Maria Knoblich

This paper presents an advanced exposure system that, we believe, for the first time, enables annular lithography to create micro- and mesostructures on curved surfaces. The principle of ring creation is based on an axicon zoom system whereas the exposure tool exhibits several features that enhance its performance over previous models. Its optical design features a high numerical aperture, resulting in narrow ring widths and high resolution. Additionally, the ring diameter, which can be varied between 400 µm and 8 mm, remains constant along the optical axis due to telecentric imaging. Further functional components include an observation unit for alignment and monitoring, as well as an integrated autofocus system. In addition to determining the exposure ring width (∼ 10 µm), diffractive lenses on planar substrates as well as on a spherical lens with a radius of curvature of 68.8 mm were structured. The periods of the diffractive structure varied between 60 µm and 460 µm.

Annular lithography is a recently introduced, flexible technique that has been tailored to the fabrication of rotationally symmetric optical structures in the meso and micro range. The optical concept for the exposure tool is based on a combination of axicons with movable components that create a ring-shaped light distribution with variable diameter in the image plane. This contribution demonstrates for the first time the use of gray tone exposure in annular lithography to fabricate continuous relief structures, overcoming the previous limitation using binary structures. For the controlled exposure of the continuous relief structures, the sensitivity curve of the resist, the exposure dose decreasing with increasing ring diameter, and the exposure time have to be considered. A control and simulation tool is introduced to provide radius-dependent exposure data and, furthermore, to control and iteratively improve the fabricated structures. To demonstrate the gray tone capabilities, various diffractive elements as well as refractive spherical and aspherical elements with a maximum diameter of ~6 mm and a maximum height of 4 µm are shown as examples. Profile shape measurements of fabricated elements show good agreement with the expectations.

This paper presents concept, optical design, and the implementation of a novel, to the best of our knowledge, lithographic exposure tool for the fabrication of rotationally symmetric meso- and microscale optical structures using a variable ring-shaped light distribution. Compared to the conventional lithographic technique of direct writing in Cartesian coordinates, which is intrinsically suboptimal for the fabrication of rotationally symmetric optical structures, this approach allows for fast exposure and avoids disturbing stitching effects. The diameter of the exposure ring varies between 1.6 and 6.5 mm, and the ring width measures ∼ 75 µ m full width at half-maximum for all diameters. The basic capabilities of the exposure tool are demonstrated by the fabrication of exemplary meso- and microscale structures such as diffractive axicon elements, phase rings, Fresnel zone plates and zone plate arrays.