A low-cost fabrication method for a high-surface-quality glass microlens array (MLA) was proposed using a glass molding technique with a vitreous carbon (VC) mold. A VC mold with a high-surface-quality MLA cavity was fabricated, and the glass MLA with a root mean square surface roughness of 4.59 nm was replicated using the VC mold. To obtain the glass MLA with high replication quality, the effects of molding conditions were examined. The surface quality was not degraded during the proposed VC mold fabrication method and glass molding process. The focused beam spot of the glass molded MLA was analyzed; it showed a diffraction-limited characteristic of the glass molded MLA.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The microlens array (MLA) is an important optical component in various application fields such as data storage , optical communication , imaging , illumination , and display systems . A polymer replication process including injection molding , hot embossing , ultraviolet (UV) molding , and roll-to-roll replication  has been used for mass production of MLA because of its cost effectiveness. However, the application of polymer MLA is limited when high optical, thermochemical, or mechanical properties are required. A glass MLA exhibits many advantages over polymer MLA, such as wide selection of refractive index, high transmittance, low temperature coefficient of refractive index, low thermal expansion coefficient, and excellent acid, moisture, and wear resistance [10–12]. Glass MLA has been fabricated by etching or machining processes on glass substrate, including the pattern transfer method with a lens-shaped photoresist barrier  and laser machining [11, 12]. However, it is very difficult to obtain a precise glass MLA at low cost using these methods, because the machining (etching) rate of glass is considerably low and is highly sensitive to process conditions.
The glass molding process can be regarded as the most promising method for mass-production of precise glass MLA, in which a glass substrate and mold are heated to the glass softening temperature and press to replicate the shape of the mold cavity on the glass substrate. Y. Chen et al. fabricated glass MLA using a reflow process of a glass molded micro-cylinder array with a vitreous carbon (VC) mold having a micro-hole array . G. C. Firestone et al. fabricated a glass molded MLA using an incomplete filling technique with a tungsten carbide (WC) mold having a micro-hole array . Although these methods used a mold with a micro-hole array, they obtained an MLA shape using the reflow and incomplete filling techniques. However, these additional techniques could not provide high reproducibility, and it was difficult to control the shape precisely.
To obtain high reproducibility and precision of the glass molded MLA, a mold with a negative shape of the designed glass MLA is essentially required. W. J. Choi et al. fabricated a WC mold having a negative-shape MLA cavity using powder pressure forming against the silicon MLA master, followed by a sintering process. However, the surface roughness of the mold was limited because of the size of the initial WC powder . In this study, a VC mold having a negative-shape MLA cavity with high surface quality was fabricated by carbonization of a replicated furan precursor [16, 17], and a glass MLA on a soda-lime glass substrate was obtained by a glass molding process. To fabricate the VC mold, an MLA master was fabricated by a photoresist (PR) reflow method. To protect the reflowed PR master in the next furan replication process, the MLA structure was transferred to a polymer master by a double-replication method. A furan precursor was replicated from the polymer master, and a VC mold with negative shape MLA cavity was obtained by carbonization of the furan precursor. To verify the feasibility of the proposed method, a glass MLA was fabricated by the glass molding process with soda-lime glass. The glass molding process conditions were optimized to obtain high replication quality. Finally, the surface roughness and the optical property of the fabricated glass MLA were examined.
2. Fabrication of the vitreous carbon mold
Because the mold used in the glass molding process must have superior thermal resistance, sufficient mechanical strength, and good release properties, the selection of mold materials is limited. Some materials, such as WC , silicon carbide , nickel alloy , and VC [21–23] have been proposed as the mold materials for glass molding. Among these mold materials, VC is regarded as the most suitable material because it can provide high temperature resistance, high hot hardness, extreme resistance to chemical attack, and low adhesion to the glass, which are the key characteristics required for the mold material of the glass molding process. Focused ion beam machining , electrochemical machining , and laser machining  have been used for fabricating micro- and nano-structured VC molds. However, it is extremely difficult to obtain an MLA cavity shape using these machining methods.
A progressive glass molding system (also called a die molding transfer system) has been commonly used for the mass production of glass molded optical components because of its high production rate . For the progressive glass molding system, multiple molds are essentially required. Therefore, a low-cost mold fabrication method is the most important issue for mass production of glass molded MLA.
In this study, we developed a low-cost fabrication method for VC molds having MLA cavities and applied it to the glass molding process as shown in Fig. 1. An MLA master pattern was fabricated by a thermal reflow process of a photoresist pattern. To protect the master pattern, a polymer MLA master was obtained by a double-replication process (polydimethylsiloxane (PDMS) molding and UV imprinting). A mixture of furan resin was poured on the polymer master and solidified by a thermal curing process. A VC mold was obtained by carbonizing the cured furan precursor. Finally, the glass MLA was replicated by the glass molding process using the fabricated VC mold.
