Technical Brief

Fabrication of Polymer-Derived Silicon Oxycarbide Microparts and Their Mechanical Characteristics

[+] Author and Article Information
Takahiro Namazu

Associate Professor
Division of Mechanical Systems,
Department of Mechanical and Systems Engineering,
University of Hyogo
2167 Shosha, Himeji,
Hyogo 671-2201, Japan;
Japan Science and Technology Agency,
4-1-8, Honcho, Kawaguchi,
Saitama 332-0012, Japan
e-mail: namazu@eng.u-hyogo.ac.jp

Hiroyuki Kudara

Division of Mechanical Systems,
Department of Mechanical and Systems Engineering,
University of Hyogo,
2167 Shosha, Himeji,
Hyogo 671-2201, Japan

Yoshio Hasegawa

R&D Division,
3135-20, Muramatsu,
Tokai-mura, Naka-gun,
Ibaraki 319-1112, Japan

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received March 10, 2014; final manuscript received May 20, 2014; published online July 9, 2014. Assoc. Editor: Nicholas Fang.

J. Micro Nano-Manuf 2(4), 044501 (Jul 09, 2014) (7 pages) Paper No: JMNM-14-1013; doi: 10.1115/1.4027734 History: Received March 10, 2014; Revised May 20, 2014

In this paper, fabrication and mechanical characterization of silicon oxycarbide (SiOC) microparts made from polycarbosilane (PCS) precursor is described. The developed fabrication technique is a combination of ultraviolet thick photoresist lithography and slip casting. The slips consisting of β-SiC nanoparticles and a PCS solution are cast into SU-8 photoresist micromold, fabricated on a porous tungsten carbide plate. The plate works as a filter for solid–liquid separation. The cast slips are fired at 1000 °C in N2 gas flow for an hour. During the firing, the SiOC body can be released from the mold because of SU-8 vaporization at 450 °C. By using the technique, we have successfully produced SiOC microgears with diameters ranging from 0.5 mm to 2 mm. To improve the mechanical reliability, the polymer infiltration and pyrolysis (PIP) process is carried out. The influence of the PIP process is evaluated by means of the nanoindentation test. The Young's modulus and hardness are increased with increasing PIP process cycles. From energy dispersive X-ray measurement results, it is found that their distributions are related to the amount of oxygen and the carbon-to-silicon ratio.

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Fig. 1

A schematic of fabrication process for PCS-derived SiOC micromechanical parts. The combination of UV thick resist photolithography and slip casting is named microslip casting. First SU-8 sheet is laminated onto a WC plate. Then, photolithography, baking, and development are conducted to fabricate SU-8 micromold with gear pattern. After that, the slips are cast into the mold. During casting, we apply a pressure and a depressure over and under the mold, respectively, for smooth solid–liquid separation. To make the cast body denser, the casting and pressuring–depressuring process is repeated several times. After casting, the cast body is fired. During firing, the mold is evaporated. By applying a small shear force to SiOC gears, they can be easily released.

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Fig. 2

Photographs of the polished and blasted WC plates along with their magnified pictures. These were used as a filter for solid–liquid separation. The blasted plate is better for the fabrication of crack-less SiOC parts because lots of pores are distributed more uniformly on the surface.

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Fig. 3

Examples of microgear mold made of SU-8 photoresist. When applying the best lithography condition for SU-8 on Si wafer to SU-8 on WC plate, a gear pattern could not be finely made. When the best condition for SU-8 on the plate was found, finely shaped gear mold could be realized.

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Fig. 4

Photographs of the topside and backside of SiOC microgears fabricated through the CPD process of 2, 3, and 6 cycles. In total the amount of the slips used per one fabrication was 600 μl, which was maintained for all the fabrications. When the CPD process cycles were twice, which indicates that the slips of 300 μl were cast twice, many lost cogs and cracks were found. With increasing the cycles, these were reduced. At 6 cycles, no lost cogs were found. Multiple CPD process cycles are effective for finely shaped SiOC parts.

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Fig. 5

Photograph of fabricated several types of SiOC microgears after 6 times casting processes. Even gears with the diameter less than 1 mm could be finely made. The diameter of the smallest gear is approximately 300 μm.

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Fig. 6

Photographs of the backside of SiOC microgears after 6, 9, and 12 PIP cycles. With increasing PIP cycles, a dark-colored portion appears to increase. This indicates that the gear is getting denser by conducting multiple PIP cycles.

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Fig. 7

Relationship between the number of PIP cycles and the density of SiOC. No large differences between polished and blasted WC plates are seen.

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Fig. 8

Process flow for fabricating the specimen for the nanoindentation test. A groove with the width and depth of 250 μm and 200 μm, respectively, was fabricated using a dicing cutter onto 300 μm-thick diced silicon chip. Epoxy glue was applied onto the silicon chip, and then half-cut SiOC gear was put onto the groove for bonding. At the same time, two silicon blocks were also bonded for the support of the gear. The cut face was investigated by the nanoindentation test.

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Fig. 9

Nanoindentation-derived hardness and Young's modulus maps on SiOC surfaces after 6, 9, and 12 PIP cycles along with O and C/Si maps derived from EDX. Darker portion in each map is indicative of higher portion in each parameter. It is found that higher Young's modulus and hardness were obtained where less O content and the C/Si ratio closer to 1.

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Fig. 10

Relationships between O content, C/Si content ratio, and SiOC Young's modulus. With increasing PIP cycles, O content was reduced and C/Si ratio was getting close to 1. As the result an increase of the Young's modulus was provided.




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