Fabrication of SiCN ceramic MEMS using injectable polymer-precursor technique

Sensors and Actuators A 89 (2001) 64±70

Fabrication of SiCN ceramic MEMS using injectable polymer-precursor technique Li-Anne Liew*, Wenge Zhang, Victor M. Bright, Linan An, Martin L. Dunn, Rishi Raj 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

Abstract In this paper, a novel and cost-effective technology for the fabrication of high-temperature MEMS based on injectable polymer-derived ceramics is described. Micro-molds are fabricated out of SU-8 photoresist using standard UV-photolithographic processes. Liquid-phase polymers are then cast into the molds and converted into monolithic, fully-dense ceramics by thermal decomposition. The resultant ceramic, based on the amorphous alloys of silicon, carbon and nitrogen, possess excellent mechanical and physical properties for high-temperature applications. This capability for micro-casting is demonstrated in the fabrication of simple single-layered, high aspect ratio SiCN microstructures. A polymer-based bonding technique for creating more complex three-dimensional structures is also presented. # 2001 Published by Elsevier Science B.V. Keywords: MEMS; High-temperature; Polymer; Ceramic

1. Introduction MEMS for extreme temperature environments have attracted much attention due to their many potential applications, such as optical MEMS for high power laser applications [1] and microcombustors [2] for MEMS power sources. However, the high temperature environments remain a signi®cant challenge to current MEMS technology. The fabrication of MEMS for high-temperature applications is a two-fold problem: selecting suitable refractory materials and developing appropriate microfabrication techniques. Traditional microfabrication processes, such as surface micromachining and LIGA, rely on polysilicon and plastic/nickel as structural materials, respectively. These materials cannot operate at high temperatures for extended periods of time. At the same time, state-of-the-art ceramics that are designed for high-temperature environments cannot be easily processed using existing microfabrication techniques. Moreover, CVD of SiC [3], a technique currently under development for high-temperature MEMS, is timeconsuming and expensive. Also, the planar nature of CVD prevents the fabrication of complex three-dimensional *

Corresponding author. Tel.: ‡1-303-492-3842; fax: ‡1-303-492-3498; URL: http://mems.colorado.edu. E-mail address: [email protected] (L.-A. Liew). 0924-4247/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 5 4 5 - 8

structures. Therefore, the development of new materials and appropriate microfabrication techniques for high-temperature MEMS is of both scienti®c and practical interest to the MEMS community. Fig. 1 compares the aspect ratios achieved and maximum operating temperature of traditionally-microfabricated MEMS with that of the polymerderived SiCN MEMS that are described in this paper. 2. Injectable polymer derived ceramics A novel fabrication technique for high-temperature MEMS using an injectable polymer-derived SiCN ceramic has been developed. This new technology is based on the recently developed polymer-derived ceramics, which are bulk ceramics fabricated by the thermal decomposition of compacted crosslinked polymer powders [4]. The polymerderived ceramics are amorphous alloys of silicon, carbon, and nitrogen (SiCN) which remain thermally stable up to 15008C. The compositions of the new ceramics can be varied through the use of different polymer precursors, and can be tailored to produce SiCN with excellent thermal and mechanical properties. Table 1 compares the physical properties of SiCN with those of Si and SiC. Young's modulus, Poisson's ratio, and density are in the same range as those of SiC and Si. The creep resistance of SiCN is

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Table 1 Comparison of physical properties of SiCN, Si, and SiC

3

Density (g/cm ) E modulus (GPa) Poisson's ratio CTE  10ÿ6 (K) Hardness (GPa) Strength (MPa) Toughness (MPa m1/2)

Fig. 1. Diagram of microstructure aspect ratio against maximum use temperature for different MEMS materials and processes.

comparable to that of SiC and Si3N4 [5,6], while its oxidation resistance exceeds that of the same materials [7]. In addition, the thermal shock resistance of SiCN appears very promising for high-temperature applications.

SiCN

Si

SiC

2.20 158 0.18 0.5 15 250 3.5

2.33 163 0.22 2.5 11.2 175 0.9

3.17 405 0.14 3.8 30 418 4±6

SiCN may be obtained from liquid- or powder-based polymer precursors. However, the SiCN obtained from the powder-route shows relatively low strength and hardness due to the high porosity of powder-derived ceramics in general (typically 10 vol.% porosity). In addition, powder-processing cannot be easily integrated into existing microfabrication techniques. Therefore, liquid polymer precursors were used to develop a novel micro-casting technique for the fabrication of SiCN MEMS structures. Fig. 2 compares the microstructure, strength, and hardness of SiCN samples obtained through powder- and casting-routes.

