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Application of microforging to SiCN MEMS fabrication
Sensors and Actuators A 95 (2002) 143±151
Application of microforging to SiCN MEMS fabrication Yiping Liua,*, Li-Anne Liewa, Ruiling Luoa, Linan Anb, Martin L. Dunna, Victor M. Brighta, John W. Dailya, Rishi Raja a
Department of Mechanical Engineering, NSF Center for Advanced Manufacturing and Packaging of Microwave, Optical, and Digital Electronics (CAMPMODE), University of Colorado, Boulder, CO 80309-427, USA b Department of Mechanical, Materials and Aerospace Engineering (MMAE), Advanced Materials Processing and Analysis Center (AMPAC), University of Central Florida, Orlando, FL 32826, USA
Abstract Ceramics and polymers are attractive candidate materials for MEMS applications because of the wide range of properties that can be obtained, and the promise of improved performance as compared to the existing materials set for MEMS. A challenge in the fabrication of ceramic MEMS is prohibiting cracking that can occur during processing. For example, this is signi®cant in the development of a microcasting fabrication technique from a polymer precursor for silicon carbonitride (SiCN) MEMS. In this case, shrinkage mismatch between the SiCN structure and the microfabricated mold during thermal processes leads to signi®cant stresses that can crack the ceramic structure. Here, we propose an approach to overcome this problem that relies on demolding prior to the large shrinkage mismatch thermal processes, which itself is a nontrivial challenge. To this end, we propose and describe a microforging process that facilitates demolding and show representative results for numerous SiCN ceramic microstructures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: SiCN; MEMS; Microcast; Microforge; Microform; Ceramic
1. Introduction Ceramics are attractive candidate materials for MEMS applications because of the wide range of properties that can be obtained and the promise of improved performance as compared to the existing materials set for MEMS. With the development and use of new materials, new processing techniques arise, and with these, new issues and challenges emerge. Here, we treat such a problem that has emerged in our development of a microcasting fabrication technique for polymer precursor derived silicon carbonitride (Si1.7C1.0N1.6) high-temperature MEMS [1]. Conventional ceramics such SiC and Si3N4 are attractive for high-temperature applications. The fabrication of SiC- or Si3N4-based ceramic structures usually involves sintering of ceramic powder at a temperature higher than 15008C [1]. The necessary sintering additives can degrade ceramic mechanical properties at such a high-temperature. Moreover, the powder route is not compatible with available micropatterning techniques. Chemical vapor deposition (CVD) of SiC is a new technique which involves surface micromachining and can be used to fabricate SiC high-temperature
* Corresponding author. E-mail address: [email protected] (Y. Liu).
MEMS [2]. However, the CVD and micromachining of SiC are time-consuming and expensive. Due to the planar nature of CVD, it is dif®cult to fabricate high aspect ratio MEMS structures. A recently developed processing technique of injectable polymer SiCN ceramic has been applied to implement hightemperature MEMS components with higher aspect ratio and lower cost than that of CVD SiC approach [3]. Instead of using a cross-linked polymer powder as the starting material, a liquid polymer precursor is directly cast into micropatterned molds and then thermoset to form polymer MEMS structures. Finally, bulk SiCN ceramic structures are obtained by cross-linking and thermal decomposition of the polymer at temperatures as low as 10008C. In MEMS applications, microstructure dimensions are relatively small and often at least one dimension is in the range of hundreds of microns. With additional isostatic pressure during cross-linking, the released gaseous by-products are likely to diffuse out before they build up high enough pressure to form voids inside the structure. So the direct microcasting method is especially suitable for MEMS fabrication. The resulting single phase and amorphous SiCN ceramic microstructure surfaces are shiny and no microcracks and few voids are observed on SEM images of the cross-section [3]. The linear shrinkage can reach up to 30%, which suggests that a large degree of densi®cation occurred
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during the thermal treatment. The bulk density of SiCN MEMS samples is about 2.35 g/cm3 which is in good agreement with the solid phase density deduced form Hgpressure porosimetry (2.33 g/cm3) and from water pycnometry of SiCN powder (2.30 g/cm3) [4]. Therefore, it can be concluded that the resulting microcast SiCN structure is fully densi®ed with virtually no nanoporosity. By comparison, SiCN compacts fabricated through the powder route contain more pores. Even applying high pressure (640 MPa) when the green body is formed, the open porosity is still larger than 6% [4]. The microcast route can signi®cantly improve the microstructure as well material properties of SiCN micro components. The high-temperature properties of microcast SiCN are comparable to those of SiC and superior to those of silicon [3]. Both Vickers hardness (15 GPa) and elastic modulus (158 GPa) have been measured using a microindentation system. The amorphous SiCN monoliths decompose at about 14008C in vacuum and are thermally stable in nitrogen up to 17008C [5]. Furthermore, it has oxidation resistance up to 16008C in air due to the formation of a highly pure silica oxidation layer [6]. SiCN, like SiC, also exhibits good creep resistance at high-temperature [3]. The combination of the attractive high-temperature properties of SiCN compositions, and the unique fabrication capabilities of the microcasting approach make SiCN MEMS attractive alternatives for a wide range of applications, e.g. microscale combustion chambers and turbine engines, where materials and structures are required to resist devitri®cation and decomposition, exhibit good oxidation resistance, and maintain good mechanical properties at hightemperatures. Furthermore, their mechanical properties can be tailored to some degree via the molecular level design of the polymer precursor, thus, broadening the spectrum of available properties and freeing design constraints [5]. The SiCN microcasting process involves three key steps [3]: (1) an inorganic polymer precursor is cast into a microfabricated mold and thermally set at about 2508C to form an infusible transparent polymer solid structure; (2) the solid structure is then cross-linked at about 4008C under isostatic pressure; (3) the pre-ceramic microstructure is then pyrolyzed at about 10008C to form a dense, monolithic SiCN ceramic structure. Due to the nature of microcasting, high aspect ratio 3D MEMS structure can be fabricated. Since the pre-ceramic polymer is infusible after thermoset and the shrinkage during decomposition is linear, the shape and de®nition of the microstructure will be retained after pyrolysis, except the dimensions are smaller than that of original mold. We typically use standard lithographic processes to fabricate soft polymeric molds, e.g. from photoresist. A signi®cant practical challenge in this process is the prevention of component cracking during the cross-linking and/or pyrolysis processes where the relatively weak component undergoes large shrinkage. If the shrinkage is overly constrained by the mold, the component may crack. An approach to overcome this potential cracking is to demold
