Aug 1, 2000 - Satellite Venture Business Laboratory, The University of Tokushima, 2-1 Minamijyosanjima, Tokushima 770-8506, Japan. Takeshi Kawakami ... photon energy is too small to excite the initiator in a ... scanned with a 3D computer-controlled piezoelectric ... nation, diagnosis, repair, and treatment in narrow.
1110
OPTICS LETTERS / Vol. 25, No. 15 / August 1, 2000
Real three-dimensional microstructures fabricated by photopolymerization of resins through two-photon absorption Hong-Bo Sun Satellite Venture Business Laboratory, The University of Tokushima, 2-1 Minamijyosanjima, Tokushima 770-8506, Japan
Takeshi Kawakami, Ying Xu, Jia-Yu Ye, Shigeki Matuso, and Hiroaki Misawa Department of Ecosystem Engineering, Graduate School of Engineering, The University of Tokushima, 2-1 Minamijyosanjima, Tokushima 770-8506, Japan
Masafumi Miwa and Reizo Kaneko Department of Opt-Mechatronics, Faculty of System Engineering, Wakayama University, Sakaedani, Wakayama 640-8510, Japan Received March 27, 2000 Effective energy windows for two-photon absorption (TPA) photopolymerization of resins were investigated and, with a properly selected laser pulse energy, exquisite three-dimensional (3D) microstructures with submicrometer spatial resolution were achieved. The results show the inherent utility of TPA in the fabrication of real 3D patterns. In particular, we propose and utilize a resin pre-exposure technique by which freely movable components affixed to an axle are built, demonstrating a new application of TPA in laser microfabrication. © 2000 Optical Society of America OCIS codes: 220.3740, 220.4000, 230.4000, 160.4330, 190.4180, 160.5470.
Conventional photolithography, which is a well-known technique for converting patterns of light into physical structures, is limited to the formation of twodimensional (2D) structures. Therefore it cannot satisfy the requirements of modern applications such as microelectromechanical systems1 – 3 and photonic crystals4 to real three-dimensional (3D) patterns. Two-photon absorption5,6 (TPA) laser microfabrication has paved the way to direct laser writing of 3D microstructures.7,8 This method uses laser pulses whose photon energy is too small to excite the initiator in a photopolymer by conventional one-photon absorption (1PA). When the beam is tightly focused with an objective lens of high magnif ication, however, the peak intensity in a small volume near the focal region is suff icient to expose the light-curling materials by simultaneously absorbing two photons. By scanning the focal volume in all dimensions, one can fabricate 3D structures with pinpoint accuracy. Furthermore, because of the quadratic dependence of the TPA rate on the laser pulse energy (LPE), solidification is confined to a highly localized area, which permits spatial resolution smaller than the laser diffraction limits, to the submicrometer range. Because most known molecular two-photon absorptivities are too small for use in practical fabrications that involve common lasers, two trends have emerged that have furthered laser microfabrication with TPA. One is the design of novel compounds with large TPA cross sections.9,10 Larger TPA cross sections have been observed for bis-(diphenylamino) substituted polyenes, PPV oligomers, and, most recently, dendrimers based on bis-(diphenylamino) stilbene repeat units. The second trend is the use of common photopolymers that are exposed to ultrashort 0146-9592/00/151110-03$15.00/0
laser pulses with high peak power.11 Various 3D microstructures have been achieved by both methods. Some of the present authors previously reported, for what we believe was the first time, the TPA fabrication of 3D photonic crystals with submicrometer spatial resolutions by use of an intense femtosecond laser.11 Notably, the success of the fabrications was evidenced by pronounced photonic bandgap effects. However, for constructing more-complicated structures, one immediately meets the problem of component drift and distortion. To solve this problem we proposed and utilized a pre-exposure technique by which the viscosity of resins was increased and the solidif ied elements were therefore tightly conf ined at the exposure sites. Here we first investigate features of TPA photopolymerization, including the determination of the effective energy windows available for TPA fabrications, and then we present some fabrication examples from a general and a pre-exposure TPA photopolymerization. A laser of 400-nm wavelength, 150-fs pulse width, and 1-kHz repetition rate was focused into liquid resins by an objective lens of 1003 magnification and a 1.35 numerical aperture (N.A.). The sample was scanned with a 3D computer-controlled piezoelectric translator; for experimental details, see Ref. 11. The resins that we used are commercially available photopolymers: Nopcocure 800 (San Nopco, NOP-800 for brevity) and NOA series optical adhesive (Norland Products, Inc.). 1PA bands of NOP-800 and NOA-68 start from 300 and 370 nm, respectively, and go to shorter wavelengths. They are highly transparent across the entire visible spectral region, and neither the resins nor the initiators have any 1PA at 400 nm, the fabrication wavelength. © 2000 Optical Society of America
