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Parallel assembly of microsystems using Si micro electro mechanical systems
Microelectronic Engineering 67–68 (2003) 445–452 www.elsevier.com / locate / mee
Parallel assembly of microsystems using Si micro electro mechanical systems G. Skidmore, M. Ellis, A. Geisberger, K. Tsui, R. Saini, T. Huang, J. Randall* Zyvex Corporation, 1321 North Plano Road, Richardson, TX 75081, USA
Abstract The remarkable advances in miniaturization have been achieved largely by the monolithic integration techniques developed by the integrated circuit industry. However, monolithic integration frequently results in compromised performance and for complete systems frequently there is ‘‘some assembly required’’. Assembly is typically an expensive procedure that is carried out serially either by human hands or by automated machinery. However, with the growing demand for microsystems, there is an opportunity to drastically reduce assembly costs by arraying both the parts and microsystems, so that parallel assembly can be used. This paper describes an approach to parallel assembly that makes use of silicon micro electro mechanical systems that should achieve low assembly costs while maintaining high precision and a clear path to downscaling. 2003 Elsevier Science B.V. All rights reserved. Keywords: Parallel assembly; Micro-systems; MEMS
1. Introduction The past several decades have brought a general trend of miniaturization with huge economic benefits and great convenience. One of the technologies most often sited as enabling continued miniaturization is the ‘‘system on a chip’’ concept. This approach promises integration of all system functionality onto a single chip. However, there are limits to monolithic integration. For instance, optical and high-frequency functions are better accomplished with material systems other than silicon. For many systems, heterogeneous integration will provide superior results, however, most suffer the higher costs associated with assembly. The single biggest barrier to low cost assembly manufacturing is the fact that parts are handled one at a time. Described in this paper is an approach to parallel assembly of microsystems that uses silicon micro electro mechanical systems (MEMSs) as an integral part of the assembly process. In order to exploit * Corresponding author. E-mail address: [email protected] (J. Randall). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00100-X
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MEMSs for parallel assembly purposes, several technologies need to be developed: releasable, but tethered parts; MEMS grippers; MEMS connectors; and integration with high precision robotics systems.
2. Electrothermal actuators Thermal actuation in microscale devices has been demonstrated as a compact, high-force actuation technique which compliments electrostatic actuation [1–3]. Resistive Joule heating is typically used to power these actuators, which generally operate at lower and more desirable voltages than electrostatic devices. In addition, the available work per volume available with thermal actuation is approximately 155% over that of electrostatic actuation, under ideal conditions [4]. However, electrothermal actuators consume considerably more power than electrostatic devices. Common electrothermal actuator designs include the U-beam and V-beam actuators [5], shown in Fig. 1. The U-beam actuator amplifies thermal expansion in the thin, ‘‘hot arm’’, beam by connecting it to a wide, ‘‘cold arm’’, beam which has a flexure at its base. By passing current through the U-beam actuator the thin, higher resistance beam is heated more than the wide, low resistance, beam. This asymmetry in heating creates a bimorph effect and achieves motion at the tip of the actuator. The V-beam actuator uses thermal expansion of a beam with both ends restrained and an offset in the central region to initiate actuation direction. Examples of applications utilizing electrothermal actuation include: linear and rotary microengines
Fig. 1. As fabricated and actuated shape of (a) the U-beam actuator; and (b) the V-beam actuator.
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[6]; in plane and out of plane actuators for optical switching, scanning, and mirror arrays [7–9]; and gripper actuation for microassembly operations [10], which will be discussed in more detail later.
