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Fabrication of three-dimensional SiC-based ceramic micropatterns using a sequential micromolding-and-pyrolysis process
Microelectronic Engineering 83 (2006) 2475–2481 www.elsevier.com/locate/mee
Fabrication of three-dimensional SiC-based ceramic micropatterns using a sequential micromolding-and-pyrolysis process Tae Woo Lim a, Sang Hu Park a, Dong-Yol Yang a,*, Tuan Anh Pham b, Dong Hoon Lee b, Dong-Pyo Kim b,*, Sung-Il Chang c, Jun-Bo Yoon c a
c
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea b Department of Fine Chemical Engineering and Chemistry, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea Received 17 March 2006; accepted 29 May 2006 Available online 3 July 2006
Abstract A sequential micromolding and pyrolysis process is presented for fabricating three-dimensional (3D) SiC-based ceramic micropatterns with a submicron scale resolution using preceramic resins, which is a promising technique for diverse applications such as tribological micro-stamps of hot embossing. Firstly, a diffuser lithography process (DLP) and a two-photon polymerization (TPP) process have been employed to create master patterns, which are utilized in the fabrication of molds. In the DLP, various hemispheric-concave master shapes were built readily by exposing UV-light onto a thick positive photoresist film through a diffuser, which randomizes the paths of incident UV-light. Alternatively, the TPP process based on two-photon polymerization was used for the creation of real 3D master patterns with a two-photon sensitive resin mixture. Subsequently, the preceramic polymer micropatterns were fabricated via a micromolding process using polydimethylsiloxane (PDMS) molds replicated from the masters. Finally, the UV-cured preceramic micropatterns were transformed into SiC-based ceramic microstructures when pyrolyzed at 800 C under inert atmosphere. 2006 Elsevier B.V. All rights reserved. Keywords: 3D Ceramic microstructures; Micromolding; Diffuser lithography; Two-photon process
1. Introduction In recent years, there is a rapid progress in nano/microfabrication for the purpose of highly functionalized devices with superior mechanical and chemical properties, and it becomes interest-attracting issue continuously for neo-conceptive applications. The majority of microfabrications have been carried out, thus far, using various organic polymers, for example, SU-8, AZ9260, urethane acrylate, and many others [1,2]. However, the general properties of these kinds of organic polymer are not sufficient for devices applicable to harsh environments requiring a tolerance * Corresponding authors. Tel.: +82 42 869 3214; fax: +82 42 869 3210 (D.-Y. Yang); Tel.: +82 42 821 6695; fax: +82 42 823 6665 (D.-P. Kim). E-mail addresses: [email protected] (D.-Y. Yang), [email protected] (D.-P. Kim).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.05.010
for high temperatures as well as a resistance to corrosion. Therefore, it is certain that there will be an increasing demand on the development of novel and economic fabrication processes for ceramic microstructures. Until now, a machining process has been widely utilized for structuring various engineering materials from metals to nonmetals, and it has also played an important role in fabricating ceramic microstructures [3]. However, the current machining processes are mostly quite cost consuming, which is not sufficient to meet the required resolution of the versatile MEMS and NEMS devices. Besides, the machinable ceramic materials are limitedly available. As an alternative to the machining process, the molding and microstereolithography (lSL) processes using a slurry of ceramic powder and organic binder have been utilized in the creation of three-dimensional (3D) ceramic microstructures with a 100 lm scale resolution [4–6]. These fabrication
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processes consisted of a micromolding of the slurry using the molds created by SL and subsequent sintering step at temperature range 900–1550 C to form a dense ceramic microstructure [5]. It is readily expected that resolution of the microstructures is dependent on the accuracy of the molds and the grain size of ceramic powders. In order to improve the resolution of ceramic structures in the micromolding process, preceramic polymers as precursors of non-oxide ceramics must be useful with versatile processibility as demonstrated by Whitesides group [7]. The polymeric patterns were created with polydimethylsiloxane (PDMS) molds with various relief structures obtained from soft lithography method [7–9]. Then the cured and molded polymers were transformed into ceramic phases by pyrolysis upto 1200 C as the highest under inert atmosphere. Therefore, it is one of important issue that the resolution of the fabricated structures is dependent on the precision of molds. In this study, a sequential micromolding and pyrolysis process is presented for fabricating 3D SiC-based ceramic micropatterns with a submicron scale resolution using preceramic resin. At first, the precise masters with a high resolution were created using two approaches; a diffuser lithography process (DLP) [10] and a two-photon polymerization (TPP) process [11,12]. The novel DLP as developed by our group is a simple way for the creation of various hemispherical patterns in a large area, while the TPP process enables microfabrication of various 3D shapes with a high spatial resolution. Subsequently, the preceramic polymer patterns were fabricated via a micromolding process using PDMS molds replicated from the masters. Finally, the UV-cured polymeric patterns were transformed into SiC-based ceramic microstructures by pyrolyzing at 800 C under inert atmosphere. These 3D ceramic microstructures are applicable to unique MEMS devices requiring a tolerance to high temperatures or/and corrosive conditions as well as tribological micro-stamps of hot embossing [6,13]. 