We developed new photopolymers for use in the formation of three-dimensional (3-D) carbon microstructures via two-photon microfabrication and microtransfer molding. The photopolymers contain the epoxy resorcinol diglycidyl ether. They have a high carbon content and a high bond energy, ensuring structural fidelity of the microstructures after pyrolysis. A cationic photoinitiator is incorporated into one of the new photopolymers and an additional radical photoinitiator into another. These two photopolymers are found to be ideal for two-photon microfabrication and microtransfer molding, respectively, with complex 3-D carbon microstructures such as a bunny and pyramidal models being formed. Potential applications of the new photopolymers include 3-D carbon electrodes for fuel cells or batteries and interfaces for biosensors.
©2013 Optical Society of America
Carbon materials have attracted wide attention as there are many varieties of carbon allotropes, such as diamond, graphite, fullerenes, and amorphous carbon [1–4]. In particular, amorphous and glassy carbon materials have been widely used as electrode and mechanical materials for versatile applications such as energy devices, microelectromechanical systems (MEMS) and biosensors, owing to their highly desirable properties, including good conductivity, a large surface area and biocompatibility [1,2]. Various types of fabrication methods for carbon micro/nano structures, that use polymer precursors or photoresists have been proposed and developed [5–12]. For example, soft lithography using elastomeric molds has been used to make glassy carbon microstructures with a furfuryl alcohol-based resin [5,6]. Another promising method is based on the pyrolysis of photoresists patterned by photolithography [7–12]. In this approach, photopatterned resists such as SU-8 and AZ P4620 are converted to carbon patterns by heat treatment in an inert environment . Owing to the intrinsic features of photolithography, this technique can provide large-scale, reproducible, carbon microstructures with high precision. However, patterning via a photo-mask generally limits the structures that can be formed to simple column and mesh structures. Recently, multi-step exposure and grayscale lithography have been employed to produce simple three-dimensional (3-D) structures such as step-like, overhanging and cantilever structures [11,12]. However, fabrication of more complex 3-D carbon microstructures remains a challenge.
One of the most promising ways to overcome the intrinsic limits of traditional photolithography is patterning by direct laser writing. The formation of complex 3-D microstructures has been successfully demonstrated using a femtosecond-pulsed laser two-photon microfabrication technique in recent years [13–17]. Using this technique, 3-D polymeric microstructures are fabricated by scanning the laser beam inside a photopolymer layer deposited on a substrate. In addition, the 3-D polymeric microstructure is used as a master mold for microtransfer molding techniques. Complex 3-D microstructures including not only overhanging but also freely movable microparts can be reproduced by using elastomeric molds [18–21].
In this study, we fabricate 3-D carbon microstructures using a two-photon microfabrication technique followed by carbonization in an inert atmosphere. Commercially available photopolymers are not suitable for the formation of carbon microstructures via carbonization. Here we instead develop new photopolymers, which are found to be suitable for this process. Using our new photopolymers, sophisticated 3-D carbon microstructures are fabricated by direct laser writing and transfer molding patterning techniques.
2. Development of photopolymers for carbonization
2.1 Pyrolysis of commercial photopolymers
Initial pyrolysis experiments were carried out using the commercially available epoxy-type photopolymers TSR-820, TSR-828 (C-MET Inc.), and SCR-701 (D-MEC Ltd.). These photopolymers have previously been used in two-photon microfabrication and single-photon microstereolithography [22,23]. The same processing conditions were used for all the polymers. Preliminary experiments were carried out using polymeric pellets formed by ultraviolet (UV) light curing of the photopolymers in poly(dimethylsiloxane) (PDMS) wells (diameter: 5 mm, depth: 1.5 mm). An example pellets is shown in Fig. 1(a).
The polymeric pellets were carbonized under nitrogen using a thermogravimetry / differential thermal analysis (TG / DTA) measurement system (Shimazu Corp., DTG-60H). The pellets were heated from room temperature to 700 °C at a heating rate of 10 °C/min. After this carbonization process, the pellets were seen to turn black and decrease in size. An example of a carbonized SCR-701 pellet is shown in Fig. 1(b). These results indicate that commercial photopolymers easily decompose when heated under a nitrogen atmosphere. They are therefore not suitable as precursor materials for the formation of carbon microstructures.
2.2 Development of new photopolymers for carbonization
The new photopolymers were developed using the epoxy compound resorcinol diglycidyl ether (Fig. 2(a)). Compared to the epoxies used in commercial photopolymers, such as 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (Fig. 2(b)) and hydrogenated bisphenol-A diglycidyl ether (Fig. 2(c)). Resorcinol diglycidyl ether has high carbon content. It also contains a benzene ring, which has a high bond energy. In addition, resorcinol diglycidyl ether does not contain aliphatic and alicyclic groups, which are easily combustible. The low viscosity of resorcinol diglycidyl ether makes it easy to process for both 3-D direct laser writing and microtransfer molding. These properties make our resorcinol diglycidyl ether photopolymers ideal materials to use as carbon precursors.
