Abstract

Interest in micro-optical components for applications ranging from telecommunications to life sciences has driven the need for accessible, low-cost fabrication techniques. Many microlens fabrication processes are unsuitable for applications requiring 100% fill factor, apertures 1000μm with high numerical aperture, and scalability to large areas (e.g., tens of centimeters to meters) with millions of lenses. We report on a flexible, low-cost mold fabrication technique that utilizes a combination of milling and microforging. The technique involves first performing a rough cut with a ball-end mill. Final shape and sag height are then achieved by pressing a sphere of equal diameter into the milled divot. Using this process, we have fabricated molds for rectangular arrays of 1–10,000 lenses with apertures of 251600μm, sag heights of 3130μm, interlens spacings of 2502000μm, and fill factors up to 100%. Mold profiles have a roughness and figure error of 68nm and 354nm, respectively, for 100% fill factor, 1000μm aperture lenses. The required forging force was modeled as a modified open-die forging process and experimentally verified to increase nearly linearly with surface area. The optical performance of lens arrays injection molded from microforged molds was characterized by imaging the point spread function and was found to be in the range of theoretical values. The process can be easily adapted to lenticular arrays as well. Limitations include milling machine range and accuracy.

© 2007 Optical Society of America

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References

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    [CrossRef]
  8. Y.-C. Lee, C.-M. Chen, and C.-Y. Wu, "Spherical and aspheric microlenses fabricated by excimer laser LIGA-like process," J. Manuf. Sci. Eng. 129, 126-134 (2007).
    [CrossRef]
  9. S. Kalpakjian and S. Schmid, Manufacturing Engineering and Technology (Academic, 2001).
  10. E. Hecht, Optics (Academic, 2001).

2007 (2)

S. Morgenthaler and W. G. Thilly, "Summed multi-allelic risk: logical and statistical models for discovery of carrier genes in human populations," Mutat. Res. 615, 28-56 (2007).
[CrossRef]

Y.-C. Lee, C.-M. Chen, and C.-Y. Wu, "Spherical and aspheric microlenses fabricated by excimer laser LIGA-like process," J. Manuf. Sci. Eng. 129, 126-134 (2007).
[CrossRef]

2004 (1)

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. Micromec. Microeng. 14, 1197-1204 (2004).
[CrossRef]

1998 (1)

1994 (2)

T. R. Jay and M. B. Stern, "Preshaping photoresist for refractive microlens fabrication," Opt. Eng. 33, 3552-3555 (1994).
[CrossRef]

D. L. MacFarlane, V. Narayan, J. A. Tatum, W. R. Cox, T. Chen, and D. J. Hayes, "Microjet fabrication of microlens arrays," IEEE Photon. Technol. Lett. 9, 1112-1114 (1994).
[CrossRef]

1988 (1)

1983 (1)

Appl. Opt. (2)

IEEE Photon. Technol. Lett. (1)

D. L. MacFarlane, V. Narayan, J. A. Tatum, W. R. Cox, T. Chen, and D. J. Hayes, "Microjet fabrication of microlens arrays," IEEE Photon. Technol. Lett. 9, 1112-1114 (1994).
[CrossRef]

J. Manuf. Sci. Eng. (1)

Y.-C. Lee, C.-M. Chen, and C.-Y. Wu, "Spherical and aspheric microlenses fabricated by excimer laser LIGA-like process," J. Manuf. Sci. Eng. 129, 126-134 (2007).
[CrossRef]

J. Micromec. Microeng. (1)

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. Micromec. Microeng. 14, 1197-1204 (2004).
[CrossRef]

Mutat. Res. (1)

S. Morgenthaler and W. G. Thilly, "Summed multi-allelic risk: logical and statistical models for discovery of carrier genes in human populations," Mutat. Res. 615, 28-56 (2007).
[CrossRef]

Opt. Eng. (1)

T. R. Jay and M. B. Stern, "Preshaping photoresist for refractive microlens fabrication," Opt. Eng. 33, 3552-3555 (1994).
[CrossRef]

Opt. Lett. (1)

Other (2)

S. Kalpakjian and S. Schmid, Manufacturing Engineering and Technology (Academic, 2001).

E. Hecht, Optics (Academic, 2001).

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Figures (5)

Fig. 1
Fig. 1

(Color online) Schematic of the lens array mold manufacturing process and photographs of a 100 lens array mold and molded part. (a) Blank aluminum mold is faced and polished. (b) Rotating ball-end mill is used to cut an array of divots. (c) Tungsten-carbide sphere is lowered onto the surface to deform the divots to the final shape. (d) Mold is used to injection mold lens arrays.

Fig. 2
Fig. 2

(Color online) Photograph of a rectangular array of 10,000 lenses injection molded from a milled∕microforged mold in PMMA. The spherical lenses each have a 1000 μm square aperture, 2.5 mm radius of curvature, 100 μm sag height, and 100% fill factor. The central feature, a sprue, is an artifact of the injection molding process.

Fig. 3
Fig. 3

(Color online) Forging force theory and experimental measurements versus molded surface area.

Fig. 4
Fig. 4

Figure error (left) and roughness (right) of lens array molds. All lenses have 120 μm total sag height, so increasing milled depth implies that a larger percentage of the final figure was determined by milling. (left) As milled depth increases, the figure error generally improves while the roughness increases (degrades) somewhat to a plateau, but is still small.

Fig. 5
Fig. 5

Theoretical and experimental Airy region sizes for the tested lenses. The glass and forged lenses have round apertures, while the microforged and milled lens arrays have square apertures ( ϕ = 100 % ) .

Equations (8)

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F 0 = Y f π r 2 ( 1 + 2 μ r 3 h )
F = Y f A s ( 1 + μ d 3 z ) ,
z ( d , R ) = R R 2 d 2 2 ,
A s ( d , R ) = 2 π R 2 ( 1 R 2 d 2 2 R ) ,
I c ( x ) = | 2 J 1 ( 2 π ( NA ) x 2 λ ) 2 π ( NA ) x 2 λ | 2 ,
I sq ( x ) = | 2 sin ( 2 π ( NA ) x 2 λ ) 2 π ( NA ) x 2 λ | 2 ,
f = R n 1 ,
d Airy = λ NA .

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