Fig. 2. (a) Scanning electron micrographs showing the microstructure of polymer-derived SiCN from powder-route and (b) from casting-route. This figure shows that the material resulting from casting is fully dense in that no defects can be seen despite a higher magnification than that in (a). (c) Comparison of strength and hardness (which are not directly related to each other) of samples obtained from powder- and casting-routes.

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3. Microfabrication of SiCN MEMS Fig. 3 outlines the process ¯ow in the fabrication of SiCN MEMS. First, a mold is fabricated using standard photolithographic techniques. The liquid polymer precursor is then cast into the mold, and the mold and polymer precursor are then heated or ``thermal-set'' at 2508C to solidify the polymer. After thermal-setting, the polymer becomes a transparent solid, and may be separated from the mold if suitable techniques are used. After thermal-setting, the polymer part is crosslinked by heating to 4008C under isostatic pressure. After crosslinking, the polymer becomes infusible, remaining transparent. In the ®nal stage (pyrolysis), the crosslinked polymer part is heat-treated at 10008C to convert it to a monolithic ceramic part. The commercially-available Ceraset (CerasetTM, Lanxide Company, USA) was used as the starting polymer. A catalyst (Dicumyl peroxide, Aldrich Chemical, Milwaukee WI, USA) is added to lower the thermosetting temperature to 1408C, so that it is compatible with photoresist processing. The mold was made using SU-8 photoresist [8] from MicroChem Corp. SU-8 is a well-known negative, epoxy-type photoresist based on EPON SU-8 resin. Its key features are its ability to produce high-aspect ratio structures and low optical absorption in the near-UV range, which leads to straight vertical sidewalls. Other advantages of using SU-8 as the mold are: (1) standard UV-photolithography can produce high-aspect ratio structures; (2) batch fabrication; (3) the SU-8 photoresist decomposes during pyrolysis,

Fig. 3. General processing steps in the fabrication of injectable polymerderived ceramics.

Fig. 4. Fabrication process for SiCN MEMS.

enabling the release of the structures using the ``lost mold technique''. Fig. 4 outlines the micro-casting technique in more detail. First, the photoresist is spun onto a substrate (a). Then, the photoresist is patterned using standard UV-lithography and developed, producing cavities of desired shapes (b). The liquid precursor (Ceraset) is then cast into the cavities by spinning, and the wafer is thermal-set in an oven for 20 min. At the end of this step, the Ceraset is solid and there is a thin layer of it covering the entire wafer (c). This thin top layer of Ceraset is polished-off (d). The wafer is then crosslinked under isostatic pressure (e), and during pyrolysis the SU-8 decomposes and the SiCN part no longer adheres to the substrate (f). The ®nal result is a free-standing high aspectratio microstructure (g). Of course, this schematic shows only one structure being fabricated; in reality, multiple structures are fabricated at once on a wafer, which is one of the attractions to microcasting using photoresist molds. Fig. 5(a) shows an SU-8 mold for a micro-gear. Fig. 5(b) shows the same mold when ®lled with the polymer precursor. For small cavities (less than 20 mm wide), often the liquid does not fully ®ll the cavity due to the surface tension of the liquid and/or air bubbles in the mold cavity. This can be solved by placing the ®lled mold in vacuum to remove most of the air bubbles, and then pressing a thin ®lm of Te¯on over the wafer to force the liquid into the cavities. This ®lm is left on the mold during thermosetting, and also

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Fig. 5. (a) Photoresist mold (200 mm diameter and 75 mm thick); (b) mold filled with Ceraset.

