Open Access
Add to BibSonomyAbstract
A thermoplastic polymer refractive microlens array has been rapidly fabricated by contactless hot embossing technology through the stainless steel template with micro through-holes array, which has a diameter of 150 µm and a pitch of 185 µm. By optimizing the technical parameters including heating and demoulding temperature, loading pressure, loading and pressure holding time, a series of high quality microlenses arrays of different sags could be obtained. In addition, the sag and the radius of curvature of the microlens are controllable. The geometrical and optical properties of the microlenses are measured and the influence of temperature and pressure duration on the optical properties of the microlenses are analysed. The results show good surface features and optical performances. Unlike previous contactless hot embossing, a low cost and durable stainless steel template was utilized instead of silicon or nickel mold to avoid valuable equipments and complicated fabrication procedure. Besides, the whole contactless hot embossing process was absence of vacuum equipment. We think that the technology could be an attractive high flexibility method for enhancing efficiency and reducing cost.
© 2015 Optical Society of America
1. Introduction
Microlens arrays are widely used for collimation, focusing and imaging of light. For all applications where miniaturization of optical device is necessary, the microlenses are an interesting alternative. As the most typical microlens, refractive microlens which refracts the incident light according to Snell’s law has gained widespread application. Therefore, its producing method is obviously an issue of concern [1–4].
Previously, refractive microlenses made of fused silica or Silicon is fabricated by standard semiconductor technologies including photolithography and etching. Although the wafer-based fabricating technology allows accurate shaping profile and precise positioning of the microlenses, the wafer process should be taken in cleanroom environment and depend on extremely valuable equipment [5–9]. In recent years, the emergence of diverse polymeric optical materials provides micro molding and micro jetting with opportunities to manufacture microlenses [10–12]. Micro molding is a potentially mass production technology and its crucial issue is the fabrication quality of the mold. The duplicated microlenses entirely depend on the morphology and surface quality of the mold [13, 14]. In order to reduce dependence on the quality of mold inner relief, IMM (Institute of Microtechnology Mainz GmbH) first developed the contactless embossing technology [15–17]. The basic mechanism is to form spherical microlenses according to surface tension caused by the applied pressure on the micro hole outside region. After the contactless embossing technology was put forward, another similar ways have been proposed, their most differences and formed results are illustrated in Table 1. The original Ni template made by LIGA with blind holes was embossed on PMMA (Polymethylmetacrylate), and the resultant f-number was larger than 2 [15, 16]. When the blind holes went through the whole template, the f-number would reduce to 1.35. Furthermore, the f-number would finally reduce to 1.2 when the vacuum device was equipped during embossing [17]. In [18,19], the Si template fabricated by DRIE (Deep reactive ion etching) with blind holes array was adopted. Without vacuum environment, the f-number of the formed PC (Polycarbonate) microlens array was larger than 1.64. The UV curable polymer microlens array fabricated by PDMS (Polydimethylsiloxane) soft mold was with blind micro holes. The minimum f-number was 1.58 [20,21]. Among these methods, for obtaining low f-number and deep sag, the through micro holes array and the additional vacuum equipment are two effective methods. Nickel mold fabricated by LIGA and Si template with blind holes fabricated by wafer process are usually applied. However, the Si through hole manufacturing technology and LIGA extremely rely on a variety of expensive equipments and a technically complex procedure. In addition, due to the fragility, Si mold possesses great possibilities to be destroyed during the loading process. Besides, additional vacuum equipment will result in structure and operation complexity and greatly increase the whole cost. Therefore, a simple made, effective and low cost template used on hot embossing equipment without vacuum apparatus is an ideal fabrication technique.
Table 1. Contactless hot embossing and their features
This paper proposed a novel contactless hot embossing technology with low cost stainless steel micro through-holes array and without additional vacuum equipment. A series of microlenses arrays with different geometrical properties can be obtained by adjusting the technical parameters. Optical and geometrical measurements of microlenses were made. The effects of processing conditions on the shape of formed microlens are also investigated.