the structure after the thermoset process, but this is not trivial. We have used three conventional demolding methods: direct mechanical demolding, lost mold, and dissolved mold, but none of these has been completely satisfactory. As a result, we propose a new method to achieve demolding which uses a microforged thin foil structure as the mold and mechanical peeling to facilitate demolding. During this process, we also use a NaCl single crystal substrate which facilitates the separation of fragile microstructures from the substrate by dissolution of the substrate. The result is a freestanding thermoset component that is not subject to constraint from the mold when it shrinks during the subsequent cross-linking and pyrolysis steps, and thus, it undergoes no damage during these processes. 2. Fabrication of microforged molds We fabricate SiCN ceramic MEMS by microcasting a liquid polymer precursor, typically the commercially-available CERASETTM (from Lanxide Company, USA), into SU8 (from Micro Chem Corp.) photoresist micromolds made by standard photolithographic methods. The bonds between the thermoset precursor, the mold, and the substrate are typically quite strong. As a result, during direct mechanical demolding, the weak thermoset structure may fail before the interfaces between the thermoset structure and the mold and/ or substrate. The weak thermoset precursor may stick to the walls of the mold and the surface of substrate, thus, damaging the microstructure. During cross-linking and pyrolysis, CERASET will undergo a shrinkage as high as about 30%, while the baked SU-8 photoresist will experience less shrinkage and the substrate none at all. When subjected to such a large shrinkage mismatch, the thermoset structure is highly stressed due to the restriction of the mold walls and substrate, and these stresses can lead to microstructure damage by cracking of the brittle solid. To avoid such cracking damage, we propose a method based on the use of microforged molds and mechanical peeling to demold the microstructure after thermosetting, rendering it free from stress-inducing constraint during cross-linking and pyrolysis. High-precision forging is a microforming method that has become increasingly popular in modern plastic working where many strategies have been employed and process models developed and validated [7]. However, these techniques have not yet to seen application in ceramic MEMS fabrication. In this study, we employ a relatively simple soft microforging technique to demonstrate the potential of the approach; future work will consider more sophisticated techniques and process optimization. Microforged molds can be divided into two categories by the way the prototype is designed. If a positive prototype is used, which resembles the real structure, a positive forging setup is made. While for negative prototypes, a corresponding negative prototype forging procedure is followed.
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The simpli®ed soft microforge technique shares similarities with hot embossing microfabrication methods [8]. The fabrication processes of microforged molds using both positive and negative prototypes are described below. 2.1. Positive microforge assembly First, as shown in Fig. 1, we fabricate a positive SU-8 photoresist prototype on a silicon substrate, which has the shape desired for the SiCN microstructure, but is larger than the SiCN microstructure to compensate for the shrinkage caused by cross-linking and pyrolysis. Then a layer of thin aluminum foil (20±40 mm in this work) is applied to the top of the prototype, and assembly is placed into a pressure cylinder, which uses plasticine as the medium through which pressure is applied. A compressive force is gradually applied, during which the thin aluminum foil is forged into a negative mold and the plasticine layer serves as a mold holder. The de®nition of the aluminum mold depends on the foil thickness, the applied pressure, and the rigidity of the baked photoresist prototype. Although we have not studied these interrelationships in depth, Figs. 2 and 3 show
Fig. 1. Soft microforge of aluminum foil mold using positive prototype.
examples that illustrate these relationships over a range of foil thickness and applied pressure that we have found to be practically feasible. The forged negative molds have the inverse shape of the positive prototype. It is apparent that the deformation of the prototype is small. With a decrease of the foil thickness and/
Fig. 2. Effect of foil thickness and forge pressure on the definition of the aluminum foil mold (scale is in mm).
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Fig. 3. Effect of foil thickness and forge pressure on the sharp edges and corners of the aluminum foil molds.
or an increase of the forge pressure, the de®nition of the aluminum mold is improved. Of course, there are limits to the de®nition that can be obtained and presumably the process parameters can be tailored to achieve optimized de®nition. Under increased pressure, the SU-8 prototype deforms further, primarily at sharp corners and rectangular edges which are stress raisers. Increasing the SU-8 prototype hardness can also minimize the deformation. This can be accomplished by controlling the SU-8 post-bake temperature and time. Actually, with different combinations of temperature and time, the SU-8 can be baked into a color spectrum from transparent to yellow, brown and black. The hardness of SU-8 is gradually increased accordingly. However, the brown prototype tends to crack while the black one is brittle. So the yellow SU-8 prototype is a good choice compared to the other baked states. Fig. 4 shows the SEM images of microforged molds based on transparent and overbaked yellow prototypes, respectively. The rectangular edge de®nition of the molds is greatly improved by using overbaked prototypes.
Fig. 5 compares the prototype, the aluminum foil mold and a free-standing thermoset structure. The sharp edges and corners of the four-arm structure are rounded, indicating a reduction in de®nition. However, this rounding may be advantageous in situations where the sharp corners and edges may lead to severe stress raisers in the ®nal structure in its intended use environment. Also, if the prototype edge is extremely sharp and hard, it is likely to tear the aluminum foil during forging. So controlling the prototype hardness is necessary for microforging. 2.2. Negative microforge assembly The setup of negative microforge assembly, as shown in Fig. 6, is only slightly different from the positive one. Since we want to use the outside of the aluminum mold, we put a layer of thin plastic ®lm between the plasticine and aluminum foil. This thin plastic ®lm not only facilitates the separation of the plasticine and aluminum mold after forging, but also protects the mold surface from plasticine contamination.
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Fig. 4. Effect of the SU-8 prototype hardness on the definition of aluminum foil molds, which are for tensile test specimens.
In this con®guration, the silicon substrate and over baked SU-8 negative prototype gives the thin foil mold a stronger support than that of the plasticine alone described in positive forging case. However, the disadvantage of this case is also apparent. Because the aluminum foil has a thickness around 20±40 mm, the foil mold dimensions are no longer the same as the prototype dimension. Along with the shrinkage of polymer precursor, it will further complicate the design of
MEMS structures. As a result, the positive microforge method is preferred. Since the aluminum foil is forged at room temperature, the aspect ratio of the resulting structures is about unity. Our primary intent here is to demonstrate the potential possibility of using microforged thin foil molds to facilitate the demolding of thermoset pre-ceramic structure. In the future, the microforging techniques can be improved in many ways
Fig. 5. Definition comparison of original prototype, forged mold, and free-standing thermoset structure.
