本研究以無光罩式數位微影技術為基礎，利用步進與灰階曝光的方式，在光阻表面製作出三維的立體微結構。研究方法是藉由控制數位微反射鏡裝置 (Digital Micro-mirror Device, DMD) 的脈衝寬度調變 (Pulse-Width Modulation, PWM)，精準地調控紫外 (UV) 光源投影在光阻表面的劑量，並且依據光阻的特性曲線，將目標結構的深度轉換成所需的UV劑量。結合本研究開發的數值模擬和最佳化，藉由光點的卷積疊加，精確地預估 UV 曝光劑量，以大幅降低實驗的次數。
此一步進式灰階曝光演算法，先針對正型光阻材料，以正面曝光的方式成功地在面積 35x30 mm²範圍內，製作出週期性的擴散片和非球面微透鏡陣列。為了抑制結構表面的粗糙度，引入熱回流技術 (Thermal Reflow)，將表面粗糙度 Ra 值從21.1 nm 降低到10.6 nm，同時保持整體結構形貌的精確度。
最後，本研究也針對負型光阻材料，開發出背後曝光的技術，在石英基板上塗佈的光阻層上直接製作出二種不同型貌的非球面微透鏡陣列。為了驗證微透鏡陣列的光學聚焦品質，本研究建立一套光學聚焦檢測系統，得出兩種非球面微透鏡陣列聚焦光點的半峰全寬 (FWHM) 分別為15 和 5 μm，驗證本研究有能力製作光學元件等級的三維微結構。

This study is based on maskless digital lithography technology and a stepwise grayscale exposure method to fabricate three-dimensional (3D) microstructures on a photoresist (PR) layer. It involves precise control of the spatial distribution of ultraviolet (UV) light dose projected onto the photoresist surface by using pulse-width modulation (PWM) of a digital micro-mirror device (DMD). Numerical simulation and optimization algorithm are developed to determine the UV dose distribution and the DMD’s grayscale images based on deconvolution calculation and the PR’s characteristic curve.
The proposed stepwise grayscale exposure method is applied for fabricating two kinds of periodic microstructures, namely, diffusers and aspherical microlens arrays. A positive PR is used with UV exposure on the PR surface directly. Accurate 3D profiles are successfully obtained over an area size of 35x30 mm². Thermal reflow technology is used to reduce the average surface roughness (Ra) from originally 21.1 nm to 10.6 nm.
As for negative types of PR, a back-side exposure technique is developed in which the UV light is incident from the PR’s coated substrate. Two kinds of aspherical microlens arrays are directly fabricated on a quartz substrate. To verify the optical focusing quality of these fabricated microlens arrays, an optical focusing inspection system is established. The full width at half maximum (FWHM) of the focused light spots for the two types of non-spherical microlens arrays are measured to be 15 μm and 5 μm, respectively. This validates the capability of the methods developed in this research to fabricate three-dimensional microstructures with the optical qualities that can be used as optical components.

[1] K. Takahashi and J. Setoyama, “A UV-exposure system using DMD.” Electron. Commun. Jpn. Part II: Electron., vol. 83, pp. 56-58, 2000.
[2] K. Keskinbora, C. Grévent, M. Bechtel, M. Weigand, E. Goering, A. Nadzeyka, L. Peto, S. Rehbein, G. Schneider, R. Follath, J. V.-Comamala, H. Yan, and G. Schütz, “Ion beam lithography for fresnel zone plates in x-ray microscopy,” Opt. Express., vol. 21, pp 11747-11756, 2013.
[3] J. Niu, M. Zhang, Y. Li, S. Long, H. Lv, Q. Liu, and M. Liu, “Highly scalable resistive switching memory in metal nanowire crossbar arrays fabricated by electron beam lithography,” J. Vac. Sci. & Technol. B, vol. 34, 02G105, 2016.
[4] R. Barbucha, M. Kocik, J. Mizeraczyk, G. Kozioł, and J. Borecki, “Laser direct imaging of tracks on PCB covered with laser photoresist,” Bull. Pol. Acad. Sci. Tech. Sci., vol. 56, 2008.
[5] B. Du, H. Zhang, J. Xia, J. Wu, H. Ding, and G. Tong, “Super-resolution imaging with direct laser writing-printed microstructures,” J. Phys. Chem. A, vol. 124, pp. 7211–7216, 2020.
[6] Texas Instruments, “DLP6500 0.65 1080p MVSP Type A DMD,” DLPS040A datasheet, Oct. 2014 [Revised Oct. 2016].
[7] 簡弘量, “光點陣列斜掃描無光罩式微影系統開發,” 國立成功大學機械工程學系博士論文, 2020.
[8] L. Huang, C. Liu, H. Zhang, S. Zhao, M. Tan, M. Liu, Z. Jia, R. Zhai, and H. Liu, “Technology of static oblique lithography used to improve the fidelity of lithography pattern based on DMD projection lithography,” Opt. & Laser Technol., vol. 157, 108666, 2023.
[9] K. F. Chan, Z. Feng, R. Yang, A. Ishikawa, and W. Mei, “High-resolution maskless lithography,” J. Microlith. Microfab. Microsyst., vol. 2, pp. 331-339, 2003.
[10] C. Peng, Z. Zhang, J. Zou, and W. Chi, “A high-speed exposure method for digital micromirror device based scanning maskless lithography system,” Optik, vol. 185, pp. 1036-1044, 2019.
[11] K. Kim, S. Han, J. Yoon, S. Kwon, H.-K. Park, and W. Park, “Lithographic resolution enhancement of a maskless lithography system based on a wobulation technique for flow lithography,” Appl. Phys. Lett., vol. 109, 234101, 2016.
