The Materials Challenge in Diffraction‐Unlimited Direct‐Laser‐Writing Optical Lithography

Abstract

two-photon excitation fluoscence a) The concept of stimulated emission depletion (STED) fl uorescence microscopy introduced by Stefan W. Hell has recently revolutionized the fi eld of optical microscopy [ 1–4 ] and has led to important applications in biology. [ 5 , 6 ] Spectacular lateral spatial resolutions down to less than 10 nm have been reported. [ 7 ] Conceptually, in sharp contrast to what was commonly believed for decades, the “diffraction limit” of far-fi eld optical microscopy is no longer a limit. Obviously, it would be highly desirable to translate this tremendous resolution enhancement in laser-scanning optical microscopy into a corresponding advance in optical lithography , especially in two-photon direct-laser-writing (DLW). [ 8–10 ] DLW can be viewed as the three-dimensional analogue of two-dimensional (planar) electron-beam lithography. If arbitrary nanostructures could be made by STED-DLW with feature sizes of 10 nm in all three dimensions, a nanotechnologists’ dream would come true. However, the step from microscopy to lithography is not simple at all and particularly requires developing suitable photoresist material systems. [ 3 ] Early work on DLW inspired by STED has used one-photon absorption [ 11 ] or one-color schemes. [ 12 ] Both results have been highly encouraging, but they do not yet allow for complete fl exibility in three-dimensional lithography. In contrast to this work, [ 11 , 12 ] our approach is to directly beat the inter-system crossing rate of the photoinitiator molecules by effi cient stimulated emission. As we will argue and show by experiments below, commercially available photoinitiators and photoresists containing them are far from ideal for the purpose of STED-DLW. Thus, in this communication, we present a novel class of photoresist systems, an example of which is successfully applied to DLW. In regular DLW, femtosecond laser pulses are very tightly focused into the volume of a photoresist. [ 8–10 ] By means of twophoton absorption, only a tiny volume is suffi ciently exposed by the light. By computer-controlled scanning of the relative positions of focus and resist via piezoelectric actuators, almost