2. Contactless hot embossing procedure
Avoiding the direct contact of the formed microlens array surface and inner relief of the template is the prominent characteristic of contactless hot embossing. When above the glass transition temperature (Tg), the thermoplastic polymer will form the microlens like shape under the combined force of the surface tension and applied pressure, as Fig. 1 shown. Firstly, the spherical profile of the microlens will gradually come into being when the microlens sag is increasing, then the steady state will achieved as soon as the sag is no longer increasing, as Fig. 1(b) shown. Through the above steps, the microlenses of a series of different sags can be made by precisely adjusting the embossing process parameters. Furthermore, if the pressure is continue to apply, the sag will not change anymore but the height of the additional cylinder will keeping on increasing until the pressure is released.
3. Experimental setup
3.1 Fabrication of the stainless steel template with micro through-holes array
The stainless steel template was fabricated by silk-screen printing and wet etching process. The silk-screen printing aims to transfer the hole pattern on the mold to the photoresist over the stainless steel sheet. After exposure and development, the sample could enter etching procedure. The wet etching solution is ferric chloride adding 0.5~1.5% hydrogen chloride. The etching rate is about 100μm/min, and the etching processing time is only about 5 minutes. Figure 2 shows the optical micrographs of the full view and × 200 zoomed view. The micro through-holes array has a diameter 150 µm and the pitch is 185 µm, the thickness is 500 µm.
3.2 Contactless hot embossing process and equipment
PMMA is the most common used optical thermoplastics polymer, which possesses three states such as glass state, rubber elastic state and viscoelastic state. When the temperature is below Tg, polymer is at glass state as hard elastic, brittle and rigid, which is applicable for optical use. When the temperature is at about glass transition temperature, polymer shows rubber elastic but no plastic flow. According to memory effect of the thermoplastics polymer, it can be embossed with large pressure but the formed shape is hard to sustain. The third state is viscoelastic state, namely plastic state, apparently shows viscous-elastic and plastic flow and is the appropriate status for hot embossing [22,23].
The glass transition temperature of PMMA is 95°C. In the range of 95~105°C, PMMA is in rubber elastic state, the distortion is reversible. When in 105~125°C, PMMA is in viscoelastic state and is in the optimal appropriate embossing status. Over high temperature will result in demoulding difficulty. Thus, the proper demoulding temperature is important. When the demoulding temperature is above 105°C, the microlens array profile will change along with the demoulding because PMMA is still in viscoelastic state. Then the height of the microlens will reduce because of memory effect. On the contrary, when the demoulding temperature is about room temperature, the template and the PMMA sample will most likely set together and give rise to demoulding difficulty. The larger demoulding force easily leads to damage on the surface of the microlens.
Figure 3 shows the self-developed hot embossing apparatus. The above-mentioned stainless steel template is installed in the holder through vacuum clamp. A sheet of PMMA of 2 mm in thickness is placed on the heating stage and underneath the template. The hot embossing process is shown as Table 2. Firstly, the template should be downward pressed to tightly contact with the PMMA sheet for ensuring uniformity in the pattern structure. Then, the process is heating. The temperature is raised from room temperature to the desired processing temperature, which should be higher than the glass transition point of the PMMA sheet (95°C). As Table 2 illustrated, the heating time is about 5, 6, 6.5, 7 min from room temperature to 100°C, 110°C, 115°C and 120°C, according to the prototype demonstration. Upon each temperature, the load will be applied on 10, 15, 20 kg/cm2 respectively. During the load process, the applied pressure and temperature are kept constant for about 20~30 min. This process is the most crucial phase that the thermoplastic polymer deforms to the spherical shape inside the stainless steel template. The next step is cooing and pressure-holding. The temperature is reduced to a demoulding point, which is set at 65°C and the cooling time is about 25, 29.5, 31.5 and 33 min. It needs to be emphasized that the pressure should be kept holding on the PMMA sample during the whole cooling phase to avoid damaging of the formed microlens. In addition, the slow cooling rate has contributed to reduce thermal stresses, so the aided cooling methods, such as forced air or water cooling, are not required. When the temperature reaches to the demoulding point, the template will be lifted and the PMMA microlens array sample will continue to drop to the room temperature. At this moment the sample can be taken out and the hot embossing process is completed.
