Esashi Masayoshi

Itoh Toru

Li J.-F.

Tanaka Shuji

Watanabe Ryuzo

Journal of Microelectromechanical Systems

Itoh, Toru

Tanaka, Shuji

Li, J.-F.

Watanabe, Ryuzo

Esashi, Masayoshi

Institutional Repositories DataBase (IRDB)

Silicon-Carbide Microfabrication by Silicon Lost Molding for Glass-Press Molds

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Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Silicon-Carbide Microfabrication by SiliconLost Molding for Glass-Press Molds著者 江刺  正喜journal orpublication titleJournal of microelectromechanical systemsvolume 15number 4page range 859-863year 2006URL http://hdl.handle.net/10097/34845JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 4, AUGUST 2006 859Silicon-Carbide Microfabrication by Silicon LostMolding for Glass-Press MoldsToru Itoh, Shuji Tanaka, Jing-Feng Li, Ryuzo Watanabe, and Masayoshi EsashiAbstract—This paper describes two silicon carbide (SiC) micro-fabrication processes for SiC glass-press molds. One is silicon lostmolding combined with SiC chemical-vapor deposition (CVD) andSiC reaction sintering (RS). The other is silicon lost molding com-bined with SiC CVD and SiC solid-state reaction bonding (SSRB).In both of these processes, an original pattern on a silicon substrateis transferred to a CVD SiC film, and then the film is backed bybulk SiC to obtain rigidity and robustness against pressing force.Finally, the silicon substrate is etched away to release a SiC mold.In the process using SiC CVD and RS, an original pattern on asilicon substrate was transferred to a SiC mold, but the surfaceroughness of the SiC mold was 0.05–0.08 mRa, and worse thanrequired by the glass-press mold. This was caused by the transfor-mation of amorphous SiC to polycrystalline SiC in RS, which wasconfirmed by the X-ray diffraction (XRD) data of the CVD SiC filmbefore and after RS. In the process using SiC CVD and SSRB, thesurface of the SiC mold was smooth (0.004–0.008 mRa) withoutthe crystallization of the amorphous CVD SiC film. The SiC moldwas pressed to Pyrex glass to demonstrate its high-temperaturestrength. The Pyrex glass was deformed by the SiC mold at 850 Cwithout a void, and no significant deformation of the SiC mold wasobserved. [1572]I. INTRODUCTIONSILICON CARBIDE (SiC) has heat resistance, chemicalinertness, and high hardness, and its microfabrication tech-nology has various potential applications in harsh environments[1]. One of the promising applications is a micromold to pressvarious glasses at higher temperature . Glass presstechnology is being used to mass produce nonspherical lenses,and it can be also used for microoptics, microreactors, micro-electromechanical systems (MEMS), etc., if microstructurescan be formed on molds. Especially, microoptics such as lightguides, pick-up optics, and microlens arrays are important dueto the continuous development and widespread use of opticalcommunication and data storage. Currently, such microopticsare made from plastics by injection molding or press moldingat low cost. The optical characteristics of plastics are, however,inferior to glasses. For example, the dispersion and doublerefraction of the plastics often become problems for opticalManuscript received April 15, 2005; revised August 17, 2005. This work wassupported by the Iwate Foundation under Japan Science and Technology Cor-poration (JST) through the Regional Science Promoter Program. Subject EditorM. Mehregany.T. Itoh, S. Tanaka, and M. Esashi are with the Department of Nanomechanics,Tohoku University, Aoba-ku, Sendai 980-8579, Japan (e-mail: shuji@cc.mech.tohoku.ac.jp).J.