2.1 Fabrication of the MLA master
Among the various methods for fabricating MLA master patterns, thermal reflow of photoresist was used in this study. In the thermal reflow process, the photoresist pattern made by photolithography was thermally treated so that the surface tension converted the pattern into the lens shape. AZ6612 positive photoresist was spin-coated on a silicon wafer at 2500 rpm for 40 s, and the spin-coated photoresist was soft-baked at 95 °C for 90 s on a hot plate. A stepper (LD-5011iA, Hitachi Ltd., Japan) with a magnification of 5:1 and an exposing intensity of 6.67 mW/cm2 was used in the lithography process. The exposure time was 0.55 s, and the post-exposure baking conditions were 115 °C for 90 s. The photoresist pedestal was reflowed at 180 °C for 180 s on a hot plate. Figure 2(a) shows the 3D surface profile obtained by atomic force microscopy (AFM) and measured scanning electron microscopy (SEM) image of the reflowed MLA master with a pitch of 12.7 μm, diameter 10.6 μm, and sag height 1.057 μm.
2.2 Fabrication of the polymer master
To obtain the VC mold with a negative-shape MLA cavity, a furan precursor with negative-shape MLA cavity is required. Although the furan precursor can be directly replicated from the reflowed MLA master, we fabricated a replicated polymer master with an MLA shape to avoid damage to the reflowed master pattern during the furan replication process. The polymer master was obtained by a double-replication process. The first replication process was performed using PDMS (Silgard 184, Dow Corning Co., USA). The base material (Silgard A) and the curing agent (Silgard B) were mixed at a ratio of 10 to 1, poured on the reflowed silicon master at a thickness of approximately 5 mm, and then cured at room temperature for 24 h. To obtain a polymer master, the second UV imprinting process was carried out from the PDMS mold. A UV-curable polyurethane acrylate (UP088, SKC Co., Republic of Korea) was poured on the PDMS mold, and primer treated polyester (PET) film (SH34, SKC Co. Ltd., Republic of Korea) was covered on the UV resin. The UV resin was exposed to UV light for 3 min. Figure 2(b) shows the 3D surface profile obtained by AFM measurement result and measured SEM image of the polymer master with a pitch of 12.7 μm, diameter 10.6 μm, and sag height 1.048 μm.
2.3 Fabrication of a Furan precursor
A VC is a non-graphitizing carbon that combines glassy and ceramic properties with those of graphite. The VC is obtained by pyrolysis of a polymer precursor with a high carbon yield, such as furan resin, phenolic resins, or cellulose [17,25]. A furan resin (KC-5302, Kangnam Chemical Co. Ltd., Republic of Korea) was selected as a polymer precursor material in this study because of its high carbon yield (~40–-50%) to minimize the shrinkage occurring in the carbonization process. A furan mixture was prepared using 89.8 wt% furan resin, 0.2 wt% of p-toluenesulfonic acid monohydrate (PTSA, Kanto Chemical Co. Inc., Japan) and 10 wt% ethanol. The furan mixture was degassed to remove the air bubbles, created during the mixing process, in a vacuum chamber for 2 h. Because the rapid curing process of furan resin can increase internal stress and block the gas escape path, a two-step curing process was employed in the replication of the furan precursor in order to minimize the warpage and air bubble defects. After pouring the furan mixture on the polymer master, the first curing process was conducted at room temperature for five days under atmospheric conditions, and the second curing was carried out at 100 °C in a convection oven. In the second curing process, the rate of temperature increase was set to 0.1 °C/min, and the temperature was maintained for 60 min for every 5 °C temperature increment until the maximum temperature was reached. After the second curing process, the back of the furan precursor was polished to obtain the desired thickness and flatness. Figure 2(c) shows the 3D surface profile obtained by AFM measurement result and the measured SEM image of the furan precursor with a pitch of 12.7 μm, a diameter of 10.6 μm, and a sag height of 1.045 μm.
