Fabrication of arrays of microlenses with controlled profiles using gray-scale microlens projection photolithography

This paper demonstrates the use of microlens projection lithography using gray-scale masks to fabricate arrays of microstructures in photoresist. In microlens projection lithography, an array of microlenses (diameter d = 1-1000 mum) reduces a common, centimeter-scale pattern in an illuminated mask to a corresponding pattern of micrometer-scale images in its image plane. The pattern of intensity projected by the array of microlenses depends on the shape and gray-level distribution of the pattern on the illuminated mask and on the shape and pattern of the lenses. The distribution of intensity in the microimages could be adjusted using gray-scale masks. After the recording of this intensity distribution in layers of photoresist and developing, the developed resist showed arrays of 3D microstructures over areas larger than 10 cm(2). We used these arrays of 3D microstructures as masters and cast transparent elastomer onto them to generate complementary replicas. For a specific microlens array and a fixed light source, the profile of the 3D microstructures generated by this method depended on the pattern on the illuminated mask and on the distance of the mask from the lens array. An appropriate mask with noncircular, gray-level patterns generated arrays of 3D microstructures that acted as lenses. This technique generates arrays of noncircular microlenses over areas larger than 10 cm(2) in a single exposure.