Assembly and manipulation of micro devices—A state of the art survey

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Robotics and Computer-Integrated Manufacturing 23 (2007) 580–588 www.elsevier.com/locate/rcim

Assembly and manipulation of micro devices—A state of the art survey J. Cecil, Derek Powell, Daniel Vasquez Virtual Enterprise Engineering Laboratory (VEEL), Center for Information Based Manufacturing (CINBM), Department of Industrial Engineering, New Mexico State University, Las Cruces, NM 88003, USA Received 11 June 2004; received in revised form 14 August 2005; accepted 1 May 2006

Abstract This paper provides a comprehensive review of research efforts in the emerging field of micro devices assembly (MDA) as well as identifies future directions for research. The general domain dealing with both manual and automated assembly of micro devices can be referred to as MDA. The study of computer-based methods to accomplish the assembly of micron-sized parts can be described as Automated MDA (AMDA). The primary focus of this paper is to provide an overview of concepts related to MDA as well as a review of various segments of MDA research including study of the role of interactive forces at the micro level, the design of innovative gripping and assembly techniques as well as the use of information technology (IT) based approaches. r 2006 Elsevier Ltd. All rights reserved. Keywords: Micro assembly; Micron sized parts; Robots; Micro grippers

1. Introduction and background Micro-electrical mechanical system (MEMS) are mechanisms that generally incorporate silicon-based mechanical and electrical components. MEMS technology exploits the existing microelectronics infrastructure to create complex machines with micron feature sizes. These machines can perform complex functions including communication, actuation and sensing (Fig. 1 shows micro gears). Currently, the most common techniques used to fabricate MEMS devices involve monolithic techniques that require no or little assembly. Typical products manufactured utilizing this technique are accelerometers, and inkjet printer heads. However, micron sized devices that have incompatible processes, different materials, or complex geometries, have to be ‘assembled’. Manual assembly involves a highly skilled human operator pick and place micro-parts manually using high power microscopes and micro-tweezers. This method of assembly is difficult, tedious and time consuming. Innovative computer-based automated assembly methods must be developed to increase efficiency, reliability, and reduce cost. This Corresponding author.

E-mail address: [email protected] (J. Cecil). 0736-5845/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.rcim.2006.05.010

emerging field involving the study of computer-based methods to accomplish the assembly of micron-sized parts can be described as automated micro devices assembly (AMDA). The general domain dealing with both manual and automated assembly of micro devices can be referred to as micro devices assembly (MDA). This paper reviews research efforts in this field of growing importance as well as identifies future directions for research. The assembly of micro devices involves handling of parts that are extremely small (in the order of 10 6 m). While the exact configuration of physical micro assembly cells may vary, a typical cell will comprise of computer-controlled devices (such as robots, grippers, etc.), components and fixtures that can functionally accomplish or support the accomplishment of one or all of the following tasks: (a) grasping of a target part (or micro device), (b) manipulation and placement/assembly of parts, (c) part and object recognition before, during and after assembly, (d) planning and control of the actuators and grippers to accomplish the physical assembly. The main segments of current and past research have dealt with addressing the automation of the manual micro

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Computer assisted

Vision

Eyes Cameras

Cameras

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Arms

Stages micro positioners

Tools

Tweezers

Grippers Automated tweezers Interfaces

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Section 5 provides an overview of future research directions. This paper concludes in Section 6. 2. Interactive forces at the micro level

Fig. 1. Functional themes underlying manual and computer assisted micro assembly.

Fig. 2. An example of a linear stage used for micro positioning applications.