August 1, 2000 / Vol. 25, No. 15 / OPTICS LETTERS
On exposure by a tightly focused laser beam, polymerization of monomers initiated by photogenerated radicals was begun inside liquid resins. The size of solidif ied volume elements (voxels) formed by the entangling of high-concentration oligomers and polymers at the focal point depended not directly on the width of the beam waist but on the light-intensity profile from where the TPA solidif ication commences. It was further increased by the diffusion of oligomers. A 400-nm laser has a beam diameter 共F兲 of 361 nm as evaluated by the Rayleigh criterion, F 苷 1.22l兾N.A. However, voxel size increased with an increase in LPE, as shown in Fig. 1. The data were abstracted from atomic-force microscope scanning of series of solidif ied rods. Voxel size did not depend solely on the LPE; rather, it also relied on the duration of exposure and the repetition rate of the lasers and was closely related to focusing conditions (dry or wet contacting; magnification and N.A. of the objective lens). It can be seen from Fig. 1 that for different resins the dependence of voxel size on LPE is similar, and the size differences at the same relative energy may be related to the difference in, e.g., generation eff iciency and diffusion velocity of radicals in the materials used. Under a certain fabrication condition, the size of solidif ied voxels can be continuously varied by adjustment of the input laser power. However, the available LPE range was limited by an effective energy window def ined by the TPA solidif ication threshold, 共TPA兲TH , and the threshold at which boiling that results from the laser-induced breakdown begins. Under the current focusing condition, the two-photon exposure thresholds for NOP-800 and NOA 68 are approximately 共TPA兲TH 苷 0.2 and 共TPA兲TH 苷 0.5 mJ, and those for breakdown are 0.5 and 1.4 mJ, respectively. Thus the window [ratio of threshold for breakdown to 共TPA兲TH ] is a factor of 2.5 in width for NOP-800 and 2.8 in width for NOA 68. Pulse energy beyond the boiling threshold causes out-of-control photopolymerization, and the resultant structures are commonly incomplete or deconstructed. A wider window can be obtained by optimization of the fabrication wavelengths, because the breakdown was not inf luenced by, but the TPA threshold is highly sensitive to, the wavelength.8 Also, as the LPE for TPA fabrication is determined by the TPA polymerization threshold, and ultimately by the TPA cross sections of resins, one can broaden the energy window greatly by using resins with large two-photon absorptivities and a scaled-down LPE. When one uses LPE near but greater than the TPA threshold, f ine-scaled two-dimensional and 3D microelectromechanical structures can be fabricated. The spatial resolution achieved with TPA is highly desirable in technologies that require micromachined systems with advanced functions, including examination, diagnosis, repair, and treatment in narrow spaces, in vivo or in an apparatus. Here we give several examples to demonstrate the unique capability of TPA for microfabrication. Figure 2A shows a scanning-electron microscope image of a micro gear wheel. A LPE of 1.2 共TPA兲TH was used in its fabrication. Laser scanning started from the inner circle, and
1111
its outline was def ined by a prof ile function. The thickness of the wheel resulted from layer-by-layer solidif ication, where the focal point was raised at a step of 0.5 mm. The external and internal diameters were 7.0 and 1.6 mm, respectively, and the thickness was 2.3 mm. What is noteworthy is the notch, which is measured to have a tip width of not more than 0.5 mm. Such spatial resolution and structures that are well controlled in more than one dimension have not been reported before to our knowledge. Surface f latness is favored by the high stability of peak-to-peak LPE’s and of the translator movement. In a similar process, the gear-wheel pair shown in Fig. 2B was obtained. A 1-kHz repetition rate and a scanning
Fig. 1. LPE-dependent spatial resolutions of two kinds of resin. The data are from atomic-force microscope scanning of solidified rods. The LPE’s were normalized to the TPA exposure threshold of each resin. For NOP-800 and NOA-68, the values are 共TPA兲TH 苷 0.2 mJ and 共TPA兲TH 苷 0.5 mJ, respectively.
Fig. 2. Scanning-electron microscope images of A, a micro gear wheel and B, micro gear-wheel pairs.
