puddle Formation • Material Issues 30.4 Fabrication of Microdroplet Generators Multiple Pieces • Monolithic Fabrication 30.5 Characterization of Droplet Generation Droplet Trajectory • Ejection Direction • Ejection Sequence/velocity and Droplet Volume • Flow Field Visualization 30.6 Applications Inkjet Printing • Biomedical and Chemical Sample Handling • Fuel Injection and Mixing Control • Direct Writing and Packaging • Optical Component Fabrication and Integration • Solid Freeforming • Manufacturing Process • Integrated Circuit Cooling 30.7 Concluding Remarks 30.1 Introduction Microdroplet generators are becoming an important research area in microelectromechanical systems (MEMS), not only because of the valuable marketing device—inkjet printhead—but also because of the many other emerging applications for precise or micro-amount fluidic control. There has been a long history of development of microdroplet generators ever since the initial inception by Sweet (1964, 1971), who used piezo actuation, and by Hewlett-Packard and Cannon [Nielsen et al., 1985] in the late 1970s, who used thermal bubble actuation. Tremendous research activities regarding inkjet applications have been devoted to this exciting field. Emerging applications in the biomedical, fuel-injection, chemical, pharmaceutical, electronic fabrication, microoptical device, integrated circuit cooling, and solid freeform fields have fueled these research activities. Thus, many new operation principles, designs, fabrication processes and materials related to microdroplet generation have been explored and developed recently, supported by micromachining technology. In this chapter, microdroplet generators are defined as droplet generators generating microsized droplets in a controllable manner; that is, droplet size and number can be accurately controlled and counted. Thus, atomizer, traditional fuel-injector or similar droplet-generation devices that do not offer such control are not discussed here. Fan-Gang Tseng National Tsing Hua University . shifted the acronym from representing structures to microelectromechanical systems . Microsystems encompass microoptical systems (microoptoelectromechanical systems, MOEMS), microfluidics. powering • Microrobot communication 28.7 Conclusion and Discussion 28.1 Introduction 28.1.1 MEMS as the Motivation for Robot Miniaturization The microelectromechanical systems (MEMS) field has. Pump Scaling Positive-Displacement Pumps • Kinetic Pumps • Capture Pumps • Pump-Down and Ultimate Pressures for MEMS Vacuum Systems • Operating Pressures and Requirements in MEMS