Open Access
Add to BibSonomyAbstract
We describe a novel fabrication method of three-dimensional (3D) microstructures using local electrophoresis deposition together with laser trapping. A liquid cell consisting of two-faced conductive substrates was filled with a colloidal solution of Au nanoparticles. The nanoparticles were trapped by a laser spot and positioned on the bottom substrate, then deposited onto the surface by the application of electrical voltage between the two substrates. By moving the liquid cell downward while maintaining the deposition, 3D microstructures were successfully fabricated. The smallest diameter of the fabricated pillar was 500 nm, almost the same as that of the Airy disc. The Young’s modulus of the fabricated structure was 1.5 GPa.
© 2014 Optical Society of America
1. Introduction
Fabrication technologies of three-dimensional (3D) microstructures have been studied in the effort to develop microdevices such as micromachines and microelectro mechanical systems. In particular, in optics, it is very attractive issue to fabricate 3D micro-optic structures such as photonic crystals [1] and plasmonic metamaterials [2, 3].
Photolithography is one of the most popular fabrication techniques; however, as it requires expensive facilities such as cleanrooms and vacuum equipment, an alternative process using maskless patterning has been sought. Focused ion beam chemical vapor deposition [4–7] has been used for fabrication of 3D microstructures by drawing the ion beam with a single stroke. Although the technique fabricates microstructures without mask patterning, it also requires expensive equipment and vacuum conditions. Local photocemical reactions such as phtopolymerization of photo-curing resins [8–10] and phtoreduction of silver ion using photocatalyst [11] were reported as 3D microfabrication techniques. The processes of fabrication, in which the laser beam is also drawn with a single stroke, are quite simple and inexpensive. Materials for the photochemical reactions by these methods, however, are still limited.
Laser trapping based on photon pressure has been widely used for manipulation of micro-and nanoparticles in a liquid condition [12–17]. The trapping laser beam can gather and localize a number of nanoparticles into a focused spot in a colloidal solution. However, due to repulsion of the electrical double layer on the particle surfaces, the gathered particles never aggregate, even in the tiny space of the focused spot. Thus, in general, it is difficult to fabricate solid structures by aggregation of colloid nanoparticles using a laser trapping technique.
There are a few methods to fix the trapped nanoparticles on the substrates utilizing photo-induced interaction [18–20]. However, to fix the trapped particles, some adhesive materials should be mixed in the particle solution, and another excitation laser is setup to induce photochemical reactions to deposit the fine particles on the substrate. Simple techniques without chemical reactions and additives in the solution are needed.
Recently, we developed a deposition technique for colloidal nanoparticles using laser trapping coupled with electrophoresis deposition [21]. The nanoparticles trapped by the laser spot positioned on the substrate can be fixed on the substrate by electrophoresis deposition. We have successfully deposited Au nanoparticles in dot arrays and line patterning two-dimensionally on the substrate.
In this paper, we describe a novel fabrication technique for 3D microstructures achieved by the developed method that uses a combination of local electrophoresis deposition and laser trapping. The 3D microstructures were fabricated by moving the substrate vertically while maintaining the local electrophoresis deposition. The method is relatively simple and it is possible to realize the fabrication with minor modifications of a liquid cell in a conventional laser trapping setup. Here, fabrication properties of the microstructures consisting of Au nanoparticles were investigated. In addition, mechanical properties of the spring constant and Young’s modulus of the fabricated pillar were evaluated.
2. Method and experimental setup
2.1. Fabrication method of 3D structures
Figure 1(a) is a schematic representation of the electrophoresis deposition method together with laser trapping for the fabrication of micro 3D structures. The fabrication process is as follows. First, a liquid cell consisting of an upper cover glass and a lower substrate, with a spacer sandwiched between them, is filled with a colloidal solution of dispersed nanoparticles. To enable application of electrical voltage for electrophoresis, the facing surfaces of both the upper cover glass and lower substrate should be conductive. When a laser beam is introduced into the cell and focused on the lower substrate, nanoparticles around the laser spot are gathered into the spot on the substrate. However, these gathered nanoparticles will not be deposited due to the repulsion generated by the electric double layers on the surfaces of both the colloidal particles and the substrate. By applying the appropriate voltage between the upper cover glass and lower substrate, however, the trapped particles in the laser spot can be locally fixed on the substrate due to electrophoresis deposition. Here, to fabricate 3D structures, the liquid cell is then moved downward while steady deposition continues. As a result, 3D structures such as micropillars can be fabricated, as shown in Fig. 1(a).
