Direct laser writing is the central technology for the production of almost any complex micro- and nanostructures. For this purpose, ultrashort laser pulses are focused with objectives of high numerical aperture into a photosensitive material that is transparent for the laser wavelength itself. Due to the strong focusing, such high intensities are achieved in the focus that the material is locally polymerized or crosslinked via two- or multi-photon absorption. Precise focusing in the material by means of a three-axis piezo table or a fast galvo-mirror then allows the creation of arbitrarily connected, cross-linked areas in the photosensitive material. In a subsequent development step, the non-crosslinked areas are removed. This technology has been continuously improved over the last ten years and is now commercially available through our spin-off Nanoscribe GmbH.
The installation of spatial light modulators expands the system's possibilities, as they can correct aberrations and, for example, write with several focuses in parallel. The addition of a second laser beam also allows the implementation of STED-inspired lithography (STED stands for STimulated Emission Depletion).
The structures produced in this way can either be used directly for experiments or serve as templates for further process steps. For this purpose, we have developed two processes which either mould the structure directly or its inverse structure in other materials. Examples of materials are silicon or gold. In order to be able to deposit materials evenly in these templates in three dimensions, common techniques such as thermal evaporation or sputtering are not applicable. Atomic layer deposition, which can deposit a variety of materials with monolayer accuracy, chemical vapor phase epitaxy and electrochemical deposition of metals are used.
The quality of the structures is so high that they can be used as calibration standards in optical metrology. In order to further expand the range of applications, we are developing photoresist systems that allow biomaterials such as cellulose and chitin or metals such as silver and nickel to be written directly.