OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 5 — Mar. 3, 2008
  • pp: 3041–3048
« Show journal navigation

Fabrication of large area resin microlens arrays using gas-assisted ultraviolet embossing

Po-Hsun Huang, Tzu-Chien Huang, Yi-Ting Sun, and Sen-Yeu Yang  »View Author Affiliations


Optics Express, Vol. 16, Issue 5, pp. 3041-3048 (2008)
http://dx.doi.org/10.1364/OE.16.003041


View Full Text Article

Acrobat PDF (850 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Large-area microlens arrays are becoming important components in many applications such as LCD-TV diffusers. This paper reports a uniform pressure, low temperature process for their fabrication. The process integrates gas-assisted embossing and UV-curing embossing. During the process, the 230mm×203mm PMMA substrate is pressed against the stainless-steel stamper coated with UV-curable resin. Under the gas pressuring and UV irradiating, a large array of microlens can be formed. By using this process, high embossing temperature and high embossing pressure can be avoided. Little residual stress is observed in the embossed PMMA substrate. The uniformity of large-area fabrication and optical properties of fabricated resin microlens array have been verified. This study has successfully shown the potential of this gas-assisted UV embossing process for the replication of large-area microstructures.

© 2008 Optical Society of America

1. Introduction

This study is devoted to developing a process for effective fabrication of large-area microlens arrays at room temperature and with low pressure. This process integrates the gasassisted embossing process and the UV-curing embossing process. The gas-assisted embossing provides the uniform pressing pressure over the whole large area, while the UVcuring embossing enables the process to perform without heating and cooling and under low pressure. By using gas-assisted and UV-curing mechanisms, the high temperature and high pressure can be avoided. Moreover, the problems of non-uniform pressure and residual stress can be addressed.

In this paper, the facility and process for fabrication of large-area microlens arrays based on gas-assisted UV-curing embossing principle is implemented and tested. Using photolithography and wet etching, the micro-holes array is first formed on a stainless-steel plate of 0.8mm thickness, which is used as the stamper. During the gas-assisted UV-curing embossing process, the PMMA substrate is pressed against the stainless-steel stamper coated with UV-curable resin. Under the gas pressuring and UV irradiating, a large array of microlens can be formed. The uniformity and optical properties of the microlenses array over whole substrate are verified with an optical microscope, a surface profiler, a strain viewer, and a light focusing system.

2. Experiment setup

Figure 1 schematically shows the apparatus and fabrication procedure of the gas-assisted UV embossing apparatus for fabricating large-area of microlens array. The embossing apparatus is composed of a upper chamber, a lower chamber, a UV-LED lamp, and a seal film. For upper chamber, nitrogen gas can be introduced to produce a pressing pressure. The lower chamber is vacuumed to reduce air bubble during embossing. Since the traditional UV-lamp is a linear light source and is difficult to provide a uniform irradiation over large-area, a UV-LED array (14×9) lamp as shown in Fig. 2 has been designed and implemented for completely covering, irradiating, and curing a large-area. The wavelength of the UV-LED lamp is between 375–395nm. Poly(ethylene terephthalate) (PET) film with a thickness of 180µm is used as the seal film. The seal film is needed to separate the nitrogen gas in the upper and lower chambers and to pressurize the stamper/substrate stack. 0.8mm-thick PMMA plates (230mm×203mm) are used as the substrates in the experiment. An ultraviolet-curable resin with a refractive index of 1.56 is employed.

In this study, the large-area stainless-steel stamper (270mm×195mm) with a micro-holes array is supplied by Industrial Technology Research Institute of Taiwan. The stainless-steel stamper has a very large array (over 1500×1500) of holes and is fabricated by photolithography and wet etching. As shown in Fig. 3, the average diameter of the microholes is 120.5µm. The shapes of the etched micro-holes are varying and not uniform. The average depth of the micro-holes is 7.95µm.

The gas-assisted UV-curing embossing process is composed of five steps:

  1. The stainless-steel stamper with micro-holes array cavity is coated with the UVcurable liquid resin using a scraper and is fixed on the holder of lower chamber.
  2. The upper and lower chambers are closed. At the same time, the PMMA substrate is brought into contact with the UV-curable resin coated on the stainless-steel stamper. Subsequently, the lower chamber is vacuumed and the nitrogen gas is introduced into upper chamber to produce a pressing pressure. The stack of stamper and substrate is pressed for a specific duration.
  3. The liquid resin is transferred onto the PMMA substrate and is cured by the UV irradiation at room temperature.
  4. After curing, the nitrogen gas is introduced into lower chamber to separate the stamper and the substrate.
  5. The upper and lower chambers are opened. The stainless-steel stamper is removed from the PMMA substrate, and the PMMA substrate with microlens array on its surface can be obtained.
Fig. 1. Schematic of the gas-assisted UV-curing embossing process for the fabrication of large area microlens array.
Fig. 2. Photograph of the UV-LED array lamp.
Fig. 3. OM image of the micro-holes array on the stainless steel stamper.