2. Fabrication of three-dimensional ceramic microstructures Fig. 1(a)–(h) shows a schematic illustration of the fabrication of 3D ceramic micropatterns via the sequential process of micromolding and pyrolysis. The process consists of four sequential steps: (i) fabricating photoresist master patterns, (ii) replicating PDMS molds, (iii) micromolding using preceramic polymer, and (iv) pyrolysis process. The following parts will explain them in detail. 2.1. Mold with hemispherical pattern by diffuser lithography process (DLP) DLP has been recently developed by our group to fabricate readily hemispherical arrays with a high fill factor and a pre-defined shape over a large area [10]. Contrast to conventional photolithography in which only a collimated UV-light source is used to obtain photoresist patterns with
a
UV light
Diffuser
Mask
d
AZ9260 (Photoresist) Substrate
Direct fabrication of 3D complicated microstructures using two-photon polymerization process system as shown in fig. 2
Replica molding (70°C for 1h)
b
PDMS (convex) 3D Master Pattern AZ9260 (Photoresist)
Substrate
Substrate Replica molding (70°C for 1h)
c
PDMS (concave)
PDMS Mold fabricated from the master pattern using diffuser-lithography
Substrate PDMS Mold fabricated from the master pattern using two-photon polymerization process
PDMS (concave) Mold Microtransfer Molding (Pour and cure pre-ceramic polymer)
g
Preceramic polymer structure
e
PDMS (concave)
PDMS (embossment)
f
Replica molding (70°C for 1h)
PDMS (concave) Mold Substrate
SiCN ceramic structure
h
Pyrolysis at 800°C under Nitrogen
Substrate
Fig. 1. Schematic illustration of the complete procedure; the fabrication of the polymer master patterns and the PDMS molds, the micromolding using preceramic polymer, and the pyrolysis process.
a rectangular cross-section, this DLP process utilized the randomized light to form the circular or elliptical shape of cross-sections in the photoresist as shown in Fig. 1(a). After spin-coating a thick AZ9260 positive photoresist under 1500 rpm for 1.5 s (target thickness: 45 lm) onto a substrate, then the thick layer was soft-baked for 1 h at 85 C. UV-light was exposed on the photoresist layer through the diffuser and mask to generate hemispherical patterns. The diffuser randomized the direction of UVlight, so the exposed region forms hemispherical concave shapes with a circular or elliptical cross-section. The diffuser used in this work was a 10 · 10 cm2 of soda-lime glass plate with thickness of 5 mm, of which a 450 lm thick opaque opal layer was coated onto one-side (F43-719, Edmund Optics Ltd.). The sizes of the hemispheres were changed by controlling the UV-dose in the diffuser lithography. Additionally, hemispheric patterns with a fill-factor of 100% were fabricated as the adjacent patterns were merged together with an excessive exposure dose. In order to fabricate PDMS molds from the master pattern, the mixture of Sylgard silicone elastomer 184 and curing agent (Dow Corning) with the ratio of 10:1 was used. The mixture was poured on the photoresist master pattern
T.W. Lim et al. / Microelectronic Engineering 83 (2006) 2475–2481
and solidified under the condition of 70 C for 1 h. Then, the cured PDMS layer, on which convex replicated patterns from the master exist, was peeled off carefully from the master pattern, as illustrated in Fig. 1(b). For the creation of concave PDMS molds, the molding process was iterated as shown in Fig. 1(c) and (f). 2.2. Mold with three-dimensional patterns by two-photon polymerization (TPP) process The DLP has a plenty of merits; for instance, simple, easily controllable, and mass-productive process. However, the created pattern was limited to the shapes of hemispheres. At this point, the TPP process has been recognized as a unique process for producing 3D complicated nanoand microstructures. A highly localized region at the center of a focal plane was selectively polymerized by two-photon absorption (TPP) mechanism to form a solid structure with a high spatial resolution under the diffraction limit of light [11]. Fig. 2 shows a schematic illustration of the developed system for the two-photon polymerization process. A mode-locked Ti:Sapphire laser with a wavelength of 780 nm, an 80 MHz repetition, and an ultrashort pulsewidth of less than 100 fs was used as a beam source for the TPP. The laser was tightly focused with an objective lens (1.4 numerical aperture, ·100) on the two-photon polymerizable resin. The used resin was a mixture of a commercially available resin SCR500 (JSR) and a twophoton absorbing photosensitizer (TP-Flu-TP2, 0.1 wt%) [14]. The beam was scanned on the focal plane with the use of a galvano-scanner, and was vertically positioned using a piezoelectric stage. A galvano-shutter that functioned as an on/off switch for the beam could be operated under the maximal frequency of 1.0 kHz. A high-magnification charge-coupled-device (CCD) camera was used for the optical adjustment of the focused beam and for monitoring the fabrication process. In the TPP process, 3D complicated microstructure was created by accumulating layers, which were obtained from 3D CAD data. After the entire 3D microstructure was fabricated, the remaining liquid resin was rinsed using a developing solvent ethanol.
Fig. 2. Schematic diagram of the system for two-photon polymerization process.