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2.
3.
Fig. 6. Soft microforge of aluminum foil mold using negative prototype.
including raising the forging temperature, using a lubricant, matched dies, and/or superplastic alloy foil, controlling the forging force, and chilling the die. 3. Microcasting processes Based on a positive prototype, the complete microcasting process, including details of the proposed new demolding method, are shown in Fig. 7. Brie¯y, the process consists of the following steps: 1. Fabricate a positive SU-8 photoresist prototype on a silicon substrate. After post-exposure, baking and development, the SU-8 photoresist prototype is mechanically
4.
5. 6. 7.
stiff enough to resist moderate forging pressures without appreciable deformation. The prototype can be used repeatedly. Use the simplified soft forge method as described in the last section to make a negative aluminum foil mold. We use plasticine underneath the mold to hold and support the foil mold during subsequent steps. Microcast the liquid polymer precursor in the aluminum foil mold and apply a single crystal NaCl plate as a top cover. Since the liquid precursor is sensitive to air and humidity, this step is done in an argon atmosphere. We use a NaCl plate instead of a silicon wafer to facilitate easier release of the precursor structure after it is demolded from the aluminum foil mold. To ensure tight contact, a mass is applied on top of the NaCl plate. Thermoset the liquid polymer precursor resulting in a solid, but weak, polymer structure. Although the liquid polymer precursor will shrink slightly during thermosetting, it seems to cause only elastic deformation in the structure and mold. The stresses developed are not enough to damage the structure. Remove the plasticine supporting layer, and then mechanically peel the foil mold to leave free polymer structures adhering to the NaCl plate. Apply a layer of wax to the polymer structure side to serve as a handling wafer for the entire batch of microstructures. Release the wax handling wafer together with the attached polymer microstructures from the NaCl plate using water as a penetration solvent. The release time is
Fig. 7. Fabrication processes of free-standing precursor structure after thermoset.
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only several seconds and the water does not impact the polymer precursor. 8. Melt or burn away the wax during the subsequent heat treatment which consists of cross-linking and pyrolysis. The result is a batch of free-standing, crack free SiCN MEMS structures. With this method, the key to successful demolding is the nature of the stress state imposed on the thermoset structure during the peeling process. If the thickness of the aluminum foil is much thinner than the minimum feature size of the MEMS structure, the foil is relatively ¯exible and can be easily peeled. Because the stress is highly localized at the tip of the interface crack between the thermoset microstructure and the aluminum foil, force equilibrium dictates that the average stress applied to the polymer structure is much lower than the critical fracture stress. This situation is in contrast to the stress state developed during direct pull or push demolding where the friction force and bonding force between the mold wall and the polymer structure will impede demolding. So the average stress inside the structure can exceed the critical fracture stress and result in cracking of the microstructure.
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4. Results and discussion Using the microforging process described in the previous sections we have successfully fabricated a wide variety of SiCN ceramic MEMS structures that have passed through the thermoset, demolding, cross-linking, and pyrolysis steps free of damage. Some of these are shown in Fig. 8. They include a gear, a fan blade, and microstructures with various channels including rectangular and hexagonal channels. Compared to lost mold methods [9] which may require days to slowly burn off the molds, e.g. PMMA molds, the mechanical peeling demolding method is ef®cient in terms of both time and cost. As mentioned previously, another way to demold is to dissolve or etch away the mold. This is problematic with the SiCN polymer precursor at the thermoset stage, though, because the ceramic precursor itself may be attacked by the demolding etchant. However, it is possible that the metal foil mold may also be etched as a sacri®cial layer in order to release the pre-ceramic structure. The primary result here is the demonstration of the viability of the microforging mold process to yield
Fig. 8. Various ceramic structures made by the microcasting process using microforged molds (the thickness of the structures is around 50 mm).
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damage-free ceramic structures via microcasting of a polymeric precursor. Of course, this is an initial step in the development of reliable technologies for the mass production of ceramic MEMS. Many improvements in addition to our preliminary efforts are expected in order to obtain better de®nition and higher achievable aspect ratio for the ceramic microstructures. These include: 1. Using an advanced superplastic material for the foil mold. There are potentially two advantages: (a) superplastic deformation will give high aspect ratio and more precise definition to the object; (b) if a low melting alloy is used (e.g. indium-based) then it is possible that the metal foil mold can be removed by melting it away leaving behind a self standing cross-linked CERASET structure. 2. Using a prototype with higher stiffness than the SU-8 photoresist. An obvious possibility is a silicon or a metal prototype that can be fabricated by established MEMS bulk and/or surface micromachining techniques. 3. Using matched dies for microforging. High-accuracy dies and an alignment frame will facilitate the development of desirable near-net shape microstructures. 4. Microforging at elevated temperatures. The ductility of the metal foil increases with temperature, but at the same time the stiffness of metal prototype decreases. Therefore, the temperature should be controlled to optimize these competing effects. 5. Microforging with a lubricant. This will potentially improve the process, but this is a challenging problem at MEMS length scale. 5. Conclusions We proposed a method of microforging-based mold fabrication and demolding as an approach to eliminate cracking in the fabrication of polymer-derived ceramic MEMS during thermal heat treatments. We described the microforging process and the subsequent demolding process necessary to yield free-standing pre-ceramic structures prior to the ®nal cross-linking and pyrolysis. As a result, the shrinkage that occurs during these processes is unconstrained, substantial stresses, thus, do not develop, and crack free ceramic MEMS structures are obtained. Here, we reported our initial efforts that demonstrate the viability of the process, and we also pointed out process improvements that are expected to lead to improved de®nition of ceramic MEMS. Acknowledgements This work is supported by the Defense Advanced Research Projects Agency (DARPA) and US Air Force under contract #F30602-99-2-0543.