Table 2. Technological parameters of contactless hot embossing
4. Results and discussion
4.1 Effect of process conditions on the replication quality of microlens arrays
Under different process conditions as Table 2, the corresponding formed PMMA microlens arrays can be seen in Figs. 4–7. The optical micrographs show the effect of process parameters on the replication quality and surface shapes of the microlens arrays. Figure 4 shows the optical micrographs of 100°C under 10, 15, 20 kg/cm2. It is clearly seen that the surface profile has not formed to spherical shape and the focus facula cannot be observed in Figs. 4(a) and 4(b). When the pressure was elevated to 20 kg/cm2, the focus facula came into being, which indicated the spherical shape formed. But the edge in contact with the circle profile of the micro hole is not clear. It shows that PMMA is in rubber elastic state. Although PMMA microlens array will form under larger pressure, excellent geography is far more difficult to come into being. With the increase of heating temperature, PMMA gradually evolves into the viscoelastic state. From Figs. 5–7, it is easily observed that the temperature range from 110~120°C is appropriate for embossing microlens. However under 10 kg/cm2, the microlens edge seems a little damage with the temperature of 110°C and 115°C, as Figs. 5(a) and Fig. 6(a) shown. With 120°C under the same pressure of 10 kg/cm2, the edge is perfect as Fig. 7(a) seen. It can be concluded that the heating temperature is the prominent influencing factor on hot embossing technology. When PMMA is in rubber elastic state, the microlens shape can form under larger pressure, but the recovery to flat shape and the reduction of the lens height are inevitable. When PMMA is in viscoelastic state, the spherical shape tends to forming and less obviously depends on the magnitude of the force, which having possible relation with the lens sag of microlens. The Taylor Hobson Form Talysurf PGI Dimension 3 was used to determine surface profile and lens sag. Figure 8 displays the profile of the formed microlens array (diameter = 150μm, lens sag = 48μm) shown in Fig. 7(c).
4.2 Surface roughness and optical property of replicated microlens
The scanning probe microscope (SPM, Veeco NanoScope MultiMode) was used to measure the surface roughness of the microlenses. Five sampling areas of 5 × 5 μm were randomly selected for measuring. Figure 9 shows roughness analysis data of two areas among these regions. Table 3 illustrates the arithmetical mean deviation of the profile Ra and the mean standard deviation (Mean Std Dev). The average Ra is 0.2348 nm, the Mean Std Dev is 0.00487 nm. The resultant surface shows an excellent optical surface smoothness.
Table 3. Arithmetical mean deviation of the profile Ra and the mean standard deviation
The optical property of the fabricated microlens array is further measured using a profiler. The profiler is composed of a collimator, a star orifice plate, a CCD and an image grabbing card. The focus of the ultra short focal parallel collimator(SSFC-IV) is 51.48 nm. Figure 10 shows a portion of the spot patterns produced by the formed microlens array shown in Fig. 7(c). The image reveals that the pitch and the intensity of the focused light spots are uniform.
