One-Step Mask-Based Diffraction Lithography for the Fabrication of 3D Suspended Structures
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We propose a novel one-step exposure method for fabricating three-dimensional (3D) suspended structures, utilizing the diffraction of mask patterns with small line width. An optical model of the exposure process is built, and the 3D light intensity distribution in the photoresist is calculated based on Fresnel-Kirchhoff diffraction formulation. Several 3D suspended photoresist structures have been achieved, such as beams, meshes, word patterns, and multilayer structures. After the pyrolysis of SU-8 structures, suspended and free-standing 3D carbon structures are further obtained, which show great potential in the application of transparent electrode, semitransparent solar cells, and energy storage devices.
KeywordsThree-dimensional suspended structure Diffraction Carbon microelectromechanical systems Pyrolytic carbon structures
Carbon microelectromechanical systems
3D carbon microelectromechanical system (C-MEMS) structures have drawn more and more attentions owing to their excellent chemical stability, electrochemical activity, and biocompatibility [1, 2, 3, 4, 5]. Suspended carbon structures are the typical 3D C-MEMS structures free of any intermolecularity , presenting significant advantages in sensors [6, 7], microelectrodes [8, 9], and energy storage applications . Various C-MEMS microstructures have been achieved through pyrolysis of polymer, in which SU-8 is the most widely used precursor for pyrolytic carbon structures [10, 11]. With respect to its low light absorption, it is easy to fabricate high aspect ratio microstructures with SU-8 . However, it is still a great challenge to obtain suspended polymer template.
Diverse approaches have been developed to fabricate suspended microstructures, such as E-beam writer [13, 14, 15], X-ray [10, 16], and two-photon lithography [17, 18, 19]. Two-photon lithography is a feasible way for achieving complex suspended structures, such as suspended hollow microtubes, with great accuracy but low efficiency . Taking the efficiency and cost into account, UV lithography could be a better choice for fabricating photoresist precursor. Multi-step lithography process with controlled exposure dose for fabricating suspended structures has been demonstrated [3, 6, 7, 20]. Lim et al.  fabricated suspended nanowires and nanomeshes using a two-step UV lithography process and obtained glassy carbon nanostructures through a pyrolysis process. Some one-step lithography methods have also been proposed. No et al.  achieved suspended microstructures by a single exposure process, during which an optical diffuser film was put on the Cr-masks. The diffuser film had a significant impact on the exposure process, leading to the deformation of photoresist patterns. Long et al.  successfully fabricated 3D suspended structures by controlling the exposure dose and air gap between the photoresist and photomask during the proximity exposure process, whereas the proximity exposure mode limited the fabricating resolution. Grayscale photolithography has also been applied in fabricating suspended structures with grayscale masks or maskless lithography systems [11, 23]. Since SU-8 is almost transparent when the light wavelength is above 350 nm , it is very difficult to control the accuracy of the thickness of the suspended layer by adjusting the exposure dose [8, 10]. Hemanth et al.  optimized the UV wavelength in the exposure process according to the properties of SU-8. They chose the UV wavelength of 405 nm for the high ratio microstructures and 313 nm for the suspended layer. However, the combination of exposure with different UV light wavelengths increases the costs and difficulties of the whole fabrication process.
In this study, we demonstrate a novel one-step mask-based diffraction lithography process that is compatible with most kinds of photoresist to fabricate 3D suspended structures. A 3D light intensity distribution is simulated in the photoresist according to Kirchhoff’s diffraction theory and further verified by experiments. The thickness of the suspended structures is controlled by the width of the patterns, and the suspended beams are broadened by stacking several line patterns side by side with proper spacing. Complex 3D suspended structures, such as beams with gradient thickness and full suspended meshes with word patterns, can be achieved by the one-step lithography process. Finally, the suspended carbon beams, meshes, and free-standing carbon meshes have also been obtained via a pyrolysis process.
Methods and Experiments
Optical Model of Diffraction Lithography
where (x, y, z) equals the coordinate of P1.
Results and Discussions
Light Intensity Distribution
Suspended Photoresist Structures
Then, we introduce absorption coefficient α in optical model and perform the calculations with formula (8). The results under α = 0.0374 μm−1 (the absorption coefficient of NR21-25000P at 365 nm, tested by a UV-visible spectrophotometer, UV 2600, Shimadzu Co., Ltd.) are shown in Fig. 4l, where the fitted line with R2 = 0.986 agrees well with the experimental results of NR26-25000P. Thus, our method is available for almost all kinds of thick negative photoresist to fabricate suspended structures with one-step exposure, in which the exposure depth can be guided through simulations.
Compared with previous works [2, 11, 22, 23], we form a 3D light intensity distribution model in the photoresist by utilizing the diffraction of the small mask patterns. The 3D suspended structures can be well controlled and forecasted by simulations. The absorption coefficient of the photoresist is also taken into account here. Suspended structures with various thicknesses, such as gradient beams, are formed easily through the one-step exposure. Moreover, the exposure process is performed with an ordinary mask in a typical contact exposure mode, and no special masks or equipment is needed, exhibiting excellent compatibility with high fabrication resolution.
Pyrolytic Carbon Structures
In summary, we demonstrated the fabrication of suspended structures via a novel one-step mask-based diffraction lithography method. The 3D light intensity distribution in the photoresist was simulated, showing that the exposure depth increased with the increase of the width of the line patterns under d < 5 μm. This phenomenon could be utilized to fabricate suspended structures with defined thickness of SU-8 photoresist, which was almost transparent and hard to form suspended structures with grayscale lithography. The corresponding experiments were also conducted here. We found that the thickness of the suspended SU-8 beams was very close to the simulation results, while that of the NR26-25000P was much thinner than the exposure depth in the simulations. This was caused by the high light absorption property of NR26-25000P. Then, the absorption coefficient of photoresist was introduced in the optical model, and the simulation results agreed well with the experiments. Three different cross connection patterns were designed for fabricating suspended 3D meshes with or without supporting pillars, and the surface textures were well replicated. Meshes with pillars and full suspended meshes were also successfully achieved. Other complex 3D suspended photoresist structures, including suspended beams with gradient thickness, suspended concentric rings, and suspended word structures, were obtained through the one-step mask-based diffraction lithography.
Carbon suspended structures and free-standing carbon meshes were further fabricated with a typical two-step pyrolysis process. The suspended 3D carbon structures could be applied in electrochemical electrode, supercapacitor, and sensors owing to their large surface area. The free-standing meshes exhibited excellent conductivity, flexibility, and high transparency. Thus, we developed a simplified and promising method for the fabrications of 3D suspended structures and carbon meshes, which showed great potential in the applications of transparent electrode, semitransparent solar cells, and energy storage devices.
The authors acknowledge the Micro and Nano Fabrication and Measurement Laboratory of Collaborative Innovation Center for Digital Intelligent Manufacturing Technology and Application for the support in SEM test. Thanks to Mr. Huang Guang in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support of MJB4 operation.
This work is supported by the National Natural Science Foundation of China (Grant Nos. 51805195, 51675209, and 51675210) and the China Postdoctoral Science Foundation (Grant Nos. 2017M612448 and 2016M602283).
Availability of Data and Materials
The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.
XHT and GLL designed the experiments. XHT and JBL performed the experiments and the calculation. XHT and GLL drafted the manuscript. Other authors contributed to the data analysis and the manuscript modification. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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