assembly activities. These themes are reflected in Fig. 1 and can be categorized functionally into three main areas: in general, the underlying motivation has been to attempt to mimic and replace the functionalities embodied in a skilled human who can perform micro assembly. The first area of interest deals with visually sensing the positions of the target parts, associated pins, obstacles, etc; the second area of interest focuses on the ability to actually perform the difficult micro assembly task using stages and other positioners (while a manual process may involve using arms and hands to reach and maneuver around a target assembly area, an automated approach can seek to use precision micro stages as shown in Fig. 2; the third area deals with designing and building various grippers and interfaces which would enable accomplishing assembly rapidly (one of the most common manual approaches is the use of tweezers, which requires considerable skill). In summary, while manual assembly involves using eyes, microscopes, fingers, arms as well as tools such as tweezers, computer assisted or automated methods (which are the focus of research efforts in MDA) have explored the use of sensors, cameras, grippers and stages (an example of a stage is shown in Fig. 2). This paper is divided into several sections of research. In Section 2, the study of forces, which come into play at the micro level, is discussed. Research efforts dealing with gripping and assembly techniques are reviewed in Section 3. Innovative methods, based on use of machine vision techniques as well as virtual reality (VR) technology to aid in micro assembly tasks, are delineated in Section 4.

In the macro-world of assembly, gravity dominates. However, in the micro domain, gravity becomes negligible, and adhesive forces dominate pick and place operations. Releasing a part from the grasp of micro-gripper is no simple task as a part may stick to the gripper (due to the presence of these interactive forces). In [1], a thorough overview of micro assembly issues is provided. Other topics reviewed deal with the role of the interactive forces at the micro level including sticking effects, adhesion, electrostatic forces, and dielectrics. Various research efforts have attempted to measure forces in the micro domain. In [2], a method to measure micro level forces is outlined using a cantilever beam and cameras. Using a known spring constant and the displacement of the cantilever beam, the force involved is calculated. Zhou [3] summarized the various categories of forces that are dominant in MEMS. Because of the small size, MEMS are susceptible to molecular interactions, electrical, and magnetic forces. The interactive forces are a function of material properties, geometry, and size of the micro objects. The interactive forces investigated include van der Waals, Casimir attractive, capillary, electrostatic, and magnetic forces. A quantitative comparison of the interaction forces between bodies is also provided: Electrostatic (30 V) was shown to be the strongest, followed by Capillary forces, van der Waals, and finally magnetic forces. A more fundamental discussion of Intermolecular and Surface Forces is discussed elaborately in [4]. The discussion includes an overview of the fundamental theory associated with the van der Waals energy model between molecules, dispersion self energy of a molecule in a medium and other aspects of van der Waals forces such as anisotropy, non-additivity, and retardation effects. Other issues discussed deal with the approach required to calculate the van der Waals energy between two solid geometries. Other relevant background information such as the Hamaker constants and energy equations are also elaborated in this publication. In [5], a theoretical overview of adhesive forces is provided; adhesion models pertaining to a sphere and a flat surface is also discussed. Adhesion oriented interactions is classified into several categories. The first category includes long-range attractive interactions that bring particles to surfaces and establish adhesion contact. These forces include van der Waals, electrostatic, and magnetic forces. The second category of forces focus on adhesion including diffusion, condensation, diffusive mixing, mutual dissolution, liquid and solid bridges between particles and surfaces, and capillary forces. The third category includes very short-range interactions, which can contribute to adhesion only after adhesion contact area has been

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established. These forces include chemical bonds, intermediate bonds and hydrogen bonds. The results of experiments conducted in an effort to understand adhesive forces are also elaborated including the use of ultrasonic, and mega sonic agitations of chemical solutions. In [6], a novel integration technique is proposed to model the interaction energies (van der Waals, electrostatic double layer, and acid-base) of a sphere in an infinite cylindrical pore in an aqueous system. The technique proposed is called surface element integration (SEI) technique and is compared extensively to the modified Deryaguin approximation to model interaction energies. Results indicate that the SEI model fares better than the MDA model when compared to the exact solutions. Gu and Dongqing [7] presented discussions relating to numerical comparisons of van der Waals forces between two models: a sphere and a finite sized cylinder, and a sphere and an infinitely long cylinder. The two models are compared non-dimensionally to van der Waals models that simulate the interaction between a sphere and a flat plate. Montgomery et al. [8] sought to establish first-principle based models for van der Waals interactions of several macro geometric cases involving an infinite cylinder and an opposing geometry. This paper also presented a method to calculate the van der Waals interaction for energy and force between a molecule/atom and an infinite cylinder. Subsequently, the paper develops the models for four basic cases: (1) sphere and infinite cylinder, (2) disk and infinite cylinder (3) disk at 901 and an infinite cylinder, and (4) a slice and an infinite cylinder. Kitchener [9] provided a quick overview of the surface forces that influence the control of particles smaller than about 1 mm. An overview of a deposition and adhesion process is provided; deposition occurs when particles attach physically, and adhesion comes into existence when particles attach through a mechanical force. Deposition is determined by long-range forces and adhesion by shortrange forces. Adhesion is also influenced by the amount of pressure applied to the contact surfaces on a particle, and the solubility of the particle surface in a liquid environment, which can dissolve small contact points and then recrystallize them to form larger contact surfaces. 3. Micro-gripping and micro assembly techniques Micro assembly requires the design of innovative gripping tools as well as the use of ‘stages’ capable of micron resolution and motion. An example of a micro stage is shown in Fig. 2. In this section, a review of micro gripping strategies is discussed. The design of a LIGA manufactured micro-gripper instrumented with semiconductor strain gauges is discussed in [10]. The micro-manipulation system used consists of a micro-gripper, 3 DOF micro-positioners, a PC, and Phantom 1.0 haptic device. The location of the strain gauges that were placed on the LIGA grippers was determined through FEM analysis, and the grippers were