4.3 Radius of curvature, focal length and f-number of the microlenses
The basic optical properties of a microlens are the lens diameter D, equal to the diameter of the through micro holes in this case, and effective focal length f. Additional parameters are the sag at the microlens vertex δ, the radius of curvature R, and the refractive index n of the bulk material [24,25]. As Fig. 11 shown, an optical microscope is used to measure the microlens sag. Firstly, the optical microscope slide platform is adjusted to observe the apex of the microlens, then the platform is adjusted to observe the top surface of the PMMA sheet, the altitude difference is the microlens sag, that is δ. According to the fundamental optical and geometrical equation
The radius of curvature R can be figured out as
Another method also can experimentally measure the optical properties of the microlenses by observing the focal points from top and bottom light sources. Figure 12 shows the schematic diagram [20]. The applied optical microscope has dual light sources from both the top and bottom. When the light is emitting from the top, a focal point can be found at a position inside the PMMA microlens sheet and is denoted as M. When the light is emitting from the bottom, a focal point can be observed above of the lens and is denoted as L. The distance fL and fM are the distances between the focal point L and M with the top surface of the mold respectively. The radius of curvature R can be expressed as
where, na and np are the refractive index of air and the PMMA sheet, respectively.Then, the focal length f and the f-number can be calculated as
Upon different processing conditions, the optical parameters keep changing typically associated with temperature and press. To quantitatively analyze the optical performance of the microlens, 30 microlenses of each sample shown in Figs. 4-7 were randomly chosen to be measured. Characteristicses of lens sag, radius of curvature, focal length and f-number are shown in Table 4. Based on these experimental results, the maximum and minimum microlens sags are 6.3 and 48.2 μm. The corresponding mean standard deviations are 0.064 and 0.602 μm. Because PMMA of 100°C is in rubber elastic state, the formed shape usually are unstable. The other parameters such as the radius of curvature, the focal length and the f-number under 100°C would not been discussed. From 110~120°C, the maximum and minimum radius of curvature are 107.99 and 82.46 μm. The corresponding mean standard deviations are 0.994 and 0.428 μm. The focal length is between 168.28 to 220.39 μm, and the f-number is between 1.12 and 1.47. The small deviation as measured indicates good reliability and reproducibility of the process.Table 4. Lens sag, radius of curvature, focal length and f-number during different temperatures and presses
4.4 Influences of temperature and pressure duration on the optical properties of microlens
The operating parameters of heating temperature and pressure are two important influences on the formation of the optical properties of microlens, such as the lens sag, the radius of curvature, the focal length, the f-number, and so on. There are also critical parameters to evaluate optical performance of the microlens. Figure 13 shows the temperature dependent curves from 100, 110, 115 to 120°C under the press of 10, 15, 20 kg/cm2. The resulting microlens sags increase from 6.3 to 48.2 μm, the radius of curvature decreases from 107.99 to 82.46 μm limited to 110~120°C. The general trends are obviously that the microlens sag increases but the radius of curvature decreases as the viscosity and surface tension of polymer melt reduces with the heating process. At a certain temperature, the radius of curvature will gradually decrease when the applied pressure is increasing. Therefore, the height of the microlens can be controlled by adjusting the pressure. Beside, the pressure plays a great impact on the microlens shape under lower temperature range 100~115°C. However, under higher temperature above 115°C, the variety of microlens sizes can be made small substantially by different pressures. Therefore, for obtaining a series of microlenses with different parameters, the temperature range of 110~115°C has contributed to accurate control of pressure.
Figure 14 shows the press dependent microlens sag from 10, 15, 20 kg/cm2 under the temperature of 100, 110, 115 to 120°C. The general trend is that the microlens sag increases but the radius of curvature decreases as the viscosity and surface tension of polymer melt decreases at high temperature. With the microlens sag increasing from 6.3 to 48.2 μm, the radius of curvature decreases from 107.99 to 82.46 μm limited to 110~120°C. Especially, the heating temperature is the prominent influencing factor on hot embossing technology. As shown in Fig. 14(a), the difference of the lens sags under 115°C and 120°C is not as distinct as under lower temperature. The difference values are considerably larger than the same value of 100°C.
Figure 15 shows the radius of curvature and f-number under various microlens sags. With the microlens sag increasing from 30.3 to 48.2 μm, the radius of curvature decreases from 107.99 to 82.46 μm. The focal length decreased from 220.39 to 168.28 μm and the f-number decreased from 1.47 to 1.12 accordingly. The upper portion of the curves of the radius of curvature from 107.99 to 87.44 μm and the f-number from 1.47 to 1.19 drop dramatically. However, there is a little change in the shape of spherical profile according to the rest portion of the curves. So the radius of curvature and the f-number are a few changes in Fig. 15. The result also indicates that the geometry of the formed microlens is strongly depending on the heating temperature as mentioned above. In higher temperature, the variation of force might have affected microlens optical parameters less. For obtaining a series of microlenses with different parameters, the temperature range of 110~115°C has contributed to accurate control of pressure.