Fig. 1 (a) The schematic of the fabrication method for 3D structures. Pillar structures can be fabricated by moving the stage downward while deposition is underway. (b) The schematic of the experimental setup for local electrophoresis deposition assisted by a laser trapping technique.
In this research, the surfaces of upper cover glass and lower substrate of the liquid cell were coated with indium thin oxide (ITO). The thickness of the spacer was 160 μm. A colloidal suspension of Au nanoparticles (3 nm in diameter, 0.26 wt%) was used as a solution to fill the cell.
Figure 1(b) shows the actual experimental setup for fabrication of micro 3D structures. A laser beam from a solid-state laser (wavelength, 488 nm) was expanded, and then introduced into an objective lens (×60, numerical aperture (NA) = 1.20) for the laser trapping of the nanoparticles in the cell. The deposition process was monitored by a CCD video camera. To position the laser spot on the substrate, a Z-axis stage driven with a piezo motor and an XY-axis piezo stage were used. To fabricate 3D structures without disconnections and constrictions while moving the substrate downward under deposition, a pantograph-type piezo-electric actuator was employed to realize Z-axis smooth movement of the cell. All the stages and the applied voltage were controlled by a personal computer.
After deposition, the fabricated structures on the substrate were observed with a scanning electron microscope (SEM).
2.2. Evaluation of stiffness of the fabricated structures
Stiffness measures such as the spring constant and Young’s modulus of the fabricated pillar were evaluated by measuring the deflection of the pillar when a calibrated load was applied (Fig. 2). A commercially available cantilever made of Si3N4 for an atomic force microscope (AFM) was used to apply a standard force to the fabricated pillars. The spring constant of the cantilever was calculated from its known Young’s modulus and dimensions. The length and width of the cantilever were measured by the SEM. The thickness of the cantilever was derived from the mechanical resonant frequency that was measured by a network analyzer. The calculated spring constant of the cantilever was 0.04 N/m.
Fig. 2 Schematic of measurement method for the spring constant of a fabricated pillar using an AFM cantilever.
To position the cantilever relative to the fabricated pillars, a custom-designed micromanipulator was operated under SEM observation [22].
3. Experimental results
3.1. Control of pillar length by changing the stage displacement
Figure 3 shows the SEM images of the fabricated pillar structures. The applied voltage was 1.6 V, which corresponded to the electrical field of 10.0 kV/m with the cell spacer of 160 μm in thickness. The condition was experimentally optimized as an appropriate electrical field to deposit only those nanoparticles trapped by the laser spot. The laser intensity was 1.5 mW, which was the minimum intensity for fabrication of 3D structures in this research. The moving velocity of the Z-axis piezo-actuator was 0.67 μm/s, which was the maximum velocity at which it was possible to maintain deposition of the nanoparticles for 3D fabrication at 1.5 mW laser intensity. Figures 3(a), 3(b) and 3(c) show the fabricated pillars with different lengths of 10 μm, 25 μm and 50 μm, respectively. The length of each pillar was the same as the piezo-actuator displacement, where the laser beam was shut off under movement of the actuator. Thus, it is possible to control the length of the pillar by the traveling displacement of the laser spot.
Fig. 3 SEM images of the fabricated structures. (a)–(c) Changing the piezo-actuator displacement where the laser beam was shut off under movement of the actuator. (d)–(f) Magnified images of dashed-line squares (I), (II) and (III) in image (c), respectively.
Figures 3(d), 3(e) and 3(f) are magnified images of the dashed line squares (I), (II) and (III) in Fig. 3(c), respectively. The average diameter of the pillar was approximately 650 nm. Thus a 3D structure submicrometer in diameter was successfully fabricated. From the images, it may be seen that the diameters were entirely uniform from the lower to the upper part of the pillar, which means that the deposition rate was constant under constant velocity of the Z-axis piezo-actuator.