References [1] R. Riedel, G. Passing, H. Schonfelder, R.J. Brook, Synthesis of dense silicon-based ceramics at low temperatures, Nature 355 (6362) (1992) 714±717. [2] M. Mehregany, A. Heuer, S.M. Philips, A multidisciplinary research proposal for advanced electromechanical microsystems, Semi-annual Technical Report to DARPA, 1998. [3] L. An, W. Zhang, V.M. Bright, M.L. Dunn, R. Raj, Development of injectable polymer-derived ceramics for high-temperature MEMS (MEMS'2000), 2000, pp. 619±623. [4] R. Riedel, et al., Polymer-derived silicon-based bulk ceramics. Part I. Preparation, processing and properties, J. Eur. Ceram. Soc. 15 (1995) 703±715. [5] R. Riedel, A. Kienzle, W. Dressler, L. Ruwisch, J. Bill, F. Aldinger, A silicoboron carbonitride ceramic stable to 20008C, Nature 382 (6594) (1996) 796±798. [6] H. Schonfelder, F. Aldinger, R. Riedel, Silicon carbonitrides Ð a novel class of materials, J. Phys. 3 (1993) 1293±1298. [7] D.B. Shan, Z. Wang, Y. Lu, Study on isothermal precision forging technology for a cylindrical aluminum-alloy housing, J. Mater. Process. Technol. 72 (3) (1997) 403±406. [8] H. Becker, U. Heim, Hot embossing as a method for the fabrication of polymer high aspect ratio structures, Sens. Actuators A: Phys. 83 (1±3) (2000) 130±135. [9] H. Freimuth, V. Hessel, H. Kolle, M. Lacher, W. Ehrfeld, T. Vaahs, M. Bruck, Formation of complex ceramic miniaturized structures by pyrolysis of poly(vinylsilazane), J. Am. Ceram. Soc. 79 (6) (1996) 1457±1465.
Biographies Yiping Liu received her BS and MS degrees in materials engineering from the Shanghai Jiao Tong University of China in 1990 and 1993, respectively. She is now a doctoral research assistant in the Department of Mechanical Engineering at the University of Colorado, Center for Advanced Manufacturing and Packaging of Microwave, Optical and Digital Electronics (CAMPMODE). Her research interests are in the design, fabrication and testing of MEMS for high-temperature environments. Li-Anne Liew received her BS and MS degrees in mechanical engineering from the University of Colorado at Boulder in 1998 and 2000, respectively. Currently, she is a doctoral research assistant in the Department of Mechanical Engineering at the University of Colorado, Center for Advanced Manufacturing and Packaging of Microwave, Optical and Digital Electronics (CAMPMODE). Her research interests are in the design and packaging of MEMS sensors, MEMS for biomedical applications, and the design and fabrication of MEMS for hightemperature environments. Ruiling Luo received his BS and MS degrees in materials engineering from the Shanghai Jiao Tong University of China in 1993 and 1996, respectively. He received another MS degree in mechanical engineering from the University of Colorado at Boulder in 2000. His research interests are in the design, fabrication and testing of MEMS for high-temperature environments. Linan An is an assistant professor in Advanced Materials Processing and Analysis Center (AMPAC) and Department of Mechanical, Materials and Aerospace Engineering (MMAE) at the University of Central Florida, Orlando. Prior to joining UCF, Dr. An was a post-doctor research associate at University of Colorado at Boulder. His research interests are in the general area of processing±structure±property relationships of ceramic materials. Most recently, his research is focused on polymer-derived
Y. Liu et al. / Sensors and Actuators A 95 (2002) 143±151 ceramics and their applications in high-temperature MEMS. Dr. An is pioneer in the microfabrication of polymer-derived ceramic MEMS. He has more than 20 papers on materials science and microfabrication technologies. Martin L. Dunn is an associate professor of mechanical engineering. He received the PhD in mechanical engineering in 1992 from the University of Washington, USA. Professor Dunn's research interests include: (i) the micromechanical behavior (fracture and deformation) of materials and structures for microelectromechanical systems applications, (ii) the micromechanics and physics of heterogeneous media, including defects, polycrystals, and composites, with emphasis on piezoelectric solids, and (iii) the acoustic characterization of material microstructure. His research is based on a strong coupling between theoretical and experimental efforts. Professor Dunn has published over 60 articles in refereed journals on these subjects and has been the principal or co-principal investigator on grants and contracts in these areas from NSF, DOE, ONR, NIST, DARPA, and Sandia National Laboratories. Victor M. Bright is an associate professor of mechanical engineering and the director of the MEMS R&D Laboratory, University of Colorado at Boulder. Prior to joining the University, he was an associate professor and the director of the Microelectronics Research Laboratory in the Department of Electrical and Computer Engineering, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio (June 1992 to December 1997). Professor Bright's research includes MEMS, silicon micromachining, microsensors, microactuators, MEMS self-assembly, MEMS packaging, opto-electronics, and semiconductor device physics. Dr. Bright received the following awards in the area of MEMS: best paper
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of the MCM'98 Ð International Conference and Exhibition on Multichip Modules and High Density Packaging, 1998; R.F. Bunshah best paper award at the 1996 International Conference on Metallurgical Coatings and Thin Films. Dr. Bright has authored and co-authored more than 70 papers in the areas of MEMS. He is a member of IEEE, ASME, and SPIE. He serves on the Executive Committee for ASME MEMS Sub-division. John W. Daily is currently professor of mechanical engineering and director of the Center for Combustion and Environmental Research at the University of Colorado at Boulder. He studied mechanical engineering at the University of Michigan (BS 1968, MS 1969) and at Stanford University (PhD 1975.). After receiving the PhD, he was a faculty member at the University of California at Berkeley until 1988, attaining the rank of full professor of mechanical engineering. Most of his academic career has been devoted to the field of combustion and environmental studies. Recently, he has been working in the areas of MEMS applications in combustion. He has over 100 scientific and technical publications. Rishi Raj. Prior to joining University of Colorado, Boulder in 1996, he had served on the faculty of Materials Science and Engineering at Cornell University for 21 years. He received the PhD in applied physics from Harvard University in 1970. His research is in processing of hightemperature metals, composites, and ceramics. His most significant contributions have been in understanding how interfaces control hightemperature processing and mechanical behavior. He has over 200 publications of which 150 are in refereed journals, and include 14 US Patents. He has been a Guggenheim fellow, an Alenxander von Humboldt senior scientist awardee and a Matthias scholar at Los Alamos National Laboratory.