Three different photopolymers were prepared from the resorcinol diglycidyl ether epoxy compound. The epoxy was mixed with oxetane, different photoinitiators and acrylate resins. To prepare the photopolymers, resorcinol diglycidyl ether is melted by heating at 50 °C. Then, the ingredients except for the photoinitiators are thoroughly mixed by a magnetic stirrer, followed by addition of the photoinitiators and mixing for a few hours. The compositions of the new photopolymers are given in Table 1. TSR-DA1 contains resorcinol diglycidyl ether, oxetane and a cation photoinitiator. TSR-DA2 and TSR-DA3 contain additional acrylate resins and a radical photoinitiator to enhance the sensitivity for two-photon initiated polymerization. The composition of the precursors was chosen so that the resulting materials had the sensitivity required for two-photon initiated polymerization and could be used to form high fidelity carbon structures.
Figure 3 shows the TSR-DA1, TSR-DA2, TSR-DA3 and SU-8 pellets after carbonization. The SU-8 photoresist is typically used for carbon MEMS and was, therefore, used as a reference material. The TSR-DA1, TSR-DA2, TSR-DA3 pellets all keep their original shape, unlike that of the commercial photopolymer SCR-701 (Fig. 1(b)).
To elucidate the material microstructure and graphitization of the carbon microstructure, we have measured the Raman spectrum and X-ray diffraction spectrum of carbon pellets. The Raman spectrum was measured using a laser Raman spectrophotometer (JASCO, Inc., RMP-300). Figure 4(a) shows the Raman spectrum of a carbon pellet prepared from TSR-DA1. The measured Raman spectrum demonstrates that the carbon structure exhibits broad D and G bands, suggesting amorphous carbon [9,24,25]. The X-ray diffraction spectrum of the carbon material was also measured by using an X-ray diffractometer (Rigaku Corp., Ultima IV). For this measurement, we prepared a powder sample by grinding the carbon pellets derived from TSR-DA1. Figure 4(b) shows the X-ray spectrum of the TSR-DA1-derived carbon material. Because there are two broad diffraction peaks located at 2θ = 22° and 43°, corresponding to the (002) and (101) diffractions of the graphitic structure, the carbon consists mainly of amorphous incompletely graphitized structures .
We also measured the resistance of the carbon structures at room temperature by the four-point probe method. We found that the resistivities of TSR-DA1 and TSR-DA3 were around 0.13Ω cm and 0.19 Ω cm, respectively. Because these values are comparable to those of other photoresist precursors such as SU-8, the photopolymers can be used as electrode materials .
In addition, the mass loss of each pellet was evaluated by thermogravimetric measurement. Figure 5 shows thermogravimetric measurements of TSR-DA1. Shrinkage of pellets was also evaluated by shrinkage ratio determined by the diameter ratio of a carbon pellet to the initial polymeric pellet. The shrinkage and mass loss of the pellets during carbonization are summarized in Table 2. The shrinkage ratios of TSR-DA2 and TSR-DA3 are larger than that of SU-8, while TSR-DA1 and SU-8 have the same shrinkage ratio. The mass loss seen for TSR-DA1, TSR-DA2 and TSR-DA3 is smaller than that observed for SU-8. TSR-DA1 has the smallest mass change with a decrease of only 68% observed. We think that the small weight loss of TSR-DA1 is useful for making fine, complex 3-D micro/nano structures via a simple heating process in a nitrogen atmosphere. TSR-DA1 is therefore the most promising material to use in the formation of carbon 3-D microstructures via UV nanoimprinting , or 3-D microtransfer molding processes with UV exposure [18–21].
2.3 Two-photon initiated photopolymerization sensitivity
The suitability of TSR-DA1, TSR-DA2 and TSR-DA3 for patterning via two-photon initiated polymerization was investigated. A self-made two-photon microfabrication system was used. In our self-made optical system, a Ti:sapphire laser (Newport Corp., Mai Tai, wavelength: 750 nm, repetition frequency: 80 MHz) was used as the light source. The femtosecond pulsed laser beam was collimated using a beam expander (10x magnification) and introduced into an upright microscope (Olympus Corp., MX-51). The laser beam was focused on the sample using an objective lens with a numerical aperture of 1.35. At the sample stage, the photopolymer sample was sandwiched between a cover glass and a non-reactive, heat-resistant silicon substrate with a spacer (Lens cleaning paper, thickness: around 50 μm). The thickness of the spacer is smaller than the working distance of the objective lens so that the laser beam can be focused on the silicon substrate through the cover glass. The 3-D microstructures were designed using computer-aided design software. The sample was placed on a 3-D piezo stage (PI-Japan Corp., P-563.3CD) and scanned according to the design specifications. After fabrication, the silicon substrate with fabricated polymeric structures was rinsed in a rinse solution (Olympus Corp., EE-4210), followed by rinsing in ethanol.