3.2. Control of pillar diameters by changing the laser intensity
We attempted to change the diameter of the pillars by changing the intensity of the laser beam. Figures 4(a)–4(f) show the SEM images of typical pillar structures fabricated with different laser intensities. The laser intensity was changed from 1.50 mW to 2.75 mW in increments of 0.25 mW. The applied electric voltage and moving velocity of the Z-axis piezo actuator were 1.6 V and 0.67 μm/s, respectively. Figure 4(g) shows the relationship between the diameter of the pillars and the laser intensity. To evaluate the reproducibility of the fabrication, a series of the six pillars was fabricated by changing the laser intensity in each liquid cell, and totally 10 liquid cells were processed. Thus, for the graph, error bar at each intensity represents mean and maximum/minimum for 10 pillars fabricated in the different liquid cells. As shown in the figures, pillar diameter decreased with decreasing laser intensity. At the intensity of 1.5 mW, the smallest diameter of the fabricated pillar was 500 nm as shown in Fig. 4(h). With respect to the diameter of the fabricated pillar, the spot size of the laser can be estimated as 496 nm in diameter from the formula of an Airy disc, d = 1.22λ/NA, where λ is the wavelength of the light (488 nm) and NA is the numerical aperture (1.20) of the objective lens. Thus, the smallest diameter of the fabricated pillar was almost the same as that of the estimated laser spot. The nanoparticles trapped by the laser spot are confined within the Airy disc. In the case of higher laser intensity, a significant number of the trapped nanoparticles are deposited at the laser spot, which might induce the spatial extent of the deposited area to expand slightly comparing with the Airy disc. By decreasing the laser intensity, the trapping potential becomes shallower [23]; thus, the number of the trapped nanoparticles are decreased by the shallower potential. As a result of the deposition, the diameter of the pillar decreases, and it eventually might be the same size of the Airy disc due to the deposition within the laser spot. Therefore, it is possible to control the diameter of the pillar by altering the laser intensity.
Fig. 4 (a)–(f) SEM images of the typical fabricated pillars by changing laser intensity from 1.50 mW to 2.75 mW in increments of 0.25 mW, respectively; (g) relationship between the diameter of the pillars and the laser intensity. (h) SEM image of the fabricated pillar whose diameter was the smallest one in this experiment.
3.3. Evaluation of mechanical properties of the pillar
We attempted to evaluate the mechanical properties of a spring constant and Young’s modulus of the fabricated pillar. The fabricated pillar was deflected by application of a loading force with the AFM cantilever under SEM observation. Figure 5(a) shows the SEM image of the fabricated pillar used for these measurements. The laser intensity, applied voltage and moving velocity of the Z-axis piezo actuator for fabrication of the pillar were 2.75 mW, 1.6 V and 0.67 μm/s, respectively. Figure 5(b) shows the pillar deflected with the cantilever by applying the loading force. In the figure, a significant deflection is evident. In Fig. 5(c), the images of Fig. 5(a) and 5(b) are superimposed. As indicated by the arrowheads, the deflection of the pillar is 3.3 μm. Figure 5(d) shows the superimposed image of the cantilever captured before and after the loading force was applied to the fabricated pillar. Again, as indicated by the arrowheads, the deflection of the cantilever is 4.2 μm, which means that a loading force of 168 nN, calculated from the spring constant (0.04 N/m), was applied to the fabricated pillar. From the deflection of the pillar and the loading force, the spring constant of the pillar was calculated to be 0.05 N/m. From the value of the spring constant k, Young’s modulus E was calculated as 1.5 GPa using the following equation:
where L is the length of the pillar and I is a second moment of area. However, the value of Young’s modulus was smaller than that of Au bulk (78 GPa). The colloidal solution used for the fabrication included mercaptosuccinic acid as a protective agent to prevent the nanoparticles from aggregating. The nanoparticles with protective agents were deposited and aggregated on the substrate by electrophoresis deposition. As a result, the pillar fabricated under the process consisted of the aggregated Au nanoparticles and the protective agents, which were physically adsorbed onto each other by the deposition. Therefore the Young’s modulus of the pillar could be expected to be smaller than that of Au bulk. By post treatments such annealing processes, the stiffness will be improved in future research.Fig. 5 SEM images of measurement of the deflection of the fabricated pillar and the cantilever: (a) The cantilever and the fabricated pillar used in the experiment; (b) the fabricated pillar deflected with the cantilever; (c) a superimposed image of Figs. 5(a) and (b); (d) superimposed image of the cantilever captured before and after applying a loading force to the fabricated pillar.
