Design and fabrication of a tip-on-aperture probe for resolution enhancement of optical patterning

• We proposed a plasmonic probe with a specially shaped tip on an aperture NSOM probe.
• A right triangle tip was selected as the optimal tip shape for optical resolution enhancement.
• The designed probe was effectively fabricated by FIB milling at different angles.
• Isolated line segments as narrow as 50 nm were produced on photoresist film.

A variety of near-field scanning optical microscope (NSOM)-based lithography techniques have been developed for different applications through the use of a sub-wavelength aperture probe. The light transmission efficiency of an NSOM probe decreases markedly as the aperture diameter decreases. Apertureless NSOM yields a much higher resolution by concentrating the light field near the tip apex. However, far-field illumination by a focused laser beam deteriorates the resolution in optical patterning process. In this report, a tip-on-aperture (TOA) probe was fabricated to achieve high pattern resolution beyond that obtained from conventional NSOM probes. The shape of the probes was designed using the results of numerical analysis based on a finite-difference time-domain (FDTD) algorithm. The results showed that a probe with a triangular shape tip confined the electromagnetic energy with a 6.7 times larger intensity than the value obtained from a probe with a cylindrical shape tip. The TOA probes were fabricated by generating a triangular tip on a metal-coated NSOM probe using a focused ion beam (FIB) process. The resolution enhancement was experimentally demonstrated by the formation of the developed pattern on photo-resist.

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