[12] J. Liu, J. Liu, Q. Deng, J. Feng, S. Zhou, and S. Hu, “Intensity modulation based optical proximity optimization for the maskless lithography,” Opt. Express, vol. 28, pp. 548-557, 2020.
[13] Q. Deng, Y. Yang, H. Gao, Y. Zhou, Y. He, and S. Hu, “Fabrication of micro-optics elements with arbitrary surface profiles based on one-step maskless grayscale lithography,” Micromachines, vol. 8, 314, 2017.
[14] K. Zhong, Y. Gao, F. Li, N. Luo, and W. Zhang, “Fabrication of continuous relief micro-optic elements using real-time maskless lithography technique based on DMD,” Opt. & Laser Technol., vol. 56, pp. 367-371, 2014.
[15] X. Ma, Y. Kato, F. Kempen, Y. Hirai, T. Tsuchiya, F. Keulen, and O. Tabata, “Multiple patterning with process optimization method for maskless DMD-based grayscale lithography,” Procedia Eng., vol. 120, pp. 1091-1094, 2015.
[16] N. Luo, and Z. Zhang, “Fabrication of a curved microlens array using double gray-scale digital maskless lithography,” J. Micromech. Microeng., vol. 27, 035015, 2017.
[17] A. Kaltashov, P. K. Parameshwar, N. Lin, and C. Moraes, “Accessible, large-area, uniform dose photolithography using a moving light source,” J. Micromech. Microeng., vol. 32, 027001, 2021.
[18] H. Chien and Y. Lee, “Three dimensional maskless ultraviolet exposure system based on digital light processing,” Int. J. Precis. Eng. Manuf., vol. 21, pp. 937-945, 2020.
[19] M. N. Hasan, D.-H. Dinh, H.-L. Chien, and Y.-C. Lee, “Maskless beam pen lithography based on integrated microlens array and spatial-filter array,” Opt. Eng., vol. 56, 115104, 2017.
[20] D.-H. Dinh, H.-L. Chien, and Y.-C. Lee, “Maskless lithography based on digital micromirror device (DMD) and double sided microlens and spatial filter array,” Opt. & Laser Technol., vol. 113, pp. 407-415, 2019.
[21] M. A. Smith, S. Berry, L. Parameswaran, C. Holtsberg, N. Siegel, R. Lockwood, M. P. Chrisp, D. Freeman, and M. Rothschild, “Design, simulation, and fabrication of three-dimensional microsystem components using grayscale photolithography,” J. Micro/Nanolithogr., MEMS, MOEMS, vol. 18, 043507, 2019.
[22] L. Mosher, C. M. Waits, B. Morgan, and R. Ghodssi, “Double-exposure grayscale photolithography,” J. Microelectromech. Syst., vol. 18, pp. 308-315, 2009.
[23] B. Badawi, O. Sayadi, I. Eisele, and C. Kutter, “Three-state lithography model: an enhanced mathematical approach to predict resist characteristics in grayscale lithography processes,” J. Micro/Nanopatterning, Mater. Metrol., vol. 20, 014601, 2021.
[24] Y. Hirai, K. Sugano, T. Tsuchiya, and O. Tabata, “A three-dimensional microstructuring technique exploiting the positive photoresist property,” J. Micromech. Microeng., vol. 20, 065005, 2010.
[25] X. Ma, Y. Kato, F. Kempen, Y. Hirai, T. Tsuchiya, F. Keulen, and O. Tabata, “Multiple patterning with process optimization method for maskless DMD-based grayscale lithography,” Procedia Eng., vol. 120, pp. 1091-1094, 2015.
[26] N. Xiang, H. Yi, K. Chen, S. Wang, and Z. Ni, “Investigation of the maskless lithography technique for the rapid and cost-effective prototyping of microfluidic devices in laboratories,” J. Micromech. Microeng., vol. 23, 025016, 2013.
[27] Z.-J. Lian, S.-Y. CHung, M.-H. Shen and H. Yang, “Maskless lithography based on digital micromirror device (DMD) and double sided microlens and spatial filter array,” Microelectronic Eng., vol. 115, pp. 46-50, 2014.
[28] S. Huang, M. Li, L. Shen, J. Qiu, and Y. Zhou, “Fabrication of high quality aspheric microlens array by dose-modulated lithography and surface thermal reflow,” Opt. & Laser Technol., vol. 100, pp. 298-303, 2018.
[29] C. Yuan, K. Kowsari, S. Panjwani, Z. Chen, D. Wang, B. Zhang, C. J.-X. Ng, P. V. Y. Alvarado, and Q. Ge, “Ultrafast three-dimensional printing of optically smooth microlens arrays by oscillation-assisted digital light processing,” ACS Appl. Mater. & Interfaces, vol. 11, pp. 40662-40668, 2019.
[30] C. Sun, N. Fang, D. Wu, and X. Zhang, “Projection micro-stereolithography using digital micro-mirror dynamic mask,” Sens. Actuator A: Phys., vol. 121, pp. 113-120, 2005.
[31] A. Rammohan, P. K. Dwivedi, R. M-Duarte, H. Katepalli, M. J. Madou, and A. Sharma, “One-step maskless grayscale lithography for the fabrication of 3-dimensional structures in SU-8,” Sens. Actuators B: Chem., vol. 153, pp. 125-134, 2011.
[32] D. G. Kasi et al., “Rapid prototyping of organ-on-a-chip devices using maskless photolithography,” Micromachines, vol. 13, 49, 2022.
[33] 許永昕, “利用斜掃描與頻閃技術之高精度無光罩式微影系統的開發與應用,” 國立成功大學機械工程學系博士論文, 2023.
[34] J. A. Nelder and R. Mead, “A simplex method for function minimization,” J. Comput., vol. 7, pp. 308-313, 1965.