3. Constraining released MEMS parts with tethers Performing assembly of MEMS fabricated parts and components requires a method for constraining parts after the release procedure (HF etching of silicon dioxide for example). Mechanical devices for accomplishing this constraining function are known generically as tethers. A desirable tether should possess the following characteristics: constrain components or parts during and after the release procedure with 100% yield, constrain the parts to known locations in all dimensions with the highest possible accuracy and precision, be easily removable on demand, leave the parts completely unaffected once removed, and remain constrained once removed as to not interfere with assembly operations. The MEMS process used to produce the components controls the type of tethering possible and inhibits obtaining some of the above characteristics. Tethering has been accomplished in MEMSs for a number of years. The simplest tethers possible, and the only ones available to a one-released-layer MEMS process is a solid connection. While this type of tether satisfies the first characteristic, it fails the others. This solid connection must be broken in some way to release the components, and once broken does not constrain well in the out-of-plane direction, is prone to leave shards on the part and / or the wafer. The break can be accomplished forcefully with a tungsten probe as is commonly used to electrically contact devices. The break can also be accomplished by passing current to melt the tether mimicking the operating principle of an electrical fuse. More complicated and effective tethering can be accomplished with MEMS processes allowing two-released mechanical layers as is available in the well-known MUMPs姠 process. These tethers (several differing types demonstrated in varied multiple-released layer processes) accomplish all of the above mentioned desirable characteristics. A two-layer positional tether built using the MUMPs process is shown in Fig. 2. These positional tethers leave the part completely unaffected by the tethering process. The use of two layer positional tethers allows arrays of released parts to be held in place in known orientations.
Fig. 2. Positional tethers produced by the MUMPs process are shown constraining the lower Si ‘‘Lego’’ brick, while the tethers have been moved to free the upper ‘‘Lego’’ brick.
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Fig. 3. As fabricated (a) and actuated (b) shape of the amplified V-beam gripper.
4. Grippers We have designed and analyzed micro-grippers utilizing electrothermal actuation [11,12]. The main focus in designing these structures is to amplify the actuator motion using some compliant structure. In addition, the gripper must be capable of delivering the desired force, or in some cases work, to the device it is gripping. Fig. 3 demonstrates a successful design under development. The amplified V-beam gripper shown in Fig. 3 makes use of both the U- and V-beam actuators. Placing the U-beam actuator at the apex of the V-beam actuator amplifies motion at the gripper tips and makes use of Joule heating in both actuators as current passes between the mounting pads. Using this design, grippers have been fabricated from 100 mm thick single crystal silicon covering a surface area of approximately 1.5 mm 2 and capable of opening 50 mm. When these grippers are open, the available mechanical potential energy is approximately 18 nJ.
5. Si snap connectors Pick-and-place assembly has spawned extensive interests in integrating sub-systems that are difficult or impossible to fabricate monolithically [13]. However, existing pick-and-place mechanisms do not suffice for many applications that demand micron-scale precision. Consequently, manual application of adhesives and manual realignment are often required, significantly increasing manufacturing costs. We are developing a micro-assembly technology utilizing parts with built-in, self-aligning mechanisms. A major advantage of this approach is that the precision of the assembly is not limited to the external robotic system’s accuracy. In fact, the final assembly can have better alignment than that achievable with the same robotic system using standard parts (i.e., parts without built-in compliances). In addition, the relaxation of the required placement accuracy of the robot lends itself well to scaling to a parallel assembly operation. Snap connectors are micro-mechanical devices with built-in compliant systems that, once assembled, can accurately self-align—reposition themselves to their designed, stable, equilibrium positions, independent of the initial placement of the part by the external robot. Because, these compliant systems are typically symmetrical, we call this self-alignment ‘‘self-centering’’. Fig. 4
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Fig. 4. Self-centering of a one DOF snap connector assembly.
demonstrates the principle of self-aligned, micro-assembly using a snap connector with a one degree-of-freedom (DOF), self-centering mechanism. The assembly operation consists of four steps. First, the MEMS gripper attached to the external robotic system moves close to the part; the gripper arms close, compress the compliant springs on the snap connector and pick up the part. Second, the robotic system moves the part to roughly align it to its destination (i.e., the center of the mating receptacle). Third, the gripper arms open, the compliant springs on the snap connector are released and subsequently make contact with the mating receptacle. Fourth, the gripper arms are removed. The self-alignment of the assembled system occurs immediately after the last assembly step; that is to say, after the robotic system is released and removed, the contact forces between the springs and the receptacle align and secure the relative position of the part with respect to the receptacle. The symmetry of the compliant springs ensures the centering of the part, independent of the initial gripper placement. The aforementioned mechanism can be extended to include other dimensions, thereby achieving self-centering with multiple DOFs. Currently, the parts we have assembled are fabricated with built-in snap connectors. These snap connectors will be used as modular carriers containing pre-assembled parts that are incompatible with MEMS fabrication. In addition, when combined with the electrical-mechanical interconnect technology developed at Zyvex, the snap connector technology enables reliable mechanical and electrical integration of heterogeneous components.