2.4 Fabrication of a VC mold
The furan precursor was carbonized in a temperature history controllable tube furnace with a maximum temperature of 1200 °C (modified MIR- TB1001-2, Mirfurnace Co. Ltd., Republic of Korea). The carbonization process was conducted in a vacuum environment to protect the furan precursor against oxidation. To minimize warpage of the VC mold and escape the generated gas slowly from the precursor in the carbonization process, the heating rate was set to 0.5 °C/min up to a temperature of 600 °C, and was changed to 1 °C/min to a maximum temperature of 1000 °C under 2 × 10−2 Torr, because most of the thermal decomposition of the furan precursor occurred below 600 °C. After a 10-h holding time at the maximum temperature, the carbonized VC mold was cooled in natural cooling conditions for 24 h. The VC mold having an MLA cavity with a pitch of 9.9 μm, diameter of 8.4 μm, and sag height of 0.704 μm was obtained as shown in the AFM and SEM measurement results in Fig. 2(d). Figure 3 shows the cross-sectional surface profiles of the reflowed MLA master, polymer master, furan precursor (inverted), and VC mold (inverted) obtained by AFM measurement results. The measured pitches, diameters, and sag heights of the reflowed MLA master, polymer master, and furan precursor were similar; however, a significant decrease was observed in the VC mold because of thermal decomposition in the carbonization process. The shrinkage rate values for the pitch, diameter, and sag height of the VC mold compared with the reflowed MLA master were 22.0%, 22.2%, and 33.4%, respectively.
We also measured the surface roughness of the fabricated samples at the microlens surface. A flattening process was conducted to remove the curvature of microlens from the measured profile. We selected inspection area with a circular shape (Φ10.6 μm for reflowed master, polymer master and furan precursor, and Φ 8.4 μm for VC mold) to cover whole microlens region. The average root mean square (RMS) surface roughness of four randomly selected microlens in each sample were 2.17 nm, 14.50 nm, 15.70 nm, and 4.78 nm on the reflowed master, polymer master, furan precursor, and VC mold, respectively. Although the surface roughness increased for the polymer master and the furan precursor because of the chain size of the polymer materials, it decreased during carbonization owing to the inherent shrinkage. The measured surface roughness of the fabricated VC mold was the acceptable range for conventional optical applications. This represents clear evidence that the proposed fabrication process can provide a high-surface-quality VC mold for MLA.
3. Glass molding process
A glass MLA was molded using the fabricated VC mold. A glass molding system consisting of an infrared (IR) heater, a motor-driven pressure module, and a controller was designed and constructed. The system allows precise temperature and pressure control up to 1100 °C and 140 kgf. To prevent oxidation of the mold and glass material, the glass molding process was conducted in a vacuum environment. A low-iron soda-lime glass having a softening temperature of 726 °C and high optical transmittance (Hanglas Co. Ltd., Republic of Korea) was selected as the glass molding material. In the glass molding process, the VC mold and the glass substrate were heated to the molding temperature with a maximum heating rate of 70 °C/min, and the temperature was maintained for 10 min (holding time) in order to obtain uniform temperature distribution. After the holding time, a molding pressure was applied for 20 min. After cooling to room temperature, the glass molded MLA was removed from the VC mold. To optimize the processing condition in the glass molding, the effects of glass molding temperature and pressure on the replication quality of glass MLA were analyzed. Figure 4 (a) shows the effects of glass molding temperature on the measured sag height of glass MLA with a fixed molding pressure of 2 MPa. The sag height of glass molded MLA was measured using a confocal microscopy (OLS-4000, Olympus Co. Japan). It clearly shows the sag height of glass MLA was increased as increasing temperature, and reached to the value of VC mold cavity depth at the temperature higher than 720 °C. Since macro bubble defects occurred on the glass molded parts at 730 °C as shown in Fig. 4(b), the molding temperature of 720 °C was selected as the optimum condition. The effect of molding pressure on the replication quality of the glass molded MLA was examined at a molding temperature of 720 °C. Figure 5 shows a comparison of cross-sectional surface profiles obtained by AFM measurement between the VC mold (inverted) and the glass molded MLA with applied molding pressures of 1 MPa and 2 MPa. Although the diameters of the glass molded MLAs were the same as that of the VC mold, the sag height was 0.6 μm and 0.7 μm at molding pressures of 1 MPa and 2 MPa, respectively. The deviation of cross-sectional surface profiles between the glass molded MLA with a pressure of 2 MPa and the VC mold was negligible. This shows that the glass molding conditions with a temperature of 720 °C and a pressure of 2 MPa was sufficient to fill the MLA cavity.
Figure 6 shows the 3D surface profile obtained by AFM measurement result and the SEM image of the glass molded MLA. The measured pitch, diameter, and sag height of the glass molded MLA at the optimum glass molding conditions were 9.9 μm, 8.4 μm, and 0.699 μm, respectively, which values were nearly the same as those value of the VC mold. Figure 6 also shows that a uniform glass molded MLA with high surface quality was fabricated. The measured average RMS surface roughness value in whole microlens was 4.59 nm, which is in the acceptable range for conventional optical applications. This is clear evidence that the proposed VC mold fabrication process can provide a high-surface-quality glass MLA.