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Then the PDMS mold for ceramic molding was obtained using the 3D master as shown in Fig. 1(d)–(f). 2.3. Fabrication of ceramic structures using micromolding and pyrolysis Polyvinylsilazane (Kion VL20) was used as a precursor of SiC-based ceramic in this work. It is well documented that the polymeric precursor transformed into an amorphous SiCN ceramic phase at 800 C or less; then, it was crystallized into SiC and Si3N4 phase at 1600 C [15]. A photoinitiator (Irgacure 500) was added to the polyvinylsilazane at the ratio of 5 wt% to enhance the solidification reaction with UV exposure. The mixed precursor was poured into two types of the PDMS molds, which were replicated, from the masters. An overflowed precursor was scraped away using a flat PDMS bar, leaving fully filled precursor into the PDMS mold. The filled mold was then brought into contact with a Si wafer, and the polymeric precursor was cured under a UV-light exposure for 20 min with the intensity of 10 mW/cm2, as shown in Fig. 1(g). Following these operations, the PDMS mold was physically removed by peeling off from the substrate. The molded preceramic microstructures were pyrolyzed by heating to 800 C at a heating rate of 2 C/min in a tube furnace to convert into SiCN ceramic structures. The overall process was carried out under an inert atmosphere due to moisture sensitivity of the preceramic polymer. 2.4. Mechanical test of polymer-derived ceramic film A nano-indentation test has been conducted to evaluate the variation of mechanical strength of polymer-derived ceramic materials, depending on the pyrolysis temperature to 800 C. For the test, a spin-coated film on a Si wafer was used as a specimen. The detail procedures for preparing a specimen are the followings; a polyvinylsilazane solution of 50 wt% in toluene was dropped onto a Si(1 0 0) wafer and then spun at 2000 rpm for 30 s using a model PM101DT-R485 spinner (Head-way Research Inc.); the coating process was carried out inside a glove bag in an N2 atmosphere to avoid an exposure to moisture; the polymer films were cured by UV exposure at 10 mW/cm2 for 20 min, then annealed at various temperatures (from 100 to 800 C) in a tube furnace in an N2 atmosphere at a heating rate of 2 C/min. Fig. 3 shows the variation of Young’s modulus according to the pyrolysis temperatures. The modulus of the specimen was kept at approximately 4 GPa until heated to 500 C with minor change of less than 1 GPa. However, there was a sudden increase of Young’s modulus to more than 30 GPa for the sample annealed at near 600 C. This clearly indicates that the polymer pyrolyzed over 600 C became a ceramic phase in terms of mechanical behavior. Moreover, the film strength was further increased to 61 GPa at 800 C, which is similar to the strength of the reported polymer-derived ceramics [16]. Therefore, the
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Fig. 3. Dependence of Young’s modulus on spin-coated polymer films, UV-cured and pyrolyzed at various temperatures.
fabricated ceramic structures can be applied to microdevices operated under harsh conditions at high temperature and high mechanical stress. 3. Results and discussions The used preceramic polymer, polyvinylsilazane, has been utilized in a variety of ceramics applications [17,18]. The polymer containing a repeat unit of silicon-nitrogen bond showed a high ceramic yield 76% which is advantageous for obtaining a dense ceramic product. It is well-documented that conversion chemistry of the polymer to ceramics revealed to form an amorphous SiCN ceramic phase upon heating to 1000 C, then to develop a mixed crystalline phase of SiC and Si3N4 ceramics by pyrolysis at elevated temperatures over 1400 C [15,19]. In addition, as studied by the mechanical behavior of the spin-coated films, the polymer exhibited ceramic-like strength when pyrolyzed at higher temperatures than 600 C. temperatures. Through the micromolding of the polymer and subsequent pyrolysis process, various 3D ceramic micropatterns
have been fabricated with a submicron resolution. Fig. 4(a) and (b) shows the SiC-based ceramic microstructures fabricated using hemispherical masters which were created by the DLP. The hemispherical ceramic microstructures with a fill-factor of 100% could be fabricated uniformly in large area by simple process. The surface profiles of the ceramic pattern and corresponding PDMS mold were evaluated using an atomic force microscopy (AFM), as shown in Fig. 5(a) and (b). In the test results, the ceramic hemispheres became gentler compared to the PDMS master patterns. And the diameter of the hemispheres in base is preserved as 10 lm with no change even after the pyrolysis, because the bottom surface is strongly adhered on a substrate. However, the height of ceramic structures is 1.23 lm, which is considerably smaller than 1.85 lm of PDMS mold, presumably due to the volume shrinkage during pyrolysis. It might be caused by the phase conversion from low density polymer to high density ceramic phase with some extent of mass loss [9]. Despite severe change of the pattern height after the pyrolysis, the entire profile of patterns is nearly identical to that of PDMS mold. Therefore, the final dimension of a ceramic pattern can be estimated by considering the extent of shrinkage for precise creation of ceramic patterns. In addition, this weakness of the preceramic polymer route can be improved to some degree using alternative high-ceramic yielding precursors and considering a precompensative way in the CAD modeling step [20]. Moreover, diverse hemispheric ceramic patterns can be also fabricated, as shown in Fig. 6(a)–(d), using various masters which are created by controlling the hole size and pitch between holes of photomasks in the diffuser lithography. The hole diameter and the distance between the holes in the photomasks were four pair cases: (10 lm and 10 lm), (10 lm and 5 lm), (5 lm and 10 lm), and (5 lm and 5 lm). A slightly different geometry of the hemisphere was obtained, depending on the dimensions of photomask, under an identical UV-dose for the fabrication of the masters. The larger diameter of the holes in the photomask resulted in a gentler slope of the hemispherical
Fig. 4. SEM images of SiCN ceramic structures fabricated by soft lithography, whose mold was obtained from a diffuser lithography process in variable sizes with an inclined view: (a) in low magnification and (b) in high magnification.