-F. Li is with the Department of Material Processing, Tohoku University,Aoba-ku, Sendai 980-8579, Japan. He is also with the Department of MaterialsScience and Engineering, Tsinghua University, Beijing 100084, China.R. Watanabe is with the Department of Material Processing, Tohoku Univer-sity, Aoba-ku, Sendai 980-8579, Japan.Digital Object Identifier 10.1109/JMEMS.2006.872231communication. Thus, the demand for the micromolding ofoptical glass is increasing.In glass press molding, micropatterns on a mold are trans-ferred to glass around its softening temperature. For example,the press molding of optical glass (BK7) and quartz glass is per-formed approximately at 700 and 1400 , respectively, [2].In such high temperature process, SiC maintains hardness andinertness to some extent, and can be used as the mold materialwith glassy carbon antisticking coating [2]. Conventional mi-crofabrication techniques, however, have limitations, when ap-plied to SiC due to its hardness and chemical inertness. For ex-ample, grinding and electric discharge machining have the limi-tation of shape resolution by tool size, and suffer from the rapidwear of tools. The etching of SiC is also limited. The reactiveion etching (RIE) [3]–[9] and deep RIE [10], [11] of SiC werestudied, but it is generally difficult to mirror-finish the surface.Isotropic and crystal-orientation-dependent wet etching cannotbe used for SiC unlike silicon to microfabricate various features.Rajan et al. [12] reported silicon lost molding combined withSiC chemical vapor deposition (CVD) for SiC microfabrica-tion with shape resolution on the order of micrometers. Theydeveloped an excellent CVD system, which enables the rapiddeposition of low-stressed thick SiC films. However, it is gen-erally difficult to deposit low-stressed SiC films with thicknessof a few hundreds micrometers or thicker at a practical deposi-tion rate by CVD. Tanaka et al. [13] developed “microreactionsintering process,” which combines silicon micromachining, re-action sintering (RS) by hot isotropic pressing (HIP) and sil-icon lost molding. This process allows us to produce millimeter-sized SiC parts with microstructures, but the surface is too rough(2–4 ) to be used for glass-press molds. In spite of theseproblems, we thought that the silicon lost molding was usefulto fabricate micromolds for glass press, because we can usewell-established silicon microfabrication technology to produceoriginal patterns. In this paper, we developed the fabrication pro-cesses of SiC micromolds using lithography-based technologyand silicon lost molding.II. MICROFABRICATION PROCESSES OF SIC FORGLASS-PRESS MOLDSTo fabricate a glass-press mold using SiC as a material, ahigh-precision, high-resolution SiC microfabrication processwhich can mirror-finish the surface is needed. In addition, themold pattern should be formed on a thick SiC bulk, because themold must be rigid and robust to stand pressing force. To satisfythe requirements, we have developed two microfabrication pro-cesses shown in Fig. 1. One is silicon lost molding combinedwith SiC CVD and SiC RS [Fig. 1(a)], and the other is siliconlost molding combined with SiC CVD and SiC solid-state re-action bonding (SSRB) [Fig. 1(b)]. In both of these processes,1057-7157/$20.00 © 2006 IEEE860 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 4, AUGUST 2006Fig. 1. Microfabrication processes for SiC molds. (a) Silicon lost molding com-bined with SiC-CVD and RS. (b) Silicon lost molding combined with SiC-CVDand SSRB.a SiC film is deposited on an original pattern fabricated on asilicon substrate, and then backed with bulk SiC by SiC RSor SiC SSRB. Finally, the silicon substrate is etched away toreveal the CVD SiC surface. Both processes are based on thesilicon lost molding, and can produce SiC microstructures ona thick SiC bulk. In this paper, we fabricated SiC micromoldswhich have triangular-section gratings with pitch of 5–20 .A. Silicon Lost Molding Combined With SiC CVD and RS1) Experimental Method: A silicon mold, which has tri-angular-section gratings with pitch of 5–20 , is fabricatedby a standard silicon wet etching process