3. Results and discussion

3.1 Observation of residual stress for the fabricated large area microlens arrays

To observe and inspect the residual stress, a strain viewer (Compact, Sharples, UK), based on the polarized light and birefringence, was employed. Figure 4 shows the observation of photoelastic stress. There is little residual stress observed. The result demonstrates that negligible residual stress is induced in the large-area PMMA substrate using proposed process. By using proposed process in this study, the high temperature and high pressure can be avoided due to gas-assisted and UV-curing mechanisms. Accordingly, the substrate is expected to be free of residual stress.

Fig. 4. The observed photoelastic image from (a) central area and (b) side area of PMMA substrate.

3.2 Profile of fabricated microlens and uniformity of large area fabrication

As shown in Fig. 5, a vary large array (over 1500×1500) of resin microlenses has been successfully fabricated onto a 203mm×230mm PMMA substrate by gas-assisted UV-curing embossing process. The result demonstrates the capacity of this proposed process for largearea fabrication of microlens arrays. Figure 6 shows the optical microscope (OM) image and the surface profile of the fabricated microlens array under the processing conditions of 5 Kgf/cm2 of pressing pressure, 60 seconds of pressing duration (including UV irradiating time), and 20 seconds of UV irradiating time. A large array of resin microlenses has been successfully fabricated over the whole PMMA substrate. To inspect and verify the fabrication uniformity of resin microlenses in large-area, the whole fabricated area was divided into nine areas as shown in Fig. 5. The surface profiles of 270 microlenses (30 microlenses were randomly selected in each area from nine areas) from a single process run are measured. Table 1 shows the average diameter and sag height of the fabricated microlenses measured from the nine areas. The average diameter is 119.77µm with a standard deviation of 1.73µm, while the average sag height is 7.89µm with a standard deviation of 0.47µm. The difference between averages and standard deviations in the nine areas must result from the non-uniformity of the micro-holes in the original stamper, not from the replication process. In the proposed process, the stamper is first coated and the micro-holes in it are completely filled with resin. The shape of the etched micro-holes is replicated with fidelity onto the PMMA substrate upon embossing.

Figure 6(c) shows the stylus scan of the five neighboring replicated microlenses. It is noted that the shape of lens is not identical and variation is observed especially at the lens vertices. Upgrading the uniformity of micro-holes in the stamper is needed to improve the quality of the microlenses produced.

Fig. 5. The PMMA substrate (203mm×230mm×0.8 mm) with fabricated microlens array.
Fig. 6. OM images and surface profile of the fabricated microlens array on PMMA substrate.

Table 1. Large-area uniformity of fabricated microlens array.

table-icon
View This Table

3.3 The optical property of the molded microlens arrays

In order to verify the optical property of the fabricated microlens array using this process, a light focusing system is employed. Figure 7(a) schematically shows the light focusing system which is composed of expanding lenses, an optical attenuator, a filter, a micrometer scale resolution Z-stage, a microscope, a CCD system and a 633 nm wavelength laser light source. The measured average focal length is 418µm for the fabricated resin microlens with an average diameter of 119.77µm and an average sag height of 7.89µm. Based on the geometry and optical theory, the theoretical radius of curvature (R), focal length (f), and numerical aperture (NA) can be calculated using the equations shown below.

R=D2+4h28h,f=Rn1,NA=D2f

where D, h and n are the diameter, the sag height, and the refractive index of the UV-curable resin, respectively. The calculated average radius of curvature, the average focal length, and the average numerical aperture of the formed resin microlenses are 232.1µm, 414.46µm, and 0.145, respectively. The calculated average focal length agrees well with the measured average focal length. Results indicate good uniformity of the fabricated microlenses. Figure 7(b) shows a portion of the focused light spots observed by this system. The image reveals that the uniformity of pitch and intensity of the focused light spots are acceptable. Variation in the spot shape is caused by the non-uniformity in the replicated microlenses, which is due to the limitation in the uniformity of wet-etched micro-holes. To further improve the uniformity of spot pattern, the quality of the original stamper will be upgraded.

Fig. 7. (a) Schematic setup for measuring the optical property of formed polymeric microlens array and (b) the light spots pattern of a randomly selected area of formed microlens array.

