Three-dimensional laser lithography: Finer features faster

Summary form only given. Direct laser writing is a versatile technique for the fabrication of three-dimensional nano- and microstructures and has become recently commercially available [1]. Lateral feature sizes down to 100 nm are routinely achieved due to two-photon absorption and the threshold behaviour of the photoresists. The finite numerical aperture of the objective lens used in most systems (typically NA 1.4, oil immersion) results in features which are axially elongated by more than a factor of 2.6 for conventional photoresists. This aspect ratio between axial and lateral feature size hinders small lattice constants, e.g., in the fabrication of three-dimensional photonic structures [2]. Here, we employ a spatial light modulator to modify amplitude and phase distribution on the back focal plane of the objective lens. This allows (i) for the correction of remaining aberrations in the setup and (ii) for displaying so called shaded-ring-filters (SRF), clearly reducing the aspect ratio from 3.2 in the uncorrected case (not shown) down to 1.9 with aberration correction and SRF [3] (see Fig. 1 (a)). By focusing deep into the volume of photoresists with refractive index different from the immersion system, spherical aberrations usually counteract this correction already for small writing depths. To circumvent this, we employ an index matched photoresist, which is additionally used as immersion system. This so-called dip-in configuration allows photonic crystals with several hundred layers without any observable degradation in feature size. To reduce the overall writing time, two strategies are followed: 1. Writing with several voxels in parallel reduces writing times by about an order of magnitude. It is crucial to employ algorithms taking into account phase and amplitude simultaneously to achieve truly identical voxels. Common phase-only algorithms fail for voxel distances below a wavelength of light (see Fig. 1 (b)) due to interference effects. 2. Scanning the be- m instead of the sample reduces the writing time by another two orders of magnitude. The overall increase in writing speed, while maintaining the improved feature sizes, might open the door for industrial applications of three-dimensional laser lithography.