4. Conclusion
In this paper, the contactless hot embossing technology through the stainless steel template with micro through-holes array for fabricating thermoplastic refractive microlens array is reported. By optimizing the processing parameters, a series of high quality microlenses arrays with a diameter of 150 μm, a pitch of 185 µm and the maximum sag up to 48.2 µm has been successfully fabricated. Micro through holes template is helpful for producing refractive microlenses with small f-number close to 1.12. In previous contactless hot embossing, the abandon additional vacuum equipment is necessary for this result. The measured surface roughness of the microlens Ra is 0.2348 nm. The results microlens array shows good surface features and optical performances. In addition, the temperature range of 110~115°C has contributed to accurate control of pressure for obtaining a series of microlenses with different parameters. The contactless hot embossing technology is a very flexible and cost effective for fabricating thermoplastic refractive microlens array.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 51475400 and Grant No. 51105321), the Provincial Natural Science Foundation of Fujian (Grant No. 2013J05084) and the Xiamen Municipal Science and Technology Plan Project (Grant No. 3502Z20143032).The experimental work is mainly carried out at the Institute of Micro-Systems in School of Mechanical Science and Engineering in Huazhong University of Science and Technology. The financial and technique supports are gratefully acknowledged.
References and links
1. V. Bardinal, E. Daran, T. Leïchlé, C. Vergnenègre, C. Levallois, T. Camps, V. Conedera, J. B. Doucet, F. Carcenac, H. Ottevaere, and H. Thienpont, “Fabrication and characterization of microlens arrays using a cantilever-based spotter,” Opt. Express 15(11), 6900–6907 (2007). [CrossRef] [PubMed]
2. A. Akatay and H. Urey, “Design and optimization of microlens array based high resolution beam steering system,” Opt. Express 15(8), 4523–4529 (2007). [CrossRef] [PubMed]
3. L. Li and A. Y. Yi, “Design and fabrication of a freeform microlens array for a compact large-field-of-view compound-eye camera,” Appl. Opt. 51(12), 1843–1852 (2012). [CrossRef] [PubMed]
4. M. He, X. C. Yuan, N. Q. Ngo, J. Bu, and S. H. Tao, “Low-cost and efficient coupling technique using reflowed sol-gel microlens,” Opt. Express 11(14), 1621–1627 (2003). [CrossRef] [PubMed]
5. Y. L. Li, T. H. Li, G. H. Jiao, B. W. Hu, J. M. Huo, and L. L. Wang, “Research on micro-optical lenses fabrication technology,” Optik (Stuttg.) 118(8), 395–401 (2007). [CrossRef]
6. M. Sokuler, D. Aronov, G. Rosenman, and L. A. Gheber, “Tailored polymer microlenses on treated glass surfaces,” Appl. Phys. Lett. 90(20), 203106 (2007). [CrossRef]
7. F. Li, S. H. Chen, H. Luo, Y. F. Zhou, J. J. Lai, and Y. Q. Gao, “Fabrication and characterization of polydimethylsiloxane concave microlens array,” Opt. Laser Technol. 44(4), 1054–1059 (2012). [CrossRef]
8. W. Moench and H. Zappe, “Fabrication and testing of micro-lens arrays by all-liquid techniques,” J. Opt. A, Pure Appl. Opt. 6(4), 330–337 (2004). [CrossRef]
9. P. C. Xie, P. He, Y. C. Yen, K. J. Kwak, D. G. Perez, L. Q. Chang, W. C. Liao, A. Yi, and L. J. Lee, “Rapid hot embossing of polymer microstructures using carbide-bonded graphene coating on silicon stampers,” Surf. Coat. Tech. 258, 174–180 (2014). [CrossRef]