4. Conclusions

This paper reports a novel and effective process to fabricate large-area microlens arrays on thin PMMA substrates by gas-assisted UV-curing embossing with a stainless-steel stamper and a UV-LED lamp. This process takes advantage of the uniform pressing pressure of gasassisted embossing and the low pressure and fast curing of the UV embossing. During this process, by pressing the PMMA substrate against the stainless-steel stamper coated with UVcurable resin, the resin microlens array can be fabricated. A large array (over 1500×1500) of microlenses has been successfully fabricated onto the whole PMMA substrate (203mm×230mm) with 5Kgf/cm2 of pressing pressure, 60 seconds of pressing duration, and 20 seconds of UV irradiating duration. With this process, no residual stress is observed in the embossed PMMA substrate. The uniformity of large-area fabrication and optical properties of fabricated resin microlens array have been proven. This process has proven its potential for the large-area fabrication of microstru ctures.

Acknowledgement

This work was partially supported by the Industrial Technology Research Institute of Taiwan (Under the grant of 96E1046). The financial and technique supports are acknowledged. Help and encouragement from co-workers at the Grace Laboratory for polymer processing at National Taiwan University are greatly appreciated.

References and links

1.

D. Daly, R. F. Stevens, M. C. Hutley, and N. Davies, “The manufacture of microlenses by melting photoresist,” Meas. Sci. Technol. 1, 759–766 (1990). [CrossRef]

2.

M. He, X.-C. Yuan, N. Q. Ngo, J. Bu, and V. Kudryashov, “Simple reflow technique for fabrication of a microlens array in solgel glass,” Opt. Lett. 28, 731–733 (2003). [CrossRef] [PubMed]

3.

C.P. Lin, H. Yang, and C.K. Chao, “A new microlens array fabrication method using UV proximity printing,” J. Micromech. Microeng. 13, 748–757 (2003). [CrossRef]

4.

X.-C. Yuan, W. X. Yu, N. Q. Ngo, and W. C. Cheong, “Cost-effective fabrication of microlenses on hybrid sol-gel glass with a high-energy beam-sensitive gray-scale mask,” Opt. Express 10, 303–308 (2002). [PubMed]

5.

W. X. Yu and X. -C. Yuan, “UV induced controllable volume growth in hybrid sol-gel glass for fabrication of a refractive microlens by use of a grayscale mask,” Opt. Express 11, 2253–2258 (2003). [CrossRef] [PubMed]

6.

V. Bardinal, E. Daran, T. Leïchlé, C. Vergnenègre, C. Levallois, T. Camps, V. Conedera, J. B. Doucet, and F. Carcenac, “Fabrication and characterization of microlens arrays using a cantilever-based spotter,” Opt. Express 15, 6900–6907 (2007). [CrossRef] [PubMed]

7.

W. L. Chang and P. K. Wei, “Fabrication of a close-packed hemispherical submicron lens array and its application in photolithography,” Opt. Express 15, 6774–6783 (2007). [CrossRef] [PubMed]

8.

S.-I. Chang and J.-B. Yoon, “Shape-controlled, high fill-factor microlens arrays fabricated by a 3D diffuser lithography and plastic replication method,” Opt. Express 12, 6366–6371 (2004). [CrossRef] [PubMed]

9.

R. Guo, S. Xiao, X. Zhai, J. Li, A. Xia, and W. Huang, “Micro lens fabrication by means of femtosecond two photon photopolymerization,” Opt. Express 14, 810–816 (2006). [CrossRef] [PubMed]

10.

B.K. Lee, D.S. Kim, and T.H. Kwon, “Replication of microlens arrays by injection molding”, Microsystem Technologies ,” 10 (2004) 531–535. [CrossRef]

11.

N.S. Ong, Y.H. Koh, and Y.Q. Fu, “Microlens array produced using hot embossing process,” Microelectron. Eng. 60 (2002) 365–379. [CrossRef]

12.

W. Pan, X. Shen, and L. Lin, “Micro-plastic lens array fabricated by a hot intrusion process,” J. Micromech. Microeng. 13, 1063–1071 (2004).

13.

L. T. Jiang, T. C. Huang, J. R. Ciou, C. Y. Chang, and S. Y. Yang, “Fabrication of plastic microlens arrays using hybrid extrusion rolling embossing with a metallic cylinder mold fabricated using dry film resist,” Opt. Express 15, 12088–12094 (2007). [CrossRef] [PubMed]

14.