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Fig. 5. (a) AFM images of the PDMS mold whose master was fabricated by diffuser lithography. (b) The replicated ceramic structures using the PDMS mold. The hemispheres became gentler due to shrinkage during pyrolysis. The width was maintained by the substrate, but the height became 66% smaller.
Fig. 6. SEM images of the SiCN ceramic structures fabricated from various diffuser lithography masters. The hole diameter and the distances between the holes are: (a) 10 lm and 10 lm, (b) 10 lm and 5 lm, (c) 5 lm and 10 lm, and (d) 5 lm and 5 lm, respectively.
segment, as the height of the hemisphere was in proportion solely to the UV-dose. In addition, as the distance between the holes in the photomask decreased, the hemispheres with gentler slopes were produced because of the influence of the UV light diffused from the adjacent holes. Eventually, this
process produced ceramic hemispheres with various fill-factors and shapes, which were relatively controllable. More complicated 3D-shaped ceramic patterns could be fabricated by employing a TPP process for making the master patterns. Fig. 7(a) and (c) shows the PDMS molds
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Fig. 7. SEM images of fabricated PDMS molds, and SiCN ceramic structures fabricated by soft molding. (a) and (c) The figures on the left side are PDMS molds, while (b) and (d) on the right side are replicated SiCN ceramic structures.
whose master patterns were fabricated using the TPP process, and Fig. 7(b) and (d) shows the SiCN ceramic structures created by the micro-transfer molding using the PDMS molds. It is obviously demonstrated that the precise shape of SiCN ceramic patterns could be replicated from the PDMS molds within the resolution of submicron using a preceramic polymer. For the sake of illustration, multi-scale hemisphere ceramic patterns with a diameter of 1–4 lm were reproduced from the corresponding concave patterns of PDMS mold. And also pyramidal shape of ceramic patterns was successfully replicated from the PDMS mold as shown in Fig. 7(c) and (d). In particular, the curved sides of the pyramid patterns were presented with welldeveloped sharpness. Therefore, it is worthy to noting that the use of preceramic polymers readily infiltrates into the complicated parts of the mold structures, resulting in great improvement of the replication resolution of patterns compared to other types of patterning materials [14]. 4. Conclusions The micromolding and pyrolysis process using preceramic polymer is especially valuable for the fabrication of 3D ceramic microstructures with high resolutions. For the fabrication of complex 3D master patterns, novel methods are suggested that employ a DLP and a TPP process. Using the master patterns fabricated by DLP, hemispherical SiC-based ceramic micropatterns, of which the shapes and sizes were controllable, could be produced in a large area. Additionally, diverse and complicated 3D ceramic
micropatterns can be created using the master patterns fabricated by the TPP process. However, it was observed that the dimensional change was observed due to the volume shrinkage during pyrolysis. Finally, the 3D SiC-based ceramic micropatterns fabricated in this study can be applied to various areas, such as the tribological mold of a hot embossing process or mechanical and chemical MEMS devices used in harsh environments. Acknowledgments The authors give thanks to Korean Ministry of Science and Technology (project of research for development of fundamental nanotechnology) and National Research Laboratory (M10400000061-04J0000-06110) for financial supports. References [1] A.L. Bogdanov, S.S. Peredkov, Microelectron. Eng. 53 (2000) 493. [2] J. Tlusty, S. Smith, C. Zamudia, Sensors Actuators 104 (2003) 275. [3] M. Mendes, R. Vilar, Appl. Surf. Sci. 217 (2003) 149. [4] X. Zhang, X.N. Jiang, Sensors Actuators 77 (1999) 149. [5] C. Provin, S. Monneret, H.L. Gall, S. Corbel, Adv. Mater. 15 (2003) 994. [6] U.P. Pchonholzer, R. Hummel, L.J. Gauckler, Adv. Mater. 12 (2000) 1261. [7] H. Yang, P. Deschatelets, S.T. Brittain, G.M. Whitesides, Adv. Mater. 13 (2001) 54. [8] L.A. Liew, W. Zhang, V.M. Bright, L. An, M.L. Dunn, R. Raj, Sensors Actuators 89 (2001) 64.
T.W. Lim et al. / Microelectronic Engineering 83 (2006) 2475–2481 [9] Y. Liu, L.A. Liew, Y. Liu, R. Luo, L. An, M.L. Dunn, V.M. Bright, J.W. Daily, R. Raj, Sensor. Actuat. A 95 (2002) 143. [10] S.Il. Chang, J.B. Yoon, Opt. Exp. 12 (2004) 6366. [11] S. Kawata, H.B. Sun, T. Tanaka, K. Takada, Nature 412 (2001) 697. [12] S.H. Park, S.H. Lee, D.Y. Yang, H.J. Kong, K.S. Lee, Appl. Phys. Lett. 87 (2005) 154108. [13] L.J. Heyderman, H. Schift, C. David, B. Ketterer, M.A. Maur, J. Gobrecht, Microelectron. Eng. 57 (2003) 375. [14] S.H. Park, T.W. Lim, D.-Y. Yang, H.J. Kong, R.H. Kim, K.S. Kim, K.-S. Lee, Bull. Korean Chem. Soc. 25 (2004) 1119.