using tetra-methylammonium hydroxide (TMAH). A SiC film is deposited usinga home-made atmospheric pressure CVD (APCVD) system.Fig. 2 shows the diagram of the APCVD system. It consists ofa 100-mm diameter double-walled quartz tube, a SiC-coatedgraphite suscepter, an inductively heated coil, a water coolingline, and gas lines. The source gas is tetra-methyl silane(TMS) and the carrier gas is hydrogen. In this experiment, a30- -thick SiC film was deposited at 1100 in 300 min.The TMS flow rate is 0.5 mL/min, and the hydrogen flow rateis 2 L/min. Fig. 3 shows the cross section of the silicon moldcovered with a 3- -thick SiC film. The film is amorphousSiC, confirmed by the X-ray diffraction (XRD) data shown inFig. 4.Fig. 5 illustrates the preparation process of SiC RS forbacking the SiC film. This process was done, basically fol-lowing the method reported in a previous paper [13]. First, thesilicon mold is set in a silicon frame and filled with a materialpowder ( -SiC: 60 wt%, graphite: 30 wt%, phenol resin:10 wt%) by cold isotropic pressing (CIP). Next, the frame iswrapped in silicon powder and subsequently in boron nitride(BN) powder by CIP. Finally, the sample is encapsulated inFig. 2. APCVD system for SiC deposition.Fig. 3. Cross section of the silicon mold covered with a 3-m-thick SiC film.vacuum using a Pyrex glass tube. The BN powder prevents thesample from reacting with the glass tube.SiC RS was performed by HIP at 100 MPa and 1700 for2 h. Fig. 6 shows the profiles of temperature and pressure duringthe HIP process. In this step, the silicon powder melts, infiltratesinto the material powder and reacts with graphite powder to gen-erate SiC. The original SiC powder is bonded with this newlygenerated SiC, and dense SiC backing is realized. After the HIPprocess, the sample is taken out from the capsule. The siliconframe and the silicon mold are etched away to release a SiC moldusing the mixture of hydrofluoric acid and nitric acid (1:2).2) Results: Fig. 7 shows the cross section of a fractured SiCRS sample before silicon etching. The CVD SiC film is success-fully backed with reaction-sintered SiC, and the shape of the tri-angular-section grating is maintained. Fig. 8 shows the surfaceof the SiC mold after silicon was etched away. The shape of thesilicon mold is transferred to the SiC mold. The surface rough-ness was 0.05–0.08 , which is approximately one fiftiethITOH et al.: SILICON-CARBIDE MICROFABRICATION BY SILICON LOST MOLDING FOR GLASS-PRESS MOLDS 861Fig. 4. CVD SiC film data. (a) XRD. (b) XPS.of that fabricated by our previous process without the CVD film[13], but worse than required by the glass-press mold.The close-up view in Fig. 8 suggests that the rough surfacewas caused partly by the transformation of amorphous SiC topolycrystalline SiC during the HIP process, and this was con-firmed by XRD patterns of the CVD SiC film before and afterthe HIP shown in Fig. 9. Another possible reasons of the sur-face roughing is the reaction between the CVD SiC film and thesilicon mold during the HIP process. The reaction might resultin a silicon-rich SiC surface which could be etched away in wetetching.B. Silicon Lost Molding Combined With SiC CVD and SSRB1) Experimental Method: First, a silicon mold is fabricated inthe foresaid process. Next, a 50- -thick SiC film is depositedon the silicon mold using the APCVD system, and the surface ispolished to be mirror-finished using a 1–3 diamond slurryand a Kemet lapping plate, which is rotated at 100 rpm. TheSiC film is backed with a 500- -thick SiC ceramic substrateby SSRB [14]. In this paper, the bonding was performed usinga 0.5- -thick nickel interface layer at 900 and a pressingpressure of 0.1 MPa for 0.5 h using a simple hot-press systemshown in Fig. 10. The nickel interface layer diffuses and reactswith SiC, realizing mechanically and thermally stable bonding[14]. After