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Realization of refractive microoptics through grayscale lithographic patterning of photosensitive hybrid glass

Jeremy D. Rogers, Ari H. O. Kärkkäinen, Tomasz Tkaczyk, Juha T. Rantala, and Michael R. Descour

Open Access Open Access

Abstract

Refractive microlenses with more than 50 µm sag are fabricated using grayscale lithography. Mechanical assembly features are made simultaneously alongside the microlenses to facilitate high precision assembly of miniature optical systems. The microlens elements are formed using lithographic patterning of photosensitive hybrid sol-gel glass requiring no etch transfer to the substrate material. Grayscale lithography enables the straightforward patterning of aspheric lenses and arbitrary surfaces within the material depth. Lessons learned in the design of a grayscale photomask are described. Characterization of the fabricated lens elements is reported including lens shape, surface quality, and image quality of a complete assembled imaging system.

©2004 Optical Society of America

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Figures (9)
Fig. 1.
Fig. 1. Material response for hybrid glass showing surface height as a function of optical density for one set of exposure and development parameters.

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Fig. 2.
Fig. 2. Optical density versus radial distance dependence for the designed lens element grayscale photomask.

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Fig. 3.
Fig. 3. A schematic diagram of the optical components for the miniature imaging system (top) and the optical layout (bottom).

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Fig. 4.
Fig. 4. Comparison of two ways to encode lens shape in grayscale mask. The lens on the left has a uniform step height (uniform change in OD), but very narrow zones at the edge (varying zone size). The lens on the right has varying step heights, but uniform step widths making mask fabrication simpler and more reliable.

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Fig. 5.
Fig. 5. Comparison of a grayscale photomask written using annular zones (a-c) and a grayscale photomask written with pixels (d-f). Shown on the left (a, d) are schematics of the two techniques. In the center (b, e) are images from each photomask taken with a 10x microscope objective. On the right (c, f) are SEM images of the lens elements fabricated using the respective photomasks. Note the annular zone mask (top row) creates a radial spoke pattern visible in the mask (b) and in the fabricated lenses (c).

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Fig. 6.
Fig. 6. A scanning electron microscope image of the lens element surrounded by mechanical alignment features used for positioning the element in the complete system. The superimposed dotted-line represents the cuts that would be made by a dicing saw or scribe.

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Fig. 7.
Fig. 7. Profile of designed lens in red, actual measured profiles of fabricated lenses in green and blue, and a linearly scaled version of the designed profile in pink.

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Fig. 8.
Fig. 8. A schematic diagram of the optical components for the miniature imaging system (left) and an assembled prototype (right).

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Fig. 9.
Fig. 9. The central 100×100 µm area of an image formed by the 4M device of a 600 lp/mm grating (left). Right is an image of 10-30 µm diameter glass microspheres as imaged by 4M device in reflection mode.

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Tables (1)

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Table 1. Measurements of lens curvatures

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