Sub-micrometer direct laser writing using an optimized binary-amplitude zone plate lens.

Direct laser writing (DLW) is a versatile and reliable lithography method widely used in many micro and nano fabrication areas. However, the resolution of DLW is limited by the optical diffraction limit. Many methods have been proposed to improve the lithography resolution, but with either high cost or increasing the complexity of the system. Here, we propose a high numerical aperture binary-amplitude-type zone plate lens that can achieve a sub-wavelength focal spot with a large depth of focus and long working distance. The critical dimension of such a lens is set at micrometer scale for ease of fabrication. By integrating the as-designed planar lens into a DLW system, we experimentally demonstrate less than 300 nm lithography resolution with exposure depth larger than 500 nm. Our results show the possibility of writing sub-micrometer scale structures with the integration of a planar lens into the DLW system, which enables miniaturization and compactness of lithography instruments for many applications.