The two-photon induced polymerization of TSR-DA1, TSR-DA2 and TSR-DA3 was investigated by changing the scanning speed (10 to 70 μm/s) and power (20 to 80 mW) of the femtosecond pulsed laser beam. The results are summarized in Table 3. The results show that TSR-DA2 and TSR-DA3 can be polymerized using two-photon initiated polymerization. The acryl moiety found in these photopolymers reacts rapidly. This allows the polymer to be loosely solidified in the vicinity of the laser beam focal spot. TSR-DA3 is most sensitive to the laser beam irradiation and can be polymerized at a laser power of only 30 mW. Polymerization of TSR-DA3 can be achieved at scanning speeds of 70 μm/s for a 70 mW powered laser.
TSR-DA1 could not be polymerized using the two-photon polymerization technique and is therefore not suitable for use in two-photon microfabrication. One reason for this may be that the rate of polymerization for TSR-DA1 is much slower than the irradiation time used in the experiments. TSR-DA1 does, however, have excellent properties and exhibits the small shrinkage ratio and mass loss useful for nanoimprinting and transfer molding as mentioned in Section 2.2.
3. 3-D carbon microstructures formed via two-photon microfabrication
3-D polymeric microstructures were fabricated from TSR-DA3 using a two-photon microfabrication technique. Scanning electron microscopic (SEM) images of the carbonized 3-D microstructures can be seen in Fig. 6. The microstructures were carbonized in a nitrogen atmosphere by heating the samples from room temperature to 800 °C, at rate of 10 °C/min. A vacuum electric furnace (ADVANTEC TOYO, Co., Ltd., FUA112DB) was used for this process. Complex 3-D carbon microstructures were successfully produced by via two-photon microfabrication and subsequent pyrolysis. The shrinkage ratio of the table structure after carbonization was about 42% (Fig. 6(a)).
4. 3-D carbon microstructures formed via microtransfer molding
3-D carbon microstructures were also formed via a PDMS microtransfer molding technique as shown in Fig. 7. In this process, two-photon microfabrication was used to form a master of the 3-D structure, which was then used to create a 3-D PDMS mold. Then, a PDMS well was attached to the PDMS mold to prepare a polymeric film on the 3-D microstructures, as shown in step (4). Next, TSR-DA1 was injected into the PDMS mold and exposed to UV light. The 3-D polymeric replica on the polymeric film was then demolded from the PDMS mold. Finally, after pyrolysis of the polymeric replicas using the method described in section 3, 3-D carbon microstructures were formed on the carbon film. Therefore, because the polymeric film also shrank during pyrolysis, the spaces in the pyramidal models were smaller than those of the original polymeric models. The shrinkage not only of the 3-D microsturctures but also of the base film will be useful for miniaturization of micro/nano 3-D patterns.
Figure 8 shows examples of the carbonized TSR-DA1 3-D microstructures formed using this method. Figures 8(a) and 8(b) show the pyramidal microstructures before and after carbonization. The shrinkage ratio was about 30%. In contrast to previously reported multi-step exposure patterning techniques , this molding process can be used to easily form high-aspect ratio layered structures using a single UV light exposure step. Figures 8(c) and 8(d) show TSR-DA1 bunny structures before and after carbonization. The results demonstrate that complicated 3-D polymeric microstructures with overhanging parts can be replicated using the highly flexible PDMS molds.
We developed novel photopolymers and demonstrated their suitability for the formation of 3-D carbon microstructures. The new photopolymer TSR-DA3 is shown to be an ideal material for use in two-photon microfabrication systems. Using TSR-DA3 3-D microstructures were successfully carbonized by pyrolysis in a nitrogen atmosphere. A second photopolymer TSR-DA1 is shown to be suitable for 3-D patterning via a microtransfer molding and UV curing process. Two-photon microfabrication can be used to form sophisticated 3-D microstructures as well as mechanical microstructures . Membrane-assisted transfer molding [19,20] allows the mass-production of low cost, complex 3-D carbon MEMS. Our new photopolymers are ideal materials for the production of practical 3-D carbon MEMS, fuel cells, and biosensors.
We are grateful to Dr. Shiguo Zhang and Professor Masayoshi Watanabe for measuring Raman spectrum and X-ray diffraction spectrum of carbon materials.
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