bonding, the silicon mold is etched away to release aFig. 5. Preparation process for SiC RS.Fig. 6. Profiles of temperature and pressure during the HIP process.SiC mold using the mixture of hydrofluoric acid and nitric acid(1:2).2) Results: Fig. 11 shows the released SiC mold. The CVDfilm was completely backed with the SiC ceramic substrate bySSRB, and its surface was smooth (0.004–0.008 ) com-pared with the CVD SiC film backed by SiC RS. The crystal-lization of the amorphous SiC does not occur during the bondingprocess at 900 .The SiC mold was pressed to a glass to confirm its high-tem-perature strength. Using the hot-press system, the SiC mold waspressed to a 1-mm-thick Pyrex glass at 850 and 1 MPa. Inthis experiment, the SiC mold was not separated from the Pyrexglass due to the restriction of the experimental setup. Fig. 12shows the interface between the SiC mold and the pressed glass.The Pyrex glass was deformed by the SiC mold without a void,and no significant deformation of the SiC mold was observed.862 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 4, AUGUST 2006Fig. 7. Cross section of a fractured SiC RS sample before silicon etching.Fig. 8. Surface of the SiC mold produced by RS.This result encourages the application of this SiC mold to glasspress up to 850 , which is higher than the softening temper-ature of Corning 7740 Pyrex glass (820 ) and BK7 opticalglass (718 ).III. CONCLUSIONTwo SiC microfabrication processes for SiC glass-pressmolds were developed. One is silicon lost molding combinedwith SiC CVD and SiC RS. The other is silicon lost moldingcombined with SiC CVD and SiC SSRB. In both of these pro-cesses, an original pattern on a silicon substrate is transferredto a CVD SiC film, and then the film is backed by bulk SiC toobtain rigidity and robustness against pressing force. Finally,the silicon substrate is etched away to release a SiC mold.In the process using SiC CVD and RS, an amorphous SiC filmwas deposited on a micromachined silicon mold, and backedwith RS SiC by HIP at 100 MPa and 1700 . The surfaceFig. 9. XRD patterns of the CVD SiC film. (a) Before HIP. (b) After HIP.Fig. 10. Schematic of the hot-press system.roughness of the SiC mold was 0.05–0.08 , and worsethan required by the glass-press mold. This was caused by thetransformation of amorphous SiC to polycrystalline SiC duringthe HIP process, which was confirmed by the XRD patterns ofthe CVD SiC film before and after the HIP process.In the process using SiC CVD and SSRB, a CVD SiC filmon the micromachined silicon mold was bonded with a SiC ce-ramic substrate using a nickel interface layer at 900 and aITOH et al.: SILICON-CARBIDE MICROFABRICATION BY SILICON LOST MOLDING FOR GLASS-PRESS MOLDS 863Fig. 11. Released SiC mold produced by SSRB.Fig. 12. Interface between the SiC mold and the pressed Pyrex glass.pressing pressure of 0.1 MPa. The surface of the SiC mold wassmooth (0.004–0.008 ) without the crystallization of theamorphous CVD SiC film. The SiC mold was pressed to Pyrexglass to demonstrate its high-temperature strength. The Pyrexglass was deformed by the SiC mold at 850 without a void,and no significant deformation of the SiC mold was observed.From this result, we conclude that this process has potential formaking a SiC mold for glass press.REFERENCES[1] M. Megregany, C. A. Zorman, N. Rajan, and C. H. Wu, “Silicon car-bide MEMS for harsh environments,” in Proc. IEEE, 1998, vol. 86, pp.1594–1610.[2] H. Maehara and H. Murakoshi, “Quartz glass molding by precisionglass molding method,” (in [in Japanese]) Trans. Inst. Elect. Eng.Japan, vol. 122-E, pp. 494–497, 2002.[3] J. Sugiura, W.-J. Lu, K. C. Cadien, and J. Steckl, “Reactive ion etchingof SiC thin films using fluorinated gases,” J. Vac. Sci. Technol. B, vol.4, pp. 349–354, 1986.[4] A. J. Fleischman, C. A. Zorman, and M. Mehregany, “Etching of3C-SiC using CHF =O and CHF =O =He plasmas at 1.75 Torr,”J. Vac. Sci. Technol. B, vol. 16, pp. 536–539, 1998.