3.4. Fabrication of the spring structure
As a demonstration of a 3D structure, we fabricated a spring structure using the proposed method. Figure 6(a) shows the SEM image of the fabricated spring structure. The spring structure was fabricated by moving the substrate downward using the Z-axis piezo actuator while adding circular motion to the XY-axis piezo stage (see illustration in Fig. 6). The fabrication parameters of laser intensity and applied voltage were 2.25 mW and 1.6 V, respectively. The moving velocities of the substrate to form the structure were 0.3 μm/s for vertical movement and 0.4 μm/s for circular movement, with 40 s rotation periods. As shown in the image, a spring shape was successfully fabricated. Figure 6(b) shows the side view of the spring structure. As indicated with arrowheads in the image, pitch A, B and C of the spring structure were the same length, 12 μm, which was consistent with the vertical distance of the laser spot moved relative to the substrate during the rotation period. The diameter of the spring structure was 5 μm, which means it was approximately 16 μm in circumference, consistent with the distance of the lateral trajectory of the laser spot moved relative to the substrate during the rotation period. Therefore, using the proposed method, it was possible to fabricate 3D structures following the trajectory of the laser spot exactly. In this study, the structures were fabricated one by one using a single beam spot. In future studies, to improve the speed of the fabrication, multiple beam spots should be formed by improving the setup with installing optical devices such as a galvanometer mirror [24], microlens arrays [25] and a spatial light phase modulator [26,27]. By combination with such multiple beam techniques and a computer aided manufacturing, it might be possible to fabricate more complex 3D microstructures.
Fig. 6 SEM images of the spring structure: (a) SEM image of the spring structure; (b) side view of the spring structure. Every pitch of the spring structure was same length of 12 μm.
For laser trapping, it is possible to deal with various nanomaterials, thus, the fabrication technique might be applicable to various applications in many fields. In optics, nanoparicles assembling might be applicable to fabrication of plasmonic metamaterilas and photonic devices. Furthermore it is possible to trap nanomaterials with lower laser intensity, and deposit them as they are without chemical reactions, which would be suitable for biological applications such as bioassembly and biomaterials for tissue engineering.
4. Conclusion
A novel local deposition technique for the fabrication of 3D microstructures was developed using an electrophoresis deposition method in conjunction with laser trapping. Au colloidal nanoparticles trapped by a laser spot were electrophoretically deposited on a substrate. The 3D structure was fabricated by moving the substrate vertically while maintaining steady local electrophoresis deposition. By changing the laser intensity, it was possible to control the diameter of the fabricated pillars. The smallest diameter of the fabricated pillar was 500 nm. The Young’s modulus of the pillar measured using an AFM cantilever was 1.5 GPa. As a demonstration, a micro spring structure was successfully fabricated.
This fabrication method could potentially be applied to various nanomaterials, such as nanoparticles and molecules. In future research, we intend to try various materials and improve the system to explore the possibility of the method for various applications such as micro-optic structures and bioassembly.
Acknowledgments
This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT) and from CREST of the Japan Science and Technology Agency (JST).
References and links
1. A. L. Rogach, N. A. Kotov, D. S. Koktysh, J. W. Ostrander, and G. A. Ragoiha, “Electrophoretic deposition of latex-based 3D colloidal photonic crystals: a technique for rapid production of high-quality opals,” Chem. Mater. 12(9), 2721–2726 (2000). [CrossRef]
2. J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85(18), 3966–3969 (2000). [CrossRef] [PubMed]
3. A. Ishikawa, T. Tanaka, and S. Kawata, “Negative magnetic permeability in the visible light region,” Phys. Rev. Lett. 95, 237401 (2005). [CrossRef] [PubMed]
4. S. Matsui, T. Kaito, J. Fujita, M. Komuro, K. Kanda, and Y. Haruyama, “Three-dimensional nanostructure fabrication by focused-ion-beam chemical vapor deposition,” J. Vac. Sci. Technol. B 18, 3181 (2000). [CrossRef]