Application of microforging to SiCN MEMS fabrication Yiping Liua,*, Li-Anne Liewa, Ruiling Luoa, Linan Anb, Martin L. Dunna, Victor M. Brighta, John W. Dailya, Rishi Raja a
Department of Mechanical Engineering, NSF Center for Advanced Manufacturing and Packaging of Microwave, Optical, and Digital Electronics (CAMPMODE), University of Colorado, Boulder, CO 80309-427, USA b Department of Mechanical, Materials and Aerospace Engineering (MMAE), Advanced Materials Processing and Analysis Center (AMPAC), University of Central Florida, Orlando, FL 32826, USA
Abstract Ceramics and polymers are attractive candidate materials for MEMS applications because of the wide range of properties that can be obtained, and the promise of improved performance as compared to the existing materials set for MEMS. A challenge in the fabrication of ceramic MEMS is prohibiting cracking that can occur during processing. For example, this is signi®cant in the development of a microcasting fabrication technique from a polymer precursor for silicon carbonitride (SiCN) MEMS. In this case, shrinkage mismatch between the SiCN structure and the microfabricated mold during thermal processes leads to signi®cant stresses that can crack the ceramic structure. Here, we propose an approach to overcome this problem that relies on demolding prior to the large shrinkage mismatch thermal processes, which itself is a nontrivial challenge. To this end, we propose and describe a microforging process that facilitates demolding and show representative results for numerous SiCN ceramic microstructures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: SiCN; MEMS; Microcast; Microforge; Microform; Ceramic
1. Introduction Ceramics are attractive candidate materials for MEMS applications because of the wide range of properties that can be obtained and the promise of improved performance as compared to the existing materials set for MEMS. With the development and use of new materials, new processing techniques arise, and with these, new issues and challenges emerge. Here, we treat such a problem that has emerged in our development of a microcasting fabrication technique for polymer precursor derived silicon carbonitride (Si1.7C1.0N1.6) high-temperature MEMS [1]. Conventional ceramics such SiC and Si3N4 are attractive for high-temperature applications. The fabrication of SiC- or Si3N4-based ceramic structures usually involves sintering of ceramic powder at a temperature higher than 15008C [1]. The necessary sintering additives can degrade ceramic mechanical properties at such a high-temperature. Moreover, the powder route is not compatible with available micropatterning techniques. Chemical vapor deposition (CVD) of SiC is a new technique which involves surface micromachining and can be used to fabricate SiC high-temperature
* Corresponding author. E-mail address: [email protected] (Y. Liu).
MEMS [2]. However, the CVD and micromachining of SiC are time-consuming and expensive. Due to the planar nature of CVD, it is dif®cult to fabricate high aspect ratio MEMS structures. A recently developed processing technique of injectable polymer SiCN ceramic has been applied to implement hightemperature MEMS components with higher aspect ratio and lower cost than that of CVD SiC approach [3]. Instead of using a cross-linked polymer powder as the starting material, a liquid polymer precursor is directly cast into micropatterned molds and then thermoset to form polymer MEMS structures. Finally, bulk SiCN ceramic structures are obtained by cross-linking and thermal decomposition of the polymer at temperatures as low as 10008C. In MEMS applications, microstructure dimensions are relatively small and often at least one dimension is in the range of hundreds of microns. With additional isostatic pressure during cross-linking, the released gaseous by-products are likely to diffuse out before they build up high enough pressure to form voids inside the structure. So the direct microcasting method is especially suitable for MEMS fabrication. The resulting single phase and amorphous SiCN ceramic microstructure surfaces are shiny and no microcracks and few voids are observed on SEM images of the cross-section [3]. The linear shrinkage can reach up to 30%, which suggests that a large degree of densi®cation occurred
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during the thermal treatment. The bulk density of SiCN MEMS samples is about 2.35 g/cm3 which is in good agreement with the solid phase density deduced form Hgpressure porosimetry (2.33 g/cm3) and from water pycnometry of SiCN powder (2.30 g/cm3) [4]. Therefore, it can be concluded that the resulting microcast SiCN structure is fully densi®ed with virtually no nanoporosity. By comparison, SiCN compacts fabricated through the powder route contain more pores. Even applying high pressure (640 MPa) when the green body is formed, the open porosity is still larger than 6% [4]. The microcast route can signi®cantly improve the microstructure as well material properties of SiCN micro components. The high-temperature properties of microcast SiCN are comparable to those of SiC and superior to those of silicon [3]. Both Vickers hardness (15 GPa) and elastic modulus (158 GPa) have been measured using a microindentation system. The amorphous SiCN monoliths decompose at about 14008C in vacuum and are thermally stable in nitrogen up to 17008C [5]. Furthermore, it has oxidation resistance up to 16008C in air due to the formation of a highly pure silica oxidation layer [6]. SiCN, like SiC, also exhibits good creep resistance at high-temperature [3]. The combination of the attractive high-temperature properties of SiCN compositions, and the unique fabrication capabilities of the microcasting approach make SiCN MEMS attractive alternatives for a wide range of applications, e.g. microscale combustion chambers and turbine engines, where materials and structures are required to resist devitri®cation and decomposition, exhibit good oxidation resistance, and maintain good mechanical properties at hightemperatures. Furthermore, their mechanical properties can be tailored to some degree via the molecular level design of the polymer precursor, thus, broadening the spectrum of available properties and freeing design constraints [5]. The SiCN microcasting process involves three key steps [3]: (1) an inorganic polymer precursor is cast into a microfabricated mold and thermally set at about 2508C to form an infusible transparent polymer solid structure; (2) the solid structure is then cross-linked at about 4008C under isostatic pressure; (3) the pre-ceramic microstructure is then pyrolyzed at about 10008C to form a dense, monolithic SiCN ceramic structure. Due to the nature of microcasting, high aspect ratio 3D MEMS structure can be fabricated. Since the pre-ceramic polymer is infusible after thermoset and the shrinkage during decomposition is linear, the shape and de®nition of the microstructure will be retained after pyrolysis, except the dimensions are smaller than that of original mold. We typically use standard lithographic processes to fabricate soft polymeric molds, e.g. from photoresist. A signi®cant practical challenge in this process is the prevention of component cracking during the cross-linking and/or pyrolysis processes where the relatively weak component undergoes large shrinkage. If the shrinkage is overly constrained by the mold, the component may crack. An approach to overcome this potential cracking is to demold
