Femtosecond-Laser-Coupled Near-Field Scanning Optical Microscopy Patterning Using Self-Assembled Monolayers

Abstract

Investigations into the fabrication of nanoscale patterns on gold substrates via direct scanning near-field lithography were carried out. A laser beam is scanned over the workpiece where the pattern formation is desired by near-field scanning optical microscopy (NSOM). A pipette-type nanoprobe having a 100 nm aperture at its apex is used with the NSOM and a frequency-doubled Ti:sapphire femtosecond laser at a wavelength of 395 nm as the illumination source to prevent pulse dispersion of the femtosecond laser pulse. Self-assembled monolayers (SAMs) formed by the adsorption of alkanethiols onto the gold substrate are employed as very thin photoresists. The process underlying photopatterning of SAMs on gold is well-known at the phenomenological level. Alkanethiolates formed by the adsorption of alkanethiols are oxidized to alkylsulfonates upon exposure to UV light in the presence of air. Specifically, it is known that deep-UV light of wavelength less than 200 nm is necessary for oxidation to occur. The results show that an ultrafast laser can replace a deep-UV laser source for the photopatterning of thin organic films. Femtosecond-laser photolithography may be applied to create the patterning of a surface chemical structure or a three-dimensional nanostructure by combination with suitable etching methods.