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[15] I.K. Sung, Cristian, M. Mitchell, D.P. Kim, P.J.A. Kenis, Adv. Funct. Mater. 15 (2004) 1336. [16] Q.D. Nghiem, D.P. Kim, J. Mater. Chem. 15 (2005) 2188. [17] M. Horz, A. Zern, F. Berger, J. Haug, K. Mueller, F. Aldinger, M. Weinmann, J. Eur. Ceram. Soc. 25 (2005) 99. [18] M. Zemanova, E. Lecomte, P. Sajgalik, R. Riedel, J. Eur. Ceram. Soc. 22 (2002) 2963. [19] E. Kroke, Y.-L. Li, C. Konetschny, E. Lecomte, C. Fasel, A. Riedel, Mater. Sci. Eng. R26 (2000) 97. [20] S.B. Sun, T. Suwa, K. Takada, R.P. Zaccaria, M.S. Kim, K.S. Lee, S. Kawata, Appl. Phys. Lett. 85 (2004) 3708.
Fabrication of three-dimensional SiC-based ceramic micropatterns using a sequential micromolding-and-pyrolysis process Tae Woo Lim a, Sang Hu Park a, Dong-Yol Yang a,*, Tuan Anh Pham b, Dong Hoon Lee b, Dong-Pyo Kim b,*, Sung-Il Chang c, Jun-Bo Yoon c a
c
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea b Department of Fine Chemical Engineering and Chemistry, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea Received 17 March 2006; accepted 29 May 2006 Available online 3 July 2006
Abstract A sequential micromolding and pyrolysis process is presented for fabricating three-dimensional (3D) SiC-based ceramic micropatterns with a submicron scale resolution using preceramic resins, which is a promising technique for diverse applications such as tribological micro-stamps of hot embossing. Firstly, a diffuser lithography process (DLP) and a two-photon polymerization (TPP) process have been employed to create master patterns, which are utilized in the fabrication of molds. In the DLP, various hemispheric-concave master shapes were built readily by exposing UV-light onto a thick positive photoresist film through a diffuser, which randomizes the paths of incident UV-light. Alternatively, the TPP process based on two-photon polymerization was used for the creation of real 3D master patterns with a two-photon sensitive resin mixture. Subsequently, the preceramic polymer micropatterns were fabricated via a micromolding process using polydimethylsiloxane (PDMS) molds replicated from the masters. Finally, the UV-cured preceramic micropatterns were transformed into SiC-based ceramic microstructures when pyrolyzed at 800 C under inert atmosphere. 2006 Elsevier B.V. All rights reserved. Keywords: 3D Ceramic microstructures; Micromolding; Diffuser lithography; Two-photon process
1. Introduction In recent years, there is a rapid progress in nano/microfabrication for the purpose of highly functionalized devices with superior mechanical and chemical properties, and it becomes interest-attracting issue continuously for neo-conceptive applications. The majority of microfabrications have been carried out, thus far, using various organic polymers, for example, SU-8, AZ9260, urethane acrylate, and many others [1,2]. However, the general properties of these kinds of organic polymer are not sufficient for devices applicable to harsh environments requiring a tolerance * Corresponding authors. Tel.: +82 42 869 3214; fax: +82 42 869 3210 (D.-Y. Yang); Tel.: +82 42 821 6695; fax: +82 42 823 6665 (D.-P. Kim). E-mail addresses: [email protected] (D.-Y. Yang), [email protected] (D.-P. Kim).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.05.010
for high temperatures as well as a resistance to corrosion. Therefore, it is certain that there will be an increasing demand on the development of novel and economic fabrication processes for ceramic microstructures. Until now, a machining process has been widely utilized for structuring various engineering materials from metals to nonmetals, and it has also played an important role in fabricating ceramic microstructures [3]. However, the current machining processes are mostly quite cost consuming, which is not sufficient to meet the required resolution of the versatile MEMS and NEMS devices. Besides, the machinable ceramic materials are limitedly available. As an alternative to the machining process, the molding and microstereolithography (lSL) processes using a slurry of ceramic powder and organic binder have been utilized in the creation of three-dimensional (3D) ceramic microstructures with a 100 lm scale resolution [4–6]. These fabrication
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processes consisted of a micromolding of the slurry using the molds created by SL and subsequent sintering step at temperature range 900–1550 C to form a dense ceramic microstructure [5]. It is readily expected that resolution of the microstructures is dependent on the accuracy of the molds and the grain size of ceramic powders. In order to improve the resolution of ceramic structures in the micromolding process, preceramic polymers as precursors of non-oxide ceramics must be useful with versatile processibility as demonstrated by Whitesides group [7]. The polymeric patterns were created with polydimethylsiloxane (PDMS) molds with various relief structures obtained from soft lithography method [7–9]. Then the cured and molded polymers were transformed into ceramic phases by pyrolysis upto 1200 C as the highest under inert atmosphere. Therefore, it is one of important issue that the resolution of the fabricated structures is dependent on the precision of molds. In this study, a sequential micromolding and pyrolysis process is presented for fabricating 3D SiC-based ceramic micropatterns with a submicron scale resolution using preceramic resin. At first, the precise masters with a high resolution were created using two approaches; a diffuser lithography process (DLP) [10] and a two-photon polymerization (TPP) process [11,12]. The novel DLP as developed by our group is a simple way for the creation of various hemispherical patterns in a large area, while the TPP process enables microfabrication of various 3D shapes with a high spatial resolution. Subsequently, the preceramic polymer patterns were fabricated via a micromolding process using PDMS molds replicated from the masters. Finally, the UV-cured polymeric patterns were transformed into SiC-based ceramic microstructures by pyrolyzing at 800 C under inert atmosphere. These 3D ceramic microstructures are applicable to unique MEMS devices requiring a tolerance to high temperatures or/and corrosive conditions as well as tribological micro-stamps of hot embossing [6,13]. 