Improved spatial resolution and surface roughness in photopolymerization-based laser nanowriting Kenji TakadaHong-Bo SunSatoshi Kawata
Citation: Appl. Phys. Lett. 86, 071122 (2005); doi: 10.1063/1.1864249 View online: http://dx.doi.org/10.1063/1.1864249 View Table of Contents: http://aip.scitation.org/toc/apl/86/7 Published by the American Institute of Physics
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APPLIED PHYSICS LETTERS 86, 071122 共2005兲
Improved spatial resolution and surface roughness in photopolymerizationbased laser nanowriting Kenji Takada Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan
Hong-Bo Suna兲 Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan and PRESTO, Japan Science and Technology Agency (JST), Japan
Satoshi Kawata Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan and JST-CREST, Japan and RIKEN, Hirosawa, Wako, Saitama 351-0198, Japan
共Received 20 October 2004; accepted 17 December 2004; published online 11 February 2005兲 The fundamental issues on the smallest possible processing accuracy and the best feasible surface smoothness in pinpoint polymerization-based laser fabrication were experimentally investigated. The lateral spatial resolution is improved from the previously reported value, 120 nm, to around 100 nm by intentionally introducing radical quenchers in the resin. The roughness measured from 10 m ⫻ 10 m surface areas were averaged to 4 – 11 nm, which is found slightly affected by the laser pulse energy but independent on the scanning pitch when it is smaller than a critical value. The surface quality of this level could fully satisfy the requirement of various photonic elements and devices. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1864249兴 The past several years has witnessed a rapid progress and a wide spread of femtosecond 共fs兲 laser nanofabrication technology.1–4 Particularly pinpoint two-photon absorption induced photopolymerization has been established as currently the sole tool that is capable of producing threedimensional 共3D兲 photonic crystals with arbitrarily designed primitive units, affording a new approach for precisely tailoring the distribution of electromagnetic modes in both real and k spaces.2,5,6 The present research interest in the community is, in material aspect, introduction of functional compositions to photopolymerizable resins so that molecular attributes could be either directly imparted to device performance7 or coupled with fine structures of the fabrication to create functions.8 In the optical aspect, a parallel processing system that utilizes a microlens array for beam splitting has made it possible to simultaneously manufacture more than two hundreds of 3D micro-objects, meaning a two-order increase of the processing efficiency.9 Despite these tremendous efforts towards more complex construction, better performance and higher yield of production, some fundamental issues on the pinpoint two-photon photopolymerization remain open problems. For example, a 120 nm lateral spatial resolution has been achieved in our previous study as a historical record of subdiffraction-limited laser fabrication by use of radical quenching effect.10,11 It is interesting to know to what degree the processing accuracy could be further improved and what are the constraint factors. Moreover, the quality of the surface and interface is essential for optical elements, on which the similar questions may be asked: how smooth of a pinpoint photopolymerized surface or interface could reach and how the roughness may be minimized? The answer to these questions needs deep insight into the microscale photopolymerization process, which will not only promote the application of the technology to broader fields, but also possibly initiate breakthroughs a兲
Author to whom correspondence should be addressed; electronic mail:
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to the technology itself. In this letter, we present a systematic study on the extremity issues of the spatial resolution and the surface roughness, which led to answers to the above questions. Figures 1共a兲 and 1共b兲 show the scanning electron microscopic 共SEM兲 images of voxels that were two-photon polymerized from thin films of the liquid resin 共SCR500, JSR兲. The layer is estimated to be 7 m according to the interference fringe exhibited in the voxels in Fig. 1共b兲, a thickness within the diffusion length of oxygen molecules in polyurethane acrylate resin, ⬃30 m.12 An enhanced radical
FIG. 1. Spatial resolution of two-photon polymerization after the function of radical quenchers. The exposure was conducted with 780 nm wavelength, 80 fs pulsewidth, 80 MHz repetition rate Ti: sapphire laser. The laser beam was tightly focused by a high numerical aperture 共NA, ⬃1.4兲 objective lens: 共a兲 and 共b兲 voxels produced in thin resin film of different thicknesses; 共c兲 the exposure duration dependent lateral resolution under different resin compositions: 共夝兲 and 共䊏兲 are with 0.1 wt % and 0.01 wt % the additional initiator, respectively; 共兲 general resin, and 共쎲兲 with 0.8 wt % the additional radical quencher; 共d兲 the 100-nm-width line achieved with optimized quencher concentration. The inset shows the focusing geometry, by which the full width of the rods could be correctly reflected by SEM observation.