serves to minimize the thickness of the extra top layer of Ceraset that needs to be polished-off. 4. Crosslinking and pyrolysis The key to the success of the casting-route is the application of isostatic pressure during crosslinking. The heattreatment during crosslinking generates gaseous by-products. This out-gassing may cause the formation of microcracks which disintegrate the sample [4]. In the casting approach, the applied isostatic pressure provides an obstacle to the nucleation of bubbles and microcracks. The gaseous by-products would need to build up suf®cient pressure to overcome the applied pressure before bubbles and microcracks can nucleate. Due to the open structure of the polymer, the gases will diffuse out of the sample before they can build up a pressure that is high enough to overcome the applied pressure. Thus, transparent and defect-free crosslinked polymers can be obtained by the application of isostatic pressure during crosslinking. The fact that no defects can be seen in the ®nal ceramics indicates that the infusible polymer network has enough strength to withstand the further heat-treatment during pyrolysis. The crosslinked samples are then pyrolyzed at 10008C for 8 h to convert the polymer into a ceramic. Very low heating/ cooling rates are used: the heating rate is about 18C/min to 4008C and 258C/h to 10008C; the cooling rate is 18C/min to

room temperature. Higher heating rates result in the production of most of the gaseous by-products at a narrow temperature range (H2 at 600±7008C, and CH4 at 6008C), which could generate defects in the pyrolyzed sample and degrade the mechanical strength of the structures. Examples of microstructures fabricated from this casting process are shown in Fig. 6. 5. Demolding Initially during pyrolysis, the SU-8 photoresist mostly decomposed but left a thick, hard residue on the walls of the samples, as can be seen in Fig. 6 and also in Fig. 7(a). These SU-8 by-products, being chemically resistant to etchants and strongly attached to the SiCN, could not be easily removed through either chemical or physical means. Attempts to burn-off the residue at 10008C in an oxygen atmosphere were also not successful. The by-products are the result of reactions between the liquid Ceraset and the polymer mold at the interface, producing unknown compounds. Many attempts were made to de-mold the parts before pyrolysis in order to prevent the reactions. One drawback of using SU-8 is that it is not easily removed, even when using the solvent provided by the manufacturer. It was found that the SU-8 could be stripped if the solvent was heated to about 908C. However, before pyrolysis, the Ceraset is highly reactive and chemically

Fig. 6. (a) Gear (200 mm diameter and 45 mm thick) made from the mold shown in Fig. 8; (b) cantilever beam (137 mm thick); (c) tensile test sample (142 mm thick).

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Fig. 7. (a) SiCN cantilever beam encased in SU-8 residue; (b) SiCN gear tooth (75 m thick), obtained from SU-8 mold that had undergone post-fabrication treatment to prevent residue formation.

reacts with many polymers. Various photoresists that could be easily stripped were experimented with as alternatives to SU-8, but the highly reactive nature of Ceraset still proved to be the limiting factor. This problem was ®nally solved through a combination of an extended bake and extended UV-exposure of the SU-8, following the mold fabrication but prior to casting. This ensured that the SU-8, being a branched polymer, was completely crosslinked before the Ceraset was cast into it. Results were encouraging. SU-8 thus remained the material of choice for mold fabrication, and the ``lost mold'' technique was pursued following this post-fabrication treatment of the mold prior to casting. Fig. 7(b) shows the marked improvement in surface quality following this treatment. Despite the success of the post-fabrication treatment of the mold in producing SiCN parts that were clean and residue-free, it is still advantageous to de-mold prior to pyrolysis, especially if polymer-based bonding (decribed later in this paper) is to be used to fabricate multi-layer structures.

crack at the crosslinking and pyrolysis stages if the length exceeds a certain critical value, which is a function of the sample thickness, being larger for thicker samples. Ideally, there should not be any limit on the sample size, which is proven by casting in macro-sized Te¯on molds that have been conventionally machined. In this case, the thermallyset samples are released from the Te¯on mold and crosslinked and pyrolyzed as free-standing samples. The diameter of these samples can be up to 10 mm (which is limited by the size of our furnace). The reason for the observed scale limit for the micro-cast samples is explained with the aid of Fig. 8. The Ceraset experiences volume shrinkages of 5 and 25% during crosslinking and pyrolysis, respectively. However, at the same time the Ceraset adhesion to the silicon substrate results in a tensile stress, s, in Ceraset structure. This tensile stress in turn induces a shear stress, t, at the silicon/Ceraset

6. Size limitations We have studied the size range of samples that can be successfully cast. It has been found that the samples will

Fig. 8. Induced stresses during heat-treatment of Ceraset on silicon.

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interface. Assuming a disk-shaped sample, the tensile and shear stresses are related by r sˆ t 2h where s increases with the progress of heat-treatment. Let tc and sc be the critical values for which the sample will peeloff from the substrate and fracture, respectively. Then, if the ratio r/h is too large (which corresponds to thin samples), then sc will be reached first and sample will crack. If the sample is thick, then tc will be reached first and sample will peel-off from the substrate without cracking. The problem can be prevented by using crosslinked Ceraset as the substrate, because it will shrink together with the microstructures.