2. Fabrication of three-dimensional ceramic microstructures Fig. 1(a)–(h) shows a schematic illustration of the fabrication of 3D ceramic micropatterns via the sequential process of micromolding and pyrolysis. The process consists of four sequential steps: (i) fabricating photoresist master patterns, (ii) replicating PDMS molds, (iii) micromolding using preceramic polymer, and (iv) pyrolysis process. The following parts will explain them in detail. 2.1. Mold with hemispherical pattern by diffuser lithography process (DLP) DLP has been recently developed by our group to fabricate readily hemispherical arrays with a high fill factor and a pre-defined shape over a large area [10]. Contrast to conventional photolithography in which only a collimated UV-light source is used to obtain photoresist patterns with
a
Diffuser
Mask
d
AZ9260 (Photoresist) Substrate
Direct fabrication of 3D complicated microstructures using two-photon polymerization process system as shown in fig. 2
Replica molding (70°C for 1h)
b
PDMS (convex) 3D Master Pattern AZ9260 (Photoresist)
Substrate
Substrate Replica molding (70°C for 1h)
c
PDMS (concave)
PDMS Mold fabricated from the master pattern using diffuser-lithography
Substrate PDMS Mold fabricated from the master pattern using two-photon polymerization process
PDMS (concave) Mold Microtransfer Molding (Pour and cure pre-ceramic polymer)
g
Preceramic polymer structure
e
PDMS (concave)
PDMS (embossment)
f
Replica molding (70°C for 1h)
PDMS (concave) Mold Substrate
SiCN ceramic structure
h
Pyrolysis at 800°C under Nitrogen
Substrate
Fig. 1. Schematic illustration of the complete procedure; the fabrication of the polymer master patterns and the PDMS molds, the micromolding using preceramic polymer, and the pyrolysis process.
a rectangular cross-section, this DLP process utilized the randomized light to form the circular or elliptical shape of cross-sections in the photoresist as shown in Fig. 1(a). After spin-coating a thick AZ9260 positive photoresist under 1500 rpm for 1.5 s (target thickness: 45 lm) onto a substrate, then the thick layer was soft-baked for 1 h at 85 C. UV-light was exposed on the photoresist layer through the diffuser and mask to generate hemispherical patterns. The diffuser randomized the direction of UVlight, so the exposed region forms hemispherical concave shapes with a circular or elliptical cross-section. The diffuser used in this work was a 10 · 10 cm2 of soda-lime glass plate with thickness of 5 mm, of which a 450 lm thick opaque opal layer was coated onto one-side (F43-719, Edmund Optics Ltd.). The sizes of the hemispheres were changed by controlling the UV-dose in the diffuser lithography. Additionally, hemispheric patterns with a fill-factor of 100% were fabricated as the adjacent patterns were merged together with an excessive exposure dose. In order to fabricate PDMS molds from the master pattern, the mixture of Sylgard silicone elastomer 184 and curing agent (Dow Corning) with the ratio of 10:1 was used. The mixture was poured on the photoresist master pattern
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and solidified under the condition of 70 C for 1 h. Then, the cured PDMS layer, on which convex replicated patterns from the master exist, was peeled off carefully from the master pattern, as illustrated in Fig. 1(b). For the creation of concave PDMS molds, the molding process was iterated as shown in Fig. 1(c) and (f). 2.2. Mold with three-dimensional patterns by two-photon polymerization (TPP) process The DLP has a plenty of merits; for instance, simple, easily controllable, and mass-productive process. However, the created pattern was limited to the shapes of hemispheres. At this point, the TPP process has been recognized as a unique process for producing 3D complicated nanoand microstructures. A highly localized region at the center of a focal plane was selectively polymerized by two-photon absorption (TPP) mechanism to form a solid structure with a high spatial resolution under the diffraction limit of light [11]. Fig. 2 shows a schematic illustration of the developed system for the two-photon polymerization process. A mode-locked Ti:Sapphire laser with a wavelength of 780 nm, an 80 MHz repetition, and an ultrashort pulsewidth of less than 100 fs was used as a beam source for the TPP. The laser was tightly focused with an objective lens (1.4 numerical aperture, ·100) on the two-photon polymerizable resin. The used resin was a mixture of a commercially available resin SCR500 (JSR) and a twophoton absorbing photosensitizer (TP-Flu-TP2, 0.1 wt%) [14]. The beam was scanned on the focal plane with the use of a galvano-scanner, and was vertically positioned using a piezoelectric stage. A galvano-shutter that functioned as an on/off switch for the beam could be operated under the maximal frequency of 1.0 kHz. A high-magnification charge-coupled-device (CCD) camera was used for the optical adjustment of the focused beam and for monitoring the fabrication process. In the TPP process, 3D complicated microstructure was created by accumulating layers, which were obtained from 3D CAD data. After the entire 3D microstructure was fabricated, the remaining liquid resin was rinsed using a developing solvent ethanol.