6. Si assembly substrate (Si optical bench) We are developing a silicon optical bench technology that will make automated assembly of optical components feasible. High cost savings are possible by replacing labor intensive serial assembly with
450
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Fig. 5. SEM of an active socket. The mirror is 300 mm square.
automated parallel assembly [14,15]. The concept of a silicon optical bench is very similar to that of an electric circuit board. Optical components (mirrors, lenses, thin film filters, etc.) are assembled at a predefined location on an SCS device layer like electrical components are assembled on a circuit board. In addition to passive receptacles for our snap connectors, we developed a forced aligning receptacle that opens up when powered and aligns the placed part accurately when power is turned off. We call this receptacle an ‘‘active socket’’. Fig. 5 shows the scanning electron microscopy (SEM) results for an active socket. When current flows through the bent beams and bimorph, the shuttle moves about 45 mm from its initial position creating a cavity for component placement. The part (a mirror with etch holes in this case) is then dropped inside the opening with the help of MEMS grippers and the current in the socket is turned off. During its retreat to the initial position, the shuttle aligns the placed component accurately by flush mounting its edges with the surface of the inserted component. This is a very accurate way of self aligning assembled optical components on an optical bench. The active socket of Fig. 5 has an angular misalignment of less that 0.18 which is a result of the etch profile created by deep silicon etching. We have also designed a proprietary method to micro-position the component that can lock in with power off position to compensate this error in misalignment.
7. Assembly robotics Pick-and-place assembly of Si micro-components has been demonstrated previously using microgrippers and vacuum suction. Several other techniques for self assembly include hinged structures [17], fluidic self assembly [20], plastic deformation [16], solder reflow [18], and surface tension [19]. However, these techniques have several drawbacks prohibiting the possibility of heterogeneous assembly of micro-scale components. The proposed assembly technique uses a 5-DOF robotic system (MEMbler) composed of highprecision Newport stages that is used as the development platform for a parallel pick-and-place assembly approach (Fig. 6a). MEMS grippers have been successfully packaged and mounted on the
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Fig. 6. (a) 5-DOF robotic system, (b) MEMS gripper picking up component off substrate, and (c) parallel assembly operation with an array of components and build locations.
MEMbler arm, and a computer-controlled automated serial assembly sequence has been demonstrated. This automated assembly sequence has assembled Si micro-machined mirrors into mating receptacles perpendicular to the device substrate. The Si parts to be assembled were first de-tethered on the substrate to fully release the components. The component is then picked up off the substrate by the MEMS gripper (Fig. 6b). An appropriate rotation of the part is made followed by insertion of the part into a mating receptacle on the build substrate. By using MEMS grippers to pick and place other MEMS components we fully utilize the benefits of batch processing, creating arrays of both components and grippers in known orientations. This approach allows greater flexibility in components’ assembly as well as the particular assembly operations, while still providing high positional and orientation precision. This approach can be parallelized by using an array of MEMS grippers that are packaged onto the arm of the MEMbler in order to pick and place an array of parts simultaneously on the substrate in a known orientation (Fig. 6c). Work is in progress at Zyvex to develop this parallel assembly process.
8. Conclusions As the demand for microsystems grows, there will be an increasing need for low cost, high-yield, high-precision, automated assembly. We believe that parallel assembly is the only real avenue for low-cost assembly, and that an approach using MEMSs as an integral part has significant advantages including high precision, low cost, and a downscaling roadmap. While there are many industries that should benefit from a parallel assembly approach, we believe that fiber-optic components, RF systems, and biomedical applications are likely candidates.