the structure after the thermoset process, but this is not trivial. We have used three conventional demolding methods: direct mechanical demolding, lost mold, and dissolved mold, but none of these has been completely satisfactory. As a result, we propose a new method to achieve demolding which uses a microforged thin foil structure as the mold and mechanical peeling to facilitate demolding. During this process, we also use a NaCl single crystal substrate which facilitates the separation of fragile microstructures from the substrate by dissolution of the substrate. The result is a freestanding thermoset component that is not subject to constraint from the mold when it shrinks during the subsequent cross-linking and pyrolysis steps, and thus, it undergoes no damage during these processes. 2. Fabrication of microforged molds We fabricate SiCN ceramic MEMS by microcasting a liquid polymer precursor, typically the commercially-available CERASETTM (from Lanxide Company, USA), into SU8 (from Micro Chem Corp.) photoresist micromolds made by standard photolithographic methods. The bonds between the thermoset precursor, the mold, and the substrate are typically quite strong. As a result, during direct mechanical demolding, the weak thermoset structure may fail before the interfaces between the thermoset structure and the mold and/ or substrate. The weak thermoset precursor may stick to the walls of the mold and the surface of substrate, thus, damaging the microstructure. During cross-linking and pyrolysis, CERASET will undergo a shrinkage as high as about 30%, while the baked SU-8 photoresist will experience less shrinkage and the substrate none at all. When subjected to such a large shrinkage mismatch, the thermoset structure is highly stressed due to the restriction of the mold walls and substrate, and these stresses can lead to microstructure damage by cracking of the brittle solid. To avoid such cracking damage, we propose a method based on the use of microforged molds and mechanical peeling to demold the microstructure after thermosetting, rendering it free from stress-inducing constraint during cross-linking and pyrolysis. High-precision forging is a microforming method that has become increasingly popular in modern plastic working where many strategies have been employed and process models developed and validated [7]. However, these techniques have not yet to seen application in ceramic MEMS fabrication. In this study, we employ a relatively simple soft microforging technique to demonstrate the potential of the approach; future work will consider more sophisticated techniques and process optimization. Microforged molds can be divided into two categories by the way the prototype is designed. If a positive prototype is used, which resembles the real structure, a positive forging setup is made. While for negative prototypes, a corresponding negative prototype forging procedure is followed.
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The simpli®ed soft microforge technique shares similarities with hot embossing microfabrication methods [8]. The fabrication processes of microforged molds using both positive and negative prototypes are described below. 2.1. Positive microforge assembly First, as shown in Fig. 1, we fabricate a positive SU-8 photoresist prototype on a silicon substrate, which has the shape desired for the SiCN microstructure, but is larger than the SiCN microstructure to compensate for the shrinkage caused by cross-linking and pyrolysis. Then a layer of thin aluminum foil (20±40 mm in this work) is applied to the top of the prototype, and assembly is placed into a pressure cylinder, which uses plasticine as the medium through which pressure is applied. A compressive force is gradually applied, during which the thin aluminum foil is forged into a negative mold and the plasticine layer serves as a mold holder. The de®nition of the aluminum mold depends on the foil thickness, the applied pressure, and the rigidity of the baked photoresist prototype. Although we have not studied these interrelationships in depth, Figs. 2 and 3 show
Fig. 1. Soft microforge of aluminum foil mold using positive prototype.
examples that illustrate these relationships over a range of foil thickness and applied pressure that we have found to be practically feasible. The forged negative molds have the inverse shape of the positive prototype. It is apparent that the deformation of the prototype is small. With a decrease of the foil thickness and/
Fig. 2. Effect of foil thickness and forge pressure on the definition of the aluminum foil mold (scale is in mm).
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Fig. 3. Effect of foil thickness and forge pressure on the sharp edges and corners of the aluminum foil molds.
or an increase of the forge pressure, the de®nition of the aluminum mold is improved. Of course, there are limits to the de®nition that can be obtained and presumably the process parameters can be tailored to achieve optimized de®nition. Under increased pressure, the SU-8 prototype deforms further, primarily at sharp corners and rectangular edges which are stress raisers. Increasing the SU-8 prototype hardness can also minimize the deformation. This can be accomplished by controlling the SU-8 post-bake temperature and time. Actually, with different combinations of temperature and time, the SU-8 can be baked into a color spectrum from transparent to yellow, brown and black. The hardness of SU-8 is gradually increased accordingly. However, the brown prototype tends to crack while the black one is brittle. So the yellow SU-8 prototype is a good choice compared to the other baked states. Fig. 4 shows the SEM images of microforged molds based on transparent and overbaked yellow prototypes, respectively. The rectangular edge de®nition of the molds is greatly improved by using overbaked prototypes.
Fig. 5 compares the prototype, the aluminum foil mold and a free-standing thermoset structure. The sharp edges and corners of the four-arm structure are rounded, indicating a reduction in de®nition. However, this rounding may be advantageous in situations where the sharp corners and edges may lead to severe stress raisers in the ®nal structure in its intended use environment. Also, if the prototype edge is extremely sharp and hard, it is likely to tear the aluminum foil during forging. So controlling the prototype hardness is necessary for microforging. 2.2. Negative microforge assembly The setup of negative microforge assembly, as shown in Fig. 6, is only slightly different from the positive one. Since we want to use the outside of the aluminum mold, we put a layer of thin plastic ®lm between the plasticine and aluminum foil. This thin plastic ®lm not only facilitates the separation of the plasticine and aluminum mold after forging, but also protects the mold surface from plasticine contamination.
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Fig. 4. Effect of the SU-8 prototype hardness on the definition of aluminum foil molds, which are for tensile test specimens.
In this con®guration, the silicon substrate and over baked SU-8 negative prototype gives the thin foil mold a stronger support than that of the plasticine alone described in positive forging case. However, the disadvantage of this case is also apparent. Because the aluminum foil has a thickness around 20±40 mm, the foil mold dimensions are no longer the same as the prototype dimension. Along with the shrinkage of polymer precursor, it will further complicate the design of
MEMS structures. As a result, the positive microforge method is preferred. Since the aluminum foil is forged at room temperature, the aspect ratio of the resulting structures is about unity. Our primary intent here is to demonstrate the potential possibility of using microforged thin foil molds to facilitate the demolding of thermoset pre-ceramic structure. In the future, the microforging techniques can be improved in many ways
Fig. 5. Definition comparison of original prototype, forged mold, and free-standing thermoset structure.
