Three-dimensional optical laser lithography beyond the diffraction limit

Direct laser writing has become a versatile and routine tool for the mask-free fabrication of polymer structures with lateral linewidths down to less than 100 nm. In contrast to its planar counterpart, electron-beam lithography, direct laser writing also allows for the making of three-dimensional structures. However, its spatial resolution has been restricted by diffraction. Clearly, linewidths and resolutions on the scale of few tens of nanometers and below are highly desirable for various applications in nanotechnology. In visible-light far-field fluorescence microscopy, the concept of stimulated emission depletion (STED) introduced in 1994 has led to spectacular record resolutions down to 5.6 nm in 2009. This review addresses approaches aiming at translating this success in optical microscopy to optical lithography. After explaining basic principles and limitations, possible depletion mechanisms and recent lithography experiments by various groups are summarized. Today, Abbe's diffraction barrier as well as the generalized two-photon Sparrow criterion have been broken in far-field optical lithography. For further future progress in resolution, the development of novel tailored photoresists in combination with attractive laser sources is of utmost importance.