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FIG. 2. Microstructures fabricated with optimized resin composition: 共a兲 a microchain; 共b兲 microbull of different size: 共A兲 ⬃10 m, 共B兲 ⬃7 m, and 共C兲 ⬃4 m in length. The magnified 4 m microbull is shown in 共c兲.
quenching effect due to the relative high concentration of dissolved atmospheric oxygen is then expected, as is actually the purpose of the current experiment. However, the expected reduction of the minimum voxel size has not been reproducibly acquired although an increase of polymerization threshold is observed from time to time, a proof of the quenching effect. This result reveals the necessity to alter materials composition, and therefore their characteristics, in a well-controlled manner. For precise regulation to the volume of photopolymerization around the 3D focal spot, we add an additional radical quencher and an initiatior to the resin. It is understandable that the concentration decrease of EA4BPA,13 the initiator, from 0.1 wt %, 0.01 wt %, to 0%, reduces the minimum lateral voxel size from 404 nm 共 ⬃ 1 ms兲, 317 nm 共16 ms兲, to 154 nm 共2 ms兲 关Fig. 1共c兲兴 because the volume where radicals survive the scavenging gradually shrinks, where is the exposure duration per voxel. It is worth mentioning that in order to increase the robustness of polymerized voxels against developing, we made continuous voxels scanning 关Fig. 1共d兲兴. Special care has been taken to guarantee the center of the focal spot is above the substrate so that lateral size of voxels is correctly measured.14 Use of the radical quencher keeps the tendency of voxel size reduction and the smallest lateral resolution15 that could be obtained is ⬃100 nm 关Fig. 1共d兲兴, when the concentrations of the additional initiator and quencher are 0% and 0.8 wt %, respectively. The delicate control to voxel size involves three factors: absorbed photon number and their spatial distribution, which are governed by and NA, respectively, and the concentrations of the initiator and the quencher. In a static model, small voxels seems to be realizable by reducing either laser pulse energy or the initiator concentration. However, if dynamic processes like radical and quencher diffusion are considered, the use of quenchers is quite advantageous to eliminate the diffused radical for a tighter confinement to the resultant photopolymerization reaction. The nearly 20% improvement of the fabrication accuracy would reduce the dimension of polymerized objects in principle by the same percentage. Figure 2 shows the SEM images of a microchain and the microbulls fabricated with the improved precision. In case of the bull 关Fig. 2共b兲兴, its size reduces from the original 10 m length 共the inset A and also Ref. 10兲, to 7 m 共B兲 and 4 m 共C兲. 100 nm is not considered as a limit of the resolution. Voxel size reduction challenges the mechanical strength of a
FIG. 3. Cube structures for testing the surface roughness under different voxel distance or depicting pitch: 共a兲 400 nm⫻ 400 nm, 共b兲 300 nm ⫻ 300 nm, 共c兲 200 nm⫻ 200 nm, 共d兲 100 nm⫻ 100 nm, 共e兲 50 nm ⫻ 50 nm, 共f兲 tilted view of the 50 nm cube.
