Abstract

This paper describes a simple and effective method to fabricate a plastic microlens array with controllable shape and high fill-factor, which utilizes the conventional lithography and plastic replication. The only difference from conventional lithography is the insertion of a diffuser that randomizes paths of the incident ultraviolet (UV) light to form lens-like 3D latent image in a thick positive photoresist. After replication of the developed concave microlens mold onto the polydimethylsiloxane (PDMS), the focal length of the fabricated hemispherical microlens was observed to be 13–88 µm depending on the UV exposure dose. Two PDMS curing conditions were tested, where the elevated temperature of 85 °C resulted in smoother surface roughness of 2.6 nm in RMS value in the microlens mold. The proposed method can be extensively applied for microlens fabrication with other plastic materials due to its simplicity and versatility.

© 2004 Optical Society of America

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References

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  1. D. Daly, ???Applications and Fabrication Technologies??? in Microlens Array (Talyer and Francis, London 2001).
  2. M. V. Kunnavakkam, F. M. Houlihan, M. Schlax, J. A. Liddle, P. Kolodner, O. Nalamasu, and J. A Rodgers, ???Low-cost, low-loss microlens arrays fabricated by soft-lithography replication process,??? Appl. Phys. Lett. 82, 1152-1154 (2003).
    [CrossRef]
  3. C. R. King, L. Y. Lin, and M. C. Wu, ???Out-of-Plane Refractive Microlens Fabricated by Surface Micromachining,??? IEEE Photonics Technol. Lett. 8, 1349-1351 (1996).
    [CrossRef]
  4. M.-H. Wu and G. M. Whitesides, ???Fabrication of two-dimensional arrays of microlenses and their applications in photolithography,??? J. Micromech. Microeng. 12, 747-758 (2002)
    [CrossRef]
  5. M.-K. Wei and I -L. Su, ???Method to evaluate the enhancement of luminance efficiency in planar OLED light emitting devices for microlens array,??? Opt. Express 12, 5777-5782 (2004).
    [CrossRef] [PubMed]
  6. J. Chen, W. Wang, J. Fang, and K. Varahramyan, ???Variable-focusing microlens with microfluidic chip,??? J. Micromech. Microeng. 14, 675-680 (2004).
    [CrossRef]
  7. H. Yang, C.-K. Chao, M.-K. Wei and C.-P. Lin, ???High fill-factor microlens array mold insert fabrication using a thermal reflow process,??? J. Micromech. Microeng. 14, 1197-1204 (2004).
    [CrossRef]
  8. Z. L. Liau, V. Diaduik, J. N. Walpole and D. E. Mull, ???Gallium phosphide microlenses by mass transport,??? Appl. Phys. Lett. 55, 97-99 (1989).
    [CrossRef]
  9. Z. L. Liau, D. E. Mull, C. L. Dennis, R. C. Williamson and R. G. Waarts, ???Large-numerical-aperture microlens fabrication by one-step etching and mass-transport smoothing,??? Appl. Phys. Lett. 64, 1484-1486 (1994).
    [CrossRef]
  10. A. W. Lohmann and S. Singzinger, ???Array of doublet lenslets,??? Optik 93, 84-86 (1993)
  11. R. Yang and W. Wang, ???Out-of-plane polymer refractive microlens fabricated based on direct lithography of SU-8,??? Sensor. Actuat. A-Phys. 113, 71-77 (2004).
    [CrossRef]
  12. N. Chronis, G. L. Liu, K.-H. Jeong, and L. P. Lee, ???Tunable liquid-filled microlens array integrated with microfluidic network,??? Opt. Express 11, 2370-2378 (2003).
    [CrossRef] [PubMed]
  13. J.-B. Yoon, B.-I. Kim, Y.-S. Choi, and E. Yoon, ???3-D Construction of Monolithic Passive Components for RF and Microwave ICs Using Thick-Metal Surface Micromachining Technology,??? IEEE T. Microw Theory 51, 279-288 (2003).
    [CrossRef]
  14. B. D. Gates and G. M. Whitesides, ???Replication of Vertical Features Smaller than 2 nm by Soft Lithography,??? J. Am. Chem. Soc. 125, 14986-14987 (2003).
    [CrossRef] [PubMed]
  15. P. Nussbaum, R. Völkel, H. P. Herszig, M. Eisner, and S. Haselbeck, ???Design, fabrication and testing of microlens arrays for sensors and microsystems,??? Pure Appl. Opt. 6, 617-636 (1997)
    [CrossRef]

Appl. Phys. Lett. (3)

M. V. Kunnavakkam, F. M. Houlihan, M. Schlax, J. A. Liddle, P. Kolodner, O. Nalamasu, and J. A Rodgers, ???Low-cost, low-loss microlens arrays fabricated by soft-lithography replication process,??? Appl. Phys. Lett. 82, 1152-1154 (2003).
[CrossRef]