1112
OPTICS LETTERS / Vol. 25, No. 15 / August 1, 2000
Fig. 3. Micro gear-wheel aff ixed to a shaft. The gear wheel is freely movable.
speed of 3.8 mm兾s result in spacing between exposed sites of 3.8 nm, much smaller than the voxel size. Therefore neighboring voxels overlap laterally in more than 99% of their area, enabling smoothly solidif ied rods or planes to be made. Structures such as those shown in Fig. 2 are fabricated by a multilayer packing technique. They are also achievable by conventional photolithography, except that they have a much smaller feature size. In Fig. 3 we show a real 3D microstructure, a micro gear wheel aff ixed to a shaft, which can be created only by TPA processing. To build such structures we first constructed a shaft on a hemispherical base by layer-by-layer packing. The gear wheel was located over the top end of the shaft, with their axle centers coinciding. After careful in situ development, the gear wheel fell and was trapped by the axle. For this kind of fabrication it is important to prevent the spatially separated devices from f loating away. For this purpose we employ a pre-exposure technique. The basic idea is to partly expose the resin by 1PA before TPA fabrication. Short-chain photopolymerization is expected during this process, which increases the viscosity of resins. Because photons cannot penetrate the surface to a significant depth (in 1PA wavelength), vigorous stirring of the resist can produce uniform prepolymerization. In the current fabrication method the NOA-800 is pre-exposed under a 150-W xenon lamp for 60 s with a quick stir. We can verify that the solidif ied components were tightly clamped on the exposure sites during fabrication; their movement was small enough to be neglected. The success of this fabrication is unambiguously shown in Fig. 3. The gear wheel was installed in the axle during and after developing (i.e., removal of the unsolidif ied liquid resin by acetone), and a change of its orientation relative to the axis was clearly observable under a microscope. General 3D TPA fabrication was achieved through layer-by-layer packing, and new layers always needed to be anchored upon existing surfaces. It was quite difficult to create structures with complicated lateral size distributions along the fabrication direction because of problems of f loating and distortion. In the pre-exposure technique the clamping of high-viscosity resins made it possible to introduce spatially isolated components and then assemble them. This fabrica-
tion process was effective for all the resins that we investigated, and it was much simpler than conventional techniques, such as the Lithographie Galvanik Abformung technique and surface micromachining, which require sacrificial layers. However, a problem inherent in the increase in viscosity was an increase in difficulties in developing, especially for structures with interior microholes or cavities. Therefore an appropriate degree of pre-exposure (determined by time and intensity of light) should be the least enhancement of viscosity at which solidif ied components will be anchored in the liquid resins. Such a degree is related primarily to material properties and is only slightly inf luenced by structure size. In summary, we have demonstrated exquisite microelectromechanical devices achieved by TPA photopolymerization. The feature size (inner circles and tips of gear wheels) in our structure is much smaller than those that were reported previously. Furthermore, freely movable components have been achieved, for the first time to our knowledge, by TPA microfabrication with a resin pre-exposure technique, which opens the door to the creation of real 3D spatial structures with complicated geometries. This research was supported in part by the Satellite Venture Business Laboratory of the University of Tokushima. H. Misawa’s e-mail address is misawa@ eco.tokushima-u.ac. References 1. See, for example, S. Maruo and S. Kawata, in Proceedings of the IEEE International Workshop on Micro Electro Mechanical Systems (MEMS’97) (Institute of Electrical and Electronics Engineers, Piscataway, N.J., 1997), p. 169. 2. S. Maruo and S. Kawata, J. Microelectromech. Syst. 7, 411 (1998). 3. K. Ikuta, S. Maruo, Y. Fukaya, and T. Fujisawa, in Proceedings of the IEEE International Workshop on Micro Electro Mechanical Systems (MEMS’98) (Institute of Electrical and Electronics Engineers, Piscataway, N.J. 1998), p. 131. 4. E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987). 5. W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73 (1990). 6. Y. R. Shen, The Principles of Nonlinear Optics (Wiley, New York, 1984). 7. S. Maruo, O. Nakamura, and S. Kawada, Opt. Lett. 22, 132 (1997). 8. G. Witzgall, R. Vrijen, E. Yablonovitch, V. Doan, and B. J. Schwartz, Opt. Lett. 23, 1745 (1998). 9. M. Albota, D. Beljonne, J.-L. Bradas, J. E. Ehrlich, J.-Y. Fu, A. A. Heikal, S. E. Hess, T. Kogel, M. D. Levin, S. R. Marder, D. McCord-Maughon, J. W. Perry, H. Rackel, M. Rumi, G. Subramaniam, W. W. Webb, X.-L. Wu, and C. Xu, Science 281, 1653 (1998). 10. B. H. Cumpston, S. P. Anaanthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I.-Y. S. Lee, D. McCord-Maughon, J. Qin, H. Röckel, M. Rumi, X.-L. Wu, S. R. Marder, and J. W. Perry, Nature 398, 51 (1999). 11. H.-B. Sun, S. Matsuo, and H. Misawa, Appl. Phys. Lett. 74, 786 (1999).