Fig. 2. Schematic diagram of the system for two-photon polymerization process.
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Then the PDMS mold for ceramic molding was obtained using the 3D master as shown in Fig. 1(d)–(f). 2.3. Fabrication of ceramic structures using micromolding and pyrolysis Polyvinylsilazane (Kion VL20) was used as a precursor of SiC-based ceramic in this work. It is well documented that the polymeric precursor transformed into an amorphous SiCN ceramic phase at 800 C or less; then, it was crystallized into SiC and Si3N4 phase at 1600 C [15]. A photoinitiator (Irgacure 500) was added to the polyvinylsilazane at the ratio of 5 wt% to enhance the solidification reaction with UV exposure. The mixed precursor was poured into two types of the PDMS molds, which were replicated, from the masters. An overflowed precursor was scraped away using a flat PDMS bar, leaving fully filled precursor into the PDMS mold. The filled mold was then brought into contact with a Si wafer, and the polymeric precursor was cured under a UV-light exposure for 20 min with the intensity of 10 mW/cm2, as shown in Fig. 1(g). Following these operations, the PDMS mold was physically removed by peeling off from the substrate. The molded preceramic microstructures were pyrolyzed by heating to 800 C at a heating rate of 2 C/min in a tube furnace to convert into SiCN ceramic structures. The overall process was carried out under an inert atmosphere due to moisture sensitivity of the preceramic polymer. 2.4. Mechanical test of polymer-derived ceramic film A nano-indentation test has been conducted to evaluate the variation of mechanical strength of polymer-derived ceramic materials, depending on the pyrolysis temperature to 800 C. For the test, a spin-coated film on a Si wafer was used as a specimen. The detail procedures for preparing a specimen are the followings; a polyvinylsilazane solution of 50 wt% in toluene was dropped onto a Si(1 0 0) wafer and then spun at 2000 rpm for 30 s using a model PM101DT-R485 spinner (Head-way Research Inc.); the coating process was carried out inside a glove bag in an N2 atmosphere to avoid an exposure to moisture; the polymer films were cured by UV exposure at 10 mW/cm2 for 20 min, then annealed at various temperatures (from 100 to 800 C) in a tube furnace in an N2 atmosphere at a heating rate of 2 C/min. Fig. 3 shows the variation of Young’s modulus according to the pyrolysis temperatures. The modulus of the specimen was kept at approximately 4 GPa until heated to 500 C with minor change of less than 1 GPa. However, there was a sudden increase of Young’s modulus to more than 30 GPa for the sample annealed at near 600 C. This clearly indicates that the polymer pyrolyzed over 600 C became a ceramic phase in terms of mechanical behavior. Moreover, the film strength was further increased to 61 GPa at 800 C, which is similar to the strength of the reported polymer-derived ceramics [16]. Therefore, the
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Fig. 3. Dependence of Young’s modulus on spin-coated polymer films, UV-cured and pyrolyzed at various temperatures.
fabricated ceramic structures can be applied to microdevices operated under harsh conditions at high temperature and high mechanical stress. 3. Results and discussions The used preceramic polymer, polyvinylsilazane, has been utilized in a variety of ceramics applications [17,18]. The polymer containing a repeat unit of silicon-nitrogen bond showed a high ceramic yield 76% which is advantageous for obtaining a dense ceramic product. It is well-documented that conversion chemistry of the polymer to ceramics revealed to form an amorphous SiCN ceramic phase upon heating to 1000 C, then to develop a mixed crystalline phase of SiC and Si3N4 ceramics by pyrolysis at elevated temperatures over 1400 C [15,19]. In addition, as studied by the mechanical behavior of the spin-coated films, the polymer exhibited ceramic-like strength when pyrolyzed at higher temperatures than 600 C. temperatures. Through the micromolding of the polymer and subsequent pyrolysis process, various 3D ceramic micropatterns
have been fabricated with a submicron resolution. Fig. 4(a) and (b) shows the SiC-based ceramic microstructures fabricated using hemispherical masters which were created by the DLP. The hemispherical ceramic microstructures with a fill-factor of 100% could be fabricated uniformly in large area by simple process. The surface profiles of the ceramic pattern and corresponding PDMS mold were evaluated using an atomic force microscopy (AFM), as shown in Fig. 5(a) and (b). In the test results, the ceramic hemispheres became gentler compared to the PDMS master patterns. And the diameter of the hemispheres in base is preserved as 10 lm with no change even after the pyrolysis, because the bottom surface is strongly adhered on a substrate. However, the height of ceramic structures is 1.23 lm, which is considerably smaller than 1.85 lm of PDMS mold, presumably due to the volume shrinkage during pyrolysis. It might be caused by the phase conversion from low density polymer to high density ceramic phase with some extent of mass loss [9]. Despite severe change of the pattern height after the pyrolysis, the entire profile of patterns is nearly identical to that of PDMS mold. Therefore, the final dimension of a ceramic pattern can be estimated by considering the extent of shrinkage for precise creation of ceramic patterns. In addition, this weakness of the preceramic polymer route can be improved to some degree using alternative high-ceramic yielding precursors and considering a precompensative way in the CAD modeling step [20]. Moreover, diverse hemispheric ceramic patterns can be also fabricated, as shown in Fig. 6(a)–(d), using various masters which are created by controlling the hole size and pitch between holes of photomasks in the diffuser lithography. The hole diameter and the distance between the holes in the photomasks were four pair cases: (10 lm and 10 lm), (10 lm and 5 lm), (5 lm and 10 lm), and (5 lm and 5 lm). A slightly different geometry of the hemisphere was obtained, depending on the dimensions of photomask, under an identical UV-dose for the fabrication of the masters. The larger diameter of the holes in the photomask resulted in a gentler slope of the hemispherical
Fig. 4. SEM images of SiCN ceramic structures fabricated by soft lithography, whose mold was obtained from a diffuser lithography process in variable sizes with an inclined view: (a) in low magnification and (b) in high magnification.