4. Analysis of optical properties
To examine the optical property of the fabricated glass MLA, a focused beam spot of glass MLA was analyzed by measuring the energy intensity distribution at the focal plane. An optical bench was composed of a laser source with a wavelength of 665 nm as an illumination unit, positioning stages, jigs, and a CCD camera having a pixel size of 9.875 μm × 9.875 μm as a detector with a magnification of 200. Figure 7 shows the measured light intensity profile of the glass molded MLA at the focal plane. It was noted that the pitch of the focused light spot is uniform and the same as the pitch of the VC mold, and the spots have uniform intensity. The measured focused beam spot diameter of the glass molded MLA was 0.95 μm, which was smaller than the theoretical spot size (1.17 μm).
A VC mold with high-surface-quality MLA cavity was fabricated by carbonization of replicated furan precursor from the reflowed MLA master, and a glass MLA with a pitch of 9.9 μm, a diameter of 8.4 μm, a sag height of 0.7 μm, and a surface roughness (RMS) of 4.59 nm was replicated using the VC mold. The effects of glass molding temperature and pressure on the replication quality were analyzed and the optimum molding temperature of 720 °C and pressure of 2 MPa was selected. To examine the optical property of the fabricated glass MLA, the intensity profile of the focused light of the glass molded MLA was measured. The glass MLA exhibited diffraction limited optical characteristics. The results show that the proposed VC mold fabrication method and glass molding process can fabricate high-surface-quality glass MLAs at low cost. Although the VC material might have lower durability than the conventional mold materials (i.e. WC), we expect the VC mold can be used at least few hundreds times at optimum molding conditions. This is acceptable for mass production of glass micro optics because multiple VC molds can be obtained from single MLA master. In addition, the VC mold does not need the repeated anti-adhesion layer (protected layer) coating process which is required in conventional WC mold for glass molding.
A National Research Foundation of Korea (NRF) grants funded by the Korean Government (MSIP) (No. 2015R1A5A1037668 and NRF-2017R1A2B4011149), the Technology Innovation Program (No. 10051636) of Korea Evaluation Institute of Industrial Technology (KEIT) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea, and the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government - Ministry of Trade Industry and Energy(MOTIE) (No. N0001075).
References and links
1. S. M. Kim, H. M. Kim, J. S. Lim, S. I. Kang, Y. H. Kim, R. Henderiks, A. Kastelijn, and C. Busch, “Elimination of Jitter in Microlens illuminated Optical Probe Array Using a Filtering Layer for the Optical Read Only Memory Card System,” Jpn. J. Appl. Phys. 45(2B), 1162–1166 (2006). [CrossRef]
2. H. Hamam, “A two-way optical interconnection network using a single mode fiber array,” Opt. Commun. 150(1-6), 270–276 (1998). [CrossRef]
3. O. Matoba, E. Tajahuerce, and B. Javidi, “Three-dimensional object recognition based on multiple perspectives imaging with microlens arrays,” in 14th Annual Meeting of the IEEE Lasers and Electro-Optics Society (2001), pp. 495–496. [CrossRef]
4. E. Bonet, P. Andrés, J. C. Barreiro, and A. Pons, “Self-imaging properties of a periodic microlens array: versatile array illuminator realization,” Opt. Commun. 106(1-3), 39–44 (1994). [CrossRef]
6. M. Chakrabarti, C. Dam-Hansen, J. Stubager, T. F. Pedersen, and H. C. Pedersen, “Replication of optical microlens array using photoresist coated molds,” Opt. Express 24(9), 9528–9540 (2016). [CrossRef] [PubMed]
7. C. Y. Chang and C. H. Yu, “A basic experimental study of ultrasonic assisted hot embossing process for rapid fabrication of microlens arrays,” J. Micromech. Microeng. 25(2), 1–11 (2015). [CrossRef]
8. J. Chen, J. Cheng, D. Zhang, and S. C. Chen, “Precision UV imprinting system for parallel fabrication of large-area micro-lens arrays on non-planar surfaces,” Precis. Eng. 44, 70–74 (2016). [CrossRef]
9. C. Y. Chang and M. H. Tsai, “Development of a continuous roll-to-roll processing system for mass production of plastic optical film,” J. Micromech. Microeng. 25(12), 1–10 (2015). [CrossRef]
10. X. Huang, P. Wang, E. Lin, J. Jiao, X. Wang, Y. Li, Y. Hou, and Q. Zhao, “Fabrication of the glass microlens arrays and the collimating property on nanolaser,” Appl. Phys., A Mater. Sci. Process. 122(649), 1–6 (2016).