[5] P. H. Yih and A. J. Steckl, “Residue-free reactive ion etching of 3C-SiCand 6H-SiC in fluorinated mixture plasmas,” J. Electrochem. Soc., vol.142, pp. 2853–2860, 1995.[6] G. F. McLane and J. R. Flemish, “High etch rates of SiC in magnetronenhanced SF plasmas,” Appl. Phys. Lett., vol. 68, pp. 3755–3757,1996.[7] W. Reichert, D. Stefan, E. Obermeier, and W. Wondrak, “Fabricationof smooth -SiC surfaces by reactive ion etching using a graphite elec-trode,” Mat. Sci. Eng., vol. B46, pp. 190–194, 1997.[8] C. Richter, K. Espertshuber, C. Wagner, M. Eickhoff, and G. Krötz,“Rapid plasma etching of cubic SiC using NF =O gas mixtures,”Mat. Sci. Eng., vol. B46, pp. 160–163, 1997.[9] F. A. Khan and I. Adesida, “High rate etching of SiC using inductivelycoupled plasma reactive ion etching in SF -based gas mixture,” Appl.Phys. Lett., vol. 75, pp. 2268–2270, 1999.[10] P. Chabert, “Deep etching of silicon carbide for micromachining appli-cations: Etch rates and etch mechanism,” J. Vac. Sci. Technol. B, vol.19, pp. 1339–1345, 2001.[11] S. Tanaka, K. Rajanna, T. Abe, and M. Esashi, “Deep reactive ionetching of silicon carbide,” J. Vac. Sci. Technol. B, vol. 19, pp.2173–2176, 2001.[12] N. Rajan, M. Mehregany, C. A. Zorman, S. Stefanescu, and T. P.Kicher, “Fabrication and testing of micromachined silicon carbide andnickel fuel atomizers for gas turbine engines,” J. Microelectromech.Syst., vol. 8, pp. 251–257, 1999.[13] S. Tanaka, S. Sugimoto, J.-F. Li, R. Watanabe, and M. Esashi, “Siliconcarbide micro-reaction-sintering using micromachined silicon molds,”J. Microelectromech. Syst., vol. 10, pp. 55–61, 2001.[14] K. Bhanumurthy and R. Schmid-Fetzer, “Solid-state reaction bondingof silicon carbide (HIP-SiC) below 1000 C,” Mater. Sci. Eng., vol.A220, pp. 35–40, 1996.Toru Itoh received the B.E. and M.E. degrees fromTohoku University, Sendai, Japan, in 2001 and2003, in mechatronics and precision engineering,respectively.He has been an engineer with Alps Electric Co.,Ltd., since 2003.Shuji Tanaka received the B.E., M.E., and Dr.E. de-grees, all in mechanical engineering, from The Uni-versity of Tokyo, Tokyo, Japan, in 1994, 1996, and1999, respectively.From 1996 to 1999, he was a Research Fellowof the Japan Society for the Promotion of Science.He was a Research Associate with the Departmentof Mechatronics and Precision Engineering, TohokuUniversity, Sendai, Japan, from 1999 to 2001, andan Assistant Professor from 2001 to 2003. He is cur-rently an Associate Professor with the Departmentof Nanomechanics, Tohoku University. He is also a Fellow of the Center forResearch and Development Strategy, Japan Science and Technology Agency.His research interests include power MEMS, MEMS technology for harshenvironments, and microturbo machinery.Jing-Feng Li, photograph and biography not available at the time of publication.Ryuzo Watanabe, photograph and biography not available at the time ofpublication.Masayoshi Esashi received the B.E. and Dr.E. de-grees in electronic engineering from Tohoku Univer-sity, Sendai, Japan, in 1971 and 1976, respectively.He was a Research Associate with the Departmentof Electronic Engineering, Tohoku University, from1976 to 1981, and an Associate Professor from 1981to 1990. He was a Professor with the Departmentof Mechatronics and Precision Engineering, To-hoku University, from 1990 to 1998, and moved toNew Industry Creation Hatchery Center, TohokuUniversity, until 2004. He is currently a Professorwith the Department of Nanomechanics, Tohoku University. His researchinterests include MEMS, integrated sensors, micromachining technology, andpackaging.

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Silicon-Carbide Microfabrication by Silicon Lost Molding for Glass-Press Molds

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