- Email: [email protected]
Micro rapid prototyping system for micro components
Thin Solid Films 420 – 421 (2002) 515–523
Micro rapid prototyping system for micro components Xiaochun Li*, Hongseok Choi, Yong Yang Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA
Abstract Similarities between silicon-based micro-electro-mechanical systems (MEMS) and Shape Deposition Manufacturing (SDM) processes are obvious: both integrate additive and subtractive processes and use part and sacrificial materials to obtain functional structures. These MEMS techniques are two-dimensional (2-D) processes for a limited number of materials while SDM enables the building of parts that have traditionally been impossible to fabricate because of their complex shapes or of their variety in materials. This work presents initial results on the development of a micro rapid prototyping system that adapts SDM methodology to micro-fabrication. This system is designed to incorporate microdeposition and laser micromachining. In the hope of obtaining a precise microdeposition, an ultrasonic-based micro powder-feeding mechanism was developed in order to form thin patterns of dry powders that can be cladded or sintered onto a substrate by a micro-sized laser beam. Furthermore, experimental results on laser micromachining using a laser beam with a wavelength of 355 nm are also presented. After further improvement, the developed micro manufacturing system could take computer-aided design (CAD) output to reproduce 3-D heterogeneous microcomponents from a wide selection of materials. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Rapid prototyping; Micro-component; Micro-feeding; Micromachining
1. Introduction Silicon-based micro-electro-mechanical system (MEMS) techniques are very popular for the manufacture of micro sensors, actuators, and components w1,2x. However, these MEMS techniques are two-dimensional (2-D) processes with multiple steps and require complex processing procedures in a cleanroom environment. Only a limited number of materials can be processed by these techniques. With 2-D processes, it is very difficult, if not impossible, to fabricate an enclosed volume of arbitrary shape and composition without the use of microassembly. Furthermore, the reliability, as well as the performance, of MEMS deserves more attention due to the increasing use of integrated microsystems w3x. Having reliable mechanical properties of micro components is critical to the safety and function of these complex MEMS. MEMS should not only be built in three-dimensions (3-D), but should also use a wider selection of materials, including alloys, polymers, ceramics, and heterogeneous materials. Micro-compo*Corresponding author. Tel.: q1-608-262-6142; fax: q1-608-2652316. E-mail address: [email protected] (X. Li).
nents with high aspect ratios, complex geometries, and complex microstructures are essential in many applications and can deliver a new generation of functionality and performance. However, little work has been done to successfully attain efficient micro manufacturing techniques for the fabrication of functionally and geometrically complex heterogeneous micro components. Concurrently with MEMS development, solid freeform fabrication (SFF) (also called ‘layered manufacturing’ or ‘rapid prototyping’) has emerged in the last decade as a popular manufacturing technology for accelerated product creation w4–8x. A general system is schematically presented in Fig. 1. A machine for SFF builds a part layer by layer directly from a CAD model without the fixturingytooling or human intervention demanded of conventional processes. This novel manufacturing technology enables the building of parts that have traditionally been impossible to fabricate because of their complex shapes or of their variety in materials. Different processes have also been used to create multimaterial parts w9–13x. Three-dimensional printing has been applied to build parts with local composition control. Selective laser sintering (SLS) has been used to build multi-material and functionally gradient mate-
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Fig. 1. Solid freeform fabrication process.
rials, while LENS (laser engineered net shape) has been used to tailor certain physical properties. Research has been directed at efforts that use several layered manufacturing processes to create 3-D micro scale components. Micro-stereolithography has been extensively studied and complex 3-D microstructures have been demonstrated w14–19x. Movable microstructures were made by the use of two-photon 3-D microfabrication with submicron resolution. EFAB (Electrochemical FABrication) specializes in fabrication of dense micro metal parts by electroplating w20x. However, these micro SFF processes are not suitable to build 3-D heterogeneous components due to their limited flexibility in changing material composition in situ. Laser-assisted Shape Deposition Manufacturing (SDM), an emerging SFF process, was developed to fabricate macro scale structures w21–23x. Unlike most additive SFF processes, SDM uses sequential additive (deposition of part materials and sacrificial materials) and subtractive (material removal) steps to form 3-D structures. Silicon-based MEMS and SDM processes both integrate additive and subtractive processes, and use part and sacrificial materials to obtain functional structures. SDM allows control of material location and properties in 3-D space. SDM has been used to build complex 3-D macro shapes with internal cooling channels, parts with continuously varying material properties, mechanisms, and heterogeneous parts with embedded sensors and actuators. If we adapt SDM methodology to microfabrication, it becomes possible to create functionally and geometrically complex heterogeneous micro components in a rapid fashion. However, little work has been done to fabricate 3-D micro components by use of micro SDM. To scale the SDM process down to the micro-world, it is essential to use tools that are capable of realizing both additive and subtractive processes at a micro-scale. Laser has been used for heating, melting, and ablation. Extensive work has been done in the arena of laser micromachining and microdeposition. Laser micromachining relies on the process of ablation w24x. Laser micromachining, especially with an excimer laser, has been used on a wide range of materials (polymers,
Fig. 2. Laser-based Micro-SDM system.
ceramics, semiconductors, and metals) w25–28x. In recent years, frequency-tripled or quadrupled Nd:YAG lasers at wavelengths of 355 nm and 266 nm have been used for micro-machining of different materials w29– 32x. While laser micro-machining is a subtractive process, laser micro-deposition is an additive process. Laser Particle Guidance (LPG), developed by Reen w33x, can deposit materials at 10 mm line width. MAPLE DW can deposit materials (polymers, ceramics, and metals) with a resolution approximately 10 mm w34x. Thus, a laserassisted micro-SDM process might be developed to integrate additive (laser micro-cladding) and subtractive (laser micro-machining) to form 3-D heterogeneous micro components. Laser based additiveysubtractive processes offer many advantages, including: no contact with substrate, no chemicals, flexible feature size and shape, high precision, work in air and at room temperature. 2. System design Micro-scale laser materials processing has become popular for microfabrication. Laser micro-machining processes can create 2-D and simple 3-D MEMS from a spectrum of homogeneous materials. However, few laser microdeposition processes are capable of in situ local composition control, which is vital for the fabrication of heterogeneous microstructures. This limitation is mainly due to the lack of a micro-powder feeding system that is capable of mixing and delivering various sub-micronynano powders without additional chemical mixtures. Therefore, an effective micro powder-feeding device is needed. Thin patterns of dry powders can be sintered or cladded by a laser beam. Different laser wavelengths are likely needed since different materials show different absorption characteristics. Thus, a pulsed Nd:YAG laser with different harmonic modes (1064 nm, 532 nm, 355 nm and 266 nm) seems an ideal match for the micro-deposition and micro-machining of various materials.
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Fig. 3. Manufacturing cycle of heterogeneous MEMS.
The micro-manufacturing system developed is depicted as shown in Fig. 2. A pulsed Nd:YAG laser (20Hz at all four harmonic modes), model PRO-290-20-NRF from Spectra-Physics, served as a micro additive and subtractive tool in this micro-SDM system. The energy levels are: 1.95 Jypulse at 1064 nm, 0.95 Jypulse at 532 nm, 0.40 Jypulse at 355 nm, and 0.175 Jypulse at 266 nm. A laser beam attenuator was used to control laser intensity. Moreover, a PC-controlled three-axis microstage, LW-7 XY and Anoride 7-4 Z from Anorad Inc., with a 30-nm resolution and a speed of up to 200 mmy s were also implemented. The CNC-2000 software accepts G-code that is generated by the UNIGRAPHICS CADyCAM software to control the movement of the micro-stage. The pulsed laser can be controlled spatially and temporally to obtain precise micro cladding and machining. In addition, an ultrasonic-based micro-powder feeding mechanism was developed and implemented to deposit microynano powders. An optical system with CCD camera and monitor was employed to monitor the micro-fabrication process with a maximum magnification of 3200. Image acquisition hardware and software from National Instruments were installed in a Dell PC. Based on the system design, the manufacturing cycle for the micro-SDM process can be depicted as shown in Fig. 3. The ultimate goal of this micro-manufacturing system is to provide a solid foundation for a seamless integration from CAD to the realization of complex 3D heterogeneous micro components in a wide selection of materials. 3. Experimental results and discussion Since it is still at an early stage of development, this session presents only initial results on the study of a micro-powder feeding device and on the characterization of laser micro-machining. It should be pointed out that
further experiments are being carried out to improve and integrate the system and will be reported in the future. 3.1. Study on micro-powder feeding mechanism Current laser-assisted SDM uses powder feeders and feed nozzles (with an inner diameter in the order of 1 mm) to control the material composition by mixing various powders (normally larger than 50 mm). Linoya et al. suggested that a vibrating system is more suitable to feed dry powders in sizes less than 100 mm w35x. Much finer powders (for instance, -1000 nm) could form agglomerates that prevent continuous feeding through a capillary tube. Ultrasonic vibration has been investigated to assist the feeding of fine powders w36– 39x. Capillary tubes with approximately 500-mm inner diameters have been studied to feed submicron alumna powders at rates of milligrams per second. Ultrasonic vibration at 20 kHz was generated by a piezoelectric transducer and applied to the capillary tube. It was observed that spherical powders flowed better than irregular shaped powders. In this study, an ultrasonic-based micro-feeding device was developed to deposit dry micro powders. Fig. 4 shows the schematic of the experimental set-up. A micro capillary tube with taper hole was assembled into a small aluminum block. A PZT plate, from Piezo System Inc., was glued on the top surface of the aluminum block. As a feasibility test, a low-cost micro capillary tube with an inner diameter of 125 mm and a length of 16 mm was used due to commercial availability. A function generator and a power amplifier were used to control the frequency and amplitude of the ultrasonic wave, which was generated by the thin PZT plate. It was detected that the resonant frequency of the PZT plate was 49 KHz. Thus, an ultrasonic wave of 49 KHz
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Fig. 4. Schematic of the micro-feeding and monitoring system.
was applied to feed dry powders through the capillary tube. The ultrasonic wave was effectively coupled into the glass capillary from the aluminum block with a tight fit. A video microscope system was used to study the mechanism that governs the micro-feeding of dry powders. The powder flow rate is important for the microdeposition since it can affect the continuation, width, and thickness of a deposited powder line. To measure the flow rates, a highly sensitive electric-balance was used to measure the mass of discharged powders. Powder flow rates for spherical copper powders (3 mm nominal in diameter) and stainless steel powers (3 mm nominal in diameter) were measured against the voltage applied on the PZT plate. Fig. 5 shows the curves of flow rates as functions of applied voltage.
Continuous discharge of the copper and stainless steel powders was achieved at a flow rate of approximately 10y5 gys. As shown in Fig. 5, the flow rates increase as the applied voltage increases until the voltage reaches approximately 280 V. However, the flow rates decrease quickly as the applied voltage surpasses 280 V. This is probably caused by saturation and higher temperature induced in the PZT plate at higher voltages. The difference in the flow rates between copper and stainless steel powders might have resulted from the difference in their properties. High quality for the deposited thin pattern of dry powders is critical for the rapid micro-fabrication of micro components. To characterize the quality of powder deposition, a series of straight lines of copper and stainless steel powders were deposited on a silicon substrate. The input voltage to the PZT plate was set to 280 V. To achieve thin, continuous and smooth lines, the parameters (moving velocity of the substrate, and gap between the feeding tip and the substrate) are important. Fig. 6 displays images of those deposited lines with various combinations of gap distances and velocities. The results showed that the feeding device could deposit a thin and continuous powder line with an optimized combination of a gap distance of 85 mm and a moving velocity of 4 mmys. It should also be noted that a twisted and broken line was also demonstrated with a combination of a gap distance of 170 mm and a moving velocity of 4 mmys. A minimum line width of 127 mm (almost the same as the inner diameter of the capillary tube) was obtained. The mechanism of ultrasonic-based micro-powder feeding device was designed to stimulate motions of the
Fig. 5. Flow rates of powers against the voltage applied to the PZT plate.
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Fig. 6. Images of the deposited lines of dry powders.
Fig. 7. Mechanism for micro-powder feeding: (a) top view of the inner surface particle motion; (b) section view of the inner surface particle motion; and (c) the motion orbit of the discharged micro powder.