polymer because any useful mechanical properties are a consequence of their high molecular weight.16 Further decrease of voxels size only means softer polymerized parts and less clear interface between the liquid and solid phases. Therefore the smallest voxel size one may use in practical fabrication depends on needs and material characteristics. The physical and chemical properties of a solid polymer is strictly determined by the nature and the structure of the oligomer, which constitutes the backbone of the polymer network after curing. A monomer acts as a diluent to reduce the resin viscosity and participates in the polymerization process. These two components generally occupy a large percentage, e.g., ⬃98 wt % of a resin. Therefore as a general solution towards higher spatial resolution, one may enhance the polymer rigidity by high-degree crosslinking, or by designing polymer chains with strong intermolecular forces, for example, polyamides and polyesters develop larger strength at lower molecular weights than polymers having weaker intermolecular forces, for example, polyethylene. Surface roughness is another important issue to address since it significantly affects the surface forces that dominate in nanomechanical devices and micromachines, and as for photonic elements like waveguides, mirrors, and switches, coarseness-induced scattering is the principal source of power dissipation. These devices, however complicated their designs are, are realized through point-by-point scanning; therefore, the vestige of voxels is reasonably considered as the prime factor to cause the surface roughness. As a test model, a series of cube structures of 12 m ⫻ 12 m surface area, a size range of interest for micronanosystems, were designed and polymerized with different planar voxel distance 共D兲, or pitch 共Fig. 3兲. It is obvious that when D is larger than a certain value, e.g., ⬃100 nm for the current condition: NA= 1.4 focusing, photon flux density p
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FIG. 4. Mechanism of roughness formation: 共a兲 R–D plot. The insets are AFM images of tube surface with 共A兲 D = 300 nm, 共B兲 100 nm, 共C兲 50 nm, and 共D兲 a spincoated polymer surface; 共b兲 illustrative model to show the voxel-induced roughness cannot propagate to the sample surface when D ⬍ Ds; 共c兲 therefore ambient and material factors should be responsible for the surface unflatness.
= 2.0 MW/ cm2, and exposure duration, = 1 ms/ voxel, the scanning trace governs the surface quality. That is the case of Figs. 3共a兲–3共c兲. In order to make quantitative characterization, we use atomic force microscope 共AFM兲 to analyze the sample surface, for which the sampling was conducted at 256⫻ 256 points on 10 m ⫻ 10 m area. Moreover, we used the definition of the surface roughness, R, as followed: R=
1 S0
冕冕
122%, 243%, and 486% for 共a兲–共e兲, respectively. On the other hand, liquid resins in the concaves boarded by voxel ends 关Fig. 4共b兲兴 is difficult to remove due to the surface Gibbs free energy that is minimized by the negative curvature. As a result, the roughness propagated to the polymerized surface becomes negligible compared with those caused by other factors when the overlapping degree is high enough, or equivalently the pitch is sufficiently small. This is the origin of the critical scanning pitch, which is obviously a function of NA, laser pulse energy and exposure duration. The roughness of a surface spin-coated with the same resin is ⬃1 nm 关Fig. 4 共A兲-D兴, while surfaces scanned at varied laser wavelengths, focusing, and exposure conditions were at the range of 4 – 11 nm. The Rs was found insensitive to the variation of the above parameters but slightly affected by the laser power, e.g., Rs = 6.7, 7.5, and 8.3 nm for the photon flux density of 1.7, 6.2, and 8.9 MW/ cm2, respectively. The larger roughness