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Fig. 5. (a) AFM images of the PDMS mold whose master was fabricated by diffuser lithography. (b) The replicated ceramic structures using the PDMS mold. The hemispheres became gentler due to shrinkage during pyrolysis. The width was maintained by the substrate, but the height became 66% smaller.
Fig. 6. SEM images of the SiCN ceramic structures fabricated from various diffuser lithography masters. The hole diameter and the distances between the holes are: (a) 10 lm and 10 lm, (b) 10 lm and 5 lm, (c) 5 lm and 10 lm, and (d) 5 lm and 5 lm, respectively.
segment, as the height of the hemisphere was in proportion solely to the UV-dose. In addition, as the distance between the holes in the photomask decreased, the hemispheres with gentler slopes were produced because of the influence of the UV light diffused from the adjacent holes. Eventually, this
process produced ceramic hemispheres with various fill-factors and shapes, which were relatively controllable. More complicated 3D-shaped ceramic patterns could be fabricated by employing a TPP process for making the master patterns. Fig. 7(a) and (c) shows the PDMS molds
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Fig. 7. SEM images of fabricated PDMS molds, and SiCN ceramic structures fabricated by soft molding. (a) and (c) The figures on the left side are PDMS molds, while (b) and (d) on the right side are replicated SiCN ceramic structures.
whose master patterns were fabricated using the TPP process, and Fig. 7(b) and (d) shows the SiCN ceramic structures created by the micro-transfer molding using the PDMS molds. It is obviously demonstrated that the precise shape of SiCN ceramic patterns could be replicated from the PDMS molds within the resolution of submicron using a preceramic polymer. For the sake of illustration, multi-scale hemisphere ceramic patterns with a diameter of 1–4 lm were reproduced from the corresponding concave patterns of PDMS mold. And also pyramidal shape of ceramic patterns was successfully replicated from the PDMS mold as shown in Fig. 7(c) and (d). In particular, the curved sides of the pyramid patterns were presented with welldeveloped sharpness. Therefore, it is worthy to noting that the use of preceramic polymers readily infiltrates into the complicated parts of the mold structures, resulting in great improvement of the replication resolution of patterns compared to other types of patterning materials [14]. 4. Conclusions The micromolding and pyrolysis process using preceramic polymer is especially valuable for the fabrication of 3D ceramic microstructures with high resolutions. For the fabrication of complex 3D master patterns, novel methods are suggested that employ a DLP and a TPP process. Using the master patterns fabricated by DLP, hemispherical SiC-based ceramic micropatterns, of which the shapes and sizes were controllable, could be produced in a large area. Additionally, diverse and complicated 3D ceramic
micropatterns can be created using the master patterns fabricated by the TPP process. However, it was observed that the dimensional change was observed due to the volume shrinkage during pyrolysis. Finally, the 3D SiC-based ceramic micropatterns fabricated in this study can be applied to various areas, such as the tribological mold of a hot embossing process or mechanical and chemical MEMS devices used in harsh environments. Acknowledgments The authors give thanks to Korean Ministry of Science and Technology (project of research for development of fundamental nanotechnology) and National Research Laboratory (M10400000061-04J0000-06110) for financial supports. References [1] A.L. Bogdanov, S.S. Peredkov, Microelectron. Eng. 53 (2000) 493. [2] J. Tlusty, S. Smith, C. Zamudia, Sensors Actuators 104 (2003) 275. [3] M. Mendes, R. Vilar, Appl. Surf. Sci. 217 (2003) 149. [4] X. Zhang, X.N. Jiang, Sensors Actuators 77 (1999) 149. [5] C. Provin, S. Monneret, H.L. Gall, S. Corbel, Adv. Mater. 15 (2003) 994. [6] U.P. Pchonholzer, R. Hummel, L.J. Gauckler, Adv. Mater. 12 (2000) 1261. [7] H. Yang, P. Deschatelets, S.T. Brittain, G.M. Whitesides, Adv. Mater. 13 (2001) 54. [8] L.A. Liew, W. Zhang, V.M. Bright, L. An, M.L. Dunn, R. Raj, Sensors Actuators 89 (2001) 64.
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