Acknowledgements The authors would like to thank Jim Von Ehr, Ted Khoury, and Grady Roberts for their encouragement and useful discussions. We also thank Joon-Won Kang, Ken Yang, Jorg Pilchowski,
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Dan Ruiz, Craig Johnson, and John Klein of Honeywell International—Redmond for MEMS fabrication. This work was performed under the support of the US Department of Commerce, National Institute of Standards and Technology, Advanced Technology Program, Cooperative Agreement Number 70NANB1H3021.
References [1] J. Comtois, V. Bright, Sensors Actuators A 58 (1997) 19–25. [2] L. Que, J. Park, Y. Gianchandani, in: MEMS‘99, International Conference on Microelectro Mechanical Systems, Orlando, FL, 1999, pp. 31–36. [3] M. Sinclair, in: Inter Society Conference on Thermal Phenomena, IEEE, 2000, pp. 127–132. [4] P. Krulevitch, A. Lee, P. Philip, B. Ramsey, J. Trevino, J. Hamilton, M. Northrup, J. Microelectromech. Syst. 5 (4) (1996) 270–281. [5] J. Maloney, D. DeVoe, D. Schreiber, in: Micro-Electro-Mechanical-Systems (MEMS)—ASME 2000, Vol. 2, 2000, pp. 233–240. [6] J.-S. Park, L. Chu, A. Oliver, Y. Gianchandani, J. Microelectromech. Syst. 10 (2) (2001) 255–262. [7] W.-C. Chen, J. Hsieh, W. Fang, in: MEMS2002, The 15th International Conference on Micro Electro Mechanical Systems, 2002, pp. 693–697. [8] M. Sinclair, in: MEMS2002, The 15th International Conference on Micro Electro Mechanical Systems, 2002, pp. 698–701. [9] A. Tuantranont, V. Bright, L.-A. Liew, W. Zhang, Y. Lee, in: Proceedings of the 13th Annual International Conference on Micro Electromechanical Systems (MEMS 2000), 2000, pp. 455–460. [10] A. Geisberger, M. Ellis, G. Skidmore, in: The 5th International Conference on Integrated Nano / Micro / Biotechnology for Space and Medical and Commercial Applications, 2002. [11] H. Du, C. Su, M. Lim, W. Jin, Smart Mater. Struct. 8 (1999) 616–622. [12] N. Mankame, G. Ananthasuresh, in: Technical Proceedings of the 2000 International Conference on Modeling and Simulation of Microsystems, Vol. 3, 2000, pp. 609–612. [13] M. Cohn, K. Bohringer, J. Noworolski, A. Singh, C. Keller, K. Goldberg, R. Howe, in: Proceedings of the SPIE Microfluidic Devices Conference, Vol. 3515, September 1998, pp. 2–16. [14] J.T. Feddema, T.R. Christenson, Parallel assembly of high aspect ratio microstructures, in: Proceedings of SPIE— Microrobotics and Microassembly, Vol. 3834, 1999, pp. 153–164. [15] E. Hui, R. Howe, M. Rodgers, Single-step assembly of complex 3D microstructures, in: MEMS 2000, Miyazaki, 2000. ´ J. Microelectromech. Syst. 10 (2) (2001) 302–309. [16] J. Zou, J. Chen, C. Liu, J. Schutt-Aine, [17] Y. Yi, C. Liu, J. Microelectromech. Syst. 8 (No. 1) (1999) 10–17. [18] P. Green, R. Syms, E. Yeatman, J. Microelectromech. Syst. 4 (4) (1995) 170–176. [19] R. Syms, J. Microelectromech. Syst. 8 (4) (1999) 448–455. [20] U. Srinivasan, D. Liepmann, R. Howe, J. Microelectromech. Syst. 10 (1) (2001) 17–24.
Parallel assembly of microsystems using Si micro electro mechanical systems G. Skidmore, M. Ellis, A. Geisberger, K. Tsui, R. Saini, T. Huang, J. Randall* Zyvex Corporation, 1321 North Plano Road, Richardson, TX 75081, USA
Abstract The remarkable advances in miniaturization have been achieved largely by the monolithic integration techniques developed by the integrated circuit industry. However, monolithic integration frequently results in compromised performance and for complete systems frequently there is ‘‘some assembly required’’. Assembly is typically an expensive procedure that is carried out serially either by human hands or by automated machinery. However, with the growing demand for microsystems, there is an opportunity to drastically reduce assembly costs by arraying both the parts and microsystems, so that parallel assembly can be used. This paper describes an approach to parallel assembly that makes use of silicon micro electro mechanical systems that should achieve low assembly costs while maintaining high precision and a clear path to downscaling. 2003 Elsevier Science B.V. All rights reserved. Keywords: Parallel assembly; Micro-systems; MEMS
1. Introduction The past several decades have brought a general trend of miniaturization with huge economic benefits and great convenience. One of the technologies most often sited as enabling continued miniaturization is the ‘‘system on a chip’’ concept. This approach promises integration of all system functionality onto a single chip. However, there are limits to monolithic integration. For instance, optical and high-frequency functions are better accomplished with material systems other than silicon. For many systems, heterogeneous integration will provide superior results, however, most suffer the higher costs associated with assembly. The single biggest barrier to low cost assembly manufacturing is the fact that parts are handled one at a time. Described in this paper is an approach to parallel assembly of microsystems that uses silicon micro electro mechanical systems (MEMSs) as an integral part of the assembly process. In order to exploit * Corresponding author. E-mail address: [email protected] (J. Randall). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00100-X
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G. Skidmore et al. / Microelectronic Engineering 67–68 (2003) 445–452
MEMSs for parallel assembly purposes, several technologies need to be developed: releasable, but tethered parts; MEMS grippers; MEMS connectors; and integration with high precision robotics systems.
2. Electrothermal actuators Thermal actuation in microscale devices has been demonstrated as a compact, high-force actuation technique which compliments electrostatic actuation [1–3]. Resistive Joule heating is typically used to power these actuators, which generally operate at lower and more desirable voltages than electrostatic devices. In addition, the available work per volume available with thermal actuation is approximately 155% over that of electrostatic actuation, under ideal conditions [4]. However, electrothermal actuators consume considerably more power than electrostatic devices. Common electrothermal actuator designs include the U-beam and V-beam actuators [5], shown in Fig. 1. The U-beam actuator amplifies thermal expansion in the thin, ‘‘hot arm’’, beam by connecting it to a wide, ‘‘cold arm’’, beam which has a flexure at its base. By passing current through the U-beam actuator the thin, higher resistance beam is heated more than the wide, low resistance, beam. This asymmetry in heating creates a bimorph effect and achieves motion at the tip of the actuator. The V-beam actuator uses thermal expansion of a beam with both ends restrained and an offset in the central region to initiate actuation direction. Examples of applications utilizing electrothermal actuation include: linear and rotary microengines
Fig. 1. As fabricated and actuated shape of (a) the U-beam actuator; and (b) the V-beam actuator.
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[6]; in plane and out of plane actuators for optical switching, scanning, and mirror arrays [7–9]; and gripper actuation for microassembly operations [10], which will be discussed in more detail later.
3. Constraining released MEMS parts with tethers Performing assembly of MEMS fabricated parts and components requires a method for constraining parts after the release procedure (HF etching of silicon dioxide for example). Mechanical devices for accomplishing this constraining function are known generically as tethers. A desirable tether should possess the following characteristics: constrain components or parts during and after the release procedure with 100% yield, constrain the parts to known locations in all dimensions with the highest possible accuracy and precision, be easily removable on demand, leave the parts completely unaffected once removed, and remain constrained once removed as to not interfere with assembly operations. The MEMS process used to produce the components controls the type of tethering possible and inhibits obtaining some of the above characteristics. Tethering has been accomplished in MEMSs for a number of years. The simplest tethers possible, and the only ones available to a one-released-layer MEMS process is a solid connection. While this type of tether satisfies the first characteristic, it fails the others. This solid connection must be broken in some way to release the components, and once broken does not constrain well in the out-of-plane direction, is prone to leave shards on the part and / or the wafer. The break can be accomplished forcefully with a tungsten probe as is commonly used to electrically contact devices. The break can also be accomplished by passing current to melt the tether mimicking the operating principle of an electrical fuse. More complicated and effective tethering can be accomplished with MEMS processes allowing two-released mechanical layers as is available in the well-known MUMPs姠 process. These tethers (several differing types demonstrated in varied multiple-released layer processes) accomplish all of the above mentioned desirable characteristics. A two-layer positional tether built using the MUMPs process is shown in Fig. 2. These positional tethers leave the part completely unaffected by the tethering process. The use of two layer positional tethers allows arrays of released parts to be held in place in known orientations.