5. T. Morita, R. Kometani, K. Watanabe, K. Kanda, Y. Haruyama, T. Hoshino, K. Kondo, T. Kaito, T. Ishihashi, J. Fujita, M. Ishida, Y. Ochiai, T. Tajima, and S. Matsui, “Free-space-wiring fabrication in nano-space by focused-ion-beam chemical vapor deposition,” J. Vac. Sci. Technol. B 21, 2737 (2003). [CrossRef]
6. J. Fujita, M. Ishida, T. Ichihashi, Y. Ochiai, T. Kaito, and S. Matsui, “Growth of three-dimensional nano-structures using FIB-CVD and its mechanical properties,” Nucl. Instrum. Methods Phys. Res. B 206, 472–477 (2003). [CrossRef]
7. D. Guo, R. Kometani, S. Warisawa, and S. Ishihara, “Growth of ultra-long free-space-nanowire by the real-time feedback control of the scanning speed on focused-ion-beam chemical vapor deposition,” J. Vac. Sci. Technol. B 31, 061601 (2013). [CrossRef]
8. S. Maruo, O. Nakamura, and S. Kawata, “Three-dimensional microfabrication with two-photon-absorbed photopolymerization,” Opt. Lett. 22(2), 132–134 (1997). [CrossRef] [PubMed]
9. S. Maruo and K. Ikuta, “Three-dimensional microfabrication by use of single-photon-absorbed polymerization,” Appl. Phys. Lett. 76(19), 2656–2658 (2000). [CrossRef]
10. S. Maruo and H. Inoue, “Optically driven micropump produced by three-dimensional two-photon microfabrication,” Appl. Phys. Lett. 89(14), 144101 (2006). [CrossRef]
11. K. Matsuda, S. Takahashi, and K. Takamasu, “Development of in-process visualization system for laser-assisted three-dimensional microfabrication using photocatalyst nanoparticles,” Int. J. Precis. Eng. Manuf. 11(6), 811–815 (2010). [CrossRef]
12. A. Ashkin, “Acceleration and trapping of particles by radiation pressure,” Phys. Rev. Lett. 24(4), 156–159 (1970). [CrossRef]
13. A. Ashkin, “Applicationf of laser radiation pressure,” Science 210(4474), 1081–1088 (1980). [CrossRef] [PubMed]
14. A. Ashkin, J. M. Dziedzic, and T. Yamane, “Optical trapping and manipulation of single cells using infrared laser beams,” Nature 330, 769–771 (1987). [CrossRef] [PubMed]
15. K. Svoboda and S. M. Block, “Optical trapping of metallic rayleigh particles,” Opt. Lett. 19(13), 930–932 (1994). [CrossRef] [PubMed]
16. T. T. Perkins, S. R. Quake, D. E. Smith, and S. Chu, “Relaxation of a single DNA molecule observed by optical microscopy,” Science 264(5160), 822–826 (1994). [CrossRef] [PubMed]
17. D. G. Grier, “Optical tweezers in colloid and interface science,” Curr. Opin. Colloid Interface Sci. 2(3), 264–270 (1997). [CrossRef]
18. J. Won, T. Inaba, H. Masuhara, H. Fujiwara, K. Sasaki, S. Miyawaki, and S. Sato, “Photothermal fixation of laser-trapped polymer microparticles on polymer substrates,” Appl. Phys. Lett. 75(11), 1506–1508 (1999). [CrossRef]
19. S. Ito, H. Yoshikawa, and H. masuhara, “Optical patterning and photochemical fixation of polymer nanoparticles on glass substrates,” Appl. Phys. Lett. 78(17), 2556–2568 (2001). [CrossRef]
20. S. Ito, H. Yoshikawa, and H. masuhara, “Laser manipulation and fixation of single gold nanoparticles in solution at room temperature,” Appl. Phys. Lett. 80(3), 482–484 (2002). [CrossRef]
21. F. Iwata, M. Kaji, A. Suzuki, and S. Ito, “Local electrophoresis deposition of nanomaterials deposition by a laser trapping technique,” Nanotechnol. 20, 235303 (2009). [CrossRef]
22. F. Iwata, Y. Mizuguchi, H. Ko, and T. Ushiki, “Nanomanipulation of biological samples using a compact atomic force microscope under scanning electron microscope observation,” J. Electron Microsc. 60(6) 359–366 (2011). [CrossRef]
23. A. Ashkin and J. M. Dziedzic, “Optical trapping and manipulation of viruses and bacteria,” Science 235(4795), 1517–1520 (1987). [CrossRef] [PubMed]
24. K. Sasaki, M. Koshioka, H. Misawa, N. Kitamura, and H. Masuhara, “Pattern formation and flow control of fine particles by laser-scanning micromanipulation,” Opt. Lett. 16(19), 1463–1465 (1991). [CrossRef] [PubMed]
25. C. H. Sow, A. A. Bettiol, Y.Y. G. Lee, F. C. Cheong, C.T. Lim, and F. Watt, “Multiple-spot optical tweezers created with microlens arrays fabricated by proton beam writing,” Appl. Phys . B 78(6), 705–709 (2004). [CrossRef]
26. J. Liesener, M. Reicherter, T. Haist, and H. J. Tiziani, “Multi-functional optical tweezers using computer-generated holograms,” Opt. Commun. 185, 77–82 (2000). [CrossRef]
27. R. L. Eriksen, V. R. Daria, and J. Glück, “Fully dynamic multiple-beam optical tweezers,” Opt. Express 10(14), 597–602 (2002). [CrossRef] [PubMed]