placed at the points of maximum stress. Experiments were performed using optical fibers with various diameters. A force control strategy is discussed and a model for the micro-gripper in an idling condition is also developed. Will and Coutinho [11] discussed a sensor less control algorithm for moving parts as it pertains to Intelligent Motion Surfaces. Intelligent Motion Surfaces use an array of manipulators to have the effect of a force field on the object. Appropriate force fields can have translation, rotation, and alignment effects on both macro and micro scale parts. A dynamic squeeze force field is used to move the parts along paths composed of straight-line segments. Roch et al. [12] designed, fabricated, and assembled a micro gripper (SU-8) utilizing a monolithic hybrid process. The fabrication process created shape memory properties that are inherent in the actuation of the micro-gripper. The micro gripper is driven by current induced thermal expansion, and has been shown to open to 500 mm for a 0.9 A current. Petrovic et al. [13] presented a classic mechanical micro gripper design as a means to pick and place MEMS parts in an assembly. The gripper is described as having the approximate dimensions of 100 mm long  150 mm wide and 50 mm thick. The fingers of the gripper constructed from spring steel open parallel to each other, and can handle parts that range in size from 10 to 2 mm. Force feedback from the gripper is provided via an integrated optical sensor that consists of a LED that is then interpreted by a photosensitive element. Nakao et al. [14] introduces three types of nontweezing micromanipulators: a pneumatically aspirating pipe tip, an electro statically charged needle, and a pipe tip that uses water surface tension to retain micro parts. The three micromanipulators are capable of handling parts as small as 10–500 mm. All three tools utilize vibration to assist in the release of a micro part. A comparison of the three micromanipulators was conducted using four different micro objects: a silicon block, a copper wire, alumina granules, and stainless powder. Experimental results showed that the electrostatically charged needle was the most successful tool in pick and place operations, followed by the pneumatic tool, and the surface tension tool. Keoschkerjan and Wurmus [15] designed and fabricated a micro gripper using UV-lithographic techniques and wet chemical etching from micro structural photosensitive glass. The micro gripper is designed to avoid picking up cylindrical micro devices in a rotational fashion, but instead to grab such parts utilizing parallel motion. Actuation is achieved through a piezoactuator; analytical and numerical comparisons are used to approximate the maximum movement of the gripping arms. Micro-grippers with force control and sensors are discussed in [16,17]. Kasaya et al. [16] describes an automated task involving the manipulation of micro objects performed using visual and force control. Algorithms are developed to manipulate micro objects scattered randomly on a substrate into a given configuration. In [17], a micro-gripper is described which is made from molded

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polysilicon with thermal expansion beams and polysilicon piezoresistive strain gauges. The usefulness of Hexsil tweezers was demonstrated to accomplish simple pick and place operations. Zhou and Nelson [18] delineated the design of a force controlled micro gripper using optical beam deflection techniques. A theoretical model is developed to represent the total adhesive forces that come into play between two objects including van der Waals, electrostatic, and surface tension). The Van der Waals force between a flat plane and a sphere is modeled using H (the Lifshitz–van der Waals constant), D (the diameter of the sphere), and d, which is the adhesion contact distance between the sphere and the plane. Experiments were conducted which led to the conclusion that material properties such as surface roughness and conductivity were important influencing factors in the existence of adhesion forces between a sphere and a plane. In [19], the use of micro-electro discharge machining techniques (fine wire EDM and micro-die sinking EDM) to fabricate micro grippers and suction micromanipulators is elaborated. Modular micro-grippers and vacuum manipulation systems were built using these technologies. The development of a two-dimensional force sensing system is the focus of the research elaborated in [20]. The development of a polyvinylidence fluoride (PVDF) multi-directional force sensor is outlined. PVDF is a polymeric piezoelectric material, which is very flexible and can be used to construct force sensors based on its output voltage. A description of the sensing system created for using this force sensor is provided along with a discussion of experimental results. In [21,22], a summary of micro assembly methods and techniques is provided [23–31]. In [27], a reconfigurable micro assembly system for photonic applications is discussed. The use of shape memory alloy grippers as part of a micro assembly system is described in [31]. Thompson and Fearing [28] described a micro assembly system with two linear positioners orthogonal to each other and a 3axis stage, which was used to locate and move parts using force feedback. Kim et al. [26] outlined an integrated teleoperated assembly system with a vision-based approach, which was used to assemble micron-sized parts using a haptic device. In [32], the use of a MEMS cantilever beam to sense nano-level forces with the help of a nano probe is described. An analytical model is developed based on the role of van der Waals forces, where the cantilever tip is modeled as a sphere. The creation of miniaturized factories for precision assembly is proposed in [24]. In [25], a study of the assembly of lithography galvonoforming abforming (LIGA) ratchet drive mechanisms is presented; these mechanisms consist of pawls, cam, and springs and are assembled using pins cut from 170 mm wire ranging in length from 500 to 1000 mm. Operatorguided insertion and assembly is accomplished using hybrid/position control algorithms. Feddema and Simon [23] investigated fine motion planning for micro-assembly

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Fig. 3. The zyvex micro gripper being used to pick up a micro object.

activities as well as investigated role of the van der Waals forces in micro assembly. Micro tweezers have been used by several researchers at Sandia to pick and place micronsized pins. The design, testing, and operation of an optically transparent electrostatic micro gripper are described in [33]. The gripping force of the micro gripper is measured as a function of voltage, and compared to a parallel plate capacitor. Cecil and Gobinath [34] outlined the creation of a virtual and physical environment to support the assembly of micro devices; in this environment, a micro gripper from Zyvex Corporation was used to accomplish various manipulation tasks; Fig. 3 illustrates the use of the Zyvex micro gripper to pick up a micron sized chip. Wang et al. [35] developed a three-probe nanogripper designed to handle and characterize nanoscopic objects. The nanogripper has three shanks; each shank consists of two parts: a micro fabricated probe and a distal endeffector. The distance between the nanogripper tips ends is approximately 0.3–2 mm. Two techniques of fabrication were utilized to construct the shanks, nanowire growing using a focused ion beam (FIB) microscope and ion beam milling. Thermal bimetallic actuation was used to achieve out-of-plane displacement (20 mm) of the middle digit. 4. Machine vision and virtual reality based techniques to facilitate micro assembly The use of Machine vision techniques and VR technology has been explored at varying levels of application by various researchers to aid in the assembly of micro devices. Machine vision approaches have been used to facilitate identification of objects in a scene as well as to measure the gripping forces during assembly. VR based frameworks have been proposed to enable users to propose and visualize assembly solutions prior to physical assembly. Monferrer and Bonyuet [36] proposed a system to control robots in difficult or dangerous tasks using a VR based framework. A set of guidelines is proposed to define an ideal user interface that would use VR to help an operator control a robot. The case of an underwater robot