Fig. 2. Positional tethers produced by the MUMPs process are shown constraining the lower Si ‘‘Lego’’ brick, while the tethers have been moved to free the upper ‘‘Lego’’ brick.
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Fig. 3. As fabricated (a) and actuated (b) shape of the amplified V-beam gripper.
4. Grippers We have designed and analyzed micro-grippers utilizing electrothermal actuation [11,12]. The main focus in designing these structures is to amplify the actuator motion using some compliant structure. In addition, the gripper must be capable of delivering the desired force, or in some cases work, to the device it is gripping. Fig. 3 demonstrates a successful design under development. The amplified V-beam gripper shown in Fig. 3 makes use of both the U- and V-beam actuators. Placing the U-beam actuator at the apex of the V-beam actuator amplifies motion at the gripper tips and makes use of Joule heating in both actuators as current passes between the mounting pads. Using this design, grippers have been fabricated from 100 mm thick single crystal silicon covering a surface area of approximately 1.5 mm 2 and capable of opening 50 mm. When these grippers are open, the available mechanical potential energy is approximately 18 nJ.
5. Si snap connectors Pick-and-place assembly has spawned extensive interests in integrating sub-systems that are difficult or impossible to fabricate monolithically [13]. However, existing pick-and-place mechanisms do not suffice for many applications that demand micron-scale precision. Consequently, manual application of adhesives and manual realignment are often required, significantly increasing manufacturing costs. We are developing a micro-assembly technology utilizing parts with built-in, self-aligning mechanisms. A major advantage of this approach is that the precision of the assembly is not limited to the external robotic system’s accuracy. In fact, the final assembly can have better alignment than that achievable with the same robotic system using standard parts (i.e., parts without built-in compliances). In addition, the relaxation of the required placement accuracy of the robot lends itself well to scaling to a parallel assembly operation. Snap connectors are micro-mechanical devices with built-in compliant systems that, once assembled, can accurately self-align—reposition themselves to their designed, stable, equilibrium positions, independent of the initial placement of the part by the external robot. Because, these compliant systems are typically symmetrical, we call this self-alignment ‘‘self-centering’’. Fig. 4
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Fig. 4. Self-centering of a one DOF snap connector assembly.
demonstrates the principle of self-aligned, micro-assembly using a snap connector with a one degree-of-freedom (DOF), self-centering mechanism. The assembly operation consists of four steps. First, the MEMS gripper attached to the external robotic system moves close to the part; the gripper arms close, compress the compliant springs on the snap connector and pick up the part. Second, the robotic system moves the part to roughly align it to its destination (i.e., the center of the mating receptacle). Third, the gripper arms open, the compliant springs on the snap connector are released and subsequently make contact with the mating receptacle. Fourth, the gripper arms are removed. The self-alignment of the assembled system occurs immediately after the last assembly step; that is to say, after the robotic system is released and removed, the contact forces between the springs and the receptacle align and secure the relative position of the part with respect to the receptacle. The symmetry of the compliant springs ensures the centering of the part, independent of the initial gripper placement. The aforementioned mechanism can be extended to include other dimensions, thereby achieving self-centering with multiple DOFs. Currently, the parts we have assembled are fabricated with built-in snap connectors. These snap connectors will be used as modular carriers containing pre-assembled parts that are incompatible with MEMS fabrication. In addition, when combined with the electrical-mechanical interconnect technology developed at Zyvex, the snap connector technology enables reliable mechanical and electrical integration of heterogeneous components.