may arise from 共i兲 nonuniform surface relief occurring during the material solidification, to which the surface protrusions in Fig. 4共a兲-C, features much larger than voxel size, could be attributed; 共ii兲 ambient factors, e.g., vibration may cause voxel displacement 关Fig. 4共c兲兴; and 共iii兲 jittering of the laser output induces the voxel size fluctuation 关Fig. 4共c兲兴, which should be the origin of the weak power dependence In summary, a 100-nm-lateral voxel size has been achieved in pinpoint two-photon polymerization based laser nanofabrication, which is not considered as the lower limit of the processing accuracy. The photopolymerized surface roughness tend to be minimized at several nanometers, or 1 / 50– 1 / 100 visible or near-infrared wavelengths, which could satisfy the requirements of various photonic and optoelectronic devices to their surface and interfaces. S. Maruo, O. Nakamura, and S. Kawata, Opt. Lett. 22, 132 共1997兲. H.-B. Sun, S. Matsuo, and H. Misawa, Appl. Phys. Lett. 74, 786 共1999兲. J. R. Qiu, Chem. Record 4, 50 共2004兲. 4 H.-B. Sun, Y. Xu, S. Juodkazis, K. Sun, M. Watanabe, S. Matsuo, H. Misawa, and J. Nishii, Opt. Lett. 26, 325 共2001兲. 5 K. Kaneko, H.-B. Sun, X.-M. Duan, and S. Kawata, Appl. Phys. Lett. 84, 2091 共2003兲. 6 C. M. Soukoulis, Photonic Crystals and Light Localization in the 21st Century, NATO Science Series, Ser. 3, Vol. 563 共Kluwer Academic, Dordrecht, 2000兲. 7 F. Stellacci, C. A. Bauer, T. Meyer-Friedrichsen, W. Wenseleers, V. Alain, S. M. Kuebler, S. J. K. Pond, Y. D. Zhang, S. R. Marder, and J. W. Perry, Adv. Mater. 共Weinheim, Ger.兲 14, 194 共2002兲. 8 S. Yokoyama, T. Nakahama, M. Miki, and S. Mashiko, Appl. Phys. Lett. 82, 3221 共2003兲. 9 J. Kato, N. Takeyasu, Y. Adachi, H.-B. Sun, and S. Kawata, Appl. Phys. Lett. 86, 044102 共2005兲. 10 S. Kawata, H.-B. Sun, T. Tanaka, and T. Kenji, Nature 共London兲 412, 697 共2001兲. 11 T. Tanaka, H.-B. Sun, and S. Kawata, Appl. Phys. Lett. 80, 312 共2002兲. 12 C. Decker and K. Moussa, Makromol. Chem. 189, 2381 共1988兲. 13 H.-B. Sun, T. Suwa, K. Takada, R. P. Zaccaria, M. S. Kim, K.-S. Lee, and S. Kawata, Appl. Phys. Lett. 85, 3708 共2004兲. 14 H.-B. Sun, T. Tanaka, and H.-B. Sun, Appl. Phys. Lett. 80, 3673 共2002兲. 15 Here only the lateral spatial resolution is discussed since the longitudinal one is not independently adjustable. At the near-threshold level, the later is roughly 3 times of the former 共Refs. 14 and 17兲. 16 D. I. Bower, An Introduction to Polymer Physics 共Cambridge University Press, Cambridge, 2002兲. 17 H.-B. Sun, K. Takada, M. S. Kim, K. S. Lee, and S. Kawata, Appl. Phys. Lett. 83, 1104 共2003兲. 1 2 3
兩f共x,y兲 − z0兩dxdy,
共1兲
where S0 is the scanned area, f共x , y兲 is the height at each point, and z0 the average height. From Fig. 4共a兲, we confirmed the pitch-dependent roughness, for example, R falls rapidly from 76 to 9 nm as the pitch decreases from 400 to 100 nm. It is also interesting to notice that the roughness tends to saturate, to a value we may define as Rs, when the pitch is smaller than the critical value Dc. Both SEM and AFM measurement demonstrate the surfaces scanned with 100 nm 关Figs. 3共d兲 and the inset of 4共a兲-B兴 and 50 nm 关Figs. 3共e兲 and 4共a兲-C兴 pitches are almost identical. Judging from Fig. 4共a兲, Rs ⬃ 8 nm. The existence of the critical pitch, Dc, and saturated roughness, Rs could be interpreted by examining how a surface is piled out of voxels in two dimensions. Voxels take ellipsoid shape that is associated with the NA of the focusing lens, and their size is delicately determined by the laser pulse energy and the exposure time.17 Therefore, the roughness dependence on the pitch could be attributed as different degrees of voxels overlapping. The gradually improved surface quality in Fig. 3 corresponds to the overlapping degrees, defined as the ratio of voxel size to the pitch,5 from 61%, 81%,