Z. L. Liau, V. Diaduik, J. N. Walpole and D. E. Mull, ???Gallium phosphide microlenses by mass transport,??? Appl. Phys. Lett. 55, 97-99 (1989).
[CrossRef]

Z. L. Liau, D. E. Mull, C. L. Dennis, R. C. Williamson and R. G. Waarts, ???Large-numerical-aperture microlens fabrication by one-step etching and mass-transport smoothing,??? Appl. Phys. Lett. 64, 1484-1486 (1994).
[CrossRef]

IEEE Photonics Technol. Lett. (1)

C. R. King, L. Y. Lin, and M. C. Wu, ???Out-of-Plane Refractive Microlens Fabricated by Surface Micromachining,??? IEEE Photonics Technol. Lett. 8, 1349-1351 (1996).
[CrossRef]

IEEE T. Microw Theory (1)

J.-B. Yoon, B.-I. Kim, Y.-S. Choi, and E. Yoon, ???3-D Construction of Monolithic Passive Components for RF and Microwave ICs Using Thick-Metal Surface Micromachining Technology,??? IEEE T. Microw Theory 51, 279-288 (2003).
[CrossRef]

J. Am. Chem. Soc. (1)

B. D. Gates and G. M. Whitesides, ???Replication of Vertical Features Smaller than 2 nm by Soft Lithography,??? J. Am. Chem. Soc. 125, 14986-14987 (2003).
[CrossRef] [PubMed]

J. Micromech. Microeng. (3)

J. Chen, W. Wang, J. Fang, and K. Varahramyan, ???Variable-focusing microlens with microfluidic chip,??? J. Micromech. Microeng. 14, 675-680 (2004).
[CrossRef]

H. Yang, C.-K. Chao, M.-K. Wei and C.-P. Lin, ???High fill-factor microlens array mold insert fabrication using a thermal reflow process,??? J. Micromech. Microeng. 14, 1197-1204 (2004).
[CrossRef]

M.-H. Wu and G. M. Whitesides, ???Fabrication of two-dimensional arrays of microlenses and their applications in photolithography,??? J. Micromech. Microeng. 12, 747-758 (2002)
[CrossRef]

Microlens Array (1)

D. Daly, ???Applications and Fabrication Technologies??? in Microlens Array (Talyer and Francis, London 2001).

Opt. Express (2)

Optik (1)

A. W. Lohmann and S. Singzinger, ???Array of doublet lenslets,??? Optik 93, 84-86 (1993)

Pure Appl. Opt. (1)

P. Nussbaum, R. Völkel, H. P. Herszig, M. Eisner, and S. Haselbeck, ???Design, fabrication and testing of microlens arrays for sensors and microsystems,??? Pure Appl. Opt. 6, 617-636 (1997)
[CrossRef]

Sensor. Actuat. A-Phys. (1)

R. Yang and W. Wang, ???Out-of-plane polymer refractive microlens fabricated based on direct lithography of SU-8,??? Sensor. Actuat. A-Phys. 113, 71-77 (2004).
[CrossRef]

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Figures (5)

Fig. 1.
Fig. 1.

Process flow of the proposed plastic microlens fabrication. (a) UV exposure through a diffuser and a mask. (b) Development. (c) Liquid PDMS casting and curing. (d) Replicated PDMS microlens peel-off.

Fig. 2.
Fig. 2.

SEM photographs and schematic cross-sections of the photoresist patterns. (a) Photoresist pattern cross-section with type-I diffuser and an exposure dose of 300 mJ/cm2. (b) Cross-sectional photoresist profiles (solid line: type-I diffuser, dotted line: type-II diffuser, dashed line: no diffuser). (c) Photoresist pattern cross-section with type-I diffuser and an exposure dose of 1200 mJ/cm2. (d) Cross-sectional photoresist profiles with different exposure doses.

Fig. 3.
Fig. 3.

SEM photographs of the photoresist pattern cross-sections. (a) A circular cross-section with a 1.5 µm diameter and (b) with an 80 µm diameter. (c) A concave cross-section. (d) An elliptical cross-section. (e) A cross-section with a very high density.

Fig. 4.
Fig. 4.

SEM photographs of the fabricated plastic microlens array. (a) Conventional convex microlens. (b) Hemispherical microlens. (c) Ellipsoidal microlens. (d) Microlens array with a high fill-factor (~100 %).

Fig. 5.
Fig. 5.

Optical photograph of the magnified gold patterns on a glass through the fabricated microlens array.

Tables (1)

Tables Icon

Table I. Characteristic comparison of the fabricated PDMS microlenses and photoresist molds cured at two different conditions.

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