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2.
3.
Fig. 6. Soft microforge of aluminum foil mold using negative prototype.
including raising the forging temperature, using a lubricant, matched dies, and/or superplastic alloy foil, controlling the forging force, and chilling the die. 3. Microcasting processes Based on a positive prototype, the complete microcasting process, including details of the proposed new demolding method, are shown in Fig. 7. Brie¯y, the process consists of the following steps: 1. Fabricate a positive SU-8 photoresist prototype on a silicon substrate. After post-exposure, baking and development, the SU-8 photoresist prototype is mechanically
4.
5. 6. 7.
stiff enough to resist moderate forging pressures without appreciable deformation. The prototype can be used repeatedly. Use the simplified soft forge method as described in the last section to make a negative aluminum foil mold. We use plasticine underneath the mold to hold and support the foil mold during subsequent steps. Microcast the liquid polymer precursor in the aluminum foil mold and apply a single crystal NaCl plate as a top cover. Since the liquid precursor is sensitive to air and humidity, this step is done in an argon atmosphere. We use a NaCl plate instead of a silicon wafer to facilitate easier release of the precursor structure after it is demolded from the aluminum foil mold. To ensure tight contact, a mass is applied on top of the NaCl plate. Thermoset the liquid polymer precursor resulting in a solid, but weak, polymer structure. Although the liquid polymer precursor will shrink slightly during thermosetting, it seems to cause only elastic deformation in the structure and mold. The stresses developed are not enough to damage the structure. Remove the plasticine supporting layer, and then mechanically peel the foil mold to leave free polymer structures adhering to the NaCl plate. Apply a layer of wax to the polymer structure side to serve as a handling wafer for the entire batch of microstructures. Release the wax handling wafer together with the attached polymer microstructures from the NaCl plate using water as a penetration solvent. The release time is
Fig. 7. Fabrication processes of free-standing precursor structure after thermoset.
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only several seconds and the water does not impact the polymer precursor. 8. Melt or burn away the wax during the subsequent heat treatment which consists of cross-linking and pyrolysis. The result is a batch of free-standing, crack free SiCN MEMS structures. With this method, the key to successful demolding is the nature of the stress state imposed on the thermoset structure during the peeling process. If the thickness of the aluminum foil is much thinner than the minimum feature size of the MEMS structure, the foil is relatively ¯exible and can be easily peeled. Because the stress is highly localized at the tip of the interface crack between the thermoset microstructure and the aluminum foil, force equilibrium dictates that the average stress applied to the polymer structure is much lower than the critical fracture stress. This situation is in contrast to the stress state developed during direct pull or push demolding where the friction force and bonding force between the mold wall and the polymer structure will impede demolding. So the average stress inside the structure can exceed the critical fracture stress and result in cracking of the microstructure.
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4. Results and discussion Using the microforging process described in the previous sections we have successfully fabricated a wide variety of SiCN ceramic MEMS structures that have passed through the thermoset, demolding, cross-linking, and pyrolysis steps free of damage. Some of these are shown in Fig. 8. They include a gear, a fan blade, and microstructures with various channels including rectangular and hexagonal channels. Compared to lost mold methods [9] which may require days to slowly burn off the molds, e.g. PMMA molds, the mechanical peeling demolding method is ef®cient in terms of both time and cost. As mentioned previously, another way to demold is to dissolve or etch away the mold. This is problematic with the SiCN polymer precursor at the thermoset stage, though, because the ceramic precursor itself may be attacked by the demolding etchant. However, it is possible that the metal foil mold may also be etched as a sacri®cial layer in order to release the pre-ceramic structure. The primary result here is the demonstration of the viability of the microforging mold process to yield
Fig. 8. Various ceramic structures made by the microcasting process using microforged molds (the thickness of the structures is around 50 mm).
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damage-free ceramic structures via microcasting of a polymeric precursor. Of course, this is an initial step in the development of reliable technologies for the mass production of ceramic MEMS. Many improvements in addition to our preliminary efforts are expected in order to obtain better de®nition and higher achievable aspect ratio for the ceramic microstructures. These include: 1. Using an advanced superplastic material for the foil mold. There are potentially two advantages: (a) superplastic deformation will give high aspect ratio and more precise definition to the object; (b) if a low melting alloy is used (e.g. indium-based) then it is possible that the metal foil mold can be removed by melting it away leaving behind a self standing cross-linked CERASET structure. 2. Using a prototype with higher stiffness than the SU-8 photoresist. An obvious possibility is a silicon or a metal prototype that can be fabricated by established MEMS bulk and/or surface micromachining techniques. 3. Using matched dies for microforging. High-accuracy dies and an alignment frame will facilitate the development of desirable near-net shape microstructures. 4. Microforging at elevated temperatures. The ductility of the metal foil increases with temperature, but at the same time the stiffness of metal prototype decreases. Therefore, the temperature should be controlled to optimize these competing effects. 5. Microforging with a lubricant. This will potentially improve the process, but this is a challenging problem at MEMS length scale. 5. Conclusions We proposed a method of microforging-based mold fabrication and demolding as an approach to eliminate cracking in the fabrication of polymer-derived ceramic MEMS during thermal heat treatments. We described the microforging process and the subsequent demolding process necessary to yield free-standing pre-ceramic structures prior to the ®nal cross-linking and pyrolysis. As a result, the shrinkage that occurs during these processes is unconstrained, substantial stresses, thus, do not develop, and crack free ceramic MEMS structures are obtained. Here, we reported our initial efforts that demonstrate the viability of the process, and we also pointed out process improvements that are expected to lead to improved de®nition of ceramic MEMS. Acknowledgements This work is supported by the Defense Advanced Research Projects Agency (DARPA) and US Air Force under contract #F30602-99-2-0543.
References [1] R. Riedel, G. Passing, H. Schonfelder, R.J. Brook, Synthesis of dense silicon-based ceramics at low temperatures, Nature 355 (6362) (1992) 714±717. [2] M. Mehregany, A. Heuer, S.M. Philips, A multidisciplinary research proposal for advanced electromechanical microsystems, Semi-annual Technical Report to DARPA, 1998. [3] L. An, W. Zhang, V.M. Bright, M.L. Dunn, R. Raj, Development of injectable polymer-derived ceramics for high-temperature MEMS (MEMS'2000), 2000, pp. 619±623. [4] R. Riedel, et al., Polymer-derived silicon-based bulk ceramics. Part I. Preparation, processing and properties, J. Eur. Ceram. Soc. 15 (1995) 703±715. [5] R. Riedel, A. Kienzle, W. Dressler, L. Ruwisch, J. Bill, F. Aldinger, A silicoboron carbonitride ceramic stable to 20008C, Nature 382 (6594) (1996) 796±798. [6] H. Schonfelder, F. Aldinger, R. Riedel, Silicon carbonitrides Ð a novel class of materials, J. Phys. 3 (1993) 1293±1298. [7] D.B. Shan, Z. Wang, Y. Lu, Study on isothermal precision forging technology for a cylindrical aluminum-alloy housing, J. Mater. Process. Technol. 72 (3) (1997) 403±406. [8] H. Becker, U. Heim, Hot embossing as a method for the fabrication of polymer high aspect ratio structures, Sens. Actuators A: Phys. 83 (1±3) (2000) 130±135. [9] H. Freimuth, V. Hessel, H. Kolle, M. Lacher, W. Ehrfeld, T. Vaahs, M. Bruck, Formation of complex ceramic miniaturized structures by pyrolysis of poly(vinylsilazane), J. Am. Ceram. Soc. 79 (6) (1996) 1457±1465.
Biographies Yiping Liu received her BS and MS degrees in materials engineering from the Shanghai Jiao Tong University of China in 1990 and 1993, respectively. She is now a doctoral research assistant in the Department of Mechanical Engineering at the University of Colorado, Center for Advanced Manufacturing and Packaging of Microwave, Optical and Digital Electronics (CAMPMODE). Her research interests are in the design, fabrication and testing of MEMS for high-temperature environments. Li-Anne Liew received her BS and MS degrees in mechanical engineering from the University of Colorado at Boulder in 1998 and 2000, respectively. Currently, she is a doctoral research assistant in the Department of Mechanical Engineering at the University of Colorado, Center for Advanced Manufacturing and Packaging of Microwave, Optical and Digital Electronics (CAMPMODE). Her research interests are in the design and packaging of MEMS sensors, MEMS for biomedical applications, and the design and fabrication of MEMS for hightemperature environments. Ruiling Luo received his BS and MS degrees in materials engineering from the Shanghai Jiao Tong University of China in 1993 and 1996, respectively. He received another MS degree in mechanical engineering from the University of Colorado at Boulder in 2000. His research interests are in the design, fabrication and testing of MEMS for high-temperature environments. Linan An is an assistant professor in Advanced Materials Processing and Analysis Center (AMPAC) and Department of Mechanical, Materials and Aerospace Engineering (MMAE) at the University of Central Florida, Orlando. Prior to joining UCF, Dr. An was a post-doctor research associate at University of Colorado at Boulder. His research interests are in the general area of processing±structure±property relationships of ceramic materials. Most recently, his research is focused on polymer-derived
Y. Liu et al. / Sensors and Actuators A 95 (2002) 143±151 ceramics and their applications in high-temperature MEMS. Dr. An is pioneer in the microfabrication of polymer-derived ceramic MEMS. He has more than 20 papers on materials science and microfabrication technologies. Martin L. Dunn is an associate professor of mechanical engineering. He received the PhD in mechanical engineering in 1992 from the University of Washington, USA. Professor Dunn's research interests include: (i) the micromechanical behavior (fracture and deformation) of materials and structures for microelectromechanical systems applications, (ii) the micromechanics and physics of heterogeneous media, including defects, polycrystals, and composites, with emphasis on piezoelectric solids, and (iii) the acoustic characterization of material microstructure. His research is based on a strong coupling between theoretical and experimental efforts. Professor Dunn has published over 60 articles in refereed journals on these subjects and has been the principal or co-principal investigator on grants and contracts in these areas from NSF, DOE, ONR, NIST, DARPA, and Sandia National Laboratories. Victor M. Bright is an associate professor of mechanical engineering and the director of the MEMS R&D Laboratory, University of Colorado at Boulder. Prior to joining the University, he was an associate professor and the director of the Microelectronics Research Laboratory in the Department of Electrical and Computer Engineering, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio (June 1992 to December 1997). Professor Bright's research includes MEMS, silicon micromachining, microsensors, microactuators, MEMS self-assembly, MEMS packaging, opto-electronics, and semiconductor device physics. Dr. Bright received the following awards in the area of MEMS: best paper
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of the MCM'98 Ð International Conference and Exhibition on Multichip Modules and High Density Packaging, 1998; R.F. Bunshah best paper award at the 1996 International Conference on Metallurgical Coatings and Thin Films. Dr. Bright has authored and co-authored more than 70 papers in the areas of MEMS. He is a member of IEEE, ASME, and SPIE. He serves on the Executive Committee for ASME MEMS Sub-division. John W. Daily is currently professor of mechanical engineering and director of the Center for Combustion and Environmental Research at the University of Colorado at Boulder. He studied mechanical engineering at the University of Michigan (BS 1968, MS 1969) and at Stanford University (PhD 1975.). After receiving the PhD, he was a faculty member at the University of California at Berkeley until 1988, attaining the rank of full professor of mechanical engineering. Most of his academic career has been devoted to the field of combustion and environmental studies. Recently, he has been working in the areas of MEMS applications in combustion. He has over 100 scientific and technical publications. Rishi Raj. Prior to joining University of Colorado, Boulder in 1996, he had served on the faculty of Materials Science and Engineering at Cornell University for 21 years. He received the PhD in applied physics from Harvard University in 1970. His research is in processing of hightemperature metals, composites, and ceramics. His most significant contributions have been in understanding how interfaces control hightemperature processing and mechanical behavior. He has over 200 publications of which 150 are in refereed journals, and include 14 US Patents. He has been a Guggenheim fellow, an Alenxander von Humboldt senior scientist awardee and a Matthias scholar at Los Alamos National Laboratory.