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interfaces between the adhesive layer and the solid polymer parts. The thermal-set or crosslinked components are then aligned and held together (b). The assembled structure is then thermal-set again to solidify the adhesive layer, and then crosslinked under isostatic pressure (c) to establish chemical bonds between the polymer in the adhesive layer and the polymer in the original components. Therefore, after crosslinking, the whole structure becomes a solid piece of polymer without a noticeable interface. This bonded structure is identical to a single cast piece. The sample is then pyrolyzed (d). Further examination of parts produced by this method indicated that there are no observable defects at the bonding interface. 8. Conclusion

7. Polymer-based bonding Another advantage of using a liquid precursor is the ability to use it as an adhesive layer to bond two or more SiCN parts together. This ``polymer-based bonding'' technique thus may be used as a means of fabricating monolithic multi-layer structures, whose ®nal thicknesses might not otherwise be achievable due to the size limitations described above. The basic bonding process is shown schematically in Fig. 9. First, two thermal-set or crosslinked solid parts are fabricated by casting (a). The same liquid polymer is then spread on the desired location in the same way that glue is spread on components to be attached. The liquid nature of the polymer precursor allows atomic level contact at the

A novel micro-casting process based on injectable polymer-derived SiCN has been developed. The resulting ceramic structures exhibit excellent mechanical and thermal properties, making this process very promising for fabricating MEMS for high-temperature applications. SU-8 photoresist molds enable the use of low-cost microfabrication facilities. A polymer-based bonding technique can be used whereby SiCN parts are bonded together to produce complex, three-dimensional monolithic ceramic components. Compared to the existing MEMS materials and processes, the new injectable polymer-precursor technique substantially enhances the manufacturability of MEMS for hightemperature applications. Acknowledgements This work is supported by the Defense Advanced Research Projects Agency (DARPA) and U.S. Air Force under contract # F30602-99-2-0543. Also, thanks to Mr. Tsali Cross for developing the polymer-based bonding technique.

References

Fig. 9. Illustration of process steps for polymer-based bonding of SiCN parts: (a) two solid polymer parts; (b) bonding parts together with a layer of liquid polymer; (c) crosslinked structure, and (d) ceramic multi-layer component.

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Biographies Li-Anne Liew received her BS and MS degrees in Mechanical Engineering from the University of Colorado at Boulder in 1998 and 2000, respectively. She is currently 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. Wenge Zhang received his BSME degree from Dalian University of Technology in Dalian, China, in 1982 and his MS degree in Mechanical Engineering from the University of Colorado, Boulder, in 1995. He joined the University in 1991 and is currently a research associate for the Center for Advanced Manufacturing and Packaging for Microwave, Optical and Digital Electronics, at the University of Colorado. His research interests include low-cost prototyping and thermal management of MCMs, thermosonic flip-chip bonding and optoelectronics packaging. He has 14 years of research experience in mechanical design, computer control systems and logic circuit design, and 5 years of research experience in thermosonic flip-chip bonding, MCM substrate fabrication, and optoelectronics packaging. Dr. 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± December 1997). Prof. 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 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. Dr. Linan An obtained his PhD degree from Lehigh University in the field of materials science and engineering. His research includes processing of ceramic materials, microstructural design of ceramics, high temperature behavior, oxidation and corrosion of ceramic materials, and mechanisms of fracture in brittle solids. Dr. Martin L. Dunn is an Associate Professor of Mechanical Engineering. He received the PhD in Mechanical Engineering in 1992 from the University of Washington. Prior to joining the faculty at the University of Colorado, he was a postdoctoral appointee in a solid mechanics group at Sanida National Laboratories, and a design engineer in a transducers group at the Boeing commercial Airplane Company. 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. Dr. Rishi Raj prior to joining C.U. Boulder in 1996, 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 high-temperature metals, composites, and ceramics. His most significant contributions have been in understanding how interfaces control high-temperature processing and mechanical behavior. He has over 200 publications of which 150 are in refereed journals, and include 14 U.S. Patents. He has been a Guggenheim Fellow, an Alenxander von Humboldt Senior Scientist Awardee, and a Matthias Scholar at Los Alamos National Laboratory.