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Fig. 9. Relationship between laser fluence and depth of drilled hole. Fig. 8. Relationship between laser shots and depth with laser fluence of 3.056 Jycm2.
surface particles at the inner wall of the capillary. With a suitable motion of the inner surface of the capillary, the friction and adhesive forces between the powders and the inner wall of the capillary can effectively discharge the powders. The motion of the inner surface is induced by a Rayleigh wave w40x, which is propagating on the elastic material surface. Fig. 7a shows the top view of the motion of the surface particles on the inner wall of the capillary. Surface particles near the circle move in elliptic locus. This motion was verified by powder spinning inside the capillary, which was observed with the video system. Fig. 7b is a longitudinal section view of the capillary. It indicates that the motions of the surface particles are moving in elliptic locus along the length of the inner wall. With these two surface motions, powders were driven to flow out the feeding tip of the capillary in a helix pattern, as depicted in Fig. 7c, which is verified by the image of the twisted broken line in Fig. 6. When the capillary was placed horizontally, powders were still discharged. This phenomenon verified that the powders were driven to the feeding tip of the capillary tube by friction and adhesive forces instead of by gravity. We believe that the Rayleigh wave can be further applied to feed powders through capillary tubes with smaller inner diameters (10–25 mm).
nesses of the foils, this paper focuses on the results that were achieved on surface-polished stainless steel 316L sheets with thickness of 0.762 mm. The relationship between the number of laser shots and the depth of the hole machined is shown in Fig. 8. The drilled depth on stainless steel is almost linearly proportional to the number of laser shots. A depth of approximately 200 nm per pulse can be achieved. This allows accurate control of the depth of drilled holes. The laser intensity (laser fluence) significantly affects the depth of drilling. Experiments were completed with a single pulse of controlled laser intensity. Fig. 9 shows that the depth of drilled hole increases rapidly with laser fluence after the ablation threshold. Then ablation rates remain almost constant, approximately 0.8 mmypulse, as the laser fluency increases until up to 100 Jycm2. The ablation rates then increase continuously with increasing laser fluence. It was observed under optical microscope that the recast area, a layer of debris on the surface of the material caused by the molten metal during laser micro-machining, increases with the laser fluence. Fig. 10 shows the relationship between the laser influence and the recast area. The location of focal plane related to the top surface of the stainless steel foils was found to be very important
3.2. Study on laser micromachining Laser micromachining was studied with a laser wavelength of 355 nm. All experiments were completed in air. An optical lens with a nominal focus length of 135 mm was used. The beam diameter on the lens was set to 6.0 mm. A larger focus length or a larger beam size on the lens can be used to further decrease the focus spot size. Stainless steel 316L foils with various thicknesses from 0.05 to 0.762 mm were used. Due to the similarities of experimental results on different thick-
Fig. 10. Relationship between recast area and laser fluence.
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time saving. The taper angles are found to be quite large, up to 208. The large taper angles may be caused by the beam quality (0.7 fit to Gaussian beam) and intensity variation inside the beam. It is believed that taper angles would be controlled to 3–58 if a beam homogenizer was used. 4. Conclusion
Fig. 11. Relationship between laser focal plane location and machining depth (445 pulses at a laser fluence of 40.74 Jycm2).
for laser micromachining of micro components with high aspect ratio. Fig. 11 shows the relationship between the focal position and drilled depth. With the same laser fluence and the same number of laser pulses, the maximum depth is obtained when the focal plane of the laser beam was located at the bottom surface of the stainless steel foil. Fig. 12 shows the shape and depth of the drilled holes under various focal plane positions. The moving velocity of the substrate related to the laser beam affected the shape and the depth of the machined channels, as shown in Fig. 13. The two channels were machined under two moving velocities of 0.1 and 0.2 mmys, respectively, under the same laser fluence. The slower moving velocity, 0.1 mmys, results in a deeper depth, but also a wider channel. The optimum speed is necessary for accurate machining and
This work discusses the development of a micro-rapid prototyping system that adapts SDM methodology to microfabrication. Initial results on the study of a micro powder-feeding device and on the characterization of laser micromachining are presented. By incorporating microdeposition and laser micromachining, the developed system could take computer-aided design (CAD) output from a computer to reproduce micro components. An ultrasonic-based micro powder-feeding mechanism was developed in order to form patterns of dry powders that can be cladded or sintered onto a substrate by a micro-sized laser beam. The ultrasonic wave was effectively coupled into a glass capillary from an aluminum block through a tight fit. Continuous discharges of the copper and stainless steel powders were achieved at a rate of approximately 10y5 gys. It was verified that the powders were driven to the feeding tip of the capillary tube by friction and adhesive forces rather than by gravity. Currently, a minimum line width of 127 mm (almost the same as the inner diameter of the capillary tube, 125 mm) was obtained. The principle of powder feeding with a Rayleigh wave promises to feed microy
Fig. 12. Polished cross-section of drilled holes with various focal plane locations from top surface of the stainless steel sheets.
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Fig. 13. Relationship between the machining speed and the machined depth (beam spot diameters38 mm, powers400 mW): (a) 0.1 mmys; and (b) 0.2 mmys.
nano powders through capillary tubes with smaller inner diameters (10–25 mm). Laser micromachining was studied with a laser wavelength of 355 nm. The drilling depth is almost linearly proportional to the number of laser shots. A depth of as small as 0.1–0.4 mm can be obtained with a single laser pulse. Laser fluence significantly influenced the depth of machined holes. The recast area increases with laser fluence. Moreover, with the same laser fluence and same number of laser pulses, the maximum depth was obtained when the focal plane of the laser beam was located at the bottom surface of the stainless steel foil. Finally, experiments showed that the moving velocity of the substrate affected the shape and the depth of the machined channels. Acknowledgments The authors are grateful to the support from the Wisconsin Alumni Research Foundation and National Science Foundation.