10. Z. G. Zang, X. S. Tang, X. Liu, X. Lei, and W. Chen, “Fabrication of high quality and low cost microlenses on a glass substrate by direct printing technique,” Appl. Opt. 53(33), 7868–7871 (2014). [CrossRef] [PubMed]
11. D. Xie, H. H. Zhang, X. Y. Shu, and J. F. Xiao, “Fabrication of polymer micro-lens array with pneumatically diaphragm-driven drop-on-demand inkjet technology,” Opt. Express 20(14), 15186–15195 (2012). [CrossRef] [PubMed]
12. J. T. Wu, Y. T. Chu, S. Y. Yang, and C. C. Li, “Low-temperature embossing technique for fabrication of large-area polymeric microlens array with supercritical carbon dioxide,” Microelectron. Eng. 87(12), 2620–2624 (2010). [CrossRef]
13. N. S. Ong, Y. H. Koh, and Y. Q. Fu, “Microlens array produced using hot embossing process,” Microelectron. Eng. 60(3), 365–379 (2002). [CrossRef]
14. C. Peng, X. G. Liang, Z. L. Fu, and S. Y. Chou, “High fidelity fabrication of microlens arrays by nanoimprint using conformal mold duplication and low-pressure liquid material curing,” J. Vac. Sci. Technol. B 25(2), 410–414 (2007). [CrossRef]
15. J. Schulze, W. Ehrfeld, H. Muller, and A. Picard, “Compact self-aligning assemblies with refractive microlens arrays made by contactless embossing,” Proc. SPIE 3289, 22–32 (1998). [CrossRef]
16. J. Schulze, W. Ehrfeld, H. Lowe, A. Michel, and A. Picard, “Contactless embossing of microlenses—a new technology for manufacturing refractive microlenses,” Proc. SPIE 3099, 89–98 (1997). [CrossRef]
17. Z. Slawomir, F. Ines, K. Henryk, and K. Stefan, “Contactless embossing of microlenses—a parameter study,” Opt. Eng. 42, 1053–1055 (2003).
18. X. J. Shen, L. W. Pan, and L. W. Lin, “Microplastic embossing process: experimental and theoretical characterizations,” Sensor. Acturat. A. Phys. 97, 428–433 (2002). [CrossRef]
19. L. W. Pan, X. J. Shen, and L. W. Lin, “Micro-plastic lens array fabricated by a hot intrusion process,” J. Micromech. Microeng. 13(6), 1063–1071 (2004). [CrossRef]
20. C. Y. Chang, S. Y. Yang, L. S. Huang, and K. H. Hsieh, “Fabrication of polymer microlens arrays using capillary forming with a soft mold of micro-holes array and UV-curable polymer,” Opt. Express 14(13), 6253–6258 (2006), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-13-6253. [CrossRef] [PubMed]
21. C. Y. Chang, S. Y. Yang, and M. H. Chu, “Rapid fabrication of ultraviolet-cured polymer microlens arrays by soft roller stamping process,” Microelectron. Eng. 84(2), 355–361 (2007). [CrossRef]
22. L. Aretxabaleta, J. Aurrekoetxea, I. Urrutibeascoa, and M. Sánchez-Soto, “Characterisation of the impact behaviour of polymer thermoplastics,” Polym. Test. 24(2), 145–151 (2005). [CrossRef]
23. J. Liu, X. Jin, T. Sun, Z. Xu, C. Liu, J. Wang, L. Chen, and L. Wang, “Hot embossing of polymer nanochannels using PMMA moulds,” Microsyst. Technol. 19(4), 629–634 (2013). [CrossRef]
24. R. Stevens and T. Miyashita, “Review of standards for microlenses and microlens arrays,” Imaging. Sci. J 58(4), 202–212 (2010). [CrossRef]
25. X. Zhu, S. Hu, and L. Zhao, “Focal length measurement of a microlens-array by grating shearing interferometry,” Appl. Opt. 53(29), 6663–6669 (2014). [CrossRef] [PubMed]