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is presented as an example. VR helps to solve a problem called ‘‘special representation’’ that occurs when operators used only video images. It is concluded that the human user should be considered the most important input in the system and that the collaborative environment needs additional development. Cassier et al. [37] discussed the concept of a desktop micro-assembly factory, which is controlled through the interaction of a user and a VR environment. The micro-robotic work cell station consists of two motion manipulators equipped with a tool holder, a coarse motion worktable, a multi-degree of freedom fine positioning system, and a micro-conveyer. Force sensing using a buckling-type force sensor is discussed along with a visually servoed position/force controller. A combination of vision servoing and VR-based simulation allows for assembly tasks to be planned and verified before any physical assembly takes place. Alex et al. [38] integrated VR environments with visual servoing micromanipulation to perform tasks in a real (or physical) micro assembly work cell. The VR interface allows an operator to manipulate objects in the VR world while obtaining feedback from the physical work cell

Fig. 4. A view of the VIRAM-S environment developed at CINBM to support MDA (the positioning stages, work piece supporting platen and other components can be seen).

environment. The interface system is designed to allow remote teleoperation over the Internet using VRML and Java programming languages. Another framework involving use of VR is discussed in [39]; in this approach, simulation of the assembly of micron parts is accomplished prior to performing the actual assembly process. Cecil et al. [21,22] delineated the design of two VR-based frameworks for use in conjunction with a physical micro assembly system. The frameworks are implemented on Windows and Unix platforms respectively; the Windows based system is referred to as VIRAM (an acronym for virtual environment for assembly of micro devices) while the Unix based environment (implemented on a Silicon Graphics workstation) is referred to as VIRAM-S. The Windows based framework (see Fig. 7) uses Perl and VRML 2.0 to support an immersive VR based environment [34]; the Unix based environment is implemented on a Silicon Graphics workstation (see Fig. 4) and is linked to motion sensors, 3 D stereovision eye wear and an advanced Wand (for immersive applications). The VR environment is relative advanced compared to earlier research efforts and is linked to a physical micro assembly cell consisting of a three-axis integrated stage (comprising of three micro positioners) on which rests a work piece platen, a linear stage supporting the gripper, a micro gripper, a video camera, a potentiometer (to control the micro gripper) and a control computer (this physical cell is shown in Fig. 5). A commercially available micro gripper (from Zyvex Corporation) was used in the physical cell (as shown in Fig. 6). In Fig. 3 (for example), this micro gripper is shown picking up a micron sized chip as part of a series of experiments conducted to understand pick and place operations. Ralis et al. [40] described an approach to visual servoing for micro-assembly operations. Multiple visual sensor arrays are used to control the motion of micro-assembly tasks. Workspace is examined ‘‘globally’’ to obtain a rough estimate of the needed position and then ‘‘locally’’ in order to obtain a higher precision view. A technique of depth from focus is used to visually servo along the optical axis of

Color Video Camera Video Microscope

Line Generator High Resolution Monitor

High Intensity Illuminator Micro Gripper

Vibration Isolation Table

Linear Translation Stage (LTS)

Integrated Translation Stae (LTS)

Fig. 5. A view of the physical assembly cell for the assembly of micro devices at the Center for Information Based Manufacturing at New Mexico State University.

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Gripper finger

500 µm

Target pin

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Leads to potentiometer supply

Fig. 6. A closer view of the Zyvex micro gripper (left) and the interface to control its activation (right).

the camera in order to obtain full 3D control of positioning. Conclusions from this research are that image-based visual servoing using optical flow and image intensity can be used to control movement with micron scale repeatability. A real time visual tracking algorithm is described in [41] to visually servo the movement of a robotic arm. Experimental results are presented for a moving train model, where a camera follows the path of a model train as it traveled in an elliptical path. The system that was built addresses three individual problems that are encountered in real-time motion tracking: (a) the fast computation of 3D parameters, (b) predictive control, and (c) planning related to grasp. Optic-flow is a technique used to measure image velocity at each pixel in the image. While the general results were acceptable, it was noted that the positional errors were greater at higher velocities. Lee et al. [42] used multiple CCD video cameras with a zoom lens and a stereo-microscope to perform micromanipulation tasks. The goal of their research was to develop a teleoperated assembly system for the construction of micron sized structures. Image processing is used to gain information about displacement of micro-parts in the workspace. Nelson et al. [43] discussed the use of visual feedback in assembly processes. In order for automatic robotic assembly to work successfully, the authors stressed that a system must be precisely calibrated and be able to place parts to within thousandths of an inch. With the use of a vision sensor, such a system can be less dependent on initial calibration and be more dynamically adjustable through use of feedback from the vision sensor to modify positioning tasks. The authors conclude that with the improvement of vision systems and technology, such an approach will become an invaluable part of future assembly operations and work cells. A detailed discussion