11. F. Chen, H. Liu, Q. Yang, X. Wang, C. Hou, H. Bian, W. Liang, J. Si, and X. Hou, “Maskless fabrication of concave microlens arrays on silica glasses by a femtosecond-laser-enhanced local wet etching method,” Opt. Express 18(19), 20334–20343 (2010). [CrossRef] [PubMed]
12. H. K. Choi, M. S. Ahsan, D. Y. Yoo, I. B. Sohn, Y. C. Noh, J. T. Kim, D. O. Jung, J. H. Kim, and H. M. Kang, “Formation of cylindrical micro-lens array on fused silica glass surface using CO2 laser assisted reshaping technique,” Opt. Laser Technol. 75, 63–70 (2015). [CrossRef]
13. Y. Chen, A. Y. Yi, D. Yao, F. Klocke, and G. Pongs, “A reflow process for glass microlens array fabrication by use of precision compression molding,” J. Micromech. Microeng. 18(5), 1–8 (2008). [CrossRef]
14. C. Y. Huang, W. T. Hsiao, K. C. Huang, K. S. Chang, H. Y. Chou, and C. P. Chou, “Fabrication of a double-sided micro-lens array by a glass molding technique,” J. Micromech. Microeng. 21(085020), 1–6 (2011).
15. W. J. Choi, J. Y. Lee, W. B. Kim, B. K. Min, S. I. Kang, and S. J. Lee, “Design and fabrication of tungsten carbide mould with micro patterns imprinted by micro lithography,” J. Micromech. Microeng. 14(11), 1519–1525 (2004). [CrossRef]
16. H. J. Jang, M. R. Haq, Y. K. Kim, J. Kim, P. H. Oh, J. H. Ju, S. M. Kim, and J. S. Lim, “Fabrication of Glass Microchannel via Glass Imprinting using a Vitreous Carbon Mold for Flow Focusing Droplet Generator,” Sensors (Basel) 18(83), 1–9 (2018).
17. J. H. Ju, S. L. Lim, J. W. Seok, and S. M. Kim, “A method to fabricate Low-Cost and large area vitreous carbon mold for glass molded microstructures,” Int. J. Precis. Eng. Manuf. 16(2), 287–291 (2015). [CrossRef]
18. Z. Li, G. Jin, F. Fang, H. Gong, and H. Jia, “Ultrasonically Assisted Single Point Diamond Turning of Optical Mold of Tungsten Carbide,” Micromachines (Basel) 9(77), 1–11 (2018).
19. C.-Y. Huang, C.-H. Kuo, W.-T. Hsiao, K.-C. Huang, S.-F. Tseng, and C.-P. Chou, “Glass biochip fabrication by laser micromachining and glass-molding process,” J. Mater. Process. Technol. 212(3), 633–639 (2012). [CrossRef]
20. T. Zhou, J. Yan, Z. Liang, X. Wang, R. Kobayashi, and T. Kuriyagawa, “Development of polycrystalline Ni–P mold by heat treatment for glass microgroove forming,” Precis. Eng. 39, 25–30 (2015). [CrossRef]
21. S. W. Youn, M. Takahashi, H. Goto, and R. Maeda, “A study on focused ion beam milling of glassy carbon molds for the thermal imprinting of quartz and borosilicate glasses,” J. Micromech. Microeng. 16(12), 2576–2584 (2006). [CrossRef]
22. E. Nam, C. Y. Lee, M. B. Jun, and B. K. Min, “Ductile mode electrochemical oxidation assisted micromachining for glassy carbon,” J. Micromech. Microeng. 25(045021), 1–8 (2015).
23. S. F. Tseng, M. F. Chen, W. T. Hsiao, C. Y. Huang, C. H. Yang, and Y. S. Chen, “Laser micromilling of convex microfluidic channels onto glassy carbon for glass molding dies,” Opt. Lasers Eng. 57, 58–63 (2014). [CrossRef]
24. S. H. Chang, Y. M. Lee, K. H. Shin, and Y. M. Heo, “A study of the Aspheric Glass Lens Forming Analysis in the Progressive GMP Process,” J. Opt. Soc. Korea 11(3), 85–92 (2007). [CrossRef]
25. F. C. Cowlard and J. C. Lewis, “Vitreous carbon—a new form of Carbon,” J. Mater. Sci. 2(6), 507–512 (1967). [CrossRef]