Advances in lithography on non-planar surfaces

Laser lithography on non-planar surfaces is a technology which has been investigated at the IOF for more than 5 years. A special lithography system was developed for this purpose, allowing to structure spherical substrates with radii of curvature of ≥ 10 mm . Binary or gray scale exposures with a minimum feature size of about 1 μm covering a field of view (FOV) of ±10° are possible in standard operation mode. We present two approaches which will overcome certain disadvantages of the writing strategy at the expense of an increase in minimum feature size. First, we propose an exposure strategy which allows for an extension of the FOV to up to ±20° with satisfying accuracy of the structures. The according data is decomposed into concentric circles and projected onto the sphere so the exposure is not perpendiculary to the surface any more. Here we make use of the ability of our laser lithography system to adapt to a wide range of possible substrate thicknesses. On substrates with a flat edge, even layer to layer alignment for the curved structures is possible with an accuracy of ≤ 2μm. Example structures as well as prospects and limitations of this exposure strategy are presented. Secondly, similar to planar lithography, the use of a mask to produce multiple copies of a master sample is possible. Evidently the spherical mask needs to have the opposite radius of curvature than the desired substrate, and additional problems arising from the curved geometry have to be taken into consideration. Despite a lowered contrast due to back reflections and a varying distance between mask and substrate exposure results of sufficient quality are achieved with the help of an adapted aperture.