Fig. 7. A view of the virtual environment in VIRAM.

on the general issues relating to visual servo control is presented in [44]. A basic conceptual framework is outlined including basic concepts from computer vision and robotics. Important parts of visual servo control include image extraction, image processing, and closed loop operation. In [34], an integrated approach to micro assembly is discussed. The VIRAM system described uses a genetic algorithm to determine assembly sequences as well uses a 3D motion planner; after various micro assembly plans are proposed, their feasibility are compared in a PC based virtual environment. After analysis, a selected plan is downloaded to the physical environment where a target micro assembly task is implemented. Fig. 7 provides a close-up view of the VIRAM virtual environment.

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Wang et al. [45] summarized their investigation into the stiffness and mechanics of rigid and flexible micro objects using compliant objects. A visual force sensing approach is outlined which used a finite element technique that assumes that the location of the applied force (for the compliant mechanism) and the fixed portions of the mechanism are known. The validation of the force sensing technique involved using a macro-sized mechanism (140 mm  114 mm  3 mm) made of polypropylene, which was actuated using a digital linear step motor (which served to apply an input force). The mechanism was marked with black nodal markers that served as tracking points when observed via a CCD camera. A helical compression spring was placed between the jaws of the mechanism to establish a known force measurement. As the mechanism was actuated, the finite element analysis (FEA) program written in Matlab predicted force inputs based on the displacement of the nodal markers. Position control was experimented with a mechanism made of polypropylene, with two attached actuators. Clark et al. [46,47] have developed an algorithm designed to analyze MEMS known as SUGAR. SUGAR is a collection of MATLAB algorithms that can perform static, steady state and transient analysis of three-dimensional mechanical structures and electrical circuits. The modeling preformed using SUGAR includes mode analysis, residual stress effects, thermal expansion, non-linear deflections, time varying electrostatic forces, process sensitivities, induced currents, and transient performance in accelerated reference frames. SUGAR utilizes a modified nodal analysis approach to formulate a system of Ordinary Differential Equations solved by static, steady state and transient solvers. Jing et al. [48] developed a numerical analytical method known as nodal design of actuators and sensors (NODAS) to model non-linear beam deflection. NODAS is demonstrated in two cases: first, modeling of the deflection of a micro-sized cantilever beam, and subsequently, modeling of the deflection a micro fixed-fixed beam with an applied central point load. The NODAS deflection model of the cantilever beam was plotted and compared to a finite element solution based on Elasticas, and showed agreeable results. The fixed–fixed beam NODAS deflection model was compared to Abaquss, which demonstrated agreeable results. Kamalian et al. [49] presented a contrast of synthesis tools for MEMS. Multi-objective algorithms (MOGA) and single objective genetic algorithm (SOGA) are compared against simulated annealing (SA) based optimization in two cases; the first case was the synthesis of a meandering resonator, and the second case was the optimal synthesis of an electrostatic actuator/spring device. Results for example one showed that SA provided a quicker solution than SOGA. SA required only 279 iterations in 1 h; SOGA required a 30-generation run and 4–5 h. For the second example, MOGA was used to determine an optimal solution in 9 h, but SA was unable to reach an optimal solution after 10 000 iterations and 17 plus hours. The

authors conclude that SA can develop valid solutions faster then GA algorithms but does not handle multiple objectives well and has difficulty in finding optimal design. MacDonald [50] described a manufacturing process (related to the production of microelectronics components) known as single crystal reactive etching and metallization (SCREAM). The SCREAM process utilizes a single crystal silicon substrate, silicon oxidation, photo resist, optical lithography, etching, and metallization to create suspended, high aspect ratio microstructures such as picofarad sensing capacitors and high force actuators, materials testing instruments, and micro-scanning tunneling microscopes (STM). Other similar methods of micro-fabrication are described such as cantilevers by oxidation for mechanical beams and tips (COMBAT), and SCREAM I, a low temperature version of SCREAM. 5. Future research directions Further research is needed to address several issues of importance. Additional research is necessary related to gripper designs, creation of micro factories, modeling of interactive forces, and development of simulation based environments. These are summarized in this section. The design of effective grippers and gripping strategies to support the high volume rapid assembly of micro devices needs to be addressed. As it is difficult to release parts after gripping, the role of innovative fixtures and adaptive devices to help facilitate rapid release of micro devices accurately needs to be adequately studied. Further, the use of tool changers that can support the quick exchange of various grippers and end effectors may be necessary to realize the flexible assembly of micro devices. The design of Information Technology-based frameworks and architectures to support quick turn-around of varying customer requirements and changing part designs will become increasingly important. Such frameworks must be designed with a view towards providing an agile assembly capability in a modern manufacturing context. Another major issue that can be explored is the design of micro factories, which can be composed of a variety of automated micro assembly work cells. These work cells can be comprised of micro robots, conveyors, grippers, motion detectors and other sensors. By working as a cohesive set of resources within a ‘micro’ factory, the automated activities can be integrated to ensure assembly of complex part designs. Each work cell, in turn, can focus on completing the assembly of a segment of the final product. In this context, the design and development of innovative planning, control and collaborative approaches needs to be researched; appropriate control architectures have to be proposed to support an integrated and automated approach to the assembly of micron-sized parts in a flexible manufacturing environment. Although there have been initial research efforts aimed at studying the role of various interactive forces, there is a need to continue the development of analytical models to

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enable the prediction of the impact of micro level forces for varying manipulator and gripping strategies and configurations. The impact of the nature of gripping surfaces on reducing the interactive forces (such as van der Waals) needs to be researched. The design of virtual environments and simulation based design approaches are areas of research that need additional investigation. The development of high-fidelity physics based VR environments is an important focus area which will enable the comparison of candidate gripping approaches, gripper configurations as well as simulation of path planning and manipulation strategies. 6. Conclusion This paper provided a review of research efforts in the emerging area of micro assembly. As manual assembly of micron-sized parts is difficult and time consuming, there is a need to develop computer-controlled approaches to support rapid micro devices assembly. A discussion of various research investigations dealing with exploring the role of interactive forces at the micro level was provided. These micro level forces include van der Waals, surface tension and electrostatic forces. Further, a review of innovative micro gripping and micro assembly techniques and devices was also presented. Other research efforts reviewed included the design and development of computer vision based techniques to facilitate assembly as well as the creation of VR based frameworks to propose, compare and visualize micro assembly alternatives. An outline of future research directions was also presented. Acknowledgements The research work outlined in this paper was accomplished as part of research and educational projects funded by Sandia National Laboratories and the National Science Foundation (Grant numbers 0443533 and 0423907); their assistance is gratefully acknowledged. References [1] Bohringer K, Fearing K, Goldberg K. A chapter on micro assembly. The Handbook of Industrial Robotics. 2nd ed.; 1998, /http:// www.ee.washington.edu/research/mems/publications/1999/chapters/ industrialrobotics-bohringer-99.pdfS; [retrieved 7.02.2003]. [2] Greminger M, Yang G, Nelson B. Sensing nanonewton level forces by visually tracking structural deformations. In: Proceedings of the 2002 IEEE international conference on robotics and automation; 2002, /http://www.menet.umn.edu/grem/papers/icra2002.pdfS; [retrieved 14.06.03]. [3] Zhou S-A. On forces in microelectromechanical systems. Int J Eng Sci 2003;41:313–35. [4] Israelachvili J. Intermolecular surface forces. 2nd ed. New York: Academic Press; 1992. p. 83–107. [5] Bowling RA. Theoretical review of particle adhesion. In: Mittal IKL, editor. Particles on surfaces; 1999. p. 129–42. [6] Bhattacharjee S, Ashutosh S. A polar, polar, and electrostatic interactions of spherical particles in cylindrical pores. J Coll Interface Sci 1997;187:83–95.

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