J. H. Chang, F. S. Cheng, C. C. Chao, Y. C. Weng, S. Y. Yang, and L. A. Wang, “Direct imprinting using soft mold and gas pressure for large area and curved surfaces,” J. Vac. Sci. Technol. A 23, 1687–1690 (2005). [CrossRef]

15.

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, 6253–6258 (2006). [CrossRef] [PubMed]

OCIS Codes
(220.0220) Optical design and fabrication : Optical design and fabrication
(220.4000) Optical design and fabrication : Microstructure fabrication
(230.0230) Optical devices : Optical devices
(230.3990) Optical devices : Micro-optical devices

ToC Category:
Optical Design and Fabrication

History
Original Manuscript: January 3, 2008
Revised Manuscript: February 10, 2008
Manuscript Accepted: February 14, 2008
Published: February 20, 2008

Citation
Po-Hsun Huang, Tzu-Chien Huang, Yi-Ting Sun, and Sen-Yeu Yang, "Fabrication of large area resin microlens arrays using gas-assisted ultraviolet embossing," Opt. Express 16, 3041-3048 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-5-3041


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. D. Daly, R. F. Stevens, M. C. Hutley, and N. Davies, "The manufacture of microlenses by melting photoresist," Meas. Sci. Technol. 1, 759-766 (1990). [CrossRef]
  2. M. He, X.-C. Yuan, N. Q. Ngo, J. Bu, and V. Kudryashov, "Simple reflow technique for fabrication of a microlens array in solgel glass," Opt. Lett. 28, 731-733 (2003). [CrossRef] [PubMed]
  3. C. P. Lin, H. Yang and C. K. Chao, "A new microlens array fabrication method using UV proximity printing," J. Micromech. Microeng. 13, 748-757 (2003). [CrossRef]
  4. X.-C. Yuan, W. X. Yu, N. Q. Ngo, and W. C. Cheong, "Cost-effective fabrication of microlenses on hybrid sol-gel glass with a high-energy beam-sensitive gray-scale mask," Opt. Express 10, 303-308 (2002). [PubMed]
  5. W. X. Yu, and X. -C. Yuan, "UV induced controllable volume growth in hybrid sol-gel glass for fabrication of a refractive microlens by use of a grayscale mask," Opt. Express 11, 2253-2258 (2003). [CrossRef] [PubMed]
  6. V. Bardinal, E. Daran, T. Leïchlé, C. Vergnenègre, C. Levallois, T. Camps, V. Conedera, J. B. Doucet, and F. Carcenac, "Fabrication and characterization of microlens arrays using a cantilever-based spotter," Opt. Express 15, 6900-6907 (2007). [CrossRef] [PubMed]
  7. W. L. Chang, and P. K. Wei, "Fabrication of a close-packed hemispherical submicron lens array and its application in photolithography," Opt. Express 15, 6774-6783 (2007). [CrossRef] [PubMed]
  8. S.-I. Chang, and J.-B. Yoon, "Shape-controlled, high fill-factor microlens arrays fabricated by a 3D diffuser lithography and plastic replication method," Opt. Express 12, 6366-6371 (2004). [CrossRef] [PubMed]
  9. R. Guo, S. Xiao, X. Zhai, J. Li, A. Xia, and W. Huang, "Micro lens fabrication by means of femtosecond two photon photopolymerization," Opt. Express 14, 810-816 (2006). [CrossRef] [PubMed]
  10. B. K. Lee, D. S. Kim and T. H. Kwon, "Replication of microlens arrays by injection molding", Microsystem Technologies," 10 (2004) 531-535. [CrossRef]
  11. N. S. Ong, Y. H. Koh and Y.Q. Fu, "Microlens array produced using hot embossing process," Microelectron. Eng. 60 (2002) 365-379. [CrossRef]
  12. W. Pan, X. Shen, and L. Lin, "Micro-plastic lens array fabricated by a hot intrusion process," J. Micromech. Microeng. 13, 1063-1071 (2004).
  13. L. T. Jiang, T. C. Huang, J. R. Ciou, C. Y. Chang, and S. Y. Yang, "Fabrication of plastic microlens arrays using hybrid extrusion rolling embossing with a metallic cylinder mold fabricated using dry film resist," Opt. Express 15, 12088-12094 (2007). [CrossRef] [PubMed]
  14. J. H. Chang, F. S. Cheng, C. C. Chao, Y. C. Weng, S. Y. Yang, and L. A. Wang, "Direct imprinting using soft mold and gas pressure for large area and curved surfaces," J. Vac. Sci. Technol. A 23, 1687-1690 (2005). [CrossRef]
  15. 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, 6253-6258 (2006). [CrossRef] [PubMed]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited