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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 4, Iss. 12 — Nov. 10, 2009
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A high numerical aperture, polymer-based, planar microlens array

Anurag Tripathi, Trushal Vijaykumar Chokshi, and Nikos Chronis  »View Author Affiliations


Optics Express, Vol. 17, Issue 22, pp. 19908-19918 (2009)
http://dx.doi.org/10.1364/OE.17.019908


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Abstract

We present a novel microfabrication approach for obtaining arrays of planar, polymer-based microlenses of high numerical aperture. The proposed microlenses arrays consist of deformable, elastomeric membranes that are supported by polymer-filled microchambers. Each membrane/microchamber assembly is converted into a solid microlens when the supporting UV–curable polymer is pressurized and cured. By modifying the microlens diameter (40-60 μm) and curing pressure (7.5-30 psi), we demonstrated that it is possible to fabricate microlenses with a wide range of effective focal lengths (100–400 μm) and numerical apertures (0.05-0.3). We obtained a maximum numerical aperture of 0.3 and transverse resolution of 2.8 μm for 60 μm diameter microlenses cured at 30 psi. These values were found to be in agreement with values obtained from opto-mechanical simulations. We envision the use of these high numerical microlenses arrays in optical applications where light collection efficiency is important.

© 2009 OSA

1. Introduction

Microlenses are used in optical communication [1

1. F. B. McCormick, F. A. P. Tooley, T. J. Cloonan, J. M. Sasian, H. S. Hinton, K. O. Mersereau, and A. Y. Feldblum, “Optical interconnections using microlens arrays,” Opt. Quantum Electron. 24(4), S465–S477 (1992). [CrossRef]

,2

2. H. Hamam, “A two-way optical interconnection network using a single mode fiber array,” Opt. Commun. 150(1-6), 270–276 (1998). [CrossRef]

], displays [3

3. Y.-P. Huang, H.-P. D. Shieh, and S.-T. Wu, “Applications of Multidirectional Asymmetrical Microlens-Array Light-Control Films on Reflective Liquid-Crystal Displays for Image Quality Enhancement,” Appl. Opt. 43(18), 3656–3663 (2004). [CrossRef] [PubMed]

,4

4. H. Peng, Y. L. Ho, X.-J. Yu, M. Wong, and H.-S. Kwok, “Coupling Efficiency Enhancement in Organic Light-Emitting Devices Using Microlens Array-Theory and Experiment,” J. Display Technol. 1(2), 278–282 (2005). [CrossRef]

], optical sensors [5

5. K. Cai-Jun, Y. Xin-Jian, L. Jian-Jun, and C. Si-Hai, “Fabrication, Testing and Integration Technologies of Polymer Microlens for Pt/Si Schottky-Barrier Infrared Charge Coupled Device Applications,” Chin. Phys. Lett. 22(1), 135–138 (2005). [CrossRef]

,6

6. B. Javidi, S.-H. Hong, and O. Matoba, “Multidimensional optical sensor and imaging system,” Appl. Opt. 45(13), 2986–2994 (2006). [CrossRef] [PubMed]

], photolithographic systems [7

7. M.-H. Wu, K. E. Paul, J. Yang, and G. M. Whitesides, “Fabrication of frequency-selective surfaces using microlens projection photolithography,” Appl. Phys. Lett. 80(19), 3500–3502 (2002). [CrossRef]

,8

8. K. F. Chan, Z. Feng, R. Yang, A. Ishikawa, and W. Mei, “High-resolution maskless lithography,” J. Microlitho. Microfab. Microsyst. 2(4), 331–339 (2003). [CrossRef]

] as well as in biomedical imaging applications [9

9. M. Eisner, N. Lindlein, and J. Schwider, “Confocal microscopy with a refractive microlens-pinhole array,” Opt. Lett. 23(10), 748–749 (1998). [CrossRef]

11

11. K. Aljasem, D. Mader, A. Werber, H. Zappe, and S. Reichelt, “Pneumatically-actuated tunable microlens for endoscopic optical coherence tomography Transducers 2007 - 2007 International Solid-State Sensors Actuators and Microsystems Conference,” (2007), pp. 2557–2560.

]. Recent advances in micromachining technology led to the development of a variety of microlens microfabrication approaches including photoresist-reflow and transfer methods [12

12. M. H. Wu and G. M. Whitesides, “Fabrication of two-dimensional arrays of microlenses and their applications in photolithography,” J. Micromech. Microeng. 12(6), 747–758 (2002). [CrossRef]

,13

13. D. A. Fletcher, K. B. Crozier, K. W. Guarini, S. C. Minne, G. S. Kino, C. F. Quate, and K. E. Goodson, “Microfabricated silicon solid immersion lens,” Microelectromechanical Systems, Journalism 10, 450–459 (2001). [CrossRef]

], ink jet processes of UV curable polymers [14

14. S. Biehl, R. Danzebrink, P. Oliveira, and M. A. Aegerter, “Refractive Microlens Fabrication by Ink-Jet Process,” J. Sol-Gel Sci. Technol. 13(1/3), 177–182 (1998). [CrossRef]

], hot embossing techniques [15

15. N. S. Ong, Y. H. Koh, and Y. Q. Fu, “Microlens array produced using hot embossing process,” Microelectron. Eng. 60(3-4), 365–379 (2002). [CrossRef]

], micromolding using silicon substrates [16

16. S.-d Moon, S. Kang, and J.-U. Bu, “Fabrication of polymeric microlens of hemispherical shape using micromolding,” Opt. Eng. 41(9), 2267–2270 (2002). [CrossRef]

,17

17. J. Albero, L. Nieradko, C. Gorecki, H. Ottevaere, V. Gomez, H. Thienpont, J. Pietarinen, B. Päivänranta, and N. Passilly, “Fabrication of spherical microlenses by a combination of isotropic wet etching of silicon and molding techniques,” Opt. Express 17(8), 6283–6292 (2009). [CrossRef] [PubMed]

], soft lithography-based replication processes by molding various materials against rigid or elastomeric molds [18

18. P. Nussbaum, I. Philipoussis, A. Husser, and H. P. Herzig, “Simple technique for replication of micro-optical elements,” Opt. Eng. 37(6), 1804–1808 (1998). [CrossRef]

20

20. X. C. Yuan, W. X. Yu, M. He, J. Bu, W. C. Cheong, H. B. Niu, and X. Peng, “Soft-lithography-enabled fabrication of large numerical aperture refractive microlens array in hybrid SiO[sub 2]–TiO[sub 2] sol-gel glass,” Appl. Phys. Lett. 86(11), 114102–114103 (2005). [CrossRef]

]. Photoresist-reflow methods rely on the surface tension of the photoresist to form a smooth microlens surface. These methods require accurate control of the microfabrication parameters (photoresist thickness, hydrophobicity) and produce microlenses with small numerical aperture (NA) due to the small aspect ratio (thickness vs diameter) of the patterned photoresist. Ink-jet methods are serial processes that require an elaborate experimental setup for accurately dispensing small drops of the optical material onto a rigid substrate. The properties of ink-jet processed microlenses depend on the rheological properties of the dispensing material (viscosity, surface tension), making the fabrication of microlenses with small diameters (<100 µm) and large aspect ratio (to achieve high NA), a challenging task. Hot embossing techniques produce microlenses with rough surfaces and suffer from post-embossing shrinkage. Fabrication of silicon molds requires isotropic wet etching of silicon and deposition of silicon oxide/nitride as the etchant mask. These fabrication steps are extremely time consuming and expensive. Soft lithography-based replication processes provide low-cost microlens arrays whose properties depend on the quality of the master mold. These processes take advantage of the excellent optical [21

21. S. Kopetz, D. Cai, E. Rabe, and A. Neyer, “PDMS-based optical waveguide layer for integration in electrical-optical circuit boards,” AEU, Int. J. Electron. Commun. 61(3), 163–167 (2007). [CrossRef]

] and mechanical [22

22. F. Schneider, T. Fellner, J. Wilde, and U. Wallrabe, “Mechanical properties of silicones for MEMS,” J. Micromech. Microeng. 18(6), 065008 (2008). [CrossRef]

,23

23. K. L. Mills, X. Y. Zhu, S. C. Takayama, and M. D. Thouless, “The mechanical properties of a surface-modified layer on poly(dimethylsiloxane),” J. Mater. Res. 23(1), 37–48 (2008). [CrossRef] [PubMed]

] properties of the polydimethylsiloxane (PDMS) elastomer. PDMS has low elastic modulus, minimum light absorption in the visible spectrum and extremely low autofluorescence, it is therefore an ideal optical material for microlens molding and replication processes [24

24. X. Yu, Z. Wang, and Y. Han, “Microlenses fabricated by discontinuous dewetting and soft lithography,” Microelectron. Eng. 85(9), 1878–1881 (2008). [CrossRef]

,25

25. N. Chronis, G. Liu, K.-H. Jeong, and L. Lee, “Tunable liquid-filled microlens array integrated with microfluidic network,” Opt. Express 11(19), 2370–2378 (2003). [CrossRef] [PubMed]

].

In this work, we describe a novel array of high numerical aperture (NA~0.3), polymer-based, solid microlenses. Key element is the microfabrication of an ‘inflatable’, polymer-filled PDMS mold that consists of an array of circular microchambers enclosed by flexible membranes. When the mold is inflated, the microchamber/membrane array is converted into a microlens array. Using a UV-curable polymer as a filling medium, the microlens arrays can be subsequently solidified when the desired focal length is obtained. The proposed approach has three major advantages: a) the microlens focal length can be adjusted by regulating the pressure applied to the filling polymer during curing, b) the use of PDMS as the mold material enables large membrane deflections that result in microlenses with high NA, and c) the optical properties (e.g. index of refraction) of the microlenses can be varied as there is a large collection of commercially available curable materials. The novel microfabrication process is particularly useful for obtaining small diameter (<100 µm) microlens arrays with high light collection efficiency (high NA). We envision such microlens arrays to play a key role in various lab-on-chip detection systems for imaging micron-size objects (cells, viruses, etc.) [26

26. X. Cheng, A. Gupta, C. Chen, R. G. Tompkins, W. Rodriguez, and M. Toner, “Enhancing the performance of a point-of-care CD4+ T-cell counting microchip through monocyte depletion for HIV/AIDS diagnostics,” Lab Chip 9(10), 1357–1364 (2009). [CrossRef] [PubMed]

,27

27. D. S. Reichmuth, S. K. Wang, L. M. Barrett, D. J. Throckmorton, W. Einfeld, and A. K. Singh, “Rapid microchip-based electrophoretic immunoassays for the detection of swine influenza virus,” Lab Chip 8(8), 1319–1324 (2008). [CrossRef] [PubMed]

].

2. Design and microfabrication of the microlens array

The microfabrication process of the microlens array involves (Fig. 2
Fig. 2 The microfabrication process of the microlens device.
): i) the fabrication of two SU-8 master molds (for each of the two PDMS layers) using standard photolithographic processes, ii) the fabrication of the inflatable PDMS mold from the two SU-8 master molds using soft-lithography, and iii) the injection and curing of the UV-curable polymer into the inflatable PDMS mold.

The fabrication of the first SU-8 master mold (corresponding to the microlens PDMS layer) is performed in a two-step photolithographic process [25

25. N. Chronis, G. Liu, K.-H. Jeong, and L. Lee, “Tunable liquid-filled microlens array integrated with microfluidic network,” Opt. Express 11(19), 2370–2378 (2003). [CrossRef] [PubMed]

]. A 13 µm thick film of SU-8 photoresist is patterned on a silicon wafer to define the microfluidic network, followed by the patterning of a second, 40 µm thick, SU-8 film to form the circular microchambers. A 20:1 PDMS mixture is then spun at 1750 rpm and cured on the two-step SU-8 mold to obtain a ~52 μm thick PDMS layer. Such a PDMS composition results in a layer of low elastic modulus (<1MPa [28

28. E. Wilder, M. Fasolka, S. Guo, C. Stafford, and S. Lin-Gibson, “Measuring the modulus of soft polymer networks via a buckling-based metrology,” Macromolecules 39(12), 4138–4143 (2006). [CrossRef]

]), which is necessary to achieve large membrane deflections, and therefore high NA microlenses. By accurately measuring the thickness of SU-8 circular microchambers and the PDMS microlens layer, we estimated the PDMS membrane thickness to be 12.1 ± 1.4 µm (16 measurements were taken from different runs using a profilometer).

The second SU-8 mold (corresponding to the support PDMS layer) is fabricated by patterning a 50 μm thick SU-8 film. The design incorporates an array of 200 μm x 200 μm square microwells, each one sitting on top of a single microlens. A thick (~1 mm) PDMS layer (the support layer) is subsequently cured, peeled off and air-plasma bonded (90 W, 35 sec) to the PDMS microlens layer. Finally, the two-layer PDMS assembly is detached from the first SU-8 mold and bonded to a 175 µm thick glass slide to create the sealed inflatable PDMS mold. The use of the support PDMS layer is critical for peeling off the thin microlens layer as it provides structural integrity and eliminates the microlens membranes from tearing apart. The inflatable PDMS mold is then filled with a UV curable polymer, pressurized and cured.

3. Results

To obtain the optical properties of the proposed high NA, solid microlenses, we characterized an array of 40 μm and 60 μm diameter microlenses following the microfabrication process described above. A high index of refraction UV-curable polymer (Norland 60, index of refraction n = 1.56) was injected into the inflatable PDMS mold and cured (10 min, at 365 nm (Entela UV lamp)) under constant pressure to ensure uniform deflection across the membrane array. The effective focal length (EFL) of the cured microlens device was measured for a range of curing pressures (7.5-30 psi) using a custom-made optical setup (see Fig. 8
Fig. 8 Schematic of the experimental setup for characterizing the optical propereties (effective focal length (EFL), point spread function (PSF)) of the microlens device. A laser (532 nm) was used to obtain the EFL while a white light source was used to obtain the PSF.
). The EFL corresponds to the distance between the top surface of a microlens and the best-focused image formed when passing a laser beam (532 nm) through the microlens. We followed a 2-step sequence to determine the EFL: i) the top surface of a microlens was visually identified and set as the reference plane (plane I), and ii) the plane containing the image of the focused laser beam was brought into the imaging plane of the microscope objective (plane II) by vertically moving the microlens. The distance between these two planes corresponded to the EFL of the microlens. Plane II was identified by extracting the plane of the maximum light intensity through image analysis software (Metamorph®).

The EFL of both the 40 μm and 60 μm diameter microlenses exhibited a strong dependence on the curing pressure (Fig. 3
Fig. 3 Effective Focal Length (EFL) versus curing pressure for (A) 40 µm and (B) 60 µm diameter microlenses. Experimental and opto-mechanically simulated EFL values are obtained for a pressure range of 7.5-30 psi. Each data point is the average of 16 measurements from 2 microlens devices (8 microlenses/device). The measured EFL values have standard deviation of 4.31% and 4.10% for 40 µm and 60 µm diameter microlenses respectively.
). EFL changes up to ~200% were measured over the entire pressure range. For low pressures (< 7.5 psi), the circular membranes undergo small (sub-µm) deflection. These microlenses weakly focus the laser beam resulting in a large depth of focus (several mm’s) that makes it difficult to assign a single value to the focal length. High pressures (>30 psi) resulted in breakage of the microlens membranes. Experimental EFL data were also compared with opto-mechanical simulation values. The pressure-dependent microlens profiles were simulated using commercially available finite element analysis software (ANSYS). The deflected membrane profiles were imported into an optical design software (OSLO LT) to determine the corresponding EFLs. Simulations were performed for a membrane thickness of 10.7 µm and 13.5 µm to capture the upper and lower limit of the corresponding measured values. The elastic modulus and Poisson ratio of the PDMS membrane was set to 3 MPa [23

23. K. L. Mills, X. Y. Zhu, S. C. Takayama, and M. D. Thouless, “The mechanical properties of a surface-modified layer on poly(dimethylsiloxane),” J. Mater. Res. 23(1), 37–48 (2008). [CrossRef] [PubMed]

] and 0.49 respectively. A refractive index of 1.41, 1.56 and 1.51 was used for the PDMS, the UV-curable polymer and the glass slide respectively. The experimental EFL values for both the 40 µm and 60 µm diameter microlenses were within the simulated upper and lower EFL bounds.

The numerical aperture (NA) of the microlenses (for use in air) was estimated from [29

29. M. Oikawa, H. Nemoto, K. Hamanaka, and E. Okuda, “High numerical aperture planar microlens with swelled structure,” Appl. Opt. 29(28), 4077–4080 (1990). [CrossRef] [PubMed]

] where a and F represent the microlens radius and EFL respectively.
NA=sin[tan1(aF)]
(1)
Based on the EFL values reported above, a maximum NA of ~0.17 and ~0.3 was obtained for 40 µm and 60 µm diameter microlenses cured at 30 psi respectively (Fig. 4
Fig. 4 Numerical Aperture (NA) versus curing pressure for (A) 40 µm and (B) 60 µm diameter microlenses. NA values are calculated using the EFL values presented in Fig. 3. 60 µm diameter microlenses (with lower EFL) have higher NA than 40 µm diameter microlenses (with higher EFL).
). The light loss through a single microlens was obtained by measuring the decrease in the light intensity of white light passing through the microlens device. For microlenses cured at 30 psi, the light loss was found to be 0.86 dB (18%).

To characterize the surface profile of a single microlens from the array (Fig. 5(a)
Fig. 5 (A) Surface profile of a 60 µm diameter microlens (cured at 30 psi) obtained using a white light interferometer. (B) A 2D microlens profile measured along its midline (cross section A-A’ in A). The profile has been curve fitted with a sixth order polynomial.
we used a white-light interferometer (Veeco NT9100). We obtain direct optical access to the array by immersing the microlens device in acetone for 10 minutes and manually detaching the support layer. A 25 nm thick, gold layer was subsequently evaporated on the microlens surface to acquire a clean interferometric image.

The PDMS membrane profile of a 60 µm diameter solid microlens showed a maximum deflection of ~6 µm when cured at 30 psi, corresponding to a microlens with an aspect ratio of 10:1 (Fig. 5(b)). The microlens profile (Fig. 5) allows us to determine the minimum microlens pitch (microlens center-to-center distance). For 60 µm microlenses cured at 30 psi, the membrane deformation extends up to ~40 µm from the center of the microlens, and therefore they can be placed as close as 80 µm apart. Microlenses of smaller diameter or cured at lower pressures will have smaller pitch. The depth of focus and the transverse resolution of such a microlens (60 µm diameter solid microlens, cured at 30 psi) were calculated by measuring its axial intensity profile and point spread function (PSF) respectively (Fig. 6
Fig. 6 (A) Relative intensity (maximum intensity for a given axial plane normalized with the maximum intensity measured at the best focused plane) along the optical axis of a 60 µm diameter microlens as a function of the distance from the best-focused plane. The pictures depict the focused laser beam as imaged at the focal plane and at two out-of focus planes. Scale bar, 10 µm. (B) The PSF of the same microlens represented as the in-plane intensity distribution (I[x,y] – I0) at the best-focused plane, normalized with respect to the background intensity I0.
). To obtain the axial intensity profile, we extracted the maximum intensity from imaging planes above and below the best focused plane in an optical setup similar to the one used for obtaining the EFL. The theoretical value of the depth of focus (DOF) was estimated to be ~6 µm [30

30. S. Inoue, and K. R. Spring, Video Microscopy The Fundamentals (Plenum Publishing Corporation, New York, 1997).

]. Within that distance, the maximum intensity dropped by 5%. To obtain the PSF of the microlens, we measured the background-corrected in-plane intensity distribution (I[x,y]- I0) at the best-focused plane and normalized that value by the background intensity (I0). The transverse resolution of the microlens, defined as the full width at half maximum (FWHM) of the PSF [31

31. J. R. Polimeni, D. Granquist-Fraser, R. J. Wood, and E. L. Schwartz, “Physical limits to spatial resolution of optical recording: clarifying the spatial structure of cortical hypercolumns,” Proc. Natl. Acad. Sci. U.S.A. 102(11), 4158–4163 (2005). [CrossRef] [PubMed]

], was found to be 2.8 µm.

Finally, we demonstrated that it is possible to use the fabricated microlenses to magnify and image micron-size resolution features patterned on a chrome mask (Fig. 7
Fig. 7 Micron-size resolution patterns imaged: (A) without using a microlens, and (B) through a 60 µm diameter microlens (NA ~0.3). Equally-spaced, 3 µm wide lines are magnified by a factor of ~2 and are clearly visible. Scale bar, 50 µm (A) and 25 µm (B).
). 3 µm wide line features were clearly visible after placing the microlens device between the chrome mask and a 1x microscope objective (we used Olympus SZX16 stereo microscope). Minimum distortion was observed at the center region of the microlens.

4. Conclusions

We demonstrate a novel microfabrication process for obtaining polymer-based, planar microlens arrays with high numerical aperture. The microlens array comprises of polymer-filled circular microchambers enclosed by deformable, elastomeric membranes. Each microchamber is transformed into a solid microlens when the UV-curable polymer is pressurized and cured. By varying the curing pressure (7.5-30 psi), microlenses with a wide range of EFL (100-400 µm) and NA (0.05-0.3) have been fabricated from 40 µm and 60 µm diameter membranes. We have obtained a NA of 0.3 and transverse resolution of 2.8 µm for 60 µm diameter microlenses (cured at 30 psi). Smaller diameter (40 µm) microlenses have lower NA than larger diameter (60 µm) microlenses for similar curing conditions. We envision the use of these high NA planar microlenses in lab-on-chip detection systems for imaging micron-sized biological specimens. Moreover, the proposed microlens design is compatible with a wide variety of curable polymers and therefore it can be used to obtain microlenses with different optical properties.

Appendix

Acknowledgements

We would like to thank Tom Stout (Veeco Corporation, AZ) for performing the white light interferometric measurements. We also thank Suhasa Kodandaramaiah and Mostafa Rezaie for useful discussions. We thank Peter Diehr for helping with the light loss measurements. All the devices were fabricated at the Lurie Nanofabrication Facility at the University of Michigan.

References and links

1.

F. B. McCormick, F. A. P. Tooley, T. J. Cloonan, J. M. Sasian, H. S. Hinton, K. O. Mersereau, and A. Y. Feldblum, “Optical interconnections using microlens arrays,” Opt. Quantum Electron. 24(4), S465–S477 (1992). [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.

Y.-P. Huang, H.-P. D. Shieh, and S.-T. Wu, “Applications of Multidirectional Asymmetrical Microlens-Array Light-Control Films on Reflective Liquid-Crystal Displays for Image Quality Enhancement,” Appl. Opt. 43(18), 3656–3663 (2004). [CrossRef] [PubMed]

4.

H. Peng, Y. L. Ho, X.-J. Yu, M. Wong, and H.-S. Kwok, “Coupling Efficiency Enhancement in Organic Light-Emitting Devices Using Microlens Array-Theory and Experiment,” J. Display Technol. 1(2), 278–282 (2005). [CrossRef]

5.

K. Cai-Jun, Y. Xin-Jian, L. Jian-Jun, and C. Si-Hai, “Fabrication, Testing and Integration Technologies of Polymer Microlens for Pt/Si Schottky-Barrier Infrared Charge Coupled Device Applications,” Chin. Phys. Lett. 22(1), 135–138 (2005). [CrossRef]

6.

B. Javidi, S.-H. Hong, and O. Matoba, “Multidimensional optical sensor and imaging system,” Appl. Opt. 45(13), 2986–2994 (2006). [CrossRef] [PubMed]

7.

M.-H. Wu, K. E. Paul, J. Yang, and G. M. Whitesides, “Fabrication of frequency-selective surfaces using microlens projection photolithography,” Appl. Phys. Lett. 80(19), 3500–3502 (2002). [CrossRef]

8.

K. F. Chan, Z. Feng, R. Yang, A. Ishikawa, and W. Mei, “High-resolution maskless lithography,” J. Microlitho. Microfab. Microsyst. 2(4), 331–339 (2003). [CrossRef]

9.

M. Eisner, N. Lindlein, and J. Schwider, “Confocal microscopy with a refractive microlens-pinhole array,” Opt. Lett. 23(10), 748–749 (1998). [CrossRef]

10.

J. C. Roulet, R. Volkel, H. P. Herzig, E. Verpoorte, N. F. de Rooij, and R. Dandliker, “Fabrication of multilayer systems combining microfluidic and microoptical elements for fluorescence detection,” J. Microelectromech. Syst. 10(4), 482–491 (2001). [CrossRef]

11.

K. Aljasem, D. Mader, A. Werber, H. Zappe, and S. Reichelt, “Pneumatically-actuated tunable microlens for endoscopic optical coherence tomography Transducers 2007 - 2007 International Solid-State Sensors Actuators and Microsystems Conference,” (2007), pp. 2557–2560.

12.

M. H. Wu and G. M. Whitesides, “Fabrication of two-dimensional arrays of microlenses and their applications in photolithography,” J. Micromech. Microeng. 12(6), 747–758 (2002). [CrossRef]

13.

D. A. Fletcher, K. B. Crozier, K. W. Guarini, S. C. Minne, G. S. Kino, C. F. Quate, and K. E. Goodson, “Microfabricated silicon solid immersion lens,” Microelectromechanical Systems, Journalism 10, 450–459 (2001). [CrossRef]

14.

S. Biehl, R. Danzebrink, P. Oliveira, and M. A. Aegerter, “Refractive Microlens Fabrication by Ink-Jet Process,” J. Sol-Gel Sci. Technol. 13(1/3), 177–182 (1998). [CrossRef]

15.

N. S. Ong, Y. H. Koh, and Y. Q. Fu, “Microlens array produced using hot embossing process,” Microelectron. Eng. 60(3-4), 365–379 (2002). [CrossRef]

16.

S.-d Moon, S. Kang, and J.-U. Bu, “Fabrication of polymeric microlens of hemispherical shape using micromolding,” Opt. Eng. 41(9), 2267–2270 (2002). [CrossRef]

17.

J. Albero, L. Nieradko, C. Gorecki, H. Ottevaere, V. Gomez, H. Thienpont, J. Pietarinen, B. Päivänranta, and N. Passilly, “Fabrication of spherical microlenses by a combination of isotropic wet etching of silicon and molding techniques,” Opt. Express 17(8), 6283–6292 (2009). [CrossRef] [PubMed]

18.

P. Nussbaum, I. Philipoussis, A. Husser, and H. P. Herzig, “Simple technique for replication of micro-optical elements,” Opt. Eng. 37(6), 1804–1808 (1998). [CrossRef]

19.

M. V. Kunnavakkam, F. M. Houlihan, M. Schlax, J. A. Liddle, P. Kolodner, O. Nalamasu, and J. A. Rogers, “Low-cost, low-loss microlens arrays fabricated by soft-lithography replication process,” Appl. Phys. Lett. 82(8), 1152–1154 (2003). [CrossRef]

20.

X. C. Yuan, W. X. Yu, M. He, J. Bu, W. C. Cheong, H. B. Niu, and X. Peng, “Soft-lithography-enabled fabrication of large numerical aperture refractive microlens array in hybrid SiO[sub 2]–TiO[sub 2] sol-gel glass,” Appl. Phys. Lett. 86(11), 114102–114103 (2005). [CrossRef]

21.

S. Kopetz, D. Cai, E. Rabe, and A. Neyer, “PDMS-based optical waveguide layer for integration in electrical-optical circuit boards,” AEU, Int. J. Electron. Commun. 61(3), 163–167 (2007). [CrossRef]

22.

F. Schneider, T. Fellner, J. Wilde, and U. Wallrabe, “Mechanical properties of silicones for MEMS,” J. Micromech. Microeng. 18(6), 065008 (2008). [CrossRef]

23.

K. L. Mills, X. Y. Zhu, S. C. Takayama, and M. D. Thouless, “The mechanical properties of a surface-modified layer on poly(dimethylsiloxane),” J. Mater. Res. 23(1), 37–48 (2008). [CrossRef] [PubMed]

24.

X. Yu, Z. Wang, and Y. Han, “Microlenses fabricated by discontinuous dewetting and soft lithography,” Microelectron. Eng. 85(9), 1878–1881 (2008). [CrossRef]

25.

N. Chronis, G. Liu, K.-H. Jeong, and L. Lee, “Tunable liquid-filled microlens array integrated with microfluidic network,” Opt. Express 11(19), 2370–2378 (2003). [CrossRef] [PubMed]

26.

X. Cheng, A. Gupta, C. Chen, R. G. Tompkins, W. Rodriguez, and M. Toner, “Enhancing the performance of a point-of-care CD4+ T-cell counting microchip through monocyte depletion for HIV/AIDS diagnostics,” Lab Chip 9(10), 1357–1364 (2009). [CrossRef] [PubMed]

27.

D. S. Reichmuth, S. K. Wang, L. M. Barrett, D. J. Throckmorton, W. Einfeld, and A. K. Singh, “Rapid microchip-based electrophoretic immunoassays for the detection of swine influenza virus,” Lab Chip 8(8), 1319–1324 (2008). [CrossRef] [PubMed]

28.

E. Wilder, M. Fasolka, S. Guo, C. Stafford, and S. Lin-Gibson, “Measuring the modulus of soft polymer networks via a buckling-based metrology,” Macromolecules 39(12), 4138–4143 (2006). [CrossRef]

29.

M. Oikawa, H. Nemoto, K. Hamanaka, and E. Okuda, “High numerical aperture planar microlens with swelled structure,” Appl. Opt. 29(28), 4077–4080 (1990). [CrossRef] [PubMed]

30.

S. Inoue, and K. R. Spring, Video Microscopy The Fundamentals (Plenum Publishing Corporation, New York, 1997).

31.

J. R. Polimeni, D. Granquist-Fraser, R. J. Wood, and E. L. Schwartz, “Physical limits to spatial resolution of optical recording: clarifying the spatial structure of cortical hypercolumns,” Proc. Natl. Acad. Sci. U.S.A. 102(11), 4158–4163 (2005). [CrossRef] [PubMed]

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

ToC Category:
Optical Design and Fabrication

History
Original Manuscript: July 29, 2009
Revised Manuscript: October 6, 2009
Manuscript Accepted: October 6, 2009
Published: October 19, 2009

Virtual Issues
Vol. 4, Iss. 12 Virtual Journal for Biomedical Optics

Citation
Anurag Tripathi, Trushal Vijaykumar Chokshi, and Nikos Chronis, "A high numerical aperture, polymer-based, planar microlens array," Opt. Express 17, 19908-19918 (2009)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-17-22-19908


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References

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  28. E. Wilder, M. Fasolka, S. Guo, C. Stafford, and S. Lin-Gibson, “Measuring the modulus of soft polymer networks via a buckling-based metrology,” Macromolecules 39(12), 4138–4143 (2006). [CrossRef]
  29. M. Oikawa, H. Nemoto, K. Hamanaka, and E. Okuda, “High numerical aperture planar microlens with swelled structure,” Appl. Opt. 29(28), 4077–4080 (1990). [CrossRef] [PubMed]
  30. S. Inoue, and K. R. Spring, Video Microscopy The Fundamentals (Plenum Publishing Corporation, New York, 1997).
  31. J. R. Polimeni, D. Granquist-Fraser, R. J. Wood, and E. L. Schwartz, “Physical limits to spatial resolution of optical recording: clarifying the spatial structure of cortical hypercolumns,” Proc. Natl. Acad. Sci. U.S.A. 102(11), 4158–4163 (2005). [CrossRef] [PubMed]

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