References w1x S. Fatikow, U. Rembold, Microsystem Technology and Microrobotics, Springer-Verlag, Berlin Heidelberg, 1997, pp. 46–102. w2x M. Tabib-Azar, Microactuators: Electrical, Magnetic, Thermal, Optical, Mechanical, Chemical and Smart Structures, Kluwer Academic Publishers, New York, 1998. w3x J. Ding, Y. Meng, S. Wen, Annual Proceedings of Reliability Physics Symposium, n 01, CH3716, pp. 106–111. w4x F.B. Prinz, Process overviews, Rapid Prototyping in Europe and Japan, vol. I, Analytical Chapters, International Technology Research Institute, Loyola College, Maryland, 1997, pp. 5–20. w5x S. Au, P.K. Wright, Intelligent Concurrent Design: Fundamentals, Methodology, Modeling, and Practice, DE, 66, ASME, New Orleans, Louisiana, 1993, pp. 73–82. w6x P. Barkan, M. Iasiti, Concurrent Engineering: Research and Applications, 1, 1993, pp. 125–134. w7x J.G. Conley, H.L. Marcus, ASME Trans. J. Manuf. Sci. Eng. 119 (4B) (1997) 811–816. w8x J.J. Beaman, J.W. Barlow, D.L. Bourell, R.H. Crawford, H.L. Marcus, K.P. McAlea, Solid Freeform Fabrication—A New Direction in Manufacturing, Kluwer Academic Publishers, New York, 1997. w9x T.R. Jackson, H. Liu, N.M. Patrikalakis, E.M. Sachs, M.J. Cima, Mater. Des. 20 (2y3) (1999) 63–75.
X. Li et al. / Thin Solid Films 420 – 421 (2002) 515–523 w10x M. Sachs, N.M. Patrikalakis, Quarterly of the Rapid Prototyping Association of the Society of Manufacturing Engineers, 7(2), Second Quarter Issue, Dearborn, MI, 2001, pp. 1–5. w11x L. Jepson, J.J. Beaman, D. Bourell, K. Wood, Proceedings of the 1998 NSF Design and Manufacturing Grantees Conference, Monterrey, Mexico, 1998, pp. 497–498. w12x B. Jackson, K. Wood, J.J. Beaman, Solid Freeform Fabrication Proceedings, The University of Texas at Austin, Austin, TX, 2000, pp. 176–182. w13x M. Griffith, L. Harwell, J. Romero, E. Schlienger, C. Atwood, J. Smugeresly, Solid Freeform Fabrication Symposium, Austin, Texas, 1997, pp. 387–393. w14x X. Jiang, C. Sun, X. Zhang, B. Xu, Y.H. Ye, Sens. Actuat. A 87 (2000) 72–77. w15x X. Zhang, X. Jiang, C. Sun, Sens. Actuat. A 77 (2) (1999) 149–156. w16x V.K. Varadan, V.V. Varadan, S. Motojima, Proceedings of SPIE 2722 (1996) 156–164. w17x V.K. Varadan, V.V. Varadan, SPIE Proceedings 3990 (2000) 246–254. w18x S. Maruo, K. Ikuta, Proceedings of the International Symposium on Micro Machine and Human Science, 1999, pp. 173–178. w19x S. Maruo, K. Ikuta, Appl. Phys. Lett. 76 (19) (2000) 2656–2658. w20x A. Cohen, G. Zhang, F. Tseng, F. Mansfeld, U. Frodis, P. Will, Solid Freeform Fabrication Proceedings, Austin, TX, 1998, pp. 161–168. w21x R. Merz, F. Prinz, K. Ramaswami, K. Terk, L. Weiss, Proceedings of the Solid Freeform Fabrication Symposium, University of Texas at Austin, Austin, Texas, 1994, pp. 1–8. w22x R. Merz, Shape deposition manufacturing, Dissertation, Technical University of Vienna, Vienna, Austria, 1994. w23x K. Ramaswami, Process planning for shape deposition manufacturing, PhD Thesis, Stanford University, Palo Alto, California, 1997. w24x J.C. Miller, in: J.C. Miller (Ed.), Laser Ablation, SpringerVerlag, Berlin Heidelberg, 1994.
523
w25x E. Swenson, Y. Sun, C. Dunsky, SPIE Proceeding V 4095 (2000) 118–132. w26x M.C. Gower, in: R.C. Crafer, P.J. Oakley (Eds.), Laser Processing in Manufacturing, Chapman & Hall, MA, 1994. w27x J. Ramos, R. Estrela, L. Pitarch, J. Julia, B. Pascual, SPIE Proceeding V 3822 (1999) 207–213. w28x J. Li, G. Ananthasuresh, J. Micromech. Microeng. 11 (1) (2001) 38–47. w29x K. Hartke, K. King, D. Farson, K. Ely, SPIE Proceeding V 4088 (2000) 248–251. w30x U. Stamm, M. Fiebig, S. Govorkov, E. Mayer, R. Osmanov, M. Scaggs, E. Slobodchikov, A. Wiessner, D. Basting, SPIE Proceedings V 4088 (2000) 118–123. w31x H. Toenshoff, A.F. Von, A. Ostendorf, G. Willmann, T. Wagner, SPIE Proceedings V 3680 (1999) 536–545. w32x E.K. Illy, M.J. Withford, D. Brown, J.A. Piper, Conference Proceedings of the Lasers and Electro-Optics Society Annual Meeting CLEO, 11, IEEE, Piscataway, NJ, USA, 1997, p. 359, 97CH36110. w33x M.J. Renn, O. Vdovin, C.E. Weiman, D.Z. Anderson, E.A. Cornell, Phys. Rev. Lett. 75 (1995) 3253–3256. w34x A. Pique, D.B. Chrisey, J.M. FitzGerald, R.A McGill, R.C.Y. Auyeung, H.D. Wu, S. Lakeou, V. Nguyen, R. Chung, M.T. Duignan, J. Mater. Res. 15 (9) (2000) 1872–1875. w35x K. Linoya, K. Gotoh, K. Higashitani, in: K. Linoya, K. Gotoh, K. Higashitani (Eds.), Powder Technology Handbook, Marcel Dekker Inc, New York, 1991, p. 553. w36x S. Matsusaka, M. Urakawa, H. Masuda, Adv. Powder Technol. 6 (4) (1995) 283–293. w37x S. Matsusaka, K. Yamamoto, H. Masuda, Adv. Powder Technol. 7 (2) (1996) 141–151. w38x M. Egashira, N. Shinya, H. Saito, SPIE Proceedings 3673 (1999) 141–147. w39x T. Takano, Y. Tomikawa, Smart Mater. Struct. 7 (3) (1998) 417–421. w40x N. Osakabe, M. Kurosawa, T. Higuchi, O. Shinoura, Proceedings of the IEEE Workshop on Micro Electro Mechanical Systems, Heidelberg, 1998.
