Abstract
We report microstructures of SU-8 photo-sensitive polymer with high-aspect-ratio, which is defined as the ratio of height to in-plane feature size. The highest aspect ratio achieved in this work exceeds 250. A multi-layer and single-photon lithography approach is used in this work to expose SU-8 photoresist of thickness up to 100 μm. Here, multi-layer and time-lapsed writing is the key concept that enables nanometer localised controlled photo-induced polymerisation. We use a converging monochromatic laser beam of 405 nm wavelength with a controllable aperture. The reflection of the converging optics from the silicon substrate underneath is responsible for a trapezoidal edge profile of SU-8 microstructure. The reflection induced interfered point-spread-function and multi-layer-single-photon exposure helps to achieve sub-wavelength feature sizes. We obtained a 75 nm tip diameter on a pyramid shaped microstructure. The converging beam profile determines the number of multiple optical focal planes along the depth of field. These focal planes are scanned and exposed non-concurrently with varying energy dosage. It is notable that an un-automated height axis control is sufficient for this method. All of these contribute to realising super-high-aspect-ratio and 3D micro-/nanostructures using SU-8. Finally, we also address the critical problems of photoresist-based micro-/nanofabrication and their solutions.
Highlights
We report microstructures of SU-8 photo-sensitive polymer with high-aspect-ratio, which is defined as the ratio of height to in-plane feature size
Along with multi-layer and single-photon lithography technique, we demonstrate an interfered point spread function (PSF) that was generated using incident and reflected laser beam from silicon surface underneath the SU-8 film
For higher aspect-ratio than 200, we report a feature size of 75 nm at an atomic force microscopy (AFM) probe like sharp tip-end, which is applicable for near-field scanning optical microscopy (NSOM) of single molecules and nanoparticles[18,19,20]
Summary
We report microstructures of SU-8 photo-sensitive polymer with high-aspect-ratio, which is defined as the ratio of height to in-plane feature size. We introduce a multi-layer and single-photon lithography process to obtain high-aspect-ratio micro-/nanostructures. Along with multi-layer and single-photon lithography technique, we demonstrate an interfered point spread function (PSF) that was generated using incident and reflected laser beam from silicon surface underneath the SU-8 film. Such interfered PSF was used by Schnitzbauer et al.[14] in a modified 4-π fluorescence microscopy to achieve high axial resolution of Stefan Hell developed typical ‘Double-confocal scanning microscopy[15]. Considering photolithographic technique, such nanoscale feature size is very small These kinds of high-aspect-ratio micro-nanostructures are immediate interests of different www.nature.com/scientificreports/. For higher aspect-ratio than 200, we report a feature size of 75 nm at an AFM probe like sharp tip-end, which is applicable for near-field scanning optical microscopy (NSOM) of single molecules and nanoparticles[18,19,20]
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