6. Si assembly substrate (Si optical bench) We are developing a silicon optical bench technology that will make automated assembly of optical components feasible. High cost savings are possible by replacing labor intensive serial assembly with
450
G. Skidmore et al. / Microelectronic Engineering 67–68 (2003) 445–452
Fig. 5. SEM of an active socket. The mirror is 300 mm square.
automated parallel assembly [14,15]. The concept of a silicon optical bench is very similar to that of an electric circuit board. Optical components (mirrors, lenses, thin film filters, etc.) are assembled at a predefined location on an SCS device layer like electrical components are assembled on a circuit board. In addition to passive receptacles for our snap connectors, we developed a forced aligning receptacle that opens up when powered and aligns the placed part accurately when power is turned off. We call this receptacle an ‘‘active socket’’. Fig. 5 shows the scanning electron microscopy (SEM) results for an active socket. When current flows through the bent beams and bimorph, the shuttle moves about 45 mm from its initial position creating a cavity for component placement. The part (a mirror with etch holes in this case) is then dropped inside the opening with the help of MEMS grippers and the current in the socket is turned off. During its retreat to the initial position, the shuttle aligns the placed component accurately by flush mounting its edges with the surface of the inserted component. This is a very accurate way of self aligning assembled optical components on an optical bench. The active socket of Fig. 5 has an angular misalignment of less that 0.18 which is a result of the etch profile created by deep silicon etching. We have also designed a proprietary method to micro-position the component that can lock in with power off position to compensate this error in misalignment.
7. Assembly robotics Pick-and-place assembly of Si micro-components has been demonstrated previously using microgrippers and vacuum suction. Several other techniques for self assembly include hinged structures [17], fluidic self assembly [20], plastic deformation [16], solder reflow [18], and surface tension [19]. However, these techniques have several drawbacks prohibiting the possibility of heterogeneous assembly of micro-scale components. The proposed assembly technique uses a 5-DOF robotic system (MEMbler) composed of highprecision Newport stages that is used as the development platform for a parallel pick-and-place assembly approach (Fig. 6a). MEMS grippers have been successfully packaged and mounted on the
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Fig. 6. (a) 5-DOF robotic system, (b) MEMS gripper picking up component off substrate, and (c) parallel assembly operation with an array of components and build locations.
MEMbler arm, and a computer-controlled automated serial assembly sequence has been demonstrated. This automated assembly sequence has assembled Si micro-machined mirrors into mating receptacles perpendicular to the device substrate. The Si parts to be assembled were first de-tethered on the substrate to fully release the components. The component is then picked up off the substrate by the MEMS gripper (Fig. 6b). An appropriate rotation of the part is made followed by insertion of the part into a mating receptacle on the build substrate. By using MEMS grippers to pick and place other MEMS components we fully utilize the benefits of batch processing, creating arrays of both components and grippers in known orientations. This approach allows greater flexibility in components’ assembly as well as the particular assembly operations, while still providing high positional and orientation precision. This approach can be parallelized by using an array of MEMS grippers that are packaged onto the arm of the MEMbler in order to pick and place an array of parts simultaneously on the substrate in a known orientation (Fig. 6c). Work is in progress at Zyvex to develop this parallel assembly process.
8. Conclusions As the demand for microsystems grows, there will be an increasing need for low cost, high-yield, high-precision, automated assembly. We believe that parallel assembly is the only real avenue for low-cost assembly, and that an approach using MEMSs as an integral part has significant advantages including high precision, low cost, and a downscaling roadmap. While there are many industries that should benefit from a parallel assembly approach, we believe that fiber-optic components, RF systems, and biomedical applications are likely candidates.
Acknowledgements The authors would like to thank Jim Von Ehr, Ted Khoury, and Grady Roberts for their encouragement and useful discussions. We also thank Joon-Won Kang, Ken Yang, Jorg Pilchowski,
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G. Skidmore et al. / Microelectronic Engineering 67–68 (2003) 445–452
Dan Ruiz, Craig Johnson, and John Klein of Honeywell International—Redmond for MEMS fabrication. This work was performed under the support of the US Department of Commerce, National Institute of Standards and Technology, Advanced Technology Program, Cooperative Agreement Number 70NANB1H3021.
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