Micro rapid prototyping system for micro components Xiaochun Li*, Hongseok Choi, Yong Yang Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA
Abstract Similarities between silicon-based micro-electro-mechanical systems (MEMS) and Shape Deposition Manufacturing (SDM) processes are obvious: both integrate additive and subtractive processes and use part and sacrificial materials to obtain functional structures. These MEMS techniques are two-dimensional (2-D) processes for a limited number of materials while SDM enables the building of parts that have traditionally been impossible to fabricate because of their complex shapes or of their variety in materials. This work presents initial results on the development of a micro rapid prototyping system that adapts SDM methodology to micro-fabrication. This system is designed to incorporate microdeposition and laser micromachining. In the hope of obtaining a precise microdeposition, an ultrasonic-based micro powder-feeding mechanism was developed in order to form thin patterns of dry powders that can be cladded or sintered onto a substrate by a micro-sized laser beam. Furthermore, experimental results on laser micromachining using a laser beam with a wavelength of 355 nm are also presented. After further improvement, the developed micro manufacturing system could take computer-aided design (CAD) output to reproduce 3-D heterogeneous microcomponents from a wide selection of materials. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Rapid prototyping; Micro-component; Micro-feeding; Micromachining
1. Introduction Silicon-based micro-electro-mechanical system (MEMS) techniques are very popular for the manufacture of micro sensors, actuators, and components w1,2x. However, these MEMS techniques are two-dimensional (2-D) processes with multiple steps and require complex processing procedures in a cleanroom environment. Only a limited number of materials can be processed by these techniques. With 2-D processes, it is very difficult, if not impossible, to fabricate an enclosed volume of arbitrary shape and composition without the use of microassembly. Furthermore, the reliability, as well as the performance, of MEMS deserves more attention due to the increasing use of integrated microsystems w3x. Having reliable mechanical properties of micro components is critical to the safety and function of these complex MEMS. MEMS should not only be built in three-dimensions (3-D), but should also use a wider selection of materials, including alloys, polymers, ceramics, and heterogeneous materials. Micro-compo*Corresponding author. Tel.: q1-608-262-6142; fax: q1-608-2652316. E-mail address: [email protected] (X. Li).
nents with high aspect ratios, complex geometries, and complex microstructures are essential in many applications and can deliver a new generation of functionality and performance. However, little work has been done to successfully attain efficient micro manufacturing techniques for the fabrication of functionally and geometrically complex heterogeneous micro components. Concurrently with MEMS development, solid freeform fabrication (SFF) (also called ‘layered manufacturing’ or ‘rapid prototyping’) has emerged in the last decade as a popular manufacturing technology for accelerated product creation w4–8x. A general system is schematically presented in Fig. 1. A machine for SFF builds a part layer by layer directly from a CAD model without the fixturingytooling or human intervention demanded of conventional processes. This novel manufacturing technology enables the building of parts that have traditionally been impossible to fabricate because of their complex shapes or of their variety in materials. Different processes have also been used to create multimaterial parts w9–13x. Three-dimensional printing has been applied to build parts with local composition control. Selective laser sintering (SLS) has been used to build multi-material and functionally gradient mate-
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 9 3 5 - 5
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Fig. 1. Solid freeform fabrication process.
rials, while LENS (laser engineered net shape) has been used to tailor certain physical properties. Research has been directed at efforts that use several layered manufacturing processes to create 3-D micro scale components. Micro-stereolithography has been extensively studied and complex 3-D microstructures have been demonstrated w14–19x. Movable microstructures were made by the use of two-photon 3-D microfabrication with submicron resolution. EFAB (Electrochemical FABrication) specializes in fabrication of dense micro metal parts by electroplating w20x. However, these micro SFF processes are not suitable to build 3-D heterogeneous components due to their limited flexibility in changing material composition in situ. Laser-assisted Shape Deposition Manufacturing (SDM), an emerging SFF process, was developed to fabricate macro scale structures w21–23x. Unlike most additive SFF processes, SDM uses sequential additive (deposition of part materials and sacrificial materials) and subtractive (material removal) steps to form 3-D structures. Silicon-based MEMS and SDM processes both integrate additive and subtractive processes, and use part and sacrificial materials to obtain functional structures. SDM allows control of material location and properties in 3-D space. SDM has been used to build complex 3-D macro shapes with internal cooling channels, parts with continuously varying material properties, mechanisms, and heterogeneous parts with embedded sensors and actuators. If we adapt SDM methodology to microfabrication, it becomes possible to create functionally and geometrically complex heterogeneous micro components in a rapid fashion. However, little work has been done to fabricate 3-D micro components by use of micro SDM. To scale the SDM process down to the micro-world, it is essential to use tools that are capable of realizing both additive and subtractive processes at a micro-scale. Laser has been used for heating, melting, and ablation. Extensive work has been done in the arena of laser micromachining and microdeposition. Laser micromachining relies on the process of ablation w24x. Laser micromachining, especially with an excimer laser, has been used on a wide range of materials (polymers,
Fig. 2. Laser-based Micro-SDM system.
ceramics, semiconductors, and metals) w25–28x. In recent years, frequency-tripled or quadrupled Nd:YAG lasers at wavelengths of 355 nm and 266 nm have been used for micro-machining of different materials w29– 32x. While laser micro-machining is a subtractive process, laser micro-deposition is an additive process. Laser Particle Guidance (LPG), developed by Reen w33x, can deposit materials at 10 mm line width. MAPLE DW can deposit materials (polymers, ceramics, and metals) with a resolution approximately 10 mm w34x. Thus, a laserassisted micro-SDM process might be developed to integrate additive (laser micro-cladding) and subtractive (laser micro-machining) to form 3-D heterogeneous micro components. Laser based additiveysubtractive processes offer many advantages, including: no contact with substrate, no chemicals, flexible feature size and shape, high precision, work in air and at room temperature. 2. System design Micro-scale laser materials processing has become popular for microfabrication. Laser micro-machining processes can create 2-D and simple 3-D MEMS from a spectrum of homogeneous materials. However, few laser microdeposition processes are capable of in situ local composition control, which is vital for the fabrication of heterogeneous microstructures. This limitation is mainly due to the lack of a micro-powder feeding system that is capable of mixing and delivering various sub-micronynano powders without additional chemical mixtures. Therefore, an effective micro powder-feeding device is needed. Thin patterns of dry powders can be sintered or cladded by a laser beam. Different laser wavelengths are likely needed since different materials show different absorption characteristics. Thus, a pulsed Nd:YAG laser with different harmonic modes (1064 nm, 532 nm, 355 nm and 266 nm) seems an ideal match for the micro-deposition and micro-machining of various materials.
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Fig. 3. Manufacturing cycle of heterogeneous MEMS.
The micro-manufacturing system developed is depicted as shown in Fig. 2. A pulsed Nd:YAG laser (20Hz at all four harmonic modes), model PRO-290-20-NRF from Spectra-Physics, served as a micro additive and subtractive tool in this micro-SDM system. The energy levels are: 1.95 Jypulse at 1064 nm, 0.95 Jypulse at 532 nm, 0.40 Jypulse at 355 nm, and 0.175 Jypulse at 266 nm. A laser beam attenuator was used to control laser intensity. Moreover, a PC-controlled three-axis microstage, LW-7 XY and Anoride 7-4 Z from Anorad Inc., with a 30-nm resolution and a speed of up to 200 mmy s were also implemented. The CNC-2000 software accepts G-code that is generated by the UNIGRAPHICS CADyCAM software to control the movement of the micro-stage. The pulsed laser can be controlled spatially and temporally to obtain precise micro cladding and machining. In addition, an ultrasonic-based micro-powder feeding mechanism was developed and implemented to deposit microynano powders. An optical system with CCD camera and monitor was employed to monitor the micro-fabrication process with a maximum magnification of 3200. Image acquisition hardware and software from National Instruments were installed in a Dell PC. Based on the system design, the manufacturing cycle for the micro-SDM process can be depicted as shown in Fig. 3. The ultimate goal of this micro-manufacturing system is to provide a solid foundation for a seamless integration from CAD to the realization of complex 3D heterogeneous micro components in a wide selection of materials. 3. Experimental results and discussion Since it is still at an early stage of development, this session presents only initial results on the study of a micro-powder feeding device and on the characterization of laser micro-machining. It should be pointed out that
further experiments are being carried out to improve and integrate the system and will be reported in the future. 3.1. Study on micro-powder feeding mechanism Current laser-assisted SDM uses powder feeders and feed nozzles (with an inner diameter in the order of 1 mm) to control the material composition by mixing various powders (normally larger than 50 mm). Linoya et al. suggested that a vibrating system is more suitable to feed dry powders in sizes less than 100 mm w35x. Much finer powders (for instance, -1000 nm) could form agglomerates that prevent continuous feeding through a capillary tube. Ultrasonic vibration has been investigated to assist the feeding of fine powders w36– 39x. Capillary tubes with approximately 500-mm inner diameters have been studied to feed submicron alumna powders at rates of milligrams per second. Ultrasonic vibration at 20 kHz was generated by a piezoelectric transducer and applied to the capillary tube. It was observed that spherical powders flowed better than irregular shaped powders. In this study, an ultrasonic-based micro-feeding device was developed to deposit dry micro powders. Fig. 4 shows the schematic of the experimental set-up. A micro capillary tube with taper hole was assembled into a small aluminum block. A PZT plate, from Piezo System Inc., was glued on the top surface of the aluminum block. As a feasibility test, a low-cost micro capillary tube with an inner diameter of 125 mm and a length of 16 mm was used due to commercial availability. A function generator and a power amplifier were used to control the frequency and amplitude of the ultrasonic wave, which was generated by the thin PZT plate. It was detected that the resonant frequency of the PZT plate was 49 KHz. Thus, an ultrasonic wave of 49 KHz
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Fig. 4. Schematic of the micro-feeding and monitoring system.
was applied to feed dry powders through the capillary tube. The ultrasonic wave was effectively coupled into the glass capillary from the aluminum block with a tight fit. A video microscope system was used to study the mechanism that governs the micro-feeding of dry powders. The powder flow rate is important for the microdeposition since it can affect the continuation, width, and thickness of a deposited powder line. To measure the flow rates, a highly sensitive electric-balance was used to measure the mass of discharged powders. Powder flow rates for spherical copper powders (3 mm nominal in diameter) and stainless steel powers (3 mm nominal in diameter) were measured against the voltage applied on the PZT plate. Fig. 5 shows the curves of flow rates as functions of applied voltage.
Continuous discharge of the copper and stainless steel powders was achieved at a flow rate of approximately 10y5 gys. As shown in Fig. 5, the flow rates increase as the applied voltage increases until the voltage reaches approximately 280 V. However, the flow rates decrease quickly as the applied voltage surpasses 280 V. This is probably caused by saturation and higher temperature induced in the PZT plate at higher voltages. The difference in the flow rates between copper and stainless steel powders might have resulted from the difference in their properties. High quality for the deposited thin pattern of dry powders is critical for the rapid micro-fabrication of micro components. To characterize the quality of powder deposition, a series of straight lines of copper and stainless steel powders were deposited on a silicon substrate. The input voltage to the PZT plate was set to 280 V. To achieve thin, continuous and smooth lines, the parameters (moving velocity of the substrate, and gap between the feeding tip and the substrate) are important. Fig. 6 displays images of those deposited lines with various combinations of gap distances and velocities. The results showed that the feeding device could deposit a thin and continuous powder line with an optimized combination of a gap distance of 85 mm and a moving velocity of 4 mmys. It should also be noted that a twisted and broken line was also demonstrated with a combination of a gap distance of 170 mm and a moving velocity of 4 mmys. A minimum line width of 127 mm (almost the same as the inner diameter of the capillary tube) was obtained. The mechanism of ultrasonic-based micro-powder feeding device was designed to stimulate motions of the
Fig. 5. Flow rates of powers against the voltage applied to the PZT plate.
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Fig. 6. Images of the deposited lines of dry powders.
Fig. 7. Mechanism for micro-powder feeding: (a) top view of the inner surface particle motion; (b) section view of the inner surface particle motion; and (c) the motion orbit of the discharged micro powder.
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Fig. 9. Relationship between laser fluence and depth of drilled hole. Fig. 8. Relationship between laser shots and depth with laser fluence of 3.056 Jycm2.
surface particles at the inner wall of the capillary. With a suitable motion of the inner surface of the capillary, the friction and adhesive forces between the powders and the inner wall of the capillary can effectively discharge the powders. The motion of the inner surface is induced by a Rayleigh wave w40x, which is propagating on the elastic material surface. Fig. 7a shows the top view of the motion of the surface particles on the inner wall of the capillary. Surface particles near the circle move in elliptic locus. This motion was verified by powder spinning inside the capillary, which was observed with the video system. Fig. 7b is a longitudinal section view of the capillary. It indicates that the motions of the surface particles are moving in elliptic locus along the length of the inner wall. With these two surface motions, powders were driven to flow out the feeding tip of the capillary in a helix pattern, as depicted in Fig. 7c, which is verified by the image of the twisted broken line in Fig. 6. When the capillary was placed horizontally, powders were still discharged. This phenomenon verified that the powders were driven to the feeding tip of the capillary tube by friction and adhesive forces instead of by gravity. We believe that the Rayleigh wave can be further applied to feed powders through capillary tubes with smaller inner diameters (10–25 mm).
nesses of the foils, this paper focuses on the results that were achieved on surface-polished stainless steel 316L sheets with thickness of 0.762 mm. The relationship between the number of laser shots and the depth of the hole machined is shown in Fig. 8. The drilled depth on stainless steel is almost linearly proportional to the number of laser shots. A depth of approximately 200 nm per pulse can be achieved. This allows accurate control of the depth of drilled holes. The laser intensity (laser fluence) significantly affects the depth of drilling. Experiments were completed with a single pulse of controlled laser intensity. Fig. 9 shows that the depth of drilled hole increases rapidly with laser fluence after the ablation threshold. Then ablation rates remain almost constant, approximately 0.8 mmypulse, as the laser fluency increases until up to 100 Jycm2. The ablation rates then increase continuously with increasing laser fluence. It was observed under optical microscope that the recast area, a layer of debris on the surface of the material caused by the molten metal during laser micro-machining, increases with the laser fluence. Fig. 10 shows the relationship between the laser influence and the recast area. The location of focal plane related to the top surface of the stainless steel foils was found to be very important
3.2. Study on laser micromachining Laser micromachining was studied with a laser wavelength of 355 nm. All experiments were completed in air. An optical lens with a nominal focus length of 135 mm was used. The beam diameter on the lens was set to 6.0 mm. A larger focus length or a larger beam size on the lens can be used to further decrease the focus spot size. Stainless steel 316L foils with various thicknesses from 0.05 to 0.762 mm were used. Due to the similarities of experimental results on different thick-
Fig. 10. Relationship between recast area and laser fluence.
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time saving. The taper angles are found to be quite large, up to 208. The large taper angles may be caused by the beam quality (0.7 fit to Gaussian beam) and intensity variation inside the beam. It is believed that taper angles would be controlled to 3–58 if a beam homogenizer was used. 4. Conclusion
Fig. 11. Relationship between laser focal plane location and machining depth (445 pulses at a laser fluence of 40.74 Jycm2).
for laser micromachining of micro components with high aspect ratio. Fig. 11 shows the relationship between the focal position and drilled depth. With the same laser fluence and the same number of laser pulses, the maximum depth is obtained when the focal plane of the laser beam was located at the bottom surface of the stainless steel foil. Fig. 12 shows the shape and depth of the drilled holes under various focal plane positions. The moving velocity of the substrate related to the laser beam affected the shape and the depth of the machined channels, as shown in Fig. 13. The two channels were machined under two moving velocities of 0.1 and 0.2 mmys, respectively, under the same laser fluence. The slower moving velocity, 0.1 mmys, results in a deeper depth, but also a wider channel. The optimum speed is necessary for accurate machining and
This work discusses the development of a micro-rapid prototyping system that adapts SDM methodology to microfabrication. Initial results on the study of a micro powder-feeding device and on the characterization of laser micromachining are presented. By incorporating microdeposition and laser micromachining, the developed system could take computer-aided design (CAD) output from a computer to reproduce micro components. An ultrasonic-based micro powder-feeding mechanism was developed in order to form patterns of dry powders that can be cladded or sintered onto a substrate by a micro-sized laser beam. The ultrasonic wave was effectively coupled into a glass capillary from an aluminum block through a tight fit. Continuous discharges of the copper and stainless steel powders were achieved at a rate of approximately 10y5 gys. It was verified that the powders were driven to the feeding tip of the capillary tube by friction and adhesive forces rather than by gravity. Currently, a minimum line width of 127 mm (almost the same as the inner diameter of the capillary tube, 125 mm) was obtained. The principle of powder feeding with a Rayleigh wave promises to feed microy
Fig. 12. Polished cross-section of drilled holes with various focal plane locations from top surface of the stainless steel sheets.
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Fig. 13. Relationship between the machining speed and the machined depth (beam spot diameters38 mm, powers400 mW): (a) 0.1 mmys; and (b) 0.2 mmys.
nano powders through capillary tubes with smaller inner diameters (10–25 mm). Laser micromachining was studied with a laser wavelength of 355 nm. The drilling depth is almost linearly proportional to the number of laser shots. A depth of as small as 0.1–0.4 mm can be obtained with a single laser pulse. Laser fluence significantly influenced the depth of machined holes. The recast area increases with laser fluence. Moreover, with the same laser fluence and same number of laser pulses, the maximum depth was obtained when the focal plane of the laser beam was located at the bottom surface of the stainless steel foil. Finally, experiments showed that the moving velocity of the substrate affected the shape and the depth of the machined channels. Acknowledgments The authors are grateful to the support from the Wisconsin Alumni Research Foundation and National Science Foundation.
References w1x S. Fatikow, U. Rembold, Microsystem Technology and Microrobotics, Springer-Verlag, Berlin Heidelberg, 1997, pp. 46–102. w2x M. Tabib-Azar, Microactuators: Electrical, Magnetic, Thermal, Optical, Mechanical, Chemical and Smart Structures, Kluwer Academic Publishers, New York, 1998. w3x J. Ding, Y. Meng, S. Wen, Annual Proceedings of Reliability Physics Symposium, n 01, CH3716, pp. 106–111. w4x F.B. Prinz, Process overviews, Rapid Prototyping in Europe and Japan, vol. I, Analytical Chapters, International Technology Research Institute, Loyola College, Maryland, 1997, pp. 5–20. w5x S. Au, P.K. Wright, Intelligent Concurrent Design: Fundamentals, Methodology, Modeling, and Practice, DE, 66, ASME, New Orleans, Louisiana, 1993, pp. 73–82. w6x P. Barkan, M. Iasiti, Concurrent Engineering: Research and Applications, 1, 1993, pp. 125–134. w7x J.G. Conley, H.L. Marcus, ASME Trans. J. Manuf. Sci. Eng. 119 (4B) (1997) 811–816. w8x J.J. Beaman, J.W. Barlow, D.L. Bourell, R.H. Crawford, H.L. Marcus, K.P. McAlea, Solid Freeform Fabrication—A New Direction in Manufacturing, Kluwer Academic Publishers, New York, 1997. w9x T.R. Jackson, H. Liu, N.M. Patrikalakis, E.M. Sachs, M.J. Cima, Mater. Des. 20 (2y3) (1999) 63–75.
X. Li et al. / Thin Solid Films 420 – 421 (2002) 515–523 w10x M. Sachs, N.M. Patrikalakis, Quarterly of the Rapid Prototyping Association of the Society of Manufacturing Engineers, 7(2), Second Quarter Issue, Dearborn, MI, 2001, pp. 1–5. w11x L. Jepson, J.J. Beaman, D. Bourell, K. Wood, Proceedings of the 1998 NSF Design and Manufacturing Grantees Conference, Monterrey, Mexico, 1998, pp. 497–498. w12x B. Jackson, K. Wood, J.J. Beaman, Solid Freeform Fabrication Proceedings, The University of Texas at Austin, Austin, TX, 2000, pp. 176–182. w13x M. Griffith, L. Harwell, J. Romero, E. Schlienger, C. Atwood, J. Smugeresly, Solid Freeform Fabrication Symposium, Austin, Texas, 1997, pp. 387–393. w14x X. Jiang, C. Sun, X. Zhang, B. Xu, Y.H. Ye, Sens. Actuat. A 87 (2000) 72–77. w15x X. Zhang, X. Jiang, C. Sun, Sens. Actuat. A 77 (2) (1999) 149–156. w16x V.K. Varadan, V.V. Varadan, S. Motojima, Proceedings of SPIE 2722 (1996) 156–164. w17x V.K. Varadan, V.V. Varadan, SPIE Proceedings 3990 (2000) 246–254. w18x S. Maruo, K. Ikuta, Proceedings of the International Symposium on Micro Machine and Human Science, 1999, pp. 173–178. w19x S. Maruo, K. Ikuta, Appl. Phys. Lett. 76 (19) (2000) 2656–2658. w20x A. Cohen, G. Zhang, F. Tseng, F. Mansfeld, U. Frodis, P. Will, Solid Freeform Fabrication Proceedings, Austin, TX, 1998, pp. 161–168. w21x R. Merz, F. Prinz, K. Ramaswami, K. Terk, L. Weiss, Proceedings of the Solid Freeform Fabrication Symposium, University of Texas at Austin, Austin, Texas, 1994, pp. 1–8. w22x R. Merz, Shape deposition manufacturing, Dissertation, Technical University of Vienna, Vienna, Austria, 1994. w23x K. Ramaswami, Process planning for shape deposition manufacturing, PhD Thesis, Stanford University, Palo Alto, California, 1997. w24x J.C. Miller, in: J.C. Miller (Ed.), Laser Ablation, SpringerVerlag, Berlin Heidelberg, 1994.
523
w25x E. Swenson, Y. Sun, C. Dunsky, SPIE Proceeding V 4095 (2000) 118–132. w26x M.C. Gower, in: R.C. Crafer, P.J. Oakley (Eds.), Laser Processing in Manufacturing, Chapman & Hall, MA, 1994. w27x J. Ramos, R. Estrela, L. Pitarch, J. Julia, B. Pascual, SPIE Proceeding V 3822 (1999) 207–213. w28x J. Li, G. Ananthasuresh, J. Micromech. Microeng. 11 (1) (2001) 38–47. w29x K. Hartke, K. King, D. Farson, K. Ely, SPIE Proceeding V 4088 (2000) 248–251. w30x U. Stamm, M. Fiebig, S. Govorkov, E. Mayer, R. Osmanov, M. Scaggs, E. Slobodchikov, A. Wiessner, D. Basting, SPIE Proceedings V 4088 (2000) 118–123. w31x H. Toenshoff, A.F. Von, A. Ostendorf, G. Willmann, T. Wagner, SPIE Proceedings V 3680 (1999) 536–545. w32x E.K. Illy, M.J. Withford, D. Brown, J.A. Piper, Conference Proceedings of the Lasers and Electro-Optics Society Annual Meeting CLEO, 11, IEEE, Piscataway, NJ, USA, 1997, p. 359, 97CH36110. w33x M.J. Renn, O. Vdovin, C.E. Weiman, D.Z. Anderson, E.A. Cornell, Phys. Rev. Lett. 75 (1995) 3253–3256. w34x A. Pique, D.B. Chrisey, J.M. FitzGerald, R.A McGill, R.C.Y. Auyeung, H.D. Wu, S. Lakeou, V. Nguyen, R. Chung, M.T. Duignan, J. Mater. Res. 15 (9) (2000) 1872–1875. w35x K. Linoya, K. Gotoh, K. Higashitani, in: K. Linoya, K. Gotoh, K. Higashitani (Eds.), Powder Technology Handbook, Marcel Dekker Inc, New York, 1991, p. 553. w36x S. Matsusaka, M. Urakawa, H. Masuda, Adv. Powder Technol. 6 (4) (1995) 283–293. w37x S. Matsusaka, K. Yamamoto, H. Masuda, Adv. Powder Technol. 7 (2) (1996) 141–151. w38x M. Egashira, N. Shinya, H. Saito, SPIE Proceedings 3673 (1999) 141–147. w39x T. Takano, Y. Tomikawa, Smart Mater. Struct. 7 (3) (1998) 417–421. w40x N. Osakabe, M. Kurosawa, T. Higuchi, O. Shinoura, Proceedings of the IEEE Workshop on Micro Electro Mechanical Systems, Heidelberg, 1998.